Wednesday, August 14, 2013

Thermodynamic interpretation of reduction of Iron Ore in Blast Furnace.

An article published in quarterly magazine of Pakistan Steel on the inaugural ceremony of Blast Furnace no. 1 on August 31st, 1981.

Friday, August 9, 2013

Iron Making Department Pakistan Steel

Iron Making Department  
(Pakistan Steel).

Iron Making Department of Pakistan Steel is comprised of two Blast Furnaces. The  volume of each Blast furnace is 1033 m3 and the production capacity of each is  1,750 Tons per day or 5,50,000 Tons pig iron annually to attain (Phase-I ) production of 1.1 million tonne pig iron per year.

Blast furnace #1 was commissioned on August 14, 1981 while Blast furnace # 2 was inaugurated in 1984. The foundation of third Blast furnace for enhancing the production capacity  to 2.2 million tonne (Phase-II ) was made available by Tyazhpromexport, the principle Russian company that constructed the Pakistan Steel.

The Blast Furnace Construction :

The Blast furnace  has a vertical cylindrical structure externally covered with a shell of thick steel plate and internally lined with refractories.

The refractory structure is cooled by water circulated metal components called staves, which are embedded between the shell and the refractories.

A prerequisite for long campaign life is an effective furnace shell cooling.
The cooling system can be constructed in different variants. Compact plate coolers or plate coolers with intermediate flat coolers, cast iron staves with integrated refractory material or the newly developed copper staves are used.

Compared with cast iron staves, copper staves permit significantly more intensive heat removal thus forming a stable protective layer by the burden materials. The use of expensive refractory materials to increase the service life of the copper staves can be dispensed with.

 A well balance over all operation of B.F is ensured by integrating the properties of the different zones of B.F shell, the cooling elements and refractory lining.

Once a Blast furnace is started it will continuously run for 12 -15 years with only short stops to perform planned maintenance. 

Finally the refractory brick linings are worn out and at that stage the process is stopped and the furnace goes for capital repair and relined with new refractory bricks, ready to begin its next campaign.

A Blast furnace capital repair & relining work is one of the most comprehensive and labour intensive projects undertaken by a steel mill and requires an extensive quantity of materials. During capital repair, several important jobs including modifications or replacements are done.

Any unanticipated part that was not in the original plan, but necessary when engineers discovered any faulty internal component, needed to be replaced like valves, various pipes,burners, switches, electrical cables and connections, motors etc.

An inside view of large B.F. showing installation of scaffolding platform for bricks laying work.

Blowing- in:
Blowing in of a newly lined Blast furnace is one of the most important operations connected with its commissioning after capital repair.

The main aim at the beginning ( pre-heating period of the blowing –in) is heating of the brick work to the temperature which ensures normal commencement of ore reduction reaction.

The Blast Furnace Main Equipments & Iron Making Process.

B.F. Stock House & Materials Charging System:
The iron-bearing materials are mainly comprised of sized iron ore & sinter. The coke is added to provide the main chemical reagents (carbon and carbon monoxide) for the iron ore reduction. The flux ( limestone and dolomite) is added to combine with ash in the coke and gangue materials in the ores, to produce a slag.

Above is back view of Iron making department of Pakistan Steel showing Blast furnace  #1 &   and stock house where charge materials (Iron ore, Sinter, Coke, Limestone)are stocked to form the blast furnace burden.

The stock house is facilitated with weighing, screening, batching, conveying and charging systems. The skip cars are used to deliver the blast furnace charge (burden) to the top of the Blast  furnace for charging.

Blast-Furnace Gas: 
When hot air blast with temperatures 1000 – 1200 0C passes over coke  and iron ore in Blast Furnace, the off gas product generated is known as Blast Furnace Gas.

Blast furnace Gas consists of about 55 – 58% Nitrogen, 15 - 20% Carbon dioxide, and some Oxygen, which are not flammable. The rest is comprised of 20 -30%Carbon monoxide, upto 0.5% methan, 1 – 4% Hydrogen  which are flammable.

The waste gas from Blast Furnace is generated in enormous quantities, 2.5 – 3.5 per ton of pig iron produced at higher pressure and at temperatures about 110 - 150 °C. The temperature of the gas is dropped down during dust removing and cleaning process.  

With the above chemical composition, the B.F Gas has a fairly low heating value 850 -1100 kcal/m3 and is utilized as fuel gas in preheating the B.F. Blast stoves discussed below and other places  in the mill.

Blast Furnace Gas Cleaning System:

 There are four uptakes pipes where the hot dirty BF gas exits the furnace dome.
 The escaping Blast Furnace gas has a high content of extremely abrasive dust. As the gas flows up, the two uptakes merge into "off takes" on each sides. The two off- takes then merge into the "downcomers" as shown below.

At the extreme top of the off takes there are "bleeder valves" which may release extra gas pressure and protect the top of the furnace from sudden gas surges. 

The BF gas descends in the downcomer to the "dustcatcher", where coarse particles settle out, accumulate and are dumped into a railroad car or truck for disposal.

 The BF gas then flows through a "Venturi Scrubber"  which removes the finer particles and finally into a "gas cooler" where water sprays reduce the temperature of the hot but clean BF gas, ready for burning as fuel.

Thermal Power Plant & Turbo blower station of Pakistan Steel has electrically driven compressors which provide cold blast at the high desired flow and pressure to feed into the hot blast stoves of Blast furnaces where it is heated.

After leaving the stoves, the hot blast enters the base of the blast furnace via bustle pipe and the tuyeres. It passes up through the Blast furnace, reacting with the coke, ores and fluxes, and emerges as top gas, containing mainly CO and CO2 in the iron making process.

Hot blast stoves are regenerative heat exchangers. Each stove consists of a combustion chamber and a regenerative chamber. Each of them represents a cylindrical structure filled with multiple course grid ( checker-work) which is made of special shaped refractory bricks. Checker-work is the basic constructive element of hot stove, which defines the process of heat transfer from combustion products to cold blast.

 Each of the two Blast furnaces of Pakistan Steel has three cylinderical stoves, so that at any time one stove is on-blast while the others are on-gas or boxed. A boxed stove has been heated up to temperature and sealed, so that it is ready to go on-blast. The three stoves are operated on alternate cycles, providing a continuous source of hot blast to the Blast furnace. If one stove is down for repair, it is possible to run on just two stoves.
  Full working period of regenerative hot stoves consists of two operating cycles:

       - Cycle of checker-work heating when products of combustion of fuel gas ( clean and hot blast furnace gas or mixed gas) enter from the top and, passing through the checker-work, they heat the refractory checker-work  up;

       - Blast cycle  when air (cold blast) enters the previously heated refractory checker-work from the bottom and passing through it upwards, is heated up. The checker-work is thus cooled down.
The rise in the hot blast temperature through hot stoves leads to the reduction in the specific coke consumption rate and enhancement of the specific productivity of blast furnaces, which essentially reduces the operating cost.

Supplemental fuels such as natural gas, pulverized coal, and oil, are also typically injected through a lance inserted into the blowpipe leading up to the tuyere to further enhance the efficiency of the process.

Bustle Pipe & the Tuyeres: 

The hot blast main pipe enters into a doughnut shaped pipe called the "Bustle pipe".  Bustle Pipe encircles the blast furnace and delivers the hot blast air from the hot blast line to the tuyeres of Blast furnace. 

The tuyeres are made up of water cooled copper since the temperature directly in front of the them may be 3600°F to 4200°F. They are equally spaced around the circumference of the furnace , located at the top of the Blast Furnace hearth which starts immediately below tuyere level. 

A view of Bustle Pipe and Tuyeres of a modern large Blast Furnace.

Blast Furnace Hearth:

The hearth is an intricately constructed crucible-like vessel upon which the vertical shaft portion of the furnace sits. All the molten metal and slag collect in the hearth before being drained.

A control room  from where, by means of the installed automation and control devices, the operation of the blast furnace is monitored and controlled. 
  Appearance of Blast Furnace control room.

The efficient operation of the modern Blast Furnace requires a high degree of automation in conjunction with computerized monitoring and control systems, allowing Blast Furnace operators to optimize the hot metal production.

Thermodynamic interpretation of reduction of Iron from Iron Ore in Blast Furnace.

Tapping a Blast Furnace.

The openings in the furnace hearth for draining and removing the molten slag & metal from the furnace are called the "iron & slag notches".

 A large drill mounted on a pivoting base called the "tap hole drill", swings up to the iron notch and drills a hole through the refractory clay plug into the liquid iron.

Once the tap hole (iron notch) is drilled open, liquid iron and slag flow down a deep trench called a "trough" . Set across and into the trough is a block of refractory, called a "skimmer", which has a small opening underneath it.

 The hot metal flows through the skimmer opening, over the "iron dam" and down the "iron runners". The hot metal continues to flow down the bend runner from which it is diverted into individual hot metal ladles.

 The hot metal ladle is a refractory lined 110 Ton capacity container. It transports hot metal from Blast Furnace to Steel Making Department and Pig Casting Machines.

 Since the slag is less dense than iron, it floats on top of the iron, down the trough, hits the skimmer and is diverted into the "slag runners". The liquid slag flows into "slag pots"  or into slag pits (not shown here) and the liquid iron flows into refractory lined "ladles".

When the BF hot metal tapping is finished, the tap hole is closed with hydraulically operated mud gun.

After finishing each B.F.tapping, the Cast house area where molten pig iron is cast,  the frozen skulls, build up on runner walls, bottom , necks , dams and iron troughs are cleaned. The fresh casting sand is rammed and the cast house is readied for subsequent B.F. tapping which may occur in 2 hours.

Blast Furnace Slag:
The chemical composition of  B.F. slag varies considerably depending on the composition of the raw materials in the iron production process.

Slag is formed from the gangue materials of the burden ( charge materials) and the ashes of the coke and other auxiliary reductants.

Four main components (, CaO 34-42%, MgO 6-12% , SiO2  28-38% and  Al2O3  8-20% ) make up about 96% of the slag.  The minor components are MnO, TiO2, K2O, Na2O, S and P. The basicity of Blast furnace slag is kept controlled in the range of 1.1- 1.3 which is the ratio of Basic to Acidic oxides in the Blast furnace slag.

The nature, composition and amount of the final slag in the hearth control the composition of the hot metal and the productivity of the blast furnace.

The BF slag is taken to the slag dump yard where it is processed to sell in market.

 The iron ladles are taken either to the Steel Making Department for converting into steel or to the Pig Casting Machine for casting Foundry grade Pig Iron.

Common Pig iron usually consists of: about 92 - 94 percent iron, 3 - 4 percent carbon, 0.5 - 3 percent silicon, 0.5 - 6 percent manganese, phosphorous 0.1 - 2 percent and traces of sulphur 0.01 - 0.05 percent.

The iron making Blast furnaces of  Pakistan Steel produce two types of Pig Iron - Foundry Grade Pig Iron (F.P.I) and Conversion Grade Pig Iron (C.P.I). Normal practice is to produce Conversion Grade Pig Iron (CPI) for steel making purpose. 

Foundry grade pig iron:
Pig iron intended for foundries, the Si content is usually high, from 1.25% to 3.6%, and the C content is higher than 3.3%. The high Si content requires a high operating temperature in the blast furnace.

The degree of reduction of silicon depends mainly on the coke expended; for every extra percent of silicon in the cast iron, the expenditure of coke is increased 5–7 percent, which also increases the amount of hot gas in the furnace and causes the blast-furnace shaft to overheat. Therefore, the price of foundry pig iron is usually higher than conversion grade  pig iron.

The molten foundry grade iron is poured into the moulds of Pig Casting Machine where it acquires a shape of pigs shown below which are pushed under water sprays until they become cold, solidify and discharge.

 The foundry grade pig iron is supplied to various foundries where it is remelted to produce mainly Grey iron and Nodular iron in Cupola or Induction furnaces, widely used for casting vehicle engines, machinery parts, and many other products.

Cast iron properties depend largely on chemical composition shown above and the subsequent annealing process used.

Ductile iron is produced by treating molten low sulfur base iron with magnesium. Under close controlled conditions, the free graphite in ductile iron being deposited in spheroidal or nodular form instead of flake form as in gray iron.

With the free graphite in nodular form, the continuity of the metal matrix is at a maximum, accounting for the formation of a far stronger, tougher ductile material greatly exceeding gray iron in strength, ductility, wear resistance, fatigue resistance and in impact characteristics.

An example of above is Bolan Castings Limited Hub Lasbella Balochistan, a downstream industry of Pakistan steel uses pig iron to produce Nodular cast iron ( where Mg is added as grains refiner ) for casting the body of engines of Millat Tractors Limited.

Conversion grade iron.
Conversion grade pig iron is used for converting into steel  in Steel Making Department in LD-Converter to produce different grades of steel.

The Si content in Conversion grade Pig Iron is lower than that in foundry grade pig iron. The required chemical composition for CPI as per Pakistan Steel's specification is as under:

·         Carbon not less than 4%
        ·         Mn : 0.5 – 1.0%
·         Si : 0.4 – 0.8%
·         P : 0.2% max
·         S : 0.04% max

The specific range of Chemical composition & Physical temperature of hot metal (conversion grade pig iron) delivered to Steel making department, have important role to play in the production of desired steel grades and quantity of scrap to be melted which will be discussed subsequently under the heading of Steel Making.

Recent Past Induction of New And Renewed Blast furnaces in Industrially Developed  Countries.

 Hyundai Motor Group chairman Chung Mong-koo fires up Hyundai Steel’s new blast furnace at the company’s plant in Dangjin, South Chungcheong Province in 2010 (Hyundai Steel).
 As such it was planned, 6.5 million tons of the plant’s 8 million ton crude steel output will be used to produce cold rolled products with automobile products forming the majority.

POSCO restarts operation of renewed blast furnace (US)

Posted on 13-10-2010  

POSCO Chairman Chung Joon-yang ignites an upgraded blast furnace in Pohang, North Gyeongsang Province. The refurbished facility is the nation’s biggest and fourth-largest in the world, with an annual capacity of 5.3 million tons. (Credit  Photo @ POSCO)
The firm is expected to have an annual production capacity of 41 million tons this year after completion of maintenance and expansion projects.

Blast furnace goes digital - after 50 years of operation
SSAB Oxelösund with its two blast furnaces is the only integrated steel works in Sweden. Blast furnace 2 which has been in service for more than 50 years is now more modern than ever after being overhauled in 2006: Digital Profibus technology and some 400 PROFIBUS PA field devices (ABB) are now keeping a watchful eye on the production of pig iron 

Approximately 300 temperature transmitters, 50 flow transmitters and 50 pressure transmitters have been installed, all with a Profibus PA connection. That is about twice as many measuring points than in previous furnaces, giving the operator more information from the process.

Conclusion And Suggestions:

Amongst all the iron making processes, the Blast Furnace technology, which has been around the longest, still holds the dominant position ( 70% of all) and competitive owing to the continuous and several innovative developments in its design, automation, control on wastage of energy of exhaust gases, leakages, emission and refractory lining pattern etc. that have taken place since its inception.

Careful preparation of raw materials for physical and chemical consistency is an effective way of stabilizing operation over long periods. It is also necessary to understand the physical and chemical behavior accurately in each part of the furnace during operation. For this purpose, monitoring the conditions inside the furnace and applying artificial intelligence for data processing and judgment have been put into practical use with great success in advanced countries.

Since blast-furnace relining is enormously expensive, total production costs can be reduced substantially by extending furnace campaign life. Technological advances in Refractory lining pattern, operation and maintenance to date have extended the life of Blast Furnaces to as long as 16 years (current record), but further technical development is desired to extend furnace life to twenty years or more.

I myself was part of struggle in construction to successful operation during 22years of my service in this national institution. A sad state of affairs is because of corrupt bureaucracy. This is not a small foundry or re-rolling mill which was reluctantly handed over and left on the discretion of  contractor Lalgee and now the employees are facing the consequences.

Only sanctioning bailout packages will not bring steel mill out of crisis. Identification of  critical areas of the steel mill where the investment is essentially needed are of prime importance to obtain long term benefits and high value market oriented flexible products. Pakistan steel has vast potential to increase its profit as much as five times more than present capability and this needs sincere result oriented efforts.