Effect of Borax-, KOH-, and NaOH-Treated Coal on Reducing Carbon Waste and Activated Carbon Production in Synthetic Rutile Production from Ilmenite

Abstract

Coal is commonly used as both fuel and reducing agent in producing synthetic rutile from ilmenite (FeTiO₃) via the Becher process, which upgrades ilmenite to high-purity TiO₂ (>88%). However, coal-based reduction generates significant carbon waste. This study investigated the effect of adding 1–5% w/w potassium hydroxide (KOH), sodium hydroxide (NaOH), and sodium tetraborate (borax) to coal during ilmenite reduction to improve metallisation and reduce carbon burn-off. Results showed that 1% w/w additives significantly increased metallisation to 96% (KOH), 95% (NaOH), and 93% (borax), compared to 80% without additives, while higher concentrations (3–5% w/w) decreased metallisation. Scanning electron microscopy (SEM)analysis showed cleaner particle surfaces and optimal metallisation at 1% w/w, whereas higher additive levels caused agglomeration or sintering due to elevated silica and alumina activity. Additive type also influenced TiO₂ quality, with KOH enhancing TiO₂ at low concentrations but causing negative effects at higher levels, while NaOH and borax reduced TiO₂ quality via sodium-based compound formation. All additives reduced carbon burn-off, with KOH producing the greatest reduction. The iodine number of the carbon residue increased with higher additive concentrations, with KOH achieving 710 mg/g at 1% w/w and 900 mg/g at 5% w/w, making the residue suitable for water treatment. Overall, KOH is the most effective additive for producing high-quality synthetic rutile while minimising carbon waste.

1. Introduction

Coal is commonly used as both a fuel and the primary reducing agent in producing synthetic rutile from titanium-bearing minerals [1,2]. However, growing interest in sustainable practices has led to efforts to minimise carbon use by adopting alternative reductants like renewable carbon or entirely replacing carbon with green hydrogen [3,4]. Traditional methods of synthetic rutile (SR) production often generate large amounts of solid carbon waste, which raises environmental concerns. Reducing carbon waste would offer both environmental and industrial benefits. This study focuses on reducing carbon waste by introducing alkaline additives in SR production and assessing their impact on SR quality.

Ilmenite, a mineral containing titanium dioxide (TiO₂) and iron oxide (FeO), is widely found in placer and beach sands. The typical composition of natural pure ilmenite includes approximately 53% TiO₂ and 47% FeO. The stoichiometric formula of ilmenite (FeTiO₃) indicates that one mole contains one mole of titanium, one mole of iron, and three moles of oxygen. Therefore, the molar ratio of titanium (Ti), iron (Fe), and oxygen (O) in ilmenite is 1:1:3. Ilmenite, containing >52% TiO₂, is upgraded to over 88% TiO₂ through the Becher process to produce SR [1,2]. This process involves two main processing steps: first, ilmenite mixed with coal is heated to around 1100 °C to reduce the iron content. The reduced ilmenite (RI) then undergoes accelerated aeration and acid leaching, producing fine iron oxide, which is easily separated. The resulting SR is a high-value product used as a substitute for natural rutile in pigment production.

Titanium is considered a critical mineral by several countries, including Australia, the United States of America, the European Union, India, Japan, and South Korea, but not by the United Kingdom. According to Geoscience Australia [5], in 2022, Australia produced 700 kt of ilmenite and 200 kt of rutile. Globally, titanium resources were over 1 million kt of ilmenite and nearly 56,000 kt of rutile, with production reaching 14,900 kt of ilmenite and 600 kt of rutile [5]. Australia has substantial reserves of beach sand deposits containing titanium-bearing minerals, particularly ilmenite (FeTiO₃), leucoxene (secondary rutile), and rutile (TiO₂) [5]. The limited supply of high-grade rutile makes upgrading ilmenite essential to meet the growing demand for high-purity titanium materials. Converting lower-grade ilmenite into SR enhances the economic value of Australia’s titanium industry, as SR commands a premium price and is ideal for producing titanium metal and pigments.

Previous studies have investigated the effect of alkali phosphate on iron-based oxygen carriers, including ilmenite, hematite, iron oxides, and iron mill scale [6,7,8]. However, limited attention has been given to their reactivity, with most research focusing on agglomeration behaviour. It has previously been observed that monopotassium phosphate (KH₂PO₄) forms alkali phosphate-rich melts with dissolved Fe, which coats oxygen carrier particles and induces defluidisation at lower quantities than other potassium salts [9]. The studies used different atmospheres (air, CO, H₂O, H₂), temperatures (850–950 °C), and potassium contents (0–10% w/w) [6]. These studies did not consider the influence of carbon, revealing a knowledge gap regarding its role as a strong reducing agent in the carbothermic reduction of chemically doped ilmenite.

The production of activated carbon from biomass using alkaline chemical activation is a well-established and widely practiced method. Strong bases such as potassium hydroxide (KOH) and sodium hydroxide (NaOH) are the most used activating agents, producing activated carbon with high surface area, well-developed microporosity, and excellent adsorption capacity [10,11]. These materials are extensively applied in water treatment, air purification, and various metallurgical and chemical processes due to their superior efficiency in removing impurities. In comparison, the use of boron-containing compounds, such as sodium tetraborate (borax), remains limited; only a few studies have explored its potential as an activating agent, indicating that it can promote carbonisation and pore development [12,13]. Using chemical activation in activated carbon production provides the advantage of higher carbon yield and reduced carbon loss, as the chemicals promote efficient carbonisation and limit the formation of volatile byproducts.

A recent study investigated the effect of alkali chlorides, such as potassium chloride (KCl) and sodium chloride (NaCl), on natural and synthetic ilmenite during chemical looping combustion in a lab-scale fluidised bed reactor [14]. Charcoal impregnated with KCl, NaCl, and a mixture of both was tested with methane and syngas. The study found that KCl- and NaCl-impregnated charcoal improved the reduction rates of natural ilmenite, while synthetic ilmenite exhibited lower reduction rates. For natural ilmenite, KCl formed K-titanate phases, while NaCl interacted with Si and Ti, forming Na₂O₅SiTi. A combination of sodium chloride (NaCl) and potassium chloride (KCl) produced both K-titanate and titanosilicates Na₂O₅SiTi phases. However, no significant K-titanate or Na₂O₅SiTi phases were detected in synthetic ilmenite samples [14]. The study was limited to the reduction of ilmenite during chemical looping combustion, with no further test work conducted on RI.

Previous research [15] explored the effect of Na₂B₄O₇ on the carbothermic reduction of Panzhihua ilmenite, which contains 46% TiO₂ and 36% Fe₂O₃. Their findings indicated that low concentrations of sodium metaborate (Na₂B₄O₇) enhance the metallisation rate, while higher concentrations negatively impact it [15]. The study was limited to the reduction process and did not investigate the impact of Na₂B₄O₇ on the production or quality of SR. Further research is needed to evaluate its influence on SR properties and quality.

All the studies reviewed here support the hypothesis that the presence of alkaline materials in ilmenite improves its reactivity. A previous study [11] reported that the increase in ilmenite reactivity is mainly due to increased porosity within ilmenite particles. It is well known that smaller ionic alkaline metals, such as potassium and sodium, are prone to diffusion, promote pore structure, and increase surface area and pore development during activated carbon production. The reactivity of coal is significantly improved when doped with alkaline metals [11]. Research to date has not yet determined the effects of impregnating coal with borax also known as sodium tetraborate (Na₂B₄O₇∙5H₂O), KOH, or NaOH on minimising carbon waste during ilmenite reduction and improving SR production and quality.

The current study investigated the effects of three additives in SR production, including borax, KOH, and NaOH, focusing on minimising carbon wastage in the ilmenite reduction process. Borax is a mild alkaline mineral used in various industries, while KOH and NaOH are strong bases with highly corrosive properties. They are commonly used in chemical manufacturing and as reagents in different industrial processes. The aim of this work was to produce SR particles suitable as feed material for pigment production, obtained through a carbothermic reduction followed by chemical leaching. During the reduction process, the carbon undergoes chemical treatment, producing activated carbon that has several environmental applications, including water treatment.

2. Materials and Methods

2.1. Sample Preparation

In this study, a representative ilmenite sample from Western Australia was used. The sample originated from a beach sands deposit, and ilmenite was produced through a series of mineral processing steps, including drying, electrostatic separation, and magnetic separation. The received ilmenite sample was dried in an oven (Thermo Fisher, Waltham, MA, USA) at 110 ± 2 °C for 4 h to remove moisture.

A representative bulk coal sample was received from the Collie coal mine in Western Australia. The sample, as received, was crushed using a laboratory jaw crusher (Essa, Perth, Australia) to reduce the particle passing size (P₈₀) from 100 mm to P₈₀ 6 mm. The crushed sample was then screened using 5 mm and 6 mm sieves, producing one fraction −6 mm + 5 mm. The coal sample was oven-dried at 110 ± 2 °C for 24 h to remove moisture. To minimise particle size effects, only the −6 mm + 5 mm fraction was used in the study.

2.2. Chemical Treatment of Coal with Borax, KOH, and NaOH

Coal samples were chemically impregnated with borax (Na₂B₄O₇∙5H₂O), KOH, and NaOH using the wet impregnation method. High-purity reagents (>99.9 mass%, particle size < 5 µm) were sourced from Rowe Scientific (Perth, WA, Australia). Impregnation was performed at chemical-to-coal mass ratios of 0%, 1%, 3%, and 5% w, with each reagent dissolved in 50 mL of deionised water. The prepared chemical solution was mixed with the coal sample at room temperature, and the mixtures were dried at 110 ± 2 °C for 24 h. The dried coal was cooled to room temperature and sieved to 6 mm to remove any fines generated during processing.

2.3. Ilmenite Reduction

The ilmenite reduction was performed in a rotating laboratory-scale kiln furnace (Nanyang Xinda Electo-Mechanical Co, Nanyang, China) at 10 rpm. A mixture of 100 g of ilmenite and 50 g of coal was heated from 30 °C to 1050 °C at 5 °C/min. The furnace maintained a temperature of 1050 °C for two hours before cooling to 30 °C at the same rate. Post-reduction, the resulting RI was separated from char particles using sieves with a mesh size of 53 and 500 μm.

2.4. Hydrochloric Acid Leaching of Reduced Ilmenites

A 20 g sample of RI and 20 mL of analytical-grade (AR) hydrochloric acid were placed in a 250 mL beaker. The acid leaching was performed on a hot plate equipped with a magnetic stirrer for 60 min. Following leaching, the slurry was cooled and screened using a 53 μm sieve to separate fine iron particles (<38 μm) from the coarse solid residue (SR) product (>60 μm). The fine fraction, comprising iron and waste, was discarded, while the coarse fraction was oven-dried for subsequent chemical analysis.

2.5. Analytical Methods

2.5.1. Proximate and Ultimate Analyses

A proximate analysis was conducted with a thermogravimetric analytical instrument (Leco TGA701 (Leco Corporation, St. Joseph, MI, USA)) to assess moisture, ash, volatile matter, and fixed carbon. Additionally, an ultimate analysis for carbon, hydrogen, oxygen, nitrogen, and sulphur was carried out using isotope ratio mass spectrometry (IRMS (Thermo Fisher Scientific, Bremen, Germany)) in conjunction with an elemental analyser.

2.5.2. Chemical Analysis

Elemental compositions of the reductants and products were determined using a Panalytical Axios^MAX advanced X-ray fluorescence (XRF) instrument (Malvern Panalytical, Almelo, Netherlands) with a copper radiation source. Chemical composition analysis was conducted using Panalytical SuperQ software (version 6).

2.5.3. Metallic Iron Determination

Metallisation refers to the rate at which ferric iron is converted to ferrous iron (Fe) in a sample. The metallisation effectively reflects the iron conversion rate in reduced ilmenite samples. It is determined using the following formula:

$$\mathrm{Metallisation} (\%) = {\displaystyle \frac{\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{d} \mathrm{F}\mathrm{e} \mathrm{i}\mathrm{n} \mathrm{r}\mathrm{e}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d} \mathrm{i}\mathrm{l}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}{\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l} \mathrm{F}\mathrm{e} \mathrm{v}\mathrm{a}\mathrm{l}\mathrm{u}\mathrm{e} \mathrm{i}\mathrm{n} \mathrm{r}\mathrm{e}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d} \mathrm{i}\mathrm{l}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}\times 100$$

(1)

Metallic iron in reduced ilmenite is determined using a diluted copper sulphate solution, where the sample is treated with the boiling solution, causing the metallic iron to dissolve and form ferrous sulphate. After filtering out insoluble materials and removing excess copper by precipitation with aluminium, the ferrous iron is titrated using the standard procedure. AR-grade reagents, including copper sulphate, aluminium powder, and titration chemicals, were purchased from Rowe Scientific (Perth, WA, Australia).

2.5.4. Iodine Number Determination

The iodine number was measured following ASTM International standard D4607 [16]. AR-grade reagents, including iodine, sodium thiosulfate, and hydrochloric acid, were sourced from Rowe Scientific (Perth) in Western Australia.

2.5.5. Mineralogical Analysis

A Rigaku SmartLab X-ray diffraction instrument (Rigaku Corporation, Tokyo, Japan), operated with a copper radiation source, an X-ray slit width of 1°, and a detector slit width of 0.1°, was used to analyse the samples.

Sample surface properties and morphology were examined using a Joel NeoScope benchtop scanning electron microscope (SEM) (Joel NeoScope, Tokyo, Japan) with an energy-dispersive spectroscopy (EDS) detector (Joel NeoScope, Tokyo, Japan). Before examination, the samples were oven-dried and carbon-coated, and the SEM-EDS was operated within a 5 to 30 kV voltage range.

2.6. Theory and Reactions

Ilmenite is effectively reduced using carbon, hydrogen, or a combination of both [3,4]. Coal is the most commonly used in industry as it serves both as a reducing agent and a fuel to maintain kiln temperatures. While solid–solid reduction of iron by carbon is slow, the solid–gas heterogeneous reaction between iron and carbon monoxide is faster and more efficient for ilmenite reduction. Several key reactions in a rotary kiln involve carbon reacting with oxygen to produce CO₂ and CO, as shown in Reactions (2)–(7).

C(s) + O₂(g) → CO₂(g)

(2)

C(s) + 0.5O₂(g) → CO(g)

(3)

C(s) + CO₂(g) → 2CO(g)

(4)

3Fe₂O₃(s) + CO(g) → 2Fe₃O₄(s) + CO₂(g)

(5)

Fe₃O₄(s) + CO(g) → 3FeO(s) + CO₂(g)

(6)

FeO(s) + CO(g) → Fe(s) + CO₂(g)

(7)

Carbon reacts with oxygen, as shown in Reactions (2) and (3). The reaction between carbon and carbon dioxide, known as the Boudouard reaction, is shown in Reaction (4). This reaction plays a critical role in ilmenite reduction, enabling the stepwise transformation of Fe₂O₃ to Fe₃O₄, FeO, and finally to Fe (metallic iron), as detailed in Reactions (5)–(7).

A recent study reported the preparation of activated carbon from biomass using NaOH, where two main Reactions (8) and (9) occurred during activation. NaOH reacted with SiO₂ ash [17]:

2NaOH(l) + SiO₂(s) → Na₂SiO₃(s) +H₂O(g)

(8)

6NaOH(l) + 2C(s) → 2Na(l) + 2Na₂CO₃(s) + 3H₂(g)

(9)

Table 1 lists the main reactions during KOH activation of biomass carbon from a recent investigation [18], showing how KOH interacts with surface functional groups and carbon to generate gases, metallic potassium, and pores, ultimately contributing to the development of the activated carbon’s porous structure.

Table 1. Principal reactions during KOH activation of biomass carbon [18].

Reaction Number	Reaction	Description
(10)	KOH + -COOH → -COO-K+ + H2O(g)	Reaction with carboxyl groups on biomass carbon
(11)	KOH + -OH → -O-K+ + H2O(g)	Reaction with hydroxyl groups on biomass carbon
(12)	2KOH → K2O + H2O(g)	Decomposition of KOH at 300–417 °C
(13)	KOH + -COO-K+ → K2CO3 + ½H2 (g)	Reaction with carboxylate groups generating hydrogen
(14)	4KOH + -CH2- → K2CO3 + K2O + 3H2 (g)	Reaction with methylene groups forming gases and oxides
(15)	CH2· + 2H → CH4(g)	Methane formation from surface radicals
(16)	½K2CO3 + -C- → K + 3/2CO(g)	High-temperature reaction at carbon defects producing K vapour
(17)	K2O + -C- → 2K + CO(g)	K2O reacting with carbon defects to form metallic potassium
(18)	H2O + -C- → CO(g) + H2(g)	Steam-carbon reaction contributing to pore formation

Table 1. Principal reactions during KOH activation of biomass carbon [18].

Reaction Number	Reaction	Description
(10)	KOH + -COOH → -COO-K+ + H2O(g)	Reaction with carboxyl groups on biomass carbon
(11)	KOH + -OH → -O-K+ + H2O(g)	Reaction with hydroxyl groups on biomass carbon
(12)	2KOH → K2O + H2O(g)	Decomposition of KOH at 300–417 °C
(13)	KOH + -COO-K+ → K2CO3 + ½H2 (g)	Reaction with carboxylate groups generating hydrogen
(14)	4KOH + -CH2- → K2CO3 + K2O + 3H2 (g)	Reaction with methylene groups forming gases and oxides
(15)	CH2· + 2H → CH4(g)	Methane formation from surface radicals
(16)	½K2CO3 + -C- → K + 3/2CO(g)	High-temperature reaction at carbon defects producing K vapour
(17)	K2O + -C- → 2K + CO(g)	K2O reacting with carbon defects to form metallic potassium
(18)	H2O + -C- → CO(g) + H2(g)	Steam-carbon reaction contributing to pore formation

Borax can act as a chemical activating agent during the pyrolysis of biomass [12]. Upon thermal treatment under an inert atmosphere, it facilitates dehydration, cross-linking, and aromatization of the carbonaceous matrix, enhancing carbon retention and promoting the development of a porous structure. Simultaneously, borax can react with carbon and inorganics to form boron oxides and sodium-containing byproducts, which can be removed post-pyrolysis to yield activated carbon with high surface area and porosity.

Simplified Reaction (19):

$$\mathrm{Biomass} + {\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7 \stackrel{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t},\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{t} \mathrm{g}\mathrm{a}\mathrm{s}}{\to }{\mathrm{C}}_{\mathrm{activated}} + {\mathrm{B}}_2{\mathrm{O}}_3 + {\mathrm{Na}}_2\mathrm{O} + \mathrm{Volatile} \mathrm{compounds}$$

(19)

The key Reactions (20)–(24) occurring during the hydrochloric acid leaching of RI involve the selective dissolution of iron oxides and chlorination processes, ultimately facilitating the production of synthetic rutile while leaving titanium predominantly in the solid phase [19].

Leaching of pre-reduced ilmenite in HCl:

TiO₂(s) + Fe(s) + 4H⁺(aq) → Ti²⁺(aq) + Fe²⁺(aq) + 2H₂O(l)

(20)

Oxidation of Ti²⁺ to TiO₂:

Ti²⁺(aq) + 0.5O₂(g) + H₂O(l) → TiO₂(s) + 2H⁺(aq)

(21)

Oxidation of Fe²⁺ to Fe³⁺:

Fe²⁺(aq) + 2H⁺(aq) + 0.5O₂(g) → Fe³⁺(aq) + H₂O(l)

(22)

Formation of ferrous chloride:

Fe²⁺(aq) + 2Cl⁻(aq) + 0.5O₂(g) → FeCl₂(aq) + H₂O(l)

(23)

Formation of ferric chloride:

Fe²⁺(aq) + 2Cl⁻(aq) + 0.25O₂(g) → FeCl₃(aq) + 0.5H₂O(l)

(24)

The key reactions involved in the formation of sodium and potassium titanates and borates during additive-assisted ilmenite reduction are presented in

Table 2. Key reactions forming sodium and potassium titanates and borates [20].

Reaction Number	Reaction	Phase Formed	Comments
(25)	Na2O + B2O3 → 2NaBO2	Sodium metaborate	Forms from sodium oxide + boron oxide; borax decomposition at high temperature
(26)	Na2O + TiO2 + B2O3 → Na2TiB2O7	Sodium titanium borate	Forms in presence of TiO2 and boron
(27)	K2O + TiO2 → K2TiO3	Potassium titanate	Forms a titanate compound; acts as a flux similar to sodium titanates
(28)	K2O + B2O3 → KBO2	Potassium metaborate	Analogous to NaBO2 formation
(29)	K2O + TiO2 + B2O3 → K2TiB2O7	Potassium titanium borate	Similar structure to sodium counterpart

. At high temperatures, sodium and potassium oxides react with titanium dioxide to form various titanates. During borax decomposition, sodium oxide reacts with boron oxide to form sodium metaborate, which can further react with titanium dioxide to form borate.

3. Results and Discussion

3.1. Characterisation of Samples

3.1.1. Proximate and Ultimate Analyses of Coal Sample

Table 3. Proximate, ultimate analyses and calorific value of coal (air-dried).

Parameter	Value (mass%)
Calorific Value (MJ/kg)	23.7± 0.5
Proximate Analysis	-
Moisture	3.6 ± 0.2
Volatile Matter	37.3 ± 0.8
Ash	2.5 ± 0.3
Fixed Carbon	56.5 ± 0.9
Ultimate Analysis	-
Carbon (C)	63.4 ± 0.4
Hydrogen (H)	7.1 ± 0.2
Nitrogen (N)	1.3 ± 0.1
Oxygen (O *)	27.3 ± 0.2
Sulphur (S)	0.9 ± 0.1

* Oxygen was calculated by the difference.

shows the proximate and ultimate analyses and the calorific value of the coal sample used in the current study. The semi-bituminous Collie coal sample exhibits a moderate calorific value of 23.7 MJ/kg, with low ash content (2.5%) and a high fixed carbon percentage (56.5%). Ultimate analysis reveals a significant carbon content (63.4%) alongside a moderate hydrogen value (7.1%). The low sulphur content (0.9%) suggests limited potential for sulphur emissions during combustion, making it a relatively cleaner coal option.

3.1.2. Mineralogical Analysis of Coal and Ilmenite

The XRF analysis results of the ilmenite and coal ash samples are presented in

Table 4. X-ray fluorescence analysis results of ilmenite and coal ash samples.

Oxides	Ilmenite	Coal Ash
	%(w/w)	%(w/w)
TiO2	59.28	8.14
Fe2O3	34.16	19.60
MnO	1.06	0.18
Al2O3	0.70	19.12
SiO2	0.60	43.01
MgO	0.27	1.68
V2O5	0.25	0.04
Cr2O3	0.18	0.11
Nb2O5	0.15	0.02
P2O5	0.10	0.24
ZrO2	0.10	0.09
PbO2	0.04	0.01
CaO	0.03	1.26
CeO2	0.02	0.06
La2O3	0.01	0.03
SO3	0.01	0.68
Others	3.04	5.11

. Ilmenite predominantly comprises TiO₂ (59.3%) and Fe₂O₃ (34.2%). This sample had impurities, including MnO, MgO, Cr₂O₃, and aluminosilicates (Al₂O₃ and SiO₂). The present ilmenite sample had slightly higher TiO₂ content than natural ilmenite, indicating an upgraded TiO₂ concentration due to natural weathering.

The mineralogical matter of the ashed coal sample is dominated by silica and alumina, with a high SiO₂ content (43.0%), Al₂O₃ (19.1%), and Fe₂O₃ (19.6%). Minor impurities in the ashed coal sample include the basic oxides MgO and CaO and the acidic oxide SO₃.

Figure 1 shows SEM images of ilmenite grains, displaying various morphologies. Most particles range from rounded to sub-rounded, with a few angular grains observed. Rounded grains with pitted and fractured surfaces indicate mechanical interactions, likely from particle collisions during transport, while flat surfaces suggest erosion. SEM-EDS analysis confirms the deposition of fine rutile particles (Figure 1b). These findings support the hypothesis that the observed pits and grooves on the grain surfaces likely result from chemical dissolution or leaching in reactive environments.

Figure 2 shows SEM images of coal samples at magnifications of 200 µm and 50 µm. The analysis showed surface cracks of varying lengths and the presence of mineral matter on the coal surfaces. As shown in Figure 2b, kaolinite is visible on the surface. In low-rank bituminous coals, the inorganic components primarily comprise distinct mineral grains, with minimal amounts of organically bound material. Coal typically contains kaolinite, quartz, pyrite, and calcite minerals [21]. These minerals can occur in the coal as isolated particles within mineral-rich partings, secondary infillings of plant pores, fracture or cleat fillings, or as finely dispersed individual grains [22].

3.2. Characterisation of Products

3.2.1. Metallisation Analysis of Reduced Ilmenite

Figure 3 shows the metallisation results of reduced ilmenite against the concentration of coal-impregnated additives (mass%). Without any additive, the reduced ilmenite achieved a metallisation of 80%. The metallisation percentage increases significantly at a lower additive concentration of 1% w/w NaOH, KOH, or borax, achieving the highest values of 96% for KOH, 95% for NaOH, and 93% for borax. However, metallisation decreases at higher additive concentrations (3 and 5% w/w), with KOH consistently yielding the highest metallisation across all concentrations.

Table 5. Chemical analysis results of reduced ilmenite samples.

Oxide	Additive	NaOH			KOH			Borax
-	0%	1%	3%	5%	1%	3%	5%	1%	3%	5%
TiO2	64.96	66.34	66.24	65.95	66.46	66.30	66.06	66.21	66.02	65.86
Fe2O3	6.84	1.78	2.22	3.59	1.23	1.98	3.04	2.36	3.25	4.00
Fe	19.10	22.63	22.32	21.37	23.02	22.49	21.75	22.23	21.61	21.08
MnO	1.16	1.18	1.18	1.17	1.18	1.18	1.17	1.17	1.17	1.17
Al2O3	0.76	0.78	0.80	0.82	0.78	0.79	0.81	0.78	0.85	0.92
SiO2	0.66	0.67	0.71	0.75	0.67	0.70	0.72	0.74	0.77	0.79
MgO	0.29	0.30	0.30	0.30	0.30	0.30	0.30	0.30	0.30	0.30
V2O5	0.27	0.28	0.28	0.28	0.28	0.28	0.28	0.28	0.28	0.28
Cr2O3	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20	0.20
Others	3.74	3.80	3.83	3.86	3.79	3.81	3.83	3.80	3.86	3.92

shows the chemical analysis of reduced ilmenite with and without additives. The data show the major oxide concentrations of TiO₂, Fe₂O₃, and other minor oxides. During reduction, the removal of oxygen increases the metallised Fe content. This reduction and associated mass loss also led to a relative increase in TiO₂ content.

3.2.2. Mineralogical Analysis of Reduced Ilmenite Samples

Figure 4 shows the XRD (X-ray diffraction) spectra of ilmenite samples reduced with varying concentrations (1%, 3%, and 5%) of coal-impregnated additives, including borax, KOH, and NaOH. The results indicate that the reduced ilmenite samples predominantly consisted of rutile and metallic iron, with minor phases of ilmenite and pseudobrookite (FeTi₂O₅). The primary diffraction peak for metallic iron was observed in the 2θ range of 44.6° to 44.8°, with a secondary peak at 65°. The main rutile diffraction peak appears at a 2θ value of 27.5°, and its sharpness indicates that the rutile phase is highly crystalline.

Figure 5, Figure A1, and Figure A2 show SEM images of reduced ilmenite samples, showing the influence of different additive concentrations. The same coal was used as the reducing agent, but different additive levels caused significant morphological changes in the metallic iron features. Higher magnification images (Figure 5b,d,f, Figure A1b,d,f and Figure A2b,d,f) reveal variations in metallic iron morphology with increasing additive concentration. Additive levels of 5% NaOH led to the agglomeration of finer reduced ilmenite particles, while 5% borax caused sintering of the particles. These effects are likely due to the enhanced activity of SiO₂ and Al₂O₃ at elevated additive concentrations. In contrast, samples with 1% additive displayed cleaner particle surfaces compared to those treated with higher concentrations.

3.2.3. Chemical Analysis of Synthetic Rutile

Figure 6 shows the weight per cent of TiO₂ and Fe₂O₃ in the SR from leached RI, plotted against the concentration of coal-impregnated additives (mass%). It is apparent from this figure that, without any additives, SR contained 81% w/w TiO₂ and 7% w/w Fe₂O₃. SR produced from ilmenite using coal with 1% w/w additives achieve the highest TiO₂ content, with values of 96%, 95%, and 94% for 1% w/w KOH, 1% w/w NaOH, and 1% w/w Borax, respectively. The Fe₂O₃ content is lowest at 1% for 1% KOH, increasing to over 2% for 1% NaOH and 1% Borax, respectively. The data shows that as additive concentrations increase, TiO₂ content decreases slightly, while Fe₂O₃ content increases.

3.2.4. Scanning Electron Microscopy Analysis of Synthetic Rutile Samples

SEM analysis was performed on SR samples made from ilmenite reduced with coal-impregnated additives. The results at different magnifications are shown in Figure 7, Figure A3 and Figure A4. The samples appeared visually clean, with noticeable variations in appearance, highlighting the effect of the coal additives. SR samples with 1% w/w borax and NaOH as coal-impregnated additives show surface cracking activity (Figure 7d,h). Increasing the additive concentration results in a completely altered surface appearance, indicating a significant chemical attack on the particles (Figure A4b,f).

The SR samples show a honeycomb-shaped appearance with interconnected pores, indicating a high surface area, in the presence of 1% w/w KOH (Figure 7d) and 3% w/w KOH (Figure A3d). At 5% w/w KOH, pitting becomes evident (Figure A4d), suggesting enhanced leaching activity. Additionally, a significant number of fine particles containing >98% TiO₂ were observed on the surface, as confirmed by scanning electron microscopy—energy-dispersive X-ray spectroscopy (SEM-EDX) analysis.

3.3. Effect of Additives on Synthetic Rutile Quality

3.3.1. Effect of Borax

The mineralogical analysis of SR produced from ilmenite reduced with coal, with and without borax additive, are shown in Figure 8. Without any additive, the main phases in the SR were ilmenite and rutile, with the primary XRD peak for the rutile phase, observed at 27.5°, alongside secondary peaks, as marked in Figure 8. The spectra indicate that the conversion of ilmenite to SR is incomplete without an additive. As the borax additive concentration increases, the intensity of the main rutile peak decreases, while the intensity of several secondary rutile peaks increases. At a borax concentration of 5% w/w, the presence of NaBO₂ (sodium metaborate) and Na₂TiB₂O₇ (sodium titanium borate) becomes evident.

The addition of borax influences the phase composition and crystallinity of SR, as evident from the XRD analysis. As borax concentration increases, the formation of sodium-based compounds such as NaBO₂ and Na₂TiB₂O₇ is promoted, indicating interactions between borax and titanium phases. These interactions disrupt the rutile structure, resulting in a decrease in TiO₂ grades and an increase in Fe₂O₃ grades, as shown in the chemical analysis in Figure 8. The reduction in rutile crystallinity is attributed to the formation of these secondary phases, which alter the mineralogical composition. This observation suggests that borax concentrations above 1% w/w negatively affect SR quality by forming disruptive sodium-based phases.

3.3.2. Effect of KOH

Figure 9 shows the mineralogical analysis of SR produced by reducing ilmenite with coal, both with and without the addition of KOH. The primary XRD peak for the rutile phase was observed at 27.5°, along with secondary peaks as marked. Without the KOH additive, residual ilmenite peaks are evident, indicating incomplete conversion of ilmenite to SR. At 1% w/w KOH, a sharp rutile peak is observed, suggesting improved crystallinity. As the KOH content increases, minor peaks corresponding to potassium-titanium interactions emerge. This was particularly noticeable at 5% w/w KOH, where the formation of potassium titanate (K₂TiO₃) was identified in the XRD spectra, indicating the incorporation of potassium into TiO₂ phases.

The addition of KOH significantly affects the crystallinity and phase composition of SR, similar to borax. The increase in KOH concentration above 1% w/w leads to the formation of disruptive potassium-based phases, which negatively impact SR quality. Chemical analysis in Figure 6 shows that this results in a decrease in TiO₂ grades and an increase in Fe₂O₃ grades, highlighting the detrimental effect of high KOH concentrations on the mineralogical composition. However, the KOH additive consistently produced higher TiO₂ grades compared to borax and NaOH.

3.3.3. Effect of NaOH

Figure 10 shows the mineralogical analysis of SR produced by reducing ilmenite with coal, both with and without the addition of NaOH. The primary rutile XRD peak was observed at 27.5°, with several secondary rutile peaks labelled. As the NaOH content increased, several minor and overlapping peaks were noted, primarily due to the interaction of sodium with TiO₂. Notably, at NaOH concentrations of 3% w/w and 5% w/w, the formation of sodium titanate (Na₂TiO₃) was observed, indicating the impact of NaOH on the mineralogical composition of the product.

The presence of NaOH in coal negatively affects the crystallinity and phase composition of SR, even at concentrations as low as 1% w/w. As the NaOH content increases beyond 1% w/w, the interaction with TiO₂ intensifies, leading to the formation of sodium-based titanium phases. Some sodium becomes chemically bound with TiO₂ and Fe₂O₃, as evidenced by the chemical analysis, which results in a reduction in the TiO₂ and Fe₂O₃ grades of the SR, as shown in Figure 6. Based on the current results, in terms of positively affecting SR TiO₂ grade, coal impregnated with KOH is more effective than NaOH, and NaOH, in turn, is more effective than borax. This suggests that KOH has the most beneficial impact on TiO₂ grade among the additives tested.

3.4. Effect of Additives on Reducing Carbon Burn-Off in Synthetic Rutile Production

Figure 11 shows the effect of various coal-impregnated additives (NaOH, KOH, and borax) on the percent carbon burn-off at different additive concentrations (0, 1, 3, and 5% w/w). At 0% w/w additive, the carbon burn-off percentage was recorded at 71%. This baseline value indicates the natural carbon burn-off rate without the influence of any additives. As the additive concentration increased to 1% w/w, a noticeable reduction in carbon burn-off was observed across all three additives. The burn-off percentages were 63, 53, and 58% for borax, KOH, and NaOH, respectively. This indicates that the presence of additives starts to inhibit the carbon burn-off process, though borax exhibited the least reduction in carbon burn-off among the three.

The reduction in carbon burn-off became more significant at higher concentrations (3 and 5% w/w). At 3% w/w, burn-off percentages decreased to 46, 41, and 50% for NaOH, KOH, and borax, respectively. At 5% w/w, the burn-off further decreased to 29% for NaOH, 26% for KOH, and 35% for borax. The results highlight that the additives significantly reduce carbon burn-off, with the inhibition effect increasing as the concentration of the additives rises. Borax was found to be the least effective additive in comparison to NaOH and KOH, showing a smaller reduction in burn-off. As the concentration of the additives increases, they likely promote stronger interactions with the carbon, which could lead to more controlled combustion at elevated temperatures, potentially stabilising the carbon structure and reducing its reactivity.

With 1% w/w KOH, the carbon burn-off was reduced by nearly 19% compared to the baseline case without any additive. This study found that at 1% w/w KOH concentration, the SR product grade and quality were higher compared to NaOH and borax. KOH could play an important role in SR production by minimising carbon burn-off while maintaining product grade and quality.

3.5. Effect of Additive on Activated Carbon in Synthetic Rutile Production

The effect of additive concentration on the percentage of carbon residue for NaOH, KOH, and borax is shown in Figure 12. The results show a clear increase in carbon residue with higher additive concentrations, indicating the effectiveness of additives in carbon retention. Carbon residue rose from 15% at 0% w/w additive to 36% with NaOH, 37% with KOH, and 33% with borax at 5% w/w additive. KOH showed the highest retention, indicating higher catalytic effects. Borax also improved carbon retention but was less effective than the alkali-based additives. The performance of NaOH and KOH is attributed to their stronger chemical interactions with carbon-containing compounds, with KOH slightly outperforming NaOH due to its greater catalytic efficiency.

The effect of coal-impregnated additives (NaOH, KOH, and borax) concentrations on the iodine number of carbon residue is shown in Figure 13. The results indicate that iodine number of carbon residue increases with the concentration of additives, with NaOH, KOH, and borax showing similar trends. For NaOH, the iodine number rises from 410 to 790 mg/g as the additive concentration increases from 0 to 5% w/w. KOH shows a slightly higher increase, starting at 410 and reaching 900 mg/g at an additive concentration of 5% w/w. Borax exhibits a moderate increase, from 410 at 0 concentration to 645 mg/g at 5% w/w.

At 1% w/w KOH, the iodine number of the carbon residue reaches 710 mg/g, which is considered suitable for water treatment applications. Activated carbon with an iodine number greater than 700 is generally effective for this purpose, indicating the potential of KOH-treated carbon for water filtration. Higher additive concentrations increase iodine number but negatively impact the quality of SR products. At minimal concentrations, KOH proves to be promising for both SR production and activated carbon production, balancing performance with product quality.

3.6. Environmental Impact of Additives

Table 6. Environmental and safety comparison of KOH, NaOH, and borax.

Property	NaOH	KOH	Borax
Electricity requirement (kWh/kg product)	2.2	3.0	1.2 (mining/refining equivalent)
Carbon footprint (current, kg CO2 e/kg) *	1.1	1.5	0.6
Carbon footprint (green electricity, kg CO2 e/kg) *	~0.05–0.10	~0.05–0.12	0.20–0.40
Energy demand	High; energy-intensive electrolysis process	High; similar to NaOH	High; mining and refining processes
Recycle for activated carbon production	No	Yes	No
Toxicity: Corrosivity/Acute hazard	10	10	2
Toxicity: Acute oral	4	3	2
Toxicity: Chronic	2	2	8

* CO₂ emissions calculated as: Electricity × CO₂ factor. Grid average = 0.5 kg/kWh, Green = 0.02 kg/kWh.

shows the environmental impact, energy use, and safety of KOH, NaOH, and borax. Both NaOH and KOH require considerable energy for production via electrolysis, with electricity demands of 2.2–3.0 kWh/kg [23,24], whereas borax has a lower energy requirement of 1.2 kWh/kg [25]. The current carbon footprints are greatest for KOH (1.5 kg CO₂ e/kg) and NaOH (1.1 kg CO₂ e/kg), while Borax remains moderately carbon-intensive (0.6 kg CO₂ e/kg [25,26,27]. The production of KOH and NaOH can significantly reduce CO₂ emissions when powered by clean, green energy sources, as the electricity used in the electrolysis processes would result in close to zero emissions. However, the environmental impact still exists in terms of raw material extraction, transportation, and some process-specific emissions. The production of borax involves the extraction of boron from ores, followed by refining, which is both energy-intensive and associated with CO₂ emissions [28]. The process of refining borax often involves high temperatures, contributing to emissions unless clean energy is used.

Borax is not biodegradable and can have toxic effects [29], making it less sustainable in the long term due to the significant environmental impacts associated with its mining and processing. KOH is commonly recommended than NaOH for the sustainable production of activated carbon, as it is more effective and can be recovered and reused [11,21]. While both KOH and NaOH exhibit toxic properties, the recyclability of KOH renders it a comparatively more environmentally sustainable option. A recent study on a coconut shell-based activated carbon production compared KOH and NaOH activation routes, assessing environmental impacts across eighteen metrics [30]. For 1 kg of activated carbon, KOH activation required slightly more energy (7.87 kWh) than NaOH (7.52 kWh) and had a marginally higher carbon footprint (1.26 vs. 1.21 kg CO₂ eq.). When evaluated per adsorption capacity, the KOH pathway demonstrated higher efficiency, achieving 5% better energy performance and 6% lower carbon emissions than NaOH due to its greater adsorption capacity. Both borax and NaOH pose greater environmental challenges in production and disposal, positioning KOH as the greener alternative when sustainably sourced.

4. Conclusions

Coal is currently used as a fuel and a reducing agent in producing synthetic rutile from ilmenite through the Becher process. The process produces large amounts of carbon waste, causing environmental concerns. This study explored the use of three different additives, namely potassium hydroxide, sodium hydroxide and sodium tetraborate, on minimising carbon wastage in the ilmenite reduction process and hence environmental impact.

The study shows that the metallisation of ilmenite improves with the addition of 1% w/w NaOH, KOH, and borax, reaching 96%, 95%, and 93%, respectively, while higher additive concentrations (3–5% w/w) result in reduced metallisation. KOH consistently produces the highest metallisation across all concentrations. SEM analysis indicates that 5% w/w NaOH causes particle agglomeration, and 5% w/w borax induces sintering, likely due to elevated SiO₂ and Al₂O₃, whereas 1% w/w additives maintain clean particle surfaces, reflecting optimal metallisation. The study further shows that borax and NaOH can reduce TiO₂ quality through sodium-based compound formation, while KOH enhances TiO₂ quality at low concentrations but negatively affects it at higher concentrations due to potassium compound formation.

Coal-impregnation additives (NaOH, KOH, and borax) significantly reduce carbon burn-off, making the effect more noticeable at higher concentrations. Among the additives, KOH exhibited the highest reduction in burn-off and provided the best synthetic rutile quality at 1% w/w. The iodine number of carbon residue increases with higher concentrations of coal-impregnated additives, with KOH showing the highest rise, reaching 900 mg/g at 5% w/w. At 1% w/w KOH, the iodine number reaches 710 mg/g, making the carbon residue suitable for water treatment applications, while higher concentrations of additives improve iodine number but reduce synthetic rutile quality.

Overall, KOH demonstrated higher catalytic effects in increasing carbon residue retention and iodine number, producing the best TiO₂ quality among the additives tested. This makes it the most effective option for increasing synthetic rutile production, potentially producing high-quality activated carbon suitable for water treatment applications.