New route for the extraction of crude zirconia from zircon

Article abstract of DOI:10.1111/j.1151-2916.2002.tb00438.x

A commercial grade of zircon (ZrSiO₄) concentrate was mechanically milled with MgO for up to 100 h in a laboratory-scale mill. The resultant powders were subjected to thermal processing, chemical leaching, and X-ray diffraction (XRD). There was no direct evidence of reaction during the milling step, with no new phases evident from XRD. Leaching of the powder showed that a reaction had occurred, and increased solubility with milling time was attributed to the formation of a nanostructured Mg-Zr-Si oxide, which dissolved congruently. Heating the powders resulted in a number of thermal events, including the formation/crystallization of ZrO₂ and Mg₂SiO₄. Thermal treatment of the milled powders allowed selective chemical leaching of the magnesium and silicon, leaving a powder containing ～90% ZrO₂.

Full text of DOI:10.1111/j.1151-2916.2002.tb00438.x

J. Am. Ceram. Soc., 85 [9] 2217–21 (2002)
New Route for the Extraction of Crude Zirconia from Zircon
Nicholas J. Welham†
journal
Department of Applied Mathematics, Research School of Physical Sciences and Engineering,
Australian National University, Canberra ACT 0200, Australia
A commercial grade of zircon (ZrSiO4) concentrate was
mechanically milled with MgO for up to 100 h in a laboratory-
scale mill. The resultant powders were subjected to thermal
processing, chemical leaching, and X-ray diffraction (XRD).
There was no direct evidence of reaction during the milling
step, with no new phases evident from XRD. Leaching of the
powder showed that a reaction had occurred, and increased
solubility with milling time was attributed to the formation of
a nanostructured Mg-Zr-Si oxide, which dissolved congru-
ently. Heating the powders resulted in a number of thermal
events, including the formation/crystallization of ZrO2 and
Mg2SiO4. Thermal treatment of the milled powders allowed
selective chemical leaching of the magnesium and silicon,
leaving a powder containing ϳ90% ZrO2.
from the zirconium. Magnesium was proposed to be the most
suitable candidate for the alkaline earth oxide, because of its high
solubility for a low fraction of the mill feed, its almost negligible
rates of hydration and carbonation (unlike those of the other
oxides), and its ready bulk availability in the form of the mineral
periclase.
The objective of this paper is to examine in more detail the use
of magnesium oxide (MgO) in the processing of zircon by
mechanochemistry.
II. Experimental Procedure
The zircon had a particle size range of 200–300 ␮m, and
microscopic examination showed that zircon was predominant,
with Ͻ1% of particles being other minerals, the majority of which
were ilmenite, rutile, and quartz. Reactions are known to occur
between MgO and both ilmenite6 and rutile;7 however, the low
abundance of those materials in the feed stock was not expected to
affect the results significantly.
I. Introduction
ONSIDERATION of the novel technologies and methods for
C
extracting valuable materials from ores and concentrates is
In the previous paper,3 a 2:1 molar ratio of MgO:ZrSiO4 was
shown to give a 97% dissolution after milling for 200 h, and this
ratio is used in accordance with reaction (1):
becoming increasingly important as the life-cycle approach to
manufacturing becomes increasingly important. This approach
makes it important to reduce the overall environmental impact of
the recovery process, rather than simply optimizing a series of
independent unit operations. In many cases, upgrading a concen-
trate to a salable product involves the greatest challenge in
reducing the environmental impact, because large volumes of
hazardous chemicals usually are used.
ZrSiO4 ϩ 2MgO ^ ZrO2 ϩ Mg2SiO4
(1)
Samples of 7.00 g were prepared in accordance with the required
stoichiometry and milled in air for up to 200 h, at room temper-
ature, in a vertical 316S stainless steel ball mill, using five
25.4-mm-diameter 420C stainless steel balls confined in the
vertical plane, which gave a ball:powder mass ratio of 43:1. A
further sample of powder, comprising unmilled zircon and MgO,
was placed in a mill with no balls and rotated for 100 h; this
well-mixed powder was considered to have a milling time of zero.
Differential thermal analysis (DTA) was performed on the
as-milled powders. Powder was loaded into an alumina crucible,
which was tapped to consolidate the powders. The crucible was
allowed to equilibrate within the instrument and then heated in air
to 1000°C, at 20°C/min. Thermogravimetric analysis (TGA) of
selected powders was also performed, using the same parameters.
The as-milled powders were removed from the mills and
subjected to an agitated leach, at 1% slurry density, for 6 h, at room
temperature, in 18% HCl. The solutions were separated by
centrifugation, decanted, washed with water, dried for 24 h at
110°C in air, and the mass loss determined. The zircon solubility
was calculated from the mass loss during leaching, by assuming
that the bulk composition of the powder removed from the mill
was the same as that of the feed powder and that the insoluble
residue was pure zircon. Thus, the solubilized zircon was simply
the difference between the known mass of zircon in the powder
and the mass of residue. This method did not account for iron
contamination from milling and made no attempt to distinguish
between the solubility of zircon resulting from nonreactive milling
and that resulting from reaction.
The route for producing high-purity ZrO2 is usually dissolution
in alkaline fluoride and precipitation by pH adjustment. Lower
grades of ZrO2 are made by fuming off the silica (SiO2), after
dissociation at 2400°C in an arc furnace, and leaching the
remaining SiO2 away with caustic soda.1 In both routes, the
necessity to dispose of the highly alkaline solution remains a major
drawback to the processes.
Because the main problem element in zircon is silicon, a way
must be found to separate silicon from zirconium. Once this
separation is achieved, selectivity is possible. Fusion of sodium
hydroxide with zircon is known to occur at Ͼ600°C, forming
sodium zirconate and sodium silicate,2 which are both soluble. A
mechanochemical process previously was reported as unsuccess-
ful, but that system used a solution of hydroxide rather than solid
hydroxide.2
Earlier papers3–5 showed that reaction between alkaline earth
metal oxides and zircon occurred during extended ball milling,
leading to high solubility of zirconium and silicon in relatively
mild acid conditions. It was speculated3 that appropriate thermal
treatment of the as-milled powders would lead to crystallization of
the phases present, possibly allowing the separation of the silicon
T. A. Parthasarathy—contributing editor
Selected powder products were analyzed by X-ray diffraction
(XRD), using monochromatized CoK␣ radiation (␭ ϭ 1.78896 Å),
with a count time of 1 s per 0.025° step. The crystallite size was
estimated as the average of the peak broadening of as many peaks
as were evident for each phase, using the Scherrer formula.8
Manuscript No. 188119. Received November 29, 2000; approved November 14,
2001.
†Now with Murdoch University, Murdoch WA 6150, Australia.
2217

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Journal of the American Ceramic Society—Welham
Vol. 85, No. 9
The powders were also analyzed by X-ray fluorescence (XRF),
is slightly substoichiometric, compared with reaction (1), which
indicates that zircon probably will remain after any reaction.
After the dehydration endotherm, an exothermic region oc-
curred in all samples milled for 1–50 h. No such events were
present in the unmilled and 100 h milled powders. This exotherm
seemed to result from milling the zircon, with similar events
present in the sample of pure zircon milled for 5 h but absent from
unmilled zircon (as demonstrated by the sample containing un-
milled zircon). No distinct changes appear in the XRD traces of
powders heated below and above the exotherm, although some
recrystallization and/or strain relief probably occurred. However,
for the 50 h sample, the dehydration occurred concomitantly with
the first exotherm. The overall exothermic nature of this event
indicates that the reaction released enough energy to also drive off
the remaining water from the system, thus removing the endotherm
noted in samples milled for Ͻ10 h.
No further thermal events were observed that would indicate
chemical reaction between the zircon and MgO, except in the two
samples milled for the longest times. In these samples, two distinct
exotherms were observed at 800°–900°C. XRD traces for the 50 h
powder heated below, between, and above these exotherms are
shown in Fig. 2.
From the traces in Fig. 2, the first exotherm is associated with
the formation of a broad peak at ϳ35.8° and the second with the
narrowing, intensification, and shift to a slightly lower 2␪ value
(35.5°) of this peak. Similar results were obtained for the 100 h
milled powder. The peak which appeared was probably a result of
the formation of zirconia,3 and the movement of the peak position
was confirmed by a sample heated to 1000°C, which showed that
the peak had moved to 35.3°, indicating the formation of tetragonal
zirconia (t-ZrO2). The movement of the peak may be attributable
to a change in the crystalline form of the zirconia from metastable
to stable.
within a scanning electron microscope (Model 6400, JEOL,
Tokyo, Japan). The samples were embedded in resin, polished, and
then run for 300 s at 20 kV, using a beam current of 100 nA; the
results obtained were compared with those for standard samples of
ZrO2, zircon, and Mg2SiO4.
III. Results and Discussion
DTA results for the as-milled powders are shown in Fig. 1,
along with traces for MgO and a sample of zircon milled without
MgO for 5 h (as-received zircon showed no thermal events up to
1000°C). Endothermic events clearly occurred at ϳ300°–400°C in
samples milled for up to 5 h.
These events were paralleled by a similar event in the nominally
pure MgO. TGA of the powders (TGA results for the 5 and 50 h
milled powders are shown as broken lines in Fig. 1) indicates
association with a mass loss. The pure zircon showed no mass loss,
and this event is considered the result of dehydration of Mg(OH)2,
which was present as an impurity in the MgO. Beyond 5 h, no such
endotherm occurred for any sample; clearly, milling had an effect
on the dehydration of Mg(OH)2. The TGA trace shown in Fig. 1
for the 50 h milled powder indicates a mass loss of similar
magnitude, but with a higher onset temperature and over a wider
temperature range than in the 5 h milled powder. Thus, milling the
powders together seems to have led to a change in the state of the
water, by increasing the binding strength of the water within the
system. The nature of this change could not be determined. The
presence of Mg(OH)2 in the TGA results indicates that the system
The onset temperature of the thermal events did not change
substantially with ball milling. This finding suggests that the
reactions are not governed by solid-state diffusion; were this the
case, then the onset temperatures would be expected to decrease as
the milling time increased to a constant value.9 Thus, the reaction
between zircon and MgO apparently is not controlled by solid-
state diffusion but rather by chemical reaction, which would be
unaffected by the intimate mixing induced by extended milling.
Nonetheless, milling seems to modify the bonding of water in the
system with increasing milling, leading to an increase in the
temperature at which water is evolved.
Also shown in Fig. 2 are XRD traces for 1, 10, and 100 h
as-milled powders heated to 1000°C during DTA. These traces
verify that increasing the milling time leads to an increased
conversion of zircon to zirconia. A magnesium silicate phase is
only evident after milling for 100 h, confirming that the longer
milling time aids in the formation of this particular product.
However, a magnesium- and/or silicon-bearing product must also
exist after the formation of zirconia in the shorter milled samples;
the phase(s) may simply be below the detection limit of the XRD.
In all powders, a small peak remains at ϳ50° for MgO, which
indicates that the reaction is incomplete. A peak for zircon is also
present; however, this peak is expected, because insufficient MgO
was present as a result of its hydration to Mg(OH)2.
The fractional conversion can be estimated from the ratio of the
areas of the main peaks for zircon (ϳ31.4°) and zirconia (ϳ35°–
36°). This conversion is shown in Fig. 3, along with the estimate
for samples heated for 1 h at 1000°C. Holding the sample at
1000°C for 1 h clearly resulted in a substantially more complete
reaction for all milling times, with Ͼ80% conversion for milling
times of Ͼ2 h; only the 50 and 100 h milled powders had the same
extent of reaction under DTA. Thus, from an energy point of view,
extending the heating stage is more economical than using longer
milling times.
For the 5 h milled powder, a series of time and temperature runs
were made to assess the relative conversion. The results of these
runs are summarized by the dotted lines in Fig. 4. The absence of
reaction after 1 h at 600°C confirms that the exothermic peaks
observed by DTA below this temperature were not associated with
Fig. 1. DTA traces of mixtures of zircon and MgO milled for up to 100 h;
also shown are DTA traces for the MgO used and 5 h milled zircon
((– – –) TGA traces of the 5 and 50 h milled powders).

September 2002
New Route for the Extraction of Crude Zirconia from Zircon
2219
Fig. 2. XRD traces for the 50 h milled powder heated to 750°, 840°, and 900°C, then cooled; also shown are traces for 1, 10, and 100 h milled powders
heated to 1000°C, then cooled.
reaction but with structural rearrangements within the as-milled
powder. Reasonable correlation between two separate runs is
evident; the 5 h milled samples were held at 1000°C for 1 h, where
the conversions to ZrO2 were assessed to be 75% and 80%,
respectively.
The dissolution of as-milled and variously heat-treated powders
is plotted in Fig. 4. The horizontal broken line is the initial mass of
MgO used, which is the minimum expected solubility, because
MgO is completely soluble in the acid used. Nevertheless, the
unmilled powder apparently is more soluble than its MgO content;
this result is attributed to poor sampling, because of the large
difference in particle sizes between the zircon and MgO used.
The solubilities of the 1 and 2 h samples are more realistic and
very close to the minimum, suggesting little, if any, effect of
milling over the first 2 h. The initial 2 h of milling has been shown to
cause size reduction to an approximately constant value,10 and the
absence of increased dissolution suggests the same case here for
zircon, with little or no apparent conversion into a soluble form.
Beyond the initial 2 h of milling, the dissolution within the
system increased, confirming that some reaction must have oc-
curred to form soluble phases; the extent of this reaction increased
with milling time. The continuous increase in dissolution is
indicative of a reaction which proceeds slowly during milling and
does not seem to require an incubation time, other than that
necessary for size reduction. Other reactions involving alkaline
earth oxides have also shown an increasing extent of reaction with
milling time,3,6,7 and this feature may be typical of mechanochem-
ical reaction involving these phases.
Fig. 3. Relative fractions of zirconium in zircon and zirconia, based on the peak areas of the main peaks for samples heated to 1000°C and then cooled
and for samples heated for 1 h at 1000°C.

2220
Journal of the American Ceramic Society—Welham
Vol. 85, No. 9
Fig. 4. Conversion to ZrO2 (⅐⅐⅐⅐) and subsequent solubility (—) of a 5 h milled powder thermally treated for (a) 1 h at 600°–1200°C and (b) 0.25–2 h at
1000°C in air ((– – –) content, by mass, of MgO).
For all powders, the leach residue consisted only of zircon. No
peaks became evident after leaching, indicating that any phases
that formed during milling were either soluble or nanocrystalline.
The zirconium:silicon ratio in the insoluble residue was the same
as that in the original powder, indicating no segregation of
zirconium and silicon during milling.
formation, during milling, of an amorphous phase, which was
absent after leaching in the solution used here. Milling zircon alone
led to an amorphous component that was insoluble,2,11 whereas
milling with MgO leads to the formation of an amorphous phase
that is soluble, implying that the MgO is an essential component of
this phase.
In an attempt to enhance the separation of the magnesium,
silicon, and zirconium, samples of the 5 h milled powder were
annealed for 1 h in the temperature range 600°–1200°C and at
1000°C for up to 2 h. The leaching results are shown by the solid
lines in Fig. 4; the initial percentage of MgO is shown by the
broken horizontal line. For both sets of thermally treated powders,
the leaching results were somewhat disappointing, showing no
apparent correlation between the conversion to ZrO2 (dotted lines)
and the amount dissolved. These results suggest that the products
of the reaction were slightly less soluble than the phases present in
the as-milled powder.
XRF analyses of the residues after leaching were performed on
the heat-treated samples previously milled for 50 h. For each major
element, the percentage remaining in the solid phase after leaching
was calculated, and these results are presented in Table I. Directly
after milling with MgO, no separation of zirconium and silicon is
shown, with 60% of each reporting to solution. Clearly, the
dissolution was of an essentially stoichiometric zircon; this result
apparently contradicts the findings of previous works,2–4,11 which
showed that zircon milled alone for up to 500 h was essentially
insoluble in the solution used here. Therefore, part of the zircon
must have reacted with the MgO to form a congruently soluble
phase. Preliminary work on the MgO–zircon system4 indicated the
After the as-milled powder was heated for 1 h at 800°–1200°C,
the extent of dissolution was approximately halved. The percent-
age of the initial zirconium remaining in the solid phase was
Ͼ90%, indicating that thermal processing had led to the formation
of an insoluble zirconium phase. The percentage of silicon
remaining was also greater than in the as-milled powder, but it was
significantly lower than for zirconium, indicating that a separation
of the elements had occurred during heat treatment. An increasing
percentage of magnesium remained in the solid phase as the
temperature increased; clearly, insoluble compounds had formed.
A further sample was milled for 50 h, using a 3:1 ratio of
MgO:ZrSiO4 to ensure sufficient MgO for any reactions. DTA and
TGA of this powder (not shown) showed that the onset and peak
width of the DTA traces and the onset of mass loss were
essentially identical in form to that observed for the 50 h sample
shown in Fig. 1. However, the exothermic peaks were somewhat
more intense for the higher ratio DTA, and the mass loss also was
somewhat greater. The greater mass loss was expected, because
more Mg(OH)2 would have been present in the higher ratio
powder. The greater DTA energies were somewhat unexpected but
probably reflect a greater extent of reaction during this low-
temperature reaction stage. Previous work on the carbothermic
reduction of ilmenite12 showed that, for identical milling and
Table I. Percentage of Elements Remaining in Insoluble Residue after Various
Preleaching Treatments on the 2:1 and 3:1 MgO:ZrSiO4 Molar Ratios
Amount of element remaining in residue (%)
Amount leached
Si:Zr ratio
in residues
Treatment conditions
(%)
Zirconium
Silicon
Magnesium
Unleached
MgO:ZrSiO4 ϭ 2:1
As-milled
800°C
1.05
61.2
31.0
36.1
29.2
40.0
94.1
91.3
94.8
39.9
66.4
43.9
53.0
1.3
26.5
33.4
44.3
1.05
0.74
0.50
0.58
1000°C
1200°C
MgO:ZrSiO4 ϭ 3:1
As-milled
800°C
73.7
50.2
53.4
51.5
39.8
81.2
96.7
95.3
40.1
61.1
9.5
0.4
3.2
1.06
0.79
0.10
0.15
1000°C
7.6
1200°C
13.8
12.1

September 2002
New Route for the Extraction of Crude Zirconia from Zircon
2221
annealing conditions, the extent of ilmenite reduction under
identical conditions increased with the stoichiometric excess of the
carbon; the same phenomenon may well hold true here, also, with
the excess of MgO.
to occur. Increasing the milling time resulted in an increased extent
of reaction. Thermal analysis of the powders showed a change
from a net endothermic to a net exothermic state after milling for
10 h. Zircon recrystallization was evident as an exothermic peak at
400°–550°C for all samples except that milled for 100 h. The mass
loss associated with the small amount of Mg(OH)2 present shifted
to higher temperature as the milling time increased, indicating that
the reaction was increasing the binding strength of the water. For
50 and 100 h milling, two exothermic peaks also occurred at
800°–900°C, both apparently associated with the emergence of
ZrO2.
Leaching of the as-milled powders showed increasing dissolu-
tion with milling time, although the soluble phase could not be
positively identified because of a lack of crystallinity. However,
the only phase remaining after leaching was zircon, and the
silicon:zirconium ratio was the same as that of the feed powder,
implying that a complex nanostructured phase composed of
zirconium, silicon, and magnesium formed during milling and was
congruently soluble.
XRD of this powder heated at 800°C showed peaks for zircon
and MgO and a broad, weak peak for ZrO2 (essentially identical to
that in the sample heated to 840°C in Fig. 2). Clearly, although the
MgO content was greater, the reaction during milling was not
complete. Heating the powder to 1000°C showed the presence of
t-ZrO2 as the major phase, with somewhat weaker peaks evident
for Mg2SiO4. No evidence supported the presence of either zircon
or MgO. Clearly, a thermal reaction must have occurred between
zircon and MgO, between 800°C and 1000°C, to form ZrO2 and
Mg2SiO4. This reaction correlates with that observed for the
exothermic events in the 50 and 100 h milled powders in Fig. 1,
implying that the increase in the MgO content did not alter the
progress or the extent of the reaction; thus, chemical control must
have been the rate-determining step.
The percentage of each element reporting to the solids after
leaching is presented in Table I, along with the silicon:zirconium
molar ratios. For the unleached powder, the ratio is slightly greater
than the unity expected for zircon; this result is probably attribut-
able to the presence of small amounts of silica in the feed powder.
When the as-milled powder was heated, the total solubility
decreased to 50%–54%. However, the percentage of the various
elements remaining in the insoluble solid changed dramatically.
After the powder was heated at 800°C, the percentage of zirconium
increased from 40% to ϳ81%, whereas the silicon increased from
40% to 61%. This result suggests that heating the powder did not
substantially change the soluble percentage, although some sepa-
ration did occur, with the silicon:zirconium molar ratio decreasing
from 1.05:1 to 0.79:1. Heating to 1000°C caused the most dramatic
changes, with almost 97% of the zirconium reporting to the solid,
along with only 9.5% of the silicon. After 1 h at 1200°C, a similar
result occurred, with 95% of zirconium in the solid; however, more
silicon and magnesium were present in the solid than after heating
to 1000°C. Clearly, heating at or above 1000°C caused substantial
separation of zirconium and silicon, although heating to 1200°C
increased the percentage of both silicon and magnesium in
insoluble phases. The powder obtained after heating to 1000°C
contained ϳ90 wt% ZrO2, 4.5% SiO2, and 5.5% MgO, clearly a
more appropriate starting material for chlorination than pure
zircon.
Thermal processing of the as-milled powders induced crystal-
lization of ZrO2 and Mg2SiO4, of which the latter was soluble,
resulting in separation of the zirconium and silicon. Using a greater
fraction of MgO led to a greater extent of reaction and improved
separation, with Ͼ90% of silicon and magnesium solubilized,
while Ͻ5% zirconium went into solution. The final powder was
ϳ90% ZrO2, 4.5% SiO2, and 5.5% MgO.
References
1F. Farnworth, S. L. Jones, and I. McAlpine, “The Production, Properties and Uses
of Zirconium Chemicals”; pp. 248–79 in Speciality Inorganic Chemicals. Edited by
R. Thompson. Royal Society of Chemistry, London, U.K., 1981.
2T. Puclin, W. A. Kaczmarek, and B. W. Ninham, “Mechanochemical Processing
of ZrSiO4,” Mater. Chem. Phys., 40, 73–81 (1995).
3N. J. Welham, “Investigation of Mechanochemical Reactions between Zircon
(ZrSiO4) and Alkaline Earth Metal Oxides,” Metall. Mater. Trans. B, B29, 603–10
(1998).
4N. J. Welham, “Enhancing Zircon (ZrSiO4) Dissolution by Ambient Temperature
Processing,” AusIMM Proc., 305, 1–3 (2000).
5N. J. Welham and L. Walmsley, “Solubilisation of Zircon by Mechanically-
Induced Solid-State Cation Exchange with Alkaline Earth Oxides,” Trans. Inst. Min.
Metall., Sect. C, 109, C57–C60 (2000).
6N. J. Welham, “Ambient-Temperature Formation of Metal Titanates from Ilmenite
(FeTiO3),” J. Mater. Sci., 33, 1795–99 (1998).
7N. J. Welham, “Mechanically Induced Reaction between Alkaline Earth Metal
Oxides and TiO2,” J. Mater. Res., 13, 1607–13 (1998).
8B. E. Warren, X-Ray Diffraction; pp. 251–314. Dover Press, New York, 1990.
9N. J. Welham, “Mechanical Activation of the Solid-State Reaction between Al and
TiO2,” Mater. Sci. Eng. A, A255, 81–89 (1998).
10N. J. Welham and D. J. Llewellyn, “Mechanical Enhancement of the Dissolution
of Ilmenite,” Min. Eng., 11, 827–41 (1998).
IV. Conclusions
11T. Puclin, W. A. Kaczmarek, and B. W. Ninham, “Dissolution of ZrSiO4 after
Mechanical Milling with Al2O3,” Mater. Chem. Phys., 40, 105–109 (1995).
12N. J. Welham, “A Parametric Study of the Mechanically Activated Carbothermic
Zircon was milled with MgO for up to 100 h, which resulted in
a solid-state reaction. No apparent reaction occurred during the
initial 2 h of milling, during which time size reduction was thought
Reduction of Ilmenite,” Min. Eng., 9, 1189–200 (1996).
Ⅺ