METHOD FOR THE OXIDATION AND HYDROTHERMAL DISSOCIATION OF METAL CHLORIDES FOR THE SEPARATION OF METALS AND HYDROCHLORIC ACID

Abstract

A process is disclosed for the oxidation and thermal decomposition of metal chlorides, leading to an efficient and effective separation of nuisance elements such as iron and aluminum 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.

Claims

1. A process for the oxidation of ferrous iron in chloride solutions and recovery of hydrochloric acid, comprising: i. feeding a solution containing ferrous chloride and hydrochloric acid into a reactor having an anode and a cathode; ii. applying a current to the anode and cathode to cause oxidation of the hydrochloric acid forming reactive monatomic chlorine, which immediately reacts with the ferrous iron oxidising it to ferric; iii. heating of the so-formed ferric chloride-containing solution to effect hydrothermal decomposition of the metal chlorides contained in the solution, evolving hydrochloric acid and forming a mixture of metal oxides and basic chlorides; iv. quenching of the so-formed decomposition slurry in dilute hydrochloric acid, wherein the basic metal chlorides re-dissolve; and v. proceeding with solid-liquid separation of the quench slurry for the recovery of metal oxides.

2. The process of claim 1, wherein in (i), a molar ratio of ferrous iron to hydrochloric acid is >1.

3. The process of claim 2, wherein an excess hydrochloric acid is used to maintain the pH <2.0 to prevent subsequent ferric iron hydrolysis.

4. The process of claim 1, wherein in (ii), a residual ferrous iron concentration is maintained in the range 0.5-5.0 g/L.

5. The process of claim 4, wherein the residual ferrous iron concentration is maintained in the range 0.5-1.0 g/L.

6. The process of claim 1, wherein in (ii), a feed temperature is from ambient to boiling.

7. The process of claim 1, wherein in (ii), the current has a density of from 50-500 A/m.sup.2.

8. The process of claim 7, wherein the current density is 300-350 A/m.sup.2.

9. The process of claim 1, wherein in (iii), the ferric solution also contains a metal chloride which remains liquid at a temperature of 180-190 C.

10. The process of claim 9, wherein the metal chloride is magnesium.

11. The process of claim 9, wherein the metal chloride is calcium.

12. The process of claim 9, wherein the metal chloride is zinc.

13. The process of claim 1, wherein in (iii), the solution also contains one, any or all of aluminum, cobalt, nickel, copper, lead, manganese, titanium, or vanadium.

14. The process of claim 1, wherein in (iii), the temperature is raised to 180-190 C.

15. The process of claim 1, wherein in (iii), trivalent and higher valent metals form their oxides, which are insoluble in dilute hydrochloric acid.

16. The process of claim 15, wherein iron forms hematite and aluminum forms alumina.

17. The process of claim 1, wherein in (iii), divalent metals form their basic metal chlorides, which are readily soluble in dilute hydrochloric acid.

18. The process of claim 1, wherein in (iii), alkali metal chlorides and calcium chloride remain as chlorides, and zinc remains as a chloride.

19. The process of claim 1, wherein in (iii), the hydrochloric acid is condensed and recycled within the process.

20. The process of claim 1, wherein in (iii), the reaction is allowed to go to completion, denoted by no more HCl gas being evolved.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The above and other aspects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which:

[0048] FIG. 1 shows a schematic for the oxidation of ferrous iron.

[0049] FIG. 2 shows a schematic for the hydrothermal decomposition of metal chlorides and recovery of hydrochloric acid.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0050] A novel process will be described hereinafter. Although the invention is described in terms of specific illustrative embodiments, it is to be understood that the embodiments described herein are by way of example only and that the scope of the invention is not intended to be limited thereby.

[0051] The terminology used herein is in accordance with definitions set out below.

[0052] As used herein % or wt. % means weight % unless otherwise indicated. When used herein % refers to weight % as compared to the total weight percent of the phase or composition that is being discussed.

[0053] By about, it is meant that the value of weight % (wt. %), time, pH, volume or temperature can vary within a certain range depending on the margin of error of the method or device used to evaluate such weight %, time, pH, volume or temperature. A margin of error of 10% is generally accepted.

[0054] The description which follows, and the embodiments described therein are provided by way of illustration of an example of particular embodiments of principles and aspects of the present invention. These examples are provided for the purposes of explanation and not of limitation, of those principles of the invention. In the description that follows, like parts and/or steps are marked throughout the specification and the drawing with the same respective reference numerals.

[0055] In accordance with a broad aspect of the present 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 have been generated by treating any base, precious or light metal-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 metal requiring oxidation, the principals and practice equally apply to other metals requiring oxidation such as, but not limited to, copper or manganese.

[0056] 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.

[0057] Ferrous chloride solution, on its own (i.e. no other ions present), cannot be raised to a temperature 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 promotes the reaction wherein one third of the iron is converted to hematite solids. Handling such solids can be problematical, especially in terms of scaling and abrasion of valves, such as encountered by SMS Siemag in the publication referenced above. Hematite, especially in the nickel laterite industry, is well-known for its propensity to cause scaling.

[0058] To avoid these problems, namely the need for pre-concentration or the use of an autoclave, along with the formation of abrasive solids, the present invention makes use of the fact that free hydrochloric acid in the ferrous 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 equation (1):

2HCl.fwdarw.Cl.sub.2+H.sub.2(1).

[0059] 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.

[0060] The reactive chlorine reacts instantaneously with ferrous iron to form ferric iron, according to equation (2):

2FeCl.sub.2+Cl.sub.2.fwdarw.2FeCl.sub.3(2).

[0061] 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 hematite solids, and also without the need for any elevated temperature.

[0062] However, care has to be taken, since an additional reaction may take place at the cathode, as shown in equation (3), namely the formation of metallic iron:

FeCl.sub.2.fwdarw.Fe+Cl.sub.2(3)

[0063] 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 equation (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.

[0064] 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. temperature 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 subsequent 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.

[0065] Referring to FIG. 1, feed solution 10 containing some ferrous iron is fed into an electrolytic oxidation reactor 11. The temperature of the feed solution may be from ambient to boiling, being whatever the process step which generated it operates at. The oxidation reaction is exothermic, however, and under steady state conditions, the temperature of the reactor will operate at 100-160 C. or higher, depending on the initial iron concentration and temperature of the feed solution 10. The presence of the formed ferric iron permits the temperature to exceed the boiling point of pure ferrous chloride solution.

[0066] 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 ion to effect the oxidation. Ideally, the excess hydrochloric acid will be 5-25%, sufficient to maintain the pH of the resultant ferric chloride at 2.0 in order to prevent premature ferric iron hydrolysis.

[0067] 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.

[0068] The anodic current density 12 should be in the range 50-500 A/m.sup.2, the actual value being dependent upon the ferrous iron concentration and the desired kinetics. Typically, the value will be 300-350 A/m.sup.2.

[0069] 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 collected by any conventional means, such as absorption by palladium metal. The predominant purpose of the air is to depolarise the cathode, and therefore lower the power consumption.

[0070] Oxidised solution 15 is withdrawn from the anodic compartment of the cell.

[0071] Turning to FIG. 2, there is shown a schematic representation of a method for hydrothermally decomposing an oxidised metal chloride solution. In the present embodiment, the feed solution 20 is one that might result form the leaching of a laterite or polymetallic base metal sulphide ore.

[0072] The feed solution 20 is fed into a hydrothermal decomposer reactor 21 wherein the temperature is raised to 170-200 C., preferably 175-185 C. It is a condition of the invention that the feed solution contains one of, all of, or a combination thereof of magnesium, calcium or zinc, since the presence of these metals do not decompose under these conditions, and will ensure that the solution does dry out in the decomposer. These metals should comprise at least 10%, and preferably >30% of the overall metal concentration.

[0073] The hydrothermal decomposer 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 scaling on the walls. In practice, a cascade of several reactors is required to ensure sufficient residence 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 generation 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.

[0074] Raising the temperature causes the thermal decomposition of the metal chlorides. The temperature 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 metal chlorides decompose, 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 equations show the reactions for iron, aluminum (trivalent metals), copper and nickel (divalent metals):

2FeCl.sub.3+3H.sub.20.fwdarw.Fe.sub.20.sub.3+6HCl(4)

2AlCl.sub.3+3H.sub.2O.fwdarw.Al.sub.2O.sub.3+6HCl(5)

2CuCl.sub.2+3H.sub.2O.fwdarw.Cu(OH).sub.2.Cu(OH)Cl+3HCl(6)

2NiCl.sub.2+3H.sub.2O.fwdarw.Ni(OH).sub.2.Ni(OH)Cl+3HCl(7).

[0075] Theoretically, it is possible to selectively decompose 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 re-dissolve in dilute hydrochloric acid.

[0076] As the metals decompose, the non-reactive metal chlorides (calcium, magnesium 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 metal oxides do not, and in this way, copper and nickel are effectively separated from iron and aluminum, and the associated hydrochloric acid recovered for recycle.

[0077] The strength of the dilute hydrochloric acid is sufficient to re-dissolve the base metals. The background metal chlorides which had not decomposed are allowed to build up to a suitable concentration to allow further processing. For example, in the case of magnesium, this would be 300-350 g/L MgCl.sub.2, and for zinc chloride 200-250 g/L.

[0078] Solid-liquid separation 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 metal oxides, primarily, but not limited to, hematite and alumina. The solution 29 contains base metals and the non-decomposable metal chlorides, which may be processed by conventional means for the recovery of the separate metals.

[0079] 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 present invention, solid-liquid separation is carried out at ambient and atmospheric temperatures, which is a very simple and effective operation, whereas in the other processes, it has/had to be carried out at 170-180 C., with the attendant potential for freezing, particularly of the various valves involved.

[0080] The objective of this process has been to have an effective and efficient separation of value metals such as nickel and cobalt, from nuisance elements such as iron and aluminum, and at the same time recover the associated hydrochloric acid for recycle.

[0081] The principles of the present invention are illustrated by the following examples, which are provided by way of illustration, but should not be taken as limiting the scope of the invention.

Example 1

[0082] A saturated solution of ferrous chloride was prepared at room temperature, and de-aerated 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/m.sup.2 was applied, and the ferrous iron concentration was monitored via titration. No chlorine evolution 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 evolution continued as long as ferrous iron was observed in solution, and ceased once there was no detectable ferrous iron in solution. Concurrently, chlorine evolution at the anode was noted, and after the test was stopped, a thin plate of iron foil was noted on the cathode.

[0083] 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.

Example 2

[0084] A solution containing 282 g/L ferric iron, 10.5 g/L Al, 9.96 g/L Cu, 9.61 g/L Co, 9.96 g/L Ni and 11.4 g/L Mg was heated up to 177 C. 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.

[0085] This demonstrates the efficiency of separating iron and aluminum from base metals, and at the same recovering hydrochloric acid.

Example 3

[0086] A solution similar to that in Example 2 was heated to a temperature of 186 C., but allowed to react for 648 minutes. This time, there were no detectable 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.

[0087] While illustrative and presently preferred embodiments of the invention have been described in detail hereinabove, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.