Hydrometallurgisten syanidivapaiden kultaprosessien arvioiminen simulaatioon perustuvalla elinkaarianalyysillä

The public defense on 3rd April 2020 at 12:00 is available via remote technology. Link: https://aalto.zoom.us/j/650442383 Meeting ID: 650 442 383 Zoom Quick Guide: https://www.aalto.fi/en/services/zoom-quick-guideIn this doctoral dissertation simulation based life cycle assessment was adapted in evaluation of process performance and environmental impacts of cyanide-free gold leaching. This methodology enables the evaluation of industrial scale process operation combined with early stage estimation of environmental impacts of the processes. The simulations are based on hydrometallurgical expertise, and can be adjusted according to experimental data. Adapting such a methodology in to evaluation of hydrometallurgical gold processes, is relevant for two reasons. Firstly, the challenges in up-scaling the metallurgical processes from laboratory to industrial scale require that the new technologies are assessed already in development stage, for both process performance and environmental impacts. Secondly, this is extremely timely in gold production, where cyanidation is the dominant processing method and faces several challenges as the gold ore grades diminish and mineralogy of the ores become increasingly complicated, in addition to health and safety risks related to cyanide. Gold production should utilize the best available technology, due to which the research has started to focus on more environmentally friendly alternatives in order to replace cyanidation. In the current study the simulations have been constructed using HSC-Sim, linked to GaBi software. HSC-Sim enables flowsheet simulation of hydrometallurgical processes to create accurate life cycle inventory data that can be used in life cycle assessment. This study presents simulation of six gold extraction processes, including pressure oxidation, cyanidation, chlorination, halogen leaching and cupric chloride leaching. The main challenges for cyanide-free chloride based processes are water balance, in situ method for recovery of gold, and improved gold recovery. However, the faster leaching kinetics, and the simultaneous oxidation of sulfide mineral and gold extraction could offer competitive edge for chloride processes. The estimated environmental impact categories in this work include global warming, acidification and eutrophication potential as well as water resource depletion. Cyanidation shows lowest indicator values in all categories, however by optimization of the chloride processes and improved recovery the indicator values can be brought to same level. The simulation based life cycle assessment was found to be a suitable methodology for evaluation of critical points in development stage processes and estimation of their early stage environmental indicator values. This work does not only contribute to research of cyanide-free processing of gold, but also critically evaluates the existing data and provides more accurate inventory data for other life cycle assessment researchers. Finally, the methodology utilized can provide the supportive information necessary to introduce more sustainable processing methods, not only for gold industry but to any metallurgical processing.Tässä väitöskirjatutkimuksessa käytettiin simulaatioon perustuvaa elinkaarianalyysia syanidivapaan kullan prosessoinnin suorituskyvyn ja ympäristövaikutusten arviointiin. Tämä menetelmä mahdollistaa kehitysvaiheessa olevien teollisen mittakaavan prosessien arvioinnin yhdistettynä ympäristövaikutusten arviointiin. Simulaatiot perustuvat hydrometallurgiseen asiantuntemukseen, ja niitä voidaan säätää kokeellisten tietojen perusteella. Tällaisen metodologian sovittaminen hydrometallurgisten kultaprosessien arviointiin on olennaista kahdesta syystä. Ensinnäkin metallurgisten prosessien teollisen mittakaavan käyttöönotto voi olla haasteellista jonka vuoksi kehityksen alla olevia prosessien toimintaa ja ympäristövaikutuksia on arvioitava jo aikaisessa vaiheessa Toiseksi, syanidivapaat prosessivaihtoehdot ovat ajankohtaista kullantuotannossa, jossa syanidointi on hallitseva prosessointimenetelmä. Syanidaatioon liittyy useita haasteita, kun tarve prosessoida köyhiä ja vaikeasti liuotettavia malmeja kasvaa, ja syanidin käyttöön liittyy terveys- ja turvallisuusriskejä. Kullantuotannossa olisi käytettävä parasta käytettävissä olevaa tekniikkaa, minkä seurauksena tutkimus on keskittynyt ympäristöystävällisempiin prosessivaihtoehtoihin syanidin korvaamiseksi. Tässä väitöskirjatutkimuksessa simulaatiot on rakennettu käyttämällä HSC-Simiä, joka on yhteensopiva GaBi -ohjelmiston kanssa. HSC-Sim mahdollistaa hydrometallurgisten prosessien vuokaavioiden simuloinnin, jotta saadaan tarkka inventaarioanalyysi käytettäväksi elinkaarianalyysissä. Tässä väitöskirjatutkimuksessa simuloitiin kuusi prosessia, mukaan lukien refraktorisen kultamalmin painehapetus, syanidaatio (CIP/CIL), klorinaatio, halogeeniliuotus ja kuparikloridiliuotus. Syanidivapaiden kloridiprosessien päähaasteet ovat vesitasapaino, in situ-menetelmä kullan talteenottamiseksi ja talteenoton parantaminen. Kloridiprosessien nopeampi liuotuskinetiikka ja sulfidimineraalien hapetus ja kullan liuottaminen samanaikaisesti voivat tarjota kilpailuedun verrattuna syanidaatioon. Arvioituihin ympäristövaikutusluokkiin kuuluvat ilmaston lämpenemis-, happamoitumis- ja rehevöitymispotentiaali sekä vesivarojen ehtyminen. Syanidaation indikaattoriarvot ovat kaikissa kategorioissa alhaisimmat, mutta optimoimalla kloridiprosesseja ja parantamalla talteenottoa indikaattoriarvot voidaan saattaa samaan tasoon. Simulaatioon perustuva elinkaarianalyysi on todettu sopivaksi menetelmäksi kehitysvaiheen prosessien kriittisten pisteiden arvioimiseksi ja vertailukelpoisten ympäristöindikaattorien arvojen tuottamiseksi. Tässä työssä on kullan syanidivapaan prosessoinnin tutkimuksen ohella tuotettu tarkempaa inventaarioanalyysidataa, jota muut elinkaarianalyysi-tutkijat voivat hyödyntää. Lisäksi käytetty metodologia tuottaa tietoa, joka mahdollistaa kestävämpien prosessien kehittämisen niin kullantuotantoon kuin mihin tahansa metallurgiseen prosessointiin

Leaching of pyrite in cupric chloride solution

Pyrite (FeS2) is the most abundant sulphide mineral in the world and may contain significant amounts of gold locked in its structure. Traditionally, pyrite has to be oxidized in order to break down the mineral matrix for better gold liberation by e.g. roasting, pressure leaching and bioleaching [1]. Due to environmental and health concerns, as well as public opinion, alternative methods are developed to replace gold cyanide leaching, which has been the preferred gold leaching process. Gold is known to form a stable complex with chloride as [AuCl4]- or [AuCl2]-. In chloride leaching of pyrite in the presence of [Cu2+] which acts as a catalyst/oxidant, the pyrite structure can be simultaneously broken down and the gold dissolves into a solution from which it can be later recovered [2, 3]. In this study, dissolution of the pyrite concentrate in cupric chloride solutions was investigated. The effect of cupric ion concentration (16, 32 g/L), temperature, (60, 75, 90 °C), chloride ion concentration (50, 150 g/L), pH (1, 1.5) and mixed chloride-bromide ion concentration (75+75 g/L) on pyrite dissolution were studied during 6 hour batch leaching experiments. Redox potentials between 0.495-0.511 V vs. Ag/AgCl resulted in pyrite dissolution of 31-34%, whereas redox potentials between 0.523-0.573 V vs. Ag/AgCl resulted in 45-68% pyrite dissolution

Flowsheet design and environmental impacts of cobalt co-product recovery from complex Au-Co ores

Funding Information: This study was supported by BATCircle2.0 project (grant number 44886/31/2020), financed by Business Finland, and the School of Chemical Engineering, Aalto University. The authors have made use of Finland’s RawMatTERS Finland Infrastructure (RAMI). The authors are grateful to Mawson Gold Ltd for discussions and invaluable help in formulating the research questions. Publisher Copyright: © 2023 The AuthorsThe increasing demand for battery metals, such as cobalt, increases the attractiveness of conventionally overlooked raw materials, such as small multi-metal deposits. With the length of mining and extractive metallurgy projects, it is advantageous to evaluate possible processing routes well in advance with a variety of tools, such as experimental, piloting, simulation, and life cycle assessment. In this study, simulation-based life cycle assessment was used to compare the baseline of only recovering gold from sulfidic Au-Co-(Cu-As) ores to five scenarios where both gold and cobalt were recovered, the latter as battery grade cobalt sulfate heptahydrate. Technical performance and several impact categories affecting climate change, ecosystems, and human health, were investigated to assess the advantages and drawbacks of each option. The environmental competitiveness varied between different studied scenarios vs. observed impact categories. The baseline process - with no cobalt recovery - had the lowest energy and global warming impacts but fared worse in toxicity categories. Pressure leaching scenario achieved low toxicities but was shown to be environmentally highly intensive in all other categories. The pyrometallurgical-hydrometallurgical roasting-leaching process resulted in the lowest environmental impacts in most of the studied categories. However, in terms of acidification and photochemical oxidant creation, the fine grinding-leaching process was the best option. In all studied scenarios the major technical challenge was found to be the stabilization of arsenic, yet the technical issues can effectively be resolved by treating the arsenical and non-arsenical concentrates in separate processes. In the current results, the temporal boundary, allocation choice, and current lack of experimental work remain major sources of uncertainty.Peer reviewe

Process simulation and environmental footprint of gold chlorination and cyanidation processes

Chlorination was applied widely in gold processing throughout the 1800s as vat and barrel leaching, with chlorine gas acting as an oxidant. In the early 1900s, hydrometallurgical gold process technology changed from chlorination to cyanidation, which replaced chlorination as the predominant gold leaching process due to technical and financial advantages. The challenges in the chlorination process were related to the highly corrosive leaching chemicals and associated high reagent consumption. In the past, new gold extraction technologies have not been evaluated by their environmental footprint, but rather by the financial, technological and operational advantages. In order to determine the associated ecological impact and sustainability, new transformative technologies need to be evaluated from the environmental footprint point of view prior commercialization. In this study, the environmental indicator category chosen for evaluation was global warming potential (GWP) and the other impact categories, such ashuman toxicity potential and acidification were left out of the scope. The global warming potentials (GWP) were determined both for the historical chlorination process and for a state-of art cyanidation process by modelling with HSC Chemistry 8.0 HSC-Sim module combined with lifecycle analysis by GaBi 6.0. This analysis provides a baseline scenario for comparison, which can be used to support the future life cycle assessment research for development stage gold processes. The combination of HSC-Sim and GaBi was shown to be an efficient way for the investigation of the environmental footprint of the historical and current gold processes. The GWP of the cyanidation process was 455 kg CO2 equivalent and the GWP of chlorination process was 10500 kg CO2 equivalent. The main emission source in cyanidation was found to be electricity, whereas in chlorination the main emissions originated from the chlorine gas, although, it is worth noting that environmental impacts are not defined for all chemical products in the GaBi database. Additionally, some compromises had to be made when the LCA Equivalents were applied to the process streams modelled in HSC-Sim. The corresponding or most appropriate LCA Equivalents were defined from the HSC/GaBi database and the LCA group was defined for all the inputs and outputs within the scope of the process.Peer reviewe

Life cycle assessment and process simulation of prospective battery-grade cobalt sulfate production from Co-Au ores in Finland

Funding Information: Open access funding provided by Aalto University. This study was supported by the School of Chemical Engineering (Aalto University); the BATCircle project, grant number 4853/31/2018, funded by Business Finland; and the Academy of Finland-funded “RawMatTERS Finland Infrastructure” (RAMI). The raw material was provided by Mawson Gold Ltd. Publisher Copyright: © 2021, The Author(s).Purpose: The soaring demand for cobalt for lithium-ion batteries has increased interest in the utilization of non-conventional cobalt sources. Such raw materials include complex ores containing minerals such as cobaltite and skutterudite, which, while rare, occur around the world, including in Finland, Canada, and the USA. The goal of this study was to evaluate the cradle-to-gate impacts of cobalt sulfate recovery from unutilized cobalt- and gold-bearing ores with the use of process simulation. Methods: A literature analysis was conducted to establish the state-of-the-art processing methods for complex cobalt ores containing significant amounts of gold. The drafted process was simulated using HSC Sim software to obtain a mass and energy balance, which was compiled into a life cycle inventory (LCI). The environmental impact categories (global warming, acidification, eutrophication, ozone depletion, photochemical smog creation, water use) were calculated in GaBi software. Uncertainty regarding the possible future raw material composition was studied, and the simulation was used to investigate process performance and to evaluate the effect of variation in the process parameters on the environmental impact indicators. Results and discussion: The results indicated that the main cobalt mineral type (cobaltite, linnaeite) had only minor effects on the evaluated impact categories. With cobaltite-dominated ores (High As case), the global warming potential (GWP) was estimated to be 20.9 kg CO2-eq, of which 12.7 kg CO2-eq was attributed to the hydrometallurgical process. With linnaeite-dominated ores, the equivalent values were 20.4 kg CO2-eq and 11.0 kg CO2-eq. The production of a high grade concentrate was observed to greatly decrease the impacts of the hydrometallurgical process, but the cobalt losses in the beneficiation stage and the mineral processing impacts would likely increase. The simulation showed that there is still potential to improve the cobalt recovery (to approximately 96%), which would also affect the indicator values. Conclusions: The impacts were estimated prior to intensive metallurgical testing to determine the possible high impact areas in the process. Based on this, it is suggested that, during hydrometallurgical processing, improved treatment of cobalt-containing wash waters and the optimization of oxygen utilization efficiency in pressure leaching are the most significant ways to decrease the environmental impacts. Optimal solutions for the concentrate could be found when experimental data on the minerals processing steps becomes available.Peer reviewe

Process simulation based life cycle assessment of cyanide-free refractory gold concentrate processing – Case study : Cupric chloride leaching

The development of cyanide-free gold leaching methods is becoming increasingly important due to the treatment of complex ores, where treatment by cyanidation is not economically viable. Cupric chloride leaching provides an alternative leaching process to replace cyanidation. A detailed simulation of refractory gold concentrate processing by cupric chloride leaching is presented in this study. The simulation with mass and energy balances was built to be used as life cycle inventory data to evaluate the environmental impacts of the development stage cupric chloride process. Three cases, the Base Case (125 g/L Cl-), Mild Case (50 g/L Cl-), and Extremely Mild Case (20 g/L Cl-), were investigated in two flowsheet options. Loss of gold to wash waters was observed in the Flowsheet 1 cases, and therefore Flowsheet 2, with the recirculation of wash water to solvent extraction, was developed and investigated in order to achieve higher gold recovery. The gold extraction improved from around 85% to 99%. Chemical consumption (NaCl, NaBr, CuCl2) was greatly affected by the leaching conditions, higher concentrations consuming more initial chemicals. In milder conditions, efficient recycling of chlorides could be obtained in the process, and no addition of NaCl was required. In the Extremely Mild Case, the chloride concentration was close to sea water conditions (20 g/L), where sea water could be used to provide chlorides for the process, and the effluent waters could be disposed of in the sea after purification. The global warming potential was estimated to be 12.5 t CO2-e/kg Au in chloride leaching and was further decreased to 10.6 t CO2-e/kg Au in the mildest conditions (20 g/L Cl-). The milder chloride leaching conditions (20 g/L Cl- and 50 g/L Cl-) were shown to decrease the acidification potential, eutrophication potential, and water depletion.Peer reviewe