PCB Blog - New Development of Waste PCB Board Reuse Technology

1 Overview

1.1 The necessity of carrying out the reuse of waste PCB board

At present, around the EU's WWW Directive and the implementation (or about to begin full implementation) of similar regulations in various regions and countries around the world, the recycling and reuse of waste electronic products have become a very important task. In this work, the recycling and reuse of the PCB board in waste electronic products are one of the most important issues. This paper mainly summarizes the technological progress in this area in Japan in recent years. In recent years, relevant Japanese authorities have investigated the amount of PCB boards contained in waste products of major household appliances and the amount of disposal in Japan. The rate (by mass) is below 10%. Electronic products with a high PCB content rate are computer products. The PCB board occupancy rate in the computer is 20-30% (mass ratio). According to the statistics of the Computer 3R Promotion Business Committee of the Japan Electronics Technology Industry Association, there are currently about 65,000 corporate computers in Japan every year, which need to be recycled and solved for reuse. According to the survey and speculation of the Japan Electromagnetic Information Technology Industry Association, the amount of domestic computer waste in Japan was about 10,000 tons in 2003, 50,000 tons in 2013, and 80,000 tons in 2015.

1.2 The principles to be grasped in the recycling of waste PCB board

The various components of the printed circuit board, taking the computer motherboard as an example, it is composed of three major materials: metal materials (about 50.8% of the total mass of the board): glass fiber cloth (about 16.3%), and organic Resin (about 32.8%). The separation of these three components of the PCB board is a more difficult task. For this reason, in the past, landfill methods were often used to dispose of these wastes. However, this treatment method fails to meet the requirements of the EU "WEEE" directive (the WEEE directive proposes that the regeneration rate of IT and small household electronic products should reach more than 75%; the recycling rate should reach more than 65%). Therefore, efforts must be made to recycle and reuse. For the reuse of waste PCB boards, companies should generally consider the benefits from three aspects in the adoption of technology: (1) Obtain environmental benefits. To achieve the benefits of sexual or regional environmental protection, that is, the air, water quality, soil, and human health are no longer affected. (2) Reuse benefits. To achieve the reuse and re-commercialization of waste PCB boards, and to reduce the amount of waste that cannot be reused to the limit as much as possible. (3) Cost-effectiveness. The processing cost for the reuse of waste PCB boards should below. Obtaining this low-cost benefit is closely related to the process technology used in the processing.

1.3 Main aspects of recycling waste PCB board

The recovery and reuse of waste PCB board in the world have been carried out early, and it is the recovery of the metal parts it contains. This work is mainly carried out by some metal smelting plants. Metals such as copper, gold, and nickel are used in circuits, terminals, etc. of waste PCB boards. Various metals such as lead and tin have been used in order to join components on the PCB. The electronic components mounted on the PCB also contain precious metals such as gold, silver, and palladium. According to the research and statistics of relevant institutions, the mainboards used in desktop computers mainly contain various metal material components, and their proportions are copper 66.9%, tin 10.7%, iron 10.3%, lead 7.61%, gold 0.11%, palladium 0.02%, other metal components 4.41%. In the raw ore containing gold and palladium, their content is only less than 10ppm, while the content of gold and palladium in the waste PCB board is 2-20 times as much as that in the raw ore. Therefore, the recovery and reuse of gold and palladium metals have great development prospects. A flame retardant component is also contained in the resin for PCB board. The flame retardants are mainly bromine-containing compounds, antimony oxide, phosphorus compounds, and so on. These impurities must be separated before the copper refining and recycling of waste PCB board, which is also the key to ensuring safety in the metal recycling process. In recent years, the world's research on the reuse of waste PCB boards has mainly focused on three aspects. That is, metal recycling technology, the decomposition technology of the whole PCB board, and the technology of removing halogen components from waste PCB boards.

2. Metal in waste PCB board

At present, metal recovery from waste PCB board mainly adopts the metal smelting method (this method is referred to as the dry method). In addition, other recovery methods such as the method of dissolving metal by chemical method (this method is abbreviated as the wet method) and biological method have also appeared.

2.1 Dry recovery of metals from waste PCB board

The metal smelting method is used to recover metals in waste PCB board, and pre-treatment is required before smelting and processing. Its pretreatment uses dry distillation or pulverization to separate metals and electronic components mounted on PCB board. This dismantling, separation, and screening project is relatively easy to carry out. The separation and screening of waste PCB board components can be separated by the difference in particle size and relative density. It can also be screened and separated by static electricity, magnetic force, wind force, etc. But the main purpose is always to improve the recovery rate of metals. It should be noted that a certain amount of gas will be generated by the method of dry distillation. There will be brominated flame retardants in these gases, so they need to be completely burned in the secondary combustion furnace. In order to prevent the possible generation of bromide in high combustion, the gas after combustion should be rapidly cooled with cold water. In terms of treatment facilities for discharging and recycling wastewater, special attention should be paid to achieving the standard requirements of pollution-free liquids. Through screening and separation means, the metal content of the recovered material is increased, and then it is put into the refining process of copper. Using the copper smelting method, the extraction process of copper components in waste PCB boards is to first put the waste PCB boards into a self-melting furnace, then smelt through a converter and a refining furnace, and extract the copper by electrolysis. The slag obtained during the smelting contains a large amount of SiO 2 components present in the glass fibers and is recovered to be used as a raw material for adhesives (filler), materials for paving, and other recycled products. For the recycling and reuse of metals in discarded mobile phones, a very popular treatment method is the dry method. It is to first remove the battery in the mobile phone, and then recycle the entire body. At present, in the research and development of the recycling and reuse of waste PCB boards in Japan, the focus is on the pre-processing project of recycling. Research how to remove components other than metals by effective means to improve the efficiency of metal recovery.

2.2 Wet recycling of metals from waste PCB board

The wet method is used to smash the osprey in the waste PCB board. United States: 2713231) proposed to use an acid solution to ionize and dissolve the metal on the PCB board, and then add alkali to precipitate silver, etc., to achieve the purpose of recovery. In the research content of this aspect, the method of immersing the broken material of the waste PCB board into the mixed solution of mineral acid and hydrogen peroxide, and dissolving the metal material to be obtained is also proposed.

3. Separation of components in waste PCB board

In recent years, in addition to the recycling of metals in waste PCB boards, the research on the reuse of other components of PCB board has also made considerable progress, which is highlighted in the separation and purification of the recovered components. In the manufacture of substrate materials for PCB board, in order to achieve high reliability and high interlayer adhesion of PCB boards, the commonly used resin adhesives are thermosetting resins such as epoxy resin and phenolic resin. These thermosetting resins become insoluble and infusible polymers after curing and molding, which makes it difficult to separate, remove and reuse them. For the separation of thermosetting resins that have become integrated into the PCB board, the current mainstream separation process in Japan is to first separate the thermosetting resin from the PCB board and then separate the remaining metal and glass fiber components. It heats and burns the PCB board, and then recycles the residue produced after the combustion. This test is carried out in a rotary LPG-type combustion furnace. The inner cavity volume of the combustion furnace is 1.3m × 0.5m × 0.5m. Put the waste PCB board sample to be recycled into the furnace (can be put in 5kg) and heat it to 1173-1223°F (ie 634-662°C). In this study, a comparative test was also conducted on the relationship between different sizes of recycled PCB board and the recovery rate. The obtained test result is that the PCB board sample with a large size has a higher metal recovery rate than the recycled PCB board sample with a small size and size (see Figure 4). This is because of the small size of the PCB board sample, more generated gas will be scattered during the combustion process. However, in the recycling and processing of micro-sized PCB board samples, the energy consumption and recycling costs are relatively low.

4. Removal of halogen components in waste PCB board

In general electrical products, household appliances, etc., for the sake of safety, flame retardants such as halogen and antimony compounds are added to the resin of the PCB board. It is currently considered to be harmful to the environment and the human body. Therefore, in the research of recycling waste PCB board, the removal of halogen components in waste PCB board has become an important research and development work. The test results of relevant foreign research departments show that: in the PCB board for desktop computers produced in the 1990s, the bromine content accounts for about 9% (the weight ratio of the PCB board is 100). The PCB board of TV, office electronic products (OE), etc. (generally, phenolic-paper-based copper-clad laminate is used as the base material), its bromine content accounts for 4.4%-5.3%, and the antimony content accounts for 0.4%-0.9%. The removal of halogen components in waste PCB board is mainly to remove the bromine substances present in the PCB board as flame retardants. Since the 1960s, the world's research work on halogen removal technology has begun to rise. The current work of removing halogen is mainly to achieve the "harmless" recycling and reuse of waste PCB board. The removal of halogens contained in the PCB board is to separate the halogen atoms bound to the benzene ring from the benzene ring. Theoretically, the separation between the chlorine atom and the benzene ring requires 916KJ/mol of energy, while It takes 879 KJ/mol of energy to separate the bromine atom from the benzene ring. In the method of removal treatment, solution method, dry solid-phase reaction method, etc.

(1) Hydrogenation method

Among the various dehalogenation inventions that have appeared recently, the process route of hydrogenation is the most numerous. This method of removing halogens in PCB boards by hydrogenation reaction is mainly in the presence of metal catalysts, the PCB board to be treated are contacted with hydrogen to generate a reaction between halogen atoms and hydrogen atoms. The metal catalysts used are generally noble metals such as palladium supported by activated carbon, and some research results have used hydrogenation catalysts such as lithium-based aluminum hydride. The hydrogen administration agent is mostly formic acid or formate.

(2) Thermal decomposition method

Thermal decomposition of the resin containing halogen flame retardant in waste PCB board, so that the resin containing halogen can be decomposed in liquid phase or lead to oily (crude oil recycling), and then the free halogen compound is combined with the added alkalinity. The compound reacts to extract the halogen-containing species. In some of these kinds of removal of halogens (such as JP 10-24274, JP 2001-172426, JP 9-262565, JP 9-249581, etc.), the types of basic catalysts and reaction temperatures that are in contact with each other are as follows: the difference. Halogen-containing resins in waste PCB board are mostly bromine-containing epoxy resins. After the bromine is separated by thermal decomposition, the added basic catalyst (potassium salt) also reacts with the separated bromine to form potassium bromide which can be easily recovered. This thermal decomposition method is more rigorous than the hydrogenation method in terms of recovering halogens. Its free halogen is more completely recycled.

(3) Solid-phase reaction method

The metal compound is used for dry mixing with the resin in the waste PCB board, and a solid-phase reaction occurs so that the halogen existing in the resin of the waste PCB board is separated and extracted in the form of a halide salt. This solid-phase reaction method for removing halogen components in waste PCB board mainly uses calcium oxide, iron oxide, silicon dioxide, and aluminum oxide as the metal compounds used.

(4) Biochemical method

The use of microorganisms to remove halogens from waste PCB board has also been proposed in recent years. However, using this method to remove and process the halogen concentration to ppm level takes a long time (several days or even months), so it is not suitable for the removal of halogens in large quantities of waste PCB board.

(5) Electrolysis

Relevant research results put forward the method of an electrolytic method to remove halogen in the PCB board. And it is considered that this is a kind of process method which has the possibility of being widely adopted in the future. It dissolves halogen compounds in the resin of waste PCB board in organic solvents and separates them through an electrolytic reaction.