Ours is a low-grade refractory gold mineral resource distribution is more extensive national, has been proven geological reserves of gold, the left and right about 1000t belong refractory gold resources, accounting for total proved reserves of gold (4634t) of 1/4. With the large-scale exploitation of easy-to-select leaching gold mines and the depletion of resources, research and development of efficient extraction of valuable metals in refractory gold mines has become an important research topic for comprehensive utilization of mineral resources and environmental protection. At present, the methods for treating refractory gold ore are mainly oxidative pretreatment-cyanide, enhanced cyanidation and non-cyanide leaching. The oxidative pretreatment technology is widely used at home and abroad, mainly including oxidative roasting method and pressurized oxidation. Method, chemical oxidation and biological oxidation. Biooxidation has become one of the most promising methods. It has the advantages of no pollution to the environment, simple process, low investment and low cost. Cyanide gold extraction - Bacterial Pre - oxidation preoxidation disseminated paper type refractory gold-containing arsenic were fine.
First, the nature of the ore
The results of multiple analysis of ore chemistry are shown in Table 1. The valuable metal in the ore is gold, and the harmful element has a high arsenic content, and contains harmful impurities such as strontium and carbon. The main metal ore minerals pyrite, stibnite, realgar (primary), orpiment, occasionally arsenopyrite, as shown in Table 2. Gold exists in the form of ultramicroscopic, disseminated, mainly related to pyrite. The gold-bearing minerals are very fine, mostly between 1 and 5 μm. More than 90% of the gold in the ore is in the form of gold in the package. Among them, the sulfide-coated gold accounts for 30.96%, and the other packaged gold accounts for 59.53%. It belongs to the sulphur-rich high-arsenic fine-dip-dyed refractory gold ore.
Table 1 Multiple analysis results of ore chemistry (mass score) /%
Au(g/t) | Ag(g/t) | As | TFe | TS | Organic C | TC | Sb | TiO 2 | K 2 O |
23.30 | 2.72 | 4.30 | 4.04 | 5.54 | 0.21 | 1.85 | 0.26 | 0.48 | 2.00 |
CaP | MgO | SiO 2 | Al 2 O 3 | FeO | Na 2 O | Cu | Pb | Zn | |
2.78 | 1.10 | 50.95 | 15.09 | 4.74 | 0.50 | 0.012 | 0.004 | 0.022 |
Table 2 Relative content (mass fraction) of major mineral components in ore
quartz | Dolomite, calcite | Sericite | Realgar, orpiment | Chlorite | Ground stone | Pyrite | Glow mine | Arsenopyrite |
43.6 | 14.3 | 11.1 | 11.1 | 6.3 | 6.3 | 4.5 | 2.8 | Occasionally |
Second, the principle of chemical oxidation and bacterial oxidation
During the chemical oxidation pretreatment of alkaline medium, sulfur, arsenic and iron in sulfide minerals such as pyrite and arsenopyrite are oxidized to sulfate, arsenate and hematite, respectively, thereby destroying the lattice structure of sulfide minerals. To expose the gold wrapped by it, the main chemical reactions are as follows:
2FeS 2 +8NaOH+15/20 2 →Fe 2 0 3 +4Na 2 S0 4 +4H 2 0 (1)
2FeAsS+lONaOH+70 2 →Fe 2 0 3 +2Na 3 As0 4 +5H 2 0+2Na 2 S0 4 (2)
The pre-oxidation of sulphide ore is a complex process. Chemical oxidation, bio-oxidation and primary battery reactions occur simultaneously. Sulfur, arsenic, iron and antimony in sulfide minerals are oxidized to sulfate, arsenate, citrate and iron, respectively. The hydroxide or slag, etc., eventually destroys the sulfide crystals, exposing the encapsulated gold, and recovering it by cyanidation. The main reactions occur as follows:
2FeS 2 +70 2 +2H 2 0→2FeS0 4 +2H 2 S0 4 (3)
FeS 2 +2Fe 3 + →3Fe 2+ +2S 0 (4)
FeS 2 +14Fe 3 + +8H 2 0→15Fe 2+ +2S0 4 2- +16H + (5)
4Fe 2+ +O 2 +4H + →4Fe 3+ +2H 2 0 (6)
2FeAsS+70 2 +H 2 SO 4 +2H 2 0→2H 3 As0 4 +Fe 2 (S0 4 ) 3 (7)
FeAsS+7Fe 3 + +4H 2 0→8Fe 2+ +H 3 As0 4 +S o +5H + (8)
FeAsS+5Fe 3 + →S o +As 3 + +6Fe 2 + (9)
2S 0 +30 2 +2H 2 0→2H 2 S0 4 (10)
For the mechanism of bacterial oxidation of pyrite, there are three main viewpoints: direct action, indirect action and combined leaching (reaction formulas (3) to (4), (6), see Figure 1). Boon proposes a two-step biooxidation mechanism ( In the reaction formulas (5) to (6)), the oxidation of the arsenopyrite is mainly an indirect action (reaction formulae (7) to (10), (6)). The bacterial oxidation process of pyrite and arsenopyrite is aerobic reaction. The oxidation process of pyrite is acidogenic reaction, and the oxidation of arsenopyrite is acid-consuming reaction. From a thermodynamic point of view, the poisonous sand is oxidized prior to pyrite.
Third, the leaching conditions test
(1) Chemical pretreatment - cyanide immersion gold test
The ore is treated by flotation process. The recovery rate of gold in gold concentrate is 27.83%, and the gold grade is not enriched. The direct leaching rate of gold in the original mud is very low, only 5.62%, and must be pre-oxidized. In order to effectively expose the gold, it is recycled by cyanidation.
Under the conditions of ore particle size of -0.048mm, 90% of pulp, 33% of NaOH, 10% of NaOH, and reaction temperature of 65 °C, the chemical leaching rate was 65.4%, which was higher than the total mud cyanidation. 60%, it has a certain effect. However, the gold grade of tailings is 8.03g/t, and a large part of the packaged gold is not leached.
(2) Bacterial pre-oxidation-cyanide leaching test
The purpose of bacterial oxidation pretreatment is to oxidize the gold-bearing minerals, destroying the crystal lattice of the arsenopyrite and pyrite, and dissociating the gold encapsulated therein to facilitate the next cyanide leaching.
The medium-temperature bacteria used in the experiment is the domesticated strain collected from a hot spring in Fujian. After the ore was crushed and finely ground to a certain particle size, the bacteria were pre-oxidized. The effects of grinding fineness, slurry concentration, oxidation time and oxidation temperature on the gold leaching rate were investigated. The oxidized slag is subjected to cyanidation leaching and gold extraction at a normal temperature by a carbon leaching method (CIL). The conditions for cyanidation were as follows: slurry concentration 33%, NaCN dosage 6 kg/t, activated carbon dosage 50 kg/t, pH 11.5, cyanidation time 24 h.
The pulp oxidation-reduction potential and pH value during bacterial oxidation pretreatment are the main reaction parameters. The trend of slurry potential and pH in the process of bacterial pre-oxidation is shown in Figure 2. From the thermodynamic point of view, the smaller the potential of the mineral is, the better the leaching is. The standard electrode potential of N-type and p-type pyrite at 25 °C is 0.458V and 0.368V respectively, indicating that the pyrite can be oxidized and dissolved when higher potential is required. Pyrite is more difficult to dip than other sulfide ore. In order to dissolve pyrite, the crystal lattice must be destroyed. According to the valence bond theory, the surface of the pyrite loses electrons, and the valence bond is not destroyed. It only increases the surface potential, and the valence bond is destroyed when the decomposition potential is reached. Under the oxidation of bacteria, maintaining a higher potential in the solution can ensure that the surface of the mineral is continuously losing electrons, the potential is increased accordingly, and the dissolution of pyrite is promoted. Therefore, the potential is a key factor affecting the dissolution of pyrite. Due to the strong oxidation of the bacterial flora, the ratio of Fe 2+ /Fe 3+ in the solution increased sharply and the potential increased, especially in the first 60 h. The potential change tended to be gentle after 120 h, and the pulp was in the middle and late stages of the oxidation process. The potential is always maintained above a higher level of 790 mV (SHE), which is beneficial to the oxidation of pyrite.
The pH value tends to decrease at the beginning of the initial stage. At the beginning, due to the dissolution of acid soluble substances (calcium, magnesium carbonate, etc.) in the ore, the oxidation of Fe 2+ is also an acid-consuming reaction, and the pH value increases; The oxidation of low-priced sulfur into sulfate in minerals (see reaction formula (9)) shows a significant downward trend at 48 h. After a certain value, it remains stable for a period of time and then falls again. This is due to the oxidation of sulfur. A certain amount of sulfuric acid, and when [Fe 3+ ] and [S0 4 2- ] in the solution reach the solubility product required for the formation of jarosite, the jarosite is precipitated in the solution due to the jarosite The formation consumes a large amount of OH - , resulting in an increase in [H + ] in the solution and a decrease in pH.
1. Influence of grinding grain size
The effect of different grinding grain size on gold leaching rate is shown in Fig. 3 under the conditions of 10% bacterial inoculum, 15% pulp concentration, pH=1.8±0.2, stirring strength 180r/min and reaction temperature 45°C.
It can be seen from Fig. 3 that the gold leaching rate is 89.24% when the fine leaching rate of the fine-grained fine gold is increased to -0.074 to 95%, and the effect of fine grinding on the gold leaching is not significant. Since biooxidation is a contact oxidation, if the particle size of the oxidized mineral is too coarse, the surface area available for adsorption by the bacteria is small, the total amount of bacteria adsorbed onto the solid particles is small, and the oxidation rate of the mineral is low. Taking into account the cost, the grinding size is selected from -0.074 to 95%.
2. Influence of slurry concentration
In the grinding particle size -0.074mm particle size 95%, bacterial inoculum 10%, pH = 1.8 ± 0.2, stirring strength 180r / min, reaction temperature 45 ° C conditions for different pulp concentration pre-oxidation test, the results shown in Figure 4.
It can be seen from Fig. 4 that the concentration of pulp has a great influence on the gold leaching rate, and the leaching rate increases with the increase of the pulp concentration, but the gold leaching rate decreases after the concentration is too high. At higher pulp concentrations, the shearing force generated by the shaking culture is larger, which not only makes the bacteria difficult to adsorb to the mineral surface, but also easily damages the bacterial cell wall. Tests have shown that the optimum slurry concentration is 15% while maintaining a high leaching rate.
3. Effect of oxidation time and oxidation temperature
The oxidation time and oxidation temperature were tested under the conditions of grinding particle size -0.074mm particle size 95%, pulp concentration 15%, inoculum size 10%, pH=1.8±0.2, stirring intensity 180r/min. The results are shown in Fig. 5. . It can be seen from Fig. 5 that with the prolongation of bacterial oxidation time, the leaching rate of gold increases with the consideration of cost factors, and the preoxidation time is preferably 7d. The leaching rate of gold increased with the increase of reaction temperature, the reaction temperature was 45 °C, and the leaching rate of gold was 89.24% when the bacteria was oxidized for 7 days, which was higher than the leaching rate of gold at 40 °C. The reason is that the strain used in the experiment is a medium-temperature bacteria, mainly composed of Thiobacillus and Microspirulina, which can tolerate higher temperatures. The bacteria grow faster at 45 °C, and the oxidation activity is higher, which is beneficial to pyrite. The destruction of the crystal lattice and the decomposition of minerals increase the leaching rate of gold.
Fourth, the conclusion
90% of the gold in the arsenic-containing fine-disseminated gold ore studied exists in the form of gold, which cannot be enriched by flotation process. Direct all-cyanide can not effectively extract gold. Through the comparison of chemical oxidation and bacterial pre-oxidation process, it is concluded that the bacterial pre-oxidation-cyanide process can effectively treat the gold ore. The optimum process conditions are: grinding particle size -0.074mm95%, pulp concentration 15%, bacterial inoculum volume 10 %, pH = 1.8 ± 0.2, stirring strength 180 r / min, oxidation at 45 ° C for 7d. Under the process conditions, the gold leaching rate reached 89.24%, and the smokeless gas was polluted during the process.
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