Coal Processing Principles


Coal as mined from the working face of the coal seam is an inhomogeneous mixture of numerous types of metamorphosed plant materials originally laid down in swamplands several hundred million years ago. These materials are analysed and categorized by visual, microscopic and analytical methods into the various types of coals graded from lignite to anthracite. Depending on the composition of the coal seam, in which several types of inorganic intrusions may be present due to sedimentation and volcanic activity, and depending on the mining method employed, the run-of-mine coal will commonly contain rock, shale and other undesirable contaminants.

Coal beneficiation is the process of removal of the contaminants and the lower grade coal to achieve a product quality which is suitable to the application of the end user - either as an energy source or as a chemical agent or feedstock. A common term for this process is coal "washing" or "cleaning".

Coal processing is a broader term used to describe the complete process of sizing, handling and washing of the run-of-mine coal.

While the coal preparation engineer will require a full understanding of all the classification methods and properties of coal, it is mainly the relative density distribution of the mined material and its relationship to ash, volatile matter, moisture content and fixed carbon (collectively known as the proximate analysis) with which he will mostly work. Calorific value and sulphur content are also important parameters which relate to the relative density distribution.

This analytical method is termed "washability analysis" and it describes and quantifies the opportunity to upgrade a particular coal to a desired quality by gravity concentration methods.

By study of the washability of the coal the preparation engineer will decide at what specific gravity to separate the product from the discards to obtain the correct specification for the client.

A perfect separation as may be achieved under laboratory conditions would be a step-shaped graph as shown below where all particles lighter than the cut-point density would report to the product while none of the particles heavier than the cut-point would report to the product. In an operating process plant, some of the particles will be misplaced and report to the wrong stream, leading to a separation curve as shown, known as a partition curve or Tromp curve after its designer.

TYPICAL PARTITION CURVE

It is the objective of the design engineers to construct a plant which will achieve the sharpest possible separations over long periods of trouble free operation so that both maximum throughput and maximum recovery efficiency will be obtained.

Portaclone process plants have a long proven record of achieving and maintaining these objectives in widely varied applications and remaining highly efficient and reliable over many years of operation.