Bauxite is an aluminum-rich sedimentary rock. It is a principal ore of aluminum. Aluminum in bauxite is hosted by aluminum hydroxide minerals, mostly gibbsite. The major impurities are iron oxides and hydroxides (which give reddish color to most bauxites) and clay minerals. Bauxite is a weathering product of aluminum-bearing rocks.
Aluminum starts out as bauxite ore – an aluminum ore formed from laterite soil. Bauxite is the world’s primary source of aluminum. Before it can become aluminum, however, bauxite destined for use as aluminum must first be processed into alumina.
In 2015, the US Geological Survey estimated that over 95% of bauxite was converted to alumina, with the remainder going toward a variety of products such as abrasives, chemicals, proppants, and more.
The primary approach to transforming bauxite ore to alumina is known as the Bayer Process.
The Bayer Process
The Bayer process is not easily explained in brief. This method of obtaining alumina from bauxite ore is complex and involves a lengthy succession of chemical reactions, with the process varying slightly depending on the makeup of the unique source of bauxite ore.
Crushed and washed bauxite ore must first go through a process to remove any silica found in the bauxite, which would otherwise hinder the process and result in a product of lesser quality.
In a process known as digestion, after any silica has been removed, the remaining bauxite ore is combined with a hot caustic soda material in a heated pressure vessel in order to dissolve the aluminum-bearing minerals, yielding a sodium aluminate solution:
Al2O3 + 2 NaOH > 2 NaAlO2 + H2O
Once this reaction has occurred, bauxite residue can be separated from the solution through a sedimentation process.
The alumina can then be crystallized from the solution via a precipitation process which carries out the following reaction:
Al(OH)4– + Na+ → Al(OH)3 + Na+ + OH–
Coarse crystals are then removed through classification, and processed in a calciner or rotary kiln to remove bound moisture and yield alumina in the following reaction:
2Al(OH)3 → Al2O3 + 3H2O
The red mud problem
One cannot discuss the Bayer process without also discussing the associated red mud challenge.
Red mud, sometimes also referred to as red sludge, is a by-product of the Bayer process. In addition to incurring disposal costs, red mud is an environmental hazard due to its composition and high alkalinity. In 2010, red mud from an alumina plant in Hungary broke free from a retention pond, contaminating the surrounding area, killing several people, and injuring many more.
For every ton of metallic aluminum produced, around two tons of red mud are also produced, with annual production at around 30 million tons per year (dry basis).¹
Efforts are underway to find beneficial reuse opportunities for red mud, with potential applications on the horizon.
Alternative uses for bauxite
While bauxite primarily serves as the world’s supply of aluminum, it offers other uses as well, namely, refractory and proppants.
Bauxite is processed in a rotary kiln to remove bound moisture so that it can be used as a refractory, a heat resistant material used in a variety of applications.
Bauxite has also seen growing use as a proppant in recent years. Proppants are used in the hydraulic fracturing process to “prop” open rock fissures allowing natural gas or oil to flow out. Bauxite is sintered in a rotary kiln in order to harden the material so that it can withstand the extreme pressures it will be subjected to.
Luckily, bauxite is in no shortage; experts estimate that the world has centuries worth of bauxite reserves available.
Alumina, or aluminum oxide (Al2O3– the material resulting from the Bayer Process – is a chemical compound that can be refined to produce aluminum.
Refining alumina into aluminium
The conversion of alumina to aluminum is carried out via a smelting method known as the Hall-Heroult Process.
This process entails dissolving the alumina in cryolite, a molten solvent. An electrical current is run through the mixture, causing the carbon from the carbon anode to attach to the oxygen component in the alumina, yielding aluminum and carbon dioxide:
2 Al2O3 + 3 C > 4 Al + 3 CO2
This process takes place at temperatures between 940-980º C and yields an aluminum of high purity.
Alternitive uses for alumina
While most alumina (around 90%) produced goes toward the production of aluminum, alumina boasts a number of characteristics that lend it to a diverse range of applications outside of aluminum production as well. Alumina is abrasive, has a high thermal conductivity, and is very hard.
Due to these characteristics, in addition to aluminum, alumina is frequently used in the production of ceramics, chemicals, refractories, and more. Alumina is even sometimes used in dental and bone implants.
Similar to the production of many bauxite products, alumina is often processed in a rotary kiln to achieve the desired material characteristics necessary for the intended end use.
The aluminum resulting from the Hall-Heroult process is in a molten form. This molten aluminum can be cast into ingots, the currency of the aluminum industry and the starting material for many of the products we rely on every day.
Once aluminum has been produced, it is infinitely recyclable. Unlike its complex origins, the recycling of aluminum requires comparatively less processing; aluminum is broken down into shredded scrap, processed in a decoating kiln to remove any coatings or lacquers, and then melted and cast into ingots, ready for reuse.
/sandatlas/ Carrie Carlson,Alex Ebbenis-FEECO International, Inc.