The process where metals are obtained from the ores is called metal extraction. Since different metals have different properties, there is a difference in the process of extracting different metals.
Some non-reactive metals like gold, platinum etc. are found in their pure state while less and more reactive metals are usually found in their compound states like oxides, sulﬁdes, nitrates, carbonates etc. For this reason, the compounds of reactive metals are extracted by reduction or electrolysis process.
There are a number of steps of metal extraction from their ores:
- Crushing the ores
- Condensation of ores
- Conversion of condensed ore to oxides
- Conversion of metallic oxides to free metals
- Puriﬁcation of metals
However, the steps of metal extraction depend on the properties of the ores. All steps are not required to all ores.
In this part, we will discuss on the first three steps and rest will be discussed in Metal Extraction: Part- 2.
1. Crushing the Ores
If the stones of ore that are excavated from the mines are big, they need to be crushed in the Joe crusher to smaller pieces to turn into powder or small lattices in the Ball Crusher.
2. Condensation of Ores
Usually, the ores contain a mixture of substances apart from the metal is called impurities. The crushed powder of ores still contains the impurities mixed with them. For example, bauxite ores contain sand as an impurity when it is excavated from the mine. The process that is employed to separate these impurities from the intended metal is known as condensation of ores.
Condensation is of various kinds like Hydrolytic method, Magnetic separation, Froth Floatation, Chemical method etc.
This method is usually applied to oxide ores. In the process, the ore is poured on a slanted, chambered table and water is ﬂown over the ore. The particles of oxide are comparatively heavy while the impurities are light. Here, the heavy ore becomes condensed and goes inside the chamber while the impurities are washed away with water. This is how the ores are condensed.
Froth Floatation Method
This method is applied to sulﬁde ore. The ore is ﬁrst taken inside a large tank and water is added to it. Later, oil is slowly added to the mixture. When air is blown on the ore, the sulﬁde ore dissolves in the oil and ﬂoats up as froth. Then the froth is collected while the impurities remain at the bottom of the tank.
Magnetic Separation Method
The process is applied if either of the ores or gangue has magnetic property. Crushed ores are passed through a plastic conveyor belt and the outer wheel of the conveyor belt has the magnetic property. It attracts the magnetic ores or gangue and they are gathered below and near the wheel. The non-magnetic portion of the mixture gathers separately a little afar. As a result, the ore is separated from the gangue. The process is used to separate the magnetic ores like chromites FeO.Cr2O3, rutile TiO2 etc.
This method is applied if a particular substance undergoes reaction with either of the ores or gangue. As for example, Bauxite contains a mixture of impurities like iron oxide, titanium oxide, sand and etc. To obtain aluminium oxide ores from bauxite, the chemical method is applied.
(NaOH) with bauxite (Al2O3.2H2O) and heating it up to 1500-2000ºC will produce sodium aluminate (NaAlO2).
When hot sodium aluminate is let to react with water, it produces aluminium hydroxide and sodium hydroxide. Sodium hydroxide gets the bauxite dissolved while the iron oxide, titanium oxide, sand and etc. will not be dissolved.
Finally, heating the separated aluminium hydroxide at 1100°C will produce pure aluminium oxide and water. This pure aluminium oxide is the output of condensation.
3. Conversion of Condensed Ore to Oxides
Condensed ore converts into metal oxide by calcination or roasting process.
Calcination of Ores
The condensed ore is heated at a temperature below the melting point in absence of air, organic substances and moisture will be removed from the ore. This process is called calcination process.
The calcinations enforce water, carbonate, bicarbonate, hydroxide etc. like impurities to be removed in the form of carbon dioxide or water from the ore. If these impurities are not removed at this stage, it will be tougher to remove later.
Generally, the roasting process is applied for sulﬁde ores. If sulﬁde ore is heated at the temperature below the melting point in presence of air, organic substances and moisture will be removed from the ore as oxides.
*References- NCTB Books and Wikipedia