Article pubs.acs.org/IECR
Mineralization of CO2 Using Natural K‑Feldspar and Industrial Solid Waste to Produce Soluble Potassium Chao Wang,† Hairong Yue,*,† Chun Li,† Bin Liang,*,† Jiahua Zhu,† and Heping Xie‡ †
Multiphases Mass Transfer and Reaction Engineering Laboratory, College of Chemical Engineering, and ‡Center of CCUS and CO2 Mineralization and Utilization, Sichuan University, Chengdu 610065, China ABSTRACT: This article describes a novel CO2 mineralization approach using natural insoluble K-feldspar and phosphogypsum for the emission of CO2, reduction of phosphogypsum waste, and production of soluble potash. K-feldspar was activated with CaSO4 at high temperature and then mineralized with CO2 to extract potassium under hydrothermal conditions. Activation and mineralization conditions (e.g., ore/CaSO4 mass ratio, calcination and mineralization temperatures, initial pressure of CO2) were systematically investigated with an optical potassium extraction ratio of ∼87% and a CO2 mineralization ratio of ∼7.7%. A reaction mechanism was proposed based on the experimental results and the characterizations, such as polarized light microscopy, X-ray diffraction, and thermogravimetric and differential thermal analyses. This new methodology is a promising process and has the potential to reduce emissions of CO2 and phosphogypsum from a practical point of view. 100−280 million tons per year.34 However, only 15% of the PG is used, as a set retarder of cement or as a component of gypsum plaster and bricks.34−39 The remaining huge amount of PG is mostly discarded in large stockpiles without any disposal treatment, resulting in land occupation and environment damage.33,34 Herein, a new CO2 mineralization method is proposed to reduce emissions of CO2 and industrial waste of PG while extracting soluble potassium fertilizer from natural K-feldspar ore. This is a coupled process comprising the activation of Kfeldspar with PG at high temperature to extract K2SO4 and CO2 mineralization of the solid residue (Figure 1). The PG used in
1. INTRODUCTION Carbon dioxide in the atmosphere had risen to 31.6 billion tons as of 2013.1 The increase in CO2 emissions arguably contributes to the warming climate due to the “greenhouse effect”.2 Governments and scientists have made many efforts to curb CO2 emissions and develop efficient CO2 capture and utilization systems in the past couple of decades.3 Large-scale CO2 capture and storage (CCS) as a terminal approach mainly includes geological storage,4,5 ocean storage, and mineralization storage. Geological storage and ocean storage could lead to the risk of leaks, geological disasters, groundwater pollution, and other secondary disasters.6 Since Seifritz first reported the mineralization storage of CO2 by fixing CO2 with natural alkaline ores in 1990,7 numerous efforts have been made to mineralize CO2 with sodium-, calcium-, or magnesium-rich ores, such as albite,8 dolomite,9−15 magnesite, wollastonite,4,13,14,16,17 olivine,18−20 and serpentine21−23 to stable carbonate minerals. Although these approaches for the mineralization of CO2 can stably store CO2 for thousands of years, economic infeasibility limits the application of these methods. Potash fertilizer provides elemental nutrients to plants and is largely consumed globally to fertilize farmland. The global consumption of potassium exceeded 27 million tons in 2011.24 At present, the global production of potash fertilizer is primarily based on soluble hoevellite resource because plants cannot absorb insoluble potash ores.25 However, the soluble hoevellite resource is scarce (6.3%. Although use of a higher temperature (e.g, 1225 °C) could lead to a higher yield of soluble K and higher conversions of K and CO2, it is not practicable for commercial application, because the bulk melting in the calcination furnace could form furnace rings and reduce the efficiency of the process. Therefore, calcination temperatures above 1200 °C were not investigated in this research. Calcination is a high-temperature reaction for K-feldspar with CaSO4, where K+ exchanges with Ca2+ to become soluble
Table 3. Effects of Ore Particle Size and Molding Pressure ξc (%)
ηd (%)
69.60 87.59 83.78
3.36 6.36 6.86
72.23 76.28 87.59 82.76 92.73
3.62 3.4 6.36 5.24 5.52
a
ore particle size (μm) 109−150 48−75 38−48 molding pressureb (MPa) 2 4 6 8 10
Calcination conditions: 6 MPa; 2 h; 1:2; 1200 °C. Mineralization conditions: 2 h; 150 °C; initial pressure of CO2, 4 MPa. bCalcination conditions: 48−75 μm; 2 h; 1:2; 1200 °C. Mineralization conditions: 2 h; 150 °C; initial pressure of CO2, 4 MPa. cξ is extraction ratio of potassium dη is CO2 mineralization ratio a
The stoichiometric KAlSi3O8/CaSO4 mass ratio was about 4:1 in the activation reaction. The dispersion of particles is critical for the solid−solid reaction, because a low KAlSi3O8/ CaSO4 mass ratio can enhance the dispersion of K-feldspar particles with CaSO4 species and lead to a high conversion of potassium. Table 4 presents the effects of the ore/CaSO4 mass ratio on the extraction ratio of potassium and the mineralization ratio of CO2. The results indicate that the extraction ratio of potassium and the mineralization ratio of CO 2 were significantly affected by the ore/CaSO4 mass ratio, as both increased with decreasing ore/CaSO4 mass ratio. When the ore/CaSO4 mass ratio exceeded 1:2.0, the extraction ratio of potassium and the CO2 mineralization ratio reached ∼87% and ∼6.2%, respectively. The mass of CaSO4 used in the reaction Table 4. Effects of Calcination and Mineralization Conditions activation
mineralization
ore/CaSO4 mass ratio
calcination temperature (°C)
mineralization temperature (°C)
ξ (%)
η (%)
1:1.4 1:1.6 1:1.8 1:2.0 1:2.2 1:2.0
1200
2
4
150
2
4
150
1:2.0
1100 1125 1150 1175 1200 1200
4
150
1:2.0
1200
0.5 1 1.5 2 2.5 2
4
1:2.0
1200
2
50 75 100 125 150 100
64.7 70.67 77.66 87.59 88.88 24.21 22.34 26.09 57.64 87.59 42.86 63.73 70.72 87.59 87.36 87.59
0.94 4.02 5.94 6.36 6.21 6.85 6.05 4.40 5.39 6.37 0.68 2.33 4.04 6.36 6.53 0.76 4.87 7.70 6.43 6.36 3.20 4.50 6.33 5.77 7.70
calcination time (h)
initial pressure of CO2 (MPa)
0.3 1 2 3 4 7973
dx.doi.org/10.1021/ie5003284 | Ind. Eng. Chem. Res. 2014, 53, 7971−7978
Industrial & Engineering Chemistry Research
Article
Figure 2. PLM images of K-feldspar: (a) perpendicularly polarized light (+), (b) plane polarized light (−).
K2SO4.41 The solid-phase reaction is rather slow, and the reaction time greatly affects the conversions and extraction. The effects of the calcination time on the extraction of potassium and the mineralization of CO2 are included in Table 4. In the initial 2 h, the potassium extraction ratio and the CO2 mineralization ratio increased linearly with the calcination time to ∼87% and ∼6.3%, respectively. In addition, glass melt was observed in the calcination products by polarized light microscopy (PLM), as shown in Figure 3 below. The mineralization reaction is a gas−liquid−solid multiphase reaction, so the pressure of CO2 and the mineralization temperature are the primary parameters conspicuously influencing the mineralization rate. The effects of the mineralization reaction parameters are also included in Table 4. The CO2 mineralization ratio increased with the initial pressure of CO2 from 0.3 to 4 MPa. High pressures of CO2 enhanced the reaction rate because of high solubility, with the highest CO2 mineralization ratio of ∼7.7% at 4 MPa. The optimal mineralization temperature was 100 °C, and a much higher temperature would lead to a low CO2 mineralization ratio, because the higher temperature would reduce the solubility of CO2 in the liquid and hinder the mineralization reaction. Based on the experimental results, the optimized parameters of the reaction were obtained as follows: ore particle size of 48−75 μm, molding pressure of 6 MPa, ore/CaSO4 mass ratio of 1:2.0, calcination temperature of 1200 °C, calcination time of 2 h, mineralization temperature of 100 °C, and initial CO2 pressure of 4 MPa. Under this optimal operating conditions, a high K extraction ratio of ∼87% could be obtained, and the CO2 mineralization ratio reached ∼7.7%. 3.3. Characterizations. 3.3.1. Polarized Light Microscopy (PLM) Analysis. Because crystals exhibit different optical properties, such as anisotropy and dichroism, the samples can be identified with PLM by observing the different interference colors under perpendicularly polarized light. Glass is noncrystalline and isotropic and always appears black under perpendicularly polarized light. As shown in Figure 2, Kfeldspar was observed to be gray and white with a granular texture under polarizing microscope. The major component of the K-feldspar was microcline, which was colorless and transparent under plane polarized light. The minor component was unevenly distributed quartz, and the trace components were calcite and sericite.
Glasses are generally formed during the rapid cooling of the melted phase in calcination reactions.42,43 The morphologies of samples with an ore/CaSO4 mass ratio of 1:2.0 calcined at different temperatures is are shown in Figure 3. A glass phase is distributed in the gaps between the solid particles. It is hard to detect in Figure 3a,b. However, it was detected in a small amount when the temperature reached 1150 °C (see Figure 3c,d) and was observed in an increasing amount at 1200° (see Figure 3e,f). 3.3.2. XRD Analysis. Figure 4 presents the XRD spectrum of raw K-feldspar. Only microcline (PDF 84-0708) and quartz (PDF 46-1045) were detected in the XRD analyses, which is in agreement with the PLM images. Because of low contents or high dispersion, minerals such as calcite and sericite were not detected in the ore sample. The XRD patterns of the samples (ore/CaSO4 mass ratio of 1:0.3) calcined at temperatures of 1000, 1100, and 1150 °C are shown in Figure 5a. No change was observed in the XRD patterns until the reaction temperature reached 1000 °C. Above 1100 °C, the peaks of KAlSi3O8 gradually weakened, and the peaks of CaSiO3 (PDF 27-0088), KAlSi2O6 (PDF 38-1423) and CaAl2Si2O8 (PDF 70-0287) appeared. In the sample with an ore/CaSO 4 mass ratio of 1:1.0, the calciolangbeinite [K2Ca2(SO4)3, PDF 74-0404] phase was detected in the calcination products at 1200 °C. However, it disappeared (Figure 5b) after the mineralization process. To clarify the formation process of calciolangbeinite, a stoichiometric mixture of calcium sulfate and potassium sulfate was calcined at 1200 °C. The calcined slag sample was washed with deionized water and characterized by XRD. As shown in Figure 5c1, the appearance of calciolangbeinite peaks indicated the formation reaction between CaSO4 with K2SO4 was at high temperature (e.g., 1200 °C). In Figure 5c2, the peaks of K2Ca2(SO4)3 disappeared, and only the peaks of the CaSO4· 0.5H2O (PDF 81-1848) appeared, suggesting the dissolution of K2Ca2(SO4)3 in the water. The XRD patterns of the dried sample after CO 2 mineralization are shown in Figure 6. The diffraction peaks of CaCO3 (PDF 41-1475) and some residual CaSO4 were detected in the mineralization products. The XRD patterns indicate that K2Ca2(SO4)3 was leached out in the filtering separation. To further verify the CO2 mineralization ratio, thermogravimetric analy sis was employed to investigate the decomposition 7974
dx.doi.org/10.1021/ie5003284 | Ind. Eng. Chem. Res. 2014, 53, 7971−7978
Industrial & Engineering Chemistry Research
Article
Figure 3. PLM images of the calcination products: (a) 1100 °C, +; (b) 1100 °C, −; (c) 1150 °C, +; (d) 1150 °C, −; (e) 1200 °C, +; (f) 1200 °C, −.
4. DISCUSSION From the experimental results, it was observed that the K2SO4 extraction ratio was mainly affected by the calcination conditions, including the calcination temperature, ore/CaSO4 mass ratio, calcination time, ore particle size, and molding pressure. The temperature is the most important factor in the calcination reaction. Solid−solid reactions can be sharply accelerated at temperatures near the melting points of the reactants.40 Temperatures are expected in the range from particle surface melting (half-melting) to the melting point to maintain the fluidity of the reactants in commercial production.
temperature range of calcium carbonate. Thermogravimetric and differential thermal analyses (TG/DTA) were conducted in N2 atmosphere with a flow rate of 50 mL/min (see Figure 7). The TG/DTA profiles of the CaCO3-rich slag sample (
