BACKGROUND

Bauxite residue, also known as “red mud,” is a by-product of aluminum production, and in conventional approaches is typically discarded. Red mud is often dumped as waste, which has triggered environmental concerns in recent years. Industrial processes such as alumina production generate substantial quantities of raw red mud, which often end up in landfills and has a serious impact on the environment. The high alkali content in red mud restricts its use as a building material, soil aggregate, and wastewater treatment reagent and even makes the valuable metals recovery processes for iron, aluminum and titanium complicated and expensive.

SUMMARY

A method for recovering bauxite residue includes mixing a solution of hydrochloric acid (HCL) according to a predetermined concentration, and adding the HCL solution to a quantity of raw red mud recovered from industrial operations as waste material. The highly alkaline property of the bauxite residue, commonly known as red mud is at least partially neutralized from the HCL, and makes the resulting washed red mud more amenable to subsequent uses in various applications in fields such as construction, wastewater treatment, and metal recovery processes. The process recovers washed red mud from the red mud and HCL solution by filtering the raw red mud and HCL solution for generating a stream of leach liquor from the filtrate and the recovered washed red mud from the residue.

Configurations herein are based, in part, on the observation that aluminum production generates substantial quantities of bauxite residue as a by-product. Bauxite reside contains small amounts of valuable metals, such as Ni, Mg and Co often used in batteries as well as other rare earths. However, the bauxite residue tends to be highly alkaline in pH, and its high soda content restricts further use such as in brick making and iron smelting industries. Furthermore, the hazardous nature of bauxite residue makes transportation and shipping to the recycling facilities difficult and expensive. Unfortunately, conventional approaches to bauxite recycling typically focus on extraction of these trace amounts of metals, rather than recycling of the larger volume of solids contained in the bauxite residue. Accordingly, configurations herein substantially overcome the shortcomings of conventional bauxite recycling by providing a process and method of neutralizing bauxite residue for transport and use in applications such as construction and materials processing.

In a particular configuration, red mud, also known as bauxite residue or simply bauxite, is neutralized with a mild hydrochloric acid (0.5 - I M) wash, followed by filtration to obtain neutralized red mud. Acid washing, which may also include sulfuric acid, is carried out with a pulp density of 10 - 15 % for 10 - 15 minutes duration. Solid-liquid separation is performed using bag filters with a cut off size of 10 and 1 microns. Physical agitation of the slurry increases the solid to liquid contact ratio and accelerates the neutralization process. The pregnant leach liquor can be further processed to recover alumina via an acid-base neutralization route.

The washed red mud has increased iron composition and reduced sodium and calcium than the raw red mud prior to washing, in addition to other changes. Further applications include retrieving the leach liquor from the HCL solution used for the washed red mud, and recovering alumina and other solutes from the leach liquor via acid-base neutralization. In a typical configuration the HCL solution has a concentration between 0.5 - 1.0 M, and preferably around 0.7 M.

In further detail, a method for recovering and recycling bauxite residue as discussed further below includes mixing an acid wash solution according to a predetermined concentration, and neutralizing a quantity of bauxite residue recovered from industrial operations as waste material by combining and washing the bauxite residue with the acid wash solution. The result is neutralized bauxite residue from the bauxite residue and the acid wash solution, where the neutralized bauxite residue has a substantially neutral pH around 6 or 7. Subsequent leaching also provides input to magnetite production.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features will be apparent from the following description of particular embodiments disclosed herein, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Fig. 1 is a context diagram of acid washing of bauxite residue for producing yields as disclosed in configurations herein;

Fig. 2 is an XRD (X-Ray Diffraction) of a bauxite residue sample received in conjunction with configurations herein;

Fig. 3 is a process flow of bauxite residue processing using a sample as in Fig. 2;

Figs. 4 A and 4B show elemental composition and XRD spectra of the bauxite residue sample of Fig. 2 following processing as in Fig. 3;

Fig. 5 shows a comparison of HCL and sulfuric acid for acid washing; and Fig. 6 shows incremental elemental composition along the steps of Fig. 3.

DETAILED DESCRIPTION

Depicted below is an example of red mud washing for neutralizing the caustic properties and leaching constituent metals and useful elements from the former waste material. Red mud, also known as bauxite residue or simply bauxite, results from aluminum production, which generates substantial quantities that often end up in landfills and may have a serious impact on the environment. The high alkali content in red mud restricts its use as a building material, soil aggregate, and wastewater treatment reagent and even makes the valuable metals recovery processes for iron, aluminum and titanium complicated and expensive. Configurations below neutralize the red mud and reduce the alkali content to below 1 % through an acid washing process. Leaching behavior of different elements (Ca, Si, Ti, Na, Al, Fe) in HCl and H ₂ SO ₄ solution is performed. The experimental and characterization results show the separation of more than 90 % Na and Ca, 40 % Al, 60 % Si into the solution after acid washing with HCl or H ₂ SO ₄ .

Conventional approaches to red mud processing attempt an acid-base reaction, and carbonation including bubbling of CO ₂ containing flue gas through a slurry to form carbonic acid in an aqueous solution which neutralizes the basic components. These conventional approaches result in separation of only 20 - 30 % alkali content (free sodium), however, industrial application of red mud in construction and concrete industries requires alkali content to be as low as below 1 %. Roasting with sodium-based flux (sodium carbonate, hydroxide, sulfate) may be efficient in removing Al and Na via the formation of water soluble sodium silicate and sodium aluminum silicate phases. However, these require substantial energy and external flux (NaOH) for the heat treatment and is often not economical.

Fig. 1 is a context diagram of acid washing of bauxite residue for producing yields as disclosed in configurations herein. Referring to Fig. 1, the method for recovering and recycling bauxite residue having a composition 10 as disclosed herein includes mixing an acid wash solution according to a predetermined concentration at step 11, and neutralizing the quantity of bauxite residue 12 recovered from industrial operations as waste material by combining and washing the bauxite residue with the acid wash solution, at step 14. Typical configurations receive the bauxite residue as a byproduct of an aluminum production process. Filtration 16 recovers neutralized bauxite residue 18 from the bauxite residue (solid) and the acid wash solution 20 (liquid), such that the neutralized bauxite residue has a pH < 7.0 and the composition 30. The initial bauxite residue 10 has a pH greater than 10.0 and the washed bauxite residue has a pH around 6.0 or lower, in contrast to conventional approaches. The acid wash solution 11 may be mixed using at least one of hydrochloric or sulfuric acid, or other suitable acid solution as needed.

Leaching processes have been performed in a 500 mL Pyrex beaker using a magnetic stirrer. The solid residue was separated from the leach solution by vacuum filtration using Whatman (grade 1) qualitative filter paper. The solid residue was dried in a laboratory oven at 95 °C for 12 h. Solid residue and leach solution were further analyzed to determine the metal content and the corresponding dissolution value. The elemental analysis of the samples was earned out using PerkinElmer® Optima 8000 Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP - OES). Solid samples were fused using borate flux at 1000 °C for 1 h and dissolved in 25 % nitric acid and further diluted with 2 % nitric acid for ICP - OES (inductively coupled plasma - optical emission spectrometer) analysis. In contrast, liquid samples were analyzed after proper dilution.

Mineral phase analysis was carried out using the PANalytical Empyrean X- ray diffractometer (XRD) using Cr tube line focus. The diffraction data were recorded in the 2-theta range of 10 - 80° with a scanning rate of 2 °/min and a step size of 0.02°. The quantitative phase analysis was conducted by presuming triangular peaks and the underlying peak area depicting the phase quantity.

Bauxite residue used in this study is mainly composed of Fe (21.3%), Al (9.98%) and Na (3.4 %), along with Ca (2 %), Si (2.8 %) and Ti (3.4 %). The complete elemental composition analysis is shown in Table I. The pH value of the raw bauxite residue slurry in water was determined as 10.5, reflecting high basic nature. Referring to Fig 2, an XRD (X-Ray Diffraction) rendering of a bauxite residue sample received in conjunction with configurations herein is shown. The XRD spectrum in Fig. 2 reveals that the Fe values are present in the form of hematite (Fe ₂ O ₃ ) phase, Al values are associated with boehmite (Al(OOH)) and gibbsite (Al(OH) ₃ ), whereas Ti is present in the form of anatase ( Ti O ₂ ). Calcium carbonate (CaCO ₃ ), quartz (SiO ₂ ), and sodium aluminum silicate hydrate (Na ₈ Al ₆ Si ₆ O ₂₄ (OH) ₂ (H ₂ O) ₂ ) are the other mineral phases present in the bulk bauxite residue sample.

TABLE I

Fig. 3 is a process flow 300 of bauxite residue using a sample as in Fig. 2. The basic compositional improvement is achieved through lowering of sodium and calcium and increase of iron oxide content. Referring to Fig. 3, a metric ton (MT) of bauxite residue 301 is neutralized with a mild hydrochloric acid (0.5 - I M) wash 310, forming a slurry, followed by filtration to obtain neutralized red mud. Respective quantities are noted. Acid washing is carried out with a pulp density of 10 - 15 % for 10 - 15 minutes duration. Solid-liquid separation is performed using bag filters with a cut off size of 10 and 1 microns. Physical agitation of the slurry may increase the solid to liquid contact ratio and accelerate the neutralization process. The pregnant leach liquor 314 can be further processed to recover alumina via an acid-base neutralization route.

The neutralized bauxite residue undergoes a further leach phase to generate a leach solution including the neutralized bauxite residue and a leach agent, such that the leach agent has a concentration selected based on an iron content in the neutralized bauxite residue. In a particular configuration, the leach agent is oxalic acid having a concentration between 1.5 and 2.5 molar, and further includes leaching the slurry at around 95° C for between 2 and 3 hours. In the example of Fig. 3, the neutralized bauxite residue 312 is further leached 316 with oxalic acid 318 at an acid concentration of 2 Molar, 95 °C temperature for 2.5 hour. The acid concentration can be variable based on the Fe ₂ O ₃ content in the neutralized bauxite residue. The neutralized bauxite residue forms a slurry from the leach solution and is filtered to generate a leach liquor and a solid residue of washed bauxite residue. The reacted slurry is filtered through a filter press to obtain leach liquor 320 and solid residue 322, thus separating the leach liquor from the slurry. The solid residue can be stored and further processed to recover TiO ₂ in downstream processing. The leach liquor is photochemically reduced using ultraviolent light 330 (250 - 500 nm wavelength) to precipitate out ferrous oxalate 332. Directing UV (ultraviolet light) at the leach liquor 320 induces a photochemical reduction for precipitating the iron in the form of ferrous oxalate via photochemical reduction. The photochemical reduction time will depend on the quantity of the liquid processed, the power of the light source and equipment design. The photochemical reduction can also be performed by using direct sunlight eliminating power requirements. The liquid leach 338 from the oxalic acid solution contains Ti which may be precipitated/harvested. Filtering the ferrous oxalate forms a magnetite precursor. This can be decomposed in an inert atmosphere heated to around 500° C for around 1 hour to form magnetite. The ferrous oxalate precipitate 334 is recovered from the reduced solution via filtration and further decomposed to convert into magnetite powder 340. The decomposition heating 336 reaction is carried out in an inert atmosphere of N ₂ gas at a process temperature of 500 °C for Ih in a tubular furnace. Alternative heating and/or decomposition may be employed. Thus, the neutralized bauxite residue is processed to recover high purity magnetite powder with wide industrial applicability.

Figs. 4 A and 4B show elemental composition and XRD spectra of the bauxite residue sample of Fig. 2 following processing as in Fig. 3. Referring to Figs. 1, 4A and 4B, the basic oxides (NaOH, CaCO ₃ ) in the bauxite residue can be separated after washing with mild hydrochloric acid (HCl). The chemical reaction for different components present in bauxite residue with HCl is shown in Eq. (1) to (5) below. The thermodynamic equilibrium calculations for the reaction of different phases present in bauxite residue was carried out using HSC Chemistry 5.11 software, and the Gibbs free energy at 40 °C was found to be negative for Na and Ca phases. The more negative value of Gibbs free energy for NaOH and CaCO ₃ indicates a spontaneous equilibrium reaction compared with Al(OH) ₃ and Fe ₂ O ₃ . The stoichiometric requirement of HCl was calculated at 0.26 g per gram of bauxite residue for the complete dissolution of Na and Ca in bauxite residue, and therefore, preliminary experiments were conducted at 0.7 M HCl concentration at 10% pulp density and 40 °C for 15 min. Approximately 67% Na, 60% Ca, 8% Si, and 1% Fe were leached out. Furthermore, increasing the acid concentration to 1M HCl recovered more than 95% Na and Ca along with approximately 40% Al and 75% Si. Dissolution of Al and Si, along with Na and Ca, increases the acid demand during neutralization, and therefore high Na and Ca removal efficiency is achieved at acid dosage excess to the stoichiometric requirement.

The composition (wt%) of feed 402, 0.5 M 404, and 1 M 406 HCl washed bauxite residue is shown in Fig. 4A. The Na and Ca content dropped to 0.67 % and 0.18 %, respectively, after 0.5 M and 1 M HCl wash. On the other hand, Fe concentrated up to 26 % in the residue. The XRD spectra of bauxite residue 410 and 1 M 408 HCl washed bauxite residue shown in Fig. 4B, shows that the peaks intensity corresponding to sodium aluminum silicate (2 theta = 20.8°, 52.39°) and calcite phase (2 theta = 44.4°) dropped to zero after HCl wash. The XRD analysis shows that HCl wash predominantly dissociated sodium aluminum silicate and calcium carbonate phases and dissolved corresponding Na, Al, Si, and Ca values. Based on preliminary experiments, it was found that Na and Ca are leached into the solution in a concise duration (~ 15 min) and extended leaching results in the dissolution of other elements. Therefore, the leaching time was fixed at 15 min. Furthermore, to optimize other leaching parameters (acid concentration, S/L ratio, and temperature) and to study the corresponding impact on elemental dissolution, the statistical design of experiments was pursued. Equations applicable to the HCL based neutralization are as follows:

NaOH + HCl → NaCl + H ₂ O ΔG _40°C = -115.3 kj/mol (1) CaCO ₃ + 2HCI → CaCl ₂ + H ₂ O + CO ₂ ΔG _40°C = —4.49kJ /mol (2) Al(OH) ₃ + 3HCI → AlCl ₃ + 3H ₂ O ΔG _40°C = 175.8 kj/mol (3) Fe ₂ O ₃ + 6HCI → 2FeCl ₃ + 3H ₂ O ΔG _40°C = 124.8k//moZ(4) SiO ₂ + 2H ₂ O → H ₄ SiO ₄ = 14.13k/ /mol (5)

The dissolution of various elements was as follows; 15 - 22 % Fe, 40 - 56 % Al, 60 - 75 % Si, and more than 85 % Na and Ca. The Ti values behaved inert during HCl wash and reports to the residue. The leach solution at high acid concentration formed gelatinous liquid due to silica gel formation. The silica soluble in the form of silicic acid (Si(OH) ₄ ) monomer connected through the Si - O - Si bond and form polysilicic acid, which further causes gelation. The presence of free H ⁺ accelerates the polymerization rate and therefore, highly depends on the pH, temperature, and ionic strength. A final pH value of approximately 2.45 was obtained at the end of the washing. The bauxite residue washing resulted in the dissolution of approximately 42% Al, 5% Fe, 96% Ca, 40% Si, and 94% Na. The composition of bauxite residue washed at the optimized conditions is shown in Table II. Table

Sulfuric acid was also tested for neutralization of alkali in the bauxite residue. Sulfuric acid being diprotic acid, it is expected that the required acid concentration should be half of that of hydrochloric acid. The Gibbs free energy for reaction of different species present in red mud with H ₂ SO ₄ is shown in Eq. (6) to (9). The Gibbs free energy is negative for all the elements (Na, Ca, Al, Fe), showing thermodynamic feasibility for dissolution in the acid solution. Leaching experiments were carried out using H ₂ SO ₄ with a pulp density and the leaching time of 10 % and 15 min, respectively, while the acid concentration was varied between 0.1 M to 1.5 M. Calcium was separated as insoluble calcium sulfate in the residue. The pH at the end of the washing was 1.7. Na dissolution dropped to 80 %, with a decrease in acid concentration to 0.25 M at 10 % S/L. The pulp density was further increased to 13 % and approximately 94 % Na, 25 % Ca, 81 % Si, 1 % Ti, 9 % Fe, 45 % Al were recovered into the solution, and the slurry's final pH was 2.94. Further increase in pulp density caused the silica to polymenze and form silica, causing difficulty in filtration; therefore, 13 % S/L was found optimal.

2NaOH + H ₂ SO ₄ → H ₂ SO ₄ + 2H ₂ O ΔG _40°C = -295.1 kJ /mol (6)

CaCO ₃ + H ₂ SO ₄ → CaS0 ₄ + H ₂ O + CO ₂ ΔG _40°C = —140.77kJ /mol (7)

2Al(OH) ₃ + 3H ₂ SO ₄ → AZ ₂ (SO ₄ ) ₃ + 6H ₂ O ΔG _40°C = -175.98 kJ/ mol (8)

Fe ₂ O ₃ + 3H ₂ SO ₄ → Fe ₂ (SO ₄ ) ₃ + 3H ₂ O ΔG _40°C = -162.7 kJ /mol (9)

The composition for bauxite residue neutralized in Sulfuric acid is shown in Table III.

Table III

Based on the acid washing experiments carried out, the comparison between HCl and H ₂ SO ₄ as a reagent to actively separate alkali (Na, Ca) from bauxite residue is shown in Fig. 5. It was found that Na removal depends upon the concentration of H ⁺ ions in the solution, therefore, 1 M HCl solution resulted in similar result as with 0.5 M H ₂ SO ₄ . The bauxite residue slurry pH was reduced from -10.5 to -2.5 along with the dissolution of approx. 38% solid mass with HCl and 27 % with H ₂ SO ₄ . Both the acids completely dissociated the sodium aluminum silicate phase at the optimized conditions and separated more than 90 % Na, 40 - 45 % Al, 60 % Si and less than 10 % Fe. The key difference between the two acid is the limited separation of Ca with H ₂ SO ₄ due to formation of insoluble CaSO ₄ . The leach solution from both HCl and H ₂ SO ₄ wash consist of high amount of Al (~ 4500 ppm), along with Si (~ 4400 ppm), Na (4000 ppm), and Ca (500 - 3000 ppm). The pH adjustment of the leach solution can be employed to recover the alumina values, and the remaining solution can be recirculated for acid washing. The costs of HCl and H ₂ SO ₄ may be a factor in selecting one over the other. Based on the optimized conditions, cost of HCl and H ₂ SO ₄ required for alkali separation is 7.56 $ - 15.75 $ and 5.00 $ - 6.62 $ per 100 kg of bauxite residue, respectively. Based on the material balance and cost analysis H ₂ SO ₄ seems to be a better reagent with separation of more than 90 % Na, and retains around 73 % of solid mass after washing, compared to 62 % with HCl. However, for separation of Ca, washing with HCl is recommended.

The acid- washed alkali reduced material is therefore suitable for use as a building material (additive in cement, mortar, concrete, geopolymers), as a soil aggregate, wastewater treatment, glass ceramics, and for valuable metal recovery. Differences between HCL and H ₂ SO ₄ neutralization of Figs. 1 and 3 is shown in Fig. 5

Fig. 6 shows incremental elemental composition along the steps of Fig. 3. Referring to Figs. 3 and 6, the bauxite residue and acid wash yields neutralized solids 612 and liquid 614 products, having a respective composition 612’ and 614’, from initial bauxite residue composition 601’. The subsequent oxalic acid leach also generates solid 622 and liquid 620 products, having respective compositions 622’ and 620’. Photochemical reduction of the oxalic acid leach generates magnetite precursor solids 632 with composition 632’, and leach liquor 638 including titanium at a concentration of at least 400 ppm, in composition 638’.

While the system and methods defined herein have been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.