Electrolyte purity

Zinc electrowinning requires high electrolyte purity.186 Cementation with powdered zinc to remove heavy metals such as Cd, Hg, and Pb, and other precipitation techniques are most... 

A word of caution is in order at this point. The success or failure of these electrocrystallization experiments depends on many factors. Solvent and electrolyte purity have been found to be crucial. Slight impurities can and will inhibit the successful crystallization of these materials. There is evidence suggesting that the porosity of the frit used to separate the two compartments of the electrocrystallization cell is also important. The lowest porosity seems to work best. Even when all conditions are controlled as best as possible it is not unusual to have a particular run to fail. 

The success of the SX process chemistry can be assessed with reference to the typical quality of the zinc cathode produced, which consistently exceeds SHG grade due to the high electrolyte purity. Table 5.5 shows the specification for SHG cathode and a typical analysis of the Skorpion Zinc product. 

The commonly used pretreatment protocols for activating solid electrodes are reviewed in this chapter. Specifically, the pietreatment of carbon, metal, and semiconductor electrodes (thin conducting oxides) is discussed. Details of how the different electrode materials are produced, how the particular pretreatment works, and what effect it has on electron-transfer kinetics and voltammetric background current are given, since these factors determine the electroanalytical utility of an electrode. Issues associated with cell design and electrode placement (Chapter 2), solvent and electrolyte purity (Chapter 3), and uncompensated ohmic resistance (Chapter 1) are discussed elsewhere in this book. This... 

A wide variation is found in the detailed design and operating parameters of Moebius-type cells but the typical range of conditions may be summarized A cell may have between four and twenty cathodes operating at 100- 500 A and from -1 5 to —2 8 V The cells arc usually operated near ambient temperature with a cathode current density of 20--40 mA cm While the cathode silver purity is dependent upon anode quality, electrolyte purity and operating conditions >99 9% wt Ag is usually obtained, with 99.99% wt being realized in some cases. 

It seems clear that interest in stochastic (vs. ensemble) electrochemistry will continue and, with time, progress to similar experiments with large biomolecules and eventually to single molecules of ordinary dimensions. Such studies result in deeper insights into the interactions of molecules with electrode surfaces and the nature of electron transfer processes at interfaces. Such advances will rely on smaller, nm dimension, UMEs and surfaces that show much lower background currents (or other responses). Such studies will also place great demands on solvent and electrolyte purity and instrument sensitivity. The challenges are considerable, but with continued advances as have been seen over the last decades, success is assured. 

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

Silver is also recovered during electrolytic refining of copper. Commercial fine silver contains at least 99.9% silver. Purities of 99.999+% are available commercially. 

Conversion of aqueous NaCl to Cl and NaOH is achieved in three types of electrolytic cells the diaphragm cell, the membrane cell, and the mercury cell. The distinguishing feature of these cells is the manner by which the electrolysis products are prevented from mixing with each other, thus ensuring generation of products having proper purity. 

Selective Reduction. In aqueous solution, europium(III) [22541 -18-0] reduction to europium(II) [16910-54-6] is carried out by treatment with amalgams or zinc, or by continuous electrolytic reduction. Photochemical reduction has also been proposed. When reduced to the divalent state, europium exhibits chemical properties similar to the alkaline-earth elements and can be selectively precipitated as a sulfate, for example. This process is highly selective and allows production of high purity europium fromlow europium content solutions (see Calcium compounds Strontiumand strontium compounds). 

Zinc. The electrowinning of zinc on a commercial scale started in 1915. Most newer faciUties are electrolytic plants. The success of the process results from the abiUty to handle complex ores and to produce, after purification of the electrolyte, high purity zinc cathodes at an acceptable cost. Over the years, there have been only minor changes in the chemistry of the process to improve zinc recovery and solution purification. Improvements have been made in the areas of process instmmentation and control, automation, and prevention of water pollution. 

Other Meta.Is, Although most cobalt is refined by chemical methods, some is electrorefined. Lead and tin are fire refined, but a better removal of impurities is achieved by electrorefining. Very high purity lead is produced by an electrochemical process using a fluosiUcate electrolyte. A sulfate bath is used for purifying tin. Silver is produced mainly by electrorefining in a nitrate electrolyte, and gold is refined by chemical methods or by electrolysis in a chloride bath. 

The electrorefining of many metals can be carried out using molten salt electrolytes, but these processes are usually expensive and have found Httie commercial use in spite of possible technical advantages. The only appHcation on an industrial scale is the electrorefining of aluminum by the three-layer process. The density of the molten salt electrolyte is adjusted so that a pure molten aluminum cathode floats on the electrolyte, which in turn floats on the impure anode consisting of a molten copper—aluminum alloy. The process is used to manufacture high purity aluminum. 

Nickel Fluoroborate. Fluoroboric acid and nickel carbonate form nickel fluoroborate [14708-14-6] Ni(BF 2 6H20. Upon crystallization, the high purity product is obtained (47). Nickel fluoroborate is used as the electrolyte ia specialty high speed nickel plating. It is available commercially as a concentrated solution. 

A newer approach developed for producing commercial quantities of high purity AP (8,36) involves the electrolytic conversion of chloric acid [7790-93 ] to perchloric acid, which is neutralized by using ammonia gas ... 

The first commercial plant to use CYANEX 272 became operational in 1985. An additional three plants were constmcted between 1985 and 1989. Of the four, one is in South America and three in Europe. An additional three plants have been built two in Europe (1994) and one in North America (1995). Approximately 50% of the Western world s cobalt is processed using CYANEX 272. Both high purity salts and electrolytic cobalt metal are recovered from solutions ranging in composition from 30 g/L each of cobalt and nickel to 0.2 g/L Co, 95 g/L Ni Operating companies usually regard use of CYANEX 272 as confidential for competitive reasons and identities cannot be disclosed. CYANEX 272 is being evaluated on the pilot-plant scale in many additional projects involving the recovery of cobalt and other metals. 

Preparation of Plutonium Metal from Fluorides. Plutonium fluoride, PuF or PuF, is reduced to the metal with calcium (31). Although the reactions of Ca with both fluorides are exothermic, iodine is added to provide additional heat. The thermodynamics of the process have been described (133). The purity of production-grade Pu metal by this method is ca 99.87 wt % (134). Metal of greater than 99.99 wt % purity can be produced by electrorefining, which is appHcable for Pu alloys as well as to purify Pu metal. The electrorefining has been conducted at 740°C in a NaCl—KCl electrolyte containing PuCl [13569-62-5], PuF, or PuF. Processing was done routinely on a 4-kg Pu batch basis (135). 

Cryolite. Cryofite [15096-52-3] Na AlF, is the primary constituent of the HaH-Hfiroult cell electrolyte. High purity, natural cryofite is found in Greenland, but its rarity and cost have caused the aluminum industry to substitute synthetic cryofite. The latter is produced by the reaction of hydrofluoric acid [7664-39-3] HE, with sodium aluminate [1302-42-7] NaA102, from the Bayer process... 

This process used an all-fluoride electrolyte, a portion of which was frozen on the carbon sidewalls to prevent short circuiting through the wads. One version of the cell operated at 20,000 A and 950—1000°C. The highest purity aluminum produced was 99.98%. A summary of the cell characteristics is given in Table 9. 

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