Characterisation and beneficiation of Narsingdi clay

Abstract

A sample of clay was collected from Narsingdi, near Dhaka for the purpose of this study. The sample of clay was characterized by its moisture content, bulk density, organic matter content, pH, alkalinity, liquid limit, plastic limit, firing properties etc. X-ray fluorescence analysis, X-ray diffraction analysis, Thermogravimetric analysis and Differential thermal analysis were also done for the characterization of the sample. The average moisture content (%m) in the clay was 3.27 % which is rather low. Bulk density of the clay, as measured by pycnometer method, was found to be 2.28 g/cc. The organic matter content in the clay was about 6.31% and the pH of the clay was 5.16 which means that the clay is acidic in nature. The alkalinity of the clay was found about 4.5 (wt.%). The average liquid limit is 45.27, the plastic limit is 33.81 and the plasticity index is 11.46. The x-ray diffraction analysis showed that the clay contained mostly quartz and a small amount of kaolinite. X-ray fluorescence analysis showed that the clay sample contained 8.15% iron oxide, 27.49% alumina and 59.45 % of silica and minor amounts of other elements. The thermogravimetric analysis curves showed a mass loss till 500OC due to loss of moisture and organic matter. Subsequently a gain in mass was noted and the reasons for this gain in mass could not be identified. Four sets of bricks were prepared and fired at four different temperatures. The bricks fired at highest temperature (i.e., 1100OC) showed maximum bulk density (2.25 gm/cm3), lowest water absorption (4.64%) and maximum shrinkage (23.3% ). It also showed the maximum cold crushing strength which was 40.724 N/mm2. The clay was leached in oxalic acid and in hydrochloric acid. Process variables, effects of concentration of acids, time and temperature and effect of stirring speed on the rate of dissolution of iron, contained in the clay sample was investigated. Temperatures studied in this investigation were in the range of 400C to 1000C, concentrations of both acids were in the range of 0.2M to 2M and the stirring speed was in the range of 0 rpm to 1000 rpm. While finding out the kinetic models that fit the leaching reactions, the Mixed controlled process i.e., t/τ = [(1-2X/3)-(1-X)2/3 ]+b[ 1-(1-X)1/3 ] seemed to be the most appropriate one to fit the kinetic data of leaching of clay in oxalic acid and the diffusion controlled reaction model i.e., t/τ = (1-2X/3)-(1-X)2/3 seemed to be the most appropriate while leaching in hydrochloric acid.. The Arrhenius activation energy for leaching in oxalic acid was found to be 41.03 kJ/mole and for hydrochloric acid was 50.82 kJ/mole. Alumina content was found to be affected by leaching in both the acids. The x-ray diffraction analysis showed that leaching did not cause any change in phases. The beneficiated clay was again characterized by the Atterberg limit, organic matter content, cold crushing strength, thermogravimetric analysis and X-ray diffraction analysis. The average liquid limit for the clay leached in oxalic acid was 41.72 and the plastic limit was 28.37. The plasticity index was 13.35. On the other hand the liquid limit for the clay leached in hydrochloric was 40.46 and the plastic limit was 29.1. The plasticity index was 11.36. The sample leached in oxalic acid contained 4.58% of organic matter and the sample leached in hydrochloric acid contained 3.1% of organic matter. Bricks were made by using the beneficiated clay. These bricks were fired at 1100OC and then crushed. The cold crushing strength of these bricks was found 37.26 N/mm2 (i.e., clay leached in hydrochloric acid) and 40.9 N/mm2 (i.e., clay leached in oxalic acid).