CO-PRODUCTION OF CHEMICALS IN
MINING BY MECHANOCHEMISTRY
Conor Young
ABSTRACT
Grinding of ores in mineral processing consumes approximately 3% of electricity
worldwide. The efficiency of grinding is low when compared with the theoretical
energy required to create new surface area; as a result much of the energy input is
converted to heat. Localised high temperatures and pressures caused by grinding have
been shown to increase the rate of catalytic chemical reactions, providing an
opportunity for value-adding chemical reactions to be facilitated by controlling the
chemical environment inside the grinding mill. Various oxide ores were ground in a
butane atmosphere and headspace samples analysed using gas chromatography (GC)
to determine if any reaction had taken place. Whilst partial oxidation products were
detected in some experiments, these could not be attributed to reactions involving
butane. The study was limited by access to suitable experimental and analytical
equipment. Avoiding significant air leakage into the gas samples proved difficult and
the planned grinding of sulfide ores could not be completed due to lack of access to
suitable
equipment
to
detect
the
anticipated
product
gases.
SCHOOL OF CHEMICAL ENGINEERING
1
INTRODUCTION
1.1 ENERGY USE IN MINING
The most energy intensive step in mineral processing is
grinding (milling), accounting for 35%1 to 53%2 of mine
site energy consumption and 3% of global electricity
consumption2. Compared with the theoretical energy
required to create new surface, the energy efficiency of
grinding can be as low as 1%3.
The Bond work index is the most commonly used
parameter to describe ore grindability and estimate ball
milling work input (kWh/t) for a given size reduction.
1
1
𝑊 = 10𝑊𝑖 (
−
)
√𝑃80 √𝐹80
Equation 1: Bond Ball Mill Work Index, JKTech, 2015
W is work input (kWh/t), Wi is Bond work index (kWh/t),
P80 is 80% passing size of the product (µm) and F80 is 80%
passing size of the feed (µm).
The Bond work index is equal to the work input required
for ore particle size reduction from very large down to
100µm, and is typically 10 to 20kWh/t. The desired
reduced particle size is dependent on ore type and grade
but is generally 70-110µm. The Bond work index gives a
reasonable estimate of total comminution energy input.
1.2 MECHANOCHEMISTRY
Deformation and fracture of solids causes increased
surface area and energy as well as localised high
temperatures. Mechanical activation, which occurs
during grinding, causes short lived surface structures
which are chemically reactive. Thiessen’s magma-plasma
model of mechanochemical interaction (Figure 1)
proposes that a high energy concentration exists at the
contact point between particles which forms a plasmatic
state. Electrons and photons are emitted and local
temperatures can transiently exceed 10 000oC4.
Figure 1: Magma-plasma model, Balaz, 2000
Mechanical work input can increase the rate of reaction
of catalytic chemical reactions. In situ ball milling of
various metal oxide catalysts (NiO, Cr2O3, Pt/Al2O3,
Co3O4) has been shown to cause a several order of
magnitude increase in the rate of oxidation of CO,
showing that the activity of the surface to reduction by
CO was increased during milling5. The rate of reaction
steadily increased during several hours of milling, but
rapidly decreased again as soon as milling ceased.
Hydrogenation of CO to methane over amorphous NiZr
powder was shown to occur during milling, with
conversion peaking after approximately 10 hours6.
Similarly, hydrogenation of CO2 to methane was
performed during ball milling of various MgO-mixed
catalysts at elevated pressure and temperature for one
hour7. It was found that additional milling of the catalyst
prior to introduction of reactant gases had little effect on
reaction yield, showing that in situ milling is primarily
responsible7.
Grinding of cellulose from various sources with kaolinite
and bentonite clays has been shown to be an effective
mechanocatalytic method to cause depolymerisation
into glucose and other useful precursor chemicals8. The
layered structure of clays is particularly effective as a
mechanocatalyst as the weak interlayer bonds are
broken during grinding to create an active and high
surface area8.
The implementation of fine grinding (5-20µm) has been
shown to improve efficiency in hydrometallurgical
processes. Mechanical activation is achieved by grinding
ore to 5-20µm prior to leaching, providing substantial
increases in yield9.
oxidation of the alkane to syngas. The reaction and
potential yield is outlined below:
1.3 TECHNOECONOMIC
POTENTIAL
12Fe2O3 + C4H10 --> 4CO + 5H2 + 8Fe3O4
The energy consumed in grinding represents a sizable
potential resource to exploit. Given the low efficiency of
the grinding process, up to 99% of energy is consumed in
generating heat and disordered surface structures. The
rate of electricity consumed in comminution globally
may be as high as 65GW (3% world electricity
consumption10) and worth $57B annually (@$0.1/kWh).
Hematite has a bond work index of 14.25kWh/t
(51.3MJ/t). Assuming 99% of grinding energy goes into
reaction, 82 mol/t of reaction will proceed per tonne of
hematite ground. The products would consist of 9.17kg
CO, 0.8kg H2 and 152kg Fe3O4.
High volume commodity ores cost US$50 to US$150 per
tonne11. With comminution work input of 10-20 kWh/t
and commercial electricity cost of $0.1/kWh, electrical
energy input represents 1-4% of sale price. Ores of
interest are those which have high production levels in
Australia and globally, such that implementation of
energy saving and value adding measures to their
comminution process would have significant impact.
The value of the chemical reaction performed must
justify mill modification expense and additional milling
energy required if comminution work input is increased.
1.4 PROPOSAL
It is proposed that a portion of the milling energy may be
used to drive reactions by creating a mechanocatalytic
environment of intense localised heat, pressure and
highly-active, short-lived surface structures. By
controlling the chemical environment in the grinding mill
it is hoped that value-adding chemical reactions may be
facilitated.
Ores which are of interest to the study are those which
have high production volume and have been identified
as potential catalysts in the literature. A full list of ores
of interest can be found in Appendix A. Ores which were
tested in preliminary experiments were hematite (Fe2O3),
ilmenite (FeTiO3) and a nickel ore (NiS, NiO). Mixed sands
and gravels were also tested.
Extended milling of hematite has been shown to increase
conversion and cause a reduction in the temperature
required for ore reduction by hydrogen gas12. One
hypothesis is that the milling of oxide ores in an alkane
gas will drive the reduction of the ore and partial
ΔH = ΣHf0 products - ΣHf0 reactants = 627 kJ/mol (at 1atm,
298K)
Assuming 100% reaction efficiency, 15% of ore is
reduced by normal milling work input (to 100µm). In
practice the products will likely also include CO2, H2O and
unreacted butane.
To test the hypothesis of partial oxidation of alkanes in
the presence of oxide ores and milling work input, a
preliminary round of experiments were performed.
Butane was used as the alkane as it is more reactive than
methane. Methane would ultimately be a more
economically practical choice due to its abundance in
natural gas.
There is also interest in sulfide ores and the potential to
synthesise thiol compounds. Sulfide ores (NiS, CuS and
CuFeS2) may be tested if suitable methods can be utilised.
2 METHODS
2.1 ORE PREPARATION
Ores were generally used in their natural form, as
supplied. As initial particle size varied 0.1mm (ilmenite)
to 100mm (hematite), particles larger than 50mm
diameter had to be first crushed using a mortar and
pestle to fit the grinding chamber. Approximate initial
ore sizes are listed in Table 3. 50g of specimen was used
in each milling experiment.
As experiments sought to investigate oxide ore reactions
the ‘Kambalda nickel ore’ sample, identified to
predominantly contain nickel sulfide, was heated to
800oC in 1atm air for 24 hours prior to milling. Nickel
sulfide has been shown to react with oxygen to form
nickel oxide at temperatures above 700oC13.
A mixed gravel sample was used as a model containing a
diverse mixture of solid oxides. The gravel pebbles were
After milling a Malvern Hydro Mastersizer 2000 was used
to determine the product size. Particle size distribution
plots were produced and the P80 measurement used to
characterise each specimen.
2.2 GRINDING
A 5.8kW Siebtechnik vibratory pulveriser mill was used
as the source of mechanical work input. The lid of the
steel milling chamber was modified to incorporate two
gas taps for filling and sampling of the headspace.
The 350mL milling chamber was purged with 2L of feed
gas prior to milling by injecting gas from a sample bag.
Each specimen was ground for 40 minutes and the
external chamber temperature recorded at the
beginning and end of each run using an infrared
thermometer.
2.3 GAS ANALYSIS
The feed gas used in preliminary experiments was a
predominantly n-butane fuel gas which also contained
propane and isobutane. Argon was substituted in control
experiments.
After milling, a syringe was used to extract 100mL of gas
from the milling chamber which was then transported in
a polymer gas sample bag to the gas chromatograph.
Typical transport times were 60 minutes.
2.3.1 OXIDATION PRODUCT
DETECTION (H 2 , CO, CO 2 )
Gas samples were analysed to determine the
concentration of gas reaction products. H2, CO and CO2,
O2 and N2 levels were measured.
Nitrogen, oxygen and hydrogen were detected using a
Shimadzu GC2014 with argon carrier gas, 5A molecular
sieve column and TCD detector, as seen in Figure 2. This
is an effective setup to measure low molecular weight
gases, however, due to the pore size of the zeolite
molecular sieve the column does not retain
hydrocarbons larger than ethane and is unsuitable for
use with CO2 as it becomes permanently trapped on the
column14.
Detector intensity, mV
mixed composition and colour. Sand, believed to be
predominantly quartz, was also milled.
120
100
80
60
40
20
0
0
1
2
Time, minutes
hydrogen 10%
oxygen
air
nitrogen
3
4
Figure 2: Standard Gases
Carbon dioxide and carbon monoxide were detected
using a Shimadzu GC2010 with nitrogen carrier gas,
Shincarbon ST micropacked column and FID methanizer
detector.
2.3.2 ALKANE DETECTION
Propane, isobutane, n-butane and their partial oxidation
products were detected using helium carrier gas,
Shincarbon ST micropacked column, FID methanizer and
TCD detector. The apparatus was used to determine the
content of known products of partial oxidation of
alkanes but also more complex potential products. Initial
column temperature is set to 40oC for 5 min whilst light
molecules elute before a ramp to 200oC to accelerate the
elution of heavier molecules such as butane.
3 RESULTS AND DISCUSSION
originated from water present in the ore prior to milling,
which oxidised the ore during grinding.
140
Detector intensity, mV
3.1 CO & CO 2 PRODUCTION
Of the ores milled in butane, ilmenite was the only one
which showed significant CO and CO2 in product gas
samples. The sub-atmospheric levels of CO2 in hematite,
gravel and nickel ore are to be expected given the
leakage of air into those samples.
Further testing was performed on ilmenite to determine
the origin of CO2 in product gases. It was found that even
higher levels of CO and CO2 were present when ilmenite
was milled in argon gas, rejecting the hypothesis that
these could be products of partial or total oxidation of
hydrocarbons.
An ilmenite sample was pre-cooked at 600oC for 24
hours prior to milling and showed sub atmospheric levels
of CO and CO2. This may indicate that the CO and CO2 is
adsorbed to or contained within the ilmenite prior to
milling but can be desorbed by heating.
3.2 HYDROGEN PRODUCTION
Hydrogen was measured in the product gases of all
milled ores except hematite. High levels were measured
from milling sand both in butane and argon gas, 14% and
5% respectively.
No H2 was measured in sand which was pre-heated prior
to milling, as shown in Figure 3. This suggests that the
hydrogen measured in product gas samples likely
Hydrogen
120
100
80
60
40
Oxygen
Nitrogen
20
0
0
Butane
1
2
3
Time, minutes
4
Butane, pre-cooked
5
Argon
Figure 3: Milled Sand Samples
3.3 FEED GAS CONSUMPTION
Air concentration increase was compared with reduction
in concentration of the butane fuel mix feed gas
(comprised of propane, isobutane and n-butane) to
estimate whether significant amounts of feed gas were
consumed in chemical reactions during milling of an
ilmenite and sand mixture, as seen in Table 2.
Table 2: Gas Concentration Changes
Concentration
Pre milling
Post milling
Delta
Butane fuel mix
91%
80%
11%
Air
12%
21%
9%
Table 1: Milling headspace GC analysis
Ore
Ilmenite
Ilmenite
Ilmenite
Hematite
Nickel ore
Gravel
Sand
Sand
Sand
Ilmenite, sand 1:1
Pre-treatment
600degC, 24hr
800degC, 24hr
600degC, 24hr
Reactant
Gas
butane
argon
butane
butane
butane
butane
butane
argon
argon
butane
Air Leak
++
+
+++
++
++
+
+
++
+
Products
H2
CO2 ppm
0.2%
3458
0.6%
9755
298
0%
357
0.1%
136
2%
161
14%
5%
0.02%
CO
++
++
+
+
+
+
Key: Air leak: + <33%, ++ <66%, +++ >66%; CO: + trace amount, ++ significant amount
Butane consumption
2%
Detector Intensity, mV
100
Unidentified
elution
0.4
0.3
CO2
0.2
0.1
CO
O2, N2
80
0
0
5
10
15
20
Time, minutes
60
Post Milling
Propane
40
Pre Milling
Air
Figure 6: Unidentified elution, FID, ilmenite and sand, butane
20
iso-butane
n-butane
3.4 PARTICLE SIZE REDUCTION
AND ENERGY EFFICIENCY
0
0
50
100
Time, minutes
From can
Post Milling
Pre Milling
Figure 4: TCD, ilmenite and sand, butane
An unidentified elution was detected at low
concentrations at six minutes by both TCD and FID
detectors as seen in Figure 5 and Figure 6 respectively.
The peak is much smaller than the main gas constituents.
3
Detector Intensity, mV
0.5
Detector Intensity, V
The 2% concentration discrepancy between air
infiltration and feed gas reduction could indicate
reaction, however, as the 95% confidence intervals of
the feed gas and air concentrations were ±6% and 12%
respectively, this cannot be confirmed. Figure 4 shows
the TCD chromatograms from this experiment.
2.5
Unidentified
elution
2
1.5
Table 3 shows the p50 (50% sample passing size) and p80
of ores after milling. Whilst a more rigorous method
needs to be used to characterise feed size, the difference
between feed and product particle size can be used as
an estimate of milling work input using the Bond Work
Index equation. It can be seen that regardless of initial
feed size all of the ores were reduced to sub 100µm after
40 minutes of grinding. Whilst the ilmenite was labelled
as 106µm, it is likely that this was a minimum size rather
than P80 specification, as the product was visibly many
times finer than the feed.
Table 3: Ore particle size
Feed size,
µm
CO2
1
0.5
0
0
5
10
15
20
Time, minutes
Post Milling
Pre Milling
Air
Figure 5: Unidentified elution, TCD, ilmenite and sand, butane
Hematite
Ilmenite
Nickel
Gravel
30000
106
5000
30000
Product size, µm
P80
P50
30.0
7.9
82.3
14.7
46.6
12.0
26.5
6.4
Table 4 shows the estimated work input from milling,
based on the measured particle size reduction. This is
likely an underestimation of the actual work input as the
Bond index is used for ball milling, a more efficient
grinding method than vibratory milling. Also the size
reduction may have slowed or stopped before the end of
the 40 minute milling procedure. Typically the vibratory
mill is only used for several minutes at a time to reach
desired particle size reduction.
Table 4: Estimated work input
Bond Index, kWh/t Work input, kWh/t
hematite
11
19.4
ilmenite
15
2.0
nickel
15
19.9
gravel
17.7
33.4
The energy consumption of each 40 minute milling
operation based on the actual energy usage of the mill
was 3.9kWh, giving a work input of 77MWh/t. This figure
is three orders of magnitude larger than typical energy
inputs and thus likely infeasible for commercial
operations to adopt.
3.5 AIR INFILTRATION
Air infiltration was an issue throughout the experimental
process. Efforts were made to reduce leakage by
modifying experimental practice and equipment.
Considerable infiltration occurred injecting gas into the
sample bag and then from the sample bag into GC (7%).
The majority of infiltration (15%) occurred when
injecting and sampling gas to and from the milling
chamber, whilst an additional 7% of air leaked into the
chamber during milling. Considerable and variable air
leakage was measured between gas specimens tested
immediately after milling and 24 hours later due to
leakage
Air infiltration could be reduced by using a vacuum pump
to evacuate the chamber prior to injecting gas rather
than the purging method used. Use of a syringe with a
tap would likely reduce the infiltration in each operation
of transferring gas between stages of testing. Time
between taking samples and GC measurement must be
kept to a minimum due to air infiltration during sample
bag storage.
4 CONCLUSION
Several high-volume commodity oxide ores as well as
sand and gravel mixtures were subjected to intense
mechanical work input in various, controlled gas
atmospheres. The intention of screening experiments
were to find out whether chemical reactions were
readily observed and rapid under typical conditions.
Partial oxidation products (H2, CO, CO2) were detected
after grinding particular ores with butane gas however
these products could not be attributed to reactions with
the alkane gas as they also occurred when grinding in an
inert argon atmosphere. The hydrogen produced was
attributed to moisture in the ores prior to milling as precooked ore samples showed no hydrogen in headspace
samples. Limited experimental results showed that no
significant amount of butane was consumed during
milling processes.
Further testing may uncover opportunities for this
potential resource to be utilised. Sulfide minerals
represent a large proportion of commodity ores in
Australia and globally and were not tested in this body of
work.
5
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Identified
Australian
6 APPENDICES
6.1 APPENDIX A: ORES OF IMPORTANCE
Table 5: Major Commodity Ores in Australia
Mine production 2013, kt15
Bond work index,
kWh/t16
Chemical formulae of common
species15
Price, USD/t11
Fe2O3, Fe3O4
45
Australia
World
Iron ore
609000
3024000
10.99, 14.25
Black coal
538000
6926000
14.3
Bauxite
81100
263000
9.68
Brown coal
73000
905000
14.3
Manganese ore
7447
48000
Phosphate
rock(w)
2580
Zinc
57
AL2O3.3H2O, AL2O3.H2O
58
13.4
MnO2, MnCO3
120
224000
10.9
Ca5(F, Cl, OH)(PO4)3, P2O5
1520
13600
12.7
Zn-FeS, ZnCO3
Ilmenite
1152
12270
Copper
1000
17900
Lead
710
Magnesite
FeTiO3
115
14.03
CuFeS2, Cu5FeS4, Cu2S
300
5400
12.93
PbS, PbCO3, PbSO4
290
504
21160
12.27
MgCO3, MgO
Zircon
333
1440
Rutile
244
570
Nickel
234
2480
Kaolin
155
39000
Al2Si2O5(OH)4
Bentonite
135
14600
Al2O34SiO2H2O
Chromium
94.2
26000
FeCr2O4
Tin
6.5
231
12.02
SnO2
Uranium
6.4
58.4
17
UO2, U3O8
Cobalt
6.4
117
CoAs2, CoAsS, Co(AsO4)2.8H2O
Antimony
5
163
Sb2S3
ZrSiO4
1700*
13.98
TiO2
900*
15.05
NiS
20-75
*Indicates ore price is for a concentrated product
90-300*