Sulphuric acid processing of phosphorite slime and activated sludge into organomineral fertilizers

Phosphorite slime from Central Kyzylkum ore beneficiation and activated sludge from municipal wastewater treatment were jointly processed to obtain two complementary organomineral fertilizers. The study evaluated the effects of sludge proportion and sulphuric acid dosage on the reaction slurry composition and the distribution of nutrients and humic substances after filtration. Mixtures containing 10–80 wt.% activated sludge were treated with 30% sulphuric acid at 50–90% of the calculated acid demand of calcium oxide and then separated by vacuum filtration. The phosphorite slime supplied most of the phosphorus and calcium, whereas increasing the sludge proportion increased organic matter, humic acids, fulvic acids and potassium. The slurry's fulvic-to-humic acid ratio remained close to 2.43. Filtration transferred most fulvic acids and water-soluble organic matter into the liquid, while 88.9–95.3% of humic acids and 86.0–91.9% of calcium remained in the solid. At a slime-to-sludge ratio of 60:40, increasing acid dosage from 50% to 80% raised phosphorus transfer to the filtrate from 37.1% to 50.0%. The method provides a fulvate-rich liquid and a humic- and calcium-rich solid fertilizer, pending product safety and agronomic evaluation.

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