Blast Furnace Irregularities during Operation
Blast Furnace Irregularities during Operation
Smooth and uniform movement of burden materials downward and movement of furnace gases in the upward direction is very important for a stable and efficient operation of the blast furnace (BF). For ensuring this, a lot of work has been carried out in the recent past. This includes (i) improvement in the characteristics of the burden materials, (ii) improvement in the furnace charging system, (iii) improvement in the BF cooling system, (iv) adequate automation and control of the BF operation to eliminate human errors, and (v) improvement in the furnace operating procedures. In spite these improvements, BF does not always run as smoothly as the casual observer can be led to believe and irregularities during the operation do occur. However, the furnace irregularities are not as frequent as they were in former years, but still the upsets in the BF operations are there which cause considerable concern and frequently need quick thinking and the use of good judgement and skill as well as timely corrective actions on the part of the operator to prevent serious trouble.
The main reasons for the BF operational irregularities are (i) faulty mechanical devices such as leaking cooler, and defective valves etc., (ii) faulty operations such as faulty charging, and delays in tapping etc., and (iii) abnormal physical-chemical changes which are taking place inside the BF. The major BF operational irregularities are described below.
Furnace hanging and slipping
Hanging is the phenomenon in the BF which takes place, when the burden materials charged at the top of the BF do not move continuously towards the hearth of the furnace. Hanging originates when the burden, on its way down, meets a very high resistance resulting into the stoppage of the movement of the burden. Hanging takes place due to the bridging of the burden materials in the stack of the furnace. When it occurs, the material below the hang continues to move downward, forming a space which is void of solid material but filled with hot gas at very high pressure. This space continues to grow until the hang finally collapses.
When the hanging collapses then the material fall down because of the gap which has been created below the hanging. The collapsing of the hang is a phenomenon called ‘slipping’ which results into irregular working of the BF resulting into a non-uniform gas distribution with its implications on the BF parameters. During the furnace slipping, the charged materials fall uncontrollably toward the hearth of the furnace in a thermally unprepared state which leads to the furnace getting cold. It also forces the hot gases upward with the a very high force. In severe cases, the sudden downward thrust of the hanging material forces the hot gas upward with the force of an explosion. This sudden rush of gas opens the top gas bleeders and sometimes is so great that it causes severe damage to the furnace top equipment’s.
The hanging which precedes slipping is caused by any of a number of different conditions in which the permeability of the charge is decreased since some of the material closes up the voids between the charged particles and bonds them loosely together. When there is a high percentage of fines in the burden and the velocity of the furnace gas is relatively high, the fines close the openings between the other particles and cause hanging. In some instances, slag which has been melted is blown upward in droplets and when it subsequently contacts colder burden material it resolidifies and closes up the openings between the particles and tends to cement them together.
In some cases, the carbon monoxide (CO) decomposition reaction 2CO = CO2 + C is catalyzed, and the carbon (C) is deposited as soot. This closes the openings between the particles and holds the particles together. In some other cases, where the alkali content of the burden is high, the alkali compounds get reduced to alkali vapour which ascends with the furnace gas and condenses in the cooler portion of the charge to cause the same type of hanging condition.
Another type of hanging sometimes occurs in the BFs which are being run very efficiently and are being pushed to their best production rate. Under these conditions, if there is a slightly unfavourable change in the gas distribution, the strength of the coke, or the particle size of the burden, then the iron oxide of the ferrous burden does not reduce to metallic iron rapidly enough, resulting into iron oxide to melt and run down as a liquid onto the coke particles. When this occurs, the liquid iron oxide gets reduced to solid iron and considerable heat is consumed by the reduction. Hence, the coke particles are cemented together and the permeability of the moving mass in the BF is considerably decreased resulting into the hanging of the furnace.
A similar type of hanging can also occur if the BF is being operated at too high a flame temperature for the quality (particularly the reducibility) of the burden material. When the high temperature isotherms expand far enough up the furnace they can begin to melt unreduced material, and when that material descends into a more reducing environment it reduces and, depending on the temperature, it can solidify (the melting point of FeO is around 1,370 deg C and pure iron is around 1,535 deg C), and close the burden.
When the burden is not moving properly through the furnace and there is sluggish movement of the material through the furnace, the operator is required to take corrective measures immediately to avoid a major slip which can be a very disastrous event. Under very extreme condition a slip can lead to a chilled furnace. Every hanging and slipping is to be properly analyzed to determine the causes of the hanging so that changes can be made in the operating procedures to prevent the hanging from recurring.
There are normally two types of hangings which are normally taking place in a BF. These are (i) top hanging which occur in the top of the stack and mainly take place because of the carbon deposition reaction and alkali vapour condensation, and (ii) bottom hanging which occurs in the lower stack, belly, and bosh areas and take place because of the voids getting generated in the stack.
The remedial actions for removal of hanging in the BF are (i) use of large lump lime stone, calcining of which in the BF produces CO2 (carbon di-oxide) which forces the solution loss reaction to take place and improves the permeability of the bed, and (ii) reduction in the blast temperature and pressure so as to improve the distribution and flow of gases in the furnace. In case of prolonged heavy hanging, the pressure of the hot blast is brought down drastically for a few moments. The shock created due to this sudden reduction in the pressure of hot blast makes the furnace slip. This slip is normally heavy and hence, this remedial action is to be carried out only after tapping the furnace when the furnace hearth has minimum of liquid metal and slag. In an extreme case, a persistent hanging can be cured by blowing down the furnace to bosh level and filling it with coke blank.
The term scaffolding is used when accretions or scabs build up on the furnace walls and cause a decrease in the cross sectional area of the stack of the BF. Scaffolds are normally made up of a solid shell on the inner side of the BF and a layer of loose burden material between this shell and the wall of BF. Scaffolding can occur relatively at the higher level of the stack of the blast furnace or relatively low in the stack, near the top of the bosh. It is difficult to generalize the types of scaffolds since there is very little in common between the structure and location of scaffolds from different BFs. However, scaffolds can be generally arranged in two groups. These groups are (i) laminated scaffolds, and (ii) non laminated scaffolds. Scaffolds with laminated structure consist of alternate layer of metallic iron (Fe) and burden rich in alkalis. Scaffolds can cause hanging in the BF. Typical formation of a large scaffold in a BF is shown in Fig 1.
Fig 1 Typical formation of a large scaffold in a BF
The scaffold formation near the top of the bosh frequently results because of excessive fines in the burden material and a higher than normal lime chemical composition of the slag (reflected by the higher basicity of the slag). The solution of lime into the slags formed in the furnace stack increases the melting point of the slag. Since the slag frequently carries some of the fines particles from the burden in suspension, the increase in the melting point can cause this mixture of fines and slag to adhere to the upper bosh walls. This build up in the upper bosh wall deflects the hot furnaces gases farther to the centre of the furnace. With lesser volume of hot gases along the walls, the accretions tend to cool down and solidify completely. These scabs then can grow until they block a large percentage of the cross sectional area of the BF.
Prerequisites for stable and harmful scaffold formation are (i) presence of suitable material in the BF burden to build the scaffold (e.g. fines, poorly screened burden, sinter with inferior low temperature reduction degradation characteristics, use of long time stored, wet and cold sinter, or small size coke etc.), (ii) presence of agglomerating (cementing) material for the agglomeration of the burden material, (iii) presence of a fixing (anchoring) mechanism to build the scaffold on the shaft wall of the BF which can be a chemical bond with the lining material, physical anchoring around the cooling plates, arch building towards the bosh walls, or simply condensation of the agglomerating material on the wall, (iv) continuous supply taking place of the adhering components, and (v) formed scaffold is strong enough to withstand the wearing forces of the descending materials.
The place where the scaffold is located depends on the agglomerating material, adhering material, burden materials, furnace operation, and furnace constructional features such as cooling elements and lining material. It can be located at various levels in the BF such as the shaft, the bosh, or the belly.
Alkali or zinc compounds are reduced to metallic vapours near the bottom of the BF. These vapours rises with the furnace gases to the cooler top portion where they are reoxidized to very fine solid particles. These fine particles adhere to the furnace wall along with other fine materials entrapped in it. This is also the another cause of starting of the formation of a scaffold.
The blockage due to the scaffolding reduces the area available for smelting of the iron bearing materials. Scaffolds distorts the gas flow inside the furnace and increases the fuel rate while promoting hanging and slipping of the furnace. It also decreases the furnace productivity. Due to higher fuel rate, lower furnace fuel efficiency results. When the scaffolds dislodge from the walls, it descend into the hearth. This causes serious furnace upsets and reduces the quality of hot metal. In case the size of scab is too big, it can cause chilling of the BF.
The phenomenon of channeling happens when the ascending gases in the furnace does not properly get uniformly distributed both radially and circumferentially in the furnace and find a passasge of least resistance. The different causes for channeling to occur in the blast furnace are charging of excessive fines, improper distribution of the burden material inside the furnace and high level of liquid iron and liquid slag in the hearth. Channeling upsets the heating and reduction processes which in turn affects the quality of the hot metal.
The indications of the channeling are (i) BF accepts blast without increase in the pressure drop, (ii) Temperature of top gas leaving the BF is high, (iii) CO / CO2 ratio is high, (iv) top gas has high content of flue dust, and (v) there is an increase in the coke rate.
In case of fines charging, the channeling leads to the increase of the heat load at the walls of the BF which results in an unstable BF operation and reduction in the production. Due to the fines, the ascending gasses gets diverted from the area and channel around the fines. This diversion of the ascending gases upset the preheat of the materials and the reduction process. It causes unscheduled bleeder opening, off chemistry of the hot metal, unstable production of the BF and reduction in the furnace productivity. If the channeling can be predicted effectively, then the BF heat load can be reduced by improving the quality of the raw materials or by adjusting the BF operation.
The important aspects in case of channeling in a BF are (i) BF charge has non-uniformity both with respect to size and the charge distribution, (ii) critical gas velocity can be exceeded locally, (iii) lighter particles (coke) are blown out of those regions and deposited in regions of low velocity and the heavier ores settle down preferentially (ore shift), (iv) the phenomena taking place at (iii) contribute to the compactness of less permeable region and make the radial pressure drop more uneven, (v) gas in the furnace then flows through a system of distinct channel which is known as channeling, and (vi) restoring of blast rate to previous value is not a solution because of ‘hysteresis effect’.
Precaution needed for the control of channeling include (i) use of burden materials having higher strength, narrow size distribution, and optimum size, and (ii) keeping of the top pressure at a high level.
A ‘breakout’ is the term used to denote the conditions and results of the escape of gas and coke, or slag, or iron, from the bosh, tuyere breast, or hearth of a BF. Breakouts can occur at any point below the fusion zone in the furnace, but the most of the severe breakouts are of liquid slag and of liquid iron. Liquid iron breakout takes place at a level below the surface of iron lying in the hearth, and are either through the hearth walls and cooling stave or into the hearth bottom and out under the hearth cooling stave. BF breakout ia an infrequent and insidious hazard in the operation of BF. Breakout can take place at the bosh level, at the tuyre stock (breast cooler, blow pipe, or eyesight), or at the hearth.
Slag break outs are normally not as serious as iron break outs, because there are not as much danger from explosions as in the case when liquid iron and water come into contact. With either type of breakout, it is necessary, if at all possible, to open the tap hole and drain out as much liquid material as possible, and to take the furnace off blast.
In case of a slag breakout, the breakout can be chilled by stream of water, and the hole where the breakout occurred can be closed by replacing the refractory bricks, or pumping fireclay grout in the opening or ramming a plastic cement or putting asbetos rope into it.
In case of iron breakout, there is practically no control. The hot metal is to run out of the hole until the furnace is dry. After the accumulated iron has been cleared away, a suitable refractory can be used for closing the hole. If the iron break out is severe then a complete hearth repair is normally needed. In case of non severe break out it is frequently necessary to change the damaged hearth cooling staves.
Breakouts are caused by failures of the walls of the hearth, with the result that liquid iron or liquid slag or both can flow in an uncontrolled way out of the furnace and surrounding auxiliaries. The danger of hearth breakouts has been reduced remarkably in recent times since hearth has received much attention, and heavier, stronger, and more expensive construction of the hearth has been developed.
Slag breakouts occur from the top of the hearth cooling stave and upto the level of the tuyeres. They are seldom dangerous but can cause some damage to the brick lining and are a considerable nuisance because of the resulting delay for repair and the time needed for clean up the mess caused by them.
Blast, gas, and coke breakouts, normally known as bosh breakouts are almost a thing of the past. Their elimination can be attributed to the improvements in the operational practice, smoother work on increasingly inferior ores, less violent ‘slips’, and to the strengthening of the bosh in general. With the present control over furnace operation, the bosh does not fail except at infrequent intervals.
The causes of the bosh breakouts are (i) by conditions inside the furnace, such as high pressure of blast, very heavy slips, or severe working on the hearth walls, all of which can lead to breakout, (ii) breaking of the hearth bands, ejection of the cooling plates, or parts of brickwork between the band and the plate, or (iii) cracking and opening of the bosh cooling staves.
Safe methods of practice are of little avail in preventing tuyere breast, or bosh breakouts if by faulty design or construction, weakly built, insufficiently reinforced, or improperly cooled segments of brickwork have been incorporated in this part of the BF. Modifications methods of practice is of little help because of suddenness with which such a breakout occur. Possibly 95 % of prevention lies in construction and 5 % in experience, resource fullness, and arrangement of the cast house for the accessibility of signals and possibilities of escape.
During the past few years serious breakouts have occurred more frequently at the hearth than at the bosh and the tuyere breast. In fact this has always been so, but with small amount of hotmetal in the hearth, the breakouts were not necessarily serious, specially as the blast pressure has not being high. With increasing tonnage and fast driving, the breakouts have assumed serious proportions, sometimes wrecking the furnace, occasionally costing lives, and almost always causing bad messes, delays, and inconvenience.
The destructive agencies within the hearth walls responsible of the hearth breakouts are (i) erosion of the hearth walls by the hot air blast especially over the tap hole, (ii) disintegration of brickwork by the chemical action of the liquid iron and liquid slag, and (iii) mechanical action of liquid iron in penetrating the joints of the brickwork. Fig 2 shows typical wear mechanism of BF hearth.
Fig 2 Typical wear mechanism of BF hearth
This is a very serious disorder since it affects tapping adversely. It can result because of low fuel input, excessive moisture in the blast, and water leaking from tuyeres etc. If it is due to these reasons chilling is gradual and can be rectified before it becomes serious. Heavy slip also can cause chilling of the BF. The chilling due to slip is sudden.
Common reasons for chilling of BF are normally the long unprepared stops. Even prepared stops can also result into chilled hearth conditions during the restart. The chilling of the BF can take place due to several reasons, which include operating irregularities, improper charging of the burden, major equipment break downs, serious water leakage, and many more.
The BF can get cold when insufficient coke or other fuels are present at the tuyeres for sustaining the normal reduction and melting process. If the BF shows the symptoms of chilling, the BF operator is confronted with a difficult choice. If he continues to blow wind, liquids continue to get produced which cannot be drained. High level of liquids in the hearth can result into burning of the tuyeres and the blow pipes. On the other hand, if the blowing is stopped, slag enters the tuyeres and blow pipes and gets solidified resulting into significant damage to those parts. Further, time is needed to repair this damage, which causes the furnace to cool down further and making the recovery even more difficult.
During the regular operation, BF normally provides warning signals before the furnace shows the symptoms of chilling. The warning signals normally consist of (i) reduction in wind volume and slow burden movement due to the furnace running cold, (ii) frequent hanging and slipping in the furnace, (iii) temperature of the tapped hot metal and liquid slag is lower than the normal temperature, (iv) tapped liquid slag is viscous and not freely moving in the slag runner, (v) water coming out from the tap hole, (vi) blocking of tuyeres and blow pipes with slag or slag-metal mixture, (vii) excessive build-up of hot metal and slag in the furnace due to either insufficient draining of the hot metal and slag during tapping and / or delay in opening of the tap hole, and (viii) very less coke in the dead man area. When the furnace starts giving warning signals, it is necessary to take remedial actions to avoid approaching of the BF towards a chilling. The corrective actions are several but it is advisable to run the furnace on the hotter side by increasing the coke in the charge.
In short, the reasons for the chilling of the BF can be (i) excessive water leakage, (ii) out of specification furnace burden materials (raw materials), (iii) vast fluctuations in the quality of the burden materials, (iv) instruments and measuring devices are either not calibrated properly or malfunctioning, (v) BF operator is not able to read properly the happening inside the furnace from the data available to him, (vi) BF operator is either not reacting or reacting late to the problems being noticed during the operation, (vii) early warning signals are ignored and not reported to higher ups, (viii) violations of the technological discipline with respect to inspection of water leakage, cast house practice, and blanking of tuyeres etc., (viii) lack of experience of operating personnel, (ix) unprepared furnace stoppages due to sudden break downs of key equipments such as charging system, hot blast system ,and gas collecting and cleaning system requiring major repair and long time for repair, and (x) tap hole or hearth break out needing a long time for recovery.
If the blast is unable to penetrate right upto the centre of the furnace it can lead to the formation of a cold central column of the stock with an annular hot zone around it. This is known as pillaring. A bar inserted right through a tuyere hole shows red hot portion at both ends and a cold middle portion if pillaring exists in the furnace. The extent of cold middle portion of this rod indicates the extent of pillar existing in the furnace. Pillaring can be eliminated by increasing the blast pressure which can penetrate more and heat up the pillar.
Choking of gas uptakes
BF operation has to be suspended if dust gets accumulated in the uptakes and down comer and it can be resumed only after the clean-up. This happens because of faulty gas uptake design, particularly the inadequate cross section and improper joints.
Flooding and coke ejection through tap holes
In bosh, liquid metal and slag trickle through the permeable coke bed against the upward thrust of the ascending gases. An increase in gas or liquid flow can prevent the liquid metal and slag from flowing downwards, causing it to accumulate in the coke interstices until weight of the liquid overcomes the upward thrust of the gases and descents suddenly into the hearth. This phenomenon is known as flooding which can be minimized by having a high voidage i.e. by using higher mean size of coke. Better quality coke is also beneficial since the degradation inside the furnace is reduced and consequently the permeability in the bosh region is improved.
Anything which imposes a constraint on the tuyere raceway volume causes hold up and subsequent tendency to flooding. The interruption in uniform blowing rate causes the raceway to collapse and when it is resumed again the small particles of coke cannot re-enter the raceway and consequently descend in to the hearth instead of burning in the tuyere area, resulting in a choke hearth thereby causing the well-known phenomenon of coke ejection from the slag and the iron notches during tapping. This frequently leads to the unjustified criticism of coke quality. Uniform blowing of the furnace is the best remedy to avoid this.
Leaking tuyeres, tap holes and coolers
In spite of proper designs, the water cooled parts of the furnace can give way and these are to be immediately replaced or rectified, as far as possible. The monkey is the troublesome part and often needs frequent replacements. If it is not possible to rectify or replace the faulty cooler it has got to be cut off from the water mains and put out of use.
Leaking tuyeres or coolers in the lower part of the furnace can have disastrous effects if these are not rectified in time. The leaking tap-hole coolers lead to generation of steam which on coming in contact with the carbon hearth, erodes the hearth lining, and the campaign has to be stopped for capital repairs. The maintenance instructions for tap holes are to be scrupulously followed to minimize these troubles.