What is the sustainable production and recycling of metal materials?

Circular economy


With the current requirement for social change, such as carbon neutrality, Life Cycle Assessment (LCA), which evaluates the environmental impact of products and services, is becoming increasingly important. However, conventional LCA methods have several challenges, such as not sufficiently taking into account the impact on society as a whole. We asked Ichiro Daigo, who is developing a new LCA method to lay the groundwork for the future of industry, about the challenges of conventional LCA, the assessment method he is developing, and what he has learned from it.

醍醐 市朗DAIGO Ichiro

Associate Professor, Research Center for Advanced Science and Technology, The University of Tokyo

He is involved in Pre-emptive LCA research based on material flow analysis (MFA), which reveals the dynamics of material flows and stocks.

Please give us an overview of your research.

I specialize in materials engineering and am developing evaluation methods to clarify how materials can be selected and used to make them and less environmental impacts for sustainable development.

Another important theme is to identify how end-of-life products should be recycled for sustainable resource use. Recycling tends to be viewed as “a process of waste management,” but from the perspective of those who produce feedstock for material production, recycling is “raw material production”. Our goal is to change recycling from “waste management” to “manufacturing,” and to advance the development of technologies for recycling from the perspective of raw material production. We are also developing evaluation methods for this purpose.

Similar to the metabolism in human life, industry uses materials and energy to carry out economic activities while emitting CO2 and waste. Improving the efficiency of this industrial metabolism will help to reduce CO2 emissions and waste generation. As with the human body, we must recognize problems as soon as possible before they occur, but to do so, we need to monitor and “visualize” the actual state of industrial metabolism. The methods we are developing, such as Life Cycle Assessment (LCA) and Material Flow Analysis (MFA), are used to evaluate whether the actual situation is healthy or not. One of the important purposes of our research is to predict what will happen in the future based on these assessment methods and to avoid risks.

What is the difference between LCA and MFA?

LCA is an evaluation method that quantifies the environmental impact associated with a product through its lifetime (life cycle), from resource extraction to manufacturing, use, recycling, and disposal. It is often expressed as “cradle to grave”.

MFA, on the other hand, focuses on a specific material and analyzes its stock (the amount of materials in use) and flow. For example, steel materials are used in various applications such as automobiles, buildings, and home appliances, and after use, they are collected as scrap and recycled again at steel mills, etc. MFA analyzes how much steel material is used annually in each application and how much is recovered as steel scrap, etc. Thus, LCA and MFA are complementary, and MFA can be used as a reference when assessing the environmental impact of a product in LCA (Figure 1).

Figure 1 LCA and MFA

LCA evaluates the environmental impact of a product through its life cycle, whereas MFA quantifies the stock and flow of materials.

How do we assess the environmental impact of the future?

What challenges do you see with traditional LCA?

Society is now moving toward carbon neutrality and a circular economy. Technology development needs to be aligned with these new social directions. However, advanced technologies and materials are still in their early stage and will mature in the future. Then, even if there are technologies and materials that look promising in the future, they must be evaluated in their immature state at this point in time, and how to evaluate them has been a challenge for conventional LCA.

In general, LCA evaluates the environmental impact per “functional unit” of the product in scope. In the case of automobiles, for example, the functional unit is “100,000 km of driving distance in 10 years of use,” and the environmental impact of each vehicle can be compared. However, what happens if we multiply this by the tens of millions of automobiles running in Japan as a whole? It is possible that critical raw materials will be in short supply, or that the environmental impacts will be higher than the evaluation based on the function unit. This is what conventional LCA cannot capture.

The whole system perspective is also necessary when thinking about the future. Energy resources may shift to hydrogen and biomass in order to achieve carbon neutrality, but there are various constraints, such as how to procure large amounts of hydrogen and it is not acceptable to be deforested in order to obtain large amounts of biomass. For recycling, for example, even if we try to change the production of steel materials from iron ore to scrap, large amounts of scrap is not always available. Such things cannot be evaluated with conventional LCA. We are required to develop a new LCA method that takes into account the constraints of the entire system and present the evaluation results.

What specific methods have you developed and what evaluation results have you obtained?

Materials perform their functions when they are in use, and the amount of material in use is called “in-use stock “. Let me give you an example of steel. We estimated the in-use stock of steel to be 1 to 1.1 billion tons for Japan as a whole, or about 9 tons per capita. Estimates for the developed countries showed that all countries head at around 10 tons per capita.

Until now, forecasts of future demand for materials have been estimated from data on production and demand in the past. However, we found that the stock of steel saturates at a certain point, so we changed our thinking and proposed a “stock-driven model” that estimates future demand based on the historical data of in-use stock (Figure 2).

Figure 2: Future Demand Estimation Model

Since the amount of material stock in use saturates at a certain level, we proposed a “stock-driven model” to estimate future demand from the in-use stock.

Based on this model, even developing countries, which currently stock around 1 ton of steel per capita, are expected to increase their steel stocks toward 10 tons, which is on par with developed countries. Based on this assumption, future demand for steel is estimated to be about 2.5 billion tons worldwide in 2100, while only 1.5 billion tons of steel scrap is expected to be available. The remaining 1 billion tons will have to be made from iron ore. Based on this analysis, it is possible to propose that cities that will be developed in the future will have an urban structure in which the amount of steel stock per capita is not 10 tons, but only 5 tons, so that infrastructure can be developed.

MFA is important in the recycling of metals.

What kind of research are you doing on metal recycling?

Metals are widely used in automotive components, and while reducing the weight of a component will improve fuel efficiency, there is a tradeoff: high-strength materials for weight reduction often have a greater environmental impact in the manufacturing process. Conventional LCA allows the selection of materials for components, but the problem is that many components are recovered as scrap after use and used in the next product lifecycle. Conventional LCA assumes that only the first user bears the environmental burden, and the environmental burden of those who procure the materials as scrap is zero. There has been a long-running debate as to whether the environmental impact can be assessed as zero, even though product who use the scrap can also use it as a raw material for metals.

In relation to this issue, we studied how many times metal materials are recycled. Applying the stochastic process theory called “Markov chain model” to the MFA of steel and copper, we analyzed the average number of times these materials are used. As a result, steel are used about 5 times and copper materials about 2 times. These estimates of the number of times these materials are used are referred to as one indicator in the promotion of the circular economy. In addition, the fact that steel material is used five times means that only the first user bears the environmental burden, while the remaining four times are free-riding. Therefore, we are proposing a methodology in which the environmental burden is passed on to the second and subsequent users when the material is recycled, and the first users are relieved of their environmental burden.

What other issues do you see regarding the recycling of metal materials?

In recycling, the quality of the scrap collected is also important. We performed MFA on aluminum to clarify the problems of contamination of other elements during recycling.

There are approximately 200 different alloys in the wrought aluminum products distributed in the world. Even beverage cans are made of different aluminum alloys for the bodies and lids, and the aluminum recovered for recycling is not uniform. For this reason, aluminum recovered from end-of-life management is currently used as a raw material for casting materials that do not require very high purity, especially for automobile parts. About half of the automobiles produced in Japan are exported.

In other words, the material flow of aluminum (Figure 3) consists of (1) purchasing highpurity primary ingots from other countries, (2) alloying them to make wrought products, (3) collecting them after use to make castings (automobile parts), and (4) exporting them as automobiles. This analysis shows that if automobiles were no longer exported from Japan, the aluminum recycling system would instantly fail. Neither the aluminum industry nor the automobile industry was aware of these problems, but we have foreseen that the current recycling system will eventually reach a dead end, and we have been sounding the alarm. This research would have provided the impetus for a national project to promote advanced recycling of aluminum starting in FY2021.

Figure 3 Results of material flow analysis of aluminum

In aluminum recycling, aluminum is recovered without consideration of alloy type, so much of it is exported as castings to be used as automobile parts.

In addition, looking at the world as a whole, it is estimated that the supply of aluminum scrap for casting will overtake the demand for casting material in about 2030 (Figure 4). This means that demand for aluminum scrap will decrease and the amount discarded will increase. One factor contributing to this is the shift to electric vehicles. The engine block, a key component of the engine, is made of cast aluminum. Electric vehicles use motors instead of engines, which reduces the demand for casting materials. While the shift to electric vehicles will result in lower carbon emissions, it will also result in the waste of aluminum scrap. To avoid this, it will be necessary to review the current system worldwide.

Figure 4: Estimated future global aluminum material production

The supply of scrap is expected to exceed the production of castings around 2030.

How much used material can be recovered with high quality?

What is needed to create a sustainable recycling system?

It is important to recover as much end-of-life materials as possible and to recover them in high quality with minimal impurities. In particular, in the case of steel, copper impurities as small as 0.4% can cause hot-surface shortness, so manufacturers are very careful about the impurities in the scrap materials they use. However, the actual quality of obsolete steel scrap is not well known in the world as a whole. So we looked into it.

Since it is difficult to obtain data on impurities in recycled materials from all companies, we actually went to the field and conducted a field survey (Figure 5 left). In addition to measuring copper concentrations in various steel materials in Japan (Figure 5 upper right), we also examined concentrations of copper, tin, nickel, chromium, and molybdenum in recycled materials in Western Europe, Ukraine, China, and Vietnam (Figure 5 lower right). In the future, we intend to investigate the causes of the differences between countries and to clarify which process is responsible for the contamination of impurities in scrap to prevent impurities contamination in the steel cycle.

Figure 5: Impurity concentration in steel

Top: Data collection by field survey. Bottom left: Copper concentration in steel in Japan. Bottom right: Comparison of impurity concentrations in recycled steel in different countries.

We are also developing a system that uses image analysis technology to estimate the chemical composition from images of steel scrap without melting. Since the thicker the steel scrap is, the higher the yield, the thicker the scrap is in the market trade, the higher the price. However, as mentioned earlier, the quality of scrap is greatly affected by the presence of impurity elements such as copper. If the chemical composition in scrap can be easily determined through image analysis, this can be reflected in the price. We hope that this will lead to the creation of a distribution system that allows steel scrap to be traded at more reasonable prices.

What are your future prospects for research?

As we have discussed so far, we have clarified the flow and stock of materials such as iron, copper, and aluminum, as well as their uses such as automobiles, and evaluated their environmental impact. However, these are one-dimensional analyses. In reality, everything is connected, and they interact with each other in the overall social system. In the future, we would like to create a model that encompasses all LCAs for various products and MFAs for various materials.

Once such a model is created, it will be possible to see where and how changing any part of the model will have an impact. Then, looking at society as a whole, we can see what kind of technology can really reduce the environmental burden, and what kind of policies should be enforced and what kind of technology should be developed. I believe that adding a time horizon of “the future” to this model is what is desperately needed now. It is not an easy task, but I would like to move forward with haste so that we can provide a guideline for manufacturing toward a sustainable society.

(Reporting by Chisato Hata)

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