A FTIR STUDY OF DISSOLUTION AND PRECIPITA-TION OF SILICA IN BAYER PROCESS

A quantitative Fourier transform infrared (FTIR) technique mas established by comparison with chemical analysis and quantitative X-ray diffraction (XRD) analysis. By the quantitative FTIR analysis of bauxite during dissolution , the process of dissolution and precipitation of silica was found to occur in three stages. In the first stage, there was no precipitation of desilication product (DSP). All the silica dissolved from kaolinite went into the solution. In the second stage, the silica concentration in solution attained a maximum and then dropped very quickly. The precipitation of DSP was the main cause of the reduction of the silica concentration in solution. In the third stage, the concentration of DSP reached a constant level. The dissolution and precipitation process gradually attained an equilibrium. The mechanism of silica dissolution and DSP precipitation was discussed on the basis of the FTIR quantitative analysis of kaolinite and DSP in bauxite.

Unraveling the size distributions of surface properties for purple soil and yellow soil

Soils contain diverse colloidal particles whose properties are pertinent to ecological and human health, whereas few investigations systematically analyze the surface properties of these particles. The objective of this study was to elucidate the surface properties of particles within targeted size ranges（i.e. 〉 10, 1-10, 0.5-1, 0.2-0.5 and 〈 0.2 μm） for a purple soil（Entisol） and a yellow soil（Ultisol） using the combined determination method. The mineralogy of corresponding particle-size fractions was determined by X-ray diffraction.We found that up to 80% of the specific surface area and 85% of the surface charge of the entire soil came from colloidal-sized particles（〈 1 μm）, and almost half of the specific surface area and surface charge came from the smallest particles（〈 0.2 μm）. Vermiculite,illite, montmorillonite and mica dominated in the colloidal-sized particles, of which the smallest particles had the highest proportion of vermiculite and montmorillonite. For a given size fraction, the purple soil had a larger specific surface area, stronger electrostatic field, and higher surface charge than the yellow soil due to differences in mineralogy.Likewise, the differences in surface properties among the various particle-size fractions can also be ascribed to mineralogy. Our results indicated that soil surface properties were essentially determined by the colloidal-sized particles, and the 〈 0.2 μm nanoparticles made the largest contribution to soil properties. The composition of clay minerals within the diverse particle-size fractions could fully explain the size distributions of surface properties.