- THE MAGAZINE
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.
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.
Experiments: Round 1The 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.
Experiments: Round 2In 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.
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.
High-Strength BricksWith 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.