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COKING COAL BENEFICIATION – Part 2
A coal preparation plant even though very well designed, cannot give good results unless proper care is taken in running it with a well-planned operation and management schedule. The operational problems can be divided into two categories – one due to designed defects and the other due to defective operation. The problems faced while operating the washeries and their remedies have been analysed. The objective is to achieve rated capacity of the plant, thus, it is necessary to know how it can be achieved.
The rated capacity of a washery is hardly attained due to non-availability of essential spares and replacement as well as lack of proper supervision and/or co-ordination. The quality of raw coal has changed since the washeries were installed. This has adversely affected the quality of washed products as well as the capacity utilisation of the plants.
The scenario of coal preparation in India is quite different from that of many other countries. There are so many constraints which are briefly narrated hereunder:
- Quality of raw coal to be treated is deteriorating gradually due to depletion of better quality reserves.
- More quantity of fines is generated because of more mechanisation in the method of mining and handling.
- Because of low output from individual seams of mines, generally central washery is built. The size consist and Beneficiation characteristics of coal feed are divergent. It is difficult to wash mixed coals of different sources. Thus, proper blending facilities are required to be provided.
- For central type of washery, raw coal is to be transported to the plant from long distances, by that more cost is involved and size degradation takes place due to repeated handling.
- Because of continuous degradation of raw coal, adequate facility becomes necessary in the capacity of individual circuit/and units so that rated throughput of the plant is achieved.
- Indian coals yield large quantity of middlings and rejects, subsequently, disposal of these products pose problems because their less heat value.
Suggestions for improvement
Corrective measures both during the design and operation of the coal washeries need to be speedily implemented. Built-in capacity of the different sections of a plant should have extra allowance to take care of anticipated variation of size and washability characteristics of raw coal. All the equipment should be easily accessible for maintenance and overhaul for reducing downtime. Design deficiencies of existing washeries should be rectified as soon as possible. Appropriate modification should also be done in existing washeries to accommodate the deteriorating quality of raw coal feed. Latest technique of coal preparation should be adopted in future washeries and also in existing washeries in the form of modification schemes. In addition to foregoing, the following major areas need special consideration.
The performance of a washery is evaluated by the examination of the extent of fulfilment of different duties of the plant such as (Kumar, D., 1995):
- Handling the required hourly throughput capacity of the plant.
- Effective working hours over a shift or day.
- Efficiency parameter of individual processing and handling units.
- Yield and quality of washed products.
- Consumption of magnetite, reagents, water, power, spares for machinery etc.
- Cost of Beneficiation.
While evaluating the performance study, the characteristics of raw coal feed to the washery has to be maintained within a reasonable degree of consistency. The actual quantity and quality of clean coal should be compared with those expected from theoretical assessment. In other words, the organic efficiency of the plant as an integrated unit should be determined. The periodic performance study and evaluation is essential for taking corrective steps.
In addition, there are other types of reasons for idle time like non-availability of wagons for off-take of washed products, and subsequently, there is no adequate place for stocking the products. In the analysis of four washeries (Figure 9.1), it is observed that washeries B and C suffer greatly from lost period. Linkage of raw coal to the washeries is a pre-requisite for proper utilization of the washeries. Similarly, for supply of wagon, proper co-ordination is necessary with the railway authority.
Control of operations
The performance of coal preparation plants can be properly controlled by taking recourse to the following measures:
- Control on the quality of raw coal feed.
- Blending of raw coal feed from different sources.
- Efficient operation of individual equipment and the whole system.
- Proper maintenances and supply of spares and replaceable parts.
- Regular supply of power, water, magnetite, and other consumable materials.
- Regular receipt of required quantity of raw coal and despatch of washed products.
- Plant management, supervisory control and training of personnel.
- Quality control of washed products by statistical control charts.
- Distributed control system of automation by programmable logic controller (PLC).
- Design of optimum flow sheet, selection of the right type of equipment and standardisation.
- Use of proper type of liners against abrasion.
- Provision for upgradation of raw magnetite in the plant and design of proper type of magnetite circuit.
- Protective measures against power tripping.
- Economic design of water circuit.
- Pollution control against dusts, slurry, and noise.
- Proper disposal of rejects and their possible utilization.
- Modification of existing washeries on feedback from research and development.
Crushing of middlings in the existing washeries
In the composite beneficiation system of coking coal in India, usually three types of products are obtained – clean coal, middlings, and rejects. It is observed that the proportion of middlings is rising day by day on one hand, and on the other the yield of clean coal is decreasing in the washeries because superior grade raw coal is depleting gradually. This phenomenon results in short supply of coking coal to the steel industry. The middlings which are used presently for power generation contain a good amount of coking coal. Satisfactory separation of coal from mineral matter is not possible by conventional methods of separation as Indian coals are of high intergrown nature. The ash content of middlings is usually 30 to 38%, while the production varying from 20 to 30%. This substantial amount of middlings present an excellent potential source of coking coal in view of conservation of this scarce raw material of the nation. The coking coal resources can be augmented by recovering extra low ash coking coal from washery middlings by suitable beneficiation processes (Jha, 2012).This possibility has been established from tests. The middlings were crushed to 13 mm and 3 mm and thereafter subjected to float and sink tests. The results are presented in Figure 10.0 (a).
The recoverable weight of clean coal varies from 3.1 to 29.5% for middlings crushed to 13 mm and the same varies from 3.9 to 35% for middlings crushed to 3 mm depending upon the nature of coal. In pilot plant tests, the middlings samples were crushed to 3 mm and the size (3-0. 5) fraction was treated in HM cyclone and (-0.5 mm) fraction in flotation. The results proved to be quite encouraging. Thus, it can be inferred that the cyclone-cum-flotation scheme could be a feasible solution to this problem. This can be incorporated in the existing Beneficiation system as shown in Figure 10.0 (b). As per this scheme, the middlings of the existing washeries will be crushed to 3 mm and then screened at 0.5 mm. The coarser size will be treated in heavy medium cyclone and the fines by flotation process. The fines of the existing system can also be put into the new flotation scheme. The cleans produced from the modified scheme can be mixed with the clean coal of the existing washeries.
It is anticipated that the proportion of middlings may further rise to as high as 35 to 40% of total raw coal feed to washery. This will mean lesser availability of washed metallurgical coal. This situation calls for immediate remedial measures. It is, however, believed that this scheme should hold good more or less for coals to be washed in the existing washeries of India. This integrated approach will go a long way in solving the problem of acute shortage of coking coal in the country.
Adoption of standard flow sheets in comparison to diverse systems of Beneficiation as they prevail in India can offer many benefits (Kumar, D., 1988):
- Standardisation will help in the development of manufacture of equipment and spares
- Cost will reduce in the design, manufacture and installation of washeries.
- Stores inventory of spares will reduce as the demand will be limited to a few standard designs.
- Training of personnel for operation and maintenance of washery will be easier as they can be interchanged.
- Operation and maintenance of washery will be easier due to ready availability of spares and equipment
- Interchangeability of equipment and the spares becomes possible
- Time schedule for the construction of new washeries will be less.
Most coal washeries in India are not working properly resulting in deterioration both in the quality and quantity of washed clean coal. The low performance of coal washeries causes a great damage to the national economy because of increased import of coking coal. It is necessary to analyse the problems very critically so that the present situation can be improved. If the real causes of failures are identified, proper action can be taken whether of technical character or management techniques. The latest technology of management science should be introduced in this industry as it is done in other industries.
Liberation of coal substance is not complete unless coal is crushed to a certain level of top size. There is every likelihood of losing some combustibles along with rejects produced from the washery if the coaly matter is not made free before beneficiation. It is suggested that the raw coals should be crushed to 6 mm for liberation of more combustibles (Kumar, 1982).
- Jha, N. C., 2012. Sustainable Development of Indian Coal Industry – Daunting Challenges, Proceedings of 4th Asian Mining Congress, 29-31 January, MGMI, Kolkata, India, pp. 1-8.
- Kumar, V., Saxena, V. K., 2014. Studies on the Variation in Coal Properties of Low Volatile Coking Coal after Beneficiation, IJCER, Vol. 04, Issue, 1, January, pp. 39-51.
- Kumar, D. 1984. Flotation von Kohlenschlämmen in Indien, Glückauf, 120, Nr. 20, pp. 1336-1340.
- Kumar, D., 1981. Beneficiation of coking coal in India with special reference to conservation, Urja, Feb., pp. 69-11.
- Kumar, D. 1982. Aufbereitung von Kokskohle in Indien, Autbereitungs-Technik, Nr. 11, pp. 610-614.
- Kumar, D., 1988. The Standard Flowsheet for Preparation of Indian Coking Coal, Aufbereitungs – Technik. Nr. 1, pp. 16-21.
- Kumar, D., 1995. Potential of Performance Improvement of Indian Coal Washeries, Coal Prep International, Lexington, Kentucky, May, 2-4, pp. 303-319.
- Nicol, S. K., 2001. Fine coal beneficiation, in Swanson, A.R., (Ed), Australian Coal Preparation Monograph Series, Volume IV, Part 9.
- Ofori, P., O’Brien, G., Firth, B. and Jenkins, B., 2004. Flotation process diagnosis and modelling by coal grain analysis, in Wembrey. In: WB. (Ed.), Proceedings of the Tenth Australian Coal Preparation Conference, C9.
- O’Brien, G. Firth, B. and Adair, B., 2011. Application of the coal grain analysis method to coal liberation studies, International Journal of Coal Preparation and Utilization, 31, pp. 96-111.
- Sengupta, A. K., Senapati, G. 2014. Improvement in Washed Coal Yield in BCCL Washeries by up-gradation of Washeries, Proceedings of 5th Asian Mining Congress, 13-15 February, MGMI, Kolkata, India, pp. 1-1.
- Vince, A., 2013. Post-treatment of coal. In: Osborne, D (Ed.) The Coal Handbook, Towards cleaner production Volume 1: Coal production, pp. 461-528.
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About the books:
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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