Coal Washing Waste as a Source of Rare Earth Elements

Summary

Rare earth elements (REE) are known as the vitamin of industry, and demand for these elements is growing daily. Despite the term "rare" in their name, they are not rare. In the Earth's crust, most of them are found in greater quantities than copper, gold and silver. However, to recover these values economically, they must be found in quite high concentrations, and therefore mining these metals is very difficult. Recovery of REE from new or existing reserves, as well as from recycled products and as by-products from other mineral sources, is one of the subjects that requires investigation. Coal is known to contain REE along with many trace metals, and most of these elements are found together with inorganic minerals in coal. In our country, the quantities of coal washing plants are now very large and will become an even greater problem in the future. Due to the valuable metals they contain, evaluation of these waste types through recovery processes will create significant added value, and work in these areas will encourage both academic institutions and investors.

Subject

Atom REE refers to a group of 17 metallic elements consisting of 15 lanthanides with atomic numbers between 57 (lanthanum) and 71 (lutetium), together with yttrium and scandium. Due to their use in a wide range of advanced technology products, REE are referred to as the strategic and indispensable elements of the century. Materials containing REE as additives are stable, high-temperature and corrosion-resistant lightweight materials. With these properties, REE are used in computers, hybrid vehicles, rechargeable batteries, mobile phones, flat-screen televisions, laptops, wind turbines, medical imaging equipment, radar systems, catalytic converters, more corrosion-resistant metal alloys, aircraft engines, medicine, ceramics, glass production, and oil refining. Production of REE, enrichment, and conversion of enriched concentrates to their intended use require knowledge, technology, experience and, most importantly, expertise. REE production increased significantly in scale starting in the 1980s with rapid technological development. This increase progresses regularly in line with the global economy, and production rates decline during years of economic crisis. China produces a large portion of the world's neodymium-iron-boron (NdFeB) and samarium cobalt (SmCo) magnets. World REE production is approximately 130×103 tonnes/year, with the monetary value of this production around USD 4×109/year, and China alone accounts for 95% of world REE production. As long as people desire more comfortable living, demand for REE will continue to increase (Yıldız, 2016). With developing technology, the increase in the variety of metals used in industry and their accompanying price increases, countries are being directed to research their own underground resources more effectively and to procure the minerals needed for domestic consumption either from their own sources or through imports. In parallel with the developing global industry, our country, which is rapidly developing and growing, will be able to increase the variety of minerals used in industry only through more effective identification and extraction of our underground resources.

As a result of increasing world energy demand due to population growth and technological developments, the use of fuels such as coal and oil as energy sources remains important today. Despite increasing interest in renewable energy sources, coal's share of global energy continues to grow.

As expected from a material with a long and varied geological history, coal contains many elements from the periodic table. Researchers point out that rare elements concentrated in coal ash can have very different chemical properties. Among these elements are beryllium, strontium, barium, boron, scandium, yttrium, lanthanum and lanthanides, zirconium, vanadium, cobalt, nickel, molybdenum, uranium, copper, zinc, gallium, germanium, arsenic, antimony, cadmium, tin, iodine, lead, bismuth; silver, gold, rhodium, palladium, platinum, thorium, indium, thallium, selenium, and tellurium. In most cases, all or most of these elements concentrate in the ash. In the literature, there are generally studies on the recovery of REE that can be found in high concentrations in fly ash obtained from combustion processes in thermal power plants. As a result of coal combustion, minerals containing REE pass into the gas phase in fine particles or are carried away by the exhaust and come into contact with porous fly ash. Similarly, as a result of combustion processes, the concentration of REE increases in the incineration bottom ash within the unburned inorganic product (Gupta and Krishnamurthy, 1992). The selection of primary energy sources from domestic or imported sources is such that it will change economic balances. Because energy has strategic importance from an economic perspective, it is necessary to turn to domestic sources to the extent possible. Coal extracted from mines contains, along with coal particles of various sizes, various types and quantities of inorganic substances such as non-coal quartz, clay minerals, carbonate minerals, sulphur minerals, etc. For the production of coal meeting the properties required by industry and with low ash and sulphur content that does not cause air pollution, it is necessary to remove these impurities present in coal. In parallel with coal use, the waste generated is also increasing. Residues formed during coal washing (washing) and preparation have different characteristics. In coal operations, coarse-grained residues are generally stored in open piles, while more aqueous and fine-grained mud-like residues are sent directly to waste impoundments or after filtration.

In general, all residues resulting from coal processing are called coal residue or shale. Recovery of coal contained in these waste materials will reduce waste storage problems and costs. As a result, coal in the residue is an energy source.

Direct REE production from coal extracted through mining activities is not a logical approach. Since it is known that REE concentrate in the unburned inorganic portion, recovery of these values from residues resulting from coal washing operations using appropriate methods would provide significant benefits. In the literature, there are some studies, though limited, applying physical and physicochemical methods such as sizing, separation by specific gravity difference, magnetic separation, and flotation regarding REE recovery from coal by-products as secondary sources. As is known, ore preparation and beneficiation provide both technological and economic benefits in recovering metals from primary and secondary sources, and produced raw materials are sent to metallurgical processes after being enriched by appropriate methods. This also provides both economic and technological benefits. Recovery of REE from alternative sources such as coal by-products and residues, while ensuring reliable supply of advanced technology materials, carries the potential to make coal mining economically sustainable.

The advantages of using coal by-products as feedstock for REE production are as follows: (1) large and reliable sources, (2) already being mined (no new mining licence required), (3) providing potential environmental and health benefits as they are not newly extracted ores, and (4) utilization of potential waste products.

The average total REE concentration worldwide in coal is approximately 68 g/t. In the Far East coal mines in Russia, there are coal deposits with content of 300 to 1,000 g/t, in the Fire Clay coal deposit in Eastern Kentucky approximately 500 g/t, and in the Sydney Basin in Nova Scotia, Canada, coal deposits with high REE content between 72-483 g/t. Turkish coals have average REE contents of 116 ppm, which is almost twice the world coal average. In short, products resulting from every stage of coal production (extraction, washing, and combustion) are a rich source in terms of REE (Kislyakov and Shchetochkin, 2000; Dai et al., 2008). It is very important for our country to develop technologies and methods that will enable selective recovery of coal and REE from coal and coal by-products, an indispensable, reliable and cheap energy source for us, taking into account environmental and industrial impacts. From this perspective, coal shales—which must be evaluated economically, stand ready for processing in waste dumps and storage pools with their high REE content, and are in a sense REE mines—should prioritize carbon recovery first, followed by recovery of these important values, which will serve the national interest. Moreover, evaluation of these waste materials, which are seen as a major threat on national and international basis, as a secondary source will produce new processes that reduce environmental impacts. Assoc. Prof. Dr. Fırat Burat Istanbul Technical University Faculty of Mines Department of Mineral Processing Engineering Recovery, Separation and Beneficiation Research Group  

References •Dai, S., Y. Jiang, C. R. Ward, L. Gu, V. V. Seredin, H. Liu, D. Zhou, X. Wang, Y. Sun, J. Zou, and D. Ren. 2012. "Mineralogical and geochemical compositions of the coal in the Guanbanwusu Mine, Inner Mongolia, China: Further evidence for the existence of an Al (Ga and REE) ore deposit in the Jungar Coalfield", International Journal of Coal Geology, 98, 10–40. •Gupta, C. K., Krishnamurthy, N. 2016. "Extractive metallurgy of rare earths", International Materials Reviews, 37, 197-248. •Kislyakov, Ya.M., Shchetochkin, V.N., 2000. "Hydrogenic Ore Formation. Geoinformmark, Moscow. •Yıldız, N. 2016. "Nadir Toprak Elementleri", ISBN 978- 605 -01- 0912-2, Maden Müh. Odası yayını, 74 sayfa.