SYNTHESIS OF CALCIUM FLUORIDE BY DRY METHOD FROM CALCIUM CARBONATE AND AMMONIUM FLUORIDE

Abstract

The proposed scheme for producing CaF2 according to the “dry” method is based on the following stages: hydrolysis of hydrofluoric acid to produce white carbon black and a 12-18% NH4F solution and subsequent production of CaF2 from CaCO3 at a temperature of 200-400 °C in a fluidized bed or drum furnace in a continuous mode, while ammonia from the exhaust gas is sent to the stage of hydrolysis of hydrofluoric acid. In the article the features of synthetic calcium fluoride obtaining by the "dry" method from calcium carbonate and ammonium fluoride are studied. It is shown that the heating of the mixture proceeds with a step-by-step decomposition of ammonium fluoride, and decomposition products interact with calcium carbonate forming a synthetic calcium fluoride. These factors determine the behavior of a solid mixture of calcium carbonate and ammonium fluoride under heating, which allows us to evaluate the kinetic factors of the interaction of reagents and products and of their thermal transformation. Conditions of receiving synthetic calcium fluoride from the waste products of phosphoric acid and synthetic calcium carbonate by “dry” method are determined. The calcium fluoride can be recognized as a basic product in solving the problem of the waste catalyst hydrofluoric acid utilization. In the article, the features of synthetic calcium fluoride obtaining by the “dry” method from calcium carbonate and ammonium fluoride are studied. A synchronous thermal analysis of this mixture was carried out with the analysis of exhaust gases by the IR-Fourier method at samplesheating rates of 1.5 ; 5; 10 °С /min in an air atmosphere. It was shown that heating of this mixture proceeds with stepwise decomposition of ammonium fluoride, the products of which interact with calcium carbonate, form synthetic calcium fluoride. In the first stage, the reactions of calcium carbonate with ammonium fluoride and hydrogen fluoride occur in the kinetic region (Еа = 94.22 kJ/mol), and in the second, the reaction of interaction of CaCO3 and HF (Еа = 30.09 kJ/mol) is limited diffusion of HF to the CaCO3 core in the temperature range of 250-300 °С. These factors determine the behavior a solid mixture of calcium carbonate and ammonium fluoride under heating conditions, which allows us to evaluate the kinetic laws of the process of interaction of reagents and products of their thermal transformation. The conditions for obtaining by “dry” method of synthetic calcium fluoride from waste products of phosphoric acid and synthetic calcium carbonate have been identified, and calcium fluoride can be recognized as the base product in solving the problem of disposal of waste hydrofluoric acid.