American process

The direct process is noted for its simplicity, low cost, and excellent thermal efficiency. It consists of an initial high-temperature reduction (1000-1200 °C) of a zinc-containing material (as oxide), the reducing agent being coal. Reduction takes place according to Boudouard s equations  

The zinc vapor and the CO gas are then oxidized to zinc oxide and carbon dioxide above the reaction bed or at the furnace exit. Various zinc-containing materials are used, e.g. zinc concentrates (mainly in China), metalhzation residues, skimmings from casting furnaces, oxidic residues from the indirect process and above all zinc ashes from hot dip galvanization. The ash from galvanization must first be treated to remove chloride and lead by heating at ca. 1000 °C in rotary kilns. 

Only rotary kilns are now used in the EU for the direct process the use of static furnaces has been discontinued. The zinc content of raw materials is between 60 and 75%. There are two types of rotary kiln  

A rotary kiln that is shorter (5 m) and has a larger diameter (ca. 3 m).Charging is continuous, but the dezincified residues are removed batchwise. 

In both cases, operating conditions are controlled to obtain a high yield and to give the required particle shape and size. Provided no contamination is introduced, chemical purity is determined solely by the composition of the raw materials used. 

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The direct American process is basically a metallurgical operation. Ziac oxide ores are reduced usiag coal (qv) ia a rotary kiln at temperatures of 1000—1200°C to 2iac metal. At these temperatures the produced 2iac metal evaporates and is oxidi2ed with air to ZnO ia the upper part of the kiln. 

Leaded Zinc Oxide. Oxides containing more than 5 wt % basic lead sulfate are classified as leaded and are made ia the American process from high lead materials, usually lead sulfide mineral, or by blending ziac oxide and basic lead sulfate. There is only one manufacturer ia the United States and the product contains 20—28 wt % basic lead sulfate. Leaded oxides are used only ia mbber ia the United States. 

Zinc oxide is made either by the oxidation of the metal in oxygen (the indirect, IP, or French process), by the direct decomposition of zinc ores in air (the direct or American process) or by the thermal decomposition of zinc salts (TD zinc oxide). IP zinc oxides differ from TD zinc oxides in that their surfaces do not contain absorbed water. Also, whereas TD zinc oxide reacts with plain eugenol, IP zinc oxide hardly reacts unless activated by an acetic add or zinc acetate accelerator (Table 9.2). 

Both these methods give zinc oxides of low activity. Zinc oxide from the American process can have a varying sulphur content, dependant upon the ore s source, and unless known and allowed for, this can affect the compound vulcanisation rate. 

French process material, in general, tends to have a blockier particle shape with a relatively narrow particle size range in comparison with the American process product. The French process oxides also exhibit a finer particle size and hence have a higher surface area. 

American process by amalgamation.—This method, invented in Mexico by Bakthqlomeo de Medina in... 

This small consumption of mercury compared with that of the American process arises from the fact that in the latter the chloride of silver is reduced by the mercury, and the chloride of mercury thus formed passes wholly into the residues, whereas in the Saxon process this reduction is effected by the iron. 

Around 1850, S. Wetherill of the New Jersey Zinc Company perfected a roasting furnace in which a grate was charged with coal and then covered with a mixture of zinc ore and coal. The zinc was reduced by the partial combustion of the coal and reoxidized at the furnace exit (direct or American process). These furnaces were subsequently improved but are now no longer used. During the second half of the 19th century, the use of ZnO in rubber was introduced to reduce the time needed for vulcanization. The discovery of the first organic accelerators for vulcanization in 1906 added to the importance of ZnO, which acts as an activator in these materials. 

US 4,011,279 (American) Process for making General Electric Block copolymer... 

Campanella, O. H., Popplewell, L. M., Rosenau, J. R., and Peleg, M. 1987. Elongational viscosity measurements of melting American process cheese. J. Food Sci. 52 1249-1251. 

Italian Method. During the war. Professor Contardi proposed a method of preparation much more simple than the American process just described. In studying a new process for manufacturing diphenylamine, he observed that the hydrochloride of this base is completely dissociated into hydrochloric acid and diphenylamine when heated to slightly over 100° C. He studied the possibility of using this reaction to prepare phenarsaane chloride by starting from arsenious oxide and diphenylamine hydrochloride, instead of arsenic trichloride and diphenylamine. The equation of this reaction is as follows ... 

This process differs from the American method more particularly in saving a considerable proportion of the hydrochloric acid (more than two-thirds) and of the arsenious oxide, and also makes it unnecessary to prepare arsenic trichloride. Moreover, all the difficulties attendant on the necessity for utilising or disposing of the large quantities of arsenical products which are invariably obtained in the American process are obviated. 

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