Control of metal dusting

Building on the understanding discussed in the previous sections, one can consider three approaches for controlling metal dusting corrosion. One approach is based on alloy compositions that have the intrinsic ability to form protective 

21 SEM cross-section of 446 steel after metal dusting corrosion at 650°C showing protective surface oxide and intermetallic precipitates 

1 Alloys with intrinsic metal dusting resistance 

As discussed in Section 5.5, chromia forming alloys that are widely considered for use in metal dusting environments suffer from the fact that in many cases a fully protective surface oxide is unable to form, or defected regions in the oxide are unable to reheal themselves for kinetic reasons. While high chromium ferritic alloys exhibit a better tendency for protective chromia film formation, the poor mechanical properties of such alloys render them unsuitable as bulk 

23 Parabolic rate constants for the growth of MnO, MnCr204 and Cr203 as a function of temperature. 

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Cement plants in the United States are now carehiUy monitored for compliance with Environmental Protection Agency (EPA) standards for emissions of particulates, SO, NO, and hydrocarbons. AH plants incorporate particulate collection devices such as baghouses and electrostatic precipitators (see Air POLLUTION CONTROL methods). The particulates removed from stack emissions are called cement kiln dust (CKD). It has been shown that CKD is characterized by low concentrations of metals which leach from the CKD at levels far below regulatory limits (63,64). Environmental issues continue to be of concern as the use of waste fuel in cement kilns becomes more widespread. 

Fabric filters can be more costly to operate and maintain than electrostatic precipitators, cyclones, and scrubbers however, fabric filters are more practicable for filtration of specific dusts. For example fabric systems are the typical control method for toxic dusts from insecticide manufacturing processes, salt fumes from heat treating, metallic fumes from metallurgical processes, and other applications. Any other control method may not be as efficient, nor economically feasible for such applications. 

Air pollution control (APC) dust and sludge Metal dusts (consisting of iron particulate, zinc, and other metals associated with the scrap, and flux) Slag Kish... 

Cleaned steel products (e.g., sheets, plates, bars, pipe) Process wastewater containing mill scale, oils, other pollutants, and low levels of metals Wastewater sludge Air pollution control (APC) dust Spent pickle liquor (K062)... 

X-ray fluorescence can be used to analyse all types of samples. Its applications are numerous, whether in research and development or in quality control of production. Initially, X-ray fluorescence was used in industries that treat metals of primary fusion or alloys and, more generally, in the mineral industry (for use one ceramics, cements, steel, glass, etc.). Because of the ease of use of common X-ray fluorescence instruments, its scope of application has expanded into other areas the photographic industry and semi-conductors (for impurity control in silicon chips), the petroleum industry, geology, paper mills, gas analyses (such as nitrogen), toxicology and environmental applications (dust, fumes from combustion, heavy metals, and dangerous materials in waste such as Pb, As, Cr, Cd, etc.). 

Many different chemical treatment systems have been developed to reduce the leachability of lead and cadmium compounds in flue dust. These systems usually rely on stabilization/solidification, adsorption, chemical reduction, or pH control. Chemical reduction employing the use of metallic iron has been successful in reducing the leachability of lead to below EP-Toxicity levels. Adding a 5 percent by weight dose of iron filings to cupola furnace emissions control sludge, for instance, reduced lead leaching from 28.6 mg/1 to less than 0.1 mg/1 (Stephens 1984). 

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