«GOSPODARKA SUROWCAMI MINERALNYMI Tom 23 2007 Zeszyt 4 VLADIMIR CABLIK* Characterization and applications of red mud from bauxite processing Key words ...»
GOSPODARKA SUROWCAMI MINERALNYMI
Tom 23 2007 Zeszyt 4
Characterization and applications of red mud from bauxite processing
Red mud, pigment, bauxite
This work describes the characterization of red mud – a waste generated by the Bayer process in the aluminium
industry – which causes environmental problems. Residue of the alumina leaching from bauxite was analyzed for mineral compositions of the mineral ore and its residue for chemical composition, density, and grain-size composition. The residue was calcinated and finally tested as a pigment for use in the building material industry.
The test blocks were tested on the compressive strength.
Introduction The Birac Alumina Industry is located in eastern Bosnia, 25 km west of Tuzla and 85 km SW of Sabac, Serbia. Bauxite is transported to the Industry by road and railway mainly from local sources (Vlasenica, Krunici, Mrkonjic) and from Niksic, Montenegro, or from India.
The Birac Industry extracts alumina mainly from monohydrate bauxites by the Bayer process. A large amount of the red mud waste that pollutes streams is an environmental problem for the industry and the local community.
Bauxite residue (also known as red mud) is a by-product of the Bayer process. The amount of the residue generated, per ton of the alumina processed, varies greatly with the type of the bauxite ore used from 0.3 ton to 2.5 tons for high and very low-grade bauxites, respectively.
* Ph.D. Eng., VSB-Technical University of Ostrava, Faculty of Mining and Geology, Ostrava-Poruba, Czech Republic; email@example.com Reviewed by Prof. Barbara Tora The Birac factory is located in the Zvornik industrial area (Karakaj), eastern Bosnia.
Located only one kilometre from the Drina river, its red mud waste is conveyed by pipes to the dump area at Ðuliæi, some ten kilometres from the Industry. This waste dump contains at present about ten million tons of red mud and is a big concern of the Industry.
The basic properties of the waste are: very high pH (12.2) of the suspension conveyed to the dump, and extremely fine solids of the suspension. The solids are highly migrative during the slow precipitation and complex in the chemical composition (due to dissolved salts).
Red mud is a serious pollutant of the environment both by its chemical composition and the amount generated every year. A part of this waste (cleared water) is conveyed back to the industry and most of the solids remain settled out in the dump.
Red mud is classified for its complex character into the waste unsuitable for treatment and disposal. For many years now researches have been made in the use of this waste and in a safe disposal, neutralization or recultivation of old dumps.
Red mud can be permanently disposed off or used:
— in ceramic industry as an additive to make special ceramics, — dewatered (ferro-alumina) as a raw material in cement manufacture, — leached to produce TiO2, — in cement industry (Lafarge plants), — in building material industry as a raw material in manufacture of building and pavement blocks and road surfacing, — micronized and calcinated as a pigment, — in agriculture to improve soil quality, spread on the ground to lower the rate of phosphorus leaching by rainwater, etc.
This work also presents a research in the use of red mud as pigment in the building material industry for manufacture of blocks, bricks, and pavement and road surfaces.
1. Aluminium minerals and ores
The principal source of aluminium is bauxite. The ore, first used in 1845 by Dufrenoy, is named after the locality (Les Beaux-de-Provence, near Arles) in France where Pierre Bertier discovered it in 1821. By their mineral composition, the bauxites in ex Yugoslavia, like similar bauxites elsewhere, are dominantly monohydroxide minerals – either boehmite or diaspore. The commonest bauxite minerals that vary depending on the derivation are given in Table 1. Besides the essential minerals, bauxites contain many other elements: Na, K, P, Cr, V, Ga, Zn, Pb, Cu, Ni, etc.
Bauxite ores from ex Yugoslavia are worldwide known for their quality. Potential sources of bauxites are large. The bauxites from Bosnia and Herzegovina (Vlasenica, Bosanska Krupa, Mostar, Kljuc, Krunici, etc.) and Montenegro (Niksic), processed in the Biraè Alumina Industry, contain about 55% of Al2O3 and about 5% SiO2.
Chemical Mainly used in water treatment plant chemicals, after conversion of the alumina minerals Grade to aluminium sulphate (low iron gibbsitic bauxites preferred)
The Birac Alumina Industry was designed for production of monohydrate bauxite of the average silicate modulus 8.
Bauxite is treated by the Bayer process through the following stages:
1. Bauxite preparation (crushing and grinding).
4. Sedimentation and red mud rinsing.
5. Hydrate dissociation and treatment.
Milling sizes for bauxite processed in this industry are: on-sieve particles 0.147 mm to 4%, on-sieve particles 0.058 mm to 25%, and particles of ten microns. The final waste of the Bayer process is the bauxite residue – red mud.
Red mud properties. Repeatedly rinsed red mud suspension, of a density between
1.2 and 1.3 g/cm3 and dry matter concentration from 250 to 350 g/l, is disposed as a waste.
The liquid phase of the suspension contains about 7 g/l of Na2O, a serious pollutant, which is dumped in selected and prepared areas. Red mud from the Biraè Industry is dumped in an open field – a natural dump area. The only environmental control work so far has been a reinforced-concrete dam wall.
3. Test sample
A sample of red mud was taken in June 2005 in the Birac Alumina Industry from a transverse cut in the effluent pulp at the ultimate thickener.
During the sample treatment in laboratory, the suspension was left to sediment and then the liquid phase was decanted and the solid phase dried forty hours at the temperature of 105°C. Secondary, representative samples were separated from the dewatered residue and further tested for characterization of the waste material (Fig. 1). Samples were taken for determination of the chemical and mineral compositions, density, grain sizes, calcination and other tests. A portion of the sample was laboratory-dried for two months to study changes in the sample. The average density of the representative dry red mud sample was
Fig. 1. Visual appearance of the autoclaved red mud after being dried, pulverized and calcinated Rys. 1. Widok próbek czerwonych mu³ów po wysuszeniu (a), rozdrobnieniu (b), kalcynacji (c)
Chemical composition was analyzed in the central laboratory of the Birac Alumina Industry, Zvornik. Red mud as a complex multiphase waste was short analyzed on the following compounds: Al2O3, SiO2, Fe2O3, TiO2, CaO, Na2 tot, P2O5, V2O5, ZnO, MgO, MnO, K2O, and loss of ignition. Chemical analysis is given in Table 4.
The above chemical analysis of the Birac Alumina Industry red mud indicates an expected high percent of compound Fe2O3 (48.5%) and high silica (11.53%) and Al2O3 (14.14%). The high loss of Al2O3 through the bauxite processing suggests inadequate mineral liberation in milling (grind fineness). Of rare metals, notable was the presence of TiO2 (5.42%), whereas vanadium was much lower (0.116%). Rare metals seldom occur as native minerals in bauxites. They are dispersed in the essential metal structure (aluminium, iron or silicon).
The following data may give an idea of the wide ranges of bauxite constituents (Fe2O3 30–60%, Al2O3 10–20%, SiO2 3–50%, Na2O 2–10%, CaO 2–8%, and TiO2 trace to 10%.
Red mud very rich in iron can be used as an inexpensive pigment for coloured concrete.
The red coloration can be enhanced by calcination within the range from 900 to 950°C.
Red mud calcination was conducted at a very high temperature to achieve decomposition and evaporation of certain components. At a temperature between 900 and 950°C, water (of crystallization and of constitution) was driven off; organic matter burnt, and carbon dioxide produced by disintegration (CaCO3 – CaO + CO2) also was driven off into the atmosphere. Calcination led to the oxidation of iron, manganese and sulphide. The ignition loss varied from 4.93 to 5%.
Mean density of the representative red mud sample from the Birac Industry was
3.05 g/cm3, and the ignition loss for the temperatures of 900 and 950°C was 4.93 and 5%, respectively. It should be noted that after the calcination and the conversion of the lower into higher oxides, and after decomposition of the carbonates, the red mud sample acquired the characteristic red colour that makes it usable as a pigment.
Mineral composition of the red mud depends on the mineral composition of the source material – bauxite. Bauxite is a multiphase ore that may contain, according to some references, as many as more than hundred minerals. Its essential constituents, however, are the minerals of aluminium, iron, silicon, titanium, calcium, magnesium, etc. The accessory minerals are those of many other elements: Na, K, P, Cr, V, Ga, Zr, Zn, Pb, Cu, Ni, Mn, Co, etc. Depending on the type of mineral deposits, the amounts of the essential and accessory minerals may vary within wide ranges. Not infrequently, the variations are notable within one and the same deposit.
Aluminium is contained in bauxite in the form of hydrous oxides: hydrargillite, boehmite and diaspore, and at lower rates as corundum (Al2O3) or various aluminosilicates. Aluminium minerals are naturally concentrated and mixed in ores with many metals and petrogenic minerals.
The most abundant gangue mineral in bauxites is free silica (various forms of crystalline SiO2 – quartz, quartzite, chalcedony, or amorphous SiO2 – opal) or bound silicon oxide (in the form of aluminosilicates, commonly kaolinite). Iron in bauxite occurs in various minerals forming the principal waste (red mud) component. Iron minerals are hematite, magnetite, hydrohematite, goethite, limonite. Its principal carbonates are siderite and ankerite; silicate – chamosite; sulphides and sulphates – pyrite, melanterite, boutlerit, jarosite, etc.
Commonest in bauxite are hematite and goethite, and less common are magnetite and limonite. Titanium is almost always found in bauxites, in the form of rutile, anabase or brucite, of which anabase is the commonest. Bound titanium dioxide may be contained in bauxites in the form of sphene, perovskite or ilmenite. Carbonate constituents are calcite, magnesite, dolomite, hydrous magnesite, ankerite, malachite and azurite. The red mud considered in this work is largely depending on chemical and mineral compositions of the mineral ore, grind fineness and effective leaching (decomposition).
5. Testing pigment production from red mud
The idea to use red mud as pigment in the building material industry was based on the extremely fine particles (4% on sieve 0.147 mm, 25% on sieve 0.058 mm and the remaining percentage of about ten microns) and the characteristic red colour.
Dewatered and ground red mud (waste) was preliminary tested for use as pigment in the building material industry. Similar researches were reported by the Building Research Institute (BRI) of Jamaica; Tübitak Marmara Research Center Gebze-Kacaeli, Turkey;
Afyon Kocatepo University, Turkey.
For this test, building blocks were made of the standard mixture (crushed limestone Class 3.3+0 mm, cement and water) with the addition of red-mud pigment in various proportions.
Before being added, red mud was homogenized and dewatered and then used as a raw material unground or fine-ground. At the feeding point, the water content varied within the range from 9 to 14 wt%, and the red mud content was between 1 and 33%. The cement types used in the test were white cement (CEM 152.5 N) from the Lukavac Cement Plant and Portland cement (PC 42.5 N) from the Titan Cement Plant of Kosjeriæ. The components were mixed and homogenized and water was gradually added to the desired mixture density. Then the mixture (sample) was poured into a mould, 8.5 × 4.5 × 15.5 cm in size, and manually pressed. The exact proportions of the constituents are given in Table 7.
A number of blocks were tested for compressive strength after 17-14-28 days of rest at the room temperature (18°to 23°C). The compression test was conducted to the JU Standards for the given materials. The tests were carried out in the Laboratory of Mine Material Testing, using AMSLER Zürich press for determination of the mechanical strength. The test data, given in Table 5, indicate satisfactory compressive strengths. Test blocks are shown in Figure 2.
Compressive strengths, given in the table above, vary from 14.83 to 27.77 MPa. Test block 1 has the lowest compressive strength due to the high red mud constituent (100 g) and to its form (unground). Test block 7 has the highest compressive strength due to the lowest red mud constituent (5 g) and its form (ground). In relation to the compressive strength, all tested samples were satisfactory.
TABLE 5 Composition of test blocks TABELA 5 Sk³ad badanych bloczków
Rys. 2. Porównanie testowanych bloczków betonowych z dodatkiem pigmentu.
Kolor zale¿y od iloœci dodawanego pigmentu, sposobu jego przygotowania i wytrzyma³oœci mechanicznej The preliminary test data approved the usability of red mud (as pigment) in the building material industry, viz.: blocks, bricks, pavement surfacing, etc. Of course, for practical application, additional tests will be necessary.