Abstract:

Ammonium dihydrogen phosphate with high purity can be used as water-soluble fertilizer, flame retardant, feed and food additives and recently it has been studied for preparation of phosphate optical glass, cathode materials for lithium batteries and nonlinear optical materials.Due to its broad application prospect in the fields of agriculture, fire protection, food and materials, the demand for high purity ammonium dihydrogen phosphate is increasing.High purity ammonium dihydrogen phosphate can be fabricated via thermal phosphoric acid and extraction-purified wet-process phosphoric acid route.The formerroute is simple and can get high quality of product, but it is restricted to the influence of resources, energy and environment.While the latter route is complex and needs large investment.Hence, more researchers focus on the direct preparing high purity ammonium dihydrogen phosphate with raw wet-process phosphoric acid.But the lower P₂O₅ yield of product（25%~40%）inhibited its development due to the formation of a large amount of phosphorus-containing precipitates（e.g., metallic phosphate） during ammoniation.Therefore, herein modified-process with sodium fluoride was used to selectively precipitate metalcations to compounds containing no phosphorus（e.g., （NH₄）_x（Mg）_y（Al）_zF₆（OH）₆·2H₂O, NaMgAlF₆·XH₂O, MgAl₂F₈, MgAlF₅）before ammoniation and thus to promote P₂O₅ yield of the product.Meanwhile, the dosage of NaF was optimized.The influence of NaF on the XRD patterns and morphologies of precipitates, removal of cations and the residual fluoride content in phosphoric acid was analyzed.When m_NaF/m（mass ratio of NaF to H₃PO₄ solution） was lower than 3.5%, precipitates mainly existed as NaMgAl（F, OH）₆·H₂O, XMgAlF₆（X=K+or NH₄ ⁺）.While m_NaF/m was increased, the diffraction peaks intensity of NaMgAlF₆ gradually increased.When m_NaF/m was further increased to 6.0%, calcium-containing silicate or aluminate salts phases appeared.Significantly, the adding amount of NaF also influenced the filtration performance.The content of metal cations in phosphoric acid decreased with the increase of NaF content.When m_NaF/m was 6.0%, the content of Mg, Al, Fe, K, Na and Ca was decreased to 0.029%, 0.002 7%, 0.012%, 0.014%, 0.018% and 0.037%, respectively.Albeit higher m_NaF/m would result in higher removal efficiency of metal cations, the effect of NaF on the residual fluoride content in phosphoric acid was significant.When m_NaF/m was in the range of 1.2%~2.5%, there were no obvious differences in residual fluorine content of phosphoric acid, but further increased to 5%, the residual fluorine content in phosphoric acid was sharply increased from 0.148% to 0.331%.Hence, balancing the removal efficiency of metal cations and residual fluorine content in phosphoric acid, m_NaF/m was fixed at 2.5% for the following fabrication of ammonium dihydrogen phosphate.Ammonium dihydrogen phosphate with a purity of 98.53% was produced with a high P₂O₅ recovery ratio of 86.2% and low residual fluorine content of 0.23%, which met the class Ⅱ standard of industrial grade ammonium dihydrogen phosphate.

Key words: wet-process phosphoric acid, ammonium dihydrogen phosphate, removal of impurities