Sulfidic Gold Ore Research Articles

Effect of Temperature on Column Bioleaching of a Refractory Gold Ore

Low-grade, finely disseminated refractory sulfide gold ores associated with high arsenic are ubiquitous resources all over the world. Since heap bio-oxidation is an economic and promising biotechnology to recover gold, low grade, high organic carbon and arsenic bearing gold ores from Zhesang Mines in China were chosen for this purpose to study the key factors that would affect biooxidation. Pyrite and arsenopyrite (particle size 0.002-0.22 mm) were the main minerals from Mineral Liberation Analysis (MLA). Column biooxidation and cyanidation of mineral size < 10 mm were evaluated for its potential for gold extraction. Results showed that temperature was the main factor influencing sulfide oxidation. 58-67 % of sulfide was oxidized at 35-45°C after > 240 days of biooxidation with mixed mesophiles, while higher sulfide-S dissolution (77%) was obtained at 60°C. Sulfide-S fraction distribution revealed higher mineral decomposition, finer fractions and eventually higher sulfide oxidation at 60°C. Jarosite and scorodite were found from the residues at 60°C by SEM and EDX, which implies higher temperature accelerated arsenic precipitation. No elemental sulfur was detected during the biooxidation at 35-60°C. After bio-oxidation, column cyanidation was successfully demonstrated recovery of gold from the residues, with gold extraction rate reaching 66%.

Eucalyptus oils as green collectors in gold flotation

In this study, the potential of Eucalyptus oils as green collectors in gold flotation was examined. Flotation tests were conducted on a sample of Boddington’s copper–gold sulfide ores. Eucalyptus oil compositions were determined by gas chromatography–mass spectrometry. Gold association of flotation products was investigated by Mineral Liberation Analyzer. It was found that Eucalyptus oils have the potential to replace potassium amyl xanthate as a collector in free gold flotation. The collecting power of Eucalyptus oils is correlated to the percentage of non-polar and weakly polar constituents, and in particular 1,8-cineole, phellendrene and pipertone. Eucalyptus oils possess strong selectivity for free gold rather than gold carriers, and therefore achieve high gold grades at the head of the flotation circuit.

Effects of dissolved oxygen on the biooxidation process of refractory gold ores

While multiple theories exist regarding the effect of dissolved oxygen (DO) on the biooxidation of minerals, few studies have been performed the cellular or molecular scale (e.g., genetics) and the mechanism remains unclear. In this paper, the effects of DO concentration on the biooxidation process of refractory sulfide gold ores by Acidithiobacillus ferrooxidans were investigated in the experimental stirred tank bioreactors (STRs). The results indicated that higher biooxidation and cell growth rates were correlated with higher DO concentration. The biooxidation process was restricted at 1.2 ppm DO due to oxygen limitation. Furthermore, the effects of DO on cellular and molecular scale were studied for the first time. The results demonstrated that the oxygen uptake rate (OUR), the Fe(2+) oxidation activity and the rus gene expression of A. ferrooxidans all increased with the DO concentration, which might be responsible for the increase of the biooxidation rates with the DO concentration. This study provides insight into the potential impact of molecular-level mechanisms of DO in the biooxidation process of minerals.

The Oxidation Pretreatment of a Sulfidic Refractory Gold Ore with Pyrolusite

An attempt has been made to study the pretreatment of sulfidic refractory gold ore with pyrolusite. The influences of various parameters were studied to optimize the conditions. The optimum leaching conditions were pyrolusite/gold ore mass ratio of 3.0, H2SO4concentration of 3.0 M, temperature of 100°C, particle size of 45μm, stirring speed of 200 rpm. After the pretreatment, gold and silver were exposed, these made the extraction of the precious metal in the gold ore much easier.

Integrating geologic and satellite imagery data for high-resolution mapping and gold exploration targets in the South Eastern Desert, Egypt

The granitoid-greenstone belts of the Arabian–Nubian Shield are well-endowed with lode gold and massive sulfide ores. Although generally characterized by excellent outcrops and arid desert realm, poor accessibility and lack of finance have been always retardant to detailed geologic mapping of vast areas of the shield. Lack of comprehensive geological information and maps at appropriate scales would definitely hinder serious exploration programs.In this study, band ratioing, principal component analysis (PCA), false-color composition (FCC), and frequency filtering (FFT-RWT) of ASTER and ETM+ data have substantially improved visual interpretation for detailed mapping of the Gebel Egat area in South Eastern Desert of Egypt. By compiling field, petrographic and spectral data, controls on gold mineralization have been assessed in terms of association of gold lodes with particular lithological units and structures. Contacts between foliated island arc metavolcanics and ophiolites or diorite are likely to be favorable loci for auriferous quartz veins, especially where the NW–SE foliation is deflected into steeply dipping NNW-trending shear planes. High-resolution mapping of the greenstone belt, structures and alteration zones associated with gold lodes in the study area suggests that dilatation by foliation deflection was related to emplacement of the Egat granitic intrusion, attendant with a sinistral transpression regime (i.e., ∼640–550Ma?). Gold mineralization associated with granitoid intrusions in transpression-induced pull-apart structures elsewhere in the Eastern Desert (e.g., Fawakhir, Sukari and Hangaliya mines) emphasize the reliability of this setting as a model for gold exploration targets in greenstone terrains of Egypt, and may be elsewhere in the Arabian–Nubian Shield.

First findings of palladium-gold-REE ore mineralization in precambrian carbonaceous shales on the western slope of the Southern Urals

866 During the last 20 years, a new type of veindissem� inated goldsulfide ore mineralization with dispersed gold and platinum group minerals has been identified. These deposits are located within rift structures in areas of weakly manifested magmatism and localized in ancient rocks enriched in organic carbon matter. Black shale sequences form extended regional zones, but the deposits within them are associated with local fields, where dynamothermal processes and certain types of metasomatic alterations are manifested. On the western slope of the Southern Urals, Riphean black shale deposits are widespread. Testing of these mineralized black shale deposits showed the presence of abnormally high (up to industrial) gold content. Scientific and research works carried out by the authors in 2005-2009 within the northern area of the anticlinoriums of Mayardak and Yamantau (Fig. 1) made it possible to study the Precambrian carbon� aceous deposits on noble metals, tungsten, molybde� num, and vanadium (Table 1). The most interesting results were obtained as a result of study of carbon� aceous shales of the ZigazinoKomarovskaya suite. The geological traverses carried out in the area of this suite showed that it is composed of chlorite, sericite, quartz, mica-quartz, mica-feldspar-quartz shales, quartz siltstones, and sandstones in various propor� tions, enriched to varying degrees in carbon matter. In shales sulfidization (from single disseminated grains of pyrite to sulfide veinlets up to 1 cm) at a sulfide con� tent in the rocks of up to 25-30% is very often observed. Most outcrops and eluvial-deluvial placers are presented by more or less weathered rocks in which, instead of the primary sulfides, one can observe cavities of cubic habit or intensively ferruginous zones with cavernous textures. During further weathering, brown ironstones formed instead of these rocks. All this, in our opinion, indicates that this stratigraphic level is potentially a noble metalbearing area. This horizon has to be taken into account as the main object for organization in this area searching works for Au and PGE. The contents of Au, Pt, Rh, and Ir were determined in the chemical laboratory of the Institute

10.1007/s11477-008-1002-6

The concentration levels and distribution features of the platinum group elements (PGE) in quartz-sulfide and base-metal ores in deposits of the Sayan-Baikal Fold Region (SBFR) are discussed. Microfire assay neutron activation analysis (MF-NAA), which enables one to work on a nondestructive basis and allows one to avoid inaccuracies related to chemical sample preparation, was used as the main analytical technique. Three types of hydrothermal mineralization with elevated grades of PGE (especially Pt, Pd, and Ru) have been identified: (1) pyrite-pyrrhotite (massive sulfide) mineralization hosted in black shales of the Il’chir Sequence; (2) gold-sulfide ores of the Zun-Kholba, Tainsky, Kamenny, and some other gold deposits; and (3) silver-basemetal ores of the Dzhida-Vitim Zone. The PGE contents significantly vary, from global average values to tens of grams per ton. An absence of PGE minerals implies that these elements are finely dispersed in sulfide minerals and native gold. Taking into account difficulties in conversion of PGE into analytical forms, their nonuniform distribution in sulfide minerals, their high affinity to coordination compounds, and experimental results, cluster species of Pt and Pd in major minerals are suggested for the gold-sulfide and silver-base-metal ores in deposits, which are related to suprasubduction ophiolites and island-arc and intraplate settings in the SBFR.

Oxidative pre-treatment in thiosulphate leaching of sulphide gold ores

Abstract A diagnostic leaching showed that partial oxidation of the sulphide minerals in a gold ore was beneficial for thiosulphate leaching of gold. A pre-treatment process with oxidative ammoniacal solution enhanced the thiosulphate leaching of the sulphide ore, while the thiosulphate consumption was substantially reduced. The sulphide minerals partially decomposed in the pre-treatment process, exposing gold to the leach solution. Oxygen input by air bubbling and a longer contact time enhanced the oxidative ammonia pre-treatment process and hence accelerated subsequent thiosulphate leaching of the sulphide ore. Gold extraction in 0.8 M ammonia and 0.1 M thiosulphate solution after 24 h increased from 69% without pre-treatment to 81%, 84%, 90% and 94% respectively after 1, 3, 7 and 22 h pre-treatment. The consumption of sodium thiosulphate was 2.37 kg/t after 24 h leaching without pre-treatment, but was negligible after over 1 h oxidative ammonia pre-treatment. A counter-current leaching process was conducted in the leaching of the sulphide ore. The fresh leachant still gave higher leaching rates in contact with the pre-leached ore, while the pre-used leachant had significantly lower leaching kinetics and overall gold extraction in contact with the fresh ore. This 2-step counter-current leaching process proved that the leachant, other than the passivation, was the determinant factor causing the gold leaching rates to decrease after a certain time of leaching. The findings enable the thiosulphate leaching of high sulphide containing gold ores to be more efficient at lower thiosulphate consumption following the oxidative ammoniacal pre-treatment.

Comparative leaching of a sulfidic gold ore in ionic liquid and aqueous acid with thiourea and halides using Fe(III) or HSO 5− oxidant

The leaching of gold, silver and base metals from a sulfidic gold ore in the presence of an oxidant (peroxomonosulfate (HSO 5 −) or iron(III)) and leaching agent (thiourea, chloride, bromide or iodide) is compared in 1-butyl-3-methylimidazolium hydrogen sulfate (bmimHSO 4) and chloride (bmimCl) ionic liquids, as well as in aqueous saturated K 2SO 4 as the solvent medium. Over 85% of gold and silver was recovered in the presence of HSO 5 −/ thiourea at 25–50 °C in both bmimHSO 4 and bmimCl, with silver recovery significantly enhanced compared with that from aqueous sulfate medium. The leaching efficiency with HSO 5 − was similar to that with Fe(III) as oxidant in bmimHSO 4 and was far superior in bmimCl. With HSO 5 − /halide ion (Cl −, Br −, I −) as leaching agent, gold and silver recovery in bmimHSO 4, bmimCl or saturated aqueous K 2SO 4 improved from Cl − to Br − to I −, but only I − gave a high recovery in the bmimCl ionic liquid due to the particular stability of the iodo complex anion in this medium. However, recovery was significantly higher than in an aqueous medium. Negligible recovery of base metals occurred in the ionic liquid medium, making it highly selective for Au and Ag. Concentration dependence studies with respect to halide and oxidant have defined optimum conditions for gold and silver recovery.

Comparative study of inorganic arsenic resistance of several strains of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans

The growth characteristics of several strains of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans were studied in the presence of soluble inorganic arsenic(III) and (V) with regard to media pH changes, total bacterial populations and sulfur oxidation rates. Most of these bacteria could reach large populations and have strong sulfur oxidation activity in the absence of arsenic. However, in the presence of up to 120 mM arsenite or arsenate, different strains showed different inorganic arsenic resistance. A. thiooxidans LYS and A. ferrooxidans BY-3 were two of the best performers which showed high arsenite resistance: up to 80 mM and 60 mM, respectively. On the other hand, A. thiooxidans JY and A. ferrooxidans TKY-2 could adapt up to 120 mM and 100 mM arsenate, respectively. These bacteria strains may play key roles in the bioleaching of arsenopyrite or in the bio-oxidation pretreatment of arsenic-bearing refractory gold sulfide ores and concentrates.

Oxidative Capacity of Native Strains from Copahue Geothermal System in the Pretreatment of a Gold Sulfide Ore

In this report we evaluate the oxidative capacity of native microorganisms from Copahue geothermal system applied to a gold concentrate (71.1 g/t) compared to that showed by some culture collection strains. The sulphide ore, in which gold is present as submicroscopic particles contained in a pyrite matrix, came from several reservoirs of polymetallic ores located in Neuquén, Patagonia Argentina. The ore was processed in Andacollo treatment plant for gold concentration. After biooxidation tests, the highest percentages of solubilised iron were reached in the inoculated system (100% mixed native cultures, 47.8% Leptospirillum ferrooxidans ATCC 29047 and 28.5% for Acidithiobacillus ferrooxidans DSM 11477) while in the sterile control it was only 8.8%. The gold recovery was 96.3% and 45.3% for the treated and untreated concentrate respectively.

Mineralogy of the Precambrian southern Kostomuksha gold prospect in Karelia

The southern Kostomuksha gold-sulfide prospect with a grade of 0.2–30 g/t Au belongs to the gold-pyrrhotite-arsenopyrite mineral type and is localized in the metasomatically altered shear zone at the southern flank of the Kostomuksha iron deposit. The Au-bearing pyrite ore is commonly characterized by a low grade (0.02–1.0 g/t Au). The grade of Au-bearing mineralization composed of arsenopyrite, loellingite, and electrum (4.28–15.31 wt % Ag and up to 0.99–2.16 wt % Hg) is higher; pyrrhotite, chalcopyrite, galena, maldonite, aurostibite, and native bismuth are additional components of this mineral assemblage. The ore mineralization is hosted in the near-latitudinal shear zone close to the contact between the folded and metamorphosed banded iron formation (BIF) and halleflinta. The early stage of collision-related HP-HT metamorphism resulted in the formation of a garnet-amphibole-biotite assemblage (T = 680-750°C) and microcline. After an abrupt drop m pressure, metasomatic alteration and ore mineralization took place. The ore-forming process started at 510–440°C with deposition of arsenopyrite. Galena and electrum were formed at a lower temperature. The temperature continued to decline down to the stage of ore oxidation and deposition of colloform marcasite. Ore minerals precipitated from acid chloride aqueous solutions admixed with methane at the initial stage and from diluted aqueous solutions at the final stage. The character of wall-rock alteration and the gain of K, Rb, and B show that the ore-forming process postdated the emplacement of potassium granite. The occurrence of Cu, Zn, Pb, As, and Ni and other heterogeneous elements indicates a complex metamorphic-metasomatic source and an additional supply of Au, As, Bi, Sb, and Te under conditions of sulfur deficiency. The gold mineralization at the southern Kostomuksha prospect is classified as gold-sulfide (arsenopyrite) ore type related to shear zones in the BIF.

Pedrolampi: A precambrian gold sulfide deposit, Karelia

The Precambrian gold deposit Pedrolampi belongs to the structure controlled type and is located within the intersection between the submeridianal and northwestern schistosity and rock metasomatical alteration zones of the Elmus area in Central Karelia. Structural and mineralogical investigations performed allowed us to confirm that gold-sulfide ore formation occurred in the Late Archean within the submeridianal zone, and more late silver and copper mineralization overlapped along the zones of NW schistosity in the post-Yatulian period. Submeridianally striking quartz veins of the Pedrolampi deposit are boudinaged by NW deformations, and along these zones chlorite‐tourmaline-bearing metasomatites with Ag‐Au‐Cu mineralization developed. The formation temperature of metasomatites, veins, and ores descended from 400 up to 110 ° C and lower in the zone of oxidation. The ore-formation temperature is close for both types of ores and accounts for 270‐ 290 ° C (ore type-1) and 240 ° C (ore type-2). The simple mineral composition (gold‐pyrite), the presence of platinoids and Hg, and the low content of Ag in gold in ore type-1 prove their more high-temperature character and additional source of metals at the early ore-formation stage. Gold‐chalcopyrite mineralization in the ores of ore type-2 formed under lower temperatures and was accompanied by Cu and Ag afflux. Pedrolampi is a Precambrian mesothermal genetically polystage small gold deposit with resources accounting for about 10 t. The Precambrian gold deposit Pedrolampi is located in the intersection between submeridianal and northeasternly directed zones of schistosity and metasomatical alteration of Central Karelia rocks. New data on the geology and magmatism of the Elmus area belonging to the Segozersk-Vedlozersky greenstone belt, ore mineralogy, and ore-boundary alterations of the Pedrolampi area make it possible to consider it as a polygenetic mesothermal gold-sulfide deposit (with resources accounting for about 10 t) formed in the late AR and altered in the early PR. The geological structure of the Elmus area is composed of Neoarchean and Paleoproterozoic rocks (Fig. 1). Upper Archean (Lopian) strata strike in the submeridianal direction. Metabasalts and tuffs in the lower part of the section are overlapped by volcanogenic sedimentary rocks (banded and aglomerated tuffs and volcanites represented by quartz‐carbonate‐mica‐chlorite, chlorite‐cericite shales, sometimes with veinletimpregnated pyrite ores). In the upper part of the section occur metasandstones and polymictic conglomerates. These conglomerates belong to the molasse formations (accumulated in the basin of a strike-slip fault nature) similar to the strata occurring in the Koykarsky structure located to the south. Weakly rounded pebbles of 1‐ 10 cm in size within these conglomerates are presented by apo-tuff schists, porphires, and quartz. Archean strata are broken by bodies and dikes of pyroxenites, gabbro, rhyodazites, granite-porphyres, diorites, and

Platinum group elements in gold-sulfide and base-metal ores of the Sayan-Baikal Fold Region and possible platinum and palladium speciation in sulfides

The concentration levels and distribution features of the platinum group elements (PGE) in quartz-sulfide and base-metal ores in deposits of the Sayan-Baikal Fold Region (SBFR) are discussed. Microfire assay neutron activation analysis (MF-NAA), which enables one to work on a nondestructive basis and allows one to avoid inaccuracies related to chemical sample preparation, was used as the main analytical technique. Three types of hydrothermal mineralization with elevated grades of PGE (especially Pt, Pd, and Ru) have been identified: (1) pyrite-pyrrhotite (massive sulfide) mineralization hosted in black shales of the Il’chir Sequence; (2) gold-sulfide ores of the Zun-Kholba, Tainsky, Kamenny, and some other gold deposits; and (3) silver-basemetal ores of the Dzhida-Vitim Zone. The PGE contents significantly vary, from global average values to tens of grams per ton. An absence of PGE minerals implies that these elements are finely dispersed in sulfide minerals and native gold. Taking into account difficulties in conversion of PGE into analytical forms, their nonuniform distribution in sulfide minerals, their high affinity to coordination compounds, and experimental results, cluster species of Pt and Pd in major minerals are suggested for the gold-sulfide and silver-base-metal ores in deposits, which are related to suprasubduction ophiolites and island-arc and intraplate settings in the SBFR.

The behaviour of carbonaceous matter in cyanide leaching of gold

Carbonaceous matter was extracted from ore obtained from Stawell Gold Mine and characterised by microscopic methods and XRD. The carbonaceous matter was predominantly hexagonal crystalline graphite mixed with some minor semi-crystalline graphite or amorphous carbon. It was found that the natural carbonaceous matter had a low gold adsorption capability, dependent on cyanide concentration. No adsorption occurred with > 250 mg/L KCN. Auger studies demonstrated that during grinding, the carbonaceous matter preferentially smeared on iron sulphide rather than alumino-silicate surfaces present in the gold ore. The carbonaceous matter coated unevenly, preferentially at the edges and the defect sites. XPS analysis indicated that when gold foil was ground with the ore, the carbonaceous matter coated on gold surfaces mainly in the form of elemental carbon or graphite. Very limited amounts of organic carbon compounds smeared on the gold foils during grinding. Sulphur species were also present on the gold surfaces after grinding with sulphide ores. The current work shows that the carbonaceous coating had a significant detrimental effect on gold dissolution when gold was pre-ground with the carbonaceous ore. Sulphide coating on gold surfaces appeared to hinder gold dissolution to a lesser extent. The dissolution of gold foils artificially coated with the natural carbonaceous matter extracted from Stawell ore was also substantially retarded. In comparison, graphite had a much lower coating ability on gold. The addition of free carbonaceous matter to pre-ground non-carbonaceous sulphide gold ores did not have much effect on gold cyanidation. However, the carbonaceous matter ground with the non-carbonaceous sulphide ores significantly reduced gold leaching kinetics and recoveries.

HEAP BIOOXIDATION OF REFRACTORY GOLD ORES: CURRENT STATE OF THE ART

Heap leaching of low-grade refractory sulfidic gold ores has generated much interest during the last decade. Attention has focused primarily on improving gold extraction, with less concern about understanding how the multiple phenomena that are involved determine the overall sulfide oxidation rate. Varying the conventional operating parameters (flow rate, particle size, height) has had little impact on sulfide oxidation beyond the average of 10–15%/month. Variable aeration flow, adequate supply of carbon dioxide, finetuning of temperatures, well-defined particle kinetics, and a careful balance between coarse and fine ore particles may hold the key to successful commercial practice.

Adsorption and desorption of arsenic on an oxisol and its constituents

The present work investigates the adsorption and mobility (desorption) of As(III) and As(V) on an oxisol, and its main mineral constituents, as part of a broader project aimed at selecting a soil liner to be used in tailings dams at a sulfidic gold ore plant. Emphasis was given to a quantitative comparison of As mobility—here assessed by the amount of As leached from the loaded samples—under different experimental conditions. From among the soil constituents, goethite was the most efficient adsorbent with regard to arsenic adsorption, 12.4 mg g −1 for As(V) and 7.5 mg g −1 for As(III), respectively. Gibbsite also presented a relevant adsorption capacity (4.6 mg g −1 for As(V) and 3.3 mg g −1 for As(III)); adsorption on kaolinite was negligible (<0.23 mg g −1 for As(V) and As(III)). Desorption of the arsenic was shown to vary largely with the arsenic oxidation state, the adsorbents and the leaching solutions. While only 1–2% max. of As(V) was released from the loaded samples, leaching the A(III) reached 32%, the highest values corresponding to the solutions containing sulfate ions. Oxisol and goethite were superior to gibbsite with respect to As immobilization. Adsorption and mobility were also discussed with the help of electrophoretic mobility and isoelectric points (IEP) determined prior and following arsenic adsorption on goethite and gibbsite. The results indicated that As(V) is mainly adsorbed as an inner sphere complex. As(III) may be adsorbed as an inner or an outer neutral complex.

Distribution of the silver in Bulgaria ore deposits

The content of silver in the different, in terms of genesis and macrochemism, endogenic mineralisations in Bulgaria varies from 1-2 to 200 g/t, most frequently 5-35 g/t. The highest concentrations of this element are registered in the West Balkan region (lead-silver ores near Chiprovtsi, gold-sulphide ores near Govezhda and polymetallic ores Iskar-Vratsa ore region as well as Kremikovtsi deposit); in the Central Rhodopes region (lead-zinc deposits in Laki, Madan and Davidkovo ore fields, Ardino silver-polymetallic deposit); in the lead-zinc ores in Ossogovo ore region; in the East Rhodopes ore region (gold-silver-polymetallic ores in Madzharovo ore field and Popsko, silver-gold ores in Sedefche and Sarnak); in the East Srednogorie (gold-copper-polymetallic ores near Bakadzhik and Zidarovo). Some of the more significant favourable geological prerequisites for the accumulation of silver in the ores are: Late Paleozoic or Tertiary (Oligocene-Miocene) age, geodynamic environments of collision and zones with thick earth crust, anticlinoria and big horst-like uplifts, WNW and NW host structures, close to the surface or flange sections of the mineralisations. The mineralisations with higher content of silver are, as a rule, related to granites. Their host rocks are carbonates with development of metasomatic processes such as dolomitisation and granitoids with periore alterations of the quartz-sericite, quartz-mica (beresite), adular-sericite or quartz-adularia metasomatites and argillizites. The composition of the ores includes lithophile-chalcophile geochemical associations with prevalence of antimony. A favourable factor for the accumulation of silver is the lead-zinc macrochemism with prevalence of lead in zones with thick earth crust and the polymetallic - in zones with basificated (subocean type) earth crust. The mineral composition of the ores includes galena, sulphosalt silver minerals, the especially high silver-bearing tetrahedrite-tennantite series with prevalence of tetrahedrite and participation of magnesium and iron carbonates (siderite, ankerite, dolomite). The character of the silver-bearing mineralisations in Bulgaria is medium to low-temperature and, according to their peculiarities, they can be related to the mesothermal, epithermal or telethermal genetic classes.

Characterizing gold in refractory sulfide gold ores and residues

Gold is present in refractory sulfide gold ores mainly in arsenian pyrite and arsenopyrite, where it occurs in both the chemically bonded state and as nano-size grains of metallic gold. During roasting or pressure oxidation, the sulfide matrix is destroyed and essentially all the gold is converted to the metallic form. The liberated gold is readily dissolved in conventional cyanide media, although a few residual gold particles, commonly encapsulated by films of silica-rich gel or by calcium sulfate, are detected. Chloridecontaining gold ores can generate soluble gold chlorocomplexes during hydrometallurgical processing and the dissolved gold can be sequestered on the surfaces of associated carbon particles, resulting in reduced gold recoveries.

Preg-robbing phenomena in the cyanidation of sulphide gold ores

The ability of pyrite and chalcopyrite to adsorb gold was examined. Experiments were conducted in cyanide deficient solutions with and without activated carbon. Chalcopyrite was shown to be very strongly preg-robbing. It competed with activated carbon to remove the majority of gold from solution. Pyrite was also strongly preg-robbing, and in cyanide deficient solutions adsorbed the majority of the gold in preference to activated carbon. The degree of adsorption onto the ore or the activated carbon was a function of the kinetics of the adsorption process. The role of the cyanide complexes of copper, silver, zinc, nickel and iron was also examined. It was found that these complexes counteract preg-robbing, and serve to stabilise the aurocyanide complex by precipitating prior to gold precipitation. When cyanide or cyanide-species were present in solution, the adsorption onto activated carbon was found in general to take preference to the adsorption onto the minerals. The ability of a sulphide ore to adsorb Au(CN) 2 − from solution was found to be a function of how rapidly the ore consumed cyanide and precipitated metal–cyanide complexes. A mechanism was proposed where the gold is reduced at the chalcopyrite surface through the formation of an adsorbed intermediate. The gold was reduced, along with the oxidation of chalcopyrite to form copper–cyanide complexes in solution. Reduction of gold on the pyrite was observed to occur with the release of zinc as an impurity into solution, and a mechanism for this reaction was proposed.