This review looks at the necessity of coal cleaning and its various stages, along with information about books, consultants, suppliers and research institutes.
Coal preparation is the process where raw coal from mine is cleaned using different physical processes, with or without any chemical reagents, to get a product which could be sold in the market or can be used for different purposes as coke making or power generation (Albrecht, 1980).
The reasons for cleaning coal are to:
- remove inorganic material (ash) thus reducing ash handling in coal plants
- increase the heating value of coal
- reduce the transportation cost per unit weight of coal
- meet customers' specifications
Coal preparation is a physical process where coal is washed in a water based medium in order to decrease the ash content and thus producing three different streams of products, namely clean coal, middlings, and rejects.
The basic principle for coal preparation is to first liberate the coal particles from the gangue (quartz) and then, by applying different water based techniques, separate the coal particles and gangue particles. The most common techniques for coal preparation are gravity separation and flotation.
There are four basic types of operations used when cleaning the coal. To these may be added a number of auxiliary operations, which are not directly involved in coal cleaning. The four basic operations are comminution, sizing, concentration and dewatering.
This is the stage where coal lumps are converted into small manageable sizes so that they can be processed by different equipment. In most cases this is achieved by installing a rotary breaker or a roll crusher. Impact crushers like hammer mills and ring granulators are also used.
The rotary breaker acts as a crusher as well as a screen. Coal, being softer, gets broken and passed through the aperture of the breaker and the shale particles remain unchanged due to their hardness and come out from the other end of the rotary breaker.
For the grindability tests of coal, the Hardgrove Grindability Index test is performed using the specially designed ball-and-race mill. This index gives the hardness of the coal. Usually coal is very soft and depending upon the amount of gangue, it may be hard or medium hard.
Sizing is the separation of coal into products characterized by differences in size. This can be accomplished by screening or by classifying. The classifiers are generally used for fine coal particles. Generally, the medium used is water.
Sizing of the coal is mostly achieved by incorporating vibrating screens, either a single or double deck screen, depending upon the size fraction required for further processing and hydrocyclones.
Generally, the size ranges obtained are:
- Coarse: >10mm
- Intermediate: < 10mm but > 0.6mm
- Fines: < 0.6mm
These sizes are not fixed and change for different plants at different locations.
Once the coal is classified into different size fractions, each size fraction is subjected to different unit operations for cleaning.
The cleaning results for a gravity separator (jig, dense medium baths, dense medium cyclones) are predicted using float and sink analysis and plotting washability curves. The efficiency of coal cleaning is predicted by the Tromp partition curves, whereas release analysis is used to study the flotation results.
Coarse coal cleaning is usually achieved in a jig or dense medium bath, however sometimes a dense medium cyclone can also be used. Intermediate coal cleaning is performed in a dense medium cyclone. The separation medium is magnetite slurry in water.
Fine coal cleaning is achieved by flotation, as coal is naturally hydrophobic. Some reagents like diesel oil (as collector) and Methyl Isobutyl Carbinol (MIBC) (as frother) are used.
All these processes divide the raw coal into the following three categories:
- Clean coal, having ash content approx. 7 - 9%
- Middlings, having ash content approx. 25 - 30%
- Rejects, having ash content approx. 65 - 70%
Here also, the ash content of products depends on the raw coal characteristics and customer specifications.
Coal preparation is carried out in water and thus a lot of moisture is added into the system. The total moisture content of the final coal product has become a major quality control factor, and the removal of moisture from both the coarse and the fine coal are serious technical issue.
The benefits of lower moisture can be listed as (Lockhart, 1996):
" Reduction in transportation cost as no water will be transported
" Increase in higher calorific value as no energy will be wasted in water removal
" Improvement in the handling ability
" Avoidance of penalties or inability to sell more coal
Coarse coal dewatering is achieved by dewatering screens or centrifuges, whereas fine coal dewatering is done using filters (belt filters/ vacuum filters). Sometimes thermal drying is also utilized for fine coal dewatering.
HEAVY MEDIA RECOVERY
It is one of the most important steps in coal preparation where the heavy media used in the concentration process is recovered for reuse. This is the stage where magnetite used in the process of coal beneficiation is recovered using magnetic separators. Magnetite is added to the coarse coal for the separation of clean coal, middling and reject. The dilute medium from rinse screens is passed through a series of magnetic separators in order to recovery of media. And, the magnetite recovered from the separators is reutilized for the media formation. Most commonly used magnetic separator is the magnetic drum separator.
Auxiliary operations are by nature quite diverse. They involve storing material in bins, silos or open piles; transporting material by conveyors, feeders, elevator or pumps; sampling, weighing, chemical reagent feeding, feed distribution and such operations needed to move or control the material from one cleaning operation to another.
A general coal preparation flowsheet is shown below:
The best book available for the basic study for coal preparation is Coal Preparation by Joseph III Leonard.
Other books with further information on coal preparation are:
There is also some very good information provided at following links:
A very good video on coal preparation is given at this link:
There are several courses on EduMine related to coal preparation and comminution:
All the equipment used in coal preparation plants is supplied by numerous suppliers. Below is a list of various equipment used in coal preparation with links to equipment suppliers listed on the SupplyMine section of InfoMine:
Some popular consultants for building and operating a coal preparation plant are:
Popular research institutes where studies on coal are very prominent include:
There is also an online forum called the Coal Research Forum, based in the UK.
Their three main aims are:
- To provide a forum for the exchange of information between technology providers and key stakeholders.
- To support the utilisation of coal in the UK as a secure primary source of energy, and,
- To promote UK research and expertise on the utilisation of coal, biomass and other fossil fuels in the UK and to a worldwide audience.
RECENT DEVELOPMENTS AND RESEARCH
Most of the current research is based on the development of the air based cleaning system, improving the efficiency of air medium separation and decreasing the environmental concerns. Also, research is happening on the flotation of oxidized fine coal and reagents for improved efficiency and economy.
Sulfur in coal is also a problem, and sulfur removal is a major research focus area. (http://scholar.lib.vt.edu/vtpubs/spectrum/sp960829/3c.html)
Clean coal technology is also a current area of research. There are various technologies being developed to reduce the environmental impact of generating energy from coal.
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