Refinement of the Low-Grade Amakusa Pottery Stone by Hydrothermal Treatment

Abstract

The major mineral compositions of Amakusa pottery stone are quartz, sericite and kaolin mineral. The refinement of the low-grade of Amakusa pottery stone has been required because of the recent shortage of the resources of high-grade one. This paper concerns an optimum hydrothermal condition to refine the low-grade Amakusa pottery stone of which impurities are feldspar, limonite, siderite and calcite. The results obtained are as follows:(1) Feldspar included in the sample is converted into kaolin mineral with 0.5wt% HCl solution at the temperature below 200°C and about 15atm. The formation of kaolin mineral is observed to be increased with increases in volume of the solution, temperature, and the treatment time.(2) Conversion of the feldspar into the kaolin mineral is promorted by adding aluminium chloride under the condition concerned. At the temperature lower than 180°C, no formation of kaolin mineral is observed. Aluminum chloride is considered to supply the insufficient alumium during the conversion of feldspar to kaolin mineral.(3) A product having SK value higher than 26 and Fe2O3 content lower than 0.4wt% is obtained from a raw material with SK value less than 15 by the hydrothermal treatment.(4) A plasticity index of a product is measured with the Pfefferkorn method to be higher than that of a raw material.(5) The refined pottery stone obtained in the present study has smaller green strength and higher firing shrinkage than those of common Amakusa pottery clay. This is considered to be due to the difference of its surface state from that of a raw material.