How much ore do we need? Assuming an alumina content of 50%, it takes two pounds of bauxite to make one pound of alumina, and two pounds of alumina to make one pound of elemental aluminum. (EA)

Bauxite Confidential How easy is it to find and mine bauxite? Bauxite was discovered in 1821 by Pierre Berthier. Berthier was an amateur geologist, on holiday in Provence, and his attention was caught by a conspicuous band of red earth in the white face of a limestone ridge near Les Baux. (Raymond, 220-1) In 1858, the prominent chemist Henri Deville, who had developed the first commercial process for making aluminum, was sent a sample of an "iron ore" which an engineer in Marseilles had been unable to smelt. Deville identified it as high-grade bauxite. The sample came from nearby Les Baux, and was found in a kilometers-long exposed seam-much like the one which Berthier had found previously. (Raymond, 224).

Germany. While Germany is not even listed in Brubaker's 1963 overview of bauxite countries, a German website says that in 1918, Germany was the "world's fourth largest bauxite producer." It adds, "The pit "Eiserne Hose" near the town of Lich has been in production until 1975. Miocene basalt is weathered to depths up to 100 m and overlain by bauxite." A photograph shows a roadside falloff on which a red soil is exposed. http://mindepos.bg.tu-berlin.de/mk/mkb1.htm Rest of the world. About 80% of world bauxite production is from surface mines, usually exploiting "blanket deposits." (International Aluminum Institute, IAI) Blanket deposits may be exposed, or covered with some kind of overburden which must be removed by open-pit techniques.

According to IAI, "large blanket deposits are found in West Africa, Australia, South America and India.

These deposits occur as flat layers lying near the surface and may extend over an area covering many kilometers. Thickness may vary from a meter or less to 40 meters in exceptional cases although 4-6 meters are average." That sounds promising.

In British Guiana and Dutch Guiana (Suriname), we can find blanket deposits up to forty feet thick, and covered by up to sixty feet of overburden (typically sand and clay, not rock). The ore is 58-63% alumina, 2-5% silica, and 3-6% ferric oxide. The Gold Coast deposits are fairly similar: average thickness is 33 feet, maximum 60 feet; and as much as 64% alumina. (Bateman, 558-9) Lancashire says that Jamaican bauxite is also near the surface (usually not more than 100 feet underground). Moreover, the overburden is soft, and thus easily removed. A map of bauxite mining areas shows that they cover about half of the central third of the island (the region earmarked by theHammond Citation World Atlas as being of interest).

According to IAI, in southern Europe and Hungary, bauxite is most often found in pockets. These may need to be reached by tunneling.

Extracting Alumina The alumina can be isolated from bauxite by the 1888 Bayer process. There is a general description of this process in both theEncyclopedia Americana and the modernEncyclopedia Britannica . The bauxite is washed with a hot sodium hydroxide solution, converting the aluminum oxide to a "green liquor"

of saturated sodium aluminate. The bauxite impurities (silica, iron oxides, and titanium dioxide) are less soluble and to some degree are filtered out, using cloth filters, as a "red mud."

Crystal "seeds" are added to the liquor, and the solution is allowed to cool, so that an aluminum hydrate (hydroxide) precipitates out. The aluminum hydroxide is then heated (EA says to 1093.3 deg. C (2000 deg. F) to produce the purified aluminum oxide (alumina) in the form of a sugar-like powder. The 1911EB provides some additional information, such as the specific gravity of the sodium hydroxide solution (from which a chemist can calculate its concentration).

The modernEncyclopedia Britannica says that alumina, after purification for smelting purposes, usually contains less than 0.1% of other oxides.

Alumina Confidential Our heroes will be pleasantly surprised to discover that most bauxite will not require a great deal of processing. If there is a lot of clay mixed in, it can be removed by "washing, wet screening, cycloning or hand picking." (International Aluminum Institute) The ore should also be crushed so as to increase the surface area over which the dissolution can take place.

The necessary temperatures are dependent on the nature of the bauxite mineral. Gibbsite requires just 135-150 deg. C; Boehmite, 205-245; and diaspore, even higher temperatures. Sodium hydroxide concentrations may also need to be increased to complete extraction of the more stubborn minerals.

(Lancaster) The real bugbear is silica content. Lancaster says that ores with more than 7% silica cannot be economically processed, but of course that depends on the prices and availability of alumina and aluminum. Bateman says the allowable silica is only 4.5%. (554) Essentially, the problem is that the same sodium hydroxide which dissolves the alumina (aluminum oxide) also dissolves the silica (silicon dioxide). The dissolved silica reacts with sodium aluminate to form sodium hydroaluminosilicate, which is essentially a waste product. "As a result, 0.666 kg NaOH and 0.85 kg Al2O3 per 1 kg of silica are lost irrevocably." (Rayzman) Silica content varies from one deposit to the next, and hence it would be prudent to assay it before beginning mining operations.

There is a trick for processing high-silica ores, and it is mentioned in the modern Encyclopedia Britannica. The infamous red mud is heated with limestone (calcium carbonate) and soda to regenerate sodium aluminate, and the solution is fed back into the Bayer process. The residue, rich in silicate, is called "brown mud."

Another possible problem impurity is ferric oxide. Bateman says that bauxite ore should have not more than 6.5%. (554) The amount of "red mud" waste generated for each ton of alumina produced depends on the ore, being just 0.33 tons for Surinamese bauxite, one ton for the Jamaican, and two tons for the Arkansan. Efforts have been made to find uses for it, or alternatively to treat as a low grade iron, titanium or aluminum ore and extract metal from it. (Chandra, Waste Materials Used in Concrete Manufacturing, 292)

From Alumina to Metallic Aluminum: The Hall-Heroult Process In nature, aluminum exists in an "oxidized" state (combined with other elements, especially oxygen). Toobtain the metal, the aluminum must be "reduced," usually in an electrolytic cell.

The cell (pot) is the reverse of a battery; a battery uses a chemical reaction to create an electric current; an electrolytic cell uses a current to force a chemical reaction to occur.

Inside the cell is an electrolyte, a fluid medium in which ions and electrons can move. Like a battery, a cell has two poles. The cathode provides the electrons, and they leave the cell at the anode. Reduction occurs at the cathode and oxidation at the anode.

There is a decent description of the Hall-Heroult process in theEncyclopedia Americana . The electrolyte is a melted (982 deg. C) solution of alumina in cryolite; no water is involved. (Cryolite is needed because the melting point of pure alumina is 2050 deg. C.) The cryolite is held in a carbon-lined cast-iron shell, whose bottom serves as the cathode. Carbon rods are suspended in the melt; they are the anode. Current is passed from the anode to the cathode, reducing the aluminum oxide to aluminum, and releasing oxygen (which attacks the carbon rods, producing carbon dioxide).

The Hall-Heroult process was developed in 1886, and by 1892, it was routinely producing aluminum which was over 99% pure. (Wallace, 9) If you need material of, say, 99.9% purity, you will need to further refine it.

The principal inputs are: alumina (aluminum oxide), cryolite, electricity, and carbon (for the rods). We have already discussed alumina. What about cryolite?

Cryolite The 1911EB says that, except for "mechanical losses," the initial charge of cryolite would last indefinitely.

That makes sense, because cryolite acts a flux, reducing the melting point of the alumina, not a reactant.

So, the amount of cryolite you need should be dictated just by how much alumina you want to process at one time.

In practice, as the encyclopedia implies, some cryolite is lost. However, there are some tricks (which Grantville must re-invent) for regenerating it once smelting is underway.

Natural Cryolite. Cryolite ("frost stone") is a mineral, sodium aluminum fluoride (Na3AlF6, so NaF:AlF3 is 3:1). 1911EB reveals that cryolite can be found "almost exclusively at Ivigtut (sometimes written Evigtok) on the Arksut Fjord in SW. Greenland." The article on "Greenland" notes that the mines are in the district of Frederickshaab, and provides a lovely map showing the location of Ivigtut, "Arsuk"

Fjord, and a prominent landmark, Cape Desolation.

This information will be meaningful to down-time mariners, at least the whalers who frequented Greenlandic waters. Both the Cape, and a fjord of the correct shape, are shown on a map made by William Barents and published in 1598. (Braat) Once our shivering crew of geologists is disembarked at Ivigtut, they know that they want to look for a "granitic vein running through gneiss," and that the cryolite is "accompanied by quartz, siderite, galena, blende, [and] chalcopyrite." Once they locate the correct formation, they can look for the actual mineral. 1911EB sets forth its color, crystal form, cleavage, hardness, specific gravity and "flame test" result. Most distinctively, it is "nearly transparent on immersion in water."

A picture would still be nice, and there we are in luck. There is one in Hochleitner,Minerals: Identifying, Classifying, and Collecting Them(1992), which also mentions that it is found in pegmatites (probably more accurate than "granitic veins"), in association with siderite, fluorite, topaz and quartz.

There are more photos inThe Audubon Society Field Guide to North American Minerals, and Eyewitness Handbooks: Rocks and Minerals .

Whittaker states that cryolite "was traded as early as the beginning of the 18th Century amongst the native people of the western coast of Greenland." It is possible that the mineral was already known in 1632 to the Inuit Eskimos, in which they can be paid to guide an expedition to the source.

Still, it doesn't take much imagination to appreciate that mining cryolite in Greenland will be arduous and perhaps dangerous. According to 1911 EB, Deville toyed with the idea of using cryolite as an aluminum ore, but, "finding the yield of metal to be low, receiving a report of the difficulties experienced in mining the ore, and fearing to cripple his new industry by basing it upon the employment of a mineral of such uncertain supply," decided to produce aluminum instead by chemical reduction of aluminum chloride (see below).

Anyone seeking to raise money for a cryolite expedition will have to explain away Deville's mid-nineteenth century objections. One possible excuse would be that Deville was influenced by the French supply of bauxite, which might not be made available to other powers.

Synthetic cryolite. The uptimers know that cryolite can be synthesized (EA), and they at least know its chemical formula (Na3A1F6).

There are several chemists in Grantville and each will have a personal library of chemistry texts. It is within the realm of possibility that even a general chemistry book will explain how cryolite is made. For example, my own library includes a 1993 introductory college chemistry text which suggests adding sodium hydroxide and hydrogen fluoride to aluminum hydroxide (from the Bayer process). The reaction is 3NOH + Al(OH)3 + 6HF -> Na3AlF6 + 6H2O. (Ebling, 880) But I think that a good chemist could probably work it out without this help.

Cryolite Confidential More on Natural Cryolite. There are contradictory accounts about early mining at Ivittuut. What I think is most accurate is that there were two separate operations there. Julius Thomsen and George Horwitz obtained a license to mine cryolite in 1854, but didn't commence large-scale mining until 1859. In the meantime, in 1854-55, two other entrepreneurs extracted forty tons of silver-bearing galena (itself lead sulphide). (Greenland BMP) They decided, after six months, that the formation wasn't rich enough to warrant further work. (Whittaker) About 3.7 million tons of cryolite ore, with an average cryolite content of 58%, were mined in the period 1854-1987. The mine closed because it had become uneconomic to compete with synthetic cryolite (seebelow).

Cryolite was initially wanted for use in a new process for production of soda (sodium carbonate), which Thomsen had patented in 1853. Thomsen decomposed cryolite with calcium hydroxide into calcium fluoride and sodium aluminate. He filtered off the former, and added carbon dioxide to get aluminum hydroxide and sodium carbonate. (Hornburg 31) The Thomsen synthesis was used until the 1890s, when it was superseded by the 1864 Solvay ammonia-soda process. (Hornburg) The handwriting had been on the wall for several years, of course, and Thomsen's company, Oresund Chemiske Fabriker, had been trying to develop other markets: soap factories, manufacture of enamel, insecticide ("cryocide"), abrasives. (Hornburg; Grossman) The advent of the Hall-Heroult process, which used cryolite as a flux, was extremely fortunate for the Danish-owned cryolite operation. In 1904, about 25% of cryolite was sold for this purpose, while, by 1939, it was 82% of their market. (Travis, 335) Cryolite was also used, in the period 1855-64, as an aluminum ore in the Rose-Dick process (see Alternatives to Hall-Heroult Process, below). However, only about fifteen tons was exported from Greenland for this purpose.

According to a University of the Arctic course, "it was mined in an open-pit with easy accessibility and an ice-free harbor." However, while Hurlbut'sMinerals and Man (88) admits that the cryolite is "easily extracted," it warns that the harbor was only free of ice for a "brief period," and asserts that "mining is difficult because of the harsh climate."

It is certainly promising that in OTL, cryolite was mined for 128 years. The very volume of the industry tells us mining cryolite was quite practical in the late nineteenth and early twentieth centuries. Production was 14,000 tons in 1857-67; 70,000 in 1867-77 (Johnson's) Obviously, we're talking about more than one shipload a year.

However we do need to ask whether, in 1859, when the cryolite operation began, Europeans were better equipped to sail in Arctic waters (e.g., better maps and navigational equipment) and to live and work under Arctic conditions (e.g., Burberry garbardine windproofs) than they would be in the 1632 Universe. Indeed, by the end of the nineteenth century, they could have used steel-hulled steam ships.

Moreover, our ability to reliably access cryolite will be hindered by both war and piracy, neither of which were serious concerns for Thomsen in 1859.

Even the climate is probably worse; the 1630's fall within the Little Ice Age (which lasted until 1850).

Brian Fagan, in his book of that title, refers to the seventeenth century as "bitterly cold." A volcanic cold spike is due to occur in 1641.

The fact that cryolite has significant uses, other than as a flux in the Hall-Heroult process, will make it easier to attract investors for a cryolite mining venture; they can turn a profit even if the aluminum industry dies stillborn. However, bear in mind that sodium carbonate, while a very important industrial chemical, can be made not only by the Thomsen cryolite process, but also by both the earlier Le Blanc process (1791) and the later Solvay process, both described in Grantville encyclopedias.

I think natural cryolite will be of greater interest to the French, rather than the up-timers, since they are less likely to figure out how to synthesize cryolite, and hence will need natural cryolite as a flux for alumina. More on Synthetic Cryolite. Hall didn't use imported cryolite, he made it himself. Which in turn implies that the process isn't real complicated. After all, Hall was working in the woodshed behind his family home. And he had very little formal training in chemistry. (Oberlin) Synthetic cryolite was commercially available at least as early as 1900, and, by 1930, was offering "serious competition" to the natural product. (Travis, 335) Safety. It may appear that a big disadvantage of synthetic production of cryolite is that hydrofluoric acid, which is quite nasty stuff, is directly or indirectly involved.

However, even if natural cryolite is used, it is standard practice to add other fluorides to improve the properties of the liquid-and these may also be made using hydrofluoric acid. Moreover, processing cryolite itself is not without risks-fluorosis was discovered in the workers at the cryolite-to-soda factory in Copenhagen.

Electricity It will be obvious to anyone reading the encyclopedias that aluminum smelting is energy intensive, although the sources disagree somewhat as to how much electricity is needed. The 1911EB reports an aluminum yield of one pound per twelve e.h.p. hours (nine kilowatt hours). According to the Encyclopedia Americana, it takes about ten kilowatt hours (kWh) to produce one pound of aluminum.

The modern EB reports that while, in 1930, it required 12 kWh to produce one pound of aluminum, this had dropped to 4.5 by the early 1980s. (IEP 391) I'll use the 10 kWh figure for now.

The Grantville power plant is a 200 megawatt steam turbine plant, operating at 58% load at the time of the Ring of Fire (Loren Jones, "Power to the People,Ring of Fire ). The plant is expected to fail in eighteen to twenty-four months, by which point they expect to have a steam engine and generator running which will supply ten to fifteen megawatts.

According to DOE, in 2001 the average electrical consumption in the United States was 10,656 kilowatt-hours per household. There were about 1,000 households in the Grantville area transported by the Ring of Fire.

The current power plant can produce a maximum of 1,752,000 megawatt hours annually. But the immediate post-Ring of Fire load, considering only the remaining residential customers, is just 10,656 megawatt hours-less than one percent of the production capacity.

Once the power plant gears down from 200 megawatts to ten megawatts, it still can output 87,600 megawatt hours/year. And the residential load would absorb just 12% of that. The remainder is still sufficient to smelt over 700,000 pounds/year aluminum-not that we'll be making that much anytime soon!

With most of its customer base left behind, and a payroll to meet, the power plant needs to find new customers, fast. Power companies love smelters because they exert a steady, high demand for electricity.

Electricity Confidential In 1995, the electricity required by modern plants-not counting generation and transmission losses-was 13 kWh/kg (1 kg = 2.2 pounds). The theoretical minimum is 6 kWh/kg(Choate, 25).

However, it is probably more meaningful to look at early practice; the first commercial cells in Pennsylvania (Hall) and Switzerland (Heroult) were drawing over 40 kWh/kg. (Id.) That's about twice as high as the most conservative encyclopedia value.

Carbon Carbon is needed, both to line the electrolytic cell, and for the anodes. The lining protects the pot from corrosion by molten aluminum and fluorides.

Carbon Confidential In Grantville, there isno design information on either the linings or the anodes, so that will all have to be worked out empirically.

One important design parameter is the thickness of the carbon lining. This must vary so the electrolyte (the cryolite) freezes on the inner walls but not on the bottom. (Kirk-Othmer, 195) The advantage of this "ledge" is that it protects the lining from corrosion. Cryolite also freezes to form a crust at the top of the pot, which helps retain heat (and reduce the heating bill).

Another parameter is the chemical nature of the lining. The bottom lining acts, initially, as the cathode.

(Once the process has commenced, a pool of aluminum collects at the bottom, and that becomes the true cathode.) Since the carbon lining must be conductive, it is either natural graphite, or carbon baked to form a "graphitic" structure (carbon atoms mostly connected in closely-spaced layers). Anthracite can be used, as high purity is not as important as in the anodes. (Totten, 2:38) With regard to the carbon anodes, there are two major designs in modern use. "Prebaked anodes" are made by baking blocks of petroleum coke and coal tar pitch ("paste") at 1,000-1,200 deg. C. The advantage of petroleum coke is that it is low in ash (silica, iron oxide, etc.); as the anode is consumed, any silicon or iron would deposit in the aluminum. Coke can be made from coal, but then it has to be purified to remove the ash.

Soderberg anodes are "continuously self-baking." What that means is that the operators are continuously feeding petroleum coke and coal tar pitch into a casing. These materials are baked by the heat of the pot, forming the carbon anode at the bottom of the casing. A smelter using Soderberg anodes doesn't need a carbon baking facility.

The prebaked anodes are about 30% more conductive, but Soderberg anodes are baked using waste heat. Overall, smelters using prebaked anodes are about 3% more energy efficient. (Brubaker) Prebaked anodes are also more environmentally friendly; the hydrocarbon waste gases are collectedmore readily in a carbon-baking facility than at the pot.

Small operators may prefer Soderberg anodes because they don't require capital investment in a carbon baking facility and have slightly lower labor requirements. (Beck; Brubaker, 91-96) The problem for Grantville is not so much the choice between prebaked and Soderberg anodes, but rather appreciating the advantage of using low-ash carbon. The ash content of bituminous and anthracite coals is 1-10%, while that of a typical oil distillate is 0.5-1.5%. (Scurlock)

Grantville Knowledge of the Hall-Heroult Process The electrical current has two functions: (1) heating the cryolite-alumina mixture so it melts, thus forming the electrolyte solution, and (2) reducing the aluminum. Curiously, while the current density was given, the total current wasn't. The working temperature is said (by EB11-Al) to be 750-850 deg. C., and of course this must be maintained for the reduction to continue.

The voltage applied to each cell was 3-5 volts, and 10-12 such cells were connected in series, so the total voltage drop was 30-60 volts. The essayist recognized that part of the voltage was needed to overcome the resistance of the electrolyte, and that, in doing so, it heated it. I will say more about that in the next section.

We next turn to the modernEncyclopedia Britannica . This, of course, is describing more recent practice. The normal pot voltage is about the same (4-6 volts), but pot lines are now 50-150 cells, requiring a total line voltage of 200-900 volts. The currents are much higher, 50,000 to 100,000 amperes.

The reported pot temperature is 950 deg. C (higher than before), and the alumina is apparently preheated at this temperature to drive off moisture.

A figure shows an alumina supply hopper over the cell, with the anodes flanking the hopper outlet. The molten aluminum is periodically siphoned off the bottom of the cell. While not shown, there is reference to the addition of aluminum fluoride to "restore the chemical composition of the bath." (EB-IEP 390-1)

Hall-Heroult Process Confidential In the cell, aluminum oxide is being reduced to aluminum at the cathode (requiring 2.2 volts), while carbon is oxidized to carbon dioxide at the anode (supplying 1.0 volts), for a net minimum requirement of 1.2 volts direct current (at 960 deg. C.) to drive the reaction.

Higher voltages are used because there are electrical energy losses as a result of the electrical resistance of the cryolite, the anodes, the deposited aluminum, and so forth.Each resistance requires a certain voltage to overcome it, and that voltage drop equals the product of the resistance and the current. The biggest voltage drops are in the anode (0.3-0.42 volts), cathode (0.45-0.68 volts), and electrolyte (1.75-1.535 volts). Also, overvoltages at the anode (0.51) and cathode (0.08) help to drive the reaction.(Prasad; Kirk-Othmer 2:197-8; Choate, 31) * * *

Electrical energy is converted by the resistance into heat energy at a rate equal to the voltage times the current. Thus, the energy bill exacted by the resistance of the materials is proportional to thesquare of the current.

Some of this heat is put to good use; it keeps the electrolyte molten, or, if the anodes are of the Soderberg type, it bakes them. The rest is just waste heat. Heat loss occurs at the top of the bath (through the cryolite crust and the carbon anodes), and at the sides and bottom (through the cryolite "side freeze" and the carbon lining). Some heat loss at the sides is desirable to form that "side freeze," which reduces corrosion of the lining. The aluminum metal protects the carbon at the bottom.

It follows that both the electrical conductivity of the electrolyte, and its melting point, are of concern.

Density is also important, as the produced aluminum must be able to sink through it to the bottom of the pot, lest it short out the anode rods.

The 1911 EB says that cryolite dissolves about 30% of its weight of pure alumina (EB11-Al, 78), implying an alumina content of 23% of the melt. That's way too high for economical production. The cryolite-alumina solution with the lowest melting point (960 deg. C) is one which is 89.5% cryolite, 10.5% alumina. (Kirk-Othmer) The solubility of the alumina increases with temperature, being about 15% at 1050 deg. C. Of course, the higher the temperature, the higher the energy requirements, so you want to limit the alumina content.

Usually, other fluoride salts are added to improve the characteristics of the electrolyte. Each addition can affect the melting point, the solubility of the alumina, the electrical conductivity, and the density. Reducing melting point or increasing conductivity saves electricity. A high solubility makes it easier to maintain proper alumina levels. The density must be less than that of aluminum, so the metal sinks.

The addition of other salts reduces the solubility, perhaps to about 6%, but also reducing the melting point to as low as 920 deg. C. (Prasad; Aurbach, Nonaqueous Electrochemistry, 503) A typical electrolyte is 2-8% alumina, 5-7% calcium fluoride, 5-7% excess aluminum fluoride, 0-7% lithium fluoride, and 80-85% cryolite. (Kirk-Othmer 2:193, 11:282) The calcium fluoride usually is not added intentionally, because it reduces conductivity and increases density (so there is a risk that aluminum will float to the top and get re-oxidized, or short out the cell). It is derived from calcium oxide in the alumina. The level in alumina is perhaps 0.4%, but the calcium accumulates in the melt. Calcium fluoride does offer one benefit; it reduces the melting point.

The effects of aluminum fluoride are similar. However, it's deliberately added for several reasons. First, to compensate for loss of the aluminum component of the cryolite. Molten cryolite dissociates to some degree into NaF and NaAlF4. The latter vaporizes more readily, reducing the aluminum-sodium ratio.

Also, the moisture in the air reacts with the cryolite to form sodium fluoride, hydrofluoric acid, and alumina, thus further worsening the ratio. (Kirk-Othmer, 11:280) Another reason to add aluminum fluoride is to eliminate sodium oxide (Na2O), an alumina impurity (~0.6%). The reaction is 3Na2O + 4AlF3 -> 2Na3AlF6 + Al2O3 (Kirk-Othmer, 2:192) Conveniently, this reaction also helps replace cryolite lost as a result of vaporization, absorption by the pot lining, etc. The conductivity of the electrolyte is improved by adding lithium fluoride, which also reduces both melting point and density.

In the course of the operation, the level of alumina will drop. If it drops below 1-2%, the pot voltage rises, and the cryolite is itself electrolyzed ("anode effect"). The fluorides react with the carbon anodes to form polyfluorocarbons, which are carcinogenic. (Beck) If, to avoid or stop an anode effect, you add too much alumina (concentration over 4%), some alumina is deposited, forming a sludge which reduces the current flow. Lowering the pot temperature below the normal 960 deg. C. saves energy, but narrows the "safe" alumina concentration range (Choate, 35) The pot must be agitated to keep the alumina solubilized in the cryolite. However, if you agitate too aggressively, you run the risk of short-circuiting the pot (by bringing the aluminum in contact with the carbon rods).

There are just two basic ways of increasing production: increasing the current running through the pots, and adding more pots (either to an existing pot line, or to a new one).

The theoretical production rate per cell is proportional to the current; if the current is 180,000 amperes, Faraday's Law predicts that the cell will produce 1,450 kilograms aluminum per day.

Hall's first commercial cell used only 1,750 amperes, and in the Thirties, Wallace thought that the maximum amperage was 30,000. Modern cells can reach as high as 500,000 amperes, but 180,000 is quite respectable.