LATE STAGE MAGMATIC FLUIDS PROVIDE THE KEY TO THE SILEACH™ PROCESS FOR RECOVERING LITHIUM FROM SILICATES
Lithium Australia NL (ASX:LIT)
Lithium Australia NL has been developing a number of hydrometallurgical process flowsheets for the recovery of lithium from silicates. The most advanced, the halogen based SiLeach™ process, was developed specifically for the digestion of spodumene, a refractory lithium pyroxene previously only processed by roasting followed by sulphation bake and water leach. The SiLeach™ process, which relies on the reaction of halogens with Si=O bonds, is widely applicable to the dissolution of all categories of silicate minerals and has found practical application in the processing of lithium micas. SiLeach™ is currently being commercialised utilising pilot facilities established at ANSTO Minerals (a division of the Australian Nuclear Science and Technology Organisation).
The halogens can be added to the process slurry in a number of ways however the preferred method is by way of the addition of ground fluoride minerals, which may include the lithium micas themselves, with the process slurry, prior to sulphuric acid addition. Due to the kinetics of competing reactions, this sequence allows the momentary generation of F- in solution, and its almost instantaneous reaction with the silicates, without any accumulation of HF in the slurry. Process plant operations can be accomplished without the hazards often considered to be the risk of fluorine based processes.
The SiLeach™ process is capable of recovering a wide range of by-products, as all metals in the target silicate are dissolved. The low energy requirements of the process, and by-product credits, enhance the economics which may result in lithium production costs using SiLeach™ to be amongst the lowest.
SiLeach™ has the potential to provide access to a wide range of plant feed previously not considered viable. These include low-grade spodumene concentrates and micas as primary feed sources. Furthermore, vast quantities of lithium minerals, currently being discharged as tailings from non-lithium mining operations, create very attractive targets for application of this type of technology. Utilisation of these materials as lithium sources will greatly enhance the sustainability of the lithium industry.
Keywords: SiLeach™, lithium, spodumene, mica, halogen, fluorine, hydrometallurgy
There are two historic sources of lithium: hard-rock and brine occurrences. Each of these presently account for about half of global lithium demand (USGS, 2017). Lithium bearing “clays” are also a promising source and a number are currently under investigation for future production (Ausenco, 2016).
Regardless of type, all commercially significant lithium sources have a common provenance – they result from magmas that have accumulated a high concentration of volatile elements. The volatile elements, including lithium, reach the near-surface, exploitable accumulations by:
• emplacement of pegmatites,
• development of greisens,
• degassing of aqueous components, or
• generation of hydrothermal fluids.
Very specific tectonic conditions are required to provide an environment which accumulates volatile components and lithium. It is the chemistry of the last stages of volatile magma emplacement that provides many of the keys to dissolution of silicate minerals under commercial conditions. Indeed, under the unique conditions encountered in these intrusions, which invariably solidify at shallow crustal levels or generate violent explosive volcanic events, natural fluxes and solvents accumulate in abundance.
The high quantities of water in these melts depresses the liquidous, and the high pressures (relative to atmospheric) prevent the water separating as a single phase – the conditions are beyond the critical point of water. Other accumulated fluxing elements further depress the solidus, in particular Li, B and P. As the magmas rise and pressures are reduced in these melts, the distinction between magmatic and hydrothermal fluid blurs.
The final fluid phases are of great interest as their behaviour demonstrates the possibility of recreating similar conditions within commercial processing systems that are capable of reversing the natural sequence and take minerals back into solution. The SiLeach™ process has achieved this reversal and can be considered a digestion process rather than a simple leach.
THE PYROMETALLURGICAL/HYDROMETALLURGICAL TRANSITION
In terms of extractive metallurgy the definition of “pyro” and “hydro” are widely separated, but in the bowels of the earth, the processes cross – there is no distinct boundary. The extreme pressures are capable of generating a process continuum. But other factors are at work. The fluxing characteristic of many of the “incompatibles” result hydrous melts of specific chemistry, having very low melting points. At the extreme, the water saturated melts approach the operating conditions that can be achieved in mineral processing autoclaves. That being the case, the phase chemistry of these late-stage magmatic products can provide some of the keys as to how the processes may be reversed to exploit the ability to return metals to an aqueous phase.
Late stage magmas, and magmatic fluids accumulate the incompatible elements – those elements that don’t readily fit into the lattice of the common rock forming minerals. Some of those elements are too big to conveniently fit into most silicates and they get a free ride into the late stage fluids, particularly as the temperatures and pressures drop below the critical point of water, which decompresses to become an aggressive solvent harbouring the incompatible elements.
Other elements, such as fluorine, generate such a depression of the solidus that they become the self-fulfilling component – the component that remains in the molten state (morphing into the late stage aqueous fluids) because they act as fluxing agents and their ever-increasing concentration creates an ever-reducing solidus temperature. As fractional crystallization in the magma chamber advances, the fluid phase composition trends towards a complex eutectic point and the remaining magma has relatively low viscosity. High-level intrusives solidifying within a few kilometres of the earth’s surface, may transgress the critical point of water, explosively propelling the remaining low-viscosity fluids into brittle fractures of the surrounding country rock. This is the birth of the pegmatite.
It is the behaviour of water, and the fluxing agents that provide the key to reversing the process in a synthetic, controlled environment, to result in the dissolution of silicate minerals at low temperatures and pressures.
VULNERABILITY OF SILICATES TO HYDROTHERMAL PROCESSES
Examination of naturally occurring, low-temperature magmatic processes may provide empirical evidence as to what minerals might be best suited to attack by hydrometallurgical processes and the conditions under which attack may be effectively controlled.
TAKING NATURE ONE STEP FURTHER
The dissolution of ore minerals in sulphuric acid systems is commonplace. Sulphides are oxidised at atmospheric or low-pressure conditions, to a large extent, mimicking weathering processes. Taking these natural processes and exaggerating the corrosion conditions in a synthetic system may amplify the kinetics. This amplification may be of such magnitude as to make the process of metal extraction a commercial reality within an acceptable residence time.
The HPAL processes, used for nickel extraction, reduce the dissolution and deportment of chemical component from millions of years, to hours or minutes. The natural secrets of these chemical processes remain preserved in the lateritic profile for the astute student to observe.
The dissolution of silicates may be considered a reversal of the crystallization processes we find preserved in the geological record as pegmatites and greisens. To artificially reverse the process, water has to be added to the system, and other volatiles/fluxes added to the mineral slurry. The halogens, principally F and Cl, have an important role in natural systems. Indeed, at such elevated temperatures and pressures, chlorine may well be a significant factor in transporting other elemental species. At lower temperatures and pressures the dominant reactive halogen is F, as recorded in the composition of mineral species, mica in particular, as crystallization proceeds.
The effects of increasing concentration of F in late stage pegmatite fluids is often observed as fluorite (CaF2) overgrowths on earlier F bearing phases such as topaz or tourmaline. Both of these crystallize earlier than fluorite and contain significantly less F than fluorite. This attests the rising concentration of this important halogen in the final stages of pegmatite crystallization. In many cases the miarolitic cavities within, which the fluorite crystalizes, are the silent witnesses to the relatively low pressure conditions under which the pregnant F liquors may carry soluble components. Jacobson (2013) summarised many of these occurrences as follows:
The anorogenic…….granites formed from the melting of lower crustal rocks during crustal thinning due to tension, not during compression related to continental collisions or subduction. The miarolitic, vuggy character of these granites is due to the upper part of the magma rising to a shallow emplacement in the crust. This shallower, lower pressure environment allowed water to exsolve from the magma as the pressure decreased and to form bubble-like pegmatite cavities. Almost any NYF [niobium yttrium fluorine] granite with miarolitic pegmatites may contain attractive fluorite crystals.
Albeit miarolitic pegmatite of the NYF association is a very specific example, it does serve to demonstrate that aqueous F solutions are the medium for mass transfer in natural systems with temperatures as low as 200oC and pressures of 200-250Mpa (London et al, 2012).
Without resorting to the pressures of geological systems, the activity of F can be harnessed to break the Si=O bond (Kuang et at, 2012) that is the very building block of the silicate minerals. Moderate concentrations of F are required in solution, and elevated temperature. Lithium Australia NL has been developing the 100% owned SiLeach™ process to achieve that goal.
Spodumene has long been the only commercially processed hard-rock lithium mineral. Conventional processing relies upon a phase conversion from low-temperature α-spodumene, to the higher temperature polymorph, β-spodumene to enhance leach performance in sulphuric acid. The process effectively rejects the gangue elements, leaving lithium as the principal revenue source. The conventional process for recovering lithium from spodumene is energy intensive but its selectivity is an advantage resulting in simple processing steps following the initial phase conversion. Lack of significant by-product credits is a significant commercial disadvantage with conventional processing of spodumene.
The cost of producing lithium carbonate from a 7% Li2O spodumene concentrate is about US$4,500/t (Roskill, 2016). It follows that the cost of producing lithium carbonate, using similar processes, but starting with a lithium mica, at half the grade, should be twice as much. Deutsche Bank (2016) has confirmed the veracity of this assumption indicating Chinese producers, using a lithium mica feed, have operating costs approaching US$8000/t (Figure 1 – far righthand bar).
Figure 1 Lithium carbonate cost curve after Deutsche Bank (2016). If conventional processing technology is used, the lowest cost producers are the brines, followed by spodumene (and petalite) and lastly the lithium micas.
THE DEVELOPMENT OF THE SILEACH™ PROCESS
• many hard-rock lithium occurrences demonstrate crystallization of lithium minerals from magmatic phases with high levels of water and fluorine,
• many of the late stage, relatively low temperature and pressure minerals contain F as a major constituent of the crystal lattice,
• Li and F increase in the magma towards the eutectic which arises from partial crystallization of fertile magmas.
Fluorine chemistry is a critical component in the genesis of lithium ore minerals and that chemistry remains critical in the redissolution processes if high temperatures and high pressures are to be avoided during the extraction of lithium from lithium minerals.
The SiLeach™ process is a broad-spectrum, halogen-based, digestion process developed to recover metals from silicates without the requirement to roast. SiLeach™ is a reversal of the last stages of fertile magma crystallisation. Lithium recovery is the principal SiLeach™ target although other alkali metals may be commercially important under some circumstances, with Al and Si also available to produce valuable by-products.
LITHIUM MINERAL ACTIVITY SERIES
Lithium Australia studied a number of lithium minerals, and also a number of non-lithium minerals to determine digestion characteristics under comparable conditions using sulphuric acid at elevated temperature (Figure 2).
The reactivity of the micas is dependent upon their structure and the way in which the structure presents active lattice sites, or exposed chemical bonds, to the lixiviant. Reactions that liberate metals from phyllosilicates include:
• ion exchange,
• protonation of anions, or
• destruction/replacement of Si=O, or other tetrahedrally co-ordinated oxygen bonds
The mica group can be divided into dioctahedral structures and trioctahedral structures. The latter have more reactive sites per lattice unit that the dioctahedral micas and hence have greater ion exchange capacity and greater reactivity in acids. The orientation of hydroxyl dipoles also influences the mineral reactivity.
In strong sulphuric acid solutions, the ion exchange reaction mechanisms are less important than the protonation of anions, OH- and F- in particular. These are only exposed on surfaces perpendicular to the minerals’ perfect cleavage. Indeed, viewed perpendicular to the C-axis, the micas expose only outward facing oxygen atoms being the base of silica tetrahedra. The exposed atoms, and the connecting Si=O bonds are inert to sulphuric acid, and consequently the reaction kinetics is controlled largely by the surfaces exposed perpendicular to the cleavage planes.
As the aspect ratio of exposed surfaces in the micas (C:A or C:B crystallographic axes) is very large. As the large surface area is dominated by oxygen at the termination of Si=O bonds, most of the exposed surfaces are unreactive. The abundance of the reactive OH- bonds is low, due to their exposure only on the relatively small surface areas exposed perpendicular to the cleavage. This results in long reaction times in sulphuric acid, as few reactive species, OH- and F-, are exposed to the attack of H+. The reaction of the acid on the mica generally proceeds slowly, but as it progresses, it releases F- from the lattice which is then available to attach Si=O bonds which form the framework of the mica (Figure 3).
If the amount of F- in solution is increased by the addition from a source not derived from the mica, the reaction kinetics can be changed. For example, the stoichiometric addition of F- to lepidolite, resulting in the same concentrations that would be expected from zinnwaldite will create a digestion rate of lepidolite that is similar to that of zinnwaldite in sulphuric acid.
Taken to extremes, spodumene, a lithium pyroxene, can also be made to react at a similar rate and it is the control of these reaction rates that are the basis of the SiLeach™ process (Figure 4).
SiLeach™ is a non-selective, halogen-based digestion system that provides access to cations in the silicate lattice, by the destruction of Si=O bonds, to release all metals into solution. This provides the opportunity to recover metals other than lithium from minerals digested using the SiLeach™ process. In the case of the lithium micas, the framework of the lattice provides abundant Si and Al whereas the octahedral sites harbour abundant Group 1 metals including K, Li, Rb, Cs. To a lesser extent Na, Mn and Fe may also be present in the lattice. The potential for abundant by-product credits and elimination of the requirement to roast are key elements of the SiLeach™ processing strategy. Process flowsheet specifics vary with mineral species however the flowsheet required for micas provide a good example of the process steps involved (Figure 5).
Clearly conventional processing is an alternative to SiLeach™ but it has a high energy cost and has revenue restricted largely to lithium. In the case of micas, release of fluorine, in the form of HF vapour, not only presents a safety issue, but materials of construction and corrosion of equipment is a practical concern.
Hydrometallurgical alternatives are few, and leaching in sulphuric acid alone is kinetically restrictive on all but the most reactive lithium minerals.
Elevating temperature and pressure, as has been the domain of the platinum producers, and lateritic nickel operations is an alternative. The application of pressurised reaction vessels certainly presents another dimension to the processing of lithium silicates, allowing the options of both acid and caustic systems. Lithium Australia, together with ANSTO Minerals (a division of the Australian Nuclear Science and Technology Organisation) has been developing the LieNA™ process – a moderate temperature, caustic leach which requires pressure containment. LieNA™ is capable of digesting all of the common lithium minerals, including spodumene and provides project specific process optimisation which can capitalize on unique factors such as local logistics, reagent costs, and energy casts.
The reactivity of most lithium minerals in sulphuric acid is kinetically limited. The kinetics can be improved by the addition of fluorine to the processing system, and this can be done in such a way as to minimise the production of volatile fluorine species, HF in particular, that would otherwise present an occupational health and safety risk. The operational perfection of the fluorine assisted leaching is the essence of Lithium Australia’s 100% owned SiLeach™ process.
The addition of temperature, and elevation of pressures beyond atmospheric, provide the flexibility to digest lithium silicates in either acidic or caustic environments. Caustic digestion is the foundation of the LieNA™ process which Lithium Australia currently has under development.
The author acknowledges the contributions of Lithium Australia, its staff, and consultants. In particular Work conducted at ANSTO Minerals and Murdoch University partially funded by two Innovation Connections Grants under the Entrepreneur’s Programme run by the Department of Industry, Innovation and Science and one grant provided by the Minerals Research Institute of Western Australia.
The hydrometallurgical work of pioneers such as Arthur J Moxham (1925) laid the foundation of much of the modern sulphuric acid process development and has been an inspiration to us all. Enej Catovic lead the Lithium Australia team’s research, and Andrew Skalski has commenced the task of turning research into commercial reality.
Finally, the support of Lithium Australia’s shareholders, who have not only been owners, but also financiers, have been instrumental in the outcome.
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