Preparation of HCl Gas Scavenger from Blast Furnace Slag through Alkali Fusion

Abstract

Blast furnace (BF) slag, one of the byproducts of iron- and steel-making plants, was converted to a product, including a hydrogrossular, through the alkali fusion method for HCl gas removal. BF slag was transformed to the alkali-fused slag with reactive phases via alkali fusion, and then, the fused slag was added to distilled water and stirred at room temperature to prepare the precursor for the synthesis of the product including a hydrogrossular by heating. The effects of the mixing ratio of NaOH to slag (NaOH/slag ratio), fusion temperature, ratio of the fused slag mass to distilled water volume (W/V ratio), stirring time, heating time, and heating temperature of the product phase were investigated, and the HCl gas removal ability of the obtained product was determined. The optimal conditions for hydrogrossular synthesis are NaOH/slag ratio of 1.6, fusion temperature of 600°C, W/V ratio of 125 g/L, stirring time of 24 h, heating temperature of 80°C, and heating time of 3–6 h. The product removed more HCl gas than the BF slag and showed higher Cl fixation than lime. These results suggest that a novel scavenger for HCl gas removal at high temperature can be synthesized from the BF slag through alkali fusion.

1. Introduction

Blast furnace (BF) slag is a by-product generated in the iron making process in which approximately 290 kg of BF slag are produced per ton of pig iron.¹⁾ Approximately 80 million tons of pig iron is produced annually in Japan, generating approximately 23 million tons of BF slag as the by-product.²⁾ The BF slag has been mostly reused as a raw material for cement, roadbed, and concrete aggregate. However, the use of the BF slag in these fields tends to be saturated; therefore, exploring new uses of the BF slag has become increasingly important.

Synthesis of functional materials from the BF slag has been widely attempted. Takazawa et al.³⁾ synthesized zeolite-X by an alkali reaction from the precursor that was obtained from the BF slag via acid treatment. Moreover, Sugano et al.⁴⁾ synthesized zeolite-A from the BF slag by an alkali hydrothermal treatment using a ball mill-type reaction vessel. Nakayama et al.⁵⁾ synthesized monolithic tobermorite from the BF slag by hydrothermal synthesis, whereas Wajima et al.⁶⁾ synthesized 11 Å tobermorite from the BF slag by using an alkali hydrothermal reaction with ethylenediaminetetraacetic acid (EDTA), and Hongo⁷⁾ synthesized allophane from the BF slag. Kuwahara et al.⁸⁾ reported the synthesis of a hydrotalcite-like compound from the BF slag through co-precipitation via an acid treatment. In these studies, the contents of Si, Al and Ca in BF slag was dissolved into the solution using hydrothermal condition or acid reaching pre-treatment to occur the synthesis reaction between these contents of BF slag. However, hydrothermal method is partly conversion of BF slag due to the reaction on the surface of BF slag, and acid leaching method is complicated process using both acid and alkali solutions. In previous studies, Wajima et al.^9,10,11) synthesized a hydrocalumite-like compound from the BF slag through alkali fusion to remove harmful anions. Alkali fusion is one of the attractive method for value-added conversion of BF slag because a large amount of BF slag can be treated continuously using a rotary kiln and there is a possibility to use waste heat of steelworks for this dry process.

Incineration is one of the most environmentally benign methods for hazardous waste disposal, provided the problems associated with incineration are assessed and dealt with. In particular, air pollutants such as hydrogen chloride (HCl), sulfur oxides (SO_x), nitrogen oxides (NO_x), and other contaminants must be removed. In recent years, considerable attention has been devoted to reducing the level of atmospheric pollution caused by HCl derived from the incineration of poly (vinyl chloride) plastics. The present scrubbing technologies for the removal of HCl that use calcium hydroxide [Ca(OH)₂] or calcium carbonate (CaCO₃) sorbents to capture HCl as CaCl₂ are relatively simple, easy to operate, and have low capital costs.^12,13,14) However, CaCl₂ scales are generally deposited on the reactor wall, leading to problems in the incineration, and the elution of CaCl₂ from the ash after incineration at the dumping sites causes environmental problems. Therefore, it is important to develop materials other than Ca(OH)₂ or CaCO₃ for HCl emission control.

The reaction of HCl gas with hydrogrossular is interesting for controlling acid gas emission from combustion processes, most notably municipal waste combustion and hazardous waste incineration.¹⁵⁾ The hydrogrossular group [Ca₃Al₂(SiO₄)_3−x(OH)_4x (0 < x < 3)] consists of solid solution compounds ranging from grossular (x = 0, Ca₃Al₂Si₃O₁₂) to katoite (x = 3, Ca₃Al₂(OH)₁₂). Katoite is the only thermodynamically stable calcium aluminate hydrate formed in calcium aluminate cements and consists of a three-dimensional framework constructed of Al(OH)₆ octahedra and Ca(OH)₈ dodecahedra. The 4(OH)⁻ group may be substituted by (SiO₄)⁴⁻ in the structure, resulting in the formation of intermediate compositions between Ca₃Al₂Si₃O₁₂ and Ca₃Al₂(OH)₁₂. This katoite hydrogrossular is a new solid sorbent candidate for the HCl gas. It is a hydration product in solidified cement pastes^16,17,18,19) and can also be synthesized by the hydrothermal treatment of by-products such as coal ash or molten slag.^20,21) It was found that hydrogrossular sorbents in a fixed-bed reactor can reduce the level of HCl gas emission to near zero at 400–950°C, and the reaction with HCl converts hydrogrossular to wadalite (Ca₁₂Al₁₀Si₄O₃₂Cl₆) and CaCl₂ at >400°C,¹⁵⁾ as follows; 5Ca₃Al₂(SiO₄)_0.8(OH)_8.8 + 12HCl → Ca₁₂Al₁₀Si₄O₃₂Cl₆ + 3CaCl₂ + 19H₂O. Chloride ions are fixed in the wadalite structure after heating at high temperatures. Additionally, such high-temperature removal of HCl can lead to the control of the downstream formation of hazardous by-products such as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs).²²⁾ Mayenite (Ca₁₂Al₁₀Si₄O₃₂(OH)₆), which is formed by the calcination of hydrogrossular at the temperatures above 680°C (5Ca₃Al₂(SiO₄)_0.8(OH)_8.8 → Ca₁₂Al₁₀Si₄O₃₂(OH)₆ + 3CaO + 19H₂O), was found to combust volatile organic compounds (VOCs) such as benzene, toluene, and propylene at above 400°C and decomposed other VOCs (chlorobenzene) to carbon oxides (CO₂ and CO) and H₂O.²³⁾ Chloride ions formed from the decomposition of chlorobenzene were captured by mayenite, resulting in the formation of wadalite,^23,24) as follows; Ca₁₂Al₁₀Si₄O₃₂(OH)₆ + 6HCl → Ca₁₂Al₁₀Si₄O₃₂Cl₆ + 6H₂O. Therefore, hydrogrossular is more promising than conventional HCl sorbents because of its favorable performance for both HCl fixation and hydrocarbon combustion.

In recent years, many studies have been devoted to investigating the ability of hydrogrossular to remove HCl gas at high temperatures, providing a new approach to convert the slag to functional materials. There is a possibility to synthesize hydrogrossular from BF slag with Ca, Si and Al contents. However, there have been no reports on the synthesis of a hydrogrossular compound from the BF slag via alkali fusion, and no information is available on the optimal synthesis conditions. Elucidation of the optimal reaction conditions is important for designing manufacturing equipment and operating conditions.

In this study, conversion of the BF slag into a hydrogrossular compound by alkali fusion was investigated. The main purpose of this study is to determine the optimal synthesis conditions and the feasibility of utilization of the product including hydrogrossular as an HCl scavenger. The investigated synthesis conditions include the fusion temperature, ratio of the NaOH powder to slag (NaOH/slag), ratio of the fused powder mass to solution volume (W/V ratio), stirring time, heating temperature, and heating time. In addition, the ability of the product to remove HCl gas at high temperature was examined for its possible application in effluent gas treatment.

2. Experimental

2.1. Materials

The BF slag can be classified into air- and water-cooled slag according to the cooling method, with these slags consisting of crystalline and amorphous phases, respectively. To ensure that the phases in the slag were converted to the functional phases by alkali fusion, the BF slag used in this study (received from a steel-making plant in Japan) was air-cooled. The BF slag was ground in a mill and sieved to under 1 mm. The chemical and mineralogical compositions of the raw slag were determined by X-ray fluorescence spectrometry (XRF; XRF-1700, Shimadzu, Japan) and X-ray diffraction (XRD; RINT-2500, Rigaku, Japan), respectively (Table 1, Fig. 1). Raw slag was mainly composed of CaO (41.5%), SiO₂ (33.9%), and Al₂O₃ (14.2%) found as calcite (CaCO₃), gehlenite (Ca₂Al₂SiO₇), and larnite (Ca₂SiO₄). Other oxides such as MgO, SO₃, Fe₂O₃, TiO₂, K₂O, and MnO were present in smaller amounts.

Table 1. Chemical composition of raw slag, fused slag, precursor, and product.

Oxide (wt.%)	Raw slag	Fused slag	Precursor	Product
CaO	41.5	15.8	41.7	42.4
SiO2	33.9	12.7	31.0	33.3
Al2O3	14.2	5.3	12.5	13.5
MgO	6.6	2.0	6.6	6.6
K2O	0.3	0.2	0.1	0.1
Fe2O3	1.1	0.8	1.0	0.8
SO3	1.3	1.0	0.2	0.1
Na2O	–	60.9	6.1	2.3

Fig. 1.

XRD patterns of BF slag.

2.2. Synthesis

To investigate the relationship between the synthesis conditions (fusion temperature, NaOH/slag ratio, W/V ratio, stirring time, heating time, and heating temperature) and product phases, a three-step synthesis process comprised of alkali fusion, aging, and crystallization was carried out, as shown in Fig. 2.

Fig. 2.

Flow chart of the experiment.

In the alkali fusion process, the slag (10 g) was mixed with NaOH (4, 8, 12, 16, and 20 g) for NaOH/slag ratios of 0.4, 0.8, 1.2, 1.6, and 2.0 g/g, respectively, and was ground to obtain a homogeneous mixture. The mixture was then heated in a nickel crucible in air at 200, 400, 600, and 800°C for 1 h. The obtained fused mixture was cooled to room temperature and ground again to obtain the alkali-fused slag.

In the aging process, the fused slag (1.25, 2.5, or 5 g) was added to distilled water (20 mL, W/V ratios of 62.5, 125 and 250 g/L, respectively) in a 50 mL polymethylpentene bottle and stirred with a magnetic stirrer at room temperature for 3, 6, 12, 24, and 48 h to prepare the precursor solution. It is noted that atmospheric carbon dioxide was dissolved into the solution on this process.

In the crystallization process, the precursor solution (200 mL) was prepared from the slag fused at 600°C with 1.6 times NaOH after 24 h-stirring at a W/V ratio of 125 g/L. To examine the effect of the heating temperature, the prepared precursor solution (10 mL) was added into the pressure vessel, and then placed in an electric furnace at 80, 120, and 160°C for 6 h. After heating, the solid product was filtered, washed with distilled water, and dried in a drying oven at 60°C overnight. To examine the effect of the heating time, the prepared precursor solution (200 mL) was heated in a water bath at 80°C with stirring. During stirring, a part of the slurry (2 mL) was collected, and the solid product was filtered, washed with distilled water, and dried in the drying oven at 60°C overnight, and the filtrate solution at each collecting time was obtained.

The product phases and morphologies were analyzed by XRD and scanning electron microscopy (SEM; S-4500, Hitachi, Japan), respectively. Concentrations of Si, Al, and Ca in the filtrate were determined by inductively coupled plasma (ICP, SPS3000, Seiko instruments, Japan).

The change in the amount of the mineralogical phases in the product during the crystallization process was represented as relative crystallinity calculated using the intensity of the major XRD peaks in the main products at the: hydrogrossular (4 2 0), hydrocalumite (0 0 2), and calcite (1 0 4) given diffraction planes, as follows   

$$\begin{array}{l}\mathrm{R}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{\hspace{0.17em} } \mathrm{c}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{\hspace{0.17em} } (\%)=\\ (\mathrm{I}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{\hspace{0.17em} } \mathrm{o}\mathrm{f}\mathrm{\hspace{0.17em} } \mathrm{m}\mathrm{a}\mathrm{j}\mathrm{o}\mathrm{r}\mathrm{\hspace{0.17em} } \mathrm{X}\mathrm{R}\mathrm{D}\mathrm{\hspace{0.17em} } \mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k}\mathrm{s}\mathrm{\hspace{0.17em} } \mathrm{i}\mathrm{n}\mathrm{\hspace{0.17em} } \mathrm{t}\mathrm{h}\mathrm{e}\mathrm{\hspace{0.17em} } \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t})/\\ (\mathrm{I}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{\hspace{0.17em} } \mathrm{o}\mathrm{f}\mathrm{\hspace{0.17em} } \mathrm{m}\mathrm{a}\mathrm{j}\mathrm{o}\mathrm{r}\mathrm{\hspace{0.17em} } \mathrm{X}\mathrm{R}\mathrm{D}\mathrm{\hspace{0.17em} } \mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k}\mathrm{s}\mathrm{\hspace{0.17em} } \mathrm{i}\mathrm{n}\mathrm{\hspace{0.17em} } \mathrm{t}\mathrm{h}\mathrm{e}\mathrm{\hspace{0.17em} } \text{standard})\times 100\end{array}$$

Hydrogrossular and hydrocalumite prepared using the procedure reported by Fujita et al.²⁵⁾ and Wajima,²⁶⁾ respectively, and commercial calcite (Wako, Japan) were used as the standards.

2.3. HCl Fixation

The experiments for HCl gas fixation at high temperature using raw slag, lime (Wako, Japan), and the products synthesized at 80, 120, and 160°C were performed in a vertical reactor, as shown schematically in Fig. 3. The reactor was a fused silica tube (17 mm i.d., 1-m long). The reactor was surrounded by an electric furnace to control its temperature. The sample (0.1 g) was placed in the reactor, and then N₂ was passed through the reactor at a rate of 1 L/min (for the volume measured at NTP) to replace the atmosphere in the reactor with N₂. The reactor was then heated to 800°C (this temperature is typical for a waste incinerator) with N₂ flowing through the reactor at 1 L/min. Once the required temperature was reached, the reaction gas (1000-ppmv HCl in N₂) was fed at 500 mL/min into the fixed-bed reactor. An HCl concentration of 1000 ppmv is similar to the average concentration in the exhaust gas of an incinerator in Japan. The gas exiting the reactor was passed through distilled water (500 mL) with 0.1% NaOH solution by bubbling, which was sufficient for all of the HCl in the gas to be dissolved. The pH of the solution was measured with a pH meter (D-53, Horiba, Japan) to determine the amount of the HCl gas fixed by the sample. The HCl feed gas was stopped at the end of the experiment, and N₂ was fed into the reactor until the reactor cooled to room temperature. Then, the solid sample in the reactor was collected, the mineralogical phases of the product after the HCl removal were analyzed, and the Cl elution test for each solid sample that was used in the HCl removal experiments was carried out. An aliquot (0.1 g) of the sample was added to distilled water (40 mL) in a 50-mL centrifuge tube, and the tube was shaken using a reciprocal shaker for 30 min. Then, the tube was centrifuged for 10 min, and the Cl⁻ concentration in the supernatant was determined (LAQUA, Horiba, Japan) in order to calculate the soluble Cl⁻ content of the solid sample.

Fig. 3.

Experimental apparatus for the HCl gas removal test.

3. Results and Discussion

3.1. Synthesis of Hydrogrossular

Figure 4 shows the XRD patterns of the BF slag, fused slag, precursor, and product. It is noted that the fusion conditions are the NaOH/slag ratio of 1.6 and fusion temperature of 600°C, the aging conditions are the W/V ratio of 125 g/L and stirring time of 24 h, and the crystallization conditions are the heating temperature of 80°C and heating time of 6 h. The slag was composed of the gehlenite, calcite, and larnite crystalline phases (Fig. 4(a)). After alkali fusion, most of the crystalline phases were converted to lime [CaO], portlandite [Ca(OH)₂], and soluble sodium salts such as sodium silicate hydrate [Na₂SiO₃‧9H₂O] and sodium aluminate silicate hydrate [Na₄Al₂Si₆O₁₇‧2H₂O] (Fig. 4(b)), and the precursor including calcite and hydrocalumite was formed after the aging process (Fig. 4(c)). Finally, by heating at 80°C for the crystallization of hydrogrossular was formed, and the product that included the hydrogrossular, hydrocalumite, and calcite crystalline phases was synthesized (Fig. 4(d)).

Fig. 4.

XRD patterns of (a) BF slag, (b) fused slag, (c) precursor, and (d) product.

Figure 5 shows the SEM micrographs of the raw slag, fused slag, precursor, and product. Although the slag is composed of rock-like particles (Fig. 5(a)), the fused slag consists of particles with a melted surface resulting from the formation of sodium salts by alkali fusion (Fig. 5(b)). After the aging process, the aggregates of thin regular hexagonal crystals such as the hydrocalumite-type compound were confirmed to be present in the precursor (Fig. 5(c)). Finally, the octahedral crystals of hydrogrossular are observed on the surface of the product (Fig. 5(d)).

Fig. 5.

SEM photographs of (a) BF slag, (b) fused slag, (c) precursor, and (d) product.

Table 1 show the results for the chemical composition of the BF slag, fused slag, precursor, and product. The slag mainly contains CaO (41.5 wt.%), SiO₂ (33.9 wt.%), and Al₂O₃ (14.2 wt.%), and some minor elements. After alkali fusion, the obtained fused slag contains higher amounts of Na₂O (60.9 wt%) and lower amounts of CaO (15.8 wt.%), SiO₂ (12.7 wt.%), and Al₂O₃ (5.3 wt%) than the raw slag due to the NaOH addition. After agitation and synthesis, the precursor and product have almost chemical composition identical to that of the raw material, implying that most elements in the slag were converted to new phases in the product. It is noted that the mixture of slag (10 g) and NaOH powder (16 g) was converted to the fused slag (24 g) to form a precursor (12.1 g) and to synthesize the product including hydrogrossular (11.6 g).

3.2. Alkali Fusion

The effect of alkali fusion conditions (NaOH/slag ratio and fusion temperature) on the hydrogrossular synthesis was examined. Aging and crystallization conditions were fixed at a W/V ratio of 125 g/L, stirring time of 24 h, heating temperature of 80°C, and heating time of 6 h. Figure 6 shows the XRD patterns of the product after alkali fusion with various NaOH/slag ratios (0.4, 0.8, 1.2, 1.6, and 2.0) of the added NaOH/slag mixtures, at 600°C, and crystallinities of the product phases in the product. Hydrogrossular, katoite [Ca₃Al₂(SiO₄)_x(OH)_3−x], hydrocalumite [Ca₄Al₂O₆(OH)₂‧11H₂O], and calcite [CaCO₃] were present in all products. At the NaOH/slag ratio of 0.4, high gehlenite peaks were still observed for the product. At the NaOH/slag ratios greater than 0.8, the gehlenite concentration decreased and a mixture of hydrogrossular, hydrocalumite, and calcite was obtained. The alkali fusion reaction between the BF slag and NaOH powder occurs to completion at the NaOH/slag ratios greater than 0.8, and the product obtained at the NaOH/slag ratio of 1.6 showed the highest crystallinity of hydrogrossular. The crystallinities of hydrocalumite and calcite are almost same at the ratio of higher than 0.8.

Fig. 6.

(a) XRD patterns of the product from BF slag after alkali fusion with NaOH at 600°C, and (b) crystallinities of the product phases in the product. The ratios of NaOH to BF slag are 0.4, 0.8, 1.2, 1.6, and 2.0.

Figure 7 shows the XRD patterns of the products at various alkali fusion heating temperatures (200°C, 400°C, 600°C, and 800°C) at a NaOH/slag ratio of 1.6, and the crystallinities of the product phases in the product. Aging and crystallization conditions were also fixed at the same above-mentioned conditions. At 200°C, high gehlenite peaks were still observed for the product, but these then decreased with a corresponding increase in the hydrogrossular peaks at the higher temperature of 400°C. This occurs because the alkali fusion reaction between the BF slag and NaOH powder is promoted by an increase in the fusion temperature. At 800°C, the intensities of the hydrogrossular peaks decreased. It was reported that alkali metal (sodium) release into the atmosphere occurs at approximately 800°C.²⁷⁾ It was considered that the sodium conditions such as content and chemical species in fused slag at 800°C were not suitable for the synthesis of hydrogrossular. The product obtained from the fused slag after alkali fusion at 600°C showed the highest and lowest peaks for hydrogrossular and gehlenite, respectively. The crystallinities of hydrocalumite and calcite are almost same regardless of fusion temperature.

Fig. 7.

(a) XRD patterns of the product from BF slag after alkali fusion with NaOH at a NaOH/slag ratio of 1.6 at 200°C, 400°C, 600°C, and 800°C, and (b) crystallinities of the product phases in the product.

Thus, the optimal fusion conditions for hydrogrossular synthesis were identified as the NaOH/slag ratio of 1.6 and fusion temperature of 600°C.

3.3. Aging

Figure 8 shows the XRD patterns of the products produced at the various W/V ratios and the optimal fusion conditions mentioned above for the stirring time of 24 h, heating temperature of 80°C, heating time of 6 h, and crystallinities of the product phases in the product. A mixture of hydrogrossular, hydrocalumite, and calcite was synthesized at the W/V ratios of 62.5 g/L and 125 g/L, while the crystallinity of the hydrogrossular decreased together with the appearance of portlandite peaks for the product at the W/V ratio of 250 g/L. The crystallinity of hydrocalumite gradually increases, and that of calcite is almost constant, with increasing the W/V ratio. The optimal W/V ratio for hydrogrossular synthesis was found to be 125 g/L because it is possible to synthesize hydrogrossular using smaller amount of water.

Fig. 8.

(a) XRD patterns of the product from the fused slag (NaOH/BF slag = 1.6, fusion temperature 600°C), and (b) crystallinities of the product phases in the product. The ratios of the fused slag weight to distilled water volume are 62.5 g/L, 125 g/L, and 250 g/L.

Figure 9 shows the XRD patterns of the product (NaOH/slag ratio = 1.6, fusion temperature = 600°C, W/V ratio = 125 g/L, heating temperature = 80°C, and heating time = 6 h) at various stirring times, and crystallinities of the product phases in the product. Regardless of the stirring time, a mixture of hydrogrossular, hydrocalumite, and calcite was obtained under all conditions. With increasing stirring time, the crystallinity of the hydrogrossular first increased for 24 h, and then decreased. The crystallinities of hydrocalumite and calcite are almost same regardless of stirring time.

Fig. 9.

(a) XRD patterns of the product from the fused slag (NaOH/BF slag = 1.6, fusion temperature 600°C), and (b) crystallinities of the product phases in the product. The stirring times are 0 h, 3 h, 6 h, 12 h, 24 h, and 48 h.

Thus, the optimal aging conditions for hydrogrossular synthesis were identified as the W/V ratio of 125 g/L and stirring time of 24 h.

3.4. Crystallization

Figure 10 shows the XRD patterns of the product obtained from the BF slag at various heating temperatures, optimal fusion, and aging conditions as mentioned above, and at a heating time of 6 h, and crystallinities of the product phases in the product. The product including hydrogrossular and calcite was obtained for the synthesis at 80°C, while a mixture of hydrogrossular, calcite, and 11 Å tobermorite [Ca₅Si₆(OH)₁₈‧5H₂O] was obtained for the synthesis at 120°C and 160°C. This result is in good agreement with the results of the previous studies^28,29,30,31) regarding the importance of the breakdown of hydrogrossular for the formation of 11 Å tobermorite in metakaolin, lime, quartz, and water systems, and indicates that the amount of hydrogrossular decreases with increasing reaction temperature, while that of 11 Å tobermorite increases concurrently. Therefore, the product including a high amount of hydrogrossular can be synthesized at 80°C.

Fig. 10.

(a) XRD patterns of the product from the fused slag (NaOH/BF slag = 1.6, fusion temperature 600°C) after 24 h aging (W/V = 125 g/L) by heating at 80°C, 120°C, and 160°C, and (b) crystallinities of the product phases in the product.

Figure 11 shows the XRD patterns of the product obtained from the BF slag under the above-mentioned optimal fusion and aging conditions and the heating temperature of 80°C during the crystallization process. After stirring prior to heating (0 h), hydrocalumite and calcite appeared in the product. After heating for 0.5 h, hydrogrossular appeared; with increasing heating time, the intensities of the hydrogrossular and hydrocalumite peaks increased, while those of calcite peaks remained almost unchanged. It is noted that the chemical composition of the hydrogrossular calculating the d value is Ca₃Al₂(SiO₄)_0.8(OH)_8.8, belonging to katoite structure.

Fig. 11.

XRD patterns of the product from the fused slag (NaOH/BF slag = 1.6, fusion temperature 600°C, 125 g/L) after 24 h aging (W/V = 125 g/L) by heating at 80°C during the crystallization process.

The reaction was monitored by analyzing the Ca, Si, and Al solution contents and the crystallinities of the product hydrogrossular, hydrocalumite, and calcite phases in the solid during the reaction, as shown in Fig. 12. In the initial crystallization stage, the Si and Al contents in the solution were 1500 and 2300 mg/L, respectively. By contrast, the Ca content was lower than 100 mg/L during the crystallization due to the precipitation of the crystalline phases such as hydrocalumite and calcite, and amorphous phases formed by the reaction among Ca, Si, Al and other elements (Mg, Fe, Na, etc.).^32,33,34) With increasing reaction time, the Si concentration decreased rapidly and reached a steady state after 1 h, the Al concentration decreased gradually and reached a steady state after 6 h, while the Ca concentration decreased to zero for 2 h reaction. The hydrogrossular peak intensity increased in the early stage and was almost constant after 3 h of reaction, while the intensities of the hydrocalumite and calcite peaks were almost constant during the reaction. Changes in the Si, Al, and Ca concentrations in the solution were correlated to the formation of the hydrogrossular crystal. This means that the Si and Al in the solution reacted with the Ca contents mainly in the solid phase to synthesize the hydrogrossular crystals. In addition, the Ca reacted with CO₂ dissolved from the atmosphere into the alkali solution to form calcite prior to heating and remained during the reaction due to the high stability of the calcite structure.

Fig. 12.

(a) Concentrations of Ca, Si, and Al in solution and (b) crystallinities of the product phases in the solid during the synthesis.

Therefore, the optimal crystallization conditions for hydrogrossular synthesis were identified as the heating temperature of 80°C and heating time of 3–6 h.

3.5. Removal and Fixation of HCl Gas

Figure 13 shows the results for the removal of the HCl gas using raw slag, lime (Ca(OH)₂), and the products. It is noted that the products used in this experiment were synthesized at 80°C (product-80), 120°C (product-120), and 160°C (product-160). For blank and raw slag, the pH of the solution drastically decreased from 10 to 3–4 for 15 min, implying that the HCl gas was dissolved in the solution after passing through the reactor without removal using raw slag. By using the products, the pH of the solution was almost 10 for 20 min and then decreased, while the pH remained almost constant for 30 min and then decreased because of the use of lime. Therefore, the products can remove the HCl gas, but the amounts of the HCl removal are lower for the products than for lime. It is noted that product-80 shows higher HCl removal than product-120 and product-160 due to its higher hydrogrossular content.

Fig. 13.

Removal behavior of the HCl gas using raw slag, lime, and the products.

Table 2 shows the soluble Cl contents of the products and lime. Lime shows a high HCl removal ability but Cl was included as soluble Cl after the removal, which is unfavorable for landfill disposal and cement production. The soluble Cl content in the product was 5–10 mg/g which is approximately 20–30 times lower than that in lime after the HCl removal.

Table 2. Soluble Cl contents of the products and lime.

	Solubility (mg/g)
	Product-80	Product-120	Product-160	Ca(OH)2
Before HCl removal	< 0.1	< 0.1	< 0.1	0.3
After HCl removal	10.2	5.2	6.0	180.0

These results indicate that the product including hydrogrossular can remove HCl gas at high temperature and fix Cl in the product. Therefore, the product can be used as a scavenger for HCl gas treatment.

4. Conclusion

The chemical conversion of the BF slag to a scavenger including a hydrogrossular compound through alkali fusion was performed. The slag was transformed to a precursor with reactive phases through alkali fusion and aging in distilled water, and the product was synthesized by heating at 80–160°C. The optimal hydrogrossular synthesis conditions are NaOH/slag ratio of 1.6, fusion temperature of 600°C, W/V ratio of 125 g/L, stirring time of 24 h, heating temperature of 80°C, and heating time of 3–6 h. The product can remove HCl at high temperature and showed lower solubility of Cl after the HCl removal compared to lime. These results suggest that the product can be applied for the HCl gas removal from a high-temperature effluent gas and be reclaimed or used for cement production after the HCl removal.

Acknowledgment

This research was supported by a Research Promotion Grant from the Iron and Steel Institute of Japan. I would like to thank Editage (www.editage.com) for English language editing.

References

• 1)   Y.  Kuwahara,  T.  Ohmichi,  T.  Kamegawa,  K.  Mori and  H.  Yamashita: J. Mater. Chem., 20 (2010), 5052.
• 2)  The Annual Report of Statistics of Iron and Steel Slag 2018, ed. by Nippon Slag Association, http://www.slg.jp/statistics/report.html, (accessed 2019-12-10).
• 3)   Y.  Takazawa,  F.  Iizuka,  Y.  Tomosue and  A.  Yamazaki: Abstr. MMIJ Fall Meet., (1996), 39.
• 4)   Y.  Sugano,  R.  Sahara,  T.  Murakami,  T.  Narushima,  Y.  Iguchi and  C.  Ouchi: ISIJ Int., 45 (2005), 937.
• 5)   A.  Nakahira,  H.  Naganuma,  T.  Kubo and  Y.  Yamasaki: J. Ceram. Soc. Jpn., 116 (2008), 500.
• 6)   T.  Wajima and  K.  Munakata: J. Ion Exch., 21 (2010), 171.
• 7)   T.  Hongo: ISIJ Int., 55 (2015), 2700.
• 8)   Y.  Kuwahara,  T.  Ohmichi,  T.  Kamegawa,  K.  Mori and  H.  Yamashita: Bull. Chem. Soc. Jpn., 89 (2016), 472.
• 9)   T.  Wajima,  K.  Oya,  A.  Shibayama,  K.  Sugawara and  K.  Munakata: ISIJ Int., 51 (2011), 1179.
• 10)   T.  Wajima,  K.  Oya,  A.  Shibayama and  K.  Munakata: Int. J. Soc. Mater. Eng. Resour., 18 (2012), 59.
• 11)   T.  Wajima: ESTEEM Acad. J., 13 (2017), 1.
• 12)   M.  Daoudi and  J. K.  Walters: Chem. Eng. J., 47 (1991), 11.
• 13)   G.  Mura and  A.  Lallai: Chem. Eng. J., 47 (1992), 2407.
• 14)   C. E.  Weinell,  P. I.  Jensen,  K.  Dam-Johansen and  H.  Livbjerg: Ind. Eng. Chem. Res., 31 (1992), 164.
• 15)   S.  Fujita,  K.  Suzuki,  M.  Ohkawa,  Y.  Shibasaki and  T.  Mori: Chem. Mater., 13 (2001), 2523.
• 16)   S.  Kobayashi and  T.  Shoji: Mineral. J., 11 (1983), 331.
• 17)   G. A.  Lager,  T.  Armbruster,  F. J.  Rotella and  G. R.  Rossman: Am. Mineral., 74 (1989), 840.
• 18)   E.  Passaglia and  R.  Rinaldi: Bull. Mineral., 107 (1984), 605.
• 19)   M.  Sacerdoti and  E.  Passaglia: Bull. Mineral., 108 (1985), 1.
• 20)   S.  Fujita,  K.  Suzuki and  Y.  Shibasaki: J. Mater. Cycles Waste Manag., 4 (2002), 41.
• 21)   S.  Fujita,  K.  Suzuki,  Y.  Shibasaki and  Y.  Mori: J. Mater. Cycles Waste Manag., 4 (2002), 70.
• 22)   L.  Stieglitz,  G.  Zwick,  J.  Beck,  H.  Bautz and  W.  Roth: Chemosphere, 19 (1989), 283.
• 23)   S.  Fujita,  K.  Suzuki,  M.  Ohkawa,  T.  Mori,  Y.  Iida,  Y.  Miwa,  H.  Masuda and  S.  Shimada: Chem. Mater., 15 (2003), 255.
• 24)   K.  Suzuki,  S.  Fujita,  Y.  Shibasaki,  N.  Ogawa,  T.  Yamasaki,  T.  Fukuda and  S.  Sataka: Organohalog. Compd., 50 (2001), 480.
• 25)   S.  Fujita,  N.  Ogawa,  T.  Yamasaki,  T.  Fukuda,  S.  Sataka,  K.  Suzuki,  Y.  Shibasaki and  T.  Mori: Chem. Eng. J., 102 (2004), 99.
• 26)   T.  Wajima: Proc. 5th Int. Arsenic Symp. Miyazaki 2018, (2018), 100.
• 27)   T.  Kato,  Y.  Ami,  T.  Wajima,  K.  Murakami,  K.  Sugawara and  T.  Sugawara: Clay Sci., 14 (2008), 1.
• 28)   E. I.  Al-Wakeel and  S. A.  El-Korashy: J. Mater. Sci., 31 (1996), 1909.
• 29)   D. S.  Klimesch and  A.  Ray: Cem. Concr. Res., 28 (1998), 1309.
• 30)   D. S.  Klimesch and  A.  Ray: Cem. Concr. Res., 28 (1998), 1317.
• 31)   C. A.  Ríos,  C. D.  Williams and  M. A.  Fullen: Appl. Clay Sci., 43 (2009), 228.
• 32)   T.  Kuwabara,  K.  Arakawa,  T.  Sato and  Y.  Onodera: J. Soc. Inorg. Mater. Jpn., 14 (2007), 104 (in Japanese).
• 33)   T.  Kuwabara,  K.  Kikutani,  T.  Sato and  Y.  Onodera: J. Soc. Inorg. Mater. Jpn., 17 (2010), 81 (in Japanese).
• 34)   T.  Kuwabara,  A.  Matsumura,  M.  Maeno and  T.  Sato: J. Soc. Inorg. Mater. Jpn., 23 (2016), 15 (in Japanese).