Observation of sodium titanate and sodium aluminate silicate hydrate layers on diaspore particles in high-temperature Bayer digestion

doi:10.1016/j.hydromet.2020.105255

Hydrometallurgy

Volume 192, March 2020, 105255

https://doi.org/10.1016/j.hydromet.2020.105255 Get rights and content

Highlights

•
Direct observation of a sodium titanate layer on Al-bearing minerals.
•
Ti-O-Al bond promotes the covering of sodium titanate on disapore.
•
Sodium aluminate silicate hydrate layer also inhibits the diaspore digestion.
•
A better method to observe the changes at mineral-solution interface.

Abstract

During the high-temperature Bayer process, the inhibitory effect of titanium-containing minerals on diaspore digestion is believed to be resulted from the formation of a dense sodium titanate layer covering the diaspore particles, however, there is a lack of direct observational evidence to date. This work adopted X-ray photoelectron spectroscopy (XPS), cross-section SEM and in-situ analysis to clarify the inhibitory mechanism by investigating the interactions between Al-/Ti-/Si-bearing minerals in the simulated high-temperature Bayer digestion process. The observation of a sodium titanate layer provided direct evidence for the solid layer theory. XPS result further indicates that the Ti-O-Al bond is a significant factor in promoting the covering of sodium titanate on alumina-bearing mineral particles. Cross-section SEM and in-situ analyses illustrate that the dissolution of diaspore is carried out not only on the surface of diaspore particles but also on the inner surface such as pores and gaps of particles. Meanwhile, as similar to Ti-bearing minerals, the Si-bearing minerals also inhibit the diaspore digestion by forming a sodium aluminate silicate hydrate layer at an elevated temperature.

Introduction

The high-temperature Bayer process is currently the dominant process for producing alumina from the diasporic bauxite. As one of the most common compositions of bauxite, the Ti-bearing minerals, especially anatase, strongly inhibit alumina dissolution during the high-temperature Bayer digestion process. Only a small quantity of Ti-bearing minerals can reduce the digestion ratio of alumina to <20% even at a digestion temperature of 260–270 °C (Bi et al., 1996; Li et al., 2014a, Li et al., 2014b). To eliminate the inhibition, excess lime (10–13%, mass ratio of bauxite) has to be added in the industrial digestion process (Whittington, 1996; Whittington and Cardile, 1996), resulting in detrimental consequences for alumina recovery (~5% decrease) and the discharge of red mud (200–300 kg increase per ton of alumina) (Li et al., 2017). Therefore, extensive efforts were made in recent years for eliminating the inhibition and simultanously reducing the red mud discharge. Undoubtedly, clarifying the inhibitory mechanism of Ti-bearing minerals on diaspore digestion is helpful to the practical application of an efficient Bayer digestion process.

Generally, it is believed that the inhibitory effect is due to the formation of a dense layer of sodium titanate covering the surface of diaspore particles, thus blocking the interaction of the diaspore and Bayer liquor. In 1957, researchers from the former Soviet Union first put forward this point and calculated the thickness of the layer of ~1.8 nm (Chen and Peng, 1997). More recently, Chester et al. (2009) investigated the dissolution of various titanium oxide phases in Bayer liquor. They found that anatase is found to have an anomalous behavior when gibbsite co-precipitates. In that case, the dissolution of anatase does not follow the expected free caustic trend and the most probable cause is an impervious aluminum/sodium titanate surface layer hindering further dissolution. Ireland et al. (2014) reported that the dissolved titanate adsorbs onto the surface of the boehmite and thus limits its dissolution. Their results of energy-filtered transmission electron microscopy showed an enrichment of titanium on the boehmite crystals, and the aluminum also becomes associated with some of the Ti-rich particles.

However, some other researchers (Gu et al., 1993) suggested that the coating is not a solid sodium titanate but a dissolved titanate ion. This liquid-layer theory is supported by the fact that all the digestion additives (Paspaliaris and Karalis, 1993; Kuznetsova et al., 2008) such as lime, magnesium oxide, strontium chloride, and barium salt decrease the concentration of titanate ion in Bayer liquor at elevated temperatures. Therefore, the inhibitory effects of titanium-bearing minerals may also be contributed by the strong adsorption layer of titanate ion on the surface of aluminum minerals.

Our previous work (Li et al., 2014a, Li et al., 2014b) indicated that the dissolved sodium titanate concentration is primarily dependent on the free alkali and alumina concentrations during the digestion process at elevated temperatures. With the decrease of free alkali concentrations, the dissolved sodium titanate has more potential to precipitate as a solid layer. However, it is not known whether the precipitation on different mineral particles is selective.

In summary, the mechanism of inhibiting the dissolution of aluminum minerals by titanium minerals has not reached a consensus due to lack of observational evidence. Meanwhile, the composition and structure of the possible sodium titanate layer are not clear. Besides, as both the Ti- and Si-bearing minerals in diasporic bauxite retard the conversion of iron-bearing minerals (Murray et al., 2009; Whittington et al., 1998; Kwon et al., 2005), it is reasonable that the Si-bearing minerals can also inhibit the dissolution of Al-bearing minerals. Therefore, the objective of this study is to investigate the microscopic interactions of Ti-/Si-bearing minerals at the interface between the diaspore particles and the Bayer liquor, as well as the resulting macrostructure changes, so as to clarify the inhibitory mechanism of Ti-/Si-bearing minerals during the high-temperature Bayer digestion process.

In this paper, X-ray photoelectron spectroscopy (XPS), cross-section SEM and in-situ analysis were adopted in order to investigate the interactions between Al-/Ti-/Si-bearing minerals in the Bayer digestion process. The experimental work presented here provides one of the exact investigations into how the Ti-/Si-bearing minerals inhibit the diaspore dissolution.

Section snippets

Materials

Corundum powder used to simulate the diaspore in bauxite was obtained by roasting aluminum hydroxide (AR) at 1200 °C for 180 min. Anatase powder used to simulate the dominant Ti-bearing mineral in bauxite was of analytical purity. Diasporic bauxite was sourced from Henan provinces, China. Bauxite was dried in an oven at 90 °C for 12 h, ground, and then sieved through a 100-mesh screen (pore size, 150 μm). Mineralogical and chemical analyses of the coarse bauxite samples (>150 μm) were performed

Observation of sodium titanate layer and XPS analysis

Generally, the TiO₂/Al₂O₃ mass ratio in diasporic bauxite ranges from 4% to 8%. To simulate the reaction of natural minerals during the high-temperature Bayer digestion process, the experiment was conducted with 10 g corundum powders and 0.8 g anatase (8% mass ratio of TiO₂/Al₂O₃) in 100 mL Bayer liquor at 260 °C for 90 min. The resulting product was characterized using SEM and EDX, as shown in Fig. 1.

From the backscattered electron image of the polished product (Fig. 1(a)), a continuous dense

Conclusions

1)
Ti-bearing minerals react with Bayer liquor to produce a solid sodium titanate layer, and its thickness depends on the Ti/Al ratio under the simulated Bayer digestion conditions. The direct observation of sodium titanate layer provided evidence for the solid layer blocking the diaspore digestion.
2)
Cross-section BSEM and in-situ analyses illustrate the mineralogical conversion during high-temperature Bayer digestion process. The dissolution of diaspore is not only carried out on the surface of

Author statement

On behalf of my co-authors, I declare that this work described has neither been published previously nor under consideration for publication elsewhere. All authors have seen the manuscript and approved to submit to your journal.

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

This work was financially supported by the China Postdoctoral Science Foundation (No. 2019M652799) and the National Natural Science Foundation of China (No. 51604309).

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Observation of sodium titanate and sodium aluminate silicate hydrate layers on diaspore particles in high-temperature Bayer digestion

Highlights

Abstract

Introduction

Section snippets

Materials

Observation of sodium titanate layer and XPS analysis

Conclusions

Author statement

Declaration of Competing Interest

Acknowledgments

Hydrometallurgy

Hydrometallurgy

Hydrometallurgy

Trans. Nonferr. Metal Soc.

Trans. Nonferr. Metal Soc.

Hydrometallurgy

Int. J. Miner. Process.

Hydrometallurgy

The study on the reaction of rutile in Bayer liquor

Strength Digestion Methods of Diasporic Bauxite