US 6,402,824 B1 · 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 CaCO3 pigment to build slurry solids can be reduced or even eliminated by a facile made in a decanting operation within the inventive process flow.

Claims23

Dependent claims

A process for the preparation of a precipitated calcium carbonate (PCC) composition, comprising the steps of wet grinding an aqueous slurry of PCC, said grinding being done in a plurality of separate grinding stages;

partially dewatering said ground PCC slurry in the presence of an organic dispersant in a solid bowl centrifuge;
and subjecting said centrifuged PCC slurry in the presence of an organic dispersant to fluid shearing forces in a rotor and stator mixer.

A process for the preparation of an aqueous slurry of precipitated calcium carbonate (PCC) suitable for paper coating compositions, comprising the steps of:

(a) precipitating calcium carbonate by carbonating an aqueous slurry of calcium hydroxide to produce a PCC slurry comprised of an aqueous. suspension of PCC predominantly in calcite crystal form;
(b) wet grinding the PCC slurry in a plurality of separate grinding stages effective to produce a ground slurry comprising PCC particles having a 75/25 slope value of not more than approximately 1.8, and a median particle size of between approximately 0.25 to 2.0 μm, said grinding being carried out on a slurry that is essentially free of the presence of organic dispersant;
(c) adding an organic dispersant with mixing to said ground PCC slurry, where the ground PCC slurry has a solids content less than approximately 25 wt %;
(d) centrifuging said ground PCC slurry in a solid bowl centrifuge effective to partially dewater said slurry to obtain a filtered PCC slurry having a solids content of at least about 65 wt %;
(e) adding an organic dispersant with mixing to said partially dewatered slurry to provide a dispersed partially dewatered PCC slurry having a Hercules shear viscosity;
and
(f) subjecting said PCC slurry to fluid shearing forces in an in-line rotor and stationary stator mixer effective to reduce the Hercules shear viscosity of said dispersed partially dewatered slurry while maintaining the slope of the PCC within 5% of that present at the completion of step (b).

4 more elements

Dependencies are inferred from the claim text.

Processes for preparing precipitated calcium carbonate compositions and the products thereof

Life of this patent

Processes for preparing precipitated calcium carbonate compositions and the products thereof

Life of this patent

Abstract

Claims23