Lithium type low silicon aluminum X-type zeolite molecular sieve (LSX) is a widely used air separation agent. It has the advantages of large nitrogen adsorption capacity, high helium oxygen separation coefficient, and easy desorption. In processes such as pressure swing adsorption separation (PSA) and vacuum pressure swing adsorption separation Leaving (VSA) ? All have applications. The water solution exchange method is in industrial preparation processes. Although this method has the advantages of mild exchange conditions and easy operation, there are several issues during the exchange process. It includes the high salt utilization rate (about 12%) in the exchange solution, difficulty in achieving high exchange degree, and difficulty in lithium heat recovery. In view of this, this paper proposes a new method of combining aqueous solution exchange and high-temperature solid-phase exchange. It not only improves the exchange degree of lithium ions (969), but also reduces the amount of lithium salt used. At the same time, the process is simple and easy to achieve industrialization.
The conclusion of the paper for lix molecular sieve
The main conclusions of this paper are as follows:
The optimal exchange conditions for aqueous solution exchange are: under the condition of a water bath at 90 ℃, the exchange concentration of LiC solution is 0.4mol/L. Whats more, the exchange frequency is 4 times, and the exchange time for each exchange is 2 hours. Under this exchange condition, the LSX molecule has a Ru exchange degree of over 96%. This basically meets the requirements for industrial applications. In the investigation of water solution exchange conditions, after two exchanges, the ion exchange degree in the zeolite molecular sieve reached 86.6%. And the lithium ion utilization rates in the third and fourth exchanges were both very low, less than 10%. Using the zeolite molecular storage obtained from two water-soluble wave exchanges as raw material, the solid-state exchange method was to achieve lithium ion exchange at the remaining cation positions. The optimal exchange conditions were as follows. Under the condition of a ratio of 1 to 5 (the ratio of sodium ions in the zeolite molecular sieve to lithium ions in lithium xenon monohydrate), ion exchange was out according to the following procedure. At room temperature, the temperature was up to 120 ℃ at a gradient of 1 ℃ c/min for 120 minutes. Heat up to 200 ℃ at a temperature gradient of 1.33 ℃/min and maintain the temperature for 120 minutes. Then raise the temperature in a gradient of 2.5 ℃/min to 350 ℃, maintain a constant temperature for 180 minutes. And then naturally cool down. On this basis, FT-IR, TG, DTA, XRD, and SEM were to analyze the samples.
The Charateristics of The Structure
The characterization of the structure is as follows:
1. Through FT-IR characterization of the sample, the basic structural units that make up the zeolite molecule did not change. And were still control by S1.0.AI. However, due to the smaller size of lithium ions compared to sodium ions, their influence on the skeleton vibration was slightly different. For example, the peak representing the stretching vibration inside the tetrahedron was 664 and 75 cm. The peaks at 1 and 696, 37cm’1 have shifted to 668, 70cm. A small shoulder peak appeared near 696cm. While the peak representing the external stretching vibration of the tetrahedron shifted from 742.42cm to 746.53cm. The characteristic peak of the double ring vibration changed from 557.48cml to 584.95cm. There are double peaks of varying sizes appearing nearby, etc;
- Through TG and DTA analysis, compared with the results reported in the literature, the high-temperature direction of the dehydration peak shifted. And the water absorption capacity of the zeolite molecules was stronger after exchange. The collapse temperature of the skeleton increased from less than 600 ℃ to around 700 ℃,. Indicating that the thermal stability of the exchanged zeolite molecular sieve has been improve.
The Test Analysis Results
- XRD analysis results showed that after exchange, the characteristic peaks of the molecules generally shifted towards larger angles. It indicates that the interplanar spacing (d value) was smaller. This is due to the small radius of lithium ions, which can fully enter the smaller cages in the zeolite molecular sieve. On the other hand, under the same charge, the smaller ion radius weakens the interaction force between ions and skeleton elements, causing the skeleton to contract and the unit cell to shrink
- Through SEM images, it can be observed that the particle size of the molecules remains unchanged before and after exchange, and the distribution range is relatively narrow (~5pm) with clear contours, indicating that the ion exchange method has little effect on their morphology; 5. Obtained by comparing the water-soluble wave exchange method and solid-phase exchange method The analysis and characterization results of LSX zeolite molecular storage, FT-IR, TG-DTA, XRD, and SEM show that there is no difference in the structural properties of the samples prepared by these two methods.
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