manganese hydroxide crystals in Couette–Taylor crystallizer

Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 31–39

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Agglomeration of nickel/cobalt/manganese hydroxide crystals in Couette–Taylor crystallizer Jong-Min Kim a , Sang-Mok Chang a , Joon Hyun Chang b , Woo-Sik Kim b,∗ a b

Department of Chemical Engineering, Dong-A University, Busan 602-714, Republic of Korea Department of Chemical Engineering, ILRI, Kyung Hee University, Yoing Kiheung-ku Seochun-dong 1, Kyungki-do 449-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 29 December 2010 Received in revised form 26 February 2011 Accepted 2 March 2011 Available online 10 March 2011 Keywords: Couette–Taylor vortex Metal hydroxide Reaction crystallization Agglomeration Continuous crystallizer

a b s t r a c t The crystal agglomeration of Ni/Co/Mn hydroxide (NCM hydroxide) in a continuous Couette–Taylor (CT) crystallizer was experimentally studied. The NCM hydroxide crystals produced by the reaction crystallization were simultaneously coagulated to form agglomerate particles via the consecutive steps of physical adhesion of the crystals and molecular growth. Thus, the hydrodynamic conditions and supersaturation profile in the CT crystallizer were the most critical influencing parameters determining the agglomerate particle size and shape. The particle size was reduced when increasing the rotation speed of the inner cylinder in the CT crystallizer, as enhancing the intensity of the Taylor vortex broke and re-dispersed the physically adhered crystals. In addition, a high Taylor vortex changed the irregularly shaped agglomerate particles into spherical ones. The supersaturation profile in the CT crystallizer was prolonged when increasing the mean residence time and decreasing the feed concentration, thereby enlarging the particle size. Similarly, a chelating reaction also prolonged the supersaturation profile in the CT crystallizer, resulting in a larger particle size. In the agglomeration, the individual crystals created a hexagonal system stacked on the (0 0 1) face, meaning that the particles were mostly covered by the (1 0 1) face. In general, under the tested operating conditions, the CT crystallizer was able to produce agglomerate particles that were narrowly distributed and smaller than 5 ␮m in size within 10 min of the mean residence time. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Agglomeration, a key crystallization phenomenon, is frequently encountered during the processes of powder production, separation, and purification when using crystallization methods. In the case of powder production, the particle size, distribution, and shape, the most important characteristics determining the powder properties, are unpredictably changed by agglomeration. In addition, impurities from the mother liquor are sometimes entrapped in the agglomerate particles, thwarting the purification process during crystallization. Therefore, much effort has been invested in studying and controlling agglomeration and the agglomeration process during crystallization. It is generally known that agglomeration occurs via the consecutive steps of physical adhesion of the crystals (called aggregation) and molecular growth of the aggregates [1–5]. Based on this agglomeration mechanism, the key operating factors of crystallization controlling agglomeration have already been investigated.

∗ Corresponding author. Current address: 343 Olin Hall, School of Chemical Eng. & Biomole. Eng., Cornell University, Ithaca, NY 14853-5210, USA. Tel.: +82 31 201 2576; fax: +82 31 273 2970. E-mail address: [email protected] (W.-S. Kim). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.03.016

For example, in a mixing tank crystallizer, agglomeration is predominantly dictated by the turbulent agitation, as the turbulent fluid motion of the suspension causes crystal collisions resulting in agglomeration [6–8]. Simultaneously, the turbulent fluid motion also breaks and re-disperses the adhered crystals, meanwhile promoting mass transfer for molecular growth. Thus, the effect of turbulent agitation on agglomeration is quite complicated. When using a semi-batch mode in a mixing tank crystallizer, the agglomerate particle size normally increases when increasing the agitation speed up to a certain level and then decreases with any further increase of the agitation speed [7,9,10]. However, in the case of cerium carbonate, ferric hydroxide, and alumino-humic floc, the agglomerate particle size is monotonically reduced regardless of the agitation speed. This different agglomeration behavior according to the agitation originates from the physic-chemical properties of the crystal materials. In reaction crystallization using a batch, semi-batch, or continuous mode, several studies have demonstrated the important role of supersaturation for agglomeration [4,11]. Increasing the supersaturation promotes the agglomeration of crystals due to the facilitation of molecular growth, resulting in larger agglomerate particles. Supersaturation also contributes to crystal agglomeration by generating a larger particle population, thereby increasing the possibility of crystal collisions in an agitated suspension [7,12,13].

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Plus, no further agglomeration occurs when the reactant feed is stopped during reaction crystallization in a semi-batch mode, as the supersaturation is depleted. A salt additive has also been used to control agglomeration [4], where the salt ions are adsorbed on the crystals and create a surface charge that induces a repulsive interaction between the crystals. As a result, crystal agglomeration is inhibited, which reduces the agglomerate particle size. Meanwhile, the influence of the solvent on crystal agglomeration has been studied using seed crystals of paracetamol [14,15]. When using a solvent with a low polarity, the crystals are significantly agglomerated. However, the crystal agglomeration is reduced when increasing the solvent polarity, as the polar solvent is strongly adsorbed on the crystals, inhibiting crystal adhesion. In the present study, a Couette–Taylor vortex is applied to control the crystal agglomeration of nickel, cobalt, and manganese hydroxide (NCM hydroxide) during continuous crystallization. The Couette-Talyor fluid motion is an interesting and well-defined flow regime with a unique flow behavior and features [16]. A stable Couette flow motion is induced when slowly rotating the inner cylinder of two co-axial cylinders. This motion then changes to a periodic unstable turbulent motion, called a Taylor vortex, when the rotation speed of the inner cylinder increases beyond a certain level, called the critical Taylor number (TaC ). Thus, a laminar Taylor vortex flow is induced at 1 < Ta/TaC < 3, a wavy vortex flow at 3 < Ta/TaC < 13.3, a quasi-periodic wavy vortex flow at 13.3 < Ta/TaC < 18, a weakly turbulent vortex flow at 18 < Ta/TaC < 33, a turbulent Taylor vortex flow at 33 < Ta/TaC < 160, and a turbulent flow at Ta/TaC > 160 [17,18]. Owing to its rigorous radial vortex combined with a small axial dispersion, a Taylor vortex provides a homogeneous intensity for turbulent mixing, which has many beneficial applications in crystallization, polymerization, ceramic membranes, photocatalytic reactions, filtration, and biological systems [19–25]. For example, in the continuous CO2 –Ca(OH)2 reaction crystallization of calcium carbonate, a Taylor vortex promotes CO2 absorption, inducing a high and uniform supersaturation around the inlet region of the crystallizer, which results in a narrow size distribution and uniform shaped crystals in the product suspension of the outlet steam [13,26]. Also, spindle calcite, which is produced at a low rotation speed, becomes rhombohedral when increasing the rotation speed, as the excess Ca2+ ions in the solution at the low rotation speed create stoichiometric conditions for the reaction and crystallization [27,28]. In addition, the high mass transfer and homogeneous mixing in the phase transformation and material synthesis were achieved by the Taylor vortex [21,22]. Accordingly, the present study applied a Taylor vortex to the reaction crystallization of NCM hydroxide, a precursor of the cathodic material (NCM oxide) used in batteries. Here, the particle size and shape of the NCM hydroxide are the most critical factors determining the electrical properties of the cathodic materials. The generation of nano-scale flake-shaped crystals is usually unsuitable for downstream processes, such as powder processing. Thus, the preference is agglomerate particles of NCM hydroxide crystals appropriately sized to enable a high packing density and specific surface area for the charge transfer as a cathodic material. Currently, the preparation of agglomerate particles of NCM hydroxide is mostly performed in a MSMPR crystallizer with a long mean residence time, over 10–20 h, signifying an extremely low productivity from the crystallization process [29,30]. Notwithstanding, it is still difficult to achieve an agglomerate particle size smaller than 10 ␮m to enable a high packing density and specific area as a cathodic material. Besides, using a binder or additive, the tumbling and fluidized beds were frequently adopted for the granulation/agglomeration of the powders in industries. However, they might be hardly applicable for the production of the agglomerate particles of NCM hydroxide because any binder and additive

in the agglomerates deteriorated electrical property of cathodic material. Therefore, using a Couette–Taylor crystallizer, the present study investigates the effect of a Taylor vortex on the agglomeration and agglomeration process of the NCM hydroxide reaction crystallization in order to control the size and shape of the agglomerate particles, such as less than 10 ␮m in size and spherical ones in shape, which might be acceptable for the industrial application. In addition, the key operating factors promoting the agglomeration process in the crystallization are identified in order for the design of the productive process. 2. Prediction of agglomerate particle size The agglomerate particle size is frequently limited by the hydrodynamic conditions of the suspension, as the aggregate particles are broken and re-dispersed by the fluid shear before they are converted to agglomerates. As regards the hydrodynamic conditions for crystal agglomeration, according to Parker et al. [33] and Jarvis et al. [34], the agglomerate particle size (dmax ) can be described in terms of the average shear rate of the fluid motion (G) as, dmax = CG−

(1)

where C is the particle strength coefficient and  is the stable particle size exponent, both of which depend on the physical properties of the materials. The average shear rate in a turbulent flow has been suggested as [35,36],



G=

ε 

(2)

where ε is the energy dissipation of the turbulent fluid motion per unit mass of fluid and  is the kinematic viscosity. In a Taylor vortex, the energy dissipation is derived as [37,38], ε=

LC ri4 ωi3 f

(3)

VCT

where LC indicates the length of the CT crystallizer, ri is the radius of the inner cylinder of the CT crystallizer, ωi is the angular velocity of the inner cylinder and VCT is the volume of the CT crystallizer. The friction factor, f, depending on the Reynolds number (Re = ri ωD/) when Re is above the critical value (ReC ) is suggested as,

 D 0.35

f = 0.80

ri

Re−0.35

for ReC ≤ Re

(4)

where D is the gap between the inner and outer cylinders. Using above equation set of Eqs. (1)–(4), it would be possible to predict the relationship between the agglomerate particle size and the hydrodynamics condition of the Taylor vortex. 3. Experiment The metal (Ni, Co, Mn) hydroxide co-precipitation was carried out based on the reaction of metal sulfates (nickel sulfate, cobalt sulfate, and manganese sulfate) with sodium hydroxide. Nickel sulfate hexahydrate (>98.5%, Samchun Co., Korea), cobalt sulfate heptahydrate (>99%, Samchun Co., Korea), and manganese sulfate monohydrate (>99%, Daejung Co., Korea) were used as the metal ion sources of nickel, cobalt, and manganese, respectively. The sodium hydroxide (>99%) was supplied by Samchun Co, Korea. Ammonia (32%, Samchun Co., Korea) was applied as the chelating agent for the co-precipitation. All the reagents were used without further purification. In the crystallization, the reagent molar ratio of nickel sulfate, cobalt sulfate, and manganese sulfate in the feed solutions was always fixed at 1:1:1. To modify the stoichiometric conditions of the crystallization, the concentrations were equally varied from

J.-M. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 31–39

0.5 to 3.0 mol/l, while the corresponding sodium hydroxide concentration was changed from 1.0 to 6.0 mol/l. As a by-product of the reaction, sodium sulfate salt was produced, which can influence the crystallization by changing the electric charge of the metal hydroxide particles, plus its concentration changed according to the reactant concentration. Thus, in the present study, the sodium sulfate salt concentration during the crystallization was set at 1.5 mol/l of the product suspension to provide an equal salt effect on the crystallization. To achieve this, extra sodium sulfate was added to the feed solutions to compensate for any difference in the by-product (sodium sulfate) concentration with the different feed concentrations. In the crystallizer, the crystallization was always run at 45 ◦ C and the pH of the output stream was controlled between 10.0 and 12.0 by slightly adjusting the flow rate of the sodium hydroxide solution. The Couette–Taylor crystallizer was composed of inner and outer co-axial cylinders made of stainless steel, as shown in Fig. 1. The diameter of the outer (ID) and inner (OD) cylinders was 84 and 76 mm, respectively, and their length was 250 mm. Thus, the gap between the two cylinders was 4 mm and the working volume of the crystallizer was 250 ml. Initially, the gap between the two cylinders was filled with the distilled water, and the co-precipitation initiated by injecting the reactant solutions of the metal sulfates and sodium hydroxide into the crystallizer. The metal hydroxide feed solutions were injected through an inlet port positioned at

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one axial end of the crystallizer, while the sodium hydroxide solution was simultaneously fed though another inlet port positioned at the same axial end, yet 180◦ across from the inlet port for the metal hydroxide feed in order to minimize any direct contact of the two reactants before being homogeneously mixed in the crystallizer. The mean residence time in the crystallizer was varied from 1 to 30 min. Here, the two feed solutions of the metal sulfates and sodium hydroxide flowed equally into the crystallizer. Ammonia as the chelating agent was also injected into the crystallizer using another inlet port at the same axial end, and the amount was controlled between 0 and 1.5 mol/l by adjusting the injection flow rate. The maximum flow rate of the chelating agent was 1/10 of the feed flow rate. The rotational motion of the inner cylinder was driven by an AC motor (Siemens, 1LA7083, Germany) to induce turbulent Taylor vortices in the solution in the gap between the two cylinders, and was varied from 300 to 1500 rpm. In a steady state, a sample of the product suspension was taken from the output stream located at the other axial end of the crystallizer. The 5 ml suspension sample was quickly diluted with 45 ml of distilled water to quench the crystallization and measure the particle size in the suspension. The suspension sample was also filtered using filter paper with a 5 ␮m pore size (ADVENTEC 5 C, U.S.A.) to obtain the powder for a microscopic analysis. Here, the filtered power was washed with distilled water 2–3 times during the filtration, and then dried in a convection oven (OV-11, JeioTech, Korea)

Fig. 1. Schematic diagram of (a) experimental system for reaction crystallization of metal hydroxide in continuous Couette–Taylor crystallizer and (b) Taylor vortex induced in Couette–Taylor crystallizer.

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15

25

1: 300 rpm, 10 min

700rpm 10min

Volume Distribution (%)

Particle Diameter (µm)

2: 1500 rpm, 10 min

1500rpm 10min

12

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9

6

3

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20

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4 1

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0 0

2

4

6

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τ(reaction time/mean residence time)

0.1

1

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Agglomerate Particle Size (µm)

Fig. 2. Transient behavior of reaction crystallization of Ni/Co/Mn hydroxide in continuous Couette–Taylor crystallizer. The feed concentrations for the reaction crystallization were always fixed at 3.0 mol/l of (Ni/Co/Mn)SO4 and 6.0 mol/l of NaOH.

Fig. 4. Typical particle size distribution with various crystallization conditions in continuous Couette–Taylor crystallizer. The feed concentrations for the reaction crystallization were always fixed at 3.0 mol/l of (Ni/Co/Mn)SO4 and 6.0 mol/l of NaOH.

at 110 ◦ C for 24 h. SEM (LEO SUPRA 55, Carl Zeiss, Germany) and ˚ M18XHF-SRA, Mac Scipower pattern XRD (Cu-K␣ ray (1.54056 A), ence, Japan) were used to measure the particle shape and structure, respectively. The crystal structure of the primary crystals contained in the particles was determined by selected area diffraction using a TEM (JEM-2100F, JEOL, Japan).

4. Results and discussion The reaction crystallization of the metal hydroxide ((Ni,Co,Mn(OH)2 = NCM hydroxide) was initiated by injecting the metal sulfate reactants (nickel sulfate, cobalt sulfate and manganese sulfate, (=NCM sulfates)), sodium hydroxide reactant,

Fig. 3. Typical morphology of Ni/Co/Mn hydroxide particles produced with various crystallization conditions in continuous Couette–Taylor crystallizer: (a) mean residence time of 10 min and rotation speed of 300 rpm, (b) mean residence time of 10 min and rotation speed of 1500 rpm, (c) mean residence time of 30 min and rotation speed of 300 rpm, and (d) mean residence time of 30 min and rotation speed of 1500 rpm. The feed concentrations for the reaction crystallization were always fixed at 3.0 mol/l of (Ni/Co/Mn)SO4 and 6.0 mol/l of NaOH.

J.-M. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 31–39

a

15

Agglomerate Particle Size (µm)

and ammonia as the chelating agent into the Couette–Taylor (CT) crystallizer initially loaded with pure water. As shown in Fig. 2, the particle size of the product suspension was quite large during the early stage of the precipitation and then quickly dropped after 1–2 times the mean residence time. Thereafter, it leveled off after about 7–8 times the mean residence time, indicating a steady state condition in the continuous CT crystallizer. The experimental results also revealed that the particle size in the steady state varied according to the operational conditions of the CT crystallizer. As such, a higher rotation speed of the CT crystallizer produced a smaller particle size, while a longer mean residence time produced a larger particle size in the product suspension, implying that these were the key operating parameters controlling the particle size of the NCM hydroxide in the continuous CT crystallizer. In a steady state, the typical particle shape and size distribution were as shown in Figs. 3 and 4, respectively. At a low rotation speed of 300 rpm, the particle shape appeared irregular, due to the agglomeration of crystals (Fig. 3(a)). Also, the particles were broadly distributed, as shown in Fig. 4. However, when increasing the rotation speed, the crystals became round in shape and

12

3 min 10min 30 min 9

6

3

0 0

300

600

900

1200

1500

Rotation Speed (RPM)

b

were narrowly distributed with a small size. The particle shape also became like a sphere when increasing the mean residence time (Fig. 3(d)). The variation in the particle shape and distribution according to the rotation speed originated from the agglomeration of the crystals controlled by the Taylor vortex fluid motion of the suspension. When considering that the agglomeration of crystals involves the consecutive steps of crystal adhesion followed by molecular growth, the fluid motion had a direct influence on both steps. As such, the crystals collided and adhered to form aggregates in the vortex fluid motion, and then were concretely bound by molecular growth to become solid agglomerates. However, the vortex fluid motion did not facilitate only crystal adhesion by collision but also the simultaneous breakage of the aggregates by the fluid shear. Thus, when the fluid shear was not high enough to break the interaction between the crystals in the aggregates, the adhesion of the crystals predominantly determined the agglomerate particle shape and size. Therefore, at a low rotation speed of 300 rpm, the agglomerate particles were large in size and irregular in shape. However, when the fluid shear was greater than the physical interaction between the crystals, the aggregate particles were trimmed by the turbulent shear, becoming round in shape, as shown at a high rotation speed of 1500 rpm. In addition, when increasing the mean residence time, the particles became more spherical, due to a longer exposure to the high fluid shear. As shown in Fig. 5(a), the agglomerate particle size was monotonically reduced when increasing the rotation speed from 300 to 1500 rpm, indicating that the fluid shear predominantly controlled the crystal agglomeration. Notwithstanding, the particles were enlarged when increasing the mean residence time, despite being exposed longer to the fluid shear, as shown in Fig. 5(b). This can be explained in terms of the effect of supersaturation on crystal agglomeration, which is an essential factor for molecular growth to form crystal agglomerates. As such, a longer mean residence time implied a slower feed flow rate into the crystallizer, resulting in a lower initial nucleation of crystals and slower drop in the supersaturation level along the axial direction of the crystallizer. This supersaturation profile was advantageous for the molecular growth needed for agglomeration in the crystallizer, and is consistent with the previous observations of Mersmann and Kind [31] and Lindenberg et al. [32]. In their studies, a low supersaturation profile was demonstrated to be good for single crystal growth without the induction of serious nucleation [31], plus supersaturation was found to be a key factor limiting the agglomerate size [32].

15 300 rpm

50

700 rpm

12

1500 rpm

Agglomerate Particle Size (µm)

Agglomerate Particle Size (µm)

35

9

6

3

0 0

5

10

15

20

25

K ol m

ogro

v Mi

c ros

3min 10min 30min cale

10

5

30

Mean Residence Time (min) Fig. 5. Influence of crystallization conditions on particle size: (a) rotation speed and (b) mean residence time. The feed concentrations for the reaction crystallization were always fixed at 3.0 mol/l of (Ni/Co/Mn)SO4 and 6.0 mol/l of NaOH.

1000

5000

10000

Average Shear Rate, G (1/s) Fig. 6. Correlation of particle size with fluid shear of Taylor vortex in crystallizer.

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In the present study, the influence of the supersaturation on the agglomeration and crystal growth will be discussed in detail later. When compared with the long holding time of 10–20 h required for the production of agglomerate particles around 10–15 ␮m in a MSMPR crystallizer of semi-batch mode [29,30], the operating conditions of the continuous CT crystallizer, including a mean residence time of around 10 min and moderate rotation speed, appeared much more efficient for the production of agglomerate particles less than 5 ␮m, which are suitable for the high packing density and specific surface area required for a cathode. The agglomerate particle size under the turbulent flow was frequently predicted in terms of turbulent shear, as suggested in Eq. (1) by Parker et al. [33] and Jarvis et al. [34]. Thus, using Eq. (1), the agglomerate particle size produced in the CT crystallizer was linearly aligned on a log dmax − log G plot, as shown in Fig. 6. From the plot of the NCM hydroxide particles, the stable particle size exponent () was identified as 0.21 ± 0.03 and the particle strength coefficient (log C) was 1.45 ± 0.03. These values were quite comparable with other agglomerations of ferric hydroxide ( = 0.29 and

log C = 1.9) and alumino-humic floc ( = 0.44 and log C = 3.1) [34]. Here, the smaller value of  for the NCM hydroxide particles indicated a lower sensitivity of the particle size to the fluid shear, implying a stronger interaction between the aggregated crystals. The selected area diffraction (SAD) was used to define the simple crystal structure and faces of the NCM hydroxide formed during the crystallization. An individual crystal consisting of agglomerated particles was typically shaped as shown (left top) in Fig. 7(a). According to the diffraction pattern, the crystal system of the NCM hydroxide was hexagonal (˛ = ˇ = 90◦ and  = 120◦ ), resulting in the hexagonal morphology of the crystal shown in the image [29,39–41]. In addition, the front large face of the crystals was confirmed as a (0 0 1) plane normally surrounded by six symmetric faces with a {1 0 0} plane. This hexagonal crystal system of the NCM hydroxide was identical with the crystal systems of a single metal hydroxide, such as nickel hydroxide, cobalt hydroxide, and manganese hydroxide (JCPDS Card number: 03-0177, 30-0443, 12-0696). When considering that nickel, cobalt, and manganese have very similar ionic radii, it is reasonable that the NCM hydrox-

Fig. 7. Crystal structure analysis using electron beam: (a) selected area diffraction of (Ni/Co/Mn) hydroxide crystal (TEM image, top left), (b) SEM image of (Ni/Co/Mn) hydroxide particle obtained with mean residence time of 10 min and rotation speed of 300 rpm, and (c) SEM image of (Ni/Co/Mn) hydroxide particle obtained with mean residence time of 10 min and rotation speed of 1500 rpm. The feed concentrations for the reaction crystallization were always fixed at 3.0 mol/l of (Ni/Co/Mn)SO4 and 6.0 mol/l of NaOH.

J.-M. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 384 (2011) 31–39

ide exhibited a similar crystal system. Among the three metal hydroxides, the crystallographic geometry of the NCM hydroxide was closest to that of a cobalt hydroxide crystal. For example, the spacing distances of the (1 0 0) and (1 1 0) planes of the metal hydroxide were calculated as 2.754 A˚ (d1 0 0 ) and 1.589 A˚ (d1 1 0 ), respectively. Plus, the spacing distance of the (1 0 1) plane was esti˚ This crystallographic data is identical to that for mated as 2.39 A. ˚ and a cobalt hydroxide crystal (d1 0 0 = 2.753 A˚ and d1 1 0 = 1.5895 A, ˚ respectively). Also, as shown in Fig. 7(b) and (c), the d1 0 1 = 2.369 A, crystals in the agglomerate particles were stacked on the (0 0 1) plane, while the particle surface was mostly covered by the (1 0 1) plane of the crystals. Furthermore, the (1 0 1) crystal face varied according to the crystallization conditions. As such, the crystal area of the (1 0 1) plane was larger with a higher rotation speed, due to the higher growth rate. The facial growth of individual crystals in the particles was measured when varying the crystallization conditions, i.e. the rotation speed and mean residence time, as shown in Fig. 8, where the quantitative measurement of facial growth was based on the characteristic peak ratio between the (1 0 1) and (0 0 1) planes of the crystal. When increasing the rotation speed, the peak ratio increased, as shown in Fig. 8(b), indicating that the (0 0 1) plane was more favorable to growth than the (1 0 1) plane. As a result, the (1 0 1) plane was larger, as shown in Fig. 8(b). In the microscopic images, the length scale of both the (1 0 1) and (0 0 1) planes increased due to an enhanced mass transfer rate when increasing

a

37

the rotation speed. However, since the (0 0 1) plane of the crystal was less stable (higher surface energy) than the (1 0 1) plane, the growth rate of the (0 0 1) plane was faster than that of the (1 0 1) plane, appearing as preferential facial growth. Similarly, preferential growth occurred when increasing the mean residence time, as the higher mean residence time provided a better supersaturation profile for molecular growth in the crystallizer, as mentioned above. Thus, since the (0 0 1) plane grew faster than the (1 0 1) plane, the characteristic peak ratio between the (010) and (0 0 1) planes also increased when increasing the mean residence time (Fig. 8(d)). Using the ICP, it was confirmed that the agglomerate particles of NCM hydroxide were purely composed at the ratio of 1:1:1 of nickel, cobalt and manganese ions, which might provide the optimum metallic composition for the cathodic property of Li(NCM)oxide. Also, the EDX ionic mapping demonstrated that the metallic ions of nickel, cobalt and manganese were homogeneously dispersed in the agglomerate particles (Supplementary material S1). The influence of the feed concentration on the particle size was also monitored, as shown in Fig. 9. In reaction crystallization, the supersaturation level is directly related with the feed concentration. Thus, the injection of a high feed concentration generated a high peak supersaturation around the inlet region of the CT crystallizer, resulting in abundant nucleation of crystals. This was then followed by a rapid drop in the supersaturation level due to the instantaneous rapid consumption of the supersaturation by the

b

MRT 6min

0.36 0.34

Intensity (a.u.)

3500 3000 2500

300rpm

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(101)/(001)

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0.32 0.30 0.28 0.26

1500rpm 300

0 20

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Intensity (a.u.)

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0 20

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Mean Residence Time (min)

2θ Fig. 8. XRD patterns of (Ni/Co/Mn) hydroxide particle obtained when varying: (a) rotation speed, (b) corresponding characteristic peak ratio of (1 0 1) and (0 0 1) planes, (c) mean residence time, and (d) corresponding characteristic peak ratio of (1 0 1) and (0 0 1) planes. The feed concentrations for the reaction crystallization were always fixed at 3.0 mol/l of (Ni/Co/Mn)SO4 and 6.0 mol/l of NaOH.

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10

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Agglomerate Particle Size (µm)

Agglomerate Particle Size (μm)

700rpm, 10min 700rpm, 30min

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0.0

Feed Concentration of Metal Solution (mol/L)

M+ (aq) + xNH4 OH(aq) → [M(NH3 )n 2+ ](aq) + nH2 O + (x − n)NH4 OH(aq)

(5)

[M(NH3 )n 2+ ](aq) + yOH− + zH2 O → M(OH)2 (s) + zNH4 OH + (n − z)NH3

(6)

Here, the ammonium ions combine with the metal ions to form a metal–ammonium complex before producing the metal hydroxide. Thus, when compared with the very fast and direct reaction of the metal sulfate with sodium hydroxide, the chelating reaction of ammonia with the metal ions was relatively slow, playing a similar role to a slow supply of supersaturation after the initial nucleation, promoting the agglomeration of crystals. As a result, the particles were enlarged when increasing the amount of ammonia fed into the crystallizer, as shown in Fig. 10. Previous studies have also reported that the chelating reaction contributes to the production of large and dense agglomerates of metal hydroxide, as found with the present experimental results [26]. However, in the case of a high feed of ammonia above 1.5 mol/l, obtaining stoichiometric particles (1:1:1 composition of Ni, Co, and Mn) became impossible, as most of the nickel ions were dissolved out into the ammonia solution.

1.0

1.5

Concentration of Ammonia (mol/L)

Fig. 9. Influence of feed concentration on particle size in Couette–Taylor crystallizer.

high number of nucleated crystals. As regards crystal aggregation, a high population of crystals can be advantageous as it enhances the frequency of crystal collisions. However, the low supersaturation level following the initial nucleation reduced the molecular growth rate, which then retarded the agglomeration in the crystallizer. In contrast, a reduced feed concentration produced a moderate peak supersaturation and low nucleation around the inlet region of the crystallizer, followed by a relatively slow drop in the supersaturation level over a certain period. Therefore, supersaturation conditions were maintained for molecular growth, which positively contributed to the formation of agglomerate particles. As a result, the particle size increased when reducing the feed concentration. This influence of the supersaturation profile in the crystallizer was confirmed using a chelating reaction during the crystallization, where ammonia was used as the chelating agent for the metal hydroxide reaction as [30],

0.5

Fig. 10. Influence of ammonia on particle size in Couette–Taylor crystallizer. The feed concentrations for the reaction crystallization were always fixed at 3.0 mol/l of (Ni/Co/Mn)SO4 and 6.0 mol/l of NaOH.

5. Conclusion In the reaction crystallization of metal (Ni, Co, Mn) hydroxides (NCM hydroxide) in a Couette–Taylor (CT) crystallizer, the particles were formed by crystal agglomeration, which involved the consecutive steps of crystal adhesion and molecular growth. Thus, the particle size was directly controlled by the fluid motion and supersaturation. The crystal aggregates were effectively broken and re-dispersed by the Taylor vortex induced by the inner cylinder rotation before being converted to concrete agglomerates. Therefore, the particle size was reduced when increasing the rotation speed of the inner cylinder of the CT crystallizer. Meanwhile, the conversion of the aggregates into agglomerates via molecular growth was promoted by a properly elongated supersaturation profile in the CT crystallizer. As the result, the particle size was enlarged when increasing the mean residence time. Similarly, the crystal agglomeration was enhanced, i.e. the particle size was increased, when decreasing the feed concentration within a certain range. The crystal agglomeration was also controlled using a chelating reaction. When using ammonia as the chelating agent, a NCM–ammonium complex was initially formed, which was then converted to NCM hydroxide. This also provided an appropriate supersaturation profile in the CT crystallizer, thereby enhancing the particle size. In the agglomeration, the individual crystals of NCM hydroxide created a hexagonal system stacked on the (0 0 1) plane, which was the largest surface of the crystals. As a result, the apparent outer surface of the agglomerate particles was mostly covered by (1 0 1) plane crystal faces. Plus, due to the high surface energy of the crystal faces, the face growth of the (0 0 1) planes was faster than that of the (1 0 1) planes, resulting in a larger surface area when increasing the rotation speed and mean residence time. When compared with agglomeration in an MSMPR crystallizer, which requires a long mean residence time of over 10–20 h, the CT crystallizer demonstrated effective production of agglomerate particles less than 5 ␮m in size within about 10 min of the mean residence time. Acknowledgement This study was financially supported by the NRF Research Fund (Korea, 2010-0017993).

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