Process design considerations for the fluidized bed technology applications in the nickel industry
A number of fluidized bed applications in nickel extraction have been developed for commercial use. Five important processes have gained particular acceptance, namely, sulphide ore roasting, nickel sulphide (matte) roasting, nickel oxide reduction, laterite ore pre-reduction, and nickel chloride pyrohydrolysis. These applications all benefit from the fast reaction kinetics, high heat and mass transfer rates, and the excellent controllability of fluidized bed reactors. This paper examines the design and modelling issues surrounding the above-mentioned applications. Based on the open-literature data, computerized modelling is used to provide generic flowsheets that illustrate the key process features.
Most of the nickel applications use the so-called ‘bubbling’ fluidized beds. Bubbling fluidization which occurs when the gas velocity through the bed is more than the minimum to suspend it, with most of the excess gas moving through in bubble-like pockets. Bubbling beds normally operate at 3 to 30 times the minimum fluidization velocity, which increases with the particle size and decreases with temperature.
Partial Roasting of Sulphide Ores
Partial roasting of sulphide ores has been extensively used by the industry as most of the developed nickel deposits are of the sulphide type. Fluid bed roasting oxidizes the maximum amount of iron sulphide to iron oxide, while leaving the nickel and copper as sulphides.
3 Fe7 S8 + 38 O2 Æ 7 Fe3O4 + 24 SO2
The roaster product is fed to a smelter, where iron oxide reacts with a siliceous flux to form a molten slag that is separated from the Ni/Cu sulphide matte. In one process, the matte is slow-cooled to segregate the sulphide phases and the nickel sulphide phase is recovered by physical beneficiation as a fine concentrate.
Nickel Sulphide Roasting
The nickel sulphide concentrate can be roasted to nickel oxide, using either air or oxygen-enriched air, in a fluid bed.
2 Ni3S2 (s) + 7 O2 Æ 6 NiO (s) + 4 SO2 (g)
Nickel Oxide Reduction
From nickel oxide, a metallic product can be formed by direct reduction in another fluid bed. This reaction occurs at temperatures as low as 350°C and can use carbon monoxide and hydrogen. In the case of hydrogen, the following reaction occurs:
NiO (s) + H2 Æ Ni (s) + H2O (g)
Laterite Ore Pre-reduction
Oxide ores (e.g. laterite) can be smelted for metal recovery, using electric furnaces to produce ferronickel alloys. To conserve electricity, the ore is dried, calcined, and partially reduced before the furnace. Traditionally, the ore preheating and pre-reduction is done in large rotary kilns, however, excellent heat and mass transfer in the fluid beds is encouraging its utilization in the new plants. The dominant pre-reduction reactions are:
NiO (s) + CO (g) Æ Ni (s) + CO2 (g)
Fe2O3 (s) + CO (g) Æ 2 FeO (s) + CO2 (g)
Nickel Chloride Pyrohydrolysis
Nickel ores can also be processed by hydrometallurgical means resulting in an aqueous nickel chloride product. Fluidized bed treatment (pyrohydrolysis) of nickel chloride has been used to yield nickel oxide and recover the hydrochloric acid reagent.
NiCl2 (a) + H2O Æ NiO (s) + 2HCl (g)