Presentation on theme: "Prospects of Non-coking Beneficiation in India Kalyan Sen, Emeritus Scientist, Bengal Engineering & Science University, Shibpur, Howrah. Institution of."— Presentation transcript:
Prospects of Non-coking Beneficiation in India Kalyan Sen, Emeritus Scientist, Bengal Engineering & Science University, Shibpur, Howrah. Institution of Engineers(India) International Coal Congress & Expo, Delhi,2006
INTRODUCTION The choice of washers for effective cleaning of high ash Indian Coals primarily depends on the Washability characteristics of the coal vis-a-vis need of the industry. The present scenario requires thermal power plant feed to be prepared below 34% ash. The same for Cement plants varies from 30 to 32% ash and for Sponge iron industries below 28% ash. Though there is no particle size restriction for Pulverized coal fired plants, others prefer +6mm. Moisture (total) in washed coal should be restricted to 8 to 12%.,.
Coal quality requirements for Power plants Sl.No.CharacteristicsRequirement i.Total Moisture Content Max. 8 to 12% ii.Volatile Matter, (air dry basis) Min. 19 percent by mass iii.Ash percent by massMax. 34% (annual average) iv.Sulpur, percent by mass Max. 0.8 v.Chloride, percent by massMax. 0.01 vi.Size, mmMax. 250
Coal quality requirements for cement industry IS 12770: 1989 Sl.No.CharacteristicsRequirement i.Total Moisture Content (at 60% RH and 40 degree C) Percent by Mass Max. 8 ii.Volatile Matter, (air dry basis) percent by mass Min. 24 iii.Ash percent by mass a) dry process Max. 27 b) wet process Max. 24 iv.Sulpur, percent by mass Max. 0.8 v.Chloride, percent by mass Max. 0.01 vi.Size, mm Max. 250
Coal Quality requirements for Sponge Iron Sl.No.CharacteristicsRequirement i.Total Moisture Content (at 60% RH and 40 degree C) Percent by Mass 6 ii.Grade & UHV kcal/kgB/C, 4940-6200 iii.Fixed Carbonabove 42 iv.Volatile matter (air dry)above 30 v.Ash percentage by mass22 - 25 vi.Initial deformation temp.> 1280 deg. C Vii.Reactivity> 1.7 cc of CO per gm of ‘C’ viii. Size, mm-25 + 3 Source: OSI
Crushing coal before or after washing Crushing raw coal to finer size before washing leads to more energy requirement, products de-watering and water effluent clarification problems than crushing the washed coal to the size as per the requirement of the consumers. Washing, preferably at larger size i.e. at 100mm top size seems to be beneficial considering the poor liberation characteristics of Indian non-coking coals.
Table : 3 Washability Characterization of Coal under Study
Figure 1: Liberation study of Talcher coal (Whole coal basis) x100mm :100-3mm(93.7%, a 41.4%) x25mm :25-3mm(84.3%, a 40.9%)
Choice of washers Cement or sponge iron making warrants deep beneficiation of coal to an ash range 24-27%. Whereas dry or wet deshaling of coal may be acceptable for achieving 34% ash for power plants. Theoretical recovery of washed coal is calculated from Washability data which are shown in Fig 1. Practical yield of clean coal obtained from a coal of definite size consist in dynamic conditions differs from the theoretical yield. The magnitude of difference depends upon the efficiency of separation of the washing unit.
Economical Aspect of washing process 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. Heavy medium washing plants cost more to build and operate, the cleaning efficiency is comparatively more with more yield of cleans. 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.
Conclusion Washability characteristics of the raw coal should be prime criteria in selecting the washing circuit. Considering the poor liberation characteristics Indian non-coking coals should be washed, preferably, at larger size and after washing cleans should be crushed as per requirement of the consumers. Heavy medium baths/cyclones are the most efficient separator for high ash Indian coals having high near-gravity material at separation point and can be operated at wide range of gravity from 1.30 to 1.80 with minimum cleans misplacement. Requirement of low ash coal (around 25 percent) for Cement and Sponge Iron Industry can be met by Heavy medium separators.