Tenor of ore

Tenor of ore states that the lowest permissible metallic deposit in an ore. 

• The lowest amount of metal that ore can contain.
• The Higher the content of metal, the better the tenor is.
• It is generally expressed in % age of metal.
• Any ore must contain some percentage of metal in it.

An ore is a naturally occurring solid material from which a metal can be extracted chiefly and profitably.

Types of Ore Deposit

• Magmatic
  Crystallisation of minerals within a body of magma
• Hydrothermal
  Hot fluids released as magma cools, minerals precipitate from fluids

Sedimentary

Precipitation of minerals from a lake or ocean water

Placer Deposits

Deposition of metals in a river or stream

Residual

Minerals produced by weathering and chemical reaction with oxygen and water. For example, commercial ores of iron consist of grade

• High grade ore – Fe content > 65%
• Average and medium grade ore – Fe content 62-64%
• Low grade ore – Fe content < 52%

Places Ore is Found

• Hematite ( 60 – 70%)
  Dharma Sedimentary rocks, Suddhat sedimentary rocks
• Magnetite ( 60 – 65%)
  Igneous or metamorphic
• Lignite (5% Fe)
  Danuta series, Narwhal, Kangra valley, Mizar

1. The grade of ore is the concentration of the desired material it contains
2. Used interchangeably with tenor
3. High grade, low grade and medium grade.

Among metals, aluminium is the metal that is most widely available. It is the third most widely found element in the earth’s crust (8.3% approx. by weight). It is a significant component of many igneous minerals, which also includes mica and clays. Many gemstones are impure forms of Al2O3,  and the impurities range from Cr (in ‘ruby’) to Co (in ‘sapphire’). The second most widely found metal in the earth’s crust is iron. It forms various compounds, and its multiple uses make it very important.

Water is important in the formation of most sedimentary rock. Water plays a major role  in forming  the minerals in chemical sedimentary rocks. Detrital sedimentary rock is a collection of grains that are produced by weathering and erosion by water.  There are several special properties which make water an incredibly unique substance that is integral to sediment production. 

Types of Ore

There are mainly four types of ore: oxides, carbonate, sulphide, and halite ores. The standard method of finding apexes of which the drill holes are placed is multiplying the surface by the average depth of the ore. Mineral ores, therefore, are made up of a combination of different substances and contain only a certain amount of metal. In most cases, copper ores contain between 0.6 and 1 percent of copper.

Metallurgy

The complete scientific and technological process used for isolation of the metal from its ores is known as metallurgy. Metallurgy is of three types –

• Pyrometallurgy: In this type of metallurgy, heat is used as the main component for the extraction of elements from their ore. Copper, zinc, iron, tin etc., are extracted by this method.
• Hydrometallurgy: In this type of metallurgy, metals are extracted by using their aqueous solutions. Silver and gold are extracted by using this method.
• Electrometallurgy: In this type of metallurgy, a method of electrolysis is used to extract metals. Sodium, potassium, lithium, calcium, etc., are extracted from their molten salt solution through electrolytic method. 

Crushing of Ore

The big lumps of ore are crushed into smaller pieces with the help of jaw-crushers. The process of grinding the crushed ore into fine powder with the help of the stamp mills is called Pulverisation.

Concentration of Ore

Removal of unwanted materials(e.g. Sand, clays, etc.)  from the ore is known as ore concentration, ore dressing or ore benefaction. It can be carried out in various ways depending upon the nature of the ore . The various techniques are – 

• Hydraulic Washing/Gravity Separation/Levigation – This is a process in which the lighter impurities from the ore are removed from the heavier ore particles by simply washing with water. This method is based on the difference in densities of the particles.
• Froth Flotation – This method is based on the preferential wetting of ore particles by oil and that of gangue by water. As a result, the ore particles become light and rise to the top in the form of froth while the gangue particles become heavy and settle down.
• Electromagnetic Separation – This method of concentration is employed when either the ore or the impurities associated with it are magnetic in nature, e.g. chromite containing magnetic silicious gangue and wolframite can be separated by this method.

Isolation of metals from concentrated ore involves two major steps:

• Conversion to Oxide
• Calcination: involves heating when the volatile matter escapes, leaving behind the metal oxide
• Roasting: ore is heated in the presence of air.
• Reduction of the Oxide to Metal

The roasted or calcined ore is then converted to the free metal by reduction. Reduction method depends on the activity of the metals.

Metals which are low in activity series ( copper, mercury, gold) are obtained  by heating their compounds in air ; metals which are in the middle of the activity series ( iron, zinc, nickel, tin) are obtained by heating their oxides with carbon while metals  which are  very high in the activity series (sodium, potassium, calcium, magnesium, aluminium) are obtained by electrolytic reduction method.

Conclusion

Tenor of ore states is the lowest permissible metallic deposit in an ore. An ore is a naturally occurring solid material from which a metal can be extracted chiefly and profitably. The higher the content of metal, the better the tenor. Among metals, aluminium is the metal most widely available. The second most widely found metal in the earth’s crust is iron. A number  of methods are applied to extract metals from their ores.