Different process involved in metalurgical process

Metallurgy is a domain of Material Science and Material Engineering. It is a study of the physical and chemical behavior of metallic elements and their mixtures, called alloys. Metallurgists study the microscopic mechanisms that cause a metal or alloy to behave in the way that it does, the changes that occur at the atomic level, that affect the metal's (or alloy's) macroscopic properties. The various processes involved in the extraction of metals from their ores and refining are known as metallurgy. The compounds of various metals mixed with impurities and found in nature are called minerals. The naturally occurring metals from which metal can be extracted profitably and conveniently are called ores.
Different process involved in metalurgical process

The process of extracting a metal from its ore and refining it, is called metallurgical process or simply as metallurgy.

The actual process of extraction of a metal from its ore depends upon the nature of the ore and the metal. There is no universally operational method for the extraction of metals. Certain common steps however, are involved in all metallurgical processes.

Steps involved in a metallurgical process

The extraction of a metal from its ore involves the following steps:

Mining of ore

Most ores generally occur deep inside the Earth. Some may occur only a few metres under the earth's surface. 'Mining' is the process of taking out the ores from the mines. When an ore occurs near the surface of the Earth, it can be directly dug out. Such mining is termed as open-pit mining. When an ore is taken out from greater depths, then the mining is termed deep-mining.

Crushing of the ore

Extracted ore often occurs in big lumps. It is essential to break it into smaller pieces The lumps are crushed to smaller pieces by hammering in a hammer mill or by help of a jaw-crusher.

Grinding and pulverization of the crushed ore

The crushed ore is then finally pulverized to fine powder state in a stamp mill or a pulveriser.

Concentration of the ore (ore dressing

)

The removal of the undesired foreign impurities i.e., gangue, from the ore is called concentration (or beneficiation) of the ore. Either of the following methods is used for concentrating the ores:

Hand picking

If the impurities present are quite distinct from the ore, and are of large size, these may be removed by hand picking. This method is slow and is generally adopted in the initial stages of concentration.

Gravity or levigation method

When the ore particles are heavier than the gangue particles, the ore is fed into a running stream of water and impurities are washed away. This separation is by way of gravity or levigation method and is commonly used for oxide ores such as hematite and native ore of Au, Ag, etc. In order to concentrate the ore in bulk, a slanting vibrating wooden table with wooden strips called riffles is introduced in the process. Such tables are termed Wilfley tables. The ore is continuously washed with a fine spray of water and the rocking motion sieves the heavier portions, while allowing the impurities to filter away.

Fig: 10.2 - Wilfley table for washing of the ore

Sometimes in the gravity method, a hydraulic classifier based on the gravity method is used. Ore is agitated by a powerful current of water pushing upwards through the bottom of a conical reservoir. The heavier ore particles settle down and are continuously removed from another opening near the bottom, while the lighter particles are washed away by water.

Fig: 10.3 - Hydraulic classifier

Magnetic separation

Magnetic separation is done especially in the case of haematite ore, whereby the powdered ore is dropped on to leather or brass conveyer belt, which moves over two rollers one of these rollers, is magnetic. When the ore passes over the magnetic roller, it sticks to the belt due to the force of attraction and falls nearer due to the force of attraction of the magnetized roller. The gangue falls over readily, further away. The ore and the magnetic impurity are collected as two separate heaps.

Fig: 10.4 - Magnetic separation

Froth flotation process

This process is used for concentrating sulphide ores, as such ores are preferentially wetted by oil while the gangue particles are wetted by water. Powdered ore is mixed with water and a little pine oil and the mixture is vigorously stirred by passing compressed air. The froth, which is produced rises to the surface and carries the ore particles along with it. The gangue is left behind

Fig: 10.5 - The froth flotation process

Leaching process

In this method, the ore is treated chemically with a suitable reagent that preferentially dissolves the active component of the ore. The concentrated ore form is then recovered from the solution by a suitable chemical method.

A typical example of ore concentration by leaching process is the purification of bauxite using NaOH solution as a leachant. The Bauxite is digested with concentrated solution of caustic soda at 150°C in an autoclave. The Aluminium oxide dissolves in NaOH leaving behind the insoluble impurities, which are removed by filtration.

The solution of NaAlO₂ (sodium meta-alumiinate) is then treated with freshly prepared Al(OH)₃ when the entire aluminium in the solution gets precipitated as Al(OH)₃

The precipitate of Al(OH)₃ is removed, washed and dried to get Al₂O₃.

Leaching of silver ore

Leaching process is also employed in the recovery of some precious metals. Silver is extracted from its ores (argentite, Ag₂S; horn silver, AgCl) by cyanide process. The finely powdered concentrated ore is treated with a dilute aqueous solution of NaCN (sodium cyanide) and a current of air is passed through the solution. Silver present in the ore gets dissolved due to the formation of soluble sodium argento-cyanide complex, Na[Ag(CN)₂] viz.,

Na₂S so formed gets oxidized (by air) to Na₂SO₃, Na₂SO₄ and thus allow the reaction to go in the forward direction. The solution of Na[Ag(CN)₂) is then treated with zinc scrap to recover silver.

With horn silver (AgCl), the reaction with NaCN can be written as,

Leaching of gold ore

Gold-containing ore gets dissolved in KCN solution in the presence of air to give a solution containing K[Au(CN)₂]. Gold can then be recovered from this solution by either precipitation or electrolytic method.

Electrostatic concentration and liquation are other methods of concentrating of ores. The usage of these methods depend on the nature of the ores and the type of impurities present.

Calcination

The concentrated ore is converted into oxide by calcination i.e., heating it strongly in the absence of air or roasting (heating it strongly in presence of air). This helps in removing volatile impurities like CO₂, SO₂, organic matter, and moisture from the ore. For example,

• It removes moisture from bauxite.

• It removes CO₂ from carbonate ores e.g.,

Fig:10.6 - A reverberatory furnace

Calcination is done on the hearth of a reverberatory furnace.

Roasting

In this process the ore (usually sulphide) is heated strongly, in the presence of excess of air but below its melting temperature. The result is

• It removes moisture, CO₂, SO₂ and organic matter.

• The sulphide ore is converted partly into its oxide or sulphate i.e.,

Similarly,

Roasting is done in a reverberatory furnace or in a blast furnace.

Problems

2. How is the mixture of lead sulphide and Zinc sulphide ores concentrated?

Solution

The mixture of lead sulphide and Zinc sulphide ores are concentrated by the method of electrostatic concentration. The powdered ore is fed upon a roller in a thin layer and subjected to the influence of a electrostatic field. Lead sulphide being a good conductor gets charged immediately and is thrown away from the roller. Zinc sulphide being a poor conductor, falls vertically from the roller.

3. What is liquation and when is it used?

Solution

Liquation is a method for concentrating ores, which have a lower melting point than the impurities. The ores of antimony are concentrated by this method. The powdered ore is heated upon a sloping floor of the furnace. The temperature is raised above the melting point of the ore: this causes the ore to melt and flow down the floor and the impurities are left behind.

4. How is calcination used in limonite?

Solution

Calcination is used in limonite to remove the water of hydration.

5. How are the impurities of sulphur, arsenic and phosphorous removed?

Solution

The impurities of sulphur, arsenic and phosphorous are removed by roasting, where they are removed as their volatile oxides.