Kaolinite transformation in high molar KOH solutions

Experiments measuring kaolinite dissolution and recrystallisation rates in KOH solutions were carried out using a batch reactor at 35° and 80°C. An untreated, sized kaolinite from St. Austell was used. No potential catalysts or inhibitors were present in solution. Each reactor was charged with 1 g of kaolinite of the ≤2 μm fraction and 80, 160, 240 ml of 0.1–4 m KOH solution. In these experiments the solution composition and mineralogy were monitored as a function of time for up to one year. A dissolution dominant stage was followed by a precipitation dominant period. The dissolution is not linear as a function of time but linear as a function of (log) time. This apparent relationship is explained by an affinity effect (approach to equilibrium). Under these high pH conditions, the sequence of reaction products observed is: illite, followed by KI-zeolite, phillipsite and finally by the stable product K-feldspar precipitation. The point in time at which crystallisation controls the concentration of Al and Si in solution and the persistence of the various metastable phases is temperature dependent. Concerning the stability of clay barriers in an alkaline solution context, low temperatures allow more material to enter solution before the crystallisation of metastable phases controls the solution composition. Also at low temperatures the dissolution phase continues for longer periods of time. Hence, low temperatures favour an actively aggressive solution (dissolving clays).