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Electrostatic sorting
Figure 4 Schematic diagram of the freeze pulverizing device
Non-ferrous metal scrap pretreatment refers to non-ferrous scrap and scrap state becomes possible to effectively process subsequent metallurgical processes. This process includes: achieving various physical dimensions and weight standards for various waste materials and waste materials; separating non-ferrous metals from ferrous metals; removing non-metallic inclusions, moisture, oil, and the like. Fine and high-quality preparation of waste non-ferrous metals, suitable for metallurgical processes, can reduce the loss of non-ferrous metals to a minimum, reduce the unit consumption of fuel, electricity and flux, and make efficient use of metallurgical equipment and transportation tools. And improve labor productivity and the quality of non-ferrous metals and alloy products.
Pretreatment of non-ferrous metal scraps and scraps includes the following main processes: sorting, cutting, packing, briquetting, crushing, grinding, magnetic separation, drying, degreasing, etc. Special recycled raw material (waste battery, motor waste, scrap wire, scrap iron horse) pretreatment, using a special production line.
The All-Russian Recycling Non-Ferrous Metals Scientific Research Institute has studied the general process flow of waste non-ferrous metal pretreatment (Fig. 1), starting from non-ferrous scrap and scrap into the workshop, and until the finished product is sent to the customer's plant.
It is suitable to use a cylindrical electrostatic separator for removing insulation from thin wires of copper or aluminum having a diameter of 0.4 mm or less. The principle of this sorter is to take advantage of the role of the corona field. The particles in the corona field, regardless of their composition, carry a charge, the magnitude of which depends on the polarity of the corona electrode. The maximum charge (Qmax) that each particle can obtain is calculated by:
Qmax=4πε0a 2 kE (1)
Where ε0-dielectric constant, ε0-8.8542×10 -12 coulomb/farm·m; a-electrically equivalent rotating ellipsoid radius (m); E-electric field voltage (volts/meter); k- according to rotating ellipse A constant that varies by the ratio between the bodies c, a and the dielectric constant ε. [next]
According to equation (1), copper or aluminum particles can achieve a larger Qmax than insulator particles. When the charged particles are in contact with the grounding conductor (the sorter drum), the particles emit their own charge. The amount of charge that is released depends on the resistance, capacitance, and particle contact time. The charge received by the particles and the charge emitted from the particles to the ground conductor can be quickly balanced. The equilibrium charge Q can be calculated by:
Q
=
1+
Ki
-
√
(
1+
Ki
)
-1
(2)
Qmax
Bk
Bk
Where, Ki - the value that varies with the size of the corona field: B k = 10 14 /R k (R k - the equivalent total resistance of the particles).
For copper and aluminum particles, 10 14 /R k tends to infinity due to their lower total resistance, and Q/Qmax tends to zero, that is, the charge obtained by these particles is rapidly released. At the same time, the insulator particles remain on the grounding separator because they retain their own charge for a longer period of time. Thus, the metal particles on the surface of the drum instantaneously release their own electric charge and fall into the reservoir 8 (Fig. 2). The non-conductive particles are rotated with the drum to remain on the surface of the drum and then fall into the reservoir 7 (intermediate product) or the reservoir 6 (tailings-insulator).
The potential difference between the corona electrode and the roller depends mainly on the core diameter of the waste cable, which is generally 25 to 35 kV. The increased distance between the electrodes reduces the amount of copper entering the tailings, but it is necessary to increase the potential difference at the same time. If the pole pitch is shortened below the specified limit, current breakdown occurs due to air temperature and metal dust. The distance between the electrode and the roller is 45 to 70 mm. If the moisture content in the waste cable reaches 1.0 to 1.5%, the sorting cannot be performed, so the moisture content of the material sent for electrostatic separation should not exceed 0.5%.
The position of the stun electrode relative to the sorter drum (see Figure 2) has an effect on concentrate quality and copper recovery. The optimum angle α is 55°. The auxiliary electrode mounted under the corona electrode for increasing the electrostatic field can increase the deflection angle of the metal particles, thereby facilitating the separation of the metal particles from the insulator particles. With the aid of the auxiliary electrode, the copper content in the insulator tailings can be reduced to less than 0.1%. When handling a thick core waste cable, the utility of the auxiliary electrode is significantly reduced.
Figure 2 Electrostatic sorting process
1-broken cable compartment; 2-ground roller; 3-halo electrode; 4-auxiliary electrode;
5-brush; 6-insulator reservoir; 7-intermediate product reservoir; 8-purity metal reservoir
The process of electrostatically separating the plastic insulation layer and the rubber insulation waste cable can be expressed by the process flow (Fig. 3). Through electrostatic separation, concentrates containing 95.0 to 99.5% of copper or aluminum can be obtained, and the metal recovery rate is 90 to 95%. Electrostatic sorting has been listed as one of the waste cable machining processes. It can reduce the loss of non-ferrous metals with waste insulation, but it is not suitable for the separation of large-diameter waste wire insulation.
Figure 3 Waste cable processing process (plan)
The regenerative atomic energy, such as waste cables and wires, waste electric motors and generators, scrap cars and household appliances, used in the freezing process is being continuously promoted. [next]
The essence of the freezing method is the use of refrigerant to treat waste and waste. The chilled insulating material, ferrous metal, zinc and zinc alloy become very brittle and easily broken. Copper and aluminum still have plasticity. Thereafter, the crushed or finely ground material can be sorted into corresponding products by magnetic separation, sieving, hydraulic classification, heavy medium sorting, or the like.
The refrigerant generally employs a mixture of liquid nitrogen (liquid nitrogen having a boiling point of 195.8 ° C and a melting point of -210 ° C), solid CO 2 ("dry ice") and certain liquids.
The carbonic acid gas liquefies quickly at 20 ° C and a pressure of 57.06 · 10 5 Pa, and is converted into a colorless flowing liquid. When liquid CO 2 flows out, a lot of heat is absorbed by evaporation, so that liquid CO 2 becomes solid - "dry ice". "Dry ice" sublimates at 78.47 °C.
The following are the melting points and boiling points of certain materials that can be used to prepare the low temperatures required to process precious metal containing waste:
Methyl chloride ethanol methanol ethyl chloride
CH 2 Cl C 2 H 5 OH CH 3 OH C 2 H 5 Cl
t melt (°C) —103.6 —117.3 —97.3 —142.5
t boiling (°C) —24.2 —78.3 64.4 12.2
Under atmospheric pressure, an excess of solid CO 2 is mixed with the above liquid to obtain the following temperature: mixed with methyl chloride - 82 ° C; mixed with methanol - 80 ° C; mixed with ethanol - 72 ° C; Ethane mixed - 60 ° C. Liquid nitrogen, "ice" and the liquid mixture of methanol and of CO 2, has been used in various refrigeration apparatuses. A refrigerant mixed with an organic substance other than methanol has a high toxicity because of its toxicity and is currently not industrially usable.
Most foreign refrigeration units are frozen with liquid nitrogen. The (former) Soviet domestic cryogenic refrigeration unit is based on a turbo air freezer (TXM). This type of freezer can produce - (115 ~ 125) ° C cold air. Practice has shown that the use of cold air freezing by the turbo air freezing mechanism is more suitable for reducing nitrogen consumption from the viewpoint of reducing energy consumption and saving costs.
Table 1 is the embrittlement temperature of some insulating materials calculated according to the data of the All-Su Renewable Non-Ferrous Metal Scientific Research Institute.
Table 1 Embrittlement temperature of various insulating materials
Brand
t crisp (°C)
Brand
t crisp (°C)
Brand
t crisp (°C)
rubber
Polyvinyl chloride
Polyethylene
CKT
—70
И40-13
—40
315303-003
—120
CKTC
—109
И50-13
—50
315303-002
—120
PШ-1
—50
И60-12
—60
015105-002
—110
PKH-1
—30
И045-12
—45
316305-005
—110
CKБH
—70
О-50
—50
316603-011
—100
CKCH
—75
ОMБ-60
—60
317803-015
—100
The principle of the (front) West German Palla type freezer is shown in Fig. 43. The unit has an airtight insulation chamber that maintains the necessary low temperatures. The frozen material can be pulverized by a pulverizer or a roller crusher , and then the metal is removed from the light granule product by pneumatic grading or hydraulic grading.
The heavy-grain grade product contains 99% metal and 99% non-metallic material in the light-weight grade. The refrigeration consumes about 400 liters/ton of liquid nitrogen. One of the great advantages of the freezing method is that it has no harmful gas emissions and low processing costs.
1-receiving bin; 2-gate; 3-pre-cooling silo; 4-spiral feeder;
5-air-conditioning double circulation fan; 6-broken product warehouse; 7-vibration grinding machine; 8- regulating valve; 9-liquid nitrogen container
Some countries have developed and put into use a variety of liquid nitrogen freezing methods to treat low quality waste. The waste materials to be treated include waste electric motors containing a large amount of iron, waste generators, waste starters, and the like.
The waste is charged into a liquid nitrogen freezing cylinder (1.6 to 2 meters in diameter and 8 to 12 meters in length). The weight ratio of waste to liquid nitrogen should be 3:1. In the cylinder, the waste was frozen to -98 °C. Then it is sent to the hammer crusher for crushing. The crushed product with a particle size of 30-100 mm is magnetically selected to select steel scrap.
The non-magnetic component (non-ferrous metal) is first sent to a particle size of 3 to 25 mm and then subjected to secondary magnetic separation. The non-ferrous metals after the secondary magnetic separation are also subjected to heavy medium sorting. The heavy medium is a suspension made of a ferrosilicon alloy as a weighting agent. As a result, a product containing copper (copper content 94.0 to 96%, aluminum content 2 to 3%) and aluminum (aluminum content 93 to 95%, copper content 2 to 3%) was obtained, in addition to scrap steel. After the waste is frozen and subsequently treated, the insulation contained therein becomes wasteable sludge.
The capitalist countries process about 15 million cars and refrigerators a year to recover non-ferrous metals and steel. According to the total weight of a medium-sized car (1625 kg), the content (%) of metal and non-metal substances in each car or refrigerator is: Fe-78, Zn-1.5, Al-1.4, Pb -0.5 to 4.5, glass, rubber and plastic -15 or less. Freezing and crushing is also one of the methods of dealing with scrapped cars.
The packaged scrap car (package block section 600×600 mm or 700×700 mm, length varies) is first placed in the pre-cooling chamber, and the room is supplied with nitrogen to cool down to -20 °C. Then enter the main chamber with liquid nitrogen inside. The frozen material is then crushed and magnetically selected. Among the non-magnetic components accounting for 7% of the weight of the car, according to the estimation of the US Bureau of Mines, containing (%): Cu-3.3; Zn-1.3; Al-3.8; Pb-1.3; Fe-11.9, rubber and plastic - 24.4%; glass -42.3. The above materials can be processed by pneumatic sorting, hydraulic classification, screening and other sorting methods. The liquid nitrogen consumption of waste vehicles is close to 500 liters/ton of waste.
By deep-frozen treatment of different reclaimed raw materials, non-ferrous metals can be sorted from ferrous and non-metallic materials to prepare semi-finished products with high specific gravity and suitable for the production of alloys. The advantages of various refrigeration devices are that there are no environmentally hazardous waste and exhaust emissions. However, the widespread introduction of the plant freezing method will be limited by the high cost of liquid nitrogen, the precision of the insulation equipment, and the necessity of using steel materials suitable for low temperature operation.
When selecting the waste cable pretreatment method, if the technical and economic indicators are similar, then the mechanical method with the simplest equipment structure and no toxic waste discharge to the environment should be selected.