This document describes a new process for CO2 mitigation and metals recovery using a regenerable solvent-precipitant system. The system uses a tertiary amine and acid that can undergo a pH swing via a change in temperature. Specifically, various tertiary amines and acids were tested to identify a combination that can adjust the pH between 10 and 2, which is suitable for metal hydroxide precipitation and metal leaching from magnesium solids. The researchers found that a triethylamine-sulfuric acid-water system could achieve a pH swing between 10.5 and 1.9 with temperature changes, meeting the criteria. This combination uses triethylamine to alkalize the solution to pH > 10.5 and uses sulfur

1. International Journal of Greenhouse Gas Control 42 (2015) 379–387
Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control
journal homepage: www.elsevier.com/locate/ijggc
A regenerable precipitant-solvent system for CO2 mitigation and
metals recovery
Reydick D. Balucan, Karen M. Steel∗
School of Chemical Engineering, The University of Queensland, St. Lucia, QLD 4067, Australia
a r t i c l e i n f o
Article history:
Received 8 October 2014
Received in revised form 4 August 2015
Accepted 4 August 2015
Keywords:
pH swing
Mineralisation
CO2 sequestration
Metals recovery
a b s t r a c t
This work describes a new resource engineering process for greenhouse gas control and metals recovery,
and reports initial progress in its development. Central to this process is a regenerable solvent-precipitant
system that enables pH-swing via a change in operational temperatures. In this particular contribu-
tion, various liquid-phase aliphatic tertiary amines and acids were examined to identify the suitable
components that afford pH control between 10 and 2, deemed necessary for metal hydroxide precipita-
tion and leaching metals from magnesium-bearing solids, respectively. Within the desired operational
constraints (Tmax ≤ 110 ◦
C, Pmax ≤ 1 bar, tmax ≤ 2 h), we found the triethylamine–sulphuric acid aqueous
system (Et3N–H2SO4–H2O) has sufficiently covered the pH regimes of interest, affording a pH swing (at
25 ◦
C) between 10.5 and 1.9. Triethylamine (Et3N) provided the highest acid neutralising capacity of 7 mol
H+
L−1
Et3N, capable of aqueous alkalisation to pH > 10.5 (25 ◦
C, 1 bar), and recoverable at ≤105 ◦
C (1.0 bar)
via fractional distillation. As for the acid component, H2SO4 allows for the formation of an acidic aqueous
solution (pH 1.9, 25 ◦
C, 1 bar) resulting from the partial liberation of Et3N from the acid-bound Et3N·H+
and HSO4
−
counter anion dissociation.
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Mineralisation of anthropogenic CO2 into carbonates not only
presents a safe and long-term storage solution (Sipilä et al., 2008;
Lackner et al., 1995; IPCC, 2005; Huijgen and Comans, 2003,
2005) but could also stimulate economic activity (Geerlings and
Zevenhoven, 2013; Brent et al., 2012) as a consequence of produc-
ing value-added materials (i.e., silica, iron oxides, metal hydroxide
precipitates) and derived materials (i.e., magnesium carbonate
aggregates) for use in land reclamation (Geerlings and Zevenhoven,
2013; Khoo et al., 2011), in agriculture as soil modifiers, or as com-
ponent materials for the built environment. Generally, any CO2
mineralisation technology requires multiple steps (Sipilä et al.,
2008; Geerlings and Zevenhoven, 2013). Extraction of Mg from
natural feedstocks (i.e., serpentinites, peridotitic minerals, lateric
ores) and alkaline solid wastes (i.e., asbestos-containing wastes)
would usually require acidic conditions and elevated temperatures.
Subsequent controlled precipitation of desired precipitates such as
carbonates (MgCO3·xH2O), brucite (MgOH)2, and mixed hydroxide
precipitates (MHP) from tailings (i.e., MgSO4, serpentine mine
∗ Corresponding author.
E-mail address: karen.steel@uq.edu.au (K.M. Steel).
tailings) and leachates from the magnesium silicate dissolution
stage would then require alkaline conditions at or near ambient
conditions (Steel et al., 2013a; Park and Fan, 2004).
Whilst the capability to extract and selectively precipitate
metal-bearing compounds are highly desirable, the energy inten-
sity and process economics of earlier proposed pH swing methods
were industrially unattractive (Sipilä et al., 2008). Conventional
methods such as the use of strong acids and bases suffer from poor
economics and high energy requisites as the process chemicals are
either deemed lost (i.e., salt formation) or is impractical to recy-
cle at a scale required for CO2 sequestration (Sipilä et al., 2008;
Huijgen and Comans, 2003, 2005). In a pH-swing process using
HCl or HNO3 and NaOH to convert serpentine into hydromagne-
site [Mg5(CO3)4(OH)2·4H2O], it was estimated that recovering HCl
or HNO3 would generate 2.6 and 3.5 tonnes of CO2 for each tonne
of CO2 stored, respectively (Teir et al., 2009). For pH-swing type
processes to be economically viable, significant reduction to heat,
power and chemical demand is therefore necessary (Sipilä et al.,
2008; IPCC, 2005; Huijgen and Comans, 2003, 2005; Khoo and Tan,
2006).
Relatively newer CO2 mineralisation processes using recyclable
chemicals (Wang and Maroto-Valer, 2011a, 2011b, 2011c, 2011d;
Wang, 2011; Sanna et al., 2013; Romao et al., 2013; Nduagu et al.,
2013, 2012a, 2012b, 2012c; Nduagu, 2008) offer better chemical
resource economics. In particular, these processes are based on
http://dx.doi.org/10.1016/j.ijggc.2015.08.004
1750-5836/© 2015 Elsevier Ltd. All rights reserved.

2. 380 R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387
recyclable ammonium salts such as NH4HSO4 (Wang and Maroto-
Valer, 2011a, 2011b, 2011c, 2011d; Wang, 2011; Pundsack, 1967)
and (NH4)2SO4 (Romao et al., 2013; Nduagu et al., 2013, 2012a,
2012b, 2012c; Nduagu, 2008). Although the required chemicals
for these processes are at a minimum, the energy requirements
are high due to their need for temperatures well above 300 ◦C
and water evaporation. Alternatively, a new processing pathway
based on a regenerable system (Steel et al., 2013a, 2013b; Steel,
2013) whose energy demand can be supplied via low temperature
(≤110 ◦C) sources can be explored and developed. Furthermore, if
this new processing pathway can utilise existing infrastructure,
it would have the potential for easier integration to the exist-
ing energy generation and transport sectors as well as provide an
alternative source for the production of metals and construction
materials.
Our current research work examines a potentially low energy
means to perform pH-swing (Steel et al., 2013a, 2013b; Steel, 2013)
and combines metals recovery with CO2 mitigation. CO2 miner-
alisation could be performed by either precipitating magnesium
carbonates (i.e., nesquehonite, MgCO3·3H2O; hydromagnesite,
Mg5(CO3)4(OH)2·4H2O) via contacting the alkalised leachate (i.e.,
devoid of metals except Mg) with CO2 (supplied from a capture and
separation plant) or simply precipitating brucite, Mg(OH)2 for use
in subsequent mineralisation. The latter option allows for the direct
combined capture and storage of CO2 from point sources, where
Mg(OH)2 can be brought on-site and contacted with flue gases,
converting them into magnesium-based minerals. Furthermore,
brucite production and deployment for mineralising other flue
gases besides CO2 (i.e., SOx and NOx) could pave the way towards
a wider utilisation in various industries and applications including
those in the transport (i.e., mineralising tail-pipe emissions), agri-
cultural (i.e., passive CO2-absorber, soil pH modification), industrial
(i.e., nickel, cement, steel, aluminium production) and in various
energy generation sectors. In addition, it could be used for air
capture mineralisation, where brucite could be spread over spe-
cific areas allowing for passive carbonation over time. This passive
mineralisation capability could in the future facilitate for a market-
based framework for carbon trading, bridging the gap between the
CO2 generators and mitigators.
Fig. 1 shows the proposed combined direct CO2
capture–mineralisation and metals recovery process. Central
to this process is a regenerable solvent-precipitant system that
enables the pH-swing. The regenerable system is to consist of a
suitable aliphatic tertiary amine (R3N) and an acid, which via a
change in operational temperature, serves the dual purposes of
aqueous acidification and alkalisation. This process is different to
that reported by Liu et al., wherein the regeneration of the tertiary
amine in their process requires the addition of, and thereby the
consumption of, Ca(OH)2 (Liu et al., 2014).
The process may commence with either magnesium sulphate
tailings (MgSO4·xH2O) or with silicate feedstocks. In the dissolu-
tion stage, proton exchange with the hydroxides and oxides in the
feedstock would cause the liberation of Mg2+ and other metal ions,
producing a leachate which can then be separated from a solid
product that is largely amorphous silica. The leachate pH is then
raised via a controlled R3N addition to precipitate, firstly metal
hydroxides (Al, Fe) at pH ∼ 4, and then either MgCO3 at pH 8 via
concomitant CO2 injection or, in the absence of CO2, Mg(OH)2 at
pH > 10. The resultant solution that is devoid of precipitates and
now laden with the protonated amines (R3N·H+), is then heated to
regenerate both the acid and R3N components for recycling. When
considering tailings from nickel processing as the feedstock, the
process is essentially a retrofit which regenerates the acid and base
required for the process. Aqueous MgSO4·xH2O-rich tailings (i.e.,
from a nickel processing plant) are contacted with R3N to precip-
itate MgCO3 or Mg(OH)2 which can be, firstly, used as the base
needed to neutralise excess acid used in nickel processing to replace
the calcrete or other base that is currently used and, secondly, either
Fig. 1. Generalised schematic diagram of a resource engineering process based on a novel pH swing method for CO2 mitigation (direct CO2 capture-mineralisation) and metal
recovery process. R3N represents the tertiary amine component of the regenerable precipitant–solvent system.

3. R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387 381
stored in the case of MgCO3 or sold as a CO2 mineralising compound
in the case of Mg(OH)2.
This paper examines the ability of various tertiary alkylamines
to adjust the pH of an acidic solution to >8 to enable MgCO3 and
Mg(OH)2 precipitation, and the ability to thermally recover both
amine and acid back from the protonated amine. In particular,
this study focused on examining the liquid-phase aliphatic ter-
tiary amines and various types of acids, in order to identify the
combination that is suitable for: (i) precipitating metal hydrox-
ides, (ii) dissolving silicate feedstocks and (iii) regenerable within
the desired operational conditions. A suitable amine-acid combi-
nation for this regenerable solvent-precipitant system must cover
the pH regimes of interest (i.e., pH ≥ 10 for product precipitation
and pH ≤ 2 for feedstock dissolution) within the desired operational
constraints (T < 110 ◦C, P < 1 bar, t < 2 h).
2. Experimental
2.1. Materials
Tertiary amines (R3Ns) that were considered and studied in this
paper include triethylamine (Et3N, ≥99% purity), tripropylamine
(Pr3N, ≥98% purity), tributylamine (Bu3N, >98.5% purity), tripenty-
lamine (Pe3N, 98% purity), trihexylamine (Hx3N, 96% purity) and
trioctylamine (Oc3N, 98% purity). For the acid component, citric
acid (H3Ci, 99% purity), hydrochloric acid (0.1 M HCl, AR-grade)
and standard sulphuric acid (0.5 M H2SO4, AR-grade) were studied.
In addition to the acids mentioned, gaseous CO2 of 99.9% purity
was used to generate carbonic acid, H2CO3. These amines and acids
were selected having considered their availability, flammability
and the likelihood of generating explosive reactions (i.e., hypergolic
reaction of HNO3 and triethylamine, Et3N), such that trihepty-
lamine (Hp3N), trimethylamine (Me3N) and nitric acid (HNO3)
were excluded in this study, respectively.
2.2. Equipment
Potentiometric titrations and pH measurements were obtained
with a pH meter fitted with a high temperature KNO3 gel-based pH
probe and thermometer to allow compensation for temperature
effects. In determining the amount of amine recovered, a Met-
tler Toledo T50 autotitrator, operated with a glass pH combination
probe, was used in neutralisation titrations of the sample aliquots
at ambient conditions with standard 0.1 M HCl as the titrant.
In all of the titration and volatilisation/distillation experiments,
a 250 mL, three-necked round bottomed flask served as the main
reactor, where its three openings afforded titrant delivery, assem-
bly of distilling implements and probe insertions. Solutions were
continuously mixed via magnetic stirring at 500 rpm whilst an oil
bath maintained the solution temperatures at the desired value
within ±5 ◦C. In the Et3N volatilisation and recovery experiments
via fractional distillation, a 3-sectioned glass Snyder fractionat-
ing column was added in between the distilling head and receiver
assembly. Floating glass ball valves prevented the backflow of vola-
tised components back into the distilling flask.
2.3. pKa predictions and thermodynamic modelling
The pKa values for protonated amines (R3N·H+) and acids were
predicted using Marvin 6.1.0 (Marvin calculator plugin, 2013)
(build date 2013-09-05) via the pKa plug-in (macro mode, static
acid/base prefix). Molecular hydrophobicity (MHP) or, R3N’s mis-
cibility with water was estimated via the Marvin’s log P plug-in
using the Viswanadhan and Ghose (VG) method. Predicted pKas and
molecular properties of the candidate amines are found in the Sup-
porting Information. The titration profiles for HCl, H2SO4 and H3Ci
against Et3N as well as the aqueous pH of antigorite and chrysotile
were simulated using OLI Stream Analyzer 3.2 (OLI Stream Analyzer
Studio, 2013) (aqueous framework model).
3. Results and discussion
3.1. Acid neutralisation and aqueous alkalisation
In practical terms, the amount of protons required for the disso-
lution stage must exceed the stoichiometric requirement in order
to maintain acidic conditions. Reaction (1) shows the reaction
between serpentine and acid. High acid concentration, in addition
to an excess of acid, is necessary to ensure a high rate of Mg2+
dissolution. Following extraction and separation of the Mg2+, the
purpose of the amine is to bind with protons from the excess acid
(Reaction (2)) and then, in order to raise pH, bind protons from the
self-ionisation of water (Reaction (3)). The reaction product of Reac-
tion (2) is referred to as acid-protonated amine, R3N·H+–Hx−1Acid−,
whilst that of Reaction (3) is referred to as water-protonated amine,
R3N·H+–OH−.
Mg3Si2O5(OH)4(s) + 6H+
(aq) → 3Mg2+
(aq) + 2SiO2(s) + 5H2O(l) (1)
R3N(aq) + HxAcid(aq) R3N·H+
(aq) + Hx−1Acid−
(aq) (2)
R3N(aq) + H2O(l) R3N·H+
(aq) + OH−
(aq) (3)
Fig. 2 shows the titration profiles of the tertiary amines (R3Ns)
against 0.1 M HCl (25 mL) at 25 ± 2 ◦C. From the neutralisation pro-
files, the estimated acid neutralisation capacities, ANC, in terms of
mol H+ L−1 R3N are: 7.1 (Et3N), 5.3 (Pr3N), 4.5 (Bu3N), 3.6 (Pe3N), 3.1
(Hx3N) and 2.6 (Oc3N). Clearly, Et3N is most efficient in immobilis-
ing excess protons. Of the total amount of dispensed amine, a vast
majority was needed for acid neutralisation (to bind protons from
excess acid) whilst only a small portion was required for aqueous
alkalisation (to bind with protons from ionised water). Of the R3Ns
tested, Bu3N can raise system pH above neutral, and to some extent
cause metal hydroxide precipitation. However, only Et3N and Pr3N
are capable of aqueous alkalisation at the range applicable for CO2
mineralisation.
Aqueous alkalisation occurs when protons from the self-
ionisation of water form coordinate covalent bonds with R3N’s
nitrogen atom, and thus in effect, render an aqueous system replete
with hydroxide anions (OH−1). This occurrence which produces
water-protonated R3Ns, is dictated by the aqueous solubility of
R3Ns. R3Ns aqueous solubility is an inherent property which gener-
ally decreases with increasing alkyl chain length. With increasing
chain length, the N site becomes more sterically-hindered and thus,
effectively diminishing the molecular polarisability of the R3N.
Unless protons of sufficient amount greater than those produced
from the self-ionisation of water are introduced, R3Ns’ miscibil-
ity with water decreases with increasing chain length (Marvin
calculator plugin, 2013). Whilst protonation increases the aqueous
miscibility of the amine, it does not enhance its alkalising capacity.
This is because the acid-protonated amines are already paired to
the conjugate base of the acid, thereby unable to bind with protons
released from the self-ionisation of water and as such, unable to
free up OH− ions.
3.2. R3N regeneration via phase separation
The possibility of regenerating protonated R3Ns via heating
and/or solvent extraction to induce phase separation was investi-
gated via colorimetric visualisation test using a universal indicator
as the mixture was heated. Tests were performed under three sys-
tem configurations; binary, ternary and quarternary and at various
R3N concentrations (i.e., ∼5%, ∼10%, ∼15% w/w R3N). The binary

4. 382 R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387
Fig. 2. Titration profiles of 0.1 M HCl against candidate aliphatic tertiary amines, R3N, at ∼1.0 bar and at 25 ± 2 ◦
C. The pH regions of particular interest to the envisioned
process are included for reference.
system comprised the candidate R3N and H2O, whilst the ternary
system included an acid component, H2SO4. Kerosene was used
as the fourth component in the quarternary system to enhance
phase separation. In the ternary and quarternary systems, 5, 10
and 15% w/w R3N corresponded to a proton-dominated (insuffi-
cient R3N), neutral (full acid neutralisation with R3N) and alkaline
(R3N in excess), respectively. By comparison to the binary sys-
tem, this set-up provided insights into the ease by which R3N·H+
could phase separate and be recovered as R3N. In each experi-
ment, 3 × 30 mL glass bottles containing approximately 20 mL of a
particular R3N system of varying amine concentrations were simul-
taneously heated up to 70 ◦C. A fourth bottle filled with H2O and
fitted with a thermometer approximated the temperature readings
of all the three systems.
These colorimetric visualisation tests found that regeneration of
R3Ns via heating and/or solvent extraction to induce phase sepa-
ration without volatilising any component (Reactions (4) and (5)),
was found to occur only in the binary system R3N–H2O (Reaction
(3)), or in ternary and quarternary systems, only if there was an
excess of the R3N component (i.e., R3N well above the stoichio-
metric amount for acid neutralisation). The latter’s occurrence is in
essence a binary system (R3N–H2O), wherein the excess R3N inter-
acts with H2O and with temperature increases effectively reversing
their interaction.
Et3N·H+
(aq) + Hx−1Acid−
(aq) Et3N(aq) + HX Acid(aq) (4)
R3N(aq) R3N(org) (5)
The results suggests that acid-protonated R3Ns,
R3N·H+–Hx−1Acid−, are irrecoverable via temperature-induced
phase separation or by enhanced temperature-induced phase
separation (by introducing an organic phase to aid in R3N–H2O par-
titioning). On the contrary, water-protonated R3Ns, R3N·H+–OH−,
are recoverable via simple heating. It does appear that heating
to deprotonate acid-protonated R3Ns is unlikely to occur at
conditions well below the volatilisation temperature of any of its
components. Furthermore, the results established that aqueous
alkalisation (Reaction (3)) is generally reversible even with mild
heating, an indication of decreased R3N aqueous miscibility with T.
3.3. Counter-anion effect on R3N regeneration via volatilisation
When the temperature-induced phase separation proved
unable to regenerate amine from its acid-protonated state,
component volatilisation was explored (Reactions (4)–(6)). A fully-
neutralised system (i.e., no excess Et3N or acid) comprising Et3N
paired to different acids of various acidity strengths were tested to
check whether Et3N can be recovered from Et3N·H+–Hx−1Acid−,
and ascertain the ease by which the amine component can be
removed from these systems.
Et3N(org) Et3N(g) (6)
The influence of counter anions on Et3N volatilisation was
examined by heating 25 mL aqueous solutions of Et3N (2.31 mmol,
0.33 mL) which had been previously fully neutralised (equivalent
amount of H+ = 2.27 mmol) by various proton sources (i.e., H3Ci,
H2CO3, HCl). These solutions that contained no excess Et3Ns (i.e.,
all Et3N·H+ are paired to the acid’s conjugate base) were heated to
boil in the 250 mL round bottomed flask (distillation vessel). The
volatilised component was condensed and collected into another
250 mL round bottom flask (receiver vessel). The receiver vessel
contained 25 mL aqueous solution of HCl providing at least 5% of
the equivalent amount of Et3N in the distillation vessel. This set-
up allows for an “autotitration” of the acidic solution and provides
the “breakthrough titration curve” once more than ∼5% of Et3N
(>0.116 mmol Et3N) has been volatilised and recovered. It must be
noted that recovered Et3N comes from Et3N·H+ paired to the acid’s
conjugate base (i.e., Et3N·H+–Hx−1Acid−), as neither hydroxide-
paired triethylammonium (i.e., Et3N·H+–OH−) nor free Et3N exist
in this system. Further confirmatory tests were performed by alka-
lising the receiver flask with NaOH pellets to check for amine gas
evolution as manifested by inflation of an attached balloon.
In these tests, Et3N was chosen as the amine component,
because besides being the most promising candidate based on
its acid neutralisation and aqueous alkalisation capabilities, it
is the most volatile. As for the perceived counter anions, the
following were represented by OH− for the Et3N–H2O system,
HCO3
− for Et3N–H2O–CO2, Ci− for Et3N–H2O–H3Ci and Cl− for
Et3N–H2O–HCl. These systems are of increasing acid strength
relative to OH− (i.e., counter anion in aqueous alkalisation). The

5. R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387 383
Fig. 3. Breakthrough autotitration curves generated as volatilised Et3N migrates from the distillation flask and collects onto an acid-containing receiver vessel. The “break-
through” indicates the recovery via volatilisation of >5% w/w of Et3N from the acid-protonated amine, Et3N·H+
−Hx−1Acid−
.
Et3N–H2SO4–H2O system was not included, with Et3N–H2O–HCl
already representing the strong acid counter anion system.
Fig. 3 shows the breakthrough titration curves that depict the
regeneration of >5% w/w Et3N that was liberated from the acid-
protonated Et3N (Et3N·H+–Hx−1Acid−). The results clearly indicate
that the counter anion influences the ease by which Et3N is recov-
ered, demonstrating the reversibility of Reactions (4)–(6). Whilst it
appears that Et3N was unrecoverable from Et3N·H+–Cl− by heating
at 80 ◦C for more than 3 h, alkalisation of the receiver flask with
NaOH pellets resulted in gas evolution sufficient to inflate the bal-
loon attached to the receiver’s opening. This gas evolution indicates
that Et3N was volatilised, albeit at a minimal amount. It is likely that
the system needed much longer time 3 h or higher T to achieve
>5% recovery.
Based on these results, the relative ease of Et3N recovery
from its protonated state via volatilisation is of the order:
Et3N–H2O > Et3N–CO2–H2O > Et3N–H3Ci–H2O Et3N–HCl–H2O.
With Et3N generally regenerable from its protonated state in
weakly acidic systems, the subsequent sections explore Et3N recov-
ery/acid generation from strongly acidic systems; Et3N–HCl–H2O
and Et3N–H2SO4–H2O.
3.4. Et3N–H2O system
Capitalising on Et3N’s low boiling point and the formation of a
type II (Negi and Anand, 1985) partially miscible system with water,
recovery is likely achievable via phase partitioning and then amine
volatilisation. The Et3N–H2O system is defined by a lower critical
solution temperature (LCST, see Fig. 4), where above this curve, the
components are partially miscible thus forming a 2-phase system
comprising of organic and aqueous layers. For instance, at 50:50
mass ratio of Et3N to H2O, the LCST occurs at 18 ◦C which is the
approximate minima in the LCST curve. The LCST increases to a
maximum at about 60 ◦C for which the amine phase is approxi-
mately 98 wt% pure (Negi and Anand, 1985; Davison et al., 1960).
In general, heating the Et3N–H2O system would cause Et3N ( Et3N =
0.73 g mL−1
) to form a separate layer above that of water when the
temperature goes beyond 60 ◦C. This layer could then be volatilised
with either none or minimal co-evaporation of water.
Fig. 4 shows the dew point and bubble point curves as well as the
reconstructed lower critical solution temperature (LCST) curve for
the Et3N–H2O system. We can see that once the LCST is breached
and after the Et3N concentrates above that of the aqueous layer,
Et3N could volatilise at ∼55 ◦C (if Et3N component ∼15% w/w). In
fact, when the system temperature exceeds 60 ◦C, phase separa-
tion enables Et3N to concentrate and volatilise. This suggests that
initially, pure Et3N is liberated until the azeotropic temperature
and composition is reached. It must also be realised that as this
system forms an azeotrope at 74 ◦C (90% Et3N and 10% H2O), it is
to be expected that water will always be co-evaporated, albeit at a
minimal amount.
3.5. Et3N–H2O–HxAcid system
Acid–Et3N aqueous interactions were investigated experimen-
tally via replicate titrations of 25 mL 0.1 M acid (i.e., HCl, H2SO4,
H3Ci) against Et3N. Fig. 5 shows the experimental curves for HCl,
H2SO4 and H3Ci (citric acid) against Et3N and compares these to
the OLI-predicted theoretical curves. A good agreement between
experimental and thermodynamic simulation curves is observed
for both acid neutralisation and aqueous alkalisation regions in
the Et3N–H2O–HCl and Et3N–H2O–H2SO4 systems. Meanwhile, in
the Et3N–H2O–H3Ci system, we see that beyond the 0.1 M Et3N
mark, the experimental curve diverges from the simulation in both
the neutralisation and alkalisation regions. This is as expected due
to the limited rate at which H3Ci’s carboxylic sites deprotonate.
For H3Ci, the predicted 2nd deprotonation reaction (pKa2 = 4.67,
Table 1) roughly corresponds to the pH at which the simulated and
experimental neutralisation curves begun to diverge (4.5).
Table 1 is a summary of the predicted pKa of protonated Et3N at
various temperatures, and compares this to the acids in this study.
With the relatively high pKa values for Et3N·H+, an option can be
made to introduce serpentine minerals (i.e., antigorite, chrysotile)
into the regeneration stage to aid in amine recovery, by inducing
Et3N·H+ to deprotonate and thereby making it relatively easier to
volatise.
Introducing serpentine minerals into the regeneration step, or
perhaps the integration of both regeneration and dissolution steps

6. 384 R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387
Fig. 4. Temperature–composition-miscibility diagram for the Et3N-H2O system. The lower critical solution temperature (LCST) curve is replotted from the data obtained
from Davison et al. (1960). The bubble point and dew point curves were simulated using OLI stream analyser 3.2.
Fig. 5. Comparison of the aqueous titration curves for hydrochloric acid (HCl), sul-
phuric acid (H2SO4) and citric acid (H3Ci) against triethylamine (Et3N) at 25 ◦
C and
1 bar. Dotted lines are theoretical titration curves simulated using OLI Stream Anal-
yser 3.2.
into one could be an option. With the predicted aqueous pH at 75 ◦C
for antigorite at 9.3 whilst that for chrysotile at 8.9, neutralisation
reaction with Et3N·H+ could further enhance the thermal regener-
ation of the amine. It must also be realised that the deprotonation
reaction of Et3N·H+ alone may be insufficient to dissolve silicate
minerals. And as such, a counter anion that is capable of releasing
protons, such as the bisulfate ion (HSO4
−), is expedient in aiding, if
not providing for the bulk of the dissolution work.
3.6. Acid component
A suitable acid, which enables the recovery of the amine (amine
regeneration) as well as being able to impart sufficient acidity to the
system (acid generation) when the amine is deprotonated, needed
to be found. The strong acids, HCl and H2SO4, yield pH systems
lower than 2, and are especially suitable for silicate dissolution.
However, amine regeneration-acid generation is yet to be demon-
strated for these strong acid-based systems for Et3N.
On the other hand, weak acids such as citric acid and carbonic
acid, allows for the Et3N regeneration via volatisation, but the
potentially achievable pH upon regeneration may be insufficient for
dissolving feedstocks. As such, the use of H3Ci as an acid component
Table 1
Predicted pKas of protonated Et3N·H+
and acids.
T (◦
C) Et3N·H+
H3Ci (citric acid) H2SO4 H2CO3 HCl
pKa pKa1 pKa2 pKa3 pKa1 pKa2 pKa1 pKa2 pKa
25.0 9.57 3.05 4.67 5.39 −3.03 1.90 6.05 10.6 −7.00
50.0 8.90 3.14 4.65 5.31 −2.47 2.08 5.92 10.2 −6.13
75.0 8.33 3.23 4.62 5.24 −1.98 2.24 5.80 9.73 −5.38
95.0 NA 3.29 4.60 5.19 −1.64 2.35 5.72 9.43 −4.85

7. R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387 385
Fig. 6. Aqueous pH trend of the Et3N−HCl−H2O system with increasing temperature, T. The residual volume, % Vres at the end of the reaction time, ttotal, min, is included.
Note that the residual volume, % Vres at the end of the reaction time, ttotal, min, of 10% signifies excessive water evaporation.
will be expanded by evaluating the efficacy of this weak acid in dis-
solving serpentinite in another study. With regards to CO2 source of
the acid component, this appears unlikely as CO2 dissolution is tem-
perature dependent. As seen in the breakthrough titration curve
(Fig. 3), regenerated amine would be unable to raise oncoming
leachate stream to above pH 8. Furthermore, unless a high pres-
sure reactor is used in the dissolution stage with concomitant CO2
delivery, low pH is highly unlikely to be achievable.
Strong polyprotic acids, such as H2SO4 present an interesting
candidate in that their subsequent deprotonation reactions are
likened to weak acid systems. Take for instance, the bisulfate ion
HSO4
−
(aq) which has a pKa value of 2.35 at 95 ◦C. This value is only
slightly lower than H3Ci’s 1st deprotonation at 95 ◦C, of pKa = 3.29,
making this acid slightly stronger than citric acid. This property
makes acid generation and amine recovery potentially feasible.
The subsequent section explores this possibility by tracking the
pH evolution of the Et3N–HCl–H2O and Et3N–H2SO4–H2O systems
with T, the latter being of particular interest due to the potential
for HSO4
−
(aq) ion generation. With water co-evaporation likely to
occur due to the Et3N–H2O azeotrope (i.e., excess Et3N reacting
with water to alkalise the system), particular focus is apportioned
to finding the best possible set-up that minimises water evapo-
ration whilst simultaneously generating the necessary acidity and
recovering sufficient Et3N for use in the subsequent alkalisation of
leachates.
3.7. pH evolution within the Et3N–HCl–H2O system
The pH evolution experiments probing the Et3N–HCl–H2O sys-
tem involved heating ∼25 mL of an aqueous solution of fully
neutralised HCl (2.5 mmol; 25 mL 0.1 M HCl) with excess Et3N
(3.5 mmol, 0.5 mL 99% purity Et3N) from 25 ◦C to boiling tempera-
ture (90 ◦C). Fig. 6 shows the pH evolution for the Et3N–HCl–H2O
system with T. As the system temperature ramps up towards 60 ◦C,
a slight drop in pH from 10 to 9.8 signified a decrease in Et3N
aqueous miscibility. This reduced miscibility with water effec-
tively enables the partitioning of excess Et3N, in which at 60 ◦C the
aqueous pH drops to below pH 9. With the partitioned Et3N con-
centrating at the upper layers of the mixture, rising temperatures
(60–90 ◦C) effectively renders the amine susceptible to volatilisa-
tion.
The volatilisation of Et3N and water co-evaporation, would
have altered the dissociation equilibrium in Reaction (5) towards
forming Et3N(aq) from Et3NH+–Cl−. This however, occurs at the
expense of significant water volatilisation, such that the total solu-
tion volume decreased to 10% of its original. Whilst the pH swing
from pH 10 to pH > 3 was an indication that Et3N could indeed
also be recovered from strong acid systems, the associated energy
for effecting the change is practically cost-prohibitive. Whilst the
co-evaporation of water is unavoidable due to the Et3N–H2O
azeotrope, the minimal co-evaporation of water and Et3N to effect
pH swing from alkaline to acidic conditions is desired in order to
attain both technological feasibility and operational cost efficacy.
3.8. pH evolution with T in the Et3N–H2SO4–H2O system
Fig. 7 shows the pH evolution of the Et3N–H2SO4–H2O sys-
tem under single stage and multistage (fractional) distillation
arrangements. These experiments involved heating 30 mL aqueous
solution of fully neutralised H2SO4 (20 mmol; 20 mL 0.5 M H2SO4)
with excess Et3N (71 mmol; 10 mL 99% purity Et3N) from 25 ◦C to
boiling temperature (i.e., 105 ◦C). Condensates were collected dur-
ing these experiments to ascertain the amine recovery efficiency
and are reported in Table 2.
The single stage pH evolution profile is similar to that of HCl
with the pH decreasing to approximately 7 as the temperature
increases before dropping significantly to pH 3 while the tem-
perature remains constant during amine volatilisation. The main
difference was the constant boiling point temperature of 105 ◦C and
the reduced co-evaporation of water. The solution volume reduced
to 32% as opposed to 10% for the HCl. It is likely that the higher
boiling temperature aided more selective separation. When frac-
tional distillation was applied, the selectivity improved with the
residual solution volume now 65% of the original. Given that the
Et3N comprises approximately 33% of the system’s initial volume,
the selectivity achieved with multiple stage distillation is excellent.
From the Et3N recovery values, we could also see that the collected
condensates during fractional distillation were all pure Et3N.

8. 386 R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387
Fig. 7. Aqueous pH trends of Et3N−H2SO4−H2O system with respect to temperature in simple and multistage (fractional) distillation arrangements. The pH of the cooled
residues, residual volumes, % Vres, and total time, ttotal, min, are included for comparison. Note that the volume fraction of Et3N originally in the system corresponds to 33%.
Table 2
Recovery of Et3N based on the condensate analysis. Of the mL Et3N–H2SO4–H2O distilled, 10 mL comprises Et3N whilst 20 mL was 0.5 M H2SO4.
Condensate no. Condensate V (mL) Condensate pH (25 ◦
C) Et3N content (%v/v) Et3N regenerated (mL)
Simple distillation
1 4.30 11.7 96.9 4.20
2 1.80 11.8 100 1.80
3 6.30 11.7 12.5 0.80
4 2.80 11.6 12.5 0.30
Total 15.2 7.10
Fractional distillation
1 3.00 11.6 100 3.00
2 0.50 11.6 100 0.50
3 3.20 11.2 100 3.20
4 0.20 11.2 100 0.20
5 0.50 11.2 100 0.50
Total 7.40 7.40
We attribute the acid that is being generated during regenera-
tion from the Et3N–H2SO4–H2O system is HSO4
−
(aq) (Reaction (7)),
and consequently the liberation of Et3N (Reaction (5) and (6)). Spe-
ciation studies in OLI analyser 3.2 indicate that at pH 1.5 the species
in equilibrium are largely (Et3NH)2SO4 with smaller amounts of
regenerated HSO4
− and H+. As this solution that would be recycled
for Mg2+ dissolution, recovery of both amine and acid need not be
high, but enough to provide sufficient acidity for Mg2+ dissolution.
2Et3N·H+
(aq) + SO4
2−
(aq) Et3N·H+
(aq) + HSO4
−
(aq) + Et3N(aq) (7)
HSO4
−
(aq) SO4
2−
(aq) + H+
(aq) (8)
The pH variance of the resulting solution at high tempera-
ture and ambient condition was attributed to the temperature
dependence of HSO4
− dissociation (Reaction (8)). At elevated tem-
peratures, acidity generation resulting from HSO4
− dissociation is
reduced (Table 1) such that pH readings at ambient conditions are
expected to be lower than those at elevated temperatures. As seen
in Fig. 7, the resultant pH during multiple stage distillation at ambi-
ent pressure was 2.7 at ∼105 ◦C, which, when cooled to ambient
temperature measured, pH 1.9.
To further check if reduced pressure conditions could enhance
separation and amine recovery, experiments where the system
was continuously evacuated and maintained at −0.5 bar were
conducted. These experiments neither enhanced separation nor
minimised water co-evaporation. Based on these results, frac-
tional distillation at ambient pressure best provides for the pH
swing from 10 to 2 with minimal water evaporation within
2 h.
At this current stage in research and development, the use of
Et3N–H2SO4–H2O system is considered most promising in provid-
ing the pH swing capability to our envisioned chemical process.
Process optimisation of the amine-recovery and acid generation
with this system as well as energy cost evaluation are underway.
Future efforts could also consider acids with even higher boiling
temperatures to improve the selectivity of evaporation and min-
imise the co-evaporation of water. However, higher temperatures
would increase energy requirements. It also appears that, provided
multiple stage distillation is used, high selectivity is achievable.
There also does not appear to be any reason why lower pH levels
cannot be achieved during regeneration by further increasing the
number of stages. Future work will also focus on demonstrating the
recyclability of the Et3N–H2SO4–H2O system, provide dissolution

9. R.D. Balucan, K.M. Steel / International Journal of Greenhouse Gas Control 42 (2015) 379–387 387
kinetics data for various silicates and solid waste materials, as well
as conduct detailed precipitation studies.
4. Conclusion
This study has identified Et3N–H2SO4–H2O as the most promis-
ing system to function as the regenerable precipitant-solvent for
this new resource engineering process. Within the imposed opera-
tional constraints, this system affords pH control between 10 and 2.
Triethylamine provides the highest neutralisation capacity, capa-
ble of adequate alkalisation needed for brucite precipitation, and is
regenerable via volatilisation amongst the selected candidate R3Ns.
All other R3Ns that were tested are neither alkalising nor poten-
tially regenerable via volatilisation. Sulphuric acid, when paired to
Et3N, allows for the partial removal of the amine from a fully neu-
tralised Et3N–H2SO4–H2O system, yielding a low pH (<2). Based on
the reaction time, residual volume and resultant pH, we found that
fractional distillation of the Et3N–H2SO4–H2O system under ambi-
ent conditions best provides high Et3N recovery whist minimising
the co-evaporation of water.
Acknowledgements
The authors thank Sirius Minerals Plc for the financial assistance.
RD Balucan acknowledges the postdoctoral research fellowship
from The University of Queensland. Assistance from Ms Fox and
Mr Chagas is appreciated.
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