This paper describes a process to desilicate either Bayer pregnant liquor or Bayer spent liquor supersaturated in silica, it applies also to other sodium aluminate solutions, silica supersaturated.
Silica in Bayer liquor is an undesirable impurity for two main reasons: first, it co-precipitates with alumina trihydrate and contaminates the product; second, it precipitates as sodium aluminum silicate (sodalite/cancrinite, desilication products, DSP) on surface of heat exchangers, thus reducing the heat transfer coefficient.
This process removes dissolved silica from a Bayer solution by contacting the solution in a moving packed bed of coarse quartz particles in a reactor, whereby the dissolved silica precipitates as desilication products. This process is carried out at temperatures between 60 and 140oC.
The solution can be filtered in situ and the filtrate, less supersaturated in silica, is immediately available either as pregnant liquor for alumina trihydrate precipitation or as spent liquor to be preheated in heat exchangers before being fed to the digestion. The coarse solids, which contain the desilication products, are withdrawn from the reactor. The process is fully autogenous.
The recovered desilication product can then be processed to recover valuable soda and alumina or be available for sale.