Pickling process reactions

The corrosive effects to be considered (mainly simple corrosion of metals) are, as would be expected from the edible nature of foodstuffs which are not excessively either acidic or basic but which may contain sulphur, less severe than those often encountered with inedible materials containing reactive substances. The importance of corrosive efiects where foodstuffs are concerned lies not so much in the action of the foodstuffs on the metal involved as in the resultant metal contamination of the foodstuff itself, which may give rise to off-flavours, in the acceleration of other undesirable changes (by the Maillard reaction for example), and in the possible formation of toxic metallic salts. Metal ions generally have threshold values of content for incipient taste effect in different liquid foodstuffs. Except in the case of the manufacture of fruit juices and pickles, process plant failure through corrosion must be rare. Nevertheless all foodstuffs, particularly liquid ones, should be regarded as potentially corrosive and capable of metal pick-up which may be undesirable. 

CERNOX [Ceramic NOx] A process for destroying NOx by reaction with ammonia, catalyzed by a zeolite. Developed by Steuler Industrie Werke in the 1950s for treating vapors from nitric acid baths used for pickling stainless steel. The zeolite was developed for this process by Mobil Corporation and is still proprietary. The process was introduced to the market in 1982, and by 1988 100 units had been installed in Europe alone. 

F-S [Ferrous sulfate] A process for removing ammonia, hydrogen sulfide, and hydrogen cyanide from coke-oven gas by scrubbing with aqueous ferrous sulfate solution obtained from steel pickling. A complex series of reactions in various parts of the absorption tower yield ammonium sulfate crystals and hydrogen sulfide (for conversion to sulfur or sulfuric acid) as the end products. Developed in Germany by F. J. Collin A.G. 

Summary Chloroform can be prepared in a unusual manner by electrolyzing a solution of pickling salt and isopropyl alcohol in the presence of a small amount of sulfuric acid and potassium dichromate using graphite or lead electrodes. The process develops low yields of chloroform, and takes considerable amounts of time, but is effective at producing small amounts of chloroform. After the initial reaction, the mixture is extracted with benzene, toluene, or xylene, and the resulting solvent extracts are then combined, and then distilled to recover the chloroform. 

Hydrochloric acid is of great importance and is widely used in industrial processes. The largest use is in the metallurgical industry for the pickling of metals to remove oxide scales. Other uses include the production of inorganic halides from metals, oxides or carbonates, extraction of metals from ores in metallurgical processes, and various organic reactions. 

Pickhng inhibitors used in practice are seldom pure compounds. They are usually mixtures, which may be a byproduct, for example, of some industrial chemical process for which the active constituent is unknown. They are added to a pickling acid in small concentration, usually on the order of 0.01-0.1%. The typical effect of concentration of an inhibitor on the reaction between steel and 5% H2SO4 is illustrated [28] in Fig. 17.4, showing that, above a relatively low concentration, presumably that necessary to form an adsorbed monolayer, an additional amount of inhibitor has little effect on further reducing the rate. 

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