Aluminate effect on desilication product phase transformation

It remains a grand challenge in the treatment of bauxite residue to physically separate zeolite (also known as the DeSilication Product, DSP) from other unleached mineral phases owing to their fine sizes, typically less than 2 µm in diameter. In this study, the effect of aluminate concentration on DSP phase transformation was investigated from synthetic NaOH-NaAl(OH)4-Na2SiO3-H2O solution at 90 °C. The results show how at relatively low aluminate concentrations of 0.125 and 0.25 M NaAl(OH)4(aq), larger zeolite crystals of up to 60 µm in diameter are formed during homogenous precipitation from aqueous solution. The precipitation process involves the conversion of zeolite LTA to sodalite via the formation of an intermediate phase, octahedral LTN. Initial amorphous solids precipitate as aggregates in which cubic zeolite LTA nucleation occurs. As the reaction proceeds, truncated octahedral LTN appears with the cubic zeolite LTA and finally sodalite crystallizes on the LTA-LTN aggregates resulting in coarsened particles.