Note: Descriptions are shown in the official language in which they were submitted.
Outolcumpu Oy, Outokumpu
73 0391 ~ S
Process for the production of an iron-free chromium(III)
compound
The present invention relates to a process for the precipitation,
at an elevated temperature and pressure/ of a substantially
iron-free chromium(III) compound from an acid aqu`eous solution
which contains iron and chromium.
Chromium chemicals are currently produced industrially only
from chromium or ferrochromium by means of oxidizin~ calefaction.
The alkali chromate is separated out from the product subjected
to calefaction, by dissolving it in water, and after several
puriication and washing steps a dichromate conversion is
carried but with sodium bisulfate derived from chromium
trioxide production, with sul~uric acid, with carbon dioxide
or the like. Also other chromium salts, e.g. potassium,
ammonium, zinc and lead chromates, chromium trioxide, Cr2O3,
Cr(OH)C12, CrO2, etc., and metallic chromium are obtained
from the purified alkali chromate solution directly or by
a suitable treatment of alkali dichromate.
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The prior known processes are both complicated and expensive.
Since hexavalent chromium is toxic, these ~rior known
processes have always involved environmental and health hazards.
The object of the present invention is to eliminate the
above disadvantages and to provide a process for the precipitation,
at an elevated temperature and pressure, of a substantially iron-
free chromium(III) compound from an acid aqueous solution which
contains iron and chromium, without the co-precipitation of iron.
The raw material used in the process according to the invention can
be chromium-poor or chromium-rich chromite, ferrochromium, chromium-
bearing residues, etc.
According to the present invention, there is provided a
process for the precipitation, at an elevated pressure and a
temperature of approximately 110-300C, of a substantially iron-
free chromium(III) compound from an acid aqueous solution having a
pH of 0.5 at minimum which contains iron and chromium, comprising
performing the precipitation from a solution which also contains
bivalent chromium 2 g/l at minimum in order to prevent co-precipit-
ation of iron, the chromium(III) compound being preclpitated as at
least chromium oxyhydrate.
In the process according to the invention, the co-precipit-
ation of iron is prevented by carrying out the precipitation from a
solution which also contains bivalent chromium at minimum 2 g/l,
preferably approx. 5-15 g/l. The pH of the solution is preferably
at minimum 0.5, most preferably 1.5-2.2, and temperature approx.
110-300 C, preferably 180-250C. Quite many acids can be used.
Those which are most interesting technically are, of course,
primarily H2SO4 and/or HCl.
Depending on the raw material used, the beginning of the
process can vary greatly. For example, SiO2, calcium, sulfate,
sulfite, bivalent nickel, trivalent aluminum, etc., can be separated
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during the pretreatment stages. The use of so-called low-valence
processes has been prevented by the fact that the separation of
iron and chromium from each other, in particular, has been difficult.
This difficulty has now been overcome by the process according to
the present invention and without uslng any external reagent. This
is achieved by performing the precipitation from a solution which
also contains bivalent chromium at minimum 2 g/l and preferably
trivalent chromium at minimum approx. 70-90 g/l. In this manner it
has been possible to precipitate the chromium as chromium oxyhydrate
and/or alkalic sulfate without the co-precipitation of iron in the
final product. At the same time -the acid combined with chromium can
be regenerated and returned to the initial stages of the process.
The concentration of iron in the circulating solution is maintained
at a suitable level, for
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example within the range 40-70 g/1, by crystallizing iron
sul~ate out ~rom the solution and by hydrolysing the solution
at an elevated temperature. The precipitation temperature
of CrOOH depends on the concentration of hydrochloric acid in
the solution. The upper temperature limit for pure sulfate
is 250 C, but if hydrochloric acid is present th~s upper
temperature limit is higher.
The upper limit for temperature is due to the fact that above
this temperature the precipitating iron compound is
no longer soluble in water. It is also obvious that the
treatment temperature must not be too low because in that case
the advantage of the regeneration of the acid is lost, not to
speak of a lowered yield of the product.
The solution containing iron and chromium, used in the process
according to the invention, can be prepared from chromite.
In the production of metallic chromium or chromium oxide to
be used for dyes, the initial solution can also contain aluminum,
since the precipitating jarosite-type aluminum sulfate
decomposes during calcination. In this manner the color tone
of the chromium oxide obtained can be regulated. When metallic
chromium is produced by aluminothermy, a small amount of-
aluminum oxide in chromium oxide is, however, not detrimental~
Bivalent chromium is best obtained directly from the dissolving
of ferrochromium, but some other reducing agent or electrolysis
can also be used. This, of course, also holds true when the
initial raw material is something other than a metal-alloy
type material, for example chromite. In this case a metal
alloy similar to ferrochromium can simultaneously serve not
only as a reduding agent but also as a neutralizer of the
residual acid from chromite dissolving, for example. Since
the operation takes place, as regards Cr , in a non-equilibrium
system E (Cr3 -~pCr2+) ~ -410 mV, as compared with the
hydrogen electrode, it is self-evident that there should be
no delay between the dissolving and the recipitation of a
pure Cr compound.
,
Nevertheless, in industrial applications of the process
it is often expedient, for example, in order to separate
the dissolution residue or for temperature control, to carry
out the dissolving and the precipitation of the pure Cr
compound separately.
The separation is preferably carried out from a solution
having a pH of 0.5 at minimum, but the pH of the initial
solution must not rise to the precipitation range (pH over 3)
of alkalic trivalent chromium, except in cases in which the
dissolving and the precipitation are carried out simultaneously,
in which case the upper limit for the pH is determined, for
example, on the basis of the precipitation of alkalic Fe2+
compounds and is in the order of 3.5-4Ø
It is evident to an expert in the art that an iron-free
chromium(III) compound can be precipitated out from a solution
which contains iron and chromium, in accordance with the
present invention, at a pH of over 0.5 and at an elevated
temperature and pressure, by using highly varied raw materials,
such as chromium-bearing scraps and wastes, chromite and
ferrochromium. Using the process according to the invention
in combination with calcination it is possible to produce
pure or alloyed hydrous chromium oxyhydrate, calcinated
chromium oxyhydrate, chromium oxide, and metallic chromium
from these.
The invention is described below in more detail with reference
to the accompanying drawings, in which Figures 1-3 depict three
alternative flow diagrams for carrying out the process according
to the invention.
In the embodiment according to Figure l, chromium oxyhydrate
is precipitated in accordance with the present invention,
and thereafter the chromium oxyhydrate or an alkalic salt of
chromium is se~arated out from the solution, which is
directed to crystallization. The sulfates of nickel, cobalt
and iron, having a chromium concentration of 0.5 %,-are thereafter
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B91 5
separated out from the solution, which contains bivalent iron
10-15 g/l, trivalent chromium approx. 40 g/l, and sulfuric
acid over 100 g/l. The chromium-bearing alloy is mixed with
this solution and possibly also electrolysed in order to
increase the concentration of bivalent chromium in the solution
to the level required by the subsequent precipitation of
chromium oxyhydrate, if the initial material contains
problematic oxidation catalysts of Cr2~.
In the embodiment according to Figure 2, the solution derived
from chromium oxyhydrate precipitation is exposed to the
separation of iron in the presence of oxyg.en in an autoclave.
From the filtr.ation, a trivalent iron hydroxysulfate having
a chromium concentration of 4-10 ~ is obtained. Sulfuric acid
is a~ded to the solution, which contains trivalent iron approx.
20 g/l, at such a rate that the concentration of sulfuric
acid is above 100 gjl, whereafter the solution ls directed
to the dissolving of the chromium-bearing raw material in
order to increase the concentration of bivalent chromium in
the solution to a level sufficiently high for the subsequent
precipitation of chromium oxyhydrate.
In the embodiment according to Figure 3, all or part of the
solution derived from the crystallization and separation is
directed toithe dissolvir~ of chromite, whereafter the solution
plus any subsidiary flow are subjected to electrolysis, or
ferrochromium is dissolved in it in order to raise the
concentration of bivalent chromium to the desired level.
The accompanying Figure 4 depicts the percentage concentration
of iron in the obtained chromium oxyhydrate precipitate as a
function of the concentration of bi.~alent chromium (g/l).
It can be seen in Figure 4 that the concentration of iron in
the precipitate drops very sharply when the concentration of
bivalent chromium in the solution increases~ The shape of the.
curve shown in Figure 4 remains the same even under changed
precipitation conditions. The curve can, however, move within
the limits indicated by dotted lines in the figure, depending
on the crystal nuclei used and on the other substances present
.
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in the solu-tion, or on whether or not the dissolving and
precipitation are perEormed simultaneously.
The invention is described below in more detail with
reference to the accompanying examples.
Example 1
Ferrochromium was dissolved in sulfuric acid so that a solution
was obtained directly which contained trivalent chromium
70 g/l, bivalent iron 40 g/l, and bivalent chromium 5 g/l.
The pH of the solution was 2.0 in this case. The solution was
enclosed in a pressure vessel at 235 C for 30 minutes
(nitrogen atmosphere). The obtained chromium oxyhydrate
precipitate was washed with water and dried. The concentration
of iron in the precipitate was 0.15 %. The chromium yield of
the precipitate was 65 %.
Example 2
The solution according to Example 1 was oxygenated so that
its concentration of bivalent chromium dxopped to 2 g/l,
whereafter precipitation was performed at 235 C for 30
minutes. The concentration of iron in the precipitate was
0.5 % in this case. Carbon 1 % was mixed with the product, and
it was kept in a 1000 C chlorine gas flow for 30 minutes.
The concentration of iron in the chromium oxide obtained was
less than 0.1 %.
~xample 3
The concentration of bivalent chromiwn in the solution
according to Example 1 was decreased to 0.5 g/l and the solution
was kept at 235 C for 30 minutes. The obtained product
contained iron 4.3 %.
Example 4
Ferrochromium was dissolved in a mixture of sulfuric acid and
hydrochloric acid. A solution was obtained which had a pH of
2.0, a total chromium concentration of 100 g/l, an iron
concentration of 63 g/l and a chloride concentration of 50 g/l.
By keeping the solution at 230 C for 60 minutes, chromium
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oxyhydrate was obtained with a yield o~ 40 %, the initial
concen-tration of bivalent chromium in th~ solution being 6.7
g/l. The concentration of iron in the product was 0,08 %.
Example 5
The concentration of bivalent chromium in the solution according
to Example 4 was increased to 8.9 g/l by additional dissolving,
its pH was increased to 2.1, and chromium oxyhydrate nuclei
were added at a rate of 2 g/l. The solution was kept at 230
C for 60 minutes, whereby 48 % of the chromium precipitated
as chromium oxyhydrate from the solution. The concentration
of iron in the product was 0.03 %.
Example 6
The pH of the solution according to Example 4 was decreased
to 1.5 by means of sulfuric acid, and the concentration of
bivalent chromium at the same time decreased to 3.4 g/l.
The solution was kept at 230 C for 60 minutes, and the yield
of chromium oxyhydrate was 35 %.
Example 7
Chromate was dissolved in sulfuric acid. Part of the bivalent
iron, bivalent m~gnesium and trivalent aluminum was removed
from the obtained solution by crystallization. Ferrochromium
was added to the obtained solution in order to increase
the concentration of bivalent chromium, and so the product
was a solution in which the total concentration of chromium
was 92 g/l, the concentration of bivalent chromium 5.2, of
bivalent iron 43 g/l, of aluminum 1 g/l, of magnesium 1.5 g/l,
and the pH was 2.2. The solution was kept at 240 C for 30
minutes, whereby a chromium oxyhydrate was obtained in which
the concentration of iron was 0.15 ~, of aluminium 1.8 %,
of magnesium 0.04 % and of chromium 53 %.
Example 8
A solution which contained bivalent iron 43 g/l, trivalent
chromium 70 g/l and bivalent chromium 3 g/l and which had a
pH of 2, was kept at a temperature of 200 C
a) for an hour, in which case the yield of chromium oxyhydrate
,
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was 10 % and the concentration of iron 0.4 ~,
b) for 16 minu-tes, in which case the yield of chromium
oxyhydrate was 6.5 ~ and the concentration of iron in the
precipitate 0.2 %.
Example 9
A solution which contained bivalent iron 71 g/l, trivalent
chromium 90 g/l, and bivalent chromium 5 g/l and which had
a pH of 2 was kept at 250 C for 30 minutes, in which case
the yield of chromium oxyhydrate was 74 % and the concentra-
tion of iron in the precipitate was 4.5 ~.
Example 10
A solution which contained bivalent iron 43 g/l, trivalent
chromium 70 g/l, bivalent chromium 15 g/l, magnesium 10 g/l
and which had a p~ of 2, was treated for an hour at 235 C,
whereby a chromium hydroxide precipitate having an iron
concentration of 0.03 % was obtained.
Example 11
A finely-divided ferrochromium, extremely low in Si, was
enclosed, together with H2SO4, N2 and CrOO~I nuclei, in a
pressure vessel with good mixing. The pressure was lovered
by discharging gas. The temperature was allowed to rise to
210 C. At its best, approx. 1/4 of the chromium was in
Cr2 form. After four hours the solution and the precipitate
were removed and filtered. The pH of the FeSO4 solution was
1.4 and its Fe concentration 83 g/l. After slight grinding,
magnetic separation, an acid wash, and drying, the
concentration of Fe in the CrOOH product was 0.18 %.
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