metal precipitation

These are dissolved metals mainly found in surface water treatment effluents, mine water, hydrometallurgy leachates, wash water used on gas produced from coal and household waste incineration…

The most frequently used process consists in precipitating these metals as hydroxides through simple neutralisation of these acid effluents. As the maximum precipitation pH for all these metals is not the same, we look for the optimum pH zone. Sometimes, two pH adjustments and two floc separation stages will be required.

One favourable case is the preponderant presence of a metal (at least 5 times more concentrated as mole/mole) which, as it precipitates, "drags" the others to its optimum precipitation pH.

If, however, the solubility of the hydroxide does not allow it to sufficiently insolubilise the metals present, we can look for better elimination in the following form:

• carbonate or rather hydroxycarbonate (e.g. Pb²⁺);
• phosphate or rather hydroxyapatite (e.g. PO₄ (Ca, Zn);
• sulphides (case of bivalent metals) and this is achieved by adding:
  • Na₂S; however, the excess S^2- then has to be precipitated and the sulphides that are formed will appear as very fine colloids that have to be coagulated and flocculated in turn;
  • or organic derivatives of S^2– as polymers that also act as flocculants (thus much easier to use, at least when the quantities of metals to be precipitated are not too high).

It should be noted that, in the presence of natural (e.g. humic acids) or artificial (e.g. EDTA) complexing agents, the above reagents will not be enough: preliminary treatment using a powerful oxidant such as ozone will be essential to destroy the complexing agent and, consequently, "releasing" the metal.