Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 256

Insoluble impurities in the blister copper anode fall to the bottom forming a sludge that
collects under the anodes called anode slime. What happens to the anode slime? The slime
contains gold, silver, platinum, and tin all elements that are insoluble in the electrolytic refin-
ing and so do not deposit on the cathode. The anode slime is a valuable by-product that is
further processed in specialized smelters to recover these valuable metals ( Case 6.1 ).

The anode slime is a valuable
by-product that is further
processed in specialized smelters
to recover valuable metals.

6.2 DISSOLVING ORE MINERALS FROM
GANGUE - HYDROMETALLURGICAL MINERAL
PROCESSING ILLUSTRATED

Since the mid-1980s, ore leaching has been developed on a larger scale to recover copper
from an entirely different set of ores and mining by-products than is possible by smelting;
namely, oxidized materials. These may be mined copper minerals that are in an oxidized
form - minerals such as Azurite (2CuCO 3 Cu(OH) 3 ), Brochantite (CuSO 4 ), Chrysocolla

(CuSiO 3 2H 2 O), and Cuprite (Cu 2 O), residual copper in old mine waste dumps whose

sulphide minerals have been oxidized by exposure to the air or sulphide copper minerals
that have been oxidized by bacterial leaching, a technology discussed below.

Leaching of Oxide Ores

Leaching is the process by which metals are liberated from minerals by dissolving them
away from solids; it constitutes the hydrometallurgical branch in extractive metallurgy.
Leaching operations resemble chemical plants. The chemical process industries also use
leaching but the process is usually called extraction, and organic solvents are often used.
An easily understood example of liberating materials by dissolving them away from sol-
ids is removing salt (the solute) from salty sand (the ore) by extraction with water (the sol-
vent or the extractant). Using again the example of copper extraction, Figure 6.6 illustrates
the two steps involved for single stage leaching: (1) contact of solid (copper ore) and solvent
(dilute sulphuric acid) for transfer of solute (copper) to solvent; and (2) separation of result-
ing solution from the residual solid. The extract is the solvent phase, now copper bearing
(the pregnant leach solution or PLS, also termed pregnant liquor); the raffinate is the solid
material, leached ore (also termed spent ore), and its adhering solution. While this process
is designed for copper, other metals can be extracted simultaneously.

Leaching is the process by
which metals are liberated from
minerals by dissolving them away
from solids.

CASE 6.1
The Unknown Fort Knoxes of this Earth

Gold is a common associate of copper, and eventually
accumulates in the anode slime during electro-refi ning
of copper, together with silver and other valuable metals.
Some copper smelters incorporate special smelting and
refi ning units to process produced anode slimes and to
extract these valuables metals. These specialized smelters
also often process anode slimes from other copper smelters
that lack the anode slime processing step.

Assuming that copper concentrates contain gold at
a concentration of say 1/100 of one percent a copper
smelter with a production capacity of 100,000t/y may
produce about 1 ton of gold per month. It is alleged
that gold produced at the PASAR copper smelter in
the Philippines contributed to the wealth of the former
President Marcos.