Page 173 - Introduction to Colloid and Surface Chemistry

162 The solid-liquid interface
The world-wide treatment of crude ore by flotation amounts to c.
9
10 tonne per year. By concentrating metal ores on site, considerable
saving of transportation costs can be effected, thus permitting the
exploitation of low-grade ores (as little as 1 per cent metal content)
which would otherwise be uneconomic.
After mining, the crude ore is crushed and then ground into a
slurry in water (with particle diameters typically in the range 0.01-
0.1 mm). A small quantity of collector oil is added during the
grinding stage. The collector oil adsorbs strongly on the surfaces of
the metal ore particles, with the result that the contact angle at the
solid-liquid-air boundary increases to the point where flotation is
possible. The collector oil does not adsorb so strongly on siliceous
material, which remains wetted by water and does not float.
The collector oil molecules are amphiphilic, with their polar groups
exhibiting some affinity for particular metal ores, and so create a
hydrophobic particle surface on adsorption. They can be anionic,
cationic or non-ionic. Organic xanthates and thiophosphates are
often used for sulphide ores and long-chain fatty acids for oxide and
carbonate ores.
A foaming agent, such as crude cresol or pine oil (soap is
unsuitable, as it lowers 6 too much), is added to the suspension of
ground ore and collector oil in water and the pH is adjusted to give
the particles low zeta potentials and, therefore, minimise electrostatic
repulsions. Air is forced through a fine sieve at the bottom of the
vessel. The particles of metal ore become attached to the air bubbles,
which carry them to the surface (Figure 6.7), where they collect as a
metal-rich foam which can be skimmed off.
Contact angles of at least 50-75° are required for satisfactory
flotation. This can often be achieved with as little as 5 per cent surface
coverage, so that the amount of collector oil used is fairly small. In a
typical metal sulphide ore flotation, the slurry will contain approx-
imately 3 tonne of water, 50 g (maximum) of collector oil and 50 g of
foaming agent for each tonne of crude ore, and recovery of c. 90 per
cent of the metal content would normally be achieved.
Sometimes the ore must be pretreated before it will adsorb the
additive satisfactorily; for example, zinc sulphide must be pretreated
with dilute copper sulphate solution, which deposits copper on the
ore surface by electrochemical action. Specificity of flotation may also
be achieved by the addition of depressants; for example, cyanide ions
prevent ferrous sulphide and zinc sulphide from floating but allow