Behavior Of Hydrated Calcium Silicate In The Sodium Aluminate Solution

The key project in alumina production is the separation of aluminum and silicon. Calcium silicate hydrate is an important compound containing silicon in the process of alkaline alumina production. Due to the silica of C-S-H combining no alumina and soda more and more importance has been attached to it in alumina field along with the impoverishing of bauxite. For example, in order to separate aluminum and silicon, the silicon-containing compounds with no or less alumina, such as C-S-H, are formed in High-pressure Hydro-chemical Process under very rigorous condition of high temperature and high caustic concentration. However C-S-H can only exist stably in aluminate solution with rigorous condition. In Sintering Process of alumina production, the secondary loss of alumina and soda is actually caused by the reactions between calcium silicate hydrate and aluminate solution in the operation of leaching sinter. Therefore it is of great importance for alumina production to know the reaction law between C-S-H and aluminate solution. In this case, the following aspects have been studied systematically in the dissertation:1) The main reactions between C-S-H and aluminate solution have been discovered after theoretically analysis, then thermodynamic calculation has been carried out. The results show that C-S-H mainly reacts with Al(OH)₄^-and the reactions between C-S-H with NaOH and Na₂CO₃ are limited in aluminate solution. The stability sequence of different C-S-H in aluminate solution is :6CaO·6SiO₂·H₂O >CaO·SiO₂·H₂O>2CaO·SiO₂·1.17H₂O.2) The hydro-thermally synthesis of calcium silicate hydrates. Using calcium hydroxide and sodium silicate as starting materials, six kinds of calcium silicate hydrates were synthesized at different conditions. The XRD patterns reveal that the basic structure units of all the calcium silicate hydrates synthesized with CaO/SiO₂=1(mol) and the temperature 80℃, 120℃ 160℃ respective are the same, and the crystal states of these calcium silicate hydrates are also the same, which means that the main substance of them is the same, and conclusion can be made that the main substance is CaO·SiO₂·H₂O after analysis. The law of the calcium silicate hydrates synthesized with CaO/SiO₂=2(mol) and the temperature 80℃ 120℃ 160℃ respective is the same, and the main substance of these calcium silicate hydrates is 2CaO·SiO₂·1.17H₂O.3) The reaction rules between calcium silicate hydrates and aluminate solution have been studied. The results show that CaO·SiO₂·H₂O is more stable than 2CaO·SiO₂·1.17H₂O in aluminate solution and the stability of C-S-H increases with the elevation of temperature, which is consistent with thermodynamic results. The concentration of Na₂CO₃ and NaOH have less influence on the stability of calcium silicate hydrates, meanwhile the Al(OH)₄^- concentration in aluminate solution affects those remarkablythrough formation of hydro-garnet and sodium alumino-silicate hydrate. Al(OH)4'contributes more significantly than NaOH and Na2CC>3 on the high SiC>2 concentration in the solution after reaction of calcium silicate hydrates with aluminate solution. These conclusions are consistent with thermodynamic calculation results.4) XRD patterns have been used to study the substance after calcium silicate hydrate reacting with aluminate solution. The results suggest that DSP emerges at the beginning of the reaction, which means that the in situ reaction formatting hydro-garnet and DSP directly has happened.