Recovery of cerium from glass polishing slurry

1. JOURNAL OF RARE EARTHS, Vol. 29, No. 11, Nov. 2011, P. 1075
Foundation item: Project supported by the Energy & Resource R&D Program under the Ministry of Knowledge Economy, Republic of Korea (2008-R-RU02-P-02-0-000)
Corresponding author: Jong-Young Kim (E-mail: jykim@kicet.re.kr; Tel.: +82-31-645-1423)
DOI: 10.1016/S1002-0721(10)60601-1
Recovery of cerium from glass polishing slurry
Jong-Young Kim1
, Ung-Soo Kim1
, Myeong-Seop Byeon1
, Woo-Kyu Kang2
, Kwang-Teak Hwang1
, Woo-Seok Cho1
(1. Icheon Branch, Korea Institute of Ceramic Engineering and Technology, Gyeongchung Rd., Sindun-Myeon, Icheon-si, Kyeonggi-do 467-843, Korea; 2. RANCO,
Cheongju-si, Chungcheongbuk-do, Korea)
Received 20 February 2011; revised 20 June 2011
Abstract: Cerium was extracted from rare earth slurry waste used for polishing a glass substrate. Initially, glass frit and flocculant were re-
moved by froth flotation and dissolution. The recovered rare earth slurry exhibited almost the same particle size distribution as original slurry,
which could be reused as slurry for glass polishing. From the rare earth slurry, the cerium solution was obtained by an oxidative thermal treatment
and subsequent chemical leaching. The cerium solution was further purified up to 94% by selective precipitation of rare earth species.
Keywords: cerium; separation; waste; polishing slurry; rare earths
Due to its specific functional applications, cerium is the
most important one of the rare earth elements. Rare earth
oxides have been widely investigated as catalytic promoters
to improve the activity and thermal stability of catalysts[1]
.
Cerium plays an important role in three-way catalysis and
fluid catalytic cracking, two catalytic processes that are sig-
nificant because of their economic relevance and tonnage[2]
.
Cerium and other rare earth oxides have also been studied as
possible heterogeneous catalysts for selective oxidation of
hydrocarbons.
Cerium compounds are also used as abrasive materials for
polishing glass and silicon wafers. However, an enormous
amount of abrasive materials is wasted after being used for
polishing glass for display applications[3]
. The slurry for
glass polishing is a mineral containing a high concentration
of cerium and other elements such as the lanthanides (La, Nd,
Pr), which are mixed with the elements such as Fe, Al, and
Si in the polishing process. The heterogeneous nature of
slurry waste makes it difficult to refine cerium from the
waste, but after being refined, cerium of a high purity can be
obtained at a significantly reduced cost.
The separation of a rare earth mixture into individual ele-
ments is very difficult to achieve, due to their similar physi-
cal and chemical properties[4–8]
. In an aqueous solution, rare
earth elements are present as trivalent cations. Cerium is
most likely to be oxidized to a tetravalent state and, given
this property, its separation is generally the easiest. In the
tetravalent form, the cerium ion exhibits chemical behavior
markedly different from other trivalent rare earth ions[9,10]
. In
alkaline solutions, the trivalent cerium is readily oxidized to
the tetravalent ceric ion either by bubbling oxygen during
rare earth hydroxide precipitation or afterwards by drying the
rare earth hydroxide in the presence of air[11]
. In acidic solu-
tions, the oxidation of Ce(III) to Ce(IV) may occur by
chemical oxidation with strong oxidants, such as persulfate,
permanganate, bismuth, lead oxide or silver oxide, or by
electrochemical oxidation or photochemical oxidation[12–14]
.
The separation of cerium from the rare earth elements can
be carried out by selective dissolution of the trivalent rare
earth hydroxides, keeping the cerium(IV) hydroxide in its
insoluble form, or through its selective precipitation from
acid solution. In either case, the cerium separation is possible
due to the solubility difference between the Ce(IV) hydrox-
ide (Ksp~10–54
) and the RE(III) hydroxide (Ksp~10–22
). How-
ever, oxidation through permanganate solution leads to si-
multaneous precipitation of Ce(OH)4 and MnO2. After pre-
cipitation, the solids need to be purified if a high purity
product is desired[15]
. Based on this work, we showed that
rare earth slurry with ~40 wt.% of cerium could be recycled
after froth flotation, which could be used as polishing slurry.
Furthermore, cerium solution of a high purity could be ob-
tained from the recycled slurry by subsequent separation
process such as solvent extraction.
In this work, we attempted to separate the cerium from the
slurry waste for glass polishing by selective precipitation of
rare earth element, in which tetravalent cerium remains in
acidic solution. As a first stage, glass frit and flocculant were
separated by froth flotation and chemical dissolution. Ce-
rium was extracted from the resulting rare earth slurry by
oxidative roasting and a subsequent acid leaching process,
and purified further by removing the rare earth elements
through selective precipitation.
1 Experimental
The rare earth slurry waste used in this experiment was

2. 1076 JOURNAL OF RARE EARTHS, Vol. 29, No. 11, Nov. 2011
supplied by Ashai glass. The slurry waste was precipitated
by flocculant and dried by filter press. Table 1 represents the
chemical composition of the slurry waste. The slurry waste
contains rare earth elements (cerium, lanthanum, praseo-
dymium), a glass frit component, mainly consisting of SiO2,
and flocculant such as poly-aluminium chloride. Initially, 12
kg of the waste was mixed with 1 kg of oxalic acid (or citric
acid) and 22 kg of water (pH~1.5) and stirred for 4 h at room
temperature to remove the glass frit and flocculant. The re-
action was carried out in a froth flotation vessel and ultra-
sonic wave (28 MHz) was applied. After the acid treatment,
the remaining glass frit was dissolved by 33 kg of 3% so-
dium hydroxide solution (pH~11.5). The resulting slurry was
sieved (1450 mesh) and washed with distilled water several
times, and then dried for 24 h at 90 ºC.
The dried powder was roasted at 600 ºC for 2 h in a rotary
kiln-type furnace in air. The roasted powder was leached by
sulfuric acid solution (6 N) with a concentration of 100 g/L.
To eliminate the rare earth elements apart from cerium, the
resulting solution was reacted with Na2SO4 at 50 ºC for 4 h.
The ratio of Na2SO4 to TREO was varied from 0.5 to 1.0.
The dissolved cerium solution was analyzed by Inductive
Coupled Plasma spectroscopy. Powder XRD patterns were
recorded at room temperature on a Rigaku RINT-2000 dif-
fractometer with a Cu K radiation source. The X-ray tube
voltage and current were at 40 kV and 200 mA, respectively.
2 Results and discussion
Abrasive materials used for polishing glass, which is used
as a substrate for LCD and PDP displays are usually miner-
als with a high concentration of rare earth elements, such as
cerium. Consequently, the slurry waste contains glass frit
originating from the glass substrate, which exists as mixed
powder or adsorbed on rare earth abrasive materials[3]
. Be-
Table 1 Results of chemical analysis of the raw materials and
leaching solution
Elements Raw materials
(wt.%, XRF)
Leaching at 60 ºC
(roasted by Muffle
furnace)/(mg/L)
Leaching at60ºC
(roasted by rotary
kiln)/(mg/L)
Leaching at80ºC
(roasted by rotary
kiln)/(mg/L)
Si 0.27 77 31 295
Al 4.30 28 – 68
Ce 39.40 48958 56570 19457
La 20.00 23905 23740 353
Ca 0.22 62 107 52
Na 0.27 45 – 48
Fe 0.60 – 122 22
P 0.30 – – –
Ti 0.01 – – –
S 0.40 – – –
Mg 0.11 – – –
Zn 0.04 – – –
Ni 0.02 – – –
Li 0.15 – – –
Cu 0.01 – – –
F 3.68 – – –
sides glass frit, flocculants containing aluminum or iron are
included because they are used to precipitate the solid. Poly
aluminum chloride or FeCl3·xH2O can be used as a floccu-
lant to make the sludge cake by lowering the pH. Therefore,
we attempted to remove the flocculant by chemical dissolution
and then separate the glass frit from the rare earth materials by
froth flotation using their difference in density and surface
properties even though their sizes are similar (<10 m)[16]
.
Initially, inorganic flocculant was dissolved by mixing
organic acid, which does not form any salt with cerium, with
the slurry waste. Citric acid or oxalic acid was used as the
organic acid. The slurry waste was reacted with organic acid
solution at room temperature at a pH of 1.0–6.0 to turn the
flocculant into dissolved species. With organic acid, the
glass frit floats in a separation vessel with air bubbles and
ultrasonication because of its hydrophobic surface and low
density (d=1.4), while rare earth particles, that have a higher
density (d=7.0), will precipitate.
For more efficient separation, ultrasonic waves were ap-
plied during froth flotation to separate the adsorbed glass frit
from the cerium abrasive. Furthermore, air bubbles were
dispersed from the bottom of the vessel to remove the glass
frit efficiently. After the flotation process, the remaining
glass frit was dissolved by an alkaline solution at a pH value
from 9.0 to 13.0. The cerium abrasive was sieved (1450
mesh) and dried at 90 ºC for 24 h. Fig. 1 shows the particle
size distribution for original polishing slurry, recycled slurry,
and recycled and sonicated slurry, respectively; the figure
reveals that recycled slurry exhibits almost the same particle
size distribution as the original slurry. Table 2 summarizes
the characteristics for particle size distribution. Figs. 2(a), (b),
and (c) show the SEM (scanning electron microscopy) im-
ages for the original polishing slurry, recycled slurry, and
recycled and sonicated slurry, respectively. The recovered
rare earth slurry exhibits almost the same particle size dis-
tribution as the original slurry, which can be reused as slurry
for glass polishing[16]
.
Table 1 shows the composition of the ceria abrasive waste
before acid leaching. The cerium content is about 39% and
the major impurity is lanthanum (~20%). According to XRD
pattern after oxidation by rotary kiln (Fig. 3), cerium oxide
exists as CeO2, which means that cerium was oxidized to a
Fig. 1 Particle size distribution for original slurry, recycled slurry,
recycled and sonicated slurry

3. Jong-Young Kim et al., Recovery of cerium from glass polishing slurry 1077
Fig. 2 Scanning electron microscopic images for original slurry (a), recycled slurry (b), recycled and sonicated slurry (c)
Table 2 Particle size distribution of recycled slurry
Slurry D10 / m D50 / m D90/ m D100 / m
Original 0.725 1.689 3.704 8.71
Recycled 0.800 2.029 5.448 34.674
Recycled (sonicated) 0.751 1.767 3.855 8.71
Fig. 3 X-ray diffraction patterns of slurry waste roasted by rotary
kiln at 600 ºC
tetravalent state by oxidative roasting at 600 ºC for 2 h. As
the temperature rises up, the yield of the leaching increases,
however, the temperature was set at 600 ºC because fluoride
ions should be involved in the subsequent precipitation
process[3]
. Fluorine is present in raw materials as shown in
Table 2.
Table 3 shows the comparison of the roasting process be-
tween in an electrical muffle furnace (condition 1) and in a
rotary kiln (condition 2). Because fresh air is provided in the
rotary kiln during the heat treatment, cerium is more easily
oxidized to a tetravalent state, which results in an increased
yield from the acid leaching process. As shown in Table 3,
the yield of the acid leaching for the rotary kiln is larger than
for the muffle furnace by ~11%. Furthermore, the La/Ce ra-
Table 3 Results of chemical analysis of leaching solution ac-
cording to the roasting and leaching condition
Condition 1 2 3
Roasting condition Muffle furnace Rotary kiln Rotary kiln
Leaching condition 60 ºC, 3h 60 ºC, 3 h 80 ºC, 3 h
Yield of Ce 71.26% 82.34% 28.32%
Purity of Ce 69.99% 70.21% 95.98%
La/Ce ratio 0.4883 0.4197 0.0181
tio for the rotary kiln is less than that for the muffle furnace
because lanthanum is unaffected by oxidation.
Table 3 also shows the effect of temperature during the
acid leaching process on the recovery of the cerium. The pu-
rity for acid leaching at 80 ºC (condition 3) is higher than
that at 60 ºC (condition 2); however, the yield at 80 ºC is
lower than that at 60 ºC. The recovery yield decreases with
an increasing temperature as shown in Table 3, which is well
consistent with the previous leaching result. The dissolution
of lanthanide sulfates in water is exothermic, so their solubil-
ity decreases with an increase in temperature[17]
. The ratio of
La/Ce at 80 ºC is significantly smaller than that at 60 ºC,
which means that leaching of the lanthanum is more affected
by temperature than cerium. While the purity increases with
the increase of temperature, the concentration of cerium de-
creases so that the subsequent precipitation process is needed
for further purification.
The cerium solution was further purified by selective pre-
cipitation with Na2SO4. A leach solution produced under op-
timum roasting and leaching conditions was used in the pre-
cipitation experiment. The aim of precipitation is to produce
a reasonably pure Ce(IV) solution using selective precipita-
tion of rare earth elements forming sodium rare earth double
salt. The solubility of the double salt in water is low particu-
larly for the trivalent lanthanides of La, Ce, Pr, Nd, Pm, Sm,
Eu, and Sc. Leach liquor typically contains cerium and a low
concentration of lanthanides of La and other impurities as
shown in Table 2. The purification was based upon the fol-
lowing reaction[3]
:
[CeF6]2–
+RE3+
+2Na2SO4 [CeF6]2–
+RENa(SO4)2 +3Na+
(1)
As the temperature rises, the precipitation rate increases,
but the reaction rate exhibits no significant change above 50 ºC
and crystallization equilibrium is achieved in 90 m[18]
. The
reaction temperature and time were set at 50 ºC and 90 m,
respectively, in order to effectively remove RE3+
such as
La3+
.
Sodium sulfate was added to 0.5 to 1.0 times the lantha-
num concentration, and its effect on the yield is shown in
Table 4. When the amount of sodium sulfate is small in rela-
tion to cerium, the purity level cannot be high since the pre-
cipitation of the rare earth element is not enough. However,
the surplus amount of sodium sulfate should be the source of
impurity, and therefore, the ratio of Na2SO4/Re should be
controlled. As shown in Table 4, no lanthanum was observed

4. 1078 JOURNAL OF RARE EARTHS, Vol. 29, No. 11, Nov. 2011
when Na2SO4/RE is larger than 0.625. The purity of cerium
decreased when the ratio becomes 0.625. Therefore, the
equivalent amount of Na2SO4 lies between 0.5 and 0.625.
When Na2SO4/RE is 0.5, the purity is highest and the yield is
~63%; therefore, the optimum precipitation condition is
Na2SO4/RE=0.5. Such impurities as lanthanum, sodium can
be separated by subsequent solvent extraction.
Table 4 Purity and yield of cerium solution according to the
Na2SO4/RE ratio after selective precipitation
Na2SO4/RE 0.5 0.625 0.75 0.875 1
Purity of Ce/% 94.37 94.18 85.97 87.61 86.79
La/Ce ratio 0.0307 0 0 0 0
Na/Ce ratio 0.0226 0.0567 0.1544 0.1355 0.1465
Yield/% 62.91 65.11 40.12 57.57 68.13
Table 5 Results of chemical analysis of cerium solution accord-
ing to the Na2SO4/RE ratio after selective precipitation
Na2SO4/RE=
0.5
Na2SO4/RE=
0.625
Na2SO4/RE=
0.75
Na2SO4/RE=
0.875
Na2SO4/RE=
1
Element
Conc./
(mg/L)
Element
Conc./
(mg/L)
Element
Conc./
(mg/L)
Element
Conc./
(mg/L)
Element
Conc./
(mg/L)
Ce 50900 Ce 55320 Ce 37870 Ce 51490 Ce 64310
La 1562 La – La – La – La –
Na 1150 Na 3135 Na 5848 Na 6975 Na 9422
Fe 172 Fe 156 Fe 151 Fe 158 Fe 176
Si 110 Si 117 Si 144 Si 145 Si 162
Ca 42 Ca 9 Ca 37 Ca – Ca 31
3 Conclusions
Cerium was extracted from the rare earth slurry waste for
polishing glass, which can be used as raw materials for pro-
ducing high purity cerium materials. Flocculant and glass frit
were removed by froth flotation and chemical dissolution.
The recovered rare earth slurry exhibited almost the same
particle size distribution as original slurry, which could be
reused as slurry for glass polishing. Cerium was leached by
sulfuric acid from the resulting slurry oxidized at 600 ºC.
When the raw material was roasted in a rotary kiln, the
leaching yield was higher than that roasted in a closed
chamber. When the leaching temperature was increased
from 60 to 80 ºC, the yield of leaching decreased, but the pu-
rity of the cerium increased because the leaching of lantha-
num decreased more drastically. By selective precipitation,
the purity of the cerium increased significantly and lantha-
num was removed completely. The optimum conditions for
the precipitation of a rare earth salt with sodium sulfate were
an amount of Na2SO4/RE=0.5, and the yield was found to be
~60%.
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