Processes for preparing precipitated calcium carbonate compositions and the products thereof

Abstract

Processes for making high quality PCC compositions, and the resulting PCC products thereof. A precipitated calcium carbonate (PCC) composition is prepared by a process comprising the step of wet media mill grinding an aqueous slurry of PCC made by carbonation of lime milk, with the grinding being done at a low solids content in a plurality of separate grinding stages, which can done while essentially free of the presence of organic dispersant. After the multi-stage wet grinding step, the ground PCC slurry is partially dewatered in the presence of an organic dispersant in a solid bowl centrifuge. Then, the partially dewatered PCC slurry is subjected, in the presence of an organic dispersant, to fluid shearing forces created in a rotor/stator type mixer. While these three steps are advantageously used together within a single PCC process flow for optimal effect, the individual steps also can be incorporated singly or in lesser combinations thereof into different PCC manufacturing schemes to beneficial effect. The resulting PCC compositions are endowed with reduced high-shear viscosity in aqueous slurry form at high solids content and contain PCC particles of narrower particle size distribution. PCC slurries are provided that are particularly well-suited for paper coatings applications including medium and low coating weight, woodfree paper coatings, although it has wider applicability and is by no means limited thereto. As another aspect, the additional cost otherwise associated with the use of evaporative-based dehydration procedures to increase the PCC solids content in a PCC slurry or associated with the adding of dry CaCO pigment to build slurry solids can be reduced or even eliminated by a facile made in a decanting operation within the inventive process flow.