Research on the removal of radium from uranium effluent by air-aeration hydrated manganese hydroxide adsorption
3.4. Comparison with other methods
As the experiments above show, removal of radium from uranium effluent by air aeration hydrated manganese hydroxide adsorption is one way to treat uranium effluent. Its main advantage is to use the manganese ions in the effluent to treat the effluent, i.e. using the waste to treat the waste.
To further evaluate this method, comparative systems are employed as the reference. Under the Coordinated Research project (CRP) agreement with the Agency, the following work was done. They are: (i) neutralize effluent without aeration at pH 5, 8, and 11, in three tanks:
duration 1 hour per tank; (ii) neutralize effluent with aeration at pH 5, 8, and 11, in three tanks:
duration 1 hour per tank; (iii) neutralize effluent with barium chloride at pH 5, 8, and 11, in three tanks: duration 1 hour per tank.
The original effluent pH is 1.54, U = 11.8 mg/l, Ra = 32 Bq/l, SO42– = 3.36 g/l, Mn2+= 89.25 mg/l. The solution pH is adjusted by 10% lime cream. The test results are shown in Tables 14, 15, and 16 and Figures 5, 6, and 7.
Table XIV. Neutralize effluent without aeration at pH 5, 8, 11 by lime milk pH
Conditions
5 8 11
Stirring time, min 60 60 60
Classification time, h 4 4 4
U, mg/l, processed effluent 0.88 0.3 0.3 Removal efficiency rate, U 95.5% 97.5% 97.5%
Ra, Bq/l, processed effluent 24 10 8.6 Removal efficiency rate, Ra 25% 68.75% 73.13%
Table XV. Neutralize effluent with barium chloride at pH 5, 8, 11 pH
Conditions
5 8 11
Stirring time, min 60 60 60
Classification time, h 20 20 20 Ra, Bq/l, processed effluent 2.1 1.4 0.81 Removal efficiency rate, Ra 93.44% 95.63% 97.47%
Table XVI. Neutralize effluent with aeration at pH 5, 8, 11 pH
Conditions
5 8 11
Mn2+, mg/l 300 300 300
Aeration time 60 min 60 min 60 min
Classification time, h 4 4 4
Ra, Bq/l, processed effluent 13 Bq/l 6.8 Bq/l 3.6 Bq/l Removal efficiency rate, Ra 59.38% 78.75% 88.75%
Classification time, h 48 48 48 Ra, Bq/l, processed effluent 13 Bq/l 6.5 Bq/l 2.7 Bq/l Removal efficiency rate, Ra 59.38% 79.68% 91.56%
Classification time, 7 days 7 days 7 days Ra, Bq/l, processed effluent 13 Bq/l 5.3 Bq/l 1.0 Bq/l Removal efficiency rate, Ra 59.38% 83.44% 96.87%
Classification time, 15 days 15 days 15 days Ra, Bq/l, processed effluent 13 Bq/l 4.8 Bq/l 0.31 Bq/l Removal efficiency rate, Ra 59.38% 85% 99.03%
0 5 10 15 20 25 30 35 40
0 5 10 15 20 30 60 120 180 240 Times, minutes
Height,cm pH=5
pH=8 pH=11
IG. 5. The settling curve by alkali deposit.
0 5 10 15 20 25 30 35 40
0 5 10 15 20 30 60 120 180 240 Time, minutes
Height, cm
pH=5 pH=8 pH=11
FIG. 6. The settling curve by barium chloride.
F
0 5 10 15 20 25 30 35
5 10 15 20 30 60 120 180 240 Time, minutes
Height, cm
pH=5 pH=8 pH=11
FIG. 7. The settling curve by air aeration hydrated manganese hydroxide.
Experiment results in Tables 14, 15 and 16 indicate that removal of radium from uranium effluent by air aeration hydrated manganese hydroxide adsorption can effectively remove radium from effluent. Under the experimental condition, the aerated effluent needs much long time for settling. As Table 16 shows, in seven days, the radium in the processed effluent can reach 1.0 Bq/l which meets the industrial discharge standard. In the real effluent, the higher concentration of aluminum, silicon and magnesium can greatly influence the removal efficiency (please see 3.1.4). Air aeration forms much smaller, stickier alkali particle of the hydrated manganese hydroxide, which needs long time for settling. Under the comparative tests, neutralizing by lime and barium chloride is effective too. The clarification time is shorter, but still needs 20 hours for settling. At all the experimental conditions, uranium in the effluent can be easily removed. It can be seen from Figures 5, 6 and 7, the higher the pH of the effluent, the longer the settling time. This is because under the experimental conditions of stirring and air aeration, an alkali gel solution is formed.
3.5. X-ray diffraction diagrams of hydrated manganese hydroxide, and two samples of conventional uranium tailing sludge, 1# and 2# are shown in Figures 8, 9 and 10. It is shown that the amorphous hydrated manganese hydroxide exists in the lime precipitation-air aeration sludge.
4. Conclusions
The advantage of lime precipitation-air aeration-hydrated manganese hydroxide adsorption to process uranium effluent is application of the waste ions in effluent to treat the effluent. This is beneficial for the environmental protection. And it is also an effective method to process the uranium effluent. The following points can be drawn from research and experiments:
FIG. 8. X-ray diffraction diagram of hydrated manganese hydroxide, Mn Content: 5.6%.
FIG. 9. X-ray diffraction diagram of conventional tailing sludge, 1#, Mn content: 0.38%.
FIG. 10. X-ray diffraction diagram of conventional tailing sludge, 2#, Mn content: 0.4%.
1. The effluent pH of the air aeration to produce hydrated manganese hydroxide to adsorb radium and remove other harmful elements must be adjusted to about 11 by lime milk.
Manganese ion concentration in effluent between 100–300 mg/l can meet the requirement to remove the radium. The contents of manganese depend on the amount of radium in original effluent and other impurities.
2. The longer the air aeration time, the better the result for radium removal because as aeration time increases, production rate of hydrated manganese hydroxide is raised.
Normally 30 minutes of air aeration time can meet the requirement.
3. Impurity elements of aluminum, silicon, and magnesium have a great effect on the lime precipitation-air aeration-hydrated manganese hydroxide to process acidic uranium effluent. This is a big disadvantage. If higher contents of these elements exist in effluent, longer time for clarification is needed or two steps are required. First step to remove the impurity from the effluent by lime adjusting pH to 8, second step to remove radium by air aeration at pH about 11.
4. The longer clarification time benefits the effluent process. The clarification time is related to impurity levels in the effluent. For low concentrations of aluminum, silicon and magnesium in effluent, 1~5 hours can meet the requirement. For higher concentrations, the clarification time may be as long as 8–9 days.
5. Under the experimental conditions, the radium adsorption by hydrated manganese hydroxide sludge produced with lime precipitation-air aeration process is stable. There is no obvious release of radium from the sludge. If the processed alkaline effluent is stored in a tailings dam, there is no need to neutralize the effluent. It will be neutralized by adsorbing CO2 from the air. This will save reagent cost.
6. pH has a great influence on the adsorption of radium by the hydrated manganese hydroxide complex. The adsorption pH choice can be between 6.5 and 9. At this effluent pH, the adsorption residence time can be 5 minutes. Also, the hydrated manganese
hydroxide complex shows good re-adsorption ability at radium removal in uranium effluent.
7. Compared with neutralizing effluent without aeration and the barium chloride process at pH 5,8, and 11, removal of radium from uranium effluent by air aeration hydrated manganese hydroxide adsorption can effectively remove radium from effluent. Of course, neutralizing by lime and barium chloride deposit is effective too. The higher the pH of the effluent, the longer the time for settling.
ACKNOWLEDGEMENTS
The authors wish to thank Li Yanping for X-ray diffraction analysis for this project and Xu Jiazhong for supporting this project.
REFERENCES
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[2] CHENG, SHIAN., LIU, JIAN., YU, KUI., Treatment of wastewater in tailings dam of a uranium mill by lime precipitation-air aeration, Uranium mining & Metallurgy, 2(2), (1983) p. 52.
[3] NIRDOSH, I., MUTHUSWAMI, S.V., BAIRD, M.H.I., Radium in uranium mill tailings - some observation on retention and removal, Hydrometallurgy 12, (1984) p. 151–176.
[4] NIRDOSH, I., TRENBLEY, W.B., JOHNSON, C.R., Adsorption-desorption studies on the
226Ra-hydrated metal oxide systems, Hydrometallurgy 24, (1990) p. 237–248.