Investigation on the effects of fluid intensification based preconditioning process on the decarburization enhancement of fly ash

Danlong Li,Yannan Liang,Hainan Wang,Ruoqian Zhou,Xiaokang Yan,Lijun Wang,Haijun Zhang,*

1 National Engineering Research Center of Coal Preparation and Purification,China University of Mining and Technology,Xuzhou 221116,China

2 School of Chemical Engineering and Technology,China University of Mining and Technology,Xuzhou 221116,China

3 School of Electrical and Engineering,China University of Mining and Technology,Xuzhou 221116,China

Keywords:Fly ash Waste treatment Preconditioning energy input Fluid intensification Flotation Kinetic modeling

ABSTRACT Fly ash (FA) is a complex and abundant solid waste created by humans,and has caused environmental issues,for which flotation is an effective technique employed before its comprehensive utilization.However,the complex and hydrophilic characteristics of FA particles cannot naturally fulfill the selective separation by common flotation.Therefore,this study aims to provide an insight into fluid intensification effects on flotation to achieve the enhancement of FA surface property and decarburization.The relevant effects and mechanisms are investigated,based on the measurements of zeta potential,infrared spectroscopy,contact/wrap angle,induction time,size distribution and scanning electron microscopy–energy dispersive spectrometry.Experimental results manifested that the maximum unburned carbon recovery(73.25%) and flotation rate (0.2037 s-1) were achieved with preconditioning energy inputs of 14.23 and 6.57 W?kg-1 respectively.With increasing preconditioning energy inputs,fluid intensification effects could promote the inter-particle collision/attrition,detachment of hydrophilic existence and collector adsorption on particles.Correspondingly,absorbance of some hydrophobic and hydrophilic functional groups was strengthened and weakened respectively,which accounted for the improved interfacial properties,reflected as the increased contact and wrap angles,together with declined induction time.Overall,this article revealed the positive influences of fluid intensification based preconditioning process on rendering particle surface hydrophobic and improving separation performance.

1.Introduction

Fly ash(FA)is a by-product generated during the combustion of pulverized coal for energy production and a main waste material derived from thermal power plants [1].According to some statistics,the current production of FA worldwide is estimated up to 600–800 million tons on an annual basis,while the proportion of global comprehensive utilization is only 25% [2,3].Consequently,if not properly handled,most FA must pile up on the ground for a long time,which will not only occupy substantial land resources,but also pollute soils and ground water due to the composition of some hazardous materials [4].With the improvement of people’s environmental awareness,the concerns over FA have gained increasing attention,and hence the comprehensive utilization of FA is of great importance [5].

FA has been introduced in many aspects,such as soil conditioner [6],agricultural utilization [7],and adsorbents [8].In addition,FA is commonly used as a mineral admixture in cement or concrete for construction applications[9–11].However,FA usually has such a high quantity of unburned carbon(UC)that it cannot be directly applied in cement or concrete such as the employment of collected fly ash microspheres as a kind of inorganic dispersant[12,13].Basically,the loss on ignition (LOI) is used to determine the UC content.In DIN EN 450-1,the maximum allowable LOI value lies between 5.0% and 9.0%,depending on the category[13].GB/T 1596-2017 recommends that for use in concrete,the LOI value of FA should be less than 8% [14].So far,several studies have shown that the UC in FA may be valuable as a source of activated carbon and a low-cost adsorbent [15].Therefore,with the decarburization process fulfilled,not only can the separated UC be recycled elsewhere,but the leftover ash will find its way.

Flotation has been proven to be an effective separation technique aimed at the removal of UC from FA[16].This process mainly utilizes the differences in the surface characteristics of different particles [17,18].However,according to the report by Yanget al.,there are poor hydrophobicity and floatability properties of FA particles,as their contact angle is below 41° [14].As is known,the intensification of difference in hydrophobicity is usually achieved by a preconditioning process,which has been applied to the flotation of nickel,copper sulphides,sphalerite and coal [19–22].The preconditioning prior to the flotation process plays significant roles in keeping particles suspending,scrubbing particle surface,and prompting particle–collector interactions [23–25].In previous literatures,many researchers have demonstrated that sufficient energy input during preconditioning could make a great difference to the pulp-mixing efficiency and subsequent flotation performance[26–28].Chenet al.[19]indicated that substantial improvements of flotation recovery and selectivity were strongly influenced by sufficient energy input during conditioning stage,which was helpful for the selective collector adsorption.Tabosa and Rubio [21] focused on the high intensity conditioning,and the improved flotation kinetics and concentrate grade were found due to the occurrence of particle aggregation.Feng and Aldrich[26]attributed the enhancement of flotation rates and combustible matter recovery to the partial removal of the sliming coating and the oxidation layer by high intensity conditioning.Yanget al.[28] suggested that modulating turbulent flow patterns could significantly intensify the adsorption morphology (density and height) of collector on particle surface and rendered it more hydrophobic,which contributed to enhancing flotation performance.Basically,the energy input in preconditioning provides energy sources for sustaining specific hydrodynamic conditions,wherein as a continuous phase,fluids play significant roles in flotation process intensification [29].However,despite some articles underlying the influence of preconditioning on flotation process,there are few studies on hydrodynamics intensifying FA flotation and lack of researches that systematically discuss the relevant effects,namely the changed interfacial properties of FA and responsive influences on decarburization performance.

Therefore,this study aims to provide an insight into modulating preconditioning process to achieve the fluid intensification based preconditioning process for decarburization enhancement of FA.The pertinent effects were investigated,based on the flotation tests,and methods related to the particle characterization,including measurements of zeta potential,Fourier transform infrared spectroscopy(FTIR)and scanning electron microscopy–energy dispersive spectrometry (SEM–EDS).The outstanding mechanisms of fluid intensification were discussed and revealed in respect of inter-particle and particle–reagent interactions.Fundamentally,under the action of fluid intensification,the pulp is readily homogenized,wherein both particles and reagents achieve uniform mixing and dispersion.When two particles or fluid elements move at different velocities,attrition or shear force will occur,hence contributing to the detachment of fine impurities on particle surfaces and the exposure of natural hydrophobic moieties of UC.The collector molecules of light diesel oil exactly tend to interact with the hydrophobic surfaces,which further strengthens the surface hydrophobicity and is helpful for successive particle–bubble interaction process.Consequently,the improved surface characteristics and enhanced decarburization performance of FA can be fulfilled.

2.Materials and Methods

2.1.Materials and reagents

The samples of FA were collected from a thermal power plant in Hunan province,China.The raw sample was initially dried,and then sieved through a 0.5 mm standard sieve.Subsequently,100 g of -0.5 mm size fractions was conducted wet sieving through the standard sieve of 0.045,0.074,0.125,and 0.25 mm.The collected products were dried until they reached constant weights,which were employed to calculate particle size distribution.To characterize the mineral phase of FA,part of -0.074 mm size fractions was introduced to the measurements of X-ray fluorescence (S8 TIGER,Bruker,Germany) and X-ray diffraction (D8 ADVANCE,Bruker,Germany).With respect to flotation reagents,light diesel oil and 730-flotation non-ionic reagent (prepared by mixing polyethylene glycol and 2-octanol at an appropriate ratio)were employed as the collector and frother,which were bought from Suzhou Tongyi Oil Co.,Ltd.(China) and a treatment plant of FA decarbonization in Hunan (China),respectively.

2.2.Preconditioning-flotation tests

The preconditioning-flotation tests based on varying preconditioning energy inputs(PEIs)were performed in a laboratory single flotation cell of XFD-1.0 L with a solid concentration of 0.1 kg?L-1.Schematic diagram of the flotation cell and process flow chart of preconditioning-flotation tests were displayed in Fig.1(a) and(b),respectively.Based on previous experiments,the reagent regime was determined with the respective dosage of collector and frother being 1.2 and 1.4 kg?t-1.

Fig.1.(a) Schematic diagram of the experimental flotation cell;and (b) process flow chart of preconditioning-flotation tests.

The slurry was first stirred for 3 min,and then mixed with the light diesel oil for an additional 3 min under the various PEIs,as displayed in Fig.2.The power (P,W) dissipated in the cell was measured using a power monitor,and PEI (ε,W?kg-1) was calculated by Eq.(1).

where ρldenotes the density of water,andVrepresents the volume swept by the impeller.With increasing PEIs,the eddies dissipated in pulp will become smaller,which are expected to strengthen the micro-interactions in this stage [30].

Subsequently,the impeller speed was fixed at 1800 r?min-1,and meanwhile,the frother was added.After 2 min,air was introduced to the flotation system at a flow rate of 1.67 L?min-1.The flotation process lasted for 5 min,and froth products were collected at the cumulative times of 10,30,90,180,and 300 s respectively.The concentrates and tailings were filtered,dried at 80°C until they reached constant mass,and cooled in a dry atmosphere.Finally,the LOI values and removal rate of UC (RUC) can be calculated using Eqs.(2) and (3),respectively.

whereWaandWbare the mass of the sample after and before burning (g),respectively;LOICand LOIRare the LOI of the concentrates and the raw FA,respectively (%);andYCis the yield of the concentrates (%) [14].The second-order model with rectangular distribution of floatability was employed to fit the results under different conditions for the PEI into the flotation system:

where γ,γ∞andkdenote the fractional recovery at timet,maximum cumulative recovery of UC and rate constants respectively[31–33].

Fig.2.Energy input introduced to preconditioning with different impeller speed.

2.3.Zeta potential measurements

Zeta PALS potential and particle size analyzer (Brookhaven,USA) was employed to observe the electrokinetic phenomena of FA suspension.Each suspension collected from the corresponding PEI was left undisturbed for 12 h,and the supernatant was transferred into a cuvette.Then the electrode of SZ770 was inserted into it,and the whole was placed in measurement system.Each measurement was repeated five times at room temperature,and the final values were averaged.

2.4.FTIR spectrum measurements

The changes of surface functional groups of FA conditioned under five levels of PEI were recorded using FTIR(Vertex 80v,Bruker,Germany)with wavelengths of 4000–400 cm-1.First,100 g of FA and 1.0 L of water were placed in the flotation cell and stirred for 3 min.Then,they were mixed with the collector dosage of 1.2 kg?t-1and stirred for another 3 min under different PEIs.Subsequently,the slurry was filtered and dried at approximately 40°C to obtain the conditioned samples.Finally,the samples were transferred into FTIR measurements,and the results could be analyzed.

2.5.Contact angle measurements

In order to study the changes in the hydrophobicity of the FA,a K100 (KRUSS,SIBER HEGENER LTD,Germany) tensiometer was employed to conduct contact angle measurements of FA.First,2.0 g of each conditioned sample was weighed and placed in a Washburn tube.Then,with all the preparatory work completed,the software could determine the capillary constant of each sample relative ton-hexane.Finally,the corresponding contact angle to water for each capillary constant was obtained.All the measurements were repeated and the values were averaged as the results.

2.6.Induction time and wrap angle measurements

The induction time and wrap angle measurements were conducted using 2015EZ induction timer instrument (Alberta University,Canada).First,1.0 g of the prepared sample was placed in a vessel to form a flat particle bed.Then,a bubble generated from the capillary tube was made to interact with the particle bed at a set time.When the capture probability was 50%,the corresponding contact time was identified as induction time.Similar measurements were repeated several times and the average values were selected.Subsequently,enough time was provided for particle–bubble contact,and with the particle–laden bubble elevated,the wrap angle was determined from the graph.

2.7.Particle size measurements

The particle size distributions of the samples coming from the flotation products were determined using a laser diffraction particle size analyzer (S3500,Microtrac Inc.,USA).In the repetitive flotation tests under various PEIs,the first froth products and the tailings were collected in turn as the measurement samples.In each test,part suspension of the flotation product was fetched to the sample delivery system using a dropper,which contained deionized water.With each measurement completed,the fluids were drained,and fresh entered the sample delivery system again,which made preparations for the next sample.Finally,the results were obtained using Microtrac FLEX software.

2.8.SEM–EDS measurements

Samples of SEM–EDS (Quanta 250,FEI,USA) measurements were collected from the raw FA,concentrates,and tailings under the two PEIs(0.89 and 6.57 W?kg-1).Before the tests,each sample was adhered to the conductive tape on the sample holder,forming a thin particle layer.Then,the layer was gilded in a vacuum system to be equipped with electrical conductivity.The prepared samples were used in scanning tests under a vacuum environment.Finally,several points were selected and illustrated using graphs under proper magnification powers,and the corresponding sample surface element composition was displayed.

3.Results and Discussion

3.1.Physicochemical property analysis of raw FA

Fig.3(a) displayed the wet sieving results of the FA.Fine particles,especially those with a size fraction less than 0.045 mm,made up the majority of the whole.The UC distribution had a growing tendency with decreasing size fraction,indicating that the UC mainly gathered at the size fraction of less than 0.25 mm.The content of residual carbon in FA was measured at 22.38%,directly manifesting the imperfect combustion of coal and the necessity of effective carbon-ash separation before comprehensive utilization.The results of X-ray fluorescence and X-ray diffraction were presented in Fig.3(b) and (c),respectively,indicating that FA mainly consisted of SiO2and Al2O3,corresponding to quartz,mullite,and sillimanite components.Fig.3(d) showed the microstructures of the raw FA.Obviously,the raw sample mainly comprised the inorganic granules that appeared as fine spherical particles,agglomerates,and hollow fly ash grains,while UC particles primarily consisted of block-shaped particles,together with a portion of porous granules.These were in accordance with the studies by Yaoet al.[5] and Terzi?et al.[34].It was obvious that there were some particles with complex characteristics,such as the ones coated with fine impurities and the interlocked carbon-mineral aggregates,which belonged to the refractory part and require attention in flotation.

3.2.Zeta potential analysis

Fig.3.Basic physicochemical characteristics of fly ash sample:(a) particle size distribution;(b) inorganic composition;(c) mineral phase;and (d) surface morphology.

Fig.4.Effect of fluid intensification on zeta potentials of fly ash suspension.

Fig.5.FTIR spectra of fly ash conditioned under different preconditioning energy inputs.

Zeta potentials were measured to characterize the effect of preconditioning process on inter-particle interaction,and the results were displayed in Fig.4.According to the previous literature,some inorganic minerals have highly positive zeta potentials,while UC particles usually present negative zeta potentials[35,36].Their difference in charge character can cause the formation of organo–mineral aggregates,namely the so-called slime coating.Based on the results that increasing PEIs contributed to the reduction of negative zeta potentials,it could be inferred that strong shear force from fluid intensification effect facilitated the detachment of ash particles and promoted their migration into supernatant,reflected as‘‘surface cleaning”,because they mostly existed in fine size fractions (-0.045 mm) and tended to keep suspending [37].Consequently,the detached mineral particles neutralized the negative zeta potentials to some extent,and with the PEI growing from 0.89 to 14.23 W?kg-1,the zeta potentials increased from -13.14 to -8.49 mV.

3.3.Surface modification analysis

FTIR measurements were performed to probe the effect of fluid intensification on surface functional groups of FA.The spectral absorbance and infrared spectra of the FA conditioned with flotation collector under different PEIs were presented in Fig.5.There were several hydrophobic groups such as the -C-H of the aromatic nucleus or benzene rings located at 700–900 cm-1,namely the wavelengths of 797 and 833 cm-1[38].The improved absorbance indicated the enhanced collector adsorption and the exposure of fresh surface of residual carbon for aromatic nucleus were the typical structure of coal.The removal hydrophilic existence (i.e.,the oxidation layer and fine impurities) that prevented particle–collector interaction and even particle–bubble adhesion could promote the exposure of fresh UC surface and was beneficial for flotation enhancement [39,40].The IR peaks centered at approximately 3435 and 1630 cm-1could be attributed to the O-H stretching and O-H bending,respectively,both of which belonged to the hydrophilic groups on the sample surface[6].With increasing PEIs,the absorbance derived from two types of hydrophilic groups decreased,further manifesting the effective adsorption of collector molecules.It is also noted that the infrared absorption peak located at 1088 cm-1belonging to inorganic minerals exhibited an outstanding increase under strong conditioning intensity [41].Given the prepared samples coming from-0.074 mm,the changes closely related to the detachment of fine particles through surface cleaning.

The contact angle and induction time are closely related to the surface properties of solid particles,while the wrap angle characterizes the particle–bubble attachment and detachment process to some degree,all of which could be used to describe the changes of physical condition and chemical moieties of FA surface components,including the microstructure and the surface functional groups with an increase in PEIs [42,43].It is widely accepted that the successful events of particle–bubble aggregates will occur only when the contacting time is longer than the induction time.Therefore,during the wrap angle measurements,enough time was supplied to make the attachment possibility as large as possible,and hence the obtained values could find some relation to the actual flotation process.As shown in Fig.6,the contact and wrap angles similarly showed a growing trend.With increasing PEIs from 0.89 to 6.57 W?kg-1,the former increased from 44.83° to 52.42°,and the latter raised from 45.13° to 86.85°.Meanwhile,the induction time decreased from 700 to 250 ms.Based on the formula shown in Eq.(5),enhanced particle surface hydrophobicity could decrease the energy barrier and increase the separation distance(Tc) at the point of film rupture,thereby promoting the formation of particle–bubble aggregates [44].

Fig.6.Effect of fluid intensification on surface hydrophobicity of fly ash:(a) contact angle and induction time;and (b) wrap angle.

where γ is the interfacial tension,mN?m-1,and θ denotes the contact angle,(°).

Without any exception,aforementioned information identically demonstrated the enhanced hydrophobicity of the conditioned FA under a higher PEI,which agreed well with the results of the FTIR analysis.With the shear force under proper conditioning intensity,collector was easily broken into oil droplets,and the hydrophilic impurities coating on UC particles tended to be partially removed as well.As the collector used for FA flotation is insoluble,its uniform diffusion makes the pulp homogenize and increases the interfacial area of reagent molecules,which is helpful for the particle–reagent contact and successive interaction procedures [45].According to the report by Safari and Deglon [46],the attachment rate constant (ka) could be described by Eq.(6):

wherec1andc2are empirical coefficients;andn1andn2are empiricalexponents.Obviously,the improved hydrophobicity of UC particles is helpful for particle–bubble attachment process.With the increasedkaunder higher PEIs,it is indicated that flotation rate constant (k) will exhibit an increment.

3.4.Flotation behavior analysis

Fig.7.Comparison of bubbles under different preconditioning energy inputs:(a) preconditioning energy input 0.89 W?kg-1 and (b) preconditioning energy input 6.57 W?kg-1.

Fig.8.Particle size distributions:(a)and(b)fraction and cumulative yields of the initially collected concentrates,respectively;(c)and(d)fraction and cumulative yields of the tailings,respectively.

As for the flotation behavior of FA,comparable photos of the flotation foam with two contrastive PEI levels were presented in Fig.7,showing that the overall size of mineralized bubbles under the PEI of 6.57 W?kg-1was larger than that under a PEI of 0.89 W?kg-1.Throughout the fluid intensification,the surface of UC particles was rendered more hydrophobic,fully reflected as the variations in contact/wrap angle and induction time (Fig.6).According to the report by Dippenaar [47] and Ata [48],after the attachment to a bubble,they tended to bridge both interfaces of the bubble and the other to bring about the bubble coalescence.Additionally,based on the findings by Parket al.[49],particles with sufficient hydrophobicity experienced great Laplace pressure,the factor governing the process of liquid film drainage,which resulted in the increase of hydrodynamic pressure in the film,and hence contributed to film thinning,accompanied by the bubble coalescence.Its occurrence was intimately associated with the improved surface hydrophobicity of FA particles,which in turn forced lesser hydrophobic particles dropping off for the lack of bubble surface area.Therefore,the decarburization performance with higher PEIs was expected to increase to some extent.

Fig.9.Effect of fluid intensification on the decarburization performance of fly ash flotation.Note:γ∞i and ki(i=1,2,3,4,5)denote the maximum cumulative recovery of unburned carbon and flotation rate constant under five PEI levels,respectively.

The size distributions of the initially collected concentrates and the tailings under various PEIs were shown in Fig.8.Obviously,the froth products obtained during first period shown in Fig.8(a) and(b)represented the portion with greatest floatability.With increasing PEIs,the peak gradually moved to the right side and the average particle diameters increased for the first concentrates,implying that a higher PEI could improve the flotation rate of coarser particles at the beginning.Especially,with the PEIs elevating from 0.89 to 6.57 W?kg-1,the content of medium-sized (0.25–0.045 mm) particles that exhibited a higher LOI increased from 60.69% to 66.82%,which characterized their outstanding flotation rates and could be associated with the enhanced surface characteristics of this size fraction.Based on the Fig.8(c) and (d),under higher PEIs,the curve flotation tailings moved towards the left,reflected as the decreased average particle diameters.Expressed in other words,there were more fine particles migrating to flotation tailings,part of which may be the products after surface cleaning.Due to the fine particles with high interfacial free energy and low momentum,they tend to show low particle–bubble collision efficiency,hence causing rare occurrence of subsequent attachment stages [21,30].

3.5.Decarburization performance analysis

For different PEIs,the flotation results were presented in Fig.9.When PEIs were less than 3.41 W?kg-1,few changes were observed.This implied it was when the PEI reached a threshold that increasing PEIs could modify the interfacial properties of FA to attain better RUC values.According to the findings by Yuet al.[37],sufficient conditioning intensity can promote the recovery of desired components,while mild agitation exactly provided the kinetic energy to break inter-particle energy barriers,thus aggravating sliming coating.Therefore,the positive influences of the PEI below 3.41 W?kg-1were restricted.When the PEI reached 6.57 W?kg-1,the flotation performance was moving toward an optimum,after which it declined.The results derived from Fig.9 showed that this turning point occurred between 6.57 and 14.23 W?kg-1.Excessive intensification can cause immoderate inter-particle collision and surface attrition,accompanied with the destruction of adsorbed reagent on UC particles,thereby deteriorating particle surface modification [14,40].

A generally held perspective implies that kinetics approach is helpful for better acquiring reasonably accurate predictions [50].The second-order model with a rectangular distribution of flotabilities was selected to model this flotation process.Thus,the values ofkand γ∞could be jointly obtained by fitting curve,which were illustrated in Fig.9.It is obvious that the cumulative RUC value rapidly increased at the beginning but then leveled off to some extent.According to the changed slope,the flotation rate exhibited a progressive decrease,corresponding to the variation in floatability of the concentrates.On the basis of fluid intensification,the γ∞value increased from 66.49% to 73.16% with the PEIs increasing from 0.89 to 14.23 W?kg-1,and the maximum flotation rate constant (0.2037 s-1) was obtained with the PEI of 6.57 W?kg-1.The results manifested positive influences of modulating preconditioning process on enhancing flotation kinetics and decarburization performance,which strongly coincided with the improved surface characteristics of FA and firmly indicated the significance of fluid intensification during the preconditioning-flotation process.

Fig.10.Microstructures and surface element compositions:(a)and(b)concentrates and tailings at the preconditioning energy input of 0.89 W?kg-1,respectively;(c)and(d)concentrates and tailings at the preconditioning energy input of 6.57 W?kg-1,respectively.

3.6.Microstructure and element composition characteristics

SEM–EDS is an effective tool for microstructure observation of some materials,as well as an excellent approach to observe the changes in the microstructures of FA under different conditioning intensities.The surface morphology of flotation products obtained with different hydrodynamic environment directly clarified the effect of fluid intensification on particle surface cleaning.Fig.10 showed the SEM–EDS results for the concentrates and tailings at the two comparable levels(0.89 and 6.57 W?kg-1).Compared with that in Fig.10(a)and(b),the amount of fine spherical particles that existed in isolation according to Fig.10(c) and (d) was obviously larger,which was in good agreement with the results of zeta potential and particle size distribution analysis.Most of these fine granules originally existed on particle surface or in the pores of UC,which generated interlocked assemblages and was known as the so-called slime coating.Basically,they broke up mainly due to the inter-particle collision/attrition or shear effect under the action of fluid intensification,which were closely associated with the different movement velocities of particles or fluid elements,respectively [30].Therefore,the particles of flotation products obtained with higher energy input tended to exhibit relatively homogeneous surfaces with few slimes coating,further clarified the significance of optimized hydrodynamic conditions.

4.Conclusions

This research has probed into the effects of modulating preconditioning process.The UC primarily consisted of block-shaped and porous particles,while the tailing ash mainly comprised fine spherical grains,agglomerates,and hollow grains.Due to the surface cleaning and enhanced particle–reagent interaction,the infrared absorbance of some hydrophobic and hydrophilic groups was strengthened and weakened,respectively,which accounted for the increased contact/wrap angle,and the decreased induction time of the FA conditioned with higher PEIs.Consequently,the maximum UC recovery (73.16%) and flotation rate constant(0.2037 s-1) were obtained with PEIs of 14.23 and 6.57 W?kg-1,respectively.The intermediate particles showed faster flotation rates under a higher PEI that meanwhile contributed to the detachment of fine granules from particle surface or pores,and generated a greater likelihood of their migration into tailings.The findings provide an insight into the influences of fluid intensification based preconditioning process on the surface modification and decarburization enhancement of FA,and could meanwhile serve as an important reference for actual events in some sense.For prospective studies,other methods of process intensification (e.g.,highefficiency reagents or technologies) maybe allied with the fluid intensification to jointly tackle these refractory individuals.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge financial support from National Natural Science Foundation of China (51722405,51974310),National Key Research and Development Project of China (2019YFC1904301).