Patent ID: 7713416

Claim:
A process for treating sludge comprising: a) disinfecting the sludge by heating the sludge at a temperature of 70° C.; b) feeding the heated sludge to a static mixer and adding a mineral acid comprising phosphoric acid in order to decrease the pH of the sludge to less than 2.0, thus destroying pathogen microorganisms present in the sludge; c) feeding the sludge obtained in step b) to an agitated tank and stirring the mix in order to further disinfect the sludge; d) neutralizing the mineral acid added in step b) by feeding a lime slurry enriched with magnesium hydroxide until the pH of the sludge reaches a final value of between 7.3 to 7.5, and transforming the phosphoric acid into a mix of tri-calcium phosphate and di-calcium phosphate; e) filtering the product obtained in step d) in order to remove water and reduce the product's water content, and recuperate a hard filter cake containing insoluble matter; f) crumbling the cake obtained in step e) in order to break the hard filter cake consistency; g) further reducing the water content of the crumbled cake by producing an exothermic reaction between the crumbled cake containing tri-calcium phosphate and bi-calcium phosphate, and sulfuric acid, thus evaporating 90% of the water content and transforming the tri-calcium phosphate and bi-calcium phosphate into phosphoric acid and calcium sulfate; h) feeding the product obtained in step g) to a mixer, adding grinded recycled fines and thick matter produced by a granulator system, and mixing the product; i) feeding the product obtained in step h) to a granulator with anhydride ammonia, potassium salts and pre-neutralized phosphoric acid, and granulating the product; j) drying the granulated product obtained in step i) in order to reduce the water content to 3.0% by weight; and k) sieving the granulated product obtained in step j), classifying the final sieved product by a desired size, and recycling the product not having the desired size to step h).