Carbon-bearing gold ore experimental research program

I. Mineral chemical characteristics of carbonaceous gold deposits

Produced in black (or carbon-containing) gold mine rock series in sedimentary rocks and is one of the world's major gold deposit type. Since the beginning of this century, the gold industry has recognized the harmful effects of carbonaceous materials in gold mines on cyanide leaching. From the perspective of gold extraction and smelting, “carbonaceous gold ore” was originally defined as a hard-impregnated ore containing organic carbon. The organic carbon in the ore interacts with the gold-cyanide complex and cannot be treated by conventional cyanidation. The most famous carbonaceous gold deposits include the Karin Gold Mine in the United States and the Mulong Gold Mine in Uzbekistan. A large number of large carbonaceous gold deposits have been discovered in Canada, Australia, New Zealand and China.

At home and abroad, it is believed that carbon quality plays an important role in the formation of primary deposits of fine-grained gold deposits, especially in the formation of gold deposits with sedimentary rocks as ore-bearing rocks. Some gold and organic carbons are contained in the Karin Gold Mine, the Dongzhai Gold Mine in Sichuan Province, and the gold deposits in the Golden Triangle of Guizhou. The analysis results of carbon and organic carbon in the ore of several finely immersed beam type gold deposits in China are shown in Table 1.

Table 1 Carbon and organic carbon in China's main fine disseminated gold ore

It is generally believed that when the primary ore contains organic carbon compounds above 0.2%, it will seriously interfere with the cyanidation extraction process of gold. This type of ore is called carbonaceous gold ore. In addition to the detrimental effects of carbonaceous materials, carbonaceous gold ore also has mineralogical properties that are generally difficult to leach, such as gold in vulcanization or in the presence of fine (microscopic and ultramicroscopic) inclusions in clay minerals.

Most carbonaceous gold gold and pyrite or other sulfur (arsenic) compounds symbiosis. In some fine-grained and metamorphic rock type gold deposits, carbon is one of the main gold-bearing deposits. In the Jerrit Cayon gold mine in Carlin, USA, most of the gold is present in the carbonaceous material with submicroscopic particle size; the carbonaceous single mineral in the gold deposit of China is 53.6g/t; the carbonaceous material of the Shanta gold deposit The gold content in the gold is 27.32g/t; in some mineral samples of the Getang gold mine, the gold encapsulated in the carbonaceous material accounts for 46.5% of the gold in the package, and the gold content in the carbonaceous material can be as high as 100g/ton in individual samples. . However, for most refractory carbonaceous gold deposits, the proportion of gold present in carbonaceous is small, and most of the gold is closely symbiotic with sulfides such as pyrite.

There are three main types of carbonaceous carbonaceous gold minerals: solid (elemental) carbon, a mixture of high molecular hydrocarbons and organic acids similar to humic acids, the latter two being collectively referred to as organic carbon. The presence of carbon in the ore is generally believed to be the result of a small amount of organic matter (possibly including hydrocarbons) during the hydrothermal period.

The elemental carbon has three kinds of structural forms: graphite , amorphous amorphous carbon and crystallite dysplastic pseudo-graphite (both amorphous and graphite). The main component is carbon, which generally does not contain gold. Due to bulk carbon, especially amorphous carbon, it has the property of activated carbon during the cyanidation leaching process, and adsorbs the soaked gold cyanide complex ions. The organic composition in the carbonaceous ore consists of a long-chain hydrocarbon that does not interact with the cyanide complex ion, and an organic acid that forms a complex with the cyanate salt (similar to humic acid). Some researchers in China have divided the organic carbon in the ore of the fine-grained gold deposit into two forms of soluble organic matter and insoluble organic matter (kerogen) in cyanide. Under the influence of diagenesis, deep pyrolysis and metamorphism, the volatile matter decreased, the carbon content increased, and the hydrogen, oxygen and nitrogen contents decreased. The analysis shows that the kerogen in the mineralized main rock of several finely disseminated gold deposits has a carbon content of 72.58% to 83.60%, a hydrogen content of 0.71% to 2.18%, an oxygen content of 2.54% to 9.11%, and a nitrogen content of 0.39%. 0.84%. The single mineral analysis of the chemical phase method of six mining areas, such as Louge Ditch, Getang and Jinya, showed that the gold content in kerogen was 0.14~147.46g/t. The experimental study of the separated components found that the main component of the carbonaceous material in the Jinya gold mine is the activated carbon component, followed by the humic acid, the ester and hydrocarbon similar to the "robbery" effect. Nothing. The pretreatment method of carbonaceous gold ore is divided into two types: removing or decomposing carbonaceous materials in the ore; or causing the carbonaceous materials to lose adsorption activity during cyanidation. The latter method only eliminates the harmful effects of carbonaceous materials during the cyanidation leaching process, does not destroy the ore carbonaceous matter, and thus does not dissociate the gold originally contained in the carbonaceous material.

China's proven carbonaceous gold resources account for a relatively high proportion of gold industry reserves, basically in the western part of China, to solve the pretreatment process technology of carbonaceous gold mines, the sustainable development of China's gold industry and the economic development of the west. Has a decisive role. The aqueous solution chlorination process is an effective non-cyanide gold extraction process for carbon refractory gold ore. The following is the research results in this area.

Second, the chemical principle of aqueous solution chlorination

Before the end of the 19th century, when cyanidation was used to extract gold, chlorine was always an important leaching agent for gold. Up to now, chlorine as a pretreatment process and leaching agent before conventional cyanidation leaching of refractory gold ore still plays an important role in the gold extraction industry.

Aqueous chlorination is one of the most effective wet oxidation pretreatment methods and direct leaching methods for carbonaceous gold ore. Chlorine slurry chlorination and hypochlorous acid oxidation are two of the most used, and the oxidative leaching pretreatment of FeCl ₃ , CuCl ₂ , and HCl-NaCl systems has also been studied. It was found that a considerable part of gold was dissolved during the chlorination pretreatment process, and a direct gold extraction process by chlorination was proposed.

Since gold and chloride ions can form relatively stable complex ions, hypochlorous acid is a strong oxidant. In the sodium chloride solution, hypochlorous acid can be used to immerse gold. The total reaction can be expressed as:

3ClO ^- +2Au+5Cl ^- +3H ₂ O→2AuCl ₄ ^- +6OH ^-

The chemical thermodynamic equilibrium calculation shows that the stable complex of gold under the experimental conditions is a complex of Au ^{3 +} .

On the other hand, the strong oxidizing property of hypochlorite can not only passivate the adsorption of carbon on gold, but also oxidize sulfide minerals to make the gold coating bare. Hypochlorite is suitable for leaching carbon refractory gold ore. . The oxidation reaction of sulfide minerals can be simply expressed as

MS+4ClO ^- →M ^{2 +} +4Cl ^- +SO ₄ ^{2 -}

or

MS+4ClO ^- +2OH ^- →M(OH) ₂ +SO ₄ ^{2 -} +4Cl ^-

The non-cyanide gold extraction process of aqueous solution leaching gold has the advantages of simple process and fast leaching speed. When used for easy leaching ore, the production cost is slightly higher than the cyanidation method, and the environmental hazard is less than the cyanidation method; for the refractory ore, the production cost is lower than the cyanidation method because no pretreatment process is required.

Third, sodium hypochlorite solution immersion gold test

The hypochlorite chlorination process is more suitable for carbonaceous gold ore containing higher carbonate minerals because leaching is carried out in a weakly alkaline medium. A mineral in Guizhou is in the sedimentary carbonate-mudstone fault gold-bearing alteration zone, and the bedrock is mainly argillaceous limestone and marl . The average grade of gold is 5.06 g/t. The gold ore is ultra-fine and micro, the particle size of gold is less than 0.01 μm, and the ore contains 0.1% to 0.7% of carbon. The multi-element analysis of the ore samples for the test is shown in Table 2.

Table 2 Elemental analysis of ore (%)

*Cr-total carbonaceous, refers to all other types of carbon except carbonate, including the sum of various elements and organic carbon.

The main mineral components in the ore include dolomite, calcite , pyrite, arsenic-containing pyrite, quartz and kaolin clay minerals. Starting from the gold extraction process, the main mineral components are three types: carbonate, sulfur (arsenic) and silicate. The mineral content is 68.1%, 3.5% and 28.4%, respectively, determined by chemical phase dissolution ore method. The gold in the three types of minerals is 2.0% to 3.4%, 45.8% to 47.3%, and 10.5% to 14.1%, respectively. Wherein the silicate phase comprises a carbonaceous phase. The ore dressing test shows that the gold mine has poor flotation indicators. In this study, full mud leaching was used, and the direct cyanidation leaching rate of gold in the ore was about 16%. After the ore is pretreated by oxidation, the cyanidation leaching rate of gold can reach about 90%. Oxidative destruction of gold-bearing minerals is the key to increasing gold leaching rates.

The results of the leaching test of sodium hypochlorite showed that the main factors in the leaching process were the concentration of the leaching agent and the reaction temperature, and the factors such as the pH of the solution and the initial chloride ion concentration had no obvious effect, indicating the influence of various related factors in the leaching process. The leaching agent is related to the reaction of the sulfide.

As shown in Figure 1, the leaching test of different initial concentrations of sodium hypochlorite showed that when the concentration of hypochlorous acid exceeded 3%, the leaching rate of gold was very fast, and the gold could be leached more than 75% in 1 hour, and the rate was significantly higher than cyanide. Dip.

Figure 1 Gold immersion at different hypochlorous acid concentrations

The leaching speed and leaching rate of gold increased with the increase of the concentration of the leaching agent, but the gold leaching rate did not increase substantially after 4 hours of leaching (Fig. 1). When the initial concentration of sodium hypochlorite in the leachate is greater than 3% of available chlorine, the gold leaching curve is very similar. After 1 hour of leaching, the rate of increase of gold leaching rate tends to be slow. The test results show that when the initial effective chlorine concentration is higher than 2%, the leaching rate of gold within 8h is basically equal. About 80%.

Although the immersion gold reaction formula indicates that the chloride leaching process requires the participation of chloride ions, the test results show that during the sodium hypochlorite leaching process, the initial chloride ion concentration in the solution (added to NaCl adjustment) does not substantially affect the gold leaching (Fig. 2). . Chloride ions produced by hypochlorous acid oxidation are sufficient to satisfy the gold reaction.

Figure 2 Effect of initial chloride ion concentration on immersion gold

The initial pH has little effect on the final leaching rate of gold, but at low pH leaching, the leaching rate increases (Figure 3).

Figure 3 Effect of solution pH on gold leaching rate

When the reaction temperature is lower than 50 ° C, there is almost no difference in the leaching curve at different temperatures (Fig. 4); when the reaction temperature is higher than 50 ° C (such as 60 ° C), the gold leaching rate decreases.

Figure 4 Leaching curve at different temperatures

During the leaching process of sodium hypochlorite, it mainly contains two basic processes: oxidative decomposition of sulfide and complexation and leaching of gold by chlorination. The reaction is slightly different at different stages of the process. As shown in Fig. 5, in the initial stage of leaching, the oxidation reaction of sulfide is dominant, and the gold leaching reaction hardly occurs. Then, after the oxidation rate of the sulfide exceeds 30%, the oxidative decomposition of the sulfide is substantially synchronous with the leaching of gold, and as the sulfide is oxidized, the leaching rate of gold rapidly increases; in the final stage of leaching, it is converted again. For the oxidative decomposition of sulfides, the leaching rate of gold has been substantially no longer increased with the further oxidation of sulfides.

Figure 5 Correlation between sulfur oxidation rate and gold leaching rate in different leaching periods

Under the optimal conditions, the primary leaching rate of gold is 80% when the fine sodium solution is leached into the fine particle-disseminated gold ore. Compared with the pre-oxidation-cyanide leaching results, the leaching rate of sodium hypochlorite was lower than that of the latter, indicating that some gold was purified during the leaching process of sodium hypochlorite. In order to eliminate the passivation of gold particles, the leaching residue was treated differently.

In a 3% effective chlorine sodium hypochlorite solution, after a period of 4 hours of leaching at room temperature, two-stage leaching of different methods was tested, including re-grinding of a section of leaching residue and slurry leaching of activated carbon or ion exchange resin. In Table 3.

Table 3 Two-stage leaching results of sodium hypochlorite solution leaching residue

In the second paragraph of the table, in the method, A is leached from the slurry, and sodium hypochlorite is added to 3% effective chlorine, and then leached for 4 hours. B is leached from the slurry and sodium cyanide (the same amount and concentration are the same as the standard method) for 24 hours; C is the slurry filtration, drying, and re-grinding, adding 3% effective chlorine sodium hypochlorite solution for further immersion for 4h; D is leaching slurry plus 60g/t of activated carbon, and then leaching for 4h; E is leaching filter After re-grinding, add 3% effective chlorine sodium hypochlorite solution and 50 kg / t resin, and then leaching for 4 h.

The above results indicate that it is feasible to directly leaching fine-particle carbon refractory gold ore under normal temperature and pressure at normal temperature and pressure. When leaching, the initial effective chlorine concentration in the solution is more than 3%, and the gold leaching rate is 80. %; two-stage leaching can achieve a gold leaching rate of 85% to 88%. After the leaching residue is ground again, the second stage leaching is carried out with sodium hypochlorite, and the total leaching rate is equivalent to the cyanidation leaching rate of the ore. The sodium hypochlorite chlorination gold extraction process is an effective method for such refractory gold ore. It has the advantages of non-cyanide, no toxicity, no environmental pollution, simple process, fast leaching speed, etc., and has industrial application prospects.

4. Electrooxidation generation and regeneration of hypochlorite

The electrochemical generation and regeneration techniques of sodium hypochlorite leaching agent were studied experimentally. The ion-exchange membrane is used to isolate the electrolytic cell, the purified solution containing NaCl is introduced into the anode chamber, and the NaOH solution is introduced into the cathode chamber to produce a solution of up to 15% NaClO at the anode, which can be used for immersion gold. Therefore, the gold leaching process of sodium hypochlorite solution consumes only alkali, sodium chloride and electricity.

The main reaction on the anode is

2Cl ^- →Cl ₂ +2e ^-

6ClO ^- +3H ₂ O→2ClO ₃ -+4Cl ^- +6H ⁺ +1.5O ₂ +6e ^-

2H ₂ O→O ₂ +4H ⁺ +4e ^-

Cl ₂ +H ₂ O→HClO+Cl ^- +H ⁺

The electrolysis efficiency is generally above 95%. The natural decomposition of hypochlorous acid in solution is the main reason for the low current efficiency. The main reactions are:

HClO→H ⁺ +ClO ^-

6ClO ^- +3H ₂ O→2ClO ₃ ^- +4Cl ^- +6H ⁺ +1.5O ₂ +6e ^-

ClO ^- +H ₂ O+2e ^- →Cl ^- +2OH ^-

2HClO+ClO ^- →ClO ₃ ^- +2Cl ^- +2H ⁺

2ClO ^- →O ₂ +2Cl ^-

Mainly oxygen precipitation reaction on the cathode

Since the electrolysis of the sodium chloride solution produces hypochlorite, the suspension gold leaching technique can be used to treat the carbonaceous gold ore. It was found in the test that the electroplating effects of the anode and cathode chambers were separated and the electrolysis was not separated in the two types of electrolytic cells. The oxidation effect in the anode and anode isolation electrolytic cells is better, which is related to the prevention of hypochlorous acid reduction in the cathode. Due to the low concentration of hypochlorous acid produced during suspension electroleaching, it takes a long time to suspend electrolytic leaching (such as 14 ~ 16h), and the leaching rate of gold is still not higher than 65%, which is significantly lower than that of hypochlorite. Direct leaching.

V. Conclusion

The aqueous solution chlorination method is a more effective gold extraction method for carbonaceous fine particles difficult to treat gold ore, and the following conclusions are obtained through experiments.

(1) The sodium hypochlorite solution can effectively leach the carbonaceous fine-grain-impregnated gold deposit. When the effective chlorine concentration is not less than 3%, the primary leaching rate can reach 80%, and the secondary leaching rate after the leaching residue is 85%. the above. Compared with the cyanidation method, the method has the advantages of high leaching speed, no need for pretreatment, low cost and simple process.

(2) The main factor affecting the leaching rate and final leaching rate of gold is the initial concentration of hypochlorite. As the initial concentration increases, the immersion rate increases almost linearly. The effect of temperature on the leaching process is not large, and normal temperature leaching can be used. However, when the temperature is higher than 50 °C, the leaching rate of gold decreases. The pH of the solution and the initial concentration of chloride have essentially no effect on the leaching.

(3) The sodium hypochlorite solution for immersion gold can be produced or regenerated by the diaphragm electrolysis method of sodium chloride solution.