Dross is a mass of solid impurities floating on a molten metal or dispersed in the metal, such as in wrought iron. It forms on the surface of low-melting-point metals such as tin, lead, zinc or aluminium or alloys by oxidation of the metal. For higher melting point metals such as steel, oxidized impurities melt and float making them easy to pour off.
With wrought iron, hammering and later rolling removed some dross. With tin and lead the dross can be removed by adding sodium hydroxidepellets, which dissolve the oxides and form a slag. If floating, dross can also be skimmed off.
Dross, as a solid, is distinguished from slag, which is a liquid. Dross product is not entirely waste material; for example, aluminium dross can be recycled and is used in secondary steelmaking for slag deoxidation.
Silver is found in virtually every electronic device. If it has an on/off button, it’s likely that silver is inside. Silver’s excellent electrical conductivity makes it a natural choice for everything from printed circuit boards to switches and TV screens.
Silver membrane switches, which require only a light touch, are used in buttons on televisions, telephones, microwave ovens, children’s toys and computer keyboards. These switches are highly reliable and last for millions of on/off cycles. Silver is also used in conventional switches likes those used for controlling room lights.
For printed circuit boards, used in consumer items from mobile phones to computers, silver-based inks and films are applied to composite boards to create electrical pathways. Similarly, silver-based inks produce RFID tags (radio frequency identification) antennas used in hundreds of millions of products to prevent theft and allow easy inventory control. RFID’s are also used in prepaid toll road passes. Many plasma display panels are also fabricated using silver and silver’s use in the evolving 5G deployment is a plus for silver.
History
Silver was discovered after gold and copper about 4000 BCE, when it was used in jewelry and as a medium of exchange. The earliest known workings of significant size were those of the pre-Hittites of Cappadocia in eastern Anatolia. Silver is generally found in the combined state in nature, usually in copper or lead mineralization, and by 2000 BCE mining and smelting of silver-bearing lead ores was under way. Lead ores were smelted to obtain an impure lead-silver alloy, which was then fire refined by cupellation. The best-known of the ancient mines were located at the Laurium silver-lead deposit in Greece; this was actively mined from 500 BCE to 100 CE. Spanish mines were also a major source.
Precious Metal Refining
Dore is a mixture of gold and silver typically containing less than 5% base metal impurities. The exact composition varies widely depending on its source and processing history. Dore producers, in deciding whether or not to refine their dore, can custom design a facility around a single feedstock. Refiners, in contrast, must handle a variety of feedstocks. The ratio of gold to silver and the type and relative amounts of impurities determine the most effective refining sequence.
The typical strategy adopted by large refiners is base metal removal followed by the separation of gold and silver (known as parting), culminating with fine gold production. Platinum group metals are recovered and separated after gold. Figure 2 shows how three types of dore bullion can be integrated into this general scheme. Since it is all encompassing, the processing path for low-silver high-copper dore is described below.
Smelting
After melting incoming dore for homogenization and sampling (discussed in a later section), most base metals are removed prior to parting gold and silver. This step is generally done pyrometallurgically, sometimes in the same equipment used for initial melting. In some cases, this step may be performed at the mine rather than at the refinery (for example, on-site retorting for zinc or mercury removal).
Typical of these pyrometallurgical operations is “dore furnace” refining as practiced by the copper producers. Decopperized tankhouse slime is placed in a small reverberatory furnace to form a slag layer which is skimmed off. Antimony is volatilized and collected as flue dust. Selenium and tellurium are partially volatilized, but are mainly collected into an alkaline slag.
A low-silver high-copper dore probably contains sufficient copper to warrant an oxidative smelt known as cupellation to form a copper oxide dross. This can be accomplished in a reverberatory furnace by oxygen lancing until the batch contains less than 10% copper. Dross consists principally of base metal oxides, but does contain enough precious metal to require further treatment, usually by leaching.
Silver Refining
Bullion from cupellation is cast into anodes for electrolytic refining. In this process, silver dissolves into a dilute nitric acid electrolyte and plates out on a cathodic surface (stainless steel or graphite) in a very dendritic, crystalline deposit. Gold and principal platinum group metals form slimes which are collected in anode bags. After drying, these slimes are sent to the gold refining area. Copper, the main electrolyte contaminant, is allowed to build to relatively high levels before solutions are removed and treated for silver recovery. Silver crystal is better than 0.999 fine and, after rinsing and drying, is cast into 1,000 ounce ingots for marketing.
