Commercial electrolytic processes

The chemistry of metals is characterized by their ability to donate electrons to form ions. Because metals are typically very good reducing agents, most are found in nature in ores, mixtures of ionic compounds often containing oxide, sulfide, and silicate anions. The noble metals, such as gold, silver, and platinum, are more difficult to oxidize and are often found as pure metals. 

The breakthrough came in 1886 when two men, Charles M. Hall (Fig. 11.21) in the United States and Paul Heroult in France, almost simultaneously discovered a practical electrolytic process for producing aluminum. The key factor in the Hall-Heroult process is the use of molten cryolite (Na3AlF6) as the solvent for the aluminum oxide. 

Electrolysis is possible only if ions can move to the electrodes. A common method for producing ion mobility is dissolving the substance to be electrolyzed in water. This method cannot be used for aluminum because water is more easily reduced than Al3+, as the following standard reduction potentials show  

Thus aluminum metal cannot be plated out of an aqueous solution of Al3 +.  

Bauxite is not pure aluminum oxide (called alumina) but also contains the oxides of iron, silicon, and titanium, and various silicate materials. The pure hydrated alumina (A1203 wH20) is obtained by treating the crude bauxite with aqueous sodium hydroxide. Being amphoteric, alumina dissolves in the basic solution  

Charles Martin Hall (1863-1914) was a student at Oberlin College In Ohio when he first became Interested In aluminum. One of his professors commented that anyone who could manufacture aluminum cheaply would make a fortune, so Hall decided to give It a try. The 21-year-old Hall worked In a wooden shed near his house with an Iron frying pan as a container, a blacksmith s forge as a heat source, and galvanic cells constructed from fruit jars. Using these crude galvanic cells. 

Hall found that he could produce aluminum by passing a current through a molten Al207/Na3AIF6 mixture. By a strange coincidence, Paul Heroult, a Frenchman who was born and died In the same years as Hall, made the same discovery at about the same time. 

Unless otherwise noted, all art on this page is O Cengage Learning 2014. 

Silver coins and tankards salvaged from the wreck of the Atocha. 

Both the limestone formation and the corrosion had to be dealt with. Since CaCOj contains the basic anion acid dissolves limestone  

Soaking the mass of coins in a buffered acidic bath for several hours allowed the individual pieces to be separated, and the black Ag2 on the surfaces was revealed. An abrasive could not be used to remove this corrosion it would have destroyed the details of the engraving—a very valuable feature of the coins to a historian ora collector—and it would have washed away some of the silver. Instead, the corrosion reaction was reversed through electrolytic reduction. The coins were connected to the cathode of an electrolytic cell in a dilute sodium hydroxide solution as represented in the figure. 

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Sodium is produced by an electrolytic process, similar to the other alkali earth metals. (See figure 4.1). The difference is the electrolyte, which is molten sodium chloride (NaCl, common table salt). A high temperature is required to melt the salt, allowing the sodium cations to collect at the cathode as liquid metallic sodium, while the chlorine anions are liberated as chlorine gas at the anode 2NaCl (salt) + electrolysis —> Cl T (gas) + 2Na (sodium metal). The commercial electrolytic process is referred to as a Downs cell, and at temperatures over 800°C, the liquid sodium metal is drained off as it is produced at the cathode. After chlorine, sodium is the most abundant element found in solution in seawater. 

BASF in Germany operated a number of commercial, electrolytic processes [44, 56, 57] using a strategy based on the availability of a reliable and simple cell design and then noting the chemistry that can be carried out within this cell. In this cell, a series of horizontal carbon disks (diameter 1 m) were stacked with a separation of 1 mm (maintained by polymer spacers) and the electrolyte was pumped outward from the center of the disks. The cell is operated undivided and as a bipolar stack with bypass currents minimized because the stack is not immersed in electrolyte. The cell is well suited to methoxylation reactions carried out in methanol as the solvent. For example, BASF have carried out the following conversions ... 

Commercial Electrolytic Processes Production of Aluminum Electrorefining of Metals... 

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