Note: Descriptions are shown in the official language in which they were submitted.
1.0~ 1 ~73
Zinc i5 the twenty-fourth most abundant element
in the earth's crust and finds many industrial applications,
the most important being the oxidation-resistant treatment of
iron surfaces, and others being in variGus fields, including
topical medicines, chemical reagents, etc.
Zinc is not found in the metallic state in nature.
Its chief ore is zine blend or sphalerite tZnS) which is the
source of ninety percent of the zinc produced today. The zinc
production methods employed today have heavy treatment costs
and consequently zinc metal producers demand high-grade con~
centrates.
There are two main methods of zinc recovery from its
ores, i.e., thermal reduction and electrolytic deposition, the
latter requiring the availability of relatively inexpensive
electrical power in view of the fact that the production of
one ton of zinc requires approximately 4500 kilowatt-hours.
The purest zinc (99.99%) is achieved by the electrolytic
methods.
The current world production of zinc is about
3,800,000 metric tons per year, 47~ by electrolytic methods
and the balance by thermal methods.
The thermal methods involve the following general
reactions:
heat
ZnS ~ ZnO
heat
2 ZnO + C ~ 2 Zn + CO2
The electrolytic methods generally involve the
following reactions:
-- 1 --
g~-
10~1973
heat
ZnS ~ > ZnO
2nO + H2SO4 3 Zn SO4 + H2O
Zn SO4 ~ Zn + H2SO4
Electrolytic zinc plants utilize four operations:
(1) roasting of zinc sul~ide concentrate; (2) leaching of
the roasted concentrate or calcine to extract the soluble zinc;
(3) purification of the resulting solution; and (4) electrolysis
of the solution to obtain metallic zinc.
Zinc electrolyte typically contains impurities of
copper, cobalt, nickel and cadmium that are detrimental to
the plating of 7inc and must be removed prior to electrolysis.
These elements are removed by a hot copper sulfate/arsenic
trioxidefzinc dust purification procedure.
The precise mechanism of the hot copper sulfate/arsenic
trioxide/zinc dust purification technique is not thoroughly
understood. ~owever, a plausible explanation is as follows:
Zinc dust displaces copper and arsenic from solutions which
are thought to precipitate as a metallic couple. Zinc dust
ordinarily does not displace cobalt and nickel from solution,
but in the presence of the copper/arsenic couple such metals
are quantitati~ely precipitated. The abo~e copper sulfate
addition may not be necessary if sufficient copper is already
present in the impure zinc electrolyte.
The byproduct of the purification procedure is a
cement copper cake residue containing, in addition to copper,
varying amounts of zinc, cadmium, cobalt, nickel and arsenic.
The market value of such cake is primarily dependent on the
percentage of copper contained therein.
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10~1973
There are several disadvantages to the above
described purification procedure.
~1) The process requires the addition of arsenic
trioxide and possibly copper sulfate, which affects the
economics of the overall process.
(2) The cement copper cake residue, because of its
arsenic content, has a greatly reduced market value.
(3) The zinc, cadmium and cobalt values in the
cement copper cake are not reflected in the marketable value
of the latter and consequently reflect losses in the overall
process economics.
The present invention relates to the electrolytic
production of zinc metal and involves the treatment of residue
from such production processes to provide copper arsenate to be
recycled for use in the preliminary purification of the zinc
electrolyte, and simultaneously to upgrade the copper and
reduce the arsenic content in the treated residue to increase
the market value thereof and thereby process economics.
Thus, the present invention provides , in a process
for electrowinning zinc metal wherein copper, cadmium and
cobalt contaminants are removed from impure electrolyte
prior to electrolysis, includlng the formation of a cement
copper cake residue fraction contalning zinc, cadmium, cobalt
and arsenic, the improvement comprising:
(a) subjecting said cement copper cake to an acid leach
to form a slurry, followed by an alkali addition to adjust the
pH of the slurry to about 3.5 to 4.0 and to extract zinc,
cadmium and cobalt in-solution and provide a copper-enriched
residue;
(b) precipitating cobalt from the extraction solution
of step (a);
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(c) separating the precipitated cobalt from the solution
in step (b);
(d) subjecting said copper-enriched residue of step (a)
to a caustic leach to extract arsenic in solution therefrom
and provide a copper-enriched cement copper cake product;
(e) reacting the arsenic-containing solution from
step (d) with a copper salt to form copper arsenate residue
and leave a zinc-containing solution;
(f) treating said impure electrolyte with said copper
arsenate residue to remove said contaminants and to prepare
purified zinc electrolyte.
In the aspect which forms the principal subject
of this divisional specification, the present invention provides
a process of purifying zinc electrolyte containing cobalt
contaminant prior to electrolysis thereof in the electrowinning
of metallic zinc, comprising adding to said electrolyte zinc
dust in an amount of about 1.5 to 2.5 grams per liter and
copper arsenate in an amount of about 0.25 grams per liter,
maintaining the temperature of the electrolyte at about 90
to 95C until the cobalt level of said electrolyte is less
than 0.1 ppm, adjusting the pH to about 4, and then removing
the solids from said electrolyte.
In a preferred embodiment the invention provides such
a process of purifying zinc electrolyte containing cobalt
contaminant up to 330 ppm prior to electrolysis thereof in the
electrowinning of metallic zinc comprising adding to said
electrolyte, at a temperature of about 80 to 90C, zinc dust
in an amount of about 1.5 to 2.5 grams/liter and copper
arsenate in an amount of about 0.25 grams/liter, then raising
the temperature to and maintaining it in the range or about 90
to 95C until the cobalt level of said electrolyte is less than
.~.................................................................. .
" ~0~73
0.1 ppm, and adjusting the pH to about 4 and then removing
the solids from said electrolyte.
Upgrading of cement copper cake and recovery of
arsenic is accomplished in four basic operations: (1) Acid
leaching; (2) cobalt removal; (3) caustic leach; and (4) arsenic
removal. The acid leach is conducted under optimum conditions
for the dissolution of zinc, cadmium and cobalt while at the
same time suppressing copper extraction.
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10~ 973
The solution and residue of the acid leach are separated
by filtration for further processing. In order to make
a zinc/cadmium solution suitable for recycle to the
zinc plant, cobalt is removed from the acid leach solution.
The copper and arsenic containing residue from the acid
leach is subjected to a caustic leach to dissolve the
arsenic. The caustic leach slurry is then filtered. This
leaves a residue containing 60 to 80 percent copper and
less than 1 percent arsenic,providing an improved market-
able product for its copper values.
Arsenic is removed from the caustic leachsolution by precipitation as copper arsenate which is used
as a substitute for arsenic trioxide and copper sulfate
in the first stage purification of zinc electrolyte.
The drawing is a schematic flow diagram of the
overall process of the invention.
As can be seen in reference to the drawing,
showing the process of the present invention in schematic
form, the process involves the electrolytic refining of
zinc in zinc plant 10. Not shown in the drawing is the
preliminary processing of the zinc ore which is-conventional.
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~0~3~11973
Zinc sulfide ore is ln accordance with known procedures
roasted to form zinc oxide, then leached with sulfuric acid to
form zinc sulfate. The zinc sulfate leach solution also
includes impurities that must be removed prior to electrolysis
in order to avoid contamination of the final zinc product.
The impure feed solution typically contains 0,5 - 1.0 grams
per liter copper, 20-30 parts per million cobalt, 1-2 parts per
million nickel and in addition may also contain cadmium.
The impure feed solution 11 is fed to the zinc
electrolyte purification section 12 where it is treated in
accordance with the present invention with zinc dust and copper
arsenate as described in more detail below in order to
precipitate the aforementioned contaminating impurities using
the cobalt level in the solution as a control. The final
cobalt level must be less than 0.1 parts per million in order
to ensure sufficient purity of the electrolyte for the
electrolysis step.
~ he residue from the purification step 12 is separated
by any conventional means such as filtration at 13 and con-
stitutes what is known as cement copper cake 14 which typically
has the following composition:
43~4% copper
6.64~ zinc
1.89% cadmium
1.25% cobalt
o.05% nickel
6.89~ arsenic
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~1.0~973
The purified zinc electrolyte filtrate isdelivered through line 15 to the electrolytic zinc plant
10 for electrolysis. The cement copper cake 14 is treated
in order to (a) upgrade the copper content and purity of
the residue to increase the market value thereof and (b)
to recover the arsenic content and convert the latter to
copper arsenate and recycle the copper arsenate to the
purification step 12 and use it in conjunction with zinc
dust for the previously described precipitation step in
place of the prior art copper sulfate/arsenic trioxide
reagents.
The cement copper cake 14 is subjected to an
acid leaching step 16 in order to recover zinc, cadmium and
cobalt constituents therefrom as a filtrate and provide
a residue separated by filters 17. The filtrate is
delivered to the cobalt removal stage 18 to separate the
cobalt therefrom so that the remaining zinc/cadmium solution
can be delivered through line 19 to recycle line 20 through
which it is delivered to the electrolytic zinc plant 10
for reuse. The residue from cobalt removal stage 18 has
a high cobalt content which has market value.
The residue from filtration step 17 is subjected
to a caustic leach 21 to dissolve the arsenic and leave
a high copper residue which is separated in filtration
step 22. The latter residue, designated as the treated
cement copper cake, has an enhanced copper content which
increases its market value. The filtrate from step 22 is
treated with copper sulfate in arsenic removal stage 22 to
provide copper arsenate which may be recycled through
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return line 24 to the electrolytic purification step
previously described. The remaining zinc containing solution
is delivered to recycle line 20 for delivery to the
electrolytic zinc plant 10.
Acid Leaching
Temperature of the acid leach slurry in step 16
is maintained at about 95C with 20 percent initial solids
loading and 15 to 20 grams per liter initial sulfuric
acid addition. After about two hours, the acid leach
slurry is neutralized to pH 3.5 to 4.0 with sodium hydroxide
to precipitate any leached copper. The quantity of sodium
hydroxide used in this step varies with different copper
cake samples, and so also does the final pH. However,
control is easily maintained by observing the color of the
leach solution. Copper precipitation is complete when
the solution loses its blue color. Consumption of sodium
hydroxide is generally 75 to 150 pounds per dry ton of
copper cake. Fresh copper cake consumes more sodium
hydroxide than stockpiled copper cake. It should be noted
that some degree of attritioning to break up lumps is
necessary either before or during the acid leaching of
stockpiled copper cake. Attrition is not needed for fresh
cake.
The acid leach slurry is filtered at 17 and the
arsenic rich residue displacement washed. The residue
is black and finely divided. It is amenable to ~iltration
by filter press, and behaves similarly to regular copper
cake.
-- 8 --
10~ 973
Typical products of the acid leach, based on an
average sample of copper cake assaying 6.24~ Zn, 1.89% Cd,
1.25% Co, 0.71~ Mn, 43.4% Cu, 6.89% As, O.lC% Na and 15.5%
SO~, have the following analysis.
TABLE I
Results of Acid Leach Extractions
Stockleiled Cake
Recovery (as %
Filtrate metal value
10comPonent (grams/liter)Residue % in feed) _ _
Zn 11.8 2.16 71.4
Cd 3.95 0.46 81.9
Co 2.77 0.22 87.0
Mn 1.71 0.05 94.3
Cu 0.22 57.8 0.20
As 1.99 8.14 11.4
Na 6.52 0.13 96.4 s
SO4 42.2 6.81 76.4
Fresh Cake
.
Recovery (as %
Filtrate metal value
Component (~rams/liter)Residue % in feed*)
Zn 13.6 1.21 85.5
Cd 3.27 0.84 67.9
Co 1.87 0.70 58.8
Mn 1.76 0.03 97.0
Cu 55 ppm 57.9 0.05
As 1.68 8.30 9.62
Na 11.5 0.32 94.5
SO4 N/A N/A N/A
* Copper cake plus added reagents.
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Cobalt Removal
Cobalt removal is carried out at 18 under the
following conditions. Filtrate 23 from the acid leach
process stage 16 is heated to about 95C and solid potassium
permanganate is added until a slight excess of permanganate
is noted, as seen by a deep purple color. Sodium
hydroxide is then added in an amount sufficient to maintain
pH of the resulting slurry at about 3.0 to 3.5 These
conditions are maintained for about two hours. Total
cobalt removal may be accomplished by using a longer
reaction time but is not mandatory in this process. The
slurry is filtered at 25 and the residue displacement washed.
Average reagent consumption per pound of cobalt
rem~ved is 3~8 pounds of potassium permanganate and 2.0
pounds of sodium hydroxide. These figures constitute
a considerable excess over the theoretical requirement.
The reason for this is that potassium permanganate is
consumed in oxidizing manganese and arsenic in addition
to cobalt in the acid leach solution.
Typical products of the cobalt removal stage
obtained from as typical acid leach solution analyze as
shown below in Tahle II. The filtrate is returnable to
the zinc plant for recovery of zinc and cadmium. The amount
of zinc and cadmium reporting to the cobalt rich residue
is largely dependent on washing efficiency. This residue
requires a thorough displacement wash to remove zinc and
cadmium.
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10~3'~973
TABLE II
Results_of Cobalt ~em~
Recovery (as %
Filtrate metal value
s Component (grams~llter) Residue % in feed*)
Zn 11.0 2.45 5
Cd 3.48 1.64 10
* Solution from acid leach stage plus added reagents.
Results of Cobalt Removal
Recovery (as %
Filtrate metal value
Component (grams/liter) Residue %in feed_*)
Co less than S ppm 11.5 100
Mn less than 5 ppm 22.4 100
Cu 0.11 0.45 50
As 0.20 7.45 90
Na 9.11 2.03 5
* Solution from acid leach stage plus added reagents.
Caustic L~ach
The residue of high arsenic content from the acid
leach process stage 16 is leached with caustic at 21 to dissolve ~ -
the arsenic. Temperature of the caustic leach slurry is
maintained at a~out 95C with 10% initial loading of residue
from the acid leach stage and addition of 5~ sodium hydroxide
solution. Air is continuously added to the slurry at the rate
of approximately 500 standard cuhic feet per minute per dry
ton of acid leach residue. These reaction conditions are
maintained for about six hours. The slurry is then filtered
at 22 and the residue displacement washed. The residue from
the filtration stage 22 is usually brown or green and is slower
filtering than other residues in this process.
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1~9~973
Typical products of the caustic leach stage obtained
from the feed to this stage analyze as follows:
TABLE III
Results of Caustic Leach
Recovery (as %
Filtrate metal value
Component (grams/liter) Residue % in feed*)
Zn 0.51 2.07 21.4
Cd less than 1 ppm 0.56 0
Co less than 1 ppm ~.26 0
Mn less than 1 ppm 0.07 0
Cu less than 60 ppm 70.4 less than 0.1
As 8.29 0.75 92.4
Na 28.0 0.32 99~0
SO4 7.16 0.42 95.0
* Residue from acid leach stage plus added reagents.
Many other caustic leaches using slightly different
conditions also extract more than 90% of the arsenic.
Arsenic Removal
Arsenic compounds are removed from the caustic leach
solution as an insoluble solid at 23. The purified solution 26
is ret~rnable to the zinc plant. In the preferred embodiment,
the following conditions are used. The filtrate 27 from the
caustic leach stage is heated to 75-80C. Because formation
of copper arsenate is not complete if copper sulfate is added
directly to the solution, sufficient sulfuric acid is first
added to bring the solution to about pH 8. Copper sulfate is
then added; about 60 pounds of CuSO4-5H2O per water ton o.
solution is required. The
1091~973
amount of copper sulfate required may vary depending on the
arsenic content of the solution. The resulting slurry is
maintained for about two hours. A small amount of lime,
usually about 3 pounds per water ton, is then added to
ensure a final pH above 4. The slurry is then filtered.
Arsenic removal is usually greater than 95% by this method.
The filtrate is returnable to the zinc plant. The residue,
a pale blue quick filtering solid, is returnable through
line 24 for use as a substitute for arsenic trioxide and
copper sulfate in first stage purification of zinc
electrolyte. Typical products of the arsenic removal stage
obtained from the feed to this stage analyze as follows:
TABLE IV
Results of Arsenic Removal
Recovery (as %
Filtrate metal value
Component (grams/liter)Resi ue ~ in feed*)
Zn 0.12 1.30 75.0
Cd 1 ppm 10 ppm -
Co 1 ppm 10 ppm - -~
Mn 1 ppm 10 ppm
Cu 0.32 24.6 95.7
As 0.18 27.6 97.7
Na 25.9 4.33 4.54
Ca 0.5 2.33 56.0
* Solution from caustic leach stage plus added reagents.
Electrolyte Purification
The copper arsenate product from the
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arsenic removal stage 23 is sued as a substitute for
copper sulfate and arsenic trioxide in the removal of
copper, cobalt and nickel from impure zinc electrolyte
at stage 12. Impure zinc electrolyte solution typically
contains 0.5-1.0 grams/liter copper, 20-30 parts per million
cobalt, and 1-2 parts per million nickel. Electrolyte
¦ purification is performed as follows: 1.5 to 2.5 grams/liter
of zinc dust and 0.25 grams (dry)/liter of copper arsenate
product are added to impure zinc electrolyte solution
at about 80 to 90C. The temperature is then maintained
at about 90 to 95C until the cobalt level in the solution
is less than 0.1 ppm. When this cobalt level is reached,
the pH of the solution is adjusted to approximately pH 4
and the slurry is then filtered. Total reaction time is
usually about two hours. The filtrate is fed to the zinc
plant for electrolytic recovery of metallic zinc. The
cement copper cake residue is treated according to the
present invention to remove and recycle its arsenic con-
tent, with attendant upgrading of the copper cake.
Example 1
Six plant runs were made using the electrolyte
purification process of the present invention. Impure
zinc electrolyte solution of pH about 4 and at about
80-90C was treated with 1.5 to 2.5 grams/liter of zinc
dust and 0.25 grams (dry)/liter of copper arsenate product
from the arsenic removal stage. The temperature was
maintained at about 30 to 95C until the cobalt level
in the solution was less than 0.1 ppm. The results are
summarized below:
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109~973
TABLE V
Results of Use of Copper Arsenate Product In
Removal of Cobalt from Impure Zinc Electrolyte
>
Amount Time
Initial of Copper Taken
Cobalt Amount Arsenate For Cobalt
Level In of Zinc Product To Reach
Impure Dust Added (55% Less Than
Run No. Feed Added moisture) 0.1 ppm
1 16 ppm 550 pounds 160 pounds 85 minutes
2 17 650 160 110
3 19 650 160 90
4 19 650 172 41
19 650 172 80
6 19 650 172 115
The laboratory procedure differs somewhat from
the plant procedure. It was found that on a small (1 liter) ~ -
scale the reagent requirements are greater and the
purification is more difficult to control. The following
example illustrates the laboratory procedure:
Example 2
Four 1 liter samples of impure zinc electrolyte
containing 16 ppm Co were treated with copper arsenate
and zinc dust in the following way. The samples were heated
to 90C in beakers; 1 g dry powdered copper arsenate product
was added to each beaker together with 2, 3, 4 and 5 g,
respectively, of zinc dust. The beakers were stirred for
1 hour, and solution samples were removed from each at
10 minute inter~als. The solutions treated with 2 and 3 g
of zinc dust did not achieve the required cobalt level of
less than 0.1 Fpm. In those treated with 4 and 5 g of zinc
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dust the co~alt level fell to less than 0.1 ppm after
20 minutes.
It is thought that performance on a plant scale
is more effective than on a bench scale because the reducing
conditions required for the purification are more easily
maintained in a large tank than in a 1 liter beaker.
As can be seen from the above description, accord-
ing to the present invention a copper arsenate product is
made from cement copper cake residue and reused in
purification of zinc electrolyte. Two superior features
of this invention are:
1. Arsenic is recycled; therefore only
a small makeup of supply of arsenic
trioxide is required: and
2. The cement copper residue is
considerably upgraded during
treatment, the final residue
containing more than 70~ copper
and less than 1% arsenic.
The copper arsenate product is derived from zinc
plant cement copper residue in a four stage process compris-
ing:
1. An acid leach in which about 74% of
the zinc, 82% of the cadmium and
87% of the cobalt are extracted.
2. A cobalt removal stage in which
cobalt is precipitated from the
acid leach filtrate leaving a
zinc/cadmium solution suitable
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lOq~73 "
for recycle to the zinc plant.
3. A hot aerated caustic leach in
which over 90% of the arsenic is
extracted from the acid leach
residue.
4. An arsenic removal stage in which
arsenic is precipitated as copper
arsenate for the caustic leach
filtrate, again leaving a solution
which is returnable to the zinc
plant~
Expected benefits from the present invention
include:
1. Reduced expenditure for arsenic
trioxide as a purification reagent.
2. Reduced dependence on outside
sources for supply of arsenic
trioxide. -
3. Increased value of upgraded copper
2~ residue.
While one embodiment of the present invention has
been shown and described herein, it is to be understood
that certain changes and/or additions may be made thereto
by those skilled in the art without departing from the
scope and spirit of the invention.
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