New method for carbon dioxide mineralization based on phosphogypsum and aluminium-rich industrial wastes resulting in valuable carbonated by-products

1 New method for carbon dioxide mineralization based on phosphogypsum and aluminiumrich industrial wastes resulting in valuable carbonated by-products I. Romero-Hermida, 1 A. Santos, 2 R. Pérez-López, 3 R. García-Tenorio, 4 L. Esquivias, 5,6 V. Morales-Flórez, 5,6,* 1 Departamento de Química-Física, Facultad de Ciencias, Universidad de Cádiz, Av. República Saharaui s/n, 11510 Puerto Real, Spain 2 Departamento de Ciencias de la Tierra, CASEM, Universidad de Cádiz, Av. República Saharaui s/n, 11510 Puerto Real, Spain 3 Departamento de Ciencias de la Tierra, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus ‘El Carmen’ s/n, 21071 Huelva, Spain 4 Departamento de Física Aplicada, ETS Arquitectura, Universidad de Sevilla, Av. Reina Mercedes, s/n, 41012, Seville, Spain 5 Departamento de Física de la Materia Condensada, Facultad de Física, Universidad de Sevilla, Av. Reina Mercedes, s/n, 41012, Seville, Spain 6 Instituto de Ciencia de Materiales de Sevilla (CSIC/US), Av. Américo Vespucio, 49, 41092, Seville, Spain * Corresponding Author: V. Morales-Florez. Tel: +34 954 550 946. Fax: +34 954 552 870. E-mail address: [email protected] 2 ABSTRACT A new carbon mineralization method was designed based on a sequestration agent synthesised exclusively from industrial wastes. Phosphogypsum waste from the fertiliser industry was dissolved into caustic aqueous waste from the aluminium anodising industry. The resulting precipitate consisted of katoite (Ca 3 Al 2 (OH) 12 , a Si-free hydrogrossular solid solution end-member of the Al-containing hydrogarnet) and thenardite (Na 2 SO 4 );the latter easily removed by rinsing with water. The carbonation performance of this katoite-rich sequestration agent was evaluated using two different methods: by bubbling in aqueous media, and by weathering. Both procedures yielded high carbonation efficiencies (80% and 100%, respectively), and resulted in a solid precipitate composed primarily of calcite (CaCO 3 ) and aluminium hydroxide (Al(OH) 3 ). Priority attention was given to the transfer of trace elements and radionuclides of the uranium series typically present in the phosphogypsum. Results confirmed that the traces were transferred to resulting final solid carbonate at concentrations similar to those present in the raw phosphogypsum. In conclusion, these carbonated minerals would trap substantial amounts of CO 2 and produce final materials with similar civil engineering uses to those proposed for current phosphogypsum wastes. This work offers new methods for jointly manage specific industrial wastes oriented to more sustainable industrial processes and to control CO 2 emissions. KEYWORDS Carbon sequestration · Industrial waste · phosphogypsum · aluminum industry · katoite 1. Introduction The anthropogenic emissions of greenhouse gases, especially CO 2 , are among the most hazardous actions that impact the Earth's climate and ecosystems, which have the potential to cause serious, negative impacts on human welfare [1−3]. These threats are motivating large research efforts on both carbon sequestration technologies and strategies to reduce these emissions. Nevertheless, it is unlikely that a single strategy achieves the necessary decrease in CO 2 concentrations, considering the colossal magnitude of global CO 2 emissions is currently 38 Gt/yr [4]. It has been suggested that seven emission reducing activities or “stabilization wedges”, each capable of decreasing emissions by 3.7 Gt CO 2 /yr, could achieve stabilisation of atmospheric CO 2 concentrations at ~500 parts per million (ppm) [5]. One of these wedges is based on carbon capture and geological storage (CCS). However, this technology could be more broadly interpreted to include carbon mineralisation [4] (also known as mineral sequestration). This process, which captures CO 2 as a solid carbonate phase [6,7], considers as sequestration agents not only natural minerals from geological formations but also other feedstocks [8-10]. In the last years, the research on carbon mineralization focused mainly on three aspects: decreasing the energy requirements, enhancing the slow kinetics, and searching for alternative feedstocks as sequestration agents [11]. Precisely, the “accelerated carbonation” [12,13] involves examining industrial 3 wastes containing substantial quantities of available carbonation reactants, predominantly calcium and magnesium oxides, as alternative feedstock. Currently, the carbonation of wastes is already being recognised as a potential CO 2 sink [8,14−17], and several wastes have been proposed for reducing the energy requirements and to design cost-effective methods. Furthermore, the economic viability of the process could be improved, considering that the carbonation by-products are carbonate minerals that could be sold, as well as the financial advantages over carbon taxes [18−20]. Consequently, carbon capture, storage and utilization (CCSU) technologies are nowadays being developed to convert the CO 2 emissions in an opportunity [21,22]. It has been shown that the microstructure and high reactive surface area of the sequestration agent implies high mineralisation kinetics and full carbonation efficiency. This facilitates dissolution and, furthermore, the “passivating layer” caused by the surface precipitation of secondary phases such as carbonates or silica is avoided [23,24]. Additionally, comminution or other pre-treatments to increase kinetics and efficiency through the microstructure may not be necessary, as many wastes are produced with a particle size suitable for rapid and efficient carbonation [25−27]. Criteria similar to that applied to the selection of natural minerals for carbon mineralization are also used to select of alkaline wastes, namely, a substantial content in alkaline earth metals such as calcium and magnesium, a high degree of alkalinity, or the proximity to CO 2 sources [4,8,11,14,16]. For example, suitable mineral feedstock for carbon sequestration has been found in phosphogypsum (PG) waste stockpiles derived from the fertiliser industry [28−30]. These wastes have been considered for a two-step (“indirect”) mineralisation process under atmospheric conditions. According to this strategy, in our previous work [28] the PG is first transformed into a carbon sequestration agent (calcium hydroxide) by alkaline dissolution with soda (reagent grade). Then, this agent rapidly and completely reacts with CO 2 giving carbonates. In south-west Spain, there are 120 Mt of PG stockpiled in waste from the activity of the fertiliser industry for decades, and a similar amount is produced annually in different parts of the world. Thus, 30 Mt of CO 2 could potentially be sequestered using this PG feedstock upon a simple engineering implementation. In addition, some of the environmental issues associated with these wastes (e.g. mobility of the heavy metals or radionuclides) would ultimately be resolved in this methodology, as these traces would remain immobilised in the final carbonate [28,29]. Previous to this research, only alkaline reagent-grade solutions had been studied for PG carbonation [28,31]. However, alkaline industrial liquid wastes should be considered in order to not penalize the economic aspects of this two-step mineralisation process. One of these is the caustic liquid waste of the aluminium industry. Aluminium finishing plants primarily produce different effluents from the anodising and etching sections. Anodising is usually performed in sulphuric acid and etching is performed in sodium hydroxide solutions with concentrations ranging between 50 and 200 g/L. Typically, alkaline waste streams from the etching operation are treated by mixing them with the acid wastewaters from the anodising step. However, this neutralization process requires the incorporation of an additional 4 neutralising agent (NaOH or lime) due to the overall acid balance. Nevertheless, various methods to recover and recycle etching solutions have been proposed [32], such as that based on dawsonite synthesis [33]. In this work we have evaluated the viability of the indirect PG carbonation by caustic dissolution with liquid wastes of the aluminium anodising industry. Previously reported procedures based on PG [28,29,34] considered the use of a pure soda reagent, so costs and energy consumption of its production suppose an important drawback for its viability. The current study proposes a new management strategy for PG and waste streams of the aluminium anodising industry with two environmental objectives: (1) to synthesise katoite (member of the isomorphous hydrogrossular series [35-37]) by recycling both PG and aluminium wastes and (2) to explore the potential of katoite as a CO 2 sequestration agent by aqueous carbonation. In addition, special attention was paid to the transfer of hazardous traces (heavy metals or radionuclides) typically present in PG wastes, throughout the methodology. 2. Experimental procedure The main steps of the experimental procedure are summarised in Fig. 1. 2.1. Starting materials 5 kg of raw phosphogypsum (‘PG’) were supplied by Fertiberia (Huelva, Spain). The PG was homogenised, ground and oven-dried at 40 ºC in order to preserve the structural water of gypsum. The caustic liquid Na-Al waste was supplied by Verinsur SA (Jerez de la Frontera, Spain). This waste was derived from the finishing process of the aluminium industry, and, regarding the supplier, ca. 200 t are produced annually. It is a transparent solution with pH ~14 and contains a small amount of reddish precipitate. In this work, the raw waste was filtered and the reddish precipitate was discarded. In addition, the filtered liquid was dried to evaporation at 110 ºC (in a hot plate) and the dried precipitate (‘A (S) ’) was analysed. 2.2. Synthesis of the carbon sequestration agent The sequestration agent was synthesised by alkaline dissolution of the PG using the Na-Al-rich liquid waste. The process started by the addition of 12.5 g of phosphogypsum in 25.0 mL of liquid waste upon magnetic stirring in a closed beaker for 3 h under room conditions. During the process, the pH remained at 12.0. A precipitate was formed (‘PGA’), which was separated by centrifuging for 5 min at 3000 rpm. Then, it was put inside a stove at 80 ºC to quickly dry the sample in order to avoid non-controlled carbonation by weathering or further hydration. The corresponding supernatant ‘PGA (L) ’ was not considered for the carbon sequestration procedure. Based on the chemical composition of the residues (see section 3.1), this specific mass ratio corresponded to a stoichiometric molar ratio of Ca:Al = 1.5, targeting the following chemical reaction: 5 3(CaSO 4 ·2H 2 O) + 6Na +(aq) +12OH -(aq) +2Al 3+(aq) ↔ 3Na 2 SO 4 + Ca 3 Al 2 (OH) 12 + 6H 2 O [Eq. 1] Other molar ratios, as well as the addition of extra distilled water, were studied. Only the conditions that yielded the best results have been presented. Fig. 1. Experimental methodology of the synthesis of the carbon sequestration agent and the two carbon capture procedures. 2.3. Carbon sequestration procedures Two procedures were considered to evaluate the carbon capture efficiency of the new sequestration agent, PGA: (1) bubbling pure CO 2 into an aqueous suspension of the sequestration agent, and (2) using the “weathering pools” [25]. 2.3.1. Bubbling This procedure started with the dispersion of 2 g of the PGA sample in 40 mL of distilled water. The dispersion was then subjected to a continuous pure CO 2 flux (~1 bar, 20 cm 3 /s) under magnetic stirring and room conditions, for 90 min. Under this flux, the pH rapidly decreased to 6.7. The expected carbonation reaction is: Ca 3 Al 2 (OH) 12 + 3CO 2 ↔ 3CaCO 3 + 2Al(OH) 3 + 3H 2 O [Eq. 2] This procedure resulted in a new solid-liquid suspension, which was separated by centrifugation (3000 rpm, 5 min) into a solid and liquid phases, hereafter referred to as ‘PGAB’ and ‘PGAB (L) ’, respectively. The solid phase was analysed, and the liquid phase was discarded. This step was repeated four times to check reproducibility. 6 2.3.2. Weathering The carbonation of the sample PGA was also studied using the atmospheric CO 2 by the weathering pools procedure [25]. The weathering pools consist of very shallow, flat, ponds on which a thin film of the sequestration agent is spread. In this work, 11 g of sample PGA were spread over a flat surface of 20x20 cm 2 . Distilled water (20 mL) was initially added, and the carbonation of the sample occurred during drying under room conditions. Distilled water (20 mL) was added every two weeks to enhance the capture performance. Typically, the complete process took up to 2 months. The final solid obtained is hereafter referred to as ‘PGAW’. 2.4. Characterisation techniques Chemical analyses were made by X-ray fluorescence (AXIOS Panalytical, with an Rh tube) using the semi-quantitative method for major elements, and pseudo-total acid-digestion followed by analysis with inductively coupled plasma mass-spectrometry (ICP-MS; HP-4500 instrument) for trace elements. The chemical analyses of the liquid waste were performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Horiba-Jobin-Yvon, Ultima 2). Activity concentrations of radionuclides belonging to the uranium series were determined in the raw materials and in the various solid and liquid fractions obtained by α-particle spectrometry with 450 mm 2 PIPIS detectors. Previously, the isotopes were sequentially isolated using a validated radiochemical procedure based on extraction chromatographic resins (UTEVA resins, Triskem Int.) and on the preparation of the counting source by electrodeposition [38]. Crystalline phases of the samples were identified by X-ray diffraction (XRD) (BRUKER D8- Advance, Cu-Kα radiation, λ α1 =0.154 nm), from 10.00º to 70.00º (∆(2θ) = 0.015º, counting time = 80 s). Sample morphology was characterised by scanning electron microscopy (SEM-FEG, Hitachi S5200) using an acceleration voltage of 5 kV. Particle size was estimated by laser diffraction using a Mastersizer 2000 system. Nitrogen physisorption experiments (ASAP 2420) were performed at 77.35 K to reveal nanostructural features. Samples were degasified at 150 ºC for 2 h prior to the experiment. The carbonation degree of the samples was estimated by thermogravimetric analyses (TGA) in an STD Q600 experimental device. The experiments were conducted under a nitrogen flux of 100.0 mL/min, from an initial ambient temperature up to 1000 ºC, at 10 ºC/min. As a reference, note that for a pure calcite sample the stoichiometric weight loss due to decarbonation is 44%, over the range 550−900 ºC. To compare the experimental results with this theoretical maximum, the mass of the samples from the TGA experiments was normalised to the weight of the dehydrated sample (at T = 120 ºC). Table S1 (Supplementary Material) summarizes the samples and techniques considered in this work. 7 3. Results and discussion 3.1. Characterisation of the starting materials XRD confirmed that gypsum was the only crystalline phase present in the raw sample PG (Fig. 2). Laser diffraction revealed a typical grain size of 40 µm with > 90% below 110 µm (Fig. S1, Supplementary Material). The caustic liquid waste had a density of 1.32 g/cm 3 , pH ~ 14, and a composition of [Na] = 110 ± 2 g/L (4.78 ± 0.09 M) and [Al] = 52 ± 2 g/L (1.93 ± 0.07 M). In the precipitate, A (S) , hydrated sodium carbonate (PDF: 01-070-0845, Na 2 CO 3 (H 2 O)) and gibbsite (PDF: 00- 012-0460, Al(OH) 3 ) were identified (Fig. 2). The presence of Na 2 CO 3 is due to the carbonation of the sodium by atmospheric CO 2 during the drying process itself. But the Na 2 CO 3 is a very soluble phase that does not guarantee the permanent fixation of the CO 2 (in comparison to CaCO 3 ), so this particular carbonation was not considered as part of the carbon sequestration potentialities of the method. Fig. 2. X-ray diffraction patterns of samples ‘PG’, ‘A (S) ’, and ‘PGA’, as-synthesised and washed. Maximum intensities reached 44860 counts for PG, 8110 for A (S) , 2860 for PGA, and 3650 for PGA (washed). The major elements of the starting materials obtained from XRF are shown in Table 1. Sample PG was mainly composed of Ca and S, in a molar ratio very close to unity (0.993), as expected for gypsum, and an 18.4 wt.% of loss of ignition (LOI) due to the gypsum dehydration, which departs from stoichiometric values due to the presence of some major impurities inherited from the industrial process 8 such as SiO 2 (2.52 wt.%) and P 2 O 5 (0.65 wt.%). Thus, the PG sample was composed of gypsum with purity > 96%. These results are similar to other analyses of samples from the same stockpiled PG [28,29]. On the other hand, the solid precipitate from the liquid waste (sample A (S) ) was composed of mainly Al and Na (>99 wt.%), while major detected impurities were Ca, S, Si, Fe, and K. The presence of trace elements and radionuclides in the PG is discussed in sections 3.4 and 3.5, respectively. 3.2. Carbon sequestration agent The dissolution of the PG with the liquid waste generated a solid precipitate, PGA, and the corresponding supernatant, PGA (L) , which was discarded. Chemical analysis of the discarded liquid revealed the presence of Na ([Na] = 91 ± 6 g/L), S ([S] = 39.6 ± 0.4 g/L), Al ([Al] = 23 ± 2 g/L) and negligible Ca ([Ca] < 0.1 mg/L) amount. The density of the supernatant was 1.34 ± 0.1 g/cm 3 . The constituent crystalline phases of PGA were identified by XRD (Fig. 2), and the two observed phases were hydrogrossularite (PDF: 00-002-1124, Ca 3 Al 2 (OH) 12 , katoite * ) and thenardite (PDF: 01-074-2036, Na 2 SO 4 , sodium sulphate). No residual gypsum was detected. In accordance with this, the chemical composition of PGA (Table 1) revealed Ca, Al, S and Na as the major components, in relative molar ratios Ca:Al and Na:S equal to those of the katoite (which was the target ratio of the experimental procedure) and thenardite, respectively. Table 1 Major elements of the various solids obtained from the experimental procedure. LOI: Loss of ignition at 1000 ºC; n. d.: not detected. Raw materials Dissolution Carbonation Major elements (wt. %) PG A (S) PGA PGAB CaO 32 ± 1 0.15 ± 0.01 21 ± 1 32 ± 2 Al 2 O 3 0.20 ± 0.01 32.3 ± 0.6 12.7 ± 0.2 14.5 ± 0.3 SO 3 46 ± 3 0.07 ± 0.01 27 ± 2 14.4 ± 0.9 Na 2 O 0.01 ± 0.01 46 ± 2 20.5 ± 0.7 2.29 ± 0.08 SiO 2 2.52 ± 0.03 0.06 ± 0.01 1.81 ± 0.02 2.66 ± 0.03 P 2 O 5 0.65 ± 0.02 n. d. 0.38 ± 0.01 0.47 ± 0.01 K 2 O 0.02 ± 0.01 0.02 ± 0.01 0.02 ± 0.01 0.03 ± 0.01 Fe 2 O 3 n. d. 0.03 ± 0.01 n. d. 0.01 ± 0.01 LOI 18.4 ± 0.4 21.4 ± 0.2 16.4 ± 0.2 33 ± 1 Molar ratio Ca:Al 1.51 2.01 Molar ratio Ca:S 1.11 3.43 Molar ratio Na:S 1.96 0.41 * This nomenclature was approved by the Commission of the International Mineralogical Association. 9 Katoite is included in a group of minerals known as hydrogarnet (Ca 3 (Al,Fe) 2 (SiO 4 ) y (OH) 4(3−y) ; 0<y<3), where the SiO 44− tetrahedra are partially or completely replaced by OH − . The Al-containing hydrogarnet includes the hydrogrossular solid solution (Ca 3 Al 2 (SiO 4 ) y (OH) 4(3−y) , 0<y<3), being the katoite the Si-free end-member (C 3 AH 6 in cement notation † ) [36,39]. Thenardite is highly soluble in water under room conditions, so gentle rinsing with pure water led to the separation of the thenardite and katoite (“PGA washed” in Fig. 2; Ca 3 Al 2 (OH) 12 ) within a purity of 95% (XRF, see Table S2, Supplementary Material). TGA analysis (Fig. 3) confirmed that a weight loss of 12 wt.% occurred in a two-step process between 250 ºC and 400 ºC, which can be explained in terms of the reported two-step katoite dehydration [40]. No other substantial weight losses were detected in the TGA. The morphology of the thenardite and katoite present in PGA was observed by SEM. In Fig. 4, micron-sized octahedral habit of the katoite was easily recognised, as reported previously [41,42]. In addition, elongated-shaped thenardite crystals could be readily observed. Fig. 3. TGA experiments of the sequestration agent PGA, and carbonated samples PGAB and PGAW. Fig. 4. SEM images of sample PGA, where ~2 micron sized katoite octahedra and elongatedshaped thenardite crystals were readily identified. † Cement short-hand notation: C=CaO, A=Al 2 O 3 and H=H 2 O. 16 present in the final carbonate by-product were low enough to enable their use in selected applications in civil engineering similar to those currently proposed for phosphogypsum. In summary, the procedure presented in this work represents an environmental proposal to jointly manage the controversial phosphogypsum stockpiles and the aluminium industry caustic waste, enabling an efficient carbon dioxide sequestration technology. Acknowledgements This work was supported by the Spanish Government through the research project MAT2013-42934- R and the Andalusia Government through the research project P12-RNM-2260. IR-H wants to especially thank the funding support of the project MAT2013-42934-R. 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Morales-Florez. Tel: +34 954 550 946. Fax: +34 954 552 870. E-mail address: [email protected] SUPPLEMENTARY MATERIAL Figure S1. Particle size distribution of the sample PG Figure S2. XRD pattern of a poorly carbonated PGAB sample, where the polymorphs of calcium carbonate are identified as V: vaterite (PDF: 00-024-0030), Q: quartz alpha (PDF: 01-089-8936), C: calcite (PDF: 01-086-2340). Other crystalline phases are also identified: B: bassanite (CaSO 4 ·0.5H 2 O, PDF: 00-041-0224), COH: calcium aluminum oxide carbonate hydrate (Ca 4 Al 2 CO 9 ·11H 2 O, PDF: 00-014-0083). Figure S3. XRD pattern of sample PGAB heated at 800 ºC. CO: Calcium Oxide (CaO, PDF: 00-043-1001); M: Mayenite (Ca 12 Al 14 O 33 , PDF: 00-009-0413); CA: Calcium Aluminum Oxide Sulfate (Ca 4 Al 6 O 12 SO 4 , PDF: 00-042-1478). Table S1 : Description and sample identifiers used in this work . S ummary of the experimental techniques. X-ray diffraction (XRD), X-ray fluorescence (XRF), total acid digestion followed by i nductively coupled plasma mass-spectrometry (ICP-MS), i nductively coupled plasma (ICP), s canning electron microscopy (SEM-FEG) , laser diffraction, N 2 physisorption, thermogravimetric analyses (TGA), and α–Spectrometry (αSP). Sample Sample characteristic Technique PG Original phosphogypsum sample XRD, XRF, ICP-MS, laser diffraction, αSP A (L) Na-Al-rich caustic liquid waste ICP-AES, αSP A (S) Solid precipitate from the liquid waste XRD, XRF PGA Carbon sequestration agent XRD, XRF, TGA, SEM, ICP-MS, αSP, N 2 physisorption PGA (L) Liquid phase (supernatant of PGA) ICP-AES, αSP PGAB PGA carbonated (by bubbling CO 2 ) XRD, XRF, ICP-MS, αSP PGAB (L) Liquid phase (supernatant of PGAB) ICP-AES, αSP PGAW Weathered PGA sample XRD, TGA Table S2. Chemical composition of sample “PGA washed”. LOI: Loss of ignition at 1000 ºC. Raw material Major elements (wt. %) PGA washed CaO 47.5 Al 2 O 3 18.9 SO 3 4.8 Na 2 O 0.3 SiO 2 1.2 P 2 O 5 0.6 F 2 O 0.9 LOI 25.8