Raw material sustainability
The main ingredient in coagulants is typically either aluminium or iron. Total production is almost evenly split between the two metals. Both elements are abundant on earth with iron extracted from ores such as magnetite and hematite. Aluminium is typically produced from bauxite ore.
Many inorganic coagulants, particularly those based on iron salts, are produced from by-products of other industrial processes. This avoids the use of virgin raw materials and results in a lower carbon footprint for these coagulants.
ALUMIMIUN SALT COAGULANTS
IRON SALTS COAGULANTS
Aluminium is the most common element in the earth’s crust although much of it is in a complex form in the soil. Bauxite ore is the most common source of ore due to its high aluminium content.
Aluminium salts that are a by-product of aluminium profile production and aluminium galvanisation processes can also be used as raw materials. However, due to relatively high concentrations of iron and other impurities, the by-product cannot be included in coagulants used for drinking water treatment or in the paper industry. As a result, relatively few aluminium-salt based coagulants can be produced from the by-products of other production processes.
For applications which require very pure aluminium salt, the Bayer Process is used. During the process, the bauxite is refined to pure aluminium oxide and the aluminium hydroxide is separated out. Dissolving the pure aluminium hydroxide in sulphuric or hydrochloric acid produces aluminium sulfate and aluminium chloride. If the aluminium hydroxide is dissolved in caustic soda, sodium aluminate is produced.
The main raw materials in iron-salt based coagulants are by-products from other production processes. The most commonly used by-product is ferrous sulfate which is also known as copperas. Copperas is produced in large quantities during the pickling of steel and the manufacture of titanium dioxide.
Iron salt by-products are in bivalent form. In some cases they can be directly used to treat sewage water.
If iron salts are oxidised with chlorine or oxygen they produce a trivalent product. Trivalent iron salts have more applications in water treatment as their higher valency makes them a more effective coagulant.