Patent Number: 047626461
Section: description

The apparatus of FIG. 1 comprises a number of vessels all formed of or provided with an inner wall of stainless steel such as INOX 314 or 316. A receiving vessel 1 has a hollow wall 2 to receive and circulate coolant liquid such as water. A pipe 3 connects the outlet 4 of the vessel 1 and a holding tank 5, the pipe 3 incorporating a control valve 6. Each of vessels 1 and 5 incorporates a stirring device 7. A pipe 8 leads from the outlet 9 of the tank 6 to the roof 10 of an atomizer dryer 11 of the type known as F10 or P6 available from NIRO Atomizer, France. A vacuum pump 12 is present in the pipe 8. The dryer 11 has an upper portion 13 of constant diameter and a lower portion 14 of conical shape. A rotary turbine 15 extends downwardly from the roof 10 of the dryer 11 and is arranged to rotate at a speed of about 18,000 to 24,000 revolutions/minute. Air is supplied to an electric heater 16 having a capacity of about 140 KW and the heated air is supplied via a pipe 17 to the dryer 11. A pipe 18 leads from the outlet of the dryer 11 to a first filter 19. The filter incorporates filter elements 20. The lower outlet 21 of the filter 19 leads to a fluidized bed 22 and a side outlet 23 leads to a second filter 24 which leads to a ventilator extractor 25. The exit end of the bed 22 leads to heat unit 26 through which pass solid particles and a thermo-hardenable resin below which is a storage area 27. In use, low level radioactive waste liquid is introduced into the vessel 1. A neutralizing agent, such as a solution of potassium hydroxide in water is added while coolant is circulated through the hollow wall 2 and the stirring device 7 is actuated. The pH of the liquid is monitored until a value of between about 6 and about 8, preferably about 6.7 is attained. The neutralized liquid is then passed to the holding tank 2. Air heated by heater 16 is passed via pipe 17 to the dryer 11. The neutralizer liquid is pumped to the rotary turbine 15 which is rotated at about 18,000 to 24,000 r.p.m. to form droplets within the dryer 11 and the heated air atomizes the droplets to form particles and water vapour which deposits as a powder on the inside wall of the dryer 11. The air then passes the powder to the filter 19 to separate water vapour from the particles which are passed over the fluidized bed 22 to the heater 26 to be encapsulated under vacuum and heat in resin. The method is simple to operate and the apparatus is not prone to corrosion. The volume of the liquid is reduced substantially to provide a satisfactory stable end product of high density and low moisture content. The apparatus shown in FIG. 2 is the apparatus of FIG. 1 mounted on a trailer 30 having wheels 31. The trailer may be moved from site to site so that low level radioactive waste may be treated on site. A radiation proof shield 32 covers the exterior of the apparatus. The invention is further illustrated with reference to the following examples. EXAMPLE I Different components which had been subject to a "swimming bath" contamination were decontaminated electrolytically by reaction with a solution formed from a 50/50% by weight mixture of phosphoric acid and sulphuric acid, and then rinsed. A suspension containing 125 g/l of H.sub.2 SO.sub.4, 125 g/l of H.sub.3 PO.sub.4 and 3.3 g/l of metallic ions was collected and was subjected to the process according to the invention in an installation capable of treating approximately 80 l/h of suspension. The suspension was first neutralized to a pH of6.7 by means of a lixiviate at 450 g/l of KOH, while maintaining a temperature below 90.degree. C. A suspension at 438 g/l total salinity was collected, this was then treated in an atomizer equipped with a turbine rotating at 18,000 r.p.m., on the inside of which circulated an output of air of 980 m.sup.3 /h entering at 450.degree. C. and leaving at 110.degree. C. The filtrate was collected off the filters, and about 35 kg/h of particles of 26 micron mean granulometry, 0.57 density and containing less than 0.05% humidity were collected. The content of gaseous waste particles was less than 0.01 mg/Nm.sup.3. These solid particles were mixed with 15 kg of low-density polyethylene of 300 micron granulometry and the mixture placed in polyethylene packings in which was created a relative vacuum of 250 Pa and which were heated to 130.degree. C. The product to be encasked represented 50 dm.sup.3. EXAMPLE II A solution, representative of low level radioactive waste liquid, was made up as follows: ______________________________________ H.sub.3 PO4 686 g/l H.sub.2 SO4 387 g/l Fe 20 g/l Cr 4.75 g/l Ni 2.8 g/l ______________________________________ 100 ml of the solution was diluted with 100 ml of water and to form a mixture which had a pH of about 0.5. The mixture was neutralized with a solution of potash (1.5 potash beads in 4 parts water) to a pH of 6.5. During the course of neutralization a green crystalline precipitate was formed and this was kept in suspension by simple agitation. The neutralized solution was treated using apparatus according to FIG. 1. The heated air entered in the atomiser dryer at 500.degree. C. and exited at 120.degree. C. The turbine was rotated at 20,000 revolutions/minute and the drying time was about 45 minutes. The dryer was opened, and a powdery deposit about 10% humidity was observed on the lower part of the dryer. After drying the moisture content fell to 3%. The sieve analysis showed that 10% of the product was below 14 micron, 50% below 41 micron and 90% below 86 micron. EXAMPLE III The method of Example I was repeated at an inlet temperature of 425.degree. C. and an outlet temperature of 130.degree. C.; the speed of turbine rotation was 24,000 revolutions/minute and the drying took about 2.5 hours. The sieve analysis showed that 10% of the product was below 9 micron, 50% below 30 micron and 90% below 63 micron. The apparatus of the invention may be cleaned out using demineralized water. Because the method of the invention provides a non corrosive form of the radioactive materials and because the inner lining of the vessels is a stainless steel, there is little or no build up of radioactive material in the apparatus so that it will have a long and safe life.