Surface Modified Silica Gel And Alumina And Their Adsorption Properties For Au(S₂O₃)₂^3- With The Mechanism

Thiosulfate gold extraction technology,as a green and environmental non-cyanide gold extraction process,is one of the research hotspots in hydrometallurgy.The research of thiosulfate gold extraction technology mainly focuse on three aspects:gold dissolution mechanism,improvement of leaching process and gold recovery.Among them,the technology has not been industrialized due to the lack of effective and low-cost gold recovery method in leaching solution.Therefore,it is of great significance to study the problem of recovering gold from leaching solution.In this thesis,gold recovery from thiosulfate leaching solution by adsorption method was studied.Modified silica gel(SG-SH)and modified alumina(Al₂O₃-SH)which can adsorb gold in thiosulfate leaching solution were prepared.Bonding mechanism and mode in the process of surface modification were analyzed.Adsorption properties of SG-SH and Al₂O₃-SH for gold in thiosulfate solution were discussed.Mechanism of each species in leaching components affecting the gold adsorption were elucidated.Desorption-regeneration process of gold loading silica gel or alumina and its practical application were also studied.Results showed that surface properties of SG and Al₂O₃ had been changed by surface modification which enhanced gold adsorption capacity of SG and Al₂O₃ in thiosulfate solution,especially when 3-mercaptopropyltrimethoxysilane(MPTMS)was used as modifier.Mercapto content and gold loading capacity of SG-SH and Al₂O₃-SH were 1.1 mmol/g,0.34 mmol/g and 13.51 kg/t,13.41 kg/t,respectively.The modification of SG is a process of chemical bonding,in which modifier molecule can react with the silanol by condensation on the surface of SG,to form Si-O-Si-C bond which released methanol.The bottom of each mercapto group could form new bond of Si-O-Si-C with 1-3.In thiosulfate solution,p H of the adsorption solution has an obvious effect on the gold adsorption,H⁺is released during the adsorption process,and alkalescence condition is favorable to the adsorption.Thiosulfate exhibits an inhibitory impact on the gold adsorption,and the adsorption process is an endothermic process.The gold adsorption in thiosulfate solution by SG-SH and Al₂O₃-SH involves a ligand exchange process.The gold(I)is adsorbed on the surface of the material.The mercapto groups on the material surface undergo deprotonation and to form a mercaptan-gold complex with the gold(I).Gold leaching reagents affected the gold adsorption of the SG-SH in the solution.Among them,Cu(II)inhibited the adsorption of gold,while the addition of NH₃ or ethanediamine(en)promoted the adsorption by the SG-SH.Cu(NH₃)₄²⁺or Cu(en)₂²⁺complexes formed by Cu(II)complex with NH₃ or en are the main species that affect the adsorption of gold on SG-SH.The mercapto groups on the surface of SG-SH are oxidized into disulfide bonds,which reduces the adsorption capacity of the SG-SH for gold in solution.In the study of desorption-regeneration process of the SG-SH and Al₂O₃-SH,it was found that the acidic solution of thiourea and sodium thiosulfate solution could be used for the elution of gold loading silica gel and alumina,and the eluted material could be directly used for re-adsorption,cycle performance is excellent.The adsorption properties of SG-SH and Al₂O₃-SH in the actual leaching solution were also investigated.It was found that the SG-SH and Al₂O₃-SH could realize gold recovery from Cu(II)-NH₃-S₂O₃^2-and Cu(II)-en-S₂O₃^2-system by multistage adsorption.Finally,the theoretical calculation was used to analyze the properties of Au(S₂O₃)₂^3-complex and the gold adsorption mechanism on material surface,the results show that S₂O₃^2-of Au(S₂O₃)₂^3-is a metastable ligand.When coordination with Au(I),the bond length formed between Au and S is 2.42A,bond angle of S-Au-S bond is 176.02°,the binding energy of Au(S₂O₃)₂^3-complex is-0.44 hartree.The charge transfer between the thiosulfate(ligand)and Au(I)occurs mainly from the 3s and 3p orbitals of the ligand S atom in thiosulfate to the 6s orbital of Au(I).The reason for the modified material can be used to recover gold from thiosulfate leaching solution is that the interaction between Au(I)and mercapto/mercaptan are stronger than that of thiosulfate.Moreover,mercaptan has a stronger binding ability to Au(I)than mercapto group,and the complex formed with mercaptan is more stable.Therefore,the mercapto group on the material surface will be converted into mercaptan during the adsorption process,and then releasing H⁺,which resulted the solution p H dropped.When gold(I)is adsorbed on the prepared material surface,the 6s empty orbital of Au(I)form?bond with ligand S,and the S atom on the material plays a leading role in the adsorption process.This thesis affords a new method and idea for gold recovery from thiosulfate leaching solution and provides a theoretical basis and technical support to the development of thiosulfate gold extraction technology.