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.
