OA20636A - Process for the recovery of metals from polymetallic nodules. - Google Patents

Abstract

The present disclosure concerns a process for the recovery of valuable metals from polymetallic nodules. A two-stage process using SO2 in an acidic aqueous media is disclosed. In a first step, performed in mildly acidic conditions, Mn, Ni, and Co are dissolved; in a second, more acidic step, Cu is dissolved. Under these conditions, the leachate of the first step contains most of the Mn, Ni, and Co, while being nearly Cu-free. The Ni and Co are precipitated as sulfides; the Mn can be recovered as sulfate by crystallization. Cu, which is leached in the second step, is selectively precipitated, also as sulfide.

Description

Process for the recovery of metals from polymetallic nodules

The présent disclosure concerns a process for the recovery of valuable metals from polymetallic nodules. Polymetallic nodules, also called deep sea manganèse nodules, are rock concrétions formed of concentric layers of iron and manganèse hydroxides at the bottom of océans.

To date, the most economically interesting nodules hâve been found in the Clarion Clipperton Fracture Zone (CCFZ). Nodules in this area typically contain 27% Mn, 1.3% Ni, 1.1% Cu, 0.2% Co, 6% Fe, 6.5% Si, and 3% Al. Other éléments of économie interest are Zn, Mo and rare earths.

Since the seventies, many processes hâve been investigated to treat polymetallic nodules. A recent comprehensive review of the available processes can be found in a paper by T. Abramovski et al., Journal of Chemical Technology and Metallurgy, 52, 2, 2017, 258-269.

Kennecott and INCO attempted to develop industrial processes. Kennecott developed the Cuprion ammoniacal process, while several companies developed hydrometallurgy processes in sulfate, chloride and more recently nitrate media. INCO studied pyrometallurgical process with production of a matte. More recently, production of an alloy has been proposed. None of these processes went further than the piloting scale.

The Cuprion process faces issues with low Co recovery, slow réduction of the nodules by CO gas, and low value of the manganèse residue. Sulfate processes derived from lateritic processes making use of autoclave leaching to reject Mn and Fe in the leach residue, face technological issues in the leaching, as well as poor valorization of the Mn. Other sulfate-based processes lead to huge reagent consumption and/or production of fatal ammonium sulfate. Chloride and nitrate routes hâve high energy consumption for the régénération of the reagents by pyro-hydrolysis and pyrolysis. Drying of nodules before pyrometallurgy processing leads also to high energy consumption.

The disclosed process intends to solve these issues by means of proven and scalable leaching technologies. This process involves the use of SO2 as leaching agent.

In this context, it should be noted US 3,906,075 discloses a single-step leaching process using SO2 and sulfuric acid. Mn, Ni, Co, and Cu are leached simultaneously. This document also illustrâtes the crystallization of manganèse as MnS04, followed by its décomposition to oxide, thereby generating SO2 for re-use in the leaching step. Cu needs to be extracted from the single leachate stream. Liquid-liquid extraction is typically used, even though the cost and complexity of this process are considérable in view of the volumes to be treated.

In a similar context, US 4,138,465 discloses that selectivity between Ni, Mn, Co versus Cu can be achieved under well controlled leaching conditions, in particular when using a metered quantity of SO2 fed as sulfurons acid to the leaching step. Noteworthy is that selectivity is only achieved when the material is finely crushed to 100 mesh or finer. Mn, Ni, and Co leach first, leaving a Cu-bearing residue, which is separated and subjected to a second leaching step using a mixture of CO2 and NH3. This process is not particularly robust from an industrial point of view as the précisé amount of sulfurons acid needed to dissolve the Ni, Mn, and Co while leaving Cu untouched dépends upon the métal content of the material, which is inevitably variable. Any departure from the optimal conditions will lead to impure leachate streams, or to a residue requiring further processing.

The aim of the présent disclosure is to provide a process offering selectivity between Mn, Ni, and Co versus Cu in an industrially robust way. This can be achieved using a two-step leaching.

Figure 1 illustrâtes the process. The main unit operations are:

L1 : First leaching

PI : First précipitation

L2: Second leaching

P2: Second précipitation.

Also shown are the optional recycle of the metal-bearing aqueous phase (8) to the first leaching step (Ll), and the optional forwarding of excess of reagent (12) to the first leaching step (Ll). These options are represented with dotted lines.

The disclosed process for the recovery of the metals Mn, Ni, Co, and Cu from polymetallic nodules (1), comprises the steps of leaching said metals using an SO2-bearing gas as leaching agent in acidic aqueous conditions, and is characterized in that the leaching is performed according to a two-stage process comprising the steps of:

- a first leaching (Ll), wherein a major part of the Mn, Ni, and Co is dissolved by contacting the nodules with a first quantity of SO₂-bearing gas (10) in a first acidic aqueous solution of sulfuric acid (11) at a pH of 2 to 4, thereby producing a first leach solution (2) and a first leach residue (3), which are separated; and,

- a second leaching (L2), wherein a major part of the Cu is dissolved by contacting the first leach residue (3) with a second quantity of SO₂-bearing gas (12) in a second acidic aqueous solution of sulfuric acid (13) at a pH of less than 1.5, thereby producing a second leach solution (4) and a second residue (5), which are separated.

The first leaching step is performed in mildly acidic conditions, by contacting the nodules (1) with SO₂ in a solution of diluted sulfuric acid (11). This will lead to the dissolution of most of Mn, Co, and Ni, while Cu remains essentially in the residue (3). The pH of 2 to 4 is obtained by dosing either of the sulfuric acid in the aqueous solution (11), the SO3 optionally présent in the SO2 (10), or the metal-bearing aqueous phase (8) that is optionally recirculated from the second précipitation (P2).

The second leaching step is performed at a more acidic pH, by contacting the first leach residue (3) with SO₂ in a solution of diluted sulfuric acid (13). This will lead to the exhaustion of the residue (3) and to the dissolution of Cu in particular. The pH of less than 1.5 is obtained by dosing the sulfuric acid in the aqueous solution (13), or the SO3 optionally présent in the SO₂ (12).

This approach uses mildly acidic conditions in the first step (Ll), ensuring that most of the Mn, Co, and Ni dissolve, while avoiding the dissolution of Cu. This first leach solution (2) preferably contains less than 0.2 g/L Cu, or less than 10% of the Cu in the nodules; it contains more preferably less than 0.1 g/1 Cu, or less than 5% of the Cu in the nodules. Thanks to the absence of any significant amount of Cu, the first précipitation step (PI) results in a Co and Ni precipitate nearly fiee of Cu. Most of the Cu, together with residual Mn, Ni, Co, but also Fe, is dissolved in the second more acidic leaching step (L2).

Second residue (5) is exhausted in leachable metals. It will mainly contain less valuable minerais such as silica and alumina.

It should be noted that a stochiometric excess of SO₂ may be helpful to enhance the yield and the kinetics in the leaching steps (Ll, L2). Similarly, a stoichiometric excess of sulfides may be helpful to enhance the yield and the kinetics of the précipitation reactions in PI.

The expression “major part”, when related to an élément, désignâtes a fraction of more than 50 weight % of that element, with respect to its total amount fed to the process.

According to an advantageous embodiment, the process further comprises the step of:

- first précipitation (PI) of Ni and Co from the first leach solution (2), using a first sulfide précipitation agent (14), at a pH of 2 to 4, thereby obtaining a Mn-bearing aqueous phase (6) and a Ni- and Co-bearing solid phase (7), which are separated. The first sulfide précipitation agent (14) is preferably H2S.

Sulfide précipitation is indeed sélective towards Ni, and Co, but any Cu will unavoidably also precipitate. The low Cu content of the solution ensures that a concentrated Ni and Co product is obtained, nearly free of Cu. Such a product is suitable in applications where Cu is undesired, such as for the manufacture of cathode materials for Li-ion batteries.

According to an advantageous embodiment, the process further comprises the steps of: - second précipitation (P2) of Cu from the second leach solution (4), using a second sulfide précipitation agent (15), at a pH of 0.5 to 1.5, thereby obtaining a metal-bearing aqueous phase (8) and a Cu-bearing solid phase (9), which are separated; and,

- recirculation of a major part of the metal-bearing aqueous phase (8) to the first leaching step (Ll), for use as the first acidic aqueous solution (11). The second sulfide précipitation agent (15) is preferably H₂S and/or a mixture of elemental sulfur and SO₂.

Cu can be selectively precipitated from the obtained second leach solution (4), thus leaving Mn, Ni, Co, but also Fe as solutés in the metal-bearing aqueous phase (8).

The recirculation of this metal-bearing aqueous phase (8) to the first leaching step (Ll) has several benefits. As described above, most of the residual Mn, Ni and Co métal that is not leached in the first step (Ll) will dissolve in the more acidic second leach (L2). These 3 recovered metals will be recirculated to the first leaching step (Ll) and will report to the first leach solution (2). An advantage of this embodiment is thus the enhanced yield. An even more pronounced yield enhancement is obtained for Fe, in particular as Fe is mainly dissolved in the second leaching step (L2). Fe foliows the path of Mn, which is bénéficiai as both éléments find a common use in the Steel industry. A further advantage is that the recirculated metal-bearing aqueous phase (8) provides for at least part of the acid consumed by the nodules in the fïrst leaching step (Ll). The pH of the second leach solution (4) should preferably not be below 0.5 in case of recirculation, as the acid needs of the first leaching step (Ll) may otherwise be exceeded.

The expression “major part”, when related to a stream, désignâtes a fraction of more than 50 volume % of that stream.

According to an advantageous embodiment, the process further comprises the steps of:

- crystallization of the Mn from the Mn-bearing phase (6) by heating and/or by évaporation of water, thereby obtaining a Mn-bearing solid, which is separated;

- pyrolysis of the Mn-bearing solid by heating at a température of more than 550 °C, preferably more than 850 °C, thereby forming Mn oxide and an SOi-bearing gas, which is separated; and - recirculation of the SCh-bearing gas to either one or both leaching steps (Ll, L2) for use as leaching agent (10,12).

It is preferred to crystallize the dissolved manganèse as sulfate and/or dithionate, and to subject it to pyrolysis, thereby producing a mixture of SO2 and SO3 that is suitable for recirculation to the leaching stages. Décomposition may start at 550 °C when a reducing agent such as coal is admixed; otherwise a température of at least 850 °C is needed. Mn represents by far the most abundant métal in typical nodules. Recirculating the sulfur présent in the manganèse sulfate and/or dithionate will therefore fulfill the needs of the leaching stages to a large extent.

According to an advantageous embodiment, an excess of SOi-bearing gas is fed as leaching agent (12) to the second leaching step (L2), thereby obtaining a stream of unreacted SO2, for use as leaching agent (10) in the first leaching (Ll) step.

The skilled person will readily détermine the amounts of acid and of sulfur dioxide needed in the leaching steps based on the stoichiometry according to the below-mentioned reactions. In a preferred embodiment, a stoichiometric excess of SO2 is introduced in the second leaching stage (L2) only. The excess will leave the reactor of the second leaching stage (L2). It is circulated to the first leaching step (Ll).

According to an advantageous embodiment, the SO2-bearing gas also contains SO3.

By SO₂-bearing gas is meant a gas that contains a significant amount of SO₂, preferably more than 10% by volume, more preferably more than 40%. The volume of SO2-bearing gas to be injected in the leaching steps could otherwise become impractical. Other main constituents of the gas may comprise N₂, and the combustion products of the fuel used in the step of pyrolysis of the Mn-bearing solid.

The mixture of SO2 and SO3 may be obtained from an extemal source such as from the combustion of sulfur. In that spécifie case, the mixture will primarily contain SO2 and only traces ofSO₃.

The amount of SO2 needed is essentially dictated by the leaching stoichiometry. In the first leaching step ai a pH of 2 to 4, a major part of each of Ni, Mn, and Co reacts according to: MnÜ2 + SO2 MnSO4 and, MnÛ2 + 2 SO2 —> MnS₂Oe

NiO + H2SO4 -> NiSO₄ + H₂O

CoO + H2SO4 C0S04 + H₂O

With respect to the feed to the process, the major part of the Ni, Mn, and Co, is leached. Cu remains essentially in the first residue, together with minor amounts of Ni, Mn, and Co.

In the second leaching step, at a pH below 1.5, almost ail undissolved Ni, Mn, and Co will dissolve, as well as Cu according to:

MnO₂ + SO2 -> MnSC>4 and MnO₂ + 2 SO2 -> MnS₂Û6

NiO + H2SO4 NiSO₄ + H₂O

COO + H2SO4 -4- COSO4+H2O

CuO + H2SO4 -> CuSO₄ + H₂o

With respect to the feed to the process, the major part of the Cu is leached. The minor amounts of Ni, Mn, and Co left in the first residue are recovered in this step. The second residue is thus depleted in Ni, Mn, Co, and Cu.

In an optional pyrolysis, a mixture of SO₂ and SO3 is produced according to the reactions: MnSO₄ —> MnO₂ + SO₂

MnS₂Oe —> MnO₂ + 2 SO₂

SO₂+O₂ —> 2 SO₃.

Some impurities such as Na, K, and Mg may accumulate with time when the process is run continuously using recirculation. This problem is solved according to known means by providing a bleed stream, thereby limiting the fraction of the recirculated amount so somewhat less than 100%. The bleed stream is treated separately for removal of impurities; it will however also contain some sulfur-bearing species. This loss of sulfur is relatively minor but could be compensated by adding SO2, SO3, or sulfuric acid from extemal sources.

Example 1

This Example illustrâtes the two-step leaching process without recirculation of the metal-bearing aqueous phase (8). The first leaching step (Ll) is therefore performed in an essentially pure aqueous acidic solution.

In the first leaching step (Ll), 1 kg (dry) ground nodules havîng a mean particle diameter (D50) of 100 pm, is blended in 3.9 L of a slightly acid solution containing 18 g/L H₂SO₄. The slurry is continuously stirred (500 rpm) and heated to 95 °C. For 1.5 hours, SO₂ gas, for a total amount of 550 g, is injected into the slurry. At the end of the reaction and after obtaining a pH of 3, the slurry is separated by filtration. The residue (3) is fully washed with water and dried.

In the second leaching step (L2), the residue (3) is repulped in 1.52 L water. The slurry is continuously stirred (500 rpm) and heated to 80 °C. For 1.5 hours, a total amount of 400 g SO₂ and 150 g H₂SO₄ are gradually added to the slurry. About 100 g SO₂ gas is effectively consumed. At the end of the reaction and after obtaining pH 0.9, the slurry is separated by filtration. The residue (5) is fully washed with water and dried.

The filtrate (2) of the first leaching step is treated for précipitation of Ni and Co (PI). To this end, the filtrate is brought to 80 °C and is continuously stirred (300 rpm) while blowing Ar over the liquid surface. For the 3.9 L filtrate, 363 mL NaSH (34 g S/L) is needed to precipitate Ni and Co (i.e. 160% of the stoichiometric needs for Ni, Co, Cu, and Zn). NaSH is slowly added at 2 g/min. The slurry is filtrated, and the solids are washed with water and dried in a vacuum stove at 40 °C.

The filtrate (4) of the second leaching step is similarly treated for précipitation of Cu (P2). To this end, the filtrate is heated to 60 °C and is continuously stirred (300 rpm) while blowing Ar over the liquid surface. Cu is precipitated by slowly adding 162 mL NaSH (34 g S/L) to 1.75L fîltrate (i.e. 100% of the stoichiometric needs for Cu) at 2g/min. The slurry is filtrated, and the solids are washed with water and dried in a vacuum stove at 40 °C.

The métal composition and quantity of the different filtrâtes and residues are given in Table 1 A. The yields are reported in Table IB.

Table 1 A: Elemental analysis of streams according to Example 1

Stream N°	Stream Type	Weight (g)	Volume (L)	Weight % (solids) or g/L (liquide)
Mn	Ni	Co	Cu	Fe	Si	Al
1	Nodules	1000		29	1.3	0.25	1.2	6.2	6.3	2.7
2	Filtrate		4.76	53	2.3	0.46	0.03	2.9	0	0.2
3	Residue	440		9.2	0.56	0.09	2.7	11	14	5.9
4	Filtrate		2.09	18	1.1	0.15	5.2	17	0	1.5
5	Residue	290		1.0	0.09	0.02	0.33	4.2	22	7.9
6	Filtrate		4.67	53	0	0	0	3	0	0.23
7	Residue	25		10	43	9	0	3	0	0
8	Filtrate		2.09	18	1.1	0.15	0.0	17	0.0	1.5
9	Residue	16		0	0	0	66	0	0	0

Table IB: Yield vs. process inputs of leaching steps according to Example 1

Step	Yield (%) ra. Nodules
Mn	Ni	Co	Cu	Fe	Si	Al
L1	86	81	85	1	22	0	4
L2	13	17	13	91	59	0	34

During the first leaching step, Mn, Ni and Co are selectively leached vs. Cu (resp. 86%, 81% and 85% vs. 1%). The first leach solution (2) is therefore nearly Cu-free. This is advantageous, as any Cu would unavoidably precipitate even before Ni and Co in the first sulfide précipitation step (P 1). A pure Ni and Co sulfide isthusobtained.

In the second sulfïde précipitation step (P2), the proper dosing of sulfides easily achieves the sélective précipitation of Cu, while Co and Ni remain in solution. A pure Cu sulfïde is thus obtained.

Example 2

This Example illustrâtes the two-step leaching process with recirculation of the metal-bearing aqueous phase (8). The first leaching step (Ll) is therefore performed in an aqueous acidic solution also containing significant quantities of solutés. It is assumed that the process opérâtes in a continuous way and that equilibrium conditions hâve been reached.

In the first leaching step (Ll), 1 kg (dry) ground nodules having a mean particle diameter (D50) of 100 pm is blended in 2.09 L of filtrate (8) from P2, to which 1.81 L water is added. The slurry is continuously stirred (500 rpm) and heated to 95 °C. For 1.5 hours, SO2 gas, for a total of 550 g, is injected into the reactor. At the end of the réaction and after obtaining pH 3, the slurry is separated by décantation.

The underflow (3), which contains the solid residue as such and permeating liquid, resp. tagged as 3 S and 3L in Table 2A, is fed to the second leaching step. The overflow goes to the first précipitation step.

In the second leaching step (L2), 0.22 L water is added to the underflow (3). The slurry is continuously stirred (500 rpm) and heated to 80 °C. For 1.5 hours, a total amount of 400 g SO2 and 150 g H2SO4 are gradually added to the slurry. About 100 g SO₂ gas is effectively consumed. At the end of the reaction and after obtaining pH 0.9, the slurry is separated by filtration. The residue (5) is fully washed with water and dried.

The filtrate (2) of the first leaching step is treated for précipitation of Ni and Co (PI). To this end, the filtrate is brought to 80 °C and is continuously stirred (300 rpm) while blowing Ar over the liquid surface. For the 3.11 L filtrate, 428 mL NaSH (34 g S/l) is needed to precipitate Ni and Co (i.e. 160% of the stoichiometric needs for Ni, Co, Cu, and Zn). NaSH is slowly added at 1 g/min. The slurry is fîltrated, and the solids are washed with water and dried in a vacuum stove at40 °C.

The filtrate (4) of the second leaching step is similarly treated for précipitation of Cu (P2). To this end, the filtrate is heated to 60°C and is continuously stirred (300 rpm) while blowing Ar over the liquid surface. Cu is precipitated by slowly adding 163 mL NaSH (34 g S/l) to 2.09L filtrate (i.e. 100% of the stoichiometric needs for Cu) at 1 g/min. The slurry is filtrated, and the 5 solids are washed with water and dried in a vacuum stove at 40 °C. The metal-bearing aqueous phase (8) is re-used in the first leaching step.

The métal composition and quantity of the different filtrâtes and residues are given in Table 2 A.

The yields are reported in Table 2B.

Table 2A: Elemental analysis of streams according to Example 2

Stream N°	Stream Type	Weight (g)	Volume (L)	Weight % (solids) or g/L (liquids)
Mn	Ni	Co	Cu	Fe	Si	Al
1	Nodules	1000		29	1.3	0.25	1.2	6.2	6.3	2.7
2	Filtrate		3.11	92	4.1	0.78	0.03	16.0	0	1.3
3S	Residue	440		9.2	0.56	0.09	2.7	11	14	5.9
3L	Filtrate		1.56	92	4.1	0.78	0.03	16.0	0	1.3
4	Filtrate		2.09	87	4.1	0.73	5.2	29	0	2.5
5	Residue	290		1.0	0.09	0.02	0.33	4.2	22	7.9
6	Filtrate		3.11	91	0	0	0	15	0	1.35
7	Residue	32		9	40	8	0	8	0	0
8->ll	Filtrate		2.09	87	4.1	0.73	0.0	29	0.0	2.5
9	Residue	16		0	0	0	66	0	0	0

Table 2B: Yield vs. process inputs of leaching steps according to Example 2

Step	Yield (%) vs. Nodules
Mn	Ni	Co	Cu	Fe	Si	Al
L1	86	81	85	1	22	0	4
L2	13	17	13	91	59	0	12

Example 2 demonstrates the advantages of usîng recirculation when compared to the two-step leaching without recirculation according to Example 1 :

- enhanced global (L1 + L2) yields for Mn, Ni, and Co, reaching respectively 99%, 98%, and

98%;

- increased recovery of Fe, in particular thanks to the second leaching step, resulting in a global recovery (L1 + L2) of 81%;

- reduced acid consumption, as the acid needed in the first leaching step is provided by the recirculated metal-bearing aqueous phase (8).

The invention is not limited to the embodiment/s illustrated in the drawings. Accordingly, it should be understood that where features mentioned in the appended daims are followed by reference signs, such signs are included solely for the purpose of enhancing the intelligibility of 10 the daims and are in no way limiting on the scope of the daims.

Claims (8)

1. Process for the recovery of the metals Mn, Ni, Co, and Cu from polymetallic nodules (1), comprising the steps of leaching said metals using an S02-bcaring gas as leaching agent in acidic aqueous conditions, characterîzed in that the leaching is performed according to a two-stage process comprising the steps of:

- a first leaching (Ll), wherein a major part of the Mn, Ni, and Co is dissolved by contacting the nodules with a first quantity of SOi-bearing gas (10) in a first acidic aqueous solution of sulfuric acid (11) at a pH of 2 to 4, thereby producing a first leach solution (2) and a first leach residue (3), which are separated; and,

- a second leaching (L2), wherein a major part of the Cu is dissolved by contacting the first leach residue (3) with a second quantity of SO2-bearing gas (12) in a second acidic aqueous solution of sulfuric acid (13) at a pH of less than 1.5, thereby producing a second leach solution (4) and a second residue (5), which are separated.

2. Process according to claim 1, further comprising the steps of:

- first précipitation (PI) of Ni and Co from the first leach solution (2), using a first sulfide précipitation agent (14), at a pH of 2 to 4, thereby obtaining a Mn-bearing aqueous phase (6) and a Ni- and Co-bearing solid phase (7), which are separated.

3. Process according to claim 2, wherein the first sulfide précipitation agent (14) is H2S.

4. Process according to any one of claims 1 to 3, further comprising the steps of:

- second précipitation (P2) of Cu from the second leach solution (4), using a second sulfide précipitation agent (15), at a pH of 0.5 to 1.5, thereby obtaining a metal-bearing aqueous phase (8) and a Cu-bearing solid phase (9), which are separated; and,

- recirculation of a major part of the metal-bearing aqueous phase (8) to the first leaching step (Ll), for use as the first acidic aqueous solution (11).

5. Process according to claim 4 wherein the second sulfide précipitation agent is H2S and/or a mixture of elemental sulfiir and SO2 (15).

6. Process according to any one of claims 1 to 5, further comprising the steps of:

- crystallization of the Mn from the Mn-bearing phase (6) by heating and/or by évaporation of water, thereby obtaining a Mn-bearing solid, which is separated;

- pyrolysîs of the Mn-bearing solid by heating at a température of more than 850 °C, thereby

5 forming Mn oxide and an SO2-bearing gas, which is separated; and

- recirculation of the SO2-bearing gas to either one or both leaching steps (Ll, L2) for use as leaching agent ( 10,12).

•

7. Process according to claim 6, whereîn an excess of SO2-bearing gas is fed as leaching

10 agent (12) to the second leaching step (L2), thereby obtaining a stream of unreacted SO₂, for use as leaching agent (10) in the first leaching (Ll) step.

8. Process according to any one of claims 1 to 7, wherein the SO₂-bearing gas also contains SO₃.