Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FUR UTILIZING PH CONTROL IN THE
RECOVERY OF METAL AND CHEMICAL
VALUES FROM INDUSTRIAL WASTE STREAMS
STATEMENT OF RELATED APPLICATIONS
This application is a continuation-in-part of application Serial Number
08/439,352 filed on May 11, 1995, currently pending.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a process for the recovery of metal
and chemical values including, for example, essentially pure zinc oxide and an
iron-
carbon residual, from industrial waste streams comprising zinc compounds and
iron
compounds. The present invention relates more specifically to controlling the
pH in a
process which subjects a waste materials stream comprising zinc compounds
and/or
iron compounds, such as electric arc furnace (EAF) dust, to a combination of
reducing and leaching steps in a recycling operation which recycles process
solutions
for reuse, and recovers metal and/or chemical values.
2. Related Art
U.S. Patent No. 3,849,121 to Burrows, now expired but which was assigned to
a principal of the assignee of the present invention, discloses a method for
the
selective recovery of zinc oxide from industrial waste. The Burrows method
comprises leaching a waste material with an ammonium chloride solution at
elevated
temperatures, separating iron from solution, treating the solution with zinc
metal and
cooling the solution to precipitate zinc oxide. The Burrows patent discloses a
method
to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a
series of
steps, to separate out the iron oxides and other metals. However, the material
obtained in the last step is a mixture of a small amount of zinc oxide,
hydrated zinc
phases which can include hydrates of zinc oxide and zinc hydroxide, as well as
other
phases, and a large amount of diamino zinc dichloride Zn(NHj)ZCI, or other
similar
compounds containing zinc and chlorine ions. Currently, the Burrows method is
not
economically viable because of Environmental Protection Agency guidelines
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established subsequent to the issuance of the Burrows patent. Additionally,
the
Burrows method is not a continuous method and, therefore, is not economical as
a
continuous process.
The first step in the Burrows patent is the treating of the EAP dust with an
ammonium chloride solution. The action of the treatment is the leaching of
zinc
oxide, lead oxide and cadmium oxide in the solution without any leaching of
the iron
oxides present. Burrows does not teach the control of the solubility of zinc
compounds in the ammonium chloride solution, other than by temperature
variation.
As a result, the Burrows method does not disclose or contemplate controlling
solubility by controlling pH.
U.S. Patent No. 4,071,357 to Peters discloses a method for recovering metal
values which includes a steam distillation step and a calcining step to
precipitate zinc
carbonate and to convert the zinc carbonate to zinc oxide, respectively.
Peters further
discloses the use of a solution containing approximately equal amounts of
ammonia
and carbon to leach the flue dust at room temperature, resulting in the
extraction of
only about half of the zinc in the dust, almost 7% of the iron, less than 5%
of the lead,
and less than half of the cadmium.
Steam distillation precipitates zinc carbonate, other carbonates and iron
impurities. Steam distillation disadvantageously results in an increase in
temperature
which drives off ammonia and carbon dioxide, resulting in the precipitation of
iron
impurities and then zinc carbonate and other dissolved metals. Temperature
lowering
advantageously precipitates a number of crystalline zinc compounds. The purity
of
the zinc carbonate obtained depends on the rate of steam distillation and the
efficiency of solids separation as a function of time. Peters does not
disclose or
contemplate controlling solubility by controlling pH to control the amount and
effect
of precipitation. In addition to the advantages of temperature lowering, the
present
process also employs steps to control the solubility of the product solution
by the
variation of the pH of the product solution.
Copending application Serial No. 08/439,352, of which this application is a
continuation-in-part, describes a method for the recovery of zinc products
from
industrial waste streams by treating the waste streams with carbon and an
ammonium
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_ chloride solution and crystallizing the zinc products to remove them from
the
solution. However, the crystallization of zinc compounds can be unpredictable
and
difficult to control at times. For example, the solubility of the zinc
compounds may
vary depending upon the composition of the waste stream. Increased solubility
~ renders it more difficult to crystallize the compounds in the
crystallization step. On
the other hand, decreased solubility may lead to premature crystallization of
the
compounds. Both problems reduce the operational efficiency and economic
viability
of the process.
Thus, there exists a need for a method of recovering metal and chemical
values which provides for the enhanced control of the solubility of zinc
compounds in
an ammonium chloride solution. There also exists a need for an improved method
of
purifying zinc oxide which utilizes controlled precipitation of zinc oxide
from an
ammonium chloride solution. Further, this need also relates to processes for
producing iron-based feedstocks from industrial waste streams.
BRIEF SUMMARY OF THE INVENTION
The present invention satisfies these needs in a method which recovers metal
and/or chemical values from waste materials containing, inter alia, zinc or
zinc oxide
and/or iron or iron oxide, in which the solubility of certain zinc compounds
is
controlled through control of the pl-I of the product solution. Essentially
pure zinc
oxide can be recovered, along with zinc metal if desired, and values of other
metallic
elements contained in the waste material such as lead, copper, silver, and
cadmium.
The solutions used in the process are recycled such that the process does not
have any
liquid wastes. The solids recovered from the process, namely, the zinc oxide,
zinc,
metal values, and other residues all can be used in other processes. For
example,
some residue may be used directly as the feedstock for the typical iron or
steel
production process.
Briefly, the waste material, typically a fly ash, baghouse dust, or flue dust
such as EAF dust, is heated and reduced to decompose franklinite to zinc oxide
and to
reduce any iron oxide present to direct reduced iron. The fumes from the
heated
waste material, which typically comprise the majority of the solids from the
waste
materials, then are leached with an ammonium chloride solution resulting in a
product
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solution and undissolved materials. The product solution and the undissolved
materials are separated, with both the product solution and the undissolved
materials
being further treated to recover valuable components. Zinc metal is added to
the
product solution to cement out any lead and cadmium contained in the product
solution. The remaining product solution is rich in zinc compounds, which can
be
recovered via a crystallization step. The undissolved materials contain inerts
and
some alkali salts.
Waste material streams and the fumes from the heated waste material streams
commonly contain chloride in the form of alkali chlorides (sodium and
potassium),
zinc chlorides, lead chlorides, and other complex metal salts. Leaching these
waste
material streams and/or fumes in an ammonium chloride solution may result in a
product solution with a low pH (less than about G.3). The lower pH renders the
zinc
compounds, namely, diamino zinc dichloride, more soluble at a given
concentration
and temperature, and, therefore, more difficult to crystallize in the
crystallization step.
If the pH is higher, on the other hand, the solubility of the zinc compounds
at a given
concentration and temperature is decreased, which may lead to premature
crystallization of the zinc compounds.
Therefore, the solubility of zinc compounds at a given concentration and
temperature can be controlled by monitoring and adjustment of the pH at
various
stages of the process. Preferably, the pH of the product solution is kept low
by the
addition of a suitable acid until the product solution reaches the
crystallizer. This will
prevent crystallization from occurring prematurely. Once the product solution
reaches the crystallizer, a suitable base, such as ammonia, is added to
decrease zinc
compound solubility and facilitate crystallization.
The remaining product solution then can be treated in two manners. First, the
remaining product solution can be cooled thereby precipitating the zinc
components
from the product solution as a mixture of crystallized zinc compounds. These
crystallized zinc compounds are separated from the product solution, washed
and then
dried at elevated temperatures, resulting in a zinc oxide product of 99% or
greater
purity. Second, the remaining product solution can be subjected to
electrolysis in
which zinc metal plates onto the cathode of the electrolysis cell. Any
remaining
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- product solution after crystallization or electrolysis is recycled back to
treat incoming
waste material. Both of these processes can be carried out in a continuous
manner.
Therefore, it is an object of the present invention is to provide a method for
recovering zinc metal, zinc oxide, DRI, and/or iron oxide which is economical,
quick
and efficient.
It is another object of the present invention is to provide a zinc oxide
purification process which utilizes controlled precipitation of zinc oxide out
of an
ammonium chloride solution.
These objects and other objects, features and advantages of the present
invention will become apparent to one skilled in the art when the following
Detailed
Description of a Preferred Embodiment is read in conjunction with the attached
figures.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The terminology used in this specification is for the purpose of describing
particular embodiments only and is not intended to be limiting. As used in the
specification and the appended claims, the singular forms "a," "an," and "the"
include
plural references unless the context clearly dictates otherwise. The
"effective
amount" of a compound as provided in this specification is meant to be a
sufficient
amount of the compound to provide the desired result. As will be pointed out
below,
the exact amount required will vary, depending on the composition of the waste
stream and the compound employed. Thus, it is not always possible to specify
an
exact "effective amount." However, an appropriate effective amount may be
determined by one of ordinary skill in the art using only routine
experimentation. The
term "suitable" is used to refer to a moiety which is compatible with the
appropriate
compounds for the stated purpose. Suitability for the stated purpose also may
be
determined by one of ordinary skill in the art using only routine
experimentation.
The method for recovering metal and chemical values disclosed herein is
carried out in its best mode in recovering these values from the waste streams
of
industrial or other processes. A typical industrial waste stream used is a
flue dust
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where the charge contains galvanized steel, having the following percent
composition:
TABLE I
Analysis of Flue Dust
Component Weight Percent
zinc oxide 30.00
iron oxide 40.00
lead oxide and lead chloride 6.48
inert materials' 9,1 p
sodium oxide and sodium chloride 5.00
calcium oxide 2.80
potassium oxide and potassium chloride 3.00
manganese oxide 1.29
tin oxide 1.13
aluminum oxide 0.38
magnesium oxide 0.33
chromium oxide 0.16
copper oxide 0.06
silver 0.05
unidentified materials' 0.22
Generally, the present process is an improved continuous method for the
recovery of metal and/or chemical values from waste material streams which
comprise zinc and iron compounds, siliceous material and salts comprising the
steps
of:
a. heating the waste material at an elevated temperature and in a
reducing atmosphere;
'siliceous material, such as slag, with carbon granules occluded.
=molybdenum, antimony, indium, cadmium, germanium, bismuth, titanium, nickel
and boron.
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b. treating the fumes from the heating step with an ammonium
chloride solution at an elevated temperature to form a product solution which
comprises dissolved zinc compounds, and an undissolved material which
comprises
siliceous material salts, and, depending on the feed composition, small
amounts of
iron compounds;
c. separating the product solution from the undissolved materials;
d. determining the pH of the product solution and if the pH of the
product solution is above 6.3 then adding an effective amount of a suitable
compound
to the product solution to obtain a pH of less than about 6.3;
e. adding zinc metal to the product solution whereby any lead and
cadmium ions contained within the product solution are displaced by the zinc
metal
and precipitate out of the product solution as lead and cadmium metals;
f. separating the product solution from the lead and cadmium
metals;
g. adding an effective amount of a suitable compound to the
product solution to obtain a pH of about 6.5 to about 7.0;
h. lowering the temperature of the product solution thereby
precipitating the zinc component as a mixture of crystallized zinc compounds;
and
i. separating the precipitated zinc compounds from the product
solution.
The precipitated zinc compounds can be further treated to produce one or
more high purity zinc-based products. The undissolved materials can be used as
is or
further treated to be used for the feedstock to iron and/or steel making
processes. The
remaining product solution also can be further treated to recover additional
metal
and/or chemical values.
The initial leaching step can be performed either on fumes produced by the
reducing step or on raw dust. One skilled in the art will recognize that
various
reduce-leach-reduce or leach-reduce-leach combinations are compatible with
this
method, as described in related applications. A two-stage leaching process
will
provide greater yields of zinc oxide, and a two-stage heating (reducing)
process will
provide greater yields of iron-based feedstocks.
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An ammonium chloride solution in water is prepared in known quantities and
concentrations. If the two-stage leaching process is used (leach-reduce-
leach), the
feed material, such as the waste material flue dust described in Table I or
any other
feed material source which contains zinc, zinc oxide, iron and/or iron oxide
mixed
with other metals, is added to the ammonium chloride solution at a temperature
of
about 90°C or above. Otherwise, the feed material first is reduced by
heating the feed
material in a reducing atmosphere. The iron oxide is reduced to ferrous oxide
(Fe0)
in the reducing step to ensure no iron solubility. The zinc and/or zinc oxide
dissolves
in the ammonium chloride solution along with other metal oxides, such as lead
oxide
and cadmium oxide. The ferrous oxide and the remaining iron oxide does not
dissolve in the ammonium chloride solution. The solubility of zinc oxide in
ammonium chloride solutions is shown in Table II.
TABLE II
Solubility of Zn0 in 23% NH4C1 solution
Temperature °C ~ Dissolved/100 ~
HO
90 14.6
80 13.3
70 8.4
60 5.0
SO 3.7
40 2.3
An 18%-23% by weight ammonium chloride solution in water at a
temperature of at least 90°C provides the best solubility of zinc
oxide, with a 23% by
weight ammonium chloride solution being the most preferred. Concentrations of
ammonium chloride below about 23% do not dissolve the maximum amount of zinc
oxide from the flue dust, and concentrations of ammonium chloride above about
23%
tend to precipitate out ammonium chloride along with the zinc oxide when the
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solution is cooled. Iron oxide and inert materials such as silicates will not
dissolve in
the preferred solution.
At least a portion of the zinc oxide, as well as smaller concentrations of
lead
or cadmium oxide, may be removed from the initial dust by the dissolution in
the
S ammonium chloride solution in a first leaching step. The solid remaining
after this
leaching step contains zinc, iron, lead and cadmium, and possibly some other
impurities. The remaining solid then is heated in a reducing atmosphere,
typically at
a temperature greater than 420°C and often at 700°C to
900°C. Preferably, the feed
material is first heated in a reducing atmosphere, at the previously mentioned
temperature. The reducing atmosphere can be created by using hydrogen gas,
simple
carbon species gases such as carbon dioxide, or by heating the material in an
oxygen
containing gas in the presence of elemental carbon. The carbon preferably is
in the
form of dust or pellets. Typical heating times are from 30 minutes to 4 hours.
The typical feed dust contains from 15% - 30% zinc by weight. X-ray
diffraction indicates the existence of certain crystalline phases in this
dust,
specifically zinc oxide. The positive identification of the iron phase is
complicated
by the possible structural types (i.e. spinet type iron phases showing almost
identical
diffraction patterns). The combination of chemical analysis and x-ray
diffraction
indicates that the feed dusts typically comprise a combination of magnetite
(iron
oxide: Fe304). Both of these phases have very similar spinet type structures.
The zinc
within the franklinite, (Fe, Mn, Zn)(FeMn)z04, cannot be removed by
dissolution with
ammonium chloride. In addition, no simple extraction process will remove zinc
from
this stable oxide phase. Although franklinite is very stable to oxidation (all
elements
in the highest oxidation state), it is relatively easy to destroy this
compound by
reduction at elevated temperatures.
The reducing step can be carried out prior to the initial leaching step, or
between a first and second leaching step. The waste dust is heated to
temperatures
greater than 500°C. This temperature causes a reaction which causes a
decomposition
of the stable franklinite phase into zinc oxide and other components, and yet
does not
allow for the complete reduction of zinc oxide to zinc metal. The resulting
zinc oxide
can be removed by sublimation or extraction with an ammonium chloride
solution,
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_ such as by following the steps detailed above under the general process. The
resulting material after extraction has less than 1 % by weight zinc. Further,
when
heated in a reducing atmosphere, iron oxide will be reduced to form direct
reduced
iron.
5 The dust can be reduced using many conventional reduction processes, such
as, for example, direct or indirect heating and the passing of hot gases
through the
dust. For example, non-explosive mixtures of reducing gases, such as for
example
hydrogen gas and nitrogen or carbon dioxide, can be passed through the dust.
I-lydrogen gas is not the only species that may be used for reductive
decomposition of
10 franklinite and the reduction of iron oxide. It is possible to use carbon
or simple
carbon containing species, including carbon-containing reducing gases and
elemental
carbon. Heterogeneous gas phase reductions are faster than solid state
reductions at
lower temperatures and therefore suggest the use of carbon monoxide. The
carbon
monoxide can be generated in situ by mixing the franklinite powder with carbon
and
heating in the presence of oxygen at elevated temperatures. The oxygen
concentration is controlled to optimize CO production. The carbon monoxide may
be
introduced as a separate source to more clearly separate the rate of carbon
monoxide
preparation from the rate of Franklinite decomposition. The prepared zinc
oxide then
can be removed by either ammonium chloride extraction or sublimation.
The reduction process also can be performed to complete reduction by using
carbon at high temperatures and collecting zinc metal that will melt at very
low
temperatures (420°C) and boil at 907°C. In this process, zinc
metal is obtained that,
if desired, can be converted readily to the oxide by air roasting.
After the dust has been reduced by heating in a reducing atmosphere, the
fumes created by the heating step typically comprise the majority of the
solids from
the dust and are subjected to a leaching step in the 18% - 23% ammonium
chloride
solution in water at a temperature of at least 90°C. Any zinc or zinc
oxide formed
during the reducing step dissolves in the ammonium chloride solution. The zinc
oxide and ammonium chloride solution then is filtered to remove any
undissolved
material, including the iron oxide.
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Because the temperature at which zinc compounds (specifically, for example,
diamino zinc chloride) will crystallize at a given zinc concentration
decreases with
pH, the pH of the product solution is kept below about 6.3 prior to reaching
the
crystallizer. The lower pH will prevent premature crystallization of the zinc
compounds. Premature crystallization reduces the zinc recovery rate and may
clog
transfer lines or other parts of the recovery apparatus. The pH can be
determined
either through knowledge of the composition of the waste material stream, by
direct
measurement, or by other known methods. As stated above, the product solution
pH
often will be low due to chlorides present in the waste material stream.
However, if it
is necessary to further reduce the pH of the product solution, one skilled in
the art will
recognize suitable compounds to add to the product solution in an effective
amount to
reduce the pH. The presently preferred compound is hydrochloric acid (HC1).
For example, zinc rich fume dust from a rotary hearth furnace has a typical
approximate composition of:
70% Zn0
6% Pb
3% Na
3% K
1 I% C1
3% Insoluble
4% Other
When this dust is leached with 20% ammonium chloride solution, the resulting
product solution will have a pH of 5.9-6.3. This product solution will not
prematurely crystallize prior to the crystallizes due to its lower pH, and
thus needs no
pH adjustment. However, as discussed below, an upward pH adjustment will be
necessary at the crystallizes.
While one skilled in the art will recognize that the pH of the product
solution
may be adjusted at various stages of the process, care should be taken in
doing so
before the undissolved materials including iron oxide are removed. If the pH
is
strongly acidic at this point, the iron oxide may dissolve into the solution.
It is
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preferable to keep the iron oxide, and any other iron compounds, out of
solution for
ease of recovery and use in other processes.
To recover the zinc oxide, while the filtered zinc oxide and ammonium
chloride solution is still hot, that is at a temperature of 90°C or
above, finely
powdered zinc metal is added to the solution. Through an electrochemical
reaction,
any lead metal and cadmium in solution plates out onto the surfaces of the
zinc metal
particles. The addition of sufficient powdered zinc metal results in the
removal of
virtually all of the lead of the solution. The solution then is filtered to
remove the
solid lead, copper, zinc and cadmium.
Powdered zinc metal alone may be added to the zinc oxide and ammonium
chloride solution in order to remove the solid lead and cadmium. However, the
zinc
powder typically aggregates to form large clumps in the solution which sink to
the
bottom of the vessel. Rapid agitation typically will not prevent this
aggregation from
occurring; however, mixing with high shear forces may. To keep the zinc powder
I 5 suspended in the zinc oxide and ammonium chloride solution, any one of a
number of
water soluble polymers which act as antiflocculants or dispersants also may be
used.
In addition, a number of surface active materials also will act to keep the
zinc powder
suspended, as will many compounds used in scale control. These materials only
need
be present in concentrations of 10 - 1000 ppm. Various suitable materials
include
water soluble polymer dispersants, scale controllers, and surfactants, such as
lignosulfonates, polyphosphates, polyacrylates, polymethacrylates, malefic
anhydride
copolymers, polymaleic anhydride, phosphate esters and phosponates. A
discussion
of these various materials can be found in the literature, such as Drew,
Principles of
Industrial Waste Treatment, pages 79-84, which is incorporated herein by
reference.
Flocon 100 and other members of the Flocon series of malefic-based acrylic
oligomers
of various molecular weights of water soluble polymers, produced by FMC
Corporation, also are effective. Adding the dispersants to a very high ionic
strength
solution containing a wide variety of ionic species is anathema to standard
practice as
dispersants often are not soluble in such high ionic strength solutions.
At this stage there is a filtrate rich in zinc compounds and a precipitate of
lead,
cadmium and other products. The filtrate and precipitate are separated, with
the
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_ precipitate being further treated, if desired, to capture chemical values.
The filtrate
may be treated in several manners, two of which are preferred. First, the
filtrate may
be cooled resulting in the crystallization and recovery of zinc oxide. Second,
the
filtrate may be subjected to electrolysis resulting in the generation and
recovery of
metallic zinc.
To facilitate crystallization, the product solution pH is raised to about 6.5
to
about 7.0 immediately prior to crystallization. Increasing the pH of the
product
solution reduces the solubility of the zinc compounds at a given concentration
and
temperature, and thereby increases crystallization. One skilled in the art
will
recognize suitable compounds to add to the product solution in an effective
amount to
increase the pH. The presently preferred compounds are ammonium hydroxide and
ammonia. Addition of these compounds results in the formation of additional
ammonium chloride which increases the pH.
The filtrate can then be treated to crystallize out diamino zinc dichloride
and
other complex compounds. This can be done in either a batch or continuous
crystallizer by cooling the filtrate to between 20°C and 60°C.
The crystallized
diamino zinc dichloride then is added to 25°C to 100°C water to
decompose it into
zinc oxide and ammonium chloride. The amount and temperature of the water
controls the decomposition of the diamino salt to zinc oxide and thus,
influences
particle size and chloride content as described in related applications. The
solid
hydrated zinc oxide species are filtered from the solution and dried at a
temperature of
100°C - 350°C for 5 seconds to 5 minutes.
As the zinc, lead and cadmium contained in the feed materials are amphoteric
species, by using ammonium chloride solution these species will go into
solution,
while any iron oxide present in the feed material will not go into solution.
Other
solutions, such as strong basic solutions having a pH greater than about 10 or
strong
acidic solutions having a pI-I less than about 3, also can be used to dissolve
the zinc,
lead and cadmium species; however, if strong acidic solutions are used, iron
oxide
will dissolve into the solution, and if strong basic solutions are used, iron
oxide will
become gelatinous. The lead and cadmium can be removed from the ammonium
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chloride solution through an electrochemical reaction which results in the
precipitation of lead and cadmium in elemental form.
The difference in solubility between diamino zinc dichloride and zinc oxide in
water and in ammonium chloride solutions allows the selective dissolution of
the
diamino zinc dichloride such that pure zinc oxide can be recovered. This also
can be
used in the crystallization step to improve the relative amounts of diamino
zinc
dichloride and zinc oxide species form. Significantly, all of the zinc can be
recycled
so that all of the zinc eventually will be converted into zinc oxide. The
crystallization
step can be done continuously in order to increase the throughput and maximize
the
zinc oxide yield after the washing and drying step.
During the crystallization step, it is preferable to use a reverse natural
cooling
profile. Such a profile is the opposite shape as that which is observed by
natural
cooling. In a reverse natural cooling profile, the cooling is slower at the
beginning
and faster at the end; in a natural cooling profile, the cooling is faster at
the beginning
1 ~ and slower at the end. Controlling the temperature with a reverse natural
cooling
curve results in a larger average crystal size than by linear cooling or
natural cooling
which improves the filtration rate.
To produce pure zinc oxide from waste dust containing zinc efficiently and in
a safe and cost effective way, the process recycles all zinc which is not
removed from
the leachate in the crystallization step. In addition, the diamino zinc
dichloride and
ammonium chloride which is redissolved in water in the washing step also is
recycled, fhe recycle of zinc increases the overall zinc concentration in
liquid
solution in the process. This allows the crystallizer to operate at a higher
temperature
due to the rapid change in zinc oxide solubility with temperature in ammonium
chloride solution.
The recycle has the advantage in that the solution may become saturated
relative to certain materials present in the dust, such as CaO. When this
occurs, Ca0
no longer is leached from the dust but remains with the iron. This increases
the value
since Ca0 is still present and will not have to be added when the iron cake is
fed to a
furnace in steel making. Another important advantage in that there is no
liquid
effluent in this process. The only products are solid (iron cake, zinc oxide,
and other
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metals), which are then sold for use in various industrial processes. No waste
is
produced since all liquid is recycled.
The process also can be operated to produce a high-quality iron-carbon cake
as a residual product. The iron oxide contained in the waste stream does not
go into
solution in the ammonium chloride solution, but is filtered from the product
solution
as undissolved material. This iron oxide cake can be used as is as the
feedstock to a
steel mill; however, it becomes more valuable if reduced by reaction with
elemental
carbon to produce an iron-carbon or direct-reduced iron product. One preferred
method for producing such an iron-carbon or direct-reduced iron product from
the
waste material is to add carbon to the product solution whereby the carbon
will not go
into solution, and then separate the product solution from any undissolved
materials
present in the product solution including any of the iron oxide and the
carbon.
Combining carbon and iron oxide results in the reduction of the iron oxide,
producing
direct-reduced iron (DRI). Generally the iron oxide and carbon product is
pressed
into a cake for ease of handling and use.
The reducing process produces vapors, from the zinc, lead and cadmium and
other impurities, that have to be condensed into dust. These impurities can be
sent to
the baghouse at the end of the steel making process, mixed into the original
waste
dust, and then sent to the first leaching step, in a recycle fashion.
Alternatively, the
exhaust vapors and dust from the reducing step may be sent to a separate
baghouse at
a stand alone facility.
The fumes exhausting from the steel mill furnace and the reduction furnace
typically are iron poor, but comprise other valuable components. The furnace
exhaust
fumes are an excellent source of iron poor waste materials useful for recovery
in the
present process. The exhaust fumes may be filtered in a baghouse, with the
resulting
filtrate being added to the waste stream feed of the present process, or with
the
resulting filtrate being the primary waste stream feed of the present process.
The
exhaust fumes also may be scrubbed in a wet scrubber, with the resulting
loaded
scrubbing solution being added to the ammonium chloride leachant of the
present
process. If an ammonium chloride scrubbing solution is used instead of water,
the
CA 02293687 1999-10-13
WO 98/48066 PCT/US98/07880
16
loaded ammonium chloride scrubbing solution may be used as the primary
leachant
of the present process.
The process also can be operated to recover zinc metal by replacing the
crystallization steps with an electrolysis step. One preferred method for the
recovery
of zinc oxide from waste material streams which comprise zinc compounds using
electrolysis comprises subjecting the final product solution to electrolysis
to extract
zinc metal from said combined product solution. The product solution from the
leaching steps comprises zinc ions in solution as Zn2+. When the product
solution is
subjected to electrolysis in an electrolytic cell containing an anode and a
cathode, the
zinc metal is electrodeposited on the cathode. Although it is preferable to
have the
cathode made from zinc metal, cathodes of other material also will allow the
electrodeposition of zinc metal from the combined product solution. Any of the
electrolysis cells discussed in the literature are suitable, as long as such
cells are
configured for the electrolysis of zinc ion containing solutions.
The product solution also contains sodium, potassium, magnesium, calcium,
and other solubles in solution. These solubles can be recovered by introducing
an
electrolyte either in the leaching step or in the ammonium chloride storage
tanks
receiving the recycled product solution. As ammonium chloride is used as the
leachant, ammonium salts in solution is the preferred electrolyte. For
example, if
some ammonium sulfate is added, one could precipitate out calcium sulfate.
Ammonium sulfate is a preferred electrolyte to add because the process already
uses
ammonium in the form of ammonium chloride. The preferred electrolytes include
ammonium sulfate, ammonium hydroxide, or ammonium carbonate to precipitate out
various solubles.
The above description sets forth the best mode of the invention as known to
the inventor at this time, as it is obvious to one skilled in the art to make
modifications to this process without departing from the spirit and scope of
the
invention and its equivalents as set forth in the appended claims.
