When it comes to the actual phase conversion of spodumene, the process has been known since the 1950s. The process has been patented and, still nowadays, heavily dominates the lithium production industry. This process starts with the crushing of spodumene ore. The cause behind the grinding of the spodumene is an acceleration of the heat transfer between the surrounding atmosphere and the mineral. The crushed mineral is then heated in a furnace at, at least, 1000 °C for 30 min. It is stated that almost any kind of furnace will do for this part of the process. The thermal treatment will allow the α-spodumene to decrepitate into β-spodumene. However, nowhere in this process is it stated that γ-spodumene exists. Therefore, the data concerning the spodumene phase transitions is not complete. This process is stated to be exclusive to spodumene. The other lithium-bearing minerals being impossible to decrepitate using this method. It was the first process to efficiently extract lithium from spodumene (85% to 90% lithium yield at the time) and was scaled up shortly after. The lithium extraction went from total digestion of minerals such as lepidolite (K(Li,Al)3(Si,Al)4O10(F,OH)2) or amblygonite ((Li,Na)AlPO4(F,OH)) followed by complex purification to selective extraction of lithium.
The process is based on the higher reactivity of β-spodumene towards sulphuric acid. The acid is brought into contact with the β-spodumene and heated at about 250 °C. It is reported that the temperature can go as low as 200 °C but cannot reach higher than 300 °C, temperature at which the sulphuric acid starts to decompose. The acid excess must be at least 30% to ensure the availability of the protons after reactions with impurities such as potassium or sodium. Depending on the grade of the ore, acid excess can go up to 140%. The reaction between the sulphuric acid and the spodumene is presented below.
2 LiAlSi2O6 (s)+ H2SO4 (l) →2 HAlSi2O6 (s)+Li2SO4(s)
After reaction between the acid and the concentrate, the lithium sulphate is leached by water in which it dissolved while leaving the leached concentrate in its solid state. The lithium sulphate can thereafter be precipitated as is, or transformed into lithium chloride or lithium carbonate.
The authors of this patent have come to three statements, all of them being in favor of a diffusion mechanism allowing the protons to diffuse through the β-spodumene to allow the cationic exchange.
1. α-spodumene is almost completely resilient toward acid roasting contrary to β-spodumene
2. β-spodumene density is significantly lower than that of α-spodumene.
3. The structure of the leached β-spodumene is very similar to that of β-spodumene.
Hence, β-spodumene has a more open structure. This structure would allow the diffusion of ions through its matrix via a pseudo-Brownian movement. This statement was later confirmed by crystallographic studies which confirmed that the structure of β-spodumene presents pseudo-zeolithic channels in which protons and lithium cations are free to move. The aluminosilicate portion of β-spodumene is in fact isostructural to keatite, which presents those channels. An important heat production is observed during the acid roasting around 175 °C. This exothermic reaction is linked to the formation of liquid lithium bisulfate (LiHSO4) as a reaction intermediate since it has a melting point of around 170 °C.
This process was so efficient and easy to implement that it has been considered the one and only method of extracting lithium from spodumene in the lithium industry. From there, two major steps can be pointed out. The first one is the lithium extraction from β-spodumene and the second is the decrepitation of α-spodumene.
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