Abstract: The invention concerns a process for sulfuric acid treatment of clays or aluminiferous minerals to obtain pure aluminum oxide via a basic intermediate. The process finds application in the production of aluminum through fusion electrolysis and for the production of different types of alumina. The process provides a mass production method which is economically and technically superior over the prior art methods.The mechanically and/or thermally pretreated raw material is subjected to a two-step pressure treatment with sulfuric acid solution. The aluminum sulfate solution is treated to reduce the iron with SO.sub.2 and hydrolytic precipitation of basic aluminum sulfates. After separation of the salts from the mother liquor, which is returned to the process, the aluminum sulfate is thermally decomposed. The SO.sub.2 /SO.sub.3 -containing waste gases from the decomposition are used to renew the treatment acid.