GB860959A - Process for preparing precipitated products - Google Patents

Abstract

Pigmentary calcium carbonate is precipitated by contacting calcium ions and carbonate ions in an aqueous solution having a pH of at least 8.5 and the resulting precipitate is subjected to the shearing action of a stator and rotor of which the minimum peripheral relative speed is 1160 feet per minute. The precipitation is preferably effective at 25 DEG to 60 DEG C. and the calcium ions may be furnished by milk of lime or calcium <PICT:0860959/III/1> chloride and the carbonate ions by carbon dioxide or sodium carbonate. Crystals produced in this temperature range are elongated in shape and calcitic in structure. If precipitation is effected at - 10 DEG to 25 DEG C., the calcium carbonate crystals are cubic in shape; at above 60 DEG C. the crystals are needle-like in shape and are aragonite in structure. A suitable apparatus is shown in Fig. 2 where a reactor formed by cylindrical side-walls 16 and a hemispherical base 14 is provided with a rotor and stator arrangement generally indicated by 24. A stationary annular disc 32 has an outer and inner peripheral ring of teeth 34 and 36, embracing the upward extending teeth 40 of a rotor member 38 which is joined to the central shaft 44 by pumping blades or vanes 42. A solution bearing calcium ions is fed through a conduit 50 and the solution of carbonate ions is fed through conduit 56 which ends in a gas sparger 60. Product calcium carbonate is withdrawn at 68. Examples refer to the production of calcium carbonate by feeding milk of lime and carbon dioxide gas and to preparation by feeding calcium chloride solution with a solution containing sodium carbonate and sodium bicarbonate. A comparative example illustrates that the reaction between calcium chloride and a sodium carbonate/bicarbonate mixture will result in a calcium carbonate gel if vigorous agitation is not applied.