CA1086075A - Reductive leach of oxidic mixtures - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

Addition of a small amount of elemental sulfur to an aqueous slurry of oxidic material, such as a limonitic ore, enables efficient leaching of Fe, Ni and Co from the material by treating the slurry with sulfur dioxide at moderate temperature and pressure.

Description

6~ 5 The present invention relates to a hydrometal-lurgical procesis for recovering iron, nickel and cobalt from oxidic mixtures containing these metals, and more - particularly to a process wherein the metals are solubilized by means of a leach conducted under reducing conditions.
While the invention is particularly suitable for the pro-cessing of lateritic ores, such as limonite, it is not restricted to such an application and may be usefully adopted for the treatmient of calcines or other oxidic mixtures.
Hydrometallurgical processing has been looked - at, in recent years, more and more as a potential candidate for replacing the established pyrometallurgical techniques for recovering such metals as nickel in particular. The emphasis on hydrometallurgy can be explained in part by considerations of environmental pollution, and also in ; part by the likelihood of a greater future dependence on -~I lateritic deposits as sources of such metals, rather than on sul~idic ores the sulfur content of which can contribute to the economy of pyrometallurgical techniques. Lateritic deposits containing oxidic ores of iron, nickel and cobalt include such ores as limonite which is characterized by a - relative~y high ratio of iron to nickel or cobalt, a typlcal composition containing, by weight, of the order of 1.5~ nickel, 0.1~ cobalt, with 35% or more of iron.
One prior art approach to the processing of such an ore 1s described in U.S. Patent 3,772,423 and involves roastlng the ore under reducing conditions at about 1,000-1,800F,and then subjecting an aqueous slurry of the reduced ore to an ammionia leach. Two disadvantages :' 1 .,',` ' ~~ ~

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s inherent in this approach are the undesirability of using ammonia on an industrial scale, and the need to resort to a preliminary roasting. An alternative scheme is described in Canadian Patant 748,374, where preferen-tial leaching of the nickel and cobalt values is aimed at. In this scheme, the ore is firs~ roasted under conditions which are controlled so as to reduce nickel and cobalt to thei~ metallic form, while iron is converted to magnetite. Subsequent to this roast, the material is leached by passin`g a sulfur dioxide - oxygen mixture through an aqueous slurry thereof to solubilize the nickel and cobalt while leaving the magnetite undissolved.
A fu~ther approach to the problem is described in U.S. Patent 2,584,700, wherein the need for a preliminary reductive roast is obviated. In this case the necessary reduction is accomplished during the leach itselfj which `~ leach involves tXeatment of the ore with sulfuric acid and sulfur dioxide. The drawbacks of such a process are:
a) the overall acid consumption is high, so that a sulfuric acid plant of substantial size is needed; and b) more e~icient acid utilization, coupled with high metal extractions can be achieved only by resorting to long leaching times and carrying out the leach at elevated sulfur dioxide pressure in an autoclave.
Yet another alternative process is described in Canadian Patent Application Serial No. 229,006, filed on June 10, 1975 and assigned in Common with the present , -. .

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invention. The process described in that co-pending application involves leaching with sulfuric acid in the presence of a sulfide, ~ypically hydrogen sulfide.

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While the process enables both rapid dissolution and complete acid utilization to be realized, its implemen-tation requires provision of a hydrogen sulfide plant ,, as well as a sulfuric acid plant.
, The present invention aims to provide an im-proved process, whereby an oxidic material such as a lateritic ore can be leached efficiently without resort-ing to a preliminary roasting treatment.
It is a further ob~ect of the invention to provide such a leaching process which utilizes inexpen-sive reagents.
Yet another object of the invention is the defining of a leaching process which can be implemented commercially without the need for a sulfuric acid manu-facturing plant.
Accordingly the present invention provides a process for reductively leaching an oxidic material containing oxides or hydroxides of iron and of at least one nonferrous metal selected from the group consisting of nickel and cobalt, to dissolve substantially all of the iron and nonferrous metalts), wherein the improvement comprises ~orming an aqueous slurry of the material and elemental sulfur in an amount corresponding to at least about 5~ of the total weight of the oxides or hydroxides, and introducing sulfur dioxide into the slurry until the desired dissolution has been effected.

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~ , : ,, ' ' ,, , , We have found that the addition of such a small ; amount of sulfur to the material to be leached enables the leaching to be carried out without an addition of sulfuric acid, i.e. relying entirely on acid formed in situ. Apart from this elimination of the need to supply acid, it has been found that incorporating elemental sul~ur in the aqueous slurry results in greatly improved rates ;; of dissolutibn and enables excellent metal extractions to be achieved in a reasonable time. This is achieved without resorting to the expense and inconvenienoe of high-pressure autoclaves. It is of course possible, thouyh unnecessary, to resort to high temperatures and sulfur dioxide pressures.
Thus the process can be carried out under approximately atmospheric pressure (i.e. an overpressure of 0.05 MPa or less), and the slurry temperature is preferably maintained within the range of about 50-100C during leaching.
To ensure the desired satisfactory leaching, the slurry must include at least about S% of elemental sulfur by weight of the oxides or hydroxides to be leached, i.e.
at least 50 g/kg of oxide. Of course when the oxidic material to be treated consists almost entirely of the oxides to be leached, the elemental sulfur minimum re-quirement can be expresYed as 50 grams per kilo~ram of the material. On the other hand where the oxidic material is one that contains a substantial amount of insolubles, as would be the case with a filter cake comprising metal hydroxides mixed with gypsum, the sulfur requirement corresponding to 50 g/ky of hydroxides would be only 3 or 4~ if expressed in relation to the overall weight of the cake. Preferably a higher amount than the minimum -`
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6(~5 requirement of sulfur is used, e.g~, between 10~ and - 250 g/kg of oxides or hydroxides to be leached. Both the material itself and the added sulfur should be of relatively ,'r small particle size to ensure rapid leaching. Thus pre-ferably the material should be ground, if necessary, to ; ensure a particle size of not more than about 600 microns, while the elemental sulfur should be in the form of particles of not more than about 80 microns.
The slurry density is chosen in accordance with - 10 the overall p~ocessing scheme. In general a high slurry density will be pre~erred for economic reasons, and we have found that the reductive leach of limonitic ores can proceed satisfactorily to completion with a slurry contain-ing o~ the order of 40% solids by weight. However at such high solid densities, the soluhility of ferrous sulate in the final liquor may be exceeded, leading to ferrous sulfate cry~tals in the leached slurry. To avoid this slurries containing no more than about 20% solids by weight should be used. However lf, as described below, a solid-liquid separation is not performed immediately after the reductive leach, higher slurry densitieq can be used to advantage.
The leaah is carried out by heating the aqueous `` ~lurry to the desired temperature, and supplying sulfur dioxide to it on demand at a relatively low overpressure, e.g. about 0.02 MPa overpressure. The leaching time needed will depend on the slurry temperature, and in general it will-be of the order of a few hours.
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, The reductive leachin~ process of the invention can be practised on a Variety o~ materials including nok only oxidic ores and concentrates but also metallurgical intermediates such as calcine mixtures and hydroxide precipitate mixtures. An important application of the process is of course the treatment of lateritic ores. In general such ores contain a predominantly fine fraction of limonite as well as a somewhat coarser serpentinic fraction, and it is only the former fraction which it is desirable to treat by the reductive leaching process.
However in accordance with a pre~erred ~eature o~ the ` invention, the reductive leach may be incorporated as one stage of a multistage process wherein the serpentinic as well as the limonitic fraction are processed. Such a proce~s utilizes the fact that at the end of the reductive leaching process, the resulting slurry or liquor can be used as an acid source for carrying out an oxidative pressure-leaching process. As is well known such oxidative - pressure-leaching is applicable to a serpentinic ore or ` 20 to a further ~upply of limonitic ore.
When a limonltlc ore i~ reductively leached in the presence of elemental sulfur and sulfur dioxlde substantially all of the iron, nickel, cobalt, manganese and aluminum present therein are solubilized. Chromite, which is often present in admixture with limonite, is unaffected by such a leach and can be recovered from the residue as a by-product. Furthermore elemental sulfur present in the residue can be recovered by flotation and , ~
~,` recycled for leaching further ore. The leach liquor, after :,.. '` .
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~ 136~3~7Si .' ; separation from the solids residue, can be treated to re-cover the nonferrous metal, e.g., by precipitation or sol-vent extraction techniques, leaving a solution from which ferrous sulfate can be crystallized out. This ferrous sulfate can then be thermally decomposed to produce iron oxide and sulfur dioxide and the latter can be recycled to the reductive leaching ves~el.
Alternatively instead of treating the liquor from the reductive leach to recover its metal values, this liquor can be added to either a further batch of limonitic ore or to a batch of serpentinic ore, and the resulting ~ slurry heated under pressure in an oxidizing atmosphere.
; When this i8 done it is not essential to resort to solid-liquid separation after the reductive leach and prior to `~ the oxidative leach. Instead the whole of the slurry resulting from the reductive leach may be combined with ` the limonite or serpentine to be leached in the second stage. In this way, as well as utilizing the ferrous sulfate in the liquor as an acid source, any elemental sulfur pre~ent i~n the first stage slurry serves to generate leaching acid during the second stage, Some examples of the invention will now be described. (Unless otherwise specified all percentages referred to are percentages by weight).

A series of leaching tests were carrled out on samples of a limonitic ore which contained 1.22~ nickel, O.lS~ cobalt and 46.1% iron. The ore was screened to minus 20 Mesh (Tyler). A first test (A) was carried out for the;purpose of comparison, without adding .~ .
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- elemental sulfur to the ore. Four tests (1-4) were carried out in accordance with the inyention by adding to the ore the amounts o~ elemental sulfur corresponding respectively to 5, 10, 15 and 20% by weight of the ore.
- In all five tests the ore, or ore and sul~ur mixture, was slurried with water to a 20% solids consistency, and the slurry was hea~ed to 100C in a vessel to which sul~ur dioxide was supplied on demand to maintain an overpressure of 0.05 MPa. The leach was carried out for 6 hours after ; 10 which the residue was separated and analyzed. Table 1 below shows the weight and composition of the leach residue in each case, while the calculated extractions are given in Table 2.

~t of ~nt of Res~due Co~~sition (% b~w2igh~t , Test S added~ sidue* _ Fe Ni _ Co S -A _ 94.047.4 1.13 0.06 _ 1 S S4.538.0 1.23 0.05 2.1 ,' 2 10 26.3 5.5 0.15 ~ 0.02 59 ; 20 3 15 29.8 2.8 0.10 C 0.02 69 4 20 3~.~ 2.8 0.04 0.02 75 '~ * expressed as percentage by weight of the ore.

Ext raction (~) ; Test Ee _N1 Co i~' A 4.2 48 6852 ~` 2 96 96 96 ~' ,, ` -8-. ~ .

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6~75 The above r~sults ~how the marked improvements resulting from the use of as little elemental sulfur as 5% by weight of the ore. The use of 10% or more of sulfur led to virtually complete extraction of the three metals of interest. (It should be noted that 10%
S by weight of ore in this example corresponded to about 11.2% S by weight of the soluble oxide fraction of the ore).

To illustrate the applicability of the process of the invention to oxidic materials other than limonite, a feed was used which consisted of a calcined intermediate fractlon from an existing process and contained 19.5~
copper, 27.3% nickel, 7.72% cobalt and 3~14% iron. A
sample of this feed was mixed with elemental sulfur in -~ an amount corresponding to 13.3% by weight of the sample, slurried in water to a consistency of about 30% solids, and leached for 5 hours at 100C under 0.05 MPa sulur dioxide overpressure. By way of comparison a similar test was carried out in which no elemental sulfur was added to the slurry. Table 3 below shows the extractions achieved in the test in accordance with the invention (S) and the Comparative Test (B).

hxNnt o ~nt of Extractions (%) est S added ~ sidue * Ch Nl ~ e ~- B 0 71 0.5 66 56 9 ~' 5 I~ ~ ~7 C 0.1 77 96 63 *expressed as percentage of the weight of the calc~ sample.
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The above results show that the process of the invention not only enables the efficient extraction of nickel, cobalt and iron but also provides a very selective in situ separation of these metals from copper which is retained essentially as a sulfide.

A further test in accordance wikh the invention was carried out on a feed material which consisted of a mixture of metal hydroxides and gypsum in the form of a filter cake analyziny 5.85% copper, 3.67% nickel, 0.08% cobalt and 1.09% iron. A sample of this cake was slurried in water at 20% solids and heated to 90C.
Sulfur dioxide was sparged through the slurry until its pH had been lowered to 3, elemental sulfur in an amount corresponding to 3.9% of the weight of cake (i.e. 11%
of the weight of hydroxides) was added to the slurry, and the sulfur dioxide sparging was continued for a further 5 hours at 90C. A final residue was obtained which analyzed 7.17% copper, 0.63% nickel, 0.007% cobalt and 0.47% iron. This corresponds to extractions of 84%, 92%
and 59% for nickel, cobalt and iron respectively, whereas less than 0.01% of the copper was extracted.

A sample of limonite analyzing: 1.22~ Ni, 0.15%
Co, 46.1% Fe, 3.75~ ~l and 0.94~ Mg was reductively leached in accordance wlth the invention for 6 hours at 100C
under a 0.05 MPa sulfur dioxide pressure. After separation of the residue the leach liquor was found to contain the following dissolved values~

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2.4 g/l of nickel 0.3 g/l of cobalt 88 g/l of iron i 5.4 g/l of aluminum 1.8 g/l of magnesium.
The amount of ferrous sulfate represented by thP
; above assay is equivalent to 154.5 g/l of sulfuric acid. The liquor can therefore be put to good use as a lixiviant for a leaching process carried out under oxidizing ; 10 conditions. To illustrate this,two tests were carried out using synthetic solutions of substantially the same compo-sition as the above-mentioned liquor.
In the first of these te5t8 the oxidative leach was carried cut on a further sample of limonitic ore which contained 1.45% Ni and 48.2% Fe. This sample was mixed with an amount of the synthetic liquor equivalent to 18%
sulfuric acid;by weight of the limonite. The slurry was diluted to a consistency of 26% solids and leached for ` 2 hours at 240C under an oxygen overpressure of 0.7 MPa.
At the end of thi~ period solid-liquid separation yielded a residue contalning mo~t of the iron in the ore and lnitlal liquor. The final leach 901ution, after dilution, was found - to analyze : 2.59 g/l of Ni with only 0.41 g/l of iron. This ' xepre~ents an a~all extraction of 94% o~ the nidkel from thR linLnite.
:~ In the second such test an oxidative leach was perfo~med on a sample of serpentinic ore. This sample which had been ground to pass through a 200 Me~h Tyler screen, was mixed with a synthetic solution similar to that obtained from leaching a limonitic ore. The relative amounts of serpentine and solution mixed were .
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:.-such that the ferrous sulfate in solution was equivalent to 48% of sulfuric acid by weight of the ~erpentine. The resultant slurry was leached at 150C for 2 hours under a 0.7 MPa oxygen overpressure. The results of this leach are glven in Table 4.

g71 in % in g/l in ~ in Initial Leach Final Metal Se~tme Solution Resi~ue Solution % Extrac Ni 2.27 2.40 0.54 5.1 67 Co 0.13; 0.32 0.02 0.50 80 ; Mg 11.2 1.9 0.8 ~4.0 90 Fe 22.3 85.5 36.0 1.65 _ It hould be pointed out that the above test ~, was not carried out under optimized conditions for the ~,' oxidative leaching, but nevertheless demonstrate~ the practicability of the two-stage leaching advocated. Such two-~tage leaching can be readily ~een to have the following Attractlons:
~ 2~ i) It enables known oxidative leach proces~es .' to be carried out without the necessity for ~- providing local acid manufacturing faailities. ;

ii) It enabies an integrated pro~ess to be designed , ., - .
for treating both the limonitic and serpen- ~
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tinic fractions of a laterite ore.
iii) It can yield a final solution which has a high nonferrous metal content coupled with a very ~; low iron content in view of the oxidation of iron in the second leaching stage.

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- It will be understood that various modifications can be made to the details of the preferred embodiment~
described above. Thus whereas only batch tests were described in the examples, the process can be u~efully implemented in~ a continuous manner. When this is done feed i8 continuously introduced into, and reacted slurry continuously withdrawn from, the leaching vessel. The reacted slurry is then separated into its pregnant solution and solids components. The solids will contain a substantial amount of elemental sulfur as will be readily seen from the results of Table 1 for example. By resorting .
to a suitable technique such as flotation, the elemental sulfur can be separated from the remainder of the solids, and recycled a required to the leaching vessel. In this way, apart from the sulfur addition needed at the start-up of operations, the sulfur requirement can be satisfied by the elemental sulfur formed during the reductive leach.
5uch modifications and others may be resorted to without departing from the scope of the invention, which is defined by the appende~ claims.

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Claims (12)

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

1. A process for reductively leaching an oxidic material, which contains oxides or hydroxides of iron and of at least one nonferrous metal selected from the group consisting of nickel and cobalt, to dissolve substantially all of the iron and nonferrous metal(s) wherein the improvement comprises forming an aqueous slurry of the material and elemental sulfur in an amount corresponding to at least about 5% of the total weight of the oxides or hydroxides, and introducing sulfur dioxide into the slurry until the desired dissolution has been effected.

2. A process as claimed in claim 1 wherein the slurried material has a particle size of not more than about 600 microns.

3. A process as claimed in claim 2 wherein the elemental sulfur is present in the form of particles not larger than about 80 microns, and in an amount correspond-ing to about 10 to 25% by weight of the oxides or hydroxides.

4. A process as claimed in claim 3 wherein the leach is carried out at about 50 to 100°C and at substantially atmospheric pressure.

5. A process as claimed in claim 4 wherein the sulfur dioxide is introduced into the slurry on demand to maintain an overpressure of about 0.02 MPa.

6. A process as claimed in claim 1 wherein the reacted slurry is separated into a pregnant solution and a solids residue, the pregnant solution is treated with a precipitating agent effective to precipitate only the nonferrous metals dissolved therein, and the precipitated metals are separated from the iron bearing liquor.

7. A process as claimed in claim 6 including the further step of crystallizing ferrous sulfate from the liquor, and thermally decomposing the sulfate to generate sulfur dioxide for recycling to perform the reductive leach.

8. A process as claimed in claim 1 wherein the oxidic material also contains copper, whereby selective dissolution of iron and nickel or cobalt effects separation of the copper therefrom.

9. A process as claimed in claim 1 wherein the material comprises a limonitic ore or concentrate.

10. A process as claimed in claim 9 wherein the reacted slurry is separated into a pregnant solution and a solids residue, the pregnant solution is mixed with a particulate nonferrous metal bearing lateritic ore or concentrate and the resultant slurry is heated under pressure in an oxidizing atmosphere for a period sufficient to leach at least some of the nonferrous metal values from the lateritic ore.

11 A process as claimed in claim 10 wherein the lateritic ore or concentrate comprises a further batch of the limonitic ore or concentrate.

12. A process as claimed in claim 10 wherein the lateritic ore or concentrate comprises a serpentinic ore or concentrate.