Thermal reduction

In a different attempt made by Zhang et al. to fabricate Au NPs-graphene composites via redox reaction between Au salt solution and GO nanosheets, an aqueous solution of HAuCl was mixed with GO and subsequently the solution was heated at 84°C for 15 min. The formation of Au NPs-rGO composites and the variation of their morphology with concentration of the reactants were executed using transmission electron 

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Thermal Reduction. Magnesium metal can also be formed by the thermal reduction of magnesium oxide with a reactive metal, such as siUcon [7440-21-3] which forms a stable oxide. 

Three basic processes exist for the thermal reduction of magnesium oxide the Pidgeon process, the Magnetherm process, and the BoKano process. 

Pldgeon Process. The Pidgeon (46—49) process (Fig. 6) was the first commercial thermal reduction process usiag siUcoa, and was developed ia the 1940s. This process is used by Timminco (Haley, Oatario, Canada) and Ube Industries (Japan). The overall reaction for this process is... 

Silicon Tetrachloride. Most commercially available sihcon tetrachloride is made as a by-product of the production of alkylchlorosilanes and trichlorosilane and from the production of semiconductor-grade sihcon by thermal reduction of trichlorosilane. 

Sir Humphry Davy first isolated metallic sodium ia 1807 by the electrolytic decomposition of sodium hydroxide. Later, the metal was produced experimentally by thermal reduction of the hydroxide with iron. In 1855, commercial production was started usiag the DeviUe process, ia which sodium carbonate was reduced with carbon at 1100°C. In 1886 a process for the thermal reduction of sodium hydroxide with carbon was developed. Later sodium was made on a commercial scale by the electrolysis of sodium hydroxide (1,2). The process for the electrolytic decomposition of fused sodium chloride, patented ia 1924 (2,3), has been the preferred process siace iastallation of the first electrolysis cells at Niagara Falls ia 1925. Sodium chloride decomposition is widely used throughout the world (see Sodium compounds). 

Thermal Reduction. MetaUic sodium is produced by thermal reduction of several of its compounds. The eadiest commercial processes were based on the carbon reduction of sodium carbonate (46—49) or sodium hydroxide (1,8,50) ... 

A number of other thermal reductions are described in the Hterature (8), but it is doubtful that any have been carried out on commercial scale. 

A process development known as NOXSO (DuPont) (165,166) uses sodium to purify power plant combustion flue gas for removal of nitrogen oxide, NO, and sulfur, SO compounds. This technology reHes on sodium metal generated in situ via thermal reduction of sodium compound-coated media contained within a flue-gas purification device, and subsequent flue-gas component reactions with sodium. The process also includes downstream separation and regeneration of spent media for recoating and circulation back to the gas purification device. A full-scale commercial demonstration project was under constmction in 1995. 

This reaction occurs in a vacuum and the gaseous metal is condensed in a cooler part of the apparatus. AH strontium metal is produced commercially by the thermal reduction process in aUoy steel retorts. 

Barium is prepared commercially by the thermal reduction of barium oxide with aluminum. Barium metal is highly reactive, a property which accounts for its principal uses as a getter for removing residual gases from vacuum systems and as a deoxidiser for steel and other metals. 

These reactions are thermodynamically unfavorable at temperatures below ca 1500°C. However, at temperatures in the range from 1000 to 1200°C a small but finite equiUbrium pressure of barium vapor is formed at the reaction site. By means of a vacuum pump, the barium vapor can be transported to a cooled region of the reactor where condensation takes place. This destroys the equiUbrium at the reaction site and allows more barium vapor to be formed. The process is completely analogous to that used in the thermal reduction of CaO with aluminum to produce metallic calcium (see Calcium AND CALCIUM alloys). 

Manufacture. An outline of the black ash process for BaCO manufacture is shown ki Figure 1. It is from the appearance of the product exiting the thermal reduction step that the process derives its name. 

Thermal Reduction. Thermal reduction is usually accompHshed ki a high temperature countercurrent rotary kiln. "Hot zone," a region near the kiln spik, temperature is usually controlled at 1100—1200°C. The reaction rate has been shown to be only slighdy lower at 1050°C than at 1130°C (9). About 6% of the feed BaSO remains unreacted after 30 min at 1050°C. Reaction completion is approached ki less than 10 min at 1100°C (10). 

Only two processes for the manufacture of Be are of industrial importance (i). the thermal reduction of BeF2 using Mg, and (ii) the electrolysis of BeCl2 in a molten chloride electrolyte. Direct reduction of the oxide is ineffective because of its thermodynamic stability only Ca reduces BeO to the metal unfortunately, Ca cannot be used since it forms a stable intermetallic compound with Be, BejjCa. 

Other methods include thermal reduction of BeO, using either Ti or Zr, thermal reduction of BeCl2, using either Li, Na, K, Mg or Ca, and thermal decomposition of... 

Metallic Mg is produced industrially using both electrolytic and thermal reduction methods. The electrolytic processes differ primarily in the choice of electrolyte—anhyd MgClj, partially hydrated MgClj x HjO and MgO. The more important thermal reduction processes use FeSi, A1 alloys or C as reducing agents. 

The most widely employed thermal reduction process for preparing Mg metal uses PeSi as reducing agent. Mixtures of the substrate, usually calcined dolomite (i.e., MgO, CaO) and PeSi are fabricated into briquettes with a hydrocarbon binder and loaded into Ni-Cr steel (15/28) retorts. After evacuation the retort is subjected to a temperature gradient Mg distills from the hot mixture (at 1150°C) and high-purity Mg crystals collect at the water-cooled end of the retort ... 

Aluminum or one of its alloys (Al-Si, Al-Ca-Si, Al-Ca-Pe-Si) can be substituted for PeSi in this process. The alternative thermal reduction process is based on the reduction of pure MgO by carbon in an arc furnace at temperatures above 1800°C. Since the reaction ... 

Electrolytic Mg is less pure than Mg obtained by thermal reduction methods and is purified commercially by sublimation under vacuum, a process inherent in thermal reduction. Typical analyses for crude electrolytic Mg (assay 99.8%), for sublimed material (assay 99.95%) and for twice sublimed material (assay 99.99%) are collected in Table l. Sublimation reduces most impurity levels (by 10 ) but leads to increased Zn content. 

In the principal thermal reduction process, CaO, obtained by calcination of high-purity CaCOj, is reduced with A1 or one of its alloys ... 

Briquettes of CaO with 5-20% excess powdered A1 are heated under vacuum to 1170°C in a Ni-Cr steel (15/28) retort in which the Ca vapor, produced by reduction of solid CaO by A1 vapor, is condensed in a zone at 680-740 C. Any Mg impurity is condensed in a zone at 275-350°C a mixture of the two metals condenses in an intermediate zone. The A1 content of the product can be reduced by passing the metal vapor, before it condenses, through a vessel filled with solid CaO. The adaptation of the FeSi thermal reduction process for Mg production (see 7.2.3.2.1) to Ca manufacture has also been described but is not economically viable in comparison with the above process. The thermal reduction of CaO with carbon has been proposed as for Mg production, however, the reversibility of the equilibrium ... 

The relatively impure crude Ca obtained from both thermal reduction and electrolytic sources (97-98%) is distilled to give a 99% pure product. Volatile impurities such as the alkali metals are removed in a predistillation mode at 800°C subsequent distillation of the bulk metal at 825-850°C under vacuum removes most of the involatile impurities, such as Al, Cl, Fe and Si. The N content is often not reduced because of atmospheric contamination after distillation. Unfortunately, these commercial methods have no effect on Mg, which is the major impurity (up to 1 wt%). Typical analytical data for Ca samples prepared by electrolysis, thermal reduction (using Al) and distillation are collated in Table 1. 

Metallic Sr can be prepared by methods similar to those used for Ca manufacture (sec 7.2.3.3.1). Thus thermal reduction of SrO using A1 and electrolysis of anhyd SrCU both yield pure Sr. Reduction of SrO by CH4 is also successful on a pilot scale. Metallic Ba is produced by thermal reduction of BaO by Al alternative reducing agents arc Na, Mg, Si and FeSi. Electrolysis of fused anhydrous halides (c.g., BaCK) is not applicable since the reaction yields a subhalide rather than the pure metal. 

A laboratory adaptation of the commercial thermal reduction method utilizes reduction of SrO (BaO) by A1 powder ... 

The thermal reduction by Np(V) is a slow reaction of complex kinetics, but it proceeds readily under the influence of light with kinetics ... 

All of the preparation procedures for the oxide promoted catalysts (T-O shared one common feature, heat-treatment of the oxide impregnated Ft on carbon catalysts in an inert atmosphere at elevated temperature, usually around 900 C. If an "alloy" phase of Ft with the metal of the metal oxide is formed by this heat-treatment, thermal reduction would have to occur with carbon as reducing agent, e.g. 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Based on the molecular design of precursor metal complexes, the solvent-free controlled thermolysis of metal complexes may cause the thermal reduction and simultaneous attachment of organic moiety on the growing metal nuclei and give us a solution of the defects of ordinary... 

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