Desilication Kinetics And Mechanism Of Hydroxy-sodalite In High Alkali Sodium Aluminate Solution

Compared with the Bayer method,high-pressure hydro-chemical method has great advantages in dealing with low-grade aluminum-containing resources such as red mud,fly ash and diaspore bauxite.However,in high-temperature,high-pressure and high-alkali environment,silicon will also be dissolved into the solution,the silicate ions in the solution will affect the purity of the product.At the same time,it will also cause pipeline crusting and increase energy consumption.The Bayer process generally dilutes the leachate to below below 250 g/L,and then desilicate it in a low-alkali solution.In this paper,the hydroxy-sodalite desilication agent for high alkali sodium aluminate solution was synthesized firstly.It can be directly desilicated in high alkali sodium aluminate solution,so that the liquid can precipitate hydrated sodium aluminate better.It is no longer necessary to dilute and then evaporate the solution,which simplifies the process and saves energy.The high-alkali sodium aluminate solution can precipitate sodium aluminate hydrate crystal after desilication.The effect of initial silica concentration,experimental temperature and alumina temperature are investigated for the precipitation of sodium aluminate hydrate SAH in high-alkali sodium aluminate solution.The experimental results show that the temperature has a very different effect on the change of silica concentration in sodium aluminate solution with different initial silica concentration The silica content in the SAH crystallization solution should preferably be desilicated to about 2.0 g/L or less.When desilication is carried out in high-alkali sodium aluminate solution,the dosage of the desilication agent should not be too low,otherwise the desilication process cannot be carried out;at the same time,the alumina concentration should not be higher than 150 g/L,and should be lower than 80 g/L,otherwise the desilication agent will act as a seed crystal of sodium aluminate hydrate.The desilication effect of NaCaHSiO4,zeolite,C3AH6,CaO,Ca(OH)2,synthetic sodium aluminosilicate and other substances in high-alkali solution was investigated.The experimental results show that the synthetic sodalite-type sodium aluminosilicate has the best desilication effect in the high-alkali sodium aluminate solution.The structure infomation of hydroxy-sodalite type aluminosilicates synthesized at different temperatures were investigated by XRD,FTIR and NMR.The effects of stirring,dosage of desilication agent,alkali concentration,initial silica concentration,desilication temperature and synthesis time on the desilication process of hydroxy-sodalite in high-alkali solution were investigated.Using bauxite and fly ash,two economically common low-grade aluminum resources,to synthesize desilication agent suitable for the high-alkali sodium aluminate solution.The main phases of the desilication agent sod-bauxite and sod-fly ash were both identified as sodalite.Using sod-bauxite and/or sod-fly ash as the desilication agent,the silicate content in the aluminate solution containing 500 g/L Na2O was successfully reduced to about 2 g/L in 16 hours at 100 ?.For the synthesized hydroxy-sodalite desilication agent,the relationship between the solubility and the experimental temperature,and the relationship between the solubility and the Na2O concentration was investigated.The experimental results show that the solubility increases with the experimental temperature and the concentration of Na2O.The relative supersaturation of silica(?)in a high-alkali sodium aluminate solution is the driving force for the sodalite precipitation that promotes the desilication process.The apparent activation energy Ea of the desilication process in 470 g/L high-alkali sodium aluminate solution is 94.04 kJ/mol,and the order of desilication is n=2.47±0.47.The apparent activation energy Ea of the desilication process in 500 g/L high-alkali sodium aluminate solution is 144.21 kJ/mol,and the order of desilication is n=4.27±0.18.The reaction crystallization should be the desilication mechanism using hydroxy-sodalite desilication agent in a high-alkali sodium aluminate solution.