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Concentration of Ores

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  • Difficulty Level : Medium
  • Last Updated : 10 Nov, 2021

Metallurgy is the process of extracting pure metals from their ores. On the basis of their reactivity, methods are utilised to extract metals from their ores. Electrolysis is used to extract highly reactive metals from their ores, whereas calcination, roasting, and reduction processes are used to remove metals with medium reactivity. Roasting and refining are used to recover low-reactivity metals from their ores. Although the majority of less reactive metals, such as gold, are found in their free form, as are silver and mercury (As amalgam).

Concentration of ores is done before using all of these methods of metal extraction based on their reactivity. Many impurities, such as rock fragments, sand, and other impurities, can be present in the ore. Matrix or gauge refers to the impurities in a mineral. Concentration ore refers to the process of removing matrix or gauge from an ore.

Occurrence of Metals

Some elements, such as carbon, sulphur, gold, and noble gases, are found in their free state in the earth’s crust, while others are found in mixed forms. The amount of elements varies. The most plentiful metal on the planet is aluminium. In fact, it is the earth’s crust’s third most plentiful element (8.3% approx. by weight). It’s found in a lot of igneous minerals, including mica and clays.

Many gemstones contain impure Al2O3 forms. In the earth’s crust, iron is the second most plentiful metal. It is a very significant element since it produces a variety of compounds and has a wide range of applications. It’s also one of the most important components in biological systems. To obtain a certain metal, we must first hunt for minerals, which are chemical components found naturally in the earth’s crust and collected through mining. Only a few minerals in which a metal can be found are suitable for use as a source of that metal. Such minerals are known as ores.

A given element can be found in a wide range of compounds. The method for isolating an element from its compound should be both chemically and commercially feasible. Bauxite is the material of choice for extracting aluminium. Iron is usually extracted from oxide ores, which are plentiful and do not emit harmful gases (such as SO2, which is produced by iron pyrites). 

Metallurgy refers to the complete scientific and technological process of isolating metal from its ore. 

Various chemical principles are used in the extraction and isolation of an element from its mixed form.

Following are some principal ores of their corresponding important metals:

Metal

Ores

AluminiumBauxite, Kaolinite
IronHematite, Magnetite, Siderite, Iron Pyrite
CopperCopper Pyrite, Malachite, Cuprite, Copper glance
ZincZinc Blende, Calamine, Zincite

Concentration of Ores

Concentration, dressing, or benefaction are terms used to describe the process of removing undesired elements (such as sand, clays, and other minerals) from ore. 

Ores are graded and crushed to an acceptable size before being concentrated. The concentration of ores entails multiple processes, the order of which is determined by the variations in physical attributes between the metal complex and the gangue. The type of metal, the facilities available, and environmental variables are all taken into account. The following are some of the most important ore concentration techniques. 

Hydraulic Washing

This is due to the ore and gangue particles having different gravities. As a result, it’s a type of gravity separation. The powdered ore is washed with an upward stream of flowing water in one such operation. The lighter gangue particles are washed away, leaving behind the heavier ores.

Magnetic Separation

This is due to changes in the ore components’ magnetic characteristics. If either the ore or the gangue (one of these two) can be attracted by a magnetic field, such separations can be done (e.g., in the case of iron ores). A conveyer belt transports the ground ore, which passes across a magnetic roller.

Froth Floatation Method

The gangue from sulphide ores is removed using this process. The powdered ore is suspended in water during this procedure. It is supplemented with collectors and froth stabilizers. Collectors (e.g., pine oils, fatty acids, xanthates, etc.) improve the mineral particles’ non-wettability, whereas froth stabilizers (e.g., cresols, aniline) keep the froth stable. Oils wet the mineral particles, while water wets the gangue particles. A revolving paddle agitates and sucks air into the mixture. As a result, froth forms, carrying the mineral particles with it.

The foam is minimal and easily removed. The ore particles are then dried for recovery. Separating two sulphide ores can sometimes be done by changing the proportion of oil to water or by utilising depressants. The depressant utilised in an ore containing ZnS and PbS, for example, is NaCN. It selectively prohibits ZnS from reaching the froth while allowing PbS to do so.

Leaching

If the ore is soluble in a suitable solvent, leaching is frequently utilised. The process is demonstrated in the following examples:

Leaching of alumina from bauxite

The primary ore of aluminium is bauxite. Impurities such as SiO2, iron oxides, and titanium oxide (TiO2) are common. Heating the powdered ore with a concentrated solution of NaOH at 473–523 K and 35–36 bar pressure is used to concentrate it. Digestion is the term for this procedure. Al2O3 is extracted as sodium aluminate in this manner. The impurity SiO2 dissolves as well, resulting in sodium silicate. Other contaminants are left in the process.

Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2Na[Al(OH)4](aq)

By passing CO2 gas through the solution, the sodium aluminate is neutralized, and hydrated Al2O3 is precipitated. A little amount of freshly prepared hydrated Al2O3 is added to the solution at this point. This is referred to as seeding. It is the cause of the precipitation.

2Na[Al(OH)4](aq) + 2CO2 (g) → Al2O3 .xH2O(s) + 2NaHCO3 (aq)

The solution contains sodium silicate, and the hydrated alumina is filtered, dried, and heated to get pure Al2O3.

Sample Questions

Question 1: Name some ores of iron.

Answer:

The ores of iron are Hematite, Magnetite, Siderite, and Iron Pyrite.

Question 2: What are ores?

Answer:

To obtain a certain metal, we must first hunt for minerals, which are chemical components found naturally in the earth’s crust and collected through mining. Only a few minerals in which a metal can be found are suitable for use as a source of that metal. Such minerals are known as ores.

Question 3: Define metallurgy.

Answer:

Metallurgy refers to the complete scientific and technological process of isolating metal from its ore. 

Question 4: What is benefaction?

Answer:

Benefaction is a term used to describe the process of removing undesired elements (such as sand, clays, and other minerals) from ore. 

Question 5: On what principle, is hydraulic washing based?

Answer:

It is based on the differences in gravities of the ore and the gangue particles. 

Question 6:  On what principle, is magnetic separation based?

Answer:

It is based on the changes in the ore components’ magnetic characteristics.

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