Electrolytic Reduction of Concentrate

Since chemical reduction means gain of electrons, electrolysis is the most direct way of recovering a metal from its ores, as long as these can be handled in a fluid state. Consideration of E° values for reactive metal halfcells such as Na+(aq)/Na(s), Mg (aq)/Mg(s), and Al +(aq)/Al(s) (—2.71, —2.36, and —1.67 V, respectively) shows that these metals can never be obtained by electrolysis of aqueous solutions of their salts, as H2 would be produced instead, but they can often be obtained by electrolysis of suitable molten salts such as NaCl and MgCl2  

It is usual to use a mixture of salts (in the case of sodium, 60% NaCl + 40% CaCl2) so as to depress the melting point and permit electrolysis at a lower temperature (600°C, instead of 801 °C for pure NaCl), but obviously the second metal ion (Ca +) must be less readily reduced than the one of interest. 

Consumption of the graphite is quite acceptable inert anode materials for these conditions are hard to find, but in any event involvement of C in reaction 17.20 reduces the electricity requirement of the energy-intensive electrolysis by nearly half. Other, less commonly used processes that involve lower temperature electrolysis of molten AICI3 (mp 183 °C, performed in a closed vessel to prevent sublimation) have been developed. In these, the aluminum is formed as a solid (mp 660 °C), and the anodes are not consumed. Of course, one has to convert AI2O3 to AICI3 first, and this is usually done using graphite (coke) anyway  

Insoluble impurities fall to the floor of the cell as anode slime. Despite the derogatory name, this material contains precious metals such as gold, silver, and platinum. Anode slime from the electrorefining of nickel at Sudbury, Ontario, is a significant source of platinum and palladium as byproducts ( 0.34 g Pt and 0.36 g Pd per metric ton of ore), whereas deposits in the Bushveld complex (Transvaal, South Africa) are so rich in platinum-group metals (Ru, Os, Rh, Ir, Pd, Pt) that the associated Co, Ni, and Cu recovered are considered to be by-products of the lucrative platinum production (4.78 g Pt and 2.03 g Pd per metric ton of ore).  

---

Electroplating. Aluminum can be electroplated by the electrolytic reduction of cryoHte, which is trisodium aluminum hexafluoride [13775-53-6] Na AlE, containing alumina. Brass (see COPPERALLOYS) can be electroplated from aqueous cyanide solutions which contain cyano complexes of zinc(II) and copper(I). The soft CN stabilizes the copper as copper(I) and the two cyano complexes have comparable potentials. Without CN the potentials of aqueous zinc(II) and copper(I), as weU as those of zinc(II) and copper(II), are over one volt apart thus only the copper plates out. Careful control of concentration and pH also enables brass to be deposited from solutions of citrate and tartrate. The noble metals are often plated from solutions in which coordination compounds help provide fine, even deposits (see Electroplating). 

The electrolytic processing of concentrated ore to form the metal depends on the specific chemical properties of the metallic compound. To produce aluminum about 2 to 6 percent of purified aluminum oxide is dissolved in ciyolite (sodium alumi-no-fliioride, Na AlF ) at about 960°C. The reduction of the alumina occurs at a carbon (graphite) anode ... 

Several studies have been concerned with the chemistry of the + ni oxidation state of these elements, and the characterization of the first tantalum(iii) compounds has been claimed. The diamagnetic dimer [TaCl3(MeCN)2]2 has been prepared and used to obtain [TaClafphen)], [TaCljfbipy)], and tris-(dibenzoylmethanato)tantalum(ni). NbFa has been characterized as the product of the reaction of Nb and NbF (1 1) at 750 °C under pressure. Electrolytic reduction of niobium(v) in ethanol,formamide, and dimethylformamide can afford preparative concentrations of niobium(iii) and the new compound niobium(iii) trilactate has been obtained from ethanol. 

In 1898, Cowper-Coles 2 claimed to have successfully effected the electrolytic reduction of an acid solution of vanadium pentoxide to metallic vanadium, but the product was subsequently shown by Fischer 3 to have been a deposit of platinum hydride. Fischer, in a series of over three hundred experiments, varied the temperature, current density, cathode material, concentration, electrolyte, addition agent, and construction of cell, but in not one instance was the formation of any metallic vanadium observed. In most cases reduction ceased at the tetravalent state (blue). At temperatures above 90° C. reduction appeared to proceed to the divalent state (lavender). The use of carbon electrodes led to the trivalent state (green), but only lead electrodes produced the trivalent state at temperatures below 90° C. Platinum electrodes reduced the electrolyte to the blue vanadyl salt below 90° C. using a divided cell and temperatures above 90° C. the lavender salt was obtained. 

By the electrolytic reduction of a solution of niobium pentoxide in (a) concentrated hydrochloric acid and (b) dilute hydrochloric acid, and subsequent evaporation in vacuo or in an atmosphere of carbon dioxide, two compounds having the following compositions have been reported 3... 

The important result of these experiments is that nearly all nitro-bodies with an unoccupied para-position are converted by electrolytic reduction in concentrated sulphuric acid into p-amidophenol derivatives, i.e. not only is the nitro-group reduced completely to the amido-group, but in most cases the hydrogen atom in p-position to the amido-group is simultaneously substituted by the hydroxyl group. 

Various attempts to obtain the carbonyl-pentacyano species [Cr(CN)5(CO)] " have failed (ref. 33, p. 39), but a pentacyano-nitrosyl derivative [Cr(CN)s(NO)] " may be obtained by electrolytic reduction of Kj[Cr(CN)5(N0)] H20 on a mercury cathode, in an alkaline solution containing cyanide ions. The reaction conditions are adjusted to give an approximately O.IOM concentration for the Cr(I) complex, KOH and KCN addition of EtOH to the reduced solution, with rigorous exclusion of air, produces blue-green crystals of K4[Cr(CN)5(NO)] 2 HjO. 

The corresponding hydroxide, W(OH)j, has been prepared by the electrolytic reduction of solutions of tungsten trioxide in hydrochloric or hydrofluoric acid. It is a brown powder, insoluble in sodium hydro.xide, sulphuric acid, or acetic acid, but soluble in concentrated hydrochloric acid, yielding a greenish solution which rapidly becomes blue owing to oxidation of tetravalent tungsten to the pentavalent condition. 

The use of hexamethylphophoramide (HMPA) in the electrolytic reduction of substrates, such as benzene derivatives, aliphatic ketones, amides, and olefins, involves the intermediate formation of solvated electrons, since very negative potentials must be reached. The substrates are normally difficult to reduce. However, it is important not to add so much proton donor to the solvent that the rate of its reaction with the solvated electrons becomes too high. Generally, ethanol can be used as a cosolvent [311], and the best stability of the intermediate solvated electron combined with an optimum protonating efficiency was found to be the mxiture of HMPA-ethanol (33-66 mol moP ) the relatively high concentration of EtOH does not protonate ens because HMPA appears to be selectively adsorbed on the cathode interface and the reaction of Cys with the substrated is fast. With such a ratio of ethanol, a maximum current efficiency was found [312]. 

Hydrocarbon formation is more interesting in the electrochemical reduction of COj, since multielectron transfer is required in this process. In the electrochemical reduction of concentrated COj in the COj-methanol medium, the major products are still the two-electron transfer products, CO and methyl formate, at the Cu electrode, when tetrabutylammonium salts are used (Table 3). However, when tetraethylammonium salt was used as the supporting electrolyte, efficient formation of methane and ethylene was observed with good reproducibility. We defined the hydrocarbon selectivity as the ratio of the... 

Pu(IV) reduction rate were studied Pig.8 shows that at large excess of U and low concentration of Pu, no effect of acid concentration on Pu(IT) reduction rate could he observed. After a period of electrolysis of less than 30 seconds, nearly all of the Pu(IV) could be converted to Pu(III). This fact corresponds to the change of the potential of the electrolyte solution with time, which drops very rapidly after the start of the electrolysis. The effect of 0-concentration on the reduction rate of Pu(IV) is shown in Fig.9, from which it is clearly shown that the reduction rate of Pu(IT) depends very much on the amount of U relative to that of Pu in the electrolyte solution. The upper two curves showed that if the weight ratio of U/Pu is near or more than one, the reduction rate of Pu(IV) could be greatly accelerated. This fact indicates clearly that here U(IV) plays an important role in the reduction of Pu(IV). On the other hand, if the U-oontent in the solution is small compared to that of Pu, the rate of reduction of Pu(IV) is determined chiefly by the electrolytic reduction of Pu(IV) itself which is rather slow. This fact should be borne in mind in designing electrolytic reduction equipments in the purex process. 

Experiments on the electrolytic reduction of U and Pu in the aqueous phase in presence of hydrazine were carried out to investigate the effect of various factors influencing the rate of reduction. The potentials of the aqueous solution, which can serve to indicate the course of the reduction process, were measured and operating parameters such as acid concentration, hydrazine concentration, applied potential on the cathode, etc., were investigated. Experimental results indicated that, on Ti-cathode nitric acid could be reduced to nitrous even when there is no HN02 in the initial HNO3 solution and, with a u/Pu ratio ranging from 10 2 to 102, Pu(IV) can be reduced readily when the U/Pu ratio is near or more than 1 at low concentration of Pu. In this case, obviously TT(IV) formed in the process plays an important role in the reduction of Pu(IV). 

Derivation (1) By reduction of p-nitrophenol with iron filings and hydrochloric acid (2) by electrolytic reduction of nitrobenzene in concentrated sulfuric acid and treatment with an alkali to free the base. Also available as the hydrochloride. 

Confirmation of the reaction mechanism is provided by kinetic data dependent upon the same pK for the N- l coordinated ruthenium(III) complex (see Figure 6) Owing to the instability of l-[(Ado)(NH3)3Ru(II)l and severe restrictions required of the oxidizing partner isomerization kinetic rates were derived from cyclic voltammetric data using the method of Nicholson and Shain " " after forming the N-1 coordinated Ru(II) complex at the electrode surface by electrolytic reduction of the N- 6 bound Ru(III) species. Since the specific rates estimated by this method were independent of concentration the rate law is taken to be first order in the complex. 

Reduction of the hydroxyimino group with acetylation of the resulting amino group can be effected by zinc dust in the presence of acetic acid and acetic anhydride diethyl (acetylamino)malonate, which is important for amino acid syntheses, has been prepared in this way.97 Zinc dust reduces oximes of diaryl and arylalkyl ketones better in concentrated ammoniacal than in acid solution, giving the amines as very pure free bases whilst hardly any higher alkylated product is formed.98 Aminoacetone," diaminoacetone,100 and 2-afnino-3-pentanone" have been obtained by reducing oximes with tin or tin(n) chloride in alcoholic hydrochloric acid solution. In individual cases oximes have been reduced by sodium dithionite or aluminum amalgam.198 Further, the patent literature contains examples of electrolytic reduction of oximes.198... 

Aminobutyric acid has been prepared by the electrolytic reduction of succinimide to pyrrolidone and hydrolysis of the latter by means of barium hydroxide,1 by the oxidation of piperylurethan with fuming nitric acid and the treatment of the resulting product with concentrated hydrochloric acid in sealed tubes at 100°,3 and by the hydrolysis of the condensation product from 2V-(/3-bromoethyl)-phthalimide and sodiomalonic ester.4 The present method is a slight modification of that of Gabriel.2... 

---