Bauxite is the primary source of aluminum worldwide. Without Bauxite, we could not cover up our leftover food with tin foil, have ladders to climb on or be able to cool our food in a refrigerator or cool our houses in the summer. A major part of the bauxite is the metallurgical grade, which after primary screening and washing, is processed in a refinery. The alumina (Al2O3) is extracted via chemical digestion using caustic soda, following the Bayer process. The waste product of this process is called red mud. The refined alumina is finally converted into metallic aluminum. This process takes place in smelters with a large number of electrolytic baths following the so-called “Hall-Heroult” process.
The efficiency and economics of every step in this process, as for any other ore-to-metal process, is defined by both the chemical and the mineralogical properties and composition of bauxite and alumina.
In the series: “The value of mineralogical monitoring for the aluminum industry” we will showcase the added value of adequate mineralogy control at every step on the way from bauxite ore to the aluminum metal.
Step I. Bauxite mining: Bauxite Ore Grades
Over 70% of the mined bauxite deposits are lateritic in nature (Australia, Brazil, South East Asia). The major constituent is gibbsite, Al(OH)3, with smaller amounts of boehmite, γ-AlO(OH), and diaspore, α-AlO(OH). Apart from the major Al-bearing phases, bauxite contain impurities, like iron oxides (hematite Fe2O3, goethite FeOOH), silica (quartz SiO2, clays such as kaolinite Al2Si2O5(OH)4 ) and titania (rutile TiO2, anatase TiO2).
The ratio between Al-bearing phases and impurities will define the maximum alumina yield (total available alumina) and digestion parameters such as temperature or caustic soda consumption
The digestion temperature is determined by the ratio between gibbsite, boehmite and diaspore. Whereas about 140-150 °C is sufficient to extract alumina from gibbsite, boehmite and diaspore digestion requires 240-260 °C.
The caustic soda consumption per ton of bauxite ore is directly proportional to the amount of reactive silica present (silica, which will react with caustic soda). At lower temperatures, only the silica of the clay minerals is reactive during the digestion. If higher temperatures are applied, quartz will also react and increase the caustic soda consumption.
Bauxite ore high in gibbsite and low in boehmite and silica content has the highest grade as it can be digested at the lower temperature with the lowest caustic soda consumption. Ore high in silica and iron oxides is considered as low grade or waste. Intermediate grade ore, high in gibbsite and boehmite and diaspore, can be refined, but at higher temperature.
To summarize, the most important parameter for bauxite mining and processing are:
- Available alumina
- Reactive silica
- Ratio between Al-bearing phases
These parameters define the overall economics and efficiency of further downstream processing.
How is ore grade defined?
During exploration and mining, bauxite ore bodies are divided into grade blocks. Traditionally this was done using chemical analyses (X-ray fluorescence and wet chemistry) and geological prospecting. However, neither of these methods are comprehensive as accurate mineralogical probs. Wrong determination of one grade block is a loss of about 0.5-1 million USD downstream. Therefore, today more accurate tools, like X-ray diffraction (XRD) and near-infrared spectroscopy (NIR) are used for ore grading.
How accurate is bauxite ore grading using X-ray diffraction (XRD)?
Before going to the real bauxite mine case study, let’s check the accuracy of XRD for bauxite analyses by measuring a set of bauxite certified reference materials from ALCAN.
Figure 1 shows a typical result of XRD analyses of bauxite. Any XRD pattern is a set of diffraction peaks of different intensities, located at certain diffraction angles (2θ), specific to a certain mineralogical phase. The peak positions allow the identification of the mineral phases. The relative intensities of each mineral contribution to the XRD pattern allow quantifying the relative amount of each mineral using the full-pattern Rietveld method .
In the example shown in Figure 1, one of the ALCAN CRMs, BXT-02, was analyzed. This sample consists of nearly 60% of gibbsite and nearly 40% of impurities, including iron and titanium oxides, clays and quartz.
How accurate are these numbers?
Unfortunately, phase composition is not yet being reported for bauxite CRMs; however, using our XRD results, we can calculate the chemical composition and compare it with the certified values. We get very good agreement (Figure 2) between the certified values and XRD results, including the total alumina (total present Al2O3) and total available alumina values (total extractable Al2O3).
Actual bauxite mine case study using XRD
The results of the measurements with the bauxite CRMs show that XRD is an accurate and fast technique to monitor bauxite mineralogy, providing not only the complete mineralogy of a bauxite ore but also the important process parameters. The following case study trials the method on a set of samples collected from an operating bauxite mine.
Figure 3. Comparison of total available alumina and reactive silica as obtained by XRD with the reference values (wet chemistry)
20 samples from a Northern Brazil bauxite mine were analyzed. The mineralogical composition was quantified and used for the calculation of the total available alumina and reactive silica. The comparison of the XRD results with the reference values (wet chemistry) is plotted in Figure 3, showing a good correlation between wet chemistry and XRD results.
The graphs show that both methods are capable techniques for this application. However, each XRD measurement in Figure 3 took on average 15 minutes, including automatic sample preparation, scanning and quantification; while each wet chemistry data point was manually obtained, took hours and required use of hazardous substances.
Apart from reactive silica, total alumina and available alumina, XRD analyzed the complete mineralogical composition, including the ratio between gibbsite, diaspore and boehmite. All parameters together are available after a single sample test of 10-15 minutes.
The ability to get a comprehensive set of process parameters, speed of the analyses, increased safety for the operators, and the possibility to automate, makes XRD a much more economically attractive alternative to wet chemistry .
Bauxite ore grading using NIR
Figure 4. NIR spectra of main bauxite phases and associated minerals
Almost all bauxite minerals (quartz excluding) are spectrally active, therefore they can be identified and quantified using near-infrared spectroscopy (NIR). Figure 4 gives an example of NIR spectra of main bauxite phases as well as common accompanying and alteration phases. All bauxite phases have distinct spectral features and can be easily be distinguished from one another.
Quantitative mineralogical analyses using NIR is different than using XRD. NIR quantification requires a statistical model built using over 100 reference samples, mineralogical quantification of which was done using another primary technique, like XRD. Therefore, NIR can never be more accurate than a primary technique. The creation and modification of a quantification model is a more labor-intensive process compared to XRD.
Although NIR lacks the accuracy and flexibility of XRD, it has a great advantage – design simplicity. On one hand, this makes NIR devices very portable, on the other hand, NIR technology can be easily implemented as an on-line sensor over a conveyor belt. Both portability and on-line implementation are translated into extremely short feedback loops and fast decision making.
Portability makes NIR an ideal tool for the initial exploration of bauxite deposits as well as on the spot troubleshooting during ongoing mine operation. On-line NIR sensor will enable 24/7 bulk monitoring of bauxite run-of-mine. And it can also be used for the calculation of total alumina, reactive silica and total available alumina for low temperature digestive process.
In our next blog, we will discuss the added value of mineralogical analyses for the next step in the bauxite-to-aluminium process: the extraction of alumina from bauxite at a refinery.
Written by: Uwe König, Posted by: Malvern Panalytical – (www,materials-talks.com)