Study On The Release Of Elemental Sulfur From Gold-Bearing Sulfides Under Microwave Irradiation And Its Mechanism Of Action In Gold Leaching

At present,cyanidation is the dominant method for gold extraction,but the highly toxic nature of cyanide has brought huge safety and environmental pressure to gold mining enterprises.Alkaline sulfur-containing lixiviants are considered to be the most promising alternative to cyanide,which can be prepared by using elemental sulfur dissolved in alkaline solutions.In view of the serious environmental pollution of cyanidation process,this research brings forward a new non-cyanide leaching method using elemental sulfur generated from the thermal decomposition of gold-bearing sulfides in the microwave field.The thermodynamic analysis of the release of elemental sulfur and essential law of phase transition during in-situ heating of pyrite was investigated.The mechanism for the formation of elemental sulfur under microwave irradiation and the beneficial effect of microwave heating on the sulfur conversion and release was revealed.In addition,the mechanism of the action of generated elemental sulfur in alkaline solutions was studied by using the quartz crystal microbalance with dissipation monitoring(QCM-D).All these works have improved the foundation and theoretical system of this novel leaching technique.The main research results are as follows:Pyrite is the main source of elemental sulfur during thermal decomposition of gold-bearing sulfides.The in-situ phase transition of pyrite occures in the temperature range of 500～600℃,and the effect of temperature on the v(S-S)stretching vibration is more significant.As the temperature increased,the pyrite crystal underwent linear thermal expansion with the average volume expansion coefficient is 5.64×10-3 K-1,and the expansion of pyrrhotite crystals was faster along the z axis.The imaginary part of the complex permittivity of high-sulfur gold concentrate was determined to be 9.99 at 2.45 GHz,which increased by 4.76 times when the temperature reached 700℃.Although it decreased to 27.76 at 900℃,the value of the imaginary part was still rather than the value of 11.40 at 500℃.When the sample temperature reached 450℃ under microwave irradiation,pyrrhotite phases were found in the products,and the sulfur conversion rate reached 46%at 900℃.With the temperature increasing,the pyrite in cubic system gradually transformed into pyrrhotite in hexagonal system.The chemical state of sulfur was first converted to Sn2-from S22-in pyrite,and then it further transformed to S2-with the increasing temperature,in which the presence of Sn2-indicated the formation of S0 in the products.After pyrolysis,a large number of cavities and pore structures were found inside the mineral particles,forming nano-sized pyrrhotite poly crystalline aggregates.The porosity increased from 32.9%to 48.9%,and the total pore area increased by 2.16 times,when the sample was heated to 700℃ for 30 min under nitrogen atmosphere.The gold particles encapsulated by the sulfides were exposed,increasing the chance of contact with leaching gold agents.After comparative study,it is found that microwave radiation heating reduced the phase transformation temperature during the thermal decomposition of pyrite,promoted the transformations of sulfur forms,reduced the activation energy of pyrite decomposition by 27.14 kJ·mol-1.The Fe-S bond is more likely to break than S-S bond,and the ortho-octahedral structure[FeS6]gradually disintegrates with increasing temperature.The applied electric field results in a small displacement of the electron cloud in pyrite lattice,which weakens the interatomic interaction forces,and the applied magnetic field also produces the facilitating effect on the thermal decmopsition reations of pyrite to produce elemental sulfur.It has been confirmed that the self-leaching of gold-bearing sulfides using the elemental sulfur generated during the microwave pyrolysis process,and the recovery of gold can reach over 90%.An in-situ study on the mechanism of action of generated elemental sulfur in gold leaching process was conducted by using the quartz crystal microbalance with dissipation(QCM-D).The results indicate that the source of elemental sulfur has little effect on the gold leaching performance,and the elemental sulfur generated during the microwave pyrolysis of gold-bearing sulfides and sublimated sulfur had the same leaching ability in alkaline solutions,and the gold leaching rate is 0.59 ng·cm-2·s-1 and 0.68 ng·cm-2·s-1,respectively.Since the copper-ammonia complex ion could eliminate the adsorption of sulfur-containing species on the gold surface,the dissolution rate of gold significantly increased.The sulfur-containing species resulting from the dissolution of elemental sulfur in alkaline solutions takes place viscoelastic adsorption on the gold surface,and the higher the concentration,the stronger the flexibility of the adsorption layer,the more serious the passivation of gold surface.Furthermore,the gold leaching reactions of sulfur-containing reagents occurs at the interface of gold and alkaline sulfide solutions to form Au-S complexes,and then dissolves into leaching solution.