Abstract: Calcium sulfate is regenerated to lime at a relatively low temperature and in a short time. In a method of regenerating lime by reducing calcium sulfate by flowing a reducing gas containing 0.5 to 20% by volume of CO and 0.03 to 30% by volume of CO2, the reduction is carried out in the presence of an aid selected from the group consisting of Fe2O3, MgSO4 and mixtures thereof at a temperature of 900 to 1000° C. In cases where the aid is Fe2O3, the amount thereof is 0.2 to 10% by weight based on the weight of the calcium sulfate in terms of anhydrous calcium sulfate. In cases where the aid is MgSO4, the amount thereof is preferably 10 to 50% by weight based on the weight of the calcium sulfate in terms of anhydrous calcium sulfate.

Abstract: A process for producing hydroxyapatite fibers is disclosed. The process comprises the steps of preparing a mixture of (1) calcium metaphosphate fibers and (2) calcium oxide, calcium hydroxide and/or calcium carbonate powder; heating the mixture to convert the calcium metaphosphate to hydroxyapatite; and removing calcium oxide in the heat-treated mixture by dissolving off from the resulting mixture.

Abstract: Disclosed is a novel process of producing plate-shaped calcium carbonate particles. The process comprises the steps of adding aqueous calcium hydrogen carbonate solution with a temperature of less than 50.degree. C. into an aqueous medium bath held at a temperature of not less than 50.degree. C. to generate plate-shaped calcium carbonate particles in the bath, the temperature of the bath being kept not less than 50.degree. C. during the addition of the aqueous calcium hydrogen carbonate solution; and recovering the generated plate-shape calcium carbonate particles.

Abstract: Disclosed is a novel process of producing needle-shaped calcium carbonate particles. In the process of the present invention, aqueous calcium hydroxide solution is added to an aqueous medium bath with a temperature of not less than 60.degree. C. into which carbon dioxide gas or a carbon dioxide-containing gas is being blown to generate needle-shaped calcium carbonate particles. The temperature of the aqueous medium bath is kept not less than 60.degree. C. during the addition of the aqueous calcium hydroxide solution. Then the generated needle-shaped calcium carbonate particles are recovered.