Energy recovery method and system for combined sulphuric acid and phosphoric acid manufacturing plant

In a process for the manufacture of concentrated phosphoric acid in which sulphuric acid and phosphate rock are reacted to produce a slurry of calcium sulphate in dilute phosphoric acid, the slurry being filtered to produce a dilute stream of phosphoric acid and the phosphoric acid then being concentrated by evaporation of excess water in at least first and second evaporator stages, and in which said sulphuric acid is produced in a sulphuric acid plant physically adjacent the phosphoric acid plant, said sulphuric acid plant having means for producing a stream of gas containing oxygen, means for reacting said stream of gas to produce sulphur dioxide therein, a converter to convert said sulphur dioxide to sulphur trioxide, drying means for absorbing, in a first portion of sulphuric acid, moisture from said gas stream upstream of said converter, absorbing means for absorbing in a second portion of sulphuric acid, sulphur trioxide from said converter, said first and second portions of sulphuric acid having temperatures in the range from approximately 180.degree. F. to 230.degree. F. before cooling, cooling means for cooling at least said second sulphuric acid portion, and means for producing high quality steam, the improvement comprising the steps of recovering in a heat recovery fluid heat from said cooling means, and using said heat as essentially the sole source of heat to evaporate water from the phosphoric acid in at least said first and second evaporator stages by flowing said heat recovery fluid first to said second evaporator stage and passing it in countercurrent flow with said phosphoric acid in a heat exchanger of said second evaporator stage to operate said second evaporator stage at a selected temperature, and then flowing said heat recovery fluid from said second evaporator stage to said first evaporator stage and passing it in countercurrent flow with said phosphoric acid in a heat exchanger of said first evaporator stage to operate said first evaporation stage at a temperature substantially lower than said selected temperature, maintaining the phosphoric acid in said first evaporator stage at a concentration which does not exceed approximately 40% P.sub.2 O.sub.5 , flowing phosphoric acid from said first evaporator stage to said second evaporator stage and maintaining the phosphoric acid in said second evaporator stage at a concentration which substantially exceeds that in said first evaporator stage operating said first evaporator stage at a very low pressure by a barometric condenser and operating said second evaporator stage at essentially the same very low pressure as said first evaporator stage to enable said evaporation, thereby allowing said steam to be used for other purposes.