Abstract: There is disclosed a dry process for preparing magnesium sulfate by heating one surface of a reaction mixture of asbestos tailings to a temperature of from 350.degree. to 400.degree. C. whereby there is provided a gradient temperature between the heated zone, an intermediate zone and the outside zone where the ammonium bisulfate formed can condense in one of the last two zones which are at a temperature lower than 250.degree. C. and react with ammonia to provide a new supply of ammonium sulfate for further reaction with the asbestos tailings and recovering the magnesium sulfate thus obtained.