Metallurgy, extractive

Production of a metal is usually achieved by a sequence of chemical processes represented as a flow sheet. A limited number of unit processes are commonly used in extractive metallurgy. The combination of these steps and the precise conditions of operations vary significantly from metal to metal, and even for the same metal these steps vary with the type of ore or raw material. The technology of extraction processes was developed in an empirical way, and technical innovations often preceded scientific understanding of the processes. 

The scientific basis of extractive metallurgy is inorganic physical chemistry, mainly chemical thermodynamics and kinetics (see Thermodynamic properties). Metallurgical engineering reties on basic chemical engineering science, material and energy balances, and heat and mass transport. Metallurgical systems, however, are often complex. Scale-up from the bench to the commercial plant is more difficult than for other chemical processes. 

Eurther progress was made in the eighteenth and early nineteenth centuries. Many metals were discovered upon the development of experimental chemistry. The modem metallurgical industry was bom with the invention of steelmaking in 1856 (see Steel). Industrial processes for making zinc (see Zinc and zinc alloys), aluminum (see Aluminumand aluminum alloys), and copper followed before the end of the nineteenth century. These processes made possible the industrial revolution and the development of an industrial society relying heavily on the use of metals. 

AH metals come originally from natural deposits present in the earth s cmst. These ore deposits result from a geological concentration process, and consist mainly of metallic oxides and sulfides from which metals can be extracted. Seawater and brines are another natural source of metals, eg, magnesium (see Chemicals frombrine Magnesium and magnesium alloys Ocean raw materials). Metal extracted from a natural source is called primary metal. 

Scrap is also an important raw material for the metallurgical industry. The metal produced by a recycling (qv) process is called secondary metal. 

Extraction OF metals from minerals1-7 has many points in common with corrosion control. Redox chemistry is applied to wrest metals from Nature, just as it is used to prevent her reclaiming them through corrosion. There is an additional factor to be considered, however. Most ores contain only minor amounts of the metal of interest, and, before final reduction or refining, the desired metal or its compounds must be concentrated and separated from other metals that may be present. 

S seeks to join M S2-lattice of sulfide, not Al/Si-O lattice of gangue 

Hydrometallurgical methods4,5 use reactions in aqueous solution (often involving metal complex formation) to concentrate and/or separate the metal ions of interest. A commercially important example is the heap leaching of low-grade copper ores with acid. 

Gold particles in crushed rock can be brought into solution as Au(CN)2 ion by leaching the rock with aerated sodium cyanide solution (see Exercise 15.2) the aluminosilicates remain undissolved  

The reaction is rather slow, requiring several days, after which the gold may be recovered by electrolysis6 (Section 15.7) or by reaction with zinc7 ( cementation )  

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Young, R. S. Chemical Analysis in Extractive Metallurgy, Griffen London, 1971, pp. 302-304. 

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