An Eco-Friendly Chemistry for Gold extraction Using Supercritical Carbon Dioxide
A number of exciting challenges, opportunities and unanswered questions remain concerning the chemistry, geochemistry, medical, industrial and analytical applications of gold and its compounds. Due to their technological and apparent therapeutic properties, there is a renewed interest in gold colloids, nanoparticles and thin layers. Understanding how gold moves between the solution and solid states, and the roles of oxidation states and complex species, is of fundamental interest in earth and environmental sciences. Recent geochemical studies of gold solubility and speciation in hydrothermal solutions have highlighted the importance of supercritical conditions and sulphur species in the deposition of gold in natural environments. Geochemists are seeking to understand gold transport and deposition mechanisms in the formation of mesothermal- and porphyry-type ore deposits. Gold solubility and speciation studies in hydrothermal solutions reveal an important role for AuHS0 species in the deposition of gold in natural environments. Gold solubility in supercritical brines is far higher than previously supposed while interest in the high content of carbon dioxide in the mineralizing fluids is growing.
In recent years there has been a tremendous amount of attention paid to the area of supercritical fluid extraction (SFE) of metal ions. This interest has been fuelled by the introduction of more stringent environmental control and the subsequent need for cleaner solvents and processes to replace established ones. Carbon dioxide is the substance of choice for SFE because of its moderate critical parameters (Tc = 31oC, Pc = 73atm), inertness and availability in pure form. It is an excellent extraction medium for non-polar and moderately polar species, which with careful ligand design, can be used for analytical and process scale metal extraction from solution and solid phases. Among more recent examples of the potential of chelation in supercritical CO2 are the extraction of metals from flyash, the extraction of actinides from solution and soils, plutonium and americium from soil, and uranium and lanthanide extraction for nuclear processing.
In this paper, we show that supercritical conditions and complexation with sulphur donor atoms, important in the natural deposition of gold, forms the basis of a clean, eco-friendly chemistry for gold extraction and deposition. We demonstrate that gold, in a variety of matrices, can be effectively complexed and extracted by supercritical carbon dioxide containing new fluorinated macrocyclic calixarenes and linear thiourea ligands. With reagent solubilities greater than 0.1 mol L-1 at 60 oC and 250 atm, the extraction of Au(III) from spiked cellulose paper and from Au(0) powder, following addition of oxidant, is demonstrated using atomic absorption spectrometric analysis of extracts. The more readily isolatable Ag(I) complexes are characterised by FTIR and elemental analysis, providing evidence of coordination through the sulphur atom. Thermally treated Ag complexes, deposited from supercritical carbon dioxide, onto ceramic supports, are further characterised using energy dispersive X-ray spectrometry (EDX) and scanning electron microscopy (SEM). Furthermore, the extraction of gold from ore reference material using supercritical CO2 is demonstrated, and proposed as a clean, eco-friendly chemistry for gold extraction.
supercritical, Gold, Carbon dioxide, Extraction