5.8 Metallurgical Methods for Metals Recovery
147
5.8.3.6 Cementation
Cementation is a type of precipitation in which an electrochemical reaction occurs
between the cementing metal and the ion of the precipitated metal. Recovery of gold
from cyanide solutions by zinc cementation is known as the Merrill–Crowe Process
and has been used commercially for many years. In this process, zinc powder is
added to the cyanide solution and results in the cathodic deposition of gold and
anodic corrosion of zinc. The cathodic and anodic reactions are simplified as follows
[20, 31]:
Au(CN)
−
2 + e
−
→ Au + 2CN
−
(5.15)
Zn + 4CN
−
→ Zn(CN)
2−
4 + 2e
−
(5.16)
The reaction is stable in the pH range of 8–10. However, the presence of impurities
such as lead, copper, nickel and sulfur have detrimental effects on gold cementation
[20]. The efficiency of the cementation process depends on the contact time, initial
metal ion concentration and the reaction temperature. This process has the advantages
of simple operation and control and the ability to recover valuable metals. However,
the solution needs to remove solids and oxygen by clarification and degassing prior
to the cementation process [31].
5.8.3.7 Supercritical Fluids Extraction
Supercritical fluids extraction is a new and green technology for the extraction of
metal ions based on their size, boiling point, polarity and molecular weight. Supercritical carbon dioxide (ScCO 2 ) is the most common solvent for metals extraction
due to its moderate critical point and the inert, non-toxic and non-flammable characteristics [12, 32]. Chelating agents are usually added to the supercritical fluids
to neutralize the charges of metal ions and organometallic compounds and solubilize them in supercritical fluids by forming a metal chelate with metal species.
Without using the chelating agents, it is impossible to extract the metal ions from
pure supercritical fluids. Selecting a suitable chelating agent is very important for the
efficiency of the process. The most commonly used chelating agents are dithiocarbamates, β-diketones, organophosphorus agent and macrocyclic compounds. The
chelation supercritical fluid extraction of metals depends on the solubility of the
chelating agents and metal chelates, the effects of water and pH, temperature and
pressure of the reaction, reaction time, use of a solvent modifier and the matrix effect
[32]. The method of supercritical fluids extraction has the advantages of simple operation, good selectivity, easily recovered products, no secondary waste production,
low surface tension, low energy consumption and non-toxicity, while it is a relatively
costly process due to the consumption of the expensive chelating agents [12, 32].
147
5.8.3.6 Cementation
Cementation is a type of precipitation in which an electrochemical reaction occurs
between the cementing metal and the ion of the precipitated metal. Recovery of gold
from cyanide solutions by zinc cementation is known as the Merrill–Crowe Process
and has been used commercially for many years. In this process, zinc powder is
added to the cyanide solution and results in the cathodic deposition of gold and
anodic corrosion of zinc. The cathodic and anodic reactions are simplified as follows
[20, 31]:
Au(CN)
−
2 + e
−
→ Au + 2CN
−
(5.15)
Zn + 4CN
−
→ Zn(CN)
2−
4 + 2e
−
(5.16)
The reaction is stable in the pH range of 8–10. However, the presence of impurities
such as lead, copper, nickel and sulfur have detrimental effects on gold cementation
[20]. The efficiency of the cementation process depends on the contact time, initial
metal ion concentration and the reaction temperature. This process has the advantages
of simple operation and control and the ability to recover valuable metals. However,
the solution needs to remove solids and oxygen by clarification and degassing prior
to the cementation process [31].
5.8.3.7 Supercritical Fluids Extraction
Supercritical fluids extraction is a new and green technology for the extraction of
metal ions based on their size, boiling point, polarity and molecular weight. Supercritical carbon dioxide (ScCO 2 ) is the most common solvent for metals extraction
due to its moderate critical point and the inert, non-toxic and non-flammable characteristics [12, 32]. Chelating agents are usually added to the supercritical fluids
to neutralize the charges of metal ions and organometallic compounds and solubilize them in supercritical fluids by forming a metal chelate with metal species.
Without using the chelating agents, it is impossible to extract the metal ions from
pure supercritical fluids. Selecting a suitable chelating agent is very important for the
efficiency of the process. The most commonly used chelating agents are dithiocarbamates, β-diketones, organophosphorus agent and macrocyclic compounds. The
chelation supercritical fluid extraction of metals depends on the solubility of the
chelating agents and metal chelates, the effects of water and pH, temperature and
pressure of the reaction, reaction time, use of a solvent modifier and the matrix effect
[32]. The method of supercritical fluids extraction has the advantages of simple operation, good selectivity, easily recovered products, no secondary waste production,
low surface tension, low energy consumption and non-toxicity, while it is a relatively
costly process due to the consumption of the expensive chelating agents [12, 32].