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Mechanical and Chemical Dissolution Methods Versus Selective Fragmentation for Mineral and Fossil Separation and Concentrates from Select Geologic Materials

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Abstract

Selective fragmentation of geologic materials was performed on samples including granodiorite, quartz pebble conglomerate, amphibolite, ash flow tuff, and fossiliferous carbonate. Minerals and fossils of interest include zircon (igneous and detrital grains), apatite, amphibole, monazite, titanite, sanidine, and conodont tests. Each sample has corresponding splits reduced by traditional methods (jaw crusher and vertical grinder or acid-buffer dissolution for carbonates). Both the selective fragmentation splits and traditional splits were further processed for mineral and fossil concentrations of interest by using magnetic separation and heavy liquids. Splits of selective fragmentation and traditionally reduced methods were examined visually by binocular scope and in more detail by electron microscopy. Some initial observations for selective fragmentation include rapid comminution (<10 minutes/sample), more composite grains, more fully intact grains, and less fine-grained material (e.g. dust). Traditional sample reduction methods observations include, heavy dust generation, many fractured grains of interest, but few composite grains. Buffered acetic acid digestion of carbonate rocks yields almost complete recovery of conodonts and other residue that can typically be concentrated in magnetic and heavy liquid separation splits relatively easily. The separation of condont by selective fragmentation requires large efforts in post-processing methods including large sample throughput in magnetic separation and heavy liquid separation.
- Quantitative analysis of heavy mineral size and c-axis length.
- Determination of selective fragmentation as a means for reducing samples to undergo
fluid inclusion analysis.
- Determine the effectiveness of selective fragmentation on a greater variety of geologic
samples.
- Quantitative analysis of mineral fractures propagated or exploited by selective
fragmentation.
- Determine the effectiveness of selective fragmentation in ridding glassy material from
mineral grains, such as sanidine and other felsic minerals.
Abstract
Mechanical and Chemical Dissolution Methods Versus Selective Fragmentation for Mineral
and Fossil Separation and Concentrates from Select Geologic Materials
STRONG, Thomas1, BALA, Sarah A.2, HOLM-DENOMA, Christopher S.3, ROMEIJN, Eva4, PARVAZ, Daniel4and ROBERTS, Jason E.5
(1)Metropolitan State University of Denver, MS 973 Box 25046, Denver, CO 80204, (2)Department of Earth and Atmospheric Sciences, Metro State University of Denver, Denver, CO 80217-3362, (3)Central Mineral and Environmental Resources Science Center, United States
Geological Survey, Box 25046, MS 973, Denver, CO 80225-0046, (4)Selfrag AG, Biberenzelgli 18, Kerzers, CH-3210, Switzerland, (5)Chemistry, University of Colorado-Denver, Denver, CO 80204, tstrong4@msudenver.edu
Observations
Figures
References:
Dostal, J., Kontak, D.J., Hanley, J., and Owen, V. 2011. Geological investigation of rare earth element and uranium deposits of the Bokan Mountain Complex, Prince of Wales Island, Southeastern Alaska. U.S.
Geological Survey Mineral ResourcesExternal Research Report G09AP00039, p. 122.
Jordens, Adam., Cheng, Ying Ping., Waters, Kristian.,A review of the beneficiation of rare earth element bearing minerals. Department of Mining and Materials Engineering, McGill University, 3610 University,
Montreal, Quebec, Canada.12 December 2012
Iget Software Terra Softa, Inc., authored by M. J. Carr.
GCDKit 3.0Janoušek, V., Farrow, C. M. & Erban, V. 2006. Interpretation of whole-rock geochemical data in igneous geochemistry: introducing Geochemical Data Toolkit (GCDkit). Journal of Petrology
47(6):1255-1259
http://mrdata.usgs.gov/wms/namag-1.1.1.xml
Historically, the first steps in mineral and fossil comminution/separation in rocks have involved mechanical
force (crushing and grinding) or chemical dissolution. Selective fragmentation is a technology that uses high
voltage pulse power fragmentation in an insulating medium (water) to progressively break materials along
grain boundaries. Some advantages of selective fragmentation include low dust production, fewer fractured
grains, coarser mineral separates, and no hazardous chemical exposure or waste. This study attempts to
determine qualitative differences between selective fragmentation and traditional comminution practices in a
variety of samples that represent commonly processed rocks at the USGS.
Selective fragmentation of geologic materials was performed on samples including granodiorite, quartz
pebble conglomerate, amphibolite, ash flow tuff, and fossiliferous carbonate. Minerals and fossils of interest
include zircon (igneous and detrital grains), apatite, amphibole, monazite, titanite, sanidine, and conodont
tests. Each sample has corresponding splits reduced by traditional methods (jaw crusher and vertical grinder
or acid-buffer dissolution for carbonates). Both the selective fragmentation splits and traditional splits were
further processed for mineral and fossil concentrations of interest by using magnetic separation and heavy
liquids. Splits of selective fragmentation and traditionally reduced methods were examined visually by
binocular scope and in more detail by electron microscopy. Some initial observations for selective
fragmentation include rapid comminution (<10 minutes/sample), more composite grains, more fully intact
grains, and less fine-grained material (e.g. dust). Traditional sample reduction methods observations include,
heavy dust generation, many fractured grains of interest, but few composite grains. Buffered acetic acid
digestion of carbonate rocks yields almost complete recovery of conodonts and other residue that can
typically be concentrated in magnetic and heavy liquid separation splits relatively easily. The separation of
condont by selective fragmentation requires large efforts in post-processing methods including large sample
throughput in magnetic separation and heavy liquid separation.
Diorite Sample (Temora 2): Selective fragmentation fractions produced zircon grains free of
mineral intergrowth and often free of inclusions. However, the presence of metallic
inclusions attracted the discharge track, fracturing zircon grains. The traditional fraction did
not fracture, in general, at inclusion sites, but produced many small fragments through
mechanical comminution.
Carbonate rock (CO-ML-L1): Selective fragmentation wielded many small fragments of
micro-fossils, as well as fragments in situ. Traditional methods, obversely, wield intact
micro-fossils and has a near 100% recovery.
Quartz-pebble conglomerate (CON-X): Selective fragmentation preserves detrital zircon and
apatite grains, as well as surficial provenance features that can be utilized during myriad
analyses. Traditional methods can preserve the features noted in the selective
fragmentation fraction, but to a lesser degree (i.e. some grains are broken/fractured).
74AR15: Selective fragmentation fractions show preserved apatite and zircon grains, as well
as other heavy minerals, and a retention of provenance features. Traditional methods
yielded similar results, yet there were a greater degree of broken or fractured grains.
Methods
Each sample was reduced by the two methods mentioned- selective fragmentation and mechanical
comminution. Selective fragmentation reduces whole rock to its constituent minerals by inducing high-
voltage pulse power that separates the minerals along their natural physical boundaries. The shockwave
wave that follows the discharge track (illustrated in Figure 1) acts as a comminutive force by physically
separating the constituents. Selective fragmentation of the geologic minerals was undertaken at the
SEFRAG Lab in Switzerland, where a lab technician determined the correct operational parameters for
selective fragmentation of each individual sample. The following parameters were used:
- Temora 2: 8 cycles @ 190-200kV, 5 Hz, 50 pulses
-74AR15:10 cycles @ 195 &200 kV, 5 Hz, 6 cycles @ 60 pulses, 4 cycles at 100 pulses
- Conglomerate: 8 cycles @ 194 &200 kV, 5 Hz, 50 pulses
Mechanical comminution was used on representative fractions of each of the geologic samples, as well;
chemical dissolution using an acid-buffer solution for the carbonate rock For this reduction process whole
rock was first reduced by ajaw crusher, then further reduced by a disk mill, before being sieved to a size
range of 44-300 microns (325-50 mesh). Once sieved, the bulk material was then processed on a Wilfley
water table, which effectively separates and concentrates heavy minerals (>3.0 SG). Selective
fragmentation fractions were also sieved to this size range, but not concentrated by the Wilfley table. Each
of the separately-processed fractions were then separated using a Frantz isodynamic magnetic separator,
where subsequent paramagnetic charges (0.4A, 0.8A, 1.8A) were used to reduce the bulk material in stages.
Once a non-magnetic fraction was obtained for each process (usually containing heavy minerals such as
zircon and apatite) it underwent a gravity separation utilizing methylene iodide (~3.32 SG) as a density
medium.
Upon acquiring non-magnetic, heavy mineral fractions for each process qualitative observations were
made via visual discrepancy and comparison of the fractions under binocular microscope and scanning
electron microscopy (SEM). The binocular microscope afforded a general overview of the effectiveness of
each process, as well as an understanding of the behavior of the mineral phases. SEM imagery, both
backscatter electron (BSE) and secondary electron imaging (SEI), provided a visual test of the surface
expressions of mineral grains (SEI) and contrast of mineral grains and inclusions (BSE). When BSE imagery
indicated inclusions at fracture planes electron dispersive X-ray spectroscopy (EDS) was used in order to
obtain a qualitative elemental composition of the inclusion.
74AR15
Selective fragmentation fraction (Temora 2): SEI image of
typical zircon grains liberated. Traditional fraction (Temora 2): SEI image of typical zircon
grains liberated.
Selective fragmentation fraction (Temora 2): SEI image of
fractured, euhedral zircon grains free of intergrowths or
inclusions.
Traditional fraction (Temora 2): SEI image of fractured zircon
grains with plagioclase intergrowths at the surfaces.
Illustration of the preferential attraction of the discharge track to
metallic, or high dielectric constant, inclusions (as evidenced by
the image to the left.)
Selective fragmentation fraction (Temora 2): BSE image of
zircon grain fractured at the contact of two XX inclusions. See
illustration (right).
Selective fragmentation fraction (Temora 2): Reflected light
image of zircons liberated. Note the near absence of in-place
inclusions, of which can be seen lying around the zircons.
Traditional fraction (Temora 2): Reflected light image of
liberated zircons. Note that nearly all inclusions have remained
within the zircon grains.
Selective fragmentation fraction (CO-ML-L1): SEI image of
fractured, in situ microfossil. Selective fragmentation fraction (CO-ML-L1): Reflected light
image of carbonate rock material. Note the fragments of
microfossil (dark grains) and euhedral calcite grains.
Selective fragmentation fraction (XXXX): BSE image of zircons
with preserved inclusions and provenance features (rounding,
surface pitting.)
Selective fragmentation fraction (74AR15): Reflected light
image showing euhedral apatite and fragments liberated.
Selective fragmentation fraction (XXXX): Reflected light image
showing preserved, rounded apatite (colorless) and detrital
zircon (pink).
Images (selective fragmentation): Top left - Image of the Selfrag Lab used to
reduce samples. Top right - cross-sectional view of the Selfrag Lab showing
high-voltage pulse being introduced to the processing vessel. Bottom -
Illustration showing the discharge track as it travels through the material
following natural boundaries.
Images (traditional methods): Top left - Jaw crusher. Top middle - Disk mill. Top right - Sieve shaker.
Bottom left - Wilfley table. Bottom middle - Frantz magnetic separator. Bottom right - Gravity
separation via methylene iodide (heavy liquid).
The use of selective fragmentation to reduce geologic samples has many advantages that
may or may not be found in traditional methods. Where traditional methods produce dust
and fine material, fractured or broken grains, sample loss, and take several days to finish the
process, selective fragmentation shines. Selective fragmentation produces very little dust, is
quite fast (<10 minutes), and produces intact grains while retaining all material. The number
of processes used traditionally can add to the risk of sample contamination, whereas
selective fragmentation lessens and nearly omits the chance of contamination.
SEM imagery shows that grains liberated by selective fragmentation are virtually free
of intergrowths and surficial remnants, while traditional methods produce grains with
intergrowths and a greater degree of mineral fragments. Due to the highly inclusive zircon
grains of the Temora 2 sample, several fragments were produced by selective
fragmentation’s preferential attraction to metallic inclusions (which may be quite useful for
large-scale comminution of ore rock). This was not seen in selective fragmentation fractions
where zircon inclusions were non-metallic.
The traditional method of chemical dissolution of the carbonate rock using an acid-
buffer solution proves much more effective at liberating intact microfossils than selective
fragmentation. Selective fragmentation did not adequately preserve or separate micro
fossils from the host rock; many of the microfossils were found to be in small fragments,
and/or in situ.
Overall, selective fragmentation proves particularly effective as a reduction method,
especially when dealing with a small sample and material retention is critical, but also on
larger samples in which the mineral(s) of interest may be scarce. Further research can
provide a qualitative and quantitative analysis comparing the two processes among other
types of material, geologic or otherwise.
Conclusions
Future Research
... TEMORA 2 Zircon, Strong et al., 2015;Rubatto, 2007Rubatto, , 2009 HV Fragmentation • Coarser zircon recovery • Improved knowledge of grain size and zonation • Less loss of crystal surface features • 3D crystal study possible From Baxter and Scherer, 2013;Christensen et al, 1989. Effects of Inclusions • Preferential partitioning of Sm, Nd, Lu, Hf into garnet or inclusions alters bulk gt geochemistry • Incorporated low Sm/Nd phase-> false young age • Contamination from equilibrated low Sm/Nd phases-> false old age ...
Presentation
Full-text available
Analytical capability increases year on year as both technology and methodologies improve analytical detection limits, accuracy and precision of scientific instrumentation. Sample preparation methods, however, have remained largely unchanged for the past century. Jaw crushing and milling are dust generating and highly destructive, losing or breaking large proportions of the target material, or smearing metallic particles, destroying information on crystal structure. Selective dissolution or cryogenic methods to liberate fragile particles are time and labour intensive. Without improving preparation technique, it may not be possible to extract the full value from this increased precision and accuracy. High voltage (90 – 200kV) electrical pulses (HVP) serve to disaggregate a material along phase boundaries by creation of tensile stresses within the material, pulling the rock apart quickly and with minimum fine generation or breakage of individual phases. This work showcases recent research and applications of HVP in non-destructive liberation of zircon, garnet, native metals and microfossils from the host rock for the purpose of improving chemical and isotopic analysis.
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