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 theWashability characteristics of the coal vis-a-vis need of the industry.The present scenario requires thermal power plantfeed to be prepared below 34% ash. The same forCement 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%.

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Coal quality requirements for Power plants

Sl.No. Characteristics Requirement

i. Total Moisture Content Max. 8 to 12%

ii. Volatile Matter, (air dry basis) Min. 19percent by mass

iii. Ash percent by mass Max. 34%(annual average)

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 cement industryIS 12770: 1989

Sl.No. Characteristics Requirement

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. Characteristics Requirementi. Total Moisture Content (at 60% RH and 40 degree C) Percent by Mass 6

ii. Grade & UHV kcal/kg B/C, 4940-6200

iii. Fixed Carbon above 42 iv. Volatile matter (air dry) above 30

v. Ash percentage by mass 22 - 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 leadsto more energy requirement, products de-wateringand water effluent clarification problems thancrushing the washed coal to the size as per therequirement of the consumers.

Washing, preferably at larger size i.e. at 100mm topsize seems to be beneficial considering the poor

liberation characteristics of Indian non-coking coals.

Size Analysis

ROM Coal Crushed to 100 ROM Coal Crushed to 25 mmSize,mm Wt.% Ash% (dry) Size,mm Wt.% Ash% (dry)100-75 29.7 42.5 25-13 57.4 42.275-50 25.0 40.8 13-10 8.3 41.350-25 19.0 41.1 10-3 18.6 39.325-13 11.2 39.7 -3 15.7 38.813-6 6.0 37.8 100.0 416-3 2.8 37.33-0.5 3.7 34.9-0.5 2.6 41.0

100.0 40.6

Proximate Analysis(Air dried) (dry basis)

Moist% 6.5 - IDT >1400 deg CAsh% 37.8 40.4V.M. % 25.9 27.7 Equlibrated 4.7F.C.% 29.8 31.9 Moist.%

Table : 3 Washability Characterization of Coal under Study

0

10

20

30

40

50

60

70

80

90

100

20 25 30 35 40

Ash%

Rec

ove

ry %

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 coalto an ash range 24-27%.Whereas dry or wet deshaling of coal may be acceptablefor 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 sizeconsist in dynamic conditions differs from the theoretical yield.

The magnitude of difference depends upon the efficiency of

separation of the washing unit.

Jigs are used more frequently than heavy-medium vessels becauseof their large capacities, low cost, adoptability to wide size rangeof coal and simple technique.

But they have the limitation in operating efficiently below 1.60 sp.gravity.

Cleaning non-coking coal at desired ash level for both cement andsponge iron industries may be achieved in Heavy mediumseparator.

A possible circuit may be, screening the coal to 25/13mm andwashing the coarser fraction in HM bath followed by washing thefiner fraction (-25/13 +3mm) in HM Cyclone.

Choice of Washer (Contd....)

Figure 2: Practically Achievable Yield of Washed coal in different washers (Whole coal basis)

0

20

40

60

80

100

20 25 30 35 40Ash %

Rec

ove

ry %

Theoretical

Baum Jig (Im 0.18)

Batac Jig (Im 0.12)

HM (Im 0.05)

Raw Coal (100-3mm): 93.7%, Ash 41.0%

Rotary Breaker

Figure 3: Practically Achievable Yield of Washed coal with different washers (Whole coal basis)

0

10

20

30

40

50

60

70

80

90

100

20 25 30 35 40Ash %

Rec

ove

ry %

Theoretical

Baum Jig (Im 0.18)

Batac Jig (Im 0.12)

HMC (Im 0.05)

Raw Coal (25-3mm): 84.3%, Ash 40.9%

WOC

Rotary Breaker

Economical Aspect of washing process

A key element in process selection is maximum recovery of theproduct from raw coal at lowest possible cost.

Selection of suitable circuit must be backed up by computersimulated washability data for prediction of yield gain over othercircuits.

Heavy medium washing plants cost more to build and operate, the

cleaning efficiency is comparatively more with more yield ofcleans.

HM circuit may yield sometimes 6-8% extra washed coal atparticular ash level when compared to the water only basedwashing units like jig ,water only cyclones or hydro- separators.

Conclusion

Washability characteristics of the raw coal should be prime criteriain selecting the washing circuit.

Considering the poor liberation characteristics Indian non-cokingcoals should be washed, preferably, at larger size and after washingcleans should be crushed as per requirement of the consumers.

Heavy medium baths/cyclones are the most efficient separator forhigh ash Indian coals having high near-gravity material atseparation point and can be operated at wide range of gravity from1.30 to 1.80 with minimum cleans misplacement.

Requirement of low ash coal (around 25 percent) for Cement andSponge Iron Industry can be met by Heavy medium separators.