Note: Descriptions are shown in the official language in which they were submitted.
~.0~68
This invention relates to the recovery of
chromium together with nickel and associated metal values
from nickel lateritic ores while producing an iron ore
tailings having utility for the recovery of iron there-
from.
State of the Art
Generally speaking, chromium is recovered from
high grade ores containing by weight 50% to 60% chromium
as chromium oxide together with about 12% to 25% iron in
the form of ferrous oxide. The recovery of chromium from
low grade chromium ores has not been economically feasi~e.
- One method has been proposed for recovering
` chromium from nickel lateritic ores which generally contain
less than 5% chromium. Such a method is disclosed in U.S.
Patent No. 3,082,080 (March 19, 1963) which describes a
process for recovering chromium and nickel from iron con-
tained in lateritic, limonite ores. The ore is mixed with
a tetravalent manganic oxide mineral in an amount suffi-
cient stoichiometrically to oxidize the chromium, the ore
' 20 being leached in an aqueous acid solution containing sul-
} ~uric acid equalling at least 40/O of the dry weight of
f - the ore, the amount of acid in the slurry being predeter-
mined so as not to dissolve any substantial proportion of
irsn present. A temperature of at leas~ about 475F is
used at superatmospheric pressure and an iron or~ ~ailings
results from the treatment.
~,
t -2-
,, ,
~4~468
However, the foregoing treatment does not pro-
duce an iron ore tailings sufficiently low in associzted
metal values, such as chromium, among other elements,
for use in the recovery of iron therefrom.
Relative to the foregoing problem, we have
developed a method which enables the substantially com-
plete removal of chromium, together with associated metal
values, from nickel lateritic ores.
; We have found that chromium appears in the ore
in two forms, soluble chromium and insoluble chromium.
Thus, the use of prior methods generally results in the
insoluble chromium appearing in the iron tailings which
decreases its utility in steel making.
Obiects of the Inv~ntion
It is thus the object of the invention to pro-
vide a method for recovering chromium from particulate
iron-containing ores in an economical manner while produc-
ing an iron ore by-product having utility in the manufac-
ture of steel.
Another object is to recover chromium from
nickel and cobalt containing lateritic ores which also
~ contain iron, wherein the nickel and cobalt are recovered
! together with chromium, and wherein the iron is recovered
in the tailings suitable for use in the recovery of iron
therefrom.
,. ..
,
,~ ~, , , , , ; " ' ' , ' : . ,
1~44468
These and other objects will more clearly ap-
pear from the following disclosure and the appended
claims~
Statement of the Invention
:`
Our discovery is based on the fact that substan-
tially all of the soluble chromium appears in a size frac-
tion of the particulate or comminuted ore of very fine
average size which may comprise a size less than about
20 microns, preferably less than lO microns. By sizing
the comminuted ore to produce a desired size fraction in
which the chromium is substantially acid-soluble and then
leaching the metal values therefrom, an iron ore tailing
i9 obtained that is acceptable for use in the recovery of
' iron therefrom. It is preferred that the ore be one in
.
which the size fraction containing the soluble chromium
.,
constitutes at least about 50~/O of the ore batch to be
treated. Generally, the fines fraction may range from
about 50/O to 85% by weight of the total ore batch. Tetra-
,. ~ --1 -
valent manganese ion, such as manganese dioxide tor MnO4
or S2O8 ), is preferably added to the size fraction as
an oxidizing agent to oxidize the chromium to a higher
` valence and reduce its tendency to hydrolyze out of solu-
tion. ~
-4-
.,
,~ , i
.
10~468
Thus, the invention resides in a method of
separating chromium from particulate lateritic ores con-
taining substantial amounts of iron, which comprises as
a first step, sizing said ore to produce a fines fraction
in which substantially all of the chromium is in the
soluble form. In the case of ~ew Caledonia ore, it has
been found that the fines fraction may be minus 20
microns, preferably minus 10 microns. As stated herein-
- above, it is preferred to add an amount of tetravalent
manganese oxide material or other of the aforementioned
oxidizing agents in an amount at least stoichiometrically
suficient to oxidize the chromium to a higher valence.
The additional steps comprise forming an aqueous slurry
of the ore containing an amount of sulfuric acid corres-
ponding to about 0.1 to 0.5 parts by weight of acid per
part of dry ore, and then subjecting the ore to high pre~
sure leaching at an elevated temperature, thereby extract-
ing said chromium and associated metal values from the
iron which reports in the tailings, the tailings being
, 20 very low in the associated metal values. The chromium
in the tailings is reduced to less than 0.2% and generally
less than 0.1%.
,~
68
Details of the Invention
The cut size of the ore may depend upon the
size distribution of the soluble and the insoluble forms
of chromium. The size distribution may be determined
by a procedure in which 0.5 gram of sample is mLxed with
1 cc of 36 ~ H2S04, 10 cc of 12 ~ HCl and 2 cc of io N HF
in a 100 cc polyprolyene flask. The ore sample is allowed
to decompose for 1 hour at 100C in a water bath. Then
75 ml of 5% boric acid solution is added and the volume
brought up to 200 cc. The soluble chromium analysis is
determined by atomic absorption. The foregoing is carried
out on various cuts to determine-that fines fraction in
which substantially the chromium is acid-soluble. As sta-
ted earlier, the fines fraction should preferably consti-
tute at least about 50/0 by weight of the ore batch.
The total chromium content in the size fraction
is dFtermined in the usual manner by ~a202 fusion and the
percent soluble chromium in the fraction determined rela-
tive to the total chromium content.
$ 20 The cut slze of the ore following the determina-
tion of the soluble chromium distribution selected for
the leach feed material generally contains about 100% of
tbe soluble chromium. As stated above, in the case of
--6--
J
~r, ' ' ' '' ," ",
... . .. . . . . . . . .
~044468
New Caledonia lateritic ores, the chromium in a cut size
`~ of minùs 10 microns has been found to be ~ore than 9~h
soluble chromium. This is important as it enables the
production of iron ore tailings following leaching which
contains very little or no chromium. The cut size or
fines fraction is easily produced by using a hydrocyclone.
It was found relative to New Caledonia ore that
a coarser cut, such as 400 mesh, generally lowers the
soluble chromium relative to total chromium to about 60~/o
`~ 10 to 70%. This should be avoided since the leach residue
would then contain chromium higher than the maximum
generally permitted for iron ore.
By determining the proper fines fraction in
which substantially all of the contained chromium is
soluble, an acceptable iron ore tailing is generally
obtainable containing less than about 0.1% chromium.
In order to insure that the soluble chromium
i leached stays in solution, it is preferred that the
leaching be effected under oxidizing conditions by using, for
- 20 exa~ple, tetravalent manganese oxide material containing
MnO2 or its equivalent.
Since New Caledonia lateritic ores contain
nickel and cobalt as valuable metal values together with
associated metal values, such as chromium, then these
metals are also recovered together with chromium. Fol-
,
,;, .
, , :
~7~
r" ~ . . ; . ` . ;. . . : . .
lU4~468
lowing leaching, the various metal values can be separa-
ted by well known chemical precipitation techniques.
The leaching temperatures generally range from
about 200C to 300C, e.g. about 230C to 280C, the t~m-
perature preferably being about 250C, the retention time
being usually one hour and higher. The superatmospheric
pressure under which the leaching is conducted generally
ranges from about 400 psig to 1000 psig.
The ore is slurried in an aqueous acid solu-
tion of sulfuric acid at a pulp density of over about
2~/o so~ids and preferably from about 3~/O to 50~/O. The
acid-to-ore ratio (dry ore) on the weight basis ranges
from about 0.1 to 0.5. The MnO2 to ore ratio will vary
with the ore but should be at least stoichiometrically
sufficient to oxidize the chromium to the higher valence.
When the soluble chromium is leached from the nickel
lateritic ores, the nickel and the cobalt extractions
are very close to 100%, with the tails or leach residue
meeting the specification requirements of iron ore. The
iron tails can be subjected to such well known processes
as pelletizing, roasting or other processes to produce
acceptable iron ore material for the metallurgical produc-
tion of steel.
Aa illustrative of the advantages of the in-
vention, the following example is given:
, ,. ' ~
z , .
-8-
.
E
xample 1
An ore sample containing 1.44% Ni, 0.11% Co,
2.25% Cr, 0~80~o Mn, 44~2~/o Fe, each in the form of oxides
and the balance oxides of silicon, aluminum, etc. is
sized using a hydrocyclone to provide a size fraction of
minus 10 microns, the size fraction constituting about
; 63% by weight of the total ore sample.
The chemical analysis of the size fraction as
to the important metal values was as follows: 1.43% Ni,
0~04% CO, 0~90% Cr, 0.40% Mn and 48~0% Fe. The total
chromium in the fines fraction includes both the soluble
and insoluble chromium, the soluble chromium being over
90/O of the total chromium present.
A given weight of the fines fraction is formed
into a leach pulp with a 35% solids content, the acid-to-
`~ ore ratio being about 0~25 on the dry ore basis. The
pulp was subjected to leaching in an autoclave for about
one hour at 250C and a pressure of about 575 psig.
The soluble chromium leached out of the ore
i 20 ~ corresponded to about 0.85% chromium in the fines frac-
.
tion, the total chromium content in the fines fraction
being 0.9/O. Thus, the ratio of the soluble chromium to
the total chromium on the percent extraction basis cal-
.
g
, ,, , i , ,, . , i . , ,
' ,' - , ' ' ~ ' ' ' ,. ' ', , , ' ' , , ' . ' ' ' .' ,,: . ' . ,, ' :, `
~044468
culates to about 94%. This results in an iron ore tail-
ings containing 0.06% chromium, the iron content of the
tailings being about 52%. This is acceptable ore grade.
By comparison, minus 400 mesh fraction of the
ore was similarly treated. This fraction had a total
chromium content of 1.32% of which 0.80% is soluble chro-
mium. The ratio of soluble chromium to total chromium
on the percent basis corresponds to only 61% of the total
chromium. Leaching this fraction under the same condi-
tions as above results in an iron ore residue assaying
0.57% chromium which is not acceptable grade where a
substantially chromium-free product is desired.
In a size fraction ranging from about 65 to
400 mesh having a total chromium content of 7.48%~ of
lS which 1~75% is soluble chromium, the iron ore residue
` resulting from the leach assayed about 6.3% chromium
which renders the tailings even more unacceptable as iron
ore for the recovery of iron therefrom.
In another fines fraction of minus 10 mi,~rons,
all of the chromium was soluble, the amount assaying
about 0.95%. The iron ore residue after leaching con-
tained substantially no chromium.
The chromium in the ore is in the trivalent
form and tends to hydrolyze when the emf values of the
solution are reducing. However, hyflrolysis of Cr+3 czn
be prevented at all pH vàlues if the solution emf values
--10--
,, ,,, ., .~, .. . . .
. , : ' ,,., " : :,. . . . . . . .
- . ... , ,.: , . : . . .
104~68
are sufficiently oxidizing, for example, by adding tetra-
valent manganese ion (e.g. MnO2) or its equivalent, such
as MnO4 or S208 . It takes 3 mols of Mn+4 to oxidize
2 mols of Cr+3 to 2 mols of Cr+6. Thus, the MnO2 addi-
` 5 tion (or its equivalent) should be at least stoichio-
metrically sufficient to oxidize the chromium to hexa-
valent chromium. The ratio by weight of Mn to Cr
may range from about 1.5:1 to 3:1. The stable chromium
ion produced is Cr2O7 which is stable over a broad pH
range.
Thus, by leaching feed material in which sub-
; ~tantially all of the chromium is soluble and by using
oxidizing conditions to convert the chromium from the
trivalent state to the hexavalent state, the leach resi-
due will be practically free of chromium and provide an
acceptable iron ore tailings. However, the chromium may
be kept in the trivalent form where it is to be removed
by precipitation following the chromium leaching step.
The foregoing method is applicable to nickel
lateritic ores cQptaining: about 0.5% to 2.5% nickel;
about 0.05% to 1.0% cobalt; about 0.25% to 5.0/O manganese;
about 0.3% to 15% chromium; about 0.2% to 10% aluminum;
about 0.1% to 15% magnesium; about 2% to 45% silica (sio2);
and about 10% to 55% iron. The metal values are present
as oxides.
. . .
,, , ... .. ,. ~ ......... .. .
. .
468
In removing the chromium by pressure leaching
as described herein using an aqueous sulfuric acid solu-
tion, generally at least 95% of the nickel and cobalt is
removed with the soluble chromium together with amounts
of manganese, aluminum, magnesium, etc. The chromium in
this instance may be in the trivalent state.
The metal values are separated by methods well
known in the art. For example, the pregnant liquor which
has a low pH may be neutralized with coral mud to a pH
of about 2.5 to 2.8 and the nickel and cobalt removed by
precipitation with H2S in an autoclave at about 122C and
a pressure of about 150 psig. The sulfide precipitate is
separated from the effluent chromium-containing solution
for further treatment.
The effluent solution may then be adjusted to
a pH of about 5.3 to precipitate chromium (trivalent),
aluminum and any residual iron in solution. The chromium
" can be recovered from the precipitate by any known method,
such as by alumino-thermic smelting.
Any foreign ions remaining in solution following
~ ~he foregoing precipitation, such as lead, copper, zinc,-~ etc., can be separated by other precipitation techniques.
One method is to lower the pH of the solution to about
l.S and then precipitate these metals as sulfides using
H2S.
'J ' ' ' ' . , , " '', ' ,, ~,' ' ', ' ; ' , ' ' ' ' ' ' .
468
T~e important aspect of the invention resides
in a technique for removing substantially all of the
chromium from particulate ore which is first cut sized
to provide a fines fraction in which substantially all
S of the chromium is acid-soluble, such that the iron tail-
ings remaining after the chromium leaching step is sub-
qtantially chromium-free.
Where the chromium is recovered in the hexa-
valent state (Cr2O7 ), it may be desirable to recover
it selectively as a dichromate salt. One method for do-
; ing this comprises passing the solution containing the
chromium in the hexavalent state (to strip out such cations
as Cr+3 and ~i+2) through cationic exchangers containing
an ion exchange resin in which the active member is a
~;
; j 15 strongly acidic sulfonic acid type. Examples of such
resins are: Dowex-50 which is a strongly acidic nuclea
sulfonic acid type in a resin matrix of divinylbenzene;
Amberlite -200 which is defined as being strongly acidic
sulfonated macroreticular, the resin being styrene divinyl-
benzene; and Amberlite IR-120 which is a strongly acid
~ulfonic acid type cationic exchanger, with the resin a
, polystyrene base. The effluent from this ion exchange
sy~tem is then neutralized to pH 8, where the oxidizing
potential of the solution is so low that little oxidation
: ' .
' -13-
. , ,
- 104~68
of an anion resin can occur. The neutralized solution
is then treated with an anion exchange resin. When the
anion resin is saturated with chromium, it is eluted
with sodium hydroxide solution to produce a three percent
solution of sodium chromate.
Although the present invention has been
described in conjunction with preferred embodiments, it
is to be understood that modifications and variations may
be resorted to without departing from the spirit and
scope of the invention as those skilled in the art will
readily understand. Such modifications and variations
are considered to be within the purview and scope of the
invention and the appended claims.
,
-14-
.. .. .
