Yu.D. Ivakin, M.N. Danchevskaya, G.P. Muravieva
Induced formation of corundum crystals in supercritical water fluid
The formation of fine-crystalline corundum from hydrargillite at 400 °C in supercritical
water fluid (SCWF) at 26.8 MPa in the presence of corundum particles is investigated.
It is found that the addition of fused corundum particles into the reaction medium
accelerates the formation of fine-crystalline corundum by the increase of the nucleation
rate. The process is accompanied by both the build-up of added fused corundum
grains, and the generation of new corundum crystals from boehmite. The formation of
new crystals depends on the distance between the grains of the additive. Depending on
this distance, the newly formed crystals consist of two or three fractions with different
average particle sizes. It is concluded that the build-up of the fused corundum particles
in the medium of SCWF occurs due to the uptake of corundum nuclei formed in the
surrounding layer of boehmite. For the added particles of fused corundum with average
size of 2.82 μm the layer from which the build-up occurs has a thickness of 17.47 μm.
The corundum nuclei formed at large distances from the growing particles become the
centers of the growth of new corundum crystals of the smallest sizes. The role of SCWF
is to increase the spatial and structural mobility of the reactants.
Key words: fine-crystalline corundum, supercritical water fluid, solid phase transformation, induced nucleation, build-up of crystals