Remediation of Metal-Bearing Aqueous Waste Streams via Direct

Direct carbonation using liquid carbon dioxide can be used for the remediation of aqueous streams or slurries while sequestering carbon dioxide in the...
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Energy & Fuels 2001, 15, 256-262

Remediation of Metal-Bearing Aqueous Waste Streams via Direct Carbonation Robert M. Enick,* Eric J. Beckman, Chunmei Shi, and Jianhang Xu Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261

Lalit Chordia Thar Industries, 730 William Pitt Way, Pittsburgh, Pennsylvania 15238 Received October 25, 2000. Revised Manuscript Received January 4, 2001

Direct carbonation using liquid carbon dioxide can be used for the remediation of aqueous streams or slurries while sequestering carbon dioxide in the form of metal carbonates. Carbonic acid (pH ) 2.9), which formed when the aqueous phase was contacted with excess liquid carbon dioxide at ambient temperature (295 K) and elevated pressure (6.89 MPa), reacted with the metal cations to form metal carbonates in the agitated vessel. These metal carbonates precipitated out of solution as the pH returned to neutral when the system was depressurized. Electric arc furnace K061 dust, red mud from the Bayer process of alumina manufacture, and metal-bearing wastewater streams were amenable to this treatment. The treatment of a K061-dust slurry from a steel plant was semi-continuous. The dust particles were retained in the high-pressure reaction vessel as fresh water was continuously injected and high-pressure, metal carbonate-bearing water was withdrawn. The water residence time in the reactor was 12 min. About 30% of the metal in the K061 dust was extracted into a metal carbonate product, and 98% of the metal in the carbonate product was zinc. Unfortunately, lead was not selectively extracted from the dust. Red mud was neutralized in batch experiments that lasted 5-15 min. The pH of a 45 wt % red mud/55 wt % water slurry was reduced from 12.5 to 7. A post-treatment pH elevation to 9.5 was attributed to slow desilication reactions that occurred over 1-2 weeks at ambient temperature and pressure. A plating bath wastewater stream containing aluminum (666 ppm) and zinc (40 ppm) was contacted with excess liquid carbon dioxide for 5 min. The aluminum and zinc concentrations were reduced by 89% and 90%, respectively, and the metal carbonate precipitate was easily filtered. Although the combined sequestration potential of these wastes is small, the ability to effectively remediate waste streams could lead to an industrial interest in the development of direct carbonation technology.

Introduction Global energy use and concentrations of CO2 in the atmosphere are expected to increase during the 21st century. In 1995, the Intergovernmental Panel on Climate Change predicted that global anthropogenic emissions of CO2 will rise from the 1997 level of 7.4 Gt C/year to 26 Gt C/year by 2100. The IPCC also predicted a doubling of atmospheric CO2 concentration by 2050. Such an increase could have deleterious environmental consequences.1 Strategies for reducing the atmospheric concentration of CO2 include more efficient energy use, an increase in the use of low-carbon and carbon-free fuels, and CO2 sequestration. The DOE goal is “to have the potential to sequester a significant fraction of 1 Gt C/year in 2025 and 4 Gt C/year in 2050”.1 The sequestration of CO2 has been proposed in ways that include advanced biological processes, advanced chemical ap* Corresponding author. Tel: (412) 624-9649. Fax: (412) 624-9639. E-mail: [email protected]. (1) Office of Science, Office of Fossil Energy, US DOE. Carbon Sequestration Research and Development; December 1999.

proaches, and introduction of CO2 into oceans, terrestrial ecosystems, and geologic formations. Worldwide emission has been estimated to be 21.8 Gt CO2/year, or 5.9 Gt C/year. The United States generates 4.8 Gt CO2/ year, or 22% of these emissions. The U.S. CO2 output from all electric generating plants is 35% of this total, 0.46 Gt C/year, or 1.7Gt /year of CO2, or 30.6 TSCF/ year of CO2.1 The use of mineral carbonates as carbon dioxide sinks has been proposed as a sequestration technology.2-8 This technology is considered as one of the “Advanced Chemical Approaches to Sequestration” by the US (2) Goff, F.; Guthrie, G.; Lackner, K. Carbon Dioxide Sequestering Potential of Ultramafic Rocks; 23rd International Technical Conference on Coal Utilization and Fuel Systems, March 1998, pp 603-613. (3) O’Connor, W. K.; Dahlin, D. C.; Walters, R. P.; Turner, P. C. Carbon Dioxide Sequestration by Ex-Situ Mineral Carbonation; Proceedings of the Second Annual Dixie Lee Ray Symposium, ASME, Washington, DC, August 29-September 2, 1999. (4) O’Connor, W. K.; Dahlin, D. C.; Walters, R. P.; Turner, P. C. Carbon Dioxide Sequestration by Direct Mineral Carbonation with Carbonic Acid; Proceedings of the 25th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 6-9, 2000.

10.1021/ef000245x CCC: $20.00 © 2001 American Chemical Society Published on Web 02/02/2001

Remediation of Metal-Bearing Aqueous Waste Streams

DOE.1 The reaction of carbon dioxide with magnesiumor calcium-bearing minerals to form magnesite or calcite may be a viable means of sequestration because these carbonates are inert and stable solids that could be landfilled. Enormous deposits of magnesium-bearing, natural silicate minerals have been identified near population centers that could be used to sequester large volumes of anthropogenic carbon dioxide. Serpentinite and peridotite rocks are of particular interest because they contain 35-45 wt % MgO in the form of forsterite, Mg2SiO4, or serpentine, Mg3Si2O5(OH)4. Olivine, a magnesium-iron silicate, (Mg,Fe)2SiO4, has also been suggested as a candidate mineral for carbon sequestration.7 Although olivine deposits are not as prolific as serpentine, there occur in sufficient magnitude to make them viable candidates for sequestration. Unlike serpentine, olivine does not require high-temperature pretreatment to drive off hydroxyl groups. Two carbonation processes are currently being considered. During “extraction-reaction”, magnesium is extracted from the rock using hydrochloric acid. The magnesium chloride is then reacted with water to yield Mg(OH)Cl and is then processed in water to yield magnesium hydroxide and magnesium chloride. The magnesium hydroxide is then carbonated in a gas-solid or aqueous system reaction.7,8 “Direct carbonation” is a simpler carbonation process, but it may have more significant reaction rate barriers to overcome. The minerals are crushed into a fine powder, and mixed with water. The aqueous slurry is then contacted with an excess of liquid or supercritical carbon dioxide, which forms a second fluid phase within the reactor. The formation of carbonic acid lowers the pH of the aqueous phase9 (e.g., 2.80-2.95 at 25-70 °C and 7-20 MPa), which leads to the formation of metal carbonates. Although the carbonates may be insoluble in the low pH aqueous phase, some of the carbonates may dissolve. This technology requires high-pressure apparatuses and has exhibited relatively low conversions of about 30%.8 Recent work at DOE-NETL, in conjunction with researchers at ARC, LANL, ASU, and SAIC was conducted with olivine and serpentine.10 Carbonation was very slow at ambient temperature, therefore most experiments were conducted at temperatures above 450 K and pressures greater than 11 MPa. Reaction times at these conditions were of the order of hours to days. The overall reaction rate was also enhanced by the use of a 0.5 M NaHCO3 bicarbonate aqueous solution, rather than distilled water, during the direct carbonation. The (5) Lackner, K. S.; Butt, D. P.; Wendt, C. H.; Sharp, D. H. Carbon Dioxide Disposal in Solid Form; Proceedings of the 21st International Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 18-21, 1996. (6) Lackner, K. S.; Butt, D. P.; Wendt, C. H. Los Alamos National Laboratory Report LA-UR-97-660, 1997. (7) Lackner, K. S.; Butt, D.; Wendt, C.; Ziock, H. Los Alamos National Laboratory Report LA-UR-98-4530, 1998. (8) Walters, R. P.; Chen, Z.-Y.; Goldberg, P.; Lackner, K.; McKelvy, M.; Ziock, H. Mineral Carbonation: A Viable Method for CO2 Sequestration, Program Plan and Approach, 1999, Albany Research Center, SAIC, FETC, Los Alamos National Lab, Arizona State University. (9) Toews, K. L.; Shroll, R. M.; Wai, C. M. Anal. Chem. 1995, 67, 4040-4043. (10) Fauth, D. J.; Goldberg, P. M.; Knoer, J. P.; Soong, Y.; O’Connor, W. K.; Dahlin, D. C.; Nilsen, D. N.; Walters, R. P.; Lackner, K. S.; Ziock, H.; McKelvy, M. J.; Chen, Z. Carbon Dioxide Storage as Mineral Carbonates; Proceedings of the ACS National Meeting, Washington, DC, August 2000.

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increased rate was attributed to the increased solubility of carbon dioxide in the aqueous phase. Increasing temperature and pressure were shown to induce significant increases in rate and conversion, although the reactions still required hours to achieve significant carbonation. Mass transfer effects were also evidenced by an increased conversion with decreased particle size and increased surface area, and improved conversion upon vigorous agitation of the reaction vessel. The objective of our study was to use anthropogenic, metal-bearing waste streams from industry, rather than natural minerals, as a feed stream for a continuous direct carbonation reactor. Three examples considered as candidates for carbonation include red mud, electric arc furnace dust, and wastewater from plating facilities. Each of these remediation processes would sequester carbon dioxide in a metal carbonate form. Unlike the carbonation of natural silicate minerals, the carbonation reaction was expected to proceed rapidly at ambient temperature without the need for pretreatment. In some cases, it was desirable to separate and recover the metal carbonates. These three target feeds have a very low sequestration capacity relative to these amounts of anthropogenic carbon dioxide. Therefore we are proposing that this technology should be considered as a means of industrial waste stream remediation while sequestering small amounts of carbon dioxide. (The sequestration estimates provided for these waste streams are optimistic, order-of-magnitude assessments based on mass balances. If energy requirements, which are the ultimate fate of the treated waste and process economics are also considered, their sequestration capacities will be diminished.) This would provide an immediate incentive toward the development of high-pressure direct carbonation technology for processing these waste streams. The technology could be subsequently applied to the large-scale direct carbonation of natural minerals, which would be processed in a similar manner. The major byproduct from the Bayer process is red mud, the insoluble residue of the alumina extraction from the bauxite.11,12 The Bayer process, developed by Karl Josef Bayer 110 years ago, remains the most widely used means of manufacturing calcined commercial alumina from bauxite (40-60% Al2O3).13 Aluminum is subsequently produced by the electrolysis of alumina. Approximately 70 million tons of red mud is generated annually throughout the world. Red mud disposal methods include traditional closed cycle disposal (CCD) methods and modified closed cycle disposal (MCCD) and a new class of dry stacking (DS) technology. Problems associated with the disposal of red mud waste include its high pH (12-13), alkali seepage into underground water, safety in storage, and alkaline air-borne dust impact on plant life. Efforts to ameliorate red mud typically and possibly use it as a raw material usually incorporate a pH-reduction processing step.14-17 Various (11) Hind A. R.; Bhargava, S. K.; Grocott, S. C. Colloids Surf. 1999, 146 (1-3), 359-374. (12) Prasad, P. M.; Chandwani, H. K.; Mahadevan, H. Trans. Indian Inst. Metals 1996, 49 (6), 817-839. (13) Anderson, W. A.; Haupin, W. E. Aluminum and Aluminum Alloys. In Kirk-Othmer Concise Encyclopedia of Chemical Technology; Wiley and Sons: New York, 1985. (14) Wong, J.; Ho, G. Soil Sci. 1994, 158 (2), 115-123. (15) Vachon, P.; Tyagi, R.; Auclair, J.; Wilkinson, K. Environ. Sci. Technol. 1994, 28 (1), 26-30.

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Figure 1. Extraction of metals from dust using water and liquid CO2.

aqueous acidic solutions have been considered for this application, including acidic industrial wastewater.14 The use of carbonic acid has also been considered. Gasphase CO2 or CO2-containing flue gas has been bubbled through aqueous slurries to form carbonic acid in the aqueous phase.18 The carbonic acid results with basic components of the red mud, lowering its pH. However, the pH of water exposed to gaseous CO2 is not likely to drop below 5.5 (approximately), and hence the rate of reaction/neutralization of the solids in the aqueous slurry is typically not fast enough to satisfy industrial needs. At the short contact times which industrial process rates demand, only a fraction of the alkaline material in red mud is neutralized using gaseous CO2. Hence although the pH of the aqueous phase drops rapidly upon exposure to CO2 gas, it soon rises again to unacceptable levels as additional alkaline material leaches from the mud. The focus of this study is the use of high-pressure liquid or supercritical carbon dioxide, rather than vapor phase carbon dioxide, for the pH reduction of red mud and sequestration of CO2. The pH of water in contact with liquid CO2 is 2.80-2.95 over the 298-343 K temperature range and 7-20 MPa pressure range. These pH vales are significantly lower than the pH of 5.0-5.6 that can be attained with gasphase carbon dioxide. Therefore, we expected that a more rapid and effective neutralization of the red mud would be achieved using liquid carbon dioxide. Domestic steel manufacturers such as Nucor generate approximately 2 million tons of metal-rich, electric arc furnace dust each year. This iron-rich, iron oxide-rich dust can also contain oxides of zinc, lead, chromium, cadmium, copper, and nickel. The dust is classified as a hazardous waste, RCRA K061, if it contains cadmium (16) Koumanova, B.; Drame, M.; Popangelova, M. Resour. Conserv. Recycl. 1997, 19 (1), 11-20. (17) Rodriguez, G.; Rivera, F.; Pendas, S. Boletin de Espanola De Ceramica Y Vidrio 1999, 38 (3), 220-226. (18) Szirmai, E.; Babusek, S.; Balogh, G.; Nedves, A.; Horvath, G.; Lebenyi, Z.; Pinter, J. U.S. Patent 5 053 144, Oct. 1, 1991

or lead.19 If the cadmium and lead could be selectively extracted, the waste would be more benign. Zinc can be present in substantial amounts (up to 44% ZnO), and its extraction and recovery could be profitable for the manufacturer. We have recognized that if the zinc carbonate would be used as a recycle source of zinc for alloy production or was sold to a user that would react the zinc carbonate, the CO2 “sequestered” during the extraction would be released again as the zinc carbonate was processed. The development of a high-pressure direct carbonation unit that would be developed by the steel industry, even for the sole purpose of zinc recovery, would accelerate direct carbonation technology. The objective of our study was to determine which of the metals in the K061 dust could be extracted as a lowpH, high-pressure, water-soluble metal carbonate using direct carbonation. Typically, 70 wt % of the particles are less than 5 µm in diameter, 25% are in the 5-44 mµ range, and 5% are greater than 44 mµ in diameter. Numerous manufacturers generate wastewater streams containing dissolved metal cations. We anticipated that these streams would be the best candidates for remediation via direct carbonation because there would be no mass transfer limitations associated with solid particles because the metals were already in solution. The absence of solids also facilitated the introduction and withdrawal of the water from a highpressure carbonation reactor. Finally, the dissolved carbonates could be recovered via depressurization of the water and filtering of the carbonate precipitate. The general configuration of the proposed technology is illustrated in Figure 1, which illustrates the extraction of metals from steel dust. K061 dust is a dry feed, therefore water must be introduced to make a slurry that can be pumped into the extraction reactor. The reactor may be a cylindrical reactor with static mixers (as shown in Figure 1), a three-phase slurry bubble column reactor, or an agitated tank reactor. In this (19) US EPA, EPA Report EPA/310-R-95-010, U.S. Environmental Protection Agency, Washington, DC, September 1995.

Remediation of Metal-Bearing Aqueous Waste Streams Table 1. Composition of the Alcoa Red Mud compound

formula

hematite alumino-geothite sodalite tricalcium aluminate anatase/rutile calcite quartz boehmite gibbsite kaolinite muscovite

content (wt %) MW

Fe2O3 10-30 (Fe,Al)OOH 10-30 3Na2O‚Al2O3‚6SiO2‚Na2SO4 4-40 3CaO‚Al2O3‚6H2O 2-20 TiO2 2-15 CaCO3 2-10 SiO2 0-30 Al2O3‚H2O 0-20 Al2O3‚3H2O 0-5 Al2O3‚2SiO2‚2H2O 0-5 K2O‚3Al2O3‚6SiO2‚2H2O 0-15

160 75 994 378 80 100 60 120 156 258 796

extractor, the aqueous phase pH reduces to approximately 3 as carbonic acid forms. Metal carbonates form and dissolve (to some extent) in the water. There is an excess of CO2, therefore the pressure drop due to the loss of CO2 to the formation of carbonates is small. The high-pressure, three-phase mixture (aqueous phase containing dissolved metal carbonates, excess carbon dioxide phase, solid particles) exits the reactor and then enters a high-pressure cyclone. The CO2 phase is less dense than water, therefore it will exit through the vortex finder with most of the water. Some of the highpressure water and all of the dust particles exit the cyclone apex. Most of the dissolved metal carbonates are recovered as the overflow from the first high-pressure cyclone is depressurized and fed to a low-pressure cyclone. The solubility reduction and pH elevation that accompanies the pressure reduction of this stream causes the metal carbonates to precipitate and be recovered from the apex of the low-pressure cyclone. Dissolved carbonates in the water of the slurry that exited the apex of the first high-pressure cyclone are recovered by sending the slurry to another high-pressure cyclone, and the depressurizing the overflow stream. Conventional flash drums and solid-liquid separation techniques are then used to obtain the four streams: metal carbonates, extracted dust, water, and CO2. Both the water and CO2 can be recycled. Experimental Section Waste Streams. Red Mud. Dewatered red mud was provided by Alcoa. Its approximate composition is provided in Table 1. K061 Dust. Nucor Steel provided us with a sample of K061 dust composed of 31.90% iron, 14.95% zinc, 1.41% lead, 0.33% copper, 0.23% chromium, and 0.03% cadmium. Wastewater. Superior Industries provided us with a wastewater sample from the plating baths associated with the manufacture of automobile wheels. This stream was slightly basic, pH ) 9, and the predominant metallic cations were aluminum (666 ppm), zinc (40 ppm), and iron (5 ppm). Carbonation Apparatus. The apparatus illustrated in Figure 2 was used to carbonate the waste streams. This semicontinuous unit was operated at ambient temperature, 295 K. A specified mass of the steel plant dust slurry, the red mud slurry, or the wastewater was charged to the reactor at the beginning of the experiment. The one-liter reactor was then pressurized with liquid carbon dioxide to about 6.89 MPa (greater than the vapor pressure of 5.99 MPa), providing a layer of liquid carbon dioxide above the aqueous slurry. The vessel was agitated with axial flow impellers rotating at several hundred rpm. High-pressure water (6.89 MPa) was introduced to the reactor as high-pressure water containing dissolved metal carbonates and carbon dioxide was withdrawn.

Energy & Fuels, Vol. 15, No. 2, 2001 259 The flow rate correlated to a residence time on the order of several minutes. The single-phase, high-pressure water effluent was depressurized as it passed through a valve leading to a cyclone. Metal carbonate powder, which came out of solution immediately upon depressurization, was recovered through the apex while water and water and CO2 vapor exited through the vortex finder. The extraction was continued until carbonate recovery ceased. Metals Content Analysis. Results were quantified by material balance on the entire sample and the metals of particular interest. For example, the metals content of the steel plant dust, the extracted dust, the carbonate product, and the effluent low-pressure water were determined by EPA Solid Waste Method 846 6010 B for the dust samples and EPA Method 200.7 for the wastewater (Microbac Laboratories) in order to determine the efficiency of the process for the steel plant dust extraction. X-ray diffraction analysis was conducted at West Virginia University on the steel plant dust before and after the extraction to determine if an increase in Fe2O3 occurred during the extraction. The metals content of the wastewater, the metal carbonate product, and the effluent water was measured for the wastewater treatment experiments. Only the pH reduction and CO2 consumption (sequestration) were monitored for the red mud because there was no industrial interest in recovering the carbonates.

Results Red Mud. Approximately 900 g of the 45 wt % red mud/55 wt % water slurry was introduced to this vessel, which was then blanketed with liquid carbon dioxide. The aqueous slurry and the liquid carbon dioxide were mixed at 295 K and 6.7 MPa using several axial flow impellers spinning at 180 rpm. During the reaction, the pressure dropped as CO2 was transported from the fluid phase to form the carbonate. Therefore, CO2 consumption was monitored by measuring the amount of carbon dioxide added to the experiment to maintain constant pressure. The red mud was exposed to the high-pressure carbon dioxide for 5-15 min. The neutralization results are shown in Figure 3. These results show that there was a reduction in pH to neutral (pH ) 7) immediately following the exposure to liquid CO2. The pH slowly rose and then leveled off following the treatment due to the release of bound soda via desilication. The red mud slurries approached an equilibrium pH of approximately 9.5-10 after 1-2 weeks, and approximately 2.3 g of CO2 were sequestered for each 100 g of dewatered red mud. The annual, worldwide production rate of red mud (dry basis) is approximately 30 000 000 t/year. Based on our preliminary results and the composition of red mud, the order of magnitude of CO2 sequestration is 1 wt % or the red mud (dry basis). Therefore, 300 000 t/year of CO2, or about 82 000 t C/year could be sequestered in every aluminum manufacturing site in the world employed this technology. Currently, there is not a regulatory mandate to reduce the red mud pH to a specified level prior to disposal. Therefore, it is unlikely that this technology would be considered unless such guidelines are introduced, despite the successful reduction in pH that was achieved in this study. These results illustrate that red mud can be quickly neutralized via contact with liquid carbon dioxide. Although the process yields a neutral red mud (pH ) 7), slow desilication reactions cause the pH to slowly rise after treatment to a stable pH of 10. The sequestra-

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Figure 2. Semi-continuous extraction apparatus; batch feed of slurry, continuous feed, and withdrawal of CO2 and H2O.

Figure 3. pH reduction and posttreatment pH rise of 45% red mud slurries, treated in a 1-L vessel, with vigorous mixing with impellers; 900 g of slurry contacted with 130 g of CO2 at 295 K and 6.7 MPa, 5 and 15 min residence time.

tion capacity of this process is small because of the relatively small amount of red mud available for treatment, the small ratio of CO2 sequestered to red mud, and the current lack of economic incentive to invest in the equipment required to treat red mud with highpressure carbon dioxide. K061 Dust. The extraction of Nucor Steel K061 dust was performed at 295 K and 6.9 MPa. A turbine impellor (300 rpm) near the bottom of the vessel provided the agitation. The lower 300 mL of the vessel were occupied by the slurry, with a 700 mL blanket of carbon dioxide residing above the slurry. Freshwater and high-pressure metal carbonate-bearing water were introduced to and withdrawn from the reactor, respectively, at a rate of 25 mL/min. This resulted in a residence time of 12 min for the water. The dust particles were retained within the vessel during the extraction. One hundred grams of the dust remained within the reactor while 2600 g of water was introduced to and withdrawn from the reaction vessel. The results of the direct carbonation of a Nucor steel dust sample are provided in Table 2. The concentration of each metal in the K061 dust is listed in the “UD” (untreated dust) column. During the extraction, the effluent high-pressure water was depressurized, yielding water and a solid carbonate precipitate. This effluent

was fractionated into four samples as it was produced, with C1-C4 corresponding to the carbonate particles filtered from the water samples W1-W4, respectively. The combined carbonate samples and combined water samples are listed as Ctot and Wtot, respectively. The amount of each metal in each of these samples is found in the column below the sample’s designation. The experiment was stopped when it was obvious that no additional carbonates were recovered from the depressurized water. A total of 13.8 g of carbonates was recovered, along with 2333 g of water and 81.01 g of processed dust. The “total” column accounts for all of the metal at the end of the process found in the treated dust, the carbonates, and the low-pressure water. The material balance column compares this recovered amount with the amount of metal that entered the process in the K061 dust at the beginning of the experiment. The final column, %ex, provides the percentage of the metal originally in the K061 dust that was extracted into the carbonate samples. The results clearly demonstrate that this process selectively extracts zinc while concentrating iron in the treated dust. Consider the 17.95 g of zinc in the 100 g sample of Nucor dust; 2.229 g of zinc were recovered in the first carbonate sample, obtained via the depressurization of the first 603.8 g of water that passed through the reactor, and 1.263 g, 0.828 g, and 0.416 g were recovered during the depressurization of the subsequent 646.7 g, 639.1 g, and 443.7 g of water. Therefore a total of 4.735 g of zinc, in the form of zinc carbonate, was extracted. This was 31.7% of the zinc in the Nucor dust. This was a significant amount of extraction for a low -temperature, moderate-pressure, and short-residence-time reaction. The solubility of zinc carbonate in the high-pressure, carbon dioxide-saturated water appears to be on the order of 1 wt % (2.229 g of zinc, corresponding to 4.29 g of zinc carbonate obtained in the first 603.8 g water, 0.7 wt %). Only zinc appeared in a significant concentration (0.035 g Zn/603 g water, or 58 ppm, in the first water sample) in the depressurized water samples. The extraction was also preferential to cadmium and zinc, although the small amount of cadmium in the feed requires that one take caution in generalizing this result. This extraction experiment was successfully reproduced. The extraction

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Table 2. Results of Nucor Steel Plant Dust Direct Carbonation Extraction, the Amounts of Each Metal (in grams) Present in Each Sample Are Listed UD 100 g

C1 7.58 g

C2 3.19 g

C3 2.05 g

C4 0.98 g

Ctot 13.8 g

W1 603 g

W2 646 g

W3 639 g

W4 444 g

Wtot 2333

ED 81.01

total metal

MB err.%

%ex

metal Cd Cr Cu Fe Pb Ni Zn

0.033 0.226 0.325 31.90 1.410 0.022 14.950

0.02 5E-4 0.014 0.045 0.018 1E-04 2.229

0.001 1E-4 0.009 0.003 0.014 7E-05 1.263

0.001 8E-5 0.01 0.001 0.008 8E-5 0.828

7E-4 2E-4 0.008 0.009 0.007 7E-5 0.416

0.023 9E-4 0.041 0.058 0.048 3E-4 4.735

5E-4 6E-4 3E-5 6E-6 6E-5 5E-5 0.035

4E-5 6E-6 1E-5 6E-6 6E-5 6E-5 0.051

6E-6 6E-6 6E-6 6E-6 6E-5 3E-5 0.006

7E-5 4E-6 9E-6 4E-6 4E-5 4E-5 0.045

6E-4 6E-4 6E-5 2E-5 2E-4 2E-4 0.137

0.014 0.267 0.363 33.21 1.588 0.018 13.2

0.037 0.269 0.404 33.27 1.636 0.018 18.08

12.1 19.0 24.3 4.3 16.0 -18.2 20.9

69.7 0.4 12.6 0.2 3.4 1.4 31.7

of 100% of the cadmium, 36% of the zinc, and 10% of the copper was achieved in the second experiment. Although iron was the metal most prevalent in the feed, iron was the least significant component of the carbonate product. There were mass transfer limitations to the extraction, as evidenced by the large amounts of metal remaining in the extracted dust. These limitations may be attributable to the formation of iron-rich passivating layers that form on the dust particles during the extraction.10 Table 1 indicates that the concentration of iron in the treated dust increased because no iron was extracted, while other metals such as zinc and copper were extracted. XRD measurements indicated that the composition Fe2O3 in the dust increased from 66% Fe2O3 to 76% Fe2O3 after the extraction, which corresponded to an increase from 46% Fe to 53% Fe. Although the XRD results [WVU] provided slightly higher iron concentrations that the metals analysis results [Microbac Laboratories] of 32% to 41%, both analyses clearly indicate that iron becomes concentrated in the dust during this extraction. The domestic production rate of K061 dust is approximately 2 000 000 t/year. If one assumes that 50% of the dust is metal, 2 mol of CO2 are sequestered for each mole of metal, mass transfer limitations can be reduced, and that 5-50% of the metals in the dust could be converted into carbonates, then about 88 000880 000 tCO2/year or 24 000-240 000 t C/year could be sequestered by treating this dust with direct carbonation. Unfortunately, the dust treated by this process would not result in the reclassification of the K061 dust because, as shown in Table 2, both the extracted dust and the carbonate product would contain significant amounts of lead. These results demonstrate that the carbonation of a K061 dust slurry with liquid carbon dioxide can be conducted at ambient temperature and sufficient pressure to maintain carbon dioxide in the liquid phase. The reaction was conducted in a semi-continuous manner. The dust remained in the reactor while the highpressure water was being introduced to the reaction vessel at the same rate that the water containing dissolved carbonate was being withdrawn. The flowing water had a residence time of 12 min. The ratio of water to dust required to complete the extraction was about 25 (mass basis). Approximately one-third of the zinc was extracted from the dust and recovered as a zinc carbonate-rich product. Cadmium, zinc, and copper were selectively extracted while iron was not extracted. The mass transfer limitations on the extraction may be attributable to passivating layers of Fe2O3 on the particle surfaces. The inability to selectively extract lead from the dust precluded this process from successfully declassifying the dust as a K061 waste, however.

Table 3. Results of Superior Wastewater Extraction metal

feed wastewater ppm

post-treatment ppm

extraction %

Al Cr Fe Pb Zn

666 0.43 5.29 0.28 40

38 0.20 0.83 0.10 4.27

94% 53% 84% 64% 89%

Therefore the viability of this process would be related to the identification of an industrial need for zinc carbonate. Wastewater. The results correspond to a batch reaction of the wastewater (pH ) 9) with excess liquid CO2 for five minutes. The system was then depressurized, and the carbonate powder filtered from the wastewater. Table 3 illustrates the extraction results. These promising results were obtained for a singlestage extraction. The metals present in greatest concentration, aluminum and zinc, were extracted with efficiencies of 94% and 89%, respectively. The annual production rate of metal bearing-wastewater is on the order of 10 000 000 t/year, with a representative dissolved metals content of 1000 ppm. The most common treatment process for the wastewater is precipitation with lime or caustic, followed by filtration. The CO2 sequestration potential of all of these metal-bearing streams would be only 4400 t CO2/year, or 1200 t C/year. Although this process was the most promising with respect to the efficiency and ease of the remediation of the waste stream, the sequestration capacity of the process would be insignificant. Relative to the red mud and K061 extractions, the wastewater feed was the most amenable to the carbonation reaction. The extraction experiment was the simplest extraction to conduct due to the absence of solids in the feed or the reactor. Solid carbonate product was formed only after the effluent water stream was depressurized. Further, the degree of carbonation of the highly concentrated metals was approximately 90% because of the absence of solid-liquid mass transfer limitations in the extraction vessel. Unfortunately, the sequestration potential of this process is extremely small. Discussion Direct carbonation has been shown to be a viable technique for the remediation of metal-bearing wastewater streams or slurries, while sequestering carbon dioxide in the form of metal carbonates. This process has been tested at 295 K and sufficient pressure to maintain an excess liquid carbon dioxide phase. No preprocessing of the waste stream was required. Reaction times on the order of minutes were required to

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attain significant carbonation. The pH of red mud was reduced from 12.5 to 9.5, 31% of the zinc from K061 electric furnace arc dust was selectively extracted, and 89-94% of the predominant dissolved metal cations from a plating operation wastewater were precipitated and filtered. The combined sequestration potential of these three waste streams is only 100 000-1 000 000 ton CO2/year, however. Although the direct carbonation of these waste streams will not make a significant impact on the sequestration of anthropogenic carbon dioxide, this technology may provide an incentive for the immediate commercial development of direct carbonation technology. The remediation of the wastewater stream exhib-

Enick et al.

ited the most technical and practical promise, exhibiting efficient metals recovery and processing ease due to the absence of solids in the feed stream to the carbonation reactor. Acknowledgment. We are grateful for the financial support of Normex International. We also express our appreciation to Superior, US Steel, Nucor, and Alcoa for providing samples. Dan Fauth of the US DOE NETL provided many useful suggestions and ideas for this study. Dr. Renton of West Virginia University provided the XRD analysis of several dusts. EF000245X