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Chemical and industrial processes requiring the use of lime embrace a widely diversified field, involving causticization, coagulation, distillation, precipitation, neutralization, etc. Such major industries as steel, iron and steel products, paper, water treating, leather, sandlime brick, calcium carbide and cyanamide, sugar, bleach, glue and gelatin, soda, insecticides, fungicides, disinfectants, pigments, etc., employ for the most part (either directly or indirectly) large tonnages of high-calcium quicklimes. Generally such quicklimes contain a minimum of 90 per cent CaO and less than 3 per cent CaCO3. In some instances the quicklime is first hydrated to a dry powder consisting essentially of Ca(OH)2. For all chemical reactions the CaO must first take on the hydroxyl ion, or in other words, CaO is converted into Ca(OH)2. Many processes, therefore, require that quicklime be first slaked with water to form a milk of lime before it can be effectively utilized. Leather chemists maintain that only the Ca(OH)2 actually in solution (not suspension) is readily effective in the removal of hair from hides and otherwise conditioning them for the tanning process. Therefore, many leather chemists require a water-soluble test on lime as a measure of its availability. Also, almost invariably they insist that the milk of lime holds up well in suspension. Realizing that CaO is sparingly soluble in cold water (approximately one part of CaO to one thousand parts of water by weight), no doubt a more or less continued suspension of Ca(OH)2 particles is deemed necessary to facilitate solubility and maintain saturation. Whether or not the Ca(OH)2 must actually go into solution in order to be a readily effective reagent, it is only logical to assume that a continued suspension facilitates chemical reaction through the medium of fine particles having low densities. The settling rates and the final volumes of solids should in a measure at least be indicative of chemical reactivity and serve as a basis for comparing various high-calcium quicklimes.
Washburn, D. E.
Chief Chemist, American Lime and Stone Co., Bellefonte, Pa.