Autogenes

On a laboratory scale, hydrotliennal syntliesis is usually carried out in Teflon-coated, stainless-steel autoclaves under autogenous pressure. A typical syntliesis mixture consists of up to four major constituents, a T-atoni source (silicon and aluminium, otlier elements may also be incoriiorated as indicated above), a solvent (almost exclusively... 

In earlier studies (24), the reaction was carried out at temperatures above 200°C under autogenous pressure conditions usiag alkaU metal hydroxide or alkoxide catalysts significant amounts of carboxyUc acid, RCH2COOH, were formed as were other by-products. More recent reports describe catalysts which minimize by-products MgO—K CO —CUC2O2 (25), less basic but stiU requiring high temperatures Rh, Ir, Pt, or Ru complexes (26) and an alkaU metal alkoxide plus Ni or Pd (27), effective at much lower temperatures. 

The dried charge is pneumatically fed to a burner mounted on the roof of the reaction shaft. Normally oxygen is used to get an autogenous smelt. The offgases analy2e around 70% SO2 and can be used for the manufacture of sulfuric acid after conventional gas cleaning. 

Eig. 1. Three basic types of size reduction circuits (a) conventional, (b) autogenous, and (c) autogenous plus separate fine grinding (6). 

In the first step of the polymerization process, a prepolymer is prepared as a slurry in water. Excess diamine is added to control the degree of polymerization, eg, degree of polymerization = 6-14 (158). This prepolymerization step is conducted at approximately 200°C under autogenous pressure for less than 90 min. 

Commercial Manufacture of Specific Pyridine Bases. Condensation of paraldehyde with ammonia at 230°C and autogenous pressure (eq. 22) is used to manufacture 5-ethyl-2-methylpyridine (7). This is one of the few Hquid-phase processes used in the industry to make relatively simple aLkylpyridines, and one of the few processes known to make a single alkylpyridine product selectively. 

Another of the few selective syntheses of aLkylpyridines is one for a-picoline (2) (76). This is a two-step process (eq. 24) where acrylonitrile is used to monocyanoethylate acetone in the Hquid phase at 180°C and at autogenous pressure, 2 MPa (300 psig). The monoadduct, 5-cyano-2-pentanone, is then passed over a palladium-containing catalyst to reduce, cyclize, and dehydrogenate, in sequence. 

Solvent name PEL, ppm Flash point, °C LEL, vol % UEL, vol % Evaporation b rate Autogenous ignition temperature, °C Vapor density... 

Pure zirconium tetrachloride is obtained by the fractional distillation of the anhydrous tetrachlorides in a high pressure system (58). Commercial operation of the fractional distillation process in a batch mode was proposed by Ishizuka Research Institute (59). The mixed tetrachlorides are heated above 437°C, the triple point of zirconium tetrachloride. AH of the hafnium tetrachloride and some of the zirconium tetrachloride are distiUed, leaving pure zirconium tetrachloride. The innovative aspect of this operation is the use of a double-sheU reactor. The autogenous pressure of 3—4.5 MPa (30—45 atm) inside the heated reactor is balanced by the nitrogen pressure contained in the cold outer reactor (60). However, previous evaluation in the former USSR of the binary distiUation process (61) has cast doubt on the feasibHity of also producing zirconium-free hafnium tetrachloride by this method because of the limited range of operating temperature imposed by the smaH difference in temperature between the triple point, 433°C, and critical temperature, 453°C, a hafnium tetrachloride. 

Pressure and Temperature. The pressure and temperature are relatively low, typicaUy less than about 2 MPa (20 atm) and 150°C. High pressures are used in some apphcations, but they require expensive thick-waUed reactors. High temperatures are largely avoided because they result in high autogenous pressures and because many of the catalysts are unstable at high temperatures. 

Raw material for dry process plants is ground in closed-circuit ball mills with air separators, which may be set for any desired fineness. Drying is usually carried out in separate units, but waste heat can be utilized directiy in the mill by coupling the raw mill to the kiln. Autogenous mills, which operate without grinding media are not widely used. For suspension preheater-type kilns, a roUer mill utilizes the exit gas from the preheater to dry the material in suspension in the mill. 

The pebble mill is a tube mill with flint or ceramic pebbles as the grinding medium and may be lined with ceramic or other nonmetaUic liners. The rock-pebble mill is an autogenous mill in which the... 

FIG. 20-57 Autogenous mill circuit. Simphfied flow diagram of the Cleveland-Cliffs Iron Co. Empire iron-mine concentrator with two autogenous wet-grinding stages. To convert inches to centimeters, multiply hy 2.54 to convert feet to centimeters, multiply hy 30.5. 

With other ores there has been a problem of buildup of intermediate-sized particles, but this has been solved either by adding a small load of steel balls, thus converting to a semiautogenous grinding system (SAG), or by sending the scalped intermediate-sized particles through a cone crusher. A flow sheet for a typical wet autogenous circuit is shown in Fig. 20-57. 

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