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Mineral carbonation of yellow phosphorus slag and characterization of carbonated product
Energy 188 (2019) 116102
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Mineral carbonation of yellow phosphorus slag and characterization of carbonated product Qiuju Chen a, b, Wenjin Ding a, b, *, Hongjuan Sun a, b, Tongjiang Peng a, b a
Key Laboratory of Ministry of Education for Solid Waste Treatment and Resource Recycle, Southwest University of Science and Technology, Mianyang, 621010, China b Sichuan Engineering Lab of Non-metallic Mineral Powder Modification and High-value Utilization, Southwest University of Science and Technology, Mianyang, 621010, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2019 Received in revised form 2 September 2019 Accepted 8 September 2019 Available online 12 September 2019
A procedure for producing high-purity CaCO3 by yellow phosphorus slag carbonation was proposed. The impacts of technological parameters on acid-leaching and carbonation results were discussed. The calcium leaching rate was 99.8%, while the acid-leaching product was amorphous silicon dioxide with granular morphology and some pores in the surface. The carbonation ratio was 98.10%: 1 t of yellow phosphorus slag can be used to produce 881 kg of CaCO3 and sequester 388 kg of CO2 under the optimized conditions. The CaCO3 satisfied the Chinese recommended industrial standard of HG/T 2226-2010. The products with a single calcite or vaterite structure, or binary and trinary mixed structure of calcite, vaterite and aragonite were all successfully prepared. A preliminary economic evaluation predicted that our yellow phosphorus slag carbonation procedure held a promising application for CO2 sequestration. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Yellow phosphorus slag Mineral carbonation Carbonation product Polymorphism Economic evaluation
1. Introduction Human activities play an important role in global climate change over the last few centuries [1]. Modernization and industrialization of various countries are changing our living environment. Human development will undoubtedly be accompanied by a large amount of energy consumption [2], thus tremendous carbon dioxide emission released by fossil fuels. According to the report of the United Nations environment programme, the global carbon dioxide emission was increased to 53.5 billion tonnes in 2017 due to excessive fossil fuel consumption. Atmospheric concentration of CO2 surged to 410 ppm which was last seen on earth three million years ago, while this figure was 280 ppm in pre-industrial time [3]. The CO2 anthropogenically generated in the coming centuries cannot be totally consumed by the global carbon cycle [4], so it is urgent to develop the adaptive technologies to decrease the atmospheric CO2 level. In these schemes, using clean energy in largescale and sequestering superfluous CO2 are two main candidates.
* Corresponding author. Key Laboratory of Ministry of Education for Solid Waste Treatment and Resource Recycle, Southwest University of Science and Technology, Mianyang, 621010, China. E-mail address: [email protected] (W. Ding). https://doi.org/10.1016/j.energy.2019.116102 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
For a foreseeable future, fossil fuels are expected to continue to be the biggest energy sources because of the economic and social reasons. For the adaptation technologies, geological [5,6], biological [7e9] and mineral sequestration [10,11], and terrestrial carbon sinks are now being considered by developed countries. Various solid wastes such as steel slag [12], mining residues [13], coal ash [14], waste gypsum [15,16] and blast furnace slag [17] were used as raw material in the mineral carbonation, which makes it a leading CO2 sequestration method. The yellow phosphorus slag was a solid byproduct of yellow phosphorus production using dry-process. About 8000e10000 kg of yellow phosphorus slag will be produced when 1000 kg of yellow phosphorus is made [18]. The slag rich in calcium was amorphous and it has high reaction activity, it can be suitable for CO2 sequestration as a new material. China produce approximately 8 Mt of yellow phosphorus slag per year, which can sequester about 1.5e2 Mt of CO2 in theory. While yellow phosphorus slag is used in the fields, including the preparation of sintered brick [19], glass-ceramics [20] and concrete [21]. To the best of our knowledge, there have no reports about yellow phosphorus slag carbonation for CO2 sequestration. Yellow phosphorus slag was used as raw material for CO2 sequestration under the action of HCl and NH4OH. According to the previous studies [22,23], carbonation conditions could affect the
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polymorphism of carbonated product. Most of the products were calcite and vaterite without using any chemical additives. In this paper, the effect of experimental conditions on carbonation ratio and polymorphism of products was systematically investigated. Fibrous carbonated product was expected to be prepared. Meanwhile we reduced the economic cost of yellow phosphorus slag carbonation by recovering the by-products and improving the performance of the carbonation product.
Table 1 Composition of yellow phosphorus slag, leaching product and carbonation product (wt%). Name
CaO
MgO
SiO2
P2O5
Al2O3
Fe2O3
Others
Yellow phosphorus slag Leaching product Carbonation product
50.31 0.27 55.82
2.93 0.05 0.12
34.70 98.12 0.03
3.80 0.17 0.02
3.47 0.31 0.01
3.12 0.14 e
1.67 0.94 44
Note: The chemical elements are in the form of oxide.
2.2. Methods 2. Materials and methods 2.1. Materials The granular yellow phosphorus slag used in this study was from Sichuan province (Fig. 1b), with no crystal phase in it (Fig. 1a). The composition of yellow phosphorus slag was detailed in Table 1. CaO and SiO2 were major constituents, and Fe2O3, P2O5 and Al2O3 were three leading impurities. Prior to the experiment, yellow phosphorus slag was pulverized into particles less than 150 mm. No further purification was performed on the reaction auxiliary, HCl and NH4OH, and industry-grade 99.9% CO2.
The acid-leaching of yellow phosphorus slag was performed by adding 160 mL HCl solution to 20 g of slag sample in a 250 mL reaction vessel. A magnetic stir bar was used to ensure the adequate mixing throughout the reaction. The mixture was heated in a water bath to maintain a desired reaction temperature, and then chilled down to room temperature after the determined time. Then, the solid product was filtrated out from the obtained mixture, and washed and dried for further use. The carbonation was completed by adding NH4OH and CO2 into the neutralized leaching solution. The CO2 was injected with a predetermined flow rate at atmospheric pressure. The solids were obtained by filtrating the solution after the desired reaction time, and then washed with distilled water and baked for 24 h at 80 C. The carbonation filtrate is evaporated to recycle NH4Cl. Factors and levels of acid-leaching experiments for yellow phosphorus slag and carbonation experiments for Ca2þ leaching solution are listed in Tables 2 and 3. The overall reactions involved in the proposed mineral carbonation process are as follows [24]: CaSiO3(amorphous) þ 2HCl /CaCl2 þ H2O þ SiO2
(1)
CaCl2 þ 2NH4OH / Ca(OH)2 þ 2NH4Cl
(2)
Ca(OH)2 þ CO2 / CaCO3 þ H2O
(3)
2.3. Data analysis The sample composition was determined by a chemical titration and X-ray fluorescence. The X-ray diffraction spectra were collected with a DX-2700 X-ray diffractometer using Cu Ka radiation (l ¼ 0.15406 nm), at 40 kV, 40 mA with a scanning speed of 0.02 deg/s. The samples were observed using a scanning electron microanalyzer with an accelerating voltage of 5 kV. Specific surface area analysis was conducted using an Autosorb-1MP full-automatic specific surface area analyzer (BET and BJH) of Quantachrome Instruments (USA). The whiteness of samples was tested by HY-BDY. The Ca2þ leaching rate and the purity of the obtained calcium carbonate were determined by chemical titration and inductively coupled plasma (ICP). The carbonation ratio (h) can be calculated from:
h¼
Fig. 1. (a) XRD pattern and (b) SEM image of yellow phosphorus slag.
MWCa MWCaCO3
M
CV
100%
(4)
where C and V are the percentage of calcium in salt-leaching filtrate and the filtrate volume，and where M is the carbonation product mass, V is the filtrate volume, and C is the calcium concentration of the filtrate. M and V could be directly weighted, while C was measured by an EDTA titration. The mole fractions (mol%) of aragonite (XA), calcite (XC) and vaterite (Xv) in the mixture were determined through the XRD
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Table 2 Factors and levels of acid-leaching experiments for yellow phosphorus slag. No.
Yellow phosphorus slag (g)
HCl concentration (mol/L)
Temperature ( C)
Liquid to solid ratio (mL/g)
Time (min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
1 2 3 4 6 3 3 3 3 3 3 3 3 3 3 3 3 3
60 60 60 60 60 30 40 50 60 80 60 60 60 60 60 60 60 60
10 10 10 10 10 10 10 10 10 10 3 5 8 10 8 8 8 8
60 60 60 60 60 60 60 60 60 60 60 60 60 60 30 60 90 120
Table 3 Factors and levels of carbonation experiments for Ca2þ leaching solution. No.
Volume of Ca2þ leaching solution (mL)
Ammonia dosage (mL)
CO2 flow rate (mL/min)
Temperature ( C)
Time (min)
CO2 purity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 25
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200
5 10 15 20 25 30 25 25 25 25 25 25 25 25 25 25 25 25
100 100 100 100 100 100 50 100 150 200 100 100 100 100 100 100 100 100
30 30 30 30 30 30 30 30 30 30 20 30 40 50 30 30 30 30
60 60 60 60 60 60 60 60 60 60 60 60 60 60 30 60 90 120
99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9
intensities of the (221) plane of aragonite (IA221 ), the (104) plane of calcite (IC104 ) and the (110) plane of vaterite (IV110 ) [25]: For a mixture of aragonite and calcite:
XA ¼
3:157IA221 IC104
þ 3:157IA221
XC ¼ 1 XA
(5)
(6)
For a mixture of vaterite and calcite:
XV ¼
7:691IV110 IC104
þ
7:691IV110
XC ¼ 1 XV
(7)
(8)
3:157IA221 IC104
þ 3:157IA221 þ 7:691IV110
IC104 XA 3:157IA221
XV ¼ 1:0 XA XC
(10)
(11)
3. Results and discussion 3.1. Extraction of calcium from yellow phosphorus slag
For a ternary mixture:
XA ¼
XC ¼
(9)
The influences of experimental conditions on the leaching rate of calcium were studied with several selected parameters, and Fig. 2 presented our results. An upward tendency of the calcium leaching rate was observed when the concentration of HCl was increased (Fig. 2a). The calcium leaching rate was 42.5% for 1 mol/L HCl, and it reached to 99.8% as the HCl concentration increases to 3 mol/L. When the HCl concentration has been further increased to 4 and 6 mol/L, this figure underwent a slight change; which indicates the leaching experiment has finished. Fig. 2b displays the reaction temperature impact on the leaching rate of calcium. The calcium leaching rate of 39.8% was achieved at 30 C, and it reached to 99.8% as the temperature increased to 60 C. Because the ions in
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Fig. 2. Technological conditions on Ca2þ leaching rate for yellow phosphorus slag: (a) concentration of HCl, (b) temperature, (c) liquid to solid ratio, (d) time and (e) error bar of leaching results.
the reaction system become more reactive with the increasing the reaction temperature. Amorphous calcium silicate in yellow phosphorus slag is gradually dissolved, then the concentration of calcium increased. This figure hardly changed when the temperature was further elevated to 80 and 100 C, the calcium leaching rate hardly changes; this is because the leaching reaction has almost finished. The curves of calcium leaching rate vs. temperature, time, and liquid to solid ratio show similar variation trends (Fig. 2c and d). The optimized time and liquid to solid ratio were 60 min and 8 mL/g, respectively. Therefore, the optimal conditions were found to be 3 mol/L, 60 C, 8 mL/g and 60 min. All acid-leaching experiments were done three times. The error bar of leaching rate was shown in Fig. 2e. The error was small from Fig. 2e, which indicated the leaching experiment has good repeatability. Fig. 3 shows the SEM image and XRD pattern of the corresponding acid leaching product. Crystal phase was not observed in Fig. 3a. The main
ingredient of leaching product is SiO2 from Table 1. It indicates almost all amorphous calcium silicate in yellow phosphorus slag is dissolved, on which the following carbonation experiments were based. The morphology of the product is granular with some pores in the surface (Fig. 3b). The content of SiO2 in leaching product is 98.12% and the whiteness of leaching product is 65.4%. The specific surface area of leaching product is 74.24 m2/g, it indicates the leaching product has high chemical reactivity. Thus, it can be used directly in packing field, and it is also a good raw material for white carbon black production. 3.2. Yellow phosphorus slag carbonation process To determine the impact of the NH4OH dosage on the carbonated reaction process, the carbonation ratio was measured during the optimizations of the CO2 flow rate, temperature and reaction
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temperature and time were 30 C and 60 min. All carbonation experiments were done three times. The error bar of leaching rate was shown in Fig. 4e. The error was small from Fig. 4e, which indicated the carbonation experiment has good repeatability. The so-formed carbonation filtrate can be evaporated for ammonia chloride production due to rich Cl and NHþ 4 . The XRD and SEM were used to investigate the reaction product of the yellow phosphorus slag carbonation. The only crystal phase of carbonation product is calcite (Fig. 5a), indicating that the yellow phosphorus slag carbonation with the action of HCl and NH4OH at ambient pressure and the low temperature has taken place. The microstructure and morphology of the products produced in the carbonation are illustrated in Fig. 5b. The agglomeration of many cubic granules can be observed in carbonation product. The size of the granule is ~500 nm in diameter (Fig. 5b). The obtained CaCO3 has a purity of 99.68% (Table 1). The CaCO3 sample has a whiteness of 95.7% and its features fulfilling the Chinese recommended industrial standard of HG/T 2226-2010 “Ordinary precipitated calcium carbonate for industrial use”. Therefore, our prepared CaCO3 is suitable to be used in the packaging field. 3.3. Control of polymorph and morphology of CaCO3
Fig. 3. (a) XRD pattern of leaching product and (b) SEM image of leaching product for yellow phosphorus slag.
time, and the results were shown in Fig. 4. Obviously, the trends of the effect of technological parameters on carbonation ratio are almost identical. A carbonation ratio of 26.15% was achieved with 5 mL of NH4OH. It correspondingly increased to 92.42% and 98.1% for increased NH4OH dosages of 20 and 25 mL, respectively, indicating the uncompleted carbonation reactions because of the insufficient ammonia in the cases of 5, 10, 15 and 20 mL. The ratio has changed little as the NH4OH dosage was further increased to 30 mL (Fig. 4a). The effect which was evaluated by considering four selected CO2 flow rates of 50, 100, 150 and 200 mL/min was considered in the evaluation of the CO2 flow rate impact on the yellow phosphorus slag carbonation (Fig. 4b). At 200 mL/min, the carbonation ratio is 98.77%. As the CO2 flow rate decreases to 150 mL/min and 100 mL/min, the carbonation ratio is 98.4% and 98.1%, respectively. It undergoes a slight change, indicating the carbonation reaction has completed after 60 min. Further decreasing the flow rate to 50 mL/min, the carbonation ratio is 77.46%. This indicates that the carbonation reaction can be accelerated by the increase of CO2 flow rate. The temperature has little influence on carbonation ratio from Fig. 4c. The variation trend of carbonation ratio is the same among ammonia dosage, CO2 flow rate, and reaction time from Fig. 4a, b and d. The optimal
The precipitated CaCO3 have been widely utilized in functional applications such as cosmetic manufacturing [26], function material [27], coating [28], food [29], rubber [30], pharmaceuticals [31] and paper-making [32]. The crystal properties were mainly determined by the crystal type and the morphology. As an example, different particle properties can be realized for one substance with different forms [33]. Therefore, the polymorph and morphology of the carbonation product play a crucial role in the process here investigated. We demonstrate that the polymorph and morphology of CaCO3 can be well-designed and precisely controlled using our method. The obtained products with various structures were presented Figs. 6e9. The promising applications of CaCO3 aragonite with fibrous morphology were wider and the economic value was greater. Through the action of HCl and NH4OH in the above yellow phosphorus slag carbonation, pure CaCO3 was prepared during the CO2 sequestration. The polymorphism of CaCO3 was studied under various reaction conditions. Fig. 6a showed the NH4OH dosage effect on product phases. Pure calcite was prepared with different NH4OH dosages (Fig. 6a). Spherical vaterite composed with cubic calcite were achieved with 5 mL ammonia (Fig. 6a and c), and the molar contents of vaterite and calcite were 91.8% and 8.2% (Fig. 6b). The mole fraction of calcite increased from 26.23% to 40.51% and that of vaterite decreased correspondingly from 73.77% to 59.49% (Fig. 6b) as the amount of ammonia decreased from 10 to 15 mL, and the product was mixed of cubic calcite and spherical vaterite (Fig. 6a, d and e). If the ammonia dosage was further increased to 20, 25 and 30 mL, the only crystal phase of the product was cubic calcite (Fig. 6a, feh). This is because thermodynamic instability vaterite can be produced when there is not enough Ca2þ in the solution. With the increase of NH4OH, the amount of Ca(OH)2 increases gradually in the solution. Then, vaterite in the solution gradually transforms to thermodynamic stability calcite [34]. Pure spherical vaterite was produced when the CO2 flow rate was 50 mL/min (Fig. 7a and c). With the CO2 flow rate increased to 100 and 200 mL/min, pure cubic calcite was produced (Fig. 7a, def). The molar content of vaterite at different CO2 flow rate was 100%, 0%, 0% and 0%, and the molar content of calcite at different CO2 flow rate was 0%, 100%, 100% and 100%, respectively (Fig. 7b). The CO2 dissolved in the solution went up in unit time with the increase of CO2 flow rate. The amount of Ca(OH)2 is constant in the solution with a certain amount of NH4OH, vaterite is firstly produced, then,
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Fig. 4. Technological conditions on the carbonation ratio for leaching solution: (a) the ammonia dosage, (b) CO2 flow rate, (c) temperature, (d) time and (e) error bar of carbonation ratio.
it gradually transforms into calcite with the increasing of CO2 3 amount involved in the reaction [35]. So, the molar content of calcite in the product increased with the added CO2 flow rate. CaCO3 has three crystal types: calcite, aragonite, and vaterite. The thermodynamic stability of these three crystal types decreases in the following order: calcite > aragonite > vaterite. Furthermore, the polymorph and morphology of CaCO3 can be influenced by reaction temperature (Fig. 8). Pure spherical vaterite was achieved at 20 C (Fig. 8a, c), which maintained when the temperature increased from 20 to 30 C, without new crystal type formed (Fig. 8a, d). Spherical vaterite transforms into fibrous aragonite and cubic calcite in the product synthesized at 40e50 C (Fig. 8a, e and f). Meanwhile, the molar contents were 2.16% and 11.34% for aragonite, and 97.84% and 88.66% for calcite, respectively (Fig. 8b). The product comprising cubic calcite, spherical vaterite, and fibrous
aragonite was prepared when the temperature was increased to 60 and 80 C (Fig. 8a, g and h). The molar contents were 2.5% and 5.23% for vaterite, 87.58% and 41.38% for calcite, and 9.92% and 53.39% for aragonite, respectively (Fig. 8b). The vaterite disappeared when the temperature was further increased to 100 C, resulting the cubic calcite and fibrous aragonite mixed product (Fig. 8a, i). Meanwhile, the calcite and aragonite molar content were 22.81% and 77.19%, respectively (Fig. 8b). This indicated that the transformation of vaterite to more stable aragonite and calcite occurred at a higher temperature [36]. It is a good method to synthesize aragonite without any chemical additive. We also discussed carbonation time impact on carbonated product, as shown in Fig. 9. Spherical vaterite product with some cubic particles was prepared for a 30-min reaction (Fig. 9a, c). Meanwhile, the molar contents of vaterite and calcite were 51.96%
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Fig. 5. (a) XRD pattern and (b) SEM image of carbonation product under optimum conditions.
and 48.04%, respectively (Fig. 9b). Spherical vaterite and cubic calcite was replaced by cubic particles upon prolonging the time (Fig. 9a, def). The product with single cubic calcite was produced for the reactions of 60, 90 and 120 min (Fig. 9a, def). This indicated that the transformation of vaterite to more stable calcite occurred if the reaction time was increased, which was consistent with previous study [37].
3.4. Preliminary economic assessment According to the above research, the CO2 sequestration using yellow phosphorus slag was presented. A carbonation ratio (h) of 98.10% was reached with our optimized process: 881 kg of highpurity CaCO3 can be produced with 1000 kg of yellow phosphorus slag, sequestering 388 kg of CO2. The amount of CO2 sequestering
Fig. 6. Polymorph and morphology controllable preparation of carbonation product with different ammonia dosage: (a) XRD patterns of carbonation product, (b) composition of carboination procucts: calcite and vaterite, SEM images of carbonation product: (c) 5 mL, (d) 10 mL, (e) 15 mL, (f) 20 mL, (g) 25 mL and (h) 30 mL.
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Fig. 7. Polymorph and morphology controllable preparation of carbonation product with different CO2 flow rate: (a) XRD patterns of carbonation product, (b) composition of carbonation products: calcite and vaterite, SEM images of carbonation product: (c) 50 mL/min, (d) 100 mL/min, (e) 150 mL/min and (f) 200 mL/min.
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Fig. 8. Polymorph and morphology controllable preparation of carbonation product with different temperature: (a) XRD patterns of carbonation product, (b) composition of carbonation products: calcite, argonite and vaterite, SEM images of carbonation product: (c) 20 C, (d) 30 C, (e) 40 C, (f) 50 C, (g) 60 C, (h) 80 C and (i) 100 C.
by yellow phosphorus slag was more than the amount of CO2 sequestering by the other solid wastes (Table 4). The performance indexes of carbonation product fulfilled the Chinese recommended industrial standard of HG/T 2226-2010. CaCO3 with different polymorphism and morphology was produced during yellow phosphorus slag carbonation process by adjusting the experimental conditions without any chemical additive. The product, which has major fibrous particles, was also successfully prepared, and single fibrous aragonite product may be produced by further optimizing process conditions in the follow-up study. All of these were greater than previously obtained by the carbonation with the other solid wastes (Table 4). The material consumption of yellow phosphorus slag carbonation in Fig. 10 based on processing 100 kg of yellow phosphorus slag, where yellow phosphorus slag was milled to minus 150 mm, and mixed with 200 L of HCl and 600 L of H2O and approximate 800 L of Ca2þ leaching solution and ~39 kg of SiO2 slag were produced after 60 min reaction at 60 C. The leaching solution was cooled to room temperature, then, 26 L of NH4OH was used to adjust the pH value of solution. At the same time, 4 kg of Fe(OH)3 can be obtained. The neutralized leaching solution was used to react with NH4OH and CO2 in mineral carbonation, to yield 88 kg of carbonated product and permanently fix approximate 39 kg of CO2. The resulted leachate of carbonation was neutralized with HCl, evaporated, and crystallized to recover NH4Cl for further using. During the whole carbonation process, a certain amount of HCl and NH4OH were volatilized.
According to the material consumption, an economic assessment was conducted to provide evidence for the promising utilization and the market value of our scheme. About 88 kg of Ca-rich product consisting of 99.68% CaCO3 and 39 kg of Si-rich product consisting of 98.12% SiO2 could be produced through leaching of raw material and processing 100 kg of yellow phosphorus slag. These products can replace 88 kg of light calcium carbonate and 39 kg of activated silica in the composite material production. The unit prices of light calcium carbonate and activated silica are 145 $/t and 97 $/t, respectively, thus, the $17 income can be expected. Approximately 4 kg Fe(OH)3 and 93 kg of NH4Cl could also be obtained through recovering the by-product during the whole carbonation process. These products could be used as a raw material for industrial manufacture. Its market value would be $16. Approximately 159 L of HCl, 104 L of NH4OH and 607 L of H2O were required under the whole process. And the marketing value would be $24. Thus, a $9 income from the entire process can be obtained without considering energy consumption and equipment costs. 4. Conclusions In this study, we experimentally investigated the mineral carbonation of yellow phosphorus slag for CO2 sequestration with HCl and NH4OH. Under optimized conditions (3 mol/L, 60 C, 8 mL/ g and 60 min), the calcium leaching rate was 99.8%. The acidleaching product was amorphous silicon dioxide with granular morphology and some pores in the surface. Under the optimal
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Fig. 9. Polymorph and morphology controllable preparation of carbonation product with different time: (a) XRD patterns of carbonation product, (b) composition of carbonation products: calcite and vaterite, SEM images of carbonation product: (c) 30 min, (d) 60 min, (e) 90 min and (f) 120 min.
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Table 4 Comparisons of CO2 sequestration by solid wastes carbonation. Raw material
Process conditions
Results
(NH4)2SO4 and NH3 were used as agents, roasting at 400 C, carbonation at 25 C and atmospheric pressure
302 kg (CO2)/t The purity of CaCO3 in carbonated product was 47.14%. Chlorinated tailing (NH4)2SO4 and NH3 were used as agents, roasting at 330e450 C 279.8 kg (CO2)/t The purity of CaCO3 in carbonated product was 59.60%. Fly ash Steam was used as agents, carbonation at 400 C and atmospheric pressure 60 kg (CO2)/t The purity of CaCO3 in carbonated product was 13.64%. Desulfurization NH4OH was used as agent, carbonation at room temperature and atmospheric pressure 373 kg (CO2)/t gypsum The whiteness of CaCO3 in carbonated product was 82%. 253 kg (CO2)/t Red gypsum NH4HCO3 was used as agent, carbonation at 70 C and atmospheric pressure The purity of CaCO3 in carbonated product was 99.05%. Phosphogypsum NaOH, HCl and NH4OH were used as agent, carbonation at low temperature and atmospheric 228 kg (CO2)/t pressure The purity of CaCO3 in carbonated product was 99.4%. Yellow phosphorus HCl and NH4OH were used as agent, carbonation at low temperature and atmospheric 388 kg (CO2)/t slag pressure The purity of CaCO3 in carbonated product was 99.68%.
Blast furnace slag
References [38]
[39]
[40]
[41]
[42]
[43]
This work
Fig. 10. Material consumption of the yellow phosphorus slag carbonation process described in this paper.
conditions (25 mL, 100 mL/min, 30 C, 60 min, and normal pressure), a carbonation ratio (h) of 98.10% with 881 kg byproduct of high-purity CaCO3 and 388 kg sequestered CO2 for manufacturing 1000 kg of yellow phosphorus slag was achieved. The cubic calcite carbonation product satisfied the Chinese recommended industrial standard of HG/T 2226-2010. The structure of carbonation product was controlled by adjusting the experimental conditions; the product structures such as a single calcite or vaterite structure, and binary and trinary mixed structure of calcite, vaterite and aragonite, were well controlled in this work. Material consumption was determined according to the experimental results. An income of $9 can be obtained by selling product and byproduct by a preliminary economic evaluation. Acknowledgments This work was supported by the National Key R&D Program of China (2018YFC1802902), the doctoral foundation of Southwest University of Science and Technology (17zx7117), and the Longshan academic talent research and Innovation Team Project of SWUST
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[27] He X, Sun H, Zhu M, Yaseen M, Liao D, Cui X, et al. N-Doped porous graphitic carbon with multi-flaky shell hollow structure prepared using a green and 'useful' template of CaCO3 for VOC fast adsorption and small peptide enrichment. Chem Commun 2017;53:3442e5. [28] Sun S, Ding H, Hou X, Chen D, Yu S, Zhou H, et al. Effects of organic modifiers on the properties of TiO2-coated CaCO3 composite pigments prepared by the hydrophobic aggregation of particles. Appl Surf Sci 2018;456:923e31. [29] Kuo D, Nishimura T, Kajiyama S, Kato T. Bioinspired environmentally friendly amorphous CaCO3-based transparent composites comprising cellulose nanofibers. ACS Omega 2018;3:12722e9. [30] Rungpin N, Pavasupree S, Prasassarakich P, Poompradub S. Production of nano-calcium carbonate from shells of the freshwater channeled applesnail, Pomacea canaliculata, by hydrothermal treatment and its application with polyvinyl chloride. Polym Compos 2015;36:1620e8. [31] Zhou Y, Li H, Liu J, Xu Y, Wang Y, Ren H, et al. Acetate chitosan with CaCO3 doping form tough hydrogel for hemostasis and wound healing. Polym Adv Technol 2019;30:143e52. [32] Chen F-F, Zhu Y-J, Xiong Z-C, Sun T-W, Shen Y-Q. Highly flexible superhydrophobic and fire-resistant layered inorganic paper. ACS Appl Mater Interfaces 2016;8:34715e24. [33] Wang D, Xu L, Nai J, Bai X, Sun T. Morphology-controllable synthesis of nanocarbons and their application in advanced symmetric supercapacitor in ionic liquid electrolyte. Appl Surf Sci 2019;473:1014e23. [34] Watanabe H, Mizuno Y, Endo T, Wang X, Fuji M, Takahashi M. Effect of initial pH on formation of hollow calcium carbonate particles by continuous CO2 gas bubbling into CaCl2 aqueous solution. Adv Powder Technol 2009;20:89e93. [35] Sheng Han Y, Hadiko G, Fuji M, Takahashi M. Crystallization and transformation of vaterite at controlled pH. J Cryst Growth 2006;289:269e74. [36] Gopi S, Subramanian VK, Palanisamy K. Aragonite-calcite-vaterite: a temperature influenced sequential polymorphic transformation of CaCO3 in the presence of DTPA. Mater Res Bull 2013;48:1906e12. [37] Rodriguez-Blanco JD, Shaw S, Benning LG. The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite. Nanoscale 2011;3:265e71. [38] Liu W, Yin S, Luo D, Zhang G, Yue H, Liang B, et al. Optimising the recovery of high-value-added ammonium alum during mineral carbonation of blast furnace slag. J Alloy Comp 2019;774:1151e9. [39] Chu G, Wang L, Liu W, Zhang G, Luo D, Wang L, et al. Indirect mineral carbonation of chlorinated tailing derived from Ti-bearing blast-furnace slag coupled with simultaneous dechlorination and recovery of multiple valueadded products. Greenh Gases-Sci Technol 2019;9:52e66. [40] Liu W, Su S, Xu K, Chen Q, Xu J, Sun Z, et al. CO2 sequestration by direct gassolid carbonation of fly ash with steam addition. J Clean Prod 2018;178: 98e107. [41] Ding W, Yang H, Ouyang J. Mineral carbonation of a desulfurization residue for CO2 sequestration. RSC Adv 2015;5:67184e94. [42] Azdarpour A, Karaei MA, Hamidi H, Mohammadian E, Barati M, Honarvar B. CO2 sequestration using red gypsum via pH-swing process: effect of carbonation temperature and NH4HCO3 on the process efficiency. Int J Miner Process 2017;169:27e34. [43] Ding W, Chen Q, Sun H, Peng T. Modified phosphogypsum sequestrating CO2 and characteristics of the carbonation product. Energy 2019;182:224e35.
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Energy journal homepage: www.elsevier.com/locate/energy
Mineral carbonation of yellow phosphorus slag and characterization of carbonated product Qiuju Chen a, b, Wenjin Ding a, b, *, Hongjuan Sun a, b, Tongjiang Peng a, b a
Key Laboratory of Ministry of Education for Solid Waste Treatment and Resource Recycle, Southwest University of Science and Technology, Mianyang, 621010, China b Sichuan Engineering Lab of Non-metallic Mineral Powder Modification and High-value Utilization, Southwest University of Science and Technology, Mianyang, 621010, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2019 Received in revised form 2 September 2019 Accepted 8 September 2019 Available online 12 September 2019
A procedure for producing high-purity CaCO3 by yellow phosphorus slag carbonation was proposed. The impacts of technological parameters on acid-leaching and carbonation results were discussed. The calcium leaching rate was 99.8%, while the acid-leaching product was amorphous silicon dioxide with granular morphology and some pores in the surface. The carbonation ratio was 98.10%: 1 t of yellow phosphorus slag can be used to produce 881 kg of CaCO3 and sequester 388 kg of CO2 under the optimized conditions. The CaCO3 satisfied the Chinese recommended industrial standard of HG/T 2226-2010. The products with a single calcite or vaterite structure, or binary and trinary mixed structure of calcite, vaterite and aragonite were all successfully prepared. A preliminary economic evaluation predicted that our yellow phosphorus slag carbonation procedure held a promising application for CO2 sequestration. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Yellow phosphorus slag Mineral carbonation Carbonation product Polymorphism Economic evaluation
1. Introduction Human activities play an important role in global climate change over the last few centuries [1]. Modernization and industrialization of various countries are changing our living environment. Human development will undoubtedly be accompanied by a large amount of energy consumption [2], thus tremendous carbon dioxide emission released by fossil fuels. According to the report of the United Nations environment programme, the global carbon dioxide emission was increased to 53.5 billion tonnes in 2017 due to excessive fossil fuel consumption. Atmospheric concentration of CO2 surged to 410 ppm which was last seen on earth three million years ago, while this figure was 280 ppm in pre-industrial time [3]. The CO2 anthropogenically generated in the coming centuries cannot be totally consumed by the global carbon cycle [4], so it is urgent to develop the adaptive technologies to decrease the atmospheric CO2 level. In these schemes, using clean energy in largescale and sequestering superfluous CO2 are two main candidates.
* Corresponding author. Key Laboratory of Ministry of Education for Solid Waste Treatment and Resource Recycle, Southwest University of Science and Technology, Mianyang, 621010, China. E-mail address: [email protected] (W. Ding). https://doi.org/10.1016/j.energy.2019.116102 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
For a foreseeable future, fossil fuels are expected to continue to be the biggest energy sources because of the economic and social reasons. For the adaptation technologies, geological [5,6], biological [7e9] and mineral sequestration [10,11], and terrestrial carbon sinks are now being considered by developed countries. Various solid wastes such as steel slag [12], mining residues [13], coal ash [14], waste gypsum [15,16] and blast furnace slag [17] were used as raw material in the mineral carbonation, which makes it a leading CO2 sequestration method. The yellow phosphorus slag was a solid byproduct of yellow phosphorus production using dry-process. About 8000e10000 kg of yellow phosphorus slag will be produced when 1000 kg of yellow phosphorus is made [18]. The slag rich in calcium was amorphous and it has high reaction activity, it can be suitable for CO2 sequestration as a new material. China produce approximately 8 Mt of yellow phosphorus slag per year, which can sequester about 1.5e2 Mt of CO2 in theory. While yellow phosphorus slag is used in the fields, including the preparation of sintered brick [19], glass-ceramics [20] and concrete [21]. To the best of our knowledge, there have no reports about yellow phosphorus slag carbonation for CO2 sequestration. Yellow phosphorus slag was used as raw material for CO2 sequestration under the action of HCl and NH4OH. According to the previous studies [22,23], carbonation conditions could affect the
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polymorphism of carbonated product. Most of the products were calcite and vaterite without using any chemical additives. In this paper, the effect of experimental conditions on carbonation ratio and polymorphism of products was systematically investigated. Fibrous carbonated product was expected to be prepared. Meanwhile we reduced the economic cost of yellow phosphorus slag carbonation by recovering the by-products and improving the performance of the carbonation product.
Table 1 Composition of yellow phosphorus slag, leaching product and carbonation product (wt%). Name
CaO
MgO
SiO2
P2O5
Al2O3
Fe2O3
Others
Yellow phosphorus slag Leaching product Carbonation product
50.31 0.27 55.82
2.93 0.05 0.12
34.70 98.12 0.03
3.80 0.17 0.02
3.47 0.31 0.01
3.12 0.14 e
1.67 0.94 44
Note: The chemical elements are in the form of oxide.
2.2. Methods 2. Materials and methods 2.1. Materials The granular yellow phosphorus slag used in this study was from Sichuan province (Fig. 1b), with no crystal phase in it (Fig. 1a). The composition of yellow phosphorus slag was detailed in Table 1. CaO and SiO2 were major constituents, and Fe2O3, P2O5 and Al2O3 were three leading impurities. Prior to the experiment, yellow phosphorus slag was pulverized into particles less than 150 mm. No further purification was performed on the reaction auxiliary, HCl and NH4OH, and industry-grade 99.9% CO2.
The acid-leaching of yellow phosphorus slag was performed by adding 160 mL HCl solution to 20 g of slag sample in a 250 mL reaction vessel. A magnetic stir bar was used to ensure the adequate mixing throughout the reaction. The mixture was heated in a water bath to maintain a desired reaction temperature, and then chilled down to room temperature after the determined time. Then, the solid product was filtrated out from the obtained mixture, and washed and dried for further use. The carbonation was completed by adding NH4OH and CO2 into the neutralized leaching solution. The CO2 was injected with a predetermined flow rate at atmospheric pressure. The solids were obtained by filtrating the solution after the desired reaction time, and then washed with distilled water and baked for 24 h at 80 C. The carbonation filtrate is evaporated to recycle NH4Cl. Factors and levels of acid-leaching experiments for yellow phosphorus slag and carbonation experiments for Ca2þ leaching solution are listed in Tables 2 and 3. The overall reactions involved in the proposed mineral carbonation process are as follows [24]: CaSiO3(amorphous) þ 2HCl /CaCl2 þ H2O þ SiO2
(1)
CaCl2 þ 2NH4OH / Ca(OH)2 þ 2NH4Cl
(2)
Ca(OH)2 þ CO2 / CaCO3 þ H2O
(3)
2.3. Data analysis The sample composition was determined by a chemical titration and X-ray fluorescence. The X-ray diffraction spectra were collected with a DX-2700 X-ray diffractometer using Cu Ka radiation (l ¼ 0.15406 nm), at 40 kV, 40 mA with a scanning speed of 0.02 deg/s. The samples were observed using a scanning electron microanalyzer with an accelerating voltage of 5 kV. Specific surface area analysis was conducted using an Autosorb-1MP full-automatic specific surface area analyzer (BET and BJH) of Quantachrome Instruments (USA). The whiteness of samples was tested by HY-BDY. The Ca2þ leaching rate and the purity of the obtained calcium carbonate were determined by chemical titration and inductively coupled plasma (ICP). The carbonation ratio (h) can be calculated from:
h¼
Fig. 1. (a) XRD pattern and (b) SEM image of yellow phosphorus slag.
MWCa MWCaCO3
M
CV
100%
(4)
where C and V are the percentage of calcium in salt-leaching filtrate and the filtrate volume，and where M is the carbonation product mass, V is the filtrate volume, and C is the calcium concentration of the filtrate. M and V could be directly weighted, while C was measured by an EDTA titration. The mole fractions (mol%) of aragonite (XA), calcite (XC) and vaterite (Xv) in the mixture were determined through the XRD
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Table 2 Factors and levels of acid-leaching experiments for yellow phosphorus slag. No.
Yellow phosphorus slag (g)
HCl concentration (mol/L)
Temperature ( C)
Liquid to solid ratio (mL/g)
Time (min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
1 2 3 4 6 3 3 3 3 3 3 3 3 3 3 3 3 3
60 60 60 60 60 30 40 50 60 80 60 60 60 60 60 60 60 60
10 10 10 10 10 10 10 10 10 10 3 5 8 10 8 8 8 8
60 60 60 60 60 60 60 60 60 60 60 60 60 60 30 60 90 120
Table 3 Factors and levels of carbonation experiments for Ca2þ leaching solution. No.
Volume of Ca2þ leaching solution (mL)
Ammonia dosage (mL)
CO2 flow rate (mL/min)
Temperature ( C)
Time (min)
CO2 purity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 25
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200
5 10 15 20 25 30 25 25 25 25 25 25 25 25 25 25 25 25
100 100 100 100 100 100 50 100 150 200 100 100 100 100 100 100 100 100
30 30 30 30 30 30 30 30 30 30 20 30 40 50 30 30 30 30
60 60 60 60 60 60 60 60 60 60 60 60 60 60 30 60 90 120
99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9
intensities of the (221) plane of aragonite (IA221 ), the (104) plane of calcite (IC104 ) and the (110) plane of vaterite (IV110 ) [25]: For a mixture of aragonite and calcite:
XA ¼
3:157IA221 IC104
þ 3:157IA221
XC ¼ 1 XA
(5)
(6)
For a mixture of vaterite and calcite:
XV ¼
7:691IV110 IC104
þ
7:691IV110
XC ¼ 1 XV
(7)
(8)
3:157IA221 IC104
þ 3:157IA221 þ 7:691IV110
IC104 XA 3:157IA221
XV ¼ 1:0 XA XC
(10)
(11)
3. Results and discussion 3.1. Extraction of calcium from yellow phosphorus slag
For a ternary mixture:
XA ¼
XC ¼
(9)
The influences of experimental conditions on the leaching rate of calcium were studied with several selected parameters, and Fig. 2 presented our results. An upward tendency of the calcium leaching rate was observed when the concentration of HCl was increased (Fig. 2a). The calcium leaching rate was 42.5% for 1 mol/L HCl, and it reached to 99.8% as the HCl concentration increases to 3 mol/L. When the HCl concentration has been further increased to 4 and 6 mol/L, this figure underwent a slight change; which indicates the leaching experiment has finished. Fig. 2b displays the reaction temperature impact on the leaching rate of calcium. The calcium leaching rate of 39.8% was achieved at 30 C, and it reached to 99.8% as the temperature increased to 60 C. Because the ions in
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Fig. 2. Technological conditions on Ca2þ leaching rate for yellow phosphorus slag: (a) concentration of HCl, (b) temperature, (c) liquid to solid ratio, (d) time and (e) error bar of leaching results.
the reaction system become more reactive with the increasing the reaction temperature. Amorphous calcium silicate in yellow phosphorus slag is gradually dissolved, then the concentration of calcium increased. This figure hardly changed when the temperature was further elevated to 80 and 100 C, the calcium leaching rate hardly changes; this is because the leaching reaction has almost finished. The curves of calcium leaching rate vs. temperature, time, and liquid to solid ratio show similar variation trends (Fig. 2c and d). The optimized time and liquid to solid ratio were 60 min and 8 mL/g, respectively. Therefore, the optimal conditions were found to be 3 mol/L, 60 C, 8 mL/g and 60 min. All acid-leaching experiments were done three times. The error bar of leaching rate was shown in Fig. 2e. The error was small from Fig. 2e, which indicated the leaching experiment has good repeatability. Fig. 3 shows the SEM image and XRD pattern of the corresponding acid leaching product. Crystal phase was not observed in Fig. 3a. The main
ingredient of leaching product is SiO2 from Table 1. It indicates almost all amorphous calcium silicate in yellow phosphorus slag is dissolved, on which the following carbonation experiments were based. The morphology of the product is granular with some pores in the surface (Fig. 3b). The content of SiO2 in leaching product is 98.12% and the whiteness of leaching product is 65.4%. The specific surface area of leaching product is 74.24 m2/g, it indicates the leaching product has high chemical reactivity. Thus, it can be used directly in packing field, and it is also a good raw material for white carbon black production. 3.2. Yellow phosphorus slag carbonation process To determine the impact of the NH4OH dosage on the carbonated reaction process, the carbonation ratio was measured during the optimizations of the CO2 flow rate, temperature and reaction
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temperature and time were 30 C and 60 min. All carbonation experiments were done three times. The error bar of leaching rate was shown in Fig. 4e. The error was small from Fig. 4e, which indicated the carbonation experiment has good repeatability. The so-formed carbonation filtrate can be evaporated for ammonia chloride production due to rich Cl and NHþ 4 . The XRD and SEM were used to investigate the reaction product of the yellow phosphorus slag carbonation. The only crystal phase of carbonation product is calcite (Fig. 5a), indicating that the yellow phosphorus slag carbonation with the action of HCl and NH4OH at ambient pressure and the low temperature has taken place. The microstructure and morphology of the products produced in the carbonation are illustrated in Fig. 5b. The agglomeration of many cubic granules can be observed in carbonation product. The size of the granule is ~500 nm in diameter (Fig. 5b). The obtained CaCO3 has a purity of 99.68% (Table 1). The CaCO3 sample has a whiteness of 95.7% and its features fulfilling the Chinese recommended industrial standard of HG/T 2226-2010 “Ordinary precipitated calcium carbonate for industrial use”. Therefore, our prepared CaCO3 is suitable to be used in the packaging field. 3.3. Control of polymorph and morphology of CaCO3
Fig. 3. (a) XRD pattern of leaching product and (b) SEM image of leaching product for yellow phosphorus slag.
time, and the results were shown in Fig. 4. Obviously, the trends of the effect of technological parameters on carbonation ratio are almost identical. A carbonation ratio of 26.15% was achieved with 5 mL of NH4OH. It correspondingly increased to 92.42% and 98.1% for increased NH4OH dosages of 20 and 25 mL, respectively, indicating the uncompleted carbonation reactions because of the insufficient ammonia in the cases of 5, 10, 15 and 20 mL. The ratio has changed little as the NH4OH dosage was further increased to 30 mL (Fig. 4a). The effect which was evaluated by considering four selected CO2 flow rates of 50, 100, 150 and 200 mL/min was considered in the evaluation of the CO2 flow rate impact on the yellow phosphorus slag carbonation (Fig. 4b). At 200 mL/min, the carbonation ratio is 98.77%. As the CO2 flow rate decreases to 150 mL/min and 100 mL/min, the carbonation ratio is 98.4% and 98.1%, respectively. It undergoes a slight change, indicating the carbonation reaction has completed after 60 min. Further decreasing the flow rate to 50 mL/min, the carbonation ratio is 77.46%. This indicates that the carbonation reaction can be accelerated by the increase of CO2 flow rate. The temperature has little influence on carbonation ratio from Fig. 4c. The variation trend of carbonation ratio is the same among ammonia dosage, CO2 flow rate, and reaction time from Fig. 4a, b and d. The optimal
The precipitated CaCO3 have been widely utilized in functional applications such as cosmetic manufacturing [26], function material [27], coating [28], food [29], rubber [30], pharmaceuticals [31] and paper-making [32]. The crystal properties were mainly determined by the crystal type and the morphology. As an example, different particle properties can be realized for one substance with different forms [33]. Therefore, the polymorph and morphology of the carbonation product play a crucial role in the process here investigated. We demonstrate that the polymorph and morphology of CaCO3 can be well-designed and precisely controlled using our method. The obtained products with various structures were presented Figs. 6e9. The promising applications of CaCO3 aragonite with fibrous morphology were wider and the economic value was greater. Through the action of HCl and NH4OH in the above yellow phosphorus slag carbonation, pure CaCO3 was prepared during the CO2 sequestration. The polymorphism of CaCO3 was studied under various reaction conditions. Fig. 6a showed the NH4OH dosage effect on product phases. Pure calcite was prepared with different NH4OH dosages (Fig. 6a). Spherical vaterite composed with cubic calcite were achieved with 5 mL ammonia (Fig. 6a and c), and the molar contents of vaterite and calcite were 91.8% and 8.2% (Fig. 6b). The mole fraction of calcite increased from 26.23% to 40.51% and that of vaterite decreased correspondingly from 73.77% to 59.49% (Fig. 6b) as the amount of ammonia decreased from 10 to 15 mL, and the product was mixed of cubic calcite and spherical vaterite (Fig. 6a, d and e). If the ammonia dosage was further increased to 20, 25 and 30 mL, the only crystal phase of the product was cubic calcite (Fig. 6a, feh). This is because thermodynamic instability vaterite can be produced when there is not enough Ca2þ in the solution. With the increase of NH4OH, the amount of Ca(OH)2 increases gradually in the solution. Then, vaterite in the solution gradually transforms to thermodynamic stability calcite [34]. Pure spherical vaterite was produced when the CO2 flow rate was 50 mL/min (Fig. 7a and c). With the CO2 flow rate increased to 100 and 200 mL/min, pure cubic calcite was produced (Fig. 7a, def). The molar content of vaterite at different CO2 flow rate was 100%, 0%, 0% and 0%, and the molar content of calcite at different CO2 flow rate was 0%, 100%, 100% and 100%, respectively (Fig. 7b). The CO2 dissolved in the solution went up in unit time with the increase of CO2 flow rate. The amount of Ca(OH)2 is constant in the solution with a certain amount of NH4OH, vaterite is firstly produced, then,
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Fig. 4. Technological conditions on the carbonation ratio for leaching solution: (a) the ammonia dosage, (b) CO2 flow rate, (c) temperature, (d) time and (e) error bar of carbonation ratio.
it gradually transforms into calcite with the increasing of CO2 3 amount involved in the reaction [35]. So, the molar content of calcite in the product increased with the added CO2 flow rate. CaCO3 has three crystal types: calcite, aragonite, and vaterite. The thermodynamic stability of these three crystal types decreases in the following order: calcite > aragonite > vaterite. Furthermore, the polymorph and morphology of CaCO3 can be influenced by reaction temperature (Fig. 8). Pure spherical vaterite was achieved at 20 C (Fig. 8a, c), which maintained when the temperature increased from 20 to 30 C, without new crystal type formed (Fig. 8a, d). Spherical vaterite transforms into fibrous aragonite and cubic calcite in the product synthesized at 40e50 C (Fig. 8a, e and f). Meanwhile, the molar contents were 2.16% and 11.34% for aragonite, and 97.84% and 88.66% for calcite, respectively (Fig. 8b). The product comprising cubic calcite, spherical vaterite, and fibrous
aragonite was prepared when the temperature was increased to 60 and 80 C (Fig. 8a, g and h). The molar contents were 2.5% and 5.23% for vaterite, 87.58% and 41.38% for calcite, and 9.92% and 53.39% for aragonite, respectively (Fig. 8b). The vaterite disappeared when the temperature was further increased to 100 C, resulting the cubic calcite and fibrous aragonite mixed product (Fig. 8a, i). Meanwhile, the calcite and aragonite molar content were 22.81% and 77.19%, respectively (Fig. 8b). This indicated that the transformation of vaterite to more stable aragonite and calcite occurred at a higher temperature [36]. It is a good method to synthesize aragonite without any chemical additive. We also discussed carbonation time impact on carbonated product, as shown in Fig. 9. Spherical vaterite product with some cubic particles was prepared for a 30-min reaction (Fig. 9a, c). Meanwhile, the molar contents of vaterite and calcite were 51.96%
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Fig. 5. (a) XRD pattern and (b) SEM image of carbonation product under optimum conditions.
and 48.04%, respectively (Fig. 9b). Spherical vaterite and cubic calcite was replaced by cubic particles upon prolonging the time (Fig. 9a, def). The product with single cubic calcite was produced for the reactions of 60, 90 and 120 min (Fig. 9a, def). This indicated that the transformation of vaterite to more stable calcite occurred if the reaction time was increased, which was consistent with previous study [37].
3.4. Preliminary economic assessment According to the above research, the CO2 sequestration using yellow phosphorus slag was presented. A carbonation ratio (h) of 98.10% was reached with our optimized process: 881 kg of highpurity CaCO3 can be produced with 1000 kg of yellow phosphorus slag, sequestering 388 kg of CO2. The amount of CO2 sequestering
Fig. 6. Polymorph and morphology controllable preparation of carbonation product with different ammonia dosage: (a) XRD patterns of carbonation product, (b) composition of carboination procucts: calcite and vaterite, SEM images of carbonation product: (c) 5 mL, (d) 10 mL, (e) 15 mL, (f) 20 mL, (g) 25 mL and (h) 30 mL.
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Fig. 7. Polymorph and morphology controllable preparation of carbonation product with different CO2 flow rate: (a) XRD patterns of carbonation product, (b) composition of carbonation products: calcite and vaterite, SEM images of carbonation product: (c) 50 mL/min, (d) 100 mL/min, (e) 150 mL/min and (f) 200 mL/min.
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Fig. 8. Polymorph and morphology controllable preparation of carbonation product with different temperature: (a) XRD patterns of carbonation product, (b) composition of carbonation products: calcite, argonite and vaterite, SEM images of carbonation product: (c) 20 C, (d) 30 C, (e) 40 C, (f) 50 C, (g) 60 C, (h) 80 C and (i) 100 C.
by yellow phosphorus slag was more than the amount of CO2 sequestering by the other solid wastes (Table 4). The performance indexes of carbonation product fulfilled the Chinese recommended industrial standard of HG/T 2226-2010. CaCO3 with different polymorphism and morphology was produced during yellow phosphorus slag carbonation process by adjusting the experimental conditions without any chemical additive. The product, which has major fibrous particles, was also successfully prepared, and single fibrous aragonite product may be produced by further optimizing process conditions in the follow-up study. All of these were greater than previously obtained by the carbonation with the other solid wastes (Table 4). The material consumption of yellow phosphorus slag carbonation in Fig. 10 based on processing 100 kg of yellow phosphorus slag, where yellow phosphorus slag was milled to minus 150 mm, and mixed with 200 L of HCl and 600 L of H2O and approximate 800 L of Ca2þ leaching solution and ~39 kg of SiO2 slag were produced after 60 min reaction at 60 C. The leaching solution was cooled to room temperature, then, 26 L of NH4OH was used to adjust the pH value of solution. At the same time, 4 kg of Fe(OH)3 can be obtained. The neutralized leaching solution was used to react with NH4OH and CO2 in mineral carbonation, to yield 88 kg of carbonated product and permanently fix approximate 39 kg of CO2. The resulted leachate of carbonation was neutralized with HCl, evaporated, and crystallized to recover NH4Cl for further using. During the whole carbonation process, a certain amount of HCl and NH4OH were volatilized.
According to the material consumption, an economic assessment was conducted to provide evidence for the promising utilization and the market value of our scheme. About 88 kg of Ca-rich product consisting of 99.68% CaCO3 and 39 kg of Si-rich product consisting of 98.12% SiO2 could be produced through leaching of raw material and processing 100 kg of yellow phosphorus slag. These products can replace 88 kg of light calcium carbonate and 39 kg of activated silica in the composite material production. The unit prices of light calcium carbonate and activated silica are 145 $/t and 97 $/t, respectively, thus, the $17 income can be expected. Approximately 4 kg Fe(OH)3 and 93 kg of NH4Cl could also be obtained through recovering the by-product during the whole carbonation process. These products could be used as a raw material for industrial manufacture. Its market value would be $16. Approximately 159 L of HCl, 104 L of NH4OH and 607 L of H2O were required under the whole process. And the marketing value would be $24. Thus, a $9 income from the entire process can be obtained without considering energy consumption and equipment costs. 4. Conclusions In this study, we experimentally investigated the mineral carbonation of yellow phosphorus slag for CO2 sequestration with HCl and NH4OH. Under optimized conditions (3 mol/L, 60 C, 8 mL/ g and 60 min), the calcium leaching rate was 99.8%. The acidleaching product was amorphous silicon dioxide with granular morphology and some pores in the surface. Under the optimal
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Fig. 9. Polymorph and morphology controllable preparation of carbonation product with different time: (a) XRD patterns of carbonation product, (b) composition of carbonation products: calcite and vaterite, SEM images of carbonation product: (c) 30 min, (d) 60 min, (e) 90 min and (f) 120 min.
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Table 4 Comparisons of CO2 sequestration by solid wastes carbonation. Raw material
Process conditions
Results
(NH4)2SO4 and NH3 were used as agents, roasting at 400 C, carbonation at 25 C and atmospheric pressure
302 kg (CO2)/t The purity of CaCO3 in carbonated product was 47.14%. Chlorinated tailing (NH4)2SO4 and NH3 were used as agents, roasting at 330e450 C 279.8 kg (CO2)/t The purity of CaCO3 in carbonated product was 59.60%. Fly ash Steam was used as agents, carbonation at 400 C and atmospheric pressure 60 kg (CO2)/t The purity of CaCO3 in carbonated product was 13.64%. Desulfurization NH4OH was used as agent, carbonation at room temperature and atmospheric pressure 373 kg (CO2)/t gypsum The whiteness of CaCO3 in carbonated product was 82%. 253 kg (CO2)/t Red gypsum NH4HCO3 was used as agent, carbonation at 70 C and atmospheric pressure The purity of CaCO3 in carbonated product was 99.05%. Phosphogypsum NaOH, HCl and NH4OH were used as agent, carbonation at low temperature and atmospheric 228 kg (CO2)/t pressure The purity of CaCO3 in carbonated product was 99.4%. Yellow phosphorus HCl and NH4OH were used as agent, carbonation at low temperature and atmospheric 388 kg (CO2)/t slag pressure The purity of CaCO3 in carbonated product was 99.68%.
Blast furnace slag
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Fig. 10. Material consumption of the yellow phosphorus slag carbonation process described in this paper.
conditions (25 mL, 100 mL/min, 30 C, 60 min, and normal pressure), a carbonation ratio (h) of 98.10% with 881 kg byproduct of high-purity CaCO3 and 388 kg sequestered CO2 for manufacturing 1000 kg of yellow phosphorus slag was achieved. The cubic calcite carbonation product satisfied the Chinese recommended industrial standard of HG/T 2226-2010. The structure of carbonation product was controlled by adjusting the experimental conditions; the product structures such as a single calcite or vaterite structure, and binary and trinary mixed structure of calcite, vaterite and aragonite, were well controlled in this work. Material consumption was determined according to the experimental results. An income of $9 can be obtained by selling product and byproduct by a preliminary economic evaluation. Acknowledgments This work was supported by the National Key R&D Program of China (2018YFC1802902), the doctoral foundation of Southwest University of Science and Technology (17zx7117), and the Longshan academic talent research and Innovation Team Project of SWUST
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