Physical And Chemical Behavior Of Niobium In Hydrothermal Process With KOH Alkaline

China has abundant reserves of niobium,but the grade of ore is low.Compared to traditional hydrofluoric acid leaching,the alkaline hydrothermal process is more suitable to extract niobium from the low grade niobium-bearing concentrate with higher recovery rate of niobium and no pollution and no corrosion of equipments,which has broad application prospects.At present,there are still few studies on the physicochemical behavior of niobium in KOH alkaline hydrothermal method.Therefore,it is extremely significant to do some fundamental research on niobium extraction by the alkaline hydrothermal process and provide theoretical basis for the actual production and experiment.In this paper,pure Nb₂O₅ and artificial niobium-bearing concentrate were studied as the starting materials.The dissolution behavior of niobium in KOH solution with different pH values were investigated by the calculation of high-temperature filling ratio and water evaporation ratio,thermodynamic calculation,E-pH plotting and other theorical analysis,through atmospheric dissolution and leaching test,combining with X-ray diffraction(XRD),scanning electron microscopy(SEM)and inductively coupled plasma(ICP)analysis.In addition,the kinetic model of niobium leaching was constructed and the mechanism of kinetic reaction and limited link were discussed in this study.The related research achievements are listed as follows:(1)The calculation of water evaporation ratio and high-temperature filling ratio shows that when the temperature remains constant,the water evaporation ratio,which has a greater impact on the concentration of KOH solution,increases with the decreasing of the filling ratio at room temperature;The higher the temperature,the higher the filling ratio at high temperature.And the greater the filling ratio at room temperature,the greater changes of filling ratio at high temperature with changing of temperature.For the safety of operation and the accuracy of the test,25% of the filling ratio at room temperature is suitable.(2)E-pH diagrams show that under the alkaline conditions of pH>9,niobium is leachable as NbO₃^-but iron and titanium are stable in solid state which mean that the leaching of niobium could be achieved by selecting the appropriate alkali leaching process and preventing iron and titanium from entering the leachate could be achieved.These prove that the KOH alkaline hydrothermal leaching of niobium concentrates is theoretically feasible and good for the recovery of niobium.(3)Combined with phase diagram,X-ray diffraction analysis and scanning electron microscopy analysis,the composition of artificial niobium-bearing concentrates are FeNbO₄(Fe2O₃-Nb₂O₅),Fe₂Ti₃O₉(Fe₂O₃·3TiO₂),SiO₂,Fe₂O₃,Nb₂O₅ and so on.(4)Experimental results of dissolution at room temperature indicates:The potassium niobate exists as K₈Nb₆O₁₉ in the solution obtained by dissolving pure Nb₂O₅ powder in KOH solution under atmospheric pressure when the pH of KOH solution is greater than 14.When the pH of solution is less than 14,the content of K and Nb in the leachate solution still need further investigation due to the limitation of the detection method.The equation of reaction that KOH solution dissolved pure Nb₂O₅ is obtained as 8OH^-(aq)+3Nb₂O₅(s)=Nb₆O₁₉^8-(aq)+4H₂O(5)The kinetic analysis show that the leaching process of niobium concentrate dissolved in KOH solution is consistent with the unreacted core model controlled by diffusion of the solid product layer.The XRD and SEM-EDS analysis show that the solid product layer is formed.[Nb₆O₁₉]^8-ion diffused through the solid product layer is the limiting link of the leaching process.The kinetic equation of niobium-bearing concentrate dissolved in KOH solution under the conditions of reaction temperature of 120?180?,KOH initial concentration of 6mol·L^-1 and alkali-ore ratio of 4:1 is obtained as 1+2(1-X)-3(1-X)^2/3=12.83·exp(-27649/RT)·tThe apparent activation energy of the leaching process is 27.649kJ·mol^-1.