It's a long, long way along the path to pure metal when recycling precious metals. Probably the most important milestone along the way are the results of the analysis – as the foundation for the actual recycling of gold, silver and the rest.
Items that contain precious metals may have different shapes and origins. The trick to arriving at uniform metal content is to smelt and homogenise these pieces. The major difference between batches of scrap is clear when you consider how different the precious metal can be from the alloys.
In the dental area alone, there are currently in the region of 1000 different precious metal dental alloys in Germany, all of which eventually end up in the recycling cycle.
Today, scrap is liquefied using state-of-the-art induction technology whereby an induction field is used to heat the scrap to the necessary temperature. This functions similar to the way a microwave works. However, the temperatures differ from one smelting process to the next. The scrap is heated to between 1200°C and 1800°C.
Therefore, we need powerful smelters (30–120 kW). It takes about 1 kW of energy to smelt 1kg of material, meaning that the process is relatively efficient and environmentally friendly, given the extremely high temperatures. The induction needed for the smelting process depends largely on the kind and volume of the scrap melt.
The smelter's oven consists of a water-cooled induction coil in a heat-insulated housing. Inside the coil is the graphite crucible that holds the melt. The induction field heats both the crucible and the metallic melt directly, blending and homogenising the melt.
High smelting losses occur in smelting. At these high temperatures, all the combustible impurities and attachments such as oil, paper and plastic burn. Ceramic veneers also melt and then float to the surface as slag because of their lesser density. Where possible, any base metal parts from dental work are removed from the melt.
The volume of melt also shrinks due to the partial oxidation of the base metals in alloys such as zinc and copper. These metal oxides are absorbed by the flux and brought to the surface as slag. Consequently, alloys with a lot of non-precious metal content can be expected to result in more loss of melt volume.
The melt is poured into a rectangular mould to cool. Once solidified, all impurities and slag are removed from the surface of the mass. Only when this has been done can the precious metals be visually inspected to ensure that they are properly homogenised.
After weighing the mass, samples are taken to determine the precious metal content. This is usually done by drilling diagonal burrs on the top and bottom of the mass. Drilling twice ensures that the results are as representative of the entire mass as possible. It is also possible to take samples by sawing cross-sections of the entire mass. This is quite difficult, however, and only makes sense when the mass is very heavy.
In most cases, the mass is analysed for its content of four elements, that is, gold, silver, platinum and palladium. The process of determining gold content has been practised and standardised over centuries. This analysis, called the fire assay, quickly measures the gold content with unbeatable precision – better than +/- one part per thousand, even when the gold content is high.
Gravimetric analysis is used to determine platinum and palladium content; this procedure varies in terms of time and work, because it is based on chemical precipitation reactions. Depending on the material, it may cost between €100 and €325 per analysis. It is therefore wise to check whether a platinum or palladium analysis is cost effective beforehand. To do so, simply multiply the estimated content of platinum or palladium by the volume of scrap and the price of the precious metal. It only makes sense to do the analysis if the product of this multiplication is higher than the cost.
Depending on the analysis results, the mass is further processed to recover the precious metal content. This final phase is also known as separation. As part of the separation process, the metal will be entirely dissolved in suitable acid mixtures.
It will then be subjected to selective electrochemical processes, precipitation and reduction reactions and annealing and melting procedures. Precautions are taken in each of these processes to protect the environment.