Iron and steel are the most used metals in industry. In 2021, the crude steel production in the world was 1,951 million tons, with an average growth rate of 4.06% from 2000 to 2021. In the process of steel making, 12–15% of the steel output is waste slag (Shen H et al., 2003; Wang Q et al., 2013; Guo J et al., 2018). Therefore, iron and steel smelting slags are the metallurgical solid waste products produced in the largest amounts, which results in severe environmental pollution and a waste of resources without proper disposal. In addition to using steelmaking slag in construction, environmental management, and agriculture after simple treatment, the recovery of valuable substances in steelmaking slag is another essential measure for environmental protection and to achieve a circular world economy (Suvendu D et al., 2020; Giulio D et al., 2021; Pan S et al., 2017; Aiyuan Met et al., 2018). Among these valuable substances, kish graphite, which precipitates because the solubility of excess carbon in molten iron decreases as the iron cools (Naraghi R et al., 2014), has remarkable recycling value.
Kish graphite in the steel industry’s desulphurized slag and dust has a high degree of crystallinity comparable to that of natural flake graphite (Das B et al., 2006; Jiang Y et al., 2018; Lung K S et al., 2022; Mahieux P Y et al., 2008; Branca T A et al., 2020). Walker (JUN et al., 1957) and Eitaro (EITARO M et al., 1959) studied the crystal structure of kish graphite and compared it with those of other graphite types. It was found that the layer spacing of kish graphite is smaller than that of natural flake graphite, and the crystal structure of kish graphite is well crystallized. In 1963, Walker and Bunergee (JUN et al., 1963) studied the morphology of kish graphite and the effect of air oxidation. They found that there were some holes on the surface of the purified graphite sheet. They speculated that kish graphite would form a crystal structure with Fe and other impurity elements as the core. Hydrochloric acid alone could not completely remove the impurities in kish graphite.
In 1996, Nishimoto (Nishimoto H et al., 1996) studied the photoelectronic properties of kish graphite, showing that the initial states of the kish graphite photoelectrons are symmetric. In 2001, Bourelle (Bourelle E et al., 2001) studied the surface morphology of kish graphite by scanning electron microscopy (SEM), and an iron doping test was carried out at 2800°C to measure the resistivity before and after doping. The results showed that the doped iron occurred in two forms. When iron clusters spread on the graphite matrix to form pores, the resistivity was low. When iron clusters diffused on the graphite matrix to form small mounds, the resistivity was high.
Many researchers have carried out kish graphite recycling research to obtain a substitute for natural graphite, a critical raw material. Laverty (Laverty P D et al., 1994) proposed the separation and purification process of sieving, hydraulic classification, flotation, and pickling. A graphite concentrate of over 70% grade was obtained after flotation and 95% after pickling. In 2008, Kazmi (Kazmi K R et al., 2008) optimized the leaching parameters, including the acid concentration, liquid–solid ratio, time, and temperature. The graphite grade was 92.48% when hydrochloric acid was used, and it was 99.38% when hydrofluoric acid was further used. Li (Jihui L et al., 2021) designed an ideal physical separation process for kish graphite, which involved dry separation, flotation, ultrasonic dissociation, and magnetic separation. Graphite concentrates with grades above 95% were obtained after this physical recycling and above 99% after acid leaching.
Kish graphite’s applications have attracted many researchers’ attention. An (An J et al., 2015) prepared a graphene nanofilm with kish graphite and sulfuric acid as an effective intercalating agent. He noted that high-grade graphite could prepare large-diameter thin graphene nanofilms. The average diameter reached 6.7 mm, and the thickness reached 5 nm. Li (Jihui L et al., 2021) found that the pore structure of kish-graphite-based expanded graphite provided it with excellent oil absorption capabilities. Kumari (Kumari T S D et al., 2016) and Wang (Shutao W et al., 2017) successfully prepared a graphite anode material in a battery using modified kish graphite.
In the graphite flotation process, a large number of coarse and heavy impurity particles in steel slag will sink to the bottom of the flotation tank, affecting the feasibility of the process. Thus, gravity separation with a large processing capacity should be added for pretreatment to solve this problem. In terms of practical application, the above methods are either too small to be applied for large quantities or they require too high of a graphite purity, which will undoubtedly increase the cost of the recycling and purification of kish graphite. Graphite-based carburant is a high-quality carburizing material that is frequently utilized in steelmaking. Kish graphite comes from iron and steel smelting. After sorting and purification, its main impurity is iron, and other harmful elements are not present. Thus, kish graphite has advantages as the raw material of carburant.
This study aimed to develop a separation process that combined teeter bed separator (TBS) pretreatment and flotation to recycle kish graphite with a high recovery rate and high quality from steelmaking slag and to prepare kish-graphite-based carburant that met the requirements of industrial standards. Based on the results of this paper, we aim to provide a theoretical foundation for the scientific research and practice of the recycling and application of valuable substances in steelmaking slags.