Silicon In Sodium Aluminate Solution In The Rapid Decomposition Process Behavior

Accelerating precipitation of Al(OH)₃ in Bayer process is an interestingtopics in alumina industry. However, impurities co-precipitate withGibbsite in the rapid decomposition process of supersaturated sodiumaluminate, especially silicon affect the quality of alumina. So, it isessential to research the behavior of silicon in the rapid precipitation ofsupersaturated sodium aluminate.in this paper the effect of silicon onseeded precipitation rate and particle size distribution (PSD) in ionmembrane electrolysis process have been investigated. The conclusionswere made as follows:(1) In the process of ion membrane electrolysis of supersaturatedsodium aluminate solutions, decomposition ratio can get as high as 50％in one hour; temperature influence pricipitation rate obviously. It can beconcluded that electric field accelerates precipition of Gibbsiteintensively; even in ion membrane electrolysis, the decomposition ofsupersaturated sodium aluminate solution is contolled by chemicalreaction.(2) In the process of intensifying precipitation of sodium aluminatesolution with ion membrane electrolysis, silicon dioxide may influencethe precipitation rate of Gibbsite as well as the quanlity of Gibbsite. Asthe silicon dioxide concentration was under 0.60g/l, the influence isneglective. As the silicon dioxide concentration is more then 0.60g/l,silicon effects on the decomposition rate and particle size distribution ofGibbsite obviously, when the silicon dioxide concentration is 0.90g/l, at60℃, decomposition rate was slow down; the product displayedthree-modal particle size distribution: fine -(＜1μm )-intermediate(～10μm)-coarse (～75μm) while the product displayed one-modaldistribution in blank sample. Three-modal particle size distribution wasalso observed in the solution with dioxide concentration being 0.75g/1,but it became one-modal (～80μm) distribution after 24h. SEM-EDAXanalysis indicated that there were sodium silicate on the surface ofAl(OH)₃ crystals. It was also found that, the effect of silicon on Gibbsiteprecipitation rate and PSD would be reduced by increasing the concentration of Na₂O, temperatur and adding more seed.(3) The mechanism of co-precipitation of silicon with Gibbsite mayundertake three steps: silicon co-precipitating with Al-containing anions,silicon absorbing on the surface of Al(OH)₃ and forming sodiumaluminum silicate. The network structure polymer was formed byinteraction between Al-O-Si band and Al-O-Al band,which mayslowdown the polymerization of growth units of Gibbsite and inhibit theprecipitation of Al(OH)₃.