Characterization and Processing of Low-Grade Middle Group 2 Chromite Ore by Gravity Shaking Table and a Comparative SLon Magnetic Separation: A Case Study

Abstract

Chromite is considered a strategic mineral in the global economy. It is mainly used as an essential raw material in the production of stainless steel and other metal alloys due to its corrosion and heat resistance properties. High-grade chromite resources are gradually depleting; with the increasing chromite demand in metallurgical applications, studies have focused on exploring low-grade and alternative chromite sources. This study proposes a cost-effective processing flowsheet for the low-grade middle group 2 (MG2) chromite layer, a poorly explored chromatite seam within the South African bushveld igneous complex (BIC). The study involved mineralogical characterization followed by gravity and magnetic separation of the low-grade MG2 ore at 18.18% Cr₂O₃. Characterization by XRD and Auto-SEM revealed that the ore mainly consists of pyroxene, chromite, and feldspar, with other minerals in trace quantities. The gravity separation test by shaking table upgraded the chromite (Cr₂O₃) to 42.0% at high chromite recoveries, whereas the laboratory Slon wet high-intensity magnetic separation method (SLon WHIMS) upgraded the chromite in the feed to 42.95% grade at lower chromite recoveries. Desliming the sample before the gravity and magnetic separation tests significantly improved the separation. The magnetic separation tests further demonstrated that chromite within the MG2 layer is sensitive to magnetic separation due to its high iron content. The adapted flowsheet is proposed as a cost-effective flowsheet for processing the low-grade MG2 layer. The flow sheet can be optimized by conducting the SLon WHIMS tests at high intensities followed by fine gravity tests by spiral circuits to maximize the chromite recovery while achieving commercial chromite grades and a Cr:Fe ratio greater than 1.5.

1. Introduction

Chromium, primarily extracted from chromite occurring as ferrochrome, is an essential raw material in the production of steel, stainless steel, plating metals, alloy steel, and other alloying metals. It is one of the most versatile metals with applications across various industries, including refractory, metallurgical, chemical [1], catalysis, lithium-ion batteries, etc.

Chromium increases strength as well as heat and corrosion resistance in steel applications [2]. About 90% of mined chromite is concentrated into different grades of ferrochrome [3]. Approximately 80% of this ferrochrome (mostly high carbon/charge grade) is used as a raw material in stainless steel and steel making [3]. The Bushveld Igneous Complex (BIC) of South Africa hosts the largest viable stratified chromite reserves globally in several seams, approximated at three-quarters of the world’s viable chromite deposits [4]. The chromatite resource in the BIC is hosted by a series of laterally continuous and mineralogically similar chromatite layers in the Critical Zone (CZ) of the Rustenburg Layer Suit (RLS) [5]. The chromatite layers within the BIC are divided into three groups: upper group (UG1—2), middle group (MG1—4), and lower group (LG1—7) [6]. Most upper and lower group layers have been explored extensively for both chromite and PGM recovery, while less attention has been paid to layers within the middle group.

Studies have shown that over the years, only three seams have been proven economically viable due to high average chromite concentrations requiring minimal processing steps; these are UG2 (upper group 2), LG6 (lower group), and MG1 (middle group) [7]. UG2 seam is primarily used as a PGE source, and the chromite is rejected as tailings [7]. Due to massive chromite within this seam, approximated between 60–90 vol % with an average Cr/Fe ratio between 1.3 and 1.4, UG2 has been considered the most economical seam for both chromite and PGM extraction [8]. Recently, the layers within the middle group have been gaining attention, mostly the MG2–MG4 seams, for chromite extraction. This is due to dwindling high-grade chromite reserves. Other studies focus on reprocessing the chromite tailings to maximize recovery and meet the rising stainless-steel demand. This study focuses on recovering chromite from a low-grade MG2 ore. Two techniques, magnetic versus gravity, were compared with the aim of determining a more effective processing route.

The middle group layers occur in the critical zone of BIC, with MG2 occurring directly above the MG2 layer, as seen in Figure 1. The thickness of the MG1 layer within the BIC ranges between 1.6 m and 1.87 m [7,9], while that of the MG2 seam ranges between 1.85 m and 2.5 m (Glencore, Baar, Switzerland).

Matrix Reference Material, 2024, indicated that, the MG2 seam comprises three chromatite layers separated by feldspathic pyroxenite and is subdivided into MG2 A, B, and C. MG2A and MG2B usually occur as one layer but are differentiated by their chemical compositions. Of the three subdivisions, MG2C contains the highest content of PGMs, followed by MG2A. According to Tharisa’s integrated annual report 2023, MG2B has a much lower PGM content in comparison. Typical chromite content within the middle group layer ranges between 20 and 35 wt% [7].

Chromite production in South Africa has increased over the years due to the growing demand of ferrochrome for steel production in China and rising prices per tonnage. Figure 2 presents the data extracted from U.S. Geological surveys showing chromite production over the years from 2010 to 2023.

South Africa has consistently contributed the largest portion, over 40% of global chromite production over the years, at an annual average of 44,500 tons between 2010 and 2023 [10]. The global chromite market is expected to increase at an annual growth rate of 6.61% between 2023 and 2030. This rise in the chromite market is driven by the growing demand for ferrochrome use, with more than 90% of chrome used for metallurgical purposes. Other demand is derived from chemical, foundry, and refractory industries, and the development of new technology, such as the use of chromium as a cathode material in lithium-ion batteries, contributes to the growing chromite demand. As a result, the easy-to-access (high-grade) chromite deposits are slowly depleting, and the chromite extraction from low-grade reserves is gaining attention among chromite production mines, especially in South Africa. Some mines have started to explore other chromite layers within the middle group, including MG2, MG3, and MG4 [11]. Additionally, more research has been directed towards processing fine tailings for chromite recovery in order to sustain and possibly extend the life of the mines. According to Glencore’s resources and reserves report 2023, their eastern chrome mines within the eastern limb of BIC have both MG1 and MG2 resources. The estimated total MG2 reserves are 2.3 Mt at 26.70% Cr₂O₃, while the MG1 reserves are estimated at 25.3 Mt averaging at 34.68% Cr₂O₃ and 5 Mt tailings averaging at 18.81% Cr₂O₃. Currently, Glencore targets LG6/LG6A for economic exploitation in their western operations, while the MG1 layer is mined in their eastern operations. According to Glencore’s resource and reserves report, 2023, alternative layers, including MG2, are being investigated on a continuous basis. Samancor Chrome EIA/EMPr Report, 2017, states that Samancor Chrome currently explores LG6 as the most economically viable seam due to its high chrome content and thickness consistency. The middle group seams (MG2, MG3, and MG4) are being investigated as potential viable chromite sources, further indicating the importance of investigating MG2 as a viable chromite source. Therefore, there is a need to develop robust and cost-effective processing flowsheet for the chromatite layers within the middle group seam.

Several studies have indicated that gravity separation is well-established for chromite recovery [12,13,14,15]. A typical chromite processing flowsheet follows the crushing, screening, and milling of the ore; classification by wet-screening or desliming hydrocyclones; dense media separation (DMS cyclone for coarse processing) or spiral circuits and tabling [16]. Chromite is paramagnetic and can be separated from the associated gangue by magnetic separation at high intensities (HIMS). However, the reported literature indicates that magnetic separation in chromite processing has mainly been used to improve the Cr:Fe ratio of the concentrate [3,17]. Currently, the literature does not indicate magnetic separation as a potential pre-concentration or alternative to gravity separation for chromite minerals.

Although South African mines have started to explore the layers within the middle group, there is limited reported literature on the processing of the middle group 2 seam (MG2). This study focused on developing a cost-effective flowsheet for processing a low-grade MG2 ore from the Bushveld complex. Magnetic separation was compared to gravitational separation.

2. Materials and Methods

About 250 kg of South African low-grade MG2 chromite run-of-mine (ROM) ore at 150 mm top size was obtained from a mine. The ore was dry-screened at 1 mm using a Sweco vibrating screen to remove natural fines before crushing. Approximately 25.43% of the ore was finer than 1 mm. The screen oversize was stage-crushed using a combination of jaw and cone crushers to ensure that 100% of the sample passed 1 mm screen in order to liberate locked chromite mineral from gangue material. Natural fines and crushed ore were blended, and the composite was split by a combination of rifle and rotary splitters to remove various representative subsamples for characterization, gravity concentration by shaking table, and magnetic separation tests by a laboratory SLon wet high-intensity magnetic separation (SLon WHIMS).

Feed characterization involved duplicate head chemical analysis on 200 g; particle size distribution (PSD) using screens from 850 to 25 µm; assay by size; and mineralogical characterization by X-ray diffraction (XRD), automated scanning electron microscopy (Auto-SEM), and electron probe micro analysis (EPMA). The particle size distribution was conducted in duplicates on 3 kg samples to ensure consistency; an average PSD was taken. The following screens were used to conduct the PSD: 850, 600, 425, 300, 212, 106, 75, 53, 38, and 25 µm. During the PSD, each sample was initially wet-screened at 25 µm; a vibrating screen was used to liberate any agglomerated particles and ensure that the −25 µm fines were effectively screened out. The oversized fraction was then dried and screened on a Pascal Test Sieve Shaker staked with screens from 850 to 25 µm. The resulting mass accumulated on each screen was recorded. Representative subsamples from each screen were taken and chemically analyzed to determine chromite and major gangue deportment across size classes (assay by size). Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) ferrochrome slag (FeCr_Slag) method was used for all chemical analyses.

After the characterization tests, approximately 15 kg of crushed ore was subjected to classification by wet-screening at 75 µm and subsequent hydrocycloning. A 3-stage hydrocyclone was conducted on the wet-screen undersize stream. The aim of the multi-stage cyclone was to maximize fines removal, while wet-screening was aimed at minimizing particle size range subjected to the cyclone. The cyclone test was conducted progressively in three stages such that the underflow from the previous step was fed to the next cyclone. The cyclone underflow from the last stage was blended with the screen oversize (−1000 + 75 µm) to prepare a composite for shaking table and SLon WHIMS tests. Figure 3 summarizes the process followed in the flowsheet.

2.1. Mineralogical Evaluation

The sample was characterized by X-ray diffraction (XRD), automated scanning electron microscope (auto-SEM), and an electron probe micro analyzer (EPMA) to determine mineral phases, mineral abundance (mass %), chromite liberation, and chemical composition.

In preparation for mineral characterization, a representative 2 kg of crushed sample was prepared. Three representative aliquots were subsampled from the 2 kg and prepared for analysis by XRD, Auto-SEM, and EPMA. The XRD aliquot was prepared into pucks, while Auto-SEM and EMPA aliquots were prepared into 90° bottle-mounted polished sections. XRD (Bruker D8 Advance powder X-ray diffractometer with a Lynxeye detector with Fe-filtered Co-Kα radiation type) was used to identify mineral phases. Auto-SEM analysis utilizing particle mineral analysis (PMA) measurement mode was used to determine the bulk modal composition of the sample. Auto-SEM analysis also determined the Cr-bearing minerals, their relative modal proportions, and liberations. EPMA (JEOL 8230 Super Probe) equipped with wavelength dispersive spectrometers (WDS) for quantitative compositional determinations as well as an energy dispersive spectrometer (EDS) for qualitative elemental abundances was used to determine chemical composition and thus theoretical chromite and iron content from chromium-bearing mineral phases. Finally, inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis was conducted to reconcile bulk modal mineralogy results.

2.2. Classification by Wet-Screening and Desliming Hydrocyclone

Fines are often generated during crushing stages; if not minimized, they result in inefficiencies during downstream processing. Both wet-screening and hydrocyclone were used to remove fines from the sample sequentially. The crushed sample was initially wet-screened at 75 µm to reduce the particle size range in the feed to the cyclone while breaking any agglomerated particles. The sample finer than 75 µm (screen undersize) was fed to a 3-stage hydrocyclone operated at different pressures. The cyclone was aimed at targeting and removing 10 µm particles to the overflow stream. During the test, a slurry with 20%–25% pulp density was prepared and pumped to the cyclone operated in batch stages at three pressure gauges, i.e., 200 kPa, 150 kPa, and 100 kPa cutpoints progressively. The cyclone was operated such that the underflow stream from each cyclone was fed to the next pressure cyclone operated at a different pressure gauge. The final underflow stream was blended with the oversize (−1000 + 75 µm) fraction from wet-screening in preparation for gravity and magnetic separation tests.

2.3. Shaking Table Gravity Separation Tests

A shaking table is considered an efficient gravity separation technique for fine chromite processing [15]. In chrome operations, shaking tables are often used to benchmark spiral circuits. They separate ore particles primarily based on differing densities by means of mechanical shaking and water flushing on an inclined riffled deck. The riffled deck, vibration, and cross-flowing water promote particle separation such that heavy particles collect towards one end while light particles are carried by water to the other end of the deck [18]. This unit is typically used when there is a significant difference in the specific gravity of target mineral and associated gangue [19]; the differential density facilitates the separation. Other factors, such as particle shape and size, may influence separation [20].

During the test, approximately 7 kg of deslimed representative sample was prepared and processed on a standard 1/8 Wilfley shaking table at feed rate and water flowrate conditions described in

Table 1. Shaking table technical parameters and operating conditions.

Parameter	Value
Table speed [rpm]	250
Pulsation [stroke/s]	5
Tilt angle	3–4°
Dry feed rate [kg/h]	13.61
Water in cone [L/min]	200
Water in mixing box [L/min]	200
Spray water [L/min]	300

. The tilt angle was adjusted between 3 and 4° based on the separation profile on the riffled deck. The data obtained from the tests can be used to establish grade and recovery profiles and thus determine the best attainable chromite grades and recoveries that can be achieved in a closed spiral circuit operation.

The products were collected, filtered, and dried; masses recorded; and subsamples taken and pulverized for chemical analysis by ICP_FeCr slag method.

2.4. Slon Wet High Intensity Magnetic Separation Tests

Due to the paramagnetic nature of chromium and the presence of iron in ferrochrome, most chromite ores are susceptible to magnetic separation; however, high intensities (HIMS) are required to recover chromite. One such unit is a vertical ring pulsating SLon wet high-intensity magnetic separator (SLon WHIMS). An electromagnetic separator equipped with a feed box, an actuated diaphragm (which provides the pulsating motion), and matrix rod inside the vertical separating zone/ring connected to the power supply. The SLon magnetic separator works on the principle that a combination of the generated magnetic field by a direct current, a pulsating fluid motion, and a gravitational force separates paramagnetic particles from non-magnetic particles [21]. In this unit, a slurry is introduced to the matrix through the feed box. As it passes through the magnetized separation zone, the non-magnetic particles pass through the magnetized matrix and are collected in a bucket, while the magnetic particles are attracted to the matrix. The water is continuously flushed down the matrix to separate any agglomerated particles [18]. The pulsating motion of the actuated diaphragm inside the separating zone keeps the particles loose, minimizing entrainment, thus improving separation. Once all non-magnetic particles have been collected, the current supplying the energy to the electromagnetic coils is turned off. As a result, the matrix loses the magnetic field, and the magnetic particles loosen; as the water flushes, they fall off the matrix by gravity and are collected on a separate bucket. This laboratory unit is designed for processing samples finer than 1 mm, typically between 100 and 300 g per test depending on the density and the proportion of magnetics in the feed sample. Figure 4 shows the schematic diagram of the SLon-100 whims used in the tests.

During the test, various parameters, including cooling water pressure, field intensity (adjusted each time), and pulsating frequencies, were set at desired points. The water level inside the mixing chamber of the Slon WHIMS was adjusted, and the deslimed dry sample was fed (500 g per intensity) in 100 g consignments per test such that a slurry of 15%–20% solids was prepared inside the feed box; the current generated a magnetic field inside the separating zone (matrix rod inside feed box). Multiple tests (5 times) were conducted to ensure consistent mass splits between magnetic and non-magnetic streams under the same conditions. This unit was operated at conditions summarized in

Table 2. SLon WHIMS operating conditions.

Feed Size (mm)	Feed Solids (%)	Pulsating Stroke (mm)	Pulsating Frequency (Hz)	Cooling Water Pressure (MPa)	Field Intensities (A)	Matrix Size (mm)
−1	15%–20%	10	60	0.4	200–1200	1.5

. The magnetic particles were attracted to the rods of the matrix, while the non-magnetic particles were allowed to pass through and were collected in buckets. The current was switched off, and the magnetic particles were collected in different buckets. At each intensity, two products, viz. magnetics and non-magnetics, were obtained, filtered, and dried. The mass splits were recorded, and representative subsamples were pulverized and prepared for chemical analysis.

3. Results and Discussions

3.1. Head Chemical Analysis, Particle Size Distribution, and Assay by Size

Table 3. Head chemical analysis results by ICP_FeCr_Slag method.

Al2O3	CaO	Cr2O3	FeO	MgO	SiO2
%	%	%	%	%	%
12.68	4.29	18.18	16.45	14.98	33.80

shows the head chemical analysis of the bulk sample. The sample contained approximately 18.18% Cr₂O₃ (12.43% Cr), with SiO₂ as the dominant constituent at 33.8%. With FeO at 16.45% (11.14% Fe), the resulting Cr:Fe ratio is 1.12. Silica occurs mostly in pyroxene, which is the most abundant mineral in the sample, and is also distributed in minerals such as quartz, feldspar, chlorite, and talc (refer to bulk mineralogy in supplementary material). Both chromium (Cr) and iron (Fe) occur in chromite, although traces of Fe are also distributed in other minerals such as pyroxene, chlorite, and mica.

Figure 5 shows a particle size distribution (PSD) of the sample crushed to 100% passing 1 mm. The results show that about 8% of the screened sample was finer than 38 µm. Approximately 80% (P₈₀) passed 350 µm, and similarly, 50% (P₅₀) reported below 180 µm.

The discrete mass, Cr₂O₃ grade, and Cr₂O₃ deportment across size classes (assay by size) results are illustrated in Figure 6. The results show that Cr₂O₃ is dominant between −425 µm and +75 µm sizes, with an overall mass of approximately 68% being reported within this size range. Cr₂O₃ deportment closely followed the discrete mass, with lesser deportment on the coarser than 425 µm and finer than 75 µm sizes, indicating that chromite minerals were neither fine-grained nor too coarse-grained.

3.2. Mineralogical Characterization

3.2.1. Bulk Mineralogy by Auto-SEM

The polished section of the MG2 sample was studied to determine the bulk modal mineralogy by Auto-SEM. The analysis showed that the sample was primarily composed of chromite, pyroxene, and feldspar, with quartz, chlorite, talc, mica, and other minerals in trace amounts. The mineral abundance of the sample in mass percentage is given in Figure 7. The results show that pyroxene (Ca, Na, Fe²⁺, Mg) Si₂O₆) is the dominant mineral at 40.51% mass, followed by chromite (Cr₂FeO₄) at 35.34% mass and feldspar (KAlSi₃O₈-NaAlSi₃O₈-CaAl₂Si₂O₈) at 18.68% mass. As expected, pyroxene was the most abundant mineral; this is because the middle group chromatite layers fall under the pyroxenite (anorthosite) lithology [22]. Furthermore, the MG2 seam comprises three chromatite layers separated by feldspathic pyroxenite, therefore, resulting in high pyroxene and feldspar concentrations in the sample. It was observed that silica (Si) was present in all minerals detected in the sample except in chromite; this justified its dominance in the head chemical analysis.

3.2.2. Chromite Liberation and Mineralogical Characteristics

The grain size distribution presented in

Table 4. Chromite grain size distribution.

Class (µm)	Mass %	Grains
<50	24.3	14,003
50–70	6.2	3086
70–100	12.2	3360
100–150	21.6	2509
150–200	13.5	436
200–300	12.7	124
300–400	3.1	26
≥400	6.4	34

shows that approximately 60 mass % of chromite grains in the sample ranged between 70 and 300 µm. The grain distribution also shows that about 24 mass % of chromite grains reported below 50 µm, thus becoming distinctively fine-grained. These results correspond to the assay-by-size results presented in Figure 4.

The sample exhibited 70 mass % liberation of the chromite grains at 80% liberation, which was reasonably high. The liberation across different size classes was studied, and the results are presented in Figure 8. The fraction coarser than 600 µm was the least liberated. The overall chromite grains larger than +212 µm were notably less liberated than the grains under 212 µm size classes. Chromite grains below 212 µm were mostly liberated and ranged from 75 to 84 mass % higher than 80% liberated.

The finer-grained chromite particles below 75 µm were notably poorly liberated compared to grains within the −212 + 75 µm range. Additionally, the fine-grained chromite particles were notably locked within larger particles of gangue, thus reducing the grade and recovery. Finer milling may be considered to liberate more chromite and improve its liberation. However, milling will not improve the Cr recovery, and over-milling may also result in a loss of grade. Figure 9 shows the backscattered image of the sample by Auto-SEM.

The mineralogical characteristics of the sample show that the ore is predominantly composed of pyroxene and chromite with lesser feldspar, talc, and other accessory mineral phases. This was observed in both quantification (Figure 7) and backscattered images (Figure 9). This sample exhibits slightly higher Fe content compared to other seams within the middle group; however, the Fe range was still within the typical range for MG Bushveld chromite [8,23]. It has been demonstrated that there is an increase in the Fe content in this chromatite layer, resulting in a direct increase in magnetic susceptibility [24,25]. Therefore, it has a greater potential for chromite recovery from gangue by magnetic separation.

3.2.3. Electron Probe Micro Analyzer (EPMA)

The microprobe analysis was conducted on the chrome-rich spinel phase in the sample with the aim of determining the theoretical chromium and iron compositions. The average EPMA data is provided in

Table 5. Chemical composition by electron probe micro analysis results (EPMA).

Oxides	Count	Min	Max	Mean	1σ
TiO2	54	0.6	1.5	0.8	0.1
Al2O3	54	12.8	16.1	14.9	0.6
Cr2O3	54	43.4	47.0	45.9	0.6
VO2	54	0.3	0.5	0.4	0.1
FeO	54	27.7	30.9	28.5	0.6
MnO	54	0.1	0.2	0.2	0.0
MgO	54	7.0	9.0	8.4	0.4
NiO	54	0.1	0.2	0.1	0.0
Total	54	98.5	99.7	99.2	0.3

. Chromite grains were examined, and their compositions were grouped together and averaged based on a typical spinel compositional formula given by AB₂O₄, where A = divalent cations (Fe²⁺, Mg) and B= trivalent cations (Fe³⁺, Al, and Cr) [26]. The average composition of the chrome-spinel in the sample was as follows: 45.9% wt. Cr₂O₃, 28.5% wt. FeO, 0.8% wt. TiO₂, 14.9% wt. Al₂O₃, 8.4% wt. MgO, etc. The resulting chromium-to-iron (Cr:Fe) ratio for chrome-rich spinel is 1.63. These results indicate that this ore can be processed to commercial chromite concentrate grades (>40%) with a Cr:Fe ratio commercial specification of ≥1.5 [27,28,29].

3.2.4. X-Ray Diffraction (XRD)

Semi-quantitative XRD analysis was conducted; and the results corroborated the mineral abundances determined by Auto-SEM bulk modal mineralogy. The analysis results show that the MG2 sample contained mainly chromite, pyroxene, and feldspar, with less chlorite and other minerals. Additionally, amphibole mineral, present in trace amounts of approximately 1% in the sample, was detected. Figure 10 shows the XRD diffraction pattern.

3.3. Wet-Screening and Laboratory Desliming Cyclone

The wet-screening results presented in

Table 6. Wet-screening results at 75 µm.

		Grade (%)			Recovery (%)
Stream	Mass (%)	Cr2O3	FeO	SiO2	Cr2O3 (%)	FeO	SiO2
Oversize	75.1	16.94	16.50	34.18	69.76	72.91	83.58
Undersize	24.9	22.15	18.49	20.25	30.24	27.09	16.42
Total	100	18.24	17.01	30.71	100	100	100

show that approximately 75.1% of feed was coarser than 75 µm; this was also observed from the particle size distribution in Figure 5 and Figure 6. The grade analysis showed that the oversize stream contained approximately 16.94% Cr₂O₃, with the undersize at approximately 22.15% Cr₂O₃. The undersize stream was subjected to the desliming cyclone for fines rejection.

The desliming cyclone results presented in

Table 7. Desliming cyclone results at three discrete pressure gauges.

			Grade (%)			Recovery (%)
	Pressure (kPa)	Mass (%)	Cr2O3	FeO	SiO2	Cr2O3	FeO	SiO2
Overflow	200	14.92	3.88	8.70	40.25	2.61	7.02	29.66
	150	5.93	5.75	8.94	39.88	1.54	2.87	11.67
	100	3.18	6.79	10.47	32.31	0.97	1.80	5.07
Underflow	100	75.97	27.67	21.50	14.28	94.87	88.31	53.59
Total		100	22.15	18.49	20.25	100	100	100

show that approximately 24.9% of the feed sample subjected to the cyclone reported to the overflow stream, while 76% reported to the underflow stream. The cyclone upgraded chromite from 22.15% to 27.67% Cr₂O₃ at 94.9% Cr₂O₃ recovery to the underflow stream. FeO tended to upgrade in the underflow in a similar manner as Cr₂O₃, with an upgrade from 18.49% to 21.50% FeO, while SiO₂ reduced from 20.25% to 14.28% in the underflow stream. As expected, SiO₂ upgraded to overflow from 20.25% to an average of 37.80%, while Cr₂O₃ and FeO significantly reduced to negligible concentrations of 5.47% Cr₂O₃ and 9.37% FeO in the overflow streams, indicating negligible Cr and Fe loss.

Overall, a combination of wet-screening and desliming slightly upgraded the feed for the shaking table and Slon whims from 18.18% to 22.28% Cr₂O₃ and FeO from 16.45% to 21.50%, with silica (SiO₂) reduction from 33.8% to 25.41%. The use of both classification techniques maximized slime rejection, such that silica was significantly reduced at low Fe and Cr losses.

3.4. Shaking Table Gravity Test Results

Figure 11 summarizes the shaking table chromite cumulative grade versus recovery results. The results indicate that approximately 27.60% of the feed sample reported the concentrate product at an cumulative chromite grade of 45.15% and recovery of 49.64%. About 56.60% was rejected to the tailings at a Cr₂O₃ grade of 10.86%, while 15.9% reported to middlings at 40.96% Cr₂O₃ grade and 43.62% recovery. A standard metallurgical grade [30] of 42% Cr₂O₃ was achieved at a reasonably high Cr₂O₃ recovery of approximately 76.79% and a mass yield of 43.50% by combining the concentrate and middle products. The chromite upgrade to a standard commercial concentrate grade from a low-grade MG2 ore at high chromite recoveries indicate that a low-grade MG2 ore can be used as an additional viable chromite resource. Additionally, the results indicate that spiral circuits can easily upgrade MG2 to desired chromite grades at high recoveries. As seen before, FeO tended to upgrade with chromite (as indicated in Figure 11 and Figure 12), such that a Cr:Fe ratio of 1.34 was achieved in the concentrate product at 42% Cr₂O₃. This is an indication that the majority of iron-bearing mineral phases are closely associated with chromite minerals [31]. The SiO₂ grade in the concentrate at 42% Cr₂O₃ is low at 1.34% grade and 29.94% recovery. The majority (75.96%) of the SiO₂ is recovered in tailings and slimes at 35.93% SiO₂ grade. Figure 11, Figure 12 and Figure 13 summarize the cumulative curves for the shaking table.

Figure 13 shows silica cumulative grades and recovery for the shaking table. Silica is significantly low (<1% SiO₂) in the concentrate product. At 43.45% cumulative mass, 43.62% Cr₂O₃ grade, 1.44 Cr:Fe, and 75.53% Cr₂O₃ recovery, a clean concentrate is obtained, with SiO₂ at 1.48%, indicating an effective chromite separation by shaking table. As expected, high silica levels were noted in the tailings stream.

The shaking table results show efficient separation and recovery of chromite from the gangue minerals. Can et al. (2019) [32] observed a similar trend for chromite tailings and recommended using a shaking in conjunction with a spiral circuit to achieve similar results in an industrial setup.

3.5. SLon Wet High-Intensity Magnetic Separation Results

Based on Haggerty studies [15,33,34], the chromite formula can vary in this form (Fe, Mg)O(Cr, Al, Fe)₂O₃, where the magnetic susceptibility of the ore is directly linked to the elemental composition. Therefore, higher iron content in the chromite’s crystal structure results in higher magnetic susceptibility. Both bulk mineralogy and head assay have shown that the sample contains significant iron. For this reason, SLon magnetic separation tests were conducted. During the tests, chromite was attracted to the magnetized matrix of the magnetic separator, thus achieving the separation of chromite from the gangue minerals. The SLon WHIMS test has shown that chromite in the sample was sensitive to variations in magnetic field intensity. Both chromite grades and recoveries increased with increasing intensities. The best results in terms of chromite and iron grades and recoveries were obtained at a field intensity of 500 Amperes (7500 gauss) (refer to Figure 14 and Figure 15). The optimum conditions were selected where a standard commercial chromite concentrate grade (>40% [27,28]) was achieved at the highest chromite recovery.

The overall mass yield and chromite recoveries increased with field intensity to 59.29% mass yield and 71.12% Cr₂O₃ recovery at 29.50% Cr₂O₃ grade. This observation corresponds to a study by Khakmardan et al. (2020) [15] on the recovery of chromite by magnetic separation from shaking table tailings. The reducing grade at higher intensities suggests that very weakly magnetic particles (aluminium and magnesium oxides) may also be recovered at high intensities. Gangue minerals may also be pulled into the magnetic stream by association with magnetic minerals, thus lowering the Cr₂O₃ grade. Bulk modal mineralogy has shown that Fe occurs in various minerals in the sample, including pyroxene (Ca, Na, Fe₂+, Mg)Si₂O₆), chlorite (Mg, Fe)₃(Si, Al)₄O₁₀(OH)₂·(Mg, Fe)₃(OH)₆), and mica ((K, Na, Ca)₂ (Al, Fe, Mg)_4–6 (Si, Al)₈ O₂₀ (OH, F)₄); therefore, the iron in these minerals becomes activated and is attracted to the electromagnet at high intensities, further lowering the Cr₂O₃ and FeO contents in the concentrate. Figure 14 and Figure 15 summarize these results. As shown, after 500 A, both Fe and Cr grades reduce while their recoveries increase. However, from 600 A, the Fe recovery rate stabilizes around the same point; this may be due to significant iron reduction in the tailings, as the majority of the iron accumulates to the magnetic stream. Due to a low chromite recovery of 41.86% at optimum grade conditions (42.95%), SLon unit would be more effective as a pre-concentration unit rather than a concentration unit.

Figure 16 shows the grade and recovery curves for silica at varied Slon WHIMS intensities. As expected, an opposite trend was observed for silica, where the silica content reduced with increasing field intensities. From 450A, both SiO₂ grades and recoveries slightly increased in the magnetic stream; this was due to the silica association with Fe-bearing minerals. This increase may also be due to particle entrainment as the paramagnetic minerals are being pulled to the magnetized matrix at high magnetic force.

According to Slon Magnetic Separator Co. Ltd. (n.d.), a single SLon unit can handle a capacity of up to 950 tonnes per hour. Therefore, in industrial plants, a single SLon WHIMS unit could be used as an efficient pre-concentration step for the MG2 ore at 1200 Amperes and higher to maximize chromite recovery. Özyurta et al. (2022) [12] deduced that using SLon WHIMS as a pre-concentration step can significantly reduce the downstream units required for processing. The preconcentrate from SLon WHIMS can be further processed on a shaking table or an equivalent gravity concentrator unit to achieve the desired chromite specifications. Altin et al. (2018) [13] could achieve a commercial chromite concentrate from tailings using a shaking table after pre-concentration by SLon WHIMS. This study has therefore demonstrated that SLon WHIMS can effectively maximize chromite recovery from a low-grade MG2 material, while gravity tests were more effective in concentrating the ore to commercial grades. Therefore, a combination of SLon WHIMS and gravity test (fine processing), such as spiral concentrators, could maximize the overall chromite recovery and produce the desired concentrate specifications.

4. Conclusions

Chromite is considered a strategic mineral in the global economy. With the constant decline in chromite ore grades, sustainable mining is essential to balance the rapidly increasing ferrochrome demand for stainless steel and other metallurgical applications. This study investigated the potential for a low-grade MG2, a poorly explored chromatite seam within the Bushveld complex, to be upgraded to standard metallurgical commercial grades. The results demonstrated that a low-grade (at 18.18% Cr₂O₃) MG2 chromatite layer can be processed to standard commercial chromite concentrate, thus providing a viable chromite resource. This seam contains reasonably high iron content, thus making it more susceptible to magnetic separation, and with the high-density difference between chromite and gangue minerals, the ore is also amenable to gravity separation. Both gravity and magnetic separation tests achieved the standard commercial grade. However, due to low recoveries, a SLon WHIMS at high intensities could be more effective as a pre-concentration step.

Mineralogical characterization showed that the ore is dominated by pyroxene, chromite, and feldspar, where chromite grains were more liberated after crushing the ore to −1 mm, which facilitated separation by either method. The results further indicated that milling the sample finer would not improve the chromite grades and would instead reduce its recovery. As a result, energy-intensive milling can be avoided, thus reducing the operational costs. Gravity tests by shaking table achieved a commercial metallurgical grade of 42% Cr₂O₃ at high Cr₂O₃ recovery of 76.79% and a Cr:Fe ratio of 1.34. SLon WHIMS achieved approximately the same chromite upgrade and a higher Cr:Fe ratio of 1.53, however at poor chromite recovery (41.86%). However, chromite recovery improved with increasing intensities to 71.12% at 1200 A (12,000 gauss). Classification by wet-screening and desliming cyclone played a vital role in maximizing chrome recovery by shaking table and SLon whims by significantly reducing the slimes. Based on the results, the adapted flowsheet is proposed as a cost-effective flowsheet for MG2 processing. The flow sheet can be optimized by conducting SLon WHIMS tests at high intensities (>12,000 gauss) before fine gravity tests to maximize chromite recovery while achieving commercial chromite grades and a Cr:Fe ratio > 1.5.