XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1767
Detection of Hydroxyl Radicals
Active oxygen species including hydroxyl radicals are
expected to be formed during the ultrasound leaching pro-
cess. These hydroxyl radicals are strong oxidising agents
capable of initiating pyritic oxidation. They can be detected
by the use of methylene blue dye or by identifying hydrox-
ylated products of benzoic acid when the benzoic acid is
used as a chemical probe in the presence of hydroxyl radi-
cals. The presence of hydroxyl radicals had to be confirmed
in order to postulate their role in the oxidation of pyrite
and subsequently causing the breakdown of the sulphide
matrix, releasing locked gold and increasing the gold yield
in solution. The presence of hydroxyl radicals can be indi-
rectly determined through the detection of these hydroxyl-
ated benzoic acids (HBAs) using FITR and UV-vis. Exactly
50 ml of aqueous solution containing 0.12M benzoic acid
was added to 400 ml of distilled water in ultrasonic reac-
tor. After 2hrs of sonication, the solution was transferred to
a volumetric flask and 30 ml of distilled ethyl acetate was
added to extract the organic compound. After stirring the
solution for 15 min, the organic materials were extracted
with ethyl acetate using a separating funnel. The ethyl ace-
tate solution was dried over anhydrous Na2SO4 to get rid
of water, and then it was evaporated to dryness in a rotary
evaporator. Finally, a white solid material was obtained, and
taken for FITR analysis.
RESULTS AND DISCUSSION
Sample Characterization
A mineralogical and chemical analysis of the collected sam-
ple was conducted. The chemical analysis revealed a gold
content of 0.585g/t while XRD results revealed that more
than 80% of the sample was composed of quartz. The full
XRD result is presented in Table 1. A mineralogical analysis
conducted using QEMSCAN showed that 6.52% of gold
was present as free milling, 28.99% was associated with
pyrite while 6.52% was associated with quartz (Table 2)
Changes in Solution pH
The change of H+ concentration under ultrasound pro-
cessing was measured by a pH meter. Figure 1 shows that
there was notable pH drop during ultrasonic pretreatment
and ultrasonic leaching which was caused by the sono-
lytic oxidation of the pyrite. Sonolytic oxidation occurs
when ultrasound-induced cavitation produces reactive
species such as hydroxyl radicals, which react with pyrite
(FeS2) to produce sulfide radicals, which then react with
molecular oxygen (O2) to generate sulfuric acid (H2SO4).
Decomposition of H2O under ultrasonic treatment releases
hydrogen and hydroxyl radicals as shown by reaction 1,
(Hoffmann, M.R., Hua, I. and Höchemer, R., 1996). The
presence of hydroxyl radicals, which are strong oxidizing
agents, have been postulated to help with the oxidation of
pyrite. Equations 2 and 3 below indicate that the oxidation
of pyrite under ultrasound can lead to the release of hydro-
gen ions, hence leading to a drop in pH.
H2O → ·OH +·H (1)
FeS2 +·OH → SO4 2– +H+ +Fe2+ (2)
FeS2 +·OH → S2O3 2– +H+ +Fe2+ (3)
Since the pH in cyanide leaching must be maintained
between 10.5 and 11.5, Ca(OH)2 was added to correct
Table 1. Mineralogy of the tailing sample
Mineral Chemical formulae Mass%
Quartz SiO2 81.80
Muscovite KAl
2 (Si
3 Al)O
10 (OH,F)
2 5.00
Chlorite (Mg,Fe2+)
5 AlSi
3 Al
2 O
10 (OH)
8 3.78
Pyrite FeS2 3.06
Pyrophyllite Al2Si4O10(OH)2 2.36
Goethite Fe3+O(OH) 1.74
Rutile TiO
2 0.61
K-feldspar KAlSi3O8 0.41
Biotite K(Mg,Fe++)3[AlSi3O10(OH,F)2 0.34
Diopside CaMgSi
2 O
6 0.18
Plagioclase (Na,Ca)(Si,Al)
4 O
8 0.44
Others 0.37
Total 100
Table 2. Mineralogical analysis
Mineral
Ergo Feed
Gold
Free Surface 6,52
Arsenopyrite 3,62
Mixed Cu-Fe-S phase *17,39
Pyrite 28,99
Pyrrhotite 3,26
Other Sulphides* 1,45
Microcline 0,36
Plagioclase 0,00
Quartz 6,52
Fe Silicate 0,72
Other Silicates* 11,23
Other Oxides 0,36
Rutile 8,70
U-bearing phases
(Uraninite and Coffinite)
7,97
Monazite 2,90
Total 100,00
Detection of Hydroxyl Radicals
Active oxygen species including hydroxyl radicals are
expected to be formed during the ultrasound leaching pro-
cess. These hydroxyl radicals are strong oxidising agents
capable of initiating pyritic oxidation. They can be detected
by the use of methylene blue dye or by identifying hydrox-
ylated products of benzoic acid when the benzoic acid is
used as a chemical probe in the presence of hydroxyl radi-
cals. The presence of hydroxyl radicals had to be confirmed
in order to postulate their role in the oxidation of pyrite
and subsequently causing the breakdown of the sulphide
matrix, releasing locked gold and increasing the gold yield
in solution. The presence of hydroxyl radicals can be indi-
rectly determined through the detection of these hydroxyl-
ated benzoic acids (HBAs) using FITR and UV-vis. Exactly
50 ml of aqueous solution containing 0.12M benzoic acid
was added to 400 ml of distilled water in ultrasonic reac-
tor. After 2hrs of sonication, the solution was transferred to
a volumetric flask and 30 ml of distilled ethyl acetate was
added to extract the organic compound. After stirring the
solution for 15 min, the organic materials were extracted
with ethyl acetate using a separating funnel. The ethyl ace-
tate solution was dried over anhydrous Na2SO4 to get rid
of water, and then it was evaporated to dryness in a rotary
evaporator. Finally, a white solid material was obtained, and
taken for FITR analysis.
RESULTS AND DISCUSSION
Sample Characterization
A mineralogical and chemical analysis of the collected sam-
ple was conducted. The chemical analysis revealed a gold
content of 0.585g/t while XRD results revealed that more
than 80% of the sample was composed of quartz. The full
XRD result is presented in Table 1. A mineralogical analysis
conducted using QEMSCAN showed that 6.52% of gold
was present as free milling, 28.99% was associated with
pyrite while 6.52% was associated with quartz (Table 2)
Changes in Solution pH
The change of H+ concentration under ultrasound pro-
cessing was measured by a pH meter. Figure 1 shows that
there was notable pH drop during ultrasonic pretreatment
and ultrasonic leaching which was caused by the sono-
lytic oxidation of the pyrite. Sonolytic oxidation occurs
when ultrasound-induced cavitation produces reactive
species such as hydroxyl radicals, which react with pyrite
(FeS2) to produce sulfide radicals, which then react with
molecular oxygen (O2) to generate sulfuric acid (H2SO4).
Decomposition of H2O under ultrasonic treatment releases
hydrogen and hydroxyl radicals as shown by reaction 1,
(Hoffmann, M.R., Hua, I. and Höchemer, R., 1996). The
presence of hydroxyl radicals, which are strong oxidizing
agents, have been postulated to help with the oxidation of
pyrite. Equations 2 and 3 below indicate that the oxidation
of pyrite under ultrasound can lead to the release of hydro-
gen ions, hence leading to a drop in pH.
H2O → ·OH +·H (1)
FeS2 +·OH → SO4 2– +H+ +Fe2+ (2)
FeS2 +·OH → S2O3 2– +H+ +Fe2+ (3)
Since the pH in cyanide leaching must be maintained
between 10.5 and 11.5, Ca(OH)2 was added to correct
Table 1. Mineralogy of the tailing sample
Mineral Chemical formulae Mass%
Quartz SiO2 81.80
Muscovite KAl
2 (Si
3 Al)O
10 (OH,F)
2 5.00
Chlorite (Mg,Fe2+)
5 AlSi
3 Al
2 O
10 (OH)
8 3.78
Pyrite FeS2 3.06
Pyrophyllite Al2Si4O10(OH)2 2.36
Goethite Fe3+O(OH) 1.74
Rutile TiO
2 0.61
K-feldspar KAlSi3O8 0.41
Biotite K(Mg,Fe++)3[AlSi3O10(OH,F)2 0.34
Diopside CaMgSi
2 O
6 0.18
Plagioclase (Na,Ca)(Si,Al)
4 O
8 0.44
Others 0.37
Total 100
Table 2. Mineralogical analysis
Mineral
Ergo Feed
Gold
Free Surface 6,52
Arsenopyrite 3,62
Mixed Cu-Fe-S phase *17,39
Pyrite 28,99
Pyrrhotite 3,26
Other Sulphides* 1,45
Microcline 0,36
Plagioclase 0,00
Quartz 6,52
Fe Silicate 0,72
Other Silicates* 11,23
Other Oxides 0,36
Rutile 8,70
U-bearing phases
(Uraninite and Coffinite)
7,97
Monazite 2,90
Total 100,00