Chromium economic importance

However, the object of this section is to outline the principles which govern atmospheric corrosion, and the emphasis is placed on metals whose atmospheric corrosion is of economic importance. These include iron and steel, zinc, copper, lead, aluminium and chromium. 

Basic lead silicochromate (4(PbCr04 PbO) -t 3(Si02 4 PbO)) reached certain industrial importance. It was developed to replace red lead. The pigment is a so-called core pigment, in which the active pigment substance (PbCr04) is precipitated onto an inert core (Si02). Because this anticorrosive contains both lead and hexavalent chromium, it has lost its previous economic importance as well [5.53, 5.56]. 

Because of its economic importance, far more work has been done with phosphate than with other ions. However, continuing reaction occurs with many other ions and is the norm rather than the exception. Some examples are molybdate [68], fluoride [69], sulfate [70,71], arsenate [72], zinc [73] and chromium, cadmium and mercury [74]. It has also been shown that incubating copper with soil decreases its subsequent availability to plants [75]. 

Substantial problems of economic importance due to deficiencies of chromium, nickel, tin and vanadium are unlikely, and the issue of their essentiality is largely of academic importance. Such elements must function in biological systems in exceptionally low concentrations and for practical purposes, can be regarded as non-essential. 

There are many methods for selectively oxidizing an alcohol to the corresponding aldehyde. Of course you have to think about the stability of your molecule when you decide which procedure to choose, but ecological and economic aspects are also very important. Chromium-free oxidations such as Swern conditions or Dess-Martin periodinane (DMP) are preferred over the classic methods that employ Jones reagent, Collins reagent or PCC/PDC. For the mechanism of the here used Swern protocol, see Chapter 5, p. 86. 

In the case of industrial wastewater, several other compounds can also be involved, such as sulphide, hexavalent chromium or organic molecules. This last group of compounds is considered hereafter with regard to their economic and/or environmental importance. For example, Fig. 5 presents several specific compounds encountered in refinery and petrochemical wastewater. Thus, phenol, EPA (ethylpropylacrolein), TBC (tertiobutylcatechol), NMP (/V-methylpyrolidone) and nitrite can be detected in effluents or process water [21], Moreover, the estimation of complementary aggregate parameters, such as total oxygen demand (TOD), is possible from the estimation of one of the previous organic compounds [3] (Fig. 6). 

The oxides of carbon are poisons to the iron-based ammonia synthesis catalyst and mnst be removed from the synthesis gas before use. An important reaction that affected the economics of the manufacture of synthesis gas was the conversion of carbon monoxide into equal volumes of hydrogen and carbon dioxide by reaction with steam. This procedure, now cotrunonly referred to as the water-gas shift reaction required a catalyst based on iron and chromium oxides. 

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