February 19, 2020

Process characteristics of zeolite

(1) Ion exchange performance Ion exchangeability is one of the important properties of zeolite rock. In the cavity (cavity) in the zeolite lattice, the cations and water molecules such as K, Na, Ca, etc. are not tightly bound to the grid, and it is easy to exchange cations with the surrounding aqueous solution. The lattice structure of the exchanged zeolite is also Not destroyed. It is verified that the total exchange capacity of domestic clinoptilolites for NH4+ ions varies from 50 to 220 mmol/100 g, and the total exchange capacity for K+ ions is generally 9-26 mg/g, and very few are below 9 mg/g. The total exchange capacity of mordenite rocks for NH4+ ions is generally 50~188.73 mmol/100g, and the total exchange capacity of K+ ions is generally 1~9mg/g, and very few are above 9mg/g. Therefore, the NH4+ ion exchange capacity of both the clinoptilolite and the mordenite rock is higher, but the K+ ion exchange capacity of the mordenite rock is much lower than that of the clinoptilolite, which is determined by its internal structure and other characteristics.
According to some domestic and foreign literature, clinoptilolite and mordenite selectivity cation exchange order are: â‘  alkali metal exchange sequence Cs +1> Rb +1> NH 4+
> K +1> Na +1> The Li +1 2 alkaline earth metal exchange sequence is Ba +2 >Sr +2 >Ca +2 >Mg +2 .
The clinoptilolite can selectively exchange some heavy metals, and the order of Na is Ba>Pb>Cd>Zn>Cu, which can be used to remove heavy metal ions.
The selective exchange capacity of clinoptilolite for Cs and Sr allows it to be used to remove Cs 137 and Sr 90 from radioactive waste. The selective exchange capacity of Cs 137 in the Shishikou clinoptilolite in Hebei Province is 0.15~0.25mg/g, and it can separate and recover Cs 137 with a purity of 99.5%.
Na+ in the analcime is easily exchanged by Ag, Ti, Pb or the like. The Shifang Commune of Jilin Province, the Late Jurassic saline-alkali volcanic clastic rock analcime, with an analcite content of 30-50%, a pore size of 2.6 Ã…, smaller than the diameter of NH4+ ions (NH 4 + ion diameter 2.86 Ã…), Therefore, the NH4+ exchange capacity is very low (only 10~30mmol/100g). However, Cu, Cr, Cd, etc. having an ion diameter of <2.60 Ã… have a high cation exchange capacity. It is reported that Japan has used Fang zeolite to remove Cu 2+ , Cd 2+ and the like. [next]
(2) Adsorption performance Zeolite has a large specific surface area (500-1000 m 2 /g) and thus can generate a large diffusion force, so it can be used as an excellent adsorbent. There are many holes and channels of uniform size inside the zeolite lattice, and the holes are connected to each other through the open channels and communicate with the outside world. The volume of the pores and channels is more than 50% of the volume of the zeolite crystal, in which there is a lot of desorbed free zeolite water. The amount of zeolite water can vary with changes in ambient temperature and humidity.
The pores and channels inside the zeolite have a precise and fixed diameter (about 3~11Ã…) under certain physical and chemical conditions. Different kinds of zeolites have different diameters, and substances smaller than this diameter can be adsorbed by them. Substances larger than this diameter are excluded. This phenomenon is known as the "molecular sieve" effect. However, not all zeolites can function as molecular sieves. Some zeolite pores are too small to function as sieving molecules. Zeolite not only has the property of adsorbing water, but also has the properties of adsorbing calcium oxide, SO 2 , F 2 , nitrogen, ammonium, methanol and adsorbing radioactive substances. Taking Zhejiang yunyun mordenite as an example, the adsorbed calcium oxide is 150.01 mg/g. The amount of ammonium absorbed was 177.47mmol/100g, SO 2 was 120mg/g, the nitrogen adsorption capacity (STP) was 12.93~17.11 mg/g, and methanol was 55.4 mg/g.
(3) Catalytic performance Since zeolite has a large adsorption surface and can accommodate a considerable amount of adsorbed substances, it can promote chemical reactions on its surface, so zeolite acts as an effective catalyst and catalytic carrier. Further there is an aluminosilicate zeolite framework charge, and a charge balancing ion, having a local high electric field and produce acid position grid frame, which can be used to accelerate the reaction of carbon ionic. In addition, it can exchange catalytically active metals (such as Pt, Pa, etc.) to maximize dispersion, maintain high activity and reduce the amount of precious metals. [next]
(4) Thermal stability The thermal stability of zeolite rock is related to the type of cations contained in the zeolite rock, the silica-alumina ratio of the zeolite, and the internal structure of the zeolite. In terms of thermal stability, general mordenite is superior to clinoptilolite and analcime, and potassium or sodium type zeolite is superior to calcareous or potassium-calcium type clinoptilolite (China's clinoptilolite belongs to the latter), high silica zeolite Better than low silica zeolite (China's zeolite belongs to high silica zeolite). The thermal stability of domestic zeolite rocks varies from place to place. After the continuous heating of 650 °C for 12 hours, the original structure of the tiger head clinoptilolite in Jinyun County, Zhejiang Province was maintained, but the structure was completely destroyed when heated to 800 °C. However, the zeolite rock in the paddock of Hebei Province was heated to 350-450 °C, and the crystal structure of the zeolite was destroyed. The zeolite rock of Heshikou, Heshikou, Hebei Province was heated to 600-700 °C (insulation for 2 hours), and the crystal structure of the zeolite was completely destroyed.
(5) Acid-resistant zeolite has good acid resistance. For example, the strains of clinoptilolite and mordenite in Shandong Province are treated with different concentrations of hydrochloric acid at 100 ° C for 2 hours. The test proves that both have strong acid resistance, among which The zeolite was treated in 4N hydrochloric acid, and the mordenite was treated in 10 mol/L hydrochloric acid, and the crystal structure was not destroyed.
The isoflavite rock of Xintian Ladder in Henan Province was immersed in 4 mol/L hydrochloric acid for 24 hours (normal temperature), and the crystal structure was not destroyed by X-ray diffractometry.
Zeolites also have process properties such as chemical reactivity, far infrared radiation, and reversible dehydration.

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