CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201300516

Design of a Continuous Process Setup for Precipitated Calcium Carbonate Production from Steel Converter Slag Hannu-Petteri Mattila* and Ron Zevenhoven*[a] A mineral carbonation process “slag2PCC” for carbon capture, utilization, and storage is discussed. Ca is extracted from steel slag by an ammonium salt solvent and carbonated with gaseous CO2 after the separation of the residual slag. The solvent is reused after regeneration. The effects of slag properties such as the content of free lime, fractions of Ca, Si, Fe, and V, particle size, and slag storage on the Ca extraction efficiency are studied. Small particles with a high free-lime content and

minor fractions of Si and V are the most suitable. To limit the amount of impurities in the process, the slag-to-liquid ratio should remain below a certain value, which depends on the slag composition. Also, the design of a continuous test setup (total volume ~ 75 L) is described, which enables quick process variations needed to adapt the system to the varying slag quality. Different precipitated calcium carbonate crystals (calcite and vaterite) are generated in different parts of the setup.

Introduction CO2 emissions from human activities contribute to global climate change.[1] A significant source of industrial CO2 emissions is the iron and steel industry, in which CO2 is produced both as a result of the high temperatures and high coke consumption in the processes. The sector has accounted for 4–7 %[2, 3] of global CO2 emissions (in total ~ 30 Gt)[4] during recent years. However, several options are available for the steelmakers to lower this value. As an example, by changing the feedstock and increasing material recycling, Ruukki Metals in Finland was able to avoid 0.6 Mt of CO2 emissions in 2012 compared to the use of virgin raw materials. The total CO2 emissions of the company were 3.8 Mt in the same year.[5] Another method for CO2 emission reduction is so-called carbon capture, utilization, and storage (CCUS), which actually comprises several technologies. At the moment there is no financial incentive to use CCUS because the CO2 emission costs are low, at approximately 5 E per ton emitted (EUA, European Union Allowance price).[6] The cost of CO2 capture is generally estimated to be 20 E per ton or higher, which depends on the chosen process.[7, 8] Also, viable end-treatment solutions for the captured gas are often absent or excluded from the calculations, which further increases the total cost of CCUS. With so-called mineral carbonation it would be possible to convert gaseous CO2 to solid, stable carbonates, which can be either used for marketable products or landfilled.[9, 10] Both natural minerals and industrial waste materials that contain elements suitable for carbonation such as Ca or Mg can be utilized as raw materials for mineral carbonation processes. However, industrial waste materials provide a more readily reactive [a] H.-P. Mattila, Prof. R. Zevenhoven Thermal and Flow Engineering Laboratory bo Akademi University Piispankatu 8, FI-20500 Turku (Finland) E-mail: [email protected] [email protected]

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source of these elements because they are often in a more processed and active form. However, the potential availability of the waste materials is limited compared to the amount of CO2 released.[11] In this paper, a mineral carbonation method suitable specifically for steelmaking industries is discussed. The concept, also known as slag2PCC, that uses steel converter slag as a raw material to produce precipitated calcium carbonate (PCC) to be used as, for example, paper filler, is under development from the level of theoretical process modeling[12] and batch experiments[10, 13, 14] towards a demonstration-scale continuous process. The most important features of the process concept are the direct use of purified flue gases, that is, the redundancy of a separate capture stage, as well as the possibility to operate the system at ambient temperature and pressure, which thus reduces the energy demand. Current trends in PCC manufacture through various technologies are summarized in a recent overview.[15] The markets for the PCC product are limited; in 2011 the worldwide consumption was estimated at 14 Mt. Together with ground calcium carbonate (GCC), which is of lower economic value compared to PCC, the market increases to 74 Mt.[16, 17] Measured in Mt of CO2 captured, if all PCC and GCC production was replaced by the slag2PCC method, 32.5 Mt of CO2 (6.2 Mt for PCC only) could be chemically bound to calcium carbonate product. Of all the alkaline waste materials available, steel slags are ideal for mineral carbonation applications because of the high Ca content.[15, 18] Still, the amount of produced steel converter slag forms a limitation for the process applicability, even though alternative sources of Ca (e.g., blast furnace slag)[10] can to some extent be utilized as well. Notably, in the case of the USA only, even the most widely available industrial waste materials could only account for a few percent of USA CO2 emissions.[11] Globally, in 2012 crude steel production was estimated to be 1.5 Gt,[19] whereas the ChemSusChem 2014, 7, 903 – 913

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amount of basic oxygen furnace Table 1. XRF results of the various slag fractions. (BOF) slag produced is approxiSlag fraction Component [%] mately 126 kg per ton of steel.[20] Na2O P S K2O Ti Cr rest CaO Fe SiO2 Mn MgO Al2O3 V If we assume that the CO2 capture capacity of  0.1 kg CO2 per 2008 45.2 14.9 12.4 2.4 1.7 1.5 1.4 0.1 0.4 0.1 0.1 0.6 0.2 19.0 2011 44.9 12.0 22.7 2.6 1.6 2.6 2.5 0.2 0.4 0.1 0.2 0.8 – 9.4 kg slag reported previously[21, 22] 2011 (aged, small) 44.5 13.4 23.0 2.6 1.8 2.6 2.6 0.3 0.4 0.1 0.2 0.8 0.2 7.5 can be maintained, independent 2011 (aged, large) 42.2 17.6 22.5 2.4 1.7 2.4 2.4 0.2 0.4 0.1 0.2 0.7 0.2 7.0 of the variations in slag quality, 2012 42.4 17.7 11.9 2.1 1.4 1.5 1.3 0.1 0.3 0.1 0.1 0.7 0.3 20.2 the slag2PCC process would be 2012b 47.5 17.4 11.7 2.6 1.8 1.4 1.3 0.1 0.3 0.1 0.1 0.7 0.2 14.8 able to capture 19.1 Mt CO2 annually. Thus, theoretically the limiting factor of the discussed technology would be the size of the PCC market, or if the scale of the produced materials is widened, the amount of generated slag. In conclusion, although the amounts of CO2 handled by this Ca-based process may never become significant for CO2 emissions reduction on a global scale, for a particular steel manufacturer the process could still be beneficial with respect to minimizing the amount of waste materials generated to proFigure 1. The process concept. duce a marketable product. These kinds of economic benefits are recognized to be essential for the early implementation of which are the extraction of Ca from steel converter slag and mineral carbonation technologies.[11] the carbonation of the dissolved Ca with gaseous CO2 The slag2PCC process can convert two low-value byproducts (Figure 1). The governing reaction for Ca extraction is simplior waste streams (slag and CO2) into a profitable product, which avoids disposal and emission costs. In the case of fied as Reaction (R1), which describes the effect of the ammoRuukki Metals steel mill with an annual steel production of nium salt solvent on the dissolution of calcium oxide. 2.3 Mt in 2012,[23] this would mean, by using the same average CaOðsÞ þ2 NH4 XðaqÞ þH2 OðlÞ ! CaX2ðaqÞ þ2 NH4 OHðaqÞ ðR1Þ values for slag production as for the global estimates, approximately 30 kt CO2 captured with BOF slag to produce 69 kt of In this context, CaO represents all water-soluble Ca regardPCC annually. less of the actual crystal form in which it is bound. As disCurrently, the research on slag2PCC has focused on the efcussed in earlier studies,[12, 13] the crystal forms from which Ca fects of input slag properties and other parameters (e.g., solvent concentration, slag-to-liquid ratio) on the process chemisis most easily extracted are free calcium oxide or hydroxide try and the resulting PCC quality. As steel converter slag con(CaO/Ca(OH)2) and dicalcium silicate (Ca2SiO4). The crystalline tains several elements besides Ca (Table 1), it is essential for compounds identified by XRD analysis are listed in Table 2. product purity to ensure the efficient but also selective extracFor the carbonation step, the main reactions can be sumtion of Ca only. For this purpose, different solvent concentramarized as Reactions (R2) and (R3): tions, different slag-to-liquid ratios, and differently processed 2 NH4 OHðaqÞ þCO2ðgÞ $ ðNH4 Þ2 CO3ðaqÞ þH2 OðlÞ ðR2Þ and stored slag fractions have been studied experimentally. Moreover, the experimental validation of the most suitable ðNH4 Þ2 CO3ðaqÞ þCaX2ðaqÞ $ CaCO3ðsÞ þ2 NH4 XðaqÞ ðR3Þ values for process variables, such as residence times of the reactor steps, is ongoing based on earlier modeling work.[12] X in Reactions (R1)–(R3) represents CH3COO , NO3 , or Cl Most importantly, our research efforts concentrate on ensuring ions, even though only ammonium chloride was utilized as process operability in such a way that a specified PCC particle size, crystal morphology, and purity are obtained on a continuTable 2. XRD results of the various slag fractions. ous basis by using a subpilotscale setup, which is also disSlag fraction Component [%] cussed in this paper. srebrodolskite calcium magnesium iron silicate quartz other lime portlandite larnite Process concept The slag2PCC process is based on the recirculation of an aqueous ammonium salt solution as a solvent between two stages,

2008 2011[a] 2012 2012b

(CaO) (Ca(OH)2)

(Ca2SiO4) (Ca2Fe2O5)

(Ca2Fe1.2Mg0.4Si0.4O5)

(SiO2)

5 – 12 9

59 yes – –

– – 65 38

– – 6 –

– – 17 10

26 yes – –

10 yes – 43

[a] No quantitative XRD analysis was performed on this material

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a solvent in the experiments reported here, the choice of which was motivated by the low cost of the salt.[13] A recent comparison of the three solvents is given in Ref. [24]. From Reactions (R1)–(R3) it can be seen that, theoretically, the ammonium salt spent in extraction is stoichiometrically regenerated in the carbonation step. Also in lab-scale experiments, it has been possible to reuse the solvent to a large extent.[13] For good process economics the recyclability should be as high as possible, that is, losses from the process should be minimized. As also known from earlier studies, important features that lower the process equipment cost and requirements are the overall exothermicity of both reaction steps as well as the operability at ambient pressure and temperature. Figure 2. Extraction efficiencies of Ca, Na, and K from different slag fractions. Values calculated from comparison of ICP-OES and XRF measurements of solvents and solid slag.

Results and Discussion Effects of differences in slag properties The effects of variations in slag production and storage processes on the extraction step conversion efficiencies were studied by nine experiments, each with a different slag sample. The tested slags were obtained from Ruukki Metals Oy Raahe steelworks in Finland, and consisted of three chemically different materials. Slag labeled 2008 contained both free lime and dicalcium silicate, 2011 slag had no free lime, but high Si and V contents, and slag 2012 had high amounts of free calcium oxide and hydroxide.[13, 24] Detailed compositions and mineralogies for each material are given in Tables 1 and 2. Data for slag 2012b are also given, although it was not used until later for tests with the continuous process setup. The slag fractions also differed in particle size both before and after milling, that is, the fractions named large in Table 3 consisted originally of larger (> 300 mm) particles than fractions labeled small (10–50 mm). Both of these fractions were milled to 75–125 and 125–250 mm particles that were used in extraction tests. The fractions named aged were stored outdoors for approximately 16 months to study the effect of chemical changes caused by, for example, leaching with rainwater as described in Refs. [25, 26]. The extraction efficiencies, shown in Figures 2 and 3 for various elements, are calculated by comparing the concentrations in liquid samples after extraction to the total amount present

Figure 3. Extraction efficiencies of V and Mg from different slag fractions. Values calculated from comparison of ICP-OES and XRF measurements of solvents and solid slag.

in fresh slag prior to experiments. As mentioned above, not all Ca in slag is present in a reactive form, but because the overall degree of Ca conversion is of high interest, all the efficiency calculations in this paper are made according to this method. In addition to the elements shown in Figures 2 and 3, only small amounts of Si were measured in the solutions as extraction conversions less than 1.0 %. The slag fractions with a high amount of free lime or calcium hydroxide were able to release

Table 3. Mass losses and experimental parameters in the experiments with different slag fractions. Entry

Slag fraction

Slag amount [g] initial final

Weight loss [%]

Volume [L]

NH4Cl molarity [mol L1]

1 2 3 4 5 6 7 8 9

2008 2011

25.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 49.5

24.4 7.5 7.0 11.1 8.6 10.3 12.9 11.0 26.7

0.25

2

0.5

1

2011 (aged) 2011 (aged, small) 2011 (aged, large) 2012

18.9 46.3 46.5 44.5 45.7 44.9 43.5 44.5 36.3

Solid-to-liquid [g L1]

100

T[a] [8C]

Particle size [mm]

Reaction time [min]

ambient

< 250 125–250 125–250 75–125 125–250 75–125 125–250 75–125 125–250

60 60 68 67 66 78 66 86 85

[a] Ambient temperature in these experiments varied between 20 and 25 8C.

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www.chemsuschem.org was also assessed with these experiments. Slag 2012 (Tables 1 and 2) with a high content of free calcium oxide was used in these tests because it was expected to release large amounts of Ca in pure water. The measured concentrations are presented together with concentrations calculated from the measured final pH values by stoichiometric solubility limitations in Table 5. The solubility

more Ca than those that contained Ca in a more bound form, as expected.[10, 25] Also, the smaller the slag particles used in the experiments, the better the extraction. However, the original particle size before milling did not have a significant effect. Storage outdoors decreased the extraction efficiencies slightly, but no significant differences were observed, in accordance with the literature.[27] As the slag used in the outdoor storage experiments contained no free lime, the tests only studied the ageing behavior of dicalcium silicate, and the results could have been different for a more lime-rich slag. In addition to Ca, K and Na were also extracted efficiently (up to 30 %) from the slag, but because the amounts of these elements present in the fresh slag are small, only 0.2–0.5 % in total, the concentrations remained low in the aqueous solutions. Up to 3 % V and 0.1–1.4 % Mg were extracted (Figure 3). As expected, more V was released from the high-V-content slag 2011 than from slags 2008 and 2012. The extraction yield of V was higher from smaller particles, but for Mg the results were the opposite; more Mg was extracted from the larger (125–250 mm) particles. Nonetheless, the accumulation of the minor elements in the process solutions must be accurately followed. Most of the experimental pH values leveled off at approximately the same value (9.3–9.5). All the 2011 slag experiments with the smaller particle size (75–125 mm), as well as one of the 125–250 mm tests (different mixing compared to other 125–250 mm tests) and the 2008 slag experiment showed this behavior. The 2012 slag resulted in a higher final pH (10.2), which is understandable based on the high CaO/Ca(OH)2 content. However, 2011 slag with larger particle sizes (125– 250 mm) yielded pH values of 8.7–9.0. Thus, two main reasons for different end pH values can be identified based on these experiments; the content of free calcium oxide/hydroxide and the particle size of the input slag in the experiments. Particle size effects can to some extent be compensated for with enhanced mixing. The ageing history and the size of the original slag particles before milling did not affect the pH values significantly.

Table 5. Ca extraction efficiencies [mol L1 and %], measured with Ca-selective ISE (ELIT 8041, NICO 2000 Ltd.) and calculated from stoichiometry. Entry 1 2 3 4

[Ca2+] [mmol L1] measured predicted

difference

Ca extracted % of fresh

pH end

0.39 0.44 1.26 7.08

0.03 0.04 0.16 36.72[a]

0.2 0.2 0.6 3.8

13.0 13.0 12.8 12.0

0.36 0.40 1.10 43.80

[a] Not enough reactive Ca available, thus equilibrium calculation was not valid.

information was obtained from HSC Chemistry 5.11.[28] The amounts of dissolved Ca remained small at only 0.2–3.8 % of the total Ca available in fresh slag. This is because of the lack of a buffer effect of ammonium salts; if the solution pH approaches 13, the Ca solubility is reduced noticeably. As a result, Ca was first dissolved from the slag but reprecipitated as calcium hydroxide before a sample was withdrawn for concentration measurements. For the PCC production process this would be detrimental because the Ca could not be separated efficiently from steel slag and transferred in dissolved form from the extraction to the carbonation step. In traditional PCC manufacture, in which calcium hydroxide is used as a Ca source, this is not a problem because no impurities are included in the solid material, and the unreacted calcium hydroxide can be removed by screening as a result of the different particle sizes.[29] Actually, the rate of calcium hydroxide addition can also be used to steer the particle morphology of the produced PCC.[30, 31]

Effect of ammonium salts on Ca extraction Process conditions that promote selective and efficient Ca extraction and the production of pure PCC

To confirm the potential and advantages of ammonium salt solvents in the extraction of Ca from steel converter slag, another series of experiments was performed with distilled water as a solvent and by reusing the slag from the first test in the three other tests (Table 4). Thus, the effect of slag recycling

The effects of higher concentrations of ammonium salt (up to 4.9 mol L1) and higher solid-to-liquid ratios (up to 300 g L1) on selective Ca extraction were studied with additional experi-

Table 4. Mass losses and experimental parameters in the experiments with distilled water as a solvent. Entry

Slag fraction

Slag amount [g] initial final

Weight loss [%]

1 2 3 4

2012 from previous from previous from previous

5.00 4.46 3.81 3.35

10.8 14.6 12.1 27.8

4.46 3.81 3.35 2.42

Volume [L]

0.2

NH4Cl molarity [mol L1]

Solid-to-liquid [g L1]

0

25.0 22.3 19.1 16.8

T[a] [8C]

Particle size [mm]

ambient

Reaction time [min]

125–250

93 107 68 92

[a] Ambient temperature in these experiments varied between 20 and 25 8C.

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Table 6. Mass losses and experimental parameters in the experiments with varying solvent molarity and solid-to-liquid ratio (slag fraction 2008). Entry

Slag amount [g] initial final

Weight loss [%]

Volume [L]

NH4Cl molarity [mol L1]

Solid-to-liquid [g L1]

T[a] [8C]

1 2 3 4 5 6 7 8 9 10

15 25 50 75 25 50 75 50 25 25

31.3 24.4 19.3 19.0 20.9 19.5 15.8 17.0 23.9 15.8

0.75

0.5 1.8 1.8 1.8 3.5 3.5 3.5 4.9 4.9 4.9

20 100 200 300 100 200 300 200 100 100

30

10.3 18.9 40.4 60.8 19.8 40.3 63.2 41.5 19.0 21.1

0.25

20

Particle size [mm] 125–250

< 250

Reaction time [min] 120

60

[a] Temperature was controlled by immersing the reactor in a water bath at the specified temperature.

Figure 4. Ca extraction efficiency, % of total amount in fresh slag, as a function of NH4Cl solvent molarity [m] and solid-to-liquid ratio [g L1].

Figure 5. Si extraction efficiency, % of total amount in fresh slag, as a function of NH4Cl solvent molarity [m] and solid-to-liquid ratio [g L1].

ments. The extraction efficiencies of Ca, Si, and Mg are presented in Figures 4–6, respectively, obtained from the experiments listed in Table 6. It must be mentioned explicitly that the 20 g L1 experiment was performed with a 125–250 mm slag size fraction, whereas in the other experiments the < 125 mm particles were included in the test material. Moreover, this experiment was performed at 30 8C, whereas in all the other experiments the temperature was 20 8C. Also the reaction time was twice as long as in the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 6. Mg extraction efficiency, % of total amount in fresh slag, as a function of NH4Cl solvent molarity [m] and solid-to-liquid ratio [g L1].

other tests, and thus the experiment is not fully comparable with the other tests. Besides the mentioned Ca, Si, and Mg, no other elements were detected from the solutions by inductively coupled plasma optical emission spectroscopy (ICP-OES), although, for example, Mn precipitates were visibly present in the plastic sample containers, especially after tests with salt concentrations higher than 1.8 mol L1.[32] The extraction efficiencies are calculated by comparing the concentrations in liquid samples after extraction (measured by ICP-OES) to the total amount present in fresh slag prior to the experiments (measured by Xray fluorescence spectroscopy; XRF). As reported earlier,[24, 33] the increase of the solid-to-liquid ratio from 20 to 300 g L1 reduces the extraction efficiency of Ca from ~ 50 % to roughly 35–40 %. In those studies, the reason for this behavior was unclear. However, if the dissolved Si concentrations in different experiments (Figure 5) are compared with the Ca levels, it can be observed that the changes in extraction efficiencies of Ca and Si follow similar trends from experiment to experiment. According to Refs. [34, 35], Si can be dissolved in water in concentrations of typically around 100 ppm, which depends on the origin of the Si, the pH of the solution, etc. However, both ammonium and calcium ions are used as coagulants for Si to aid in its precipitation from solutions. Moreover, ammonia in aqueous solutions is known to reduce the rate of Si dissoluChemSusChem 2014, 7, 903 – 913

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CHEMSUSCHEM FULL PAPERS tion.[23] In the current process, all of the mentioned compounds are present, partly also in high concentrations. Thus, it seems clear that if both Ca and Si concentrations in the slurry are increased by using high solid-to-liquid ratios, Ca dissolution is hindered because of the limited solubility of Si. This reduces the Ca yield of the extraction step. An increase of the pH counteracts this phenomenon by enhancing Si dissolution.[35] The effect of the ammonium salt concentration is not equally clear, although, as seen in Figures 4–6, at high concentrations the extraction efficiencies are slightly decreased compared to lower ammonium salt concentrations. As a result of these effects, the observed experimental Si concentrations remain between 3–15 ppm, which is low compared to the literature value of 100 ppm for pure water. It can be concluded that depending on the content of Sibound Ca in slag, solid-to-liquid ratios in the extraction step should not significantly exceed 100 g L1 because higher values result in a rapid decrease of the Ca extraction efficiency (Figure 4). The fraction of free lime (CaO) is an essential parameter to choose the best solid-to-liquid ratio for efficient Ca extraction because the dissolution of CaO is not affected by the Si concentration in the solution. If the slag contains only small amounts of free lime, the solid-to-liquid ratio should be preferably as low as 20 g L1. This will require larger process equipment[24] and becomes subject to the cost optimization of the overall process. Moreover, even though it is beneficial to have ammonium salts present in solution to aid Ca dissolution as discussed in the previous section, too high concentrations should also be avoided as these result in the adverse extraction of impurities as seen, for example, in the case of Mg (Figure 6). As reported earlier, product purity also suffers from ammonium salt precipitation that occurs at salt concentrations above 1.8 mol L1[32] because PCC should not contain, for example, any compounds of ammonia or chlorine. These observations are summarized in Figure 7, and the most beneficial area of process operation from the point of view of selective Ca extraction is indicated by the double

www.chemsuschem.org arrow in the bottom left of the graph. However, too low solvent concentrations result in slow kinetics,[12] again not beneficial for the overall process. The stoichiometric limit in Figure 7 is calculated for a slag that contains 45 % CaO, of which 50 % is in a reactive form. The Ca extraction efficiencies in all the experiments discussed are plotted against a combination of the identified significant variables (z) in Figure 8. z [mm g L1] is calculated from

Figure 8. Ca extraction efficiency, % of total amount in fresh slag, as a function of the parameter z. Triangles show experiments with substoichiometric ammonium concentrations.

the slag properties according to Equation (1), in which mslag is the solid-to-liquid ratio of the slag and ammonium salt solvent (g L1), d¯slag is the arithmetic mean diameter of the slag particles [mm] (Tables 3, 4, and 6), and fSi, fV, fFe, and fCa are mass fractions of Si, V, Fe, and Ca in fresh slag based on XRF measurements, respectively (Table 1). As the reactivity of other Ca compounds is lower than that of free lime, an additional parameter group is included to adapt Equation (1) for slags with various fractions of bound Ca. This group consists of the sum of an empirical constant 0.5 and the cubic root of free Ca fraction, fCa, free, calculated according to Equation (2) based on XRD analysis (Table 2). Interestingly, the ammonium salt concentration does not seem to affect the Ca extraction efficiency directly. However, as mentioned in earlier studies,[12] the kinetic rates of extraction reactions are affected by the solvent concentration. Triangles represent the experiments in which the used solvent concentration is below the expected stoichiometric calcium concentration in Figure 8, which are thus not comparable with the other experiments in which Ca extraction did not meet this limitation. The range of validity and applicability of Equation (1) need to be followed up as a part of our future work.

z¼

Figure 7. Observed limitations for selective Ca extraction with ammonium chloride solvent, the experiments listed in Table 6 are marked with circles.

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mslag  dslag  fSi  fV  fFe pffiffiffiffiffiffiffiffiffiffiffi fCa ð0:5 þ 3 fCa;free Þ

ð1Þ

fCaO þ fCaðOHÞ2 fCa;total;XRD

ð2Þ

fCa;free ¼

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Figure 9. Process scheme and picture of the constructed demonstration setup.

Continuous process setup Based on the observations summarized in previous sections, it was considered essential to ensure that the process can be run in a continuous reactor system to reduce repeated operational changes and to be able to rapidly adapt the process to changing input slag quality. At the same time, the produced PCC amounts should be large enough to guarantee a constant and good product quality, in terms of product purity as well as crystal properties. For these purposes a subpilot-scale continuous reactor setup was constructed, the detailed process scheme of which is shown in Figure 9. Extractor design The extraction reactor (on the left in Figure 9) consists of a cylindrical Perspex tube (height 650 mm, width 240 mm, total volume 29.4 L), inside which a mechanical pitched blade paddle stirrer with two paddles (width 150 mm, distance between paddles 260 mm, distance from bottom to lower paddle 30 mm) is placed. The stirrer works according to the radial flow principle, that is, the slurry in the reactor is directed towards the walls at the paddle height. This was verified with a test experiment by using inert glass beads of size fraction 125–  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

212 mm and a density of ~ 2600 g L1,[36] which are similar to the actual reactive slag particles. The mixing efficiency was estimated by calculating the theoretical solids concentration in the reactor and measuring the actual solids concentration in the reactor outflow. The measurement was performed by pumping slurry from the reactor and filtering, drying, and weighing the solid particles in a known volume. At the height of the outlet in the reactor (80 mm from the reactor bottom), the particle concentration was observed to be 120 g L1, whereas the calculated value was 100 g L1. Thus, as also confirmed visually, the reactor volume was not uniformly utilized, and a lower particle concentration was maintained at approximately 100 mm (550 mm/ 650 mm  100 g L1/120 g L1) below the reactor lid. To prevent vortex formation on the liquid surface, however, this stirrer configuration was considered to be satisfactory. To reduce the tendency for the complete rotation of the reactor contents, three baffles of 20 mm width were attached to the reactor walls at 1208 intervals. Also, a 7 mm gap was left between the reactor wall and the baffles to prevent particle accumulation behind the baffles. The design mainly followed the guidelines for a standard geometry-agitated tank,[37] modified by practical limitations set by, for example, material availability.

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Power dissipation e [kW; Eq. (3)] in mixing can be estimated in a similar manner as for lab-scale experiments presented in earlier work.[38] Po is the power number, 1 is the slurry density [kg m3], n is the frequency of rotation of the mixer [s1], and d is the diameter of the mixer [m]. e¼

Po  1  n3  d 5 1000

ð3Þ

"

The power number is estimated from the Reynolds number of mixing, which is obtained according to Equation (4), in which hmixture is the viscosity of the slag and ammonium salt solution mixture of 1.102 mPa s.[38] Rev ¼

1  n  d2 hmixture

The initial gas bubble diameter (db, i) in a bubble column can be estimated from Equation (5),[40] in which s is the surface tension of the liquid phase (73 mN m1[41]), d0 is the orifice diameter of the gas distributor (mean diameter 39 mm), g is the gravitational acceleration (9.81 m s2), and 1l and 1g are the densities of liquid and gaseous phases, respectively (1015 and 2 kg m3).

ð4Þ

Together with 1 = 1074 kg m3, and with experimental values n = 5.8 s1 and d = 0.15 m, Rev = 127 184. In other words, the mixing situation is clearly turbulent, and the volumetric discharge rate from one paddle becomes roughly 10 L s1.[37] The power number in a vessel without baffles for a turbine stirrer is then estimated to 1.4 without baffles, and 0.98  1.4 = 1.38 with three baffles.[37] Moreover, the configuration of dual pitched paddles lowers this value by a factor of 0.7  1.38  0.96, which results finally in an energy dissipation of 15 W or 0.456 W kgsolution1 for the extraction reactor with 1.0 mol L1 NH4Cl solution and a 100 g L1 slag-to-liquid ratio.[37] These are the highest concentration levels under which the process will be operated based on information presented in the previous sections. The obtained energy dissipation is of a similar order of magnitude as estimated earlier for lab-scale (1 L) experiments (0.369 W kgsolution1), in which the difference originates from a larger impeller diameter compared to the reactor diameter.

db;i

6  s  d0  ¼  g 1l  1g

#1 3

ð5Þ

On introduction of the numerical values, an initial bubble size of approximately 1.2 mm is obtained under near atmospheric pressures and ambient temperature. With Stokes’ law [Eq. (6)] and the viscosity of water hl = 1.003 mPa s, this yields the initial bubble rise speed of approximately 0.79 m s1. 

 1l  1g g  db2 vffi 18h

ð6Þ

For the gas distributor design, it is important to estimate the bubble size under steady-state conditions in the upper part of the reactor, not only at the feed point. Approximations for the Sauter mean bubble diameter dsm [m] distant from the gas inlet and gas hold-up H [m3 gas m3 total] in the reaction vessel can be obtained by iteration with Equations (7) and (8).[37] In addition to the parameters already introduced, Vl is liquid volume [m3] and us is the superficial gas velocity calculated from the inlet gas flow by assuming an even gas distribution over the total reactor area [m s1]. dsm ¼ 4:15  103 H1=2



 s0:6 3 0:4 0:2 þ 0:9  10 ðe=Vl Þ 1l

 H ¼ 4:24

1   1 2 2 us  hl  H ðe=Vl Þ0:4 10:2 us  hl l þ0:916 0:6 2 2 s gð1l  1g Þ*dsm gð1l  1g Þ*dsm

Carbonator design The carbonation reactor (on the right in Figure 9) is a Perspex tube identical to the extraction reactor. At a height of 5 cm from the reactor bottom, a porous polyethylene sheet (SPC PE3530)[39] of 3.0  0.10 mm thickness and a mean pore size of 39 mm is installed. CO2 gas is introduced into the reactor through the porous sheet from a gas bottle of 99.9996 % CO2 (Oy Aga Ab). The gas line is equipped with both a manometer and a flow meter (Kytçl LH-4CR-HR). The distance between the mechanical stirrer and the reactor bottom is 70 mm instead of 30 mm as in the extraction reactor because of the gas inlet. Besides the mixing provided by the gas flow and the mechanical stirring, a circulation pump capable of pumping 1.5 L min1 is installed in the reactor, with the intake at the top and the outlet at the bottom, which generates a co-current flow with respect to gas flow and ensures the presence of PCC seed crystals in the reactor (except at the start of the reaction). By default, the unreacted gas is vented from the system, but the vent can also be closed to provide an overpressure of approximately 0.015 bar.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ð7Þ

ð8Þ From Equations (3) and (4) with 1 = 1031 kg m3, hmixture = 1.038 MPa s, n = 0.8 s1, d = 0.15 m, and Po = 0.93 (corrected for a CO2 gas flow of 475 mL min1 according to Ref. [37]), the power dissipation in the carbonation reactor is estimated as e = 0.04 W. Together with Vl = 29 L, us = 0.18 mm s1, and hl = 1.003 mPa s, Equations (7) and (8) yield a mean bubble diameter of 2.7 mm (rise speed 4.0 m s1) and a gas hold-up of 1  104 m3 m3. Based on the experimental observations, these results seem reasonable. These values should be considered as approximations because several simplifications are made in the calculations, for example, the effect of the circulation pump is not included in the power dissipation, and the effects of chemical reactions and the precipitation of solid carbonates are excluded from the formulas used here. Also, even if high electrolyte concentration is known to reduce bubble coalescence,[42] only changes in surface tension and viscosity are included, which thus yields a higher value for dsm than with a lower coalesChemSusChem 2014, 7, 903 – 913

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CHEMSUSCHEM FULL PAPERS cence rate. In the calculation of slurry densities and viscosities, the highest measured value for the solid content (~ 40 g L1 PCC in batch experiments) is used because this yields the largest power consumption. Nevertheless, it can be concluded that the designed gas distributor can provide the system with smaller bubbles (1.2 mm) than that maintained in the reactor under the operational conditions. Thus, the mass transfer rate between the gas and liquid cannot be increased by, for example, further reduction of the pore size in the distributor sheet. Process control equipment and principles To control the flow rates in the process, five to seven peristaltic pumps are used, as shown in Figure 9. These pumps provide an adjustable flow up to 1.5 L min1. Also, the bypass stream is realized with a three-way valve, which enables a variable fraction of the extractor outflow to be fed directly to the PCC settler. The temperature and pH of the process solution are continuously monitored and recorded from four different locations; on top of both the extraction and carbonation reactors as well as on top of the inclined settlers. As a data logging system, an ELIT 9808 eight-channel recorder is used. The temperature and pH in the extraction reactor can be controlled by changing the feed rate of fresh slag. For the carbonation reactor, the most straightforward control method is to change the CO2 gas flow rate. Also, because the system is designed for a recirculating solvent, it is of importance to follow and adjust the quality of solvent that is returned to the extraction reactor. This is performed according to principles discussed in earlier work;[38] a bypass stream is taken from the slag settler to the PCC settler to precipitate as much as possible of the carbonate and bicarbonate ions present in the solution and thus prevent them from entering the extraction reactor. This inhibits carbonate precipitation on slag particles in the higher pH solution of the extraction reactor, which would reduce the Ca extraction efficiency and thus the efficiency of the entire process. Handling of solid matter Slag is fed to the extraction reactor in small batches at a known rate based on the calculated outflow of solids from the reactor, the target of which is a near-constant solid-toliquid ratio in the reactor. As both the residual slag as well as the produced PCC must be removed from the process streams continuously, the separation equipment consists of two inclined settlers,[36, 43] one for each solid, combined with more traditional barrier filters. Based on promising results with separation efficiencies of 99 % on a mass basis during earlier studies with similar equipment,[36] Perspex tubes with an inner diameter of 10 cm have been installed as settlers. For slag separation the tube length is 50 cm and for PCC 110 cm. As shown in Figure 9, additional filter units are placed after both settlers. These filters (HOH Pure 1 mm/5“, BWT Separtec Oy) are used to ensure that no small slag particles enter the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org carbonation reactor, in which they would reduce the purity of the produced PCC, and similarly that no PCC particles enter the extraction reactor, in which the valuable product would get mixed with slag particles and leave the system with the residual slag stream. Two filter units are used to collect the produced PCC from the underflow of the inclined settler after the carbonation reactor. These filters are installed in parallel to give the possibility to clean one filter while the other is in use. All the filter units are equipped with a manometer to follow the pressure drop over the filter element during operation. Each filter unit can be separated from the system with two valves. Residual slag is collected from the extraction reactor to a separate 10 L vessel, from which it can be removed during operation, if necessary.

Experimental results from the continuous setup The measured changes in pH in the four vessels during the course of the experiment are presented in Figure 10. In the extractor, the pH was maintained at approximately 9.5 after the

Figure 10. pH in the different reactors during the course of the experiment.

pumps were switched on, whereas in the carbonator, fluctuations between 8.3 and 8.9 were observed, which indicates that the system was not able to reach a steady state during the time interval. Samples of the carbonate product were collected both from the carbonator and the PCC settler after 60 min of gas feed. As expected, the crystal properties varied noticeably, which depended on where the sample was taken from. In the carbonator, the particles were in the form of rhombohedral calcite (Figure 11), whereas vaterite was formed in the PCC settler (Figure 12). In total 614 g of PCC was produced, which corresponds to 15 % of the mass of the input slag amount (or 17 % of the total Ca). In earlier batch experiments this value varied between 5 and 30 %, which depends on the input slag quality.[13] As impurities, only Si and Cl were detected in small amounts (< 1 wt %) by SEM and energy-dispersive X-ray spectroscopy (EDX).

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www.chemsuschem.org ric amounts with respect to the amount of Ca, a parameter combination of the slag-to-liquid ratio, the mean diameter of slag particles, and the fractions of Si, V, Fe, and Ca can be constructed and used to predict the total extraction percentage of Ca. A continuous four-step reactor system has been built for the process. The first test results show that the process can be successfully run in a continuous mode to produce several types of PCC particles in different process stages. In our future work, the focus will be on the control of the properties and purity of the produced PCC.

Figure 11. Rhombohedral calcite from the carbonation reactor after 60 min of carbonation. Scale bar 10 mm.

Experimental Section A specified amount of slag was mixed with a magnetic or an overhead stirrer at approximately 200 rpm together with an ammonium chloride solution for a certain time. The temperature and pH of the solution were recorded during the experiments. After the experiments, the residual slag was separated from the solution by filtration through Whatman 589/3 filter papers and dried overnight at 105 8C. The detailed experimental conditions are listed in Tables 3, 4, and 6. The solid concentrations were measured by XRF, whereas the liquid samples were analyzed by using ICP-OES. In the experiments listed in Table 4, the Ca concentration was measured by using an ion-selective electrode (ELIT 9808).

Figure 12. Vaterite from the PCC settler after 60 min of carbonation. Scale bar 10 mm.

After it is shown that the process can also be run in a continuous mode and that the crystal properties can be changed by changing the process conditions, our future research will concentrate on the control of the PCC crystal morphology and also product purity. It is likely that an additional washing step must be included in the system to reduce the level of impurities, which consist primarily of chloride. Another alternative would be to change the solvent to, for example, ammonium nitrate.[14]

Conclusions A CO2 utilization and storage concept “slag2PCC” is advanced from lab-scale experiments to a larger scale. Even though the scale in which the process could be applied into practice is relatively small, at maximum approximately 20 Mt of CO2 can be captured per year, it would still be beneficial for a steel manufacturer to produce a marketable product from waste materials. For the overall process efficiency, it is of importance that the input slag releases Ca to as large an extent as possible. Two main factors that affect this behavior were observed to be the content of free lime in the slag and the particle size to which the slag is milled before extraction. Ammonium salt solvents enhance the Ca dissolution by introducing a buffer effect, which is absent from systems that use only pure water as solvent. If ammonium salts are present in over-stoichiomet 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

To test the continuous setup, an experiment was conducted by using NH4Cl (1.0 m) as a solvent with a 100 g L1 slag-to-liquid ratio in the extractor (slag 2012b in Tables 1 and 2) and a CO2 flow of 0.5 L min1. In the start-up phase, the slag was allowed to react with the solvent until the pH value stabilized (50 min). After that the pumps were switched on in such a way that the residence time for slag in extractor became 60 min. The gas feed was started at the same time. The total amount of slag used in the experiment was 4200 g, of which 2800 g was added at the start, and the remaining 1400 g was added in two batches of 700 g at 70 min and 100 min after the start of the experiment, respectively. After 130 min from the start, the gas feed was switched off, and the system was cleaned.

Acknowledgements This work was carried out in the Carbon Capture and Storage Program (CCSP) research program coordinated by CLEEN Ltd. with funding from the Finnish Funding Agency for Technology and Innovation, Tekes. H.-P.M. also acknowledges the Graduate School in Chemical Engineering for support for his work. The XRD/XRF analyses were performed by Ruukki Metals Oy, ICP-OES by Sten Lindholm at Analytical Chemistry/AU, and SEM/EDX by Linus Silvander at Inorganic Chemistry/AU. Also, Jimmy Dahlqvist, Alf Hermanson, Daniel Legendre, and Martin Slotte are acknowledged of their work on the construction of the experimental setup. SPC Technologies Ltd. provided samples of their porous plastic sheet products to be tested in the setup. Keywords: calcium · carbon storage · environmental chemistry · industrial chemistry · waste prevention [1] IPCC, IPCC Special Report on Carbon Dioxide Capture and Storage, Cambridge University Press, New York, 2005.

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www.chemsuschem.org [24] A. Said, H.-P. Mattila, M. Jrvinen, R. Zevenhoven, Appl. Energy 2013, 112, 765 – 771. [25] J. Waligora, D. Bulteel, P. Degrugilliers, D. Damidot, J. L. Potdevin, M. Measson, Mater. Charact. 2010, 61, 39 – 48. [26] Possible Uses of Steelmaking Slag in Agriculture: An Overview. T. Branca, V. Colla in Material Recycling—Trends and Perspectives, InTech, Rijeka, Croatia, 2012. [27] M. L. Cunha, C. S. Gahan, N. Menad, . Sandstrçm, Miner. Eng. 2008, 21, 38 – 47. [28] A. Roine, Outokumpu Research Oy, 2002. [29] Hosokawa Alpine AG, Powder and Particle Processing – Applications – Minerals – PCC, 2010. [30] M. Ukrainczyk, J. Kontrec, V. Babic´-Ivancˇic´, L. Brecˇevic´, D. Kralj, Powder Technol. 2007, 171, 192 – 199. [31] W. Jung, S. Hoon Kang, K. Kim, W. Kim, C. Kyun Choi, J. Cryst. Growth 2010, 312, 3331 – 3339. [32] H.-P. Mattila, I. Grigaliu¯naite˙, A. Said, C.-J. Fogelholm, R. Zevenhoven, Proceedings of SCANMET IV—4th International Conference on Process Development in Iron and Steelmaking 2012, 2, 19 – 28. [33] S. Eloneva, A. Said, C.-J. Fogelholm, R. Zevenhoven, Proceedings of International Conference on Applied Energy (ICAE) 2010, 169 – 178. [34] J. J. Chen, J. J. Thomas, H. F. W. Taylor, H. M. Jennings, Cem. Concr. Res. 2004, 34, 1499 – 1519. [35] R. K. Iler, The Chemistry of Silica; Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry, Wiley, 1979. [36] S. Filppula, Continuous Separation of Steelmaking Slag and PCC Particles from Aqueous Streams using an Inclined Settler, M.Sc. Thesis, bo Akademi University, Turku, Finland, 2012. [37] N. P. Cheremisinoff Handbook of Chemical Processing Equipment, Butterworth-Heinemann, Woburn, MA, USA, 2000. [38] H.-P. Mattila, I. Grigaliu¯naite˙, A. Said, S. Filppula, C.-J. Fogelholm, R. Zevenhoven, Proceedings of ECOS 2012—The 25th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, June 26 – 29, Perugia, Italy, 2012. [39] SPC Technologies Ltd, Porous plastic components - SPC Technologies, 2012. [40] J. Nielsen, J. Villadsen, G. Lidn, Bioreaction Engineering Principles, Second Edition, Kluwer Academic/Plenum Publishers, New York, USA, 2003. [41] M. Yamada, S. Fukusako, T. Kawanami, I. Sawada, A. Horibe, Int. J. Thermophys. 1997, 18, 1483 – 1493. [42] V. S. J. Craig, B. W. Ninham, R. M. Pashley, J. Phys. Chem. 1993, 97, 10192 – 10197. [43] R. H. Davis, H. Gecol, Int. J. Multiphase Flow 1996, 22, 563 – 574.

Received: May 27, 2013 Revised: September 30, 2013

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