Abstract
Slags coming from stainless steel (SS) and ferrochromium (FeCr) production generally contain between 1 and 10% Cr, mostly present in entrapped metallic particles (Fe–Cr alloys) and in spinel structures. To recover Cr from these slags, magnetic and gravity separation techniques were tested for up-concentrating Cr in a fraction for further processing. In case of SS slag and low carbon (LC) FeCr slag a wet high intensity magnetic separation can up-concentrate Cr in the SS slag (fraction <150 µm) from 2.3 wt.% to almost 9 wt.% with a yield of 7 wt.%, and in the LC FeCr slag from 3.1 wt.% to 11 wt.% with a yield of 3 wt.%. Different behavior of Cr-containing spinel’s in the two slag types observed during magnetic separation can be explained by the presence or absence of Fe in the lattice of the Cr-containing spinel’s, which affects their magnetic susceptibility. The Cr content of the concentrates is low compared to chromium ores, indicating that additional processing steps are necessary for a recovery process. In the case of high carbon (HC) FeCr slag, a Cr up-concentration by a factor of more than three (from 9 wt.% to 28 wt.%) can be achieved on the as received slag, after a single dry low intensity magnetic separation step, due to the well-liberated Cr-rich compounds present in this slag. After gravity separation of the HC FeCr slag, a fraction with a Cr content close to high grade Cr ores (≥50% Cr2O3) can be obtained. This fraction represents 12 wt.% of the HC FeCr slag, and can probably be used directly in traditional smelting processes.
Highlights
In 2018, 50.7 Mt of stainless steel (SS) was produced globally, with an average annual growth of 5.8% since 1950 [1]
Cr content is still low for FeCr production, this can be considered as a pre-treatment step prior to hydrometallurgical Cr recovery
In case of HC FeCr slag, both magnetic and gravity separation resulted in an up-concentrated fraction with a Cr content comparable with commercial primary Cr ores
Summary
In 2018, 50.7 Mt of stainless steel (SS) was produced globally, with an average annual growth of 5.8% since 1950 [1]. It is estimated that approximately 0.3 tons of slag per ton of SS is produced as a by-product [2]; about 17 Mt of SS slag is produced annually. In case of FeCr production, the amount of generated slag depends on the type of produced FeCr. During high-carbon (HC) FeCr production, 1.1–1.6 tons of slag per ton of HC FeCr is produced with a global annual production of. In the case of low-carbon (LC) FeCr, the amount of produced slag per ton of LC FeCr is 2.4–2.5 tons with a global annual production 0.9 Mt in 2018 [3], which corresponds to 2.2 Mt of LC FeCr slag. A significant part of the SS and FeCr slags produced in Europe are being valorized mainly in the construction industry (road construction, addition in cement and concrete, etc.) [4,5], the relatively high Cr content in these slags (1–10 wt.%) [3,6,7] represent a serious obstacle in their full valorization
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