Effects of Boron Oxide Addition on Chromium

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Effects of Boron Oxide Addition on Chromium Distribution and Emission of Hexavalent Chromium in Stainless-Steel Slag Wanli Li†,‡ and Xiangxin Xue*,†,‡ †

School of Metallurgy, Northeastern University, Shenyang, 110819, China Liaoning Key Laboratory of Metallurgical Resources Recycling Science, Shenyang, 110819, China

‡

ABSTRACT: Stainless-steel slag is considered a hazardous waste because it contains significant levels of hexavalent chromium. In the present study, the effect of boron oxide addition on the chromium distribution in Cr-bearing phase and emission of hexavalent chromium has been investigated using 2, 4, and 8 wt % addition at three different temperatures. Thermogravimetric differential scanning calorimetry (TG-DSC) and the binary phase diagram of CaO-Cr2O3 were used to find out the temperature range for the formation of hexavalent chromium. Besides, the treated samples were characterized using X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometer (SEM-EDS), NIH ImageJ software and thermodynamic package FactSage 6.4, respectively. Additionally, emission of hexavalent chromium in the treated samples was detected used modified HJ 687−2014 and GB/T 15555.4−1995 after three high-temperature treatments. Finally, the research showed that chromium was enriched in the stable spinel phase and a lower addition of boron oxide content makes a lower emission of hexavalent chromium.

1. INTRODUCTION A consequence of intensive human activities and rapid industrial growth, various types of waste have emerged in the environment. Due to long-term disposal, many landfills are overloaded and thus represent a latent environmental threat. According to the statistics, a data set from CSSC,1 China has produced 21.56 and 24.94 million tons stainless-steel in the past two years. Every 0.25−0.33 ton stainless-steel slag will yield during one stainless steel production.2,3 As apparent consumption of stainless steel is a substantial increase, management of waste substances becomes an urgent problem due to its huge space and ecological stress to the environment. It is well-known that some waste materials from the steelmaking industry can be recycled.4,5 The main environmental impact of the reuse of waste materials is the leaching of potentially toxic substances into the terrestrial and aquatic environment. Chromium, as valuable strategic resources in stainless-steel slag, its great challenge is the enrichment and separation from mineralogical phases. However, trivalent chromium existed in the unstable phase of slag has enormous potential to get oxidized to the virulent hexavalent state (Cr6+) if exposed to the acidic and oxygen-rich environment.6 Hexavalent chromium is toxic and easily leachable from the slag, leading to water pollution and economic and ecologic issues. This causes ecological risk and limits the high-efficient reutilization of Cr-containing steel slag. © XXXX American Chemical Society

Recently, low basicity was said to favor the formation of spinel.7 Controlling slag composition by adding SiO2 to stainless-steel slag was effective for preventing Ca2 SiO4 formation and chromium elution into seawater. According to the previous study,8 an addition of alumina was found to be beneficial to spinel formation during solidification. The lowest chromium concentration levels in the leaching liquor were corresponding to MgO-based slag owing to the stable binding of chromium in spinel with MgO.9 Some10 clarified the effect of FeO on the formation of spinel phases and chromium distribution in CaO-SiO2−MgO−Al2O3−Cr2O3 slag system and found that the addition of FeO could enhance the content of chromium in spinel phases and reduce the content of chromium in soluble minerals. The present work is intended for finding an approach to enrich the chromium in spinel phases and lower the emission of hexavalent chromium by means of adding different content of boron oxide. As an amphoteric modifier in the melt, boron oxide (B2O3) acts as both network former and breaker.11 It can decrease the molten flux viscosity and apparent activation energy and simplify isolated network structure.12,13 Boron oxide Received: Revised: Accepted: Published: A

January 31, 2018 March 16, 2018 March 21, 2018 March 21, 2018 DOI: 10.1021/acs.iecr.8b00499 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Table 1. Chemical Composition of Stainless-Steel Slag

a

composition

CaO

MgO

SiO2

Al2O3

Cr2O3

TFe

FeO

MFe

Ra

wt %

46.22

5.81

23.60

1.99

6.36

8.45

6.17

1.40

1.96

R, basicity. R = CaO%/SiO2%.

can greatly increase the acidity of slag and stabilized the slag.14 To prevent environmental hazards, the safe disposal or reuse of stainless-steel slag is required. Chromium leaching must be restricted for slag to be taken into consideration environmentally friendly. Therefore, it is worth investigating the effect of boron oxide addition on the chromium distribution and the emission of hexavalent chromium in stainless-steel slag.

cooled down to the room temperature. The detailed heattreatment routine was set out in Figure 1.

2. METHODS 2.1. Materials and Sample Preparation. Stainless-steel slag was obtained from Baosteel Group Corporation, China. Slag was crushed with a jaw crusher, GJ-I sealing sample pulverizer, to −400 mesh granularity before modifying. The chemical composition of stainless-steel slag was established by X-ray fluorescence (XRF). Its composition was given in Table 1. Ingredient of the modified slag was also listed in Table 2.

Figure 1. Heat treatment regimes in this experiment.

After the crushing, modified HJ 687−2014 method was used to prepare the filtrate for the subsequent test. In the preliminary evaluation of the Cr (VI) concentration value, 2.5 g of dry powder was added to a 250 mL round-bottom flask together with 400 mg of MgCl2. After the addition of 50.0 mL of Na2CO3−NaOH mixtures and 50.0 mL of KH2PO4−K2HPO4 buffer solution, the solution was heated to 90−95 °C and kept for 1 h with the stirrer running and polyethylene film sealing. After the digestion and the cooling, the pH value of filtrate was set to proper values for the subsequent test. 2.3. Methods for Characterization. The chemical composition of stainless-steel slag was established by X-ray fluorescence (XRF). X-ray diffraction analysis (XRD) was performed on the pulverized material using PANalytical B.V. X Pertpro automatic diffractometer with a step and continuous scanning device. Diffraction patterns were measured in a 2θ range of 5−90° using (Cu Kα) radiation of 50 kV and 30 mA. Phase identification was made by reference patterns in an evaluation program supplied by the manufacturer of the equipment. The mineralogical phases in as-quenched samples were investigated using backscattered electron image signal (SEM), German Carl Zeiss Company Ultra Plus, equipped with energy dispersive spectrometry (EDS). Before SEM examination, samples were embedded in resin, ground, polished and covered with a conductive layer of gold. The mineralogical phases in the matrix phase were investigated using electron probe microanalyzer (EPMA) with wavelength dispersive spectroscopy (WDS), Shimadzu EPMA 1600. WDS analysis was accurate enough to study the chromium distribution in the phases present because of its low concentration. TG-DSC was used to study the thermal behavior of the stainless-steel slag at a heating rate of 10 °C/min from 25 to 1500 °C. The TG-DSC curves were detected by NETZSCH (STA 449F3A-1456-M). Samples were positioned in the platinum crucible in an air atmosphere as heating to a definite temperature. The levels of hexavalent chromium in the treated samples were determined using two national standard procedures: solid waste-determination of hexavalent chromium-by alkaline digestion/flame atomic absorption spectrophotometric (HJ 687−2014) and solid waste-determination of chromium(VI)-1,

Table 2. Slag Weight of Samples weight of mixtures (g) sample

description

slag

B2O3

total

(a) (b) (c)

2 wt % B2O3 4 wt % B2O3 8 wt % B2O3

15 15 15

0.30 0.60 1.20

15.30 15.60 16.20

2.2. Experimental Procedure. Fine powders of reagent B2O3 of analytical grade were used to be the addition of slag. Before modifying, B2O3 powders were heat-treated at 383 K (110 °C) for 10 h to remove any moisture. The amount of B2O3 powders added in samples a−c was 2, 4, and 8 wt %, respectively. Weighting some analytical reagents precisely which were presented in Table 2 and mixing with stainlesssteel slag into a milling equipment for 10 h. About 15 g of powder mixture for each slag sample was pressed into a specimen of 30 mm in diameter and 10 mm in thickness. Samples were placed in Mo crucibles (covered by high purity graphite crucible to prevent oxidation), which were placed in the MoSi2 furnace and soaked at 1600 °C for 2 h for sample melting. The slag then remained inside the furnace to cool down to room temperature naturally. Cool materials were crushed and pressed into a briquetting of 8 mm in diameter and 5 to 7 mm in thickness. These samples were placed in Mo crucibles and soaked at the required temperature for subsequent work. High-temperature box furnace was equipped with MoSi2 heating elements. The experimental temperature was monitored by a Eurotherm P.I.D. controller equipped with a B-type (Pt−Rh 30 pct. /Pt−Rh 6 pct.) thermocouple as the sensor. The even-temperature zone of the furnace extended to about 80 mm at the center of the furnace and temperature deviation in this zone was controlled to be less than 2 °C. The furnace was heated at a certain rate in an air atmosphere. Samples were equilibrated at 1600 °C for 2 h and cooled to a certain temperature (1300, 1200, 700) at 10 K/min. After being soaked at the targeted temperature for 2 h to promote the formation of Cr6+, Mo crucibles were withdrawn from the furnace and B

DOI: 10.1021/acs.iecr.8b00499 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

As is well-known, chromium in the stainless-steel slag mainly exists in the presence of low-valence cations,15 but it has great potential to get oxidized to the virulent hexavalent state (Cr6+). Conversion of Cr6+ to a lower-valence state would avoid generating hazardous waste where CaCr2O4 is reportedly less stable in leaching aspects than MgCr2O4.16 Alkaline environment, free oxygen, and water content are the main factor to promote the transformation at room temperature.17 The raw materials were ground and sieved into fractions. About 2.5 g of the −200 mesh fraction was used for each hexavalent chromium solubility test. The specified size fraction was used to avoid the influence of varying particle size on reaction kinetics. The levels of hexavalent chromium in raw material were detected using national standard procedure: modified solid waste-determination of hexavalent chromium-by alkaline digestion/flame atomic absorption spectrophotometric (HJ 687−2014).18 This method is reliable when Cr (VI) concentrations are higher than 8 mg/kg although the detection limit of the method for Cr (VI) is 2 mg/kg. The detection range of Cr (VI) concentration is between 8 mg/kg-320 mg/kg when weighing 2.5 g of solid waste first and making digestion liquid 100 mL at the end before the test. The concentration of the standard solution to get standard curve line was modified from 0.00 to 8.00 mg/L to 0.00−2.50 mg/L after three pretests as most of Cr existed as Cr (III) in the CaCr2O4 and MgCr2O4 phase (Figures 2 and 3). In the preliminary evaluation of Cr (VI) concentration, the modified HJ 687−2014 analysis shows that the stainless-steel slag leachate detected Cr (VI) is 18 mg/ kg, higher than the limit for inert material at 0.5 mg/kg compared with the European limitation for inert landfill, which is beyond the Cr (VI) specification for hazardous materials. 3.2. Temperature Effect on the Formation of Cr (VI) in Stainless-Steel Slag. TG-DSC was used to study the thermal behavior of the stainless-steel slag, as shown in Figure 5. By analyzing the curves of TG-DSC, the entire thermal process has two mass loss steps, a strong loss step around 700 °C and a fine loss step around 1200 °C. The increasing step (considering the cooling regime in this study) is mainly due to the reaction between Cr3+ and O2, as shown in eq 1.19−21 Increasing Cr3+ content gives increasing amounts of Cr6+.

5-diphenylcarbohydrazide spectrophotometric method (GB/T 15555.4−1995).

3. RESULTS AND DISCUSSION 3.1. Phase Composition and Initial Hexavalent Chromium in Raw Stainless-Steel Slag. Mineralogical phases present in the stainless-steel slag were studied using XRD and SEM in the current work. The phase composition results were in great accordance with XRD and SEM analysis, as shown in Figures 2 and 3. Chromium distribution in stainless-

Figure 2. Phase composition analyzed by XRD diffraction pattern of stainless-steel slag.

(Cr 3 +) + 5/2(O2 −) + 3/4O2 = (CrO4 2 −)

(1)

The binary phase diagram22 of CaO-Cr2O3 can be used as a guide to analyzing the formation of Cr6+ and the loss step in TG-DSC. The CaO-Cr2O3 binary phase diagram in Figure 6 illustrates the distribution of Cr (VI) in some certain phases. The Cr6+ only exists in three phases: 9CaO·4CrO3·Cr2O3, 3CaO·2CrO3·2Cr2O3, and CaO·CrO3. The stable temperature region and the mole ratio of Cr6+ to total Cr of certain phases are given in Table 3. As shown the high CaO content in Table 1, the only phase with Cr6+ is 9CaO·4CrO3·Cr2O3 in temperature ranging from 1174 to 800 °C and CaO·CrO3 for temperature below 800 °C. Setting the predesignated temperature target at 1300, 1200, and 700 °C is to identify the effect of B2O3 addition on dissolved chromium and emission of hexavalent chromium in stainless-steel slag. 3.3. Effect of B2O3 Addition on Phase Transformation in Treated Samples and Chromium Content in CrBearing Phase. The main phase composition in stainless-steel slag can be divided into two categories, stable phase and soluble phase, according to its different dissolving capacity. Chromium in stainless-steel slag mainly distributes in certain phases such as Cr2O3, MgO·Cr2O3, CaCr2O4, CaCrO4, and so on. Chromium

Figure 3. Main phase composition in stainless-steel slag analyzed by SEM.

steel slag was analyzed by EDS mapping, as shown in Figure 4. According to Figure 2, five crystalline phases were identified in the sample: dicalcium silicate (Ca2SiO4), spinels (Mg(Cr, Al)2O4), brownmillerite (Ca2(Al, Fe)2O5), CaCr2O4, and lime (CaO). Three mineral phases were identified as the main phase according to Figures 2 and 3 because of its strongest peak in XRD analysis and most view occupation in the field of SEM photos. The chromium in stainless-steel slag existed as Cr (III) in CaCr2O4 and spinel phases, which can be illustrated in Figure 4. Because only minerals containing chromium can leach chromium, only these minerals need to be stabilized or eliminated. C

DOI: 10.1021/acs.iecr.8b00499 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 4. Chromium distribution in stainless-steel slag analyzed by EDS mapping.

Table 3. Stable Phases and Mole Fraction at a Given Temperature Range pure stable phase

T range (°C)

Cr6+/total Cr

CaO 9CaO•4CrO3·Cr2O3 3CaO·2CrO3·2Cr2O3 CaO·CrO3 Cr2O3

1228−800 1022−900