Abstract:Sulfur-containing gold ore is the main raw material for gold extraction in recent years. However, the cyanidation effect is usually suboptimal due to the passive film being easily formed on the gold surface, and the leaching agents, i.e. cyanide and dissolved oxygen, are severely consumed by sulfide for sulfide ore ores. Ultrasound is a mechanical wave higher than 20 kHz of frequency. The introduction of ultrasound in the liquid will produce cavitation accompanied by the formation of strong oxidizing ?OH radicals, local high pressure and high temperature, shock waves, and high-speed jets. Ultrasound-assisted leaching has been used as an effective method of metal extraction. To improve the low cyanide rates of sulfur-containing gold ore, ultrasound was introduced into conventional cyanide leaching and the influence of ultrasound on gold cyanide leaching rate and kinetics was investigated to further explore the intensification mechanism. The leaching process was carried out in a 500 mL glass reactor placed in a temperature-controlled water bath. Sodium cyanide solution with certain volume and concentration was put into the glass reactor and reacted with 50 g gold ore. A magnetic stirrer was used to stir the pulp at a rate of 500 r/min to completely suspend solid particles. The pH value of the pulp was adjusted by sodium hydroxide and nitric acid. A probe-type ultrasonic generator emitting 20 kHz ultrasound was used for ultrasound-assisted leaching. The horn of the ultrasonic generator was immersed about 2 cm below the pulp level. The concentration of thiocyanide was determined by an ultraviolet-visible spectrophotometer at the wavelength of 460 nm after coloration with ferric nitrate. The concentration of free cyanide was analyzed by silver nitrate titration. Gold content in solution was determined using an atomic absorption spectrometer (AAS). Phase compositions of gold ore and leaching residue were identified by an X-ray diffraction (XRD). The Brunauer-Emmett-Teller (BET) was used to determine the specific surface area and pore volume. The size distributions of gold ore and leaching residue were detected by Laser Particle Size Analyzer. The surface morphologies of the gold ore and leaching residue were observed using a scanning electron microscope (SEM). The effect of the liquid-solid ratio on the gold extraction was examined ranging from 3 to 7 mL/g. The liquid-solid ratio of 5 mL/g was selected because the external diffusion resistance of cyanide could be eliminated at this liquid-solid ratio. The leaching rate at ultrasonic power of 360 W is approximately 17% higher than that of conventional leaching. The gold extraction under the ultrasonic action rapidly rises from 43.5% to 67.5% with an increase of initial cyanide concentration from 8.1 to 32.5 mmol/L and then slowly raises to 68.4% when cyanide concentration reaches 40.8 mmol/L. Improving the pH value (9.8 to 11.5) can favor the gold leaching, and gold leaching is less affected by pH value when pH value is higher than 11.5 with or without ultrasound. The gold extraction rises with the increase of reaction temperature. Moreover, the gold extraction of ultrasonic-assisted leaching is about 10% higher than that of conventional leaching. The results show that at 35 ℃, 5 mL/g liquid-solid ratio, 360 W ultrasonic power, 40.8 mmol/L cyanide concentration, initial pH value of 12.5, gold extraction is 53.3% and 68.4% after ultrasound-assisted leaching two hours and six hours respectively, while 51.3% of gold is extracted in conventional cyanidation after six hours. The leaching rate of gold rises by 17% under the action of ultrasonic wave, and the time to reach 51% is shortened to 1/3 of that of conventional leaching. According to the above analysis, the gold cyanidation is controlled by the internal diffusion step, and the leaching process is affected by temperature, ultrasonic power, and cyanide concentration. Furthermore, the kinetic data of gold extractions fits the internal diffusion model with and without ultrasound. The Arrhenius equation is used to calculate the activation energy of the gold cyanidation in the presence and absence of ultrasound. The activation energy drops from 21.83 kJ/mol of conventional cyanidation to 14.07 kJ/mol with ultrasound, which are slightly above 12 kJ/mol and significantly below 42 kJ/mol indicating that the leaching process is mainly controlled by internal diffusion. The results of the SEM, BET surface areas, and particle size distribution further indicate that the enhancement of cyanide leaching by ultrasound is attributed to the thermal and mechanical effects brought by cavitation, which promote the solid-liquid mass transfer and accelerate the reaction process. It is mainly reflected that the pulp temperature rises, the specific surface area of particles increases, the particle size decreases, and pores and gullies appear in mineral particles under the ultrasonic action.