Phase equilibria study of the CaO-Al2O3-SiO2-(Na2O/B2O3) slag system to recover valuable metals from e-waste
Creators: name: Islam, Md Khairul
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 349999
subject: Other chemical sciences not elsewhere classified
classificationCode: 401903
subject: Hydrometallurgy
classificationCode: 401908
subject: Pyrometallurgy

description: The research presented in this thesis deals with the recovery of valuable metals from antiquated electronics known as electronic waste (e-waste) using a pyrometallurgical route. The phase equilibria and liquidus temperatures of slags comprising the major oxide components of e-waste printed circuit boards (PCBs) smelting slag was systematically studied with the addition of different fluxing materials. The effect of the flux additives on the liquidus temperature and the phase assemblages was determined. An estimation of e-waste resources in Bangladesh generated from 4 different electronic gadgets (mobile phones, TVs, desktop computers, laptop/tablets) which are most widely used across the country was also made. Finally, the deportment of targeted metals in alloy and slag phase during smelting using one of the slag systems studied is presented and discussed.  

CaO-Al2O3-SiO2 based synthetic master slags were prepared with CaO/SiO2 (C/S) ratios of 0.3, 0.6 and 1.0 while keeping the Al2O3 content close to 20 wt.%. These master slags were doped separately with 3 different fluxing agents (Na2O, B2O3, Na2B4O7) at level ranging from 5-20 wt.% to prepare 36 sub-slag compositions. Master slags and different series of sub-slags were thermally equilibrated in a vertical tube furnace at different temperatures (above and below the predicted liquidus) and rapidly quenched in a water bath to retain the high temperature slag structure. The quenched slags were examined with optical microscope and Scanning Electron Microscope (SEM) to observe the equilibrium phases, and the chemical composition of the equilibrium phases was determined with the aid of an Electron Probe Microanalyzer (EPMA). The liquidus temperature of master slags and sub-slags were bracketed within ±20 °C uncertainty using an iterative approach. 

Three CaO-Al2O3-SiO2 master slags and 12- Na2O doped quaternary CaO-Al2O3-SiO2-Na2O slag samples were equilibrated at various temperatures to determine the liquidus temperatures and their equilibrium phase assemblages were identified. Anorthite (CaO.Al2O3.2SiO2), pseudowollastonite (CaO.SiO2), larnite (2CaO.SiO2), and gehlenite (2CaO.Al2O3.SiO2) primary solid phases were found in equilibrium with liquid phase. In general, liquidus temperature decreased with increasing Na2O content and a gradual shift in the primary phase field was observed. The lowest liquidus temperature (1110 °C) was obtained with 16.5 wt.% Na2O in master slag with a C/S ratio of 0.6 and 15.7 wt.% Al2O3. A reduction in liquidus by 225 °C was obtained by Na2O doping with the master slag of 1335 °C liquidus temperature.  However, liquidus temperature first decreased and then increased with further Na2O doping when the primary phase field moved to larnite for slag series with C/S ratio 1.0. This increase was due to the higher thermal stability of larnite. 

Phase equilibria study of quaternary CaO-Al2O3-SiO2-B2O3 slag system showed the effect of B2O3 doping (5-20 wt.%) keeping the C/S ratio in the range of 0.3 to 1.0. A gradual decline of the liquidus was common for every three series of B2O3 doped slags except an increase in the liquidus for slag with C/S ratio 1.0 and 15.6 wt.% Al2O3 beyond 14.9 wt.% B2O3. B2O3 flux showed a stronger effect on reducing the liquidus of CaO-Al2O3-SiO2 master slags with progressive doping compared to the equivalent Na2O doping. The lowest liquidus temperature in this slag series was obtained as 900 °C which is 435 °C lower than that of the undoped master slag (1335 °C) with C/S ratio 0.6 and 15.8 wt.% Al2O3. Unlike Na2O doped slags, primary phases of the slags with C/S ratio 0.3 and 0.6 did not change, while for slag with C/S ratio 1.0 primary phases shifted from gehlenite to pseudowollastonite at lower level of B2O3 doping. At 14.9 and 18.8 wt.% B2O3 doping two solid phases (anorthite and pseudowollastonite) were in equilibrium with liquid. At lower temperatures tridymite (SiO2) primary phase precipitated out of the solution in slags with C/S ratio 0.3. 

The combined effect of Na2O and B2O3 in the quinary CaO-Al2O3-SiO2-B2O3-Na2O slag was investigated by doping with anhydride borax (Na2B4O7) within the ternary CaO-Al2O3-SiO2 slag. Borax doping reduced the liquidus temperature of the quinary slag system of three series (C/S ratio 0.3, 0.6 and 1.0) with primary phase field shifting. However, contrary to Na2O and B2O3 doped slags, the lowest liquidus was obtained in slag series with C/S ratio 0.3. With 20 wt.% borax doping the slag liquidus dropped to 1050 ±20 °C from 1345 °C (undoped master slag) by an overall reduction of 270 °C. The primary phase remained unchanged for lower level of borax doping (up to 10 wt.%) and then shifted to pseudowollastonite for slags with C/S ratios 0.3 and 0.6. However, the shifting to pseudowollastonite from gehlenite primary phase begins at 5 wt.% borax doping and continued up to 20 wt.%. EPMA results showed limited Na and B in solid solution within the primary crystals while major portion of these remains in the liquid phase. 

The general order of flux for reducing the slag liquidus temperature was found as B2O3&gt;Na2O&gt;Na2B4O7 although the degree of reduction slightly varies depending on the C/S ratio and the primary phase of a particular slag in equilibrium. 

Based on the liquidus study smelting of real e-waste PCBs were designed to simulate a slag of CaO-Al2O3-SiO2-B2O3. Kilogram-scale smelting test was performed using an induction furnace equipped with flue gas treatment facilities and under nitrogen blanket. The char content withing the pyrolyzed PCBs helps maintain a reducing environment to protect the metals from oxidising.  Smelting under these conditions shows the distribution of metals in the alloy stream and the slag stream. Almost all the valuable metals reported in the alloy phase while the more refractory ones reported to the slag phase at 1350 °C.