All electronic scrap is converted into base metal bullion by a series of steps which includes chopping, incineration, milling, screening, melting, and casting anodes. Incoming circuit boards assay from 0.02% to 0.05% in gold, 0.04% to 0.3% in silver, with small quantities of palladium and platinum. Integrated circuits contain 0.005% to 0.007% gold, 0.07% to 0.5% silver and platinum group metals. Anodes are used in the hazardous waste recycling process.
For example, one sample of metal hydroxide sludge was treated in a sludge digester with 50% sulphuric acid, to pH 1. The slurry was pumped to a filter press. The washed filter cake consisting of lead sulphate, calcium sulphate, silica, from filer aid and other salts. A sample of the wet filer cake was assayed and reported 3.2 oz/t gold, 32 oz/t silver and 1.1% platinum group metals. The entire cake is dried and converted to impure Dore bar. All of the inerts are slagged out and part of lead can be recovered when is possible. This is a very saleable commodity with precious metals.
The filtrate is sent to intermediate product hold. After laboratory analysis, it can be processed. The filtrate is placed in a glassed steel reactor designed for distillation. Enough sulphuric acid is added to react metathetically with all metals and salts to produced essentially sulphate species in the reactor. The distillation after addition of sulphuric acid is designed to remove most hydrochloric and nitric acids. When most of the volatile acids have been removed, the reactor is cooled and the solution pumped to intermediate hold electrolyte.
By employing this leaching process is possible to recover copper by electrowinning. Some electrolytic cells heated by steam and the number of cathodes and anodes are 30 and 29, respectively. Basically, depending on the electrolyte and whether it is desirable to reduce the electrolyte in copper or hold the copper content constant, the anodes are fabricated of lead or copper containing gold and silver obtained from electronic waste. When this waste material is employed as anodes, gold and silver are deposited at the bottom of the cell as anode mud or slimes. The latter contains copper, gold, silver, platinum group metals, lead sulphate and other minor metals found in the anode are separated by screening, typically 28 mesh. In this way is possible to separate large pieces of anode copper and the resulting slurry is filtered, the cake is dried and melted with sodium carbonate, borax flux. It has been noted that depending on the metallic content, the refining process must be selected. For example, if high in silver, electrowinning can be practiced on this material. Finally, the electrolyte from the copper recovery cell is processed to remove the remaining metals.