the object of the invention comprises a process for the preparation of fertilizers by solubilizing phosphate rocks containing impurities , which make processing thereof difficult . the process is a simple and direct one . the product thus prepared is easily handled , can be directly used as a powdered fertilizer and dispenses with any post - treatment steps , such as purification , filtration , etc . the rock is solubilized with acid ammonium sulfate or a mixture thereof with sulfuric acid . this acid ammonium sulfate can be formed &# 34 ; in situ &# 34 ;, by the reaction of ammonium sulfate and sulfuric acid . reaction may take place in a mixing reactor , on a belt conveyor or in a granulator .