Patent Number: 055641032
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, a storage tank for a contaminated, depleted ion exchange resin bead slurry feed is shown as 10. This slurry feed can contain from 5 weight % to 70 weight % solids and may originate at a nuclear reactor power plant, hospital or any other governmental or commercial operation where ion exchange beads are utilized to capture contaminants. The slurry feed could also originate in any facility to remove ionic and dissolved impurities in a liquid media, as in water filtration plants and the like. The method of this invention is particularly applicable to a slurry feed containing at least 10 volume % of depleted ion exchange resin beads, and is particularly advantageous for contents of 20 volume % to 70 volume % of resin beads based on total solids content. These beads have a particle size range from 300 micrometers to 1000 micrometers diameter. Most commonly, nuclear grade resin beads have an average particle size of about 400 to 600 micrometers diameter and we have found that they are of generally uniform spherical shape such that 50%, that is at least one half (1/2) of the number of the beads are within .+-.10% from the average particle size of the beads taken as a whole. Therefore, if the average diameter of the beads is 500 micrometers, at least one half of the beads will be in the range of 450 to 550 micrometers. The packing factor or efficiency for these beads was found to be not much more than 0.52--the same as a simple cubic pack, which resists volume reduction. By packing factor is meant the volume of the spheres divided by the cell volume enclosing the spheres and is a reflection of packing density. Body centered cubic lattices have packing factors of 0.68, face centered cubic lattices have packing factors of 0.74, and hexagonal close parked lattices have packing factors of 0.74. Other components of the slurry feed can include ion exchange resin powder, soil, and the like, all of which are easily compressible to volume reductions of 25 volume %. In ion exchange, a solid phase contains bound groups that carry an ionic charge, either positive or negative, in conjunction with free ions of opposite charge that can be displaced. Most ion exchangers currently in large-scale use are based on synthetic resins, for example polystyrene copolymerized with divinylbenzene (to provide the requisite amount of cross linking), and they are permeable only at molecular dimensions, except when a network of coarser pores is deliberately superimposed. Cation-exchange resins can contain bound sulfonic acid groups; less commonly, these groups are carboxylic, phosphonic, phosphinic, etc. Anionic resins can contain quaternary ammonium groups (strongly basic) or other amino groups (weakly basic). Of the two types, the cross linked cation exchange resins are more difficult to process in the grinding step of this invention. Useful resinous materials effective to attract metal cations generally have structures such as a sulfonated, polystyrene divinyl benzene (strong acid); carboxylic acrylics (weak acid); quaternary ammonium gels (strong base); or a polystyrene-polyamine (weak base). In addition, a chelating functionality group may be incorporated into the resin to produce a greater affinity for metals. Any ion exchange bead resin that can capture radionucleides; cationic materials having an atomic weight greater than 22, for example, Na, Mg, Al, Ca, Mn, Co, Ni, Rb, Sr, Zr, Mo, Pb; or anionic materials such as bicarbonate, carbonate, hydroxide, sulfate, chloride, nitrate, phosphate and silica, can benefit from the resin grinding of the invention to better insure volume reduction. Of particular interest as an ion exchange feed are those having nuclear grade properties, such as those sold under the Tradename PUROLITE NRW-100, NRW-600 and NRW-37, of strong acid/cation gel, strong base/anion gel, and mixed bed strong acid gel/strong base gel, respectively. The slurry feed 10, containing the ion exchange resin beads and other solid components is preferably filtered in gross filter 11, to remove any hard objects that could harm grinder blades, and then transferred through piping 12 by pump 13 to blending tank 14. The blending tank will also receive an amount of water to insure a somewhat consistent volume of resin beads into the grinder 16. There should be a steady amount of resin beads solids, rather than an erratic flow of dilute feed followed by very viscous feed so that grinding can be consistent. The blending tank is preferably agitated as by stirring to help provide a homogeneous slurry feed into the grinder. The blending tank volume can range from about 757 liters (200 gal.) to about 15,120 liters (4000 gal). The process can be run continuously, or as a batch operation where the feed is periodically fed from tank 10 to tank 14. Preferably, the concentration of resin beads into the grinder will vary from about 30 volume % to about 50 volume % as determined from a variety of factors, such as whether the beads are the harder to process cation type beads. The homogeneous slurry from the blending tank is then passed into bead grinder 16. Pump 17 is a variable speed positive displacement pump to ensure feedrate control. The grinder will be in an enclosure to contain and eliminate all potential for airborne contamination and will operate in a manner effective to provide a processed slurry with fractured ion exchange beads having a particle size distribution of from 10 micrometers to 1000 micrometers diameters, preferably from 100 micrometers to 300 micrometers diameter. Grinding the beads to below 10 micrometers would interfere with efficient dewatering. It is important that no more than 33% of the fractured beads in the processed slurry have a relatively similar diameter, that is no more than about one third (1/3) of the number of fractured beads are within .+-.10% of the average particle size of the fractured beads taken as a whole. Therefore, if the average ground particle size is 200 micrometers, not more than one third of the fractured beads will be in the range of 180 to 220 micrometers. Ideally there will be an even distribution within the preferred range between 100 micrometers and 300 micrometers, with, for example, 10% at 100 micrometers, 10% at 120 micrometers and so on, as this would allow close random packing, allowing packing factors from 0.62 to 0.93. Other components of the processed slurry will also be ground finer but that is of minor importance to this invention which is mainly concerned with providing a compressible particle size range for the normally incompressible ion exchange beads. The grinding means can be of any type with an adjustable gap or grinding setting that can handle both high and low solid slurries of plastic like materials in a continuous fashion. Once the beads are ground the water coating on the fractured particles still serves to cause repelling action between the particles and such water must now be removed. The dewatering system can be of any type but is preferably of the type which includes a shell for providing primary containment for the slurry compression means in the shell and surrounding the slurry for compressing fluid from the slurry, filter means disposed in the shell for containing the slurry and for filtering the fluid compressed from the slurry, and vacuum means connected to the filter means for suctioning the fluid from the filter means and for producing a pressure differential across the compression means and for collapsing the compression means about the filter means and the slurry for compressing the slurry. U.S. Pat. No. 5,143,615 (Roy et al.), herein incorporated by reference, can be referred to for a complete description if such a dewatering system. Compressing dewatering alone can allow about a 5 volume % reduction, but when used in conjunction with prior grinding, the total effect can be a 20 volume % reduction, or higher. After dewatering, the effective result of this invention will be an approximate 20 volume % reduction in the final waste volume or an approximate 20 weight % increase of spent bead resin in a standard waste container. The process provides: minimal material hold-up to reduce radiation exposure to operating personnel; a totally enclosed, liquid slurry grinding process which eliminates all potential for airborne contamination; a process capable of handling wide swings in process flow rate and slurry concentration as are typically seen in nuclear plant radwaste transfer systems; and a process which minimizes the generation of particles &lt;10 micrometers which would interfere with efficient dewatering. Water or other fluid recovered in the dewatering process can be fed through line 20 for further processing. The compressed material, including dewatered, contaminated, depleted, fractured, ion exchange resin beads and other components, can be fed through line 22 to a suitable waste container system 24.