Criteria:

These criteria are listed below and are not necessarily in the order of their importance.

i) The components of the organic phase should be immiscible with the aqueous phase.

ii) It should have good stability. This will amount to the fact that the organic phase should be capable of withstanding many recycling operations in a solvent extraction circuit without degeneration.

iii) A difference in densities of the contacting phases is essential and should be as great as possible.

iv) Low power requirement for pumping and agitating, rapid extraction and settling of dispersion and high heat and mass transfer rates are corollaries of low viscosity of the organic phase. Hence, low viscosity is a desirable property. Dissolution of the extractants in low viscosity diluents modifies this property to a favourable degree.

v) The interfacial tension should be high for rapid coalescence.

vi) The solvent should cause no corrosion difficulties with common materials of construction to reduce the cost of equipment.

vii) The solvent should have low toxicity, high boiling and flash points. These mainly avoid environmental pollution and fire hazards.

viii) It should have high metal loading capacity.

ix) It should be easily stripped of the loaded metal.

x) The extractant should have high extraction and regeneration power.

xi) The extractant should be selective to extract one component of a solution in preference to the others.

xii) It should have good kinetics of extractions.

xiii) It should be relatively inexpensive. Low cost and ready availability in adequate quantities parallel each other and are of course essential solvent attributes.

xiv) The extractant should be highly soluble in aliphatic and aromatic hydrocarbons.

Of all the desirable properties described selectivity, recoverability, interfacial tension and density are essential for the process even to be envisaged for detailed investigation.