Recycling of valuable metals
Recovery of hardmetal scrap means the selective dissolution of the cobalt by aqueous acetic acid and the recovery of both valuables tungstencarbide skeleton and the binder phase cobalt. The acetic acid together with pure oxygen penetrates the scrap pieces from the surface and results in an selective leaching of the binder phase within a couple of days (depending on the size of the individual scrap pieces). The remaining carbides can be crushed and used again as starting material for new hardmetals. The resulting cobalt-acetate can be used as precursor for the preparation of nanocrystalline Co-metal powders by high pressure precipitation methods.
The advantages of this method lie in the fact that it is a "green" process using harmless chemicals and that the carbides e.g. already include the grain growth inhibitors homogeneously.
A second possibility for recycling of hardmetals and heavy metal scrap is given by the use of molten salts. Here the pieces are contacted with soda and/or soda/sodiumsulphate at temperatures above 900°C. Bulk pieces of several cm length can be easily dissolved completely within a couple of minutes. Sodium-tungstate and cobalt oxide are formed as products.
Recovery of tungsten from W/Cu scrap can be performed by an ammoniac and residue-free dissolution of compact scrap pieces and chippings and subsequent separation of Cu by hydroxide formation, sulphide formation, cation-excahnge and S/X respectively.
Another possibility for processing tungsten-containing materials is the so-called "zinc reclaim" process, in which liquid zinc penetrates the scrap pieces and forms various intermetallic compounds with the binder. This results in a change in the volume of the scrap parts, which means that they can easily disintegrate after the zinc is distilled off. What remains is a powder that corresponds exactly to the initial composition.
Precious metals - as the epitome of valuable metals - have been the focus of the group's research for more than a decade. The investigations focus on the recovery of Pt, Pd, Rh and Ir from spent Pt and Pt alloys, which are mainly used in the glass industry. For this purpose, the Pt-based structural components, including unwanted base metal impurities, are dissolved in hot aqua regia, followed by separation of the various precious metal components by S/X as well as precipitation methods resulting in pure liquid streams. The precious metals are then obtained as pure powders by thermal decomposition of the respective salts or by addition of a suitable reducing agent such as hydrazine hydrate. The resulting metal powders are then suitable for processing into ODS-Pt materials. Alternatively, refining can be done by vacuum induction melting, taking advantage of the different vapour pressures of the impurities. Undesirable and harmful base metals such as Ni, Cu, Zn, Sn, Ti, W, Sb, As, etc. can be easily removed. Suitable materials are obtained where the impurities are below the critical concentrations (mainly below 10 ppm).
Magnesiumchloride MgCl2 (including additives such as NaCl and KCl) is used as a refining salt as well as a barrier against the oxidation of a molten Magnesium. Secondary Magnesium production produces large amounts of dross, which is mainly landfilled. Since the dross contains Magnesiumoxides as well as Magnesiumchloride as a valuable material, the recovery by selective leaching with ammoniumchloride was investigated. The main focus is on the recovery of anhydrous Magnesiumchloride, which can be achieved by thermal decomposition of the leaching intermediate ammoniumcarnallite NH4MgCl3*6H2O.