Optimization Of Hemihydrate-Dihydrate Wet Process Phosphoric Acid And Control Of Phosphogypsum Crystallization

The traditional dihydrate wet process produces low phosphoric acid concentration and high content of phosphogypsum impurities,which limits its further application and development.The hemi-dihydrate process can produce high-concentration phosphoric acid,and the by-product phosphogypsum is of high quality.However,in industrial operation,the hemihydrate phosphogypsum（HPG）often has small particle sizes and fine morphology in the hemi-dihydrate process,resulting in difficult filtration of phosphoric acid,reduced phosphorus recovery rate and deterioration of phosphogypsum quality.Based on the operation problem of hemi-dihydrate wet process,this paper uses typical mid-and low-grade phosphate ore as raw materials to carry out the optimization of operating conditions and phosphogypsum crystallization regulation.The flow direction of elements and the changes in the form of occurrence were clarified.The influence of different conditions on the crystal phase and morphology of HPG was systematically studied,and the optimized process conditions for hemihydrate reaction were determined.The optimization conditions for phase conversion of HPG and the formation and inhibition mechanism of eutectic phosphorus were further clarified.which provided a basis for the stable continuous operation of hemi-dihydrate process and the quality improvement of phosphogypsum.The main research contents and conclusions are as follows:（1）The flow direction and transformation of the form of the elements in the hemi-dihydrate process were studied,the changes of solid-liquid phase during the hemihydrate reaction and phase transformation process were analyzed,and the distribution of impurity elements in phosphogypsum and phosphoric acid in phosphate ore was clarified.The results showed that in the hemihydrate reaction stage,phosphate ore decomposed into phosphoric acid（>40 wt.%P₂O₅）and HPG,Al,Mg and F in phosphate ore entered phosphoric acid in ionic or complex state,Na,K and Si entered HPG in the form of poorly soluble fluorosilicate and Si O₂,and the total phosphorus content in HPG reached 16.40 wt.%P₂O₅.In the phase transformation stage,the phosphorus content of phosphogypsum decreased to 1.09 wt.%P₂O₅,the morphology was diamond-shaped flakes,and the overall phosphorus recovery rate of the process was about94.55%.（2）Using typical mid-and low-grade phosphate ore as raw materials,the optimization of hemihydrate reaction process and the regulation of HPG crystallization were carried out.The process conditions such as SO₄^2-concentration,P₂O₅ concentration and reaction temperature in the hemihydrate reaction process were optimized.The results showed that with the higher concentration of SO₄^2-and P₂O₅,the HPG gradually changed from coarse columnar to slender rods,the particle size decreased,and Ca SO₄ and Ca SO₄?2H₂O appeared.Optimized process conditions:40 wt.%P₂O₅,2.5 wt.%SO₄^2-,reaction temperature 95°C,stirring rate 300 rpm,HPG could grow to 70μm,and the morphology was long columnar,which is conducive to concentrated phosphoric acid filtration,and the phosphorus content in HPG was reduced to 1.39wt.%P₂O₅.（3）The influence of liquid phosphoric acid and sulfuric acid concentration on the crystal phase transformation was systematically studied,and the phosphorus release and conversion mechanism in the conversion process was clarified.The results showed that during the phase conversion process,HPG dissolved and water-soluble phosphorus dissolved,and HPO₄^2-entered the dihydrate in the form of adsorption and lattice doping to form eutectic phosphorus.The phase transformation process was optimized by using on-site HPG as raw material,and the analysis of pure Ca SO₄?0.5H₂O was used to monitor the recrystallization process of dihydrate.When the concentration of phosphoric acid increased to 2.0 wt.%H₃PO₄,the average particle size of the crystal decreased to 19.04μm,and the phosphorus content increased to 0.34%P₂O₅.The system then added sulfuric acid,by reducing the crystal supersaturation and critical nucleation particle size,slowing down the influence of phosphoric acid,effectively reducing the phosphoric acid group lattice substitution and adsorption,when the sulfuric acid concentration reached 8.0 wt.%H₂SO₄,the crystalline phosphorus content of dihydrate decreased to 0.10%P₂O₅.