Important aspects of design of Blast Furnace and associated ancillary equipments
Important aspects of design of Blast Furnace and associated ancillary equipments
The design of the blast furnace (BF) proper and its associated and ancillary equipments (Fig 1) immediately upstream and downstream of the furnace is important for the efficient running of the BF. Besides the furnace proper, the immediate associated equipments include (i) the stock house, (ii) the charging equipment, (iii) the furnace top, (iv) the cooling system, and (v) the cast house area equipment.
Fig 1 Blast furnace with its associated and ancillary equipments
BF ironmaking consists of a system comprised of several components which are functioning in harmony. Proper application and operation of these components is necessary to support the ironmaking process. Selection of specific components is dependent upon such factors as existing conditions, physical constraints, production requirements, cost, schedule, reliability, and maintainability. Inter-dependence of components is as important to the successful operation of the system as their individual capability. There are major requirements and ‘normal’ practices for each area or component. There are also some alternative technologies which are commercially available which have inherent advantages and disadvantages.
BF converts iron bearing ore burden into the liquid iron (hot metal). Associated with the BF are coke ovens batteries which convert coal into coke, and sinter and pellet plants, which prepare iron ore for the BF. The BF converts these prepared raw materials into a product of higher value. Hot metal from some of the BFs operations is used in foundries for the production of iron castings. Other operations produce a lower silicon hot metal which is converted into steel in steel melting shop. Some of the hot metal is converted into pig iron in the pig casting machines. The by-products of BF are slag, BF top gas, flue dust, and filter cake. These by-products can have either positive or negative economic impact, depending on the local possibilities for utilization.
Some of the several aspects affecting every consideration for the design or re-design of BF are (i) profit, (ii) employees’ health and safety, (iii) environmental protection, (iv) statutory regulations, (v) needs of the market and downstream processing, (vi) available human, construction, and maintenance resources, (vii) changing technologies and equipment obsolescence, (viii) available raw materials, utilities, and other materials, and (ix) so on. Any serious constraint in any of these aspects can put at risk the viability of the BF unit (or even a steel plant) or preclude or necessitate the construction of a new BF.
BFs are normally grouped by size. Mini BFs produce less than 1,500 tons of hot metal per day (tHM/day), small BFs produce in the range of around 2,500 tHM/day to 5,000 tHM/day, medium BFs produce in the range of around 6,000 tHM/day to 8,000 tHM/day, and large BFs produce around 9,000 tHM/day to 12,000 tHM per day. In an integrated steel plant, a number and sizes of BFs are needed to provide the hot metal required for the production of steel. Integrated steel plants with a number of BFs are less affected when a BF undergoes relining repairs or when there are furnace control problems. Small BFs have shorter relining repairs than large BFs and are considered to be easier to operate. However, the cost of hot metal from small BFs is higher. The integrated steel plant is required to operate the minimum number of cost effective furnaces. In some cases, upgrades are made in order to reduce the number of furnaces in operation.
BFs are relined periodically after the end of its campaign (time between relining repair). In the past, this involved the replacement of the internal brick lining of the main furnace. In recent times, extensive component rebuilding, replacement, and the preventive maintenance are performed at the same time. With this practice, the more efficient steel plant with fewer large BFs lose a higher percent of its production during the BF relining repair than the plant with more number small furnaces. In order to have both the low operating cost and the minimum interference during the BF relines, the industry has worked to maximize the campaign of the BFs and to reduce the duration of the relining repair. The present day clear trend in the integrated steel plants is to operate fewer large furnaces and to utilize techniques and designs which extend the BFs campaign indefinitely.
At the same time, the reduction in product variability has become more important, and hence investments are being made in automation which improve monitoring and control of the process. BF operators, maintenance personnel, designers, and research personnel have applied modern technology and analytical methods to the BF process in order to better monitor and control the process. As a result, the standard deviation of the hot metal quality has been reduced. Improved data collection systems also provide increased information for suppliers and manufacturers. This has improved the materials selection and the design of the BFs and associated equipment. Campaign lengths have increased to around 20 years previously from 5 years to 10 years.
BF ironmaking is a 430 plus year old technology. Even so, the use of BF hot metal is still the most common method used in the integrated steel plants for the production of steel. Present day integrated steel plant processes rely upon the BF to provide on schedule predictable quantities of hot metal of consistent quality. Variation in any of the aspects of the supply of hot metal has a serious impact on the rest of the steel production processes. Hence, the BF continues to be a key process in the modern integrated steel plants.
Sometimes it is being stated that the BF process technology is at the end of its useful life. This is not so. The study of operating data of 23 years (1970 to 1993) from a typical pair of medium size BFs has shown that there is an average increase in productivity of 3 % per annum (Fig 2a). At the same time, the average reduction in fuel rate has been 1 % per annum (Fig 2b). Also, the productive time between the BF relining (the campaign) has been extended through improvements in equipment, materials, and design. As a result, the overall cost of producing hot metal, corrected for inflation, has improved even more than the operating data indicates. Hence, the BF process technology is not dead though it is a 430 plus year old technology. It is still progressing in every area at a considerable rate. The BF remains today a dynamic science supported by constantly improving technologies.
Fig 2 Operating data from a typical pair of medium size blast furnaces
Layout of the BF
The layout of a BF is essentially an exercise of integrating the equipment needed to handle the different materials needed to make hot metal and the resulting product and by-products. The most efficient design properly accommodates the entire process and its effectiveness is judged from the standpoint of both the initial capital investment and ongoing operating costs. BF shop layout is dependent upon several factors such as (i) site terrain, (ii) climatic conditions, (iii) raw material delivery method, (iv) in-plant raw material processing systems and locations, (v) downstream processing systems and locations, (vi) quantity / flow requirements for hot metal, (vii) type and size of the hot metal delivery fleet, and (viii) so on. Fig 3 shows a simplified layout of a BF plant.
Fig 3 Simplified layout of a blast furnace plant
A BF plant consists typically of several sections. These sections are (i) raw material storage, handling, and reclaiming, (ii) stock house, (iii) charging system, (iv) furnace proper, (v) cast house, (vi) slag processing and handling, (vii) hot metal handling, (viii) hot blast stoves and hot blast system, (ix) gas cleaning plant, (x) utilities, (xi) automation and control system, (xii) maintenance facilities, and (xiii) personnel support facilities.
Raw material storage and handling
Raw materials such as iron ore, sinter, pellets, fluxes, coal, and coke etc. are received either from external sources or are produced within the integrated steel plant. These raw materials need sufficient controlled storage to support the BF operations. Storage capacity is needed in the event of predictable delivery disruptions or unpredictable disruptions. Additional storage capacity can be needed in case of possible changes in the source of certain raw materials. Separate storage locations are needed because of different physical or chemical characteristics in similar materials. Mixing of similar materials can cause process control / metallurgical problems. The storage piles are to be separated to prevent intermixing of the dissimilar materials. The piles are to be placed on prepared beds to enable the raw material reclaiming equipment operators to distinguish between prime and tramp material. Piles are laid out to minimize material degradation and to prevent wind pick-up of fines. Water sprays and cocooning agents can be used to minimize dust pick-up / carry-off by winds.
Several different techniques are available for raw material lay down and retrieval. Lay down techniques include wagon tippling, ore bridges, stacking conveyors, scrapers etc. Retrieval techniques include bucket wheel reclaimers, bender reclaimers, front-end loaders, scrapers, directly from bin or pile bottoms etc. It is obvious that the lay down and retrieval systems are to be sized to ensure the through-put needed for the BF plant.
The stock house is the BF operator’s storage unit for direct feed of the burden to the furnace. Storage bins are provided for each of the burden materials for the BF. Individual bins are provided for similar materials (i.e. sinter, pellets) having different metallurgical properties. The stock house provides adequate capacity for the various burden materials in the event of short term disruption of supply from the raw material storage areas. Typical stock house bin throughput capacities, in the event of loss of raw material feed are (i) coke – 2 hours to 8 hours, (ii) iron bearing materials (ore, sinter, and pellet) – 4 hours to 16 hours, and fluxes and other miscellaneous materials – 8 hours to 24 hours. These capacities are based on rated furnace production and vary depending upon the reliability and the access time for their replacement from inventory or from a supplier.
Burden materials tend to degrade due to climatic conditions and repeated handling. The higher is the number of times that the material is handled (stockpiling, reclaiming, dumping, conveyor chutes, ore bridge buckets, etc.), the higher the percent of fines in the burden. The BF process needs controlled permeability and hence controlled burden. The charging of excessive fines, either normally throughout the charge or concentrated over specific short charging periods, can be disruptive to the BF process and damaging to the furnace equipment. The stock house provides the last reasonable opportunity for removal of fines prior to charging into the furnace. Wherever possible, vibrating screens are installed after the coke, ore, sinter, and pellet storage bins to eliminate the major portion of the fines. The removed fines are collected for recycling. Some BF operators charge fines to specific areas in the furnace to adjust local furnace permeability and control heat loads on the furnace walls.
Moisture gauges are frequently provided in the stock house to monitor the actual water quantities charged to the furnace. This information permits adjustments to the charging quantities to compensate for the varying ambient conditions (i.e. higher coke moisture during the period of rains).
Since different types and varying quantities of the burden material are needed to support the continuous operation of the BF, the burden materials are to be provided in a specific sequence (which itself can be changed frequently to support the varying furnace operating parameters). Hence, the stock house is to be provided with reliable equipment for extracting and feeding accurate quantities of specific burden materials for meeting a specific schedule. Fig 4 shows simplified equipment arrangement in the stock house.
Fig 4 Simplified equipment arrangement in the stock house
Earlier, the most common type of stock house has been the highline type. This type of stock house is located directly adjacent to the furnace. Rail cars or bridge cranes feed the storage bin while the storage bins feed directly to a travelling scale car. A scale car operator manually controls the bin discharge gate to feed specific quantities of material into the scale-equipped hopper located in the scale car. After collecting the proper types and quantities of material, the operator moves the scale car to a position above the ‘skip pit’ and dumps the burden, through a chute, into a waiting skip car. The skip car then is hoisted to the furnace top.
The highline type of stock house, in conjunction with a scale car, has presented few options for the provision of ferrous charge (ore, sinter, and pellets) screening. As knowledge of the BF process has increased, more stringent requirements for the burden have developed. The concept of ‘engineered burden’ is well recognized today in the industry. It is normally accepted that there are limits to the flexibility and adaptability of the highline stock house to support this requirement. Hence the highline type of the stock yard has been replaced by the automated conveyorized stock house for supplying charge materials to the BF.
The automated stock house is normally of two distinct and different types. The first type is the replacement of the scale car under the raw material bins with a feeder and conveyor belt system. Separate conveyors are provided for each type of raw materials (coke, iron bearing materials, and flux materials and additives etc.) over which rows of storage bins are mounted, with vibrating feeders to discharge burden materials from storage bins to conveyors. For the coke and iron bearing materials, a vibrating screen is located at the discharge of each conveyor to screen the material and feed the screened material into the weigh hoppers. This type of system continues to feed weigh hoppers ahead of the skip cars.
The second type of the automated stock house is a large structure of storage bins built entirely above ground and quite away from the BF. This is normally done for the BFs where belt conveyor is used to carry the burden materials to the top of the furnace instead of the skip cars. The method of filling the storage bins is normally by a conveyor belt system. The raw materials are drawn from the storage bins by vibrating feeders and belt conveyors into weighing hoppers. The weigh hoppers in turn discharge the material onto the main conveyor by means of a collecting conveyor. The weighing hoppers are programmed to weigh the raw materials in the correct order onto the main conveyor belt to the top of the furnace.
Provision of an automated stock house can provide more efficient feed of raw material to the stock house and more efficient selection, screening, weighing, and delivery of the burden to the furnace. The automated stack house can be located directly adjacent to the furnace feeding skip car, or can be located remote to the furnace for charging through a conveyor belt.
Automation of the stock house has considerably increased production capability, improved operating efficiency, and eliminated operating variances caused by operators and equipment. However, in practice a modern, automated stock house can be quite complex. The stock house itself can be fed by conveyors, which in turn discharge onto tripping conveyors to distribute materials to various bins. The layout of conveyors and equipment in the stock house can be arranged in a large number of ways. Placement of the stock house adjacent to the furnace frequently results in layout congestion and restricts flexibility for future modifications.
The burden materials are normally hoisted to the BF top with the help of skip cars or by conveyor belt.
Skip car hoisting – The use of skip car for BF evolved from the mining industry. BF skip cars are sized to suit the furnace throughput. Obviously, several factors such as hoist capacity, skip bridge design, and so on, have their own influences or constraints on the skip size.
Normally, two skips operate in opposing fashion (to reduce hoisting power needed) on a common hoist. Skips travel on rails on a skip bridge, normally installed at an around inclination of 60 degrees to 80 degrees from horizontal. The full skip accelerates slowly as it leaves the skip pit, accelerates as quickly as possible reaching and traveling at maximum speed for most of the lift. The hoist slows the skip down as it approaches the top of the skip bridge. The wheels of the skip are guided by the dumping and horn rails as the skip is overturned into the furnace top charging equipment. As the hoisting skip reaches and stops at the final dumping position, the empty skip (descending at the same speeds) is just reaching the bottom of its travel into the skip pit, awaiting filling. The skip charging system is a reliable and effective technique for delivering the burden to the furnace top. However, it lacks flexibility for the operator in that the skips can only hold a specific quantity of material (overloading results in overfilling or excessive hoist loads) or becomes inefficient if small volumes of specific burden are needed. Fig 5 shows schematic of skip car hoisting.
Fig 5 Schematic of skip car hoisting
Furnace charging conveyor – With the conveyorization of the stock house has come the conveyorization of the hoisting system. It is now common for the stock houses to be located remote from the furnace and one large conveyor belt carries the burden to the furnace top. If the furnace top is around 60 meters high, then a conveyor belt installed at 8 degree inclination to horizontal position, the stock house is to be located at least 427 meters from the furnace. Steeper belt inclinations are normally avoided to minimize material roll back. It is normal to charge miscellaneous materials directly over and after the end of a ferrous charge on the conveyor belt in order to hold the ferrous materials in place until they reach the furnace top.
Charging system at furnace top
The furnace proper is operated with positive high top pressure. BF gas consisting partially of carbon monoxide, carbon dioxide, and nitrogen is generated by the BF process along with large quantities of entrained dust. The BF operator is to maintain the top pressure because of the process benefits and to contain the gases and dust (both for fuel value and environmental control purposes). However, the operator is to regularly place burden material inside the top of the furnace in order to replenish the internal process, without losing the furnace top pressure.
Bell type top – For several years, the most common type of furnace top has been the two-bell top (Fig 6). As the burden reaches the furnace top (by skip or by conveyor), it falls into a receiving hopper and into the small bell hopper. The small bell (conical shaped steel casting around 2.6 meters in diameter and 1.4 meters high for a 5,000 tHM per day BF) lowers and permits the burden to fall into the large bell hopper. The small bell is lifted and seals against a fixed seat on the small bell hopper. Depending on the volume of the large bell hopper, additional loads of burden are sequenced into the large bell hopper by the small bell. Throughout this process, the large bell has remained closed, sealing the furnace. When the correct numbers of load of the burden have been collected, the large bell (conical shaped steel casting around 5.5 meters in diameter and around 3.5 meters high for a 5,000 tHM per day BF) lowers and allows the burden to slide down the bell into the top of the furnace proper. After the burden discharges the large bell is raised and seals against the underside of the large bell hopper. Fig 6 shows bell type BF top charging systems.
Fig 6 Bell type blast furnace top charging systems
Obviously, the burden distribution control with the furnace for this type of the top is limited by how evenly the burden is placed on the large bell (skip dumping results in uneven placement of burden into this type of the top) and the falling curves of the specific burden materials (i.e. coke or iron burden) as they slide and fall off the large bell. Further, the two bell top is susceptible to loss of sealing of the large and small bells and of the packing between the large bell rod and small bell tube. Bell leakage results from the abrasion by the burden material sliding over the bell sealing surfaces. The rod packing leakage is a result of abrasion from fines either from within the furnace or from collecting on the large bell rod after burden is dumped in the receiving hopper.
In an effort to minimize wear of the large bell sealing surface, BF gas taken from within the furnace is introduced between the bells to equalize the space (reducing the pressure differential across the large bell sealing surface). This gas is relieved to atmosphere prior to opening the small bell to permit introduction of more burden. Some of the options available to improve the limitations of the two bell type top system are given below.
The McKEE distributor – The McKEE distributor (Fig 6) for several years was the main burden distribution improvement available for the two-bell type top. However, it is quickly being replaced by other technologies. Its design incorporates the ability to rotate the small bell and small bell hopper together while the skip car is discharging. Burden is evenly distributed into the small bell hopper, hence improving the even placement of burden onto the large bell. This type of top is prone to small and large bell wear and subsequent loss of sealing effect.
CRM universal rotary distributor top – CRM (Centre Recherches Metallurgiques – Belgium) universal rotary distributor (Fig 6) was developed to eliminate the loss of small bell sealing effect. Two bells (a sealing bell and a material bell) are installed in place of the normal small bell. A revolving burden hopper is mounted on the material bell. The sealing bell is located beneath the material bell and seals against a fixed seat. During skip discharge, the burden hopper and the close material bell are rotated to evenly fill the hopper. When filling is complete, the hopper rotation stops. When it is time to dump onto the large bell, the revolving hopper, material bell, and sealing bell are lowered. The sealing bell lowers below the- fixed seat. Part way through the lowering process, the hopper descent is stopped and the material bell and sealing bell continue to descend until they reach their stop position. As the gap opens between the material bell and the hopper, the burden discharges evenly into the large bell hopper. As the burden leaves the gap, it does not come into contact with the sealing valve seating surface, hence maintaining the top sealing capability. This style of top is capable of maintaining 0.2 MPa of internal pressure.
The CRM top improves the sealing capability and longevity of the two bell top. It does not provide however a dramatic furnace burden distribution improvement over the McKEE distributor and does not eliminate the vulnerability of the large bell sealing surface.
The GHH lock hopper top – The GHH lock hopper top (Fig 6) is the modification to the two-bell top. It reduces dependency on the large bell to maintain a gas seal. The addition of lock hoppers with separate seal valves for each skip dump location provides an additional capacity for sealing the top. The large bell can be operated with no differential pressure across its sealing surface (i.e., furnace top pressure equals large bell hopper pressure). The operation is given below.
A skip dumps the load of burden into the lock hopper through a receiving hopper and open seal valve. The burden is placed on the rotating small bell and uniformly fills the rotating distributor hopper above the small bell. A seal between the lock hopper and the rotating small bell hopper is open while the rotation is underway. When the burden discharge from the skip is finished, the seal valve and the seal between the lock hopper and small bell hopper are closed. Equalizing gas is introduced and the lock hopper is pressurized to furnace top pressure. The small bell is then lowered to introduce the burden into the large bell hopper. The small bell closes and the pressure from the lock hopper is relieved to atmosphere. The seal valve on the opposite side (i.e. at the other skip dumping position) is opened. The seal between the lock hopper and the small bell hopper is opened. Rotation of the small bell and hopper commences. The top is now able to accept burden from the other skip.
This type of top improves the sealing capability and the longevity of the two bell top. The lock hopper top, however, does not provide a dramatic furnace burden distribution improvement over either the McKEE or CRM top. Although the large bell no longer is needed to perform a sealing function, the small bell sealing effect longevity is still critical.
Movable armour – The major step taken to improve the burden distribution of the bell type top was the development of movable armour (Fig 7). Adjustable deflectors are installed in the throat area of the furnace to deflect the burden after it slides off the large bell. The movable armour is adjusted depending upon the specific burden material being discharged and where the operator wants to place the burden within the furnace.
Several manufacturers provide different types of movable armour. Individual armour segments can be moved uniformly (simultaneously and equally) inside the furnace to place the burden in an annular pattern. Other types of movable armour are available to provide individual control of the armour plates in order to achieve non-circular distribution pattern.
Some disadvantages associated with movable armour are (i) majority of the mechanical and wear components lie with the harsh environment of the furnace top cone, (ii) some loss of internal working volume is needed to provide clearance between the movable armour and the design stockline level (although this area of the furnace cannot be classified as a rough productivity zone for furnace working volume consideration, and (iii) limited capability to deflect burden to the very centre of the furnace, particularly when the stockline level is already high. The rolling characteristic of pellets frequently negates the limited displacement of the movable armour.
Fig 7 Stockline armour and bell less top charging equipment
Bell less top charging system – In the early 1970s, Paul Wurth S.A. of Luxembourg developed the bell less top (BLT) charging system (Fig 7). This type of the furnace top is a radical departure from the bell type top. Burden can be placed within the furnace in any pattern needed by the furnace operator. Annular rings, spirals, segment, and point placement are common pattern achievable by synchronized or independent tilting and rotation of a burden distribution chute located with the top cone of the furnace.
Furnace top sealing is maintained throughout the campaign of the furnace. Maintenance activities are simple and of short duration. Normally, the BLT consists of a receiving chute or hopper (receiving burden from the skips or from a conveyor belt), a lock hopper with upper and lower seal valves, a material flow control gate, a main chute drive gearbox (a water or gas-cooled unit used for chute rotation and tilting), and the burden distribution chute. There are three main types of BLT namely (i) parallel hopper, (ii) central feed, and (iii) compact type.
Typically, the parallel type incorporates two lock hoppers (the hoppers have been installed on some furnaces for throughput and backup purposes; a one ‘eccentric’ hopper type has been installed for an application with restricted clearance). Since the early 1980s, several BFs have selected the ‘central feed’ single lock hopper type for its improvements in burden segregation and burden distribution control resulting in improved furnace operation.
A ‘compact’ type of BLT top has been developed for small to mid-sized furnaces to permit the introduction of the BLT (and its advantages) to furnaces where the other larger types of BLTs cannot be used due to cost or physical constraints. The steps in BLT operation for a central feed type are (i) burden is discharged from a skip or conveyor belt through a receiving chute or hopper past an open seal valve into the lock hopper, (ii) after the burden is received in the lock hopper, the upper seal valve is closed and equalizing gas is introduced to pressurize the lock hopper to furnace pressure, (iii) the lower seal valve opens, (iv) the burden discharges from the lock hopper as the material gate has been set to the preselected opening to suit the specific burden material to be discharged, (v) the burden drops vertically though the feeder spout with the main transmission gear box and falls onto the burden distribution chute, (vi) the burden distribution chute directs the burden to the required point(s) within the furnace, (vii) when the lock hopper is fully discharged (monitored by load cells and / or acoustic monitoring), the lower seal valve is closed, (viii) a relief valve is opened to exhaust the lock hopper to atmosphere (or through an energy recovery unit), and (ix) the upper seal valve opens and the sequence is repeated.
The advantages of BLT over other top charging systems include higher top pressure capability (i.e. 0.25 MPa), fuel savings, increased production, more stable operation, reduced maintenance in terms of cost and time, increased furnace campaign life, and improved furnace operational control when employing high coal injection rates at the tuyeres.
Gimbal system of charging – The purpose of the Gimbal system of charging is to facilitate controlled distribution of charge material into the BF via a Gimbal type oscillating chute through a holding hopper and variable material gate opening such that the pressurized charging system above can operate independently of the distribution system. It utilizes a conical distribution chute, supported by rings in a Gimbal arrangement, producing independent and combined tilting of the chute axis. The Gimbal distributor, as part of the overall BF top charging system, offers a fully integrated charging solution, generating considerable improvement in BF operation and maintenance cost. The Gimbal system utilizes a conical distribution chute, supported by rings in a Gimbal arrangement, producing independent, and combined tilting of the chute axis.
The Gimbal top incorporates a full complimentary range of furnace top distribution equipment including distribution rockers, upper seal valves, hoppers, lower seal valves, material flow gates and goggle valve assemblies, all discharging through hydraulically driven distribution chutes. The tilting chute is driven by two hydraulic cylinders, mounted 90 degree apart. This type of suspension and drive arrangement results not in a rotation of the tilting chute, but in a circular path by superposition of both tilting motions. Independent or combined operation of the cylinders allows the chute axis to be directed to any angle, or even along any path. Motion is supplied by two hydraulic cylinders, each operating through a shaft, connecting rod, and universal joint in order to drive the Gimbal rings. Through the movement of the hydraulic cylinders, the distribution chute allows precise material distribution with potential for an infinite number of charging patterns at varying speeds. These include ring, spiral, centre, spot, segment or sector charging, providing complete control of material charging into the furnace.
The whole distributor assembly is enclosed in a gas tight housing, which is mounted directly onto the top flange of the BF top cone. The housing contains a fixed inlet chute and a tilting distribution chute supported by rings in a Gimbal arrangement allowing independent and combined tilting of the chute axis. The assembly is made from a combination of stainless and carbon steel material with the fixed inlet chute and tilting chute body lined with ceramic material to give superior wear protection. A closed-circuit water cooling system supplies cooling water through the main shafts, Gimbal bearings, and universal joint bearings in order to cool the moving elements of the Gimbal distribution system.
The key features of the Gimbal design are (i) simple, rugged design, using levers driven by the hydraulic cylinders, (ii) drive cylinders are mounted outside pressure envelope, hence not subject to hot and dusty service conditions, (iii) Gimbal ring arrangement gives simple tilting motion in two planes, which when superimposed gives 360 degrees distribution, and (iv) wear on the tilting chute is equalized around its circumference giving a long extended operational life.
The BF Gimbal top is an automated, computer-controlled pressurized charging system designed to (i) receive charges of ore, coke, and miscellaneous materials in the holding hopper, independently of the distribution system below, (ii) release those discharges, as needed, to a dynamic distribution chute located below the holding hopper, and (iii) distribute material in prescribed patterns to the furnace stock-line in accordance with a predetermined charging matrix. Control of the Gimbal distribution chute is fully integrated into the overall furnace charging software. The system provides a high level of accuracy and control for the Gimbal movements and hence the positioning of the distribution chute. Gimbal material distributor is shown in Fig 8.
Fig 8 Gimbal material distributor and charging system
The furnace proper is the main reactor vessel of the BF ironmaking process. Its internal lines are designed to support the internal process. Its external lines are designed to provide the necessary systems to contain, maintain, monitor, support, and adjust the internal process.
The BF process is a counter flow process. The process comprises of (i) burden at ambient conditions is placed in the furnace top onto the column of burden within the furnace, (ii) as the burden descends with the burden column, it is heated, chemically modified, and finally melted, (iii) further chemical modifications occur with the molten material, (iv) the molten products are extracted near the bottom, (v) melting of the burden material and extraction result in the descent of the burden column and the need for replenishment of the burden at the top, (vi) hot blast air is introduced through tuyeres near the bottom, (vii) BF gases are generated in front of the tuyeres and ascend through the burden and chemically modify the descending burden as well as themselves get chemically modified and cooled, (viii) BF gas (and dust) is extracted near the top of the furnace, and (ix) heat is extracted from the vessel in all directions (primarily through the lining cooling system) and along with the BF gas, liquid iron and liquid slag. Fig 9 shows cross section and types of BF.
Fig 9 Cross section and types of blast furnace
Furnace type – Furnaces are constructed to be mantle supported or free standing. Mantle supported BFs characteristically have a ring girder (mantle) located at the bottom of the lower stack of the furnace. The mantle is supported in turn by columns which are on the main furnace foundation. The hearth, tuyere breast, and bosh are also supported by the foundation. Furnaces with mantle support column tend to have restricted access and reduced flexibility for improvements in the mantle, bosh, and tuyere breast areas.
Since thermal expansion is a major consideration in furnace shell design, the mantle style of furnace provides an interesting design consideration. The mantle support columns are relatively cool. The mantle tends to maintain a constant height, throughout the furnace campaign with respect to the furnace foundation. Thermal expansion of the stack due to process heat is considered to be based at the ‘fixed’ mantle (i.e. the top of the furnace raises with respect to the mantle). The effective height of the bosh, tuyere breast, and hearth wall shells (supported on the furnace foundation) increases due to the thermal expansion of the shell caused by the process heat. The lower portion of the furnace lifts upwards towards the fixed mantle. Hence, the provision of an expansion joint of some type is needed at the bosh / mantle connection or somewhere appropriately located in the lower portion of the furnace.
Free standing furnaces have been developed to eliminate the column and permit the installation of major equipment and furnace cooling improvements. This furnace type has a thicker shell for structural support. Installation and maintenance of a reliable cooling and lining system is necessary in order to sustain the structural longevity of the shell.
Two variations of the free standing furnace have been used. One type provides for a separate structural support tower to carry the furnace off-gas system and charging / hoisting system load. The other type (while it does employ a separate support tower for shell replacement purposes during relines) uses the furnace proper to support the off-gas system and charging / hoisting system loads. Special consideration to the furnace shell design is to be made regardless of the furnace type. The furnace vessel is subjected to internal pressures from the blast and gas, burden, liquid iron and slag. Dead and live load during all operating, maintenance, and reline stages are to be considered as well.
Furnace zones – The major zones of the furnace proper are (i) top cone, (ii) throat, (iii) stack, (iv) mantle / belly, (v) bosh, (vi) tuyere breast, (vii) hearth walls, (viii) hearth bottom, (ix) foundation.
Top cone – The top cone or dome is the uppermost part of the furnace proper. It supports the furnace top charging equipment, and the BF top gas collection system. Stock rods (stockline recorders or gauges) are normally placed here to monitor the upper level of the burden in the furnace. These devices are the units which provide the permissive or indication signals to charge the next scheduled burden input to the furnace. Typically, they are weights lowered by special winches, or microwave units. Some furnaces incorporate radio-active isotope emitters and detectors mounted in the furnace throat to monitor the burden level. Infrared camera can be installed in the top cone to monitor the BF top gas temperature distribution as it escapes the furnace burden stockline.
The top cone is the coolest zone of the furnace proper but can be exposed to extremely high temperatures if burden ‘slips’ (rapid, uncontrolled burden descent after a period of unusual lack of descent). The newly charged burden falls through this zone and the BF top gas is carried away from this section.
Throat – Steel wear plates or armour are installed in this zone. Here, abrasion of the furnace lining from the charged burden is the prime cause of deterioration. Furnace operators work to maintain the upper level of the burden (the stockline) in this region. Movable armour can be installed in this area in order to deflect the burden falling from a large bell. With the installation of the BLT, wear of the stockline area can be greatly reduced. Some BF users select to eliminate the armour plates and use an abrasion resistant refractory lining instead.
Stack – The stack (sometimes called shaft or the ‘in-wall’) is the zone between the mantle or belly on a free standing furnace and the stockline area. Smooth, uniform lines (the process ‘working surface’) of the stack are essential for uniform and predictable burden descent, BF gas ascent, and stable process control throughout the furnace campaign. Process considerations dictate a larger diameter at the base of the stack than at the top. Typical stack angles are in the range of around 85 degrees from the horizontal.
Mantle / belly – The mantle or belly area provides the transition between the expanded stack and bosh sections. Maintenance of the effectiveness of the cooling / lining system is particularly important for the mantle type furnace in order to protect the mantle structure. Thermal protection is important for the free standing furnace type as well. However, the free standing design is less complicated and more accessible in this area.
Bosh – The bosh area lies between the tuyere breast and the mantle / belly of the furnace. The bosh diameter increases from bottom to top. The inclination of the bosh permits the efficient ascent of the process gases and has been found to be necessary in order to provide the needed zone service life (the process gases are extremely hot and internal chemical attack conditions are severe). Typical bosh angles are in the range of around 80 degrees from the horizontal. Boshes are of two basic types, namely (i) banded and (ii) sealed. They can be cooled by different techniques.
Banded boshes are found in older mantle supported furnaces (they cannot be applied to free standing furnaces). A number of steel bands are placed in incrementally increasing diameters (smallest at the bottom of the bosh and largest at the top) and are tied together with connecting strips. Gap between the bands permits the introduction of copper cooling plates. Ceramic brick lining is to be used as air infiltration results in oxidation of carbon based linings. Gas leakage through the banded bosh can be high. This type is not suitable for BFs with high blast pressure / high top pressure. Banded boshes provide adequate flexibility to eliminate the requirement for a shell expansion joint in the lower portion of the furnace.
Sealed boshes, using continuous steel shell plate instead of separate bands, are employed to permit the use of improved cooling / lining systems, higher furnace operating pressures, and the free standing furnace type. Sealed boshes retain valuable gases with the furnace, hence improving the metallurgical process. As well, the seal bosh, since it precludes air entry into the lining, supports the use of carbon based refractories.
Tuyere breast – Hot blast air is introduced to the furnace through tuyeres (water-cooled copper units) located within the tuyere breast. The number of tuyeres needed depends upon the size (production capacity) of the furnace. The tuyere breast diameter, tuyere spacing, and number of tuyeres are influenced by the expected raceway zone size in front of each tuyere.
Tuyere stocks (Fig 10) convey the hot blast air from the bustle pipe to the tuyeres. The tuyeres are supported by tuyere coolers (water-cooled copper units) which are in turn supported by steel tuyere cooler holders (either welded or bolted to the furnace shell). Special consideration are to be made in the tuyere breast shell and lining design in order to maintain effective sealing of the different components in order to prevent escape and loss of the furnace gases.
Fig 10 Tuyere stock assembly and blast furnace hearth
Hearth – The hearth (Fig 10) is the crucible of the furnace. Here, hot metal and liquid slag are collected and held until the furnace is tapped. The hearth wall is penetrated by tap holes (frequently called iron notches) for the removal of the collected hot metal and liquid slag. The number of tap holes is dependent upon the size of the furnace, hot metal, and liquid slag handling requirements, physical and capital constraints etc.
Several furnaces are equipped with a slag or cinder notch (normally one per furnace, although some furnaces can have two). The slag notch opening elevation is normally sufficiently higher than the iron notch elevation. In earlier days, when slag volumes were high, the slag was flushed from the slag notch periodically. This simplified the iron / slag separation process in the cast house. More commonly now, however, the slag notch is retained solely for initial furnace set-up procedures or for emergency use in case of iron notch or other furnace operating problems.
Hearth bottom – The hearth bottom supports the hearth walls and is flooded by the iron within the furnace. As the campaign progresses, the hearth bottom lining wears away to a fixed equilibrium point. The remaining refractory contains the process and with sufficient cooling or inherent insulation value protects the furnace pad and foundation.
The application of specific cooling techniques to individual furnace zones is dependent upon several factors such as campaign life expectancy, furnace operational philosophy, burden types, refractories, cost constraints, physical constraints, available cooling media, and preferences etc. Different cooling techniques can be provided for different zones to assist the lining to resist the specific zone deterioration factors. Normally, the provision of adequate cooling capacity is necessary in each of the applicable furnace zones if the lining system located there is to survive. Where the thermal, chemical, and to some extent the abrasive conditions of the process are extreme, sufficient cooling is to be provided to maintain the necessary uniform interior lines of the furnace and to protect the furnace shell.
Typically, the top cone and throat areas of the furnace are not cooled. The hearth bottom can be ‘actively’ cooled by under hearth cooling (air, water, or oil media) or ‘passively’ cooled by heat conduction though the hearth bottom lining to the hearth wall. The basic cooling options for the balance of the furnace are (i) no cooling (typically the upper portion of the stack is not cooled in several furnaces, (ii) shower or spray cooling, (iii) jacket or channel cooling, (iv) plate cooling, and (v) stave cooling.
Shower or spray cooling – Water is directed by sprays or by overflow troughs and descends in a film over the shell plate. Effective spray nozzle design, numbers and positioning are important for proper coverage and to minimize rebound. Proper deflector plate design is necessary to ensure efficient cooling water distribution and to minimize splashing. Shower cooling is frequently employed in the bosh and hearth wall areas. Spray cooling is normally applied for emergency or back-up cooling, primarily in the stack area. Exterior shell plate corrosion and organic fouling are common problems which can disrupt water flow or insulate the shell from the cooling effect of the surface applied cooling. Water treatment is an important consideration to retain effective cooling.
Jacket or channel cooling – Fabricated cooling chambers or indeed structural steel channels or angles are welded directly to the outside of the shell plate. Water flows at low velocity though the cooling elements in order to cool the shell and the lining. Jacket or channel cooling is frequently applied to the hearth walls, tuyere breast, and bosh areas. Scale build-up on the furnace shell and debris collection in the bottoms of the external cooling elements can compromise the cooling effectiveness. Hence periodic cleaning of the cooling elements is necessary.
The critical area of concern in the cooling schemes mentioned so far is the necessity for the shell plate to act as a cooling element. If extreme heat loads are acting upon the inside face of the shell, then there exist an extremely high thermal gradient across the shell. This effect results in high thermally induced shell stresses and eventual cracking. The cracks start from the inside of the furnace and propagate to the outside. The cracks remain invisible (other than a ‘hot spot’) until they fully penetrate the shell plate. Through cracking of the shell plate results in the leak of the BF gas, exposed shell carburization, and disruption of the cooling effect (particularly spray or shower cooling). Shell cracking into a sealed cooling jacket or channel is difficult to locate and can result in long furnace outage time for repair. Entry of water into the furnace (frequently when the furnace is off-line and internal furnace gas pressure cannot prevent entry of cooling water though shell cracks) can have detrimental effect upon the furnace lining. Water in the furnace can be potentially dangerous due to explosion risk (steam or hydrogen). Since shower and jacket cooling rely on the shell plate to conduct the process heat to the cooling media, the plate and stave cooling are configured to isolate the shell from process.
Plate and cigar cooling – Installation of cooling elements though the shell of the furnace (Fig 11) has been a major furnace design improvement resulting in effective cooling of the furnace lining and protection of the shell plate. Cooling is provided along the length of the cooling element penetration into the lining. The inserted elements provide positive mechanical support for the refractory lining. Typical cooling plate manufacture is cast high conductivity copper. Single or multiple passes of cooling water can be incorporated. Cooling boxes with larger vertical section have been produced from cast steel, iron, or copper.
Fig 11 Plate cooler and cigar cooler
Cigar type (cylindrical) coolers (Fig 11) of steel and /or copper have also been successfully used. The philosophy of dense plate cooling (i.e. vertical pitch of 350 mm to 400 mm centre-to- centre, and horizontal pitch of 600 mm (centre-to-centre) has improved the cooling effect and increased lining life.
Copper cooling plates have traditionally been anchored in the shell plate with retainer bars or bolted connections to permit ready replacement if plate leakage occurs. More recently, plates have been designed with steel sections at the rear of the plate for welding directly to the steel shell. While sometimes taking longer to replace, this type provides a positive seal against BF gas leakage. Plate coolers are typically installed in areas above where the liquid iron collects in the furnace. Hence the mid-point of the tuyere breast, right up to the underside of the throat armour is the range of application.
Stave cooling – Cast iron cooling elements (Shannon plates or staves) have been used for several years in the bosh and hearth wall areas. These castings have cored cooling passages of large cross-section. While their service life have been not remarkable in the bosh, multiple campaigns have been normal for the hearth wall. These staves frequently suffered from low flow rates of marginal quality cooling water (scaling and debris deposition / build-up) and sometimes casting porosity. Water leaks into the hearth wall can be a considerable problem.
In the 1950s, the then USSR developed a new type of stave cooler (Fig 12) and ‘natural evaporative stave cooling’. For this design, castings were of gray cast iron containing steel pipes for water passes. The pipes were coated prior to casting to prevent carburization of the cooling pipe and metallurgical contact with the stave body material. The staves were installed in horizontal rows with the furnace and the cooling pipes projected through the shell. Vertical column of staves were formed by the inter-connection of the projecting pipes from one stave up to the corresponding stave in the next row. Staves can be applied to all the walls in the zones below the armour. Staves in the hearth wall and tuyere breast are supplied with smooth faces. Staves in the bosh, mantle / belly and stack normally have rib recesses for the installation of refractory.
Evolution of the stave cooler design has been dramatic. Staves in the higher heat load areas are now typically cast from ductile iron for improved thermal conductivity and crack resistance. While early stave design used castable refractory (installed after stave installation within the furnace), ribs now normally incorporate refractory bricks, either cast in place (with the stave body at the foundry) or slid and mortared in place prior to installation in the furnace. Fig 12 shows stave cooler and generations of stave development.
Fig 12 Stave cooler and generations of stave development
Staves are normally expected to retain a refractory lining in front for some time. After loss (expected) of the lining the staves are designed to resist the abrasive effects of descending burden and ascending dirty gas. As well, they are to absorb the expected process heat load and resist thermal load cycling and shock. Four generations of staves (Fig 12) are normally recognized in the industry.
First generation staves are no longer normally used. These staves have four cooling body circuits (with long radius bends which do not effectively cool the stave comers. These staves are made of gray iron castings with castable rib refractory. Second generation staves have four cooling body circuits with short radius bends for improved comer cooling. These staves are made of ductile iron castings with cast-in or glued-in rib bricks. Third generation staves have two-layer body cooling incorporating four or six cooling body circuits (stave hot face) and one or two serpentine cold face circuits (stave cold face) for additional or back-up cooling in the event of hot face circuit loss. These staves have additional edge cooling (top and bottom). The more frequent use of these staves is as cooled ledges to support a refractory lining. These staves have cast-in or mortared-in rib bricks. Fourth generation staves have two-layer cooling (similar to third generation). These staves have cooled ledges and cast-in wall brick lining eliminating the need for a manually placed interior brick lining.
Staves incorporating hot face ledges are more effective in retaining a brick lining than the smoother rib faced bricks. However, once the brick lining disappears, the ledges are very exposed within the furnace. The ledges disrupt burden descent and gas ascent. Exposed ledges tend to fail quickly. They are frequently serviced by cooling water separate from the main stave cooling circuit(s). In this way leaking ledge circuits can be more easily located or isolated. Some stave manufacturers are now providing separate ledge castings so that ledge cracking and loss does not damage the parent staves. As well, there is some present change in philosophy to abandon the application of ledges entirely.
Variations of the basic stave generation types are common. For example, staves of fourth generation type utilizing a refractory castable for the wall ling have been employed successfully. Alternatively, brick linings have been anchored to the stave bodies. Such approaches can be used to substitute for brick support ledges.
A ‘fifth’ generation of staves design has been the developed. It is the copper stave (Fig 13). The development of copper staves was carried out both in Japan and Germany for use in the region of bosh, belly, and lower stack to cope with high heat loads and large fluctuations of temperatures. While Japan has gone for cast copper staves, German copper staves are rolled copper plates having close outer tolerarnces and with drilling done for cooling passages. Drilled and plugged copper staves are typically designed for four water pipes in a straight line at the top and four water pipes in a stright line at the bottom. Materials for internal pipe coils include monel, copper, or steel. Unlike cast iron staves, copper staves are intended to be bonded to the cooling pipe.
The development of the cast-in copper stave has considered the following aspects. As per the first aspect, for the prevention of deformation, appropriate design of the stave length and bolt constrained points is important. The first aspect is that the use of the cast-in steel pipe copper stave with its own design is beneficial for effectively reducing the risk of deformation. Fig 13 shows the constrained points of a rolled copper stave and the cast-in steel pipe copper stave. A rolled copper stave is constrained to the shell by mounting bolts and pins. To prevent the weld at the base of a rising pipe from being damaged by stresses, rising piping is connected to the shell by an expansion joint. Due to this structure, the upper and lower ends of the staves are freely displaced, causing the staves to be easily deformed. The large thermal load which is repeatedly applied to the copper stave in the course of the fluctuation in the BF operations etc., causes plastic strain to be gradually accumulated, and results in large deformation. There are cases in which the deformation at the upper end has reached 50 mm or more and a weld has been broken, under the condition of an overly long stave, an in-appropriate bolt position, or high heat load exceeding the design condition.
Fig 13 Constrained points of rolled and the cast-in pipe copper stave
Presently, the most popular type of copper stave is the rolled copper stave, the manufacturing process of which involves drilling holes on a copper plate. The water channel ends of these staves are plug-welded. The cast-in steel pipe copper stave, which has been developed, is made by casting bent steel pipes into the copper, a completely different manufacturing process from that of the conventional rolled copper stave. This unique manufacturing method has enabled achieving high energy efficiency and long life of BFs, which cannot be achieved using the rolled copper stave.
Natural evaporative stave cooling (NEVC) is a technique where boiler quality water is introduced into the bottom row of staves and flows by natural mean up the vertical cooling circuits. As the process heat conducts through the stave and cooling pipe into the water, the water in turn heats up. As the water warms, it expands. Since cooler water is being introduced below, the warm water tends to move upwards. At some point in the vertical cooling circuit, the water is at the boiling point. As the water changes its phase to steam, due to the latent heat of vapourization, additional heat is absorbed (driving the phase change). After boiling begins, two-phase flow (water and steam mixture) ascends the cooling pipes to the top of the furnace. Normally located on the furnace top platform are steam separator drums used to extract and vent the steam to atmosphere. Make-up water is introduced to the drum (to replace the discharged steam). The water is piped back by gravity to the furnace bottom and is fed once more to the staves. This cooling technique is very efficient and has low operating costs. There is no pumping equipment. The improvements in this system has been to boost the flow of the cooling water with recirculating pumps (forced evaporative cooling, FEVC) in order to ensure uniform cooling water flow and to cool the recirculating water (forced cold water cooling – FCWC). Both of these approaches have resulted in improved stave and lining life.
Staves provide an excellent protection for the shell plate throughout their service life (which is extended while the interior brick lining remains in place). Stave application has been implemented in all areas of the furnace from hearth wall up to and including the upper stack.
One drawback for conversion of an existing plate cooled furnace to stave cooling can be the cost of a new shell. However, if the existing shell is already in distress and is to be replaced in any event, the conversion cost is not a major factor.
The cast house (Fig 14) is the area or areas at the BF where equipment is placed to safely extract the hot metal and liquid slag from the furnace, separate them, and direct them to the appropriate handling equipment or facilities. The hot metal and liquid slag are removed from the furnace through the tap hole. Only infrequently today slag is flushed from the slag notch. The equipment for tap hole is to be reliable and need minimum maintenance. Furnaces typically tap eight times to eleven times per day.
Fig 14 Cast house with four tap holes and two tap holes
Mud gun – The mud gun is used to close the tap hole after tapping is complete. A quantity of the tap hole mass is pushed by the mud gun to fill the worn hole and to maintain a quantity of the tap hole mass (the mushroom) within the hearth. The mud gun is normally held in place on the tap hole until the tap hole mass cures and the tap hole is securely plugged. A hydraulic mud gun uses hydraulic power to swing, hold, and push the tap hole mass. Typical injection pressure of tap hole mass is of the order of 20 MPa to 25 MPa, permitting it to push viscous mass into the furnace operating at high pressures. The hydraulic gun is held against the furnace with the equivalent of 15 tons to 35 tons of force. This type of mud gun can be swung into place in one motion.
An electro-mechanical gun has three separate electric drives for unit swing, barrel positioning, and ramming. Hence several separate motions are needed for accurate positioning of the mud gun at the tap hole. Tap hole mass injection pressure is in the range of only 5 MPa to 8 MPa. The electro-mechanical mud gun is latched to the furnace to keep it in place during plugging.
Tap hole drill – Tap hole drill is used to bore a hole though the tap hole clay into the hearth of the furnace. A drill unit is swung into place hydraulically and held hydraulically in the working position. A pneumatic motor feeds the hammer drill unit (with an attached drill rod and bit) into the hole. Compressed air is fed down the centre of the drill rod and the drill bit to cool the bit and blowout the removed tap hole mass. When the tap hole drill rod has penetrated into the hearth, the drill rod is retracted and the drill swings clear of the hot metal stream.
Soaking bar technique – The application of the soaking bar practice has improved the tapping process. When the tap hole mass is still pliable after plugging, a steel bar is driven into the tap hole by the tap hole drill. While the bar sits in place during the time between casts, it heats up by conduction from the hearth hot metal. This permits curing of the tap hole mass along its entire length (as opposed to curing with the furnace and setting at the outside near the furnace cooling elements). The cured tap hole mass is more resistant to erosion during tapping, hence improving tap flow rate control. Less tap hole mass is needed to replug the hole. When the tap hole is to be opened, a clamping device and a back hammering device on the tap hole drill extract the rod. The timing for tap hole opening can be more easily controlled (predicted) than by conventional drilling. This feature is important for smooth furnace operation and for scheduling of hot metal delivery to downstream facilities.
Same side tap hole equipment – Mud gun and drills have normally been installed on opposite sides of the tap hole. Design development has permitted installation of these equipments on one side of the tap hole. The drill swings over the mud gun or vice versa. This type of installation facilitates improved access for tap hole and trough maintenance and the improved application of trough and tap hole area flue collection.
With the advent of tuyere access platforms to facilitate tuyere and tuyere stock inspection and replacement, the headroom available for the tap hole equipment has diminished. However, same side tap hole equipment installations can be achieved with low headroom (for example 2.2 metres).
Trough and runner system – Typical hot metal and slag tapping rates are in the range of 4 to 6 tons per minute and 3 to 5 tons per minute, respectively. The trough and runner systems are to be designed to properly separate the iron and slag and to convey them away from the furnace for flow rates within the normal flow rate range and for unusual peak flow rates.
The hot metal trough (Fig 15) is a refractory lined tundish located in the cast house floor and designed to collect iron and slag after discharge from the furnace. The hot metal flows down the trough, under a skimmer and over a dam into the hot metal runner system. The hot metal level in the trough is dictated by the dam. Proper dam design submerges the lowest portion of the skimmer in the hot metal pool. The slag, being lighter than the hot metal, floats down the trough on top of the hot metal pool. Since it cannot sink into the hot metal and through the skimmer opening, it pools on top of the hot metal until sufficient volume collects to overflow a slag dam and run down the slag runner. At the end of the tapping, the slag runner dam height is lowered to drain off most of the slag. The residual hot metal is retained in the trough to prevent oxidation and thermal shock of the trough refractory lining.
Fig 15 Hot metal trough and tilting runner
When maintenance of the trough lining is needed, the hot metal pool can be dumped by removing the hot metal dam, or by opening a trough drain gate, or by drilling into the side of the trough (at its lowest point) with a drain drill. The trough bottom is normally designed with a 2 % (minimum) slope for effective draining. Trough cross-section and length design are important for effective iron and slag flow pattern, retention, and separation. A good trough design results in hot metal yield improvements. Effective trough lining and cooling techniques are important for lining life, hot metal temperature, and cast house structural steel and concrete heat protection considerations. Troughs traditionally were contained in steel boxes ‘buried in sand’ in the cast house floor system. Improved trough design incorporates forced or natural air convection or water-cooling.
Cast house practice needs the runners to be as short as possible. This minimizes temperature loss of hot metal and reduces runner maintenance and flue generation. Shorter runners can also result in reduced capital expenditure for cast house building installation or modification. Since the runners are to slope away from the furnace, the cast house floor normally follows the same slope as the runners.
Slag runners are normally designed with a 7 % (minimum) slope. Slag can be directed to (i) slag pots for railway or mobile equipment haulage to a remote site for dumping, (ii) slag pits adjacent to the furnace for air cooling and water quenching prior to excavation by mobile equipment, and (iii) granulation facilities adjacent to the furnace for conversion of the liquid slag to granulated slag. Granulation units are provided with systems to eliminate flue emissions associated with environmental issues.
Hot metal runners are normally designed with a 3 % (minimum) slope. Hot metal is normally directed to hot metal transfer ladles (torpedo cars / open top ladles) for movement to the steel melting shop or pig casting machines. While normal practice used is to have one iron runner system with diverter gates directing the hot metal to different pouring positions, each with a ladle. Application of the tilting runner practice has been beneficial. A tilting runner is normally with an electrical motor-driven actuator (with a manual hand wheel back-up), and is tilted at around 5 degrees to divert the hot metal. A pool of hot metal is held in the tilting runner to minimize splashing and refractory wear. When one hot metal ladle has been filled, the runner is tilted to the opposite side to fill the other ladle. If needed, a locomotive removes the filled ladle and spots an empty ladle in its place. This operation can be done without plugging the furnace. When the tapping is finished, the tilting runner is tilted an additional 5 degrees to dump its pool of hot metal into the ladle.
Modern cast house design includes flat floors, where the runner is fully covered and is fitted flush with the floor. This allows safer and easier use of mobile vehicles in the cast house area. The use of radio controlled equipment and other devices have helped to reform cast house work, and these, along with effective emission control systems, have improved working conditions. As the BF hearth diameter is increased, there is a resulting need to increase the size of the cast-house. Large BF are normally designed with four tap holes (Fig 14). With a four top-hole configuration, the cast-house arrangement needs to provide sufficient space for movement around the floor itself. There is no design issues associated with this requirement as long as there is the necessary space provided in the site plan. Increasing the size of the cast-house in terms of floor plan does not represent a radical change in design philosophy which can pose a challenge the furnace designer. An efficient and strictly controlled tapping is necessary for ensuring a stable operation and high productivity of the BF.
Fume collection requirements and applications appear to vary considerably around the globe. BFs presently have full, partial, or even no cast house fume collection system. Exhaust fan and bag house capacity of the order of 9,000 cubic meter per minute (cum/min) to 11,500 cum/min (depending upon operation and design practices) is typical for full flue capture of a two tap hole cast house installation (for trough runners and tilting runners).
Proper design and application of flue collection runner covers can facilitate cast house access (i.e. flat floor configuration using steel slabs or plates) for personnel and mobile equipment crossover. Runner covers can also reduce hot metal temperature loss and improve runner refractory longevity.
Some furnaces use flame suppression which eliminates the oxygen in the air directly over the trough and iron runners. Products of combustion prevent oxidation of the hot metal surface reducing visible particulate and flues.
Other aspects of BF design
It is necessary that complete study of every element in the process chain, from raw materials delivery to hot metal consumption is made to ensure that there are no ‘bottle necks’ in the system which can prevent the furnace from meeting the goals of its installation. While designing the BF, thought process is to be used to develop the furnace design and some alternatives are to be considered before freezing the design. During the designing of the furnace, those furnace equipments are to be selected which best meet the needs of the furnace operation.
The furnace design is to ensure (i) the furnace is capable of meeting the operational goals of production, productivity (tons per day per cubic metre of working volume), specific consumptions (kilograms per ton of hot metal), and product quality in cost effective manner, (ii) the furnace has the flexibility to accept and absorb the changes in the quality of the raw materials, and (iii) the furnace is capable of achieving the desired campaign life both with respect to time and the total production.
Financial justification is the over-riding consideration for the design. For this purpose, the economic study is to be very extensive. Further, the furnace design is to include latest technological developments so that the furnace does not become technologically outdated during its entire campaign.
The working volume of the furnace is the internal volume of the furnace calculated between the tuyeres and the stock line. Hearth productivity of the furnace is rated in tons per day per cubic metre of active hearth volume. Active hearth volume is the internal volume of the furnace calculated between the tuyeres and the tap hole. Active hearth volume is a measure of the holding capacity of the furnace for the liquids produced in the working volume (above the tuyeres). Hence, the tons per day per cubic metre of active hearth volume is a measure of the specific capacity (through-put per unit volume) of the hearth of the BF.
The design of the hearth is very important since it has a strong effect on the furnace operation. The furnace operation gets affected since the hearth liquid levels change rapidly which cause variations in gas flow pattern, gas utilization, and blast pressure. Also, because of these rapid changes in liquid level, there can be jamming / burning of the tuyeres which affect the blowing of the furnace.
The furnace hearth volume also determines the controls the operator is to exercise during furnace operation. For a very good hearth liquid level control, the high through-put furnace need around 90 % time spent in tapping. To make this time of tapping possible, cast floor is to be designed properly.
The furnace lining and cooling system needs special attention so that it does not pose any problem during the entire campaign of the BF. The selection of refractories, cooling elements, and internal furnace geometry is very important in this respect. Copper staves in this respect are expensive but they are very economical in comparison to the alternatives. Carbon lining of the hearth is very important for the long life of the hearth. The refractory lining thickness of the stack has implication on the furnace working volume.
The production needed normally determines the size of the BF. However, for the sizing of the BF, the raw materials, the product chemistry, and even operating philosophy are important. While the furnace size has implication on the capital cost, the productivity improvement has implication on the operating cost. The specific productivity of the furnace is to be determined for the determination of the size of the BF needed to produce the required quantity of hot metal. From the wide range of possible operating rates, the working volume of the furnace is to be calculated. Productivity and hence the furnace size is also to be based on the fuel rate. The fuel rate is dependent on the quality of raw materials, hot blast parameters, hot metal quality, and the operating philosophy.
The size is the most important factor for the determination of the BF productivity. However, there are other factors which also influence the BF productivity. The most important of these factors include (i) hot blast temperature and pressure, (ii) high top pressure, (iii) oxygen enrichment of the air blast, (iv) injection of auxiliary fuel at the tuyere, (v) prepared burden (sinter, pellets etc.), (vi) Fe content of the ferrous burden, (vii) ash in coke, (viii) quality of the coke, (ix) moisture content of the burden, (x) direct charging of fluxes (lime stone, dolomite etc.) in the BF, (xi) content of fines in the burden, (xii) quality of hot metal to be produced, (xiii) burden distribution control in the furnace, and (xiv) level of automation and control in the furnace.
The availability of furnace equipment, provision of stand-by equipment, and the maintenance philosophy are important factors which have high influence on the annual production from the BF. Further, incorporation of safe and healthy working practices during the operation of BF in the design of the BF is important which has a high influence on the furnace productions. In this regards, safety interlocks are to be provided at all the places where there exist a chance of unsafe operating practices to take place.