Ammonium phosphate slurries are produced from impure phosphoric acid that contains Fe(Ill), AI(III) and Mg(II) ions. The insolubility of these metal ions and the onset of solid formation determined as a function of pH or mole ratio (MR) of ammonia to phosphoric acid were consistent with the trend for the pH of formation of the first hydrolysis product that decreases in the following order: Fe(III)< Al(III)< Mg(II). The hydrolysis products of Fe(III) formed at pH > 2.0 or MR > 0.5 initiate ammonium phosphate crystallization, reduce the size of particles formed and generate attractive interparticle forces. Similarly, the Al(III) hydrolysis products formed later at pH > 2.6 MR > 0.7), will also initiate further crystallization, adsorb on particles and produce attractive forces. The attractive forces and the high number concentration of particle-particle interactions are responsible for the increased viscosity and non-Newtonian flow behavior displayed at increasing Fe(Ill) and Al(III) concentration. Mg(II) ions are not hydrolyzed at MR < 1.0 so its effect on theology is negligible and its effect at MR < 1.0 is also small as its concentration is much smaller than that of Fe(Ill) and AI(III) ions. The change in slurry viscosity with the degree of neutralization is also explained in terms of particle size distribution, solubility and solids concentration variations. (c) 2006 Elsevier Ltd. All rights reserved.