Note: Descriptions are shown in the official language in which they were submitted.
~04Q8~7
The present invention relates to improvements
in a process for the separation of iron from zinc solutions
or slurries.
In the hydrometallurgical production of zinc
a zinc sulfate liquor is produced by leaching the "calcine"
(essentially zinc oxide) resulting from the roasting of
sulfide ores. This liquor usually contains some iron
and prior to the electrowinning of zinc from the liquor it
is necessary to remove any iron present. In general, the
removal of iron involves firstly the oxidation of any
ferrous iron to the ferric state and subsequently preci-
pitation of the ferric iron.
A known method of oxidizing ferrous iron to
ferric iron in ~olutions such as nickel and cobalt sulfates
involves the use of sulfur dioxide/oxygen mixtures. This
method is described in U.S. Patent No. 2,816,819. However
while this method is well-known in the context of the
extraction of nickel or cobalt, it has never been considered
in the context of zinc production. This may be due to the
2Q fear of build up of sulfate ions in the electrolyte which
is continuously recirculated during the leaching/electro-
winning process.
Whatever the reasons may be, it is a fact thatthe oxidation process described in U.S. Patent No. 2,816,819,
has never been advocated for use in zinc solutions or slurries
and thus its effectiveness in enabling the removal of iron
down to the desired very low levels in such media has never
heretofore been determined. Instead in present zinc
refining practice the oxidation of ferrous iron is commonly
performed with the aid of manganese dioxide. While the cost
,, ' ,
104~)8f~
of this oxidant is low, a practical disadvantage of the
procesQ is the tendency for the liquor to pick up manga-
nese which subsequently precipitates anodically during
the electrowinning.
Alternative oxidants for achieving the ~errous
to ferric oxidation include potassium permanganate but
this has not gained widespread use for economic reasons.
The use of air as an oxidant ha~ been considered but it
has been reported that practicable rates of oxidàtion are
achieved only at pH values of 5 or more. Working at
such pH's is undesirable due to the possibility of hydro-
lizing any copper values present.
It has been no~ found that suitably chosen gaseous
mixtures of sulfur dioxide and oxygen are capable of
oxidizing the iron in a zinc sulfate liquor or slurry at
a practicable rate, provide an improved method of oxidizing
the iron to enable its removal down to very low levels
without introducing undesirable ions, such as manganese,
into the system. Moreover it has been found that the use
of such gaseous mixtures does not cause significant suIfate
ion build up, and that any build up which does occur aan
be controlled bleeding such as precipitation of calcium
sulfate, or preferably precipitation of some or all of the
oxidized iron as a basic ferric sulfate.
According to the invention, in a process for
separating iron from a zinc sulfate solution or slurry
wherein any ferrous iron present is converted to ferric
iron and thereafter separated by precipitation, the im- -
provement comprises adjusting the solution pH, if necessary,
to a value between 0.5 and 4.0 and the solution temperature
104~)8~7
to at least 50C while contacting the solution with a
gaseous mixture containing oxygen and sulfur dioxide,
the volume ratio of oxygen to sulfur dioxide being between
400:1 and 1:1, to effect oxidation of the iron.
The sulfur dioxide~oxygen mixture is effective
for oxidizing iron whether the lattex be entirely in
solution or wholly or partly in ~olid form as a precipitate
in the slurry. While a wide range of gas mixture composi-
tions can be used for the oxidation, i.e. an oxygen to
sulfur dioxide ratio from as low as 400:1 to as high as
the theoretical maximum of 1:1, the reaction kinetics may
be unsatisfactory at very low sulfur dioxide contents. A
preferred gas composition is one where the oxygen to sulfur
dioxide ratio ~ls betweeh 39:1 and 9:1, e.g. a gas
mixture containing 5~ by volume of sulfur dioxide, the
balance being oxygen.
The solution should be maintained at a temperature
of at least 50C, commensurate with a satisfactory reaction
rate. In general temperatures within the range 85C-95a~.
Advantageously the pH of the solution is adjusted to a
value of from 1.8 to 3Ø
Where the pH of the solution is maintained at
a value of 3.5 or higher during the oxidation, the ferric
iron will precipitate in the course of the oxidation as a
hydroxide. Where the pH is maintained during the oxidation
at a value of say 1.8-~.0, it is necessary to adjust the
pH subsequent to oxidation in order to precipitate the
oxidized iron. This adjustment may consist of adding
calcine or otheswise raising the pH to 3.5 to precipitate
~erric h~xxldo, Alternatively, the solution can be
.
~, . ,, , , ~
1040867
acidified to a pH of 1.5 or less in the presence of
potassium, sodium or ammonium ions thereby precipitating
a basic ferric sulfate.
According to one aspect of the present invention
substantially all the oxidized iron is precipitated as
a hydroxide. If this process is used on a continuous
basis whereby the spent solution is recycled to the
leaching operation, there is a need for preventing sulfate
build up in the solution. This tends to occur because of
the presence of some zinc sulfate in the calcine, and,
possibly to a slight extent, because of the use of sulfur
dioxide/oxygen mixtures for the oxidation. In order to
control the sulfate ion level in the recycled solution,
recourse is had to selective bleeding operations where
sulfate is precipitated, for example as gypsum by adding
lime to the solution, or as a basic ferric sulfate as
described above.
According to another aspect of the invention
substantially all the iron present is precipitated as a
basic ferrisulfate. The conditions for effecting such a
precipitation are well ~nown and are described, for example,
in U.S. Patent No. 3,434,947. Such a precipitation causes
the liquid to become depleted with respect to sulfate ions.
Therefore, if the liquid is to be recirculated continuously
in the leaching/electrowinning cycle, it will be necessary
from time to time to replenish the electrolyte by addition
of sulfuric acid thereto. ~-
According to yet another aspect of the invention,
the iron present is precipitated a~ basic ferrisulfate
only to an extent sufficient to maintain the sulfate ion
--4--
: . , ,
1046)867
concentration in the electrolyte constant at the desired
level. According to this aspect of the invention a
sufficient amount of basic ferri~ulfate is precipitated
to counteract the tendency for sulfate build up. The
remainder of the iron is precipitated as a hydroxide by
raising the pH of the liquid to about 3.5 at the completion
of the oxidizing step.
The invention will now be particularly described
with reference to specific examples.
EXAMPLE 1
A clear solution was used, containing approximately
100 g/l of zinc, 2 g/l of manganese and 2 g/l of ammonium
ions in a sulfate medium. U~ing 1 litre of this solution
a gaseous mixture having the composition of 5-10% by volume
of sulfur dioxide, balance oxygen, was bubbled through the
solution at a flow rate corresponding to 200 ml/minute of
oxygen. The temperature was maintained at 85C during the
oxidation process which lasted two hours. The solution
pH dropped in the course of the process from an initial
value of 2.7 to a final value of 1.8.
The ferrous ions concentration was reduced in
the course of the proce~s from 5.9 g/l to trace level. At
the end of the oxidation proces~ the pH was increased to
3.5 by addition of calcine. Following precipitation of the
ferric ions, the solution was analyzed for iron and found
to contain only a trace level thereof.
EXAMPLES 2-6
In these examples a slurry was used which comprised
a liquor having a ~imilar composition to the solution of
Example 1 and containing the solid residue from a calcine
leach.
" , : :
,,' ~ ' ' ' ' ., . , ' ' :' . . . :,
~)4l~867
The details of the oxidation conditions are
~hown in the following Table 1, unspecified conditions
being identical to those of Example 1.
TABLE 1
% S2 in Duration
~Temp gas mix- of oxida- pH ~e - (g/l)
,E~le C ture ,tion (hrs) Initial ~inal Initial nal
2 85 5-10 1.0 2.4 2.4 1.18 0.083
3 85 2.5 2.5 2.3 2.7 2.95 0.012
4 952.5-5.0 1.1 2.5 2.5 4.17 2.57
952.5-10 0.75 3.0 3.0 2.57 1.22
6 95 5.0 0.5 3.0 3.0 0.232 Trace -
It will be seen that in all cases the ferrous ion
concentration was substantially reduced.
In the caqe of Ex~mple 6, it was possible to dis- '~
continue the gas flow after half an hour since continuous
monitoring of the Redox potential (using a platinum electrode
and a standard calomel electrode) showed a change from
+ 200 to + 630 mV, indicating a very high ferric to ferrous
ratio. This was confirmed by analysi~ which revealed only
a trace level of ferrous ions.
A measure of the effectiveness of the gas mixture
can be derived by calculating the oxidation rate (grams of
ferrous ions per litre per hour). This figure can then be ,~
compared with the theoretical oxidation rate derived from
' '"
. .
1~40867
the flow rate of gas mixture and its composition. The
results of such comparisons for each of the Examples
1-6 are expressed as "efficiency of gaseous mixture (~)"
in Table 2 below.
TABLE 2
Efficiency
Oxidation of Gaseous
Example Rate ~g/l hr)Mixture (~)
l 2.95 78
2 1.10 29
3 0.74 100 approx.
4 1.40 63
1.80 62
6 0.46 16.
It will be seen from the above that sulfur dioxide/oxygen
mixtures can be very effective for the oxidation of ferrous
ions in zinc sulfate solutions or slurries.
It will be understood that where the precipita-
tion of basic sulfate is ùsed to remove all the iron
from the solution, the resulting depletion of the solution
with respect to sulfate ions may necessitate addition of
the latter.
Alternately the iron can be removed after its
oxidation by precipi~ation of both hydroxide and basic :
sulfate, in balanced amounts, so as to maintain the
desired sulfate level in the electrolyte.
' ' ~ .
.. ... .
, . . . . . . .
