With applications in everything from energy, infrastructure, and aerospace, to automotive, packaging, and more, aluminum has become a cornerstone of modern society.
Aluminum’s widespread use might infer ease of accessibility, but such is not the case; aluminum is not found freely in nature, requiring the beginnings of aluminum – bauxite ore – to go through a complex conversion process in order to produce the lightweight metal we so heavily rely on.
An overview of this conversion process and the materials along the way are described here.
Bauxite
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.
Refractory
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.
Proppants
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
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 Aluminum
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.
Alternative 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.
Aluminum
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.
Conclusion
The journey from bauxite ore to aluminum is complex. What starts out as bauxite ore is transformed into an incredibly diverse metal, useful throughout various industries for thousands of applications.
And while most bauxite is slated to become aluminum, both bauxite and alumina possess advantageous characteristics that can be used in a variety of other applications. In short, we have bauxite to thank not only for aluminum, but for a number of other products and materials as well.
FEECO is an industry leader in thermal processing. We offer custom rotary dryers and kilns for processing bauxite and alumina, as well as decoating kilns for aluminum. In addition, we offer a variety of custom agglomeration and material handling equipment to complement the needs of these veritable industries.
Our Innovation Center, a unique testing facility where we can test materials at both batch and pilot scale, offers testing options for bauxite, alumina, and red mud.
For more information, contact us today!
- 1.Ayres, Robert U., John Holmberg, and Björn Andersson. “Materials and the Global Environment: Waste Mining in the 21st Century.” June 2001: 477-80. MRS Bulletin. Materials Research Society. Web. Feb. 2017.
Image By James St. John (Chertified-pisolitic-bauxite3) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons.