Characterization of a waste washing solid product of mining granitic tin-bearing sands and its application as ceramic raw material

Abstract

A waste washing solid product of mining granitic tin-bearing sands has been characterised by mineralogical, chemical and thermoanalytical studies. Kaolinite was found as the main mineralogical component, accompanied by minor amounts of potassium mica (illite), quartz and feldspars. Chemical data, microscopy (SEM-EDX) and thermal analysis (including dilatometry), were in accordance with this mineral composition. Iron oxides, as amorphous gels, were also evidenced. Particle size analysis showed a high proportion (ca. 75 wt%) of clay fraction (equivalent spherical diameter of less than 2 μm), in which more than 90 wt% was less than 63 μm, being produced by washing the altered granitic rock. Determination of ceramic properties enabled assessment of the main applications of the dry solid waste. Cold-pressed cylindrical test specimens were prepared and fired in the temperature range 1000–1300°C, with 2 h of soaking time. The temperature of maximum bulk density (2.54 g/cm 3 ) was 1150°C, which is also the vitrification temperature (open porosity almost zero). The firing range was 1150–1250°C, mullite crystals forming after firing (determined as 31 wt% at 1300°C by quantitative X-ray diffraction) accompanied by glassy phase. The vitrified ceramic bodies could be used as ornamental tiles and gres or stoneware products. The resultant porous bodies obtained by firing at temperatures ca. 1000°C, or lower, could be used in the manufacture of porous tiles, with apparent porosity ca. 30 vol%, or alternatively used as supports for glazes. The combination of an adequate mineral composition, highly rich in kaolinite, the chemical composition, and very low average particle size, makes this waste product useful in the manufacture of certain ceramics, producing added-value materials.