Electrometallurgy

The recovery of metals from their sources in the earth is the science of extractive metallurgy, a discipline that draws on chemistry, physics, and engineering for its methods. As a science it is a comparatively recent subject, but its beginnings, which were evidently in the Near East about 6000 years ago, marked the emergence of humanity from the Stone Age. The earliest known metals were undoubtedly gold, silver, and copper because they could be found in their native (elemental) states (Fig. 17.15). Gold and silver were valued for their ornamental uses, but they are too soft to have been made into tools. Iron was also found in elemental form— although rarely—in meteorites. 

Metal Oxides Arranged According to Ease of Reduction 

The standard free energies of formation of the metal oxides in kJ moF are adjusted for fair comparison by dividing them by n, the total decrease in oxidation state required to reduce the metal atoms contained in the oxide to oxidation states of 0. Thus, the reduction of Cr203 involves a change in the oxidation state of two chromium atoms from + 3 to 0, so n = 2 X 3 = 6. 

Aluminum is the third most abundant element in the earth s crust (after oxygen and silicon), accounting for 8.2% of the total mass. It occurs most commonly in association with silicon in the aluminosilicates of feldspars and micas and in clays, the products of weathering of these rocks. The most important ore for aluminum production is bauxite, a hydrated aluminum oxide that contains 50% to 60% AI2O3 1% to 20% FeiOs 1% to 10% silica minor concentrations of titanium, zirconium, vanadium, and other transition-metal oxides and the balance (20% to 30%) water. Bauxite is purified via the Bayer process, which takes advantage of the fact that the amphoteric oxide alumina is soluble in strong bases but iron(III) oxide is not. Crude bauxite is dissolved in sodium hydroxide 

The water of hydration is removed by calcining at high temperature (1200°C). 

When chlor-alkali electrolysis is conducted in an undivided cell with mild-steel cathode, the chlorine generated anodically will react with the alkali produced cathodically, and a solution of sodium hypochlorite NaClO is formed. Hypochlorite ions are readily oxidized at the anode to chlorate ions this is the basis for electrolytic chlorate production. Perchlorates can also be obtained electrochemically. 

A process involving water electrolysis is the production of heavy water. During cathodic polarization the relative rates of deuterium discharge and evolution are lower than those of the normal hydrogen isotope. Hence, during electrolysis the solution is enriched in heavy water. When the process is performed repeatedly, water with a D2O content of up to 99.7% can be produced. Electrochemical methods are also used widely in the manufacture of a variety of other inorganic and organic substances. 

Electrolytic aluminum production is the most important process in both volume and significance. World production is about 15 megatons per year, consuming about 240 billion kilowatthours of electrical energy. Aluminum oxide (alumina), AI2O3, is subjected to electrolysis at a temperature of 950°C to this end it is dissolved in molten cryolite NujAlFg, with which it forms a eutectic melting at about 940°C. Carbon anodes that are anodically oxidized to CO2 in the process are employed. The overall electrolysis reaction can be written as 

Solution First we calculate the number of coulombs of electricity that pass through the cell  

we convert number of coulombs to number of moles of electrons 

From the oxidiation half-reaction we see that 1 mol 02—4 mol e. Therefore, the number of moles of O2 generated is 

The procedure for hydrogen is similar. To simplify, we combine the first two steps to calculate the number of moles of H2 generated  

Practice Exercise A constant current is passed through an electrolytic cell containing molten MgCl2 for 18 h. If 4.8 X 10 g of CI2 are obtained, what is the current in amperes  

Electroplating (EP) is employed for electrodeposition of metal coatings onto another metal or alloy. 

Molten salt electrolysis (MSB) is a high temperature electrowinning operation for producing metals that cannot be electrowon due to water decomposition, which promotes hydrogen evolution at the cathode before metal deposition occurs. Metals produced or recovered using this technique are Al, Mg, Be, Ce, iVa, K, Li, U, Pu, etc.. 

---

Electrometallurgy. A term covering the various electrical processes for the working of metals, eg, electro deposition, electrorefining and electro winning, and operations in electric furnaces. 

Smee, A., Elements of Electrometallurgy, Longmans Green, London, 2nd edn (1843)... 

Electrochemical systems with reacting metal electrodes are widely used in batteries, electrometallurgy, electroplating, and other areas. Corrosion of metals is a typical example of processes occurring at reacting metal electrodes. 

Cathodic deposition (electrocrystallization) of metals is the basic process in electrometallurgy and electroplating. 

---