Conversion Processes of Spodumene
The critical step of the process has always been considered to be the extraction step. Since the beginning of the selective extraction of lithium, decrepitation has been considered a minor step of the process where only minor improvements could be made. Hence, there is very little data on the behavior of spodumene with regards to the granulometry of the sample, the temperature of decrepitation, etc. There has been very little research on how to improve the decrepitation step of spodumene, many considering that the furnace heating in a rotary kiln is optimized. An equation describing the conversion of spodumene was derived from the Avrami equation and is presented below.
1− ω=exp(−Kt)
Where ω is the converted spodumene, K the rate of conversion and t the treatment time. This equation was developed from the Avrami equation and is considered an approximation of the phase transition, since the kinetic model only considers one molecule. The results presented by this study suggest that above 950 °C, the phase transition is almost instantaneous. The downside is that it does not consider the γ-spodumene. A compilation of spodumene studies shows that the decrepitation temperature differs greatly between the samples, with temperatures ranging from 950 to 1050 °C. Moreover, it was found that the temperature range suitable for the decrepitation becomes narrower with the increasing amount of impurities. This was further later confirmed using the Delmon theory. According to this theory, the activation energy of the spodumene conversion is 296 kJ/mol ± 6 kJ/mol and is independent of the rate of impurities. This value fits the results found from the Avrami equation between 950 and 1050 °C. It was then concluded that the temperature of conversion was indeed influenced by the amount of impurities in the spodumene.
Despite the general consensus that the traditional method of decrepitating spodumene is the best, there has recently been some research about alternative methods to the rotary kiln. One still uses traditional heating but emphasizes on the grinding of the spodumene beforehand. It is stated that grinding cleaves the Li–O, Al–O, and Si–O–Si bonds from the tetrahedral site. It shows that the α-spodumene turns into γ-spodumene between 700 and 900 °C before transitioning into β-spodumene between 900 and 950 °C. This would mean a decrease in the energy necessary to decrepitate the spodumene but does not consider the amount of energy needed to grind the material. One of the few alternative processes uses microwaves to decrepitate spodumene ore. This study found that the critical temperature for α-spodumene to start absorbing microwaves is about 634 °C. But it also states that the temperature is not uniform, with measures ranging from 650 to 1250 °C depending on the area of measurement. A significant amount of β-spodumene melts using this method, while the intermediate layer is composed of β-spodumene and γ-spodumene and the top layer is still in α-form. Hybrid microwaving has also been studied using SiC susceptor tubes. It can allow the decrepitation to occur, despite a higher temperature of 1197 °C. The upside of this method is a shorter treatment time of only 170 s. It also shows that γ-spodumene is formed if the temperature is not high enough and that this method also melts the concentrate. When it comes to the melting of the samples, it has been reported that the behavior of a spodumene sample differs greatly based on the impurities in the sample and their quantity, with melting temperatures ranging from 1000 to 1400 °C. For example, the melting point of the spodumene can go down as low as 930 °C if the sample is made of 50/50 α-spodumene/(Quartz–Albite–Microcline 10–20–20).
If the decrepitation of the spodumene was studied, the effect of the phenomena on the size of the particle is not mentioned. It has been proven that decrepitation was able to grind coarse particles (size ranging from 2 mm to 2 cm) into micrometric particles suitable for acid roasting. This study shows that heating a coarse spodumene concentrate at 1050 °C for 30 min decrepitates the α-spodumene into β-spodumene and γ-spodumene with 65% of the initial mass becoming smaller than 180 µm and that some impurities were not affected by the thermal treatment, making them easy to remove from the converted concentrate. This study was realized without prior grinding or flotation of the mineral and found out that the lithium in the finer fraction was being extracted at a 99% yield using the traditional method, meaning that the need for grinding and flotation could be reduced, thus saving time and energy.
As can be seen above, when it comes to research about the decrepitation of the spodumene, γ-spodumene is taken into account in the recent studies while this is not the case with lithium extraction and older decrepitation studies. This shows that despite the disappearance of α-spodumene, the conversion of spodumene does not give only β-spodumene. It could mean that the limitations may not have originated only from the sulphuric acid behavior toward β-spodumene but also from the behavior of γ-spodumene itself.
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