2000 Volume 11 Issue 2 Pages 169-187
Numerical simulations, based on both batch and continuously stirred tank reactor (CSTR) models, have been made to assess the effects of flow on the dissolution and growth processes of kaolin minerals under acid hydrothermal conditions of the Lake Ohyunuma, using the kinetic data of the minerals determined experimentally.
Our previous geochemical and mineralogical studies showed that the Ohyunuma lake water forms two layers with distinct stratification of temperature and chemicals. The bottom sediments contain both halloysite and kaolinite. The upper layer water has been kept undersaturated with respect to the two minerals for longer years than the duration at which all particles of the minerals have been completely dissolved if it were a batch system. This contradiction is caused not only by continuous supply of the solid materials, but also by continuous inflows of undersaturated solutions from the outside of the lake. The existence of flows assists kinetically the stabilization of kaolin minerals even within undersaturated solutions such as the upper layer water of the Ohyunuma.
The lower layer water is supersaturated with respect to kaolinite and undersaturated with respect to halloysite. Relatively coarse-grained particles of halloysite which have deposited on the lake floor are undergoing further dissolution under the lower layer water conditions, so supplying material for nucleation and subsequent growth of kaolinite. The entire transformation process of halloysite to kaolinite taking place in the Lake Ohyunuma is basically controlled by the growth kinetics of kaolinite. The transformation time, defined as the time when all halloysite particles disappear, was estimated to be a couple of years when the lower layer was assumed to be a batch system. In the CSTR system, however, the transformation time increases with increasing the supersaturation of inflowing solutions because the dissolution rate of halloysite decreases. The terminal supersaturation of solution at the point which the transformation is completed has been situated at intermediate values between the equilibrium solubility products of halloysite and kaolinite, similar to the observed supersaturation values of the lower layer waters, though it is strongly dependent on the supersaturation of inflowing solution and the residence time of the waters. Consequently, the Al concentration of Lake Ohyunuma can be said to be controlled by the dynamic dissolution and growth processes of minerals involving halloysite, kaolinite, and possibly alunite in the flow system, while the Si concentrations obey the solubility curve of amorphous silica.
