Platinum metals Refining

On the other hand, the recovery of some metals (notably those of the platinum group) by cementation is particularly difficult. This applies particularly to their removal from the waste streams of the refineries. In a typical process for platinum-metal refining,416 the metals are dissolved as their cbloro complexes and, although reactions such as... 

The metals are obtained from the metallic phase of the sulphide matte or the anode slime from electrolytic refining of nickel. In the traditional process for the platinum metals, their separation was facilitated by their solubility in aqua regia and convertibility into PdCl - or PtCl - salts. Nowadays, substantial amounts are obtained using solvent extraction. 

For the noble metals used in oxidation, the loading is about 0.1 oz per car, with calls for a million ounces per year. The current world production rates of platinum, palladium, and rhodium are 1.9, 1.6, and 0.076 million ounces respectively the current U,S. demand for platinum, palladium, rhodium, and ruthenium are 0.52, 0.72, 0.045, and 0.017 million ounces respectively (72, 73). The supply problem would double if NO reduction requires an equal amount of noble metal. Pollution conscious Japan has adopted a set of automobile emission rules that are the same as the U.S., and Western Europe may follow this creates a demand for new car catalysts approaching the U.S. total. The bulk of world production and potential new mines are in the Soviet Union and South Africa. The importation of these metals, assuming the current price of platinum at 155/oz and palladium at 78/oz, would pose a balance of payment problem. The recovery of platinum contained in spent catalysts delivered to the door of precious metal refiners should be above 95% the value of platinum in spent catalysts is greater than the value of lead in old batteries, and should provide a sufficient incentive for scavengers. 

The platinum metals are valuable by-products from the extraction of common metals such as copper and nickel. The anodic residue that results from copper refining is a particularly important source. The chemistry involved in their purification is too complicated to describe here, except to note that the final reduction step involves reaction of molecular hydrogen with metal halide complexes. 

Platinum is used as a catalyst for nitric and sulphuric acid production, in petroleum refining and in catalytic mufflers to control air pollution. Platinum salts can cause respiratory complaints, asthma, and platinosis , an allergic response. Allergic dermatitis may also result from exposure to soluble platinum salts and once subjects have been sensitized it generally precludes continued occupational exposure at any level. The 8 hr TWA OEL for platinum metal is 5 mg/m but for soluble platinum salts it is only 0.002 mg/m. Handling precautions must include containment where possible, ventilation, personal protection, and the screening out of individuals who have become sensitized. 

When rhodium is combined with platinum and palladium, the elements together form the internal metals of automobile catalytic converters, which convert hot unburned hydrocarbon exhaust gases to less harmful CO and H O. Similar alloys are used to manufacture high-temperature products such as electric coils for metal refining furnaces and high-temperature spark plugs. 

Palladium was discovered in 1803 by W.H. Wollaston during refining and purification oP platinum metal. This new metal was found in the aqua regia extract of native platinum and was detected in solution after platinum was precipitated. It was removed as ammonium chloroplatinate. Treating this solution with mercurous cyanide precipitated a yellow palladium complex salt. The precipitate was washed and ignited to form palladium metal. Wollaston named the element palladium after the newly discovered asteroid Pallas. 

Ruthenium is derived from platinum metal ores. Method of production depends on the type of ore. However, the extraction processes are simdar to those of other nohle metals (see Platinum, Rhodium and Iridium). Ruthenium, like Rhodium, may he obtained from accumulated anode sludges in electrolytic refining of nickel or copper from certain types of ores. Also, residues from refining nickel by Mond carbonyl process contain ruthenium and other precious metals at very low concentrations. The extraction processes are very lengthy, involving smelting with suitable fluxes and acid treatments. 

Because of their unique properties and in spite of their high initial cost, the platinum metals find many applications in industry. Since used platinum metals retain a large portion of their initial value, many scrap materials arc a major source of recoverable platinum metals. Practically every application of platinum generates scrap in some form, which is eventually returned to the platinum refiner for recycling. Although there are ample mine reserves, they soon would be depleted without constant scrap recycling. 

Ruthenium is obtained by separating it from other platinum metals, such as platinum, palladium, and osmium, with which it occurs. These metals are usually obtained as by-products during the refining of nickel metal. They are then separated from each other by a series of chemical reactions. 

The name comes from the Greek osme, meaning smell or stink, after the smell of the residue from which the element was refined. Osmium was discovered and named in 1803 by Smithson Tennant (1761-1815). Osmium is found in trace amounts with other platinum metals such as rhodium and palladium, and these in turn are found in nickel ores. Osmium metal is primarily used as a hardening alloy for other metals in the platinum group and is thus used in pen nibs, phonograph needles, jewelry, and catalysts. 

Fig. 5.1 Platinum group metal refining unit -closer to a...

Lead ores, except those from the Mississippi Valley, are an important source of silver. Silver is collected in the crude lead and removed in the refining process, as also are other metals such as copper, arsenic, antimony, bismuth, zinc, cadmium, nickel, tin, gold and platinum. The refining of lead is complex and a number of processes and techniques are involved. There are two methodologies utilized one is electrolysis, in which the crude lead is anodicaUy dissolved and precipitated on cathodes the other is pyrometallurgical, including cupellation or distillation processes. Lead is sold as refined lead with a purity of99.99%. 

---