Aluminium sulfate, also known as cake alum or aluminium salt, is a chemical matter with the formula Al2(SO4)3. It is applied in various industries, such as waste water treatment industry, water purification and paper industry. Aluminium sulfate appears as white crystalline solid and is odourless. It is hygroscopic and hence, able to absorb and retain water from the surroundings. It is also water soluble, non-volatile and non-flammable. It is extremely acidic such that it can burn human skin and also corrodes metal. When dissolved in water, pH can be less than two.
Aluminium sulfate is dangerous if inhaled or swallowed. The inhalation will cause coughing and potentially shortness of breath. Eating or swallowing aluminium sulfate can cause severe irritation of intestines and stomach. Ingestion will induce vomiting, nausea, and diarrhoea. Upon contact with skin or eyes, it will cause irritation, redness, itching, and pain. Besides that, adding water to aluminium sulfate can cause sulfuric acid to form.
In industrial product, the synthesis of aluminium sulfate 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 sulfate 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 sulfate from water
Concentrated aluminium sulfate solution is cooled and followed by evaporation to separate aluminium sulfate from water.
Aluminium sulfate 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 sulfate can combat this problem by reducing the phosphorus concentrations and also, maintaining the normal phosphorus concentration.
Aluminium sulfate 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 sulfate to increase the acidity of soil.
Aluminium sulfate 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, molluscicide, 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 sulfate is widely applied in the textile industry as well.