Variation Of The Iron Concentration In Sulfur-bearing Sodium Aluminate Solution And Study Of Iron Removal

High-quality bauxite resources have become increasingly scanty in China, so economic utilization of abundant domestic high-sulfur bauxite resources has been a current research focus. In the treatment of high-sulfur bauxite by the Bayer process, the sulfur, especially S2-ion, usually accumulates in the circulating spent liquor. It will have a serious negative impact on the alumina production process such as extra caustic consumption, equipment corrosion and severe scale, and it will also causes the circulating spent liquor and digestion liquor darken or black, and thus the green or dark green aluminum trihydroxide will be prepared because of the excessive iron present in the product. As the pollution mechanism of sodium aluminate solution is not understood clearly, there is no economic approach to solve the problem of excessive iron in the industrial sodium aluminate solution with high sulfur species content. Therefore, the study of the iron concentration variation in the sodium aluminate solution with high sulfur species concentration and the iron removal experiments are of great importance for the industrial use of the high-sulfur bauxite.As the composition of sodium aluminate solution is rather complicated, and thus it is very difficult to study the influence of various factors, the simplified system was employed in this thesis to study the effects of S2-concentration, caustic concentration, solution temperature, heat preservation time and the impurities on the iron concentration in the sodium aluminate solution. The results demonstrate that:(1) increasing S2-and caustic concentration can significantly increase the iron concentration in the solution;(2) the iron concentration in the solution is very low in the temperature range of the seeded precipitation in Bayer process;(3) when the mole ratio of Na2O to alumina (αk) is approximately3.0, the iron concentration increases slightly with the increase of Na2CO3concentration and obviously with the increase of Na2SO4and Na2C2O4concentration;(4) increasing ak can improve the iron concentration in the aluminate solution, and the iron concentration rapidly increases from-20mg·L-1to80mg·L-1with ak increasing from2.0to2.5;(5) the digested aluminate solution with low value of αk (1.4～2.3) contains much unstable "filtering iron" which quickly aggregates and deposits with the heat preservation time, and thus the solution iron concentration reduces sharply; while the digested aluminate solution with high value of ak (2.6～3.5) bears less "filtering iron" and the iron concentration keeps a relatively stable state of80mg·L-1with the extension of heat preservation time; and (6) the iron-bearing substance deposited from the aluminate solution is amorphous with no specific crystal structure after drying.In addition, preliminary experiments of removing iron from the digestion liquor by adding lime, other organic additives and seeds were carried out in this paper. The results show that lime and seeds have a slight positive effect on the iron removal, that the effect of organic additives on the iron removal is unstable, and that there is no obvious synergic effect with the additive of lime and organic additives to remove iron in the solution.