Silvering process

In contrast to the silver process, all of the formaldehyde is made by the exothermic reaction (eq. 23) at essentially atmospheric pressure and at 300—400°C. By proper temperature control, a methanol conversion greater than 99% can be maintained. By-products are carbon monoxide and dimethyl ether, in addition to small amounts of carbon dioxide and formic acid. Overall plant yields are 88—92%. 

Recovery Process. In past years iodine was recovered at Long Beach, California from oil field brine and from natural brines near Shreveport, Louisiana (36,37). The silver process was used. Silver nitrate reacts with sodium iodide to precipitate silver iodide. Added iron forms ferrous iodide and free silver. The ferrous iodide then reacts with chlorine gas to release free iodine. After 1966, the silver process was replaced with the blowing-out process similar to the bromine process. 

M. C. Euerstenau andj. L. Hendrix, eds.,Mdvances in Gold and Silver Processing Proceedings, Society for Mining, Metallurgy, Exploration, Inc.,... 

Chloride Cl Interferes with silvering processes and increase TDS. 

Limiting currents measured for a deposition reaction may be excessively high due to surface roughness formation near the limiting current. Rough deposits in the case of copper deposition have been mentioned several times in previous sections, since this reaction is one commonly used in limiting-current measurements. However, many other metals form dendritic or powdery deposits under limiting-current conditions, for example, zinc (N lb) and silver. Processes of electrolytic metal powder formation have been reviewed by Ibl (12). 

E. Brinckman, G. Delzenne, A. Pootand j. Willems, Unconventional /waging Systems, Focal Press (1978). This short book provides a good account of diazo imaging systems, set in the context of other non-silver processes. 

The silver process is the older one and is still used in many formaldehyde manufacturing plants today. Yields of about 90 mol. % are reported, and combustion to carbon dioxide and water is the main side reaction. 

The diazotype process is an example of a non-silver system with wide commercial application. In general, it is used in areas which require a monochrome or neutral high contrast image rather than a full colour system. For this reason, the image hue is not as critical, except for aesthetic reasons, as in the three-colour silver processes. Diazotype materials are primarily used in making direct reproductions of written, typed or drawn materials, particularly in the drafting, engineering and architectural professions. 

Production of formaldehyde as an aqueous solution, for handling convenience, using the silver process gives high yields [34]. In this process, air and vaporized methanol are combined with steam and recycled gas, and then are passed over hot silver grains at ambient pressure. Methanol is converted to formaldehyde by partial oxidation and by reduction at a temperature of 600°C. 

The difficulty for secondary plants is that, whilst the de-silverizing process (outlined in Section 15.3) can be used for the removal of low levels of silver in secondary lead bullion (i.e., up to 0.01 wt.%), the same amount of zinc must be added as for primary bullion that contains up to 0.5 wt.% silver. With this method, silver cannot be removed economically from recycled lead, as the cost of zinc alone... 

Speculum metal (Latin speculum, a mirror) or white bronze contains 2 of copper and i of tin. It is whiter even than tin, extremely brittle, and takes a high polish. It was used in Roman days for making mirrors and in more recent times found application in reflectors for telescopes. Later it was, of course, replaced by the well-known silvering process (p. 116). 

Uhde Inventa-FIscher Formaldehyde Methanol Silver process 60 1989... 

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