What is Activated Alumina?
You might’ve heard of this term about activated alumina, or might even be familiar with the term. But this activated alumina might have benefited you a number of times over the years. It might be sitting right in the water filter under your sink.
So, what is activated alumina?
Activated alumina is manufactured from aluminum hydroxide by dehydroxylation in a way which makes it a highly porous material. This material has a surface area of around 200 m2/g. This compound is used as a desiccant in many industries. The aluminum oxide in this particular form is so porous that it is actually “tunnelled” at the microscopic level. This quality makes activated alumina useful for different applications.
The most important thing to know about activated alumina is that it adsorbs, not absorbs. For example, A regular sponge will absorb water but the water molecules won’t bond with the sponge and so, the water can leak or get evaporated after a while.
On the other hand, the moisture or water that comes in contact with activated alumina will actually bond with the solid material. Once it bonds, it cannot be separated without a considerable amount of heating stimuli. Activated alumina can absorb gases or liquids without changing their forms. As air passes through alumina, it will be dried out because the water molecules will bond to the activated alumina. The original absorption capacity of activated alumina can be restored by heating it to a temperature ranging from 350° to 600°F. When the activated alumina desiccant is heated, it will be restored. That means it can be used again and again.
Activated alumina has a high affinity for water compared to any other desiccant material used. That makes it useful for applications where a lot of moisture needs to be dealt with. It’s high crust strength also allows it to withstand the high pressure of industrial applications.
The only con of activated alumina is that unlike molecular sieves, the size of pores present in activated alumina cannot be tailored and they are not uniform.
Production of Activated Alumina:
Mostly activated alumina starts as aluminum hydroxide. It is obtained through a series of chemical reactions in the bayer process which is used to transform bauxite into alumina.
Activated Alumina Calcination:
After the aluminum hydroxide is generated, it is thermally treated by calcination in a rotatory assembly known as a kiln. This step serves to dehydrate or remove the bond mixture from aluminum hydroxide in order to produce the thing we are looking for, alumina or aluminum oxide. This is a phase in which alumina obtains its highly porous structure, which is obtained by heating it to a specific temperature specific range. Process parameters such as residence time and temperature profiles are used to control the properties of end results.
Activated Alumina Agglomeration:
Depending on what the activated alumina is going to be used for, the process of agglomeration is varied. This process is especially important when activated alumina is used as an adsorbent or catalyst. The agglomeration of activated alumina allows for a high level of customization for specific applications.
The basic characteristics that are controlled or customized using this process are:
- The particle size distribution of the molecules
- Bulk density
- Crush strength
- Amount of attraction/potential for dust generation
There are various ways to create activated alumina beads, as they are mostly known. They consist of agglomerator, pin mixer, disc pelletizer, or a combination of these various techniques.
Applications of Activated Alumina:
1. As Catalyst:
Activated alumina is used for a wide range of applications.
It includes adsorption of catalyst in polyethene production and in hydrogen peroxide production. It is also used as a selective adsorbent for many chemicals like fluoride, arsenic, and sulfur removal from fluid streams which is also known as Claus catalyst processing.
2. As Desiccant
As a desiccant, it works on a process known as adsorption. The water in the air sticks to the structure of alumina and will pass out a dried stream of gas. This process is also reversible.
The desiccant is heated to a particular temperature of around 200°C. This process will release the trapped water in the stream. This process is called regenerating the desiccant.
3. As Fluoride Absorbent:
Activated alumina is widely used to remove fluoride from the water.
In the United States, there are programs to add fluoride in the drinking water. But in some regions, for example, Rajasthan in India, the fluoride level in water which can lead to a condition known as fluorosis, which is a bone-related disease caused by the excess level of fluoride in the body.
The activated alumina fibers can easily reduce the level of fluoride in water from 10 ppm to 1 ppm. The amount of fluoride filtered by activated alumina depends on the time for which the water is in contact with activated alumina. So, if the activated alumina is in the water for a long time it’ll reduce the level of fluoride in the water in the final, filtered water.
Water at lower temperatures and lower pH levels is filtered more effectively. Ideal pH level is around 5.5, which will provide around 95% of fluoride removal.
4. In vacuum systems:
It is used in high vacuum applications.
Activated alumina is used as a change in the material in fore-line traps for preventing the oil generated by rotary vane pumps from the back stream in the systems.
Activated alumina has a very specific mechanical property and non-reactivity property.
These properties allow it to be an ideal material to cover surfaces in friction in the body. For example, hip to shoulder prostheses.
Defluoridation is a process of a down adjusting the level of fluoride present in water. Activated alumina is the most widely used method for defluoridation of drinking water. Due to the capabilities of activated alumina as adsorbent, catalyst, desiccant and much more, it is a very important tool in industrial process settings.
Activated alumina is highly customizable and is often agglomerated to improve performance and handling characteristics. Feasibility testing is often an important aspect while working with activated alumina products.
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