Cryogenic Grinding

- Jul 19, 2019-

  Low-temperature crushing refers to the process of breaking the material cooled to the brittle point temperature into particles or powders with smaller particle size under the action of external force. Low temperature crushing technology can ensure that the crushed materials, such as natural products, will not be damaged during the crushing process.

Introduction 

Low-temperature crushing technology is not a new technology. As early as 1948, it has been industrialized. It has a long history of research on recycling and utilization of waste rubber, plastics, food, etc., and has relatively mature technologies and processes [6]. However, the application of low temperature grinding in the recycling of waste circuit boards has only started in the last ten years. For example, the mechanical treatment process of waste circuit boards in the research center of daimler-benzulm in Germany [7-8], although the investment in liquid nitrogen has been increased, this process can still achieve good economic benefits due to the high metal recovery and plastic reuse. In addition, the low-temperature grinding system of AirProduct (the us branch of Microtec, Germany) can crush tough materials to 0.075mm after embrittlement at low temperature, completely separating metal and non-metal. Research on waste circuit board pulverized at low temperature in China has just begun. Introduction of foreign advanced technology and equipment not only needs a lot of money, but also because of different national conditions, such as waste treatment costs and liquid nitrogen costs, even after the introduction of foreign equipment in operating costs may not be feasible, in the process route and the selection of equipment need to be adjusted to different degrees. Therefore, it is necessary and meaningful to develop research on low temperature grinding technology. In this paper, waste circuit board without components is taken as the object to compare the cumulative yield, morphology, dissociation degree and energy consumption of samples under different low temperature conditions, and study the effect of low-temperature crushing circuit board.

Test materials and methods

1.1 test materials

The material used in the test is the edge material of the mobile phone circuit board, which is crushed to 4 ~ 0.9mm by the shearing crusher, and the average particle size of the raw material is 2.658mm. Each sample weighs 200g.

1.2 test equipment

Self-designed test device is adopted, as shown in figure 1. In this test, a fixed pre-cooling chamber is used, which is connected with the entrance of the crusher through a closed pipe. After pre-cooling, materials are crushed in the crusher through the disc valve under the action of gravity from the pre-cooling chamber, and the crushed products are collected in cloth bags. The pre-cooling room is equipped with a annular liquid nitrogen nozzle and an intelligent temperature control system, which can accurately control the temperature according to the set temperature rise program; After the temperature program starts running, the amount of liquid nitrogen entering the precooling chamber is controlled by the solenoid valve. The grinder is also equipped with temperature control system and acuvim electric instrument, which are respectively used to measure the temperature inside the grinder and the power, current and voltage during the crushing process.

According to the results of cryogenic mechanics test, the impact toughness of the circuit board is the lowest at -40℃, that is to say, the resin material reaches the full embrittlement. On this basis, -60, -120 and -180℃ were selected for the test to study the influence of low temperature on the crushing effect of circuit board, and to determine the temperature range of material under this process to obtain better crushing effect. Heat transfer between liquid nitrogen and gaseous cryogenic nitrogen and materials occurs simultaneously in the precooling chamber. Unsteady heat transfer was estimated.

device

The low temperature crushing device is composed of two rotating cylinder blocks, motor, carrier platform and frame. The cylinder body is divided into two layers: shell and inner liner, which is filled with foaming insulation material, and a number of grinding rollers are placed in the inner liner. When in use, apply appropriate amount of liquid nitrogen to make the material to be milled in a low temperature state. The motor drives the cylinder and the roller.

advantage

In the recycling technology of waste circuit board, mechanical treatment technology has attracted much attention due to its low environmental pollution, simple operation, high treatment efficiency and good economic benefits, and has been widely developed in the past 20a. In mechanical processing technology, the most critical step is the crushing step, because it directly determines the efficiency of subsequent separation steps, metal yield and purity. Circuit board substrates usually contain a large number of thermosetting resin, these resin polymers have the characteristics of hard and strong, high temperature resistance, at room temperature has a certain toughness, can not be crushed by the general impact and direct extrusion crushing mode. And room temperature crushing recycling all kinds of problems, such as impact pyrolysis, harmful gases, dry shattered in the dust pollution, product particle shape heterogeneity, and long time operation easy to generate a lot of heat, cause local overheating, equipment material bonding, plugging equipment problems, these problems will cause environmental pollution, reduce the crushing efficiency, resulting in a loss of subsequent separation efficiency. The typical solution is to turn these non-brittle substances into brittle substances by freezing and then crushing them [5]. Therefore, low-temperature grinding technology has a very broad research prospect in the waste circuit board treatment.