Historical Development of Aluminum Production and Application

The historian who would trace the story of aluminum has a comparatively easy task. It is not referred to in the writings of the "ancients", nor in classical records; aluminum is a product of the present age. In 1886, in a woodshed at Oberlin, Ohio, and in a tannery at Gentilly, France, two young men almost simultaneously unlocked the door to low-cost production of aluminum from its oxide. Charles Martin Hall, in February 1886, and Paul Louis Toussaint Heroult, in April 1886, determined that molten cryolite was an effective solvent for the electrolysis of alumina. The age of light metals was born.

The historian who would trace the story of aluminum has a comparatively easy task. It is not referred to in the writings of the "ancients", nor in classical records; aluminum is a product of the present age.

In 1886, in a woodshed at Oberlin, Ohio, and in a tannery at Gentilly, France, two young men almost simultaneously unlocked the door to low-cost production of aluminum from its oxide. Charles Martin Hall, in February 1886, and Paul Louis Toussaint Heroult, in April 1886, determined that molten cryolite was an effective solvent for the electrolysis of alumina. The age of light metals was born.

From the first moment that aluminum was isolated and its properties determined, there was no doubt in the minds of the early scientists that it was a material of significant potential value. The advantageous characteristics of aluminum are partly inherent in the material, and are partly the result of extensive research and development work in industry laboratories. One of its chief advantages is lightness; others include resistance to corrosion, nontoxicity, workability, high strength-weight ratio in alloys, attractive appearance, susceptibility to an almost infinite range of finishes, high electrical conductivity, high conductivity for heat, efficient reflection of light and heat, nonmagnetism, and availability in a wide range of basic forms.

In addition to having favorable properties, aluminum has experienced a remarkably stable price history from the time it was first made available at a price, to make volume use possible. In 1854, aluminum was almost literally worth its weight in gold, selling at $17 an ounce. In 1960-ies it was sold in primary ingot form at less than 25 cents per pound.

Whether the selected starting point is the mid-1800´s –its first availability as a metal, in tiny globules– or the late 1800´s –when it first became available in ingot quantity– the history of aluminum is relatively brief. It reflects a growth paralleled by few major materials used by man. Aluminum today is second in production among metals, behind only iron and steel.

Many researchers tried to find optimal and economic method for obtaining aluminum. We listed here some of production treatments used before invention of Bayer method treatment which is today´s common used method for production of aluminum from bauxite.

Sainte-Claire Deville felt that aluminum should be available in bar and ingot form, and started work toward this goal. He received a grant from the French Academy for research on the production of aluminum. Intrigued by the potential of aluminum as a material for fabricating helmets and armor, Napoleon III financed Sainte-Claire Deville to continue his experiments on a larger scale. The work was carried on at the Javel Chemical Works; the process appeared practical but expensive.

Because sodium was a key element in the process and vastly more expensive than the aluminum ores, Sainte-Claire Deville began as early as 1854 to experiment with processes for extracting sodium.

Ironically, the solution to the lower-cost production of sodium was found in the 1880´s, on the eve of the discovery of electrolytic smelting processes that were to eliminate the sodium process from commercial consideration. The Aluminum Company Limited, started production at Oldbury, near Birmingham, England, in 1888, using Castner´s sodium process and Sainte-Claire Deville´s reduction method.

There were scores of other projects involved in the early, intense investigation of aluminum and methods to extract the metal from ores. The electrolytic smelting technique by no means came as an instantaneous flash of light in the late 1800´s: As early as 1854, both Sainte-Claire Deville and Robert Wilhelm Bunsen coincidentally explored electrolytic production methods for aluminum. At that time, batteries were the only source of current, and the cost was prohibitive. The later development of dynamo sources of electrical energy again turned the eyes of researchers to this approach.

The first American aluminum production was by Cowles Electric Smelting and Aluminum Company, in Cleveland. In 1885, Eugene and Alfred Cowles patented a process of producing aluminum alloys by electro thermal reduction of a mixture of alumina, carbon, and a heavy metal, such as copper. Alloys containing up to 40% aluminum were made, but the average aluminum content of their product was much lower.

The epochal discovery by Charle Martin Hall is in the full tradition of Edison, Steinmetz, and the legion of other young scientific geniuses who led the United States and the world into the twentieth century and the modern industrial age. Driven only by his imagination and determination, working in literally a woodshed laboratory with makeshift equipment, Hall found at 22 a secret that for decades had eluded top scientists.

After graduating from Oberlin in 1885, Hall continued full time his prior part-time experiments with alumina reduction. Hall correctly analyzed the initial problem: Alumina was readily available at low cost and in high-purity form. Its high melting point, 2050°C (3722°F), deterred its electrolysis in the fused condition. Hall believed that he should seek a fused salt that would dissolve alumina in substantial quantities, assuming that he could then electrolyze the oxide in solution. A requirement was that the salts have higher stability than alumina, so that the solvent would not decompose before or during the reduction.

Aluminum is the most abundant of all metallic elements. Of the solid portion of the earth´s crust to a depth of ten miles, an estimated 8.05% is aluminum. Only oxygen (46.68%) and silicon (27.60%) are more abundant. Next to aluminum is iron (5.03%), followed by calcium (3.63%), sodium (2.72%), potassium (2.56%), and magnesium (2.07%).

Aluminum is an important constituent of virtually all common rocks. It is especially abundant in clay, shale, slate, schist, granite, syenite, and anorthosite. In a content ranging from 15 to 40%, alumina (Al2O3) is present in feldspars, micas, and clay, the most abundant minerals with high alumina content.

The pre-eminent aluminum ore was discovered in 1821 by the French chemist P. Berthier, who analyzed specimens found near Les Baux, in southern France. The name "bauxite" was later applied to the material, from the name of the area in which it was discovered.

Bauxite cannot be precisely defined; it was long used widely in a general sense to identify the various kinds of aluminum ores found in the various parts of the world. Perhaps the best definition is that bauxite is an aluminum ore of a varying degree of impurity, in which the aluminum is present largely as hydrated oxide, the single major constituent.

The bauxites of various areas have similar composition and impurities. The physical appearance of bauxite, however, varies greatly, as does its chemical composition: It may be dense and hard, or it may be soft and earthy. The color of bauxite is related primarily to iron oxide content. If low in iron, bauxite is white, gray, Or cream; with moderate iron content, it is pink, yellow, light brown, or light red; if high in iron, it is dark red or brown.

In ore-field processing of bauxite, crushing, washing, drying, pulverizing, and calcining may be involved. The nature of the ore itself, and the subsequent processes and their location determine which, if any, of these are involved at a given site. Bauxite is sometimes refined to alumina at the ore field, and it is sometimes transported to a refining plant adjacent to a single smelter or centrally located to serve several smelters.

There is a wide range of alkaline, acid, and electro thermal methods of refining bauxite, clay, or other ores to obtain alumina. The method employed commercially today by the aluminum industry in the United States is the "original" Bayer process -and the modified combination process.

In the Bayer process, patented in Germany in 1888 by Karl Josef Bayer, bauxite is digested under pressure with hot sodium hydroxide solution, forming dissolved sodium aluminate. After filtering (the residue is discarded as "red mud"), the solution is cooled and agitated with a seed charge of alumina hydrate. Dissolved hydrate crystallizes out, is filtered and washed, and heated in kilns to drive off the combined water, converting it to alumina.

Three approaches to the production of aluminum metal from ores or from its oxide have been the subject of intensive research. The first of these is the sodiothermic technique, characterized by the work of Sainte-Claire Deville. The second is the electrolytic process, reflected in the discoveries of Hall and Heroult. The third approach is carbothermic smelting.

Sodiothermic methods have been commercially abandoned, because of the greater economies of the electrolytic process. Carbothermic techniques, on the other hand, have continued to be a subject of intense interest to research scientists both within and outside the aluminum industry. Their great attraction is the possibility of bypassing the conventional alumina refining cycle and of starting with ores less rich in aluminum than bauxite. These processes reduce the aluminum first to high-iron-silicon alloys, and then decrease the impurities in the second stage.

The search for an economical and satisfactory carbothermic smelting technique has literally been unceasing. Hall himself began investigating this technique virtually the first moment his electrolytic process was in successful operation. Despite the long years, intensive effort, and considerable investment represented in this effort, carbothermic smelting is not yet commercially useful. Now every pound of aluminum produced commercially in the world was produced by the Hall-Heroult electrolytic method.

In a modern smelter, alumina is dissolved in smelting pots filled with molten cryolite. High-amperage low-voltage direct current is passed through the bath. Metallic aluminum is deposited at the bottom of the pot (which serves as a cathode), while the freed oxygen combines with the carbon anode and is released as carbon dioxide.

The smelting process is continuous. Periodically, molten aluminum is siphoned from the cell. The metal is either cast into ingot, charged into holding furnaces for pre-casting alloying, or transported to users in molten form. As the alumina in the bath is consumed by reduction, additional quantities are added.

As necessary, aluminum fluoride is added to restore the chemical composition of the bath, since some aluminum fluoride is lost by combining with soda in the alumina and by hydrolysis from moisture in the air. Heat generated by the electric current maintains the cryolite bath in a molten condition, so new alumina charges are dissolved as introduced.

Smelting pots are deep, rectangular steel shells lined with carbon, through which the electric current flows. When the smelting process is taking place, the layer of metallic aluminum deposited on the bottom of the pot serves as cathode. Current is introduced to the pot through anodes – a set of carbon blocks suspended in the pot. Some smelting operations employ the Soderberg anode, a self-baking continuous carbon anode formed from a controlled supply of paste.

March, 2007
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