Materials Engineering: Little-known facts about precious metals

The melting points and densities of the so-called Noble Metals, coupled with their resistance to chemical attack, sets them apart from most other materials. Edited by Jean M. Hoffman Unless you work with precious metal (PM) frequently, your knowledge of these metals is probably limited. The 15-min discussion devoted to PMs in college materials-science classes likely did not do justice to the engineering value for these eight elements. If you did not know there were eight (No, titanium is not one of them.), read on to find out what your professor probably did not know either. A precious metal is a rare metallic element of high economic value. PMs are also referred to as Noble Metals because they resist most types of environmental and chemical attack. One of the few chemical solutions that attacks them (with the exception of iridium) is aqua regia. Only copper and PMs are found in nature in their metallic state. All other metals are processed from minerals or ores into metals which are inherently unstable and have a tendency to revert to their more stable mineral forms. PMs, as a group, have a set of physical and chemical properties that are unrivaled by many other materials. If PMs were more available (in both quantitative and economic terms), there would be far more applications overall. Though typical applications use only small amounts, PMs may be used in large quantities when there is no feasible substitute. The eight PMs — gold (Au), silver (Ag), platinum (Pt), iridium (Ir), palladium (Pd), rhodium (Rh), ruthenium (Ru), and osmium (Os) — are conveniently grouped together in the periodic table. A subset of this group is called the Platinum Group Metals (PGM), which includes all but Au and Ag. Typically the PGMs are found combined together in rich ore. Chemical processing extracts