Ichiro Daigo
The University of Tokyo
Sustainable use of materials is my goal. So far, many MFA studies have been conducted. Markov chain model (stochastic model), pinch technique (optimization tool), population balance model (probability distribution) and some models have been applied in the MFA field. Recycling of metals are one of my great interests. Recently, resilience, supply chain risks, and criticality are within my scope.
Steel is known as one of the most highly recyclable materials. When steel is recycled after recovery from end-of-life products as a scrap, other materials are unintentionally mixed into the recovered steel. Due to the thermodynamics, some elements contained in these other materials can often not be extracted during steelmaking processes. This results in recycled steel containing some impurity elements.
On a global level, the impurities can be diluted to a sufficient degree by virgin steel. However, future saturation of steel demand and stock will lead to steel being made mainly from steel scrap, and increased steel recycling rates. A limited input of virgin steel cannot dilute the impurities found in scrap recovered from end-of-life products. Thus, the repeated cycling of steel may increase the level of impurities to the point where it could restrict recycling.
So far very limited knowledge exists on the impurities content of steel. This study aims to compare impurities contents in steel produced from scraps amongst different countries. Our research question is: how sustainable is the current steel recycling system and of how can we make it more sustainable?
We analyzed chemical contents of Ni, Cr, Cu, Mo, As and Sn by using optical emission spectrochemical analysis or X-ray fluorescence analysis. Based on this data and materials flow analysis we conducted two analyses; (1) observing current level of impurities contents in steel in different countries, and (2) estimating sources of the impurities. Steel products were sampled from China, Japan, Netherlands, Ukraine and Vietnam, using more than 100 samples for each country.
These countries, except for Vietnam, were selected for two reasons. First, low imports of steel scrap, because the contents in steel products have to reflect the contents in domestically recovered steel scrap. Second, low import of steel products, because alloying elements in the steel products should be identified. In other words, a more closed system of the steel cycle was preferred. In the case of Netherlands, the steel cycle system within Netherlands, Germany, France and Belgium was almost closed. The results for Netherlands were regarded as a representative of the four countries in west Europe.
Our results show that countries with higher economic growth showed higher impurities contents. One of the reasons our findings is the change in recycling processes that goes with increasing welfare, from dismantling by human hands to shredding and mechanical sorting. This is highlighted when comparing results for Japan and Vietnam. Vietnamese steel products were produced from steel scrap generated in Japan after non-ferrous metals were manually separated from the scrap.
Impurities can also occur because various types of steel alloys are intentionally mixed during recycling. Chemical compositions analysis and material flow analysis allowed us to distinguish the source of impurities in recycled steel. We found that the two thirds of impurities came from other materials unintentionally mixed in steel scrap.
• Socio-economic metabolism and material flow analysis , • Circular economy
