THE USE OF QEMSCAN FOR ORE CHARACTERIZATION

D. Connelly

Director/Principal Consulting Engineer Engineering Technical Services Pty Ltd (METS) PO BOX 3211, Perth, 6832, WA ([email protected])

ABSTRACT

The QEMSCAN system is the third generation of automated mineral analysis systems that began with the QEM*SEM at CSIRO 30 years ago (Butcher et al. 2000). It is now a successful commercial instrument with 39 instruments in operation around the world. Back Scattered Electrons (BSE) and Energy Dispersive (EDS) X-Ray spectra are used to create digital mineral images with mineral identification occurring online. Low-count EDS spectra are used preferentially to BSE brightness, allowing to be accurately identified by chemical composition. Individual minerals or groups of similar compositions are identified by comparison to a comprehensive mineral database incorporated into the QEMSCAN software. Optical mineralogy and Scanning Electron Microscopy is a metallurgists’ tool for determining the ore process mineralogy and delivering a better flowsheet. These techniques have limitations, provide limited information and sometimes fail to provide representative information. In contrast, QEMSCAN provides information on the nature and occurrence of all mineral species present plus the liberation characteristics, looking at thousands of particles in the sample presented. Some of the data presented includes: • Particle and grain sizes • Dominant mineral species • Particle shape • Calculated metal content • Density • Deportment of elements of interest between the mineral phases • Quantitative mineralogy • Mineral associations, locking and liberation • Textural relationships (particle images) It can also be used on concentrates, polished sections or drill core. This is a significant improvement in ore characterisation providing lithology, mineralogy, liberation, ore grade and from a sample. Theoretical grade-recovery curves can be created to be mineralogically limited from QEMSCAN analysis which, when compared with metallurgical results, are valuable in evaluating whether liberation or chemical selectivity is driving flotation performance. A number of case study examples are given where the QEMSCAN had a significant bearing on future project development at an early stage of exploration and mine development.

INTRODUCTION

Qemscan is a mineral analysis solution based on work conducted by the Commonwealth Scientific and industrial research Organisation (CSIRO). It uses electron beam measurement and sophisticated software to analyse mineral samples. QEMSCAN® uses advanced e-beam technology from Carl Zeiss; combines this with high resolution Backscatter (BSE) and Secondary Electron (SE) imaging and state-of-the-art liquid Nitrogen-free Energy Dispersive Spectrometers (EDS); and then integrates these using QEM*SEM® electronic control systems and the unique iDiscover™ software suite to provide quantitative information on the mineralogy in seconds.

MAGNETITE IRON ORE EXAMPLE

QEMSCAN is solution providing quantitative analysis of minerals. QEMSCAN is an abbreviation standing for Quantitative Evaluation of Minerals by SCANning electron microscopy and can identify most and ore-forming minerals.

It has been used in this testwork programme to identify the minerals and gangue associations in the selected YGDD001 and YGDD002 diamond drill core. The QEMSCAN investigation was performed on two polished sections from the YGDD001 drill core and five polished sections from drill core. In both the samples, magnetite and quartz were detected as the most abundant minerals. For YGDD001, amphibole and carbonates were found to be minor gangue minerals. Amphibole and olivine/pyroxene were found to be minor gangue minerals in YGDD002 polished sections. Table 1 provides a quantitative summary.

Magnetite was the main iron bearing mineral in both samples. The iron deportment to magnetite was reported to be greater than 90% in all polished samples and is seen in 2. The QEMSCAN showed that magnetite existed in two forms, coarse grained and finely disseminated within the quartz host. The presence of coarse size grain suggested that the samples may be upgraded by magnetic separation at a coarse size. The silicon was predominantly present as quartz in YGDD001 and YGDD002 polished sections. Amphibole was found to be a minor source of silicon across all samples, while olivine/pyroxene was found to also occur in the YGDD002 mineralogy. Amphibole minerals were seen as lamella structures within the magnetite host. High Pressure Grinding Rolls (HPGR) may provide an attractive comminution tool. This technique tends to generate breakage along the lamella grain boundaries, which will unlock the amphibole and other gangue minerals from the magnetite host. This is shown in Figure 2.

Some of the deleterious minerals were observed to be finely disseminated within the quartz host. This indicated that fine grinding followed by cleaner magnetic separation would most likely be required for the rejection of the contaminants. See figure 3.

A majority of the carbonate material is seen in YGDD001 at a coarse size and is closely associated with the amphibole material. The presence of carbonate material in the sample may contribute to the positive LOI values seen in the head assays.

Phosphorus was found to be mainly present as apatite. Despite the high association of the phosphorus with the magnetite grain boundaries in YGDD001 samples, the overall amount of apatite detected in the sample was minimal. The head assay results also showed this close association of the iron and phosphorus. The phosphorus deportment can be seen in Figure 4.

Pyrrhotite was the main source of sulfur in the YGDD002 polished sections while negligible pyrrhotite was detected in the YGDD001 samples as shown in 5. Pyrrhotite is a ferromagnetic iron sulfide mineral; thus is recoverable by magnetic separation. Due to this, some upgrading in the sulfur content would be expected during magnetic separation. Flotation may be needed as a polishing step for the sulfur rejection (depending on the sulfur level in the concentrate). Minor traces of copper and lead sulfides were the main source of detectable sulfur in YGDD001 which are not recovered through magnetic separation.

Figure 1 - Mass fractions of minerals in all polished sections

Figure 2 - Iron deportment for YGDD001 and YGDD002 QEMSCAN

Figure 3 - Silicon deportment for YGDD001 and YGDD002 QEMSCAN

Figure 4 - Phosphorus deportment for YGDD001 and YGDD002 QEMSCAN

Figure 5 - Sulfur deportment for YGDD001 and YGDD002 QEMSCAN

EPITHERMAL GOLD EXAMPLE

The master composite, or low Ag grade sample, was submitted for QEMSCAN to analyse the mineralogy of the sample. Both the gravity tail and gravity concentrate were analysed during the QEMSCAN.

The samples tested contained a high percentage of quartz, muscovite and alkali feldspars (73.9% total). Minor species identified included biotite / chlorite, carbonates and pyrite (10.6, 6.4 and 5.2% respectively). Argentite, a gold and silver based mineral, was also detected in very small amounts. High levels of quartz and feldspars are common gangue material encountered during processing.

The QEMSCAN identified 40 grains of gold and silver in the concentrate and 15 in the tail. The high number of grains identified allows more confidence to be applied to the interpretation of the results to a broader sample base.

Only two gold containing grains were detected in the gravity concentrate as electrum. The gold electrum detected was exposed, associated with quartz and pyrite. The silver was present as argentite, electrum as above and native silver. The liberated Au-Ag minerals identified in Table are exclusively native silver.

The argentite is associated with several species including native silver, pyrite, quartz and minor sulphides and is either exposed or encapsulated.

The following tables taken from the provide QEMSCAN report (full report is in Appendix B) indicate the liberation breakdown of the various Au-Ag minerals, as well as pyrite. The pyrite liberation breakdown is included as this species is a target for sulphide flotation.

Table 1 - Summary of mineral abundance

Table 2 - Au-Ag mineral locking

Locking Area % Liberated (>90% by vol) 94.07 Exposed (Au-Ag at surface of particle) 4.28 Encapsulated (Au-Ag is occluded) 1.66

Following from the liberation breakdown of Au-Ag species a majority of the argentite is seen to be contained within the range of 0< x <90 liberated (

Table 3 - Argentite liberation

The native silver is contained within the 60< x = 100 range as shown in Table, with an additional 21.3% being located in the lowest 0< x <10 class. The detected major gold silver species are seen to be mostly liberated. This indicates that the leaching performance, at P80 106 micron as used in the QEMSCAN analysis, will provide good extractions.

Table 4 - Native silver liberation

The liberation of pyrite, Table 5, shows that the pyrite is mostly contained in the top two liberation classes in the gravity concentrate. Including the tail and viewing the combined breakdown it can be seen that a majority of the pyrite in the system is above 60% liberated. Flotation of this pyrite would allow for a clean concentrate with little pyrite being heavily encased in other species.

Table 5 - Pyrite liberation

LEAD ZINC EXAMPLE

The analytical method used in the QEMSCAN was a Particle Mineralogical Analysis (PMA), which involved scanning the surface and analysing particles at random locations of the sample. The samples were ground to P80 106 μm and the two separate fractions were studied. The size fraction of most significance is the -106 µm fraction. Composites A and B contain 26-30% sulphide minerals. Sphalerite was the dominant economic sulphide mineral found. The distributions of the sulphide mineralisation are shown in Table 6. Table 6 - Mass percentage of sulphide minerals in the -106 µm fraction

Mineral Formula Composite A, % Composite B, % Sphalerite (Zn, Fe)S 12.6 14.05 Galena PbS 1.15 1.34 Fe-sulphides 16.60 10.97 Total 30.35 26.36

Feldspar and carbonates were the major gangue material detected. The presence of these gangue materials are not expected to cause processing issues.

Table 7 - Mass percentage of gangue minerals in the -106 µm fraction

Gangue Formula Composite A, % Composite B, % Feldspar Predominately. alkali feldspar 23.43 22.25 Carbonates Predominately. dolomite 36.73 34.83 Quartz SiO2 5.10 7.61

Liberation of the key minerals, sphalerite, galena and iron sulphides can be summarised as follows:

• There was a high level of liberation of the economic minerals in the -106 µm size fraction. • Approximately 69% of the sphalerite in Composite A was more than 90% liberated in the -106 µm size fraction. This level of liberation is likely to improve at finer particle sizes. • Similarly, for Composite B, 57% of the sphalerite was more than 90% liberated in the -106 µm fraction. The liberation of sphalerite will improve at finer grind sizes. • The 90% liberation of galena in Composite A and B was ~50% in the -106 µm size fraction. • There was a moderate to low level of liberation of other iron sulphides.

Locking data determined for the -106 µm fraction showed Sphalerite was found to be well to moderately liberated.

• In composite A, 70% of the Sphalerite in the -106 µm was contained in particles of over 90% sphalerite. • In composite B, 58% of the sphalerite in the -106 µm was contained in particles of over 90% sphalerite. • 22% of sphalerite in Composite A and 29% of sphalerite in Composite B was locked in binary particles with carbonates or silicates.

Galena was only moderately liberated.

• In composite A, 51.7% of the galena in the -106 µm was contained in particles of over 90% galena. • In Composite B 50.2% of the galena in the -106 µm fraction was contained in particles of over 90% galena. • In Composite A and B, 22% and 35% of the galena, respectively, was locked in binary particles with carbonates and silicates.

Over 50% of the Fe-sulphides were barren of any economic sulphides.

• Only 4-8% of the mass of the Fe-sulphides was found to be locked in binary particles with carbonates and silicates.

The grain size of the sulphide mineralisation was found to be fine (7 – 12 µm), however this technique typically under-estimates grain size. The mineralogical characteristics of the Myrtle samples indicate that it is a fine grained ore. It is therefore worthy to mention that the QEMSCAN technique has some limitations distinguishing between individual grains and co-joined grains at fine sizes and such grains can be reported as larger grains by this technique. Based on the QEMSCAN data it appeared that liberation at the 106 µm size could be an issue with flotation. Thus a finer size of P80 53 µm was selected for the sighter flotation testwork.

CHROMITE EXAMPLE

The QEMSCAN results showed very pure chromite particles in the concentrate. The percentage of chromite in the concentrate was 97.2% with the main impurities being magnesium silicates (serpentine, tremolite and chlorite) and iron oxides. The presence of tremolite is potentially a health issue, though it is present in very small quantities. Tremolite is a potential asbestiform mineral.

The chromite particles are essentially liberated with only a minor amount of occluded gangue minerals. These inclusions are identified as intergrowths of iron oxides and quartz within the chromite particles and are not separable by any simple physical separation (Figure 6).

Figure 6 - Chromite particles with included gangue minerals

The composition of the chromite particles, as measured by Energy Dispersive Spectroscopy (EDS) is shown in Table 8 where it is compared to calculated compositions from previous QEMSCAN analyses. The chromium content of the Co Dinh chromite particles is higher than previously estimated from other samples. This could result from:

• variable chromite grades within the deposit • accuracy of the element deportment and mineral percentages reported by QEMSCAN • accuracy of the EDS analysis (unchecked by chemical assay)

The iron content of the various chromites are consistent which could indicate that the chromium content is variable. Some chromites are known to show enrichment of chromium through surface oxidation and along cracks and fissures. This alteration is observable optically and well as chemically1. This can occur through the depletion of magnesium and aluminium with an increase in chromium and iron, forming an altered rim around unaltered chromite.

Table 8 - Composition of chromites from Co Dinh deposit

Sample Cr Fe Mg Al Si V Co Dinh concentrate (EDS) 42.7 23.6 3.2 3.8 0.2 0.4 Gravity Tails (calculated) 35.3 24.4 N/A* N/A N/A N/A Sample 2B (calculated) 29.5 24.5 N/A N/A N/A N/A Sample 2C (calculated) 28.6 21.0 N/A N/A N/A N/A Pure chromite (stoichiometry) 46.5 25.0 0 0 0 0 * N/A – not analysed

The presence of magnesium and aluminium in the chromite mineral occurs through replacement of magnesium for iron (2+) and aluminium for chromium and iron (3+) in the chromite spinel.

Calculated Grain Size

The average particle size in the concentrate is 132 microns which is close to the grain size of the chromite at 128 microns. This is another indication of the high degree of liberation of the chromite. The grain sizes of the other minerals are substantially smaller than the average particle size and are potentially not as well liberated. Nevertheless, there are liberated particles of all minerals in the smaller size fractions that contaminate the concentrate (

Figure 7 - Liberated and composite particles of tremolite

Pyrrhotite was the main source of sulfur in the YGDD002 polished sections while negligible pyrrhotite was detected in the YGDD001 samples as shown in 7. Pyrrhotite is a ferromagnetic iron sulfide mineral; thus is recoverable by magnetic separation. Due to this, some upgrading in the sulfur content would be expected during magnetic separation. Flotation may be needed as a polishing step for the sulfur rejection (depending on the sulfur level in the concentrate). Minor traces of copper and lead sulfides were the main source of detectable sulfur in YGDD001 which are not recovered through magnetic separation.

Elemental Deportment

The deportment of chromium, iron, aluminium, silicon and magnesium indicate the distribution of each element among the contained minerals. Chromium is contained almost exclusively, 99.9%, in the chromite mineral. Chromium is identified in some of the silica minerals (chlorite, ~2.8%, serpentine, 0.9%) but due to the low levels of these minerals in the concentrate, they contribute only trace amounts of the total chromium in the sample. Similarly, the majority of the iron, 97.3%, occurs in the chromite phase with 2.4% of the iron in 1.2% of the mass as other iron oxides. The chromite also contains 99.3% of the aluminium and 92.5% of the magnesium, presumably as lattice substituted atoms. Magnesium silicates account for the remainder of the magnesium. Roughly 26% of the silicon in the sample is contained in the chromite. The percentage of silica in the sample however is less than 0.4% and thus represents a small contaminant. This silica is occurring as fine inclusions within the chromite phase as indicated in Figure 8.

Figure 8 - Mineral map showing fine inclusions of quartz (pink) and other minerals within chromite and iron oxide particles

CONCLUSIONS

Qemscan is a very powerful diagnostic and quantitative tool for describing the mineralogical characteristics of the previously referred to ore samples. Qemscan looks at thousands of particles in 3D whereas conventional optical microscopy can only look at up to 100 particles and may not find any ore minerals. Qemscan can see a sample of magnetite with silica inclusions whereas the optical mineralogist reports a particle of magnetite. The optical mineralogist has difficulty recognising some minerals whereas qemscan is very accurate in its recognition and description. It provides detailed knowledge on the ore mineralogy, liberation which includes gold, uranium and base metals (copper, iron, nickel and lead-zinc). The application of qemscan mineralogy for accessing the complexity of mineral texture and their implication on liberation proves invaluable in determining an optimised suitable process flowsheet.. Furthermore, the limitations in the traditional microscope approach as well as the progress made in the development of advanced instrumental techniques such as QEMSCAN means that this type of work should be undertaken very early in the development of a project. The previously listed case studies highlight the application of process mineralogy to solving problems in mineral processing and extractive metallurgy for complex ores.. ACKNOWLEDGEMENTS

The author would like to thank various companies, all colleagues, engineers at various sites, METS staff and other consultants for their contribution and the management of METS for their permission to publish this paper and constructive criticism of various drafts. Thanks is also due to the laboratory staff who carried out the work on behalf of METS.

REFERENCES

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CSIRO, 2008, “Technology for rapid mineralogical analysis”, Solution, < http://www.csiro.au/solutions/QEMSCAN.html>.

D. Connelly, 2000, “Engineering the Perfect Plant”, Ballarat Gold Conference, Ballarat, VIC, Australia.

D. Connelly, 2010, “Process Plant Design Considerations for High Silver Gold Ores”, Precious Metals Conference, Falmouth, England.

D. Connelly, , 2010, “Pitfalls When Undertaking Gold Testwork”, ALTA Conference, Perth WA.

D. Connelly, , 2004, “The Mina Sertão Gold Project – From Drill Core to Production”, AusIMM MetPlant Conference, Perth WA.

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