Electrolysis, purification preparation

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

Preparation. The simplest method of preparation is a combination of the elements at a suitable temperature, usually ia the range of 1100—2000°C. On a commercial scale, borides are prepared by the reduction of mixtures of metallic and boron oxides usiag aluminum, magnesium, carbon, boron, or boron carbide, followed by purification. Borides can also be synthesized by vapor-phase reaction or electrolysis. 

The products of this electrolysis have a variety of uses. Chlorine is used to purify drinking water large quantities of it are consumed in making plastics such as polyvinyl chloride (PVC). Hydrogen, prepared in this and many other industrial processes, is used chiefly in the synthesis of ammonia (Chapter 12). Sodium hydroxide (lye), obtained on evaporation of the electrolyte, is used in processing pulp and paper, in the purification of aluminum ore, in the manufacture of glass and textiles, and for many other purposes. 

A large number of trialkylacetic acid esters have been prepared by mixed Kolbe electrolysis of ethyl malonates [164]. Crossed-coupling is also used for chain extension. Extension by two carbon atoms is achieved with benzyl succinates [153, 180-182], whereby the purification of the chain extended fatty acid is simphfied by using the benzyl half ester [181a]. 

Preparation. Industrially, silver is usually a by-product of processes of extraction of other metals such as copper, lead, zinc. The so-called anode slimes from the electrolytic purification of copper contain silver and the involved process is often finished by an electrolysis of a nitrate solution containing silver. 

Electrolytic purification of metals is considered at length in Chapter 17. In essence, metals can be deposited in high purity from solution on a cathodic surface, by careful control of the voltage and other parameters. The anode can be a billet of the impure metal, and the impurities will either stay in solution or form an insoluble anode slime here, both dissolution and reprecipitation of the desired metal are accomplished in a single electrolytic step. Alternatively, a crude solution of the metal ion might be prepared by some other means, and the pure metal deposited on a cathode with an anode of some inert material the product of electrolysis at the anode will normally be oxygen gas. 

Uses Of the Stassfurt salts.—The magnesium compounds in the Stassfurt salts are used for the preparation of magnesium and of its salts. The potash salts are an essential constituent of many fertilizers used in agriculture, etc. 22 and potassium chloride is the starting-point for the manufacture of the many different kinds of potassium salts used in commerce—carbonate, hydroxide, nitrate, chlorate, chromate, alum, ferrocyanide, cyanide, iodide, bromide, etc. Chlorine and bromine are extracted by electrolysis and other processes from the mother liquids obtained in the purification of the potash salts. Boric acid and borax are prepared from boracite. Caesium and rubidium are recovered from the crude carnallite and sylvite. 

Metals prepared by reduction usually need further treatment to remove impurities. The extent of purification, of course, depends on how the metal will be used. Three common purification procedures are distillation, electrolysis, and zone refining. 

In Fig. 9.13, the heat treatments are necessary to improve the efficiency of the sulphation step. The latter can be engineered in several alternative types of plant. Alternatives are available for the subsequent steps to pure oxide, but usually based upon precipitation and crystallization, as is the one shown in Fig. 9.13. The precipitation of beryllium hydroxide by boiling an alkaline solution of sodium beryllate, is a particularly valuable purification step, and is also used in Fig. 9.14. Chlorination of oxide mixed with carbon is a standard type of operation as used for the preparation of chloride intermediates of other metals. Molten salt electrolysis is one of the two alternative commercial routes to pure beryllium metal, the other being shown in Fig. 9.14. 

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