Process And Mechanism Study On Recovery Of Vanadium And Molybdenum From Vanadium-Molybdenum Wastewater Deposited Residue By Low-Temperature Roasting,Water Leaching,and Stepwise Precipitation

The vanadium-molybdenum wastewater deposited residue（VMD residue）is mainly produced in the process of vanadium-titanium-magnetite smelting wastewater treatment,with a large annual discharge.Due to containing rare metals such as V and Mo,it has become an important secondary resource of vanadium-molybdenum.This paper based on circular economy concept,around issues of VMD residue utilization,conducted extracting vanadium and molybdenum by potassium carbonate low-temperature roasting-water leaching,precipitate vanadium in weakly alkaline solution-precipitate molybdenum by hydrothermal vulcanization,and evaporation concentration-cooling crystallization to recover potassium.An efficient recovery process of vanadium and molybdenum from VMD residue by low temperature roasting and water leaching-fractional precipitation was developed.The main conclusions of the paper are as follows:（1）Thermodynamic analysis shows that,compared with sodium carbonate,the temperature required for the conversion reaction of calcium vanadate and calcium molybdate is lower with the participation of potassium carbonate.The carbon to calcium ratio decreases from 2.66 to 1.5,and the reaction temperature decreases from 600℃to200℃.The low temperature conversion of calcium vanadate and calcium molybdate to potassium vanadate and potassium molybdate can be achieved.Ferrochrome spinel still exists at this temperature,molybdenum vanadate and chromium show a separation trend which was conducive to the subsequent precipitation recovery of vanadium and molybdenum.Under the conditions of roasting temperature 200℃,potassium carbonate dosage n（K₂CO₃）/n(Ca²⁺)=1.5,roasting time 180 min,water leaching temperature 95℃,water leaching time 250 min,the vanadium and molybdenum leaching rates could reach83.14%and 99.94%respectively,and chromium leaching rates were lower than 0.35%.（2）Under the conditions of ammonium coefficient 4,vanadium precipitation temperature 50℃and vanadium precipitation time 120 min,the vanadium precipitation rate is up to 99.19%,and the purity of 98.28%V₂O₅ product can be obtained by the weak basic ammonium salt precipitation method.The mother liquor was further treated by hydrothermal vulcanization method.Under the conditions of initial molybdenum precipitation p H=8,temperature 200℃and time 120 min,the molybdenum precipitation rate could reach 99.80%,and the purity of 98.65%Mo S₂ product was obtained.When the evaporation rate is 33.33%,41.85%K⁺and 79.75%SO₄^2-can be recovered.The final crystallization product is potassium sulfate,whose purity reaches the premium grade standard of agricultural potassium sulfate（GB/T 20406-2017）.The remaining concentrated liquid is recycled for leaching and water washing.（3）Calcium vanadate and calcium molybdate in VMD residue are converted to potassium vanadate and potassium molybdate which are easily soluble in water by potassium carbonate during roasting.However,17%of vanadium still exists in chromite spinel in the form of isomorphism,resulting in the vanadium leaching rate cannot be further improved.Finally,the total vanadium recovery rate reached 82.47%,the total molybdenum recovery rate reached 98.88%,and the potassium recovery rate reached40.30%.The leaching slag could be returned to the main process of vanadium extraction from vanadium slag.The pharmaceutical cost,energy consumption cost and product value of the whole process were estimated.The estimated gross profit of resource utilization of VMD residue was 30537 RMB/t.The process can significantly reduce the environmental harm of VMD residue,the product has high added value,and realize the low carbon and efficient recycling of VMD residue.