Metal cost problems

Corrosion is the process whereby a metal deteriorates. Corrosion caused by atmospheric oxygen is a widespread and costly problem. About one-quarter of the steel produced in the United States, for example, goes into replacing corroded iron at a cost of billions of dollars annually. Iron corrodes when it reacts with atmospheric oxygen and water to form iron oxide trihydrate, which is the naturally occurring reddish-brown substance you know as rust, shown in Figure 11.17 ... 

Abrasion of the mining and grinding equipment can also be a costly problem. Excess abrasion is caused by high concentrations or large particle size of hard minerals such as quartz. During combustion, chlorine in the coal may form hydrochloric acid and contribute to corrosion of metal surfaces in coal-fired boilers. 

Glass is overwhelmingly the preferred reinforcing fibre in marine applications although aramid (e.g. Kevlar ) and carbon are used where cost permits. Where carbon is used, there is a theoretical possibility of dangerous corrosion cells being set up with exposed carbon fibres electrically connected to metals. This problem is easily avoided. 

Eddy-current non-destructive evaluation is widely used in the aerospace and nuclear power industries for the detection and characterisation of defects in metal components. The ability to predict the probe response to various types of defect is highly valuable since it enables the influence of particular parameters to be studied without recourse to costly and time consuming experiments. The solution of forward problems is also essential in the process of inverting experimental data. 

The most common method of converting iron ore to metallic iron utilizes a blast furnace wherein the material is melted to form hot metal (pig iron). Approximately 96% of the world s iron is produced this way (see Iron). However, in the blast furnace process energy costs are relatively high, pollution problems of associated equipment are quite severe, and capital investment requirements are often prohibitively expensive. In comparison to the blast furnace method, direct reduction permits a wider choice of fuels, is environmentally clean, and requires a much lower capital investment. 

Mercury from these accumulated wastes is generally best recovered by total degradation in stills, where metallic mercury is condensed and collected. The recovery costs are amply compensated by the value of the metal recovered. Moreover, disposal problems are either eliminated or severely diminished. 

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

Production processes are given in Table 12. Electrolytic processes are dominant because of lower cost and fewer environmental problems. Production of slab zinc in Tennessee commenced in 1978 when Jersey Miniere Zinc Company began operation of its new 90,000 t/yr electrolytic smelter at Clarksville. Today, Tennessee is the leading producer of slab zinc. Although the U.S. demand for zinc metal in the past 16 years has increased by 47%, smelting capacity has declined by almost 50%. Plants closed because they were obsolete and could not meet environmental standards or obtain sufficient concentrate. Consequendy, slab zinc has replaced concentrates as the principal import form. This situation is expected to prevail up to the year 2000 (69-71). 

Contaminants in fuels, especially alkali-metal ions, vanadium, and sulfur compounds, tend to react in the combustion zone to form molten fluxes which dissolve the protective oxide film on stainless steels, allowing oxidation to proceed at a rapid rate. This problem is becoming more common as the high cost and short supply of natural gas and distillate fuel oils force increased usage of residual fuel oils and coal. 

If the critical impurities are known, then only a selected list of elements need to be examined, with some improvement in the cost effectiveness of the analysis. However, the list of elements to be included in the qualification analysis is often historical and related to the limitations of the analytical methods previously used for qualification rather than for technological reasons related to the end use of the metal. As a result, problems in application can arise for no obvious reason. The time and cost of extending the impurity list for GDMS analysis to include essentially all elements is minimal, considering the additional information gained. 

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