Selectively Leaching Gold From Waste Printed Circuit Boards With Thiourea

With the shortage of non-ferrous metal resources and ever increasing of electronic wastes,metal recycling from e-waste has been attracting much attention from the viewpoints of environmental protection and resource utilization.The contents of 18 metal elements in waste printed crcuit boards(PCB)were determined by means of ICP-AES assisted by FAAS and iodimetry after microwave digestion.Basing on the analysis result,twice oxidizing-acid leaching pretreatment has been applied to waste printed circuit boards after mechanical pretreatment for designing a recovery process for valuable metal such as copper.The separation of gold and silver was attained through another two-stage:acid thiourea leaching and Zn/Fe powder cementation.The result shows that twice oxidizing-acid leaching pretreatment makes less hazardous air pollutants of nitrogen oxides than traditional oxidizing-acid leaching one,the process is high efficient,environment-benign and moderate,at the same time,the recovery of Ag is available.The leaching rate of Cu is 97.6%. The gold leaching process is optimized designed and the leaching ratio of gold is 95.1%,silver is 80.5%and in the Zn/Fe cementation experiment, the recovery ratio of Au and Ag is about 90%.The determination by ICP-AES with microwave digestion of the 18 metal elements in the printed circuit board scrap is speedy and simple, and the data is reliable.The recovery ratio obtained by standard addition method ranged between 91%¹11%,and relative standard deviation is below 3.7%(n=6).That brings it can be used in simultaneous determination of different metals of different contents in electronic wastes and electric-electronic appliances.In the pretreatment,primary lixiviation is practiced by using nitric acid-sulfuric acid-mixing system.In the optimized process of system, fewer amounts of nitric acid is used which reduced NO_x release from the craddle and decrease the amount of silver in the primary lixiviation.The nitrogen oxides produced in the experiment are absorbed by water to gain the dilute nitric acid.The result showed when using ten grams of metal particles at the solid-liquid ratio of 1:10,the best copper leaching rate is 89.2%under the condition that the concentration of nitric acid is 1.2mol/L and sulfuric acid is 3 mol/L,leaching time is 1.5 hours,stirring speed is 400r/min and leaching temperature is 50℃.Aiming at the 90% copper leaching rate of primary lixiviating process,secondary dilute nitric acid leaching pretreatment is posed to reduce or eliminate the negative influences of base metals on gold leaching,the lixiviating rate of Cu is up to 97.6%.At the same time,the recovery of Ag in secondary dilute nitric acid leaching solution is available and also use water to absorb the harmful gases.The resault of gold leaching tests showed that the concentration of thiourea is the most important factor in leaching system,besides,the concentration of oxidants,pH,leaching time,stirring condition and temperature is also considered.By using pre-treated 2g sample with the solid-liquid ratio of 1:10,gold and silver leaching rate is 95.1%and 80.5%respectively,on the condition that stirring speed is 300r/min,the concentration of thiourea is 12g/l,Fe₂(SO₄)₃ is 0.8%,leaching time is 60 minites,pH is 1.5 and temperature is 32℃. The recovery experiments showed that both Zn and Fe powders can successfully replace the gold and silver in the leaching solutions and the recovery ratio is about 90%.However by contrast to the cementation of cyanide leaching solution,the presence of oxidants of Fe³⁺ion and acidic leaching medium caused the dosage of base metals increase.According to the paper,F^- ion can be used to mask the negative influences caused by Fe³⁺ion,which can both reduce the amount of base metals and enhance the ratio of recovery.Aiming at the weakness of acid thiourea system,by application of stabilizers and optimized selection of oxidants,proper redox potential ferric complexes were selected to replace Fe³⁺as the system oxidants,and selected Fe(CN)₆^3-/Fe(CN)₆^4-and FeF₆^3-/FeF₅^2-as redox couple,which theoretically extended the pH circumstance of gold leaching with thiourea. By using pre-treated 1 g sample with the solid-liquid ratio of 1:20,gold leaching rate is 38.6%under the condition that stirring speed is 300r/min, pH is 12,the content of potassium ferrocyanide reagent oxidants is 0.25mol/L,thiurea concentration is 0.2 mol/L,stabilizer sodium sulfite is 0.05tmol/L,leaching time is 3 hours.In the moderate weak acid thiourea system,the addition of F^- ion adjusts the redox couple of the solution and being the assisted oxidizer,and inhibited the hydrolysis of Fe³⁺ion which improved the recovery ratio by 3.7%.