Preparation of activated carbon from rice husk

They are highly complex porous structures, with high values of specific surface area (ssa) and porous sizes mainly in the range of micropores.

Activated carbons are very effective adsorbents due to their textural parameters: ssa and microporous structure.

Activated Carbon

Almost any carbonaceous material, with high carbon content and low proportion of inorganic components, can be used as precursors for the preparation of activated carbons.

The major constituents of rice husk are cellulose, lignin, and mineral components. The content of each of them depends on rice variety, climatic conditions, and even the geographic localization of the culture.

Introduction

When rice husk is subjected to combustion at moderate temperature, an amorphous ash is obtained; a 95% white silica powder could be produced after calcination at 700 °C for 6 h.

Silica production

They are obtained by two different processes: the “physical” or “thermal” activation and the “chemical” activation. In the former carbonization is followed by char activation; in the second one, carbonization and activation are performed in a single step, using a chemical agent.

Preparation of activated carbon

To avoid the negative influence of the raw material ash content, leaching steps are included in the preparation process, either before or after the activation step.

As ashes are mostly constituted by silica, hydrofluoric acid is used as the leaching agent, whose efficiency to dissolve silica is well-known.

Preparation

A sample of rice husk is calcined in a batch type retort like stainless steel reactor , under inert nitrogen atmosphere from room temperature to about 773K and maintained for about 2 hours.

Method-Carbonization

Experimental Scheme and Sample Identification

The sample in the form of powder is heated from room temperature to about 1123K with nitrogen gas flowing at some rate.

When the temperature reaches 1123K the nitrogen is replaced by water vapor which acts as the activating agent.

This treatment is for about 100min.

Activation

Hydrofluoric acid is used at room temperature.The acid solution volume to solid mass ratio is 2.5:1, and

two different concentrations of acid (25% and 50%, w/w) are used.

The slurry is magnetically stirred for 30 min. The solid is separated and washed with distilled water until neutralization, and then oven-dried at 373 K during 16 h.

Leaching of the activated samples (CALx samples) produced solids that presented the highest values of textural parameters.

The leaching process increases the specific surface area of the activated samples by 3.5 or 4 times, for concentration of the acid of 25% or 50% respectively, referenced to CA.

Leaching

From the point of view of the development of the ssa and the VDR, the process sequence carbonization-activation-leaching is more efficient than the sequence carbonization-leaching-activation.

Method

Ultimate Analysis for Rice Husk and Derived Products

Textural Parameters and Yields for Activated CarbonsObtained from Rice Husk

For the activated silica, rice husk samples are pre-and post-treated using HCl, H2SO4 and NaOH solutions. Samples are then incinerated at 600C under static air and owing atmospheres (air, argon and oxygen). The product is characterized in terms of silica content, particle size distribution and morphology, specifc surface area and porosity.

Activated Silica

They are washed with water to remove dirt and other contaminants present in them and then dried in an oven-dried at about 110C for 24 h.

Pre treatment

Acid leaching was performed by reflux boiling in 3% HCl and 10%H2SO4 for about 2 hours.

Alkaline leaching with 3%NaOH at room temperature for 24 Hours.

After leaching the sample is thoroughly washed with distilled water at 110°C.

Chemical treatment

By literature the incineration temp of 600°C is chosen and incinerated in a muffle furnace.

Incineration

First, incineration took place in a porcelain crucible for 4 h in static air.

incineration was performed in a stainless steel tubular reactor under flowing argon and then with oxygen.

Third one in the same way with air flowing.The husks were burned in the same reactor

under owing oxygen.

4 different incineration methods

Reactor used

A silica specimen that was obtained by burning the rice husks at 600C after leaching of this raw material in HCl, presented a homogeneous particle size distribution compared to the other sample preparation techniques.

Conclusion