Note: Descriptions are shown in the official language in which they were submitted.
~r~z K-0502-KCC
This invention related to the hydrometallurgical
recovery of nitric acid soluble metals, such as copper,
iron, molybdenum, silver, nickel, cobalt, and zinc,
from a sulfur or sulfur and iron containing ore. More
particularly, it relates to a nitric acid leaching
procedure which ifi potentiated by sparging oxygen gas
through the reacting leach solution.
Nitric acid leaching liquors for hydrometallurgi-
cally recovering metal values from sulfidic ores are
well known in the art as evidenced, for example, by
U.S. Patent Numbers 3,793,429 to P.B. Queneau et al.,
3,965,239 to Posel, and 3,888,748 to Brennecke. In
such systems, during leaching, the nitric acid is
degraded to nitrogen dioxide (NO2), nitrous oxide
(N2O), nitric oxide (NO), and/or nitrogen tetroxide
(N2O4). The leach liquor also oxidizes the sulfidic
content of the minerals to produce elemental sulfur
and/or sulfate. The oxidation enables the metal values
of interest to become solubilized in the leach liquor,
typically as sulfates, from which they can be recovered.
The nitric oxide produced can be readily oxidized to
nitrogen dioxide which, in turn, may be treated in a
conventional nitric acid plant to regenerate HNO3.
It is well known that as the concentration of
nitric acid is increased, the consumption of this
expensive oxidant also increases per unit of copper or
other metal leached. On the other hand, low nitric
acid concentrations are not as effective as high concen-
trations in leaching metal values from ores or concen-
lllm~
trates. Another undesirable effect of using high
nitric acid concentXations is that increased volumes of
reaction off-gases are produced which must be captured
and recycled. It is ~elieved that the foregoing phenom-
enon results, at lea$t in part, from a change in the
leaching stoichiometry as the nitric acid concentration
varies. Thus, in the case of copper sulfide, when the
concentration of nitric acid is low (e.g. below 150
g/l) cupric lons, elemental sulfur, and nitric oxide
are produced in accordance with the equation:
3CuS ~ 2HNO3 + 3H2SO4 ) 3CuSO4 + 2NO + 4H2O
On the other hand, when the nitric acid concentration
is high (e.g. 150-700 g/l) in addition to the nitric
oxide produced, nitrogen dioxide ls produced in accord-
ance with the equation:
CuS + 2HNO3 + H2SO4 ~ CuSO4 + S + 2NO2 + 2H2O
Furthermore, as can be seen from the reactions set
forth above, when the concentration of nitric acid is
high, one equivalent of HNO3 is consumed per one-half
equivalent of copper leached; whereas, when the concen-
tration is lower, one equivalent of nitric acid can
produce 1.5 equivalents of soluble copper. At high
nitric acid concentrations, the mole ratio of off-gas
produced to copper leached is 2.0; whereas, at lower
concentrations, the mole ratio of off-gas to copper
leached is 0.66.
Since the-economics of the leaching system are
most favoxa~le when: 1~ the ratias of NO3 consumed and
of~-gas produced to metal leached is 1QW; 2~ the volume
~11~77Z
of nitrogen oxides which must be recycled is low; and,
3) the rate of oxidation and percent metal leached is
high, it can be seen that neither high nor low initial
nitric acid concentration ratios are ideal. Because of
the foregoing phenomena, and because nitric acid is a
relatively expensive reagent, nitric acid leaching
procedures have not been wideIy practiced on a commer-
cial scale.
It is well known that the oxides of nitrogen
produced during this leaching can be reconverted to
nitric acid; however, the foregoing equations show that
during leaching, hydrogen ions are consumed when leach
liquors containing either high or low nitric acid
concentrations are employed. Since there are many
sources of hydrogen ions that are less expensive than
nitric acid, other mineral acids are employed in combin-
ation with nitric acid to supply these hydrogen ions.
Since sulfuric acid is relatively inexpensive and since
sulfates are produced during leaching, the preferred
source of hydrogen ions is sulfuric acid. of course
there are other acids such as hydrochloric acid and
phosphoric acid that can be employed to supply hydrogen
ions. These acids would change the leaching chemistry
somewhat.
A leach liquor containing only nitric acid can be
employed, but, hydrogen ions would be consu~ed as
sulfides become oxidized. This hydrogen ion consump-
tion would eventually result in a pH rise to a level at
which the reaction would proceed very slowly.
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It has now been discovered that the efficiency of the
known nitric acid leaching system may be signif-icantly
enhanced if certain quantities of an oxygen containing gas
are thoroughly mixed with the nitric acid leach liquor
during the course of the leaching procedure. By utilizing
such a technique with leach liquors containing high con-
centrations of nitric acid, it is possible to decrease the
ratio of nitric acid (or nitrate) consumed to metal value
leached, reduce the volume of the off-gas which otherwise
must be treated and recycled, reduce the production of
gases such as elemental nitrogen and nitrous oxide, and
essentially eliminate nitric oxide as an off-gas component.
Eliminating nitric oxide in the off-gas obviates the need
for a separate reactor to oxidize the nitric oxide prior
to its introduction into a nitric acid plant.
In accordance with the invention, an aqueous leach
solution containing nitric acid is mixed with a sulfidic
ore, concentrate, etc., and, oxygen, preferably in suffic-
ient quantities to eliminate the appearance of NO in the
reactor off-gas, is sparged and intimately mixed through-
out the reacting leach solution. Either oxygen gas or
an oxygen containing gas such as air may be used.
Specifically, the invention provides a process for
leaching metal values from a sulfide which contains nitric
acid soluble metal values comprising the steps of:
A. providing an aqueous nitric acid leach solution
containing at least 100 g/l of nitric acid;
B
1~1777;2
B. adding the sulfide which contains the nitric
acid soluble metal values to said leach solution to
form a slurry and leach metal values;
C. sparging oxygen throughout the slurry of step
B during leaching of the metal values in sufficient
quantities and with sufficient mixing to eliminate
nitric oxide in the off-gas as a product; and,
D. separating a metal bearing pregnant liquor
from said slurry,
said sparging of oxygen decreasing the ratio of nitrate
consumed to metal values leached and reducing the
volume of oxides of nitrogen present in the off-gas.
In the practice of the invention, it is imperative
that oxygen be sparged through and intimately contacted
with the reacting slurry. An atmosphere of oxygen
above the slurry, even if under pressure, is insufficient
to induce the improvements disclosed herein and will
give results essentially identical to the prior art
nitric acid leaching processes. It should be noted
that the process is not simply an in-situ regeneration
of nitrate. Instead, the effect of the process is to
improve oxidant efficiency. That the chemistry of the
process involves more than nitrate regeneration is
demonstrated by the fact that nitrate concentration
decreases as leaching proceeds.
In prefexred embodiments an inorganic acid such as
sulfuric acid is added to the slurry to maintain a low
pH and provide a source of hydrogen ions. It is also
111~77~
preferable to sparge enough oxygen and to admix oxygen
sufficiently so that all the gaseous effluent nitrogen
oxides can be oxidized to nitric acid. While a large
variety of different ores, concentrates, and the like,
may be leached with the process of the invention,
chalcopyrite, chalcocite, bornite, covellite, digenite,
concentrates thereof, and mixtures thereof are preferred.
Accordingly, it is an object of the invention to
improve the well known nitric acid hydrometallurgical
recovery procedure for leaching nitric acid soluble
metàls such as copper, silver, nickel, cobalt, and zinc
from sulfur or sulfur and iron containing ores of the
same.
Another object of the invention is to provide a
nitric acid leaching procedure wherein the volume of
off-gases produced is reduced.
Yet another object of the invention is to provide
a nitric acid leaching system wherein the ratio of
nitrate consumed to metal leached remains at a relatively
low, constant level during the course of the leaching
operation.
Another object is to provide a nitric acid leaching
process wherein the ratio of nitrate consumed to nitric
acid soluble ions leached is decreased as compared with
prior art techniques.
Fi~. 1 is a graph of the ratio of HNO3 consumed to
copper extracted vs. initial HNO3 concentration for a
conventional HNO3 leaching process and for a leaching
process in accordance with the present invention;
11~7772
Fig. 2 is a graph of relative N2O flow vs. time
for a conventional HNO3 leaching process and a process
in accordance with the present invention;
Fig. 3 is a graph of percent copper extracted vs.
time for a leaching process in accordance with the
present invention and for a leaching process employing
a system containing H2SO4 and oxygen (no HNO3); and
Fig. 4 is a schematic diagram illustrating one
metal recovery process employing a leaching process in
accordance with the present invention.
It is well known that the mole ratio of nitric
acid consumed to metal value leached from a sulfidic
ore, e.g., the mole ratio of HNO3 consumed to copper
produced, increases as the nitric acid concentration
increases. This phenomenon cannot be overcome simply
by decreasing the nitric acid concentration because
such leach liquors fail to leach the relatively large
percentages of sulfidic minerals available in the
system as required by sound economics~ A demonstration
of this phenomenon is set forth in Fig. 3 or U.S.
Patent No. 3,793,429 to P. B. Queneau et al. which is
reproduced in Fig. 1 of the drawing as curve A.
According to the foregoing patent, the curve
labelled A (Fig. 1) was produced by subjecting an ore
concentrate containing 25% copper, 25~ iron and 31%
sul~ur to an aqueous leach liquor containing 2M H2SO4
and various initial nitric acid concentrations. The
curve labelled B shows results which are consistent
with those of Queneau et al. which were obtained in a
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1~17772
controlled experiment. Throughout this specification
and claims, all percentages are weight percentages
unless otherwise specified.
As can be seen from curve A of Fig. 1, a 10%
nitric acid concentration results in a ratio of NO3
consumed to Cu++ produced of about 3.3. When the
nitric acid concentration was raised to about 30%, the
ratio of nitrate consumed to copper leached increased
to about 5.2. However, when oxygen gas was thoroughly
mixed with a leach liquor otherwise identical to that
shown in curve A in sufficient quantities such that
nitric oxide was eliminated as a net reaction product,
the HNO3 consumed/copper leached ratio was substantially
constant (curve C). As is shown by curve C at 10~, 15~
and 30% NO3, the ratio of NO3 consumed to Cu++ produced
remained substantially constant at about 3.
The volume of nitrous oxide produced in the off-
gases of such a leaching system ~which represents a
loss since N2O cannot be readily converted to HNO3) is
also greatly reduced if oxygen is sparged through the
leach liquor as disclosed above. For example, Fig. 2
shows a plot of nitrous oxide produced versus time for
two leaching procedures, identical except that one
leach procedure was conducted while sparging oxygen gas
throughout the leach liquor; whereas, the other procedure
was conducted in the absence of oxygen. From Fig. 2 it
can be observed that nitrous oxide evolution in systems
containing oxxgen is only about one-half of that which
occurs in leach systems which do not contain oxygen.
lilm~
The curves in Fig. 2 correspond to experiments IV and V
of Table I below.
In addition to the dramatic reduction of nitrous
oxide (N2Ol which results by practicing the invention,
the production of nitric oxide (NO~ in the off-gas is
also essentially eliminated since any nitric oxide
produced is oxidized in situ by the oxygen. Accordingly,
the need for a separate off-gas oxidizing reactor to
convert nltric oxide to nitrogen dioxide (NO2~ is
eliminated. In fact, if oxygen in excess of that
required to eliminate the production of nitric oxide is
added, nitrogen dioxide can be oxidized in situ to
nitric acid. Accordingly, the need for a separate
nitric acid plant may be eliminated or a plant of
reduced size may be employed.
As a control, oxygen gas was sparged through a
sulfuric acid leaching liquor containing no nitric acid
and compared with a similar leaching liquor containing
nitric acid. The results of this experiment, expressed
as percent copper extracted versus time, are shown in
Fig. 3. It should be noted that H2SO4 by itself will
not oxidize copper sulfide under the reaction conditions
disclosed herein: but with the addition of oxygen,
leaching in the absence of nitric acid occurs very
slowly.
It should also be noted that the concentration of
nitric acid and the amount of oxygen sparged through
the leach liquor can ~ary as required for the particular
sulfidic m~nerals to be treated. The rate of oxygen
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sparging should be at least sufficient to eliminate
nitric oxide as a reaction product. Opexable and
perferred ranges for acid concentration ratios as well
as temperature and pressure parameters appear below:
Initial H2SO4: 25-200 g/l
(50-100 g~l preferred)
Initial HNO3: 100-7Q0 g/l
t200-5Q0 g/l preferred)
Reaction temp.: ~115C, the melting point
of elemental sulfur,
85-100C preferred.
Oxygen pressure: 15-30 psi, absolute
tl5-20 psi absolute
preferxed)
Another advantage of the leaching process of the
invention is that it enables relatively dilute solutions
of nitric acid to retain considerable leaching ability.
Thus, it becomes economical to recycle the leach liquor
and thereby to provide for efficient use of this rela-
tively expensive reagent.
Fig. 4 represents a schematic illustration of a
metal recovery system embodying the improved leaching
process of the invention. A sulfidic ore is introduced
at 10 into a leaching tank 12 as a slurry and is mixed
with a sulfuric acid-nitric acid aqueous leach liquor
14. The residence time of the sulfidic ore in the
leaching tank 12 depends on the temperature at which
the leach is conducted and on the relative concentration
o~ the acids in the leach solution to the sulfidic ore.
Oxygen containing gas is thoroughIy mixed into the
leach liquor by sparging and passes upwardly therethrough
as a multipl~city of bubbles 16. During the course of
~17772
the leach, copper and other nitric acid soluble metals
are solubilized in the leach liquor and eIemental
sulfur is produced. Excess oxygen gas and oxides of
nitrogen exit via conduit 18; and, at the conclusion of
the leach, the pregnant liquor and solids exit via
conduit 20.
The pregnant liquor containing the slurry in
conduit 20 is then transferred to liquid/solid separation
tank 22 ~here the solids, predominantly elemental
sulfur and gangue, are separated from the metal bearing
liquor. The aqueous metal bearing liquor is delivered
to conventional means 24 for winning and/or separating
the solubilized metal values from the pregnant liquor.
Such sepa~ation means may be an organic extractant
separation system or an electrowinning system. The
metal depleted leach liquor, after removal of unwanted
ions by conventional techniques through purge system
26, is delivered back into the leaching tank 12 for
further nitric acid utilization via conduit 28.
The off-gases which are removed via conduit 18
contain essentially no nitric oxide and relatively
small amounts of nitrous oxide. As is conventional
practice, any NO2 and/or N2O4 produced is contacted
with cool water in the presence of excess oxygen to
regener~te nitric acid in nitric acid plant 30. It
should be noted that if oxygen in excess of that re-
quired to eliminate NO as an effluent is used, the
nitxic acid plant ma~ be eliminated since NO2 will be
oxidized to nitric acid in-si-tu. ~re~erably, the
-12-
nitric acid produced in plant 30 is mixed with the
previously purged, metal depleted leach li~uor 28.
Small make-up amounts of nitric acid and sulfuric acid
are introduced into the liquid stream via conduit 31.
The reconstituted leach liquor is reintroduced into
leaching tank 12 for further reaction.
The invention will be further understood from the
following nonlimiting example.
A series of nitric acid leach experiments was
conducted under varying conditions using 600 ml of
leach solution having a solids density of 167 g/l. The
composition o$ the solids used for leaching was 20.9%
copper, 27.4% iron, 30.2% sulfur and 21.5% gangue. The
total reaction time (reagent mixing to termination) was
120 minutes. The leach reactions were performed by
cooling the acid leach liquor to 5C and then adding
100 grams of concentrate (ore) to achieve the desired
slurry density. Heat was applied to the reactor until
the reaction temperature of 90-100C was achieved (10-
15 min.).
The reaction slurry was maintained at 90-100C
until 120 minutes total time had elapsed. At this
point, the reactor was cooled, the slurry filtered and
washed, and the residual solids analyzed for copper,
iron, elemental sul~ur, and total sulfur. Additionally,
the pregnant leach liquor was analyzed for Cu++, Fe
total, and nitrate. The results of these experiments,
and the conditions under which they were conducted, are
set forth in Table I beIow.
1~17772
.
+ +l +l +l +l +l +l +l
~71+ ¦ ¦ ~ N 1~ IJ') ~r N _I
Z ~ N
Z
~ _, ~ ~
~ N N N N N N N
5~ +1 +1 +1 +1 ~1 +1 +1
~ ~J ~ N 1` ~' CO ~D O
~ X c~
_l
~ O --I~r N N ~ N O el'
~1 Z ~ oa~ o
_i N N N N _I ~1
r~
~~ O O O O O O O
rl O _I O O O O 0 1~ 0
N~ ~
,_
a) ~ ~ I o I O ~ a
O ~d-- I I u~ N Lt
O O O O O O O
Q a~ o o ~ a~
C~
I ~ H H H~ ~' ~ H
~17772
Fro~ a study o~ Table I, the improvements character-
istic of the process of the invention may be appreciated.
A comparisQn of Experiments I and II indicates that the
ratio of nitrate consumed to copper leached increases
(3.3 + .3 to 5.2 ~ .4).
A co~parison of experiments III and II demonstrates
that under the conditions of the leach, if 5Q0 ml per
minute of oxy~en gas is thoroughIy mixed with the
reacting leach liquor, the percent copper extracted is
improved (97 + 2% vs. 92 + 2%~, the amount of nitric
acid consumed is decreased (116 g~l VS. 213 g/l~, and
the ratio of nitrate consumed to copper leached is
decreased (2.7 ~ .3 vs. 5.2 ~ .4). Experiments IV and
V demonstrate that the improved results are obtainable
when the leaching operation is conducted at 100C as
well as at 90C.
In expeximent VI, the depleted leach liquors of
experiment III and V were used to extract copper values.
This leach liquor containing a reduced sulfuric acid
concentration and only 250 ml per minute oxygen was
sparged therethrough, yet 86 + 2% of the available
copper was solubillzed.
The in~ention may be embodied in other specific
forms with~ut departing from the spirit or essential
characteri~tics thereof. The present embodiments are
therefore to be considered in all xespects as illustrative
and not restrict~ye, the scope of the in~ention being
indicated by t~e appended claims rather than by the
foregoing descr~ption, and all changes which come
lllm~
within the meaning and range of e~ui~alency of the
claims ~re therefore intended to be embraced therein.
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