Electronic waste includes waste products, scraps, and discarded printed circuit boards from the manufacturing of electronic components and parts, as well as obsolete computers, appliances, and mobile phones. Every year, millions of computers, televisions, and other household appliances, and tens of millions of mobile phones are discarded, containing significant amounts of precious metals. Roughly estimated, 5 million computers contain approximately 4.2 tons of Au, 30 tons of Ag, and 1.5 tons of Pd. 50 million mobile phones contain approximately 140 kg of Au, 1 t of Ag, and 50 kg of Pd, totaling 4.34 tons of Au, 31 tons of Ag, 1.55 tons of Pd, and a small amount of Pt. This represents a substantial amount of resources, and many small businesses and workshops in China are actively engaged in recycling. However, the primary focus of electronic waste recycling is on Au, Ag, Pd, and Pt, with the latter accounting for only a small percentage of the total secondary resource recycling volume.
Electronic waste is characterized by its large volume, low-grade precious metals, and complex composition, containing significant amounts of Cu, Sn, Pb, as well as bakelite and plastics. Workshop-style processing mainly uses incineration to decompose bakelite and plastics. The slag is dissolved in calcined acid or aqua regia, and precious metals are then separated and recovered from the solution. This method has low material utilization and precious metal recovery rates, and causes serious environmental pollution. A comprehensive approach suggests establishing specialized dismantling plants in designated areas to dismantle and classify materials as follows:
- Reusable components and easily recyclable plastics, wires, etc.
- Materials that easily cause environmental pollution
- Waste components and circuit boards containing non-ferrous and precious metals. For the latter, the most effective processing technology is closed rotary kiln roasting → smelting in a blast furnace or electric arc furnace → obtaining a copper-dominant metallic phase containing precious metals for electrolysis, recovering Au, Ag, Pd, and Pt from the copper anode slime.
Yao Hong’s research method is as follows: Electronic waste → crushing, grinding, and sieving → magnetic separation of coarse material to separate iron-containing components → dissolving fine powder (less than 40 mesh) twice with HCl + H2O2 → adding an appropriate amount of Na2SO4 to the precious metal solution to precipitate lead → iron powder replacement → leaching of precious mud containing >50% Cu and 2% precious metals twice with HCl + NaClO3 at 65℃ → mixed solution of Pt, Pd, Au, Cu, Pb, Sn, and Fe → separation and refining into precious metal products. Processing 35 tons of waste components yielded 28.18 kg of Pd, 5.2 kg of Pt, and 150 kg of Au. The purity of P and Pd was 99.95%, with recovery rates of 97.4% and 96.5%, respectively.
Methods for recovering gold, palladium, and silver from discarded mobile phones. After separating the outer casing and battery, the motherboard was crushed. Light bakelite powder was washed with water to obtain a sediment containing 0.92% Au, 0.38% Pd, and 1.67% Ag. First, non-metallic components were separated by calcination at 600 °C. Ag and Pd were dissolved in 25% HNO3 solution at 60–80 °C, with a solubility rate >98.5%. NaCl was added to the solution to precipitate AgCl, and palladium was recovered from the silver precipitation mother liquor. The nitric acid-insoluble residue was dissolved in aqua regia for gold dissolution, and then reduced to crude gold with sodium sulfite. The overall recovery rates of gold, palladium, and silver were all >95%.
A special gold leaching technique was studied, involving the addition of MnO2 and NaSCN solution, leaching gold at room temperature (pH 1–2), achieving a leaching rate of 96%. This material requires comprehensive utilization, but the leaching behavior of other metals was not investigated.
