CA1117772A - Nitric acid-oxygen leaching of sulfidic minerals - Google Patents

Abstract

Abstract of the Disclosure The leaching action of a nitric acid leach liquor on a sulfidic ore is potentiated by intimately admixing an oxygen containing gas through the reacting slurry during treatment. This procedure results in the ratio of nitrate consumed to metal values leached remaining substantially constant at a relatively low level through-out the leaching procedure and in a significant reduction in the volume of off-gases normally produced. The process also reduces or eliminates the evolution of nitric oxide.

Description

~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.

111~772 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 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.

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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 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 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.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. 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. 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 quan-tities 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.

2. The process as set forth in claim 1 wherein said leach solution provided in step A also includes sulfuric acid.

3. The process as set forth in claim 2 wherein said sulfide is selected from the group consisting of sulfides of Cu, Fe, Mo, Ag, Ni, Co, Zn, and mixtures thereof.

4. The process as set forth in claim 2 wherein said sulfide is a mineral selected from the group consisting of chalcopyrite, chalcocite, bornite, covel-lite, digenite, concentrates thereof, and mixtures thereof.

5. The process as set forth in claim 2 wherein NO2 is produced during said sparging step and sufficient O2 is mixed thoroughly with the reacting slurry to oxidize NO2 to nitric acid.

6. The process as set forth in claim 2 wherein NO2 is produced during said sparging step and the NO2 is recycled to a nitric acid plant for HNO3 regeneration.

7. The process as set forth in claim 2 wherein said sulfide is a copper sulfide.

8. The process as set forth in claim 2 wherein the leach solution provided in step A contains between 100 and 700 g/l nitric acid.