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Thermally Assisted Beneficiation of a Low-Grade Iron Ore Powder in a Pilot-Scale Drop Tube Reactor: Effects on Ore Upgrading, Mineralogy and Chemical-Physical Characteristics
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Abstract
This study delves into the thermally assisted beneficiation of low-grade iron ore powder using a pilot-scale drop tube reactor, focusing on ore upgrading, mineralogy, and chemical-physical characteristics. Key topics include the effects of heating on ore conversion, mineralogical changes, and the impact on separation performance and recovery. The research compares rapid heating in a drop tube reactor with slow heating in a thermal analyzer, highlighting differences in thermal decomposition behaviors and their implications. Notably, the study achieves significant iron upgrade and recovery, demonstrating the potential of this novel process for enhancing iron ore quality and supporting green steel production. The findings offer promising insights for improving downstream gas-solid reduction processes and optimizing iron ore beneficiation techniques.
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Abstract
This study presents an in-depth analysis of thermally assisted beneficiation of a low-grade iron ore (goethite/hematite, 54.8 wt pct Fe) powder sourced from the Pilbara region in Western Australia. It was calcined in an electrically heated pilot-scale drop tube reactor (DTR) at relatively low temperatures (300 °C, 400 °C, and 500 °C), coupled with wet high-intensity magnetic separation to determine iron upgrade and recovery, and impurities removal. Various characterization techniques, including thermogravimetric analysis (TGA), X-ray diffraction, nitrogen adsorption, scanning electron microscope with energy dispersive X-ray spectroscopy, and X-ray fluorescence, were used to study the impact of flash heating on ore conversion, changes in ore mineralogy, and chemical-physical characteristics. It was found that the degree of conversion (calcination via goethite dehydroxylation) and mineralogy are strongly influenced by both heating rate (HR) and particle residence time (τ); samples heated under conditions relevant to flash heating in the DTR (HR of 30 to 50 °C/s; τ in the order of minutes) exhibited different thermal decomposition behaviors to samples heated slowly in a TGA (HR of 5 to 40 °C/min; τ in the order of hours). The analysis also shows that the proposed thermally assisted beneficiation method produces a hematite-rich concentrate with an iron upgrade and recovery of up to 60 wt pct and 94 pct Fe, respectively, and a high specific surface area (fivefold increase).
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Introduction
In the global effort to achieve net zero carbon emission targets, the steelmaking sector is transitioning towards green steel production using processes such as direct reduction and electric arc furnace (EAF). These processes will favor the use of high-grade iron ores (HGIO), either raw or beneficiated, owing to the adverse impact of slag in the EAF.[1] Australia is presently the global leader in terms of both iron ore resources and annual production volume; in 2024, Australia had 24,405 Mt of iron ore reserves (31 pct of the world iron ore resources) and produced 953 Mt (38 pct of the world iron ore production).[2] However, Australia is facing a steady decline in the availability of high-grade iron resources for direct shipping. There are huge volumes of low-grade iron ores (LGIO) in Australia and globally, typically at- or close to- the surface. However, low-grade ores (typically < 59 wt pct Fe) are unsuitable for direct use in ironmaking and steelmaking processes due to their high impurities content (also called gangue), fine grain size, and complex mineralogy, which negatively impacts iron and steelmaking costs, emissions, efficiency, and productivity.[1,3] The current quality requirement for iron ore export is > 62 wt pct iron (Fe), < 4 wt pct silica (SiO2), < 2.25 wt pct alumina (Al2O3), < 0.09 wt pct phosphorous (P), and < 0.02 wt pct sulfur (S), where 62 wt pct Fe sinter fines is the benchmark for iron ore prices.[1,4] Australia’s plentiful reserves of LGIOs can be beneficiated to increase Fe levels and reduce gangue minerals to meet these export specifications, achieve better productivity in ironmaking and steelmaking processes, while also serving future demand of HGIOs for green steel.[1] However, this is challenging to achieve (at low cost) as traditional beneficiation techniques, such as gravity and magnetic separation, have limited applications with many Australian LGIOs due to their complex mineralogy.[3] Therefore, there is a need to develop novel, cost-effective, efficient, and low-carbon beneficiation technologies and pathways to upgrade ore quality, facilitate iron recovery, and support emerging green steel production routes.
Among the different proposed alternative beneficiation pathways to date, thermally assisted beneficiation coupled with magnetic separation is a promising route.[5‐7] Heating can be applied to induce thermal ore decomposition, via releasing adsorbed and chemically bound volatile components, and/or to facilitate phase transformation.[8,9] Goethite (FeOOH) is often the predominant metal oxide in LGIO, with the latter also comprised of other iron oxides, notably hematite (Fe2O3), and minerals rich in silicon and aluminum, such as kaolinite and quartz.[10] Heating can be provided in a neutral or reduction roasting environment. For neutral roasting of goethite, the adsorbed water (H2O) on the outer and inner surfaces of the ore is removed at a temperature of 120 °C to 140 °C.[11,12] Goethite then undergoes transformation to hematite at 250 °C to 500 °C through endothermic decomposition of structural hydroxyl (OH) units (Eq. [1]), known as dehydroxylation.[10,13]
Previous neutral roasting studies demonstrated utilization of various methods for heating, including convective heating from conventional furnaces (e.g., muffle furnace), and dielectric heating via radiofrequency or microwaves.[14‐16] One benefit of thermally treating LGIO is the resulting improvement in the ore magnetic properties, e.g., via conversion of weakly magnetic goethite into hematite or magnetite, which allows Fe to be more effectively separated from the non-magnetic fraction in a magnetic separation process.[3,17] In reduction roasting (also called magnetizing roasting), a reducing agent (gaseous or solid) is used in combination with ore heating to either partially or fully reduce the ore and obtain a magnetite-rich product that is easier to recover.[3,18,19] Most of the work to date in thermally assisted beneficiation has been performed at laboratory-scale, with a summary of key studies reported in Table I. Large-scale testing has also been undertaken, mainly for magnetizing roasting, using shaft furnace (SF), rotary kiln (RK), and fluidized bed (FB) reactor technologies. As shown in Table II, these three technologies operate with relatively high roasting/reduction temperatures. SFs and RKs have long roasting times and low processing capacity. Additionally, RKs have high roasting energy consumption, short maintenance cycles (of every 30 to 75 days), and can lead to ring formation which negatively impacts the product yield and kiln operation. In comparison, FBs use fine particles which eliminates the need for energy- and carbon-intensive pelletizing and sintering processes. They also promote particle mixing, resulting in a homogenous product, high mass and heat transfer due to contact between fluid and solid particles, high Fe recovery, and shorter residence times compared to RKs. Additionally, FBs are easier to control due to the absence of moving parts within the reactor, contributing to longer maintenance cycles.[3] Despite this, iron ore fines are prone to agglomeration which can cause defluidization of the bed and inhibit further particle calcination/reduction. FBs are energy-intensive due to the high fluid velocities needed for particle suspension, and also require a relatively uniform particle size distribution of the processed ore as differences in residence time results in non-uniform conversion of particles.[20] To this end, there is a need to support the development of alternative reactor technologies (at scale) that can efficiently provide heating for LGIO roasting, while also addressing present limitations of more established and commercially advanced technologies, such as reducing energy consumption.
Table I
Summary of Key Studies for Laboratory-Scale Roasting of Low-Grade Iron Ores
Here, MW is microwave, MF is muffle furnace, LIMS is low-intensity magnetic separation, BFD is blast furnace dust, TG-DTA is thermogravimetric and differential thermal analysis, WHIMS is wet high-intensity magnetic separation, and S-F is sink and float
Table II
Summary of Large-Scale Technologies for Roasting of Low-Grade Iron Ores, Including Shaft Furnace, Rotary Kiln, and Fluidized Bed Technologies[3]
Process
Shaft Furnace
Rotary Kiln
Multistage Circulating Fluidized Bed
Flash Roasting Furnace
Suspension Roasting Furnace
Feed Size
(mm)
15 to 75 (lump)
25 (lump)
Fines
< 0.5 (fines)
< 0.8 (fines)
Energy Consumption
(GJ/t)
1.35
1.76
1.06
0.94
1.23
Roasting Time
6 to 10 h
3 to 4 h
—
5 to 60 s
—
Processing Capacity
(t/h)
25
45
12
70
200
Roasting/Reduction Temperature
(°C)
700 to 850
750 to 1000
450
750 to 950
700 to 900/ 450 to 600
Initial Fe
(wt pct Fe)
31.8 (hematite)
26.8 to 40.7
33 (limonite ore)
15.1 to 43.5
32.5 to 41.6
Upgraded Fe
(wt pct Fe)
65.8
58.2 to 65.9
> 57
53.3 to 60.7
60.2 to 66.8
Fe Recovery
(pct)
78.4
78.9 to 93.9
93 to 95
70.5 to 94.3
81.8 to 98
The drop tube reactor (DTR), which is a particular class of flash heating technology, is among the reactor options under development to produce iron products from fine (< 6 mm) and ultrafine (< 150 μm) particles, which require minimum upstream processing prior to beneficiation, avoiding the need for the pellet making process.[27] A continuous flow of particles is injected at the top of the DTR and are heated as they descend through the vertical reactor tube. Similar to FB technology, which are further advanced commercially, the use of fine and ultrafine particles allows the DTR to have fast reactions and offers the potential for shorter and improved control of residence time.[28] The lower roasting temperatures and shorter contact time in the DTR also offers the potential to reduce complications related to sticking, which is often seen in FB technology. Although being pre-commercial in the iron and steel sector, flash reactors are well established in both cement and alumina calcination processes. In the iron ore sector, flash reduction has been successfully demonstrated in DTRs at both laboratory and pilot-scale using different reducing agents, including hydrogen.[29‐31] In addition, Calix, a technology developer, has recently introduced a novel DTR technology, named Zero Emission Steel Technology (ZESTY), which operates at temperatures below 1000 °C and offers significant potential advantages in terms of lower energy requirements and material challenges.[8] The scaled-up commercial demonstration plant proposed by Calix, for direct reduction of iron ore with renewable sources, targets an annual production capacity of 30,000 mt of direct reduced iron (DRI) using an electric reactor for the calcination and reduction processes.[27] Whilst this technology is mainly being investigated and developed for DRI, it is also potentially applicable for upstream iron processes such as neutral roasting of LGIO. However, there is no experimental data available to date on thermally assisted beneficiation of LGIO in a DTR, let alone at pilot-scale, hence new data and knowledge are required to confirm the potential applicability and feasibility of DTRs in the beneficiation context.
As shown in Tables I and II, temperature is a key driving variable in neutral roasting of LGIO as it influences transformation of the ore via dehydration and dehydroxylation. Previous studies have shown that goethite undergoes a complex transformation during dehydroxylation, featuring an expansion that causes crack formation at the particle surface which then propagates into the particle.[32] Additionally, hydrohematite is formed, which is a porous intermediate product between stoichiometric goethite and hematite; goethite and hematite, respectively, contain 10.14 and 0 wt pct H2O, and Fe occupancy in hematite (Feocc) of 0.75 and 1, whereas natural hydrohematite (Fe1.8O2.4(OH)0.6 to Fe1.6O1.8(OH)1.2) contains 3.63 to 7.8 wt pct H2O and Feocc of 0.8 to 0.9.[33] Hydrohematite is formed at mid-to-high temperatures (above 300 °C), followed by rearrangement into a more crystalline hematite at higher temperatures of up to 1000 °C.[13] Therefore, temperature significantly impacts the chemical-physical properties of thermally beneficiated LGIO, which subsequently affects the performance of downstream separation processes, notably magnetic separation, in terms of Fe upgrade and recovery, and gangue removal. However, no previous studies assessing the impact of temperature on mineralogy and chemical-physical properties of calcined LGIO in a pilot-scale DTR have been reported, hence new work is needed to support the development and optimization of this technology.
In light of the aforementioned gaps and needs, the present study aims to provide an investigation on utilization of DTR technology at pilot-scale to perform thermally assisted beneficiation of a LGIO powder and assess its feasibility through beneficiation performance analysis when coupled with magnetic separation. In addition, the study also aims to provide an in-depth analysis on the impact of mild roasting temperatures on separation performance and recovery, mineralogical phases, and microstructure of the iron products.
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Methodology
Material
The iron ore sample selected in this study is a low-grade goethite/hematite (54.8 wt pct Fe) from the Pilbara region of Western Australia. The mineral composition of the sample was determined through X-ray fluorescence (XRF) analysis, as shown in Table III. With reference to market specifications, the sample contains high SiO2, Al2O3 and S, moderate loss-on-ignition (LOI1000), and low P.
Table III
Mineral Composition of the Untreated Iron Ore Sample as Determined Through X-ray Fluorescence
Sample
Fe
(wt Pct)
SiO2
(wt pct)
Al2O3
(wt pct)
Mn
(wt pct)
TiO2
(wt pct)
P
(wt pct)
S
(wt pct)
LOI1000
(wt pct)
Other
(wt pct)
Untreated
54.8
5.90
3.37
0.693
0.213
0.054
0.025
8.3
0.209
‘Other’ refers to the collective percentage of minerals CaO, K2O, Na2O, MgO, Zn, and Cl
The as-received untreated sample had a 100 pct passing particle size (D100) of 1 mm. The untreated sample was heated at three wall temperatures of 300 °C, 400 °C, and 500 °C in a pilot-scale DTR (described in the subsequent section), and did not undergo any crushing prior to heating. Particle size distribution (PSD) analysis was undertaken on each sample for the initial size fraction of 0 to 1 mm. The samples were sieved for 15 minutes in an electronic sieve machine (Retsch Sieve Shaker AS 200 Control), using mesh sizes of 1000, 500, 250, 125, 75, and 45 μm. The material retained on each sieve was weighed and the D-values were calculated for 90, 80, 50, and 10 pct passing particle sizes.
The experimental process undertaken in this study for LGIO beneficiation via neutral roasting and magnetic separation is displayed in Figure 1. Qualitative and quantitative techniques were used in combination for characterization of the material phases, analysis of microstructure, and separation performance and recovery of the untreated and thermally beneficiated samples in the − 125 + 75 μm size fraction, with the analysis undertaken at the University of Adelaide, Adelaide Microscopy, and Bureau Veritas facilities.
Fig. 1
Experimental process undertaken in this study to beneficiate low-grade iron ore (LGIO) using heat treatment (neutral roasting) in a pilot-scale drop tube reactor (DTR) at average wall temperatures (\( \overline{T}_{{{\text{wall}}}} \)) of 300 °C, 400 °C, and 500 °C, coupled with wet high-intensity magnetic separation (WHIMS). Solid arrows describe the mass flow. Broken arrows indicate analysis performed on the sample, including loss-on-ignition (LOI), X-ray fluorescence (XRF), particle size distribution (PSD), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area, and scanning electron microscope (SEM). MAGS and NON MAGS represent the magnetic and non-magnetic output streams from the WHIMS process
Flash neutral roasting tests were undertaken in the indirect electrically heated pilot-scale Calix Flash Calciner (BATMn reactor) at Calix’s Bacchus Marsh facility in Victoria, Australia (Figure 2). The BATMn reactor, henceforth referred to as DTR, is a vertical reactor tube with an internal diameter of 0.2 m, and heated length of 18 m which is comprised of eighteen independently controlled electric furnace heating zones of 1 m length. A thermocouple is located in each heating zone to measure the reactor wall temperature, allowing precise control of temperature along the reactor length. The reactor is capable of operating at temperatures from ambient to 1100 °C, with temperature variations of ± 5 °C within each heating zone. The low-grade goethite/hematite fines (0 to 1 mm) were introduced at the top of the reactor at a feed rate of 60 kg/h from a semi-continuous screw feeder. The fines travel downwards through the reactor within a short residence time (in the order of 60 seconds). Calcination occurs in an air environment, and three average reactor wall temperatures (\( \overline{T}_{{{\text{wall}}}} \)) of 300 °C, 400 °C, and 500 °C were tested. The reactor tube is water cooled between the final heated section and collection vessel to facilitate quenching of the calcined product, which is then collected at the base of the reactor in sealed collection vessels. These samples are henceforth referred to as DTR-300°C, DTR-400°C and DTR-500°C, with experimental conditions provided in Table V (tests 2 to 4). The off gas also passes through a dedusting system and is discharged from the top of the reactor.
Fig. 2
Electrically heated pilot-scale drop tube reactor at Calix’s Bacchus Marsh facility in Victoria, Australia
To the best of the authors’ knowledge, in situ experimental data of particle temperature and heat transfer rates within large-scale (pilot and commercial) DTRs are not available to date due to the complexity associated with these measurements, as well as physical and optical limitations. To allow a back-to-back comparison with thermogravimetric analysis, it was assumed in this study that the mean (bulk) final particle temperature at the exit of the drop tube approaches that of the average reactor wall temperature (i.e., \({{\overline{T} }_{p,f}\approx \overline{T} }_{\text{wall}}\)). This assumption is justified by available data from literature on smaller scale drop tubes. In particular, Johnstone, et al.[34] performed experiments under similar conditions to those in this study within a sub-pilot drop tube furnace, including particles of similar size and density, together with dilute particle cloud condition, terminal velocity, residence time, and reactor wall temperature. It was found that steady state was established between the reactor wall and the gas within the order of seconds, and the ratio of thermal resistance outside the particle to that inside was within the range where temperature gradients within the particle may be neglected. Using the correlation between heat transfer and wall temperature experimentally derived by Johnstone, et al.,[34] it was determined that \({{\overline{T} }_{p,f}\approx \overline{T} }_{\text{wall}}\) for the present study, as shown in Appendix A.
For a sufficiently long drop tube with a particle cloud falling through the heated section, a first order analysis was performed to estimate mean particle residence time (\(\tau \)) and mean heating rate (\(HR\)) for the three flash heating conditions investigated. These calculated values of τ and HR were used to allow back-to-back comparison with samples heated slowly in a laboratory-scale thermal analyzer. For the DTR, τ is based on the reaction zone length \((L)\) and the hindered terminal velocity through the reactor for a cloud of particles (\({{U}_{t}}^{\prime}\)), given by Eq. [2], while HR is dependent on τ and the difference in the DTR average wall temperature (\({\overline{T} }_{\text{wall}}\)) and ambient temperature of the sample (\({T}_{p,0}\)), as shown in Eq. [3].[35,36] As previously mentioned, it is assumed the mean particle cloud temperature reaches \({\overline{T} }_{\text{wall}}\) within the residence time (i.e., \({{\overline{T} }_{p,f}\approx \overline{T} }_{\text{wall}}\)). The estimated values for τ and HR are provided in Table IV, and details of the calculations are reported in Appendix B.
Table IV
Estimated Values of Mean Residence Time (τ) and Mean Heating Rate (HR) for Each Heating Condition in the Drop Tube Reactor (DTR) Using First Order Analysis
A NETZSCH Simultaneous Thermal Analyzer 449 F5 Jupiter TGA/DSC was used for heating the untreated sample at temperatures of 300 °C, 400 °C, and 500 °C, with slow HRs of 10 °C/min. For each test, 30 mg of untreated sample was placed in an open alumina crucible, and then heated in an air purge environment (to mimic the roasting conditions of the DTR) with an air flow rate of 50 mL/min. Approximately 0.3 g of sample from each heating condition was collected for subsequent X-ray diffraction (XRD) and nitrogen adsorption analysis to compare the phase transformation and Brunauer-Emmett-Teller (BET) specific surface area achieved with the thermal analyzer and DTR. These samples are henceforth referred to as TGA-300°C, TGA-400°C, and TGA-500°C, with experimental conditions provided in Table V (tests 5 to 7).
Table V
Heating Conditions for Tests Performed in a Drop Tube Reactor (DTR) and Thermal Analyzer/Thermogravimetric Analysis (TGA) at Temperatures (T) of 300 °C, 400 °C, and 500 °C
Purpose
Test No.
Sample Used for Treatment
Heating Method
T (°C)
Treatment Conditions
Sample Name After Treatment
XRD
BET
SEM
XRF
WHIMS
Heating/
Beneficiation Comparison
1
Untreated
DTR
—
—
—
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
2
Untreated
300
Air
DTR-300°C
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
3
Untreated
400
Air
DTR-400°C
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
4
Untreated
500
Air
DTR-500°C
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
\(\checkmark\)
5
Untreated
TGA
300
Air, 10 °C/min
TGA-300°C
\(\checkmark\)
\(\checkmark\)
—
—
—
6
Untreated
400
Air, 10 °C/min
TGA-400°C
\(\checkmark\)
\(\checkmark\)
—
—
—
7
Untreated
500
Air, 10 °C/min
TGA-500°C
\(\checkmark\)
\(\checkmark\)
—
—
—
Thermal Decomposition
8
Untreated
TGA
1000
N2, 10 °C/min
TGA-10°C/min
—
—
—
—
—
9
DTR-300°C
1000
N2, 10 °C/min
—
—
—
—
—
—
10
DTR-400°C
1000
N2, 10 °C/min
—
—
—
—
—
—
11
DTR-500°C
1000
N2, 10 °C/min
—
—
—
—
—
—
12
Untreated
TGA
1000
N2, 5 °C/min
TGA-5°C/min
—
—
—
—
—
13
Untreated
1000
N2, 20 °C/min
TGA-20°C/min
—
—
—
—
—
14
Untreated
1000
N2, 40 °C/min
TGA-40°C/min
—
—
—
—
—
Note: T refers to the average wall temperature for DTR heating, and sample temperature for TGA heating
Analysis performed on each sample, including X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area, scanning electron microscope (SEM), X-ray fluorescence (XRF), and wet high-intensity magnetic separation (WHIMS), is indicated by ‘\(\checkmark\)’. Thermal decomposition tests performed in the TGA on heated samples from the DTR, as well as the untreated sample at various heating rates, are also provided
Characterization
Thermogravimetric analysis
Thermogravimetric analysis (TGA) was conducted with a NETZSCH STA 449 F5 Jupiter TGA/DSC to study thermal decomposition of the untreated sample when heated at TGA temperatures (TTGA) of 40 °C to 1000 °C, with a slow HR of 10 °C/min. For each test, 10 to 15 mg of sample was placed in an open alumina crucible and then heated in a nitrogen purge environment, with a flow rate of 50 mL/min. Nitrogen was also used as the protective gas for the balance, with a flow rate of 20 mL/min. The sample weight was continuously recorded over the duration of the experiment to generate a TGA curve, which was then used to identify weight loss attributed to reactions at certain temperatures, as well as the maximum weight loss (LOI1000) in the untreated sample (Table V, test 8). Similarly, the heat-treated samples from the DTR were exposed to TGA temperatures of 40 °C to 1000 °C to determine the residual weight loss (Table V, tests 9 to 11).
Additionally, the untreated sample was heated at three HRs of 5, 20, and 40 °C/min, with experimental conditions provided in Table V (tests 12 to 14), to determine the impact of HR on the sample conversion (volatile removal). From the weight loss data at temperatures of 300 °C, 400 °C, and 500 °C, conversion achieved with slow heating in the TGA could be calculated for comparison with flash heating in the DTR. All TGA tests were repeated a minimum of three times.
The reaction extent, also termed the conversion fraction, \(X\), is given by Eq. [4]. Here, \({w}_{0}\) is the sample weight before heating, \({w}_{t}\) is the sample weight after time (or temperature), \(t\), and \({\Delta w}_{\text{max}}\) is the maximum weight loss.[37‐39]\({\Delta w}_{\text{max}}\) is calculated from \({(w}_{0}-{w}_{\infty })\), where \({w}_{\infty }\) is the sample weight after heating at 1000 °C which is calculated from LOI1000 data for heating in the DTR (as determined by XRF analysis), and from weight loss data at 1000 °C for heating in the TGA.
X-ray diffraction patterns were recorded using the Rigaku MiniFlex 600 X-ray diffractometer for the untreated and heat-treated samples from the DTR and TGA (Table V, tests 1 to 7), as well as pure kaolinite (Al2O7·Si2·2H2O), goethite and hematite. The powder samples were placed in a glass sample holder, which has a depth of 0.5 mm. The X-ray tube voltage and current were set to 40 kV and 15 mA, respectively. The XRD was configured to analyze a scan region of 10 to 90 deg, with a step size of 0.02 deg, and scan rate of 2 deg/min for the iron ore samples. Unique spectra were generated for each iron ore sample and were compared to spectra obtained for pure kaolinite, goethite, hematite, as well as quartz (low SiO2) which was collected from the database in PDXL software, to identify mineral phases present within each iron ore sample which assists in understanding the phase changes during calcination. XRD measurements were performed three times on each iron ore sample.
Microstructure
Adsorption
Adsorption measurements were performed with the Micromeritics 3Flex adsorption analyzer and cryogenic system to determine the BET specific surface area, pore size, and pore volume of the samples. Prior to adsorption measurements, 20 to 50 mg of the untreated and heat-treated samples from the DTR and TGA (Table V, tests 1 to 7) were degassed at 150 °C for 2 hours under vacuum to remove all adsorbed moisture. Nitrogen adsorptive and 77.35 K bath temperature was used for the subsequent adsorption analysis. Adsorption tests were repeated three times for each sample.
Micron-scale analysis and imaging
A Quanta 450 FEI FEG Environmental Scanning Electron Microscope (SEM), equipped with an Oxford Ultim Max Large Area SSD Energy Dispersive X-ray Spectroscopy (EDS) detector, was utilized to visualize the impact of DTR heating on mineralogy. High-resolution microscopic imaging was performed in backscattered electron beam (BSE) mode to identify mineral phases and their mutual textures and morphologies (e.g., cracking, fracturing). EDS spot analysis and element mapping enabled identification of mineral compositions within each sample. Images were taken under high vacuum conditions, with a working distance of 10 mm, beam voltage of 15 to 20 kV, and resolution of 1024 × 884 pixels. Prior to imaging, the untreated and heat-treated samples from the DTR were placed in resin and polished, both manually and with Struers TegraPol Automatic Polishing System, and then coated with carbon to prevent charging under the electron beam.
Separation Performance and Recovery
X-ray fluorescence
X-ray fluorescence was performed on the untreated and DTR heat-treated samples to measure the mass fraction of minerals within each sample, such as Fe, Al2O3, SiO2 and other impurities. The XRF analysis had a detection limit of ± 0.01 wt pct for Fe, SiO2, Al2O3, CaO, MgO, LOI1000, and ± 0.001 wt pct for P and S. The samples were also heated to 1000 °C for LOI1000 determination, following ISO standard 11536:2015(E).[40]
Wet high-intensity magnetic separation
Wet high-intensity magnetic separation (WHIMS), a downstream separation process, was performed on approximately 40 g of the untreated and DTR heat-treated samples, in the − 125 + 75 μm size fraction, using the Eriez WHIMS L4-20. A magnetic intensity of 1 Tesla (1T) was used to separate magnetic ferrous material from weakly magnetic gangue minerals in order to upgrade Fe levels in the concentrate. XRF was performed on the magnetic stream after WHIMS for each sample, the data of which were then used to determine the recovery of elemental Fe (\(R{e}_{\text{Fe}}\), Eq. [5]) and removal of impurities (\(\text{Removal}\), Eq. [6]) for the entire beneficiation pathway.[23] Note, Eq. [6] applies only to Al2O3 and SiO2, which are the predominant phases in gangue for this sample. Here, \({\text{Fe}}_{0}\text{, }{\text{Gangue}}_{0}\), and \({m}_{0}\) are, respectively, wt pct Fe, wt pct (Al2O3 + SiO2), and mass of the untreated sample before heating, while \({\text{Fe}}_{\text{MAGS}}\text{, }{\text{Gangue}}_{\text{MAGS}}\), and \({m}_{\text{MAGS}}\) are, respectively, the wt pct Fe, wt pct (Al2O3 + SiO2), and recovered sample mass after 1T WHIMS. These are represented by the ‘0’ and ‘MAGS’ streams in Figure 1.
Weight loss curves from TGA tests are displayed in Figure 3 for the untreated and DTR heat-treated samples. As shown in Figure 3(b), the total weight loss in the untreated sample is 8.52 pct when heated to 1000 °C (which is comparable to the LOI1000 determined by XRF in Figure 4 for this sample). Also displayed here are four distinct weight loss steps, taken at points of inflection in the TGA curve, which are attributed to various stages of thermal decomposition. The first weight loss step (0.43 pct) occurs between 40 °C and 167 °C. This is a result of desorption of adsorbed water, which was observed at temperatures between ambient and 172 °C for natural goethite in Liu, et al.,[13] and similarly has been reported to commence at 120 °C to 140 °C in McCann, et al.[11] The second weight loss step (4.42 pct) is the largest and occurs between 167 °C and 304 °C. This weight loss is due to dehydroxylation of goethite to hematite, which involves removal of the hydroxyl group in the form of a water molecule during heating. Dehydroxylation has been reported to occur between temperatures of 172 °C and 310 °C for natural goethite.[13] The third weight loss step (2.83 pct) occurs between 304 °C and 448 °C, and is attributed to the formation of intermediate product hydrohematite at temperatures greater than 300 °C.[13] Additionally, hydrohematite experiences moisture removal at 120 °C to 200 °C and 200 °C to 350 °C, and is fully transformed to hematite at 700 °C.[33] The fourth weight loss step (0.85 pct) occurs between 448 °C and 1000 °C. This is consistent with the findings of Liu, et al.[13] and results from the decomposition of remnant structural hydroxyl and nonstoichiometric hydroxyl groups in goethite.
Fig. 3
Thermogravimetric analysis (TGA) curves of weight loss as a function of sample temperature in the TGA (TTGA) up to 1000 °C for (a) untreated and heat-treated iron ore samples from the drop tube reactor (DTR), and key weight loss steps for the (b) untreated, (c) DTR-300°C, (d) DTR-400°C, and (e) DTR-500°C samples
Loss-on-ignition (LOI1000) of untreated and drop tube reactor heat-treated samples at average wall temperatures of 300 °C, 400 °C, and 500 °C (\( \overline{T}_{{{\text{wall}}}} \)), as determined from X-ray fluorescence
The residual weight loss decreases with an increase in the roasting temperature for samples heat-treated in the DTR, as shown in Figures 3(c) through (e). The total weight loss of DTR-300°C, DTR-400°C, and DTR-500°C samples are, respectively, 6.86, 3.58, and 3.04 pct when heated to 1000 °C in the TGA (which is consistent with the LOI1000 values found in XRF analysis, shown in Figure 4). This decrease in weight loss is attributed to the release of volatiles during heating in the DTR, such as moisture removal and goethite dehydroxylation, resulting in lower volatile removal when subsequently heated in the TGA. Furthermore, the TGA curve for the DTR-300°C sample is similar to the untreated, indicating a minor degree of conversion for this case in comparison with the DTR-400°C and DTR-500°C samples, which instead exhibit significant dehydroxylation, as shown in Figure 3(a). Therefore, it can be seen that the residual LOI1000 decreases with increasing the pre-treatment temperature in the DTR. Also shown in Figure 3(a), weight change occurs at similar TGA temperatures for each sample, notably at approximately 300 °C, which is associated with goethite dehydroxylation into hematite.
Conversion
Conversion (X) of the heat-treated samples at temperatures of 300 °C, 400 °C, and 500 °C was estimated using the DTR LOI1000 data for HRs of 29.7, 40.6, and 51.5 °C/s, respectively, and the TGA weight loss data for HRs in the range of 5 to 40 °C/min, with results reported in Table VI. As shown in Figure 5(a), samples heat-treated in the TGA exhibit an increase in X with higher temperatures and slower HRs. In particular, the X values achieved for roasting at 400 °C to 500 °C are similar regardless of the HRs (e.g., heating to 500 °C at 40 °C/min provides 89 pct conversion, which is similar to the 95 pct conversion at 5 °C/min), however, the HR has a significant impact on X at 300 °C, with lower X values at higher HRs (eg., 36 pct conversion for heating at 40 °C/min, whereas > 70 pct conversion for 5 °C/min). Similarly, for samples heat-treated in the DTR, X increases with higher roasting temperatures (and consequently faster HRs), being 9.4 and 59.1 pct at 300 °C and 500 °C, respectively.
Table VI
Mean Heating Rate (HR), Mean Residence Time (τ), and Conversion (X) Values for Heating Low-Grade Iron Ore in Either a Drop Tube Reactor (DTR) or Thermal Analyzer (TGA) at 300 °C, 400 °C, and 500 °C
T (°C)
Heating
Test No.
HR
τ
X (pct)
300
DTR
2
29.7 °C/s
9.25 s
9.4
TGA
14
40 °C/min
9.7 min
36.5
TGA
13
20 °C/min
13.1 min
43.1
TGA
8
10 °C/min
26.1 min
53.6
TGA
12
5 °C/min
52.2 min
71.7
400
DTR
3
40.6 °C/s
9.24 s
47.4
TGA
14
40 °C/min
11.9 min
83.7
TGA
13
20 °C/min
18.0 min
85.3
TGA
8
10 °C/min
36.2 min
86.3
TGA
12
5 °C/min
72.2 min
91.7
500
DTR
4
51.5 °C/s
9.23 s
59.1
TGA
14
40 °C/min
14.0 min
89.0
TGA
13
20 °C/min
23.1 min
91.5
TGA
8
10 °C/min
46.2 min
93.9
TGA
12
5 °C/min
92.2 min
95.3
Note: T refers to the average wall temperature for DTR heating, and sample temperature for TGA heating
Reference to Table V is indicated by the test number
Fig. 5
Results from heating low-grade iron ore in the drop tube reactor (DTR) and thermal analyzer (TGA), showing (a) conversion (X) of the sample at temperatures (T) of 300 °C, 400 °C, and 500 °C, (b) X at the mean residence time (τ) corresponding to each T, and (c) the mean heating rate (HR) for each T. Note, T refers to sample temperature in the TGA, and average reactor wall temperature for the DTR samples. Best fit curves for each case are also included for ease of comparison
A comparison of the X achieved for the two heating methods indicates that the slow heating regime (TGA) achieves higher X than flash heating (DTR) for the range of temperatures and HRs considered in this study. For instance, heating the sample to 300 °C at 10 °C/min in a TGA achieves similar X as heating to 500 °C at 51.5 °C/s in the DTR, with X values of 54 and 59 pct, respectively. Additionally, at 300 °C, the DTR heat-treated samples exhibit only a minor conversion in comparison with TGA heating, which can be attributed to the differing residence times and HRs within the two reactors, as shown in Figures 5(b) and (c). In particular, Figure 5(b) indicates that for TGA tests, τ (time required to achieve the final steady-state X value for a given heating condition) increases for slower HRs, with heating to 400 °C at 40 °C/min and 5 °C/min requiring a τ of 11.9 and 72.2 minutes, respectively. Meanwhile, for DTR heating, the estimated mean τ is similar for all three roasting temperatures, of approximately 9 seconds, with the HR being two orders of magnitude higher than that of the TGA [Figure 5(c)]. This suggests that the minor conversion achieved at 300 °C is attributed to insufficient time within the DTR, with τ being shorter than the kinetic time required for reaction. At higher temperatures, the time is sufficient to promote conversion due to faster reaction kinetics, however τ is still not sufficiently long enough to achieve X values as high as the TGA cases. Nevertheless, for a given X, faster heating associated with higher HRs allows for the use of a more compact reactor (short τ), which can positively impact plant economics [Figure 5(b)].
XRD
The XRD patterns of the untreated and heat-treated samples from the DTR and TGA tests are shown in Figure 6. The peaks in the untreated sample are identified predominantly as goethite, as well as hematite and quartz. For heating in the DTR, the goethite peaks in the untreated sample decrease in intensity with temperature, mainly observed at DTR-300°C, which corresponds with previous works and is attributable to desorption of adsorbed water on the outer and inner surfaces, occurring between ambient and 172 °C for natural goethite.[13] Some goethite peaks disappear at DTR-400°C and are replaced by hematite peaks, as observed at 2 theta (2θ) angles of 33.2, 35.7, 40.9, and 62.5 deg, which is a result of goethite transforming into hematite via dehydroxylation. Natural goethite has been observed to undergo dehydroxylation at 172 °C to 310 °C,[13] while other iron ores experience goethite dehydroxylation at 250 °C to 350 °C,[11] and even up to temperatures of 400 °C.[10] Characteristic hematite peaks increase in intensity with higher temperatures up to DTR-500°C, which is mainly observed for hematite peaks formed from goethite during heating. There is no visible change in intensity for the hematite peaks from the untreated sample after DTR heating. In addition, residual goethite still exists at DTR-500°C for 2θ of 21.3, 36.7, 53.3, and 59.1 deg, indicating goethite has not completely transformed into hematite. This is consistent with the conversion trends presented in Figure 5, also noting that short τ could result in the particle temperature being lower than the inner reactor wall temperature. In contrast, heating the sample in the TGA at a slow heating rate of 10 °C/min promoted dehydroxylation of goethite to hematite at lower temperatures, as indicated by the primarily hematite peaks at TGA-300°C. Further heating to 400 °C and 500 °C in the TGA causes the hematite peaks to increase in intensity, which was also observed for the DTR-500°C sample and can be seen at 2θ of 33.7 and 36.2 deg. Previous studies attributed this to crystallinity improvement and increasing crystal size of newly formed hematite.[13] The peaks at 2θ of 26.8 and 75.8 deg are attributed to quartz-related impurities, which remained unaffected at low to mild temperature heating. This is consistent with the fact that quartz is transformed into other SiO2 polymorphs at a temperature of 573 °C.[41] Note that shifting of DTR-300°C and DTR-400°C spectra towards higher angles, and TGA-300°C and TGA-400°C spectra towards lower angles, was also observed in comparison to the TGA-500°C spectra but only by less than ±0.5 deg (2θ).
Fig. 6
X-ray diffraction patterns of the untreated and heat-treated samples from the drop tube reactor (DTR) and thermal analyzer (TGA, heating rate of 10 °C/min) at roasting temperatures of 300 °C, 400 °C, and 500 °C
As shown in Figure 7(a), the specific surface area (SSA) of heat-treated samples is greater than that of the untreated sample for both heating methods. For the DTR samples, the SSA increases from 13.1 ± 0.17 m2/g to a maximum of 61.7 ± 0.44 m2/g at 400 °C, and then decreases to 37.9 ± 0.23 m2/g at 500 °C. The measured SSA values for TGA samples exhibit a temperature dependence similar to that observed in the DTR tests, although higher SSA values were obtained for all temperatures investigated. The maximum SSA for both heating methods occurs at a roasting temperature of 400 °C, with the TGA sample exhibiting a SSA of 83.5 ± 0.22 m2/g, which is approximately 30 pct higher than the DTR-400°C sample. The largest SSA difference between the TGA and DTR samples occurs at 300 °C, with SSA of the TGA-300°C sample being 65.2 ± 0.11 m2/g, approximately three times higher than the DTR-300°C sample, with the latter exhibiting a SSA similar to that of the untreated case (this is consistent with the relative minor conversion observed in Figure 5). A decrease in SSA at 500 °C was observed for both heating methods, with the DTR samples exhibiting approximately 50 pct reduction in SSA when heated from 400 °C to 500 °C, while this difference is only approximately 15 pct for TGA samples. Overall, the trends are consistent with those reported in previous studies for thermally treated natural goethite,[13] with SSA undergoing a sudden increase due to the formation of highly porous hydrohematite (via dehydroxylation) at low roasting temperatures, followed by a decrease in SSA upon further heating at higher temperatures due to hematite recrystallization. In addition, it was found that the SSA correlates with the X in both heating methods, as shown in Figure 7(c). In particular, it can be seen that samples undergoing similar X, such as heating to 300 °C at 10 °C/min in a TGA and heating at 400 °C in the DTR, also feature very similar SSA values.
Fig. 7
Adsorption results of the untreated and heat-treated samples from the drop tube reactor (DTR) and thermal analyzer (TGA, heating rate of 10 °C/min) at roasting temperatures of 300 °C, 400 °C, and 500 °C, showing (a) Brunauer-Emmett-Teller (BET) specific surface area (SSA), (b) incremental pore volume as a function of pore width, and (c) conversion (X) as a function of BET SSA. Note, temperature (T) refers to sample temperature in the TGA, and average reactor wall temperature for the DTR samples
As shown in Figure 7(b), the pore volume becomes larger as temperature increases for both the DTR and TGA. The peak pore width of the untreated sample is 5.88 nm, which is similar to that of the DTR sample heated to 300 °C, further indicating minor changes (i.e., X) upon heating for this case. The DTR-400°C sample features higher pore volume at lower pore widths than the untreated sample, with peaks at 2.95 and 5.04 nm. Both TGA and DTR samples exhibit similar trends upon heating, although the peak pore widths are smaller in TGA samples compared to DTR samples, and also contain larger pore volume for the 300 °C and 500 °C cases. The TGA-300°C sample contains a peak at the pore width corresponding to the untreated sample, and also at a smaller pore width of 3.18 nm. The peak pore width for the TGA-400°C and TGA-500°C samples is also similar to the DTR-400°C sample, with values of 2.52 and 2.73 nm, respectively. The observed reduction in pore widths and increase in pore volume for high roasting temperatures can be attributed to hematite recrystallization.
Micron-scale mineralogy and textures
Figure 8 shows a series of representative BSE images of the untreated sample and analogous images of the same sample after heat treatment at 300 °C, 400 °C, and 500 °C in the DTR. Together, the images and EDS data enable an appreciation of which minerals are present in each sample and provide indications of transformations induced by heating. It is important to note that the particles in the untreated sample [Figures 8(a) through (c)] display heterogeneity with widespread micron-scale intergrowths between goethite and hematite, and of iron (hydr)oxides with different minerals. Moreover, the relatively low-grade of the ore is consistent with the observed abundance of clay minerals, quartz, and other accessory minerals. Among these are the inclusions of ilmenite (FeTiO3) in hematite and manganese (hydr)oxides of uncertain speciation. EDS spectra for goethite and hematite also indicate structurally bound aluminum and phosphorous, consistent with XRF data (Table III). Lastly, goethite and hematite both occur in a wide range of different morphologies within the untreated sample. This is typical of iron ores from the Pilbara region due to their prolonged geological history and the different hypogene and supergene processes involved in their formation.[42]
Fig. 8
Backscatter electron images of untreated low-grade goethite/hematite iron ore [(a) thorough (c)] and thermally treated samples at a drop tube reactor (DTR) temperature of 300 °C [(d) through (f)], 400 °C [(g) through (i)], and 500 °C [(j) through (l)]. (a) Shows intergrowths of hematite and goethite, while (b) is representative of fine hematite and goethite banding in the untreated sample. Note silicates, mostly clay minerals, are intergrown with the Fe-(hydr)oxides. (c) Distinctive hematite morphology comprising microcrystals with inclusions of quartz and ilmenite (yellow circle). (d) Micron-scale goethite-hematite intergrowths resembling those in untreated sample. (e) Relatively homogeneous idiomorphic hematite; note preservation of fine microcrystalline fabric (yellow circle). (f) botryoidal goethite featuring a conspicuous fracture pattern readily attributable to volume loss, as well as characteristic 120 deg triple junctions. (g) Particle containing a combination of primary inclusions and thermally induced features. The rounded features around the grain boundary (top right) are suggestive of new-formed crystalline hematite (light gray swirls) following goethite dehydroxylation. More enigmatic, however, is the presence of goethite growing along the margins of the particles and along cracks that extend across the grain (yellow arrows). (h) Hematite crystals forming and growing in a goethite matrix. (i) Recrystallization with cracking predominantly around the grain boundary (white arrow). (j) Thermally induced fracturing (yellow arrows) and rounded features attributable to annealing. (l) is an enlargement of (k). (Color figure online)
EDS analysis (noting that H2O and OH cannot be measured and all data are normalized to 100 pct) allows identification of goethite as containing less iron (< 60 wt pct Fe) and greater concentrations of impurities (1 to 2 wt pct each of Al and Si) compared to hematite (> 67 wt pct Fe, < 1 wt pct Al and Si). Figure 8(a) shows hematite inclusions (lighter) in a matrix of goethite (darker) with sharp boundaries between the phases. A similar distinction between phases is seen in Figure 8(b) with alternating bands of fine hematite and goethite, illustrating the textural diversity in the untreated sample is large primary hematite with inclusions of clay minerals distributed along the outside of the grain. Hematite is also observed as fine-grained euhedral crystals growing into small vugs, as well as spherical hematite suggestive of a biogenic origin.
The sample heated at 300 °C in the DTR is similar to the untreated sample in that the same minerals and textures are observed. There is minimal indication of any modification due to heating. Primary hematite occurs predominantly within a goethite matrix, where mutual boundaries indicate cycles of replacement of hematite by goethite, and of goethite by hematite over geological time. As shown in Figure 8(d), goethite and hematite remain intergrown within one another at the micron-scale. This is a primary feature inherent to the sample. Hematite grains display a broad range of textures, ranging from coarser, relatively homogeneous idiomorphic crystals, as shown in Figure 8(e), although grain margins are suggestive of limited recrystallization. The sample also contains ferrosilite (FeSiO3) and cryptomelane [K(Mn4+,Mn2+)8O16], which are unaffected by heating. Figure 8(f) illustrates botryoidal goethite featuring a conspicuous fracture pattern readily attributable to volume loss, as well as characteristic 120 deg triple junctions (so called ‘foam-textures’) resulting from annealing. Both features are suggestive of a transformation induced by heating.
Several lines of evidence for a transformation of goethite to hematite are noted in the sample heated at 400 °C in the DTR, including the propagation of cracks and what appears to be new-formed crystalline hematite. Figure 8(g) shows a particle containing a combination of primary inclusions and thermally induced features, including new-formed crystalline hematite following goethite dehydroxylation. Similarly, Figure 8(h) shows hematite crystals forming and growing in a goethite matrix. This sample also contains microcrystalline hematite in a goethite matrix. Recrystallization into high-purity iron oxide (73 wt pct Fe) and minimal gangue [Figure 8(i)] is seen, with cracking predominantly around the grain boundary. However, some particles appear very similar to those in the untreated sample. This observation, along with other observed textures, suggest that reactions are incomplete and that equilibrium conditions are not reached during heating or subsequent cooling.
The sample heated at 500 °C in the DTR shows the most convincing evidence for goethite dehydroxylation and replacement by hematite, and of the recrystallization of hematite into coarser, more homogeneous crystals. Interestingly, some of the hematite replacing goethite appears to inherit the higher impurity content of the hydroxide (~ 2 wt pct each of Al and Si). A greater abundance of thermally induced fracturing is also seen, as well as rounded features attributable to annealing [Figures 8(j) through (l)]. While the abundance of hematite is clearly higher in this sample and there is less goethite present, the reaction remains incomplete. The observation that not all goethite is fully transformed to hematite is nevertheless consistent with the presence of residual goethite in the XRD spectra of the 500 °C sample (Figure 6). This would suggest that roasting is of too short duration within the DTR to attain complete conversion of goethite at this temperature. Other components, notably clay minerals, quartz, and the aforementioned Ti- and Mn-minerals, are unaffected by heating. The findings from SEM analysis are nevertheless consistent with those from the conversion plot for the DTR (Figure 5).
Separation Performance and Recovery
Both the untreated and heat-treated samples from the DTR were subjected to one-stage WHIMS at a magnetic field intensity of 1T. XRF analysis was performed before and after heat treatment, as well as on both magnetic and non-magnetic streams from the subsequent WHIMS process. The wt pct for Fe and the two residual impurities (Al2O3 and SiO2) before and after heating, and in the final concentrate, are shown in Figure 9 and Table VII. It can be seen from Figure 9(a) that heating increases Fe levels in the sample, from 54.8 wt pct in the untreated sample to 55.6 wt pct and 57.9 wt pct when heated at 300 °C and 400 °C in the DTR, respectively, corresponding to a Fe upgrade of 0.8 wt pct and 3.1 wt pct, respectively. Heating at 500 °C resulted in a 3.3 wt pct Fe increase in comparison with the untreated ore, upgrading to 58.1 wt pct Fe. It can also be seen from Figure 9(a) that coupling heating at 500 °C with WHIMS further upgraded the sample, with Fe reaching approximately 60 wt pct. WHIMS alone (without heat pre-treatment) is unable to significantly beneficiate the sample due to the high goethite content in the untreated ore, with Fe levels only increasing by 1.7 wt pct, from 54.8 to 56.5 wt pct Fe, if heating is not provided prior to WHIMS.[43]
Fig. 9
X-ray fluorescence results for (a) iron (Fe) content and (b) alumina and silica (Al2O3 + SiO2) content before and after heat treatment in the drop tube reactor (DTR) at average wall temperatures (\( \overline{T}_{{{\text{wall}}}} \)) of 300 °C, 400 °C, and 500 °C, followed by one-stage wet high-intensity magnetic separation (WHIMS), as well as (c) Fe recovery (RFe), and Al2O3 + SiO2 removal for the entire beneficiation pathway. Note, recovered mass from the magnetic stream after WHIMS for the 300 °C sample was adjusted (due to a laboratory error) to allow values for RFe and Al2O3 + SiO2 removal to be consistent with literature
X-ray Fluorescence Results for the Iron Ore Sample Before and After Heat Treatment in the Drop Tube Reactor (DTR) at Average Wall Temperatures of 300 °C, 400 °C, and 500 °C (Stream ‘0’ in Fig. 1), As Well As for the Magnetic Material Recovered from the Subsequent One-Stage Wet High-Intensity Magnetic Separation Process (Stream ‘MAGS’ in Fig. 1)
Treatment
Stream
Fe (wt pct)
SiO2 (wt pct)
Al2O3 (wt pct)
P (wt pct)
S (wt pct)
LOI1000 (wt pct)
RFe
(pct)
Al2O3 + SiO2 Removal (pct)
MS
—
> 62
< 4
< 2.25
< 0.09
< 0.02
—
—
—
Untreated
0
54.8
5.90
3.37
0.054
0.025
8.3
—
—
MAGS
56.7
5.57
3.38
0.055
0.025
8.5
86.5
19.2
DTR-300°C
0
55.6
5.79
3.43
0.055
0.027
7.5
—
—
MAGS
57.1
5.76
3.47
0.056
0.027
7.6
86.9
17.0
DTR-400°C
0
57.9
5.98
3.55
0.058
0.025
4.4
—
—
MAGS
59.1
5.88
3.56
0.058
0.025
4.4
90.2
14.9
DTR-500°C
0
58.1
6.3
3.64
0.058
0.019
3.4
—
—
MAGS
59.5
6.22
3.65
0.059
0.019
3.4
93.5
8.4
The XRF results are accurate to ± 0.01 wt pct for Fe, SiO2, Al2O3, and LOI1000, and ± 0.001 wt pct for P and S. Iron recovery (RFe), and alumina and silica (Al2O3 + SiO2) removal values are also tabulated. The market specifications (MS) for iron ore export are provided for comparison.[4]
Figure 9(b) also shows that impurities, such as Al2O3 and SiO2, do not respond to heating which is consistent with the mineralogical analysis. The slight wt pct increase in Al2O3+SiO2 in the sample upon heating (approximately 0.7 wt pct at 500 °C) is due to volatile removal, which causes sample mass loss and hence, a subsequent increase in the gangue concentration. Nevertheless, this effect is less significant on the final concentrate when WHIMS is also used.
Figure 9(c) presents the Fe recovery (RFe) and Al2O3 + SiO2 removal (in pct) of the proposed beneficiation pathway, notably DTR heat treatment followed by WHIMS. It can be seen that the RFe is high for all conditions investigated (> 80 pct), ranging from approximately 87 pct with WHIMS only operations up to 94 pct when WHIMS is coupled with heating at 500 °C, indicating a positive effect of heating on RFe. Conversely, impurities removal exhibits a decreasing trend with an increase in temperature, which is in line with the trends observed in Figure 9(b); removal of Al2O3 and SiO2 is highest for the untreated sample (19.2 pct) and decreases to 8.4 pct in the 500 °C sample. With reference to Table VII, after heating and one-stage WHIMS all samples remain above market specification for SiO2 and Al2O3, and below specification for Fe levels. In particular, samples heated at 400 °C and 500 °C followed by magnetic separation were upgraded from a LGIO type (< 59 wt pct Fe) to higher grade (59 to 63.5 wt pct Fe, mid-grade iron ore) products with economic value.[1]
Summary of Findings
A summary of quantitative findings is provided in Table VIII for the low-grade iron ore sample heated at temperatures of 300 °C, 400 °C, and 500 °C using rapid heating in a DTR, followed by one-stage WHIMS, compared to slow heating in a TGA.
Table VIII
Summary of Results for the Low-Grade Iron Ore Sample in Either a Drop Tube Reactor (DTR) or Thermal Analyzer (TGA) at 300 °C, 400 °C, and 500 °C (T)
T (°C)
Treatment
HR
τ
X (pct)
BET SSA (m2/g)
Fe
(wt pct)
RFe (pct)
Al2O3 + SiO2
Removal (pct)
Untreated
—
—
—
0
13.1
54.8
—
—
WHIMS
—
—
—
—
56.7
86.5
19.2
300
DTR
29.7
°C/s
9.25
s
9.4
20.5
55.6
—
—
DTR+WHIMS
—
—
—
—
57.1
86.9
17.0
TGA
40
°C/min
9.7
min
36.5
—
—
—
—
TGA
20
°C/min
13.1
min
43.1
—
—
—
—
TGA
10
°C/min
26.1
min
53.6
65.2
—
—
—
TGA
5
°C/min
52.2
min
71.7
—
—
—
—
400
DTR
40.6
°C/s
9.24
s
47.4
61.7
57.9
—
—
DTR+WHIMS
—
—
—
—
59.1
90.2
14.9
TGA
40
°C/min
11.9
min
83.7
—
—
—
—
TGA
20
°C/min
18.0
min
85.3
—
—
—
—
TGA
10
°C/min
36.2
min
86.3
83.5
—
—
—
TGA
5
°C/min
72.2
min
91.7
—
—
—
—
500
DTR
51.5
°C/s
9.23
s
59.1
37.9
58.1
—
—
DTR+WHIMS
—
—
—
—
59.5
93.5
8.4
TGA
40
°C/min
14.0
min
89.0
—
—
—
—
TGA
20
°C/min
23.1
min
91.5
—
—
—
—
TGA
10
°C/min
46.2
min
93.9
73.5
—
—
—
TGA
5
°C/min
92.2
min
95.3
—
—
—
—
Note: T refers to the average wall temperature for DTR heating, and sample temperature for TGA heating. The mean heating rate (HR), mean residence time (τ), conversion (X), and Brunauer-Emmett-Teller (BET) specific surface area (SSA) values are included
Additionally, the iron content (Fe), iron recovery (RFe), and alumina and silica Al2O3 + SiO2 removal values for DTR heating followed by one-stage wet high-intensity magnetic separation (WHIMS) are also provided
Conclusions
The performance of a novel thermally assisted beneficiation process at pilot-scale, combining flash heating in a drop tube reactor with wet high-intensity magnetic separation, was reported for a low-grade iron ore powder (initial Fe of 54.8 wt pct), together with an in-depth analysis of the impact of flash heating on ore conversion, and associated changes in ore mineralogy and chemical-physical characteristics. The main findings from the experimental investigation are summarized as follows:
The proposed process can produce a hematite-rich concentrate with moderate heating at 400 °C to 500 °C, demonstrating effective ore upgrade performance. Notably, an iron upgrade and recovery of up to 60 wt pct Fe and > 90 pct, respectively, was achieved, together with a high specific surface area of up to five times the initial SSA value of the untreated ore. This is promising for downstream gas-solid reduction as porosity can have an impact on the reduction kinetics.
Ore conversion, mineralogy, and chemical-physical changes due to heating were significantly impacted by both the heating rate and particle residence time; samples heated under conditions relevant to flash heating in the DTR exhibited different thermal decomposition behaviors to those heated under slow heating rates in a TGA. This resulted in differences in mineralogy and chemical-physical properties of the products, such as the measured SSA values and SSA-conversion trends. Additionally, it was found that the degree of conversion (calcination) was lower in samples heated in the DTR compared to those heated in a TGA. This suggests that the time available for reaction in the DTR was insufficient to achieve complete conversion of goethite into hematite within the temperature range investigated, although relatively high conversion can be attained in a DTR within a much shorter time when compared with traditional heating methods.
Acknowledgments
The authors acknowledge the Australian Federal Government and Department of Education and Training for providing financial support for this research through the Australian Government Research Training Program (RTP) stipend. The work has also been supported by the Heavy Industry Low-carbon Transition Cooperative Research Centre whose activities are funded by its industry, research and government partners along with the Australian Government’s Cooperative Research Centre Program. The authors acknowledge the instruments and expertise of Microscopy Australia (ROR: 042mm0k03) at Adelaide Microscopy, University of Adelaide, enabled by NCRIS, university, and state government support. The authors would also like to acknowledge Yun Xia and Matt Boot-Handford from Calix Limited, Maddingley, VIC 3340, Australia, for their collaboration.
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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Thermally Assisted Beneficiation of a Low-Grade Iron Ore Powder in a Pilot-Scale Drop Tube Reactor: Effects on Ore Upgrading, Mineralogy and Chemical-Physical Characteristics
Authors
Renae Lillian O’Hara
Nigel John Cook
Elliott William Lewis
Maziar Arjomandi
Geoffrey Brooks
Alfonso Chinnici
Appendix A: Calculation for Mean Final Particle Temperature
For the present study, it is assumed that the mean (bulk) final particle temperature at the exit of the drop tube approaches that of the average reactor wall temperature. This can be proven by determining the temperature difference between the reactor and particle using the experimentally derived logarithmic correlation between heat transfer and wall temperature in Johnstone, et al.,[34] shown by Eq. [7], for a constant value of overall heat transfer coefficient. Here, \({\overline{T} }_{\text{wall}}\) is average reactor wall temperature, \({T}_{p,0}\) is initial particle temperature (top of reactor), \({\overline{T} }_{p,f}\) is mean final particle temperature (bottom of reactor), \({A}_{p}\) is particle surface area, \(c\) is specific heat of gas, and \(\tau \) is mean particle residence time.
Johnstone, et al.[34] performed experiments within a sub-pilot drop tube furnace under conditions similar to those in this study, including particle size and density, residence time, and reactor wall temperature. Hence, the values for average particle mass, \({w}_{p}\), and overall heat transfer coefficient, \({h}_{m}\), in Eq. [7] can be approximated by using the experimental values from Johnstone, et al.[34] The following is an example of the calculation for mean final particle temperature under conditions relevant to the present study: \({\overline{T} }_{\text{wall}}\) = 500 °C, \({T}_{p,0}\) = 25 °C, mean particle diameter of 296 μm (for calculation of \({A}_{p}\)), \(c\) = 1.092 kJ/kg K (air), \(\tau \) = 9.2 seconds (see Appendix B and Table IV), \({w}_{p}\) = 0.035 mg, and \({h}_{m}\) = 141.9 W/m2 K (from Johnstone, et al.,[34] for conditions closely related to the present study). Using these values in Eq. [7], it was found that \({{\overline{T} }_{p,f}\approx \overline{T} }_{\text{wall}}\) within 0.005 pct for the present study. Note, a sensitivity analysis was undertaken for variables \({h}_{m}\) and \(\tau \) in Eq. [7]. It was found that varying \({h}_{m}\) by ±100 W/m2 K from the reference value (for a given \(\tau \)) resulted in a difference between \({\overline{T} }_{p,f}\) and \({\overline{T} }_{\text{wall}}\) of only up to 3.9 pct, whereas this difference was up to 25 pct when varying \(\tau \) by ± 8 seconds from the reference value (for a constant value of \({h}_{m}\)). Hence, approximating the value of \({h}_{m}\) by using experimental data from Johnstone, et al.[34] does not have a significant impact on the value of \({\overline{T} }_{p,f}\) for the conditions investigated in the present study. The sensitivity analysis also suggests that the estimated mean particle residence time of approximately 9 seconds is sufficiently long for the particle temperature to equilibrize with the wall temperature.
Appendix B: Calculation for Hindered Terminal Velocity
The DTR contains a central cylindrical pipe with small particles flowing downwards in stagnant air. The mean particle velocity is estimated using a force balance on the particle in the reactor, which includes gravitational force and drag force from the surrounding fluid. Terminal velocity is where these forces are equal and is the maximum speed achieved by a free-falling particle. The presence of other particles hinders the settling of particles and hence, reduces the terminal velocity.
The settling regime for a single particle is determined by criterion \(K\) which is given in Eq. [8], where \({d}_{p}\) is mean particle diameter of the untreated sample, \(g\) is gravitational acceleration, \({\rho }_{f}\) is fluid density, \({\rho }_{p}\) is particle density, and \(\mu \) is fluid dynamic viscosity.[44] The criterion \(K\) is used to distinguish between the Stokes’ law (\(K\) < 3.3), intermediate (3.3 < \(K\) < 43.6), and Newton’s law regimes (43.6 < \(K\) < 2360).[45] Here, the flow is in the intermediate regime.
The terminal velocity of a single particle, \({U}_{t}\), in the intermediate regime is given by Eq. [9], where coefficients \(b\) and \(n\) are 18.5 and 0.6, respectively.[45,46]
The hindered terminal velocity for a cloud of particles, \({U}_{t}\)′, is calculated by Eq. [10], where \(\varepsilon \) is volume fraction of solid in suspension, and \(m\) is a function of the Reynolds number.[46]
Reynolds number of the particle, \({\text{Re}}_{p}\), is determined by Eq. [11], and \(m\) is calculated by Eq. [12] for the intermediate regime (i.e., 0.3 < \({\text{Re}}_{p}\) < 1000).[46]
\({U}_{t}\)′ is determined using an iterative approach as outlined by the following: The reactor is divided into a number of increments along its length, and the volume of each section is calculated. This, along with the known flow rate of particles into the top of the reactor and an initial estimation of residence time within a section, is used to provide an estimation of \(\varepsilon \) and \({U}_{t}\)′. This estimation of \({U}_{t}\)′ is used to estimate the residence time in the subsequent reactor section. The calculation is iterated until convergence of \({U}_{t}\)′ is achieved along the reactor length. This can be used to determine the mean particle residence time through the reactor, as well as the mean heating rate (Eqs. [2, 3]).
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