Figure 1. X-ray diffraction of the red mud sample.
Table 1. Chemical analysis of the red mud sample.
The Bayer Process is widely used throughout the world to produce alumina and aluminum. In this process, ground bauxite is reacted with caustic soda under heat and pressure. The yield of this treatment, a sodium alumina solution, is then cleaned and decomposed. The aluminum hydrate decomposition product is then transformed into alumina by a calcination process. Finally, liquid aluminum is produced from alumina in electrolysis cells using direct current.1,2
At the end of the Bayer Process, a waste material called red mud is separated from the sodium alumina solution. This byproduct contains iron, titanium oxides and undissolved sodium alumina, and, depending on the nature of the starting raw material, may also contain elements such as Ca, Mg, V, Zr, Th, U, La and Se.
Approximately 35% to 40% per ton of bauxite treated using the Bayer Process ends up as red mud waste. In other words, for each ton of alumina or half ton of aluminum, approximately one ton of red mud (based on dry weight) is produced. Disposal of the red mud is a major problem due to the large amounts that are created1-in Seydisehir, Turkey, for example, more than 3 million tons of red mud have accumulated over the past few decades. Dust from the red mud, which contains caustic soda, pollutes the air and creates severe environmental problems.
Many researchers have examined this issue. Most of the studies so far have focused on dehydrating the red mud waste to reduce its detrimental effects on the environment, or on extracting all or some of the compounds present in the red mud for use in the civil engineering, metallurgy and chemical industries (see Figure 1).3 However, many of these processes have proved uneconomical, and research in this field continues.4-12
Meanwhile, a different approach may hold the answer: using the red mud as a raw material in the manufacture of brick and structural clay products.
Table 2. DTA analysis results of the red mud sample.*
What About Quality?
Quality is a logical question when using a waste product to manufacture other products-one that several researchers have endeavored to answer. Studies examining the mechanical properties of structural brick and clay products made with red mud had three different starting points in terms of the raw material used. In one group of studies, red mud was used with no additives or extraction from the as-received state.6 A firing temperature range between 1000-1100°C was used. This method generally gave higher porosity and lower strength values compared to the other methods.
A greater number of studies used red mud that contained mineral additions, such as sand, calcium carbonate or fly ash;7 slag, volcanic ash, refractory clay, brick clay, amorphous silicic acid or boric acid;8 caolinitic clay;9,10 or serpantine, wolastonite, silimanite, zirconia, fosterite, ortoclas or anortite.11 In these studies, a somewhat wider firing temperature range, between 950-1400°C, was used, and considerably higher strength and lower porosity values were achieved.
The third approach to manufacturing construction materials from red mud required the extraction of Na2O from the raw material before it was processed. This regeneration treatment enhanced the resulting mechanical properties and gave the lowest water absorption ratios.12
However, adding and extracting minerals adds to the cost of the material, making it less desirable for use in brick manufacturing. For this reason, some of the researchers decided to try to improve upon the first method, using the red mud without additives or extractions.
Table 3. Grain size distribution of the red mud sample.
Experiments: Round 1
The Etibank Aluminum Plant in Seydieshir, Turkey, supplied the red mud samples for the experiments in this study. The samples were taken from the pulp as it came out of the thickener, before it was pumped into the waste storage facility. The chemical analysis of the samples is shown in Table 1.
To remove the organic matter and other undesirable residues, the red mud, in the form of pulp, was "washed"-i.e., diluted with water, sieved through a 45-mm sieve, precipitated, decantated, and finally dried. Bar specimens with dimensions of 5 x 10 x 60 mm were compacted in a laboratory-scale steel die. The samples were molded at room temperature using a uniaxial hydraulic press at a constant pressure of 32 kg/cm2, and were fired in an electric furnace at 950-1150°C, at a heating rate of 300°C per hour.
Figure 2. The mechanical properties of red mud samples fired at various temperatures (molding
pressure: 32 kg/cm2).
The thermal weight loss of the red mud samples was measured using Netzsch DTA-TG (Differential Thermal Analysis/Thermal Gravimetry) and XRD analysis. These numbers are given in Table 2 and Figure 1, respectively.
Figure 3a. SEM micrograph of the red mud fired at 950°C.
A Philips 1050/25 SW X-ray diffraction (XRD) instrument, a Jeol 840 JSM scanning electron microscope (SEM), and a Perkin Elmer 983 infrared spectroscopy (IR) instrument were used to determine the constituent minerals of the red mud samples. Hematite and sodium aluminum hydrosilicate minerals were shown to be the major components of the red mud used in this study. The grain size distribution of the dried samples, determined by Micromeritics Sedigraph 5000D, showed that 94% of the red mud sample was finer than 10 mm (see Table 3).
Figure 3b. SEM micrograph of the red mud fired at 1050°C .
Mechanical tests were carried out using an Instron 1115 test machine. The load during the compression tests was applied onto the larger surface of the samples, with the dimensions of 5 x 10 x 30 mm. The mechanical properties and water absorption ratios of the samples are shown in Figure 2. As seen from this figure, a noticeable increase in the mechanical properties occurs around the firing temperature of 1050°C.
Figure 3c. SEM micrograph of the red mud fired at 1150°C.
As seen in Table 1, red mud contains oxides, such as Na2O, SiO2, CaO. These oxides form a glassy phase after sintering-a property that can be exploited to develop strong ceramics. The high amorphous content of red mud tends to reduce this strength, but in these experiments, the amorphous hematite was crystallized in the heating and cooling cycles, rendering it harmless.4
Figure 4. X-ray diffraction of the red mud sample fired at 1000µC (a) and 1150µC (b).
The microstructural evolution can be followed through the SEM micrographs given in Figure 3. In each micrograph pair, the one on the left is a secondary electron image, and the one on the right is a back-scattered electron image representing the compositional variation. These electron images show that sintering and the formation of the glassy phase take place more effectively at 1150°C, confirming the findings of the mechanical tests. The ultrafine particles seen in the micrograph of the sample fired at 950°C are no longer present in the sample fired at 1150°C.
Figure 5. The mechanical properties of red mud samples molded at various pressures (firing temperature: 1150µC). (a) Variation of the compressive and three-point bending strengths with the molding pressure. (b) Variation of the water absorption ratio, porosity and density with the molding pressure.
A more distinctive compositional image was also obtained from the sample fired at a higher temperature, as the smearing effect of the ultrafine particles have been removed (i.e., regions of different contrast are better delineated). The glassy phase was thought to be nepheline, as its presence was revealed by X-ray diffraction (see Figure 4).
Figure 6a. SEM micrograph of the red mud samples at 20 kg/cm2.
Experiments: Round 2
In the second set of experiments, the red mud samples were molded at various pressures (20 to 60 kg/cm2) and were fired at 1150°C, the temperature that gave the highest strength values in the previous set of experiments. The mechanical properties and water absorption ratios of these samples are shown in Figure 5.
Figure 6. SEM micrographs of the red mud samples at 40 kg/cm2.
SEM micrographs of the samples pressed at 20, 40 and 60 kg/cm2 are given in Figure 6. Again, the back-scattered and secondary electron images are given in the first and the second halves of each photograph, respectively. It appears from the figures that increasing the pressure does not change the compositional images of the samples; that is, compositionally similar particles do not necessarily agglomerate together. As expected, however, higher pressure gave lower porosity and higher strengths due to better packing of the particles, and also enabled the glassy phase formed during firing to bind the particles more effectively.
Figure 6. SEM micrographs of the red mud samples at 60 kg/cm2.
In the experiments with washed red mud, the compression strength was found to increase as the firing temperature and the molding pressure increased. For instance, 49I6 MPa compressive strength and about 9% water absorption ratio were achieved when 60 kg/cm2 molding pressure and 1150°C firing temperature were used.
In many applications, a maximum 18% water absorption ratio is required for porous and non-porous bricks. When molding pressures higher than 30 kg/cm2 are used, water absorption values generally lower than 18% have been achieved with washed red mud.
With the correct molding pressure and firing temperature, red mud-a waste product-can be successfully used to make quality bricks. These experiments showed that the bricks made from washed red mud have higher strength values than red mud that has not undergone the washing process. To reduce costs even further, it is possible to make bricks using the red mud as-received, without washing. In either case, the achievable compressive strength levels are higher than those in the related Turkish standards of bricks (on average 5-15 MPa and 30 MPa, respectively13). Moreover, the water absorption ratios are lower (18%) than those given in the related Turkish standards.13
It may also be possible to make high-strength ceramic materials from red mud. However, further research is needed on this issue.
For More Information
For more information about using red mud to manufacture bricks, contact Mustafa Kara at the Tubitak-Marmara Research Center, P.O. Box 21, TR-41470, Gebze-Kocaeli, Turkey; phone: (90) 262-6412300; fax: (90) 262-6412309; e-mail email@example.com.