Natural Resources Extraction: Stressors and Impact Management - Sustainable Practices in Geoenvironmental Engineering - page 142

Bulk ore

Nutrients and
wastes

Bioreactors

Leachate

Slurry metal
preparation

Grinding
Crushing

Recovery

Waste solids Purified metal

FIGURE 5.2
Flowsheet of a biometallurgical process.

include low-grade ores, mine tailings, and sediments from lagoons or abandoned sites.
Low pH values lead to solubilization of the metals in the mining ores. Elemental sulfur or
ferrous iron may be added as bacterial substrates. Reactors such as Pachuca tanks, rollings
reactors, or propeller vessels have been used (Tyagi et al., 1991).
Heap leaching is more common since it allows the large volume of wastes to be treated
in place (Boon, 2000). To enhance this process, aeration can be forced through the pile.
Alternatively, hydrophilic sulfur compounds can be added (Tichý, 2000). Thiobacilli bacte-
ria are responsible for the oxidation of inorganic sulfur compounds. Applications include
metal dissolution of low-grade sulide ores, generation of acidic ferric sulfate leachates
for hydrometallurgical purposes, and removal of gold by oxidation of pyrite by bacterial
sulide production. The extraction of metals from low grade metals ores and refractory
gold ores is a multibillion-dollar business worldwide (Rawlings, 1997). Bacterial solubiliza-
tion by oxidation of the sulide minerals, pyrite, and arsenopyrite, enhances gold extrac-
tion by the traditional method of cyanidation. The solubilization mechanisms have been
debated extensively, however.
Biohydrometallurgical processes are highly eficient and cause fewer environmental
problems than chemical methods (Torma and Bosecker, 1982). For slurry processes, oxida-
tion rate per reactor volume, pH, temperature, particle size, bacterial strain, slurry density,
ferric and ferrous iron concentrations need to be optimized. Bioleaching is very effective
for recovery of gold from refractory gold pyrite and copper from chalcopyrite. The fea-
sibility of the recovery of metals from a mining residue has been shown by Aspergillus
niger , which exhibits good potential in generating a variety of organic acids effective for
metal solubilization (Mulligan and Kamali, 2003). Organic acid extraction effectiveness
is enhanced when sulfuric acid was added to the medium. The study by Mulligan and