Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
111~21~
The Ln~tention relates to a process for the preparation of an aqueous~olution of zinc sulphate for use as feedstock in a process for the
electrolytic preparation of zinc.
Of the zinc ores which occur naturally, the sulphidic ores (blendes)
currently form the principal source of raw materials for zinc metallurgy.
In a~most all cases the ores have too low a zinc content for direct
processing. They are therefore first freed by flotation of a large
part of he useless secondary rock, thereby yieldine ore concentrate.
Whereas the zinc content of an ore can be for example 4 to gdO ~y weight,
the zinc content o~ a concentrate is generally between 48 and 62d by weight.
The next step in the preparation of zinc i3 the roasting of the
ore concentrate with oxygen (air), during which the principal reaction is:
2ZnS + 32 ~ - -t 2ZnO + 2S02.
The product of this step, i.e. the "roasted material, has a zinc
content which is generally between 55 and 73% by weight.
For the electrolytic preparation of zinc, the roasted material
is treated with sulphuric acid in order to dissolve the zino oxide:
ZnO + H2S04 ~ ZnS04 + H20.
This is known as "leaching" the roasted material.
The resultant aqueous solution of zinc sulphate is electrolyzed in
electrolytic cells:
2ZnS04 + 2H2 ~ 2Zn + 2 H2S4 + 2-
The zinc precipitates on the cathode and can be removed thererrom.
As the reaction equation shows, sulphuric acid is also formed during
the electrolysis. Said acid is present in the liquid which is drained
from the electrolytic cells upon completion of the electrolysis. The acid
content and its origin from the electrolytic cells explain the name
"battery acid" which is generally given to said liquid.
Electrolysi~ i~ generally started from a neutral zinc sulphate
solution containing about 120-220 grams Or zinc per litre."Battery
acid" generally contains about 100-300 grams of H2S04 per litre and
30-70 grams of zinc per litre. It follows that not all the zinc is
precipitated from the solution during electrolysis.
l;lJ ~214
Because the roasted material does not consist of pure ZnO, in practice
the process is not as simple as indicated above. The compounds present in
the roasted material can be classified into the following groups:
1) Zinc compounds which are readily soluble in sulphurio acid.
These include in the first place ZnO. In addition the roasted material
may already contain some ZnS04.
2) Zinc compounds which are le~s readily soluble in sulphuric acid,
i.e. which only dissolve in rather strong sulphuric acid at elevated
temperature. The main compound of this type is zinc ferrite, which
may be represented by the formula ZnO.Fe203. Substantially all the
iron present in the ore concentrate is converted into zinc ferrite
by the conventional roasting methods.
3) Compounds which are not soluble in sulphuric acid. There are a great
many compounds of this type, such as PbS04, AgCl, SiO2, CaS04.
4) Compounds (not of zinc) which dissolve in sulphuric acid. These
include compounds of a large number of elements such as Co, Cu, Cd,
Sb, As, Mg, Mn.
During the leaching treatment, the compounds mentioned in (1), (3) and
(4) present few problems.
The compounds referred to in (1) dissolve readily.
The compounds referred to in (3) do not dissolve and are discharged
as "lea¢hing residue".
The compounds referred to in (4) dissolve and enter the zinc sulphate
solution. Compounds of certaln elements ~Mg, Mn) have llttle if any
disturbing effect on the electrolysis, whereas compounds of other elements
(Co, Cu, Cd, Sb, As) are extremely disturbing. For that reason in
practice a purification step is always used between leaching and electrolysis,
ln order to remove the compounds of the disturbing elements from the
zinc sulphate 301utlon. This known purification step does not require
3 discussion here as the invention relates to the carrying out of the
leaching. Moreover, the compounds referred to in (4) play a negligible
role.
214
The problems in leaching are caused by the compounds referred to in
(2). If the leach~ng is carried out under such comparatively mild
conditions that the zinc oxide does dissolve while the zinc ferrite
does not, the solid matter remaining after leaching (the "leaching
residue") will still contain zinc, so that the "leaching efficiency"
(i.e. the percentage of the zinc that was present in the roasted material
and that has been dissolved) is comparatively low. If the 'eaching is
carried out under more severe conditions, so that both the zinc oxide and
the zinc ferrite dissolve, the latter according to the equation
Fe203 + 4 H2S4 ) ZnS04 ~ Fe2(S04)3 ~ 4H 0,
a solution is obtained which contains a comparatively large amount
of iron in addition to ~inc. However, a zinc sulphate solution from
which zinc has to be made by electrolysis should contain only a small
proportion of iron.
In the past it was customary to carry out the leaching under conditions
whereby the dissolving of the zinc ferrite was minimized. This is
achieved by leaching at a low degree of acidity (pH approximately 4
or higher). This leaching treatment is known as "neutral leaching".
The acid used is generally "battery acid". If this acid is neutralized
with zinc oxide, the resultant zinc sulphate solution will automatically
have the zinc sulphate concentration desired for- the electrolysis.
At the said low degree of acidity, a small proportion of iron that
has nevertheless been dissolved precipitates as hydroxide. To thls
end it i~ lmportant, however, that the iron should be in the
ferric and not in the ferrous form. For that rea~on often an
oxidant is added (generally air, occasionally manganese or even
potas~ium permanganate).
The neutral leaching is generally carried out at room temperature
or at slightly elevated temperature (mostly not hi8her than about
60_70C), in which respect it may be noted that raising the temperature
may sometimes be beneficial for the precipitation of the iron.
3Z14
`~eutral leaching can be carried out either batchwise or continuously,
and in one or more steps, in the latter case optionally according to
the countercurrent principle.
After the neutral leaching (and, in the case of multi-step operation,
optionally after each step) a solid/liquid separation follows,
which may be carried out according to any desired technique, but which
in practice is mostly done with one or more "thickeners". The latter
are vessels into which the solid/liquid mixture is introduced and
subsequently the solids are left to settle. From the top of the thickener
clear liquid is withdrawn as overflow. From the bottom of the thickener
a slurry of solids in liquid is withdrawn as underflow. The fact that
a slurry is withdrawn means that the solid~liquid separation is
not complete. However, this has the practical advantage that a
slurry can still be run and pumped as a liquid. Of course, after
the use of a thickener the separation can be further perfected,
for example by filtering or centrifuging the slurry. This will be done
in particular when one component of the slurry is useless (for example
such a useless component consists of solids that can be discharged
as a waste product), while the other component is valuable (for
example such a valuable component consists of liquid which is still
usuable and can be recycled in the process).
The liquid obtained in the solid/liquid separation is the desired
aqueous solution of zinc sulphate. The solids are the leaohlng resldue
of the neutral leachlng.
Said leaching residue contains the insoluble compounds referred to
in (3), the undissolved zinc ferrite and some zinc oxide, because
during neutral leaching it is almost always impossible to dissolve
the zinc oxide quantitatively from the roasting charge. Furthermore,
the leaching residue may contain some iron hydroxide, precipitated durin~
3 the neutral leaching.
lllt~Z14
-- 6 _
Formerly, the neutral leaching treatment terminated the leaching
process, and the leaching resldue was discharged.
Although it would be possible, as has been explained above, to
also dissolve the zinc ferrite by leaching under more severe conditions,
this would at the same time cause a rather large amount of iron to
dissolve.
If said iron is precipitated as Fe (OH)3 in the arorementioned manner,
a large amount of a voluminous deposit is obtained, separation of which-
from the liquid encounters practically insurmountable difficulties.
Later, however, methods were found for selectively precipitating
the iron from solutions containing a rather large amount of iron
in addition to zinc, in the form Or a precipitate which can be readily
separated from liquid. One of said methods is the selective precipitation
of iron as jarosite.'Said "~arosite process" is described in the
article "Die Eisenfallung als Jarosiet und ihre Anwendung in der Nassmetallurgiedes Zinks" (the precipitation of iron as jarosite and its use in
the hydrometallurgy Or zinc) by G. Steintveit in "Erzmetall" 23 (1970),
pages 532-539. It is now therefore possible to treat the leaching
residue of the neutral leaching so vigorously with sulphuric acid,
that the zinc ferrite dissolves (and with it of course the zinc oxide
and iron hydroxide present in the residue as well). The leaching residue
of the neutral leaching is then treated at elevated temperature (mostly
80-110C, preferably approximately 95C) with an exoess Or rather
strong sulphuric acid. According to the literature mentioned, the strength
Or said sulphuric acid should be about 180-250 grams of H2S04 per litre,
but in practice it has been found that it is sufficient to use sulphuric
acid having a strength Or about 130-250 grams of H2S04 per litre.
For this purpose "battery acid" may be used~ if desired made up with
fresh strong sulphuric acid.- On completion of the treatment the
3 liquid should still be rather strongly acid; accordlng to the afore-
mentioned literature approximately 80-120 grams of H2S04 per litre,
should still be present but in practice it has been found that about
40-120 grams of H2S04 per iitre is adequate. This treatment is known
as "hot-acid leaching", a name which is explained by the high temperature
and high de8ree of acidity. In common with neutral leaching, hot-acid
leaching may be carried out either batchwise or contlnuously, and in one
or more steps.
The hot-acid leaching is followed by a solid/liquid separation,
in common with the neutral leaching. The solids can be discharged
as leaching residue. The liquid contains the dissolved sulphates
of zinc and iron and the unused excess sulphuric acid.
The liquid is now passed to the "~arosite precipitation" where
the iron is selectively precipitated as jarosite, at elevated
temperature (approximately the same temperature as in the hot-acid
leaching). The formula of jarosite is X2~Fe6(S04)4 (OH)12], wherein
X is an ammonium- or alkalimetal ion. It immediately follows that
ammonium- or alkalimetal-ions should generally be added during the
jarosite precipitation, for example in the form of a salt or a base.
In the case of addition Or ammonium-ions in the form of ammonia,
the reaction equation may be written as:
3Fe2(S04)3 1- 2 NH40H + 10 H20 ~ (NH4)2~Fe6(S04 4 12~ 2 4
In order not to contaminate the liquid needlessly, not more ammonium-
or alkalimetal-ions are added than needed for the formation of
~arosite. It is possible to use less than the equivalent quantity.
A proportion of the iron then precipitates as (H30)2~Fe6(S04)4 (OH)12~.
In this case, however, the precipltation proceeds somewhat more
slowly.
From the given reaction equation it appears that sulphuric acid
i8 formed during the reaction. Moreover, the liquid contains the excess
sulphuric acid not used during the hot-acid leaching. Thls total
quantity of sulphuric acid 19 too large for good ~arosite precipitation.
In order to obtain the rastest and fullest possible precipitation of
iron as ~arosite (and therefore the lowest possible iron content in
the remaining l~quid), it is generally ensured that said precipitation
ultimately terminates at a pH of approximately 1.0_1.5, although
it is also possible to work with a somewhat higher final degree of
acidity, depending on the available time and/or the accepted iron content
of the remainine liquid. In any case, however, it is necessary to add
a basic neutralizing agent. In order to obviate loss of acid, a
basic zinc compound is used. Pure ZnO would be ideal, but it is too
expensive, so that in practice roasted material is used.
2~
-- 8 --
Optionally, the jarosite precipitation may be divided into two
parts, namely a "pre-neutrallzation" and the "actual jarosite precipitation".
In the pre-neutralization, a pro?ortion of the roasted material
re~ulred as neutralizing agent is then added to the liquid, so that
the pH is already brou~ht close to the desired value. In the actual
jarosite precipitation, the rest of the required roasted material
and ~he ammonium- or alkalimetal-ions are then added.
,lfter the jarosite precipitation there follows another solid~Liquid
separation. (If the jarosite precipitation is divided into a pre-
neutralizatior. and an actual jarosite precipitation, it is of coursepossible to fo'low each step by a solid-liquid separation, in which
the residue of the pre-neutraliz~tion can be recycled to the hot-acid
leaching).
The liqu.d iq an aqueous solution of zinc sulphate which, in view
of the pH during the jarosite precipitation, still contains some acid.
Said liquid is generally recycled to the neutral leaching, where said
ac-d is neutralized.
The solid matter is known as the "jarosite residue". It contains
primarily the precipitated jarosite. However, it also contains other
constituents, which originate from the roasted material added as
neutralizing aeent during the jarosite precipitation. Of said roasted
material, substantlally only the zinc oxide will have dissolved
(although not quantitatively) during the jarosite precipitation, whlle
the zinc ferrite ?re3ent in thc roasted material wlll have remained
substantially undissolved and the insoluble components will of course
have remained undissolved. All these undissolved materials are therefore
present in the jarosite residue.
As has been proposed n the aforementioned article by Steintveit, it
is possible to improve the leaching efficiency even further by treatine
3 the jarosite residue again with sulphuric acid in order to dissolve the
zinc ferrites. This "jarosite acid wash" is carried out under conditions
which are somewhat milder than those of the hot-acid leaching.
After a solid/liquid separation, a substantially
zinc-free solid matter which can be discharged and an acid
liquid containing sulphates of iron and zinc are obtained.
From said liquid the iron has to be removed, which is of
course most readily done by recycling the liquid to the
jarosite precipitation.
In this way it is therefore achieved (at least
theoretically) that no residue is discharged from the process
which still contains soluble zinc.
Thus in accordance with the invention there is
provided a process for the preparation of an aqueous zinc
sulphate solution for use as feedstock in a process for the
electrolytic preparation of zinc, by means of leaching of a
material obtained by roasting a sulphidic zinc ore concentrate,
comprising a neutral leaching with a sulphuric acid bearing
solution, a hot-acid leaching with a sulphuric acid bearing
solution and a separate jarosite precipitation, each of these
operations followed hy a solid/liquid separation and recovering
zinc sulphate solution as a product stream by withdrawing the
liquid stream from the solid/liquid separation effected sub-
sequently to the neutral leaching operation, in which process
the residue of the jarosite precipitation is recycled to the
hot-acid leaching.
The invention is illustrated by reference to the
accompaying drawings in which:
Figure 1 illustrates diagrammatically a known
continuous process for preparing a zinc
sulphate solution,
Figure 2 illustrates diagrammatically a continuous
process of the invention,
B
Figu:le 3 illustrates diag:r~ rlmatically a modi.fica-
tion of the process illustrated in
Figure 2, and
Figure 4 illustrates diagrammatically an embodiment
of the process illustrated in Figure 3.
- 3a -
- 10 -
In Figure 1 the afore-mentioned most evolved, known process is
represented dia~rammatically as a continuous process. In said figure
the neutral leaching is designated as I and for the sake of simplicity
represented as a single-step operation. To it are supplied fresh roasted
material throu~h a line 1, sulphuric acid through a line 2, and air
through a line 3. The product of the neutral leaching passes through
a line 4 to a thickener II, where separation takes place into a ~inc
sulphate solution and a slurry of solids. The æinc sulphate solution
is discharged through a line 5 as overflow from the thickener II. The
slurry is discharged through a line 6 as underflow from the thickener II
and passes to the hot-acid leaching, designated as III and for the sake
of simplicity represented as a single-stage operation. To it sulphuric
acid is supplied through a line 7. The product of the hot-acid leaching
passes throu2h a line 8 to a thickener IV, where separation takes place
into a liquid and a slurry of solids. The liquid is discharged from the
thickener IV through a line g as overflow and passes to the jarosite
precipitation, designated as V and for the sake of simplicity represented
as a single-stage operation. ~rom the thickener IV the slurry is discharged
as underflow through a line 10. Said slurry passes to the filter VI,
where it is separated into solids and liquid. The separated solids
undergo a secondary washing treatment on the filter with water
supplied through a line 11, and is subsequently discharged through
a line 12. All of the liquid originating from the filter is passed
through a line 13 to the jarosite precipitation V. To the jarosite
precipitation are supplied ammonia through a line 14 and roasted
material through a line 15. The product of the jarosite precipitation
passes through a line 16 to a thickener VII, where separation takes place
3214
into a liquid and a slurry of solid matter. The liquid is discharged
from the thickener VII as overflow through a line 17 and returns to
the neutral leaching. The slurry is discharged from the thickener VII
as underflow through a line 18 and passe~ to the jarosite acid wash,
designated as VIII and ~or the sake of simplicity represented as
single-step o?erat on. To it sulphuric acid is supplied through a
line 19. The product of the jarosite acid wash passes through a line 20
to a thickener IX, where separation takes place into a liquid and a
slurry ol solid matter. The liquid is discharged from the thickener IX
as overflow through a line 21. The slurry is discharged from the
thickener I~ as underflow through a line 22 and passes to a filter X,
where the slurry s separated into solid matter and liquid. The
separated solid matter undergoes a aecondary wash on the filter
with water supplied through a line 23 and is subsequently discharged
through a line 24. All of the liquid originating from the filter is
combined via a line 25 with the liquid from the line 21. The combined
liquid stream returns through a line 26 to the jarosite precipitation V.
The sulphuric acid used in practice is principally "battery acid",
but the addition o, some fresh sulphuric acid (or sulphate ions in
any other form) is necessary in order to maintain the sulphate balance
of the system, because sulphate (in the form of jarosite) is continuously
discharged from the system. This is most effectively done by introducing
fresh concentrated sulphuric acid into the hot-acid leaching.
Now the present invention is based on the principle that it is
possible to omit the ~arosite acld wash and nevertheless attaln the same
leaching efficiency, provided the entire ~arosite residue i9
recycled to the hot-acid leaching. The zinc compounds present in the
jarosite residue dissolve during the hot-acid leaching. The jarosite
itself only redissolves to a very sllght extent, so that by far the
3 greater proportion oP the ~arosite can be dischareed with the residue.
It has been found that no complications occur as a result of the
presence of minor quantities of other contaminating metals.
T"e proce:3s according .o the invention is diagrammatically
represented as a continuous process in Figure 2. After the explanati.on
of Figure 1 whlch has been given above it would seen superfluous to
give an explanation of Figure 2.
The i.nvention therefore relates to a process for the preparation
of an aqueous solution of zinc sulphate for use as feedstock
in a ?rocess for the electrolyt-.c preparation G9 zinc, by means
of leaching of roasted material, comprising a r.eutra]. leaching,
a hot-ac d leaching and a jarosite precipi.tation, each follol.~ed by
13 ~ solid/liquid separation, in wh ch process the residue of` the jarosite
precipitation is recycled to the hot-acid leachine.
~ further improvemcnt can be attained by ?~ssing the residue of
the neutral leaching not to the hot-acid 1.eaching but to the jarosite
precipitation. Said improvernent is represented -.n Figure 3, a further
explanation of which is also superfluous. Said improvement is based
on the fact that the residue of the neutral leaching still contains
zinc oxide and so~e Fe(0~)3. ~y passing these substances not
(as in Figure 2) to the hot-acid leaching, less sulphuric acid is
required in the hot-acid leaching in Figure 3 than in Figure 2.
By passing these substances to the jarosite precipitation, less fresh
roasted material is required to be added as neutralizing agent to
the jarosite precipitation. The result of these circumstances i.9
that less liquid circulate, in the III-IV-V-VII cLrcuit -Ln Figure 3
than in the corresponding circuit in Figure 2, so that in the case
of Figure 3 the relevant hardware can be smaller and therefore
less expensive.
In Figure 4 the process of Figure 3 is represented in another
fashion. Figure 4 brings out more clearly the simplicity of the
process.
3 ~ le
This example may be read with reference to Figure 3 or 4. To the
neutral leaching (60 C, average residence time 2 hours) there is
passed:
- 13 -
.hrou~h the line 1:
22.8 ton~hour of roasted material (containing 59% by weight of
;.n, 1~g by ~leight of Fe an~ 1.8~, of S04)
throu~h ~he line 2:
72.7 rn3~hour o~ "battery acid" (containing 159 grarr.s of H2S04 per l~tre,
52 grams of Zn per litre) and 25 m3/hour of zinc-containing waste l quid
rom various departments Or the zinc plant (containing 53.1 grams of
H2S0~l per litre and 100 gram~ of Z~ per '.tre~
througi1 the line 3:
air
F rOrQ the thickener II there is discharged as overflow through the
line 5 an amount of 200 m3/hour of neutral ~inc sulphate solution (contain ng
139.5 rrrams of Zn per litre~. Through the line 6 there passes as underflow
a slurry consisting of 30 m3/hour of liquid and 12,152 ke/hour of solid
matter. Said solid matter contains 29.6% by weigt1t of Zn and 22.55% by -~eight
of Fe.
Said slurry passes to the jarosite precipitation (95 C, average
residence time 3.8 hours), whither are also passed 200 kg/hour of ammonia
(in the form of gas) throush the line 14 and 7 ton/hour of roasted material
through the line 15.
Fro;n the 'ch~ckener VII there is disch~rged through the line 17 as
underflow 145 m3/hour of acid zinc sulphate solution (cont;ainine
110 grarns of Zn per litre, 6.5 gram3 of H2S0l~ per litre, 1 &ram of
Fe3 per litre and 0.6 erams of Fe per 1itre~. Sald 30lution returns
to the neutral leachine. Through the line 18 there passes as underflow
a slurry consisting of 40 rn /hour of liquid and 18,270 kg/hour of
solid matter. Said solid matter contains 13.79g by weight of Zn, 29.20
by weight of Fe and 15.65'~ by weight of S0!l.
Said slurry passes to the hot-acid leaching (95 C, average
res dence time 3 hours), whither are also passed 83.3 m3/hour
of "battery acid" and 1,419 litres/hour of fresh 96d by weight sulphur
acid through the line 7.
From the thickener IV there is discharged through the line 9 as
overflow 87 m3/hour of liquid ~containing 50.8 grams of H2S04 per litre,
Zl~
- 14 -
81.0 grams of Zn per litre, 15.7 grams of ~e3 per litre. Said l quid
passes to the jarosite precipitation. From the thickener IV there
is discharged throush the line 10 as unaerflow a slurry consisting
of 50 m3/hour of liquid and 11,500 kg~hour of solid matter. Said solid
matter contains 1.34~ by weight of Zn, 28.24% by weight of Fe and 24.79%
by weight of S04.
Said slurry passes to filter VI, where the separated solid matter
undergoes a secondary wash by spraying with 23 m3thour of water
supplied through the line 11. Through the line 12, 11,500 kg/hour
of solid matter leaves the process. Said solid matter is still wetted
with 5 m3/hour of liquid (containing 11.~ gram of H~S04 per litre,
28 grams of Zn per litre, 6 gram~ of Fe3~ per litre;.
From the filter VI, 68 m3/hour of liquid (oontaining 36.5 grams of
H2S04 per litre, 57.5 grams of Zn per litre, 11.1 grams of Fe3 per litre)
is obtained. Said liquid passes through the line 13 to the ~arosite precipitation.
The leaching efficiency is 98.3~.
