Total salt content

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Total salt content is a measure of the presence of mainly sulfide salts with associated aluminium, magnesium, sodium, calcium and potassium cations or, more generally, any soluble anion or cation of the leaching solution. When the total salt content is in excess of about 80 g/L to 120 g/L, microbial activity is inhibited to an increasing extent. Microbial inhibition may however occur at lower levels of total salt content in the presence of particular cations and anions which cause specific inhibition (rather than non-specific ionic strength and osmotic potential inhibition), such as chlorides, nitrates, aluminium, fluoride and arsenic.

In a heap leaching system, the target pH of the pregnant liquid solution is typically in the range 1.5 to 2.2. Acid is used principally to dissolve acid-soluble copper and maintain it in solution, and to create an environment conducive to microbial growth and activity. The gangue minerals, however, are often acid-consuming and can react with the acid contained in the solution which is percolated through the heap. This reaction results in the release of salts, typically sulphate salts with associated aluminium, potassium and magnesium cations, that are carried as dissolved species in the solution. The concentration of such dissolved salts increases over time as the heap leaching process progresses and due to the concentrating effect of evaporation.

The increase in organic salts, in the aforementioned manner, results in increasing levels of inhibition of microbial activity. This can be a non-specific inhibition such as that caused by high ionic strength (high osmotic potential) which results in reduced water activity which, in turn, results in lowered microbial activity. Alternatively or additionally, the inhibition may be caused by specific inorganic compounds such as nitrate, chloride, aluminium, fluoride and arsenic. A common type of microbial inhibition (or sub-optimal microbial activity) encountered in a heap leaching operation is due to high total salt content which results in lowered water activity and non-specific microbial inhibition.



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