I started my career at British Steel in September 2015 as a technical apprentice. Throughout my apprenticeship, I had the opportunity to rotate my placements around different plant areas and this helped me discover my passion – ironmaking at the blast furnaces.
So what actually is a blast furnace? In scientific terms it’s a counter current thermal reactor, but in layman’s terms it’s a smelting furnace which produces molten iron. There are four blast furnaces at British Steel – known as the four queens of ironmaking. Personally, I think they are named after women as, like me, they can be very temperamental at times and they definitely have different personalities.
Raw materials including iron ore, sinter and coke are charged into the top of the furnace. These are charged in specific layers – a ferrous layer of iron ore and sinter and then a coke layer which is a fuel. These layers make up what is called a burden. The burden slowly descends as the chemical reactions occur within the furnace.
Hot blast at temperatures around 1100 degrees is injected through the bottom of the furnace through copper nozzles called tuyeres. This hot blast reacts with coal that’s injected through the tuyeres to produce carbon monoxide gas, which travels up the furnace and reacts with the raw materials to produce molten iron.
The molten iron works its way to the bottom of the furnace where it is tapped out and transported to the steelmaking plant.
On to the main event – what are alkali scabs?
Scabs, also known as scaffolds, are alkali build-ups within the furnace which adhere themselves to the furnace wall. Alkalis naturally cycle within the blast furnace as they are present in the raw materials charged into the furnace and to some extent are good for the furnace. Imagine an actual scab which forms when you’ve hurt yourself – it’s there to protect you. The same goes for alkali build-up in a blast furnace – a little is great for protecting the furnace refractory bricks from premature wear, however a large build-up can cause significant issues.
There are a few main causes for scab build-up – excess sinter fines within the furnace (which is sinter smaller than -5mm) and a high lime chemistry of the slag (which is the by-product of the ironmaking process). A high lime chemistry increases the melting point of the slag, which can mix with the excess fines to form a sticky mixture which can adhere to the furnace wall.
What issues can scabs cause?
The ideal blast furnace burden should have a large cross-sectional area for the carbon monoxide gas to do its magic and react with the raw materials. However, when a large scab forms, this distorts the path the gas takes and can reduce the amount of reactions happening.
Worst case scenario is that the scab can break off and fall into the molten iron below which can rapidly cool the iron and cause it to solidify. Imagine you’ve just made a hot bowl of soup and then you drop an ice cube in it. This will cool the soup rapidly – and the same thing can happen in a furnace.
So, how can scabs be treated?
The burden is usually distributed so that the gas flows through the centre of the blast furnace. However, when a scab is present, the burden can be distributed in such a way that the gas favours a pathway up the side of the furnace. This is called wall working, where the gas slowly reacts with the scab to break it down in a controlled manner.
Another way they can be treated is distributing more sinter to the walls of the furnace – this causes an abrasive effect. In a way, you can imagine the sinter acting like a pumice stone – slowly breaking down the scab in a controlled manner which won’t have a significant operational impact on the furnace.
How can scabs be monitored?
During my first placement at the blast furnaces, a scab was present on the Queen Anne furnace. I carried out the weekly stack thermography around the whole furnace shell on three levels and recorded all of the temperatures. Using this information, I created a weekly heat map of the furnace. This consisted of a spreadsheet with conditional formatting tools to turn the cells red, amber and blue depending on the temperature range. The spreadsheet was printed out every week and rolled up into a furnace shell shape to indicate exactly where the blue, cold areas were – indicating where the scab was located.
The changes in the scab size were observed weekly so the correct changes to the burden and furnace operations could be made to effectively reduce the scabs in a manner that would prevent them falling off into the hearth. This method is still an effective way to monitor build up today.