Sustainability footprint of steelmaking byproducts

Abstract

A steel mill is a large logistical hub, with raw materials and energy as input and steel plus some other materials as output. The vocabulary to designate the latter is fuzzy and changing, and it depends on who speaks about them: co-products, byproducts, residues, waste, emissions, pollutants, discharge, etc., are used in different contexts. This profusion of names echoes the conceptual hurdles that block the way when numbers have to be attributed to the co-products, related to either environmental footprints or economic values, especially if both are mixed. The paper focuses on the example of blast furnace slag, which is sold to the cement industry in large quantities as a substitute to clinker. The practice is a lively example of an industrial ecology synergy between two economic sectors: both sectors, collectively, decrease their environmental footprints in terms of energy consumption, greenhouse gas emissions, resource depletion, etc., in an unambiguous way. Many issues arise, however, when exact figures have to be worked out to allocate a footprint to each partner in the synergy. Life cycle assessment seems like a good candidate to do that job, but from a practical standpoint, the method can be implemented in so many versions and flavours that the answers end up in a series of very different figures, which confuse rather than clarify the issue. What is argued here is that these difficulties are due to the fact that the underlying problems are not yet solved, and that it is naive to ask a technology like life cycle assessment to solve issues related to the allocation of the cost of the climate change externality to commodity materials like cement and steel. Unsolved problems are due to the uncertainties related to this process. Until these issues are cleared, it is proposed either to focus mainly on the synergistic benefits of the cooperation between the two sectors or to accept different estimates of the footprint of the co-products in the two sectors. This example is a typical case in point related to what we have called the collision between the ecosphere and the anthroposphere.