Pyro technology expert Timothy Kyle looks at the process of lithium extraction and the factors that need to be considered when choosing between kiln and flash calciner technologies.
Q: What are the fundamentals of kiln and flash calciner technologies when it comes to lithium extraction?
TK: The ability to recover lithium from spodumene concentrate begins with the process of converting alpha spodumene to beta spodumene in the calcining equipment. This process unlocks the lithium contained within the spodumene concentrate, making it available for extraction after the subsequent acid roasting process (which produces water soluble lithium sulfate). Without proper conversion, the lithium remains entrapped within the structure of the spodumene.
The alpha to beta conversion generally takes place around 1075 C with minor variation depending on the raw material. This conversion temperature can be achieved both in a rotary kiln system and in a gas suspension calciner but the key in either case is the finite control of that temperature in the calcining system and control of the time at the conversion temperature.
The other key is to apply the correct technology based on the particle size distribution of the spodumene concentrate. Both technologies will work when properly designed and controlled, and in that case, we have demonstrated consistently high conversions typically 95% to 99%.
Q: What factors should a customer consider when looking at kiln versus flash calciner technologies? What makes one more suitable than the other?
TK: Both the rotary kiln approach and the suspension calciner effectively convert spodumene. The primary consideration of which approach is best is the concentrate particle size distribution (PSD). Concentrate produced via dense media separation (DMS) often results in feed size from 3mm to as large as 12mm or sometimes larger.
This PSD is too large for a suspension calciner and so the rotary kiln system is better suited for these concentrates. Although it is possible that, even with the appropriate PSD, some concentrates may contain contaminant minerals or have other properties that result in flowability issues or that otherwise prevent full alpha to beta conversion, in which case, a kiln can still be used. The kiln system provides the gradual heating and extended retention time (normally ~30 minutes) needed to effect the conversion on large particles.
Conversely, a gas suspension calciner (GSC) is best suited for spodumene concentrate that has a PSD less than 1mm, which is common for concentrates produced via the floatation process. As the feed size is very small, it takes very little time to heat the sub 1mm particles to conversion temperature and once temperature is reached, the conversion happens almost immediately. Therefore, unlike the rotary kiln process, the retention time of the material in the GSC is only a matter of seconds and yet the conversion is achieved just the same.
Q: What about concentrates that fall in the middle?
TK: For those concentrates with significant fraction less than 1mm and also a significant fraction above 1mm, we can look at several different approaches. One approach is to screen the concentrate and then grind the +1mm fraction to less than 1mm and utilise a gas suspension calciner. This involves a grinding step at the beginning of the process but allows for the most efficient thermal conversion.
Alternatively, we can design a rotary kiln type system without any pregrinding in a configuration suitable for processing the feed containing both coarse and fine particles. This is done by altering kiln size to lower gas velocities, altering preheater cyclones for increased efficiency and other steps to prevent excessive dust losses and recirculating loads. It then becomes a matter of determining which approach is the most efficient in terms of CAPEX and OPEX, and reaching a consensus with the customer.
Q: What is the process involved in flash calcination?
TK: For customers dealing specifically with fine ores, we designed the Gas Suspension Calciner (GSC) technology – something we have supplied since the beginning of the 1970s. It consists of a series of preheating cyclones, a calciner vessel and a series of cooling cyclones.
Hot air is introduced into the system via an external air heater. The raw feed is introduced at the top stage preheat cyclone, entrained in the hot gases, and carried through subsequent preheat cyclones after which it enters the calciner vessel for final conversion. Additional fuel is directly injected in the calciner vessel to complete the conversion in seconds.
The turbulent swirling mixture of combustion gases, fuel and material produces a highly uniform temperature profile throughout the calciner. Processed ore and gas continues on to several cooling cyclones after which the ore reports to a fluid bed cooler for final temperature control and dust reports to the main baghouse after which it is recovered and put back in the process.
The number of stages in the system is custom designed based on the materials being processed by the customer, as well as process requirements, thermal and system efficiency optimisation and capacity. The key advantages of the GSC (relative to a rotary kiln) are very low specific energy consumption due to full heat recovery, limited maintenance due to no moving parts, small footprint, and low energy consumption.
Q: What is the process involved in rotary kiln calcination?
TK: Having provided well over 6,000 rotary kilns in our history, FLSmidth has a solid claim to be the world leader and for spodumene conversion, our tried and tested approach is one where we integrate two to three stages of preheat cyclones ahead of the rotary kiln inlet, a rotary kiln, followed by a rotary cooler.
The wet spodumene concentrate (normally ~6% moisture) is fed into the top stage cyclone where it is dried after which it passes through the other cyclone(s) and is preheated prior to entering the rotary kiln. In this way, we recover the heat from the hot kiln gas and reduce the size of the kiln. The preheated spodumene then enters the kiln where the temperature gradually increases to the conversion temperature.
Once converted, the hot beta spodumene then discharges into a rotary cooler where the temperature is reduced via a combination of direct and indirect water cooling. It is then discharged and sent to the next step in the process. The rotary kiln system is well suited for converting larger particle sizes and the integration of the cyclone preheater greatly improves specific energy consumption over just a straight kiln. It also reduces the size of the kiln and makes treatment of off gases very simple because the heat is removed prior to treatment.
Q: Once the spodumene is converted, what happens next and what solutions are available to customers?
TK: While there can be some variation, the most common subsequent step in the process is a step called acid roasting, in which the beta spodumene is mixed with concentrated sulfuric acid, roasted in an indirect fired kiln and converted into water soluble lithium sulfate.
Following acid roasting, FLSmidth Hydromet take over the process, which involves impurity removal and extraction. FLSmidth Pyromet offers the complete pyroprocessing system.
Downstream of the calcined spodumene cooling equipment, we can provide a ball mill system to prepare the beta spodumene for acid roasting, Pneumatic Transport systems to transfer the beta spodumene, mixing equipment to combine it with the acid and the indirect fired acid roaster unit for the conversion to lithium sulfate, the rotary cooler and all necessary gas treatment equipment.