Slag Reaction

I’ve seen or heard of the consequences of slag reaction in the BOS vessel firsthand only twice in my life, once at Iscor around 1987, and a second time last year in Port Talbot. The first time there were several fatalities, the second time it was a miracle that no-one got hurt.

First a short explanation of the BOS (Basic Oxygen Steelmaking) process and how it can lead to a slag reaction. In the BOS process, the standard procedure is to charge about 20% scrap and the remainder hot metal (i.e. liquid high-carbon iron) from the blast furnaces, and then blow oxygen through a lance which burns off most of the carbon and other impurities. The carbon is evacuated as a mixture of carbon monoxide and carbon dioxide, whereas the other impurities form a liquid layer of oxides on top of the steel which is called slag.

After the oxygen blow is completed the steel gets tapped into a ladle, followed by a separate tap of the slag into a slag pot. After both taps are completed there is still some slag left in the vessel, and the next charge of scrap and hot metal can be made for the next blow operation. So far so standard. Except that at times (and I don’t know the exact reasons in both instances) the decision can be taken to charge less scrap, and in the two cases I’m aware of, the operation was using a hot metal only charge (i.e. without any scrap charge whatsoever).

The situation in Iscor was as follows: it was a Saturday and visitors were trooping into the pulpit of the steel plant, when a hot metal charge was about to start in the vessel furthest away from them. The hot metal was poured on top of what remained of the slag in the vessel, and the force of the slag reaction forced the hot metal out of the vessel, where it hit the pouring ladle and splashed sideways for a considerable distance. The group of visitors had only partly entered the pulpit and the back of the queue got struck by the spray of liquid metal, causing several of them to be killed. A few days later I passed the site on my way to the morning meeting and could see the remains of the splash against the pulpit wall. The thought occurred to me : “that could have been me.”

The second instance, in Port Talbot, did not involve me to the same extent, but I have been able to find out more of the details that led up to the slag reaction. The new manager director at the time, Mr. Jha, had insisted that Port Talbot try out a practice which he claimed was quite common in India, which is to leave out the scrap charge altogether. Not sure why this was suggested, it may have been cheaper or made better use of raw materials or maybe it’s a faster process. Whatever the case, the previous cast had to have several reblows, meaning that the remaining slag was overoxidised.

It’s not hard to see what would happen if a carbon-rich liquid meets an oxygen rich slag: it’s a bit like throwing water on a hot plate, only far more violent. The instantaneous formation of carbon monoxide gas at the interface between hot metal and slag would want to escape somehow and as such force the hot metal back out. I don’t know the exact circumstances, but presumably Port Talbot was following the practice of clearing the casting platform whenever a hot metal charge was made, and as such there was no-one about to get hurt.

Still, the spray of hot metal caused quite a bit of damage and started some fires, which fortunately were contained before they reached the carbon carbide stores at the desulphurisation station, otherwise that would have caused a massive blow that would have caused fatalities and far more substantial damage.

So why is a hot metal only practice more vulnerable to a slag reaction ? My opinion is that an overoxidised slag is less of a problem when scrap is charged first because (1) the slag is cooled by the scrap; and (2) being solid, the scrap is less likely to be thrown out of the vessel by a slag reaction. I’m not sure if the two were related, but not too long afterwards Mr. Jha was replaced as managing director for Strip Products UK.


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