颜翠平,李 杨,李世龙,严绍文,张明星,李 红

(西南科技大学 环境与资源学院,固体废物处理与资源化教育部重点实验室,四川 绵阳 621000)

引用格式:

DOI:10.13732 / j.issn.1008-5548.2024.03.013

收稿日期:2023-10-30,修回日期:2023-12-14,上线日期:2024-04-16。

基金项目:国家自然科学基金项目,编号:52204286。

第一作者简介:颜翠平(1981—),女,副教授,博士,硕士生导师,研究方向为工业通风与除尘技术。 E-mail: cuipingy_2004@163.com。

摘要:【目的】为了制备锂离子电池正极导电浆料中水分含量(质量分数,下同)极低( <10^-3 )的碳纳米管,研究工艺过程中分级机转速、引风机转速、喷嘴喉部直径、粉碎腔体积的改变对碳纳米管物料水分含量的影响。 【方法】利用 LNJ-12A 型气流磨对碳纳米管湿物料进行超细粉碎干燥;通过改变气流磨的分级机转速、引风机转速、喷嘴喉部直径、粉碎腔体积,分析不同参数对碳纳米管物料水分含量的影响。【结果】在综合考虑能耗和实际工况下,当单因素变量为分级机转速 4800r/ min、引风机转速 2400r/ min、直喷嘴喉部直径4.5mm、粉碎腔体积从1. 14×10^-2m³增加至23. 56×10^-2m³时,碳纳米管干燥后的水分含量分别为 8.45×10^-4 、6.68×10^-4 、6.88×10^-4 、5. 89×10^-4。【结论】适当增大分级机转速,减小引风机转速,使用合适直径的直喷嘴,增大气流磨粉碎腔体积,对碳纳米管的水分干燥效果产生有益影响,相同干燥次数下碳纳米管物料水分含量更低,且达到水分要求时的干燥次数更少。

Abstract

Objective In the production of lithium-ion batteries positive electrode conductive paste ,the addition of carbon nanotubes with extremely low moisture content(moisture content<1 000×10^-6) is necessary. Currently, the Chemical Vapor Deposition (CVD) method is commonly used in industry to prepare carbon nanotubes, and a double cone rotary vacuum dryer is required for drying. However, this process has the disadvantages such as a low filling rate and high energy consumption.This work proposes the use of jet mill for the moisture drying process of carbon nanotubes, and analyzes the influence of process parameters on moisture content, providing a reference for the industrial-scale moisture drying of carbon nanotubes.

Methods This paper uses LNJ-12A jet mill to ultrafine crush and dry carbon nanotubes wet materials.The effects ofvarious parameters, including the speed of the classifier, the speed of the induced draft fan, the diameter of the nozzle throat, and the volume of the grinding chamber on the water content of carbon nanotubes, were investigated by a series of single factor experi ments. Finally, the parameter combination of carbon nanotubes moisture drying to achieve the optimal effect is obtained.

Results and Discussion Increasing the speed of the classifier enhances the drying effect of carbon nanotube moisture. As the clas sifier speed increases, the coarse particles experience heightened forces from the blades, resulting in increased centrifugal force that propels them outwards from the classification wheel. Subsequently, these particles descend into the crushing area for further refinement and drying. Moreover, an elevated classifier speed amplifies the blowing effect of the classifier blades, extending the residence time of carbon nanotube materials within the crushing chamber. However, an excessive speed of the classifier can also lead to increased energy consumption, and it is necessary to increase the speed of the classifier reasonably. Reducing the speed of the induced draft fan has been observed to enhance the drying effect of carbon nanotubes. An escalation in the speed of the induced draft fan induces a corresponding increase in the axial velocity of the airflow within the classification area. This, in turn, diminishes the descent rate of coarse particles impeded by the impeller. As a consequence, the residence time of these coarse particles in the classification area extends, leading to an augmented concentration. This heightened concentration facilitates the particles’ facile descent into the dust collector through the classification machine. Additionally, the reduced drying extent of moisture in the coarse particles contributes to a higher proportion of coarse particles in the carbon nanotube material collected in the dust collector. Consequently, this results in an elevated moisture content of the overall carbon nanotube material. However, if the speed of the induced draft fan is too low, the negative pressure provided by it is insufficient, which can lead to difficulty in feeding. Therefore, the speed of the induced draft fan should not be too low. The diameter of the nozzle throat will affect the jet velocity of the material in the crushing chamber. When the gas source pressure is constant, a smaller nozzle throat diameter and higher jet velocity facilitate the entry of carbon nanotubes into the classification zone. This, in turn, reduces the residence time of carbon nanotubes in the crushing chamber, resulting in incomplete internal water release and elevated water content in the final carbon nanotube product. Conversely, a larger nozzle throat diameter lowers the jet velocity at the nozzle, diminishing the kinetic energy applied to carbon nanotubes by the airflow. This reduction in kinetic energy leads to a decrease in the collision, shear, and fragmentation of carbon nanotubes within the crushing chamber. The inadequate release of internal moisture in turn results in higher moisture content in the finished product. When the volume of the crushing chamber increases, the migration path of the material from the crushing zone to the classification zone is extended, the retention time of the material in the crushing chamber is increased, and the moisture drying effect of the carbon nanotubes is better.Taking into account energy consumption and actual operating conditions, when the single factor variables are the classifier speed of 4 800 r/min, the induced draft fan speed of 2 400 r/min, the straight nozzle throat diameter of 4.5 mm, and the crushing chamber of 23.56×10^-2 m³, the drying effect of carbon nanotubes is better, with the moisture content of 845×10^-6,668×10^-6,688×10^-6 and 589×10^-6, respectively.

Conclusion In this paper, optimizing the process by appropriately increasing the speed of the classifier, reducing the speed of the induced draft fan, using a suitable diameter straight nozzle, and increasing the volume of the airflow grinding chamber is found to have a beneficial effect on the moisture drying effect of carbon nanotubes. With consistent drying times, this approach results in lower moisture content in carbon nanotubes materials, meeting moisture requirements more efficiently.

Keywords: carbon nanotube; ultrafine powdering;jet milling

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