Role of the Amorphous Phase during Sodium Aluminosilicate Precipitation

In the Bayer process, reactive silica associated with bauxite dissolves into alkaline solution and subsequently precipitates as a sodium aluminosilicate desilication product (DSP). Multiple DSP phases can be formed, which include an amorphous solid as well as crystalline zeolite A, sodalite, and cancrinite. Even though the ability to control DSP particle size would be of great practical benefit, there remains limited fundamental understanding of the precipitation process, especially during the early stages of the reaction. In this study, DSP was precipitated from synthetic NaOH-NaAl(OH)(4)-Na2SiO3 H2O solution at 75 and 90 degrees C. Four distinct reaction stages were observed: (1) precipitation of the amorphous solid, (2) dissolution of the amorphous solid, (3) growth of crystalline phases, and (4) maturation of the crystalline phases. In addition to the amorphous phase, the nucleation of both particles and two-dimensional crystal planes occurs in the early stages of the reaction. Subsequently, the amorphous phase dissolves, which feeds the early growth of the crystalline DSP phases. After the amorphous phase disappears, the crystal phases continue to grow with reactants from the bulk solution until equilibrium is approached. In the maturation stage, the solution silicate concentration decreases only slightly; however, during this time the sizes of individual crystals decrease slightly while the overall particle size increases via crystal agglomeration, which is promoted at higher temperature.