The wet process phosphoric acid is produced by decomposition of phosphate raw materials with sulfuric acid. This gives a large amount of waste product - phosphogypsum. The thermochemical decomposition of phosphogypsum to lime and sulfur dioxide provides possibilities to utilize its main components CaO and S and, at the same time, to solve some ecological problems. The main problem arising with this method is that it needs a large consumption of primary energy carrier - natural gas. The aim of the present work is to study the process with the exergy method and find new effective methods and technological schemes to reduce energy consumption. It is suggested to decompose phosphogypsum using the products from the incomplete combustion of the natural gas with oxygen enriched air and using a preliminarily prepared reducing gas mixture.