Hazardous area Classifying Equipment is a rigorous method of determining where an explosive environment may be present. The codes and standards used in this process provide guidance for selecting, building, and installing electrical equipment in that area. An explosive atmosphere can be created by a gas or vapor, dust, or even fibers. Methane-based hazards dominate the gas-fired generation sector, whereas spontaneous combustion of coal dust plagues plants fired by that fuel. The different zones defined by codes for classifying particular hazardous areas are listed and discussed in the online supplement to this article.

The probability that any hazard exists in combustible concentration is determined by the specifics of the plant or system under consideration. For example, a natural gas vent line is much more likely to contain such a hazard than a lube oil line—unless, of course, the oil line's flanged joint is leaky. Finally, plant design also must protect against auto-ignition of combustible substances. A good example of this type of hazard is a flammable gas coming into contact with a hot surface. Codes define various temperature classes to guide designers as they specify equipment.

The National Electrical Code (NEC), NFPA 70, defines hazardous (classified) areas as those where fire or explosion hazards may exist due to the presence of flammable gases, vapors, or liquids, combustible dust, or ignitable fibers. Electrical area classification (EAC) is the process of determining the existence and extent of hazardous locations in a facility containing any of those substances. The result of the process is usually called the EAC of the facility. In the context of electrical equipment, the following terms—area classification, hazardous locations, hazardous (classified) locations, and classified areas—are all synonymous with EAC. In the following guidelines, the term EAC is used to avoid confusion with other nonelectrical classifications.

Once an area has been classified, the NEC provides very specific and stringent requirements about the electrical equipment and associated wiring that can be installed within that area. The requirements are intended to prevent electrical equipment from being the ignition source for a flammable mixture. Accordingly, the installation itself must be explosion-proof.

The formation of dioxins and furans is unavoidable due to the use of halogenated flame retardants, which presents environmental problems; thus, off-gas treatment is a prerequisite. Compared with pyrometallurgical processes, Hydrometallurgy Equipment processes offer a relatively low capital cost, reduced environmental impact and high metal recoveries. These processes are relatively suitable for small-scale applications. These attributes make the hydrometallurgical process a potential alternative for the treatment of waste EEE. Hydrometallurgical processes involve the dissolution of metals in alkaline or acid medium. Several studies have reported the use of nitric acid (HNO3), HCl, sulfuric acid (H2SO4) and aqua regia for the recovery of metals from waste PCBs. Reagents such as cyanide, halide, thiosulfate, and thiourea have also been commonly used for the recovery of precious metals. Most of the above-mentioned reports used powdered/pulverized waste PCBs for metal recovery. However, little is known about the use of large pieces of PCBs for the recovery of metals.

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