The silver process

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. 

As significant amounts of hydrogen are formed, it has long been assumed that formaldehyde is essentially formed by dehydrogenation of methanol, accelerated by the combustion of a large part of the liberated hydrogen. Recently, however, several authors explain the kinetics on the basis of direct interaction of methanol with oxygen. 

The reaction is carried out over a silver gauze or low surface supported catalyst at 600—700°C, indicating a very fast chemical reaction. This implies that determination of the intrinsic reaction rate in laboratory reactors is complicated by the interference of heat and mass transfer limitations. To avoid this problem, studies have been made at much lower temperatures, which in turn run the risk of being non-representative. 

A Langmuir—Hinshelwood type of model is suggested by Robb and Harriott [266] who studied the reaction at 420°C. They find that the intrinsic kinetics can be represented by 

The equation reflects dual site reversible adsorption. Methanol and formaldehyde compete for sites, while oxygen is dissociatively adsorbed on different sites. At a not-too-low oxygen pressure ( 0.01 atm) the coverage of the oxygen sites is complete and the equation reduces to 

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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. 

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. 

Most immersion silver in large production is applied in conveyorized equipment. The cleaner and microetch may use flood or spray application, but the silver solution is applied using flood. Because the silver process is very mild to the PCB materials, rinsing is very important to ensure that contamination from all prior fabrication steps is removed. Often, silver is apphed in part form after the PCB has been routed, depanelized, and electrically tested. This is to preserve the silver surface, which can be sensitive to handling, contamination, and fingerprints. In smaller-scale production and prototype manufacturing, silver can be apphed in vertical process tanks. 

C (Mond process). The silver-white metal is ccp. The metal is not tarnished by air but is attacked by acids (except cone. HNO3). It is resistant to Fj. It is used extensively in alloys, particularly in steels and cast iron and as a coinage metal. Used in glass (green) in catalysts (particularly for hydrogenation). Western world production 1981 662 000 tonnes. 

It is clear that the freight value depends on the type and amount of film and how the production is spread over time. Figure 3 shows 2 simulations of silver concentration in fixer for different processing regimes. In one case, 10 m of film is processed over 3 hours. In another case, 10 m of film is processed over 16 hours. It is clear that the silver level in the fixer not only depends on the amount of film processed, but also on the distribution of the working load. The simulated daily freight values are 44 and 16 mgW respectively. ... 

The process is designed such that virtually all silver is fixed in the first fixing step. The fixer in the first fixer section has a typical silver concentration of 7 g/1. The silver in the second fixer section originates from the carry-over from the first fixing section, and a typical silver concentration is 0,4 g/1. Hence, the silver carried over to the rinsing section will be substantially lower. 

Although the data for the silver halides suggest that silver(I) fluoride is likely to be more soluble than the other silver halides (which is in fact the case), the hydration enthalpies for the sodium halides almost exactly balance the lattice energies. What then is the driving force which makes these salts soluble, and which indeed must be responsible for the solution process where this is endothermic We have seen on p. 66 the relationship AG = — TAS and... 

Ethyl a-naphthylacetate is prepared as follows. To a solution of 10 g. of the diazo ketone in 150 ml. of ethanol at 55-60°, add a small amount of aslurry of silver oxide, prepared from 10 ml. of 10 per cent, aqueous silver nitrate and stirred with 25 ml. of ethanol. As soon as the evolution of nitrogen subsides, introduce more of the silver oxide and continue the process until all the slurry has been added. Reflux the mixture for 15 minutes, add 2-3 g. of decolourising carbon, filter and evaporate the alcohol on a water bath. Distil the residue and collect the ethyl a-naph-thylacetate at 176-178°/ 1 mm. the yield is 9 g. 

Selectivity Due to the chemical nature of the precipitation process, precipitants are usually not selective for a single analyte. For example, silver is not a selective precipitant for chloride because it also forms precipitates with bromide and iodide. Consequently, interferents are often a serious problem that must be considered if accurate results are to be obtained. 

Silver Catalyst Process. In early formaldehyde plants methanol was oxidized over a copper catalyst, but this has been almost completely replaced with silver (75). The silver-catalyzed reactions occur at essentially atmospheric pressure and 600 to 650°C (76) and can be represented by two simultaneous reactions ... 

The requirements for the material of constmction are the same as for the silver catalyst process except the use of alloys to protect the catalyst is not as important. 

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