The Fundamental Mechanisms Of Seawater On The Flotation Of Molybdenite

Due to the scarcity of fresh water resources and stringent environmental regulations for the quality of discharged water,incremental concentrators have resorted to multiple water sources including seawater,recycled water and even underground water containing high concentration of electrolytes as flotation media to save fresh water usage.While the flotation process with hydrolytic ions(i.e.Ca²⁺,Mg²⁺)is more complex and multifaceted than that in pure water,affecting the mineral surface properties and the particle�bubble attachment,thereby influencing flotation efficiency.Although many investigators studied the effects of salt ions on sulfide flotation and mineral surface properties,these reported results varied and even conflicted,keeping underlying mechanisms unclear.In this thesis,the flotation behaviors and kinetics of the naturally hydrophobic mineral�molybdenite were investigated in the absence or presence of dispersants?sodium silicate or sodium hexametaphosphate?,where four solutions including purewater,seawater,0.05M MgCl₂ and 0.01M CaCl₂ were chosen as the research objects.Based on many measurements?e.g.,surface hydrophobicity,Zeta potential and XPS?and methods such as solution chemistry study and EDLVO theory calculation,the changes on the surface properties of molybdenite treated in different solutions were investigated to understand the action mechanisms of sea cations and dispersants in molybdenite flotation process,further providing a theoretical foundation for the efficient flotation of molybdenite using seawater.The following results were obtained from this thesis:The flotation experiments indicated that molybdneite shows a higher flotation recovery in purewater,while a lower recovery in seawater,0.05 M MgCl₂ solution and 0.01 M CaCl₂ solution.Specially,the depressing effects in three salt solutions are more significant with the increase of pulp pH.In addition,the flotation recovery of molybdenite in 0.01 M CaCl₂ is higher than that in seawater while the recovery in seawater is higher than that in 0.05 M MgCl₂,which illustrated the depression of seawater on molybdenite flotation is mainly due to the presence of Mg²⁺.After the addition of dispersants,molybdenite recovery is slightly changed in purewater while significantly increased in three salt solutions.Specially,the effect of SS on improving the flotation recovery was weaker than that of SH.The flotation kinetics indicated that first-order model with rectangular distribution of flotation was considered to be the best fit for the molybdneite flotation process in purewater,seawater and 0.01 M CaCl₂ solution,while no one models of kinetics was suitable for fitting the molybdneite flotation process in 0.05M MgCl₂ solution.The orders of flotation rate constant obtained from the fitted first-order model with rectangular distribution was K_Purewater>K_CaCl2>K_Seawater>K_MgCl2,successively,revealing that the flotation rate was decreased in salt solutions and even the 0.05 M MgCl₂ showed the most significant negative effect.The most ideal kinetics model of flotation was still first-order model with rectangular distribution after the addition of dispersants.In addition,the presence of dispersant also increased the flotation rate and the theoretical maximum cumulative recovery of molybdenite,where the role of SH was more obvious than that of SS.Experimental result indicated that the depression of seawater on molybdenite flotation was due to the formation of abundant magnesium hydroxide precipitation,a bit of calcium carbonate precipitation and trace of calcium molybdate precipitation.These hydrophilic precipitations were adsorbed on the molybdenite surface through electrostatic attraction,resulting in a heterocoagulation phenomenon and the reduction of the surface hydrophobicity of molybdenite,thereby depressing the molybdneite flotability.In addition,the adsorption of these precipitations also changed the electric properties on molybdenite surface.The addition of dispersants,including SS and SH,reversed the total interaction force between molybdenite particles and magnesium hydroxide colloids from attraction to repulsion,further preventing the adsorption of magnesium hydroxide colloids on mineral surface,thereby restoring the depressed molybdenite flotability.In addition,SH can combine with Mg²⁺and Ca²⁺to form dissolvable complexes,thereby reducing insoluble Mg²⁺and Ca²⁺compounds or precipitation.