Journal of Scienti fic & Industrial Research Vo1.60, January 200 1 , pp 40-47

Design of Cost-effective Coal Washery Effluent Treatment Plant

for Clean Environment

M K Ghose

Centre of Min ing Environment,Indian School of Mines, Dhanbad 826 004

Received:24 July 2000; accepted 06 September 2000

Coal washery eft1uents contain large amounts of suspended solids and high COD values and they create serious water pollution problem in which they are discharged. The solids in suspension are colloidal in nature and possess h igh coking value. Thus in addition to water pollution and si ltation on the river bed, good qual ity of coking coal is being lost. I t shows the i neffec­tiveness of the present effluent treatment system. One large coal washery has been surveyed and the characteristics of the eftluent have been cvaluated. Treatability study for the removal of suspended solids has been conducted. The effectiveness of the use of di fferent conventional and synthetic polymeric flocculates for the removal of suspended solids have been evaluated. S tudies also i nvolved the identification of a suitable flocculent and to develop a methodology for the effect ive removal of suspended sol ids from the et"tluent. A treatment scheme has been suggested which will make it feasible to design a cost-effecti ve treatment plant for coal washery and the supernatant liquid can be recycled or safely discharged without causing surface water pol lution. I t would maintain the acceptable balance between environmental management and sustainable development of coal washeries in the region.

Introduction

Raw coal contains non-coal minerals that are re­leased as pollution discharges at some point in the ex­traction and use . Indian coals are believed to be of drift origin and so it is necessary to wash in washery l . A huge quantity of water is required in coal washing. A plant with a capacity of 1 00 tonnes of coal/h would re­quire in the course of 8h shift from 600,000 to 2,000,000 gallons of water2• Thi s feed water in the working pro­cess comes out from the coal preparation plant in the form of effluents, containing the dirty minerals and im­purities associated with the raw coal'. The fines pro­duced during crushing and other processes are mixed with water and other l iquid medium used in different processes which lead to the formation of slurry. The slurry formed is thickened in thickeners and the thick­ener underflow is filtered and normally mixed with clean coal . Thickener overflow along with the filtrate from the filter section and spill water from other parts of the plant form the washery effluent. The effluent contains large amount of suspended solids and high COD values and they create serious surface water pollution problem in which they are discharged4 .The present paper dis­cusses the status of water pollution due to coal washery effluents, characteristics of the pollutants, their removal

processes and design of cost-effective effluent treatment plant for sustainable development.

Working Principle of Washery

Coal washing is mainly based on the differences in specific gravities between the coal and impurities present in the coal . Since most of the impurities are heavier than the coal itself the coal is separated from its impurities by designed uses of these differences. Specific gravity of pure coal is between 1 .23 to 1 .7 . A change in specific gravity is exhibitedwith change in ash content. Higher ash content gives h igher specific gravity. The raw coal is crushed into smaller sizes and is introduced into a sus­pension or solution where specific gravity has been ad­justed so that the coal floats at the surface, whereas the refuse sinks to the bottom. A washing process flow dia­gram for upgrading of the inferior coal is shown in Fig­ure I Impact on the River Water Quality

The river Damodar originates from the h i l l s of Palamau district of Bihar and ends at West Bengal where it meets the river Hooghly. It passes through the coal fields and is the main source of water supply for domest ic , irri -

GHOSE: DESIGN OF COAL-EFFECIIVE COAL WASHERY EFFLUENT TREATMENT PLANT 4 1

R.O.M . ( -450 mm ) �

C ru s h ing Rotatory b reaker t I Rej e ct J Stock ing & Blending

125-20/13mm Sc reen ing 20/13 m m

I D e sh al ing J � I C ru s h ing I

Deshaling , - 0.5 m m

J I I H . M . C yc lon e T h i c k n e r , � .

D ewatering I Float a t i o n I H . M . Cyclone I I

1 RID S c r e ening t � �

I Dewate r i ng

I T h ickner

D ewate r ing Dewatering Discfilter Centr i f uge RID Screen t

� - D e w ate r i n g

D ewatering H.· Sc reen

Centrifuge t t

To Tai l ing pond I Clean CoalJ l Mi ddling J I Reje ct ion I

Figure I -Process now sheet of Sudamdih coal washery

gation and industrial uses for the towns and industries s ituated in the vicinity of the river (Figure 2). The river Damodar i s extremely pol luted between Bokaro and Durgapur where metal lurgica l , chemical and coal washeries discharge their waste i nto it . At present there are 23 coal washeries with an installed capaci ty of over 45 Mt of raw coaP.Most of them face acute problem in

dewatering of coal fines. None of these washeries has liquid effluent treatment provi sions. Out of 23 coal

• washeries, 1 3 are located in the vicinity of the Damodar river only in the Jharia Coalfield (JCF) of Bharat Cok­ing Coal Ltd . (BCCL)6.

The future availability of prime coking coal is l ikely to be a serious constrain. The import of coking coal at

42 J SCI IND RES VOL.60 JAN UARY 2001

B

Pa lamu H az ari bag h

R an ch i ... . . " : " �

G i r i d i h

H A

'. : .. . ....... .. . . ... . .

S a n t h al Pa rgana .. .....

. . .. . . .8 .

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D h anb a d ........ /.)/ _. _ 'j--.."" I 11 10 13 ',; "\i 'j .J

[' . , ' . ....

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West Beng al

5 10 15 20 Km f I ! I

N

A C O A L WA S H E R I E S

SC A L E

1 Giddi coal w ashery 6 Dugdha coal wash e r y ( P hase J ) 1 1 Jamodaba coal washer y

12 Pathardih coal washe r y l Kandla coal wash e r y 7 Ougdha coal washery ( Phase lJ l

3 Swang coal washE'r y 8 Lodhna coal washery 1 3 8h ojudih coal was h e r y

4 Kargali coal washe r y 9 Sudamdih coal washery

5 Kath ra coal wa sh e r y 1 0 Chasn ala coal wash e r y

@ Washery under s t u d y

Figure 2-Location of some of the coal washeries along the Damodar river

present is to the extent of 25 to 30 per cent of the total requirement1. Mineable reserve of coking coal is est i­mated to be 6,630 Mt in lCF. To upgrade the coals, their washing has to be done and to meet the demand, more washeries wi l l be insta l led in the area. This wi l l create more serious water pol lution problems. Coal washery effluents contain huge amount of suspended solids. The solids in suspension are ultrafractions of coalfines which are col loidal in nature and possess high coking value. Thus, a good qual i ty of coking coal i s be ing lost along wilh the effluents. The loss of huge amounts of coal fines shows the ineffectiveness of the present effluent treat­ment system used i n the washeries . S ince most of washeries are situated near the river Damodar and dis­charge their effluents in it containing significant amounts of suspended sol ids the river looks black due to the depo­sition of coal finesx. Apart from the water pol lution and si ltation on the ri ver bed a good qual ity of coking coal is being lost in the river. It is a great national loss in the context of energy cris is . Suspended sol ids, present in the washery effluents, also hinder the movement and development of aquatic l i fe and make the river unsuit­able for breed ing fish . These also create problems faced by the down stream water works in sett l ing and fi ltering the suspended sol ids from pol luted waterX 'J•

Details of the Study Area

A large coal washery of BCCL was chosen for the study. It is situated on the bank of the ri ver Damodar with its i nstal lation capacity of 700 TPD, within lati­tude 33"38' to 23"40' N and longitude 86"22' to 86"30'E (Figure 2). It was instal led in the year I 978 and takes water from the river Damodar and discharges i ts waste­water again into the river. Detai ls of the process flow sheet are g iven in Figure I . The average ash per cent of raw coal used were between 27 to 30 per cent. The clean coal obtained as a product of the washery contain 1 8 .5 to 1 9 .5 per cent ash. Quantity of feed coal used was 3000 to 3500 TPD. The middl ings and rejects produced were 1 350 TPD and 900 TPD, respecti ve ly.

The washery requires 9 1 kl/h for the process ing which is pumped from the ri ver. The feed water comes out from the washery as dirty blackened colour effluent. The effluent quantity from the washery ranges from 300 M' to 500 M3/d. The suspended sol ids in the washery effluent were monitored'O on regu lar bas is" and were found to range from 5000 to 30000 mg/ l with an aver­age value of 20 1 00 mg/ l (ref. I 2). The maximum permis­sible l imit to discharge the industrial effluent in the in­land surface water is 1 00 mg/1 . Present treatment method

GHOSE: DESIGN OF COAL-EFFECIIVE COAL WASHERY EFFLUENT TREATMENT PLANT 43

involves settling of washery effluent in nine settl ing ponds of different sizes. No coagulants are being used for the separation of suspended solids. Sun dried settled fines are recovered from the ponds. Two ponds are gen­erally used in series. One pond contains suspended sol­ids having h igher ash per cent and are mixed with mid­dlings to improve the quality, whereas the other which contains coal fines of lower ash per cent are mixed with clean coal. The total area uti l ised for settling ponds are 36,800 m2 • The system was found to be far from satisfactiory. The loss of huge amount of coal fines show the ineffectiveness of the present effluent treatment sys­tem. It has been estimated that about 6 to 1 0 tonnes of coal fines are being lost from this washery every day.

Treatability Study of the Washery Effluent

According to the present environmental practice, i t is necessary to treat the effluent water coming out of the tai ling pond to remove the suspended solids and be recycled preferably without creating any pollution prob­lem in the downstream water bodies and to maintain sat­isfactory water balance. The size of the suspended sol­ids in the washery effluent were found to be less than 0.3 MPD �m and they possess all the properties of col­loids and they do not settle down easily I J . They tend to remain suspended in surface water for an indefinite du­ration. The colloids are destabil ised or destroyed by al­lowing them to agglomerate or coagulate into larger par­ticles and thus they can be effectively removed 1 4 . The salt of AI and Fe are found to be suitable for this pur­pose. The synthetic poly-electrolytes are also very ef­fective f10cculants for removal of suspended solids in aqueous medium. They combine with charged particles causing rapid flocculation and an increased rate of co­agulation and settling. The precipitation of a colloid is effected by that ion on an electrolyte which has the charge opposite in sign to that of the col loidal particles.

The treatment for the removal of suspended sol id is based on the principle of an extended sedimentation. An attempt was made to .increase the settling rate by us­ing conventional coagulants such as l ime, alum, and lime mixed with alum!. ' . Synthetic flocculent 'True-floc' was also used which is an anionic copolymer in powder form l6.

The effluent containing known concentrations was sampled in a I L graduated cyl inder. The natural settling behaviour was read by noting the height of interference between clear liquid and solid (settled sludge) at known interval . The zone settl ing curves were drawn with

heights of the interference and the corresponding time intervals. S imilar studies were conducted with different concentrations of suspended solid content in the efflu­ent with varying doses of different coagulants. The zone of settling curves was drawn. The water samples from the upper zone of the cylinder were taken at known in­tervals. by using series of graduated cyl inders and sus­pended solid contents were estimated. The percentage removal of suspended solids was calculated.The results of the settling characteristics of the suspended sol ids by using different types of coagulents are shown in Figures 3 to 7 .

Results and Discussion

Natural settling of suspendeJ solids was found to be very slow and it took approximately 8 h for settl ing 17 • The present study has shown that the supernatant after 8 h of settling was not free from suspended solids. The average settling rate was estimated to be 0.094 cm/min and the s ludge volume being 350 ml . The sett l ing behaviour, using l ime as a coagulant, was 0.39 cm/min. The suspend solid after 2 h of sett ling time amounted to 642 mg/1 .

Alum dosages of 400 mg/ l 00 g, 1 .6g/ l 00 g, 2 .4g/ 1 00 g,and 8g/ l OOg, J 6g/ l OOg,respectively, of suspended solids were used for the effluent containing 25 g/I of SS. The settling rates in cm/min were found to be 0. 1 6, 0.27,

L,()

r 30 E .:! '" u .. 1: '" :s 20 o L en '0; I

10

o

%�--�--�--�3--�4----�5--�6�--�7--�8 Tim(' (Hrs.) --+

Figure 3-Natural settling characterstics

44 J SCI IND RES VOL.60 JANUARY 200 1

LO

E u

.. � i 20

10

40

10

Sudamdih Coal Washery effluenl

Suspended solid: 25 gil Lime dose: 1 6 9 /100g suspended solid

°0�---+-30�--�6�0--�9�0----�12�0��'5�0�--1�BO�--�210 Time ( M inute) --

Figure 4--Settling characteristics using time

Sudamdih Coal Washery effluent

Suspended solid: 25 gIl Alum dose : 1.6 g/100 9 suspended solid

o

°0�---+-30----�60�--�9�0----1�20�--7.1��--�1�80��210 Time ( M inute) -

Figure 5-SettJing characteristics using time

.[ � � ., :;; (; ;:: '"

';;; :r

30

20

10

Sudamdih Coal Washery effluent

Suspended solid: 25 9f1 Dose : 1 2 9 Lime + 2. 4 g Alum/100g suspended solid

o

°0�--�30�--���--�9�0----�12�0--�1�50�--�18�0--�21'0 Time ( Minule) _

Figure 6-Settling characterstics using alum mixed with

0.25, 0.24and 0.26, respectively. The settling rate of 1 .6 g of alum/ I 00 g SS was found to be an optimum dose. Suspended solids in the supernatant were 642 mg/l and the sludge volume was 405 m\ . The settling curves by mixed use of alum and l ime were found to be 0. 1 7 cm/ min and the sludge volume was 475 m\. The applied dosage was 1 2 g l ime + 2.4 g alum/ I 00 g of SS.

Synthetic flocculent 'True-floc' was used at dose of I mgl1 00 SS, 1 .25 mg / 1 00 g of SS and 2 mg/ l OO g SS,respectively . . The settling rates were found to be 2.75 cm/min, 2.2 cm/min. , 1 .5 cm.lmin. for the effluent con­taining 1 0 g/ I SS, 20 g/ I SS and 25 gi l SS, respectively. It was found that "True-floc" is the most effective in clarification of approximately 94 per cent suspended solids within. 10 min, and the settling rate was enhanced by 1 5 to 28 times. After retention for 1 h suspended sol­ids were 97 mg/ l (Table I ) ,

A c lean environment has to be maintained. At­tempts have been made for the effective removal of sus­pended solids from coal washery effluents in order to recover not only the good quality of coal fines, but also to reduce water pollution and the si ltation problem on the river bed.

Lime and alum were found to be equally effective in clarification of effluent. But 'True-floc ' was found to be the most suitable for the treatment of washery efflu-

GHOSE: DESIGN OF COAL-EFFECIIVE COAL WAS HERY EFFLUENT TREATMENT PLANT 45

Table 1 - Material balance of the coal washery

40

30

f E . u

.. u .l! ... 1l 20 · .5.

"0

:c .R' .0; :x:

10

Item

Clean coals

Middlings

Rejects

Suspend solids

(Fine coals)

Feed coal

Throughput

(tid)

700

1 ,350

900

1 0

2960

COill Wilshl!ry ItfflUl!nt SuSpl!ndl!d solid: 20 gIl T rueflOc doS!!: 1 .25 mg/ 100 g Su5p!!l'ld1!d solid

30 40 50 60 Timlt � lo4inut� ---+

Figure 7-Settling characteristic of suspended solids using truetloc

ent among those tested. The laboratory study data are provided in Figure 7 and the process will work on indus­trial scale at the various sites. The study shows that ex­isting method of washery effluent treatment is not ad­equate to reduce the suspended solids to an acceptable limit. This study has been utilised in designing coal washery treatment plant which is of much smaller in size as the retention time is much less as compared to the conventional treatment processes. In addition to this

Yeld Ash in fraction

(per cent) (per cent)

23.7 1 8.5- 1 9.5

45.6 29.5-3 1 .5

30.4 33 . 1 -35.9

0.3 1 3 .2- 1 4.8

1 00.0 27-30

substantial amounts of fine quality coal can be recov­ered and the supernatant liquid can be recycled or safely discharged in the river without causing pollution prob­lem.

Cost Estimates

Lime : Rs 1 20 / kg.C$ t = Rs 43);Alum : Rs 1 30/ kg; and True-floc :Rs 280 / kg.

The optimum dosages are:

Lime: 1 Og/ l 00 g . S S ;Alum : 1 .6g/ l OOg .SS ;and True-floc : 1 .2 mg/ 1 00 g SS.

The cost of treatment of the washery effluent with the aid of coagulant is :

Lime : Rs 1 9,200 /Mt of SS ;Alum:Rs 2080 /Mt of SS; and True-floc : Rs 3 .5 1Mt SS .

Thus, i t is evident that the treatment of washery effluent with the aid of 'True-floc' is the most effective than those of alum and lime.

Design of Clarifier

The rate at which the interface subsides is equal to the slope of the curve at that point'x . According to this methodology critical area for thickening is given by :

A = Q I V,

where, A = Area in m2 ; V = Subsidence velocity in m]/ h ;and Q = Overflow rate in m3/h.

Q and V were calculated with the data on output from tailing pond in m3/h and settling test result.

*Basic Data

Output from the tailing pond = 2 1 m'lh .

46 J SCI IND RES VOL.60 JANUARY 200 1

Table 2 --Settling characteristics with the use of synthetic coagulant "true-tloc"

Suspended solids :20g/ l ,Dose : 1 .25 mgll OOg SS

Time, Height of Sett l ing Suspended Percentage min interface(cm) rate( cm/mi n) solids(g/l ) removal

0 40.0 0.00 20.0 0.00 5 1 6.0 5 .00 1 0 4.0 3 .7 0.80 1 9 1 .0 1 5 3 . 1 2.5 20 2.4 1 .93 25 2.3 1 .54 30 2.0 1 .30 0.454 95.46 50 2.0 0.78 0.325 98.37

60 2.0 0.65

Average settling rate : 2.2 ern/min, sludge volume : 98.0 ml

TAUNG

PRESENT SCHEME

COAL COLLfCT lON PONDS IN SERIES TO BE MANUALLY C L EARED AfTER DRYING

(TOTAL PONOS AVAIAL ABLf ARE NINO

OVER FLOW

PRO POSED SCHEME

POND OVER FLOW

WATER FOR DISCHARGE

C L A RIFIED WATfR FOR RECYCLING OR DISPOSAL

TO RIVER

DRI E D COAL PARTI C L E S FOR MIXING TO C L E ANS & MIDDLINGS

Figure 8--Scheme for treatment of coal washery etIluent

Subsidence velocity V from hindered settl ing por­tion of the curve:

* Area required for clarification :

(O.4m - 0. 1 42m) V=-------

50/60 h

*Overflow rate:

2 1 m'ih x (0.40 m - 0.02m) Q = --

0.40 III

0.258

0.833

2 1 x O.38

DAD

= 0.3 ] m/h.

= 1 9 .95 m '/h.

Q 1 9 .95 m'/h A = -- = --------- = 64. 1 35m�.

V 0.3 1 mlh

Results of Clarifier Design

The test was also used for the slurry where sus­pended solid was 20 gil and the dose of True-floc was 1 .25 mg/ l 00 g of ss. The advance of the interface be­tween the water and settled solids was recorded with time and a graph was plotted between the height of ef-

GHOSE: DESIGN OF COAL-EFFECIIVE COAL WAS HERY EFFLUENT TREATMENT PLANT 47

fluent slurry vs time. The results are given in Table 2 and the curve plotted is shown in Figure 7 . Using the data the area of decant pond of the washery effluent was calculated. Based on the results it was observed that the output of effluent from the tail ing pond was 2 1 m3/h . Ini­tially the height of the suspended coal fines in the cyl in­der was 40 cm. After the addition of coagulant the height of the coal fines was reduced by 25.8 cm. It may be seen from the curve that if the initial rate of settlement was continued the desired level of settlement would have been achieved after 8 min. But the rate of settlement contin­ued to decl ine with the passage of time. A l inear rate of settlement could be observed after 8 min, but after the lapse of 1 4 min the rate of settlement further reduced and the ultimate of 37.5 cm could be achieved after 25 min . It may be observed from the curve that, had there be no reduction in the rate of settlement after 8 min, ultimate settlement should have been achieved in 1 7 min. The cylinder test result also showed that the subsidence velocity V was 0.3 1 m/h,and' overflow rate Q was found to 7 .875 m3/h . The area required being 64.35 m2.At present the total area uti l ised for settl ing ponds was 36,800 m\ which is much more than the actually require, as calculated. With the addition of the synthetic floccu­lent suspended solids were removed in the clarifier. The sludge removed from underflow of the clarifier could be mixed with the clean coal and the c larified water could be recycled or disposed off in the river safely. Figure 8 shows the proposed scheme for design of the washery effluent treatment.

Conclusions

The effectiveness of different conventional and synthetic. polymeric f10cculants for the removal of sus­pended solids present in the coal washery effluents has been evaluated. Studies also involved the identification of a suitable f10cculant and to develop a methodology for the effective removal of suspended solids from the washery effluent .The optimum dosages of l i me, alum,lime+alum,and "True-floc" were found to be 16 g/ 1 00g, 1 .6 g/ I OOg, 1 2 g lime +2.4 g alum/ I OOg and 1 .25 mg/ I OOg,respeclively, of suspended solids. By adopting the suggested treatment scheme it will be feas ible to design cost-effective effluent treatment plant for coal washery and the supernatant l iquid can be recycled or safely discharged without causing surface water pollu­tion. It will help to maintain a clean environment .

Acknowledgements The author is thankful to the Ministry of Environ­

ment and Forests, Government of India for supporting grants for infrastructral faci l i ties at Centre of Mining Environment, Indian School of Mines, Dhanbad. Assis­tance taken from Dr P K Sen and Shri Ashok Kumar at different stages of the work is also acknowledged .

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