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Metallurgy & Alloys

Gold as nuggets Why gold is available in native state as nuggets? Gold is the noblest metal among all the metals in periodic table. It has high reduction potential and can not be tarnished through oxidation. Due to these reasons Gold is available as nuggets, in its native (elemental) form. It often occurs as an alloy with other noble metals such as platinum and silver. Recently a monster gold nugget weighing 3.66kg was discovered in the Victoria Gold Fields, Australia.

Learning Objectives

After completing the topic, the student will be able to:

  • Define metallurgy and explain the terms minerals and ores.
  • Discuss the processes, concentration and benefaction.
  • Give the details of the processes, calcination, roasting, refining, etc.
  • Describe various extraction and refining methods of metals from their chief ores.
  • Understand the process of corrosion and suggest suitable methods for prevention of corrosion.
  • Define alloy and list alloys of some metals and their uses.
Metal ores Metal ores A variety of minerals – galena, calcite, pyrite, azurite, orpiment and rhodochrosite. Galena (the gray cubes) is the most important ore of lead. Pyrite, popularly known as fool's gold, is an important source of sulfur. The blue azurite is an ore of copper, and the yellow orpiment contains arsenic. The white calcite and the pink rhodochrosite are both carbonate minerals. Rhodochrosite gets its pink color from manganese.
Metallurgy

It is a science and technology that deals with the study of properties of metals and alloys , extraction and processing of metals , and producing new products from metals for consumer use.

Only a few metals gold and platinum are two examples appear in nature in metallic form. Deposits of these natural, also known as native, metals are quite rare. For the most part, metals are found in nature as chemical compounds. Iron, for example, is most frequently found as iron oxide, Fe2O3, and copper is found as chalcopyrite, CuFeS2.

Geologic deposits containing relatively high concentrations of metal – containing compounds are called ores. The metals industry mines these ores from the ground, as shown in the Figure, and then processes them into metals. Although metal – containing compounds occur just about everywhere, only ores are concentrated enough to make the extraction of the metal economically feasible.

Metal Salts and Ores

Metal ions bond with only five major types of negatively charged ions, shown in the Figure. Consequently, metal–containing compounds are classified according to which type of negative ion they contain. Iron oxide is classified as an oxide and chalcopyrite is classified as a sulfide.

Halides, such as sodium chloride and magnesium chloride, are commonly referred to as salts. They have good solubility in water and so are readily washed away by the action of either surface water or groundwater. Most of these and other water–soluble metal–containing compounds therefore end up in the ocean. These compounds are recovered by evaporating seawater.

Alternatively, water–soluble compounds may end up in land basins, such as the Bonneville salt flats of Utah, where they are readily mined. In some regions, such as along the Gulf of Mexico, vast deposits of halides remain undissolved hundreds of meters below the surface, where groundwater cannot reach.

The compounds in these deposits tend to be very pure, which makes deep mining excavations like the one shown in the below Figure worthwhile.

In contrast to halides, compounds containing carbonate, phosphate, oxide or sulfide ions tend to have relatively low solubilities in water. Hence, their ores tend to stay put and are found in more diverse geologic locations. The form in which a metal is most likely to be found in nature is a function of its position in the periodic table.

Metals of group 1 tend to be found mostly as halides, group 2 metals as carbonates and group 3 metals and lanthanides mostly as phosphates. Most metals from groups IV to VIII along with aluminum Al, and tin Sn, tend to be found as oxides and most metals from groups IX to XV along with molybdenum Mo, tend to be found as sulfides.

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