Research On The Extraction Of Gold From Gold-bearing Pyrite Cinder By High-temperature Chlorination Method

More than 8 000 000 tons of gold-bearing pyrite cinder is produced each year in China,and 22.8 tons of gold in such pyrite cinder could not be recovered,resulting in the huge waste of gold resources.Gold recovery rate is not satisfactory when such pyrite cinder is dealt with common hydrometallurgical methods because the gold is commonly wrapped.High-temperature chloridizing method has strong adaptability to raw materials.It could be applied to the extraction of gold from gold-bearing pyrite cinder due to its high separation efficiency.However,there are a series of problems for the application of conventional high-temperature chlorination process.The pulverization,scaffolding and ringing during roasting could affect the working rate.Heating procedure could not meet the technological requirements of process.Gold volatilization rate,the corrosion and adhesion of refractory is not satisfying.Higher demands on the chlorination process and chlorination reactor are drawn.Two typical gold-bearing pyrite cinders are selected as the research materials in the present paper to carry on the fundamental and applied research of the extraction of gold by high-temperature chlorination method.Optimal operating conditions are determined through the researches on the reaction mechanisms and gold chloridizing volatilization behaviour when CaCl₂ is used as the chlorinating agent,and the changes of pellet properties.The structural design of high-temperature chlorinationreactor is done by simulating computation.And appropriate refractory is selected by experiments.The experimental and computation results are verified by expanding test.Industrialization design is carried out based on the results of expanding test.The following conclusions are drawn.Research on the mechanism of the high-temperature chlorination-volatilization process for Au-CaCl₂ system was carried out.The results show that Au could be chlorinated by solid CaCl₂ only in the presence of gold-carrying minerals such as SiO2 or FeS2.Part of CaCl₂ evaporates at temperature above liquidus.Gaseous CaCl₂ has stronger chemical activity and can react with O2/H₂O to form Cl₂/HCI or chlorinate Au directly.SiO2 is the gold-carrying mineral that has the most impact on the high-temperature chlorination of Au.The reaction between SiO2 and CaCl₂ starts at 578-604℃,and the reaction has a turning point at 700 ℃.Au volatilization start temperatures are 350℃ and 750℃ when gold-bearing pyrite cinder and CaCl₂ pellets roasted in dry and humid air,respectively.Au volatilization rate reaches the maximum value when the heating temperature is 1100℃.The element volatilization rates become smaller if the heating up time is longer.More CaCl₂ addition or the addition of CaCO3 could reduce the impact of heating up time on the volatilization rate.Therefore,a satisfying gold volatilization rate could be ensured with short heating up time,large CaCl₂ adding amount and some addition agents like CaCO3.The research on the chlorination pellets made from gold-bearing pyrite cinder shows that grinding and adding organic or inorganic binders could ensure the quality of green ball and dry pellets.The strength of roasted pellets gradually decreases to less than 10%of that of dried pellets when temperature is higher than 400℃ during heating.The low mid-temperature strength is the main reason of pulverization of pellets during chlorination-volatilization process.Less addition of CaCl₂ and improvement of the strength of dry pellets could increase the mid-temperature strength.An appropriate CaCl₂ dosage and the existence of Fe3O4 are helpful for the forming of Ca-Fe-Si systematic liquid slag,which could fill the interstices between particles and increase the strength of pellets roasted at high temperature.However,too much more CaCl₂（>6wt%）could generate much more interstices and decrease the pellet strength.The structural design of high-temperature chlorination reactor is done by simulating computation.And the results show that there are optimal values for length-width ratio and height of shaft furnace,which are 0.95 and 1.75 respectively,to ensure the elemental volatilization rates and strength of roasted pellets.The usage rates of high temperature gas for different structures corresponding to the optimal values are the highest.The effects of furnace burden and Cl₂ on Al-Si systematic refractory are investigated.The results show that silicon in monomer silica and mullite could react with furnace burden.Ca-Fe-Si-Al-O systematic calcium ferrite is generated and leads to the adhesion between furnace burden and refractory during the chlorination process.α-Al2O3 has a stronger stability,and could react with iron oxides to generate solid solutions only when the temperature is high.Therefore,refractory with low Si content and high Al content is recommended for lining of high-temperature chlorination reactor.Based on the results of experimental research,high-temperature chlorination reactor for the recovery of Au from gold-bearing pyrite cinder,and expansion test with a production capacity of 20 thousand tons each year was conducted in Wuxi city.Ideal results were obtained.The results show that the pulverization ratio of dried pellets and roasted pellets are smaller.The quality of the roasted pellets is stable,and the mean compressive strength of roasted pellets could meet the technical requirements of oxidized pellets.The chlorination rates are 88%and 66%for Au and Ag respectively.No sign of corrosion and adhesion was found after long term use of refractory with low Si content and high Al content.Expansion test offer a technological support and guarantee to the industrialization.Based on the laboratory investigation and expansion test（20 000 t/a in Wuxi）,demonstration project of high-temperature chlorination process for the recovery of Au from gold-bearing pyrite cinder were built（150 000 t/a in Taian Shandong）.No waste water and residue are discharged and high recovery of gold is obtained in the project.Considerable economical,environmental and social benefits are predicted.