Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR THE RECOVERY OF GROUP IA SALTS DURING
TREATMENT OF INDUSTRIAL PROCESS WASTE STREAMS
STATEMENT OF RELATED APPLICATIONS
This application is a continuation-in-part of application Serial Number
08/604,178, filed February 21, 1996.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a process for the recovery of
usable
economically valuable products from industrial waste streams typically
comprising
zinc compounds and iron compounds. The present invention relates specifically
to a
process for the recovery of Group IA salts from industrial waste streams
comprising
Group IA compounds along with zinc compounds and iron compounds, in an overall
process in which a relatively pure iron or direct reduced iron product
feedstock and a
very pure zinc oxide product are produced.
The specific improvement of the present invention is an additional process for
recovering sodium chloride and potassium chloride from a waste material cake
resulting after a waste stream from a metals-related industrial process has
been treated
to remove a significant portion of any iron and zinc compounds.
2. Prior Art
Industrial waste streams typically contain components which have economic
value if they can be recovered in an economic fashion. For example, 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
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out and discard the iron oxides and waste metals, so that the resulting zinc-
compound-
rich solution can be further treated to recover the zinc compounds.
Waste metal process dust typically has varying amounts of other components,
in various forms, such as Group IA elements including sodium and potassium,
contained in the dust. The Burrows patent does not teach the treatment or
recovery of
any values from the discarded iron oxide containing precipitates, and does not
discuss
any method of recovering Group IA salts, such as sodium chloride and potassium
chloride, from the process.
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. However, Peters does not disclose a method
for
further treating the removed components not containing zinc compounds, nor of
recovering Group IA salts from the process.
As can be seen, there exists a need for a method which will allow the
continuous treatment of exhausts and fumes from reduction furnaces or the like
to
recover Group IA salt values. This need is addressed by the present invention.
BRIEF SUMMARY OF THE INVENTION
The present invention satisfies this need in a method which recovers Group IA
salts in conjunction with the recovery of a relatively pure iron product from
a waste
material or a combination of waste materials from industrial processes, such
as waste
streams from electric arc furnaces, typically containing zinc or zinc oxide
and iron or
iron oxide, and exhaust fumes from reduction furnaces, which typically are
iron-poor.
The non-iron solids and feed and product solutions used and/or produced in the
overall process can be recycled such that the process has minimal solid or
liquid
wastes. Other solids can be recovered by treating other compounds in the waste
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materials, for example zinc oxide, zinc, metal values, and other residues, all
of which
can be used in other processes. As an alternative embodiment, iron-rich waste
products, such as for example mill scale and used batteries, also can be added
to the
waste stream feed of the present process.
A waste materials stream such as electric arc furnace (EAF) dust or tha flue
dust disclosed in Table I herein, is subjected to a combination of processing
steps,
resulting ultimately in the recovery of certain Group IA salts. An enriched
iron
compound (an enriched iron cake or EIC) which can be used as a feedstock for
steel
mills, and other products of value, also can be produced from the general
process, and
are disclosed in and/or covered by other patent applications and patents
assigned to
Metals Recycling Technologies Corporation of Atlanta, Georgia US, the assignee
of
this invention. The EIC typically is rich in direct reduced iron (DRI).
Preferably, the
precipitate containing iron oxides is removed from a process for the recovery
of zinc
oxide and zinc metal from industrial waste streams. During the recovery
process,
I S carbon compounds can be added to the waste stream, and a cake product is
produced
from the undissolved iron and carbon compounds, which also can be used as a
feedstock for steel mills.
In a preferred embodiment of the process, the waste material stream is heated
in a reducing atmosphere in a reduction furnace, resulting in the reduction of
the iron
compounds into DRI, and the production of combustion products. The DRI can be
fed directly to a steel mill as a feed source, and the combustion products,
typically in
the form of exhaust dusts and fumes, are recovered in a filter means, such as
a
baghouse or wet scrubber. The exhaust dusts and fumes comprise the majority of
the
Group IA salt constituents, and the non-iron compounds, such as zinc, cadmium,
copper, lead, and calcium compounds.
EAF dust, either alone or in combination with iron-rich waste materials, mill
scale, used batteries, or other iron-rich or iron-poor waste materials may be
used as
the initial feed for the process. This combined waste first is heated in a
reducing
atmosphere, reducing any iron oxides present to usable DRI. The exhaust vapor
from
the DRI process is condensed and comprises mainly zinc, lead and cadmium
oxides
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and Group IA chlorides. This waste material then is leached with an ammonium
chloride solution resulting in a product solution (leachate) and undissolved
materials
(precipitate). At steady state the Group IA species reach saturation in the
ammonium
chloride solution and therefore do not dissolve, remaining with the solids in
the filter
cake.
In the leaching step, the Group IA salt constituents reach their saturation
concentration in the ammonium chloride solution and precipitate out. The
leachate
comprises metal oxides contained in the waste material, such as lead oxide and
cadmium oxide, and zinc and/or zinc oxide. The product solution and the
undissolved
materials are separated, with the product solution and the undissolved
materials being
further treated to recover Group IA salts and other valuable components, as
appropriate. For example, the remaining product solution can be treated to
produce a
zinc oxide product of 99% or greater purity. Alternatively, the remaining
product
solution can be subjected to electrolysis in which zinc metal plates onto the
cathode of
the electrolysis cell. Any remaining product solution after crystallization or
electrolysis can be recycled back to treat incoming waste material.
When processing EAF dusts, zinc-containing wastes and fumes from rotary
hearth furnaces, upon reaching steady state, the filter cake obtained after
the first
leaching step contains sodium chloride and potassium chloride since these have
reached their saturation concentration in the ammonium chloride solution. The
filter
cake comprises true insolubles, which are mainly silicates, and water soluble
salts,
which are mainly sodium chloride and potassium chloride. The salts can be
recovered
by:
1. Washing the filter cake with water, dissolving all water soluble salts;
2. Optionally cementing aut heavy metals such as lead using powdered
zinc; and
3. Crystallizing out sodium chloride and potassium chloride salts either
singly or mixed by selective evaporative crystallization or spray drying.
The salts then can be dried and bagged and sold.
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Therefore, it is an object of the present invention is to provide a waste
material
recovery process which recovers chemical values including Group IA salts from
industrial waste streams, recycles exhaust fumes from furnaces such as
electric arc
furnaces and reduction furnaces, recycles exhaust fumes from industrial
processes
such as iron and steel making processes, and recycles other waste materials,
including
both iron-rich and iron-poor waste materials, to produce valuable products.
Another object of the present invention is to provide a method for recovering
Group IA salts from the precipitate from an ammonium chloride leach used to
recover
zinc oxide.
These objects and other objects, features and advantages of the present
invention will become apparent to one skilled in the art after reading the
following
Detailed Description of a Preferred Embodiment.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a schematic of a representative process which includes the present
invention.
Fig. 2 is a flow chart of the process of the present invention.
Fig. 3 is a flow chart of an alternate process of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The method disclosed herein is carried out in its best mode in treating the
waste material from the waste streams of metal-making processes, industrial or
other
processes. These waste materials may be combined with other waste materials
recovered from furnace exhaust streams. Many processes produce an iron poor
waste
stream, such as reduction furnaces and iron and steel making processes. Many
other
processes produce an iron rich waste stream. Other processes remove iron rich
materials prior to processing. The iron poor materials can be combined with a
typical
industrial waste stream which, after treatment, results in an iron rich
material suitable
for use as a feedstock to a steel mill. Iron rich materials also can be
combined with
the typical industrial waste stream and the iron poor waste stream.
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A typical industrial waste stream used is a flue gas where the charge contains
galvanized steel, having the percent composition shown in Table I:
TABLEI
Analysis of Flue Dust
Component Percent By Weight
Zinc Oxide 30
Iron Oxide 40
Lead Oxide and Chloride 6.48
Inert Materials 9.10
Sodium Oxide and Chloride 5.00
Calcium Oxide 2.80
Potassium Oxide and 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
A second typical industrial waste stream used is a zinc rich fume from a
rotary
hearth furnace used in an iron-making or steel-making process, having the
percent
composition shown in Table II:
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TABLE II
Analysis of Rotary Hearth Furnace Fume
Component Percent By Weight
Zinc Oxide ~0
Lead
Sodium
Potassium 3
Chloride 11
I 0 Insoluble 3
Miscellaneous
General Process Description
Generally, the present process is a novel addition to a continuous method for
the recovery of chemical and metal values from waste material streams. The
basic
process steps for recovering Group IA salts from such a method are shown as a
flow
chart in Fig. 2 and comprise:
a. Heating a typical industrial process waste material stream comprising
Group IA compounds such as from a metal or metal product process, in a
reducing
atmosphere to produce an exhaust stream (typically fumes);
b. Treating the exhaust stream which may be a waste material
combination comprising other waste streams, with an ammonium chloride solution
at
an elevated temperature to form a product solution and an undissolved
precipitate
comprising Group IA salts;
c. Separating the product solution from the undissolved precipitate
comprising the Group IA salts;
d. Washing the undissolved precipitate to form a salt solution which
comprises the Group IA salts and an undissolved solid; and
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e. Crystallizing out the Group IA salts from the salt solution by
evaporative crystallization or spray drying.
An alternative set of process steps for recovering Group IA salts from such a
method are shown as a flow chart in Fig. 3 and comprise:
a. Adding a fume from a rotary hearth furnace comprising Group IA salts
to water to dissolve the Group IA salts;
b. Filtering out the components of the fume which do not dissolve in the
water as undissolved solids; and
c. Crystallizing out the Group IA salts from the salt solution by
evaporative crystallization or spray drying.
The salt solution may be subjected to a cementation step to remove other
compounds. The undissolved solids may be sent to a leaching solution to
recover
other chemical and/or metal values.
Preferred Embodiment
Referring to Fig. 1, a preferred embodiment of an overall waste stream
recovery process is shown. Subprocess 100, the digestion and filtration steps,
generally comprises the process disclosed and claimed in U.S. Patent No.
5,464,596.
Subprocess 200, the DRI production steps, generally comprises the process
disclosed
and claimed in U.S. applications Serial Nos. 08/348,446 and 08/665,043.
Subprocess
300, the chemical values recovery steps, when combined with subprocess 100,
generally comprises the process disclosed and claimed in U.S. Patent No.
5,453,111.
Subprocess 400, the enhanced DRI production steps, when combined with
subprocess
200, generally comprises the process disclosed and claimed in U.S. Patent No.
5,571,306. Each of subprocesses 200, 300, and 400 may be added to the general
process. The U.S. patents mentioned in this paragraph are incorporated herein
by this
reference.
Subprocess 200 comprises the leaching steps. Subprocess 500 comprises the
feed process and includes the relevant step of heating the waste stream in a
reducing
atmosphere. Feed streams such as iron poor waste fume streams from electric
arc
furnaces 12 and other furnaces such as reduction furnaces or smelters 14 are
filtered in
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a baghouse 16. Other feed streams such as iron rich DRI and pig iron, as well
as scrap
iron and steel, are subjected to the iron or steel making process. Exhaust
fumes from
such processes, which typically include an electric arc furnace or other
reduction
furnace, also are filtered in a baghouse 16. The constituents filtered out in
baghouse
S 16 comprise the waste stream feed to subprocess 100.
In subprocess 500, the waste feed stream is heated in a reducing atmosphere,
resulting in the reduction of the iron compounds into DRI. This heating
typically
occurs at between about S00°C and 1315°C, and preferably at
between 980°C and
1260°C. The DRI can be fed directly back into the industrial process,
such as a steel
making process. Exhausts from the heating step are recovered in a capture
means,
such as baghouse 16, and then subjected to the leaching and other chemical
values
recovery steps.
In subprocess 100, the waste stream feed is leached in digester 18 with
ammonium chloride, preferably at approximately 90°C and approximately
18-23% by
1 S weight concentration. Constituents soluble in ammonium chloride go into
solution,
while constituents insoluble in ammonium chloride, such as iron oxides, do not
dissolve. At steady state the Group IA salts reach their saturation
concentration in the
solution and do not dissolve. The precipitates are filtered from the solution
in filter
20. The filtered solution is sent to cementer 22, and subjected to subprocess
200 to
recover other chemical values. The precipitate, which typically is a filter
cake, is
further treated to recover the Group IA salts and/or is sent to subprocess
300.
If the precipitate is sent to subprocess 300 prior to or instead of recovering
the
Group IA salts, the precipitate is dried and crushed in dryer/crusher 24.
Exhaust gases
from dryer/crusher 24 may be sent to a baghouse such as baghouse 16, but more
typically are sent to an air scrubber such as air scrubber 26 for cleaning, as
the exhaust
gases from dryer/crusher 24 typically do not have a significant quantity of
recoverable
constituents. The dried and crushed precipitates are compacted in compactor 28
and
sent to a reduction furnace or smelter 14. In reduction furnace 14, the dried
and
crushed iron cake is heated at between 980°C and 1315°C,
producing an enriched iron
cake (EIC) which can comprise DItI and pig iron, which can be in liquid form.
The
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EIC can be compacted in a second compactor 30, and then cooled by cooling
water in
a cooling conveyor 32, to produce the DRI. The DRI then can be used as the
feed to a
steel mill EAF, and the process cycle starts over.
Exhaust fumes from the reduction furnace 14 are sent to scrubber 34, which
5 preferably is a recirculating wet scrubber using water or an aqueous
ammonium
chloride solution. Exhaust fumes from EAFs such as EAF 12 also can be sent to
scrubber 34. In scrubber 34, the exhaust fumes are scrubbed and the scrubbed
off gas
released. The water or aqueous ammonium chloride solution containing the
constituents scrubbed from the exhaust fumes is sent either to cementer 22 or
digester
10 18, depending on purity; more pure solutions typically are sent to digester
18, while
less pure solutions typically are sent to cementer 22.
In one embodiment, the furnace 12, 14 off gases comprise Zn0 and other
particulate impurities. If the off gases are scrubbed in scrubber 34, the
water balance
is maintained using a temperature control such as heat exchanger 36.
Additionally,
the concentration of Zn0 and other solubles in the scrubbing liquid may be
controlled
by the addition of water W to the cementer 22, or ammonium chloride to the
scrubber
34. If an ammonium chloride solution is used as the scrubbing liquid, it is
preferred
to maintain the solution at approximately 90°C and approximately 23%
NH4C1.
Heating In A Reducing Atmosphere
The heating step can be carried out prior to the initial leaching step, and
also
optionally between a first and second leaching step. The waste stream is
heated to
temperatures greater than 500°C, but typically no greater than
1315°C. This
temperature causes a reaction which causes a decomposition of the stable
franklinite
phase contained in the waste stream into zinc oxide and other components. The
resulting zinc oxide can be removed by sublimation or extraction with an
ammonium
chloride solution, such as by following the steps detailed above under the
general
process. The resulting material after extraction has less than 1 % by weight
zinc.
The solid waste material can be reduced using many conventional 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
hydrogen
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gas and nitrogen or carbon dioxide, can be passed through the waste material.
Hydrogen gas is not the only species that may be used for reductive
decomposition of
franklinite. 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.
Leaching Treatment
The exhaust stream from the reducing step (typically fumes) and, optionally a
portion of the waste material, is subjected to an ammonium chloride leach. An
ammonium chloride solution in water is prepared in known quantities and
concentrations. If the two-stage leaching process is used, the feed material,
such as
the exhaust stream and waste material flue dust described in Table I combined
with
any other feed material source which contains iron oxide, is added to the
ammonium
chloride solution. Otherwise, the feed material first is heated in a reducing
atmosphere. The majority of the waste mixture, including any zinc and/or zinc
oxide,
lead oxide, cadmium oxide, and other metal oxides, dissolves in the ammonium
chloride solution. The iron oxide does not dissolve in the ammonium chloride
solution. At steady state, the Group IA salts reach their saturation
concentration in the
solution and do not dissolve.
It has been found that an 18 - 23% by weight ammonium chloride solution in
water at a temperature of at least 90°C provides the best solubility
for such a waste
mixture. Concentrations of ammonium chloride below this range do not dissolve
the
maximum amount of zinc oxide from the waste mixture, and concentrations of
ammonium chloride above about this range tend to precipitate out ammonium
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chloride along with the zinc oxide when the solution is cooled. Therefore, 18 -
23%
has been chosen as the preferred ammonium chloride solution concentration. The
iron
oxide and inert materials such as silicates will not dissolve in the preferred
solution.
Ammonium sulfate may be added to the leaching solution to reduce and/or
remove excess calcium build-up during the process. The ammonium sulfate can be
added to the leach tank prior to charging with dust. The calcium sulfate which
forms
will be filtered out with the iron cake and returned to the steel making
furnace. The
calcium will calcine to calcium oxide when it is heated during the steel
making
process. This method can also be used in using a rotary hearth furnace in the
first
step. The enriched dust in this process contains small amounts of calcium so
that
treatment will still be necessary on a smaller scale. The precipitated calcium
sulfate
along with unleached solids will be returned to the rotary hearth furnace. The
calcium
sulfate will form calcium oxide and return with the iron units to the steel
making.
The zinc oxide, as well as smaller concentrations of lead or cadmium oxide,
I S are removed from the waste mixture by the dissolution in the ammonium
chloride
solution. The solid remaining after this leaching step contains Group IA
salts, iron
oxides and some impurities including zinc, lead, cadmium, and possibly some
other
impurities. By subjecting the leachate to evaporation, the leachate can be
concentrated, thus precipitating out Group IA salts. As the ammonium chloride
concentration rises, the Group IA salt solubility falls, causing additional
precipitation.
Recovery of Group IA Salts
When processing these EAF dusts, zinc-containing wastes and fumes from
rotary hearth furnaces, upon reaching steady state, the filter cake obtained
after the
first leaching step contains sodium chloride and potassium chloride since
these have
reached their saturation concentration in the ammonium chloride solution. The
filter
cake comprises true insolubles, which are mainly silicates, and water soluble
salts,
which are mainly sodium chloride and potassium chloride. The salts can be
recovered
by:
1. Washing the filter cake with water, dissolving all water soluble salts;
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2. Optionally cementing out heavy metals such as lead using powdered
zinc; and
3. Crystallizing out sodium chloride and potassium chloride salts either
singly or mixed by selective evaporative crystallization or spray drying.
These steps preferably are carried out in combination with a complete-waste
stream recycling operation as disclosed herein. If the optional cementation
step is
carried out, the heavy metals cemented out are filtered from the aqueous
solution and
sent on to a mixed metals separation step, such as Subprocess 300.
The production of Group IA salts can be carried out to continuously remove
the sodium chloride and potassium chloride salts during each cycle of an
overall waste
stream recycling process, such as that disclosed herein, so that the filter
cake would
not contain any significant amount of these salts. This can be done by taking
the
recycle stream going to the evaporator condensor and evaporating the stream to
an
ammonium chloride concentration which results in the precipitation of the
sodium
chloride and potassium chloride, since the salts' solubility goes down as the
ammonium chloride concentration goes up. The precipitated solid salts then can
be
filtered from the solution, dried and bagged. The salts then can be subjected
to further
separation into specific salts by another crystallization step.
Optional Carbon Addition Step
The present 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, as previously discussed, 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 comprises the steps of,
after first
heating the waste stream in a reducing atmosphere:
a. treating the waste material with an ammonium chloride solution
at an elevated temperature to form a product solution which comprises
dissolved zinc
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and dissolved zinc oxide whereby any iron oxide in the waste material will not
go into
solution and Group IA salts will precipitate out;
b. adding carbon to the product solution whereby the carbon will
not go into solution;
c. separating the product solution from the undissolved materials
present in the product solution including the Group IA salts, any of the iron
oxide and
the carbon; and
d. washing the undissolved materials to dissolve and remove the
Group IA salts for recovery, leaving the iron oxide and the carbon as an
undissolved
solid.
A mixture of iron oxide and carbon is used by the steel industry as a
feedstock
for electric arc furnaces. The iron oxide cake which is removed as undissolved
material from the leaching step is primarily iron oxide, being a mixture of
FeZ03 and
Fe304. The iron oxide cake can be made into the mixture of iron oxide and
carbon by
adding elemental carbon to the iron oxide cake in several manners. First,
carbon can
be added to the leaching tank at the end of the leaching step but before the
undissolved materials are separated from the product solution. Since the
carbon is not
soluble in the ammonium chloride solution and will not react in an aqueous
solution,
the iron oxide cake and the carbon can be separated from the product solution
and
made into a hard cake. Different size carbon, such as dust, granules, or
pellets, may
be used depending on the desires of the steel makers. Second, the carbon can
be
added to the iron oxide after the iron oxide has been separated from the
product
solution. The dried iron oxide and the carbon can be ribbon blended in a
separate
process. Combining carbon and iron oxide in a reducing atmosphere and at an
elevated temperature results in the reduction of the iron oxide, producing
DRI.
Generally the iron oxide and carbon product is pressed into a cake for ease of
handling and use. The cake typically contains approximately 82% solids, but
may
range from 78% to 86% solids and be easily handled and used. Although cakes of
less than 78% solids can be formed, the other 22%+ of material would be
product
solution which, if the cake is used as a feedstock to a steel mill, would be
reintroduced
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to the steel-making process, which is uneconomical. Likewise, drying the cake
to
have more than 86% solids can be uneconomical.
The iron oxide cake can be treated in three manners. First, the iron oxide-
carbon cake can go directly to the steel mill and, if it goes directly to the
steel mill,
5 then the reduction of the iron oxide would take place in the steel mill
furnace.
Second, the iron oxide-carbon cake can be palletized and roasted in a
reduction
furnace to form direct reduced iron. The iron oxide precipitate, which
typically
contains around 80% solids, is ground up with carbon and formed into pellets,
briquettes or cubes and then heated. These pellets, briquettes or cubes then
can be
10 introduced to a steel making furnace. The difference in the material that
would be
introduced to the furnace from the first manner and the second manner is that
in the
second manner, direct reduced iron is introduced to the steel making furnace,
while in
the first manner, a combination of iron oxide and carbon is introduced to the
steel
making furnace. The iron oxide plus carbon can be supplied to the steel mill
as is.
15 When this carbon enriched iron oxide is melted, it forms a foamy slag, and
a foamy
slag is desirable in steel making. Third, the carbon can be added through a
ribbon
blender, and then the iron oxide-carbon cake can be introduced either directly
into the
furnace or, preferably roasted in a reduction furnace first to form direct
reduced iron,
which would be preferred for steel making.
In order of preference, the first manner is the least preferable, that is
adding
the material itself as a mixture of carbon and iron oxide without any reducing
agents
mixed in with it. The second most preferable is the third manner, adding the
material
with carbon added to it either through the leaching step or through a ribbon
blender
and put directly into the furnace. The most preferable is the second mariner,
where
carbon is added either though the leaching step or a ribbon blender,
palletizing or
briquetting it, roasting it, and introducing it to the steel furnace.
In any manner, the fumes exhausting from the steel mill furnace and the
reduction furnace typically are iron poor, but comprise Group IA salt
constituents and
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
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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 loaded ammonium chloride
scrubbing
solution may be used as the primary leachant of the present process.
Optional Recovery of Zinc Oxide From Product Solution
To recover the zinc oxide from the product solution in subprocess 300, while
the filtered zinc oxide and ammonium chloride solution is still 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, 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 typically will.
Alternatively, to
keep the zinc powder suspended in the zinc oxide and ammonium chloride
solution,
any one of a number of water soluble polymers which act as antiflocculants or
dispersants 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. Flocon 100 and other members of the Flocon series of
malefic-based acrylic oligomers of various molecular weights of water soluble
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17
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 comprising zinc compounds, and a precipitate
of
Group IA salts, lead, cadmium and other products. The filtrate and precipitate
are
separated, with the precipitate being further treated to capture the Group IA
salts and
other chemical values. The filtrate also may be cooled resulting in the
crystallization
and recovery of zinc oxide and/or subjected to electrolysis resulting in the
generation
and recovery of metallic zinc.
The filtrate then can be treated to crystallize out the complex salt diamino
zinc
dichloride. This can be done in a conventional crystallizer by cooling the
filtrate to
the proper temperature, generally between about 20°C and 60°C.
The crystallized
diamino zinc dichloride then can be mixed with 25°C-100°C water
to decompose the
diamino zinc dichloride into zinc oxide and ammonium chloride. Particle size
may be
controlled as described in related specifications.
The zinc oxide then can be dried using a ring dryer or other drying means.
Iron By-Product Recycle
Iron-rich by-products produced during the recovery process can be processed
further to obtain an end product which can be recycled back into the leaching
step of
the recovery process of the present invention. The iron-rich by-products
preferably
are reduced to DRI in a reduction furnace. During the reduction process,
exhausts
fumes which consists primarily of zinc, lead and cadmium are produced in the
reduction furnace.
In accordance with a first embodiment, the DRI is sent to a steel mill where
it
is used in the production of steel. The steel production process results in
exhaust
fumes which are processed through the baghouse or/and a wet scrubber, either
or both
of which can be located at the steel milt. Fumes processed through the
baghouse are
filtered, and the captured solid residuum, along with an added amount of EAF
dust, is
recycled back into the waste materials stream whereby it is returned to the
leaching
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WO 98/48063 PCT/US98/07914
18
step of the recovery process. Fumes processed through the wet scrubber are
scrubbed
in a liquid stream and the residual impurities obtained from the scrubbing
process are
discharged from the wet scrubber directly into the ammonium chloride solution
of the
leaching step.
In accordance with a second embodiment, the fumes exhausted from the
reduction furnace used to produce the DRI are processed through the baghouse
or/and
the wet scrubber. Fumes processed through the baghouse are filtered, and the
captured solid residuum is recycled back into the waste material stream,
whereby it is
returned to the ammonium chloride solution of the leaching step. In this
embodiment,
no EAF dust need be added in with the solid residuum. Fumes processed through
the
wet scrubber are scrubbed in a liquid stream and the residual impurities
obtained from
the filtering process are discharged from the wet scrubber directly into the
ammonium
chloride solution of the leaching step.
The above detailed description of a preferred embodiment is for illustrative
purposes only and is not intended to limit the spirit or scope of the
invention, or its
equivalents, as defined in the appended claims.
