Sequestration of CO2 in Mixtures of Bauxite Residue and Saline

Energy & Fuels 2008, 22, 343–353

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Sequestration of CO2 in Mixtures of Bauxite Residue and Saline Wastewater† Robert Dilmore,‡ Peng Lu,§ Douglas Allen,⊥ Yee Soong,*,‡ Sheila Hedges,‡ Jaw K. Fu,4 Charles L. Dobbs,4 Angelo Degalbo,‡ and Chen Zhu§ National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236, Department of Geological Sciences, Indiana UniVersity, 1001 East 10th Street, Bloomington, Indiana 47405, Salem State College, 352 Lafayette Street, Salem, Massachuetts 01970, and ALCOA Technical Center, 100 Technical DriVe, Alcoa Center, PennsylVania 15069-0001 ReceiVed July 10, 2007. ReVised Manuscript ReceiVed October 18, 2007

Experiments were conducted to explore the concept of beneficially utilizing mixtures of caustic bauxite residue slurry (pH 13) and produced oil-field brine to sequester carbon dioxide from flue gas generated from industrial point sources. Data presented herein provide a preliminary assessment of the overall feasibility of this treatment concept. The Carbonation capacity of bauxite residue/brine mixtures was considered over the full range of reactant mixture combinations in 10% increments by volume. A bauxite residue/brine mixture of 90/10 by volume exhibited a CO2 sequestration capacity of greater than 9.5 g/L when exposed to pure CO2 at 20 °C and 0.689 MPa (100 psig). Dawsonite and calcite formation were predicted to be the dominant products of bauxite/brine mixture carbonation. It is demonstrated that CO2 sequestration is augmented by adding bauxite residue as a caustic agent to acidic brine solutions and that trapping is accomplished through both mineralization and solubilization. The product mixture solution was, in nearly all mixtures, neutralized following carbonation. However, in samples (bauxite residue/brine mixture of 90/10 by volume) containing bauxite residue solids, the pH was observed to gradually increase to as high as 9.7 after aging for 33 days, suggesting that the CO2 sequestration capacity of the samples increases with aging. Our geochemical models generally predicted the experimental results of carbon sequestration capacities and solution pH.

Introduction Atmospheric CO2 Rise and Proposed Methods of Mitigation. In recent years, a great deal of concern has been expressed with regard to global climate change and its link to growing atmospheric concentrations of carbon dioxide (CO2). Researchers have noted a correlation between a rise in atmospheric CO2 and increasing global mean temperature since the advent of the industrial era. An ever-increasing body of scientific evidence suggests that anthropogenic release of CO2 has led to a rise in global temperatures over the past several hundred years.1,2 If this hypothesis holds true, unabated release of greenhouse gases will result in global warming that may lead to significant and potentially catastrophic alteration of global hydrologic cycles and climate patterns. In order to decrease the impact of atmospheric CO2 on global climate, several strategies are under development to sequester CO2 released from stationary and mobile sources.3,4 Principle modes of carbon management include: (i) increasing the efficiency of energy conversion; (ii) using low-carbon or † Disclaimer: Reference in this paper to any specific commercial product, process, or service is to facilitate understanding and does not imply endorsement by the United States Department of Energy. * To whom correspondence should be addressed. Phone: 412-386-4925. Fax: 412-386-4806. E-mail: [email protected]. ‡ U.S. Department of Energy. § Indiana University. ⊥ Salem State College. 4 ALCOA Technical Center. (1) Crowley, T. J. Causes of Climate Change Over the Past 1,000 Years. Science 2000, 289, 270–277. (2) Bradley, R. S. Many Citations Support Global Warming Trend. Science 2001, 292 (5524), 2011.

carbon-free energy sources; and (iii) capturing and sequestering anthropogenic CO2 emissions. The latter strategy, termed “CO2 sequestration”, would permit continued use of fossil fuels for the generation of electric power while ensuring CO2 emission reductions and has gained increased attention in recent years. Terrestrial, geologic, advanced biological processes, and advanced chemical CO2 sequestration approaches are currently being studied. Each of these options has significant technical and economic hurdles that need to be addressed before being considered feasible for full-scale application. Geologic sequestration involves the capture of CO2 from large point sources (such as fossil fuel-fired power plants) and the long-term storage of CO2 in underground, brine-bearing geologic formations.5 Previous work on carbon sequestration in saline fluids has demonstrated that conversion of gaseous CO2 into stable carbonate minerals in natural brine solutions is largely a function of pH, with observed increase in sequestration capacity observed at higher initial fluid pH values.6 Oil and Gas Brine Produced Water. Twenty to thirty billion barrels of saline produced water are generated annually in the (3) Herzog, H.; Drake, E.; Adams, J. E. CO2 capture, reuse, and storage technologies for mitigating global climate change: A White Paper; Final Report, DOE contract DE-AF22-96PC01257, Department of Energy: Washington, D.C., 1997. (4) Carbon Sequestration Research and DeVelopment; U.S. DOE report, DOE/SC/FE-1, Department of Energy: Washington, D.C., December 1999. (5) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W. Separation and Capture of CO2 from large stationary sources and sequestration in geological formations-coalbeds and deep saline aquifers. J. Air Waste Manage. Assoc. 2003, 53, 645–715.

10.1021/ef7003943 CCC: $40.75  2008 American Chemical Society Published on Web 12/11/2007

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USA7 as a byproduct of oil and gas production. About 65% of this water is reinjected into reservoirs for pressure maintenance, and the remaining water is treated and discharged to surface water bodies. In Pennsylvania, for example, treatment costs for brine wastewater can reach as high as $3.00/barrel.8 Some natural brine solutions have high concentrations of Ca2+, Mg2+, and Fe2+ in addition to the dominant Na+ and Cl- ions. Under favorable conditions (pH, temperature, and pressure), the Ca2+, Mg2+, and Fe2+ from brine could react with CO2 to produce CaCO3(s), MgCO3(s), Fe2CO3(s), and other mineral products that would safely and permanently store CO2. However, the pH of subsurface aquifer brines is typically low (approximately 3–5). The primary issue affecting solubility trapping is the limited absorptive capacity of brine. This low pH precludes significant dissolution of CO2 and prevents carbonate formation. As dictated by the carbonate system, a solution pH of 7.8 or higher is required to achieve substantial dissolution of CO2 and subsequent mineral carbonate formation.6 Therefore, to favor CO2 dissolution and precipitation of mineral carbonates, the pH of the brine must be modified. Alumina Production and Bauxite Residue. Worldwide, over 70 million dry metric tons of bauxite residues are generated annually when aluminum is extracted from the principal ore called bauxite.9 The physical, chemical, and mineralogical characteristics of bauxite residues vary considerably as a function of the composition of the bauxite ore and, to a lesser extent, the specifications of ore processing. Typically, iron and titanium oxides, silica, calcium carbonate, and unrecoverable alumina and caustic soda (NaOH) comprise much of bauxite residue solids; as such, the residue slurry is highly alkaline. The pH of the liquid reaches values of 13 or higher, and the solids and solid surfaces also contain high alkalinity.10 The caustic nature of the byproduct yields concerns of long-term environmental liability and impact because leakage of this alkaline liquid from impoundments into groundwater can result in mobilization of several constituents of concern, including iron, aluminum, and hydroxyl ion. Worldwide, there are in excess of 200 million tons of bauxite residues, the vast majority of which are stored in tailings ponds.9 Numerous methods have been attempted to mitigate the potential environmental impacts of the residue, including washing with seawater,11,12 land application as a soil amendment,13,14 beneficial use as an admixture in (6) Soong, Y.; Jones, J. R.; Goodman, A. L.; Baltrus, J. P. Experimental and simulation Study on Mineral Trapping of CO2 with Brine. Energy ConVers. Manage. 2004, 45, 1845–1859. (7) Kharaka Y. K.; Leong, L. Y. C.; Doran, F. G.; Breit, G. N. Can produced water be reclaimed? Experience with Placerita oil field, CA. Proceedings of the 5th International Petroleum EnVironmental Conference, Albuquerque, NM, Oct 20–23, 1998. (8) Stefanik, A. Personal communication. Cabot Oil & Gas Corp.: Houston, TX, 2002. (9) Aluminum Association. Technology Roadmap for Bauxite Residue Treatment and Utilization. Workshop, Energetics Inc., Washington, D.C., February 2000. (10) Nikraz, H. R.; Bodley, A. J.; Cooling, D. J.; Kong, P. Y. L.; Soomro, M. Comparison of Physical Properties between Treated and Untreated Bauxite Residue Mud. J. Mater. CiV. Eng. 2007, 19 (1), 2–9. (11) Ward, S. C.; Summers, R. N. Modifying sandy soils with the fine residue from bauxite refining to retain phosphorus and increase plant yield. Nutr. Cycl. Agroecosyst. 1993, 36 (2), . (12) Menzies, N. W.; Fulton, I. M.; Morrell, W. J. Seawater Neutralization of Alkaline Bauxite Residue and Implications for Revegetation. J. EnVironm. Qual. 2004, 33, 1877–1884. (13) Lombi, E.; Zhao, F. J.; Zhang, G.; Sun, B.; Fitz, W.; Zhang, H.; McGrath, S. P. In situ fixation of metals in soils using bauxite residue: chemical assessment. EnViron. Pollut. 2002, 118, 435–443. (14) Hughes, J. Effect of soil amendment with bauxite Bayer process residue (red mud) on the availability of phosphorus in very sandy soils. Aus. J. Soil Res. 2003.

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cementitious materials,15,16 treatment of acidic mine drainage,17 and sewage effluent treatment.18 However, large-volume, economically viable applications for bauxite residue have not yet been identified. Bauxite Residue Treatment and Disposal. Bauxite residue carbonation using concentrated CO2 is being considered as a cost-effective means of treating the caustic byproduct slurry to reduce its pH to a less hazardous level.10 In the United States, bauxite residue is exempt from hazardous listing (Resource Conservation Recovery Act, Subtitle C) per amendment section 3001(b)(3)(A)(ii), commonly known as the Bevill exclusion. Other nations do not exclude this waste; Australia, with one of the world’s largest bauxite reserves, requires that bauxite residue be treated to pH below 12.5 for it to be considered not hazardous. Residue carbonation decreases the potential impact that would result from a release of the byproduct and diminishes long-term liabilities associated with maintaining impoundments that contain large amounts of leachable alkalinity. In this direct-carbonation bauxite residue neutralization scheme, the partially dewatered bauxite residue slurry is pumped to a carbonation reactor, where pressurized CO2 is applied in completely mixed batch reactions. This carbonation reaction is carried out using a concentrated CO2 stream under relatively low pressure (approximately 0.689 MPa or 100 psig) and at ambient temperature for an unspecified duration (less than a 1-h reaction time). The primary source of alkalinity in bauxite residue slurry is NaOH remaining from the Bayer extraction of alumina.14 Other, more resilient, sources of alkalinity associated with the solid phase of the slurry, such as tricalcium aluminate, also impact the effectiveness of bauxite residue neutralization by direct carbonation.10 Unlike dissolved or highly soluble sources of alkalinity, such as NaOH, which readily react with applied CO2, sources of solid-phase alkalinity react far more slowly. The pH of carbonated bauxite residue has been observed to drift to pH values as high as 10.6 as a result of CO2 degassing and reaction over time with solid-phase alkalinity.10 Carbonate Chemistry Overview. Dissolution of CO2 in water results in the formation of carbonic acid (H2CO3*) that dissociates to HCO3- and CO32- ions, releasing H+ to the fluid: CO2(g) + H2O ) H2CO3*(aq)

(1)

H2CO*(aq) ) HCO3-(aq) + H+(aq)

(2)

HCO3-(aq) ) CO3-(aq) + H+(aq)

(3)

resulting in a decrease in solution pH. The decrease in pH is further enhanced when carbonate minerals precipitate from ions in solution via reactions such as: Ca2+ + CO2 + H2O ) CaCO3(s) + 2H+

(4)

Mg2+ + CO2 + H2O ) MgCO3(s) + 2H+

(5)

Ca2+ + Mg++ + 2HCO3- ) CaMg(CO3)2(s) + 2H+

(6)

Fe2+ + CO2 + H2O ) FeCO3(s) + 2H+

(7)

(15) Singh, M.; Upadhayay, S. N.; Prasad, P. M. Preparation of iron rich cements using red mud. Cem. Concr. Res. 1997, 27 (7), 1037–1046. (16) Pan, Z.; Li, D.; Yu, J.; Yang, N. Properties and microstructure of the hardened alkali-activated red mud-slag cementitious material. Cem. Concr. Res. 2003, 33, 1437–1441. (17) Doye, I.; Duchesne, J. Neutralisation of acid mine drainage with alkaline industrial residues: laboratory investigation using batch-leaching tests. Appl. Geochem. 2003, 18, 1197–1213. (18) Lopéz, E.; Soto, B.; Arias, M.; Núñez, A.; Rubinos, D.; Barral, M. T. Adsorbent properties of red mud and its use for watewater treatment. Water Res. 1998, 32 (4), 1314–1322.

Sequestration of CO2 in Bauxite/Brine

Energy & Fuels, Vol. 22, No. 1, 2008 345 Table 1. Chemical composition of the Air-Exposed Oriskany Brine and Reynolds Bauxite Residue Filtratea

Al Ba Ca Fe K Mg Mn Na P Si Sr ClBrtot S

Figure 1. Illustration of bauxite residue/brine mixture CO2 sequestration process concept designed to achieve CO2 sequestration while achieving full neutralization of caustic bauxite residue and promoting mineral carbonate formation.

Thus, carbonate precipitation in the absence of buffered pH will be limited by the generation of H+, which acts to consume alkalinity resulting in increased solubility of calcite and other carbonate minerals. Effective sequestration of CO2 is enhanced by fixing the pH at relatively high values in order to counteract the loss of alkalinity (production of H+) during mineral precipitation and CO2 dissolution. In geological sequestration, alkalinity buffering can be accomplished through mineral reaction within the host rocks.19 However, due to slow reaction rates, these mineral reactions are likely to take a long time. Above-ground carbon sequestration may be more feasible because of the ability to buffer pH at relatively high values through means of caustic additives. The objective of the study described herein is to evaluate the concept of beneficial use of bauxite residue and oil field brine mixtures for carbon sequestration. In this treatment scheme, bauxite residue serves as a caustic source and brine serves as a source of calcium and, to a lesser extent, magnesium ions to promote mineral carbonation and as a means of decreasing mixture viscosity to facilitate mass transfer and gas/ slurry interaction. In addition to achieving CO2 sequestration, the proposed treatment method neutralizes the bauxite residue reactant, reducing the potential impact of the caustic byproduct to the surrounding environment. A simplified schematic diagram of the treatment concept is illustrated in Figure 1. Materials and Methods Solution and Solid Digestate Ion Analyses. Liquid solution before and after the experiments (reactants and products) were prepared for analysis by filtration through a 0.45 µm membrane filter (type HA, Millipore Corporation-Billerica, MA) facilitated by reduced pressure provided by a water aspirator. The collected solids were rinsed with deionized water on the membrane and dried in a nitrogen-purged oven at 110 °C. The filtered solutions were acidified (pH < 2.0) with trace metal grade nitric acid. Due to the high concentration of alkali and alkaline earth metals in solutions, 200-fold dilutions were prepared using distilled, deionized water. Metal concentrations were then determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a PerkinElmer Optima 3000 ICP spectrometer. Filtrate solutions were (19) Allen, D. E.; Strazisar, B. R.; Soong, Y.; Hedges, S. W. Modeling carbon dioxide sequestration in saline aquifers: Significance of elevated pressures and salinities. Fuel Process. Technol. 2005, 86, 1569–1580.

units

mean brine composition

mean bauxite residue composition

pH mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

2.72 b 847 25133 165 1930 1540 5 48733 b 7 9503 122929 797 23

13 2000 b 2 4 77 b b 4865 b 155 b 2576 b 799

a All values were determined by inductively coupled plasma analysis, except chloride, bromide, and sulfate, which were determined by ion chromatography. b Under detection limits.

analyzed for cations Al, Ba, Ca, Fe, K, Mg, Mn, Na, Ni, and Sr. Analysis for selected anions (Cl, Br, and SO4) was performed using a Dionex DX-100 ion chromatograph equipped with a conductivity detector. The columns and suppressor used for this analysis were also provided by Dionex. Solid samples were microwave digested in an aqua regia solution to leach extractable metals as prescribed in EPA Method 3051.20 Digestate samples were also analyzed for cation concentrations. X-ray Diffraction. Powder X-ray diffraction analysis of the mineral composition of solids was carried out using a PANalytical X’Pert PRO Theta-Theta multipurpose Diffractometer, equipped with a Cu anode at 45 kV and 40 mA, a divergent beam monochromator, and an X’Celerator detector. Samples were prepared for analysis by grinding the solids gently in an agate motar/ pestle. Phase identification was verified by comparison to the ICDD inorganic compound powder diffraction database. Oriskany Sandstone Formation Brine. Brine samples were collected from a well in the Oriskany Sandstone aquifer, in Indiana County, Pennsylvaniasoperated by Cabot Oil & Gas Corporation. Brine samples were collected directly from the well after purging at a formation depth of 2800 m and transported in polyethylene bottles with airtight caps to minimize oxidation. Sampling methods followed those described by Lico and others.21 Samples were stored at 4 °C for several months before being used in the experiments described herein. During this period, exposure to air occurred as a result of diffusion through high-density polyethylene storage bottles. Sample oxidation resulted in precipitation of ferric iron hydroxide and a gradual decrease in brine pH over time. However, the treatment concept under consideration involves ex-situ mixing of all reactants. Reaction with brine that has not been exposed to air would be unrealistic. Brine sample analysis was conducted regularly throughout experimentation to ensure that the filtered sample composition did not change significantly as a result of continued air exposure. Aged Bauxite Residue. Aged bauxite residue was obtained from a residue impoundment located at the Sherwin Alumina Company (formerly Reynolds Metals Company) of Corpus Christi, TX. This bauxite residue sample was a slurry with approximately 40% solid and 60% liquid content by mass and a pH of approximately 13. Residue solids were acid digested by EPA Method 3051. Mean concentrations of dissolved constituents in bauxite residue filtrate (0.45 µm paper filter) are listed in Table 1, and mean chemical (20) United States EnVironmental Protection Agency Method 3051: MicrowaVe Assisted Acid Digestion of Sediments, Sludges, Solids, and Oils. US EPA: Washington, D.C. (21) Lico, M. S.; Kharaka, Y. K.; Carothers, W. W.; Wright, V. A. Methods for collection and analysis of geopressured geothermal and oil field waters; Water-Supply Paper 2194, US Geological Survey: Reston, VA, 1982; p 21.


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Table 2. Cation Concentration of Sherwin Aged Bauxite Residue Solids Digestate in Bauxite Residue As Determined by ICP-AES Analysis Bauxite Residue Solid Digestate µg/g µg/g µg/g µg/g µg/g µg/g µg/g µg/g µg/g µg/g µg/g µg/g

Al Ba Ca Fe K Mg Mn Na P S Si Sr

65000 180 69700 266000 332 1020 8140 30700 4520 3440 53400 548

composition of the bauxite residue solid component digestate is listed in Table 2. Finally, XRD analysis of aged bauxite residue solids (Figure 2), considered in combination with total solid chemical analysis data (Table 2), yielded the estimated mineral composition shown in Figure 3. To quantify the relative percentages of crystalline phases of aged bauxite residue sample (see Figure 4), Rietveld refinement was performed with the Bruker advanced X-ray solutions program TOPAS v 2.1. The zero error of the instrument was aligned to 0 using standard sample material. All crystal structural parameters were adopted from published data. Only unit-cell parameters (nm), background, crystalline size (nm), microstrain, specimen displacement (mm), and preferred orientation were refined. The sample displacement was -0.113 ((0.003) mm. The weighted profile residual (Rwp) of the refinement is 17.3 calculated by eq 8. Rwp )

∑ w (Y - Y ∑w Y m

o,m

2 c,m)

(8)

m o,m

Where, Yo,m and Yc,m are the observed and calculated data respectively at data point m and wm is the weighting given to data point m which for counting statistics is given by wm ) 1/σ(Yo,m)2 where σ(Yo,m) is the error in Yo,m. Carbonation Experiments. Bauxite residue/brine mixture carbonation experiments were carried out in a 1/2-L autoclave (Hastelloy C-276) manufactured by Progressive Equipment Corp. A simplified schematic representation of the autoclave reactor apparatus is shown in Figure 5. In a representative experiment, the 1/2-L reactor was loaded with 180 mL of premixed reactant (brine and bauxite residue mixture) and the headspace (0.18 L) was purged with 0.1 MPa carbon dioxide three times to remove all room air from the headspace. A prescribed initial pressure of CO2 (typically 0.689 MPa) was then charged into the reactor headspace. Following CO2 loading, valves were closed and the bauxite residue/brine/CO2 mixture was agitated at a mixer speed of 500 rpm to prevent settling of solids and to promote gas/slurry mass transfer. Reactions were allowed to continue for 30 min after headspace pressure stabilization was observed, with pressure stabilization being taken to indicate that the reactor contents had achieved solubility equilibrium. Upon completion of each experiment, the remaining headspace pressure was noted and then vented. The slurry was removed from the reactor and filtered to separate the solids from the aqueous solution. A Sentron-1001 digital pH meter was used to determine the pH of the solution before and after reaction. A series of reactions (0.689 MPa initial pressure, room temperature) were carried out across the full range of bauxite residue/brine mixtures in 10% by volume increments.

Results and Discussion Experimental. Carbonation Capacity and Rate. Carbonation experiments were carried out over the full range of bauxite residue/brine mixtures in 10% increments by volume. As

described earlier, this series of reactions was carried out at room temperature (typically 21 °C) with the 0.185 L headspace initially charged with 0.689 MPa of pure CO2. Reactions were allowed to continue for 30 min after headspace pressure stabilization was observed (headspace pressure stabilization being taken to indicate that the reactor contents had achieved solubility equilibrium). Pressure change between the initial application and final equilibrium condition was used, in conjunction with initial and final system temperature, to estimate the short-term capacity of the bauxite residue/brine mixture to sequester CO2, assuming ideal behavior of gas under the applied conditions. The rate of CO2 absorption into reactive mixtures was estimated based on the rate of pressure change with initiation of carbonation reaction. After the autoclave was charged with CO2 pressure and the initial pressure noted, the mixer motor was startedsthis was considered to be the time of reaction initiation. The headspace pressure was recorded in 5 to 15 s intervals initially and less frequently as the rate of pressure change slowed. Following completion of the experiment, the initial rate of headspace pressure change was determined graphically by fitting a line to the first several pressure measurements as plotted vs reaction time and calculating the slope in pounds per square inch per minute. Results, summarized in Figure 6, suggest that the capacity of reactant mixtures to absorb CO2 is, primarily, a function of reactant mixture pH, with carbonation capacity decreasing with decreasing mixture bauxite residue concentration (increasing mixture brine concentration). In contrast, the rate of CO2 dissolution in the reactive mixture was observed to decrease with increasing bauxite residue composition (increasing percent brine) when mixed at 500 rpm, as was done for all batch experiment described herein. This trend illustrates that the increasing mixture viscosity with increasing bauxite residue composition requires an increased total energy input to achieve complete CO2 gas reaction. Without conducting a detailed rheological evaluation of bauxite residue/brine mixtures, this information suggests the potential benefit of mixing brine and bauxite residue to decrease slurry viscosity and increase overall reaction rate. Bauxite Residue Neutralization. It has been demonstrated that pure CO2 carbonation of brine/bauxite residue in mixtures with as little as 20% Oriskany brine can result in initial mixture neutralization (Figure 7). In contrast, bauxite can not be neutralized by mixing with brine in any proportions (Figure 7). This finding is in agreement with work reported by McConchie and colleagues.22 Brine dilution of bauxite residue is believed to be a poor bauxite residue treatment alternative. Carbonation of pure bauxite residue slurry using pure CO2 shows the advantages of the proposed treatment method. Results shown in Figure 7 illustrate that, while initial bauxite residue/brine mixtures do not achieve neutralization, carbonation of bauxite residue/brine mixtures regularly results in initial product neutralization for most of the bauxite residue/brine slurry combinations considered. The product filtrate pH was initially slightly higher than that of the unfiltered product. When the product slurries were first removed from the reactor following CO2 degassing, CO2 was not fully in equilibrium with the room air (as described by the Henry’s Law partitioning between gas and aqueous CO2). When samples are filtered, the perturbation of the solution as it passes through the 0.45 µm filter promotes additional degassing of CO2 (22) McConchie, D. The use of seawater-neutralised bauxite refinery residues (red mud) in environmental remediation programs. Proceedings of the 1999 Global Symposium on Recycling, Waste Treatment and Clean Technology, The Minerals, Metals and Materials Society; Gaballah, J., et al., Eds.; 1999; Vol. 1, pp 391–400.


Energy & Fuels, Vol. 22, No. 1, 2008 347

Figure 2. X-ray diffraction pattern of aged bauxite residue solids: HsHematite, syn (Fe2O3, ICDD 89-0599); CsCalcite (CaCO3, ICDD 05-0586); GsGibbsite (Al(OH)3, ICDD 76-1782); CansCancrinite (Na6(Al6Si6O24)(CaCO3) · 2(H2O), ICDD 71-0776); AsAnatase (TiO2, ICDD 89-4203); KsKaolinite (Al4(OH)8(Si4O10), ICDD 78-2110); BsBoehmite (AlO(OH), ICDD 83-1505); GosGoethite, syn (FeO(OH), ICDD 81-0464). Possible trace amounts of sodalite (Na4Al3Si3O12Cl, ICDD 37-0476), gypsum (CaSO4 · 2H2O, ICDD 36-0432), perovskite (CaTiO3, ICDD 22-0153), and Lime, syn (CaO, ICDD 37-1497), are not marked in this pattern for clarity.

Figure 3. XRD analysis of aged bauxite residue solids from the residue impoundment located at the Sherwin Alumina Company of Corpus Christi, TX.

(similar to stirring a recently opened carbonated soda). As a result of this degassing, the pH of filtered solutions immediately following filtration is, in most cases, slightly higher than that of unfiltered samples. Geochemical Modeling. A simplified geochemical model was used to interpret experimental results and predict the effectiveness of bauxite residue neutralization and potential CO2 mineral sequestration capacity when reacted with brine or a combination of brine and CO2. The model was constructed using the code of PHREEQC.23 Considering the high ionic strength of the saline brines, the Pitzer ion-interaction model was used to calculate solution chemistry and mineral solubilities. The Pitzer ion-interaction parameters used in the model were mostly from Plummer et al.,24 but those for Al and Si, which are not available in the database of Plummer et al.,24 are listed in Table 3. (23) Parkhurst, D. L.; Appello, A. A. J. User’s guide to PHREEQC (Version 2)-A computer program for speciation, batch-reaction, one dimensional transport, and inVerse geochemical modeling; Water-Resource Investigation Report 99-4259, US Geological Survey: Reston, VA, 1999; pp 312.

The model described herein determines which mineral forms when the reactive system is at equilibrium and does not take into account reaction kinetics. Nonetheless, the model can be used to provide an estimate of maximum CO2 sequestration and mineralization capacity. Two sets of simulations were conducted at 25 °C: (a) mixing of brine with bauxite residue liquid without the addition of CO2(g), and (b) mixing of brine, bauxite residue liquid, and CO2(g) (p CO2 ) 1 bar). For the initial simulations, solid components in bauxite residue were not considered. Results, therefore, represent capacity to sequester CO2 by the reactions of gas and liquid phases only. Addition of solids is expected to enhance the ultimate capacity to sequester CO2 as a result of reaction with solid-phase alkalinity associated with crystalline and amorphous components of the bauxite residue solids. The initial solutions used in the model are from Table 1. However, Cl and Na concentrations were adjusted for electric neutralities of brine and bauxite residue solutions, respectively. Dawsonite (NaAlCO3(OH)2), gibbsite (Al(OH)3), calcite (CaCO3), dolomite (CaMg(CO3)2), barite (BaSO4), muscovite (KAl3Si3O10(OH)2), witherite (BaCO3), brucite (Mg(OH)2), magnesite (MgCO3), celestite (SrSO4), and portlandite (Ca(OH)2) were considered in equilibrium with the solution and allowed to precipitate. The equilibrium constants of minerals used are listed in Table 4. (24) Plummer, L. N.; Parkhurst, D. L.; Fleming, G. W.; Dunkle, S. A., A Computer Program Incorporating Pitzer’s Equations for Calculation of Geochemical Reactions in Brines; USGS Water-Resources Investigations Report 88-4153, US Geological Survey: Reston, VA, 1988. (25) Pitzer, K. S.; Mayorga, G. Thermodynamics of Electrolytes. II Activity and Osmotic Coefficients for Strong Electrolytes with One or Both Ions Univalent. J. Phys. Chem. 1973, 77, 2300. (26) Greenspan, L. Humidity fixed points of binary saturated aqueous solutions. J. Res. Natl. Bur. Stand. 1977, 81A, 89–96. (27) Christov, C. Thermodynamic Study of Quaternary Systems with Participation of Ammonium and Sodium Alums and Chromium Alums. Calphad 2002, 26 (3), 341–352. (28) Christov, C. Thermodynamic study of the K-Mg-Al-Cl-SO4-H2O system at the temperature 298.15 K. Calphad 2001, 25 (3), 445–454. (29) Robie, R. A.; Hemingway, B. S.; Fisher, J. R. Thermodynamic properties of minerals and related substances at 298. 15 K and 1 bar (105 Pascals) pressure and at higher temperatures. U.S. Geol. SurV. Bull. 1979, 1452, 1456. (30) Plummer, L. N.; Busenberg, E. The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90°C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O. Geochim. Cosmochim. Acta 1982, 46, 1011–1040.


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Figure 4. Rietveld refinement results of aged bauxite residue sample. Relative percentages of crystalline phases are shown in the upper-right corner. The blue line is the original pattern; the red line is the calculated one; the gray line shows the errors; and the dashed dots with different colors show the peak positions of the corresponding phases in the upper-right corner.

Figure 5. Simplified schematic of continuously stirred batch reactor experimental apparatus. Figure 7. Value of pH of reactant bauxite residue/Oriskany brine mixtures and CO2 carbonated product in 10% increments by volume compared with modeling results (dashed and solid lines, respectively; see the section on modeling). The experiments were conducted at room temperature with 0.689 MPa of initial CO2 headspace pressure. Table 3. Pitzer Ion-Interaction Parameters for Al and Si Used in the Model types of parameters B0 B1 C0 Θ

Figure 6. Experimental sequestration capacities and initial CO2 adsorption rates as a function of weight percentage of bauxite residue in mixture. The solubility of CO2 was observed to increase with increasing reactant mixture percent bauxite residue. However, increased viscosity with increasing percent bauxite residue was found to slow overall CO2 reaction at mixing speeds of 500 rpm. Predicted sequestration capacity (solid line) was calculated from geochemical modeling as described in detail below.

Mixing without CO2 is shown for comparison. Mixing brine with bauxite residue helps lower the solution pH. Simulation results compared with experimental data can be seen in Figure 7. Our model can generally predict the pH evolution trends. The modeling results agree with experiments that, even though the initial pH of the residue is lowered, the fluid mixture remains

Λ

Ζ Ψ

pairs

values at 25 °C and 1 bar

ref

Al3+ ClNa+ AlO2Al3+ ClNa+ AlO2Al3+ ClNa+ AlO2Al3+ Na+ AlO2- OHSiO2 Mg2+ SiO2 Na+ SiO2 ClSiO2 SO42SiO2 Mg2+ ClSiO2 Na+ ClAl3+ K+ ClAl3+ Mg2+ ClNa+ AlO2- OH-

0.6993 4.523 × 10-2 5.8447 0.3067 0.00273 -3.037 × 10-4 -0.07 0.014 0.08211 -7.89 × 10-2 0.142 0.077599 -0.05147 -8.48 × 10-15 -0.06 -0.024 -0.0048

25 26 25 26 25 26 27 26 26 26 26 26 26 26 28 28 26

quite alkaline at high fractions of brine. The pH decreases gradually and alkalinity decreases linearly with the increase of brine fraction when not considering mineral precipitation (Figure 8). However, when considering mineral precipitation, the pH variations as a function of the mixing fraction of brine in brine/



Table 4. Equilibrium Constants of Minerals Used in the Model minerals

log K (25 °C and 1 bar)

reactions

NaAlCO3(OH)2 + 3H+ ) Al3+ + HCO3- + Na+ + 2H2O calcite CaCO3 ) CO32- + Ca2+ gibbsite Al(OH)3 + 3H+ ) Al3+ + 3H2O dolomite CaMg(CO3)2 ) Ca2+ + Mg2+ + 2CO32barite BaSO4 ) Ba2+ + SO42muscovite KAl3Si3O10(OH)2 + 10H+ ) K+ + 3 Al3+ + 3SiO2 + 6 H2O witherite BaCO3 + H+ ) Ba2+ + HCO3brucite Mg(OH)2 ) Mg2+ + 2OHcelestite SrSO4 ) Sr2+ + SO42Portlandite Ca(OH)2 ) Ca2+ + 2OHMagnesite MgCO3 ) CO32- + Mg2+

4.3464a

dawsonite

e

a Reference 29. Reference 33.

b

Reference 30.

c

Reference 31.

-8.406b 7.7560c -17.083a -9.9711c 13.5858c -2.9965c -10.88d -6.630a -5.190e -7.834d d

Reference 32.

Figure 9. Amounts of mineral precipitation as a function of the mixing fraction of brine in brine/bauxite residue solution.

Figure 8. Comparison of effects of mixing with and without mineral precipitation and without the presence of CO2 on the variations of pH (Y-axis) and alkalinity (secondary Y-axis) as a function of brine fraction.

bauxite residue solution show 2 plateaus: pH ∼12 (brine fraction from 0.1 to 0.5), and pH ∼9 (brine fraction from 0.6 to 0.98), respectively (Figure 8). The trends of alkalinity variations are generally in accordance with pH variation. After an initial decrease in alkalinities (brine fraction from 0 to 0.1), the first alkalinity plateau appears (∼0.04 eg/L, brine fraction from 0.1 to 0.5) (Figure 8). The second alkalinity plateau (∼0, brine fraction from 0.6 to 1) appears after the second decrease in alkalinity (brine fraction from 0.5 to 0.6) (Figure 8). The alkalinities of the solution were almost exhausted when the fraction of brine was changed from 0.5 to 0.6 due to neutralization by the increasing percentages of acidic brine. This may explain the drop in pH (∼2 units) when the brine fraction was changed from 0.5 to 0.6. The modeling results are consistent with experimental data. The high alkalinity in bauxite residue makes it difficult to lower its pH to neutral, even when mixed with a large amount of acidic brine. Figure 9 shows the modeled mineral precipitation as a result of mixing brine with bauxite residue. Gibbsite, portlandite, brucite, celestite, muscovite, and Barite precipitated. Portlandite and gibbsite precipitation processes are dominant when less than 35% brine is in the mixture; brucite and gibbsite precipitation are dominant when larger than 35% brine is in the mixture. The second set of mixing simulations considered the addition of CO2 to bauxite residue/brine reactive mixtures at p CO2 ) 1 bar to help further decrease the mixture pH. Since the brine/ bauxite residue liquid mixtures contain dissolved cations, there

Figure 10. Comparison of the effects of mixing brine, bauxite residue liquid and CO2(g) (p CO2 ) 1 bar) with and without mineral precipitation on the variations of pH (Y-axis) and alkalinity (secondary Y-axis) as a function of brine fraction.

is the potential to precipitate carbonate minerals as a result of carbonate addition. Figure 7 indicates that the addition of CO2 helps to dramatically decrease the reactive mixture pH. The solution is lowered to near neutral pH (6.7) when only 1% brine is in the mixture. Model simulation can generally predict the pH evolution trends. pH decreases gradually and alkalinity decreases linearly with the increase of the brine fraction when not considering mineral precipitation (Figure 10). In contrast, when considering mineral precipitation, the pH decreases gradually (fraction of brine 0.1-0.85) after an initial sharp decrease (brine fraction 0-0.1) and then decreases rapidly when the fraction of brine changes from 0.85-1.0 (Figure 10). The trends of alkalinity variation are generally in accordance with pH variation (Figure 10). The model also predicts the formation of a significant amount of calcite (Figure 11) from the dissolved Ca introduced with the brine solution (Figure 12a). Moreover, the mineral Dawsonite, a Na-Al-CO2-bearing mineral, is shown to form significantly as a result of the reaction of carbonate ions with dissolved Na and Al in the mixture, an added benefit of the 3-component mixture that would not be possible with simple brine/CO2 mixtures. Dawsonite and calcite precipitation are the dominant processes with a less than 90% mix of brine with bauxite residue; whereas, witherite precipitation is dominant when the fraction


Figure 11. Predicted mineral precipitation as a result of mixing brine/ bauxite residue and CO2(g) (p CO2 ) 1 bar). Dawsonite, calcite, dolomite, and witherite are the C-bearing minerals formed as the result of brine/bauxite residue mixture carbonation.

Figure 12. Evolution of total aqueous components as a function of mixing fraction of brine in brine/bauxite residue and CO2(g) (p CO2 ) 1 bar). (a) C, Ba, Ca, and Mg; (b) S, Sr, Si, and Al. Solid lines denote mixing with mineral precipitation, and dashed lines are mixing without mineral precipitation.

of brine in the mixture is larger than 90% (Figure 11). The formation of significant amounts of C-bearing minerals (dawsonite, calcite, and witherite) indicates the efficiency of mineral trapping. In addition, small amount of dolomite, celestite, muscovite, and barite are also precipitated as the results of mixing (Figure 11).

Dilmore et al.

Figure 13. Carbonate speciation as a function of the mixing fraction of brine in mixtures of brine/bauxite residue and CO2(g) (p CO2 ) 1 bar).

Figure 12 shows the evolution of total aqueous components (C, Ba, Ca, Mg, S, Sr, Si, and Al) as a function of the mixing fraction of brine in brine/bauxite residue solution at p CO2 ) 1 bar. When there are no mineral precipitates, total aqueous concentrations of Ca, Mg, and Ba increase and C, S, Sr, Si, and Al decrease linearly with the increasing fraction of brine in the brine/bauxite residue mixture (Figure 12, dashed lines). When considering mineral precipitation, total aqueous concentrations deviate from the linear decrease (C, S, Sr, Si, and Al) or increase curves (Ca, Mg, and Ba) (Figure 12, solid lines). In addition to the predicted formation of mineral carbonate, the product mixture should also contain significant quantities of dissolved CO2(aq) and HCO3- (Figure 13). Simulated carbon sequestration capacities of bauxite residue/brine mixtures are in accordance with experimental data (Figure 6). Figure 14a shows the distribution of sequestered CO2 calculated with our model. Mineral trapping, aided with Ca, Na, Mg, and Ba from brine, is the dominant process when the fraction of brine in the mixture is from 0.05 to 0.85 (Figure 14b). Thus, the simulation predicts significant mineral trapping, which is brought about from mixing with brine. It is worth emphasizing that the model results reflect predictions of equilibrium conditions, but experiments described herein are not expected to approach equilibrium as a result of the short observed reaction time with respect to the expected small rates of mineral formation. Mineralogical Analyses. Selected bauxite residue/brine product samples were analyzed by X-ray diffraction (XRD) in an attempt to identify changes in crystalline content of carbonation product solids. However, because of the high amorphous content of bauxite residue solids and due to the complex mineralogical makeup of the processed bauxite ore, the results of XRD analyses provide only limited information and cannot be used for reliable quantitative solids characterization. In addition, crystalline products that might be generated by reactant mixture carbonation would be formed at only very low concentrations as compared to the total mass of product solids. Considering the results of the simulated equilibrium conditions, it is possible to estimate the increase in solids content of the slurry assuming all of the predicted carbonate formation occurred. For example, a 20% brine/80% bauxite residue mixture was predicted to result in the formation of dawsonite, calcite, celestite, witherite, and muscovite (Figure 11). Considering, then, the dawsonite (5.82 × 10-2 mol/kg or 8.38 g/kg), calcite (5.1 × 10-2 mol/kg or 5.1 g/kg solution), and witherite



Figure 15. XRD-based estimate of mineral composition of solid precipitate generated as a result of mixture of 53% bauxite residue liquor by mass with 47% Oriskany brine by mass (left-hand side) and the carbonated product of that mixture (right-hand side) which illustrates the increase in mineral carbonate formation in the precipitate of the carbonated mixture. Table 5. Cation Concentration of Digestate of the Precipitate Formed As a Result of 53% Bauxite Residue Liquor by Mass with 47% Oriskany Brine, As Determined by ICP-AES Analysis

Figure 14. Distribution of sequestered CO2 in brine/bauxite residue and CO2(g) (p CO2 ) 1 bar). (a) Concentration of total carbon sequestered, carbon sequestered in aqueous solution, and carbon sequestered in minerals as a function of the fraction of brine in the brine/bauxite residue mixture. (b) Percentage of carbon sequestered in aqueous solution and minerals as a function of fraction of brine in the brine/bauxite residue mixture.

(1.58 × 10-3 mol/kg or 0.31 g/kg), we can add the mass of the minerals to determine the total mass of mineral carbonate formed per kilogram of reactive solution (approximately 13.8 g/kg solution). Considering that this mixture of solution corresponds to a slurry with a solids content of approximately 32% solids by mass, we can estimate that the product solids, assuming all of the predicted solution mineralization occurs and no reaction of the slurry solid occurs, will increase to 33.4%, or an about 1.4% increase in total solids. As mentioned in the methods section, XRD analysis is not capable of identifying such small changes in crystalline composition. Even if the predicted mineralization proceeds completely over the short reaction duration, the changes in mineral composition of the solids will not be detectable using this analytical method. Predicted mineralization as a result of reactive solution carbonation would result in mineral sequestration of 0.11 mol of CO2/kg reactive solution. In light of this analytical limitation, selected CO2 carbonation reactions were conducted using mixtures of brine and bauxite residue liquor (no solids). In one example, 47% by weight Oriskany brine was mixed with 53% by weight bauxite residue liquor and the resulting mixture was reacted with an initial headspace pressure of 1.378 MPa (200 psig) of CO2. Precipitate formed as a result of this reaction was analyzed both by XRD analysis and cation analysis of the solid digestate, as described in the methods section. Filtered solids were not rinsed prior to XRD analysis to avoid dissolution of constituents that might

specie

units

Al Ba Ca Fe K Mg Mn Na P S Sr

wt wt wt wt wt wt wt wt wt wt wt

% % % % % % % % % % %

concentration 6.60 1.64 9.67 0.63 0.39 1.54 0.011 0 0.003 0.38 3.25

have been soluble in distilled water. Results of diffraction mineralogical analysis, illustrated in Figure 15, show that approximately 35%, by mass, of the generated solids, was calcite/magnesite, approximately 30% by mass was halite and dawsonite, while other minor mineral constituents comprised approximately 15% of the total mass (including CaSO4, SiO2, and SrCl2), and around 20% of the generated solids was amorphous content. Analysis of solid digestate of this product (Table 5) shows that, in addition to the Ca/Mg, Fe, Na, and Sr, a significant concentration of aluminum (6.6 wt %) was found in this carbonation product. This result suggests that carbonation of brine/bauxite liquid does form aluminum-bearing precipitate. This solid-phase Al might be associated with dawsonite. Precipitation of magnesite (MgCO3) was not predicted by our model (see the sectin on geochemical modeling) but was observed in the X-ray diffraction results. This may be due to the fact that the magnesite precipitation process suppressed in the equilibrium model was actually ongoing kinetically. As noted before, the equilibrium simulation predicts that both mineral and solubility trapping of CO2 in the carbonated bauxite residue/brine product are significant. As such, the controlling factor influencing the overall CO2-bearing capacity of the bauxite residue/brine reactive mixture is the pH of solution. As CO2 dissolves into these alkaline mixtures and hydrates, carbonic acid releases a hydrogen ion that reacts immediately with a hydroxyl ion in solution and neutralizes the solution of the reactant mixture. The degree to which the less reactive solidphase alkalinity is consumed over the short reaction times


Figure 16. Value of pH of filtered and unfiltered product of 90% bauxite residue slurry/10% Oriskany brine mixture carbonation. The unfiltered pH was observed to drift to a value of 9.7 after 33 days, while the filtered product pH remained relatively stable, suggesting bulk solution/ slurry solids interaction over time. The data points are connected with lines to show the trend.

considered herein was not evaluated. A single long-term reaction (CO2 loading to a 90% bauxite residue slurry/10% brine mixture by volume of bauxite residue/brine) showed an upward drift in solution pH that agrees with this hypothesis (Figure 16). Product pH Drift. As a means of preliminarily evaluating the proposed resilient solid-phase alkalinity that reacts over time in the carbonated product, product slurry pH of one sample was measured over a period of several weeks. The pH of a carbonated mixture of 90% bauxite residue/10% brine by volume was periodically measured. At the same time, a portion of the product sample was filtered and the filtrate pH was monitored over the same period. Figure 16 illustrates that, as time progresses, the filtered solution pH value changes very little but the unfiltered carbonation product increases rapidly over the first several days and more slowly as a maximum pH value is approached. In the case of the 90% bauxite residue/10% brine carbonation mixture product, a pH of 9.70 was reached 33 days after reaction. As noted before, the CO2 sequestration capacities of the bauxite residue/brine mixtures are related to pH. The increase of pH with time suggests the increase of CO2 sequestration capacity when aging. The finding that the pH of carbonated brine (filtered product sample) did not increase significantly following solids removal and equilibration with room air at 1 atm and room temperature gives evidence that a solid/liquid reaction occurring over time affects the product slurry pH. The carbonate system buffers the solution between pH values of bicarbonate and carbonate following degassing of CO2 in excess equilibrium conditions. Were the solid component of the bauxite residue/brine mixture inert with respect to product pH, the pH of the unfiltered product, similarly, would change little (remain circum-neutral) following removal from CO2 loading. However, as is illustrated in Figure 16, the pH of the product continued to rise following sample removal from the reactor. It is believed that this gradual increase in bauxite residue carbonation product pH occurs as a result of either slow reaction of residual solid-phase alkalinity or pore watersassociated with bauxite residue solids. Estimate of Annual CO2 Sequestration Capacity. On the basis of the experimental results described herein, a first estimate of carbon sequestration capacity was developed. This estimate assumes the following: 70 million metric tons of dry bauxite residue is generated annually. A mixture of 90% bauxite residue slurry/10% brine by volume is used for large-scale carbonation.

Dilmore et al.

The density of the prepared mixture is 1.373 g/mL, and the mixture percent solids (all solids from the bauxite residue slurry) is approximately 36% by mass. Assumed values are based on typical experimentally determined values for the bauxite residue slurry considered herein (data not shown). Using these values, a total bauxite residue/brine slurry volume of approximately 1.42 × 1011 L of reactive mixture would be carbonated per year resulting in sequestration of 1.3 million metric tons per year of CO2. Using this 90/10 volume ratio of bauxite residue slurry/ brine, approximately 14.2 billion L (121 million barrels) of produced brine and all of the bauxite residue-associated water could be neutralized using this process. This estimate of 1.3 million metric tons of initial carbonation capacity per year can be compared with the approximately 3.3 million metric tons of CO2 emitted per year from a 550 MWe net output pulverized coal power plant.34 This comparison points out that the ex-situ carbonation scheme described herein is not a primary candidate for large-scale CO2 sequestration to manage anthropogenic emissions from large point sources. Rather, the benefits of the treatment scheme derive from the potential to neutralize and mitigate the hazardous properties of large volumes of caustic bauxite residue and acidic brine. The sequestration of modest quantities of CO2 is a additional benefit that may be significant to niche markets with CO2 emissions rates that are small as compared to large fossil fuel-fired power generation plants. Remaining Technical Issues. Several potential drawbacks to the treatment process must also be addressed before this treatment scheme could be implemented on a large scale. A significant amount of energy would be required to mix the reactive slurry to ensure CO2 mass transfer into the viscous slurry. The neutralized supernatant from the carbonated bauxite residue/brine mixture will have elevated salinity which may require treatment before discharge or reuse. Finally, impoundment of the dewatered solids of the carbonated reaction product may contain residual alkalinity such that they would still require impoundment following treatment. Summary and Conclusions In summary, experimental results show the following: (1) Mixtures of oil/gas wastewater brine and bauxite residue can be used as a CO2 sink, achieving sequestration through both mineral and solubility trapping of up to 1.3 million metric tons per year. (2) Neutralization of bauxite residue liquids is possible through reaction with brine and CO2, decreasing the potential impact of the residue to the environment. (3) Precarbonation bauxite residue/Oriskany brine mixtures were lowered to pH values between 12.5 and 8.6. Mixing of bauxite residue liquid and brine alone would not be effective to neutralize bauxite residue liquid. The carbonation of mixtures of these solutions is shown to achieve complete neutralization. (31) Helgeson, H. C.; Delany, J. M.; Nesbitt, H. W.; Bird, D. K. Summary and critique of the thermodynamic properties of rock-forming minerals. Am. J. Sci. 1978, 278a, 229. (32) Plummer, L. N.; Vacher, H. L.; Mackenzie, F. T.; Bricker, O. P.; Land, L. S. Hydrogeochemistry of Bermuda - Case history of groundwater diagenesis of biocalcarenites. Geol. Soc. Am. Bull. 1976, 879, 1301–1316. (33) Harvie, C. E.; Moller, N.; Weare, J. H. The prediction of mineral solubilities in natural waters: The Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3CO2-H2O system to high ionic strengths at 25°C. Geochim. Cosmochim. Acta 1984, 48, 723–751. (34) Klara, J. M. Fossil Energy Power Plant Desk Reference: Bituminous Coal and Natural Gas to Electricity; Report No. DOE/NETL-2007/1282, Department of Energy: Washington, D.C., May 2007.


(4) A gradual increase in unfiltered carbonation product pH to values as high as 9.7 suggests the presence of a slow-reacting solid-phase alkaline constituent or the slow reaction of water associated with the pore space of the bauxite residue solids. (5) The described brine/CO2 neutralization of bauxite residue generates a much lower volume of brine wastewater than does the previously proposed mixing of seawater with bauxite residue. In addition, the use of produced saline wastewater from oil or gas wells means that there is no net increase in saline wastewater generated. (6) X-ray diffraction analyses verify mineral carbonation and mineral trapping of CO2 in the form of dawsonite and calcite.


(7) Our simplified geochemical models generally predicted the experimental results of carbon sequestration capacities and solution pH. Acknowledgment. The authors wish to thank the following researchers for their analytical support: Bret Howard, Robert Thompson, John Baltrus, Thomas Simonyi, and Elizabeth Frommell. David Bish helped with XRD interpretations. In addition, technical support was provided by James P. Knoer and Clement J. Lacher. This research was partially funded by Alcoa’s Front End Innovation Program. R.D. and D.A. receive funding through the U.S. DOE Oak Ridge Institute for Science and Education fellowship program. EF7003943

Sequestration of CO2 in Mixtures of Bauxite Residue and Saline

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