- Email: [email protected]
MgO-ZrO2 refractory ceramics based on recycled magnesia-carbon bricks
Construction and Building Materials 231 (2020) 117084
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
MgO-ZrO2 refractory ceramics based on recycled magnesia-carbon bricks Robert Kusiorowski Łukasiewicz Research Network, Institute of Ceramics and Building Materials, Refractory Materials Division in Gliwice, ul. Toszecka 99, 44-100 Gliwice, Poland
h i g h l i g h t s Zirconia addition have positive effect on properties of magnesia refractories. Improvement of magnesia clinker properties coming from spent magnesia-carbon refractories by fusion with zirconia. Obtaining of refractories with good properties based on recycled spent magnesia refractories.
a r t i c l e
i n f o
Article history: Received 15 May 2018 Received in revised form 20 August 2019 Accepted 25 September 2019
Keywords: Magnesia Zirconia Refractories Spent magnesia-carbon refractories Fusion
a b s t r a c t Magnesia-carbon (MgO-C) refractories are used in the steel industry, where, after their life cycle, they are replaced by new ones. Consequently, a large amount of spent MgO-C is produced and stored in landfills. This waste may be re-used as a potential source of secondary magnesia. In this study, calcined MgO-C waste was used to prepare magnesia-zirconia refractories. It was applied as an individual component of raw materials mix or in the form of fused magnesia-zirconia co-clinker, from which new refractories were produced. After firing at 1660 °C, properties like linear shrinkage, open porosity, apparent density, cold crushing strength, thermal shock resistance, pore size distribution, microstructure as well as resistance to corrosion were measured and determined. The research results indicate that recycling of spent MgO-C refractories is possible as we have obtained a material with satisfactory properties, which can be additionally improved by zirconia addition. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Refractories are materials designed to withstand a variety of severe service conditions, in particular, in combination with hightemperature conditions. The characteristic feature of all types of refractories is their gradual destruction under operating conditions, so they are replaced by new ones when they have reached the end of their service life, they are replaced by new ones. Nowdays a lot of the raw materials that are used in refractory manufacturing are considered critical and vitally important for the EU [1]. This is reflected in a visible increase of prices on the raw materials market [2,3]. The union regulations regarding closed economy are based on the effective usage of natural resources, including secondary raw materials, through recycling technology from spent materials. Given the total global production of refractories, which in 2015 was ca 30–35 million tons [4–6], the amount of spent refractories produced annually is significant, because under operating conditions, approximately one third of a refractory materials mass is irreversibly destroyed
E-mail addresses: [email protected], [email protected] https://doi.org/10.1016/j.conbuildmat.2019.117084 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
[7]. The remaining part turns into wastes, which are freely dumped in landfills. This causes ecological and social difficulties. On the other hand, such spent refractories can be interesting secondary raw materials, which should be re-used. This work deals with magnesia-type refractories, for which the main applications include: (a) iron and steel industry, where they are used as wear and permanent lining in open hearth and electric arc furnaces as well as in pig iron mixers and linings in basic oxygen furnaces, (b) furnaces used in the nonferrous metal industry, (c) checker chambers, port areas, recuperators etc. in glass melting furnaces, (d) furnaces (tunnel, rotary) used by the non-metallic industry, for example rotary cement kilns [8–13]. In recent years, issues related to magnesia refractories recycling have been the subject of numerous studies [14]. From the technological point of view, a very important factor influencing the use of spent refractories is their phase and chemical composition. It is recommended that all grain fractions of the crushed spent recycled material should be analysed before its re-use [6,15,16]. Spent magnesia or magnesia-carbon refractory materials can be re-used as repair materials or as a foamy slag additive [17]. However, what causes concern in this case is mainly their quality [18] and
2
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
availability for larger applications [19,20]. The results obtained in the work [15] indicate that the addition of up to 30 wt% of recycled aggregates from fired magnesia-carbon bricks had no negative effects on the properties of magnesia refractories. In this case, however, only coarse grains were used, because more impurities were contained in the recycled aggregates’ finer fractions than in the coarse grains. The possibility of using crushed spent magnesia refractories as new refractory ceramic masses is also mentioned in other works [16,21]. Due to the magnesite belongs to a group of critical raw materials, spent MgO and MgO-C refractories could be a valuable source of magnesium oxide for the production of basic refractories. The purpose of this work is to show the possibility of using and recycling magnesia-carbon spent refractories as potentially useful raw materials for the production of zirconia-modified magnesia refractories. The properties of the obtained refractory materials based on secondary magnesia clinker from fired spent MgO-C with or without an addition of zirconia (in a traditional way or as fused MgO-ZrO2 co-clinker) were compared with each other and with a material obtained from commercially available magnesia clinker. The obtained results will allow answering the question whether refractory waste can be used as a secondary raw material in the refractory industry and applied in the glass-making or cement industries, which are recipients of magnesia refractory products. 2. Experimental 2.1. Raw materials Spent magnesia-carbon bricks, sintered magnesia clinker and zirconia were used as base raw materials for different refractory mixes. Magnesium lignosulphonate water solution was chosen as a binder. The spent magnesia-carbon material was sampled from the main ladle slag line in one of Polish steelworks. It can be assumed that this material is representative of scrap magnesia-carbon materials from other steelworks. To produce a recycled material from the spent bricks, the penetrated layers were cut and the unreacted parts were crushed; due to the presence of carbon in these spent bricks, the material was subjected to additional thermal treatment. It was fired at 1400 °C for 8 h and, finally, secondary magnesia clinker was obtained and used in further investigations. Other raw materials used in the study were sintered magnesia (Magnesita; Brazil) and commercially available zirconia. Magnesia clinker came from the process of natural magnesite double firing and was characterized by a high content (98 wt%) of MgO. The particle size of the applied magnesia clinker was <5 mm. Zirconia was used in a non-stabilised form, i.e. the monoclinic form of zirconium oxide. It was added to the ceramic mixes in medium-large size (median grain size: d50 = 100 mm). As part of the study, fused magnesia-zirconia co-clinker was also prepared, where the assumed zirconia content reached 4 wt %. It was made from zirconia and secondary magnesia clinker from fired spent MgO-C refractories in the melting process in an arcresistance furnace. Detailed conditions in which the melting process was conducted in a laboratory arc-resistance furnace with graphite electrodes were presented in the previous work [12]. The total fusion time was 150 min. As a result, a block of fused MgOZrO2 co-clinker was obtained (Fig. 1) and crushed for use as a material to prepare refractory samples. 2.2. Sample preparation The types of different batches prepared in the study has been shown in Table 1. The grain size distribution of all types of ceramic
Fig. 1. Block of fused MgO-ZrO2 co-clinker.
mixes was selected according to the Dinger and Funk model for the grain distribution coefficient n equal to 0.37. Ceramic mixes were prepared by weighing a required amount of necessary raw materials and their mixing in a laboratory mixer. When preparing the ceramic mix with the thickest fraction of magnesia clinker, sulphite lye was added in the amount of 3.5 wt% in relation to other dry components. Next, samples from so prepared mixes were made in the form of cylinders having the diameter and height of 50 mm by the method of single-axis pressing. To vent the mix, a two-stage pressing cycle was applied i.e. with a venting pressure of 60 MPa and, next, under a specific pressure of 120 MPa. After forming, the shapes were conditioned for 24 h at ambient temperature and, next, at 120 °C for 4 h. The formed shapes were fired in a gas furnace at the pre-set temperature of 1660 °C and kept at this temperature for a period of 4 h. The preparation course schematic flow chart is presented in Fig. 2. 2.3. Corrosion study Investigations into resistance to corrosion were conducted by the crucible method, i.e. holes having the diameter of 20 mm and the depth of 20 mm were cut in the obtained samples. Approximately 7 g of the corrosion set was placed and manually pressed in the hole. Two corrosion sets (named as ‘‘glassy” and ‘‘cementitious”) were prepared and their composition has been given in Table 2. Samples with the corrosion set were fired at 1400 °C for 6 h (set II, glassy) or at 1500 °C for 10 hrs (set I, cementitious). After cooling the samples, they were cut along the cylinder height. The cut fragments were washed with water so as to clean off the dust and make the corroded surface visible; next they were dried and the corrosion area was planimetred. The percentage share of uncorroded sample surface was assumed to be a measure of material’s corrosive resistance, i.e. the higher value was achieved by planimetring, the higher was the value of a given material’s corrosive resistance. 2.4. Methods The raw materials used in the investigations were tested for their chemical composition (XRF, PANalitycal Magix PW2424 spectrometer) and quality phase composition (XRD, PANalytical X’pert Pro diffractometer operating at 40 kV and 30 mA, with CuKa radiation, Ni filter and X’Celerator detector). In the case of magnesia
3
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084 Table 1 Composition of the prepared batches. Composition Sintered magnesia
Coarse grains (<5 mm) Fine grains (<0.1 mm) Coarse grains (<5 mm) Fine grains (<0.1 mm)
Sintered magnesia from a spent fired MgO-C material Zirconia Fused MgO-ZrO2 co-clinker
Coarse grains (<5 mm) Fine grains (<0.1 mm)
K0
K4Z
W0
W4Z
FC4Z
77 23 – – – – –
73 23 – – 4 – –
– – 72.6 27.4 – – –
– – 68.6 27.4 4 – –
– – – – – 74 26
Fig. 2. Schematic flow chart of the preparation course.
Table 2 Chemical composition of corrosive agents (wt%). Corrosive agent I (cementitious set) SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
LOI
9.1
2.3
1.1
29.1
0.4
24.0
0.1
26.7
7.0
Corrosive agent II (glassy set) SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
MnO
SO3
LOI
33.0
2.7
0.1
3.4
2.4
0.1
25.2
1.6
13.4
18.1
LOI = loss on ignition.
clinkers, also apparent density and open porosity were determined by the hydrostatic method (using paraffin oil as a working liquid). Moreover, differential thermal analysis (DTA) and thermogravimetric analysis (TG) combined with evolved gas analysis (EGA) were conducted for MgO-C waste (STA 409 PC NETZSCH thermal analyser, which was coupled with a QMS 403C Aëolos quadrupole
mass spectrometer; a total of 60 mg of the samples was placed in an alumina crucible; the heating rate of the samples was 10 K∙min 1; tests were performed in air with a flow rate of 10 ml∙min 1). After the sintering process, various ceramic properties were measured, including: linear firing shrinkage according to diameter
4
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Table 3 Chemical analysis (XRF) of raw materials (wt%) and main properties of magnesia materials. Oxides
Zirconia
Commercial sintered magnesia clinker
Spent MgO-C refractories
Spent fired MgO-C refractories
Fused MgO-ZrO2 co-clinker
SiO2 Al2O3 Fe2O3 TiO2 CaO MgO K2O Na2O MnO P2O5 Cr2O3 ZrO2 HfO2 LOI C/S molar ratio Bulk density [g/cm3] Apparent porosity [%]
0.37 0.20 0.12 0.10 0.53 0.21 <0.01 <0.01 – 0.30 – 96.29 1.62 0.20 – – –
0.37 0.81 0.41 0.01 0.87 97.65 <0.01 <0.01 0.09 0.05 – – – 0.15 2.52 3.14 4.7
1.03 0.90 0.56 0.03 1.78 82.41 0.07 <0.01 0.07 0.07 0.07 – – 12.99 1.85 2.73 12.3
1.56 1.42 0.73 0.02 2.17 93.72 <0.01 <0.01 – 0.08 – – – 0.45 1.49 2.74 19.0
1.28 1.32 0.63 0.02 2.01 91.02 <0.01 <0.01 – – – 3.97 0.04 0.13 1.68 3.16 8.6
LOI = loss on ignition.
and height measurement, bulk density and open porosity based on the Archimedes method, using kerosene as the immersion medium to avoid hydration (according to PN-EN 993-1:1998 standard), as well as cold crushing strength (according to PN-EN 993-5:2001 standard). The obtained refractory materials were also examined with regard to their thermal shock resistance, which involved their cyclical heating (950 °C/25 min) and cooling (water; 3 min) until the sample lost 20% of its original mass. Each parameter of the material was determined for minimum three samples, and the final result was the average value of partial results. Moreover, investigations into the pore size distribution and SEM observations were performed to characterize the microstructure of the obtained ceramics. Porosimetric investigations were carried out using an Autopore 9500 mercury porosimeter by Micromeritics for testing porosity and pore size distribution within a range of 0.006–450 lm. Scanning electron microscopy (SEM) was performed using a Tescan Mira III microscope in combination with
an Energy Dispersive Spectroscopy (EDS) system with AZtec Automated software (Oxford Instruments). 3. Results and discussion 3.1. Characteristics of raw materials The chemical analysis as well as the phase composition of the raw materials applied have been presented in Table 3 and Fig. 3, respectively. In the spent refractories mainly periclase (ICDD-PDF 01-071-4938) and graphite (ICDD-PDF 04-007-8496) with some insignificant traces of merwinite (ICDD-PDF 00-035-0591), monticellite (ICDD-PDF 00-011-0129) and spinel (ICDD-PDF 04-0083488) were identified. They were also characterized by a high value of loss on ignition, which reached ca 13 wt%. It results from the combustion process of carbon substances, i.e. graphite present in this recycled material (Fig. 3, Fig. 5). Combustion of carbon
Fig. 3. XRD patterns of raw materials and the obtained magnesia-zirconia co-clinker.
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
occurred in several stages in a wide temperature range, which was confirmed by a thermal analysis study (Fig. 4a). On the TG and DTG curves of raw MgO-C material, a significant mass change (~12 wt%) was observed within a temperature range of 500 to above 1000 °C. The course of the DTG curve shows that this process has three stages, confirmed by peaks with maximums at ~640, 820 and 870 °C, and correlated with an evolution of CO2 observed on the EGA curve. A small mass change (~1 wt%) at a temperature of ca 1200 °C resulted from sulphur compounds’ decomposition. In order to eliminate graphite from the recycled aggregates, the spent MgO-C refractory was heated at 1400 °C for 8 h. As a result of this thermal pre-treatment, secondary magnesia clinker was obtained. The firing process – although economically expensive – was incorporated in the research program, because the melting of magnesia-carbon material in an arc furnace is problematic and can be dangerous. The following reactions may occur during the process of magnesia reduction by carbon: (1) direct reduction [magnesia(s) + carbon(s) = magnesium(g) + carbon oxide (g)] as well as (2) indirect reduction: [magnesia(s) + carbon oxide(g) = magnesium(g) + carbon dioxide(g)] [22,23].
5
Its phase composition was similar to that of spent raw refractories, but it was deprived of graphite (Fig. 3). The carbon compound was practically fully combusted and the secondary MgO clinker was characterized by a negligible loss on ignition (Table 3; Fig. 4b). The characteristic flake-like structures of graphite present in the raw material (Fig. 5a) were absent as a result of combustion, and periclase grains with some porosity were obtained (Fig. 5b). This secondary magnesia clinker was characterized by significantly lower quality in comparison with commercial MgO clinker. The amount of MgO was smaller (93.7 vs 97.6 wt%) and contained higher amounts of undesirable compounds, such as CaO, Al2O3, Fe2O3. They reduce the refractoriness of magnesia clinker, especially when C/S molar ratio (CaO/SiO2) is below 2, contributing to the development of low-melting phases (like merwinite or monticellite) in the system. The C/S is an important ratio as it indicates the presence of other mineral phases in magnesia-containing refractories. When the molar ratio of CaO/SiO2 is >2, mainly dicalcium silicate, characterized by one of the highest values of refractoriness among silicate phases, coexists with periclase [22,24]. Moreover, this secondary magnesia clinker also has a visibly lower
Fig. 4. DTA-TG-EGA curves of spent MgO-C material before (a) and after (b) thermal pre-treatment.
6
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Fig. 4 (continued)
bulk density (2.73 vs 3.14 g/cm3) and higher apparent porosity (19.0 vs 4.7%), respectively, determining its lower quality, which should be improved, among others, by fusion with zirconia. In the case of magnesia-zirconia fused co-clinker, the assumed amount of zircon oxide in the material was obtained. The chemical analysis results show that the content of ZrO2 was around 4 wt% (Table 3). Moreover, this co-clinker contained undesirable compounds similar to secondary clinker. The C/S molar ratio was still below 2, but it is worth noting that the added ZrO2 easily reacts with Ca-compounds, and Ca-stabilized cubic zirconia or calcium zirconate (CaZrO3) – with an excess of calcium compounds – may be formed. CaZrO3 is characterized by a very high temperature of melting (ca 2350 °C), which is very desirable in the case of refractory materials application. The fusion process resulted in the formation of a new phase. On the co-clinker diffraction pattern, the main diffraction reflexes from the stabilized cubic form of ZrO2 instead of the monoclinic (baddelyite) form of zirconium oxide (Fig. 3) were observed. This form is favourable in the structure because it allows avoiding the disadvantageous phase transformation of zirconia at ~1200 °C (monoclinic M tetragonal). This transformation is connected with a considerable volume change, which reduces the refractory material’s strength [25,26]. The
obtained fused co-clinker was characterized by much bigger magnesia grains connected together by the zirconate phase (Fig. 5c). This is a desirable property because grains with larger dimensions reduce magnesia’s reactivity with corrosive agents. 3.2. Properties of the obtained refractory materials The results of the measured physico-mechanical properties of the refractory samples are presented in Figs. 6–9, while the microstructure characteristics are shown in Fig. 10 and in Table 4. The corrosion test results have been given in Fig. 11. All the obtained refractory samples have stable dimensions after the firing process (Fig. 6) due to their low firing shrinkage. The values of this parameter do not exceed 1.0–1.5% in relation to the original size of the samples. Due to the presence of bigger amounts of undesirable compounds, which lowered the melting point of secondary magnesia clinker, the linear shrinkage of the sample based on this raw material was noticeably higher (for example sample W0 vs K0). Fig. 7 shows the measured bulk density and apparent porosity of the obtained refractory materials. As can be seen, the sample based on secondary magnesia clinker (sample W0) was
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
7
Fig. 5. SEM images of raw spent MgO-C material (a), after thermal pre-treatment and combustion of graphite (b) and after fusion with ZrO2 in an electric arc furnace (c).
Fig. 6. Linear shrinkage of the obtained refractory samples.
8
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Fig. 7. Bulk densities and apparent porosities of the obtained refractory samples.
Fig. 8. Cold crushing strength of the obtained refractory samples.
Fig. 9. Thermal shock resistance of the obtained refractory samples.
characterized by the worst technical parameters. This material had the highest porosity (~19%) and the lowest bulk density (2.78 g/cm3). Such results were expected due to the low quality of the clinker itself (Table 3). An addition of zirconia (4 wt%) to magnesia clinker (both commercial and secondary magnesia clinker) clearly contributed to the improvement of main ceramic properties. In comparison with respective samples (K4Z vs K0 or
W4Z vs W0), higher density and lower open porosity were observed. Probably, the sintering process of MgO-ZrO2 ceramics largely dependent on solid phase sintering. Therefore, the densification of material depends on the manner of matter transport i.e. surface diffusion. Thus mechanism may be discussed by depicting the defect model [27–30]. For example, open porosity decreased from 19 to 16.5% for W4Z sample. This is a desirable
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
9
Fig. 10. SEM images of fired ceramic samples: K0 (a), K4Z (b), W0 (c), W4Z (d), FC4Z (e).
effect as it may contribute to increased corrosion resistance of a material in its future work. From this point of view, the best results were obtained for the refractory sample of fused
magnesia-zirconia co-clinker. In the case of FC4Z sample, open porosity achieved the lowest value, i.e. 13%, whereas its bulk density was the highest, reaching nearly 3.0 g/cm3.
10
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Table 4 Pore size distribution of the obtained ceramic samples (%).
>60 lm 30–60 lm 20–30 lm 1–20 lm <1 lm
K0
K4Z
W0
W4Z
FC4Z
4.26 0.28 0.19 91.73 3.54
3.34 0.41 0.98 92.89 2.38
2.86 1.12 10.12 83.65 2.25
3.86 3.93 34.13 55.79 2.29
8.38 34.05 23.70 32.20 1.67
Fig. 11. Results of tests of the obtained refractory samples’ corrosion resistance against two different corrosive agents.
The use of zirconia has a positive influence on the obtained ceramics’ mechanical strength, expressed as cold crushing strength (Fig. 8). A comparison of the results obtained for samples without zirconia addition with materials based on 4 wt% addition of ZrO2 shows an increase in the CCS values. Replacement of sintered magnesia clinker (commercial or secondary clinker from spent fired MgO-C materials) with the obtained fused MgO-ZrO2 co-clinker resulted in a further significant increase of cold crushing strength. For sample FC4Z it reached the highest value – around 65 MPa. It was higher by ca 30% than in the case of a material made from spent fired refractory aggregates or commercial magnesia clinker.
An addition of zirconia alone or in the form of fused MgO-ZrO2 co-clinker is also responsible for a slight but observable improvement of thermal shock resistance, which increased from ca 6–7 to 10 water cycles for materials with and without zirconia addition, respectively (Fig. 9). This favourable effect of ZrO2 addition can be associated with lower thermal expansion coefficient of ZrO2 in comparison with magnesia (~10∙10 6 °C 1 vs 13– 15∙10 6 °C 1, respectively). The above described investigations were supplemented with microstructure observations (Fig. 10) and pore size distribution (Table 4) in the obtained ceramic materials. Samples based on commercial magnesia (samples K0 and K4Z) are characterized by similar microstructure (Fig. 10a,b) and pore size distribution (Table 4). In SEM micrographs there are visible coarse grains of magnesia embedded in a matrix built of finer MgO fractions. In the case of sample K4Z, one can also observe areas with additional grains of a phase rich in ZrO2. These materials have similar pore size distribution, in which pores ranging from 1 to 20 lm are dominant (>90%). Replacement of commercial magnesia clinker by a recycled material (secondary magnesia clinker) having much worse parameters than commercial magnesia (Table 3), contributed to obtaining products with a different structure, in which there was a visible increase in the number of larger pores (with dimensions >20 lm), both in material W0 and with ZrO2 addition (sample W4Z). In both cases (sample W0 and W4Z, Fig. 10c,d), SEM pictures show higher amount of oblong, tubular pores, which are frequently connected with each other. This confirms the previously determined open porosity and weaker mechanical strength of ceramic samples, based on secondary magnesia clinker. The differences in the nature of the pores result from the difference in the chemical purity of the magnesia clinkers used in this study. In the case of W0 and W4Z samples which were obtained from spent MgO-C material, the content of Ca, Fe, and Si compounds is clearly higher than in the case of samples based on commercial raw magnesia clinker (Table 3). The sum of these admixtures for samples from ‘‘W” series is more than 2 times higher (5.88 wt% for spent fired MgO-C refractories vs 2.46 wt%
Fig. 12. Corrosion resistance of a sample expressed as percentage of non-corroded area after a corrosion test.
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
for commercial sintered magnesia clinker). This will make the ‘‘W” material more easily sintered as a result of the formation of lowmelting eutectics from the C-M-S system. This was confirmed in SEM pictures (Fig. 10), where the presence of the intercrystalline phases (Ca,Si-rich phase) is clearly greater in the case of the ‘‘W” series samples (Fig. 10c vs Fig. 10a). In addition, the obtained W0 sample shows no fine-grained matrix, which was sintered during the firing process of the material. In consequence we observe MgO grain growth and agglomeration of fissure pores. On the other hand, sample FC4Z has a different microstructure. The lower linear shrinkage of the FC4Z sample (in comparison with W4Z sample), in principle, results from the lower reactivity of large fused clinkers grains. This phenomenon is due to the formation of large periclase crystals (Fig. 5c) with less porosity and thus less surface development. In the case of fused magnesia-zirconia coclinker the partial closure of the Ca,Si-rich phases inside the periclase crystals was also observed (Fig. 5c). As in the case of the ‘‘W” series samples, the presence of more pores with larger sizes in FC4Z sample is the effect of sintering of the matrix. Moreover, the presence of large grains of fused raw material (MgO-ZrO2 coclinker) with lower reactivity blocked the sintering effect macroscopically, which still progressed in the product matrix under firing process. Hence, in this case the largest and widest pores with the absence of fine grains matrix were observed at SEM pictures. Although pore size distribution of FC4Z sample is less favourable (this material contains the highest number of pores with dimensions >30 lm), the microstructure indicates a more homogenous, uniform dispersion of the zirconia phase, which results from the prior process of melting and obtaining MgO-ZrO2 fused coclinker. In the further part of investigations, this may improve the products’ corrosive resistance, as the presence of ZrO2 will stop the progressing chemical corrosion of the product. In the case of corrosion with a ‘‘cementitious” set, it will be the effect of forming high-melting refractory calcium zirconate CaZrO3, and in the case of corrosion with a ‘‘glassy” set, the liquid phase formed will increase its viscosity, hindering the corrosive agent’s penetration inside the material. In respect to the corrosion study, the concept behind the composition of the ‘‘glassy” set was to recreate conditions in glass furnace regenerators, whereas the ‘‘cementitious” set was selected to recreate the conditions in a cement furnace – taking into consideration both the components of cement clinker and components from the furnace atmosphere, which also influences the corrosion processes and is, among others, connected with the circulation of alkalis [10]. In the atmosphere of cement kilns potassium prevails over sodium [10], hence the composition of this corrosion set includes potassium sulphate, enriching the system with alkalis. The contents of K2O and SO3, which in the atmosphere of the kiln circulates as SO2, have been considerably increased so as to create a drastic effect of these two components. The cross-section of samples after corrosion tests confirmed a positive effect of zirconia addition on the refractory samples’ corrosion resistance (Fig. 11), because deep penetration determined for both corrosion agents was visibly lower in comparison with the reference samples, i.e. commercial magnesia (sample K0) or secondary magnesia clinker (sample W0). An attempt to express this resistance in a semi-quantitative way has been shown in Fig. 12, where the results of corrosion resistance test were expressed as percentage of non-corroded surfaces of the materials. Due to a notably alkaline character of the cementitious corrosive agent, the obtained materials’ corrosion resistance was high, reaching a value around 95–97%. Under measurement conditions, the applied glassy corrosive agent, in the form of molten slag, penetrates deeper and easily infiltrates the porous materials. The determined corrosion resistance reached a level ranging from ~55% (for samples without zirconia) to ~70% (for samples with an addition of zirco-
11
nia). The best result of corrosion resistance against a glassy agent was observed in the case of the material based on fused MgOZrO2 co-clinker; it resulted directly from the technological parameters of the obtained raw-material, i.e. the lowest value of open porosity of FC4Z sample (Fig. 7) as well as good homogenization and dispersion of zirconia compounds in the fused material (Fig. 10e). 4. Conclusions In the presented study, MgO- and MgO-ZrO2 refractory ceramics made from secondary magnesia clinker obtained from fired spent magnesia-carbon refractories were compared with materials obtained in a traditional way – from commercial raw materials. It has been found that spent MgO-C refractories can be successfully recycled, but it is worth bearing in mind that the obtained secondary magnesia clinker is characterized by lower quality. This quality can be considerably improved by adding zirconia and/or producing fused MgO-ZrO2 co-clinker, which reduces open porosity and increases bulk density. Moreover, zirconia addition increases the cold crushing strength of ceramics, especially in the case of fused magnesia-zirconia co-clinker, and improves corrosion resistance. Consequently, these materials can be used in the future as a valuable substitute of refractory ceramics obtained in a traditional way, from natural sources. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is the result of statutory activity at the Institute of Ceramics and Building Materials, Refractory Materials Division in Gliwice supported by the Polish Ministry of Science and Higher Education. The author thanks Mr. A. S´liwa for their help in mercury porosimetric analysis. References [1] G. Bertrand, D. Cassard, N. Arvanitidis, G. Stanley, Map of critical raw material deposits in Europe, Energy Procedia 97 (2016) 44–50. [2] M. O’Driscoll, MagForum 2017, Refract. Worldforum 9 (2017) 69–73. [3] S. Shaw, China’s increasing environmental focus sends shockwaves through the refractory industry, Refract. Worldforum 11 (2019) 58–62. [4] J. Czechowski, J. Witek, A. S´liwa, Przemysł materiałów ogniotrwałych – problematyka badan´ i rozwoju (Refractory industry – research and development), Szkło i Ceramika 2 (2017) 26–27 (in Polish). [5] C.E. Semler, Review of refractoris markets & research – 2016, Refract. Worldforum 9 (2017) 30–34. [6] B. Plešinerová, P. Vadász, R. Kamensky´, J. Bounziová, J. Derd’ak, D. Medved, Spent magnesia-carbon refractory bricks from steel production: potentiality of MgO-clinker recovery, Acta Montanistica Slov. 23 (2018) 39–45. [7] A. Volcaert, Raw materials for refractories: the European perspective, in: D.G. Goski, J.D. Smith (Eds.), Proceedings of the Unified International Technical Conference on Refractories (13th biennial worldwide congress on refractories) UNITECR’13, The American Ceramic Society, 2013, pp. 84–90. [8] G. Routschka, Refractory Materials – Pocket Manual, 2nd ed., Vulkan-Verlag, Essen, 2004. [9] A.O. Surendranathan, An Introduction to Ceramics and Refractories, CRC Press, Boca Raton, 2015. [10] J. Szczerba, Modyfied magnesia refractory materials, Ceramics 99 (2007) 1– 204 (in Polish). [11] R. Kusiorowski, J. Wojsa, B. Psiuk, T. Wala, Influence of zirconia addition on the properties of magnesia refractories, Ceram. Int. 42 (2016) 11373–11386. [12] R. Kusiorowski, B. Psiuk, Fused magnesia-zirconia co-clinker for fired refractories, Ceram. Int. 43 (2017) 14263–14270. [13] E.A. Rodríguez, G.-Alan Castillo, T.K. Das, R. Puente-Ornelas, Y. González, A.M. Arato, J.A. Aguilar-Martínez, MgAl2O4 spinel as an effective ceramic bonding in a MgO-CaZrO3 refractory, J. Eur. Ceram. Soc. 33 (2013) 2764–2774.
12
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
[14] L. Horckmans, P. Nielsen, P. Dierckx, A. Ducastel, Recycling of refractory bticks used in basic steelmaking: a review, Resour. Conserv. Recycl. 140 (2019) 297– 304. [15] F. Arianpour, F. Kzemi, F.G. Fard, Characterization, microstructure and corrosion behaviour of magnesia refractories produced from recycled refractory aggregates, Miner. Eng. 23 (2010) 273–276. [16] A.N. Conejo, R.G. Lule, F. Lopéz, R. Rodriguez, Recycling MgO-C refractory in electric arc furnaces, Resour. Conserv. Recycl. 49 (2006) 14–31. [17] A.P. Luz, D.O. Vivaldini, F. Lopéz, P.O.R.C. Brant, V.C. Pandolfelli, Recycling MgO-C refractories and dolomite fines as slag foaming conditioners: experimental thermodynamic evaluations, Ceram. Int. 39 (2013) 8079–8085. [18] B. Plešinerová, P. Vadász, G. Sucˇik, I. Olejár, Specification of MgO-concentrate obtained from post mortem MgO-C refractory materials, Ceramics-Silikáty 62 (2018) 389–395. [19] K.S. Kwong, J.P. Bennett, Recycling practices of spent MgO-C refractories, J. Miner. Mater. Charact. Eng. 1 (2002) 69–78. [20] S. Hanagiri, T. Matsui, A. Shimpo, S. Aso, T. Inuzuka, T. Matsuda, S. Sakaki, H. Nakagawa, Recent improvement of recycling technology for refractories, Nippon Steel Technical Report 98 (2008) 93–98. [21] A.G.M. Othman, W.M.N. Nour, Recycling of spent magnesite and ZAS bricks for the production of new basic refractories, Ceram. Int. 31 (2005) 1053–1059. [22] L. Rongti, P. Wei, M. Sano, Kinetics and mechanism of carbothermic reduction of magnesia, Metall. Mater. Trans. B 34B (2003) 433–437.
[23] G. Brooks, S. Trang, P. Witt, M.N.H. Khan, M. Nagle, The carbotheric route to magnesium, JOM 58 (2006) 51–55. _ [24] I. Jastrze˛bska, J. Szczerba, R. Prorok, E. S´niezek, An experimental study on hydration of various magnesia raw materials, Ceramics-Silikáty 59 (2015) 48– 58. [25] A.P. Bechepeche, O. Treu Jr, E. Longo, C.O. Paiva-Santos, J.A. Varela, Experimental and theoretical aspects of the stabilization of zirconia, J. Mater. Sci. 34 (1999) 2751–2756. [26] E.H. Kisi, C.J. Howard, Crystal structure of zirconia phases and their interrelation, Key Eng. Mater. 153–154 (1998) 1–36. _ [27] E. S´niezek, J. Szczerba, P. Stoch, R. Prorok, I. Jastrze˛bska, W. Bodnar, E. Burkel, Structural properties of MgO-ZrO2 ceramics obtained by conventional sintering, arc melting and field assisted sintering technique, Mater. Des. 99 (2016) 412–420. [28] R.S. Nasar, M. Cerqueira, E. Longo, J.A. Varela, Sintering mechanism of ZrO2MgO with addition of TiO2 and CuO, Ceram. Int. 30 (2004) 571–577. [29] K.C. Chou, X. Hou, M. Chen, B. Zhao, Controllable preparation of Al2O3MgOAl2O3-CaO6Al2O3 (AMC) composite with improved slag penetration resistance, Int. J. Appl. Ceram. Technol. 13 (2016) 33–40. [30] S. Yao, E. Wang, J. Chen, K.C. Chou, X. Hou, Effectively controlling the crystal growth of Cr2O3 using SiO2 as the second phase, J. Am. Ceram. Soc. 102 (2019) 2187–2194.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
MgO-ZrO2 refractory ceramics based on recycled magnesia-carbon bricks Robert Kusiorowski Łukasiewicz Research Network, Institute of Ceramics and Building Materials, Refractory Materials Division in Gliwice, ul. Toszecka 99, 44-100 Gliwice, Poland
h i g h l i g h t s Zirconia addition have positive effect on properties of magnesia refractories. Improvement of magnesia clinker properties coming from spent magnesia-carbon refractories by fusion with zirconia. Obtaining of refractories with good properties based on recycled spent magnesia refractories.
a r t i c l e
i n f o
Article history: Received 15 May 2018 Received in revised form 20 August 2019 Accepted 25 September 2019
Keywords: Magnesia Zirconia Refractories Spent magnesia-carbon refractories Fusion
a b s t r a c t Magnesia-carbon (MgO-C) refractories are used in the steel industry, where, after their life cycle, they are replaced by new ones. Consequently, a large amount of spent MgO-C is produced and stored in landfills. This waste may be re-used as a potential source of secondary magnesia. In this study, calcined MgO-C waste was used to prepare magnesia-zirconia refractories. It was applied as an individual component of raw materials mix or in the form of fused magnesia-zirconia co-clinker, from which new refractories were produced. After firing at 1660 °C, properties like linear shrinkage, open porosity, apparent density, cold crushing strength, thermal shock resistance, pore size distribution, microstructure as well as resistance to corrosion were measured and determined. The research results indicate that recycling of spent MgO-C refractories is possible as we have obtained a material with satisfactory properties, which can be additionally improved by zirconia addition. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Refractories are materials designed to withstand a variety of severe service conditions, in particular, in combination with hightemperature conditions. The characteristic feature of all types of refractories is their gradual destruction under operating conditions, so they are replaced by new ones when they have reached the end of their service life, they are replaced by new ones. Nowdays a lot of the raw materials that are used in refractory manufacturing are considered critical and vitally important for the EU [1]. This is reflected in a visible increase of prices on the raw materials market [2,3]. The union regulations regarding closed economy are based on the effective usage of natural resources, including secondary raw materials, through recycling technology from spent materials. Given the total global production of refractories, which in 2015 was ca 30–35 million tons [4–6], the amount of spent refractories produced annually is significant, because under operating conditions, approximately one third of a refractory materials mass is irreversibly destroyed
E-mail addresses: [email protected], [email protected] https://doi.org/10.1016/j.conbuildmat.2019.117084 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
[7]. The remaining part turns into wastes, which are freely dumped in landfills. This causes ecological and social difficulties. On the other hand, such spent refractories can be interesting secondary raw materials, which should be re-used. This work deals with magnesia-type refractories, for which the main applications include: (a) iron and steel industry, where they are used as wear and permanent lining in open hearth and electric arc furnaces as well as in pig iron mixers and linings in basic oxygen furnaces, (b) furnaces used in the nonferrous metal industry, (c) checker chambers, port areas, recuperators etc. in glass melting furnaces, (d) furnaces (tunnel, rotary) used by the non-metallic industry, for example rotary cement kilns [8–13]. In recent years, issues related to magnesia refractories recycling have been the subject of numerous studies [14]. From the technological point of view, a very important factor influencing the use of spent refractories is their phase and chemical composition. It is recommended that all grain fractions of the crushed spent recycled material should be analysed before its re-use [6,15,16]. Spent magnesia or magnesia-carbon refractory materials can be re-used as repair materials or as a foamy slag additive [17]. However, what causes concern in this case is mainly their quality [18] and
2
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
availability for larger applications [19,20]. The results obtained in the work [15] indicate that the addition of up to 30 wt% of recycled aggregates from fired magnesia-carbon bricks had no negative effects on the properties of magnesia refractories. In this case, however, only coarse grains were used, because more impurities were contained in the recycled aggregates’ finer fractions than in the coarse grains. The possibility of using crushed spent magnesia refractories as new refractory ceramic masses is also mentioned in other works [16,21]. Due to the magnesite belongs to a group of critical raw materials, spent MgO and MgO-C refractories could be a valuable source of magnesium oxide for the production of basic refractories. The purpose of this work is to show the possibility of using and recycling magnesia-carbon spent refractories as potentially useful raw materials for the production of zirconia-modified magnesia refractories. The properties of the obtained refractory materials based on secondary magnesia clinker from fired spent MgO-C with or without an addition of zirconia (in a traditional way or as fused MgO-ZrO2 co-clinker) were compared with each other and with a material obtained from commercially available magnesia clinker. The obtained results will allow answering the question whether refractory waste can be used as a secondary raw material in the refractory industry and applied in the glass-making or cement industries, which are recipients of magnesia refractory products. 2. Experimental 2.1. Raw materials Spent magnesia-carbon bricks, sintered magnesia clinker and zirconia were used as base raw materials for different refractory mixes. Magnesium lignosulphonate water solution was chosen as a binder. The spent magnesia-carbon material was sampled from the main ladle slag line in one of Polish steelworks. It can be assumed that this material is representative of scrap magnesia-carbon materials from other steelworks. To produce a recycled material from the spent bricks, the penetrated layers were cut and the unreacted parts were crushed; due to the presence of carbon in these spent bricks, the material was subjected to additional thermal treatment. It was fired at 1400 °C for 8 h and, finally, secondary magnesia clinker was obtained and used in further investigations. Other raw materials used in the study were sintered magnesia (Magnesita; Brazil) and commercially available zirconia. Magnesia clinker came from the process of natural magnesite double firing and was characterized by a high content (98 wt%) of MgO. The particle size of the applied magnesia clinker was <5 mm. Zirconia was used in a non-stabilised form, i.e. the monoclinic form of zirconium oxide. It was added to the ceramic mixes in medium-large size (median grain size: d50 = 100 mm). As part of the study, fused magnesia-zirconia co-clinker was also prepared, where the assumed zirconia content reached 4 wt %. It was made from zirconia and secondary magnesia clinker from fired spent MgO-C refractories in the melting process in an arcresistance furnace. Detailed conditions in which the melting process was conducted in a laboratory arc-resistance furnace with graphite electrodes were presented in the previous work [12]. The total fusion time was 150 min. As a result, a block of fused MgOZrO2 co-clinker was obtained (Fig. 1) and crushed for use as a material to prepare refractory samples. 2.2. Sample preparation The types of different batches prepared in the study has been shown in Table 1. The grain size distribution of all types of ceramic
Fig. 1. Block of fused MgO-ZrO2 co-clinker.
mixes was selected according to the Dinger and Funk model for the grain distribution coefficient n equal to 0.37. Ceramic mixes were prepared by weighing a required amount of necessary raw materials and their mixing in a laboratory mixer. When preparing the ceramic mix with the thickest fraction of magnesia clinker, sulphite lye was added in the amount of 3.5 wt% in relation to other dry components. Next, samples from so prepared mixes were made in the form of cylinders having the diameter and height of 50 mm by the method of single-axis pressing. To vent the mix, a two-stage pressing cycle was applied i.e. with a venting pressure of 60 MPa and, next, under a specific pressure of 120 MPa. After forming, the shapes were conditioned for 24 h at ambient temperature and, next, at 120 °C for 4 h. The formed shapes were fired in a gas furnace at the pre-set temperature of 1660 °C and kept at this temperature for a period of 4 h. The preparation course schematic flow chart is presented in Fig. 2. 2.3. Corrosion study Investigations into resistance to corrosion were conducted by the crucible method, i.e. holes having the diameter of 20 mm and the depth of 20 mm were cut in the obtained samples. Approximately 7 g of the corrosion set was placed and manually pressed in the hole. Two corrosion sets (named as ‘‘glassy” and ‘‘cementitious”) were prepared and their composition has been given in Table 2. Samples with the corrosion set were fired at 1400 °C for 6 h (set II, glassy) or at 1500 °C for 10 hrs (set I, cementitious). After cooling the samples, they were cut along the cylinder height. The cut fragments were washed with water so as to clean off the dust and make the corroded surface visible; next they were dried and the corrosion area was planimetred. The percentage share of uncorroded sample surface was assumed to be a measure of material’s corrosive resistance, i.e. the higher value was achieved by planimetring, the higher was the value of a given material’s corrosive resistance. 2.4. Methods The raw materials used in the investigations were tested for their chemical composition (XRF, PANalitycal Magix PW2424 spectrometer) and quality phase composition (XRD, PANalytical X’pert Pro diffractometer operating at 40 kV and 30 mA, with CuKa radiation, Ni filter and X’Celerator detector). In the case of magnesia
3
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084 Table 1 Composition of the prepared batches. Composition Sintered magnesia
Coarse grains (<5 mm) Fine grains (<0.1 mm) Coarse grains (<5 mm) Fine grains (<0.1 mm)
Sintered magnesia from a spent fired MgO-C material Zirconia Fused MgO-ZrO2 co-clinker
Coarse grains (<5 mm) Fine grains (<0.1 mm)
K0
K4Z
W0
W4Z
FC4Z
77 23 – – – – –
73 23 – – 4 – –
– – 72.6 27.4 – – –
– – 68.6 27.4 4 – –
– – – – – 74 26
Fig. 2. Schematic flow chart of the preparation course.
Table 2 Chemical composition of corrosive agents (wt%). Corrosive agent I (cementitious set) SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
LOI
9.1
2.3
1.1
29.1
0.4
24.0
0.1
26.7
7.0
Corrosive agent II (glassy set) SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
MnO
SO3
LOI
33.0
2.7
0.1
3.4
2.4
0.1
25.2
1.6
13.4
18.1
LOI = loss on ignition.
clinkers, also apparent density and open porosity were determined by the hydrostatic method (using paraffin oil as a working liquid). Moreover, differential thermal analysis (DTA) and thermogravimetric analysis (TG) combined with evolved gas analysis (EGA) were conducted for MgO-C waste (STA 409 PC NETZSCH thermal analyser, which was coupled with a QMS 403C Aëolos quadrupole
mass spectrometer; a total of 60 mg of the samples was placed in an alumina crucible; the heating rate of the samples was 10 K∙min 1; tests were performed in air with a flow rate of 10 ml∙min 1). After the sintering process, various ceramic properties were measured, including: linear firing shrinkage according to diameter
4
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Table 3 Chemical analysis (XRF) of raw materials (wt%) and main properties of magnesia materials. Oxides
Zirconia
Commercial sintered magnesia clinker
Spent MgO-C refractories
Spent fired MgO-C refractories
Fused MgO-ZrO2 co-clinker
SiO2 Al2O3 Fe2O3 TiO2 CaO MgO K2O Na2O MnO P2O5 Cr2O3 ZrO2 HfO2 LOI C/S molar ratio Bulk density [g/cm3] Apparent porosity [%]
0.37 0.20 0.12 0.10 0.53 0.21 <0.01 <0.01 – 0.30 – 96.29 1.62 0.20 – – –
0.37 0.81 0.41 0.01 0.87 97.65 <0.01 <0.01 0.09 0.05 – – – 0.15 2.52 3.14 4.7
1.03 0.90 0.56 0.03 1.78 82.41 0.07 <0.01 0.07 0.07 0.07 – – 12.99 1.85 2.73 12.3
1.56 1.42 0.73 0.02 2.17 93.72 <0.01 <0.01 – 0.08 – – – 0.45 1.49 2.74 19.0
1.28 1.32 0.63 0.02 2.01 91.02 <0.01 <0.01 – – – 3.97 0.04 0.13 1.68 3.16 8.6
LOI = loss on ignition.
and height measurement, bulk density and open porosity based on the Archimedes method, using kerosene as the immersion medium to avoid hydration (according to PN-EN 993-1:1998 standard), as well as cold crushing strength (according to PN-EN 993-5:2001 standard). The obtained refractory materials were also examined with regard to their thermal shock resistance, which involved their cyclical heating (950 °C/25 min) and cooling (water; 3 min) until the sample lost 20% of its original mass. Each parameter of the material was determined for minimum three samples, and the final result was the average value of partial results. Moreover, investigations into the pore size distribution and SEM observations were performed to characterize the microstructure of the obtained ceramics. Porosimetric investigations were carried out using an Autopore 9500 mercury porosimeter by Micromeritics for testing porosity and pore size distribution within a range of 0.006–450 lm. Scanning electron microscopy (SEM) was performed using a Tescan Mira III microscope in combination with
an Energy Dispersive Spectroscopy (EDS) system with AZtec Automated software (Oxford Instruments). 3. Results and discussion 3.1. Characteristics of raw materials The chemical analysis as well as the phase composition of the raw materials applied have been presented in Table 3 and Fig. 3, respectively. In the spent refractories mainly periclase (ICDD-PDF 01-071-4938) and graphite (ICDD-PDF 04-007-8496) with some insignificant traces of merwinite (ICDD-PDF 00-035-0591), monticellite (ICDD-PDF 00-011-0129) and spinel (ICDD-PDF 04-0083488) were identified. They were also characterized by a high value of loss on ignition, which reached ca 13 wt%. It results from the combustion process of carbon substances, i.e. graphite present in this recycled material (Fig. 3, Fig. 5). Combustion of carbon
Fig. 3. XRD patterns of raw materials and the obtained magnesia-zirconia co-clinker.
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
occurred in several stages in a wide temperature range, which was confirmed by a thermal analysis study (Fig. 4a). On the TG and DTG curves of raw MgO-C material, a significant mass change (~12 wt%) was observed within a temperature range of 500 to above 1000 °C. The course of the DTG curve shows that this process has three stages, confirmed by peaks with maximums at ~640, 820 and 870 °C, and correlated with an evolution of CO2 observed on the EGA curve. A small mass change (~1 wt%) at a temperature of ca 1200 °C resulted from sulphur compounds’ decomposition. In order to eliminate graphite from the recycled aggregates, the spent MgO-C refractory was heated at 1400 °C for 8 h. As a result of this thermal pre-treatment, secondary magnesia clinker was obtained. The firing process – although economically expensive – was incorporated in the research program, because the melting of magnesia-carbon material in an arc furnace is problematic and can be dangerous. The following reactions may occur during the process of magnesia reduction by carbon: (1) direct reduction [magnesia(s) + carbon(s) = magnesium(g) + carbon oxide (g)] as well as (2) indirect reduction: [magnesia(s) + carbon oxide(g) = magnesium(g) + carbon dioxide(g)] [22,23].
5
Its phase composition was similar to that of spent raw refractories, but it was deprived of graphite (Fig. 3). The carbon compound was practically fully combusted and the secondary MgO clinker was characterized by a negligible loss on ignition (Table 3; Fig. 4b). The characteristic flake-like structures of graphite present in the raw material (Fig. 5a) were absent as a result of combustion, and periclase grains with some porosity were obtained (Fig. 5b). This secondary magnesia clinker was characterized by significantly lower quality in comparison with commercial MgO clinker. The amount of MgO was smaller (93.7 vs 97.6 wt%) and contained higher amounts of undesirable compounds, such as CaO, Al2O3, Fe2O3. They reduce the refractoriness of magnesia clinker, especially when C/S molar ratio (CaO/SiO2) is below 2, contributing to the development of low-melting phases (like merwinite or monticellite) in the system. The C/S is an important ratio as it indicates the presence of other mineral phases in magnesia-containing refractories. When the molar ratio of CaO/SiO2 is >2, mainly dicalcium silicate, characterized by one of the highest values of refractoriness among silicate phases, coexists with periclase [22,24]. Moreover, this secondary magnesia clinker also has a visibly lower
Fig. 4. DTA-TG-EGA curves of spent MgO-C material before (a) and after (b) thermal pre-treatment.
6
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Fig. 4 (continued)
bulk density (2.73 vs 3.14 g/cm3) and higher apparent porosity (19.0 vs 4.7%), respectively, determining its lower quality, which should be improved, among others, by fusion with zirconia. In the case of magnesia-zirconia fused co-clinker, the assumed amount of zircon oxide in the material was obtained. The chemical analysis results show that the content of ZrO2 was around 4 wt% (Table 3). Moreover, this co-clinker contained undesirable compounds similar to secondary clinker. The C/S molar ratio was still below 2, but it is worth noting that the added ZrO2 easily reacts with Ca-compounds, and Ca-stabilized cubic zirconia or calcium zirconate (CaZrO3) – with an excess of calcium compounds – may be formed. CaZrO3 is characterized by a very high temperature of melting (ca 2350 °C), which is very desirable in the case of refractory materials application. The fusion process resulted in the formation of a new phase. On the co-clinker diffraction pattern, the main diffraction reflexes from the stabilized cubic form of ZrO2 instead of the monoclinic (baddelyite) form of zirconium oxide (Fig. 3) were observed. This form is favourable in the structure because it allows avoiding the disadvantageous phase transformation of zirconia at ~1200 °C (monoclinic M tetragonal). This transformation is connected with a considerable volume change, which reduces the refractory material’s strength [25,26]. The
obtained fused co-clinker was characterized by much bigger magnesia grains connected together by the zirconate phase (Fig. 5c). This is a desirable property because grains with larger dimensions reduce magnesia’s reactivity with corrosive agents. 3.2. Properties of the obtained refractory materials The results of the measured physico-mechanical properties of the refractory samples are presented in Figs. 6–9, while the microstructure characteristics are shown in Fig. 10 and in Table 4. The corrosion test results have been given in Fig. 11. All the obtained refractory samples have stable dimensions after the firing process (Fig. 6) due to their low firing shrinkage. The values of this parameter do not exceed 1.0–1.5% in relation to the original size of the samples. Due to the presence of bigger amounts of undesirable compounds, which lowered the melting point of secondary magnesia clinker, the linear shrinkage of the sample based on this raw material was noticeably higher (for example sample W0 vs K0). Fig. 7 shows the measured bulk density and apparent porosity of the obtained refractory materials. As can be seen, the sample based on secondary magnesia clinker (sample W0) was
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
7
Fig. 5. SEM images of raw spent MgO-C material (a), after thermal pre-treatment and combustion of graphite (b) and after fusion with ZrO2 in an electric arc furnace (c).
Fig. 6. Linear shrinkage of the obtained refractory samples.
8
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Fig. 7. Bulk densities and apparent porosities of the obtained refractory samples.
Fig. 8. Cold crushing strength of the obtained refractory samples.
Fig. 9. Thermal shock resistance of the obtained refractory samples.
characterized by the worst technical parameters. This material had the highest porosity (~19%) and the lowest bulk density (2.78 g/cm3). Such results were expected due to the low quality of the clinker itself (Table 3). An addition of zirconia (4 wt%) to magnesia clinker (both commercial and secondary magnesia clinker) clearly contributed to the improvement of main ceramic properties. In comparison with respective samples (K4Z vs K0 or
W4Z vs W0), higher density and lower open porosity were observed. Probably, the sintering process of MgO-ZrO2 ceramics largely dependent on solid phase sintering. Therefore, the densification of material depends on the manner of matter transport i.e. surface diffusion. Thus mechanism may be discussed by depicting the defect model [27–30]. For example, open porosity decreased from 19 to 16.5% for W4Z sample. This is a desirable
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
9
Fig. 10. SEM images of fired ceramic samples: K0 (a), K4Z (b), W0 (c), W4Z (d), FC4Z (e).
effect as it may contribute to increased corrosion resistance of a material in its future work. From this point of view, the best results were obtained for the refractory sample of fused
magnesia-zirconia co-clinker. In the case of FC4Z sample, open porosity achieved the lowest value, i.e. 13%, whereas its bulk density was the highest, reaching nearly 3.0 g/cm3.
10
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
Table 4 Pore size distribution of the obtained ceramic samples (%).
>60 lm 30–60 lm 20–30 lm 1–20 lm <1 lm
K0
K4Z
W0
W4Z
FC4Z
4.26 0.28 0.19 91.73 3.54
3.34 0.41 0.98 92.89 2.38
2.86 1.12 10.12 83.65 2.25
3.86 3.93 34.13 55.79 2.29
8.38 34.05 23.70 32.20 1.67
Fig. 11. Results of tests of the obtained refractory samples’ corrosion resistance against two different corrosive agents.
The use of zirconia has a positive influence on the obtained ceramics’ mechanical strength, expressed as cold crushing strength (Fig. 8). A comparison of the results obtained for samples without zirconia addition with materials based on 4 wt% addition of ZrO2 shows an increase in the CCS values. Replacement of sintered magnesia clinker (commercial or secondary clinker from spent fired MgO-C materials) with the obtained fused MgO-ZrO2 co-clinker resulted in a further significant increase of cold crushing strength. For sample FC4Z it reached the highest value – around 65 MPa. It was higher by ca 30% than in the case of a material made from spent fired refractory aggregates or commercial magnesia clinker.
An addition of zirconia alone or in the form of fused MgO-ZrO2 co-clinker is also responsible for a slight but observable improvement of thermal shock resistance, which increased from ca 6–7 to 10 water cycles for materials with and without zirconia addition, respectively (Fig. 9). This favourable effect of ZrO2 addition can be associated with lower thermal expansion coefficient of ZrO2 in comparison with magnesia (~10∙10 6 °C 1 vs 13– 15∙10 6 °C 1, respectively). The above described investigations were supplemented with microstructure observations (Fig. 10) and pore size distribution (Table 4) in the obtained ceramic materials. Samples based on commercial magnesia (samples K0 and K4Z) are characterized by similar microstructure (Fig. 10a,b) and pore size distribution (Table 4). In SEM micrographs there are visible coarse grains of magnesia embedded in a matrix built of finer MgO fractions. In the case of sample K4Z, one can also observe areas with additional grains of a phase rich in ZrO2. These materials have similar pore size distribution, in which pores ranging from 1 to 20 lm are dominant (>90%). Replacement of commercial magnesia clinker by a recycled material (secondary magnesia clinker) having much worse parameters than commercial magnesia (Table 3), contributed to obtaining products with a different structure, in which there was a visible increase in the number of larger pores (with dimensions >20 lm), both in material W0 and with ZrO2 addition (sample W4Z). In both cases (sample W0 and W4Z, Fig. 10c,d), SEM pictures show higher amount of oblong, tubular pores, which are frequently connected with each other. This confirms the previously determined open porosity and weaker mechanical strength of ceramic samples, based on secondary magnesia clinker. The differences in the nature of the pores result from the difference in the chemical purity of the magnesia clinkers used in this study. In the case of W0 and W4Z samples which were obtained from spent MgO-C material, the content of Ca, Fe, and Si compounds is clearly higher than in the case of samples based on commercial raw magnesia clinker (Table 3). The sum of these admixtures for samples from ‘‘W” series is more than 2 times higher (5.88 wt% for spent fired MgO-C refractories vs 2.46 wt%
Fig. 12. Corrosion resistance of a sample expressed as percentage of non-corroded area after a corrosion test.
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
for commercial sintered magnesia clinker). This will make the ‘‘W” material more easily sintered as a result of the formation of lowmelting eutectics from the C-M-S system. This was confirmed in SEM pictures (Fig. 10), where the presence of the intercrystalline phases (Ca,Si-rich phase) is clearly greater in the case of the ‘‘W” series samples (Fig. 10c vs Fig. 10a). In addition, the obtained W0 sample shows no fine-grained matrix, which was sintered during the firing process of the material. In consequence we observe MgO grain growth and agglomeration of fissure pores. On the other hand, sample FC4Z has a different microstructure. The lower linear shrinkage of the FC4Z sample (in comparison with W4Z sample), in principle, results from the lower reactivity of large fused clinkers grains. This phenomenon is due to the formation of large periclase crystals (Fig. 5c) with less porosity and thus less surface development. In the case of fused magnesia-zirconia coclinker the partial closure of the Ca,Si-rich phases inside the periclase crystals was also observed (Fig. 5c). As in the case of the ‘‘W” series samples, the presence of more pores with larger sizes in FC4Z sample is the effect of sintering of the matrix. Moreover, the presence of large grains of fused raw material (MgO-ZrO2 coclinker) with lower reactivity blocked the sintering effect macroscopically, which still progressed in the product matrix under firing process. Hence, in this case the largest and widest pores with the absence of fine grains matrix were observed at SEM pictures. Although pore size distribution of FC4Z sample is less favourable (this material contains the highest number of pores with dimensions >30 lm), the microstructure indicates a more homogenous, uniform dispersion of the zirconia phase, which results from the prior process of melting and obtaining MgO-ZrO2 fused coclinker. In the further part of investigations, this may improve the products’ corrosive resistance, as the presence of ZrO2 will stop the progressing chemical corrosion of the product. In the case of corrosion with a ‘‘cementitious” set, it will be the effect of forming high-melting refractory calcium zirconate CaZrO3, and in the case of corrosion with a ‘‘glassy” set, the liquid phase formed will increase its viscosity, hindering the corrosive agent’s penetration inside the material. In respect to the corrosion study, the concept behind the composition of the ‘‘glassy” set was to recreate conditions in glass furnace regenerators, whereas the ‘‘cementitious” set was selected to recreate the conditions in a cement furnace – taking into consideration both the components of cement clinker and components from the furnace atmosphere, which also influences the corrosion processes and is, among others, connected with the circulation of alkalis [10]. In the atmosphere of cement kilns potassium prevails over sodium [10], hence the composition of this corrosion set includes potassium sulphate, enriching the system with alkalis. The contents of K2O and SO3, which in the atmosphere of the kiln circulates as SO2, have been considerably increased so as to create a drastic effect of these two components. The cross-section of samples after corrosion tests confirmed a positive effect of zirconia addition on the refractory samples’ corrosion resistance (Fig. 11), because deep penetration determined for both corrosion agents was visibly lower in comparison with the reference samples, i.e. commercial magnesia (sample K0) or secondary magnesia clinker (sample W0). An attempt to express this resistance in a semi-quantitative way has been shown in Fig. 12, where the results of corrosion resistance test were expressed as percentage of non-corroded surfaces of the materials. Due to a notably alkaline character of the cementitious corrosive agent, the obtained materials’ corrosion resistance was high, reaching a value around 95–97%. Under measurement conditions, the applied glassy corrosive agent, in the form of molten slag, penetrates deeper and easily infiltrates the porous materials. The determined corrosion resistance reached a level ranging from ~55% (for samples without zirconia) to ~70% (for samples with an addition of zirco-
11
nia). The best result of corrosion resistance against a glassy agent was observed in the case of the material based on fused MgOZrO2 co-clinker; it resulted directly from the technological parameters of the obtained raw-material, i.e. the lowest value of open porosity of FC4Z sample (Fig. 7) as well as good homogenization and dispersion of zirconia compounds in the fused material (Fig. 10e). 4. Conclusions In the presented study, MgO- and MgO-ZrO2 refractory ceramics made from secondary magnesia clinker obtained from fired spent magnesia-carbon refractories were compared with materials obtained in a traditional way – from commercial raw materials. It has been found that spent MgO-C refractories can be successfully recycled, but it is worth bearing in mind that the obtained secondary magnesia clinker is characterized by lower quality. This quality can be considerably improved by adding zirconia and/or producing fused MgO-ZrO2 co-clinker, which reduces open porosity and increases bulk density. Moreover, zirconia addition increases the cold crushing strength of ceramics, especially in the case of fused magnesia-zirconia co-clinker, and improves corrosion resistance. Consequently, these materials can be used in the future as a valuable substitute of refractory ceramics obtained in a traditional way, from natural sources. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is the result of statutory activity at the Institute of Ceramics and Building Materials, Refractory Materials Division in Gliwice supported by the Polish Ministry of Science and Higher Education. The author thanks Mr. A. S´liwa for their help in mercury porosimetric analysis. References [1] G. Bertrand, D. Cassard, N. Arvanitidis, G. Stanley, Map of critical raw material deposits in Europe, Energy Procedia 97 (2016) 44–50. [2] M. O’Driscoll, MagForum 2017, Refract. Worldforum 9 (2017) 69–73. [3] S. Shaw, China’s increasing environmental focus sends shockwaves through the refractory industry, Refract. Worldforum 11 (2019) 58–62. [4] J. Czechowski, J. Witek, A. S´liwa, Przemysł materiałów ogniotrwałych – problematyka badan´ i rozwoju (Refractory industry – research and development), Szkło i Ceramika 2 (2017) 26–27 (in Polish). [5] C.E. Semler, Review of refractoris markets & research – 2016, Refract. Worldforum 9 (2017) 30–34. [6] B. Plešinerová, P. Vadász, R. Kamensky´, J. Bounziová, J. Derd’ak, D. Medved, Spent magnesia-carbon refractory bricks from steel production: potentiality of MgO-clinker recovery, Acta Montanistica Slov. 23 (2018) 39–45. [7] A. Volcaert, Raw materials for refractories: the European perspective, in: D.G. Goski, J.D. Smith (Eds.), Proceedings of the Unified International Technical Conference on Refractories (13th biennial worldwide congress on refractories) UNITECR’13, The American Ceramic Society, 2013, pp. 84–90. [8] G. Routschka, Refractory Materials – Pocket Manual, 2nd ed., Vulkan-Verlag, Essen, 2004. [9] A.O. Surendranathan, An Introduction to Ceramics and Refractories, CRC Press, Boca Raton, 2015. [10] J. Szczerba, Modyfied magnesia refractory materials, Ceramics 99 (2007) 1– 204 (in Polish). [11] R. Kusiorowski, J. Wojsa, B. Psiuk, T. Wala, Influence of zirconia addition on the properties of magnesia refractories, Ceram. Int. 42 (2016) 11373–11386. [12] R. Kusiorowski, B. Psiuk, Fused magnesia-zirconia co-clinker for fired refractories, Ceram. Int. 43 (2017) 14263–14270. [13] E.A. Rodríguez, G.-Alan Castillo, T.K. Das, R. Puente-Ornelas, Y. González, A.M. Arato, J.A. Aguilar-Martínez, MgAl2O4 spinel as an effective ceramic bonding in a MgO-CaZrO3 refractory, J. Eur. Ceram. Soc. 33 (2013) 2764–2774.
12
R. Kusiorowski / Construction and Building Materials 231 (2020) 117084
[14] L. Horckmans, P. Nielsen, P. Dierckx, A. Ducastel, Recycling of refractory bticks used in basic steelmaking: a review, Resour. Conserv. Recycl. 140 (2019) 297– 304. [15] F. Arianpour, F. Kzemi, F.G. Fard, Characterization, microstructure and corrosion behaviour of magnesia refractories produced from recycled refractory aggregates, Miner. Eng. 23 (2010) 273–276. [16] A.N. Conejo, R.G. Lule, F. Lopéz, R. Rodriguez, Recycling MgO-C refractory in electric arc furnaces, Resour. Conserv. Recycl. 49 (2006) 14–31. [17] A.P. Luz, D.O. Vivaldini, F. Lopéz, P.O.R.C. Brant, V.C. Pandolfelli, Recycling MgO-C refractories and dolomite fines as slag foaming conditioners: experimental thermodynamic evaluations, Ceram. Int. 39 (2013) 8079–8085. [18] B. Plešinerová, P. Vadász, G. Sucˇik, I. Olejár, Specification of MgO-concentrate obtained from post mortem MgO-C refractory materials, Ceramics-Silikáty 62 (2018) 389–395. [19] K.S. Kwong, J.P. Bennett, Recycling practices of spent MgO-C refractories, J. Miner. Mater. Charact. Eng. 1 (2002) 69–78. [20] S. Hanagiri, T. Matsui, A. Shimpo, S. Aso, T. Inuzuka, T. Matsuda, S. Sakaki, H. Nakagawa, Recent improvement of recycling technology for refractories, Nippon Steel Technical Report 98 (2008) 93–98. [21] A.G.M. Othman, W.M.N. Nour, Recycling of spent magnesite and ZAS bricks for the production of new basic refractories, Ceram. Int. 31 (2005) 1053–1059. [22] L. Rongti, P. Wei, M. Sano, Kinetics and mechanism of carbothermic reduction of magnesia, Metall. Mater. Trans. B 34B (2003) 433–437.
[23] G. Brooks, S. Trang, P. Witt, M.N.H. Khan, M. Nagle, The carbotheric route to magnesium, JOM 58 (2006) 51–55. _ [24] I. Jastrze˛bska, J. Szczerba, R. Prorok, E. S´niezek, An experimental study on hydration of various magnesia raw materials, Ceramics-Silikáty 59 (2015) 48– 58. [25] A.P. Bechepeche, O. Treu Jr, E. Longo, C.O. Paiva-Santos, J.A. Varela, Experimental and theoretical aspects of the stabilization of zirconia, J. Mater. Sci. 34 (1999) 2751–2756. [26] E.H. Kisi, C.J. Howard, Crystal structure of zirconia phases and their interrelation, Key Eng. Mater. 153–154 (1998) 1–36. _ [27] E. S´niezek, J. Szczerba, P. Stoch, R. Prorok, I. Jastrze˛bska, W. Bodnar, E. Burkel, Structural properties of MgO-ZrO2 ceramics obtained by conventional sintering, arc melting and field assisted sintering technique, Mater. Des. 99 (2016) 412–420. [28] R.S. Nasar, M. Cerqueira, E. Longo, J.A. Varela, Sintering mechanism of ZrO2MgO with addition of TiO2 and CuO, Ceram. Int. 30 (2004) 571–577. [29] K.C. Chou, X. Hou, M. Chen, B. Zhao, Controllable preparation of Al2O3MgOAl2O3-CaO6Al2O3 (AMC) composite with improved slag penetration resistance, Int. J. Appl. Ceram. Technol. 13 (2016) 33–40. [30] S. Yao, E. Wang, J. Chen, K.C. Chou, X. Hou, Effectively controlling the crystal growth of Cr2O3 using SiO2 as the second phase, J. Am. Ceram. Soc. 102 (2019) 2187–2194.