Aluminum electrowinning

In the aluminum electrowinning process a phenomenon called the anode effect is normally encountered when the alumina content in the electrolyte falls below 2%. The anode gets partially covered with a gas blanket and as a consequence, sparking occurs and the cell voltage fluctuates considerably due to frequent breaking and reestablishment of local contact between the anode and the electrolyte. A heavy current passes through the anode area... 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

Zhang M, Kamavaram V, Reddy RG (2005) Aluminum electrowinning in ionic liquids at low temperature. Light Met 2005 583-588... 

Advantageously, electroanalytical techniques may also be employed for in situ analysis and control of industrial processes, e.g., as currently practiced for anode effect control in aluminum electrowinning/ ... 

Cases exist, however, where for fundamental reasons aqueous solutions cannot be used. One such case is that of devices in which electrochemical processes take place at elevated temperatures (above 180 to 200°C) for example, the electrowinning of aluminum performed at temperatures close to 1000°C. Another case is that of devices in which electrodes consisting of alkali metals are used, which are unstable in aqueous solutions, such as batteries with a lithium negative electrode. 

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

A number of electrolytic processes are used for the industrial production of metals. Some metals such as zinc, copper, manganese, gallium, chromium, etc. are electrowon from aqueous baths. Another common electrolytic process used is molten salt electrolysis. The most important application of molten salt electrolysis till now has been in the electrowinning of metals. Today aluminum, magnesium, lithium, sodium, calcium, boron, cerium, tantalum, and mischmetal are produced in tonnage quantities by molten salt electrolysis. As a representative example, the electrowinning process for aluminum is taken up. 

The electrowinning of aluminum, magnesium, sodium, and zinc and the electric furnace processing of ferrochrome and ferromanganese require large amounts of electrical... 

Zinc electrowinning takes place in an electrolytic cell and involves running an electric current from a lead-silver alloy anode through the aqueous zinc solution. This process charges the suspended zinc and forces it to deposit onto an aluminum cathode (a plate with an opposite charge) that is immersed in the solution. Every 24 to 48 h, each cell is shut down, the zinc-coated cathodes removed and rinsed, and the zinc mechanically stripped from the aluminum plates. The zinc concentrate is then melted and cast into ingots, and is often as high as 99.995% pure. 

Zinc electrowinning Zinc in a sulfuric acid/ aqueous solution, lead-silver alloy anodes, aluminum cathodes, barium carbonate, or strontium, colloidal additives... 

Also, the influence of aluminum sulfate, animal glue, and an extract of horse-chestnut nuts on zinc electrowiiming from a weak acidic sulfate electrolyte prepared from an industrial waste product was investigated [401]. The use of additives mixture has a beneficial effect on zinc electrowinning and results in smooth, slightly bright zinc deposits. 

Electrowinning of metals in aqueous solutions is applicable to those metals that possess high electrochemical reduction potentials, such as silver, copper, cadmium, and zinc. Magnesium, aluminum, and sodium, like other reactive metals, are electro-produced from molten salt baths, such as NaCl/CaCh mixture at ca. 600 °C for sodium and MgCl2/NaCl/CaCl2 eutectic mixture at ca. 750 °C for magnesium. 

Metals are important resources and have a wide range of applications. Metals are often extracted from ores. Once the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. Pyrometallurgy uses high temperatures to convert ore into raw metals, while hydrometalluigy employs aqueous chemistry for the same purpose. The methods used depend on the metal and their contaminants. Most metals are obtained by hydrometallurgical processes such as aqueous acids or alkalis are predominantly used to dissolve the metal oxides, sulfides, or silicates. Electrowinning and solvent extraction are frequently used to recover and concentrate the metals. A limited number of high-temperature molten salts have also been used for the recovery of refractory metals, such as titanium and aluminum, from their ores... 

Electrolytic gas evolution is a significant and complicated phenomenon in most electrochemical processes and devices. In the Hall process for aluminum production, for example, bubbles evolved on the downward-facing carbon anodes stir the bath and resist the current, both of which directly affect the heat balance and the cell voltage. Bubbles appear as a result of primary electrode reactions in chlorine and water electrolysis, and as the result of side reactions in the charging of lead-acid batteries and some metal electrowinning. Stirring of the electrolyte by gas evolution is an important phenomenon in chlorate production. Electrolytically evolved bubbles have also been used in mineral flotation. Relatively few major electrochemical processes do not evolve gas. 

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