New ideas in the field of lithium extraction from fly ash

1、 Overview
Coal energy plays an important role in both daily life and production, and is a globally important source of energy and chemical raw materials. Coal resources in China have the characteristics of large reserves and wide distribution. The majority of energy supply in China is obtained through burning coal. Fly ash is the main industrial waste after coal combustion, and it is currently one of the largest industrial solid waste emissions in China. A large amount of fly ash has caused serious harm to the environment. At present, the recycling and utilization of fly ash mainly focuses on low value-added fields such as preparing cement, concrete building materials, and synthetic zeolite. In addition to containing a large amount of elements such as aluminum and silicon, fly ash also contains abundant rare metal elements such as lithium, rubidium, and gallium. By extracting and utilizing these rare metals, high value-added utilization of fly ash can be achieved. If efficient lithium extraction from fly ash can be achieved, it will provide a new way to source lithium resources while solving the problem of fly ash pollution.

2、 The forms of lithium in fly ash
The type of lithium deposit in coal belongs to sedimentary lithium deposits. Due to the low atomic number and low content of lithium in coal, it is difficult to study the reasons for lithium enrichment in coal. Lithium in coal mainly exists in clay minerals as a carrier of silicate substances, and a small portion also exists in phosphate or organic forms. The common clay mineral carriers for lithium in coal include kaolinite and lithium chlorite. The lithium content in most coal in nature is not high. In special geographical environments, the grade of lithium deposits in coal measures can reach a level equivalent to that of traditional strategic metal deposits. The coal extracted from these high lithium content coal fields is enriched with lithium after combustion, providing feasibility for extracting lithium from fly ash.
3、 Extraction process of lithium from fly ash
3.1 Preprocessing
The mineral composition of fly ash is mainly composed of mullite, corundum, vitreous and quartz, with lithium mainly occurring in the vitreous. Its chemical composition is relatively complex, mainly consisting of SiO2 and Al2O3, as well as oxides of metals such as iron, calcium, magnesium, titanium, and other unburned carbon. Before carrying out the roasting step, it is necessary to pretreat the fly ash, including two steps: desilication and magnetic separation. The purpose of desilication is to improve the utilization rate of silicon in fly ash while reducing the amount of low value silicon containing solid waste and the material flow rate in the process. The purpose of magnetic separation is to remove iron oxides from fly ash. After the above two steps, the relative content of lithium in fly ash has increased, and the activity of fly ash has also been improved, thereby achieving more efficient lithium extraction from fly ash.
3.2 Baking
Roasting refers to the reaction of pre treated fly ash with specific sintering agents under high temperature conditions, and the roasted fly ash is further activated. Common sintering agents include sodium carbonate, calcium carbonate, etc.
3.3 Extraction
The roasted fly ash is leached with acid or alkali, and the lithium ions in the fly ash are transferred to the leaching solution, further enriching the lithium ions in the fly ash. The acid method involves cooling the sintered activated fly ash and acidification roasting it with sulfuric acid. The roasted sample is then acid leached with hydrochloric acid. Under the optimal experimental conditions, the lithium extraction rate of this process is 96.69%. The alkaline method involves using sodium carbonate as a sintering agent to activate fly ash, and then transferring it to an alkaline solution for alkaline leaching for 2 hours. Under these conditions, the leaching rate of lithium is 85.30%.
3.4 Lithium deposition process
A. The principle of carbonate precipitation method is to add one or more appropriate precipitants to the lithium enrichment solution for precipitation to obtain lithium carbonate, and then further purify the lithium carbonate. After removing impurities and precipitating aluminum from the acid leaching solution, the obtained lithium mother liquor is further evaporated, concentrated, and crystallized to purify impurities. After adding a precipitant, lithium containing carbonate precipitate is obtained. After carbonization, the alkaline leaching solution is concentrated by evaporation, and finally lithium is precipitated.
The advantages of carbonate precipitation process are simple operation and mature technology. However, due to the complex metal ion impurities in fly ash, co precipitation of metals may occur during precipitation. The cost of separation and purification is high, which affects the economic benefits of the process.
B. Adsorption method
The adsorption method mainly uses ion sieves or various different resins to adsorb and extract lithium from alkaline solutions of fly ash.
C. Solvent extraction method
The principle of extraction method for lithium extraction is based on the different solubility of lithium ions in two immiscible solvents, allowing lithium ions to transfer from solvents with lower solubility to solvents with higher solubility. The solvent extraction method has advantages such as continuous operation, high separation rate, and simple equipment. However, there is a problem of some organic extractants polluting the environment, and continuous optimization and improvement are needed to improve the selection of extractants.
4、 Summary
Extracting lithium from fly ash can not only improve the production and supply capacity of China's existing lithium resources, but also provide ideas for solving the environmental pollution problem caused by fly ash accumulation. In acidic environments, currently more mature lithium extraction processes include solvent extraction, hydrochloric acid leaching, and adsorption. The use of extraction method for lithium extraction in acidic media poses challenges such as high requirements for extraction materials, high toxicity, and strong corrosion to equipment. Therefore, from an environmental perspective, the extraction method for lithium extraction in acidic media poses serious problems of pollution and consumables. Under alkaline conditions, there are mainly two methods: precipitation and adsorption. Compared with other technologies, the precipitation method technology is relatively mature, but due to the high amount of metal impurities in it, co precipitation is prone to occur, leading to separation difficulties. With the rapid growth of global lithium resource consumption, fly ash lithium extraction is expected to become a new source of lithium resources.