Production of citrate soluble phosphates by calcination of secondary phosphate sources with a sodium-sulfuric compound

The method according to the preceding claims, wherein CaNaPO4 is a main phosphate phase.

The method according to one of the preceding claims, wherein the alkaline-sulfuric compound is selected from a sodium-sulfuric compound and/or a potassium sulphuric compound.

The method according to one of the preceding claims, wherein an amount of the alkaline-sulfuric compound is correlated to a content of a phosphorus and a content of a silicon in the raw material.

The method according to one of the preceding claims, wherein providing the alkaline-sulfuric compound is adapted to obtain in the mixture a molar ratio of Na:P of 1.1-1.8, preferably 1.3-1.6 and a molar ratio of Na:Si of 0.2-0.8, preferably 0.3-0.5, corresponding to an amount of 10 to 40 wt.-%, preferably 20-36 wt.-%, of the mixture.

The method according to one of the preceding claims, further comprising:

• providing a carbonaceous reducing agent as an additive.

The method according to the preceding claims, wherein a sewage sludge ash contained in the raw material is completely substituted by a carbonaceous reducing agent comprising phosphorus containing material.

The method according to one of the preceding claims, further comprising:

• controlling an atmosphere with respect to an oxygen concentration during calcining and/or during a cooling, wherein a content of sulfur in the product substantially corresponds to the content of sulfur in the mixture.

The method according to one of the preceding claims, wherein calcining is carried out at a temperature of 800-1100° C., preferably 850-1000° C. to obtain a stable operation condition by avoiding a baking of the product.

The method according to one of the preceding claims, wherein the raw material is an ash, selected from a sewage sludge ash or a biomass ash.

The method according to one of the preceding claims, further comprising:

• removing a heavy metal contained in the raw material by vaporization and sublimation of the reduced heavy metal species.

The method according to one of the preceding claims, wherein the alkaline-sulfuric compound is selected from a by-product of an industrial process selected from a rayon fiber production, a pulp manufacture, a paper manufacture, and/or the alkaline-sulfuric compound is selected from biomass, sludge, refuse derived fuel and waste incineration.

The method according to one of the preceding claims, wherein the alkaline-sulfuric compound is selected from a product of treating a gas, a waste water, a battery acid, or from rayon production.

A citrate soluble phosphate compound obtained from a P-containing raw material, comprising a P-phase selected from whitlockite Ca9(Mg,Fe2+)[PO3(OH)|(PO4)6], iron phosphate FePO4, aluminium phosphate AlPO4 and/or fluorapatite Ca5(PO4)3F, the process comprising:

• determining in the raw material a molar ratio of Na/Si and a molar ratio of Na/P;
• determining a quantity of a sodium-sulfuric compound based on the determined molar ratios;
• calcining under reducing conditions the raw material together with the determined quantity of the sodium-sulfuric compound to obtain the citrate soluble phosphate compound.