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Introduction
Volume 2 of the ‘Flotation Reagents Handbook’ is a continuation of Volume 1, and presents fundamental and practical knowledge on flotation of gold, platinum group minerals and the major oxide minerals, as well as rare earths. Rather than reiterating what is well known about flotation of gold, PGMs and oxide minerals, emphasis has been placed on the separation methods which are not so effective when using conventional treatment processes. These difficult separation methods are largely attributed to problems with selectivity between valuable minerals and gangue minerals, 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, the main problem arises from the presence of gangue minerals in the ore, which have flotation properties similar to those of valuable minerals. These minerals have a greater floatability than that of pyrochlore or columbite. In the beneficiation of oxide minerals, finding a selectivity solution is a major task. This volume of the Handbook is devoted to the beneficiation of gold, platinum group minerals and, most important, oxide minerals. The book contains details on flotation properties of the major minerals. The fundamental research carried out by a number of research 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 mineral processors that are faced with the problem of beneficiation of difficult-to-treat ores, with a comprehensive digest of information available, thus enabling them to carry out their development testwork in a more systematic manner and to assist in the control of operating plants. This book will also provide valuable background information for researchers, university students and professors. The book contains comprehensive references of worldwide literature on the subject. New technologies for most of the oxide minerals included in this volume were developed by the author.
ix
– 17 –
Flotation of Gold Ores
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.
2.
3.
4.
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. 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). 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. The flotation method is a technique widely used for the recovery of gold from goldcontaining 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 as their exceedingly low concentrations in the ore, is one of the principal reasons 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
2
17.
Flotation of Gold Ores
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 bene ficiation 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 Ore type
Description
Magmatic
Gold occurs as an alloy with copper, nickel and platinum group metals. Typically contains low amount of gold Placer deposits, in general conglomerates, which contain quartz, sericite, 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 This type contains a variety of ores, including(a) gold-pyrite ores, (b) goldcopper 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. Sometimes are very complex and refractory gold ores. Normally the ores are composed of quartz, sericite, chlorites, calcite and magnetite. Sometimes the ore contains wolframite and scheelite
Ores in clastic sedimentary rock Hydrothermal
Metasomatic or scarn ores
17.3
Flotation Properties of Gold Minerals and Factors Affecting Floatability
3
Table 17.2 Major gold minerals Group
Mineral
Chemical formula
Impurity content
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
Tellurides
Calaverite Sylvanite Petzite Magyazite
AuTe3 (Au,Ag)Te2 (Au,Ag)Te Au(Pb,Sb,Fe)(S,Te11)
Unstable
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
Gold associated with platinum group metals
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.
4
17.
Flotation of Gold Ores
30
Adsorption of xanthate (%)
3 25 20
2
15 10 1 5 0 0
10
20
30
40
50
60
70
80
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 concentra tion, 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
17.6
Flotation of Carbonaceous Clay-Containing Gold Ores
5
some cases, flotation is also conducted on the cyanidation tailing. The reagent combina tion 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 sulphidi zer, 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 low molecular-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 pyrometallur gical or hydrometallurgical treatment. There are several flotation methods used for beneficiation of this ore type. Some of the most important methods are described below.
6
17.
Flotation of Gold Ores
Feed Grind 1 Classification 1 Classification 2
Grind 2
Scalp Float
Classification Flotation 1 Cleaner
Classification Grind 3 Flotation 2
Cleaner 1 Cleaner 2 Cleaner 3
Final tailing
Concentrate to smelter
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
Single-stage grind-flotation Two-stage grind-flotation Three-stage grind-flotation
% Recovery in concentrate
Tailing assays (%, g/t)
Au
Ag
Sb
As
S
Au
Ag
Sb
As
S
88.1 92.2 95.3
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
Flotation of Carbonaceous Clay-Containing Gold Ores
17.6.1
7
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 combi nation 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 carbo naceous 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
CuSO4 + xanthate Amine modified CuSO4 + xanthate
Product
Gold sulphide concentrate Gold sulphide tail Head Gold sulphide concentrate Gold sulphide tail Head
Weight (%)
30.11 69.89 100.00 26.30 73.70 100.00
Assays (%, g/t) Au
S
9.63 1.86 4.20 13.2 0.85 4.10
4.50 0.49 1.70 5.80 0.21 1.68
% Distribution Au 69.1 30.9 100.0 84.7 15.3 100.0
S 79.7 20.3 100.0 90.8 9.2 100.0
8
17.
17.7
Flotation of Gold Ores
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 dithiopho sphate 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 empha sized, 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.
Gold recovery in Cu cleaner conc. (%)
100
80
60 1 40 2
20
0 0
1
2
3 4 5 6 7 8 Pyrite content of ore (%)
9
10
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).
17.7
Flotation of Gold-Containing Copper Ores
Flotation feed
9
Bulk scavenger
Bulk rougher
Regrind High-intensity conditioning
Cu-Au rougher
Cu-Au cleaner 1
Cu-Au scavenger
Combined tailing
Cu-Au cleaner 2 Cu-Au cleaner 3 Cu-Au cleaner concentrate
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
Conventional (sequential Cu/Au) Bulk (Figure 17.4)
Product
Cu/Au concentrate Cu/Au tail Head Cu/Au concentrate Cu/Au ail Head
Weight (%)
2.28 97.72 100.00 2.32 97.68 100.00
Assays (%, g/t)
% Distribution
Au
S
Au
S
27.6 0.031 0.66 27.1 0.032 0.66
32.97 0.23 0.98 36.94 0.14 0.96
95.4 4.6 100.0 95.2 4.8 100.0
76.7 23.3 100.0 85.8 14.2 100.0
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 chlor ites and aluminosilicates, as well as preactivated quartz. Sodium silicate, carboxy methylcellulose 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.
10
17.
17.8
Flotation of Gold Ores
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.
2.
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. 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] Reagent used
Product
Weight (%)
Assays (%, g/t) Au
Na2S = 2500 g/t PAXa = 200 g/t Na2S = 2500 g/t PAXa/PM230 (1:1) = 200 g/t a
PAX = potassium amyl xanthate.
Copper Cl concentrate Copper tail Feed Copper Cl concentrate Copper tail Feed
9.36 90.64 100.00 10.20 89.80 100.00
% Distribution S
33.3 14.15 1.61 1.46 4.65 2.65 39.5 21.79 0.61 0.42 0.61 0.42
Au
S
67.0 50.0 33.0 50.0 100.0 100.0 88.0 85.5 12.0 14.5 12.0 14.5
17.10
Flotation of Arsenical Gold Ores
11 Table 17.7
Metallurgical results obtained using a sequential flotation method Product
Weight (%)
Gold concentrate Stibnite concentrate Tailing Feed
2.34 4.04 93.62 100.00
Assays (%, g/t)
% Distribution
Au
Ag
Sb
Au
Ag
Sb
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 Plant metallurgical results obtained using a bulk flotation method Product
Weight (%)
Gold concentrate Antimony concentrate Middlings Bulk concentrate Tailing Feed
1.80 1.80 0.50 4.10 95.90 100.00
Assays (%, g/t)
% Distribution
Au
Ag
Sb
Au
Ag
Sb
91.1 13.0 46.6 51.7 0.6 2.7
248.8 684.2 248.8 440.0 3.1 21.0
1.5 51.3 20.0 29.0 0.2 1.3
61.0 9.0 8.6 78.6 21.4 100.0
31.3 58.6 6.0 85.9 14.1 100.0
2.0 75.0 8.0 85.0 15.0 100.0
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 goldbearing 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
12
17.
Flotation of Gold Ores
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 goldcontaining 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 arseno pyrite depression and pyrite flotation, and consists of the following steps: 1.
2.
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. 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 tempera ture 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
Pyrite/arsenopyrite recovery (%)
100 Pyrite 80 60 40 Arsenopyrite 20 0 0
20 40 60 Heating temperature (°C)
80
Figure 17.5 Effect of temperature on separation of pyrite and arsenopyrite from a bulk pyrite/ arsenopyrite concentrate.
17.11
Flotation of Gold From Base Metal Sulphide Ores
13
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 cya nidation 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 signifi cant 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 leadzinc or copper lead-zinc ore. This is attributed to two main factors: when selecting a reagent
17.
Gold recovery in Lead concentrate (%)
14
Flotation of Gold Ores
70 Greens Creek ore (Alaska)
60 50 40
Grum ore Yukon (Canada)
30 20 10 0 0
100
200
300
400
500
ZnSO4 · 7 H2O (g/t)
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 combina tions 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) Au
ZnSO4, NaCN, CaO Cu concentrate 3.10 pH 8.5 Cu, 10.5 Zn Zn concentrate 5.34 Tailings 91.56 Feed 100.00 3.05 Na2SO3, NaHS, CaO Cu concentrate pH 8.5 Cu, 10.5 Zn Zn concentrate 5.65 Tailings 91.30 Feed 100.00 Courtesy of Esso Canada Resources.
Ag
% Distribution Sb
Au
Ag
Sb
20.4 26.2 330 45.1 85.6 2.8 1.20 0.61 55.4 4.6 3.4 82.2 0.77 0.11 0.58 50.3 11.0 15.0 1.4 0.95 3.60 100.0 100.0 100.0 32.5 28.1 2.80 68.3 87.4 2.3 1.20 0.55 54.8 4.7 3.2 84.6 0.43 0.10 0.52 27.0 9.4 13.1 1.45 0.98 3.66 100.0 100.0 100.0
17.12
Conclusions
17.11.3
15
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 metal lurgical results obtained with an Aerophine collector to those obtained with a dithio phosphate 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 character istics. 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.
16
Table 17.10 Effect of collector type on Cu–Pb–Zn–Au metallurgical results from a high-lead ore, Crandon (USA) Collector
30 g/t xanthate 20 g/t dithiophosphate 3477
30 g/t xanthate 20 g/t aerophine 3418A
Cu concentrate Pb concentrate Zn concentrate Tailing Feed Cu concentrate Pb concentrate Zn concentrate Tailing Feed
Weight (%)
2.47 1.80 13.94 81.79 100.00 2.52 1.92 13.91 81.65 100.00
Assays (%, g/t)
% Distribution
Au
Cu
Pb
Zn
Au
Cu
Pb
Zn
22.4 2.50 1.10 0.71 1.33 31.3 2.80 0.90 0.41 1.30
25.5 0.80 0.60 0.089 0.80 26.1 0.90 0.50 0.093 0.82
1.20 51.5 0.80 0.28 1.30 1.10 51.1 0.72 0.30 1.35
4.50 8.30 58.2 0.52 8.80 5.00 9.20 58.5 0.44 8.80
41.6 3.4 11.5 43.5 100.0 60.6 4.1 9.6 25.7 100.0
78.6 1.8 10.4 9.1 100.0 80.1 2.1 8.5 9.3 100.0
2.3 71.3 8.6 17.8 100.0 2.1 72.5 7.4 18.0 100.0
1.3 1.7 92.2 4.8 100.0 1.4 2.0 92.5 4.1 100.0 17.
Courtesy of Exxon coal.
Product
Flotation of Gold Ores
References
17
REFERENCES 1. Kudryk, V., Carigan, D.A., and Liang, W.W., Precious Metals, Mining Extraction and Processing. AIME, 1982. 2. Martins, V., Dunne, R.C., and Gelfi, P., Treatment of Partially Refractory Gold Ores, Randol Gold Forum, Australia, 1991. 3. Baum, W., Mineralogy as a Metallurgical Tool in Refractory Ore, Progress Selection and Optimization, Randol Gold Forum, Squaw Valley, 1990. 4. Fishman, M.A., and Zelenov, B.I., Practice in Treatment of Sulphides and Precious Metal Ores, Izdatelstro Nedra (Russian), Moscow, Vol. 5, pp. 22–101, 1967. 5. Sristinov, N.B., The Effect of the Use of Stage Grinding in Processing of Refractory ClayContaining Gold Ore, Kolima, No. 1, pp. 34–40, 1964. 6. Bulatovic, S.M., and Wyslouzil, D.M., Proceedings of the 2nd International Gold Symposium, Flotation Behaviour of Gold During Processing of Porphyry Copper-Gold Ores and Refractory Gold-Bearing Sulphides, Lima, Peru, 1996. 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 –
Flotation of Platinum Group Metal Ores
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. There fore, 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
20
18.
Flotation of Platinum Group Metal Ores
Table 18.1 List of platinum group minerals and their compositions PGM
Ideal formula
Other elements present
Anduoite Arsenopalladinite Atheneite Atokite Borovskite Braggite Cooperite Daomanite Erlichamanite Froodite Genkinite Geversite Guanglinite Hollingworthite Hongshlite Iravsite Iridium Isoferroplatinum Kotulskite Majakite Monochelite Nigglite Omelite Osmium Palarstanide Palladium Platiniridium Rhodium Ruthenium Ruthenosmiridium Sperrilite Temagamite Uvantserite Vysotaskite Xingzhongite Zvyagintsevite
RuAs2 Pd8As2.5Sb0.5 (PdHg)3As PdSn 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
18.3
21
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 Stillwater 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 McCal lum 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 hydrothermally 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
22
18.
Flotation of Platinum Group Metal Ores
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 sur rounded by secrecy, therefore, neither commercial processes nor research work on recovery of PGM is publically available. Long-term research work conducted by a number of research organizations and data collected from a number of operating plants are summarized in this chapter. From a processing point of view, PGM-containing ores can be divided into three general groups as follows: 1. 2. 3.
ores amenable to gravity preconcentration, ores amenable to flotation and 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, isoferro platinum (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.
18.5
Copper-Nickel and Nickel Sulphide Deposits with PGM as a By-Product
23
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 associa tions. 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).
24
18.5.2
18.
Flotation of Platinum Group Metal Ores
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, coop erate (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. 18.5.4
Other deposits
Other deposits of significant value as PGM carriers include the Kambalda district in Western Australia, the Pipe Mine in Thomson Manitoba, Canada, and the Hitura deposit in Finland. The mineralization of PGM in these ores is similar to that of the Sudbury region ores.
18.6
CHROMIUM DEPOSITS WITH PGM
There are a number of deposits of this type with different origins. The geological environ ments are well described in the literature [8,10]. Most economical PGM chromite deposits are described as follows: (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.
18.7
Flotation of PGM-Containing Ores
25 Table 18.2
Average chemical analyses of PGM from various areas of the UG2 deposits Area
Group
Marikoma Brits Hoekfontein South-western region Bushveld complex Moandagchoek North-eastern region Bushveld complex
18.7 18.7.1
A1 A2 A3 A4 A5 – A B C D E F
Assays (g/t) Pt
Pd
Rh
Ru
Ir
Au
Total
1.58 3.09 2.91 2.85 2.55 2.61 2.67 3.04 4.33 5.25 3.14 4.31
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
