At present, many countries in the world have conducted in-depth research on the recycling of waste printed circuit boards, and have achieved certain results. Recycling treatment technologies for waste printed circuit boards include: mechanical treatment methods such as disassembly, crushing, sorting, etc .; heat treatment methods such as high temperature pyrolysis to produce liquid oil <2>, etc .; chemical treatment methods such as dissolution and washing methods. Germany adopts advanced low-temperature freezing and crushing technology for efficient treatment <3>. Domestic researchers have used electrical separation methods to treat waste printed circuit boards, and have achieved relatively satisfactory results <4>.
This paper discusses the air separation of waste printed circuit boards by using the mature theory of mineral processing <5>. Air flow sorting uses air as the sorting medium, and solid particles are sorted according to density and particle size under the action of air flow. When the gravity or centrifugal force of the particles is equal to the resistance of air movement, the relative velocity v 0 (final settling velocity) between the particles and air movement is: v 0 = Ï€dÏs 6ψÏg (1) where: d? The diameter of the particle; The density of air; the fund project can be seen from formula (1), when the particle density is the same, the particles with large diameter have a large final velocity of settling; when the particle size is fixed, the particles with a large density have a large final velocity of settling. Since the final settling velocity of particles is also related to the density, size and shape of the particles, in the same medium, particles with different density, size and shape can have the same settling velocity under specific conditions. Such corresponding particles are called equal-drop particles. Among them, the ratio of the particle size d r1 of the low-density particles to the particle size d r2 of the high-density particles is called the equal-drop ratio, expressed as e 0, that is, e 0 = d r1 d r2 (2) Both theory and practice show that e 0 It will decrease as the particle size becomes finer. When the particles reach the final settling velocity in the ascending airflow, the settling velocity v 0 of the particles is equal to the difference between the relative velocity of the particles to the medium and the ascending air velocity μ0, that is, v 0 = v-μ0 (3) From equation (3), we can see that the ascent Airflow can shorten the time for particles to reach the final settlement speed and the falling distance, so ascending airflow is often used in airflow sorting. Before the particles reach a certain final sedimentation velocity v 0, the denser particles fall faster than the smaller ones. If the sedimentation speed of the two particles is always different during the descent, and the sedimentation speed of particle A is always less than that of particle B Settling velocity, then the separation of the two materials can be achieved within a certain range of rising air velocity.
In order to improve the sorting efficiency, the crushed materials must be classified by particle size before the airflow separation, and then the metal and nonmetal are separated by the airflow to finally achieve the mechanical separation of the waste printed circuit board and calculate the metals of different sizes content.
1 Test method In this test, an abandoned mobile phone motherboard provided by an electronics company was selected. Because the test materials are mixed with more ferromagnetic substances and paper scraps (labels affixed to the mobile phone motherboard), in order to achieve a better sorting effect, the magnetic separation and flotation process are first used to pre-treat them. The purpose is to remove the ferromagnetic substance mixed in the test material and avoid excessive wear of the cutter of the crusher, and the flotation is to remove the mixed paper scraps.
The raw materials are sent to the crushing process after magnetic separation and flotation. After the first-level crushing and second-level crushing, the mixed materials are sieved to obtain + 30 mesh,-30 mesh ~ +40 mesh,-40 mesh ~ + 45 Mesh,-45 mesh ~ +50 mesh,-50 mesh ~ + 60 mesh,-60 mesh ~ + 70 mesh,-70 mesh ~ + 80 mesh,-80 mesh 8 grades of materials were weighed separately after weighing, test By adjusting the air volume and feeding speed to achieve a better separation effect, the technical parameters are shown in Table 1. The materials are sorted according to density under the action of ascending airflow, and the heavy components (metals) are discharged from the bottom of the air separator, light weight The components (glass fiber, epoxy resin) are brought into the cyclone under the action of the air flow, and the gas-solid separation is performed by the cyclone. Finally, the air is discharged from the top of the cyclone and the light component is discharged from the bottom.
Airflow sorting technical parameters mesh + 30-30 ~ + 40-40 ~ + 45-45 ~ + 50 Air volume V / (m 3?
h-1) 500 400 feeding speed v / (kg?
min-1) 0. 050 0. 025 0. 050 mesh-60 ~ + 70-70 ~ + 80-80 air volume V / (m 3?
h-1) 300 feeding speed v / (kg?
min-1) 0. 090 0. 065 0. 170 Weigh the metals and non-metals of different particle sizes obtained after sorting separately, analyze their distribution rules, and use the flotation method to determine their metal content, the specific process is as follows Figure 1.
Air flow separation process flow 2 Results and discussion 2.1 Distribution rules of materials and metals in different particle size ranges In order to reduce the test error, a total of three sets of separation tests were carried out, and the average value of the mass fractions of materials and metals in different particle size ranges was calculated See Table 2. The distribution rules of materials and metals in different particle size ranges obtained by sorting can be seen intuitively from Figure 2.
It can be seen from the data that, under the condition that the crushing time is not limited, after the materials with different particle sizes are sieved through the standard inspection sieve, the mass fraction of each mesh material and the metal mass fraction show a relatively stable trend. -The 80-mesh material accounts for the largest mass fraction of the total materials, and the -50-mesh to + 60-mesh materials account for the smallest mass fraction; although the -80-mesh material quantity is the largest in the total materials, its metal content accounts for the material However, the mass fraction of metal is the smallest. The largest metal fraction of each mesh material is +30 mesh metal;-60 mesh ~ +70 mesh metal accounts for the largest mass fraction in the total material,-50 mesh ~ +60 mesh Metal has the smallest mass fraction.
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