April 27, 2011 (Coal Geology): Coal preparation is the process of removing undesirable material from the Run-of-Mine (ROM) coal to make it suitable for a specific purpose using various separation processes which are able to differentiate between the physical and surface properties of the coal and the impurities. Coal preparation typically includes crushing, screening, conventional cleaning, deep cleaning, blending and dedusting. The goal of the coal preparation is to produce the desired output as uniform in nature as possible.
In the previous article “Effects of Coal mining on Earth and Human Health”, we have briefly described the potential of contamination coming from the coal preparation plants mainly deteriorating surface and groundwater quality. In this article, we will discuss the coal processing in details and evaluate contamination potential and learn about preventive measures.
Underground coal also includes some amount of roof and floor material. They are commonly referred to as “out of seam dilution”. Continuous miners or longwall shearers normally cut more than the thickness of the coal seam. Some underground mine also require a minimum mine height needed for equipment movements. Certain amount of roof or floor material is cut along with the seam where the seam thickness is than the mining height. Such activities increase the “undesired” content in the ROM coal.
The end user of coal such as power plants often require certain composition of ash content, sulfur, heat content etc. in the coal for optimal combustion. Coal preparation is of vital importance to meet such strict standards.
The coal preparation is important for other reasons too. For example, reduction of ash content also reduces the amount of particulates generated at the power plants. It also reduces the ash handling requirements at power stations. Coal becomes more uniform in nature in terms of heat value and improves plant efficiency following the preparation process.
Crushing: ROM could come from both surface mine and underground mine. Underground mining typically generates finer coals than the surface mining. ROM coal is usually crushed to a finer level suitable for cleaning purpose or direct use.
Screening: Screens are used to group process particles into ranges by size. Dewatering screens are used to remove water from the product coal. Screens can be static, or mechanically vibrated. Screen decks can be made from different materials such as high tensile steel, stainless steel, or polyethelene.
Washability: Present practice of coal preparation favors wet processes over dry methods. The selection of logical coal separation process is derived from washability study of the ROM coal (raw coal). Samples of coal sent to the lab during the exploratory drilling of the reserve area. The raw coals then subject to float and sink analyses using different specific gravity and quality of each fraction of coal is reported.
The main types of coal cleaning process used by the industry today are:
Dense medium separation: process of cleaning raw coal, coarse or fine, by immersing in a fluid that has density intermediate between the coal seam (usually 1.3-1.7 g/cc) and the reject (usually 2 g/cc). In this separation process the desired coal part floats while the rejects sinks at the bottom. Dense medium gravity separation method often use magnetite to form medium denser than water.
Hydraulic separation: Hydraulic separation using jigs are used for coarse coal. The jigging process creates particle stratification arranged by increasing density from top to bottom. Coal stratifies near the top in this process. Cyclones, hydrorotators, feldspar jigs, Baum jig, under-air jig could be used for this process.
Froth flotation: Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several processing industries including coal. Historically this was first used in the mining industry. Froth flotation uses the selective adhesion of coal and refuses material to air and water. Froth flotation follows crushing and grinding which increases the surface area of the coal for subsequent. At the end, the coal separates from the waste material as finely disseminated air bubbles are passed through a feed of coal slurry.
Agglomeration: In this method a bridging liquid is used to agglomerate coal particles in a suspension. The coal is recovered while the aqueous suspension with reject material is disposed.
Contamination from Coal Handling and Preparation Plant (CHPP):
Potential water contamination: The process of coal preparation uses water. The slurry coming out from the CHPP going to the impoundment area contains water too. Mining below the water table also requires water to be pumped out to keep the working section dry (which can be reused at the CHPP). The chemistry of the water depends on the chemical nature of the coal and the immediate overburden material. Elemental analyses and leach test are very important in determine the geochemistry of the coal seam and the refuse. Periodic monitoring of the slurry is also required to evaluate water quality. If any potential problem exists, such as potential for acid formation, the effluent must be treated before allowed to mix with the natural surface water.
Dewatering of the “product coal” is a common practice which reduces the mass of the coal and minimizes runoff from the stockpile. Various methods are used for dewatering of the product coal such as – a) coarse coal centrifuge; b) screen bowl centrifuge; c) slurry screens; d) dewatering cyclones; e) horizontal belt filters etc.
Potential air contamination: Preparation plant may increase the level of dust at the mine site if proper dust control measures are not practiced. Wet scrubbers, electrical precipitators and filters are commonly used to control potential air contamination at a CHPP.
Potential contamination from coal refuse: Coal slurry is a by-product from the CHPP. Coal slurry contains both solid and liquid waste – fine coal refuse and water. Refuse disposal impoundments are commonly constructed for the permanent disposal of reject materials. In many cases, depending upon the chemistry of the reject material, liner might have to be constructed under the impoundment area to stop any seepage of water to the aquifer system below. Refuse materials usually contain small amount of coal, bone, shale etc. Majority of the reject impoundments in the United States are located in the state of West Virginia, Pennsylvania, Kentucky and Virginia. Coal mined in the western part of the United States does not go through a CHPP plant eliminating any need of reject impoundment areas. Coarse refuse material is used to build embankment for the impoundment area. Impoundment receives the coal slurry and promotes settlement of suspended solid materials. The relatively cleaner water near the top is often reused at the CHPP or used for dust suppression.
Composition of coal slurry
Coal slurry contains a large range of constituents, including dissolved minerals that have been leached or washed out of the coal and other rocks. In addition, the slurry contains chemicals added to facilitate the washing or water re-use processes. One of these chemicals is acrylamide. Other chemicals found in the slurry and sludge include the following:
- Benzyl alcohol
- Butyl benzyl phthalate
- Dibutyl phtalate
- Diethyl phthalate
- Dimethyl phthalate
- 4-Bromophenyl phenyl ether
- 4-Chhlorophenyl phenyl ether
Example of coal slurry disasters:
In February 1972, three dams holding a mixture of coal slurry and water in Logan County, West Virginia failed in succession: 130,000,000 US gallons (490,000 m3) of coal slurry were released in the Buffalo Creek Flood. 125 people were killed in this accident, 1,121 were injured, and over 4,000 were left homeless. The flood caused 50 million dollars in property damages.
In 2002, 900 feet high and 2,000 feet long valley fill in Lyburn, West Virginia failed and slid into a sediment pond at the toe of the valley-fill. The accident generated a large wave of water and sediment that destroyed several cars and houses.
Permitting process usually requires proper chemical understanding of the coal and the reject materials supported by laboratory results. Adequate number of samples should be collected for the coal, and the overburden material. Location of the boreholes should be selected in a way that sampling points are evenly distributed over the project area. Any guideline, if exist, for sampling and analyses should be followed. Hydrology of the reserve area should be evaluated. Flow path of the surface and ground water system should be mapped. In general, most of the mining companies hire experienced professionals from consulting firms such as Marshall Miller and Associate (www.mma1.com). Such geological firms help the mining company to develop proper planning for the complete mining operations.