Iron Removal from Clays by Acid-Treatment and Reversal of the Clay's Plasticity

Clay has many applications; however, its utility depends on its physical and chemical composition. High-grade clays in the country are lacking due to the presence of some inherent impurities such as iron. The presence of iron in clay minerals has a profound effect on their physico-chemical properties. Structural and colloidal iron notoriously affects the colour of clays when they are fired. High percentage of hematite rules out the application of clay in paper industries and in the production of white ware like porcelain due to the red colouring imparted by Fe2O3 unless the clay is beneficiated. This study aimed at finding out whether the level of iron can be reduced by suitable acid treatment and whether the property of plasticity of the acid-washed clays was reversed by treatment with a suitable base. Studies were carried out on representative samples, which were taken from iron bearing clays from Mukurwe-ini, Nyeri County. Characterization of the clay was done in its natural form, and after acid treatment to determine its mineralogical and chemical composition. Natural clay was refluxed separately under sulphuric, hydrochloric, and oxalic acids of different concentrations at 100º and 200ºC for 2 hours followed by thorough washing in distilled water to remove the acid matrix. The acid treated clays were exposed to suitably chosen bases for 24 hours to reverse the effect of acid treatment. Atomic absorption spectroscopy, X-ray fluorescence spectroscopy, X-Ray diffraction, and Ethylenenediaminetetraacetic acid analysis techniques were used to determine the physico-chemical characteristics of natural and acid leached clay. Plasticity index was used to measure plasticity of raw and chemically treated samples. The results indicate that SiO2, Al2O3, and Fe2O3 are the major components of Mukurwe-ini clay while MgO, CaO, K2O, TiO2, MnO, and Na2O are present in trace amounts. Iron content was reduced to less than 1% in the acid washed samples. XRD characterization showed that Mukurwe-ini clay consists primarily of quartz (57.9%), kaolinite (20.8%), albite (5.3%), and microcline (16.1%). Phase quantification after acid treatment was quartz (74.7%) microcline (19.9%) and kaolinite (5.4%). Dissolution of the minerals in the acid converted the clay into mostly amorphous silica and consequently loss of its plasticity. XRD phase analysis of base acid-activated clays gave the following quantities of minerals; quartz (61.4-61.9%), kaolinite (15.7-16.3%) and microcline (22.3%). The Atterberg limits of the base treated samples were comparable with those of the natural clay. PI of raw clay between 22.3-30.7%. The samples exposed to base treatment had PI between 22 to 22.9 %, an indication that the property plasticity of the acid washed clay was restored by base treatment of acid washed samples. (author)