METHOD FOR THE PRECIPITATION OF NICKEL

PROCEDE DE PRECIPITATION DE NICKEL

English Abstract

A method (10) for the recovery of nickel and cobalt from leach solutions in
the presence of iron and/or chrome, the method comprising the steps of: i)
adding a reductant (13) to a leach solution containing nickel, cobalt and
iron, such that any iron present as ferric sulphate is reduced to ferrous
sulphate and/or any hexavalent chrome is reduced to trivalent chrome; ii)
neutralising (14) at least a portion of the free acid through addition of a
neutralising agent; iii) further addition of the reducing agent (15) to ensure
all iron present remains in the ferrous form and/or any chrome remains in the
trivalent form; iv) heating the solution prior to mixed sulphide
precipitation; v) adding a mixed sulphide seed (21) and hydrogen sulphide (22)
to effect precipitation (20) of the nickel and cobalt in the form of a mixed
sulphide product (24); and vi) maintaining this mixture in the presence of
hydrogen sulphide (22) for the required residence time to effect complete
precipitation of the mixed sulphide product (24).

French Abstract

L'invention concerne un procédé (10) de récupération de nickel et de cobalt dans des solutions de lixiviation en présence de fer ou de chrome, ledit procédé consistant i) à ajouter un agent (13) de réduction à une solution de lixiviation contenant du nickel, du cobalt et du fer, de manière que toute quantité de fer sous forme de sulfate ferrique soit réduite en sulfate ferreux et/ou toute quantité de chrome hexavalent soit réduite en chrome trivalent; ii) à neutraliser (14) au moins une partie de l'acide libre par ajout d'un agent de neutralisation; ii) à ajouter un agent (15) de réduction afin de garantir que tout le fer présent conserve sa forme ferreuse et/ou le chrome conserve sa forme trivalente; iv) à chauffer la solution avant la précipitation du sulfure mélangé; v) à ajouter un germe (21) de sulfure mélangé et un sulfure d'hydrogène (22) pour réaliser la précipitation (20) du nickel et du cobalt sous la forme d'un produit de sulfure mélangé (24); et (vi) à maintenir ce mélange en présence du sulfure d'hydrogène (22) pendant le temps de séjour nécessaire, afin de permettre la précipitation complète du produit de sulfure mélangé (24).

Claims

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The Claims Defining the Invention are as Follows:
1. A method for the recovery of nickel and cobalt from leach solutions in the
presence of iron and/or chrome, the method comprising the steps of:
i) adding a reductant to a leach solution containing nickel,
cobalt and iron, such that any iron present as ferric sulphate
is reduced to ferrous sulphate and/or any hexavalent chrome
is reduced to trivalent chrome;
ii) neutralising at least a portion of the free acid through
addition of a neutralising agent;
iii) further addition of the reducing agent to ensure all iron
present remains in the ferrous form and/or any chrome
remains in the trivalent form;
iv) heating the solution prior to mixed sulphide precipitation;
v) adding a mixed sulphide seed and hydrogen sulphide to
effect precipitation of the nickel and cobalt in the form of a
mixed sulphide; and
vi) maintaining this mixture in the presence of hydrogen
sulphide for the required residence time to effect complete
precipitation of the mixed sulphide product.
2. A method according to claim 1, wherein the reductant in step (i) and step
(iii)
comprises one or more of hydrogen sulphide, sodium hydrogen sulphide, or
sulphur dioxide.
3. A method according to claim 1 or 2, wherein the free acid concentration is
within the range of about 0.5 g/L to 3.5 g/L, after neutralisation.

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4. A method according to any one of claims 1 to 3, wherein the neutralising
agent of step (ii) comprises any one or more of limestone, lime and calcrete.
5. A method according to any one of the preceding claims, wherein the
reduction
of step (i) occurs at less than about 100 °C and ambient pressure.
6. A method according to any one of the preceding claims, wherein the ferric
sulphate concentration of the resulting solution from step (iii) is less than
1 g/L.
7. A method according to any one of the preceding claims, wherein the
oxidation
potential of the solution resulting from step (i) through to (vi) is
maintained
between about 300mV and 400mV (measured against a Pt-Ag/AgCl reference
electrode).
8. A method according to any one of the preceding claims, wherein the solution
temperature is in the range of about 80°C to 120°C when sulphide
seed is
added.
9. A method according to any one of the preceding claims, wherein the
concentration of sulphide seed in solution is in the range of about 10g/L to
100g/L, such that the total seed surface area is between about 1m2/L and
10m2/L.
10. A method according to any one of the preceding claims, wherein the
hydrogen sulphide overpressure in step (v) and step (vi) is maintained within
about 100kPa and 400kPa.
11. A method according to any one of the preceding claims, wherein the
residence time of step (vi) is between about 0.25 to 4 hours.
12. A method according to any one of the preceding claims, wherein the
concentration of nickel and cobalt in the leach solution is in the range of
about
1g/L to 10g/L and about 0.1 g/L to 2 g/L, respectively for a nickel laterite

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solution, or within the range of 10 g/L to 50 g/L and 2g/L to 10 g/L,
respectively for a nickel sulphide solution.
13. A method according to any one of the preceding claims, wherein the iron
concentration in the leach solution is within the range of about 0.5g/L to
15g/L.
14. A hydrometallurgical method for the for the recovery of nickel and cobalt
from
leach solutions in the presence of iron and/or chrome, substantially as
hereinbefore described with reference to Figure 1.
15. A hydrometallurgical method for the for the recovery of nickel and cobalt
from
leach solutions in the presence of iron and/or chrome, substantially as
hereinbefore described with reference to Example 1.

Description

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"Method for the Precipitation of Nickel"
Field of the Invention
The present invention relates to a method for the precipitation of nickel.
More
particularly, the present invention is a hydrometallurgical method for the
preferential precipitation of nickel and cobalt sulphides from solutions
containing
iron and/or chrome. Additionally, the method is intended to substantially
avoid the
formation of sulphide scale during the precipitation process.
Background Art
Nickel and cobalt are typically recovered from leach solutions by contacting
the
pregnant liquors with a suitable reductant such as hydrogen sulphide. It is
known
that iron and chrome will tend to co-precipitate as a sulphide under
conventional
hydrogen sulphide precipitation conditions. Such co-precipitation is
undesirable
for the detrimental effect on product quality and the demands placed on
downstream processing of the mixed sulphide product.
In metallurgical circuits incorporating the high pressure acid leaching of
nickel
laterites, iron is most often rejected as a ferric oxyhydroxide (typically as
a
goethite) and as a hematite product . In some situations iron is also rejected
as a
jarosite product.
Unfortunately, the rejection of iron as a ferric oxyhydroxide results in
significant
co-precipitation of the valuable nickel and cobalt products. This either
results in
metallurgical losses or necessitates the reprocessing of the iron residue in
order
to recover the valuable nickel and cobalt.
Hematite is the most acceptable iron product for intermediate storage or
disposal,
because of its high thermodynamic stability, its high density (4.9 to 5.3
g/cm3), its
high iron content (60% - 70%) and its low adsorption of water and base metals.

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However, the rejection of iron as a hematite product in the high pressure acid
leaching processes used for nickel laterites necessitates the use of
temperatures
in the order of 250 C and pressures in the order of 45 Bar. This process by
its
very nature involves capital intensive equipment which is itself expensive to
maintain and has associated high operating costs.
The rejection of iron as jarosite, whilst resulting in lower losses of nickel
and
cobalt from the improved separation coefficients, is an expensive process as
it
typically requires the use of a suitable cation (such as ammonia) together
with
elevated temperatures and pressures.
In addition to the losses of nickel and cobalt associated with ferric
oxyhydroxide
and ferric hydroxide formation, the major problems of this process are the
lower
filtration rates, thickener settling characteristics and the thickener
underflow
densities achievable. Ferric hydroxide in particular, and to a lesser extent
ferric
oxyhydroxide, tends to have an open structure. This results in the
incorporation of
large amounts of base metals during settling. The solids produced also have a
low density and low thickener underflow densities impact on both downstream
processing equipment and the volume of tailings which needs to be disposed of.
In International Patent Application PCT/AU2003/001037 (WO 2004/016816) there
is disclosed a process for preferential precipitation of nickel and cobalt
from
solutions containing iron through the addition of a reductant to reduce ferric
ions
to ferrous ions. This reaction generates acid which must be neutralised,
before
adding seed particles in the presence of further reductant to precipitate the
nickel
and cobalt. However, typically with this process significant scaling can
occur,
which adversely affects both the precipitation kinetics and recovery of the
nickel
and cobalt sulphide product.
The present method has as one objective thereof to substantially overcome the
problem of scaling, whilst also providing the advantage that the incidence of
iron
sulphide co-precipitation is reduced, or to at least provide a useful
alternative to
prior art methods.

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The preceding discussion of the background art is intended to facilitate an
understanding of the present invention only. It should be appreciated that the
discussion is not an acknowledgement or admission that any of the material
referred to was part of the common general knowledge in Australia as at the
priority date of the application.
Throughout the specification, unless the context requires otherwise, the word
"comprise" or variations such as "comprises" or "comprising", will be
understood to
imply the inclusion of a stated integer or group of integers but not the
exclusion of
any other integer or group of integers. Further, reference to "seed surface
area"
or variations thereof will be understood as based on the assumption that the
seed
particles are perfect spheres at the D50 particle diameter.
Disclosure of the Invention
In accordance with the present invention there is provided a method for the
recovery of nickel and cobalt from leach solutions in the presence of iron
and/or
chrome, the method comprising the steps of:
(i) adding a reductant to a leach solution containing nickel, cobalt and
iron, such that any iron present as ferric sulphate is reduced to
ferrous sulphate and/or any hexavalent chrome is reduced to
trivalent chrome;
(ii) neutralising at least a portion of the free acid through addition of a
neutralising agent;
(iii) further addition of the reducing agent to ensure all iron present
remains in the ferrous form and/or any chrome remains in the
trivalent form;
(iv) heating the solution prior to mixed sulphide precipitation;

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(v) adding a mixed sulphide seed and hydrogen sulphide to effect
precipitation of the nickel and cobalt in the form of a mixed sulphide;
and
(vi) maintaining this mixture in the presence of hydrogen sulphide for the
required residence time to effect complete precipitation of the mixed
sulphide product.
Preferably, the reductant in (i) and (iii) comprises one or more of hydrogen
sulphide, sodium hydrogen sulphide, or sulphur dioxide.
After neutralisation, free acid concentration is preferably within the range
of about
0.5 g/L to 3.5 g/L.
The neutralising agent of step (ii) may comprise any one or more of limestone,
lime and calcrete.
The reduction of step (i) preferably occurs at less than about 100 C and
ambient
pressure.
Preferably, the resulting ferric sulphate concentration of the resulting
solution of
step (iii) is less than about 1 g/L.
Preferably, the oxidation potential of the solution resulting from step (i)
through to
(vi) is maintained between about 300mV and 400mV (measured against a Pt-
Ag/AgCl reference electrode) to ensure no oxidation of ferrous sulphate to
ferric
sulphate occurs.
Preferably, the solution temperature is in the range of about 80 C to 120 C
when
sulphide seed is added.
Still preferably, the concentration of seed in solution is in the range of
about 10g/L
to 100g/L and the total seed surface area is between about 1 m2/L and 10m2/L.

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Hydrogen sulphide overpressure in step (v) and step (vi) is preferably
maintained
between about lOOkPa and 400kPa in order to produce the mixed sulphide
product.
Preferably, the residence time of step (vi) is between about 0.25 to 4 hours
is
employed to ensure complete precipitation of the mixed sulphide. Still
preferably,
the residence is between about 0.5 and 1.5 hours.
The concentration of nickel in the leach solution is preferably in the range
of about
1 g/L to 50 g/L, and cobalt within about 0.1 g/L to 10 g/L.
Still preferably, the concentration of nickel is in the range of about 1 g/L
to 10 g/L
for a nickel laterite solution, or between about 10 g/L and 50 g/L for a
nickel
sulphide solution. Cobalt concentrations are preferably within the range of
about
0.1 g/L to 2 g/L, and about 2 g/L to 10 g/L respectively.
Iron concentration in the leach solution is preferably in the range of about
0.5g/L
to 15gIL.
Brief Description of the Drawings
The present invention will now be described, by way of example only, with
reference to one embodiment thereof and the accompanying drawing, in which:-
Figure 1 is a diagrammatic representation of a flow sheet depicting a
method for the precipitation of nickel and cobalt from leach solutions in the
presence of iron in accordance with the present invention.
Best Mode(s) for Carrying Out the Invention
In Figure 1 there is shown a hydrometallurgical method 10 for precipitating
nickel
and cobalt from pregnant leach solutions, also containing iron, obtained from
the
high pressure acid leach of a nickel laterite ore. The nickel concentration is
in the
range of 1 g/L to 10 g/L. Cobalt concentrations are within the range of 0.1
g/L to 2
g/L. Iron concentration in the leach solution is in the range of about 0.5-
15g/L.

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The method 10 of the present invention comprises passing such a pregnant leach
solution to a pre-reduction step 12 in which hydrogen sulphide gas 13 is
sparged
through the solution at a temperature of less than 100 C. Iron present as
ferric
sulphate (Fe2(SO4)3) is reduced to ferrous sulphate (FeSO4) such that the
resulting ferric concentration is less than 1 g/L.
The solution from the pre-reduction circuit 12 then undergoes neutralisation
14
using a calcrete slurry to reduce the free acid (FA) concentration to between
about 0.5 and 3.5 g/L. It is understood that if the oxidation potential of the
solution
is not controlled, then ferric can form during neutralisation 14.
Consequently,
following neutralisation 14 a stream of hydrogen sulphide gas 15 is again
passed
through the solution from the pre-reduction circuit 12 in an additional
reduction
step 16, to ensure that the oxidation potential is within the range of about
300 to
400 mV (Pt-Ag/AgCl reference electrode), for example 350-380 mV.
A pre-heating step 18 raises the temperature of the solution from the
reduction
step 16 to between about 80 and 120 C in preparation for a subsequent
precipitation step 20. A mixed sulphide seed 21 in the range of 10 g/L to 100
g/L
is introduced to the solution prior to the introduction of hydrogen sulphide
gas 22.
The total seed surface area is between about 1 m2/L and 10m2 /L. Hydrogen
sulphide gas 22 is introduced at an overpressure of 100-400 kPa to precipitate
a
mixed sulphide product 24. This is maintained for the duration of the
residence
time, between about 0.25 to 4 hours, for example 0.5 to 1.5 hours, in order to
effect complete conversion to the mixed sulphide product 24.
The pre-heating step 18 allows the precipitation step 20 to occur within
acceptable
commercial parameters by increasing the kinetics of the precipitation
reactions
and also allows dissolved H2S to be driven off.
Neutralisation may also be effected using any one of lime, limestone, ammonia
or
caustic.
The mixed sulphide precipitate contains nickel in the range of 50 to 55%,
cobalt at
3 to 5% and iron at 1 to 3% wt/wt.

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The method of the present invention will now be described with reference to an
example that is to be understood as non-limiting.
Example 1- Reduction of Scaling
Two feed solutions containing high iron levels were treated by pre-reduction
with
H2S gas then passed through a neutralisation circuit to reduced residual free
acid
concentration to 1.4 g/L using calcrete slurry. The composition of the feed
solutions is set out in Table 1 below:
Table 1: Composition of Mixed Sulphide Feed Solution 1.
Element Concentration (mg/L)
Solution 1 Solution 2
Ni 4092 3993
Co 324 274
Fe (total) 12478 11700
Fe (ferrous) 12371 11700
Solution 1 proceeded directly to mixed sulphide precipitation, whilst the Eh
of
Solution 2 was first reduced to 350-380 mV (Pt-Ag/AgCI reference electrode)
with
H2S to ensure all iron in ferric form was converted to ferrous before heating
the
solution in preparation for sulphide precipitation.
A further test was employed with Solution 2 in which a higher addition of
mixed
sulphide seed was added, refer to Table 2 below.
The results for scale formation during precipitation from each solution are
given in
Table 2. It is quite clear that the rate of scale growth is significantly
reduced when

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the second reduction step is incorporated into the flowsheet. This effect is
enhanced by a further addition of seed to the solution.
Table 2: Effect on Scale Growth with Second Reduction Step and Seed Addition.
Solution Seed Addition m2/L Scale Growth Rate
PLS mm/week
Solution 1 0.78 11.1
Solution 2 0.88 3.7
Solution 2 3.07 0.3
It is envisaged that the method of the present invention may be applied to the
recovery of nickel and cobalt from nickel sulphide leach solutions. In such
circumstances it is typical that the pregnant leach solution have a nickel
concentration in the range of about 10 g/L to 50 g/L, and a cobalt
concentration of
about 2 g/L to 10 g/L. Again, iron concentration is typically in the range of
about
0.5 to 15g/L.
It is to be understood that the method of the present invention is equally
applicable to leach solutions containing chrome.
Modifications and variations such as would be apparent to the skilled
addressee
are considered to fall within the scope of the present invention.

Representative Drawing