Patent ID: 7175683

Claim:
A process for treating sludge comprising the steps of: a) disinfecting the sludge by heating it at a temperature of between about 50° C. to about 100° C. and adding phosphoric acid, and one or more mineral acids selected from the group consisting of sulfuric acid, nitric acid, hydrochloric acid, and mixtures thereof; b) neutralizing the acids added in step a) by adding a magnesium and calcium slurry to react the phosphoric acid with calcium hydroxide and magnesium hydroxide to obtain insoluble calcium and magnesium phosphate to reach a final pH value of 8.5 and to liberate ammonia which reacts with the magnesium phosphate to produce apatite hydroxyl; c) filtering the product obtained in step b) with the apatite hydroxyl to reduce its water content and recuperate a cake containing insoluble matter; d) eliminating the water contained in the cake obtained in step c) by feeding the cake to a cone reactor system to which are fed phosphoric acid and sulfuric acid to obtain an exothermic reaction between the apatite hydroxyl and the sulfuric acid to evaporate the water present in the cake and transform the insoluble calcium phosphate into assimilable soluble calcium sulfate; e) feeding the product obtained in step d) to a granulator-ammoniator to which are fed ammonia to neutralize the phosphoric acid and maintain granulation thus causing an exothermic reaction between the ammonia and the apatite hydroxyl which further dries the product, and a granulated product is obtained containing 35% to 40% or greater by weight of nutrients expressed as nitrogen, P 2 O 5 and K 2 O, while achieving a 90% saving in energy; and, f) drying and sieving the granulated product obtained in step e) in order to classify particles by size.