Electrolytic recovery of metals

Electrolytic recovery of metals is an alternative to precipitation where metallic impurities are removed at the point of origin. Electrodeposition of metals has distinct benefits over precipitation and ion exchange methods the metals are recovered at lower capital and operating costs, no additional chemicals are needed, and sludge formation is eliminated. 

It is not possible to review all the work concerning sulfur electrode reactions in molten salts but only to highlight the main features.Much of this work has been motivated by interests in high-temperature batteries and in the electrolytic recovery of metals from their sulfides. 8olvents such as LiCl-KCl and KCN8 have been employed, but many studies have concerned pure molten polysulfide melts (cf. the Na-8 battery). 

Lopez-Cacicedo CL. The electrolytic recovery of metals from diluent elBuent streams. J Separ Proc Technol 2(1) 34—39,... 

Most commercial tellurium is recovered from electrolytic copper refinery slimes (8—16). The tellurium content of slimes can range from a trace up to 10% (see Seleniumand selenium compounds). Most of the original processes developed for the recovery of metals of value from slimes resulted in tellurium being the last and least important metal produced. In recent years, many refineries have changed their slimes treatment processes for faster recovery of precious metals (17,18). The new processes have in common the need to remove the copper in slimes by autoclave leaching to low levels (<1%). In addition, this autoclave pretreatment dissolves a large amount of the tellurium, and the separation of the tellurium and copper from the solution which then follows places tellurium recovery at the beginning of the slimes treatment process. 

The two electrolytic processes, one with the metallic sulfide and the other with the metal itself, are industrial processes. The first one allows simultaneous recovery of metal and sulfur, and second one allows purification of impure metal. 

A. K. Suri and C. K. Gupta, Electrolytic Recovery of Molybdenum from Molybdic Oxide and Molybdenum Sesquisulfide, Metall. Trans. B, Vol. 6B, p. 453,1975. 

Lopez-Cacicedo C (1974) Recovery of metals from rinse waters in Chemelec electrolytic cells, Trans Inst Met Finish 55, Pt 2 74 Chem Abstr 84 (1976) 10365w... 

Lithium metal is produced commercially by electrolysis of a fused eutectic mixture of hthium chloride-potassium chloride (45% LiCl) at 400 to 450°C. The eutectic mixture melts at 352°C in comparison to the pure LiCl melting at 606°C. Also, the eutectic melt is a superior electrolyte to LiCl melt. (Landolt, P.E. and C. A. Hampel. 1968. Lithium. In Encyclopedia of Chemical Elements.C. A. Hampel, Ed. Reinhold Book Corp. New York.) Electrolysis is carried out using graphite anodes and steel cathodes. Any sodium impurity in hthium chloride may be removed by vaporizing sodium under vacuum at elevated temperatures. All commercial processes nowadays are based on electrolytic recovery of the metal. Chemical reduction processes do not yield high purity-grade metal. Lithium can be stored indefinitely under airtight conditions. It usually is stored under mineral oil in metal drums. 

Recovery of metals from treatment sludges by off-site commercial plants is an option that is employed at the present time to some extent in California, and that will likely see rapid growth in the near future. Types of off-site recovery services that presently exist in California or elsewhe. in the United States include those offered by smelters, hydrometallurgical plants, and ion exchange/electrolytic recovery plants. 

Electrolytic Recovery of Mercury Metal from a Mercuric Chloride-Containing Waste... 

Fig. 10.4 Experimental set-ups for different plasma (electro)chemical experiments (a) DC set-up of Klemenc, (b) set-up for the recovery of metal from slags, (c) vapor-phase electrolytic deposition set-up of Ogumi et al.

Generation of photocurrent at the semiconductor/electrolyte interface upon its illumination makes it possible to carry out photoelectrochemical reactions which can be used either for chemical fuel production, or purification of waters. Principles of operation of electrochemical cells with semiconductor electrodes for solar energy conversion to electrical and chemical energy are formulated. Most efficient cells for electricity and hydrogen production are surveyed. Certain processes for photo-destruction of pollutants, recovery of metals, etc. with making use of semiconductor dispersions are briefly discussed. 

Recovery of valuable metals from secondary sources. At the present state of development the more promising metal recovery processes based on SIR systems appear to be in the following applications in terms of both process performance and economic considerations (a) Recovery of metals from dilute solutions, particularly where such solutions are available at low cost (e.g., waste solution from other processes, mine waters, or dump leaching solutions) (b) separation of metals from concentrated solutions obtained by hydrometallurgical processing of complex ores, concentrates, mattes, and scraps and purification of process solutions (such as electrolytes) which may contain a variety of metals that have been only partially recovered in the conventional processing steps (c) separation and purification of met-... 

The direct metallothermic reduction of pollucite ore with sodium metal is the primary commercial source of cesium metal. In the process, raw pollucite ore is reduced with sodium molten metal at ca. 650"C to form a sodium-cesium alloy containing some rubidium as impurity. Fractional distillation of this alloy in a distillation column at ca. 700"C produces 99.9 wt.% pure cesium metal. Cesium can also be obtained pyrometallurgiccdly reducing the chloride CsCl with calcium metal or the hydroxide CsOH with magnesium metal. Nevertheless, the electrolytic recovery of a cesium amalgam from an aqueous solution of cesium chloride can be achieved in a process similar to the chlor-alkali production with a mercury cathode. Afterwards, the cesium is removed from the amalgam by vacuum distillation. However, cesium metal is produced in rather limited amounts because of its relatively high cost (US 40,800 /kg)... 

In membrane electrol sis an electrolysis process is combined w ilh a membrane separation process. The classical example is the chlor-alkali process in which sodium chloride is converted into chlorine and caustic soda. Other examples are the electrolytic recovery of (hea 7) metals and the production of acid and base from the corresponding salts. 

Lopez-Cacicedo, C. L., 1975 Recovery of metals from rinse waters in chemelec electrolytic cells, Trans. Inst. Met. Finish., 55 74-77. 

Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

Metals less noble than copper, such as iron, nickel, and lead, dissolve from the anode. The lead precipitates as lead sulfate in the slimes. Other impurities such as arsenic, antimony, and bismuth remain partiy as insoluble compounds in the slimes and partiy as soluble complexes in the electrolyte. Precious metals, such as gold and silver, remain as metals in the anode slimes. The bulk of the slimes consist of particles of copper falling from the anode, and insoluble sulfides, selenides, or teUurides. These slimes are processed further for the recovery of the various constituents. Metals less noble than copper do not deposit but accumulate in solution. This requires periodic purification of the electrolyte to remove nickel sulfate, arsenic, and other impurities. 

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. 

J. E. Hoffmann, "Recovery of Selenium from Electrolytic Copper Refinery Slimes," in V. Kudryk, D. A. Corrigan, and W. W. Liang, eds.. Precious Metals Mining Extraction and Processing H, TMS, Warrendale, Pa., 1983. 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

The mud or slime is coUected from the bottom of the electrolytic ceUs and pumped to the sUver refinery, where it is processed for recovery of copper, precious metals, selenium, and, in many cases, teUurium. The anode slime contains 2—20% selenium as copper and sUver selenides, whereas gold exists as the metal and in combination with teUurium. A flow diagram is shown in Figure 8. 

Hydrometallurigcal Processes. In hydrometaHurgical processes, metal values and by-products are recovered from aqueous solution by chemical or electrolytic processes. Values are solubilized by treating waste, ore, or concentrates. Leaching of copper ores in place by rain or natural streams and the subsequent recovery of copper from mnoff mine water as impure cement copper have been practiced since Roman times. Most hydrometaHurgical treatments have been appHed to ores or overburden in which the copper was present as oxide, mixed oxide—sulfide, or native copper. PyrometaHurgical and hydrometaHurgical processes are compared in Reference 34. 

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