Research On Two-stage Digestion For Mixed-bauxite

The mixed-bauxite from Australia is one of the main imported bauxites in our country, which mainly contains gibbsite and a small amount of boehmite. The traditional technology for such kind of mixed-bauxite in Bayer process is one-stage digestion at high temperature(220～240℃), which consumes excessive energy and results in high mass ratio of Na2O/SiO2and Al2O3/SiO2in the red mud leading to extra loss of sodium oxide and alumina. How to extract alumina from the mix-bauxite economically and efficiently has become one of the urgent tasks in alumina production of Bayer process.According to the different digesting characteristics of gibbsite and boehmite, this paper adopted a new technology of two-stage process to deal with the mixed-bauxite, in which the mixed-bauxite was first digested at the first stage of low temperature to extract the gibbsite, and then the residue with un-attacked boehmite was digested at the second stage of high temperature. Without adding lime, this paper has studied the influences of digestion conditions, components of spent liquor and impurities on the mass ratio of Na2O/SiO2and Al2O3/SiO2in red mud, as well as the distribution of organics at the first stage of low temperature.The results show that:1) In the first stage of low temperature, increasing digestion temperature, caustic concentration and prolonging digestion time increase the mass ratio of Na2O/SiO2, decrease the mass ratio of A2O3/SiO2. During the low temperature digestion, more than80%of organics enter the red mud.2) In the second stage of high temperature, the mass ratio of Na2O/SiO2increases and the mass ratio of Al2O3/SiO2decreases along with the increasing of caustic concentration and molar ratio of mixing material, and prolonging the digestion time. The mass ratio of Na2O/SiO2decreases along with the increasing of caustic molar ratio increasing. When the concentration of Na2SO4and Na2CO3in spent solution increases, the mass ratio of Na2O/SiO2and Al2O3/SiO2increases.3) Based on the above mentioned researches, this study proposed suitable conditions for the new process:in the low temperature stage, digestion temperature100～110℃,the caustic molar ratio of spent solution3.0, p(Na2Ok) about170g·L-1, digestion time30min; in the high temperature stage, temperature of digestion240℃, the caustic molar ratio of pregnant solution2.44～2.64, p(Na2Ok) about170g·L-1, digestion time30min. Adopting the two-stage process to deal with the mixed-bauxite, the quantity of slurry in the high temperature digestion can be reduced60～70%and thereby decreases the energy consumption. The ratio of Na2O/SiO2and the Al2O3/SiO2is about0.4and1.0respectively. Compared with the traditional one stage digestion, the loss of sodium oxide and alumina reduces about20%and17～23%respectively.