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① Preparation method
At present, there are 7 main methods for preparing molecular sieves from fly ash
.
a.
Traditional hydrothermal synthesis method The traditional hydrothermal synthesis method is to mix fly ash with a certain concentration of lye, and adjust the reaction conditions (liquid-solid ratio, silicon-aluminum ratio, stirring speed, reaction temperature, reaction time, etc.
), and then The reaction is carried out in a glass or stainless steel reactor with a polytetrafluoroethylene lining by self-increasing pressure, or in an open system to synthesize different types of zeolites
.
b.
Two-step hydrothermal synthesis method In order to solve the problem of low purity of the one-step synthesis product, Hollman et al.
proposed a two-step synthesis method
.
In the experiment, 500g fly ash was mixed with 2mol/L NaOH and kept at 90℃ for 6 hours.
c.
Microwave-assisted synthesis method The microwave-assisted synthesis method is similar to the above-mentioned traditional hydrothermal synthesis method, except that there is microwave assistance during crystallization, which can increase the reaction speed and greatly shorten the synthesis time
.
d.
The seeding method prepares the zeolite seed crystals to be synthesized according to the ratio, and then mixes an appropriate amount of the seed crystals with fly ash and alkali source to crystallize at a lower temperature to obtain the zeolite
.
In this method, the quartz and mullite in fly ash cannot be completely converted
e.
Alkali-dissolving method.
Traditional hydrothermal synthesis method.
The crystals of quartz and mullite in fly ash are difficult to dissolve in alkaline solution.
In order to improve the yield and purity of the product, Shigemoto et al.
proposed and used water in the experiment.
The method of introducing alkali fusion before the thermal reaction
.
A certain amount of NaOH, sodium aluminate and 10g fly ash are mixed, heated to 773K in a platinum crucible, kept at constant temperature for 1h, the mixture is cooled to room temperature, grind, add 100mL distilled water, mix and stir for 12h, put in a container and react at 373K for 6h , Synthetic zeolite is mainly crystalline phase NaX, the content is as high as 62%
f.
Salt-heat (melt-salt) synthesis method.
In the above-mentioned synthesis method, it is found that water is required as a reaction reagent in the synthesis process, and a higher liquid-solid ratio is required, so the waste liquid treatment problem is inevitably generated
.
In order to improve this situation, Park et al.
g.
Related links: physicochemical properties and preparation methods of zeolite (3)
