Platinum arsenides

A few compounds of the platinum metals are met, but always in rare minerals. Sperrylite Is platinum arsenide, PtAs2, which is found in rare samples of the nickel-copper ores of Ontario and of copper ores at Rambler Mine, Laramie, Wyoming. Laurite is a rare ore composed mainly of ruthenium sulfide, RuS2. 

PGM Concentration. The ore mined from the Merensky Reef in South Africa has a maximum PGM content of 8.1 g/1, of which 50—60% is platinum, and 20—25% palladium. The PGMs are in the form of a ferroplatinum alloy, or as their sulfides, arsenides, or teUurides. The aim of the concentration process is to separate from the ore a cmde metal concentrate, having a PGM content of 60%. The majority of other metals, such as nickel and copper, are separated out at this stage for further refining. 

More than 200 ores are known to contain cobalt but only a few are of commercial value. The more important are arsenides and sulfides such as smaltite, C0AS2, cobaltite (or cobalt glance), CoAsS, and linnaeite, C03S4. These are invariably associated with nickel, and often also with copper and lead, and it is usually obtained as a byproduct or coproduct in the recovery of these metals. The world s major sources of cobalt are the African continent and Canada with smaller reserves in Australia and the former USSR. All the platinum metals are generally associated with each other and rhodium and iridium therefore occur wherever the other platinum metals are found. However, the relative proportions of the individual metals are by no means constant and the more important sources of rhodium are the nickel-copper-sulfide ores found in South Africa and in Sudbury, Canada, which contain about 0.1% Rh. Iridium is usually obtained from native osmiridium (Ir 50%) or iridiosmium (Ir 70%) found chiefiy in Alaska as well as South Africa. 

Although estimates of their abundances vary considerably, Pd and Pt (approximately 0.015 and 0.01 ppm respectively) are much rarer than Ni. They are generally associated with the other platinum metals and occur either native in placer (i.e. alluvial) deposits or as sulfides or arsenides in Ni, Cu and Fe sulfide ores. Until the 1820s all platinum metals came from South America, but in 1819 the first of a series of rich placer deposits which were to make Russia the chief source of the metals for the next century, was discovered in the Urals. More recently however, the copper-nickel ores in South Africa and Russia (where the Noril sk-Talnakh deposits are well inside the Arctic Circle) have become the major sources, supplemented by supplies from Sudbury. 

Arsenic does not combine directly with carbon, silicon or boron. The reaction with metals to form definite arsenides or alloys is described no pp. 57-78. The presence of small quantities of arsenic or of its compounds in certain catalysts has a poisoning effect. The first traces added to the catalyst have the greatest effect thus the activity of 0-35 g. of platinum was reduced linearly by the addition of arsenic up to 0-7 mg., this quantity reducing the catalytic activity to 45 per cent, of its original value the addition of 10 mg. of arsenic, however, depressed the activity only to 26 per cent, of the original value.3 Vanadium catalysts are poisoned by the presence of arsenic, although the action is slow arsenic pentoxide is formed.4... 

Rammelsbergite is readily oxidised by aerated waters, the oxidation proceeding much more rapidly than in the case of niccolite and other arsenide minerals 3 the oxidation is accelerated by the presence of iron pyrites, manganese dioxide or platinum black.4... 

When arsine is passed over a heated metal, such as the alkali and alkaline earth metals, zinc or tin, the decomposition of the gas is accelerated and the arsenide of the metal is formed. If platinum is used, the removal of arsenic from the gas is complete.3 The action of sodium or potassium on arsine in liquid ammonia yields 4 the dihydrogen arsenide (MHgAs). Heated alkali hydroxides in the solid form quickly decompose the gas, forming arsenites, and at higher temperatures arsenates and arsenides of the metals.5 The aqueous and alcoholic solutions have no appreciable action.6 When the gas is passed over heated calcium oxide the amount of decomposition is not more than that due to the action of heat alone. Heated barium oxide, however, is converted into a dark brown mixture of barium arsenite and arsenate, hydrogen being liberated.7 The gas is absorbed by soda-lime.8... 

The reaction commences at a temperature above 400° C., before the melting temperature is reached, and the fused product therefore always contains some arsenious oxide.1 When heated in hydrogen, the pentoxide is reduced first to arsenious oxide and then to free arsenic. Similar reduction occurs when it is heated with carbon or phosphorus with sulphur, arsenious sulphide is formed. Arsenic and metallic arsenides result when the pentoxide is heated with alkali metals,2 zinc, lead, iron or most other heavy metals mercury and silver react only at high temperature gold and platinum do not react. 

A series of complex silico-arsenides has been obtained 6 by melting metals with silicon and an excess of arsenic under a layer of molten cryolite and sodium chloride. The following have thus been prepared copper silico-arsenide, a grey crystalline brittle mass zinc silico-arsenide, which behaved as above with hydrochloric acid iron, cobalt and nickel siMco-arsenides, of composition M2SisAs4, similar in appearance to the copper compound. When platinum was treated in the same way, a hard white product of indefinite composition was obtained, almost insoluble in nitric acid. 

The transfer gates 4 and the charge transfer gates 5 may be formed as MIS structures or as Schottky barrier structures using aluminium or platinum. Furthermore, the gallium arsenide substrate may be substituted for a silicon substrate. 

An arsenide of platinum, PtAs2, occurs in nature as the rare mineral sperryliie. and may be formed in the laboratory by heating platinum sponge with excess of arsenic, when the two elements unite with incandescence. 

Ruthenium, osmium, rhodium, iridium, palladium and platinum are the six heaviest members of Group VII1. They are rare elements platinum itself is the commonest with an abundance of about 10-6% whereas the others have abundances of the order of 10"7 % of the earth s crust. They occur in Nature as metals, often as alloys such as osmiridium, and in arsenide, sulfide and other ores. The elements are usually associated not only with one another but also with the coinage metals copper, silver and gold. The main suppliers are South Africa, Canada and the USSR. 

Nickel is often used as matrix modifier in the determinations of arsenic and selenium. Nickel stabilizes arsenic up to about 1700 K, and selenium up to about 1500 K. This is presumably due to the formation of thermally stable nickel arsenide and nickel selenide. Nickel can also be used to stabilize bismuth, antimony, and tellurium. In addition, copper, silver, molybdenum, palladium, and platinum salts have been proposed as stabilizers for these elements. Palladium and platinum are the most suitable matrix modifiers for both inorganically and organically bound tellurium. Pretreatment temperatures up to 1320 K can be used. The effect of different matrix modifiers on the determination of antimony are shown in Figure 74. Without matrix modifiers the losses of antimony begin at 1000 K. Palladium, molybdenum, and nickel are the most suitable to stabilize antimony and with these modifiers pretreatment temperatures up to about 1600 K can be used. 

Substrates, Films are usually prepared on platinum or gold electrodes which are inert, but semiconducting materials including indium tin oxide, n-type polycrystalline silicon, gallium arsenide, cadmium sulphide and cadmium selenide, graphite [38, 59], and oxide covered metals [60] have also been used. In the majority of cases, the films are produced readily and the only serious limitations are the potential and the nucleophilic nature of the solution. 

Wohler obtained phosphorus by strongly heating a mixture of sand and bone-black (calcium phosphate and carbon) — the modern process, now carried out in the electric furnace. He prepared artificial nickel arsenide and worked on the separation of iridium and osmium in a pound of platinum residues given him by Dumas on a visit to Paris in 1833. A supposed silver suboxide, Ag40, prepared by Wohler by the action of caustic potash solution on a supposed subcitrate made by heating silver citrate in hydrogen at 100°, was not analysed and was probably a mixture of silver and silver oxide AggO the red colour of the solution of the subcitrate was probably due to colloidal silver. ... 

Platinum has a certain tendency to form sulfides and arsenides, as for instance in the minerals cooperite PtS and sperrylite PtASj. The incHnation of platinum to form sulfides has the consequence that platinum often occurs with copper and nickel ores, especially pendandite. The metals are, however, mainly found as metallic phases, alloys with each other, with gold and with iron. The important alloy osmiridium with high contents of osmium and iridium also occurs in nature. 

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