Bauxite residue, the so-called red mud, is a waste product produced out of the aluminium industry. Due to its high sodium content, it is difficult to use unprocessed bauxite residue in the steel industry as a substitute for iron ore. Therefore, a common way until now is to deposit the untreated red mud in various places, although this represents a significant burden on the environment due to the footprint requested, safety surrounding the needs for big reservoirs, and its exceeding limit values on e.g. sodium content.
Focusing on this poor resource exploitation, the industry faces here more and more important market challenges, since the volume of red mud is constantly increasing and the places for landfilling or deposition are exhausted and are reaching their limits.
In order to relieve the landfills (reduce volume), but also to achieve significant cost savings for the operators, improvements on the way how red mud is being handled have become a necessity. For several years already, the de-sanding of the red mud has become a field of investigation and development for AKW Equipment + Process Design process experts.
For the challenging conditions of red mud desanding, special designed hydrocylcones AKA-VORTEX, arranged as primary and secondary stage, followed by a specially designed washing/dewatering screen unit have proven over time to be an effective solution.
An explanation of the installation and operation of such highly specialized desanding unit is presented in the following.
Bauxite residue is a by-product generated from the Bayer process for refining bauxite to alumina, it is highly alkaline with pH around 13. The amount of bauxite residue produced by an alumina plants or refinery is mainly dependent on the bauxite composition (country of origin) and on the extraction conditions. The bauxite residue is highly influenced by different factors such as the aluminum content, the type of aluminum oxide, the type of hydroxide present (especially gypsum, diaspore or boehmite), but also the temperature and the pressure level used in the transformation of the bauxite into aluminium. As a matter of fact, bauxite residue after aluminium production process can vary from 0.5 to as high as 2.5 tons of residue per ton of alumina produced, though typically it lies between 1 and 2 tons of residue per ton of alumina produced.
Since 1893 the Bayer process has been mostly used and there are approximately more than a hundred plants running with this process around the world, with more than 60 plants in China only. Especially there, the number of alumina refineries continues increasing rapidly, to fulfill the rising demand of aluminum that enjoys an average growth rate of 6 %. Therefore, the annual increase of the bauxite residues is expected to reach 120 million tons, to be added to the already existing quantity of old bauxite residue, estimated at nearly three billion tons.
2. Composition of Bauxite Residue
Bauxite has residues consisting of iron oxide, titanium oxide, silica and undissolved alumina. These materials are found mixed with some other oxides depending on the country of origin of the raw bauxite. The high concentration of iron compounds present in the bauxite residues is responsible for the redish colour and therefore for the name “red mud”. In table 1 we get to know the typical chemical composition, and in table 2 we get to know the typical mineral composition.
Table 1. Chemical composition
Table 2. Mineralogical composition
3. Processing of Bauxite Residue
By the red mud settling the contained high amount of caustic should be recovered. The caustic solution would overflow with the resulting supernatant, whereas the thickened red mud is being discharged as underflow. However, such discharges suffer from severe pipe blocking issues due to the settling of still present sand particles in the mud.
The also intended dry stacking of red mud requires the treatment by filter systems, in order to reduce the moisture content significantly. In order to protect these filters and the related pumps against the high wear, caused by the undissolved particles, the de-sanding of the red mud is a significant advantage.
Many patents have been issued and there have also been thousands of experiments done on the various uses of bauxite residue. Appropriate marketing plans have been worked out for these applications, and the biggest challenge is how to match the emerging annual tonnage (about 150 million tons/year) with potential and commercial applications. Several patents filed involve the use of bauxite residues in the construction or agricultural industries. There are also approximately two million tons that are recycled annually within a variety of industries, including refractories, cement, soil improvement, and covering landfills.
4. Overall Process and Integration of AKW A+V Equipment
Bauxite residue slurry generated from the Bayer alumina refineries will be diluted with the liquor process stream and washed in number of washing silos to reduce the soda concentration to the range of 70–80 g/l with 7–10 % solids concentration. The washing process temperature is usually in the range of 70–80 °C. Such an exemplary red mud stream is composed of e. g. approx. 80 % sludge and approx. 20 % undissolved silica particles above 100 µm.
Through a first stage of hydrocyclones the undissolved silica particles in the bauxite residue will be separated and up-concentrated in the underflow, while the overflow is recovered and reinjected into the washing liquor going into the washing silos. A second stage of hydrocyclones will achieve a further up-concentration of the silica fraction, and a separation of the unwanted fines.
The highly concentrated sand slurry can then be discharged onto a special designed linear moving dewatering screen. Suitable screen mesh, appropriate to the occurring conditions, and spray bars for wash water application completes the task of this dewatering screen. Due to the severe process conditions, the hydrocyclones system is made out of high wear and temperature resistant polyurethane. The modular and flawless design of the AKA-VORTEX hydrocyclones makes it perfectly suitable for these process conditions.
Figure 1. Exemplary process flow for test unit scale
Such concept, that has already shown success at specific site, is completed with a fully equipped monitoring and controlling system, allowing to adjust the process flow: flow meters, pressure gauges and pressure regulators, temperature sensors as well as sampling points before and after each stage can be integrated in the solution, based on customers’ requirements or wishes.
Figure 2. Full scale unit – design sketch and executed installation
5. Hydrocyclone AKA-VORTEX Principle and Selection
In principle, the hydrocyclone is a solid bowl centrifuge of “slim” design. It can be compared to a high-speed tubular centrifuge but also with a long-tube decanter. In the case of the hydrocyclone, however, the body is stationary. A rotation is initiated by the flow arising from the tangential feed under pump pressure. The fluid subjected to centrifugal forces, that lead to the creation of a primary vortex, which is directed downwards, and an internal secondary vortex directed upwards. This leads to a segregation effect of the particles presented in the fluid, due to a radially outward sedimentation.
Figure 3. Operation principle / different sections of a hydrocyclone AKA-VORTEX.
The AKA-VORTEX hydrocyclone consists of several replaceable sections connected by means of clamps and flanges
Figure 4. (a) Exemplary installation drawing, (b) 3D visualization, (c) capacity curves (m³/h) vs. ∆p (bar).
6. Selection of Dewatering Screens
Regarding the selection, sizing and functionality of the dewatering screen several aspects need to be considered.
Dewatering screens are generally known for further dewatering of up-concentrated particle slurries to reach a bulky material level that can be easily transported by conveyor belts or also by trucks.
Contrary to the circular moving classification screen, the dewatering screen has a linear movement pattern. The vibrations created generate a material bed where the liquid is drained through the screen mesh and the bulky material is transported to the discharge chute. Specifically for the red mud de-sanding application, a caustic and temperature resistant material for the screen frame as well as for the screen mesh need to be selected. Therefore, stainless steel is the appropriate material to resist against these conditions.
For the handling of the separated sand on the screen a further washing with process water has to be considered. This is aimed at further reducing the caustic content in the sand and it allows to use the sand in several applications. Such a washing concept will be realized by so-called spray bars which attach process water to the surface of the material bed, so the process water can wash the sand by going through it. The number of spray bars equipped with special spray nozzles should be related to the wanted wash effect and the needed water amount. Also, this additional wash water amount needs to be considered by sizing the required screen mesh area in order to reach the requested dewatering effect.
Figure 5. Dewatering screen, discharge of separated and dewatered sand
In this paper, the treatment of bauxite residue (red mud), which is a hazardous waste generated during the Bayer process of alumina production was reviewed. These residues are conserved because of their long-term environmental impact, while at the same time suitable applications for bauxite residues are greatly lacking.
However, this residue can be dealt with in a number of ways, and especially through proper treatment significantly reduced. AKW A+V has gained such knowledge over several years of tight cooperation with major aluminium refineries and was able to develop solutions that offer appropriate treatments for the bauxite value chain, by focusing on the handling of the waste generated and targeting a reduction of the environmental impact and reusability of part of the waste flow. As such, these unique process solutions do contribute to integrating the concept of circular economy into this demanding and growing industry.
The AKA-VORTEX hydrocyclones, made of high wear and temperature resistant polyurethane (PU), are an important addition to the alumina industry. Their unique designs ensure a specific cut size for the separation, while offering the needed flexibility to adjust the separation specificities in order to maximize the overall production yield and minimize the energy demand.
- S.A. Bibanaeva and N.A. Sabirzyanov (2020), Promising Methods for Red Mud Processing, IV Congress “Fundamental research and applied developing of recycling and utilization processes of technogenic formations”
- Zhong, L., Zhang, Y. and Yi, Z. (2009). Extraction of Alumina and Sodium Oxide from Red Mud by a Mild Hydro-Chemical Process. Journal of Hazardous Materials, issue 172(2-3), pp. 1629–