Causes and Recovery of Chilled Blast Furnace
Causes and Recovery of Chilled Blast Furnace
A chilled blast furnace (BF) in an integrated steel plant is the cause of serious disruption in the production. It can take sometimes several days or even weeks to bring the BF back to normal production. The chilling of a BF hearth occurs when the temperature of the hot metal (HM) which is tapped from the furnace is very low. Due to the low temperature of the HM, both the HM and the liquid slag become very viscous. This causes difficulties with the drainage of the HM from the furnace. If the temperature is low enough, the HM can also begin to solidify.
The chilling of a BF is a very serious phenomenon which happens due to the abnormal operation of the BF. The revival of a chilled BF is a big and herculean job and it takes a very long time and needs a lot of patience to bring back a chilled BF to normal working condition. In case of the chilling of the hearth, there is no possibility of the tapping of the liquid slag and the HM. This happens since the temperature in lower part of the BF becomes very low and the connection between the tap hole and tuyere is lost. The BF chilling takes place due to the solidification of the liquid slag which obstructs the liquids flow to the hearth. If a chill is very severe, the remaining HM in the hearth can also start solidifying as well.
The chilling of the BF can be of varying degree ranging from minor to very severe. The chilling is considered to be minor when the connection between the tuyere and tap hole is lost but there is sufficient heat in the hearth. In case of severe chilling, the HM also starts solidifying. In case of the most severe chill the HM in the hearth gets totally solidified.
Since the chilling is a very serious type of the outage of the BF and its potential consequences are very large, it is very essential that there is a standard procedure for the recovery of the furnace from chilling in every plant and the BF operators are well trained in the recovery process. Fig 1 gives a view of severe chill as visible from the tap hole.
Fig 1 Severe chill as seen from a tap hole
Causes of the chilling in a BF
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 many 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 be 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 generally 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 HM 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 HM and slag in the furnace due to either insufficient draining of the HM 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 many but it is advisable to run the furnace on the hotter side by increasing the coke in the charge.
The solidification of liquid slag can start when the conditions inside the BF favours it. The main conditions are temperature (1350 deg C -1390 deg C) and slag composition (ratio of CaO + MgO, SiO2, and Al2O3). In addition, FeO (unreduced iron oxide) also plays an important role. Factors affecting the viscosity of liquid slag are important since the viscous slag is difficult to drain from the furnace. It is also to be remembered that in a cold BF, the slag is having composition which has a high viscosity and a high melting temperature. The melting temperature is higher than that of iron. Hence when the BF starts showing symptoms of chilling then it is very necessary to drain the BF from liquid slag as much as feasible, though it a difficult thing to do at this condition of the BF. A tuyere of BF filled up with slag is shown in Fig 2.
Fig 2 A tuyere of BF filled up with slag
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.
Actions needed to avoid severe chilling of the BF
The normal practice which is to be followed by a BF operator is that when the furnace start showing the signs of near chill situation, he is to continue operations at reduced wind volume, while trying to improve the excavation of liquid slag and HM by drilling and/or oxygen (O2) lancing. If draining is not successful then HM and liquid slag continues to get build up inside the furnace. It is very likely that the BF has suffered with hanging and slipping during this period. At this stage when the BF condition has seriously deteriorated, the charging of the additional fuel though desirable but may not be of much help. In case situation is not improving, then it becomes necessary to stop the BF for safety reasons since postponement of such decision for long can results into liquid levels inside the furnace continue to increase leading to burning of the tuyeres, coolers and blow pipes. Such a situation leads to the stoppage of the BF and makes the BF very unsafe.
The condition of the BF in such a situation is indicated by (i) the opening of the tap hole is not possible either by drilling or lancing, (ii) tuyeres and blow pipes are filled with slag, (iii) some tuyeres are having solidified material in front of them, (iv) the permeability of the BF has deteriorated and the layered structure of the stack is disturbed due to the frequent slips, (v) slag/ metal has accumulated upto half way of the bosh or even higher, (vi) remedial actions taken of increasing the fuel in the charge gets disturbed due to process upset, and (vii) additional water leakage takes place due to the process upset.
In the present day practice of running the BF with fuel injection at the tuyere, it is more difficult to restart the furnace even after a few days, if the furnace has been stopped with unprepared burden. For restarting the furnace after a few days of unplanned stoppage, it is necessary that enough coke is available in front of the tuyeres.
It is always desirable that remedial actions for the recovery are to start as soon as the symptoms of the furnace chilling are visible. It is advisable not to experiment at this stage and take the help of an expert. However in the extreme case when it becomes very difficult to recover then the BF needs to be raked out. Such a situation can arise when the BF contains excessive amount of solidified slag, there is a lot of water ingress, and there remains very little coke in the deadman zone.
For making a good recovery plan it is essential to know the status of the BF at the start of the recovery. The status of the BF can be known by the following several steps as (i) estimation of the burden composition along with the required coke rate, (ii) estimation of the slag and the HM levels inside the furnace, (iii) knowledge of the composition of the burden materials and slag as well as the burden ratio, (iv) determination of whether the water leakages in the furnace are continuing or have stopped, (v) identifying of the tap hole which is better for the recovery to start, (vi) determination of the coke rate and the basicity of the burden to be charged, (vii) cleaning of all the tuyeres and the blow pipes and to plug them, and (viii) determination of the number of tuyeres with which the BF is going to restart.
In case where the hearth has been drained out empty as in the case of a heath break out, then the recovery of the BF is relatively easier as compared to the case when the BF is full of slag and water. A BF with process related problems is more difficult to revive.
The process of recovery
For the recovery of a BF which has chilled, it is necessary to re-establish connections between the tap hole and the tuyeres. Maximum heat is to be brought into the furnace to remelt the solidified slag and the metal. It is also necessary to drain the melted materials on a continuous basis. These steps are to be built into the plan. It is essential that the things are not hurried up and the next step is to taken only after assessing the success of the previous step. Here the experience and the judgment of the BF operator are very important.
For the recovery process to be successful, it is necessary that, there is enough coke in the centre of the hearth (deadman) and there is natural draft of air is established throughout the deadman zone. Also, the BF is to be free of the water leakage before the recovery is attempted. Any water entering the BF during the initial stage adversely affects the recovery and can even cause the recovery impossible.
The tap hole-tuyere connection can be re-established by hand lancing or using thermal lances. This is the traditional method. In this method, the working environment is hot, very harsh and full of hazards. The amount of heat, which can be introduced by this method, is small and is generally enough to remelt slag in a narrow passage from tuyeres to the tap hole. But since no air is blown in the BF, the furnace continues to cool down.
Further the BF cannot be started without the connection between tap hole and the tuyere. Once the connection between the tap hole and tuyere is made, the furnace is to be started with air blown in one or two tuyeres only. It is to be done very carefully and with ample care to avoid re-blocking the open connection. After stabilizing blowing in these one or two tuyeres adjacent tuyeres are opened if the operating conditions are favourable. For this a continuous monitoring is needed for the open tuyeres by the visual observations through the peep holes.
It is important that during the recovery process, tuyeres do not get self opened. This can start production of liquids at these tuyeres which can result into burning of the tuyeres, blow pipes and coolers. This also causes unnecessary delays during first critical operation of the recovery process. The tuyeres are to be opened only as per plan and hence proper plug-in of the tuyeres after cleaning is very important. A plugged tuyere is shown in Fig 3.
Fig 3 A plugged tuyere
Use of oxy-fuel lances
Traditional recovery methods try to establish a connection between the tuyeres and the tap hole by hand lancing or burning with thermal lances. This is generally done by very experienced operators. This method involves very hard and hot work, which are not only hazardous but can also do a lot of potential damage to the refractories in that area. It is even more difficult to establish such connection by hand lancing in the modern large capacity furnace with large hearth volume and bigger distance between the tap hole and the tuyeres.
The furnace recovery is very slow by this traditional method. However now a days for faster recovery oxy-fuel technology is being used. In this technology the connection between tap hole and tuyere is achieved by introducing a large amount of heat below the tuyeres by burning O2 and a fuel gas which are introduced in the furnace through lances placed in the holes which are drilled 3.5 metres to 4.5 metres inside the BF hearth. Most of the heat is generated by burning of the coke of deadman zone with the O2. Heating by using oxy-fuel lances is shown in Fig 4.
Fig 4 Heating with oxy fuel lances and a few tuyeres