1830 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Kremer and Chokshi, 1989 Zhang, 2012]. So, recovering
REEs from phosphate processing streams is very important
for the USA to enhance its self-sufficiency for these critical
materials.
Phosphoric acid sludge is a byproduct in phosphoric
acid manufacturing process. In phosphoric acid manufac-
turing, phosphate rock reacts with sulfuric acid to form a
slurry of phosphoric acid and calcium sulfate (phosphogyp-
sum). This slurry is filtered to obtain the filter acid product
containing 28–30% P2O5, which is further concentrated
to about 54% of P2O5 by evaporation. During the evapo-
ration process, impurities of aluminum, iron, magnesium,
calcium and rare earth elements (REEs) precipitate out
of solution gradually as P2O5 content increases [Becker,
1983]. The concentrated phosphoric acid slurry containing
the solid precipitates is treated by either centrifugal sepa-
ration or settling to achieve solid-liquid separation. The
resulting thick material is called phosphoric acid sludge.
Much of the REEs in the filtered acid ends up in the solid
fraction of the sludge, which has the highest REE concen-
tration (2000 ppm) among phosphate mining and pro-
cessing streams [Zhang, 2014 Marzougui et al., 2019]. On
the other hand, P2O5 content in the sludge usually reaches
15%–20%, which is the other value to be recovered.
Leaching with mineral acid (hydrometallurgical
approach) is a common approach to recover REEs and
phosphorus from the phosphate processing streams [Liang
et al., 2017 Liang and Zhang et al., 2017 Zhang et al.,
2018 Wu et al., 2018, Liang et al., 2018]. During leach-
ing the REEs and phosphorus enter the liquid, and then
they are separated from the residue by filtration. After that
the REEs were further separated through solvent extrac-
tion, and the phosphorus can be recycled to the phosphoric
acid production process. As is discussed below in the paper,
acid leaching alone cannot get complete recovery of REEs
from phosphoric acid sludge, because part of them exists in
the crystal lattice of gypsum as substitute for calcium ions,
which cannot be released into solution during leaching.
Reduction roasting had been the original method for
human beings to produce phosphorus from bone, then it
was applied to phosphate rock and has survived to this day
[Lapple, 1966]. During roasting apatite reacts with reduc-
tant (usually coal or natural gas) and additive of silica as
follows:
2Ca PO 10C 6SiO
6CaSiO 10CO
3 4 2 2
3 4
"
--
++
++P
^h
(1200–1500°C) (1)
The main purpose of the addition of silica is to lower reac-
tion temperature.
Research on phosphogypsum [Ragin and Brooks,
1951] has demonstrated that gypsum can be decomposed
by reduction roasting in the temperature range of 850–
1000°C, which means REEs in gypsum crystal lattice might
be released by roasting phosphoric acid sludge. Therefore,
in this study a thermal processing was also investigated to
recover both REEs and phosphorus from phosphoric acid
sludge in higher efficiencies.
METHODOLOGY
The original phosphoric acid sludge sample, with an average
solids content of about 20% provided by Mosaic Company
(Florida, USA), was a mixture of liquid phosphoric acid
(P2O5 content about 54%) and sludge. After settling for a
few days, the supernatant was decanted, and the settlings,
a thick and sticky mixture of solid particle and phosphoric
acid, was obtained, which, herein called feedstock sludge,
was used in our research.
Continuous Separation of Solid from Feedstock Sludge
The feedstock sludge usually contains only 25%–40% of
solid dependent on the settling time. If the phosphoric
acid in the mixture can be separated, it can return to the
dihydrate phosphoric acid production as a commercial
commodity. So, removing more acid from the feedstock
sludge is critical to reducing the overall cots for REE recov-
ery. In this research a continuous-flow decanter centrifuge
(CFDC) was employed to achieve a highly efficient solid/
liquid separation. The CFDC had a decumbent deep bowl
which consisted of a cylinder (about two thirds of the
depth, 60 mm internal diameter) and a cone (about one
third of the depth). The bowl was made of a kind of alloy
plate which could rotate at high speeds. While the CFDC
was in operation, the feedstock sludge entered the device
through a feed pipe to about the middle of bowl depth,
then it began to rotate with the wall and generated centrifu-
gal force. Under the centrifugal effect, the mineral particles
moved toward the wall, and passed through holes in the
wall then on the other side of the wall, it was transported
by a screw conveyor to the conical section, where the solid
was dewatered further and subsequently pushed to the solid
discharge outlet meanwhile, the acid was squeezed out
from the space between solid particles and moved oppo-
sitely to the liquid exit. In this way the solid and liquid
were separated continuously and produced two products,
a solid-rich stream (a dense mixture, mainly solid mineral
particles) and a liquid-rich stream (mainly phosphoric acid
with P2O5 content of about 50%). Figure 1 shows the setup
of continuous decenter testing system.
Kremer and Chokshi, 1989 Zhang, 2012]. So, recovering
REEs from phosphate processing streams is very important
for the USA to enhance its self-sufficiency for these critical
materials.
Phosphoric acid sludge is a byproduct in phosphoric
acid manufacturing process. In phosphoric acid manufac-
turing, phosphate rock reacts with sulfuric acid to form a
slurry of phosphoric acid and calcium sulfate (phosphogyp-
sum). This slurry is filtered to obtain the filter acid product
containing 28–30% P2O5, which is further concentrated
to about 54% of P2O5 by evaporation. During the evapo-
ration process, impurities of aluminum, iron, magnesium,
calcium and rare earth elements (REEs) precipitate out
of solution gradually as P2O5 content increases [Becker,
1983]. The concentrated phosphoric acid slurry containing
the solid precipitates is treated by either centrifugal sepa-
ration or settling to achieve solid-liquid separation. The
resulting thick material is called phosphoric acid sludge.
Much of the REEs in the filtered acid ends up in the solid
fraction of the sludge, which has the highest REE concen-
tration (2000 ppm) among phosphate mining and pro-
cessing streams [Zhang, 2014 Marzougui et al., 2019]. On
the other hand, P2O5 content in the sludge usually reaches
15%–20%, which is the other value to be recovered.
Leaching with mineral acid (hydrometallurgical
approach) is a common approach to recover REEs and
phosphorus from the phosphate processing streams [Liang
et al., 2017 Liang and Zhang et al., 2017 Zhang et al.,
2018 Wu et al., 2018, Liang et al., 2018]. During leach-
ing the REEs and phosphorus enter the liquid, and then
they are separated from the residue by filtration. After that
the REEs were further separated through solvent extrac-
tion, and the phosphorus can be recycled to the phosphoric
acid production process. As is discussed below in the paper,
acid leaching alone cannot get complete recovery of REEs
from phosphoric acid sludge, because part of them exists in
the crystal lattice of gypsum as substitute for calcium ions,
which cannot be released into solution during leaching.
Reduction roasting had been the original method for
human beings to produce phosphorus from bone, then it
was applied to phosphate rock and has survived to this day
[Lapple, 1966]. During roasting apatite reacts with reduc-
tant (usually coal or natural gas) and additive of silica as
follows:
2Ca PO 10C 6SiO
6CaSiO 10CO
3 4 2 2
3 4
"
--
++
++P
^h
(1200–1500°C) (1)
The main purpose of the addition of silica is to lower reac-
tion temperature.
Research on phosphogypsum [Ragin and Brooks,
1951] has demonstrated that gypsum can be decomposed
by reduction roasting in the temperature range of 850–
1000°C, which means REEs in gypsum crystal lattice might
be released by roasting phosphoric acid sludge. Therefore,
in this study a thermal processing was also investigated to
recover both REEs and phosphorus from phosphoric acid
sludge in higher efficiencies.
METHODOLOGY
The original phosphoric acid sludge sample, with an average
solids content of about 20% provided by Mosaic Company
(Florida, USA), was a mixture of liquid phosphoric acid
(P2O5 content about 54%) and sludge. After settling for a
few days, the supernatant was decanted, and the settlings,
a thick and sticky mixture of solid particle and phosphoric
acid, was obtained, which, herein called feedstock sludge,
was used in our research.
Continuous Separation of Solid from Feedstock Sludge
The feedstock sludge usually contains only 25%–40% of
solid dependent on the settling time. If the phosphoric
acid in the mixture can be separated, it can return to the
dihydrate phosphoric acid production as a commercial
commodity. So, removing more acid from the feedstock
sludge is critical to reducing the overall cots for REE recov-
ery. In this research a continuous-flow decanter centrifuge
(CFDC) was employed to achieve a highly efficient solid/
liquid separation. The CFDC had a decumbent deep bowl
which consisted of a cylinder (about two thirds of the
depth, 60 mm internal diameter) and a cone (about one
third of the depth). The bowl was made of a kind of alloy
plate which could rotate at high speeds. While the CFDC
was in operation, the feedstock sludge entered the device
through a feed pipe to about the middle of bowl depth,
then it began to rotate with the wall and generated centrifu-
gal force. Under the centrifugal effect, the mineral particles
moved toward the wall, and passed through holes in the
wall then on the other side of the wall, it was transported
by a screw conveyor to the conical section, where the solid
was dewatered further and subsequently pushed to the solid
discharge outlet meanwhile, the acid was squeezed out
from the space between solid particles and moved oppo-
sitely to the liquid exit. In this way the solid and liquid
were separated continuously and produced two products,
a solid-rich stream (a dense mixture, mainly solid mineral
particles) and a liquid-rich stream (mainly phosphoric acid
with P2O5 content of about 50%). Figure 1 shows the setup
of continuous decenter testing system.