Different process involved in metalurgical process
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:
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 oreSometimes 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 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
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.
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.
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.,
Roasting is done in a reverberatory furnace or in a blast furnace.
2. How is the mixture of lead sulphide and Zinc sulphide ores concentrated?
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?
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?
Calcination is used in limonite to remove the water of hydration.
5. How are the impurities of sulphur, arsenic and phosphorous removed?
The impurities of sulphur, arsenic and phosphorous are removed by roasting, where they are removed as their volatile oxides.