hf~mls ~&ec~~. Vol. 10, No. 1997 9. pp. 895-908, 8 1997 Elsevia Science Lid Pergamon

!30892-6875(97)ooo7%1 Rioted in Great Britain. All righu mad

0892+%751p7 $17.oo+o.00

FLOTATION BEHAVIOUR OF GOLD DURING PROCESSING OF PORPHYRY COPPER-GOLD ORES AND REFRACTORY GOLD-BEARING

SULPHIDES

S.M. BULATOVIC

Lakefield Research Limited, Lakefield, Ontario, Canada (Received 2 April 1997; accepted 5 June 1997)

ABSTFtACT

Porphyrv copper ores and gold-bearing sulphides are significant contributors to overall world gold production Flotation is the principal process for pre-concentration of the sulphides, for subsequent smelting, roasting or hydrometallurgical treatment.

Lakefield Research has had the opportunity to study oresfrom many large &posits, both new dixoveries and existing operations. Research was aimed at improved metal recoveries by applying new concepts inflowsheet design or by introducing more specijic flotation reagents. This paper highlights the variability of porphyry copper deposits that have been tested and suggests mineral processing techniques that can be used to overcome the effects posed by the direrent ore matrices. Some of the innovative flotation practices are discussed. 0 1997 Elsevier Science Ltd

Keywonds Cold ores, flotation

The flotation characteristics of gold or gold minerals found in porphyry copper ores and in refractory

depressants, flotation collectors, mineral processing

INTRODUCTION

sulphide ores have not been described in detail in the literature, whereas the advancements made in the extraction of gold from flotation products [1,21 have been described abundantly by many researchers. The sparse distribution of discrete gold minerals and particles, as well as their exceedingly low concentration in ores, are the principal reasons for the lack of fundamental work on gold flotation.

During the recovery of copper from large porphyry deposits, the emphasis is placed on producing marketable copper concentrates. Cold recovery, although important, is often not considered when optimizing the copper circuit. The need to reject gangue and iron sulphides during cleaning of the copper concentrate, invariably leads to losses of gold. The detrimental effects of depressants, which are introduced in the cleaner circuit, can often be overcome by changing the flowsheet and by adding supplementary collectors. Results of research and development over the past 10 years are summarized in the following sections.

895

896 S. M. Bulatovic

GOLD MINERALS

Over 90% of the gold in porphyry copper ores is metallic gold with variable size and association. Alloys of gold with transition elements such as silver, copper and iron are common. Calaverite and pet&e are common gold tellurides.

GOLD MINERALOGY AND ITS EPPECT ON PLOTATION

The distribution of gold in the ore plays an important role in selecting the type of flotation process that can be applied to recover the gold [3,4]. Usually, gold is contained in low concentrations as minute particles. New techniques are available to the mineralogist, which make it possible to give an accurate account of the association of the gold, the principal host minerals, sulphides and gangue. However, the evaluation of many flotation products would be time-consuming and hence impractical for process control. Since floatability data of the actual gold minerals are virtually non existent, they have to be derived from systematic flotation tests. From the combined effort of flotation studies and mineralogical examinations, some pertinent information on the floatability of gold minerals can be elucidated.

. The flotation properties of elemental gold and electrum depend strongly on the deformation and final shape of the particles after primary grinding and mgrinding. Because of their high ductility, gold particles can form platelets that are difficult to float. Small particles of less than 2Opm are readily recovered by flotation.

. The surface of gold particles can become coated with precipitates of iron compounds, either naturally or during the process of grinding. Flotation of coated particles, even with a high dosage of collector, is weak and erratic.

. Cold minerals, such as aurocupride (AuCu3), float readily in the primary copper circuit. However, if cyanide or bisulphite are then used as depressants for iron minerals in the cleaners, aurocupride recovery is reduced.

. Gold tellurides float well, but their recovery is compromised by the presence of soluble heavy metal salts [5].

. Gold-pyrite middlings can be selectively concentrated with appropriate flowsheet adjustments and selectivity-assisting agents.

Successful concentration of gold in refractory sulphide ores [6,7,8] is almost exclusively dependent on the association of the gold with the sulphides. Gold can be found as metal or alloy (silver, copper, bismuth), as a distinct mineral in combination with tellurium, or it can be contained in carbonaceous ma&al, pyrite, marcasite, arsenopyrite and siliceous gaugue. Therefore, each ore requires a special treatment procedure to achieve a satisfactory concentration of gold. Clays and graphitic carbon are the most troublesome accessory components in an ore, as far as gold concentration is concerned.

GOLD FLOTATION PROM PORPHYRY COPPER-GOLD ORES

Ore Types and Processing Characteristics of Ores

The processing characteristic of porphyry copper-gold ores vary from ore to ore and are closely related to the mineralogical composition of the ore. Based on composition, the ores can be classitkd into several distinct groups:

. Porphyry copper-gold ores containing pyrite. In these ores, gold occurs as elemental gold, some of which can be enclosed within the pyrite or in the copper minerals. Chalcopyrite is the predominant copper mineral, but it can be accompanied by secondary copper minerals and arsenical

Flotation hehaviom of gold 897

. Porphyry copper-gold ores with negligible pyrite content. These deposits are u8uaIly of low grade, but with ,a highly complex mineralogical matrix. Clay may be present in high concentrations and with a wide range of compositions. Cold is present as metal, as aurocupride or associated with sulphosalts. Copper is found as mixtures of chalcopyrite, bomite, covellite and chalcocite.

. Altered slupergene copper-gold ores. ‘lhese ores belong to a group called bdifficult-to-treat ore& The copper minerals include native copper, chalcocite and malachite. Large portions of the gold (up to 50%) can be associated with gangue minerals. Cold may also be present in the unaltered copper sulphide minerals. Invariably, these ores have a high clay content. Clays are the main reason for low recoveries of copper and gold by flotation.

Processing Options

Selection of the treatment process for porphyry copper-gold ores is based on the nature of the ore and the gold association with the contained minerals.

Operating plants such as Ok Tedi [9] and Freeport [lo] use a lime circuit with selective collectors. This choice is based on the presence of variable amounts of pyrite in the deposits and the emphasis on maintaining high-grade copper concentrates. The recovery of gold is not optimum under these conditions. Research work has shown that the overall gold recovery is not entirely dependent on the collector used, but could be enhanced by some modifiers, or by the configuration of the flowsheet. The effect of the flowsheet configuration on the metallurgy is often neglected in the design of a new plant.

Selection of Reagent Scheme

Effect of col.kctors

Most of the operating plants treating porpbyry copper-gold ores use various types of xanthate as the primary collector, in combination with dithiophosphate as a secondary collector. In most instances, these combinations give: satisfactory metallurgical result with respect to concentrate grade and copper and gold recoveries.

During the treatment of ores with pyrite, dithiophosphates are employed, with little or no xanthate added to the scavenger flotation operation. When using xanthate with ores containing clays, a dry froth is produced, requiring the addition of specific frothers or a blend of two or three diierent frothers to maintain a workable froth.

Laboratory studies on a number of copper-gold ores with pyrite have shown that gold recovery in rougher /scavenger flotation is a function of the pyrite recovered in the concentrate. Figure 1 shows the relationship between gold and pyrite recoveries in the copper concentrate, using xanthate and dithiophospbate alone, or in combination. In these experiments, total collector addition was 40 g/t and was

80

60

40

20

0 0204060 80 100

Pyrite recobwy in cu-Au mm. [%I

Fig.1 Relationship between gold and pyrite recoveries in the copper rougher concentrate

898 S. M. Bulatovic

kept constant. A series of concentrates was removed and analyzed. These tests showed that the dithiophosphate collectors were more selective than xanthate alone. The relationship between copper concentrate grade and gold recovery in the copper concentrate using different collectors is illustrated in Figure 2. The experiments were performed using the same ore as the experiments in Figure 1. With only xanthate as the collector, gold recovery in the copper concentrate was significantly reduced with increasing concentrate grade. Dithiophosphate LSB70 gave the highest gold recovery at the highest copper concentrate

- A LSB70 (j& 0 Pax+R3477 (1:l)

- OPAX

50; 16 18 20 22 24 26 28 30 32

Copper concentrate grade, [%I

Fig.2 Relationship between copper concentrate grade and gold recovery in the copper concentrate using different collectors

During treatment of ore with low pyrite content, the gold recovery in the copper concentrate was strongly related to the type of xanthate selected. Figure 3 shows the efficiency of different xanthates on the recovery of gold in the copper concentrate. Results obtained on this ore showed that xanthates with a longer carbon chain length achieved higher gold recovery.

3 80

%

p 70

@ a 60

0 Na ethyl xanthate 0 K amyl xanthatc A Na iso-amyl xanthate

50 ,,,,I~,,,I,,,,I,,,,I”‘~ 0 10 20 30 40 !

Collector addition, [g/t] 1

Fig.3 Effect of type of collector on gold recovery from copper-gold ore with high clay content

New collectors manufactnred by Senmin (South Africa), such as SN127 and PM304 [ 111 were extensively tested on oxidized, supergene, copper-gold ore from the Red Dome (Australia) operation. Because a significant portion of the gold in the ore is associated with gangue minerals, the process development work was directed towards complete removal of the copper to the f?otation concentrate, so that the remaining gold could be treated by cya+ation. Work conducted with !4emnin collectors +howqd~~ia~p~ovements in gold and copper recoveries were possible.

Flotation behaviour of gold 899

Figure 4 compares the effects of collectors SN127, PM304 and K amyl xantbate. The experiments were performed after sulpbidizing of the oxide copper minerals.

ii! 80 z 8 8 60

B

+O

$ 20

0 i i k i Ib

Flotation time, [minutes]

Fig.4 Effect of type of collector on copper and gold flotation from Red Dome (Australia) oxide ore

Effect of Modiikrs and Selectivity Assiiting Reagents

An important consideration when selecting the reagent scheme for a particular ore is the choice of modifiers. In most operating plants, lime is used for pH control. Lime is satisfactory with pyritic, porphyry copper ores [ 121, but it increases the pulp viscosity when clay minerals are present. High pulp viscosity retards the rate of flotation of gold. Sodium hydroxide gives better results for these ores, but is more expensive than lime. Figure 5 compares rates of gold flotation when lime, sodium carbonate or sodium hydroxide are used. The effect of pH using lime and sodium hydroxide was also examined on a high-clay ore. Figure 6 shows the effect of pH on gold recovery. At a low pH, (pH 7 to 91, sodium hydroxide improved the recovery of gold, but there was litile difference between the two alkalis above pH 10.

Soda ash Lime Sodium hydroxide

0 2 4 6 8 : Flotation time, [minutes]

Fig.5 Effect of type of pH modifier on the rate of gold flotation at pH 9.2

900 S. M. Bulatovic

E 2 80 8

k60 8 .f

PO > 0 Sodium hydroxide

i 20 B w

0 6 7 8 9 10 11

Flotation pH

Fig.6 Effect of pH on gold recovery into the copper concentrate with different pH modifiers, using xanthate and dithiophosphate as collectors

Lime-Organic Acid System

Two organic acid modiliers were tested on ore containing pyrite, using lime as the pH regulator (pH 11.5) and PAX+R3477 as collector. Figure 7 shows the effects of citric and oxalic acids on the recovery of gold in the copper concentrate. Mineralogical examination of the copper concentrate obtained in the test with citric acid suggested that the additional gold recovery was due to an improved middling recovery. The organic acids may have acted as a surface cleanser, by complexing iron stains or 6lms.

0 Oxalic acid 0 Citric Acid

Insol Controlling Reagents

0 loo 200 300 4ilo 5 0 Organic acid additions, [g/t]

Fig.7 Effect of different organic acids on gold recovery into the copper concentrate

Porphyry copper-gold ores usually contain some gangue components that are highly flotable, which contaminate the copper concentrate. Maintenance of a high copper concentrate grade requires that gangue depressants be used. Silicates, guars and carboxy methyl cellulose are the common depressants. The effects of different depressants are shown in Figures 8 atid “9. In Figure 8 the floatable gangue was an aluminosilicate, in Figure 9 the gangue was biotite and talc.

Flotation behavhr of gold 901

g 16

0 14 E 2 12 8 g 10

ks

86 .B

n:

0 50 loo 150 2 Reagent addition, [g/t]

Fig.8 Effect of sodium silicate and CMC (carboxy methyl cellulose) on insol content in copper

0 CMC/guar mixture 1: 1

00 0 50 loo 150 2

Reagent addition, [g/t]

Fig.9 Effect alf CMC and guar on gangue rejection during upgrading of a copper-gold concentrate

Selection of Flowsbeet

Although the flowsheet configuration for the treatment of these ores appears to be simple, some features of the cotiguration can affect the recoveries of copper and gold. During the development work on a variety of porphyry copper-gold ores, this aspect was extensively studied in locked cycle bench tests and continuous pilot plant operations. A schematic of the most frequently used flowsheet is shown in Figure 10. Modifications to this flowsheet could include column flotation in the rougher or the cleaner circuits. Since the emphasis in this flowsheet is usually the recovery of copper at a high grade, pyrite, when present, is depressed, and gold recovery is not optimum. An alternative to thii flowsheet is shown in Figure 11. This is a bulk sulphide flotation flowsheet followed by a regrind prior to the separation of the copper and pyrite. The effectiveness of this flowsheet was studied on ore from Ok Tedi and on a pyritic ore from Peru. The results obtained on Ok Tedi ore are shown in Table 1. The results obtained with bulk flotation were superior to those obtained by selective flotation, as seen in the higher copper and gold recoveries.

Similarly, better metallurgical results were achieved on a pyritic, porphyry copper-gold ore from Peru using a bulk flotation procedure. Table 2 compares the results obtained with the two flowsheets. The same copper recovery was obtained with this ore but there was a higher gold recovery with the bulk float circuit.

902 S. M. Bulatovic

Flotation Feed

Regrind

tot. cloanor

2nd cleaner

3ld cleaner

v Cu-Au cleaner concentrate

Fig.10 Conventional flowsheet

Flotation Feed z+F7

High intmnd.

Combined Tailing

N Cu-Au cleaner concentrate

Fig.1 1 Bulk flowsheet used in treatment of pyritic copper-gold ores

TABLE 1 Comparison of Metallurgical Results on Ore from OK Tedi

Flowsheet Product Weight %

Assays %, g/t cu Au

% Distribution cu Au

Conventional Cu/Au cone 1.62 42.6 58.28 88.4 80.7 (Figure 10) Cu/Au tailing 98.38 0.091 0.23 11.6 19.3

Head 100.00 0.78 1.17. 100.0 100.0

Bulk Cu/Au cone 1.73 41.8 58.74 93.8 87.6 (Figure 11) G/Au tailing 98.27 0.049 0.14 6.2 12.4

Head 100.00 0.77 1.16 100.0 100.0

Flotation hehavicur of gold 903

TABLE 2 Comparison of Metallurgical Results on Ore from Peru

Fklwshect Product Weight Assays %, g/t % Distribution % CU AU CU AU

Conventional CulAu cone 2.28 27.6 32.97 95.4 76.7 (Figure 10) Cu/Au tailing 97.72 0.031 0.23 4.6 23.3

Head 100.00 0.66 0.98 100.0 100.0

&lk &/Au cone 2.32 27.1 36.94 95.2 85.8 (Figure 11) CufAu tailing 97.68 0.032 0.14 4.8 14.2

Head 100.00 0.66 0.96 100.0 100.0

The complexity of an ore can rapidly increase when several copper minerals are present, and when differences in liberation size exist. For example, bomite and covellite are brittle and tend to slime during grinding or regrinding of the rougher concentrate. A split-circuit was proposed for a porphyry copper-gold ore from Indonesia, to deal with some of these problems (Figure 12). Table 3 compares the metallurgical results that were obtained with tbis circuit and the conventional circuit.

Flotation Feed

Combined Tailing

Cu-Au cleaner concentrate

Fig.12 Split-circuit flowsbeet

TABLE 3 Comparison of Metahrgical Results - Indonesian Ore

Flowsheet Product Weight %

Assays %, gft

cu Au % Distribution cu Au

Conventional Cu/Au cone 1.48 36.6 26.62 93.5 80.4 (Figure 10) Cu/Au tailing 98.52 0.038 0.097 6.5 19.6

Head 100.00 0.58 0.47 100.0 100.0

Bulk &/Au cone 1.36 40.2 30.07 94.0 83.5 (Figure 12) G/Au tailing 98.64 0.035 0.08 6.0 16.5

Head 100.00 0.58 0.49 100.0 100.0

904 S. M. Bulatovic

GOLD FLOTATION FROM REFRACTORY GOLD-BEARING SULPHIDE ORES

Many gold plants apply flotation [ 13,14,15] as a means of reducing the mass of material to be treated by subsequent roasting or hydrometallurgical recovery processes. Even gold plant tailings are sometimes retreated to recover unoxidized sulphides. In many instances, high gold recoveries to the flotation concentrate can be achieved.

Complications arise when there is graphitic, carbonaceous matter in the ore, such as in the ores in Nevada (USA), Western Australia and the Pacific Rim Region.

Carbonaceous ores frequently contain clays and the gold is finely disseminated in sulphides and gangue minerals. Graphitic carbon and clays are usually the cause of poor gold recovery. Figure 13 shows the relationship between gold recovery to the concentrate and the carbon content of the ore. The gold distribution in the ores from Nevada and the Pacific Rim are shown in Table 4.

s 95- 0 Ore from Nevada 0 Ore from Indonesia

0.5 1 1.5 2 2.5 3 Graphitic carbon in ore, [%J

Fig.13 Effect of graphitic carbon in the ore on the recovery of gold

TABLE 4 Gold Distribution in Carbonaceous Sulpbide Ores

Mineral Ore from Nevada Ore fkom Pacific Region

Pyrite 50.0 50.6 Marcasite 18.5 10.2

Graphitic carbon 10.8 15.2 Non-sulphide gangue 20.7 24.0

The recovery of gold by flotation from ores that contain auriferous marcasite and non-sulphide gangue is more difficult than the recovery from ores containing aur8etou.s pyrite and graphi& carbon. Extensive test work was carried out on these two ores in order to develop a method by which a concentrate could be obtained that was suitable for subsequent hydrometallurgical processing. The main e+nphasis was placed on elimination of the harmful effects of clay and carbon, and on understanding the diffemnces in flotation responses between pyrite and marcasite.

Selection of Collector

Many collectors were examined in these studies. Satisfactory results were obtained with mixtures of xanthates with dithiophosphates collector. Figure 14 shows the effect of the quantity of collector on the

Flotation behaviour of gold 905

recovery of gold. In this series of tests, a constant amount of fuel oil (200 g/t) was added to counter the effect of the graphitic carbon in the ore.The results obtained suggested that the recovery of gold was enhanced when the portion of dithiophosphate in the collector mixture was dominant, and that high levels of collector additions were required to achieve flotation from these ores. The addition of hydrocarbon oils was important to the recovery of gold, as shown in Figure 15. The optimum level was around 200 g/t.

loo- -0 PAX - 0 36 g/t Dithiophosphatc + PAX

z go

_ A 50 g/t PAX + Dithiophosphate

!s

E 3 8o-; CJ 70-- CB

60.,,,,:,,,‘:‘1”:‘,‘,:,,,,. 0 50 100 150 200 250

Collector additions, [g/t]

Fig.14 Effect of Collector on recovery of gold in a concentrate

Fuel oil additions, [g/t] Fig.15 Effect of different hydrocarbon oils on the recovery of gold with a xanthate/dithiophosphate

collector mixture

Effect of ModilIers/Depressants

None of the conventional depressants examined [17] showed any improvement in clay depression. Pulp density was the only variable that had a significant effect on selectivity during gold flotation. Figure 16 shows the effect of pulp density during flotation on weight recovery in the flotation concentrate. Data shown in Figure 16 indicate that large quantities of material float unselectively into the concentrate at pulp densities above 25 to 30 percent.

Effect of Activators

Several activators were examined, including copper sulphate, nitric acid and ammonium chloride. The effect of these reagents is shown in Figure 17. Copper sulphate had a negative effect on the recovery of gold above 400 g/t additions. Ammonium chloride additions between 400 and 600 g/t gave the highest gold recoveries. Nitric acid was also beneficial for the ‘recovery of gold, above about 400 g/t.

906 S. M. Bulatovic

Flotation pulp density, [%]

Fig.16 Effect of pulp density on weight recovery in the sulphide concentrate

60 0 200 400 600 800 1000

Reagent additions, [g/t]

Fig.17 Effect of different activators on gold recovery from refractory gold bearing sulphide ores

Effect of Flowsbeet Configuration

The flowsheet shown in Figure 18 is normally used to float gold-bearing sulphides. When treating refractory sulphides, however, the split-circuit flowsheet, shown in Figure 19, is preferred. This flowsheet includes a carbon lx-e-float before the bulk sulphide flotation. Metallurgical results obtained with flowsheet 18 and 19 are compared in Table 5.

Feed

Total concentrate Tailing

Fig.18 Standard flowsheet without carbon pre-float

Flotation hehaviour of gold 907

Feed -

Total concentrate

Fig.19 Split-circuit flowsheet using carbon pre-float

Taifing

TABLE 5 Effect of Flowsheet Configuration

Flowsbeet Product Weight %

Assays %, glt cu Au

% Distribution cu Au

Conventional Sulphide cone 29.22 9.81 4.88 77.5 86.4 (Figure 18) Tailing 70.78 1.18 0.32 22.5 13.6

Head 100.00 3.70 1.65 100.0 100.0

Split-circuit Carbon cone 4.55 16.2 2.8 19.5 7.7 (Figure 19) Sulphide cone 22.14 11.40 6.32 67.0 84.3

C + S cone 26.69 12.22 5.72 86.5 92.0 Tailing 73.31 0.69 0.18 13.5 8.0 Head 100.00 3.77 1.66 100.0 100.0

CONCLUSIONS

. Elemental gold, electrum and tellurides are normally considered to be readily flotable. When treating :porphyry copper-gold ores, large variations in flotability are common.

. Some of’ the pyrite and gangue depressants regularly used in the industry have been found to depress gold as well. Conditioning with organic acids is quite effective in reducing this undesirable effect.

. The choice of collector and the make up of mixtures of collectors have been found to have a significant effect on gold recovery when treating porphyry or refractory supergene ores containing clay.

. It has been demonstrated that reagent selection and flowsheet configuration can contribute substantially to the recovery of gold and gold minerals.

. Increased recovery of gold from refractory, low-grade gold ores containing graphitic carbon is possible with a split-flowsheet in which graphitic carbon is pre-floated with a hydrocarbon oil collector, and sulphides am fl,oated with xanthates.

908 S. M. Bulatovic

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Engineering, 7(839), (1994). Kydras, K.A. et al., Selective Flotation of An Auriferous Bulk Pyrit+Arsenopyrite Concentrate in the Presence of Sodium Sulphoxy-salts. Mineral Engineering, 6,No. 12, 1257 (1993). Lloyd, A., OK Techi Starts Up Copper Concentrator and New Gold Plant. Eng di Min Journul, 48- 53 (Nov. 1987). McCulloch, WE., Flash Flotation for Improved Gold Recovery at Freeport, Indonesia. Minerals and Metallurgical Processing, 144-148 (1990). Bulatovic, S., New Collectors for Flotation of Platinum Group Metals. m-049 Interim Report, (1995). Bulatovic, S., Evaluation of New HD Collectors in Flotation of Pyritic Copper-Gold Gres from B.C., CanadaG. L&O29 Interim Report, (1993). Allison, S.A., Dunne, RC. & DeWaal, S.A., The Flotation of Gold and Pyrite from South African Gold Mine Residues. 14th IMPC, Toronto, pp II-9 to 11-18, (1982). Lloyd, P.J.D., The Flotation of Gold, Uranium and Pyrite from Witwatersrand Ores. J.S.A. Institute Min Metals, 41 (1981). Kyte, W.J., Gold Treatment at Peko Mines, N.L. Tennant Creek. Proc Annual Conference, Aust IMM, Paper 47, (1978). Bulatovic, S., Collector Evaluation for Flotation of Refractory Low Grade Gold Ore From Nevada, USA. L&O49 Interim Report, (1993). Bulatovic, S., Evaluation of New Clay Depressants and Dispersants. Interim R&D Report, (1992).