Method of producing ultrafine ground calcium carbonate

Claim 1

A process for the production of ultrafine ground calcium carbonate having a brightness of 95-97% at 457 nm and wherein up to 90% of the particles are less than about 2 microns in size, said calcium carbonate being suitable as a white filler or coating for paper, the process comprising the steps of: (a) providing a dry crushed calcium carbonate to be slurried; (b) slurrying said calcium carbonate with water at a constant rate in a slurry tank to produce about a 15-60 weight percent solids slurry; (c) dispersing said slurry by adding a polyacrylate dispersant thereto in an amount of about 0.2 to 0.4 weight percent, said dispersant being susceptible to being destroyed by ozone; (d) bead grinding said dispersed slurry in an attrition mill containing about 40 to 60 percent by volume of beads for a sufficient time for the output product slurry of said attrition mill to compromise particles of calcium carbonate having a size wherein about 50 to 70 percent by weight are less than 2 microns; (e) removing said output product from the attrition mill to a classifying centrifuge, adding a flocculent to the output product, and classifying the output product of said attrition mill at an elevated temperature into a product stream having 70 to 90 percent by weight of its particles of a size less than 2 microns, and an underflow stream of predominately larger particles; (f) contacting said output product with about 1.5 to 7 pounds of ozone per ton of dry calcium carbonate contained in said output product for about 10 minutes to 1.5 hours to brighten said calcium carbonate and under conditions sufficient to destroy any dispersant, flocculent, organic matter or microorganisms contained in the mixture and return the product to a non-dispersed state; and (g) dewatering the product resulting from treatment with the ozone to produce a calcium carbonate mixture having a solids content of about 65 to 769 percent by weight and a brightness of 95-97% at 457 nm, and process liquids.