Physico-chemical and mineralogical studies on some plastic kaolinitic clays

Abstract

Among the best-known and widely used important industrial minerals, ball clays are of much value since they can be used in various ceramic industries. One of the essential features of ball clays is their fine grain size compared to those of other clays which is the main reason for its plasticity and so it is used to enhance the binding property. They can be added to other clays to give strength to the moulded shape and yet maintain the white color after firing. In countries like UK, USA, Japan and Germany specially formulated ball clays are available for specific industries like sanitaryware, high tension and low tension insulators, porcelainware etc. These have been prepared by different blending techniques to make the product with the tailor-made properties. In India we are using the naturally available materials with no significant processing operations as far as ball clays are concerned. It is with a view to get an idea about the preparation of tailor-made ball clay material that this detailed study of different types of ball clays was undertaken.

In Kerala, the occurance of 1.7 million tons of economically viable ball clay has been identified so far of which the major source is the Tertiary sedimentary sequence in the Quilon District, especially the Padappakkara reserve which comprises around 1.3 million tons. The origin of these kaolinite group of minerals (from feldspathic rocks) and associated iron sulphide minerals – which exist as an impurity in the matrix- with respect to the interrelationship between sandstone and clay beds of Tertiary deposits (Warkalla formation) in Kerala. Industrially these minerals find use as binding agents because of the high plastic nature once the ferruginous impurities are removed. By sedimentation method using suitable dispersing agent the denser impurities can be removed to a certain extent.

The amount of kaolinite mineral in a clay material is of considerable importance since it is responsible for many of the properties of the material. The quantitative determination of kaolinite in clay – by considering that the degree of crystallinity may have effect on the measurement – has been done by many investigators for more than forty years. Moreover, the crystallinity of kaolinite is a parameter of great structural, geochemical and industrial significance. Besides the grey variety of ball clay from Padappakkara six other samples from different locations of the world were collected for the studies: namely Payyangadi (Kerala), Bikaneer (Rajasthan), Bimadole(Andhra Pradesh), Than(Gujarat), Westerwald(Germany) and Inagaki-kibushi(Japan). <1 micron fraction of each sample was separated by sedimentation method obeying Stoke’s law. These fractions already collected were concentrated in the clay content by removing organic matter, carbonates and iron oxides and the crystallinity indices of the purified kaolinitic materials were measured from x-ray diffraction and infrared absorption patterns(Hinckley’s and Worral’s crystallinity measurements techniques). The lattice parameters have also been computed from XRD data using the program LCLSQ least square refinement technique.

The next chapter deals with the plasticity of ball clays. The samples from Padappakkara is found to be the best plastic among all the other samples already mentioned. Plasticity is a composite property which depends on the total characteristics of the particular clay material. Hence a detailed investigation on the physical, chemical and mineralogical properties of all these samples has been carried out and finally a regression analysis was also done to correlate the plasticity parameter with other three properties as silica content, base exchange capacity and particle size. The effect of humic acid content on plasticity was also studied in this context by removing the organic matter in two methods, (1) by the oxidative decomposition with 30% H2O2 (2)by extraction with NaOH. An attempt has also been made to correlate the zeta potentials of the samples of the samples with other properties.

While doing the effect of grinding on the plasticity of Padappakkara clays, it was seen that in the XRD pattrens of the ground samples, new mineralogical phases were being introduced in the matrix. So the grinding study on the kaolinite was extended with another two clays, the plastic clay from Thonnakkal, Trivandrum District. The change in plasticity, particle size and shape, cation exchange capacity and mineralogy with grinding of ball clay from Padappakkara and has been attributed to the presence of gibbsite as an impurity. Mineralogically, no considerable change was observed in the case of the other two clays. However, the formation of alpha alumina in the former case is worthwhile as this proves to be method for making a high temperature phase by wet grinding.

The last chapters are mainly concerned with the changes taking place during their sintering. The aim of this study is to determine the relationships between raw material characteristics on the phase composition and the physical properties of the fired samples. On the basis of these relationships, some additives have been incorporated in the mix to modify the properties of the fired product of the blends. The first set of blend was prepared using Bikaneer clay by mixing it with different percentages of alumino silicate gel prepared from Zeolite 3A. The gel addition upto 10% can be used as a technique for improving fired strength. The second set of blends was prepared with a view to improve the properties of Padappakkara clay so that it can be used as a substitute of the ball clay from Bikaneer which has been widely used for making high tension insulators. Accordingly the final blended mix was tested in M/s. Bharat Heavy Electricals Ltd and the results were found to be satisfactory. Thus the ball clay from Padappakkara which otherwise could not have been used as a binding clay, inspite of its high plasticity was modified by a blending procedure to get an industrially valuable product.