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Available online at www.sciencedirect.com

Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 78–84

The study on the preparation methods and the fluidityof large rare earth oxide particles

Mei Li ∗, Zhaogang Liu, Yanhong Hu, Zhenxue ShiMaterial and Metallurgy School, Inner Mongolia University of Science and Technology,

Baotou, 014010, China

Received 8 October 2007; received in revised form 2 January 2008; accepted 13 January 2008Available online 19 January 2008

bstract

In this work large CeO2 particles were prepared using H2C2O4 and NH4HCO3 as precipitators. The effects of feeding mode and speed, stirringpeed, aging time, precipitate and calcination temperature and precipitation method on physicochemical properties of CeO2 were studied whenhe precipitator was H2C2O4. By the NH4HCO3 precipitation method, the effects of adding inoculating crystals and additives on particle size,oose density and fluidity of CeO2 were investigated. Under optimized conditions, large CeO2 particles with high loose density and good fluidityf can be prepared by either H2C2O4 or NH4HCO3 precipitation method. SEM also investigates the morphology of the particles prepared by bothethods. The results show that physicochemical properties of CeO2 particles prepared by the NH4HCO3 precipitation method are not as good as

hose of CeO2 particles prepared by the H2C2O4 precipitation method. However, both methods are suitable for industrial production due to theirimple processes, low equipment investment and ease for industrial development. Our results show that large rare earth particle can be preparedy the optimized conventional methods. This study provided a useful method to prepare of large rare earth particles.

2008 Elsevier B.V. All rights reserved.

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eywords: Cerium oxide; Particle size; Loose density; Fluidity; Precipitation

. Introduction

The rare earth composites were praised as the treasury ofaterials due to their special optical, electric and magnetic prop-

rties that were given by the specific ‘f’ electron of rare earthlements. The physical characteristics of rare earth materialave important effects on its application, so techniques con-rolling the physical properties of the rare earth products areey to the development of the rare earth industry [1]. Powdersf rare earth materials, an important part of novel materials,re required to have special physicochemical properties, suchs high homogeneity, filling properties, dispersibility, fluiditynd reactive operability, etc. The particle size of rare earthowders directly determines its application [2]. Rare earth pow-

er material of different particle sizes can be used as rawaterials to prepare specific ceramic, fluorescent, electronicaterial and so on. Usually the particle size and distribution

∗ Corresponding author. Tel.: +86 472 5954390; fax: +86 472 5951505.E-mail address: limei@imust.cn (M. Li).

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927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2008.01.021

re key factors of industrial performance of rare earth pow-er [3]. For example, the efficiency and degree of polishings directly determined by the particle size of polishing pow-er.

Recently, with the development of industrial materials, rarearth compounds with controllable particle size have a goodrospect in market. Large rare earth particles show an exten-ive application beside the specific application of fine rare eartharticles. The manufacturer of decorative products hopes tourchase CeO2 powder with particle sizes larger than 30 �mnd loose density higher than 2.0 g/ml. Rare earth oxides witharge particle size, homogenous size distribution and higheroose density are favorable in producing rare earth metals bylectrolytic methods in order to avoid splashing in electrolyticrocesses. In the glass industry, CeO2 powder of large parti-le size and high loose density is used to get a better mixturef CeO2 powder and glass instead of using the classical white

rsenic [4]. Most rare earth manufacturers produce rare earthxides with the particle sizes of 3–5 �m and loose density lesshan 1.2 g/ml. These products are primary products and cannoteet the customer demands. The investigations of preparation

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lfbovtfrate of crystal growth, which results in the generation of fineparticles. When the precipitator is added all at once, the localconcentration of precipitator is so high that it produces the aggre-gation of particles. In this case, the particle size is very large but

Table 1The particle size, loose density and fluidic index of CeO2 particles prepared bydifferent feeding modes of precipitator

M. Li et al. / Colloids and Surfaces A: P

f superfine rare earths oxide particles have been reported exten-ively [2,5–9]. However, studies about large rare earth particlesre relatively few. Xu et al. [4] prepared the CeO2 powderith loose density equal to 2.01 g/ml using oxalate precipitationethod. Higashi et al. [10] reported that CeO2 with particle size

f 3–6 �m was prepared using the oxalate precipitation method.uch remains to be learned about the preparation and proper-

ies of rare earths oxide with large particle size. The oxalaterecipitation method and the NH4HCO3 precipitation methodre commonly used methods in industry to prepare this rarearth oxide. The oxalate precipitation method has virtues ofood deposition crystal type and easy filtration. The NH4HCO3recipitation method has the virtue of lower cost. However, thearticle size of rare earth oxide is far from the demand of largearticle size. How to prepare large rare earths oxide particle istill a challenge.

The purpose of this study is to prepare larger cerium oxidearticles, which was not obtained by the traditional oxalate andhe NH4HCO3 precipitation methods. To control particle size,

any factors need to be considered as described in literatures,he nucleation speed [11], the crystal growth in solution [12,13],he temperature of precipitation and calcinations [14] etc. Here,e investigated the effect of each preparation conditions includ-

ng the feeding mode and speed, stirring speed, aging time,recipitate and calcinations temperature on the physicochemicalroperties of rare earth oxide particles. Finally the optimizedreparation conditions were established to obtain the largeare earth oxide particles that fulfilled the demand of industrialaterials.

. Materials and methods

.1. Sample preparation

0.5 M Ce(NO3)3 and Y(NO3)3 solutions were prepared byissolving Ce2(CO3)3 and Y2(CO3)3 (provided by Inner Mon-olia Baotou Steel Rare-Earth Hi-Tech Co., Ltd. of Baotouteel, CeO2/�REO ≥ 99.9%, �REO ≥ 45%). The precursorompound was prepared by adding oxalic acid solution orH4HCO3 solution into Ce(NO3)3 or Y(NO3)3 solution. Theifferent conditions of precipitation were controlled to get largeare earth particles as described in results and discussion. Therecursor turned into large CeO2 or Y2O3 particle after filtration,ashing, drying and calcination processes. All other reagents are

nalytical grade.

.2. Characterizations

The particle size distribution of sample was done with CoulterS230 laser particle size distribution equipment. The morphol-gy and dispersing character were analyzed by SEM (S360). Therystal structure and phase conformation were analyzed with

RD (Philips PW170). The loose density, angle of repose, tapensity, angle of spatula and agglutination degree of samplesere tested by BT-1000 powder comprehensive characteristics

esting instrument. Degree of compression was obtained by cal-ulation from tap density and loose density.

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ochem. Eng. Aspects 320 (2008) 78–84 79

. Results and discussion

.1. The oxalate precipitation method

The effects of precipitators’ feeding mode and speed, stirringpeed, precipitate temperature, aging time, and calcination tem-erature on particle size, loose density and fluidic index of CeO2repared by the oxalate precipitation method were investigatedn this paper.

.1.1. The effect of precipitators’ feeding mode of on theroperties of CeO2 particles

There are two general feeding modes for the precipitationethod: positive feeding which has the precipitator dropped intoe(NO3)3 solution, and reverse feeding which has the Ce(NO3)3

olution added dropwise into the precipitator solution. The par-icle size, loose density and fluidic index of CeO2 particlesrepared by using the positive and the reverse feeding modesere characterized. The results were shown in Table 1. Here,50 is the particle size in the middle of the cumulative volumeresented in the curve of the cumulative volume against particleize obtained by the laser particle size distribution equipment.able 1 shows that the particle size, loose density and fluidic

ndex of CeO2 prepared by the reverse feeding mode are muchower than those prepared by positive feeding. In reverse feed-ng, the pH of the solution is held constant, the number of particleggregates rapidly increased but the number of single crystalsas kept in a low level. The rate of nucleation is greater than

he rate of crystal growth, thus, reverse feeding generated finearticles. Of course, in contrast of the reverse feeding mode, theositive feeding mode is good for generation of large particles.ere, large particles with a size of ∼29 �m were obtained by

he positive feeding mode.

.1.2. The effect of feeding, stirring speed and aging timen the properties of CeO2 particles

The effect of precipitator feeding speed on the particle size,oose density and fluidic index of CeO2 is shown in Fig. 1a. At aeeding speed range of 5–20 ml/min, the fluidic index of CeO2arely changed. The maximum particle size and loose densityf CeO2 occurred when the feeding speed was 10 ml/min. Atery low feeding speeds the rate of crystal growth is higher thanhe rate of nucleation results in forming large particles. If theeeding speed is too fast, the rate of nucleation is larger than the

eeding mode Positive feeding Reverse feeding

50 (�m) 29.96 9.826oose density (g/ml) 2.06 1.01luidity index 77 58

80 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 78–84

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ig. 1. The effect of (a) feeding and (b) stirring speed on the particle size, looset (c) feeding speed of 10 ml/min and (d) stirring speed of 300 rpm.

here are particle aggregations, which have bigger interspacesFig. S1). Thus their loose density (1.53 g/ml) and fluidity is rel-tively lower than those prepared by continuous feeding mode.ne notes that the feeding speed of 10 ml/min gives best particleroperties considering the loose density, particle size and fluid-ty. SEM image shows the morphology of particles is four-sidedylinders (Fig. 1c).

An excessive slow stirring speed increase the instant localaturation and does not encourage larger particle sizes. Thusarticles generated by slow stirring speeds are not homogenous.uitable stirring speed is favorable to prevent local supersat-rating and improve mass transport process. In this case, anppropriate stirring speed is necessary to obtain large particles.owever the excessive fast stirring speed may destroy the con-itions for growing bigger particles by breaking up the particlesnto pieces, finally result in forming uniform and small parti-les. Thus the stirring speed directly affects the particle sizend fluidity. We investigated the properties of particles preparedsing different stirring speeds. Fig. 1b shows the results. Oneay note that the loose density of CeO2 particles increased with

he increase of the stirring speed; however there is no obvioushanging trend of particle size. In the range of 200–500 rpmtirring speed, the fluidic index of particles was barely affected.owever, as the stirring speed reached 600 rpm, the fluidic indexf CeO2 particles decreased. We believed that the rapid stirringpeed resulted in small particles of low fluidity. It is obvious

hat the particle size, loose density and fluidic index of CeO2 arell relatively higher when the stirring speed is 300 rpm. Fig. 1dhows the SEM image of the CeO2 particles derived under thisondition.

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ity and fluidic index of CeO2 particles. SEM image of CeO2 particles prepared

Aging is a process that keeps precipitation and mother liquidor a certain time after complete precipitation. Large and smallrystal particles coexist in solution after precipitation, at thisime; the solution is saturated for large particles and not satu-ated for small particles because the solubility of small particless larger than that of large particles [15]. So small particles grad-ally dissolve into solution. As the dissolution of small particleseach a certain stage, the solution is saturated for small particlesnd is supersaturated for large particles, thus the crystallizingons in solution deposit on large particles. When the solutions saturated of large particles, it becomes unsaturated for smallarticles. Then small particles dissolve continually again. Byontinuously repeating these processes, small particles gradu-lly vanish and large particles gradually grow. So, the agingrocess is favorable for getting large and uniform particles. Werepared CeO2 particles with different aging times and charac-erized the properties of these particles. The results were shownn Fig. 2a. One notes that the particle size and loose densityf CeO2 increases as aging time increases. The fluidic indexf CeO2 increases as well, but in a more moderate trend. Thiseveals that increasing the aging time benefits particles’ grow-ng, uniformity and fluidity. Fig. 2b shows the SEM image ofhe CeO2 particles prepared by aging for 24 h.

.1.3. The effect of precipitation and calcinationemperature on the properties of CeO2 particles

The temperature is one of many important factors in the pre-ipitation reaction. When the quantity of solute is constant insolution, the degree of supersaturation of the solution usu-

lly decreases with the increase of temperature. This is well

M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 78–84 81

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ig. 2. (a) The effect of aging time on the particle size, loose density and fluidor 24 h.

nown from the relationship of supersaturation and temperature.lthough the supersaturation degree will be very high when the

emperature is super low, the speed of crystal particle generationill be very low due to the lower energy of solute molecules.y increasing the temperature, the generation speed of crys-

al particles can be very high, but the supersaturation degreef the solution will decrease with the increase of temperature.t the same time, the molecular kinetic energy increases toouickly to form stable crystal particles. So choosing the rightemperature is critical to control particle size and fluidity Fig. 3a

hows the effects of precipitation temperature on particle size,oose density and fluidity of particles. It is obvious that the effectf precipitation temperature on particle size, loose density anduidity of particles are quite similar. At the temperature range

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ig. 3. The effect of (a) precipitation and (b) calcination temperature on the particle sarticles (c) precipitated at 50 ◦C and (d) sintered at 900 ◦C.

ex of CeO2 particles. (b) SEM image of the CeO2 particles obtained by aging

f 30–60 ◦C, particle size and loose density all increased withhe increase of temperature. With the temperature continuallyncreasing after that, the particle size and loose density of CeO2rst decreases and then reverts back to a stable phase. The flu-

dity of CeO2 barely changed with the change of precipitationemperature except there was a slightly decrease in the fluidityf CeO2 particles at 90 ◦C. This can be explained that the nucle-tion speed is very high at high temperature resulting in smallarticle aggregates, which decreases the fluidity of the powder.o the optimum temperature should be in the middle tempera-

ure zone between 45 and 60 ◦C. Fig. 3c shows the SEM imagef the CeO2 particles obtained by precipitated at 50 ◦C.

The effect of calcination temperature on the particle size andoose density of CeO2 is another condition we investigated in

ize, loose density and fluidic index of CeO2 particles. SEM image of the CeO2

82 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 78–84

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ig. 4. (a) Particle size distribution of large CeO2 particles, (b) SEM image andarticles. All these particles are prepared by the oxalate precipitation method.

his work. The calcination is an important factor of influenc-ng the particle size. Fig. 3b shows that the particle size, looseensity and fluidic index all increased with the increases of cal-ination temperature. It is known that crystals developed moreompletely and the particle size increased with the increasef calcination temperature. SEM image of the CeO2 particlesbtained by calcination at 900 ◦C is shown in Fig. 3d.

According to the above experimental results, we optimizedhe preparation conditions of large CeO2 particles. The CeO2articles with D50 ≥ 20 �m, loose density ≥2.0 g/ml and flu-dic index equal to 80 were obtained. Fig. 4a shows thearticle size distribution of these CeO2 particles. The particle

ize in the middle of the cumulative volume (D50) is 21 �m.he width of particle size distribution (R) is 1 calculated by= (D90 − D10)/D50. It is obvious that the particle size distribu-

ion is very narrow. The volume-particle size distribution shows

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ig. 5. (a) The effect of addition of additive on particle size, loose density and fluidicddition of 30 ml additive.

he XRD pattern of large CeO2 particles, (d) Particle size distribution of Y2O3

normal distribution implying that CeO2 particles obtained byhese conditions were homogenous. Fig. 4b shows the SEMmage of these large CeO2 particles, and it reveals the mor-hology of these large particles to be four-sided cylinders. Theiffraction peaks appeared in Fig. 4c confirmed the combina-ion of these large particles is CeO2 as same as we did in ourormer work [2]. Large Y2O3 particles were prepared with theame method. The particle size in the middle of the cumulativeolume (D50) is 22 �m and the fluidic index equal to 74. Thearticle size distribution was shown in Fig. 4d. It is obvious thatarge Y2O3 particles have a normal particle size distribution.

.2. The NH4HCO3 precipitation method

It is very difficult to obtain large precursor particles for CeO2article preparation using the NH4HCO3 precipitation method

index of CeO2 particles. (b) SEM image of the CeO2 particles prepared by the

M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 78–84 83

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ig. 6. (a) The effect of addition of inoculating crystal on particle size, looserepared by the addition of 10% inoculating crystal.

ecause the reaction generates a lot of gas. So the precipita-ion conditions must be strictly controlled. Usually the additionf inoculating crystals is necessary to induce the formation ofarge particles. At the same time, the appropriate amount ofdditive is added to accelerate the particle aggregate and grow-ng.

.2.1. The effect of the quantity of additive additionThe effect of the different amounts of additive addition on

article size, loose density and fluidity of CeO2 particles wasnvestigated as adding the same amount of inoculating crys-al. The results were shown in Fig. 5a. One notes that particle

ize, loose density and fluidic index of CeO2 particles are verymall and low without the addition of additive. Oppositely, allarameters increase with the increases of the amount of addi-ive addition. However, when the volume of additive increased

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ty and fluidic index of CeO2 particles. (b) SEM image of the CeO2 particles

rom 30 to 40 ml, the increase of all parameters is not obvi-us. The reason is because a long chain macromolecule canbsorb several particles, or because a particle can be absorbedy several macromolecules at same time. After the additionf long chain macromolecules as additive, the formation ofridging action accelerates particle congregation and growth.he macromolecular additive also had polarity clusters, whichbsorb several particles and accelerate particle congregation.hereby causing particle size, loose density and fluidic indexf CeO2 particles to increase. However, when the additive isaturated, the bridging action is not as obvious as previouslyxplained. So the suitable addition volume is 30 ml in this exper-

mental range. Fig. 5b shows the SEM image of the CeO2articles prepared by adding 30 ml additive. The morphology ofhe CeO2 particles is a cauliflower form aggregated with manymall particles.

of large CeO2 particles prepared by NH4HCO3 precipitation method.

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.2.2. The effect of the quantity of inoculating crystalddition

While the quantity of additive addition was constant, theffect of the addition of different amounts of inoculating crystalsn the properties of CeO2 particles was studied here. The exper-mental results were shown in Fig. 6a. The particle size, looseensity and fluidic index of CeO2 particles reached maximumalues when 10% of the inoculating crystals are added. This isecause the inoculating crystal is the aboriginality of precipitaterystal. In some cases, the particle size can be ensured by theddition of inoculating crystal, but the addition of a large amountf inoculating crystal could decrease the particle size and fluid-ty of CeO2 particles by increasing the collision probability ofarticles. SEM image of the CeO2 particles obtained by adding0% of the inoculating crystals is shown in Fig. 6b.

Eventually we prepared large particles of CeO2 using themproved NH4HCO3 method. The loose density of CeO2 parti-les is 1.85 g/ml. The particle size in the middle of the cumulativeolume (D50) is 23.59 �m (Fig. 7a), the fluidic index is 76. Theuidity of CeO2 particles is very good. Fig. 7a shows the particleize distribution of the large CeO2 particles. The width of par-icle size distribution is 0.8 calculated by R = (D90 − D10)/D50.his result confirmed that the large CeO2 particles prepared by

he optimized NH4HCO3 method have a narrow and normalarticle size distribution. The SEM image (Fig. 7b) shows thatarge CeO2 particles aggregate with a cauliflower form, as wells florets where some small particles are in the middle of largearticles. We need to mention that XRD pattern of these largearticles shown in Fig. 7c confirmed the combination of thesearticles is CeO2.

. Conclusion

We investigated how to obtain large rare earth oxide particlessing improved H2C2O4 and NH4HCO3 precipitation methods.he effects of feeding speed and modes, stirring speed, aging

ime, precipitate temperature, precipitate method and calcinationemperature on particle size, loose density and fluidity of CeO2ere studied when the precipitator was H2C2O4. As a result, we

ucceeded in obtaining large CeO2 particles with D50 ≥ 20 �m,oose density ≥2.0 g/ml and fluidic index equal to 80 undern optimized condition. Using the same method, we obtainedarge Y2O3 particles (D50 = 22 �m and the fluidic index = 74).

e show by SEM that the particle morphology of CeO2 particlesbtained by these conditions is of four-sided cylinders form.

In the NH4HCO3 precipitation method, the effects of inoc-lating crystal and additive on particle size, loose density and

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ochem. Eng. Aspects 320 (2008) 78–84

uidity of CeO2 were investigated. The results show that addi-ion of inoculating crystal and additive can accelerate CeO2article growth and congregation and improve the fluidity ofarticles. SEM results show that the morphology of large par-icle CeO2 particles is like a cauliflower. The loose density ofeO2 particles is 1.85 g/ml−1, D50 is 23.59 �m and its fluidic

ndex is 76. The results show that all properties of CeO2 parti-les prepared with the NH4HCO3 precipitation method were nots good as CeO2 particles prepared with the H2C2O4 precipita-ion method. However, both methods were found to be suitableor industrialized production due to their simple processes, lowquipment investment and ease for industrial development. Ouresults show that large rare earth particle can be prepared by theptimized conventional methods.

cknowledgements

We greatly acknowledge the financial support by theational Basic Research Foundation of China (Granto.2004CCA03900) and the National Natural Science Foun-ation of China (Grant No. 20566001). We also thank Meganhoy for helping us in revising the English.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.colsurfa.2008.01.021.

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