ABSTRACT
The Bayer process holds an exclusive status for alumina extraction, but a massive amount of caustic ”red mud” waste is generated. In this work, three oxalate reagents: potassium hydrogen oxalate (KHC2O4), potassium tetraoxalate (KHC2O4·H2C2O4), and oxalic acid (H2C2O4) were investigated for the Al and Fe extraction process from NIST SRM 600 – Australian Darling range bauxite ore. More than 90% of Al and Fe was extracted into the aqueous phase in less than 2 h with 0.50 M\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) for all three reagents. The Fe and Al can be selectively precipitated by hydrolyzing the aqueous phase. By acidifying the Al and Fe free filtrate, 80% of the\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) can be precipitated as KHC2O4·H2C2O4. Greater than 90% of the aqueous acid can also be recycled using a cation exchange resin. The proposed closed-loop process is an energy-efficient, cost-effective, environmentally-friendly route for extracting Al and Fe from bauxite ore.
KEYWORDS. bauxite, metal extraction, ore refining, oxalic acid, red mud, leaching
INTRODUCTION
Aluminum is a lightweight, durable, recyclable metal with its primary use in the production of high strength alloys in combination with other metals like Ni, Zn, Cu, and Mn. These alloys are used in a broad range of industries that vary from automobile manufacturing to aeronautical applications.1 The primary source of Al is aluminum oxide (alumina) present in bauxite ore. More than 90% of globally mined bauxite is used for Al production. On average, a bauxite ore contains 30-60 wt% of alumina, the rest being a mixture of iron oxides and quartz.2 Commercially, bauxite is refined using the Bayer process to produce smelter-grade alumina that is then converted to Al metal using the Hall–Héroult process.1, 3
In bauxite, Al is present in the form of aluminum oxide trihydrate like gibbsite (Al2O3·3H2O or Al(OH)3) and monohydrate minerals such as boehmite (Al2O3·H2O or γ-AlO(OH)) and diaspore (Al2O3·H2O or α-AlO(OH)). Other minerals which can be found in bauxite include hematite (Fe2O3) and quartz (SiO2). The Bayer process involves the digestion of crushed bauxite in concentrated NaOH solution at high temperatures. The Al present in the ore reacts with NaOH to form water soluble sodium aluminate (NaAlO2) leaving behind an insoluble solid residue (red mud). However, the dissolution of SiO2 and Al in concentrated NaOH solution makes the Bayer process inefficient for low-grade bauxite ores (greater than 10 wt% SiO2content).
The bauxite refining industry faces a global environmental issue because of the disposal problems associated with the caustic bauxite tailings commonly referred to as red mud.4-5 The red mud is discharged from the process as an alkaline slurry with a pH > 12. It primarily consists of bauxite tailings like iron oxide (Fe2O3) and quartz (SiO2).6 Typically, about 1.0-1.5 tons of red mud are produced per ton of alumina in the Bayer process.7 For the disposal of red mud, methods like landfills, deep-sea dumping, or storage in open ponds or reservoirs are utilized. The high alkalinity of the red mud pollutes the land and threatens plant growth and wildlife.8-9 With the growing demand for Al, the disposal methods of red mud are an issue that needs global attention. In the past two decades, there have been numerous red mud incidences because of its disposal. The most disastrous incident occurred in Hungary in 2010, when the Ajka refinery dam collapsed, resulting in red mud flooding the nearby area. The release of approximately 1 million cubic meters of red mud contaminated more than 40 square kilometers of land and led to 9 deaths and 122 severely injured.10-11
To solve the problems associated with red mud, either environmentally-friendly techniques have to be utilized to dispose of red mud responsibly, or it can be eliminated at the source by developing a closed-loop bauxite refining process. Numerous researchers have worked on the recovery of valuable metals from red mud using both pyrometallurgy and hydrometallurgy.12-16Pyrometallurgy is energy intensive, whereas hydrometallurgical processes using inorganic acids (e.g., sulfuric acid and nitric acid) poses significant environmental risk from the emission of SOXand NOX. The large amount of acid initially involved for neutralizing the caustic red mud and the handling of effluents create an additional burden on the red mud processing.10 In this work, we are investigating a closed-loop hydrometallurgical approach for bauxite refining with minimal waste to eliminate the concerns associated with red mud. In our alternative approach for bauxite refining, oxalic acid (H2C2O4) and two of its derivatives with potassium oxalate (K2C2O4·H2O): potassium hydrogen oxalate (KHC2O4) and potassium tetraoxalate (KHC2O4·H2C2O4) are investigated as reagents to extract Al and Fe from bauxite.
The oxalate ion is a bidentate ligand with excellent chelation properties. The H2C2O4and its derivatives (KHC2O4 and KHC2O4·H2C2O4) utilize the chelation property along with the acidity to extract metals from the metal oxides. Previously, H2C2O4 has been used to extract metals from various sources ranging from spent lithium-ion battery cathodes17-18 to ores such as laterite19 and scheelite.20-21Corbin et al. developed two environmentally-friendly closed loop processes for extraction of Fe and Ti from ilmenite using ammonium hydrogen oxalate (NH4HC2O4)22and trimethylammonium hydrogen oxalate ((NH(CH3)3)HC2O4).23The H2C2O4 and its derivatives can be advantageous for metal separations in aqueous medium. Most of the divalent (M2+) metal ions are known to form insoluble metal oxalate compounds, whereas monovalent (M+) and trivalent (M3+) metal ions form soluble metal oxalates.24-25 The difference in the aqueous solubility can be utilized to separate metal oxalate compounds.
In this study, a standard bauxite material from the Australian Darling range (NIST SRM600) has been used to investigate the feasibility of a closed-loop Al and Fe recovery process using H2C2O4, KHC2O4, and KHC2O4·H2C2O4. The separation of SiO2 from Al2O3 and Fe2O3 is a major advantage of using an acidic process. The Al and Fe from their respective metal oxides are leached into the aqueous phase, whereas silica remains in the solid phase. The Al and Fe extracted in the aqueous phase can be separated using selective hydrolysis, and pH conditions have been optimized for efficient separation. However, the H2C2O4 and K2C2O4 are more expensive than inorganic acids such as H2SO4 and HNO3.17 To offset the cost of oxalate-based acids and make this process economical, an ion-exchange resin and a pH-based separation have been developed to recover the oxalate-based acids in their original form. To the best of our knowledge, this is the first study on extraction of metals from bauxite using oxalic acid and its derivatives. This novel closed-loop process is an environmentally-friendly and economical route for recovering Al and Fe from bauxite ore.
EXPERIMENTAL SECTION
Materials. In this study, NIST SRM 600 – Bauxite, Australian-Darling Range, H2C2O4·2H2O (ACROS Organics, 99.5%), K2C2O4·H2O (Alfa AesarTM, 98.8%), and deionized water were used for the metal extraction experiments. Potassium hydroxide, (KOH Pellets, Fisher Chemical) and Fe metal powder (20 mesh, Alfa AesarTM, 99%) were used for metal precipitation and hydrolysis, respectively. Sulfuric acid (H2SO4, Fisher Scientific, 98%) was used for acidification and regeneration of oxalate.
Reactor Setup and Sampling. The metal extraction experiments were carried out in a 1-L glass reactor attached to a 5-neck Duran® head with two thermocouples, an electric agitator, and a reflux condenser. The reactor was enclosed in a heating jacket controlled by a set of PID temperature controllers. The reflux condenser was connected to a chiller operating at 4 °C to avoid water loss during the experiment. A detailed reactor schematic can be found in our previous work.17 The reactor temperature and agitation speed (N s) were set at 98 °C and 600 rpm, respectively, for all the experiments in this work. The temperature and agitation speed values were optimized to maximize the kinetics and avoid any diffusion limitation. Samples were withdrawn from the reactor at specific intervals using a 20 cm long needle connected to a 5 mL syringe. The withdrawn samples were centrifuged in a Falcon® tube for 5 min at N s = 4000 rpm to separate out the solids. The aqueous phase was diluted with 5 wt% nitric acid solution at a ratio of 1:10 for the measurement of Al and Fe concentrations.
Metal Extraction, Hydrolysis, and Acid Regeneration. Metal extraction experiments were carried out by mixing the oxalate reagents and heating them to a set temperature and then adding the required amount of bauxite. Aqueous H2C2O4 with or without K2C2O4 was used in each experiment. The H2C2O4and K2C2O4 molar ratio is critical for the synthesis of KHC2O4and KHC2O4·H2C2O4. The reaction parameters for an efficient hydrometallurgical extraction are the acid concentration, temperature, solid-to-liquid ratio (S/L), and agitation speed. As mentioned in the previous section, the temperature and agitation speed were kept constant, while the effect of acid concentration was studied for all three oxalate reagents (H2C2O4, KHC2O4, and KHC2O4·H2C2O4). The effect of increasing the S/L ratio was studied only with KHC2O4·H2C2O4. Once the Al and Fe metals were extracted into the aqueous phase, the Fe was precipitated as Fe(OH)3 via hydrolysis by increasing the pH using KOH. After removal of the Fe, the Al was precipitated from the solution by lowering the pH to an appropriate range by adding either H2SO4 or H2C2O4. The specific range of pH for efficient precipitation of Al and Fe is discussed in the results and discussion section. The precipitation experiments were performed at 20 °C to maximize the precipitation efficiency using minimum energy.
The oxalate reagents were regenerated using two methods. The first approach involved using a strong acid cation exchange resin such as Amberlyst-15 H-form to decrease the pH. In this work, a batch process was used for the acid regeneration by mixing the activated resins with the filtrate in a benchtop shaker while monitoring the pH. After achieving the desired pH, the resins were regenerated by soaking them in a 1 M sulfuric acid solution for 24 h. In the presence of a strong acid, the resins regain their initial H-form. The regenerated resins were washed with DI water until the effluent was pH neutral before performing another ion exchange. The washing step removed any excess acid present on the resin beads. The washed and regenerated resins can be repeatedly used for additional metal precipitation. The second method for regenerating the oxalate reagents involved acidification of the filtrate post metal precipitation to the initial pH using H2SO4. The acidification will precipitate either KHC2O4 or KHC2O4·H2C2O4,depending on the pH range. To minimize the amount of water added, 98 wt% H2SO4 was used in the acidification process. This pH-based process utilizes solubility differences for separation and is discussed in detail in the results and discussion section.
Characterization. The metal concentrations in the solid and aqueous phase were measured using an inductively coupled plasma – optical emission spectrometer (ICP-OES). A Varian/Agilent 725 ES ICP-OES with simultaneous CCD detector was used for the measurements, and a Varian/Agilent SP3 autosampler was used to sequence multiple samples. The aqueous phase samples were diluted 100 times with 5 wt% HNO3 before analyzing them by ICP. Elemental compositions of the solid phases were identified using X-Ray fluorescence (XRF) using a Malvern Panalytical Zetium (1 kW) instrument with a Rh anode and a 75 μm Be window with a duplex detector. Phase identification and crystallinity measurements were performed on a Bruker D2 phaser powder X-ray diffraction (PXRD) with a Co Kα radiation source (λ = 1.78897 Å). The source voltage and current were set at 30 kV and 10 mA, respectively. The data were collected in the 2θ range of 10−70° with a step size of 0.02° and dwell time of 0.40 s per step. XRD patterns were analyzed using MDI Jade 6 software.
RESULTS AND DISCUSSION
Synthesis of Potassium Hydrogen Oxalate and Potassium Tetraoxalate. H2C2O4 is a diprotic acid with pKa1 = 1.23 and pKa2 = 4.19 at 20 °C. Both KHC2O4 and KHC2O4·H2C2O4contain the binoxalate anion (\(\mathrm{H}\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{\mathrm{-}}\)). Based on the speciation of oxalic acid, H2C2O4 is the predominant species below a pH = 1.23,\(\mathrm{H}\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{\mathrm{-}}\)is the predominant species between pH = 1.23 and 4.19, and\(\mathrm{C}_{\mathrm{2}}\mathrm{O}_{\mathrm{4}}^{2\mathrm{-}}\) is the predominant species above pH = 4.19.24 The KHC2O4 can be synthesized using a 1:1 molar ratio of H2C2O4and K2C2O4, as shown in eq 1. The KHC2O4 is sparingly soluble in the resulting solution shown in eq 1 at 20 °C and a white precipitate was observed. The precipitate was filtered and identified as KHC2O4 using PXRD.
KHC2O4·H2C2O4is another derivative of H2C2O4 that can be synthesized by mixing H2C2O4 and K2C2O4 in a 3:1 molar ratio, as shown in eq 2. The KHC2O4·H2C2O4was also found to be sparingly soluble in the resulting solution shown in eq 2 at 20 °C. A white precipitate was observed, filtered, and identified using PXRD as KHC2O4·H2C2O4·2H2O. These two derivatives provide an alternative to H2C2O4 with moderate acidity and similar chelation properties. The low solubility of these acids in comparison to the H2C2O4 provides a convenient means to recover the acids after metal extraction. The details for the acid recovery will be discussed in the next section.