Chemical promotion of the kaolin in water treatment: preparation of new coagulant minerals
The present study emphasizes the mineral raw materials of Madagascar and their uses for the satisfaction of important local needs. As most treatments of water in Madagascar (drinking water, wastewaters) call for aluminium sulphate, which is still imported, the present work offers new products based on alumina and on silica, which allow the water to be treated by coagulation-flocculation. The aim is to control the treatment of kaolin by ammonium sulphate and to promote the use of the products of this reaction in water treatment. The raw materials used include kaolin, which exists copiously in Madagascar, and ammonium sulphate, which is going to become a locally-available industrial by-product as a result of the installation of a large plant for the extraction of nickel and cobalt in the east of Madagascar. The current processes for the preparation of alumina or some aluminium salts from kaolinite consist generally of a high-temperature thermal activation (500 to 800°C) of relatively important duration, followed by an acid attack. In our process, the kaolin is attacked by the ammonium sulphate without need of preliminary activation, at low temperatures between 200 and 350°C, for two hours at most, to obtain products containing some aluminium sulphate and also silica. It is known that silica, added in small quantities, is a flocculation additive to some aluminium sulphate preparations, and the results of our research showed that the whole product resulting from the ammonium sulphate attack allows one, without separation, to treat drinking water as well as urban and industrial wastewater. Various coagulation-flocculation trials with the final products were performed on different types of water and the analyses of the treated waters confirmed that some of these new products can be used to treat waters by coagulation-flocculation.