Several methods based on pyrometallurgy and Hydrometallurgy Equipment are currently used for the recovery of metals from waste PCBs. Pyrometallurgical processes require heating the waste EEE at high temperatures to recover valuable metals. These treatments lead to the production of hazardous gases that must be removed from the air with flue gas cleaning systems. These processes are energy intensive and high-cost and require high-grade (rich in copper and precious metals) feeds. The formation of dioxins and furans is unavoidable due to the use of halogenated flame retardants, which presents environmental problems; thus, off-gas treatment is a prerequisite. Compared with pyrometallurgical processes, hydrometallurgical processes offer a relatively low capital cost, reduced environmental impact and high metal recoveries. These processes are relatively suitable for small-scale applications. These attributes make the hydrometallurgical process a potential alternative for the treatment of waste EEE. Hydrometallurgical processes involve the dissolution of metals in alkaline or acid medium. Several studies have reported the use of nitric acid (HNO3), HCl, sulfuric acid (H2SO4) and aqua regia for the recovery of metals from waste PCBs. Reagents such as cyanide, halide, thiosulfate, and thiourea have also been commonly used for the recovery of precious metals. Most of the above-mentioned reports used powdered/pulverized waste PCBs for metal recovery. However, little is known about the use of large pieces of PCBs for the recovery of metals. The effect of the above-mentioned leaching reagents on the PCB pieces during the hydrometallurgical recycling process has also not been reported. If complete metal recovery can be achieved from large pieces of PCBs, then the remaining board (nonmetallic part) can be easily recycled, which would be difficult if pulverized PCBs were used. In this study, we propose an efficient hydrometallurgical process using large pieces of PCBs rather than pulverized PCBs to simplify the overall recycling (metallic as well as nonmetallic fraction) process. In the proposed process, the waste PCBs were pre-treated to remove the chemical coating present. Then, the PCBs were subjected to an acid leaching process for metal recovery.

Platinum is one of the precious metals with many applications, including in catalysis, electronic devices and jewelry. However, its limited resources are becoming depleted. To meet the future demand and conserve resources, it is necessary to process spent platinum-containing materials, such as catalysts, electronic scraps and used equipment. These materials are usually processed by pyro/hydrometallurgical processes consisting of thermal treatment followed by leaching, precipitation or solvent extraction. Platinum leaching from such resources using acidic and alkaline solutions in the presence of oxidizing agents, such as nitric acid and hydrogen peroxide, sodium cyanide and iodide solutions. It is described with respect to the recovery of platinum and other metals under the optimized conditions of leaching with lixiviants. Previous studies have achieved platinum recovery using aqua regia and acidic solution in the presence of chlorine to produce platinum from spent catalysts on a commercial scale; however, the process generates toxic nitrogen oxide and chlorine gases. The salient findings of efforts to replace the aqua regia with hydrogen peroxide in acidic solution, chloride salts, sodium cyanide and iodide solution to improve the economics of the existing processes and reduce the environmental pollution.

The zinc industry has been in the forefront of hydrometallurgical developments for almost a century. The development of pressure leaching in the 1980s and the development of atmospheric direct leaching in the 1990s for treating zinc-sulphide concentrates have resulted in a number of zinc-refinery expansions without an increase in roasting capacity. Likewise, solvent extraction techniques are now used for the hydrometallurgical treatment of zinc-oxide ores on an industrial scale. The treatment of poor and complex zinc-sulphide resources has been extensively studied using processes as diverse as pyrolusite leaching and heap bioleaching. Finally, the precipitation of relatively pure zinc oxide at the mine site, a process that has been proven to be technically feasible very recently, may significantly change the paradigm of primary zinc production.

Hydrometallurgy consists of a series of separations that begins with leaching of ores or concentrates and ends with fairly pure, marketable cathodes, powders, or compounds recovered from solution. Intermediate separations are conducted to recover by-products, isolate impurities, or enhance the productivity of subsequent unit operations. There is a constant search for new technologies that will: (1) increase the productivity of parts of the process; (2) reduce operating costs; (3) reduce adverse environmental impact of effluents from the process; and (4) (in the case of the need for new plant capacity) develop new, simpler, cleaner, more economic processes.

The need for copper and molybdenum continues to grow as does the need for clean efficient metallurgical technologies capable of treating mixed metal concentrates. Currently, the use of hydrometallurgical pressure oxidation for copper concentrate treatment is growing. Furthermore, with limited molybdenum roasting capacity, stringent industrial molyb-denum concentrate roaster feed specifications, poor rhenium recoveries, and the inherent environmental issues asso-ciated with pyrometallurgical treatments, hydrometallurgical options are also now being pursued for molybdenum concentrates. Moreover, given the inherent grade, recovery and cost inefficiencies in the differential flotation process normally employed for molybdenum concentrates produced as a by-product of copper mining, there is a growing need to directly treat combined bulk copper and molybdenum concentrates. This minimizes molybdenum concentrate roasting limitations, specifications and requirements, while allowing simplification of increased efficiency of and cost reductions in upstream mineral processing circuits now producing separate copper and molybdenum concentrates by differential flotation. It would also allow more direct and efficient recovery of rhenium. Finally, hydro metallurgical technology will also reduce the need for costly final molybdenum concentrate impurity treatment circuits, thereby allowing for lower grade mixed metal molybdenum concentrates to be treated directly for a greater metal value realization. In summary, industrial nitrogen species catalyzed (i.e. NSC) hydro metallurgical pressure oxidation has many advantages over conventional pressure oxidation systems and offers a tangible process route to treat mixed bulk concentrates.

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