TECHNICAL FIELD
[0001] The present invention relates broadly to a method of pyroprocessing a powder
to induce a phase change in the grains of the powder particles, and/or to avoid an
undesirable phase change.
[0002] The invention is described by using the application for processing the mineral
α-spodumene for the extraction of lithium, where the phase change is the conversion
of α-spodumene to a mixture of β-spodumene and γ-spodumene to facilitate extraction
of lithium by the known arts of hydrometallurgy.
BACKGROUND
[0003] In this invention, for the avoidance of doubt, the term “calcination” is limited to a
process in which a powder is heated with the primary purpose of inducing a chemical
reaction which releases a gaseous product such a steam or CO2; and the term
“pyroprocessing” is limited to a process in which a powder is heated with the primary
purpose of inducing a phase change; and the term “roasting” is limited to a process in
which powders of different materials are heated with the primary purpose of inducing
chemical reactions between the particles. It is recognised that a person skilled in the art
may use these terms interchangeably.
[0004] Pyroprocessing of powders is well established in industry. Most of these
processes have been developed using combustion of fossil fuels and mixing the
powder into the hot combustion gases. There is a need to replace these fuels by
renewable sources of energy, such as biomass and hydrogen, to limit global warming.
However, there is also a general need to improve the quality of pyroprocessed
materials, and this invention considers a means of pyroprocessing that can be used to
improve the product quality by a process in which the powder is not mixed with a
combustion gas. Specifically, there is a need to allow processing to occur in a gas
which has the most desired reducing, neutral or oxidation potential, which is not
generally achievable in a hot combustion gas.
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[0005] This invention is directed to the pyroprocessing of the mineral α-spodumene
to enable the subsequent extraction of lithium, as a specific, but not limited, example
which demonstrates the general application of the invention.
[0006] Lithium is required at industrial scale for the production of lithium batteries,
and the growth of that market is increasing at a rate of about 18% pa to meet the
needs for storage of electric power, particularly renewable power, for many markets
which now including batteries for electric vehicles, and stationary applications such
as load balancing electrical grids to accommodate variations from solar and wind
power. This growth of battery markets is to be sustained by ongoing reductions in
the cost of input materials, including the cost of lithium carbonate and lithium
hydroxide, which are generally used as the source of lithium by lithium battery
manufacturers. There are several sources of lithium that are used, namely from salar
brines in which the lithium has been concentrated over long periods of time, and
from a range of minerals, including spodumene, eucryptite, petalite, bikitaite as
described by Dessemond et.al in “Spodumene: The Lithium Market, Resources and
Processes” Minerals, 9, 334 (2019). In recent years, the costs of extraction from brines
has become uncompetitive compared to mineral extraction methods. Of the lithium
containing minerals, spodumene, in the form of α-spodumene, has the highest lithium
content, of 8 wt% when pure, and there are abundant mineral sources of α-spodumene
with purities ranging from about 2-6 wt % that can be exploited cost effectively. The
extraction process generally involve a mix of mineral beneficiation, pyroprocessing, acid
roasting, and hydrometallurgical extraction steps. The energy and capital costs for these
extraction processes are high, and there is a need to reduce those costs by improving
these steps to meet the growing demand and cost reductions.
[0007] The mineral α-spodumene, LiAl(SiO3)2 has a crystal structure in which the
aluminium ion is tightly bound to 6 oxygen atoms so that the density is very high, about
3.15 g/cm3
. This mineral is too dense for efficient direct hydrometallurgical extraction
of lithium, and in this dense phase the migration of the lithium ion is too slow, and
extensive grinding processes to reduce this time are too expensive. The phase diagram of
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spodumene is not well established, however it would appear the α-β phase transition
commences at temperatures as low as 520oC, but is very slow. However, by heating to
about 1000oC the α-spodumene is converted to a mixture of β-spodumene and γspodumene. Both these structures are characterised by aluminium ions that are bound to
4 oxygen atoms, and the weaker bonding is such that the density of the products is low,
about 2.37 gm/cm3
and hydrometallurgical extraction processes can take place quickly in
particles that are in the range of 50-300 microns. There have been extensive studies of
this process, as described in the review “Phase transformation mechanism of spodumene
during its calcination” by Abdullah et. al. in Minerals Engineering, 140, 1058883, 2019.
The process is now understood to occur through several mechanisms depending on the
grind size. In the early literature, it was assumed that the α-spodumene converted directly
to β-spodumene, and the γ-spodumene, a known meta-stable phase was not considered.
Nevertheless, studies have shown that lithium can be extracted from both β-spodumene
and γ-spodumene without significant differences. The work of Moore et.al, “In situ
synchrotron XRD analysis of the kinetics of spodumene phase transitions”, Phy
s.Chem.Chem.Phys., 20, 10753 (2018) conducted in air, showed that at high
temperatures, in the range of 896-940oC α-spodumene was converted to a mixture of βspodumene and γ-spodumene phases with a fraction of γ which was about 35%. They
observed a slow decrease of the γ-spodumene to β-spodumene at 981oC over 240 minutes
in muffle furnace tests in air. The particle size and impurity dependence of the onset of
pyroprocessing may be related to the grain size of the ground particles, where the phase
change propagates from the grain surfaces, and/or the lowering of the phase change
temperature from substitutional impurities within the grains or impurities at the grain
boundaries.
[0008] The process of α-spodumene transformation was patented by Ellestad et. al. in US
2516109 in 1948, and described the heating process for granules of the order of 0.5-2.5
mm as one which required heating to over 1000oC, within the heating duration of about
30 minutes. The temperature was specified to be below the decomposition temperature
of 1418oC, where the silica is liberated as a molten material. By control of temperature,
100% extraction using a hydrothermal process was described. The pyroprocessing
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methods were described as a muffle furnace (externally heated with a fixed powder bed),
a rotary furnace, and a direct fired furnace with combustion. The long residence time and
thermal losses from such devices is very high, so that there is a general need to reduce the
residence time to lower costs.
[0009] The patent WO 2011/148040 describes the advantages of using a fluidised bed for
calcination of α-spodumene particles with a size of 20-1000 microns in an oxidative gas
at 800-1000oC where oxygen was required for fuel combustion in the pyroprocessor to
provide the heat; the residence time was about 15-60 minutes; and the heat in the hot gas
and solids exhausted from the pyroprocessor is used to dry and preheat the solid
feedstock; and the need to limit the formation of molten phases to less than 15% was
specified. To a person skilled in the art, the reference to restricting the molten phases is a
reference to the melting of silica, a decomposition product of spodumene, over the
product surface, which inhibits the subsequent extraction efficiency.
[0010] Colour changes are generally induced in minerals pyroprocessed in the oxidative
comditions of a combustion gas, associated with the oxidation of multivalent impurities
such are iron, chromium, copper, nickel, manganese, or crystal defects. In certain
pyroprocessing processes there is a need to control the colour of the processed solids, and
it would be preferable to process the material in a gas where the redox potential of the gas
can be controlled to produce the desired oxidation state.
[0011] In another process, first described by G.D, White and T.N.McVay “Some aspects
of the recovery of lithium from spodumenes”, Oak Ridge National Laboratory, 1958, a
process is considered in which the silica is extracted by roasting pellets of α-spodumene
and limestone CaCO3 such that calcium silicates are formed, and the lithium forms watersoluble LiAlO2. This process has recently been carried out in a muffle furnace using
particles of about 100 microns at 1050oC for 30 minutes by Braga et.al “Alkaline process
for extracting lithium from Spodumene”, 11th International Seminar on Process
Hydrometallurgy – Hydroprocess 2019, Santiago, Chile, (2019). This roasting process
includes the calcination of limestone to lime, and has not been used commercially. It is
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noted that the subsequent processing of the β-spodumene and γ-spodumene, with lime or
sodium hydroxide is a known art to liberate the lithium from those materials.
[0012] As described above, the primary motivation for the pyroprocessing of αspodumene is to open up the particles by converting the material to the low density βspodumene and γ-spodumene phases. It is well established that the product made from
this process is porous and friable, as a result of the large density change. As a result, the
product is susceptible to decrepitation in the pyroprocessor. In commercial practice, the
pyroprocessing of α-spodumene is carried out using pyroprocessors that provide the heat
by mixing the particles with a hot combustion gas. These are rotary kilns, fluidised beds
or suspension cyclone flash calciners, each of which is a known art. It would be
appreciated by a person skilled in the art that each of these pyroprocessors carries out the
process under conditions which induce decrepitation. In rotary kilns this occurs by the
need to agitate the moving bed by rotation of the kiln and the tilt of the kiln to allow the
bed to absorb the heat from a flame. In fluidised beds the high density of the bed and the
high particle collision frequency leads to attrition, and this is very high for friable
materials. In suspension cyclone flash calciners, the high gas velocity induces collisions
throughout the process which induces decrepitation. The result is that the product quality
is poor, and difficult to control because the fines and the larger particles can have
different degrees of calcination. The different residence times of the fines and the larger
particles is such that a significant fraction of the product may be overcooked so that silica
from the fusion processes is observed. In all these examples, expensive filter systems are
required the separate the fines from the combustion gas streams. In all these systems the
powder is processed in a combustion gas, which is an oxidising environment. In fluidised
beds and rotary kilns, the residence time is sufficiently long that impurities, such as silica
can melt, or form eutectic phases, which inhibit the desired phase changes. There is a
need for a pyroprocessor which does not induce decrepitation of the friable β-spodumene
and γ-spodumene material, There is a need for a flash pyroprocessor to inhibits the
formation of silica eutectic phases, which are known to inhibit extraction.
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[0013] The grinding of the α-spodumene is optimised to enable separation of the αspodumene particles from impurities. Due to the similarity of the physical-chemical
properties of α-spodumene with the gangue minerals such as quartz, feldspar, mica,
muscovite and other aluminosilicates, this is often a challenging task. A floatation
separation efficiency of 90% has been reported by Filipov et. al. in “Spodumene
Floatation Mechanism” Minerals, 9, 2019, 372 using sodium oleate as the surfactant, with
NaOH as a pH regulator and CaCl2 as an activator, with grind size reported to be in the
range of 40-150 microns. Reports on other floatation processes suggest that a particle
size distribution with a d80 of 200 microns can be used for example in the process
described by L. Filipov et.al in “Spodumene Floatation Processes”, Minerals, 9, 372
(2019), or about 45 microns in J. Tian et. al. “A novel approach for flotation recovery of
spodumene, mica and feldspar from a lithium pegmatite ore”, J. Cleaner Production, 174,
625 (2018). It would be evident to a person skilled in the art that (a) the preferred
grinding process is dependent on the mineral impurities to be separated, and (b) it is
preferable that the calcination process should be capable of processing the powders with a
particle size distribution that is the same as derived from such an optimised flotation
separation efficiency. It is apparent from the references above that the calcination
process should be capable of processing particles in the range of 40 to 200 microns. It
would be apparent to a person skilled in the art that this range of particle sizes is too
small to be readily used by rotary kilns and fluidised bed pyroprocessors because such
particles are entrained in the combustion gas, notwithstanding decrepitation. Suspension
cyclone flash pyroprocessors are appropriate for such particles, but suffer from
decrepitation issues. There is a need for a pyroprocessor that can process particles in the
range of 40-200 microns with minimal decrepitation.
[0014] The hydrometallurgical process for extraction of the lithium is inhibited if the
particles are covered by a coating of fused materials, particularly silica from the
spodumene materials which occurs not only on the external surfaces of the particles,
but more importantly on the surfaces of the pores of the particle. This limitation is
disclosed in the prior art, and is known by persons skilled in the art. The phase
transition temperature, of about 1000oC is above the softening temperature of silica.