Effective and Economic Cleaning of High Ash Indian Coal for Cement and Sponge Iron Industries

Kalyan Sen, S.K.Mitra, N.S.Das, C.K.Das & T.Gouri Charan
Central Fuel Research Institute, Dhanbad - 828108

The cement industry is the third largest consumers of coal after power and metallurgy, requiring about 15 to 20 Mt of coal per annum. According to the report of CMIE, coal off-take by cement industry accounted to 15.6Mt during the period 1999-20002. After independence in 1947, the cement production in India took a turn from 1.5Mt per annum from 18 cement plants to about 135Mt of total installed capacity from 54 large capacity plants in private sector. India now becomes the second largest producer of cement in the world, crossing the production target of 100 Mt during fiscal 2001-20024 , and with coal as the primary source of fuel modern clean coal technologies like Sponge iron, Corex, Gasification, Liquefaction, etc.envisage better quality coal of specific characteristics. Non-coking coal (grade B/C) is the prime material required for these upcoming processes.

Coal plays a dual role in the sponge iron process by acting as reductant as well as a fuel for providing heat to maintain the requisite temperature inside the kiln at 950 -10500C. Consumption of non-coking coal in sponge iron industries alone is around 5Mt/annum. Apart from BSIL, the first commercial coal-based sponge iron plant, there are about 17 other large capacity sponge iron plants in India5. The small entrepreneurs in sponge iron manufacturing are mostly coal based. In Orissa, sponge iron industries are coming up in a big way5.

Though cement companies are one of the major consumers of coal, their traditional linkages with coal companies and inconsistent supplies lead to various 'technical problems'. To resolve these problems, the Ministry of Coal and the Government came forward to modify the coal-linkage agreements and accordingly, more than 90% of the cement producing units have signed, for the first time, individual coal-supply agreements with CIL6. However, posterior price hike by CIL of various grades of coal ranging from 4% to 12% would create a dent in their operating cost. An impact analysis7 on per bag cost of cement for the four major cement companies has been made based on the assumption that 100% of their coal requirement is met from CIL. The report reveals that a 5% hike leads to a 65 paisa rise in cost per bag of cement, whilst a 10% rise would amount to an increase of Rs1.30/bag. Among the larger cement producers, those who procure major share of coal requirement through imports, would not be adversely impacted.

Reduction in import duty from a figure of 25% to 20% on non-coking coal used for cement production as included in current budget would benefit only coastal-based plants in South and West India4. For some of these plants, non-coking coal accounts for 20-25% of total manufacturing cost. The biggest beneficiary would be cement plants in Tamil Nadu belonging to India Cements, Madras Cement and Dalmia Cements as most of the plants in the state depend on non-coking coal imported from Australia, South Africa and Indonesia.
The hikes in the freight rates on cement and coal have direct impact in increasing the cost per bag of cement. In the year 2000-01, the cement industry consumed nearly 10Mt of domestic coal. Railways also dispatched 39Mt of cement7. Over the years, the proportion of cement dispatched by railways has come down to 40% from nearly 70% in 1995.

Source: India Infoline estimates

As a matter of fact, optimal beneficiation of non-coking coal needs a special approach from that of coking coal because of contrasted nature of constraints involved in their utilisation pattern, prevalent practice, existing infrastructure, availability of equipment etc. Indian coals are difficult to wash because of their close association and intermixing of the coal and inerts (both macerals and mineral matters) resulting from geological phenomena typical to Indian coals of Gondwana origin. Consequently, the separation criteria are quite complicated for these coals, except where the quality deterioration is mainly due to addition of overburden. Maximization of the recovery of clean coal at desired ash content being the major concern to the washeries, the perfection in predicting the anticipated yield of clean coal is a pre-requisite step in treating a coal in an existing washery or in designing a new plant8. 

The paper presents a case study of typical non-coking coals from Talcher coalfield, having ash content in the range of 40-41% for exploring the possible options for beneficiation routes. The characteristics of the coal and the size analysis is presented in the table 3. 

Computer simulation study on the washability data of the coal crushed to different size ranges, namely 100-3mm and 25-3mm has been made considering normal Imperfection values of different washing units and the results are represented in figures 2 and 3. Although Jigs are used more frequently than heavy-medium vessels because of their large capacities, low cost, adaptability to wide size range of coal and simple technique, they have the limitation in operating efficiently below 1.60 sp. gravity. The practical yield curves in figures 2 & 3 exhibit that Jigs or water-only washer (like Haldex, hyro-cyclone separator, etc.) can not be suitable washers to clean coal for cement and sponge iron making. Even more efficient Batac jigs would fail to achieve desired cleaning at ash level less than 25%. 

On the other hand, cleaning non-coking coal at desired ash level for both cement and sponge iron industries may be achieved in Heavy medium separator. It is observed that the practical recovery of cleans at 25% ash level is 38%, when 100-3mm fraction is treated in Heavy media for cement industry. A possible circuit may be, screening the coal to 25/13mm and washing the coarser fraction in HM bath followed by washing the finer fraction (-25/13 +3mm) in HM Cyclone, both having almost same imperfection value (Im about 0.05). It is interesting to note that even if the coal is crushed to 25mm as is currently practiced in sponge iron industries and washed in heavy medium cyclone, the practical recovery of cleans at 25% ash level would be lesser (about 34%), along with the additional problems of higher energy consumption, products dewatering and slurry/effluent treatment. 

A key element in process selection is maximum recovery of the product from raw coal at lowest possible cost. Selection of suitable circuit must be backed up by computer simulated washability data for prediction of yield gain over other circuits. In spite of the fact that heavy medium washing plants cost more to build and operate, the cleaning efficiency is comparatively more with more yield of cleans. As it happened in the case study, HM circuit may yield sometimes 6-8% extra washed coal at particular ash level when compared to the water only based washing units like jig, water only cyclones or hydro- separators. Considering washing as the single cost component, the higher cost involved with HM Cyclone (approx. Rs. 135/t of raw coal feed) compared to that with Jig (Rs. 85/t), may be compensated by yield gain achieved with the former. Moreover, the consumption of higher electrical energy for running the dynamic water only plants is also to be considered. The techno-economic analysis of a Coal preparation plant needs case to case study. 

CFRI has undertaken R & D studies in a broad spectrum on indigenous non-coking resources. Samples have been collected from most of the major sources /projects of different coalfields of the country. The study would help in identification of suitable coals and in projecting most economically viable washing circuit for cement and sponge iron industries.