Red mud

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Red mud dump near Bützflethermoor

Red mud or bauxite residue is a waste product that arises from the extraction of aluminum oxide from aluminum-containing ores (especially bauxite ). Aluminum oxide is an intermediate product in aluminum production. The characteristic red color comes from solid particles of iron (III) compounds (e.g. iron (III) hydroxide and iron (III) oxide ) suspended in sodium hydroxide solution .

More than 95% of the Al 2 O 3 produced worldwide is generated using the Bayer process ; per ton of Al 2 O 3 , approximately 1 to 1.5 tons of bauxite residue are produced. The global production of Al 2 O 3 amounted to around 115 million tons in 2015, which at the same time led to a production of 150 million tons of bauxite residue.

Emergence

For the industrial production of metallic aluminum, bauxite is used, an ore that mainly consists of aluminum oxide and hydroxide as well as iron oxide and hydroxide. The main constituents are titanium oxide, silicates and traces of heavy metals. In order to extract Al 2 O 3 , the soluble part of the bauxite is dissolved in caustic soda at high temperatures and pressure (“digested”, the so-called Bayer process ). The aluminum compounds contained are converted into water-soluble sodium aluminate Na [Al (OH) 4 ] and separated from the water-insoluble residue by extraction . Aluminum hydroxide (Al (OH) 3 ) is precipitated from the sodium aluminate solution by dilution and cooling . This is then burned to aluminum oxide (Al 2 O 3 ) in fluidized bed systems or in rotary kilns and reduced to metal using fused- salt electrolysis ( Hall-Héroult process ) .

The iron and heavy metal compounds remain as a suspension or dispersion in a strongly alkaline solution and are known as red mud due to their reddish color . In order to operate the Bayer process as efficiently as possible and to reduce production costs, as much NaOH as possible is removed from the residue in various steps and reused. This results in a residue with lower alkalinity and easier handling.

How much red mud is produced per tonne of aluminum produced depends on the composition of the bauxite used, which varies depending on the origin: For tropical bauxite, around 1.6 tonnes and for European bauxite from 3.2 to 3.7 tonnes of moist red mud can be assumed.

composition

In principle, red mud contains the foreign substances contained in the bauxite ore. These are mainly iron and titanium oxides and various silicic acid compounds . Red mud gets its characteristic red color due to its main component iron (III) oxide . The minor components contained vary with the origin of the ore. Numerous heavy metals such as arsenic , lead , cadmium , chromium , vanadium and mercury were detected here. According to an analysis of the red mud from the aluminum smelter MAL AG (Magyar Alumínium) ( Ajka , West Hungary ) commissioned by Greenpeace , the dry matter had a content of 110  ppm arsenic, 1.3 ppm mercury and 660 ppm chromium.

An elemental analysis by the Austrian Federal Environment Agency showed that the red mud from this aluminum plant is composed of a total of 38 chemical elements , including cadmium (7 ppm), nickel (270 ppm) and antimony (40 ppm). The composition varies significantly, but is usually between:

component proportion of
Fe 2 O 3 0,5… 60%
Al 2 O 3 0,5… 30%
TiO 2 0.3 ... 15%
CaO 0,2… 14%
SiO 2 0,3… 50%
Na 2 O 0,1 ... 10%

Bauxite residue is mineralogically composed as follows:

mineral Components proportion of
Sodalite 3 Na 2 O 3 Al 2 O 3  6 SiO 2  Na 2 SO 4 04… 40%
Al- goethite 10 ... 30%
Hematite Fe 2 O 3 10 ... 30%
Quartz and amorphous SiO 2 05… 20%
Katoite 3 CaO · Al 2 O 3  · 6H 2 O 02… 20%
Boehmite AlO (OH) 00… 20%
Rutile 02… 15%
Muscovite K 2 O · 3 Al 2 O 3  · 6 SiO 2  · 2 H 2 O 00… 15%
Calcite 02… 10%
Gibbsite Al (OH) 3 00… 5%
Kaolinites Al 2 O 3  · 2 SiO 2  · 2 H 2 O 00… 5%

The formation of sodium aluminate silicates reflects that some of the aluminum compounds present react with reactive silicates within the Bayer process and thus reduce the yield of aluminum oxide.

Whereabouts

The disposal of the residue has changed significantly over the course of history. Due to the shortage of storage areas and increasing concerns about the final disposal of the sludge, the so-called dry stacking method has been used since the mid-1980s. In this method, the residues are compacted (48-55% solids content) and then stored in a way that allows drying and solidification.

A widely used method is filtration, whereby a filter cake (typically <30% residual moisture) is produced. The cake is washed with water or steam to reduce the alkalinity before shipping. The filtered residue is preferred in terms of recycling because of its lower alkalinity, cheaper transportation and easier handling.

Originally, sludge with a solid content of 20% was pumped into ponds or reservoirs, some of which were created in former bauxite mines. Alternatively, sealed landfills with dams or dykes were built. A common practice was also the disposal with the help of pipelines in rivers, estuaries or in the sea. Often the residue was also disposed of on the high seas near ocean trenches. Disposal in the sea, rivers and estuaries is no longer used today.

In Germany, the sludge is today in sealed landfills into storage until the as dispersion present hydroxides and silicates have discontinued. The caustic soda that escapes is recycled. The landfill is then covered with sand and earth and recultivated . One of the largest red mud dumps in Germany is located near Stade in Lower Saxony between Bützflethermoor and Stadermoor, 10 km northwest of Stade. The aluminum oxide stade washed the caustic soda from the red mud before it was dumped.

More recently, purified red mud has also been used as a filler in road construction and as a raw material for ceramics . Other possible uses of red mud as a raw material were discussed, but so far they have proven to be too costly and not profitable. Therefore, less than 2% is currently being recycled .

hazards

The short-term danger of red mud is primarily due to the content of caustic soda lye .

Long-term harmfulness results from the content of toxic heavy metals , depending on the origin and type of bauxite. Heavy metal oxides and heavy metal hydroxides are usually only very sparingly soluble in a basic environment. Landfilled red mud contains around 1% soluble heavy metal hydroxides. Toxic components present as anions such as fluorides , arsenates , chromates and vanadates can, however, also be washed out of the sludge in a basic environment. If the sodium hydroxide of the red mud is neutralized by strong dilution or the addition of acids , soluble compounds of other heavy metals can also be formed and thus environmental hazards. Therefore, red mud landfills should have both surface coverage and protection against contact with groundwater .

Incidents

Improper disposal or the deliberate discharge of red mud into rivers and lakes can lead to serious environmental problems . There have also been accidents in the past when storing red mud:

On October 4, 2010, when the Kolontár dam burst in Hungary, between 700,000 and 1 million cubic meters of red mud leaked from the storage facilities of an aluminum plant in the Hungarian town of Ajka .

At one of the largest bauxite mines in Brazil, Porto Trombetas in the Amazon basin , red mud is discharged into Lago Batata . This resulted in an enormous extinction of species in the lake, environmental organizations such as Rettet den Regenwald e. V. even speak of a complete death of the ecosystem. In addition, the water of the lake and the adjacent Rio Sapone can no longer be used as drinking water.

Applications

Since the Bayer process was first used in 1894, the potential value of the residue has been recognized and research has been carried out to find a reuse. Attempts have been made to recover the main constituents, especially iron. Four categories with regard to the possible use of bauxite residue can be named: Extraction of main and secondary components: iron, titanium, rare earth elements (REE); Use as the main raw material for the manufacture of products, e.g. B. cement; Use of bauxite residue as a component in building materials, e.g. B. Concrete, brick, tile; Land improvement, and converting the residue into a useful product, such as B. through the Virotec process.

The varying composition of the residue leads to a multitude of different possible applications: in cement production, use in concrete as SCM (supplementary cementitious material), for the extraction of iron and titanium, use in construction elements, in bricks, in tiles, as track ballast, for Soil improvement, as calcium and silicon fertilizer, for the extraction of lanthanoids (SEE), scandium, gallium, as an adsorber of heavy metals, as a dye, for chemical water treatment, in (glass) ceramics, foam glass, as a pigment, as a filler for PVC, as Wood substitute, in geopolymers , as catalysts, as plasma spray coating of aluminum and copper, in the production of aluminum titanate-mullite composites for temperature-resistant coatings, in flue gas desulphurisation, in arsenic and chromium disposal.

It is estimated that 2 to 3.5 million tonnes of the bauxite residue produced is reused annually:

  • Cement: 500,000 to 1,500,000 t
  • Raw material for iron and steel production 400,000 to 1,500,000 t
  • Landfill cover / roads / soil improvements: 200,000 to 500,000 t
  • Building material (bricks, tiles, ceramics, etc.) - 100,000 to 300,000 t
  • Others (refractory product, adsorber, pit drainage (Virotec), catalysts, etc.) - 100,000 t

In 2015, a European initiative with funding from the European Union for the valorisation of bauxite residue was launched. 15 PhDs were recruited as part of the European Training Network for Zero-Waste Valorization of Bauxite Residue. The main focus is on the extraction of Fe, Al, Ti and REE (including scandium) and the use of the residue as raw materials for building materials (cements, geopolymers).

literature

  • KS Sutherland, “Solid / Liquid Separation Equipment”, Wiley-VCH, Weinheim (2005).
  • Annual statistics collected and published by World Aluminum .
  • “Bauxite Residue Management: Best Practice”, available from the International Aluminum Institute, 10 King Charles II Street, London, SW1Y 4AA, UK and online .
  • Data on global production of aluminum and aluminum oxide .
  • Wanchao Liu, Jiakuan Yang, Bo Xiao, “Review on treatment and utilization of bauxite residues in China”, in Int. J. of Mineral Processing, 93 220-231 (2009), DOI: 10.1016 / j.minpro.2009.08.005 .
  • MB Cooper, “ Naturally Occurring Radioactive Material (NORM) in Australian Industries”, EnviroRad report ERS-006 prepared for the Australian Radiation Health and Safety Advisory Council (2005).
  • Agrawal, KK Sahu, BD Pandey, “Solid waste management in non-ferrous industries in India”, Resources, Conservation and Recycling 42 (2004), 99–120, DOI: 10.1016 / j.resconrec.2003.10.004 .
  • Jongyeong Hyuna, Shigehisa Endoha, Kaoru Masudaa, Heeyoung Shinb, Hitoshi Ohyaa, "Reduction of chlorine in bauxite residue by fine particle separation", Int. J. Miner. Process., 76, 1-2, (2005), 13-20.
  • Claudia Brunori, Carlo Cremisini, Paolo Massanisso, Valentina Pinto, Leonardo Torricelli, "Reuse of a treated red mud bauxite waste: studies on environmental compatibility", Journal of Hazardous Materials, 117 (1), (2005), 55-63.
  • H. Genc¸-Fuhrman, JC Tjell, D. McConchie, “Increasing the arsenate adsorption capacity of neutralized red mud (Bauxsol ™)”, J. Colloid Interface Sci. 271 (2004) 313-320, DOI: 10.1016 / j.jcis.2003.10.011 .
  • H. Genc¸-Fuhrman, JC Tjell, D. McConchie, O. Schuiling, “Adsorption of arsenate from water using neutralized red mud”, J. Colloid Interface Sci. 264 (2003) 327-334, DOI: 10.1016 / S0021-9797 (03) 00447-8 .

Web links

Commons : Rotschlamm  - Collection of images, videos and audio files

Individual evidence

  1. ↑ Annual statistics collected and published by World Aluminum .
  2. ↑ Manfred Sietz, Stefan Seuring : Life cycle assessment in operational practice , Eberhard Blottner Verlag, Taunusstein 1997 p. 103
  3. ↑ Hungary: Wrong handling of the red mud
  4. ↑ Results of the analyzes of the Hungarian red mud from Kolontar on behalf of Greenpeace ( Memento of February 2, 2014 in the Internet Archive ), accessed on October 9, 2010 (PDF; 96 kB)
  5. ↑ Toxic sludge: According to Greenpeace, increased fine dust levels. ORF , October 12, 2010, accessed on October 17, 2010 .
  6. ↑ Test report no. 1010/441 "Heavy metal screening and determination of Cr (VI) in red mud" ( Memento from February 2, 2014 in the Internet Archive ). Order A 9928 - project no. 2490, accessed on October 17, 2010 (PDF; 46 kB, created by Umweltbundesamt GmbH on behalf of Greenpeace).
  7. ↑ Test report No. 1010/431 "Determination of arsenic, mercury and chromium (total) in red mud" ( Memento of February 2, 2014 in the Internet Archive ). Order A 9928 - project no. 249, accessed on October 17, 2010 (PDF; 42 kB, created by Umweltbundesamt GmbH on behalf of Greenpeace).
  8. ^ BG Purnell, “Mud Disposal at the Burntisland Alumina Plant”. Light Metals, 157-159. (1986).
  9. ↑ HH Pohland and AJ Tielens, "Design and Operation on Non-decanted Red Mud Ponds in Ludwigshafen", Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986).
  10. ↑ EI Robinsky, "Current Status of the Sloped Thickened Tailings Disposal System", Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986).
  11. ^ JL Chandler, "The Stacking and Solar Drying Process for Disposal of Bauxite Tailings in Jamaica," Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986).
  12. ↑ “Bauxite Residue Management: Best Practice”, published by World Aluminum the European Aluminum available from the International Aluminum Institute, 10 King Charles II Street, London, SW1Y 4AA, UK and online from http://bauxite.world-aluminium.org /refining/bauxite-residue-management.html
  13. ↑ KS Sutherland, "Solid / Liquid Separation Equipment", Wiley-VCH, Weinheim (2005).
  14. ↑ K. Evans, E. Nordheim and K. Tsesmelis, “Bauxite Residue Management”, Light Metals, 63–66 (2012), DOI: 10.1007 / 978-3-319-48179-1_11 .
  15. ^ G. Power, M. Graefe and C. Klauber, “Bauxite residue issues: Current Management, Disposal and Storage Practices”, Hydrometallurgy, 108, 33-45 (2011), DOI: 10.1016 / j.hydromet.2011.02.006 .
  16. ^ Accident in Hungary - First analysis warns of toxins in red mud , Cordula Meyer on October 12, 2010 in Spiegel-Online, accessed on October 20, 2013.
  17. ↑ a b Frank Muster: Rotschlamm. Residual material from aluminum oxide production - ecological backpack or input for production processes? kassel university press GmbH, Kassel 2007, ISBN 978-3-89958-359-5 , p. 15.
  18. ↑ Aluminum production - what to do with 150 million tons of "redmud" annually? In: euronews.com . April 2, 2018, accessed December 23, 2018 .
  19. ^ Chemical accident: an environmental catastrophe looms in Hungary. In: DerWesten. October 6, 2010, accessed October 6, 2010 .
  20. ↑ regenwald.org: Aluminum , accessed on February 25, 2013.
  21. ↑ eco-world.de: drinking water or fuel? And all of this is just the tip of the iceberg ... Retrieved February 25, 2013.
  22. ↑ BK Parekh and WM Goldberger, “An assessment of technology for the possible utilization of Bayer process muds”, published by the US Environmental Protection Agency, EPA 600 / 2-76-301.
  23. ↑ Y. Pontikes and GN Angelopoulos “Bauxite residue in Cement and cementious materials”, Resourc. Conserv. Recyl. 73, 53-63 (2013), DOI: 10.1016 / j.resconrec.2013.01.005 .
  24. ↑ Y.Pontikes, GN Angelopoulos, B. Blanpain, "Radioactive elements in Bayer's process bauxite residue and Their impact in valorization options," Transportation of NORM, NORM Measurements and Strategies, Building Materials, Advances in Sci. and Tech, 45 2176-2181 (2006).
  25. ↑ WKBiswas and DJ Cooling, "Sustainability Assessment of Red SandTM as a substitute for Virgin Sand and Crushed Limestone", J. of Ind Ecology, 17 (5) 756-762 (2013). DOI: 10.1111 / jiec.12030 .
  26. ↑ H. Genc¸-Fuhrman, JC Tjell, D. McConchie, "Adsorption of arsenic from water using activated neutralized red mud", Environ. Sci. Technol. 38 (2004) 2428-2434, DOI: 10.1021 / es035207h .
  27. ↑ http://etn.redmud.org/project/