A rheological study of ammonium phosphate slurries

Phosphate Hill is a mine site located in North West Queensland. At the site, mined phosphoric rock is reacted with sulphuric acid to produce phosphoric acid. The phosphoric acid is combined with ammonia in a reactor known as the pre-neutraliser. The reaction forms a very hot and viscous slurry which is later further ammoniated in a cylindrical vessel known as a granulator, to produce solid fertiliser granules of mono-ammonium phosphate (MAP) or di-ammonium phosphate (DAP). Two important parameters used in the processing of ammonium phosphate slurries are the nitrogen to phosphorous mole ratio (MR) and the specific gravity (SG). Both of these parameters can affect the rheology of the slurry produced inside the reactor. The impurities carried over in the phosphoric acid from the reaction with the phosphate rock can also have significant effects on the rheology of the slurry. The objective of this study was to examine the rheological characteristics of the ammonium phosphate slurries formed in the pre-neutraliser (PN) and determine how the viscosity changes with mole ratio, impurity composition and specific gravity. There is a lot of variability in the literature on the rheology of ammonium phosphate slurries. Previous work did not take into account the non-Newtonian nature of the slurry, whilst also basing their findings on plant based measurements, where proper control of the slurry properties would have been difficult. A bulk quantity of phosphoric acid was collected from Phosphate Hill and used as a baseline for testing the effect of adding impurities. Laboratory grade FePO4, AlPO4 and Mg3(PO4)2 were added to the plant acid to form acids of varying impurity content. Further to the viscosity experiments, work was conducted to determine the slurry particle size, chemical composition (by XRD/XRF analysis), solubility of precipitates and physical characteristics, in an attempt to explain the mechanisms behind the observed viscosity changes. Testing the viscosity of ammonium phosphate slurry was found to be very time consuming and problematic. The experimental work was complicated by a multitude of factors, including high temperatures, slow reaction and evaporation times, the precipitation of impurities and the solidification of the slurry. The trend in the viscosity with mole ratio for the as-received acid was similar to that seen in the literature. The addition of both aluminium and iron caused an increase in the viscosity around the MAP minimum solubility point of 0.9 MR. In both cases, the formation of hydrolysis products were shown to have reduced the particle size of the precipitating ammonium phosphate crystals, thus increasing the viscosity. As the mole ratio is increased, mono-ammonium phosphate combines with the additional ammonia to form di-ammonium phosphate. The hydrolysis products for iron also changed at the same time and the resultant slurry formed particles with high interparticle attractive forces which in turn formed a flocculated suspension. Increasing the iron content not only increased the viscosity in this region, but also lowered the mole ratio whereby the increase in viscosity is seen to occur. An increase in the aluminium content had no effect on the viscosity at higher mole ratios. Based on the findings, it is recommended that slurries containing high iron be preferentially processed to make mono-ammonium phosphate and slurries with high aluminium be preferentially processed to make di-ammonium phosphate. Computational Fluid Dynamics (CFD) modelling was conducted on the mixing dynamics of the PN vessel. The CFD model showed that the upper region of the vessel was the least well mixed. The potential for stagnation of the flow field and subsequent solidification increases when the viscosity of the slurry also increases. To counteract this threat, the volume of slurry in the reactor must be decreased, or the agitation speed increased. The most effective method to ensure proper mixing dynamics in the vessel was to lower the viscosity, by preferentially processing each slurry by its impurity content as mentioned above