Envlron. Sci. Technol. 1994, 28, 26-30
Chemical and Biological Leaching of Aluminum from Red Mud Pascale Vachon, Rajeshwar D. Tyagl,' Jean-Christian Auclalr, and Kevin J. Wliklnsont
Institut National de la Recherche Scientifique-Eau,Complexe Scientifique, Universite du Quebec, 2700 Rue Einstein, C.P. 7500, Sainte-Foy, Quebec, Canada G1V 4C7
Chemical and biological leaching of aluminum (Al) from red mud, the major waste product of the alkalineextraction of A1 from bauxite, was examined. Sulfuric, citric, and oxalic acids were employed individually or as mixtures in chemicalleaching experiments. The highest concentration extracted was 13 530 mg of Al/L (96% solubilization) using a 2:l ratio of citric and oxalic acids and subsequent HzSO4 addition to lower the pH to 1.5. Despite a lower concentration of extracted A1 obtained with H2S04 alone at pH 1.0 (12 140 mg/L), it may be more economical due to the high price of organic acids. Biological leaching was carried out using sewage sludge bacteria (adapted indigenous thiobacilli) and pure strains of fungi: Aspergillus niger, Penicillum notaturn, Penicillum simplicissimum, and Trichoderma viride. All microorganisms were tested for acid-producing and -leaching capabilities in the presence of increasing amounts of red mud. In thiobacilli cultures, 6265 mg of Al/L was solubilized in the presence of 5% v/v red mud and 1% w/v sulfur by recycling the leachate. I? simplicissimum was the most efficient of the fungal cultures; 1880 mg of Al/L (56% solubilization) was solubilizedat 3 % v/vred mud initial concentration. These concentrations are not high enough to be applicable on an industrial scale. However, 75% of the A1 (10585 mg of Al/L) of a 10% v/v initial red mud concentration was solubilized using the acids produced by P.simplicissimum. The high affinity of the acids produced by this fungi to leach A1 from red mud is quite unusual when compared with pure citric acid. Leaching of A1from red mud appears feasible on an industrial scale using the addition of either sulfuric acid (chemicalextraction) or biologically produced acids (microorganism-aided extraction).
Zntroduction Red mud is a chemical waste produced by the alkaline extraction of alumina from bauxite (Bayer process). The chemical composition of the mud varies depending on the mineralogical composition of the bauxite used in the extraction, but is usually around 25% A1203, 20% Fe203, 10% Si02, and 12% Ti02 (1-3). Final deposition of red mud is complicated due to its high alkalinity (pH 12-13) and the large amount of waste produced (approximately 2 t of red mud/t of commercial aluminum). Different methods of red mud disposal are practiced throughout the world, but none of them are known to be environmentally innocuous. For example, in Germany and France, many aluminum plants pump red mud directly into the sea. Studies have demonstrated that a "dead zone" is established in the center of the red mud deposit at the sea bottom. Consequently, only organisms resistant to red mud survive near this zone ( 4 ) . Another disposal method is to dump red mud into large Present address: DBpartement de chimie minbrale, analytique et appliqube, Universitb de GenBve, Section Chimie-Sciences 11, 30 Quai Ernest-Ansermet, CH-1211Geneva, Switzerland. 28
Envlron. Scl. Technol., Vol. 28,
No. 1, 1994
settling ponds near the alumina plant to form a "red lake". This practice is commonly used in Canada and India. Litt,le information is available about the environmental impact of this disposal method, except that the large content of free alkali in red mud can adversely affect adjacent soil fertility (5). Many attempts have been made to find environmentally safe methods to dispose of or use red mud. Thakur and Sant (5, 6) have listed a number of uses for this waste, such as absorbents to remove H2S from industrial emissions; constituents in building materials such as bricks, ceramics,cement, concrete, and road materials; coagulants to remove phosphate in wastewater treatment; catalysts in coal hydrogenation or in the preparation of anticorrosive materials and pigments particularly useful in marine environments. More recently, red mud has been used in columns to remove bacteria and viruses from secondary effluents (7); as a pH modifier in heap leaching of gold bearing ores (8);and as a neutralizing agent for acid wastes such as those obtained from the production of gypsum or titanium dioxide (19). All these processes can utilize only a small fraction of the total amount of red mud produced (40 000 000 t/year). Finally, some workers have also tried torecover reusable substances and/or valuable metals from red mud by using different chemical extraction processes but, due to their high complexity and/or their elevated cost, none have been feasible on an industrial scale (5,10, 11).
The acid leaching of A1 from numerous materials has been well-documented. For example, some workers extracted A1 from acid-soluble anarthosite (12);others have worked on alternative ways to extract A1 directly from ores in place of the traditional Bayer process (13); still others have extracted A1 from fly ash by commercial or microbiologically-produced citric acid (14). Because utilization of the Bayer process is limited to high-grade bauxites containing 50-60 % alumina and less than 5 % reactive silica, it is generally accepted that other sources of A1 must be identified and other extraction processes found. Bosecker (15) studied the leaching of red mud with organic acids and extracted 5500 mg/L A1 (47 % ) using oxalic acid. The goal of our study was to increase A1 solubilization from red mud by using (i) pure and mixed organic and inorganic acids, (ii) different autotrophic or heterotrophic microbial strains grown in the presence of red mud, and/or (iii) their culture filtrates to leach Al. N
Materials and Methods Red Mud. Red mud was obtained from Alcan's alumina plant in Arvida, Quebec. The mud was dried to constant weight (16),and the chemical composition was determined by X-ray fluorescence (Bausch & Lomb/ARL Model 8420 ARF). Table 1 shows the metal oxide composition and corresponding calculated ion concentrations analyzed from the red mud used in the experiment. Chemical Leaching. All experiments were conducted in 500-mL Erlenmeyer flasks at 28 "C and at 200 rpm 0013-936X/94/0928-0026$04.50/0
0 1993 American Chemical Society
Table 1. Red Mud Metal Oxide Composition As Analyzed bv X-ray Fluorescence and Corresponding Calculated Ion cincenirations (% w/w) metal ions % wlw metal oxides % WIW Fez03
A1zOa Si02 Na2O Ti02 CaO
so3
MgO PZOb MnO K2O loss on ignition total
31.60 20.61 8.89 10.26 6.23 1.66 0.006 0.01 0.08 0.06 0.01 21.13 99.33
Fe3+ ~13+
Si4+ Na2+ Ti4+ Ca2+ S6+
MgZ+
PS+ Mn2+
K+
22.09 10.91 4.15 7.61 3.74 1.18 0.000024 0.000060 0.000350 0.000498 0.000077
agitation (G24 Model shaker from New-Brunswick Scientific). Deionized water (150 mL) was added to 25 g (dry weight) of red mud samples. pH was subsequently adjusted ( 1 < pH < 13) using appropriate volumes of sulfuric acid ( 1 2 N), citric acid (1.25 N), or oxalic acid (0.75 N). Samples were then brought to a final volume of 200 mL using deionized water. A1 leaching kinetics were studied in samples having a pH 4.5. In all chemical leaching experiments, the pH was adjusted, if necessary, using the appropriate acid at 30min intervals until stabilization. With acid mixtures, 10:20 mL, 20:lO mL, and 20:20 mL volumes of citric and oxalic acids, respectively, were added to a solution of 25 g (dry wt) of red mud and 125 mL of deionized water. The pH of the resultant mixture was then adjusted to 1.5 f 0.1 using H2S04 (12N). Final sample volumes were adjusted to 200 mL with deionized water. A sample of 15 mL of solution was sampled after 0 , 3 , 6 , 9,12,and 24 h of leaching for metal analysis. All chemical leaching experiments were replicated. Biological Leaching. All experiments were performed in 500-mL Erlenmeyer flasks at 28 "C and at 200 rpm agitation under aseptic conditions. The pH was measured at 48-h intervals until it stabilized. Indigenous Thiobacilli. Seven different municipalsludges (secondary, anaerobically digested, or aerobically digested) were used as sources of indigenous thiobacilli and nutrients. The total solids contents ranged from 7.1 to 31.4 g/L. A volume of 95 mL of each sewage sludge was added to 5 g of red mud and 1g (w/v) of elemental sulfur. A control without reduced sulfur was also inoculated with the sludge. pH was adjusted to 9.0 f 0.5 using HzS04. Growth and acid production were detected by measuring the pH decrease. In sludges where growth occurred, the minimum quantity of municipal sludge needed to support the growth of the microorganisms was established by adjusting a serial concentration containing 20, 25,30,40, 50, '75, and 95% v/v sewage sludge, 5% v/v red mud and 1 % w/vsulfur and then used for subsequent cultures. Final volumes were adjusted to 200 mL using deionized water. The minimum usable sewage sludge concentration of a culture showing the same A1 leaching efficiency as the culture having the maximum sewage sludge concentration (95% v/v) was utilized in subsequent experiments. Samples of 15-mL inocula of thiobacilli grown in the presence of 5 % v/v red mud were incubated with 5 % v/v
of fresh red mud, the previously chosen concentration of sewage sludge, and 1 % w/v sulfur. Final volume was again adjusted to 200 mL using deionized water. At the same time, another similar culture was prepared using the same inocula source, but containing incremental volumes (5% v/v) of red mud. Red mud was added until the maximum resistance capacity was detected (no growth or acid production) (17). At each transfer, 20-mL samples were taken to determine metal content. The leachate was recycled in the sludge showing the best adaptive and acid production capacity. A sample of 20 mL of culture solution was kept as inoculum for each subsequent culture; the remainder was centrifuged at 12000g for 10 min. Twenty milliliters of supernatant was recovered for metal analysis; 30% v/v municipal sludge, 5% v/v red mud, and 1 % w/v sulfur were then added to 100 mL of the leachate. The pH was adjusted to 9.0 f 0.5 using H2S04, and the final volume was adjusted to 200 mL using deionized water. Samples were recycled until maximum [All had been leached and/or growth had ceased. To determine if adsorption to a solid phase limited A1 solubility (adsorption test), the final leachate obtained after recycling was separated in two equal fractions. The first was centrifuged at 12000g for 10min, and the required volume of loo00 mg/L of ICP-grade standard A1 ( 2 % "03 matrix) was added to the supernatant in order to obtain an additional 6000 mg of Al/L in solution. The same volume of A1 was added to a second control fraction which was not centrifuged. Both fractions were agitated at 200 rpm for 24 h, after which time 20-mL samples were taken for metal analysis. Indigenous MicroorganismsfromRed Lakes. Samples were taken from red mud deposits near the alumina plant. Subsamples (5 g) were placed in 100 mL of PDB medium and incubated until the pH was reduced below 4.0, at which point they were cultured as described in the previous section. Fungal Leaching. Four strains of fungi were studied: Aspergillus niger (ATCC 10108), Penicillum notatum (ATCC 62751, Trichoderma uiride (ATCC 32098), and Penicillum simplicissimum (ATCC 48705). These fungi have been extensively employed to leach A1 and other metals from a variety of materials such as ores (18), industrial residue (14-20),rocks (211,and coals (22). The first step was to choose the best culture medium for each of the strains tested by cultivating each one on a Sabouraud (SB: 5 g of Bacto-Casitone, 5 g of Bacto-Peptamin, and 20 g of Bacto-Dextrose/L) and a Potato Dextrose Broth medium (PDB: 200 g of potato infusion and 20 g of BactoDextrose/L) in the presence of 1 % v/vred mud. Duplicate experiments allowedthe identification of the most effective growth medium (defined as the one which supported the fastest production of acid to lower the pH to a value of 2.5); it was used for subsequent experiments. Fifteen grams of the centrifuged mycelium inoculum of each fungus was added as inoculum to 150 mL of axenic liquid medium, PDB for P. simplicissimum and SB for the others, containing 1-4 % v/v red mud. The pH was then adjusted to 8.0 i 0.1 using HzS04. Sterile inoculum controls were incubated simultaneously. When growth was detected by a pH decrease ( > 2 ) , the medium was centrifuged, and 20 mL of the leachate was analyzed for its metal content. Fifteen grams of mycelium was then reintroduced in 150 mL of fresh sterile medium containing an additional 1% v/v red mud. Inoculations were repeated until growth was inhibited. For red mud concentrations greater than Envlron. Sci. Technol., Vol. 28, No. 1, 1994 27
3 % v/v, a recycling procedure was carried out using a
similar procedure as described for thiobacilli. To test the leaching capacities of microbial acids, 150 mL of acidified fungal cultures with a pH of approximately 2.5 was centrifuged at 12000g for 20 min. A total of 100 mL of supernatant was recovered and added to 25 mL of red mud and 75 mL of deionized water. The pH was adjusted to 2.2 f 0.1 (HzS04) where it was maintained for the duration of the experiment. Final volumes were brought to 250 mL using deionized water. Samples were taken hourly between 0 and 4 h. The leachate of the most efficient A1 leaching strain was recycled by adding 100 mL of an acidified fresh culture of the same fungi to 100 mL of centrifuged leachate and 25 mL of red mud. The remainder of the experiment was conducted as described previously. Recycling was continued until dissolved A1 attained a plateau. Citric and sulfuric acids were similarly tested to compare their efficiencies with those of the microbial acids. In these cases, the supernatants of the cultures were replaced with 100 mL of pure acid for citric acid (1.25 N) or the required volumes of HzS04 (2 N) to adjust pH to 2.2 f 0.1. Treatment of Samples and Metals Analysis. All 20-mL samples were centrifuged at 12000g for 10 min. A 10-mL subsample of supernatant was collected, filtered through a polycarbonate membrane (Nuclepore, 2.0 pm), arid acidified with 0.5 mL (5 % v/v) of HC1. Metal analysis was performed using inductively coupled plasma atomic absorption spectroscopy (Therm0 Jarrell Ash Corporation, Model Atom Scan 25): A1 (309.3), Fe (259.9), Si (251.61, Ti (336.11, Na (588.9), and Ca (393.3 nm). MINEQL Simulations. Calculations with the chemical equilibrium simulation model MINEQL (23) were carried out to determine if A1 precipitation could limit A1 solubility. The analytically measured elemental concentrations in the leachate of the citric acid recycling experiment were utilized t o define t h e chemical environment: 0.077 M Ca2+,0.42 M Na+, 0.0026 M Fe3+, 0.063 M H+, 0.3 M S042-, 0.16 M Sios, and 0.70 M citric acid. The concentration of AP+ was doubled from 0.42 to 0.84 M to ensure that saturating A1 conditions prevailed. The ionic strength used was 2.0 (calculated), and equilibrium constants not already present in the MINEQL database were obtained from Sillen and Martell (24) or from the database of other models (MINTEQ). Simulations were carried out in the presence and absence of citric acid (CIT) and over a pH range of 2.4-1.4 by 0.1-unit intervals.
Results and Discussion Chemical Leaching, A1 solubilization from red mud began at pH 3.0 or lower when H2S04was used as the leaching agent, while A1 solubilization started at pH values near 7.0 when citric and oxalic acids were used over 24 h of leaching (Figure 1). This is mainly due to the chelating action of organic acids in solubilizing the aluminum. This also explains the reduction of A1 solubility observed with oxalic acid and citric acid at a pH lower than 3, given that complexation is less effective at lower pH values, since the acids are being progressively protonated. However, the maximum A1 concentration solubilized with these three acids was similar (Figure 1). To decide which acid was most economical,we compared the cost (in acid) to extract 1t of A1 from red mud at 75 % w/v solubilization efficiency; €&SO4was the least expensive at $210.00 CAN (Table 2). Znorganic and Organic Acids Mixture. The 2:l ratio of citric and oxalic acids at a pH of 1.5 f0.1 (HzS04adjusted) 28
Envlron. Scl. Technol., VoI. 28, No. 1, 1994
14000
j
lZ000
.
10000
;
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8000 6000
z
'$
1
.
,g
4000
1
7
: 2000
-
0 0
2
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4
8
10
12
14
Figure 1. Leaching of aluminum from 25 g of red mud as a function of pH and acid type. Symbols: (0)citric acid, (B) oxalic acid, (A) H2S04. Table 2. Acid Estimation Cost to Extract 1 t of A1 from Red M u d at 75% w/v Solubilization Efficiency
concn (N)
acid
amt delivered
cost (per kg)
approx cost to extract 1 t of A1
18 1000 L $0.125n $210.00 oxalic acid 0.75 25 kg $1.05b $1020.00 $4.25C $2960.00 citric acid 1.25 25 kg Noranda SalesCorp.Ltd.,Toronto. StanchemMontreal. A&E Ltd., Montreal.
5
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i
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8000
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5 8 6000 6
8
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4000
0
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1 3
-
-
,
8
9
12
15
18
21
24
Time (hours)
Figure 2. Leaching of AI from 25 g of red mud using different ratios of citric and oxalic acids at pH 1.5 f 0.1 adjusted with H2S04.Symbols: (B) 2 1 , (0)1:2, (A)2:2 of citric and oxalic acids respectively.
resulted in the highest solubilization value (Student t-test; p < 0.05) for aluminum of 13530 mg/L (Figure 2). McKenzie e t al. (25)have reported that the solubilization of Ni from laterites was generally higher when a mixture of organic and inorganic acids was used rather than organic acid alone. Since approximately 1500 mg/L more A1 was measured when a mixture of organic and inorganic acids was used to leach, our results suggest the same conclusion; the combined action of chelation (organic acid) and H+ attack (inorganic acid) seems to enhance the mixture ability to leach Al. Leaching Kinetics. In general, A1 solubilization occurred rapidly; a maximum in the A1 concentration was reached after 6 and 12 h of leaching (Figure 2). Following this period, A1 remained fairly constant in solution. In fact, a good proportion of A1 was rapidly solubilized as soon as the desired pH was obtained ( t
