Removal technology of halogen components from waste PCB
For safety reasons, general electrical products and household appliances add flame retardants such as halogens and antimony compounds to the resin of PCBs.
It is currently believed that it is harmful to the environment and human body. Therefore, in the research on recycling waste PCBs, removing the halogen components in waste PCBs has become an important research and development work. The test results of relevant foreign research departments show that in the PCBs for desktop computers produced in the 1990s, the bromine content accounted for about 9% (weight ratio based on the weight of PCB as 100). The PCBs of televisions, office electronic products (0E), etc. (generally using phenolic-paper-based copper-clad laminates as the substrate) contain 4.4%-5.3% bromine and 0.4%-0.9% antimony.
Removing the halogen components from waste PCBs mainly removes the bromine substances that exist in PCBs as flame retardants. Since the 1960s, the world has begun to study the technology of removing halogens. Today’s halogen removal work is mainly to achieve the “harmless” recycling and reuse of waste PCBs. The removal of halogens contained in PCB is to separate the halogen atoms bound to the benzene ring from the benzene ring. Theoretically, the separation of chlorine atoms from the benzene ring requires 916KJ/mol of energy, while 879KJ/mol of energy is required to separate bromine atoms from the benzene ring.
In terms of the removal method, there are decomposition method, dry solid phase reaction method, etc.
(1) Hydrogenation method
Among the various dehalogenation invention patents that have appeared recently, the largest number is the hydrogenation process route. This method of removing halogens from PCB through hydrogenation reaction is mainly in the presence of a metal catalyst, the treated PCB is exposed to hydrogen, and a reaction between halogen atoms and hydrogen atoms occurs. The metal catalyst used is generally a precious metal such as palladium with activated carbon as the carrier. Some research results have also used hydrogenation catalysts such as lithium aluminum hydride. The hydrogen donor is mostly formic acid or formate.
(2) Thermal decomposition method
The resin containing halogen flame retardant in waste PCB is thermally decomposed to decompose the halogen-containing resin in liquid phase or cause oil (resource conversion of crude oil), and then the freed halogen reacts with the added alkaline compound to extract the halogen-containing substance. In some patents for removing halogen (such as: Patent No. 10-24274, Patent No. 2001-172426, Patent No. 9-262565, Patent No. 9-249581, etc.), the types of alkaline catalysts and reaction temperatures used are different. Halogen-containing resins in waste PCB are mostly bromine-containing epoxy resins. After bromine is separated by thermal decomposition, the added alkaline catalyst (potassium salt) also reacts with the separated bromine to form potassium bromide that can be easily recycled. This thermal decomposition method is more rigorous than the hydrogenation method in terms of recovering halogen substances. Its free halogen is more completely recycled.
(3) Solid phase reaction method
Metal compounds are dry mixed with the resin in the waste PCB to produce a solid phase reaction, so that the halogens in the resin of the waste PCB are separated and extracted in the form of halogenated salts. The metal compounds used in this solid phase reaction method for removing halogen components from waste PCBs are mainly calcium oxide, iron oxide, silicon dioxide, aluminum oxide, etc.
(4) Biochemical method
The method of using microorganisms to remove halogens from waste PCBs has also been proposed in recent years. However, it takes a long time (several days or even months) to remove and treat the halogens to a concentration of ppm using this method, so it is not suitable for the removal of halogens from large quantities of waste PCBs.
(5) Electrolysis method
Related research results have proposed a method for removing halogens from PCBs by electrolysis. It is believed that this is a type of process that has the potential to be widely adopted in the future. It is to dissolve the halogen compounds in the resin of the waste PCB in an organic solvent and separate them through an electrolytic reaction.