Impact of the urea nitrate content in the leaching solution on the granite weight decrease and the fluoride ion concentration

Abstract

The paper presents the results of the study on the impact of the initial concentration of urea nitrate and fluoride ion in the leaching solution on the weight decrease of granite mined at Prydniprovsk Specialized Quarry and the change in the fluoride ion concentration in the solution during intermittent and continuous agitation. During the experiments, we used urea nitrate with weight fraction of nitrate acid 41.5%, urea - 44.3%, water - 14.2%, and ammonium fluoride-bifluoride (fluorine content was 61.2%). Fluoride ion concentration was measured using a fluoride-selective electrode relatively a saturated silver chloride half-cell while the acidity of solutions was measured by titrimetry. Weight decrease was studied for a granite sample, grain fraction -2.0 + 1.0 mm, during intermittent stirring depending on the initial concentration of urea nitrate 0.25 g/dm3, 0.5 g/dm3, and 1.0 g/dm3 and fluoride ion 2.81×10-2 g/dm3 to 3.20×10-1 g/dm3 with intermittent stirring at a temperature of 20±2 °C for 7 days. The data on the change in sample weight depending on the initial concentration of fluoride ion and addition of urea nitrate were received. It was revealed that the decrease in the fluoride ion concentration ambiguously depended on the initial concentration of both urea nitrate and fluoride ion. In our opinion, the fluoride ion concentration decrease occurred with adding urea nitrate due to the fact that nitric acid urea could form adducts with both sulfuric and fluoric acids resulting in the decrease of their reactivity. We have studied the decrease of the granite sample weight and the change in the fluoride ion concentration under continuous stirring and at a temperature of 30±2 °C for 4 days. It was found that the change in the sample weight and concentration of fluoride ion during continuous stirring differed from the similar data obtained during intermittent stirring due to more intensive formation of urea nitrate adducts with sulfuric and fluoride acids. The initial dissolution rate was calculated for a granite sample, its value was 2.384×10-7 s-1 after adding urea nitrate and 2.299×10-7 s-1 without its addition.