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Cyanidation of Refractory Gold Ores: A Review Conference Paper · July 2014 DOI: 10.13140/2.1.477 10.13140/2.1.4772.6407 2.6407
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Cyanidation of Refractory Gold Ores: A Review R.K. Asamoah, R. K. Amankwah and J. Addai-Mensah Ian Wark Research Institute, The ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia Mineral Engineering Department, University of Mines and Technology, Tarkwa, Ghana
Asamoah, R. K., Amankwah, R. K. and Addai-Mensah, J. (2014), “Cyanidation of Refractory Gold Ores: A Review”, 3rd UMaT Biennial International Mining and Mineral Conference, pp204-
Abstract The insusceptibility of gold ores to conventional cyanidation is defined by its mineralogical information and exacerbated by the continuous decline in gold grade. The depletion of readily amenable gold ores and discovery of complex deposits inspire knowledge improvement and search of commercially effective technique in extracting gold from refractory gold ores. This paper reviews the causes, challenges, concerns and approach towards refractory gold ore cyanidation, considering the conceptual and historical perspectives. It considers the diverse processing approach (route)neededto render refractory gold ores amenable to cyanidation. Directions for future research in processing refractory gold ores h as been stated.
Table 1 Classification of Refractory Gold Ores Based of Degree of Refractoriness. Modified after Amankwah et al. (2013)
1 Introduction Cyanide remains the universal ligand for gold extraction. It exhibitsseveral advantages (low cost, great effectiveness for gold dissolution, and selectivity for gold over other metals) overother ligands (halides, thiosulphate, thiourea, and thiocyanate) capable ofextracting gold. The use of cyanide in leaching system is termed“cyanidation”(Habashi, 1970; Marsden and House, 2006). Despite the success of cyanidation, its applicability is limited by some gold mineralization. These types ofgoldmineralisation are referred to as “refractory gold ores”(Turney et al ., 1989). Depletion of amenable ores alert researchers to decipher the polylemma associated with treating refractory ores. The degree of refractoriness has been classified based on the proportion of gold recovered (Table 1). Cyanidation can only be useful if by it, gold present is economically recovered with high efficiency.This paperreviews the causes, challenges, concerns and approach towards refractory gold ore cyanidation, considering the conceptual and historical perspectives. The review will show evidence of variation in ore characteristics with continuous gold processing, depth and possible ore characteristics in the near future. It will also discuss previous and recent discoveries on cyanidation of complex ores to help predict the direction of future research in gold extraction.
Classification
Gold recovery
Free milling
More than 95%
Mildly refractory
80 - 95%
Moderately refractory
50 - 80%
Highly refractory
Less than 50%
2 Causes of Ore Refractoriness Gold ore refractoriness vary withgoldmineralisation. These mineral associations occur during concomitant geological leaching, concentration and deposition of gold minerals in the earth crust(McKibben, 2005).Causes of refractory gold ores can also be classified based on gangue mineral association into(Turney et al ., 1989; Afenya, 1991):
Physically locked gold; Chemically locked gold; Reactive gangue minerals; Adsorption of gold; and Passivation of gold.
Some complex gold ore deposits exhibit a combination of the above effects making them more complex for gold extraction.
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In lateritic system, gold is associated in various forms of oxides(Enzweiler and Joekes, 1991; Greffié et al ., 1996). The contact made by gold with oxide especially, iron oxide has been explained by the 2+ 3+ oxidation – reduction reactions between Fe /Fe and 3+ 0 Au /Au forming metallic gold. This model explains the intergrowth of gold and iron oxide (Mann, 1984;Greffié et al ., 1996).Greffié et al . (1996)Concluded that, in natural system, where redox processes may take place, gold will be transported as o colloidal Au adsorbed or trapped in iron oxides.
2.1 Physically Locked Gold Gold in this classification exist in the free state, occluded and/or disseminated, within the cyanideinsoluble gangue minerals (silicates, sulphides and oxides)(Fig. 1)(Bache, 1987; Spry and Thieben, 2000). 2.1.1 Formation of Physically Locked Gold
Gold can be physically locked by silicates, oxides and sulphides. Its formation is primarily by natural dissolution, concentration and precipitation processes. Evidence of these natural processes have been studied extensively (Boyle, 1979; Mann, 1984; Michel, 1987;Colin et al ., 1989; Vlassopoulos and Wood, 1990;Benedetti and Boulëgue, 1991; Greffié et al ., 1996; Bowell et al ., 1999; Mohammadnejad et al ., 2013). Mohammadnejad et al . (2013) proposed that dissolved gold is reduced and deposited on the surface of silicate minerals.Sulphides are also known to reduce dissolved gold (Marsden and House, 2006). Hydrothermal activities after deposition lead to physical locking of gold. Bowell et al . (1999)associated gold – silica intergrowth to colloidal transport of Au in hydrothermal solutions. Earlier research by Saunders (1990) showed that coagulation of colloidal silica leads to trapping of gold colloids. Frondel (1938)intensively studied the stability of colloidal silica. Gold in these minerals mostly occur as submicroscopic to sizable grains(Bowell et al ., 1999; Coetzee et al ., 2011). Various research have been done on this type of refractory ore (Deep West Orebody, Jokisivu Ore, Getchell Ore, etc)(Bowell et al ., 1999; Hausen, 2000; Liipo, 2003). Research by Liipo (2003) showed that 50% of gold in Jokisivu pilot feed and products was locked up mainly in silicates which reported at tailings after cyanidation. Gold in the Carlin-type deposit was reported to be ultramicrometer native gold inclusions, not as isomorphous substitution (Mao, 1991).
2.2 Chemically Locked Gold Gold in this type is concentrated chemically by its host. Typically, gold is lattice-bound in sulphides and gold-bearing tellurides(Cook and Chryssoulis, 1990; Cook et al ., 2009). Spectrographic techniques have proved that gold, in some deposits(Sandaowanzi deposit, Tissa-Sarkhoi province, Golden Sunlight deposit, Kassiteres-Sappes area, Emperor gold deposit, etc.), is chemically combined within sulphideand telluride minerals rather than as discrete metals inclusions (Cook and Chryssoulis, 1990; Spry et al ., 1997; Mao, 1991; Spry and Thieben, 2000; Pals et al ., 2003; Voudouris et al ., 2006; Zoheir and Akawy, 2010;González-anaya et al ., 2011; Adams, 2013; Liu et al ., 2013).The world’s biggest newly discovered gold-telluride deposit (Early Cretaceous Sandaowanzi deposit, northeast China), contains>95% gold. The highest gold grade registered was 20,000 g/t (Liu et al ., 2011; Liu et al ., 2013). 2.2.1 Formation of Chemically Locked Gold
Electronic and structural investigation by Chen et al . (2014) revealed that, gold would most likely exist in pyrite by incorporating into interstitial lattice sites and by substituting for S atoms (Fig. 2). Their 1+ calculations showed that, gold was present as Au in pyrite. The co-occurrence of tellurium with noble metals such as gold and silver was attributed to the semi-metallic nature of tellurium (Zhang et al ., 2010). Major investigations showed that, this mineralisation occur as a result of hydrothermal alteration of granites, diorites, and effusive rocks at temperature in the range 70 – 280 °C. As temperature decreases, the activity of tellurium in hydrothermal fluid increases, therefore its formation in epithermal deposits.Most telluride associations are in porphyry gold deposits where the occurrence of gold mineralization is in volcanoplutonic belt. Goldtelluride formation and phase relations (among
Fig. 1 Schematic diagram of physically locked gold in pyrite, quartz and/or iron oxide. Modified after Ellis (2003)
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tellurides, sulphides and oxides) have been studied extensively (Afifi et al ., 1988; Bortnikov et al ., 1988; Damdinov et al ., 2007;Taylor, 2009;Liu et al ., 2011; Liu et al ., 2013). Fig. 3 shows the four type possible formation mechanisms (one Au atom substituting for the Fe atom; one Au atom substituting for one S atom; two Au atoms substituting for one S2 dimer; and Au incorporation into an interstitial lattice site) of gold in a 2 × 2 × 2 pyrite supercell (Fe32S64) investigated by Chen et al . (2014).In most case telluride coexist with sulphides such as pyrite, sphalerite, galena and chalcopyrite (Fig 4).
Fig. 3 Mineralogy of Sandoawanzi gold telluride ore (a) Coexisting petzite, sylvanite and chalcopyrite in quartz veins in reflected parallel light); (b) Twins in a calaveritegrain in quartz vein (reflected polarized light); (c-f) Backscattered electron SEM images. (c) Petzite and sylvanite in association with chalcopyrite. (d) Sulvanite-hessitesymplectite. (e) Coexisting krennierite, altaite and chalcopyrite in pyritebearing quartz vein. (f) Coloradoite-petzite association. Source: Liu et al. (2013).
Fig. 2 Bulk FeS2 unit cell (a) and 2 × 2 × 2 supercell (b), and schematic diagrams of Au in pyrite crystal. One Au substituting for one Fe (Fe31S64Au) (c), one Au substituting for one S(Fe32S63Au) (d), two Au substituting for a S2 dimer (Fe32S62Au2) (e) and Au in interstitial lattice site(Fe32S64Au) (f).Source: Chen et al. (2014)
2.3 Reactive Gangue Minerals The recovery of gold in cyanidation process is often accompanied by leaching of other species(sulphides and sulph-arsenides, mainly those of copper, silver, antimony and arsenic). These side reactions deplete free cyanide and oxygen required for gold extraction. Research has shown that stoichiometric amount of free cyanide needed for most gold recovery processes is < 1 % of the total consumed(Petre et al ., 2008).These cyanide- and oxygen-consuming components are known as“cyanicides” (e.g. S, Cu, Zn, Fe, Ni, etc.). They divert the lixiviant into formation of non-valuable complexes(Bache, 1987;Fraser et al ., 1991; Linge, 1995; Hausen, 2000;
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Liipo, 2003; Amankwah et al ., 2005; Nanthakumar et al ., 2007; Petre et al ., 2008; Adams, 2013).
are usual components of meta-sedimentary rocks that are often diffused within the mineral matrix. Research shows that carbon exist in different forms in the ore, which show different affinities for the gold-cyanide complex(Stenebraten et al ., 2000).The adsorption ability exhibited by copper-sulfide minerals, clay and silicates during gold dissolution is dependent on its surface area and characteristics such as mineral surface activation (Miller et al ., 2005; Mohammadnejad et al ., 2014).
2.4 Adsorption of Gold During leaching, dissolved gold is sometimes adsorbed from leach liquor making them unavailable for concentration byactivated carbon. Gold, adsorbed by these ore components, report in the tailings. Ore components capable of adsorbing gold are carbonaceous materials and other surface active gangue minerals (e.g., clay, silicates, etc.). When aurocyanide (Au(CN)2 ) is adsorbed by carbonaceous material present in the ore, it is termed “Pregrobbing”(Dunne et al ., 2012). This behavior was detected as early as 1911 by Cowes(Menne, 2003; Miller et al ., 2005). The carbonaceous matter may occur throughout the ore body or in distinct pods or veins within a deposit. Preg-robbing capacity varies widely between carbonaceous ores. Mild pregrobbing ores may have capacity to adsorb < 1 gAu/t ore, while extreme preg-robbing ores may have the capacity to adsorb > 500 g Au/t ore(Dunne et al ., 2012).Miller et al . (2005)Comprehensively reviewed preg-robbing gold ores. The adsorption process of carbonaceous material should not be confused with encapsulation of the gold. The most renowned examples of carbonaceous ores are the Carlin trend (Nevada, USA), Ashanti trend (Ghana), Witwatersrand (South Africa), Muruntau (West Uzbekistan), Kumtor(Kyrgyzstan), and Macraes (New Zealand) (Menne, 2003; Miller et al ., 2005). Stenebraten et al . (2000) showed that the presence of carbonaceous matter in the ore may not necessarily result in poor gold recovery.Aside carbonaceous material, minerals (copper-sulphide minerals, clay, and silicates) can adsorb dissolved gold during cyanidation (Bowell et al ., 1999; Marsden and House, 2006; Miller et al ., 2005; Mohammadnejad et al ., 2014; Saunders, 1990).
2.5 Gold Passivation during Leaching During leaching, some ionic species form stable intermediate adsorbing layers and oxide layers on the surface of gold mineral(Marsden and House, 2006). Usually, for reactions where gold passivation occurs, very low rate of reaction are recorded. Minerals such as chalcocite, pyrrhotite, gold-telluride, chalcopyrite, pyrite and stibnite form gold mineral surface passivation species that retard gold dissolution during leaching. Intergrowth of the passivating species results in physical locking of gold mineral for processing. These precipitates are formedatfavourablehydrometallurgical conditions (Climo et al ., 2000;Zhang et al ., 2009; Kyle et al ., 2012). From principle, passivation increases with pH to a point of very high alkalinity from which passivation reduces with further pH increase (Bagotsky, 2005).
3 Effect of Refractory Gold Ore on Cyanidation 3.1 Processing of Physically Locked Gold Since gold mineralization of this type are not by chemical reactions, physical processes can effectively render them amenable to cyanidation. Several research has shown that physically locked gold are amenable after ultrafine grinding of gold ores (< 11 µm) (Fig. 4)(Ellis, 2003). Though the operation is expensive, Ellis (2003) showed that it was more economical to some oxidation processes ( Frondel, 1938; Saunders, 1990;Bowell et al ., 1999). Not all physically locked refractory gold ores give a large recovery improvement after fine milling. Gold locked in arsenopyrite for example does not achieve the same gold recovery as gold disseminated in pyrite due to the smaller gold particle size of the locked gold (Ellis, 2003). Fig. 4 shows the effect of ultrafine grinding on gold recovery. Gold in the Kalgoolie
2.4.1 Formation of Preg-robbing Gold Ore
The association of gold with carbonaceous matter is believed to be due to biological redistribution. However, there are convincing theories relating to the origin of carbonaceous matter from methane under severe conditions of pressure and temperature(Menne, 2003). The carbonaceous material comes from the solid-state metamorphic transformation (graphitization) of organic material originally in contact with the rock. Graphitization depends on temperature, pressure and type of carbon precursor (Miller et al ., 2005).Carbonaceous matter
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Consolidated Gold Mine (KCGM) ore is not locked in arsenopyrite.
Fig. 5 Effect of pH on the Rate of Dissolution of Gold and Silver in KCN. Source: Habashi (1970).
Fig. 4 Gold Recovery of Arsenopyrite and Pyrite Ores. Source: Ellis (2003).
3.3 Processing
3.2 Processing of Chemically Locked Gold
Gold
Associated
with
Reactive Gangue Minerals
Gold-telluride minerals dissolve more slowly in cyanide solutions compared with pure gold, resulting in less efficient gold extraction (Climo et al ., 2000; Henley et al ., 2001). The slow dissolution rate in alkaline cyanide solution has been attributed to the formation of a passivating film of H2TeO3 that protect the mineral surface from further oxidation (Eq. 1). At higher pH (>12), H2TeO3 dissolves to 2TeO3 (Eq. 2) (Climo et al ., 2000; Zhang et al ., 2009; Kyle et al ., 2012).
Most often, gold is liberated from the ore but a perceived influence of reactive gangue minerals on gold processing is a competing consumption of O 2 and CN . It has been postulated for example that pyrrhotite dissolves in oxygenated alkaline cyanide solution by oxidation to form dissolved sulphur 2 4species such as SO 4 , SCN and Fe(CN)6 , consuming O2, CN and alkalinity. The most likely mechanism of pyrrhotite interference in gold processing plants involves the precipitation of dissolved gold on pyrrhotite driven by the oxidation of surface ferrous hydroxide to ferric hydroxide (Dunn et al ., 1995; Linge, 1995; Petre et al ., 2008; Azizi et al ., 2011).
(1)
of
(2)
The calaverite leach rate during cyanidation increases with increasing pH, but the pH increase is limited by the use of lime as the pH modifier due to its solubility limit (Kyle et al ., 2012). However, most leaching systems operate between pH 10 and 11. As shown in Fig. 5, high alkalinity (pH in the range 11 – 13) decreases the rate of dissolution (Habashi, 1970; Marsden and House, 2006).
3.4 Processing
of
Gold
Associated
with
Adsorbing Minerals More recently, there has been extensive research undertaken on the use of thiosulphate as a replacement for cyanide in preg-robbing ore treatment with gold thiosulphate complex not readily adsorbing on active carbon (Dunne et al ., 2012). Tan et al . (2005)showed thatgrinding carbonaceous materials with the ore is more detrimental than grinding the ore without carbonaceous matter.
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Theyalso showed that, coating of graphite is less than that of sulphides when ground together with the ore but the detri mental effect of the graphite is more than that of sulphides. For mild preg-robbing ores, CIL circuits can be an effective processing option. Adding blinding agent, such as kerosene to foul the carbonaceous material can reduce the preg-robbing capacity, however care must be taken to ensure that activity of the activated carbon is maintained(Miller et al ., 2005; Dunne et al ., 2012). For other adsorbing minerals, surface deactivation and control grinding can reduce their gold adsorbing ability(Miller et al ., 2005).
processing technology (e.g., crushing, grinding, flotation, oxidation, etc.) is defined by the gold grade and ore characteristics. Changes in ore characteristics lead to the closure and re-opening of various mines with different processing routes. Existing mines need to introduce new techniques, modify unit operations and processes in order to register high recoveries of gold in cases of ore mineralogical variation.Flow sheets of several mines (Getchell mine, Newmont Gold Company, etc.) have been modified due to ore complexity compared with the available technology (Mason and Nanna, 1988; Kontopoulos and Stefanakis, 1989; Seymour and Ramadorai, 1989; Taylor, 2009).
3.5 Processing of Ores with Passivating
5 Conclusions
Minerals
This review showed that researchers, institutions, consulting companies and gold processing companies have to improve gold extraction techniques as the mineralogy becomes more complex. Invariably low gold grade is recently processed as cyanide amenable portion of gold deposit gets depleted. There is a need for fundamental studies to aid processing of these complex ores. This fundamental studies should not be limited to cyanidation but should include all other possible economically viable means of extracting refractory gold mineral. For example, the newly discovered chemically locked gold at Sandaowanzi, with its highest gold grade being 20, 000 g/t, needs to be processed. It is believed that, by creating conditions, that opposed the concomitant processes leading to gold deposition, gold, independent of its complexity, could be extracted. Improved understanding of the mechanism for extraction can be determined from the natural mechanism of deposition.
Galvanic interaction during leaching has been shown to improve gold recovery in some refractory ores (pyrites). However, the presence of minerals such as sphalerite, chalcocite, stibnite and chalcopyrite in leaching system cause permanently poor gold recovery in presence and absence of galvanic interactions(Azizi et al ., 2011). The effect of sphalerite was attributed to surface obstruction by passivating film(intermediate products, HS and/or 2 Sx ) formed during sphalerite dissolution. Chalcocite depleted free cyanide in forming copper cyanide and hydrosulphide. Chalcopyrite, like chalcocite, consume free cyanide. Hydrosulphide from this reaction is believed to form passivating film aroundgold. Stibnite form passivating film of antimony oxide leading to poor gold dissolution(Deschenes et al ., 2000; Azizi et al ., 2011). Kinetic studies by Petre et al . (2008) argued that sulphur species affect gold leaching reaction directly. They observed that leached sulphide ions passivates the gold surface by forming a passive Au2S layer. Research shows that, the presence of lead minerals (lead oxide, lead sulphide and lead sulphite) in ore improves gold dissolution. The effect contributed by lead during cyanidation has been investigated (Deschenes et al ., 2000; Azizi et al ., 2011).
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