Chiang Mai Journal of Science

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Ball clay is a rare mineral, found in very few places around the world. It is an important component in a wide range of goods which are essential in everyday life. Ball clay is normally used for sanitaryware, floor tiles, wall tiles, tableware, porcelain tableware, electrical and chemical porcelain. Small quantities of ball clay are also used in the heavy clay industry for the production of building bricks and pavers. Ball clay is a fine-grained sedimentary plastic clay in which the clay mineral, kaolinite, predominates, and which has a light or white colour after firing to 1200°C under oxidising conditions. The typical ball clays have specific surface areas of ~20-25 m2g-1. The aim of this work was to characterise phases which generated the high specific surface area in the ball clay. The clay was fully investigated to determine the influence of organic matter and to identify the mixed amorphous organic-inorganic coating phase. transmission electron microscopy (TEM) micrographs showed the primary particles of the clay were between 500 nm and 1 m m in width. They had an amorphous coating of thickness between 20 and 30 nm. The elements present in the crystalline and amorphous phases were predominantly Si and Al, and lesser amounts of Fe and Ti. The primary particles were extensively agglomerated. Organic matter could be removed by 5 % (w/v) aqueous hydrogen peroxide solution, and by calcination at 350°C. The specific surface area of clay after removal of organic matter simultaneously increased by ~ 7 %. Detailed gas adsorption analyses showed that the volume of pores of 2 nm dimension was slightly increased by these treatments. Nevertheless, the thickness of the coating was still in the range of 20-30 nm. Further attempt to remove the coating by treatment with aqueous sodium carbonate, sodium dithionite/citrate and ammonium citrate was able to reduce the specific surface area and 2 nm pore fraction by ~50 %. TEM micrographs of clay particles after treatments showed the thickness of the amorphous coating decreased to 5-10 nm.

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