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Sustainable Saggar-Preparation Based on Recycled Silicon Carbide and Alumina Powders

Ceramic waste from broken kiln furniture, crucibles and saggars of the technical ceramic industry accumulates each year in very large amounts. It contains highly valuable materials like silicon carbide (SiC) and alumina (Al2O3). A recycling of this materials is indispensable to reduce the raw material consumption and waste production, thus decreasing the environmental impact of this industry. In this study, the recyclability of SiC and Al2O3 containing waste has been investigated. Recycled powders have been pre-processed and alumina-based recipes have been developed with the aim to produce recycling-saggars by pressure die casting. Saggars based on the developed materials have been prepared and tested under realistic application conditions.

1 Introduction
Ceramic kiln furniture and refractory products as supports, beams, rollers, crucibles, saggars, and plates are essential to produce ceramic goods in batch or continuous furnaces. While SiC is often used for plates, beams, and rollers, due to its high mechanical strength and the high thermal conductivity, Al2O3 crucibles and saggars are often used due to its high chemical stability. Although both types of materials are well known for their high temperature stability, the continuous thermal cycling as well as the interaction with the goods and the furnace atmosphere leads to crack formation, corrosion, and/or oxidation. Therefore, the operation period is limited, and a high amount of ceramic waste is produced per year. The production of SiC and Al2O3 as raw materials for kiln furniture is connected to a high energy expenditure. In detail, a consumption of 7–8 MWh of electrical energy and the emission of 6,6 t CO2 are connected to the synthesis of 1 t high quality SiC by the common Acheson-process. The complex process chain to prepare high quality Al2O3 powders including the Bayer-process, is also connected to a high energy consumption as well as the use of large amounts of sodium hydroxide solution and sulphuric acid. On that account, a recycling of these high-value materials is mandatory. The re-use of ceramic waste for products of lower requirements is well known, as for example for refractory concrete [1], expanding agent in lightweight refractory [2], or more general as filler for refractory products [3, 4]. The use of crushed silicate bonded SiC plates for the preparation of refractories based on Mullite-bonded SiC was investigated by Ewais et al. [5], focussing on semidry pressing of mixtures including SiC waste, bauxite, and silica fumes. 5,3 mass‑% of mono-aluminium phosphate was added to the mixtures with varying amounts of SiC waste (10–50 mass-%), in order to increase the green strength and promote the mullite formation. The material is especially of interest for linings in metallurgical furnaces. In a recent project, funded by the Federal Ministry of Education and Research (BMBF), the recyclability of SiC and Al2O3 containing waste to produce saggars has been investigated. Aim of the project SiAluPor of Rösler CeramInno, Spitzer Rohstoffhandelsgesellschaft mbH and Fraunhofer IKTS was the selection of recycling material sources and the pre-processing of ceramic waste to standardised SiC and Al2O3 powders that can be used for the preparation of kiln furniture, such as saggars. 

2 Experimental
The identification of potential raw material sources was performed by Spitzer Rohstoffhandelsgesellschaft mbH/DE using standard equipment for milling and sieving, as well as specialised electromagnetic separators to clean the powder products from metal impurities. The chemical composition was investigated externally by XRF (according to DIN EN ISO 12677:2013-02) at CRB Analyse Service GmbH/DE. At Fraunhofer IKTS/DE, the characterisation of the particle size distribution was performed using a laser diffraction particle size analyser (Mastersizer 2000, Malvern/GB) and the phase composition of the recycled powders was analyses by XRD (Bruker D8). The development of material compositions to created saggar products of the recycled ceramic powders was performed in lab scale at Fraunhofer IKTS and afterwards adjusted to pilot scale by Rösler CeramInno. The shaping was done by pressure die casting using a standard Netzsch/DE machine with operation pressure of 10–20 bar and a cycle time of about 2–15 min per saggar. The slurry was pumped by a diaphragm pump. Demoulded saggars have been dried under controlled humidity and temperature for about 15 d. The firing was performed in continuous, gas-fired or electrical heated furnaces at temperatures between 1250–1600 °C, depending on the type and composition of the saggar material. The performance of the developed saggar test samples has been investigated by a 6-month test period in a continuous furnace under application related conditions at Rösler CeramInno in cooperation with Fraunhofer IKTS (1180 °C and 1230 °C). 

3 Results and discussion 

3.1 Raw material screening and pre-processing
For SiC and Al2O3 in general two different kinds of sources have been identified: production waste and worn-out kiln furniture. The most important sources are summarised in Tab. 1. The sorted raw materials have afterwards been processed in a jaw crusher, an impact crusher, and a gyratory mill. The fractionation was done in a sieve plant. Prior the packaging, the powder products must be cleaned from metallic impurities coming from the milling process via magnetic separator. A high-intensity magnetic separator can be used if the content of metallic impurities is still too high. Due to the electrostatic charge of fine powders, up to 20 % of fine ceramic powders will be loosed by this step. To reduce the loss, an additional permanent magnet can be used to separate the fine powder fraction from the impurities and reduce the material loss down to 0–3 %. Based on this pre-processing technology different fractions of SiC- and Al2O3-powder, respectively have been prepared and used for the development of recycling-based saggar test samples. The powders are characterised by narrow particle size distributions according to the fractionation (Fig. 1). Due to the standardised process, no significant variation between different batches during the project period has been observed. The composition and the content of impurities of the powders was identified by XRF-analysis. The content of impurities (SiO2 excluded) was found to be between 1–2 mass-% with only small variations during a three-year observation period (Fig. 2). The main phases identified by XRD-Analysis for the SiC- and Al2O3-material are listed in Tab. 2. 

3.2 Saggar material development
With the aim to develop saggar-materials allowing an operation temperature of up to 1300 °C, the concept of clay bonding of the SiC- and/or Al2O3-particles was chosen. The advantage of this concept is the plasticity of the slurry that allows pressure die casting. In addition, clay is a cheap and widely available raw material. For both recycling powders, two example compositions are depicted in Tab. 3. Instead of joint milling of all components as reported for slip casting by Sivov [6], the coarse particles of either SiC or Al2O3 or even in combination can be stirred directly into the slurry, reaching a solid content of up to 78 % by weight. The achieved microstructure after initial firing at 1250 °C of such compositions is shown in Fig. 3. The coarse SiC- or Al2O3- particles are well-embedded in the oxidic matrix. For the SiC material, it is visible that the recycled SiC particles originate from different kinds of SiC-sources, such as reaction- bonded SiSiC, silicate-bonded SBSiC and recrystallized RSiC. 

3.3 Saggar production by pressure die casting
The production of saggars by using the pressure die casting process is very efficient and highly flexible in terms of different and tailor-made saggar-designs, sizes, and shapes. The moulds for the saggar production have a good price-performance ratio and can be manufactured in relatively short time. Within this work, a parameter window for the slurry characteristics and the operation numbers has been identified. This includes the adjustment of the solid content and the viscosity of the slurry, in order to avoid sedimentation and separation of the coarser particles, in the diaphragm pump or the segregation of fine and coarse particles in the mould. Also, the parameters for the die pressing have been adjusted to the reach a high green density and green strength and a faster draining. Based on the adjusted parameter it is possible to produce large quantities of saggars with this technology in a comparably short time. The slurry preparation is very important to get good results and high-quality saggars. It is possible to prepare a slurry with particle sizes up to 2 mm, which is very important for the formation of a high packing density in the structure. After drying, the saggars are subjected to the first firing between 1250–1600 °C. The choice of the first firing conditions depends on the different slurry mixtures and can be adjusted to the later application requirements of the customer. 

3.4 Material behaviour under application conditions
Different saggar materials have been tested over a period of six months in a continuous furnace. In the following the results of the SiC-based saggar filled with alumina hydroxide and alumina substrates will be discussed in detail (Fig. 4 a–c). During the test run, the saggar samples have been cycled up to 250 times in a 10–12 h cold-to-cold run. While for the samples tested at a temperature of 1180 °C with alumina substrates as incinerator charge, no defect formation was observed (Fig. 5 a–b). The samples tested at a temperature of 1230 °C carrying aluminium hydroxide showed a crack formation nearly to the centre of the saggar (Fig. 5 c–d). However, the mechanical strength was still high enough and none of the samples has been broken during the test period. The microscopic analysis (Fig. 6 a–d) indicates that no significant degradation of the saggar material occurs for the lower firing temperature and the pure alumina substrates as incinerator charge. On the contrary, the same saggar material applied at a 50 K higher firing temperature in contact to aluminium hydroxide is characterised by a 10–20 μm thick reaction layer at the inner contact face of the incinerator charge and the saggar. According to the EDX analysis, the reaction layer mainly consists of aluminium, oxygen, and silicon impurities like iron, calcium, potassium, and sodium seem to be homogeneously distributed without an applicationor reaction-related gradient (Fig. 7 a–h). The continuous oxidation of the SiC particles in the oxidic matrix in combination with the thermal cycling can lead to crack formation. One reason might be the formation of cristobalite. The reversible phase transformation from β- to α-cristobalite during cooling (between 275–200 °C) is connected to a volume reduction of about 2,0 to 2,8 % respectively, which promotes crack formation at microscopic scale. The simultaneously occurring thermal stress due to the difference between the edges and the centre of the saggar lead to tension at the edges and promoted crack formation, as observed in the samples. In the post-mortem analysis of the phase composition of the two saggar samples, no significant differences have been observed, indicating no unexpected interaction of the saggar material and the fired goods (Fig. 8). 

4 Conclusion and outlook
It has been proven that recycled powders of high-value materials, such as SiC and Al2O3, can be used to produce saggars for the technical ceramics industry. Well-defined particle fractions can be provided using standard powder processing and additional cleaning to reduce the amount of abraded iron. The concept of clay-bonded SiC- or Al2O3 -materials allows a wide range of compositions and application fields. Depending on the preparation technique, even very coarse particles of 1–2 mm can be used, thus reducing the effort for the powder processing. However, such saggar materials still have the potential to be optimised regarding mechanical strength, corrosion stability, and density. A reduction of the saggar density can be reached using organic placeholders. Moreover, it is possible to design individual saggars specially developed for customer purpose in terms of corrosion resistance, thermal conductivity, or resistance to temperature changes. For example, pore-forming agents can be used to adjust a well-defined porosity (total amount and pore size) within the saggar, thus reducing the weight and the thermal inertia. An increased amount of recycled particles of up to 60 % or even 70 % by weight might be feasible by the use of additional additives influencing the flow behaviour. 

The project with the number 033RK070 has been funded by the Federal Ministry of Education and Research (BMBF) via PtJ Projektträger Jülich. 

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[2] Nobis, C.; Vollpracht, A.: Verwendung von Recyclingmaterial in der Betonproduktion – Sachstand, Forschungsbericht F 7099, RWTH Aachen, 2015 

[3] Mazzanti, F.; et al.: Exploitation of ceramic wastes by recycling in alumina mullite refractories. Adv. in Sci. and Technol. 10 (2010) 9–14 

[4] Sadik, C.; Albizane, A.; El Amrani, I.-E.: Production of porous firebrick from mixtures of clay and recycled refractory waste with expanded perlite addition. J. Mater. Environ. Sci. (2013) [6] 981–986 

[5] Ewais, E.M.M.; Al-Wakeel, M.I.K.N.; Ahmed, Y.M.Z.: New refractories made from reused silicon carbide. J. of the Ceram. Soc. Japan 112 (2004) 428–433 

[6] Sivov, V.P.: Clay-silicon carbide refractories based on technogenic materials produced by slip casting. Glass and Ceramics 58 (2001) 322–325

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