Regulation Of Lead-Iron Phase Separation During Iron Precipitation Process In Zinc Hydrometallurgy

Recycling of iron residue in zinc hydrometallurgy has attracted much attention in the field of nonferrous environmental protection.Jarosite residue is a precipitate formed during the iron precipitation process in the zinc hydrometallurgy,and belongs to hazardous waste（code:321-005-48）.More than 3000 tons of jarosite residue have been stored up.It contains a variety of valuable metals and has high comprehensive recovery value.Lead is one of the valuable metals with the highest content in jarosite residue.Owing to the complex forms of lead in jarosite residue and its stable combination with jarosite which makes them difficult to destroy,the resource recovery processes are usually complex and high cost,and lead recovery is difficult.In this study,a new idea of regulating the lead-iron phase separation during iron precipitation process was first proposed to inhibit the formation of stable lead occurrence form in jarosite residue from the"source",which could reduce the difficulty for separation and recovery of lead and iron,and provide support for the source reduction and resource recovery of jarosite residue.The main innovative results are as follows:（1）The occurrence forms of lead in jarosite residue were analyzed,and the relationship between the occurrence forms of lead and their recovery properties was expounded.The content of lead in jarosite residue was 0.03%-9.32%.In the iron precipitation process,Pb-incorporated natrojarosite,Pb SO₄-occluded natrojarosite,Pb SO₄-physically mixed natrojarosite,and Pb SO₄-chemically mixed natrojarosite can be formed.By comparing the releasing behaviors of different Pb-bearing natrojarosite under acid dissolution,we found that lead was easier to release from Pb SO₄-physically mixed natrojarosite.While it was more stable in both Pb-incorporated natrojarosite and Pb SO₄-occluded natrojarosite,which led to the difficult separation and recovery of lead from the jarosite residue.（2）The pathway of heterogeneous nucleation and growth of jarosite on anglesite during the iron precipitation process was first discovered and investigated,and the microscopic formation mechanisms of stable lead occurrence forms,i.e.,Pb-incorporated natrojarosite and Pb SO₄-occluded natrojarosite,were revealed.Anglesite slowly dissolved and released constituent ions for jarosite(i.e.,SO₄^2-and Pb²⁺),resulting in increasing surface roughness and local supersaturation,which promoted the heterogeneous nucleation and growth of jarosite.Due to the high adsorption energy which was-0.78 e V、-0.67 e V、-0.65 e V respectively and strong electron interaction of Fe SO₄⁺complex on anglesite（020）,（140）and（110）facets,jarosite preferentially nucleated on these facets.Subsequently,jarosite grew through a non-classical pathway of"crystallization by particle attachment",and Pb SO₄-occluded natrojarosite was formed finally.Meanwhile,the Pb²⁺released from anglesite was incoporated into the crystal structure of jarosite,forming Pb-incorporated natrojarosite with Fe:Pb molar ratio of 3:0.17.（3）The effects of substrate surface characteristics on the heterogeneous nucleation and growth of jarosite were investigated.The method of regulating the heterogeneous nucleation and growth of jarosite by hydrophobic modification of anglesite surface was explored and its effect on Pb SO₄-occluded natrojarosite formation was demonstrated.The roughness and hydrophobicity of anglesite surface were the key factors affecting the heterogeneous nucleation rate of jarosite.Heterogeneous nucleation rate of jarosite can be decreased by increasing hydrophobicity of anglesite,and the growth rate of jarosite can be decreased by reducing the specific surface area of anglesite,for the supersaturation in the interfacial fluid layer of anglesite surface can be lowered owing to the reduced release probability of constituent ions for jarosite.A method of hydrophobic modification of anglesite by sodium caprate was developed,and it was found that the stable combination of sodium caprate with anglesite resulted in obvious improved hydrophobicity and stablity of anglesite,which could significantly inhibit the growth of jarosite on its surface.（4）A new strategy of selective nucleation and growth of jarosite induced by competitive substrates was proposed.A method of lead-iron phase separation induced by in situ formation of amorphous iron minerals was developed,and the regulation mechanism was clarified.Regulation of lead-iron phase separation by addition of competition substrate and in situ formation of amorphous iron mineral substrate was compared.The results showed that in situ formation of amorphous iron minerals was superior,and the separation rate was mainly affected by alkali dosage,alkali addition rate,temperature and iron concentration.The amorphous iron mineral formed was identified to be schwertmannite.Its large specific surface area（37.063 m²/g）and high supersaturation in the interfacial fluid layer resulted from slow dissolution and release of Fe³⁺and SO₄^2-promoted the preferential crystallization of jarosite on its surface.Schwertmannite was dissolved and transformed into discrete jarosite and a small amount of goethite eventually,which significantly inhibited the formation of Pb SO₄-occluded natrojarosite.（5）The industrial applicability of lead-iron phase separation induced by in situ formation of amorphous iron minerals was verified.The hot acid leaching solution of zinc hydrometallurgy was simulated,and the optimal conditions for lead-iron phase separation and iron precipitation were determined as follows:alkali（5 mol/L Na OH solution）dosage of 11%,addition rate of 2 m L/min and reaction temperature of 90℃.Compared with traditional iron precipitation process,the removal rate of iron increased from 90.54%to 93.05%,and the surface iron content of anglesite decreased significantly from 24.78%to 3.81%,which confirmed the efficient separation of lead-iron phase during the iron precipitation process,indicating that the method has good potential for industrial application.