Non-cyanide leaching is a gold extraction technique

Appears cyanidation gold extraction of iodine 100 years, it has been a great development, dominant in the gold industry. The cyanide gold extraction process is simple, the adaptability is good, and the gold recovery rate is high, which is the main reason for the longevity of this method. However, the following main drawback is always accompanied by the cyanide leaching process: â‘  immersion gold slow, susceptible to interference leaching copper, iron, lead, zinc, antimony, tellurium, arsenic and sulfur impurities; â‘¡ highly toxic, environmental costs mines Large, harmful to the ecological environment; 3 pairs of fine-grained gold, high arsenic, high sulfur, refractory gold ore containing organic carbon directly leaching effect is very poor, subject to complex pretreatment process and then use cyanidation or complex Strengthening the leaching method, sometimes the gold extraction effect is still not satisfactory. Therefore, researchers continue to study non-cyanide processes and non-cyanide leaching agents. The development and application of non-cyanide non-toxic and non-polluting gold extraction technology will become the focus of future research.

I. Research progress in non-cyanide gold extraction technology

(a) thiourea immersion gold

Since the Soviet Union began research on thiourea immersion gold in the 1940s, thiourea leaching has become one of the most promising alternatives to cyanidation. Thiourea (HzNCSNH ₂ ) is an organic compound. In the presence of acidity and oxidant, thiourea forms a cationic complex with gold. The reaction is: thiourea as a ligand and ionic bonding of metals. The bond may be selectively selected by a lone electron pair or a sulfur atom of a nitrogen atom. In the immersion gold process, thiourea oxidized to various products, Mr. Cheng is a formamidine disulfide, which serves as a selective oxidant gold and silver. If the solution potential is too high, dithiocarbamate will be further oxidized to cyanamide, cyanide sulfide and elemental sulfur, so the leaching solution must be strictly controlled by thiourea leaching.

According to the literature, France began to extract gold and silver from zinc baking sand by thiourea in 1977; the Colorado Colorado mine used thiourea to treat gold-bearing tailings since 1982; Australia's New England antimony ore began to use thiourea in 1984. The method deals with gold-containing antimony concentrates; Russia and other countries have also begun to use the thiourea method in gold production in recent years. After repeated industrial tests, the thiourea iron plate replacement process developed in China has been transferred to industrial production through a national appraisal in a mine in Guangxi. Therefore, it can be considered that the new process of thiourea gold extraction has begun to enter the industrial production stage from the research stage, and the process is also improving day by day.

The characteristics of thiourea are: 1 non-toxic; 2 selectivity is better than cyanide, sensitivity to elements such as copper, zinc, arsenide and bismuth is significantly lower than cyanide; 3 gold dissolution rate is fast, 4-5 times faster than cyanide leaching Above; 4 thiourea dissolved gold is carried out in an acidic medium, which is suitable for leaching of refractory minerals which have been subjected to pretreatment which can produce acid; 5 thiourea gold complex formed in solution is cation in nature, suitable for solvent Extraction and ion exchange methods. Recover gold. However, thiourea is expensive and consumes a large amount (20% oxidized by itself, 80% is absorbed by ore), not as stable as cyanide, and it is easy to corrode equipment due to immersion in gold in an acidic medium. In recent years, some materials have classified thiourea as a suspected carcinogen. Therefore, it can be said with certainty that the thiourea method is difficult to replace the cyanidation method in the near future.

(2) Halogen and its compound method immersion gold

In the middle of the 19th century, people began to use gold to soak gold, and then stopped using it after the cyanidation method appeared. This process has been re-emphasized since the 1970s, and has developed high-temperature chlorination roasting and electrochlorination leaching methods. The chemical reaction of gold extraction by chlorination is: due to the high activity of chlorine, there is no problem that the surface of gold particles is passivated. Therefore, under the given conditions, the leaching speed of gold is very fast, generally only 1~2 h. . This method is more suitable for the treatment of carbonaceous gold ore, acid-washed gold-bearing ore, arsenic-containing concentrate, and the like.

The Jerrit Canyon gold selection plant of Freerport Mining Company of the United States uses air oxidation and chlorination immersion gold to treat arsenic-containing carbonaceous gold ore. The chlorination time is 18 h, the pulp concentration is about 55%, the temperature is 49-54 ° C, and the average chlorine consumption is At 17.5 kg/t, the gold leaching rate is 94%.

Newmont converted to a flash chlorination system in 1988, increasing the gold extraction rate by 6% and reducing chlorine consumption by 25%.

Recently, Peru and France reported a new gold salt leaching process that uses high concentrations of NaCl as an oxidant to produce elemental chlorine in solution. The latter dissolves gold quickly under the action of an aqueous solution.

The method of carbon leaching in the United States is to stir coarse-grained activated carbon with a carbon-impermeable refractory gold ore. Chlorine reacts with the slurry under acidic conditions. The gold dissolves into a gold-chloride complex and is then reduced to metal gold on the surface of the carbon particles. After the leaching is completed, the gold-loaded charcoal is sieved out of the finely ground slurry for gold recovery treatment. The method is characterized in that the pretreatment, leaching and recovery of the refractory ore are carried out in the same system. The United States has also invented a similar method, using chloride leaching, ion exchange resin gold extraction, suitable for the treatment of carbonaceous ore or mixed ore of carbonaceous ore and oxidized ore.

South Africa has commissioned a large-scale water chlorination treatment of re-election gold concentrate test plant. The concentrate is oxidized and calcined at 800 °C for desulfurization. The calcination is leached in a ventilated hydrochloric acid solution, and the gold leaching rate is as high as 99%.

The Beijing Research Institute of Mining and Metallurgy obtained a flotation gold concentrate containing 65 g/t of Au obtained from the fine-grained gold ore with high arsenic, antimony, sulfur and carbon content in Miaolong, Guizhou, after roasting to remove impurities. The calcination was leached by water chlorination, the gold leaching rate was 91.48%, and the leaching time was only 5% at the time of cyanide leaching.

The use of bromine and its compounds as gold leaching reagents is the same as chlorine, because the oxidation potential is high enough to dissolve gold when halogen is changed to halide, and the halide ion (x ^- ) is a strong ligand of Au ⁺ and Au ³⁺ from thermodynamics. In general, it is conducive to the occurrence of immersion gold reaction.

As early as 1881, Shaff published a patent for the bromine gold extraction process, but it was not until the past 10 years that due to changes in environmental protection and ore properties, it began to re-examine seriously.

Around 1990, countries such as Canada and Australia have published many articles, claiming that the bromination leaching method such as bioleaching-D method and K-method should compete with cyanide leaching method, emphasizing that these new methods have the advantage of not polluting the environment.

In the bioleaching-D method, a leaching agent called Bio-D is used, which is a leaching agent composed of sodium bromide and an oxidizing agent, which can be used to leach precious metals, and has a greater affinity for denser metals than For the less dense metal, it can be used in weakly acidic to neutral solution. The dilute solution is non-toxic, the reagent is easy to regenerate, and it has biodegradation effect. The leaching rate of most ore leaching can reach 90% at 2.5 h. However, since a considerable amount of bromine vapor escapes from the solution during the reaction, this not only increases the consumption of the reagent, but also causes serious corrosion and health problems, and is still in the laboratory and semi-industrial test stage.

The K-leaching method was invented by Kalias, Australia. It essentially uses a new process using bromide as a leaching agent. It can be immersed in gold from ore under neutral conditions, but it is still in the development stage and is industrially popularized. There are still some difficulties in using it.

It was also reported that in 1987, the Bahamian refining company in Arizona developed a new method for leaching gold and silver ore to replace the cyanidation process. The leaching agent used is essentially sodium bromide and halogen. In addition to its fast leaching speed, it also leaches at lower temperatures.

The advantages of the bromination gold extraction process can be summarized as follows: high leaching speed, non-toxicity, strong adaptability to pH changes, and low cost of environmental protection facilities. When refractory gold ore is treated, since bromine can dissolve gold in an acidic medium, bromine can be directly added to the slurry after pressurized oxidation, and the pre-neutralization treatment process is omitted.

(3) thiosulfate gold extraction

The thiosulfate is generally a sodium salt and an ammonium salt of thiosulfuric acid, which are inexpensive, have a fast immersion gold, are non-toxic, are not sensitive to impurities, and have high immersion gold index.

The kinetics of gold dissolution in sodium thiosulfate solution by Baghdad Saliang et al. showed that the dissolution rate of gold is linear with temperature in the range of 45-85 °C, but in order to avoid violent decomposition of thiosulfate, the leaching temperature should be Controlled at 65.75 ° C. Rogerzhkov et al. used a thiosulfate solution containing ammonia and an oxidant to extract gold from ore into another kinetic study, which concluded that only at higher temperature conditions in the hot press leacher ( 130 to 140 ° C), in order to achieve satisfactory speed and recovery. Kakowski and Others also found that copper ions catalyze the dissolution of gold by thiosulfate, which can increase the dissolution rate of gold by 17 to 19 times. China's Jiang Tao, Cao Changlin and others have conducted a detailed study on the mechanism of thiosulfate gold extraction.

However, due to the high requirement of the thiosulfate method and the chemical instability of the thiosulfate, this method has not been popularized and applied.

(4) Polysulfide method immersion gold

The multi-stream chelate ion has a strong complexing ability to gold ions, and the polysulfide compound can effectively dissolve gold under the cooperation of a suitable oxidizing agent or by the deuteration of the polysulfide ion itself. If the leaching process produces elemental sulphur, the sulphide can also be immersed in gold because sulfides and elemental sulphur are easily converted to polysulphide. Polysulfides generally have sodium polysulfide, calcium polysulfide, ammonium polysulfide, etc., which are suitable for the treatment of gold-containing sulfide concentrates containing arsenic and antimony. Polysulfides are characterized by strong selectivity, fast leaching, a leaching cycle of several hours, and high leaching rates, as well as low grade gold ore.

The polysulfide immersion gold process (for example, ammonium polysulfide) is a 40% ammonium polysulfide solution leached at room temperature for 1 to 24 hours, and gold is introduced into the solution as NH ₄ AuS, and (NH ₄ ) _{2 is used.} The form of SbS ₃ enters the solution, arsenic is fixed in the slag, and then gold is recovered from the solution with activated carbon. Was thermally decomposed and sulfur Sb ₂ S _3, release ammonia and hydrogen sulfide gas, and a regeneration of sublimed sulfur with ammonium polysulfide. The leaching rate of this method is 80% to 99%, and the obtained arsenic trisulfide contains only 0.07%.

China's Long Bingqing and others have conducted research on the use of polysulfide immersion gold. Zhang Jian et al. studied the stone sulphur mixture, which is essentially a mixture of polysulfide and thiosulfate. They believe that with the use of stone sulphur mixture, the leaching rate of gold and silver can reach 96% and 80% respectively, and the gold leaching period is 1/8 to 1/2 of the conventional cyanidation method.

The main drawback of the polysulfide method is its poor thermal stability, decomposition of hydrogen sulfide and ammonia, which deteriorates the production environment, and strict sealing performance of equipment during industrial production.

(5) Other non-cyanide reagent immersion gold

Other non-cyanide immersion gold reagents mainly include amino acids, cyanide compounds and humic acids.

The amino acid molecule is characterized by the fact that the molecule contains two coordination atoms of nitrogen and oxygen. From a thermodynamic point of view, they can form a favorable soluble chelate complex with gold, and thus can be used as a gold immersion reagent. The amino acid immersion gold must also be carried out in the presence of a suitable oxidizing agent. Preferably the oxidant amino acid leaching of gold in general is potassium permanganate, it can make the partial oxidation of amino acid-based compound, hindered gold dissolution and destruction of carbohydrate. The concentration of amino acid leaching gold must be higher than 5g / L, the optimum concentration of potassium permanganate is 2 ~ 4g / L, the best pH value, temperature, pulp concentration are 9 ~ 10.5, 90 ~ 95 ° C, 20% ~25%.

Examples of the cyanide-containing compound include malononitrile, cyanogen bromide, thiocyanide, and amino acid calcium. These agents are less toxic than cyanide. However, there is not much research on such agents, and there is basically no research in China.

The humic acid immersion gold reagent has a wide range of sources and is inexpensive. Generally, it has an alkaline condition of pH 10 or higher. In the presence of an oxidant, the gold concentration in the immersion gold solution can reach 10 mg/L. The humic acid after sulfonation or nitration has a leaching capacity of 15 to 16 times higher than that of natural humic acid, and the gold leaching rate can reach 87%. Although there are not many people studying this law, it is one of the economic immersion methods.

2. Method and experimental study on iodine gold extraction

(I) Research status of foreign iodization and gold extraction

1. Theoretical study Iodine is a highly oxidizing oxidant. The leaching process using iodine as the leaching agent and bromine as the leaching agent should be the same, but there are few reports of iodinated immersion gold, and there are no examples of industrial applications. However, according to the Russian Academy of Exploration noble metal complex anion [AX _2] (X is an anion) Stability comparison ^{shows: CN -> I -> Br} -> Cl -> NCS -> NCO -, gold iodide The strength of the complex is worse than that of the gold cyanide complex, but stronger than that of bromine, chlorine, thiocyanide or cyanate. And iodine is a non-toxic agent compared to cyanide. Therefore, it is suitable to study immersion of gold from ore with an iodine-iodide solution.

Among the halogen elements, the AuI ₂ ^- complex ion is most stable in an aqueous solution. Iodine can leach gold from ore at a lower concentration.

Marun et al. used the thermodynamic data of Davis, Pourbaix and Latimer to map the Eh-pH relationship of the Au-I-H20 system, and proposed that two stable complexes were formed in the water stability limit: AuI ₄ ^- and AuI ₂ ^- . Among them, AuI ₄ ^- is the most stable, and the two complexes are stable throughout the pH range, and the change of iodine concentration has little effect. When the iodine concentration is lowered and the pH is high, gold oxide species, gold and iodine are present. The network and area become smaller. At the same time, compared with the Eh-pH diagram of the Au-Cl-H20 system and the Au-Br-H20 system, it was found that both AuCl ₄ ^- or AuBr ₄ ^- were stable in a small region within the water stability limit. It can be said that AuI ₄ ^- and AuI ₂ ^- are the most suitable halides for the analysis of thermodynamic conditions.
Marun et al. also calculated the equilibrium constants of the main reactions of the Au-I-I ^- -H20 system based on the studies of Angelidis and Davis et al.

Davis et al. found the most stable species at different concentrations of iodine and iodide by interpreting the equilibrium system. At a pH of <8, the molar ratio of I ₂ to I ^- is 0.1 or 0.35, the most stable are I ₃ ^- , AuI ₂ and I ^- ; at pH > 10, the most stable is IO ₃ ^- . If the molar ratio of I ₂ to I ^- is 0.5, an insoluble gold iodide is formed at pH < 8, which inactivates the surface of gold and prevents the formation of AuI ₂ ^- . Therefore, the actual work should be such that the molar ratio of I ₂ to I ^- is less than 0.5.

2. Experimental study Marun et al. conducted two experiments on iodized immersion gold. Their sample samples were as follows: A sample containing Au was 8.29 g/t, Ag was 5.0 g/t, and Cu was 0.01. %, mainly associated with mineral gold, alumite , hematite, gold, hematite, chalcopyrite- barite , gold-silicon, sulfur-arsenic copper ore and gold-silicon-barite, present at 15nln Monomer gold; B sample is flotation concentrate, containing Au is 57.69g / t, Ag is 39.49g / t, Cu is 0.15%, the main minerals are pyrite, sphalerite, galena and brass Mineral, gold is associated with quartz , and graphite is gangue. Both samples were ground to -0.074l and the masonry grade was 95%. Dip gold with iodine and potassium iodide reagent. The test conditions were determined as follows: the initial iodine, iodide molar ratio was less than 0.3, the pH was 3 to 5, and the standard reaction time was set to 4 h. The literature does not give gold leaching rate data, but the conclusion is that the gold leaching rate of cyanide leaching is high and the leaching time is long when compared with cyanide leaching. At the same time, the gold leaching test was carried out on the leaching rich liquid. The deposition rate of gold was over 90%, and the current efficiency was 0.12%-0.13%, which was basically independent of the initial concentration of iodine and iodide.

Ce, Xenbnnxos FB et al. used iodine to conduct leaching studies on gold-bearing oxidized ore from a mine in Ural. Ore following chemical composition _{(%): 50.4 SiO 2,} 15.8 Al 2 O 3, 16.4 Fe 2 O 3, 0.75 MnO, 2.46 MgO, 1.5 CaO, 0.63 Na 2 0,2.73 K 2 0,0.21 C, 0.03 S, 0.08 As, 3.5g/t Au, 9.0 other, gold is basically in a natural state but fine in particle size (0.01~0.03mill); gold is dissolved in iodine solution with a molar ratio of I ₂ to I ^- 0.1, pH is 5.5-7.5 The ratio of solid to liquid is 1:5. When the reaction equilibrium is balanced, the recovery rate of gold is 95%, and the equilibrium speed is slower than that of bromine solution. When electroplating, the higher the concentration of gold, the faster the electrolysis speed, and the maximum deposition rate of gold can reach 95% (the concentration of gold in the electrolyzer is greater than 40mg/L).

(2) The author's research on iodine gold extraction

The research on iodinated gold leaching started late. Whether it is theoretical research or gold immersion process research, it is very imperfect and not systematic. In response to the existing problems, the author conducted a systematic study on the theory and process of iodized immersion gold.
1. Theoretical study The authors have drawn the Eh-pH diagram of the actual immersion gold system (with the participation of oxidant hydrogen peroxide) Au-I-H2O by thermodynamic calculation, which is more than the Eh-pH diagram of the Au-I-H2O system reported in foreign literature. Perfect and more practical. At the same time, the Eh-pH diagram of the Au-I-H2O system aimed at the determination of AuI precipitation was drawn. The study showed that AuI precipitation occurred in the solution when the concentration of iodine and iodide ions was high, but in the normal gold leaching process. In the middle, due to the low content of gold, the concentration of iodide ions and iodine is low, AuI precipitation does not occur in the solution.

When the kinetics of leaching gold iodide, kinetic equation derived during gold leaching gold iodide dissolved reflected formula dissolution rate of gold and I ^-, I ₃ ^- between, oxidant concentration and mixing intensity The relationship has theoretical guiding significance for the practice of iodized gold immersion.
By thermodynamic calculation mechanism of leaching gold iodide analyzed, during the immersion gold iodide, I ^- and I ₃ ^- views must simultaneously acting with gold, and gold iodide complex ions of the type generated for AuI, unified Iodide immersion gold chemical reaction formula and reaction product.

The reaction behavior of impurities in the iridized gold leaching system was analyzed. It was pointed out that the sulfides, copper minerals, antimony minerals and carbonaceous minerals which are harmful to the cyanidation process are less harmful in the process of iodination. Much, the iodization method is highly adaptable to mineral types.

In the process of iodination, as long as the oxidation potential of the oxidant is greater than 0.58V, the leaching rate and leaching rate can be increased during the iodination of gold; the equilibrium constant criterion and the reaction free energy criterion for the smooth progress of the reaction are derived. Formula, and based on this, it is judged that hydrogen peroxide can be used as the oxidant of the iodination leaching process, which can make the reaction proceed smoothly. The reason why the hydrogen peroxide promotes the gold dissolution reaction is that the dispersion is uniform, the diffusion is fast, and other minerals can be oxidized to inhibit the consumption of reagents. Going on.

2. Experimental study The authors conducted a sorting and iodination leaching process test on carbonaceous oxidized ore samples and carbonaceous primary ore samples from the Getang gold deposit in Guizhou. The gold-bearing minerals in the ore sample are dispersed, including sulfides, oxides, organic materials, gold, and gangue minerals. The gold is very finely embedded. It has been proved by flotation test that the gold grade of flotation concentrate cannot be effectively enriched, and the grade of tailings is not significantly reduced. Only the original ore should be leached or roasted. The leaching rate of cyanide direct leaching gold is less than 80%. It is leached with iodine and iodide (potassium iodide, sodium iodide and ammonium iodide) solution. The direct leaching rate of oxidized ore gold is up to 95%, with an average of 91%. Left and right, above 75.70% when cyanide leaching. The leaching time is 4 h, the liquid-solid ratio is 3:1 to 5:1, and it is leached in the neutral and acidic pulp under normal temperature conditions.

When the carbon-containing primary ore is directly leached, the leaching rate of gold is about 78%, and the index cannot meet the requirements. Through the test and calculation, the calcination temperature of 650 ° C was determined, and the leaching rate after roasting reached 92% or more, which was nearly 15% higher than that of the original ore leaching, and achieved the purpose of roasting treatment. The same temperature roasting was carried out by cyanidation, and the leaching rate was 89.16%. The optimum leaching time is 4h, and it is leached in acidic and weakly acidic pulp under normal temperature conditions.

Theoretical analysis and experimental research have shown that the zinc replacement method is theoretically feasible from the recovery of gold from iodine leaching solution, but the consumption of zinc is greater than that of the cyanide system. With the carbon adsorption method of coconut shell, the adsorption recovery rate of gold can reach more than 93%. The initial concentration of gold should be greater than 40 mg / L, the electrolytic deposition recovery rate of gold can reach more than 95%, but when the gold concentration is low (<40 mg / L), the recovery rate of one-step electrolytic gold is lower. The gold iodine complex in the solution can be reduced to the monomer gold by an acidic reducing agent such as SO ₂ , NaI-ISO ₃ or Na ₂ SO ₅ . The ideal gold recovery method for iodine leaching gold is: activated carbon adsorption and desorption liquid electrolysis.

Third, the conclusion

The non-cyanide leaching of gold is the development trend of gold extraction technology. Many scholars in the world are actively exploring gold non-cyanide leaching agents. At present, the more promising leaching agents are thiourea, bromine and iodine, but they have yet to be further improved, especially in industrial trials. Through continuous research and exploration, we will be able to find environmentally friendly gold leaching agents and non-cyanide gold extraction methods suitable for industrial applications.

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