Microwave digestion of gibbsite and bauxite in sodium hydroxide

doi:10.1016/j.hydromet.2020.105257

Hydrometallurgy

Volume 192, March 2020, 105257

https://doi.org/10.1016/j.hydromet.2020.105257 Get rights and content

Highlights

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Microwave heating digested 5–7% more material than conventional methods.
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Microwaves heated solutions to temperature in 1/10th the amount of time.
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The digestion of bauxite ore produced similar results to gibbsite.
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6 M solutions achieved 10–15% more digestion than their 1 M counterparts.

Abstract

It was hypothesized that bauxite digestion may be improved by using microwave heating as it has been shown in literature that some material processes have improved efficiency. To test this hypothesis, a set of digestion experiments were conducted using gibbsite, one of the major minerals in bauxite. Gibbsite was digested at various temperatures (50, 75, and 95 °C) in either 1 M or 6 M sodium hydroxide solutions for 30 min using either a convection oven or a 2.45 GHz microwave applicator. Results show that microwave heating provided an increase of 5–7% in the digestion after 30 min and required around 1/10th the time to heat the solutions compared to conventional heating. Electromagnetic simulations show that preferential heating occurs at the solution surface creating a temperature gradient within the solution. Although vigorous stirring of the solution was used to minimize the temperature gradient, it could still be responsible for the observed difference in digestion. Digestion of bauxite itself yielded similar results to the gibbsite.

Introduction

Aluminum metal and its oxide are two important materials used in today's society. In 2015 alone, 120 million metric tons of aluminum oxide (Bray, 2017a) and 57.5 million metric tons of aluminum were produced (Bray, 2017b). These materials are extracted from bauxite ore through the highlighted digestion step in the process shown in Fig. 1. Bauxite is made up of multiple minerals, which vary in concentration based on the region the ore came from. The ore typically contains one of three aluminum based minerals (gibbsite, boehmite, or diaspore) alongside other iron, titanium, and silicon based minerals such as hematite, anatase, quartz, and kaolinite (Sinton, 2006). During digestion the caustic soda reacts with the aluminum minerals via reactions such as the one for gibbsite shown below in Eq. (1) (Heimann, 2010; Kaußen and Friedrich, 2016). $Al {(OH)}_{3} (Gibbsite) + NaOH (aq) \to Al {(OH)}_{4}^{-} (aq) + N a^{+} (aq)$

The goal of this research was to determine if microwaves would improve the digestion of gibbsite and bauxite by either reducing the reaction temperature or time or increasing the digestion. Microwaves have been used in applications ranging from solution preparation for inductively coupled plasma mass spectrometry (ICP-MS) to the drying and sintering of ceramic parts (Barclay, 2004; Clark, 2005; Kingston and Jassie, 1988; Microwave Digestion - EPA Method 3052 on the Multiwave 3000, 2020). Some mineral digestion processes have shown improvement when using microwave heating such as the digestion of chalcopyrite (copper ore) by ferric sulfate via selective heating of the ore (Al-Harahsheh et al., 2005, Al-Harahsheh et al., 2006, Al-Harahsheh et al., 2009; Al-Harahsheh and Kingman, 2004).

There has been some work using microwaves to pre-roast diasporic bauxite to improve the digestion but overall little research has been conducted investigating how microwaves could change the digestion for other types of bauxite (Le et al., 2017). For this research the digestion reaction and experimental parameters were based on information available about bauxite digestion conditions(Controlling Alumina to Caustic Ratio, 2008; Goyal, 2015; Griffing and Overcash, 2010; Heimann, 2010; Sinton, 2006). Changes were measured by examining the difference in the percentage of the material digestion that has occurred between microwave heating and conventional heating at specific temperatures and monitoring changes to the rate of heating for the solutions.

Section snippets

Material characterization

Before starting experiments, characterization of the materials was performed using a Horiba LA-950 particle size analyzer and a Panalytical X'Pert³ Powder X-Ray Diffraction (XRD) platform. Using this equipment the particle size range was measured and the phase of the aluminum hydroxide powder was confirmed to be gibbsite. Results showed that the majority of the gibbsite powder was below 10 μm and had a bimodal distribution with one peak at 0.3 μm and a second peak at 12.3 μm with a d50 of

Microwave modeling

Fig. 3(A and B) shows the 3D model of the microwave reactor. This includes: variable sliding short used to maximize the amount of absorbed energy into the material and the metallic chokes used to attenuate the microwave energy from escaping outside the reactor and to assure inherent safety design and to comply with health and safety legislations (Industrial, scientific and medical (ISM) radio-frequency Equipment. Electromagnetic disturbance characteristics, 2007). The electromagnetic simulation

Conclusion

Using microwave heating instead of conventional heating results in a 5–7% increase in digestion and requires a tenth of the time to heat the mixtures to temperature. The digestion versus temperature trends observed were linear for all experiments with high R² values for confirmation. Results of the gibbsite digestion are 3–5% lower than the gibbsitic bauxite tested but the trends between both materials are similar. This difference is most likely due to the presence of digestible minerals other

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Funding to travel to and work at the University of Nottingham was provided though the Virginia Polytechnic Institute and State University International Research Experience for Students (IRES). The IRES program is funded though NSF award 1261162.

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Microwave digestion of gibbsite and bauxite in sodium hydroxide

Highlights

Abstract

Introduction

Section snippets

Material characterization

Microwave modeling

Conclusion

Declaration of Competing Interest

Acknowledgments

Hydrometallurgy

Miner. Eng.

Hydrometallurgy

Anal. Chim. Acta

Hydrometallurgy

Microwave digestion moves into the 21st century

Todays Chem. Work

Bauxite and Alumina 2015

Mineral Commodity Summaries 2017

Microwave Solutions for Ceramic Engineers

Controlling Alumina to Caustic Ratio

Alumina Technology