XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 1833
RR Temperature
Production of white phosphorus by reducing phosphate
rock in industrial practice is usually performed at temper-
ature of ranging from 1200 to 1500°C [Threlfall, 1951].
Our tests also verified that the suitable roasting tempera-
ture should be 1150°C or above for the RR of sludge. The
results in Table 4 shows that at RR temperature of 1000°C,
only about 5% of phosphorus was released during roasting,
but as the roasting temperature increased to 1200°C, the
emission of phosphorus (weight loss of P2O5) during roast-
ing and the leaching efficiencies of REEs and P2O5 rose
dramatically. So, the RR temperature in this research was
set at 1200°C.
Roasting Product Leaching
REE leaching was conducted in plastic bottles with weight
ratio of liquid to solid of 7:3 at room temperature (about
23°C). The bottles were clamped on a wrist action shaker,
which swayed at a frequency of 60 hertz to agitate the leach-
ing slurry. Based on preliminary tests, the leaching time was
set at 1 hour.
The effect of nitric acid concentration on REEs and
phosphorus leaching efficiencies was investigated and the
results are shown in Figure 2. Both REEs and P2O5 leach-
ing efficiencies rose with the acid concentration increase,
and they exhibited similar trends. When the acid con-
centration increased to 5.0 M, both leaching efficiencies
reached about 98% and then leveled off. So, in this research
the acid concentration for sludge roasting product leaching
was set at 5.0 M.
Addition of Carbon in RR Roasting
The addition of carbon is an important influence factor for
RR process. If carbon addition is not sufficient, not only do
the reduction reactions not perform completely, but also
the roasting product agglomerates, both of which result in
low recoveries of REEs and P2O5 in leaching. On the other
hand, excessive addition will increase the cost of roasting.
The effect of carbon addition was tested, and the results
are presented in Figures 3 and 4. Figure 3 shows that as the
carbon addition decrease (weight ratio of sludge to carbon
increased), the amount of phosphorus released from sludge
(weight loss of P2O5) exhibited a downtrend trend, and it
leveled off at weight ratio of sludge to carbon of 4.6 and
5.6 because of the agglomeration of sludge in roasting. The
sludge weight loss showed a similar trend, except for the
lower value at weight ratio of sludge to carbon of about 2.0
which may be attributed to the formation of iron carbide
(Fe3C, which can not release the C during AR process). The
increases of sludge weight loss at weight ratio of sludge to
carbon of 4.6 and 5.6 may be due to the conversion of from
CaSO4 to CaO (as shown in formula (2)), rather than from
CaSO4 to CaS (as shown in formula (3)). The total weight
of CaO would be less than that of CaS, leading to higher
sludge weight losses.
Table 3. Two-stage leaching of sludge solids
First stage
Leaching Time (min.) REE Content in Leachate (ppm) %REE Recovery
30 256 42.15
60 289 47.53
120 393 72.36
Second stage
Leaching Time (min.) REE Content in Leachate (ppm) %REE Recovery
30 180 16.90
60 174 16.30
120 170 16.00
Overall %REE recovery 89.26*
*72.36% (First stage—120 min.) +16.90% (Second stage—30 min.) =89.26%
Table 4. Results of sludge roasting and leaching tests (Weight ratio of sludge to carbon in RR
feed was 4.0:1)
RR Temperature
(°C)
Weight Loss After RR and AR (%)Leaching Efficiency (%)
Sludge P2O5 REEs P2O5
1000 41.42 5.01 89.55 85.26
1200 47.45 46.64 98.38 99.04
RR Temperature
Production of white phosphorus by reducing phosphate
rock in industrial practice is usually performed at temper-
ature of ranging from 1200 to 1500°C [Threlfall, 1951].
Our tests also verified that the suitable roasting tempera-
ture should be 1150°C or above for the RR of sludge. The
results in Table 4 shows that at RR temperature of 1000°C,
only about 5% of phosphorus was released during roasting,
but as the roasting temperature increased to 1200°C, the
emission of phosphorus (weight loss of P2O5) during roast-
ing and the leaching efficiencies of REEs and P2O5 rose
dramatically. So, the RR temperature in this research was
set at 1200°C.
Roasting Product Leaching
REE leaching was conducted in plastic bottles with weight
ratio of liquid to solid of 7:3 at room temperature (about
23°C). The bottles were clamped on a wrist action shaker,
which swayed at a frequency of 60 hertz to agitate the leach-
ing slurry. Based on preliminary tests, the leaching time was
set at 1 hour.
The effect of nitric acid concentration on REEs and
phosphorus leaching efficiencies was investigated and the
results are shown in Figure 2. Both REEs and P2O5 leach-
ing efficiencies rose with the acid concentration increase,
and they exhibited similar trends. When the acid con-
centration increased to 5.0 M, both leaching efficiencies
reached about 98% and then leveled off. So, in this research
the acid concentration for sludge roasting product leaching
was set at 5.0 M.
Addition of Carbon in RR Roasting
The addition of carbon is an important influence factor for
RR process. If carbon addition is not sufficient, not only do
the reduction reactions not perform completely, but also
the roasting product agglomerates, both of which result in
low recoveries of REEs and P2O5 in leaching. On the other
hand, excessive addition will increase the cost of roasting.
The effect of carbon addition was tested, and the results
are presented in Figures 3 and 4. Figure 3 shows that as the
carbon addition decrease (weight ratio of sludge to carbon
increased), the amount of phosphorus released from sludge
(weight loss of P2O5) exhibited a downtrend trend, and it
leveled off at weight ratio of sludge to carbon of 4.6 and
5.6 because of the agglomeration of sludge in roasting. The
sludge weight loss showed a similar trend, except for the
lower value at weight ratio of sludge to carbon of about 2.0
which may be attributed to the formation of iron carbide
(Fe3C, which can not release the C during AR process). The
increases of sludge weight loss at weight ratio of sludge to
carbon of 4.6 and 5.6 may be due to the conversion of from
CaSO4 to CaO (as shown in formula (2)), rather than from
CaSO4 to CaS (as shown in formula (3)). The total weight
of CaO would be less than that of CaS, leading to higher
sludge weight losses.
Table 3. Two-stage leaching of sludge solids
First stage
Leaching Time (min.) REE Content in Leachate (ppm) %REE Recovery
30 256 42.15
60 289 47.53
120 393 72.36
Second stage
Leaching Time (min.) REE Content in Leachate (ppm) %REE Recovery
30 180 16.90
60 174 16.30
120 170 16.00
Overall %REE recovery 89.26*
*72.36% (First stage—120 min.) +16.90% (Second stage—30 min.) =89.26%
Table 4. Results of sludge roasting and leaching tests (Weight ratio of sludge to carbon in RR
feed was 4.0:1)
RR Temperature
(°C)
Weight Loss After RR and AR (%)Leaching Efficiency (%)
Sludge P2O5 REEs P2O5
1000 41.42 5.01 89.55 85.26
1200 47.45 46.64 98.38 99.04