The two main sources of lithium are hard rock spodumene deposits and salar brines. Historically, commercial lithium production has been derived from hard rock mineral ore sources such as spodumene. Recently, however, the development of salar brines from South America has expanded rapidly.
Global supply of lithium carbonate equivalent is estimated to be around 230ktpa, of which production of lithium via spodumene is c 80ktpa, primarily from pegmatites in Western Australia (of which Greenbushes is the largest and highest grade) and from some Chinese, Chilean and Argentinian sources. Output is dominated by six operations owned by four major companies (representing 91% of total market share): Albemarle, SQM, FMC and Sichuan Tianqi Lithium.
While almost none of the lithium used in consumer batteries is completely recycled, the recycling of lithium has also grown notably since Japan opened its first lithium-ion battery recycling plant in 1992. Facilities in Belgium, Germany, Japan, the US and Canada can all now process batteries for their lithium content.
Hard rock spodumene deposits typically occur in lithium rich pegmatites as colourless to yellowish, purplish or lilac kunzite or yellowish-green or emerald-green hiddenite crystals. The largest concentrations are found in granitic pegmatites (granite-like igneous rocks composed of quartz, feldspar and mica). The most important of these minerals are spodumene, which is a pyroxene mineral (LiAl(SiO3)2 or, alternatively, Li2O, Al2O3.4SiO2) and petalite (Li2O, Al2O3.8SiO2). Sources include Afghanistan, Australia, Brazil, Madagascar, Pakistan, Quebec, North Carolina and California. Associated minerals include quartz, albite, petalite, eucryptite, beryl and lepidolite (the focus of the L-Max process).
Theoretically, spodumene contains 8.03% Li2O and is usually recovered through conventional open-pit mining methods and beneficiated via gravity techniques, whereby the ore is concentrated from a 1–2% Li2O ore grade to a c 6–7% Li2O concentrate grade (ie 75–87% spodumene).
Pre-flotation treatment (ie cleaning) followed by oleic (fatty) acid or soap flotation and de-sliming is one well-established recovery method for spodumene concentration. Alternatively, spodumene may be agitated with anionic collectors, followed by flotation. Oleic acids and soaps tend to produce a superior recovery in neutral and slightly alkaline pulps, while napthanic acids and sulphonated castor oil etc tend to work better in an acid pulp.
To convert it to globally traded lithium carbonate, the concentrate is heated to over 1,100°C in a rotary kiln, before being crushed and treated with sulphuric acid. The resulting solution is first neutralised with limestone then treated with soda ash (sodium carbonate) to produce a lithium carbonate solution. Next, this solution is evaporated prior to the addition of more soda ash to precipitate lithium carbonate.
Lithium brine deposits are formed via the leaching of volcanic rocks in basin depositional environments. Salar brines can be described as underground reservoirs that contain high concentrations of dissolved Group 1 metal salts, such as lithium, potassium and sodium, and are generally found below the surface of dried lakebeds (particularly in South America).
Lithium is extracted from brines via a process that involves pumping the brine from the sediment basin and then concentrating it via solar evaporation over a number of months or even years. Potassium is often harvested first from early ponds, while later ponds have increasingly high concentrations of lithium. When the lithium chloride in the evaporation ponds reaches an optimum concentration, the solution is pumped to a recovery plant where filtering removes any unwanted boron and/or magnesium. Purification then occurs via solvent extraction, absorption and ionic exchange with sodium carbonate (soda ash) to precipitate refined lithium carbonate (Li2CO3). Alternatively, lithium chloride is a convenient intermediate salt from which to directly produce lithium hydroxide. Finally, excess residual brines are then pumped back into the salar. Since salar brines naturally occur at high altitudes and in areas of low rainfall, solar evaporation is a very efficient method for precipitating salts and it has been estimated that the cost of extracting lithium from such sources may be half of that from hard rock sources.
Advantages of spodumene processing
Notwithstanding their typically higher cost structure, pegmatite-based projects benefit from being quicker to move into production than brines, which may have a lead time of 1.5–3 years from the start of commercial extraction, depending on evaporation rates. Another key advantage of lithium production from hard rock deposits is the purity of the lithium carbonate produced. While the battery industry requires a minimum purity of at least 99.5% lithium carbonate, the composition of the remaining 0.5% is important and commercial penalties are often imposed for lithium carbonate containing enhanced levels of deleterious elements, such as iron, magnesium, etc.
Disadvantages of spodumene processing
In contrast with salar brine sources (see above), recovery of lithium from hard rock deposits, such as spodumene, requires a wide range of hydro-metallurgical processes. Pegmatite ores containing spodumene always contain other minerals such as mica, feldspar and quartz and iron and other silicates that have a tendency to concentrate with the spodumene. Problems associated with spodumene recovery therefore include the degree to which weathering has occurred and the presence of associated gangue minerals. Weathered mineral surfaces must be thoroughly cleaned before selective flotation. In addition, weathering and surface oxidation of the rocks may also give rise to alteration products that interfere with the flotation process. Gangue minerals may interfere with selective flotation, as well as consuming process reagents.
Lepidolite as an alternative source of lithium to spodumene
Lepidolite is a lilac-grey or rose-coloured member of the mica group of minerals with the formula K(Li,Al)3(Al,Si,Rb)4O10(F,OH)2. It is a phyllosilicate mineral and a member of the polylithionite-trilithionite series. Compared to spodumene’s 8.03%, lepidolite theoretically contains 7.70% Li2O. Despite being an abundant lithium-bearing mineral, it is a secondary source of the metal, with only a few, small-scale producers in the western world exploiting lepidolite for use in the ceramics industry. Otherwise, it is also produced in China as a precursor to the production of lithium, albeit via a commercially inefficient roasting process. Consequently, there has been little or no global exploration for lithium-bearing micas, with the result that the best potential assets remain un-investigated, even at surface. Notable occurrences have been reported in Brazil, the Ural Mountains, California, Manitoba (the Tanco mine, an underground caesium and tantalum mine, owned and operated by Cabot Corporation, which is the world’s largest producer of caesium), Madagascar, the Iberian Peninsula and Zimbabwe.
Zinnwaldite was first described in 1845 in Zinnwald (Cinovec) on the German-Czech border and is a silicate mineral also in the mica group. Chemically, it may be described as potassium lithium iron aluminium silicate hydroxide fluoride with formula KLiFeAl(AlSi3)O10(OH,F)2. It occurs in greisens, pegmatite and quartz veins often associated with tin ore deposits and is commonly associated with topaz, cassiterite, wolframite, lepidolite, spodumene, beryl, tourmaline and fluorite. Compared to spodumene’s 8.03% and lepidolite’s 7.70%, zinnwaldite theoretically contains 3.42% Li2O.
Other sources of lithium, to which Lepidico’s L-Max technology may prove to be applicable, include amblygonite (7.4% Li2O).