Thiourea extraction gold and silver process

When Jiabu La atmosphere of thiourea and a comparative test method was done, the two methods is treated with carbon-containing gold concentrate pyrite arsenic 500g, at 25 ℃, liquid to solid ratio of 2, using air as the oxidant (1L /min) and each of its optimal leaching parameters: thiourea method with 1000mL of TU solution with a concentration of 0.175moVL H2SO4, 0.197mL / L (15g / L); cyanidation with 1000mL deionized water plus 4g NaCN, Add Ca(OH)2 to maintain pH=l0. Figure 1 shows that after 30 minutes of leaching by thiourea, the gold recovery rate is nearly 90%. At this time, the recovery rate of cyanidation gold is only 35%. To obtain 94% extraction rate, the cyanidation method takes 24 hours, while the thiourea only needs 1 hour. .

Goroniwald uses a sulfuric acid-thiourea solution for the leaching of gold from unpretreated or pickled ore. When the solution contains H2SO4 at a concentration of 1.0 mol/L, thiourea at 1.2 mol/L and 0.1 When mol/L hydrogen peroxide is used, the gold dissolution rate is very fast, 98.5% of gold can be recovered in 1 hour, and the consumption of thiourea is 1.4 kg/t ore.

Chen Dengwen leaches gold from refractory carbonaceous ore. After roasting and sulfuric acid pretreatment, the acid thiourea leaching gold recovery rate is 95%, thiourea and sulfuric acid consumption are 1.5-2 kg per ton of ore respectively. 70 kg.

Insitu Inc. conducted an in-situ leaching test in Victoria, Australia, in 1981. It has been reported that a mixture solution of thiourea, thiosulfate and ferricyanide was first used in a "push-pull" test to extract gold from a deep-covered alluvial deposit.

According to laboratory calculations, 1000kg dry material and 100kg wet material (35g gold), under the conditions of adding H2SO4 5kg, SO2 0.5kg, H2O2 (30%) 0.75kg, thiourea 1.05kg, the extraction rate of gold can reach 98. % (80% extraction in the leaching section and 10% extraction in the thiourea washing section). After 3 stages of carbon adsorption, the recovery rate of gold is 97.86% (3 segments are 80%, 16.37% and 1.49% respectively), and the total recovery of gold The rate is above 95%.

In short, in recent years, foreign countries have shown great interest in extracting gold from thiourea, but they are more cautious. It is more generally believed that the thiourea method has the advantages of reducing environmental pollution, accelerating the rate of gold dissolution, reducing the interference degree of copper , zinc , arsenic and antimony , short process flow, low investment and simple operation, compared with the cyanidation method; Problems involving high economic efficiency, such as high equipment costs, have yet to be resolved.

After a comprehensive analysis of a large number of theoretical and experimental research work, the Soviet scholar BB Rodriguez developed a principle process for the wet treatment of gold ore based on thiourea leaching. The process includes the following four main processes:

1 Agitated leaching of gold is carried out by using a regenerated, purified and sterilized acid thiourea solution.

2 The gold-containing leachate and the leach residue are separated by dense filtration.

3 Recover gold from the solution to obtain the corresponding product that meets the requirements of the refinery.

4 The recovered thiourea solution is further processed to regenerate the thiourea and remove impurities from the solution.

For recovering gold from solution, the following methods can be used: metal (zinc, lead , aluminum ) to replace the precipitate, alkali solution (NaOH, etc.) to destroy the complex, electrolytic deposition, adsorption on activated carbon and cation exchange resin.

The choice of recovery method depends primarily on the metal content of the solution. 1 For solutions with high gold and silver content and mass concentration greater than 500 mg/L, electrolysis should be used, which can regenerate thiourea at the same time; 2 pairs of gold mass concentration is less than 50mg/L, and silver mass concentration is 200-400. The mg/L solution has the most promising adsorption method, and the precious metal can be adsorbed on the activated carbon or on the cation exchange resin; 3 pairs of gold mass concentration is more than 50 mg/L, and silver mass concentration is more than 20 mg/L. The solution, when the adsorption method is not effective, that is, the degree of enrichment of the metal is low, the precipitation method can be replaced with a metal, and in this case, it is economically unsuitable to use the electrolysis method.

The thiourea gold extraction processes studied at present include: conventional thiourea leaching method, SKW method for introducing SO2 into leachate, iron slurry method with metal plate immersion, carbon adsorbed by activated carbon or cation exchange resin. A slurry or resin slurry method, and an electrowinning method in which a cathode and an anode plate are inserted into a leaching tank for electrolysis.

Most of the raw materials for gold extraction by thiourea are gold concentrates or calcined gold, and the operation technology is almost the same as that of cyanidation with compressed air. However, acid-resistant equipment is required. The method of recovering gold from the leached slurry mostly uses a ferrite method and a carbon slurry method.

Conventional thiourea diffusion method

This method is a conventional method for leaching gold and silver into a sulfuric acid (pH 1.5-2.5) sulfur vein slurry by blast stirring. The dissolved gold in the slurry is usually filtered and washed several times, and gold is recovered from the filtrate and the washing liquid by displacement, adsorption or electrolysis. It is similar to the CCD process of cyanidation. However, since the pulp is a strong acid sulfur vein medium, copper, lead, zinc, iron and other base metals will dissolve together with gold and silver to form sulfur vein ions, which not only makes the ion concentration in the pulp too high, but also consumes a large amount of sulfur veins. Especially when used to treat sulfur concentrates, sulfur enters the solution to form sulfides such as H2S, S, SO42-, HSO4-. Their mutual transformation can make the equilibrium concentration of H2S (liquid) in the slurry about 0.1mol, which will cause a large amount of sulfide precipitates in the metal ions. In particular, gold and silver are vulcanized and precipitated in the slurry, or sulfur adheres to the surface of the ore to form a passivation, which will reduce the leaching rate of gold and silver, so that the end point of the sulfur vein leaching operation appears prematurely, and the leaching residue contains Gold is too high and causes losses. However, since the leaching rate of silver veins to silver is much higher than that of cyanidation, since 1982, the gold and silver mines in Colorado have used sulfur veins instead of cyanidation to leach silver from silver-containing tailings, which has achieved good results. .

1) Leaching bare gold from stibnite concentrate

The Hillgrove antimony deposit in New South Wales, Australia, is an early-exploited deposit with an average width of 300-400 m. In 1969, the New East Australian Mining Company (NEAM) operated a small mine and plant.

The ore is a quartz vein type gold-bearing ore deposit. The main symbiotic minerals are pyrite, pyrrhotite, arsenopyrite, scheelite and chlorite. The ore contains Sb 4.5% and Au 9g/t. The produced ore is subjected to grinding, re-election and flotation, and the produced ore concentrate is sold to the smelting plant. The concentrate contains 30-40g/t of gold, and the smelter does not pay any compensation. In order to extract the gold, the factory has been tested by cyanidation, and the effect is not good. Later, when testing other solvents, it was found that thiourea can quickly leach the bare gold in the concentrate. In March 1982, a 1t/h small batch thiourea leaching workshop was established.

The workshop's treatment of antimony concentrates is only to recover the monomer dissociation gold, and does not seek higher gold recovery. Therefore, the higher thiourea and Fe3+ concentration is used, and the immersion liquid and the concentrate are pre-mixed and pulped, so that the pure leaching time of each batch of concentrate can be shortened to within a minute. The gold in the leaching solution is adsorbed by activated carbon, and the gold-loaded charcoal containing 6-8 kg/t of gold is directly sold. The thiourea solution after adsorption of gold is added to H2O2 to adjust the oxidation-reduction potential and then returned to the leaching process for recycling.

In the first few months of leaching with thiourea, there has been a loss of dissolved gold deposits, which was found to be caused by the adsorption of gold on the surface of chlorite minerals in the concentrate, so air addition is added during flotation. Agent 633 to inhibit chlorite and to add a small amount of diesel to the slurry prior to leaching. After these measures, the bare gold can be leached, the gold recovery rate in the concentrate is 50%-80%, and the thiourea consumption is usually below 2 kg/t.

The company also found that the arsenopyrite in the flotation tailings contained a large amount of gold, so it increased the arsate flotation circuit, and the produced arsenic concentrate contained As15%~20%, Sb 5%, Au 150-200g/t. The recovery rate of gold in tailings is 70%. To this end, a 600 t/d early tailings reprocessing plant was built in 1983 to recover 1-2.5 g of gold per ton of tailings.

2) Silver-containing raw materials for sterling silver

In order to explore a new process for preparing pure silver from silver-containing raw materials, Zhang Jian et al. carried out a new process for thiourea leaching, complex crystallization and sintered crystals of silver-containing raw materials to obtain pure silver. As a result of the test, the recovery rate of silver is over 91%, and the purity of the product silver is 99.84%.

The raw material components used in the experiment were (%): Ag 0.91, AgCl 0.29, SiO 2 61.00, CaO 15.76, MgO 0.78, Fe 2 O 3 1.81, Al 2 O 31.75, K 2 O 3 1.16, Na 2 O 0.47, H 2 O 3.30, volatiles 11.05, and other 1.72.

In the small test, the raw material is ground to -2mm, 100g is placed in a 500mL beaker, 300mL of the distillate prepared by adding the second distilled water and the reagent pure agent to carry out the single factor experiment of each condition, and the comprehensive condition experiment is carried out according to the single factor experiment result. The optimum conditions are: SCN2 HQ substance concentration is 0.52mol/L, H2SO4 is 1.18mol/L, Fe2(SO4)3 is 0.004mol/L, temperature 60°C, stirring speed 700r/min, leaching time After 2.5 h, it was filtered, washed, and the washing liquid and the filtrate were combined, and the slag was discarded. The silver leaching rate was 98.50%.

Expand the experiment Under the above conditions, use tap water and industrial pure chemicals to expand the experiment by 10 times. As a result, the leaching rates of silver were 97.23% to 98.91%, respectively, and the results of the small test using the double distilled water and the reagent pure drug were reproduced.

The silver in the thiourea leaching solution is in the state of Ag(SCN2H4)3+ complex ion. The single-factor experimental results of the crystallization of the complex showed that the temperature decreased from 15 ° C to 2 ° C, the crystallization rate increased from 70% to over 95%; the pH was between 0.5 and 3, the crystallization rate was above 80%, and the pH rose. The crystallization rate is only slightly increased. A black precipitate appeared when pH > 3.5. When the concentration of silver in the solution is 0.6~3.6g/L, the crystallization rate is slightly higher than 80%. As the concentration of silver increased, the crystallization rate decreased slightly, but there was no significant effect. On the basis of this, the selected crystallization conditions were a temperature of 2 ° C, a pH of 3, a mass concentration of silver in the stock solution of 0.78 g/L, and a crystallization ratio of silver of 93%. Among the three factors, the analysis of variance shows that the main factor affecting the crystallization rate is temperature.

The produced crystals were dried at about 100 ° C, and then heated to 1100 ° C to produce 99.84% of pure silver. If the crystals separated in the mother liquor are washed with low-temperature water to remove soluble impurities, the purity of the product can be improved. The mother liquor after crystallization is separated and can be returned for leaching of silver.

Although the experiment is a small exploratory test of 0.1-1.0kg scale, the production process is short, the process is simple, the equipment investment is small, the product purity is high, and it can be used to treat impure metallic silver, silver chloride, silver sulfide, silver ore, The angle silver mine and its mixed raw materials have industrial application prospects.

2.SKW method (also known as SO2 reduction method)

This method was developed by the former German company SÃœD Cyanide Calcium (SKW). The method of introducing the reducing agent SO2 into the thiourea leaching system based on the conventional thiourea leaching method.

This method is based on the fact that thiourea has poor stability and is easy to oxidize. In a solution containing higher Fe3+ (mass concentration of 3~6g/L), thiourea will be invalid due to the following reaction:

The above reaction was carried out in three steps. The first step is a reversible reaction. The thiourea can be oxidized to form dithiomethane, and the dithiocarbamate formed in the presence of a reducing agent can be reduced to thiourea. The second step is an irreversible reaction, in which dithiocarbamate is partially reduced to thiourea by disproportionation, and a sulfenide of unknown composition is partially formed. The third step is also an irreversible reaction, which is finally decomposed into cyanamide and elemental sulfur. The cyanamide can be further decomposed into urea. Due to this reaction, the amount of oxidative loss of thiourea in the immersion process is often many times higher than the pure consumption of the gold-soluble agent. And the elemental sulfur which is finally decomposed is sticky, and it covers the surface of all the solid materials to passivate them, so that the leaching rate of gold or the like is lowered.

In order to overcome these difficulties, the irreversible decomposition of dithiocarbamate in the above reaction should be avoided, that is, the concentration of dithiocarbamate in the solution is prevented from being too high, or a reducing agent is added to reduce the dithiocarbamate to a thiourea through a reversible reaction. This idea is the basic guiding ideology of SKW law research.

Sulfur dioxide is a highly efficient reducing agent. Under the specific conditions of thiourea immersion gold, the researchers found that as long as dithiocarbamidine is present, it will not reduce other oxidants.

In the thiourea leaching test with 0.2-0.7mm silver particles, when the SO2 is not added, the surface of the silver particles is covered with a dark film, and the leaching rate of silver is about 25%, which is obviously the presence of Fe3+ and the initial concentration is low ( 0.5g/L). If excessive SO2 is supplied to the immersion liquid, the surface of the silver particles is in a bright metallic state, and the dissolution rate of silver can reach 100%. When the amount of SO2 supplied is insufficient, the leaching rate of silver will decrease again.

When the same method is used to leaching gold particles, an unexpected phenomenon occurs, that is, when the amount of SO2 is insufficient, the surface of the gold particles is bright, the dissolution rate of gold is nearly 100%; and when the excess SO2 is supplied, the gold is The dissolution rate decreases instead. However, this phenomenon can be explained by chemical kinetics.

It is proved by experiments that in the practical application of thiourea immersion gold, the temperature of the slurry is increased to 40 °C to accelerate the oxidation of thiourea to dithiocarbamate; and the SO2 is supplied to the slurry at an appropriate speed to reduce the excess amount in the slurry. Thioformin. By controlling the supply rate of SO2 so that 50% of the total amount of thiourea in the slurry maintains the oxidation state of dithiomethyl hydrazine, high-speed leaching of gold and silver and reduction of thiourea consumption can be achieved. This measure is the key to the success of the SKW law.

Table 1 is a refractory oxidized ore containing Pb 50%, Zn 6.8%, Fe 26.5%, Ag 315 g/t, Au 10.6 g/t, respectively, using cyanidation method, conventional thiourea method and SKW method. Compare the results of the test. It can be seen from the table that the SKW method supplies 6.5 kg/t of SO2 into the thiourea leaching slurry. The leaching rate of gold and silver in 5.5 h is much higher than that of the cyanidation method and the conventional thiourea method, realizing gold and silver. The high-speed leaching reduces the consumption of thiourea to 0.57 kg/t. Since the sulphur urea consumption of the SKW method has dropped to such a low level and the leaching time is greatly shortened, it is not only economical for processing high-grade gold concentrate, but also for economical treatment of low-grade gold ore. of.

The gold and silver in the leachate can be adsorbed by activated carbon, strong acid cation exchange resin or thiol resin, and then desorbed with hot acid or thiourea solution. Since the amount of thiourea is so small, it is not necessary to consider recycling it.

The results of the SKW test show that:

1 increase the operating temperature to 40 ° C, can accelerate the oxidation of thiourea to dithiocarbamate;

2 Control the supply rate of SO2, reduce the excess dithiocarbamate to thiourea by the reduction of SO2, and keep the 50% of the total thiourea in the slurry in the dithiocarbamate state, which can prevent the passivation of the feedstock. To obtain the highest gold and silver dissolution rate;

3 The appropriate amount of SO2 is supplied to reduce the excess dithiocarbamate to thiourea to prevent the irreversible chemical oxidative degradation loss caused by the excessive concentration of dithiocarbamate in the slurry, so as to increase the regeneration and utilization of thiourea and reduce the consumption of thiourea.

According to the results of the small test, RG Schulze uses the process test procedure of Figure 2. The test feed was 1.1 t (100 kg in water), the thiourea for leaching was from the recycle liquid, 1.05 kg of new thiourea was added to the thiourea wash section, and the oxidant used H2O2. The balance values ​​of the mass concentration of thiourea TU (g/L) and Au (mg/L) and the solution flow rate (L) during operation are also shown in the figure. The technical points and indicators of the operation are:

2) Study on roasting and iron-loaded leaching of carbonaceous oxidized ore

The natural gold in the Zhangjiakou gold deposit mainly occurs in limonite, pyrite, leucite, galena, chalcopyrite and quartz. It is difficult to produce carbon, mud and alkaline minerals by flotation. Processing gold concentrates. For ore similar to this, various processes are taken before and after cyanidation to eliminate the effects of carbon and harmful impurities, but the cyanidation leaching rate is still not higher than 85%-90%, and directly or after decarbonization For thiourea immersion, the leaching rate of gold is only about 80%, and 135-180 kg of sulfuric acid per ton of concentrate is used, and thiourea is more than 5 kg. In order to improve the gold leaching rate of this concentrate, after the test, the concentrate is oxidized and calcined in advance to achieve the functions of removing carbon, decomposing carbonate and repelling the crystal water in limonite, and then immersing it with thiourea Set, obtained economic and technical indicators better than the cyanidation CCD process.

Fig. 6 is a flotation concentrate containing about 100g/t of gold, calcined at a temperature of about 680 ° C for about 20 minutes, and then finely ground to 94% - 0.043 mm (-325 mesh), and adjusted to pH by adding sulfuric acid. After 1.5-2, thiourea was added, and continuous thiourea iron immersion was carried out in six leaching tanks. Tests in 44 batches of 3 batches showed that the leaching rate of gold was 95.07%~96.40% with an average of 95.79%. The gold deposit recovery rate of gold mud on the iron plate is 98.45%-99.69%, with an average of 98.99%. The total recovery rate was 94.82% on average. Each ton of concentrate consumes 70kg of sulfuric acid, 1.5-2.2kg of thiourea, and the main material consumption is 42.42-52.48 yuan/t, which is 57%-47% lower than the cost of the same cyanide CCD process for processing the same ore of 98.66 yuan.

The process pulp does not need to be filtered, the equipment and infrastructure investment is low, the floor space is small, the operation is simple, the labor is saved, and the process is short. It is the first breakthrough of China's thiourea method for extracting gold from difficult-to-treat ore, and from research to industrial application, it has been successfully applied to the industrial production of Zhangjiakou gold mine. It lays a foundation for the research and popularization of thiourea iron slurry method in China.

3) Industrial test and popularization of iron slurry method

The raw material for the industrial test of the thiourea iron slurry method is a sulphur gold concentrate containing a small amount of oxidized mineral. The main mineral pyrite, chalcopyrite, galena, sphalerite, limonite, malachite, urea and natural gold minerals quartz, sericite, chlorite, kaolin, carbonates and the like. The concentrate component (gold/silver is g/t, others are %) is Au 80.77, Ag 50, Cu 0.71, Pb 0.6, Zn 0.18, Fe 25.09, S 26.55, As 0.046, Bi 0.0063, Ni 0.038, SiO 2 22.44 , CaO 4.08, MgO 11.80, Al2O3 3.60. The natural gold particle size - 0.038 mm (400 mesh) accounts for more than 80%, of which 0.0067-0.0033 mm accounts for about 50%.

The test was carried out in a continuous immersion operation with a scale of 1.5 t/d. The feeding mode 1 is mechanical continuous feeding, and the second is manual feeding. The flotation concentrate is dewatered by a thickener and then refined, and then milled into a closed loop composed of a cyclone and a classifier, and overflowed for leaching. The recovery of the leaching gold is to hang the iron plate in the tank and deposit gold while leaching.

The test was carried out continuously in seven tanks with a grinding particle size of 80%-85%-0.043 mm (325 mesh), a solid-liquid ratio of 1:2 thiourea initial mass fraction of 0.3%, pH 1-1.5, liquid temperature 25 °C, Insert the iron plate 3m2·m3·slot-1, immerse time 35-40h, and automatically scrape the gold mud once every 2h by the crane.

After 12 days, the comparison of the two schemes showed that the leaching rates of gold were 94.50% and 95.21%, the sediment recovery rates were 99.35% and 99.64%, and the total recovery was 93.89% and 94.86%. The gold mud (an example) component (%) was: Au 3.05, Ag 1.73, Cu 13.57, Fe 15.66, S 20.36, SiO 2 19.42, CaO 0.33, MgO 0.35, A12O3 2.95. Among them (Au+Ag) is 4.78%. In order to improve the Au and Ag grades in the gold mud, the pyrite and fine-grained slime adhered to the surface of the iron plate were washed with high-pressure water before scraping, and the gold-containing gold content was increased to 5%.

The above industrial tests show that:

Compared with the cyanide countercurrent decantation washing process, the economic and technical indicators of the thiourea iron slurry method are comparable or slightly better (as shown in Table 3), mainly due to the fact that the gold content of the concentrate used in the thiourea method is lower than that of the cyanidation method. . If the thiourea iron slurry method is compared with the cyanidation carbon slurry method, the thiourea method costs more.

2 The initial leaching speed of thiourea leaching gold is very fast. When the slurry is adjusted to the thiourea tank by adding thiourea, the dissolution rate of gold has reached more than 50%, and the dissolution rate of each leaching tank gradually decreases. Table 4 is an indicator of the leaching and replacement operation of the concentrate in each tank. Because the gold leaching and iron displacement deposition in the slurry are synchronous, the metal ion concentration and the Au dissolved potential in the slurry are relatively stable. Therefore, the gold leaching rate and the replacement rate are steadily increasing.

Roofing and Wall Panel Roll Forming Machine