Platinum-group metals abundance

Osmium occurs in nature, always associated with other platinum group metals. It usually is found in lesser abundance than other noble metals. Its most important mineral is osmiridium (or iridosmine), a naturaUy occurring mineral alloyed with iridium. 

Palladium occurs in combination with platinum and is the second most abundant platinum group metal (pgm), accounting for 38% of pgm reserves. The USSR produces over 50% of the world s palladium, which is more than double that produced in South Africa. Two major sources of the metal are braggite, a mixed sulfide of platinum, palladium and nickel, which contains 16-20% palladium, and michenerite (PdBi3). 

Rhodium, as befits an element of the platinum metal sextet, occurs mainly as a minor constituent of platinum group metal ores. Thus, it occurs as a component of the ore deposits at Sudbury, Ontario, in the Merensky reef at Rustenberg in the Republic of South Africa, and in the platinum metals ore deposits in the Ural range of the USSR. The major production centers are the Urals and South Africa, since the Sudbury ores in particular have a very low rhodium content. Minor deposits of the metal ores which contain some rhodium also occur in British Columbia, the United States of America, Columbia, Spain, Borneo and Australia. Nevertheless, rhodium is an exceedingly rare element with an abundance in the earth s crust of some 10 9%. 

Ethylenediaminetetraacetic acid or its sodium salt (EDTA) is a well-known com-plexing agent which can form an abundant variety of water-soluble complexes with all metals. EDTA is widely used in industry and environmental studies are concerned with the degradation of its complexes [2] or the bioaccumulation of platinum group metals [3]. Directly related to the aim of this book is the catalytic activity in various carbonylation reactions of the precursor [Ru(EDTA)(H20)], as reported by Taqui Khan and co-workers [4], In fact the two nitrogen and three carboxylate functions bind the ruthenium(III) center, and under a CO atmosphere this complex is reduced to the ruthenium(II) species [Ru(EDTA)(CO)] in which only one carboxylate group... 

Natural occurrence. Rhodium is one of the rarest element in the Earth s crust with an abundance of 1 pg/kg (i.e., ppb wt.). Rhodium occurs in nature as a native metal along with other platinum-group metals in the native mineral iridosmine or in sulfide ores such as rhodite, sperrylite, and some copper-nickel ores. 

THE NOBLE METALS, or precious metals, consist of gold, silver, and the platinum group metals (PGM)— platinum, palladium, iridium, rhodium, osmium, and ruthenium. These metals are known for their stability in corrosive environments, physical beauty, and unique physical and chemical properties. They command a premium price because of their low abundance in nature. The noble metals are used in many applications with great success and often with few, if any, substitutable materials. 

Control of residual metal content is outlined in ICH Q3D. It is noteworthy that the metals currently most useful for chemical catalysis (Green Chemistry Principle 9), e.g. palladium and some platinum group metals (Rh, Ru, Ir), are subject to tight restriction because of known toxicity, or the assumption of near neighbour similarity in the absence of data. All four metals have an oral concentration limit of 10 ppm, reducing to 1 ppm via parenteral administration and 0.1 ppm via inhalation. Typically, delivery of API with low residual metal content can require additional processing and careful analysis to determine levels. Some of the more abundant metals suggested... 

None of the three elements is particularly abundant in the earth s crust though several minerals contain them as major constituents. As can be seen from Table 13.1, arsenic occurs about halfway down the elements in order of abundance, grouped with several others near 2 ppm. Antimony has only one-tenth of this abundance and Bi, down by a further factor of 20 or more, is about as unabundant as several of the commoner platinum metals and gold. In common with all the post-transition-element metals. As, Sb and Bi are chalcophiles, i.e. they occur in association with the chalcogens S, Se and Te rather than as oxides and silicates. 

Ruthenium and osmium are generally found in the metallic state along with the other platinum metals and the coinage metals. The major source of the platinum metals are the nickel-copper sulfide ores found in South Africa and Sudbury (Canada), and in the river sands of the Urals in Russia. They are rare elements, ruthenium particularly so, their estimated abundances in the earth s crustal rocks being but O.OOOl (Ru) and 0.005 (Os) ppm. However, as in Group 7, there is a marked contrast between the abundances of the two heavier elements and that of the first. 

---