Beneficiation of difficult-to-wash Low Volatile Coals
of Jharia and Bokaro Coalfields
Kalyan Sen and Rahul Das Gupta
Central Fuel Research Institute
Dhanbad 828 108, INDIA
Abstract
The low volatile coking coal (LVCC) constitutes about 50% of the total coking coal reserves in India. These coals are characterised by high raw coal ash content (37 to 40%) and very difficult washability potential. The present production from LVCC is at least 16 to 18 mt per year, and is entirely despatched for non coking coal use.
If these coals are treated in the existing washeries which are generally 2 or 3-product systems, the cost of clean coal is high as the by product sale value is low. It has been proposed to produce a new type of by product which enjoys high demand, namely, foundry coal having 27-28% ash and good coking properties. As the market value of foundry coal is about 2.5 times of that of middlings, the production of clean coal by this concept of 4-product washery may be competitive to the world market.
The paper suggests the concept of 2-stage washeries where the first stage will have 3-product jig to produce rejects (25-28% yield at 60-65% ash), power house coals (18-22% yield at 33-35% ash) and the remaining as pre-cleans (50% yield at 22-24% ash) for the beneficiation in the second stage by Heavy Medium washers & flotation to produce steel plant cleans (25-28% yield at 17-18% ash) and foundry coal (22-25% yield at 27-28% ash). Thus, about 50% of LVCC can be utilized as coking coal.
After successful trials at Steel Plant (pilot) coke ovens on clean coal beneficiated at CFRI pilot plant, the MoC has been spear heading a Demonstration Plant of capacity 1 mt per annum to study utilization aspects of LVC coals.
Introduction
Generally, the lower seams of any coalfield are likely to be more matured than the upper seams and consequently exhibit lower value of volatile matter, as per Hilt’s law1. It is no wonder that the lower seams of Jharia or East Bokaro coalfields have 15 to 16% volatile matter, as received basis. These coals show higher maturity (Ro = 1.3) and are expected to occur in all coalfields of the globe at a greater depth wherever upper seams are being presently mined and utilized as prime coking coal. Fortunately, India is blessed with a huge reserve2 of such coals (15,680 mt) which can be seen from the Figure 1. If it is possible to develop an economically viable technology for use of such coals, the country may even export the knowledge to others like South Africa, Botswana, etc. where the reserves of such coals are likely to be plenty.
Favourable Mining Conditions
Geological Features Indian coals belonging to Permian period were deposited in warm and humid climates, after a period of glaciation. The glaciation left a large number of detrimental materials in the glaciated region. The rivers drifted these materials and the minerals from the provenance along with vegetal matters to the basin of deposition. As the rate of subsidence of Gondowana basins was moderately higher, preservation of coal as well as mineral matter was very common though intermixed, which resulted into thick layers of coal seams with high mineral matter content. This is a marked contrast to that of Carboniferous coals of Europe and North America where coal deposits were accumulated in tropical humid and “in-situ” conditions. As a result these coals have very low mineral matter content. The Gondowana formations also vary widely in nature, quality and its petrographic constituents. While coaly portion of Carboniferous coals are enriched in vitrinite and liptinite contents, that of Permian coals are enriched with inertinite content (even upto 60-70%). The in-situ moisture contents of Gondowana coals are relatively high due to larger proportions of high porosity inertinite materials which result in lower heating value. Shallow depth of burial and absence of major igneous activity beneath the basin resulted in less maturity of Gondowana coals than the Carboniferous coals4. |
These LVC (low volatile coking) coals outcrop near the fringe areas of any coalfield and are easy for open cast production as these are free from over laden upper seams. The present production of this coal from V-VI-VII seam of Jharia coalfield is about 8 to 10 mt per year and about 6 to 8 mt per year from East Bokaro coal-fields3. The production cost is low due to low over burden : coal ratio (upto 3) and low slope (1:15). There is every possibility of high scale mechaniza-tion like Piparwar projects for these outcropping areas.
Washability Potential
Unfortunately, due to particular geological formation these lower seam coals in Jharia and East Bokaro coalfields are of very difficult washability potential (see inset). One typical coal from eastern side of Jharia belt was crushed to two different sizes. These were deshaled at 1.8 gravity and the above 0.5 of both the sizes were tested for Float & Sink analysis. The Mayer curves of the two sizes were shown in Figure 2. Thus, liberation potential of coal/mineral particles from LVCC by progressive size reduction is insignificant5.
Existing Scenario
These coals, popularly known as NLW (Not Linked Washery grade), are despatched generally to thermal power plants and some selects to beehive coke ovens. Existing washeries which were designed for upper seam coals can not produce techno-economically acceptable product from LVCC. Most of the existing washeries is being operated basically as 2-product washeries producing steel plant cleans (40-45% yield at 19% ash) and remaining middlings at 40-45% ash with little or no rejects. When LVC coals are directly treated in these washeries, the sink will have more than 45% ash with no market potentiality. Such cases, though technically very difficult, will produce hardly 25-30% of cleans at 18% ash and 70-75% will require disposal as waste. The cost of clean coal is likely to be more than 5 times of that the raw coal6.
Likewise, a 3-product washery where the sink from the first stage washing has the possibility of being rewashed in the second circuit to yield “sweetened middlings”, cannot produce economically viable cleans from LVCC. This is because the old deshaling jigs had low reject disposal capacity (hardly 5% of the feed coals) whereas the present low volatile coking coals require at least 20% rejection. In addition, the all-cyclone washeries suffer from congenital defects of cyclones circuits and are not possible to be run under very stable condition with the aim of producing 30% overflow and consequently discharging more than 70% of solids through the underflow. If these circuits are theoretically accepted to be used for washing NLW coals, the yield of cleans will not be more than 25-30% at 18 % ash and remaining by products will be 10% rejects with 60% ash and rest middlings with 45% ash with no market outlet. Even if it is aimed to produce 20% rejects, the ash content in middlings will be about 41% and cannot be sold to distant power plants due to pollution control measures.
Pilot Scale Study at CFRI
About 1200 t of V-VI-VII-VIII seams coal from eastern side of Jharia was beneficiated in the CFRI pilot plant. The cleans at 17-18% ash content was carbonized at CFRI, RDCIS & subsequently tested at Bokaro Steel Limited test coke oven. The results were highly encouraging and for the first time possibility of blending such washed coal in coke oven charge as the prime coking coal component was established. The Petrographic and Chemical Analysis of cleans and Coke produced there of are presented in Table 1. It is apparent from the table that CSR & M40 values are very high and lower CRI & M10 values indicating that the washed coal is direct Prime coking type.
Table 1 Petrographic and Chemical Analysis of LVCC |
|
|
|
Washed Coal |
|
Petrographic Analysis |
|
Vitrinite % |
53.2 |
Semi Vitrinite % |
4.8 |
Exinite % |
0.3 |
Inertinite % |
31.5 |
Mineral matter % |
10.2 |
Mean Ro % |
1.27 |
Chemical Analysis |
|
Moisture % |
0.9 |
Ash % |
18.3 |
V. M. % (dmf) |
21.0 |
S.I. |
5.0 |
Max. Fl. (ddpm) |
150.0 |
|
|
Coke Property |
|
Porosity % |
48.5 |
CRI |
12.6 |
CSR |
72.9 |
M40 |
82.6 |
M10 |
9.8 |
Proposed Washeries
It is apparent that the circuit for beneficiation of NLW coals must have enough capacity to reject at least 30% of the raw feed. The proposed circuit (Figure 3) will, therefore, have one 3-product jig to produce:
In fact, these pre-cleans will have washability characteristics similar to upper seam coals produced from underground mines. By second stage washing of such pre-cleans in existing heavy medium and flotation circuits, only two saleable products (no rejects; no middlings; in the 2nd stage) will be obtained;
The cost of steel plant cleans will be 3 times of the raw coal costs for this case.
Acknowledgment
CFRI gratefully acknowledges the assistance of M/S Bharat Coking Coal Limited, Dhanbad for sponsoring the activities on LVCC at various stages. Financial assistance of Ministry of Coal, Government of India, as Coal Conservation & Development Advisory Council towards modernization of the Coal Washing Pilot Plant is also gratefully acknowledged. Thanks are also due to SSRC for approval of the 1 mt Demonstration Plant at the Patherdih Washery Complex on LVCC.
References