Melting accelerants

Mechanical methods which improve contact between batch particles may also accelerate melting processes. Compaction of raw materials into either dry or moist bricks, pellets, or granules increases the melting rate by improving heat transfer to the individual particles, and by providing better contact between components which will react to form eutectic melts. 

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Global Ice Melt Accelerating. Worldwatch Institute Vital Signs Online. Worldwatch Institute Web Site. Available online. URL http //www.worldwatch.org/node/4266. Accessed November 14,2006. 

K. S. Hong and R. F. Speyer, Thermal Analysis of Reactions in Soda-Lime-Silicate Glass Batches Containing Melting Accelerants I One- and Two-Component Systems , J. of the Am. Ceram. Soc., 76 (3) 605-608 (1993). 

Cerium 58 Ce Polishing powders, opacifier for porcelain coatings, glass decolouriser and melting accelerator, photographic materials, textiles, arc lamps, ferrous and non-ferrous alloys including high-temperature Mg alloys, automotive catalytic converters, misch metal... 

Flows can be classified into streamline, when particles in the fluid follow paths (streamlines) that remain constant with time, and turbulent, when vortices cause unpredictable changes in the flow pattern with time. The changeover occurs at a critical value of the Reynolds number, which is defined as the melt velocity, divided by the viscosity times the channel diameter. The high viscosity of thermoplastic melts causes velocities to be low. Hence, the Reynolds number is very low and the flows are streamline. We will consider steady flows, and ignore the start and end of injection and blow-moulding flows, when the melt accelerates and decelerates, respectively. However, in the RIM process (Section 5.6.5), turbulent flow of the low viscosity constituents in the mixing head achieves intimate mixing. 

Salt cake (Na2S04) is used as a melting accelerator (forms eutectic with Na2C03) and... 

Niobium pentoxide also is reduced to metal commercially by the aluminothermic process. The finely ground powder is mixed with atomized aluminum and an accelerator compound which gives extra heat during reaction, then is ignited. The reaction is completed quickly and, after cooling, the slag is broken loose to free the metal derby which is purified by electron-beam melting. 

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

Vinylidene Chloride Copolymer Foams. Low density, fine-celled VDC copolymer foams can be made by extmsion of a mixture of vinylidene chloride copolymer and a blowing agent at 120—150°C (190). The formulation must contain heat stabilizers, and the extmsion equipment must be made of noncatalytic metals to prevent accelerated decomposition of the polymer. The low melt viscosity of the VDC copolymer formulation limits the size of the foam sheet that can be extmded. 

Another method iavolves an electric-arc vaporizer which is >2000° C before burning (25,32). One of the features of the process is a rapid quench of the hot gas flow to yield very fine oxide particles (<0.15 nm). This product is quite reactive and imparts accelerated cure rates to mbber. Internally fired rotary kilns are used extensively ia Canada and Europe and, to a limited extent, ia the United States (24). The burning occurs ia the kiln and the heat is sufficient to melt and vaporize the ziac. Because of the lower temperatures, the particles are coarser than those produced ia the other processes. In a fourth process, ziac metal which is purified ia a vertical refining column is burned. In essence, the purification is a distillation and impure ziac can be used to make extremely pure oxide. Also, a wide range of particle sizes is possible (33). 

With vertical zone melting and horizontal zone melting without a gas bubble, simple tube rotation at a constant moderate velocity does not significantly influence 5. In those cases, accelerated cmcible rotation or spin up—spin down could be used (72—75). The tube is spun more rapidly than described above, but not at constant velocity. It may, for example, be spun rapidly, suddenly stopped, spun rapidly, etc, resulting in very vigorous stirring. 

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