Activated alumina is a common adsorbent in the market, and its most common functions are dehydration and drying. But today I will talk about another main function of activated alumina: Activated alumina is used for fluorine removal.
At present, one of the treatment methods for fluorine is adsorption filtration, and the commonly used adsorbents for this filtration method are: activated alumina, activated carbon, aluminum hydroxide etc.
In villages and towns in China, many residents drink high-fluoride water for a long time, some of which can cause dental plaque and enamel, and in severe cases, affect bone development. Drinking high fluoride water for a long time will lead to fluorosis and loss of labor. In response to this phenomenon, relevant departments have taken relevant measures one after another.
At home and abroad, activated alumina is used as the adsorbent for fluorine removal. Because activated alumina has a larger area than ordinary adsorbents, it has ion communication in water, the filter material is not hardened, and it has the advantages of strong adsorption capacity. We use the adsorption filtration method with activated alumina as the adsorbent and aluminum sulfate as the regenerator to defluoride the water supply, which has been explored for many years in this regard.
Add acid or CO2 to the high-fluorine water, and after the pH is reduced to 5.5-7.0, it is pumped into the bottom of the defluorination tank by the high-fluorine water pump, and flows through the activated alumina moving bed from bottom to top. When the raw water flows through the natural mineral filter material of the fluoride removal tank, the fluoride ions are removed through adsorption and ion communication. After activated alumina is activated and regenerated by 3%-5% aluminum sulfate solution or 1% sodium hydroxide solution, it has a good effect of adsorbing fluoride ions in water.
In the fixed bed defluorination equipment commonly used in the past, the fluorine attachment capacity of the activated alumina filter bed is the uniform adsorption fluorine capacity of the filter bed rather than the maximum adsorption fluorine capacity. This installation adopts the moving bed process. Whenever the fluorine content of the effluent reaches the maximum control limit, the activated alumina of one-eighth thickness of the inlet surface is taken out and moved to the regenerator for regeneration. Therefore, the fluorine capacity of this part of activated alumina is is the maximum fluorine absorption capacity in the filter bed. The main purpose of moving bed fluoride removal is the water production of advanced equipment and the fluorine capacity of activated alumina.
When the fluorine content in the effluent of the defluorination tank reaches 1.2 mg/L, the activated alumina stored in the regenerator is first sent back to the upper part of the defluorination tank through the feeding system, and then the lifting system is used to absorb fluorine at the bottom of the defluorination tank. The amount of activated alumina that is saturated is lifted to the regenerator to stop the regeneration. After the regenerated activated alumina is neutralized or washed with low-fluoride water, it is temporarily stored in the regenerator and is sent back to the defluorination tank before the next regeneration to continue the defluorination work. Of course, if activated alumina reaches adsorption saturation, or some indicators are affected, it is necessary to regenerate or replace the medium regularly.
The above process uses activated alumina as an anion exchanger in an acidic solution, which has great selectivity to fluorine.
1. Activated alumina can be activated with aluminum sulfate solution before use to convert it into sulfate type. The reaction is as follows:
(Al2O3)n. 2H2O + SO42- →(Al2O3)n.H2SO4 + 2OH-
2. The reaction during defluorination is:
(Al2O3)n.H2SO4 + 2F - → (Al2O3)n.2HF + SO42-
3. After the activated alumina loses its ability to remove fluorine, it can be regenerated with an aluminum sulfate solution of 1%-2%:
(Al2O3)n.2HF + SO42- →(Al2O3)n.H2SO4 + 2F-
The weight of fluorine that can be adsorbed per gram of activated alumina is generally 1.2 to 4.5 mg, which depends on: the fluorine concentration of raw water, pH value, particle size of activated alumina, etc.
Jiangxi OIM Chemical Co., Ltd has been focusing on industrial adsorbents and ceramic balls such as activated alumina, molecular sieve, and silica gel for many years, providing high-quality adsorbents and services for domestic and foreign enterprises. If you would like to know more about adsorbents, please feel free to contact us.
Tracy Chen 
