**2. Materials and methods**

#### **2.1 Electronic waste collection**

The waste PCBs are obtained from the e-waste disposal unit in India. The sample was initially cleaned manually to remove dust particles by the air blower. Later, other *Leaching Technology for Precious Heavy Metal Recapture through (HCI + HNO3)… DOI: http://dx.doi.org/10.5772/intechopen.102347*

elements such as capacitors, resisters, integrated circuits, diodes, transistors, etc., were detached with the help of mechanical tools (saw metal cutter, sheet metal cutter, metal lathe cutting tool, cutting pliers, and materials separation toolkit). This separation is not as simple due to the difference in the physical characteristics of metals and non-metals. Hence, different separation methods, such as pneumatic separation, magnetic separation, filtering, eddy current separation, electrostatic separation, etc., are used to enrich metals and non-metals [10, 14, 23, 27].

The crushed PCBs obtained from the crusher are then pulverized and further exposed to milling operations for better size reduction using a ball mill, and particles of different mesh sizes are analyzed. The weight fraction of crushed PCBs obtained from the lower screens of jaw crushers with a capacity of 80 kg hr.�<sup>1</sup> and a clearance of 10 mm is much lower, making better ion recovery impossible. Thus, it is subjected to 5 mm of clearance in the same jaw crusher, yielding samples weighing 65, 53, 48, and 36 grams for sieves with mesh sizes of 0.3, 0.18, 0.05 mm, and pan, respectively, when screened using a rotary sieve shaker at a speed of 60 rpm with a power of 0.25 HP and a single-phase 80 volt supply. As the reduction in size increases the rate of recovery of metal ions [16], the resulting crushed samples are processed into powder form using a pulverizer with a disk diameter of 175 mm operated by a 3-phase motor at 1400 rpm in a 225–445 V supply (**Figure 2** and **Table 1**).

#### **2.2 Chemical leaching experimentation with aqua regia**

#### **2.3 Leaching mediaPreparation**

The leaching media is an important factor that should be considered while extracting heavy metals from PCBs. Various sorts of leaching agents show different leaching rates with respect to the type of metals present in PCBs. H2SO4, HCI, NaCl,

#### **Figure 2.**

*Stepwise size reduction of PCBs under the various mechanical operations (jaw crusher, roll crusher, furnace and pulverized mills produced small sizes between 4 and 0.05 mm) and heavy metals presents before leaching by SEM with EDx analysis.*


#### **Table 1.**

*Analysis of PCB size reduction.*

HNO3, Na2S2O3,etc. are commonly used leaching media for the extraction of heavy metals from PCBs. Aqua regia, which is a mixture of hydrochloric acid and nitric acid, is used as a leaching agent in this study. It is prepared by mixing HCI and HNO3 in a 3:1 ratio under specified conditions of temperature, time, and surrounding conditions. Different leaching agents show different rates of recovery and metal extracted with respect to the nature of the leaching media, rate of ion exchange, degree of dissociation of ions and various parameters such as time, temperature, concentration etc. When remaining constraints are held constant, metal ions such as Cu<sup>+</sup> , Zn<sup>+</sup> , Pb<sup>+</sup> , and Sn<sup>+</sup> , among others, exhibit different ionic properties with acid medium. Even though the above studies result in significant metal recovery, they also possess demerits, such as the targeted extraction of a specific metal leads to the loss of several other valuable metals. By using aqua regia as a leaching reagent, heavy metals such as Cu, Sn, Pb, and Zn can be extracted from PCBs with a high recovery rate. Aqua regia preparation involves the mixing of strong acids. It produces heat and toxic gases, so it is necessary to follow safety protocols while preparing and handling this solution. In this experiment, HNO3 is added to HCI contained in a beaker, which is placed in a water bath in order to reduce the fume generation. The two concentrated acids are mixed in the ratio of 3:1 (HCI:HNO3); concentrated HCI is about 35% and concentrated HNO3 is about 65%. So that volume ratio will be 4 parts concentrated HCI and 1 part concentrated HNO3. The solution is kept away from organic contaminants because it leads to vigorous or violent reactions and a low temperature should be maintained.

#### **2.4 Treatment of PCBs with aqua regia**

All the experiments are carried out in a conical flask incorporated with a temperature-controlled shaker. Primary analysis was conducted by applying specific conditions to obtain a standard recovery rate. 5gm of PCB samples are allowed to react with 20 ml of leaching media inside the conical flask at 60°C and shaken in a mechanical shaker at a shaking speed of 120 rpm for 2 hrs. At the end of this effective contact time, the shaker is stopped and the solution in the conical flask is filtered using filter paper. After complete filtration, the filtrate is sent for SEM with EDX analysis to determine the composition of metals retained. The rate of leaching is affected by a

number of factors, including sample size, concentration, temperature, shaking speed, and contact time. By varying these parameters, different values for the recovery rate and composition of heavy metals are obtained.
