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Specific Refractory Gold Flotation and Bio-Oxidation Products

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Specific Refractory Gold Flotation and Bio-Oxidation Products minerals Review Specific Refractory Gold Flotation and Bio-Oxidation Products: Research Overview Richmond K. Asamoah Minerals and Resource Engineering, Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia; [email protected] Abstract: This paper presents a research overview, reconciling key and useful case study findings, towards uncovering major causes of gold refractoriness and maximising extraction performance of specific gold flotation and bio-oxidation products. Through systematic investigation of the ore mineralogical and gold deportment properties, leaching mechanisms, and kinetic behaviour and pulp rheology, it was observed that the predominant cause of the poor extraction efficacy of one bio-oxidised product is the presence of recalcitrant sulphate minerals (e.g., jarosite and gypsum) produced during the oxidation process. This was followed by carbonaceous matter and other gangue minerals such as muscovite, quartz, and rutile. The underpining leaching mechanism and kinetics coupled with the pulp rheology were influenced by the feed mineralogy/chemistry, time, agitation/shear rate, interfacial chemistry, pH modifier type, and mechano-chemical activation. For instance, surface exposure of otherwise unavailable gold particles by mechano-chemical activation enhanced the gold leaching rate and yield. This work reflect the remarkable impact of subtle deposit feature changes on extraction performance. Keywords: refractory gold ores; ore mineralogy; secondary minerals; rheology and leaching kinetics; mechano-chemical activation Citation: Asamoah, R.K. Specific Refractory Gold Flotation and 1. Introduction Bio-Oxidation Products: Research The economic significance of gold to the development of several nations (e.g., Aus- Overview. Minerals 2021, 11, 93. tralia, South Africa, USA, China, Canada, and Ghana), owing to its coveted qualities and https://doi.org/10.3390/min1101 unique applications, can be traced back to the dawn of civilization [1–6]. In a recent global 0093 gold mine reserve estimation [7], Australia hosts 16% of the world’s total 56,700 t gold (Figure1) worth AUD$ 542.48 billion, at gold price of AUD$ 1690/oz. With record high Received: 3 December 2020 gold prices in 2020 (AUD$ 2670/oz), the 16% gold deposit represents a higher value of Accepted: 13 January 2021 about AUD$ 857.05 billion. A number of developing countries (e.g., Ghana), blessed with Published: 19 January 2021 gold deposits, have substantially achieved poverty alleviation by exploiting their gold reserves in an eco-friendly manner. Gold extraction continue to represent a major livelihood Publisher’s Note: MDPI stays neutral and economic support in recent time. with regard to jurisdictional claims in Currently, a greater percentage of gold is extracted from low grade, refractory gold published maps and institutional affil- ores, following depletion of most high grade deposits [8]. Increasing complexity of these iations. refractory ores warrants improvement in our fundamental and applied knowledge under- pinning sustainable, cost-effective, commercial gold extraction process and commodity production. Despite numerous reported studies on improved gold extraction (by cyanide leaching) from complex low grade ores [9–16], there is still a lack of fundamental knowl- Copyright: © 2021 by the author. edge and understanding on the interplay between refractory ore-specific primary and Licensee MDPI, Basel, Switzerland. secondary mineral phases, solution chemistry, and the particle-solution interfacial species, This article is an open access article particle interactions, and chemical/electrochemical reactions which underpin the mech- distributed under the terms and anisms and kinetics of the leaching process. How these factors interact synergistically conditions of the Creative Commons to produce fast kinetics and high gold recovery or antagonistically (e.g., low leach rate, Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ passivation/encapsulation, poor gold recovery, and high reagent consumption) during 4.0/). alkaline cyanide leaching process is as yet not clearly established. Minerals 2021, 11, 93. https://doi.org/10.3390/min11010093 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 13 Minerals 2021,, 11,, 93x FOR PEER REVIEW 2 of 1413 Figure 1. Global gold mine reserves in 2015. Total mine gold reserve was 56,700 metric tons whilst total monetary estimate was AUD$ 3380.06 billion [7]. Currently, a greater percentage of gold is extracted from low grade, refractory gold ores, following depletion of most high grade deposits [8]. Increasing complexity of these refractory ores warrants improvement in our fundamental and applied knowledge un- derpinning sustainable, cost-effective, commercial gold extraction process and commod- ity production. Despite numerous reported studies on improved gold extraction (by cya- nide leaching) from complex low grade ores [9–16], there is still a lack of fundamental knowledge and understanding on the interplay between refractory ore-specific primary and secondary mineral phases, solution chemistry, and the particle-solution interfacial species, particle interactions, and chemical/electrochemical reactions which underpin the mechanisms and kinetics of the leaching process. How these factors interact synergisti- cally to produce fast kinetics and high gold recovery or antagonistically (e.g., low leach Figure 1. Global gold mine reserves in 2015. Total mine gold reserve was 56,700 metric tons whilst total monetary estimate Figure 1. Global gold mine reservesrate, passivation/encapsulation, in 2015. Total mine gold reserve poor was gold 56,700 recovery, metric tonsand whilsthigh reagent total monetary consumption) estimate dur- was AUD$ 3380.06 billion [7]. was AUD$ 3380.06 billion [7].ing alkaline cyanide leaching process is as yet not clearly established. Of relevance to the present work is a technological conundrum associated with gold Currently,Of relevance a greater to the present percentage work of is gold a technological is extracted conundrum from low grade, associated refractory with gold ® ores,extractionextraction following fromfrom depletion twotwo biologicallybiologically of most oxidisedhighoxidised grade (BIOX(BIOX deposits®)) flotationflotation [8]. Increasing concentratesconcentrates complexity obtainedobtained of these fromfrom refractorythethe samesame lowores grade grade warrants (<1.5 (<1.5 improvementg/t) g/t) deposit deposit (Ghana in (Ghana). our). fundamentalAlthough Although the and thesame sameapplied process process knowledge route route and andcon-un- derpinningconditionsditions are are sustainable,deployed deployed for forcost-e gold goldffective, extraction, extraction, commercial on onee bio-oxidised bio-oxidised gold extraction product product process invariably and displayedcommod-displayed ~20ity~20 production. wt.%wt.% lower lower goldDespite gold recovery recovery numerous compared compared reported with with studies the the other other on (Figure improved (Figure2). Furthermore, 2). gold Furthermore, extraction the cyanide(by the cya-cya- nideleachingnide leaching)leaching process processfrom typically complex typically requires low requires grade low slurry oreslow slurry[9–16], solid loadingsolid there loading is (~35 still wt.%) a(~35 lack wt.%) to of facilitate fundamental to facilitate pulp knowledgehandleability.pulp handleability. and Nominally, understanding Nominally, 30–40 wt.% on30–40 the gold wt.%interpla is lostgoldy tobetween is tailings lost to refractory upontailings alkaline upon ore-specific cyanidealkaline leachingprimary cyanide andleaching simultaneoussecondary and simultaneous mineral gold adsorptionphases, gold solution adsorption by activated chemistry, by carbon.activated and In thecarbon. monetary particle-solution In monetary terms, the interfacialterms, gold lost the species,togold tailings lost particle to is tailings estimated interactions, is estimated to be AUD$and to chemical/e be 5.7 AUD$ million 5.7lectrochemical permillion annum per forannum reactions treating for which treating a low underpin grade a low ore grade the of 1mechanismsore g/t of at 1 ag/t 420 at and a t/d 420 kinetics plant t/d plant throughput. of throughput. the leaching In-plant In-p proclant studiesess. studies How based thesebased on factors extanton extant literatureinteract literature synergisti- review review to callyunderstandto understand to produce the the cause fast cause kinetics of of refractoriness refractoriness and high andgold and poor recovery poor pulp pulp or handleability handleability antagonistically were were (e.g., not not conclusive conclusivelow leach rate,duedue totopassivation/encapsulation, thethe complexitycomplexity ofof thethe oresores poor andand gold concentrates.concentrates. recovery, and high reagent consumption) dur- ing alkaline cyanide leaching process is as yet not clearly established. Of relevance to the present work is a technological conundrum associated with gold extraction from two biologically oxidised (BIOX®) flotation concentrates obtained from the same low grade (<1.5 g/t) deposit (Ghana). Although the same process route and con- ditions are deployed for gold extraction, one bio-oxidised product invariably displayed ~20 wt.% lower gold recovery compared with the other (Figure 2). Furthermore, the cya- nide leaching process typically requires low slurry solid loading (~35 wt.%) to facilitate pulp handleability. Nominally, 30–40 wt.% gold is lost
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