FORMATION OF NO(X) BY DIGESTION OF PHOSPHATE ORES WITH CONCENTRATED NITRIC-ACID

The digestion of twelve phosphate ores with concentrated nitric acid has been studied in order to quantify the formation of nitrogen oxides, NO(x). The experiments were performed in a batch laboratory-scale reactor under reaction conditions which are representative of the acidulation of phosphate rock with nitric acid in the nitrophosphate process, an important process for the production of compound fertilizers. The results obtained from these experiments show that the amount of NO(x) formed during digestion is directly proportional to the organic carbon content and is only marginally related to the carbonate and fluoride contents of these phosphate ores. It is assumed that organic material in the ores is partly oxidized by nitric acid, which in its turn is reduced to NO and NO2. On the basis of our results the following correlation between the organic carbon content and the quantity of nitrogen produced in the form of nitrogen oxides is proposed: [NO(x)-N] = A . [C(org)] in which A = 0.65 if [NO(x)-N] and [C(org)] are both expressed on a weight basis and A = 46 if [NO(x)-N] and [C(org)] are expressed in mol/tonne of P2O5 and kg/tonne of P2O5, respectively. The result is important in specifying the value of phosphate ores for the nitrophosphate process because it quantifies the possible emissions of NO(x) into the at atmosphere as well as the amount of wasted nitric acid.