1 Overview
1.1 The Importance of Recycling Waste PCB Boards
Currently, with the European Union’s WEEE Directive in effect and similar regulations either implemented or on the brink of full implementation in various regions and countries worldwide, the recycling and reuse of waste electronic products have become imperative. Among these, the recycling and reuse of PCB boards from electronic waste stand out as a critical concern. This paper focuses on summarizing the technological advancements in this field, particularly in Japan in recent years.
Japanese authorities have undertaken significant investigations into the prevalence of PCB boards in waste from major household appliances and the subsequent disposal rates within Japan. Shockingly, the disposal rate remains below 10% by mass. Among electronic devices, computers exhibit a notably high PCB content rate, ranging from 20% to 30% by mass. According to data from the Computer 3R Promotion Business Committee of the Japan Electronics Technology Industry Association, approximately 65,000 corporate computers in Japan require recycling and refurbishment annually.
Furthermore, projections from the Japan Electromagnetic Information Technology Industry Association indicate a substantial increase in domestic computer waste over the years. In 2003, Japan produced about 10,000 tons of computer waste, a figure that surged to 50,000 tons by 2013, and further climbed to 80,000 tons by 2015.
1.2 Principles to be Grasped in Waste PCB Board Recycling
The printed circuit board’s (PCB) components, exemplified by the computer motherboard, comprise three primary materials: metal (constituting approximately 50.8% of the board’s total mass), glass fiber cloth (about 16.3%), and organic resin (approximately 32.8%). Separating these components poses a considerable challenge. Historically, landfilling was a common disposal method, but it fails to meet the EU’s “WEEE” directive, which mandates a regeneration rate of over 75% for IT and small household electronics and a recycling rate exceeding 65%. Therefore, there’s a pressing need to recycle and reuse PCBs. Companies aiming to reuse waste PCBs should assess technology adoption from three perspectives:
(1) Environmental Benefits: Ensuring environmental protection at local and global levels, safeguarding air, water quality, soil, and human health from adverse impacts.
(2) Reuse Benefits: Facilitating the re-commercialization of waste PCBs while minimizing non-reusable waste.
(3) Cost-effectiveness: Striving for cost-efficient processing methods, tightly linked to the chosen process technology.
1.3 Main Aspects of Recycling Waste PCBs
Globally, recycling and reusing waste PCBs, particularly extracting metals, have been ongoing for some time. Metal recovery, primarily conducted by metal smelting plants, targets metals like copper, gold, and nickel used in PCB circuits and components. Lead and tin, among others, are common in component joints. Precious metals such as gold, silver, and palladium are also found in electronic components. Research and statistics show that desktop computer mainboards predominantly contain various metals: copper (66.9%), tin (10.7%), iron (10.3%), lead (7.61%), gold (0.11%), palladium (0.02%), with other metal components making up the remainder (4.41%). The concentration of gold and palladium in waste PCBs exceeds that in raw ore by 2-20 times, indicating significant potential for their recovery. Additionally, PCB resin includes flame retardants, primarily bromine-containing compounds, antimony oxide, and phosphorus compounds. Separating these impurities before copper refining is crucial for ensuring safety during metal recycling. Recent research on waste PCB reuse focuses on metal recycling technology, whole PCB decomposition, and halogen component removal methods.
2. Metal Recovery from Waste PCBs
Currently, metal recovery from waste PCBs predominantly employs the dry method, involving metal smelting. Other methods, like chemical dissolution (wet method) and biological processes, are also emerging.
2.1 Dry Metal Recovery from Waste PCBs
Dry metal recovery involves pre-treatment before smelting, utilizing methods like dry distillation or pulverization to separate metals and electronic components. Separation techniques leverage differences in particle size, relative density, static electricity, magnetic and wind forces to enhance metal recovery rates. Dry distillation generates gases containing brominated flame retardants, necessitating secondary combustion and subsequent rapid cooling to prevent bromide formation. Compliance with pollution-free liquid standards is crucial for wastewater treatment during processing. Copper extraction from waste PCBs involves smelting in a self-melting furnace, followed by refining and electrolytic extraction. Slag produced during smelting, rich in SiO2 from glass fibers, is recycled for adhesives and paving materials. Notably, the dry method is popular for recycling discarded mobile phones. Japanese research focuses on pre-processing to enhance metal recovery efficiency by effectively removing non-metal components.
2.2 Wet Metal Recycling from Waste PCBs
The wet method involves pulverizing waste PCBs and using acid solutions to ionize and dissolve metals, followed by precipitation to recover metals like silver. Some propose immersing broken waste PCBs in mineral acid and hydrogen peroxide solutions to dissolve metal materials.
3. Component Separation in Waste PCBs
In addition to metal recycling, considerable progress has been made in recycling other PCB components, particularly through improved separation and purification methods. Thermosetting resins like epoxy and phenolic resins, commonly used in PCB substrate materials, pose challenges due to their insolubility post-curing. The mainstream separation process involves first separating thermosetting resins from the PCB, then separating remaining metal and glass fiber components. Heating and burning PCBs enable resin separation, followed by recycling of the residue. Research indicates that larger PCB samples yield higher metal recovery rates due to reduced gas dispersion during combustion. However, processing micro-sized PCBs incurs lower energy consumption and recycling costs.
4. Halogen Component Removal from Waste PCBs
Halogen flame retardants in PCB resins, though vital for safety, are considered environmentally and medically harmful. Halogen removal research aims for “harmless” recycling and reuse of waste PCBs. Methods include hydrogenation, thermal decomposition, solid-phase reactions, biochemical processes, and electrolysis. Hydrogenation, employing metal catalysts like palladium, facilitates halogen removal through a reaction with hydrogen. Thermal decomposition decomposes halogen-containing resins, followed by reaction with alkaline compounds to extract halogens. Solid-phase reactions involve mixing metal compounds like calcium oxide with waste PCB resins to extract halogens as halide salts. While biochemical methods utilizing microorganisms have been proposed, their lengthy processing times make them unsuitable for large-scale halogen removal. Electrolysis, a promising method, dissolves halogen compounds in organic solvents and separates them via electrolytic reactions.
1.1 The Importance of Recycling Waste PCB Boards
Currently, with the European Union’s WEEE Directive in effect and similar regulations either implemented or on the brink of full implementation in various regions and countries worldwide, the recycling and reuse of waste electronic products have become imperative. Among these, the recycling and reuse of PCB boards from electronic waste stand out as a critical concern. This paper focuses on summarizing the technological advancements in this field, particularly in Japan in recent years.
Japanese authorities have undertaken significant investigations into the prevalence of PCB boards in waste from major household appliances and the subsequent disposal rates within Japan. Shockingly, the disposal rate remains below 10% by mass. Among electronic devices, computers exhibit a notably high PCB content rate, ranging from 20% to 30% by mass. According to data from the Computer 3R Promotion Business Committee of the Japan Electronics Technology Industry Association, approximately 65,000 corporate computers in Japan require recycling and refurbishment annually.
Furthermore, projections from the Japan Electromagnetic Information Technology Industry Association indicate a substantial increase in domestic computer waste over the years. In 2003, Japan produced about 10,000 tons of computer waste, a figure that surged to 50,000 tons by 2013, and further climbed to 80,000 tons by 2015.
1.2 Principles to be Grasped in Waste PCB Board Recycling
The printed circuit board’s (PCB) components, exemplified by the computer motherboard, comprise three primary materials: metal (constituting approximately 50.8% of the board’s total mass), glass fiber cloth (about 16.3%), and organic resin (approximately 32.8%). Separating these components poses a considerable challenge. Historically, landfilling was a common disposal method, but it fails to meet the EU’s “WEEE” directive, which mandates a regeneration rate of over 75% for IT and small household electronics and a recycling rate exceeding 65%. Therefore, there’s a pressing need to recycle and reuse PCBs. Companies aiming to reuse waste PCBs should assess technology adoption from three perspectives:
(1) Environmental Benefits: Ensuring environmental protection at local and global levels, safeguarding air, water quality, soil, and human health from adverse impacts.
(2) Reuse Benefits: Facilitating the re-commercialization of waste PCBs while minimizing non-reusable waste.
(3) Cost-effectiveness: Striving for cost-efficient processing methods, tightly linked to the chosen process technology.
1.3 Main Aspects of Recycling Waste PCBs
Globally, recycling and reusing waste PCBs, particularly extracting metals, have been ongoing for some time. Metal recovery, primarily conducted by metal smelting plants, targets metals like copper, gold, and nickel used in PCB circuits and components. Lead and tin, among others, are common in component joints. Precious metals such as gold, silver, and palladium are also found in electronic components. Research and statistics show that desktop computer mainboards predominantly contain various metals: copper (66.9%), tin (10.7%), iron (10.3%), lead (7.61%), gold (0.11%), palladium (0.02%), with other metal components making up the remainder (4.41%). The concentration of gold and palladium in waste PCBs exceeds that in raw ore by 2-20 times, indicating significant potential for their recovery. Additionally, PCB resin includes flame retardants, primarily bromine-containing compounds, antimony oxide, and phosphorus compounds. Separating these impurities before copper refining is crucial for ensuring safety during metal recycling. Recent research on waste PCB reuse focuses on metal recycling technology, whole PCB decomposition, and halogen component removal methods.
2. Metal Recovery from Waste PCBs
Currently, metal recovery from waste PCBs predominantly employs the dry method, involving metal smelting. Other methods, like chemical dissolution (wet method) and biological processes, are also emerging.
2.1 Dry Metal Recovery from Waste PCBs
Dry metal recovery involves pre-treatment before smelting, utilizing methods like dry distillation or pulverization to separate metals and electronic components. Separation techniques leverage differences in particle size, relative density, static electricity, magnetic and wind forces to enhance metal recovery rates. Dry distillation generates gases containing brominated flame retardants, necessitating secondary combustion and subsequent rapid cooling to prevent bromide formation. Compliance with pollution-free liquid standards is crucial for wastewater treatment during processing. Copper extraction from waste PCBs involves smelting in a self-melting furnace, followed by refining and electrolytic extraction. Slag produced during smelting, rich in SiO2 from glass fibers, is recycled for adhesives and paving materials. Notably, the dry method is popular for recycling discarded mobile phones. Japanese research focuses on pre-processing to enhance metal recovery efficiency by effectively removing non-metal components.
2.2 Wet Metal Recycling from Waste PCBs
The wet method involves pulverizing waste PCBs and using acid solutions to ionize and dissolve metals, followed by precipitation to recover metals like silver. Some propose immersing broken waste PCBs in mineral acid and hydrogen peroxide solutions to dissolve metal materials.
3. Component Separation in Waste PCBs
In addition to metal recycling, considerable progress has been made in recycling other PCB components, particularly through improved separation and purification methods. Thermosetting resins like epoxy and phenolic resins, commonly used in PCB substrate materials, pose challenges due to their insolubility post-curing. The mainstream separation process involves first separating thermosetting resins from the PCB, then separating remaining metal and glass fiber components. Heating and burning PCBs enable resin separation, followed by recycling of the residue. Research indicates that larger PCB samples yield higher metal recovery rates due to reduced gas dispersion during combustion. However, processing micro-sized PCBs incurs lower energy consumption and recycling costs.
4. Halogen Component Removal from Waste PCBs
Halogen flame retardants in PCB resins, though vital for safety, are considered environmentally and medically harmful. Halogen removal research aims for “harmless” recycling and reuse of waste PCBs. Methods include hydrogenation, thermal decomposition, solid-phase reactions, biochemical processes, and electrolysis. Hydrogenation, employing metal catalysts like palladium, facilitates halogen removal through a reaction with hydrogen. Thermal decomposition decomposes halogen-containing resins, followed by reaction with alkaline compounds to extract halogens. Solid-phase reactions involve mixing metal compounds like calcium oxide with waste PCB resins to extract halogens as halide salts. While biochemical methods utilizing microorganisms have been proposed, their lengthy processing times make them unsuitable for large-scale halogen removal. Electrolysis, a promising method, dissolves halogen compounds in organic solvents and separates them via electrolytic reactions.
