New Wet Process Of Phosphoric Acid From Poor Grade Phosphate Ores And Passivation Of Sesquioxide Impurities

Along with exploitation of high-grade phosphate ores during the recent decades, the quality of phosphate ores is becoming lower and lower. It has been a serious worldwide problem that how to exploit low-grade phosphate ores availably. In China, this problem is more obvious because of lack of high grade ores while plenty of poor grade. Furthermore, China is one of the countries, which is just developing phosphate industry rapidly.Aimed at JH phosphate ores in southwest of China, which is a kind of low-grade phosphate ores with high impurities of sesquioxide, two new wet processes have been put forward and studied. The plans are different from traditional processes, which includes Digestion of Low-grade Phosphate Ores in Phosphoric Acid and Nitric Acid Process (NP-Process) and Digestion of Low-grade Phosphate Ores in Sulfuric Acid, Phosphoric Acid and Nitric Acid Process (SNP-Process). The idea of two processes is described in this dissertation in detail. Moreover, Passivation Hypothesis is suggested, in which impurities of sesquioxide can be passivated in mixture acid. And the hypothesis is proved to truth at last.The orthogonal test results show that the optimal process conditions in NP-Process include: reaction temperature 65℃, ratio of liquid to solid 14∶1, initial phosphoric acid concentration 20%, nitric acid concentration 5.00 g·L^-1, KDD concentration 5.50 g·L^-1, reaction residence time 1 hour. The yield of P₂O₅ is more than 92%, while conversion fractions of Fe₂O₃ and Al₂O₃ from solid to liquid phase are lower than 17%. NP-Process can not only enhance yield of P₂O₅, but also inhibit Fe₂O₃ and Al₂O₃ into liquid phase.In order to reduce the ratio of liquid to solid and system loading in NP-Process, SNP-Process is developed. It not only possesses merits in many hands which involves the lower loading than in NP-Process for its lower ratio of liquid to solid, but also has the higher yield of P₂O₅ and the lower conversion fraction of Fe₂O₃ and Al₂O₃ from solid to liquid phase. However, the quality of the by-product phosphogypsum in SNP-Process is not better than that in NP-Process. The optimal conditions for JH phosphate ores in SNP-Process of the laboratory include: reaction temperature 65℃, ratio of liquid to solid 3∶1, initial phosphoric acid concentration 20%, nitric acid concentration 5.00 g·L^-1, sufuric acid concentration 45-50 g·L^-1, KDD concentration 22.00 g·L^-1, reaction residence time 3 hour.Whether in NP-Process or in SNP-Process, the components of Fe₂O₃ and Al₂O₃ in the solution of the tank can be controlled in the digestion. The Passivation Hypothesis is supposed as a mechanism. In the system of mixture acid, the chemical reaction occurs between impurities of sesquioxide and mixture acid while microbattery reaction also arises. The chemical reactions prompt to transfer cations Fe³⁺ and Al³⁺ into the liquid phase, while the local microbattery reactions consume H⁺ and increase the concentration of PO₄^3-. If [PO₄^3-][Fe³⁺] = K_sp(FePO₄) and [PO₄^3-][Al³⁺]= K_sp(AlPO₄), the dense layer of FePO₄ and AlPO₄ will deposit at the surface of sesquioxide. The segregation layer keeps apart the reactions between impurities and the mixture acid. The reactive residue of the ore in NP-Process was detected by XRD. The substance FePO₄ and AlPO₄, as well as other un-reactive components containing Fe and Al, were found. The Passivation Hypothesis is thus proved.