PROCESS INTENSIFICATION ROUTES FOR MINERAL CARBONATION 1*
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R. Santos , D. François , E. Vandevelde , G. Mertens , J. Elsen , T. Van Gerven
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Department of Chemical Engineering, Katholieke Universiteit Leuven, Leuven, Belgium Department of Chemical Engineering, Katholieke Universiteit Leuven, Leuven, Belgium * corresponding author:
[email protected] 2
ABSTRACT – Mineral carbonation of AOD stainless steel slag and calcium oxide were investigated. The main goal was to sequester the maximal amount of CO2 under optimal conditions. Carbonation was conducted on humid samples in a CO2 chamber and in slurry phase. The carbonation reaction was intensified by the introduction of ultrasound as a means to accelerate the carbonation reaction by particle fragmentation and carbonate layer removal. Several process parameters that influence carbonation were studied, such as temperature, moisture content and particle size. The highest conversion achieved with AOD steel slag in the CO2 chamber was 46.5% at 30oC, 20% CO2 partial pressure and 7 days duration. Carbonation of calcium oxide in slurry reached higher conversion (84.4%) than AOD slag (26.5%) over 35 minutes. Compared to stirring, the enhancement effect of sonication was dimmed by the fast reaction kinetics of calcium oxide. The sonication effect was observed by the reduction in particle size and the eroded morphology of the sonicated particles.
INTRODUCTION To overcome the many inefficiencies that current technologies face and the feasibility barriers that hinder the applicability of new technologies, process intensification promises to be a key facet of engineering development for years to come. Process intensification invariably involves thinking in an integrated manner, seeking to bring together fundamental aspects of process engineering technology and finding the most optimum balance between them. In this work, intensification routes for mineral carbonation are explored. Mineral carbonation involves the capture of carbon dioxide in a mineral form. The principal aim and advantage of this approach is the chemical stability and storage safety of mineral carbonates, the opportunities for process integration presented by the technology, and the potential for valorisation of otherwise low-value resources (virgin or waste) into useful products. The main barriers to its deployment in industry, apart from the lack of legislative mandates in place, are one or more of: high energy intensity, low reaction conversion, slow reaction kinetics, complexities of the production chain, process adaptability, and competition for attention with alternative carbon capture technologies. Mineral carbonation routes can be classified in two branches: one-stage (Huijgen et al., 2005; Stolaroff et al., 2005; Baciocchi et al., 2009) or two-stage (Teir et al., 2007; Kodama et al., 2008) processes. An advantage of two-stage process is the possibility to produce pure carbonate products, such as PCC. A disadvantage is process complexity, especially the introduction of troublesome leaching agents. Besides capture of CO2 for emissions reduction, mineral carbonation has also been reported to be useful to stabilizing leaching behavior of steel slag (Huijgen and
Comans, 2006), and to manufacture blocks for construction application (Isoo et al., 2001). In this work, the one-stage process is chosen for investigation of carbonation of pure compounds (calcium oxide) and waste materials (AOD stainless steel slag). Two experimental approaches are tested: carbonation in water slurry phase (atmospheric conditions) and on moist samples in controlled atmosphere (CO2 chamber).
BACKGROUND Climate Change The drive to develop carbon capture technologies is centered on the perceived contribution of carbon dioxide emission to global warming due to the enhancement of the well-accepted greenhouse effect. Greenhouse gases effectively absorb thermal infrared radiation emitted by the Earth’s surface and lower atmosphere, and re-emit this energy in all direction, including downward to the Earth’s surface. An increase in the concentration of greenhouse gases leads to an increased infrared opacity of the atmosphere, and therefore to a global temperature rise (IPCC, 2007). Mann et al. (2008) have tracked global surface temperature and conclude that “recent warmth appears anomalous for at least the past 1300 years” based on proxy-based reconstruction of global surface temperatures for the past 2000 years. The NOAA (2010) has reported that the combined global land and ocean average surface temperature for several months of 2010 have been the warmest on record. Thus it appears only reasonable that CO2 emissions be controlled and reduced in st the 21 century to avert possible irreversible and catastrophic climate change. Kaya (1989) has developed an expression that summarizes the factors that lead to anthropogenic CO2 emissions, called the Kaya Identity and expressed as:
In order to reduce CO2 emissions, one or more of the multipliers on the right-handside must be reduced, or the last term must increase. Reducing the population (POP) or the standard of living (GDP/POP) is unlikely to be considered; on the contrary, these continue to increase as a result of the growth of developing nations. Hence it is necessary to address: (i) energy intensity (BTU/GDP; by efficient use of energy); (ii) carbon intensity (CO2⇑/BTU; by switching to using non-fossil fuels such as hydrogen and renewable energy); or (iii) carbon capture (CO2⇓; by development of technologies to capture and sequester more CO2) (Olajire, 2010). Process intensification can aid in all three approaches, and in particular (iii) is explored in this work. Process Intensification – Ultrasound Process intensification encompasses a broad range of engineering technologies. Some of the leading areas of development make use of some of the following technological domains: photocatalysis, magnetic fields, ultrasound, microwaves, and microfluidics. Ultrasound was investigated in the present work as a way to promote particle breakage during slurry carbonation, to remove the carbonated shell or depleted matrix layers that surround the unreacted particle core, thus reducing diffusion limitations and exposing unreacted material to the aqueous phase. As a
consequence, it is envisaged that sonication can increase the carbonation conversion and the process kinetics. The use of ultrasound in chemical processes, also termed sonochemistry, applies sound waves in the range of 16 to 100 kHz, based on the premise that as frequency is lowered, the power delivered increases. Power is delivered to a solution by inducing cavitation, that is, the formation of small cavities or microbubbles that grow and collapse rapidly. Cavitation generates turbulence/circulation, which enhances mass and heat transfer, both by improving convection mechanisms and by thinning diffusion-limiting boundary layers. The collapsing microbubbles produce high local temperature and pressure and high shear forces. These effects cause solid surface erosion, leading to the removal of passivating layers or to the eventual breakage of particles. Several works are reported on the use of ultrasound for particle size reduction such as in Lu et al. (2002) (silica particles), Franco et al. (2004) (kaolinite particles), and Raman and Abbas (2008) (aluminium oxide particles). Experimental work is also reported on the use of an ultrasound horn to enhance carbonation of fluidized bed combustion ash (Rao et al., 2007), where conversion extent was increased by threefold and particle size reduction of the unreacted materials was also observed. A sonic bath has also been used to enhance carbonation of calcium hydroxide using supercritical CO2 (López-Periago et al., 2010). Stainless Steel Slag A class of waste materials that has good potential for implementation as a feed material for mineral carbonation is steel slags. Treatment and disposal of these slags can be a costly burden on steel plants. Electric Arc Furnace (EAF) stainless steel slag is already used in construction materials applications (Geiseler, 1996). Sustainable solutions for Argon Oxygen Decarbonisation (AOD) and Ladle Metal (LM) stainless steel slags are still to be found. An integrated on-site mineral carbonation approach is envisaged as a possibly economically favorable solution. Therefore AOD stainless steel slag, sourced from the stainless steel operation of Arcelor Mittal in Genk, Belgium, is used for mineral carbonation in this work.
MATERIALS AND METHODS Materials To determine which elements and minerals are present in the AOD steel slag, XRF (Panalytical PW2400) and XRD (Philips PW1830) techniques were used. Table 1 shows the elemental composition of the slag by XRF. Table 2 shows the inferred oxide composition based on the elemental analysis. CaO and MgO, the principal compounds that can be carbonated in the slag, account for over 60wt%. Table 1: Elemental composition of steel slags determined by XRF. Elements (wt%) AOD slag
Ca 41.4
Si 14.8
Mg 4.4
Al 0.49
Cr 0.45
Fe 0.14
Table 2: Oxides (wt%) present in the steel slags inferred from XRF. Oxides (wt%) AOD new
CaO 57.9
SiO2 31.6
MgO 7.3
Al2O3 0.92
Cr2O3 0.66
Fe2O3 0.20
%CO2,uptake = 0.785(wt%CaO – 0.56wt%CaCO3 – 0.7wt%SO3) + 1.097wt%MgO + 0.71wt%Na2O + 0.468wt%K2O (1) −1 100 wt %CO 2 = + 1 %CO 2 ,uptake Figure 1 shows the XRD spectrum obtained for AOD steel slag. Table 3 shows the interpreted mineral composition determined by XRD. The main mineral phases are Merwinite (Ca3Mg(SiO4)2), γ-C2S (Ca2SiO4), Periclase (MgO), and Cuspidine (Ca4Si2O7(F,OH)2). The weight percentages were determined by the Rietveld method (Topas Academic) using added ZnO as a reference. According to the equation of Steinour (1959) (1), the maximum amount of CO2 that can be captured by the AOD stainless steel slag is 34.4 wt% of CO2.
Figure 1: XRD measurement of ‘AOD new’ stainless steel slag Table 3: Minerals (wt%) present in the AOD steel slag determined by XRD (normalized to 100% of the crystalline fraction). Mineral Merwinite γ-C2S Periclase Cuspidine Bredigite Reinhardbraunsite Calcite Wollastonite Fluorite Gypsum
Chemical formula Ca3Mg(SiO4)2 Ca2SiO4 MgO Ca4Si2O7(F,OH)2 Ca14Mg2(SiO4)8 Ca5(SiO4)2(OH,F)2 CaCO3 CaSiO3 CaF2 CaSO4
AOD slag (wt %) 36.4 18.1 9.1 22.0 5.8 2.5 2.1 1.9 1.6 0.62
Experimental Procedure CO2 Chamber To study the carbonation reaction of AOD steel slag a Sanyo CO2 incubator MCO17Al was used (Figure 2). In this chamber the partial pressure of carbon dioxide and the temperature can be controlled. The percentages CO2 of can be varied between 0 and 20 % at 1 bar and the temperature can be varied between 20 and 50°C. The humidity in the chamber is kept close to saturation (approx. 95%) by placing trays of water inside the chamber.
Figure 2: CO2 chamber (front and interior)
Figure 3: Schematic overview of sonicated slurry experimental set-up.
A strict procedure is followed to generate comparable results between carbonation experiments. The samples are prepared in Petri dishes with a certain amount of water to reach a specific liquid to solid (L/S) ratio. Experimental times varied from 1 hour to 7 days. Samples were also taken after 3 hours, 6 hours and 24 hours for intermediate analysis. Water evaporates from the samples because the exothermic carbonation reaction heats the samples, therefore water was added intermittently to maintain the L/S ratio. Carbonate and silica layers are formed during the reaction, lowering the diffusion rate. Samples were ground with a mortar and pestle during sampling times breaks down these layers and increases the reaction rate. Sonicated Slurry For sonication an ultrasonic processor Hielscher UP200S was used, which operates at 24 kHz frequency. The probe used was an S14 sonotrode, which has a tip diameter of 14 mm, maximal amplitude of 125 µm, and an acoustic power density of 105 W/cm². A PT100 temperature sensor is also connected to the device and allows the follow-up of temperature during the experiments. The experimental set-up used is illustrated in Figure 3. A common glass beaker, with a volume of one liter, filled with 800ml of distilled water was used. Typical experiments were performed with 10 to 30 grams of solids for carbonation. The suspension was mixed solely by the ultrasound horn during sonication experiments, or with a mechanical stirrer (Heidolph type RZ-R1) and straight blade impeller at 340 rpm for stirred carbonation experiments. CO2 was delivered to the solution from a compressed gas cylinder with flow controlled by a Brooks Sho-rate rotameter (R-2-15-AAA).
RESULTS CO2 Chamber Carbonation Results Carbonation tests were performed to systematically study the effect of process parameters on carbonation extent of AOD slag. Finally, the process was optimized to find the greatest carbonation conversion achievable with AOD slag using the present carbonation method. Influence of Process Parameters To reach the highest conversion at the end of the carbonation reaction the optimal conditions have to be defined. Experiments at different conditions were performed to
assess the influence of the process parameters. The parameters studied were: moisture content, temperature and particle size. The influence of moisture content was investigated by performing experiments with different initial L/S ratios. The moisture content decreases due to the temperature rise created by the exothermic carbonation reaction. One hour was chosen as the minimum period between two measurements to minimize the variation of the conditions in the chamber when the door is opened (there is a 10-15 minutes o stabilization time after closing the door). Experiments were performed at 50 C and 20% CO2 partial pressure for 6 hours duration using AOD slag. The moisture content in the samples when water is added hourly is given in Figure 4, and the resulting CO2 uptake is given in Figure 5. The moisture content was kept relatively stable and varied within small ranges. The importance of adding water to the sample can be seen, comparing the experiments when the sample is carbonated as is (AS = 0.0004 l/kg) to those where water is added. Furthermore, it is found that L/S ratios of 0.1, 0.2 and 0.3 l/kg yield similar carbonation extents, except that for 0.3 l/kg the reaction is slower in the first hour. At an L/S ratio of 0.5 l/kg the carbonation is slower and extent is less, likely due to the hindrance of CO2 diffusion into the stagnant water layer in the Petri dish.
Figure 4: Moisture content during carbonation of AOD slag; water added hourly; T=50°C, 20% CO2, L/S ratios: AS=0.0004 l/kg, 0.1, 0.2, 0.3 and 0.5 l/kg.
Figure 5: CO2 uptake of AOD slag; water added hourly; T=50°C, 20% CO2, L/S ratios: AS=0.0004 l/kg, 0.1, 0.2, 0.3 and 0.5 l/kg.
The temperature has an influence on two steps in the carbonation reaction. The first one is the dissolution of the carbon dioxide in water. The solubility of CO2 in water decreases as the temperature is increased. The temperature has also an influence on the leaching of calcium and magnesium ions from the mineral matrix. Experiments were done to see what the influence of the temperature is on the carbonation of stainless steel slag. Two temperatures were used, 30°C and 50°C. The pressure in the chamber was 20% CO2 and an L/S ratio of 0.2 l/kg with hourly water replacement for moisture control was used. The CO2 uptake is shown in Figure 6. The CO2 uptake of the sample at 30°C increased throughout the 6 hours, and reached a final value of 9wt%, while at 50°C the CO2 uptake remained nearly stable after the first hour and reached only 6wt%. It is thought that the lower temperature, which favors a higher CO2 solubility, aids in CO2 diffusion through the carbonated shell given the higher carbonate ion concentration in the solution. These results differ from Baciocchi et al. (2009), who viewed higher temperatures as helping mineral dissolution; however in their work higher CO2 pressures were used.
Figure 6: Influence of temperature on carbonation of AOD slag. Process conditions: 30°C or 50°C, 20% CO2 and an initial L/S ratio of 0.2 l/kg.
Figure 7: CO2 uptake of AOD slag as a function of initial grinding times by ball mill (50°C, L/S = 0.2 l/kg, 20% CO2).
The influence of the particle size was investigated by grinding the original material with a ball mill (SPEX catalog n°8000). Different grinding times are applied to achieve fractions with different particle size distribution. These ground samples are first investigated to determine their particle size distribution and then subjected to carbonation experiments. The original AOD stainless steel slag was ground for 1, 5, 10, 20 and 30 minutes. The mean particles sizes are given in Table 4. Carbonation conditions were 50°C, 20% CO2, initial L/S ratio of 0.2 l/kg and 24 hours duration. The CO2 uptake evolution of these experiments is given in Figure 7. When the sample is not ground, the sample reaches 2wt% CO2 uptake after 24 hours. When the grinding time is increased, a higher carbonation extent is achieved. For grinding times of 5 minutes and higher a CO2 uptake value of 9wt% is achieved after 24 hours. This indicates that grinding the sample to reduce the particle size has a positive influence on the carbonation process. Table 4: Mean values of the particle size distribution determined by laser diffraction of ground AOD slag samples with different grinding times Grinding Time Mean (µm)
0 min 38.7
1 min 32.6
5 min 25.3
10 min 27.4
20 min 22.2
30 min 19.3
With knowledge of the optimum process conditions, an experiment was performed with the optimal values to obtain accelerated carbonation. Due to the length of the test, it is performed with an initial L/S of 0.5 l/kg and daily water replacement to ensure moisture retention for longer times. A temperature of 30°C and a partial pressure of 20% CO2 were used. The material was initially milled for 5 minutes to obtain smaller particle size and the samples were re-ground daily by mortar. The resulting carbonation performance of this experiment is shown in Figure 8. After 7 days a CO2 uptake of 16wt% is obtained. While this value is significantly better than the other results obtained in this study, it is still short of the theoretical maximum of 34.4 wt% CO2 according to Equation (1). This indicates that under the present process 46.5% carbonation conversion is achievable. It is likely that better control of sample moisture (given moisture content possibly decreases in less time than the rewetting intervals used) and movement of the solids (to free unreacted surfaces) could improve carbonation efficiency in an industrial set-up.
Figure 8: pH evolution of carbonation of AOD stainless steel slag at optimal values for the process parameters.
Slurry Carbonation Results Effect of Sonication on Particle Size As was illustrated in Figure 7, the carbonation reaction can be enhanced by the use of small particles which have a large surface to volume ratio. This large surface area enhances the dissolution rate of the calcium oxide particles and allows more carbon dioxide to react immediately without having to diffuse through a possibly formed carbonate layer. Sonication was tested as a means of reducing the particle size of pure calcium oxide particles and AOD slag. The evolution of the particle size distributions by the use of ultrasound is shown in Figure 9. The volume weighted mean diameter of calcium oxide particles is reduced from 20.0 µm to 10.4 µm after 30 minutes of sonication. The effect on AOD slag is smaller, with a reduction from 31.7 µm to 29.7 µm. It is likely that materials physical properties such as hardness hinder further reduction of AOD slag particle size. Further work will focus on understanding these properties and in assessing if the particle surface is eroded sufficiently to remove carbonated layers and depleted silica layers.
Figure 9: Comparison of particle size distribution of original calcium oxide and AOD slag to sonicated samples.
Figure 10: TGA curves of carbonated calcium oxide and AOD slag after 35 minutes carbonation with sonication.
Effect of Sonication on Carbonation Carbonation tests were conducted in slurry solution using sonication and mechanical stirring. In the case of sonication, the probe was set to maximum amplitude and an immersion depth of 4 cm was used. In Figure 10 the carbonation extent after 35 minutes of reaction and sonication for CaO and AOD slag can be deduced from TGA (thermal gravimetric analysis) curves. CaO reaches nearly complete carbonation at 84.4% conversion, corresponding to 35wt% CO2 uptake. AOD slag reaches only
26.2% conversion, or 12.3wt% CO2 uptake, due to slower kinetics and the heterogeneous nature of the material particles. It should be noted, however, that this conversion is nearly as high as that obtained in the previously mentioned CO2 chamber after 7 days; therefore the slurry method is found to be potentially more efficient from an industrial feasibility basis. Further work should determine the maximum carbonation possible and the effect of process parameters such as temperature, CO2 partial pressure, additives and pH. A comparison is made for the conversion extent of carbonation between the experiments executed with stirring and sonication using pure CaO (Table 5). It is seen that sonication increases the carbonation yield, although the difference is relatively small (2.8 to 8.1 % enhancement). In general, carbonation of calcium oxide is expected to be rapid. Since the particles consist of pure calcium oxide it is not likely that the whole particle will be surrounded by a carbonated shell (that would limit diffusion) in this reaction environment. Hence, dissolution of calcium ions is not too restricted, and this probably explains why carbonation with stirring still reaches high conversions. When minerals containing silicates or other impure materials are used, the exposed surface area and the amount of calcium oxide are smaller, which leads to restricted diffusion. It is thus expected that the reduction in particle size, pitting and enhanced mass transfer using sonication will have a larger effect on these materials. Detailed work is ongoing in this area. Table 5: Comparison of the carbonation conversion of CaO between stirring and sonication methods (wt% calcite by XRD). 5 minutes 10 minutes 15 minutes 25 minutes 60 minutes
Stirring 51.0 % 61.4 % 73.1 % 86.3 % 100 %
Sonication 53.8 % 65.9 % 81.2 % 93.7 % 100 %
CONCLUSIONS In this work several aspects of the mineral carbonation of AOD stainless steel slag were investigated. The main goal was to sequester the maximal amount of CO2 under optimal conditions. The effects that influence carbonation of humid samples in a CO2 chamber were studied and the optimal conditions identified. A test performed with an initial L/S of 0.5 l/kg and daily water replacement, at 30°C with 20% CO2 partial pressure, after 5 minutes milling of the material, over 7 days, achieved 46.5% carbonation conversion, or 16wt% CO2 uptake. As a means of process intensification, sonication was tested for carbonation in a slurry process. A clear reduction in particle size of CaO was observed by using sonication, however AOD slag particle size reduction was limited. A comparison between carbonation experiments that were executed with stirring or sonication on CaO showed that ultrasound enhances the yield of carbonation; however, the difference was small due to the high reactivity of CaO. AOD slag carbonation in slurry with sonication reached 26.5% conversion, compared to 84.4% conversion for CaO carbonation, over 35 minutes reaction time. Optimization of the slurry process is ongoing.
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