| The dominant speciation of bauxite is diaspore in China, which is characterized with high ratio of silicate to aluminium. The digestion behaviour of silicone minerals in Bayer process has a great effect to the quality of alumina products. With the large-scale exploitation of bauxite in recent years, the alumina industry is confronted with the challenge of bauxite shortage. The utilization of low-grade bauxite efficiently and economically is an inevitable choice. A new technology named Mineral Beneficiation-Bayer process was successfully exploited in China, which is proved to be an effective way to make full use of low-grade bauxite. Fatty acid compounds, components of collectors, may be introduced into Bayer circle during mineral beneficiation process. The behaviors of those fatty acid compounds in the Bayer circle, including distribution, cumulation, transformation, fate and the effect on bauxite digestion and crystallization process, have not been clearly interpreted.In this paper, the digestion behavior of several widely distributed monomer silicone minerals and diaspore was investigated. The effect of collectors of fatty acid on bauxite digestion and gibbsite crystallization was probed simultaneously. The results may be a technical support and theoretical guidance for alumina production. Main conclusions are drawn as follows:1. Several kinds of widely distributed monomer silicone minerals, kaolinite, illite, pyrophyllite were characterized with XRD, SEM, IR and chemical analysis. The behavior of these monomer silicone minerals during the digestion process was investigated. The results suggest that the digestion rate of kaolinite, illite and pyrophyllite is pronouncedly influenced by temperature. The characteristics of electronic structure and reaction properties were discussed from the results of quantum chemical calculations. It is found that the characteristics of electronic structure, the bonding force of Si-O bonds on the slabs of kaolinite, illite, pyrophyllite are greatly affected by OH" in sodium aluminate solution. The surface property is changed and the bonding force of Si-O bond is decreased most effectively for kaolinite. As a result of that, kaolinite can be dissolved to release silicon in sodium aluminate solution at relately low temperatures more easily.2. The precipitation of SiO2 is favored if appropriate amount of CaO is charged into the monomer silicone minerals, hence the digestion of mineral is speeded up. The phase analysis results of sodium aluminosilicate hydrate suggest that sodalite is the dominant ingredient. The transformation of sodalite to hydrate garnet is stimulated with the addition of CaO, which leads to a drop of equilibrium concentration of SiO2. So the dissolution of mineral becomes more easily.3. The reaction of diaspore with sodium aluminate solution is severely inhibited by the coverage of TiO2. At low temperatures, the dissolution rate of diaspore is greatly improved with the addition of CaO; However, the effect of CaO on dissolution rate of diaspore is very little at high temperatures. The inhibition mechanism of TiO2 may due to the formation of shielding layer on the mineral surface, which hampers the invasion of OH- to crystal lattice of diaspore. The shielding layer may bate with the addition of CaO.4. When the concentration of organic compound in the sodium aluminate solution increases, the surface tension of sodium aluminate solution decreases rapidly, accompanied with the sharp increase of Zeta potential of aluminium hydroxide particle and decrease of conductivity of the solution. As the concentration of organic compound is 1mL/L, the surface tension of solution decreases to 32.70 mN/m, while the value of conductivity and Zeta potential get to the maximum at the same time, that is, the surface tension, Zeta potential and conductivity nearly do not change any more if the concentration of organic compound keeps going up. So it is assumed that the cmc of collectors is about 1 mL/L.5. The influence of sodium oleate and benzyl hydroxamic acid on bauxite digestion is very little, so it is the case of kaolinite, illite, pyrophyllite with the addition of CaO. Benzyl hydroxamic acid can slightly promote the digestion ratio of bauxite and sodium oleate can apparently decrease the value of N/S of the red mud, additionally, the effect is more apparent with the decrease of alkaline concentration. However, the introduction of benzylhydroxamic acid leads to the increase of N/S value, and the N/S value undergoes a track of rising at first and then dropping later with the concentration of benzylhydroxamic acid uprising. If oleate and benzyl hydroxamic acid were added jointly, no obvious variation of N/S value is observed.6. The decomposition ratio of sodium aluminate liquor increases with the decreases of concentration of collectors (organic compounds), which indicates that the cumulation of collector is disadvantagous to aluminum hydroxide crystallization. During the seeded crystallization process of sodium aluminat liquor containing organic compounds, the crystallization rate changes with the variation of seed concentration. The crystallization rate increases with the seed concentration. There is little influence of seed concentration on the content of silicone and iron in alumina products, however, the content of alkaline tends to increase. Under selected conditions, the amount of fine particles reinforces with the increase of cycle number even no collector is charged, and the situation deteriorates when collectors are introduced.7. A kinetic model of seeded crystallization of sodium aluminate is established via the analysis of material balance and particle size balance. The sub steps of the crystal growth and the physical properties of alumina product were investigated with the addition of organic compounds. The relationship between parameters of kinetic model and reaction condition was confirmed on the basis of experimental data. The effect of collector concentration on the kinetic parameter Rpq was probed subsequently. The empirical expression of the relationship between kinetic constant of seeded crystallization and experimental conditions is shown as follow:k=-7.849×10-5t2+2.595×10-3αkNk+1.638×10-2αkt-7.074×10-5StSg-8.885×10-7SgNk+1.007×10-5Sgt-1.043×10-4Nkt-0.819... |
