Chemistry of inorganic compounds – Phosphorus or compound thereof – Oxygen containing
Reexamination Certificate
2001-05-16
2004-05-25
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Phosphorus or compound thereof
Oxygen containing
C423S314000, C501S127000
Reexamination Certificate
active
06740299
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to phosphate-bonded refractory compositions and to a method of manufacture.
Phosphate-bonded refractories have been used extensively in the iron, steel, aluminum, and brass industries to line vessels for containment of molten metals and other applications for more than 50 years. These products are well known for their ability to withstand high temperatures and attack by molten metals and slags. They are further characterized by ease of forming, linear and volume stability, high strength both dried and fired, and excellent resistance to abrasion and to mechanical impact at high temperatures. The full range of phosphate-bonded refractories, including chemically bonded and fired brick, monoliths, and special shapes suitable for use in a wide range of high temperature applications, can be produced by the manufacturing method of the present invention.
It has long been recognized by individuals skilled in the design, formulation, and manufacture of refractory products that a proto aluminum phosphate chemical binder could be formed by pre-reacting a source of aluminum and phosphoric acid at elevated temperatures. Aluminum orthophosphate (AlPO
4
or Al
2
O
3
.P
2
O
5
) and aluminum metaphosphate (Al(PO
3
)
3
or Al
2
O
3
.3P
2
O
5
), formed in the product after drying and firing, are refractory compounds with melting points of 1500° C. and 1537° C., respectively. The molar ratio of Al
2
O
3
to P
2
O
5
is 1:1 in aluminum orthophosphate, and the molar ratio is 1:3 in aluminum metaphosphate. Each compound is a well-defined inorganic substance having, within limits, definite chemical, physical, and electrical properties. It has long been a practice in the refractories industry to heat the phosphoric acid and aluminum source at temperatures greater than 100° C. to activate and drive the reaction between the phosphoric acid and aluminum source to form either aluminum orthophosphate or aluminum metaphosphate in an amorphous form. The prior art references of record in this application include U.S. Pat. No. 5,496,529 (hereinafter '529) and U.S. Pat. No. 5,707,442 (hereinafter '442) to Fogel et al. and SU 1458340 to Kuz'menkov et al. (hereinafter Kuz'menkov et al.). Fogel et al., in the '529 and '442 references, describe the prior art, and demonstrate that by admixing aluminum and phosphoric acid in an Al/P molar ratio of approximately 1 at about 100° C., a very viscous suspension is formed which is difficult to dry and difficult to use. Fogel et al. observed that the sources of aluminum and phosphoric acid do not react with one another to yield aluminum phosphates without heating to temperatures above 100° C. Kuz'menkov et al. is more specific and requires mixing Al(OH)
3
with heated H
3
PO
4
at a mole ratio of 1:(0.9-1.1). The working time and bonding strength of the phosphate binder are increased in the manufacture of composite materials when the binder components are mixed at 102° C. to 112° C. in the presence of HCl. The physical characteristics of the phosphate binder obtained by Kuz'menkov et al. would unquestionably be the same as that described by Fogel et al. in the '442 and '529 references. We are in complete agreement with the Fogel et al. observations regarding the extreme difficulties encountered working with a pre-reacted proto aluminum phosphate paste.
When a source of at least one aluminum compound and phosphoric acid are pre-reacted at temperatures greater than 100° C. to form a proto aluminum phosphate chemical binder, the resulting gel or paste is highly viscous and difficult to incorporate into a refractory composition. Similarly, when preparing a refractory composition having an aluminum phosphate chemical binder, it was difficult to admix the refractory filler with the binder because of the highly viscous physical characteristic of the pre-reacted proto aluminum phosphate paste. Consequently, it would be both expensive and difficult to manufacture phosphate-bonded refractory products following the Kuz'menkov et al. practice of pre-reacting Al(OH)
3
and H
3
PO
4
at 102° C. to 112° C.
It has been common practice to decrease the viscosity of proto aluminum phosphate paste by adding water to the paste. The addition of water is made to thin the paste to form a slurry. The decreased viscosity of the binder facilitates the admixing of refractory materials or fillers to create a refractory composition. However, the dilution of the paste with water causes increased porosity and permeability that is detrimental to the quality and functionality of the final refractory composition.
Were the dry aluminum phosphate products obtained following the Fogel et al. art used in the manufacture of phosphate-bonded refractory specialty products, the aluminum phosphate would be required to be finely divided and consequently milled or ground to pass substantially a 200 mesh US Standard screen. The finely divided aluminum phosphate would be introduced to the mixer with the batch ingredients and uniformly distributed. It must be recognized that both aluminum orthophosphate and aluminum metaphosphate are insoluble in water. Consequently, even with the addition of water, the finely divided aluminum phosphate would not provide a coating for the refractory grains, either in the aggregate or the matrix, nor would the finely divided aluminum phosphate contribute to the development of the plasticity or workability required in the finished product. To develop plasticity in a refractory product formulated following the Fogel et al. art, the addition of clay, such as a plastic Tennessee ball clay, would be required. This addition of clay would decrease the refractoriness of the matrix and be undesirable.
It has long been known that the shelf life (measured by the time plasticity, workability or consistency is maintained) of refractory materials containing proto aluminum phosphate chemical binders formed by pre-reacting phosphoric acid and an aluminum source at temperatures greater than 100° C., is markedly decreased and undesirable. It has long been common practice to add citric acid, oxalic acid, or hydrochloric acid to maintain the plasticity, workability or consistency of the refractory material for a period of 30 days. Without the addition of these sequestering agents, the shelf life of the refractory composition would be decreased and unsuitable for use. These sequestering agents add substantially to the raw material costs of the product being manufactured.
The manufacture of a proto aluminum phosphate chemical binder by pre-reacting a source of aluminum with phosphoric acid at temperatures greater than 100° C. requires expensive and specialized capital equipment. This specialized equipment would include: reaction vessels which can be heated in excess of 100° C.; control hoods and scrubbers capable of withstanding highly corrosive phosphoric acid fumes at elevated temperatures; and equipment capable of moving the highly viscous pre-reacted aluminum phosphate paste from the reaction vessel into a mixer in which a refractory composition could be created. Facilities manufacturing refractory compositions that include a pre-reacted proto aluminum phosphate chemical binder are subject to regulatory scrutiny by both the Federal Environmental Protection Agency, the Federal Occupation, Safety and Health Administration, and equivalent state regulatory agencies. It is estimated that the additional cost of manufacturing a phosphate-bonded refractory composition, directly attributable to the need to pre-react a proto aluminum phosphate chemical binder in the preparation of a refractory composition, would range between 10 percent and 20 percent of the total cost of the finished refractory composition. Our invention eliminates these costs.
The parameters governing the reaction process between a source of aluminum and phosphoric acid have never been described. We have found experimentally, for example, that &agr;-alumina (&agr;-Al
2
O
3
), corundum, and the aluminum oxyhydroxides (AlO(OH)), boehmite and dias
Carini George F.
Carini, II George F.
Langel Wayne A.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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