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CHAPTER 7 LEACHING KINETICS In order to understand the kinetics of bromide leaching of gold from waste printed circuit boards, it is important to first determine a rate expression of the bromine-gold system. That expression can subsequently be utilized in creating a leaching model. Moreover, the effects of temperature and agitation speed were also investigated. Finally, a leaching kinetic model was created based on the empirical data. 7.1 Procedure In order to determine the reaction rate expression and create the leaching kinetic model for gold, a series of leaching trials were conducted in a 250 mL beaker sealed by PARAFILM sealing films. Shredded waste printed circuit boards were dissolved for 40 minutes, given that gold is readily dissolved in a bromine-sodium bromide system in a short time period. Since the gold dissolution is exothermic, the 250 mL beaker was placed in a 1 L beaker filled with 300 mL tap water to maintain the desired temperature. A leaching solution of 200 mL with the desired quantity of reagents was used for each experiment. All experiments were carried out using solutions prepared from analytical grade reagents and deionized water (18.2 MΩ∙cm, 23.5 °C). Unless specified, the solutions contained 10 grams of shredded printed circuit boards and were mixed by a stirring bar at 400 RPM. The oxidation-reduction potential (ORP) and pH were measured by the Fisher ScientificTM accumetTM XL 600 meter. Samples of the solution were taken at a fixed interval of time during the leaching experiment and filtrated by filter papers (Whatman Grade 42). The filtrate was then analyzed instantly by atomic absorption spectroscopy (AAnalyst 400, PerkinElmer Inc.) in order to determine the content of gold in the solution. The residue after leaching was then filtered and rinsed by deionized water. Subsequently, the filtrated residue was dried, weighed and treated by a 4-acid digestion (HCl-HNO3-HF-H3BO3). The procedure of the 4- acid digestion was presented in Section 6.2. The gold content was then again determined by atomic absorption spectroscopy. In addition, a triplicate analysis was conducted for each condition to make an average. 89PDF Image | HYDROMETALLURGICAL TREATMENT OF E-SCRAP
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