IMPORTANT FEATURES

MILL BUILDER: Skoda Exports, Czech Republic.ODA EXPORTS, CZECH REPUBLIC COMMISSIONED ON:28th March 1992 28TH MAR`1992 Installed Capacity:8,50,000tonnes.50,000 Tons/Year TYPE OF MILL :High Speed Continuous Two High Hot Rolling Mill.IGH SPEED CONTINUOUS TWO HIGH HOT ROLLING MILL UNIQUE FEATURE: Rolling of Parallel flanged I and H beams using universal cassettes. ROLLING OF PARALLEL FLANGED H AND I BEAMS USING UNIVERSAL CASSETTE MAXIMUM MILL SPEED : 9 m/s9999 m9m/sec ROLLING RATE : 222 tonnes/hr2 Tons/H22r

Products of MMSM Merchant products like Squares, Rounds and Flats which are further processed by customers. Structural products like Angles, Channels and Beams etc.

PRODUCTSSPECIFICATIONS

Rounds40-80

Squares40-90

Channels100-180

Equal Angles75X75X6-10 & 110X110X6-12

Unequal Angles80X60X6-10 & 100X75X6-12

Flats100X10-20 & 150X10-20

T-bars100X100

H-E Beams96X100 & 114X120

I-BeamsISMB-125,175,180 ,IPE 100-180

GRADES ROLLED MILD STEEL MEDIUM CARBON STEEL HIGH CARBON STEEL ALLOY STEEL LIKE: a)Spring Steel

b)Manganese Steel c)Chromium Steel d)Copper Steel SALIENT FEATURES

3 Charging grids of 90t/hr. capacity.

Charging conveyors to transfer the blooms.

Bloom pusher for charging the blooms into the furnace.

SALIENT FEATURES RE- HEATING FURNACESUPPLIED BY M/S STEIN HEURTEY OF FRANCE 2 x 130 T/hr top and bottom fired walking beam type re-heating furnaces .

4 moving and 4 fixed skids for transfer of blooms.

Input bloom size: 250mm X 250mm Cross section, 6 -12 meters length and weighs 2780-5560 kgs (approx).

Furnace effective length: 20 metres

Furnace inside width : 13 metres

Furnace effective area: 240 m2

Chimney height :80 metres

Type of burners: Flat flame and Long flame type

Number of burners: 59

Number of zones: 5 zones (top and bottom fired) 2 top heating + 1 bottom heating 1 top soaking + 1 bottom soaking

Fuel used: Mixed gas (coke oven gas + BF gas + LD gas) of calorific value 2000kcal/mm3

Evaporative cooling system for skids cooling.

Convective and cross flow type air and gas recuperators for pre-heating gas and air. Bloom temperature :1100-12000C

Pre-heated air temperature: 4500C

Pre-heated gas temperature:3500C

PLC based automatic furnace control system.

Blooms de-scaling with 18 MPa pressure water.

Charging of blooms is done with pusher and discharging with extractor.

Walking beam cycle time: 64 seconds

15 % of heat input requirement is met from recovering heat energy from flue gas by pre-heating combustion air and gas.

De-mineralized and De-aerated water (DMDA water) is used for skid tubes cooling (ECS system).

Steam generated from skid tubes cooling is sent to plant network.

FURNACE CROSS -SECTION

MILL PROPER:Supplied by M/S ZDAS of CZECH Republic Considering the wide range of product mix and very high capacity requirement , a fully continuous 20 stand arrangement has been adopted The mill train is divided into 3 groups Roughing mill train, Intermediate Finishing mill train.

ROUGHING MILL: 4 Two- high horizontal stands 630mm diameter 2 vertical stands 630mm diameter 2 combinational stands 630mm diameter which are replaceable by either horizontal stand or by vertical stands

INTERMEDIATE MILL: 1 Two- high horizontal stands 630mm diameter 3 combinational stands 630mm diameter which are replaceable by either horizontal stand or by vertical stands 2 horizontal stands 630mm diameter which are replaceable by two 1060mm/710mm universal stands for rolling beams.

FINISHING MILL: 2 combinational stands 500mm diameter which are replaceable by either horizontal stand or by vertical stands 4 horizontal stands 500mm diameter which are replaceable by two 900mm/600mm universal stands for rolling beams

H-- HOROZONTAL STANDV VERTICAL STANDC-COMBINATION STAND U-UNIVERSAL STAND

. MAJOR UNIT PROCESSES INVOLVED

Heating of blooms in re-heating furnace.

Rolling of heated blooms to pre-determined size and shape.

Cooling and straightening of rolled products.

Cutting and packeting.

Stacking and Despatching.

MILL PROPER

MILL STANDS

SPECIAL FEATURES OF MILL PROPER

Quick stand changing facilities to reduce set up time Automatic engagement of spindles in the working roll Automatic connection of electrical supply, grease, hydraulic, lube oil and roll cooling water connection without manual intervention Hydraulic roll balancing system Motorized drive for roll gap adjustment Roll neck bearings: Radial --Four row cylindrical roller bearings Axial----Antifriction taper-roller thrust bearings Telescopic spindles Possibility to change gear ratio Self aligned roll cooling water showers Movable rest bars for mounting guides and guards.

ROLL MATERIAL ROUGHING AND INTERMEDIATE MILL ROLLS (Dia 630 rolls)

CAST STEEL

FINISHING MILL ROLLS (Dia 500 rolls) & UNIVERSAL MILL ROLLS - SPHEROIDISED GRAPHITE CAST IRON

SHEARS

8000 KN Shear: Placed before roughing mill stand and serves to crop bad-quality bloom front end and for dividing 12 metre long blooms into halves 2500 KN Shear: Crank type flying shear located after roughing mill train to crop the front end and scrap the rolling stock in to pieces in case of failure in the following mill trains 800 KN Shear: Crank type flying shear located after intermediate mill train to crop the end portions and scrap the rolling stock in to pieces in case of failure in the following mill train or cooling bed equipment 500 KN Shear: Crank type flying shear located after finishing mill train to crop the end portions and to cut the rolled bars into cooling bed length.

COOLING BED:120 metre long double side rake type cooling bed.EQUIPPED WITH: SWITCH WITH CONVEYOR: Serves for guiding of mill products alternatively on a left side and right side of cooling bed SUPPLYING CONVEYOR: Serves for mill product transportation in to two sides of the double sided cooling bed (at 2.5 to 15 m/sec) SEPARATING BAR: Serves for separating mill products following quickly one after another BRAKING FLAPS: Serves for placing mill products from supplying conveyor to cooling grates COOLING GRATES: Rake type cooling grates having 45 notches and serves to transport mill products to outlet conveyor after cooling 120 nos of cooling fans on each side of the cooling bed are installed to cool the rolled material

ROLLER SRAIGHTNER

PLACED BEHIND EACH SIDE OF THE COOLING BED FOR STRAIGHTENING ROLLED PRODUCTS Total no of rolls :9 No of top rolls: 4 (Only axially adjusted) and motor driven No of bottom rolls : 5 (Axially and vertically adjustable) and friction driven Straightener speed: 2.5 to 12 m/sec

BATCHING OF PRODUCTS

Batching grate arrests the bar after passing through the straightener Batching of pre-selected number of bars for cutting by cold circular saws.

FINISHING SHOP

COLD CIRCULAR SAWS: 2 Cold saw lines each consisting of one stationary and 2 mobile saws for cutting the finished products to customer lengths Blade diameter 1600 to 1450mm Blade thickness: 8 to 10mm Peripheral speed of saw blade 145 m/sec Max layer width 1400mm Pressure water for saw blade : 8MPa Teeth hardening system : Flame hardening/Induction hardening .

BUNDLING OF FINISHED PRODUCTS

Bundling is done for ROUNDS.

2X24 metre bundling machines.

Max bundle weight: 10 Tons.

AUTOMATIC TYING AND WELDING of Piles/Bundles:

An unique facility in India

6 tying machines in piling and 4 tying machines in bundling machines

AUXILLIARY SYSTEMS

Hydraulic and lubrication cellars for all mill plant.

Cellar for water station of De-scaling equipment.

High pressure water station for cold saw blades cooling.

3 Electrical control rooms (ECRs) for electrical equipment control.

Ventilation systems for ECRSs and Cellars.

Fire protection systems.

13 nos of Electrical Overhead Travel cranes.

Scale flushing systems.

Pipe line distribution systems for water, compressed air, Nitrogen Oxygen and Steam etc.

AUTOMATION IN THE PLANT

STACKING

Hydraulics

Hydraulicsis a topic inapplied science and engineering dealing with the mechanical properties of fluids. On a very basic level Hydraulics is used for the generation, control, and transmission of fluids in order to produce some mechanical work or in general as a part of river engineering.

THE BASIC LAWS OF HYDRAULICS

Fluid Mechanics provides the theoretical foundation of hydraulics and focuses mainly on its engineering applications. The basic laws of fluid dynamics that govern the working of any hydraulic system are: Pascal's Law: It states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same. The siphon, Hydraulic jack, Hydraulic press and braking system for automobiles are some examples of application of this law.

The relationship between pressure, force and area is given by P=F/AT

This relationship is very useful for obtaining a mechanical advantage. As per the Pascals law the pressure in a fluid remains the same everywhere which implies that the ratio of force and area must remain constant. This fact can be used to lift greater weights with lesser forces as shown in the figure.

Rashtriya Ispat Nigam LimitedVizag Steel Plant,Visakhapatnam 530 031India

Basic components of hydraulic systems

THE CLASSIFICATION OF HYDRAULIC SYSTEMS IS NOT VERY CLEAR CUT EVEN THOUGH FOR THE SAKE OF EASE IN DISCUSSION WE WILL CENTRE OUR FOCUS ONLY ON THE INSTRUMENTS WHICH ARE MOSTLY USED IN A BASIC HYDRAULIC SYSTEM.

1.TANK OR RESERVOIR OF HYDRAULIC FLUID2. MOTOR DRIVEN PUMP3. PRESSURE GAUGE 4. PRESSURE RELIEF VALVE5. DIRECTION CONTROL VALVE 6. CYLINDER AND ACTUATOR7. AN ADDITIONAL REGENERATIVE SYSTEM IS ALSO OFTEN USED

1.TANK:IT IS THE RESERVOIR WHERE THE WORKING FLUID OF THE SYSTEM IS STORED WHEN IT IS NOT CIRCULATING IN THE SYSTEM.ITS VOLUME SHOULD BE PROPERLY CACLULATED AND MAINTAINANCE SHOULD BE PROPERLY SUPERVISED. THE MAJOR TWO TYPES OF TANKS IN TERMS OF DESIGN ARE :A) OPEN TANK B) CLOSED TANK

2. PUMPS: Pumps convert mechanical energy into fluid energy. Turbines exactly the opposite, convert fluid energy to mechanical form.Classification of pumps based on the method by which mechanical energy is transferred to the fluid Positive-displacement pumps Kinetic pumps3. PRESSURE GAUGE : A carefully calibrated pressure gauge is essential to maintain the pressure in the system or line within the safe working range . This increases the working life of equipments and results in lesser break-down allowing more available time for all the equipments.4. PRESSURE RELIEF VALVE: A pressure relief valve to maintain the desired pressure in the working line is to be deployed in conjunction with a pressure relief valve. The control of this valve is through diferrent modes e.g. spring loaded pressure relief valve,manually operated pressure relief valve.5. DIRECTION CONTROL VALVE: The direction of flow of working fluid of the system and thereby the direction of application of pressure can be controlled by the direction control valves.The direction control valve can be operated by various means e.g. solenoid operated, manually operated,spring-loaded,etc

6.CYLINDER AND ACTUATOR: A cylinder uses pressurized fluid or air to create a linear foce or motion. The working fluid is pumped into one side of the cylinder under pressure,causing that side of the cylinder to expand, and advancing the actuator or piston. The fluid on the other side of the piston must be allowed to escape freely.If the incompressible fluid was trapped the cylinder could not advance. The force the cylinder can exert is proportional to the cross sectional area of the cylinder.

7.REGENERATIVE SYSTEM: Figure shows a regenerative circuit that is used to speed up the extending speed of a double-acting hydraulic cylinder. Here the pipelines to both ends of thehydraulic cylinder are connected to pump, one end (A) through the 2 / 3 way DCV and the other end (B) directly. The operation of the cylinder during the retraction stroke is the same as that of a regular double-acting cylinder. Fluid flows through the DCV zeroposition from the actuator A side during retraction. In this position, fluid from the pump directly enters the rod end of the cylinder (direct connection). Fluid in the blank end drains back to the tank through the DCV as the cylinder retracts. When the DCV is shifted to 1 position due to manual actuation, the cylinder extends. The speed of extension is greater than that for a regular double-acting cylinder because flow from the rod end (QR) regenerates with the pump flow (QP) to provide a total flow rate (QT), which is greater than the pump flow rate to the A side of the cylinder. ( Area of blank end is more than rod end, thereby blank end provide least resistance )

REGENERATIVE SYSTEM

C= Double-acting cylinderP = PumpE = Electric MotorT = TankF = FilterR = Relief ValveD =2-position, 3 way , Manually operated andSpring return DCV

COMMON PROBLEMS WITH REMEDIES

Expanded table:

VALVES:

CYLINDERS:

Boosters:

BOOSTERS CONTINUED:

FLUID MOTORS:

FLUID MOTORS CONTD.:

VANE PUMPS:

VANE PUMPS CONTD.:

VANE PUMPS CONTD.:

VANE PUMPS CONTD.:

VANE PUMPS CONTD.:

VANE PUMPS CONTD.:

VANE PUMPS CONTD.:

RADIAL PISTON PUMPS:

RADIAL PISTON PUMPS CONTD.:

RADIAL PISTON PUMPS CONTD.:

HYDRAULICS SYSTEM:

HYDRAULIC CIRCUIT DESIGN AND ANALYSIS

A Hydraulic circuit is a group of components such as pumps, actuators, and control valves so arranged that they will perform a useful task. When analyzing or designing a hydraulic circuit, the following three important considerations must be takeninto account:

1. Safety of operation2. Performance of desired function3. Efficiency of operation

It is very important for the fluid power ( Hydraulics and Pneumatics) designer to have a working knowledge of components and how they operate in a circuit. Hydraulic circuits are developed through the use of graphical symbols for all components. The symbols have to conform to the ANSI specification.

Plant DetailsThe major plants and facilities at VSP are:

1. Raw Material and Iron Makinga. Blast Furnace 2b. Coke Plant with 3 Batteriesc. Sinter Plants 2d. Refractory Departmente. Raw Material Handling Yards and Associated facilities2. Long Producta. Medium Merchant Millb. Light Merchant Millc. LD shopd. Rebar mille. Wire and Rod Mill3. Secondary Productsa. Benzene Plantb. Ammonium Sulphate Plantc. Tar Plantd. Slag Plant4. Shared Servicesa. Power Plantb. Blowers and Pump Housesc. Industrial Gasesd. Fuel Managemente. Waste Recycling Plantf. Environment and Occupational health5. Miscellaneousa. Information Technology Servicesb. Canteen Servicesc. Safety and Ergonomics d. Security and Fire Brigadee. Research and Developmentf. Laboratoryg. Hospital

GENERAL DESCRIPTION

Basic steel is a mixture of iron, carbon and other alloying elements. There are four common grades of steel:A. low carbon steel,B. mild carbon steel,C. medium carbon steel andD. high carbon steel (the higher the carbon content, the stronger and harder the steel).

Low carbon steels used to make car bodies, whereas high carbon steels are used to make cutting tools and rails.Steel often is mixed, or "alloyed," with other elements (e.g., chromium, nickel, manganese, molybdenum, selenium, tungsten, titanium or vanadium) to give it special properties, such as increased strength, durability or resistance to heat and corrosion. Steel alloys contain from 5% to 50% of one or more alloy metals; alloys with less than 50% iron content are considered ferro-alloys rather than steel. The most common steel alloy is stainless steel (a combination of basic steel, chromium and nickel), a corrosion-resistant metal that has a variety of applications.

Integrated steel mills are typically vast complexes with three major sections:A. blast furnaces that transform iron ore, coke (a purified form of coal) and limestone into iron;B. steel-making furnaces that refine iron into semi finished steel; andC. finishing mills that turn semi finished steel into basic steel products (e.g., bars, pipes, sheets, rods or wire).

Integrated steel mills operate their own power generation facilities, water treatment plants, fire brigades and medical services. Integrated steel companies operate private iron and coal mines to supply their mills. Integrated steel mills are huge complexes that consume enormous amounts of water, coal, iron ore and limestone.

Materials and Equipment

The following are the major materials required for Steel Mills Iron ore, limestone, coal, alloy additives, scrap metal Fuel oil, Natural gas, water, oxygen. Coke ovens and coke oven gas, blast furnaces, basic oxygen furnaces, reheating furnaces, conditioning furnaces. Continuous strand casting machines, rolling mills (hot), blooming mills. Gear sets. Fans, blowers. Ladles, kettles, bins, tongs, molds. Conveyors, forklifts, cranes, railcars, rail lines Electric power generation facilities

Most of these materials are delivered to the site by rail or ship; iron ore, coal and limestone. Steel mills store up to six weeks' supply of certain raw materials (e.g., coal, limestone) in the premises.

Process Description

The steel-making process at VSP entails the following processes:

1. Sizing of Iron Ore, Coke, Dolamite, Limestone, etc

The raw materials are stored in material yards. The raw material is picked up using conveyer belts and sent for crushing. Using 2 levels of crushers (primary and secondary) the ore is crushed to the required grain using filters. Similarly, coal, limestone and Dolomite is also crushed to the required grade. Using a magnetic separator ferrous impurities are separated out.

2. Refining coal into coke

The first step in steel making is the transformation of coal into a purer form known as coke. Coke provides heat, provides permeability, supports burden materials and also acts as a reducing agent. Coke is a hard, dark grey material that burns longer, hotter and cleaner than coal and is the fuel used to smelt iron ore into iron.

Major units of Coke Ovens are

a. Coal Preparation Plant

Coal from storage yard is sent to foreign objects separation section where in Ferromagnetic articles and coal lumps of above 150 mm size are removed. Coal is transported by conveyors to 16 Nos. of storage bins (capacity 800 tons each). Coals from various sources are stored in separate storage bins and are mixed in definite proportions with the help of propositioning devices located at the bottom of each storage bin (also called silo). The coal-blend is crushed to -3mm size using impact crushers. Thus coal, crushed &blended, is conveyed to two coal towers, each of 4000 t capacities.

b. Coke Oven Batteries

A coke oven battery is a monolithic structure of refractory brickwork, primarily of high quality Silica and Fireclay bricks of different shapes and sizes. There are 3 coke oven batteries, each of 67 ovens and 7m height. Each oven has an effective volume of 41.6 M3. Coking coals after crushing and blending is subjected to destructive distillation (heating in absence of air) in the ovens. Heating is done for a pre-determined coking period (about 16-18 hrs.) up to the required temperature (1050 +/- 50 OC. Adjacent to each oven there are two heating chambers. The coal charged into the oven is heated indirectly by burning fuel gases, in the heating chamber. Conduction/Convection thus transmits heat to the coal mass through the refractory brick walls. Beneath the ovensand heating chambers, regenerators are located. The hot combustion gases from the heating chambers pass through these separators, which are filled with refractory brickwork, before they are sent out through chimney. By this, heat energy in recoveredfrom the waste gases and the same in used in heating the ovens.After continuous heating of coal for 16-18 hours coke is formed. The coke mass at a temperature of 1050 +/- 50 OC is pushed out of the coke oven by means of a pusher car and sent to Coke Dry Cooling Plant.

c. Coke Dry cooling Plant

Each battery is provided with one dry cooling installation comprising 4 cooling chambers. Each cooling chamber can cool 50-52 coke/hr. The red-hot coke from a battery is transported to the top of the cooling chamber in a bucket and charged into it.The residence time of coke in the chamber is 2 to 2.1 Hrs. Circulating gases are forced by mill fan into the distribution channels in the lower part of the cooling chamber. The gases flow upward and coke moves downward in the cooling chamber. The heated gases coming out from the cooling chambers at about 750 800OC go to primary dust catchers, waste heat boilers and secondary dust catchers. The temperature of cooled coke is in therange of 180 200 OC.

d. Coke Chemicals Recovery

The chemicals that need to be removed from coke oven gas are benzene, ammonia and benzol hydrocarbon. Tar is removed first by cooling of the coke oven gas by spraying liquor over coke gas. Nearly 60% of the tar gets condensed and is separated out. Theremaining gas flows to Primary Gas Coolers (PGC), where it is subjected to cooling water and the gas is cooled to 25C and nearly all the tar gets removed. The gas is then passed through saturators where 5% sulphuric Acid solution is maintained. Then the gas bubbles through it, ammonia reacts to form ammonium sulphateand is separated out and dried. The gas is then passed through final gas coolers that removed naphthalene and then passed into benzol scrubbers that remove benzol from the gas which is distilled to remove benzene, toluene, Sol-110 and CB-II. Passing hydrogen gas of required purity does this. Hydrogen is extracted from coal gas by using a molecular sieve.

e. Coke Sorting Plant

Coke is crushed and sorted to make it of size 25-70mm which is used in the blast furnace.

3. Sintering Plant

Sintering is the process of agglomeration of fine ore particles into porous mass due to incipient fusion caused by the combustion of fuel present within the mass. The Sinter plant manufactures sinter as per blast furnace requirement and recycles the metallurgicalwaste that is generated in the process. The usage of sinter in the furnace improves the productivity, reduces the coke requirements and allows fine control over the slag chemistry. Since the fine iron ore particles cannot be directly charged in the furnace by making sinter, natural resources are saved. Raw materials for the sintering process are Iron ore fines, Limestone, Dolomite, Coke breeze, Quartzite/River sand, Lime Fines, Metallurgical waste and Manganese Ore fines. There are 2 sinter-making machines each of 312m2 grate area. The pallet is filled with sinter mix and ignited. 2 suction fans blow in the air at 1500m3/min, which flows down from the hot top layer to the colder bottom layers. The sinter is then crushed, cooled and sent to the BF.

Raw Material composition is as below (kg per tonne of sinter)

Iron Ore Fines 848 Dolomite 109 Limestone 97 Coke Breeze - 56 Sand - 9 Lime - 7 LD Slag - 4 Met. Waste - 63

4. Blast Furnace

The raw materials for iron making (iron ore, limestone and coke) are stockpiled in outdoor storage yards. The charge travels along a conveyor belt, up a ramp, and finally is dumped into a blast furnace to smelt the charge into molten iron and slag. The 2 blastfurnaces are called Krishna and Godavari with a rated capacity of 3.4 MT annually. Before the charge is dumped into the blast furnace, a series of tank-shaped stoves next to the furnace are heated. In these stoves, air is heated to 2,000 degrees Fahrenheit andblasted into the furnace at approximately 400 miles an hour. Normally, the stoves are used on a staggered basis; while some are "on blast" (forcing hot air into the blast furnace), others will be "on gas" (heating air); this maintains a continuous rate of production.When the charge is dumped into the blast furnace, it is suspended in midair by continuous blasts of hot air rushing up from the bottom of the furnace. Continuous exposure to the extremely hot air causes the charge to melt and slowly drop to the bottom of the blast furnace. Approximately halfway down the furnace, the limestone in the charge reacts with the impurities present in the coke and iron ore, drawing them out to form a lava-like material called slag; as slag forms, pure iron is left behind. Eventually, the molten charge (which has been separated into slag and pure iron at this point) collects in a brick-lined reservoir at the bottom of the furnace, called the hearth. Because slag is lighter than iron, the slag lies above the iron in the hearth. The hot reduction gasses of combustion ascends through the furnace through the charge falling from top and imparts its heat to the charge. This gas contains high amount of carbon monoxide and is reused. When enough iron has accumulated, the hearth is tapped. First, a hole is drilled through aclay-filled taphole in the hearth above the iron to let the slag pour out. The slag is collected in a slag ladle. Once the slag has been tapped, a lower taphole is opened to allow the molten iron to flow into ladles. While most iron made at a steel plant is used to make steel, some part is cooled into crude ingots known as "pig iron" and sold directly to foundries.

The main features of the furnace are-

Circular cast house with cranes Four tapholes Cast house slag granulation Useful volume of 3200m3 with 12m diameter hearth PLC controlled PW charging system Gas expansion turbine station for gas recovery O2 enrichment

There are 4 double strand pig casting machines with capacity of 1700 tonnes per day each. There are 30 open ladles of capacity 140 tonnes and 7 TLCs of 300 tonnes capacity.

5. Steel Melting Shop (SMS)

There are 4 BOF at VSP. In the first step of the basic oxygen process, the BOF is tilted toward a charging platform where scrap steel is charged into the furnace. Then, a ladle of molten iron is moved by crane and poured into the BOF; the molten iron will make between 60% and 80% of the total charge. Fully charged, the BOF is returned to an upright position, and a long, water-cooled pipe called an "oxygen lance" is lowered into the furnace near the surface of the molten charge. Once in place, the oxygen lance blows pure oxygen into the furnace at extreme pressure and speed (in excess of the speed of sound). The molten charge, saturated with oxygen, begins to burn at a very high temperature. A measured amount of lime is then released into the furnace; the lime reacts with the charge's impurities and forms a slag, which rises to the top of the molten charge and is discarded. After the oxygen "blow" has ended (usually after 20 minutes), the BOFis tilted toward the "teeming platform" where the refined molten charge rushes through a taphole into a rail-mounted ladle. Carbon and other alloying agents (e.g., tungsten, titanium, molybdenum) are added to the charge to create molten steel. The steel moves through the curved casting machine to a horizontal position, it solidifies throughout, and roller machines flatten the steel to a predetermined thickness. Finally, at regular intervals, mechanical shears cut the continuous strand of steel into "blooms, LD gases, emitted from the mouth of the converter are collected and cleaned. Each converter is provided with a separate gas cleaning system. The gas is extracted at the converter mouth through an adjustable skirt, movable cooling hood and a fixed cooling hood where the gas is cooled down to temperature of about 1100 deg. C and then led to a two-stage venturi scrubber where the gas is cleaned and further cooled to about 70 degC with dust content below100 mg/m3 . With electrostatic precipitators, the gas can be cleaned to a dust content of