Two-step pre-processing enrichment of waste printed circuit boards: Mechanical milling and physical separation

Highlights

• High efficiency two-step mechanical and physical separation was done on PCB.

• No chemical or/and thermal treatment is needed in the process.

• The major ingredients on ceramic portion are CaO + SiO₂+Al₂O₃ in all stages.

• Volatilized materials during pyrolysis are not simple greenhouse gases.

• Physical separation led to enrichment of metallic phase up to 75%.

Abstract

Printed circuit boards (PCBs), one of the most complex components of e-waste, contain different metallic and non-metallic components. Recycling of waste PCBs is an important issue, from both aspects of hazardous waste management and recovery of valuable materials. In this study, for the first time, a mechanical-physical separation method for recovery of metallic elements of waste PCBs without any chemical or/and thermal processes was introduced. Two milling stages were applied to enhance the liberation degree, followed by a physical flotation process for enrichment. Based on the elemental analysis, the total metal concentration increased by 75% after the second milling in coarser particle size classifications and ceramic components has decreased significantly. Phase identification techniques confirmed that the ceramic portion is mainly CaO+SiO₂+Al₂O₃, which generally exist inside the boards as glass fibres. Using Fourier transform infrared spectroscopy (FTIR) spectra 3 nominees of phenoxy resin, poly vinyl acetate and vinyl chloride were suggested as the major polymer content of the crushed waste PCBs. Scanning electron microscopy (SEM) demonstrated higher degree of liberation in the higher meshes for metallic and non-metallic components but it was lower for fibre glasses. The thermal pyrolysis using infrared gas analyser proved that the most volatile substances are not simple greenhouse gases such as CO, CO₂ and CH₄. Eventually, beneficiation using bromoform + acetone resulted in two phases of high density (sunk) and low density (float), in which the former one was enriched in metals excluding Al and Sn, and the latter one was almost depleted from metals.

Introduction

The generation of e-waste is estimated to be around 20–50 million metric tons worldwide with the annual increasing rate of 4% (Rajagopal et al., 2016). Among e-waste, waste printed circuit boards (PCBs) are attracting more attention. Accounting for 4 wt% of total e-waste in Australia, waste PCBs pay off 40% (US$ 150 million) of metal recovery value from all e-waste (Golev and Corder, 2017). Embedded within waste PCBs are valuable metals (Au, Ag, Ni and Cu) with a positive market value for recovery and resale. However, waste PCBs contain a range of toxic substances such as hazardous metals and brominated flame retardants in a complex structure. This fact implies importance of waste PCBs from the perspective of valuable secondary resources and environmental regulations (Puca et al., 2017, Yang et al., 2017). Therefore, recycling of valuable metals from waste PCBs is not only economically beneficial but also has huge environmental benefits. Base on the previous investigations, it has been found that metals are easier to recycle compared to other components of e-waste; i.e., ceramics and plastics (Golev and Corder, 2017).

Coexistence of organic materials, metals and glass fibres in the structure of waste PCBs, make them a challenging component for recycling (Sohaili et al., 2012). Pre-treatment of waste PCBs is not only a reasonable solution to solve aforementioned problems, but also reduces the amount of impurities and reagent consumptions in process. Pre-treatments include mechanical dismantling (capacitors, resistors, CPU and CPU case, port locations etc.), and basic chemical, physical or physio-chemical separation. To increase the recycling efficiency and to recover metals in the form of alloy or in pure state, the metal containing ingredients are subjected to other processes such as shredding, milling, leaching, purification, thermal treatments, smelting and refining (Golev and Corder, 2017, Nekouei et al., 2018)

There have been some physical and mechanical methods for enrichment, beneficiation and liberation of metallic and non-metallic components. Schaik and Reuter (2010) described the liberation behaviour of different materials connected together in design step for different types of e-waste. Results revealed that the purer particles have more efficiency in liberation step. Besides, it was found that some parts cannot be separated after shredding, which reduce the efficiency and increase the need for further steps (2010). Kumar et al. (2015) employed a physical process including scutter crusher followed by a high-speed hammer mill and classified the product into eight different particle size classifications. Then, the metallic contents of the waste PCBs were enriched utilizing pneumatic separation and froth flotation. Results illustrated that, using froth flotation, 88% and 90% of the total metals of the DVD and vacuum cleaner's waste PCBs can be recovered, respectively. However, using pneumatic separation, 75% and 65% the total metals of the DVD and vacuum cleaner's waste PCBs can be recovered. Even though density-based separation techniques are simple and consume less energy, a further process for refining is required to increase the purity of the product (Ning et al., 2017). Eswaraiah and Soni (2015) utilized a two-stage crushing process and a circulating air classifier process to liberate and separate metal-rich and non-metal fractions of waste PCBs. To enhance the recovery and separation efficiency of metals and non-metals, the parameters of the milling procedure and supercritical air flow velocity were also optimized. Results obtained from the crushing stage had shown that liberation (enrichments) of the metals is in the range of 0.5–1.8 mm. Results of the classification stage indicated that the effect of the air flow velocity on the enrichment of the metals is more influential compared to the feed rate and rotating guide vane angle. Veit and Scherer (2014) surveyed the separation process of the metallic and non-metallic fractions of the waste PCBs using Mozley concentrator, a specific design of separation equipment. Using this process enable the recovery of 85% Cu, 95% Sn, 96% Ni, and 98% Ag. It was determined that the mechanical processing improves the liberation of the metals by enhancing the efficiency of post-processing.

In the study by Bizzo et al. (2014) PCBs have subjected to size reduction and classification using 1.18 mm particle size as the border line. It has demonstrated that in the classification of particle size less than 1.18 mm contain 84%–89% inorganic materials compare to classification of particle size more than 1.18 mm which contain 64%–73%. It was also investigated that the mechanical pre-processing of waste PCBs including milling and sieving is beneficial in terms of both metal and energy recovery (due to the inorganic material content in the fraction with smaller particle-size which is suitable for incinerations). Hanafi et al. (2012) characterized the composition of waste PCBs and employed a feasible disassembly route and mechanical method including pulverization by ball-milling and disc-milling, density and magnetic separation. In addition, a chemical method was developed to recover valuable metals from waste PCBs. This study concluded that milling method, particle size and density of the solvent are most important parameters in hydrometalurgy process to enhance the recovery of metallic elements. Ball-milling machine was found to be more effective compared to disc-milling machine and the best particle size for density separation was 149 μm.

Regarding aforementioned discussion, proposing a mechanical-physical separation method for recovery of metal contents of waste PCBs with no need for further chemical or/and thermal processes is a challenging task. To reduce the reagent consumption, increase the efficiency and investigate the selective crushing characteristics of waste PCBs, this study has focused on presenting a feasible and novel process to enhance the amount and purity of metallic portion of the waste PCBs without further chemical or/and physical process. At first, two milling stages including knife mill and ring mill were applied to increase the liberation degree; then, a physical flotation process using bromoform fluid for beneficiation and enrichment was carried out. To recognize the composition and metal content in each classification, classified (using sieve shaker) powders were characterized in each stage. Eventually, the mixture of all powder was characterized in term of its thermal behaviour and composition of exhausting gas to estimate its threat for environment.