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Introduction
A well-maintained tap-hole channel is important in order to achieve good and stable furnace operation over time. Metal and slag are regularly tapped from the furnace and may cause a lot of wear on the tapping channel, increasing the risk of lining damage and ultimately leading to a burn-through. The tap-hole area is the part of the lining that has been by far the most prone to burn-through since the startup of the furnace.
Experience shows that almost all lining problems have started with burn-through in the tap-hole area or in close proximity to the tap-hole. During tap-hole repairs, side channels with metal and slag have also been observed emanating from the main tapping channel. In addition, a shorter tapping channel will cause more violent flow. This can lead to more metal droplets being entrained in the slag, and increase the risk of the furnace not being closed properly at the end of the tap.
As a burn-through is a considerable safety risk, and also causes significant production losses, systematic work has been performed over the years to maintain the tapping channel in order to prevent burn-through.
Mechanisms of tap-hole wear
The erosion and wear of the furnace lining and tapping channel is a complex phenomenon with many factors contributing. Some of the factors thought to have an impact in the case of furnace are summarized in this section:
During SiMn production the stable carbon phase will depend on the amount of silicon (Si) in the produced alloy. At lower levels of Si, below about 18 wt.%, graphite (C) will be the stable phase, while above this level silicon carbide (SiC) will be the stable carbon phase when assuming equilibrium. A SiMn alloy with a Si content above the coexistence point for C plus SiC can potentially dissolve carbon and precipitate SiC if it is in contact with carbon. This is caused by SiC precipitation removing carbon from the solution and moving the metal composition away from graphite saturation. There is little change in the point of coexistence for C and SiC with changes in temperature in the range 1550-1600°C. Cooling the area around the tapping channel will therefore have little effect on the chemical wear caused by the mechanism described above.
In the case of oxygen lancing being used to open the furnace, this can cause significant wear on the tapping channel. Introducing pure oxygen through a steel pipe will result in a very high temperature, and can cause both thermal and chemical wear on the carbon in the tapping channel . At Eramet Norway Porsgrunn, oxygen lancing is used only when the drill gets stuck or the material blocking the tapping channel is too hard to drill through. This happens, on average, in 5 to 15% of the taps. From experience, the main reasons for having to lance the furnace open are when there is a bad fit between the mud gun nozzle and the area surrounding the tap-hole, or when the furnace is not completely drained and is closed while there still is a flow of metal. This causes metal to solidify in the tapping channel, and prevents the drill from penetrating all the way through.
Experience with furnace operation also shows that thermomechanical stresses can affect the lifetime of the lining in the tapping area. The large temperature changes in this area can cause thermal expansion and contraction in the material. Over time this can lead to gaps between the bricks around the tapping block, which can be infiltrated by slag and metal. Spalling might also occur if there is slag or metal infiltration in the refractories. These effects can in the long term lead to a burn-through. The implementation of water-cooled Cu blocks in this area has been shown to reduce this problem significantly as it reduces the variation in temperature and helps to solidify stray paths of metal and slag that infiltrate the lining.
Tap-hole repair and maintenance at furnace
There have been a lot of challenges with the lining and burn-through in the tap-hole area of furnace 10 over the years. The normal procedure for repairs has been to cool the area, removed as much of the damaged material as possible, and build up a new front, which is then filled by grouting. This can in some instances give good results, but the repaired area will never be as wear-resistant as the original material.
Through the years, several measures have been taken to ensure as long as a lifetime as possible of the lining in the tapping area. Two of the most significant changes through the history of the furnace have been:
> Introduction of weekly repairs and maintenance of the tapping channel with hot electrode paste. The procedure was started in 1988 after a long period with several burn-through events in the tapping area
> Installation of water-cooled Cu blocks around the tap-hole, which was started in 1994.
As discussed in the previous section, Cu blocks can help to prevent chemical wear by the tapped slag. They can also help to cool down and solidify metal and slag penetrating out from the tapping channel, and will result in less variation in the temperature in the area. Chemical wear from the alloy and oxygen lancing are not countered to the same extent, and the weekly procedure with maintenance with hot electrode paste is therefore necessary to prevent the area of being worn down over time. The hot electrode paste will also fill minor cracks in the tapping channel, preventing the formation of damaging sideways channels.
Tap-hole maintenance with electrode paste on Furnace
The practice of maintaining the tap-hole channel with electrode paste at furnace 10 was started in 1988. The idea behind the procedure is to inject a carbon-based material which is stronger and more wear-resistant than the normal plugging paste. Electrode paste in the form of 'A clump', a now discontinued product delivered by Elkem Carbon, was originally used for the purpose.
The procedure was established of using hot electrode paste for closing the furnace one tap on each side every week. Due to the electrode paste having a significantly higher softening point than the regular plugging paste, the paste has to be preheated prior to injection. The preheating is done at 60-70°C in a closed cabinet for two to three hours. As electrode paste emits fumes during heating, the cabinet is connected to the same off-gas suction system used to collect the tapping fumes. The heated paste is then loaded into the mud gun, and used to close the tap-hole 'as normal'. The mud gun is kept in front of the tap-hole for 15 minutes after closing in order to allow the electrode paste to start baking. To allow for proper baking of the electrode paste in the tapping channel the next two taps are performed on the opposite tap-hole.
During the tapping operation, the length of the tapping channel is visually estimated and recorded by the tapper for each tap. The data is then used by the tappers and metallurgists for following up and monitoring the health of the tapping channel. The normal drilling length is around 120 cm, and if this is decreasing noticeably over time extra maintenance with electrode paste is performed. Although not very precise, the drilling length is thought to be a good method for following the length, and thus the state, of the tapping channel on furnace.
Conclusions
The work to improve the lifetime of the lining in the tap-hole area has been ongoing since the startup of the furnace.
The major contributors to wear around the tap-hole area of furnace are thought to be chemical attack by the alloy, oxygen lancing, movement in the lining caused by changes in temperature, and to a minor extent chemical attack by the slag.
Water-cooled Cu blocks around the tapping block has been implemented to inhibit lining movement and chemical wear from reaction with the tap slag. Regular maintenance with hot electrode paste is used to combat the wear caused by the alloy and oxygen lancing. The inspection of the lining during a dig-out in 2010 showed the tap-hole area on the A side of the lining to be almost completely intact, indicating the positive effect of the implemented actions.
The change to an electrode paste with CTP-free binder was successfully carried out in 2017, reducing the exposure of personnel to carcinogenic materials.
