Removal of lead compounds from polyvinylchloride in electric wires and cables using cation-exchange resin

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Abstract

Recycling treatment of cable insulation resin generated from electric wires and cables was investigated. Conventional insulation PVC contains a lead component, tribase, as a thermal stabilizer and lead removal is necessary to recycle this PVC as insulation resin. This paper describes a solid surface adsorption method using ion exchange resin to remove the fine lead containing particles from PVC dissolved solution. Low lead concentration in the recovered PVC, complying with the requirements of RoHS, was achieved.

Highlights

► Cable insulation PVC contains lead component as thermal stabilizer. ► Lead removal is needed to recycle PVC as insulation resin. ► Lead removal by an adsorption method using ion exchange resin was investigated. ► Low lead concentration, complying with the requirements of RoHS, was achieved.

Introduction

Used electric wires and cables recovered from electric power suppliers and building wreckers in Japan are cut to liberate and separate into metal conductors and cable insulation for recycling. Cable insulation is made of several kinds of resin: polyvinylchloride (PVC), polyethylene (PE), and cross-linked polyethylene (XLPE) and the PVC is most common. The amount of collected cable insulation per year is 36,000 tons and the collection ratios of mixed plastics is high (44.1%), followed by PVC at 25.1% in Japan [1]. Material recycling of mixed plastic is difficult and most is discarded in landfills. Thus plastic–plastic separation methods have been investigated [2] and electrostatic separation techniques have been used in cable insulation plastic treatment [3]. The authors [4], [5] have applied jig separation methods to cable insulation plastics, PVC, and non-PVC and reported high separation efficiencies. Single component resin pellets of PVC and PE, crushed used cable insulation, are sold as recyclable pellets, for use in the production of flooring and seat plastics and other usages [6], since these pellets do not meet required performance criteria as cable insulation.

Mizuno and Hirukawa [7] has calculated CO2 emissions from PVC cable insulation manufacture and reported that reductions of 31% of CO2 can be achieved when used PVC is mixed with virgin PVC (virgin: recycled pellets = 7:3). Murata et al. [8] has developed a kneading method for virgin PVC and used PVC mixing and reported that the blended product can be used for only sheathing of electric wires and cables since volume resistivity is lower than the performance required for cable insulation.

The PVC cable insulation contains lead compounds as a thermal stabilizer [9] and the use of toxic substances such as lead is regulated strictly by RoHS (Restriction of Hazardous Substances in electric and electronic equipment) in the EU. In RoHS, lead concentrations in electric and electronic equipment must be below 1000 ppm. Alternatives to lead compounds like hydrotalcite have been developed [10], however the development of lead removal methods for waste PVC in cable insulation is needed to recycle used insulation as a raw material for cable insulation since formerly produced cable insulation contains lead compounds. The Vinyloop method [11] is one method to recycle PVC resin. Here PVC is first dissolved in an organic solvent and PVC and additives are recovered with a deposition method by steam injection after removal of large impurities. The used organic solvent is recovered by a salting-out method and reused. In the Vinyloop method removal of lead compounds is difficult because the lead compounds are very fine particles (average size <2 μm).

To remove the lead compounds some methods have been investigated, including filtration, leaching methods [12], and liquid–liquid interface trapping [13], but technical problems such as plugging of filters and the treatment of leaching solution are remaining. This paper describes a solid surface adsorption method using ion exchange resin to remove the fine lead containing particles from dissolved PVC solution. The treatment is divided into three steps and a pretreatment method involving ion exchange resin was developed.

Section snippets

Materials

Virgin cable insulation PVC was used in the experiments. This sample contains tribasic lead sulfate (tribase, 3PbOPbSO4·H2O, Pb concentration: 17,600 ppm) and calcium carbonate (CaCO3, Ca concentration: 120,000 ppm). Methyl ethyl ketone (MEK, Daishin Chemical Corporation) was used as organic solvent. Strong acid cation exchange resin (15JWET, H+ type, Rohm and Haas Company) was used as adsorbent. Hydrochloric acid, sodium chloride, potassium chloride, magnesium chloride, calcium chloride, and

Results and discussion

According to RoHS the lead concentration in recovered PVC must be below 1000 ppm to allow use as cable insulation. The PVC sample here contains tribase (Pb concentration: 17,600 ppm) and calcium carbonate (Ca concentration: 120,000 ppm) and the particle diameter is 1–2 μm for tribase and 4–5 μm for calcium carbonate. Most of the calcium carbonate can be removed by centrifugation at 1000 × g for 1 min. In this study removal of the remaining fine lead particles by an adsorption method was investigated.

Conclusions

Removal of fine lead particles by a solid surface adsorption method was investigated. Some adsorbent compounds were tested and strongly acidic cation exchange resin showed superior results. Electron microscopic observations showed the attachment of tribase particles on the ion exchange resin. Rinsing of ion exchange resin in the pretreatment process causes variability in the lead removal results and it was found that the electric conductivity of the MEK used for the last rinsing can be used as

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