**1.1 Health hazards caused by informal disposal of e-waste**

E-waste not only includes household and industrial electrical appliances but also includes their components such as batteries, capacitors, castings, etc. Recycling of such waste has been carried out both formally and informally in several countries like China, India, Ghana, Thailand, Vietnam, etc. [9]. Traditional recycling techniques are well developed techniques to ensure safe and efficient separation, but are highly expensive to install and run. So such techniques are not taken into consideration and cheap informal techniques are to be implemented. This may cause the release of several pollutants into the environment, which can lead to a variety of health problems [8–10]. The metals present in the PCBs are highly toxic and hazardous to living bodies. These metals follow media such as dust, air, water, and soil to reach the human frame. Exposure to metals such as lead (Pb) and cadmium (Cd) affects reproductive health, growth, and mental instability and damages human DNA [11–13]. Health symptoms like headaches, dizziness, irritation in the eyes, nose, mouth, etc. are caused by the exposure to copper (Cu) which is present in landfills [14–16]. The different e-waste sources, heavy metals, and effects are explained in **Figure 1**.

**Figure 1.** *Diagram of e-waste sources in various aspects and health effects.*

*Leaching Technology for Precious Heavy Metal Recapture through (HCI + HNO3)… DOI: http://dx.doi.org/10.5772/intechopen.102347*

Informal treatment strategies as well as innovative metal recovery approaches based on the material composition present in PCBs are accompanied by management and sustainable treatment techniques involving the reduction of waste circuit boards in developing countries. There are two techniques used to dispose of and extract heavy metals from PCBs. Incineration was used as a primary method through hightemperature PCB melting and is very hazardous as it releases strong toxic metal vapors such as polycyclic aromatic hydrocarbons and dioxins due to the emission of possible contaminants during incineration [17, 18], and the secondary method was followed at low temperature by hydrometallurgical techniques with the help of chemical reagents [19–21].

## **1.2 Pyrometallurgical method**

Pyrometallurgical processing is the most common method used for the separation of heavy metals from PCBs. The smelting process consists of the melting of waste PCBs in a high-temperature furnace (up to 1500°C) and is primarily used for the recovery of copper from used waste circuit boards. The limitations of this process are relatively low performance, high energy consumption, and difficulty in distinguishing metallic and non-metallic components [19, 22]. The pyrometallurgy process involves the heating of e-waste at a high temperature to recover precious metals. This treatment leads to the release of dangerous gases into the air, which must be extracted from the air by the flue gas cleaning system [23]. The limitations of this process are:


## **1.3 Hydrometallurgical method**

The hydrometallurgical method includes the application of aqueous solution leaching media, such as strong acid or base, oxidizing agents, and complexion agents, for the recovery of heavy metal separations. Previous studies have employed various leaching media such as strong acids (sulfuric acid, nitric acid), bases (sodium hydroxide, sodium hypochlorite), and complexing agents (cyanide, thiosulphate). This treatment approach has advantages over pyrometallurgical processes such as reduced pollution, radioactive contaminants, and moderately toxic chemicals for

environmental effects. Therefore, these various recovery methods used for the treatment of used PCBs need to be reconsidered due to the enormous amount of flammable, toxic, and corrosive reagents used and the large volume of effluents and other solid waste produced [24]. In hydrometallurgical procedures, the following steps are widely used: leaching and extraction, purification, and concentration of liquefied solutions, as well as the recovery of heavy metals. Four operations are typically implemented in these recovery operations, as shown in previous studies. The hydrometallurgical procedure, such as leaching, has shown a great deal of strength in several studies. Several leaching reagents demonstrate major improvements in metal recovery. When treated with different acidic media, aq.HNO3, aq.HCI, and aq.H2SO4, PCBs were cut to remove Cu**2**<sup>+</sup> ions and the recovery percentage of Cu**2**<sup>+</sup> was 97.5, 65, and 76.5%, respectively [20]. Only trace quantities of other metals can be extracted through this targeted extraction of copper. While using HCI as a leaching agent under specified conditions, the PCB sample size of 4�4 cm results in the separation of Cu, Zn, Sn, and Pb with a composition of 117.33, 28.97, 10.41, and 9.34 mg g�**<sup>1</sup>** , respectively [23].

The amount of Zn and Pb leached was very small when compared to the typical PCB metal content. The recoveries for Cu, Pb, Zn, and Sn were 16, 2.0, 1, and 1%, respectively, when leaching was done in crushed PCBs (size between 0.43 and 3.33 mm) using sodium cyanide solution [25]. It has the least compositional value compared to the average weight of the total metals found in it. After 480 min, various metals leached from PCB waste, such as H2SO4 and H2O2, provide 76% Zn, 85% Cu, 82% Fe, 77% Al, and 70% Ni recovery [26]. Other valuable metals are retained in the leaching solution as residue. The effective treatment of PCBs will depend on choosing the suitable recovery method. The development of new technology for the recovery of toxic heavy metal ions from waste PCBs remains an important scientific endeavor. The literature study shows the more stable and effective metal ion recovery will be achieved by two-stage chemical leaching with adsorption from waste PCBs. However, a suitable carrier has to be selected for the selective recovery of heavy metals in an effective manner.

This chapter presents an overview of current e-waste scenario, its impacts and treatment methods. The experimental studies are carried out for the the extraction of copper (Cu), tin (Sn), zinc (Zn) and lead (Pb) from PCBs by leaching using aqua regia (a mixture of HCI and HNO3 and HCI and H2SO4) for varying conditions of temperature, size of sample, contact time and shaking speed.

The main objectives of the present study are:

