As the global population expands and water quality issues continue to make headlines, never has it been more important to conserve water resources to our fullest ability, and to ensure that the water available to us is clean and contaminant-free.
Simultaneously, researchers and scientists everywhere are looking for sustainable alternatives for sources of major industrial wastes. One issue that continues to garner investigation is the treatment of red mud, a caustic process byproduct produced in the extraction of alumina from bauxite in the Bayer process.
As research continues to move forward, some experts think the toxic sludge may actually be able to aid in some water treatment efforts, solving two major sustainability problems at once.
Why Red Mud?
The use of red mud as an adsorbent is being investigated for a number of reasons:
Red Mud is an Industry Problem and Environmental Risk
Red mud, or bauxite residue as it is also known, has long been a problem for the aluminum industry; produced in staggering amounts, the management of red mud not only incurs substantial waste management costs, but its caustic nature makes it an environmental risk and a long-term liability.
News of containment ponds failing and dousing surrounding areas in caustic sludge have prompted a push for improved management of the waste. Concern over red mud is further exacerbated by the anticipated increase in demand for aluminum that is expected to come along with efforts to move toward a low-carbon economy.
Red Mud is Widely Available
While the massive amounts in which red mud is produced make it a waste management problem, this also makes it a widely available, easily accessible, low-cost feedstock for potential reuse applications.
More than ever, efforts to create a more sustainable life on Earth have prompted researchers to look for ways to repurpose waste streams and red mud is no exception; among other investigated waste streams are fly ash, FGD gypsum (synthetic gypsum), and manure.
Increasing Need for Improved Water Treatment
Water treatment initiatives have never been more important. A booming population has put an increasing strain on the commodity as the need for water in industrial processes, agriculture, and for drinking continues to grow.
Additionally, recent water contamination has shone a spotlight on drinking water quality, further emphasizing the need for improved water treatment.
Red Mud Boasts Adsorbent Characteristics
Red mud is largely comprised of oxides of Iron (Fe), Aluminum (Al), and Titanium (Ti). This, in combination with its texture, promote a high surface reactivity¹.
High surface reactivity is a critical element of an effective adsorbent in any setting.
Red Mud as an Adsorbent
With some treatment, red mud has been shown to be an effective adsorbent for a range of materials. Among the most commonly studied applications for red mud as an adsorbent are:
Heavy Metals
Heavy metals are a widespread and challenging water contaminant, capable of damaging the environment, animals, and human health. Sources of heavy metal in water can be a result of anthropogenic factors, such as effluent from industrial facilities, or it may occur naturally.
The contamination of water with heavy metals is difficult to treat, and as such, the search for effective and economic treatment methods is ongoing. Red mud has shown a promising affinity for the removal of heavy metal ions from water, prompting a growing body of research around this increasingly important application. Among the heavy metals commonly examined are:
Arsenic (As)
The dangers of arsenic to human health are well established and capable of causing a number of serious health issues. In addition to anthropogenic sources of arsenic in water, arsenic occurs naturally in soils and rock formations, subsequently allowing it to contaminate groundwater resources.
In the United States, several regional areas are recognized as hosting bedrock that naturally contains unusually high levels of arsenic. Globally, arsenic in groundwater is recognized as a problem in a number of countries as well.
Lead (Pb)
Lead is also a common water contaminant that poses dangers to human health. It is frequently found in water delivery system components such as pipes and solder, and can also be found in the effluent of some industrial processes.
Chromium
Chromium, mainly in the form of Hexavalent chromium (chromium-6), is also a common water contaminant and a known carcinogen. A recent report found that 232 million Americans, spanning all 50 states, are drinking water contaminated with chromium-6.
Phosphates
Phosphorus is a critical nutrient for all living organisms and is a key ingredient in the food security challenge amidst a growing global population. Too much phosphorus, however, can be detrimental. Coupled with diminishing economically accessible phosphate reserves, scientists are worried about the future of this key nutrient.
At the same time, valuable sources of phosphorus are going to waste, causing devastating dead zones in waterways around the world. This has experts looking at how to balance the phosphorus scales. One of the methods studied most, is the recovery of phosphorus from wastewater.
While various treatment methods exist for the recovery of phosphorus from wastewater, a reliable and economically feasible method has continued to elude the industry. Low-cost adsorbents, however, might just hold the key.
One recent study found red mud, among other industrial wastes, to have a higher adsorption capacity for phosphorus compared to zeolite and diatomite, as a result of higher amounts of iron, aluminum, and calcium in the waste.
The recovery of phosphorus from wastewater and other sources could open the door to phosphorus recycling endeavors, creating a much-needed, more balanced nutrient cycle.
Formaldehyde Removal
While not a water treatment application, the removal of formaldehyde from air via red mud adsorption has also been studied. Formaldehyde is a very common air pollutant and the industry is in search of an effective, low-cost adsorbent to address it.
A study in China found red mud modified with H2O2 and then calcined could be used as an effective adsorbent for formaldehyde, with calcination temperature having a significant impact on its effectiveness.
Treatment of Red Mud for Use as an Adsorbent
Treatment serves to “activate” the red mud, improving its characteristics as an adsorbent. As such, red mud is often referred to as activated red mud upon treatment. Prior to treatment, red mud typically goes through a pre-treatment step, which may involve washing, drying, crushing, grinding, and filtering.
Various methods have been examined for the treatment of red mud for use as an adsorbent, with neutralization sometimes preceding treatment. Perhaps the two most common approaches are acid treatment and heat treatment, or a combination of the two, though research ongoing.
Acid Treatment
Acid treatment, or chemical treatment, involves treating the red mud with an acid, often HCl.
Heat Treatment
Heat treatment of red mud involves thermally processing the red mud, typically via calcination.
Both heat and acid treatment have shown to improve the adsorption capabilities of the bauxite residue.2 Source material and treatment parameters are likely to have a significant effect on the effectiveness of the resulting adsorbent.
Red Mud Process & Product Development
The use of red mud as an adsorbent continues to be studied, with a variety of processes and objectives being researched. Testing facilities such as the FEECO Innovation Center, which offers batch- and pilot-scale testing for the thermal treatment of red mud, have grown to be in high demand.
According to Alex Ebben, FEECO Process Sales Engineer and thermal processing expert, “There is a huge push to find a better solution for red mud and producers are looking for facilities that can simulate the entire pyro-processing flowsheet and provide extensive data collection. People are excited when they hear that we can offer the testing capabilities they’re looking for. We can test the thermal treatment of red mud for its use in construction products, metal recovery applications, use as an adsorbent, and more, so we’re really seeing a wide range of inquiries.”
Conclusion
Red mud, a process by-product of the Bayer process, continues to be an industry challenge and an environmental risk. Paired with the increasing need for higher quality water, a growing body of research around the use of red mud as an adsorbent for water treatment – an endeavor that would be a major environmental win – can be seen.
The ability to reuse red mud in efforts to fight contaminated water would be especially valuable in regions that struggle with water contamination and are host to a substantial bauxite and alumina industry, providing a convenient outlet for the massive amounts of waste they must manage.
FEECO provides a range of comprehensive testing services around bauxite, alumina, and red mud. In addition, FEECO can assist in process scale-up and even manufacture a custom rotary kiln engineered around the process data gathered to achieve optimal results. For more information on our red mud processing capabilities, contact us today!
Sources:
López, E., et al. “ADSORBENT PROPERTIES OF RED MUD AND ITS USE FOR WASTEWATER TREATMENT.” Wat. Res., vol. 32, no. 4, 1998, pp. 1314–1322.