Viscosity Formation Mechanism and Viscosity Reduction Strategy for Ammoniated Slurry of Nitrophosphate Fertilizer

Abstract

Wet-process phosphoric acid (WPA) preparation by the nitric acid method (WPA-HNO 3 ) is a green technology with broad development prospects. However, in the process of preparing nitrophosphate fertilizer (NP) by WPA-HNO 3 ammonification, the NP ammoniated slurry has high viscosity and poor fluidity, resulting in the shutdown of the production process. There is an urgent problem in NP production to study the formation mechanism of the high viscosity slurry and provide effective guidance for reducing the viscosity. In this paper, based on the composition of actual WPA-HNO 3 in NP production, the equilibrium species variation with pH in the ammoniated slurry was simulated by using HSC software. The viscosity and composition of the NP slurry and the hygroscopicity and microtopography of the solid products were analyzed. The mechanism of viscosity formation and the strategy of viscosity reduction were proposed. The results showed that the micron-scale CaHPO 4 formed in the NP slurry provided nucleation sites for the crystallization of the soluble ammonium salt, which promoted the precipitation of unsaturated NH 4 NO 3 and NH 4 H 2 PO 4 in large quantities. The strong hygroscopic NH 4 NO 3 solid reduced the free water content in the slurry. In addition, the NH 4 NO 3 underwent crystalline transformation with the increase in the pH, and the specific surface area of the solid was increased. The coated CaHPO 4 particles were stripped from the ammonium salt and hydrated to form CaHPO 4 •2H 2 O. The slurry viscosity increased sharply with the increase in the solid content and the decrease in free water. The large amount of Ca 2+ in WPA-HNO 3 was the inducible factor for the formation of the high viscosity slurry. By adding 2.5% (wt) sodium hexametaphosphate as the Ca 2+ complexing agent in WPA-HNO 3 to avoid the formation of micron-sized CaHPO 4 , the maximum viscosity of the slurry decreased from 1361.00 ± 128.47 mPa• s to 351.24 ± 15.87 mPa•s.

Adsorption mechanism of Ca2+, Mg2+, Fe3+, and Al3+ ions in phosphoric acid–nitric acid solution on 001 × 7 and S957 resins

“…Phosphoric acid–nitric acid solution is the neutralization mother liquor obtained using the nitric acid resolve phosphate ore, which can be used to produce nitrogen and phosphorus compound fertilizers. 1,2 In addition to phosphoric acid (4.4 mol L −1 ) and nitric acid (1.1 mol L −1 ), this solution contains many Ca 2+ ions (1.9 mol L −1 ) and a few Mg 2+ , Fe 3+ , and Al 3+ ions (0.14–0.24 mol L −1 ). During the neutralization process, the presence of these metal ions leads to the formation of precipitates in the phosphoric acid–nitric acid solution, which makes the solution easily viscous and water-soluble phosphorus degenerate.…”