Indian Journal of Chemical Technology Vol. 9, January 2002, pp. 54-59

Physico-chemical treatment of coke plant effluents for control of water pollution in India

M K Ghose* Centre of Mining Environment, Indian School of Mines, Dhanbad 826 004. India

Received 3 !"July 2000:revised 19 June 2001; accepted 26 September 2001

Coal carbonizing industries in India are important and are growing every year. Naturally found coal is converted to coke which is suitable for metallurgical industri es. Large quantities of liquid effluents produced in thi s industry contain a large amount of suspended solids. high biochemical oxygen demand (BOD), chemical oxygen demand (COD), phenols, ammonia and other toxic substances, which are causing serious surface water pollution in the area. There is a large number of coke plants in the vicinity of Jharia Coal Field (JCF). Working principle of coke plant and the effluents produced has been described in this paper. One large coke plant was chosen to evaluate characteristics of the effluent and to suggest a proper treatment method. Present effluent treatment system was not found to be adequate and a large quantity of a very good quality coke breeze is being lost, which is also causing siltation on the riverbed in addition to surface water pollution. Physico-chemical treatment has been considered as a suitable option for the treatment of coke plant effluents. A scheme has been proposed for the treatment, which can be suitably adopted for the recycling, reuse or sa fe di sposal of the treated effluent. Various unit process and unit operations have been di scussed. The process may be useful on industrial scale for various si tes so as to maintain a clean e nvironment.

The importance of coal carbonizing industries in India is very high and growing at a rapid pace 1

• It is producing gas on the one hand and coke (for metallurgical purposes) and other by-products on the other. This industry is responsible for entire supplies of benzene, toluene, anthracene and naphthalene and coal tar products, which constitute the basic raw materials for the manufacture of synthetic dyes, drugs and high explosives2

. In coke ovens, naturally found coal is converted into coke, which is suitable for use in metallurgical industries specially in iron and steel for blast furnace, in foundry and for domestic purpose3

. Large quantities of water are used for the quenching of hot coke and also for washing gas produced from ovens. Liquid effluents thus generated during the process operations are highly polluted and difficult to handle. Effluents generated from quenching of coke mainly contain suspended matter of coke breeze. Water used for washing gas comes out as strong ammonical liqueur, which contains high concentrations of ammonia, phenol, cyanide and other toxic substances. These effluents are finally discharged into the ri ver stream and cause serious surface water pollution.

*For correspondence: (E-mai l: Ghosemrinal @hotmai l.com)

Wastewater is a misplaced resource and can be reused advantageously through proper management. A differentiation can be made between water reuse and recycling. Reuse impli es utilization of water that has been previously used for another purpose, and recycling implies reuse of water more than once for that same purpose4

. The approaches in wastewater management should essentially entail waste volume reduction, waste strength reduction and by-product recover/ . For the abatement of surface water pollution a fact-finding survey is essent ial. There are a number of coke plants in operation in the vicinity of Jharia Coal Field (JCF). In some old coke plants sufficient area pis not available for biological treatment. Land disposal for treatment of effluents is cheap and does not require an expensive wastewater treatment plant6

. But the lands available in the surrounding areas mostly belong to private owners. If the effluent is toxic it should not be used for in·igation. So there may be some limitations to dispose the coke plant effluents for irrigation purposes to the lands of private owners. For these reasons physico-chemical treatment may be considered as a suitable option for the treatment of coke plant effluents for control of water pollution. One large coke plant was chosen to evaluate the characteristics

GHOSE: PHYSICO-CHEMICAL TREATMENT OF COKE PLANT EFFLUENTS 55

COAL FROM SOURCES

COt<E FOR 8U•ST FURNACES t. FOUNDI£5

BY PRODUCT RECOVfRY .----~ TAR AMMONIA NAPTHALE~E .

t LIGHT OIL I BTX l

COAL PREHEAT

BLENDING & STORAGE

TAR STOMAGE

TAR FOR PLAtH FUEL TAR REFINERY

EXHAUSTER

RECYCLE FLUSH ING LIQUOR

PRIMARY COOLER

COKE OVEN GAS FOR PROCESS USE

HEAT EXCHANGER

SLUDGE

Fig. I -Coke oven plant wi th by-product recovery

of the effl uent, its quantity and to suggest proper treatment method to reduce waste strength for recycling or to dispose it safely to surface water without causi ng pollution problem7

•

Wurking Principle of Coke Plant Coal is transformed by carbonization into a hard,

porous mass, devoid of volati le matter called coke, Coking process essenti ally consists of heating of coal (pyrolys is) in the absence of air or with limited oxidant, it decomposes to coke liquid products and gases. Coking coals are distinguished by the fact that they begin to soften8 at about 300°C. As the temperature rises, the fluidity of the mass reaches a maximum and finally the mass solidifies to form coke. The gases and vapours evolved du ri ng the plastic stage swells the mass, causi ng the coke to have a hard porons structure9

. Non-coking coal does not soften during heating and products known as char, crumbles easi ly. The production of the coke consists of heating bituminous coal in absence of air to final

temperatures of 900- 1100°C in an oven or a retort, drivi ng volati le products that are recovered as tar, light oil and gas 10

.

Several grades of coals are blended, crushed to about 50 mesh and transferred to a storage bin for chargi ng " . The coking operation is carried out in a battery of 10-100 individual ovens designed to provide relatively un iform production of fini shed products and to recover heat in a way that minimizes the fue l requirements. During the coking period, the coke oven gas is collected through ascension pipe at the top of the oven, scrubbed in the gas collecting mai n with weak ammonia liquor to remove tar, and di scharged to the byproduct recovery operation . At the completion of coking, the oven is isolated from the gas mai 1 and the incandescent coke is pushed into rail cars fot prompt transfer to a quenching tower 12

.

Here, quench water showers the coke to cool it and prevent further loss by combustion, A typical flow diagram for a coke plant with byproduct recoveries is shown in Fig. I .

56 INDIAN J. CHEM. TECHNOL., JANUARY 2002

Effluents from Coke Plant The wastewater produced during the carbonization

and classification of fuel is of three basic types' 3:

(a) Water used for quenching the coke discharged from the retorts or ovens

(b) Waste formed during cooling and washing the gas

(c) Waste formed during the purification of by­products

Large amounts of water are used when the gas is washed but as it is only slightly contaminated it may be recyc led . The most heavily polluted liquors from the coke plants are the washes from the ammonia stills, where the condensate from the gas coolers accumulates. It contains ammonia, phenol, cyanide, sulphide that are toxic to aquatic life. The insoluble pollutants, especially tar, form a surface layer that hinders the access of oxygen from air. Large particles of suspended matter settle on the bed of the stream and cause further pollution. Fine suspended particles clog the respiratory organs of fish, making it impossible for them to live in such a stream. If there is insufficient dilution, thiocyanides, free lime and pyridine may also cause serious difficulties. Much of the dissolved oxygen is consumed by sulphur compounds, mainly sulphides which also have direct harmfu l effects on fish life depending on temperature, pH and dissolved oxygen . Unless removed from the effluent, oil and grease lead to the formation of oil slicks, which limit the diffusion of oxygen from air into water.

Details of the Study Area The coke plant studied, is situated at a distance of

10 km from Dhanbad, 1155 km from Delhi and 269 km from Calcutta at latitude from 23° 38' to 23° 40' N and longitude from 806° 22' to 86° 30' E. It was built in 1915 and is well connected by rails and roads. It takes water from a river Ekra Jore Nallah and discharges its effluent to the same river. This river later joins the river Damodar. Information on the coke plant effluent is given below.

I. Coking temperature 1050- 1 100 °c 2. Coking hours 26-28 h 3. No of batteries 2 nos. in 1915 4. Total number o f ovens 40 ovens per battery 5. Quantity of coal charged 9 t pe r oven 6. Yield of coke 7 t per oven 7. Water consumed for industri al

purpose 700 kl I day 8. Waste water generat ion 175-200 kl/day.

Methods Effluent samples from the coke plant were

collected 14 each week of Monday and Friday. Since phenols were present in the wastewater, the samples were preserved and stored unless they were analysed within 4 h after collection. It was acidified with H3P04 to a pH of 4. One gram of CuSOJI was added to inhibit biodegradation of phenol. The samples were kept at 4°C. Samples were preserved for COD by 2 mill of H2S04 and for BOD samples were refrigerated at 4°C. The samples were analysed as per "Standard Methods for the Examination of Water and Wastewater" 15

. Characteristics of the effluent from coke plant are given Table 1.

Results The pH of the effluent was found to be slightly

alkaline in nature. The dissolved oxygen (DO) was found to range from 1.42 to 3.17 with an average of 2.39 mg/1. COD value of the effluent was also found to exceed the permissible limit of 250 mg/1 as per IS 2490 due to the presence of suspend solids of coal fines and organic matter. High value of BOD showed the presence of biodegradable substances. The aerobic decomposition of organic matter takes dissolved oxygen from the river water and is responsible for depletion of DO. Average oil and grease content of the effluent was found to be 19.61 mg/1. Increase of dissolved solid content observed in comparison to the raw water indicates contamination of inorganic salts. Suspended solids in the effluent were found to be very high ranging from 1521 mg/1 to 2031 mg/1 as compared to the tolerance limi t of I 00 mg/1. Ammonical nitrogen was contaminated during the extraction of coal tar from the coke oven gas ammonical nitrogen . Contamination with nitrate nitrogen, phenolic compounds, cyanides and hardness was also observed.

Discussion Coke plant effluent contains a large amount of

suspended solids. After quenching of coke a large quantity of effluent is generated, which mainly contains suspended coke breeze. Settling tanks have been provided for the settlement of suspended solid, but it was observed that the numbers were not adequate and sufficient retention time for the separation of total suspended solid (TSS) was not maintained. So a large amount of coke breeze is being lost every day along with the effluent. This showed the ineffectiveness of the present effl uent treatment

GHOSE: PHYSICO-CHEM ICAL TREATMENT OF COKE PLANT EFFLUENTS 57

Table !--Characteri stics of coke plant effl uents from the coke plant (average results)

Parameters Min . Max. Avg. Tolerance. Tolerance limit, IS: 10,500, limit IS : 2490 Drinking water Effl uent water

standard standard

pH 7. 1 8.5 8.28 5.5-8.5 5.5-9.0

Temperature (°C) 28 .7 32.0 30.40 40°C

Dissolved oxygen 1.42 3. 17 2.39

COD 525.39 810.30 692.11 --- 250

BOD 63.90 92.30 80.60 30

Oil and grease 16.92 24.23 19.61 . 0 1 10

Dissolved solids 945.00 1287.82 1122.65 500 2 100

Ammonical nitrogen 336.93 562.30 454.95 50

Nitrate nitrogen 43.2 1 55.39 49.30 45

Total hardness 440.21 495.39 462.76 300

Ca-hardness 72.00 96.00 88.35

Mg-hardness 352.00 406.07 37 1.91

Phenols 81.20 123.60 92.82 . 001 I

Cyanide 8.2 20.7 10.3 . 05 0.2

Note: All parameters except pH are ex pressed in mg/1 un less otherwi se noted

system. At the discharge point, the river water has taken a black coloured look due to the deposition of coke breezes. So, in addition to the surface water pollution and siltation on the riverbed a huge quantity of costly coke breeze is being lost every day 16

• High BOD, COD, phenol content of effluent are causing serious surface water pollution. The test result reveals that proper treatment of the effluent is needed before discharging to surface water bodies.

Physico-Chemical Treatment of Coke Plant Effluents Means of treatment in which the application of

physical forces predominate are known as unit operations. Means of treatment in which removal of contaminants is brought by the addi tion of chemicals or by biological actiVIty are known as unit processes 17

• The unit operations and chemical unit processes occur in various combinations in different treatment systems, but the fundamental principles of their operation do not change 18

. The principles developed can be applied to design the coke plant effluent treatment 19

• The proposed scheme for the phys ico-chemi cal treatment o f coke-plant effluent is given in Fig. 2 and unit operations and unit processes involved have been discussed.

Unit Operation Mixing is an important operation where one

substance must be complete ly intermixed with

another. Lime is to be mixed in the secondary settling tanks . Chemical s are also mixed with sludge to improve the de-watering characteristic before vacuum filtration. Flocculation is to be promoted by gentle stirring with slow moving paddles. The action is to be aided by the installation of stationary stator blades, located between the moving blades, that serves to breakup the mass rotation of the liquid and promote mixing. Increased particle contact will promote floc growth. Agitation should be carefully controlled so that the floe particles will be of suitable size and will settle readily.

Sedimentation of suspended particles that are heavier than water is to be carried out by gravitational settling. This operation is to be used for chemical floc removal and for solid concentration in sludge thickeners . The primary purpose is to produce a clarified effl uent, but it is also necessary to produce sludge with solid concentration that can be easily handled and treated . [n the design of sedimentation basins, due consideration should be given to produce both a clarified effl uent and a concentrated sludge. Multimedia filtration is to be used to reduce fine suspended partic les from the wastewater before its passing the activated carbon adsorption, Mechanical and dissolved air flotation thickeners are to be used to thicken sludge. Solid concentration of 5 - 6 % or more can be obtained from mixtures of sludge. Volume reduction is very desirable. Important

58 INDIAN J. CHEM . TECHNOL., JANUARY 2002

EFFLUENT FROM PROCESS FOR TREATMENT

a: :!z oo .... -

a: .... !:::; "-Z

0 z-o>-

a: lr~

<{ ,_Vl zO u...a. :::> Vl _.-u.D ....

RECARBONATION l:~ .,a. ~<-a: Q. a:O: .... 0 ~ TANK •a: -,_. <(0

8u... 0: >-_, ._,Vl ,_. _._ D t-Vl Vl :::>u.. e< <{::>

:I: ......... "' 1-0: a: a:

:I: ~0 1-z _ .... 1-u <{

UHf SlU06f

SlOW DOWN

Fig. 2-Proposed scheme for complete treatment of coke plant effluent

considerations in designing a thickener are to provide adequate time to meet peak demands and to prevent septicity with its attendant odour problems during the thickening process.

Ammonia Stripping Stripping with steam or hot gas such as air can

remove most of the ammonia, carbon dioxide and hydrogen sulfide20

. Other materials such as phenols, cyanide and light organics can also be removed. Ammonia stripping towers can be very effective in ammonia removal. The basic equipment for an ammonia striping system includes chemical feed, stripping tower, pump and liquid spray system, forced air craft, and carbonization system. Important design parameters for the system are average and peak design flow, surface loading rate, chemical dosage, tower height and air-to- water ratio.

Ion exchange for Ammonia Removal Wastewater is to be passed through a bed of

clinoptilolite (a zeolite resin) which selectively removes the ammonium ion21

• When the resin becomes saturated, it is regenerated with a lime slurry containing sodium chloride. The lime solution after regeneration of the resin must be processed to remove the ammonium ions that the solution can be reused to remove the ammonium ions, so that the solution can

be reused. Air stripping of ammonia is a feasible method. The stripped ammonia gas is passed through an absorbed material, which has high selectivity for ammonia. Proper disposal of ammonia bearing absorber material is necessary . Major equipment includes an ion exchange bed, bed regeneration system, ammonia stripping tower and ammonia absorber material. The design parameters are average and peak design flow, ion-exchange capacity of the resin bed, concentration of ammonia, percolation rate etc6

. Singh and Prassad22 reported that the 13x molecular sieve showed more than double the adsorption capacity for ammonia compared to SA zeolite.

Carbon Adsorption Carbon adsorption is to be ·used to remove soluble

refractory organics. The process consists of entrapping organic material on the carbon surface. The most common method is the use of granulated carbon columns. The treated wastewater is percolated through the column until the column becomes saturated with organic material. It is then removed from service and burning off the organic materials in a special furnace regenerates the activation. About 5% loss of carbon can be expected with each cycle. Activated carbon can effectively remove bacteria and viruses. It also removes organometallic compounds,

GHOS E: PHYSICO-CHEMICAL TREATMENT OF COKE PLANT EFFLUENTS 59

pesticides, chlorinated compounds, chlorine and many other compounds that are not removed in the conventional biological secondary process. Major equipment commonly needed for the system are carbon column, granular activated carbon, feed and back wash pump and piping, and carbon regeneration system. Common design parameters include average and des ign flow, influent characteristics, effluent quality , contact time and adsorption capacity of carbon.

Conclusion Coke plant effluents containing a large amount of

suspended solids, high BOD, COD, phenols, ammonia and other toxic substances, are causing serious surface water pollution problem in the area. Physico-chemical treatment can be considered as a suitable option for the treatment of coke plant effluent. By adopting the suggested treatment scheme it may be feasible to design an effective treatment plant for coke plant effluent. and the treaied effluent · can be used for recycling or safely discharged to water bodies.The methodology adopted may have formed a guideline for the treatment of coke plant effluent and can be useful on industrial scale at various sites and a clean environment can be maintained.

Acknowledgements The author is thankful to Ministry of Environment

and Forests, Govt. of India, for supporting grants for infrastructral facilities at Centre of Mining Environment, Indian School of Mines, Dhanbad. Assistance from Sri S . Roy, Bihar State Pollution Control Board and from Dr. P. K. Sen, MECON Ltd. for the work is also acknowledged.

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