GALVANIC WASTE SLUDGE TREATMENT AND
MANUFACTURING OF NANO-SIZED IRON OXIDES
Field of the Invention
The present invention relates to processing waste sludge after galvanic
treatment of metals, particularly after pickling. Recycled are acids with
inhibitor additives, and provided are nano-sized iron pigments.
Background of the Invention
Pickling is a treatment of metallic surfaces before subsequent processing,
such as extrusion, painting, galvanizing, or plating, comprising removing
oxides and rust with a solution containing strong mineral acids with iron
dissilving inhibitor additive, called pickling liquer. The two acids commonly
used are hydrochloric acid and sulfuric acid. Spent pickle liquor is considered
a hazardous waste by EPA. Once, spent pickle liquors were land disposed by
steel manufacturers after lime neutralization, but nowadays it should rather
be recycled or regenerated.
When manufacturing refined steel plates, pipes, etc., the steel plate is
usually drawn through the acid base as a continuous strip. The hydrochloric
or sulfuric acid in the basin gradually looses its pickling effect and reaches a
maximum iron content, becoming a waste sludge. The basin is then emptied,
and fresh acid is fed instead. There are several industrial methods of treating
the galvanic waste sludge, which include three basic technologies. The first
technology employs neutralizing the acid by Ca(OH)2, Na2CO3, NaOH, KOH,
or NH4(OH), separating a solid precipitate, and recycling water. Such
technology is described in US 3,304,246, US 3,544,309, and US 3,800,024. In
the last reference, the ions are selectively precipitated in two steps by
adjustment of the pH value. The disadvantages of the neutralization methods
include the loss of the acid, and the complexity of separating hydroxides and
treating saline water, which also result in high costs.
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The second technology includes heating in two stages of the galvanic waste
sludge in order to evaporate water and to recycle acid (200-500°C), followed
by producing metal oxides during the calcination at high temperatures
(850°C). Such techniques are described in US 4,197,139, US 5,149,515, US
5,244,551, and US 6,451,280, relating to a process for regenerating spent acid
liquor, comprising feeding into a reactor, having a first heating zone, a
substantial portion of the liquid from the spent acid to produce acid vapors
and metal salts without decomposing the acid. The metal salts are
transferred to a second heating zone where the salts are roasted to form
metal oxides. The acid vapors from the primary roasting furnace are then
transferred to an absorption column to regenerate the acid. The first heating
zone is operated at a temperature below the decomposition temperature of
the acid and the metal salts. The second heating zone operates at a higher
temperature to completely oxidize the metal salts. The disadvantages of the
technology include very high energy consumption, production of dangerous,
very corrosive gases, and low quality of produced metal oxides.
The third technology regenerates spent pickling acid by adding fresh strong
acid to preconcentrated galvanic waste sludge, and manufactures iron salts
by crystallization. The regenerated acid is reused in the pickling process, and
the iron salt is sold as a by-product after washing. Such technology is
described in US 4,255,407, US 4,382,916, WO/2001/049901 and
WO/2009/075710. This technology is advantageous because it allows to
regenerate spent pickling acid and to remove dissolved metal (for example
iron) as a by-product (for example, iron sulfate heptahydrate). The technology
is considered to be a progressive one, for example by Green Technology
Group, which published it as an innovative closed-loop process (Published in
the WEB www.oit.doe.gov in June 2000, Douglas Olsen Green Technology
Group, Sharon, CT). Disadvantages may include high energy consumption for
the acid evaporation or waste sludge concentration, difficulties with the acids
separation and with product washing. It is therefore an object of the present
invention to regenerate hydrochloric or sulfuric acid from galvanic waste
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sludge, and to recycle it to the repeated pickling process, without the
drawbacks of the prior methods.
It is another object of this invention to regenerate hydrochloric or sulfuric
acid from galvanic waste sludge or from used pickling liquor, while utilizing
dissolved iron.
It is further an object of this invention to regenerate hydrochloric or sulfuric
acid from galvanic waste sludge or from used pickling liquor, while utilizing
dissolved iron, without neutralizing the acids or concentrating them by heatevaporation.
It is still another object of this invention to recycle dissolved iron from the
waste sludge or pickling used liquor by adding a material capable of
precipitating said iron out of the solution and separating it.
It is a further object of this invention to recycle dissolved iron from the waste
sludge or pickling liquor by precipitating and washing, followed by converting
the washed material to a useful product
Other objects and advantages of present invention will appear as description
proceeds.
Summary of the Invention
The present invention provides a process for recycling spent pickling acids,
comprising i) providing a spent pickling liquor comprising hydrochloric or
sulfuric acid, and dissolved iron; ii) measuring the concentration of said
dissolved iron; and iii) adding solid oxalic acid into said liquor; iv) stirring the
mixture and reacting said dissolved iron with said added oxalate; v) removing
the formed solid iron oxalate from said mixture; thereby obtaining a solid
iron oxalate, and regenerated acids for repeated use in pickling. Said oxalic
acid is added in a an amount corresponding to 75-80 % of the stoichiometric
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stoichiometric amount necessary for the complete reaction with said
dissolved iron. Said solid iron oxalate is preferably dried, and calcined to
provide a pure nanopowder of iron oxide. The process according to the
invention preferably employs washing said solid iron oxalate by water till the
pH of 4.5-7 in the wash water, drying said washed iron oxalate at 90-105°C
thereby obtaining a powder of iron oxalate dihydrate, and calcining the dried
iron oxalate dehydrate thereby obtaining a powder of iron oxide. In one
aspect of the invention, the recycling process further comprises washing and
drying said solid iron oxalate, and calcining the dried iron oxalate at 200-
300°C in the atmosphere of dry air, thereby obtaining a red powder of nanosized
-hematite. In other aspect of the invention, the recycling further
comprises washing and drying said solid iron oxalate, and calcining the dried
iron oxalate at 290-350°C in the atmosphere of conversion gases, or
alternatively at 450-500°C in the atmosphere of nitrogen, thereby obtaining a
black powder of magnetite. Said pickling liquor may comprise 10-15 g/l of
hydrochloric or sulfuric acid, 3-5 g/l of inhibitor additive, and 130-150 g/l of
Fe2+ as ferrous chloride or ferrous sulfate. In a preferred alternative of the
process according to the invention, carbon dioxide released during said
calcining is absorbed into NaOH, if carbon monoxide is formed, it is
preferably burned and then absorbed. Said monoxide is formed rather when
producing said magnetite. Said -hematite has usually a bulk density of 0.35-
0.40 kg/l, and a particle size of 35-50 nm. Said magnetite has usually a bulk
density of 0.45-0.50 kg/l, and a particle size of 30-60 nm. The process of the
invention is an environmentally friendly process providing i) a recycled acid
for repeated use in pickling; ii) pure nano powder of iron oxide; and iii)
sodium carbonate.
The invention provides pure -hematite having a bulk density of 0.35-0.40
kg/l, and a particle size of 35-50 nm, as well as pure magnetite having a bulk
density of 0.45-0.50 kg/l, and a particle size of 30-60 nm. The particles
usually exhibit spherical shape. The term "pure" in regard to the hematite
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and magnetite products comprises purity of at least 99.2 wt%, preferably
more than 99.2 wt%, usually more than 99.5 wt%.
Brief Description of the Drawing
The above and other characteristics and advantages of the invention will be
more readily apparent through the following examples, and with reference to
the appended drawing, wherein:
Fig. 1. is a schematic presentation of a process according to the invention.
Detailed Description of the Invention
It has now been found that galvanic waste sludge or used pickling liquor can
be easily processed, utilizing both the dissolved acids and the dissolved iron,
in a surprisingly efficient and environmentally friendly manner, while
recycling the acids and converting iron to precious nano-sized oxides. It has
been found that adding powder of oxalic acid dihydrate to a waste pickling
liquor obviates the drawbacks of the known methods, namely energy
demanding steps of liquor thickening, or expensive and dangerous steps of
neutralizing the acids. The acids, whether hydrochloric or sulfuric, can be
regenerated and reused, while saving expensive materials and adhering to
ever stricter environmental regulations. Advantageously, the process
provides precious and demanded materials – pure iron oxide nano powders,
which are used as pigments and in electronics, in cosmetics and in plastic
industry.
Thus, the method of the invention enables to regenerate acids selected from
the group consisting of hydrochloric acid or sulfuric acid, and is characterized
in that the galvanic waste sludge is mixed with dry oxalic acid dihydrate,
followed by removing the precipitate of iron (II) oxalate dihydrate. The
obtained precipitate slurry is passed to a filter for solid separation and
washing. The filtrate obtained is recycled to the pickling process. The washed
iron (II) oxalate is passed to calcining oven for decomposition and for
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obtaining nano-sized iron oxides powder. The gases released during the iron
oxalate's conversion to oxides are neutralized by NaOH.
The formation of iron oxalate can be achieved by simple reaction between
concentrated solution of FeCl2 (or FeSO4) and dry oxalic acid dihydrate. The
precipitated iron (II) oxalate dihydrate is washed and calcined while
obtaining very useful and desired nano-sized iron oxides. Regarding byproducts
obtained during the galvanic waste treatment, ordinary iron oxide
and iron (II) sulfate heptahydrate are mentioned in the published techniques.
In contrast, the present invention provides expensive nano-sized iron oxides,
such as hematite (Fe2O3) and magnetite (Fe3O4), from iron (II) oxalate. A
process of the transformation of iron (II) oxalate into iron oxides has been
described [see, for example, Rane K.S. et al.: J. Mater. Sci. 16 (1981) 2387-97;
Hermanek M. et al.: J. Mater. Chem. 16 (2006), 1273-80; Zboril R, et al.: J.
Phys. Status Solidi 1 (2004) 3583-8; Zboril R. et al.: Inernational Symposium
in the Industrial Application of the Mossbauer Effect 765 (2005).257-62;
Ashok G. K. et al.: J. Nanoscience Nanotechnol. 7 (2007) 2029-35; Angerman
F. et al.: J. Mater. Sci. 43 (2008) 5123-30]. The instant invention employs
said transformation for making nano-sized iron oxides as a by-products in the
galvanic waste sludge treatment. One of oxides, magnetite (Fe3O4) is a
pigment that is used, among others, as a pigment in transparent paints, inks,
cosmetics, as catalyst, in plastics, in electronics. Other oxide, -hematite
(alpha hematite, Fe2O3) is used as a pigment for transparent paints, inks,
cosmetics, as catalyst, and in plastics.
In one aspect of the invention, the environmentally friendly (green) process
for regeneration of pickling acids, prevents disposal of hazardous acids, and
further sequesters carbon dioxide, which may become a regulated end
product, while eventually producing useful carbonates. So, the acids from
galvanic spent liquor are returned for repeated use, dissolved iron provides
expensive pure nanomaterial, and unpopular carbon dioxide is absorbed to
provide a useful by-product; all that with minimal load on the environment,
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and without neutralizing the acids or concentrating them by heatevaporation,
without regular additions of fresh acid portions, and without
crystallizing iron salts by condensing the liquor.
The process of the present invention enables to regenerate a wide range of
hydrochloric or sulfuric acid concentrations from galvanic waste pickle liquor
(shortly pickle liquor) containing ferrous slats (chloride or sulfate), while
obtaining high-quality by-products, particularly nano-sized iron oxides.
In the method of the invention, the pickle liquor is filtered, to recover the
solid contaminants, and collected in a storage feed tank. From the feed tank
it is pumped into simple, acid resistant reactor with stirrer, the reactor
comprising, for example, polypropylene. A dry crystalline oxalic acid
dihydrate (99.6%) is added into the reactor, preferably in a weight amount
corresponding to 75-80 wt% of iron (II) quantity in the galvanic waste liquor.
The iron (II) chloride or sulfate is reacted with oxalic acid at ambient
temperature. The complete reaction takes place during 4 h. As a result of
reaction, the iron (II) oxalate dihydrate is precipitated and 75-80% of spent
pickling hydrochloric (or sulfuric) acid are regenerated. A dissolution
inhibitor (which is always in the used pickling liquor) prevents re-dissolution
of the precipitated iron oxalate by the regenerated acid until its
concentration is more 20%. The regenerated acid, about 12-14% hydrochloric
or 18-20% sulfuric acid, is ready for the pickling process; it contains residual
iron in an amount of about 20-25% iron (II) cations, and 3-5% of the inhibitor.
The inhibitor does not participate in the reaction and only inhibites acid
activity as in the galvanic basin. Besides, the acid quantity in the recyclied
liquor is defined by iron oxalate solubilty in the hydrochloric and sulfuric
acids. The produced slurry, consisting of iron (II) oxalate dihydrate
precipitate and the regenerated acid, is pumped into an acid resistance filter
device for separation. The filtrate, the regenerated hydrochloric or sulfuric
acid, is pumped into a storage tank to be reused. The cake of iron oxalate
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dihydrate is washed by fresh water till the pH of wash water is more than
4.5-5, and it is dried at +105°C in a drying oven. The dry cake of iron(II)
oxalate is calcined at +245-288°C in the air atmosphere in order to produce
nano-sized red iron oxide (Fe2O3 - hematite), or it is calcinated without the
presence of oxygen (comprising, for example, nitrogen or other gas
atmosphere) at +320-488°C in order to produce nano-sized black iron oxide
(Fe3O4 - magnetite). The produced iron oxides (red or black) do not require to
employ a milling process, they are strongly dispersed (bulk density, for
example, 0.35-0.5 kg/l), highly pure, and consisting of spherical particles with
a size of, for example, about 35-60 nm. The conversion gases may be burned
(CO), may be collected (CO2) and then neutralized, for example with NaOH
while obtaining sodium carbonate.
The hydrochloric (or sulfuric) acid regeneration and nano-sized iron oxides
manufacturing of the present invention is schematically illustrated in Fig.1,
wherein the symbols have the following meanings:
T1 – galvanic waste sludge storage tank;
T2 – galvanic waste sludge feed tank;
T3 – fresh water storage tank;
T4 – regenerated pickling liquor storage tank;
R1 – polypropylene reactor with stirrer;
P1-P6 – pumps;
F1 – galvanic waste sludge filter;
F2 – iron oxalate separation filter;
F3 – iron oxalate washing filter;
Ov1 – iron oxalate drying oven;
Ov2 – iron oxalate calcination oven (hematite production);
Ov3 – iron oxalate calcination oven (magnetite production); and
Ab1 – gas absorber with NaOH solution.
The pickle liquor usually contains 115-150 g/l of Fe2+ (as ferrous chloride (or
sulfate) cation, 10-15 g/l (1.2-1.5%) of HCl (or H2SO4) and up to 50 g/l of
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inhibitor additive. The pickle liquor is transferred by pump P1 and pump P2
through filter F1 from storage tank T1 into feed tank T2. From feed tank T2
it is transferred by pump P3 into polypropylene reactor R1 with stirrer. The
99.6% dry oxalic acid dihydrate (H2C2O4.2H2O) is added into reactor from
feeder
Fd1, in a mass quantity stoichiometrically corresponding to 75-80% of Fe2+
cation mass in the waste pickling liquor is added. The chemical reaction
takes place between ferrous chloride(1), or ferrous sulfate (2), and oxalic acid:
FeCl2 + H2C2O4 x2H2O = FeC2O4x2H2O + 2HCl (1)
FeSO4 + H2C2O4x2H2O = FeC2O4x2H2O + H2SO4 (2)
The reaction takes place at ambient temperature with stirring 150-200 rpm
and it is complete after 4 h. The produced slurry of iron (II) oxalate
dihydrate and hydrochloric or sulfuric acid is transferred by pump P4 from
reactor R1 into filter F2 for separation. The filtrate of hydrochloric or sulfuric
acid is transferred by pump 6 from filter F2 into storage tank T4 for use as a
ready pickling liquor, contained 125-200 g/l (12.5-20%) HCl or H2SO4, 35-50
g/l of ferrous cations (ferrous chloride or ferrous sulfate) and about 3-5% of
inhibitor additive. From storage tank T4 it is recycled into pickling batch.
The cake of iron (II) oxalate dihydrate is transferred from filter F2 into filter
F3 for washing by fresh water, which is pumped by pumps P5 from water
storage tank T3 into filter F3. The washing process is finished at a pH of
more than 4.5-5. The washing water is collected and it can be recycled into
tank T3 after treatment. The washed cake of iron (II) oxalate dihydrate is
transferred into drying oven Ov1 and it is dried at a temperature of 100-
105°C until a humidity of 0.5%. The cake of dried iron(II) oxalate dihydrate is
transferred into oven Ov2 for calcinating in the air atmosphere or into oven 3
for calcinating in the atmosphere of the conversion gases or nitrogen. The
conversion gases, released during the decomposition of ferrous oxalate, can
thus serve and be reused in the process, further increasing the cost-efficiency
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and environmental friendliness of the whole process according to the
invention. The iron (II) oxalate dihydrate calcination takes place in two
stages. The first stage is a dehydration at +220°C and the second stage is
complete its thermal decomposition and oxidation at +245-288°C in the air
atmosphere, or complete decomposition in atmosphere of its conversion gases
or nitrogen at 320-488°C. As a result of calcination in the oven Ov2 a nanosized
alpha-hematite (Fe2O3) is obtained. As a result of calcination in the
oven Ov3 a nano-sized magnetite (Fe3O4) is obtained. The chemical reactions
of iron (II) oxalate thermal decomposition are:
2FeC2O4 +1.5 O2 = Fe2O3 + 4CO2 + heat; (hematite production);
3FeC2O4 + heat = Fe3O4 + 2CO + 4CO2 (magnetite production);
Produced nano-sized hematite and magnetite are dispersed by their own
conversion gases in time of iron (II) oxalate decomposition, obtaining a fluffy
powder. The produced conversion gases may be burned (CO), and then
absorbed in the absorption column Ab1 by sodium hydroxide (CO2). All
experiments on the galvanic waste pickle sludge (liquor) treatment were
made on the Pilot Plant with productivity 100-150 l/day.
A method for the recycling of spent pickling acids selected from the group
consisting of hydrochloric acid or sulfuric acid, characterized in that the
galvanic waste sludge is mixed with dry oxalic acid dihydrate that is removed
the precipitate of iron (II) oxalate dihydrate and that is regenerated spent
pickling acid. The slurry obtained is passed to a filter for solid separation and
washing. The filtrate obtained is recycled to the pickling batch back. The
washed iron (II) oxalate dihydrate is passed to calcining oven for
decomposition and obtaining nano-sized iron oxides powder. The iron
oxalate's conversion gases are neutralized by NaOH.
Thus, in a preferred embodiment of the invention, a method is provided for
reprocessing spent pickling waste liquor, comprising adding oxalic acid and
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so sequestering about 75-90% of iron in insoluble oxalate, washing said solid
iron oxalate by water till the pH of 4.5-7 in the wash water, drying said
washed iron oxalate at 90-105°C thereby obtaining a powder of iron oxalate
dihydrate, and calcining the dried iron oxalate dehydrate. In the most
preferred embodiment, the process further comprises washing said solid iron
oxalate by water till the pH of the wash water is about 4.5-7 in, drying said
washed iron oxalate at 90-105°C, preferably obtaining iron oxalate
dehydrate, and calcining the dried iron oxalate at 200-300°C in the
atmosphere of dry air, thereby obtaining a red powder of nano-sized -
hematite. The aqueous galvanic waste sludge usually consists of 10-15 g/l of
hydrochloric or sulfuric acid, 3-5 g/l of inhibitor additive, and 115-150 g/l of
Fe2+ as ferrous chloride or ferrous sulfate. The sludge is preferably filtered
and filled into acid resistance reactor with stirrer at ambient temperature,
and it is mixed with quantity of dry oxalic acid dihydrate (99.6%)
stoichiometric to 75-90% of Fe2+. After adding oxalic acid, and stirred for a
time sufficient for producing iron oxalate, usually about 4 hours, iron (II)
oxalate dihydrate precipitates, whereby regenerated hydrochloric or sulfuric
acid is formed. The regenerated quantity of hydrochloric or sulfuric acid is
about 75-80% of spent pickling acid, containing residue of 30-50 g/l of Fe2+
and 3-5 g/l of inhibitor additive. The regenerated hydrochloric or sulfuric acid
is separated from slurry by filtration together with dissolved inhibitor
additive and it is reused in the pickling process. The precipitated iron (II)
oxalate dihydrate is separated from slurry by filtratioin, producing a
filtration cake, to be further processed. The cake of iron (II) oxalate is is
preferably washed by water till pH 4.5-7, and then it is preferably dried at a
temperature of from +90 to +105°C. The dried cake of iron (II) oxalate
dihydrate is calcined in an oven at 200-300°C during 3 h in the dry air
atmosphere, producing red nano-sized iron oxide (αFe2O3-hematite). The iron
oxide (Fe2O3-hematite) need not be milled, it has a bulk density of 0.35-0.4
kg/l. The particle size is 35-50 nm at narrow distribution, 90% of the particles
mass being in the said range. The purity of the product is usually 99.2-99.7%.
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Usually, iron oxide (Fe2O3-hematite) light powder consists of spherical
particles. The said dried cake of iron (II) oxalate may be calcined in an oven
at +290-350°C over about 3 hours in the atmosphere of its own conversion
gases, or alternatively at +450-500°C in the nitrogen atmosphere, providing a
black powder of super paramagnetic nano-sized iron oxide (Fe3O4-magnetite).
The purity of the product is usually 99.5-99.8%. The iron oxide needs no
milling, its bulk density being 0.45-0.5 kg/l, and its particle size being usually
about 30-60 nm, at narrow distribution, about 90% particle mass being in
said range. The powder usually consists of spherical particles. In a preferred
embodiment of the invention, CO2 gas obtained in the iron (II) oxalate
thermal decomposition is absorbed in NaOH. In other preferred embodiment
of the invention, CO gas obtained in the iron (II) oxalate thermal
decomposition is burned and the formed CO2 is absorbed in NaOH. The
Na2CO3 obtained as a result of gases absorption is a by-product for sale.
The invention will be further described and illustrated in the following
examples.
Examples
Example 1
The galvanic waste pickle liquor of the first example has the following
composition: ferrous chloride, 130 g/l of Fe2+, hydrochloric acid 15 g/l (or
1.5%), about 2.5% inhibitor additive, and balance water. After filtering 100 l
of this liquor was fed to 150 l glass reactor with stirrer, and 22.5 kg
(stoichiometric to 100 g/l of Fe2+) of oxalic acid dihydrate were added into
solution with stirring 150 rpm at ambient temperature (+24°C). After oxalic
acid dihydrate addition, the temperature of the mixture was decreased to
+21°C. The reaction between ferrous chloride and oxalic acid started
immediately and it was continued during 4 hours. The temperature of the
mixture at the end of the reaction was ambient (+24°C) again. As a result of
reaction 32.2 kg of yellow precipitate of iron (II) oxalate dihydrate was
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obtained and 126 g/l of hydrochloric acid were regenerated. The total acid
quantity in the slurry was achieved 141 g/l or 14.1%. Slurry of iron (II)
oxalate dihydrate in the water solution of 14.1% hydrochloric acid was
filtered on the membrane filter. The filtrate (82 liters) was collected and
tested for the ferrous chloride and hydrochloric acid quantities. The analysis
showed the presence, in filtrate, of 30 g/l of Fe2+, 141 g/l of free hydrochloric
acid, about 2.0% of inhibitor additive, and 80% balance water. The obtained
solution was suitable for the use in the pickling process.
The cake of filtration contained 82% of solid material – iron (II) oxalate
dihydrate and 18% of mother liquor of regenerated pickling acid. The cake of
iron(II) oxalate dihydrate was washed by fresh water until visual absence of
Cl- anions in the washing water. The Cl- anions presence was tested by
addition of 1% solution of AgNO3 into washing water (white precipitate of
AgCl after addition of several drops of AgNO3 solution into washing water).
The washed cake of iron (II) oxalate dihydrate was dried in the drying oven
at temperature +100-105°C. The weight of dry iron (II) oxalate dihydrate was
32.2 kg. It was thermally decomposed to get nano-sized iron oxides: 10 kg for
the hematite production and 10 kg for the magnetite production.
According to the made TGA-DTG-DSC analysis (laboratory of Ben Gurion
University) the produced iron (II) oxalate dihydrate is decomposed by two
stages in the air atmosphere. The first stage is dehydration in the
temperature interval +187.16 - 239.93°C (peak 201.35°C) and the second
stage is oxidation in the temperature interval +208.69 - 292.36°C (peak
288.67°C). The first stage is endothermic, and the second stage is exothermic.
As a result of thermal iron (II) oxalate decomposition, the nano-sized alphahematite
is formed, and CO2 is formed as outgoing conversion gas. The
thermal decomposition of produced iron (II) oxalate in the nitrogen
atmosphere takes place in the temperature interval +399.14 -488.15°C (peak
457.42°C). The decomposition process is endothermic. As a result of this
decomposition the nano-sized magnetite is formed and mixture CO and CO2
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is formed as a conversion outgoing gas. The thermal decomposition of iron (II)
oxalate in an atmosphere of its own conversion gases takes place in the
temperature interval +230 - 370°C (peak 320°C).
The first 10 kg of produced dry iron (II) oxalate dihydrate were put into an
oven and heated in the air atmosphere at +220°C during 1 h. After complete
dehydration, the exothermic stage was started and temperature was set at
+300°C. At this temperature the iron (II) decomposition was continued
during 2 h. All outgoing gases were transferred by pumps through column
filled with water solution of NaOH for neutralization. As a result of
calcinations, 4.4 kg of red powder were produced. According to the XRD (Xray
Difraction) analysis made in the laboratory of Ben Gurion University, the
obtained red powder was alpha-hematite. It was studied under SEM
(Scanner Electronic Microscope), which showed that according EDS (Electron
Diffraction Spectrum) the red powder contained 99.9% of Fe2O3 and it
consisted of spherical particles of a size from 35 to 50 nm. The obtained nanosized
red iron oxide is very dispersed and looks fluffy; it does not need
milling or other processing. Its bulk density was 0.35-0.4 kg/l.
The samples from gas absorption column liquid showed the presence of
Na2CO3, formed as a result of reaction between NaOH and outgoing CO2.
The second 10 kg of produced iron (II) oxalate dihydrate were filled into
completely closed metallic container which was placed into muffler. This
container was connected by pipe with absorber filled by NaOH solution. The
muffler with iron (II) oxalate dihydrate was heated at +488°C during 3 h.
After cooling it was obtained 4.3 kg of black super paramagnetic powder
which was tested under SEM (Scaner Electronic Microscope) and by XRD (XRay
Difraction). The tests results showed that powder is magnetite,
contained 99.5% of Fe3O4 and consisted of spherical particles with a size of
30-60 nm. Samples of water suspension from the absorber consisted of a
mixture of NaOH and Na2CO3. The Na2CO3 was formed as a result of
reaction between outgoing gas CO2 and NaOH inside gas absorber.
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Example 2
The galvanic waste pickle liquor of the second example has the following
composition: ferrous sulfate 130 g/l of Fe2+, free sulfuric acid 12 g/l (or 1.2%),
about 3% inhibitor additive, and balance water. After filtering 50 l of this
liquor were fed to 150 l glass reactor with stirrer and 11.25 kg (stoichiometric
to 100 g/l of Fe2+) of oxalic acid dihydrate were added into solution with
stirring 150 rpm at ambient temperature (+23°C). After oxalic acid dihydrate
addition the temperature of mixture was decreased till +19°C. The reaction
between ferrous sulfate and oxalic acid was started immediately and it was
continued for 4 hours. The temperature of the mixture at the end of the
reaction was ambient (+23°C) again. As a result of reaction 16.0 kg of yellow
precipitate of iron (II) oxalate dihydrate was obtained, and 175 g/l of sulfuric
acid was regenerated. The total acid quantity in the slurry was 187 g/l, or
18.7%. Slurry of iron (II) oxalate dihydrate in the water solution of 18.7%
sulfuric acid was filtered on the membrane filter. The filtrate (40 liters) was
collected and tested on the ferrous sulfate and sulfuric acid quantities. The
analysis showed the presence, in filtrate, of 30 g/l of Fe2+, 187 g/l of free
sulfuric acid, about 3.0% of inhibitor additive and 76% balance water. The
obtained solution is suitable for the use in the pickling process. The filtration
cake contained 85% of solid material – iron (II) oxalate dihydrate and 15% of
mother liquor of regenerated pickling sulfuric acid. The cake of iron(II)
oxalate dihydrate was washed by fresh water until visual absence of SO4-
anions in the washing water. The SO4- anions presence was tested by
addition of 1% solution of BaCl2 into washing water (it is formed a white
precipitated of BaSO4 after addition of several drops of BaCl2 solution into
washing water). The washed cake of iron (II) oxalate dihydrate was dried in
the drying oven at temperature +100-105°C. The weight of dry iron (II)
oxalate dihydrate was 16 kg. It was used for the thermal decomposition and
nano-sized iron oxides production: 5 kg for the hematite production and 5 kg
for the magnetite production.
-17-
The process of calcination of iron (II) oxalate dihydrate was the same as
described in the example 1. As a result of the first 5 kg iron (II) oxalate
dihydrate calcination in the air atmosphere at +288°C, 2.2 kg of nano-sized
alpha-hematite were obtained. According to the EDS analysis the produced
alpha-hematite is very pure (99.7% of Fe2O3), under SEM it is consisted of
spherical particles with a size of 35-50 nm. As a result of the second 5 kg iron
(II) oxalate dihydrate calcination in the atmosphere its conversion gases at
+488°C, 2.1 kg of nano-sized magnetite were obtained. According to EDS the
produced magnetite is very pure (99.4% of Fe3O4),under SEM it is consisted
of spherical particles with a size of 38-60 nm. The obtained magnetite is
super paramagnetic. The outgoing gases were neutralized inside the absorber
by reaction with NaOH.
While this invention has been described in terms of some specific examples,
many modifications and variations are possible. It is therefore understood
that within the scope of the appended claims, the invention may be realized
otherwise than as specifically described.
-18-

CLAIMS
1. A process for recycling spent pickling acids, comprising
i) providing a spent pickling liquor comprising hydrochloric or
sulfuric acid, and dissolved iron;
ii) measuring the concentration of said dissolved iron; and
iii) adding solid oxalic acid into said liquor;
iv) stirring the mixture and reacting said dissolved iron with said
added oxalate;
v) removing the formed solid iron oxalate from said mixture;
thereby obtaining a solid iron oxalate, and regenerated acids for
repeated use in pickling.
2. A process according to claim 1, wherein said oxalic acid is added in a an
amount corresponding to 75-80 % of the stoichiometric amount
necessary for the complete reaction with said dissolved iron.
3. A process according to claim 1, further comprising washing said solid
iron oxalate, drying it, and calcining it to provide a pure nanopowder of
iron oxide.
4. A process according to claim 1, further comprising washing said solid
iron oxalate by water till the pH of 4.5-7 in the wash water, drying said
washed iron oxalate at 90-105°C thereby obtaining a powder of iron
oxalate dihydrate, and calcining the dried iron oxalate dihydrate
thereby obtaining a powder of iron oxide.
5. A process according to claim 1, further comprising washing and drying
said solid iron oxalate, and calcining the dried iron oxalate at 200-300°C
in the atmosphere of dry air, thereby obtaining a red powder of nanosized
-hematite.
6. A process according to claim 1, further comprising washing and drying
said solid iron oxalate, and calcining the dried iron oxalate at 290-350°C
-19-
in the atmosphere of conversion gases or at 450-500°C in the
atmosphere of nitrogen, thereby obtaining a black powder of magnetite.
7. A process according to claim 1, wherein said pickling liquor comprises
10-15 g/l of hydrochloric or sulfuric acid, 3-5 g/l of inhibitor additive, and
130-150 g/l of Fe2+ as ferrous chloride or ferrous sulfate.
8. A process according to claim 5, further comprising absorbing carbon
dioxide released during said calcining into NaOH.
9. A process according to claim 6, further comprising burning carbon
monoxide released during said calcining to produce carbon dioxide, and
absorbing said carbon dioxide into NaOH.
10. A process according to claim 5, wherein said -hematite has a bulk
density of 0.35-0.40 kg/l, and a particle size of 35-50 nm.
11. A process according to claim 6, wherein said magnetite has a bulk
density of 0.45-0.50 kg/l, and a particle size of 30-60 nm.
12. A process according to claim 1, being an environmentally friendly
process providing
i) a recycled acid for repeated use in pickling;
ii) pure nano powder of iron oxide; and
iii) sodium carbonate.
13. Pure -hematite having a bulk density of 0.35-0.40 kg/l, and the particle
size of about 35-50 nm, produced by the process of claim 1.
-20-
14. Pure magnetite having a bulk density of 0.45-0.50 kg/l, and the particle
size of about 30-60 nm, produced by the process of claim 1.
Dated this 22nd day of March 2012.