Extractive Metallurgy and Chemistry

In recent years, the proper management and recycling of metallurgical waste have become increasingly important due to their significant environmental impact and the potential for the recovery of valuable metals in order to save natural resources. Metallurgical waste comprises large-scale waste streams, which include more than 175 million tons of red mud and approximately 7.5 million tons of electric arc furnace dust produced annually worldwide. These waste materials contain substantial amounts of valuable metals, including scandium, iron, zinc, lead, copper, aluminum, and titanium. Moreover, different polymetallic ores have been increasingly used in metallurgical production over recent years due to the depletion of rich ores. Because of their difficult chemical and mineral compositions, new approaches for the extraction of valuable elements should be investigated. This Special Issue focuses on the possibilities and challenges of extracting valuable metals from important metallurgical wastes by examining different approaches: red mud, steelmaking dust, Waelz slag, and polymetallic ore.

The primary waste generated during alumina production using the Bayer process is red mud, which is also named bauxite residue. Iron is the main component of red mud; therefore, an important goal is to develop a low-cost process for extracting iron from red mud. The isothermal kinetics of iron reduction and iron grain growth processes have been investigated using the carbothermic reduction method at 1000–1200 °C [1]. Different factors such as temperature, roasting time, and the amount of Na₂SO₄ were studied to elucidate their influence on the iron reduction rate and particle size. The study showed that more than 80% of iron can be reduced at 1200 °C by implementing 30 min of roasting, and the addition of Na₂SO₄ has a positive effect on the iron growth process. This study aims to facilitate the extraction of iron from red mud using an approach consisting of low-temperature carbothermic reduction and magnetic separation methods.

In addition to iron, red mud contains a significant amount of other valuable metals such as scandium, aluminum, and titanium. The second study [2] focuses on the recovery of these metals from iron-depleted red mud to produce valuable products. The process involves reduction roasting; magnetic separation; HCl acid leaching of the non-magnetic product; alumina precipitation from the mother liquor via its saturation with hydrochloric acid; solvent extraction of scandium from the saturated liquor; and alkaline leaching of the acid residue to obtain a titanium concentrate followed by the production of white carbon from the alkaline solution. The optimization of the entire process resulted in the identification of the best conditions. All obtained products were characterized, and a flowsheet for red mud treatment was proposed.

The third study [3] examines the recycling of the Waelz slag, an iron-containing waste generated during the processing of electric arc furnace (EAF) dust using the Waelz process. The study investigates the reduction smelting of the Waelz slag to produce iron-based alloys. Thermodynamic simulations using FactSage software were conducted to predict smelting temperature ranges, as well as slag and alloy compositions. These simulations were followed by reduction smelting experiments on a laboratory scale. Two approaches including the carbothermic smelting of Waelz slag with the addition of a SiO₂ flux without preliminary treatments and the smelting of the Waelz slag after the preliminary extraction of carbon using froth flotation were evaluated. The results have shown the possibility of the production of high-copper cast iron, low-silicon ferrosilicon, and slag suitable for the construction industry.

The last study [4] focuses on the application of a promising approach to extract base metals (Al, Cr, Fe, Ti, and V) from a complex titanomagnetite ore via fusion using ammonium phosphate salt as a flux followed by water leaching. The study showed the possibility of the extraction of these metals from ores as metaphosphate compounds. It was concluded that this method has highly selective towards metals such as iron, exhibiting a stable trivalent oxidation state. Accordingly, this approach can be used not only for complex ores but also for the extraction of different metals from various high-iron waste.

The extraction of valuable metals from industrial wastes and complex ores offers a significant opportunity for resource conservation, decreased environmental pollution, and a transition to a circular economy. However, improvements in technical processes are necessary to enhance the efficiency of waste processing. In particular, the addition of sodium sulfate to red mud enhances the kinetics of iron carbothermic reduction and promotes iron grain growth. The processing of red mud enables the recovery of scandium, aluminum, titanium, and silica. The reduction smelting of the Waelz slag demonstrates the feasibility of the production of iron-based alloys with the desired compositions. The fusion treatment of the complex titanomagnetite ore with ammonium phosphate salt as a flux followed by water leaching results in a high extraction degree of iron. Thus, the treatment and proper management of industrial wastes and complex ores hold immense potential for environmental and economic benefits. Further research and technological advancements are crucial to overcome technical barriers and implement these processes on a commercial scale.