Saturday, May 31, 2014

Factors confine the productivity of LD - Converter

Factors confine the productivity of LD-Converter (Steel Making Department Pakistan Steel). 

With the recent setting up of new ore based (BF- Converter route) integrated steel plants in China, India and Brazil along their coastal areas, the share of Converter steelmaking has stabilized and maintained at 70% of the world’s total steel production.

The LD-Converter is the key component in the steel production process in an integrated steel plant like Pakistan Steel. The productivity of a steel plant is dependent to a large extent on the availability of the Converter (BOF) for production.
A view of working platform of 130 ton LD-Converter(Elevation + 8 meter). 
 Steel Making Department of Pakistan steel is comprised of two 130 Tons LD-Converters (one in operation and second as standby) to achieve the Phase - I production of 1.1 million tons per year. Whereas the foundations for installing third 130 ton LD-Converter were made available in the steel making department as per project to enhance the Phase - II production to 2.2 million tons per year.
Tap to Tap Cycle of LD-Converter.

After every 40 - 55 minutes, a batch of molten steel (called a heat  ) is tapped from LD-Converter into a Steel ladle known as tap-to-tap cycle. Steel tapping cycle is matched/ synchronized with the speed of particular continuous casting machine in operation ( Billet, Bloom or Slab caster).

 The cyclic steps of LD-Converter operation:
Heat cycle may be interrupted due to :

-          late arrival of ladle from Ladle preparation section;

-          prolonged gunning repair; 

-          prolonged Converter’s mouth cleaning operation; 

-          unforeseen automation/ electrical/ mechanical fault cropped up in any process instrument or equipment; 

-          any re-blow for correction of chemistry/ temperature when the Converter melting shop is working without having computer controlled dynamic process model (may have a separate discussion) and Ladle Furnace facility.

After tapping a heat from Converter, the steel ladle is dispatched via an electrically driven ladle car to Continuous casting bay where a 180 ton over head crane transports it to ladle treatment station and then onward to the scheduled Continuous Caster ( Billet, Bloom or Slab),

In modern high productivity Steel making shops, Converter’s tap-to-tap cycle of a heat is typically designed closest to 35 minutes by carrying out fine-tuning of molten steel at ladle treatment station, facilitated with Ladle Furnace and Vacuum-degasser.

 At present the most common worldwide de-oxidation and alloying practice is a split practice. The major part of  de-oxidation and alloys addition is carried out during steel tapping from converter where as fine tuning of chemical composition and temperature is performed at ladle treatment station.

Ladle Furnace acts as a buffer between LD-Converter and continuous casting machine and is used to :
-          adjust the chemical composition (fine- tuning) of molten metal exactly as per required steel grade.
-      produce a wider range of steel grades.
-          de-sulphurise steel.
-          hold the molten steel for casting operation.
      -     raise the temperature of steel (superheat) in ladle required to maintain casting operation.

Experiencing the use of cold tundish boards for continuous casting, introduced in 1986 stipulated to increase the tapping temperatures to an average of 15 0C and after that induction of 6-strand Billet Caster from Voestalpine  Austria in1989 further caused an increase of average tapping temperature to 300C, required to compensate the need of superheat for prolonged casting duration of steel for smaller sizes moulds of Billet caster.

In the absence of Ladle furnace, the above modifications led to increase the overall load of higher tapping temperatures on Converter refractory lining, increased the holding time and confined the productivity of Converter.

Maintenance of LD-Converter to prolong service life:

LD-Converter’s Mouth De-Skulling.

After processing a sequence of 12 –1 5 heats, the Converter’s mouth becomes dirty due to slag and metal spatters and this skull build-up needs to be cleaned to maintain the converter’s mouth workable  for scrap and hot metal charging. 

As per project the mouth de-skulling operation was carried out by hitting the deposited skull with specially designed scrap box lifted with180 tons overhead charging crane which had been causing adverse impact on the converter’s cone and liprings and lowered down the overall productivity of Converter. Later in the year 1991, a refractory lining breaking machine was introduced which carries out de-skulling of converter mouth without engaging overhead charging crane and without imposing any adverse affect on structural integrity of Converter’s components.

During operation, the top refractory rings of converter mouth and subsequent layers of  cone bricks are subjected to severe erosion of high velocity flue gases and come in contact with molten metal and slag ejections. Excessive erosion in this area leads to collapse of top bricks and subsequent refractory layers causes exposure of refractory retaining plates and lip rings which could damage during mechanical de-skulling and put the Converter out of production cycle. To prevent this mishap, the Converter’s mouth is timely deskulled and mouth refractories are maintained by spraying gunning mass regularly.

Failure to remove skull buildup also adversely affects the center of gravity of the Converter, causing unsafe operating conditions. If the skull buildup is not removed timely, heavy deposition of metal and slag amalgam trapped in between the edge and liprings requires time-consuming extensive cleaning which may lead to delay the charging operation and hence leads to discontinue the production cycle.

LD-Converter’s Refractory Maintenance.

The refractory lining of LD-Converter works in severe conditions of high temperature and oxidizing atmosphere. After days of service, the refractory bricks start getting worn out faster in some critical areas of  Converter lining due to heavy corrosive attack of metal and slag  to the extent that timely hot repairs become indispensible to prevent premature loss of entire refractory material of a converter’s lining campaign.

Gunning Maintenance:

As per project, the lining life was 300 heats. Later in 1984, by modifying the Tar-dolomite lining scheme in cylindrical portion of Converter and establishing the regime of Oxygen blowing, the lining life was enhanced to 350 - 400 heats per campaign (in particular trunnions and slag line are very weak areas in the  Converter).

Introduction of Gunning maintenance in the year 1990, has led to a significant increase in lining life of Converter, despite simultaneous adverse affect of increased tapping temperatures due to use of cold tundishes and Billet Caster without ladle furnace.

After days of service when a lining wear of about 300 mm is observed on the trunnions, slag line and upper cone, a scheduled gunning with MgO based refractory mass is started to carry out on crumbling zones of the lining regularly (Pakistan steel’s practice) which has enhanced the lining life to 500 – 550 heats and enhance the availability of Converter for production to a reasonable extent under the prevailing operating conditions discussed above.  

Gunning mass spraying on worn out refractories of LD-Converter.

The gunning machine is comprised of a telescopic gunning lance that is mounted on a Carcass frame with electric drive, a water pump, a material tank, water and material hosepipes and a regulation valve for the remote-controlled adjustment of MgO base gunning mass and amount of water. A gunning repair is a time consuming operation and takes10- 12 minutes.

 As gunning material has a cost, so the unit price of gunning refractories should always be balanced with the cost of the left over refractories to be used in a Converter’s lining campaign. Excessive use of gunning material cannot be economical and increases cost per ton of steel.

Slag Washing Maintenance:

Slagging or slag washing maintenance is carried out frequently after tapping the steel and leaving a portion of slag (congealed with dolomite) on the bottom of converter. The vessel is rocked from one pad to another to coat a thin layer of about 2 inches of conditioned slag. Slagging operation fills small holes / cavities/ refractory wear developed on the charge pad, bottom and the tap pad but cannot cover the wear of refractories on side walls/ trunnion and two third of cone areas. Slagging maintenance takes 1- 2 minutes. 

Slag Splashing Maintenance:

A nitrogen blow slag splashing technique has been developed during past 20 years to allow slag coating especially on side walls / trunnion areas and generally the whole vessel and is currently employed in many countries where the steelmaking shops are equipped with this facility.

Slag splashing is carried out normally after steel tapping by retaining total or part of the slag on the converter bottom which is blown via a supersonic jet of nitrogen gas through the existing oxygen blowing lance. During slag splashing the molten slag is splashed on the sidewalls, cone and all over converter lining by regulating the blowing height and nitrogen flow rate. Any leftover slag is discharged. The coated slag layer acts as a consumable refractory lining which protects the erosion of Converter lining in subsequent heats.

In order to implement slag splashing technique the following shop facilities need to be studied/ revisited as first step:

-          Supply of additional amount of nitrogen gas at required pressure and pipeline from Oxygen Plant, provision of switch over / regulating valves, nitrogen gas hoses/ flanges etc.

-          Bulk material conveyors system and overhead storage bins for supplying Dolomitic lime or raw Magnesia.

-          The oxygen blowing lance and lance head configuration i.e. number of orifices/ nozzles, their angle of inclination to vertical axis of lance, diameter and divergence angle of orifice to set flow of nitrogen jet on molten slag.

-          Normally two oxygen blowing lance are provided to each of converter, one is working and the other standby. Both of the lances are remained connected to Oxygen supply line to overcome any failure during oxygen blowing. So time needed to switch over from Oxygen to Nitrogen supply line should be taken into account during a process cycle and should be added to the slag splashing total process time.  

-          Slag splashing frequency can be affected in a steel plant, producing steel grades with varying chemical compositions from high carbon to low carbon grades.

      Consequently the end point chemical composition, oxidation state, viscosity and end temperature of slag are wide-ranging and are changed with the steel grades produced.

-          Successful implementation of slag splashing technique requires a slag with low FeO content (around 10-13%) and high MgO content ( 11- 14 %),which is desirable to form high melting material MgO.Fe2O3  in slag to coat on long life vessel’s lining.
      Where as MgO  > 10 %  in slag prevents de-phosphorization and reduces yield. Also the iron content of the slag is a main source of iron loss.

-          As slag quality / properties are of prime importance to achieve promising results of slag splashing, so a compromising and most favorable composition of slag is required to satisfy the needs of individual steel plant which could achieve optimal protection of refractory lining without disturbing the production and operational goals.

-          A highly viscous / sticky slag for splashing containing high melting constituents has been reported to elevate the lance skull build up problem during slag splashing requires frequent replacement of oxygen blowing lance. Heavy accumulation of slag on oxygen lance tends to hinder the movement of lance through the lance entrance / opening in converter hood or chocking of lance nozzle may force to stop the converter production cycle.

-          Frequent skull build up on bottom is also reported which tends to reduce the effective volume of Converter and leads to upset the oxygen blowing height causes slopping, loss of yield and increases erosion of mouth refractories. In order to maintain proper blowing height, the bottom is usually flushed down which hampers the productivity of Converter.  

-           Also high production rate (e.g. when Converter feeds bigger sizes of slab caster’s moulds) may hinder slag splashing maintenance. As higher production rate means lesser time for preventive repairs.

Slag splashing technique is being employed in various steel plants world over to increase refractory life of Converter. Increase in lining life depends on the frequency of slag splashing which varies from plant to plant in accordance to the prevailing technological parameters and operating conditions of individual plant.  

The optimal tapping duration:  

The purpose of the tap hole is to allow controlled tapping of steel only, leaving the slag in the Converter and directing the metal spurt into the ladle. De-oxidation and alloy additions are made to the ladle during tapping operation to bring the raw steel to the required chemical composition.

As per project refractory life of tap hole was as low as an average of 10 heats and after that it was frequently repaired by re-piping with a shuttering of steel pipe and filling MgO base mass for producing next 09-11 heats.

Later in 1990, imported Isojet Tap hole Block system ( pre-assembled MgO- C tap hole blocks) was introduced, comprised of dual- ring tap hole assembly blocks system, in which the wear is retained within the inner concentric ring thus permitting the outer ring to maintain the integrity of the entire assembly. The inside refractory rings are changed upon wearing normally after 100 heats. The Isojet taphole block system has significantly improved the frequency of tap hole repairs and reduced down time.

The tap hole is generally repaired when tap time reaches to three minutes. The recommended tapping duration for 130 ton LD-Converter is between 37 minutes so as to attain optimal consumption of deoxidizers, consistency of alloys recovery, steel quality and productivity of converter could be maintained.

The tap area refractories are eroded due to contact of molten metal and aggressive slags. To maintain proper thickness of refractory lining in the tap area, the gunning mass is sprayed regularly which prevents any metal breakthrough from converter shell during tapping.

Prolonged tap duration reduces productivity of converter, requires higher tap temperatures which causes extensive wear of tap pad refractories and increases thermal deformation of Converter’s cone and tap pad area (shorten vessel life). 

Shorter tapping duration than recommended restricts ladle additions and de-oxidations, increases slag carryover in ladle results in excessive burning of Ferro alloys, phosphorus reversion, retards desulfurization and hence deteriorates quality of steel.

A rising demand of quality steel grades requires restraining the amount of slag carry over to the ladle at the end of tapping. Various measures are taken to prevent the final slag from flowing with metal into the ladle during tapping such as throwing into the converter a ball-shaped device or refractory cone- shaped plug called dart which is lighter than metal but heavier than slag, it floats on the metal and closes the taphole at the end of metal tapping.

Pakistan steel had tried the refractory ball slag stoppers experimentally but their performance was found unsatisfactory. These stoppers are not able to fully prevent the passage of slag but diminished the quantity of slag in the ladle metal.

Under the prevailing operating conditions, Pakistan steel has trained its steel tapping operators to handle the slag carryover problem manually by using judgment skills of their eyeballs with particular emphasis on keeping the tapping duration not less than three minutes and this practice works as good as any refractory ball stopper without any investment.

In order to achieve close control of chemical composition of hi-tech steel grades (e.g. automotive, oil/ gas pipeline, railroad steels) a pneumatic device has come into use over last 20 years which is installed at the tap hole on the outside shell of the vessel. An electromagnetic slag detector actuates a stopper which pneumatically closes the taphole before commencement of tapping and after all the steel is tapped.  

 The above mentioned device has claimed to be capable of reducing the slag content in the liquid steel to levels lower than four kg/t. It can be operated either in manual or automatic mode.

However a sophisticated device like above involves investment and it does not work stand-alone. It works in steel plants facilitated with a computer controlled dynamic model for converter operation that accurately controls the steel making process. (Turn down chemical analysis, temperatures, slopping/ ejection out of converter).

Factors affect the Performance of Refractory Lining.

-          The quality of refractories employed and their maintenance.

-          Quality of brick laying work.

-          Specific volume of Converter which varies from plant to plant.

       -     Design of oxygen blowing lance orifice tip.

Affect of Processing Method:

a)     Higher pressure of oxygen and height of oxygen blowing lance above the bath level reduce lining life.

b)     The carbon content at the end of the blowing = <  0.03% , reduces lining life faster.

c)     The metallic bath temperatures at the end of blowing >16700C, reduce lining life faster. 

d)      Higher content of Silicon in pig iron, reduces basic lining life.

e)     The CaO content in the soft burnt lime > 90% ,uphold basic lining life.

f)       Addition of dolomitic lime  in early slag to saturate MgO content 7-8 %, prevents    dissolution of MgO from refractory bricks, uphold basic lining life.

g)     Properly distributed and timely dosage of flux ( lime + dolomitic lime ) in accordance with chemical analysis of metallic charge, uphold lining life.                                                                                                                                                                              
h)     Growth of aggressive oxide FeO in early slag is needed to dissolve lime but excessive formation of FeO in slag severely reduces lining life of Converter.
i)        Careful preparation and distribution of scrap in scrap charging boxes / trays such that lighter scrap on face and recommended heavy / large sizes on back, reduces the impact  load of falling scrap on refractory lining on charging pad, upholds lining life.

      j)    Excessive amount of holding time of a heat in the vessel reduces lining life.

      k)   Minimize use of CaF2, uphold lining life.

       l)    A re-blow of 20 seconds consumes the refractory lining required for one heat.

However the above suggestions could be a guide line for ready reference but need separate comprehensive enlightenment on each action.   
 Refractory Lining of 130 ton LD-Converter:
 As per project. Pakistan steel uses magnesite chrome XM1, XM4, XM8 bricks for LD-converter safety or backup lining in the upper cone, lower cone (stadium) and cylindrical portions.

The working lining of 130 Ton LD-Converter at Pakistan Steel is comprised of Tar bounded dolomite bricks which are manufactured in Lime & Refractories Production shop Pakistan Steel by processing raw dolomite (CaCO3. MgCO3 ), available  in the vicinity of Jhimpir district Thatta.

Removable Converter bottom:
The LD-Converter of Pakistan Steel has a removable bottom. The bottom is fasten to the converter with wedge lock plates by raising and clamping the bottom underneath converter with the help of a specifically designed (Russian make) hydraulic jack car.

As per project three bottoms were made available such that two are attached with two of each Converter in operation whereas the third one remains under relining work at shop floor to get ready for use in the succeeding relining campaign.

The converter bottom is lined with heavy duty Chrome- Magnesite XMKK bricks which are stable bricks at high temperatures of steelmaking.

 The removable bottom facilitates quicker cooling, demolishing and removal of discarded bricks and assists in relining work of LD-Converter.

However there is always chance of metal penetration through the bottom joints during production cycle if a thick layer of basic slag (congealed with MgO) is not maintained over bottom bricks and sealing bricks joints.

The converter bottom height with reference to Oxygen blowing lance height is measured at least once a day to accomplish regime of oxygen blowing. Maintaining proper height of converter bottom is necessary to obtain good yield, control of lining wear, and reduce slopping and smooth converter operation.

Extensive wearing of bottom refractories is observed during production of long series of deep drawing steel grades ( carbon < 0.03%). During production process when the bottom height can not be raised by normal slagging maintenance to the required level, then it is repaired by leaving a mixture of liquid slag and broken Tar- dolomite bricks on the bottom lining for at least four hours (current practice at Pakistan Steel).

LD-Converter’s Cold Repair:

After finishing a campaign life of refractories, the Converter is stopped for demolishing the worn out refractories and to carry out relining work, known as Cold Repair.

 Demolishing of Converter refractory lining after finishing campaign life.

After finishing the life campaign, the converter is flushed out to remove Converter’s bottom with the help of hydraulic jack car.

The worn out working lining is cooled and demolished by using a lining breaking machine shown above. The discarded refractory bricks are dispatched to slag dumping yard for onward disposal.

The Converter relining work is carried out with the help of a Telescopic hoist which permits step by step elevation of foldable working platform entering from the bottom hole into the upright positioned converter to support workmen and refractory materials.
Simultaneously during Cold repair, the broken equipments/ instruments pertaining to converter operation are overhauled and any unbearable mechanical, electrical, Heat & Power and automation fault revealed during preceding operational campaign is rectified.

The relining work (Cold Repair) takes 60-72 hours which considerably increases downtime and labor cost.   

 Needs to use refractory bricks with high performing characteristics.
 Despite employing the above discussed efforts, the lining life could be increased barely an average of 550 heats per campaign which is a bottleneck to the availability of Converter for production.

Achieving high availability requires maximizing lining life and reducing the number of reline campaigns. This could be achieved by improving the quality of basic working refractory lining of Converter.

Magnesia-Carbon Bricks.

 In industrially developed countries, large high density Magnesia-Carbon bricks of varying chemical composition (Zoned lining) are used in steelmaking furnaces to minimize down time and to achieve optimal pace of quality production.

 Medium quality Magnesia- Carbon bricks were tried by Pakistan Steel during 1990 – 93 in steel making LD-Converters, imported from four European suppliers which lasted for an average campaign life of 850 heats per campaign (compared to local Tar dolomite bricks an average of 550 heats/campaign) but their higher cost made them uneconomical.

Zonal lining of LD-Converter:

As per the wear conditions of refractories during operation of LD-Converter, the lining scheme is generally divided in following different zones.

Zonal lining scheme of LD-Converter.

The refractory wear pattern varies from plant to plant as per prevailing conditions of converter operation practice. Various qualities of MgO-C bricks are designed for different zones of LD-Converter as per the data of lining wear pattern obtained from the user. Critical wear areas (impact and tap pads, slag lines and trunnion areas) in Converter are zoned with bricks of the highest quality.

Conclusion & Suggestions:

1)     In order to achieve a levelheaded quality production requirements and minimize downtime, Pakistan steel needs to enhance the life of working lining of LD-Converter to at least 2500- 3000 heats per campaign on precedence in first phase which can be achieved by making use of superior quality locally available Magnesite mineral. Without improving the life of basic working lining none of the above refractory maintenance operation could effectively further increase lining life of Converter.

2)     However slag splashing technique is being employed in various steel plants world over to increase refractory life of Converter and is also an economical re-use of Converter slag. The frequency of slag splashing varies from plant to plant and depends on the prevailing technological parameters and operating conditions of individual plant. Successful implementation of this technique needs to revisit shop’s technological conditions, investment and necessary expertise briefly discussed above.

3)     Subsequent to above up-gradation of working lining of Converter, the service life         of refractory lining can be further extended to 5000 heats per campaign or above by crafting an economical optimum balance of production, extent of refractory maintenance, refractory costs without humiliating the life of vessel’s components.

4)     None availability of Ladle furnace facility in steelmaking department is one of the bottlenecks, confining the productivity of LD-Converter. Provision of  ladle furnace would eliminate un-necessary thermal and processing burden of 15-20 minutes on the Converter’s refractories, resolve quality issues, prevent any risk of returned heat from continuous casting due to drop of temperature and put off delays/ discontinuing the process cycle.

5)     After tapping the steel from Converter, the need of vacuum degassing depends on the objectives of production of quality steels of higher degree of cleanliness currently required for manufacturing automotive grades (light weight high strength steels), rail road steel, oil/gas pipeline grades etc. Vacuum degassing involves exposing liquid steel to a low absolute vacuum to remove excess hydrogen and to produce ultra low carbon and low sulphur steels. Induction of RH type degasser in steel making department may be deemed at a later stage of revamping.

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