The CHROMIC activities can be divided between those pertaining to the technological core of the project and those addressing assessments on the impact of the technologies on the economy and – in a broad sense – on society.
The technological core encompasses all the steps of the scientific work that underlies the development of recovery processes for chromium (Cr), vanadium (V), molybdenum (Mo) and niobium (Nb) from slags, which is the primary aim of CHROMIC. These steps are grouped in dedicated work packages (WPs) and are: mineral processing (WP2), selective leaching (WP3) and selective metal recovery (WP4).
In brief: Mineral processing is about physical pre-treatment of the slags; in this WP pre-treatment and comminution techniques are applied to the slags in order to increase the concentration of the most interesting fractions. Then, in the leaching stage of WP3, hydrometallurgical processes are applied to dissolve the metals of interest from their matrix and obtain a leaching liquor, which is further treated in WP4 to recover the individual metals and/or valuable products that can be put on the market.
Click below to find out more about the objectives, activities and outcomes of the three work packages.
- WP2: Efficient mineral processing procedures for enhanced beneficiation of by-product and critical metals
- WP3: Highly efficient and selective hydrometallurgical extraction technologies for by- product and critical metals
- WP4: Highly selective metal recovery techniques for complex metal mixtures including by-product and critical metals
Valuable metals such as Cr, Nb, Mo, and V are are present in composite materials and by-products like e.g. steel slags. There are several ways to separate these metals from the material matrix: WP2 focuses on comminution and beneficiation, two pre-treatment processes that involve the crushing, milling and disintegration. The main goal of WP2 is to develop innovative processes that ensure a tailored physical separation of valuable metals from residue matrix. The materials prepared in WP2 are supplied to WP3 for further processing.
The specific objectives of WP2 are the following: performing a detailed mapping of the material’s metal recovery and mineral liberation potential; designing and testing an optimal pre-treatment route for metal recovery from fine-grained low-grade secondary resources; preparing metal-rich fractions for either direct reuse or further metal recovery by leaching in WP3.
It was found that the small size of the Cr-rich particles makes recovery by mineral processing challenging because a high degree of comminution is needed, and wet separation techniques has to be employed. Only for the high carbon ferrochromium (HC FeCr) slag the size of the metallic particles is suited for dry separation. Also, in wet chemical analysis the measured Cr content differs strongly based on the method used. This indicates that the spinel phases are highly resistant to chemical attack, and are not dissolved by all methods. Overall, it emerged that a reliable quantification of Cr in the materials before and after recovery is indispensable to calculate the recovery efficiency.
In the separation of the metallic fractions/fragments the best tested method was magnetic separation, where a remarkable up-concentration of Cr in the strongly magnetic fraction was obtained. The method will be used in larger scale processing of the carbon steel EAF (CS) and HC FeCr slags.
Due to remarkably larger liberation size of Cr containing phase, the HC FeCr slag was considered to be better suited for physical separation methods. Therefore, gravity separation by e.g. wet shaking table was tested. By combining the three heaviest fractions, 36% Cr recovery was obtained. Due to the high Cr content, this material could be possibly used as a replacement for primary Cr ores in FeCr production.
The aim of WP3 is to develop highly efficient and selective alkaline leaching processes for by-product and critical metals. Leaching is the dissolution of valuable metals from ore or concentrates by aqueous and chemical attack. To improve leaching means minimize energy and reagent consumption. To do so WP3 tests innovative technologies based on dielectric heating, roasting, ozonation and ultrasound. The metal containing leachates obtained in WP3 are used in WP4 to investigate further recovery processes.
Develop novel, highly selective leaching technologies – i.e. dielectric heating assisted leaching, heap leaching, ozonation leaching and sonication leaching, as well as roasting – able to liberate more than 90% of the valuable metals present in stainless steel (SS) and FeCr slags. The leaching technologies should also ensure valorisation possibility for the remaining mineral matrix.
With the help of conventional and microwave roasting techniques Cr liberation rates >90% were achieved for selected slags. Other techniques were less efficient. Thus, further studies will concentrate on Cr recovery with conventional and microwave roasting. It was found that using KOH instead of NaOH during roasting resulted in higher Cr recovery at lower roasting temperatures. Due to selective alkaline leaching most of the matrix was preserved and will be tested for reuse in WP5 (upscaling and validation).
In this work package, the leachates obtained in WP3 are further treated to enable highly selective recovery of Cr, Nb, V and Mo. The targets for the recovery processes, the required purity and chemical form (pure metals, salts) of the final product of WP4 are based on indications provided by WP1.
WP4 focuses on the separation (e.g. from impurities or other target metals) and enrichment of the target metals (ions) in the leachates produced in WP3. The identification of a combination of technologies to achieve selective recovery of the target metals is carried out. Moreover, the separated metal ions are transformed into a product that meets market demands for applications, or the synthesis of other products.
Because of the complexity of the separation process, WP4 had to focus not only on several separation methods but also on the combination of different separation and recovery techniques to recover as many target metals as possible. Therefore, flowsheets were developed.
It was found that a combination of different methods is needed to recover the target metals – so every method involved in a process is essential for a successful metal recovery. Precipitation (+nanofiltration+reduction) allows the recovery of Cr as a Cr(III) basic chemical and NaOH (recycling for the leaching process), but could create several Cr(VI) containing washing solutions. The sorption process shows high potential for the removal of V(V) from solution but is still limited to synthetic solutions (limitations in the ratio Cr/V). Solvent extraction allows a quick separation of the target metals from impurities like Al or Si. The electrocoagulation process gives a total and fast removal of Cr(VI) from solution.
Overall, it was shown that the recovery of Cr can be achieved by the combination of the different techniques. Futhermore, new sorption materials with a high selectivity for vanadium(V) over chromium(VI) were synthesized and developed. Beside the fact that the current materials lose their selectivity in real leachate, they still have a high potential to be part of such a process after further optimization, or to be used for the solution of other challenging separation problems. Finally, two flowsheets were developed. That enables to choose a suitable process for the different types of materials which partially process significant differences in their composition (and therefore in the leachates).