Story Title
Other chemicals in opposition
Combinations of inorganic chemicals can be used with sodium aluminate to achieve flocculation polymerisation in situ. A largley unexplored chemical mechanism. Chemists investigating drinking water purification by empirical Jar Test or pilot plant techniques need to apply sodium aluminate liquor correctly at appropriate pH levels.
A range of inorganics - ferrics, polyalums or strong acids such as sulphate and hydrochloric can be used to produce the same effect as aluminium sulphate or chloride in opposition.
Comment
The representation of the coagulation reaction in the Hydrous Alumina diagram is for a system at equilibrium and involving a fixed dose of reactants. With materials in opposition, for example alum v aluminate, a wide range of alumina polymer is achievable provided pH's close to neutral, are maintained.
In current full scale practise pH's are chosen to achieve particular results, for example below 7.0 may be used, because it is claimed to achieve colour removal. The availability of sodium aluminate liquor creates opportunities for beneficial changes to be made in pH. The material can achieve higher pH's which are still in the coagulation range pH 7.4 to 8.0 thus giving pH's which are 'buffered' when aluminate is applied. This is a special feature not available to other coagulants, organic or inorganic.
In working to a specific pH the ratio of alum to SAL is adjusted so that the pH required is achieved while maintaining the same total dose of in situ polyaluminium floc.
