User:Jgarabit/Slag

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Pile of steelmaking slag at the ArcelorMittal Indiana Harbor steelmaking facility, Indiana. Photograph by Nadine Piatak, USGS.

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Slag is a by-product of smelting (pyrometallurgy) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy or non-ferrous/base metals (by-products of recovering non-ferrous materials like copper, nickel, zinc and phosphorous). Within these general categories, slags can be further categorized by their precursor and processing conditions (e.g., Blast furnace (BF) slags, air-cooled blast furnace (ACBF) SLAG, basic oxygen furnace (BOF) slag, and electric arc furnace (EAF) slag) .

Due to the large demand for these materials, slag production has subsequently increased throughout the years despite recycling (most notably in the iron and steelmaking industries) and upcycling efforts. Moreover, according to the 2019 International Energy Agency (IEA) report, the iron and steel industry directly contributed 2.6 Gt to the global CO₂ emissions and accounted for 7% of the global energy demand. The IEA report and the World Steel Association project a continual increase in demand in the coming years prompting greater efforts toward emission mitigation, recycling, upcycling and adoption of more sustainable practices.

Environmental and Ecological Impact

Slags are transported along with slag tailings to "slag dumps", where they are exposed to weathering sparking concerns about leaching of potentially toxic elements (PTEs) and hyperalkaline runoffs into the soil and water affecting the local ecological communities. Non-ferrous or base metal slags typically have higher concentrations of PTEs. Additionally, fine slags and slag dusts generated from re-processing slags to be recycled into the smelting process or upcycled in a different industry, can be carried by the wind affecting a larger ecosystem and even ingested posing a direct health risk to the communities near the plant, mines, and disposal sites.

Ferrous Slags

In the iron and steelmaking industries, slags can be produced in different stages of the process resulting in varying degrees of crystallinity and physiochemical properties. Often, the subcategory of the ferrous slag (e.g., BOF slag, BF slag, EAF slag, etc.) can be used to very roughly determine the dominant metal oxides and degree of crystallinity. For example, Piatak et al., reported that the average elemental composition of BF slags based on the compilation of more than 100 studies is 34.2±7.0 wt% SiO₂ , 36.9±8.9 wt% CaO, 12.5 ±6.4 wt% Al₂O₃, 8.2±4.0 wt% MgO and trace amounts of other elements . The degree of crystallinity depends on the rate of cooling, with slow cooled BF slags (or air-cooled slags) having a greater degree of crystallinity than quenched BF slags making it denser and better suited as an aggregate. On the other hand, water quenched BF slags (known as ground granulated blast furnace slags (GGBFS)) have greater amorphous phases giving it latent hydraulic properties (as discovered by Emil Langen in 1862) similar to Portland cement .

Applications

Construction Industry

Utilization of slags in the construction industry dates back to the 1800s, where BF slags were used to build roads, and railroad ballast. During this time, it was also used as an aggregate and have begun being integrated into the cement industry .

Today, ground granulated blast furnace slags (GGBFS) are used in combination with Portland cement (PC) to create "slag cement". GGBFS react with a calcium byproduct created during the hydration of Portland cement to produce cementitious properties. Concrete containing ground granulated slag develops strength over a longer period, via the pozzolanic reaction, leading to reduced permeability and better durability. However, careful consideration of the slag type used is required as the high CaO and MgO content can lead to excessive volume expansion and cracking in concrete .

These hydraulic properties have also been used for soil stabilization in roads and railroads constructions.

Slag is used in the manufacture of high-performance concretes, especially those used in the construction of bridges and coastal features, where its low permeability and greater resistance to chlorides and sulfates can help reduce corrosive action and deterioration of the structure. The slag can also be used to create fibers used as an insulation material called slag wool.

Wastewater Treatment and Agriculture

Dissolution of slags generates alkalinity that can be used to precipitate out metals, sulfates, and excess nutrients (Nitrogen and Phosphorous) for wastewater treatment. Similarly, ferrous slags have been used as soil conditioners to rebalance soil pH and fertilizers as a nutrient source of Ca and Mg .

Emerging applications

Slags boasts one of the highest carbonation potential among the industrial alkaline waste due its high CaO and MgO content inspiring further studies to test its feasibility in CO₂ capture and storage (CCS) methods (e.g., direct aqueous sequestration, dry gas-solid carbonation among others). Across these CCS methods, slags can be transformed into precipitated calcium carbonates to be used in the plastic, and concrete industries and leached for metals to be used in the electronic industries .

However, high physical and chemical variability across different types of slags result in performance and yield inconsistencies. Moreover, stoichiometric based calculation of the carbonation potential can lead to overestimation that can further obfuscate the material's true potential . To this end, some have proposed performing a series of experiments testing the reactivity of a specific slag material (i.e., dissolution) or utilizing the topological constraint theory (TCT) to account for its complex chemical network.