Fire clay is a variety of plastic refractory type clay which is normally found beneath the coal bed. The composition of the clay varies drastically from aluminous type to highly siliceous type.
Fire Clay is usually used for making variety of refractory materials. The physical colour of the clay varies from greenish grey to off-white creamish colour. When it dries it crumbles down and on wetting it gets converted into plastic clay. This clay contains very high amount of free silica impurities which largely influences the properties of the clay.
Iron is the most common impurity in most fire clays, present either as pyrites or sulphides and a small portion as iron oxide. may be either be present or oxide of iron. The coarser impurities can be separated by a powerful electro magnet. If the clay is calcined between 400o to 600oC, the iron impurities can be effectively removed. Medium size iron impurities can be removed by a method known as film floatation.
Film Floatation is a method in which certain floating agents like pine oil, creosote oil, kerosene etc. are added to a slurry containing these minerals. The impurities will float on the surface as a scum which can be skimmed out. The oxides of iron cannot be removed by any economical means and hence used as such.
