Pyrite is the most widely distributed in nature sulfide minerals are often associated with other minerals, the mining value in itself is not great. However, in the process of microbial leaching of ore, pyrite is both a biological energy source and Fe 3 + for the leaching system, and a large amount of sulfuric acid is generated when dissolved in a large amount, causing certain environmental pollution. Therefore, the study of the role of microorganisms in pyrite leaching process, have important guiding significance for the hydrometallurgical. In this experiment, a uranium ore in the south was used as the object to compare the effect of microbial column leaching with and without pyrite, and to investigate the effect of pyrite on bacterial leaching of uranium.
1. Mechanism of action of pyrite in the process of microbial leaching of uranium
Microbial leaching is a method of selectively leaching useful components from uranium ore or other ores using the biochemical effects of microorganisms. According to the principle of microbial leaching, when uranium ore contains pyrite components, pyrite can be oxidized by bacteria to form sulfuric acid and high-sulfur sulfate. Sulfuric acid dissolves minerals containing uranium-pyrene ions, and high-sulfur sulfate oxidizes UO2 to UO 2 2 + [ 2] .
The biochemical reaction formula of bacterial oxidized pyrite is as follows:
(1)
The high-sulfur sulfate produced by the above reaction is a strong oxidant that oxidizes pyrite and tetravalent uranium:
(2)
(3)
(4)
The ferrous sulfate and sulfur produced by the reactions (2) to (4) can be oxidized by the bacteria as high-iron and sulfuric acid as energy sources:
(5)
(6)
The high-sulfur sulfate produced by the formula (5) can oxidize more pyrite and tetravalent uranium according to the reactions (2) to (4).
In this study, through the comparative experiment of adding pyrite in the process of microbial leaching uranium, the variation of relevant parameters during the test was analyzed, and the role of pyrite in the process of microbial leaching of uranium was discussed.
Second, the test of mineral samples and bacterial liquid
(1) Mineral sample
The uranium ore sample for the test is taken from a uranium mine in southern China. The chemical multi-element analysis results are shown in Table 1. The particle size distribution and the analysis results of the uranium content of each grain size are shown in Table 2. Pyrite from a copper mine south of the content of FeS 2 was 84%.
Table 1 Results of multi-element analysis of mineral samples
Table 2 Results of particle size analysis of mineral samples
(two) bacterial liquid
The bacteria are mixed species that have been enriched, separated and mutagenized from acid pit water in the field, and cultured and acclimated in 9K medium and ore acidification solution. The growth period of the strain in the 9K medium was 18 h, and finally acclimated in the acidification solution for 20 h. After the mixed strain is domesticated and cultured by pyrite, the utilization rate of pyrite is 90%, and the pyrite added to the ore can be well utilized.
Third, test methods and process parameters
(1) Test method
The test uses column immersion method, the main equipment is as follows:
1. Leaching column. Φ50mm×1000mm plexiglass column.
2. Ion exchange column. Φ50mm×1000mm plexiglass column.
3. Ion exchange resin. D263.
4. Peristaltic pump. BT100-2J.
The leaching column without pyrite in the test is labeled as Tl, and the leaching column added to pyrite is labeled as T2. Bacterial leaching of uranium includes two stages of acid pre-dip and fungal leaching. The acid leaching solution produced by the acid pre-dip stage leaching ore and the leaching solution produced by the leaching of the ore in the leaching stage are adsorbed by the ion exchange column, and the adsorbed tail liquid is used to culture the bacteria, and the cultured liquid is reused. Leach the ore. This cycle is to reduce the discharge of tail liquid and is conducive to environmental protection.
(2) Test process parameters
The initial pH of the prepreg in the acid prepreg stage is 0.99~1.30; the initial pH of the leaching solution in the inoculation stage is 1.5~1.9, and the pH of the control solution is 1.8~2.0. Other major process parameters of the test are shown in Table 3.
Table 3 Test main process parameters
Fourth, the test results
The test lasted for 60 days and the main results of the test are shown in Table 4. It can be seen from Table 4 that in the same uranium immersion cycle, T2 not only has a total acid consumption rate less than T1, but also has a leaching rate higher than T1. Assuming that the test is terminated when the leaching rate reaches 90%, the addition of pyrite not only shortens the time for microbial leaching of uranium, but also reduces the acid consumption rate.
Table 4 Test results
V. Discussion
(1) Pyrite can accelerate the reduction of pH during uranium leaching
The pH value is an important parameter of the uranium leaching process. It is not only related to whether the leaching bacteria can adapt to the environment of the ore sample and the leaching solution, but also to the dissolution and precipitation of iron and uranium. The pH changes of the inlet and outlet of the T1 and T2 columns are shown in Figures 1 and 2.
Figure 1 T1 column pH change
â—‹-into the liquid; â–³-liquid
Acid pre-dip is a process of gradually acidifying from the top of the column to the bottom of the column. It can be seen from Fig. 1 and Fig. 2 that in the early stage of the test, a large amount of acid-consuming substances consume acid, resulting in a higher pH value of the liquid than the pH of the liquid. As the test progressed, the acid-consuming substances were basically consumed, the ore was thoroughly matured, and the pH value of the later liquid was basically stabilized at about 2.0.
Figure 2 T2 column pH change
â—‹-into the liquid; â–³-liquid
It can be seen from Fig. 1 and Fig. 2 that in the leaching system of sulfuric acid solution and bacterial infusion solution with the same pH value, the T2 column is added to the column of pyrite, and the pH value of the initial liquid is higher than the pH value of the liquid. The increase is significantly smaller and the acid pre-dip time is much shorter. This is because some of the pyrite is dissolved in the sulfuric acid solution, resulting in SO 4 2 - shortening the acid prepreg time. In the leaching stage, since the leaching solution can maintain a high potential, it can promote the dissolution and leaching of pyrite, generate a large amount of sulfuric acid, and accelerate the pH value reduction.
(2) Pyrite can reduce the acid consumption rate of uranium leaching process
The acid consumption rate of the T1 and T2 columns is shown in Figure 3.
Figure 3 acid consumption rate
â—‹-Tl; â–³-T2
It can be seen from Fig. 3 that the acid consumption rate of the T2 column is the same as that of the T1 column in the acid prepreg stage (1~26d), but the total acid consumption rate is 3.75% due to its early entry into the infiltration stage, which is better than T1. The column is reduced by 1.2 percentage points. This is because the pyrite added to the ore is oxidized by bacteria to form sulfuric acid, which makes up for the acid consumed by some acid-consuming substances, indicating that pyrite can produce acid in the process of bioleaching, reducing the process of leaching. Acid consumption.
(3) Pyrite can increase the leaching rate of uranium
The leaching rates of the T1 and 72 columns are shown in Figure 4.
Figure 4 Liquid meter leaching rate
â—‹-Tl; â–³-T2
As can be seen from Fig. 4, the leaching rate of the T2 column was higher than that of the T1 column 10 days after the start of the test, especially during the immersion period (32 to 52d), and the leaching rate of the T2 column was significantly improved. This is because the added pyrite is oxidized by bacteria, which not only provides energy to the microorganisms, but also allows it to continuously circulate, and also provides a large amount of Fe 3 + , thereby promoting the uranium leaching process, shortening the uranium leaching cycle, and improving leaching. rate.
Conclusion
As a common sulfide mineral, pyrite can produce a large amount of sulfuric acid under the action of microorganisms, reduce the acid consumption during uranium leaching, accelerate the acidification process, shorten the leaching period, and provide Fe 3 + for the leaching system. Reduce the use of oxidants and increase the leaching rate. At the same time, during the infiltration process, pyrite provides energy to the microorganisms in the ore, allowing it to form a systemic cycle.
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