Uranium mining and metallurgical wastewater mainly comes from two parts: ore mining and uranium ore processing, including pit water, adsorption tailings, resin washing water, and precipitation mother liquor.
According to the difference of leaching medium, it can be divided into acidic and alkaline wastewater. In addition to radionuclides such as uranium, thorium and radium, acidic wastewater also contains non-radioactive nuclides such as mercury, cadmium, arsenic, lead, copper, zinc and manganese. .
Due to the selective dissolution of carbonates in alkaline wastewater, iron, aluminum, titanium, etc. are hardly dissolved, and the leaching solution only contains a small amount of molybdate, silicate, vanadate, phosphate and carbonate complexes. The radionuclide thorium is also insoluble in the alkaline leaching process, while radium dissolves by 1.5% to 3.0%.
Therefore, for alkaline leaching uranium mines, the main pollutants in wastewater are radionuclides uranium and radium.
A uranium mine adopts the alkaline leaching process. The existing process wastewater is mainly composed of four parts: mine water, adsorption tail liquid, precipitation mother liquor and resin washing water.
Wastewater is treated by pyrolusite removal – ferric chloride flocculation precipitation removal uranium process. Since the loaded resin adopts an alkaline sodium chloride solution leaching process, the lean resin is not transformed, resulting in a high Cl- concentration in the wastewater. CO32- and Cl- coexist in wastewater, the existing wastewater treatment system has poor uranium removal effect, and it is difficult to achieve discharge standards.
After experimental research, a process flow of lime alkalization – ferrous sulfate neutralization – barium chloride removal of radium – sludge recycling to treat alkaline wastewater is proposed.
1.1 Source and composition of wastewater
The experimental wastewater is the mixed wastewater of a uranium mine mine water, adsorption tail liquid, precipitation mother liquor, and resin-loaded washing water. The main components are shown in Table 1.
1.2 Test method
Take 0.5L of waste water, add lime milk with a mass fraction of 50% to adjust the pH to above 12, filter the slurry, and analyze the U and CO32- mass concentrations in the filtrate; then add ferrous sulfate to the filtrate and stir for 2h, then add barium chloride to continue stirring 0.5h, after sedimentation and clarification, the U mass concentration and Ra activity concentration of the supernatant were determined.
1.3 Analysis method
Determination of major uranium by ammonium vanadate titration; 2-(5bromo-pyridylazo)-5-diethylaminophenol spectrophotometric determination of trace uranium; determination of radium by radon sparging method; titration with EDTA standard solution Determination of calcium; determination of CO32- with standard hydrochloric acid solution titration.
2 Test principle
The main pollutants of alkaline wastewater are uranium and radium. CO32- and UO22+ have strong coordination ability (k=2×1018), and the generated UO2(CO3)34- is relatively stable, making it difficult for uranium to be removed by adsorption carrier. Therefore, the coordination effect of CO32- should be eliminated first, and the CO32- and HCO3- should be quantitatively converted into OH- with Ca(OH)2, and CaCO3 precipitated to be removed. The main reaction is as follows:
Compared with ferric sulfate, ferrous sulfate is cheaper than ferric sulfate. It is selected as a neutralizing agent. Fe2+ is oxidized and hydrolyzed under the action of air to form Fe(OH)3 precipitation, and slowly releases acid to neutralize excess OH-, so that wastewater can be discharged out. pH standard; the generated Fe(OH)3 precipitate is positively charged, which has a good adsorption effect on uranyl complex ions, and achieves the purpose of deep uranium removal.
In addition, the addition of ferrous sulfate supplements the SO42- required for the radium removal process. The main reaction is:
The addition of barium chloride reacts with SO42- in wastewater to form BaSO4 precipitation. Since Ra2+ and Ba2+ have similar ionic radii, during the process of generating BaSO4 precipitation, Ra2+ enters the lattice to form Ba(Ra)SO4 co-precipitation. The main reaction is:
Test results and discussion
3.1 Effect of lime dosage on CO32- removal
Different amounts of lime were added to remove CO32- of wastewater, and the mass concentrations of U, CO32- and Ca2+ in the filtrate were measured. The test results are shown in Table 2.
It can be seen from Table 2 that when lime removes CO32-, CaCO3 precipitation is formed to carry most of the uranium, reducing the burden of deep uranium removal in the subsequent process. The minimum dosage of Ca(OH)2 was determined to be 1.1 times the stoichiometric amount, taking the reduction of ρ(CO32-) below 20mg/L as the minimum dosage.
3.2 Effect of ferrous sulfate dosage on uranium removal
The amount of lime was 1.1 times the stoichiometric amount, and different amounts of FeSO4·7H2O were added to carry out the neutralization test, and the pH of the supernatant and the mass concentration of uranium were measured. The test results are shown in Table 3.
The test results show that the uranium concentration decreases gradually with the increase of FeSO4·7H2O dosage. When the dosage reaches 2.0g/L, the uranium mass concentration is lower than 0.05mg/L, which meets the wastewater discharge standard. Taking into account the pH requirements of the discharged wastewater, the mass concentration of FeSO4·7H2O should be greater than 5.0g/L.
3.3 Influence of barium chloride dosage on radium removal effect
Ferrous sulfate neutralizes the wastewater to make the pH drop to about 8, then adds different amounts of barium chloride for stirring, and analyzes the radium activity concentration of the filtrate. The test results are shown in Table 4.
It can be seen that with the increase of the amount of barium salt, the radium activity concentration in the wastewater gradually decreases. When the mass concentration reaches 60mg/L, the radium activity concentration in the wastewater can be reduced to 0.65Bq/L.
Therefore, using lime alkalization – ferrous sulfate neutralization – barium chloride to remove radium to treat wastewater, the mass concentration of barium chloride is 60mg/L, and the treated wastewater can be discharged up to the standard.
3.4 Validation test of wastewater treatment
A comprehensive verification test was carried out on the wastewater treatment effect. The test conditions were as follows: the amount of Ca(OH)2 was 1.1 times the stoichiometric amount, the mass concentration of FeSO4·7H2O was 2.0g/L, and the mass concentration of barium chloride was 60mg/L. The test results are shown in Table 5.
The results of parallel experiments on wastewater treatment showed that the mass concentration of uranium in the treated wastewater was lower than 0.05mg/L, and the average activity concentration of radium was 0.48Bq/L, which were all lower than the wastewater discharge standard.
3.5 Sludge Circulation Volume Reduction Test
The volume of slag produced by ferrous sulfate neutralization is large, mainly because the water content of the slag is too high.
The water content of the sludge is composed of 4 parts: void water, surface adsorption water, capillary water and internal water, of which void water accounts for about 70%. Obviously, the main purpose of reducing the volume of sludge is to remove void water.
Ferrous sulfate and barium chloride are sequentially added to the filtrate obtained by alkalization of lime for stirring, and then allowed to stand for about 22 hours, the volume of the slurry is measured, and the supernatant is poured out to complete a cycle. In the next cycle, add lime alkalization filtrate to the slurry obtained in the previous cycle, and repeat the above operation process. The test results are shown in Table 6.
The results in Table 6 show that by using the method of slag recycling, the void water between the slag is continuously removed, so that the volume of the slurry is significantly reduced, and the sedimentation speed of the slag is accelerated, which is beneficial to the filtration operation and the realization of trough discharge. The yield of sludge obtained after recycling was 5.7 g/L. After the cycle, the pH of the wastewater decreases, and it can be considered to reduce the amount of FeSO4·7H2O to save the cost of wastewater treatment.
1) The use of lime alkalization-ferrous sulfate neutralization and deep uranium removal-barium chloride removal of radium-slag recycling volume reduction process can reduce the mass concentration of uranium in wastewater to below 0.05mg/L, and the concentration of radium activity to 1.0 Below Bq/L, the treated wastewater can be discharged up to the standard.
2) Three precipitants of lime, ferrous sulfate and barium chloride are used in turn, among which lime alkalization removes most of the uranium, while ferrous sulfate has both neutralization, deep uranium removal, supplementary removal of radium required SO42- and inhibition There are 4 functions of sediment back-dissolution to achieve the best treatment effect of alkaline uranium-containing wastewater.
3) The slurry circulation operation can improve the filtration and sedimentation performance of the sludge, and improve the processing capacity of the process equipment.