Up-concentration of chromium in stainless steel slag and ferrochromium slags by magnetic and gravity separation

Currently, about 75% of the chromium (Cr) is being imported in the EU. In the value chain of stainless steel production, significant amounts of Cr are lost in ferrochromium (FeCr) slags and stainless steel (SS) slags. In a slag, Cr is mostly present in entrapped metallic particles (Fe-Cr alloys) and/or in stable spinels. Cr concentrations in SS and FeCr slags are in the range of 1-2% and 2-8%, respectively. To unlock the potential of these low-grade resources, a new approach to metal recovery must be deployed. The available technologies for recovering Cr from Cr-containing raw materials (both primary and secondary) are mostly based on roasting at high temperatures (>1000°C), what incurs high processing costs. Due to the high energy consumption, using such technologies for the recovery of Cr from low grade materials would not be economically feasible. It is obvious that in order to valorize low grade materials such as SS slags and FeCr slags, Cr up-concentration by physical separation needs to be carried out. The European H2020 project CHROMIC (GA No 730471) aims to develop a process for recovering Cr from SS and FeCr slags based on the smart integration of physical separation and subsequent pyro- or hydrometallurgical treatment. Results of physical separation show significant Cr up-concentration by using magnetic separation and gravity separation techniques, where Cr content can be increased almost by a factor of 4.


Introduction
In 2017, 48.0 Mt of stainless steel (SS) was produced globally with an average annual growth of 5.9% since 1950 (International Stainless Steel Forum (ISSF), 2018). Global FeCr production reached 13.3 Mt in 2018 with an average growth of 11% for China and 6% Rest of the World since 2011 (ICDA, 2017;Pastour, 2019).
During production of SS and FeCr, significant amounts of slags are produced as a by-product. Approximately 0.3 tonnes of slag is being generated per tonne of produced SS slag. In case of FeCr it is 1.1-1.6 tonnes of slags per 1 ton of FeCr (Niemela and Kauppi, 2007).
Concentrations of Cr in SS slag and FeCr slags are in the range of 1 to 2% and 2 to 8%, respectively (Spooren et al., 2016). These relatively high Cr contents limit direct valorization of these slags in the construction industry. Although several stabilization routes to transform soluble Cr 6+ to insoluble Cr 3+ have been investigated, large amount of waste generated by stabilization and presence of remaining Cr 6+ present major environmental concern (Kim et al., 2016a). These types of by-products could, however, become potential resources for the recycling and recovery of various valuable metals, such as Cr (Kim et al., 2016a) due to changing economic boundary conditions and technological innovations. It has been estimated that in 2000 in Europe (including Turkey) almost 74,900 t/y and 18,700 t/y of Cr was lost in FeCr and SS slags, respectively (Jhonson et al., 2006). Taking into account the increase in SS and FeCr production, nowadays, the amount of Cr lost in SS and FeCr slags will be significantly higher.
The traditional process of recovering Cr from Cr ores (containing 36-56% Cr2O3), is based on roasting with Na2CO3 at a temperature above 1000°C (Kim et al., 2016b). This process could also be used to recover Cr from SS and FeCr slags, however, since this process requires high temperatures, it is not economical for the treatment of low grade material containing only a few percentages of Cr (Kim et al., 2015).
It is evident, that in order to make Cr recovery from SS and FeCr slags economical by traditional processes, the Cr content needs to be increased significantly. This could be achieved by applying mineral processing technologies like magnetic and gravity separation. These physical separation technologies, were already thoroughly described by several authors for Cr up-concentration in primary Cr ores (Murthy et al., 2011;Tripathy et al., 2016Tripathy et al., , 2015Tripathy et al., , 2012. However, due to the depletion of reserves of high-grade primary Cr ores and increasing global demand for SS and FeCr, attention needs to be aimed to secondary low grade raw materials as well.
The main aim of this study was to investigate the potential of magnetic and gravity separation techniques in up-concentration of Cr present in SS and FeCr slags, what could significantly improve the economics of Cr recovery from these by-products.
2 Experimental procedure 2.1 Material One type of SS slag and two types of FeCr slags (Table 1) were studied in this work. Samples of Low carbon (LC) FeCr and High carbon (HC) FeCr slags were delivered as size fractions 4-9 mm and 0-5.6 mm, respectively. SS slag was delivered as a fine powder with D90 = 150 µm.

Characterisation of the samples
Representative sub-samples were taken from initial samples delivered by their producers. These subsamples were dried to constant weight at 100 °C. After drying, samples were crushed, milled and sieved. Fraction < 125 µm was used for chemical and mineralogical analysis. Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) was carried out for the original size fractions.
Chemical analyses was performed by a high performance energy dispersive X-ray Fluorescence spectrometer (EDXRF) with polarized X-ray excitation geometry (HE XEPOS, Spectro Analytical Systems, Kleve, Germany). The instrument was equipped with a 50 W tungsten end window tube (max. 60 kV, 2 mA) and a Silicon Drift Detector. For signal optimization different targets were applied. All analysis were performed under He atmosphere. The dried and finely ground (< 125 μm) samples were analysed as fused beads. The quantification was performed using a precalibrated software package for the quantitative analysis of geological fused bead.
In addition to EDXRF, a handheld XRF analyser (Niton XL3t GOLDD+) placed in a mobile test stand, was used for fast elemental analysis of intermediate and final fractions produced by magnetic or gravity separation. The XRF analyser is equipped with a Ag anode (50 kV and 0.2 mA). Measuring time was 120 seconds for every measurement. This XRF analyser was chosen for analysis based on initial comparison with ICP-OES, EDXRF (powder) and EDXRF (Fused bead) which gave similar values for Cr and Fe in particular.
Mineralogy was determined by X-ray powder diffraction (XRD), carried out with a PANalytical Empyrean system, operated at 40 kV and 45 mA, with Co tube (fine focus, wavelength = 1.7903 Å). Continuous scans with a step size rate of 0.013°/ 49.725 s were performed within a 2θ range of 5°-120° (2D-detector). The obtained diffractograms were qualitatively analyzed with the aid of HighScore Plus software (version 4.6a).
For SEM-EDS analysis, the samples were embedded in a low-viscosity epoxy resin and gradually polished down to ¼ μm diamond powder grit size. The samples were coated with Pt-Pd prior to microscopic analysis. Morphological observations were carried out by a SEM microscope FEI NOVA NANOSEM 450 with EDX analyser BRUKER QUANTAX 200 with SDD detector.

Magnetic separation
Several magnetic separation techniques were employed in order to up-concentrate Cr in the SS and FeCr slags. Based on preliminary experiments, the most suitable magnetic separation techniques were selected for a certain slag. All the magnetic separation equipment was delivered by Master Magnets Ltd.. Magnetic separation techniques and their allocations to certain slag materials are listed in Table 2.  LIMS/HIMS was carried out by spreading a dry sample of ca. 200 g into a thin layer on a flat surface. The permanent magnet (ferrite or RARE) was covered by a plastic bag for convenient removal of the magnetic fraction.
WLIMS/WHIMS was carried out on a wetchute with a permanent magnet (ferrite or RARE). A sample of 10 g was mixed with 500 mL water (L:S ratio = 50), and the suspension was fed at the top of the wetchute. Resulting fractions were dried for 24 hours at 40°C before XRF analysis.
WHIMSe experiments were carried out using WHIMSe separator (Master Magnets Ltd.). Steel wool was used as a separation matrix. The magnetic field in WHIMSe can be adjusted from 0-2 T by adjusting the electric current (0-17.5A) in the coils. As it is difficult to measure the magnetic field strength when steel wool is used as a matrix, in this paper the WHIMSe set-up is reported in Amperes. Material was first fed to the WHIMSe at the maximum current of 17.5 A and the non-magnetic fraction was washed out. The magnetic fraction was recovered by turning off the magnetic field and washing with water.
In order to find out the optimal set up of the WHIMSe for Cr up-concentration, sequential experiments were carried out by gradually decreasing electric current. In this case, after washing out the non-magnetic fraction at the highest current (17.5 A), the current was not turned off completely but reduced to 15, 10, 5, 1 and 0 A, and the non-magnetic fraction was washed out at every step. The final fractions were dried for 24 hours at 40°C and analysed by XRF.

Gravity separation
The gravity separation test was carried out using a Holman-Wilfley wet shaking table (WST). This gravity separation by WST was performed on the HC FeCr slag, fraction 1-2 mm, only. Approximately 360 g of the HC FeCr slag was used for the separation test. The stroke speed was set at 250 rpm. The washing water flow was set at 1000 L/h and the feed dilution water at 250 L/h. The final fractions were dried for 24 hours at 40°C and analysed by XRF. The WST with the indication of outputs for different fractions (fraction 1 = the lightest; fraction 5 = the heaviest) is shown in Figure 2.  Table 3 shows the content of the main elements in studied slags analysed by EDXRF. Nb mg/kg 617 (1) <LLD <20 (2) (1) Only 1 measurement above detection limit (2) Measured by ICP-OES (3 replicates)
As can be seen from SEM-EDS analysis, the matrix of the SS slag consists of Ca-silicates and Ca-Mg-Al-silicates (Figure 3a). Cr-spinel phases can be clearly identified, showing a potential for physical separation. However, due to the small dimensions of the Cr-rich particles, full liberation will require additional milling.
The SEM-EDS analysis of LC FeCr slag (Figure 3b) shows elongate grains of Ca-silicates in between of which veins of spinels are present. Small (20-40 µm) metallic FeCr particles are spread out diffusely over the material. As shown in Figure 3b, some of them are encapsulated in Ca-silicate matrix. In case of HC FeCr slag (Figure 3c), Cr-rich grains (mostly as FeCr particles) and Ca-silicates grains are already very well liberated indicating potential for their separation.

Magnetic separation -SS slag
A combination of WHIMS and WHIMSe was used to process SS slag. The final flowsheet with chemical compositions of input, intermediate and final fractions are shown in Figure 4. The SS slag, as received (< 150 μm), was treated by WHIMS, where the magnetic fraction was purified by a second WHIMS separation step and the non-magnetic fraction was processed by a single WHIMSe separation step (at 17.5A). Three output fractions (M2, NM2+M3, NM4) with different Cr contents were obtained. Cr-rich fraction (M2) contains 8.8% Cr, what represents an increase of almost a factor of 4. The weight yield of the Cr-rich fraction was 7 % with 31% of Cr recovery.

Figure 4 Processing of SS slag by combination of WHIMS and WHIMSe.
A significant up-concentration of Cr (but also V and Nb) can be observed with decreasing electric current during sequential WHIMSe separation. As shown in Figure 5, the Cr content of the magnetic fraction increases with decreasing magnetic field, indicating than Cr-rich particles are those with the highest magnetic susceptibility. The highest Cr content (8.3%) was measured in the fraction washed out at current 0 A (final magnetic fraction). In this case weight yield was 5% with 21% Cr recovery. On the other hand, fraction washed out at the highest magnetic field (at 17.5 A) with weight yield 73% contains only 0.7% of Cr representing only 22% of initial Cr, which means 78% Cr removal by using WHIMSe.

Magnetic separation -LC FeCr slag
A combination of HIMS and WHIMSe was applied for processing LC FeCr slag of size fraction < 500 µm (obtained by crushing and sieving). The processing steps with mass balance and chemical composition of different streams are shown in Figure 6. As shown in Figure 6, the < 500 µm fraction was first sieved into < 125μm and > 125μm fractions, which were treated by different techniques. HIMS was used to treat the fraction > 125μm while single WHIMSe (at 17.5A) was used for treating the fraction < 125 μm. After HIMS a slight Cr up-concentration was observed in the magnetic fraction (M1). However, the difference in Cr content before and after separation (3.1% vs 3.4%) is probably too small to justify the separation costs. On the other hand, treating the fraction < 125 μm by WHIMSe shows a more significant Cr up-concentration in the magnetic fraction (M2) compared to the fraction < 125 µm before WHIMSe (7.3% vs. 2.9%). This is caused by a better liberation of the Cr-rich phases in fraction < 125 μm as shown by SEM-EDS analysis (Figure 3b). However, the weight yield of fraction M2 was only 3% what represents only 7% Cr recovery. Figure 7 shows the results of sequential WHIMSe with decreasing electric current. Unlike in the case of SS slag, the fraction with the highest Cr content (11%) was obtained after washing at a current of 5 A. The weight yield and Cr recovery were 3% and 14%, respectively.
Based on the results shown in Figure 6 and Figure 7, it can be concluded that to up-concentrate Cr present in LC FeCr slag by magnetic separation requires a milling step to liberate Cr-rich particles from the matrix. However, considering the very low weight yield of the Cr-rich fraction and thus low Cr recovery, the economic feasibility of using a magnetic separation for LC FeCr slag is questionable.

Magnetic separation -HC FeCr slag
A combination of low and high intensity magnetic separation techniques was used to separate Cr-rich phase from Si-rich matrix. As observed in the SEM images (Figure 3c), relatively big fully liberated FeCr particles of around 500 µm diameter, can be found in the HC FeCr slag. For this reason, magnetic separation tests were performed on a wide range of particle sizes (from < 125 µm to > 2 mm). Depending on the particle size either dry or wet magnetic separation techniques were used. The fractions > 500 μm were separated by dry magnetic separation and the fractions < 500 μm by wet magnetic separation techniques. Figure 8 shows the separation process with three final output fractions.

Figure 8 Combination of low and high intensity magnetic separation used for HC FeCr slag
(LIMS and HIMS for fractions > 500 µm and WLIMS and WHIMS for fractions < 500 µm).
Before magnetic separation, the material was split into different size fractions. Every size fraction was analysed by XRF in order to see whether Cr tends to concentrate in one of the size fractions. As seen from Table 4, the Cr content is the highest in the smallest fraction (< 125 μm). As Cr is mostly present as a FeCr alloy, the Fe content follows the same trend. All size fractions were subjected to the magnetic separation according to Figure 8. As shown in Figure 9, the amount of the strongly magnetic fraction (M1) increases with decreasing particle size indicating that FeCr particles are mainly present as particles smaller than 250 µm. Figure 10 clearly shows that Cr is being concentrated in the M1 fraction up to a level comparable with Cr-ores (red dashed line in Figure 10). This indicates that 17 to 54% (depending on the particle size) of the HC FeCr slag could already be used, after a low intensity magnetic separation, as a potential replacement of primary Cr ores.
The behaviors of other elements (Fe, Si and V) are shown in Figure 11, Figure 12 and Figure 13. It is clear that, Fe and V follow the same trend as Cr, while Si concentrates more in the non-magnetic (NM2) fractions.    The final recovery of Cr, Fe and V is shown in Figure 14. The highest recovery of Cr, Fe and V (76, 86 and 76%) was obtained for size fraction 125-250 µm. This is mainly due to the better liberation of Cr (Fe and V) rich phases. A slightly lower recovery for the fraction < 125 μm was due to the presence of very fine particles, which are difficult to separate by any physical separation method.

Gravity separation -HC FeCr slag
Fraction 1-2 mm was selected as input material for the gravity separation test. Five fractions ( Figure 15) were obtained after gravity separation using a wet shaking table (WST). The mass balance of the output fractions is shown in Figure 16. All fractions were dried (24 hours at 40 °C) and milled below < 125 µm for XRF analysis. Results of XRF analysis are shown in Figure 17. Due to the very small amounts and visual similarity of fractions 4 and 5, these two fractions were combined and analysed as one fraction.   The combined fraction (4+5) contains more than 38% Cr ( Figure 17). However, this fraction represents only 0.3% of the initial material. By further combining with fraction 3, the Cr content of the combined sample would stay high (> 32% Cr) and the weight yield would increase to almost 12%, representing 36% Cr recovery. Due to high Cr content, this material could be possibly used as a replacement for primary Cr ores in FeCr production.
In the case of SS slag, the Cr content can be increased from 2.3% to 8.8% in the magnetic fraction after two WHIMS separation steps. From the perspective of matrix valorization, the non-magnetic fraction (Cr-depleted) after two WHIMS steps contains only 0.7% Cr, representing 78% Cr removal.
The Cr content in LC FeCr slag can be increased from 3.0% up to 11% by optimized WHIMSe separation. However, in this case the weight yield (3%) and total Cr recovery (14%) are relatively low. Moreover, to achieve such Cr up-concentration, the slag needs to be milled down to < 125 µm, what incurs significant costs. Considering the facts above, the economic feasibility of processing this LC FeCr slag by magnetic separation is questionable.
HC FeCr slag contains relatively big (up to 500 µm) very well liberated Cr-rich particles, mostly as FeCr, what indicates potential for magnetic and density separation. After a 1-step low intensity magnetic separation (wet or dry), the Cr content can be increased to more than 24.6%, what is considered the minimum Cr content for a potential Cr ore.
After density separation of the HC FeCr slag (1-2 mm) by WST, the Cr content increased to more than 38%. By combining all fractions with Cr content higher than 32%, a Cr-rich material representing 12% of the initial slag weight can be obtained. The total Cr recovery in the Cr-rich fraction was 36%.
Based on the results in this paper, it can be concluded that magnetic and gravity separation show significant potential for being used as pretreatment techniques for the recovery of Cr form SS and FeCr slags prior to further processing.