Handbook of flotation reagents chemistry theory and practice volume 2 flotation of gold PGM and oxide minerals

Volume 2 of the‘Flotation Reagents Handbook’ is a continuation of Volume 1, and presentsfundamental and practical knowledge on flotation of gold, platinum group minerals and themajor oxi

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Volume 2 of the‘Flotation Reagents Handbook’ is a continuation of Volume 1, and presentsfundamental and practical knowledge on flotation of gold, platinum group minerals and themajor oxide minerals, as well as rare earths.

Rather than reiterating what is well known about flotation of gold, PGMs and oxideminerals, emphasis has been placed on the separation methods which are not so effectivewhen using conventional treatment processes These difficult separation methods arelargely attributed to problems with selectivity between valuable minerals and gangueminerals, especially in the flotation of oxide ores and base metal oxides, such as copper,lead and zinc oxide ores

Literature on flotation of gold, PGMs, rare earths and various oxides is rather limited,compared to literature on treatment of sulphide-bearing ores As mentioned earlier, themain problem arises from the presence of gangue minerals in the ore, which have flotationproperties similar to those of valuable minerals These minerals have a greater floatabilitythan that of pyrochlore or columbite In the beneficiation of oxide minerals, finding aselectivity solution is a major task

This volume of the Handbook is devoted to the beneficiation of gold, platinum groupminerals and, most important, oxide minerals The book contains details on flotationproperties of the major minerals The fundamental research carried out by a number ofresearch organizations over the past several decades is also contained in this book.Commercial plant practices for most oxide minerals are also presented

The major objective of this volume of the Handbook is to provide practical mineralprocessors that are faced with the problem of beneficiation of difficult-to-treat ores, with acomprehensive digest of information available, thus enabling them to carry out theirdevelopment testwork in a more systematic manner and to assist in the control of operatingplants

This book will also provide valuable background information for researchers, universitystudents and professors The book contains comprehensive references of worldwide litera-ture on the subject

New technologies for most of the oxide minerals included in this volume were developed

by the author

ix

17.1 INTRODUCTION The recovery of gold from gold-bearing ores depends largely on the nature of the deposit, the mineralogy of the ore and the distribution of gold in the ore The methods used for the recovery of gold consist of the following unit operations:

1 The gravity preconcentration method, which is used mainly for recovery of gold from placer deposits that contain coarse native gold Gravity is often used in combination with flotation and/or cyanidation

2 Hydrometallurgical methods are normally employed for recovery of gold from oxidized deposits (heap leach), low-grade sulphide ores (cyanidation, CIP, CIL) and refractory gold ores (autoclave, biological decomposition followed by cyanidation)

3 A combination of pyrometallurgical (roasting) and hydrometallurgical route is used for highly refractory gold ores (carbonaceous sulphides, arsenical gold ores) and the ores that contain impurities that result in high consumption of cyanide, which have to

be removed before cyanidation

4 The flotation method is a technique widely used for the recovery of gold from containing copper ores, base metal ores, copper nickel ores, platinum group ores and many other ores where other processes are not applicable Flotation is also used for the removal of interfering impurities before hydrometallurgical treatment (i.e carbon prefloat), for upgrading of low-sulphide and refractory ores for further treatment Flotation is considered to be the most cost-effective method for concentrating gold Significant progress has been made over the past several decades in recovery of gold using hydrometallurgical methods, including cyanidation (CIL, resin-in-pulp), bio-oxidation, etc All of these processes are well documented in the literature [1,2] and abundantly described However, very little is known about the flotation properties of gold contained in various ores and the sulphides that carry gold The sparse distribution of discrete gold minerals, as well

gold-as their exceedingly low concentrations in the ore, is one of the principal regold-asons for the lack

of fundamental work on the flotation of gold-bearing ores

In spite of the lack of basic research on flotation of gold-bearing ores, the flotation technique is used not only for upgrading of low-grade gold ore for further treatment, but

1

also for beneficiation and separation of difficult-to-treat (refractory) gold ores Flotation is also the best method for recovery of gold from base metal ores and gold-containing PGM ores Excluding gravity preconcentration, flotation remains the most cost-effective bene­ficiation method

Gold itself is a rare metal and the average grades for low-grade deposits vary between 3 and 6 ppm Gold occurs predominantly in native form in silicate veins, alluvial and placer deposits or encapsulated in sulphides Other common occurrences of gold are alloys with copper, tellurium, antimony, selenium, platinum group metals and silver In massive sulphide ores, gold may occur in several of the above forms, which affects flotation recovery

During flotation of gold-bearing massive sulphide ores, the emphasis is generally placed

on the production of base metal concentrates and gold recovery becomes a secondary consideration In some cases, where significant quantities of gold are contained in base metal ores, the gold is floated from the base metal tailings

The flotation of gold-bearing ores is classified according to ore type (i.e gold ore, gold copper ore, gold antimony ores, etc.), because the flotation methods used for the recovery

of gold from different ores is vastly different

17.2 GEOLOGY AND GENERAL MINERALOGY OF GOLD-BEARING ORES The geology of the deposit and the mineralogy of the ore play a decisive role in the selection of the best treatment method for a particular gold ore Geology of the gold deposits [3] varies considerably not only from deposit to deposit, but also within the deposit Table 17.1 shows major genetic types of gold ores and their mineral composition More than 50% of the total world gold production comes from clastic sedimentary deposits

Table 17.1 Common genetic types of gold deposits

Magmatic Gold occurs as an alloy with copper, nickel and platinum group metals

Typically contains low amount of gold Ores in clastic Placer deposits, in general conglomerates, which contain quartz, sericite, sedimentary rock chlorite, tourmaline and sometimes rutile and graphite Gold can be

coarse Some deposits contain up to 3% pyrite Size of the gold contained

in pyrite ranges from 0.01to 0.07 μm Hydrothermal This type contains a variety of ores, including(a) gold-pyrite ores, (b) gold-

copper ores, (c) gold-polymetallic ores and (d) gold oxide ore, usually upper zone of sulphide zones The pyrite content of the ore varies from 3% to 90% Other common waste minerals are quartz, aluminosilicates, dolomite etc

Metasomatic or scarn Sometimes are very complex and refractory gold ores Normally the ores ores are composed of quartz, sericite, chlorites, calcite and magnetite

Sometimes the ore contains wolframite and scheelite

Table 17.2 Major gold minerals

Native gold and

its alloys

Native gold Electrum Cuproauride Amalgam Bismuthauride

Au Au/Ag Au/Cu Hg/Au Au/Bi

0–15% Ag

15–50% Ag 5–10% Cu 10–34% Au 2–4% Bi

Sylvanite Petzite Magyazite

AuTe3 (Au,Ag)Te2 (Au,Ag)Te Au(Pb,Sb,Fe)(S,Te11) Unstable Gold associated

with platinum

group metals

Krennerite Platinum gold Rhodite Rhodian gold Aurosmiride

AuTe2(Pt,Pl) AuPt AuRh AuRh Au,Ir,Os

Up to 10% Pt

30–40% Rh

5–11% Rh 5% Os + 5–7% Ir

In many geological ore types, several sub-types can be found including primary ores, secondary ores and oxide ores Some of the secondary ores belong to a group of highly refractory ores, such as those from Nevada (USA) and Chile (El Indio) The number of old minerals and their associations are relatively small and can be divided into the following three groups: (a) native gold and its alloys, (b) tellurides and (c) gold associated with platinum group metals Table 17.2 lists the major gold minerals and their associations

17.3 FLOTATION PROPERTIES OF GOLD MINERALS AND FACTORS

AFFECTING FLOATABILITY Native gold and its alloys, which are free from surface contaminants, are readily floatable with xanthate collectors Very often however, gold surfaces are contaminated or covered with varieties of impurities [4] The impurities present on gold surfaces may be argentite, iron oxides, galena, arsenopyrite or copper oxides The thickness of the layer may be of the order of 1–5 µm Because of this, the flotation properties of native gold and its alloys vary widely Gold covered with iron oxides or oxide copper is very difficult to float and requires special treatment to remove the contaminants

Tellurides, on the other hand, are readily floatable in the presence of small quantities of collector, and it is believed that tellurides are naturally hydrophobic Tellurides from Minnesota (USA) were floated using dithiophosphate collectors, with over 9% gold recovery

Conditioning time with xanthate (minutes)

Figure 17.1 Relationship between adsorption of xanthate on gold and conditioning time in the presence of various concentrations of xanthate

Flotation behaviour of gold associated in the platinum group metals is apparently the same as that for the platinum group minerals (PGMs) or other minerals associated with the PGMs (i.e nickel, pyrrhotite, copper and pyrite) Therefore, the reagent scheme developed for PGMs also recovers gold Normally, for the flotation of PGMs and associated gold, a combination of xanthate and dithiophosphate is used, along with gangue depressants guar gum, dextrin or modified cellulose In the South African PGM operations, gold recovery into the PGM concentrate ranges from 75% to 80%

Perhaps the most difficult problem in flotation of native gold and its alloys is the tendency of gold to plate, vein, flake and assume many shapes during grinding Particles with sharp edges tend to detach from the air bubbles, resulting in gold losses This shape factor also affects gold recovery using a gravity method

In flotation of gold-containing base metal ores, a number of modifiers normally used for selective flotation of copper lead, lead zinc and copper lead zinc have a negative effect on the floatability of gold Such modifiers include ZnSO4·7H2O, SO2, Na2S2O5 and cyanide when added in excessive amounts

The adsorption of collector on gold and its floatability is considerably improved by the presence of oxygen Figure 17.1 shows the relationship between collector adsorption, oxygen concentration in the pulp and conditioning time [4] The type of modifier and the

pH are also important parameters in flotation of gold

17.4 FLOTATION OF LOW-SULPHIDE-CONTAINING GOLD ORES The beneficiation of this ore type usually involves a combination of gravity concentra­tion, cyanidation and flotation For an ore with coarse gold, gold is often recovered by gravity and flotation, followed by cyanidation of the reground flotation concentrate In

some cases, flotation is also conducted on the cyanidation tailing The reagent combina­tion used in flotation depends on the nature of gangue present in the ore The usual collectors are xanthates, dithiophosphates and mercaptans In the scavenging section of the flotation circuit, two types of collector are used as secondary collectors In the case

of a partially oxidized ore, auxiliary collectors, such as hydrocarbon oils with sulphidi­zer, often yield improved results The preferred pH regulator is soda ash, which acts as a dispersant and also as a complexing reagent for some heavy metal cations that have a negative effect on gold flotation Use of lime often results in the depression of native gold and gold-bearing sulphides The optimum flotation pH ranges between 8.5 and 10.0 The type of frother also plays an important role in the flotation of native gold and gold-bearing sulphides Glycol esters and cyclic alcohols (pine oil) can improve gold recovery significantly

Amongst the modifying reagents (depressant), sodium silicate starch dextrins and low­molecular-weight polyacrylamides are often selected as gangue depressants Fluorosilicic acid and its salts can also have a positive effect on the floatability of gold The presence of soluble iron in a pulp is highly detrimental for gold flotation The use of small quantities of iron-complexing agents, such as polyphosphates and organic acids, can eliminate the harmful effect of iron

17.5 FLOTATION OF GOLD-CONTAINING MERCURY/ANTIMONY ORES

In general, these ores belong to a group of difficult-to-treat ores, where cyanidation usually produces poor extraction Mercury is partially soluble in cyanide, which increases consumption and reduces extraction A successful flotation method [5] has been developed using the flowsheet shown in Figure 17.2, where the best metallurgical results were obtained using a three-stage grinding and flotation approach The metallurgical results obtained with different grinding configurations are shown in Table 17.3

Flotation was carried out at an alkaline pH, controlled by lime A xanthate collector with cyclic alcohol frother (pine oil, cresylic acid) was shown to be the most effective The use

of small quantities of a dithiophosphate-type collector, together with xanthate was beneficial

17.6 FLOTATION OF CARBONACEOUS CLAY-CONTAINING GOLD ORES These ores belong to a group of refractory gold ores, where flotation techniques can be used to (a) remove interfering impurities before the hydrometallurgical treatment process

of the ore for gold recovery, and (b) to preconcentrate the ore for further pyrometallur­gical or hydrometallurgical treatment There are several flotation methods used for beneficiation of this ore type Some of the most important methods are described below

Feed

Classification 1 Grind 1

Classification 2

Final tailing Concentrate to smelter

Cleaner 3 Cleaner 2 Cleaner 1

Cleaner Classification Classification

Flotation 2

Flotation 1

Grind 3

Figure 17.2 Flotation flowsheet developed for the treatment of gold-containing mercury–antimony ore

Table 17.3 Gold recovery obtained using different flowsheets [5]

Product % Recovery in concentrate Tailing assays (%, g/t)

89.2 91.8 95.2

72.9 93.4 95.7

68.4 78.7 81.2

70.1 81.2 85.7

1.7 1.0 0.7

5.0 4.1 2.2

0.04 0.015 0.005

0.035 0.022 0.015

0.38 0.27 0.19

17.6.1 Preflotation of carbonaceous gangue and carbon

In this technique, only carbonaceous gangue and carbon are recovered by flotation, in preparation for further hydrometallurgical treatment of the float tails for gold recovery Carbonaceous gangue and carbon are naturally floatable using only a frother, or a combi­nation of a frother and a light hydrocarbon oil (fuel oil, kerosene, etc.) When the ore contains clay, regulators for clay dispersion are used Some of the more effective regulating reagents include sodium silicates and oxidized starch

17.6.2 Two-stage flotation method

In this technique, carbonaceous gangue is prefloated using the above-described method, followed by flotation of gold-containing sulphides using activator–collector combinations In extensive studies [6] conducted on carbonaceous gold-containing ores, it was established that primary amine-treated copper sulphate improved gold recovery considerably Ammonium salts and sodium sulphide (Na2S · 9H2O) also have

a positive effect on gold-bearing sulphide flotation, at a pH between 7.5 and 9.0 The metallurgical results obtained with and without modified copper sulphate are shown in Table 17.4

17.6.3 Nitrogen atmosphere flotation method

This technique uses a nitrogen atmosphere in grinding and flotation to retard oxidation

of reactive sulphides, and has been successfully applied on carbonaceous ores from Nevada (USA) The effectiveness of the method depends on (a) the amount of carbo­naceous gangue present in the ore, and (b) the amount and type of clay Ores that are high in carbon or contain high clay content (or both) are not amenable for nitrogen atmosphere flotation

Table 17.4 Effect of amine-modified CuSO4 on gold-bearing sulphide flotation from carbonaceous refractory ore Reagent used Product Weight Assays (%, g/t) % Distribution

(%)

CuSO4 + xanthate Gold sulphide concentrate 30.11 9.63 4.50 69.1 79.7

Gold sulphide tail 69.89 1.86 0.49 30.9 20.3

Amine modified Gold sulphide concentrate 26.30 13.2 5.80 84.7 90.8 CuSO4 + xanthate Gold sulphide tail 73.70 0.85 0.21 15.3 9.2

17.7 FLOTATION OF GOLD-CONTAINING COPPER ORES

The floatability of gold from gold-containing copper gold ores depends on the nature and occurrence of gold in these ores, and its association with iron sulphides

Gold in the porphyry copper ore may appear as native gold, electrum, cuproaurid and sulphosalts associated with silver During the flotation of porphyry copper-gold ores, emphasis is usually placed on the production of a marketable copper-gold concentrate and optimization of gold recovery is usually constrained by the marketability of its concentrate

The minerals that influence gold recovery in these ores are iron sulphides (i.e pyrite, marcasite, etc.), in which gold is usually associated as minute inclusions Thus, the iron sulphide content of the ore determines gold recovery in the final concentrate Figure 17.3 shows the relationship between pyrite content of the ore and gold recovery in the copper concentrate for two different ore types Most of the gold losses occur in the pyrite The reagent schemes used in commercial operations treating porphyry copper–gold ores vary considerably Some operations, where pyrite rejection is a problem, use a dithiopho­sphate collector at an alkaline pH between 9.0 and 11.8 (e.g OK Tedi/PNG Grasberg/ Indonesia) When the pyrite content in the ore is low, xanthate and dithiophosphates are used in a lime or soda ash environment

In more recent years, in the development of commercial processes for the recovery of gold from porphyry copper–gold ores, bulk flotation of all the sulphides has been empha­sized, followed by regrinding of the bulk concentrate and sequential flotation of copper– gold from pyrite Such a flowsheet (Figure 17.4) can also incorporate high-intensity conditioning in the cleaner–scavenger stage Comparison of metallurgical results using the standard sequential flotation flowsheet and the bulk flotation flowsheet are shown in Table 17.5 A considerable improvement in gold recovery was achieved using the bulk flotation flowsheet

Pyrite content of ore (%)

Figure 17.3 Effect of pyrite content of the ore on gold recovery in the copper–gold concentrate at 30% Cu concentrate grade (1: ore from Peru; 2: ore from Indonesia)

Cu-Au cleaner concentrate Cu-Au cleaner 3

Cu-Au cleaner 2 Cu-Au cleaner 1

Cu-Au rougher

High-intensity conditioning

Cu-Au scavenger Regrind

Flotation feed Bulk rougher

Combined tailing Bulk scavenger

Figure 17.4 Bulk flowsheet used in the treatment of pyritic copper–gold ores [8]

Table 17.5 Comparison of metallurgical results using conventional and bulk flotation flowsheets on ore

from peru Flowsheet used Product Weight (%) Assays (%, g/t) % Distribution

Conventional Cu/Au concentrate 2.28 27.6 32.97 95.4 76.7

During beneficiation of clay-containing copper-gold ores, the use of small quantities of

Na2S (at natural pH) improves both copper and gold metallurgy considerably

In the presence of soluble cations (e.g Fe, Cu), additions of small quantities of organic acid (e.g oxalic, tartaric) improve gold recovery in the copper concentrate

Some porphyry copper ores contain naturally floatable gangue minerals, such as chlor­ites and aluminosilicates, as well as preactivated quartz Sodium silicate, carboxy methyl-cellulose and dextrins are common depressants used to control gangue flotation

Gold recovery from massive sulphide copper–gold ores is usually much lower than that of porphyry copper–gold ores, because very often a large portion of the gold is associated with pyrite Normally, gold recovery from these ores does not exceed 60% During the treatment

of copper–gold ores containing pyrrhotite and marcasite, the use of Na2H2PO4 at alkaline pH values depresses pyrrhotite and marcasite, and also improves copper and gold metallurgy

17.8 FLOTATION OF OXIDE COPPER–GOLD ORES

Oxide copper–gold ores are usually accompanied by iron hydroxide slimes and various clay minerals There are several deposits of this ore type around the world, some of which are located in Australia (Red Dome), Brazil (Igarape Bahia) and the Soviet Union (Kalima) Treatment of these ores is difficult, and even more complicated in the presence of clay minerals

Recently, a new class of collectors, based on ester-modified xanthates, have been successfully used to treat gold-containing oxide copper ores, using a sulphidization method Table 17.6 compares the metallurgical results obtained on the Igarape Bahia ore using xanthate and a new collector (PM230, supplied by Senmin in South Africa) The modifier used in the flotation of these ores included a mixture of sodium silicate and Calgon Good selectivity was also achieved using boiled starch

17.9 FLOTATION OF GOLD–ANTIMONY ORES

Gold–antimony ores usually contain stibnite (1.5–4.0% Sb), pyrite, arsenopyrite, gold (1.5–3.0 g/t) and silver (40–150 g/t) Several plants in the United States (i.e Stibnite/ Minnesota and Bradly) and Russia have been in operation for some time There are two commercial processes available for treatment of these ores:

1 Selective flotation of gold-containing sulphides followed by flotation of stibnite with

pH change Stibnite floats well in neutral and weak acid pH, whereas in an alkaline

pH (i.e >8) it is reduced Utilizing this phenomenon, gold-bearing sulphides are floated with xanthate and alcohol frother in alkaline medium (i.e pH > 9.3) followed

by stibnite flotation at about pH 6.0, after activation with lead nitrate Typical metallurgical results using this method are shown in Table 17.7

2 Bulk flotation followed by sequential flotation of gold-bearing sulphides, and depression of stibnite This method was practiced at the Bradly concentrator (USA)

Table 17.6 Effect of collector PM230 on copper/gold recovery from Igarape Bahia oxide copper/gold ore [8]

(%, g/t)

% Distribution

Feed Copper Cl concentrate Copper tail

Feed

9.36 90.64 100.00 10.20 89.80 100.00

33.3 1.61 4.65 39.5 0.61 0.61

14.15 1.46 2.65 21.79 0.42 0.42

67.0 33.0 100.0 88.0 12.0 12.0

50.0 50.0 100.0 85.5 14.5 14.5

a PAX = potassium amyl xanthate

Table 17.7

Product

Metallurgical results obtained using a sequential flotation method

Weight (%) Assays (%, g/t) % Distribution

42.3 6.2 0.65 1.86

269.3 559.8 18.7 46.4

20.0 51.0 0.7 3.2

53

13

34 100.0

13

51

36 100.0

15

64

21 10.0

Courtesy of stibnite plant (Minnesota, 1976)

Table 17.8

Product

Plant metallurgical results obtained using a bulk flotation method

Weight (%) Assays (%, g/t) % Distribution

Courtesy of the Bradly concentrator (USA)

and consisted of the following steps: (a) bulk flotation of stibnite and gold-bearing sulphides at pH 6.5 using lead nitrate (i.e Sb activator) and xanthate, (b) the bulk concentrate is reground in the presence of NaOH (pH 10.5) and CuSO4, and the gold-bearing sulphides are refloated with additions of small quantities of xanthate, (c) cleaning of the gold concentrate in the presence of NaOH and NaHS The plant metallurgical results employing this method are shown in Table 17.8

Recent studies conducted on ore from Kazakhstan have shown that sequential flotation using thionocarbamate collector gave better metallurgical results than those obtained with xanthate

17.10 FLOTATION OF ARSENICAL GOLD ORES

There are two major groups of arsenical gold ores of economical value These are the massive base metal sulphides with arsenical gold (i.e the lead–zinc Olympias deposit, Greece) and arsenical gold ores without the presence of base metals Massive, base metal

arsenical gold ores are rare, and there are only a few deposits in the world A typical arsenical gold ore contains arsenopyrite as the major arsenic mineral However, some arsenical gold ores, such as those from Nevada in the USA (Getchel deposit), contain realgar and orpiment as the major arsenic-bearing minerals Pyrite, if present in an arsenical gold ore, may contain some gold as minute inclusions

Flotation of arsenical gold ores associated with base metals is accomplished using a sequential flotation technique, with flotation of base metals followed by flotation of gold-containing pyrite/arsenopyrite The pyrite/arsenopyrite is floated at a weakly acid pH with a xanthate collector

Arsenical gold ores that do not contain significant base metals are treated using a bulk flotation method, where all the sulphides are first recovered into a bulk concentrate In case the gold is contained either in pyrite or arsenopyrite, separation of pyrite and arsenopyrite is practiced There are two commercial methods available The first method utilizes arseno­pyrite depression and pyrite flotation, and consists of the following steps:

1 Heat the bulk concentrate to 75°C at a pH of 4.5 (controlled by H2SO4) in the presence of small quantities of potassium permaganate or disodium phosphate The temperature is maintained for about 10 min

2 Flotation of pyrite using either ethyl xanthate or potassium butyl xanthate as collector Glycol frother is also usually employed in this separation

This method is highly sensitive to temperature Figure 17.5 shows the effect of tempera­ture on pyrite/arsenopyrite separation In this particular case, most of the gold was associated with pyrite Successful pyrite/arsenopyrite separation can also be achieved with the use of potassium peroxy disulphide as the arsenopyrite depressant

The second method involves depression of pyrite and flotation of arsenopyrite In this method, the bulk concentrate is treated with high dosages of lime (i.e pH > 12), followed

by a conditioning step with CuSO4 to activate arsenopyrite The arsenopyrite is then floated using a thionocarbamate collector

Separation of arsenopyrite and pyrite is important from the point of view of reducing downstream processing costs Normally, roasting or pressure oxidation followed by cya­nidation is used to recover gold

17.11 FLOTATION OF GOLD FROM BASE METAL SULPHIDE ORES Very often lead-zinc, copper-zinc, copper-lead-zinc and copper-nickel ores contain signifi­cant quantities of gold (i.e between 1 and 9 g/t) The gold in these ore types is usually found as elemental gold A large portion of the gold in these ores is finely disseminated in pyrite, which is considered non-recoverable Because of the importance of producing commercial-grade copper, lead and zinc concentrates, little or no consideration is given

to improvement in gold recovery, although the possibility exists to optimize gold recovery in many cases Normally, gold recovery from base metal ores ranged from 30%

to 75%

In the case of a copper-zinc and copper-lead-zinc ore, gold collects in the copper concentrate During the treatment of lead-zinc ores, the gold tends to report to the lead concentrate Information regarding gold recovery from base metal ores is sparse

The most recent studies [9] conducted on various base metal ores revealed some important features of flotation behaviour of gold from these ores It has been demonstrated that gold recovery to the base metal concentrate can be substantially improved with the proper selection of reagent schemes Some of these studies are discussed below

17.11.1 Gold-containing lead-zinc ores

Some of these ores contain significant quantities of gold, ranging from 0.9 to 6.0 g/t (i.e Grum/Yukon, Canada; Greens Creek, Alaska; and Milpo, Peru) The gold recovery from these ores ranged from 35% to 75% Laboratory studies have shown that the use of high dosages of zinc sulphate, which is a common zinc depressant used in lead flotation, reduces gold floatability significantly The effect of ZnSO4 · 7H2O addition on gold recovery in the lead concentrate is illustrated in Figure 17.6

In order to improve gold recovery in the lead concentrate, an alternative depressant to ZnSO4 · 7H2O can be used Depressant combinations such as Na2S + NaCN, or Na2SO3 + NaCN, may be used The type of collector also plays an important role in gold flotation of lead-zinc ores A phosphine-based collector, in combination with xanthate, gave better gold recovery than dithiophosphates

17.11.2 Copper-zinc gold-containing ores

Gold recovery from copper-zinc ores is usually higher than that obtained from either a zinc or copper lead-zinc ore This is attributed to two main factors: when selecting a reagent

Figure 17.6 Effect of ZnSO4 additions on gold recovery from lead–zinc ores

scheme for treatment of Cu-Zn ores, there are more choices than for the other ore types, which can lead to the selection of a reagent scheme more favourable for gold flotation

In addition, a non-cyanide depressant system can be used for the treatment of these ores, which in turn results in improved gold recovery This option is not available during treatment of lead-zinc ores Table 17.9 shows the effect of different depressant combina­tions on gold recovery from a copper-zinc ore

The use of a non-cyanide depressant system resulted in a substantial improvement in gold recovery in the copper concentrate

Table 17.9 Effect of different depressant combinations on gold recovery to the copper concentrate from lower

zone Kutcho Creek ore Depressant system Product Weight (%) Assays (%, g/t) % Distribution

ZnSO4, NaCN, CaO Cu concentrate 3.10 20.4 26.2 330 45.1 85.6 2.8

pH 8.5 Cu, 10.5 Zn Zn concentrate 5.34 1.20 0.61 55.4 4.6 3.4 82.2

Tailings 91.56 0.77 0.11 0.58 50.3 11.0 15.0 Feed 100.00 1.4 0.95 3.60 100.0 100.0 100.0

Na2SO3, NaHS, CaO Cu concentrate 3.05 32.5 28.1 2.80 68.3 87.4 2.3

pH 8.5 Cu, 10.5 Zn Zn concentrate 5.65 1.20 0.55 54.8 4.7 3.2 84.6

Tailings 91.30 0.43 0.10 0.52 27.0 9.4 13.1 Feed 100.00 1.45 0.98 3.66 100.0 100.0 100.0

Courtesy of Esso Canada Resources

17.11.3 Gold-containing copper-lead-zinc ores

Because of the complex nature of these ores, and the requirement for a relatively complex reagent scheme for treatment of this ore, the gold recovery is generally lower than that achieved from a lead-zinc or copper-zinc ore One of the major problems associated with the flotation of gold from these ores is related to gold mineralogy A large portion of the gold is usually contained in pyrite, at sub-micron size If coarse elemental gold and electrum are present, the gold surfaces are often coated with iron or lead, which can result

in a substantial reduction in floatability

The type of collector and flowsheet configuration play an important role in gold recovery from these ores With a flowsheet that uses bulk Cu–Pb flotation followed by Cu–Pb separation, the gold recovery is higher than that achieved with a sequential

Cu–Pb flotation flowsheet In laboratory tests, and Aerophine collector type, in combination with xanthate, had a positive effect on gold recovery as compared to either dithiophosphate or thionocarbamate collectors Table 17.10 compares the metal­lurgical results obtained with an Aerophine collector to those obtained with a dithio­phosphate collector

Because of the complex nature of gold-containing Cu–Pb–Zn ores, the reagent schemes used are also complex Reagent modifiers such as ZnSO4, NaCN and lime have to be used, all of which have a negative effect on gold flotation

17.12 CONCLUSIONS The flotation of gold-bearing ores, whether for production of bulk concentrates for further gold recovery processes (i.e pyrometallurgy, hydrometallurgy) or for recovery of gold to base metal concentrates, is a very important method for concentrating the gold and reducing downstream costs

The flotation of elemental gold, electrum and tellurides is usually very efficient, except when these minerals are floated from base metal, massive sulphides

Flotation of gold-bearing sulphides from ores containing base metal sulphides present many challenges and should be viewed as flotation of the particular mineral that contains gold (i.e pyrite, arsenopyrite, copper, etc.), because gold is usually associated with these minerals at micron size

Selection of a flotation technique for gold preconcentration depends very much on the ore mineralogy, gangue composition and gold particle size There is no universal method for flotation of the gold-bearing minerals, and the process is tailored to the ore character­istics A specific reagent scheme and flowsheet are required for each ore

There are opportunities in most operating plants for improving gold metallurgy Most of these improvements come from selection of more effective reagent schemes, including collectors and modifiers

Perhaps the most difficult ores to treat are the clay-containing carbonaceous sulphides Significant progress has been made in treatment options for these ores New sulphide activators (i.e amine-treated CuSO4, ammonium salts) and nitrogen gas flotation are amongst the new methods available

Table 17.10 Effect of collector type on Cu–Pb–Zn–Au metallurgical results from a high-lead ore, Crandon (USA)

7 Bulatovic, S.M., Evaluation of New HD Collectors in Flotation of Pyretic Copper-Gold Ores from B.C Canada, Internal R&D Report LR029, 1993

8 Bulatovic, S.M., An Investigation of the Recover of Copper and Gold from Igarape Bahia Oxide Copper-Gold Ores, Report of Investigation LR4533, 1997

9 Bulatovic, S.M., An Investigation of Gold Flotation from Base Metal Lead-Zinc and Copper-Zinc Ores, Interim Report LR049, 1996

18.1 INTRODUCTION

In chemical terms the six main platinum group elements (PGE), ruthenium, rhodium, palladium, osmium, iridium and platinum, belong to the group VIII transition metals, to which also belong iron, nickel and cobalt These elements have long been considered, when grouped with gold and silver, as ‘precious metals’ This, in fact, is misleading because the mineralogy and geochemistry of silver and gold do not correlate with that of PGE Also, in literature, there are two terms of reference, including PGE and platinum group minerals (PGM) From a flotation point of view, PGM is the more common term There­fore, the term PGM will be used in this text

The chemical similarity between the six PGE and iron, nickel and cobalt accounts for the fact that they tend to concentrate together as a result of geological processes This is quite important not only for the formation of PGM ores, but also for beneficiation

18.2 MINERALS AND CLASSIFICATION OF PGM ORES

There are over 100 different platinum group minerals Some of the most common PGM are shown in Table 18.1 The stoichiometry of most of the PGM named [1] is known, but because these minerals are subject to a wide range of element substitution, as indicated in Table 18.1, there is little consistency between an ideal formula for the individual minerals and compositions of the given minerals from various locations

In general, PGM are concentrates in the crust found in two different ways: (a) by leaching the metal-rich lava (mantle) deposited into the crust, which is known as chemical weathering, especially in a hot climate where silica and magnesia are leached away This leaves a residue enriched in iron and nickel, which contains the PGM elements; and (b) melting a portion of the mantle may give rise to ultramafic or basalic lava, which is then squeezed upwards as a result of pressure within the earth to intrude the crust or extrude lava

on the surface This magma is not particularly rich in nickel or PGM; however, because of their siderophile nature [2], the group VIII metals are also chalcophile in nature, that is they prefer to form bonds with sulphur than oxygen

19

List of platinum

Table 18.1 group minerals and their compositions

Pd8SbTe4 (PtPd)S PtS PtCuAsS2 OsS2 PdBi2 (PtPd)4Sb3 PtSb2

Pd3As RhAsS Pt(Cu) IrAsS

Ir

Pt3Fe PtTe PtNiAs PtTe3 PtSn OsAs2

Os

Pd8(SnAs)3

Pd (IrPt)

Rh

Ru (IrOsRu) PtAs2 PdHgTe3 Pd(BiPb)2 PdS (IrCuRh)S

Pd3Pb

(RuOsIr)As (PdCu)AsSb (PdHgAuCu)AsSb (PdPt)Sn

(PdPtNiFe)SbBiTe (PtNiPd)S (PtNiPd)S (PtCuAs)S (OsRhIrPdRu)S (PdPt)Bi (PtPdRhNiCu)SbAsBi Pt(SbBi)

(Pd)As (RhPdPtIr)AsS (Pt)Cu (IrRuRhPt)AsS (IrPtFeOsRhPdNi) (PtFeCuNi) (PdPt)(TeBiSb) (PdNiAs) PtPd(TeBi) (PtBiSb)Sn (OsRuFeNiIrCo)As (OsIrRuPt) (PdPtAuCu)(AsSnSb) PdHg

(IrPtFeOsCuNi) RhPt

RuIrRhOsPdFe (IrRuOsPtRhFeNiPd) (Pt)(AsSb)

(Pt)HgTe)Bi Pd(BiPb) (PdFePt) (IrCuRhFePbPtOs)S (PdPtFeNiCu)Pb

These sulphide deposits are able to concentrate these metals by a factor of 100–1000 ppm and form PGM deposits, together with precious metals, nickel and copper Almost always the PGM deposits contain nickel minerals

The PGM deposits can be classified into the following two groups: (a) PGM-dominated deposits and (b) nickel–copper-dominated deposits Of major interest concerning this chapter will be the PGM-dominated deposits The flotation of copper–nickel-containing PGM was discussed in Volume I of this book

18.3 DESCRIPTION OF PGM-DOMINATED DEPOSITS

According to the processing characteristics of PGM-dominated deposits, they can be divided into the following three groups: (a) Morensky type, (b) hydrothermal deposits and (c) placer deposits Each type of deposit is briefly described below

18.3.1 Morensky-type deposits

The Morensky-type deposits can be found in very large bodies of basaltic magma, which were intruded into stable continental rock An example includes the Busheld Complex in South Africa and the Great Dyke of Zimbabwe Mineralization similar to the above is also found in the Stillwater Complex in Montana, USA

The Busheld Complex consists of varieties of ore types, including high-chromium ores, ore with floatable gangue minerals and small but significant quantities of ultrafine slimes that are important from a processing point of view

The Stillwater Complex consists of a sequence of differential layers of mafic and ultramafic rocks, which extend for a strike length of up to 40 km and has a maximum exposed thickness of about 7.4 m [3] There are several mineralization zones at the Still-water Complex, including a PGM-rich zone and a low-grade zone The Stillwater ore that is processed nowadays contains olivine, plagioclase, as well as plagioclase-brauzite, all of which are naturally hydrophobic gangue minerals

Another similar origin deposit is Lac des Illes in Canada This complex is apparently contrary to a somewhat general rule in that of intrusion and is regarded as Archean age and may be therefore intruded prior to the Kenora origin into a technically unstable environment

18.3.2 Hydrothermal deposits

An example of a hydrothermal deposit is the New Rambler deposit, described by McCal­lum et al [4] in the Medicine Bow Mountains in south-western Wyoming, USA, which contains a significant amount of PGM The ore occurs in irregular pods that are hydro-thermally decomposed into metadiorite and metagabbro zones Pyroxenite and peridotite are reported to be intersected at a depth beneath the ore zone All have been affected by supergene alteration The main sulphides in the ore include pyrite, chalcopyrite, pyrrhotite, covellite and marcasite with associations of electrum, pentlandite and PGM

There is no evidence that the depth may be a result of an alteration in the original concentration of magmatic sulphides It may be a result of concentration of hydrothermal solutions

18.3.3 Placer deposits

The eluvial and alluvial PGM deposits have been processed in the Soviet Union, Canada, Columbia and the United States Most of these deposits are associated with Alaskan-type ultrafamic rocks, which are, themselves, enriched in PGM, in particular, in the vicinity of

concentration of chromite and with alpine ultrafamic bodies As a process of weathering, there is a marked change in Pt/(Pt–Pd) ratio as compared to the source becoming greatly increased in the former due to the greater ease with which Pd dissolves and is removed in a weathering enrichment Examples of this include the placer related to the Norilsk sulphide deposits and deposits found in Ural region USSR

18.4 EFFECT OF MINERALOGY ON RECOVERY OF PLATINUM GROUP

MINERALS The recovery of PGM minerals is a subject which has received very little attention in published literature This is mainly due to the fact that major PGM producers are sur­rounded by secrecy, therefore, neither commercial processes nor research work on recovery

1 ores amenable to gravity preconcentration,

2 ores amenable to flotation and

3 ores that can only be treated using a hydrometallurgical route

18.4.1 Ores amenable to gravity preconcentration

The most important features of these ores are (a) the valuable constituents occur as minerals

of high density, (b) they do not have middlings and (c) the grain-size distribution falls in a region where a gravity technique can be adopted successfully

Ore types where gravity preconcentration is used include Alaskan-type deposits, alluvial and fossil placer deposits

In the Alaskan-type deposits, the principal PGM minerals include Pt–Fe alloys, isoferro­platinum (Pt2Fe) and platiniridium (Ir,Pt) There are several producing plants that process these ores, mainly in rural mountain areas (USSR)

The alluvial deposits were treated in the early 20th century The PGM in these deposits occur as alloys, usually as Pt rich in the form of loose grains and nuggets These deposits have been mined in a number of countries, including Russia, Columbia and South Africa Although there is a comprehensive review of the placer deposits [5], very little is known about PGM recovery using a gravity preconcentration method Some of these deposits contain clay minerals, which require pretreatment before preconcentration It should be mentioned that the PGM ores from Alaska contain magnetite, which is removed before gravity preconcentration

The fossil placer deposits are in fact gold-bearing conglomerates that carry small amounts of PGM, together with gold, uranium and other heavy minerals However, studies conducted revealed that some of the fossil placer deposits contain about 22 PGM species, including Ir–Os–Ru alloys, sperrylite and isoferroplatinum

There are several operating mines that recover PGM and gold from fossil placer deposits, some of which include Witwatersrand and Geduld mines in South Africa

18.4.2 Ores amenable to flotation

Classification of the ores amenable to flotation

Based on flotation processing characteristics, these ores can be divided into the following major groups:

(a) PGM sulphide-dominated deposits In these deposits, PGM are in general associated with base metal sulphides, as grain boundaries between sulphides and silicates In some cases, the PGM may be present in solid solution with sulphides From these deposits, PGM are recovered in a bulk Cu/Ni/Co/PGM concentrate that

is further processed using pyrometallurgical techniques In many cases these ore types contain floatable non-opaque gangue minerals, including talc, chlorites, etc

(b) PGE-dominated deposits This in fact is a term for stratiform deposits containing sparse sulphides and PGM concentration in a range between 5 and 30 g/t These ores are typified by the Morensky Reef of the Bushveld Complex in South Africa Mineralization of a similar type is found in the Stillwater Complex in Montana, USA These deposits are characterized by a variety of different gangue minerals and high content of PGM sulphide minerals, such as cooperate (PtS), braggite [(PtPd)S] and vysotskite (PdS) Note that these minerals are rare and non-existent

in most PGM-bearing copper-nickel sulphide deposits Typical deposits that belong to this group include the Morensky Reef (South Africa), the Stillwater Complex (USA) and Lac des Illes (Canada)

18.5 COPPER-NICKEL AND NICKEL SULPHIDE DEPOSITS WITH PGM

AS A BY-PRODUCT Prior to discovery of the PGM Morensky Reef deposit, copper-nickel deposits in Ontario, Canada, and the Norilsk (USSR) were the principal sources of PGM production However, about 40% of the world’s production of PGM comes from different Cu–Ni deposits The major deposits from this group are discussed in the following sections

18.5.1 The Sudbury area in Ontario, Canada

Mineralogical examination of these ores [8] revealed a variety of PGM and their associa­tions The michenerite (PdBiTe) and sperrylite (PtAs2) are the most common platinum/ palladium minerals for many deposits in the Sudbury region Other minerals of economic value found in these deposits are moncheite (PtTe2), froodite (PdBi2), inszwaite (PtBi2), iravsite (IrAsS), niggliite (PtSn) and mertiate (PdSb3) Most of these minerals are liberated

at a relatively coarse size (40–200 μm)

18.5.2 The Norilsk Talnakh ore in Russia

In this area, the PGM are distributed in (a) disseminated sulphides, mostly in pyrrhotite, chalcopyrite and pentlandite The predominant platinum minerals are Pt–Fe alloys, coop­erate (PtS) and sperrilite (PtAs2); (b) massive sulphide ores where the predominant PGM are Pt–Fe alloys, rustenburgite (Pt3Sn) and sperrilite (PtAs2), occurring in fine inclusions in chalcopyrite and pyrrhotite; and finally (c) disseminated veins and brecia ores that may consist of mainly chalcopyrite or pyrrhotite The PGM in these ores is present as Pt-(cooperate) and Pd-(rysotkite) sulphides

18.5.3 Pechenga Cala Peninsula (USSR)

The ores from this region are of tholeiitic intrusions hosting Cu–Ni sulphides with relatively low PGM content In these ores, most of the palladium is associated with pentlandite, where the platinum and rhodium are mainly associated with pyrrhotite Only sperrilite and Pt–Fe alloys have, so far, been found in these ores

(a) Podiform chromite deposits occur in ultrafamic bodies referred to as alpine types and are located in Tibet and North-western China

(b) Stratiform chromite deposits occur in different layered intrusions, such was Bushveld (South Africa) and the Great Dyke (Zimbabwe) The best known chromite deposit, with a number of operating plants, is the UG2 Complex located below the Morensky Reef It ranges in thickness from 15 to 255 cm and dips at an angle of 5–70º towards the centre of the Bushveld Complex Mineralogically, it consists mainly of chromite (60–90%) or thopyroxene (5–25%) and plagioclase (5–15%) with only trace amounts

of base metal sulphides

PGM are usually closely associated with sulphides, such as laurite (RuS2), cooperate (PtS), braggite [(PtPd)S], Pt–Fe alloys, sperrilite (PtAs2) and vysotskite (PdS)

The average chemical analyses of the PGM from various areas are shown in Table 18.2

Average chemical analyses of

Table 18.2 PGM from various areas of the UG2 deposits

1.29 0.77 0.99 1.34 0.23 1.87 1.53 2.50 3.92 3.53 3.09 2.43

0.49 0.51 0.28 0.49 0.40 0.49 0.51 0.56 0.95 0.73 0.81 0.91

0.72 0.90 1.17 1.06 0.86 0.99 0.93 1.00 1.22 1.40 0.97 1.51

<0.5

<0.5 0.16

<0.1 0.45 0.09

<0.2

<0.2 0.06 0.03

<0.1 0.17 0.03 0.07 0.07

<0.1 0.09 0.02

4.08 5.27 5.41 5.77 4.04 6.18 5.68 7.17 10.65 10.91 8.55 9.30

18.7 FLOTATION OF PGM-CONTAINING ORES

18.7.1 Introduction

There is little published data on the flotation of PGM-containing ores Development work

on beneficiation of PGM ores has been conducted by mining companies themselves and by

a few research organizations close to the mining companies, which produce PGM Many operating plants treating PGM ores use conventional flotation techniques and the metallurgical results are below optimum in a number of these plants

Each ore type described in Section 18.6.2 require different flowsheets and reagent schemes, which is dictated by the mineral composition of the ore and the geological setting, as well as the type of PGM carrier minerals

During the past 10 years of research work, a new technology has been developed to cope with difficult-to-treat ores, such as chromium-containing PGM ore and PGE-dominated ores

The following sections discuss the flotation properties and practices of the different ore types

18.7.2 Flotation properties of PGM from sulphide-dominated deposits

Most of the current commercial operations that treat PGM from sulphide-dominated deposits are located in South Africa (Morensky Reef), Stillwater mines (Montana, USA) and Lac des Illes (Ontario, Canada) From a processing point of view, most of these ore types contain hydrophobic gangue minerals, including talc, which has a negative effect on PGM recoveries Other major factor that affects flotation recovery of PGM is the presence of a variety of sulphide minerals, including pyrrhotite, pentlandite, chalcopyrite, violarite and pyrite, where

the PGM are associated with all sulphides In addition, in some operating plants, a portion of the PGM is represented by braggite, vysotkite, monchelite and Pt–Fe alloys

In general, the flotation properties of PGM from sulphide-dominated deposits are very dependent on the ratio of the individual sulphide minerals present in the ore and the nature and occurrence of hydrophobic gangue minerals present in the ore

Each of the sulphide minerals, which are PGM carriers (i.e pyrrhotite, pyrite, pentlan­dite, etc.) have different flotation properties under some flotation conditions The selectiv­ity between sulphide minerals and gangue minerals is relatively poor in principle, and in the majority of cases, a hydrophobic gangue depressant has to be used

The flotation behaviour of the individual sulphide minerals contained in PGM sulphide­dominated ores can be described as follows:

Pyrrhotite is a relatively slow floating mineral, especially monoclinic pyrrhotite, which

is usually present in these ore types The floatability of pyrrhotite also decreases when using certain hydrophobic mineral depressants, such as guars and dextrins The flotation of pyrrhotite may improve with small additions of copper sulphate (CuSO4)

Chalcopyrite and pentlandite float well using a xanthate collector and in certain opera­tions, the recovery can reach greater than 90%

Violarite is the least floatable mineral of all the sulphides and represents a major loss of PGM in the flotation tailing from a number of operations

Figure 18.1 shows the rate of flotation of different sulphides from operation A (UG2 Complex) In these experiments, xanthate was used as the primary collector with dithiopho­sphate as the secondary collector

Valleriite

Flotation time (minutes) Figure 18.1 Rate of flotation of different sulphides from the Morensky Reef operation on a mill feed ore

One of the major problems associated with beneficiation of PGM from sulphide-dominated deposits is the presence of hydrophobic gangues, such as talc, chlorites, carbonates and aluminosilicates The concentrates produced in most of the Morensky Reef operations (South Africa) varies from 80 to 150 g/t of combined PGM, where most of the contaminants are silicates, aluminosilicates and talc (i.e up to 60%) The major hydrophobic gangue depressants used are carboxymethyl cellulose (CMC) and different modifications of guar gums

In recent years, a new line of hydrophobic gangue depressants were developed, based on

a mixture of guar gums and low-molecular-weight polyacrylates modified with organic acid, which are extremely effective With the use of these depressants, the grade of the PGM concentrate could increase from 100 up to 40 g/t without any loss in recovery

18.7.3 Reagent practice in flotation of PGM sulphide-dominated ores

There is very little published information available on flotation of PGM ores in general, especially for the operating plants in the Morensky Reef and the UG2 operations Most operations treating PGM sulphide-dominated ores have similar reagent schemes, with maybe a different choice of hydrophobic gangue depressants Most of these plants use CuSO4 as the principal sulphide activator

In the past 10 years, extensive research was carried out by a number of research organizations with the objective of developing new technology for the beneficiation of these ore types The main research work was directed towards finding better gangue depressants

Reagent schemes – Collectors and activators

The principal sulphide activator used in most operating plants is small additions of CuSO4, normally added to the secondary rind and scavenger flotation stages Although CuSO4

improves PGM recovery, it may also reduce the concentrate grade because an excess of CuSO4 will activate the gangue minerals Figure 18.2 shows the effect of level of CuSO4 on the PGM grade–recovery relationship from the Morensky Reef Plant A ore

In these experiments, carboxymethyl cellulose (CMC) was used as the main gangue depressant

In recent years, a number of alternative activators were examined It was found that organic acids along with a mixture of organic acid and thiourea can replace CuSO4 with significant improvement in PGM recovery and selectivity The results obtained using different activators on the Morensky Operation B ore are compared in Table 18.3 The highest concentrate grade and PGM recoveries were achieved using a mixture of oxalic acid and thiourea The use of CuSO4 as an activator was examined in relation to the point of addition and type of depressant used [11] It was concluded that the point of reagent addition played an important role in PGM recovery

The primary collector used in PGM flotation is xanthate As a choice of secondary collectors, dithiophosphates and mercaptans are used in some operating plants

The type of xanthate has a significant effect on PGM recoveries Studies conducted on the Stillwater Complex by the US Bureau of Mines [12] indicated that the type of xanthate had a significant effect on PGM recovery (Table 18.4)

100 g/t CuSO4

200 g/t CuSO4

concentrate concentrate tail concentrate concentrate tail concentrate concentrate tail

1.67 6.90 93.10 100.00 1.10 5.70 94.3 100.00 0.87 4.02 95.98 100.00

120 35.5 0.45 2.87

198 44.4 0.35 2.87

250 66.6 0.23 2.90

61.8 18.2 0.31 1.54

101 23.8 0.19 1.54

132 35.9 0.11 1.55

8.26 2.20 0.08 0.23 13.2 2.77 0.08 0.23 19.9 3.78 0.07 0.22

70.0 85.3 14.7 100.0 74.2 88.5 11.5 100.0 75.0 92.3 7.7 100.0

67.0 81.4 18.6 100.0 70.5 88.4 11.6 100.0 74.0 93.3 6.7 100.0

60.0 66.3 33.7 100.0 60.6 67.8 32.2 100.0 63.0 69.1 30.9 100.0

The highest PGM recovery was achieved using sodium amyl and sodium isobutyl xanthate Using a mercaptan collector alone gave poor PGM recovery However, when using xanthate with mercaptan, substantial improvement in PGM recoveries was achieved

Table 18.4 Effect of type of xanthate on PGM recovery from the Stillwater ore (USA)

89.9 80.6 77.5 114.7 83.7

4.96 3.72 3.70 6.51 –

Table 18.5 Effect of collectors from the PM series on PGM recovery from the Morensky operation A ore

PGM cleaner concentrate PGM rougher concentrate Assays (g/t) %

60.5 98.5 100.3

128 62.3

71 82.3 76.6 73.3 73.1

65 80.6

74 71.8 70.0

36.2 65.2 45.3 67.2 37.2

17.8 36.1 24.1 37.7 19.6

84.3 94.4 87.4 86.6 85.5

82.2

94 86.8 84.3 84.0

a

Cytec dithiophosphate

In recent studies, a new line of PGM collectors had been developed [13] known as the

PM series These collectors are an ester-modified mixture of xanthate + mercaptan The reaction product forms an oily greenish-coloured liquid The results obtained using the PM series of collectors are shown in Table 18.5 High PGM recovery was obtained using a combination of sodium amyl xanthate plus collector PM301

Collector PM306 was the most selective collector from the PM300 series

Choice of hydrophobic gangue depressants

Choosing a depressant for hydrophobic gangue depression is dependent on the type of gangue present in the ore During treatment of ores that contain talc, carboxymethyl cellulose (CMC) is normally used as the gangue depressant, or in some operations, guar gum + CMC Typical examples of talc-containing ores are the Stillwater Complex (USA) and Lac des Illes (Canada) Both operations use CMC for talc depression In the Stillwater operation, the additions of CMC are relatively high (i.e up to 600 g/t) and are added to the ball mill, the PGM roughers and cleaners

Laboratory and pilot plant studies [14] on the Stillwater ore showed that the molecular weight of the CMC affected both PGM grade and recovery Figure 18.3 shows the effect of molecular weight of CMC on PGM grade–recovery relationship

The best results were obtained using CMC with an average 300,000 molecular weight, corresponding to a viscosity of over 3000 cps

Studies conducted by the University of Cape Town (South Africa) researchers indicated that the point of CMC addition [15] had a significant effect on sulphides (PGM carriers) grade and recovery

Figure 18.3 Effect of CMC molecular weight on PGM grade–recovery relationship

It should be noted that in several operating plants from the Morensky Reef and Stillwater Complex, from which plant metallurgical results are available, the total PGM recoveries ranges from 82% to 85% PGM The grade of concentrate from the Morensky operations ranges from 80 to about 120 g/t (Plants A and B) Most of the contaminants are silicates and talc

18.7.4 Reagent practice in flotation of Cu–Ni and Ni ores with PGM as the

by-product

The flotation of Cu–Ni and Ni ores is discussed in Chapter 16 (Volume 1) In most operating plants, the emphasis is usually placed on Cu–Ni and Ni recovery and concentrate grade, and most of the research on these ores was directed towards improvement in Cu–Ni recovery and pentlandite–pyrrhotite separation, whereas little or no attention was paid to improvement in recovery of PGM In operations from the Sudbury Region (Canada), PGM are recovered as by-products of Cu–Ni concentrates The idealized flowsheet of the Inco Metal PGM recovery flowsheet is shown in Figure 18.4

Laboratory studies conducted on Falconbridge ores, also from the Sudbury Region, during 1980 [16] showed that PGM recovery can be improved with the use of a secondary collector Figure 18.5 shows the effect of level of secondary collector on PGM recovery in a Cu–Ni bulk concentrate The highest PGM recoveries were achieved using isobutyl dithiophosphate (Minerec 2087) as the secondary collector

Plant data from the Copper Cliff Mine showed that about 85% of the platinum was recovered in a Cu–Ni concentrate, most of which was from the nickel concentrate The

Tails PGM Bulk concentrate

Rough pyrrhotite concentrate Tails

Copper cliff secondary mill

PGM residue

Figure 18.4 Idealized flowsheet used at the Inco metals operation (Sudbury, Canada) for recovery

xanthate + dithiophosphate (Minerec 2087)

Flotation time (minutes) Figure 18.5 Effect of secondary collectors on PGM recovery in a bulk Cu–Ni concentrate

Table 18.6 Platinum recovery in the Copper Cliff plant

29.2 2.28 0.93 4.58

0.91 12.8 0.22 4.42

1.80 3.04 0.41 1.39

83.0 14.0 3.0 100.0

3.0 85.0 12.0 100.0

17.0 65.0 18.0 100.0

Improvement in overall PGM recoveries was obtained using xanthate as the primary collector and dithiophosphate as the secondary collector A slight improvement in metal­lurgical results was achieved when using mercaptan as the secondary collector

18.7.5 Reagent practice in flotation of PGM from chromium-containing ores

The major problem associated with processing of high-chromium ores includes the following:

Effect of secondary collectors on PGM

Table 18.7 from the Norilsk (Russia) disseminated Cu/Ni-PGM ore

10.25 5.58 15.83 84.17 100.00 10.60 6.45 17.05 82.95 100.00 11.49 6.29 17.78 82.22 100.00

29.6 2.0 19.88 0.18 3.3 30.3 1.32 19.34 0.13 3.4 27.5 1.16 18.19 0.31 3.5

0.8 12.8 5.03 0.12 0.9 0.7 11.41 4.81 0.12 0.92 1.2 9.83 4.26 0.18 0.91

6.5 55.0 26.95 0.63 4.8 5.8 58.98 25.92 0.40 4.75 6.1 52.4 22.47 0.86 4.7

55.0

188 101.9 1.62 17.5 49.5 180.9 98.97 0.27 17.1 52.0 165.5 93.05 0.92 17.3

92.0 3.4 95.4 4.6 100.0 94.5 2.5 97.0 3.0 100.0 90.3 2.1 92.4 7.6 100.0

9.1 79.4 88.5 11.5 100.0 9.2 80.0 89.2 10.8 100.0 15.1 68.0 83.1 16.9 100.0

13.9 75.0 88.9 11.1 100.0 12.9 80.1 93.0 7.0 100.0 14.9 70.1 85.0 15.0 100.0

32.2 60.0 92.2 7.8 100.0 30.7 68.0 98.7 1.3 100.0 35.4 60.2 95.6 4.4 100.0

Table 18.8 Chemical analyses of UG2 high-chromium ore

In recent years, extensive research [18] has been conducted on these ore types with the objective of finding a more effective PGM collector and chromium depressant Research work was conducted on UG2 high-chromium ore Detailed chemical analyses of the high-chromium ore used in this research are presented in Table 18.8

The PGM carriers in this ore include a variety of PGM minerals (sperrilite) and its alloys The main problems identified associated with processing this ore type were (a) poor concentrate grade, (b) low rate of PGM flotation, (c) excessive chromium reporting to the PGM concentrate and (d) high collector consumption

It was established that the reason for high collector consumptions was the presence of small, but significant, quantities of clay-like slimes The high collector consumption was the principal reason for the excessive amount of chromium reporting to the PGM concen­trate (mainly as fines)

Types of secondary collectors were extensively examined in research work Figure 18.6 shows the effect of secondary collectors on the PGM grade–recovery relationship The highest PGM recovery was achieved using collector PM443, which is an amine + ester-modified xanthate Among the chromium slime depressants evaluated, modified mixtures of organic acids, RQ depressants and a low-molecular-weight polyacrylic acid + pyrophosphate mixture were there The effect of different chromium depressants on chromium assays of the PGM concentrate are illustrated in Figure 18.7

Significant improvement in chromium depression has been achieved using depressants from the KM series, representing mixtures of organic acid and low-molecular-weight acrylic acid mixtures It is, therefore, possible to depress chromium during PGM flotation and at the same time reduce collector consumption The relationship between the level of collector and level of KM3 depressant is shown in Table 18.9

xanthate + PM 443

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Depressant Dosage

300 g/t RQ1

300 g/t KM1

Rougher

Figure 18.6 Effect of different secondary collectors on PGM grade–recovery relationship

Figure 18.7 Effect of different depressants on chromium assays of the PGM concentrate

Table 18.9 Effect of depressant KM3 on collector consumption during PGM flotation from UG2

high-chromium ore Reagent (g/t) PGM cleaner concentrate PGM rougher concentrate

Assays (%, g/t) % Distribution Assays (%, g/t) % Distribution Collector Depressant Pt Pd Cr Pt Pd Cr Pt Pd Cr Pt Pd Cr

Comparative continuous locked cycle tests were conducted using the reagent scheme currently used in an operating plant and the new reagent scheme developed during the research on ore from the Waterval plant (South Africa) These results are compared

in Table 18.10 A substantial improvement in metallurgical results was achieved using the new reagent scheme

This new reagent scheme included collector PM443 and depressant KM3

The collector type plays a significant role in PGM recovery from high-chromium ores Collectors were examined in detail [19] on several high-chromium ores, where new collectors from the PM series were included in the evaluation These collectors are

Table 18.10 Comparison of results using the new and standard plant reagent scheme from Waterval Plant

(South Africa) Reagent scheme Product Weight (%) Assays (%, g/t) % Distribution

Newly developed scheme PGM Cl concentrate 2.08 89.54 55.54 1.02 89.0 86.1

PGM comb tail 97.92 0.24 0.19 – 11.0 13.9 Feed (calc) 100.00 2.10 1.34 – 100.0 100.0 Standard pant scheme PGM Cl concentrate 2.01 86.01 49.08 2.72 79.8 76.7

PGM comb tail 97.99 0.45 0.31 – 20.2 23.3 Feed (calc) 100.00 2.16 1.29 – 100.0 100.0

Table 18.11 Effect of type of collector on PGM rougher–scavenger flotation from high-chromium ores Collector type PGE rougher concentrate PGE rougher + Scavenger

concentrate Assays (g/t) % Distribution Assays (g/t)

PAXa, R3477b 120.4 98.5 66.3 64.2 44.3 39.8 84.8 83.5 PAXa, R404b 110.1 97.0 64.3 62.1 46.3 41.1 85.2 83.6 PAXa, PM301 116.6 94.5 70.2 70.0 42.3 38.0 88.5 86.2 PAXa, PM305 113.8 96.3 80.2 80.0 43.3 39.6 92.5 91.1 SIBXa, PM303 122.4 97.9 82.2 81.0 44.6 40.1 92.3 92.1 a

18.7.6 Flotation of oxide PGM ores

There are only a few known oxidized PGM deposits in which the ore is in the development stage These deposits can be found in Brazil and Australia The PGM in these ores is usually represented by different PGM minerals and alloys, finely disseminated in a gangue matrix Using a flotation method with conventional reagent schemes, results in low PGM recoveries, ranging from 65% to 70% PGM Recent studies conducted on an ore from Brazil [20] indicated that a mixture of organic acid and thiourea has a positive effect on PGM recovery from oxidized ores Figure 18.8 shows the effect of organic-acid-modified thiourea on PGM flotation from oxidized PGM ore This data show that substantial improvement in PGM grade and recovery was achieved using organic-acid-modified thiourea

18.8 PLANT PRACTICE IN TREATMENT OF PGM ORES

In contrast to other sulphide-treatment flowsheets and reagent schemes, which are rela­tively simple, the flowsheet and reagent schemes for treatment of PGM ores can be highly complex, and varies from one ore type to the next

In general, the type of flowsheet used to treat PGM ores largely depends on the type of ore For example, ores that are sulphide dominated have the simplest flowsheet but