With the general trend across all commodities towards the treatment of lower grade ores, it is becoming increasingly important to develop robust protocols for comprehensive mineralogical characterisation. The tendency for the complexity of the mineral assemblages in the ore to increase requires the use of more sophisticated tools in order to characterise the ore in the context of the implications of the mineralogy on its process response. Some of the key mineralogical attributes that are used to inform process selection and infer process response include: which minerals host the valuable element (elemental deportment); the type and relative proportions of the minerals present (modal mineralogy); and the grain size, association and liberation characteristics of the valuable minerals.
Silver ores are typically complex in terms of their mineralogical characterisation. There are a wide range of minerals that contain silver in different proportions which can make it difficult to identify them. Additional challenges include the relatively low concentrations of silver (ppm) within an ore, the large number of silver-bearing minerals that can occur in a single deposit and the potential for silver to occur in solid solution in wide range of sulphide minerals. Therefore, understanding the mineralogical attributes of an ore, particularly the deportment of the silver within the ore, is critical in developing an effective flotation processing strategy.
Automated SEM-based systems are a commonly used tool to quantify these attributes for an ore, but the low-grade together with the large number of minerals that are potential hosts for silver means that often, complementary analytical tools must be used in order to properly account for the valuable element.
This thesis aims to develop an appropriate methodology to characterise complex low-grade silver ores for the purpose of developing the most appropriate flotation strategy. As result of this investigation a novel methodology is proposed for the mineralogical characterisation and it consists of three different levels of characterisation using sophisticated analytical techniques. Level 1, the simplest (which included chemical assays, XRD, oxide characterisation of lead and zinc, optical microscopy and MLA), was applied to Toldos ore (oxide ore) and successfully characterised the mineralogy. Level 2, which included laser ablation inductively coupled plasma mass spectroscopy, was needed for Tesorera ore (sulphide ore). Jayula ore (supergene oxide ore) required the added sophistication of Level 3. In addition to the techniques of Level 1 and Level 2, Level 3 included the use of electron micro probe and synchrotron XRD, XRF and XANES methods to estimate the mineralogical attributes of this ore. The insights from the mineralogical characterisation were then used to inform the metallurgical testing that was undertaken, for example, selective or bulk flotation.
The ore characterisation for Toldos identified the presence of at least eight silver-bearing minerals including coarse grained chlorargyrite and acanthite. Mineralogical analysis of preflotation test samples indicated that acanthite exhibited natural hydrophobicity. These results, together with the mineralogical characterisation, indicated that selective flotation would be an appropriate processing route for this ore. The coarse grained silver minerals required a P80 of 100 microns and a specific dosage of a mix ofcollectors. The final flow sheet produced a rougher concentrate that contained 4404 ppm of Ag, at a recovery of 83.8%.
In the Tesorera ore, the majority of the silver (>99%) was contained in pyrite, which itself represented approximately 4% of the ore. A bulk flotation strategy using sulphidisation agents and the introduction of mainstream inert grinding (MIG) to generate a P80 of 56 microns, was necessary to recover pyrite minerals to achieve a rougher concentrate of 485 ppm Ag at a recovery of 87.2%.
The mineralogical characterisation of Jayula, during which a previously unreported association was found between silver and barite, accounting for more than 20% of silver in this ore with most of the remainder occurring as fine grained acanthite helped to guide the metallurgical characterisation. The fine grind required a final flow sheet with a P80 of 25 microns and a produced concentrate for which the silver recovery was 80.8% at a grade of 1709 ppm Ag. Again, as with the previous ores, the mineralogical characterisation guided the metallurgical test work.
The outcomes of this research include:
- A systematic method that enables the development of flotation strategies to achieve >80% silver recovery in laboratory rougher separation. Analyses of flotation products were performed on unsized, size-by-size and size-by-liberation bases, opening a broad understanding of the behavior of complex low-grade silver ores.
- The identification of a unique chemical association between silver and barite, which has not been previously reported in the literature and was shown to account for up to 20% of the silver in one of the ores studied.
- A framework for assessing the ‘refractoriness’ of silver ores based on the mineralogical characteristics of the ore and the metallurgical performance.
In summary this work provides a clear demonstration of how powerful a detailed mineralogical study at the onset of a project can be to guide the metallurgical test work for improved recoveries.