*3.2.1 Effectof concentration*

By varying the concentration, the leaching process shows a significant change in the recovery rate. The recovery rate increases with an increase in the concentration of the sample with respect to time. After attaining an equilibrium state, the rate of leaching becomes constant. 20 ml of aqua regia is used to leach heavy metals from 2, 4,

**Figure 3.** *Graphical representation of size reduction in different operation.*

**Figure 4.** *Presents of metal components from PCBs by the EDXs.*

**Figure 5.** *Graphical representation of % recovery of metals with concentration.*

6, 8, and 10 g of 0.3 mm sized PCB samples at standard conditions of 80°C of temperature and 200 rpm of speed for 2 hrs. The graph shows an increase in the recovery rate of metals with an increase in the concentration of PCB. The metals recovered in the decreasing order of Cu, Pb, Sn, and Zn were recovered. Copper is the most recovered metal, whereas zinc is the least recovered. When the concentrations were increased by 0.1 g ml˗<sup>1</sup> , all metals showed a slight increase in recovery rate.

As there is no decrease in the percentage of metals recovered, it is confirmed that the metallic distribution of powdered PCBs is uniform. When the concentration is 0.5 g/ml, the graph shows the maximum recovery with metallic composition as 92.06% of Cu, 55.42% of Sn, 48.27% of Zn, and 78.42% of Pb. Based on the previous studies of metal recoveries [28, 29].

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

**Figure 6.** *Graphical representation of % recovery of metals with size.*

#### *3.2.2 Effect of size*

The different sieve size particles are leached using aqua regia and the weight fraction of metallic components is analyzed. 5 gm of particles with sizes of 4, 2.3, 0.6, 0.3, and 0.05 mm are leached for hours at 80°C with a shaking speed of 200 rpm. **Figure 6** explains the relationship between size and recovery. It shows recovery increases with an increase in contacting surface.

The graph shows an appreciable increase in the percentage of metals recovered with a decrease in the size of the sample. The higher recovery rate is shown for the PCB sample at the lowest size, which is 0.05 mm. The uniformity of metallic distribution is also conserved here. Copper is the major component present in the leached sample and zinc is the minimum. It results in a percentage recovery of copper, tin, zinc, and lead of 83.49, 58.72, 57.75, and 78.42%, respectively.

#### *3.2.3 Effect of temperatire*

5 gm of PCB samples of size 0.3 mm are treated with aqua regia in a conical flask and shaken at a speed of 200 rpm for 2 hrs. Five samples of the same condition are maintained at varying temperatures of 40, 60, 80, 100, and 120°C. After completion of effective time, the leached PCB sample is analyzed. The data obtained is represented graphically and the relations between recovery and temperature are studied. The graph shows an appreciable increase in recovery rate with an increase in temperature for a particular point of temperature [30, 31]. When the sample is leached at a temperature above 80°C, there is no appreciable change in recovery, which is negligible. It indicates that the leaching depends on temperature only for a particular limit, and that after a certain point of temperature, leaching is independent of temperature. At 80°C, the result shows a metallic composition of 89.84% of Cu, 69.05% of Sn, 65.51% of Zn, and 82.45% of Pb (**Figure 7**).

**Figure 7.** *Graphical representation of % recovery of metals with temperature.*

#### *3.2.4 Effect of time*

To determine the effective leaching time, the sample is allowed to be leached for different intervals of time. The persistent condition is maintained as a 5gm sample of size 0.3 mm shaken with 20 ml of aqua regia in a conical flask and shaken at a speed of 200 rpm while the temperature is maintained at 80°C. Then it is allowed to be leached for 1, 2, 3, 4 and 5 hrs, respectively. The data collected is graphically represented in **Figure 8** below. It shows that the recovery percentage is almost constant when a sample is leached for more time after a certain period of time. The graph shows that the recovery of metals increases with an increase in time for a certain period, and after a particular point of time, the recovery becomes almost constant. That means all the

**Figure 8.** *Graphical representation of % recovery of metals with time.*

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

metals in contact are leached from the sample within a particular time period, and there is no use in leaving the system under leaching condition after a certain period of time. The results show the maximum recovery when the sample is leached for 3 hrs. When the sample is leached for 3 hrs, the result shows the recovery of metals as 91.36, 69.43, 72.41, and 83.22% of Cu, Sn, Zn, and Pb, respectively.

## **3.3 Optimum conditional parameter studies**

Once all the results for recovery with respect to various parameters are evaluated and studied as explained above, we get the optimum condition to obtain maximum recovery of metals. The optimum condition is the value of concentration, size, temperature, shaking speed, and time at which the maximum recovery is obtained. The results obtained at optimum conditions show that the recovery of heavy metals is as high as 99.9% of copper, 98.3% of lead, 96.8% of tin and 93.1% of zinc, respectively. In this study, specific conditions of 800°C, 0.05 mm of thickness, 3 hours of contacting time, 80 rpm shaking speed, and pulp density of PCB sample of 20gm L�<sup>1</sup> were met in both stages, with a 3:1 ratio of first stage HCI and HNO3 and second stage HCI and H2SO4) prepared as a leaching agent. The experimental results were obtained under the above mentioned conditions and have been shown in (**Figure 9)** and (**Table 2**). Results found that the optimum recovery rate for stage I Cu was 89.5%,

**Figure 9.** *EDXs spectrum analysis for metal ions obtained after leaching.*


#### **Table 2.**

*Metallic composition of leached PCBs at optimum conditions by stage-I & II.*

Sn 64.4%, Zn 63.4%, Pb 80.9%, and stage II Cu was 99.0%, Sn 96.8%, Zn 93.1%, and Pb 98.3%, respectively.
