Melting refractory materials

REFRACTORY Having a high melting point, resistant to melting. Refractory materials... 

Refractory Linings. The refractory linings (2,3) for the hearth and lower wads of furnaces designed for melting ferrous materials may be acidic, basic, or neutral (see Refractories). Sdica has been widely used in the past, and is stid being used in a number of iron and steel foundries. Alumina, a neutral refractory, is normally used for furnace roofs and in the wads for iron foundries, but basic brick can also be used in roofs (4). 

Refractory Materials. Extremely high melting metals and those that are more resistant to deformation when hot are considered refractory... 

Plasmas are used extensively to melt materials for a variety of purposes. In many cases, the materials are introduced as a powder into the gas stream in a plasma torch. The molten droplets can be used to grow crystals of refractory materials such as niobium (166). 

Fusion-cast refractories are formed by first melting the material in an electric arc. The Uquid is poured into a mold and allowed to cool. Various annealing processes and refining techniques ensure a uniform and stmcturaUy sound shape. The case form is then cut or ground to size. 

Any refractory material that does not decompose or vaporize can be used for melt spraying. Particles do not coalesce within the spray. The temperature of the particles and the extent to which they melt depend on the flame temperature, which can be controlled by the fueLoxidizer ratio or electrical input, gas flow rate, residence time of the particle in the heat zone, the particle-size distribution of the powders, and the melting point and thermal conductivity of the particle. Quenching rates are very high, and the time required for the molten particle to soHdify after impingement is typically to... 

High density tungsten alloy machine chips are recovered by oxidation at about 850°C, foUowed by reduction in hydrogen at 700—900°C. Typically, the resultant powders are about 3-p.m grain size and resinter readily. There can be some pickup of refractory materials used in furnace constmction, which must be controUed. This process is important commercially. Eor materials that may be contaminated with other metals or impurities, the preferred recovery process is the wet chemical conversion process used for recovery of tungsten from ores and process wastes. Materials can always be considered for use as additions in alloy steel melting. 

Plasma spraying Metals, or refractory materials or composites are applied by melting in an ionised inert gas M, tantalum, molybdenum, alumina, zirconia M2 a variety of metallic substrates... 

Niobium carbide, also known as columbium carbide, is a important refractory material with a high melting point. It is used as a CVD coating mostly on an experimental basis. Niobium carbide has two phases Nb2C and the monocarbide NbC. The latter is the only phase of industrial importance and the only one reviewed here. Its characteristics and properties are summarized in Table 9.5. 

It is impossible to find inert containers owing to both the high mp of borides (> 2000°C) and their reactivity with refractory materials. The problem is solved by melting borides either in water-cooled boats or pedestals. Water-cooled, metal... 

Reactions of FeO-bearing melts with refractory materials... 

Bromine trifluoride calorimetry has considerable development potential both with respect to improved accuracy and with respect to the range of materials which can be examined. Thus the more insoluble or refractory materials could be reacted in "acid or "basic solutions in BrF3 or even in molten acids or bases at higher temperatures. (The, 80/180 ratio in rocks can be measured on the oxygen released by dissolving minerals in such melts.)... 

Heating/ Melting Tundish Material Ceramics/Refractory... 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

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