| Waste Electrical and Electronic Equipment (WEEE), an important component of "urban resources", contained abundant and valuable secondary resources——nonferrous metals such as copper, tin, lead, and precious metals such as gold, silver and palladium. Mobile phone had become an indispensible tool in daily communication. With innovation of science and technology, mobile phones updated much more frequent which leaded to a great amount of waste phones. Occupying a big part of WEEE, waste mobile phones had a high recycling value of precious metals such as gold, silver and palladium. Therefore, the recycling routes on precious metals gold and silver from printed circuit boards(PCBs) in waste mobile phones was established based on investigating relevant researches on precious metals recovery in mobile phones and other electronic wastes. The first step focused on gold recycling through pretreatment for gold accumulation, aqua regia leaching, organic solvent extraction/stripping to get high-concentration gold solution. The second step centered on silver recycling through nitric acid leaching, chloride precipitation, aqueous ammonia dissolution and zinc reduction to get black silver powder. By systematically studying the recycling technology in each phase, a high-efficient gold and silver recycling technology from PCBs in waste mobile phones was proposed and got good recovery efficiency.Firstly, the leaching conditions of nitric acid pretreatment of copper and silver separation were studied after clarifying the main components and contents of metals in PCBs. And then chloride gold leaching system with chlorate, hypochlorite and nitric aicid-hydrochloric acid gold leaching system were systematically discussed setting residues as samples after pretreating with nitric acid. The researching results indicated that the fold leaching rate was less than60%with chloride gold leaching system (with chlorate and hypochlorite), and the leaching efficiency in the system was not perfect; the gold rate could raise to85%in nitric acid-hydrochloride leaching system with different proportions of nitric acid and hydrochloride acid. But the optimal gold leaching methods was aqua regia system in which gold could be leached out totally. It could be explained that gold existed as gold-based alloy in mobile phones PCBs and gold could not be oxidized when using a single oxidant such as chlorate or hypochlorite. At last, an optimal leaching conditions of PCBs pretreatment and auqa regia gold leaching were determined. The gold leaching rate was100%and silver was99.64%.Setting gold containing solution as researching objective after leaching with auqa regia, the key influencing factors on extraction and stripping rates were discussed by gold extraction with tributyl phosphate (TBP) and stripping with acidic thiourea solution. The optimal gold extraction conditions were showed as follows:organic phase was20%TBP and80%kerosene, phase ratio (A/O) was1:1, solution acidity was1.5mol/L, shaking time was3min and standing time was5min.The extraction rate of gold was99.68%. The optimal stripping conditions were showed as follows:the thiourea concentration was0.5mol/L and the acidity of striping solvent was1.0mol/L. After calculating, the total recycling rate of gold in waste mobile phones PCBs was98.70%.In terms of silver recovery, though adding sodium chloride to silver containing nitric solution, precipitant AgCl was formed. And then the black silver powder was got after using aqueous ammonia and zinc powder for dissolution and reduction. It is indicated that the chloride ion concentration was a key factor affecting the silver precipitation rate. A high chloride ion concentration could make AgCl dissolve into solution again existing as silver-chloride complex ion. In addition, copper and lead ions could co-precipitate as CuCl and PbCl2with silver ion which affects the precipitation rate. CuCl could be remove under oxidant condition and PbCl2could be removed by heating. The results indicated that the silver precipitation rate was94.19%using chloride; reduction rate of silver was99.28%using aqueous ammonia and zinc powder. After calculating, the total recycling rate of silver in waste mobile phones PCBs was93.18%. |
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