Synthesis Of α-hydroxyalkylphosphonic Acids And Its Extraction Performance

China is the world’s largest producer of electrolytic manganese which is mainly producted by wet process.Limited by the production process of electrolytic manganese,the magnesium ions in the solution system are continuously enriched,which increases the viscosity of the electrolyte and leads to the decline of current efficiency.In addition,magnesium sulfate and ammonium sulfate form double salt crystals to wrap the cooling water pipe,which affects the heat exchange of the electrolytic system and leads to a series of problems such as the decline of the quality of electrolytic manganese products.Therefore,the purification of manganese sulfate solution is of great significance.Solvent extraction has obvious advantages in metal separation,and it has been widely used in the field of hydrometallurgy.The extractant is the core and key of solvent extraction.However,commercial extractants are difficult to realize the effective separation of manganese and magnesium,so there is an urgent need to synthesize a high-efficiency extractant for the separation of manganese and magnesium.The quantum chemical descriptors ofα-hydroxyalkylphosphonic acid were calculated by DFT method,and its extraction properties were predicted,which laid a foundation for the synthesis ofα-hydroxyalkylphosphonic acid.It was found that the number of negative charges of O atom in the PO-H group ofα-hydroxy-2-ethylhexylphosphonic acid was higher than that of P204 and P507,and the HOMO value was higher than that of P204 and P507.It suggested that the coordination ability ofα-hydroxy-2-ethylhexylphosphonic acid with metals was higher than that of P204 and P507.The synthesis conditions ofα-hydroxyalkylphosphonic acid were optimized with HCl as catalyst and hypophosphoric acid and aldehyde as raw materials.It was obtained that when 1,4-dioxane was used as the solvent,the reaction temperature was80°C,the amount ratio of hypophosphoric acid to aldehyde was 1.2:1 and the reaction time was 5 h,the yield ofα-hydroxyalkylphosphinic acids were more than 80%.The products were characterized by FTIR,MS and NMR.It was found that the synthesized compounds contained characteristic functional groups such as P-OH,P=O and C-OH.The relative molecular mass of the obtained compounds was consistent with that of the ideal molecules.The ¹H NMR,¹³C NMR and ³¹P NMR spectra showed that the chemical shifts of the obtained compounds were consistent with those of the ideal compounds,indicating thatα-hydroxyalkylphosphonic acids had been successfully prepared.The effects of extraction time,ratio（O/A）,extractant concentration,aqueous p H and saponification rate of organic phase on the extraction rate of manganese and magnesium were investigated by extraction experiments.The results showed thatα-hydroxy-2-ethylhexylphosphinic acid had fast extraction rate for metals and reached equilibrium within 10 minutes.Its extraction capacity for manganese and magnesium was greater than P204.The mechanism of extracting metal ions withα-hydroxy-2-ethylhexylphosphinic acid was analyzed by saturation capacity method,FTIR and slope method.It was found that the molar ratio ofα-hydroxy-2-ethylhexylphosphonic acid to Mn²⁺was 2.13,so it was speculated that the coordination ratio ofα-hydroxy-2-ethylhexylphosphonic acid and Mn²⁺was 2:1.Comparing the FTIR spectra of the organic phase before and after extraction metal,the characteristic absorption peak of P-OH at 2287 cm^-1disappeared obviously,indicating that H in P-OH was replaced by metal ion.Therefore,the mechanism of the reaction is mainly cation exchange.Slope method proved that the complex obtained after the reaction was[MR₂（HR）₂],and the equation of extraction of divalent metal ions was M²⁺+2H₂R₂→MR₂（HR）₂+2H⁺.Usingα-hydroxy-2-ethylhexylphosphinic acid as extractant,the effects of ratio（O/A）,extractant concentration,aqueous p H and saponification rate of organic phase on the separation of manganese and magnesium were investigated.The results showed that when the extractant concentration was 200 g/L,the phase ratio（O/A）was 1:2,the p H of the solution was 5,the saponification rate was 30%,the extraction rate of manganese was 79.74%,the extraction rate of magnesium was 12.23%,the separation coefficient was 35.77.The electrolytic manganese anode solution was treated by the process of extraction,precipitation and calcination,and the high-purity magnesium oxide product was obtained.The purity was up to 98.17%,and the recovery rate of magnesium in the raffinate was 94.76%.