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magnesia-chromite refractory interactions
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Effect of ZnO level in secondary copper smelting slags on slag/magnesia-chromite refractory interactions Liugang Chen ∗ , Muxing Guo, Huayue Shi, Shuigen Huang, Peter Tom Jones, Bart Blanpain, Annelies Malfliet KU Leuven, Department of Materials Engineering, Kasteelpark Arenberg 44, BE-3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 4 December 2015 Received in revised form 1 February 2016 Accepted 2 February 2016 Available online xxx Keywords: Refractories Magnesia-chromite MgO ZnO-containing fayalite slag Corrosion
a b s t r a c t To determine the effect of ZnO in ZnO-containing fayalite (ZFS) slags on the degradation of magnesiachromite refractories, the corrosion behaviour of a direct-bonded magnesia-chromite refractory was studied by rotating refractory finger tests at 1200 ◦ C under reducing atmosphere. In addition, MgO crucible tests at these conditions were performed to investigate in particular the effect of ZnO on the slag/MgO interaction. The dissolution of MgO from periclase into the ZFS slags is not enhanced by increasing the ZnO content in the slags. (Fe,Mg,Zn)2 SiO4 olivine and (Fe,Mg,Zn)O solid solution are the only two products in the slag/MgO interfacial reactions, even when the ZnO content in the ZFS slag reaches 16.7 wt%. The diffusion of ZnO into the chromite grains to form the (Fe,Mg,Zn)(Fe,Al,Cr)2 O4 spinel was favoured with increasing the ZnO content in ZFS slags. © 2016 Published by Elsevier Ltd.
1. Introduction ZnO-containing fayalite (ZFS) slags can be generated in copper smelters during the smelting process of secondary raw materials [1,2]. In such slags, the ZnO content can be varied depending on the feed materials and/or operation conditions. For example, ZnO in the ZFS slags can be enriched using high Zn (or ZnO) content secondaries and/or reducing the loss of ZnO into the off-gas during operation. Because ZnO is a basic oxide and can provide free O2− to break the silica network [3,4], the variation in the ZnO content of ZFS slags can consequently change both the basicity and viscosity of the slags. As a result, the degradation of the lining materials of the copper smelter will be influenced. Magnesia-chromite refractories are the most suitable materials in lining the furnaces used in copper-making practices [1,5,6]. In previous work [2], the chemical corrosion behaviour of magnesiachromite refractories by a ZFS slag with ∼9.0 wt% ZnO has been studied. It can be concluded that similar to the degradation mechanisms contacting with ZnO free fayalite slags, the periclase phase in the magnesia-chromite refractories is dominantly corroded by the ZFS slag because of the MgO dissolution into the slag. ZnO along with FeO and SiO2 from the ZFS slag react with the periclase in the magnesia-chromite refractory, forming a (Fe,Mg,Zn)2 SiO4 olivine
and a (Fe,Mg,Zn)O solid solution [2]. Concurrently, ZnO and FeO diffuse into the chromite grains, generating a (Zn,Mg,Fe)(Fe,Al,Cr)2 O4 spinel. Although in this first study the refractory degradation mechanisms were identified for a fixed ZnO content, it is anticipated that a higher ZnO content can influence the solubility of MgO in the slag, thereby changing the dissolution behaviour of the periclase grains in magnesia-chromite refractories. Additionally, the reaction products from slag/refractory interactions may be also affected, leading to the formation of (Zn,Fe,Mg)2 SiO4 willemite and (Zn,Fe,Mg)O zincite [2,7,8]. Therefore, it is important to quantify the effect of ZnO in the ZFS slag on the magnesia-chromite refractory degradation. However, only limited experimental data can be found with respect to this topic. To complement the authors’ prior work on the degradation of magnesia-chromite refractories by ZFS slag [2], this analysis examines the effect of the ZnO level in fayalite slags on the slag/magnesia-chromite refractory interactions. This is performed by varying the ZnO content in ZFS slags from 0 to 20 wt% in contact with magnesia-chromite refractories through rotating refractory finger and MgO crucible corrosion tests. The impact on the MgO dissolution behaviour and the refractory/slag interfacial reactions were determined by microstructural and chemical analyses using Electron Probe Micro Analysis (EPMA) and Scanning Electron Microscopy (SEM). The experimental data were supported by thermodynamic calculations using the FactSage software.
∗ Corresponding author. Fax: +32 16321991. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.004 0955-2219/© 2016 Published by Elsevier Ltd.
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Table 1 Chemical compositions of the as-prepared industrial fayalite-based ZnO-containing fayalite slags before tests, as determined by XRF (in wt%). Exp. no.
ZnO
“FeO”a
SiO2
Al2 O3
CaO
MgO
MoO3
Others
I00 I10 I20
0.1 10.2 21.2
51.6 48.1 44.0
37.7 32.4 27.0
5.0 4.0 3.3
1.0 0.8 0.5
0.5 0.4 0.4
0.0 0.0 0.0
4.1 4.1 3.5
a All in the slag Fe is calculated in the forms of “FeO” for the purpose of presentation.
Table 2 Chemical compositions of the as-prepared synthetic fayalite-based ZnO-containing fayalite slags before tests (in wt%). Exp. no.
ZnO
“FeO”
SiO2
Al2 O3
CaO
S00 S10 S20
0.0 10.0 20.0
53.6 48.3 42.9
38.3 34.5 30.7
6.9 6.2 5.5
1.1 1.0 0.9
2. Experimental 2.1. Materials preparation 2.1.1. ZnO-containing fayalite slags Industrial and synthetic fayalite slags were used as master slags for the rotating magnesia-chromite refractory finger tests and MgO crucible tests, respectively. The former slag was provided by a copper-making company, and the latter slag was prepared in a method similar to that reported by Fukuyama et al. [9,10]. Powders of high purity iron (Fe > 99.9 wt%, supplied by Chempur), hematite (Fe2 O3 > 99.9 wt%, supplied by Sigma–Aldrich) and silica (SiO2 > 98.0 wt%, supplied Sibelco Benelux) were mixed for 24 h in ethanol in a shake mill, using ZrO2 balls ( 5 mm) with a balls/powder ratio of 3/1. The resultant slurry was dried at 80 ◦ C for 24 h to remove ethanol. The details for the mixing have been described in previous work [11,12]. The synthetic fayalite slag was prepared with a ‘FeO’/SiO2 weight ratio of 1.4. The weight of ‘FeO’ was calculated by summing the weights of Fe2 O3 and Fe based on the chemical reaction between Fe2 O3 and Fe (1): Fe2 O3 + Fe → 2FeO
(1)
The synthetic fayalite slag was obtained by calcining the mixed powder mixture in an iron crucible at 1150 ◦ C for 12 h. Purified Ar gas (passing the gas through silica gel and Mg turnings furnace operating at 500 ◦ C to remove traces of moisture and oxygen) was blown into the furnace with a flow rate of ∼0.2 l·min−1 to maintain a low oxygen partial pressure in the furnace. The oxygen partial pressure is measured lower than 10−18 atm, in accordance to previously published data [2]. Afterwards, both the industrial and synthetic fayalite slags were ground into powders with a particle size of less than 80 m. Various ZnO-containing fayalite (ZnO–FeO–SiO2 –Al2 O3 –CaO, ZFS) slags were prepared by mixing the ground industrial or synthetic fayalite slags with 0, 10.0 and 20.0 wt% of reagent grade ZnO (99.8 wt%, supplied by Umicore) using shake mill for 24 h in the same method [11,12]. CaO calcined from CaCO3 (99.8 wt%, supplied by Sigma–Aldrich) at 1000 ◦ C for 24 h, and Al2 O3 (>99.0 wt%, supplied by Sasol North America, Inc.) were added to simulate the CaO/SiO2 and Al2 O3 /SiO2 ratios of the industrial fayalite slag. The as-prepared industrial and synthetic fayalite-based ZFS slags were denoted as I00, I10 and I20, and S00, S10 and S20, respectively. Tables 1 and 2 illustrate the compositions of the prepared industrial and synthetic fayalite-based ZFS slags, respectively.
2.1.2. Refractory finger and MgO crucible samples The cylindrical refractory fingers (diameter = about 15 mm; length = around 50 mm) were cut from a commercially available direct-bonded magnesia-chromite brick. The detailed chemical composition and physical properties of the investigated refectory can be found in the previous report [2]. Commercially available MgO crucibles (inside diameter = 14.0 mm; height = 40.0 mm, supplied by Ozark Technical Ceramics, Inc.) were used to investigate the MgO/slag interfacial reactions. The MgO crucible consisted of >97.0 wt% MgO, 2.0 wt% Y2 O3 as sintering additive and a small amount of SiO2 and CaO impurities. 2.2. Experimental set-up and procedure 2.2.1. Rotating refractory finger tests The details for experimental set-up and the procedure for rotating magnesia-chromite refractory finger tests have been reported in previous work [2]. Approximately 170 g of the as-prepared industrial fayalite-based ZFS slag (I00, I10 or I20) was filled in a Mo crucible (inside diameter = 40 mm; height = 80 mm), heated in a vertical tube furnace (GERO HTRV 100-250/18, with MoSi2 heating elements) to 1200 ◦ C, and kept at that temperature for 50 min to ensure the molten state. Purified Ar gas was blown into the furnace at a flow rate of around 0.4 l·min−1 to simulate the reducing atmosphere used in the industrial process. A cylindrical refractory finger sample was moved down and maintained at a distance of 10 mm above the Mo crucible for 10 min to preheat the sample. Then the refractory finger sample was immersed into the molten industrial fayalite-based ZFS slags at 1200 ◦ C for 4 h with a rotation speed of 36 rpm. Afterwards, the refractory finger sample was removed from the molten slag and quenched in Ar gas. The slag was cooled down to room temperature in the furnace, removed from the Mo crucible and ground to a particle size smaller than 80 m for further compositional analysis. 2.2.2. MgO crucible tests Approximately 6 g of the synthetic fayalite-based ZFS slag (S00, S10 or S20) was filled in the high density MgO crucible. Fig. 1 shows the schematic diagram for the MgO-crucible tests. ZnO can be reduced and continuously removed from the slag with gas flow in an open system under reducing atmosphere. In this case, the ZnO content in the ZFS slag would gradually decrease with exposure time. In order to simulate the reducing atmosphere and to avoid the massive loss of ZnO for the 12 h dwell time in crucible tests, a piece of high purity iron foil (99.5 wt%, supplied by Goodfellow) was positioned into the MgO crucible, contacting with the slag. The MgO crucible was then sealed in a quartz glass tube with Ar gas (Fig. 1). Equilibrium with metallic iron in liquidus and/or subliquidus ZnO–FeO–SiO2 systems has been used in a number of studies, wherein the ZnO–FeO–SiO2 -based slags are sealed in an iron foil envelope [8,13]. The sealed quartz tube was suspended by a stainless steel wire (diameter = 1 mm) in the hot zone of the vertical tube furnace (GERO HTRV 100-250/18, with MoSi2 heating elements), and heated to 1200 ◦ C. After soaking for 12 h at 1200 ◦ C, the sealed quartz tube was dropped into a water bath directly beneath the furnace tube by releasing the stainless steel wire. 2.3. Sample characterization technique The worn cylindrical refractory samples were sliced with a diamond saw 10 mm below the slag line, and the quenched crucibles with their contents were cut along the vertical direction. Then, the refractory and crucible specimens were embedded in a low viscosity resin (Epofix) by vacuum impregnation. The embedded specimens were ground with SiC grinding paper, polished with diamond paste and coated with a conductive carbon layer for
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Fig. 1. Schematic drawing of the vertical tube furnace and sealed MgO crucible for the MgO crucible tests.
compositional and microstructural characterization. The samples from the refractory finger and MgO crucible tests were also called I00, I10 and I20, and S00, S10 and S20, respectively, according to the slag used in the test. The microstructure of the specimens from the refractory finger tests and crucible tests were characterized using Scanning Electron Microscopy (SEM, Philips XL-30 FEG) at 15 kV. The compositional analyses of the specimens were performed with a fully quantitative electron probe micro analysis system coupled with wavelength dispersive spectroscopy (EPMA-WDS, JEOL JXA-8530F) using an acceleration voltage of 15 kV and a probe current of 15 nA. Periclase (MgO), hematite (Fe2 O3 ), chromium oxide (Cr2 O3 ), willemite (Zn2 SiO4 ), fluorspar (CaF2 ) and obsidian ((Na,K,Al,Fe) silicate glass) were used as standards. The global slag composition after MgO-crucible tests were measured with six areas with a diameter of 200 m of the tested bulk slag. The ground slag before and after the refractory finger corrosion tests was analysed with a wavelength-dispersive X-ray fluorescence spectrometer (WDXRF) from Philips (PW2400 sequential X-ray spectrometer) using UniQuant for semi-quantitative analysis. Although both Fe2+ and Fe3+ are possibly present in the slag and refractory components, for the sake of presentation, all Fe is calculated as “FeO” in the slag and as “Fe2 O3 ” in the chromite. Likewise, although (Fe,Mg)O magnesiowüstite, (Fe,Mg)2 SiO4 olivine and (Fe,Mg)(Al,Fe,Cr)2 O4 spinel can be generated in the refractory samples in contact with ZnOfree fayalite slags, here we use (Fe,Mg,Zn)O, (Fe,Mg,Zn)2 SiO4 and ZnO-containing spinel for the sake of generality. 2.4. Methodology of thermodynamic calculation Thermodynamic calculations were performed with the FactSage software [14] (version 6.4). Equilibrium calculations were performed with the equilibrium module EQUILIB, which is based on the minimization of the Gibbs free energy, and the FT oxide, FS stel and Fact PS databases. The following possible solution phases are
chosen in the calculations: (1) FT oxide-slag (molten oxide phase); (2) FT oxide-monoxide (oxide solid solution); (3) FT oxide-spinel (spinel solid solution); (4) FT oxide-olivine; (5) FT oxide-zincite; (6) FT oxide-willemite, (7) FS stel-FCC (metallic iron) and (8) FACT PS (gas phase). 1 g of metallic iron and 100 grams of the synthetic fayalite-based ZFS slags were included in the calculations to impose iron saturation of the samples. The dissolution of MgO and the reaction path between MgO and ZFS slags were modeled at 1200 ◦ C by relative addition of pure MgO (x, in gram) into the slags ((100-x), in gram). The slag compositions for the calculation were taken from Table 2. The maximum MgO dissolution amount into the ZFS slag is determined when a solid phase precipitates from the liquid slag. 3. Results and discussion 3.1. Microstructure of the corroded refractory samples Fig. 2 shows the microstructures of the surface and centre zones of the corroded magnesia-chromite refractory finger samples. The periclase is predominantly corroded by the slags in all tests. The dissolution of MgO into ZFS slags, and the formation of (Fe,Mg,Zn)2 SiO4 and (Fe,Mg,Zn)O solid solutions (‘Oss’ in Fig. 2a, b) are the three corrosion mechanisms of periclase by ZFS slags [2]. The chromite in the refractory showed a limited reaction with the ZFS slag. The main corrosion mechanism of chromite is that ZnO and FeO diffuse into chromite grains and replace MgO from chromite grains into the ZFS slag, thereby generating a ZnO-containing spinel (‘Sp1 in Fig. 2) [2]. A Mo-containing spinel (‘MS’ in Fig. 2b) was observed in the adhered slag at the surface zone of the finger sample I20, due to the corrosion of the Mo crucible (Table 3). As shown in Fig. 2c and d, refractory finger samples were completely infiltrated by the liquid slag in all tests. The only difference between the corroded refractory samples I00, I10 and I20 is the thickness of the newly formed (Fe,Mg,Zn)O solid solution layer. A thicker Oss layer is observed in the refractory sample from the test I20, suggesting
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Fig. 2. Overview of worn samples from Tests I00 and I20, showing the surface and centre zones of corroded refractory samples: Per = magnesia (periclase); C = primary chromite; Oss = (Fe,Mg,Zn)O solid solution; CI = secondary chromite type I; CII = secondary chromite type II; Sp1 = spinel solid solution; CMS = CaO–MgO–SiO2 impurities; Ol = (Fe,Mg,Zn)2 SiO4 olivine; IS = infiltrated slag; Cus = (Cu,Fe) sulphide; MS = Mo-containing spinel; P = pore; Cra = crack. Table 3 Chemical compositions of the industrial fayalite-based ZnO-containing fayalite (ZFS) slags after refractory finger tests, as determined by XRF (in wt%).
Table 4 Chemical compositions of the synthetic fayalite-based ZFS slags after MgO crucible tests, as determined by EPMA-WDS (in wt%).
Exp. no.
ZnO
“FeO”
SiO2
Al2 O3
CaO
MgO
MoO3
Others
MgOa
Exp. no. ZnO
I00 I10 I20
0.0 8.9 18.3
51.6 47.3 41.6
36.9 33.7 30.7
4.6 4.3 3.8
1.1 0.9 0.8
1.3 1.2 0.8
0.3 0.7 0.6
4.1 2.9 3.3
0.8 0.8 0.4
S00 S10 S20
a
0.0 10.0 ± 0.3 16.7 ± 0.3
“FeO”
SiO2
Al2 O3
CaO
MgO
50.9 ± 0.3 37.6 ± 0.6 7.0 ± 0.2 1.1 ± 0.0 1.3 ± 0.1 47.5 ± 0.6 33.3 ± 0.6 6.3 ± 0.4 0.9 ± 0.1 1.3 ± 0.1 41.4 ± 0.7 31.0 ± 0.3 5.4 ± 0.3 0.8 ± 0.0 1.4 ± 0.1
wt% MgOaftertest − wt% MgObeforetest .
that the formation of (Fe,Mg,Zn)O solid solution is facilitated with a higher ZnO content in the slag. (Cu,Fe) sulphide (‘Cus’ in Fig. 2) can be found in both the adhered slag and infiltrated slag in all worn refractory samples. In brief, the corrosion behaviour of magnesia-chromite refractories in ZFS slags can be characterised by (1) the dissolution of MgO from periclase in the slag, (2) the formation of (Fe,Mg,Zn)2 SiO4 and (Fe,Mg,Zn)O solid solutions from slag/periclase interactions, (3) the replacement of MgO in chromite grains into ZFS slags by ZnO and FeO from the slags to generate a ZnO-containing spinel and (4) the infiltration of liquid ZFS slags into the refractories [2]. Since refractory finger samples were completely infiltrated by the liquid slag in all tests, therefore, the corrosion phenomena of MgO dissolution, and MgO/slag and chromite/slag interactions will be discussed in detail in the following sections to investigate the effect of ZnO in ZFS slags on the degradation of magnesia-chromite refractories. 3.2. Effect of ZnO in ZFS slags on the dissolution of MgO The global slag compositions after completion of the refractory finger tests and MgO crucible tests are provided in Tables 3 and 4, respectively. The changes of the MgO content (MgO) in the industrial fayalite-based ZnO-containing fayalite (ZFS) slags after the rotating refractory finger tests are less than 1.0 wt%. The change of the MgO content in the ZFS slag is constant at 0.8 wt% with
increasing ZnO content from 0 to 10.0 wt% in the industrial fayalitebased ZFS slag, and then slightly decreases from 0.8 wt% to 0.4 wt% when the ZnO content is raised from 10.0 wt% to 20.0 wt%. Because the increased MgO content in the ZFS slag is mainly caused by the dissolution of MgO from periclase grains [2], the observed changes of the MgO content indicate that the dissolution of MgO from periclase into ZFS slags is not enhanced by increasing the ZnO content in the slags with the fixed FeO/SiO2 ratio. As shown in Table 4, the MgO contents in the synthetic fayalite-based ZFS slags after the completion of the MgO crucible tests are stable at around 1.3 wt% when the ZnO content in the tested ZFS slag increases from 0 to 16.7 wt%. The result confirms that the increase of ZnO content in ZFS slags does not influence the dissolution of MgO from periclase into the slags. Fig. 3 shows the calculated maximum concentrations of MgO soluble (MgO solubility) in ZFS slags with different ZnO contents at 1200 ◦ C. The MgO solubility in ZFS slags slightly decreases from ∼2.3 to ∼2.0 wt% when the ZnO content in ZFS slags increases from 0 to 16.7 wt%. This reveals that the driving force for the dissolution of MgO into the ZFS slag decreases with increasing the ZnO content in the slag. It is interesting to note in Fig. 3 that the measured MgO contents are lower than the MgO solubility in the tested synthetic fayalitebased ZFS slags, implying that the maximum concentrations of MgO dissolved in the slag was not reached after corrosion for 12 h at 1200 ◦ C. This result indicates that the dissolution rate of MgO into the ZFS slags is slow. This is due most likely to the formation of
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Fig. 3. Comparison of the predicted MgO solubility at 1200 ◦ C and the measured MgO contents in the tested synthetic fayalite-based ZFS slags.
new phases from the slag/periclase interactions, slowing down the dissolution of MgO from periclase into the slags. This is discussed in detail in next section. 3.3. Effect of ZnO in ZFS slags on the slag/periclase interaction 3.3.1. (Fe,Mg,Zn)2 SiO4 olivine formation Fig. 4 illustrates the interactions at the slag/MgO interfaces from refractory finger tests (Fig. 4a and b) and MgO crucible tests (Fig. 4c and d). The (Fe,Mg,Zn)2 SiO4 solid solution (‘Ol’ in Fig. 4a and b) forms a continuous layer with a thickness of less than 20 m at the surface as a continuous layer on the Oss phase in the refractory fingers. The thickness of the (Fe,Mg,Zn)2 SiO4 solid solution layer in the sample I20 (Fig. 4b) is less compared to that in the sample I00 (Fig. 4a). Similar to the refractory finger tests, the newly
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formed (Fe,Zn,Mg)2 SiO4 solid solution exhibits different grain size after MgO crucible tests. A grain size of over 100 m can be seen in the S00 (Fig. 4c) and S10 (not shown here) samples. In the sample S20, however, a smaller grain size of less than 100 m is observed (Fig. 4d). These results indicate that the formation of the (Fe,Mg,Zn)2 SiO4 solid solution was hindered in a higher ZnO content ZFS slag. This is probably due to the lowered SiO2 concentration with increasing the ZnO content in the slags (the dilution effect). Fig. 5 presents the results of a chemical line analysis of the newly formed (Fe,Mg,Zn)2 SiO4 solid solution in the sample S20. For larger distances from the slag/olivine interface (Fig. 5a), the ZnO content is constant at ∼10.5 mol%. This ZnO concentration is less than the maximum concentration of ZnO soluble in Mg2 SiO4 forsterite or Fe2 SiO4 fayalite [2,7,8], indicating that the newly formed (Fe,Mg,Zn)2 SiO4 solid solution is an olivine group member rather than a (Zn,Fe,Mg)2 SiO4 willemite, even though the ZnO content in the tested ZFS slag reaches ∼16.7 wt%. The absence of a (Zn,Fe,Mg)2 SiO4 willemite can be explained using a thermodynamic calculation. Fig. 6 shows the reaction scheme between MgO and ZFS slags with various ZnO contents at 1200 ◦ C. When the ZnO content in the ZFS slag is lower than ∼22.5 wt%, a composition ‘X’ passes from the ‘Liquid slag’ region into the two-phase region ‘Liquid slag + Olivine’ by adding MgO. This result indicates that olivine firstly precipitates from the slag/MgO interaction once the slag is saturated with MgO. In comparison, when the ZnO content in ZFS slags is higher than ∼22.5 wt%, the composition ‘Y’ will pass from the zone ‘Liquid slag’ into the zone ‘Liquid slag + Willemite’ by increasing the MgO content in the slag. As a result, the (Zn,Fe,Mg)2 SiO4 willemite will precipitate at the slag/MgO interface. As in all tests the ZnO contents in the industrial and synthetic fayalite-based ZFS slags are lower than 22.0 wt%, only (Fe,Mg,Zn)2 SiO4 olivine can precipitate from the ZFS slag/MgO interactions. On the other hand, when the ZnO content in the ZFS slag reaches 30.0 wt%, (Zn,Fe,Mg)2 SiO4 willemite
Fig. 4. Backscattered electron (BSE) images of the MgO/slag interface after (a) and (b) refractory finger tests, and (c) and (d) MgO crucible tests, showing the impact of ZnO content on the (Fe,Mg,Zn)2 SiO4 olivine formation: Oss = (Fe,Mg,Zn)O solid solution; Ol = (Fe,Mg,Zn)2 SiO4 olivine; MS = Mo-containing spinel; P = pore; Fay = (Fe,Zn)2 SiO4 fayalite; Amor = amorphous phase. The dash line in (d) shows the olivine/slag interface.
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Fig. 5. (a) ZnO, FeO, MgO and SiO2 concentration gradient profiles of the selected ‘Line AB’ in (b), in which the newly formed (Fe, Mg, Zn)2 SiO4 solid solution in the sample S20 is shown, Fay = (Fe,Zn)2 SiO4 fayalite; Ol = (Fe,Mg,Zn)2 SiO4 olivine; Oss = (Fe,Mg,Zn)O solid solution.
Fig. 6. Phase stability regions at 1200 ◦ C predicted by FactSage calculation, showing the influence of the ZnO level in the slag on the path of the slag/MgO interfacial reaction; the light grey dash line shows the predicted minimum ZnO content in liquid slag for (Zn,Fe,Mg)2 SiO4 willemite precipitation by adding MgO.
precipitates at the slag/MgO interface (not shown here), which is generally in line with the thermodynamic calculations. 3.3.2. (Fe,Mg,Zn)O solid solution formation In order to evaluate the extent of the (Fe,Mg,Zn)O solid solution (‘Oss’ in Fig. 2a and b) formation from the slag/periclase interaction in the refractory fingers, the average thickness of the Oss layer is used. The thickness of the Oss layer was measured through BSE images at the middle of a large periclase grain with a diameter larger than 500 m, to avoid the growth of the Oss layer influenced by the infiltration slag (Fig. 2b). The average thicknesses of the Oss layers are shown in Fig. 7. The thickness of the Oss layer formed at the slag/periclase interface in worn refractory finger surfaces (Fig. 2a and b) exhibits a positive correlation with the ZnO content of the industrial fayalite-based ZFS slags (Fig. 7). Fig. 8 shows the relationship between the ZnO content of the tested synthetic fayalite-based ZFS slags and the thickness of the “Oss” layer generated at the slag/MgO interface from MgO crucible tests, which is determined based on the MgO concentration in the Oss layer (Fig. 8a). The results are illustrated in Fig. 8b. As shown in Fig. 8b, the growth of the Oss layer exhibits a linear relationship with the ZnO content increase in the tested slag, which is in good agreement with the result from the rotating refractory finger tests (Fig. 7). The thickness of the Oss layer increases from ∼70 to
Fig. 7. Effect of the ZnO content in the industrial fayalite-based ZFS slag on the layer thickness of the (Fe,Mg,Zn)O solid solution (Oss layer).
∼90 m when the ZnO content increases from 0 to 10.0 wt%, and to ∼100 m for a ZnO content of 16.7 wt% in the tested synthetic fayalite-based ZFS slag (Fig. 8). This result confirms that the formation of (Fe,Mg,Zn)O solid solution is enhanced for a higher ZnO content in the ZFS slag. As shown in Fig. 8a, the maximum ZnO concentration in the newly-formed (Fe,Mg,Zn)O solid solution from test S20 is ∼10.5 mol%. It is believed that this (Fe,Mg,Zn)O solid solution is a kind of magnesiowüstite, since the maximum ZnO concentration of ∼10.5 mol% in the (Fe,Mg,Zn)O solid solution is lower than the ZnO solubility of ∼24.0 mol% and ∼32.0 mol% in the FeO and MgO, respectively, at 1200 ◦ C [2,7,8]. 3.4. Effect of ZnO in ZFS slags on the chromite corrosion There is no obvious difference in the thickness of the newlyformed ZnO-containing spinel by changing the ZnO content in ZFS slags, because the chromite grains barely react with the ZFS slags [2]. The changed ZnO content in ZFS slags, however, may influence the chemical composition of the newly-formed ZnO-containing spinel, as the driving force for the diffusion of FeO and ZnO is changed. Fig. 9 shows the chemical composition of the newly-formed ZnO-containing spinel surrounding chromite grains. The spinel rim was selected in the chromite grains at a distance of 0.5 mm from the surface zone of the refractory finger to avoid the influence of
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Fig. 8. (a) MgO and ZnO concentration profiles in (Fe,Mg,Zn)O solid solution layers from MgO-crucible tests, and (b) the thickness of the (Fe,Mg,Zn)O solid solution layer based on the MgO concentration profile in (a): Ol = (Fe,Mg,Zn)2 SiO4 olivine; Oss = (Fe,Zn,Mg)O solid solution.
the formed Mo-containing spinel at the surface zone (Fig. 2b). The spots were selected in the spinel rim with a distance of ∼1 m from the slag/chromite interface, to avoid interaction of the surrounding slag phases. As shown in Fig. 9, the MgO content in this ZnO-containing spinel rim gradually decreases from ∼23.0 mol% to ∼11.0 mol% with increasing the ZnO content in ZFS slags from 0 to 20.0 wt%. Concurrently, the ZnO content increases at the expense of MgO in this spinel rim from 0 to ∼22.0 mol%. The result is consistent with a higher driving force for the diffusion of ZnO from the ZFS slag into chromite grains for a ZnO-richer ZFS slag. It is also interesting to note in Fig. 9 that the Fe2 O3 content in the newly-formed spinel rim slightly decreases from ∼36.0 mol% to 30.0 mol% by increasing the ZnO content in ZFS slags from 0 to 20.0 wt%. This observation indicates that the diffusion of Fe2 O3 from chromite is also favoured with increasing the ZnO content in the ZFS slag. This is because the driving force for the dissolution of Fe2 O3 is increased by the lowered Fe2 O3 concentration as the total ‘FeO’ concentration in a ZnO-rich fayalite slag decreases (Table 2). 4. Conclusion The effect of ZnO level in ZnO-containing fayalite (ZFS) slags on the chemical corrosion behaviour of magnesia-chromite refracto-
ries was studied. This was performed at 1200 ◦ C under reducing atmosphere using rotating refractory finger tests and MgO crucible tests for 4 h and 12 h, respectively. Based on the microstructural and compositional analyses, and thermodynamic calculations, the following conclusions can be drawn: (1) The dissolution of MgO from periclase into ZFS slags is not increased when increasing the ZnO content in the slags at a constant FeO/SiO2 ratio of 1.4. This concurs with the decrease of the driving force for the dissolution of periclase, as the solubility of MgO in ZFS slags decreases with increasing the ZnO content in the slags. (2) (Fe,Mg,Zn)2 SiO4 olivine group member was formed at the slag/periclase interface. There was no (Zn,Fe,Mg)2 SiO4 willemite generated in the slag/periclase interfacial reactions, even though the ZnO content in the ZFS slag reaches 16.7 wt%. The formation of (Fe,Mg,Zn)2 SiO4 olivine was inhibited, probably due to the decreased SiO2 concentration by increasing the ZnO content in ZFS slags. (3) The (Fe,Mg,Zn)O solid solution formation was intensified in a ZnO-rich ZFS slag. This is attributed to the increased driving force for the diffusion of ZnO. The growth of the (Fe,Mg,Zn)O solid solution layer exhibits a linear relationship with the ZnO contents in the tested ZFS slags. (4) The diffusion of ZnO from a ZnO-rich ZFS slag into chromite grains was also favoured, because of the higher driving force, thereby removing more MgO from chromite grains towards the ZnO-rich ZFS slag. Acknowledgements This research was performed with the financial support by the Agency for Innovation by Science and Technology (IWT, project no. 990348). The authors thank the engineers of Metallo-Chimique for their close co-operation. L. Chen and H. Shi are grateful to the China Scholarship Council (CSC) for offering scholarships (no. 20120704008 and 201208420605). The authors also acknowledge the support from the Hercules Foundation (project no. ZW09-09) in the use of the FEG-EPMA system. References
Fig. 9. MgO, ZnO and Fe2 O3 contents in the ZnO-containing spinel rim formed at 0.5 mm from the surface zone of the corroded refractory fingers, as determined by EPMA-WDS.
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Effect of ZnO level in secondary copper smelting slags on slag/magnesia-chromite refractory interactions Liugang Chen ∗ , Muxing Guo, Huayue Shi, Shuigen Huang, Peter Tom Jones, Bart Blanpain, Annelies Malfliet KU Leuven, Department of Materials Engineering, Kasteelpark Arenberg 44, BE-3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Received 4 December 2015 Received in revised form 1 February 2016 Accepted 2 February 2016 Available online xxx Keywords: Refractories Magnesia-chromite MgO ZnO-containing fayalite slag Corrosion
a b s t r a c t To determine the effect of ZnO in ZnO-containing fayalite (ZFS) slags on the degradation of magnesiachromite refractories, the corrosion behaviour of a direct-bonded magnesia-chromite refractory was studied by rotating refractory finger tests at 1200 ◦ C under reducing atmosphere. In addition, MgO crucible tests at these conditions were performed to investigate in particular the effect of ZnO on the slag/MgO interaction. The dissolution of MgO from periclase into the ZFS slags is not enhanced by increasing the ZnO content in the slags. (Fe,Mg,Zn)2 SiO4 olivine and (Fe,Mg,Zn)O solid solution are the only two products in the slag/MgO interfacial reactions, even when the ZnO content in the ZFS slag reaches 16.7 wt%. The diffusion of ZnO into the chromite grains to form the (Fe,Mg,Zn)(Fe,Al,Cr)2 O4 spinel was favoured with increasing the ZnO content in ZFS slags. © 2016 Published by Elsevier Ltd.
1. Introduction ZnO-containing fayalite (ZFS) slags can be generated in copper smelters during the smelting process of secondary raw materials [1,2]. In such slags, the ZnO content can be varied depending on the feed materials and/or operation conditions. For example, ZnO in the ZFS slags can be enriched using high Zn (or ZnO) content secondaries and/or reducing the loss of ZnO into the off-gas during operation. Because ZnO is a basic oxide and can provide free O2− to break the silica network [3,4], the variation in the ZnO content of ZFS slags can consequently change both the basicity and viscosity of the slags. As a result, the degradation of the lining materials of the copper smelter will be influenced. Magnesia-chromite refractories are the most suitable materials in lining the furnaces used in copper-making practices [1,5,6]. In previous work [2], the chemical corrosion behaviour of magnesiachromite refractories by a ZFS slag with ∼9.0 wt% ZnO has been studied. It can be concluded that similar to the degradation mechanisms contacting with ZnO free fayalite slags, the periclase phase in the magnesia-chromite refractories is dominantly corroded by the ZFS slag because of the MgO dissolution into the slag. ZnO along with FeO and SiO2 from the ZFS slag react with the periclase in the magnesia-chromite refractory, forming a (Fe,Mg,Zn)2 SiO4 olivine
and a (Fe,Mg,Zn)O solid solution [2]. Concurrently, ZnO and FeO diffuse into the chromite grains, generating a (Zn,Mg,Fe)(Fe,Al,Cr)2 O4 spinel. Although in this first study the refractory degradation mechanisms were identified for a fixed ZnO content, it is anticipated that a higher ZnO content can influence the solubility of MgO in the slag, thereby changing the dissolution behaviour of the periclase grains in magnesia-chromite refractories. Additionally, the reaction products from slag/refractory interactions may be also affected, leading to the formation of (Zn,Fe,Mg)2 SiO4 willemite and (Zn,Fe,Mg)O zincite [2,7,8]. Therefore, it is important to quantify the effect of ZnO in the ZFS slag on the magnesia-chromite refractory degradation. However, only limited experimental data can be found with respect to this topic. To complement the authors’ prior work on the degradation of magnesia-chromite refractories by ZFS slag [2], this analysis examines the effect of the ZnO level in fayalite slags on the slag/magnesia-chromite refractory interactions. This is performed by varying the ZnO content in ZFS slags from 0 to 20 wt% in contact with magnesia-chromite refractories through rotating refractory finger and MgO crucible corrosion tests. The impact on the MgO dissolution behaviour and the refractory/slag interfacial reactions were determined by microstructural and chemical analyses using Electron Probe Micro Analysis (EPMA) and Scanning Electron Microscopy (SEM). The experimental data were supported by thermodynamic calculations using the FactSage software.
∗ Corresponding author. Fax: +32 16321991. E-mail address: [email protected] (L. Chen). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.02.004 0955-2219/© 2016 Published by Elsevier Ltd.
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Table 1 Chemical compositions of the as-prepared industrial fayalite-based ZnO-containing fayalite slags before tests, as determined by XRF (in wt%). Exp. no.
ZnO
“FeO”a
SiO2
Al2 O3
CaO
MgO
MoO3
Others
I00 I10 I20
0.1 10.2 21.2
51.6 48.1 44.0
37.7 32.4 27.0
5.0 4.0 3.3
1.0 0.8 0.5
0.5 0.4 0.4
0.0 0.0 0.0
4.1 4.1 3.5
a All in the slag Fe is calculated in the forms of “FeO” for the purpose of presentation.
Table 2 Chemical compositions of the as-prepared synthetic fayalite-based ZnO-containing fayalite slags before tests (in wt%). Exp. no.
ZnO
“FeO”
SiO2
Al2 O3
CaO
S00 S10 S20
0.0 10.0 20.0
53.6 48.3 42.9
38.3 34.5 30.7
6.9 6.2 5.5
1.1 1.0 0.9
2. Experimental 2.1. Materials preparation 2.1.1. ZnO-containing fayalite slags Industrial and synthetic fayalite slags were used as master slags for the rotating magnesia-chromite refractory finger tests and MgO crucible tests, respectively. The former slag was provided by a copper-making company, and the latter slag was prepared in a method similar to that reported by Fukuyama et al. [9,10]. Powders of high purity iron (Fe > 99.9 wt%, supplied by Chempur), hematite (Fe2 O3 > 99.9 wt%, supplied by Sigma–Aldrich) and silica (SiO2 > 98.0 wt%, supplied Sibelco Benelux) were mixed for 24 h in ethanol in a shake mill, using ZrO2 balls ( 5 mm) with a balls/powder ratio of 3/1. The resultant slurry was dried at 80 ◦ C for 24 h to remove ethanol. The details for the mixing have been described in previous work [11,12]. The synthetic fayalite slag was prepared with a ‘FeO’/SiO2 weight ratio of 1.4. The weight of ‘FeO’ was calculated by summing the weights of Fe2 O3 and Fe based on the chemical reaction between Fe2 O3 and Fe (1): Fe2 O3 + Fe → 2FeO
(1)
The synthetic fayalite slag was obtained by calcining the mixed powder mixture in an iron crucible at 1150 ◦ C for 12 h. Purified Ar gas (passing the gas through silica gel and Mg turnings furnace operating at 500 ◦ C to remove traces of moisture and oxygen) was blown into the furnace with a flow rate of ∼0.2 l·min−1 to maintain a low oxygen partial pressure in the furnace. The oxygen partial pressure is measured lower than 10−18 atm, in accordance to previously published data [2]. Afterwards, both the industrial and synthetic fayalite slags were ground into powders with a particle size of less than 80 m. Various ZnO-containing fayalite (ZnO–FeO–SiO2 –Al2 O3 –CaO, ZFS) slags were prepared by mixing the ground industrial or synthetic fayalite slags with 0, 10.0 and 20.0 wt% of reagent grade ZnO (99.8 wt%, supplied by Umicore) using shake mill for 24 h in the same method [11,12]. CaO calcined from CaCO3 (99.8 wt%, supplied by Sigma–Aldrich) at 1000 ◦ C for 24 h, and Al2 O3 (>99.0 wt%, supplied by Sasol North America, Inc.) were added to simulate the CaO/SiO2 and Al2 O3 /SiO2 ratios of the industrial fayalite slag. The as-prepared industrial and synthetic fayalite-based ZFS slags were denoted as I00, I10 and I20, and S00, S10 and S20, respectively. Tables 1 and 2 illustrate the compositions of the prepared industrial and synthetic fayalite-based ZFS slags, respectively.
2.1.2. Refractory finger and MgO crucible samples The cylindrical refractory fingers (diameter = about 15 mm; length = around 50 mm) were cut from a commercially available direct-bonded magnesia-chromite brick. The detailed chemical composition and physical properties of the investigated refectory can be found in the previous report [2]. Commercially available MgO crucibles (inside diameter = 14.0 mm; height = 40.0 mm, supplied by Ozark Technical Ceramics, Inc.) were used to investigate the MgO/slag interfacial reactions. The MgO crucible consisted of >97.0 wt% MgO, 2.0 wt% Y2 O3 as sintering additive and a small amount of SiO2 and CaO impurities. 2.2. Experimental set-up and procedure 2.2.1. Rotating refractory finger tests The details for experimental set-up and the procedure for rotating magnesia-chromite refractory finger tests have been reported in previous work [2]. Approximately 170 g of the as-prepared industrial fayalite-based ZFS slag (I00, I10 or I20) was filled in a Mo crucible (inside diameter = 40 mm; height = 80 mm), heated in a vertical tube furnace (GERO HTRV 100-250/18, with MoSi2 heating elements) to 1200 ◦ C, and kept at that temperature for 50 min to ensure the molten state. Purified Ar gas was blown into the furnace at a flow rate of around 0.4 l·min−1 to simulate the reducing atmosphere used in the industrial process. A cylindrical refractory finger sample was moved down and maintained at a distance of 10 mm above the Mo crucible for 10 min to preheat the sample. Then the refractory finger sample was immersed into the molten industrial fayalite-based ZFS slags at 1200 ◦ C for 4 h with a rotation speed of 36 rpm. Afterwards, the refractory finger sample was removed from the molten slag and quenched in Ar gas. The slag was cooled down to room temperature in the furnace, removed from the Mo crucible and ground to a particle size smaller than 80 m for further compositional analysis. 2.2.2. MgO crucible tests Approximately 6 g of the synthetic fayalite-based ZFS slag (S00, S10 or S20) was filled in the high density MgO crucible. Fig. 1 shows the schematic diagram for the MgO-crucible tests. ZnO can be reduced and continuously removed from the slag with gas flow in an open system under reducing atmosphere. In this case, the ZnO content in the ZFS slag would gradually decrease with exposure time. In order to simulate the reducing atmosphere and to avoid the massive loss of ZnO for the 12 h dwell time in crucible tests, a piece of high purity iron foil (99.5 wt%, supplied by Goodfellow) was positioned into the MgO crucible, contacting with the slag. The MgO crucible was then sealed in a quartz glass tube with Ar gas (Fig. 1). Equilibrium with metallic iron in liquidus and/or subliquidus ZnO–FeO–SiO2 systems has been used in a number of studies, wherein the ZnO–FeO–SiO2 -based slags are sealed in an iron foil envelope [8,13]. The sealed quartz tube was suspended by a stainless steel wire (diameter = 1 mm) in the hot zone of the vertical tube furnace (GERO HTRV 100-250/18, with MoSi2 heating elements), and heated to 1200 ◦ C. After soaking for 12 h at 1200 ◦ C, the sealed quartz tube was dropped into a water bath directly beneath the furnace tube by releasing the stainless steel wire. 2.3. Sample characterization technique The worn cylindrical refractory samples were sliced with a diamond saw 10 mm below the slag line, and the quenched crucibles with their contents were cut along the vertical direction. Then, the refractory and crucible specimens were embedded in a low viscosity resin (Epofix) by vacuum impregnation. The embedded specimens were ground with SiC grinding paper, polished with diamond paste and coated with a conductive carbon layer for
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Fig. 1. Schematic drawing of the vertical tube furnace and sealed MgO crucible for the MgO crucible tests.
compositional and microstructural characterization. The samples from the refractory finger and MgO crucible tests were also called I00, I10 and I20, and S00, S10 and S20, respectively, according to the slag used in the test. The microstructure of the specimens from the refractory finger tests and crucible tests were characterized using Scanning Electron Microscopy (SEM, Philips XL-30 FEG) at 15 kV. The compositional analyses of the specimens were performed with a fully quantitative electron probe micro analysis system coupled with wavelength dispersive spectroscopy (EPMA-WDS, JEOL JXA-8530F) using an acceleration voltage of 15 kV and a probe current of 15 nA. Periclase (MgO), hematite (Fe2 O3 ), chromium oxide (Cr2 O3 ), willemite (Zn2 SiO4 ), fluorspar (CaF2 ) and obsidian ((Na,K,Al,Fe) silicate glass) were used as standards. The global slag composition after MgO-crucible tests were measured with six areas with a diameter of 200 m of the tested bulk slag. The ground slag before and after the refractory finger corrosion tests was analysed with a wavelength-dispersive X-ray fluorescence spectrometer (WDXRF) from Philips (PW2400 sequential X-ray spectrometer) using UniQuant for semi-quantitative analysis. Although both Fe2+ and Fe3+ are possibly present in the slag and refractory components, for the sake of presentation, all Fe is calculated as “FeO” in the slag and as “Fe2 O3 ” in the chromite. Likewise, although (Fe,Mg)O magnesiowüstite, (Fe,Mg)2 SiO4 olivine and (Fe,Mg)(Al,Fe,Cr)2 O4 spinel can be generated in the refractory samples in contact with ZnOfree fayalite slags, here we use (Fe,Mg,Zn)O, (Fe,Mg,Zn)2 SiO4 and ZnO-containing spinel for the sake of generality. 2.4. Methodology of thermodynamic calculation Thermodynamic calculations were performed with the FactSage software [14] (version 6.4). Equilibrium calculations were performed with the equilibrium module EQUILIB, which is based on the minimization of the Gibbs free energy, and the FT oxide, FS stel and Fact PS databases. The following possible solution phases are
chosen in the calculations: (1) FT oxide-slag (molten oxide phase); (2) FT oxide-monoxide (oxide solid solution); (3) FT oxide-spinel (spinel solid solution); (4) FT oxide-olivine; (5) FT oxide-zincite; (6) FT oxide-willemite, (7) FS stel-FCC (metallic iron) and (8) FACT PS (gas phase). 1 g of metallic iron and 100 grams of the synthetic fayalite-based ZFS slags were included in the calculations to impose iron saturation of the samples. The dissolution of MgO and the reaction path between MgO and ZFS slags were modeled at 1200 ◦ C by relative addition of pure MgO (x, in gram) into the slags ((100-x), in gram). The slag compositions for the calculation were taken from Table 2. The maximum MgO dissolution amount into the ZFS slag is determined when a solid phase precipitates from the liquid slag. 3. Results and discussion 3.1. Microstructure of the corroded refractory samples Fig. 2 shows the microstructures of the surface and centre zones of the corroded magnesia-chromite refractory finger samples. The periclase is predominantly corroded by the slags in all tests. The dissolution of MgO into ZFS slags, and the formation of (Fe,Mg,Zn)2 SiO4 and (Fe,Mg,Zn)O solid solutions (‘Oss’ in Fig. 2a, b) are the three corrosion mechanisms of periclase by ZFS slags [2]. The chromite in the refractory showed a limited reaction with the ZFS slag. The main corrosion mechanism of chromite is that ZnO and FeO diffuse into chromite grains and replace MgO from chromite grains into the ZFS slag, thereby generating a ZnO-containing spinel (‘Sp1 in Fig. 2) [2]. A Mo-containing spinel (‘MS’ in Fig. 2b) was observed in the adhered slag at the surface zone of the finger sample I20, due to the corrosion of the Mo crucible (Table 3). As shown in Fig. 2c and d, refractory finger samples were completely infiltrated by the liquid slag in all tests. The only difference between the corroded refractory samples I00, I10 and I20 is the thickness of the newly formed (Fe,Mg,Zn)O solid solution layer. A thicker Oss layer is observed in the refractory sample from the test I20, suggesting
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Fig. 2. Overview of worn samples from Tests I00 and I20, showing the surface and centre zones of corroded refractory samples: Per = magnesia (periclase); C = primary chromite; Oss = (Fe,Mg,Zn)O solid solution; CI = secondary chromite type I; CII = secondary chromite type II; Sp1 = spinel solid solution; CMS = CaO–MgO–SiO2 impurities; Ol = (Fe,Mg,Zn)2 SiO4 olivine; IS = infiltrated slag; Cus = (Cu,Fe) sulphide; MS = Mo-containing spinel; P = pore; Cra = crack. Table 3 Chemical compositions of the industrial fayalite-based ZnO-containing fayalite (ZFS) slags after refractory finger tests, as determined by XRF (in wt%).
Table 4 Chemical compositions of the synthetic fayalite-based ZFS slags after MgO crucible tests, as determined by EPMA-WDS (in wt%).
Exp. no.
ZnO
“FeO”
SiO2
Al2 O3
CaO
MgO
MoO3
Others
MgOa
Exp. no. ZnO
I00 I10 I20
0.0 8.9 18.3
51.6 47.3 41.6
36.9 33.7 30.7
4.6 4.3 3.8
1.1 0.9 0.8
1.3 1.2 0.8
0.3 0.7 0.6
4.1 2.9 3.3
0.8 0.8 0.4
S00 S10 S20
a
0.0 10.0 ± 0.3 16.7 ± 0.3
“FeO”
SiO2
Al2 O3
CaO
MgO
50.9 ± 0.3 37.6 ± 0.6 7.0 ± 0.2 1.1 ± 0.0 1.3 ± 0.1 47.5 ± 0.6 33.3 ± 0.6 6.3 ± 0.4 0.9 ± 0.1 1.3 ± 0.1 41.4 ± 0.7 31.0 ± 0.3 5.4 ± 0.3 0.8 ± 0.0 1.4 ± 0.1
wt% MgOaftertest − wt% MgObeforetest .
that the formation of (Fe,Mg,Zn)O solid solution is facilitated with a higher ZnO content in the slag. (Cu,Fe) sulphide (‘Cus’ in Fig. 2) can be found in both the adhered slag and infiltrated slag in all worn refractory samples. In brief, the corrosion behaviour of magnesia-chromite refractories in ZFS slags can be characterised by (1) the dissolution of MgO from periclase in the slag, (2) the formation of (Fe,Mg,Zn)2 SiO4 and (Fe,Mg,Zn)O solid solutions from slag/periclase interactions, (3) the replacement of MgO in chromite grains into ZFS slags by ZnO and FeO from the slags to generate a ZnO-containing spinel and (4) the infiltration of liquid ZFS slags into the refractories [2]. Since refractory finger samples were completely infiltrated by the liquid slag in all tests, therefore, the corrosion phenomena of MgO dissolution, and MgO/slag and chromite/slag interactions will be discussed in detail in the following sections to investigate the effect of ZnO in ZFS slags on the degradation of magnesia-chromite refractories. 3.2. Effect of ZnO in ZFS slags on the dissolution of MgO The global slag compositions after completion of the refractory finger tests and MgO crucible tests are provided in Tables 3 and 4, respectively. The changes of the MgO content (MgO) in the industrial fayalite-based ZnO-containing fayalite (ZFS) slags after the rotating refractory finger tests are less than 1.0 wt%. The change of the MgO content in the ZFS slag is constant at 0.8 wt% with
increasing ZnO content from 0 to 10.0 wt% in the industrial fayalitebased ZFS slag, and then slightly decreases from 0.8 wt% to 0.4 wt% when the ZnO content is raised from 10.0 wt% to 20.0 wt%. Because the increased MgO content in the ZFS slag is mainly caused by the dissolution of MgO from periclase grains [2], the observed changes of the MgO content indicate that the dissolution of MgO from periclase into ZFS slags is not enhanced by increasing the ZnO content in the slags with the fixed FeO/SiO2 ratio. As shown in Table 4, the MgO contents in the synthetic fayalite-based ZFS slags after the completion of the MgO crucible tests are stable at around 1.3 wt% when the ZnO content in the tested ZFS slag increases from 0 to 16.7 wt%. The result confirms that the increase of ZnO content in ZFS slags does not influence the dissolution of MgO from periclase into the slags. Fig. 3 shows the calculated maximum concentrations of MgO soluble (MgO solubility) in ZFS slags with different ZnO contents at 1200 ◦ C. The MgO solubility in ZFS slags slightly decreases from ∼2.3 to ∼2.0 wt% when the ZnO content in ZFS slags increases from 0 to 16.7 wt%. This reveals that the driving force for the dissolution of MgO into the ZFS slag decreases with increasing the ZnO content in the slag. It is interesting to note in Fig. 3 that the measured MgO contents are lower than the MgO solubility in the tested synthetic fayalitebased ZFS slags, implying that the maximum concentrations of MgO dissolved in the slag was not reached after corrosion for 12 h at 1200 ◦ C. This result indicates that the dissolution rate of MgO into the ZFS slags is slow. This is due most likely to the formation of
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Fig. 3. Comparison of the predicted MgO solubility at 1200 ◦ C and the measured MgO contents in the tested synthetic fayalite-based ZFS slags.
new phases from the slag/periclase interactions, slowing down the dissolution of MgO from periclase into the slags. This is discussed in detail in next section. 3.3. Effect of ZnO in ZFS slags on the slag/periclase interaction 3.3.1. (Fe,Mg,Zn)2 SiO4 olivine formation Fig. 4 illustrates the interactions at the slag/MgO interfaces from refractory finger tests (Fig. 4a and b) and MgO crucible tests (Fig. 4c and d). The (Fe,Mg,Zn)2 SiO4 solid solution (‘Ol’ in Fig. 4a and b) forms a continuous layer with a thickness of less than 20 m at the surface as a continuous layer on the Oss phase in the refractory fingers. The thickness of the (Fe,Mg,Zn)2 SiO4 solid solution layer in the sample I20 (Fig. 4b) is less compared to that in the sample I00 (Fig. 4a). Similar to the refractory finger tests, the newly
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formed (Fe,Zn,Mg)2 SiO4 solid solution exhibits different grain size after MgO crucible tests. A grain size of over 100 m can be seen in the S00 (Fig. 4c) and S10 (not shown here) samples. In the sample S20, however, a smaller grain size of less than 100 m is observed (Fig. 4d). These results indicate that the formation of the (Fe,Mg,Zn)2 SiO4 solid solution was hindered in a higher ZnO content ZFS slag. This is probably due to the lowered SiO2 concentration with increasing the ZnO content in the slags (the dilution effect). Fig. 5 presents the results of a chemical line analysis of the newly formed (Fe,Mg,Zn)2 SiO4 solid solution in the sample S20. For larger distances from the slag/olivine interface (Fig. 5a), the ZnO content is constant at ∼10.5 mol%. This ZnO concentration is less than the maximum concentration of ZnO soluble in Mg2 SiO4 forsterite or Fe2 SiO4 fayalite [2,7,8], indicating that the newly formed (Fe,Mg,Zn)2 SiO4 solid solution is an olivine group member rather than a (Zn,Fe,Mg)2 SiO4 willemite, even though the ZnO content in the tested ZFS slag reaches ∼16.7 wt%. The absence of a (Zn,Fe,Mg)2 SiO4 willemite can be explained using a thermodynamic calculation. Fig. 6 shows the reaction scheme between MgO and ZFS slags with various ZnO contents at 1200 ◦ C. When the ZnO content in the ZFS slag is lower than ∼22.5 wt%, a composition ‘X’ passes from the ‘Liquid slag’ region into the two-phase region ‘Liquid slag + Olivine’ by adding MgO. This result indicates that olivine firstly precipitates from the slag/MgO interaction once the slag is saturated with MgO. In comparison, when the ZnO content in ZFS slags is higher than ∼22.5 wt%, the composition ‘Y’ will pass from the zone ‘Liquid slag’ into the zone ‘Liquid slag + Willemite’ by increasing the MgO content in the slag. As a result, the (Zn,Fe,Mg)2 SiO4 willemite will precipitate at the slag/MgO interface. As in all tests the ZnO contents in the industrial and synthetic fayalite-based ZFS slags are lower than 22.0 wt%, only (Fe,Mg,Zn)2 SiO4 olivine can precipitate from the ZFS slag/MgO interactions. On the other hand, when the ZnO content in the ZFS slag reaches 30.0 wt%, (Zn,Fe,Mg)2 SiO4 willemite
Fig. 4. Backscattered electron (BSE) images of the MgO/slag interface after (a) and (b) refractory finger tests, and (c) and (d) MgO crucible tests, showing the impact of ZnO content on the (Fe,Mg,Zn)2 SiO4 olivine formation: Oss = (Fe,Mg,Zn)O solid solution; Ol = (Fe,Mg,Zn)2 SiO4 olivine; MS = Mo-containing spinel; P = pore; Fay = (Fe,Zn)2 SiO4 fayalite; Amor = amorphous phase. The dash line in (d) shows the olivine/slag interface.
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Fig. 5. (a) ZnO, FeO, MgO and SiO2 concentration gradient profiles of the selected ‘Line AB’ in (b), in which the newly formed (Fe, Mg, Zn)2 SiO4 solid solution in the sample S20 is shown, Fay = (Fe,Zn)2 SiO4 fayalite; Ol = (Fe,Mg,Zn)2 SiO4 olivine; Oss = (Fe,Mg,Zn)O solid solution.
Fig. 6. Phase stability regions at 1200 ◦ C predicted by FactSage calculation, showing the influence of the ZnO level in the slag on the path of the slag/MgO interfacial reaction; the light grey dash line shows the predicted minimum ZnO content in liquid slag for (Zn,Fe,Mg)2 SiO4 willemite precipitation by adding MgO.
precipitates at the slag/MgO interface (not shown here), which is generally in line with the thermodynamic calculations. 3.3.2. (Fe,Mg,Zn)O solid solution formation In order to evaluate the extent of the (Fe,Mg,Zn)O solid solution (‘Oss’ in Fig. 2a and b) formation from the slag/periclase interaction in the refractory fingers, the average thickness of the Oss layer is used. The thickness of the Oss layer was measured through BSE images at the middle of a large periclase grain with a diameter larger than 500 m, to avoid the growth of the Oss layer influenced by the infiltration slag (Fig. 2b). The average thicknesses of the Oss layers are shown in Fig. 7. The thickness of the Oss layer formed at the slag/periclase interface in worn refractory finger surfaces (Fig. 2a and b) exhibits a positive correlation with the ZnO content of the industrial fayalite-based ZFS slags (Fig. 7). Fig. 8 shows the relationship between the ZnO content of the tested synthetic fayalite-based ZFS slags and the thickness of the “Oss” layer generated at the slag/MgO interface from MgO crucible tests, which is determined based on the MgO concentration in the Oss layer (Fig. 8a). The results are illustrated in Fig. 8b. As shown in Fig. 8b, the growth of the Oss layer exhibits a linear relationship with the ZnO content increase in the tested slag, which is in good agreement with the result from the rotating refractory finger tests (Fig. 7). The thickness of the Oss layer increases from ∼70 to
Fig. 7. Effect of the ZnO content in the industrial fayalite-based ZFS slag on the layer thickness of the (Fe,Mg,Zn)O solid solution (Oss layer).
∼90 m when the ZnO content increases from 0 to 10.0 wt%, and to ∼100 m for a ZnO content of 16.7 wt% in the tested synthetic fayalite-based ZFS slag (Fig. 8). This result confirms that the formation of (Fe,Mg,Zn)O solid solution is enhanced for a higher ZnO content in the ZFS slag. As shown in Fig. 8a, the maximum ZnO concentration in the newly-formed (Fe,Mg,Zn)O solid solution from test S20 is ∼10.5 mol%. It is believed that this (Fe,Mg,Zn)O solid solution is a kind of magnesiowüstite, since the maximum ZnO concentration of ∼10.5 mol% in the (Fe,Mg,Zn)O solid solution is lower than the ZnO solubility of ∼24.0 mol% and ∼32.0 mol% in the FeO and MgO, respectively, at 1200 ◦ C [2,7,8]. 3.4. Effect of ZnO in ZFS slags on the chromite corrosion There is no obvious difference in the thickness of the newlyformed ZnO-containing spinel by changing the ZnO content in ZFS slags, because the chromite grains barely react with the ZFS slags [2]. The changed ZnO content in ZFS slags, however, may influence the chemical composition of the newly-formed ZnO-containing spinel, as the driving force for the diffusion of FeO and ZnO is changed. Fig. 9 shows the chemical composition of the newly-formed ZnO-containing spinel surrounding chromite grains. The spinel rim was selected in the chromite grains at a distance of 0.5 mm from the surface zone of the refractory finger to avoid the influence of
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Fig. 8. (a) MgO and ZnO concentration profiles in (Fe,Mg,Zn)O solid solution layers from MgO-crucible tests, and (b) the thickness of the (Fe,Mg,Zn)O solid solution layer based on the MgO concentration profile in (a): Ol = (Fe,Mg,Zn)2 SiO4 olivine; Oss = (Fe,Zn,Mg)O solid solution.
the formed Mo-containing spinel at the surface zone (Fig. 2b). The spots were selected in the spinel rim with a distance of ∼1 m from the slag/chromite interface, to avoid interaction of the surrounding slag phases. As shown in Fig. 9, the MgO content in this ZnO-containing spinel rim gradually decreases from ∼23.0 mol% to ∼11.0 mol% with increasing the ZnO content in ZFS slags from 0 to 20.0 wt%. Concurrently, the ZnO content increases at the expense of MgO in this spinel rim from 0 to ∼22.0 mol%. The result is consistent with a higher driving force for the diffusion of ZnO from the ZFS slag into chromite grains for a ZnO-richer ZFS slag. It is also interesting to note in Fig. 9 that the Fe2 O3 content in the newly-formed spinel rim slightly decreases from ∼36.0 mol% to 30.0 mol% by increasing the ZnO content in ZFS slags from 0 to 20.0 wt%. This observation indicates that the diffusion of Fe2 O3 from chromite is also favoured with increasing the ZnO content in the ZFS slag. This is because the driving force for the dissolution of Fe2 O3 is increased by the lowered Fe2 O3 concentration as the total ‘FeO’ concentration in a ZnO-rich fayalite slag decreases (Table 2). 4. Conclusion The effect of ZnO level in ZnO-containing fayalite (ZFS) slags on the chemical corrosion behaviour of magnesia-chromite refracto-
ries was studied. This was performed at 1200 ◦ C under reducing atmosphere using rotating refractory finger tests and MgO crucible tests for 4 h and 12 h, respectively. Based on the microstructural and compositional analyses, and thermodynamic calculations, the following conclusions can be drawn: (1) The dissolution of MgO from periclase into ZFS slags is not increased when increasing the ZnO content in the slags at a constant FeO/SiO2 ratio of 1.4. This concurs with the decrease of the driving force for the dissolution of periclase, as the solubility of MgO in ZFS slags decreases with increasing the ZnO content in the slags. (2) (Fe,Mg,Zn)2 SiO4 olivine group member was formed at the slag/periclase interface. There was no (Zn,Fe,Mg)2 SiO4 willemite generated in the slag/periclase interfacial reactions, even though the ZnO content in the ZFS slag reaches 16.7 wt%. The formation of (Fe,Mg,Zn)2 SiO4 olivine was inhibited, probably due to the decreased SiO2 concentration by increasing the ZnO content in ZFS slags. (3) The (Fe,Mg,Zn)O solid solution formation was intensified in a ZnO-rich ZFS slag. This is attributed to the increased driving force for the diffusion of ZnO. The growth of the (Fe,Mg,Zn)O solid solution layer exhibits a linear relationship with the ZnO contents in the tested ZFS slags. (4) The diffusion of ZnO from a ZnO-rich ZFS slag into chromite grains was also favoured, because of the higher driving force, thereby removing more MgO from chromite grains towards the ZnO-rich ZFS slag. Acknowledgements This research was performed with the financial support by the Agency for Innovation by Science and Technology (IWT, project no. 990348). The authors thank the engineers of Metallo-Chimique for their close co-operation. L. Chen and H. Shi are grateful to the China Scholarship Council (CSC) for offering scholarships (no. 20120704008 and 201208420605). The authors also acknowledge the support from the Hercules Foundation (project no. ZW09-09) in the use of the FEG-EPMA system. References
Fig. 9. MgO, ZnO and Fe2 O3 contents in the ZnO-containing spinel rim formed at 0.5 mm from the surface zone of the corroded refractory fingers, as determined by EPMA-WDS.
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