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COKING COAL BENEFICIATION
History of coal Beneficiation
The technological progress in coal preparation from a global perspective is indicated in Figure 1.0 (a). The history of establishment of coal washeries in India is illustrated in Figure 1.0 (b), in which the period is divided into landmarks of ten years apart starting with 1950. Prior to 1950, the coals were selectively mined and directly used after sizing for blast furnace without beneficiation. The West Bokaro washery 3 was a state-of-the-art plant befitting the Tata Steel’s tradition. It could serve as an illustrious benchmark for the coal preparation industry of India.
Figure 1.0 (a) Progression of global technological development in coal preparation
Figure 1.0 (b) History of Indian coal beneficiation
The objective of beneficiation
The basic purpose of beneficiation is to reduce the ash forming impurities from raw coal so that better coal with less ash content can be produced. If these ash forming constituents are not removed, and coal is directly charged to the coke-ovens, the ash content in coke will increase and subsequently more slag will be produced in the blast furnace thereby lowering the efficiency of the blast furnace.
Nature of impurities
The impurities associated with coals depend on two conditions i.e. nature of formation and method of exploitation. The Indian coals are of ‘drift’ origin, so mineral dirt and mineral matters are intimately mixed with most coals. These impurities are mainly of inherent type and have made the Indian coals very difficult to wash (Kumar, D., 1981).
Effect of impurities in coal
It is found in steel plants that 1% increase of ash in coking coal results in the following deteriorating effects (Kumar, D., 1982):
- Increases the slag volume involving more expenses for slag dumping.
- Decreases production of blast furnace by about 3-6%.
- Increases coke consumption by about 4-5%.
- Decreases the yield of carbon in coke resulting in more coke ovens and higher expenditure.
- Increases limestone consumption by about 5%.
- Yield of tar and gas is reduced.
The liberation size is a fundamental characteristic of the coal type (Vince, 2013). Estimating the required liberation size by physical cleaning requires the ash forming components of raw coal to be sufficiently liberated from the coal. The liberation size is conventionally derived by quantifying the size reduction necessary to reduce the gangue-coal particle conglomerates such that the individual low ash value grains prevail.
Tree flotation is a technique, particularly applicable to hydrophobic coals, which can be used to estimate the ash value of most liberated coal particles present and relate this to particle-size distribution in a given sample (Nicol, 2001).
Coal Grain Analysis (CGA) is an optical microscopic imaging method that is used for routine coal petrography assessments (maceral composition and coal rank) and for obtaining compositional information on individual coal grains. This method determines the number of mineral inclusions within the particles and is a good complement to the tree flotation and the washability by size information techniques (Ofori et al., 2004; O’Brien et al., 2011).
Quantitative evaluation of minerals by scanning electron microscope (QEMSEM) and mineral liberation analysis (MLA) are automated analytical systems provided by FEI Natural Resources to estimate detailed mineralogical information. Although they are specifically designed for metalliferous minerals, they can be used to determine the size of minerals present in a coal matrix.
To plan the Beneficiation system of any coal, the petrological composition should be studied under a microscope. Under a microscope, various constituents such as organic macerals of combustible matter and inorganic minerals of ashy matter can be seen. The petrological texture of coal indicates the possibility of liberation of different constituents by crushing and grinding. The first step of Beneficiation is to crush or grind the coal to liberate the coaly matter from ashy matter which can theoretically yield complete liberation. In the case of intergrown texture, finer grinding up to a few micron size is necessary for complete liberation whereas for free dirt ordinary crushing is sufficient (Kumar, D., 1982).
Degree of liberation
The degree of liberation of a mineral can be defined as the percentage of that mineral occurring as free particles with respect to the total of that mineral present. This can be expressed as per equation (5.2).
Where, L = degree of liberation
Nf = number of free particles
Nl = number of locked particles
The liberation information can be obtained by release analysis and float and sink tests. For a binary mineral consisting of concentrate (a) and refuse (b), the composition can be expressed in terms of weight percentages. It is assumed that the feed or raw mineral is a true physical mixture of two components. The degree of liberation can be defined as percentage of free concentrate out of total content of the ore body.
Where, L = degree of liberation, V = volume, ρ = density, and Va + Vb = 1
Evaluation of liberation
Microscopic examination of the polished section of a sample cannot give the correct information as to the optimum point of liberation but it can serve as a guide. By this study, the degree of grinding required for maximum liberation can be known roughly. The evaluation of liberation involves comminution, study under microscope and/or beneficiation tests. From different float and sink data of coal crushed to different sizes, the degree of liberation can be calculated. For coal, the degree of liberation can be expressed as per the equation (5.2.1).
Y = weight of free particles i.e. yield % of clean coal
Z = ash % of clean coal
X = ash % of raw coal
6.0 Special characteristics
The density distribution in India coals is characterised by low proportion or complete lack of low relative density fractions below 1.3 or 1.4 which is evident from the density distribution curves in Figure 6.0(a). Lack of density fraction accounts for the lack of low ash fractions. As a consequence, Indian coals, as shown in Figure 6.0 (b) are incapable of producing clean coal at low ash levels common in Northern Carboniferous coalfields. Most Indian coal seams contain a large proportion of true middlings. Even though the clean coal has a relatively high ash content, the rejects (discards) contain a substantially higher proportion of combustibles than what would normally be acceptable in Europe and North America.
Some of the typical washability characteristics of Indian coals are: (i) low recovery of clean coal (ii) more yield of middlings, (iii) poor elimination of dirt, and (iv) higher content of neat-gravity materials. Due to these factors, it is rather impossible to bring down the ash content of clean coal below 10% which is the limit of ash in coking coal as supplied to steel plants in other countries Thus, Indian steel plants have been forced to adopt the technology of steel-making with coking coal of higher ash content (17%).
It is observed that the quality of washed coal from different washeries is deteriorating because of increase in ash content of raw coal feed to washeries. So the present system of Beneficiation will not hold good any more for coal to be beneficiated in future.
Existing system of beneficiation
Till the Second World War, it was the common conviction that Indian coals were not amenable to beneficiation. This belief led to selective mining of good quality coking coals for different uses, even other than steel-making. It resulted in depletion of better quality of coking coals and now we are left with poor quality coals. As the ash content of the available coking coal reserves in the country is seldom below 17 %, Beneficiation of coals becomes a must. The average ash content in raw coal usually varies from 30 to 40 %.
It is usually found in Indian coals that the specific gravity varies from 1.45 to 1.50 when the ash contents are 16 to 18 % and for specific gravity of 1.6 to 1.8, the ash contents are between 32 and 35%. This difference of property is utilised in gravity separation of coal. In coal Beneficiation, artificial suspension of above mentioned densities are produced and coals are subjected to float and sink. The coals which float at 1.5 specific gravity are called clean coal and the sinks are put to another suspension of specific gravity of 1.7. Floats of the second suspension are called middlings and final sinks are called rejects. Thus, three products are produced by this process. Clean coal is supplied to steel plants, the middlings to thermal power stations, and the rejects are usually dumped near the washery or utilized for power generation.
Summing up, raw coal is usually crushed to 15 mm and then screened into three fractions 15-25 mm, 25-0.5 mm and – 0.5 mm. The 15-25 mm fraction is washed in heavy medium washer and the 25-0.5 mm fraction either in jigs or heavy medium cyclones. The -0.5 fraction is usually upgraded by flotation.
Problems and prospects of flotation units
It has been observed that the flotation units are not performing well in India, clean concentrates of desired quality and quantity are not produced. Thus, it is necessary to impose proper operational and control measures to achieve the desired results. For that matter, all the parameters controlling the floatability of coal should be known, actually there are many but it is not possible to control all the parameters. At least, the important parameters as enumerated below, are to be taken care assuming the other parameters as constant (Kumar, 1984):
- The coarser size fraction (+0.5 mm) should not be allowed to go along with the feed to the flotation cells as it creates trouble in the flotation circuit. The optimum size range for feed coal to flotation is 0.3 mm (48 mesh) to 0.015 mm (200 mesh). Thus, it is preferable to deslime the coal by hydrocyclone and the -0.015 mm fraction may be discarded or upgraded depending upon the quality.
- As the pulp density is one of the determining factors, this should be adjusted properly which generally varies from 10 to 12%.
- Unless sufficient retention time is provided, the performance cannot be effective.
- The reagents should be added at the right places, all the reagents should not be added at one point, and they can be distributed throughout the bank of cells.
- Conditioners ahead of pulp adjusting tanks should be there so that the coal particles are properly coated with collecting reagents.
- It is always advisable to keep watch on the proper performance of flotation cells. The pulp should flow freely from cell to cell. The aeration should be sufficient and froths should leave the cells as soon as they are formed.
- Oxidized coals are difficult to float, in that case new reagents can be tried apart from usual MIBC, diesel oil and kerosene oils along with other frothers. If these reagents are not properly dispersed, then some coal particles will be coated and others will starve and if more reagents are used to counteract this, non-coal substance will be floated. If this oil can be broken into fine droplets, then chances of mass transfer of oils onto coal surfaces increase and coal floatability will improve. Sufficient dispersion of oils is possible by emulsification of these oils.
As beneficiation of coal fines containing enriched vitrinite is essential to maintain the quality of metallurgical coke, due attention needs to be paid to revamp/renovate the fine coal circuits. At least 60% recovery of clean coal at 18% ash from washery tailings is possible. Such beneficiation project can be set up through outsourcing mode by adopting BOM/BOO model (Sengupta and Senapati, 2014).
If you found this story stimulating, you may be interested in browsing more content within this book on ScienceDirect. We are pleased to offer you a free chapter – access this content by clicking on this link – Coking Coal Washing.
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Sustainable Management of Coal Preparation explains both the upstream and downstream of coal preparation, stressing clean coal technologies for coal utilization.
Management of Coking Coal Resources provides a one-stop reference that focuses on sustainable mining practices using a four-point approach that includes the economical, governmental, societal, and environmental aspects of coal exploration, coking coal mining, and steelmaking applications.
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About the author:
Dr. Dilip Kumar obtained a Bachelor’s degree in mining engineering from the Indian Institute of Engineering Science and Technology (IIEST), Shibpur, India. He also has a Master’s degree in mineral processing from the Université de Mons, Belgium and a Doctoral degree in minerals engineering from the Montanuniversität Leoben, Austria. Dr. Kumar’s expertise is in coal preparation, and he has international professional experience in countries like India, Germany, Algeria, and Canada. He is widely published and has authored a book on the “Management of Coking Coal Resources.” Dr. Kumar was also a recipient of the Rajendra Prasad Memorial Prize of Institution of Engineers (India). At present, he is engaged in consultancy and technical writing following his retirement as the Chief Mining Scientist of Central Mine Planning and Design Institute (CMPDI) in Ranchi, India.
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