Recovery of Coal from Coal Enrichment Plant Waste and Its Use for Energy Purposes

Fossil fuels (petroleum, natural gas, coal) are not only energy raw materials; they are also raw materials from which the main inputs of many industries (paints, plastics, pharmaceuticals, cosmetics, iron-steel, aluminium, etc.) are produced. Coal is a mineral and rock with chemically and physically different structure, containing small amounts of sulfur and nitrogen, consisting mostly of carbon, hydrogen and oxygen. Its other contents are inorganic compounds and mineral substances that form ash. With its widespread occurrence on earth, its production, and the longevity of visible coal reserves at current production levels compared to other fossil fuels, price stability, ease of transport, storage facilities, safe and reliable use, cheap and continuous supply to users, coal is an indispensable energy source. Coal will continue to have an important role in the future as in the past with regards to sustainable development and energy security. Because it is a cheap and competitive fuel in electricity generation, approximately 41% of world electricity production is supplied from coal. Other uses are concentrated in heating, iron-steel and cement sectors. Therefore, coal is the most widely used fuel among fuels used for electricity generation. It is estimated that coal's quality of being the most highly used fuel in electricity generation will not change in a foreseeable future.

As energy demand increases, coal continues to be seen as a reliable energy source and developing countries like Turkey in particular plan to increase their consumption.

In our country, in order to reduce foreign dependency in electricity production, increasing the share of domestic resources, accelerating exploration activities, and producing clean fuel using advanced coal technologies have been included in 2023 targets. There is a necessity to increase production levels to reduce the coal (especially lignite) deficit that exists today and will grow to increasingly large proportions in the coming years. Coals show great variety with regards to coalification process and deposition, moisture content, ash and volatile matter content, fixed carbon amount, sulfur and mineral substance contents as well as geological, petrographic, physical, chemical and thermal properties. In Turkey, coal is generally evaluated under the headings of lignite and hard coal, with hard coal reserves managed by TTK, and lignite reserves managed by EÜAŞ, TKİ and the Private Sector. In our country, although natural gas and petroleum reserves are quite limited, there are approximately 1.3 billion tons of hard coal with 506 million tons visible, and total 14.2 billion tons of lignite reserves of which 13.9 billion tons are classified as visible reserves. As a result of exploration and reserve development activities conducted in recent years, significant reserve increases have been achieved. Work on finding new fields and increasing reserves is being carried out by MTA.

Coals extracted from mines contain coal particles of various sizes as well as non-coal inorganic substances of various types and amounts.

Non-coal substances found in run-of-mine coal can be mineral substances found within coal seams and resulting from geological structure (quartz, clay minerals, carbonate minerals, sulfur minerals, etc.), or they can be foreign substances that mix with coal from outside during coal mining. To produce coal with properties desired by industry as well as coal with low ash and sulfur content that does not create air pollution, these impurities present in coal must be removed. Lignites that constitute our country's basic energy source, with high ash and sulfur content and low calorific value, are first removed of inorganic impurities at coal washing plants for electricity generation purposes and then are fired at thermal power plant boilers after being reduced to fine particle size. Coal washing techniques can be simple or more complex and advanced techniques in accordance with the requirements of the consumption area. In coarse coal enrichment, the upper size is selected as 150 or 100 mm according to the opening of the gross screen, and is generally enriched using heavy medium systems and jigs. As fine coal enrichment methods, it is possible to mention heavy medium cyclones, fine coal jigs, shaking tables, water cyclones, chutes, spirals, cones and flotation method. Wastes generated during coal washing and preparation have different characteristics.

In coal operations, coarse-grained wastes are generally stored in open heaps, while more watery and fine-grained slurry-type wastes are sent to waste dams or filtered.

In a typical coal preparation plant, approximately 20% of fed coal and fine-sized waste (shale) completely below 0.5 mm with high coal content is produced, and generally such materials cannot be used due to high processing costs. As coal usage increases, the amount of waste generated also increases and these cause environmental problems. The type, character and discharge limits of these wastes are quite important for successful waste management. Thanks to developing coal cleaning technologies, coal contained in these wastes can be efficiently recovered with low ash and pyritic sulfur contents. Coal in waste is an energy source and its recovery can reduce waste storage problems and cost factors. Generally, for coal recovery in small-size coal washing plant wastes, devices performing physical enrichment such as 1) Coal spiral, 2) Shaking tables, 3) Water cyclone, 4) Centrifugal gravity separators, 5) Hydraulic separators can be used. In addition, alternative methods such as flotation and oil agglomeration are preferred. The flotation method in particular is applied in cleaning coals below 0.5 mm. Clean coal surface showing hydrophobic (non-wetting) properties, that is, coal's natural floatability, facilitates shale and pyrite separation. The lave (clean coal) obtained from the flotation circuit is sent directly to a vacuum filter. Wastes are discharged either to a waste pool or to a thickener to enable water recovery.

Today, especially with developments in flotation technology, it is possible to produce low ash (below 4%) clean coal from coals below 0.045 mm.

As a result of the doctoral research completed in 2011 at the İ.T.Ü. Department of Mineral Processing Engineering and belonging to the responsible author titled "Investigation of the Effects of Ultrasonic and Mechanical Vibration Mechanisms in Dewatering of Small-Sized Coals," it was understood that fine-sized waste material of clay and shale below 0.5 mm, which was discarded as shale at the coal washing plant and stored in waste pools, was separated from coal during dewatering operations performed with screening, and the dewatered material was simultaneously enriched. Given that the ash content of the fed material was 41.07% and its gross calorific value was 2760 kcal/kg, as a result of the operations performed, a reduction of approximately 11.66% in ash content was achieved and an increase of approximately 1337 kcal/kg in gross calorific value was achieved. Thus, while a lave with a gross calorific value of approximately 4100 kcal/kg was obtained, a final shale waste with a gross calorific value of 1050 kcal/kg was discarded. The purpose in all these methods is to ensure the recovery of very small-sized particles created by the crushing of coal, which is an organic rock. Generally, the inorganic material contained within this material (mostly clay, silicate, carbonate, etc.) is discarded as final waste. Coal obtained as a result of these recovery processes is used as fuel for energy production, while the material resulting as waste can be evaluated as a raw material and additive for different industries. These final wastes consisting of inorganic substances are in their natural state and generally contain clay and shales.

For example, if the clay ratio in such wastes is very high, depending on their quality, they can be used in the ceramic and brick industries.

While both obtaining coal for energy production and producing additives and raw materials for various sectors provide economic added value, the environmental impacts of these wastes can be reduced to minimum levels. Because our petroleum and natural gas reserves from fossil sources are small, coal will be given more prominence in the future than today. Not only in terms of reserve size compared to other fossil sources, but the distribution of coal deposits throughout various regions of our country, their extremely positive effects on economic, regional and cultural development, the added value resulting from their operation, cheap raw material per kW/hour in electricity generation and safe transportation, etc., make coal our country's most important fossil energy source. Establishing an energy production-consumption balance based on our own resources in the short and medium term, in other words reducing the consumption share of externally supplied fuels such as coal, gas and petroleum, and applying processes that are respectful to the environment and produce less waste is quite important.   Assoc. Prof. Dr. Fırat Burat Istanbul Technical University Faculty of Mines Department of Mineral Processing Engineering Recovery, Separation and Enrichment Research Group               Dr. Eng. Mustafa Özer Istanbul Technical University Faculty of Mines Department of Mineral Processing Engineering Recovery, Separation and Enrichment Research Group