synthesis of hydrocalumite-like compound from blast

86 RESOURCES PROCESSING

RESOURCESPROCESSING

1. Introduction

Blast furnace slag (BF slag) is the largest by-product from pig iron metallurgical furnaces, with approximately 150 million metric tons of slag produced annually worldwide1,2. In Japan, more than 25 million tons of BF slag is produced annually, which is widely used for cement pro-duction, road construction, and as a concrete ag-gregate. However, the demand for recycled BF slag for these applications is currently saturated, and the development of value-added products from BF slag has become an important issue for establishing a sustainable society3. Therefore, research into various recycling processes has recently been carried out4–7.

Waste heat is actively used in steelworks. For example, a system that generates electric power from the gas generated from a blast furnace and then supplies it to the steelwork plant has been introduced; however, using slag-waste heat in a similar manner has, to the best of our knowledge, not been reported before. Slag is discharged at 1450–1550°C and carries with it a substantial amount of high-quality waste heat; this heat amounts to 12 million tons of standard coal and is lost through natural or forcible cooling, which consumes large amounts of water and emits pol-lutants8–15.

In previous studies, we converted BF slag into a hydroxide-based adsorbent that contains calcite and hydrocalumite through alkali fusion, and con-firmed that this material is an adsorbent with high affinity for the removal of phosphate from aque-ous solutions16,17. While this adsorbent is expected

Resources Processing 67 : 86–93 (2020)

Original Paper

Synthesis of Hydrocalumite-like Compound from Blast Furnace Slag by Alkali Fusion using Waste Molten-Slag Heat, and Its Anion

Removal Ability

Takaaki WAJIMA*

Department of Urban Environment Systems, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-85222, Japan

AbstractBlast furnace (BF) slag is a byproduct of iron- and steel-making plants and is produced by

the forced cooling of the 1500°C molten state. The ability to create value-added products from BF slag has become an important sustainability issue. In this study, we converted this slag into a hydrocalumite-like compound capable of removing pollutant ions from water through alkali fusion us-ing the waste heat of the melting slag. The molten slag was transformed into a precursor with reactive phases by alkali fusion at 1500°C, after which the cooled melting slag was crushed and added to dis-tilled water containing NaAlO2 and stirred at room temperature to produce the hydrocalumite-containing product. The effects of the type of alkali salt, the Na2CO3-to-slag mixing ratio (Na2CO3/slag ratio), heating time, amount of added NaAlO2, and the ability of the product to remove certain ions from aqueous solution were examined. Na2CO3 was used to alkali fuse slag at 1500°C, and the optimal conditions for the synthesis of hydrocalumite were found to include a Na2CO3/slag ratio of 2:1, a heating time of more than 10 min, and a 60 g/L concentration of NaAlO2. The product was shown to remove more F– and PO4

3– than BF slag. These results suggest that a hydrocalumite-like adsorbent capable of removing pollutant ions from aqueous solution can be synthesized from BF slag through alkali fusion using waste slag-production heat.

Key words: Blast furnace slag, Hydrocalumite-like compound, Waste molten-slag heat, Alkali fusion, Anion removal ability

Accepted 6 November, 2020*e-mail: [email protected]

Synthesis of Hydrocalumite-like Compound from Blast Furnace Slag by Alkali Fusion using Waste Molten-Slag Heat, and Its Anion Removal Ability

87Vol. 67, No. 2 (2020)

to be useful for the removal of phosphate from aqueous systems, more information on its proper-ties is required before it can be widely applied.

Fluoride occurs naturally in groundwater at levels typically below 0.5 mg/L. However, in many parts of the world, elevated fluoride concen-trations of up to 8.0 mg/L, with some even ex-ceeding 20.0 mg/L, have been reported. Weather-ing and erosion of geological formations can cause fluoride to be released from minerals into drinking water, creating unsafe conditions. Groundwater with elevated fluoride is, to a large degree, found in locations containing volcanic igneous rock; such areas include India, China, and the southwestern desert of the United States, among others18. Although the World Health Organization (WHO) and the United States Envi-ronmental Protection Agency (US EPA) classify activated alumina adsorption among the best demonstrated available technologies (BDATs) for fluoride removal, it is relatively expensive and its adsorption capacity is affected by pH and the presence of coexisting ions in water, such as sili-cate, sulfate, bicarbonate, and phosphate19,20. Many methods can be used to remove fluoride from water, including adsorption by activated alu-mina (AA)21,22, reverse osmosis23, electrodialysis and electrosorption24,25, adsorption by limestone26, goethite27, kaolinite28, acid-treated spent bleach-ing earth29, red mud30, polyaluminum chloride31, and various other low-cost materials, including bentonite, char fines, lignite and nirmali seeds32, as well as some novel hybrid adsorbents33. Hydro-calumite is promising materials for the treatment of not only naturally occurring poor-quality groundwater with not very high fluoride but also industrial wastewater enriched highly in fluoride, due to the dissolution of hydrocalumite to precipi-tate as fluorite (CaF2) and anion exchange be-tween fluoride in solution and interlayer anion in hydrocalumite34,35. Therefore, there is a possibility for the product including hydrocalumite-like com-pound from BF slag via alkali fusion to remove fluoride ion from wastewater.

In this study, we effectively formed a highly reactive precursor by applying BF slag and slag waste heat to the alkaline fusion of melting BF slag produced at 1500°C, and then synthesized a hydrocalumite-like functional material from the precursor, which was subsequently evaluated for its ability to remove anionic pollutants, especially fluoride.

2. Experimental

2.1 Raw materialThe blast furnace slag used in these experi-

ments was obtained from a steel plant in Japan. Prior to any experiment, the BF slag was mill-ground and sieved to produce slag with diameters in the 100–500 μm range. The chemical composi-tion of the blast furnace slag was determined by X-ray fluorescence spectroscopy (XRF; Shimadzu, XRF-1700). The raw slag is composed mainly of CaO (41.5%), SiO2 (33.9%), and Al2O3 (14.2%), with other oxides, such as MgO (6.6%), SO3 (1.3%), Fe2O3 (1.1%), TiO2 (0.6%), K2O (0.3%), MnO (0.3%), SrO (0.1%), and P2O5 (0.1%), present in smaller amounts.2.2 Alkali fusion

BF slag melted at 1500°C was converted into a soluble alkali salt by fusion with sodium chloride (NaCl), sodium carbonate (Na2CO3), sodium acetate (CH3COONa), and sodium hydroxide (NaOH) as representative alkali salts.

BF slag and the alkali salt was mixed and then added to a platinum crucible. The crucible was placed in an electric furnace, and heated at 1500°C for 0–60 min. The melting slag was then naturally cooled to room temperature and pul-verized with a hammer to obtain the powdered fused slag. The mineralogical phases of the fused slag prepared in this manner were analyzed by powder X-ray diffractometry (XRD; Rigaku, MiniFlex600).2.3 Synthesis

The fused slag prepared heating a 1:2 (w/w) mixture of BF slag and Na2CO3 at 1500°C for 10 min, was used to synthesize hydrocalumite by the addition of sodium aluminate (NaAlO2). A 1.0 g sample of fused slag and 0–1.0 g of NaAlO2 was added to 8 mL of distilled water and shaken with a reciprocal shaker at room temperature. Af-ter shaking for 24 h, the solid was collected by filtration, washed with distilled water, and dried in an oven overnight to afford the product. The min-eralogical phases of the product were analyzed by XRD. The concentrations of Ca, Al, and Si in the filtrate were determined by atomic absorption spectrometry (AAS; PerkinElmer, AAnalyst200). The fluoride removal of the obtained each product was determined as follows; 0.1 g of the product was added into 20 mL of 1 mM KF solution in the 50 mL of centrifuged tube, the tube was shaken with a reciprocal shaker for 24 h, the solid was collected by filtration, washed with distilled water, and dried in a drying oven overnight. The concentration of F– in the filtrate was measured

WAJIMA


using an ion electrode, and the mineralogical phases of the solid were analyzed by XRD. The fluoride removals of CaCO3 (Wako), Ca(OH)2 (Wako) and hydrocalumite, which was synthe-sized by the method reported by Wajima36, were also determined using the same procedure.2.4 Anion removal

The fluoride removal ability of the product was examined as follows; 0.1 g of the product was added into 20 mL of 0–20 mM KF solution in the tube, the tube was shaken for 3 h, the solid was filtrated, and the concentration of F- in the filtrate was determined by an ion electrode. The removal amount of F– (qe) was calculated using the follow-ing equation;

qe = (C0 – C)‧V/w (1)

where C0 and C were initial and measured con-centrations, respectively, V (L) is the volume of the solution, and w (g) is the weight of the added sample.

The ability of the product to remove anions, such as fluoride (F–), phosphate (PO4

3–), sulfide (S2–), chloride (Cl–), and nitrate (NO3

–) were examined. A 1-mM solution of each anion was prepared using potassium fluoride (KF), potas-sium dihydrogen phosphate (KH2PO4), sodium sulfide (Na2S), potassium chloride (KCl), or sodium nitrate (NaNO3). A 0.1 g sample of the hydrocalumite-containing product was added into 20 mL of each anion solution, and shaken for 24 h. The solid was collected by filtration, washed with distilled water, and dried in an oven overnight. The mineralogical phases of the solid were analyzed by XRD. The concentration of PO4

3– in the filtrate was measured using the mo-lybdenum blue method, while those of the other anions were measured using an ion electrode.

3. Results and Discussion

3.1 Alkali fusion of the melting slagThe XRD patterns of the slag fused at 1500°C

with various alkali salts are shown in Figure 1. We note that a 1:2 (w/w) ratio of raw slag/alkali salt was used, with heating for 10 min. The slag exhibits only amorphous phases in the absence of an alkali salt. Reactive phases, such as sodium silicate, are formed when fused with Na2CO3, while the fused slag was amorphous when fused with NaCl, CH3COONa, or NaOH because the slag and these alkali salts do not react. The melt-ing points of these alkali salts are 851°C (Na2CO3), 801°C (NaCl), 318°C (NaOH), and

324°C (CH3COONa), while their boiling points are 1633°C, 1413°C, 1388°C, and 881°C, respec-tively. We conclude that NaCl, NaOH, and CH3COONa escape as vapor before reacting with the melting slag, but that it is possible to convert the melting slag into reactive sodium salts through alkali fusion with Na2CO3 at high temperature.

The XRD patterns of the slag fused at 1500°C for 10 min using various amounts of Na2CO3 are shown in Figure 2. Na2SiO3 formed first and then CaO appeared as the ratio of Na2CO3 was in-creased to 1:2, while a mixture of Na2CO3, CaO, and Na2CO3 was observed at a 1:3 ratio due to re-maining unreacted Na2CO3. Therefore, twice the weight of Na2CO3 relative to that of raw slag is

Figure 1 XRD patterns of slag fused at 1500°C (a) with-out alkali salt, and with (b) NaCl, (c) Na2CO3, (d) CH3COONa, and (e) NaOH.

Figure 2 XRD patterns of slag fused at 1500°C with various amounts of added Na2CO3: raw-slag/Na2CO3 weight ratios of: (a) 0.5, (b) 1.0, (c) 2, (d) 3, (e) 4, and (f) 5.


89Vol. 67, No. 2 (2020)

sufficient to convert the melting slag into reactive sodium salts by alkali fusion with Na2CO3 at high temperature.

The XRD patterns of the slag fused at 1500°C for various heating times are shown in Figure 3; here a 1:2 (w/w) ratio of raw slag to Na2CO3 was used. Peaks corresponding to Na2SiO3, CaO, and Na2CO3 were observed after 5 min of heating. The Na2CO3 peaks decreased and those of Na2SiO3 and CaO were clearly evident as the reaction be-tween the melting slag and Na2CO3 progressed. These results reveal that Na2CO3 reacts rapidly with the molten slag. On the basis of these results, we conclude that the melting slag can clearly be converted into the reactive precursor by reaction with Na2CO3.3.2 Hydrocalumite synthesis

The mineralogical and chemical compositions of the product synthesized from fused slag with various amounts of added NaAlO2 are shown in Figure 4 and Table 1, respectively. It is noted that Product-A, Product-B and Product-C were syn-thesized from fused slag with NaAlO2 addition of 0.1 g, 0.5 g and 1.0 g, respectively. Calcite (CaCO3) is formed when 0.1 g of NaAlO2 was added to a 1.0 g sample of fused slag in 8 mL of water. The heights of the calcite peaks decrease with increasing NaAlO2 loading, and peaks corre-sponding to a mixture of calcite, hydrocalumite (Ca2Al(OH)6‧3H2O), and portlandite (Ca(OH)2) were observed when 0.5 g of NaAlO2 was added, while a broad peak and a small calcite peak were observed when 1.0 g of NaAlO2 was added. For the main contents, CaO, SiO2 and Al2O3 in the

product, the contents of CaO and SiO2 decrease, while that of Al2O3 increases, due to the NaAlO2 addition.

The intensities of the XRD peaks correspond-ing to calcite and hydrocalumite in the product synthesized from fused slag with various amounts of NaAlO2 are shown in Figure 5; here the intensi-ties of the peaks corresponding to the (1 0 4) cal-cite and (0 0 3) hydrocalumite faces were used, since they are the most intense. The intensity of the calcite peak was observed to decrease with in-creasing NaAlO2 loading, and was effectively constant at NaAlO2 loadings above 0.5 g. The hy-drocalumite peak increased in intensity the 0.1–0.3 g NaAlO2 range, and was most intense when 0.3–0.5 g of NaAlO2 was added, but was lower in intensity when 0.6 g of NaAlO2 was added, and was effectively constant thereafter. From the above results, we conclude that the reaction in the solution can be classified into three regions de-pending on the amount of NaAlO2 added.

The concentrations of Al, Ca, and Si in the solution following synthesis are shown in Figure

Figure 3 XRD patterns of slag fused at 1500°C with Na2CO3 (raw-slag/Na2CO3 ratio of 1:2 (w/w)) for various heating times: (a) 5 min, (b) 10 min, (c) 30 min, and (d) 60 min.

Figure 4 XRD patterns of the products synthesized from fused slag with (a) 0.1 g, (b) 0.5 g, and (c) 1.0 g added NaAlO2.

Table 1 Chemical compositions of the products.

Product-A Product-B Product-CCaO 59.7 58.5 57.2SiO2 19.4 17.1 17.1Al2O3 14.3 18.5 18.7MgO 1.8 1.5 2.2Fe2O3 1.3 1.3 1.7TiO2 1.2 1.1 1.2P2O5 0.5 0.4 0.4Cl 0.2 0.1 0.2SO3 0.1 0.1

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6. This solution contained 16 mg/L of Si, 3 mg/L of Ca, and a small amount of Al when the synthe-sis was carried out without NaAlO2. The concen-tration of Al was observed to gradually increase, and that of Ca was almost constant, with increas-ing NaAlO2 loading. The concentration of Si in solution was dramatically lower when 0.1 g of NaAlO2 was added, and was essentially constant when more than 0.2 g of NaAlO2 was used. Therefore, we conclude that Si first reacts with Al, after which Ca reacts with Al to form hydrocalu-mite. Hydrocalumite crystallization does not oc-cur above a 0.6 g loading of NaAlO2 because the amount of Al in the solution is too large to form hydrocalumite crystals. On the basis of these re-sults, the hydrocalumite-containing product can be prepared from the fused slag (0.1 g) with the addition of 0.3–0.5 g of NaAlO2.3.3 Anion removal ability

The ability of the product synthesized from fused slag with various amounts of NaAlO2 to re-

move fluoride are shown in Figure 7. The product synthesized without NaAlO2 exhibited a 75% re-moval efficiency, and this only increased slightly as the NaAlO2 loading was increased to 0.2 g. The products synthesized with 0.3–0.5 g NaAlO2 ex-hibited higher fluoride removal (85–98%) than the other products. When synthesized with 0.6 g of NaAlO2, the products exhibited fluoride re-moval efficiencies of 80–85%. The fluoride re-moval of CaCO3, Ca(OH)2 and hydrocalumite are 10.5%, 29.6% and 93.2%, respectively. There-fore, the fluoride removal ability of the product mainly depends on the hydrocalumite content.

The XRD patterns of the adsorbents before and after fluoride removal are shown in Figure 8, in which the hydrocalumite-containing products were synthesized from fused slag with 0.3–0.5 g of added NaAlO2. Although the patterns of the products exhibit peaks for calcite, hydrocalumite, and portlandite before fluoride removal, the peaks corresponding to hydrocalumite were less intense, and peaks due to CaF2 appear in the patterns of the products after fluoride removal. In the solution with high fluoride content, hydrocalumite indi-cates high solubility, while calcite and portlandite indicate much lower solubility35. We conclude that fluoride is efficiently removed through the formation of CaF2, due to the dissolution of hy-drocalumite as follows;

Ca2Al(OH)6‧3H2O → 2Ca2+ + Al(OH)4

– + 2OH– + 3H2O

Ca2+ + 2F– → CaF2

The anion-removing abilities of the optimal ad-

Figure 7 Fluoride-removal efficiencies of adsorbents synthesized from fused slag and various amounts of added NaAlO2.

Figure 6 Concentrations of Al, Si, and Ca in solution as functions of added NaAlO2.

Figure 5 Intensities of the calcite and hydrocalumite peaks in the XRD patterns of the products syn-thesized from fused slag and various amounts of NaAlO2.


91Vol. 67, No. 2 (2020)

sorbent synthesized from fused slag and NaAlO2 are shown in Figure 9. It is noted that pHs of the solution after removal are 10.4–12.8. A 0.1 g sam-ple of this adsorbent removed fluoride (96%), phosphate (98%) and sulfide ion (58%) from 1 mM solutions, while nitrate and chloride were not removed.

The XRD patterns of the optimal product after removal of PO4

3– and S2– are shown in Figure 10. Hydroxyapatite (Ca5(PO4)3(OH)) was formed in the product treated with KH2PO4 solution (Figure 10 (a)). This adsorbent clearly is highly selective for the removal of phosphate, due to the dissolu-tion of hydrocalumite to form hydroxyapatite37. Gypsum (CaSO4) was formed in the product treat-ed with Na2S solution (Figure 10 (b)). It is noted The oxidation-reduction potential (ORP) of Na2S solution before and after treatment of the product are –220 mV and –122 mV, respectively. It would

be considered that a part of sulfide ion was oxi-dized to sulfate ion to form gypsum by reaction with calcium ion from the dissolution of hydro-calumite.

The isotherm of fluoride removal using the opti-mum product is shown in Figure 11. With increas-ing equilibrium concentration, fluoride removal increases to approximately 0.8 mmol/g, and be-come almost constant. This fluoride removal be-havior of the product is determined by isotherm models. Several isotherm models are available to describe the equilibrium sorption distribution in which two models are used to fit the experimental data: Langmuir and Freundlich models.

The liner forms of Langmuir and Freundlich models are given by

Ce/qe = 1/(Qmax•KL) + Ce/Qmax (2)

Figure 8 XRD patterns of adsorbents prepared with vari-ous amounts of added NaAlO2 (a) before and (b) after fluoride removal.

Figure 9 Anion-removal efficiencies of the optimal adsor-bent synthesized from fused slag and NaAlO2.

Figure 10 XRD patterns of the optimal product after removals of (a) PO4

3– and (b) S2– using the product.

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ln(qe) = ln(KF) + (1/n)•ln(Ce) (3)

where qe is fluoride adsorption amount at equi-librium (mmol/g); Qmax (mmol/g) and KL (L/mg) are Langmuir constants related to the maximum adsorption capacity corresponding to complete coverage of available adsorption sites and a meas-ure of adsorption energy (equilibrium adsorption constant), respectively. KF and n are Freundlich constants.

Our experimental results give correlation re-gression coefficient (R2), which is the measure of goodness-of-fit, equal to 0.016 and 0.930 using Langmuir and Freundlich, respectively. It can be seen that the Freundlich model has better fitting than the Langmuir one as the former has a higher correlation regression coefficient then the latter. The empirical formula of Freundlich model was found as ln(qe) = (1/0.930)∙ln(Ce) + ln(0.099).

The apparent Gibbs free energy of adsorption (ΔG0) is the fundamental criterion of spontaneity. Reaction occurs spontaneously at a given temper-ature if ΔG0 is negative. The standard Gibbs free energy change (ΔG0) for the adsorption of fluoride by the prepared adsorbent can be calculated using following their thermodynamic equation38:

ΔG0 = –nRT (4)

where T is the absolute temperature, n is the characteristic constant in Freundlich isotherm, and R is the gas constant (8.314 J/(mol∙K)). The value of standard Gibbs free energy change calcu-lated was found to be –2.30 kJ/mol. The negative sign for ΔG0 is indicative of the spontaneous na-ture of fluoride removal on the product.

4. Conclusions

BF slag was converted into a highly reactive pre-cursor in order to synthesize a hydrocalumite- containing product capable of removing anions from aqueous solutions. The results are summa-rized as follows:• BF slag was converted into a highly reactive

precursor within 10 min using Na2CO3 at 1500°C.

• Hydrocalumite-containing products were syn-thesized from the fused slag by the addition of NaAlO2.

• The hydrocalumite-containing product is a highly efficient adsorbent for the removal of fluoride, phosphate and sulfide ions from aque-ous solutions.

• The fluoride removal behavior is found to fit Freundlich isotherm model than Langmuir, and determination of thermodynamics parameters, such as Gibb’s free energy changes, shows the spontaneous nature of fluoride removal by the hydrocalumite-containing product.This study demonstrated that a novel hydro-

calumite-containing adsorbent capable of remov-ing ionic pollutants from aqueous solutions can be synthesized from BF slag through alkali fusion using waste slag-production heat.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Number JP 24710086 and the Chiba Uni-versity SEEDs Fund (Chiba University Open Recruitment for International Exchange Program). Thank Editage (www.editage.com) for English language editing.

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synthesis of hydrocalumite-like compound from blast

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