Native Copper Analysis through Digital Microscopy
O. A. J. Soto, O. F. M. Gomes*, G. A. H. Pino & S. Paciornik
Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro, PUC-Rio, Rio de
Janeiro, Brazil
* Centre for Mineral Technology, CETEM, Rio de Janeiro, Brazil
ABSTRACT: There is no established procedure to determine native copper in a copper ore. Actually, due to
its low occurrence in nature, there are not many references in the literature. This work proposes a method for
copper ore analysis based on digital microcopy, image processing and chemical etching. The present case
study was developed on an ore composed mainly of copper oxides, copper primary sulfides, copper secondary
sulfides and native copper. Initially, several fields of a polished cross section of an ore sample were
automatically imaged through a motorized optical microscope and a CCD camera connected to a computer.
Then the sample suffered successive selective etchings. After each etching step, the sample was returned to
the microscope and the same fields were imaged again, through the use of a computer-controlled x-y stage.
Thus, three images per field were generated. Finally, the corresponding image sets (before and after each
etching step) were processed and analyzed.
1 INTRODUCTION
The evaluation of head mineral for copper mining is
nowadays based in just three chemical analyses:
total copper, oxidized copper and copper soluble in
sulfuric acid. Some companies use the sequential
method for copper analysis. But none of these
methods provides the amount of native copper in the
sample. Moreover, copper recovery cannot be
determined in these ores.
The present work proposes a method for copper
ore analysis based on digital microcopy, image
processing and chemical etching. Copper ore
samples were imaged with a motorized optical
microscope, allowing the acquisition of a large
number of fields per sample. Each sample was then
submitted to an etching sequence and, after each
etching step, the sample was returned to the
microscope and the same fields were imaged again.
Thus, specific particles in each field can be located
and the effect of etching can be accurately analyzed.
This correlated approach, co-site microscopy,
improves the accuracy as compared to a traditional
approach in which uncorrelated fields are analyzed
before and after etching.
2 EXPERIMENTAL PROCEDURE
2.1 Material Selection
In the present work, an ore from Yauri Cusco, Peru,
with 1.5% of native copper was used.
The ore was classified and 4 samples (+74-100
ìm, +63-74 ìm, +37-63 ì m , and +20-37 ìm) were
studied.
2.2 Sample Preparation and Etching
The samples were cold mounted by mixing the ore
grains with low viscosity epoxy resin and
subsequently ground, polished with 3 and 1 µm
diamond paste and 0.05 µm alumina. Each sample
was then glued to a glass slide in such a way that the
polished surface was kept parallel to the glass plane.
The glass slide was used as reference to insert the
sample in the microscope sample holder. The glue
used was insensitive to the chemical etchants used in
the process. Thus, each sample could be observed,
removed, etched and brought back to the microscope
at nearly the same position, thus allowing the
observation of the same fields along the etching
sequence.
The first step of etching used citric acid (5% w/v)
and sulfuric acid (5% v/v) for 1 hour at 25 C.
Samples were then washed in deionized water and
ethanol, and dried. The second step used silver
sulfate (3 g/l) in sulfuric acid (5% v/v) solution in
the same conditions as step 1.
The first etchant leaches all the copper oxides and
part of the secondary copper sulfides. It also works
as a light etchant for native copper. The second
etchant leaches native copper completely and part of
the sulfides.
2.3 Microscopy and Image Acquisition
A Zeiss AxioPlan 2ie mot optical microscope, with a
motorized x-y stage and an Axiocam digital camera
(1300 x 1030 pixels) was used in the experiments.
Being fully computer-controlled, the system allows
for the acquisition of any number of fields at specific
x-y positions of the sample holder. Thus, within the
error of the holder stepper motors, it is possible to go
back to the same fields of a sample.
The system also allows for motorized control of
the z-axis. Thus is possible to control the focusing
for each field. An autofocus routine (Valdecasas et
al. 2001, Bastos et al. 2003) was developed,
allowing for automatic in focus image acquisition
including compensation for sample surface
inclination and local focus variation.
A set of one hundred fields was acquired from
each of the samples, using a 20X objective lens,
corresponding to a resolution of 0.53 µm/pixel. At
this magnification each field corresponds to 685 x
543 µm2. Given that native copper concentration in
these samples was 1.5 %, one can expect to
randomly find a few fields showing native copper
particles.
The same routine was used for every sample, after
each of the etching steps, leading to a total of 300
images per sample.
accuracy. In our experiments this lead to a larger
misalignment in the y-direction, of several pixels,
and a smaller misalignment in the x-direction.
Nevertheless, the misalignment corresponded to a
few percent of the field size, allowing an automated
alignment procedure.
Cross-correlation in the frequency domain
(Gonzalez & Woods 1992) was used. As the
misalignment was small and there are other
components of the ore that are not affected by
etching, the images didn’t change too much due to
the etching. Thus, there is always a clear correlation
peak and its distance from the central pixel of the
image corresponds to the displacement vector. For
certain sets of images, a previous step of contrast
normalization was used to improve the correlation
peak definition. After alignment, all images were
cropped to eliminate edge effects. The procedure
runs automatically and representative results are
shown in Figure 1.
3.2 Segmentation
The image processing and analysis sequence
involved the steps of alignment and cropping,
segmentation and measurement.
As one of the goals of the procedure is to evaluate
the leaching of native copper, this phase must be
detected at each step of the etching sequence. As the
microscope illumination is also computer-controlled;
one can expect enough color stability to allow for
stable detection thresholds.
Before the first etching, native copper and bornite
(a copper/iron sulfide) appear with nearly the same
color. To distinguish between them, a threshold
based on the HSI color model (Gonzalez & Woods
1992) was used. In some fields the threshold had to
be manually adjusted and, in some cases, it was
impossible to eliminate the bornite particles without
impairing the detection of copper particles.
Once the unetched copper particles were detected,
they were used to mask the rest of the field in the
images obtained after the etching steps.
Figure 2 shows a detail of a native copper particle
before and after the two etching steps and the
segmented regions detected by the procedure. It is
possible to detect a small change after the first
etching and the total disappearance of the particle
after the second etching, leaving behind the
unfocused region of the epoxy resin.
3.1 Image Alignment and Cropping
3.3 Measurement
As images obtained after each etching step are to be
quantitatively compared, they must be in perfect
register. However, the placement of the glass slide
on the microscope holder has limited repeatability
and the motorized stage also presents limited
Once the native copper particles are detected before
and after each etching step, several size and shape
parameters can be automatically measured.
3 IMAGE PROCESSING AND ANALYSIS
Figure 1: Image alignment through cross-correlation and cropping. The reference dashed lines highlights the vertical displacement
between the images and the alignment reached after correlation.
Figure 2: Detail of a native copper particle during etching and segmentation results.
4 RESULTS AND DISCUSSION
To evaluate the leaching of native copper, the area
of each particle was measured before and after each
etching. Table 1 shows these areas for some
representative particles. A1, A2 and A3 mean area
before first etching, after first etching and after
second etching, respectively.
Table 1: Area measurements before and after each etching step
Particle
A1
A2
A2/A1
A3
A3/A1
(µm2)
(µm2)
(%)
(µm2)
(%)
1
4683
4197
90
0
0
2
3912
3341
85
0
0
3
3781
3230
85
93
2
4
3203
2487
78
0
0
5
2490
2306
93
0
0
6
2187
2143
98
0
0
7
1315
0
0
0
0
8
1014
512
50
0
0
9
954
732
77
0
0
10
599
410
68
0
0
11
578
462
80
0
0
12
553
341
62
0
0
13
552
271
49
0
0
No correlation was found between the leaching
kinetics and particle area reduction along the etching
steps. We can point out some reasons for this
behavior.
In line 3 of Table 1, one can see a native copper
particle that apparently was not completely leached
after the two etchings. Actually, as shown in Figure
3, a small silver precipitate appeared where before
there was a native copper particle
Probably, the particle got detached during the
etching and thus this result is not representative.
Particles 8 and 9 had similar areas, but after the
first etching their area reductions were very different
(Fig.4). This was a common behavior in the database
of over 250 particles. It denotes the lack of
correlation mentioned above. Many factors
contribute to this such as porosity, polishing defects,
particle shape, cross-sectional effects, among others.
Figure 4: Particles with similar original areas and different area
reduction after the first etching.
5 CONCLUSIONS
Even though no quantitative correlation between
etching kinetics and area reduction was found, the
approach of co-site microscopy opens up new
possibilities of analysis.
For instance, once the particles are identified
before and after each etching step, other
measurements (size, shape and texture) can be
automatically obtained, and may allow a better
description of the process.
The technique allows the local observation of
specific phenomena that would be lost in the
classical multi-field statistical approach and can be
applied to several other systems. Originally similar
phases may change color during etching and can
thus be discriminated particle by particle.
6 REFERENCES
Figure 3: Native copper particle and silver precipitate after the
second etching..
Referring to the particle in line 7, even though
A2=0, indicating total leaching, this is very unlikely.
Bastos, A. L., Gomes, O. F. M., Maurício, M. H. P., Paciornik,
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Gonzalez, R. C. & Woods, R. E. (1992). Digital Image
Processing. New York: Addison-Wesley.
Valdecasas, A. G., Marshall, D., Becerra, J. M., Terrero, J. J.
(2001): On the extended depth of focus algorithms for
bright field microscopy. Micron 33: 559-569.