The effect of pH value on gold extraction for various ore types is shown in Figure 28.4. Greatly increasing pH value has a beneficial effect on gold extraction for both oxide and sulfide ores and stabilizes ∼pH 9.5–10.0, whereas for some carbonaceous ores, pH value has no apparent effect. However, the outcome will vary depending on the nature of the gold and background mineral matrix. Jeffrey et al. (2008a,b) observed that increasing the pH value from 8.5 to 10.5 increased the gold leaching efficiency from 7% to 70% from a pressure-leach residue of a refractory gold ore after 1 h in 50 mmol/L (NH4)2S2O3 and 0.5–2 mmol/L CuSO4. However, leaching of gold after 24 h at pH 8.5 was similar to that observed at higher pH value, but the leaching rate was considerably slowerInquiry Online
The increase in gold extraction can be attributed to increases in residual Cu(II) in solution with increasing pH value resulting from the increase in free NH3 (Senanayake, 2004a,b). Leaching at a higher pH value can also be beneficial in improving silver extraction and stabilizing silver in solution. In addition, operating at a higher pH value favors the hydrolysis of the thiosulfate degradation products such as tetrathionate, which has been attributed to gold and silver loss during leaching (Aylmore et al., 2014). However, the increase in pH value also causes an increase in ammonia and thiosulfate consumption. Hence, optimum operating pH setpoint for a specific ore type would need to be determined
Partially roasted free sulfide particles have been found to be present in single-stage roaster calcine but are not typically observed in two-stage roaster calcine. Partially roasted free pyrite particles display concentric rimmings of hematite [Fe2O3], maghemite [Fe2O3 with up to 8% FeO], and magnetite [Fe3O4] with a pyrite core, as shown in Figures 5.25 and 5.26 and Figures 5.27–5.29. The core of partially oxidized arsenopyrite particles is a gold-bearing pyrrhotite (Chryssoulis, 1991). The submicroscopic gold content of the pyrrhotite is comparable to that of the precursor arsenopyrite (see Figure 5.30), indicating that arsenic was removed leaving behind the gold. Low-permeability hematite/magnetite particles are characterized by well-developed concentric pores and minor radiating pores, yielding inferior permeability despite the high porosity of the particle. Good permeability of calcine particles is a key requirement in order to be able to leach out colloidal size (<1 μm) gold inclusions formed during roasting
Figure 5.26. Reaction rims of roaster calcine arsenopyrite. The core is arsenopyrite [FeAsS], the inner rim is a gold-bearing pyrrhotite [Fe(1 − x)S], and the outer rim is porous hematite [Fe2O3]. Therefore, dearsenification takes place first [(Fe,Au)AsS → (Fe,As)S] and then desulfurization [(Fe,Au)S → FeOx(Au)], with colloidal gold forming in the second step (Con mine, NWT, Canada)
Figure 5.27. Reaction rims showing the progression in the oxidation of pyrite in the roaster. The core is pyrite [FeS2], the inner rim is maghemite [Fe2O3 with up to 8% Fe], and the outer rim is porous hematite [Fe2O3]
Figure 5.28. Zoned roaster calcine particle, showing two well-defined maghemite [Fe2O3 with up to 8% Fe] zones interbedded with zones enriched in hematite [Fe2O3]. Arsenic is strongly enriched in the maghemite zones, and gold is confined the inner maghemite zone. Arsenic is in the form of ferrous pyroarsenite [FeAs2O5]. Inset: SIMS elemental maps for Fe, As, and Au with concentration scale
Figure 5.29. Porous calcine particle with discontinuous sintered rim made of maghemite-ferrous pyroarsenite [FeAs2O5]. The sintered rim could be a relic of an original Au-rich arsenian pyrite layer coating an As/Au-poor pyrite or the reaction product of As2O3 with hematite [Fe2O3]. It should be noted that the areas with high arsenic content (yellow in SIMS maps) along the sintered rim are those where maghemite is more obvious in the microscope photograph on the top. Gold is localized along the sintered rim
Glazing, also referred to as sintering of the surface layer of porous hematite particles, is the result of a higher roasting temperature or longer residence time. It does reduce permeability of the particle by sealing pores on the surface and therefore porous particles with a glazing typically have a higher gold content (see Figure 5.31)
Maghemite, which essentially is hematite with magnetic properties, is an intermediate product of magnetite to hematite oxidation during roasting. It usually carries marginally more gold than the porous hematite particles, presumably because of its visibly lower porosity. The gold content of maghemite particles increases significantly when maghemite is intergrown with ferrous pyroarsenite [Fe2AsO5]. The presence of this compound has been documented in calcines produced from high in arsenic roaster feeds. Gold in ferrous pyroarsenite is usually either in solid solution or as very fine (<0.05 nm) colloidal gold
As with leached autoclave residues there will always be some gold associated with precipitated iron hydroxyl-oxides and cyanidable gold that has been encapsulated by calcium sulfate; however, it is typically present in lower absolute (g/t) and relative (%) concentration
Roasters are very efficient at burning carbonaceous matter, but 100% burning efficiency is rarely achieved. Therefore, residual exposed carbonaceous matter can become very active as a result of the roasting process. Such carbonaceous particles are fully loaded with gold, as documented by surface microbeam analysis (Figure 5.32) and contribute to gold losses in leached roaster calcines. Speciation of adsorbed gold has identified metallic gold (Au°), gold cyanide [Au(CN)2−], and thiocyanate [Au(SCN)2−]. Traces of cyanide and thiocyanate in the reclaim water complex the readily soluble colloidal gold produced in the roaster, thereby leading to possible preg-robbing ahead of the CIL circuit
Figure 5.32. Surface gold on unburned carbonaceous matter of a roaster/CIL tails sample analyzed soon after sampling and 2 months later. The change from water-soluble to cyanide-soluble gold indicates metallization of surface gold, originally present in water-soluble form
In the coarse-grained (>100 μm) microcrystalline quartz (but also other rock mineral) particles with finely disseminated tiny (1–2 μm) gold-rich pyrite inclusions, the inclusions toward the perimeter of the particles are oxidized (as clearly indicated by their conversion into hematite); however, pyrite inclusions in the core area in some instances remain unoxidized. This is not observed in particles finer than ∼53 μm. What is more important is that the gold content of rock mineral particles with hematite inclusions is only slightly lower than that of the rock particles with pyrite inclusions indicating that the “released” gold could not be removed, most likely due to lack of sufficient accessibility by the cyanide solution. This form of gold sets the minimum attainable grade for the leached calcine
As metallic gold associated with stibnite [Sb2S3]; it will be seen from the processes used to treat such materials that the majority of the gold appears to be in the metallic form rather than interstitially in sulfide lattices
A problem often encountered in evaluating historic descriptions of processes treating antimonial ores is that the gold deportment with respect to the mineral suite present is not well defined. Clearly, mineralogy has a crucial impact on the treatment route selected
Mercury in its organic form also biomagnifies in food webs. The use of mercury for gold extraction has been responsible for widespread contamination of aquatic ecosystems and the practice continues unabated in many parts of the world. Other sources, historic and present, include industrial processes, pulp bleaching, as well as combustion and release of natural mercury from flooded sediments, especially under acidic water conditions. There is good evidence that the acidification of poorly buffered lakes receiving acidic precipitation from combustion sources results in higher levels of mercury being biologically available to aquatic biota and thence to fish-eating bird species. Historically, mercury used in pesticide seed treatments caused severe mortality in seed-eating bird species and their predators. Long-term monitoring programs in seabirds suggest that mercury contamination is increasing globally. At sublethal doses, mercury has been found to affect reproduction although there appears to be wide interspecific variation in the levels that prove embryotoxic and teratogenic. Mercury is also a neurotoxicant and, even in small doses, can modify normal behaviours in bird chicks. This has been established in the laboratory through the usual tests such as visual cliffs and open fields but, at least one intriguing example has been documented in the wild: young loons on high mercury lakes spend less time on the backs of their parents
The cyanide leaching of tellurides has a history of controversy, with some researchers maintaining that successful gold extraction could be achieved provided that certain conditions were met. Operationally, plant leaching proved to be not as successful as in the laboratory and certainly was not as robust as the leaching of free gold
Henley et al. (1995) examined the technique of using high cyanide concentrations at high pH values to determine the gold–telluride content of an ore. They took a high-grade +20 mm KCGM flotation concentrate and performed a two-stage leach. In the first stage, leaching was carried out using 0.1% NaCN at a pH value of 9.2 for 24 h. It had been suggested that the native gold would be dissolved under these conditions but not the telluride gold. A second stage using strong cyanide (2%) at pH 12.5 for 96 h was used to dissolve the gold tellurides. They found that the first stage was a good estimator of the native gold content; however, the second stage failed to account for the gold associated with tellurides. Mineralogical studies showed that 6% of the telluride gold had been dissolved in the first stage and only 11% in the second stage
The technique was further refined by Ellis (1998), who introduced an oxidative step using hypochlorite leaching before the second stage of cyanide leaching. For particles coarser than 10 mm, the oxidation and subsequent cyanidation were repeated a number of times to counter the observed effect that oxidation penetrated only a small distance into a mineral grain on each pass. When no further gold extraction was observed after oxidation, it was concluded that all the gold tellurides had been dissolved. Leach conditions were modified to a lesser cyanide concentration and lime used for pH control rather than caustic soda
Jackman and Sarbutt (1990) found that solution changes during leaching gave a dramatic improvement in gold recovery on a 74-mm gold–tellurium concentrate grading 218 g/t Au, 410 g/t Te and 41.7% S, the major tellurium mineral being montbrayite [Au2Te3]. Gold recovery improved from 92% to 98% with two solution changes. The presence of carbon-in-leach process (CIL) was shown to achieve a similar gold recovery. Lead nitrate was shown to be beneficial in reducing residue grades in all cases. Fine grinding of the concentrate to 22 mm with CIL leaching improved recovery further to 98.6%
Pleysier et al. (2002) examined the leaching behavior of gold tellurides from a KCGM flotation concentrate using both a laboratory and theoretical approach. Laboratory tests demonstrated conclusively that the concentration of tellurium in solution dropped after an initial increase in concentration. Carbon was shown to be effective in removing tellurium from solution. They concluded that the telluride-leach process was affected by the presence of sulfides, which were able to cause elemental tellurium to be precipitated from solution. Once precipitated, tellurium was unlikely to redissolved as this would involve breaking stable TeTe bonds, whereas the oxidation of calaverite involves breakage of the weaker AuTe bonds. It was postulated that such precipitates, if they occurred before the complete dissolution of the gold, could result in a passivating coating that severely limits the rate of gold dissolution
Work on leaching of calaverite in the absence of sulfides also showed precipitation of tellurium (Deschênes et al., 2006). Surface analysis of calaverite grains using X-ray photoelectron spectroscopy (XPS) identified the nature of the telluride compounds that were precipitated, some of which were postulated not to interfere with leaching
Even a very low sulfide concentration could have a significantly detrimental impact on gold-leaching kinetics and overall gold extraction. Cyanidation at Fort Knox Mine was seriously affected by a minor concentration of metallic sulfide. The economic consequences were very important. Fort Knox cyanidation plant processes 38,000 tpd of a low-grade (1.0 g/t Au), free-milling gold ore with a low metallic sulfide component (below 0.3%), by gravity (11% recovery) and by cyanidation (89% recovery). The short retention time (20 h), low temperature (17 °C), and low grade make leaching kinetics critical for the performance of the process. A new ore, which represented only 14.4% of the mill throughput (and contains slightly more sulfide minerals in the form of pyrite, arsenopyrite, and stibnite), resulted in a gold extraction decreasing from 87% to 72.6% (Hollow et al., 2003). No leaching kinetics problem was identified in the laboratory investigations on the new ore fed into the circuit. The decrease in gold extraction only occurred on leaching the blend. After 12 months of laboratory investigation, three options were identified to solve the problem: leaching at pH value lower than 10, increasing the DO of the slurry, or using lead nitrate. The significant effect of pH on gold leaching pointed to stibnite as the origin of the problem
Lead nitrate addition was selected as the most effective approach, both technically and economically. The use of a low pH was not an option in the Fort Knox mill, where operating the circuit at a reduced pH resulted in above threshold-level HCN concentrations in the tank enclosures. The enclosures must enable the circuit to be operated and maintained in a subarctic environment. Extra equipment was needed to increase the DO in the leaching slurry. The antimony content of the feed was monitored against the performance of the plant with and without addition of lead nitrate (Figure 26.10). An improvement of about 5% gold extraction was observed at low concentrations of antimony
The improvement in leaching kinetics is illustrated in Figure 26.11 (laboratory experiment). The leaching rates of the two systems were similar in the first 5 h. In the case of the system without lead nitrate, the gradual dissolution of stibnite decreased the gold-leaching kinetics after the 5-h point. The gold extraction gap between the two systems increased with time
Plant data indicated that the addition of lead nitrate increased gold extraction from 87% to 91.7% and, for the 18 months evaluated, resulted in gold production in excess of 49,000 oz higher than that estimated for the non–lead nitrate reagent scheme
The acceptance of simulation tools in the gold industry is growing. Many operations have developed simulation models of either the entire gold extraction process or discrete sections of the process. Some operations now have fully integrated pit-to-product simulations that include both discrete and continuous dynamic simulations
These simulation models are providing gold producers with both tangible and intangible benefits. The obvious tangible benefit lies in the rigorous definition of process design criteria that facilitate equipment sizing and hence estimation of capital costs but also have a bearing on operating costs. One of the more important intangible benefits is that a simulation model provides a comprehensive store of knowledge representing the process design, control, stream flows, operation, properties, etc
Advances continue to be made in process simulation software and computer processing speed. This has allowed researchers and simulation engineers to delve deeper into the processes that characterize gold operations and produce ever more accurate and predictive simulation models (for example, Askew, 2011; Evans et al., 2013)
Further fundamental investigations on gold dissolution processes involving sulfur chemistry are still required. A better understanding of the adsorption and precipitation reactions, which reduce gold extraction, also requires further investigation. The advantages the process offers over cyanidation include lower reagent costs and the potential ability to leach preg-robbing ores and other ores not amenable to cyanidation. It may also selectively leach precious metals from base-metal concentrates (Hunter et al., 1998)
One of the limitations would seem to be the bio-reduction of sulfate ion using organic substrates for bisulfide regeneration. The bisulfide process is recyclable, but oxidation to sulfate is theoretically possible. A major drawback is that H2S, which is also generated, has an occupational health standard very similar to HCN. Extremely long retention times and a closed system would be required
The nitrogen species catalyzed pressure leach process is less hazardous and has been claimed to have been successfully demonstrated on a number of pilot and laboratory scale for various concentrates. The process is most probably better suited for the extraction of multielemental systems such as base and precious metals
Environmental and public concerns over the use of cyanide in the recovery of gold have led to a drive toward developing alternative leaching technologies. Among the more promising leach systems, copper-promoted, thiosulfate-leaching technology is emerging as an alternative to cyanidation for gold extraction for certain applications. The alternative lixiviants are discussed in detail in Chapter 27; however, the implications of application to copper gold ores warrant some consideration here
Despite the extent of research conducted on alternative lixiviants, the solubility of base metals and copper associated with gold ores for the alternative lixiviants has not been extensively studied. In general, for the lixiviant systems operating under acidic conditions (e.g., halides) the lixiviant is typically nonselective for gold with higher copper solubility than for cyanide. The amino acids also tend to dissolve copper preferentially to gold. Though not providing leach selectivity for gold over copper, these systems may offer a more economic processing option for some copper–gold ores, such as those requiring pressure oxidation or copper mineral dissolution to recover refractory gold
For the thiourea system, Li and Miller (2006) indicated that the thiourea complexes with base-metal ions are much weaker than between cyanide and the metal ions, except for copper. Investigations by Lacoste-Bouchet et al. (1998) found that an acid pretreatment was required to remove copper prior to the thiourea leach, to eliminate the negative impact of copper
Copper sulfide minerals, other than chalcopyrite, dissolve readily in thiosulfate leach solutions, particularly when excess ammonia is in solution (Aylmore, 2001). The solubility of copper is also limited by the solution conditions. Thus, the selectivity of gold over copper during leaching using thiosulfate is dependent on the copper mineralogy and/or solution chemistry. Molleman and Dreisinger (2002) found for two of three copper–gold samples investigated that the copper dissolution in thiosulfate was only 25–60% of that dissolved in cyanide, with similar gold recoveries (Table 43.10). Thiosulfate consumptions were around four times (by weight) those of cyanide
Dai et al. (2013) found for three concentrate samples studied that the copper dissolution using the copper–ammonia–thiosulfate system was significantly lower than for cyanide, while gold and silver recovery and reagent consumption were dependent on the ore mineralogy (Table 43.11). Despite the lower gold recovery for the flotation concentrate, the copper dissolution and reagent consumption are both much lower for the thiosulfate system compared with cyanide
Predicting the continued and future impacts of mining operations on river and human health is a difficult process, often plagued by considerable error. Qualitatively, however, the degree of mercury pollution in the Beni-Madeira river basin is likely to increase in the future as a result of Bolivia's current political and economic instability and the high prices for gold, which when combined are likely to promote gold extraction and localized deforestation. A wider range of commodities are mined within the Rio Pilcomayo basin, and the future of mining is likely to vary between them. However, the contaminant data collected thus far indicate that trace metal contamination will remain a problem for the foreseeable future. In fact, geomorphic models of landscape and channel evolution suggest that trace metals deposited and stored during channel bed aggradation may be re-excavated in the future and transported downstream, increasing the extent of contamination. The widespread nature of trace metal contamination will make remediation of the problem extremely difficult, and will require a much more detailed understanding of ecological and human exposure to the contaminants than currently exists
Being the most common sulfide minerals, Arsenopyrite and pyrite under oxidizing conditions breaks down to release acids of As and S into the environment, leading to acid mine drainage with high concentrations of dissolved As. In this research, the dissolution of gold ore (with FeS2 and FeAsS as the main sulfides) from Tianli gold mine, Liaoning province, China was investigated. The experiments were conducted in 500mL conical flasks containing 200mL of three different media, 3% pulp density and 1.6 initial pH. The results indicated that treatment with mix culture medium resulted in the dissolution of 99% of Arsenic and 99% of iron, which was higher as compared with when treated in the same culture medium after centrifugation for 20 minutes which in turn higher in comparison with when treated in acidic and pure sterile system. The oxidation potential, Eh of the mix culture medium reached 680mV (vs.SCE) within the first three days of the experiment where as that of centrifuged, acidic and sterile media reached 580, 450 and 445mV (vs.SCE) respectively. The pH tends to increase within the first three days which eventually decreased to nearly one toward ending, courtesy of pyrite which is believed to be net acid releasing sulfide. The dissolution is suggested to be a combined effect of enzymes, ferric iron ions and organic acids. It was observed that enzymes and ferric ions played an essential role in the dissolution process.
The innovation of steam engine back in the late 19 the century resulted in an enormous growth of industrial activities globally, which in turn resulted in an increase deterioration of the ecosystem because of the discharge of highly polluted effluents in the forms of solid, liquid, and gas. The efforts have been put forward toward developing eco-friendly and sustainable processes in order to avoid rapid degradation of the ecosystem. In the fields of mineral processing and extraction of metals joint efforts are put forward to developing environmentally friendly processes . Owing to the gradual depletion of high-grade ores, attention are now being focused toward recovering metal from ores, complex and lean ores, which cannot economically be treated by conventional routes [2,3]. Bio-hydrometallurgy is a new concept that involves the use of various microorganisms to recover metals from their ores. It is also environmentally friendly unlike conventional hydro-metallurgical process . Bio-hydrometallurgy technique applies to different types of materials, so far unusable resources, by which metals can be recovered and also generates minimum effluents and therefore is preferred as green technology. For last several decades bioleaching prioritize in application for metal recovery from ores/ concentrates namely; pyrite arsenopyrite, chalcopyrite, calcite . Among the major bacteria group that are promising in bioleaching process are chemolithotrophic acidophiles namely, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Leptospirillum ferrooxidans and heterotrophs like Sulfolobus. Some of eukaryotic bioleaching microorganisms that applied in metal recovery include fungal species such as Aspergillus niger and Penicillium from industrial wastes . The recovery of free milling gold by gravity and direct cyanidation proved to be straightforward and well established, refractory ores pose a very different challenge to producers. The first challenge is determining the reason for the poor recovery by direct cyanidation, which can be caused by one or more contributors. The oldest and best understood is gold locked in sulphide, and most frequently pyrite. The second contributor to refractory behaviour is arsenic, which causes high refractoriness even at low concentrations. The presence of carbon in the ore is also a frequent cause of poor recovery. In this paper, gold ore sample (with FeS2 and FeAsS as the main sulfides) from Tianli gold mine, Liaoning province, China was treated in four different media.
The sample ore for this study was obtained from Tianli gold mine, China. Chemical analysis of the representative sample by AAS (atomic absorption spectroscopy) was conducted. The sample Ore was sieved to -0.075 mm before used for all leaching experiments. X-ray diffraction analysis of the sample showed pyrite (FeS2) as the major phase and Chemical analyses also indicated that the sample contained arsenical pyrite or arsenopyrite (FeAsS). The particle size of the sample was 80% below 75 μm (Figure 1). X-ray fluorescence spectroscopic analysis of the sample was conducted and the result is shown in table 1.
A mixed culture medium (HQ 2011) was used, the studies, which utilized ferrous ion or elemental sulphur as energy source. Cultivated by Northeastern University and mainly contain Acidithiobacillus ferrooxidans, Ferroplasma acidiphilum, Leptospirillum ferriphilum etc. The cells were incubated at 180 rpm and 37°C in sterile 9K basic salt medium containing different energy substrates  After incubation for three days, some portion of the cells was harvested by centrifugation at 10,000 rpm for 10 min.
The experiments were conducted in 500mL conical flasks containing 200mL of four different media, mix bacterial medium, mix centrifuged bacterial culture medium, in acidic medium and in a sterile medium. The pulp density was 3%. Sample of the ore mineral was added into media without Fe2+, added with culture medium (inoculated amount of 10%), and pH was adjusted to 1.6 with H2SO4 and then placed into a constant-temperature incubator (175 r/p) for days.
Measurements of pH and potential were performed every day. Samples were aseptically withdrawn from the flasks every two to three days for chemical analysis. The samples were separated by centrifugation at 5000 rpm for 10 min. the dissolve Arsenic and total iron and were measured.
At 37°C, 10% Inoculum, 175 r/p, 3% pulp density and 36 days the leaching time. The effect of pH on the sample ore bio-oxidation was studied. The results are presented in Figure 3. The pH of the centrifuge and mixed bacterial media during the dissolution of the sample showed almost similar characteristics. Within the first 3 day of the experiment, the percentage of the total Fe ion were 86% and 87 for the sample with mixed and centrifuge bacterial culture medium (Figure 4), this clearly indicates that the oxidation rate of the sample is higher at the initial stage, so the increase in pH is apparent. Furthermore, the increase in concentration of total iron (the Fe3+ ion) which partake in the oxidation process resulted in the subsequent decrease of pH (eq. (1)),  to nearly 1.1 on the 36th day. On the other hand the percentage of the dissolved total Fe ion in acidic and pure sterile media were 16% and 15% respectively, indicating that the oxidation rate is lower in the initial stage. It is believed that the decrease in the pH resulted from the dissolution of pyrite being net acid releasing sulfide and arsenopyrite presents in the ore sample.
The variations in the redox potential (Eh) with time is given in Figure 5. It can be seen that after the first day, the redox potential of the samples with mix and centrifuge bacterial medium were increased reaching the values of about 640 and 465 mV after 3 day of the experiment. On the sixth day the redox potential of the sample with mix bacterial medium reached its peak value (680 mV) which indicates that microorganisms of the mix culture medium were more active at pH of about 1.5 to 1.6, whereas that of the centrifuge bacterial medium reached it maximum value (650 mV) on the 18th day of the experiment. The higher dissolution of arsenic and total Fe could be attributed to the higher redox potential of the solution. On the other hand the oxidation potentials of pure sterile and acidic media were 310mV and 340mV indicating that the dissolution of the sulphide was much lower.
It was observed that the dissolution of the arsenic and total iron for the sample with mix and centrifuge bacterial medium increases with the gradual decrease in the pH, initially the dissolution of arsenic were 73% and 34% (Figure 6) and the dissolved total Fe (Figure 4) were 87% and 86% within the first three day of the experiment and continue to rise during the process reaching up 99% both while the dissolution of the arsenic for the sample in acidic and pure sterile medium were 28% and 18% whereas those of the total Fe were 16% and 15% within the first three day of the experiment which appear to be low at the initial pH. The oxidation potential of the sample with mix bacterial and centrifuge bacterial medium is low but continue to rise during the whole process, which indicates that the bacteria are difficult to survive at the pH of 2 or above though could be alive but less active .
The bio-oxidation of arsenic-bearing gold ore from tianli gold mine was investigated by comparing the dissolution of the sample ore in four different media, in a mix bacterial medium, mix bacterial medium after centrifugation, acidic medium and in a pure sterile medium. The experimental results showed typical oxidation characteristics for the four different media. After the first day, the redox potential of the samples with mix and centrifuge bacterial medium were increased, reaching the values of about 640 and 465mV after 3 day of the experiment. On the sixth day the redox potential of the sample with mix bacterial medium reached its peak value (680 mV) which indicates that microorganisms of the mix culture medium were more active at pH of about 1.5 to 1.6, whereas that of the centrifuge bacterial medium reached it maximum value (650 mV) on the 18th day of the experiment. The higher dissolution of arsenic and total Fe could be attributed to the higher redox potential of the solution. On the other hand the oxidation potentials of pure sterile and acidic media were 310mV and 340mV indicating that the dissolution of the sulphide was much lower. It is believed that the decrease in the pH resulted from the dissolution of pyrite being net acid releasing sulfide.
So far no account has been taken of the loss of gold which is contained in pyrites, as it has been assumed that these are saved by concentration if they are valuable, and this subject is dealt with in earlier. Nevertheless, as this gold comes under the head of non-amalgamable gold, its physical state and the causes of its disinclination to unite with mercury may conveniently be considered here. In general, pyrites yields only a small proportion of its gold contents if it is run over the amalgamated plates, and if it is ground very fine in a pan with mercury the percentage extraction is better. Among the old processes used for the amalgamation of the gold in pyrites may be mentioned the treatment in revolving wooden barrels with mercury, as practised at the St. John del Hey Mine, and the practice of leaving the pyrites to be decomposed by weathering before grinding it with mercury. This method of oxidation seems to be decidedly inferior to the alternative plan of roasting the sulphides, by which the oxidation is rendered more complete and the particles of gold agglomerated to some extent. However, the amalgamation of pyrites, even when roasted, is far from perfect, part of the gold still remaining in a condition unfit for extraction in this way. The ores, which have been met with in various parts of the world, consisting mainly of limonite or hydrated oxide of iron, and in most cases believed to be the result of decomposition of pyritic ores by atmospheric agencies, are also extremely refractory, causing the mercury to sicken rapidly, and yielding only about the same percentage of gold as can be obtained from unoxidised pyrites
The most celebrated case of this kind is that of the surface ore at Mount Morgan, in Queensland, which was an ironstone gossan consisting of siliceous brown iron ore, derived according to one view from the decomposition of pyrites. Although the gold appeared to be free, it could not be amalgamated, yielding only about 30 per cent, when crushed in batteries and subjected to prolonged grinding in pans with mercury. When the ore was dehydrated by roasting in reverberatory furnaces the extremely fine particles of gold were agglomerated, and between 80 and 90 per cent, could then be extracted by amalgamation, the remainder being presumably coated by oxides of iron. The richness of the ore, however, made even this result unsatisfactory, and a process of chlorination was adopted in practice. A similar case was noticed by Mactear in South America, where a limonite ore which only yielded from 35 to 40 per cent, of its gold when treated in Huntington pans, was made to yield between 85 and 90 per cent, by merely subjecting it to a dehydrating calcination before amalgamating it. Louis Janin, Jr., mentions another case in the ores of the Southern Cross Mine, Deer Lodge County, Montana, which consist of limonite derived from the alteration of pyrites. In panning large samples, only one or two specks of gold could be seen, although the ore contained from 1 to 2 ounces per ton. This ore yielded only about 40 per cent, on being amalgamated, but over 90 per cent, was dissolved out by leaching the raw ore with cyanide of potassium, and similar results were obtained by chlorination. Here the ore was thoroughly decomposed, but yet the gold would not amalgamate to a much greater extent than if it were still contained in the original pyrites, whilst the chemicals at once dissolved it
The balance of evidence, however, seems to be in favour of the theory that gold exists in pyrites in the metallic state. Although the metal is generally invisible in undecomposed crystals of pyrites, it becomes visible when such crystals are oxidised either by air and water in nature, or by means of nitric acid, or by being roasted or subjected to deflagration with nitre. As a result of such decomposition, particles of bright lustrous gold, angular and ragged in shape, but of considerable size, often become apparent. These particles may be separated from the oxides of iron by washing, and the use of nitric acid, followed by panning, is frequently resorted to in order to detect gold in pyrites. Moreover, although usually invisible, gold can sometimes be seen in unroasted pyrites. As long ago as the year 1874, Richard Daintree and Latta found specimens of cubical pyrites, in which gold could be seen under a microscope gilding the cleavage planes of the crystals. Again, G. Melville- Attwood, on examining crystals of auriferous pyrites from California in 1881, found that the faces of the crystals were gilded in some places, and that here and there little specks or drops of gold occurred, partially imbedded in the pyrites. These films were too thin to be detected by an ordinary lens, so that it did not seem surprising that such impalpable material could not be taken up by mercury. Louis Janin, Jr., more recently found crystals of pyrites in a porphyritic gangue from the Republic of Colombia, which had gold in small globules on their surfaces. Lastly, it has long been known that crystals of pyrites are often found adhering to an amalgamated plate, the particles of gold on their surfaces having been amalgamated. It seems likely, in view of all these facts, that most if not all of the gold is in the metallic state, and its occasional refusal to amalgamate is not very surprising, when it is remembered how completely a thin coating of certain sulphurised compounds prevents amalgamation, and how readily sulphuretted hydrogen would be evolved from decomposing pyrites. Some authorities have contended that the metallic gold is disseminated mechanically through the mass of pyrites, but the action of potassium cyanide, in dissolving the whole of the gold out of comparatively coarsely crushed pyrites, seems to point to the correctness of the view that the interior of the crystal is not auriferous, the deposition of the gold being superficial, so that the enrichment of the pyrites is confined to its crystalline faces, and possibly, but not probably, to its cleavage planes. Strong evidence of the richness of the outside of the crystals of pyrites in the Banket ore of the Transvaal is given by Caldecott in his papers on the treatment of ore in the Transvaal
“Upon the ordinary auriferous sulphide of iron, or arsenical pyrites, the solution of potassium cyanide acts readily, not by dissolving the sulphuret, but by attacking the gold upon its exposed edges, and eating its way into the cubes by a slow advance, dissolving out the gold as it goes. An examination with the microscope of the pyrites after the gold has been removed, suggests the method of the operation. A sample of very rich pyrites from a mine north of Redding, was treated with a weak solution, containing less than two-tenths of 1 per cent, of cyanide, for 168 hours; the assay showed a complete extraction of the gold; as the sulphurets showed no change in their appearance to the naked eye, some of them were placed under the microscope
“ There is no change visible in the form of the crystals as a whole; along the fractured faces the mispickel looks clean and unaltered, showing the silvery-white colour and intense refraction of the arseno-pyrite. Upon the faces of the crystals appear dark lines, short, and parallel to each other. In places they are crowded close together; in other parts they are at considerable distances, but always in parallel lines. The lines vary in length, being from four or five to over a hundred times their width ; the lines are very irregular and often broken. These lines are fissures in the pyrites, and extend so deep into it that the microscope does not reveal their depth. By using the higher powers the walls of one of the fissures were seen to be completely honeycombed, looking somewhat like two empty honeycombs set opposite each other; evidently the mineral removed was crystallised along its contact walls at least. As the raw or untreated pyrites does not show any such fissuring, but, upon the contrary, shows a surface marked only by striation lines common to pyrites, I assume that the fissuring in the treated sample is caused by the solution acting upon some soluble mineral, probably gold, arranged in plates, occurring in groups, but which, by its colour and isomorphism and the extreme tenuity of its lines, is indistinguishable from the mass of pyrites enclosing it.”
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