Energy Efficient, Low CO2 Refractories for the Foundation Industries.

Jafar Daji1*, Joshua Parkin1, James Eales2, Ebele Ahizi2, Jack Cornwall2, Paul A. Bingham2

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The need to reduce greenhouse gas emissions is critical in the Foundation Industries (FI). PSR falls within two Foundation Industries – ceramics (manufacturer) and glass (supply chain), where its operations are energy intensive. The UK refractory industry is a cornerstone of our ceramics industry and supplies other FIs, and exports, that are vital to the UK economy. Glass is one of the most important basic materials in the world and PSR and other refractory suppliers are considered key enabling technology manufacturers. For example, PSR remained open and maintained manufacturing operations throughout the Covid-19 pandemic, being an essential supplier to manufacturers of glass packaging. Now the industry is faced with the need to meet the challenges caused by climate change and the need for long-term sustainability.

It is thus, vitally important to reduce fossil fuel energy consumption in the refractory manufacturing process to achieve the target of net zero carbon emissions by , thereby supporting the global FI sector in a sustainable manner.

The UK ceramics industry (including refractories), accounts for approx. £2 billion in annual sales including £0.5 billion in exports, with 75% of companies being SMEs. The UK sector is energy-intensive and consumes around 4.7TWh of delivered energy / year, and total emissions of 1.2 million tonnes CO2.  The need to decarbonise has been highlighted in several ceramic industry decarbonisation roadmaps [UK Govt – Parsons Brinckerhoff () / UK Ceramics industry – BCC () / Ceram-Unie – EU ()], with the need for developing new ceramic mixtures or adding other components to the raw materials to improve firing efficiencies being identified. This project directly addresses these recommendations and working with our partners – Sheffield Hallam University, Ceramics UK and Innovate UK – to produce a commercially viable solution to this need.

For refractories manufacturing, the sintering stage involves firing materials in high-temperature kilns run over several days at temperatures exceeding °C in most cases, depending on the product. That means a lot of time and energy going into the making of refractories that are essential to the FI. This stage in the process is consequently rather costly to the manufacturer but more importantly it is energy and emission intensive with resultant large amounts of CO2 being produced by the industry. With PSR’s business strategy now including decarbonisation, there is a drive to find opportunities to reduce energy use, where one idea was to look for a way to lower sintering temperatures and run times. However, PSR needed to do this without compromising the appropriate level of sintering and densification of the refractory material to be suitable for its application.

The focus of the study was the use of dopant additives in alumina-zirconia-silica (AZS) refractory mixtures to enable firing at reduced sintering temperatures and times (without any detriment to the materials). The project used two approaches for using dopants – firstly, to use low melting phase dopants to enhance sintering and mass flow and secondly, to use grain boundary pinning dopants to control grain boundary growth. These effects have been explored before in an academic context where, researchers showed that sintering temperatures could be lowered by using metal oxide dopants in solid solutions which can: enhance diffusion kinetics by increasing defect populations; produce a liquid phase which can facilitate particle rearrangement; reprecipitation and increase zirconia dissociation. These processes have been shown to promote sintering at lower temperatures and reduce energy input.

The project consisted of integration of several dopants into AZS refractories as a function of and sintering temperatures (-°C). Effects on mixing were evaluated first in order to attain the viability of dopant integration with our AZS-based formulations. Dopants that passed this stage were then fired at the various sintering temperatures outlined above and tested for bulk density and apparent porosity. Materials successful in the sintering regard were then brought forward to more application-specific testing.

Due to the proprietary nature of the work, it is not possible to divulge in this piece the nature of the dopants used in the study, also, due to the large amount associated data, only the salient features of the results will be presented below.

1 – Industrial Mix Feasibility

The first major consideration for industrial mixes that is overlooked in academic journals is the effect of dopant integration on particle dispersion and flow characteristics of the resultant mixes. Industrial mixes typically consist of particles of varying size distributions and therefore optimal packing ability of these particles is paramount for both flow characteristics and properties after firing. Given the emphasis on a slip-casting production method at PSR, these factors were crucial to maintain.

Some dopants were found to be suitable for producing mixes with consistent rheological properties. These led to mixes with high stability that were easy to cast. There were other dopants however that resulted in mixes with poor dispersion and flow characteristics. The reasons for this unsuitability may be attributed to pH, dopant particle radii size, surface chemistry, which can affect particulate dispersion and rheology.

 2 – Single Dopant Mixes

The next stage of dopant candidate selection consisted of porosity and density testing. This would indicate the effect the dopants have on the sintering process and the structure of the fired product.

Generally, dopant content had a positive effect on density at lower sintering temperatures. Notably, a particular dopant, ‘Dopant 1’, at a °C sintering run gave: (1) the greatest increase in density and (2) the most substantial density values overall. This gave a 2.5% approximate improvement in bulk density, giving a comparable value to the standard AZS material that is currently fired to °C. Another dopant, ‘Dopant 2’, also exhibited improvements in bulk density values (figure 1). Although, these typically occurred at higher sintering temperatures and dopant concentrations than that of Dopant 1.

However, apparent porosity values did not see as dramatic an improvement as bulk density. Both °C and °C sintering runs gave consistent porosity values across all the dopant contents comparable to the standard AZS material. Dopant 2 had little effect on porosity values with decreasing sintering temperatures (figure 1). Overall, reducing sintering temperatures leads to an increase in porosity. Increasing dopant content resulted in largely consistent porosity values associated with that decrease in sintering temperature. The lack of effect on porosity for both dopants could be due to the increased mass flow kinetics, this leads to areas of high and low densification.

X-ray diffraction (XRD) data confirmed sintering effects occurring at lower temperatures. Firstly, zirconia dissociation was promoted at lower sintering temperatures by all dopant integrated materials mentioned. Greater dopant concentrations resulted in higher proportions of zirconia present in the final sintered product. Mullite formation was seen to occur at °C (>1wt%.) Exceeding this concentration led to further increases in mullite formation at lower temperatures, which exceed even that of the standard material.

3 – Co-Dopant Mixes

Following the initial results of the primary select dopants, the subsequent work focussed on combining them with grain boundary pinning dopants to see if further density and porosity gains could be made.

The first of these was a D1-based co-dopant mixture (Mix 1). The results showed a rather surprising result with a greater improvement in bulk density for most dopant concentrations at all sintering temperatures (figure 2). This is highlighted by the fact that all samples with dopant contents ≥1wt% exhibit greater bulk density values than the standard AZS mix, this was the case for all sintering temperatures. Samples with 2 and 4 wt% dopants were consistently achieving bulk densities above 3.20gcm-3, the greatest achieved so far with any of these dopant mixtures. Also, a major improvement over the previous dopant mixtures tested is seen in the porosity values. Where values below that of the that of the standard AZS material are observed.

Mix 2 (a D2-based co-dopant system), yet again exhibited an improvement in bulk density over the single-doped counterpart (Figure 2). Similar to the D2 material, the oC sintering run produced the most improved bulk densities and apparent porosity values compared to the standard material. A peak of 3.22 gcm-3 was recorded at 4wt% dopant concentration in this particular case. Porosity values were consistent with that of the standard AZS material, rather than a distinct improvement.

XRD data yet again showed promotion of zirconia dissociation and mullite formation for the doped materials. However, these effects were not greatly improved from the single doped materials, implying the improved porosity and density values were due to grain boundary pinning effects rather than improved sintering.

Due to these results, Mix 1 and Mix 2 co-doped mixes were progressed to further mechanical and application-specific testing.

4 – Glass Corrosion Resistance

Glass corrosion (GC) is a dominant factor affecting the lifespan of a glass-contact refractory part. Resistance to this process is dependent on both the phases present in the material and it’s microstructure. Zirconia offers a degree of chemical resistance to molten glass, so increased zirconia dissociation is a benefit dopants could provide. The occurrence of dopant-zirconia intermediate and glassy phases could also provide their own resistances to glass corrosion (Toperesu, et al. ).

Comparing the GC results of the two co-dopant systems, Mix 1 and Mix 2, overall, they displayed excellent GC performance when compared to the standard AZS material at °C sintering temperatures (figures 3 and 4). A more defined glass-refractory boundary was observed for lower sintering at the standard temperature for both doped materials. Dopant 2 exhibits greater improvements in this regard due to the intermediate phases produced, potentially producing a higher quality refractory product overall. Overall, the °C sintered samples exhibit improved performance over the standard material fired at °C. Another major benefit is the lack of glass colouration seen in these samples.

The formation of eutectics and intermediate phases leads to improved density and porosity values, these also have lower solubility in molten glass, and therefore, greater corrosion resistance and extended lifespans.

5 – Thermal Shock Resistance

Thermal shock resistance concerns the integrity of the refractory part after rapid changes in temperature. This consists of allowing the sample to reach elevated operating temperatures (oC), cooling the sample rapidly and repeating for 5-10 cycles. Observations of structural integrity are then made.

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Mix 1 and Mix 2  both exhibited hairline cracks on all samples after 5 cycles. Hairline surface cracking is typical after this degree of exposure to heat and cooling. The behaviour observed in these doped samples doesn’t diverge from our typical AZS refractories.

6 – Feasibility Study / Commercial considerations

To garner more information on the economic effects of these chemical dopants analysis of kiln firing energy use data was conducted to estimate the potential annual savings if firing to a sintering temperature of oC (down from the current oC). Some preliminary data analysis shows an approximately 15% predicted saving on PSR’s shuttle batch kilns, and a predicted saving of approximately 10% on PSR’s moving hood kilns. Consequently, this would lead to equivalent reductions in CO2 emissions of 15% and 10%, respectively. Furthermore, it would improve asset utilisation by kiln firing times being reduced by 11 to 14 hours per firing, respectively.

7 – Conclusion

The sector suffers from being highly energy intensive. Production of such materials is very costly in this regard, so even modest reductions in temperature / firing times produce sustainability and potential economic advantages. The benefit of this project offers the opportunity to produce products with a lower carbon footprint for an important foundation industry. Any benefits gained would also benefit the downstream user with a higher-value proposition – materials that have been supplied with a lower Carbon footprint / improved properties / longer lifetime.

PSR envisage their business strategy will in future introduce the use of more energy efficient technologies, to support the development of innovative energy / carbon-efficient manufacturing technologies, to create or safeguard jobs and increase competitiveness, to increase or improve manufacturing and, to move towards a more circular economy.

References:

Chandra, G.C. Das, U. Sengupta, S. Maitra, . Studies on the reaction sintered zirconia-mullite-alumina composites with titania as additive. Ceramica, Volume 59, pp. 487-494.

L.A. Xue, I.W. Chen, . Low-Temperature Sintering of Alumina with Liquid-Forming Additives. J. Am. Ceram. Soc., 74(8), pp. -.

P.M. Toperesu, G.M. Kale, J. Daji,. D. Parkinson, . Development and evolution of a novel (Zr1-xSnx)O2 toughened. Journal of the European Ceramic Society, Issue 41, pp. -.

W.E. Lee, G.P. Souza, C.J. McConville, T. Tarvornpanich, Y. Iqbal, . Mullite formation in clays and clay-derived vitreous ceramics. J. Eur. Ceram. Soc., Volume 28, pp. 465-471.

I.B. Cutler, C. Bradshaw, C.J. Christensen, E.P. Hyatt, . Sintering of Alumina at Temperatures of °C and Below. J. Am. Ceram. Soc, 40(4), pp. 134-139.

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1 Parkinson-Spencer Refractories Ltd, Holmfield, Halifax HX3 6SX, UK

2 Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, UK

* Corresponding author :

There are two major challenges that all industrial sectors must face now and in future – and this includes the glass industry: increasing energy efficiency and reducing emissions across the board.

Ambitious efforts are underway, and RATH is intensively supporting its customers. Be it innovative products for refractory linings or ceramic hot gas filter elements to filter particle-laden industrial flue gases: RATH attaches great importance to continuous research and development. A recent example of this: RATH developed a new, advanced feeder expendables series, which stands for even better product quality and longer service life.

With more than 130 years of experience, RATH is regarded as a competence leader in the refractory sector. Its customers benefit from the widest range of products and service available on the market, coupled with extensive, industry-specific expertise – which also applies to the glass industry: The specialists at RATH have the required in-depth industry knowledge and understanding of the specifics of glass processing. RATH´S precision-crafted refractory products are designed for a wide range of applications, including container glass, float glass, C-glass, E-glass, and tableware. Including the original Emhart Glass designed feeder expendables and refractory formulas, RATH offers the complete range of highest quality refractory materials for the entire glass manufacturing process. Only the best quality high-purity raw materials are used for all of RATH´S refractories, designed to achieve predictable density and resistance to erosion and corrosion. 

New, optimized feeder expendable series

The further development of refractory compositions is a challenge that RATH is taking on with great commitment which is also proven by the new RATH product series. “The new FOURATH 4xx feeder expendable series is based on an optimized formulation which has been exclusively developed and produced by RATH”, says Robert Nusszer, Managing Director of the Sales Unit Glass at RATH. “The product has been put through its paces in our laboratories and has successfully passed the first glass factory trials in the last 1.5 to 2 years.”

From improved crack and thermal shock resistance to longer service life

While the previous 3xx mixes (like 333, 315, 301, 338, 345, etc.) were produced using the slipcasting technique, the new FOURATH 4xx mixes are produced using a so-called hybrid technology: RATH uses the same mold park, but a special vibrocasting technology. “This brings some significant advantages: For example, the new feeder expendable series convinces with improved product quality – due to a more compact mix and better surface quality”, says Robert Nusszer. “We selected the highest quality raw materials in order to keep the impurity level to a minimum, to prevent blister formulation. Furthermore, with the new manufacturing technology we can avoid the additional grinding, e. g. of the tube´s outer surface – thus, we are able to increase the service life of the tube. In addition, we have been able to significantly reduce the drying time of the refractory, which gives us better flexibility in terms of lead times. “The thermal shock resistance has also significantly improved and the risk of cracking has decreased considerably”, Nusszer adds. “Our customers can switch to the new product series immediately: The same part numbers are used as before, only the last three digits will change.”

At the recent glass trade fairs in Düsseldorf and Istanbul, the following products were shown: the already well tested and released 420 (a complementary mix of 333, 315) – tubes, plungers, orifice rings and stirrers as well as the new 473 zirconia inserted spouts. The new fused silica mixes, the drain-cast 457 and the slip-cast 458 were also introduced. These mixes are used in the highest quality glass production, e. g. in melting crucibles, and the 458 is also available, for example, for crown repair of glass melting tanks, as well as suspended brick in the doghouse. “It is a very important step ahead that we have recently resumed the manufacturing of forehearth 333 channel blocks at our US-facility in Owensville, as well as producing the first front channel blocks for trial purposes in mix 420, responding to serious market demand. Furthermore, at our Budapest facility, we have set up a significant production capacity for producing high-alumina channel blocks”, Robert Nusszer adds.

Doubling of the production capacity for hot gas filter elements in Meißen

Recently, RATH has doubled its production capacity for both catalytic and non-catalytic hot gas filter elements at its Meißen production facility in Germany. As the only remaining European manufacturer of filter elements, RATH can produce 25,000 pieces of filter elements per year up to a length of six meters from its own fiber, which is also produced by RATH, at its production facility in Mönchengladbach (Germany). The new production line is already in operation and running at the highest efficiency level.

For many years, the company has been developing innovative ceramic hot gas filter elements that make it possible to filter particle-laden industrial flue gases at temperatures up to °C and reduce emissions of nitrogen and sulfur oxides, as well as reduce the emission of fine particulates. Since , all resources, capabilities and expertise in hot gas filtration have been pooled in the Meißen facility. This research and production site of the company is home to cutting-edge facilities for the manufacture of RATH´S FILTRATH® non-catalytic and FILTRATH®CAT catalytic filter elements. The latter were developed for pollutant emission control: These rigid yet highly porous and catalytically coated ceramic filter elements are used for multiple pollutant control of (fine) dust, acid gases, dioxins and nitrogen oxides in hot gas flows (at temperatures between 250 ° and 420 °C) and can easily replace conventional textile filters.

The filtration business line is one of the business lines that has seen a significant growth within the RATH Group. “It has great potential since the glass industry has to drastically reduce its footprint on environmental pollution and the greenhouse effect worldwide. RATH is committed to advancing environmental protection and helping its customers in the glass industry achieve their ambitious environmental goals. Thus, RATH continuously develops and invests in this technology”, Robert Nusszer summarizes.

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