A Multifocal Study Investigation of Pyrolyzed Printed Circuit Board Leaching

Abstract

Electric waste from numerous devices that are put out of use every day has some form of printed circuit board that contains precious and valuable metals in their components. In order to extract these metals, the printed circuit boards were crushed and pyrolyzed into powder. The fine pyrolyzed printed circuit board (PPCB) powder was separated into fractions, and the fine metallic fraction was used as a raw material for metal leaching extraction. In order to better understand how various metal species react in leaching media, several leaching agents were used (sulfuric acid, nitric acid, glycine, and acid mine drainage-AMD) alone, and with the addition of hydrogen peroxide. Additionally, the influence of the S/L ratio and leaching temperature were investigated in sulfuric acid leaching solutions, as this is the one most widely used. In one case, the reactor was heated in a thermal bath, while in the other, it was heated in an ultrasonic bath. Lastly, several experiments were conducted with a (consecutive) two-pronged leaching approach, with and without applied pretreatment. The aim of this paper is to give a multifocal and detailed approach to how metals such as Al, Cu, Co, Zn, Sn, and Ca behave when extracted from fine PPCB powder. However, some attention is given to Nd, Pd, Pb, and Ba as well. One of the main findings is that regardless of the pretreatment or the sequence of leaching media applied, consecutive two-pronged leaching cannot be used for selective metal extraction. However, AMD was found to be suitable for selective leaching with very limited applications.

Two-Stage Leaching of PCBs Using Sulfuric and Nitric Acid with the Addition of Hydrogen Peroxide and Ozone

“…In addition, leaching of Cu with sulfuric(VI) acid in the presence of dissolved oxygen occurs spontaneously and is the main copper oxidation reaction (Equation (1)). Another reaction related to the oxidation of Cu is the reaction with iron(III), in which Fe(III) etches the Cu surface, causing its oxidation (Equation (2)) [ 39 , 40 ]. 2 Cu + 2 H 2 SO 4 + O 2 → 2 CuSO 4 + 2 H 2 O Cu 0 + 2 Fe 3+ → Cu 2+ + 2 Fe 2+ …”

“…The reduction of the available oxygen in the leaching solution severely interferes with the formation of copper(II) sulfate(VI), as shown in Equation (2), which explains the decrease in Cu leaching efficiency. [ 39 , 41 ].…”

“…As can be seen from Equations (3) and (4), zinc(II) sulfide leaching requires the addition of dissolved oxygen, whereas zinc(II) oxide does not. In addition, as studies of the leaching of Zn(II) from sphalerite in an acidic environment have shown, elemental sulfur [ 39 , 40 , 41 , 42 , 43 , 44 , 45 ], as well as other insoluble salts (lead(II) sulfate(VI) and barium sulfate(VI)), may form on the surface of zinc(II) sulphide, hindering further leaching [ 39 , 46 , 47 ]. ZnO + H 2 SO 4 → ZnSO 4 + H 2 O 2 ZnS + 2 H 2 SO 4 + O 2 → 2 ZnSO 4 + 2 S + 2 H 2 O …”

“…Under the given leaching conditions (2 M H 2 SO 4 , S/L = 1/10 g/cm 3 , 353 K, 480 min), the tin could not be passivated or oxidized only to a small extent, hence the high results of dissolution of this metal in sulfuric(IV) acid. In addition, the amount of dissolved oxygen present in the leaching solution is a major factor in determining whether the reaction of tin with sulfuric(VI) acid will form soluble Sn(IV) sulfate or insoluble Sn(IV) hydroxide since the growth oxygen concentration is directly favorable for the formation of tin(IV) hydroxide [ 39 ].…”

“…In the case of zinc, nitric(V) acid dissolved the passive layer of sulfur (formed during leaching with H 2 SO 4 solution), resulting in 100% leaching of Zn(II) at 298 K. Leaching of zinc(II) oxide and zinc(II) sulphide in nitric(V) acid is shown in Equations (9) and (10) [ 39 ]: ZnO + 2 HNO 3 → Zn(NO 3 ) 2 + H 2 O 3 ZnS + 8 HNO 3 → 3 Zn(NO 3 ) 2 + 2 NO + 3 S + 4 H 2 O. …”