OA19386A - Method for the oxidation and hydrothermal dissociation of metal chlorides for the separation of metals and hydrochloric acid. - Google Patents

Abstract

A method is disclosed forthe oxidation and thermal decomposition of metal chlorides, leading to an efficient and effective separation of nuisance elements such as iron and aluminium from value metals such as copper and nickel. In the first instance, oxidation, especially for iron, is effected in an electrolytic reactor, wherein ferrous iron is oxidised to ferric. In a second embodiment, the oxidised solution is treated in a hydrothermal decomposer reactor, wherein decomposable trivalent metal chlorides form oxides and divalent metal chlorides form basic chlorides. The latter are soluble in dilute hydrochloric acid, and may be selectively re- dissolved from the hydrothermal solids, thereby effecting a clean separation. Hydrochloric acid is recovered from the hydrothermal reactor.

Description

0029 The embodiments of the présent invention shall be more clearly understood with reference to the following detailed description taken in conjunction with the accompanying drawings.

0030 In accordance with a broad aspect of the présent invention, there is a process described for oxidising ferrous iron and recovering hydrochloric acid from a chloride-based feed solution containing ferrous iron. Such solution may hâve been generated by treating any base, precious or light métal-containing material with any lixiviant comprising acid and a chloride, but in particular with hydrochloric acid generated and recycled within the process, or being derived from SPL or ZPL. It is understood that whilst the description references ferrous iron, which is by far the most common métal requiring oxidation, the principals and practice equally apply to other metals requiring oxidation such as, but not limited to, copper or manganèse.

0031 It is a particular aspect of the invention that ferrous iron oxidation is effected without either recourse to the use of an autoclave, the need to pre-evaporate the incoming solution, or without the need to use a matrix which has to be oxygenated to form an intermediate hypochlorite.

0032 Ferrous chloride solution, on its own (i.e. no other ions présent), cannot be raised to a température above 120°C under atmospheric conditions, such that oxidation with oxygen or air is both difficult and very slow. Even under favourable conditions, such as in an autoclave, oxidation with oxygen or air promûtes the reaction wherein one third of the iron is converted to hématite solids. Handling such solids can be problematical, especially in terms of sealing and abrasion of valves, such as encountered by SMS Siemag in the publication referenced above. Hématite, especially in the nickel latérite industry, is well-known for its propensity to cause sealing.

0033 To avoid these problems, namely the need for pre-concentration or the use of an autoclave, along with the formation of abrasive solids, the présent invention makes use of the fact that free hydrochloric in the ferrons solution may be electrolytically oxidised (at the anode) to form elemental chlorine. Such chlorine, the moment it is formed, is highly reactive due to being in a monatomic state, so-called “nascent” chlorine. The reaction, in a simple form, is shown in équation (1).

2HC1 -> Ch + H₂ (1)

0034 The hydrogen produced (at the cathode) is also reactive, and spontaneously reacts with dissolved oxygen in the solution to form water. Alternatively, a stream of air may be blown across the cathode to remove the hydrogen and depolarise it.

0035 The reactive chlorine reacts instantaneously with ferrous iron to form ferrie iron, according to équation (2).

2FeC12+C12 -> 2FeCh (2)

0036 It is a particular aspect of this invention that in this case, the oxidation of ferrous is effected in-situ without the formation of any hématite solids, and also without the need for any elevated température.

0037 However, care has to be taken, since an additional reaction may take place at the cathode, as shown in équation (3), namely the formation of metallic iron.

FeCh Fe+Ch (3)

0038 The formation of metallic iron is highly undesirable for two reasons, namely that it plates on the cathode, thereby reducing the effectiveness of the cathode, and secondly, it has a very high power consumption compared to équation (1). It has been found, therefore, that it is essential to maintain a residual level of ferrous iron in solution, from 0.5-5.0 g/L, optimally from 0.5-1.5 g/L.

0039 A further advantage of carrying out the ferrous iron oxidation in this manner is that there is no longer any need to adjust the solution composition to maintain the 145-155°C température range required by the current processes, whether it be by an autoclave or by the use of a matrix. This further means that the need to inject steam is no longer required, and that the composition of the feed solution may be adjusted prior to the subséquent hydrolysis reaction in such a manner as to generate the required composition of HCl directly off the reactor. In other words, the amount of water required for the hydrolysis reaction is derived entirely from the incoming feed solution, and thus the need to inject steam for the hydrolysis reaction to occur is eliminated.

0040 Referring to Figure 1, feed solution 10 containing some ferrous iron is fed into an electrolytic oxidation reactor 11. The température of the feed solution may be from ambient to boiling, being whatever the process step which generated it opérâtes at. The oxidation reaction is exothermic, however, and under steady state conditions, the température of the reactor will operate at 100-160°C or higher, depending on the initial iron concentration and température of the feed solution 10. The presence of the formed ferrie iron permits the température to exceed the boiling point of pure ferrous chloride solution.

0041 A condition is that the solution contains a molar ratio of free hydrochloric acid to ferrous iron >1 (i.e. HCl/Fe(II) >1). This is necessary in order to supply the requisite amount of chloride 5 ion to effect the oxidation. Ideally, the excess hydrochloric acid will be 5-25%, sufficient to maintain the pH ofthe résultant ferrie chloride at <2.0 in order to prevent prématuré ferrie iron hydrolysis.

0042 Any simple electrolytic cell 11 may be used, but the preferred configuration is that of a bipolar cell, with a header on the cathodic compartments to collect any hydrogen formed.

0043 The anodic current density 12 should be in the range 50-500 A/m², the actual value being 10 dépendent upon the ferrous iron concentration and the desired kinetics. Typically, the value will be 300-350 A/m².

0044 Hydrogen 14 is liberated from the cathodic compartment of the cell. Stripping of the hydrogen may be facilitated by a small stream of air blown across the faces of the cathodes into a header. Some hydrogen will react to form water with dissolved oxygen, but the balance may be 15 collected by any conventional means, such as absorption by palladium métal. The prédominant purpose of the air is to depolarise the cathode, and therefore lower the power consumption.

0045 Oxidised solution 15 is withdrawn from the anodic compartment of the cell.

0046 Tuming to Figure 2, there is shown a schematic représentation of a method for hydrothermally decomposing an oxidised métal chloride solution. In the présent embodiment, 20 the feed solution 20 is one that might resuit form the leaching of a latérite or polymetallic base métal sulphide ore.

0047 The feed solution 20 is fed into a hydrothermal décomposer reactor 21 wherein the température is raised to 170-200°C, preferably 175-185°C. It is a condition of the invention that the feed solution contains one of, ail of, or a combination thereof of magnésium, calcium or zinc, since the presence of these metals do not décomposé under these conditions, and will ensure that 25 the solution does dry out in the décomposer. These metals should comprise at least 10%, and preferably >30% of the overall métal concentration.

0048 The hydrothermal décomposer reactor 21 may be any agitated vessel, and is preferably acid-brick lined, more preferably with fused alumina. Agitation is necessary, especially if the reactor is externally heated, in order to prevent sealing on the walls. In practice, a cascade of 30 several reactors is required to ensure sufficient résidence time for the reactions of (4) and (5) below to reach completion. The end-point of the reaction is simply determined in that no further génération of HCl gas is observed. This is a very simple and easily-observed end-point, unlike what is observed with those processes discussed in the Background section.

0049 Raising the température causes the thermal décomposition of the métal chlorides. The température may be raised by heat 22 through an external heat exchanger, or by the addition of steam, or by a jacketed heated vessel. As the métal chlorides décomposé, HCl vapour 23 is formed and condensed in any suitable off-gas system. The strength of the HCl vapour is directly proportional to the decomposable metals concentration of the incoming feed solution 20. The following équations show the reactions for iron, aluminium (trivalent metals), copper and nickel (divalent metals).

2FeCh + 3H2O —» Fe2Oa + 6HC1(4)

2AICI3 + 3H2O AI2O3 + 6HCI(5)

2CuCl2 + 3H₂O Cu(OH)₂*Cu(OH)Cl + 3HC1(6)

2N1CI2 + 3H₂O Ni(OH)₂-Ni(OH)Cl + 3HC1(7)

0050 Theoretically, it is possible to selectively décomposé the metals in order, according to the order indicated by Monhemius referenced in paragraph 5. However, in practice it is difficult to do so, and nor is it necessary, since the base metals form basic chlorides, and these readily re1 g dissolve in dilute hydrochloric acid.

0051 As the metals décomposé, the non-reactive métal chlorides (calcium, magnésium and zinc) increase in composition, and the reactor is allowed to overflow into a quench reactor 24, containing dilute hydrochloric acid 25 and operating at atmospheric conditions. The basic chlorides re-dissolve, whereas the métal oxides do not, and in this way, copper and nickel are 15 effectively separated from iron and aluminium, and the associated hydrochloric acid recovered for recycle.

0052 The strength of the dilute hydrochloric acid is sufficient to re-dissolve the base metals. The background métal chlorides which had not decomposed are allowed to build up to a suitable concentration to allow further processing. For example, in the case of magnésium, this would be 300-350 g/L MgCL, and for zinc chloride 200-250 g/L.

0053 Solid-liquid séparation 27 of the quench reactor slurry 26 may be effected by any convenient means, such as, but not limited to, flocculation and thickening, filter press or vacuum belt filter. The solids 28 are a mixture of métal oxides, primarily, but not limited to, hématite and alumina. The solution 29 contains base metals and the non-decomposable métal chlorides, which may be processed by conventional means for the recovery of the separate metals.

0054 Carrying out the quench reaction in this way thereby solves the issues which were paramount with the PORI and SMS Siemag Processes, and which ultimately resulted in their downfall. In the présent invention, solid-liquid séparation is carried out at ambient and atmospheric températures, which is a very simple and effective operation, whereas in the other processes, it has/had to be carried out at 170-180C, with the attendant potential for freezing, 30 particularly of the various valves involved.

0055 The objective of this process has been to hâve an effective and efficient séparation of value metals such as nickel and cobalt, from nuisance éléments such as iron and aluminium, and at the same time recover the associated hydrochloric acid for recycle.

0056 The principles of the présent invention are illustrated by the following examples, which 5 are provided by way of illustration, but should not be taken as limiting the scope of the invention:

0057 Example 1

0058 A saturated solution of ferrous chloride was prepared at room température, and deaerated with nitrogen. The de-aeration was carried out in order to preclude any air oxidation. 200 mL of solution were placed in an electrolytic cell, containing a titanium cathode and a graphite anode. An anodic current density of 300 A/m² was applied, and the ferrous iron 10 concentration was monitored via titration. No chlorine évolution was observed from the anode, and the solution rapidly turned a red colour. Because of the de-aeration, hydrogen was initially observed to be evolved from the cathode. Hydrogen évolution continued as long as ferrous iron was observed in solution, and ceased once there was no détectable ferrous iron in solution. Concurrently, chlorine évolution at the anode was noted, and after the test was stopped, a thin 15 plate of iron foil was noted on the cathode.

0059 This test demonstrates that electrolytic oxidation proceeds, and that it is also necessary to maintain some ferrous iron in solution to prevent the plating of metallic iron.

0060 Example 2

0061 A solution containing 282 g/L ferrie iron, 10.5 g/L Al, 9.96 g/L Cu, 9.61 g/L Co, 9.96 g/L 20 Ni and 11.4 g/L Mg was heated up to 177C for a period of 110 minutes. Hydrochloric acid of 6M concentration was recovered. After quenching, solids analysing 64.4% Fe, 1.43% Al and 0.05% Cu were recovered. The other metals were not detected in the solids. 56% of the HCl and 67.2% of the iron were recovered.

0062 This demonstrates the efficiency of separating iron and aluminium from base metals, and 25 at the same recovering hydrochloric acid.

0063 Example 3

0064 A solution similar to that in Example 2 was heated to a température of 186°C, but allowed to react for 648 minutes. This time, there were no détectable base metals in the solids, and the iron content of the solids was 64.3%. 100% of the HCl was recovered at a concentration of 10.9M.

Claims (17)

1. A method for the séparation of nuisance éléments such as iron and aluminium from base metals in chloride solutions, with the simultaneous recovery of hydrochloric acid, comprising:

i. A process for the oxidation of ferrous iron in chloride solutions and recovery of hydrochloric acid.

ii. Feeding a solution containing ferrous chloride and hydrochloric acid into a reactor having an anode and a cathode.

iii. Applying a current to cause oxidation of the hydrochloric acid forming reactive monatomic chlorine, which immediately reacts with the ferrous iron oxidising it to ferrie.

iv. Heating of the so-formed ferrie chloride-containing solution to effect hydrothermal décomposition of the métal chlorides contained in the solution, evolving hydrochloric acid and forming a mixture of métal oxides and basic chlorides.

v. Quenching of the so-formed décomposition slurry in dilute hydrochloric acid, wherein the basic métal chlorides re-dissolve.

vi. Solid-liquid séparation of the quench slurry for the recovery of métal oxides.

2. The process of Claim 1 (ii) wherein the molar ratio of ferrous iron to hydrochloric acid is >1.

3. The process of Claim 2 wherein there is sufficient excess hydrochloric acid to maintain the pH < 2.0 to prevent subséquent ferrie iron hydrolysis.

4. The process of Claim l(iii) wherein a residual ferrous iron concentration is maintained in the range 0.5-5.0 g/L, preferably 0.5-1.0 g/L.

5. The process of Claim 1 (ii) wherein the feed température may be from ambient to boiling.

6. The process of Claim 1 (iii) wherein the current density is from 50-500 A/m², preferably 300350 A/m².

7. The process of Claim l(iv) wherein the ferrie solution also contains a métal chloride which remains liquid at a température of 180-190°C.

8. The process of Claim 7 wherein the métal chloride is magnésium.

9. The process of Claim 7 wherein the métal chloride is calcium.

10. The process of Claim 7 wherein the métal chloride is zinc.

11. The process of Claim l(iv) wherein the solution also contains one, any or ail of aluminium, cobalt, nickel, copper, lead, manganèse, titanium, vanadium.

12. The process of Claim l(iv) wherein the température is raised to 180-190°C.

13. The process of Claim l(iv) wherein trivalent and higher valent metals form their oxides, 5 which are insoluble in dilute hydrochloric acid. For example, iron forms hématite and aluminium forms alumina.

14. The process of Claim l(iv) wherein divalent metals form their basic métal chlorides, which are readily soluble in dilute hydrochloric acid.

15. The process of Claim l(iv) wherein alkali métal chlorides and calcium chloride remain as chlorides.

10

16. The process of Claim l(iv) wherein the hydrochloric acid is condensed and recycled within the process.

17. The process of Claim l(iv) wherein the reaction is allowed to go to completion, denoted by no more HCl gas being evolved.