Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
1999-10-08
2001-11-06
Sample, David R (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S133000, C501S154000, C432S195000
Reexamination Certificate
active
06313057
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to silica refractories and methods of producing the same. More particularly, this invention relates to silica refractory bricks and methods of producing silica refractory bricks that are resistant to degradation in alkali containing environments, such as oxygen-fuel fired furnaces.
BACKGROUND OF THE INVENTION
Silica bricks are used as a refractory in building and repairing industrial furnaces, including coke ovens, hot blast stoves and glass furnaces. U.S. Pat. Nos. 4,866, 5,310,708, and 5,496,780, which are incorporated by reference herein in their entirety, disclose the use and properties of silica refractory bricks.
There has been very little recent improvement in the manufacture of the conventional silica refractory manufactured for use in glass furnaces. Conventional silica bricks are typically manufactured by mixing quartz grains with calcium oxide binder, pressing the mixture into bricks and firing the bricks to temperatures up to 1700° C. to allow more than about 95% of the quartz to transform to cristobalite and tridymite. This transformation avoids the problems associated with expansion changes of alpha and beta quartz and allows for faster heating of the furnaces in which the brick is used. Conventional silica refractory bricks used in glass furnaces contain about 2.5% to 3% calcium which acts as a binder between the silica grains.
Environmental considerations, particularly the desire to lower the emission of nitrogen oxides from industrial furnaces, have driven the replacement of air-fuel firing of glass furnaces with oxygen-fuel firing. The change from air-fuel firing to oxygen-fuel firing has increased alkalis in the furnace environment, for example, sodium hydroxide in glass melting furnaces. Consequently, the increased alkalis in the furnace environment have caused additional corrosion of fused silica refractories. In some instances, the switch from air-fuel firing to oxygen-fuel firing increases the alkalis such as sodium hydroxide by a factor of four in the glass furnace.
Thus, alkalis in the furnace have been identified as being detrimental to silica refractory bricks. The crowns of glass furnaces are typically made of silica refractory bricks. The surface of an oxygen-fuel fired glass furnace crown made from silica refractory bricks can be degraded by about one to two inches per year. The typical life of a crown of a float glass furnace using oxygen-fuel firing is about seven years, which is a reduction of about five years compared with a furnace using air fuel-firing.
Alternatives to conventional silica refractories include alumina-zirconia-silica (AZS) refractories and amorphous silica refractories that do not contain any binder. However, one disadvantage with these alternative refractories is that they are much more expensive than the conventional silica refractory bricks made from quartz grains and contain calcium oxide binder.
There is a distinct need for a silica refractory that is resistant to alkali attack and degradation in oxygen-fuel fired furnaces, but is less expensive than AZS and amorphous silica refractory materials. It would also be advantageous to provide a refractory brick that could be utilized for long spans of glass furnace crowns and would require very little maintenance.
SUMMARY OF INVENTION
Accordingly, the present invention generally provides a new silica refractory made from quartz grains and containing calcium oxide binder in an amount less than one weight percent. Preferably, the calcium oxide binder is present in an amount between about 0.1 and 0.8 weight percent, and most preferably, the silica refractory contains between about 0.3 and 0.7 weight percent calcium oxide binder. The refractory bricks of the present invention are useful in glass manufacturing furnaces, particularly in the crowns of the furnace, and in furnaces that contain quantities of alkali vapors that are harmful to conventional silica refractories.
In another aspect, the invention includes a method of producing silica refractory brick made from quartz grains comprising adding less than one weight percent calcium oxide binder to the composition prior to firing of the brick to prevent degradation of the brick in a furnace containing alkali vapors, for example, and oxygen-fuel fired furnace. In a preferred embodiment of the present invention, the method involves adding between 0.1 and 0.8 weight percent calcium binder, and in a more preferred embodiment, the invention involves adding between 0.3 and 0.7 weight percent calcium oxide binder.
Additional features and advantages of the invention will be set forth in the description which follows. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
DETAILED DESCRIPTION
As mentioned above, conventional fused silica refractories made from quartz grains typically contain approximately 2.5 to 3 weight percent calcium oxide binder. The present invention is based on applicants' discovery that calcium oxide binder significantly contributes to the deterioration and wear of conventional silica refractories in furnace environments containing alkali vapors.
Thus, the present invention is based on the discovery that reduction of the amount of calcium oxide binder contained in the silica brick prior to firing results in a brick that is resistant to attack from alkalis in the furnace environment. The refractory material of the present invention is particularly useful for making silica bricks for glass furnaces, particularly glass furnaces that are oxygen-fuel fired.
Conventional fused silica refractory bricks are typically manufactured by mixing together the calcium oxide binder with crystalline quartz grains having the right characteristics for conversion to the high temperature crystal phases of silica. After forming the brick, which is usually done by pressing (e.g., dry pressing), the brick is fired at temperatures high enough to convert the quartz into cristobalite and tridymite, which are stable at high temperatures.
Without being bound to any particular theory, applicants believe that when conventional fused silica bricks containing the usual levels of 2.5 to 3 weight percent calcium oxide are used in furnaces, the high furnace temperatures causes the calcium to react with silica to form pseudo wollastonite or sodium metasilicate. This mineral surrounds the silica grains, and forms a continuous path which apparently acts as pathway for drawing alkali from the furnace deep into the brick. Penetration of the alkali below the face of the brick causes premature corrosion and deterioration of the brick, leading to early failure of the furnace.
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patent: 2901367 (1959-08-01), Kraner
patent: 3144345 (1964-08-01), McCreight et al.
patent: 3236665 (1966-02-01), King
patent: 3620783 (1971-11-01), Mahler et al.
patent: 3684538 (1972-08-01), Wright
patent: 3788866 (1974-01-01), Flood et al.
patent: 4866015 (1989-09-01), Koschlig et al.
patent: 5310708 (1994-05-01), Harako et al.
patent: 5496780 (1996-03-01), Ichikawa et al.
patent: 74-016087 (1974-04-01), None
patent: 50-159503 (1975-12-01), None
A. J. Faber and D. S. Verbeljen, Ceramic Engineering Science Proceedings 16(1) (1997), pp. 109-119,Refractory Corrosion Under Oxy-Fuel Firing Conditions(No Month).
Brown John Thomas
Wosinski Jean A.
Wosinski John F.
Corning Incorporated
Nwaneri Angela N
Pacella Patrick
Sample David R
Wosinski Jean A.
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