Minerals Engineering, Vol. 7, No. 7, pp. 839-849, 1994 Elsevier Science Lid

t ][~rlgarllon Printed in Great Britain 0892-6875/94 $7.00+0.00

0892-6875(94)E0028-A

THE FLOTATION OF GOLD BEARING ORES - A REVIEW

C.T. O'CONNOR and R.C. DUNNE'~

Department Of Chemical Engineering, University Of Cape Town, Rondebosch, 7700, South Africa i" Newerest Mining Limited, 179 Great Eastern Highway, Belmont, WA 6104, Australia

(Received 16 November 1993; accepted 5 Janualy 1994)

ABSTRACT

The practice of the flotation of pure gold and gold-bearing ores such as tellurides, aurostibite, kerogen, pytqte, pyrrhotite, copper-goM ores and mixed sulphMes is reviewed. The factors which itfluence the choice of collectors, pH and Eh, are discussed as well as the application of differential flotation. The importance of proper conditioning is highlighted and applications of valqous flotation cells attd circuits briefly discussed.

Keywords Flotation, Gold-bearing ores

INTRODUCTION

Gold is a relatively rare metal and the average content in the earth's crust is about 3ppb. The average gold grade of a low grade deposit is only some 3ppm and this low abundance makes the exploration, sampling, assaying and processing of gold bearing ores difficult. Gold occurs mainly in its native form and to a lesser degree as gold compounds of telluride, antimony and selenium contained within sulphide (especially pyrite and, to a lesser degree, arsenopyrite and pyrrhotite), silicate, carbonate and oxide minerals [1,2,3]. Silver is the most common metal to alloy with gold.

The best processing method for recovering gold is ultimately determined by the mineralogy and particle size distribution of the gold. Gold particles vary in size from large nuggets to particles locked in the crystal lattice of certain sulphide minerals [4]. These gold particles or components occluded typically in a sulphide or quartz matrix are usually liberated after milling to between 60 and 80% smaller than 75 p.m. Grinding ores finer than this is usually uneconomic unless the ore has a very high gold content. Often the gold bearing ores are refractory due to the smallness of the gold grains and concentration by flotation is required, followed by either roasting, bacterial leaching or pressure leaching to liberate the gold prior to cyanidation [1,4,5,6]. A notable exception to this is the treatment of carbonaceous gold bearing sulphide ores of the Carlin Trend, Nevada, where the finely disseminated nature of the minerals does not allow for any preconcentration [7].

The cyanide leaching process can be inhibited by the presence of a number of minerals such as pyrrhotite, marcasite, arsenopyrite and stibnite which consume oxygen and cyanide in varying degrees depending on their activity in solutions, active carbonaceous material which behaves like activated carbon and adsorbs gold from cyanide solutions [8], and certain copper minerals which consume large quanities of cyanide resulting in high operating costs or low gold leach rates and recoveries. Environmental and capital constraints have led to gold treatment circuits which incorporate flotation and cyanidation of only the concentrate [9,10].

839

840 C.T. O'CONNOR and R. C. DuI~NE

In the following sections the effect of chemical and physical variables on the flotation of gold bearing ores will be discussed. In view of the wide scope of the subject the treatment will rather attempt to emphasize those factors considered to be of greatest importance. The ores on which the discussion will focus are free gold, tellurides, aurostibite, kerogen, pyrite, arsenopyrite, pyrrhotite, copper-gold ores and mixed sulphides. The paper will conclude with a discussion of the effects of conditioning and flotation cells and circuits.

CHEMICAL EFFECTS

In the flotation process the main chemical effects are reagent type and pH. Recently the need to operate circuits at moderate pH levels, to improve separation efficiencies when treating complex, low grade ores, to reduce costs of reagents, to develop reagents which are stable over a wide pH range, and to take advantage of the role of synergism in the use of mixtures of collectors has led to ever-increasing research to develop new collectors for the flotation of gold bearing ores.

COLLECTORS

Gold

Native gold surfaces are normally hydrophobic due to contamination by organics in nature [1 I]. Thus floatability, especially of fine gold, is the antithesis of gravity separation. Untarnished gold of the appropriate size can be readily floated with only a frother [11,12]. Xanthates are normally used on gold flotation plants in conjunction with other collectors (promoters] to enhance gold recoveries. The recovery of gold is activated by sulphide ions and depressed by ferric ions [13]. Amines are also used commercially to recover gold [5,14].

Selectivity for gold in the presence of pyrite has been found to be enhanced in the case of alkoxy or phenoxy carbonyl alkyl thionocarbamates and thioureas, dialkyl or diaryl monothiophosphates and monothiophosphinates, glyoxalidine and anainothiophenols [ 15,16]. Monothiophosphorous acids have been shown to be able to float gold selectively from base metal sulphides [9,17].

Telluride

The limited information available [18-22] shows that gold tellurides are easily floated and in practice only a frother is required. The addition of a collector results in unselective flotation of telluride and other sulphides, usually pyrite, that are present in ores where tellurides are present. At the Wright-Hargreaves Mine in Ontario, Canada, telluride was floated with only a frother in the presence of lime and cyanide to depress the pyrite [22].

Aurost ib i te - S t ibn i te

No fundamental flotation information is available on the flotation response of aurostibite and it is assumed that its flotation characteristics are similar to those of stibnite. Lager and Forssberg have recently reviewed the processing of antimony bearing ores [23,24]. Stibnite is not an easily floated mineral and in most instances requires an activator for reasonable flotation recovery. It will float without an activator with the higher homologous series of xanthates but even then reasonable quantities of collector are required [25]. It floats well only in acid or neutral circuits.

Kerogen And Active Carbonaceous Material

Kerogen (thucolite) is a uraniferous carbonaceous ore [1] with gold concentrations of up to 300g/t and may contain as much as 30% of the gold in the feed to a flotation plant [26]. Although it ought to float readily it is thought that it is often occluded in the pyrophyllite agglomerates tbrmed when guar

The flotation of gold bearing ores--a review 841

depressants are used. The use of dispersants and kerosene collectors have been reported to increase the recovery under such circumstances [27]. Active carbon components in gold ores are known to float better when a fuel oil is used in conjunction with a frother [14]. However a coconut based carbon added to reduce gold solution loss on a gold treatment plant was not easily floated in the downstream pyrite flotation plant [28].

Pyrite

When the surface of pyrite is free of oxidation products it is oleophilic [29] but it is nevertheless necessary to use a collector to float pyrite. Thiol collectors are most commonly used [30-32]. The fundamentals of flotation of sulphide minerals using thiol collectors continues to be the subject of intensive research (e.g.31-34]. At pHs of approximately 4 all thiols are effective as collectors but the instability of xanthates at these pHs leads to a preference for mercaptobenzothiazole and dithiosphosphate. The xanthates are most widely used at alkaline pHs. Recently collectors such as dithiocarbamates and thionocarbamates and mixtures of these with xanthates and mercaptobenzothiazoles have also been successfully used [4]. The mechanism of adsorption of xanthates on pyrite surfaces has been the subject of a number of studies and has been extensively reviewed [e.g.31,34,35].

Dithiophosphates, usually used as a secondary collectors, are reported to be selective against pyrite and to a lesser extent against arsenopyrite. Amine based collectors have been reported to be capable of floating cyanided pyrite without the use of acid preconditioning [36]. Thionocarbamates, thiocarbamates and thioureas have been found to be stable over a wide pH range. It has also recently been reported that the addition of hydrolyzed polyacrylamide reduces the nonproductive consumption of butyl xanthate when floating gold bearing pyrite [37].

When the major purpose of floating gold bearing pyrite is to produce sulphuric acid by the roasting of the concentrate and the catalytic oxidation of the resultant sulphur dioxide, a sufficiently high grade of about 32 % is required to satisfy the autothermal nature of the roasting process [38]. This constraint results in decreased gold recoveries, in some cases of only 50 %, when floating pyrite from cyanide residues.

Arsenopyrite

Arsenopyrite has very similar properties to pyrite and the flotation conditions for its recovery are similar to pyrite [23]. When the arsenopyrite needs to be separated from pyrite, the former is usually depressed and this is discussed below.

Pyrrhotite

Pyrrhotite floats readily in acid and neutral pH ranges [39]. Surface coatings in the alkaline regions results in lower recovery and collector regimes are similar to those for pyrite [40].

Copper-gold Ores And Mixed Sulphide Ores

Since cyanide leaching of gold bearing copper ores and porphry copper ores, which usually contain only small quantities of gold, e.g. 0.5 g/t, is uneconomic, these are treated so as to yield a bulk copper-gold concentrate which is fed to smelters [41,42]. This flotation is carried out using xanthates and a promoter and it is frequently necessary to add a sulphidiser such as Na2S [5,43,44].

It is normal practice to regrind the rougher and scavenger concentrate before refloating in the cleaner [44] a practice not used on gold pyrite plants. In a recent application chalcopyrite was separated from pyrite/arsenopyrite without the use of cyanide to depress the latter. In this way the copper concentrate earl be treated separately by smelting and the pyrite/arsenopyrite by bacterial leaching [36].

842 C .T . O'CoNNoR and R. C. DUNNE

PROMOTERS

These are collectors added in smaller quantities than those of the primary collector and their addition results in overall better flotation recoveries. The term "synergism" is often applied to this phenomenon. The order of addition of reagents can also be important and it has been suggested, for example, that when weaker collectors are added first, they adsorb on the strong sites, followed by the stronger adsorbing collector which then adsorbs on the weak sites [45]. Dithiophosphates are probably the most widely used promoters in gold flotation [5,46].

FROTHERS

The strength and stability of the froth is important when floating free gold. There appears to be a preference for polyglycol-ether based frothers on most gold plants in conjunction with another frother. When selectivity is required or, in the case of copper gold ores, where a copper concentrate is sold to a smelter, a weaker frother such as MIBC is used [47,48].

ACTIVATORS

In gold flotation these are soluble base metal salts where the metal ion adsorbs onto the mineral surface thus changing its surface chemical properties. In this way the pH range of flotation for the mineral can be extended, the rates of flotation increased and selectivity improved.

The role of copper sulphate, which is widely used as an activator in the flotation of gold bearing pyrite [49], is still not entirely clear. It has been generally shown that when copper sulphate is used as a flotation modifier, better grades and recoveries are obtained for gold [50], telluride [22], stibnite [25], pyrite [39,40], pyrrhotite [40] and arsenopyrite [51].

It has been suggested that activation with copper sulphate increases the flotation of coarse pyrite and the overall rate of flotation [6]. The adsorption of copper onto pyrite and pyrrhotite is pH dependent [39,40], smaller quantities being adsorbed at alkaline conditions. Adding xanthate to copper sulphate, under conditions where no precipitate forms, increased the adsorption of copper onto pyrrhotite but decreased its adsorption on pyrite [40].

A survey of plant operations has shown that copper sulphate addition appears to influence mainly the froth stability and that there is an optimum dosage - too little resulting in high slime recoveries and too much resulting in froth instability [52]. Lead sulphate or nitrate is often used in preference to copper sulphate for the activation of stibnite [25].

DEPRESSANTS FOR SILICATES AND CARBONATES

A number of depressants such as guar gums, starch and carboxymethylcellulose are used in gold flotation circuits to counteract the adverse effects of talc [49], carbonaceous, aluminous, iron oxide and manganese slimes [49], pyrophyllite [5] and carbonates [53]. Selection of the correct depressant type and dosage is critical as overdosage results in both loss of free gold [49] and sulphides that contain gold [54]. The combination of collector and depressant is also important since in the flotation of pyrite, for example, guar gum will have a more adverse effect when used with mercaptobenzothiazole than with xanthate [54]. Alternatively small quantities of frother are sometimes added to the pulp and the talcaceous minerals such as pyrophyllite are floated in the rougher cells with a richer bulk sulphide concentrate being floated after this. The talc concentrate which contains 30-40 % of the gold is cyanided separately or recombined with the sulphide tailing prior to cyanidation [5]. Tannic acid has been shown to be reasonably successful in depressing chlorite [55].

The flotation of gold bearing ores--a review 843

Ores containing a high level of carbonates are often problematic in downstream bio-oxidation processes since their presence in the concentrate influences the subsequent acid requirement for maintaining the correct pH. Lignin sulphonate has been shown to be an efficient depressant for the carbonates [36].

PREFERENTIAL DEPRESSION-FLOTATION

Many gold bearing ores consist of mixtures of sulphides and hence it is often necessary to carry out a differential flotation which may involve the selective depression of one or more ores.

Pyrite-Arsenopyrite

It is often found that gold is associated with arsenopyrite in a pyrite-arsenopyrite system [56,57,58]. Although the arsenopyrite is usually depressed, there is considerable interest in selectively floating the arsenopyrite from the pyrite in order to produce a concentrate for treatment by pressure oxidative or bacterial leaching. Bacterial oxidation subsequent to flotation is however sensitive to the presence of residual flotation reagents and hence it is critical to establish the toxicity of the reagents prior to use. Bacterial leaching also removes the constraint which roasting places on minimum sulphur grade. Bacteria will often adhere to the mineral surface and the high gold concentrations sometimes observed in the troublesome bacterial leach reactor froths suggest that the bacteria may play a collecting role depending on the hydrophobie nature of the particular bacteria surfaces [59,60].

Regulating the oxidation state of the pyrite and arsenopyrite by the addition of oxidants or reductants is the key to selective separation [56]. Potassium permanganate is the preferred oxidant and when used as an arsenopyrite depressant is coupled with control of the redox potential at between 400 and 500mV [61 ]. When potassium peroxodisulphate is used it not only depresses the arsenopyrite but may also activate the pyrite [57]. Modifiers such as sodium metabisulphite, hydrazinium sulphate and magnesia can significantly enhance this separation [56]. Proper addition of copper sulphate has also resulted in better separations. A two stage process has been recently been reported which yields recoveries of arsenopyrite and pyrite of about 60 % and 20 % respectively using mixtures of dithiophosphates and dithiocarbamates [51].

Pyrrhotite - Pyrite/arsenopyrite

The selective removal of pyrrhotite to reduce cyanide and oxygen consumption in the gold leaching process has been successfully applied. Preaeration of the pulp before flotation results in a selective flotation of pyrite and arsenopyrite from pyrrhotite [12]. Conditioning with potassium permanganate in an alkaline circuit has also been also successfully tested [62]. Dithiophosphates, usually used as a secondary collector, are reported to be selective against pyrite and to a lesser extent against arsenopyrite [511.

St ibn i te - Pyr i te /a rsenopyr i te

Gold bearing ore deposits that contain stibnite (and possibly aurostibite) have arsenopyrite and pyrite present and significant amounts of gold are often associated with the latter minerals. Since antimony causes problems in the cyanide leaching process, its separation from the other sulphides is desirable. This may be achieved by floating at a higher pH, depressing the stibnite and activating the arsenopyrite/pyrite using copper sulphate [25]. Conversely, a bulk sulphide flotation at neutral pH with the addition of lead nitrate activates the stibnite. The stibnite is then separated from the other sulphides by adding sodium hydroxide and floating the copper activated arsenopyrite/pyrite [63].

Copper - Pyrite/arsenopyrite/pyrrhotite

When it is necessary to separate copper gold bearing ores from a mixed sulphide ore it is usual to depress the other sulphides by addition of cyanide and operation of the plant at high pH. In this way a selective gold bearing copper concentrate can be produced for smelting.

844 C.T. O'CoNNoR and R. C. DUNNE

pH

Control of pH is of paramount importance in flotation. The zeta potential of pyrite is dependent on pH but the values quoted in the literature are variable mainly due to the fact that the pyrite surface is very susceptible to oxidation and thus can vary from a sulphur surface to an iron oxide surface depending on the past history of the mineral [64,65]. In general it is desirable to use collectors which are not highly sensitive to pH changes since pH is difficult to control accurately. Often however the pH of flotation is dependent on the immediate upstream process. For example, when flotation follows uranium leach or acid leaching of cyanide residues, a low or neutral pH will be chosen. The gangue components will also influence the pH of choice. Pyrophyllite, for example, floats readily in the pH range 5-9 and thus a value outside this range is usually chosen. At high pH values flotation of talcaceous minerals such as pyrrophyllite is not favoured and hence this pH is desirable since it precludes the need to use expensive depressants. However pyrite flotation is not favoured at this pH and the addition of more collector and copper sulphate is required. Copper bearing sulphides are usually floated at alkaline pHs.

E h AND CONTROLLED SULPHIDIZATION

Ralston [64] has reviewed extensively the role of E h in the flotation of sulphides and has shown that when Eh data is used intelligently in conjunction with other information such as size by size recovery curves, flotation rate constants, solution and surface chemical analyses, the net information can substantially aid the identification of new, as well as lead to the improvement of existing, processing strategies. Controlled potential sulphidization has been successfully applied on plant scale to improve overall recovery of sulphides from copper-gold bearing ores [66].

PHYSICAL PARAMETERS

A number of physical factors influence the efficiency of the flotation of gold bearing ores. These include particle size, bubble size, temperature, pulp density, aeration, agitation speed and residence time in the flotation cell. Since the effects of these factors are common to most flotation systems only those features which are peculiar to the flotation of gold bearing ores will be discussed in this section.

Particle Size

Studies have been carried out to determine the influence of gold particle size on the flotation of gold from gold/quartz mixtures and ores [11,49,67,68]. The recovery of sub-12 ~m gold can be as high as 80% in plants treating milled ore. Lower recoveries in the case of tailings treatment plants is mainly due to poor recoveries of coarse "chatted" (i.e. composite gangue-pyrite) grains of pyrite as these particles contain proportionately large quantities of gold. There is usually no free gold present in current residues whilst in old residues the free gold is normally fine having largely originated via the precipitation of gold from solution. The presence of fine pyrophyllite influences the grade but not the the flotation response of the gold [6]. The control of aeration, agitation speed and pulp density in the flotation cells has also been shown to be crucial in the flotation of gold. The upper size of gold recovered efficiently under turbulent conditions has been reported to be as high as 0.71mm at > 80% recovery [67]. In practice it has been found that the coarser the auriferous pyrite, within the limitations of bubble levitation and liberation, the higher the rate of flotation and the higher the grade due to less gangue entrainment [1].

Temperature

The effect of temperature on the flotation of pyrite has been reported [69,70]. An increase in temperature up to 50C results in an increase in the rate and grades of flotation as a result of the reduced viscosity of the water draining back and cleaning the froth of gangue particles. At temperatures above 60C flotation performance decreased possibly due to collector desorption. Temperature control of the flotation pulp has been applied on some gold plants to maintain performance especially during the winter months [71,72]. The temperature is usually maintained above 25oc.

The flotation of gold bearing ores--a review 84.5

Residence T ime

Because flotation is often a rate controlled process it is important to optimize the cell residence time. It has been reported that the rate of gold and pyrite flotation is greater from plants treating coarse milled ores than from those treating finer ores or tailings. Thus shorter flotation residence times [less cell volume or greater throughput and consequently lower capital) and fewer cleaning stages are required for coarsely milled flotation feeds. Pulp densities are usually 30 - 40% solids by mass [5].

CONDITIONING PROCEDURES

The manner in which the ore is conditioned or pretreated is of great importance in the flotation of gold bearing ores. Factors such as the grinding media, pre-aeration, reagent concentration and sequence of addition, oxygen, nitrogen or sulphur dioxide conditioning, acid leaching and conditioning time all play a role in improving the flotation efficiency. It has been reported [73,74,75] that an increase in the energy input into the pulp by increased conditioning time and impeller speeds leads to an increase in the recovery and rate of flotation of pyrite. In the treatment of current or aged cyanide residue material it is necessary to acid condition the ore with sulphur dioxide or sulphuric acid in order to remove the oxidized surface layer from the gold or pyrite particles [50]. This can also sometimes be achieved by a short grind prior to flotation. The surface chemistry of the interactions between xanthate and oxidised pyrite surfaces has been studied and it is well known that it is necessary to acid treat aged pyrite ore prior to flotation [33,76].

When flotation follows cyanidation significant amounts of lime and cyanide may be present. Return water from dams can also contain cyanide. Cyanide is inherently a pyrite depressant and even traces can have a significant effect on flotation. Both of these factors require that the feed must be conditioned at acid or neutral pH in the presence of copper sulphate in order to destroy the effect of lime and cyanide [77]. However amine based collectors may be capable of floating cyanided pyrite without the use of acid pre-conditioning [23,14].

Laboratory conditioning of pyrite prior to flotation using oxygen, nitrogen ,r sulphur dioxide has been reported [65,78,79,80]. These gases will all influence the E h of the pulp which in turn has an effect on the flotation of sulphides [64,81]. In general the use of oxygen rich gas improves the flotation of pyrite under controlled conditions. These effects can be related to the redox potential of the pulp and conditioning can be controlled by monitoring this potential [79,64]. An increase in the concentration of oxygen in the pulp enhances the adsorption of xanthates [65], increases the redox potential of the pulp and improves the condition of the froth [82]. The selectivity of pyrite flotation in the presence of other sulphide minerals can be enhanced by pretreatment with nitrogen [78]. This may imply that the other minerals are depressed more than pyrite. Bubbling sulphur dioxide through the pulp to acidify it has also been reported to increase the sulphur grade at 85% recovery from 12% to 19% [80]. Moreover SO 2 addition to cyanide residues has been shown to result in precipitation of the remaining gold in solution lost during filtration. This gold was recovered subsequently in the bulk pyrite flotation circuit [71,72]. When using E h as a control parameter the proper value will depend on the oxidizing or reducing nature of the pulp. This topic has been reviewed by Poling [35].

Grinding media also affect the flotation of sulphide minerals [83,84]. Unlike other sulphide minerals, the effect of using stainless steel or autogeneous media for milling pyrite is not significant. Since pyrite is the most readily oxidized of the common sulphide minerals it is possible that the reducing conditions created during grinding in mild steel mills have little effect on the subsequent behaviour of pyrite.

FLOTATION CELLS AND CIRCUITS

The most interesting and important application of new flotation cells for gold flotation is the Skim Air [Flash] flotation cells especially in a flotation plant where the tailings are not cyanide leached (e.g.

846 c .T . O'CONNOR and R. C. DUNNE

copper-gold ores). The high relative density of gold and some gold minerals means that they are recycled in the grinding-classification circuit until they are of sufficiently small size to exit the cyclone classifier. The small particle size presents problems as with any other mineral resulting in slow flotation rates. Flotation recoveries of free gold have improved dramatically with the installation of the Skim Air flotation cells in the grinding circuit [85,86,87]. Some modifications have had to be made to circuits where viscosity is a problem. Reagent additions are relatively low and the flotation rates fast. Froth level control is important because froth stability is reduced due to the high bubble loadings and the coarse nature of the concentrate. Column cells have recently shown potential for improving gold recovery as its relatively quiescent pulp phase may enable selective collectors to be used more efficiently. They also have the potential to enable the more efficient use of conditioning gases such as nitrogen and sulphur dioxide [61].

The use of various flotation circuits has been reviewed elsewhere [ 1,5]. Flotation circuit configuration can be divided into two categories, viz. open circuits with no cleaning at all, and open and closed circuits with single and two stage cleaning. Open circuits have the advantage of avoiding the feedback of non-steady state effects. Closed and open circuit cleaning is used on plants where high grade concentrates are required for roasting and smelting. Cleaning circuit design depends on the sulphide particle size and the presence or absence of floatable gangue components.

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