Bayer Process

In situ reactions between the magnesium(II) ion and synthetic sodium aluminate have been used in our laboratory to mimic neutralization of supernatant liquor by seawater. This reaction produces fine white precipitates of hydrotalcite, which belong to the class of chemicals known as layered double hydroxides. These have interesting properties making them useful for amelioration of wastewater streams. Of particular importance is their capacity to adsorb a number of chemical species deemed toxic to the environment, including arsenates, oxalates and oxy-anions of transition metals such as vanadium and molybdenum. Laboratory work has indicated that under conditions similar to those utilized by the seawater neutralization process, only a limited number of possible crystalline forms can be synthesized and that these particles are restricted in size. It has also been found that through adjustment of other physicochemical parameters it is possible to alter the crystalline form and the rate at which toxic chemical species can be adsorbed.

Reaction with seawater to produce the layered double hydroxide hydrotalcite is an important means for amelioration of Bayer process wastewater prior to its disposal into the marine environment. This reaction has been synthesised under controlled conditions to elucidate the principal mechanisms involved, particularly those related to absorption of transition metals that are removed from solution. Magnesium–aluminium layered double hydroxides precipitated from reaction between magnesium chloride and synthetic sodium aluminate solution in the presence of transition metal oxyanions have been prepared and characterised using thermal, X-ray and microscopic techniques. The layered double hydroxide precipitated at pH 413 is found to consist of nano-sized hexagonal crystals of final magnesium–aluminium ratio of 2:1, while that precipitated at pHo9.5 has a less well defined crystal structure and final ratio of 4:1. In situ absorption of transition metals from solution into hydrotalcite during precipitation is a pH dependent process. Transition metals are found to replace carbonate in the interlayer space but have little or no overall effect on crystal size or structure. Interlayer distance remains constant in the presence of transition metals, suggesting that these species are intercalated as simple monomeric or dimeric oxyanions, rather than larger polymeric forms.

Atomic force and scanning electron microscopy have been used to study the surface topography and crystal growth mechanism of gibbsite crystals in industrial Bayer liquors. AFM images of the as-grown basal face of industrially produced gibbsite show that the surface is rough and contains diamond shaped features, whereas freshly cleaved basal planes show steps of one unit cell height. An

The purpose of seawater neutralisation of supernatant pond liquor (SNL) is to precipitate heavy metal impurities to such an extent that the neutralised SNL may be discharged out to sea, thus reducing the total tailings dam inventory. Nabalco is actively pursuing several avenues of enhancing this process to increase the rate of processing SNL and quality of effluent for disposal. CSIRO conducted bench tests that looked at the addition of a red mud slurry to the seawater, SNL mix, and concluded that the final vanadium concentration could be significantly reduced. This led to a pilot study conducted in 2000 confirmed that the addition of red mud during the neutralisation process enhanced the precipitation of vanadium from SNL. Unfortunately, separation of precipitated material remained problematic and was considered to have resulted from design irregularities in the equipment used for the trial. A large-scale plant trial using a Deep Cone Thickener was commissioned in early 2002 to validate the original findings and to improve separation of precipitated material from the effluent. The findings will be used to develop a continuous full-scale facility for the treatment of SNL. Further laboratory studies have indicated that addition of alkaline earth chemicals such as calcium and magnesium hydroxides may further enhance the neutralisation process. A long-term investigation is now underway to study their efficiency, where to best introduce them into the waste disposal system and whether they would have any detrimental effect on a mud based neutralisation system. This paper describes the progress achieved to date in all these fields of research. 1.0 Introduction Nabalco has operated an SNL/Seawater neutralisation facility called the labyrinth since 1996. This facility operates on a simple ratio of SNL and seawater flows in an open settlement basin, with a process that efficiently removes some heavy metal species. Options to increase the processing rate of SNL are limited due to environmental restrictions and the costs of processing the precipitate generated. Recent research by CSIRO, and confirmed by Nabalco in field trials, has shown that: Separation of the precipitate by gravity settling can be enhanced with polymer addition; Most heavy metal species can be efficiently extracted from the process stream with the addition of red mud slurry (more efficiently than with seawater alone); and Processing in a thickener facility is preferred due to the improved management of precipitate. The aim of the recent trial was to further investigate the neutralisation chemistry by determining the minimum amount of seawater and red mud slurry required to produce an overflow that could be discharged into the environment. Minimisation of these streams would ultimately result in a more cost effective, full-scale treatment plant. The trial made use of a deep cone thickener in order to settle and thicken the precipitate-slurry.

Controlling silica level in Bayer liquor is critical in order to prevent scaling or alumina quality issues. The conventional method of removing silicates from aluminate solutions requires the introduction of calcium oxide or calcium hydroxide. In this work, Friedel's salt (FS: 3CaO·A12O3·CaCl2·10H2O) is proved for the first time to remove up to 95% silica from sodium aluminate solution. FS is a mineral anion exchanger belonging to the layered double hydroxides (LDHs) which was prepared by adding calcium chloride to sodium aluminate over the temperature range of 50–90 °C. It was characterized by XRD, SEM, Particle Size Analyzer and TG-DSC. FS prepared at 50 °C has a relatively high desilication capacity, better than calcium oxide.Experimental parameters affecting the desilication process, such as temperature, sodium aluminate liquor composition, initial silica concentration (4–10 g/L) and FS dosage were investigated in detail and a comparison of desilication between FS and CaO was carried out. The desilication products (DSP) were mainly calcium aluminium silicates, identified by XRD to be Chabazite and Wadalite and the final chloride concentration in the sodium aluminate solution after anion exchange with FS was ∼ 0.015 g/L. The rate of desilication by FS was first order with a rate constant 2.582 × 106 min− 1. The apparent activation energy was estimated to be 57.7 kJ mol− 1 over the temperature range of 80–110 °C.

Mg-Al layered double hydroxides precipitated from reaction between magnesium chloride and synthetic sodium aluminate solution in the presence of sodium carbonate and sodium oxalate have been prepared and characterised using thermal, X-ray and microscopic techniques. The layered double hydroxide precipitated at pH >13 is found to consist of nano-sized hexagonal crystals of final Mg:Al ratio of 2:1, while that precipitated at pH <9.5 has a less well defined crystal structure and final ratio of 4:1. Carbonate concentration will control absorption of oxalate by the 2 :1 hydrotalcite. Absorption of chloride becomes the dominant process during formation of the 4:1 hydrotalcite, with subsequent loss of ability to absorb carbonate and oxalate.

Hydrotalcite is a layered double hydroxide that precipitates spontaneously when seawater is used to neutralise wastewater from the Bayer process prior to disposal; however, ability to retain intercalated anions in the marine environment remains largely unstudied. This paper reports preliminary results obtained from novel computational molecular models being developed to examine hydrogen bonding and electrostatic forces within the molecular matrix of hydrotalcite; and give insight into its long-term environmental stability. During in-situ precipitation, anionic species that may be harmful to or accumulate in biota become entrained in hydrotalcite via a number of bonding mechanisms, the most common of which is held to be intercalation. Modeling of intercalation suggests that a structured environment within the interlamellar space that hold anions principally by hydrogen bonding with either water molecules or terminal –OH groups of the Mg/Al lamellae forms. Electrostatic forces between anions and the metal hydroxide layers also exist, but the strength of these bonds is pH dependent and anion specific. Results show that while the most of anions are strongly held by hydrotalcite, leaching and ion-exchange of less strongly bonded anions may still occur. This suggests that the long-term environmental stability of hydrotalcite may not be as great as previously thought. The models show potential as useful predictive tools for other hydrotalcite bonded anionic species not investigated here, although some refinements may be required to provide closer correlation with results measured for precipitates prepared under industrial conditions.

The nano alumina have been synthesised employing a novel eco-friendly route from natural bauxite ore. The synthesis of alumina powder from natural bauxite is of great interest owing to the fact that it enables mass production without the use of expensive chemical resources and processing techniques. Employing the Bayer process, synthesis of nano alumina has been made from natural bauxite followed by sol-gel route. The ultra fine alumina powder with high surface area is obtained through the digestion of gel followed by sintering. The size of the particle and its morphology was controlled by calcination temperature and processing time. The crystallite size of the nano alumina powder has been measured using XRD pattern and the FTIR spectrum of nano powder after calcination at different temperatures (873 K and 1273 K) were measured. The results from the energy dispersive X-ray analysis (EDAX) and morphological studies reveal interesting informations. The observed results indicate that o...