Froth Flotation Of Spodumene From Spodumene-bearing Pegmatite Ores:Theoretical And Practical Aspects

Lithium（Li）is a strategic and crucial element for the 21st century energy production because it is widely used in energy storage,generation and conservation.Spodumene from pegmatite deposits is expected to be the main source of lithium from ores at present.Flotation is the most frequently utilized beneficiation technique.As such,a systematic study of spodumene flotation mechanisms was conducted and spodumene beneficiation flow-sheets were developed based on the fundamental study.It is anticipated that this work not only can contribute to a better understanding of spodumene flotation fundamentals,but also provide a promising guideline for the processing of spodumene-bearing pegmatite ores with similar mineralogical properties.（1）Crystal structure,surface stability and hydration structure of spodumeneIn this study,density functional theory（DFT）calculations using the DFT-D3 dispersion correction were applied to study the cystal structure ofα-spodumene,which is the most important lithium mineral in pegmatites.Four low index planes of spodumene were established by DFT,namely,（110）,（001）,（010）and（100）to investigate their stability.The shape of a spodumene nanoparticle was predicted via thermodynamic Wulff construction to be a columnal crystal particle with（110）facet as the dominant side plane,chiseled at its end by the（001）facet.The（110）surface was the main cleavage plane of spodumene.In this work,the surface hydration mechanism for spodumene（110）surface was also revealed from first principles calculations.The results indicated that spodumene（110）surface was strongly hydrophilic.The hydration layers on（110）surface could be divided into the chemisorbed and physisorbed water layers.Each Al atom on the surface could strongly bond with oxygen atoms of two water molecules.（2）Spodumene flotation from spodumene-bearing pegmatite ores under low-alkaline conditionsWith the aim of unraveling the sodium oleate（Na OL）flotation mechanisms of spodumene under low-alkaline conditions,we reported,here,an unprecedented investigation on the spodumene/Na OL interface by combining surface characterization measurements（i.e.,infrared spectroscopy and X-ray photoelectron spectroscopy）with ab initio molecular dynamics（AIMD）method.The results showed that the adsorption of Na OL on spodumene at p H 8～9 was the co-adsorption of inner-and outer-sphere.At low fatty acids concentration,the inner-sphere coordination of Na OL to the surface Al sites principally corresponded to chelate coordinate complexes,which was shown to be the more favorable in energy.At high fatty acids concentration,the inner-sphere adsorption of Na OL on the spodumene surface mainly corresponded to monodentate complexes.The surface aluminum sites at spodumene cleavage planes were found to be the major contributing factor for the spodumene flotation using Na OL collector at weakly alkaline p H.Thus,the flotation recovery of spodumene could be enhanced via exposing more aluminum on the surface of spodumene,improving the affinity of the surface aluminum sites towards Na OL and introducing cations as highly attractive adsorption sites for carboxylate collectors.Ion exchange was the prevailing process in the sorption of Ca²⁺on spodumene at weakly alkaline p H of 8.5.On MD simulation timescales,ion exchange between solution Ca²⁺cations and lattice Li⁺was observed for both semi-rigid（only lattice Li⁺free to move）and fully flexible（all lattice atoms free to move）spodumene（010）slabs.The pre-adsorbed Ca²⁺ions on the spodumene surface,played an active role since they served as additional chemisorption sites for Na OL collector.However,when the flotation pulp contained high concentrations of calcium ions,large amounts of residual Ca²⁺ions in the pulp were detrimental to spodumene flotation.The reason was that the residual Ca²⁺ions formed Ca（OL）⁺complexes with OL^-while Ca（OL）⁺showed poor affinity to the spodumene surface,leading to less adsorption of oleate ions on spodumene particles.Batch flotation tests of a spodumene-bearing pegmatite ore（real ore）were performed using Na OL as collector under weak alkaline p H conditions.The results showed that the performance of spodumene and flotation reagent additions required were directly affected by the hardness of flotation water.Based on the insights acquired through fundamental investigations and the mineralogy knowledge of the pegmatite spodumene,the flotation-magnetic separation flow-sheet was developed for the processing plant to product high-grade spodumene.In the case of the ore with head grade of 1.36～1.80%Li₂O,the final concentrate grade assayed>6%Li₂O at 67～72%recovery.（3）Spodumene flotation from spodumene-bearing pegmatite ores under high-alkaline conditionsIn the flotation system of spodumene at high alkaline pulp,ion adsorption is the main mechanism for calcium ions adsorption on the spodumene.The pre-adsorbed calcium species on the spodumene surface were the main attractive sites for Na OL chemisorption,thereby markedly increasing the amount of Na OL adsorbed on the spodumene surface.Calcium activation was therefore necessary for the spodumene flotation under high-alkaline conditions.Herein,we compared the adsorption of free Ca²⁺and Ca（OH）⁺hydroxy complex at the spodumene/water interface,combining three simulation methods（i.e.,static DFT,AIMD and Well-Tempered metadynamics simulations）.It was proved that calcium hydrolysis was strongly promoted at the spodumene/water interface than in bulk solution due to low dielectric constant of interfacial water.The simulations results unambiguously identified that compared to the adsorption of Ca²⁺,the adsorption of Ca（OH）⁺hydroxyl complex on the surface was more thermodynamically favorable.Our results predicted that Ca（OH）⁺hydrolyzed species dominated the spodumene/water interface.The theoretical calculation provided reasonable explanations for the adsorption behavior of calcium ions on spodumene surface.When Na OL was used as the collector for the flotation of calcium ion-activated spodumene at high-alkaline pulp p H,the hydrophobicity of spodumene depended on the structure of the Na OL absorbed on the surface.As the amount of absorbed Na OL increased,we observed monolayer adsorption having low degree lateral associations,hemimicelle adsorption,hemimicelle-bilayer adsorption and bilayer adsorption.To investigate the formation of different adsorbed structures,we computed the free energy of OL^-adsorption on the spodumene（110）surface using molecular dynamics simulations.The morphology of these adsorbed structures was characterized via density profiles and the angle distribution of OL^-.The conclusions drawn from purely theoretical computations matched remarkably well with the Na OL adsorption isothermal experiment observations and the flotation recovery data.In the flotation system of calcium-activated spodumene at high alkaline pulp,the Na OL-OHA（octylhydroxamic acid）mixtures at a molar ratio of（1～1.5）:1 showed a positive synergistic effect.The Na OL-OHA mixtures displayed a superior performance for spodumene flotation than single Na OL.Notably,the mixed collector improved the hydrophobicity of the spodumene surface,which could enhance the collection of coarse spodumene particles.The adsorption behavior of the Na OL-OHA collector mixture on the spodumene surface was ascertained through FTIR,XPS,AFM and molecular modelling.These results confirmed that the mixed collector resulted in the formation of“mixed Na OL-OHA aggregates”as the adsorption product on spodumene surfaces.The mixed Na OL-OHA aggregates were more orderly organized,consisting of higher amounts of“head-on”oriented collector ions on the surface,which therefore produced stronger hydrophobicity than single Na OL collector.Finally,a comparative study was conducted for processing a pegmatite spodumene ore from overseas.In the bench-scale investigation,the spodumene was floated from pegmatite ore at high alkaline p H using the Na OL-OHA collector mixture.A 5%increase of the overall Li₂O recovery was achieved as compared to that by the de-sliming step-low alkaline flotation process.Moreover,it was unnecessary to de-slime the ore prior to flotation using this method.