Comparative Study of Gold Concentration by Elutriation from Different Precious Metal Bearing Ores

doi:10.4236/ijnm.2013.24018

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International Journal of Nonferrous Metallurgy, 2013, 2, 121-127

http://dx.doi.org/10.4236/ijnm.2013.24018 Published Online October 2013 (http://www.scirp.org/journal/ijnm)

Comparative Study of Gold Concentration by Elutriation

from Different Precious Metal Bearing Ores

Martín A. Encinas-Romero*, Guillermo Tiburcio-Munive, Jesús L. Valenzuela-García

Departamento de Ingeniería Química y Metalurgia, Universidad de Sonora, Hermosillo, México

Email: *[email protected]

Received February 11, 2013; revised May 28, 2013; accepted June 10, 2013

tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly

cited.

ABSTRACT

Conventional methods for precious metals gravimetric concentration involve equipment such as shaking tables, centri-

fuging concentrators, jigs, trommels, or a combination of those. A less commonly used technique is elutriation, which

represents an efficient, safe and low-cost method of separation. The goal of the present investigation was to make a

comparative study of gold concentration by elutriation from different precious metal bearing ores: an oxide ore, a min-

eral consisting of a sulfide matrix, a mineral in which the precious metals are free and disseminated and a slimy and

clayey black sand material. The best recoveries of precious metals by elutriation were attained for the free disseminated

ore and for the black sands, obtaining gold recoveries of 70% and 96% respectively, with appreciable ratios of concen-

tration as well.

Keywords: Elutriation; Precious Metals; Oxides, Sulfides; Free Gold; Black Sands

1. Introduction

Elutriation is a particulate separation process in which an

upward fluid stream generally air or water is used. The

classification is made through a series of tubular or coni-

cal vessels of increasing size, so that the flow rate de-

creases successively from one of vessel to the next.

Generally, an elutriator consists of one or more “sor-

ting columns” in which the fluid flows upwards at a con-

stant velocity. Feed particles introduced into the sorting

column will be separated into two fractions, according to

their terminal velocities calculated from Stoke’s Law.

Particles with a terminal velocity smaller than the fluid

mean flow rate will overflow, whereas those with a lar-

ger velocity than the mean flow rate will sink toward the

underflow. Elutriation is carried out until either no visual

signs of a further separation are observed, or there is no

change in the weight proportions of the products [1].

Elutriation with air is faster than elutriation with a li-

quid. It also tends to be more efficient due to a lower air

resistance to the particles fall, and to a less tendency for

agglomeration of particles [1,2].

Elutriation with liquids is a process of separation or

sub-separation of particulate of different sizes within a

fluid stream, such as water. If the specific gravity of the

feed material is uniform, the resulting grades of the

product streams can be significantly uniform even for

very narrow size ranges. On the other hand, a consider-

able size variation of the product streams occurs when

there are large differences in specific gravity or particle

size of the feed even for narrow size ranges [3,4].

The main advantage of elutriation is the absence of

moving parts. It represents an economic alternative as a

method for precious metal concentration. The main dis-

advantage of elutriation is related to the velocity profile,

originating across the fluid stream due to the resistance

imposed by the vessel walls. Thus, the particles are ex-

posed to a fluid velocity field that varies with the radial

position in the vessel. In fact, these particles are carried

toward the region of high-speed flow due to the pressure

differences on their surfaces. Consequently, the high-

speed fluid captures these particles, reducing thereafter

the efficiency of the elutriator [5].

There are no recent reports on the use of the elutriation

with liquids as applied to upgrade precious metal miner-

als. Therefore, as a part of a general project on the use of

the non-conventional methods for precious metal gravity

concentration, the following study was undertaken. The

goal of this study is to compare the precious metals con-

centration by elutriation as applied to different ores. Gold

*Corresponding author.

M. A. ENCINAS-ROMERO ET AL.

122

recoveries and concentration ratios for those materials

were measured and used as comparison parameters.

2. Materials and Methods

2.1. Feed Materials

The study involved the treatment of four different types

of feed materials from regional mining deposits:

1) Oxide ore

2) Sulfide ore

3) Free gold ore, and

4) Black sands

The as-received materials were first crushed in a 170 ×

135 jaw crusher. Afterwards, they were secondary cru-

shed in a 222 mm cone crusher. Then, they were pul-

verized in a 222 mm ring pulverizer. Finally, the materi-

als were wet-screened and dried to prepare close-sized

feeding for the elutriation tests. A representative simple

for every size range was taken for gold analysis and the

results are presented in Table 1.

With the exception of black sands, chemical and X-ray

analysis were made for bulk samples of the different ores.

Also, specific gravities for every mineral studied were

determined by a picnometer. Table 2 presents the results.

The main mineral species present in the oxide ore are

quartz and Fe-bearing minerals (hematite, limonite and

jarosite). For the sulfide ore, pyrite is the main compo-

nent, whereas silicates (quartz, orthoclase, albite and

muscovite) are the most abundant species in the free-gold

ore.

2.2. Elutriation Equipment

The elutriation equipment consisted of two sorting co-

lumns of PVC as shown in Figure 1. Both columns have

diameters of 50.8 mm (2”) and the lengths for the elutria-

tion section are 1054.1 mm (41.5”) and 482.6 mm (19”),

respectively. Details of equipment facilities are given

elsewhere [6].

A typical run involved the following steps. Firstly, the

elutriator is assembled as shown in Figure 1 and the con-

centrate 1, concentrate 2 and tailings containers are posi-

tioned tightly in place. Afterwards, the water feed valve

is opened to fill up completely the first column and par-

tially the second one. The sample was mixed with water

aiming at preparing a slurry containing 30% of solids

(weight basis). This pulp is put into the feed tank and is

kept continuously stirred to maintain the solids in sus-

pension.

A test started by opening the pulp feed valve and re-

gulating its flow rate within the range 6 to 13 l/h, and the

feed water flow rate, within the range 30 to 121 l/h. The

precise velocity ratio for every particle size is adjusted

when a clear and constant separation of the two fractions

is obtained. Typically, when the particle size decreases

much slower feed and washing flow rates are used than

with coarser particles. The experiment was stopped until

no visible signs of particle separation were observed. The

products: concentrates 1 and 2 and tailings were dried at

100˚C and analyzed by fire assay for its gold contents.

3. Results and Discussion

3.1. Oxide Ore

Figures 2 and 3 show the gold distribution and ratio of

concentration for concentrates 1 and 2, respectively, for

the oxide ore. For small particles sized the greatest re-

coveries are found in the concentrate 2 (>90%). However,

as Figure 3 shows, the concentration ratios in this frac-

tion have an average value of 1:1, which means that most

of the feed ore stays in the concentrate 2 without any

appreciable upgrading. On the other hand, as the particle

Table 1. Gold head essay for different particle size of the minerals tested.

Size Distribution Gold Head Essay (g/ton)

Mesh Size Average Particle

Size (µm) Oxide Mineral Sulfide Mineral Free Gold Mineral Black Sand

−40; +60 302.5 na na na 0.72

−60; +100 200 na na na 0.39

−80; +100 165 0.3 7.07 38.2 na

−100; +150 128 0.26 66.75 36.6 0.65

−150; +200 90.5 0.20 74.94 26.65 na

−200; +270 64 0.33 102.7 23.12 na

0.270; +325 49 0.33 74.98 15.73 na

−325 45 0.46 106.6 na 0.40

na: not applicable.

M. A. ENCINAS-ROMERO ET AL. 123

Table 2. Chemical composition and X-ray difraction analysis for the different minerals tested.

Species Composition (%w/w)

Oxide Mineral Sulfide Mineral Fee Gold Mineral Black Sand

SiO2 >30 >30 >30 8.4

Fe 14.2 19.8 3.92 58.83

Cu 0.031 0.21 0.30

Pb 1.96 0.020

Zn 0.20 0.26 0.020

Cr2O3 0.04

ZrO2 0.054

Fe2O3 (hematite) <5 <5 <5

Fe2O3Fe2O3·H2O (limonite) <5 <5

KFe(SO4)2(OH)6 (jarosite) <5 <5

(NaK)Al3(SO4)2(OH)6 (natrualunite) <5

PbFe6(SO4)4(OH)12 (plumbojarosite) <5

FeS2 (pyrite) <5 <5 10 - 15

KAlSi2O8 (orthoclase) <5 <5 10 - 15

NaAlSiO3 (albite) <5

PbS (galena) <5

KAl2Si2AlO10(OH)2 (muscovite) <5 5 - 10

K-Na-Mg-Fe-Al-Si-O-H2O (iccite) <5

Al2SiO5(OH)4 (kaolinite) <5

FeO(OH) (goethite) <5

PbCO3 (cerusite) <5

Ca-Na-Mg-Fe-Al-Si-O-OH-H2O (montmorillonite) 0.20 74.49 39.43 0.36

Au-80 mesh (g/ton) 0.2

Bulk Specific Gravity (g/cm3) 3.62 3.56 3.64 5.34

Feed Inlet

Middlines

Water InletWater Inlet

Concentrate 1

Outlet Concentrate 2

Outlet

Tail ings

Outlet

Figure 1. Schematic diagram of experimental elutriator.

Numbers 1 and 2 refer to first and second elutriators.

size increases recovery in the concentrate 1 increases.

Gold recoveries of 74% were obtained for the particle

sizes of 165 μm (−80 + 100 mesh), and gold concentra-

tion ratios of the order of 5.5:1. As an important %w/w in

this ore corresponds to iron oxides, others researchers

had reported that the smaller iron oxides particles are not

easily elutriated from fluidized beds of mixed size parti-

cles [7]. These results are consistent with the results of

present study.

3.2. Sulfide Ore

Regarding the sulfide ore, Figures 4 and 5 indicate a

poor gold recovery for both concentrates (<8%). For

coarser sizes a slight increase in recovery was observed

for concentrate 1 whereas for concentrate 2 the increase

is more significant (from 10% to 60%), similar to the

behavior observed with the oxide ore. However, the in-

crease in gold distribution is accompanied by a drastic

decrease in the ratio of concentration indicating that most

f the gold values are lost in the tailings. o

M. A. ENCINAS-ROMERO ET AL.

124

1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3

100

Log[dp]

Au Recovery, %

120

160

200

Ratio of Concentration

Figure 2. Gold recovery and ratio of concentration, as a function of a particle size, for concentrate 1 of oxide mineral.

1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3

100

Log[dp]

Au Recovery, %

0.0

0.4

0.8

1.2

1.6

Ratio of Concentration

Figure 3. Gold recovery and ratio of concentration, as a function of a particle size, for concentrate 2 of oxide mineral.

1.61.71.81.92.02.12.22.3

Log[dp]

Au Recovery, %

100

150

200

250

300

Ratio of Concentration

Figure 4. Gold recovery and ratio of concentration, as a function of a particle size, for concentrate 1 of sulfide mineral.

M. A. ENCINAS-ROMERO ET AL. 125

1.61.71.81.92.02.1 2.22.3

Log[dp]

Au Recovery,

Ratio of Concentration

Figure 5. Gold recovery and ratio of concentration, as a function of a particle size, for concentrate 2 of sulfide mineral.

1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3

Log[dp]

Au Recovery, %

Ratio of Concentration

Figure 6. Gold recovery and ratio of concentration, as a function of a particle size, for concentrate 1 of free gold mineral.

1.61.7 1.81.92.02.1 2.22.3

Log[dp]

Au Recovery, %

Ratio of Concentration

Figure 7. Gold recovery and ratio of concentration, as a function of a particle size, for concentrate 2 of free gold mineral.

M. A. ENCINAS-ROMERO ET AL.

126

3.3. Free Gold recoveries were obtained, e.g. 96% gold recovery in con-

centrate 1 fraction with adequate concentration ratios, as

Figure 8 shows. For coarser particle sizes, most of the

gold reported in concentrate 2 as shown in Figure 9. Be-

cause this kind of ores contain relatively pure, well-

sorted heavy mineral concentrates, they are separated

from larger, less dense particles limiting the settling of

larger, less dense particles, but also allows smaller, dens-

er particles to settle unhindered [8].

For the free gold ore, Figures 6 and 7 depict the varia-

tion of gold recovery and ratios of concentration against

particle size for concentrates 1 and 2, respectively. Com-

pared to previous materials, here a significant difference

is observed. For coarser sizes concentrate 1 has gold

contents above 70% and reasonable ratios of concentra-

tion average 70:1. For smaller particle sized, the greatest

proportion of gold is present in concentrate 2, with ap-

preciable ratios of concentration as well. This kind of

materials showed a better response to elutriation as com-

pared to the other minerals.

Tables 3 and 4 summarize the complete comparison of

gold distribution and ratio of concentration in concen-

trates 1 and 2, from the different minerals tested in this

study.

4. Conclusions

3.4. Black Sands

In the case of black sands, a great susceptibility to this

treatment was observed. Figures 8 and 9 indicate that for

almost the whole particle size range studied, excellent

Elutriation is not suitable for gravimetric concentration

of precious metals if they are embedded in complex ox-

ide or sulfide matrix.

1.6 1.7 1.8 1.9 2.0 2.1 2.22.3

100

Log[dp]

Au Recovery,

100

Ratio of Concentration

Figure 8. Gold recovery and ratio of concentration, as a function of a particle size, for concentrate 1 of black sand mineral.

1.6 1.71.8 1.9 2.0 2.1 2.2 2.3

100

Log[dp]

Au Recovery,

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ratio of Conc en tration

Figure 9. Gold recovery and ratio of concentration as a function of a particle size, for concentrate 2 of black sand mineral.

M. A. ENCINAS-ROMERO ET AL. 127

Table 3. Gold recovery as a function of a particle size, from different minerals tested. (C-1): Concentrate 1, (C-2): Concen-

trate 2.

Average

Particle Size,

[dp] (µm)

Gold Recovery

Oxide Mineral (%) Gold Recovery

Sulfide Mineral (%) Gold Recovery

Free Gold Mineral (%) Gold Recovery

Black Sand Mineral (%)

C-1 C-2 C-1 C-2 C-1 C-2 C-1 C-2

302.5 na

na na na na na

6.0 82.22

200 na

na na na na na

76.93 17.08

165 74 22 7.5 60.21 73.9 22.4 na na

128 19 74.8 2.8 5.6 62.5 15 96.70 2.35

90.5 1.46 89 1.0 11.81 19.6 42 na na

64 - 78.8 0.5 13.61 10.6 24.7

na na

49 1 92 5.6 21.16 2.8 24.9 na na

45 2.2 94.5 - - na na 82 13.2

na: not applicable.

Table 4. Ratio of concentration as a function of a particle size, from different minerals tested. (C-1): Concentrate 1, (C-2):

Concentrate 2.

Average

Particle Size

[dp] (µm)

Ratio of Concentration

Oxide Mineral (%) Ratio of Concentration

Sulfide Mineral (%) Ratio of Concentration Free

Gold Mineral (%) Ratio of Concentration Black

Sand Mineral (%)

C-1 C-2 C-1 C-2 C-1 C-2 C-1 C-2

302.5 na na na na na na 29.4 1.1

200 na na na na na na 12.3 1.3

165 5.5 1.24 30.2 2.79 18.9 2.2 na na

128 9.3 1.13 46.8 21.21 31.3 3.7 33.1 1.9

90.5 157.5 1.01 158.2 9.02 42.7 4.9 na na

64 - 1.03 276.2 6.82 68.5 3.1 na na

49 127 1.02 36.4 2.70 54.3 6.1 na na

45 84 1.01 - - na na 50.9 2.0

na: not applicable.

cious metals if they are present in slimly and clayey

black sands or as free gold.

For the free gold ores the best gold recoveries (70%)

and concentration ratios (70:1) were obtained for coarser

particles sizes, from 90.5 to 165 µm.

For the black sands the best results (96% of gold re-

covery) were for smaller sizes, i.e. 45 to 128 µm.

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