Value recovery from mine calcine tailings

Mutimutema, Paul (2021-03)

Thesis (MMed)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: As gold mining progresses, rich and free-milling ores which are metallurgically easy to process eventually get depleted. More metallurgically complex ores are encountered. These ores yield gold recoveries below 80% and are termed refractory ores. The South African gold mining industry has left a legacy of abundant tailings dams – some of which are an environmental hazard due to the formation of acid mine drainage (AMD), discharge of metalloids, and or radioactive dust storms. Processing of tailings dams has slowly attracted the attention of investors as it is potentially cheaper to mine on the surface than shaft mining, in addition to having a lower exposure to risk associated with fatalities from mine shaft incidents. The possibility of recovering gold from a calcine tailings heap was considered in this study. Due to the refractory nature of the host ore from which they were generated, a mineralogical characterization was performed to understand the nature and occurrence of the gold. Fire assay revealed a gold grade of 2.96 ± 0.26 g/t. X-ray diffraction (XRD) and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) both confirmed that silicates were the most abundant phase followed by iron oxide. Scanning electron microscopy (SEM) showed that gold existed in submicron and micron size, as free gold, and also as refractory gold associated with arsenic, sulfur and silicates like quartz and talc. These mineralogical findings influenced the selection of extraction routes for investigation to recover the gold. Therefore, ultrafine milling, microwave roasting, microwave assisted cyanide leaching and sodium hydroxide pre-leach treatment were the methods potentially considered to recover the gold. Firstly, in order to confirm the degree of refractoriness of the tailings a series of direct cyanidation tests were conducted on the calcine tailings which resulted in average 17.3% gold recovery. Because of the confirmed high refractoriness, ultrafine milling of the original tailings (P80 — 53 μm) to P80 — 16 μm followed by cyanidation increased the gold recovery to 66.5%. Breaking of the host phases by ultrafine milling was beneficial. Microwave roasting pre-treatment on the as received sample (original calcine tailings) proved effective as compared to the ultrafinely ground calcine tailings sample. A 43.7% gold cyanidation recovery was obtained for the as received sample after microwave roasting for 60 minutes. Addition of 6% water was also effective on the as received sample, and yielded a 42.6% gold cyanidation recovery after undergoing 30 minutes of microwave roasting. A cyanidation gold recovery of 68.4% was attained after 30 minutes of microwave roasting the ultrafine sample. Microwave assisted cyanidation had rapid kinetics. In 50 minutes, 56.8% gold recovery was attained on the ultrafine sample at a sodium cyanide dose of 8 kg/t. Application of sodium hydroxide pre-leach treatment was beneficial by leaching gangue related elements in gold carrier phases (e.g. quartz and talc) into solution. Ambient temperature pre-leach at 1 M sodium hydroxide gave a 51.6% cyanidation gold recovery on the as received tailings sample after 24 hours. The ultrafine milled sample, after ambient pre-leach at 1 and 3 M sodium hydroxide, exhibited fast leaching rates, and maximum cyanidation gold recoveries of 71.9% and 77.6% respectively were obtained after 8 hours. Heat assisted pre-leach on a hot plate, however, did not result in improved gold cyanidation recoveries for both grinds and sodium hydroxide concentrations. Based on the experimental findings, three process flowsheets were considered. The gold mass balance for each flowsheet was completed. The flowsheet which incorporated sodium hydroxide pre-leach treatment was chosen as the best as it achieved the highest cyanidation gold recovery of 77.6%.

AFRIKAANSE OPSOMMING: Soos goudmynbou voortgaan, word ryk en vrymaalerts wat metallurgies maklik is om te prosesseer eventueel uitgeput. Meer komplekse metallurgiese erts word teëgekom. Hierdie erts bring goudherwinning onder 80% op en word weerbarstige erts genoem. Die Suid-Afrikaanse goudmynindustrie het ’n erfenis van oorvloedige uitskotdamme –waarvan sommige ’n omgewingsrisko is as gevolg van die formasie van suurmyndreinering (AMD), afskeiding van metalloïdes, en of radioaktiewe stofstorms. Prosessering van uitskotdamme het die aandag van beleggers stadig getrek omdat dit potensieel goedkoper is om op die oppervlak te myn as skagmynbou, saam met die laer blootstelling aan risiko geassosieer met noodlottighede van mynskaginsidente. Die moontlikheid om goud te herwin van ’n kalsienuitskothoop is oorweeg in hierdie studie. As gevolg van die weerbarstige natuur van die gasheererts waarvan dit gegenereer is, is ’n mineralogiese karakterisering uitgevoer om die natuur en voorkoms van die goud te verstaan. Vuuressai het ’n goudgraad van 2.96 ± 0.26 g/t getoon. X-straaldiffraksie (XRD) en kwantitatiewe evaluasie van minerale deur elektron mikroskopie skandering (QEMSCAN) het beide bevestig dat silikate die mees oorvloedige fase was, gevolg deur ysteroksied. Elektron mikroskopie skandering (SEM) het getoon dat goud bestaan het in submikron- en mikrongrotes, as vrye goud, en ook as weerbarstige goud geassosieer met arseen, swael en silikate soos kwarts en talk. Hierdie mineralogiese bevindinge het die keuring van ekstraksieroetes wat ondersoek is vir goudherwinning, beïnvloed. Daarom is ultrafyn malery, mikrogolfoond braaiery, mikrogolfoond-geassisteerde sianiedloging en seepsoda voorlogingsbehandeling die metodes wat potensieel oorweeg is om goud te herwin. Eerstens, om die graad van weerbarstigheid van die uitskotte te bevestig, is ’n reeks direkte sianidisasietoetse uitgevoer op die kalsienuitskotte wat op gemiddeld 17.3% goudherwinning tot gevolg gehad het. As gevolg van die bevestigde hoë weerbarstigheid, het ultrafyn malery van die oorspronklike uitskotte (P80 — 53 μm) na P80 — 16 μm gevolg deur sianidisasie die goudherwinning tot 66.5% verhoog. Die breek van die gasheerfases deur ultrafyn malery was voordelig. Mikrogolfbraaiingvoorbehandeling op die soos-ontvangde steekproef (oorspronklike kalsienuitskot) is as doeltreffend bewys in vergelyking met die ultrafyn gemaalde kalsienuitskotsteekproef. ’n 43% goud sianidisasie herwinning is verkry vir die soos-ontvangde steekproef na mikrogolfbraaiing vir 60 minute. Byvoeging van 6% water was ook doeltreffend op die soos-ontvangde steekproef, en het ’n 42.6% goud sianidisasie herwinning opgebring na 30 minute se mikrogolfbraaiing. ’n Sianidisasie goudherwinning van 68.4% is verkry nadat die ultrafyn steekproef vir 30 minute in die mikrogolf gebraai is. Mikrogolf-geassisteerde sianidisasie het vinnige kinetika gehad. In 50 minute is 56.8% goud herwinning op die ultrafynsteekproef verkry by ’n natrium-sianieddosering van 8 kg/t. Toepassing van seepsoda voorlogingbehandeling was voordeling by logings van aarsteen-verwante elemente in gouddraendefases (bv. kwarts en talk) in oplossing in. Omgewingstemperatuur voorloging by 1 M seepsoda het 51.6% sianidisasie goudherwinning op die soos-ontvangde uitskotsteekproef na 24 uur gegee. Die ultrafyn gemaalde steekproef, na omgewingsvoorloging by 1 en 3 M seepsoda, het vinnige logingstempo’s getoon, en maksimum sianidisasie goudherwinning van 71.9% 3 en 77.6% onderskeidelik verkry, na 8 ure. Hitte-geassisteerde voorloging op ’n warm plaat het egter nie tot verbeterde goud sianidisasie herwinning gelei vir beide male en seepsodakonsentrasies nie. Gebaseer op die eksperimentele bevindinge, is drie prosesvloeidiagramme oorweeg. Die goudmassabalans vir elke vloeidiagram is voltooi. Die vloeidiagram wat seepsodavoorlogingbehandeling geïnkorporeer het, is gekies as die beste omdat dit die hoogste sianidisasie goudherwinning, van 77.6%, bereik het.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/110143
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