In the previous blog about mineralogical monitoring for the aluminum industry, we discussed the added value of accurate on-line and at-line mineralogical monitoring for bauxite ore. The advantages of X-ray diffraction (XRD) and near-infrared spectroscopy (NIR) that provide information about the mineralogical composition and important process parameters of bauxite were discussed. This information is not only important for bauxite mining but also for efficient downstream processing in the alumina refinery.

Step II. Alumina refining

Process

Alumina refineries process bauxite ore to produce alumina, which is then used to extract aluminium metal. Alumina (aluminium oxide) is a white granular material.

The process to produce alumina from bauxite ore is called the “Bayer process”, developed by Carl Josef Bayer in 1888 (figure 1). It consists of four steps: digestion, clarification, precipitation and calcination.

After milling, bauxite is mixed with caustic soda (sodium hydroxide) under high temperature and pressure. Alumina dissolves from the aluminium bearing phases (excluding clays). Undissolved impurities settle down as a fine red mud, which after few recycling steps, is discarded as waste.

The solution of alumina in caustic soda (liquor) goes through further clarification, filtration and precipitation steps. Alumina crystals are recovered from the caustic solution by mechanically stirring the solution in open-top tanks. The precipitated material (called hydrate) is washed and dried at temperatures exceeding 1000°C. The dry white anhydrous aluminium oxide powder (alumina) is cooled and conveyed to storage. Alumina powder is further used to extract metallic aluminium using electrolytic baths. Caustic soda is recovered and returned to the start of the process and used again.

Let’s discuss the effect of mineralogy on the above process. Temperature required for the digestion of diaspore α-AlO(OH) and boehmite γ-AlO(OH) (both called monohydrate alumina phases, MHA) is higher than for gibbsite γ-Al(OH)3 (called trihydrate alumina, THA). Therefore, the temperature for effective digestion of bauxite depends on the ratio between the different aluminium containing mineral phases.

In addition, the consumption of caustic soda per ton of bauxite depends on the amount of silica impurities: clays and quartz. Under certain conditions, these minerals react with caustic soda and consume part of the reagents from the process. Low-temperature digestion suffers from the reagent loss to clays only, but during high-temperature digestion, both quartz and clay minerals react with caustic soda, increasing reagent consumption and costs.

Therefore, the knowledge about the mineralogical composition of bauxite is an important factor that defines the efficiency of the Bayer process.

Analysis of alumina for quality control (QC) and quality assurance (QA)

Mineralogical monitoring not only adds value for the analysis of the raw material bauxite but also for the quality control of the final product from the Bayer process, alumina. XRD is the only suitable tool to distinguish between the different modifications of alumina (eg. α-Al₂O₃, γ-Al₂O₃) which defines the quality of dry alumina powder, as well as particle size and impurities.

The knowledge of the different modifications is important to predict and optimize the behavior during the smelting process. γ-Al₂O₃ is desired for the electrolysis since it dissolves more easily during the smelting process than α-Al₂O₃. For that reason, the ratio of the different sub-α and α-Al₂O₃ modifications of alumina must be monitored. The analysis of α-Al₂O₃ can be done with a classical straight-line calibration of the α-Al₂O₃ peaks or using full pattern fitting methods. The advantage of using the full information of the XRD pattern is the simultaneous quantification of all sub-α-Al₂O₃ modifications. Even a small fraction of 0.5 wt.% α-Al₂O₃ can be detected and quantified. Figure 2 shows a measurement of dry alumina powder using X-ray diffraction. The majority of the sample consists of g-alumina, with only 0.5% of a-alumina.

Impact of particle size

Particle size impacts directly on the rate of dissolution of the alumina in the cryolite bath and is therefore another important variable. Furthermore, fines are an issue from both health and safety as well as a product transport point of view, so particle size distribution needs to be carefully controlled. The ideal particle size distribution is defined between 45μm and 150μm to prevent problems with dissolution in the cryolite bath and the accumulation of fines during processing that cause conveying and process instabilities and health issues.

The application of on-line particle size analysis allows aluminium processors to operate more efficiently and to produce a more consistent product for downstream unit operations. Economic benefits in the form of reduced waste, reduced energy consumption, reduced manpower and increased throughput are achieved. The availability of industrially relevant systems, at a cost that can be recovered in a relatively short time, makes on-line analysis an increasingly attractive option.

Red mud – monitoring of waste products using XRD

Red mud is the bauxite residue generated during the refinement of bauxite into alumina using the Bayer process. It is composed of various oxide compounds, including iron oxides which give its red color.

Depending on the bauxite grade about 1.3 to 2.2. tons of red mud are produced per ton of alumina. The treatment, long-term storage and re-use of the red mud is one of the challenges of alumina refining. The adequate analytical tools for the control of mineralogy, chemistry and physical properties of red mud are essential for the correct, environmentally friendly management of the waste product.

The complete mineralogical composition of the red mud sample determined by XRD is shown in Figure 4. The main part of the sample consists of the insoluble impurities of bauxite (titanium and iron oxides) and products of chemical reactions, which occur during the digestion steps. However, in this example, valuable aluminium bearing phases such as gibbsite, boehmite and diaspore still comprise about 25% of the red mud sample. This indicates that the processing, in particular the digestion, was not executed to the maximum level of efficiency.

Comprehensive analysis of bauxite and red mud mineralogy along with the analysis of digestion conditions should be performed to understand the root cause for the lost efficiency.

Value of analytical monitoring of mineralogy and particle size for the Bayer process

Alumina refining is a complex process the efficiency of which is directly related to the mineralogy of the bauxite ore. Fast and accurate mineralogical information throughout the entire process, speed of the XRD analyses, increased safety for the operators and the possibility to automate makes XRD a reliable and economic alternative compared to the traditional analytical methods such as wet chemistry and does justify the initial investment.

In our next blog, we will discuss the added value of mineralogical analyses for the next step in the bauxite-to-aluminum process: the conversion of refined alumina to aluminum metal.

Written by: Uwe König, Posted by: Malvern Panalytical – (www,materials-talks.com)

Hello, my name is Mark Ingham and I’m an expert on XRF. In my first blog, I’ll describe the history of WROXI reference materials and the benefits stemming from the new ISO 17034 accreditation.

Let’s take a look at why the WROXI reference materials for XRF have proved so popular over the last 15 years, and the benefits stemming from the new ISO 17034 certification.

The beginnings of WROXI

Do you ever find that inspiration only strikes when you’re faced with the same problem, day-in, day-out? It was certainly true for me when I came up with the idea for WROXI – our ready-to-use kits for making XRF calibration standards.

At the time – the early 1990s – I was working in the Analytical Geochemistry Group at the British Geological Survey (BGS) in Keyworth, near Nottingham, UK. We mostly used single-oxide standards for calibration, which was very time-consuming, and also difficult because the powders did not always fuse very easily.

A wide range of oxides

As a result of these difficulties, I began to think – why not mix the oxides of the commonest rock-forming elements together into a single set of standards? That would reduce the number of calibration samples needed, and also make it easier to fuse the powders to get clear, high-quality beads.

So that’s exactly what I did, and the ‘WROXI’ product (short for Wide-Range OXIdes) was born.

These WROXI standards were designed for our clients PANalytical. As time went by, the focus of BGS shifted away from analytical work, and in 2011 BGS decided to sell the facility to PANalytical, which I joined as a staff member.

For a while, BGS leased the laboratories to PANalytical, but when that arrangement came to an end, I moved (literally) just a mile or so down the road, from Keyworth to Tollerton. There, I helped set up a laboratory in The Coach House, which remains the ‘headquarters’ of our WROXI and other standards products today.

Why is WROXI so popular?

Through all these business changes, our WROXI standards have remained pretty much the same. (The only significant change was the introduction of a ‘Base’ set with ‘Cement’ and ‘Pro’ extensions in 2020, so customers in certain industries don’t have to measure more elements than necessary.)

So what’s the reason for WROXI’s enduring appeal? I think it comes down to four key points:

• Using WROXI standards is quick and cost-effective. Just imagine having to set up an application for (let’s say) the 11 common oxides. Buying the reference materials, working out the fusion parameters, building the application, calibration, and would take months of effort. But with WROXI most of this work is already done, meaning your Epsilon 4 or Zetium spectrometer can be ready to run your samples within a week.
• WROXI standards are totally synthetic. Unlike conventional reference materials made from stocks of geological samples, soils, fly ash or cement clinker, WROXI standards are made from high-purity, commercially available chemicals, so they’ll never run out.
• Because WROXI standards are synthetic, we can avoid measurement uncertainties caused by peak overlap between different elements (such as titanium and vanadium), by not including them in the same standard. And of course, mineralogical effects and particle-size effects are eliminated during the fusion process, giving an order-of-magnitude increase in accuracy over pressed powder pellets.
• The two extension sets make WROXI standards flexible – and we are always happy to develop custom standards containing uncommon elements or different concentration ranges. I recall an industrial recycling company a few years ago who needed to measure tungsten in their samples… as well as 20 other elements! At the time, there were only three reference materials available, none of which supported quantitation of tungsten, so we created a set of custom standards for them containing the entire suite of elements. This then allowed them to use the existing reference materials for validation.

Certification of XRF reference materials: A ‘world first’

There’s also another advantage resulting from the synthetic nature of WROXI standards – we’re able to make them gravimetrically. This opens up the possibility of certifying them to the international standard for reference materials (ISO 17034), which is what we’ve now done with the launch of our WROXI Certified Reference Materials (CRMs).

It’s not been a quick job though – it’s taken two years to go through the ISO 17034 certification process. This is partly because we were the first laboratory in the world to request this for synthetic XRF Certified Reference Materials.

What does this mean for our customers? Essentially an additional confidence boost in the WROXI product. Our laboratories at Tollerton were already accredited by UKAS to ISO/IEC 17025 (the general laboratory standard for testing), but we’ve now had to go much further, by demonstrating that all the processes we use to make our WROXI CRMs meet strict requirements, that all our measurements are traceable, and that every single bottle of the finished product has a certificate and documentation trail associated with it.

The powders now come with the major endorsement of ISO 17034 certification. After 40 years in the XRF business, achieving this is a great milestone for me personally… and of course it further strengthens the ‘analytical chain’ of XRF instrumentation, supplies and expertise from Malvern Panalytical!

Written by : Mark Ingham Posted by: Malvern Panalytical (www.materials-talk.com)