Production of phosphate fertilizers

Claim 1

A process for the continuous production of a citrate soluble phosphate fertilizer by calcination of a mixture of phosphate rock, silica, and an alkali metal compound in a flame heated rotary kiln at a temperature of 1,000* to 1,300*C, the mole ratio of the alkali metal oxide Me2O to the P2O5 of the phosphate rock being 1.2 : 1 to 1.5 : 1, the mole ratio of P2O5 : SiO2 being 1 : 0.1 to 1 : 0.9 and the amount of SiO2 in the mixture being adjusted to bind the amount of CaO which exceeds the mole ratio of 2 CaO : 1 P2O5 in the form of Ca2SiO4, including in a first stage outside of the rotary kiln treating an alkali metal hydroxide solution having a content of 30 to 60 percent by weight of alkali metal hydroxide with the hot waste gases of the rotary kiln containing carbon dioxide together with the expelled alkali metal compounds and dust particles from the previous calcination stage, concentrating and carbonating the alkali hydroxide solution which contains said alkali metal compounds and dust particles to form a granulate with the phosphate rock and the silica which can be heated without melting to the calcination temperature, and calcining said granulate in a second stage in said rotary kiln and employing the waste gases therefrom for the treatment of further alkali metal hydroxide solution in the first stage.