Environmental Engineering Reference
In-Depth Information
Spent leaching solution or barren solution (raffinate) is an acidic aqueous solution that
has been stripped of copper but still has some carryover of the organic extraction/diluent
used in the solvent extraction operation. The raffinate generated at hydrometallurgical
plants is typically stored in ponds and recycled to the dump leaching operation. As a result,
it does not become a waste until closure of the plant. Following mine closure, spent leach-
ing solutions must be treated prior to disposal.
Other wastes and materials are typical of industrial operations, and include spent sol-
vents, refuse, and used oil. As is true for all waste streams, accurate recording of wastes
generated and documentation of waste disposal operations are important.
Management of Leach Solution and Water
It is often argued that hydrometallurgical processing has very little environmental impact
because its liquid streams are contained. In the SX/EW process, all impurities are returned
to the site where they originated and the sulphuric acid is eventually neutralized by lime or
limestone to form calcium sulphate (gypsum) - a relatively insoluble substance. Although
heap leach operations are designed for zero discharge, (with the heap, its collection ditches,
the pregnant solution pond, and plant feed ponds all lined to prevent solution loss), solu-
tion losses from the heap to undetermined sources (e.g. evaporation or seepage) will occur.
Released solution potentially impacts ambient groundwater.
As for mine waste storages there is concern for the long-term integrity of the remain-
ing leached ore materials or mine wastes. At heap closure the leached ore will remain on
the pad. Closure will entail rinsing of the heap by application of water or by natural rain-
fall, often with no cover or reclamation planned for the heap pad. It is often necessary to
continue operation of an SX/EW plant to treat leachate resulting from rinsing operations.
This may be required for several years. The life of pond liners becomes important, as are
anticipated flow rates from the heap after application of lixiviant ceases. There is also the
need to establish 'trigger levels' for determining the point at which leachate collection will
no longer be required.
All solutions (leaching, organic, and electrolyte) are reused in the SX/EW operation.
Electrolytic solutions decrease in effectiveness over time due to an accumulation of impuri-
ties. The method used to remove impurities from spent electrolyte and the disposal of any
wastes are important. A monthly balance of all solutions and reagents needs to be estab-
lished indicating total consumption and total amount recycled.
At the electrowinning facility, completed copper cathodes are washed with water to
remove residual electrolyte from the sheets. The amount of wash water generated in this
operation and its disposition are also important. Waste lead anodes are of value and are
sent to a smelter for lead recycling.
Although heap leach operations
are designed for zero discharge
solution losses from the heap to
undetermined sources will occur.
6.6 COMMON TECHNIQUES TO ESTIMATE EMISSIONS
To assess potential environmental impacts of mineral processing activities, it is necessary
to estimate the quantity and quality of the various emissions that are associated with the
project. Four techniques are commonly used to estimate these emissions: (1) sampling or
direct measurement; (2) mass balance; (3) engineering calculations; and (4) application of
emission factors. While particular estimation methods are generally more suited to par-
ticular applications, the final choice will depend on factors such as costs, level of required
accuracy, nature of emission and substance, and data availability.
 
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