Refractory articles include both pre-formed products and products that are shaped in situ. Pre-formed products include shrouds, tubes, plates, and bricks. Formed products may be used as linings for vessels, tubes or channels, and are often provided as a mixture that may be rammed, gunned, trowelled, sprayed, vibrated or cast in place.
Refractory articles must resist thermal, chemical and mechanical attacks. Thermal attacks include high temperature, often above 1000 C, and thermal shock caused by quickly changing the temperature of the article. Frequently, the application in which the article is used includes or generates damaging chemicals. For example, slag present in steel casting chemically attacks the refractory articles so that articles in contact with slag often include slag-resistant oxides, such as zirconia. Similarly, refractory tubes used in aluminum-killed steels must resist a build-up of alumina that could otherwise clog the tube. Finally, the refractory article must be strong enough to resist mechanical forces, such as compressive, tensile and torsional stresses.
Commonly, refractory articles are formed from a combination of refractory aggregate and a binder. The binder holds the aggregate in place. Both the aggregate and binder can profoundly affect the properties of the article. Common aggregates include silica, zirconia, silicon carbide, alumina, magnesia, spinels, calcined dolomite, chrome magnesite, olivine, forsterite, mullite, kyanite, andalusite, chamotte, carbon, chromite, and their combinations.
Binders have fallen into two broad classes, cementitious and “chemical.” Chemical binders include organic and inorganic chemicals, such as phenols, furfural, organic resins, phosphates and silicates. The article must often be fired to activate the chemical and initiate the binder. Cementitious binders include cement or other hydratable ceramic powders, such as calcium aluminate cement or hydratable alumina. They usually do not require heating to activate the binder but do require the addition of water. Water reacts with the cementitious binder to harden the mixture. Water also serves as a dispersing medium for the fine powders. Dry powders have poor flowability and are not suitable for forming refractory articles in the absence of high pressure. Water reduces the viscosity of the mixture, thereby permitting the aggregate/binder mixture to flow. Unfortunately, the presence of water in a refractory article can have disastrous effects, namely cracking of the article when exposed to elevated temperatures and even explosive vaporization at refractory temperatures. An article having a cementitious binder often requires a drying step to eliminate residual water.
A refractory aggregate/binder mixture typically includes at least 70 wt. % aggregate and up to about 15 wt. % cement binder. Water is added to make up the balance of the mixture in a quantity sufficient to produce the desired flow for forming a refractory article. Water can be added directly or as a hydrate. For example, European Patent Application Publication No. 0064863 adds water as an inorganic hydrate that decomposes at elevated temperatures. U.S. Pat. No. 6,284,688 includes water in micro-encapsulated sodium silicate.
The porosity of the article affects the drying speed and the danger of explosive vaporization, in that pores permit water to evaporate or volatilize from the article. Prior art has increased porosity of the mixture by the addition of metal powders. JP 38154/1986 teaches a refractory mixture comprising aggregate, cement and aluminum powder. The aluminum powder reacts with added water to produce hydrogen gas. The bubbling gas forms pores through which drying can occur and steam can be released. The aluminum reaction produces copious amounts of heat that further aid in drying. Problems with aluminum powder include the strong exothermic quality of the reaction, release of inflammable hydrogen gas, formation of microcracks in the article, and limited shelf life of the aluminum powder. In order to control this reactivity, U.S. Pat. No. 5,783,510 and U.S. Pat. No. 6,117,373 teach a monolithic refractory composition comprising refractory aggregate, refractory powder, and reactive metal powder. The refractory powder includes aluminous cement to bond the aggregate, thereby imparting physical strength to an article formed by the composition. The reactive metal includes aluminum, magnesium, silicon and their alloys. The amount of reactive metal is selected to control generation of hydrogen gas and, thereby the porosity. Alternatively, Japanese Unexamined Patent Publication No, 190276/1984 teaches the use of fibers to form fine channels through which water can escape. Unfortunately, fibers are difficult to disperse uniformly in the mixture and decrease flowability. The porosity of the article is also increased with deleterious effects on physical properties of the finished article.
Refractory articles may include a chemical, that is, non-cementitious, binder that can eliminate the need for water. Viscosity is typically very high and aggregate/chemical binder mixtures often do not flow well. Chemical binders are typically activated by heating or firing at elevated temperatures, and are used, for example, in dry vibratable mixtures and many pre-formed articles. U.S. Pat. No. 6,846,763 includes granulated bitumen as a binder, along with refractory aggregate, an ignitable metal powder, and oil. Heating the mixture ignites the metal powder, which burns the oil, and melts and cokes the bitumen. The result is a carbon-bonded refractory article. A typical composition includes 70 wt. % aggregate, 6 wt. % silicon, 7 wt. % oil and 13 wt. % bitumen. Although requiring high temperature to form the carbon-bond, the article is substantially water-free. Carbon-bonded articles are not as stable as oxide-bonded articles. Unless held in a reducing atmosphere, carbon-bonded articles are also susceptible to oxidation at elevated temperature.
U.S. Pat. No. 5,366,944 teaches a refractory composition using both low temperature and high temperature binders. Water is not added to the composition. The low temperature binder includes organic binders such as phenolic resins. The high temperature binder includes a metal powder of aluminum, silicon, magnesium, their alloys and mixtures. An article can be formed from the composition and cured at low temperature to activate the low temperature binder. The low temperature binder holds the article together until the article is installed and the high temperature binder activates. The metal binder cannot activate until refractory temperatures are achieved. Advantageously, the metal binder produces an article of higher refractoriness than cement-based binders.
A need exists for a non-cement-based refractory mixture having low water content and low porosity, producing refractory articles with high strength at high temperatures.