Determination of fluorine in tungsten ore and its leaching residue by ion selective electrode method with distillation of sulfuric-phosphoric mixed acid
The accurate determination of fluorine in tungsten ore and its leaching residue is of great signifi-cance for process optimization and cost reduction.Taking advantage of strong acidity and high boiling point of sulfuric-phosphoric mixed acid,the sample was dissolved by sulfuric-phosphoric mixed acid and heated for distillation.The fluorides were converted to hydrogen fluoride.Then hydrogen fluoride was absorbed with sodium hydroxide solution and converted to sodium fluoride based on the principle of acid-base neu-tralization.Therefore,the separation of fluorine from metal ions such as aluminum and calcium were real-ized.The bromocresol green was selected as the indictor.After adjusting the pH of solution to 5-6 using hydrochloric acid and sodium acetate solution,the total ion strength buffer solution was added.The ab-sorption solution was determined by ion selective electrode method,realizing the determination of fluorine in tungsten ore and its leaching residue.The calibration curve was drawn with the mass concentration of fluorine as abscissa and its corresponding potential as ordinate.The linear correlation coefficient of calibra-tion curve was 0.999 6.The linear range was 0.025％-12.50％.The contents of fluorine in samples of tungsten ore and its leaching residue were determined for 11 times according to the proposed method,and the relative standard deviations(RSD,n=11)of determination results were between 3.0％and 5.9％.The standard solution of fluorine ion was added into samples for standard addition recovery test,and the recoveries were between 97％and 105％.The contents of fluorine in samples of tungsten ore and its leac-hing residue were simultaneously determined by the proposed method,sodium hydroxide fusion-ion selec-tive electrode method in YS/T 1115.13-2016,and perchloric acid distillation-EDTA titration in GB/T 18114.11-2010.The results showed that the differences of determination values between the proposed method and two standard methods were within the allowable ranges specified in the corresponding standard methods.
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