Equipment Description:Dispersion, Makedown and Mixing
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Coatings are applied to paper at high solids content. Coating pigments can either be supplied as a high solids dispersed suspension, usual for calcium carbonates, or as a dry or semi dry powder or lump product (often for kaolin or talc). In the latter case it is necessary to prepare a high solids suspension in the mill, a process which is referred to as makedown. |
The process of making down a mineral powder in water involves wetting, dispersion and mechanical forces. The equipment used is a mixing vessel with a carefully designed rotor or impeller. The finished suspension is referred to as a slurry. Addition of the other coating ingredients to the slurry turns it into a coating colour. The resistance to flow exhibited by the slurry is referred to as its viscosity. This is a critical property of the slurry since further handling using pumps depends on the viscosity being within a critical range. |
 Diagram courtesy of Dr John Husbands, Imerys Minerals Ltd. |
The requirements needed to prepare a slurry of kaolin or talc in water are : (a) that the particles are wetted by the water , (b) that aggregates are broken up by mechanical energy, and (c) that the particles are separated from each other and remain so during storage and use. For kaolins the wetting requirement is met naturally. For talcs, which have a hydrophobic surface, a wetting agent is required. Both minerals also require the addition of a dispersing agent, usually sodium polyacrylate or a polymeric phosphate salt. The dispersant dose is determined by adding small increments of dispersant to a slurry and measuring the viscosity. The viscosity value will show a minimum at the optimum dose. The slurry pH should be adjusted to 6.5 to 7.5 with NaOH for optimum dispersion. Following makedown the slurry needs to remain stable during storage for periods up to a few weeks. There should be minimal sedimentation to form a residue, and thickening must also be absent. |
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