Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to the dehalogenation of
o~idic zinc-bearing materials and, more particularly, to the
dehalogenation of zinc oxide fume by leaching.
In processes for the recovery of lead and zinc from
sulfide concentrates, intermediate products such as fumes and
sublimates are obtained which contain a major portion of zinc
oxide, as well as lead as lead sulfate and lead oxide, and
minor metals such as cadmium, arsenic, antimony and others.
Such products usually also contain small amounts of chloride
and fluoride, the presence of which i5 undesirable during
further processing. Several methods have therefore been
developed for dehalogenation.
In the conventional dehalogenation of the oxidic
zinc-bearing materials, the chloride and fluoride content is
reduced by roasting or calcining~ but only partial removal
is obtained. Chloride can be removed further by treating
subsequently obtained zinc-containing solutions for precipita-
tion of chloride as cuprous chloride or silver chloride, but
the fluoride content can not be reduced in such solutions to
a satisfactory low level.
It has now been found that chloride and fluoride
can be removed from oxidic zinc-beariny materials t~ the
desired low concentrations by submitting the materials to an
aqueous leach with sodium carbonate, prior to dissolving such
materials to obtain zinc-containing solution.
It is, therefore, an object of the present invention
to provide a method for the dehalogenation of oxidized zinc-
bearing materials.
It is another object to provide a process for the
leaching of zinc oxide-containing ~umes and sublimates with a
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sodium carbonate solution to extract a major portion of
contained chloride and fluoride.
Qther ~bjects will become apparent from the
following description of the process.
Accordingly, there is provided a method for the
dehalogenation of oxidic zinc-bearing materials containing
chloride and fluoride which method comprises the steps of
leaching major portions of said chloride and fluoride from
said materials by forming a reaction slurry consisting of
a mixture of said materials, sodium carbonate and water,
said sodium carhonate being present in the reaction slurry
in an amount sufficient to maintain the pH of the reaction
slurry at a value ~bove about 7; agitating said reaction
slurry during said l~aching for a period of time in the range
of about 10 minutes to 8 hours; separating reaction slurry
into a solids fraction and a liquid fraction; and recovering
said solids fraction as dehalogenated oxidic zinc-bearing
materials.
Preferably, the reaction slurry contains oxidic
zinc-bearing materials in an amount of from 15 to 25~ by
weight of the slurry; the leaching is carried out at a pH
in the range of about 8.5 to 9.0 and at ambient temperatures.
The method will now be described in detail. Oxidic
zinc-bearing materials, referred to hereinafter as oxide fume,
consist mainly ~f zinc oxide, lead oxide, lead sulfate and
minor amounts of metal halides such as chlorides and fluorides,
as well as small amounts of other compounds containing metals
such as cadmium, arsenic, antimony, germanium, thallium, etc.
The oxide fume is leached in an aqueous medium with sodium
carbonate. In the leach, the sodium carbonate reacts with the
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metal halides to form sodium halides and with lead sulfate
to form an insoluble basic lead carbonate and sodium sulfate.
The chemical reactions are believed to proceed according to
the following equations:
3Pb SO + 3 N~2 C3 + H2O ~ Pb3 ~OH)2 (C3)2 2 4 2
3Pb (Cl F) ~ 3 Na2 CO3 + H2O ~ Pb3 (OH)2 (CO3)2 2
The zinc and lead oxides remain unchanged. Some
carbon dioxide reacts with sodium carbonate to form sodium
bicarbonate according to:
C2 ~ Na2 CO3 + H2O ~ 2 Na H CO3
During the leaeh the sodium halides and sodium
sulfate, as well as any other water soluble compounds present,
are subs~antially extracted in the leach solution leaving a
solids residue of dehalogenated fume containing zinc, lead
and other insoluble compounds.
The leach may be carried out batchwise or in multiple-
stage co-current or countercurrent fashion. The leach is
carried out at ambient temperatures. If desired, elevated
temperatures may be used, but use of elevated temperatures does
not significantly increase the extraction efficiency. Oxide
fume and sodium carbonate are fed to the leach either as solids
or as aqueous slurries to form a reaction slurry. When fed in
the solid form, sodium carbonate is added to a leach vessel
containing water or reaction slurry while oxide fume is added,
both the sodium carbonate and oxide fume added at rates so as
to ensure that the desired pH of the reaction slurry, to be
discussed, is maintained.
The solids content of th~ reaction slurry must be
controlled. Some fumes, or components of fume, have character-
istics which may cause the hardening or setting of a slurry of
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such fumes and, therefore, may cause difficulties in the
handling and processing of such fumes. Too high a solids
content may also cause problems because of too high a viscosity
of the slurry. Although it is possible to treat slurries having
a solids content of as high as 50%, it is preferred to process
reaction slurries having a solids content in the range of about
15 to 25% by weight of said slurry.
The amount of sodium carbonate in the leach depends
on the nature of the oxide fume treated and is generally
proportional to the anion content of the fumeO The use of an
amount of sodium carbonate which is approximately stoichiometric
to the anion (mostly sulfate and small amounts of chloride and
fluoride) content of the fume ~see reaction equations above) is
satisfactory. If desired, a small excess of sodium carbonate
may be used.
The pH of the slurry is maintained above a value of
about 7. A pH above 7 permits the use of mild steel e~uipment.
The pH in the leach is preferably maintained at values in the
range of about 8.5 to 9Ø In this range r the solubility of
the metals in the reaction slurry, particular7y cadmium, lead
and zinc, is at a minimum. Conse~uently, the heavy metals
concentrations in the leach solution are maintained at the
lowest possible values.
The reaction slurry is agitated for a period of time
sufficient to ensure extraction of the chloride and fluoride
into the leach solution. Satisfactorv extraction of chloride
and fluoride is usually attained after a retention time of as
short as about 10 minutes, usually about 30 to 60 minutes. It
was found that prolonged agitation of the reaction slurry in
the leach alleviates any tend~ncy toward hardening or set~ing
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of the slurry. Prolonged agitation, for example up to 4 hours,
and as long as 8 hours, also appears to improve the extraction
of chloride and fluoride. Retention times, therefore, are in
the range of about 10 minutes to B hours.
The reaction slurry is discharged from the leach and
is subjected to a solids-liquid separation. Solids-liquid
separation is carried out by conventional methods, such as
thickening or filtration. The liquid fraction is removed from
the process and the solids fraction is recovered as dehalogena-
ted oxide fumeO If desired, the solids fraction may be washed
to remove any residual soluble halides. The solids may be
subjected to further treatment for the recovery of metal values.
For example, th~ solids fraction consisting of dehalogenated
oxide fume is mlxed with return acid obtained from a zinc
electrowinning pxocess for the dissolution of zinc values and
the formation of lead sulfate. The lead sulfate is separated
and further treated, and the zinc-containing solution is
purified and subjected to electrolysis for the recovery of zinc.
The advantages of the process of the present
invention are the efficient removal of major portions of
chloride and fluoride from oxide fume, the elimination of dry
dust handling, improved hygiene, simplicity of operation and
process control, and the use of relatively inexpensive
materials of construction.
The invention will now be illustrated by means of
the following non-limitative examples.
Example 1
7.5 kg of zinc oxide fume irom fuming furnaces
containing 50% Zn, 25% Pb, 2~4% S04/S, 0.13~ F and 0.07% Cl,
as well as minor-metals such as Cd, As, Sb, Ge and Tl, was
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mixed in a leach vessel with 30L of a sodium carbonate
solution. The carbonate solution contained an amount of
sodium carbonate sufficient to give a pH of 8.7 in the
reaction slurry. The sodium carbonate to fume weight ratio
was 1:14 and th~ reaction slurry contained 20% solids. The
reaction mixture was agitated and samples of the mixture
were taken after certain time intervals. The samples were
filtered and the solids fractions were analyzed. The analys~s
results are shown in Tahle 1.
Table 1
Solids Fractions _ Retention Time in Hours
Halogen Content in % 0.5 1 3 6 8
_ _ _
Cl 0.018 0.016 0.011 0.009 0.011
F 0.019 0.017 O.G16 00015 0.015
_ .
The samples filtrates were combined and analyzed, and
shown to contain 1.1 mg/L Pb, 1.2 mg/L Zn, 0.5 mg/L Cd,
1.8 mg/L As, 1.0 mg/L Sb, 1.5 mg/L Ge, 4.5 mg/L Irl and
5.76 g/L S04/S.
It was calculated that after 0.5 hour 74~ Cl and
85% F, and after 8 hours 84% Cll ~8% F as well as 96% of the
sulfate were removed from the oxide fume.
Exam~le 2
Using fume, similar to that used in Example 1, and
containing 50% Zn, 25% Pb, 2.6% S04/S, 0.11% F and 0.07% Cl,
the carbonate leach was conducted in two-stage countercurrent
fashion at ambient temperature. A 50% by weight slurry of
fume in water was fed at a rate of 283 ml/min solution +
283 g/min fume to a first-stage,agitated leach vessel. Also
added to the first leach v~ssel was 1100 ml/min overflow
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~.olution from the second-stage thickener. The solids
content in the first leach vessel was 20.5% and the pH was
in the range of 8.6 to 9Ø Reaction slurry from the first
leach vessel was sub~ected to thickening in the first-stage
thickener at a rate of 1383 ml/min solution + 270 g/min solids.
The first-stage thickener overflow was removed from the
process at a rate of 983 ml/min solution and analyzed, and the
underflow was directed to the second-stage leach vessel at a
rate of 400 ml/min solution + 270 g/min solids. Sodium
carbonate solution containing 26.4 g/L Na2 CO3 was fed to the
second-stage, agitated leach vessel at a rate of 1120 ml/min.
The pH in the second-stage leach was about 10.6. Reaction
slurry from the second-stage leach vessel was thickened in the
second-stage thickener from which the overflow was xeturned to
the first-stage leach vessel and the underflow was discharged
a~ a rate or 420 ml/min solution ~ 270 g/min solids. The
discharged underflow was washed, dried and analyzed. The
retention time of the oxide fume in the process was 40 minutes.
The first thickener overflow had a pH of 8.8 and contained
1.0 mg/L Pb, 1.3 mg/L Zn, 0.4 mg/L Cd, 2.0 mg/L As, 1O5 mg/L Sb,
1~8 mg/L Ge and 6.1 mg/L Tl. The dehalogentated fume contained
0.014% Cl, 0.013~ F and 0.2~ SO4/S. The calculated extractions
were 93~ SO4, 89~ F and 81~ Cl~
It will be understood that modifications can be made
in the embodiment of the invention illustrated and described
herein without departing from the scope and purview of the
invention as defined by the appended claims.
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