Ammonium thiosulphate leaching of gold from printed circuit board waste

Thesis (MEng)--Stellenbosch University, 2017.ENGLISH SUMMARY: Technological innovation leads to a reduced lifespan of older electrical and electronic equipment, which in turn leads to the generation of vast quantities of electronic waste (e-waste). The recycling of e-waste is becoming increasingly important as it provides certain economic benefits apart from the obvious environmental benefits. Printed circuit boards (PCBs) are found in most forms of e-waste and contain especially high concentrations of base and precious metals. Hydrometallurgy is one of the major processing routes for the recovery of valuable metals from e-waste. This processing route normally implements several leaching stages to selectively recover certain metals. A two-step base metal leaching stage was implemented that utilized two different lixiviants. The first step used nitric acid to mainly recover Pb and Fe, while the second step used sulphuric acid in combination with hydrogen peroxide to mainly recover Cu, Zn and Ni. The Au and Ag were subsequently recovered in an additional leaching stage with ammonium thiosulphate in the presence of copper(II) sulphate. This study focused on the use of a less environmentally hazardous lixiviant than the traditional alternative, cyanide, to promote the development of a more sustainable recovery process. The primary objective of this study was to determine how the variation of copper in the first stage residue will affect the gold leaching in the second stage. The extent of interactions between process conditions was also studied. These process conditions included temperature, thiosulphate concentration, ammonium concentration, cupric ion concentration, pH and pulp density. The secondary objective of this study was to determine how the degradation of thiosulphate was affected by the change in certain process conditions. The screening phase determined that only a change in S2O3 concentration, pH range and pulp density had a statistically significant effect on the Au extraction. Statistically significant interactions existed between the Cu on the PCBs and Cu(II) concentration; and the Cu on the PCBs and pulp density. These results were used together with recommendations from literature to determine what factors to include in the full factorial design. The S2O32- concentration (0.1 and 0.2 M), NH3 concentration (0.2 and 0.4 M), pH range (9 – 9.5 and 10 – 10.5) and pulp density (25 and 50 g/L) were chosen. The investigation of the S2O32- and NH3 concentrations determined that Au leaching was dependent on the S2O32-/NH3 ratio. S2O32- concentrations that were too high relative to NH3 resulted in the Cu(S2O32-)35- complex becoming more prominent, which hindered Au dissolution. NH3 concentrations that were too high resulted in a decrease in the oxidation potential of the Cu(II)-Cu(I) couple, which in turn reduced the driving force for the Au leaching reaction. NH3 concentrations that were too low reduced the amount of Cu(NH3)42+ (oxidizing agent for gold) that was available, which in turn also reduced Au leaching. The optimum S2O32-/NH3 ratio for the range of parameters that were investigated was found to be 0.5. A change in NH3 concentration was found to have a more significant effect on Au extraction at the lower pH range of 9 – 9.5. This was believed to be due to a higher concentration of NH4+ relative to NH3 being present at lower pH values, which caused faster Au leaching. The lower pH range of 9 – 9.5 also generally produced better Au leaching. An increase in pulp density from 25 to 50 g/L resulted in a decrease in Au extraction, which could be attributed to the fact that the amount of reagent per unit weight of PCB decreased. The importance of the interactions between S2O32- and NH3; and pH range and NH3 were confirmed in the statistical analysis of the full factorial design. The statistical analysis produced a model with a R2 value of 0.94 that predicted an optimum Au extraction of 78.04 % at the same conditions that produced the optimum Au extraction during testing. The predicted an optimum compared well with actual value of 78.47 %, which was obtained at 0.2 M S2O32-, 0.4 M NH3, 0.02 M Cu(II), 25 g/L, 25°C, 1 – 10 % leftover Cu and pH range of 9 – 9.5. The optimum conditions were used to determine the effect of a variation in Cu in the first stage residue, temperature and Cu(II) concentration. Au extraction decreased with an increase in Cu leftover content, temperature and Cu(II) concentration. Increased amounts of Cu inhibited Au leaching through the dissolution of Cu to Cu(NH3)2+ with the consumption of Cu(NH3)42+. Increased rates of thiosulphate consumption/degradation were encountered at higher temperatures, Cu(II) concentrations and leftover Cu.AFRIKAANS OPSOMMING: Tegnologiese vordering lei tot 'n verkorte lewensduur van ouer elektriese en elektroniese toerusting. Dit lei tot die opgaar van groot hoeveelhede van elektroniese afval (e-afval). Die herwinning van e-afval word toenemend belangrik aangesien dit sekere ekonomiese voordele benewens die ooglopende voordele vir die omgewing het. Gedrukte stroombaan borde (GSBe) is ‘n algemene vorm van e-afval en bevat veral hoë konsentrasies van basis- en edelmetale. Hidrometallurgie is een van die primêre prosesroetes vir die herwinning van waardevolle metale uit eafval. Hierdie prosesroete maak gewoonlik gebruik van 'n paar logingsfases om die metale selektief te herwin. Tydens hierdie studie is die basismetale herwin deur ‘n twee-stap logingsfase. In die eerste stap is salpetersuur aangewend om hoofsaaklik Pb en Fe te herwin, terwyl in die tweede stap swaelsuur aangewend is in kombinasie met waterstofperoksied om hoofsaaklik Cu, Zn en Ni te herwin. Die Au en Ag is daarna herwin in 'n bykomende logingsfase met ammonium tiosulfaat in die teenwoordigheid van koper (II) sulfaat. Hierdie studie fokus op die gebruik van 'n minder gevaarlike logingsoplossing as die tradisionele alternatief, sianied, met die oog om die ontwikkeling van 'n meer volhoubare herwinningsproses te bevorder. Die primêre doel van hierdie studie was om vas te stel hoe die variasie van oorblywende koper in die eerste logingsfase residu, die goud loging in die tweede logingsfase sal beïnvloed. Die mate van interaksie tussen prosestoestande is ook bestudeer. Die prosestoestande het temperatuur, tiosulfaat konsentrasie, ammonium konsentrasie, koper ioon konsentrasie, pH en pulpdightheid ingesluit. Die sekondêre doel van hierdie studie was om vas te stel hoe die degradering van tiosulfaat beïnvloed word deur die verandering in sekere prosestoestande. Die keuringsfase het bepaal dat slegs 'n verandering in S2O32- konsentrasie, pH en pulpdigtheid statistiese beduidende effekte gehad op die Au ekstraksie. Statistiese beduidende interaksies bestaan tussen die Cu op die GSB en Cu (II) konsentrasie; en die Cu op die GSB en pulpdigtheid. Hierdie resultate is saam met aanbevelings uit die literatuur gebruik om te bepaal watter prosestoestande in die faktoriaaleksperimente ingesluit moet word. Die S2O32- konsentrasie (0.1 en 0.2 M), NH3 konsentrasie (0.2 en 0.4 M), pH (9 – 9.5 en 10 – 10.5) en pulpdigtheid (25 en 50 g/L) is gekies. Die faktoriaal eksperimente het bepaal dat Au loging afhanklik is van die S2O32-/NH3 verhouding. S2O32- konsentrasies wat te hoog is in vergelyking met NH3 het veroorsaak dat die Cu(S2O32-)35- komplekse meer prominent geraak het. Dit het die Au ekstraksie belemmer. NH3 konsentrasies wat te hoog is het gelei tot 'n afname in die oksidasie potensiaal van die Cu(II) – Cu(I) koppeling, wat op sy beurt die dryfkrag vir die Au logingsreaksie verminder het. NH3 konsentrasies wat te laag is, het die hoeveelheid Cu(NH3)42+ (oksideermiddel vir goud) wat beskikbaar was verminder. Dit het gevolglik die Au loging verminder. Die optimale S2O32-/NH3 verhouding vir die toestande wat ondersoek is, is 0.5. 'n Verandering in NH3 konsentrasie het 'n meer beduidende effek op Au ekstraksie gehad by die laer pH reeks van 9 – 9.5. Daar is veronderstel dat dit te danke was aan 'n hoër konsentrasie van NH4+ relatief tot NH3, wat teenwoordig was by laer pH waardes, wat vinniger Au loging veroorsaak het. Die laer pH reeks van 9 – 9.5 het ook oor die algemeen beter Au loging produseer. 'n Toename in pulpdigtheid van 25 – 50 g/L het 'n afname in Au ekstraksie tot gevolg gehad, wat toegeskryf kan word aan die feit dat die hoeveelheid reagens per eenheidsgewig van GSBe afgeneem het. Die belangrikheid van die interaksies tussen S2O32- en NH3; en pH en NH3 is bevestig in die statistiese analise van die faktoriaal eksperimente. Die statistiese analise het 'n model produseer met 'n R2 waarde van 0.94, wat 'n optimale Au ekstraksie van 78.04 % voorspel het by dieselfde toestande wat die optimale Au ekstraksie geproduseer het tydens eksperimente. Die voorspelde optimum het goed vergelyk met die werklike optimum van 78.47 % wat behaal is by 0.2 M S2O32-, 0.4 M NH3, 0.02 M Cu(II), 25 g/L, 25°C, 1 – 10 % oorblywende Cu en ‘n pH reeks van 9 – 9.5. Die optimale toestande is gebruik om die effek van 'n verandering in oorblywende Cu in die eerste logingsfase, temperatuur en Cu(II) konsentrasie vas te stel. Au ekstraksie het afgeneem met 'n toename in oorblywende Cu, temperatuur en Cu(II) konsentrasie. Verhoogde hoeveelhede van Cu inhibeer Au loging deur die ontbinding van Cu na Cu(NH3)2+, met die verbruik van Cu(NH3)42+. ‘n Verhoogde tempo van tiosulfaat verbruik/degradering is ondervind by hoër temperature, Cu(II) konsentrasies en oorblywende Cu vlakke