Studies on CO2 Dissolution in Sodium Silicate for the Precipitation of Silica

Abstract

Rice husk is an agricultural residue abundantly available. Rice husk is a fuel having high ash content; major constituent being silica (90%). Extracting silica from Rice husk ash (RHA) not only adds value but also solves disposal problem of large amount of ash. Conventionally silica is extracted from quartz (silica rock) but this route is expensive and required more energy and RHA is the ultimate choice. Alternately Extraction of silica from rice husk ash involves dissolution of CO₂ after digesting with NaOH. Carbon dioxide is a green house gas and plays dominant role in climate change. To reduce effect of carbon dioxide on the climate and human health, it has to be captured and stored. This work is aimed at understanding the dissolution of carbon dioxide gas in sodium silicate solution and also to finds suitable temperature for dissolution. Raw materials used to produce of sodium silicate solution are Rice husk ash, caustic and carbon dioxide. Producing sodium silicate solution from RHA includes three steps, namely digestion, precipitation and regeneration. Digestion involves mixing of RHA with NaOH and water to produce sodium silicate. Sodium silicate upon infusion with CO_2, precipitates silica as solid and essential sodium carbonate can be treated with calcium hydroxide to get NaOH in the regeneration step. Sodium silicate solution from digestion process was used for study of carbon dioxide dissolution in the sodium silicate solution. For this study an absorption column was constructed of height 1m and Square cross section of 0.20m. Experimental work was carried out to study of carbon dioxide bubble diameter, positioning of bubble and finally acceleration of bubble at different temperature of sodium silica solution over the column height. Carbon dioxide is passed to absorption column through sparger, the moment gas entered into absorption column bubble formation takes place. When it moves up in column, diameter of the bubble and acceleration of the bubble gradually decreases. This shows that diameter and acceleration of bubble decreases with increase in time and bubbles reaches surface of the column and escapes. Frame wise analysis was carried High speed PHOTRON Fast Cam SA4 video camera, from experiments data it was observed that, at a temperature of 50^oC and 60^o C complete dissolution was not possible. For temperature more than 60^oC was observed that the bubble size was reduced to zero by the time it reaches top. This shows that CO₂ was completely dissolved in the liquid. From the experiment 70^oC is ideal for complete dissolution of CO₂ in sodium silicate solution.