Alumina alkaline process

Dextran is a glucan material produced by the action of the extracellular enzyme dextran sucrase on sucrose in solution and it has shown uses for flocculation in alkaline systems, such as those found in the alumina Bayer process. 

Processes rendered obsolete by the propylene ammoxidation process (51) include the ethylene cyanohydrin process (52—54) practiced commercially by American Cyanamid and Union Carbide in the United States and by I. G. Farben in Germany. The process involved the production of ethylene cyanohydrin by the base-cataly2ed addition of HCN to ethylene oxide in the liquid phase at about 60°C. A typical base catalyst used in this step was diethylamine. This was followed by liquid-phase or vapor-phase dehydration of the cyanohydrin. The Hquid-phase dehydration was performed at about 200°C using alkah metal or alkaline earth metal salts of organic acids, primarily formates and magnesium carbonate. Vapor-phase dehydration was accomphshed over alumina at about 250°C. 

Starch is a polysaccharide found in many plant species. Com and potatoes are two common sources of industrial starch. The composition of starch varies somewhat in terms of the amount of branching of the polymer chains (11). Its principal use as a flocculant is in the Bayer process for extracting aluminum from bauxite ore. The digestion of bauxite in sodium hydroxide solution produces a suspension of finely divided iron minerals and siUcates, called red mud, in a highly alkaline Hquor. Starch is used to settle the red mud so that relatively pure alumina can be produced from the clarified Hquor. It has been largely replaced by acryHc acid and acrylamide-based (11,12) polymers, although a number of plants stiH add some starch in addition to synthetic polymers to reduce the level of residual suspended soHds in the Hquor. Starch [9005-25-8] can be modified with various reagents to produce semisynthetic polymers. The principal one of these is cationic starch, which is used as a retention aid in paper production as a component of a dual system (13,14) or a microparticle system (15). 

A.lkaline Solutions. The most important example of alkaline leach is the digestion of hydrated alumina from bauxite by a sodium hydroxide solution at 160-170°C, ie, the Bayer process (see Aluminumand aluminum alloys). 

The alkylation of pyridine [110-86-1] takes place through nucleophiUc or homolytic substitution because the TT-electron-deficient pyridine nucleus does not allow electrophiUc substitution, eg, Friedel-Crafts alkylation. NucleophiUc substitution, which occurs with alkah or alkaline metal compounds, and free-radical processes are not attractive for commercial appHcations. Commercially, catalytic alkylation processes via homolytic substitution of pyridine rings are important. The catalysts effective for this reaction include boron phosphate, alumina, siHca—alurnina, and Raney nickel (122). 

Ethylamines. Mono-, di-, and triethylamines, produced by catalytic reaction of ethanol with ammonia (330), are a significant outlet for ethanol. The vapor-phase continuous process takes place at 1.38 MPa (13.6 atm) and 150—220°C over a nickel catalyst supported on alumina, siUca, or sihca—alumina. In this reductive amination under a hydrogen atmosphere, the ratio of the mono-, di-, and triethylamine product can be controlled by recycling the unwanted products. Other catalysts used include phosphoric acid and derivatives, copper and iron chlorides, sulfates, and oxides in the presence of acids or alkaline salts (331). Piperidine can be ethylated with ethanol in the presence of Raney nickel catalyst at 200°C and 10.3 MPa (102 atm), to give W-ethylpiperidine [766-09-6] (332). 

Because of the industrial magnitude of these processes, many catalysts have been examined with variations in metal distribution, pore size, and alkalinity. In most synthetic work where catalyst life and small variations in yield are not of great importance, most palladium-on-carbon or -on-alumina powder catalysts will be found satisfactory for conversion of phenols to cyclohexanones. Palladium has a relatively low tendency to reduce aliphatic ketones, and a sharp decrease in the rate of absorption occurs at about 2 mol of consumed hydrogen. Nickel may also be used but overhydrogenation is more apt to occur. 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

J. Reich 6 also patented a process based on the calcination of the alkali fluosilicate or fluoborate with an oxide of the alkaline earths. When the calcined mass is lixiviated with water, the alkali fluoride is obtained in soln. L. Schuch 6 made sodium fluoride by boiling finely powdered cryolite with a cone. soln. of sodium hydroxide—the alumina and silica pass into soln.—sodium fluoride crystallizes from the cooling soln. Sodium silicate can be used in place of the hydroxide. F. Jean made sodium fluoride by leaching a calcined mixture of fluorspar, limestone, Glauber s salt, and charcoal. 

The high stability of the aluminate ion allows the production of concentrated solutions of aluminum with the virtual exclusion of the main metallic impurity, viz. iron as an oxide residue. The resultant impure aluminate solution is clarified and its temperature reduced when the reverse of the above reaction occurs with the formation of A1203,3H20 by a slow crystallization procedure. The high-purity alumina trihydrate product is calcined and then reduced electrochemically in a molten fluoride bath by the well-known Hall-Heroult process. The major problems in the Bayer process have their origin in the coordination chemistry of aluminum in alkaline solutions. The... 

The process includes steps for isomerizing 1-butene into 2-butene at temperatures below 0 °C over alumina treated with alkaline metal and for disproportionating the 2-butene with isobutene over rhenium heptaoxide on alkaline-... 

Zeolites — are any of various natural or synthesized hydrous framework silicates, consisting of interlocked tetrahedrons of SiC>4 and AIO4, with a ratio (Si + Al)/0 equal to 0.5. Z. were firstly discovered and named in 1756, by the Swedish mineralogist Axel Fredrick Cronst-edt, in case of stilbite. Other natural z., as, e.g., analcime, heulandite, natrolite, and stilbite, are often formed near volcanic activity centers, have been formed from rocks by interaction with alkaline groundwaters. Synthetic z., produced since the 1930s, can be made by a slow crystallization process of a silica-alumina gel in the presence... 

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