Several tasks may be performed at preparation plants:

• Sizing (crushing and screening to desired particle size)
• Increasing heating value
• Removing unwanted minerals and elements
• Homogenization
• Blending

Sizing

Mined coal (called run-of-mine, ROM) is a mixture of coal and rock of different sizes. One of the functions of coal preparation is sizing. Sizing is the preparation and separation of particles by size. Different preparation technologies and end-user technologies (e.g. power-plant boilers, steel plants, etc.) are designed for specific ranges of particle sizes. Sizing is done by physically crushing and screening, sieving, milling, or classifying raw coal particles to desired sizes at different points in the preparation process (Nunenkamp, 1976; Gibbs and Hill, 1978; McCandless and Shaver, 1978; Honaker and others, 2007; Lutrell and Honaker, 2012).

In most coal preparation facilities, ROM is crushed to particle sizes of less than six inches. Then, material is processed to different sizes in different parts (called circuits) of the preparation plant:

The exact particle size that constitutes coarse, fine, etc. particles in a preparation plant varies. Some preparation facilities only define coarse and fine (combining many of the smaller sizes in the chart above) sizes. Others define and separate three (to four) finer sizes. In general, the small or fine size is approximately the size of beach sand. Fine or very fine particle sizes are similar to very fine sand to coarse silt grain sizes; which can look like a powder. Ultra-fine particles are silty to the size of particles in talcum powder. Most modern preparation plants only separate out coal particles greater than 0.15 mm (e.g. NRC, 2002); essentially the size of fine sand grains. Historically, technologies were not available for separating out fine, very fine, and ultra-fine grained particle sizes.

Cleaned coarse coal particles probably average around three inches in size when they leave a preparation plant. Some percentage of cleaned fines (small, fine, very fine) may also be included with the cleaned coal product (if separated), but too many fines will complicate handling and transport. Because of transport issues, crushing to a fine or an ultra-fine particle size may be done by the end user (e.g., power or steel plant), after transport, rather than at the preparation facility.

Increasing heating value

Enhancing the heating value of a raw (ROM) coal is achieved through removal of material that will not combust when the coal is heated. The ROM material sent to a preparation facility includes rock and mineral partings and sometimes fragments of roof and floor rock mined with the coal. The non-coal material will not combust, so removing as much of the incombustible material from the coal as is economically practical before combustion increases the relative heating value of the processed coal product. Removal of unwanted rock material is achieved using processes that exploit differences in density (specific gravity) and surface properties of the coal and rock.

Removing unwanted minerals and elements

Aside from fragments of roof and floor rock, coal beds commonly contain thin rock partings, and mineral-filled fractures (cleats), and may contain visible mineral nodules and cements. Some minerals, elements, and chemical compounds also occur at microscopic scales, intimately associated with the matrix structure of the coal. Aside from decreasing the heating value of the product coal, some of these minerals, elements, and chemical compounds create waste products and unwanted pollutants when the coal is combusted. For example, pyrite (FeS₂) is a common mineral, and is a major source of sulfur (S) in coal. When pyrite is combusted, sulfur dioxide (SO₂) forms and is liberated to the atmosphere, and then returns to the surface as acid rain. In the United States, because of historic problems from acid rain, emissions from power plants are limited to less than 1.2 pounds of sulfur per million Btu generated (U.S. EPA, 2023). Hence, one of the primary goals of coal preparation is the removal of sulfur-containing pyrite (to less than ~ 1%) before it gets to the power plant.

Through coal preparation, much of the coarse rock and minerals are removed from the washed product coal. The finest, microscopic minerals and elements remain with the product coal. They become the solid waste, or ash at a power plant, or become part of the gaseous emissions from a power plant boiler, which must be reduced or removed at the power plant (see Coal for electricity).

Homogenization

Another function of coal preparation is producing a homogenous end product; one that meets a specific range of criteria for the end user. In a mine, conditions are variable. The coal may contain rock partings in one part of the mine but not others. More roof or floor rock (non-coal) may be mined with the coal in some parts of the mine than others. The coal itself may vary in thickness, chemical makeup, and its physical characteristics (grindability, etc.). The end user, usually a power plant, wants a product that is consistent in a variety of characteristics, depending on its needs. The coal product that leaves the preparation facility generally has to meet a range of (or a minimum or maximum value of) particle size, heating value (Btu), ash content (% of non-combustible material), sulfur content, total moisture content, and in some cases, element contents (for example iron, mercury, chlorine, etc.).

Blending cleaned coal

Some preparation plants only work with one feed coal from one large mine. Other plants may process the same coal bed from several different mines. Some may receive material from different coal beds from multiple mines. The final coal products of preparation are stored and sampled to test their heating value and quality (ash content, sulfur content, etc.). In many cases, a single coal will not meet or satisfy all of the requirements of the end-user. In other cases, cleaned coals may have better quality or heating value than are needed. In these cases, different cleaned coals are blended together to achieve a desired coal product based on the requirements of the purchaser. For example, a power plant may want a coal with less than 1% sulfur. Cleaned coals from a preparation plant that have 0.5% sulfur and 2% sulfur might be blended to achieve the average 1% sulfur product. Blending can be done to achieve heating value, sulfur, ash, and other elemental constraints of the purchaser. Blending may be done at the preparation facility or at transportation facilities (load outs) where materials from different preparation facilities are received and stored prior to transport to the end user.