Cyanide and removal options from efuents in gold mining andmetallurgical processesNural Kuyucaka, Ata Akcilb,aGolder Associates Ltd., 32 Steacie Drive, Kanata, Ontario K2K 2A9, CanadabDepartment of Mining Engineering, Mineral Processing Division (Mineral-Metal Recovery and Recycling Research Group), Suleyman Demirel University, Isparta TR32260, Turkeyarti cle i nfoArticle history:Received 18 April 2013Accepted 27 May 2013Available online 28 June 2013Keywords:CyanideHydrometallurgyLeachingProcess efuentsWastewater treatmentabstractCyanide has been widely used as an essential raw material in several industries including textile, plastics,paints, photography, electroplating, agriculture, food, medicineandmining/metallurgy. Becauseofitshighafnityforgoldandsilver, cyanideisabletoselectivelyleachthesemetalsfromores. Cyanideand cyanide compounds in wastewater streams are regulated. Residues and wastewater streams contain-ing cyanide compounds have to be treated to reduce the concentration of total cyanide and free cyanidebelow the regulated limits.Natural degradation reactions can render cyanide non-toxic, resulting in carbon dioxide and nitrogencompounds. Thesenatural reactionshavebeenutilisedbytheminingindustryasthemostcommonmeans of attenuating cyanide. However, the rate of natural degradation is largely dependent on environ-mental conditions and may not produce an efuent of desirable quality in all cases year round. Technol-ogies that include chemical, biological, electrochemical and photochemical methods have beendeveloped to remove cyanide and cyanide compounds to below the regulated limits in wastewaters. Thispaper discusses commercially available and emerging methods for removing cyanide fromwastestreams, particularlyfromtailingsandtailingsreclaimwatersthataregeneratedinthegoldminingprocesses. 2013 Elsevier Ltd. All rights reserved.Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.1. Cyanide in daily life. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.2. Solution chemistry of cyanide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3. Formation of cyanide compounds and pathways for cyanide degradation/attenuation in the environment. . . . . . . . . . . . . . . . . . . . . . . . 151.3.1. Complex formation (chelation). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.2. Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.3. Adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.4. Cyanate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.5. Thiocyanate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.6. Volatilization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.3.7. Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.3.8. Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.4. Characterization of samples for cyanides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.5. Cyanide for dissolution and recovery of precious metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.6. Cyanide and cyanide compounds toxicityand guidelines and regulations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162. Treatment processes to remove cyanide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.1. Process selection criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2. Treatment processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2.1. Natural cyanide degradation in tailings ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2.2. Chemical treatment methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190892-6875/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.mineng.2013.05.027Corresponding author. Tel.: +90 246 2111321; fax: +90 246 2370859.E-mail address: ataakcil@sdu.edu.tr (A. Akcil).Minerals Engineering 5051 (2013) 1329ContentslistsavailableatSciVerseScienceDirectMinerals Engineeringj our nal homepage: www. el sevi er . com/ l ocat e/ mi neng2.2.3. Biological cyanide degradation process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.2.4. Other methods and emerging processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262.2.5. Electrolytic oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263. Process costs and selection of suitable process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281. Introduction1.1. Cyanide in daily lifeThe term cyanide refers to a chemical compound containingone atom of carbon and one atom of nitrogen. The technical deni-tionofcyanideisatriple-bondedmoleculewithanegativeonecharge, consisting of one atom of carbon in the +2 oxidation stateandoneatomof nitrogeninthe 3oxidationstate(InfoMine,2012). CyanidesmostcommonlyrefertosaltsoftheanionCN.Cyanide can naturally be found in about 2000 sources (WHO ECEH,2000). The principal human-made cyanide forms are gaseoushydrogencyanide(HCN)andsolidsodiumorpotassiumcyanide(NaCN or KCN). Humans are in close contact with cyanide in theirdaily life through food, drink, smoking, medicines and while usingproductswhichcontaincyanide(USEPA, 1976). Forinstance, asillustrated in Fig. 1, they are exposed to certain levels of cyanidedue to smoking tobacco (0.5 mg/cigarette), respiring exhaust fromvehicles (79 mg/km), handling certain type of pesticides, insecti-cidesandfertilisers,particularly forgrowingtobacco, cotton andgrapes, andusingcosmetics(e.g., eyeblush, lipstick). Somefoodproducts may either naturally contain cyanide or cyanide is usedin their production. The concentration of cyanide per kg in certainfoodproductshasbeenobservedas:tablesalt(20 mg/kg), limabeans (100300 mg/100 g plant tissue), cassava (104 mg/100 gplant tissue), wild cherries (140370 mg/100 g plant tissue),packed fruit juice (e.g., 0.0315.84 mg/L in orange and sour cher-ry), sorghum (250 mg/100 g plant tissue), almond (297 mg/kg), al-mond products (50 mg/kg). Varying quantities of cyanide may befoundinotherfruitsandvegetablessuchasapricot, strawberry,apple, peach and cauliower and alcoholic beverages (USEPA,1976; Cornell University, 2002). Some pharmaceutical drugs usedforcuringcancerandhighbloodpressurearemadeof cyanideandcyanidecompounds. Plants, microorganismsandinsectscanalso naturally produce cyanide.Although the use of cyanide is mostly perceived by the generalpublic as being associated with the mining and metallurgical pro-cesses, only about 13% of world cyanide production is consumed bytheseindustrieswiththeremainingpercentagebeingusedelse-where (WHO ECEH, 2000; Mudder and Botz, 2001; Mudder et al.,2001a,b; Akcil, 2002, 2006, 2010; Mudder and Botz, 2004). Cyanideis an essential raw material in producing synthetic silk, syntheticrubberandnylon, paints, medical andpharmaceutical products,agricultural products, coloured photographic and television lms,and cosmetics (Kirk-Othmer, 2002). Electroplating, galvanisingand jewellery making also use cyanide. Cyanide is often added asa depressant in the otation of base metal sulphide ores (e.g., ironand copper). Because of its ability to selectively dissolve gold andsilver from their ores, the recovery of gold and silver is often asso-ciated with cyanide use. Iron cyanide is often used as an anti-cak-ing agent in both table and road salts. Surface waters may containlowlevelsofcyanideduetoindustrialactivitiesassociatedwithcyanide use such as mining/metallurgical processes, road salts, reretardants, predator(coyote)poisoningdevices, shinginSouthAsia. Fig. 1showstheconcentrationofcyanidefoundinseveralproducts.Fig. 1. Cyanide content of various materials. Fig. 2. Relationship between HCN and CN with pH (25 C).14 N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 13291.2. Solution chemistry of cyanideThe cyanide ion is an anion that can be found as complexes (e.g.,weak, moderately strong and strong), as free cyanide or as simplecompounds in cyanidation solutions. The dissolution and dissocia-tion (or ionisation) of molecular or ionic cyanide in aqueous solu-tions creates free cyanides. Depending on the pH of the solution,free cyanide can be in the form of either cyanide anion (CN-) orhydrocyanic acid (or hydrogen cyanide, HCN). HCN is a relativelyweak acid and is predominantly found in waters with a pH belowapproximately 8.5, where volatilizationof cyanide takes place(Randol, 1985; Flynn and McGill, 1995). At an optimal gold extrac-tion pHof 10.5 or greater, most of the free cyanide in the solution isin the form of the cyanide anion (CN), where cyanide loss by vol-atilizationislimited. Otherwise, thecyanidationprocesswouldneitherbepracticalandsafenoreconomicallyandenvironmen-tally feasible. In natural aqueous systems that have pH values be-tween 5 and 8.5, the majority of free cyanide can be found in theformof HCNandcanbelost byvolatilization. Fig. 2illustratesthe relationship between pH levels and cyanide compounds.Salts of hydrocyanic acid such as sodium cyanide (NaCN), potas-sium cyanide (KCN) and calcium cyanide (Ca(CN)2) are simple cya-nides which dissolve completely in aqueous solutions giving freealkali earth cations and cyanide anions as shown byNaCN ! Na CN1CN H2O ! HCNOH2The weak and moderately strong cyanide complexes primarily in-clude cadmium, copper, nickel, silver and zinc cyanide complexes.These complexes form in a step-wise manner as the cyanide con-centration of the solution is increased. The solution pH and the con-centrationof cyanideandmetalsplayaroleinthequantityofcomplexformation. Intermsof chemical stability, zincandcad-mium are known to be the weakest complexes while iron and co-balt complexes are the strongest ones. These complexes maydissociate and release free cyanide in the presence of UV radiationand strong acids. The rate of dissociation is affected by several fac-tors such as the intensity of light, water temperature, pH, total dis-solvedsolidsandcomplexconcentration. HydrolysisofHCNinaneutral oracidicsolutionyieldsformateaseitherformicacidorammonium formate. Iron cyanide is known to be refractory cyanideas it is a stable typically non-reactive cyanide that passes throughthe general oxidation systems unaffected. It has been reported thatiron cyanide could break down when it is exposed to the UV radia-tion from the sun releasing free cyanide in receiving lakes and riv-ers. Inaddition, therecent treatment systems employelevatedtemperature oxidation principals potentially combined with a cata-lyst to reach low parts per billion (ppb) treatment levels (Manches-ter, 2013).A variety of cyanide related compounds can form in solutionsresulting from cyanidation, natural attenuation or treatment pro-cesses. These compounds include thiocyanate (SCN), cyanate(OCN), ammonia (i.e., free ammonia NH3or ammoniumionNH4) and nitrate (NO3).1.3. Formation of cyanide compounds and pathways for cyanidedegradation/attenuation in the environmentNatural attenuation mechanisms of cyanide may include: com-plexation(chelation), cyanidecomplexprecipitation, adsorption,oxidationtocyanate, volatilisation, bio-attenuation, formationofthiocyanateandhydrolysis/saponicationof freecyanide. Thesepathways help decrease the reactivity of cyanide in natural envi-ronment (ICMI, 2012). Since some of these mechanisms provide abasisforthedevelopment of chemical andbiological treatmentprocesses, they are briey discussed below.1.3.1. Complex formation (chelation)Cyanide forms ionic complexes with metals (e.g., Fe, Cu, Zn) thatare muchlesstoxic than hydrogen cyanide. Stability ofcyanidemetal complexes may vary depending on the type of metal. Weakacid dissociable Cu and Zn complexes are relatively unstable andcanreleasefreecyanidebacktotheenvironment;whereasbothferro- andferricyanidecomplexes, suchasthosefoundinsoilsand in aqueous solutions, can show extreme stability under mostenvironmental conditions except ultraviolet (UV) light, where theycan undergo photochemical decomposition.1.3.2. PrecipitationIron cyanide complexes form insoluble precipitates with Fe, Cu,Ni, Mn, Pb, Zn, Cd, Sn, Ag over a pH range of 211.1.3.3. AdsorptionCyanide and cyanidemetal complexes are adsorbed on organicand inorganic constituents such as oxides of aluminium, iron andmanganese, certaintypesofclays, feldsparsandorganiccarbon.As cyanide is strongly bound to organic matter, the strength of cya-nide retention on inorganic materials is unclear.1.3.4. CyanateCyanide can be oxidised to less toxic cyanate (OCN) with thehelp of a strong oxidising agent such as ozone, hydrogen peroxide,hypochlorite or gaseous oxygen. Cyanide adsorbed on both organicandinorganicmaterialsinthesoilappearstobeoxidisedundernatural conditions. Oxidationof cyanideresultsinformationofcyanatewhich isnormally found incyanidation solutions duetothe formation of hydrogen peroxide in the initial step of cyanida-tionprocess. Cyanatedoesnotaccumulateinsolutionsbecauseof its hydrolysis to ammonia and carbonate (or an ammonium saltand carbon dioxide), which incidentally has a signicantly (at least1000 times) lower inherent toxicity in comparison to that of cya-nide. The rate of cyanate hydrolysis is relatively rapid at pH levelslessthan6oratelevatedtemperatures. Sincefreeammoniacanform soluble amine complexes with heavy metals such as copper,zinc, silver and nickel, the presence of ammonia may inhibit pre-cipitation of these metals at pH values of above 9, which is knownto be an effective range for the precipitation of metal hydroxides(Kuyucak, 2001). Nitrate is the end-product of the cyanide oxida-tion process and forms as a result of chemical or biological oxida-tion of ammonia.1.3.5. ThiocyanateReaction with some sulphur species converts cyanide to aboutseventimeslesstoxicthiocyanate(SCN, ICMI, 2012). Freesul-phur, sulphide minerals (e.g., chalcopyriteCuFeS2, chalcociteCu2S and pyrrhotiteFeS) and their oxidation products (e.g., poly-sulphidesandthiosulphate)foundinsoilscanprovidepotentialsulphur sources. Thiocyanate occurs whenpyrite andpyrrhotite-bearing solutions are treated with cyanide. All sulphides minerals,except leadsulphide, havethepotential togeneratethiocyanate,where production is accelerated if inadequate aeration is allowedinconjunctionwithlowalkalinityconditions. Alargerquantityof thiocyanate is produced in the free sulphur or pyrrhotte (an ironsulphide) containing minerals. It may chemically and biologicallybreakdownproducingammoniaandnitrate, carbonateandsul-phate. Thiocyanide,that is potentially toxic itself,creates an oxi-dant demandand breakdown products typicallyinclude cyanate,ammonia andnitrate. Underlowalkalinityandreducedaerationconditions, and if pre-oxidised iron sulphide ore contains ferrousiron; ferrocyanidemayformrelativelyrapidly. Conditions thatN. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329 15could be arranged to minimize the formation of either thiocyanateor ferrocyanide are limited because a condition that may restrictthe formation of one product may promote formation of anotherundesirable product.1.3.6. VolatilizationAt the pH typical of environmental systems, free cyanide will bepredominatelyintheformof hydrogencyanide, withgaseoushydrogen cyanide evolving slowly over time. The amount of cya-nide lost through this pathway increases with decreasing pH, in-creasedaerationof solutionand withincreasing temperature.Cyanide is also lost through volatilization from soil surfaces.1.3.7. BiodegradationCyanide can degrade to ammonia by microorganisms under aer-obic conditions, which then oxidises to nitrate. Effectiveness of thisprocessislimitedtocyanideconcentrationsof upto200 ppm.Although biological degradation also occurs under anaerobic con-ditions, cyanideconcentrationsgreater than2 ppmaretoxictothese microorganisms.1.3.8. HydrolysisHydrogen cyanide can be hydrolysed to formic acid or ammo-nium formate. Although this reaction is not rapid, it may be of sig-nicance in ground water, where anaerobic conditions exist.1.4. Characterization of samples for cyanidesAnalytical methods can determine three types of cyanideincluding total cyanide, weak acid dissociable (CNWAD) and freecyanide. All cyanide compounds present in the solution includingcyanideinmetal complexes, except withcobalt, canberepre-sented by total cyanide values. WAD cyanide covers both the freeand complex forms of cyanide. Free cyanide includes the cyanideanion and molecular hydrogen cyanide. As a result, the total cya-nide level should be greater than or equal to the CNWAD cyanidelevel and the CNWAD level should be greater than or equal to thefree cyanide level. There are several methods used to characterisesamplesforcyanidesandcyanidecontainingcompounds(APHA,1998). Theappropriatemethodisselectedbasedonthesolutionchemistry requirements (e.g., potential interferences, site-specicsamples) andits reliabilityandefciency(e.g., reproducibility,time andcost). For instance, a rapidandinexpensive methodwhich can provide good estimates can be used during a eld pilottest and the results can be periodically veried with another ana-lytical method for their reliability. Samples need to be preservedas described by the analytical laboratory procedures if the samplecannot be analysed immediately. Detection limits can vary for eachcyanide compounds, type of analytical method and the laboratory.Theuseofautomatedon-linecontinuousmonitoringsystemsisbecomingwidespread. If required, cyanidecanalsobeanalysedin a solid sample.1.5. Cyanide for dissolution and recovery of precious metalsGold and silver are noble metals and as such they are not solu-ble in water. The precious metals form complexes with the cyanideanions to produce soluble derivatives, e.g., [Au(CN)2]and[Ag(CN)2]understrong alkalineconditions, typically pH > 10, inorder to maintain ionic form of cyanide and prevent volatilisationto hazardous hydrocyanic acid (HCN). In the absence of other metalcyanide complexes,a relatively dilute,100500 mg/LNaCN solu-tion at a pH greater than 10 which may contain about 50 mg/L freecyanide, can provide the maximum rate and extent of gold and sil-ver dissolution. This is called Cyanidation (Habashi, 1966; Ruboet al., 2006). Cyanidationof gold takes place in two reaction stepsand the overall reaction is known as Elsners equation (Eq. (3)):4Au 8NaCNO2 2H2O ! 4NaAuCN2 4NaOH 3Ag2S 4NaCN H2O ! 2NaAgCN2 NaSHNaOH 4The pregnant liquor containing these ions is separated from thesolids, whicharediscardedtoatailingpondorspent heap, therecoverable goldhavingbeenremoved. After goldis extracted,wastewater or process solutions may contain three principle typesof cyanidecompounds: freecyanide, weaklycomplexedcyanideand strongly complexed cyanide (WHO ECEH, 2000). Gold and cya-nidecanformastrongcomplexinrelativelyweakcyanidesolu-tions. For leaching of silver, as the silver cyanide complex isweakerthanthegoldcyanidecomplex, strongercyanidesolutionand/or longer reaction times must be employed to accomplish silverdissolutionduetotherefractorinessof silversulphideminerals.Zinc and copper are dissolved from sulphide minerals during cyan-idation to some extent. The iron minerals in pyrite and pyrrhotiteoresconsumethehighestquantityof cyanide. Aggressivecondi-tions can dissolve gold twice as rapidly as silver but, can increasethe attack on other minerals present in the ore resulting in an in-crease in cyanide consumption, higher concentrations of other met-als in the solution and a decrease in selectivity for gold and silver.Goldcanbeseparatedfromthepregnant solution bearinggoldcyanidecomplexesbyreductionwithZnor Zn-dust (zinccementation, MerrillCrowe process) or adsorption onto activatedcarbon. In the zinc precipitation process, elemental zinc powder re-duces gold ions to its free metal form in the absence of oxygen. Intheactivatedcarbonprocess, goldandsilvercyanidecomplexesare adsorbed onto active carbon sites, thereby removing gold andsilver from solution. Then, goldand silver are desorbed from theloaded carbon using 0.1% NaCN and 1% NaOH at elevated temper-atures(ZadraProcess). Goldisrecoveredusuallyfromthestripsolution by electro winning and a portion of the cyanide is recy-cled. Theuseofactivatedcarboncandecreasetheconcentrationof undesirable metals, such as mercury and copper, consequently,thetreatmentof wastewaterbecomeseasierandmoreefcient(ScottandIngles, 1987;MudderandBotz, 2001; Mudderetal.,2001a,b).Acompletediscussiononthecyanidationprocess andgoldrecovery is out of the scope of this paper. Only a brief discussionon cyanidation is given here to provide a background for toxicolog-ical, chemical and treatment aspects of the cyanide bearingsolutions.1.6. Cyanide and cyanide compounds toxicityand guidelines andregulationsCyanidedoesnotaccumulate inthebodyanddoesnotcausechronicdiseases, butbecauseofitsabilitytobindironinbloodby forming complexes, it can inhibit oxygen transfer to the cells,therebycausing suffocationof humans andanimals. Similarly,inhibition of cytochrome oxidase results in toxicity to freshwatershmainlybyblockingelectrontransportandenergyreleaseincells (Ripleyet al., 1996). Liquidor gaseous hydrogencyanideand alkali salts of cyanide can enter the body through inhalation,ingestion or absorption through the eyes and skin. The toxicity ofa substance is typically expressed as the concentration or dose thatis lethal to 50% of the exposed population (LC50 or LD50). Inhala-tionof100300 ppmgaseoushydrogencyanideresultsindeathwithin 1060 min, inhalation of 2000 ppmhydrogen cyanidecausesdeathwithinoneminute. TheLD50foringestionis50200 mg, or 13 mg/kg of body weight, calculated as hydrogen cya-nide. Forcontactwithunabradedskin, theLD50is100 mg/kgof16 N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329body weight as HCN. There is no reported bio-magnication of cya-nide in the food chain (ATSDR, 2006; ICMI, 2012).Cyanide in efuents resulting from gold mining is known to bethe prime candidate for treatment. However, according to a studybytheOntarioMinistryoftheEnvironment(Canada);ammonia,suspended solids and copper were identied as the most importantcontaminantsingoldmining efuentsrather thancyanide. Totalcyanide concentration is only controlled in drinking water, whichis0.2 mg/L(Ministryof Environment Ontario, 2013). Currently,theconcentrationof ammoniaintreatedwatersisregulatedinCanada and is required not to exceed 10 mg/L (CCME, 2013). Sincemany of treatment processes convert cyanide to ammonia and car-bon dioxide, ammonia may exceed the regulation limits. Investiga-tions are currently underway to develop methods to manageammonia and nitrate in wastewaters as well as cyanides. Althoughnitrate is a relatively non-toxic compound, at high concentrationsit may cause harm to humans, especially infants; and aquatic lifeby enhancing algae growth and thereby, decreasing dissolved oxy-gen concentration in the aquatic environment; and increasing shmortality rates. Similarly, whereas thiocyanate is reported to be se-ven times less toxic than cyanide, increased thiocyanate concentra-tions in water resources can adversely affect the sh and aquaticecosystems, and in the human body, chronic cyanide exposure det-riments the thyroid gland (Bhunia et al., 2000; ICMI, 2012).As a result of natural attenuation and dissociation phenomena,cyanide does not accumulate in the soil, water or air. UV radiationin sunlight and gaseous oxygen in air naturally degrade cyanide toC and N compounds. Cyanide in water is rst adsorbed by soil par-ticlesthenslowlydecomposeswiththehelpof microorganisms(Turkmen, 1998).Despite on-going concerns with regulatory agencies and com-munitiesregardingthecyanidecompounds, wherehealtheffectmayoccur, itisbelievedthathigherthan0.1 mg/Ltotalcyanideand0.05 mg/LCNWAD levels mayadversely affect humansandaquatic life (Randol, 1985 Donato et al.,2007; ICMI, 2012). Stan-dards or criteria are established based on determined toxicity lev-els to provide reasonable levels of safety and to promulgate theirenforcement toprotectthehumanhealthandtheenvironment(i.e., wildlife, aquaticlife) (HagelsteinandMudder, 1998). Thestandardsset fortheallowableconcentrationof ionsandcom-pounds may vary from country to country, jurisdictions to jurisdic-tions and evensite tosite (Randol,1992; ICMI,2012). While theallowablecyanideconcentrationinpotablewaterisestablishedat 0.2 mg/L by the United States and Canadian Environmental Pro-tection Agencies (USA/Canada EPA), the World Health organisation(WHO) guidelines consider0.07 mg/LCN safe forboth acute andlong-termexposureindrinkingwater. TheEuropeanUnion(EU)species0.05 mgtotalCNperlitre(Turkmen,1998;WHOECEH,2000; Akcil, 2002). Usually, standards set for both fresh and marineaquatic life are lower than the standards set for the drinking waterquality. Forinstance, Canadasetsfreshandmarinewaterstan-dards for cyanide as 0.005 and 0.001 mg/L, respectively. The WorldBank requires that the efuent from base metal and iron ore min-ing companies should meet 1 mg/L total cyanide, 0.1 mg/L free CNand0.5 mg/LCNWADlevels(WorldBankGroup, 1997;Howelland Christophersen, 2009).AnInternational Cyanide Management Institute (ICMI) wasestablished by the United Nations Environment Programme(UNEP) to administer the International Cyanide Management Codefor the Manufacture, Transport and Use of Cyanide in the Produc-tion of Gold (the Code). ICMI promotes the adoption of the Code,evaluatesitsimplementation, andmanagesthecerticationpro-cess. TheparticipationtotheCodethat exclusivelyfocuses onthe safe management of cyanide at gold mines is voluntary (Akcil,2002, 2010). Since the establishment of ICMI, the Cyanide Code hasexperienceddramaticgrowth. TheCyanideCodeisbeingimple-mentedat133goldminesincludingcompaniesthatcollectivelyproduceapproximately60%of theworldscommercially-minedgold, 20cyanideproductionandrepackagingfacilitiesand105transportoperationsin48countriesthroughouttheworld;and173 of these already have been certied in compliance (ICMI 2012).2. Treatment processes to remove cyanideTreatment aims to destroy cyanide, generally by converting itintocompounds that areinsolubleandcannot betakenupbyorganisms. Treatment methods range from rinsing heaps ofsolidtailings withwater to more complex techniques suchas alkalinechlorination, sulphur dioxide oxidation and electrochemical, whichtreatbothsolutions(spentcyanidesolutionandheaprinse)andslurries(tailings),fromremoval torecoveryof cyanides(USEPA,1994; Acheampong et al., 2010).2.1. Process selection criteriaCyanide destruction plants must be reliable all the time to pro-duce streams with acceptable water quality that can be dischargedto the environment. The treatment systems should allow recyclingofsolutionsthatdonotmeetthedischargecriteriaduringplantupsets and should be installed with a continuous on-line monitor-ing device. The treatment process should not produce undesirableby-productsandshouldbeabletoremoveharmful by-productssuchasammoniaandmetals. It shouldbeappropriatetosite-specic climatic conditions (e.g. temperature, precipitation),solutionchemistryanddischargerequirements. Thecapital andoperational costs shouldsuit theminingproductionconditionssuch as production rate and life expectancy.2.2. Treatment processesThenatureof thetreatmentprocessmayrangefromnaturaldegradationin tailingsimpoundment(viz. natural attenuationinsurface ponds) to highly sophisticated plant applications. Naturaldegradation in tailings ponds has been the most commonly usedtreatmentmethodinmostmills, includingthoseinCanada, formany years. Although natural degradation is still used for cyanideremoval, in the last two decades, several processes includingchemical, biological, electrochemical and thermal hydrolysis meth-ods have been developed to either supplement or supplant the nat-uraldegradation(Ritcey, 1989;SmithandMudder, 1991;Ripleyet al., 1996; Nelson et al., 1998; CDS, 2013). The recent tendencyis the use of highly sophisticated and automated treatment plantsinstalledwithon-linemonitoringdevicesduetotheincreasingconcerns of regulators and communities.The common cyanide treatment methods include natural degra-dation in tailings pondsand chemical and biologicaldegradationprocesses. Chemical processes may consist of alkaline chloride oxi-dation(alkalinechlorination), hydrogenperoxideoxidation, IncoSO2/Air oxidation, Hemlo/Golden Giant (copper and iron sulphate)precipitation and AcidicationVolatilizationRegeneration (AVR).Methodsof removingordestroyingcyanidearesummarizedinTable 1. Natural degradation, Inco SO2/Air and hydrogen peroxidetechniquesareknowntobewidelyused methodswhileothermethods ndlimitedapplication. Thefocusofthispaperwillbemainly on the widely used methods and the unique Homestakebiological process.2.2.1. Natural cyanide degradation in tailings pondsNatural degradationinvolvesdetainingcyanide-bearingefu-ent in tailings or holding ponds for prolonged periods. The degra-dation of cyanide results from a combination of naturallyN. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329 17occurringphysical, chemical andbiological processes includingvolatilization,photodecomposition, chemical oxidation, microbialoxidation, chemical precipitation, hydrolysis and precipitation onsolids. Of thesemechanisms, volatilizationis consideredtobethe most important. Natural degradation is inuenced by a numberof variablessuchas:cyanidespeciesandtheirconcentrationinsolution, pH, temperature, bacteriapresent, sunlight (UVradia-tion), aeration and pond conditions (e.g., area, depth, turbidity, tur-bulence, ice cover). In the 1980s, aconsiderableamount of workwas carried out by Environment Canada to better understand andmathematically model the natural degradation process of cyanideintailingsponds(Ritcey, 1989; Zaidi et al., 1987; Ripleyet al.,1996).Volatilizationof HCNfromsolutionisthedominant naturalattenuation mechanismin most surface ponds (e.g., tailingsponds). More than 90% of free cyanide is removed by volatilisation.It is believed that photolysis also contributes to degradation of cya-nide in ponds. Over time, the pH in the pond is lowered by the nat-ural uptake of carbon dioxide from the air and by the addition ofacidic rainwater. Due to the carbon dioxide uptake, an equilibriumpH ranging between 7.0 and 9.0 establishes in the tailings ponds,and the pH of the tailings slurry drops from about 10.5 to less than9.0. The drop in pH induces a change in the cyanide/hydrogen cya-nideequilibrium, favouringtheformationofHCNanditssubse-quent volatilization as hydrocyanic gas according to Eq. (5)(Fig. 2). Under aerobic conditions cyanide is initially converted tocyanate and then cyanate hydrolysis occurs to form carbon dioxideasbicarbonateandammoniaasgasandammoniumions. Thesereactions are shown by Eqs. (6) and (7) (Young and Jordan,1995). Conversion of cyanide to cyanate requires a photochemical(sunlight), bacteriological or mineralogical catalyst.CN H ! HCNaq ! HCNg5CN 1=2O2catalyst ! OCN6OCN H2O ! HCO3 NH4 OH7Cyanidedecayoccursinbothshallowanddeepsectionsof thepond. However, thereisatimelagforthedeeperpondsections.In stagnant water bodies, cyanide volatilization is directly propor-tional to the ratio of solution area to depth. Natural mixing of waterby wind action and convection currents caused by the temperaturedifferencebetweenpondwaterandair, inducesvolatilizationinponds (Zaidi et al.,1987). Temperature and aeration are the mostimportantfactorsaffectingthevolatilizationrateof freecyanideandcyano-metalcomplexesofzinc, copper, nickelandiron. Cya-nideattenuationobeysarstorderreactionwithrespecttofreecyanide and cyano-metal complexes. UV radiation affects the stabil-ity and attenuation of ironcyanide complexes in the surface ponds.Thestudiesrevealedthatdegradationofferricyanidetoferrocya-nide byphotolysis andphoto-catalysis under solar radiationisstrongly affected by the water pH, as the initial ferricyanide concen-trationdidnot showanyeffect (ArellanoandMartinez, 2009).Regardlessof thepresenceorabsenceof catalysisintheaquaticmedia, the redox reactions under sunlight exposure were stronglydependent on the water pH.When the pond surface is covered with ice during winter, cya-nide decay may be signicantly reduced. The tailings prole in atailings pond can be divided into three zones by depth: oxidisingzone (03 m); intermediate zone (315 m) and reducing zone(1535 m). Although all zones may have the same pH levels, totaldissolvedsolidsandtotalcyanidecontentsareusuallyhigherinTable 1Methods available for removing or destroying cyanide and process mechanisms involved.Cyanide removing/destroying method Process mechanismsNatural Attenuation/Degradation (Collecting and Holding In Ponds)Volatilization Biodegradation Oxidation (by UV, microorganisms)Chemical Addition Under Controlled Conditions Oxidation ProcessesSO2Air OxidationOxidationAlkaline ChlorinationConversion to cyanate Chlorine gasHydrolysis Hypochlorites Electrolytic (in situ) generationOzonationOxidationComplexationSunlight (UV) w/o catalystHydrolyisHydrogen Peroxide OxidationPrecipitation Iron Sulphide Processes Thermal treatment under high pressure Fe/Cu PrecipitationBiological Oxidation In reactors and under controlled conditions (e.g., Home stake RBC process)Degradation (to CO2 and NH3) Passive systems (Wetlands and in-pit)Oxidation to NO3; then reduction to N2) Complexation, adsorption/uptake, precipitation, etc.Conversion to Less Toxic FormsThiocyanide conversionComplexationFerrocyanide conversionCyanide Recovery byAcidicationSART, AVR, CyanisorbVolatilization & Condensation Re-neutralizationAbsorption ProcessesIon Exchange (also leads to CN recovery)ComplexationActivated CarbonIon FlotationElectrolytic ProcessesCyanide regenerationDecompositionCyanide destruction18 N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329the intermediate zone. As the total cyanide content in pore wateroftailingsremainrelativelystable, WADcontentrapidlydecays.If reduced portions and HS-and H2S species are present in anaero-bic (reducing) zones, thiocyanide (HSCN) forms as a result ofanaerobic bioattenuation of cyanide. Then, HSCN hydrolysis takesplace producing H2S, NH3 and CO2. Anaerobic bioattenuation is alesseffectivemechanismthanthatofaerobicbioattenuationbe-cause of its slowrate and the toxicity effect of cyanide on anaerobicbacteria (e.g., 2 mg/L vs. 200 mg/L).Cyanideremoval bynatural means mayproduceacceptableefuents in some cases. If natural degradation cannot produce a -nal efuent quality suitable for discharge, then natural processeswilltypicallybeusedasanintermediatestepforthepartialre-movalofcyanidepriortochemicaltreatment. Whilesomemillstreatthewholetailingsslurry, somemillstreatonlythetailingspond overows. Cyanide is especially biodegradable in warm cli-matesasaresultofnaturalattenuationprocesses. Anypotentialimpact on surface waters due to cyanide use cannot persist for asignicant period of time after the minemill shuts down (Randol,1985).2.2.2. Chemical treatment methodsInabilitytoproduceefuentsofacceptablequalitybynaturalmeans has led to the development of chemical and biological treat-ment methods (Botz et al., 2005). The processes widely used andused with limitations are discussed below.2.2.2.1. Alkalinechlorination. Alkalinechlorinationhasbeenwell-known for a long time as an effective means of treatment for indus-trial wastewaters (Cushnie, 2009). It has been applied to gold millefuents for a long time. The process destroys cyanide (CNWAD),except iron cyanide and more stable cyanidemetal complexes, byoxidationwithchlorineunderalkalineconditions. Althoughtheprocess mayalsodestroy thiocyanate,itisprimarily effectiveonfree cyanide, and where high concentrations of cyanide are pres-ent. Cyanate is also oxidised to give N2 and CO2. Chlorine can beprovidedintheformof gaseouschlorineorsodiumorcalciumhypochlorite. Lime is an alkaline agent that is commonly used forpHcontrol. Thecompleteoxidationof cyanidetonitrogenandbicarbonatetakesplaceintwostages. Intherststage, cyanideis oxidised, and then hydrolyzed to cyanateNaCNCl2 ! CNCl NaCl 8CNCl 2NaOH ! NaCNO NaCl H2O 9The Second stage of the process is the further oxidation of cya-nate to nitrogen and bicarbonate3Cl2 2NaCNO 6NaOH ! 2NaHCO3 N2 6NaCl 2H2O10The rst stage of the reaction (Eqs. (8) and (9)) takes place at a min-imum pH of 11, while the second stage (Eq. (10)) occurs at a pH of8.5. Considerablygreater reagent additionsandlonger retentiontimes are required in completing both stages at a pH level of 11. Be-low pH 11, hydrolysis of highly toxic cyanogen chloride gas will notbe completed (Eq. (9)). As a consequence of both high cost and therelatively lowtoxicity ofcyanate(1/1000 thetoxicity offree cya-nide), regulatoryagenciesmayaccept theprocesstobecarriedthrough only at the cyanate stage. The treated water may requireaeration or a polishing step for removal of toxicity due to residualchlorine. Ferrocyanide is not destroyed but, partially oxidised to fer-ricyanide. An additional process is required for removal of iron cya-nide. Theprocessdoesnot requireacatalystsuchasCu, andisreported to be easy to operate. Concentrated cyanide wastes, suchas spent plating or stripping solutions, should not be reacted withhypochlorite because the reaction can be violent, emitting chlorinegas. These wastes can be batch treated by electrolytic oxidation andthermal destruction(Cushnie, 2009). Sodiumhypochlorite con-sumption is usually estimated to be 25100% greater than the stoi-chiometric requirement (approximately 7 lb of Cl2 or 7.5 lb of NaOClper lbof CN). Theexcesschlorineisconsumedbyoxidationoforganics and metal ions present in the wastewater.During chlorination, oxidizable substances other than cyanide,suchasthiocyanate, thiosaltsandmetalsinlowoxidationstatesconsume additional chlorine. The presence of thiosalts may signif-icantly increase the chlorine demand. Several companies in Canadahaveemployedthismethodasstandard untiltheIncoprocesswas fully developed and its capabilities were demonstrated. Someindustrieshaveusedgaseouschlorineandsomehaveusedsolidcalcium hypochlorite. Although the chlorination units are usuallylocatedinthemillbuildings, theycanbeinstalledasaseparatetreatment plant adjacent to the discharge end of the tailings ponds(SGS Technical Bulletin, 2010).Giant Yellowknife Mine in Canada operated a two-stage treat-ment system for the removal of both cyanide and arsenic. Cyanideandmetalswereremovedbychlorineadditionintherststageand arsenic was precipitated by ferric sulphate addition in the sec-ondstage. Afterocculationthetreatedefuentfromthisplantwas discharged to a large polishing pond with a 15-day retentiontime. A conceptual owchart of the process is given in Fig. 3. Withthis process, the concentration of total cyanide, free cyanide, cop-perandironcouldbereducedfrom400, 250, 300and400 mg/Lto less than 5, 0.2, 0.5 and 0.3 mg/L, respectively.Although it is possible to recover cyanide by this method for re-use, it requires costlyandcomplexadditions andinstallations.Handlingof chlorinegasraisessafetyconcerns(Kilborn, 1991).Toxicity of the intermediate products that form during the chlori-nation process and residual chlorine are of concern. Therefore, inrecent years, interest inusingthealkalinechlorinationprocesshas been decreasing. Peroxygen compounds such as hydrogen per-oxide, peroxymono sulfuric acid (commonly known as Caros acid),andpersulfateshavebeenfoundtobeeffectivealternativestoalkaline chlorinationfor destroying free and complexed cyanides(US Peroxide, 2013). Detailed discussions on peroxygen oxidationare provided in Section 2.2.2.4.2.2.2.2. SO2/Air (INCO) process. This process was developed and pat-entedby theInco MetalsCompany in 1984inCanada(CanadianMining Journal, 2002). It was known to be Inco process. In the Incoprocess, a mixture of SO2 and air is used in the presence of copperasacatalyst, undercontrolledpHconditionsof 810,toselec-tively oxidise both free and complex cyanide species (CNWAD), ex-ceptironcyanide, tocyanate(CNO)(SmithandMudder, 1991;Devuyst etal. 1992; Nelson etal.,1998; Saarela and Kuokkanen,2004). Aconceptual owchart of the process is illustratedinFig. 4. Withtheexceptionof iron, metalsareprecipitatedfromsolution as hydroxides. The process also removes iron cyanide byprecipitation as insoluble copper and zinc ferrocyanide. Investiga-tionsshowthat theseprecipitatesarequitestableoverawiderangeof pH. Eq. (11) representsthecyanideoxidationreactionand products produced by the SO2/Air processCNWAD O2 SO2 Cu H2O ! CNO Cu H2SO411Neutralisation of acid generated and precipitation of metals areillustrated byH2SO4 CaOH2 ! CaSO42H2O 12Me2 CaOH2 ! MeOH2 Ca213N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329 19The amount of copper required to catalyse the reaction varies,anddependsontheconcentrationsof otherconstituentsinthewastewater. Copper requirements are signicantly reduced oreliminated for waters containing greater than 50 mg/L copper. Ontheother hand, SO2consumptiontoCNWADratiobyweight is2.46, whereCNWADistheweakaciddissociablecyanidethatin-cludes free cyanide and weakly complexed cyanides. The SO2 re-quired for the process may be supplied in a variety of forms suchasgaseousSO2, solublesulphiteormetabisulphite(Na2S2O5), orroastergasescontainingSO2. Currently, sodiumsulphiteandso-dium metabisulphite are being used more frequently.Because of its ability to handle slurries with ease and to removetotalcyanidespeciationwithlow-costreagentsinasingle-stagecontinuoustreatment, INCOProcessoffereddistinct advantagesover the two main alternative, alkaline chlorination and hydrogenperoxide methods. It has been accepted by many regulatory agen-cies throughout the world as the state-of-the-art method. The SO2/Air process received worldwide attention for the treatment of goldmill efuents and it has been applied to over 100 operations world-wide. Companies previously using alkaline chlorination have con-verted to the SO2/Air process to overcome iron removal problemsand/orhighreagentcosts. Aprocessroyaltywaschargedforitsuse until 2004 (Nelson et al., 1998). While some mills are applyingthis process to the barren bleed solutions, some are treating tail-ings slurries. Some mills employ the method to treat tailings pondoverow. Therecenttendencyistotreattailingsslurriesbeforedisposal intothetailingspondstoprotecttheenvironmentandwild life.The SO2/Air process offers an effective means to treat gold millefuents and to produce efuents containing concentrations of lessthan0.5 mg/Lfortotalcyanideandforeachofthemetalscom-monlyfoundingoldmillefuents(e.g., copper, iron, nickelandzinc). The process can be applied to either aqueous reclaim wateror directlytothetailings slurry. NorandadevelopedNorandaSO2 process where both tailings slurry and efuents can be trea-tedusingpure copper andSO2without air injection(Kilborn,1991;Carrillo-PedrozaandSoria-Agullar, 2001;InfoMine, 2012).Althoughitshowedpotential fordevelopmentandwasusedatthe Noranda operations, it has not been adapted as widely as theINCO process. Other sites also reported the use of modied INCOprocesses to destroy cyanide from mill efuents (Yang and Skryp-niuk, 2009).2.2.2.3. Iron/copper precipitation processes. Addition of ferrous ironto form insoluble ferrocyanide at pH higher than 10 is one of theoldest methods usedtodetoxify freecyanide(Manoranjan etal.,2003). In these processes, the presence of thiocyanates has shownsignicant inhibition to the complexation of cyanide with the me-tal ion, preventing the removal of cyanide to an acceptable level. Inaddition, decomposition of ferrocyanides observed under sunlightyielding HCN has raised stability concerns. Several cyanide detox-ication processes have been developed based on the principles ofFig. 3. Conceptual alkaline chlorination treatment process.Fig. 4. Conceptual INCO SO2/Air process.20 N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329insoluble metal cyanide (e.g. ferrocyanide, copper cyanide) precip-itate formation.TheGoldenGiant Mineof HemloGoldMines Inc., Ontario,Canada, developed a process for cyanide removal at their property(Konigsmann et al., 1989). The Hemlo Process reacts the untreatedwater with a pre-mixed solution of cupric sulphate and ferrous sul-phate in the rst step. Upon addition of the wastewater, the ferrousironisquicklyoxidisedandformsaferrichydroxideprecipitatewhilethecupricionis simultaneouslyreducedtothecuprousionwhichremovesfreecyanideasaninsolublecuprouscyanideprecipitate. Theremoval of freecyanidecausesthedissociationof copper, nickel and zinc cyanide complexes resulting in furtherremoval of cyanide as cuprous cyanide. Copper, nickel, zinc, anti-monyandmolybdenumareco-precipitatedfromsolutionwiththeinitiallyformedferrichydroxide. Ferricsulphateisaddedinthesecondsteptoensurecompleteprecipitationof themetals.Duetotheacidityoftheferricreagent, limeisaddedtocontrolthe pH at4. Ferrocyanide is precipitated from the solution as cupricferrocyanide. Anal polishing stageoccursinthethirdstep,where hydrogen peroxide is added at neutral pH. Thickened sludgeis disposed of to the tailings pond. A conceptual process ow chartis shown in Fig. 5. The Hemlo process can generate a treated efu-ent containing less than 0.5 mg/L total cyanide and metal ions.AnothermethoddevelopedbyManoranjanetal. (2003)con-sisted of removing free cyanide and CNWAD from efuents usinga mixture of thiosulphate and complexing agents (such as divalentcopper, divalent iron, divalent cobalt or their mixtures), and acti-vatedcarbon. Sulphatesalts wereusedtopreparecomplexingagents. ThepHoftheprocesscouldbeadjustedbetween5and10 with the help of lime or NaOH. The process could effectively re-duce total cyanide and CNWADconcentrations to less than0.5 mg/Land0.005 mg/L, respectively. Theprecipitatesresultingfrom the process has been claimed to be chemically stable.2.2.2.4. Hydrogen peroxide oxidation process. Hydrogenperoxide(H2O2) can oxidise cyanide to cyanate in the presence of a transi-tionmetal(Cu, Ag, V, Th)asacatalystinconcentrationsof5to50 mg/L. Theoxidationrequires1.26 lbH2O2perlbof cyanide.The nal products are carbonate and ammonia ions (USEPA,1994; US Peroxide, 2013). The choice of peroxygen system dependsonthereactiontimeavailable, thedesiredproducts(cyanate, orCO2 and NH3), the types of cyanides being treated (free, weak aciddissociable, or inert), and the system economics. It was observedthat thereactionratecouldbesignicantlyacceleratedbytheadditionof copper-impregnatedactivatedcarbontothecyanidebearing solution (Yeddue et al., 2011). The reaction rate increasedwiththeincreasinginitial molarratioof [H2O2]0/[CN] andde-creasedwithincreasingpHfrom8to12. Thetemperaturedidnot have a signicant effect on the kinetics of the cyanide degrada-tion and the cyanide removal kinetics t to pseudo-second orderwith respect to cyanide.A pH of 910 should be maintained to avoid release of HCN (USPeroxide, 2013). Increaseintemperatureanddosageof catalystand the use of excess H2O2 can increase the reaction rate. Photoac-tivation (UV + H2O2) can destroy stable cyanide complexes such asferricyanide. The destruction of cyanide by H2O2 is shown byCN H2O2 ! CNO H2O 14CNO 2H2O ! CO23NH415This process is more suitable for wastewaters rather than slurries,whereH2O2consumptionmightbesignicantlyhigh. Itwassuc-cessfullyusedintheUSAtoneutralisecyanideremainingintheheap leach piles (USEPA, 1994). OK TEDI Mining Limited in Papua,New Guinea, employed H2O2 to meet extremely stringent govern-ment imposed efuent quality requirements in the 1980s. The goldmill operation throughput of 22,500 tonnes per day was a giant oneby gold mill standards. The H2O2 solution was directly added to thecarbon-in-pulp (CIP) tailings slurry and reacted in a mixed tank tooxidisecyanidetocyanatebeforedischarge. Thisprocessisalsoknown as the Degussa process because Degussa, who manufacturesH2O2, developedanautomatedandcloselycontrolledprocessinwhichH2O2is efcientlyused(Degussa, 1988). AlthoughH2O2can successfully oxidise and destroy cyanide, due to handling dif-culties caused by its hazardous nature and economic reasons, it isnotfrequentlyused. Ammoniaintheresultingefuentmayposetoxicity to the sh.Newmonts Waihi Gold Mine in New Zealand uses H2O2 methodto destroy cyanide in the form of free cyanide and WAD (as Cu, Ni,Zn complexes), existed in efuents resulting from gold and silverextraction (Waihigold, 2013). Copper sulphate is added to the pro-cess as a catalyst to speed up the reaction. The cyanide destructionprocess is followed by a metal and trace ion removal process usingferricchloride,limeandocculants. Thetreatmentplant built in1999 was recently upgraded incorporating a reverse osmosis pro-cess to produce high quality water that can be discharged to theOhinemuri River.In 2003, Degussa (Cyplus) and INCO developed a new processcalledCombinOxcombiningtheadvantagesof IncoSO2/Airandhydrogen peroxide technologies (Chemeurope, 2003). The processcould reduce both cyanide and heavy metals to lowlevels. The ex-ibilityof theprocesstoaccommodatechangesinthefeedwasshown to be the key advantage. It was also claimed that, dependingon site-specic conditions, the new process could offer capital andoperational cost savings over the traditional Inco process.At the Marlin gold mine in Guatemala, World Bank and the localregulatory agencies imposed stringent standards for the dischargeofwastewaterresultingfromMerrillCroweprocesswheremer-cury (Hg) is used along with cyanide to recover gold (Howell andChristophersen, 2009). ThetailingsslurryissubjectedtoSO2/Airoxidation process for the destruction of cyanide before its disposalto the tailings pond. Excess water in the tailings impoundment tobedischargedtotheenvironmentrequiresfurtherpolishingforthe removal of metal ions including Hg and cyanide. The polishingtreatment process consists of three steps:(1)oxidation with hydrogen peroxide in the presence of coppersulphatecatalysistomeet thetotal WAD andfree cyanidelimits; removal of Hg by a chelating polymer to below detec-tion limit of 0.0002 mg/L;(2)occulation, precipitation and separation of metal and ironhydroxide sludge in a high-rate sand-blasted settling/clari-fying equipment and ltration;(3)carbon adsorption as a precaution to meet the desired levelof Hg removal.The use of Caros acid made of H2O2 and sulphuric acid can signif-icantly accelerate the reactionrate. Alternatively, persulphates(ammonium, sodiumorpotassium) canoxidisecyanidebeyondcyanate above pH 9 and in the absence of a catalyst.2.2.2.5. Ironsulphide(FeS)andsulphideprecipitationprocessesandSART. Waste barren bleed solution is treated by the addition of fer-rous sulphide (FeS) to remove both free and complexed cyanides.Finelydividedinsolubleferroussulphideparticlesarepreparedinthemill byreactingferroussulphatewithsodiumsulphide.The adsorption of cyanide on the FeS particles results in removalof cyanide. The reaction is strongly pH dependent and requires apH of approximately 7.5 and a retention time of 15 min. The FeStocyanideratioismaintainedat31. Ferroussulphatealsore-moves arsenic which dissolves from the ore. The cyanide removalN. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329 21step is not employed to produce an efuent of suitable quality tomeet the imposed efuent discharge limits but rather relies on fur-ther removal of cyanidebynatural degradationinthetailingsponds. The regulated parameters in the efuent from the tailingspond are usually below the objective limits of this process.Recently, brand name products such as DTOX have been intro-duced, based on the sulphide precipitation process principles (Min-eral Process Control, 2013). Thereactionbetweencyanideandpolysuldesulfurcontainedin DTOX converts hydrogen cyanidetothiocyanate. Inaddition, heavymetalionscanbeprecipitatedascyanide, andfreeandcomplexcyanidesareneutralised. Theend products released to the environment containcarbonatesand ammonia. It is claimed that the reaction is safer and the ratesare more rapid than those of hypochlorite. The experiments con-ducted toinvestigate thereactionin a2%(20,000 mg/L) solutionofsodiumcyaniderevealedthatthereactionwas95%completewithin 1 h, and cyanide concentrations were non-detectable with-in 2 weeks at a CN: poly S0ratio of 1:2 by weight.AprocesscalledSART(SulphidizationAcidicationRecycleThickening) that was developed by Lakeeld and Tech in Canadain early 2000s uses a reagent such as sodiumhydrosulphide(NaHS)toprecipitatecopperandzincassulphidesandconvertcyanide to HCN (Alta Metallurgical Services, Australia, 2012). Theprecipitatesareremovedforpossiblesaleorfurtherprocessing,andthesolutionisneutralisedwithsodiumhydroxideor limeandrecycledbacktotheleachingprocesstore-usethecyanide.Telfer Gold Mine in a remote location of Western Australia investi-gated SART as a cost-effective process to recover copper and cya-nide(SGSTechnicalBulletin, 2001). Atthemine, theoxidegoldore was treated in a conventional CIL circuit by blending the lowandhighgradecopper ores tominimizecyanideconsumptionand allow smooth operation. The sulphide ore was treated througha otation circuit to produce pyrite and copper concentrates. Be-cause ofeconomical and environmental reasons,it was desirabletoreducetheamountofcyanidereportingtothetailingspond.The resulting slurry, containing copper as copper sulphide precip-itate and cyanide as HCN in solution, was fed to the SART thickenerwhere they were separated. The process involved in sulphidizationand acidication reactions was presented below:2CuCN23S2 6H ! Cu2S 6HCNaq16The results of the pilot tests revealed that a pH of 5 would be suf-cient to remove over 95% of the copper from tailings efuent and re-cover over 94% of the cyanide. NaSH additions at 100%stoichiometriccopperweresufcienttoremoveover95%of thecopperfrom the efuent when sulphuric acid wasused toacidifythesolution. Acidconsumptionwas1.72.4 kgof100%sulphuricacid/kg WADcyanideto reduce the pH from about 10to pH 45.Lime consumption ranged from 0.51.3 kg/kg WAD CN at apH ofFig. 5. Conceptual Golden Giant mine efuent treatment process.22 N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 13293. The SART process has been successfully commercialised and ap-plied several sites in the last decade (SGS Technical Bulletin, 2010).2.2.2.6. Acidication process and cyanide recovery. Thisprocess wasused at the Emperor Gold Mine in Fiji to remove residual cyanidefromcyanidedandwashedcalcineaftertheotationsteptore-cover sulphides. To remove cyanide, the pH of calcine slurry is low-eredfrom 9to2.5bysprayingSO2-containingroastergasesinacounter-current decantation (CCD) system. HCN gas released dur-ingtheprocessisstrippedintowersanddecantedtoeliminatecyanide emissionto the atmosphere. The acidicationprocesswasprincipallyemployedforcyaniderecovery, notforenviron-mental protection. Therefore, informationabout the qualityofresulting efuents by this process was not reported.Since 1998, cyanide recovery from mill tailings has been prac-tised at Cerro Vanguardia in Argentina that includes multiple openpits and a hybrid Merrill-Crowe and CIL circuit to recover gold andsilver from oxide ore (Botz et al., 2004). Initially, the cyanide recov-ery plant aimed to process tailings slurry to achieve greater than90% cyanide recovery from tailings containing about 600800 mg/LWADcyanide(asCN). Relativelyhighconsumptionofsulphuric acid and sodium hydroxide leading to high rates of scaleformation in the acidication tank and stripping towers were ob-served. Changes were made to the cyanide recovery system allow-ingtheoperationofatwostageCCDcircuitwithclearsolution,whichreducedthesulphuricacidconsumptionandeliminatedscale formation in the process equipment. About 9092% sodiumcyanide recovery can be obtained from a solution containing about350 mg/LWADcyanide, whichaccountsforabouttwo-thirdsofthe total cyanide added to the leaching circuit.2.2.2.7. AcidicationVolatilizationRegeneration (AVR) or MillsCrowe process for recovery of cyanide. AcidicationVolatilizationRegeneration (AVR), or alternatively referred to as the MillsCroweprocess, involvesacidifyingwastecyanidebearingsolutionstoapH of between 2 and 3 with H2SO4 or SO2, volatilizing the resultingHCN by airushing and recovering theHCN by absorption in analkalinesolution(e.g., NaOHorCa(OH)2). Therecoveredcyanideisrecycledtothecyanidationcircuitforreuse. AspecicnameCyanisorb has also been givento AVR referring as Acidify, Vola-tilizeandReneutralize (SGSTechnical Bulletin, 2009). TheAVRprocess has alsobeencommercialisedandsuccessfullyappliedseveral miningsites(SGSTechnical Bulletin, 2010). Thisprocesswasusedat asilvermineoperatedbyCompaniaBeneciadorade Pachuca in Mexico. The SO2 gas is produced by burning elemen-tal sulphur at the site.Hudson Bay Mining and Smelting Company at FlinFlon, Mani-toba, Canada,used theMillsCroweprocess for cyanide recoveryfrom 1935 to 1978 while the company was using cyanidation forthe recovery of gold from base metal tailings. Waste barren solu-tionwasacidiedwithH2SO4solutionfromtheirzincrecoveryplant. Air was used in towers to sweep HCN from the acidied bar-ren solution and the HCN was absorbed in Ca(OH)2 for reuse in thecyanidation circuit. Ninety-one percent of the regenerable cyanide,which was the amount of cyanide released in laboratory acidicat-ion tests, was recovered by this method. Both SART and AVR pro-cesses are applied to tailings efuents rather than pulp.Therefore, thecost of solidliquidseparationmust beaddedtothe costs of cyanide recovery. This can be a signicant capital-costburden if the tailings pulp is difcult to separate into solid and li-quidcomponents asoccurswithhighclay, slimyorviscousores(SGS Technical Bulletin, 2010).2.2.3. Biological cyanide degradation processSeveral microorganisms (bacteria, fungi) andtheir enzymeshave ability to degrade cyanide and cyanide complexes includingiron cyanide complexes to less toxic compounds such as nitrogen,formicacidandformamide. Microorganismscanutilisecyanidecompounds as source of carbon and nitrogen for their own growth(SaarelaandKuokkanen, 2004; Chapmanet al., 2007; Sabatinietal., 2012). Cyanidedegradationcanoccurunderbothaerobicand anaerobic conditions. Aerobic oxidation process producesammonia(Akcil, 2003; Akcil andMudder, 2003). Four typesofenzymatic reactions can take place including substitution/addition,hydrolysis, oxidationandreduction. Hydrataseenzymedegradescyanide to formamide which is further hydrolysed by amidase en-zyme into NH4 and formic acid. The reaction pathway and the pro-cesskineticscanbeaffectedbyaerobicoranaerobicconditions,nutrients (carbon such as sugar), nitrogen, yeast, pH, oxygen andtemperature. Microorganisms tend to form a biolm on a supportmaterial in reactor applications (Maier et al., 2009). The reactionsshowingoxidationof metal cyanidecomplexesandthiocyanidebyPseudomonassp. areillustratedinEqs. (17)and(18)(Youngand Jordan, 1995; Akcil et al., 2003; Kadlec and Wallace, 2009)CN 1=2O2aq ! OCN ! OCN 3H2O! NH4 HCO3 OH17SCN 3H2O 2O2aq ! SO24NH4 HCO3 H18The mode of application can take place in either active fully con-trolled automated bioreactors or in passive systems (passive biore-actors, wetlands, pit-lakes, i.e., in-pit treatment) (MPERG, 2010).Passive systems have been investigated for their methodology andperformance, reliability, suitability for cold climates, longevity, sus-tainability and cost-effectiveness, especially for their application toremote sites. Examples are given in Section 2.2.5.3.Active biological processes have been neither widely practisednor received a great deal of attention for treating cyanide bearingwastewatersfromgoldmills. Barrenbleedandtailingsslurriespose a hostile environment to bacteria and reduced bacterial activ-itymayoccurif cyanideandmetalsareinhighconcentrations.Application of the biological process is more successful for tailingspond waters where both cyanide and metal concentrations are rel-atively low due to natural degradation and dilution. Biological pro-cesses areusuallyruledout incoldclimatecountries suchasCanada. Threefull-scaleactivebiological treatment processeshave been developed and used by Homestake Mining in the USAandCanada, arebrieydiscussedbelowinSection2.2.4.1and2.2.4.2.Success of the Homestake process has increased attention in theapplication of microorganisms for degradation of cyanides. A sig-nicant amount of research has been carried out to develop micro-biological processeswithimprovedefciencyandperformance.Forinstance, theuseofimmobilisedcellsofPseudomonasputidain air-uplift-type uidized bedreactors to convertinorganic cya-nidestocarbondioxideandammoniahasbeeninvestigatedasan alternative, but no pilot- or full-scale process have been oper-ated using this method (Babu et al., 1993). A test heap was inocu-lated with a cyanide-reducing bacteriumPseudomonas pseudoalcaligenes (UA7) in Alaska aiming to compare cyanide destructioncosts with INCO process (Nelson et al., 1998). The results indicatedthatcyanidecouldbereducedtoacceptablelevelsbythebacte-riumand, despitethehigherinitial capital cost, theoverallcostof the treatment in the long term could be lower than the chemicalmethod due to signicantly lower operating costs. A recent studyrevealed that the alkaliphilic autochthonous bacterium Pseudomo-nas pseudoalcaligenes can detoxify cyanide in two ways. Cyanidesareirreversiblyboundtometals, suchas toiron, causingirondeprivation outside the cell and metalloenzymes inhibition insidethe cell occurs resulting in a cyanide-insensitive respiratorysystemandacyanidedegradation/assimilationpathway(Luque-N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329 23Almagro et al., 2011). The cyanide assimilation pathway that gen-erates ammonium, is further incorporated into organic nitrogen.2.2.3.1. Homestakeminebiological processintheUSA. Theprocesswasdevelopedinearly1980sand, sincethen, hasbeenusedbyHomestakeMiningCompanyatLead, SouthDakotaintheUSA.Therefore, itisknownasHomestakeMiningsbiologicalprocess.TheHomestakeMininghasstill beentheonlycompanyintheworld using this method to treat their combined mine and tailingsimpoundment wastewaters. Blending relatively warm mine waterwith tailings pond water to maintain the temperature of the com-binedwastewaters at a sufcientlyelevatedtemperature yearroundis keytotheir successful operation. Theplant wascon-structed in 1984to treat 3600 US gpmat a capital cost ofUS$10 million.Cyanide, thiocyanate, copperandammonia arethemain con-taminants found in the mixed wastewater. The process takes placein rotary biological contactors (RBC) where bacteria are attached toslowly rotating discs as a biolm. About 40% of each disc is sub-merged in the tanks at all times (Maier et al., 2009). The biologicaloxidationprocessiscarriedout intwostages(Whitlock, 1995;Lawrence et al., 1998). The rst stage consists of the bacterial oxi-dation of cyanide and thiocyanate to carbon dioxide, sulphate andammonia. Inthesecondstage, bacterial nitricationoccursandammonia isconvertedto nitrate. Different strains ofbacteria areinvolvedinthevariousstagesoftheprocess. HomestakeMiningholdstwopatentsfortheuseofspecicbacteriaresponsibleforoxidation of cyanide and thiocyanate. Metals associated with cya-nide and thiocyanate are removed through adsorption by bacteria(biolm)oncethewastewaterisfreeof cyanide. Asthebiolmcontinuously sloughs off the discs, the bacteria-metal sludge is re-moved from the efuent by clarication and dual-media sand l-tration, anddisposedintothetailings pond. Incontrastwiththecompetingchemical treatments, hydrogenperoxideandSO2/Air,the efuent can be discharged directly to the environment withoutadditional treatment. Asimplied owchart of the process isshown in Fig. 6.Forty-eight RBCs with 12 foot diameter discs were used to con-tact thebacteriawithwastewater andair. TheRBCs werenothousedbutallwerewellinsulated. Therst24contactorswereemployed in the cyanide and thiocyanate removal stage. The onlyreagents added were soda ash as an inorganic carbon source to aidnitrication and phosphorus as a trace nutrient. Efuent from thebiological contactors was modied by the addition of ferric chlo-rideand/or apolymer toproduceasludgebeforeclaricationand sand ltration. The full-scale plant was operated satisfactorilyandproducedefuentsofthequalitypredictedfrompilotplanttests for about two decades. About 99% removal rate could be ob-tainedforcyanide, reducingtheinitial cyanideconcentrationof4.1 mg/Lto0.06 mg/L. Thiocyanate, ammoniaandcopper wererespectively reduced from 50.0, 6.0 and 1.0 mg/L to less than 0.1,0.1and0.05 mg/L respectivelyin thenal efuent. Although theremoval of copper and iron in the process was high, with 9598%efciency;othermetalssuchasnickel, chromiumandzincwerenot removed as effectively (Maier et al., 2009). The suspended sol-ids content in the treated efuent was 50,000 mg/LCN). How-ever, it has not yet been widely used studies to develop anelectrolytic cyanide and metal recovery (CELEC) process are under-way (Cushnie, 2009).Ozone as a strong oxidising reagent has been used to destroy CNreplacingthealkalinechlorinationprocess(Cushnie, 2009). Theozoneprocessiseffectiveinreducingcyanideconcentrationstoenvironmentally acceptable low levels. Low operational costs areshown to be the prime advantage of the ozone CN destruction pro-cess, where ozone is less expensive than chlorine or sodium hypo-chloride. Ozone is generated on-site typically by the silentelectricaldischargemethod. Productionoftoxiccompoundsasaresult of oxidation of organics does not occur as is the case in thealkalinechlorinationprocess. Anotheradvantageistheabilityofozone to destroy zinc, copper and nickel cyanide complexes. How-ever, the equipment cost including the cost of ozone generator issignicantlyhigher. Oxidation ofcyanateresultingfromtherstoxidationstepof cyaniderequiresexcessiveozonetherebytheoperatingcostissignicantlyincreased. Oxidationbyozonere-quires 1.82.0 lbs of ozone per pound of CN to reach the cyanatestage and 4.65.0 lbs to reach complete oxidation. The ozonationprocesshasalsobeencombinedwithUVradiationforthetreat-ment of halogenated organics.Processessuchas, electrolyticdecomposition, ozonation, acti-vated carbon, ion exchange (an adsorbent resin capable of bindingmetal cyanidecomplexes), hydrogenperoxideoxidationwithorwithout theuseof acatalystisandferroussulphideadsorptionandthermal hydrolysis haveshownpotential for development(Kilborn 1991; Carrillo-Pedroza and Soria-Agullar, 2001; Cushnie,2009; InfoMine,2012). Addition of carbondioxide alone withouta catalyst to replace SO2 was tested with success as an inexpensivealternative to the SO2/Air process (Randol, 1992).Removal/reductionof cyanidebyadsorptionusingactivatedcarbonfromminingwastewatershasbeenfocused(Dashetal.,2009). Due to technically difcult and costly regeneration of acti-vated carbon, the adsorption technology has not been widely used(Crini, 2006). The use of activated carbon adsorption was combinedwith other technologies to meet stringent water quality standards(HowellandChristophersen, 2009), mainlyforremovalofmetalions (e.g., Hg) and other compounds rather than CN.Ionexchangetechniqueshavenotbeenwidelypractisedde-spitepromisingresultsobtainedfromvariouspilot-plantopera-tions. As intheSARTprocess, ionexchangeresins present thepotential torecyclecyanideassociatedandtogeneraterevenuevia the sale of the copper itself (Ripley et al., 1996; SGS TechnicalBulletin, 2010). Recycling of cyanide could also decrease environ-mental risksbyreducingloadingof potentiallyharmful saltsinthe tailings and recycle water (e.g. CNS, CNO, NH3and metal cya-nides, and the need for transportation, storage and handling of cya-nide. If the revenue could offset the operating costs of the cyaniderecoveryplant, thetechnologycouldtransformuneconomicorebodies into viable mines. Main issues cited are ion selectivity, com-petition between ions, ionblinding, elution andassociatedcosts.Because of their ability to extract cyanide and metal cyanide com-plexes directly from gold plant tailings, anion exchange resins havebeen used in the resin-in-pulp (RIP) process, which is a well-devel-oped industrial process usedfor goldand uranium recovery. Theion exchange process has been considered as readily adaptable tocyanide recovery, thereby also circumventing solid/liquid separa-tionprocesses. Moreover, conventional, commercial strong-baseresinsarewell-suitedtocyaniderecoveryapplicationsincethemost common cyanide species in gold plant tailings are free cya-nide anions (usually 100500 mg/L) and the tricyano copper com-plex, both of which can be extracted directly from pulps by anionexchange resins. When the loaded resin is treated with sulphuricacid, copper remains in the resin phase as a precipitate of the com-pound, CuCN, and only two moles of cyanide are released per moleof copper. The Augment and Hannah proprietary processes, where-in CuCN is intentionally precipitated in the pores of a conventionalstrong baseresin and,in thisform,produces aregenerated resin26 N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329that is an efcient adsorbent for free cyanide ions and soluble cop-percyanide(SGSTechnical Bulletin, 2010). AsHannahreferstostrong base resin extraction of free cyanide and metal cyanide spe-cies, the Augment process is a strong base resin extraction of cop-per and cyanide (SGS Technical Bulletin, 2009). Elution of copper(or metal) and cyanide from the resin poses technical difcultiesincreasing the cost of the cyanide recovery process.It has been reported that the sunlight (UV) could function as astrongoxidizerwhenwastewaterissubjectedtosunlightinthepresenceof asemiconductor catalyst suchas titaniumdioxide(TiO2). This process was found to be suitable for the oxidation ofboth free and complexed cyanides dissolved in wastewater (Pargaet al., 2003). When exposed to the sunlight, the catalyst mixed intothe water, creating a slurry, or xed onto a lattice-type structureabsorbs the high-energy photons from the UV portion of the solarspectrumformingpowerfuloxidizersofreactivehydroxyl(OH)radicals. Theseradicalsbreakdownthecyanidewheretherstproduct of photocatalytic oxidation of cyanides in the presence ofpolycrystallineTiO2inaqueousmediumisCNO. UVlight causesmetal complexes such as ferricyanide and ferrocyanide to partiallydissociate. In the case of iron cyanide the compound is photolyzedto free cyanide and iron hydroxide. UV oxidation is limited to rel-atively clear solutions. UV in combination with ozone results in theformation of strong oxidising agent of OH-radicals that can oxidiseiron cyanide complexes. Suitable light sources emit in the range of200280 nm(nm). Ozone will absorb in this band. Additionally, theUV/ozone oxidationdoes not produce undesirable by-productssuch as ammonia (Spartan Environmental Technologies, 2013).2.2.5.2. Thermal cyanide destruction. Solid or liquid wastes contain-ing cyanides are treated at elevated temperature and pressure inbatchor continuousmode(CDS, 2013). Reactionsoccur intwosteps as given in Eqs. (25) and (26) (NMFRC, 2013). Hydrolysis isthe principle of the cyanide destruction process mechanism.Hydrolysis takes place in a heated pressure chamber. High temper-ature (450470F) and pressure (600 psi) signicantly increase thekineticsofhydrolysis, whereOCNisconvertedtoammoniaandbicarbonates/carbonates as shown inNaCNNaOCl ! NaCNO NaCl 252NaCNO 3NaOCl H2O ! 3NaCl N2 2NaHCO326This process was developed in early 1990s and has been shown tobe cost-effective for treating wastes containing high concentrationsof CN. Cyanide wastes containing 100,000 mg/LCN can be treated togive about 25 mg/LCN. The residual CN can be treated by a conven-tional method such asozone orhydrogenperoxide, to reduce theCNconcentrationtoalower, environmentallyacceptablelevel. Itis claimed that the process is able to destroy all cyanide complexes,eventhatofiron. Inausualoperation, CNwastesaretransferredintothehightemperatureandpressurevessel andtreatedonabatchbasisforabout10 handthendischargedtoaconventionaltreatment plant for polishing. For large ows, a continuous processis used and the process is controlled by adjusting the temperatureandpressure. Ammoniagas generatedbytheunit is ventedtoatmosphere. Institutions that tested the process experienced tem-peratureandpressurecontrolproblemsduetoseal leak(N2andoilseal)atthepointofagitatorentryintothevesselresultingindowntime (Cushnie, 2009).3. Process costs and selection of suitable processSince the cost of reagents and the degree of treatment require-ments may vary from site to site, cyanide treatment costs are site-specic. Thedirectcomparisonof cyanidetreatmentcostsfromonelocationtoanotherwouldbemisleading. Cost analysesforboth capital and operating costs should be carried out for each siteconsidering the actual site conditions. The type of process can beselected depending on the site climate, production rate, ore type,possibilityof theuseof natural degradationor zerodischargemethods, need for the removal of other metals or anions, and efu-ent regulations (e.g., from nil to aggressive treatment). Operationalcapacity and life expectancy of the mine should also be taken intoaccount while selecting and deciding on the appropriate treatmentplant. In some cases, if time allows, conducting a series of labora-tory and pilot tests, and scale-up studies should be considered asa useful path in making a decision.Forinstance, astudyconductedattheRyanLodeMinenearFairbanks, Alaskainvestigatedthecomparativecostsofchemicalandbiologicaldestructionofcyanideinminewatersincoldcli-mates. Specically, the capital and operating costs of the SO2/Airprocesstothebiologicaldegradationprocesswerecomparedforthe treatment of rinse water fromspent leachheaps (Nelsonet al., 1998). It was found that the biological method had a highercapital cost, but a signicantly lower operating cost resulting in alow present-worth cost. Therefore, it was suggested that bacterialdetoxication be considered as a viable alternative to affect the re-moval of cyanide from spent leach heaps. In case of Colomac Minein Yellowknife, NWT, Canada, cost benet analyses including cap-ital and operational costs and site specic condition requirements,biological RBC process due to cold temperature of the wastewaterand the cold climate of the location would require heating (Chap-man et al., 2007). After conducting a series of laboratory and pilottests, insitutreatmentinthepitwasselectedasthemostcost-effective process for the site. Alkaline breakpoint chlorination pro-cess was kept as a backup process to be able to meet the requiredefuent quality for discharge. The recent tendency is to use a pro-cesssuchasSART, AVRandionexchange torecoverandrecyclecyanide and metal for economical and environmental reasons. Be-cause SART and AVR applications enable the use of the recoveredcyanide back in the metal extraction process and subsequently re-duce the consumption of cyanide in the overall metallurgical pro-cesses, theyareconsideredtobemoreeconomical thanotherefuent treatment processes which require destruction of cyanide.4. SummaryWater is considered to be a crucial asset for mining and metal-lurgical processes. The mining industry has to deal with water inevery stage of mining, starting from excavation of the ore to the -nal product. Even after a site closure, water may need to be man-aged for many years. Underground mines or open pits should bedrained to be able to access to the ore during excavation. Later dur-ing processing of ores (e.g., extraction and recovery), water is usedas part of each process. Processing of 1 m3of ore requires a mini-mum of1.3 m3ofwater(Kuyucak,2009;Kuyucak andPalkovits,2009). The need for water grows as the mining activities increase.Therefore, water management gains a signicant importance.Waterlosseshavetobeminimizedandrecirculationandreusemustbepractisedasmuchaspossible. Thequalityofwaterhasto be ensured for its compliance before its discharge to the envi-ronment as it is essential in maintaining and sustaining the aquaticresources.As the environmental regulations have become more and morestringent with respect to cyanide, the need for developing removalprocesses has beenincreasing. Natural degradationin tailingsponds is still known to be the most common method, it has beeneithersupplementedorreplacedbyachemicalprocessinmanycases because of its limitation in producing a nal efuent of de-siredquality. Inaddition tocyanide, thetoxic effect ofammoniaN. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329 27is of concern tothe regulatory agencies and investigations to re-duce ammonia concentrations in efuents are underway. Despiteits wide use in the last two decades, the use of alkaline chlorinationprocesshasbecomelimitedduetotoxicnatureofintermediateandby-products. Uniquemethods, suchasHomestakebiologicalprocess and the Hemlo/Golden Giant copper and iron sulphate pre-cipitation processes, are mainly developed and used by theirrespective companies. In addition to the conventional SO2/Air pro-cess, SART,AVRandionexchange processeshavebeenreceivingworldwideattention. Theapplicationof hydrogenperoxidealsohasbeenincreasing. Investigationsforthedevelopment of newprocesses have been ongoing. In arid climates, evaporation is usedto obtain zero discharge.Since the cost of reagents and the degree of required treatmentmay vary from site to site, cyanide treatment costs are site specic.Some processes may require up front high capital costs and mayprovidelower operational costs (e.g., reagent cost, energyandmaintenancerequirements),while some processes canbeimple-mented to a site with small capital costs, but operation using thesemethodsmightbeexpensive. Duringtheprojectplanningstage,cost-benet analyses should be conducted for potential alternativetreatment methods to determine the most appropriate process fora given site.ReferencesAcheampong, M.A., Meulepas, J.W., Lens, P.N., 2010. Removal of heavy metals andcyanide from gold mine wastewater. Journal of Chemical Technology,Biotechnology 85, 590613.Akcil, A., 2002. First application of cyanidation process in Turkish gold mining andits environmental impacts. Minerals Engineering 15, 695699.Akcil, A., 2003. Destructionof cyanideingoldmill efuents: biological versuschemical treatments. Biotechnology Advances 21, 501511.Akcil, A., 2006. Managing cyanide: health, safety and risk management practices atTurkeys Ovacik goldsilver mine. Journal of Cleaner Production 14, 727735.Akcil, A., 2010. ANewglobal approachof cyanidemanagement: internationalcyanide management code for the manufacture, transport, and use of cyanide inthe production of gold. Mineral Processing and Extractive Metallurgy Review 31(3), 135149.Akcil,A., Mudder,T.,2003. Microbialdestruction of cyanidewastesin goldmining:process review. Biotechnology Letters 25, 445450.Akcil, A., Karahan, A.G., Ciftci, H., Sagdic, O., 2003. Biological treatment of cyanide bynatural isolated bacteria (Pseudomonas sp.). Minerals Engineering 16 (7), 643649.Alta Metallurgical Services, Australia, 2012. .American Public Health Association (APHA), 1998. Standard Methods for theExaminationof Water andWastewater.20thEd, EditedbyL.S. Cleseri, A.E.Greenberg and A.D. Eaton.Arellano,C.A.P., Martinez,S.S.,2009. Effects of pH on the degradation ofaqueousferricyanide to ferrocyanide by photolysis and photocatalysis under solarradiation. Solar Energy Materials and Solar Cells 94 (2), 327332.Agencyfor ToxicSubstances andDiseaseRegistry(ATSDR), 2006. ToxicologicalProle for Cyanide. US Department of Health and Human Services, Public HealthService, Atlanta, GA, p.298.Babu, G.R.V., Wolfram, J.H., Chapatwala, K.D., 1993. Degradation of InorganicCyanidesByPseudomonasputidaCells. InBiohydrometallurgicalTechnolgies:Fossil Energy Materials, Bioremediation, Microbial Physiology. In: Torma, A.E.,Apel, M.L., Brierley, C.L. (Eds.), Proceedings of an InternationalBiohydrometallurgy Symposium, Jackson Hole, Wyoming, USA. August 2225,1993, vol. II., pp. 159165.Bhunia, F., Saha, N.C., Kaviraj, A., 2000. Toxicityofthiocyanatetosh, plankton,worm, andaquaticecosystem. BulletinofEnvironmental ContaminationandToxicology 64 (2), 197204.Botz, M.M., Scola, J.C., Fueyo, R., de Moura, W., 2004. Cyanide recovery practice atCerroVanguardia, In: Proceedingof 2004SMEAnnual MeetingandExhibitFebruary 2325, Denver, CO.Botz, M., Mudder, T., Akcil, A., 2005, Cyanidetreatment: physical, chemical andbiological processes. In: Adams, M. (Ed.), AdvancesinGoldOreProcessing,Elsevier Ltd., Amsterdam, pp. 672700. (Chapter 15).Canadian Mining Journal, 2002. INCO R&D Process Development, ProductDevelopment and Cyanide Destruction, April 2002.Carrillo-Pedroza, F.R., Soria-Agullar, M.J., 2001. Destruction of cyanide by ozone intwo gasliquid contacting systems. European Journal of Mineral Processing andEnvironmental Protection 1, 5563.CCME, 2013. CanadianEnvironmental QualityGuidelines andSummaryTable..CDS (Cyanide Destruction Systems), 2013. .Chapman, J.T., Coedy, W., Schultz, S., Rykaart, M., Water treatment and Managementduring the Colomac Mine Closure. In: Proc. Mine Closure Conf. Santiago Chile,October 1619, 2007.Chemeurope, 2003. .Cornell University, 2002.Common Foods That Have High Levels of Cyanide. .Crini, G.,2006. Non-conventional low-costadsorbentsfor dye removal:a review.Bioresource Technology 60, 6775.Cushnie, G., 2009. Pollution Prevention and Control Technologies for PlatingOperations. Section6WastewaterTreatment. seconded., National Centerfor Manufacturing Sciences, Ann Arbor, MI, March 2009. (ISBN:978-0-615266-39-8).Dash, R.R., Balomajumder, C., Kumar, A., 2009. Removal of cyanide from water andwastewater using granular activated carbon. Chemical Engineering Journal 146,408413.Degussa, 1988. Advances inthe treatment of goldmill efuents byhydrogenperoxide. In: Presented by Andrew Grifths. The Annual Meeting of Society ofMining Engineers, Phoenix Arizona, 1988.Devuyst, E.A., Vergunst, R.D., Iamarino, P.F., Aguist, R.J., 1992. Recent application ofINCOSO2/Air Cyanide Removal process. In: Presentedat the 94thAnnualGeneral Meeting of the CIM, Montreal, PQ, April 2729, 1992.Donato, D.B., Nichols, O., Possingham, H., Moore, M., Ricci, P.F., Noller, B.N., 2007. Acritical reviewof the effects of goldcyanide-bearing tailings solutions onwildlife. Environment International 33(7), 974984(Epub2007May30)..Flynn, C.M., McGill, S.L., 1995. Cyanide Chemistry-Precious Metals Processing andWaste Treatment. US Bureau of Mines, NTIS, Publication PB96-117841.Given, B., Dixon, B., Douglas, G., Mihoc, R., Mudder, T., 1998. Combined aerobic andanaerobic biological treatment of tailings solution at the Nickel Plate Mine. In:Mudder, T.I., Botz, M. (Eds.), The Cyanide Monograph, second ed. The CyanideCompendium on CD by Mining Journal Books Limited, London, UK, pp. 391421(ISBN 0-9537-33602).Habashi, F., 1966. Thetheoryof cyanidation. Transactionsof theMineralogicalSociety of AIME 235, 236239.Hagelstein, K.A., Mudder, T.I., 1998. The Ecotoxicological Properties of Cyanide. TheCyanide Monograph, Mining Journal Books Ltd., London, United Kingdom.Howell, C., Christophersen, D., 2009. Three-Phase Mining Efuent Treatment Plantto Meet Stringent Standards. E&MJ, April 2009, pp. 4851.International Cyanide Management Code (ICMI), 2012. .InfoMine, 2012. .Kadlec, H.R., Wallace, S., 2009. Treatment Wetlands. CRC Press, pp. 535537.Kilborn Engineering Ltd., 1991. Best Available Pollution Control Technology. Reportprepared for the Ontario (Canada) Ministry of the Environment Metal MiningSector. .Kirk-Othmer Encyclopaedia of Chemical Technology, 2002. John Wiley & Sons, Inc.4th Ed.Konigsmann, E., Goodwin, E., Larsen, C., 1989. Watermanagement andefuenttreatment practiceat thegoldengiant mine. In: Presentedat theCanadianMineral Processors Conference, January 17, 1989.Kuyucak, N., 2001: AcidMineDrainage(AMD) treatment optionsfor miningefuents. Mining Environment Management Journal, March 2001.Kuyucak, N., 2002. Microorganisms in mining: generation of acid rock drainage, itsmitigation and treatment. European Journal of Mineral Processing andEnvironmental Protection 2 (3), 179196.Kuyucak, N., 2006. Selectingsuitablemethodsfortreatingminingefuents. In:Presentedand publishedinthe Proceedingsof the Water in Mining 2006:Multiple Values of Water Conference to be held on 1416 November 2006 inBrisbane, Australia.Kuyucak, N., 2009. Combiningforstrength:anintegratedapproachtowaterandmine waste. Mining Journal, 2223.Kuyucak, N., Chabot, F., 2009. Passive treatment systems successfully. In: 8th ICARDandSecuringthe Future Conference onWater Treatment Requirements inExtremely Cold Climate in Canada, Skelleftea, Sweden, June 2009.Kuyucak, N., Palkovits, F., 2009. Paste technology integrated approach for watermanagement. In: Paste2009The13th IntSeminaronPasteandThickenedTailings, Vinna Del Mar, Chile, April 2124, 2009. pp. 151161.Lawrence, R.W., Poulin, R., Kalin, M., Bechard, G., 1998. The potential ofbiotechnology in the mining industry. Mineral Processing and ExtractiveMetallurgy Review 19, 523.Luque-Almagro, V.M., Blasco, R., Martnez-Luque, M., Moreno-Vivin, C., Castillo, F.,Roldn, M.D., 2011. Bacterial cyanide degradation is under review: PseudomonaspseudoalcaligenesCECT5344,acase ofan alkaliphilic cyanotroph. BiochemicalSociety Transactions 39, 269274.Maier, R.M., Pepper, I.L., Gerba, J.P., 2009. Environmental microbiology. In: AquaticEnvironments:CaseStudy6.1BenecialBiolmRemovesCyanide, ElsevierInc. Publication, pp. 111. (Chapter 6).Manchester, 2013.Cyanide Oxidation Systems. .Manoranjan, M., Gautam, P., Kumar, J.B., 2003. CyanideDetoxicationProcess.Patent 6551514 Issued on April 22, 2003.Menne, D., 2000. Managing Cyanide in Waste Discharges. .28 N. Kuyucak, A. Akcil / Minerals Engineering 5051 (2013) 1329Mineral Process Control, 2013. info@mpcwa.com. .Ministry of Environment Ontario, 2013. Safe Drinking Water Act, 2002 andAmendment in 2006. .Mining and Petroleum Environment Research Group (MPERG), 2010. Evaluation ofthe Effectiveness of Biological Treatment of Mine Waters. MPERGReportpreparedbyLabergeEnvironmental ServicesforMPERG. Whitehorse, Yukon,Canada, March 31, 2010.Mudder, T., Botz, M., 2001. TheCyanideMonograph, seconded. MiningJournalBooks, London, UK.Mudder, T., Botz, M., 2004. Cyanideandsociety:acriticalreview. TheEuropeanJournal of Mineral Processing and Environmental Protection 4, 6274.Mudder, T., Botz, M., Smith, A., 2001a. The Chemistry and Treatment of CyanidationWastes, second ed. Mining Journal Books, London, UK.Mudder, T., Botz, M., Smith, A., 2001b. Cyanide Compendium. Mining Journal Books,London, England, United Kingdom (1000+ pages on CD).Nelson, G.M., Kroeger, E.B., Arps, P.J., 1998. Chemical and biological destruction ofcyanide: comparative costs in a cold climate. Mineral Processing and ExtractiveMetallurgy Review 19, 217226.NMFRC (National Metal Finishing Resource Center), 2013. Pollution Prevention andControl Technologies for Plating Operations. Section 6 Wastewater Treatment..Parga, J.R.,