Refractory protection

Coordination compounds are found in new materials such as onedimensional conductors and are also utilized as volatile precursors for the deposition of metal oxides in thin superconducting films or deposition of metal carbides or nitrides for refractory protective coatings and other applications. Coordination compounds have also proven to be useful precursors for the deposition under relatively mild conditions of high purity thin metal films for microelectronic applications. Historically, we consider Ni(CO)4 as the prototypical example discovered by Mond in... 

At the required oxygen potential for the formation of magnetite to occur and provide refractory protection there will generally need to be some compromise between the PbO content of slag and the residual sulfur level in bullion. 

Samples of animal bones weighing approximately 3 g are ashed at 600 °C until the entire bone is ash-white. Samples are then crushed in a mortar and pestle. A portion of the sample is digested in HCl and diluted to a known volume. The concentrations of zinc and strontium are determined by atomic absorption. The analysis for strontium illustrates the use of a protecting agent as La(N03)3 is added to prevent an interference due to the formation of refractory strontium phosphate. 

Although rocking of the furnace to intermittently cover and hence protect up to 90% of the refractory, as well as improved refractories, has done much to make the indirect-arc furnace more viable, these furnaces are becoming less common, primarily due to high operating costs as a result of erosion of the refractory by the intense arc radiation. 

Vacuum Radiation Furnaces. Vacuum furnaces are used where the work can be satisfactorily processed only in a vacuum or in a protective atmosphere. Most vacuum furnaces use molybdenum heating elements. Because all heat transfer is by radiation, metal radiation shields ate used to reduce heat transfer to the furnace casing. The casing is water-cooled and a sufficient number of radiation shields between the inner cavity and the casing reduce the heat flow to the casing to a reasonable level. These shields are substitutes for the insulating refractories used in other furnaces. 

C or higher for the kaolin-based products to 1425°C and above for the zirconium-containing materials. At temperatures above 1000°C these ceramic fibers tend to devitrify and partially crystallize. Specially prepared ceramic fibers are used to protect space vehicles on re-entry and can withstand temperatures above 1250°C (see Ablative materials Refractory fibers). 

The furnace is constmcted with a steel shell lined with high temperature refractory (see Refractories). Refractory type and thickness are deterrnined by the particular need. Where combustion products include corrosive gases such as sulfur dioxide or hydrogen chloride, furnace shell temperatures are maintained above about 150—180°C to prevent condensation and corrosion on the inside carbon steel surfaces. Where corrosive gases are not present, insulation is sized to maintain a shell temperature below 60°C to protect personnel. 

In Germany and Japan, pulverized quicklime is used in making self-fluxing sinters, partially replacing limestone. Granular dead-burned dolomite is stiU used to protect the refractory lining of open-hearth and electric furnaces, but not the basic oxygen furnace. Refractory time has declined with the... 

Siliconizing is yet another process used especially for coating of the refractory metals Ti, Nb, Ta, Cr, Mo, and W (see Refractories). These metals form siHcides which have a surface oxidation protection layer of Si02. Siliconizing is especially effective on molybdenum against air oxidation up to 1700°C. 

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel). 

Chemically Functional. Refractory coatings are used for corrosion-resistant high temperature service in gas turbine and diesel engines, components such as cmcibles, thermocouple protection tubing, valve parts, etc. 

Interstitial Compounds. Tungsten forms hard, refractory, and chemically stable interstitial compounds with nonmetals, particularly C, N, B, and Si. These compounds are used in cutting tools, stmctural elements of kilns, gas turbines, jet engines, sandblast nozzles, protective coatings, etc (see also Refractories Refractory coatings). 

Approximately 5% of the U.S. consumption of is in agriculture. Boron is a necessary trace nutrient for plants and is added in small quantities to a number of fertilizers. Borates are also used in crop sprays for fast rehef of boron deficiency. Borates, when apphed at relatively high concentration, act as nonselective herbicides. Small quantities of borates are used in the manufacture of alloys and refractories (qv). Molten borates readily dissolve other metal oxides usage as a flux in metallurgy is an important apphcation. Other important small volume apphcations for borates are in fire retardants for both plastics and ceUulosic materials, in hydrocarbon fuels for fungus control, and in automotive antifreeze for corrosion control (see Corrosion and corrosion inhibitors). Borates are used as neutron absorbers in nuclear reactors. Several borates, which are registered with the Environmental Protection Agency (EPA) can be used for insecticidal purposes, eg, TIM-BOR. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

In North America, a special, high conductivity, low permeability, "hot-pressed" carbon brick is utilized almost exclusively for hearth walls. Because of their relatively small size and special, heat setting resin cement, and because the brick is installed tightly against the cooled jacket or stave, differential thermal expansion can be accommodated without refractory cracking and effective cooling can be maintained. Additionally, the wall thickness is generally smaller than 1 m, which promotes the easy formation of a protective skull of frozen materials on its hot face. Thus hearth wall problems and breakouts because of carbon wall refractory failure are virtually nonexistent. 

The metallurgy of the cyclone equipment has in recent years focused primarily on type 304 H stainless steel. The 304 H material is durable and easy to fabricate and repair, withstands the high regenerator temperatures, and is oxidation- and corrosion-resistant. Essentially all internal surfaces of the cyclone that are subject to erosion are protected with a 2 cm layer of erosion-resistant lining. When installed and cured, most refractory linings are highly resistant to erosion. 

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