Selective Extraction Of Gold Research Articles

Basic trends in practical use of ion-and electron-exchange fibres and materials made from them

The optimum conditions for use of synthesized redox and ion-exchange fibres of different types in biosynthesis of the antibiotic heptaene, selective extraction of gold and silver from hydrometallurgical plant solutions, and sorption of radioactive iodine from a mixture of fission products were established. The basic physicochemical indexes of ion-exchange fibrous-porous materials were comprehensively evaluated in comparison to nonwoven needle-punch fabric made of VION KN-1 fibre. It was shown that fibrous-porous material (FPM) is more stable in a large number of liquid treatment cycles and is better wet with aqueous solutions of acids and bases when the sorption activity indexes closely correspond, and this facilitates regeneration and increases their efficacy. The use of a mixed fibre-resin charge in a small filter module for local treatment of waste washing waters from electroplating plants to remove heavy metal ions increased output from 250 to 500 liters/h.

‘Green’ leaching: recyclable and selective leaching of gold-bearing ore in an ionic liquid

The recovery of gold and silver from ore in an ionic liquid is reported for the first time. The 1-butyl-3-methyl-imidazolium hydrogen sulfate ionic liquid (bmim+HSO4−) was employed, with iron(III) sulfate oxidant and thiourea added. Selective extraction of gold (≥85%) and silver (≥60%) from powdered ore (of dominantly chalcopyrite/pyrite/pyrrhotite/sphalerite mineralogy) was achieved at room temperature in 50 h, with other lower-value metals present in the ore (Cu, Zn, Pb, Fe) extracted to only low percentages. Gold extraction was similar to that achieved in aqueous H2SO4/thiourea/Fe2(SO4)3, and silver extraction was significantly better. Moreover, the ionic liquid can be recycled following selective stripping of gold and silver on activated charcoal, with reuse in at least four successive treatments leading to neither ionic liquid degradation nor any loss in extraction efficiency.

Selective extraction of gold(III) in the presence of Pd(II) and Pt(IV) by salting-out of the mixture of 2-propanol and water

The mixture of 2-propanol with water has been employed to extract Au(III) along with other precious metals such as Pd(II) and Pt(IV) by using NaCl in the concentration range of 2.5–4.0 mol dm −3. Upon the addition of NaCl within this concentration range (2.5–4.0 mol dm −3) phase separation was attained. Gold(III) in aqueous phase was quantitatively extracted into the 2-propanol phase at 2.5–4.0 mol dm −3 of NaCl. The extraction of the other metals such as Pd(II) and Pt(IV) was much lower than for that of Au(III). Thus a maximal selective separation of Au(III) from these metals could be attained using the mixture of 2-propanol with water. A reaction mechanism involving the ion-pair of Na + and [AuCl 4] − has been proposed to explain this extraction.

The mobility of gold in tropical rain forest soils

The complex mineralogical and chemical system of a tropical rain forest weathering profile at the Ashanti mine, Ghana, was characterized to evaluate the role of inorganic and organic processes with the potential to mobilize gold in the rain forest soils. The concentration of gold has been measured in bulk saprolite, soil samples, and in pore-water extractions from the saprolite and soil horizons from the Sansu prospect at Ashanti. Gold concentration in the ground waters varied from below the detection limit (0.01 mu g/l) to a maximum of 84.4 mu g/l at the water table. Average gold concentration in the soil pore waters is 3.6 mu g/l. The gold in the pore waters is present as either soluble gold or as suspended particles less than 0.45 mu m in size.Despite the presence of significant gold concentrations in the soil pore waters, gold concentration in soils over the auriferous shear zones is up to 400 times greater than over the nonauriferous country rocks suggesting a limited gold mobility. Similarly, a close correlation between gold held in the solid soil fraction and in soil pore waters reveals very little in the way of supergene gold enrichment. Conversely, the gold is dissolved and reprecipitated over a range of a few centimeters. The high concentration of gold in the soil pore waters at Ashanti is a reflection both of the high gold content of the hypogene ore and of the efficiency of supergene fluids in the tropical rain forest environment to leach the gold but not retain it in a mobile form. Solubility calculations indicate that gold complexing in the soils appears to be dominated by AuOH(H 2 O), Au(CN) 2 (super -) , Au(S 2 O 3 ) 2 (super -3) , Au(NH 3 ) 2 (super +) , and a gold fulvate complex. Selective extraction of gold from organic and inorganic soil fractions reveals that the element is largely concentrated as native gold. A significant proportion (up to 28%) is concentrated in the light organic fraction of the near-surface soils which is dominated by fulvic acid. The role of gold adsorption by clays and hydrous Fe-Mn oxides appears to be small with less than 1 percent of the gold being present in an easily exchangeable form.Gold appears relatively immobile in the lower saprolite with gold values in solution being below the detection limit (0.01 mu g/l). Higher up in the saprolite, gold appears to be dissolved by hydroxide and thiosulfate complexes in the upper saprolite and by fulvate, cyanide, and hydroxide complexes in the soils. Due to the in situ nature of gold dissolution and reprecipitation, even short-lived species such as thiosulfate and cyanide can play an important role in the supergene chemistry of gold in the Ashanti soils. Halide complexes are not considered important owing to their low concentration in the ground waters (Cl, <9 mg/l) and the unrealistically high Eh-low pH conditions required. Humic acids are also not considered important in the dissolution of gold as they are only sparingly soluble in the pH range of the Ashanti soils (4.2-6.8).

PREPARATION OF ADSORBENTS FROM THE PROCESSED COMBUSTIBLE RAW MATERIALS

ABSTRACT Data are presented on the way in which petroleum residues and shale phenols can serve as raw material sources for producing high efficiency carbon adsorbents. These co-polycondensates produce mechanically strong adsorbents with a well-developed microporous structure. The new adsorbents may be useful for air cleaning air and gas cleaning, as well as selective extraction of gold from multi-component metal cyanide solutions.

Selective extraction and voltammetric determination of gold at a chemically modified carbon paste electrode

A sensitive chemically modified electrode for measuring gold by differential-pulse voltammetry is described. The electrode is prepared by dissolving triisooctylamine in carbon paste. Because of the selective extraction of gold, most of the common metal ions do not interfere with the stripping voltammetric measurements. The electrode was used to determine gold in authentic mineral samples without pre-isolation. The calibration plots are linear over the concentration range 0.1–10 µg ml–1 and the detection limit was found to be 0.03 µg ml–1 after a 15 min accumulation period. The preparation and electrochemical properties of the electrode were studied and the mechanism of the accumulation of gold at the modified electrode is discussed.

The extraction of metal cyanides by crosslinked polydiallylamine

The extraction of Au(CN) 2 −, Ni(CN) 4 −, Cu(CN) 2 − and Ag(CN) 2 − by polydiallylamine (PDAA) was examined. Crosslinking was found to have a major effect on monovalent/divalent anion selectivity. PDAA with a low degree of crosslinking displays a clear preference for Ni(CN) 4 − over Au(CN) 2 − under most conditions, with the gold species only favoured at high pH. Increasing the degree of crosslinking significantly lowers the distribution of nickel by limiting the flexibility of the resin chains, thereby reducing its ability to orient functional groups around the divalent anion. Selective extraction of gold may therefore be achieved at a lower pH. X-ray photoelectron spectroscopy (XPS) shows that Ni(CN) 4 − and Cu(CN) 2 − are adsorbed intact and that Ag(CN) 2 − adsorbs with a stoichiometry of Ag(CN) 1.5 x− . This contrasts to earlier results for Au(CN) 2 − which indicate decomposition to an AuCN species.