Central Europe Chemistry

Red mud is the by-product of the Bayer-processing of bauxites, one of the oldest large-scale industries in the world. In 1892 Karl Joseph Bayer, an Austrian chemist, applied for a patent on the digestion of bauxite, an aluminium-containing ore, by means of a concentrated sodium hydroxide solution employed at elevated temperature and pressure, which enables dissolution of the aluminium content and its separation from other bauxite components. From the caustic solution the dissolved aluminium hydroxide can be precipitated by dilution. Then the hydroxide is subjected to calcinations to form alumina, from which metallic aluminium is produced by electrolysis. The other metallic components of bauxite, such as iron, titanium, and rare earth metals, are not dissolved during digestion, so that they form a residue, which in turn is disposed of using various amounts of caustic solution. Nowadays about 90 % of bauxite is processed using the Bayer technology.
The caustic mud containing the bauxite residue is termed “red mud”. It consists of two main components, the caustic solution and the bauxite residue. Its characteristic red colour is due to its high iron content. The creation and disposal of red mud are a problem throughout the world. Over the course of 93 years, from the beginning of alumina production in 1892 until 1985, some 109 tons of bauxite residue were disposed of worldwide. This quantity then doubled in the next 15 years. The deposited bauxite residue amounts to about 3·109 tons in the world now, with an annual accumulation rate of about 120·106 tons.
The red mud leaving the Bayer processing plant in the Ajka refinery consists of 30 % bauxite residue and 70 % sodium hydroxide solution with a pH of 12 — 13. This mud has a density of about 1.5 g·cm—3, and its rheological properties are close to those of pure water. In environmental terms, the high alkalinity spells a special danger; the direct environmental impacts and serious health consequences of the disaster in Ajka are mainly attributed to the caustic ”flood”. Environmental loading due to the sedimentation of bauxite residue must not to be overlooked either, however.
There have been intensive investigations of the composition, physical and chemical properties of bauxite residue just before and after the Ajka disaster. The dissolution of different components along with their health and environmental impacts were widely studied: for several reasons, there was a diversity of results. It is important that these discrepancies and the facultative paradoxes involved be exposed through careful consideration of all important aspects, including sampling and the conditions under which the analyses were performed.
It is often asked how much sloppy slurry, in what composition, escaped from Chamber 10, and how it could spread over a territory of some 10 km2. These questions have still not been entirely answered, and further investigation by specialists in the field is required. Nevertheless, the available information does permit some conclusions. According to conservative estimates, caustic slurry of about 1.3 million m3 spilled out from the chamber. The solid content of the slurry was 7 to 8 %; that is, with a density of 1.15 — 1.2 g·cm—3 with a viscosity of 0.01 Pa·s it was less concentrated than the red mud from Ajka refinery. The force and velocity of the caustic flood is believed to be due to the high level of solution within the chamber and the very rapid breaking of the dam, so that a vast amount of low-density liquor rushed through a relatively narrow opening. Two items on the aerial photo of the dam (Figure 1) are particularly worthy of note. The first is the sharply vertical surface of the fracture, which indicates an instantaneous rupture of the dam. The second is the upper part of the fracture surface, which is white and thus was not in contact with red mud. From this we know that the dam broke from the bottom up. To summarise: in this case we can assume a very rapid breaking of the dam from the bottom up, with the thin slurry flowing through the opening with great force.


<h2>Red mud disposal</h2>
The harmful effects of the red mud flood were closely connected to disposal practices in Ajka. Some of the causes were, first, the low solid content and high alkalinity of the thin slurry. For a long time, not only in Ajka but in other alumina refineries all over the world, a caustic slurry of about 30 — 35% solid content from bauxite digestion was pumped into land-based ponds without neutralisation or thickening. This practice, called “lagooning” (Figure 3a), is the simplest and cheapest land-based disposal method, though the riskiest in environmental terms. As with all disposal methods, lagooning has both advantages and disadvantages. Advantages are:
<list list-type="order">
<list-item>
</list-item></list>it has the lowest capital costs of all land-based disposal methods;

<list-item>
it does not require additional thickening or filtration;
</list-item>
<list-item>
the dust potential is minimised;
</list-item>
<list-item>
there is no return of impurities to the Bayer technology.
</list-item>

Disadvantages are:
<list list-type="order">
<list-item>
</list-item></list>it requires substantial areas of land for disposal;

<list-item>
significant financial resources need to be allocated for long term closure and rehabilitation;
</list-item>
<list-item>
there is a loss of sodium hydroxide and alumina with entrained solution.
</list-item>

If the red mud is not neutralised before disposal further problems are encountered, such as:
<list list-type="order">
<list-item>
</list-item></list>a strongly caustic, poorly compacted mud in the pond overlaid with a highly alkaline solution;

<list-item>
a range of safety and environmental hazards;
</list-item>
<list-item>
the significant amount of engineering required for safe operation and to lower the chances of disaster;
</list-item>
<list-item>
the difficulty of closing and rehabilitating the disposal site due to the huge amount of caustic liquor that would need to be removed.
</list-item>

Some of the disadvantages can be eliminated by employing mineral acids to neutralise the slurry prior to discharge.
Under favourable geological conditions the disposal site can be developed in natural caves or hollows. If the site does not provide these, artificial ponds should be constructed with thorough consideration of all important technological and environmental factors. With Chamber 10 in Ajka, there were doubts regarding even this. The pond, whether natural or man-made, should be lined with a water impervious material to minimise infiltration of liquor into the underlying soil and ground water. The most suitable lining is a single layer of compacted clay, but other options such as polymers or geo-membranes can also be used.
The other land-based disposal method is dry disposal (Figure 3b). In this case the residue from the mud washing is filtered to form a dry cake of &gt;60% solid content. The material on the filter is also washed with water or steam to regain the soda and to decrease the alkalinity of the residue. The bauxite residue of increased solid content is transported to disposal by trucks.
Advantages of dry disposal include:
<list list-type="order">
<list-item>
</list-item></list>smaller area of land required for storage;

<list-item>
less complicated engineering structures and a lower risk of material discharge;
</list-item>
<list-item>
soda and alumina recycled into the Bayer process;
</list-item>
<list-item>
reduced environmental hazard;
</list-item>
<list-item>
facilitation of further processing of bauxite residue;
</list-item>
<list-item>
less complicated closure and rehabilitation.
</list-item>

The disadvantages of this method are that:
<list list-type="order">
<list-item>
</list-item></list>it requires sufficient land area for storage;

<list-item>
significant capital investment is needed for long-term closure and rehabilitation;
</list-item>
<list-item>
specially constructed high capacity filters are needed;
</list-item>
<list-item>
the recycled caustic solution may induce technical problems in the Bayer technology;
</list-item>
<list-item>
there is a chance of dust lift-off from the surface of the dry stack.
</list-item>

Recently, with environmental aspects gaining more attention, the regulations on disposal are becoming stricter. Thus, disposal practice is moving towards dry disposal worldwide. The biggest alumina refineries mainly use dry disposal methods. Even in the Ajka refinery dry bauxite residue is stored from February 28, 2011 beginnigly. There will be fewer soda and alumina losses, the risk of a disaster like that of October 4, 2010, will be minimised, and the negative environmental impacts of alumina production will decrease as well.


<h2>The residue as secondary raw material</h2>
In addition to deposition, the bauxite residue can be utilised as a secondary raw material (cf. Table). It contains valuable metals, such as iron, titanium, vanadium and rare metals in different concentrations. Extensive research and development on transforming bauxite residue into valuable products has been conducted in various countries, including Hungary, has taken two main directions:
The first involves utilisation of the bauxite residue itself as a material with special physical and chemical properties, or, alternatively, to separate the constituents of the bauxite residue and transform each into value-added products.
The second option, based on technical references and our extensive research on Hungarian bauxite residues, involves the many technical possibilities for selecting and processing the components of red mud. The solutions may differ significantly in volume and in technical and economic terms. We can say with certainty, however, that complex processing aiming at transformation of all components of red mud into valuable products may be feasible in technical, environmental and economic terms at one and the same time.


<h2>What we have learned</h2>
 We have learned much from the red mud disaster in Ajka — a long list of legal, technical, environmental, human, social and other lessons. Let us name just two: first, human and environmental aspects can never be made secondary to economic or any other considerations; second, during operations involving such safety hazards as a red mud pond, we must take precautions against any worst-case scenario, and the facility should be designed, constructed and operated with this aspect always in mind.
Jαnos Szépvölgyi is director of the Institute of Materials and Environmental Chemistry of the Hungarian Academy of Sciences.

Artikel

A Chemical Engineer's View of the Red Mud Disaster

Red mud is the by-product of the Bayer-processing of bauxites, one of the oldest large-scale industries in the world. In 1892 Karl Joseph Bayer, an Austrian chemist, applied for a patent on the digestion of bauxite, an aluminium-containing ore, by means of a concentrated sodium hydroxide solution...

Porous materials with pore diameters of a few tenths of a nanometre (microporous materials) and with pore diameters between a few nanometres and a few tens of nanometres (mesoporous materials) have a very wide range of applications. Their particularly important properties are large surface areas, adsorption capacities, and shape selectivity for molecules. A well-known and immensely exploited class of microporous materials are zeolites. These are aluminosilicate materials built of AlO4 and SiO4 tetrahedra, which are connected through common oxygen vertices into many different porous structures.<xref rid="bib1" ref-type="ref">1</xref> Zeolites are extensively used as catalysts in oil and gas refinement, because they contain catalytically active acid sites due to the aluminium within their frameworks.
Over the past two decades researchers have attempted to improve the efficiency of zeolites in order to extend their range of applications and to tailor their properties for specific applications. These attempts included the modification of zeolites with small amounts of (transition-) metal ions to generate redox sites within them<xref rid="bib2" ref-type="ref">2</xref> and the preparation of zeolites in the form of nano-sized particles to increase their surface areas.<xref rid="bib3" ref-type="ref">3</xref> Intensive research in the field of porous materials also led to the development of silicate mesoporous materials<xref rid="bib4" ref-type="ref">4</xref>,<xref rid="bib5" ref-type="ref">5</xref> and of porous materials with hybrid inorganic-organic composition.<xref rid="bib6" ref-type="ref">6</xref>


<h2>Catalysts</h2>
Catalysts are important to the chemical industry and environmental remediation because they can very effectively convert one chemical to another. Our recent work is focused on the development of supported catalysts, more precisely, on the development of materials that consist of porous silicate supports and catalytic components based on transition metals. The catalytic components are included either as nanoparticles of metal oxides or as metal-modified zeolitic nanoparticles. Preparation of catalytic components in the form of nanoparticles enhances the efficiency of catalysts, because it enlarges the surfaces of particles and thus increases the accessibility to catalytic sites. To avoid agglomeration of nanoparticles, it is favourable to stabilise them into/onto a support, e.g., to deposit or incorporate them into mesoporous materials.
We developed the synthesis route for the incorporation of isolated transition-metal ions (Ti, Mn, Fe) into zeolitic particles (Silicalite-1, ZSM-5, BETA) of typical dimensions of up to 100 nm. In order to understand how to prepare such nanoparticles most efficiently, detailed studies of zeolite formation and growth were carried out using spectroscopic and microscopic techniques.<xref rid="bib7" ref-type="ref">7</xref> Knowledge of the preparation of functionalised zeolitic seeds has allowed us to develop hierarchical porous structures, that is, microporous/mesoporous composite materials. For example, we organised microporous zeolitic seeds containing isolated Ti centres into mesoporous silicate structures with a hexagonal cubic mesopore arrangement (TiBeta/MCM-41, TiBeta/MCM-48, TiBeta/SBA-15).<xref rid="bib8" ref-type="ref">8</xref>—<xref rid="bib10" ref-type="ref">10</xref> These composite solids enabled the diffusion of larger molecules through their mesopores than did pure microporous zeolitic materials. The composites were also thermally more stable than pure mesoporous materials and were not prone to transition-metal leaching. This means that the materials could be readily recovered following catalytic reactions.
More recently we have started to use mesoporous supports for the immobilisation of metal oxides. We have homogeneously distributed MnOx nanoparticles onto a new mesostructured silicate support with textural porosity, KIL-2.<xref rid="bib11" ref-type="ref">11</xref> The material is a potential catalyst for environmentally friendly applications due to its convenient structural characteristics (porosity, distribution and accessibility of nanoparticles), hydrothermal stability, and its non-toxicity and cost-effectiveness. Ordered (SBA-15) and disordered (KIL-2) mesoporous silicate supports were used also for the immobilisation of TiO2 nanoparticles. TiO2 supported on SBA-15 and KIL-2 showed high photocatalytic activity towards the degradation of volatile organic compounds such as isopropanol and toluene.


<h2>Natural zeolites for wastewater treatment</h2>
We come even closer to the topic of environmental protection by investigating the potential of natural zeolitic tuffs for the removal of toxic metals from wastewater and drinking water. Natural zeolites have attracted interest because of their availability and non-toxicity, as well as by their robustness and high selectivity. In collaboration with the groups from Zagreb and Belgrade we performed detailed structural and kinetic studies of unmodified and metal-loaded zeolitic tuffs from two deposits (Vranjska Banja, Serbia, and Donje Jesenje, Croatia) in order to determine the sorption behaviour of selected zeolites for zinc, copper, manganese, nickel, chromium and arsenic.<xref rid="bib12" ref-type="ref">12</xref>—<xref rid="bib15" ref-type="ref">15</xref> The studies showed that the higher content of zeolite clinoptilolite in zeolitic tuff from Serbia (more than 70 %, vs. 50 % for Croatian tuff) resulted in its better sorption performance for selected metals. The pre-treatment of zeolites with Na+ significantly enhanced the uptake of all metal cations, while pre-treatment with Fe3+, which resulted in stable iron-oxo-species on the zeolite surface, enabled the sorption of anions. Spectroscopic and microscopic analyses (XAS, XPS, SEM, TEM) showed that metal concentrations in all samples decreased from the surfaces towards the subsurface region of crystallites suggesting that metal atoms first accumulate on the surfaces of crystallites. Metals arrange in the pores as cations coordinated with framework oxygen atoms or some water molecules (e. g. Cu2+ and Zn2+) or in the form of larger poly-oxo-complexes (e. g. Cr3+). Anionic species form smaller oxo-complexes, which are attached to the iron cations on the surface of the clinoptilolite. Post-treatment of the samples with HCl, NaCl or NH4Cl solutions revealed irreversible sorption of chromium and arsenic and mostly reversible sorption of zinc and copper.


<h2>Hydrogen storage materials</h2>
Hydrogen is foreseen as an efficient and clean energy carrier and has already been widely tested as an alternative to fossil fuels. One of the main obstacles toward the commercialisation of hydrogen technology is safe storage of H2 under moderate conditions. One family of promising materials for hydrogen storage applications is the metal-organic framework (MOF) materials,<xref rid="bib16" ref-type="ref">16</xref> which are crystalline materials composed of metal units linked through organic linkers with strong covalent bonds to form three-dimensional porous structures with exceptional adsorptive properties. At low temperatures (77 K) they already meet the 2010 U.S. Department of Energy targets for H2 storage capacity (gravimetric and volumetric storage capacity of 6 wt.% and 45 g H2/L, respectively), but at ambient conditions their storage capacities are still low.<xref rid="bib17" ref-type="ref">17</xref>
We focused our research mostly on MOF materials based on light and/or non-toxic metal atoms, such as magnesium, calcium, titanium, iron, and zinc. We developed four new 3-dimensional metal-organic open-framework materials: two zinc-based 1,3,5-benzenetricarboxylates, a magnesium succinate and a calcium pimelate. We coped with the structures of new materials by using either a CCD single-crystal diffractometer or a synchrotron facility (Soleil, Paris, France). In collaboration with the Institute Lavoisier, University of Versailles, we attempted to enhance the interaction between H2 molecules and MOF materials and thus to increase H2 storage capacity at ambient temperatures by modification of the selected MOF materials via post-synthetic or in-situ introduction of functional groups. Iron terephthalate (FeIII(bdc)OH or MIL53(Fe)) was modified by CF3, titanium terephthalate (Ti8O8(OH)4(bdc)6 or MIL125(Ti)) was modified by NH2 group, and zirconium terephthalate (Zr6O6(OH)4(bdc)6 or UiO-66(Zr)) by NH2 and CF3 groups. We discovered that modification of the organic linker caused an increase of H2 storage capacity only in the case of MIL53(Fe). Another attempted method for increasing storage capacity for H2 is to introduce charged metal species onto the pore surface. Mg2+, Ni2+ and Li+ are introduced into highly porous iron terephthalate and trimesate (MIL-101(Fe) and MIL-100(Fe)) through anionic functional groups. Determination of H2 storage capacities of these modified MOF materials is in progress.


<h2>Materials for heat storage</h2>
Development of new heat-storage systems is extremely important for broader solar-energy and waste-heat utilisation for domestic and industrial use. Efficient low-cost heat storage is particularly important when there is a gap between when a renewable energy or waste heat is available and when the energy or heat is required (e. g., solar energy is available in summer and needs to be stored until winter).<xref rid="bib18" ref-type="ref">18</xref>
Another class of promising materials for heat storage is adsorbents, among which microporous and mesoporous materials with a broad surface area play an especially important role. The utilisation of water adsorbents for heat storage is schematically explained in Figure 3. The heat collected and stored in an adsorbent during summer could be, upon adsorption by water molecules, released and used during winter.
In our group we study the interdependency of the structure of novel microporous and mesoporous materials and their energy storage abilities (energy density, thermal power, transport properties, and the temperature levels for charging and discharging). Using the knowledge obtained, we then also try to improve microporous and mesoporous storage materials. For example, we have prepared and optimised a very promising microporous aluminophosphate adsorbent. Its performance, which combines the water uptake mechanism, the heat of adsorption, and hydrothermal stability, is the best so far among the aluminophosphates for water-sorption-based heat storage applications and also for heat transformation in heat pumping systems.


<h2>Drug-delivery systems</h2>
Drug-delivery systems based on mesoporous silicates have recently attracted considerable attention, because they could enable controlled release of drugs. This means that by using such drug-delivery systems in combination with proper endogenic or exogenic physical or chemical stimuli, drugs could be released in a given part of the human body within a given time period in sufficient concentrations.<xref rid="bib19" ref-type="ref">19</xref> In our group we are studying incorporation and release of indomethacin as a model drug to and from two carriers, SBA-15 and MCM-41. Both have well-defined hexagonal pore systems, but the diameters of the pores within them are different. The diameter of pores in SBA-15 is about 9 nm, while the diameter of pores in MCM-41 is approximately 3.5 nm. Indomethacin is an anti-inflammatory drug that is especially interesting as a model drug substance because it can crystallise in several different forms.
Our study particularly focuses on the physical state of the drug within mesopores with different diameters. We are investigating how this state depends on the concentration of the drug within the pores and how it influences drug release. We are also studying the influence of the functionalisation of the silicate walls on drug incorporation and release. Detailed description requires inspection by XRD, NMR, thermal (TG and DSC) analysis, nitrogen adsorption and helium picnometry.
Initial results show very interesting and unexpected effects due to the confining of a drug within mesopores: incorporation of indomethacin proceeds differently in pores with different diameters. In SBA-15, when the amount of the incorporated drug is small, the drug forms a thin layer on the pore wall. When the amount is increased, particles extending throughout the entire pore diameter begin to grow. In MCM-41, on the other hand, particles form even at small amounts of the incorporated drug. The two porous systems also direct or favour formation of two different forms of indomethacin. SBA-15 directs the formation of indomethain alpha, while MCM-41 directs the formation of indomethacin gama.


<h2>Characterization</h2>
Characterization of the materials prepared in our laboratory is an extremely important part of our research. It allows us to understand the structure and function of the materials. We combine diffraction techniques with microscopy and with spectroscopic methods probing the local short-range arrangements of constituents. In the past years we have installed (or gained access to) valuable equipment such as three X-ray powder diffractometers (Siemens D5000, PANalytical X'Pert PRO, and PANalytical X'Pert PRO HTK), microscopes SEM (Zeiss SupraTM 3VP), TEM (200-kV field-emission gun microscope JEOL JEM 2100), and AFM (Agilent 5500), NMR spectrometer (600 MHz Varian NMR systems with several MAS probeheads), gas adsorption analyzer (Micromeritics, ASAP), system for thermal analysis (SDT 2960), and synchrotron facilities for X-ray absorption spectroscopy (DESY in Hamburg, Elettra in Trieste).
Emanuela Žunkoviè, Tomaž Èendak, Tadeja Birsa Èeliè, Darja Mauèec, Tina Ukmar, Mojca Rangus, Saša Cecowski, Matjaž Mazaj, Gregor Mali, Alenka Ristiè, Nataša Zabukovec Logar, Nataša Novak Tušar, Roman Gabrovšek, Venèeslav Kauèiè are authors of this article. Venčeslav Kaučič is a head of the Laboratory for Inorganic Chemistry and Technology at the National Institute of Chemistry in Ljubljana and since 1992 professor of Inorganic Chemistry at the University of Ljubljana. His current research interests include synthesis and characterisation of porous solids (micro- and mesoporous materials, their composites and metal-organic frameworks), with the emphasis on the use and development of crystallographic methods. He is currently the president of the Slovenian Chemical Society and the president of the Science Council of the Slovenian Science Foundation.


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Artikel

A Chemical Engineer's View of the Red Mud Disaster

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