Smelting point? An update on major refractories for glass smelters

By IM Staff
Published: Monday, 22 August 2016

Fused cast blocks have dominated the market for glass refractories since the early 20th century. Carlo Ratto takes a look at some of the alternative products which have emerged over the years and assesses the future uptake prospects for these materials.

Since the advent and rapid diffusion of fused cast refractories in the first half of the 20th century, there have not been any equally significant revolutions in the development of new refractories for glass furnaces.

The "fused cast era" laid the foundation for the evolution of glassmaking technologies and the development of new classes of advanced glasses that would not have been achievable without the availability of glass-contact refractories capable of extremely low glass-defect cession.

In almost 100 years of using fused cast glass refractories, there have been significant improvements in furnace design and glass chemistry. Better furnace handling practice and repair techniques have considerably increased furnace life. In addition to improving glass quality, the combined effect of these advances has been to increase the productivity of a typical furnace campaign by around two orders of magnitude.

Considering fused cast as a general class of refractories, characterised by a common manufacturing practice (ceramic foundry technology), its development trajectory has been one of evolving chemistries from the initial aluminas and alumina-zirconia-silica (AZS) families, to different chrome-bearing compositions, high-zirconia and alumina-magnesia spinels.

Different fused cast chemistries have filled application niches for various glass compositions and for different sections of furnaces, both in glass-contact and superstructure products, bringing the advantage of extremely dense (low porosity) refractories which exhibit high refractoriness, chemical stability and resistance to abrasion.

With the exception of the development of the last generation of large sintered pre-cast shapes, which are gaining a significant success in superstructure applications of soda-lime container furnaces, there has not been any significant newcomer in the non-fused cast glass furnaces lining or glass-contact arenas.

Until a radically new glass-smelting practice is developed, fused cast refractories will remain the leading refractory products in glassmaking.

Schematic of submerged combustion melting process

Glass1  

Source: US Dept. of Energy 2006

SCM

One of the most promising innovations with the potential to shake up the glass refractories segment is not a new class of refractory, but rather a new fusion technology known as submerged combustion melting (SCM). This involves a new kind of smelter that works through combustion inside the melted glass batch. 

Tests of this process have shown that SCM smelters can significantly improve thermal efficiency and also display a number of other advantages, including a very minor dimensional impact, rapid start-up and shut-down and the simplicity of the required change in chemistry.

Due to their design, SCM furnaces use a very minor quantity of refractories per unit of product. SCM technology was first theorised in the 1960s, but serious work on this process by research institutions and private industry did not begin until around 15 years ago. Today, only a handful of factories in Belarus and Ukraine currently employ this innovative fusion technique to produce glassrock.

The reason for such slow implementation of SCM technology is down to one large drawback intrinsic to this method of fusion. The smelted glass extracted from the furnace’s fusion tank has a high occurrence of gas blisters, making it unsuitable for use in many large end use segments such as containers, tableware and flat glass, not to mention in quality-sensitive applications like flat panel display and optical glass.

This explains why SCM furnaces are currently only being used to produce glassrock, but with some refining, the technology could be used to make higher quality types of glass. However, in order to make it suitable for use in the major glass segments, the technique requires several stages of advanced refining, which could ultimately cancel out many of its cost and simplicity advantages.

It is therefore unlikely that SCM technology will become established in container, tableware, flat and speciality glassmaking in the medium term.

Crown refractories

Crown refractories for glassmaking are the main application for silica wedges, which form a "crown" in the glass furnace. One of the main problems associated with this silica bricks application is "silica rat holing", a phenomenon whereby alkali-rich vapour condenses in the cool areas of the silica crown, causing corrosion.  A great deal of effort has been devoted to increasing the physical and chemical quality of silica refractories to withstand this corrosion. Research has also been conducted on developing adequate insulation for the silica refractories, so as to move the sulphate condensation zone outside the furnace’s dense silica layer. These measures have reduced, but not eliminated, the need of hot repairs to the silica crown during a furnace’s campaign life.

The advent of oxyfuel fusion technology, the take-up of which has been boosted by manufacturers’ responses to anti-pollution legislation, has created an opportunity for AZS and alumina-based fused-cast refractories to be installed in silica furnace crowns, although the popularity of this technique has been curbed by the higher energy cost associated with oxygen generation and a much higher crown weight. Nevertheless, a significant number of oxyfuel furnaces use high quality silica wedges instead of fused cast blocks in crowns, in spite of the rat holing problem.

The utilisation of other refractories such as zircon and spinel-based materials and other refractories in crowns is limited to furnaces making borosilicate, frits and some other specialty glasses. It is not anticipated that there will be any major shifts in these usage patterns in the medium term.

Glass-contact refractories

The biggest application area for fused cast refractories, mostly AZS and aluminas, is in smelters used to produce glass for large volume markets, like containers and flat glass. Some speciality glass production also uses high-zirconia fused cast (HZFC) refractories, typically in the making of FPD glass (particularly with Corning’s fusion technology, but also in the float process), due to the requirement for the glass to be of very high quality.

A few attempts have been made to use unshaped or pre-cast materials in these smelters, but the application of such materials is only an option for minor hot repairs, since at present there are no corrosion-resistant sinter materials comparable to fused cast. Exceptions to this are high chrome vibro-cast or iso-pressed shapes, which are suitably corrosion-resistant but have the drawback of causing glass discoloration proportional to the glass’ contact with the refractory surface. These high chrome materials are however a viable option for fibreglass and glass wool, where the colour of the glass is not a concern.

The bottom of glass fusion and refining tanks are typically lined with AZS or alpha-beta alumina fused cast tiles. In a few cases, tiles made of sintered AZS or zircon iso-pressed refractories can be used as an alternative to the fused cast material.

Glass contact reinforcements (cross-walls, DH corners, throat inlets/covers) used to be a typical application for high-zirconia AZS (41% ZrO2). In recent decades, as a consequence of moves to extend furnace campaign life by means of reinforcing weak points, it has become more common to install special refractories with greater wear resistance than high zirconia AZS in the most stressed spots of furnaces.

Among the beneficiaries of this trend were chrome-bearing refractories. Several years ago, attention centred on using chrome-bearing fused cast materials such as AZS-Cr (AZS doped with chrome) and ACr (chrome-corundum).

More recently, with the development of the iso-pressing shaping technique for large blocks, high chrome (escolaite), iso-pressed and sintered blocks have successfully replaced fused cast refractories in extremely stressed applications, such as cross walls. These have the advantage of a very homogeneous structure compared with the intrinsically inhomogeneous texture and composition of a fused cast body, prolonging the refractory’s life within the furnace.

Chrome-based materials still have the drawback of causing glass discolouration. The presence of even a few parts per million of chrome in a refractory strongly colours glass it comes into contact with, meaning these materials are of limited use in making white, extra white and some special glasses as well as tableware, illumination, float and some borosilicate tubing glass.

As an alternative to chrome-based refractories, AZS-molybdenum (Mo) composite fused cast blocks have emerged from the R&D departments of some major western refractory producers. In these materials, a molybdenum sheet is embedded into the fused cast block, a few millimetres beneath the surface. After the wearing down of the AZS surface layer, the molybdenum sheet is directly exposed to the glass contact, providing long term protection to the rest of AZS block. Molybdenum is used because it is a refractory metal with extremely high resistance to glass corrosion.

However, these advanced composite blocks are difficult and expensive to produce and can contain hidden intrinsic defects, such as poor positioning of the molybdenum sheet. Inappropriate heating of the blocks can cause them to crack, allowing oxygen to get to the metal. Any of these defects can lead the blocks to fail, with problematic consequences for the furnace’s operation.

As a result of these risks, AZS-Mo refractories are not widely used. Protecting the refractory blocks via various means, including platinum external cladding of AZS fused cast blocks, is an option to mitigate the chances of refractory failure, but generally only for borosilicate glass production, where the value of the final product makes the additional cost worthwhile.

Superstructures

Shortly after the introduction of fused cast bricks in the first half of the twentieth century, aluminas (both alpha-beta and beta forms) and AZS fused cast (generally low-zirconia with 32% ZrO2) replaced the silica, silico-alumina and zircon sinter bricks and shapes previously used in glass furnaces.

The fused cast blocks succeeded in prolonging the campaign life of furnaces, although AZS refractories tended to have the unwanted side effect of causing exudate to drip into the glass, potentially creating alumina/zirconia-enriched defects. This persistent problem motivated glassmakers and refractories manufacturers to develop improved AZS fused cast with reduced tendency to exudate. This was achieved increasing the oxidation level of the smelted ceramic components of the blocks via oxygen injection, reducing impurities in the raw materials and contamination from electrode graphite, ultimately developing special low-exudation chemistries.

In the last decade, a new generation of sinter materials produced with a pre-cast and firing technology have been installed in a number of furnaces in place of fused cast blocks. The main drawback of these sinter materials in this application is their susceptibility to erosion via nephelitic conversion (the formation of nephelite from the brick’s components under thermal and chemical aggression) and subsequent spalling of the converted layer. Since they were first introduced, advanced formulations and the use of special raw materials have significantly reduced corrosion rates so that, in several cases these materials can now withstand the full extent of furnaces’ life.

The opportunity presented by these materials to provide other advantages, like the possibility of producing extra-large shapes such as single piece arches, reducing thermal conductivity and preventing exudation, has increased glassmakers’ interest in using and developing these new pre-cast sinter refractories, although there is as yet no industry-wide consensus on their effectiveness.

Regenerator packages

Regenerative furnaces continue to represent a popular design for combustion energised furnaces which run on gas or oil. These types of furnace include smelters for producing container, float and tableware glass. For regenerative furnaces, regenerator packages require large volumes (tonnes) of refractories, comparable to those of glass-contact, superstructure and crown.

The package’s main task is to recover or exchange heat from combustion fumes in a chemically and mechanically aggressive environment with a wide range thermal cycling. Early refractories used to perform this role were special silico-aluminas, before being replaced with high temperature-fired periclase (mag-chrome and periclase-zircon). Initially installed as bricks, these were eventually replaced with tiles that greatly increased the surface area per volume of refractory and with this the thermal exchange efficiency of the package as a whole.

Some innovative producers of basic refractories have developed products with specific chemistries to be installed in different zones of the regenerator package according to the type of furnace, based on an advanced understanding of their corrosion mechanisms and thermal behaviour.

In the last few decades, one leading global manufacturer of fused cast has attempted to shake up the regenerative package segment by introducing a new family of fused cast shapes, referred to as cruciforms, with chemistries ranging from AZS to spinel.

A solid body of experimental evidence has demonstrated that these products are competitive in performance terms with traditional sinter checkers, so fused cast cruciforms have successfully gained a significant share of the regenerative package market. The proliferation of these products has however been limited by their debatable overall financial benefits and occasional instances of premature failure against their life span expectations of two furnace campaigns.

Future for fused cast

Although challenger products have demonstrated considerable improvements over incumbent fused cast refractory technology, the various drawbacks of these materials outlined above when it comes to their performance in glass furnaces, particularly with regard to the large volume glass commodities, suggest that fused cast materials will remain the dominant glass refractory for some time.

This forecast is supported by the fact that the supply of glass refractories, which was previously dominated by two major Western manufacturers, has in the last decade diversified to offer a range of products and services together with more competitive prices.

Broadly speaking, there are now three main tiers of glass refractory manufacturers.

Tier 1

The first tier of glass refractory manufacturers is dominated at the top end by Western companies which have developed market-leading technologies and services and whose main manufacturing bases continue to be based in Western countries.

These tend to be the most expensive suppliers, justifying the prices they charge for their products and services by ensuring a low level of technical risk associated with their technology. However, in an increasingly competitive industry, it is becoming more and more difficult for glassmakers to justify procuring refractories from these top level suppliers.

The lower half of this tier is characterised by Western companies with premium technologies which have relocated their main manufacturing operations to low-cost centres such as Asia. Typically, the products manufactured in these cheaper locations are not made to exactly the same standards as they are in Western factories, particularly for AZS materials, but the quality of these products is solid and still among the best in the industry and their level of technical risk is among the lowest.

As a result, products and services offered by these lower first tier suppliers are generally more affordable than those at the top end of this bracket.

Tier 2

The second tier of glass refractory makers comprises mainly low-cost independent manufacturers, generally based in Asia and producing less sophisticated products than first tier suppliers. The extent of the differences between the technology offered by this group compared to Western companies varies, but can be widely divergent in the case of alumina fused cast materials. The level of service offered by second tier companies to glassmakers is regarded as average to poor and the level of technical risk is higher, however the price of both products and services offered by this layer of supplier is significantly lower.  

Accordingly, even some major Western glass producers will occasionally buy materials from this tier, usually with the safety net of having first tier level third party service providers to hand.

Tier 3

These are low-cost independent manufacturers invariably based in Asia and offering the most basic level of refractory technology. Their low-cost manufacturing operations are generally set up to serve domestic markets rather than foreign customers.

Conclusion

The fused cast refractories sector is becoming increasingly competitive and is ripe for a leap forward in technology.

Industry observation suggests that service provision is developing more rapidly than product sophistication and a new class of third party service providers is emerging to satisfy the needs of glassmakers who are demanding more from the performance of the refractories they use.

As glass producers are increasingly tempted by lower cost products with higher levels of technical risk, better service provision could exacerbate the shrinking market share of first tier refractory suppliers. 

*Dr P. Carlo Ratto has over 35 years of experience in the fused cast refractory industry. www.fusedcast.com