CA1081471A

CA1081471A - Pressure leaching of sulfides

Info

Publication number: CA1081471A
Application number: CA267,348A
Authority: CA
Inventors: Peter J.M. Ryan; Peter G. Garritsen
Original assignee: Vale Canada Ltd
Current assignee: Vale Canada Ltd
Priority date: 1976-12-07
Filing date: 1976-12-07
Publication date: 1980-07-15
Also published as: AU3069577A; NO152453B; NO774149L; ZA776776B; AU511297B2; GB1544763A; NO152453C

Abstract

ABSTRACT OF THE DISCLOSURE:

One or more of copper, nickel and cobalt is recovered from an iron-containing sulfide concentrate by subjecting an aqueous slurry of the concentrate to a pressure-leach which is carried out in part below and in part above the melting point of sulfur.

Description

1(~81~7~ i The present invention relates to the leaching of sulfur-rich mineral concentrates, and more particularly to the oxidative pressure-leaching of one or more of the metals copper, nickel and cobalt from iron-containing sulfide concentrates.
It is known that non-ferrous metal values present in a sulfidic mineral can be recovered by subjecting such mineral to an oxidative pressure leach. This involves forming an aqueous slurry of the concentrate, from which the copper and/or nickel is to be leached, and while main-taining the slurry in an agitated and heated state bringing it into contact with a free-oxygen-containing atmosphere under superatmospheric pressure. As a result the desired non-ferrous metal value is made to dissolve,:iron present in the ore is oxidized to insoluble iron hydroxides and sulfur present initially as sulfide reports mainly as elemental sulfur, and to some extent as dissolved sulfate in the leach solution. At the termination of such a leach it is desirable to be able to resort to simple solid-liquid separation techniques, to separate a liquid containing most of the desired non-ferrous values, from solids consisting of iron h~x~ide,unreacted mineral concentrate and elemental sulfur.
In most of the prior art procedures suggested for effecting such an oxidative pressure leach, it is advocated to maintain the aqueous slurr~ at a temperature lower than the melting point of sulfur during the treatment with oxygen.
This avoidance of exceeding the melting point of sulfur stemmed from the apprehension that molten sulfur would have ' .' : ' : ' ' ' ~. :
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1~81471 a tendency to coat the sulfide particles and impede or prevent further leaching. Examples of such 'low-temperature' leaching are described in Canadian Patent No. 965,962 and in U.S. Patent 3,816,105. ~owever inherent problems encountered with such low temperature leaching are the long leach times needed to achieve sub-stantially complete extraction of the non-ferrous values and the poor settling and filtration properties of the iron hydroxide produced. In consequence such processes are characterized in practice by relatively low non-ferrous value extractions and low grade elemental sulfur separated from the leach residue. To overcome the second of these problems it has been suggested, for example in the above-mentioned Canadian patent, to heat the leach slurry àfter completion of the oxidation for a short period prior to undertaking the solid/liquid separation. Whether such post leaching heat treatment causes the iron hy,droxide and unreacted ~fides to become sulfur-coated or merely aids in agglomeration of elemental sulfu~, it does not overcome the problem of slow leaching and poor extraction.
Alternatively it has been suggested that the oxidative leaching can be performed at high-temperatures, i.e. above the melting point of sulfur. In order to overcome any tendency of unreacted sulfides to become coated with sulfur various measures have been advocated including vigorous agitation as described in U.S. Patent 2,898,196, and the use of additives as described in Canadian Patents 964,867 and 965,964. However this 'high temperature' leaching approach is not without its disadvantages. The major one of these is the tendency for substantial amounts .

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of the sulfides to be converted to sulfates. This is undesirable because sulfate formation entails wasteful consumption of oxygen and of base needed for solution neutralization, and results in a higher coolinq requirement during the leach as well as higher levels of dissolved iron in the leach solution from which the iron must be subsequently precipitated.
It is an object of the present invention to provide an improved oxidative leach process whereby a high proportion of non-ferrous values in a sulfide con-centrate can be solubilized and a residue with good settling properties obtained while minimizing sulfate formation.
According to the invention a process is provided for leaching a sulfide concentrate containing iron and at least one non-ferrou~ value selected from the group consist-ing of copper, nickel and cobalt by treating an agitated aqueous slurry of the concentrate with a free-oxygen-containing gas under superatmospheric pressure to solubilize the non-ferrous value(s) and convert the iron to iron hydroxide, wherein the slurry is maintained at a temperature below the melting point of sulfur for a period long enough to ensure dissolution of only a portion of the non-ferrous value(s), and thereafter the temperature of the slurry is increased to a value above the melting point of sulfur and maintained at that value until the desired non-ferrous value(s) has been substantially completely dissolved.
The temperature of the slurry during the initial part of the oxidation ~hould be within the range ~0 to 110C, and preferably it is about 105C. The higher temperature .. : . ,, : .
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used for the later part of the leach can be within the range 115 to 180C and preferably it is about 135CC.
~sing the dual temperature combination 105 and 135C, we have found that the precise point at which the change in temperature is made is in no way r~gidly dictated, and the leaching times can be optimized in accordance with the chemical and physical properties of the sulfide concentrate treated. The duration of the high temperature stage of the leach may represent as little as 20% or as much as 80~ of the total leaching time. In other words the ratio of the low temperature oxidation period to the high temperature oxidation period may be between 1:4 and 4:1. A ratio lower than 1:4 would lead to excessive sulfate formation, while if the ratio is too high the extraction of non-ferrous values will be unacceptably low. It i9 preferred that the ratio of the low temperature and high temperature periods be close to unity,e.g. 0.7 to 1.5.
It should be noted that despite the use of temperatures in excess of the me~ting point of sulfur during the second period of leaching, we have not found it necessary to resort to the use of any surfactants or sulfur-solvents as additives. In fact by ensuring agitation of the slurry no problem of leach inhibition by sulfur coating of unreacted sulfides was encountered.
The leach parameters, other than the temperatures used and duration~ at these temperatures, are well known in the extensive literature on this subject and need not be described in any great detail herein. In general it can be stated that the mineral concentrate to be leached should be in a finely ground form, for example, such that 80~ or "` 1(~81~71 more is of smaller particle size than 325 mesh Tyler Screen Size (TSS). The particulate solid is slurried in water to a consistency of the order of about 20 or 30% solids, and is treated in a sealed autoclave heated to the desired temperature. Oxygen or a free-oxygen-containing gas is fed into the autoclave to ensure a partial pressure of oxygen over the slurry of the order of 3 atmospheres or more so that the overall pressure in the autoclave, taking into account the steam partial pressure as well as the partial pressure of any carrier gas intro-duced with the oxygen, will typically be of the order of ~-4 or 5 atmospheres or even higher.
The process of the invention can be employed usefully in connection with various non-ferrous metal con-taining sulfide concentrates. Where such concentrates con-tain for example both nickel and copper values it will generally be preferred to leach both these values at the same time so that substantially all, or as much as practicable, of the copper and the nickel are solubilized in a single `
leaching treatment. However the invention is by no means `~
restricted to such a procedure, and the pressure used as well as the leach duration may if desired be chosen so as to solubilize substantially all of the nickel while solubilizing very little of the copper.
At the termination of the leach, the resulting slurry is amenable after cooling to standard solid/liquid separation techniques such as filtration or elutriation to separate the leach solution containing the desired solubi-lized non-ferrous value from the solids consisting essentially of elemental sulfur, iron hydroxide and unreacted minerals.

`` 1~8~71 The invention will now be specifically described by way of examples. Unless otherwise specified, all percentages quoted herein are percentages by weight.
EXAMPLE
A series of leachinq tests were conducted on a concentrate which analvzed:
Copper : 0.4~
Nickel : 4.0%
Iron : 39~
Sulfur " 24%
The concentrate was ground to ensure that at least 95% thereof ' was finer than 325 mesh (TSS), and slurried with water to a 30% solids consistency. The slurry was then leached in an autoclave while agitating it vigorously under a total pressure of about 2 Megapasca1s (MPa) of oxygen and steam. The series comprised a dual temperatUre test in accordance with the invention, and a pair of comparative tests representing a high temperature and a low temperature leach respectively. ~
For the purpose of the dual temperature test (Test 1) the '-' slurry was maintained for one hour at 105C after which time lt was heated to 135C and held at ehat temperature for a further,two hour period of oxidative leaching. In compara-tive Test A the leach was conducted for three hours at 105C,while,in comparative Test B the slurry was held at 135C for three hours.
At the end of each leaching test the slurry was transferred to a settler where th,e settling properties were determined. As Table 1 below shows the slurry obtained by the dual temperature leach in accordance with the invention exhibited settling properties not only far better than those ' -6-.
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1()8147i of the slwny leached at low temperature (A), but indeed superior to those of the slurry leached at high temperature only ~B).
TABLE : 1 TeRtType of leach Settling Rate Underflow Density ~m/h) (% Solids) .. .
1~1 hour at 105C 1.8 43 ~2 hours at 135C
A3 hour~ at 105C 0.3 37 33 hours at 135C 1.3 42 The clear leach liquor obtained from the settler was analyzed to determine the extent of solubilization of the metals as well as the sulfate content. The results obtained, ~hown in Table 2, qhowed that the copper and nickel solubilization which it was desired to maximlze was better for Test 1 than in the case of the low temperature Test A
though not quite so high as for comparative Test B. At the - same time however eXcessive amounts of sulfate found in Test B were minimized in Test 1 which gave a sulfate level only slightly higher than test A. Most significant is the fact that the undesirable dissolution of iron i~ mini-mized in Test 1 better than in elthor of the aomparative tests.
TABLE s 2 . . T ~t _ Leach Liquor Compo~ition_ ~g~l) : . -Copp~r- Nickel Iron Sulfate 11.02 18.4 7.6 72.0 A0.84 17.8 9.9 64.0 31.33 19.8 10.3 92.2 .

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The solids were also analyzed after separation from the leach liquid to determine the iron, nickel and - copper contents and the relative amounts of sulfur present as unreacted sulfide, as sulfate and as elemental sulfur.
From these measurements the overall effectiveness of the leach could be expressed in terms of the percentage extraction, i.e. dissolution of each metal,and the dis-tribution of sulfur in its various forms in the final slurry.
This data is shown below in Table 3:
TABLE : 3 -- .
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Test Extraction (%) SulI ur Distribut ion (%) _ ~opper ~b~ r Iron Elemental As Sulfate As Sulfide 1 69 93 5.4 63 32 5 A 54 81 5.7 56 30 14 74 94 7.3 50 46 4 EXAMP~E 2 A further ~e~ies of leaching tests were carried out comprising a dual temperature leach in accordance with the invention (Test 2) and a pair of comparative tests (Tests C and D). ~he sulfide to be leached in this case was a concentrate containing:
Copper : 2.5 %
Nickel : 4.5 ~
Iron : 45 %
Sulfur : 28 %.
It was ground and slurried in the same way as described in Example 1 except for the temperature and duration of the ~-leaching which were as follows:

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Test 2 : 2 hours at 105C then 2 hours at 135C
Test C : 5 hour~ at 105~
Test D : 3 hours at 135C.
Analysis of the solids and liquids resulting from each of the leaching tests gave the data in Table 4 below:
TABLE : 4 TestExtraction (~¦ Sulfur Distrlbution t%) Copper Nickel Iron Elemental As Su~Fate ~As Sulfide It will be apparent from the comparison of the data in Table 4 that whether it is ~udged by-msximization of copper or nickel dissolution, minimization of iron dissolution or maximization of the fraction of sulfur present in elemental form, the test in accordance with the invention gave results at least e~ual to, and in most cases superior to the better of the two comparative tests.
A olear benefit of the process of the invention can be seen from the results of flotation tests to separate sulfur from the remainder of the solids in the leached slurry. The purity of the sulfur separated from the slurries of Tests C and D
was 70 and 78% respectively. On the other hand similar flo-tation performed on the leach discharge slurry of Test 2 resulted in a 90~ pure sulfur product.
While the present invention has been described with reference to specific embodiments thereof, it will be .

' ' 1(~8~71 understood that many additions to, or modifications of such embodiments may be resorted to without departing from the scope of the invention which is defined by the appended claims.

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Claims

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

1. A process for leaching a sulfide concentrate containing iron and at least one non-ferrous value selected from the group consisting of copper, nickel and cobalt by treating an agitated aqueous slurry of said concentrate with a free-oxygen-containing gas under superatmospheric pressure to effect dissolution of said non-ferrous value(s) and conversion of said iron to iron hydroxide, wherein the improvement comprises maintaining said aqueous slurry at a first temperature which is lower than the melting point of sulfur for an initial oxidation period sufficiently long to ensure dissolution of only a portion of said non-ferrous value(s), and thereafter heating said slurry to a second temperature which is higher than the melting point of sulfur and maintaining it at said second temperature for a further oxidation period sufficiently long to ensure substantially complete dissolution of said or at least one of said non-ferrous value(s).

2. A process as claimed in claim 1 wherein said first temperature is about 80-110°C and said second temperature is at least 115°C.

3. A process as claimed in claim 2 wherein said first temperature is about 105°C and said second temperature is about 135°C.

4. A process as claimed in claim 1 wherein the ratio of said initial oxidation period to said further oxidation period is between 1:4 to 4:1.

5. A process as claimed in claim 4 wherein said periods are of approximately equal duration.