State-of-the-Art Refractory Linings for Float Glass Furnaces

S. Postrach, B. Schmalenbach, A. Gupta, R. Bei

A high quality refractory lining is

essential for the production of
float glass, especially when the
stringent requirements on man-
ufacturers such as product qual- a
ity, energy consumption and fur-
nace lifetime are taken into con-
sideration. In this article the lin-
ing options for float glass fur-
naces have been described. b
Fig. 1 a) Standard silica grade and b) STELLA GNL after 3 months in an oxy-fuel furnace
1 Introduction
Currently, more than 95 % of the world's It is evident that in addition to melting tech- tiated, which itself starts to dissolve the
flat glass is produced using the float glass nology and furnace construction improve- coarse silica grains. Rapidly the silica brick
process. Since its development in 1959 by ments, the necessary production develop- becomes enriched with glassy phase and
Pilkington, the technology has undergone ments can only be achieved using high qual- starts to drip. These reactions have been
significant improvements. Nearly every year ity refractory products. The following article confirmed using thermochemical calcula-
another record regarding the lifetime or ca- provides a review of the refractory lining tions [3]. Additionally, these investigations
pacity of new furnaces is established. Today, concepts for float glass furnaces to meet the demonstrated that high CaO levels in silica
float glass facility lifetimes exceeding 15 current demands and describes specific grades are detrimental for this application.
years are achieved and the largest furnaces grade characteristics. The only lime-free silica grade available with
have a capacity of 1000 t/d. In addition, suitable thermomechanical characteristics is
float glass quality requirements are continu- 2 Melter STELLA GNL, comprising 99 mass-% SiO2.
ally increasing and even the smallest glass 2.1 Crown The results of a 3-month test performed in
defects that distort product clarity cannot be The most important and established refrac- an oxy-fuel furnace to compare a standard
tolerated. However, achieving these ad- tory for the crown of regeneratively fired silica brick grade and STELLA GNL are illus-
vances is an increasing challenge for float float furnace melters is silica. High quality trated in Fig. 1. Based on the improved puri-
glass manufacturers because of additional silica materials are characterized, amongst ty, additionally STELLA GNL can be used at
demands including more and more stringent other factors [1], by: higher temperatures than standard silica
environmental requirements (e.g., energy • Low remnant quartz content – typically grades which leads to increasing of the pro-
savings and NOx reduction). 0,4 mass-% in STELLA GGS duction capacity.
• Low creep in compression (CIC) value –
Stefan Postrach, Bernhard Schmalenbach, essential for broadly spanned crowns 2.2 Superstructure
Rongxing Bei • Tight dimensional tolerances to ensure Fused cast Al2O3-ZrO2-SiO2 (AZS) is routinely
RHI AG, Industrial Division, Wiesbaden, Germany closed joints. used in the superstructure from port 1 to 4,
Practical experiences have shown that in owing to the heavy batch carry-over.
Amul Gupta highly stressed areas of the crown standard All three main types of AZS refractories,
RHI Monofrax Ltd., Falconer, USA silica materials are at their limits (for exam- differentiated by their ZrO2 and glassy phase
ple: oxy-fuel fired furnaces). The reason for contents, are used for this application.
Corresponding author: Stefan Postrach this is that the wollastonite-bonding phase The most important quality characteristic
stefan.postrach@rhi-ag.com is easily corroded by alkalis, for example of these grades is the exudation behaviour
Keywords: refractory lining options, float glass NaOH [2]. As a result of the reaction be- and the impact of the ZrO2 content on
furnaces, energy consumption tween wollastonite and NaOH, the forma- this phenomenon has previously been de-
tion of glassy phase in the silica brick is ini- scribed [1]. From port 5, β-alumina grades

80 refractories WORLDFORUM 1 (2009) [2]

are most suitable due to the low glass defect vides these advantages in glass contact are-
potential (e.g., Monofrax H). These grades as.
are described in detail in the following sec- High quality fused cast alumina refractories
tion. are manufactured by melting high purity raw
materials to more than 2000 °C in an elec-
2.3 Glass contact area tric arc furnace. The molten liquid is cast into
In the glass contact area of the melter, fused graphite moulds. Following a brief period of
cast AZS grades are also commonly applied. cooling, whereby a solid skin develops on all
The most important requirements are a low surfaces, the refractory block is removed
glass defect potential and high corrosion re- from the graphite mould and annealed in di-
sistance. REFEL and Monofrax products are rect contact with an insulating powder for a
proven for this application. The bottom of several days.
the melter is usually lined with fused cast Fused cast β-alumina is essentially a single-
pavings (32 mass-% ZrO2 content). To en- phase refractory, as can be seen from the mi-
sure tight joints the dimensional accuracy of crostructure in Fig. 2a. Since it is saturated
the pavings is extremely important (i.e., 90° with alkali (i.e., Na2O), it is chemically very
angles and tolerances). Underneath the stable in the high alkali and alkaline earth
fused cast pavings, a thin mortar settlement environment found in the superstructure. In
layer and a monolithic layer are common. service experience has shown that a 1/9,2
The main role of the monolithic layer is to ratio of Na2O/Al2O3 in fused cast β-alumina
protect the additional insulation lining be- is the most effective composition to ensure
low; therefore, it should have a good resist- the following characteristics: b
ance against glass melt. In addition, it • High thermal shock resistance
should not shrink during the heat-up process • Chemical stability during exposure to an
Fig. 2 a) Monofrax H fused cast β-alumina refractory
to avoid cracks and gaps forming around the alkali and alkaline earth oxide-rich envi-
(β ~ Na2O · 9,2 Al2O3) and
thermocouples and electrode blocks (i.e., en- ronment. b) Monofrax M fused cast α/β -alumina refractory
abling melt to infiltrate down to the insula- However, since fused cast β-alumina refrac- (α = Al2O3 and β ~ Na2O · 8,2 Al2O3)
tion layer). Depending on the actual bottom tory has a relatively high porosity (i.e.,
construction the following monolithics are ~ 4 vol.-%) and is coarse grained, it does
suitable for this application: not provide enhanced resistance to corro- walls, pavers, canals, and lipstone. In these
• RESISTIT ZM260: Ready-to-use ramming sion from batch carry-over or glass melt. applications, Monofrax M is preferable to
mix based on zircon and alumina Therefore, its application is restricted to the AZS due to its lower defect potential. The
• DIDURIT ZM465: Castable based on AZS downstream superstructure only. absence of zirconia and a viscous glassy
• RESISTIT ZS717: Ready-to-use ramming In addition to fused cast β-alumina, silica is phase in Monofrax M results in a lower ston-
mix based on zircon. a common solution for the refiner super- ing and viscous knot potential. Furthermore,
structure. However, in contrast to the fused α/β-alumina refractories produce consider-
3 Refiner area cast option the risk of glass defects are high- ably less seeds and blisters in glass than
Due to lower temperatures in the refiner er due to silica melt running down into the AZS.
compared to the melter, the demands on the glass. A critical component in the float process is
refractory lining are less severe. However, the In contrast to fused cast β-alumina, fused the lipstone. For several years, RHI Monofrax
requirements for a low glass defect potential cast α/β-alumina consists of two major has offered a complete lip spout assembly
are very high and components from the re- phases (i.e., α- and β-alumina) with a very encased in steel for convenient installation
fractory lining should not generate any glass small grain boundary phase (Fig. 2b). A in a float furnace (Fig. 3).
defects. Established grades for this applica- phase composition of approximately
tion consist of α/β- and β-alumina. The 40 mass-% α-alumina, 58 mass-% β-alu- 4 Regenerator
near absence of a glassy phase in both types mina, and a 2 mass-% boundary silicate 4.1 Regenerator checker work
of alumina refractories provides certain phase shows the best corrosion resistance In modern float furnaces the regenerators
unique properties and benefits for the end and is realized in Monofrax M. are solely equipped with thin-walled check-
user. Whereas the AZS refractories demon- Monofrax M is a fine-grained refractory with ers, namely ceramically bonded chimney
strate a characteristic glassy phase exuda- an open porosity of approximately 2 vol.-%. blocks or fused cast checkers. These types of
tion upon furnace heat up and during the This is necessary for high corrosion resist- checkers are characterized by a high thermal
subsequent campaign, both types of alumi- ance in glass contact applications, for exam- efficiency. In addition to this fundamental re-
na refractories are dry upon heat up, which ple in the glass contact area of the refiner. quirement, the corrosion resistance of
offers a significantly reduced defect poten- However, the temperatures should not ex- grades used for the demanding conditions is
tial compared to AZS for superstructure ap- ceed 1350 °C. In float furnaces, α/β-alumi- important. The conditions in the regenerator
plications. In addition, α/β-alumina pro- na refractories are used for the refiner side- vary from the charging end to the refining

refractories WORLDFORUM 1 (2009) [2] 81

 • Silica (crown) + mullite (walls)
• Silica (crown) + magnesia (walls)
• Magnesia (crown and walls)
• Silica (crown and walls).
High quality silica in the crown is a cost-effi-
cient solution with excellent thermomechan-
ical characteristics, especially high creep re-
sistance. Whilst a high degree of additional
insulation is necessary, silica has a high cor-
rosion resistance against SiO2 attack. How-
ever, a critical aspect is liquid slag formation
due to alkali attack, which can lead to a pre-
Fig. 3 Monofrax M lipstone in a spout lip assembly mature wear. In addition, the current ten-
dency towards lower temperatures and a re-
area and this must also be considered for raw materials. The coarse magnesia crystals ducing atmosphere in the regenerators is
appropriate refractory selection. are protected by a bonding matrix of detrimental for standard silica. For this situ-
The first two ports are characterized by a forsterite and zirconia that are formed dur- ation RHI offers the improved silica grade
high loading with batch dust. This mainly in- ing the firing process of the bricks. STELLA GNL. Due to the absence of lime
fluences the grade selection for the regener- Since zircon is widely used in different re- bonding this grade has a higher corrosion
ator crown and the upper courses of the fractory products, the availability of this raw resistance against alkalis compared to that
checker work since it is necessary to install material cannot be guaranteed. Therefore, of standard silica.
materials with a high resistance against the zircon-free magnesia products have become A more effective solution compared to silica
SiO2-containing batch carry-over. For oil- important. RUBINAL ESP was developed for for regenerator wall applications is mullite.
fired furnaces, vanadium oxide attack must this purpose. RUBINAL ESP is a magnesia Pure mullite grades (e.g., DURITAL
also be taken into account. brick with a very strong spinel bonding ma- E75EXTRA) exhibit excellent characteristics,
Magnesia-zircon material (i.e., RUBINAL VZ) trix whereby a part of the spinel is formed in for example high creep resistance and high
has proved to be effective in the upper situ, namely during the firing process. The resistance against SiO2 and V2O5 attack.
checker courses when aggressive SiO2 attack spinel bonding matrix is resistant to sulphate However, pure mullite refractories are expen-
occurs. In these grades periclase is protected attack and protects the periclase. Therefore sive because they are based on synthetic raw
by a fringe of forsterite and zirconia; there- RUBINAL ESP chimney blocks can be used in materials and require high firing tempera-
by, forsterite bursting is minimized. In addi- the condensation zone of the regenerator tures. Therefore, grades based on andalusite
tion, products based on 99 mass-% fused under oxidizing conditions. with corundum addition have been devel-
alumina grains (i.e., DURITAL K99EXTRA) Since the beginning of 2007, RUBINAL ESP oped (e.g., DURITAL S70), which form mul-
possess a high resistance to the corrosive has been installed in the condensation zone lite during the firing process.
agents present in the top courses. of regenerators of glass furnaces. To date the However, a disadvantage of mullite is that
Less critical is the grade selection for the performance has been satisfactory and will with decreasing sand carry-over the risk of
area below the top courses down to temper- be monitored further. alkali corrosion, especially nepheline forma-
atures of 1100 °C, especially when the fur- In the last two ports the conditions alter tion, increases.
nace is heated with natural gas. In such cas- considerably because the influence of batch In service experiences have shown that a re-
es the use of Ca2SiO4-bonded MgO (i.e., carry-over and exhaust gases is reduced. fractory lining combination of silica in the
ANKER DG1) is recommended. In cases Therefore, the use of a forsterite-bonded crown and MgO walls should be avoided
where the furnace is fired with heavy oil the magnesia grade (i.e., ANKER DG 3) in the since silica melt from the crown runs down
use of checkers with a low CaO content and condensation zone is appropriate. over the walls and corrodes the MgO. This
strong forsterite bonding, for example a results in softening of the MgO and
magnesia-zircon grade (i.e., RUBINAL VZ) is 4.2 Regenerator casing forsterite spalling.
common, because it exhibits a high resist- Due to the large dimensions of float furnace A MgO concept is a cost-efficient option for
ance against V2O5. regenerators, the casing has a significant im- the casing. ANKER DG10 is an appropriate
In the regenerator, the temperature zone be- pact on the investment costs. Many different grade for the crown and upper regions be-
tween 1100 and 800 °C is the most critical refractory lining solutions are common for cause of its excellent creep behaviour up to
region for the checker work. Condensing al- this application. Nearly every float glass 1600 °C that enables effective insulation.
kali sulphates as well as gaseous SO3 cor- manufacturer has its own philosophy and The main reason for the excellent creep re-
rode the checker material. Since more than has a positive experience with a specific lin- sistance is the use of large crystal sinter in
20 years RUBINAL EZ magnesia zircon ing strategy that is optimized to the specific this grade. However, in certain cases a par-
blocks have been installed in this area giving operating conditions. Predominantly, four ticular detrimental phenomenon has been
excellent results over long campaigns. In types of lining concepts for the upper part of observed with magnesia in the first month of
these checkers zircon (ZrSiO4) is one of the the regenerator casing exist: operation. If fine cullet is used during the

82 refractories WORLDFORUM 1 (2009) [2]

start-up this component reaches the regen- • A smooth surface and sharp edges are ob-
erator and can result in forsterite spalling. tained after grinding and drilling a) b)

This phenomenon concerns only the furnace • Thermal expansion comparable to fire clay
start-up period and causes no general prob- blocks: The design of the expansion joints
lems for further furnace operations. To avoid does not have to be modified when re-
this issue, extremely fine cullet should be not placing the fire clay with SUPRAL CA
used to fill the furnace during start-up. • Thermal conductivity is lower compared to
fire clay material
5 Tin bath • Thermal resistance is higher in comparison
5.1 Tin bath bottom blocks to fire clay blocks: SUPRAL CA can work at Zircon
The tin bath is the core of every float bath a maximum temperature of 1200 °C,
facility and refractory selection for this unit whereas fire clay starts to creep at
has an enormous influence on the glass 1100 °C Fig. 4 a) The stud hole filled with ram-
quality over the lining lifetime. The history • Hydrogen diffusivity is at a very low level ming mix based on zircon mullite, after
and development of tin bath bricks has been of approximately 10 mm WG: The forma- firing at 1100 °C: cracks formed b) The
reviewed in detail [5]. Common fire clay tin tion of bubbles under the glass ribbon stud holes filled with DIDOFLO D345, af-
bath bottom blocks exhibit the following caused by thermal transpiration during the ter firing at 1100 °C: no cracks or open
characteristics: campaign can be excluded completely. joints
• Alumina content of approximately There were thermodynamic calculations
38 mass-% showing a possibility of formation of calcium
• Glassy phase content of approximately sulphite (CaS). This phenomenon was not 6 Conclusions
30 mass-% observed in the practice, also no sulphate A high quality refractory lining is essential
• Cold crushing strength of approximately has been found in the used tin bath blocks. for the production of float glass, especially
50 N/mm2 to ensure a good surface ap- Currently, there are more than 20 float baths when the stringent requirements on manu-
pearance with a low risk of transport and in operation with SUPRAL CA bottom blocks facturers such as product quality, energy
installation damage by several leading float glassmakers around consumption and furnace lifetime are taken
• Hydrogen diffusivity in the range of 50 mm the world. The first campaign started in into consideration. In this article the lining
WG. 2004, and to date no problems have been options for float glass furnaces have been
This type of material, with an alumina con- observed. described.At RHI the prerequisite chemical
tent of 38 – 40 mass-%, has been in service and physical material properties, in addition
since the beginning of the 1980s. However, 5.2 Mixes for tin bath stud holes to the required brick specifications are
whilst in certain instances campaigns of 12 In addition to the blocks, the selection of un- achieved through tight manufacturing toler-
years or more have been reached without shaped refractory products to close the stud ances and functional quality monitoring.
problems, in most cases nepheline peeling holes in tin bath bottom blocks is important.
has occurred. The ramming mix based on zircon mullite is References
Nepheline is a reaction product of Al2O3, most common for this purpose. This practice [1] Postrach, S.: State-of-the-art refractory linings
SiO2, and Na2O and results from Na2O dif- has following disadvantages: for container glass furnaces. RHI Bull. (2007) [2]
fusing from the glass, through the tin bath, • A longer installation time. 18–21
to the bottom block. However, the time peri- • The mix is not dense enough and cracks [2] Faber, A.J.; Verheijen, O.S.: Refractory corrosion
od between installation and first peeling can be formed after heating up (Fig. 4a). under oxy-fuel firing conditions. Ceram. Eng. Sci.
varies between 5 and 10 years, depending RHI has developed a self-flowing castable Proc. 18 1(997) [1] 109–119
on the alumina and the glassy phase con- based on calcium aluminate (DIDOFLO [3] Beerkens, R.: TNO Glass Group, Eindhoven, The
tents of the refractory. To circumvent these D345) [6], which can be poured easily into Netherlands, personal communication, 2003
problems RHI developed a grade mainly con- the holes and is self-levelling. It simplifies [4] Heilemann, G.; Schmalenbach, B.; Postrach, S.;
sisting of calcium aluminate, termed SUPRAL the installation process and saves time. The Lynker, A.: Neuentwicklungen für den Kondensa-
CA. The characteristics and advantages of homogeneous structure ensures a high qual- tionsbereich von Kammergitterungen – Lösun-
this grade have previously been described in ity filling. DIDOFLO D345 is not only suitable gen bei reduzierender Atmosphäre. Paper pres-
detail [6] and include: for SUPRAL CA blocks, but also for SUPRAL ented at 81. Glastechnische Tagung der Deut-
• No chemical reaction with tin 40FG (fireclay) tin bath bottom blocks. schen Glastechnischen Gesellschaft, Aachen,
• No reaction with alkalis dissolved in the DIDOFLO D345 has the following advan- Germany, 4–6 June, 2008
tin bath tages: [5] Schmalenbach, B.; Weichert, T.; Santowski, K.:
• No influence of the reducing atmosphere • Self-flowing and therefore easily to install. Development of refractories for the tin bath bot-
present in the tin bath Volume stability: no open joints and no crack tom of float glass lines. RHI Bull. (2006) [3] 36–
• Good mechanical strength to enable per- formation (Fig. 4b) 42
fect grinding and drilling as well as to • High resistance against tin infiltration [6] Bugajski, M.: Internal Report 20070415, RHI
eliminate handling damages • High resistance against alkali attack. AG, Technology Center, Leoben, Austria, 2008

refractories WORLDFORUM 1 (2009) [2] 83