Extraction of lithium from spodumene

Spodumene is an important source of lithium, a key element of Li-ion batteries used in mobile communication and entertainment devices, hybrid and all electric cars and electric bikes. Spodumene forms three different crystal structures, the naturally occurred α- spodumene, and y and β-spodumene which are the products of heat treatment at 700 to 1100 °C. Among these three modifications β-spodumene proven suitable for lithium extraction processes, hence the production of these phases should be closely monitored in feed preparation stage. In order to extract lithium from spodumene, β form of the mineral goes through acid roasting with concentrated sulfuric acid at 250 °C. This method remined unchanged for almost 50 years and limited information on the details of the process and effect of key factors is available. This study will investigate the preparation of suitable spodumene phase for extraction process exploiting two different types of heating. Then focuses on traditional acid roasting of spodumene and the key factors of the process and further on proposing a less energy intense method of acid roasting using microwaves. Spodumene concentrate of the highest purity from Greenbushes Western Australia was studied for mineralogical changes with temperature in muffle furnace. The feed sample with particle size of 325 mm was heated at temperatures from 800 to 1100 °C for different durations of time and structural changes were closely monitored. At 950 °C after 30 minutes of heating γ phase appeared on the XRD spectra which detected while β-spodumene was produced, after 2 hours of heating at 1100 °C. The crystal structure altered from monoclinic α to hexagonal γ and finally tetragonal β-spodumene. Physical properties of product heated at different temperatures and times were analyzed. The significant change was related to the particle size. Conversion of α to γ-spodumene is accompanied by shrinkage of the crystal units leading to contraction of particles. Moreover, particles go through substantial expansion with formation of β-spodumene. This leads to cracking of the particles and their dispersion to smaller particles. This phenomenon directly causes the reduction in particle size which increases the specific surface of the sample. Specific gravity of the sample was constantly reduced with order of changes of the crystal structure. All these alterations positively affect the yields of lithium extraction from β over α-spodumene. As an alternative process of calcination, a sample of α-spodumene was subjected tomicrowave and hybrid microwave heating. The sample reached 98 °C after 10 minutes of microwave irradiation with power of 3 kW. This proved that spodumene is categorized in the group of non-absorbers of microwave. Next a hybrid microwave heating set up was designed which applied three SiC sticks to absorb microwave energy and conventionally heat the spodumene sample. After 32 minutes of hybrid microwave heating and temperature increase up to 643 °C a sudden increase in temperature was observed. Due to localized heat some spot of the sample heated up to the melting point of the spodumene and left sintered and/ or melted parts. This suggested that α-spodumene can start absorbing microwave at temperatures above 643 °C. The process was repeated for a sample of synthesized β-spodumene absorption of microwave energy started at 447 °C. This phenomenon made the complete conversion of the sample complicated. As the next part of this study the common method of extraction of lithium from spodumene was studied. Sample of synthesized β-spodumene was mixed with concentrated sulfuric acid and roasted at temperatures between 200 and 300 °C. This process was followed by water leaching at 50 °C for 1 hour. In addition to temperature, the effects of acid dosage and roasting time were investigated. The highest extraction of 98% was achieved after roasting at 250 °C for 1 hour with 80% excess acid to the stoichiometry of the reaction of spodumene and sulfuric acid. Elongated roasting, roasting at temperatures close to boiling point of the acid and very high amount of excess acid negatively affected Li extraction. The residue after water leach was identified as aluminium silicon hydrate (H2O.Al2O3.4SiO2). In order to reduce the energy consumption of the acid roast process application of microwave oven was proposed. Acid roasting of spodumene was replicated in benchtop microwave oven adjusted on 700 W power. Interestingly 96 % of lithium was extracted after 20 second of microwave irradiation in presence of 80% excess acid. After 30 seconds of roasting the extraction reduced and reached 49% after 4 minutes. The residue of the water leach was aluminium silicon hydrate (H2O.Al2O3.4SiO2) for roasting under 30 seconds and after 4 minutes peaks of β-spodumene appeared in the XRD pattern. More studies on the residue showed that the residue has ion exchange properties and at elevated temperatures in presence of lithium the H+ can be replaced with Li+. With slight grinding the excess acid could be reduced to 15%. The energy consumption of microwave acid roasting was 15.4 kJ which was 3 order of magnitude less than the energy consumption of conventional acid roasting. Microwave acid roasting of β-spodumene is a promising method with less energy consumption.