Hot water separation process for copper and material recovery from electric cable waste Shing-Wen

Recycling in the electric cable industry, especially the complete separation of metal and insulating material for reuse, has always been a problem. The present processes to recover copper and insulating material from cable waste involve the grinding of cables into smaller pieces to improve metal liberation from the insulating material. The method results in fine thread-shaped metal wedges in the insulating material, which prevents the re-use of the insulating material. A ’hot water separation process’ was used to recover metal and insulating material from cable waste, with no need to grind the cable into smaller pieces to improve the liberation of copper from the insulating material. Based on the

more

difficult

Min-Shing Tsai Department of Mineral & Petroleum Engineering, National Cheng Kung University, Tainan, Taiwan

Keywords process;

types of cable

478

are

water

separation

Mineral &

Petroleum Engineering, National Cheng Kung Taiwan 70101

University, Tainan, Received 16

April 2000, accepted

in revised form 14

April

2000

used method for the recovery of the metals from the cable waste. Such thermal recycling is no longer allowed in many countries owing to release of harmful gases, such as dioxin and hydrogen chloride, which result from PVC burning (Sijstermans 1997; Yoshino et al. 1999). In addition, it is not economic to recover only the copper, without considering the insulating material. Currently, recycling plants for scrap cables use mechanical grinders and other mechanical methods to separate the copper from the insulating material. In the cryogenic shredding and grinding process, cables are treated with liquid nitrogen to make the insulating material more brittle. After liberation, the metal particles are separated from the insulation particles by using the differences in density,

frequently

produced for requirements of electrical measurement, regulation, drives, high voltage applications, etc. Most cables are composed of inner conductive wires and outer insulating material. The most commonly used conductors are copper and aluminum, while the preferred insulating material is polyvinyl chloride (PVC), polyethylene (PE), rubber, and paper. Scraps of electric cable usually have valuable metal content and plastics. Much work has been done in cable recycling, especially the separation and re-use of metal and insulating material (Mikofalvy et al. 1993; Kertscher 1994; Krause 1994). Burning the insulating material has been a Many

Cable waste; copper; hot

insulating material; recycling

Corresponding author: Min-Shing Tsai, Department of

to recover.

Introduction

Sheih

Department of Resources Engineering, Dahan Institute of Technology, Hualien, Taiwan

differences in coefficient of thermal expansion of the insulating material and copper, the cables are cut into a suitable length and placed into a blender with hot water. The insulating material and copper are separated and collected individually. By controlling the water temperature, blending speed and cutting length, complete separation can be achieved. This process is suitable for recycling many types of cable waste, including oil-containing wires, which are

usually

insulating

magnetic and conductive properties. The cryogenic did

find wider

process

the high costs of et al. Warczok et al. 1990). 1990; liquid nitrogen (Rowson The room temperature crushing and grinding process involves a multistage grinding of cables into smaller pieces, liberating them into metal and non-metal fractions (Koyanaka et al. 1997). The metal and the insulating material are separated by particle shape separation (Koyanaka et al. 1997), air table (Warczok et al. 1990), electrostatic separators (Koehnlechner 1994), floatex density separation (Mankosa & Carver 1995), etc., into two fractions: one fraction consisting of metal and the other fraction consisting of insulating material. So this process necessarily uses an excessive amount of energy for multistage grinding of the cables into smaller pieces. Also, this method results in some impregnation of the insulating material with fine metal particles. These impregnated metals are difficult to separate from the recovered insulating material, but it is important for the re-use of insulating material to be absolutely metal-free. The oil-containing cables had to be excluded from this recycling method as the sticky grease and the removal of fine wires from the plastic make separation impossible. A new method has been proposed, with no need to grind the cable into smaller pieces called the ’hot water separation process’. The scraps of insulated cable are first cut or shredded to an appropriate length. The insulating material and the copper can be separated rapidly in hot water owing to the difference in the thermal expansion coefficient. The cutting length of the wire, temperature of the water and speed of blending have all been considered, and a simple, rapid, effective and energy efficient procedure for the recycling of all kinds of electric cable has been developed. not

application owing

to

Experimental procedures The cables used for this research consist of copper wires with insulation made of PVC plastics. The conductor consists of only one copper wire and is classified as single wire, while multiple copper wires are classified as stranded wires. The single wire with diameter (~) 0.6, 0.8, 1.0 and 1.2 mm and stranded wires with cross-section area (A) 1.25, 2.0 and 3.5 mm , respectively, were used. The composition of cable waste is shown in Table 1. All the wires mentioned were cut into the appropriate lengths, then placed into a vessel. Hot water was added into the vessel and the systems were blended with a three-blade propeller, for 5 min. After 5 min, the metal component of the wire was separated from the insulating material. A

used to control the speed of blending hand tachometer was used for examination of the rotating speed. A filter was then used to remove the bulk of the water remaining, and the cables were separated into copper, PVC and unliberated wire (copper still with PVC), respectively. Finally, the insulating material was recovered from the copper and unliberated wire by heavy liquid

voltage regulator was and

a

separation using carbon tetrachloride (CC14, specific gravity 1.5), and the copper (Cu) was recovered by using tetrabromoethane (C2H2Br4, specific gravity 2.94), then dried in the open air and individually weighed to evaluate the percentage of wire separation. The separation rate was calculated by the weight of pure copper and PVC over the

weight of cables. The operational conditions of separation were liquid-solid ratio, water temperature, cutting length of wire, and blending speed. The water volumes of 100, 200, 300, 400 and 500 ml ( 100 gt1 wire, the water temperatures of 25, 40, 50, 60, 70, 80 and 90°C, the cutting lengths of the single wire of < 1, 1 to 3, 3 to 5, 5 to 8, and 10 to 15 mm, the cutting lengths of the stranded wires of 3 to 5, 5 to 8, 10 to 15, 15 to 20, 25 to 30, and 35 to 40 mm, and the blending speeds of 1200, 1500, 1800, 2400, 3000, 3600, 4200, 4800 and 5400 r.p.m. were selected, respectively, in this study.

Results and discussion The effects of the

liquid-solid ratio realize the effect of the liquid-solid ratio, single wire with diameter 0.8 mm was cut to a length of 5 to 8 mm and then blended in 80°C water at a speed of 4200 r.p.m. for 5 min. Fig. 1 shows the blending result for 100 g of single wire. When the volume of water increased above 300 ml, no obvious effect was seen on the separation rate. Below 300 ml a slight decline in this percentage was noticed. This is because of the more rapid heat loss during blending from the smaller volume of water. For these reasons the liquid to solid ratio was used at 500 ml per 100 g of cables for further In order

to

experiment.

The effects of water temperature in separation diameter of 0.6 to 1.2 mm were lengths and placed in a blender with 500 ml of 20 to 90°C water. The blending speed was set at 4200 r.p.m. After 5 min of blending, the wire components were removed from the blender and separated by heavy media. The relationship between the separation rate and water 100 g of

cut

wire with

single

into 5 to 8

a

mm

479

Table 1. Composition of cable waste

where: Ap is the cross-section area of PVC after expansion, and Ac is the cross-section area of copper after expansion. Al is the cross-section area of PVC and A2 is the cross section of copper at room temperature, supposed that A, = A2. Thus, area

respectively.

It

was

temperature is shown in Fig. 2. It can be seen that increasing the temperature towards 90°C resulted in an increased

separation rate irrespective of diameters of wires. It can also be seen that the larger the diameter of the wire, the greater the separation rate. The increase in temperature causes different expansion of the insulating material (PVC) and copper and is shown in Fig. 3. It was assumed that at room temperature (25°C) the insulating material and copper were absolutely joined with no interspace. As the temperature increased by AT, an interspace was created between the insulating material and copper owing to the difference in their thermal expansion coefficients oc~

=

1.7 X

(thermal expansion coefficient of

copper:

10-5 °C-1, thermal expansion coefficient of 5-20 X 10-5 °C-1) (Sears et al. 1982). The

PVC: otp difference between the diameter of the PVC and copper after expansion, ds, could then be calculated according to both coefficients of area expansion and is given below, =

.

Fig. 1. The effects of liquid-solid ratio on separation rate of insulating material and copper (single copper wire).

480

assumed that d1 is the diameter of AI, and d2 is the diameter of A2 at room temperature. So di = d2 = ~. It can be found that Al A2 (7T({) /4) and so ds can be It

was

=

=

simplified:

calculated for wire with diameter 0.6, 0.8, 1.0 and 1.2 mm individually. The results are shown in Table 2. The calculated data are compared with the previous experimental results. For wire with the same diameter, ds increases with increased temperature. At a constant AT, ds varied according to wire diameter, the greater the diameter, the greater the d,. Furthermore, the greater the diameter of the wire, the

According

to

Equation 3, ds

was

2. The effects of water temperature on separation material and copper (hard-draw copper wire).

Fig.

rate

of insulating

Fig.

3. The

expansion of the insulting material and copper at the temperature

change AT. greater the difference between the mass of copper and the mass of insulating material. This difference in mass results in a great difference in motion between the two components during blending, and as a result separation becomes easier. According to these factors, the separation rate for single wire of any diameter increases with increase in water temperature,

but this increase eventually levels off at high temperatures

give

a

constant

maximum

The effects of cutting

separation

Fig. 4. The effects of the cutting length on separation rate of insulating material and copper (single copper wire) for given diameters (~).

to

rate.

length

The water temperature was set at 80°C and the other conditions were kept constant as described previously. Three diameters of single wire were used: ~ 0.6, 0.8, 1.2 mm, and cut to lengths < 1, 1 to 3, 3 to 5, 5 to 8, and 10 to 15 mm, respectively. The results as shown in Fig. 4 indicate nearly 100% separation at L/A < 3.8 (A rcz/4). When L/ A > 3.8, the relationship between separation rate (Single)’ cutting length (L, medium length of cutting wire) and crosssection area (A) is also shown in Fig. 4 and expressed in Equation 5 by using regression analysis (the dotted line). =

When running this experiment with stranded wires, six cutting lengths were used: 3 to 5, 5 to 8, 10 to 15, 15 to 20, 25 to 30, and 35 to 40 mm. The results are shown in Fig. 5. When L/A < 5, the wire separated nearly 100%, while for L/ A > 5, the relationship between the separation rate of the stranded wires (Stranded)’ cutting length and cross-section area of the copper is as follows when the regression analysis was used:

=

where: L A

=

=

cutting length (medium length of cutting wire) cross-section

area.

Table 2. The diameter difference temperatures

ds (

x

10-3 mm) of single wire

at

different

From Figs 4 and 5 it can be observed that for both single and stranded wires, the shorter the cutting length, the higher the separation rate is. For single wire cut longer than 8 mm and stranded wires longer than 15 mm, the separation rate deviates from the straight line of the Equations 5 and 6. This is possibly due to the longer length, which causes interference with the propellers of the blender by increasing the contact of propellers and wires. This increased contact may increase the difference in motion between the

insulating material and copper. By comparing Figs 4 and 5 it can be seen that the slope of the regression equation of the stranded wires is greater than that of the single wire. This could be due to the difference in the flexibility of the wires. The stranded wires appeared to be far more flexible than the single wire and so the effects of cutting length on the separation rate of the stranded wires were

greater than

The effects of To

test

on

single

wire.

blending speed

the effect of

blending speed

on

the percentage of

481

5. The effects of the cutting length on separation rate of insulating material and copper (stranded copper wires) for given cross-section area (A).

Fig.

separation, single

wire with

0.6 mm was used and L/A was varied between 14 and 22.9. The results are shown in Fig. 6. From this figure it can be seen that

separation

is increased

rate

diameter §

as

the

=

blending speed

is

increased. The separation rate becomes constant but remains incomplete when the blending speed is above 3600 r.p.m. when L/A = 14 and the blending speed is above 4200 r.p.m. when L/A = 22.4, respectively. This may be because the blender was not powerful enough to overcome the effect of

longer cutting length. By comparing Figs 4

and 6 it can be seen that complete separation was obtained with blending speed up to 3.8 and 4800 r.p.m. for L/A 4200 r.p.m. for L/A 5.1, This shows that the longer respectively. phenomenon cutting length needs the faster blending speed in order to obtain complete separation. From Fig. 7 it can be seen that the separation rate of stranded wires with cross-section area 2.0 mm2 decreases owing to decrease in the blending speed for L/A 3. By comparing Figs 6 and 7 it can be seen that the optimum separation rate was 80%, despite the large difference of L/A between stranded wires and single wire (for single wire L/ A 22.9 and stranded wires L/A 9, respectively). This phenomenon proved the aforementioned statement of the effects of the flexibility of the wires on the separation rate. =

=

=

=

=

Operation of oil-containing cable Oil-containing cables are difficult to recycle as they contain 482

Fig. 6. The effects of the blending speed on separation of insulating material and copper (single copper wire) for given L/A ratios.

sticky grease. The components of the oil were found to be straight chain hydrocarbon compounds with a melting point of 98°C. Hence, 100 g of oil-containing single wire with diameter 1.2

and L/A 3.8 was put into 500 ml of boiling water for 5 min, and then blended at 4200 r.p.m. After blending, the copper and insulating material could be separated and the separation rate was nearly 100%. The insulating material present after this separation was mm

=

absolutely clean (metal-free) and

can

be re-used.

Benefits of the ’hot water separation’ process The conventional practice to recover copper and insulating material from used wires involves multistage grinding, followed by gravity enrichment, particle shape separation, air table, etc., to liberate and separate the copper from insulating material. This approach can recover about 90% of copper (Koehnlechner 1994; Mankosa et al. 1997). A substantial portion of metal is lost along with the insulating material owing to inefficiencies in the grinding and separating procedures and sticking of the fine threadshaped copper particles to insulating material. It is important for the re-use of insulating material that the granules are absolutely metal-free. Insulating material contaminated with copper can not be re-melted as the melt screen of the extruder rejects it. It is not possible to reach these high separation rates with the usual mechanical treatment. With the help of the electrostatic or centrifugal separation process, the required cleanness can possibly be

method is more expensive than other mechanical methods owing to the high costs of liquid nitrogen but still profitable. It is obvious that recovery and re,use of copper and insulating material from used wire is economically feasible. Hot water separation batch tests have indicated that cables cut into a suitable value of L/A, followed by blending with hot water could recycle cable waste with an efficiency of nearly 100%. The recovered copper and insulating material are completely separated and pure, both of which can be reused. There is still a need for final design engineering for the pilot plant, which defines not only the processes but also unit operations and allows the economic evaluation.

Conclusions Based

Fig. 7. The effects of the blending speed on separation of and copper (stranded copper wires) for given L/A ratios.

insulating material

the results of the present conclusions can be drawn:

(1) achieved with a separation rate above 90% and the metal content decreases to less than 0.2% (Koehnlechner 1994;

However, with the hot water separation process, the cables suitable value of L/A, with no need to grind the cable into smaller pieces to improve the complete liberation of the copper from insulating material. Blending with hot water results in the recovery of all copper and insulating material with a purity grade of nearly 100%, respectively. The required cleanness of products as well as energy savings by less grinding stages can be achieved by this route. Copper scrap is recycled in copper foundries as a valuable raw material with a price closely tied to the market value of new copper (Anonymous 1998), and was about $1675 t in 1998. In cases where the cable insulating material is not reused but wasted, the cost of landfilling is up to $512 t1 (Koehnlechner 1994). The high cost of disposal means that recycling of the insulating material into new products is really profitable. Philip’s Services Corporation uses a cryogenic process to make plastics brittle enough to be hammered into a clean powder that can be separated from commingled copper and aluminum. The separated plastics can be resold at $44 to 88 t (Rubin 1997). The cryogenic into

a

(2)

studies, the following

For any type of wires of the

same length and blending, at the speed, separation rate increases with increase in temperature. Wires of large diameter need lower water temperature than wires of small diameter for effective separation. For single wire with varied diameter, 100% separation is achieved when L/A < 3.8. For L/A > 3.8 the relationship between the separation rate Ssirgl,, length (L) and cross-section area (A) is: Ssingle(%~ 104.88 - 1.08 L/ A. Stranded wires can be completely separated when L/ A < 5. For L/A values greater than 5, the relationship between the separation rate of the stranded wires, cutting length and cross-section area of the copper is: Sstranded(%) 125.05 - 6.64 L/A. An increase in blending speed results in a higher separation rate. The optimum separation rate can be obtained with a blending speed of 4200 r.p.m. The hot water separation process is suitable for recycling all types of cable waste, including oil-containing cables, where pre-extraction of oil with boiling water is needed. The recovered insulating material after the hot water separation process is absolutely metal-free and can be reused.

the

Sijstermans 1997). are cut

on

same

=

(3)

=

(4)

(5)

(6)

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Koehnlechner, R. (1994) Copper separation from cable scrap. Wire Industry , 342-343. 61 Koyanaka, S., Ohya, H., Endoh, S., Iwata, H. & Ditl, P. (1997) Recovering

483

wastes using a particle shape separation technique. Advanced Powder Technology 8, 103-111. Krause, F. E. (1994) Vinyl durables recycling. Journal of Vinyl Technology 16,

copper from electric cable

177-180. Mankosa, M. J. & Carver, R. M. (1995) Processing of chopped wire waste material using the floatex density separator. In: Queneau, P. B. & Peterson, R. D. (eds.) 3rd International Symposium on Recycling of Metals and Engineered Materials. Warrendale, PA, USA: Minerals, Metals and Materials Society, pp. 111-120. Mankosa, M. J., Carver, R. M. & Venkatraman, P. (1997) Processing techniques used to recover secondary materials. Mining Engineering

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copper/plastic scrap. In: Proceedings of the Recycling of Metalliferous Materials Conference. Birmingham, London, UK: Institution of Mining and Metallurgy, pp. 235-245.

484

Rubin,

D. K.

(1997) Philip’s Canadian high. Engineering News Record 239,

40-44. F. W., Zemansky, M. W. & Young, H. D. (1982). University Physics. Reading, MA, USA: Addison-Wesley, pp. 290-291. Sijstermans, L. F. (1997) Recycling of cable waste. In: Proceedings of the 1997 14th International Conference and Exhibition on Electricity Distribution. Birmingham, London, UK: Institution of Electrical Engineers, Part 1/3

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(of 7), Vol. 3, 8.1-8.7. Warczok, J., Pers, A. & Smieszek, Z. (1990) Processing of cable scrap in Poland. In: Proceedings of the Recycling of Metalliferous Materials Conference, Birmingham, London, UK: Institution of Mining and Metallurgy, pp. 283-289. Yoshino, A., Sawada, H., Suzuki, J., Hasegawa, M., Murayama, M. & Shinmoto, T. (1999) Halogen free new recyclable cable with low toxic gas emission. In: Hiroyuki, Y., Ryoichi, Y., Fumihiko, K., Tadatomo, S. & Yasushi (eds.) Proceedings of the Ecodesign ’99: First International Symposium on Environmentally Conscious Design and Inverse Manufacturing. Tokyo, Japan, pp. 979-984.