Mini-Encyclopedia of Papermaking Wet-End
Part Three: Equipment & Unit Operations
The word "calcining" refers to treatment of a mineral product at a very high temperature such as 800 to 1000 degrees centigrade. The word is derived from the process of converting calcium carbonate to calcium oxide. This reaction plays a key role both in the production of precipitated calcium carbonate filler and in the chemical recovery cycle of kraft pulping.
When the term calcination is used together with the word "clay," there is no calcium involved. Rather, the word "calcination" is used only because similar equipment, as used for calcium carbonate, can also be used to prepare anhydrous clay. Intense heating of kaolin clay drives off water of hydration and yields a bright, anhydrous clay product in which individual platelets of the mineral are fused together. Calcined clay is widely used in coating due to its bulking nature and brightness. Calcined clay also is abrasive. With regard to filler addition at the wet end of the paper machine, over the last 20 years economics have favored the shift from calcined clay and other clay products to calcium carbonate products.
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