SKELP MILL

INTRODUCTION:-

          The Skelp Mill was set up 1.6 MT/year expansion program of DSP. Annual rated capacity is now 2,50,000 T/year.

          The general size of the skelp roll is given as:

                   212mm × 2.5mm × 200m

                   234mm × 3.1mm × 200m

SLAB REHEATING FURNACE

          Two zone-fired side charged and side discharged type of furnace and is fired by fuel, which is a mixture of CO & Blast furnace Gas in the ratio of 1:1. Dual fired burners have been installed to fire the furnace. Oil is also sometimes used instead of gas. The heating capacity is 60 T/hr.

MILLS STANDS AND LAYOUT:

          11 horizontal & 6 vertical edgers. H₁-H₆ constitute the roughing train while H₇-H₁₁ constitute the finishing train.

STANDS	DIAMETER OF ROLLS	BARREL LENGTH
H1-H6	16”	16”
H7-H11	13”	16”
Edgers E1 & E2	26”	One pass
Edgers E3 & E4	16”	Four passes

All stands are individually driven.

MERCHANT MILL:

          In the merchant mill mainly rods and bars generally made. Merchant mill was commissioned in the year 1961. It has been modernized with a capacity of 280000 MT/yr of plain rounded and thermo-mechanically treated (TMT) bars.

          Here a special process known as Thermax Process (after modernization) makes tore steel or structural steel with ribs and lugs. Previously it was seen that structural steel made by twisting failed in case of fire accidents. In the Thermax process the steel rod is passed through tubes and water sprayed on it as a result of which the outer side gets hardened due to formation of martensite while the inner core remains ductile as tempered marensite is formed. The steel prepared by this process was seen to withstand high temperature without failing. Rounds less than 10mm diameter are called rods and greater than 10mm diameter are called bars. They are prepared from the rolled billets from the billet mills. The process is explained below.

          The billets of 100 sq. mm and 9m lengths from the billet mill are stacked quality wise in the billet stockyard. They are then transferred by cranes to the aspirating skid table which automatically de-piles batches of product into single layer.

          First the billets are reheated in a reheating furnace. The furnace is two zones fired type, and also side charged and side discharged. The capacity of the reheating furnace is 70 tons/hr. It is fired with the help of a mixed gas (CO gas + blast furnace gas). Its calorific value is 300 BTU/cu. Ft. It has got dual burners.

MILL STANDS:

The mill consists of total 13 rolling mills. Of these there are 7 roughing stands, 2 intermediate stands, 4 looping stands and the arrangement of which is given below.

·        Roughing stands – 1 to7

·        Intermediate stands – 8 to 9

·        Looping stands – 10, 11, 12 and 14.

Each stand is driven through pinion housing and reduction gearing by individual DC motor.

·        400 HP for stands 3, 4, 5.

·        500 HP for stands 1, 2, 6 and 9.

·        600 HP for stands 10 and 11.

·        800 HP for stands 12 and 14.

Four 180⁰ repeaters are provided between stands 9-10, 10-11, 11-12, & 12-14 to handle sections up to 25 mm diameter rounds & ribbed bars, beyond the finishing stands. There is drum type rotary shear, which cuts the bar to such length that can be accommodated on the cooling beds, and finally the bars are packed and automatically packed and bundled in the dispatch bay. The bars produced generally are 16, 18 and 20 mm diameter.

The major features envisaged after the modernization:

Ñ     Rapid quenching by Thermax process to produce TMT bars of high strength combined with toughness and ductility.

Ñ     CROP cums COBBLE SHEAR to crop the front end of the bar and also cropping in pieces, which is to be cobbled.

Ñ     DOUBLED STRAND ROOLING of 12–16 mm round by slit rolling process, which slits a single billet into two parts longitudinally.

AUTOMATIC BUNDLING & BINDING facilities for bar counting, weighing, bundling and binding.

BLOOMING  & BILLET MILL:

         

          This mill is also known as the Mother Mill and is responsible for catering to the needs of the other mills. The ingots produced are first sent to this mill. Top pouring in the steel melting shop produces these ingots. The ingots are first fed into a soaking pit. The ingots have to be heated and in these pits at a temperature of around 1300⁰C. These are mainly reheating furnaces. These pits can handle 14 ingots at a time and they are side fired. The heating and the soaking time is 3/2 of track time. Soaking helps in uniform heat distribution. TRACK TIME: it is the time interval between heat tapped in steel melting shop to last ingot charged. There are four soaking pits whose specifications are as following:

          After soaking is over the ingots are taken out of the furnace and then fed into the rolling mills by the means of an EOT Crane, this crane is also known as Vertical Ingot Charger. Its capacity is 35 T. The ingots are placed on the ingot chariot running on a special track along the outer row of pits for the length of the building and up to the mill-receiving tables. The bloom is prepared by rolling the heated ingots a given numbers of times in two high rolling mills reducing cross section to the required size. The Fish Tails produced during the rolling process are cut by shear and again cut to required size before being sent to dispatch. The ingots are also produced by bottom pouring method are also rolled to produce blooms, which are then sent to the Wheel & Axle Plant for the production of the axles. A brief explanation of the rolling mills employed is mentioned below.

42” BLOOMING MILL:

          The blooming mill is 42”, two high reversing type arranged for twin motor drive 3000HP DC motors at a mill speed of 0/40/80 rpm. The roll diameter varies in size from 950 to 1100 mm and hence it is called 42” rolling mill. Generally the mill rolls the ingots down to 12”×10” blooms in approximately 13 passes.

          All blooms from the 42” mill are sheared by a 1200-ton shear. The front and the back ends of the blooms which develop fish tails during rolling are cropped and discarded. The shear can cut a max cross section of 256 sq. inch.

32” INTERMEDIATE MILL:

          This is a two high reversing mill driven by one 4500 HP DC motor. The roll diameter varies from 800 mm to 925 mm. The rolls are spring balanced. The roll is driven by the main motor through pinions and universal spindles with the speed range of 0/80/120 rpm. All the blooms are cut or cropped to length at the secondary hot bloom shear with the nominal blade load of 700 tons and is of the same construction as that of the primary shear.

The size of the blooms produced are 230mm×160mm×4.4m.

          The blooms having some defects like surface cracks etc. are unsatisfactory for further rolling operation. They are removed by the process known as “SCARBING”.

BILLET MILL:

          It is continuous type of mill. It provides 8 stands I & IV stands are vertical stands where as the rest are horizontal stands. Twisting jaws have been provided to twist the material through 90⁰ C after the no. 5 & 7 stands. First four stands are known as the roughing stands and last four are known as the finishing stands. First two stands are driven by 800 HP motor whereas the next two stands are driven by 1000 HP motors. The finishing stands are run by reduction gears and pinion arrangement. A single 2500 KW generator supplies power to the 4 individual motor of the roughing stands. Just before the first stand there is up & down cut shear that is used in emergency cases.

         Billets mill rolls billets and slabs are also produced with further reduced cross section the slabs are used in the skelp mill for the production of skelp coil. Some of the billets produced are directly sold to the market, and some are sent to the merchant mill for the merchant for the production of bars and rounds.

Billet Mill dispatch: the billet mill dispatch has two bays and each bay is provided with two cranes for handling the materials. This section stacks and sends the material to the Merchant Mill, skelp mill and also dispatches the billets to the outside buyers. The size of the billets produced is 125mm×125mm×4.4m.

ROLLING MILLS

                   The Rolling Mills comprise of the various mills where the products are mainly comprise of finished products which are made by the process of hot rolling. Here different types of rolls are employed for the above purpose. Here the main plant is the Bloom and the Billet mill (B and BM) where the steel ingots produced in the Steel Melting Shop by pouring the hot metal into the ingot moulds. The ingots are stripped from the moulds in the Stripper Bay where the stripping is done by the means of an special Electric Overhead Travel (EOT) Crane with a ram fixed in its hoist to provide the pressure for separating the moulds from the ingots. These ingots are sent into the B and BM. The ingot size is 775×700×2400mm weighing approximately 8 tonnes. The main products of this department are Blooms, Billets, Slabs, Sleeper bars etc.

B.O.D PLANT:

          Liquor from tar and Benzol plant are toxic. Feeding directly into river or atmosphere may seriously damage the ecological balance; so they are treated with bacteria and then disposed off.

          NEW SYSTEMS:

1.     Electrically driven exhauster with steam drive as stand by.

2.     Auto-operation of cooling tower pumps, with the help of level switch.

Tarry liquor flushing in primary coolers.

COAL DESCRIPTION:-

The input for the coke oven is coal. In technical terms, there are two types of coal available; one is cooking coal (i.e. which gives useful metallurgical coke on carbonization) and the other is non-cooking coal. There are several types of coking coal,depending on their ash content. In India the ash content is around 24 to 32%, whereas the imported coal is divided into four major types:

a)        Primary coking coal(PCC)

b)       Medium coking coal(MCC)

c)        Blendable coking coal(BCC)

d)       Primary coking coal(IC)

So, in an integrated steel plant, we have to use the above four types in definite proportions. In DSP the percentages of the various types are as follows:-

·        PCC- 30%

·        MCC-33%

·        IC-30%

·        BCC-7%

This   proportion gives the maximum productivity and losses in washing,filtering, etc, are minimized. Also the ash content is such that the overall effect is very less in the carbonization process. There is another difference in grades of coal i.e. lump coal and washed coal. Lumped coal  is one that can be directly charged into coke ovens after giving the exact size where as washed coal has to be washed  before being charged into the coke oven.

COAL HANDLING PLANT:-

          Raw material from different mines, mostly from Jharia, Barakar, and Bishergarh is received in open box wagon and mechanically unloaded by machines called tipplers. After being unloaded all types of coal are sent to Bradford Breaker for primary crushing. Jharia coal is diverted to coal washery (which is exclusive for DSP) and the other two varieties are sent to the stock files by a system of conveyers.

          There are 14 coal beds made of concrete floor in all. Stacking is done by neat of wing tippler. In each pile different types of coal having different ash content are stacked in different layer so that while taking a coal out from the pile for use in the coke oven, these type gets mixed up and a composite mixture called Reclaimer. This blended coal is then further pulverized in hammer mills to (-1/8) size and stored in bunkers.

To cater the need(both qualitative and quantitative)for the existing coke oven batteries through the adoptive strategies    like grade wise stacking, selective coal crushing and bunker blending. Reclaimed coal from different beds is stored in blending bunkers marked as prime, medium and blendable.  Coal is drawn from blending bunkers through automatic proportioning device at predetermined rates and is sent to  selective crushing unit through first stage crusher.

                     In selective ‘crushing unit’, and fine fractions of coal is separated by pneumatic classifier. The finer fraction is sent to coal tower, the coarser fraction is crushed again in secondary mills. The output of this mill is fed again fed to pneumatic classifier in order to arrest infiltration of course fraction going into the coal tower. The final coal from the pneumatic classifier (i.e. -3mm) only is fed to the coal tower. This coal is sent through conveyers to the coal tower, the capacity of each tower being 4000T.During this crushing  a little amount of moisture (6-7%) is ensured to minimize the loss and it also helps in pollution control. This coal is now ready for carbonization.

COAL WASHERY:-

DSP is the only steel plant, which has a coal washery inside the plant. The coal from Jharia has to be washed to reduce the ash content, after primary crushing to (-3”) size. The coal is separated into two sizes, namely (+1/2”) to     (-3”) and (-1/2”); each variety is treated separately in Heavy media section and Jig section respectively.

       Heavy media section

        Coal of size (+1/2”) to (-3”) is fed to primary  DREWBOY unit. The specific  gravity of the bath is maintained at 1.41 to 1.42 with the help of magnetic slurry(300 mesh). The float of the primary unit  is the clean coal and is sent to the stock piles. The sink is further treated in secondary DREWBOY unit, specific  gravity being 1.51.The float of the secondary unit is called middlings and the sink is called rejects. Middlings is being used by the captive power plant while rejects are sold to outside parties.

Jig section :

Coal of size (-1/2”) is charged in jig box along with water at a rate of 1000GPM. By the action of air pulsation in each of the fines compartments of the jig box, a batch of coal is separated depending in the heaviness. Coal from first two and a half compartments is comparatively lighter and is collected as middlings. These are sent to their respective bunkers. Clean coal which is lighter compared  to the other two is carried over by water and subsequently separated in three stages to the stock piles by a common washed coal belt conveyer.

NEW FACILITIES AFTER MORDERNISATION

·        A new wagon tippler of 720 T/hr capacity.

·        Four new coal beds 17000T each.

·        Two new stackers 720 T/hr capacity each.

·        Three bridge type bucket wheel, which reclaims 600 T/hr each.

·        Two primary hammer mills, 400 T/hr capacity each.

·        Two secondary mills, 300 T/hr capacities each.

·        Four pneumatic classifiers, 400 T/hr each.

·        Additional Reclaiming conveyer series.

·        14 blending bunkers, 1250 T capacities each.

·        Additional conveyer for feeding coal to coal tower, 600 T/hr each.

·        24 existing conveyers are to be modified to improve their capacity & performance.

COKE OVEN BATTERY:-

There are four batteries, each battery consisting of two independent and separate blocks of 39 ovens. The ovens are of the under jet, compound, twin flue type, using BF gas as normal fuel and with facilities for burning CO gas when available. Each chamber is of rectangular shape and lined inside with refractories. Coal is charged inside the ovens and is heated in absence of air to produce coke. All the volatile matter is removed from coal as raw or flue gas and the mass that is left behind is coke. When coke formation is complete, it is pushed out and quenched with water.

There are four types of special cars, which assists the coke making are as followed:

CHARGING CAR:

          This car collect the coal from the coal tower and charges it into the individual oven through charge holes, which are open by the charging car itself.

PUSHER CAR:

          This car does the job of pushing the coke out of the oven. It also opens and closes the ovens’ door, which provide for the escape of volatile gases.

GUIDE CAR:

          This car guide the coke during pushing operation and it also helps in opening and closing of the oven door.

QUENCHING CAR:

          This car receives the hot cake for quenching.

          Other than the coke oven chamber, there are heating chamber that provide the necessary heat for carbonization of coal. Each coke chamber is flanked on either side by heating chamber. Thus for a battery of 79 ovens there are 80 heating chamber. In these heating chamber, BF gas or CO gas is sent under pressure and air is supplied under natural suction pressure (through regenerator). The gas is sent in one direction in the flues numbered one, three, five, seven etc. to two, four, six, eight etc. for 30 minutes, the cycle reverses after the lapse of 30 minutes to the opposite direction i.e. from 2,4,6,8 etc. to 1,3,5,7 etc. Thus the heat supplied to the coal inside the coke chamber via the wall through conduction. The heating temperature is around 1220-1250⁰C. The coke temperature is 1050⁰C.

SPECIFICATION:

          Input coal to oven = 17.5 T/Day

          Heat = 600 Mcal/T of dry coal

          Output crushed coke = 77-78% of dry coal

          Output BF coke = 57-58% of dry coal

          CO gas = 290 Nm³/T of dry coal

          With calorific value = 4200 Kcal/m³

OVEN DIMENSIONS:

Height = 4.45 m

Length = 13.95 m

Width = 420-480 mm

Effective Volume = 23.8 m³

COKE OVENS AFTER MODERNIZATION:

Battery Features

No. of coke oven = 4 battery of 78 ovens each

Coal tower capacity = 4000 T

Total oven available = 273

Total pushing capability = 307 ovens/day

Coal through put = 0.6688 MT/Year

Coke Output = 0.4250 MT/Year

Gas Make = 22,700 m³/Year

MAIN EMPHASIS:

1.     Mechanization of operation in the following areas:

^·         Hot and hard job like oven charging coke wharf operation, cleaning operation in even top and battery platform.

^·         Lubrication of machines and equipments motorized with “auto” mode operation.

^·         Operation of large sized gas valve (>26” Dia) motorized.

2.     Pollution control measures introduced as follow:

^·         To achieve smokeless serving of oven a system of “ON-MAIN CHARGING OF OVEN” with the help of high pressure Ammonia Liquor at 30kg/cm² introduced.

^·         Water filled A.P. lids provided on oven top to minimize gas leakages.

^·         Chimney height increased to 110 m to reduce air borne pollutant (mainly CO).

^·         Arrangement for gas transfer during charging to adjacent oven to reduce pressure inside the oven being charged.

^·         Fly-ash arrester in coke quenching tower provided.

^·         Dust extractor and dust suppression system provided in coke plant.

3.     Thermal efficiency of battery increased by providing:

·        Insulation layer at different point to reduce lot by radiation.

·        Low circuit resistance in regenerator.

·        Electro-hydraulic reversing hinged machine with “PAUSE HEATING”.

4.     Equipment safety and electrical interlocks provided in all critical areas.

5.     Better control of operation by VLC control.

COKE HANDLING PLANT:

          The quenched coke is supplied to the wharfs. These are collected into a belt conveyer system and are sent for screening. The size of the coke must be supplied to the Blast Furnace is the major aspect of the coke handling plant. Coke plays a major role in the BF, as

·        It provide the necessary fuel for the preparation of the hot metal

·        It act reducing agent, i.e. help in reduction of iron oxide to iron.

·        It help in paving way for the blast gases to rise upword due to it porosity.

The size specified for the BF is +20mm to -80mm. The under sized coke is under different name, like peace coke, nut coke and coke breeze.

PROCESS:

          The coke is first made to pass through primary screen i.e. positive type 20mm, the output is that whose size is greater than 20mm, are sent to the next screen called 80mm screen, while the under sized coke is used is Nut coke. From the 2^nd i.e. 80mm screen the coke whose size is between 20mm & 80mm is sent to secondary screen (10mm), while the coke whose size is greater than 80mm is sent to coke cutter where the cutter cut the coke, this coke is again sent to the secondary screen where the coke size (between 20mm & 80mm) is collected as Pearl Coke and those less than 10mm are collected as Breeze Coke.

COAL CHEMICAL:

INTRODUCTION:

          The gas evolved during the carbonization of coal content various chemical. For the economy of the carbonization process and also for using this gas as a fuel, the recovery of these chemicals from the gas is essential. The coal chemical plant recover these chemicals from the coke oven gas and allows the clean gas to be used in coke oven and other plant as fuel.

          The hot gas coming out of the oven through astension pipe is cooled in the goose neck by direct contact with the spray of dilute Ammonia liquor called a flushing liquor to a temperature about 80⁰C. Most of the tar is condensed from the gas and is collected in the hydraulic mains. The hydraulic mains conduct the gas to primary condenser where the gas is cooled to 30-35⁰C and through a downcomer at first and the tar and liquor discharges into a tar catch tank and the other separated liquor is again used as flushing liquor.

          The exhauster cool the gas from the oven through the primary condenser and push it through electrostatic detarrers into the other unit of the plant. In detarrers the gas is subjected to the action of a high voltage electric field, the gas is ionized and the charged particle collect on the surface of the grounded positive electrode. Detarrers remove 95-99% of tar mist from the gas passed through it. From detarrers the gas is delivered to saturators, where ammonia present in the gas is absorbed by dilute sulfuric acid. Ammonium sulfate crystals produced are ejected into centrifuge to remove adhering liquids and then dried in a rotary drier.

          The gas leaving the saturator is cooled in the secondary cooler cum primary naphthalene washer. Here a portion of the naphthalene is removed from the gas by wash oil in secondary naphthalene washer. Then the gas is cooled in final cooler to 20⁰C and then passed through Benzol washers and final naphthalene washers for removal of Benzol and remaining naphthalene from the gas. The clean coke oven gas is used as a fuel.

AMMONIUM SULPHATE PLANT

The plant is subdivided into three sections:

a)     Gas absorption

b)    Intermediate storage liquor tank

c)     Salt production section

          Gas absorption section handles 33,000N /hr of CO gas each from 1M stage & 1.6 M stage. Product liquor from both the stages is stored in buffer tank and in large evaporator feed tank. Production capacity of salt section is 98 tons/day.

BASIC REACTION:

 +      →  (  + ∆H;                  ∆H=-65440 Kcal/kmol

   RAW MATERIALS:

a)     Coke oven gas to absorber ( flow rate=33000 /hr)

b)     concentration = 6-8 g per N/  of CO gas

c)     Inlet pressure 900 to 1500 mm water gauge.

d)    Inlet temperature=40-45 degree Celsius.

e)     Composition of CO gas:

     =58.2%              =20%              = 4.2%

     =0.4%            =4.2%              Unsatd. HC= 2%

UTILITES:

a)     Sulphuric acid =98% concentration (w/w)

b)    IP steam (superheated): P=5 kg/ ,T=25  C

c)     LP steam (superheated): P=1.5 to 2.75 kg/ ,T= C

d)     Cooling  water: P=2 to 2.5 kg/ , T= C 

e)    Compressed air : P=2 to 3 kg/ , T = ambient

EQUIPMENT OF SALT HANDLING

·        Belt conveyer

·        Bucket elevator

·        Centrifuge

·        Rotary drum drier (dia=1600mm,length=10000mm)

·        Drier exhaust fan:

Flow rate =11000 N /hr, 15 Kw motor, 1450 rpm, fluid is air with entrained dust at 80 degree Celsius with suction pressure  of -225mm water(g)

·        Spray nozzle

a)     Fluid: (  solution with free sulfuric acid.

b)    Type : non-clogging full cone without atomization

c)     Temperature=55 to 60  degree Celsius; pressure=1.5 bar

d)    Specific gravity= 1.24; spray angle=60 degree

e)     Average droplet size=500-600µ

f)      Gas velocity = 2 to 2.3 m/s

The ammonium sulphate crystals obtained are white , free flowing with acidity content of 0.04 and a concentration of 20.6% of nitrogen.

          BENZOL PLANT:

After the recovery of ammonia from the CO gas, naptha and crude benzol are removed from the gas in scrubbers by direct contact with wash oil. The ratio of oil : gas is kept low in naphthalene scrubber as compared to benzol scrubbers. Napthalene enriched oil from naphthalene scrubbers is sent to naphthalene stripping unit and some crude benzol, which was absorbed along with naphthalene.

          In crude form benzol is a mobile liquid, which distills almost completely below 200⁰C. The wash oil, which has absorbed benzol from the gas, is treated in benzol plant. Benzol is separated from the oil by steam stripping and the benzol-free wash oil is sent back for circulation. The separated benzol is called crude benzol that is then fractioned (refined) in the rectification unit to get pure products like benzene, toluene, xylene and heavy and light solvent naptha. These are stored in respective storage tanks.

          TAR FRACTIONATOR AND MNCP:

          The crude tar that is separated earlier is further distilled to get the following primary products viz. light oil, naphthalene oil, creosote oil, residue oil and pitch. Napthalene oil is thus separated from tar, and is treated in Mechanized Napthalene Crystallization Plant (MNCP); it is allowed to cool and settle down in big traps in naphthalene plant, where naphthalene crystallizes out. The remaining oil is drained and stored as Drained naphthalene oil. The naphthalene flakes are pressed into cakes in a hydraulic press. The pressed naphthalene is then crushed and bagged except the initial filling of pumps.

COKE OVEN AND BY-PRODUCT RECOVERY PLANT

INTRODUCTION:

  Coke oven is an important department in an integrated steel plant. In this department coal is carbonized into coke, which is an essential raw material for the production of pig iron in blast furnaces. While carbonizing coal into coke a number of valuable by products are obtained. These are also recovered and processed  in this department.

Major sections of this department are:-

1.     Coal handling

2.     Coal washery

3.     Coke oven battery & coke handling

4.     By-products plant; it has the following sections

 a)General condensation section

 b)Benzol plant

 c)Tar distillation & Napthalene plant

 d)Sulfuric acid plant

                         e)Bacteriological liquor treatment plant