2.4Recycling Technologies2.4.1Metal Recycling Technologies

Electronic scrap consists of highly multi-component wastes containing besides steel, plastics mostly all main non-ferrous metals such as aluminium, copper, zinc, tin and lead, ferrous metals and precious metals like gold, palladium and silver. Next to the extraction of harmful components as lead, cadmium, hexavalent chromium and mercury are the high trade prices for gold, copper and silver the determining factor for recovery of the precious metals of electrical and electronic systems. Nonetheless metal recycling technologies are demanding high requirements to achieve an efficient output, a proper disposal of problematic substances and a reliable evaluation of the precious metal contents has to be provided, too.

As exemplified in table 24 precious metals are used in components such as pin connectors, contact points, silver coated wire, terminals, capacitators, plugs and relays (PCB components).

Table 24: Metal compounds in EES componentsMetal compoundMain use in EES components

Copper

Conductors (wire harness, coils, electric motors, PCB)

IronHousings and structural parts, magnetic cores (in coils, solenoids, motors, transformers)

Aluminium

Housings of components, heat sinks

LeadBattery (lead acid), piezoelectrical components (sensors + actuators), solder (PCB, junction boxes and all components soldered to wires), pyrotechnical initiators, carbon brushes of electric motors

TinSolder (PCB, junction boxes and all components soldered to wires)

NickelBattery (NiMH), PCB

ZincBattery, metal coatings

MagnesiumAluminium alloys, castings

MercuryDischarge lamps (also used in LCD screens)

SilverPCB components, batteries, heating wires of window heating

GoldPCB components and electrical contacts, relays, connectors

PalladiumPCB components

PlatinPCB components, electrodes of sensors / actuators

Metallic alloys (CuZn, CuSn)Wire harness

LithiumBattery (Li-ions)

TungstenDecandescent lamps

IndiumLCD panels

Zr (ZrO2)Ceramics (e.g. lambda control sensor, piezoelectrics)

Titanium (TiO2)Ceramics (e.g. lambda control sensor, piezoelectrics)

Other (bismuth, antimony, tantalum)PCB

In the following chapter the pyrometallurgical processes, which are applied for the recovery of the major amounts of metals from end-of-life vehicles copper, aluminium, iron and steel and the hydrometallurgical processes, which are applied for the recovery and refinement of secondary non-ferrous metals and precious metals, are described.

2.4.1.1Pyrometallurgical Processes Smelting

Smelting consists of separation and purification of specific metals by means of heating and treating the scrap fractions. The non-ferrous metals alloys can be treated by means of pyrometallurgical processes by taking advantage of their different smelting point meaning that the material is processed in high-temperature reactions to separate metals from impurities. The heavy fraction of scrap consisting of lead, copper and zinc can be extracted by selective smelting processes. In general, the extraction of metals from electroscrap is a complex process, integrated with several process lines to refine various primary and secondary metals.

The recovery of metals in a selective smelting process is done on special sweating furnaces (rotary, reverberatory, or muffle furnaces) where the metal fraction is slowly heated. The oxidation of the finely divided scrap is minimised by a multi zone temperature control, which allows the controlled melting of the different metals present [Rosseau 1991].

The TS-process (Trennschmelz-Verfahren) described by Rosseau (1991) for metal recovery is based on the selective smelting of a particular metal by partial immersion of the previously dried scrap in a bath consisting of the same metal. Consequently only one metal can be recovered in each step, starting with the lowest melting point, followed subsequently by the rest of the components. The most frequent metals recovered by this process are lead-rich (90% Pb), zinc-rich (92% Zn) and copper-rich (45 50% Cu) products by the performance of two subsequent TS-treatments.

A variety of furnaces can be used for melting metal scrap. The choice of furnace depends upon the quality and composition of the metal scrap, the desired production rate, and the mode of operation desired. Other factors influencing furnace selection are capital costs, refractory lifetime, and metal losses.

The processing of several metals by smelting processes as listed in table 25 will be described in the next paragraphs, on the basis of the different furnace descriptions :

Table 25: Overview of pyrometallurgical processes for the recovery of metals

Pyrometallurgical processMetals

SmeltingCu, Al, Zn, Pb, Mg

Reverbatory furnaceCu, Al, Zn

Electric furnacesteel, Fe

Multichamber furnaceAl, Zn, non-ferrous metals

Blast furnacesteel, Cu

Crucible furnaceZn, Mg

Gas fired furnacePb, Zn, Cu, Cd, Al

Rotary furnaceCu, Al, Zn

Induction furnaceAl, Zn

Blast oxidation furnacesteel

Electric arc furnacesteel

Reverberatory Furnace

Reverberatory furnaces heat the required metals to melting temperatures with direct fired wall mounted burners. The primary mode of heat transfer is through radiation from the refractory brick walls to the metal scrap, but convective heat transfer also provides additional heating from the burner to the metal. Reverberatory furnaces are available with capacities ranging up to 150 tons of molten metals. The advantages provided by reverberatory melters are the high volume processing rate, and low operating and maintenance costs. The disadvantages include the high metal oxidation rates, low efficiencies, and large floor space requirements.

Figure 56: Reverberatory Furnace [http://www.osha.gov/SLTC/etools/leadsmelter/popups/blastfurnacelleadtap_popup.html]

Electric Reverberatory Furnace

Electric reverberatory furnaces are used primarily as holding furnaces. These furnaces are refractory lined vessels using resistance heating elements mounted in the furnace roof above the hearth. These furnaces are used for smaller melting applications where limitations on emissions, product quality, and yield are of high priority.

Advantages over gas-fired reverberatory furnaces include low emissions, low metal oxidation, and reduced furnace cleaning. Disadvantages include high fuel costs, low production rates, higher capital costs, and frequent replacement of heating elements.

Multichamber Furnace

This type of furnace consists of two rooms. The scrap is fed into one of the chambers. In the other room, the metal is heated using a flame, identical to the reverberatory furnace. Through a system with natural or forced convection, the warm metal is transported to the room with the scrap. By circulating the liquid metal through it, the scrap is melted. This type of furnace is mostly used for melting moderately polluted types of scrap.

Blast Furnace

A blast furnace is a smelting furnace consisting of a vertical cylinder atop a crucible, into which lead-bearing charge materials are introduced at the top of the furnace and combustion air is introduced at the bottom of the cylinder. It operates in temperature greater than 980C in the combustion zone that metal compounds are chemically reduced to elemental metals (e.g. lead and elemental lead).

Figure 57: Blast Furnace [http://www.osha.gov/SLTC/etools/leadsmelter/smelting/blastfurnace.html]

Crucible Furnace

The crucible furnace consists of a crucible of refractory material in which the metal is melted by direct heating with a flame, or by an induction spiral. In order to tap the furnace, it is turned over manually or by using a hydraulic device. This type of furnace is used in the recycling industry to remelt thin-walled, clean types of scrap. Advantages provided by the electric crucible furnace is the near elimination of emissions and low metal oxidation losses. Disadvantages include increased fuel costs and size limitation.

Figure 58: Diagram of a crucible furnace with an inward radiating metal fibre burner [http://www.acotech.com/appl10.htm]

Gas Fired Crucible Furnaces

Crucible furnaces are small capacity, indirect melters/holders typically used for small melting applications or exclusively as a holding furnace. The metal scrap is placed our poured into a ceramic crucible which is contained in a circular furnace which is fired by a gas burner. The energy is applied indirectly to the metal by heating the crucible. The advantages of crucible furnaces are their ability to change alloys quickly, low oxidation losses, and their low maintenance costs. Disadvantages include low efficiency, (as low as 12%), high emissions, and size limitations [Metal Advisor 2004].

Energy efficiency can be improved by 50% by adding a ceramic matrix recuperator to the exhaust system to recover waste heat for preheating the combustion air.

Rotary Furnace

Rotary furnaces are used almost exclusively for reclaiming low grade scrap. The Furnace operates by rotating the charge through the furnace which comes in direct contact with a gas burner or with a refractory wall which was directly heated by the burner. Typical rotary furnaces have holding capacities of 2 to 5 tons and are usually charged with salt which acts as a flux to improve metal recovery and reduce oxidation.

The advantage provided by rotary furnaces is their ability to process dross and low-grade scrap which is difficult to process in other types of furnaces. The disadvantages are low efficiency, higher maintenance requirements, and considerable salt cake production which must be disposed of as hazardous waste.

Figure 59: Rotary Furnace [http://www.alliedmineral.com/products/rotaryfurnaces.htm]

Induction Furnace

There are two general types of induction furnaces: channel and coreless. Channel furnaces are used almost exclusively as holding furnaces. Channel furnaces operate at 60 Hz where the electromagnetic field heats the metal between two coils and induces a flowing pattern of the molten metal which serves to maintain uniform temperatures without mechanical stirring. Coreless furnaces heat the metal via an external primary coil. Coreless furnaces are slightly less efficient than channel furnaces, but their melt capacity per unit floor area is much higher. Coreless furnaces are used mainly for melting of finely shredded scrap where they are most cost competitive with gas-fired furnaces. Advantages of induction furnaces include high melting efficiency (5070%), low emissions, low metal oxidation losses, and high allow uniformity due to increased mixing. Disadvantages are primarily their high capital and operating costs.

Figure 60: Coreless Induction Furnace [http://www.alliedmineral.com/products/corelessinductioncopper.htm]

Figure 61: Channel Induction Furnace [http://www.alliedmineral.com/products/channelfurnacecopper.htm]

Basic Oxygen Furnace (BOF)

The BOF is a pear-shaped furnace, lined with refractory bricks, that refines molten iron from the blast furnace and scrap into steel. Up to 30% of the charge into the BOF can be scrap, with hot metal accounting for the rest. Scrap is dumped into the furnace vessel, followed by the hot metal from the blast furnace. A lance is lowered from above, through which blows a high-pressure stream of oxygen to cause chemical reactions that separate impurities as fumes or slag. Once refined, the liquid steel and slag are poured into separate containers. The main advantages include its rapid operation, lower cost and ease of control.

Figure 62: Diagram of the Basic Oxygen Furnace [http://homepage.tinet.ie/~jcelce/subjects/metalwork/pages/bop.html]

Electric Arc Furnace

An Electric Arc Furnace is a steel melting furnace in which heat is generated by an arc between graphite electrodes and the metal. The basic material is metal scrap in place of molten metal, and both carbon and alloy steels are produced. Furnaces with capacities up to 200 tonnes are now in use. The Electric Arc Furnace (EAF) offers an alternative method of bulk steel manufacture, utilising scrap as a metal source, e.g. car scrap.

The EAF has evolved into a highly efficient melting apparatus and modern designs are focused on maximising the melting capacity of the EAF. Melting is accomplished by supplying electrical or chemical energy to the furnace interior. The melting point is reached at around 1520 C and the steelmaking efficiency is about 55-65 % [Jones, 2004].

The first step in the production is to select the grade of steel to be made. Many operations add some lime and carbon in the scrap and supplement this with injection. After the scrap is loaded in the furnace, the roof is lowered and then the electrodes are lowered to strike an arc on the scrap, this commences the melting portion of the cycle.

Once the final scrap charge is melted, flat bath conditions are reached. The analysis of the bath chemistry will allow the melter to determine the amount of oxygen to be blown during refining and arrange the alloy additions to be made. Refining operations in the electric arc furnace have traditionally involved the removal of phosphorus, sulphur, aluminium, silicon, manganese and carbon from the steel with the addition of oxygen throughout the cycle, as a result some of the melting and refining operations occur simultaneously. In recent times, dissolved gases, especially hydrogen and nitrogen, has been recognised as a concern. Other operation include the oxidation of impurities by de-slagging [Jones, 2004].

Once the desired steel composition and temperature are achieved in the furnace, the tap-hole is opened, the furnace is tilted, and the steel pours into a ladle for transfer to the next batch operation (usually a ladle furnace or ladle station).

Figure 63: Electric Arc Furnace [http://www.steel.org/learning/howmade/eaf.htm]

The following section characterises the individual smelting procedures by metal type present in automotive EES:

Copper Smelting

Low-grade copper scrap is melted in either blast or rotary furnace resulting in slag and impure copper. The smelting point of Cu is approximately 1080C. In the blast furnace, the copper is charged to a converter, where the purity is increased to about 80 to 90%, and then to a reverberatory furnace, where purity levels of 99% are achieved [EPA 1995]. In these fire-refining furnaces, flux is added to the copper and air is blown upward through the mixture to oxidise impurities. Then by reduced atmosphere, cuprous oxide (CuO) is converted into copper. Fire-refined copper is cast into anodes, which are used during electrolysis. The anodes are submerged in a sulphuric acid solution containing copper sulphate. As copper is dissolved from the anodes, it deposits on the cathode, with a purity up to 99.99%, where it is extracted and recast.

The facilities for a low grade copper (less than 80% copper) need to have in addition a converter step and fire-refining to obtain high grade recycled copper. Further processes may include alloying with other metals.

Aluminium Smelting

Aluminium smelting takes place primarily in reverberatory furnaces, but also in tower, rotary and sweating furnaces. Usually these recovery facilities use batch processing in melting and refining operations. The aluminium fraction is mostly remelted under the addition of fluxing salts to prevent oxidation in a temperature range from 585C - 650C depending on the amount of alloying elements.

The induction smelting and refining process is designed to produce aluminium alloys with increased strength and hardness by blending aluminium and hardening agents in electrical induction furnace. The process include charging scrap, melting, adding and blending the hardening agent, skimming, pouring and casting into notched bars. Hardening agents include manganese and silicon [EPA 1995].

Zinc Smelting

Zinc scrap is processed by selective smelting in rotary, crucible, reverberatory, and electrical induction furnaces. Flux is used in these furnaces to trap impurities from the molten zinc. The impurities float to the surface of the melt and dross, and is subsequently skimmed from the surface while the remaining molten zinc is poured into molds or transferred to the refining operation in an molten state [EPA 1995]. A sweating furnace slowly heats the scrap containing zinc and other metals to approximately 364C. This temperature is sufficient to melt zinc but is still below the melting point of the remaining metals [Rosseau 1991]. Molten zinc collects at the bottom of the furnace and is subsequently recovered. The zinc alloys produced from pre-treated scrap during sweating and melting processes may contain small amounts of other metals like copper, aluminium, magnesium, lead or others present in the mixture.

Lead Smelting

Lead scrap with Zinc and other metals may be extracted from the heavy fraction by selective smelting. In a selective smelting lead is the first metal smelted at 350 C. In a second furnace zinc is smelted at 425 C and the remaining copper rich fraction with precious metals is further separated in a copper smelter [Dalmijn, Witteveen 1991].

Magnesium Smelting

Magnesium scrap is sorted and charged into a steel crucible furnace maintained at approx. 675 C. To control oxidation of the melt fluxes of chloride salts, magnesium, barium, magnesium oxides and calcium fluorides must be added. This fluxes float on the melt preventing air contact. The composition of the melt must be carefully monitored, once the molten metal reaches the desired levels of key components, it is poured, pumped, or ladled into ingots.

Iron and Steel Smelting

The two basic types of furnace used in steel making and recycling are the electric arc furnace, and the basic oxygen furnace. While the electric arc furnace is almost entirely designed to operate on steel scrap, the oxygen furnace is able to use only 20 to 30 % of the melt capacity of steel scrap. The remaining 70% to 80% consists of molten pig iron which is used in a blast furnace from iron ore, limestone and coke.

2.4.1.2Hydrometallurgical Processes

The hydrometallurgical recycling is characterised as a multistage process where acquired metals are dissoluted and recovered by methods like electrolysis, extraction, cementation, ion-exchange or precipitation. Therefore hydrometallurgical processes are also named under wet-chemical processes.

Figure 64: Overview of the main hydrometallurgical recovery methods[Modified from http://147.96.1.15/info/metal/qprep.htm]

Compared to pyrometallurgical processes it uses much less energy. Its industrial scale is more than 10 times smaller than the conventional pyrometallurgical smelter plants.

Dissolution

After a thermal and mechanical treatment, the metals are dissolved at pressurised vessels using water, oxygen and other substances at ambient temperature. The objective of the leaching process in the recycling process is to produce a metal from impure metal or metal compounds, which have been prepared by a pyrometallurgical process. The leaching process can be performed by two methods: the heap leaching and the agitation leaching.

In the heap leaching process the solids are brought in contact to the leaching solution by pumping or evenly spraying the solution over the top of the heap [Boyer, Gall 1985]. As the solution percolates through the heap it dissolves the metal. The agitation is either performed by bubble action using compressed air or mechanical movement using impellers. The standard equipment is the Pachuca tank. Through a central pipe an air lift pushes the solution in an upward direction increasing the contact of solids to the leaching solution. The maintenance and operating costs are lower then for mechanically working agitators.

For mechanically working agitating tanks the main difference is the type of impeller, depending on the flow pattern by the type of tank. The major types of impellers used are marine propellers, paddles and turbines. Here pressure leaching has the objective to decrease the dissolution time of metals by permitting higher operating temperatures. This supports the solubility of oxygen and further the rapid oxidation of metals. The standard equipment used for pressurised metal leaching is the autoclave (see Figure 65). They are made out of metal for strength and often are of stainless steel or titanium because of severe corrosion that can occur at high pressure and high temperatures. Most autoclaves have agitators for mixing combined with heating and cooling coils for temperature control of the process.

The schematic below shows the components of a horizontal acid leach autoclave:

Figure 65: Illustration of an autoclave for metal dissolution

(1.Motor Drive Assembly for Agitator, 2. Compartment Divider, 3. Agitator Shaft, 4. Service Nozzle, 5. Support Saddle, 6. Carbon Steel Shell/Lead and Brick Lined )

The process of the selective dissolution of silver, platinum and base metals by nitric acid is still used today. It was temporarily substituted by sulfuric acid, but because of the costly equipment and considerable off-gas problems it changed back again.

(Me)solid+ 2 HNO3+ O2 Me(NO3)2 dissoluted+ H2O

Not only the high consumption of expensive and complex reagents also the disposal of the non-metallic fraction and the treatment of solvents and sludges are concern of hydrometallurgical processes.

Recent research focus on a single dissolution stage, which excludes interstage pollution and contamination problems [Goosey, Kellner 2002]. Here various metals are dissoluted at once, with highly concentrates of HCl + NaCl electrolytes (ph1). Chlorine is leaching metals of electronic scrap via oxidative dissolution and produces a de-metallised waste:

2CL- 2Cl + 2 e-

Mscrap+ Cl- MCl-

MCl- M++Cl

Dissolution of Gold

The recovery of gold containing compartments of PCBs are whether edge connectors or gold coated assemblies (edge, pin connectors) from manually separated scrap boards. Gold is separated as a metal flake via the acidic dissolution of copper substrates or the dissolution of gold in cyanide/thiourea based leachants:

2 Au + 4NaCN + O2+ 2H2O 2 Na[Au(CN)2] + H2O2+ 2NaOH

Conventional methods for gold and silver recovery are based on the dissolution in cyanide leaching media over the entire pH range [Reprints, 1991]. The fine grinded gold is dissolved by sodium cyanide or calcium cyanide which then reacts under oxygen and lead to a cyano-complex. To increase the velocity of the solubility Lurgi proposed a process where the operation is put under pressure of 40 bar where the reaction time is reduced to 2h [Rousseau 1991]. To fasten the reaction even more an additional oxygen donor, such as hydrogen peroxide is added. These reaction processes are efficient within a pH range of 10 and 12 and in the presence of catalysts as Fe(III) and Cu(II), which enable the decomposition of hydrogen peroxide and consequently the oxidation of cyanide (CN2-) to cyanate (CN4+).

The gold recovery takes place by precipitation or electrolysis. It can also be extracted from solutions with a very low gold content ( 0,1 g/L) by ion exchange resins or activated charcoal extraction [Habashi 1997].

Ion Exchange

The ion-exchange process involves an adsorption and an elution process in an ion-exchange column. Its aim is whether to separate selectively dissolved metals or concentrate metals.

In the adsorption process metal-ions are removed from an aqueous solution while passed through a bed of resins. These resins can be cationic or anionic. The cathodic resins are strong or weak acids, which exchange H+ions. Anionic resins are strong or weak basis. Here mostly Cl-ions are exchanged with anions in solution [Boyer, Gall 1985]. The phenomena of the affinity of the resins towards specific metal ions realise the selection of specific metal ions from a complex solution.

For the recovery of copper, nickel and lead sodium-based resins weak-acid chelated resins are applied. The metal ions are captured by the resins and backwashes with an acid solution, which supplies hydrogen ions and exchanges with the metal ions and returns as metal salts in a acid holding tank.

In the REMCO metal recovery process ion exchange (MRIX) process 10 gallons of solution per cubic feet of resin is produced (ca. 1,400 litre per cubic meter resin) [http://www.remco.com/ix-procs.htm]. Some metal salts are easily removed by evaporation (sodium forms of cyanide and chromium, sulphate or chloride salts of copper and nickel), for metals as copper, nickel, zinc it is better to recover by electroplating. To prepare the column for the next operation a thorough fresh water rinse is arranged and a solution of 5% sodium hydroxide is passed through the column to replace the hydroxide ions with sodium ions. A second backwash is generated to remove the remaining caustic soda. The generation cycle takes about 3-5 h.

The ion-exchange plant require high capital costs and a large plant area, because of the large amounts of input material and enhanced process technology [Boyer, Gall 1985]. Ion exchange system are able to recover from a concentration of 10-50 ppb.

Extraction

The extraction of metals is a method to prepare the process products of preceding treatments (decomposition) for following procedures (electrolysis). It is a chemical process which selectively exchanges metal species between an impure feed solution and a pure aqueous feed solution. The metal dissolves in an organic solution and is stripped from the organic solution (alcohol, ethers, ketones etc.) by an aqueous solution. One objective is to separate the desired precious metals from interfering components. A complete extraction of a metal is normally not to achieve. A small fraction of precious metals always remains with the base metals and cannot be recovered. But to deduce the loss, the process can be arranged reversibly.

Carbon Adsorption

The use of the carbon adsorption process is exclusively to recover noble metals as gold and silver. At low temperatures the metals deposit in metallic form on the carbon. The metal leaching solution is fed to carbon columns and almost completely removed from the solution by adsorption on the solid carbon. When the carbon is loaded the metals are stripped, which can be done by passing a stripping solution over the column. The stripping is followed by electrowinning from solution of a very pure gold or silver product.

Electrolysis

The aim of the electrolysis is the decomposition of a metal solution by a supplied current on two poles. The ions are neutralised when transferred to the different charged poles. At the cathode positive valued metal ions are separated from other ions moving into the direction of the anode.

A recently developed method with resulting low capital and operation costs for the recovery of copper and zinc is the application of an slurry electrolysis cell. The electrolytic reaction initiated through an applied current (1.25-1.75 V) in an electrolytic cell leads to a redox-reaction. In the electrolysis oxygen and hydrogen is produced at the anode, initiating the leaching of copper/zinc from the cathode and producing acid.

This process is however only possible if a mixing is avoided through the separation electrolyser cell [Habashi 1997]. A mixing reaction is avoided by a membrane, inhibiting the transport of large metal ions. The electrolyser cell contains anode compartments, a cell separator on either side of the anode chamber and a cathodic compartment as illustrated in the following figure.

Figure 66: Scheme of an electrolysis cell

Anodic reaction

1. In base electrolyte: 4 OH- O2+ 2 H2O + 4 e2. in acid electrolyte: 2 H2O O2+ 4 H++ 4 e

Cathodic reaction

Men++ ne- Me

Nickel can be recovered electrolytically to give pure nickel powder. The electrolyte has to be refreshed continuously and includes also stabilisator and buffer substances [Nickel, 1996]. The anolyte solution is a NaOH solution of a pH>6 [Rousseau 1991]. At the anode chloride ions are oxidising to sodium hypochlorite and nickel is recovered from the solution by electro- deposition as a counter reaction for the chlorine generation at the cathode.

The precious metals gold, silver, platinium and palladium are recovered by this method. Precious metals are easier to separate from solvents than unprecious ones, because of the higher affinity to anions. It was observed that the metals Ag, Pd, Au, Cu, Pb, Sn of one solution can be selectively recovered under the application of varying decomposition potentials [Goosey/Kellner 2002].

Electro-refining

Metals as copper, lead and nickel, which are recovered from smelters in a purity from 9098.5 % are refined by electrolysis. The refinement of copper takes place in an electrolysis cell, where copper anodes, produced by pyrometallurgical processes are dissoluted and recovered at the cathode with a purity of 99.9%.

Impurities in the copper anode include lead, zinc, nickel, arsenic, selenium, tellurium, and several precious metals including gold and silver. Potentiostatic control is essential to prevent the dissolution of silver and disposition at the copper cathode. So that less active metals like the precious metals silver, gold, platin, palladium are not oxidised at the anode, but aggregated at the bottom of the electrolysis cell and recovered by several treatment processes. Nickel impurities of the copper fraction are dissoluted in the solvent.

Depending on the metal being plated different cathode materials can be used. For copper electrowinning titanium, stainless steel and copper itself (see Figure 67) can be used [Boyer, Gall 1985]. Aluminium is used to recover zinc and titanium, and stainless steel has been used to recover manganese and cobalt.

The electrolyte consists of an acidic solution of CuSO4. Application of a suitable voltage to the electrodes causes oxidation of copper metal at the anode and reduction of Cu2+to form copper metal at the cathode. This strategy can be used because copper is both oxidised and reduced more readily than water.

Figure 67: Electrolysis cell for refining of copper. As the anodes dissolves, the cathodes is grown in size on which the pure metal is deposited. [http://cwx.prenhall.com/bookbind/pubbooks/blb/chapter23/medialib/blb2304.html]

Once the metal has been deposited on the cathode it must be recovered. This usually involves peeling the plated metal away from the cathode.

Cementation

Cementation is a type of precipitation method implying an electrochemical mechanism. The tendency of one metal to displace or reduce another metal from solution is based on the potential of metals for reduction. A more electropositive metal will tend to reduce a less electropositive metal from solution (e.g. zinc will tend to reduce a less active or noble metal as silver or copper). The greater the difference in potential between the two metals the more complete will the reaction be. The rate at which cementation reactions occur depend on initial concentrations, temperature, agitation, polarisation, metal characteristics and agents [Boyer, Gall 1985]. An example of cementation at industrial scale is the copper reduction by metallic iron. However, the noble metals (Ag, Au and Pd), as well as As, Cd, Ga, Pb, Sb and Sn, can also be recovered in this way.

Copper

Copper cementation is a procedure where a more noble metal can be extracted with a less noble metal (e.g. iron, aluminium or Fe-Al alloys). The positioning of a less noble metal iron leads to the deposition of a more noble metal like copper in form of sludge. This occurs through the exchange of their charge, the electrons switch to the copper:

CuSO4+ Fe2 FeSO4+ Cu

Cu2++ 2 e- Cu

Fe Fe2++2e-

Concentrates produced vary from about 65 to 95 per cent copper according to the procedures used. Copper sulphates are fed to open launders or cones containing steel scrap, where copper is recovered at the bottom.

Gold

Another cementation process is the recovery of gold through the copper cementation of gold thiosulphate with the use of alkaline thiosulphate solution [Choo, Jeffrey 2004]. This involves the dissolution of the substrate metal, and the simultaneous reduction and deposition of gold from the solution. During the cementation of gold on copper, the gold thio-sulphate is reduced to gold metal. Cementation of Cu/Au occurs according to the following stoichiometry:

Au (S2O3)23-+ S2O32-+ Cu Cu (S2O3)35-+ Au

The presence of copper in these solutions introduces the problem of co-precipitation of copper during gold recovery. However, cementation offers a means of getting around this, simply by selecting an appropriate substrate metal. Of these substrates, copper is among the most promising, as the copper that goes into solution during cementation could be oxidised to Cu(II), which is the oxidant during the leaching process.

Precipitation

The removal of ionic species from a solution as solid compounds is described by the precipitation process. Precipitation can be used for the removal of impurities and concentration of the metal compound, e.g. the recovery of sulfides of nickel, copper, lead and zinc form leaching solutions. The treatment costs are low and solutions with very low concentrations of metal values can be treated.

The precipitation methods can be described by:

1. Addition of chemicals (appropriate cation or anion)

2. Changing of the pH value

(as the OH- value increases, the solution becomes more basic, solid hydroxides precipitate (iron, copper, cobalt, nickel are precipitated selectively as hydroxides in solutions by raising the pH with mild and lime)

3. Evaporation of water from the solution

Gaseous Reduction

Also reducing gases as hydrogen, carbon monoxide and sulphur dioxide can be used to reduce metals from an aqueous solution. Hydrogen is among the most widely used, because of simple reaction products. Using higher pressures of hydrogen gas and higher temperatures in an autoclave nickel is reduced from solution according to the following chemical reaction:

Ni2++ H2 Ni + H+What are low alloy steels, and what precautions should I take when welding them?Frequently Asked QuestionsLow alloy steels contain a few percent (typically between 1 and 7%) of elements such as Cr, Ni, Mo and V. This category includes chromium steels (containing up to 5% Cr and 1% Mo) and nickel steels (containing up to 5% Ni).Low alloy steels are generally weldable (seeWhat is weldability?), but it is important to know the service, joint configuration and the subgroup of the material type. Low alloy steels can be welded by most processes, as long as adequate precautions are taken to avoid defects. It is important to know the composition of the material, either from a mill sheet or a dedicated chemical analysis, as composition influences weldability significantly.With increasing carbon or alloy content, low alloy steels generally become more difficult to weld as the heat affected zone hardness increases. The need forpostweld heat treatment(PWHT) of these joints also increases. The composition is also important in identifying high, but allowable, levels of residual elements such as sulphur or phosphorus, which can lead to problems with liquation cracking ortemper embrittlementduring PWHT.To avoid fabrication hydrogen cracking, it is important to use low hydrogen processes and consumables, particularly as increasing the carbon and alloy content, and increasing the section thickness, increases the risk of hydrogen cracking. Apost-heattreatment may be required to reduce the levels of hydrogen in the weld region.1: Brittle Fracture SurfaceMaterials that do not fail in a ductile manner will fail in a brittle manner.Brittle fractures are characterised as having little or no plastic deformation prior to failure.Materials that usually fracture in a brittle manner are glasses, ceramics, and some polymers and metals. Under some circumstances some metals that are usually ductile will fail in a brittle manner, possibly with catastrophic results.Like ductile fractures, brittle fractures also have a distinctive fracture surface. The fracture surface of a brittle failure is usually reasonably smooth. The crack propagates through the material by a process called cleavage.The images below show the fracture surface of a steel that failed in a brittle manner.

CleavageCrack propagation (cleavage) in brittle materials occurs through planar sectioning of the atomic bonds between the atoms at the crack tip.

What are low alloy steels, and what precautions should I take when welding them?Frequently Asked QuestionsLow alloy steels contain a few percent (typically between 1 and 7%) of elements such as Cr, Ni, Mo and V. This category includes chromium steels (containing up to 5% Cr and 1% Mo) and nickel steels (containing up to 5% Ni).Low alloy steels are generally weldable (seeWhat is weldability?), but it is important to know the service, joint configuration and the subgroup of the material type. Low alloy steels can be welded by most processes, as long as adequate precautions are taken to avoid defects. It is important to know the composition of the material, either from a mill sheet or a dedicated chemical analysis, as composition influences weldability significantly.With increasing carbon or alloy content, low alloy steels generally become more difficult to weld as the heat affected zone hardness increases. The need forpostweld heat treatment(PWHT) of these joints also increases. The composition is also important in identifying high, but allowable, levels of residual elements such as sulphur or phosphorus, which can lead to problems with liquation cracking ortemper embrittlementduring PWHT.To avoid fabrication hydrogen cracking, it is important to use low hydrogen processes and consumables, particularly as increasing the carbon and alloy content, and increasing the section thickness, increases the risk of hydrogen cracking. Apost-heattreatment may be required to reduce the levels of hydrogen in the weld region.1: Brittle Fracture SurfaceMaterials that do not fail in a ductile manner will fail in a brittle manner.Brittle fractures are characterised as having little or no plastic deformation prior to failure.Materials that usually fracture in a brittle manner are glasses, ceramics, and some polymers and metals. Under some circumstances some metals that are usually ductile will fail in a brittle manner, possibly with catastrophic results.Like ductile fractures, brittle fractures also have a distinctive fracture surface. The fracture surface of a brittle failure is usually reasonably smooth. The crack propagates through the material by a process called cleavage.The images below show the fracture surface of a steel that failed in a brittle manner.

CleavageCrack propagation (cleavage) in brittle materials occurs through planar sectioning of the atomic bonds between the atoms at the crack tip.

What are low alloy steels, and what precautions should I take when welding them?Frequently Asked QuestionsLow alloy steels contain a few percent (typically between 1 and 7%) of elements such as Cr, Ni, Mo and V. This category includes chromium steels (containing up to 5% Cr and 1% Mo) and nickel steels (containing up to 5% Ni).Low alloy steels are generally weldable (seeWhat is weldability?), but it is important to know the service, joint configuration and the subgroup of the material type. Low alloy steels can be welded by most processes, as long as adequate precautions are taken to avoid defects. It is important to know the composition of the material, either from a mill sheet or a dedicated chemical analysis, as composition influences weldability significantly.With increasing carbon or alloy content, low alloy steels generally become more difficult to weld as the heat affected zone hardness increases. The need forpostweld heat treatment(PWHT) of these joints also increases. The composition is also important in identifying high, but allowable, levels of residual elements such as sulphur or phosphorus, which can lead to problems with liquation cracking ortemper embrittlementduring PWHT.To avoid fabrication hydrogen cracking, it is important to use low hydrogen processes and consumables, particularly as increasing the carbon and alloy content, and increasing the section thickness, increases the risk of hydrogen cracking. Apost-heattreatment may be required to reduce the levels of hydrogen in the weld region.1: Brittle Fracture SurfaceMaterials that do not fail in a ductile manner will fail in a brittle manner.Brittle fractures are characterised as having little or no plastic deformation prior to failure.Materials that usually fracture in a brittle manner are glasses, ceramics, and some polymers and metals. Under some circumstances some metals that are usually ductile will fail in a brittle manner, possibly with catastrophic results.Like ductile fractures, brittle fractures also have a distinctive fracture surface. The fracture surface of a brittle failure is usually reasonably smooth. The crack propagates through the material by a process called cleavage.The images below show the fracture surface of a steel that failed in a brittle manner.

CleavageCrack propagation (cleavage) in brittle materials occurs through planar sectioning of the atomic bonds between the atoms at the crack tip.