稀有金属(英文版) 2020,39(03),316-326

Effectively enhancing recovery of fine spodumene via aggregation flotation

He-Peng Zhou Jie Hu Yong-Bing Zhang Yi-Jun Cao Xian-Ping Luo Xue-Kun Tang

School of Chemical Engineering and Technology,ChinaUniversity of Mining and Technology

Faculty of Resource and Environmental Engineering,Jiangxi University of Science and Technology

Chinese National Engineering Research Center of Coal Preparation and Purification,China University of Mining and Technology

Henan Province Industrial Technology Research Institute of Resources and Materials,Zhengzhou University

Western Mining Co.,Ltd.

作者简介：*Yi-Jun Cao,e-mail:caoyj@cumt.edu.cn;*Xian-Ping Luo,e-mail:lxp9491@163.com;

收稿日期：25 February 2019

基金：financially supported by the National Natural Science Foundation of China (No.51574240);the Natural Science Foundation of Jiangxi Province,China (Nos. 20181BBG70050 and 20171BBG70044);

Effectively enhancing recovery of fine spodumene via aggregation flotation

He-Peng Zhou Jie Hu Yong-Bing Zhang Yi-Jun Cao Xian-Ping Luo Xue-Kun Tang

School of Chemical Engineering and Technology,ChinaUniversity of Mining and Technology

Faculty of Resource and Environmental Engineering,Jiangxi University of Science and Technology

Chinese National Engineering Research Center of Coal Preparation and Purification,China University of Mining and Technology

Henan Province Industrial Technology Research Institute of Resources and Materials,Zhengzhou University

Western Mining Co.,Ltd.

Abstract：

The effects of shear strength on aggregation flotation processes for fine spodumene(particle size less than 19 μm) were investigated in this study.Sodium oleate was used as a surfactant and collector.The shear strength was controlled by varying the agitation speed of a selfmade stirring apparatus.The aggregation process was studied by measuring the continuous transformations in the size distribution and shape of flocs.The results showed that as the shear strength increased,the distribution of fine spodumene transformed from bimodal to unimodal mode.The flocs tended to bridge more branches with a high shear strength and form globule-like flocs with very high strengths.The parameter "aggregation degree" was introduced to evaluate the aggregation process as a function of shear strength.The flotation rate of flocs formed with different shear strengths was also studied.These results demonstrated that the flotation rate was closely related to shear strength and that there was a close correlation between this and aggregation degree.These results could be used to guide the actual production of fine particles via shear aggregation flotation.

Keyword：

Shear strength; Fine spodumene; Flotation rate; Shear aggregation; Sodium oleate;

Received： 25 February 2019

1 Introduction

Lithium (Li) is the lightest metallic element in nature and has excellent electrochemical activity [ 1, 2] .Li products are widely applied in high-energy batteries [ 3] ,the aerospace industry [ 4] ,power generation via nuclear fusion,and other fields.Spodumene is a typical Li-containing mineral and,in the mineral processing field [ 5] ,is the most important raw material from which Li is extracted.Froth flotation,a physicochemical separation process based on the difference in surface properties of valuable and gangue minerals,is the method most widely used in both laboratories and industrial practice to separate spodumene from other minerals [ 6] .However,in actual ore,spodumene is closely associated with aluminosilicate gangue minerals such as mica,quartz,and feldspar [ 7] ,which,in practical forth flotation operations,easily form fine-sized particles during the crushing and grinding processes because they are brittle [ 8, 9] .

Fine-grained particles of spodumene are difficult to recover in the flotation process because their low mass and momentum lead to a much lower probability of collision with and adhesion to bubbles [ 10, 11] .Even worse is the fact that the large specific surface area of the fine-grained particles of both spodumene and gangue minerals results in a high surface energy in the froth flotation system [ 12] .Thus,it would on one hand result in a highly viscous pulp that would reduce spodumene flotation indexes [ 13] ;on the other hand,the existence of fine-grained spodumene particles would increase the consumption of flotation reagent [ 14, 15] .It is also difficult to separate fine spodumene particles from gangue minerals of the same nature,which causes a serious entrainment of fine particles in the froth flotation process [ 16] .Therefore,finding ways to reduce the negative effects of fine spodumene particles in the spodumene froth flotation process is vital.

Thus far,research on this has focused on two aspects.The first type focused on modifications of the flow of the desliming process to improve quality of slurry and particle size [ 17] .Although this solves most of the technical problems with muddy spodumene,the addition of a desliming process would increase the overall complexity and cost of the spodumene flotation process.Moreover,the spodumene in deslimed ores cannot be recovered.The second type of research focused on high alkali nondesliming processes,especially to develop and popularize highly efficient environmental agents [ 18] .However,there has been no major breakthrough on this to date.

For these reasons,it is necessary to find other ways to reduce the negative effects of fine spodumene particles in the froth flotation process.Such methods could include increasing the apparent particle size of fine spodumene to enhance recovery of fine spodumene in conventional flotation process [ 19] .Achieving this requires methods that ensure the selective hydrophobization of fine particles in slurry,so that it forms flocs when the pulp is fully dispersed.Finally,the flocs should be separable by conventional flotation processes.By Warren (1975) [ 20, 21] ,“shear flocculation”method was applied for the first time to recover scheelite in a flotation process.It has been proven that“shear flocculation”is the process by which hydrophobic fine particles are aggregated under high shear conditions [ 22] .Under these conditions,fine mineral particles are selectively aggregated into clusters because they adsorb a surfactant that destroys their hydration shell [ 23] .In the system,the clusters have a much larger apparent size than the fine particles and can thus be recovered easily in the froth flotation process [ 24] .

However,the current research on spodumene is focused on flotation reagents,particularly the collectors [ 25, 26] .There are limited researches on the shear aggregation flotation of fine spodumene.Thus,this study was based on shear aggregation flotation and modern tests such as microscopic and floc-size analyses on fine spodumene.Its purpose was to study the effects of different shear states on aggregation and flotation processes related to fine spodumene.

2 Experimental

2.1 Materials and reagents

The samples of pure spodumene were obtained from Keketuohai District in Xinjiang,China.Most of the samples were pink and white bulk crystals.They were first crushed manually,their impurities were removed,and then they were ground with a ceramic ball mill and a three-head agate grinder.The powdered samples were then wetscreened with 75,45,31,and 19μm sieves to obtain four different size fractions (45-75,31-45,19-31,and0-19μm).Finally,all samples were washed five times with distilled water using an ultrasonic cleaner before being dried at a low temperature below 50℃.X-ray diffraction(XRD,DX-2700,Fangyuan Instrument Co.,Ltd)(Fig.1)was used to determine the mineral composition of the samples,and the results showed that they were more than98%pure.

2.2 Measurements of shear-stirring strength

The shear agitation of the suspension was achieved with a homemade stirring apparatus made using a mixing tank with four baffles and a stirring device made with a fourbladed impeller,as shown in Fig.2.The agitation tank was a 60-mm-high cylinder with a diameter of 40 mm.Four baffles (40 mm×5 mm) were installed in the agitation tank.The entire paddle blade was 25 mm in diameter with four 10 mm×10 mm blades placed perpendicular to the axis of the agitation tank.The impeller,which was driven by a power-controlled agitator that regulated the agitation speed in a stepless manner from 150 to 6000 r·min^-1,was placed 10 mm from the bottom of the agitation tank.The torque required to overcome the fluid resistance was recorded by a TQ-662-type dynamic resistance strain torque sensor in the agitator [ 27] .

Fig.1 XRD pattern of spodumene sample

The slurry which had a liquid-solid mass ratio of 20:1was composed of 2 g fine spodumene and 40 ml deionized water.The suspensions were mixed for 3 min using an ultrasonic cleaner to fully disperse the fine mineral particles.Pulp was stirred uniformly for 1 min before the pH value was adjusted with the addition of HCl or NaOH (the entire pH adjustment time took 3 min).The shear-stirring time was calculated after adding NaOL.The samples used to analyze particle size distributions and flotation were extracted from the stirring apparatus.

This study aggregated the fine spodumene particles via shear agitation in the presence of NaOL.The shear-stirring strength of the slurry was calculated by the following expression [ 28, 29] :

where E is the shear-stirring strength applied to the slurry to make sure that the fine particles were combined,kJ·m^-3;T is the torque,N·m,which is the force that causes slurry to twist or turn given fluid resistance;n is agitation speed,r·min^-1;t is agitation time,s;and V is slurry volume,m³.

2.3 Particle size measurements

The particle size distribution of fine spodumene in the slurry was determined after shearing and agitation and was measured with a laser particle size analyzer based on the principle of light scattering.Each slurry was sampled twice with a pipetting gun before the two samples were mixed.The particle size distributions were determined three times for each sample,and the average of the measurement results was used to draw a relevant curve.The apparent size of fine spodumene with different shear-stirring strengths was expressed in terms of D₅₀ (the maximum particle diameter greater than the diameter of 50%samples) and D₉₀ (the maximum particle diameter greater than the diameter of 90%samples).A parameter denoted as“aggregation degree”was defined to indicate the flocculation behavior of the fine spodumene and was calculated by the following formula:

Fig.2 Schematic of agitation tank

where R_i is the aggregation degree of a specific fraction(fine size,middle size,or coarse size,hereinafter referred to as FS,MS and CS);α_i is the content of size fraction i that was not exposed to NaOL;andβ_i was the average quantity of size fraction i exposed to different shear-stirring strengths after the NaOL was added.Using this formula,the particles smaller than 19μm were subpided into three narrow size distributions of FS (0-8μm),MS (8-13μm),and CS (13-19μm) to help determine which granularity displayed the more evident aggregation.Using this parameter (R_i),the aggregation level of the fine spodumene particles (less than 19μm) can be understood intuitively as follows:when the aggregation degree of a given size fraction increases,it indicates that spodumene content of this granularity decreases after agitation.

2.4 Observation of floc microstructure

In this study,an SPF-330C optical polarizing microscope equipped with a camera was used to observe the structures of the flocs formed by the fine spodumene particles under different shear-stirring strengths at a 200×magnification.Given the same conditions as the flotation test,the flocs in the pulp floated up.After this,1 ml suspension was extracted with a large-head dropper.The suspension was then diluted to a concentration of 1%to prevent the flocs from drying up and aggregating during the image acquisition process;this was done to ensure that flocs used for observation were fully dispersed.The entire sampling process must be careful and gentle to avoid destroying the integrity of the flocs.Following this,0.2 ml diluted suspension was dropped on slide glass;a representative field of view was selected and photographed.

2.5 Micro-flotation tests

The micro-flotation tests were carried out on an XFG airinlet hanging trough inflator equipped with a 40-ml flotation cell with a spindle speed of 1600 r·min^-1.The optimal conditions were determined by basic flotation experiments on spodumene particles of different sizes.The pH of the pulp was adjusted to the desired value by adding HCl or NaOH,and the optimum amount of NaOL to be added was determined by tests under different pH values.Each flotation test involved weighing out 2 g spodumene samples accurately in a beaker,adding an appropriate amount of deionized water,and then dispersing it completely.The slurry needed to be stirred for 1 min to ensure full dispersion before the pH regulator and collector were added,respectively.The flotation time for a basic flotation test was 4 min.

For the aggregation flotation tests for fine spodumene,the shear-stirring process of slurry was carried out in the stirring apparatus.Then,pulps handled under different shear strengths were moved into flotation cell and stirred for 15 s at the agitator speed of 1600 r·min^-1 before flotation began.For each flotation under inpidual conditions,the total flotation time was 8 min.The spodumene concentrates were collected in batches;the flotation time for each batch was 20,20,20,30,30,60,60,120,and120 s.The concentrates were collected on glass dishes and dried for weighing to calculate the flotation recovery.

For the aggregation flotation tests for fine spodumene,the process of shear-stirring the slurry was carried out in the stirring apparatus.The pulps handled under different shear strengths were then moved into flotation cells and stirred for 15 s at an agitator speed of 1600 r-min^-1 before flotation began.The total flotation time was 8 min for each flotation.The spodumene concentrates were collected in batches and the flotation time for each was 20,20,20,30,30,60,60,120,and 120 s.The concentrates were collected on glass dishes and dried for weighing so that the flotation recovery could be calculated.

In this study,the classical one-level model of flotation dynamics was used to analyze the flotation of fine spodumene so that the effects on flotation of the aggregation degree of fine minerals under different shear-stirring conditions could be determined [ 30] .The classical one-level model is formulated as follows:

where ε is the flotation recovery;ε∞ is the maximum recovery that can be achieved by the flotation process;k is the flotation rate constant;and t is the flotation time.The experimental flotation rate constant under different states can be obtained by linear fitting using this formula.

3 Results and discussion

3.1 Effect of particle size on spodumene flotation

Micro-flotation tests were used to study the flotation behavior of the four particle size fractions at different pH values and NaOL dosages,and the results are illustrated in Fig.3.The range of suitable pH values for spodumene flotation was from 7 to 12 (Fig.3a).Overall,the flotation regulations for the four different particle sizes were similar.With the increase in pH,the flotation recovery increased with the pH value when it was less than 9.0.In contrast,when the pH value was above 9.0,the recovery decreased with the increase in pH value.Therefore,based on the data in Fig.3a,the optimum pH for spodumene flotation was9.0.Moreover,in the particle size range of 0-45μm,the flotation recovery increased with granularity.When the spodumene particle sizes were in the range of 31-45μm,a peak recovery of 43.19%was observed,as shown in Fig.3a.Overall,these results imply that the pH value of slurry has a considerable influence on the flotation behavior of differently sized particles.When the pH values are different,the zeta potential curve moved downward as the zero-electric point of spodumene in NaOL solution became more acidic;this was different from the conditions in the pure water solution.This indicates that negatively charged oleate ions were adsorbed on the surface of spodumene,thereby reducing the surface potential.When the pH value was approximately 9,the zeta potential on the surface of spodumene had the most significant negative movement,indicating that NaOL had the largest adsorption effect at this pH value.

The distribution of the composition of NaOL in the slurry system was considerably affected by the pH value.As this increased,the concentration of the oleate ion component (RCOO^-and RCOO₂^2-) and ion-molecular association component (RCOOH·RCOO^-) gradually increased.When the pH value was approximately 9,the concentration of the two components reached the maximum.When pH was greater than 9,the concentration of ion-molecular association component decreased gradually,and the ionic component existed mainly in the solution.In the flotation system,the ion-molecular association seemed to be twice as large as the hydrocarbon chain when compared with a single ion.Therefore,the association showed a stronger surface activity.It could enhance the hydrophobicity of the minerals significantly by getting adsorbed on their surfaces.Therefore,as the pH of the slurry increased,the recovery rate of the spodumene first increased and then decreased.The floatability was the best when the pH was 9.

Figure 3b shows that the flotation recovery for each size of particle increased as the collector dosage increased when the sodium oleate dosage was below 8×10^-4 mol·L^-1.However,when this level was exceeded,the flotation recovery barely changed.Moreover,the recovery of the coarse fraction (31-75μm) was higher than those of the intermediate (19-31μm) and fine (0-19μm) fractions.

The flotation recovery of fine spodumene was only28.51%when the dosage of NaOL was 1×10^-3 mol·L^-1;this was similar to that of intermediate particles under the same flotation conditions.Moreover,the flotation recovery increased slightly as NaOL increased.This could be attributed to the anisotropic crystal structure of spodumene [ 31] .The number of broken bonds per unit crystal plane of spodumene (110) is more than that found on a (001) plane [ 32] .Therefore,NaOL adsorption was stronger on the former than the latter.When the coarse spodumene particles began to break,they broke mainly along the (110)plane.At this time,the number of (110) crystal planes increased as the adsorption capacity of NaOL increased,resulting in a good flotation performance [ 33] .When these were ground further,the spodumene particles mainly broke along their chemical bonds and the proportion of (001)planes increased gradually [ 3] .However,the intensity of adsorption between the (001) crystal plane and the NaOL was significantly less than that between the (110) plane and the latter,which resulted in the fine spodumene having a poor flotation behavior with the same dosage of sodium oleate [ 34] .

Fig.3 Effects of a pH and b NaOL dosage on flotation behavior of spodumene

Overall,the results showed that when the pH of the slurry was 9 and the quantity of NaOL used was 8×10^-4mol·L^-1,there was a good recovery with spodumene flotation.However,the recovery rate of fine spodumene was too low.Therefore,to improve this recovery,a shear flocculation method was used to increase the apparent size of the particles.During the shear flocculation flotation process,the pH value of the slurry was 9 and the quantity of sodium oleate was 8×10^-4 mol·L^-1.The flocculation process and flotation behavior of spodumene under different shear conditions were studied.

3.2 Flotation experiments under different shear states

The flotation experiments were carried out to determine the flotation behavior and particle size distributions of fine spodumene (less than 19μm) under different shear conditions.The results are shown in Figs.4,5,6,7,and 8.

As shown in Fig.4,using a shear flocculation method to increase the apparent size of the particles results in a significant increase in the flotation speed and flotation recovery rate of the fine-grained spodumene.As the shear strength increased,a higher flotation rate and flotation recovery were obtained.

Fig.4 Flotation recovery of fine spodumene (-19μm) under different shear conditions

To study the aggregation behavior of fine particles under varied conditions,particularly the formation,growth,stabilization,and breakup of the flocs formed,we systematically investigated the relationship between the aggregation process and the flotation efficiency.The entirety of the aggregation process and performance of the corresponding flotation are discussed in stages.

Figure 5 shows the effect of performing shear aggregation flotation on the fine spodumene.It demonstrates the distribution of floc sizes and the corresponding cumulative flotation recovery as the flocs were beginning to form.We found that with an increase in stirring intensity,particle sizes of approximately 2 and 8μm decreased significantly while number of 20-μm-sized particles increased.When the shear strength input to the spodumene pulp system increased from 0 to 3.68 kJ·m^-3,particle size analysis showed that the D₉₀ increased from 14 to 17μm and that the D₅₀ increased from 5.30 to 10.37μm.The results of the particle size detection showed that shearing can cause fine spodumene particles to aggregate,thereby significantly increasing the apparent size of fine particle [ 15] .In this shear strength range,the significant growth in flocs was mainly due to the aggregation of FS and MS.

Fig.5 a Floe size distribution and b flotation behavior of Stage 1 (beginning of floe formation and a slightly increased rate of flotation)

Fig.6 a Distribution of the sizes of floes and b flotation behavior of Stage 2 (growth of floe and remarkable improvement in flotation)

Fig.7 a Distribution of size of the flocs and b flotation behavior of Stage 3 (formation of large flocs and the increase in flotation)

The cumulative recovery curve corresponding to the same shear stations shows that the flotation recovery increased from 47.73%to 51.34%,indicating that increasing the particle size can promote recovery to someextent.In other words,in this shear state,the fine spo-dumene particles began to aggregate and had a certainpromoting effect on flotation behavior.

The apparent size analysis and flotation efficiency at Stage 2,when the increase in shear-shirring intensity increased further,are shown in Fig.6.At this stage,the high peak at 8μm decreased gradually,while the peak at20μm increased;the entire peak shifted to the right when the stirring time increased further.The particle size distribution changed from a bimodal distribution to unimodal one by degrees.This was mainly due to the aggregation of small flocs formed at the first stage and provided a core for further aggregation.The core can significantly attract spodumene particles (less than 10μm);therefore,there was an evident growth in the apparent size of the flocs.During this phase,the D₉₀ varied in the range of 17-19μm,while the D₅₀ increased from 10.37 to 14.29μm.Flotation recovery increased from 51.34%to 54.42%(Fig.6b).

Fig.8 a Distribution of the size of the floes;b flotation behavior of Stage 4:stabilization of the floes and the unchanged flotation

At this range of shear agitation intensity,the apparent size distributions of spodumene particles gradually transformed from a bimodal to a unimodal mode.The peak located at 8μm began to fade when the shear strength increased to 4.60 kJ·m^-3.Therefore,the floc diameter increased because of the adhesion and aggregation of fine particles,particularly those smaller under 10μm and the FS.

Figure 7 indicates the formation of the bigger flocs and their effect on spodumene flotation.The particle size distributions of the flocs completely changed from bimodal mode to unimodal,and the peak moved from 20 to 30μmwhen the shear-stirring strength increased from 6.90 to10.12 kJ·m^-3.The D₅₀ value increased from 14.29 to18.01μm and the D₉₀ increased from 18.89 to 32.86μmwithin this range of stirring intensity.As shown in Fig.7a,this was mainly the combination of the stable flocs that had been formed in the previous stage.There was a considerable increase in flotation recovery,from 54.52%to60.5l%,and the flotation rate also increased.

The variations in particle size distributions in the previous stages showed that the initial aggregations were dominated by MS,after which FS were adsorbed on the flocs formed by the aggregation of MS;at this stage,CS were less aggregated.When the shear strength continued to increase,the larger flocs formed by bridging between the flocs had aggregated in the previous stage,between the flocs and the CS particles and between the CS particles.

Figure 8 shows that when the shear strength was very high,the large flocs formed during the earlier stages would break,to some degree,but this would have little effect on the flotation behavior,only slightly reducing the flotation rate.The larger flocs were formed by the bridging of small flocs;however,the structural strength of this bridging was weak.The size distributions clearly showed that the large flocs were destroyed and the relatively stable small flocs,with diameter of 20μm,formed again when the shear strength increased from 10.12 to 12.88 kJ·m^-3.Figure 8a clearly shows a reduction in the larger flocs,i.e.,those greater than 30μm in length,and the increase in the number of particles with size<19μm.

The fragmentation of flocs did not appear to have reduced the flotation efficiency of fine spodumene at this stage.The main reason for this is that although the large flocs were destroyed,the small ones formed had an average diameter of～20μm.The results described in Sect.3.1indicated that the maximum recovery was observed for the size range of 19-75μm.The particle size of most of the small flocs was 20μm,and that of the flocs ranged from 19to 75μm;as a result,when the shear-stirring strength was too high,the larger flocs are broken into smaller flocs but the flotation index was not affected.

3.3 Floc shapes under different shear states

Figure 9 shows a series of photomicrographs of flocs formed by fine spodumene particles under different shearstirring strengths.The fine spodumene particles were well dispersed in the pulp without the NaOL.The well-dispersed particles began to aggregate when the NaOL was added to the slurry system and shear-stirring was carried out.As the shear strength increased,the flocs grew and then formed bridges among themselves.The large unstable flocs finally broke up and formed smaller stable flocs when the shear strength exceeded that the floc could tolerate.

Fig.9 OM images of shapes of floes shape given different shear strengths:a well-dispersed fine spodumene,b formation of a floc (3.68 kJ·m^-3),c growth of a floe (6.90 kJ·m^-3),d branched floes (10.12 kJ·m^-3),and e breakup of branched floes and formation of globule-like floes(12.88 kJ·m^-3)

During the entire experiment,the change in floc morphology reflected the movement and aggregation of fine particles in the formation,growth,bridging,and breakage of the spodumene flocs under different shear-stirring strengths.The aggregation and dispersion process of fine spodumene particles can be represented by a floc growth model,as shown in Fig.10.

The results showed that the growth model obtained from the experiments could be used to qualitatively describe the changing rules of the formation,growth,and breakup of fine spodumene agglomerations under different shear strengths,as shown in Fig.10.In the model,FS (0-8μm),MS (8-13μm),and CS (13-19μm) are represented by circles of different diameters.The aggregation process under different shear strengths was simulated by aggregating and dispersing these balls.When NaOL was added to the slurry without a shear force,the fine spodumene particles were well dispersed.When a basic shear strength just able to form the fine spodumene agglomerations was applied,the MS in the fine spodumene (less than 19μm)aggregated and created the floc cores,in which a range of FS and CS particles were dispersed in slurry in the form of single particles.As the shear strength increased,the FS adhered to the floc core;however,the CS did not aggregate.Finally,as the suitable shear strength exceeded10.12 kJ·m^-3,all fractions were combined into a big floc,showing that CS aggregated into flocs by adhering to the FS and MS flocs that had already formed.This conclusion can also be supported by the fact that the flocs have many branches in the growth stage.When the flocs broke,the stability of the three fractions that make them up,given the increasing shear force,is ranked as MS>FS>CS.This shows that CS is the least stable part of the flocs and is the first to become re-dispersed in pulp given an excessive shear strength.In summary,MS acts as the core of the flocs while FS and CS form their branches during the process in which fine spodumene forms aggregates.

The process of shear aggregation can be pided into two stages:selective hydrophobic agglomeration on the surface of mineral particles and hydrophobic aggregation between the particles [ 35] .The hydrophobic portion of mineral particles is influenced by the surfactant,which in turn makes the entire surface of the mineral particle hydrophobic.Therefore,when exposed to a high shear strength (a high mixing speed and a long mixing time) in the slurry,the fine particles aggregated given the effects of hydrophobic force and mechanical energy.Thus,the most fundamental factor in shear aggregation is the hydrophobicity of the mineral particles.When NaOL was added to the pulp solution,the hydrophilic group in NaOL was adsorbed to the surface of the spodumene by a chemical action.This points the hydrophobic hydrocarbon chain to the solution and causes the surface of the particles to become hydrophobic.The hydrophobic aggregation is caused by a change in the substructure of water and the association of hydrophobic hydrocarbon chains in NaOL.First,the hydrophobic particles in the solution created a discontinuity in the hydrogen bond between water molecules.This causes the substructure of the molecule to become denser and the arrangement of the adjacent water substructure to become more ordered,leading to an increase in the free energy in the system.Furthermore,because water molecules tend to repel hydrophobic particles,thus hydrophobic particles close in close proximity to each other and form clusters.When shear force was added to the system,the collision and adhesion between the hydrophobic particles lead to contact with the drug adsorption layer on the surface of the spodumene particles.A cross-association formed between particles on the hydrophobic hydrocarbon chain and stabilized the hydrophobic group formed.Therefore,the appropriate shear strength can promote the associations among hydrocarbon chains and improve the effect of the poly group.

Fig.10 Floe growth model given different shear strengths

During shear aggregation flotation,the shear intensity of fluid pulp increased with the stronger agitation.This means that the collision energy of the mineral particles can increase while a collector with a long hydrocarbon chain can become adsorbed to the surface of a target ore,thus generating a hydrophobic bonding force [ 36] .This force can reduce the energy barrier that causes repulsion among particles,and as a result,the depressed repulsion barrier is breakable due to the collision energy of the agitation [ 37] .This results in the aggregation flotation of the fine mineral.Our study found that the different shear conditions affect flotation behavior differently.Therefore,an optimal shear stirring condition,determined via analysis of the flocculation degrees of the three narrow fractions FS (0-8μm),MS(8-13μm),and CS (13-19μm),is required to achieve the highest aggregation and flotation efficiency.

  下载原图

Table 1 Results of fitting flotation recovery curves to different shear strengths

3.4 Aggregation degree under different shear states

The experimental flotation rate constant of fine spodumene at different shear strengths can be obtained by performing linear fitting on the test data given in Sect.3.3,as presented in Table 1.The aggregation degrees of FS (0-8μm),MS(8-13μm),and CS (13-19μm) can be calculated by analyzing the spodumene particle size distribution given in Sect.3.2.The effect between shear strengths on flotation rate constant and aggregation degree is shown in Fig.11.The flotation rate constant increased with the shear strength;when the shear strength reached 10.12 kJ·m^-3,the flotation rate constant reached its maximum.Figure 11shows the relationship between the shear strength and the flotation rate constant;the variation trend in the latter was basically consistent with the flotation behavior reported in Sect.3.2.

The relationship between the shear strength and the aggregation degree of the fine spodumene particles shows that the aggregation degree of CS was～15%when shear strength was less than 10.12 kJ·m^-3.Therefore,it can be considered that the content of the CS fraction was basically unchanged under this shear state.When shear-stirring strength increased further,the flocculation degree suddenly increased to 30%and continued to strengthen while the aggregation degree decreased.All of these indicated that large-sized flocs cannot exist in a stable condition under a high shear strength that is too high and that they will break up again to form small diameter flocs.This is consistent with the change of average flocs diameter with shear strength in Fig.12.When the shear strength was less than4.60 kJ·m^-3,the flocculation degree of MS (8-13μm)was～0,indicating that no aggregation occurred.When the aggregation degree increased with shear agitation,the maximum was reached when shear strength became10.12 kJ·m^-3.At this point,the aggregation degree decreased with the increase in shear-stirring strength.The change in the trend of the aggregation degree of MS was identical to the aggregation and fragmentation processes of flocs described in Sects.3.2 and 3.3.The aggregation degree of the FS fraction first increased but then remained basically unchanged after the shear strength reached10.12 kJ·m^-3.This indicates that the fraction could be present in a stable condition in slurry given a high shear strength.

Fig.11 Flotation rate constant and aggregation degree of fine spodumene versus shear strength

Fig.12 Average diameter of floes versus shear strengths

This study indicates how to find the optimal shear strength required to achieve the most effective aggregation by monitoring the variations in the process of particle size distribution (which can be represented by flocculation degree) and the flotation rate constant.During the shear aggregation flotation of spodumene,flotation recovery and flotation rate should also be considered [ 38, 39] .Therefore,these factors need to be considered as a whole to determine the range of the shear strength required.For this system,a suitable shear strength is the one in the 9.20-11.50 kJ·m^-3range,with a flotation rate constant ranging from 0.0194 to0.0213 s^-1.

4 Conclusion

By carrying out micro-flotation on different fractions of spodumene,it was determined that flotation recovery changed as the pH increased to a maximum of 9 and that the recovery of fine spodumene (-19μm) was lower than that of other fractions of the mineral.

Fine spodumene can be aggregated via shear agitation with the addition of NaOL as a collector;the latter improves recovery significantly.The strength of the shear stirring influences the apparent size and flotation rate of the fine spodumene.These factors increase as the shear strength increases in a certain range,and the size distribution of the fine spodumene transforms from bimodal to unimodal as the shear strength changes.A higher shear strength leads to the formation of large flocs with branches;however,these have a weak resistance to breakage;however,a strength that is too high tends to lead to the fragmentation of large flocs and the formation of highly stable globule-like flocs.To achieve the most effective shear aggregation flotation,flotation rate constant and flocculation degree should be considered together to determine a suitable shear strength range.In a fine spodumene flotation system,this shear strength lies in the9.20-11.50 kJ·m^-3 range with a flotation rate constant in the range of 0.0194-0.0213 s^-1.

参考文献

[1] Zhang F,Shen F,Fan ZY,Ji X,Zhao B,Sun ZT,Xuan YY,Han XG.Ultrathin Al_2O_3-coated reduced graphene oxide membrane for stable lithium metal anode.Rare Met.2018;37(6):510.

[2] Barbosa LI,Valente G,Orosco RP,Gonzalez JA.Lithium extraction fromβ-spodumene through chlorination with chlorine gas.Miner Eng.2014;56:29.

[3] Xu LH,Hu YH,Wu HQ,Tian J,Liu J,Gao ZY,Wang L.Surface crystal chemistry of spodumene with different size fractions and implications for flotation.Sep Purif Technol.2016;169:33.

[4] Hu AM,Li M,Dali DLM,Liang KM.Crystallization and properties of a spodumene-willemite glass ceramic.Thermochim Acta.2005;437(1):110.

[5] Aylmore MG,Merigot K,Rickard WDA,Evans NJ,Mcdonald BJ,Catovic E,Spitalny P.Assessment of a spodumene ore by advanced analytical and mass spectrometry techniques to determine its amenability to processing for the extraction of lithium.Miner Eng.2018;119:137.

[6] Yu FS,Wang YH,Zhang L,Zhu GL.Role of oleic acid ionic-molecular complexes in the flotation of spodumene.Miner Eng.2015;71:7.

[7] Gruber PW,Medina PA,Keoleian GA,Kesler SE,Everson MP,Wallington TJ.Global lithium availability.J Ind Ecol.2011;15(5):16.

[8] Amarante MM,Sousa ABD,Leite MM.Processing a spodumene ore to obtain lithium concentrates for addition to glass and ceramic bodies.Miner Eng.1999;12(4):433.

[9] Wang YH,Yu FS.Effects of metallic ions on the flotation of spodumene and beryl.J China Univ Min Technol.2007;17(1):35.

[10] Miettinen T,Ralston J,Fornasiero D.The limits of fine particle flotation.Miner Eng.2010;23(5):420.

[11] Yin XH,Gupta V,Du H,Wang XM,Miller JD.Surface charge and wetting characteristics of layered silicate minerals.Adv Colloid Interface Sci.2012;179(si):43.

[12] Peltosaari O,Tanskanen P,Hautala S,Heikkinen EP,Fabritius T.Mechanical enrichment of converted spodumene by selective sieving.Miner Eng.2016;98:30.

[13] Xing YW,Xu XH,Gui XH,Cao YJ,Xu MD.Effect of kaolinite and montmorillonite on fine coal flotation.Fuel.2017;195:284.

[14] Gupta V,Miller JD.Surface force measurements at the basal planes of ordered kaolinite particles.J Colloid Interface Sci.2010;344(2):362.

[15] Gupta V,Hampton MA,Stokes JR,Nguyen AV,Miller JD.Particle interactions in kaolinite suspensions and corresponding aggregate structures.J Colloid Interface Sci.2011;359(1):95.

[16] Wang L,Peng Y,Runge K,Bradshaw D.A review of entrainment:mechanisms,contributing factors and modelling in flotation.Miner Eng.2015;70:77.

[17] Yu FS,Wang YH,Wang JM,Xie ZF,Zhang L.First-principle investigation on mechanism of Ca ion activating flotation of spodumene.Rare Met.2014;33(3):358.

[18] Wu HQ,Fang S,Xu LH,Tian J,Chen HY.Research situation of pegmatite spodumene flotation reagents and process.Met Min.2018;7:1.

[19] Leistner T,Peuker UA,Rudolph M.How gangue particle size can affect the recovery of ultrafine and fine particles during froth flotation.Miner Eng.2017;109:1.

[20] Warren LJ.Flocculation of stirred suspensions of cassiterite and tourmaline.Colloids Surf.1982;5(4):301.

[21] Warren LJ.Shear-flocculation of ultrafine scheelite in sodium oleate solutions.J Colloid Interface Sci.1975;50(2):307.

[22] Chen G,Grano S,Sobieraj S,Ralston J.The effect of high intensity conditioning on the flotation of a nickel ore,part 2:mechanisms.Miner Eng.1999;12(11):1359.

[23] Liu XW,Zhu RL,Ma JF,Ge F,Xu Y,Liu Y.Molecular dynamics simulation study of benzene adsorption to montmorillonite:influence of the hydration status.Colloids Surf A.2013;434(19):200.

[24] Xu LH,Hu YH,Tian J,Wu HQ,Yang YH,Zeng XB,Wang Z,Wang JM.Selective flotation separation of spodumene from feldspar using new mixed anionic/cationic collectors.Miner Eng.2016;89:84.

[25] Zhou HP,Zhang YB,Geng L,Yang ZZ,Luo XP.Effect of slurry stiring time on flotation process of mud-containing spodumene.Chin J Rare Met.2019.http://kns.cnki.net/kcms/detail/11.2111.TF.20191011.1516.004.html.

[26] Sadowski Z,Polowczyk I.Agglomerate flotation of fine oxide particles.Int J Miner Process.2004;74(1):85.

[27] Gui X,Liu J,Cao Y,Gan C,Li S,Lun W.Flotation process design based on energy input and distribution.Fuel Process Technol.2014;120(120):61.

[28] Chen W.Optimization Model of Flocculation Agitation Energy Consumption Distribution and Its Experimental Verification.Wuhan:Huazhong University of Science and Technology;2011.111.

[29] Yang Y.Effects of Different Stirring Intensities on Morphology of Humic Acid Flocs in Shear Field.Xi'an:Xi'an University of Architecture and Technology;2011.13.

[30] Lynch AJ.Mineral and Coal Flotation Circuits Their Simulation and Control.Amsterdam:Elsevier Press;1987.64.

[31] Xu LH,Peng TF,Tian J,Lu ZY,Hu YH,Sun W.Anisotropic surface physicochemical properties of spodumene and albite crystals:implications for flotation separation.Appl Surf Sci.2017;426:1005.

[32] Rai B,Sathish P,Tanwar J,Pradip,Moon KS,Fuerstenau DW.A molecular dynamics study of the interaction of oleate and dodecylammonium chloride surfactants with complex aluminosilicate minerals.J Colloid Interface Sci.2011;362(2):510.

[33] Koh PTL,Hao FP,Smith LK,Chau TT,Bruckard WJ.The effect of particle shape and hydrophobicity in flotation.Int J Miner Process.2009;93(2):128.

[34] Moon KS,Fuerstenau DW.Surface crystal chemistry in selective flotation of spodumene(LiAl[Si03]2)from other aluminosilicates.Int J Miner Process.2003;72(1):11.

[35] Sun Y,Xie G,Peng Y,Chen Y,Ma G.How does high intensity conditioning(HIC)affect flotation performance?Int J Coal Prep Util.2019;39(6):302.

[36] Dang FQ,Hasegawa T,Biju V,Ishikawa M,Kaji N,Yasui T,Baba Y.Spontaneous adsorption on a hydrophobic surface governed by hydrogen bonding.Langmuir ACS J Surf Colloids.2009;25(16):9296.

[37] Chipfunhu D,Zanin M,Grano S.The dependency of the critical contact angle for flotation on particle size—modelling the limits of fine particle flotation.Miner Eng.2011;24(1):50.

[38] Su XG,Li YJ,Liu J,Yuan HQ,Wane ZH,Xu XG.Shear flocculation and flotation of hematite.Adv Mater Res.2011;158:224.

[39] Yin WZ,Yang XS,Zhou DP,Li YJ.Shear hydrophobic flocculation and flotation of ultrafine Anshan hematite using sodium oleate.Trans Nonferrous Met Soc.2011;21(3):652.