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Activated alumina defluorination principle

2021-02-05
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Activated alumina has good physical properties, high strength, non-toxic, and odorless. The specific surface area of about 320m2/g enables the activated alumina to have a large contact area and thus has a good ion exchange capacity. The pore volume is above 0.4cm3/g. It has a high adsorption capacity, because it is easy to regenerate, has a long life, and is relatively cheap. It is an effective, economical and practical method among various fluoride removal methods. Then it is necessary to understand the principle of activated alumina defluorination. The removal of fluoride ions by activated alumina is divided into two functions, adsorption and ion exchange. Finally, we summarize the process parameters of activated alumina defluorination.


1. The adsorption principle of activated alumina


Activated alumina fluoride removal agent is originally a kind of adsorption material. High specific surface area and high pore volume are the basic characteristics of adsorption materials. Activated alumina has a certain role in adsorbing impurities. In fact, activated alumina can be divided into macropores (100~500A) , Mesopores (40~100A), micropores (<40A), with pore size distribution ranging from 10A to 500A. Activated alumina has a certain adsorption capacity for molecules smaller than its own pore size. The fluorine in water mainly exists in an ionic state. The fluorine removal process mainly removes the ionic fluorine from the water. The adsorption of activated alumina accounts for a small part, and the most important thing is the ion exchange capacity of activated alumina.


The chemical formula for physical adsorption and ion exchange of activated alumina:


Al2O3·Al2(SO4)3·XH2O+6F-=Al2O3·2AlF3·XH2O+3SO42-


2. The ion exchange principle of activated alumina


1. The fluorine contained in drinking water mainly exists in an ionic state, and its solubility in water is far greater than the allowable fluorine content (1mg/L). Therefore, the fluorine removal process is mainly to remove fluorine in the ion state from the water. Activated alumina is a typical amphoteric compound. Under acidic conditions, activated alumina is an anion exchanger, and its order of selecting anions in water is: OH->PO43->F->SO42->Cl-.


2. Activated alumina regenerated with aluminum sulfate is in the process of removing fluoride. However, the fact that the number of milliequivalents of SO42- ions exchanged and the sum of milliequivalents of F- and HCO3- ions removed are roughly equal, which shows that the principle of activated alumina defluorination is mainly an ion exchange mechanism. The activated alumina is treated with aluminum sulfate or sulfuric acid to convert it into R2SO4 type. According to the ion exchange sequence, the F-ion in the water is reliably exchanged for SO42- ions from the R2SO4 of the activated alumina, thereby removing the F in the water. -ion.


3. In the process of fluoride removal, the cations in the water were not removed together with F- ions. In the experiment, the following formula was established: 2NaF+R2SO4⇄2RF+Na2SO4. This shows that using activated alumina to remove fluorine, its adsorption performance is secondary. Using activated alumina R2SO4 type ion exchanger to remove fluorine, the following reaction formula is established:


R2SO4+2F-⇄2RF+SO42-


R2SO4+2HCO3-⇄2RHCO3+SO42-


4. After the activated alumina is exchanged and saturated, it is regenerated with aluminum sulfate. The regeneration process relies on the high concentration of the regenerant to overcome the selectivity, and the above reaction formula proceeds to the left, so that the activated alumina is regenerated. It can also be regenerated with 1% NaOH. After regeneration, the filter layer is alkaline and must be acidified to make the filter layer acidic to remove F-ions in the water.


5. Experiments have proved that reducing the pH value of the raw water to about 5.5 is a fluorine removal environment. Most foreign fluoride removal devices operate by adjusting the pH value of raw water to between 5.5 and 6.5. Not only the defluoridation effect is good, it is also relatively economical. In order to save the cost of the acidification process in China, the pH value of the raw water is usually lowered slightly, generally controlled at 6.5 to 7.0.


Fluoride removal process of groundwater activated alumina


3. Fluoride removal process parameters


1. The adsorption exchange capacity of activated alumina indicates the ability of activated alumina to adsorb and exchange fluoride ions in water. Generally, the number of milligrams of fluoride ion in the water per gram of activated alumina is adsorbed and exchanged (mgF-/g activated alumina). The adsorption and exchange capacity of activated alumina is affected by the fluorine content of the raw water, the pH value and alkalinity of the raw water, the filter contact time, Activated alumina filter media particle size and regeneration conditions affect. It is safest to use activated alumina's fluorine removal capacity of 1~2mg/g as its design work exchange capacity in the design.


2. In actual use, the particle size of the activated alumina filter material is considered to be 0.5~2mm, and the unevenness coefficient of the filter material K≤2. If the particle size is too large, the surface area of a certain amount of filter material is too small, which will affect the adsorption and exchange capacity; if the particle size is too small, although it can increase the surface area of the filter material and increase the adsorption and exchange capacity, it is easy to wear and break during operation, and the loss is large. .


4. The thickness of the activated alumina filter layer is generally 1 meter to 1.5 meters. If the thickness of the material layer is too small, the filter contact time will be affected, and the water flow will also be easily short-circuited during filtration.


5. Filter contact time When filtering, the contact time between raw water and activated alumina filter material has a great influence on the exchange capacity of the filter material. The longer the contact time, the greater the adsorption exchange capacity. The contact time is generally 20-30 minutes. Appropriate.


6. The regeneration cycle can take about 10 days to determine the amount of filter material.


The principle of fluorine removal by activated alumina is generally considered to be the dual role of fluorine ion exchange and physical adsorption, of which ion exchange plays an important role.

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