Coal Washery Wastes as a Rare Earth Element Source

04.03.2019

Abstract

Rare earth elements (REE) are known as industrial vitamins and have wide application in all areas of life. The need for REE in the world is increasing day by day. REE is not “rare” as its name implies. The contents in Earth’s crust are more than some elements (exp. gold, silver etc.). However, the mining of these metals is very difficult because it is not unusual to find them at sufficiently high concentrations for an economic recovery. In addition to the new or existing reserves of REE, both recovery from the recycling products and obtaining as a by-product from other mineral sources is one of the issues to be investigated. It is known that coal contains REE in addition to many trace elements, and many of these elements present together with inorganic matter in the coal. The waste amounts of coal washing plants in our country is too much and will become a bigger problem in the future. The evaluation of such wastes, which have great economic value due to the precious metals in their structure, with recycling processes will create a great added value to economy and encourage both academic institutions and investors with the studies to be carried out on these issues.

Topic

A total of 17 metallic elements consisting of 15 lanthanides and yttrium and scandium between atomic numbers 57 (lanthanum) and 71 (luteites) are called as REE.Due to their use in a wide range of high-tech products the REE is referred as the strategic and indispensable elements of the century. Materials that contain REE as additives are stable, high temperature and corrosion resistant light materials. With these characteristics, REE are used in computer, hybrid vehicles, rechargeable batteries, mobile phones, flat television screens, laptops, wind turbines, medical imaging devices, radar systems, catalytic converters, corrosion resistant metal alloys, aircraft engines, medicine, ceramics, glass and petroleum refining. The production, enrichment and adaptation of the enriched concentrates to the intended use requires knowledge, technology, experience and most importantly expertise. In the 1980’s, REE production increased significantly in keeping with the rapid development of technology. This continuously rises regularly due to the world economy, and in the years of economic crises, the production rate decreases. China produces a large portion of samarium cobalt (SmCo) magnets with neodymium-iron-boron (NdFeB) used in the world. World REE production is approximately 130x103 tons/year and the monetary value of this production is around 4x109 $/year and China performs 95% of the world’s REE production alone. As long as people desire to live more comfortably, the demand for REE will also increase (Yıldız, 2016).

The increase in the variety of metal used in the industry and the high price of the developed technological products directed the countries to investigate their underground resources more effectively and to supply the mines necessary for their domestic consumption either from their own resources or through imports.

In parallel with the developing industry in the world, our rapidly growing and developing country must increase the diversity of the mineral used in the industry by identifing and extracting our underground resources more effectively. The use of fuels such as coal and petroleum, which are energy sources due to the increase in world energy demand due to population growth and technological developments, still maintain its importance. Despite the increasing interest in renewable energy sources, the global energy share of coal continues to increase. As expected from a substance with a long and diverse geological background, coal contains many of the elements in the periodic table. The researchers note that the rare elements found in coal ashes have very different chemical properties. These elements include 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, tellurium. In most cases, all or most of these elements are concentrated in the ashes. In literature, there are studies on the recovery of REE. They can be found in high content in the flue ash and generally obtained as a result of the burning operations in thermal power plants.

The minerals in which the REE is located are passed to fine grains or gas phase and dragged to chimney and are contacted with a porous structure. Similarly, as a result of coal burning, the concentration of REE in the inorganic product that does not burn in the furnace base ash increases.

During burning, the minerals in which the REE is located are drained from the chimney by becoming fine grains or by passing to the gas phase and they are in contact with the fly ash with a porous structure. Similarly, the concentration of REE in the inorganic product that does not burn in the furnace base ash increases (Gupta and Krishnamurthy, 1992). The selection of primary energy sources from domestic or imported sources may change the economic balances. Because of the strategic importance of energy in terms of economics, as far as possible, domestic resources have to be used. The coal extracted from the mines, as well as coal grains of various sizes, contains also quartz, clay minerals, carbonate minerals, sulfur minerals, etc. as inorganic materials. These impurities in the coal must be removed for the production of coal with low ash and sulfur content. In this way, a saleable product is obtained that is desirable in the industry and the air pollution is minimized. In parallel with the use of coal, the waste amount also increases. The wastes produced during coal washing and preparation could have different characters. In coal washeries, coarse grained wastes are usually stored in open pits, while wastes in the form of more aqueous and fine grained sludge are transported to waste dams with and without filtered.

In general, all wastes generated from the coal washery process are called as coal waste or schist. The recovery of coal in these residual materials reduces waste stock problems and cost. After all, the coal is a source of energy. The production of REE from coal extracted by direct mining activities is not a logical way.

Since REE is known to be concentrated in the non-combustible inorganic part, it would be of great benefit to obtain these values in the residues resulting from coal washing processes by appropriate methods. In the literature, there are some limited studies about recovery of REE as a secondary source from coal by-products using physical and physicochemical methods such as specific gravity difference, magnetic separation, and flotation. As it is well known, Mineral Processing provides both technological and economic benefits at the point of gaining metals from primary and secondary sources, and the produced raw materials are enriched with suitable methods and then sent to metallurgical processes. This provides both economic and technological benefits. The recovery of REE from alternative sources, such as coal by-products and residues, has the potential to sustain the economic applicability of coal mining, while ensuring the reliable supply of high-tech materials. The advantages of using coal by-products as raw materials for REE production are as follows; (1) large and reliable sources, (2) already mining production (no new mining license required), (3) the potential environmental and health benefits, as they are not a newly extracted mine, and (4) the use of generated waste. The average total REE concentration in the world is approximately 68 ppm in coal. At the Far East coal mines in Russia with a content of 300 to 1000 ppm, in the East Clayton Fire Clay coal bed at a content of 500 ppm, and in the Sydney Basin of Nova Scotia, Canada, it has a high content of REE content of 72-483 ppm are found in coal deposits.

The average REE content of Turkish coals is given as 116 ppm and is almost twice the world coal average. In short, in every stage of coal production (extraction, washing and burning), the products can be called as a rich source of REE (Kislyakov and Shchetochkin, 2000; Dai et al., 2008).

Considering the environmental and industrial impacts of coal and REE from coal and coal by-products, which are indispensable, reliable and inexpensive energy sources for our country, it is very important to create technologies and methods that will provide selective production. From this point of view, REE which is inevitable for the economic evaluation, can be served in the landfills and storage ponds. Moreover, these processes, which appear to be a major threat on a national and international basis, will be evaluated as a secondary source and new processes will be produced to reduce environmental impacts. Coal residues with the high content of REE waits in the waste sites and storage pools. Primarly carbon production from the coal schists and then recovery of the REE from residual product is very important and these significant values will serve the country’s interests.

Assoc. Prof. Dr. Fırat Burat Istanbul Technical University Faculty of Mines Mineral Processing Engineering Department Recycling, Separation and Enrichment Group