Coal preparation process

Technical and commercial value of coal is defined by the amount of combustible material that it contains. Coal mined from the ground termed as “run-of-mine” (ROM) coal contains impurities and is represented by a mixture of multiple size particles and, thus, has low commercial value. The treatment of ROM coal known as coal preparation, coal washing or coal beneficiation (The Coal Resource, 2009). This process aims to increase coal quality and heating value via lowering or removing of inert material and usually results into three components (Bragina, 2011):

- concentrate which is low-ash and low-sulfur product;

- middlings (energy fuel) intermediate material which has high ash content;

- tailings.

The basic idea of coal preparation process is represented on a figure 13. Beneficiation flow is comprised by series of unit operations interconnected by a material handling system which transports the slurry (Albrecht, 2014).

Figure 13. The concept of coal preparation process.

BRIQUETTING (OPTIONAL) THERMAL DRYING

(OPTIONAL) DEWATERING CONCENTRATION

SIZING

COMMINUTION ROM coal

+ X mm

Concentrate

Concentrate

Concentrate

Tailings

Tailings

Tailings or brown coal

Middllings

Middlings + Z mm

The technological process of coal washing can be split up into the following operations (Albrecht, 2014):

1. Comminution. Raw coal is crushed into the particles of appropriate size in order preparation equipment to be able to process them. Crushing can be an independent process, for example, in crushing-sorting plants (Egorov, et al., 2006) or represent the first step in preparation process helping to get rid of mineral matter and facilitating further operations.

2. Sizing. The target of this step is to divide raw coal into different fractions according to the size of particles. Normally it is accomplished on screens to separate bulk from finer material. After sizing preparation process goes individually for classified fractions.

3. Concentration. This is a heart of coal preparation process where actual purification occurs and concentrate is separated from the tailings. Concentration methods used for coal applications can be classified into gravity methods (wet and dry preparation), flotation and magnetic separation.

Gravity methods (hydraulic) are based on difference in density of concentrate and mineral matter. Most common hydraulic methods are (Egorov, at al., 2006):

- jigging - separation on grid-plate in vertically pulsating water flow. Particles with high density are concentrated under plate when lower density fractions stay above the grid-plate and are flowed away;

- dense media separation - separation under force of gravity or centrifugal force realized in mineral suspensions. The density of such media is intermediate which facilitates sinking of dense fractions and floating of lighter ones;

- preparation on inclined panes – separation method which uses gravity, friction, water flow and water resistance forces to separate fine fractions.

Approximately 50% of totally prepared coal undergo jigging which makes it the most popular separation method. Though, the limitation of jigging is processing only coarse coal. Dense media separation is the second popular preparation method targeting to process fine coal fractions. It is usually applied for coal containing over 35% of mineral matter providing high separation quality and minimum losses. Often hydro cyclones are used for this purpose.

(Turchenko, 1995)

Preparation on inclined panes is mainly done in spiral separators or table concentrators. Under influence of previously mentioned forces upper part of water flow contains low density fraction and moves it away when high dense media slides on the bottom. (Suslina, 2012) Wet gravity methods increase moisture content (“free moisture”) and require further dewatering. Dry (pneumatic) separation which is done in pulsating air flow does not influence on moisture content. However, the main limitation of this method is difficulty in dust collection. (Klein & Vakhonina, 2011)

Flotation separation utilizes the difference in water-affinity. It is common method to separate fine fractions smaller than 0,5 mm. For coal application is used froth flotation where hydrophilous particles (mineral matter) sink and hydrophobic fraction (fine coal concentrate) sticks to the air bubbles and floats. (Egorov, et al., 2006)

4. Dewatering. This step aims to remove free moisture from coal and reclaim water back to the process. The amount of free moisture directly correlates to the size of processed particles: fine coal has larger surface area and thus contains more “free” moisture. Dewatering of large coal fractions is not demanding and done on screens. Smaller fractions of coal concentrate are usually dewatered in centrifuges or vacuum filters. Coal tailings treatment depends on water loop circulation. (Suslina, 2012)

In open-loop tailings are discharged to the tailing ponds in thickened state or without any treatment. Nowadays environmental restrictions are forcing coal producers to introduce or adjust the process towards closed water loop circulation. It aims to re-use the water to the process via more effective dewatering and clarification. In closed loop tailings are firstly thickened to remove most of fine solids from the process water. Then thickener overflow is pumped back to the preparation process and underflow is filtered and then transported to the tailings disposal area. (Laskowski, 2001)

5. Thermal drying can be considered as an optional part of dewatering process. In Russia mainly old preparation plants utilize this operation in dewatering process. It is efficient but expensive treatment. New plants normally eliminate thermal drying from technology process which raises the requirements for other operations. The reason is that without thermal drying other processes, machines and equipment have to work more efficiently to provide low moisture content.

6. Briquetting (optional). Briquetting is typically applied for the brown coals which are widely spread in Russia. Brown coals are fragile and thus are difficult to burn because the fraction easily falls down between the grate furnaces. Briquetting can be described as a process of making coal briquettes out of small coal residues with a help of pressure.

Depending on coal briquetting can be realized with a help of binders and 80 MPa pressure (fine coals, tailings or anthracite) or without the binders if the pressure exceeds 80 MPa (brown coals) (Egorov, et al., 2006). The bits and pieces left after briquetting are collected and returned back to the process. The briquettes are used as a fuel for industrial and household needs and for semi coking (Speight, 1994).

Technology methods and utilized equipment may vary from one preparation plant to another.

Typically coal preparation plant is designed to process particular coal ranks and grades with a certain predefined feed characteristics.