1832 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
RESULTS AND DISCUSSION
Continuous Separation of Solid from Feedstock Sludge
The high-speed rotation of CFDC caused high centrifugal
forces, generated from up to 1500 G gravity acceleration,
to the feedstock sludge, which led to a high-efficient separa-
tion of solid particles from the feed. A series of tests indi-
cated that a single pass of phosphoric-acid sludge through
the CFDC yielded 95% liquid recovery and 90% recovery
of REEs-containing solids from 20 to 34 wt.% solids-con-
taining sludge. A reduced order model developed for the
CFDC operation showed good agreement with experimen-
tal data, and preliminary technoeconomic analysis revealed
potential process economic feasibility.
Table 1 summarizes separation results from 8 continu-
ous tests, while Table 2 shows the fate of REE and P2O5
values.
Hydrometallurgical Processing
Parametric Leaching
Initial leaching tests using dilute (0.5 M) nitric acid
(HNO3) washing achieved expected objectives: 1) sub-
stantial removal (50%) of U and Th, 2) nearly complete
removal (98%) of P, and 3) negligible REE loss (0.3%).
Parametric leaching tests were then conducted on washed
sludge solids using nitric acid at varying nitric acid concen-
trations and temperatures. Based on the results from para-
metric leaching tests, the optimal leaching conditions were
found to be as follows:
Nitric acid concentration: 5 M
Temperature: 70–90 °C
Solid to liquid weight ratio: 3:7
Under these conditions, a REE recovery of 80% was
achieved in a single step. Two or three-step sequential
leaching was conducted later, which achieved higher REE
recovery. Large batch (2 L) leaching tests showed that much
higher nitric acid concentration hindered REE leaching
efficiency. For example, REE recovery was below 20%
using a 12 M nitric acid (HNO3) solution..
Stagewise Leaching Under Optimized Leaching
Conditions
Under the above conditions, REE leaching recovery of
about 90% was achieved by two-stage leaching as shown
in Table 3.
Thermal Processing
As shown in formulas (2)–(5), the RR is a process that
produces gases and loses weight. These two phenomena
provided clues to the progress of reduction reactions. By
monitoring the emission of gases in preliminary tests, the
RR time was determined as 2 hours. Based on our research
experience, the AR was set at temperature of 900°C and
lasted for 2 hours. Nitric acid was used for REE leaching
from the roasting product. Besides, the effects of other con-
tributing factors, including RR temperature, nitric acid
concentration and weight ratio of sludge to carbon in RR
feed, were investigated.
Table 1. Summary of solid-liquid separation test results
Test No
%Solids in
Feed
Time
(Min)
%Solids in
Liquid
%Liquid
Recovery
%Solid
Recovery
1 31.4 80 4.8 91.8 89.9
2 30.5 70 6.1 92.5 88.8
3 29.6 75 4.4 95.3 87.5
4 26.3 90 3.9 95.5 90.7
5 26.3 40 6.2 96.5 82.1
6 26.3 30 7.1 95.5 79.7
7 34.5 53 8.0 94.2 84.3
8 35.1 72 7.9 094.5 85.2
Table 2. Distribution of key-value components in separation products
Product Total REE (ppm) P
2 O
5 (wt. %)
Feed liquid 63 50.5
Feed solids 1528 30.9
Separated solids 1494 16.5
Separated liquid 56 40.0
RESULTS AND DISCUSSION
Continuous Separation of Solid from Feedstock Sludge
The high-speed rotation of CFDC caused high centrifugal
forces, generated from up to 1500 G gravity acceleration,
to the feedstock sludge, which led to a high-efficient separa-
tion of solid particles from the feed. A series of tests indi-
cated that a single pass of phosphoric-acid sludge through
the CFDC yielded 95% liquid recovery and 90% recovery
of REEs-containing solids from 20 to 34 wt.% solids-con-
taining sludge. A reduced order model developed for the
CFDC operation showed good agreement with experimen-
tal data, and preliminary technoeconomic analysis revealed
potential process economic feasibility.
Table 1 summarizes separation results from 8 continu-
ous tests, while Table 2 shows the fate of REE and P2O5
values.
Hydrometallurgical Processing
Parametric Leaching
Initial leaching tests using dilute (0.5 M) nitric acid
(HNO3) washing achieved expected objectives: 1) sub-
stantial removal (50%) of U and Th, 2) nearly complete
removal (98%) of P, and 3) negligible REE loss (0.3%).
Parametric leaching tests were then conducted on washed
sludge solids using nitric acid at varying nitric acid concen-
trations and temperatures. Based on the results from para-
metric leaching tests, the optimal leaching conditions were
found to be as follows:
Nitric acid concentration: 5 M
Temperature: 70–90 °C
Solid to liquid weight ratio: 3:7
Under these conditions, a REE recovery of 80% was
achieved in a single step. Two or three-step sequential
leaching was conducted later, which achieved higher REE
recovery. Large batch (2 L) leaching tests showed that much
higher nitric acid concentration hindered REE leaching
efficiency. For example, REE recovery was below 20%
using a 12 M nitric acid (HNO3) solution..
Stagewise Leaching Under Optimized Leaching
Conditions
Under the above conditions, REE leaching recovery of
about 90% was achieved by two-stage leaching as shown
in Table 3.
Thermal Processing
As shown in formulas (2)–(5), the RR is a process that
produces gases and loses weight. These two phenomena
provided clues to the progress of reduction reactions. By
monitoring the emission of gases in preliminary tests, the
RR time was determined as 2 hours. Based on our research
experience, the AR was set at temperature of 900°C and
lasted for 2 hours. Nitric acid was used for REE leaching
from the roasting product. Besides, the effects of other con-
tributing factors, including RR temperature, nitric acid
concentration and weight ratio of sludge to carbon in RR
feed, were investigated.
Table 1. Summary of solid-liquid separation test results
Test No
%Solids in
Feed
Time
(Min)
%Solids in
Liquid
%Liquid
Recovery
%Solid
Recovery
1 31.4 80 4.8 91.8 89.9
2 30.5 70 6.1 92.5 88.8
3 29.6 75 4.4 95.3 87.5
4 26.3 90 3.9 95.5 90.7
5 26.3 40 6.2 96.5 82.1
6 26.3 30 7.1 95.5 79.7
7 34.5 53 8.0 94.2 84.3
8 35.1 72 7.9 094.5 85.2
Table 2. Distribution of key-value components in separation products
Product Total REE (ppm) P
2 O
5 (wt. %)
Feed liquid 63 50.5
Feed solids 1528 30.9
Separated solids 1494 16.5
Separated liquid 56 40.0