Elsevier

Powder Technology

Volume 331, 15 May 2018, Pages 258-269
Powder Technology

Leaching behaviour of mechano-chemically activated bio-oxidised refractory flotation gold concentrates

https://doi.org/10.1016/j.powtec.2018.03.040Get rights and content

Highlights

  • Mitigation of bio-oxidised gold concentrate refractoriness posed by secondary mineral gold passivation.

  • Complementing ore bio-oxidation with mechano-chemical activation for improved gold extraction.

  • Increasing mechanical stress does not always increase processability of mineral ores.

Abstract

In this paper, the leaching behaviour of mechano-chemically activated, refractory, bio-oxidised flotation gold concentrates has been investigated. In particular, the effect of activation parameters (e.g., milling time and speed, and ball to pulp ratio) and cognate physico-chemical changes on the impact of the refractory mineral components in the ore was studied. The results showed that mechano-chemical activation led to reduction in the average particle size and increased surface area and reagent consumption in tandem with remarkable mineral amorphisation of the bio-oxidised products. The gold yield increased by a maximum of ~1.8 times after 24 h of cyanide leaching, irrespective of the magnitude of milling speed, milling time, ball to pulp ratio and the degree of mineral amorphisation. Furthermore, the increase in reagent consumption was consistent with increasing mineral particle stress and exposure of reactive gangue mineral phases (e.g., sulphate minerals), a factor that least influenced the gold yield. It appears that the improvement in gold extraction efficacy was due to gold surface cleaning, creation of pores/crevices that are linked to gold sites, and disintegration of gold-gangue mineral agglomerates that encapsulated gold particles. The findings were consistent with the nature of refractoriness and necessary for understanding ways of mitigating gold refractoriness in bio-oxidised flotation concentrates.

Introduction

Integration of pre-treatment methods (e.g., bio-oxidation and pressure oxidation) into gold processing flowsheets is vital for rendering complex, low grade (<2 g/t), refractory ores amenable to conventional cyanide leaching [[1], [2], [3]]. The bio-oxidation (BIOX®) method liberates gold from auriferous sulphide minerals (e.g., arsenopyrite and pyrite) [1,4]. The process, generally, leads to the creation of porous bio-oxidised products based on the mineral phase zoning and their level of susceptibility to bacterial oxidation. Notwithstanding its advantages, some intractable challenges including dissolved gold preg-robbing, high cyanide consumption and gold encapsulation by secondary sulphate minerals may still persist [1,5]. Previous process modification (e.g., application of HiTech®) focused on the elimination of preg-robbing and high cyanide consumption effects from such gold ores [[6], [7], [8], [9]]. Our previous studies [10] made it evident that the formation of refractory jarosite in bio-oxidised products could prevent over 40 wt% gold extraction. To date, there is limited information on ways of effectively mitigating the gold refractoriness posed by secondary sulphate minerals.

Mechanical activation deploys mechanical energy to improve the processability (e.g., leaching and flotation) of certain mineral particles by altering their surface and/or bulk physical and micro-structural properties. When the process leads to changes in chemical composition and/or structure (e.g., crystalline to amorphous product), it is known as mechano-chemical activation [[11], [12], [13]]. Whilst some previous studies deployed mechanical activation in synthesizing materials such as fosterite and bio-nanocomposites [[14], [15], [16]], others deployed the technique to improve mineral floatability (e.g., fine gold and copper sulphide) and oxidative leaching (e.g., chalcopyrite and sphalerite) [[17], [18], [19], [20], [21], [22]]. Hashemzadehfini, Ficeriová, Abkhoshk and Shahraki [21] showed that increasing the magnitude of activation parameters such as milling speed and milling time resulted in a corresponding increase in gold recovery from complex sulphide ore. It is, however, not clear whether higher gold recoveries may always be realized from mechano-chemically activated ores irrespective of the ore mineralogy and chemistry. Ore specific knowledge on the activated product characteristics and gold leaching performance will be useful in defining the cost effectiveness of implementing mechano-chemical activation as a pre-leach improvement step. The impact of mechano-chemical activation on gangue mineral reaction and reagent consumption, given the change in physico-chemical properties, is also unknown. Complementing eco-friendly bio-oxidation method with a cost-effective technique to improve gold extraction efficacy from refractory ores is highly desirable.

The aim of this paper, therefore, is to investigate the gold leaching behaviour of mechano-chemically activated, refractory, bio-oxidised flotation concentrate (AL). The effect of activation parameters such as milling speed, milling time and ball to pulp ratio coupled with physico-chemical variations of the as-received bio-oxidised product were studied. The key research questions addressed are as follows.

  • 1.

    How does mechano-chemical activation affect the particle size and surface area of the selected bio-oxidised product before and after cyanide leaching?

  • 2.

    How do the mineralogy and chemistry of the ore particles change with the mechano-chemical activation parameters?

  • 3.

    Does mechano-chemical activation mitigate the refractoriness of bio-oxidised gold ores containing secondary sulphate minerals and significantly improve gold recovery and yield upon cyanidation? If so, how?

  • 4.

    How do reagent (sodium cyanide and sodium hydroxide) consumption vary between the as-received and mechano-chemically activated bio-oxidised products?

  • 5.

    What is the correlation between the magnitude of mechano-chemical activation parameters and the leach reagent consumption?

  • 6.

    Overall, how does the leaching behaviour of the mechano-chemically activated bio-oxidised product reconcile with the cause of refractoriness in the as-received bio-oxidised product?

Section snippets

Materials

The as-received (AL) refractory bio-oxidised flotation concentrate was obtained from the Eburnean tectonic province of the West African Precambrian Craton, Ghana. It was reported to be polydispersed and mineralogically complex comprising mixture of silicates [muscovite, illite, albite, ephesite], sulphates [jarosite, gypsum, bassanite], and oxides [quartz, rutile] as the predominant phases [10,23]. Table 1 shows the mineralogical composition determined by semi-quantitative XRD.

Sulphides

Particle size distribution and specific surface area

Cumulative particle size distribution results of the as-received and mechano-chemically activated bio-oxidised products, before and after cyanide leaching, are shown in Fig. 2. In addition, the particle sauter mean size (d32) and BET specific surface area results are presented in Table 4. The particle size distribution data showed that mechano-chemical activation of the bio-oxidised product generally reduced the particle size distribution (Fig. 2A). The degree of impact on the particle sizes

Conclusions

The gold leaching behaviour of mechano-chemically activated, refractory, bio-oxidised flotation concentrates that contain secondary sulphate minerals has been investigated. Links were drawn to changes in physico-chemical characteristics and activation parameters.

  • 1.

    Mechano-chemical activation reduced the particle size, and increased the surface area and amorphisation of the bio-oxidised product. Also, refractory gypsum minerals were rendered soluble after activation whilst some portions of gangue

Acknowledgement

Financial support from the Research Training Program (RTP) of the Australian Government (GRC Ref: Rnd 0513) is gratefully appreciated.

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