30 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Pentlandite oxidation:
(Ni,Fe)9S8 +9H2SO4 +4.5O2 =
4.5NiSO4 +4.5FeSO4 +8S +9H2O
Pyrrhotite oxidation:
Fe7S8 +7H2SO4 +3.5O2 =7FeSO4 +8S +7H2O
Chalcopyrite oxidation:
CuFeS2 +2H2SO4 +O2 =
CuSO4 +FeSO4 +2S +2H2O
Sulfur oxidation:
S +1.5O2 +H2O =H2SO4
Iron precipitation:
Fe2(SO4)3 +3H2O =Fe2O3 +3H2SO4
The autoclave slurry discharge will contain dissolved
metal sulfates, acid, chloride and elemental sulfur and
iron precipitates along with any gangue minerals (or reac-
tion products of gangue minerals with acid). The slurry
discharge at elevated temperature and pressure is flashed
through a let-down system to cool the slurry to atmospheric
temperature and pressure. The flash steam and the vent
from the autoclave (to purge inert gases and excess oxygen)
is cleaned though a scrubber system and discharged to the
atmosphere.
The acid component of the autoclave discharge is used
to redissolve nickel, cobalt, and copper precipitates in the
primary neutralization residue, the secondary neutraliza-
tion residue and secondary mixed hydroxide residue.
The major reactions for (1) redissolution of nickel,
cobalt and copper hydroxides, (2) reaction of acid with
unreacted alkali (lime or limestone) contained in the pre-
cipitates, and (3) partial redissolution of iron and alumi-
num hydroxides is shown below:
M(OH)2 +H2SO4 =MSO4 +2H2O, M =Ni, Co, Cu
CaCO3 +H2SO4 +H2O =CaSO4·2H2O +CO2(g)
CaO +H2SO4 +H2O =CaSO4·2H2O
2M(OH)3 +3H2SO4 =M2(SO4)3 +6H2O, M =Fe, Al
The Fe/Al Precipitation Stage 1 uses an alkali (lime-
stone or lime) to neutralize acid and precipitate the major-
ity of ferric iron and aluminum in the autoclave discharge
thickener overflow. The pH of this process is controlled to
minimize any coprecipitation of nickel, cobalt, or copper
(although the circuit is designed to allow recovery of these
metals through the “releach” step using autoclave acid.
H2SO4 +CaCO3 +H2O =CaSO4·2H2O +CO2(g)
Fe2(SO4)3 +3CaCO3 +9H2O =
2Fe(OH)3 +3CaSO4·2H2O +3CO2(g)
Al2(SO4)3 +3CaCO3 +9H2O =
2Al(OH)3 +3CaSO4·2H2O +3CO2(g)
The form of iron precipitation can be as ferric hydroxide,
goethite, or jarosite, depending on the conditions of precip-
itation. The precipitation is performed at pH 2.2–3.0, and
preferably at 2.7. Sufficient time (1–8 hours) and elevated
temperature (natural temperature from the autoclave thick-
ener overflow) is used to ensure effective neutralization.
The Fe/Al Precipitation Stage 1 slurry is thickened. The
overflow goes to the copper removal tanks and the under-
flow is recycled to meet autoclave acid to ensure recovery of
nickel, cobalt, and copper.
Sodium hydrogen sulfide (NaSH) solution is dosed
to precipitate copper after removal of dissolved oxygen
through an optional nitrogen sparging process. The process
is seeded with recycle of fine copper sulfide particles to the
head of the copper removal circuit. The chemistry of the
process is shown below. The major reaction is to precipitate
CuS.
CuSO4 +NaSH =CuS +0.5Na2SO4 +0.5H2SO4
The CuS in the copper removal thickener underflow may
be filtered and washed to form a CuS saleable product.
The Fe/Al Precipitation Stage 2 removes iron and alu-
minum to low residual levels in solution using oxidation
for iron with aeration and pH adjustment using limestone.
4FeSO4 +O2 +4CaCO3 +12H2O =
4Fe(OH)3 +4CaSO4·2H2O +4CO2(g)
Al2(SO4)3 +3CaCO3 +9H2O =
2Al(OH)3 +3CaSO4·2H2O +3CO2(g)
The conditions for this stage are natural temperature (45–
75 °C) and pH 4.2–5 for 1–8 hours.
In the MHP Precipitation Stage 1, freshly prepared
MgO slurry is added to the solution to form nickel and
cobalt hydroxides.
NiSO4 +MgO +H2O =Ni(OH)2 +MgSO4
CoSO4 +MgO +H2O =Co(OH)2 +MgSO4
Any remaining iron, aluminum or copper in solution will
precipitate into the mixed hydroxide precipitate.
Pentlandite oxidation:
(Ni,Fe)9S8 +9H2SO4 +4.5O2 =
4.5NiSO4 +4.5FeSO4 +8S +9H2O
Pyrrhotite oxidation:
Fe7S8 +7H2SO4 +3.5O2 =7FeSO4 +8S +7H2O
Chalcopyrite oxidation:
CuFeS2 +2H2SO4 +O2 =
CuSO4 +FeSO4 +2S +2H2O
Sulfur oxidation:
S +1.5O2 +H2O =H2SO4
Iron precipitation:
Fe2(SO4)3 +3H2O =Fe2O3 +3H2SO4
The autoclave slurry discharge will contain dissolved
metal sulfates, acid, chloride and elemental sulfur and
iron precipitates along with any gangue minerals (or reac-
tion products of gangue minerals with acid). The slurry
discharge at elevated temperature and pressure is flashed
through a let-down system to cool the slurry to atmospheric
temperature and pressure. The flash steam and the vent
from the autoclave (to purge inert gases and excess oxygen)
is cleaned though a scrubber system and discharged to the
atmosphere.
The acid component of the autoclave discharge is used
to redissolve nickel, cobalt, and copper precipitates in the
primary neutralization residue, the secondary neutraliza-
tion residue and secondary mixed hydroxide residue.
The major reactions for (1) redissolution of nickel,
cobalt and copper hydroxides, (2) reaction of acid with
unreacted alkali (lime or limestone) contained in the pre-
cipitates, and (3) partial redissolution of iron and alumi-
num hydroxides is shown below:
M(OH)2 +H2SO4 =MSO4 +2H2O, M =Ni, Co, Cu
CaCO3 +H2SO4 +H2O =CaSO4·2H2O +CO2(g)
CaO +H2SO4 +H2O =CaSO4·2H2O
2M(OH)3 +3H2SO4 =M2(SO4)3 +6H2O, M =Fe, Al
The Fe/Al Precipitation Stage 1 uses an alkali (lime-
stone or lime) to neutralize acid and precipitate the major-
ity of ferric iron and aluminum in the autoclave discharge
thickener overflow. The pH of this process is controlled to
minimize any coprecipitation of nickel, cobalt, or copper
(although the circuit is designed to allow recovery of these
metals through the “releach” step using autoclave acid.
H2SO4 +CaCO3 +H2O =CaSO4·2H2O +CO2(g)
Fe2(SO4)3 +3CaCO3 +9H2O =
2Fe(OH)3 +3CaSO4·2H2O +3CO2(g)
Al2(SO4)3 +3CaCO3 +9H2O =
2Al(OH)3 +3CaSO4·2H2O +3CO2(g)
The form of iron precipitation can be as ferric hydroxide,
goethite, or jarosite, depending on the conditions of precip-
itation. The precipitation is performed at pH 2.2–3.0, and
preferably at 2.7. Sufficient time (1–8 hours) and elevated
temperature (natural temperature from the autoclave thick-
ener overflow) is used to ensure effective neutralization.
The Fe/Al Precipitation Stage 1 slurry is thickened. The
overflow goes to the copper removal tanks and the under-
flow is recycled to meet autoclave acid to ensure recovery of
nickel, cobalt, and copper.
Sodium hydrogen sulfide (NaSH) solution is dosed
to precipitate copper after removal of dissolved oxygen
through an optional nitrogen sparging process. The process
is seeded with recycle of fine copper sulfide particles to the
head of the copper removal circuit. The chemistry of the
process is shown below. The major reaction is to precipitate
CuS.
CuSO4 +NaSH =CuS +0.5Na2SO4 +0.5H2SO4
The CuS in the copper removal thickener underflow may
be filtered and washed to form a CuS saleable product.
The Fe/Al Precipitation Stage 2 removes iron and alu-
minum to low residual levels in solution using oxidation
for iron with aeration and pH adjustment using limestone.
4FeSO4 +O2 +4CaCO3 +12H2O =
4Fe(OH)3 +4CaSO4·2H2O +4CO2(g)
Al2(SO4)3 +3CaCO3 +9H2O =
2Al(OH)3 +3CaSO4·2H2O +3CO2(g)
The conditions for this stage are natural temperature (45–
75 °C) and pH 4.2–5 for 1–8 hours.
In the MHP Precipitation Stage 1, freshly prepared
MgO slurry is added to the solution to form nickel and
cobalt hydroxides.
NiSO4 +MgO +H2O =Ni(OH)2 +MgSO4
CoSO4 +MgO +H2O =Co(OH)2 +MgSO4
Any remaining iron, aluminum or copper in solution will
precipitate into the mixed hydroxide precipitate.