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Assessment of Raw Materials in Stainless Steelmaking-Their Energy Consumption and Greenhouse Gas Emission

Time: Thu 2021-09-16 14.00

Location:, Stockholm (English)

Subject area: Metallurgical process science

Doctoral student: Wei Wenjing , Processer

Opponent: Ye Guozhu,

Supervisor: Anders Tilliander, Processer; Peter Samuelsson, Materialvetenskap; Professor Pär Jönsson, Processer

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In stainless steelmaking, around 68% of the total greenhouse gas emissions come from the processing of raw materials. Thus, it is important for steelmakers to make efforts together with their raw material suppliers to implement low-carbon initiatives. To facilitate such initiatives, assessment of raw materials will provide guidance. In this work, the assessment of materials consists of two parts: i) different production scenarios are studied by using a static process model coupled with life cycle assessment approach to investigate the reduction potential of environmental impacts for Mo and Ni alloys; ii) assessment of the effect of trace element content (phosphorus) in stainless steel scrap on steel’s manufacturing cost, resource consumption and environmental impact using an online static process model.

The results show that the overall GHG emission of FeMo production varies between 3.16-14.79 t CO2-eq/t FeMo (i.e. 5.3-24.7 tCO2-eq/t Mo). The main variance comes from the mining and beneficiation stages and depends mainly on the ore’s beneficiation degree. However, whether molybdenum is extracted as a co-product from copper mine or not can have an even greater effect on the total GHG emission of molybdenum due to the allocation of the impacts.  In the case of nickel alloys, the GHG emissions for producing nickel metal, nickel oxide, ferronickel and nickel pig iron are 14, 30, 6 and 7 tCO2-eq/t alloy (i.e. 14, 40, 18, and 69 tCO2-eq/t Ni), respectively. Extracting sulfide ore through flash smelting process has been shown to have the least energy requirement and greenhouse gas emissions. In comparison to sulfide ore processing, oxide ore processed in an electric furnace is much more energy intensive and less environmental friendly primarily due to high content of gangue. However, by using a sustainable electricity source such as hydro-powered electricity, or applying a thermal heat recovery, it is possible to reduce the impact from electric furnace smelting of laterite. Furthermore, the use of stainless steel scraps with low phosphorous contents reduces slag amount, alloy consumption, production cost and carbon footprint. An estimation equation between phosphorous content and scrap’s value-in-use is obtained in the study to support the development of purchasing strategy.

To conclude, the application of static process model based on mass and energy balance provides the possibility to assess raw materials’ environmental impact (energy consumption and GHG emissions) and to identify potentials to realize sustainable stainless steelmaking.