Note: Descriptions are shown in the official language in which they were submitted.
~ 9383 9709-1
~ he present inventlon is dlrected to the processing of
Na2cr2o.7 or Na2CrO4 to produce a high purity Cr203. A particular
embodiment of the present lnvention is directed to the processlng
of chromium ores to obtain high purity Cr203
Chromium compounds are essential to the oil and gas
industries for corrosion control and the preparation of catalysts,
the food and beverage lndustries in refrigeration brlnes and
cleansing compoundsJ the transportation industries ln diesel loco-
motlves and automoblles, the iron and steel industries in stalnless
1~ steel metal and chromium plate, the alrcraft lndustry ~or anodizing
aluminum and pickling magnesium, the copper industry for descaling
brass and stripping copper, the electrical lndustry ln mercury-arc
rectifiers and dry cells, the pyrotechnical industries in matches
and ~ireworks, the photographic indus~ry in llthography and engraving.
However, the ma~or product ls chromic oxide or Cr203 which is used in
metallurgy and as well as a plgment.
In the production of chromium metal from chromic oxlde,
a relatively pure grade of Cr203 is desired. ~en the Cr203 material
contains significant amounts o~ sodium a~ an impurity, sodium ~aporizes
in the vacuum carbon reduction process of producing pure chromium.
mis sodium causes ~ire hazards when lt ls deposlted on the equlpment
walls and later exposed to the atmosphere.
Also, it is important in many applications that a Cr23
product be essentially ~ree of aluminum and sulfur impurities.
It is therefore an ob~ect o~ the present lnventlon to
provide a method for producing high purity Cr203 from Na2Cr207 or
Na2CrO4 or mixtures thereof.
It ls a further ob~ect of the present lnvention to provide
a method for producing high purity Cr2o3 from chromium ores.
Other ob~ects will be apparent from the following des-
cription and claims taken in con~unction -;~ith the drawlng which sho~s
a flowsheet of a method in accordance with the present invention.
,.~ ~
1~9383 Y'709-1
A method in accordance with the present lnventlon
comprises:
(i) providing an aqueous solution of Na2Cr207 or
Na2CrO4 or mixtures thereof:
(11) adding an acid to the solution of step (i) to provide
a pH of about 1 to 2;
(lii) contacting the acidified solution of step (ii) with an
o~ganic extractant to recover chromium values from the acidified
solutlon in a chromium containing organic liquid phase; ~
(iv) mixing the chromium containing organic phase with a
water solution of ~40H to obtain an aqueous solution of (NH4)2CrO4;
(v) sub~ecting the solution of (NH4)2CrO4 to evaporation
to provide a solid chromium bearing material;
(vi) ignitlng the solid chromium bearing material to obtain
Cr203 .
A p~rticular embodiment of a method in accordance with
the present invention as applied to chromium ores comprises:
(i) roastlng in a gaseous oxidizing environment a mixture
of chromium ore with Na2C03 and CaO at a~,temperature in the range o~
about 600C to 1100C for from about 0.5 to 5 hours, the amount of
Na2C03 being that which provides from about 1.4 to 4.2 pounds o~
Na2C03 per pound of Cr203 in the ore and the amount of CaO being that
which provides from about o.6 to 1 pound of CaO per pound o~ Cr203
ln the ore;
(ii) water leach~ng the roasted material obtained in step
(i) at a temperature of from about 5C to the boiling point o~ water
for from about 5 minutes to 5 hours;
(iii) adding an inorganic acid, e.g. HCl, HN03, H2S04 to the
leach llquor obtained in step (ii) to provide a pH of from about
30 3 to 9.5 to cause precipltatlon of aluminum impurlties;
--3--
~9383 ~ i J~-l
(iv) separating the precipitate frorn the leach liquor
and thereafter adding an inorganic ac~d to the leach liquor to
provide a p~ o~ about 1 to 2;
(v) Contacting the acldified leach liquor obtained in
step (iv) with an or~anic extrac~ant to recover chromium values
from the acidified leach liquor in a chromium containing organic
llquid phase;
(vi) ~lixing the chromium conta~ning organic phase with a
water solution of N~40~r. to obtain an aqueous solution of (~)2CrO4;
(vii) subjecting the solution of (NH4)2CrO4 to evaporation
to provide a solid chromium-bearing material;
(viii) igniting the solid chromium-bearing material to
obtain Cr203.
In the practice of a particular embodiment the present in-
vention, and with reference to the drawing, a natural chromium
ore, e. g. Transvaal ore (30 to 50~ Cr203J 15 to 25~ Fe, 2 to
10~ SiO2, 10 to 15/G~ Al~ less than l~o Na, 6-15~ MgO, 0.2-0.6~ Ca)
is particulated to a sultable size, e. g. 200 mesh Tyler series
and finer as indicated at 10 and mixed with Na2C03 and CaO (the
CaO may be inltially present as CaC03) and subjecting the mixture
to roasting as indicated at 20.
The amount of Na2C03 and CaO can be varied in the roast
between the limits of 1.4 and 4.2 pounds of Na2C03 and o.6 to
1.0 pounds of CaO per pound of Cr203 in the ore.
The roasting temperature can be varied from 600C to 1100C
and the roasting time can be varied from 0.5 hours to 6.o hours.
m e preferred amounts of Na2C03 and CaO are 1.9 and o.6 pounds,
respectively. m e preferred temperature and roasting time is
950C for 2 hours. The roasting is conducted in gaseous oxygen
--4--
11 19 ~ 3 9703-l
environment; by providing an excess of oxygen by passing, e.g.
air, oxygen or a combustion g~s with a sufflcient excess of
oxygen~ over the roast bed.
The following equation is representative of the roast
reaction using Na2C03to solubilize the chromium
4 (Cr203-FeO) + 8Na2C3 + 72 -~ 2Fe203 ~ 8Na2CrO4 ~ 8co2~
The calcines obtained from the roast are conventionally
water leached as shown at 30 to substantially solubilize and remove
the water soluble chromium salts from the calcines. The leaching
lG can be carried out at temperatures from 5C to the boiling point
and require a time from 5 minutes to 5 hours depending on the
chromium concentration in the leach and the temperature. Bubbling
air through the leach liquor increased the amount of chromium
extracted from the calcines.
The leach liquor, in addition to the chromium values, con-
tains aluminum and s~dium as undesired impurities, e.g. as
NaA102. Aluminum impurities are removed, as shown at 40 and ~O by
adding an acid to the leach liquor until the pH of the liquor is
reduced to the range of 3 to 9.5 preferably 8.o. The amount of
chromium coprecipitated with the Al(OH)3 at a pH of 8 is typically
about o.6% of the total chromium content in the liquor. However,
as the pH is reduced further the coprecipitation of chromium
increases. Acids other than H2S04, preferably HCl, should be
used if a low sulfur product is desired.
The aluminum left in solution after the Al(OH)3 is filtered
off is typically less than 0.7% A1203 based on the weight of
equivalent Cr203 in solution.
In order to remove the remaining impurities, e.g. Na im-
purities in the leach liquor obtained after the Al(OH)3 precip-
3o itation step, a solvent extraction procedure is used as indicatedat 60. This procedure consists of treating a dilute solution of
leach liquor with acid (other than H2S04 for low sulfur products)
so that the final pH is in the range of l to 2, preferably 1.6
--5--
~ 1119383 9709-l
and the concentration of chromium as Cr203 is suitably in
the range of l g/l to 25 g/l and preferably in the range of
5 g/l to 25 g/l with a particularly advant~geous concentration
being 8.2 g/l. The organic solvent can be either benzene,
xylene or toluene alone or mixed with an isoparafinic hydro-
carbon such as Isopar H produced by Exxon. The e~tractant
is a tertiary amine such as Alamine 336 produced by General
Mills. A 0.1 molar solution of the extractant in the solvent
was employed, however, lower or higher concentrations of C.02
to 0.3 molar can be employed. It has been found that with only
2 stages of extraction the organic phase can be loaded to 10 g/l
of Cr203 and the aqueous phase (raffinate) reduced to only
0.04 g/l or 40 parts per million. No emulsions are formed and
phase separations are rapid. The organic phase to aqueous phase
ratio can be varied from 0.33 to 1Ø
The loaded organic phase is water washed to remove traces
of aqueous leach liquor containing impurities of sodium and
others. The number of stages of washing would depend on t~e type
and efficiency of the washing step, however~ no more than l or
2 stages of washing would ordinarily be required.
The chromium values in the loaded organic phase are stripped,
as indicated at 70, by mixing with NH40H solution at approximately
1.5 molar. The resulting phase separation is rapid ( ~-l minute).
m e aqueous phase contain (NH4)2CrO4 and the stripped organic
contained r~0.04 g/l Cr203. The ratio by volume of aqueous
NH40X to organic can range from 0.5 to 10. Thus, a relatively
concentrated solution of (NH4)2CrO4 can be obtained. The stripped
organic can be recycled as indicated at 65.
The (NH4)2CrO4 solution is evaporated to dryness as indi-
cated at 80 and the resulting solid material is carefully ignitedto Cr203 in a gaseous oxygen environment~ e.g. air or oxygen, at
about 500C, after drying as indicated at 90. me solid material
--6--
~ 383 ~709-1
consists essentially of a mixture oi (N~4)2CrO4 an~ 4)2CrO207
i.e. ammonium chromate compounds.
The foregoing description commencing with the description
of the sol~ent extraction procedure is applicable to the method
of the present invention as applied to aqueous chromium-bearing
solutions prepared by dissolving, for example, a commercial
grade Na2Cr207 or Na2CrO4 in water instead of obtaining an
aqueous chromium-bearing solution of ~a2CrO4 by ore leaching.
With reference to the drawing~ as shown at 1, a commercial
grade Na2Cr207 or Na2CrO4 or mixtures thereof is dissolved in
water forming an aqueous solution having a concentravion of
chromium as Cr203 suitably in the range of 1 g/l to 25 g/l and
preferably 5 g/l to 25 g/l. If required, this solution may be
filtered to remove undissolved solids. Similar to the foregoing,
a solvent extraction procedure as shown at 60 is applied to the
aqueous solution with the remaining steps of the method of the
present invention applied as hereinbefore described.
An example of the method of the present invention as ap-
plied to a Na2Cr207 aqueous solution is as follows:
EXAMPLE I
One liter of an aqueous Na2Cr207 solution was prepared
by dissolving 11.5 grams of commercial grade Na2Cr207.2H O in
water with 12 molar HCl being introduced into the solution so as
to obtain a chromium concentration in solution based on Cr203 of
5.8 grams per liter at a pH of 1.6. The analysis of the Na2Cr207.
2H20 was as follows:
Na15.38~ (by chemical analysis)
Cr34.63~ (by chemical analysis)
S0.029~o (by chemical analysis)
Alo.oo8-o.o8~0 (by spect. analysis)
Ca0.001~0.01~ (by spect. analysis)
Mg0.001-0.01% (by spect. analysis)
V0.002-0.02~ (by spect. analysis)
No other metallic elements were detected in the analysis.
-7
1~9383 9709-1
A O.lM amine organic solution was prepared by dissolving
49 ml of Alamine 336 (tertiary amine) and 50 ml of isodecanol
in benzene so that a total of 1 liter of organic solution ~ias
made. ~hen 50 ml of the above aqueous solution and 50 ml
organic were shaken it was found that emulsions formed.
However, when a tertiary amine was used with benzene
or xylene or Aromatic 100 or 150 produced by Exxon or mixtures
of these with an isoparafinic hydrocarbon such as Isopar H
produced by Ex~on without isodecanol no emulsion formed.
An example of this is as follows: ~hen 50 ml of the
above aqueous solution is mixed and shaken with 50 ml of a
O.lM organic solvent consisting of 49 ml Alamine 336 (tertiary
amine) dissolved in benzene to provide 1 liter of solution,
no emulsions formed and separation o~ the phases is rapid (~ 1 min.).
, The organic phase was water washed twice to remove
traces of impurities.
The aqueous phase and washes were water white and contained
negligible amounts of chrom~um. ~ess than O.l~o of the chromium
ph Q~ ~
in the organic plaoc was lost in the washes.
The organic phase (50 ml) contained 5.79 grams per liter
Cr203 and the aqueous phase (50 ml) contained 0.02 grams per
liter Cr203. Thus, using an organic to aqueous ratio of 1.0,
greater than 99~ of the chromium was extracted in a single
stage extraction.
The 50 ml of loaded organic containing tertiary amine and
benzene was stripped with 10 ml of 1.5 molar NH40H using an
organic to aqueous ratio o~ 5.0 and produced about 10 ml of
strip liquor. The stripped organic was colorless and contained
0,04 grams per liter Cr203. No emulsions were observed.
--8--
~ 9383 9709-1
The strip liquor was evaporated in a stainless
steel dish and the resulting solids were ignited at
60~ C to Cr203. A typical analysis of the product
Cr23 is
Cr23 >99% (by dlfference)
Al 0.0004-0.04% ~by spect. analysis)
Ca o.oO8-0.08% (by spect. analysis)
Mg 0.002-0.02% (by spect. analysis)
Na o.o8 -o.8s~ (by spec~. analysis)
S1 0.008-0.08~ (by spect. analysis)
V 0.001-0.01~ (by spect. analysis) ;~
Through the practice of the present invention a high -~
purity (less than ~ o.o8 - o.8% Na) Cr203 product can be obtained
~rom Na2Cr207 or Na2cr4
The method of the present invention may be practiced
on solutions of commercial grade Na2Cr207 or Na2CrO4 or mixtures
o~ these two wherein the chromium concentration in so~ution based
on Cr203 is suitably 1 to 25 grams per liter. If undissolved
sollds are present in the solution, the solution may be first
23 filtered to remove them. -
An example o~ the method of the present inventions asapplied to chromium ore is as follows:
EXAMPLE Il
Transvaal chromium ore having the anal~sis shown
below in the amount of 5 lbs. was subjected to grinding
to finer than 200 mesh.
Chromium Ore Analysis (Oxides)(Chemical 'nalysis)
Cr203 44.6% '~
Total Fe20327.5%
SiO2 3.5%
CaO 0.4%
A1203 26.9%
Na20 . 1%
MgO 10.1%
g
- ~
. - . ,, , ~,
111~3 9703-1
100 g. of the ground ore was mixed with 25 grams of
03
co~mercial grade caCG and 82.8 grams of powder reagent grade
Na2C03. The mixture was placed in a Inconel X dish and roasted at
950C for 2 hours with air being forced over the mixture. After
roasting, the calcined mi~ture was leached with 2 ml. of water
per gram of calcine and then water washed with 2 ml. of water
per gram of calcine to provide leach liquor and wash combined
having the following analysis:
Cr20322.3 g/liter (by chem. anal.)
10 Al0.1-1 g/liter (by spect. anal.)
Ga0.004-0.0~ g/liter ( " " "
Mg0.0004-0.004g/liter ( " " "
Na~ 20 g/liter ( " " "
Si0.004-0.04 g/liter ( " " "
12 molar HCl was introduced into the leach ]iquor to
reduce the pH from 12.8to 8.o with resultant precipitation~of
aluminum as Al(OH)3 After removal of the precipitate by
filtration the acidified leach liquor had the following analysis:
Cr203 22.3 g/liter
Al 0.13 g/liter
Na ~ 20 g/liter
Solvent extraction feed solution having a concentration
of 5.8 g/l Cr203 was prepared by mixing 520 ml of concentrated
leach liquor (pH 12.8) with 265 ml of 12 molar HCl and H20 to
produce 2000 ml of solution having a pH of 1.6.
An organic solution was prepared by mixing isodecanol
(5~ by volume~, Isopar H, and Alamine 336 (0.1 m) to form 2000
ml. of organic solution as suggested by General Mills Chemical
Division Chromium-Liquid Ion Exchange~ ~ulletin CSDI-61. When
150 ml. of the above aqueous and 150 ml. of organic were shaken,
emulsions formed and the organic turned brown.
--10--
9709-1
~ hen the lsodecanol ~as omltted, emulsions sti'l ormed,
but the organlc phase ~ras not brot~ but orange in color suOges-
tir.g the prev~ous ~iscoloration ~.as cue to degradation of the
alcohol.
/ihen the tertiary amine ;~as used ~ith benzene or xylene
or ~arious aroma'ic diluents from Exxon, without isodecanol,
no emulsion fol~ec. ~n example of this is as foilols~ .en
100 ml of the above 2queous ,ras mixeQ ~rith 100 ml of the organic
solvent of the tertiary amine in ei4her benzene or xylene or
~raratic 100 or l~, no emulslon o~ed and separation OL the
phases was rapid ( ~1 r~nute). The orOanic phase was water
;lashec three times to re-.o-te _r.lrurit~es. The aqueous rashes
were water white and contained negl_~ble amoun~s o~ chromiur...
The amine ~id not degrace.
The or~anic phase contained ~.79 g/l Cr203 and the a~ueous
phase contained 0.019 ~/l Cr203.
100 ml of the loaded or~anlc containing tertia~y amine
and benzene ~ras stripped with 2C ml OL 1.5 molar l~'4G'T and
produced about 20 ml of strlp liquor which after eva~oration
and ignition of the resulting solids a~ 600C had the follo~inO
analysis:
2 3 (by di ference)
Al .o8 - .8% (by spect. analysls)
Ca 0.008-o.o~ (by spect. analysis)
Mg 0.0004-0.004% (by spect. analy~is)
~a 0.04 -0.4% (by spect. analysis)
Si 0.004 -0.04% (by spect. analysis)
~ 0.004 -0.04% (by spect. analysis)
I~ e.~ulsions form on a lar~e scale extraction plant, the
3 process would not be practical, thus the choice o~ the proper
dlluents or cor..blnations of the diluents for the amine lS extremely
~mportant.
--11--
`\~ ;~'
.. .
1~9383 9709-1
The recovery o~ chromium was over 99% in the extraction
step.
In the practice of the present invention a roasting
temperature of 950C led to 94~ extraction of the chromium from
the calcines, when air was bubbled into the leach liquor during
the leaching step. Without air during leaching comparable con-
ditions resulted in only 88~ extraction. Also, the ranges for
the amounts of Na2C03 and CaO are important for economic reasons,
94~ extraction was obtained with 25 g. CaO and 82.8 g. ~a2C03 per
13 100 g. chrome ore containing 44.6~ Cr20~. The use of larger
amounts 50 g. CaO and 82.8 g. Na2C03 lowered the extraction from
88~ to 83% when air was not used in the leaching step. The CaO
may be added as lime or limestone.
In the calcine leaching step the pH of ~-12 is adequate.
The pH range of 3 to 9.5 in the aluminum precipitation step is
important, with an optimum pH at 8.o. At higher pH values more
chromium is lost in the Al(OH)3 ppt. and rnore chromium is lost
at the lower pH range by dissolution of the chrome hydrate.
Through the practice of the present invention a high
purity Cr203 product, less than --0.2~ Ai and less than ~ 0.04 -
0.4~g ~a, can be obtained from natural ch-omium ores such as Transvaal
ore and other oxidic chromiu~ bearing materials.
In the practice of the present invention, the choice of
the proper diluents or combinations of the diluents for the a.~ine
i8 extremely important since if emulsions form in a large scale
extraction plant- the process would not be practical.
The par~icular organic solvents used in the present
invention are essential for the prevention of emulsions. Benzene,
xylene, Aramatic 100 or 150 produced by Exxon or mixtures of the
3~ above or a mixture of one of the above with an isoparafinic hydro-
carbon (such as Isopar H produced by Exxon) were found satisfactory.
It was found that the use of isodecanol as a solvent ex-
traction modifier was undesirable due to its reaction with the
chromlum in the solution which reaction causes the alcohol to brezk
down and contribute to emulsion formation.
-12_
:
~1~9383 9709-1
In addition, by the practice of the present invention,
an essentialiy sulfur-free product can be obtained due to the
fact that inorganic acids other than H2S04 can be readily used
in the practice of the invention and further that the solvent
extraction purification step removes substantially all sulfur
contamination which may be due to the use of H2S04 or which
may be introduced as an impurity ~ith the materials invo]ved
in the process. HCl would be a preferred acid to use in the
practice of the present invention if a sulfur-free product is
desired.
-13-
