Recovery of calcium carbonate from lime-soda process

Abstract

Economy of the production of sodium hydroxide by lime soda process in the context of Bangladesh, depends largely on the recovery of a good quality CaC03 from the sludge formed during the causticization reaction: Ca(OH)2 + Na2C03 = 2NaOH + CaC03. However, there is no published information available on the recovery of CaC03 from the sludge. The present study aims to develop a process scheme for the recovery of CaC03 from this sludge. Furthermore, it was planned. to study the effect of lime quality on the optimum reaction time for causticization of soda ash, settling time for the slurry from causticization, and characteristics of the recovered calcium carbonate. Effect of the temperature on the drying time for the recovered CaC03 cake was also studied. Causticization of soda ash solution was carried out in laboratory scale with three different grades of lime at 90°C and atmospheric pressure. The three grades of lime were designated as AR grade, Analytical grade and Commercial grade lime containing 96%, 90% and 59% Ca(OHh respectively. To gain an understanding of the optimum time for reaction, conversion data as a function of time for reaction were obtained. The optimum reaction time for all three grades of lime was approximately 2 hours, while percent conversion depended on the purity of the lime and varied from 75 to 80 percent. Study of the setthng behavior of the slurry from causticization reactor revealt:d that settling of slurry from commercial grade lime was coinpleted after 80 minutes, while that from analytical grade lime required more than 3 hours. Conventionally in lime soda process after causticization caustic soda solution is recovered by decantation and filtration and the filter cake containing CaC03 is reburnt to recover Ca(OH)2 for the process. For making the process economically feasible a scheme was developed for the recovery of CaC03 from the filter cake. The scheme basically utilizes recovery stages. Each recovery stage consists of washing of filter cake with water, further causticization by heating half an hour and subsequent vacuum filtration. It showed that including up to three recovery stages the conversion of lime to sodium hydroxide increased and at the same time the alkalinity of the recovered CaC03 decreased. Drying data for the finally recovered CaC03 cake were obtained in the temperature range 120°C to 220°C. At 120°C drying completed after 2 hours while at 220°C it took only 30 minutes to dry the filter cake. Color of the CaC03 was observed visually. No appreciable discoloration was noticed within this temperature range. It was evident that the color of the recovered CaC03 is purely dependent on the color of the lime which is used for causticization. Finally characterization of the recovered CaC03 was done. Carbonate content of the recoveredCaC03 from commercial grade lime is 87% and impurities in this powder is 10.5%. The pH of the saturated solution of this CaC03 is 10.2. This product may tlnd use as filler in plastic and rubber industries.