Bauxite analysis

Sample preparation

Once at the laboratory, sample preparation for DSO bauxites is usually limited to drying, weighing, crushing, sub-sampling and grinding the sample to less than 75 microns. For bauxites that are subject to beneficiation, sample preparation includes a screening step, additional drying and weighing steps, and the calculation of a percentage yield.

Multi-element geochemistry

ALS offers routine multi-element geochemistry with high precision and accuracy necessary for bauxite ore analysis. Bauxite ore is mainly composed of aluminium hydroxide, gibbsite, boehmite and diaspore but may also contain iron oxides, goethite and hematite, kaolinite, anatase (TiO₂), ilmenite (FeTiO₃ or FeO.TiO₂ ) and other minor or trace level impurities which can affect processing.

Bauxite analysis

Although bauxite comprises a mixture of minerals, the industry standard analytical method for reporting the composition is by elemental analysis, expressed as metal oxides. This analysis is usually determined by X-Ray Fluorescence Spectrometry (XRF) available by ALS methods ME-XRF13u/n (unnormalised and normalised options). In addition, Loss on Ignition (LOI) is also measured by furnace or Thermogravimetric Analyser (TGA). This determines the loss of mass due to volatiles that are driven off when the sample is heated from 105°C to 1,000°C (i.e., after the removal of free moisture).

HAG for bauxite

Analysis of bauxite by automated fusion/ loss on ignition system, HAG, is also available.

Impurities

Impurities are often more important to consider than the alumina grade because of their detrimental effect. Silica is the most commercially important impurity in bauxite as generally the more silica in the bauxite, the higher the amount of caustic soda consumed in the refining process, and the higher the loss of alumina to the red mud tailings in the desilication product (DSP).

Available alumina & reactive silica

ALS offers methods for analysing available alumina and reactive silica at 145ᵒC by method ME-LICP01 and at 235ᵒC with method ME-LICP02. The temperature, caustic strength and sample/caustic weight ratio maybe requested for these methods.

Mineralogy for bauxite

Mineralogy is very important as it dictates the refining conditions that must be used and has a large bearing on the economics of processing bauxite. Alumina refineries are usually classified as either high temperature (>240°C) or low temperature (~143°C -150°C). Pure gibbsite or mixed gibbsite and boehmite deposits with a low boehmite content are usually sent to low temperature refineries. However, once the deposit has more than about 6% boehmite, then it must be sent to a high temperature refinery due to economics.

Kaolinitic silica

Fourier Transform-Near Infrared Spectroscopy (FT-NIR) can be used to determine the kaolinitic content of samples. The method requires a project specific calibration using multiple samples that have been analysed by an alternative technique. This information is used to set up a chemometric algorithm which is used to quantify kaolinite in subsequent samples.

The aluminium minerals

Gibbsite - Al(OH)₃ or Al₂O₃.3H₂O: Commonly referred to as tri-hydrate due to the three lots of water. The three OH bonds are relatively weak and require the lowest refining temperature to break them, typically 143°C -150°C.

Boehmite - AlO(OH) or Al₂O₃.H₂O: Commonly referred to as monohydrate due to the single water. The single OH bond is much stronger than the three bonds in gibbsite and requires a higher refining temperature to break it, typically 240-260°C

Diaspore - AlO(OH) or Al₂O₃.H₂O: Has the same composition as boehmite but is denser and harder due to a different crystal form. Diaspore requires the highest refining temperatures to process, typically +260°C.