Theoretical And Experimental Research On Vacuum Separation And Purification Of Crude Silve

Precious metals silver and gold are strategic metals to ensure the security of the national financial system and indispensable key basic materials to support the development of modern high-tech industries.The existing crude silver purification methods have many problems such as strict requirements for raw material components,high consumption of chemical reagents and the like.Based on the advantages of vacuum metallurgy,such as short process and less pollution,and the differences of physical properties such as saturated vapor pressure,evaporation rate and condensation characteristics,the academic concept of separating crude silver by vacuum metallurgy was proposed,and the theoretical and experimental investigations on vacuum separation and purification of crude silver were carried out.The main conclusions are as follows:(1)The study of vacuum evaporation-condensation theory and evaporation kinetics of pure silver shows that the saturated vapor pressure and the theoretical maximum evaporation rate of pure silver increase significantly with the increase of temperature.With the increase of temperature and the decrease of pressure,the mean molecular free path of silver increases.The temperature of the silver triple point is 1234.78 K,and the pressure is 0.343 Pa.The relationship between the average evaporation rate of pure silver and temperature is lgω=A/T+B,and the relationship between the average evaporation rate of pure silver and pressure conforms to the Logistic model.The vacuum evaporation of pure silver is a first-order reaction,and the kinetic equation is-lnmt/m0=15.5382e-211020.9596/RT A/Vt.In the range of 1473～1673 K,the gas phase mass transfer is the rate-controlling step.With the increase of temperature,the rate controlling step gradually transforms into gas relative flow mass transfer and gas-liquid interface evaporation mass transfer.(2)The analysis of crude silver raw materials show that the matrix of crude silver is Ag-Cu-Au alloy phase and Ag-Cu-Se-Te phase composed of Ag2Te,Ag2Se,CuSe,CuTe and other compounds.The basic study of vacuum separation and purification thermodynamics indicate that the order of the saturated vapor pressure and the theoretical maximum evaporation rate of each element in crude silver is Se>Te>Sb>Bi>Pb>Ag>Cu>Au.With the increase of equilibrium pressure,the phase transition temperature of each element increases.At the same pressure,the condensation temperature of gold and copper is higher than that of silver,while the phase transition temperature of lead,antimony,bismuth,selenium and tellurium is lower than that of silver.MIVM is accurate and reliable in predicting the activity of Ag-Au,Ag-Cu,Ag-Pb,Ag-Bi,Cu-Pb,Cu-Bi,Au-Pb and Au-Bi binary alloys,and vacuum gasification can easily separate the above binary alloys except Au-Cu alloy.The calculation and drawing of the vapor-liquid equilibrium composition diagrams and vapor-liquid equilibrium phase diagrams of the above binary alloy and Ag-Cu-Au alloy can realize the prediction of product composition,as well as the correlation of temperature-composition and pressure-composition.The experimental results of vacuum separation of gas phase composition of Ag-Cu-Au alloy are similar to the calculated results.(3)The results of vacuum thermal decomposition and evaporation of silver telluride and silver selenide show that when the pressure is 1 Pa and 100 Pa,the temperature corresponding to the thermal decomposition of silver telluride to Te(g)and Ag(g)is 1476.18 K and 1887.02 K,respectively.The temperature corresponding to the decomposition of silver selenide to Se(g)and Ag(g)is 1552.68 K and 1993.63 K,respectively.With the increase of temperature,the decrease of pressure and the extension of holding time,the distribution rate of tellurium,selenium and silver in volatiles increased,the distribution rate in residues decreased,the purity of selenium and tellurium in volatiles decreased,and the purity of silver in residues increased.(4)The experimental study on the two-stage vacuum separation and purification of coarse silver shows that the content of volatile silver obtained from the high-temperature-low-temperature vacuum separation and purification of coarse silver at 1673 K,10 Pa and 90 min increased from 84.33 wt.%in the raw material to 91.84 wt.%.The gold content in the residue was 72.80 wt.%.The volatiles were mainly pure silver phase and Ag-Cu-Se-Te phase,and the residues were mainly Au-Cu alloy phase and Ag-Cu-Au alloy phase.The purity of silver in the residue was 95.11 wt.%and the distribution rate was 91.14%under the conditions of 1523 K,10 Pa and 120 min.The low-temperature-high-temperature vacuum separation and purification of crude silver was carried out at 1573 K,1 Pa and 120 min.The residue obtained in the low-temperature section of the experiment was Au-Ag-Cu alloy.The contents of gold,silver and copper were 22.49 wt.%,71.58 wt.%and 5.68 wt.%,respectively,and the distribution rates were 99.89%,27.07%and 94.34%,respectively.Using the residue as raw material,the purity of silver in the volatile obtained by high temperature section experiment at 1623 K,10 Pa and 120 min was 99.08 wt.%.(5)The experimental studies of vacuum evaporation-fractional condensation separation and purification of crude silver shows that the total evaporation rate of crude silver is 90.36%,and the distribution rates of gold and silver in the residue are 98.26%and 1.10%,respectively,which effectively realizes the separation of gold and silver.The first and second order condenser volatile were volatilized by vacuum evaporation-fractional condensation,and gold was enriched by 149 and 128 times in the residues,respectively.The purity of silver could be further improved,and the main phases of the volatiles were Ag,Ag2Se and Ag2Te.The residue was volatilized by vacuum evaporation-fractional condensation,the gold content in the residue increased from 75.71 wt.%to 83.88 wt.%,the silver content decreased from 11.90 wt.%to 4.41 wt.%,and the main phases were Au-Sb compound phase,Au-Cu alloy phase,Au-Ag-Cu alloy phase.The precious metals platinum and palladium are mainly enriched in the residue during the vacuum evaporation-directed condensation process.(6)The research on vacuum evaporation-directional condensation separation and purification of crude silver shows that the vacuum evaporation-directional condensation furnace developed successfully runs stably and the temperature gradient is controllable.Increasing the temperature of the upper temperature zone and prolonging the holding time are beneficial to reduce the content and distribution rate of selenium and tellurium in the volatiles of the upper temperature zone,but the content and distribution rate of gold and copper in the volatiles of the upper temperature zone will also increase.Under the conditions of TL=1723 K,TU=1673 K,t=5 h,p=10 Pa,ΔTL=10 K/min,the volatile in the upper temperature zone is pure silver phase containing a small amount of Ag-Cu-Te-Se phase.The contents of silver,selenium and tellurium in volatile are 97.14 wt.%,0.54 wt.%and 2.64 wt.%,respectively,and the distribution rates are 74.46%,41.55%and 27.13%,respectively.The residue in the lower temperature zone is Ag-Cu-Au alloy phase.The contents of silver,gold and copper were 16.05 wt.%,72.19 wt.%and 12.51 wt.%,respectively,and the distribution rates were 1.81%,98.53%and 63.85%,respectively.In summary,the interphase distribution behavior of components in vacuum separation of crude silver was studied in this paper.The kinetics model of vacuum evaporation of pure silver was established,and the thermodynamics of vacuum separation of crude silver were systematically calculated.The vacuum evaporation-directional condensation furnace with stable operation and controllable temperature gradient was successfully developed,and a series of experiments of vacuum separation of crude silver were carried out to clarify the distribution behavior of various elements in the gas-liquid phase.The source solves the fundamental problem of mutual wrapping of gold and silver in the process of precious metal extraction and purification,and realizes the green and efficient separation of crude silver.