6
HPAL Overall Findings
Table 5 summarizes the calculated metal extractions/deport-
ment to solution achieved in all eight HPAL tests, including
the key metals illustrated in Figures 1–3. Elements such as
magnesium were relatively easily leached in all tests, while
others were present in small quantities (i.e., Cr) and showed
larger variation from test to test, regardless of conditions
In addition to the increased pulp density in later tests
helping to reduce the overall acid addition (while main-
taining similar levels of free acidity and extractions) it also
helped to increase the concentration of the key metals in
solution (Table 6, along with the typical residue concentra-
tion) as would be expected.
The tenors at the lowest pulp density (5%) were
impacted by the reduced leach performance under oxy-
gen overpressure, but there is a large increase in the nickel,
cobalt, and copper concentrations in subsequent tests,
while the levels of iron and aluminum in solution are not
as strongly influenced. This improves the relative ratios
of Ni/Co/Cu in solution with Fe/Al prior to downstream
impurity removal. The Na present in solution is assumed to
be from the nodules themselves, due to residual dried salt
water entrained in the nodules after harvesting.
The HPAL residues remained enriched in Mn, with Si
as the next most prevalent element. The increased leaching
of Ni/Co/Cu under conditions without oxygen resulted in
very low levels of those metals in the residue (0.1% each).
The testwork demonstrated the ability of conventional
pressure acid leaching to selectively recover nickel, cobalt,
and copper from seabed nodules, with only limited co-
extractions of iron, aluminum, and manganese, leaving up
to 90% in the leach residue. The residue at 30–35% Mn is
higher grade than many manganese deposits, and could be
considered for further processing to recover the manganese,
either using a reductive leach or through pyrometallurgical
means. The process as tested has not been optimized and
significant opportunities exist to reduce acid addition via
variations in operating temperature and feed pulp density.
Nevertheless, the process conditions are in essence quite
comparable (but less aggressive) to HPAL conditions used
in the nickel laterite industry.
To confirm the amenability of the HPAL residue to
reductive leaching, a single test was conducted using sulfur
dioxide gas as the reductant, sparging in a reactor at 10%
solids maintained at ≥80°C, using residue from HPAL Test
8 as feed. Gas was injected until the pulp reached 200 mV
ORP (Ag/AgCl reference electrode) before turning off the
gas flow and holding the reactor for one hour. Nearly quan-
titative leaching of Mn was achieved (leaving 0.43% Mn in
the residue), along with 85% Fe, 13% Al, and the balance
of the remaining Ni/Co/Cu that was not leached in the
Table 5. HPAL calculated extractions
Test
ID 1 2 3 4 5 6 7 8
Ni 68 72 75 93 99 88 99 98
Co 7 12 19 69 97 84 93 92
Cu 61 73 79 94 99 84 94 94
Fe 3 7 9 27 19 16 10 9
Mg 90 92 94 99 100 89 100 98
Al 44 54 64 77 80 67 19 10
Cr 13 20 42 63 86 70 57 31
Mn 2 3 4 5 10 8 6 7
Ca 90 91 92 47 38 16 23 17
Na 91 91 90 92 92 80 77 74
Table 6. Typical Nodule Pressure Leach Solution/Residue Compositions
Final Filtrate (mg/L) Final Residue (%)
Test
Type
w O
2 (5%
solids)
w/o O
2 (20%
Solids)
w/o O
2 (30%
Solids)
Test
Type
w O
2 (5%
solids)
w/o O
2 (20%
Solids)
w/o O
2 (30%
Solids)
Ni 480 2775 5110 Ni 0.46 0.07 0.07
Co 15 413 821 Co 0.25 0.05 0.03
Cu 403 2230 4030 Cu 0.40 0.05 0.12
Zn 71 147 498 Fe 6.37 6.19 5.54
Fe 179 3075 2463 Mg 0.21 0.02 0.10
Mg 944 4085 7520 Al 1.52 0.71 2.00
Al 734 4320 3293 Cr 0.01 0.01 0.01
Cr 2 67 46 Mn 34.7 34.8 30.7
Mn 454 4720 8393 Ca 0.17 1.04 1.30
Ca 665 800 609 Si 7.98 9.37 8.29
Na 1020 4295 6507 Na 0.25 0.24 0.56
HPAL Overall Findings
Table 5 summarizes the calculated metal extractions/deport-
ment to solution achieved in all eight HPAL tests, including
the key metals illustrated in Figures 1–3. Elements such as
magnesium were relatively easily leached in all tests, while
others were present in small quantities (i.e., Cr) and showed
larger variation from test to test, regardless of conditions
In addition to the increased pulp density in later tests
helping to reduce the overall acid addition (while main-
taining similar levels of free acidity and extractions) it also
helped to increase the concentration of the key metals in
solution (Table 6, along with the typical residue concentra-
tion) as would be expected.
The tenors at the lowest pulp density (5%) were
impacted by the reduced leach performance under oxy-
gen overpressure, but there is a large increase in the nickel,
cobalt, and copper concentrations in subsequent tests,
while the levels of iron and aluminum in solution are not
as strongly influenced. This improves the relative ratios
of Ni/Co/Cu in solution with Fe/Al prior to downstream
impurity removal. The Na present in solution is assumed to
be from the nodules themselves, due to residual dried salt
water entrained in the nodules after harvesting.
The HPAL residues remained enriched in Mn, with Si
as the next most prevalent element. The increased leaching
of Ni/Co/Cu under conditions without oxygen resulted in
very low levels of those metals in the residue (0.1% each).
The testwork demonstrated the ability of conventional
pressure acid leaching to selectively recover nickel, cobalt,
and copper from seabed nodules, with only limited co-
extractions of iron, aluminum, and manganese, leaving up
to 90% in the leach residue. The residue at 30–35% Mn is
higher grade than many manganese deposits, and could be
considered for further processing to recover the manganese,
either using a reductive leach or through pyrometallurgical
means. The process as tested has not been optimized and
significant opportunities exist to reduce acid addition via
variations in operating temperature and feed pulp density.
Nevertheless, the process conditions are in essence quite
comparable (but less aggressive) to HPAL conditions used
in the nickel laterite industry.
To confirm the amenability of the HPAL residue to
reductive leaching, a single test was conducted using sulfur
dioxide gas as the reductant, sparging in a reactor at 10%
solids maintained at ≥80°C, using residue from HPAL Test
8 as feed. Gas was injected until the pulp reached 200 mV
ORP (Ag/AgCl reference electrode) before turning off the
gas flow and holding the reactor for one hour. Nearly quan-
titative leaching of Mn was achieved (leaving 0.43% Mn in
the residue), along with 85% Fe, 13% Al, and the balance
of the remaining Ni/Co/Cu that was not leached in the
Table 5. HPAL calculated extractions
Test
ID 1 2 3 4 5 6 7 8
Ni 68 72 75 93 99 88 99 98
Co 7 12 19 69 97 84 93 92
Cu 61 73 79 94 99 84 94 94
Fe 3 7 9 27 19 16 10 9
Mg 90 92 94 99 100 89 100 98
Al 44 54 64 77 80 67 19 10
Cr 13 20 42 63 86 70 57 31
Mn 2 3 4 5 10 8 6 7
Ca 90 91 92 47 38 16 23 17
Na 91 91 90 92 92 80 77 74
Table 6. Typical Nodule Pressure Leach Solution/Residue Compositions
Final Filtrate (mg/L) Final Residue (%)
Test
Type
w O
2 (5%
solids)
w/o O
2 (20%
Solids)
w/o O
2 (30%
Solids)
Test
Type
w O
2 (5%
solids)
w/o O
2 (20%
Solids)
w/o O
2 (30%
Solids)
Ni 480 2775 5110 Ni 0.46 0.07 0.07
Co 15 413 821 Co 0.25 0.05 0.03
Cu 403 2230 4030 Cu 0.40 0.05 0.12
Zn 71 147 498 Fe 6.37 6.19 5.54
Fe 179 3075 2463 Mg 0.21 0.02 0.10
Mg 944 4085 7520 Al 1.52 0.71 2.00
Al 734 4320 3293 Cr 0.01 0.01 0.01
Cr 2 67 46 Mn 34.7 34.8 30.7
Mn 454 4720 8393 Ca 0.17 1.04 1.30
Ca 665 800 609 Si 7.98 9.37 8.29
Na 1020 4295 6507 Na 0.25 0.24 0.56