KUMAR NARENDIRAN (IN)
KINHAL KRISHNA V (IN)
BHATT NIRAV P (IN)
SUBRAMANIAM PUSHPAVANAM (IN)
"Peroxides- Advances in Research and Application", Q.ASHTON ACTON, SCHOLARLY EDITIONS, 2013, ISBN: 978-1-481-67890-2
| CLAIMS: 1. A process for the recovery of tin (Sn), lead (Pb) and copper (Cu) from printed circuit boards of electrical and electronic devices, comprising of: a) adding 2.0 - 2.5 M nitric acid to said pieces of printed circuit boards to obtain a colloidal solution of stannic acid which is dispersed in an aqueous solution of lead nitrate and nitric acid; b) recovering stannic acid as precipitate by filtering, decanting or centrifuging at a high rate from the solution obtained in step (a); c) concentrating the dilute solution of lead nitrate and nitric acid by evaporating or distilling, and cooling the concentrated solution to obtain lead nitrate crystals or powder with or without adding acid of concentration above 13M; d) filtering the said precipitated lead nitrate obtained in step (c); e) physically or mechanically shaking the PCBs to separate electronic components from copper tracks of the said printed circuit boards which are present in undissolved printed circuit boards obtained from step (a); f) adding 4M - 15.7M nitric acid to the copper track of step (e), to obtain a solution of copper nitrate and nitric acid; and g) separating the copper nitrate from nitric acid through sequential steps of concentration and extraction. 2. A process for the recovery of Sn, Pb and Cu from printed circuit boards of electrical and electronic devices of claim 1, wherein the concentration process employed in step (g) is through distillation. 3. A process for the recovery of Sn, Pb and Cu from printed circuit boards of electrical and electronic devices of claim 1, wherein the NOx gases released in the steps (a), (c), (f) and (g) are sent to a scrubber in the reactor system, wherein the said NOx gases are absorbed and nitric acid is regenerated for reuse. |
The present invention relates to a method of extraction of metals from electronic waste to reduce the impact on environment. In particular, the invention relates to a method of extraction of metals from waste printed circuit boards by hydrometallurgical treatment. This method employs the principle of differential solubility of metals in different concentrations of acid.
BACKGROUND OF THE INVENTION
Increased accumulation of electronic waste, also known as e-waste, can lead to severe pollution of ground water sources by heavy metals. Significant amounts of precious/ rare metals are present in electrical and electronic waste. A large fraction of metals such as copper, lead, and tin contribute to the increased toxicity levels of ecosystem, and hence an extensive need to develop efficient metal recovery technologies exists.
Various pyrometallurgical, hydrometallurgical, and bio-hydrometallurgy approaches have been reported for metal recovery from e-waste in the prior art. Conventional processes however, may not meet the industrial feasibility because of issues such as inefficient recovery of metals, environmental pollution of process adopted, and high economic costs. Recovery of metals from e-waste is a challenging task due to the presence of a mixture of metals and alloys and in varying amounts.
Pyrometallurgical process is associated with demerits such as high temperature operation, high energy demand and release of toxic fumes to the environment, and high costs.
Use of hydrometallurgical methods to recover metals from e-waste has increased in recent years due to lower amounts of toxic gas emissions compared to pyrometallurgical process, and also reduced dust generation, less energy requirement, lower cost and significant recovery of metals. However, some of the hydrometallurgical processes of the prior art employ toxic reagents such as cyanide and thiourea in the leaching stage.
Some of the recent approaches include eco-friendly bio-hydrometallurgical process technologies such as chelation technology, ionic liquids etc.. However, bio-hydrometallurgical processes involve longer leaching time, and potential contamination possibilities. W02011130622A1 discloses a process for recycling printed wire boards by extracting metals from printed wire boards. The process involves contacting the printed wire boards with compositions containing an oxidizing agent, to selectively remove the solder and base metals.
WO2015052658A1 discloses a hydrometallurgical process for the recovery of metals from electronic boards. The process involves two steps comprising a leaching step wherein nitric acid is used, and a precipitation step wherein, different chemical reagents are used to precipitate different metals.
CN100342044C also discloses a step-by-step hydrometallurgical process to extract metals from electronic industry waste. In the first step, organic matter in the solid residue is removed by burning at high temperatures followed by the addition of nitric acid and filtration of the precipitate formed.
The existing techniques for treating, managing and handling of e-waste, use highly toxic chemical reagents which could result in polluting the atmosphere through emission of toxic and carcinogenic gases, and also cause acidification of soil. Existing techniques employ the use of several chemical reagents during the course of separation of metals which may be expensive and cumbersome to handle. When a combination of chemical reagents is used, recovering and recycling them need additional steps which can render them economically at a disadvantage. Therefore, there exists a need to develop a safe, environmentally friendly and economical scientific process for the recovery of metals from printed circuit boards (PCBs). A challenge in this area is the variation in the composition of the PCBs. To circumvent this challenge, the inventors of the present invention have developed a hydro-metallurgical process, wherein, the PCB is exposed to a single acid but with varying concentrations of nitric acid based on the principle of varying solubilities of different metals along with the ability to recycle the NO x gases or other harmful gases generated during the process.
Metal recovery and resource optimization is achieved through the present invention wherein after the recovery of the metals, the nitric acid generated is recycled in the process.
OBJECT OF THE INVENTION
The principal object of the present invention is to extract metals from printed circuit boards (PCBs) of waste electronic devices. Another object of the present invention is to sequentially extract the metals such as lead, tin and copper, using a single acid.
SUMMARY OF THE INVENTION
The present invention provides a method of extraction of metals from PCBs of electronic devices. The metals of focus in the present invention are tin, lead and copper which are the dominant components in PCBs. The metals in the PCB are extracted using nitric acid. Tin (Sn) and lead (Pb) are present in the solder. The tin (Sn) present in the solder dissolves when exposed to low concentration of acid to obtain stannic acid which is insoluble in nitric acid and can be separated by simple processes such as filtration, decantation, centrifugation and the like. The lead (Pb) present in the solder too dissolves in low concentration of nitric acid. The lead present in the PCBs comes out as an aqueous solution of lead nitrate and nitric acid at the end of this stage. The aqueous solution is evaporated or distilled to increase the concentration of nitric acid. Lead nitrate is insoluble in concentrated nitric acid, so it forms crystals or powder which can separated by simple processes such as filtration, decantation, centrifugation and the like. The electronic components attached to the copper tracks in the PCBs are separated by shaking or agitation. The copper track is then treated with high concentration of nitric acid, wherein the copper (Cu) present in copper tracks reacts with nitric acid to obtain copper nitrate which is highly soluble in nitric acid. Nitric acid can be separated from copper nitrate by distillation and extraction.
BRIEF DESCRIPTION OF THE DRAWINGS
The summary of the present invention, as well as the detailed description, is better understood when read in conjunction with the accompanying flowchart, of which: Fig. 1 illustrates the complete process for the recovery of metals from (PCBs) from electronic waste, in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION
The present invention is related to a system for the recovery of metals from PCBs of electronic devices. Generally, PCBs consist of resin boards which have a conducting copper track. These tracks are covered with an epoxy layer called solder mask. All the electronic components such as capacitors, resistors, transistors, integrated circuits, etc. are connected electrically to each other through a copper track. The electronic components are attached to the board by a soldering means that comprises Sn and Pb. Earlier versions of PCBs have a solder comprising of 63% Sn and 37% Pb. Nowadays, lead-free solders are used. But e-waste is available today primarily from old electronic equipment that contain lead as a part of solder which has to be treated. In accordance with the present invention, there is provided a process for the extraction of metals from waste PCBs. As described earlier, PCBs comprise mainly of three metals such as tin (Sn), lead (Pb), and copper (Cu). Pb and Sn are present in a solder and Cu is present as a track. Though other metals such as Fe, Ni, Ag, Au etc. are also present in the electronic components, the metals of focus in the present invention are tin, lead and copper which are the dominant components in PCBs. These three metals are extracted based on the difference in their solubility in varying concentrations of an acid. Nitric acid, which is commonly used as a strong oxidizing agent is used in the process for the extraction of metals from PCBs in the present invention. Nitric acid oxidizes metals to their salts and the formation of these products resulting from the oxidation reaction depend on the reaction temperature and the concentration of the acid used.
The metals obtained as metal salts after extraction have high commercial value and purity. Harmful and carcinogenic gases released are purified and recycled for further use. A step-by-step process involved in the extraction of lead, tin and copper metals from PCBs in accordance with the embodiments of the present invention, is described in detail herein.
Initially, PCBs are cut into small pieces, for instance into about 5x5 cm 2 in a cutting machine. If the dissolution reactor size is sufficiently big, then it is not necessary to cut the PCBs and the cutting step can be avoided. The dissolution process is done at room temperature (25 to 35 °C). These PCBs are dissolved in dilute nitric acid of concentration about 2M - 2.5M nitric acid for about an hour with a solid -liquid ratio of 1 :3 to 1 :7 grams/cm 3 . This ratio or the volume of nitric acid is adjusted in such a way that the liquid used is sufficient to submerge the PCBs. The concentration of nitric acid is kept low to prevent the reaction of Cu and other metals present in the electronic components, with nitric acid. Besides, the solder mask (epoxy layer) does not react with low concentrations of nitric acid and acts as a resist thus preventing the copper track from reacting. So, at the low concentrations of nitric acid chosen, only Sn and Pb present in the said solder dissolve. The dissolution takes about an hour and the following reactions take place.
Sn + 4HNO 3 -» H 2 Sn0 3 (precipitate)+4N02+H 2 0
Pb+ 4HN0 3 -» Pb (N0 3 ) 2 + 2N0 2 + 2H 2 0
Stannic acid (EESnCE) is formed as a precipitate in the above reaction and is insoluble in nitric acid. The precipitate is separated from the solution by decantation. The obtained stannic acid precipitate is heated to form tin oxide which is analysed using EDAX (Energy Dispersive X-Ray Analysis). This tin oxide has a composition comprising 43.76% wt. of Sn and 56.24 wt% of O when the dissolution of the PCBs is carried out in 2.5M nitric acid.
The tin precipitate obtained when PCBs are dissolved in 3M nitric acid is also analysed using ED AX and the elemental composition is as shown in Table 1.
Table 1. Elemental composition of stannic acid formed when PCB is dissolved in 3.0 M nitric acid From Table 1, it is evident that if the concentration of nitric acid is above 2.5M, fractions of other metals present in the PCBs will also be extracted along with the desired metal.
After the stannic acid separation, the aqueous solution comprising of lead nitrate and dilute nitric acid is collected. As the solubility of lead nitrate is low in concentrated nitric acid, the aqueous solution of lead nitrate and nitric acid is concentrated by evaporation, distillation, and the like. On cooling the concentrated solution, the lead nitrate crystals of high purity are formed. Lead nitrate gets precipitated or crystallized as it is insoluble in concentrated nitric acid and it can be separated by filtration. The unreacted nitric acid obtained in this step is recycled back for dissolution rendering the process economically viable. The lead nitrate crystals are also analysed using ED AX and the elemental composition is as shown in Table 2.
Table 2. ED AX elemental composition of lead nitrate crystals obtained
From Table 2, it is evident that the lead nitrate obtained is of high purity. Next, the solderless PCBs are taken out and all the electronic components present on it are separated physically by vigorous shaking. After separation of the electronic components, the resin boards containing the copper track covered with epoxy layer can be dissolved in 4M - 15.7M nitric acid. According to some of the embodiments of the present invention, the concentration range is preferably 4M -
8M as it reduces further downstream processing steps making the process simple. At these relatively higher concentrations (> 4M) of nitric acid, the epoxy layer peels off and the Cu present in it starts reacting with nitric acid as per the following reaction:
Cu + 4HN0 3 -> CU(N0 3 ) 2 + 2N0 2 + 2H 2 0
The aqueous mixture obtained from the above dissolution comprises of copper nitrate and nitric acid. Copper nitrate is highly soluble in nitric acid. Therefore, it can be separated from nitric acid by distillation followed by extraction with tributylphosphate (TBP). TBP extracts the nitric acid from the mixture of copper nitrate and nitric acid.
The gases released during the different dissolution steps in the entire process are sent to a scrubber in the reactor system, wherein these gases are absorbed by H 2 0 2 /H 2 0, and nitric acid is regenerated. The regenerated nitric acid is recycled for further use in the process disclosed by the present invention. The scrubber is filled with H2O2/H2O of sufficiently high volume. Specifically, 300ml-700ml of scrubber volume is found to be sufficient when treating 100 grams of PCB. This ensures that NOx gases do not escape into the atmosphere. The NOx gases released are dissolved in H2O2/H2O to form nitric acid. H2O2 or any other oxidizing agent can be used to oxidize the sparingly soluble nitric oxide to nitrogen dioxide which has a high solubility. Figure 2 represents a process for the recovery of NO x emissions during the treatment of PCBs with concentrated nitric acid in accordance with the embodiments of the present invention.
It is to be understood, however, that the present invention would not be limited by any means to the components, arrangements and materials that are not specifically described, and any change to the materials, variations, and modifications can be made without departing from the spirit and scope described in the present invention.