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Need for Pilot Plant Investigations and Role of a Consultant for the Design of Large Capacity Coal Washeries for Thermal Power Plants in India
kalyan sen, former Director, CFRI-cum-CMRI, Dhanbad.

Pilot plant Investigations

After lab scale experimentation, with a specific coal, Pilot Plant trials with a large sample of the coal is generally prescribed for new or virgin coal deposits. The installation of large washeries is in the offing for beneficiation of non coking coals, particularly for modern power plants. However, good quality reliable data from actually working washeries are not generally available for the outlying coal deposits like Talcher, Ib Valley, Pench Kanhan, etc. coalfields.
Most important data required for design of a large washery is the size distribution of coal. Studies and research at the Central Mining Research Institute (CMRI), Dhanbad, on the size distribution of raw coal, either blasted or sheared by picks mounted on drums, have developed simulation programs to predict and control both the top size and size distribution.
However, the size distribution can be varied by use of different types of secondary crushers like Single Roll, Double Roll, Impact Crusher, etc. For optimum design of the screening and washing circuits, the size distribution data of the feed to the plant is extremely involved and important. It should be understood, that no amount of data generated by crushing bore hole samples (-50 mm) to less than 13 mm or CHP product sample (-200 mm) subsequently crushed manually or by manual feeding to a small laboratory crusher dealing with a quantity of about 500 kg or less, cannot produce industrially achievable PSD.
RRB plot of the screen analysis data helps to identify normalcy, or otherwise, in crushing / sizing of the parent coal. Where crushing was not done following Nominal Top size concept (about 5% should be oversize above the nominal top size of crushing) or was crushed manually, the distribution pattern of over or under crushed sample shifts away from that expected. The regression analysis of the RRB equation can also be used to predict the amount of coal expected to be retained on any intermediate screen size or even by extrapolating to higher sizes screens with a limited confidence.
Normally, RRB is suitable for naturally crushed coal or many other minerals or even glass or quartz. Coal is, however, generally crushed on scalped feed basis. In such cases the straight line bends up words at those sizes which have been used as aperture sizes in scalping/ check/ control screens. There is a common tendency for the plot to depart from a straight line in the extra large sizes, as well. It is also reported that n increases as particles become very fine, so that at the fine end there may also be a departure from linearity. This departure is in the opposite direction from that at the coarse end. Thus, over a very wide size range, the RRB plot of an actual cumulative coal-particle size-distribution tends to be S-shaped. This result is not surprising, as the law is an exponential one and assumes a finite number of particles larger or smaller than any arbitrarily assigned or naturally imposed limit. In fact, an upper limit is set physically by coal bed thickness, and a lower one by micellular size, and no particles larger or smaller do, in fact, exist. Furthermore, if a linear frequency distribution curve is plotted, it usually is bimodal. The larger mode belongs to the distribution of the complement produced by the principal comminuting event, and the smaller to the size distribution of the residue and resembles the distribution produced by the original mine breakage or other previous event.
Size analyses of carefully taken samples obtained from strategic points in a coal preparation pilot plant may be used to determine the extent of degradation produced by each sizing, washing, and transporting operation, as well as by any deliberate comminuting step. Combined with a size-distribution analysis of the plant feed, these data can be used to control operations for optimum size-consist and predict with reasonable accuracy the changes in size distribution which could occur as a direct result of contemplated changes in the preparation system.
However, it is unfortunate that most of the data generated in testing laboratories or academic institutions with limited infrastructure do not usually follow the practically achievable trend. Thus, Pilot Plant experimentation at a demonstration scale, say 40 tph, seems to be the minimum capacity required for finding out the parameters of the preceding belt conveyor, chute, crusher, etc.
Subsequently, the screening of the crushed product (say -100 mm) at 13 mm or so, for most of our Deshaling plants, will also require a screen of adequate dimension to produce repeatable Screen Analysis (SA) data. The screen has to be loaded with the minimum rate (say 30 tph) per meter width of the screen unit for reliable data.
One should also consider the kinetics of screening, which determines the optimum length of the screen to achieve desired performance efficiency (less than 5 % misplacement). No amount of laboratory screening using sieves or wire mesh can produce acceptable data because of the effect of moisture, adherence of sticky fines along with the courser product, etc.
If water spraying is desired while screening, particularly at lower size say 6 or 3mm, to overcome the problem of adhering of fines with the coarser product, mainly for H.M. circuits, to reduce spoiling of the medium due to presence of large amount of coal / clay fines, the designer will ask for optimum rate of water spraying wrt tonnage of coal. Such data cannot be generated without Pilot Plant trials.
The float and sink study on a requisite amount of coal (as per IS           or ISO               ) crushed as mentioned  above  produces a set of reliable and repeatable results which are further processed by simulated computer programs  to predict and compare the achievable yields of clean coal by alternative circuits. Here again, a study conducted with a reasonably good quantity of raw coal crushed judiciously as mentioned above, will produce both washed coal for downstream  trials, like combustion study, ash erosion, clinkering, agglutinating, etc . characteristics  of ash forming minerals and last but not the least for confirmation and corroboration of the computer predicted results. 
One of the most difficult problems to be encountered while handling large scale washing of Indian power coals, is apprehended to be the pollution control arising out of the effluent generated by wet coal washing like Jig, Heavy Medium (HM), Barrel Washes, Spiral, etc. It is estimated that at least 0.5 % of the feed to the washery will produce slurry containing ultrafine particles (less than 50 µ). The recovery of ultrafine solids by application of flocculants, followed by pressure filter, micro flotation or oil agglomeration, can only be experimented fruitfully in a pilot plant. As the size of coal particles is less or even micro-fines, the capacity of a continuous pilot plant can be as low as 2 to 3 tph. This problem is likely to aggravate further as the less matured non-coking coals contain higher inherent moisture (due to presence of hydrophilic OH groups in the coal matrix). The experience at Rajrappa washery while washing non-coking coals that the colour of the Damodar river water turned blackish or tasted brackish is the case in point.
Thus, the choice of a suitable crusher, depending upon the liberation characteristics of ash forming minerals vs good coals, the choice of suitable screen of particular type and size, finalization  of the type and size of the washer, the optimum parameter and the range of variation for HM circuits, the choice of flocculants, etc. all require experimentation in a suitable size Pilot Plant. The quantity of coal required for an 8h trial is at least 500t, if the top size of feed coal is about 100mm .
It may be stressed that coals of quantity more or less 1000t were passed through the CFRI Pilot Plant in 1960s for coking coal washery designs, and since 1980s for non-coking coal washery circuits like Lalmatia,ECL (1984), Ananta,MCL (for Corex process), Dipika project (USAID), etc.
 One of the biggest constraints of a pilot plant design and operation is that requisite numbers of on-line samplers and ash-cum-moisture meters are disproportionately costly wrt the total civil and machinery cost. A C-type ash monitor for moving belt conveyors costs no less than a crore of rupees. Even then, its guaranteed accuracy is

Role of a Consultant for Designing of a Coal Washery

Perhaps the most important step to take is to engage a consultant, an engineer or an engineering organization to act for the entrepreneur in the solution of the optimum coal washery design problem. The fee of this expert advice for the design of a proper plant which is economically as well as physically suitable for the problem, will usually be a small percentage of the entire cost. The engagement of consultants who are known experts in this field will save money, as these men know all the angles, they know all the authorities, they know the laws on pollution and they know the means of solving the particular problem. They do not need to waste time studying a phase of engineering with which they’re not normally acquainted, and they know where to go to get any information they need which will help them solve the problem. They will satisfy the authorities at not a great cost to them.
Coal washery wastes are frequently very complex and although in many cases, known and common methods and processes of treatments may be found successful, a complete knowledge of the limitations and the advantages of the different recognized methods is required to be able to select that one which will best solve the problem. In many cases, industrialists have been approached with wonderful panaceas and unsuspecting guinea pigs for someone with a bright idea. Many piles of scrap iron lying around in the yards of industrial plants represent good money thrown away on unsuccessful experiments.
This discussion is not intended, by any means, as an argument against research and development of new and more economical methods of treatment of industrial wastes, but such research should be carried out by proper organizations, many of which are now functioning through the apportionment of funds by an industry to one of these to carry out work on a given subject. This is much better than spending a lot of money on a full scale plant to be operated on an un-tried process which may result in an utter failure. Subsidizing a research laboratory or school to undertake the problem in a knowledgeable way usually not only gives the industrialist the answer at a relatively low cost but will bring added prestige to the research organization which has worked out this idea. In doing this, all the kinks and the headaches in the process are ironed out in the beginning and when the final report is made to an industrialist, a full scale plant may be built with confidence in its success. In building such a plant for an untried process without the necessary preliminary research, the possibility arises that if the plant does not work or if in working out the bugs in the system, a bad effluent is turned out, fines or a lawsuit for damages may result.

 

 

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