Study On A Clean Metallurgical Process Of Tungsten Based On Hydrochloric Acid Circulation

China has the largest reserves,production,export,and consumption of tungsten resource.With the increasing exhaustion of high-quality tungsten minerals,much attention has been paid to the exploitation and extraction of low-grade complex scheelite.However,when treating this low-grade complex scheelite with Na OH or Na₂CO₃leaching,which is the main process of tungsten extraction in the tungsten metallurgical industry nowadays,there will be problems.For example,it has poor adaptability to these minerals and will generate a large amount of high-salt wastewater,and hazardous waste（alkali leaching slag）.Therefore,using low-grade scheelite with high content of phosphorus and molybdenum（hereinafter called the‘low-grade scheelite’）from Luanchuan,Henan province as raw material,this paper systematically studied the contents of hydrochloric acid pretreatment of the low-grade scheelite,hydrochloric acid decomposition of high-molybdenum scheelite concentrate from pretreatment process,purification of leaching solution from hydrochloric acid pretreatment and decomposition process,hydrochloric acid regeneration and recycling.Accordingly,a double-cycle clean metallurgical process consisting of hydrochloric acid cycle pretreatment of low-grade scheelite and hydrochloric acid cycle decomposition of high-molybdenum scheelite concentrate was proposed.This new process has broken the limit of the high-quality requirement of scheelite by the hydrochloric acid decomposition process,realized the efficient decomposition of scheelite and the resource recovery of valuable elements such as molybdenum and phosphorus associated with scheelite,in addition,basically achieved zero discharge of wastewater and waste residue.The main conclusions are as follows:（1）Hydrochloric acid pretreatment of low-grade scheelite can effectively leach calcium-bearing gangues and produce a high-molybdenum scheelite concentrate with few other impurities.The low-grade scheelite is mainly composed of scheelite（33.21%）,calcium molybdate（5.42%）,apatite（35.06%）,calcite（15.00%）,and fluorite（1.95%）.The thermodynamic analysis indicated that apatite and calcite can be leached with hydrochloric acid easily,and scheelite and calcium molybdate were leached in the form of phosphotungstic acid,phosphomolybdic acid,oxo-molybdenly cations,and chloro-molybdenly complexes,due to apatite was leached in the form of phosphoric acid and a hydrochloric acid-phosphoric acid leaching system was formed.The experimental results showed that the leaching of phosphorus,molybdenum,and tungsten was mainly affected by the final concentration of hydrochloric acid and reaction temperature,the final concentration of hydrochloric acid should be controlled at about 1.0 mol/L and the reaction temperature below 40°C.Pretreating low-grade scheelite using 4 mol/L HCl,and a liquid-solid ratio of 3 m L/g,stirring speed of 300 r/min at 20°C for 60 min,apatite and calcite were almost completely leached,and the leaching efficiency of molybdenum was 20.61%,while that of tungsten was only 0.95%.The content of tungsten（WO₃）in scheelite increased from 26.41%to 58.27%,and the grade of scheelite increased 1.2 times.As calcium molybdate is associated with scheelite in the form of isomorphism and embedded with each other,the separation effect of tungsten and molybdenum was not good.（2）Hydrochloric acid decomposition of high molybdenum scheelite concentrate can efficiently decompose scheelite and achieve tungsten and molybdenum separation to a large extent.The high molybdenum scheelite concentrate is mainly composed of scheelite（72.36%）,calcium molybdate（7.82%）,and fluorite（8.49%）.The thermodynamic analysis indicated that scheelite and calcium molybdate were converted to tungstic acid and molybdic acid through reacting with hydrochloric acid,while molybdic acid can be further leached in the form of oxo-molybdenly cations and chloro-molybdenly complexes.The experimental results showed that the decomposition efficiency of scheelite was very high with high hydrochloric concentration and high temperature.At the initial stage of the reaction,scheelite reacted rapidly with hydrochloric acid,but the reaction rate gradually slowed down due to the formation of tungstic acid that wrapped the mineral grain.Molybdenum was difficult to be completely leached,as molybdic acid and tungstic acid formed in hydrochloric acid decomposition were also embedded with each other.Under the conditions of initial hydrochloric acid concentration of 7 mol/L,a liquid-solid ratio of6 m L/g,reaction temperature of 95°C,reaction time of 90 min,and stirring speed of 300 r/min,the decomposition efficiency of scheelite was 99.8%,and leaching efficiency of calcium molybdate was 84.3%,and there was still 13.4%of calcium molybdate was converted to molybdic acid.The residue rate was only 3.9%after being dissolved by ammonia,and there was still a certain amount of tungsten in the residue,which can be returned to hydrochloric acid decomposition,so that no residue was generated during the hydrochloric acid decomposition process.The molybdenum in hydrochloric acid severely inhibits the leaching of calcium molybdate,and the concentration of molybdenum in decomposed mother liquor cannot be increased by cyclic leaching.When the concentration of SO₄^2-and Ca Cl₂in regenerated hydrochloric acid exceeds 0.01 mol/L and 0.25 mol/L,respectively,the decomposition efficiency of scheelite will be decreased.But,SO₄^2-has insignificant effect on the leaching efficiency of calcium molybdate,while Cl^-in Ca Cl₂ can greatly enhance the leaching efficiency of calcium molybdate.（3）Tungsten,molybdenum,iron,fluorine,phosphorus,silicon,and manganese in the pretreatment mother liquor can be separated and recovered successively through solvent extraction and chemical precipitation method,and a relatively pure Ca Cl₂ solution was obtained.Tungsten and molybdenum could be recovered and separated from iron in pretreatment mother liquid using TBP+P507 as extractant.About 99%of molybdenum,94%of tungsten,and 10%of iron were extracted with 24%TBP+16%P507+60%sulfonated kerosene under the conditions of A/O ratio of 3:1,temperature of 15°C,contact time of 10 min,and 2-stage countercurrent extraction;Then,85%of iron was eluted with almost no loss of tungsten and molybdenum under the conditions of 2 mol/L HCl,A/O ratio of 2:1,temperature of 15°C,elution time of 10 min;Finally,almost 100%of molybdenum and tungsten were stripped from eluted organic phase under the conditions of 5 mol/L ammonia,A/O ratio of 1:3,temperature of 15°C,contact time of 10 min,and 2-stage countercurrent stripping.Iron,fluorine,phosphorus,silicon and manganese in raffinate can be separately recovered using the fractional chemical precipitation method.And an iron residue containing rare earth elements,a Ca F₂ residue,a high quality Ca HPO₄·2H₂O,and a silicon-manganese residue were obtained,respectively.（4）Molybdenum and iron can be extracted and removed from hydrochloric acid decomposition mother liquor using solvent extraction with TBP,and a relatively pure HCl-Ca Cl₂ solution was obtained.The extraction efficiency of molybdenum and iron reached 99.8%and 99.9%respectively,with 50%TBP+20%2-ethylhexanol+30%sulfonated kerosene under the conditions of A/O ratio of 5:1,extraction temperature of 15°C,contact time of 5 min,and 2-stage countercurrent extraction,the concentration of molybdenum and iron in raffinate were only 0.01 g/L and0.002 g/L.Then,about 99.7%of molybdenum and 99.9%of iron were stripped respectively under the conditions of O/A ratio of 1:1,temperature of 15°C,contact time of 5 min,and 2-stage countercurrent stripping,the concentration of molybdenum and iron in stripping solution were about40.10 g/L and 12.29 g/L.（5）The Ca Cl₂ and HCl-Ca Cl₂ solutions generated from purification of the hydrochloric acid pretreatment mother liquor and decomposition mother liquor can be regenerated to hydrochloric acid with sulfuric acid conversion method.Using the regenerated hydrochloric acid to pretreat the low-grade scheelite or decompose high molybdenum scheelite concentrate,the effect was basically same as that of analytically pure hydrochloric acid.The solubility of calcium sulfate dihydrate,which was crystallized from hydrochloric acid regeneration process,in HCl and HCl-Ca Cl₂ solutions increased with the increase of temperature,but first increased and then decreased with the increase of hydrochloric acid concentration,and reached maximum at 3.0 mol/L and 2.0 mol/L HCl,respectively.In addition,the common ion effect of Ca Cl₂ significantly decreased the solubility of calcium sulfate dihydrate in HCl-Ca Cl₂ solution.Hydrochloric acid regeneration can be achieved by reacting with 18.4mol/L H₂SO₄ at room temperature,300 r/min for 60 min according to the concentration of hydrochloric acid to be regenerated;And the concentration of SO₄^2-in regenerated hydrochloric acid can be decreased below 0.01 mol/L,which met the requirements of cyclic decomposing scheelite,through inhibiting the dissolution of calcium sulfate dihydrate with controlling the Ca Cl₂ concentration of regenerated hydrochloric acid to 0.25 mol/L,then further removing SO₄^2-with a Ba CO₃stoichiometric ratio of 0.95.The obtained calcium sulfate dihydrate was basically free of impurities after countercurrent washing with deionized water,and the concentration of H⁺in washing filtrate was relatively high,which can be returned for hydrochloric acid regeneration after appropriate supplementation of Ca Cl₂.