Experimental And Theoretical Study On The Reaction Process Of Azurite Sulfide Catalyzed By Ammonium Salt And Enhanced Xanthate Flotatio

Copper oxide minerals,which represent an important copper resource,are typically processed using the economical sulfidization-xanthate flotation method.However,using direct sulfidization treatments,it is difficult to completely sulfidize the surfaces of these minerals such that a significant amount of the copper oxide is lost in the tailings.Hence,the copper flotation performance is poor and at a low economic benefit.The surface sulfidization of copper oxides was enhanced by pretreatment with ammonium salts prior to the use of Na₂S,to promote the adsorption of xanthate on the mineral surfaces.Sequentially,the flotation performance gets improved.However,it is still unclear to the copper oxide minerals of enhanced sulfidization process by ammonium salts.Azurite is a common copper oxide mineral for which the global reserves are second only to malachite,and both minerals are often found together.Even so,prior research concerning the flotation of oxidized copper ore has been performed primarily with malachite and there has been comparatively little work regarding azurite.On this basis,the present study performed systematic theoretical and experimental analyses of the surface sulfidization reactions of azurite and the enhanced xanthate flotation of this mineral in an ammonium salt system.This paper takes as azurite as the research object,the electronic structure and other properties of azurite crystal surfaces were evaluated using density functional theory（DFT）calculations.Species found in aqueous solutions as well as the distribution of copper ions in such solutions were also ascertained numerically through the solution chemical calculation.These calculations provided a theoretical foundation for subsequent examinations of the sulfidization and ammonium salts catalytic sulfidization flotation of azurite.These processes were characterized by micro-flotation tests,contact angle assessments,Fourier transform infrared（FT-IR）and ultraviolet（UV）-visible spectroscopy,zeta potential measurements,inductively coupled plasma mass spectrometry（ICP-MS）,X-ray photoelectron spectroscopy（XPS）and time of flight secondary ion mass spectrometry（ToF-SIMS）.The thermodynamics of the sulfidization process were also evaluated.On the basis of these characterizations,the flotation behavior of azurite,the sulfidization mechanism,the ammonium-assisted catalytic sulfidation process and the adsorption properties of the collector were all ascertained.The results of this work confirm that the addition of ammonium phosphate provides a more active azurite surface.This research also revealed the reaction mechanisms associated with ammonium salt catalytic sulfidization.Calculations showed that the intrinsic electron conductivity of ideal azurite was found to be poor.The Cu-O bond length was found to be significantly longer than the C-O bond length,meaning that the former are more likely to be broken as the azurite is crushed.The（011）cleavage surface of ideal azurite has the lowest surface energy and that the density of states（DOS）near the Fermi level is primarily associated with Cu 3d and O 2p energy levels while Cu atoms serve as active sites for subsequent surface reactions.The flotation of azurite is affected by the chemical composition of the pulp solution and the form of existence.Cu₃（OH）₄²⁺is the primary compound in the pulp solution at the optimum pH for flotation.The present work indicated that the flotation recovery of azurite initially increased following the addition of Na₂S but subsequently decreased with continued increases in the Na₂S concentration.The addition of（NH₄）₃PO₄ prior to sulfidization to pretreat the azurite while employing the same concentrations of Na₂S and Na IX was found to increase the flotation recovery by 10%-15%.The dominant species of Na₂S in an azurite flotation pulp solution is HS^-and the adsorption of this ion on the azurite decreases the surface zeta potential on the mineral surface.Hence,a high Na₂S concentration promotes the formation of copper sulfide species.However,the concentration of residual S ions in the pulp also increases at higher Na₂S concentrations and these ions inhibit the adsorption of the collector on the mineral surface.After the azurite was treated with Na₂S,active Cu-S sites were formed on the mineral surface while copper sulfide（CuS）and poly（copper sulfide）（CuSn）were also produced and some Cu（Ⅱ）species were reduced to give Cu（Ⅰ）compounds.The DFT calculations indicated that the adsorption energy was-149.42 kJ/mol following the adsorption of HS^-on the azurite（011）surface.As such,the sulfidization reaction of azurite would be expected to be spontaneous.The geometric structure of the azurite（011）surface is also changed following HS^-adsorption together with electron transfer between Cu and S atoms.The DOS of Cu and S atoms has multiple splitting peaks.The Cu 3d and S 3p energy levels were found to overlap near the Fermi level,indicating that sulfidization of the azurite surface was primarily caused by the bonding of S atoms in HS^-ions with Cu atoms on the mineral surface to produce copper sulfide species.The addition of（NH₄）₃PO₄ promoted the dissolution of copper ions on the azurite surface,based on the action of NH₄⁺ions that provided NH₃ species.This NH₃ not only reacted with Cu species in the solution to form copper ammonia ions but was also adsorbed on Cu sites on the azurite surface to produce an unstable copper ammonia complex,a portion of which dissolved in the solution.This surface dissolution behavior resulted in a higher concentration of copper ammonia complex ions near the azurite surface than in other regions of the solution,primarily in the form of[Cu（NH₃）₃]²⁺.Following the addition of Na₂S to the solution,S species adsorbed on the azurite surface to form copper sulfide while NH₃ in the copper ammonium complex ions was replaced to produce Cu-S species.In addition,S species reacted with copper ammonium complex ions near the azurite surface to form copper sulfide.This copper sulfide was highly active and was immediately adsorbed on the azurite surface to increase the thickness of the copper sulfide layer.In this process,S ions replaced NH3 in the copper ammonium complex ions and the NH₃ that was released into the solution regenerated this reactant.Hence,the NH₃in（NH₄）₃PO₄ plays the role of catalytic sulfidization.Compared with direct sulfidization,this catalytic process generates more S species in the solution that can be adsorbed on the mineral surface to generate copper sulfide.As such,the concentration of Cu（Ⅰ）species on the azurite surface increases along with the proportion of polysulfides.The present DFT calculation results also demonstrate that the adsorption energy on the azuriate（011）surface decreases after ammonium salt catalyzed sulfidization while the surface structure is greatly changed and the Cu-O bonds are weakened.The DOS of the Cu and S atoms are decreased and ammonia and sulfur species are both present on the azurite in unstable intermediate states.Thus,at the completion of sulfidization,ammonia species have dissociated into the solution while the structure of copper sulfide species on the mineral surface recombines in conjunction with changes in chemical bonds.These processes result in the formation of a stable layer of adsorbed copper sulfide.For these reasons,sulfidization of the azurite surface is enhanced by the addition of an ammonium salt.This effect promotes the adsorption of the collector on the mineral surface to increase floatation of the azurite.