XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3495
the melt in a reactive diffusion rate-limiting step. Belite has
slow while alite has fast hydration kinetics, so it is essential
to maximise alite content in the clinker. Therefore, a high
reaction temperature of at least 1,450°C is essential. It is a
particular feature of the CaO-SiO2 phase diagram that the
reactive alite phase is preserved during quenching. Partially
molten clinker forms through an aggregation process,
which requires the temperature to not exceed 1,450°C,
which would also be detrimental to the refractory lining
of the kiln.
Stoichiometrically excess CaO, or a too low firing tem-
perature, leads to “free lime” which is detrimental to the
durability of concrete. Limestone is usually required to
contain less than 3% MgO, because clinker is not capable
of absorbing more than 5% MgO. Excess MgO as periclase
in clinker has a deleterious effect on concrete durability,
causing delayed expansion. In contrast, a high MgO con-
tent in GBFS concrete enhances resistance to carbonation
(Bernal et al. 2014). It appears that MgO as part of a reac-
tive phase is beneficial in concrete, while MgO as periclase
is detrimental. There are several waste and virgin materi-
als like dolomitic limestone containing MgO levels above
what can be tolerated in clinkerization.
A reason why high MgO cannot be accommodated in
rotary kiln processes is that the quenching process is too
slow to prevent the periclase from phase separating from
the alite and belite. Disappointingly, the literature is largely
silent on the constraint of quenching rate on cementitious
phase formation, likely because it is not possible to subject
clinker to a rapid granulation process as applied to GBFS.
The amorphous phases in CFA and GBFS are reasonably
reactive, as they are the result of rapid quenching. In con-
trast, clinker requires the exact composition of alite to have
high reactivity in the slow quenching system used for clin-
ker. Clinkerization as the incumbent process for cement
production imposes a constraint on the selection of source
materials and the potential to reduce CO2 emissions in
cement production. The decision by the cement industry
to simply add CCS to rotary kilns (Schneider et al. 2023)
means that the inherent constraint of rotary kilns is not
recognized by the industry.
DEVELOPMENT OF NEW MODULAR
REACTORS
TerraCO2 in the US replaces up to 40% PC by vitrify-
ing crystalline material including silica sand, to replace
CFA (www.terraCO2.com). The advantage of TerraCO2
technology is that it can be applied to a range of locally
available source material to produce synthetic SCM. With
the constrained availability of SCM, technologies such
as TerraCO2 could be used to convert mine tailings and
waste rock into synthetic SCM. It is not known whether
TerraCO2 can adjust the composition of materials to con-
vert high melting CaO and MgO into a glassy phase.
The CSIRO in Australia has developed MagSonic ™
(www.csiro.au) for the direct reaction of MgO with car-
bon to produce Mg vapour and CO, using supersonic
expansion to cause shock quenching. Similarly, Lansell et
al. (2017) used shock quenching through a venturi to pre-
serve the glassy state in powdered products from its elec-
trically enhanced supersonic shockwave reactor (EESS). As
outlined by Van Deventer et al. (2021), these systems can
produce reactive SCM from a range of crystalline materials
with low reactivity. The shock quenching releases heat that
is recycled to pre-heat materials fed to the EESS reactor.
Rocks have high compressive strength but low tensile
strength, which is further reduced in an electric field. The
invention by Lansell et al. (2017) combines tensile stress
and electric fields, which requires low energy of fracture.
The electrical field increases the entropy of materials and is
not simply a source of energy transfer as per the Joule Effect.
Besides De Knoop et al. (2018) who demonstrated how the
electric field in a TEM can melt gold at ambient tempera-
ture by transforming a crystalline phase into a disordered
phase, the literature is silent on this phenomenon. Instead
of using an increase in temperature to decrease Gibbs free
energy, the electric field increases both entropy and temper-
ature to the same effect. Consequently, cement formation
occurs at a lower temperature, even with a high content of
CaO and MgO. High current is used here, which is dif-
ferent from the high voltage pulsing system developed by
Andres (1995), which also subjects particles to tensile frac-
ture. It is noteworthy that the EESS reactor produces fine
particle product instead of clinker, so no further grinding is
required. Figure 1 depicts a schematic diagram of the EESS
reactor. The engineering of the EESS system at scale is not
straightforward, but Lansell et al. (2017) have already dem-
onstrated the system at 10 tons/hour. The EESS overcomes
the constraints of a rotary kiln for clinkerization and offers
the flexibility of production (Table 3), as it can be imple-
mented as modules of say 50 tons/hour. Rotary kilns use a
combination of fossil fuel and high calorific value waste as
energy sources. In contrast, EESS reactors can be designed
to use a combination of such fuel sources and electrical
energy.
CONVERSION OF TAILINGS TO CEMENT
Figure 2 depicts pathways for the conversion of tailings and
mine waste into construction materials. Different fractions
of sand could be screened from tailings, or removed by
the melt in a reactive diffusion rate-limiting step. Belite has
slow while alite has fast hydration kinetics, so it is essential
to maximise alite content in the clinker. Therefore, a high
reaction temperature of at least 1,450°C is essential. It is a
particular feature of the CaO-SiO2 phase diagram that the
reactive alite phase is preserved during quenching. Partially
molten clinker forms through an aggregation process,
which requires the temperature to not exceed 1,450°C,
which would also be detrimental to the refractory lining
of the kiln.
Stoichiometrically excess CaO, or a too low firing tem-
perature, leads to “free lime” which is detrimental to the
durability of concrete. Limestone is usually required to
contain less than 3% MgO, because clinker is not capable
of absorbing more than 5% MgO. Excess MgO as periclase
in clinker has a deleterious effect on concrete durability,
causing delayed expansion. In contrast, a high MgO con-
tent in GBFS concrete enhances resistance to carbonation
(Bernal et al. 2014). It appears that MgO as part of a reac-
tive phase is beneficial in concrete, while MgO as periclase
is detrimental. There are several waste and virgin materi-
als like dolomitic limestone containing MgO levels above
what can be tolerated in clinkerization.
A reason why high MgO cannot be accommodated in
rotary kiln processes is that the quenching process is too
slow to prevent the periclase from phase separating from
the alite and belite. Disappointingly, the literature is largely
silent on the constraint of quenching rate on cementitious
phase formation, likely because it is not possible to subject
clinker to a rapid granulation process as applied to GBFS.
The amorphous phases in CFA and GBFS are reasonably
reactive, as they are the result of rapid quenching. In con-
trast, clinker requires the exact composition of alite to have
high reactivity in the slow quenching system used for clin-
ker. Clinkerization as the incumbent process for cement
production imposes a constraint on the selection of source
materials and the potential to reduce CO2 emissions in
cement production. The decision by the cement industry
to simply add CCS to rotary kilns (Schneider et al. 2023)
means that the inherent constraint of rotary kilns is not
recognized by the industry.
DEVELOPMENT OF NEW MODULAR
REACTORS
TerraCO2 in the US replaces up to 40% PC by vitrify-
ing crystalline material including silica sand, to replace
CFA (www.terraCO2.com). The advantage of TerraCO2
technology is that it can be applied to a range of locally
available source material to produce synthetic SCM. With
the constrained availability of SCM, technologies such
as TerraCO2 could be used to convert mine tailings and
waste rock into synthetic SCM. It is not known whether
TerraCO2 can adjust the composition of materials to con-
vert high melting CaO and MgO into a glassy phase.
The CSIRO in Australia has developed MagSonic ™
(www.csiro.au) for the direct reaction of MgO with car-
bon to produce Mg vapour and CO, using supersonic
expansion to cause shock quenching. Similarly, Lansell et
al. (2017) used shock quenching through a venturi to pre-
serve the glassy state in powdered products from its elec-
trically enhanced supersonic shockwave reactor (EESS). As
outlined by Van Deventer et al. (2021), these systems can
produce reactive SCM from a range of crystalline materials
with low reactivity. The shock quenching releases heat that
is recycled to pre-heat materials fed to the EESS reactor.
Rocks have high compressive strength but low tensile
strength, which is further reduced in an electric field. The
invention by Lansell et al. (2017) combines tensile stress
and electric fields, which requires low energy of fracture.
The electrical field increases the entropy of materials and is
not simply a source of energy transfer as per the Joule Effect.
Besides De Knoop et al. (2018) who demonstrated how the
electric field in a TEM can melt gold at ambient tempera-
ture by transforming a crystalline phase into a disordered
phase, the literature is silent on this phenomenon. Instead
of using an increase in temperature to decrease Gibbs free
energy, the electric field increases both entropy and temper-
ature to the same effect. Consequently, cement formation
occurs at a lower temperature, even with a high content of
CaO and MgO. High current is used here, which is dif-
ferent from the high voltage pulsing system developed by
Andres (1995), which also subjects particles to tensile frac-
ture. It is noteworthy that the EESS reactor produces fine
particle product instead of clinker, so no further grinding is
required. Figure 1 depicts a schematic diagram of the EESS
reactor. The engineering of the EESS system at scale is not
straightforward, but Lansell et al. (2017) have already dem-
onstrated the system at 10 tons/hour. The EESS overcomes
the constraints of a rotary kiln for clinkerization and offers
the flexibility of production (Table 3), as it can be imple-
mented as modules of say 50 tons/hour. Rotary kilns use a
combination of fossil fuel and high calorific value waste as
energy sources. In contrast, EESS reactors can be designed
to use a combination of such fuel sources and electrical
energy.
CONVERSION OF TAILINGS TO CEMENT
Figure 2 depicts pathways for the conversion of tailings and
mine waste into construction materials. Different fractions
of sand could be screened from tailings, or removed by