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Bioleaching due to low cost, low investment nor environmental pollution caused widespread concern in domestic and foreign researchers, it has become the very active area of research, especially in the recovery of low-grade, refractory metal ore respect bacterial leaching has Its unique advantages. In the uranium ore leaching process, sulfuric acid leaching is usually used, and hydrogen peroxide is often used as an oxidizing agent for the purpose of enhanced leaching, but is gradually replaced by another oxidizing agent due to its disadvantages such as high cost and corrosiveness. Since bacteria capable of oxidizing pyrite mineral raw sulfate, not only as the oxidant, but also save some sulfuric acid has been extensively studied. In view of the low grade, low leaching rate and long leaching time of a hard rock uranium mine, the first experimental study on the in-situ bactericidal leaching of uranium ore was carried out in China, and the leaching was carried out by using bacteria instead of hydrogen peroxide as oxidant. Good test results.
First, the material
(a) strain
The strains used in the test were obtained from the uranium mine acid pit water and were continuously screened and domesticated.
(2) Ore
The type of ore used in the test is cryptoexplosive breccia type, granite type and andesite type. The cryptoexplosive breccia type is the main ore type of the deposit, accounting for more than 95% of the total reserves of the deposit. The ore symbiosis combination includes uranium-pyrite, uranium-hematite, uranium- fluorite , uranium- calcite , uranium-chlorite, etc. The chemical composition of the ore is shown in Table 1.
Table 1 Ore chemical composition (mass fraction) /%
U
U 6+
U 4+
F -
S
FeS 2
FeO
Fe 2 O 3
0.088
0.048
0.040
0.28
0.16
0.21
0.54
1.01
The data in Table 1 show that the test contained in the ore with pyrite, conducive to bacterial growth, reproduction, favorable for leaching; but the ore F - higher levels of bacterial leaching adverse effects should be considered domesticated bacteria culture.
Second, the mechanism of bacterial leaching
The on-site expansion test is to examine and verify the process parameters obtained in the small-scale test using the scaled equipment and test scale. The on-site test was carried out on the basis of the test results obtained from the indoor sulfuric acid, hydrogen peroxide, and bacteria comparative leaching tests. The results of the indoor leaching test are shown in Table 2. On-the-spot experiments use bacteria to oxidize Fe 2+ in solution to Fe 3+ on the one hand; on the other hand, the ore-cultured bacteria are used to inject ore bodies to adhere bacteria to ore particles and oxidize yellow in ore. Iron ore that produces high-sulfur sulfate and sulfuric acid. The two aspects work together to increase the leaching rate of metal uranium, shorten the leaching time, and save some sulfuric acid. The on-site test method is to inject the domesticated and cultivated strains together with the leaching solution into the ore body and participate in the leaching process of the ore. The main chemical reaction equation is:
Table 2 Comparison results of sulfuric acid, hydrogen peroxide, and bacteria indoor leaching
Leaching conditions
Leach rate /%
Liquid meter
Slag meter
10g/LH 2 SO 4
76.9
77.3
10g/LH 2 SO 4 +0.4g/LH 2 O 2
98.7
99.5
10g/LH 2 SO 4 + bacteria
98.2
99.4
Third, the test method
In the in-situ crushing and leaching uranium industry test, through computer simulation, the appropriate amount of explosives was loaded, and the uranium ore was crushed to a certain particle size (-150mm grain size accounted for about 75%) by in-situ blasting to form a natural ore heap, which will contain microorganisms. The solution is sprayed onto the top of the heap, and the solution penetrates into the heap to leach the metal uranium in the ore. The solution containing metal uranium flows out from the bottom of the heap, is collected into a sump, and then pumped to the leaching solution transfer tank for ion exchange adsorption through the adsorption tower. After the adsorption tail liquid is regenerated, some chemical reagents (such as sulfuric acid) are added. Circulating leaching, periodic sampling and detection of uranium concentration, redox potential, sulfuric acid concentration and worldwide activity in the leachate.
The bacterial leaching field test was carried out on the basis of the test of the in-situ crushing and leaching uranium industry. In principle, the original process flow was not changed and the normal production was not affected. The bioreactor was designed and processed according to site conditions, and a bioreactor tank was constructed next to the in-situ crushing industrial test leach solution preparation tank, and the adsorption tail liquid portion was transferred to the bioreactor. Because Fe 2+ concentration in the adsorption tail liquid is relatively low, FeSO 4 ·7H 2 O needs to be added, so a FeSO 4 ·7H 2 O dissolution tank is constructed. The leaching process is shown in Figure 1.
Figure 1 Bacterial leaching process
1—leachate transfer tank; 2—adsorption tower; 3—FeSO 4 ·7H 2 O dissolution tank; 4—bioreactor; 5—leaching agent preparation tank; 6—stainless steel pump; 7—high tank; 8—cloth liquid Network; 9 - underground heap; 10 - liquid guiding well; 11 - liquid collection tank
Fourth, the test results
(1) Changes in uranium concentration in leachate after addition of bacteria
During the field test of in-situ leaching and leaching of uranium ore, the uranium concentration in the early and middle stages was higher. After leaching for a certain period of time, the uranium concentration in the leaching solution gradually decreased. During the test, when the uranium concentration was reduced to 130-150 mg/L, the cloth liquid was stopped for 10 days, and then the cloth liquid was resumed. The leaching solution was sampled and analyzed every day during the leaching test to obtain the metal uranium concentration Ï(U) of the leaching solution. The time relationship curve is shown in Figure 2. The cloth liquid was stopped for 10 days from May 22 to June 1.
Fig. 2 uranium metal concentration Ï(U) time curve
The results in Figure 2 show that in the uranium ore heap leaching process, when the concentration of uranium metal in the leaching solution drops to a certain value, the uranium concentration of the leaching solution will increase, which will save manpower and raw materials to a certain extent. Energy, etc., have certain economic benefits.
The leaching solution Ï(U)=130～150mg/L before stopping the cloth liquid, and the concentration of the liquid leaching liquid of the cloth was quickly increased from 170mg/L to 400mg/L. However, the maintenance time was not long and it quickly fell from the high peak. The bacterial leaching test was officially started on July 19, and the leaching solution Ï(U) = 120-140 mg/L before the test, and it is still in a downward trend. After the addition of bacteria, the uranium concentration of the leachate rapidly increased to 170 mg/L, and the uranium concentration was increased by about 30%, and the concentration was maintained for a long time. The curve in Fig. 2 clearly shows the effect of the bacteria.
(B) the effect of adding bacteria on the consumption of sulfuric acid
Sulfuric acid consumption (abbreviated as acid consumption) can be estimated by analyzing and measuring the concentration of sulfuric acid in the infusion and leachate before and after the test. The sulfuric acid consumption is calculated by the difference between the concentration of sulfuric acid in the leach solution and the leachate, that is, the value of the sulfuric acid concentration per liter of the leach after passing through the ore body (for example, Ï(H 2 SO 4 )=5g/L in the leach solution, When the residual sulfuric acid concentration Ï(H 2 SO 4 )=2.1g/L in the leachate, the sulfuric acid consumption is 2.9g/L), and the acid consumption calculation method is as follows:
The average concentration of sulfuric acid in the leaching agent is the ratio of the total sulfuric acid mass to the total solution volume at a certain stage, and the residual acid of the leaching solution is also used as the average value, which is the ratio of the total residual sulfuric acid mass to the total volume of the leaching solution at this stage. Field tests have shown that it takes only 1 to 2 days for the solution to flow through the entire ore body, so the calculation method is basically not affected by the retention of the solution in the ore body.
After preliminary calculation, the average acid consumption before bacterial leaching was 2.9 g/L, and the test period was 1.7 g/L, that is, the acid consumption during the bacterial leaching test was reduced by 1.2 g/L before the test, and the sulfuric acid consumption was saved by 40%. In the previous analysis of the bacterial oxidation mechanism, the idea of ​​leaching with pyrite ore to save sulfuric acid was confirmed from the bacterial leaching field test.
V. Conclusion
(1) The designed bioreactor can meet the requirements of field test. The carrier used can effectively immobilize bacteria. The bacteria can adapt well to the site environment and rapidly oxidize Fe 2+ in the solution.
(2) For the uranium ore used in the test, bacterial leaching can increase the concentration of metal uranium in the leaching solution, and reduce the acid consumption by about 40%, which brings good economic benefits to the enterprise. If the scale of the experiment is further expanded, the bacteria can be fully utilized. The economic benefits will be more obvious.