Mark C. Barnes

The mechanism of the sodalite-to-cancrinite phase transformation in synthetic spent Bayer liquor

Abstract The precipitation of zeolite, sodalite and cancrinite and subsequent phase transformations were investigated under a variety of conditions in sodium aluminate liquor. At sufficiently high SiO 2 relative supersaturation, amorphous sodium aluminosilicate and zeolite precipitated at temperatures as high as 160°C. They subsequently transformed to sodalite and finally to cancrinite. Thus the sequence of the transformation of phases is: [Aluminosilicate species]→Amorphous phase→Zeolite (Linde A)→Sodalite→Cancrinite. It was found that sodalite did not transform to cancrinite in the absence of a liquid medium. The transformation of sodalite to cancrinite was demonstrated to involve a solution-mediated mechanism with sodalite dissolution and subsequent cancrinite precipitation. Neither the amorphous phase nor the zeolite phase precipitates at typical spent Bayer liquor SiO 2 supersaturation.

High power ultrasonics as a novel tool offering new opportunities for managing wine microbiology

Industrial scale food and beverage processes that utilize microorganisms are typically faced with issues related to the exclusion, suppression or elimination of spoilage organisms. Yet the use of traditional anti-microbial treatments such as heat, chemical biocides or sterile filtration may themselves be restricted by regulations or else be undesirable due to their adverse sensory impacts on the product. High power ultrasound (HPU) is a technology whose application has been evaluated if not exploited in several food and beverage processes but has yet to be introduced into the wine industry. This review examines the research findings from related industries and highlights possible applications and likely benefits of the use of HPU in winemaking.

The solubility of sodalite and cancrinite in synthetic spent Bayer liquor

The equilibrium SiO2 solubility of sodalite and cancrinite crystals in sodium aluminate solutions containing low concentrations of aluminium (synthetic spent Bayer liquor) has been determined over a range of temperatures (90–220°C) and as a function of solution NaOH and Al(OH)3 concentration for cancrinite. The same SiO2 solubilities, within experimental error, were measured when equilibrium was approached from both above and below SiO2 solubility for spent Bayer liquors in contact with either sodalite or cancrinite crystals. The solubility of sodalite and cancrinite (expressed in terms of SiO2 concentration) increased linearly with increasing temperature. The equilibrium SiO2 solubility of sodalite was higher than that of cancrinite at all temperatures. The solubility of cancrinite increased with increasing concentration of NaOH (3.87–5.42 M) and Al(OH)3 (1.39–2.23 M) in solution. The SiO2 solubilities of four types of dimorphic sodalite/cancrinite mixed phase seed crystals synthesised from Bayer plant spent liquor and synthetic Na2CO3, Na2SO4 or NaNO3 rich NaOH solutions containing dissolved kaolinite were investigated. The solubility of the four types of mixed phase seed crystals was found to be substantially the same as the solubility for pure cancrinite.

A methodology for quantifying sodalite and cancrinite phase mixtures and the kinetics of the sodalite to cancrinite phase transformation

A simple methodology for determining the cancrinite proportion of a sodalite and cancrinite phase mixture was developed using powder X-ray diffraction. Using this methodology to quantify phase mixtures, the kinetics of the sodalite to cancrinite phase transformation were determined. The transformation reaction leading to cancrinite formation was found to be first order with respect to the relative concentration of sodalite. Over the temperature range 160–240°C, an activation energy for this process was estimated to be 133 kJ mol−1. The techniques of 29Si MAS NMR and FTIR were both found to be unsuitable for quantifying dimorphic phase mixtures due to the ambiguity of the results they produced.

The kinetics of desilication of synthetic spent Bayer liquor seeded with cancrinite and cancrinite/sodalite mixed-phase crystals

Isothermal, batch desilication kinetics of synthetic, sodium aluminate solution (spent Bayer liquor) via cancrinite and cancrinite/sodalite mixed-phase crystal growth, have been studied under conditions at which sodium aluminosilicate scale forms at the surfaces of steel heat exchangers of alumina plant. Seeding with the pure cancrinite and mixed-phase crystals results in the suppression of scale formation and a faster rate of liquor desilication in comparison with its sodalite dimorph. Cancrinite seed crystals prepared from NO−3-rich solutions exhibited crystal growth mechanism and kinetic behaviour different from dimorphic mixed-phase crystals prepared from CO2−3-rich solutions, when both were used to desilicate CO2−3-rich spent Bayer liquor. The rate of desilication due to crystal growth on CO2−3-cancrinite/sodalite mixed phase crystals followed a second-order dependence on the relative supersaturation of SiO2. An activation energy of 52 kJ mol−1 was estimated for the crystal growth process. For desilication kinetics involving NO−3-cancrinite seed crystal growth, a third-order dependence on relative supersaturation of SiO2 and an activation energy of 63 kJ mol−1 were obtained.

The mechanism of TiO2 deposition by direct current magnetron reactive sputtering

Thin films are generally thought of as being the product of a reaction between the surface and atoms and/or molecules in the gas phase. However, a relatively new theory, the theory of charged clusters (TCC), suggests that charged clusters nucleate in the gas phase and become the growth unit for a thin film. The aim of this study was to determine whether or not TiO2 thin film deposition by DC reactive sputtering occurs via this mechanism. TiO2 was deposited on unheated transmission electron microscopy grids to observe TiO2 clusters, as well as glass and silicon substrates to observe the resulting thin films. The results showed that TiO2 clusters were indeed produced in the chamber of a direct current reactive sputtering system. Furthermore, these clusters were observed as close as 50 mm away from the target. Clusters 3 nm and <2 nm in diameter were found 250 mm and 50 mm away from the target, respectively The cluster size was found to have a direct effect on the film deposited. Smaller clusters produced a facetted crystalline anatase film whereas larger clusters produced an amorphous film.

The kinetics of desilication of synthetic spent Bayer liquor and sodalite crystal growth

The kinetics of desilication of synthetic, sodium aluminosilicate solution (spent Bayer liquor) and growth of sodalite crystals have been studied under isothermal, batch crystallization conditions close to those prevailing in Bayer process heat exchangers. The desilication rate of the liquor via the formation and growth of sodalite scale on steel substrates was found to be independent of agitation rate. With sodalite seeding, the desilication rate was observed to increase dramatically due to seed crystal growth with the suppression of scale formation. An activation energy of 30 kJ mol−1 and a second order dependence of the desilication rate on relative supersaturation of SiO2 were obtained for sodalite crystal growth.

Spontaneous generation of charged clusters of a few nanometers during thermal evaporation of copper

Charged copper clusters of a few nanometers were spontaneously generated under typical copper thin film processing by evaporation at 1573 K. The deposition behavior of these clusters was drastically influenced under an electric field. After deposition for 10 s with a bias of +200, 0, and -200 V applied to the substrate, the number density of clusters was negligible on a positively biased substrate but ~1010 mm−2 on neutral and negatively biased substrates. After deposition for 5 min, the film thickness was 10, 100 and 120 nm on positive, neutral and negative substrates, respectively.

The effect of RF power on the deposition behavior of anatase clusters

In the thin-film formation process, it is generally accepted that thin film growth occurs via a reaction between the surface and atoms and/or molecules in the gas phase. The theory of charged clusters proposes instead that a thin film is a self-assembly of charged clusters that nucleate in the gas phase. It was demonstrated that similar-sized anatase clusters have quite different deposition behaviors depending on the RF power in the reaction chamber. At 180 W a highly crystalline nanostructured film was produced. However, at 90 W, a nanopowder was deposited instead. This can be explained by the theory of charged clusters, in that the clusters become charged at higher RF power, i.e. the charging efficiency of clusters increases with increasing RF power. Lower power (90 W) did not efficiently charge the anatase clusters. Cluster charging at 180 W resulted in Coulombic repulsion, which prevented the agglomeration observed at 90 W. The self-assembly characteristics of charged clusters is highlighted by the formation of a nanostructured film.