|
|
|
The principle technology to recover or remove metals from a solution is by chemical precipitation. Chemical precipitation includes two secondary removal mechanisms, co-precipitation and adsorption. Precipitation processes are characterized by the solubility of the metal to be removed. They are generally designed to precipitate trace metals to their solubility limits and obtain additional recovery by co-precipitation and adsorption during the precipitation reaction
There are many different treatment variables that affect these processes. They
include the optimum pH, the type of chemical treatments used, and the number
of treatment stages, as well as the temperature and volume of the solution,
and the chemical specifications of the metalls to be recovered. Each of these
variables directly influences treatment objectives and costs. Studies are
performed to optimize the relevant variables, so that goals are met and costs
minimized.
In theory, the precipitation process has two steps, nucleation followed by
particle growth. Nucleation is represented by the appearance of very small
particle seeds which are generally composed of 10–100 molecules. Particle
growth involves the addition of more atoms or molecules into this particle
structure. The rate and extent of this process is dependent upon the
temperature and chemical characteristics of the wastewater, such as the
concentration of metal initially present and other ionic species present,
which can compete with or form soluble complexes with the target metal
species.
Determining the optimal pH range to facilitate the maximum precipitation of
metal is a difficult task. It is typically accomplished by laboratory studies
rather than theoretical calculations. Often the real solutions behave
differently and the theoretical metal solubilities and corresponding optimal
pH ranges can vary considerably from theoretical values.
|
|
|
|
|
|
|
|
