Gold Leaching From Waste Printed Circuit Board By Thiocyanate Process

The rapid development of the electronic industry leads to a significant increase of waste printed circuit board(PCB). PCB is composed of plastic, refractory oxides and metals such as Au Ag Cu Hg Se Ni Pb Cr. The main economic driving force for the recycling of PCB is the recovery of metals, particularly precious metals like gold.In this paper, some kinds of PCB have been taken as examples to summarize the characteristics of PCB and the methods of recycling metals from PCB at home and abroad. The methods can be broadly divided into physical processes, pyrometallurgical processes, hydrometallurgical processes, pyrolysis, supercritical water oxidation, and so on. The objective of this research is to offer basic data for industrial application of gold recovery from waste PCB by thiocyanate process and to promote a sustainable development of recycling of waste PCB through proposing the non-cyanide process for gold recovery.The work described in this thesis mainly deals with three parts, i.e., Analysis of the component and content of metals in PCB; Dissolving the copper from PCB; Gold leaching from PCB by thiocyanate process.Part one: The element component and content of metals in PCB were determined by AAS and ICP-AES methods respectively. Result indicated that the circuit board was mostly composed of copper(883.5mg/g); the content of gold was 0.324mg/g.Part two: Dissolving copper from PCB by H₂SO₄—H₂O₂ system was the pretreatment for gold leaching. Experimental results obtained were listed as follows: 30℃, 40mL 2.4mol/L H₂SO₄ and 10ml H₂O₂ mixed with every 2g powder of PCB, 2.5h, which resulted in recovery of 98 % of copper.Part three: Leaching gold from pretreated powder of PCB by thiocyanate process.First, the composition characteristic for gold leaching systems, the relationship between oxidants and ligands and the thermodynamic criterion of oxidants selected for gold leaching systems t were analyzed in details. The gold leaching systems are composed of ligand and suitable oxidant. The oxidation potential of gold is controlled by the stability constant of gold-complex species and special leaching conditions. The larger stability constants, the smaller the standard electrode potential of oxidants needed for gold leaching systems. The leaching in the form of gold(III)-complex species could be performed spontaneously provided that the standard electrode potential of oxidants is larger than (1.50—0.0197lgK₂) V, and when the standard electrode potential of oxidants is larger than (1.68—0.0591lgK₁) V, the leaching in the form of gold(I)-complex species could be carried out according to thermodynamic prediction.Second, under these theoretic analyses, the effect of time, temperature, pH value, solid/liquid ratio, concentration of oxidants on the leaching of gold was discussed in detail. Experimental results obtained were listed as follows:â‘ Thiocyanate-MnO₂ system: powder of PCB : MnO₂(weight)=1: 0.7, 0.4mol/L NaSCN, solid/liquid ratio: 1/22, pH=1～2,～20℃, 3h. The recovery of gold was over 96%.â‘¡Thiocyanate-iron(III) system: 0.1mol/L Fe³⁺, 0.4mol/L NaSCN, solid/liquid ratio: 1/22, pH=2,～20℃, 3h. The recovery of gold was about 96%.Third, kinetics models have been established for the processes of gold leaching by these two systems respectively. The results indicated that the rates of the reactions were both first order; the activation energy of the thiocyanate-MnO₂ was found to be 5.3 kJ/mol; the activation energy of the thiocyanate-iron(III) system was found to be 5.1 kJ/mol and both of these two processes were considered to be diffusion-controlled.Generally speaking, the recovery of gold from waste PCB by thiocyanate process is feasible in technique and beneficial in both economy and environment.