DEPLETED SOLUTION

The cake is dissolved in nitric acid solution, a filtration is required, then the aqueous nitric solution is corrected to pH = 2.5 and uranium is precipitated as a high grade product with hydrogen peroxide. A continuous adjustment of pH is required to be carried out with ammonia otherwise pH fluctuates (decreases).

More than 95% of existent uranium in orthophosphates has been eliminated and recovered. A less costly alternative to this process is based on U(VI) reduction to U(IV) in WPA by iron scrap. In this case uranium is precipitated with heavy metals as phosphates and is separated at filtration stage. However, its content is now only 0.2-0.4% U but not at all negligible. This method seems also attractive.

In these two processes uranium is eliminated from STPP. ²²⁶Ra is also absent since it was removed at sulphuric acid attack of phosphate rock and was carried on CaSO42H2O.

3.2. Phosphoric acid production by solvent extraction (WPA purification)

A plant was built in Romania to produce phosphoric acid destined as ingredient in foodstuffs as such or as salts. The process uses as raw material WPA produced by sulphuric acid attack of phosphate rock. A solvent extraction method was considered to extract only the phosphoric acid leaving the rest of impurities in the aqueous phase. The initial WPA was clarified and purified of various undesirable impurities (F^-, SiO2, organic matter etc).

A final purification and separation is carried out with an organic solvent, which extracts only H3PO4. In Romanian process butanol was the choice as solvent. The phosphoric acid is the only extracted component. After separation of the two phases the solvent is distilled, condensed and recirculated while phosphoric acid of high purity is left unchanged.

The aqueous phase (the raffinate) left at extraction stage having a smaller volume held all inorganic impurities at higher concentration than in original WPA. However our measurements have shown that only half of uranium is found in the aqueous phase the rest of 50 mg U/L was extracted by butanol the same time with H3PO4 and at distillation stage uranium was left in H3PO4. Therefore our determinations have shown that Uranium presence in H3PO4 intended for use in foodstuffs cannot be tolerated and the plant was shut down.

3.3. Phosphogypsum obtained in phosphate fertilizer industry

In our previous papers [1,2,3,11] on phosphate fertilizer radioactivity it was shown that almost all ²²⁶Ra was carried by CaSO42H2O in the process of sulphuric acid attack of phosphate rock.

Phosphogypsum is obtained in large amounts since per each ton of phosphate reacted with sulphuric acid, 1.5 tons phosphogypsum wastes have resulted.

There are 4 fertilizer plants in Romania processing each 330 000 t/y phosphates by sulphuric attack therefore 500 000 t/y phosphogypsum is obtained in each case. These plants were in operation 20–30 years and phosphogypsum accumulated is of the order of 5–7 millions tons deposited around the plant creating big problems, due to the fact that dump site is in the vicinity of big settlements on a radius of 1–3 km. One plant is near an important Black Sea Resort. Strong winds are spreading phosphogypsum powder on large areas the same time with²²²Rn.

Our measurements have established that ²²⁶Ra content of phosphogypsum of sedimentary origin has average values 600-1000 Bq/kg exceeding the permitted limits mentioned last year

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at IAEA of 1 Bq/kg. Uranium content of 10–20 mg/kg is also exceeded compared with maximum 1 mg/kg allowed.

Phosphogypsum of volcanic origin has Ra activity (more exact from Thorium descendents) of 150 Bq/kg. Phosphogypsum was largely used as soil amendment and as raw material in building industry. The current regulations in many countries (USA) have prohibited its use therefore its accumulations and the problems are always present.

Many years ago a plant was built in Romania to process phosphogypsum in order to make it acceptable as wallboards. Some flats were built using wallboards for interior rooms. Our measurements have shown that ²²⁶Ra radioactivity of these walls was 300-400 Bq/kg and

222Rn concentration higher than in normal rooms. Therefore the plant was shut down and the use of this material forbidden.

Our studies regarding an eventual radium abatement in phosphogypsum starting from hydrocyclone processing separation led to a fine fraction of 700 Bq/kg and a coarse fraction of 200–300 Bq/kg each representing approx. 50%.

Chemical treatment of phosphogypsum to obtain either (NH4)2SO4 which is a fertilizer or Na2SO4 by conversion with corresponding carbonates cannot solve the problems due to large amounts of phosphogypsum.

The only alternative to phosphogypsum use is in our opinion restoration works of dumping sites to eliminate the radioactive contamination of the environment. In other countries [11, 12]

due steps were taken to avoid its impact on environment but in Romania no such works were involved.

At present it is considered that Radon evolution is only possible from superior layers of phosphogypsum perhaps 3–4 m depth. The inferior layers contribution of Radon evolution is minimized. Therefore a dump site levelling off and coverage with various protection layers is envisaged. A drain system is to be taken into account.

The great problem is the quantity of 150 million tons phosphogypsum accumulated worldwide each year. At present a practical use is not allowed neither in agriculture nor in building industry it is no longer dumped in rivers or sea due to radioactivity accumulation of radioactive decay products in marine life.

REFERENCES [1] BUNUS, F.T., Talanta 24 (1977) 117.

[2] BUNUS, F.T., DUMITRESCU, R., Hydrometallurgy, 16 (1986) 167.

[3] BUNUS F.T., DUMITRESCU, R., Hydrometallurgy, 28 (1992) 331.

[4] CIOROIANU, T.M., BUNUS, F.T., FILIP, D., FILIP, Gh., Environmental considerations on uranium and radium from phosphate fertilizers, Proceedings of IAEA Technical Meeting, Vienna, Sept. (1998).

[5] BUNUS, F., FILIP, G., CIOROIANU, T., FILIP, D., Uranium recovery from phosphate fertilizer industry in the form of a high purity product, Proceedings of IAEA Technical Meeting, Vienna, June (1997).

[6] BUNUS, F., MIU, I., DUMITRESCU, R., Hydrometallurgy, 35 (1994) 375.

[7] TULIU, D., NITULESCU, I., Mine, Petrol, Gaze 2,6,8 (1975) 391.

[8] FILIP, Gh., COLDEA, I., TATARU, S., VACARIU, V., Romanian Patent 76707 - (1981).

[9] BUNUS, F.T., from Pollution control in fertilizer production, hodges - Popovici editors, Marcel Dekker, New York (1995).

[10] STOLTZ, E.M. Jr., Proceedings of International Conference on Peaceful uses Atomic Energy 3 (1958) 234.

[11] JUMPEI ANDO, CAMERON J.E., from Pollution Control in Fertilizer Production, Hodge - Popovici Editors, Marcel Dekker, NY, (1995).

[12] CAMERON, J.E., O’CONNOR, J.J., Design and operating criteria for gypsum stack, Proceedings of the 2^nd International Symposium on Phosphogypsum, Miami, USA, (1986).

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