**4. Conclusions**

In this study, copper was found to be the most abundant metallic element in waste printed circuit boards, followed by iron and tin. Silver was the primary precious metal present in printed circuit boards at 513 ppm. Silver was liberated, in addition to tin, lead, and nickel. Gold was present at approximately 145 ppm and was usually found liberated or as a gold–nickel alloy.

Given that physical pretreatment was not satisfactory as a result of a significant loss of precious metals, direct bromide leaching was investigated. The thermodynamic calculations illustrated a region where bromine could dissolve gold to form stable tetrabromo gold ions. A series of leaching experiments demonstrate that the concentration of bromine and sodium bromide were the most significant factors influencing bromide leaching. It also appeared that the addition of hydrochloric acid can reduce the usage of sodium bromide. Furthermore, the kinetic results indicate that a combination of chemical and diffusion controlling is responsible for gold dissolution in waste printed circuit boards.

Further investigations will focus on the implementation of a two-step leaching, where the first step will be used to first remove base metals and then, the second step will be to dissolve precious metals by bromine and sodium bromide. Additional studies should be performed in order to eliminate the impurity effect by using a gold disc technique and correlate the existing kinetic with other factors, such as gold distribution and adsorption. Finally, more work needs to be performed to investigate and optimize the metal purification and recovery flowsheet.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-4701/10/4/462/s1. Figure S1: Eh-pH diagram of Br-Ag-H2O system at 25 ◦C ([Ag] = 10−4 M, [Br] = 0.775M) (Stabcal). Figure S2: Eh-pH diagram of Br-Pd-H2O system at 25 ◦C ([Pd] = 10−5 M, [Br] = 0.775M) (Stabcal). Figure S3: Eh-pH diagram of Br-Cu-H2O system at 25 ◦C ([Cu] = 10−3 M, [Br] = 0.775M) (Stabcal). Figure S4: Effect of bromine concentration on gold dissolution at 23.5 ◦C and 400 RPM. Figure S5: Reaction orders with bromine during gold dissolution. Figure S6: Effect of sodium bromide on gold dissolution at 23.5 ◦C and 400 RPM. Figure S7: Effect of copper on gold dissolution at 23.5 ◦C and 400 RPM. Figure S8: Arrhenius Plot for the gold-bromine leaching system.

**Author Contributions:** Conceptualization, C.A. and H.C.; methodology, C.A. and H.C.; formal analysis, H.C.; investigation, H.C.; data curation, H.C.; writing—original draft preparation, H.C.; writing—review and editing, C.A.; supervision, C.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Center for Resource, Recovery and Recycling (CR3).

**Conflicts of Interest:** The authors declare no conflict of interest.
