Pretreatment process for refractory ore and concentrate - microbial oxidative decomposition

When 伦伦维茨 found in 1900 that gold mixed with decaying plants, gold would dissolve. At that time, he believed that the dissolution of gold was the result of oxidation of plants to produce nitric acid and sulfuric acid. Later, when studying the open-pit gold mine in the Ivory Coast, Marcin discovered that living bacteria in the mine water could dissolve the vein gold and cause it to migrate and reprecipitate. Nowadays, it is known that all microorganisms which are separated from soil and water and which are capable of dissolving gold are non-toxic, so that microorganisms can be used for underground heap leaching in small mining areas. The best medium for these microorganisms is green walnut soup, peptone, dried fish meal and eucalyptus soup. When these media are used, various salts are usually added in different proportions to have different concentrations.

The experiments on the ore containing 11.1～18.3g∕t of gold from the Ivory Coast gold deposit show that the role of four microbial communities when using the green walnut soup as the medium The strongest.

The process by which bacteria dissolve gold in ore is divided into four stages. The first stage is the incubation period, about 3 to 5 weeks; the second stage is the dissolution period, about 2.5 to 3 months. At this stage, the solubility of gold increases non-uniformly, and sometimes precipitates of gold are repeatedly precipitated; the third stage is The solubility period, about half a year to one year, does not change the solubility of gold at this stage. The dissolved gold concentration is about 10 mg ∕L; the fourth stage is the final stage. At this stage, gold is mainly precipitated, and the solubility will decrease significantly. Therefore, the cycle of gold leaching by bacteria has the highest solubility of 75-90d gold.

Golden iron-ore-containing arsenopyrite (arsenopyrite) iron ore using Thiobacillus sp leaching, gold can be recovered successfully. The hydrolyzate of the nutrient yeast was treated with sodium oxychloride to leach the arsenic pyrite flotation concentrate containing 30 g of gold. After leaching for 50 days, the recovery rate of gold was 80%.

The gold leaching from the sulphide ore begins with the decomposition of S, As, and Fe, which can completely oxidize and decompose them within 2 to 6 days. The dissolved arsenic can produce stable or relatively stable arsenate precipitation. The dissolution of arsenic, sulfur and iron causes the fine natural gold particles to lose the carrier and dissociate, or the dissolution of the natural gold particles occurs due to their dissolution. Exposure is convenient for the next step of gold leaching. This is the essence of microbial pretreatment of refractory ore.

According to studies in the former Soviet Union and South Africa, the suitable temperature for the bacterium of the genus Thiobacillus is 25 to 35 ° C, and the bacteriophage temperature of the genus Sulfolobus is 50 to 80 ° C. In order to facilitate the operation, most of the researches currently used for gold leaching are concentrated on Thiobacillus ferrooxidans. Taking pyrite as an example, the role of iron thiobacillus is to oxidize sulfide to soluble sulfate by the metabolism of bacteria itself, and to obtain self-supporting energy during the oxidation of Fe ^{2 +} to Fe ^{3 +} ( Direct oxidation of microorganisms):

2FeS ₂ +2H ₂ O+7O ₂ 2FeSO ₄ +2H ₂ SO ₄

2FeSO ₄ +0.5O ₂ +H ₂ SO ₄ Fe ₂ (SO ₄ ) ₃ +H ₂ O

The second is the decomposition of pyrite by acid Fe ^{3 +} generated by bacterial oxidation (indirect oxidation of microorganisms):

FeS ₂ +Fe ₂ (SO ₄ ) ₃ 3FeSO ₄ +2S

2FeSO ₄ +0.5O ₂ +H ₂ SO ₄ Fe ₂ (SO ₄ ) ₃ +H ₂ O

2S+3O ₂ +2H ₂ O 2H ₂ SO ₄

The latter can not only achieve the oxidation of pyrite, but also become a decomposing agent for sulfide minerals such as arsenic and antimony and even some non-sulfided minerals that affect gold leaching. In addition, the microorganisms also take up CO ₂ from the air and add an appropriate amount of nitrogen (in the form of an ammonium salt) as a nutrient.

Since silver is an active fungicide, the presence of natural silver or silver ions inhibits the growth of bacteria and affects the recovery of gold. Insoluble silver sulfide such as fluorite ore does not affect the recovery rate of gold. Interestingly, by studying the oxidation of several traces of silver sulfide ore by mixed culture of Thiobacillus ferrooxidans and Thiobacillus thiooxidans, it was found that silver accumulated on the vesicles in contact with the leach residue. The silver-aggregating bacteria are collected from a solution of leaching sulfide ore from dilute potassium hydroxide, mixed and cultured, and washed. The silver film accumulated on the bacteria is Ag ₂ S crystal body, which is aggregated into large pieces after various processes of batch leaching, and the black bright deposit obtained by drying at 100 ° C has the highest silver content of 250 mg ∕g.

The microorganisms used, whether they are natural strains or artificially induced strains, must be screened and cultured for the collected microorganisms, so that they gradually adapt to the physical and chemical comprehensive conditions of the target raw material leaching process. When leaching gold ore or concentrate containing arsenic and sulfur, many elements will dissolve in large amounts and collect ions in the leachate. Usually, the iron content can reach above 15g/L, and the arsenic content can reach above 8g∕L. Among these ions, especially if the concentration of As ^{3 +} is too high, the vital activity of the microorganisms is inhibited, and the oxidative activity of the microorganisms is lowered. For this reason, in addition to collecting strains from mines with similar ore types or processing ore or concentrates with similar physical and chemical properties, the collected strains should be placed in the target raw materials for screening and cultivation, so that they gradually adapt to the physical and chemical properties of the target materials. Under the comprehensive conditions, the microbial community which can maintain stable vitality under the corresponding ion conditions and has strong activity in the oxidation process of the sulfide mineral can be finally obtained, and the decomposition speed of the microorganisms can be greatly improved. Studies by С. И. Polikin et al. have shown that when experiments are carried out using a well-screened culture of the excellent population of Thiobacillus ferrooxidans, the bacillus is firmly anchored to the surface of the sulfide mineral after 10 to 15 minutes of contact with the ore particles. The agitation leaching test of arsenic gold ore by using the trough type (large trough or Pachuca trough) bacterial leaching method showed that the oxidation rate of bacteria to minerals was much faster than the leaching and underground leaching leaching speed reported in the literature. . Concentrates used to treat much higher sulfides are also economically advantageous than ores with tailings that are low in sulfide content, and can be leached with concentrated slurry. Therefore, although Thiobacillus ferrooxidans can be used to treat concentrates, it can also be used to treat ores; it is suitable for both agitation and heap leaching. However, most of the research work so far has focused on continuous agitation of concentrates.

For example, a high arsenic gold concentrate of a mine has a particle size of 90% to 95% -0.074 mm (200 mesh), and the leaching rate of direct cyanide gold is only 10% to 32%. When using the cultured microorganism solution, one or two stages of leaching in the Pachuca air stirring leaching tank at a solid-liquid ratio of 1:5, a slurry temperature of 28 to 35 ° C, and a pH of 2.2 to 1.7 (time 60 ～ 120h), the oxidation rate of the arsenopyrite is 80% to 90%, and the slag contains As 1.3% to 1.4%. The slag is further cyanated, and the leaching rate of gold is increased to 85% to 91%. The bacteria-containing solution separated after the leaching of the microorganisms is neutralized to a pH of 2.8 to 3.2 with lime milk to regenerate the bacteria-containing liquid after precipitation of As, Fe, and S. The concentration of bacteria in the regenerant is as high as 10 ⁶ to 10 ⁹ ∕mL, which can be returned to the leaching process.

The researchers also carried out microflotation-cyanation after re-flotation of a carbonaceous arsenic gold concentrate in a mine to remove carbonaceous shale , or directly perform micro-progressive-cyanide and roasting of concentrate without re-flotation. - Comparative test of three schemes of cyanide, gold leaching rate indicators are quite close. It is indicated that these three schemes are effective for the carbonaceous arsenic gold concentrate.

According to the results of the above-mentioned small test, the microbial tank agitation proposed, the bacteria-containing liquid regeneration and the expansion test for returning use confirmed that: (1) It is reasonable to carry out the removal of As, S and Fe from a high-arsenic gold concentrate. (2) The carbon-containing arsenic gold concentrate does not need to be floated again to remove carbon. It is feasible to directly use the microbial decomposition and then cyanide. (3) The bacteria-containing solution after separation of the leaching residue is neutralized to a pH of 2.8-3.2 with good regeneration effect, and the solution containing bacteria has a high concentration (usually up to 10 ⁶ to 10 ⁹ /mL) and can be returned to use. (4) For other difficult-to-treat gold-bearing sulfide ore, microbial decomposition before cyanidation to dissociate fine-grained gold is not only effective, but also economically advantageous. (5) Although the production cost of the trough microbial decomposition method is higher than that of the fine grinding-flotation-baking process, the equipment investment and reagent cost are low, and the technical and economic indicators are reasonable. The concentrate for treating arsenic and sulfur can also avoid As. , S pollution to the atmosphere. Therefore, if this process can be widely applied, not only can many refractory gold deposits be redeveloped, but many existing companies can overcome difficulties. Taking the Failview gold mine in South Africa as an example, after the microbial oxidation method was used instead of the roasting method, the 18-month production practice showed that the recovery rate of gold was 5% higher than that of the roasting method, and the investment was less than the roasting method. 20%, 40% less than the pressurized oxidation method.

The increase in production costs of microbial oxidation is mainly due to the slow reaction kinetics of the bacterial leaching process. According to the test of the Pogira Gold Mine in Papua New Guinea, the electricity consumption for stirring accounts for 50% of the operating cost due to the leaching period of 2 to 6 days, and a precise cooling control system must be installed to ensure the liquid temperature is 30-40. Between °C and keep the pH of the system working between 1.8 and 2.2.

Another advantage of the microbial oxidative decomposition process is that it selectively and preferentially decomposes the arsenopyrite. When both the arsenopyrite and the pyrite are similar in the concentrate, it can selectively decompose the arsenopyrite and inhibit the decomposition rate of the pyrite, so that the fine gold particles present in the arsenopyrite are preferentially released. . When a large amount of toxic sand is oxidized and its product is recombined into a new substance precipitate to dilute the solution, the decomposition rate of pyrite is accelerated. This is especially effective for ores that do not require complete oxidation of the sulfide (mainly pyrite) to dissociate the gold from the package.

Of course, the preferential decomposition of the arsenopyrite may also cause As to accumulate in the immersion liquid for a certain period of time and cause bacterial poisoning, especially when the concentration of Fe ions in the immersion liquid is low. GN Stresa studied the concentration ratio of Fe and As in the microbial oxidation leaching system. It is pointed out that in the system with Fe ^{3 +} as the oxidant, the iron content of the solution can exceed 50g ∕L, but the ability of bacteria to resist As is only 20g ∕L. In order for the system to function adequately, it is necessary to maintain the Fe ^{2 +} concentration not less than 10 g ∕ L, and to uniformly distribute Fe ^{3 +} in the immersion liquid by the immersion cycle. If the main mineral arsenopyrite was leached feedstock, continuously decomposing into solution mainly As, Fe ^{2 +} insufficient amount can not be maintained Fe ^{3 +} concentration in the solution, and the need to use the medium for bacterial pyrite The Fe ^{3 +} separated in the breeder is replenished into the leachate to form a FeAsO ₄ precipitate with As to overcome the accumulation of As in the leachate. In order to obtain the optimum leaching speed of the sulfide when leaching high arsenic concentrate, the working pulp concentration (weight/volume) should be 15% to 20%. Only when the content of sulfide in the feed is low, the concentration of the slurry can be increased to 20% to 30%.

The first production plant to pretreat refractory sulphur gold concentrates using microbes was a Zimbabwe plant that was commissioned in 1988 and was designed by EIMCO Process Equipment. The plant uses two stirred reactors to treat concentrates containing 31% pyrite, 18% arsenopyrite, and 60g ∕t gold. The leaching period is 4-6 days. The process is determined by the rate at which S and As are fed to the reactor in the concentrate. When S and As are given too fast in the reactor or the feed is accidentally too high, the O ₂ in the immersion liquid is depleted and the bacteria are killed and the reaction is stopped.

The first large-scale microbial oxidation leaching plant in the United States was a pretreatment plant at the Topkin Springs gold mine in Nevada. The plant was put into operation in June 1989, with a production capacity of 1300t/d, a gold yield of 90%, and an annual gold production of 1.5t. The plant's microbial leaching factory costs $22 million and the gold production cost is expected to be $240/oz. For the leaching operation, four stainless steel stirring leaching tanks of 16 m in diameter and 13 m in height were used. The microorganisms used for leaching are particularly adaptable. As long as the temperature, pH value, air supply amount and feed rate of the system are controlled within a suitable range, the oxidation reaction can be accelerated. And as the ore composition changes, the bacteria can also perform a continuous natural variation selection process to gradually adapt to the differences in the conditions of the system.

After microbial oxidation leaching, the leaching residue is extracted by cyanide carbon slurry.

Regarding the effect of flotation reagents on bacterial growth, the Institute of Chemical Metallurgy and the Institute of Microbiology of the Chinese Academy of Sciences conducted preliminary joint experiments. Study ^# 8 ferrooxidans strain isolated from their acid wastewater yunfu Chadong arsenopyrite ore, antimony, arsenic Longshan gold concentrate contained flotation reagent showed: ethyl xanthate maximum inhibition of the strain . When the ethyl xanthate in the pulp reaches 10g ∕L, the growth rate of the bacteria is obviously inhibited and increases with the concentration of the drug. Butylamine black medicine also has a certain inhibitory effect, but butyl xanthate, isobutyl xanthate and 2 ^# oil are smaller. Therefore, the flotation concentrate should be removed as much as possible before treatment.

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