Article Cite This: Ind. Eng. Chem. Res. 2017, 56, 13602-13609
pubs.acs.org/IECR
Enhancing the Rate of Magnesium Oxide Carbothermal Reduction by Catalysis, Milling, and Vacuum Operation Boris A. Chubukov, Aaron W. Palumbo, Scott C. Rowe, Mark A. Wallace, and Alan W. Weimer* Department of Chemical and Biological Engineering, University of Colorado Boulder, 596 UCB, Boulder, Colorado 80309-0596, United States ABSTRACT: Catalysis, milling, vacuum operation, and their interactions were studied as methods for increasing the rate of carbothermal reduction (CTR) of hard-burned and soft-burned MgO powders. For CTR at 1550 °C and 10 kPa, pellets made with soft-burned MgO reached 90% conversion in 30 min without milling and achieved the same conversion in 5 min with 120 min of milling. Crystallite attrition of soft-burned MgO was not observed, and the increased reactivity was attributed to mixing and aggregate attrition. Catalytic additives improved the reactivity in the CTR of hard-burned MgO but decreased the rate of CTR for soft-burned MgO. Additives were shown to catalyze the gas−solid reaction pathway, and thermogravimetric analysis revealed that the rate of carbon oxidation by CO2 was approximately 2 orders of magnitude higher than that of MgO reduction by CO; thus, the latter reaction was rate-limiting to the gas−solid reaction pathway.
1. INTRODUCTION The carbothermal reduction (CTR) of magnesium oxide is an attractive alternative to current primary magnesium production methods (silicothermic and electrolytic) because of the low cost of carbon. C + MgO ↔ CO + Mg (1) The product magnesium metal is gaseous at the reaction temperature, and reversion of the products has prevented this chemistry from being widely adopted. Industrial CTR plants have operated by diluting and quenching product gases,1 but low yields currently prevent this technology from being profitable. High yields (>90%) have been demonstrated by supersonic quenching2 and by vacuum condensation,3 but neither method has yet been commercialized. Magnesium oxide CTR at atmospheric pressure generally requires high temperatures to achieve complete conversion: Prentice et al.4 operated at temperatures in excess of 1700 °C to reach complete conversion after a hold time of 40 min. The ability to operate at lower temperatures, while maintaining high conversions, would allow for simpler and lower-cost furnace design and would reduce the sensible heat input required. Vacuum operation has been explored theoretically and experimentally as an approach to lower the reaction temperature of MgO CTR.5−10 Lowering the partial pressures of CO and Mg(g) within the reaction chamber, by pumping or by dilution with inert gas, pushes the reaction equilibrium toward the products. The predicted equilibrium states at product gas pressures of 1, 10, and 100 kPa are shown in Figure 1. The temperature required for complete conversion, in a closed vessel, is approximately 200 °C lower for each order of magnitude reduction in pressure. If product gases are removed © 2017 American Chemical Society
Figure 1. Thermodynamic predictions for the extent of MgO CTR in a closed vessel given the indicated product gas partial pressures.
Received: Revised: Accepted: Published: 13602
July 31, 2017 September 29, 2017 October 25, 2017 October 25, 2017 DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
Article
Industrial & Engineering Chemistry Research
Previous studies demonstrated the ability of solid-state reactions to proceed quickly given the effective preprocessing of powders. Here, catalysis, milling, vacuum operation, and their interactions were examined in an effort to achieve high magnesium oxide conversion to magnesium metal by carbothermal reduction under isothermal and isobaric conditions. The mechanisms of these processes were further investigated by measuring the rates of the individual reactions comprising the gas−solid reaction pathway, reactions 2 and 3, by thermogravimetry.
from the reaction chamber, complete conversion can be achieved at temperatures lower than those depicted in Figure 1. Theoretically, operating at sufficiently low pressure can shift the equilibrium such that the reaction proceeds at any temperature, even below the boiling point of Mg. The implementation of this method experimentally suffers from two major flaws. First, for the reaction to be favorable at temperatures below 1000 °C, the pressure must be near ultrahigh vacuum (UHV, 10−7 Pa). As a noncondensable byproduct (CO) exists, the amount of pump work required to maintain a high vacuum becomes unrealistic. For operation in this regime, effectively 10 km3 of CO must be pumped for every 1 mol of Mg produced. Additionally, experiments have shown that the reduction rate increases with vacuum only to a certain pressure below which the rate decreases.3,11 The pressure at which the reaction rate is maximized depends on pellet structure (porosity, size) and reaction variables (temperature, gas velocity). This phenomenon has been attributed to the decrease in carbon and magnesium oxide interparticle contact and the propagation of the reduction by gases (CO, CO2).3 MgO + CO ↔ Mg + CO2
C + CO2 ↔ 2CO
2. METHODS 2.1. Materials. Magnesium oxide powders were obtained from Premier Magnesia LLC (soft-burned, MagOx Super) and Martin Marietta (hard-burned, MagChem10). As a carbon source, petroleum coke from the Robinson, IL, refinery was obtained from Marathon Petroleum. Carbon and magnesium oxide powders were mixed in a molar ratio of C/MgO = 2.0 for all tests. The powder properties are listed in Table 1. Table 1. Properties of Raw Materials Used
(2) (3)
The reaction mechanism consists of two parallel reaction pathways: reaction 1 and reactions 2 and 3. The overall reduction rate is thus the sum of the rates of the two pathways. Under a high vacuum, reaction 1 dominates the overall rate of reduction because the partial pressures of CO and CO2 in the reaction chamber are too low to effectively propagate the reduction by the gas−solid pathway of reactions 2 and 3. Thus, application of a vacuum can increase the reaction rate only to a certain extent, and other parameters must be considered to drive the reaction to high conversion at low temperature. Catalysis and milling can effectively increase the reactivity of C/MgO pellets to improve process performance. Previous studies demonstrated the catalysis of CTR for many oxides (MgO,12 Fe2O3,13,14 FeCO3,15 SiO2,16 ZnO,14,17,18 MnO19) by the addition of alkali metals, alkaline earth metals, transition metals, and their oxides. Some researchers20,21 postulated a reaction mechanism by oxygen spillover on the catalyst surface, in which the dissociative adsorption of CO2 on the catalyst surface allows for carbon reaction with O• on the catalyst− carbon junction. Other proposed mechanisms14 include the vapor cycle mechanism, alkali-carbon intercalates as intermediates, and molten carbonate cycling. For the industrial production of Mg by silicothermic reduction, CaF2 catalyzes the reaction by powder addition prior to briquetting.22,23 For industrial carbon gasification, K2CO3 was added to the carbon feed by Exxon in the 1980s.24 In this work, the use of these catalysts and others for MgO CTR was examined. Milling powders can substantially increase the rate of solidstate reactions and can reduce the reaction onset temperature by mixing, particle attrition, and mechanoactivation.25 Milling has been shown to increase reactivity for the CTRs of SiO2,26 Fe2O3,27 MgO,28 and Al2O3.29 Notably, Nusheh et al.28 demonstrated the CTR of MgO at temperatures as low as 600 °C by planetary ball milling of C/MgO powder. These kinetic measurements were done by thermogravimetric analysis using a temperature ramp to 1500 °C, so the extent of reaction that can be achieved under low-temperature (T ≤ 1200 °C) and isothermal conditions is unclear.
property
petroleum cokea
soft-burned MgO
hard-burned MgO
purity (%) SSAb(m2/g) CaO (%) SiO2 (%) Fe2O3 (%)
91.1 7.78 >0.1 >0.1 >0.1
98.4 147 0.9 0.3 0.1
98.2 0.642 0.9 0.4 0.2
a
Properties of petroleum coke after 1 h of milling and pyrolysis at 650 °C. bSpecific surface area.
2.2. Powder Processing. Milling of carbon and magnesium oxide powder mixtures was performed using an HD-01 attrition mill from Union Process. A 1.0 L stainless steel container was loaded with stainless steel milling medium (440c 3.175 mm) in a medium/powder mass ratio of 10:1. The milling speed was constant at 708 rpm. Wet impregnation was performed on milled (120 min) C/ MgO powders using nitrate or chloride salts of the desired metal component. The amount of salt to obtain a 5 wt % loading was dissolved in 100 mL of anhydrous ethanol. C/MgO powders were then added, and the mixture was heated to 80 °C and stirred to remove all solvent. Following pretreatment, powders were pressed at 2 MT into 6.35-mm-diameter cylindrical briquettes each weighing 0.15 g. Briquettes were pyrolyzed at 650 °C for 4 h in N2 to remove organics. After being allowed to cool, the briquettes were stored in a vacuum desiccator. 2.3. Rate Measurement. Reduction rate experiments were performed by dropping pellets into a hot silicon carbide crucible resting within an electrically heated graphite furnace. The furnace, auxiliary equipment, and experimental procedure are described in detail in ref 3. The concentration of product gas (CO) was measured to infer the reaction rate. The gas signal was corrected for gas dispersion within the vacuum system3,30 and back-reaction prior to detection. Condensed magnesium was collected on a removable graphite liner. Gas−solid reactions 2 and 3 were studied by thermogravimetric analysis. Reaction 3 was analyzed using a Netzsch STA 449 F1 Jupiter thermal analyzer. The same instrument could not be used to analyze reaction 2, as the product magnesium would damage the sensitive equipment upon condensation. Instead, weight loss measurements were made at discrete time 13603
DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
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Industrial & Engineering Chemistry Research points before and after the reaction. Powders were contained within an alumina boat inside a 5.08-cm-o.d. alumina tube, and an atmosphere of CO was maintained by a flow of 0.5 SLPM through the tube. A horizontal tubular furnace (Carbolite STF16/50/450) was used to heat the sample and tube to the desired temperature. 2.4. Analytical Analyses. The compositions of the C/ MgO pellets before and after the reaction were determined by carbon and oxygen elemental analyses. Both analyses were based on IR detection of CO and CO2. Elemental carbon analysis was by oxidation in O2 using a combustion analyzer (LECO C200), and oxygen analysis was by reduction with carbon using a total oxygen analyzer (LECO TC600). The magnesium oxide composition was calculated assuming that all of the oxygen was bound as magnesium oxide. The morphologies of the pellets and particles were characterized by field-emission scanning electron microscopy (FESEM). The surface areas of the magnesium oxide and carbon powder were calculated by Brunauer−Emmett−Teller (BET) analysis. The catalyst weight loading and recovery were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Catalyst recovery was quantified based on the amount of catalyst remaining in the carbon residue. Crystallite size was estimated based on X-ray diffraction (XRD) peak width (fwhm) after correcting for instrument broadening using the Scherrer method. Thermodynamic calculations were performed using FactSage 7.1.31
3. RESULTS AND DISCUSSION 3.1. Experimental Summary. Pellets maintained their shape after the reaction but easily crumbled into a powder when handled. The FESEM images in Figure 2 show that the pellet was initially a dense compact of C/MgO and, after the reaction, it displayed a porous structure in the carbon residue. Condensed Mg was approximately 40−100 μm in diameter, and clusters of C/MgO from reversion were present in the condensate. The yield of Mg(s) from the reaction followed trends similar to those observed previously,3,32 and high yields (>95%) were realized from reaction at 0.1 kPa. 3.2. Mechanical Milling. Mechanical milling offers a lowtemperature preprocessing step for increasing C/MgO pellet reactivity. Milling can be with a solvent or without by the use of a dry lubricant. Carbon functions as an excellent dry lubricant. The effect of milling on reactivity was studied using hardburned and soft-burned MgO by reaction at 1550 °C and 10 kPa. The higher calcination temperature for hard-burned MgO resulted in a larger initial crystallite size relative to that of the soft-burned material due to sintering. This size difference was quantified by XRD peak width analysis and was confirmed by transmission electron microscopy (TEM) (Figure 3). Pellets made with soft-burned MgO reached 90% conversion after 30 min upon hand-mixing of the powders, whereas the same extent of conversion was reached after 5 min when the powders were milled for 120 min (Figure 3). Crystallite attrition was not evident in the soft-burned MgO, so the increase in reactivity was attributed solely to mixing and aggregate attrition. Pellets made with hard-burned MgO were less reactive than those made with soft-burned MgO. Powder milling of C and hard-burned MgO powders for 120 min resulted in a rate similar to that of hand-mixed C and softburned MgO powders. The hard-burned MgO crystallites decreased in size from 28 to 14 nm after 120 min of milling, but were still larger than the soft-burned MgO crystallites (3.5 nm).
Figure 2. SEM imaging of initial pellet structure, pellet residue, and product Mg. EDS spectra are also shown for Mg and reversion products.
Therefore, the increased reactivity of hard-burned MgO was due to crystallite attrition, aggregate attrition, and mixing. Dry milling of high-surface-area carbon and magnesia resulted in the greatest reactivity. For this increased reactivity to be beneficial to the overall process, the increase in reactor efficiency must outweigh the energy requirements of milling. 3.3. Catalytic Carbothermal Reduction. The performances of several potential catalysts in terms of the rate of CTR of hard-burned MgO were evaluated by reaction at 1200 °C 13604
DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
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Figure 3. Effects of milling time on CTR at 1550 °C and 10 kPa (left) and estimated crystallite size of milled powders (right). TEM images show nonmilled powders.
At a reaction temperature of 1550 °C, transition metal additives catalyzed the CTR of hard-burned MgO when the reaction pressure was 10 kPa, whereas at 0.1 kPa, catalysis was not evident (Figure 5). Previous work3 concluded that gas− solid reactions 2 and 3 are most prevalent at high reaction pressures because of higher concentrations of reaction intermediates under these conditions, whereas at low pressure (0.1 kPa), the reduction is primarily driven by solid−solid reaction 1. Thus, the catalysis of MgO CTR proceeded by gas− solid reactions 2 and 3, as catalysis was evident only at 10 kPa. At a reaction pressure of 0.1 kPa, additives had essentially no effect on reactivity, and thus, solid−solid reaction 1 was not catalyzed by transition metals. Application of a vacuum increased the reactivity; the decrease in product gas concentration favored reaction 1. Here, the increased rate of reaction 1 with a vacuum at 0.1 kPa outperformed the catalysis of reactions 2 and 3 at 10 kPa. Use of soft-burned MgO further increased the reactivity at 1550 °C and 10 kPa, but catalytic additives decreased the reduction rate (Figure 6). Additives likely blocked C and MgO interparticle contact. In this case, catalytic additives might have enhanced the rates of reactions 2 and 3 but decreased the rate of reaction 1. 3.4. Indirect Reduction, Reactions 2 and 3. To further understand the mechanism by which additives can catalyze CTR, the gas−solid reactions 2 and 3 were analyzed by thermogravimetric analysis (TGA). Figure 7 shows that, at 800 °C, reaction 3 proceeded readily and at an increased rate with the impregnation of transition metal salts. At 1200 °C, the reaction in the TGA instrument became mass-transfer-limited, as the rates were equal for all three samples. Wet impregnation of nickel salt increased the rate of MgO reduction by CO (reaction 2), as shown in Figure 7. After being heated for 1440 min, the powders sintered completely into a single mass (SSA = 1.0 × 10−3 m2/g), and the reduction rate was fairly constant. The reduction rate of the impregnated
and 10 kPa. The weight loadings of the catalysts on the C/ MgO powders and fractions of catalyst recovered from the residue are listed in Table 2. The relative rates observed using Table 2. Catalysts Screened for MgO CTR by Reaction for 240 min at 1200 °C and 10 kPa
a
catalyst
loading (wt %)
method
catalyst recovery (%)
Co(NO3)2 Ni(NO3)2 Fe(NO3)3 CaCl2 KNO3 K2CO3 CaF2 none
5.0 5.2 2.6 2.2 3.6 3.7 5.0 −
WIa WI WI WI WI milling milling WI
100 100 100 33 0 0 N/A −
Wet impregnation.
each catalyst are shown in Figure 4. Without a catalyst, the reaction reached a maximum conversion (5.3%) after 60 min. All additives catalyzed the reaction, and the impregnation of Ni and Ca catalyzed the reduction to the greatest extent. After 240 min of reduction, all pellets with additives showed conversions of between 23% and 31%, except for pellets with CaF2, which showed only 8.5% conversion. Higher temperatures were needed to push the reaction to completion. The addition of CaF2 is thought to increase pellet reactivity by lowering the melting point of MgO through a eutectic mixture.22,33 The increase in pellet reactivity by this effect was minimal relative to catalysis by other metals in this study. The mechanism of catalysis by the transition metals was further analyzed. Catalyst recovery correlated with the volatility of the added compound. High-boiling-point transition metals completely remained in the residue, whereas lower-boiling-point additives, potassium and calcium, showed low recoveries of 0% and 33%, respectively. 13605
DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
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Figure 4. CTR of hard-burned MgO at 1200 °C and 10 kPa with the addition of transition metals (left) and alkali and alkaline earth metals and minerals (right).
Figure 5. CTR of hard-burned MgO at 1550 °C and 0.1 kPa (black) or 10 kPa (red).
Figure 6. CTR of soft-burned MgO at 1550 °C and 10 kPa: impregnation onto milled C/MgO powders (black) and powder metal addition into hand-mixed powders (red).
MgO powders was 2.4 times higher than that of the raw powders. Under an Ar flow, neither MgO thermal dissociation nor transition metal volatilization was observed, indicating that only reaction 2 occurred in a CO atmosphere. MgO was collected downstream of the reaction zone indicating that product Mg(g) reverted completely. Some researchers14,17−19,34,35 have concluded that the ratelimiting step of carbothermal reduction is carbon gasification, reaction 3, because the experimentally determined activation
energies of the two reactions are similar, ∼200 kJ/mol,18,34−41 and because experiments have shown that the catalysts used for carbon gasification also increase the rate of carbothermal reduction.21,42−44 For the case of MgO CTR, it is unlikely that reaction 3 was rate-limiting because the rate of carbon oxidation at 1200 °C is of the same order of magnitude as the rate of magnesium oxide reduction by CO at 1500 °C. Assuming an activation energy of 200 kJ/mol for reaction 3, the extrapolated rate of carbon gasification at 1500 °C is 5.7 × 10−9 mol·s−1· 13606
DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
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Industrial & Engineering Chemistry Research
Figure 7. Thermogravimetric analysis of carbon oxidation in CO2 at 800 and 1200 °C (left) and magnesium oxide reduction by CO at 1500 °C (right). Reaction rates were calculated based on the initial rate assuming a first-order dependence on the reacting gas.
m−2·Pa−1 for carbon powders and 5.0 × 10−8 mol·s−1·m−2·Pa−1 for carbon powders impregnated with Ni salt. By this reasoning, the rate of carbon gasification by CO2 is approximately 2 orders of magnitude greater than the rate of magnesium oxide reduction by CO. Given that reaction 3 proceeded quickly at 1200 °C and that the reduction of MgO by CO was relatively slow and showed catalytic behavior at 1500 °C, reaction 2 was concluded to be rate-limiting to the gas−solid reaction pathway for MgO CTR. It might be the case that, for relatively less stable oxides (Fe2O3, ZnO) in which CTR proceeds at lower temperatures (∼1000 °C), carbon gasification is indeed rate-limiting. However, for relatively stable oxides (MgO, CaO, SiO2) in which CTR requires high temperatures (>1500 °C), the rate-limiting step must be reaction 2. Intuitively, this conclusion can be reached by measuring the ratio of CO to CO2 in the product gas stream. For MgO CTR, the CO/CO2 product gas was composed almost entirely of CO. Carbon dioxide is known to react with Mg(g) during condensation,3,32 so the true CO/CO2 ratio generated by CTR was calculated to be 5.69 ± 0.46 at a reactor pressure of 100 kPa based on the ratio of C to MgO reacted within a pellet.3 For the carbothermal reduction of Fe2O3, Khaki et al.45 reported a CO/CO2 ratio between 0.5 and 1.0. The greater the concentration of CO2 in the product gas, the greater the extent of indirect reduction (reaction 2). It is important to realize that the rate-limiting step here refers to the gas−solid reaction pathway, reactions 2 and 3. There exists a parallel reaction pathway, reaction 1, that is governed by a separate rate-limiting step.
time (3.5 nm), indicating effective mixing and aggregate attrition. For the milling of hard-burned MgO, the crystallite size decreased from 28 to 14 nm over 120 min of milling. The increase in reduction rate from milling was by mixing and aggregate and/or crystallite attrition. Addition of catalytic material by wet impregnation or powder addition improved the reactivity of pellets made with hardburned MgO but decreased the rate for pellets made with softburned MgO. For hard-burned MgO, additives catalyzed the reduction at 10 kPa, but reduction at 0.1 kPa showed no catalytic activity, indicating that catalysis was by the gas−solid reaction pathway. The retarding effect of additives on the reduction of soft-burned MgO was likely due to physical blocking of C and MgO interparticle contact impeding reaction 1. For the reduction of soft-burned MgO, the hindrance of additives to reaction 1 outweighed any catalysis by reactions 2 and 3. Thermogravimetric analysis of reactions 2 and 3 showed that reduction of MgO by CO was approximately 2 orders of magnitude slower than C oxidation by CO2. Thus, for MgO CTR, the rate-limiting step of the gas−solid reaction pathway is reaction 2. Based on the current results, fast kinetics can be achieved by efficient calcination and milling rather than catalysis. Increased pellet reactivity does not inherently lead to greater process efficiency. Any energy input through preprocessing must be outweighed by the increase in reactor efficiency. The dependence of powder characteristics on the effectiveness of catalysis, milling, and vacuum operation demonstrate the complex nature of MgO CTR. The presented results are intended to serve as a guide for process engineering given a specific C and MgO feedstock.
4. CONCLUSIONS Catalysis, milling, vacuum operation, and their interaction were studied in an effort to effectively increase the rate of MgO CTR. For the milling of soft-burned MgO, the pellet reactivity increased even while crystallite size remained constant over 13607
DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
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Industrial & Engineering Chemistry Research
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Boris A. Chubukov: 0000-0002-3711-447X Notes
The authors declare the following competing financial interest(s): Boris Chubukov, Aaron Palumbo, and Scott Rowe are co-founders of Big Blue Technologies, a start-up from the University of Colorado - Boulder working to commercialize Mg production by carbothermal reduction.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Science Foundation (Award 1622824) and from the Advanced Research Projects Agency-Energy (ARPA-E) of the U.S. Department of Energy (DOE) (Award AR0000404). We thank Clint Bickmore for his comments and assistance in operating the ball mill.
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REFERENCES
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DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
Article
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DOI: 10.1021/acs.iecr.7b03175 Ind. Eng. Chem. Res. 2017, 56, 13602−13609
