Fundamental Research On Valuable Metals Recovery From Jarosite By Slurry Electrolysis

In China,currently more than 50 million tons of jarosite are piled up with an annual increasing rate of 1,500,000 tons.Jarosite is a kind of hazardous waste,harmful but resourceful.There are many studies reported on jarosite pollution control and resource recycle,as well as industrialization cases.However,its resource and environmental characteristics are remain unclear.New,environmentally friendly,short-process,high-value and efficient recycling technologies need to be developed.Slurry electrolysis has the advantages of short-process,high value,and high recycling efficiency.However,to the best knowledge,there is no research on jarosite recycling by slurry electrolysis.Therefore,starting from the resource and environmental characteristics of jarosite,this thesis first uses electric field enhanced leaching to explore the leaching behavior of valuable metals such as iron,zinc,copper,and heavy metals such as lead,cadmium and arsenic in jarosite.Then,slurry electrolysis is adopted to recover valuable metals such as iron,zinc,copper from jarosite.After that,the circulation of electrolyte in the process of jarosite resource utilization by slurry electrolysis and the migration and transformation of zinc,copper,lead,cadmium,arsenic,etc.in the whole system are discussed.The main findings are as follows:（1）The main phase of jarosite are ammonium-jarosite,gypsum and zinc ferrite,containing toxic and harmful heavy metals,such as As,Cd,Cr,Ni,Cu,Sb,Sn,especially As,up to 4.025 mg/g.Toxicity leaching concentration of Cd,Zn,Ni,Se and Be in jarosite exceeds their thresholds of hazardous waste in China,while Cd,Pb and Se exceeds their thresholds of TCLP.Therefore,jarosite is classified as hazardous waste.The sum of exchangeable&soluble,reduceable,and oxidizable fractions of Be,Cd,Ni and Se are higher than 40%,indicating that heavy metals in jarosite are easily soluble in water,thus easily migrating to the environment.In addition,jarosite shows a high bioavailability.Fe in the jarosite contributes the most ecotoxicity,3.97×10⁵ PAF·m³·day/kg_emitted;As has the greatest carcinogenicity,1.27×10^-5 cases/kg_emitted,and Pb is the second,1.64×10^-5 cases/kg_emitted;the highest noncarcinogenicity contributor is Pb,5.76×10^-3cases/kg_emitted,followed by Zn,As,Be and Cd.（2）Zn Fe₂O₄ occupies a large neutral and oxidation region in aqueous solution at room temperature,while NH₄Fe₃（SO₄）₂（OH）₆ is stable in the acidic,high oxidation potential region.The first dissolution rout of Zn Fe₂O₄ is that it can be directly converted into Fe²⁺and Zn²⁺in an acidic reduction environment.The second is to decompose into Zn²⁺and Fe₂O₃ first,and then Fe₂O₃ is further decomposed into Fe³⁺.NH₄Fe₃（SO4）₂（OH）₆ can be dissolved to Fe³⁺by increasing the concentration of H⁺or by reducing the redox potential to promote the reduction of Fe³⁺to Fe²⁺.p H is the main factor affecting the dissolution and conversion of jarosite.When the initial concentration of NH₄Fe₃（SO₄）₂（OH）₆ and Zn Fe₂O₄ is fixed,the concentration of Fe³⁺and Zn²⁺in the solution increases with the decrease of p H.p H<2 is more conducive to the dissolution of NH₄Fe₃（SO₄）₂（OH）₆ and Zn Fe₂O₄,and Fe in these two tends to be converted into positively charged soluble ions Fe²⁺/Fe³⁺under acidic conditions.Meanwhile,increasing H⁺concentration is also conducive to the dissolution of metal oxides such as As,Pb,Mn and Ag in jarosite.（3）Electric field can greatly promote the leaching of jarosite.Excessive solid-liquid ratio will affect the migration and diffusion of metal ions from the surface of solid particles to the solution.Increasing the concentration of H⁺can promote the dissolution of jarosite.Temperature can greatly increase the leaching efficiency of metals in jarosite.When the current density was 30 m A/cm²,the ratio of jarosite-to-extract volume ratio（solid-liquid ratio）was 70 g/L,and the sulfuric acid concentration was 1.5 mol/L,the leaching rate of Fe and Zn was respectively 98.12%and 79.35%for 8 h at 80°C.By electric field enhanced leaching,Fe³⁺in the ammonium-jarosite is reduced to Fe²⁺,which has a greater solubility in the acid solution.Under traditional conditions,zinc ferrite is first decomposed into Zn²⁺and Fe₂O₃,and then Fe₂O₃ is further decomposed into Fe³⁺.By electric field enhanced leaching,zinc ferrite is directly converted to Fe²⁺and Zn²⁺in an acidic reduced potential environment.During the electric field enhanced leaching,the leaching of Fe and Zn was controlled by chemical reaction and diffusion,and the apparent activation energy was respectively 16.57k J/mol and 30.30 k J/mol,and the pre-reference factors were 0.38 and 23.18,respectively.（4）Under the conditions of solid-liquid ratio of 70 g/L,p H of 2.0,Fe²⁺of 1 mol/L and 30m A/cm²,8 h,iron in the jarosite was recovered in the form of elemental iron in the cathode,with a recovery rate of 69.95%,a Zn recovery rate of 27.49%,and the recovery rate of As,Cd,Cu,Mn and Pb is 20.99%,52.20%,41.18%,36.18%and 22.95%,respectively.After the slurry electrolysis of jarosite,its environmental risk was significantly reduced,and its heavy metal leaching toxicities were lower than their limits of GB5085.3 and US TCLP;the metals in the residues after slurry electrolysis were mainly in the residual fraction（more than 90%）.Its bioavailability was greatly improved.The main growth crystal face of cathode iron obtained by slurry electrolysis is（110）face.During the slurry electrolysis,the Fe-（OH）-O bond of the[(OH,Fe³⁺)（SO₄）₂]octahedron of some ammonium-jarosite was broken,and Fe³⁺was released from the crystal lattice,migrated to the cathode and reduced to elemental iron.Zn Fe₂O₄ in the jarosite hardly dissolves due to its stable spinel structure.Therefore,the recovery rate of elemental iron by slurry electrolysis is not high.（5）The recovery rate of Fe decreases with the increase of electrolyte reuse runs,from69.95%at the first run of the electrolyte to 55.59%at the 10th run.The purity of Fe in the cathode product does not change significantly,its purity remains at 96.84～97.65%,and the impurity element is mainly Zn（1.5～3.09%）.Fe accumulates in the electrolyte during the electrolyte reuse,and rises from 67.96%to 83.35%after 10 reuse runs.Most of Fe still remains in the electrolyte.During the electrolyte reuse,most of Cd,Cu,Zn and Mn are recovered into the electrolyte;Pb and As are enriched in the anode slag.The circulation of electrolyte has little effect on the main growth crystal plane of cathode iron,and Fe（110）surface is still the mainstay.The grain size is reduced as a whole,and the crystallization of the product is denser and more stable.Based on the research on of the resource and environmental characteristics of jarosite as well as its potential impact on ecosystem and human health,metal leaching behavior of jarosite during the electric field enhanced leaching was studied,metal recovery from jarosite by slurry electrolysis was discussed,the feasibility of electrolyte recycling was investigated,the migration and conversion of heavy metals in the process of electric field enhanced leaching,slurry electrolysis and electrolyte reuse were discussed.Finally,hazardous waste jarosite was successful detoxified and valuable metals were simultaneously recycled.The research results have reference value for solving the pollution control and recycling of jarosite.