Method for producing sulfuric acid with by-product of calcium sulfate under low temperature

The invention provides a method for producing sulphuric acid at low temperature by using gypsum, especially the gypsum that is a by-product (including phosphate gypsum, fluorgypsum, flue gas desulfurization gypsum and so on). The gypsum is firstly prepared into ammonium sulfate, and then the phosphoric acid is used as reaction intermediate to decompose the ammonium sulfate into sulfuric anhydride and sulphuric acid. The phosphoric acid and ammonia can be recycled. The invention is low in equipment investment, the cost of the sulphuric acid produced by the method is far less than that of the sulfuric acid made by sulphur, which finds a good way for the large amount of gypsum that is a by-product; the invention avoids the environmental pollution and guarantees the sound development of the phosphate chemical industry.