Abstract: A controller for a kiln plant, typically a cement plant, has a thermodynamic controller which measures a number of variables including the kiln hood temperature and one or more output gas concentrations, and controls the fuel input to the kiln to maintain the hood temperature within a desired range and a main impeller of the kiln to maintain the measured gas concentrations within a predetermined range. The invention includes a quality controller wich controls the amount of 3CaO.SiO2 present in clinker produced by the plant. The controller comprises an inner controller which controls free-lime content in the clinker and a thermodynamic outer controller.

Abstract: A method for producing precipitated calcium carbonate by reacting an aqueous solution of calcium nitrate [Ca(NO.sub.3).sub.2 ] with an aqueous solution of ammonium carbonate [(NH.sub.4).sub.2 CO.sub.3 ] and allowing calcium carbonate to precipitate from the resultant mixture containing nitrate [NH.sub.4 NO.sub.3 ] in the mother liquor, the process being characterized in that: (i) the calcium nitrate [Ca(NO.sub.3).sub.2)] solution utilized in the processes is prepared by slaking lime [CaO] in water in the presence of ammonium nitrate [NH.sub.4 NO.sub.3 ] to form calcium nitrate [Ca(NO.sub.3).sub.2 ] and ammonium hydroxide [NH.sub.4 OH] in solution, filtering the solution to render it solids free, and heating the filtrate to dissociate the ammonium hydroxide [NH.sub.4 OH] and to drive ammonia gas [NH.sub.3 ] from the solution; (ii) the ammonium carbonate (NH.sub.4).sub.2 CO.sub.3 solution utilized is prepared by absorbing ammonia gas [NH.sub.3 ] and carbon dioxide gas [CO.sub.

Abstract: A method of producing relatively pure calcium carbonate from a relatively impure source of calcium oxide, characterized in that the method comprises the steps ofslaking the calcium oxide source in water to produce an aqueous hydrated lime slurry;treating the slurry by the addition thereto of a water soluble source of anions which anions are capable of forming a salt with calcium ions which salt has a dissociation constant greater than the dissociation constant of calcium hydroxide;separating the solid content of the slurry from the liquid fraction content thereof to obtain a substantially solids-free solution of calcium ions and anions;intimately contacting the solids-free liquid fraction with carbon dioxide gas at a selected rate to maintain the temperature of the solution within a specific range and to lower the pH of the solution to a pre-set value conducive for the formation of a precipitate of calcium carbonate in which one of the crystalline forms, selected from the group consisting of calcite, vaterite