A Review of Circular Economy Prospects for Stainless Steelmaking Slags PDF

Journal of Sustainable Metallurgy (2021) 7:806–817 https://doi.org/10.1007/s40831-021-00392-w THEMATIC SECTION: MOLTEN 2021: SLAGS, FLUXES, AND SALTS FOR ENVIRONMENT, RECYCLING, AND SUSTAINABILITY A Review of Circular Economy Prospects for Stainless Steelmaking Slags Lauri Holappa1...

Journal of Sustainable Metallurgy (2021) 7:806–817 https://doi.org/10.1007/s40831-021-00392-w THEMATIC SECTION: MOLTEN 2021: SLAGS, FLUXES, AND SALTS FOR ENVIRONMENT, RECYCLING, AND SUSTAINABILITY A Review of Circular Economy Prospects for Stainless Steelmaking Slags Lauri Holappa1 · Marko Kekkonen1 · Ari Jokilaakso1 · Juha Koskinen2 Received: 9 April 2021 / Accepted: 10 June 2021 / Published online: 28 June 2021 © The Author(s) 2021 Abstract The world of stainless steel production was 52 Mt in 2019, and the annual amount of slags including electric furnace, AOD converter, ladle, and casting tundish, was estimated at 15–17 Mt. Nowadays, only a minor fraction of slags from stainless steel production is utilized and a major part goes to landfilling. These slags contain high-value elements (Cr, Ni, Mo, Ti, V…) as oxides or in metallic form, some of them being environmentally problematic if dumped. Thus, any approach toward circular economy solutions for stainless steel slags would have great economic and environmental impacts. This contribution examines the slags from different process stages, and the available and new potential means to increase internal recycling and to modify slags composition and structure by optimizing their properties for reclaiming in high-value applications. Eventual methods are, e.g., fast controlled cooling and modifying additives. Means to recover valuable metals are discussed as well as potential product applications to utilize various slags with different chemical, physical, and mechanical properties. By integrating the treatments and steering of slags′ properties to the total process optimization system, the principles of circular economy could be achieved. The contributing editor for this article was Mansoor Barati. * Lauri Holappa [email protected] 1 Department of Chemical and Metallurgical Engineering, School of Chemical Engineering, Aalto University, 02150 Espoo, Finland 2 Tapojärvi Oy, 95400 Tornio, Finland 13 Vol:.(1234567890) Journal of Sustainable Metallurgy (2021) 7:806–817 807 Graphical Abstract Keywords Slags utilization · Metals recovery · Recycling · Slags properties · Productization · Environmental issues Introduction slags from stainless steel production is different: on aver- age, only a minor fraction is utilized, and a major part goes Stainless steel is the most rapidly growing metal with an to landfilling. The utilization degree varies from zero to annual growth rate of 5.33% (1980–2019). That num- 100% depending on the plant′s course of action. An appar- ber matches well with Fig. 1 which shows the recent pro- ent reason is that stainless steel plants are small compared gress from the year 2005: the production has doubled in to carbon steel plants, and the amount of slags are minor, 15 years and approached 52 Mt in 2019 belonging to the respectively. Hence, landfilling has been a simple means same category with aluminum and copper as to the volume and permitted thus far, but problems may arise in the long and value. The overall world steel production was 1869 run. Another reason is the complexity of these slags, which Mt/2019. The iron and steel production together gener- makes the treatment and utilization more demanding. It also ated a massive quantity of slags (≈ 600 Mt/year). Such vol- needs investments in equipment. Consequently, the slag umes cannot be landfilled for environmental and economic processing has not been considered economically attractive reasons, and various treatments and applications have been enough, and many steel plants have settled down to steel intensively developed. Nowadays, a high percentage is either production and marginalized secondary functions. But there recycled, reused, or valorized in different applications. The are several positive grounds as well which are highlighted total amount of slags from stainless steel production was through this article. estimated as 15–17 Mt/year including slags from different Differing from blast furnace and converter slags, stainless process stages, EAF melting, AOD & VOD converting, steelmaking slags contain high-value elements (Cr, Ni, Mo, ladle operations, and casting. The present situation of Ti, V) as oxides or in metallic form. An efficient recovery 13 808 Journal of Sustainable Metallurgy (2021) 7:806–817 for reclaiming in different high-value applications. Eventual methods are, e.g., modifying additives and fast controlled cooling. Different means to maximize the recovery of valu- able metals are reviewed as well as potential product appli- cations to utilize various slags. Slags from Different Unit Processes An overall scheme of stainless steelmaking is shown in Fig. 2. The process starts with melting stainless scrap, ordi- nary recycled steel charging, and alloying additions (FeCr, Fig. 1  The growth of world stainless steel production in the years FeMo, FeNi) in an electric arc furnace (EAF). The aim is to 2005–2019 prepare a liquid steel charge close to the final composition as for the main alloying elements and proper carbon and silicon contents for the subsequent AOD (Argon Oxygen of these metals is an economic driver and an environmental Decarburization) or alternatively VOD (Vacuum Oxygen target for saving the use of natural resources. Another envi- Decarburization) converter. Melting with arcs assisted by ronmental aspect is that some components in slags can be oxygen blowing results in a partial oxidation of [C] and most environmentally problematic if dumped. Cr (VI) is a well- of [Si] to final contents about 1–1.5% C and 0.1–0.2% Si. known risk, and its formation should be eliminated. Cr and The Cr oxidation is strived to restrict and to avoid too high Ni are also carcinogens [4, 5]. Fluorspar (­ CaF2) is commonly ­Cr2O3 content in the slag via these residual contents, espe- used as flux in stainless steel slags causing an environmen- cially [%Si]. In the case of direct VOD treatment (without tal risk. Dusting is a further problem characteristic to slags AOD process), lower [C] is required after the EAF. with high basicity. To summarize the foregoing aspects, all In the AOD converter, carbon is oxidized to low contents actions towards circular economy solutions will have a great (≤ 0.05%) by O ­ 2 + Ar ­(N2) blowing starting with 100% O ­ 2 economic and environmental potential. The aim of this con- and by stepwise lowering p­ O2 from 100% to zero and increas- tribution is to review available and new feasible means to ing ­pAr from zero to 100%, in tandem. Carbon oxidation is increase internal recycling, and to modify slags composi- preferred to Cr oxidation when ­pCO is decreased by neutral tion and structure as objectives to optimize their properties gas (Ar, ­N2) dilution. In VOD converter, ­pCO is reduced by Fig. 2  The scheme of different unit processes in stainless steel- making and formation of slags Table 1  Approximate compositions and amounts of slags from stainless steelmaking Unit process CaO SiO2 MgO Al2O3 Cr2O3 CaO/SiO2 CaO + MgO/SiO2 Amount kg/t steel Minor other components EAF 40–45 25–30 5–12 5–10 3–7 1.5–1.8 1.7–2.0 100–150 Fe, Mn, Ti, V, Ni AOD 55 25–30 5–10 1–5a 0.5–1 2 2.5 100–120 CaF2 LF-CC 55–60 20–30 5–10 1–5a 1–5 2–3 2.2–3 15–20 CaF2, Ti, Nb, V… a Even higher when the slag is reduced, and steel deoxidized with Al- or Ca-aluminate added into the slag 13 Journal of Sustainable Metallurgy (2021) 7:806–817 809 low pressure, i.e., vacuum. Anyway, towards very low [C], the basic slag material is delivered to purposes such as road some [Cr] is oxidized, and quite high ­Cr2O3 contents about and infrastructure construction. Unfortunately, most slags 25% can be found in AOD slag after the decarburization from stainless steelmaking still go to landfilling which needs period. Therefore, the next necessary stage is slag reduction space, causes loss of valuable resources, and is hazardous done by adding FeSi or eventually Al into the steel melt and to human health and the environment. As mentioned ear- stirring with argon gas. After the slag reduction stage, the Cr lier, the main risk is the eventually high Cr content which content in the slag is aimed at low contents e.g., 0.5% ­Cr2O3. can lead in contamination of soil and water in the form of Then the slag is tapped to a slag pot, and lime and fluorspar leachable Cr (VI). Cr and Ni are also carcinogens. Avoiding are added into the converter to form basic liquid slag for a negative impacts is a strong motivation for emphasized slags short desulfurization treatment with intensive Ar stirring. utilization, but there is also a great economic potential via In addition to the composition of AOD slag in Table 1, the improved recovery and slags valorization. In the following slag after reduction can contain several percent fluorspar. chapters feasible treatments for improved metals, recovery After tapping into the ladle, CaO and C ­ aF2 are added and valorized utilization of slags are surveyed. Both estab- again to form a basic slag to protect the steel from the lished methods and new innovative solutions are discussed. influence of air, to absorb deoxidation products from the In many cases, the references are from carbon steel produc- steel, and to improve steel cleanliness via ladle metallurgi- tion, whereupon the special features of slags from stainless cal (LM) treatments under Ar stirring. Trimming alloying steel making should be considered when contemplating is performed as well. It is common that the LM treatments potential applications. take place in a ladle furnace (LF) which makes tempera- ture adjustment easy. The LF slag follows on the ladle to continuous casting (CC), and after the cast end, the slag Metals Losses in Slags is poured into a slag pot. An adjunct slag used in the CC tundish is typically more acidic. Its function is thermal Metals as dispersed fine particles or dissolved as oxides in insulation and protecting steel during casting. The amount slags are difficult to recover. Let us consider our primary is minor and was not presented separately in Table 1. It interest, Cr as an example to examine which factors influ- can be incorporated in other slags. As a general comment, ence its presence in the slag. The content of oxidized Cr MgO (dolomitic lime) is added into slags to protect mag- “Cr2O3” in the end slag of the EAF process or the slag from nesia-based refractory linings. It influences the properties the AOD reduction stage depends on the oxygen potential of the slag, e.g., basicity, Cr solubility, melting tempera- (defined as p­ O2 or a­ [O]), which is determined by the ambient ture, and viscosity as well as the mineralogical structure contents (activities) of [Cr] and the controlling solutes [Si] after solidification and cooling. and eventually [C] in the EAF. Hitherto, the ambient tem- In Table 1, the main three types of slags are described. perature as well as the slag and liquid metal compositions The figures are approximate composition ranges based on influence via the activity of ­Cr2O3 and ­a[Cr], respectively. Nordic steel plants. They refer to slag compositions in situ Thermodynamic and kinetic aspects were investigated and at the end of each process stage and do not include eventual discussed, e.g., by M. Guo et al. [6, 7]. The equilibrium Cr large metal lumps. Of course, slags are factory specific and distribution between the slag and steel (%Cr)slag/[%Cr]steel can differ substantially due to various raw materials, process can be derived from the reaction equation: operation, and steel grades to be produced. In Table 1, the (1) ( ) slags from different unit processes differ both in basicity, Cr 2[Cr] + 3[O] ↔ Cr2 O3 , content, and minor impurities. Except for the oxide form- ing components in the slag, also less-oxidizable metals like aCr2 O3 Ni and Mo can be found but mostly in metallic particles K1 =. (2) a2Cr ⋅ a3O ejected from the bulk steel or endogenously formed inside the slag via the reduction process. In addition, slags can Chromium oxide was simplified here as 3-valent oxide retain macroscopic metal particles, splashes, skulls, tapping ­Cr2O3, although it is well known that in low p­ O2 conditions, remains, etc., which are not included in the slags´ composi- also 2-valent oxide CrO exists [8, 9]. In a process with oxy- tions above. Their removal and recovery in an early stage gen blowing, ­pO2 or ­a[O] is controlled by carbon oxidation of a treatment process is essential. Nowadays, a typical reaction, ­a[O] increases, and the equilibrium is approached slag processing route in a stainless steel plant consists of from left to right. The ambient top slag can become even wet grinding and metal separation. It is emphasizing metal supersaturated with oxygen via Cr oxides, especially when recovery but has quite restricted ability to slag recycling ­O2 top blowing is applied. In a reduction stage, a­ [O] is con- and productization. Depending on slag composition, cooling trolled and pressed down by adding silicon or aluminum, and method (slag pit vs. intensified water cooling), and grinding, 13 810 Journal of Sustainable Metallurgy (2021) 7:806–817 the reaction should go backwards. The gross reactions can independently, even by considering slags from production be written as follows: of different steel grades (e.g., Ni, Mo, Ti, V). Then each slag type could get its own specific post-treatment without (3) ( ) ( ) Cr2 O3 + 3∕2[Si] = 2[Cr] + 3∕2 SiO2 , getting blended into the big bulk. The recovered metals can be recycled as reverts to process. The quantity is typically (4) several percent of the slag weight. In the case of stainless ( ) ( ) Cr2 O3 + 2[Al] = 2[Cr] + Al2 O3. steel, the value of Cr is the leading factor; its content is high According to several researchers, a primary reduction in all stainless grades throughout the process stages. The mechanism is the reaction between the metal bath and the comparison of unit prices of valuable elements (€/kg) gives emulsified slag droplets due to the large surface area of the an order: Cr