Sustainable strategy for removing Ag impurities from crude gold via a vacuum distillation process to produce a 4N gold product

Gold plays an irreplaceable role in economic development, jewelry making, precision electronics, medicine and other industries and has extremely high economic value. The traditional gold refining process is limited by environmental concerns, high costs, and various issues when Ag and Au are intertwined with each other. In this study, a detailed theoretical prediction and an industrial practice for removing the Ag impurity from crude gold via a vacuum distillation process is developed. The saturated vapor pressure, separation coefficient, vapor‒liquid equilibrium diagram of the Au-Ag alloy, evaporation rate, and molecular free path for the main elements in crude gold in the vacuum distillation process were theoretically calculated. Two industrial experiments on the vacuum distillation process for removing Ag from crude gold were separately conducted. For the crude gold with an Au content of 53.33 % and an Ag content of 46.66 %, the Au content in the distilled residue was enriched to 96.62 %, and 3.38 % Ag was reserved at 1773 K with a soaking time of 3.5 h, and 96.03 % of the Ag in the crude gold raw material was removed in the form of volatile matter. For the crude gold with an Au content of 91.54 % and an Ag content of 6.40 %, 3.46 % Ag remain in the distilled residue, and gold was purified to 94.42 % at 1723 K with a soaking time of 6 h. The vacuum distillation-aqua regia leaching process had a stronger adaptability to crude gold raw materials; the yield of 4N gold after one operation was increased to over 96 % with respect to 60–80 % from the conventional aqua regia leaching process, the refining time of gold was shortened by two-thirds, and the consumption was also significantly reduced. This study contributes to the advancement of purification technologies and the sustainable production of high-purity gold for diverse industrial applications.