EXTRACTION STUDIES ON SOME PLATINUM METALS USING ORGANOPHOSPHORUS EXTRACTANTS

Abstract

here is probably no material that has shaped the progression of mankind more than metals. Metals are as old as human civilization and their history can be traced back to 6000 BC. During the ancient times, locations rich in ores tended to make the people living in that area richer and more powerful. Metal is so important in the history of mankind because of its influence on industry. Today’s fast growing technical world has witnessed many examples when a rare, lesser known and worthy metal has occupied important and indispensable position in many fields, once its particular use was realized. The later half of the twentieth century has witnessed the importance of metals in several technical discoveries which have completely revolutionized the approach of human life. Few such metals belong to Platinum Group Metals (PGMs). Platinum group metals consist of six elements–platinum, palladium, rhodium, iridium, ruthenium and osmium. PGMs together with gold and silver form the family of precious metals. PGMs are generally used in applications which depend strongly on their unique properties, such as their excellent inertness with corrosion and oxidation, biocompatibility, high melting temperature, good conductivity and electronic and catalytic properties. The higher premium paid for PGMs is justified by their superior performance in specific applications. PGMs are most often used as catalysts because of their chemical stability, but are not limited to this role. Platinum metals have become vital in many technological fields such as automotive catalytic converters, catalysts in the chemical and petroleum refining and in jewellery making, electronics, glass industry and medical sciences. The growing areas are auto catalysts and jewellery. The PGMs are widely used as catalysts for a variety of chemical reactions such as hydrogenation, isomerisation, oxidation, dehalogenation, dehydrogenation and cyclization. These metals in pure form or alloyed with other metals are used in several high tech applications. Continuous research on the properties and qualities of platinum group metals is unearthing new applications every year. These metals are extremely rare, owing in part to their low natural abundance and in part to the complex processes required for their extraction and refining. With the exception of small alluvial deposits of platinum, palladium, and iridosmine (an alloy of iridium and osmium), virtually no ore exists in which the major metal is from the platinum group. Platinum metals are usually highly disseminated in sulfide ores, particularly the nickel and copper. The most common ores of PGMs include laurite (RuS2), irarsite [(Ir,Ru,Rh,Pt)AsS], osmiridium (Ir,Os), cooperite (PtS), and braggite [(Pt,Pd)S]. This makes the mutual separation of PGMs and their separation from associated metals a must to do task. The world’s largest deposit is the Bushveld Complex of South Africa. Other major deposits include the Sudbury deposit of Ontario, Canada, and the Norilsk-Talnakh deposit of Siberia in Russia. The availability of PGMs in used scrap materials contributes great economy of PGM usage and technological importance of these metals has created a never ending demand (1). The petroleum and petrochemical industries generate considerable amounts of spent catalysts with small but significant PGM content. The PGMs are the main active ingredients of these catalysts. Spent catalysts are classified by the North-American Environment protection Agency (EPA) as hazardous wastes because they may be pyrophoric, spontaneously combustible and release toxic gases. Although the amount of these metals in a commercial catalyst is very low, it corresponds to the main cost of this product (2, 3). When regeneration is no longer possible the so-called spent catalysts are usually treated in order to recover the noble metals present. Therefore, recycling of spent catalysts is an attractive way to lower the catalyst cost (4). This necessitates the development of indigenous methods for the recovery of PGMs from available sources. Unlike other precious metals like gold and silver, which could be readily isolated in a comparatively pure state by simple fire refining, the platinum metals require complex aqueous chemical processing for their isolation and identification. Because these techniques were not available until the turn of the 19th century, the identification and isolation of the platinum group lagged behind silver and gold by thousands of years. As evident, the separation and purification of PGMs is difficult owing to their similar structures and chemical behaviours. Thus it is of paramount importance to develop efficient methods for the recovery and separation of platinum metals. This prompted the author to undertake studies on the extraction of various platinum group metals and their recovery from different matrices.