The Chemistry and Processing of Jamaican Bauxite

The first commercial extraction of alumina (Al2O3) from bauxite has been attributed to Henri Sainte-Claire Deville in about 1854.

Soon after this, in 1888, Karl Joseph Bayer described what is now known as the Bayer Process, which led to a dramatic reduction in the cost of aluminum metal. Today, it is an everyday commodity, rather than a precious metal.

Although deposits of aluminous red earth have been known to occur in the Tertiary Limestone areas (which covers two thirds of the land surface of Jamaica) since the 1820's, it was not until the 1940's that their economic significance as an ore of aluminum was recognised.

In October, 1943, Alcan was incorporated under the name Jamaica Bauxites Limited as a Jamaican company to investigate the commercial potential of Jamaican bauxite. In the same year, 2500 tonnes of ore was shipped to the USA for process investigation and it was realised that the bauxite was suitable for processing using Bayer technology.
The Kirkvine works were completed around 1952 and the first shipment of alumina was consigned to a Norwegian smelter in January 1953.

bauxite plant aerial view - ca 1980
A Bayer plant in Jamaica

The Bayer Process, which continues to be the most economical method of manufacturing alumina can be schematically summarised in a flow chart,

flow chart

and involves the following operations:

Mining

Immediately prior to mining any deposit, the land is cleared and the top soil, to a minimum of 6 inches, removed and stockpiled for later replacement when mining is completed.

The surface occurrence of the ore (usually less than 100 feet) makes the deposits suitable for mining by simple opencast methods. Due to the soft, earthy nature of the ore, no drilling or blasting is generally required.

Deposits are located in areas varying from gentle undulating to rugged, hilly terrain involving major capital expenditures in establishing a system of ore transportation.

ICENS Al info bauxite mining areas
Al and Mining areas in Jamaica
The International Centre for Environmental and Nuclear Sciences (ICENS) has an on-going programme of mapping the geochemical content of Jamaica. 'A Geochemical Atlas of Jamaica' was published in 1995 and is available from Amazon or ICENS.
The results found for Aluminium are shown above (courtesy of Prof G.C. Lalor).

Dissolution of the alumina at elevated temperatures

The exact procedure required for digestion, depends on the nature of the ore deposits.
The following Table outlines a number of minerals commonly found in bauxites:

Gibbsite (hydrargillite) α-Al2O3.3H2O
Boehmite α-Al2O3.H2O
Diaspore β-Al2O3.H2O
Hematite α-Fe2O3
Goethite α-FeOOH
Magnetite Fe3O4
Siderite FeCO3
Ilmenite FeTiO3
Anatase TiO2
Rutile TiO2
Brookite TiO2
Halloysite Al2O3. 2SiO2.3H2O
Kaolinite Al2O3. 2SiO2.2H2O
Quartz SiO2

Jamaican bauxite is mainly gibbsitic but some amount of boehmite is also present. The average grade of bauxite mined is of the order of 45% available alumina and 1.5% reactive silica.
a = 8.684
b = 5.078
c = 9.736
α = 90
β = 94.54
γ = 90
volume = 427.985
space group = P 1 21/n 1
Gibbsite

In order to remove the iron oxides and most of the silicon oxides present, the ore is first treated with sodium hydroxide. The digestion process takes advantage of the solubility of amphoteric aluminum oxides to form a solution of aluminate ions, whilst the basic iron oxides which form do not dissolve and are separated by filtration.

     Gibbsite   Al2O3.3H2O    + 2NaOH   →   2 NaAlO2  +  4 H2O  (135-150 °C) 


     Boehmiite  Al2O3.H2O     + 2NaOH   →   2 NaAlO2  +  2 H2O  (205-245 °C) 


     Diaspore   Al2O3.H2O     + 2NaOH   →   2 NaAlO2  +  2 H2O  (high T and P) 

Complete extraction from diasporic bauxite requires stronger caustic solutions, in addition to higher temperatures and pressures. In general the reaction equilibria above move to the right with increases in caustic soda concentration and temperature. In practice this means that for deposits containing the more easily recovered Gibbsite only, production costs are much lower than when Boehmite or Diaspore are present.

The control of silica in the conventional Bayer Process is most important and in fact ores having reactive silica greater than 7% cannot be economically processed.

Unlike quartz, which is considered virtually non-reactive at Gibbsite extraction temperatures, some minerals, including kaolins, dissolve rapidly and the reaction of the silica can give rise to appreciable loss of caustic soda and aluminum.

The control of silica is generally carried out during, or prior to, the digestion step, and generally involves dissolution, eg for kaolin

                Al2O3.2SiO2   +   NaOH   →   Na2SiO3

and desilication via precipitation.
                Na2SiO3  +  NaAlO2      →    Na2O.Al2O3.2SiO2

Dissolution is necessary to supersaturate the liquid to a point where the sodalite formed acts as a seed to precipitate more sodalite. The rate of precipitation is found to increase with temperature, however at 135-150 C it is significantly slower than is required for complete Gibbsite extraction which occurs within minutes. The need for desilication therefore means that the material must be held at the digestion temperature long enough to allow the silica to precipitate.

Still to come

Addition of Flocculants

Precipitation of pure Gibbsite

Regeneration of solutions

The Red Mud Ponds

red mud pond

and the latest ideas on dry stacking

Calcination


References.

see for example,
R.J. Lancashire, "Bauxite and aluminium production"
Education in Chemistry, May 1982, pps 62-64.

Production figures on Bauxite from Victoria in Australia are available at the National Resources and Environment website under 'Minerals and Petroleum'.

A preview of the book on the Global Bauxite Industry

For the latest statistics on bauxite/alumina production check the US Geological Surveys site

Production Figures

Total bauxite production figures released by the Jamaica Bauxite Institute are measured as crude bauxite plus bauxite converted into alumina and in 1997 reached 3 million tonnes for the first quarter. This compared with a 11.7 million for 1996 and 11 million tonnes in 1995.
The alumina plants were said to be working at 96% capacity for the first time in many years and for the first quarter (1997) produced 816,999 tonnes.

A description of the Hall process for electrolytic reduction of alumina to aluminum is appropriately provided at Oberlin College, from where Charles Hall had graduated only eight months prior to his successful experiment.


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The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.
Created Feb 1995. Links checked and/or last modified 25th November, 2013.
URL http://wwwchem.uwimona.edu.jm/lectures/bauxite.html