Separation and Purification Technology 118 (2013) 448–454

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Separation and Purification Technology

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Recovery of silicon powder from silicon wiresawing slurries by tuningthe particle surface potential combined with centrifugation

1383-5866/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2013.07.011

⇑ Corresponding author.E-mail address: hzhu@metall.ustb.edu.cn (H. Zhu).

Suning Liu, Kai Huang, Hongmin Zhu ⇑School of Metallurgical and Ecological Engineering and State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Xueyuan Rd. 30, HaidianDistrict, 100083 Beijing, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 January 2013Received in revised form 3 July 2013Accepted 12 July 2013Available online 20 July 2013

Keywords:Wiresawing slurrySilicon powderParticle surface potentialCentrifugation

A novel recovery process for effective separation of silicon and silicon carbide micropowders from thewiresawing slurry was proposed, based on the difference in their size and surface charging state. The sur-face charging levels of silicon and silicon carbide particles were found to be obviously different by tuningthe solution pH values, so the pre-dispersed mixed particles in the suspension will be subject to differentstatic electrical repulsion and particle mass. Under the same centrifugation filed, Si and SiC particles willhave distinctly different separation tendency. Experimental results show that the silicon microparticlestend to suspend in the upper part of the vessels whereas the silicon carbide particles will settle downto the bottom quickly, leading to the effective separation of above powders. The optimal results show thatSi content can reach 91.8% in the Si-rich powder and 4.8% in the SiC-rich powder by separating the mixedpowder with Si 24.9% and SiC 70.1% as the raw material.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Utilization of solar energy has become an important directionfor new energy exploitation to relieve the dependence on the lim-ited fossil energy such as petroleum, natural gas and coal [1,2]. Ithas attracted many scientists to develop new technologies, amongwhich the polysilicon cell is regarded as one of the most fascinatingways to transfer the solar light into the storage electrical energy[3–5]. Nowadays, more than 90% solar cells are manufactured byusing high pure polysilicon or monocrystalline silicon as the matrixmaterials [6–8]. The main process of producing silicon wafer is bymulti-wire slicing [9,10], which uses polyethylene glycol (PEG) asthe suspension agent for SiC particles as the abrasive material[11–14]. After slicing of the polysilicon ingot into wafers, about50 wt% silicon scrap powder will be produced into the wiresawingslurry and mixed with the existing cracked abrasive SiC particlesand a small amount of scrap iron and copper particles from thesteel wires will also sneak into the powders. Therefore, the wastecutting slurry is actually a viscous mixture of the above mentionedmicropowders and PEG [15–19]. The obtained wiresawing slurryusually contains about 30 wt% Si, 30 wt% SiC, 30 wt% polyethyleneglycol and water, and 5 wt% iron and other metal scraps [20–22].Silicon powder in the wiresawing slurry has high purity about6 N. But its recovery and utilization as the polycrystalline siliconfor solar energy cell is a big challenge due to the lack of effective

separation and recovery technologies up to now. So the resourcesre-utilization and the environmental protection, and also causesthe high production cost of polycrystalline silicon solar cells[23,24].

Various techniques have been proposed to separate and recoverthe silicon micropowders from the wiresawing slurry. For theeffective separation of PEG and metal scraps, the related tech-niques such as cleaning with water or acid have been studied quitewell, while for the separation and recovery of silicon powder, manystudies are still conducted in the lab scale. Obviously, a satisfactoryrecovery process should be simple in operation and economicallyacceptable. Lin used a heavy-liquid with a density between thatof silicon and silicon carbide as the centrifugation medium [25].Its separation efficiency is acceptable; however the heavy-liquidcontains methenyl bromide, which is highly poisonous. They alsodeveloped the phase-transfer separation process to recover siliconpowder [26]. Based on the different densities, surface charge andparticle size of the wiresawing silicon and silicon carbide powders,Wu studied their separation behavior in an electrical field, whichwas simple and feasible to apply and recover silicon powder fromthe wiresawing slurries [27]. Tsai modified the baffle plates of thehorizontal electrical sedimentation cell and obtained the Si-richpowder containing 92.4 wt% Si [28]. Huang applied froth flotationtechnology to separate Si and SiC and obtained the quite good re-sults, but the separation details as well as the extraction principlewere not mentioned at all [29]. Shibata also used the froth flotationtechnology, but only SiC powders were recovered and SiO2 wasformed due to the burning of Si particles [30]. So for the above

S. Liu et al. / Separation and Purification Technology 118 (2013) 448–454 449

mentioned techniques, the flaws such as using poisonous media orcomplex equipments render the separation of Si actuallyunfeasible.

In the present study, a novel process is proposed to effectivelyrecover the silicon and silicon carbide from the wiresawing slur-ries, which is based on the different particle size and surface chargein the aqueous solution, coupled with centrifugation treatment torealize the separation efficiently. Sequential separation by repeat-ing above procedures will make the recovered Si-rich powderreach a quite acceptable content.

2. Experimental

2.1. Chemical compositions of the mixed powder

The sample powder was kindly donated by Liaoyang Silicon Co.Ltd., Liaoning Province of China, in which the most of the PEG inthe cutting slurry had been removed by washing in the factory,the residual PEG was removed by water washing way. The receivedsample was treated to remove metal scraps by dissolving with HClin our lab, and the obtained powder was used as the raw materialsfor further separation study herein. Ten gram of such powder sam-ple was fed into a polyethylene beaker, and then 100 ml of 5 mol/lnitric acid and 40 ml of 22.5 mol/l hydrofluoric acid were added,and the gas bubbles would be seen giving off from the solution.When there’s no bubbling, the reaction was terminated. After fil-tering, water washing and drying, the filtered solid powder to-gether with filter paper was weighted and recorded as M1 (theweight of filter paper was designated as M2).

The content of silicon and silicon carbide can be calculatedaccording to the following equations:

Si wt:% ¼ ½10� ðM1�M2Þ�=10� 100% ð1Þ

SiC wt:% ¼ ðM1�M2Þ=10� 100% ð2Þ

2.2. Pre-dispersion treatment of the wiresawing solid powders

A typical wiresawing slurry usually contains polyethylene gly-col (PEG), scrap metal fine particles, silicon and silicon carbide

mixed solution

the first centrifuge

sedimentary layer(sample 3)

upper layersample

the secondcentrifuge

sedimentary layer(sample 4)

upper layersample

the thirdcentrifuge

SiC-rich powder(sample 5)

upper layersample

u(

Fig. 1. Proposed separation and recovery flow ch

micropowders. Before the recovery of silicon powder, PEG needsto be removed firstly. Typically, sufficient water and dilute hydro-chloride acid solution were added to the wiresawing slurry firstly,and constantly stirred for 3 h to dilute the slurry and dissolve themetal particles. Then the obtained suspension was filtrated, and re-peated above process for 4 times. The dried filtered cake was ballmilled for 20 min to crack the agglomerated mixed powders intothe separate individual particles sufficiently, which was beneficialto the sequential separation process under electrical repulsionoriginated from the surface charging and centrifugation force.

2.3. Surface charging of particles and centrifugation treatment

Ten gram of pre-dispersed mixed powders were added into thewater with the solid–liquid weight ratio set at 1:4. An aliquot of0.5 mol/L NaOH aqueous solution was added to adjust pH to8.15. The mixed solution was then treated by an ultrasonic wavetreatment for 20 min, and poured into the centrifugal tubes andcentrifuged at different rpm and time intervals.

The recovery flow sheet in this study is illustrated in Fig. 1. Afterthe first time of centrifugation, the suspension was found to beseparated to two parts. The upper suspension was collected, andthe sediment was combined with water and adjusting its pH to8.15 for the second time of centrifugation. Above centrifugal stepwas repeated totally three times, and all the upper suspensionwere collected. Finally, such-obtained upper suspension was addedwith dilute hydrochloric acid to adjust its pH to 2.0, and the pow-der settled rapidly under centrifugation, and then was dried to getthe Si-rich powder. Similarly, the SiC-rich powder can be obtainedby drying the settled powder after the third time of centrifugation.

2.4. Analysis methods

The particle size distribution was measured by LMS-30 LaserParticle Analyzer (Japan Seishin). X-ray diffraction measurementsof the samples were carried out by a Rigaku D/max-RB X-ray dif-fractometer. Scanning electron microscopy (SEM) instrument(Cambridge S-360) was used to observe the size and surface mor-phology of the particle samples.

pper layer sample 1) centrifuge

Si-rich powder(sample 2)

sedimentarysample

art of Si powder from the wiresawing slurry.

Fig. 2. Zeta potentials of Si and SiC particles in the aqueous solution at different pHvalues.

Fig. 3. Digital graphs of the suspension samples before (a) and after (b) centrifu-gation under the operation conditions: X = 2000 rpm, t = 5 min, pH = 8 (c) is thepartially enlarged figure of (b).

450 S. Liu et al. / Separation and Purification Technology 118 (2013) 448–454

3. Results and discussion

3.1. Tuning surface charging state of the particles

Fig. 3 shows the zeta potential curves of silicon and silicon car-bide particles at pH range of 2–10, and it can be found that at pH of8–10, the surface charging levels for both particles are quite large(Fig. 2), which means that at this pH range the electrostatic repul-sion force between the particles were quite strong tending to keepthe each particle in dispersing and suspending state.

Taking into account the smaller size of the silicon particles inthe actual wiresawing slurry, it can be deduced that more siliconparticles will be suspended in the upper section of the solutionthan that of the silicon carbide particle which has much larger sizeand density.

The surface charging of both Si and SiC particles were large withthe pH was around 8, And the repulsive force between them alsolarge. Assuming all the Si and SiC particles are independent anddispersed, and each of the particles in the centrifugation are af-fected by gravity, centrifugal force, buoyancy and viscous resis-tance. According to the Stokes’ law, the viscous resistance ofparticle in the solution with the condition of the particle is verysmall and move very slow.

Table 1The calculating formulas of the related parameters.

Parameters The calculating formulas

Viscous resistance (f) f = 6pgrvCentrifugal force (F) F ¼ 4

3 pr3qx2xBuoyancy (F0) F 0 ¼ 3

4 pr3q0gSedimentation coefficient (S) S ¼ 2ðq�q0 Þ

9g r2

Terminal velocity (m) m ¼ sx2x

Table 2Characteristic parameters of Si and SiC particles obtained from the wiresawing slurry.

Parameters Si SiC Unit

Density (q) 2.33 3.16 g/cm3

Peak diameter (dp) 1.0 9.5 lmAverage volume (V) 0.5 448.9 lm3

Average masses (m) 1.2 � 10�3 1.4 mgSedimentation coefficient 8.4 � 10�8 1.3 � 10�5

Terminal velocity 0.02 3.3 cm/s

The calculating formulas of the related parameters are shown inTable 1.

g is fluid viscosity coefficient, r is the radius of particle, m is thefinal settling velocity of particle, e q is particle density, x is angularvelocity, x is centrifuge diameter, T is rotating speed of the centrif-ugal machine, q0 is fluid density, S is sedimentation coefficient.

And as listed in Table 2, it can be found that the average particlediameters of these two particles is about ten times different andcombined with the density of the Si and SiC particles can estimatethe average mass of SiC is around 1000 times than Si particle. So itcan be assumed that under the same centrifugation treatment, thelarger size of SiC particles will make them tend to settle down tothe bottom of the centrifugal tube, and the smaller size of Si micro-particles will be suspended in the upper section of the tube andseldom settle down. Since the acceleration of gravity is far be lessthan the centrifugal acceleration, the former is ignored. The centri-fuge diameter is 150 mm, the fluid viscosity coefficient g is1.000 � 10�5 Pa�S, and the fluid density q0 is 0.9982 g/cm3at20 �C. The relative data and calculation results were showed inTable 2.

This phenomenon is quite beneficial to the separation of Si andSiC powder. So in the same solution and centrifugal field, the Si-rich particles will be collected in the upper suspension and theSiC-rich particles will be obtained in the bottom section of the cen-trifugal tube, as shown in Fig. 3.

3.2. Different properties between Si and SiC powder of the wiresawingslurry

The mixed powder, after pretreating by removal of PEG andmost of the metal components, was used for analysis accordingto the method described in Section 2. The analytical results are

Table 3The content of Si in the mixed powder.

1 2 3 Average content

wt% Si 24.91 24.56 25.32 24.93

Fig. 4. Particle size distribution of the mixed powder (a) and SiC powder (b).

Fig. 6. Digital graphs of the suspension samples after centrifugation at pH = 4 (a)and pH = 8 (b) under the same other operation conditions: X = 1500 rpm, t = 5 min.

S. Liu et al. / Separation and Purification Technology 118 (2013) 448–454 451

listed in Table 3. It shows that the content of Si in the mixed pow-der was around 24.9%.

The particle size distribution curve, as shown in Fig. 4a, has twopeaks corresponding to 1 lm and 8 lm at their central positions,respectively. The particle size distribution range of Si and SiCmixed powder is between 0.5 lm and 11.0 lm. In order to deter-mine the components contribution of Si and SiC particles to theabove two peaks, 5 g of mixed powder was placed in a mixed solu-tion with 50 ml 5 mol/L nitric acid and 20 ml 22 mol/L hydrofluoricacid to remove the silicon powder by complete dissolving, and thefiltrated indissoluble particles can be attributed to SiC powder,which was further analyzed by the laser particle size analyzer.The results show that the obtained SiC particles ranged from1.6 lm to 11.0 lm as shown in Fig. 4b, so it can be deduced thatthe peak of 0.5–1.6 lm can be as assigned to the existence of Siparticles mostly. Based on above results, it can be evaluated thatthe particle size of SiC is as at least ten times large as that of siliconparticles.

Fig. 5 shows the SEM graph of Si particles and SiC particles inthe mixed powder. It can be seen that the two kinds of particlesare quite different in size. Most of the silicon powder is adheredeach other or adhered onto the surface of larger SiC particles. Moresatisfactory results would be obtained by the centrifugal separa-tion when the two types of particulates can be fully dispersedfirstly. Ultrasonic waves can disperse the agglomerates to some ex-tent, making the Si and SiC particles easier to be separated underthe centrifugation filed.

Fig. 5. SEM photograph of the original slurry powder.

3.3. Effect of pH values

The different pH values of mixed solution can make significantinfluence on the surface charging state of Si and SiC particles; fur-ther more the result of centrifugal separation process. Fig. 6 showsthe mixed solution after centrifugal separation when the pH valuesare 4 and 8 respectively. At pH 4, under the experimental condi-tions of centrifugal time of 5 min at 1500 rpm, the upper suspen-sion of centrifugal tube is basically clear as shown in Fig. 6a, andthe sediment powder at the bottom can be found consisting oftwo layers, the upper layer of dark powder covered on the graypowder. Since the Zeta potentials of Si and SiC particles are about�20 mV at pH 4 (Fig. 2), the repulsion between the particles isweak, and larger SiC particles will settle down faster due to itsgreater mass. When the pH value of the mixed solution was 8, asshown in Fig. 2, the surface charging of both particles is large, sug-gesting there is a very strong repulsion force between the particles,and in this case the smaller size of Si particles makes them very dif-ficult to settle down to the centrifugal tube bottom, while for thelarger SiC particles, they are strongly prone to settle down to formthe gray sediment layer, as shown in Fig. 6b.

Fig. 7. The particle size distribution of the Si-rich powder obtained at differentcentrifugation time of 2 min (a), 5 min (b), 10 min (c) under the same otheroperation conditions: X = 1500 rpm, pH = 8.5.

Fig. 8. The particle size distribution of the Si-rich powder obtained at differentcentrifugation rates of 500 rpm (a), 1500 rpm (b) and 2500 rpm (c) under the sameother operation conditions: t = 5 min, pH = 8.

452 S. Liu et al. / Separation and Purification Technology 118 (2013) 448–454

3.4. Effect of centrifugation time

With the centrifugal time is set at 2 min, 5 min and 10 minrespectively, the pH is 8.5, the centrifugation rotation rate is1500 rpm, the particle size distribution curves for the Si-rich sam-ples are shown in Fig. 7. With the centrifugal time increases, the

Fig. 9. SEM photographs of Sample 1 (a), sample 2 (

content of Si-rich phase becomes less and less, and the averageparticle size changes from 1.8 lm to 1.0 lm correspondingly. Soit can be confirmed that the content of silicon in the small particlesincreased. Due to the effect of centrifugal force, the big particlessettled down in advance, while the smaller particles kept suspend-ing in the centrifugal process. During the further centrifugal pro-cess, for the smaller particles, more particles settled from the topof the centrifuge tube to the bottom in the longer the time, and fi-nally large particles in the upper suspension became less.

3.5. Effect of centrifugation rotation rates

The results show that the content of silicon in the Si-rich phasewas different with centrifugation rotation rates. Fig. 8 shows theparticle size distribution of Si-rich powders when the centrifuga-tion rotation rate was 500 rpm, 1500 rpm and 2500 rpm respec-tively. When the rotation rates increases from 500 rpm to2500 rpm, size range of the collected upper particle sampleschange from 0.8–3.0 lm to 0.5–1.4 lm. Since the Zeta potentialof SiC was almost the same as that of Si, at the same centrifugalrate, the larger SiC particles will have stronger tendency to settleddown.

3.6. Properties of the obtained Si-rich and SiC-rich powder

The Si-rich and SiC-rich powder was recovered by using theprocess proposed in Fig. 1, and the experimental conditions wereas follows: ultrasonic wave time = 20 min, X = 1500 rpm, centrifu-gal time = 5 min, pH = 8. Fig. 9 shows the SEM images of powder

b), sample 3 (c), sample 4 (d) and sample 5 (e).

Fig. 10. The particle size distribution of sample 1 (a), sample 2 (b), sample 3 (c),sample 4 (d) and sample 5 (e).

Fig. 11. XRD patterns of Si-rich sample (a) and SiC-rich sample (b).

Table 4Comparison of results for different separation techniques of wiresawing slurry.

References Separation methods Content ofobtained Si or SiC(%)

Recoveryratio (%)

Lin [25] Heavy-liquid centrifugation Si: 90.8 wt% Si: 74.1%Lin [26] Phase-transfer separation Si: 99.1 wt% Si: 71.1%Wu [27] Electrical field Si: 92.8 wt% –Tsai [28] Electrical field Si: 92.4 wt% –Tsai [31] Electrical field – –Huang

[29]Froth flotation Si: 96 wt%

SiC:99 wt%–

Presentstudy

Surface electric charge tuningand centrifugation

Si: 91.8 wt% Si: 70.5%SiC: 95.2 wt% SiC: 90.8%

S. Liu et al. / Separation and Purification Technology 118 (2013) 448–454 453

samples 1–5 collected according to the proposal in Fig. 1 respec-tively. A few large SiC particles are still left in the upper layerthrough the first one time of centrifugal separation as indicatedin Fig. 9a. Sample 2, obtained by one more time of centrifugationfor the supernatant of sample 1 is basically composed of uniformsilicon particles with the size of about 0.8 lm as seen in Fig. 9b.Although the sediment powders collected after each time of cen-trifugal separation still had some Si fine particles left as shown inFig. 9c–e, the number of Si particles can be found to decrease obvi-ously after each more time of centrifugation. Fig. 9e shows the par-ticle size of SiC-rich powders with the size of about 10 lm, and itcan be found that there is only very small amount of tiny Si particleleft.

Fig. 10 exhibits the particle size distribution of above samples1–5. Fig. 10a and b shows the particle size of sample 2 is smallerthan sample 1, which means that centrifugation times are quitesignificant to decrease the amount of larger SiC particles sus-pended in the upper suspension. The particle size of samples 3–5increases successively as shown in Fig. 10c–e. Above results dem-onstrate that most of the tiny Si particles can be suspended and thelarge SiC particles can settle down by tuning the particle surfacepotential combined with appropriate centrifugation force, andthe cost-effective separation process can be realized feasibly.

The XRD patterns of the final recovered particle samples arepresented in Fig. 11. Fig. 11a shows the characteristic peaks of Siand that of SiC in Fig. 11b. It can be seen that the Si and SiC powderof the wiresawing slurry were effectively enriched by the proposedrecovery flow chart. Table 3 lists the content of Si or SiC in the ob-

tained powders, and they can reach 91.8 wt% and 95.2 wt%, respec-tively. And the separation results by reported other processes werealso summarized in the table, and it can be found that the separa-tion efficiency in our work is fairly good. The recoveries of Si andSiC were above 70–90% respectively. Table 4 summarizes the re-sults of different methods to separate and recover the Si and SiCfrom the wiresawing slurries.

4. Conclusions

It is verified experimentally that silicon and SiC micropowdercan be effectively separated by surface potential tuning combined

454 S. Liu et al. / Separation and Purification Technology 118 (2013) 448–454

with centrifugation for the wiresawing slurry. When the levels ofsurface potential difference reaches maximum, Si particles in thesolution will be suspended sufficiently while the larger SiC parti-cles will settle down quickly under the centrifugation field. Resultsshow that the Si-rich powder obtained by the proposed separationflow chart contains more than 91.8 wt% Si and SiC-rich phase con-tains more than 95.2 wt% SiC, which can be used as the intermedi-ate material for further separation like high temperature vacuumrefining process [32] or directional consolidation process [33] toreach higher purity silicon materials for the manufacturing of solarcell.

Acknowledgements

This work was supported by National Science Foundation ofChina (Nos. 50934001, 21071014, 51102015 and 51004008), Na-tional High Technology Research and Development Program of Chi-na (863 Program, No. 2012AA062302).

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