Electrochemical recovery of metals

Dalrymple I and Sunderland G, The electrochemical recovery of metals and chemicals from waste sources, in ref (45a)... 

Metal removal from surface water, groundwater or wastewater streams is more straightforward than that from soils. Typically, removal is achieved by concentration of the metal within the wastestream using flocculation, complexation, and/or precipitation. For example, the use of lime or caustic soda will cause the precipitation and flocculation of metals as metal hydroxides. Alternatively, ion exchange, reverse osmosis, and electrochemical recovery of metals can be used for metal removal (Chalkley et al., 1989 Moore, 1994). Unfortunately, these techniques can be expensive, time-consuming and sometimes ineffective, depending on the metal contaminant present. 

The electrochemical recovery of metals (electrometallurgy) is an old (1. 2). The earliest literature reference to an electrochemical phenomenon Is found In Pliny s treatise on chemical subjects (3) and refers to the protection of iron with lead plating. However, it is known that various forms of metal plating were practised much earlier and remains of primitive storage batteries have been found associated with ancient Assyrian civilizations dated from around 2000 B.C. (1.4). 

With the increasing costs of raw materials and the threat of depletion of world reserves of many resources, electrochemical processes should become more attractive to reuse and recycle wastes/materials. The recovery of metals in chemical solutions is very important from both the environmental and economical view points [232]. 

Cementation, the process by which a metal is reduced from solution by the dissolution of a less-noble metal, has been used for centuries as a means for extraction of metals from solution, and is probably the oldest of the hydrometallurgical processes. It is also known by other terms such as metal displacement or contract reduction, and is widely used in the recovery of metals such as silver, gold, selenium, cadmium, copper and thallium from solution and the purification of solutions such as those used in the electrowinning of zinc. The electrochemical basis for these reactions has been well established414 and, as in leaching reactions, comprises the anodic dissolution of the less-noble metal coupled to the cathodic reduction of the more-noble metal on the surface of the corroding metals. Therefore, in the well-known and commercially exploited44 cementation of copper from sulfate solution by metallic iron, the reactions are... 

Recovery of metals such as copper, the operation of batteries (cells) in portable electronic equipment, the reprocessing of fission products in the nuclear power industry and a very wide range of gas-phase processes catalysed by condensed phase materials are applied chemical processes, other than PTC, in which chemical reactions are coupled to mass transport within phases, or across phase boundaries. Their mechanistic investigation requires special techniques, instrumentation and skills covered here in Chapter 5, but not usually encountered in undergraduate chemistry degrees. Electrochemistry generally involves reactions at phase boundaries, so there are connections here between Chapter 5 (Reaction kinetics in multiphase systems) and Chapter 6 (Electrochemical methods of investigating reaction mechanisms). 

Refs. [i] Wendt H, Kreysa G (1999) Electrochemical engineering Science and technology in chemical and other industries. Springer, Berlin, chap 11, pp 326-344 [ii] Long RB (1995) Separation processes in waste minimization. Marcel Dekker, New York, chap 10, pp 288-298 [iii] Kr-ishnan ER (1995) Recovery of metals from sludges and wastewaters. Pollution Technology Review, no 207. Noyes Data Corporation, William Andrew, pp 38-46... 

Membranes, selective to either cation or anion transport, are employed in many of the electrochemical treatment systems already discussed in this chapter. For example, in the cathodic recovery of metals from waste streams, a cation-permeable membrane can prevent the migration of anions such as Cl- from the catholyte to anolyte, where oxidation to CI2 can occur with... 

Examples of electrochemical reactors for discontinuous recovery of metal. 

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. 

Recent research has also been focusing on electrochemical recovery of cobalt from spent Li-ion cathodes. Researchers in Italy (20) and Brazil (21) investigated the recycling of cobalt from the cathodic material of Li-ion spent batteries, where the final cobalt recovery step relied on electrochemical techniques. In one example, the cathodic material (also containing nickel) was dissolved in acid, cobalt and nickel separated by solvent extraction, and the cobalt electrowon at 250 A/M, pH 4.1 and 50°C. In the other example, cathodic LiCo02 was acid leached, and the metallic cobalt deposited directly on 430 steel. The deposit was then oxidized to C03O4 in air at 850°C. 

Scott K (1988) A consideration of circulating bed electrodes for the recovery of metal from dilute solutions. J Appl Electrochem 18 504-510. doi 10.1007/ BFO1022243... 

Coueuret F (1980) The fluidized bed electrode for the continuous recovery of metals. J Appl Electrochem 10 687-696... 

A commercially available cell " uses this type of particulate turbulence promoter in the form of a fluidized bed of electrochemically inert particles, typically glass spheres. This Chemelec cell is aimed at the secondary recovery of metals and their removal from effluents. Little reliable information is available on mass transfer to spheres in a fluidized bed. Nassif, using a limited current technique, investigated mass transfer to the particles of a fluidized bed as well as to the wall enclosing them. For the latter he derived ... 

Buckle, R. (2007), The Recovery of Metals from Waste Solutions by Electrochemical Methods , Thesis, Newcastle University pp. 33-7 and 84-7. 

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. 

A minor source of metallic Ca is the recovery of the Ca that crystallizes from the liq Na produced in the electrochemical cell. This can be effected either by filtration of the liquid metal or by aleohol leaching of the residual Na-Ca sludge. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

The quality of the refined metal, and the current efficiency strongly depend on the soluble vanadium in the bath and the quality of the anode feed. As the amount of vanadium in the anode decreases, the current efficiency and the purity of the refined product also decrease. A laboratory preparation of the metal with a purity of better than 99.5%, containing low levels of nitrogen (30-50 ppm) and of oxygen (400-1000 ppm) has been possible. The purity obtainable with potassium chloride-lithium chloride-vanadium dichloride and with sodium chloride-calcium chloride-vanadium dichloride mixtures is better than that obtainable with other molten salt mixtures. The major impurities are iron and chromium. Aluminum also gets dissolved in the melt due to chemical and electrochemical reactions but its concentrations in the electrolyte and in the final product have been found to be quite low. The average current efficiency of the process is about 70%, with a metal recovery of 80 to 85%. 

Electrolytic recovery (ER) is the oldest metal recovery technique. Metal ions are plated-out of solution electrochemically by reduction at the cathode.34 There are essentially two types of cathodes used for this purpose a conventional metal cathode and a high surface area cathode (HSAC). Both cathodes can effectively plate-out metals, such as gold, zinc, cadmium, copper, and nickel.22... 

---