text: It is known to remove impurities from a liquid by passing the liquid through a filter screen that has been precoated with a thin layer of finely divided exchange resin particles. In U.S. Pat. No. 3,250,702 a method is described wherein a mixture of anion and cation exchange resin in the size range 60 to 400 mesh is precoated on a filter screen. The resins are combined in aqueous suspension causing the resins to agglomerate or "clump" with one another to form larger particles. In so doing, a greater void space is provided in the precoat layer of finely divided resin particles so that there is less resistance to flow of liquid through the precoat layer, and therefore a lower pressure drop across the filter.
  In U.S. Pat. No. 4,190,532 a method is described for removing impurities from a liquid by passing the liquid through a precoat layer which has been deposited on a filter screen. The precoat layer consists of a treated filter aid material mixed with ion exchange resin particles in the size range of 60 to 400 mesh. The filter aid material, characterized by a negative surface charge in aqueous suspension, is treated with an electrolyte-type compound that produces a positive surface charge thereon. The mixture of treated filter aid material and ion exchange resin particles produces a clumping phenomenon similar to that achieved in accordance with U.S. Pat. No. 3,250,702.
  In U.S. Pat. No. 4,238,334 a method is described for removing impurities from a liquid by passing the liquid through a precoat layer which has been deposited on a screen. The precoat screen consists of a treated fibrous filter aid material and an active particulate material. The treated filter aid material and the active particulate material have opposite surface charges in aqueous suspension, and the mixture produces the aforementioned clumping phenomenon. The filter aid material is treated with an electrolyte-type compound that produces a surface charge opposite to the normal surface charge of the filter aid material.
  It has heretofore been known to utilize small quantities of powdered anion exchange resin in the hydroxide form to enhance flocculation of hydrolyzed polyester-based precoats as described in U.S. Pat. No. 4,474,955.
  The above discussed liquid treatment methods have been widely commercially utilized in thin precoat layers (typically less than about one inch) for removal of traces of impurities from water and chemical process streams. However, because of pressure drop limitations, the utilization of such precoat materials in thicker layers or beds to remove higher concentrated impurities has heretofore not been deemed commercially viable. Further, it has been the heretofore practice to dispose of the thin precoat layers after each use rather than to regenerate it for subsequent use, as it has heretofore not been considered possible to maintain flocculation characteristics after regeneration.
  There are numerous instances of the use of ion exchange and adsorptive processes for recovering and purifying various pharmaceutical, medicinal and biological substances. These processes primarily involve separation and concentration techniques. Since most of the substances being recovered and purified are of high molecular weight, small particles of ion exchangers and adsorbents are preferred because of kinetic considerations. In most instances, the kinetic process involved in the adsorption and elution of high molecular weight species is controlled by particle diffusion which follows the relationship in which the rate is inversely proportional to the square of the particle diameter. Since the relationship between diffusion rate and molecular weight is also an inverse relationship, it is apparent that fine particles are required for the processing of many pharmaceutical products by ion exchange or adsorption.
  In small scale operations, there are essentially little problems associated with the use of fine particles of ion exchange resins. For example, 5-10 micron particles of ion exchange resins have been routinely used for a myriad of analytical procedures. On a small scale, the hydraulic problems associated with the columnar performance of fine particles is of little consequence. However, scale-up of these laboratory systems based upon fine particles is quite difficult because of the high pressure drop involved.