Noble metal recovery

Production processes are operated on a more or less regular basis - that is, all relevant problems concerning catalyst performance and separation, supply of materials, product isolation and purification, noble metal recovery, etc. have been solved. 

Noble metal films, 15 251 Noble metal nanoparticles, 26 805 Noble metal recovery, in photocatalytic water decontamination, 19 87-89 N-0 bond polymerization inhibitors,... 

The supply of noble metals for three-way catalysts and particularly the rhodium supply is of concern to manufacturers. The rhodium use in platinum-rhodium three-way catalysts exceeds the naturally occurring ratio of these metals. Automobile catalytic converters are a large user of noble metals and this imbalance in the use of platinum and rhodium can influence the price and availability of rhodium. Noble metal recovery from spent automobile exhaust catalysts is currently a source of platinum and palladium and can be expected to be a source of rhodium after 1990. 

Noble metal recovery from scrap can be difficult because of the heterogeneity of the material. Sometimes, it is more difficult to obtain metals in pure form from scrap than from ore. If proper allowances in refining are not made to account for contaminants, products may be obtained that are unsuitable for demanding applications. 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

Pemoval of Other Impurities. After softening, the impurities that may stiU remain in the lead are silver, gold, copper, tellurium, platinum metals, and bismuth. Whereas concentrations may be tolerable for some lead appHcations, the market values encourage separation and recovery. The Parkes process is used for removing noble metals and any residual copper, and the KroU-Betterton process for debismuthizing. 

Troublesome amounts of C and Q acetylenes are also produced in cracking. In the butadiene and isoprene recovery processes, the acetylenes in the feed are either hydrogenated, polymerized, or extracted and burned. Acetylene hydrogenation catalyst types include palladium on alumina, and some non-noble metals. 

Base metals frequently are used in nonsupported form, but noble metals rarely are, except in laboratory preparations. Supporting the noble metals makes a more efficient catalyst on a weight of metal basis and aids in recovery of the metal. Neither of these factors is of much importance in experimental work, but in industrial processing both have significant impact on economics. 

Eventually all catalysts become spent. At this stage they can be discarded, itself sometimes a problem, or returned to a refiner for recovery of metal values. In commercial use, noble-metal catalysts are always returned to a refiner. At the refinery, the catalyst is destroyed and the noble metals are recovered and converted to high-purity metal. In a loop system, the pure metal is converted to a suitable salt and again used for catalyst manufacture. In the entire loop, some metal will be lost and must be replaced with fresh metal. Refining is nowadays very efficient, and most metal loss will occur in the process itself, The total cost of a catalyst used in a loop is accordingly given by ... 

The rhodium complexes are excellent catalysts for hydrogenation of NBR. At low temperature and pressure, high catalyst concentrations are used to obtain a better rate of reactions. Due to higher selectivity of the reaction, pressure and temperature can be increased to very high values. Consequently the rhodium concentration can be greatly reduced, which leads to high turnover rates. The only practical drawback of Rh complex is its high cost. This has initiated the development of techniques for catalyst removal and recovery (see Section VU), as well as alternate catalyst systems based on cheaper noble metals, such as ruthenium or palladium (see Sections IV.A and B). 

When they have served their purpose or become damaged the noble metals will realise a very high proportion of their initial cost. No matter in what form they are utilised, very efficient processes are available for effecting their complete recovery. This factor often makes noble metals the most economic in use in the chemical and engineering industries. For many applications, no other metal or group of metals can fulfil their function as efficiently, combined with such a low net cost to their user. 

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. 

Besides the environmentally sound disposal of hazardous components, the recovery of ferrous, nonferrous, and noble metals is the main priority in the disposal of electrical and electronic appliances. Here, it is important to ensure that the requirements relating to scrap quality are met. 

Johnson Matthey, Catalysis and Chiral Technologies (Synetix) In licensed chiral ligands, process R D. Metal precursors and M-L complexes in technical quantities recovery of noble metals [112]. 

Three-dimensional Cu or Ni foam and reticulated carbon (for the recovery of noble metals) cathodes are used. Between each pair of cathodes an inert,... 

Osmium tetroxide is obtained as an intermediate during recovery of osmium metal from osmiridium or other noble metal minerals (See Osmium). In general, oxidation of an aqueous solution of an osmium salt or complex, such as sodium osmate with nitric acid, yields the volatile tetroxide which may be distilled out from the solution. In the laboratory, the compound can be prepared by oxidation of the osmium tetrachloride, OsCh, or other halide solutions with sodium hypochlorite followed by distdlation. 

The recovery or removal of metals from solutions derived from the leaching of minerals is an important step in any hydrometallurgical process. Precipitation by reduction to the metallic state in electrochemical cells will be discussed in Section 63.3.5 this section will cover the use of chemical reagents to control the precipitation process. Therefore, although the production of metallic powders by the reduction of metal ions with hydrogen or less-noble metals (cementation) is electrochemical in nature, it will be discussed under this heading. 

Cementation, the process by which a metal is reduced from solution by the dissolution of a less-noble metal, has been used for centuries as a means for extraction of metals from solution, and is probably the oldest of the hydrometallurgical processes. It is also known by other terms such as metal displacement or contract reduction, and is widely used in the recovery of metals such as silver, gold, selenium, cadmium, copper and thallium from solution and the purification of solutions such as those used in the electrowinning of zinc. The electrochemical basis for these reactions has been well established414 and, as in leaching reactions, comprises the anodic dissolution of the less-noble metal coupled to the cathodic reduction of the more-noble metal on the surface of the corroding metals. Therefore, in the well-known and commercially exploited44 cementation of copper from sulfate solution by metallic iron, the reactions are... 

A potential and very attractive practical application of reduction by semiconductor photocatalysis technology is the removal of harmful toxic metals and the recovery of noble metals in wastewater. Metal species, such as Hg(II), Pb(II), Cd(II), Ag(I), Ni(II) and Cr(VI), are generally nondegradable and they are very toxic when present in the environment. 

Finally, the use of the expensive metal palladium as the metal of choice in the telomerization reaction holds obvious disadvantages for the economic feasibility of large-scale processes. Catalyst recovery and reuse should therefore receive further attention in future studies, for instance, by clever reactor design, or heteroge-nization of the catalyst. Alternatively, the use of palladium might be completely avoided if non-noble metals can be prompted to perform the same reactions when a suitably designed ligand environment is offered. 

In many homogeneous catalyst-based industrial processes efficient recovery of the metal is essential for the commercial viability of the technology (see Section 1.4). This is especially true for noble metal-based homogeneous catalytic reactions. Apart from economic reasons spent catalyst recovery is also essential to prevent downstream problems, such as poisoning of other catalysts, deposition on process equipment, waste disposal, etc. Several different techniques are being followed industrially for the recovery of the catalyst from the reaction medium after the end of the reaction ... 

The widespread application of enantioselective catalysis, be it with chiral metal complexes or enzymes, raises another issue. These catalysts are often very expensive. Chiral metal complexes generally comprise expensive noble metals in combination with even more expensive chiral ligands. A key issue is, therefore, to minimise the cost contribution of the catalyst to the total cost price of the product a rule of thumb is that it should not be more than ca. 5%. This can be achieved either by developing an extremely productive catalyst, as in the metachlor example, or by efficient recovery and recycling of the catalyst. Hence, much attention has been devoted in recent years to the development of effective methods for the immobilisation of metal complexes [130, 131] and enzymes [132]. This is discussed in more detail in Chapter 9. 

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