Metal oxidation, stages

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

The first-stage catalysts for the oxidation to methacrolein are based on complex mixed metal oxides of molybdenum, bismuth, and iron, often with the addition of cobalt, nickel, antimony, tungsten, and an alkaU metal. Process optimization continues to be in the form of incremental improvements in catalyst yield and lifetime. Typically, a dilute stream, 5—10% of isobutylene tert-huty alcohol) in steam (10%) and air, is passed over the catalyst at 300—420°C. Conversion is often nearly quantitative, with selectivities to methacrolein ranging from 85% to better than 95% (114—118). Often there is accompanying selectivity to methacrylic acid of an additional 2—5%. A patent by Mitsui Toatsu Chemicals reports selectivity to methacrolein of better than 97% at conversions of 98.7% for a yield of methacrolein of nearly 96% (119). 

At a much earlier stage in the research and development cycle, fluidized-bed processes use porous sorbents containing copper oxide (82), cerium oxide (83), and other metal oxides (84). 

Zinc occurs most abundantly in tire mineral. Sphalerite, ZnS, which is roasted to produce the oxide before the metal production stage. The products of the roast are then reduced by carbon to yield zinc oxide and CO(g). In the older process, tire Belgian retort process, the metal oxide and carbon are mixed together in a reactor which allows the indirect heating of the charge to produce the gaseous products followed by tire condensation of zinc at a lower temperature in a zone of the reactor which is outside the heating chamber. The carbon monoxide is allowed to escape from the vessel and is immediately burnt in... 

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

Therefore, the use of several specific techniques while implementing the method of semiconductor sensors makes it feasible to detect and analyze emission of oxygen atoms at initial stage of metal oxidation although in case of silver it should be noted that there are no phase of silver oxide formed due to its instability at such conditions [57]. Rather, the absorption of oxygen by silver would be related to dissolution and internal oxidation. 

Apart from playing an important role in reducing many metal oxides (in some cases its use has been incorporated in commercial practice), hydrogen has also been exploited as a reductant for metal sulfides. However, the hydrogen reduction route for metal sulfides has not yet passed the laboratory development stage as the process involves interesting phyiscochemical principles. 

It is useful to treat the raffinate to recover the two acids it contains. The raffinate can first be subjected to a thermal treatment which decomposes it into insoluble metallic oxides, hydrofluoric acid and sulfuric acid. In a first scrubbing, sulfuric acid is recovered, whilst in a second stage, hydrofluoric acid is absorbed. When put into operation, this process not only causes a reduction in the amount of sludges but also leads to acid production for return to the process. 

CLC consists of two fluidised bed reactors, namely reducer and oxidiser. In the reducer reactor, fuel is fed along with the metal oxide containing oxygen, which is transferred from the metal oxide to the reactor as the combustion occurs (Figure 7). A flue gas containing over 99%v/v of C02 can be obtained by a simply condensation stage because of the fact that the exhaust gas at the reducer outlet is primarily formed by C02 and water vapour. This stream is then sent to further compression and permanent storage. 

In the second stage of the process, the metal oxide in a low-oxidation state is reoxidized by air to the original metal oxide in a high-oxidation state ... 

It has been proposed [92] that oxygen radicals may be formed in the stage of glycoxidation during the transition metal oxidation of protein enediol. 

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