SUPPLEMENTAL INFORMATION
for
Processing History and Workplace Aerosols at Vale’s (Inco’s)
IronOre Recovery Plant
Bruce R. Conard
Metallurgical Terminology
Ore: Ore is valuable solid material that is mined. Because it is impractical and uneconomic to selectively mine the specific minerals of value, ore also contains an amount of host rock having little commercial value. In the Sudbury basin several valuable metals are intimately mixed together in the sulfidic ore and notably include Cu, Ni, and Co, but also Ag, Au, Pt, Pd, Rh, Ru, Ir and Os.
Concentrate: A concentrate is a material generated by processing the ore and which increases the fraction of a valuable element by rejecting some rock as waste and producing separate material streams of companion elements that were present in the ore. Thus, a concentrator plant (or mill or mineral beneficiation or mineral processing plant) uses ore as feed and may produce a nickel concentrate, a copper concentrate and reject material (tailings). The objective of milling is to reject rock and other low value minerals prior to roasting and smelting the higher value minerals.
Roasting (also Calcining): Roasting is a chemical process of oxidizing a material at high temperature (usually in a controlled manner). The feed material is ore or concentrate and in Vale’s case contains Ni+Cu+Co+Fe sulfides. Roasting is carried out prior to smelting to oxidize Fe preferentially with the bulk of the Ni+Cu+Co remaining as sulfides. “Dead roasting” is where virtually all of the sulfur is removed during oxidation.
Selective reduction: This is a process by which certain elemental oxidation states are chemically reduced while the oxidation states of other elements are unaffected. In solid state selective reduction, the oxidizing potential of the gas phase is controlled so that oxides of some metals are chemically reduced to the metallic state (for example, nickel oxide is reduced to nickel metal), while leaving other oxides (for example, iron oxides) unaffected.
Leaching: this is a process by which a chemical solution selectively dissolves an element while leaving other elements in the solid state.
Aerosol: A disperse system of liquid or solid particles suspended in a gas—usually air. Particle size distribution and chemical composition are two important characteristics of an aerosol. Particle size determines how rapidly a particle will settle and defines whether a particle is inhalable and, if so, where in the respiratory tract it deposits, and what fraction is exhaled without deposition. The chemical identity of a particle influences how it interacts with biological tissues it contacts.
Mineral terminology
PyrrhotiteFe1-xS with end members FeS and Fe7S8
Silica (quartz)SiO2
MagnetiteFe3O4
HematiteFe2O3
Nickel spinelNiFe2O4
WustiteFeO
AmmoniaNH3
Ammonium carbonate(NH4)2CO3
Green nickel oxideNiO
Black nickel oxideNi1-xO
Notes on company names
The International Nickel Company of Canada: Was formed in 1916 by The International Nickel Company to carry out refining of nickel-copper matte in Canada. It was commonly referred to as Inco.
Vale: A Brazilian company that purchased Inco in 2006. For several years the Canadian nickel operations went under the name Vale Inco. In 2009 these operations became a division of Vale.
Figure S1: Aerial view of the Iron Ore Recovery Plant near Sudbury,Ontario.
Roasting:
The roasting reactions took pyrrhotite to either a limited amount of magnetite (Fe3O4) or to hematite (Fe2O3) according to:
3 Fe7S8 (s) + 38 O2(g) → 7 Fe3O4(s) + 24 SO2(g)
4 Fe7S8 (s) + 53 O2(g) → 14 Fe2O3(s) + 32 SO2(g).
The overall reaction of pyrrhotite can be represented as:
Fe7S8 (s) + 79/6 O2(g) → 1/3 Fe3O4 (s) + 3 Fe2O3 (s) + 8 SO2(g).
The Ni in pyrrhotite was oxidized to NiO:
Ni (in Fe7S8) +0.5 O2 (g) → NiO (s)
and the NiO combined with Fe2O3 to form nickel spinel:
NiO (s) + Fe2O3 (s) → NiFe2O4 (s).
Roasting was carried out in Fluid Bed Roasters. A schematic diagram showing how the FBRs were interfaced with waste heat boilers, calcine sizing capability, and electrostatic precipitators for gas cleaning, is shown below.
Figure S2: Arrangement of FBR and auxiliary equipment for roasting and product capture (from Boldt, 1967).
Cyclones and ESPs are gas treatment units that remove particulate. In a cyclone (see Figure S3) dust-laden gas enters a cyclindrical or conical chamber tangentially. Particulate is forced by centrifugal forces to the wall of the chamber and the larger (heavier) particles swirl downward and exit the chamber at the bottom, while gas and the lighter particles exit the top of the chamber.
Figure S3: Cut-away schematic of a cyclone (from Boldt, 1967).
ESPs are units that are designed to recover particulate (even fine particulate) at a very high efficiency (see a cut-away view in Figure S4). They work on the principle of ionizing gaseous molecules in the gas stream. These ionized molecules are then attracted toward a grounded electrode, but they collide with particles and exchange their charge with particles in the gas stream. These charged particles are also attracted to the grounded electrode and stick to it. By vibrating the grounded electrode periodically, the attached dust layer can be shaken off and falls into a collection bin underneath the electrodes.
Figure S4: Cut-away view of an electrostatic precipitator (ESP) (from Boldt, 1967).
Chemical reduction in the solid state:
Reduction of the roasted calcine was carried out in rotary kilns, a schematic of which is shown in Figure S5.
Figure S5: Schematic of an IORP reduction kiln (from Boldt, 1967).
The kiln reduced nickel ferrite to Ni metal:
NiFe2O4 (s) + CO (g) → Ni (s) + Fe2O3 (s) + CO2 (g).
It reduced hematite to magnetite:
3 Fe2O3 (s) + CO (g) → 2 Fe3O4 (s) + CO2 (g),
and reduced wustite to metallic iron:
FeO (s) + CO (g) → Fe (s) + CO2 (g)
and Ni and Fe formed an alloy:
Ni (s) + Fe (s) → NiFe (s),
but the kiln was operated in a way that limited the formation of wustite (FeO), which upon cooling would decompose to metallic iron and magnetite (metallic Fe was not desired in the kiln product because it would leach with the reduced Ni and would subsequently need to be eliminated). The control of the extent of reduction was obtained by firing the kiln in a concurrent fashion. This meant that the hottest end of the kiln (where the burner was located) was the same end where the feed entered. The feed rotated with the kiln and, due to the slight incline of the kiln, gradually moved to the exit end where the gases were also exiting the kiln. This manner of operation meant that the temperature was slowly lowered as the solids moved down the kiln and the amount of wustite forming was limited, as shown in Figure S6. The maximum temperature of the solids was maintained at about 870oC with a natural gas burner operating in an oxygen-deficient mode so that the CO2/CO ratio was about 1.3 at the hot end and about 1.6 at the cooler (700oC) end.
Figure S6: Reduction parameters along the length of an IORP reduction kiln (from Boldt, 1967).
At the discharge end of the kiln the solids drop into air-cooled coolers that rapidly dropped the temperature of the solids to 200oC. At this point 90% of the Ni was present either as an Fe-Ni alloy or as a sulfide. Upon exiting the special coolers, the reduced calcine discharged into a mechanically agitated water quench tank so that re-oxidation of the calcine would be minimized. A slurry at 30% solids was pumped from the quench tanks to the leaching building.
Off-gases from the kilns were sent through a heat-exchanged where the heat was used to pre-heat combustion air for the burner. Off-gases then went to cyclones and water scrubbers to remove dust. Then the combustibles were ignited prior to the gas being vented to the main stack.
Leaching of nickel:
Leaching of the reduced calcine was carried out using aerated ammonia-ammonium carbonate solutions in agitated cylindrical tanks. A schematic of such a tank is shown in Figure S7.
Figure S7: Cut-away view of a mechanically agitated leaching tank.
The Fe-Ni alloy present in the reduced calcine leached rapidly in the first two stages according to:
NiFe (s) + O2 (gas) + 8 NH3(aq) +3 CO2 (aq) + H2O →
Ni(NH3)6++ + Fe++ + 2 NH4+ + 3CO3--.
As the leaching progressed, the Fe++ was further oxidized to Fe+++ and precipitated in these stages:
4 Fe++ + 2 H2O + 8 OH- →4 Fe(OH)3 (s).
Each leaching stage had its oxidation potential controlled by the amount of air introduced. During alloy dissolution, this potential was about -500mV (vs. sat’d calomel electrode). Subsequent sulfidic leaching was carried out in stages 3 to 5 at -150 mV and 55oC:
4 NiSFeS + 9 O2 + 24 NH3 (aq) + 2 H2O →
4 Ni(NH3)6++ + 4 S2O3-- + 2 Fe2O3H2O.
Copper removal:
Copper was removed from the pregnant solution by adding sodium hydrosulfide:
Cu(NH3)4++ + HS- + OH- → CuS(s) + 4 NH3(aq) + H2O.
This sulfide was thickened, filtered, dried and sent to the smelter for Cu recovery.
Precipitation of nickel:
Nickel was precipitated as a basic nickel carbonate as live steam was used to remove ammonia and carbon dioxide:
2Ni(NH3)2++ + 2 Na2CO3 + H2O → Ni(OH)2NiCO3(s) + 3Na+ + 4NH3(g) + CO2 (g)
Nickel oxide formation:
Decomposition of basic nickel carbonate by heating and removing the contained water and carbon dioxide was done in a rotary kiln:
Ni(OH)2NiCO3 (s) → 2 NiO(s) + H2O+ CO2
Production of hematite pellets:
Magnetite was heated and oxidized to hematite:
2Fe3O4(s) + ½O2(g) → 3Fe2O3(s).
This was carried out in a travelling grate furnace shown below.
Figure S8: Schematic of travelling grate induration furnace for oxidizing magnetite to hematite.
Acid plant operation:
Sulfur dioxide was oxidized to sulfur trioxide using a vanadium-based catalyst:
SO2(g) + ½O2(g) → SO3(g).
The sulfur trioxide was then combined with water in sulfuric acid:
SO3(g) + H2O(l, in H2SO4) = H2SO4(l).
References
Boldt JR, Jr. 1967. The Winning of Nickel, Longmans Canada Ltd: Toronto.