Slag
Slag is a
Due to the large demand for these materials, slag production has also significantly increased throughout the years despite recycling (most notably in the iron and steelmaking industries) and upcycling efforts. The World Steel Association (WSA) estimates that 600 kg of by-products (about 90 wt% is slags) are generated per tonne of steel produced. [4]
Composition
Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals.
The major components of these slags include the oxides of calcium, magnesium, silicon, iron, and aluminium, with lesser amounts of manganese, phosphorus, and others depending on the specifics of the raw materials used. Furthermore, slag can be classified based on the abundance of iron among other major components.[1]
Ore smelting
In nature, iron,
As a co-product of steelmaking, slag is typically produced either through the blast furnace – oxygen converter route or the electric arc furnace – ladle furnace route.[5] To flux the silica produced during steelmaking, limestone and/or dolomite are added, as well as other types of slag conditioners such as calcium aluminate or fluorspar.
Classifications
There are three types of slag:
Ferrous slag
Ferrous slags are produced in different stages of the iron and steelmaking processes resulting in varying physiochemical properties. Additionally, the rate of cooling of the slag material affects its degree of
During the process of smelting iron, ferrous slag is created, but dominated by calcium and silicon compositions. Through this process, ferrous slag can be broken down into blast furnace slag (produced from iron oxides of molten iron), then steel slag (forms when steel scrap and molten iron combined). The major phases of ferrous slag contain calcium-rich olivine-group silicates and melilite-group silicates.
Slag from steel mills in ferrous smelting is designed to minimize iron loss, which gives out the significant amount of iron, following by oxides of calcium, silicon, magnesium, and aluminium. As the slag is cooled down by water, several chemical reactions from a temperature of around 2,600 °F (1,430 °C) (such as oxidization) take place within the slag.[1]
Based on a case study at the Hopewell National Historical Site in Berks and Chester counties, Pennsylvania, USA, ferrous slag usually contains lower concentration of various types of trace elements than non-ferrous slag. However, some of them, such as arsenic (As), iron, and manganese, can accumulate in groundwater and surface water to levels that can exceed environmental guidelines.[1]
Non-ferrous slag
Non-ferrous slag is produced from non-ferrous metals of natural ores. Non-ferrous slag can be characterized into copper, lead, and zinc slags due to the ores' compositions, and they have more potential to impact the environment negatively than ferrous slag. The smelting of copper, lead and bauxite in non-ferrous smelting, for instance, is designed to remove the iron and silica that often occurs with those ores, and separates them as iron-silicate-based slags.[1]
Copper slag, the waste product of smelting copper ores, was studied in an abandoned Penn Mine in California, USA. For six to eight months per year, this region is flooded and becomes a reservoir for drinking water and irrigation. Samples collected from the reservoir showed the higher concentration of cadmium (Cd) and lead (Pb) that exceeded regulatory guidelines.[1]
Applications
Slags can serve other purposes, such as assisting in the temperature control of the smelting, and minimizing any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal. In some smelting processes, such as ilmenite smelting to produce titanium dioxide, the slag can be the valuable product.[7]
Ancient uses
During the
Historically, the re-smelting of iron ore slag was common practice, as improved smelting techniques permitted greater iron yields—in some cases exceeding that which was originally achieved. During the early 20th century,
Modern uses
Construction
Utilization of slags in the
Today, ground
These hydraulic properties have also been used for soil stabilization in roads and railroad constructions.[11]
Granulated blast furnace 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 to reduce corrosive action and deterioration of the structure.
Slag can also be used to create fibers used as an insulation material called
Slag is also used as aggregate in asphalt concrete for paving roads. A 2022 study in Finland found that road surfaces containing ferrochrome slag release a highly abrasive dust that has caused car parts to wear at significantly greater than normal rates.[13]
Wastewater treatment and agriculture
Dissolution of slags generate alkalinity that can be used to precipitate out metals, sulfates, and excess nutrients (nitrogen and phosphorus) in wastewater treatment. Similarly, ferrous slags have been used as soil conditioners to rebalance soil pH and fertilizers as sources of calcium and magnesium.[14]
Because of the slowly released phosphate content in phosphorus-containing slag, and because of its liming effect, it is valued as fertilizer in gardens and farms in steel making areas. However, the most important application is construction.[15]
Emerging applications
Slags have one of the highest carbonation potential among the industrial alkaline waste due their high calcium oxide and magnesium oxide content, inspiring further studies to test its feasibility in CO2 capture and storage (
However, high physical and chemical variability across different types of slags results in performance and yield inconsistencies.
Health and environmental impact
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Slags are transported along with slag tailings to "slag dumps", where they are exposed to weathering, with the possibility of leaching of toxic elements and hyperalkaline runoffs into the soil and water, endangering the local ecological communities. Leaching concerns are typically around non-ferrous or base metal slags, which tend to have higher concentrations of toxic elements. However, ferrous and ferroalloy slags may also have them, which raises concerns about highly weathered slag dumps and upcycled materials.[22][23]
Dissolution of slags can produce highly
Fine slags and slag dusts generated from
See also
- Calcium cycle
- Circular economy
- Clinker (waste)
- Dross
- Fly ash
- Ground granulated blast furnace slag
- Heavy metals
- Mill scale
- Pozzolan
- Slag (welding)
- Spoil tip
- Tailings
References
- ^ ISSN 0883-2927.
- ISBN 978-0-309-22368-3.
- ^ "Iron and Steel Statistics and Information". www.usgs.gov. Retrieved 2021-11-27.
- ^ "worldsteel | Steel industry co-products position paper". www.worldsteel.org. Retrieved 2021-11-27.
- ISBN 0-930767-02-0.
- ISBN 978-0-12-817369-5, retrieved 2021-11-26
- ^ Pistorius, P. C. (2007). "Ilmenite smelting: the basics" (PDF). The 6th International Heavy Minerals Conference 'Back to Basics': 75–84.
- ^ "The chemical composition of glass in Ancient Egypt by Mikey Brass (1999)". Retrieved 2009-06-18.
- ISBN 978-0-08-100368-8, retrieved 2021-11-27
- ISSN 0950-0618.
- S2CID 238965391. Retrieved 2021-11-27.
- ^ "High Performance Cement for High Strength and Extreme Durability by Konstantin Sobolev". Archived from the original on 2009-08-03. Retrieved 2009-06-18.
- ^ "Autojen jakohihnojen rikkoutumisen taustalla ferrokromikuonan eli OKTO-murskeen aiheuttama kuluminen". Geological Survey of Finland. 20 September 2022. Retrieved 20 September 2022.
- S2CID 238967817, retrieved 2021-11-27
- PMID 34323725.
- ISSN 0892-6875.
- OSTI 1187926.
- S2CID 236390725.
- ISSN 2076-3417.
- ISSN 0002-8061.
- S2CID 238670674.
- ^ S2CID 238952198, retrieved 2021-11-27
- ^ S2CID 238945892, retrieved 2021-11-27
- ^ S2CID 12325820.
Further reading
- Dimitrova, S.V. (1996). "Metal sorption on blast-furnace slag". Water Research. 30 (1): 228–232. .
- Roy, D.M. (1982). "Hydration, structure, and properties of blast furnace slag cements, mortars, and concrete". ACI Journal Proceedings. 79 (6).
- Fredericci, C.; Zanotto, E.D.; Ziemath, E.C. (2000). "Crystallization mechanism and properties of a blast furnace slag glass". Journal of Non-Crystalline Solids. 273 (1–3): 64–75. .
External links