Chromium -oxidant systems

To appreciate the potential changes in food cans, it is necessary to describe briefly the steel-based materials used in modern can manufacturing operations. The tin can is made from a special grade of thin gage, low carbon, cold-rolled steel, which is generally referred to as a tin mill product. The base steel is coated with either tin, a chromium-chromium oxide system, or it is just cleaned and oiled. It may also be coated with organic coatings. 

Despite their importance in olefin polymerization reactions, little attention has been paid to the nature of the adsorbed oxygen species on supported chromium oxide systems. 

The tetravalent chromium alkyl compounds were found to give catalysts that are somewhat more active than the catalyst made from the divalent chromium counterpart, under commercial reaction conditions (90-110 °C, 0.5-1.5 mol ethylene L ). Indeed, they were among the most active organochromium catalysts tested in our laboratory. Their overall 1-h yield was usually also superior to that observed with some of the best chromium oxide on silica-titania catalysts. Even when compared with chromium oxide systems used with a cocatalyst, the catalysts made with tetravalent chromium alkyls were equal or better in activity. Unfortunately, for commercial applications, these catalysts also tend to make some oligomers and wax as well. 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

Chromium Removal System. Chlorate manufacturers must remove chromium from the chlorate solution as a result of environmental regulations. During crystallization of sodium chlorate, essentially all of the sodium dichromate is recycled back to the electrolyzer. Alternatively, hexavalent chromium, Cr, can be reduced and coprecipitated in an agitated reactor using a choice of reducing agents, eg, sodium sulfide, sulfite, thiosulfate, hydrosulfite, hydrazine, etc. The product is chromium(III) oxide [1333-82-0] (98—106). Ion exchange and solvent extraction techniques have also... 

The chrome—tin system is the only family to produce purple and maroon shades, as well as pinks. The system can be defined as pigments that are produced by the calcination of mixtures of small amounts of chromium oxide with substantial amounts of tin oxide. In addition, most formulations contain substantial amounts of siHca and calcium oxide. 

A variety of chromium (VI) oxidizing systems have been developed which allow for the oxidation of a wide range of sensitive compounds. One of the most widely used chromium (VI) reagents is the Jones reagent (/), whose use is detailed in the procedure. A related system employs acetic acid as the solvent, and an example of this reagent is also given. 

A recently discovered (2) oxidizing system promises to become very important for the oxidation of acid-sensitive compounds. The reagent is chromium trioxide-pyridine complex, which may be isolated after preparation and employed in nonaqueous solvents (usually methylene chloride). A remarkable feature of the reagent is that good yields of aldehydes are obtained by direct oxidation of primary alcohols. The preparation of the reagent and its use are given. 

Figure 12-8A. Piston rings. The piston rod is manufactured from heat-treated stainless steel and is coated with wear-resistant overlays, such as ceramic, chromium oxide, and tungsten carbide applied by plasma techniques. Piston rod cross-head attachment has mechanical preloading system for the threads. Rider rings and seal rings are manufactured from PTFE filled resins fillers are matched to the gas, piston speed, and liner specifications. Typical fillers are glass, carbon, coke, or ceramic. (Used by permission Bui. BCNA-3P100. Howden Process Compressors Incorporated. All rights reserved.)...

The corrosion of stainless steel in 0.1 mol-1 NaCl solutions at open circuit potential was studied in detail by Bruesch et al. [106] using XPS in combination with a controlled sample transfer system [38]. It was verified by XPS analysis that the passivating film contains chromium oxide. The position and the height of the Cr concentration maximum depends critically on the bulk chromium content of the steel. Significant variations in the electrode passivation properties were observed at a Cr concentration of 12%, while the film behaviour was found to be rather independent of the other components like Mo, Ni, Cu. From the fact that the film structures and... 

J. Herscovici, M.-J. Egron, and K. Antonakis, New oxidative systems for alcohols Molecular sieves with chromium-(VI) reagents, /. Chem. Soc. Perkin Trans. I (1982) 1967-1973. 

Metallocenes give polyethylene producers a long list of opportunities to work on. They have already created polyethylene copolymers that compete well in applications that have been formerly the exclusive domain of the more costly, so-called high value plastics. Further, they are augmenting the chromium oxide and Ziegler-Natta catalysts systems that have been used for HDPE and LLDPE with metallocene catalysts. That creates even further... 

Sulfur dioxide in the sample causes a negative interference of approximately 1 mole of ozone per mole of sulfur dioxide, because it reduces the iodine formed by ozone back to potassium iodide. When sulfur dioxide concentrations do not exceed those of the oxidants, a method commonly used to correct for its interference is to add the amount of sulfur dioxide determined by an independent method to the total detector response. A second method is to remove the sulfur dioxide from the sample stream with solid or liquid chromium trioxide scrubbers. Because the data on the performance or these sulfur dioxide scrubbers are inadequate, the performance for each oxidant system must be established experimentally. 

Chromium(vi) Complexes.—These studies have been virtually confined to oxyhalide and oxide systems. 

AFM has been used to study surface molecules under different conditions. Colloidal system studies by AFM AFM has allowed scientists to be able to study molecular forces between molecules at very small (almost molecular size) distances. Further, it is a very attractive and sensitive tool for such measurements. In a recent study, the colloidal force as a function of pH of Si02 immersed in the aqueous phase was reported using AFM. The force between an Si02 sphere (ca. 5 mm diameter) and a chromium oxide surface in the aqueous phase of sodium phosphate were measured (pH from 3 to 11). The Si02 sphere was attached to the AFM sensor as shown in Figure 10.3. 

Strangely enough, a combination similar to the ammonia catalyst, iron oxide plus alumina, yielded particularly good results (32). Together with Ch. Beck, the author found that other combinations such as iron oxide with chromium oxide, zinc oxide with chromium oxide, lead oxide with uranium oxide, copper oxide with zirconium oxide, manganese oxide with chromium oxide, and similar multicomponent systems were quite effective catalysts for the same reaction (33). 

The Econ-Abator system is a fluidized-bed catalytic oxidation system. Catalytic fluidized beds allow for destruction of volatile organic compounds (VOCs) at lower temperatures than conventional oxidation systems (typically 500 to 750°F). The technology uses a proprietary catalyst consisting of an aluminum oxide sphere impregnated with chromium oxide. 

The oxide systems deriving from chromium(III) are numerous. Chromium(III) oxide, Cr203, is the final product of the calcination of many chromium(III) complexes. Chromium(III) oxide has the A1203 structure (Ds) with hexagonal lattice parameters a = 4.95, c = 13.66A and c/a = 2.761.712... 

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