Enrichment Characteristics of Cr in Chromium Slag after Pre-Reduction and Melting/Magnetic Separation Treatment

Concentrating the chromium in chromium slag and improving the chromium–iron ratio is beneficial for the further utilization of chromium slag. In this paper, chromium slag obtained from a chromite lime-free roasting plant was used as the raw material. Pellets made of the chromium slag and pulverized coal were reduced at different pre-reduction temperatures and then separated by a melting separation process or magnetic separation process, respectively. The mass and composition of the metallized pellets before separation, along with the alloy and tail slag after separation, were comprehensively analyzed. The experimental results showed that the output yield of alloy, iron recovery rate, and chromium content in the alloy were all higher when using melting separation than when using magnetic separation, because of the further reduction during the melting stage. More importantly, a relatively low pre-reduction temperature and selection of magnetic separation process were found to be more beneficial for chromium enrichment in slag; the highest chromium–iron ratio in tail slag can reach 2.88.

Occurrence and probable source of chromium enrichment in Permian coals, South Africa

The trace element data pool for South African coals is limited. However, certain elements are reported as occurring in considerably higher concentrations than global average values. These elements include chromium (Cr), as well as manganese (Mn), cobalt (Co), and molybdenum (Mo), with Cr showing the most consistent enrichment. The aim of this study was to investigate the occurrence of Cr in South African coals sampled from five coalfields, and to assess the possible source of Cr. A total of 21 run-of-mine (ROM) coal samples from the Waterberg, Soutpansberg, Witbank, Highveld, and the Nongoma Coalfields were obtained from active mines. Coal characterization, mineralogy, and geochemical data were compiled. The Cr values for the Waterberg (23 to 28 ppm), Soutpansberg (4.7 to 43 ppm), Witbank (24 to 37 ppm), and Highveld (33 to 37 ppm) coal samples are higher than those for the Nongoma coals (1.2 to 2.5 ppm), and, in nearly every case, higher than the Clarke value for hard coals (17 ppm). Correlation coefficients, density fractionation, and selective leaching were used to infer (indirect) modes of occurrence. A dominant silicate (clay) affinity was determined with Cr, as well as an organic affinity in the ROM samples. The Nongoma coals, with illite and no kaolinite, have extremely low Cr values. The other samples are enriched in kaolinite and also have a high Cr content. Chromium values as high as 67 ppm were reported for 1.4 RD float fraction samples, indicating a positive correlation with organic matter. A Cr - silicate association was observed following selective leaching. These results imply multiple modes of occurrence of Cr in the South African samples, in agreement with studies conducted globally. Chromium in the South African coals could have originated from the Bushveld Complex (approximately two billion years old), the largest Cr reserve in South Africa, which predates coal formation (approximately 299 to 252 million years ago) and is proximal to the coalfields. The Cr, seemingly not associated with chromite, may have been redistributed in the peat swamps during peat deposition, accumulating in the sediments and organic material. The Nongoma Coalfield is the farthest distance from the possible Bushveld Complex Cr source, and these samples are depleted in Cr.

Stability of Chromium in Stainless Steel Slag during Cooling

The chromium elution behavior of stainless steel (SS) slag depends highly on the chromium distribution, and the molten modification process proved to effectively improve the chromium enrichment in stable phases. However, the phase transformation and variation of chromium stability during the subsequent cooling process is still poorly understood. In this work, the phase composition and chromium distribution of SS slag from different quenching temperatures were experimentally studied, and the stability of chromium-bearing phases was evaluated using standard leaching tests. The results indicated that dicalcium silicate and spinel phases had formed in the molten slag at 1600 °C, while the dicalcium silicate disappeared and the phases of merwinite and melilite precipitated when the temperature decreased from 1600 to 1300 °C (at a rate of 5 °C/min). During this cooling process, the chromium migrated from other phases into the spinel, significantly suppressing the chromium elution. The leaching results also demonstrated that the potential chromium-bearing phases of glass, dicalcium silicate and merwinite are unstable and are presumably the main source of chromium release. The treated SS slag meets the requirements for the utilization of chromium-bearing slag in the cement and brick industries.