RESUMEN
The waste water generated from the sodium tungstate ion exchange process of scheelite hydrometallurgical extraction contains a certain concentration of fluorine ion, which caused environmental pollution and harmed human health. In this study, a new method for removing fluorine from the wastewater by precipitation with addition of lanthanum chloride was proposed. In the process, fluorine was removed by from the solution as insoluble lanthanum fluoride precipitates. To explore the favourable conditions for the formation of lanthanum fluoride, thermodynamic analysis of the La-F-H2O system was conducted. Results show that lanthanum fluoride is stable when the solution pH value is between 1.0 and 10.0, and the lanthanum fluoride is gradually converted into lanthana hydroxide when the pH value is more than 10.0 at 298K. The effects of various parameters on the fluorine removal were studied, and the optimum process parameters were determined. More than 92% of the fluorine can be removed when the concentration of fluorine in the solution ranged from 60 to 400 mg/L, the dosage of lanthanum chloride was 1.3 times of the theoretical amount, the pH value was 8.0 at 60°C for 30 min. After removing fluorine from the solution, the resiual fluorine concentrtion was lower than 10 mg/L, which could meet the requirement of national wastewater discharge.
RESUMEN
The ammonium tungstate solution obtained by leaching scheelite with phosphate contains a large amount of phosphorus. For production of qualified ammonium paratungstate products, phosphorus must be deeply removed from the ammonium tungstate solution. In this study, a novel process for ammonium phosphate recovery and deep phosphorus removal from the solution was proposed. First, ammonium phosphate was crystallized and separated from the ammonium tungstate solution by blowing ammonia and cooling. Results showed that the crystallization ratio of phosphorus was above 95% under the conditions of an ammonia concentration of 4.18 mol/L, an initial phosphorus concentration ranging from 15 g/L to 30 g/L, a holding time of 60 min and the temperature of 20°C. Then, the small portion of phosphorus remaining in the ammonium tungstate solution was further deeply removed by basic magnesium carbonate percipitation. The phosphorus removal efficiency was above 99% and tungsten loss was less than 0.22% under the following conditions: the basic magnesium carbonate stoichiometric ratio was 1.5, the initial phosphorus concentration was ranging from 0.5 to 4 g/L, the reaction time was 120 min and temperature was 25°C. After phosphorus removal, the concentration of phosphorus in the ammonium tungstate solution was below 10 ppm, which meant deep phosphorus removal was achieved.