A fundamental evaluation of the atmospheric pre-leaching section of the nickel-copper matte treatment process
Date
2007-03
Authors
Lamya, Rodrick Mulenga
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
Nickel-Copper sulphide ores are the most important Platinum Group Metal bearing
ores. The South African deposits are exceptionally rich in the platinum group
metals (PGMs) and production of the PGMs is the primary purpose of treating these
ores. The methods used in the recovery of the PGMs from the nickel-copper ores
generally consists of ore concentration by physical techniques, pyrometallurgical
concentration and hydrometallurgical extraction of the base metals followed by the
PGMs. Pyrometallurgical concentration produces Ni-Cu matte, which is treated by
hydrometallurgical processes to recover the nickel, copper, cobalt and the precious
metals.
In this study, the leaching behaviour of a Ni–Cu matte in CuSO4–H2SO4 solution
during the repulping (pre-leach) stage at Impala Platinum Refineries was studied.
The repulping stage is basically a non–oxidative atmospheric leach stage, in which
nickel, iron and cobalt are partially dissolved, while the copper is precipitated. To
understand the nature of the leaching process during this stage of the base metal
refining operation, the effects of variations in the key process variables such as
temperature, stirring rate, particle size, pulp density, residence time, initial copper
and acid concentrations were investigated. The pre-leached matte was then
pressure leached to ascertain the effect of process conditions in the pre-leach stage
on the subsequent pressure leach stage.
It was found that the leaching mechanism entails dissolution of metal alloys out of
sulphide minerals with transformation of Ni3S2 to NiS. Aqueous copper precipitates
as metallic copper and as chalcocite. The matte is leached by both acid and the
cementation process, especially in the early stage when the Cu2+ ions are present.
Galvanic interaction of the sulphide minerals and/or the Ni alloy also enhances the
leaching process. The leaching kinetics of Ni was characterized with the shrinking
core model and was found to be controlled by diffusion through surface layer. An
activation energy of 31 kJ/mol was obtained, which also suggested a diffusion
controlled leaching reaction. Atmospheric leaching tests indicated that Ni extraction increased slightly in the temperature range 50 – 60 oC, however no
significant increase was observed from 60 to 80 oC, probably because the leaching
process was found to be diffusion controlled. The slight increase in nickel
dissolution at higher temperatures (>60 oC) may be attributed to the
transformation of Ni3S2 to NiS, which is easier to leach. Co extraction appeared to
be insensitive to temperature changes, while Fe extraction was low at 50 oC but
increased significantly at 60 – 80 oC. The Ni extraction increased gradually with
increase in the stirring rate from 145 to 400 rpm while Co and Fe extractions were
insensitive at 145 and 205 rpm, but increased substantially at 400 rpm. This was
probably due to increased mass transfer rate and transformation of Ni3S2 to NiS.
With pulp density, Ni and Co extractions appeared to be insensitive to changes in
the pulp density as only a slight increase in extractions was observed when the
density was reduced from 1.7 kg/L to 1.6 kg/L. Similar iron extractions were
achieved at 1.7 and 1.75 kg/L but increased significantly at 1.6 kg/L. It was found
that Ni and Co extractions were not significantly affected by changes in the particle
size, probably because metal alloys were liberated and hence exposed to the
leaching solution. Iron extraction could not be determined accurately because of
iron precipitation at pH above 3. Generally the leaching of metals did not depend
on the initial copper concentration in the investigated range of 25 - 48 g/L Cu. It
was also observed that initial acid concentration did not have an effect on Ni
extraction, probably due to the fact that most of the nickel was leached by the
process of cementation. However, Co and Fe extractions increased when the acid
increased from 90 g/L to 110 g/L, but no further increase was noted at 125 g/L. A
residence time of 5 hours was found to be adequate as there was no significant
increase in metal extractions when the residence time was increased beyond 5
hours. As much as 20% Ni, 40% Co and 80% Fe can be extracted from the Ni-Cu
matte during the repulping stage of the leaching process studied, provided the
investigated conditions prevail.
Generally the rate of Cu cementation increased with increasing temperature and
pulp density, but decreased with an increase in particle size, acid and copper concentrations. The rate of stirring did not affected Cu cementation. It was found
that aqueous copper precipitated from the solution within 90 minutes when the
temperature was raised to 80 oC. However, under the present pre-leach
temperature of about 60 oC complete Cu cementation can only be achieved after
about 5 hours. The cementation reaction was found to follow a mixed control
mechanism, with two distinct activation energies namely 18.2 kJ/mol at 70 – 80
oC and 74.6 kJ/mol at 50 – 70 oC. This suggested that the rate of cementation
reaction is probably controlled by a boundary layer diffusion mechanism at higher
temperatures. At low temperatures the rate is probably controlled by a surface
reaction mechanism.
The pressure leaching experiments, which were aimed at investigating the response
of pre-leached matte to the subsequent pressure leaching process, showed that Ni
extractions were similar for the investigated pre-leach temperature of 50 – 80 oC
and stirring rate of 145 – 400 rpm. For the pre-leach stage conditions of 60 – 80 oC
and 205 – 400 rpm Ni3S2 was transformed into NiS, which is easier to leach in the
pressure leaching stage. However, because of the aggressive conditions prevailing
in the pressure leaching stage, all the nickel minerals were leached at about the
same rate. In the case of pulp density, Ni extraction was comparable for all the
investigated pulp densities (1.6 – 1.75 kg/L). This was probably due to the fact that
Ni3S2 transformed to NiS in the pre-leach stage. It was found that Ni extraction
increased with increasing residence time for the investigated time of 1 hour to 9
hours, probably due to the changes in the mineral phases of the matte as indicated
above. The copper minerals (Cu2S and Cu1.96S) transformed into Cu2S and Cu1.8S
with aqueous copper being precipitated, and were not leached under the applied
conditions. All the cobalt and iron dissolved in the pressure leaching stage.
A semi-empirical kinetic model was developed for the pre-leaching stage. A
comparison of the model predictions and the experimental data for the dissolved
species during the batch leaching process showed that the model can satisfactorily
fit the trends in the leaching of the metals.
Description
Thesis (PhD (Process Engineering))--University of Stellenbosch, 2007.
Keywords
Smelting, Nickel, Copper, Dissertations -- Process engineering, Theses -- Process engineering