What is Activated Alumina?
You might have heard of this term about activated alumina or might even be familiar with the term, but this Activated Alumina might have benefited you several times over the years. It could be sitting right in the water filter under your sink. Activated Alumina is manufactured from aluminium hydroxide by dehydroxylation to make 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 aluminium 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 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 the 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 much moisture needs to be dealt with. Its 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 pores present in Activated Alumina cannot be tailored and are not uniform.
Production of Activated Alumina
Most Activated Alumina starts as aluminium hydroxide. It is obtained through a series of chemical reactions in the Bayer process to transform bauxite into alumina.
Activated Alumina Calcination
After the aluminium hydroxide is generated, it is thermally treated by calcination in a rotatory assembly known as a kiln. This step dehydrates or removes the bond mixture from aluminium hydroxide to produce the thing we are looking for, alumina or aluminium oxide. This phase is when 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 the results.
Activated Alumina Agglomeration
Depending on what the Activated Alumina will be used for, the agglomeration process is varied. This process is fundamental when activated alumina is used as an adsorbent or catalyst. The accumulation of Activated Alumina allows for a high level of customisation for specific applications.
The primary characteristics that are controlled or customised 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 an agglomerator, pin mixer, disc pelletizer, or a combination of these various techniques.
Applications of Activated Alumina
Activated Alumina is used for a wide range of applications. It includes the adsorption of catalysts in polyethene production and hydrogen peroxide production. It is also a selective adsorbent for many chemicals like fluoride, arsenic, and sulfur removal from fluid streams, known as Claus catalyst processing.
As a desiccant, it works on a process known as adsorption. The water in the air sticks to the alumina structure 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, called regenerating the desiccant.
Activated Alumina is widely used to remove fluoride from the water. In the United States, there are programs to add fluoride to drinking water. But in some regions, for example, Rajasthan in India, the fluoride level in water can lead to a condition known as fluorosis, a bone-related disease caused by the excess level of fluoride in the body.
Activated Alumina fibres can easily reduce fluoride in water from 10 ppm to 1 ppm. The amount of fluoride filtered by Activated Alumina depends on when the water is in contact with Activated Alumina. If the Activated Alumina is in the water for a long time, it’ll reduce fluoride levels in the final filtered water. Water at lower temperatures and lower pH levels is filtered more effectively. The ideal pH level is around 5.5, which will provide about 95% of fluoride removal.
It is used in high vacuum applications. Activated Alumina changes the material in fore-line traps to prevent the oil generated by rotary vane pumps from the backstream in the systems.
Activated Alumina has particular mechanical property and non-reactivity properties. 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 down-adjusting the level of fluoride present in water. Activated Alumina is the most widely used method for the defluoridation of drinking water. Due to the capabilities of Activated Alumina as an adsorbent, catalyst, desiccant, and much more, it is an essential tool in industrial process settings. Activated Alumina is highly customisable 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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