Aluminium sulphate, also known as cake alum or aluminium salt, is represented by Al2 (SO4)3. It is used in various industry applications, such as waste water treatment industry, water purification and paper industry. Aluminium sulphate is available in a number of forms like block, coarse and as fine powder. It is hygroscopic and hence, has the ability to absorb and retain moisture from the surroundings. It is also water soluble, non-volatile and non-flammable. It is highly acidic in nature, thus must be handled with extreme care to prevent any burns or injuries.
In industrial product, the synthesis of aluminium sulphate is carried out in several stages:
1. Mixing aluminium hydroxide with sulfuric acid
Sulfuric acid is fed into a heat-resistant and acid-resistant reaction vessel. Aluminium hydroxide is then added to the vessel and is heated to a high temperature of about 100°C to 130°C. The concentration of sulfuric acid added to the mixture must be high in order for the reaction to be efficient.
2. Adding Nitric Acid into the vessel
Recently, a more advanced method has been developed to increase the efficiency of aluminium sulphate production. This allow the reaction to be held at a lower temperature, shorter time, and can require lower concentration of sulfuric acid without a heating source. Contrary to the method mentioned above, an additional product (nitric acid or hydrogen peroxide solution) has to be added to the reaction.
3. Separate aluminium sulphate from water
Concentrated aluminium sulphate solution is cooled and followed by evaporation to separate aluminium sulphate from water.
There are other ways with which aluminium sulphate is prepared. Clay or bauxite is gently calcined, mixed with sulphuric acid and water and then heated gradually, later the clear solution is drawn off. Aluminium sulphate is also prepared from cryolite and calcium carbonate together. The mixture is heated and extracted with water later. Sodium bicarbonate or carbon dioxide is introduced to the solution, and the precipitate is finally dissolved in sulphuric acid.
2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3 + 6H2O
Aluminium sulphate is well known for waste water treatment. Because of its non-toxicity, it is used in a water treatment reactor to purify drinking water. It is also used to maintain the stability of the lake or reservoir. Excess phosphorus in the lake water induces ‘booming algae’ which aluminium sulphate can combat this problem by reducing the phosphorus concentrations and also, maintaining the normal phosphorus concentration.
Aluminium sulphate is used as a buffer solution for balancing of soil pH. The flower changes its colour when the pH of soil is different from the standard conditions. Gardeners even use aluminium sulphate to increase the acidity of soil.
When aluminium sulphate is dissolved in large amount of water with neutral pH, aluminium hydroxide is formed, which helps in dyes stick to clothes or fabrics. So aluminium sulphate is used as a ‘dye fixer’ in the textile industry.
Aluminium sulphate is used in manufacturing paper, but nowadays synthetic components are taking its place. Aluminium sulphate is used to change the level of absorbency of paper.
Aluminium sulphate is also used for waterproofing and to accelerate the setting of concrete. It is also used to produce antiperspirants, firefighting foaming, lubricating compositions, sizing paper, baking powder, material for fire retardant, catalyst, and decolourisation agent. Moreover, it is also applied as a coagulant to absorb the impurities which are removed as particulates via filtration. Aluminium sulphate is widely applied in the textile industry as well.