Insights into Coproduction of Silica Gel via Desulfurization of Steel Slag and Silica Gel Adsorption Performance.

Steel slag is a calcium-containing alkaline industrial solid waste that can replace limestone for flue gas desulfurization. It can remove SO2 and coproduce silica gel while avoiding CO2 emission from limestone in the desulfurization process. In this study, steel slag with a D 50 of 3.15 μm was used to remove SO2. At room temperature, with a solid-liquid ratio of 1:10, a stirring speed of 800 rpm, and the mixed gas introduced at a flow rate of 0.8 mL/min, 1 ton of steel slag could remove 406.7 kg of SO2, a SO2 removal efficiency typical of existing calcium-rich desulfurizers. As limestone desulfurization can release CO2, when limestone desulfurization was replaced with steel slag of equal desulfurization ratio, CO2 emissions could be reduced by 279.6 kg and limestone could be reduced by 635.5 kg. The yield of silica gel was 5.1%. Silica gel pore structure parameters were close to those of commercially available B silica gel. Products after desulfurization were mainly CaSO4 ·2H2O, CaSO4 ·0.5H2O, CaSO3 ·0.5H2O, and silica gel. With a silica gel dosage of 30 mg, a temperature of 20 °C, a pH value of 6.00, a stirring time of 0.5 h, and a methylene blue concentration of 0.020 mg/mL, the removal ratio of methylene blue adsorbed by silica gel was 98.4%.

Introduction

Nearly 100 desulfurization processes have been developed, but fewer than 10 have been applied industrially. Among the desulfurization systems in operation or under construction, wet flue gas desulfurization accounts for ∼80%. Among wet flue gas desulfurization technologies, limestone/lime-gypsum is the most widely used and mature standard desulfurization process technology worldwide. It is the basic process for flue gas desulfurization in large-unit thermal power plants.

Cheap and readily available limestone or lime was used as a desulfurization absorbent in this method. Limestone is crushed and ground into powder and mixed with water to form an absorbent slurry. When lime is used as the absorbent, lime powder is digested and water is added to make the slurry. In the absorption tower, the absorption slurry contacts and mixes with the flue gas. SO2 in flue gas, calcium carbonate in slurry, and the blown oxidizing air undergo a chemical reaction, and the final reaction product is desulfurization gypsum. Small droplets are removed from flue gas by a mist eliminator after desulfurization, heated by a heat exchanger, and discharged into a chimney. The desulfurized gypsum slurry is recovered after being dehydrated by a dehydration device.

When limestone is used as the absorbent, SO2 is converted in the absorption tower, and the reaction equation is as followsHerein, the slurry containing CaCO3 scrubbing suspension is sprayed into the flue gas from the upper part of the absorption tower. In the absorption tower, SO2 is absorbed to generate Ca(HSO3)2, which falls into the absorption tower slurry tank.

The blown air causes calcium bisulfite to be oxidized into gypsum in the absorption tower slurry tank.Thus, limestone/lime-gypsum wet flue gas desulfurization (WFGD) consumes a large amount of natural minerals, limestone, and lime and also releases greenhouse gas CO2. Therefore, based on the “carbon peak, carbon neutrality” vision, it is important to develop a desulfurization agent that can replace natural ore without emitting other pollutants. According to reports, CO2 emitted by thermal power plants accounts for about 33–40% of global emissions. When 1 ton of limestone (CaCO3 > 90%) used in the WFGD process is decomposed, it will emit ∼400 kg of CO2. In addition to CO2 produced by burning coal, a typical 500 MW power plant can emit about 2–3 million tons of CO2 per year.[1]

Steel slag (SS) is a solid waste that is produced in the steelmaking process. In 2018, China produced nearly 100 million tons of SS,[2] which accounts for nearly 50% of the global SS production. However, nearly 70% of SS was stacked in a disorderly manner, which will inevitably cause environmental pollution, waste resources, and land occupation.[2] The main mineral components in SS are Ca2SiO4, Ca3SiO5, Ca2Fe2O5, and RO, all of which are highly crystalline phases. The main chemical components in SS are CaO, Fe2O3, SiO2, MgO, Al2O3, MnO, and P2O5. Besides, some SS also contains small amounts of components such as TiO2 and V2O5. SS has attracted widespread attention due to its high content of alkaline substances, high pH value of the slurry, and the coproduction of high-value-added silica gel following desulfurization. If SS is used to replace limestone in wet flue desulfurization, CO2 emissions will be significantly reduced. In addition, as a byproduct of desulfurization, gypsum has potential as a retarder in the production of Portland cement.

Previous studies have shown that SS or other calcium-based alkaline solid wastes such as fly ash, red mud, and waste concrete can be used for flue gas desulfurization.[3−11] Meng et al.[3] proposed a method to simultaneously remove SO2 and NOx from coke oven flue gas using SS slurry. After optimizing the reaction conditions, the removal ratios of SO2 and NOx were 100% and 83.4%, respectively. Meng et al.[4] proposed a method combining ozone oxidation and (NH4)2S2O3/SS slurry to simultaneously remove SO2 and NOx from flue gas. The median diameter of the SS sample after grinding was 74 μm. Under optimal operating conditions, the removal efficiency of SO2 was close to 100%, and the removal efficiency of NOx was >78.0%. Over-exploitation of natural limestone in the wet flue gas desulfurization process has caused significant ecological damage. To reduce the consumption of natural limestone, Liu et al.[5] used waste concrete particles (WCPs) produced by a waste concrete recycling plant as an alternative desulfurization absorbent to remove SO2. The results showed that the slurry prepared by dissolving WCP in water was strongly alkaline and rich in Ca2+. The WCP slurry effectively removed SO2 from flue gas, and the desulfurization efficiency reached >98%. The desulfurization capacity of WCP was between 0.44 and 0.73 (g SO2/g WCP), a SO2 removal capacity typical of existing calcium-rich desulfurizers. Chen et al.[6] developed a method to simultaneously remove NOx–SO2–CO2 using fly ash in supergravity rotating packed bed (RPB). The most important factors influencing mass transfer were investigated, including the high gravity coefficient (β), gas–liquid ratio (GLR), and liquid–solid ratio (LSR). The optimal conditions for simultaneous removal of NOx–SO2–CO2 in RPB were β of 233.8, GLR of 69.5, and LSR of 40. Researchers explore ways to improve the desulfurization efficiency, but the extraction and application of the silica gel product have not been reported. The use of SS desulfurization to synergistically extract silica gel is not only of theoretical significance, but, more importantly, it can support the development of value-added SS desulfurization products and the comprehensive utilization of solid waste resources.

Silica gel is a nontoxic and odorless amorphous substance. It is a highly active porous absorbent material prepared by reaction of sodium silicate and sulfuric acid after a series of subsequent treatments.[12,13] The main component is silicon dioxide,[14] and the molecular formula is mSiO2·nH2O. Silica gel has a tetrahedral structure with silicon atoms at the center and oxygen atoms at the apex. The main structure of silica gel is composed of irregular stacks of these tetrahedrons. There are many Si–OH and Si–O–Si bonds on the surface of silica gel.[15] Although there are two lone pairs of electrons provided by oxygen atoms in Si–O–Si, these lone pairs of electrons form a π electron cloud, in which electrons interact with each other and cannot form strong active adsorption sites. In the silicic hydroxyl (Si–OH) groups on the surface, the hydroxyl oxygen atom has a strong electron-donating ability; hence, Si–OH groups are active adsorption sites.

As mentioned above, SS is used instead of limestone for flue gas wet desulfurization, which can reduce the mining of limestone, reduce CO2 emissions, and also produce high-value-added silica gel. However, there are no relevant reports on coproduction of silica gel following SS desulfurization, and the desulfurization mechanism remains unclear. In this study, SS was used to remove SO2 and coproduce silica gel and the desulfurization mechanism was explored. The sulfur fixation rate of SS was calculated and compared with those of existing calcium-rich desulfurizers. Extracted silica gel was characterized and applied to the adsorption of methylene blue. The yield of silica gel was calculated, and pore structure parameters were compared with those of commercial silica gel.

Experimental Section

Materials

Steel slag (SS) was a basic oxygen furnace slag (BOF) collected from Sichuan Dazhou Iron and Steel Plant (China). It was pulverized into ultrafine powder by a supersonic steam-jet smasher. The steam temperature was 270 °C, and the steam pressure was 1.2 MPa. The D50 of SS was 3.15 μm. The ultrafine SS powder was dried and then bagged for later use. In the flue gas wet desulfurization process, air needs to be blown in through a Roots blower outside the desulfurization tower to oxidize the desulfurization product calcium sulfite to calcium sulfate. Therefore, the gas used in the experiment was controlled by dual cylinders, the volume ratio of SO2 and N2 was 5%:95%, the volume ratio of O2 and N2 was 25%:75%, and the ratio of the intake flow rate of the dual cylinders was 1:1. After mixing through the gas distribution device, the mixed gas was passed into the reactor, and the inlet flow rate was 0.8 mL/min. The dialysis bag trapped molecules with a molecular weight of 8000–14 000. Methylene blue was an analytically pure reagent and used directly without purification. Deionized (DI) water was provided by the lab. The chemical components and particle sizes of SS are listed in Tables and 2, respectively.

Table 1

Chemical Composition of the SS

oxide	CaO	Fe2O3	SiO2	MgO	MnO	P2O5	Al2O3	V2O5	others
content (wt %)	37.10	20.60	15.80	7.06	3.53	3.44	2.20	0.88	8.07

Table 2

Particle Size Distribution of SS (μm)

size	SS
D10	1.22
D50	3.15
D90	6.27

Methods

Using SS to Remove SO2

First, 100.00 mL of DI water was placed in a three-necked flask on a thermostatic heating magnetic stirrer. At a solid/liquid ratio of 1:10, 10.0000 g of SS was added to the beaker and stirred at a steady rate, and a pH meter was used to record changes in pH online. After the pH value was stable, a mixed gas of SO2 and O2 at a flow ratio of 1:1 was introduced at a flow rate of 0.8 mL/min, and changes in slurry pH were monitored online to obtain a curve of slurry pH vs time. When the pH value stabilized, ventilation was stopped and the reaction was finished. The resulting slurry was vacuum-filtered to yield a solid product and a filtrate. The solid product was placed in an oven at 105 °C, dried to a constant weight, and bagged and sealed for testing. The filtrate was put in a dialysis bag and placed in water for dialysis to extract silica gel. Each experiment was performed three times, and the results are expressed as mean ± standard deviation. The experimental setup is shown in Figure .

Figure 1

Schematic diagram of the experimental apparatus. Note: (1) SO2 + N2 cylinder; (2) O2 + N2 cylinder; (3) gas flow meter; (4) gas mixing tank; (5) magnetic stirring; (6) reaction glass container; (7) condenser; (8) pH meter; (9) NaOH solution.

Silica Gel Extraction Process

The dialysis bag containing the desulfurized filtrate was soaked in a beaker containing DI water, and the water was continuously changed. Before changing the water, a conductivity meter was used to measure the conductivity of the DI water and the DI water in the dialysis bag. When the two values were similar, dialysis was considered complete. Next, the solution in the dialysis bag was divided into two parts; one part was poured into a beaker to allow most of the water to escape via steam and then placed in an oven for drying; the other part was frozen and placed in a freeze dryer. Silica gel particles obtained by both treatment methods were sealed in bags for testing. A flow diagram of SS desulfurization and extraction of silica gel is shown in Figure .

Figure 2

Flow diagram of SS desulfurization and extraction of silica gel.

Adsorption of Methylene Blue by Silica Gel

Methylene blue solution was mixed with silica gel particles and then placed on a magnetic stirrer and stirred for a set time. After stirring, the mixed solution was centrifuged for 6 min in a high-speed refrigerated centrifuge. After centrifugation, the absorbance of the supernatant was measured at 662 nm. All experiments were performed three times, and the absorbance value was averaged.

Characterization

The pH was monitored with a pH Meter (PHS-3E, China). Chemical compositions of SS were analyzed by X-ray fluorescence (S8 Tiger, Bruker, Germany), utilizing a generator voltage of 50 kV and a tube current of 40 mA. The diameter of the irradiation hole was 20 mm. Particle sizes of SS were determined by a laser particle size analyzer (Mastersizer 3000, Malvern, Germany). The mineralogical compositions of SS and desulfurization products were analyzed by an X-ray diffractometer (D2-Phaser, Bruker, Germany) with a Cu Kα source at 40 kV and 40 mA. The divergence slit was fixed at 0.38 mm, and a diffraction angle of 10–80° was scanned at a rate of 0.02° s–1. The morphologies of SS and the desulfurization products were determined with a scanning electron micrograph coupled with an energy-dispersive X-ray spectrometer (SEM-EDS, JSM-IT500HR, Japan). The sample was mounted on the copper sample holder with conducting resin, and the sample surface was coated by gold spraying. The thermal weight loss of desulfurization products was measured by thermal gravimetric analysis (Pyris 1, PerkinElmer) with an Al2O3 crucible. The N2 pressure was 0.2–0.3 MPa, and the purge gas of N2 was at 0.3 MPa. The heating rate was 10 °C/min. The heating temperature was from 50 to 850 °C. The functional groups of the materials after the desulfurization reaction was characterized by a Fourier transform infrared spectrometer (PerkinElmer 1730). The FT-IR spectra of all samples were collected over a wavenumber from 400 to 4000 cm–1 with the KBr pellet method. The sulfur contents in the raw materials and desulfurization products were determined by an automatic sulfur analyzer (Sundy, China). The absorbance was measured by an ultraviolet–visible spectrophotometer (UV-2600i, Shimadzu, Japan). The specific surface areas and pore structure parameters were obtained by measuring the N2 adsorption isotherm of silicone at liquid nitrogen temperature on a physical sorbent (ASAP2460) using the monolayer adsorption and capillary condensation theory. All samples were preprocessed before characterization, and samples were degassed for 1 and 12 h at 90 and 120 °C, respectively.

Results and Discussion

pH Changes during Desulfurization

Various reactions occurred during the desulfurization process as follows Figure (7)

Figure 3

pH–t curves of hydrolysis and desulfurization reactions.

At the SS hydrolysis stage, reaction mainly occurred, during which the concentration of OH– in the slurry increased rapidly, and the pH increased rapidly to 12.03 within 300 s. However, when the reaction time was extended, the pH remained unchanged. This may be because the mass of alkaline substances dissolved in SS reached a threshold. The desulfurization reaction is divided into five stages. In stage a, reactions mainly occurred. As SO2 dissolved into water, acid–base neutralization occurred, and the pH of the slurry decreased. In stage b, as the amount of SO2 dissolved was increased, the rate of pH decrease speeded up, and a slow pH change plateau then appeared. This is because the slurry was highly alkaline and it took a certain time for SO2 to dissolve; hence, both were too late to react. In the c stage, the pH value decreased rapidly from 9.49 to 5.64. This was because SO2 dissolved in water and acid–base neutralization reaction occurred. The main reactions in this stage were 3–9. There were small fluctuations after the rapid decrease in pH because SO2 was not added to the slurry in time. In stage d, the rate of pH decrease slowed down again. This was because the continuous introduction of SO2 reduced the amount of reactive alkaline substances in the slurry; hence, the reaction slowed down. When the reaction time reached 1800 s, the pH was 3.48, and it did not change as the reaction time was extended. At this time, the desulfurization reaction had essentially finished. In stage e, to verify whether the desulfurization reaction was complete, the introduction of SO2 was stopped at 1800 s, and the pH value of the slurry tended to increase and stabilized at around 7.10 with no further change. This showed that the alkaline substances in the slurry had reacted fully, and the desulfurization reaction was over.

Sulfur Fixation Ratio

The sulfur fixation ratio of SS was defined as the mass of SO2 removed by SS per unit mass, calculated using eq where ζ represents the desulfurization efficiency, Wproducts,SO represents the percentage of SO2 in the products, WSS represents the percentage of SS in the product, and Wraw material,SO represents the percentage of SO2 in the SS raw material.

Data measured by the sulfur analyzer were entered into eq , and the relevant parameters and calculation results are shown in Table . SS with a median diameter of 3.15 μm was used to remove SO2; the sulfur fixation ratio was 40.67%; this means that 1 ton of SS can fix 406.7 kg of SO2, a SO2 removal efficiency typical of existing calcium-rich desulfurizers. In other words, if limestone desulfurization was replaced with SS of equal desulfurization ratio, CO2 emission can be reduced by 279.6 kg and limestone can be reduced by 635.5 kg.

Table 3

Desulfurization Efficiency Parameters of SS

	sulfur content was calculated as SO3 (%)
	1	2	3	average	sulfur content was calculated as SO2 (%)	desulfurization efficiency (%)
SS	1.12	0.95	1.08	1.05	0.84	40.67
desulfurization products	38.28	36.52	36.65	37.19	29.75

Yield of Silica Gel

The yield of coproduced silica gel after desulfurization was defined as the mass of silica gel extracted per unit mass of SS after the desulfurization reaction. The yield of silica gel was calculated according to eq where η represents the yield of silica gel, msilica gel (g) represents the mass of silica gel generated after the desulfurization reaction, and mSS (g) represents the mass of SS used in the desulfurization reaction. After weighing and calculation, the yield of silica gel in this study was 5.1%. Thus, ∼51 kg of silica gel could be extracted per ton of SS after desulfurization. At present, the price of industrial-grade ordinary silica gel on the market is ∼32 000 yuan per ton, and the new value of the byproduct of desulfurization of SS is ∼1600 yuan per ton, making it of significant economic value.

Analysis and Characterization of Desulfurization Solid Products

FT-IR Analysis

Figure shows the FT-IR spectra of the solid-phase product and silica gel after desulfurization. The absorption peaks observed at 3435 and 1639 cm–1 can be attributed to the stretching vibration band of −OH in silica gel (Figure a). There was a strong absorption peak at 1087 cm–1, which was assigned to the antisymmetric vibration absorption peak of the Si–O–Si bond in silica gel. The adsorption peak at 799 cm–1 was related to the symmetric vibration absorption peak of Si–O–Si.[16−18] It can be seen from Figure that there was no obvious impurity peak in the FT-IR spectra of the prepared silica gel; hence, the prepared silica gel was of high purity. The stretching vibration absorption of O–H occurred at 3406 cm–1, and the deformation vibration of H–O–H occurred at 1620 cm–1 for crystal water in calcium sulfate (Figure b). The presence of the adsorption band at 1147 cm–1 was related to the in-plane bending vibration of SO42–, and the absorption bands at 652 and 602 cm–1 were due to the discrete peak of the out-of-plane bending vibration.[19] The absorption bands at 990 cm–1 corresponded to the vibration absorption of Si–O–Si, and the absorption bands at 953 cm–1 corresponded to the vibration absorption of Si–OH. This indicated that part of the silica gel generated after the desulfurization reaction also existed in the solid-phase products. The absorption bands observed at 1423 cm–1 can be attributed to the antisymmetric stretching vibration peak of the C–O group of CaCO3 in raw SS.[20−22]

Figure 4

FT-IR spectra of (a) liquid products of desulfurization and (b) solid products of desulfurization.

XRD Analysis

The XRD results for SS raw materials and desulfurization products are shown in Figure . Strong diffraction peaks for Ca(OH)2 and CaCO3 were detected in SS samples. Obvious diffraction peaks for dicalcium silicate (C2S) and tricalcium silicate (C3S) and relatively weak diffraction peaks for Ca2Fe2O5 and the RO phase were also detected in SS.

Figure 5

XRD patterns of (a) SS raw material and (b) solid products of desulfurization.

The diffraction peaks at 2θ of 14.6, 20.8, 25.5, and 29.8° in the desulfurization product were from CaSO4·2H2O, and the diffraction peaks at 31.8, 34.0, and 53.9° were from CaSO4·0.5H2O. The diffraction peaks at 15.9, 16.5, 18.2, 23.4, 28.1, and 36.4° were from CaSO3·0.5H2O, among which CaSO3·0.5H2O had the largest number of diffraction peaks and peaks with the strongest intensity. From the above analysis, it can be seen that after the desulfurization reaction, Ca(OH)2, CaCO3, C2S, and C3S in SS participated in the reaction and generated CaSO4·2H2O, CaSO4·0.5H2O, and CaSO3·0.5H2O.

SEM Analysis

Comparison with raw SS revealed obvious block-shaped crystals in the desulfurization product (Figure ). Combined with the EDS analysis results, we can conclude that these crystals were CaSO4. CaSO4 was tightly wrapped on the surface of SS, so that the active sites of SS that participated in the desulfurization reaction were reduced, which will inevitably hinder subsequent desulfurization reactions, which may be the main reason for reducing the sulfur fixation ratio of SS.

Figure 6

SEM images of (a) SS raw material and (b) solid products of desulfurization and (c) EDS analysis of solid products of desulfurization.

TG–DTG Analysis

To study the thermal weight loss characteristics of the solid-phase products of desulfurization, solid-phase products were analyzed by TG–DTG, and the results are shown in Figure . Solid-phase products displayed four obvious weight loss stages. In the first stage, the temperature range of weight loss was 35.5–100.2 °C, the peak temperature of weight loss was 93.9 °C, and the weight loss ratio was 3.5%. This was due to the loss of free water in solid products. In the second stage, the temperature range of weight loss was 100.2–252.6 °C, the peak temperature of weight loss was 110.1 °C, and the weight loss ratio was 4.0%. This was due to the loss of crystal water from solid products. In the third stage, the temperature range of weight loss was 329.5–433.7 °C, the peak temperature of weight loss was 391.2 °C, and the weight loss ratio was 2.8%. This was due to the thermal decomposition of Ca(OH)2 in raw SS that was not involved in the desulfurization reaction.[23,24] In the fourth stage, the temperature range of weight loss was 627.3–747.6 °C, the peak temperature of weight loss was 721.5 °C, and the weight loss ratio was 1.2%. This was due to the thermal decomposition of CaCO3 in SS that did not participate in the desulfurization reaction.[25−28] From the above analysis, we can conclude that part of Ca(OH)2 and CaCO3 did not participate in the desulfurization reaction in the system. This may be because the desulfurization product particles covered the surface of SS particles and prevented further progress of the reaction; hence, it is essential to reduce the SS particle diameter using a supersonic steam-jet smasher to expose more reactive active sites. The weight loss ratio of the whole process was 11.5%.

Figure 7

TG–DTG curves of solid products of desulfurization.

Analysis and Characterization of Silica Gel

The liquid-phase product was heated, evaporated to dryness, freeze-dried, and then analyzed by SEM-EDS, and the results are shown in Figure . The surface of the products obtained by direct evaporation is dense, and a large number of particles are randomly accumulated on the solid surface (Figure a). Combined with EDS analysis, we can conclude that these particles were SiO2 and that silica gel (mSiO2·nH2O) was formed following desulfurization. A large number of random accumulations of particulates on the surface of the freeze-dried sample are evident (Figure b). Also, EDS analysis showed that this was an amorphous SiO2 particulate. The difference is that pores appeared on the sample surface after freeze-drying. This is because silica gel is a colloidal substance that swells after absorbing water and shrinks after dehydration;[29] hence, the pores remain after freeze-drying. This shows that silica gel has the potential for adsorption.

Figure 8

SEM-EDS images of the liquid product evaporated residues (a) and freeze-dried products (b).

The results of XRD analysis of the silica gel sample are shown in Figure a. There was a significant diffused peak in the XRD spectrum, which was attributed to the amorphous SiO2 peak cluster,[30,31] indicating that the prepared silica gel sample consisted of amorphous SiO2. The silica gel had an amorphous structure. No other sharp crystal diffraction peaks were observed in the XRD spectrum. This showed that the purity of the prepared silica gel was relatively high. As shown in Figure b, the laser lamp passed cleanly through the liquid-phase product; hence, a Tindal effect occurred, which can infer the presence of colloids in the desulfurization product. Together with the results discussed above, this proved that the desulfurization product was silica gel.

Figure 9

XRD analysis of silica gel (a) and Tyndall phenomenon (b).

BET Analysis

According to IUPAC classification, the N2 adsorption and desorption isotherms of silica gel were similar to type IV isotherms (Figure a).[30,32] In the area of P/P0 < 0.6, the curve was convexed upward, similar to the type II isotherm. In the region of P/P0 > 0.6, the adsorption and desorption isotherms showed a rapid upward trend, which was caused by the capillary condensation of the adsorbate. When all pores had coalesced, adsorption only occurred on the outer surface, but because the outer surface was much smaller than the inner surface, the curve was flat. Close to a relative pressure of 1, when adsorption occured at large pores, the curve raised. The desorption and adsorption isotherms did not overlap, and the desorption isotherm was located above the adsorption isotherm; hence, a desorption hysteresis (a hysteresis loop) appeared (Figure a). The reason for this was that capillary condensation occurred in this region, causing a hysteresis, which was related to the shape of the pore and its size. The arrest loop belonged to type H3; hence, there was no obvious saturation adsorption platform in the isotherm due to the irregularity of the sample pore structure. As shown in Figure b, the aperture distribution of silica gel was mainly an interpore of 2–24 nm.

Figure 10

Adsorption–desorption isotherms of N2 (a) and pore-size distribution of silica gel (b).

The pore structure parameters of silica gel prepared in this research and commercial fine-pored silica gel and B-type silica gel were compared. The pore structure of B-type silica gel was between coarse-pored and fine-pored silica gel (Table ). The pore structure parameters of the silica gel prepared in this research were close to those of commercial B-type silica gel, but the specific surface area was slightly smaller. This may be because the prepared silica gel was not been activated at high temperatures.

Table 4

Pore Structural Parameters of Silica Gel

	SBET (m2·g)	Smic (m2·g)	Vt (cm3·g–1)	Dav (nm)
this study	334.6	20.9	0.555	6.64
commercial fine-pored silica gel	650–800		0.35–0.40	2.0–3.0
commercial B-type silica gel	450–650		0.60–0.85	4.5–7.0

TG–DTG Analysis

To investigate the thermal weight loss properties of silica gel, we performed a TG–DTG analysis of the obtained products, and the results are shown in Figure . Silica gel displayed a significant weight loss in the range of 19.7–109.6 and 109.6–194.6 °C, with weight loss ratios of 13.1 and 4.6% and weight loss peak temperatures of 56.4 and 122.5 °C, respectively. This is due to the dehydration of silica gel.[33−35] Within a temperature range of 123–1000 °C, the TG curve showed a significant downward trend with a weight loss ratio of 10.7% and no significant peak temperature on the DTG curve. This suggested that silica gel dehydration also occurred in this temperature range. This is because with an increase of temperature, the silanol group on the surface of silica gel began to dehydrate and formed the −Si–O–Si bond.

Figure 11

TG–DTG curves of silica gel.

Desulfurization Mechanism

Based on the above results, we concluded that the process of using SS to remove SO2 involves three stages. In the first stage, when the flue gas enters the SS slurry, SO2 quickly dissolves into the water to generate SO32–, HSO3–, and H2SO3.[36] SO32– is oxidized to SO42– under the action of dissolved oxygen. The specific process can be described as follows:[37] (1) SO2 and O2 diffuse from the gas phase to the gas–liquid interface; (2) SO2 and O2 dissolve in the liquid phase according to Henry’s law; (3) SO2 is hydrated with water to form sulfurous acid, and it then ionizes to produce H+, SO32–, and HSO3–; (4) H+, SO32–, HSO3–, and O2 dissolve in the liquid phase; and (5) SO32– and HSO3– are oxidized to SO42– by dissolved oxygen in the liquid phase. In the second stage, free CaO in the SS particles is hydrolyzed in the slurry to generate Ca(OH)2, which releases a large amount of Ca2+ in the slurry. In an environment where SO2 is dissolved in the slurry to generate acidic conditions, C2S, C3S, and Ca2Fe2O5 also release Ca2+. The content of Fe in the SS used in this study was 20.6% as Fe2O3, the second-most abundant element. According to previous reports,[8,38] Fe promotes the desulfurization reaction, which is another advantage of SS replacing limestone in flue gas wet desulfurization. In the third stage, Ca2+ reacts with SO32– and SO42– to generate block-shaped CaSO4·2H2O, CaSO4·0.5H2O, and CaSO3·0.5H2O, which adhere to the surface of SS particles. The generated silica gel is a colloidal substance with a particle size <100 nm, which can enter the filtrate through the pores of the filter paper. The mechanism of removing SO2 by SS is shown in Figure .

Figure 12

Schematic diagram of the mechanism of removing SO2 by SS.

Optimization of Reaction Conditions for Adsorption of Methylene Blue on Silica Gel

Influence of Silica Gel Dosage on Removal Ratio

At a methylene blue concentration of 0.012 mg/mL, room temperature, and a reaction time of 30 min, the influence of the dosage of silica gel on the removal ratio of methylene blue was investigated, and the results are shown in Figure a.

Figure 13

Effects of (a) silica gel dosage, (b) initial concentration of methylene blue, and (c) pH value on removal ratio.

At silica gel dosages of 20, 25, 30, 35 and 40 mg, the removal ratios of methylene blue were 97.4, 97.8, 98.4, 96.7, and 97.2%, respectively. The dosage of silica gel had little effect on the removal ratio of methylene blue, and the optimal dosage was 30 mg. When the dosage was <30 mg, the adsorbent dosage may be too low; hence, the removal ratio was low; when the dosage was >30 mg, because silica gel is a colloidal substance, it swells after absorbing water; hence, it will be completely suspended in the upper phase following centrifugation, resulting in high absorbance and low removal ratio.

Effect of Initial Methylene Blue Concentration on Removal Ratio

At a silica gel dosage of 30 mg/mL, room temperature, and a reaction time of 30 min, the effect of the initial concentration of methylene blue on the removal ratio was investigated, and the results are shown in Figure b. At methylene blue concentrations of 0.004, 0.008, 0.012, 0.016, and 0.020 mg/mL, the removal ratios of methylene blue were 98.0, 98.0, 95.9, 97.7, and 98.3%, respectively. The initial concentration of methylene blue had little effect on the removal ratio.

Effect of pH on Removal Ratio

At a silica gel dosage of 30 mg/mL, room temperature, and a reaction time was 30 min, the effect of methylene blue pH value on the removal ratio was investigated, and the results are shown in Figure c.

When the pH levels of methylene blue were 2.00, 4.00, 6.00, 8.00 and 10.00, the removal ratios of methylene blue were 95.8, 98.5, 98.3, 98.5, and 98.4%, respectively. The removal ratio of methylene blue was not changed significantly, except for the low removal ratio at pH 2.00. Thus, strong acidic conditions were not conducive to the adsorption of methylene blue. This may be because under acidic conditions, H+ in solution may compete with methylene blue cations for adsorption, and the adsorbent surface would become positively charged, which would generate electrostatic repulsion of methylene blue cations, resulting in low adsorption capacity and low removal ratio. When the pH is increased, hydrogen bonding between the hydroxyl groups and methylene blue cations on the surface of silica gel is strengthened, the surface of the adsorbent is negatively charged, and electrostatic attraction between the surface of the adsorbent and methylene blue cations will increase the adsorption capacity and the removal ratio.

Mechanism of Silica Gel Adsorption of Methylene Blue

The mechanism of the adsorption of methylene blue on the silica gel byproduct of SS desulfurization was considered. The adsorption capacity of a solid adsorbent depends not only on the specific surface area and pore structure but also on the surface characteristics (i.e., the functional groups on the surface of the adsorbent). There are six mechanisms for describing adsorption on a solid surface: electrostatic interaction, ion exchange, ion–dipole interaction, surface metal cation coordination, and hydrophobic interaction.[30,39] The main force between silica gel and methylene blue is electrostatic attraction. The Si–OH groups on the surface of silica gel ionize to give generate H+ and [SiO]−, which combines with the cationic dye MB+ through the attraction between positive and negative charges. Under acidic conditions, the amount of H+ in the solution is large, and this cation may compete with MB+ for adsorption on the surface of silica gel, which will reduce the adsorption rate of MB+. However, when the pH of the solution is increased, the reaction between OH– and H+ will increase the number of [SiO]− groups on the surface of the silica gel, which will inevitably increase the MB+ adsorption ratio. This is consistent with the results described above. In addition, hydrogen bonds form between the H atoms of the Si–OH groups on the surface of the silica gel and the N atoms of the methylene blue molecules. Thus, both electrostatic attraction and hydrogen bonding may occur between silica gel and methylene blue molecules. This conclusion is consistent with previous research.[39,40] A schematic diagram of the adsorption process is shown in Figure .

Figure 14

Schematic diagram of the adsorption of methylene blue on the surface of silica gel: (a) electrostatic interaction and (b) hydrogen bonding.

Conclusions

SS has a high Ca content, and the slurry is strongly alkaline; hence, it can be used for the removal of SO2 and can replace natural ore limestone in the limestone–gypsum desulfurization process of thermal power plants, thereby avoiding discharge of CO2 from limestone during the desulfurization process. This is compatible with the “use waste to treat waste” concept, and it can also produce high-value-added silica gel as a byproduct. Our study mainly examined the sulfur fixation ratio of SS for removing SO2, the yield of silica gel, and the optimal conditions for silica gel adsorption of methylene blue. The main conclusions are as follows:

At a solid–liquid ratio of 1:10, room temperature, and a constant stirring speed, SS with a D50 of 3.15 μm was used to remove SO2. The results showed that 1 ton of SS can remove 406.7 kg of SO2 and yield 51.0 kg of silica gel. In other words, 1 ton of SS can remove 406.7 kg of SO2, reduce CO2 emissions by 279.6 kg, and save 635.5 kg of limestone.

FT-IR, XRD, SEM-EDS, and TG–DTG showed that the main products following desulfurization were CaSO4·2H2O, CaSO4·0.5H2O, CaSO3·0.5H2O, and silica gel.

The optimal methylene blue adsorption conditions are a silica gel dosage of 30 mg, a temperature of 20 °C, a pH of 6.00, a stirring time of 0.5 h, and a methylene blue concentration of 0.02 mg/mL. The removal ratio of methylene blue adsorbed by silica gel was 98.4%.

Two forces (electrostatic attraction and hydrogen bonding) appear to occur between silica gel and methylene blue molecules, but the most important force is electrostatic attraction. Thus, Si–OH groups on the surface of the silica gel ionize to generate H+ and [SiO]−, which combines with the cationic dye MB+ through the adsorption of positive and negative charges. In addition, hydrogen bonds are formed between H atoms of Si–OH groups on the surface of the silica gel and N atoms of the methylene blue molecules.