Upgrading the value of BOS slag by addition of difficult to recycle glass or slags

While the steelmaking slag produced in furnaces is an important component of tarmac, by itself it doesn’t have the necessary skid resistance (or ‘polished stone value’ – PSV) to meet the safety needs for the highest grade of roads. As a result, virgin aggregate is mixed into the product to solve this problem, but comes with a range of issues relating to its environmental impact.

The Materials Processing Institute, with funding from the Transforming Foundation Industries (TFI) challenge, undertook a research and testing project to identify new methods to provide the same quality of road surface, while also reducing waste and emissions from road surface production.

“They key to modifying the slag product is to increase its silica content, thereby improving the PSV value to the level required for top grade roads. This has the advantage of replacing the benefits that the virgin aggregate brings, but without the environmental issues of quarrying, transport and importing that comes with its use,” explained Lucy Smith, Group Manager of Circular Economy at the Materials Processing Institute. “As is so often the case, the answer lies in the Foundation Industries coming together.”

This project’s objective was to use slag arising within the steel industry and streams of hard-to-recycle waste from the glass industry. “The project aimed to make a number of new products by modifying steelmaking slag with a number of alternative high silica sources. Thermodynamic modelling was to be used to define the mixtures to achieve the required mineralogy, before the slags were tested to assess the improvement in skid resistance (PSV) and abrasion resistance,” says Smith.

Researcher watching semi-solid steel pass through a rolling mill causing the release of steam

Another goal of the project was to undertake a detailed assessment of the volumes and values of waste, or difficult-to-reuse, streams of appropriate material from within the glass industry and the steel works, in order to determine their potential for use as a slag modifier.

Outlining the work of the project, Smith confirmed: “Following initial set up work, the project began by melting the steel slag and glass waste. At this stage it was evident that the iron in the mixture had begun to react with the graphite crucible itself, which impacted the results of the trial.

“The next tests were switched to a magnesium crucible instead, and results showed a small increase in the PSV, although below the aims set by the project. It’s clear there is some success to be had, and the findings of this project will influence others moving forward.

As the project reached its conclusion, successes have come through the collaboration efforts and development of the ideas established through the work. Smith added: “As a result of this investigation work, the Materials Processing Institute have developed closer relationships with various partners that will benefit future projects in this and related areas. The Institute are now planning new similar project collaborations alongside glass and steel industry partners, as well as academics, having seen the potential for success in the adaptation of slag products more widely.”

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