METHOD FOR RECOVERING ZINC FROM SOLUTION

Abstract

A method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution, the method comprising the steps of: Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-enriched organic solution of a zinc extractant; Separating the zinc-enriched organic solution of a zinc extractant from the zinc-depleted aqueous ammoniacal ammonium carbonate solution; Contacting the zinc-enriched organic solution with an aqueous acidic solution, producing a zinc-enriched aqueous acidic solution and a zinc-depleted organic solution of a zinc extractant; and Recovering zinc from the zinc-enriched aqueous acid solution.

Claims

1. A method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution, the method comprising the steps of: i. Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc- enriched organic solution of a zinc extractant; ii. Separating the zinc-enriched organic solution of a zinc extractant from the zinc-depleted aqueous ammoniacal ammonium carbonate solution; iii. Contacting the zinc-enriched organic solution with an aqueous acidic solution, producing a zinc-enriched aqueous acidic solution and a zinc-depleted organic solution of a zinc extractant; and iv. Recovering zinc from the zinc-enriched aqueous acid solution.

2. The method according to claim 1, wherein the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution is controlled to between 0.05 and 2.0.

3. The method according to claim 1, wherein a portion of the carbonate is transferred from the aqueous ammoniacal ammonium carbonate zinc solution to organic solution of a zinc extractant, such that the method more specifically comprises the steps of: i. Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc and the carbonate is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted carbonate-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-and carbonate-enriched organic solution of a zinc extractant; ii. Separating the zinc- and carbonate-enriched organic solution of a zinc extractant and zinc-depleted carbonate-depleted aqueous solution; iii. Contacting the zinc-and carbonate-enriched organic solution with an acidic solution producing a zinc-enriched aqueous acid solution, a zinc- and carbonate-depleted organic solution of a zinc extractant and carbon dioxide; and iv. Recovering zinc from the zinc-enriched aqueous acid solution.

4. The method according to claim 1, wherein the method further comprises the step of: Leaching a zinc-containing carbonate-containing ore with an ammoniacal solution to produce the aqueous ammoniacal ammonium carbonate zinc solution.

5. The method according to claim 4, wherein the zinc-containing ore is leached with an ammoniacal solution in the presence of ammonium carbonate to produce the aqueous ammoniacal ammonium carbonate zinc solution.

6. The method according to claim 3, wherein the carbon dioxide produced in the step of: Contacting the zinc-and carbonate-enriched organic solution with an acidic solution producing a zinc-enriched aqueous acid solution, a zinc- and carbonate-depleted organic solution of a zinc extractant and carbon dioxide is absorbed into an aqueous solution to produce an aqueous solution of carbon dioxide.

7. The method according to claim 6, wherein the carbon dioxide is absorbed into an aqueous solution containing ammonia.

8. The method according to claim 6 or 7, wherein the carbon dioxide is absorbed into the zinc- and carbonate-depleted aqueous ammoniacal ammonium carbonate solution produced during step (i).

9. The method according to claim 6, wherein the aqueous solution of carbon dioxide is used in the ammoniacal leaching of a zinc-containing ore to produce the aqueous ammoniacal ammonium carbonate zinc solution which is the feed to step (i).

10. The method according to claim 1, wherein the aqueous ammoniacal ammonium carbonate zinc solution contains free ammonia, such that the method more specifically comprises a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, the method comprising the steps of: i. Contacting the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia with an organic solution of a zinc extractant, such that a portion of the zinc and ammonia is transferred from the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, producing a zinc- and ammonia-depleted aqueous ammoniacal ammonium carbonate solution and a zinc- and ammonia-enriched organic solution of a zinc extractant; ii. Separating the zinc-enriched ammonia-enriched organic solution of a zinc extractant and zinc-depleted ammonia-depleted aqueous solution; iii. Contacting the zinc-enriched ammonia-enriched organic solution with an ammonia scrub solution to reduce ammonia thereby producing a zinc-rich ammonia-depleted organic Contacting the zinc-enriched ammonia-depleted organic solution with an acidic solution producing a zinc-rich acid solution and a zinc-depleted ammonia-depleted organic solution of a zinc extractant; and iv. Recovering zinc from the zinc-rich acid solution.

11. The method according to claim 10, wherein the ammonia scrub solution preferably has a pH in the range 2 to 7.

12. The method according to claim 10, wherein the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia is controlled to between 0.05 and 2.0.

13. The method according to claim 1, wherein the aqueous ammoniacal ammonium carbonate zinc solution contains impurities, such that the method more specifically comprises a method for recovering zinc from an aqueous impurity-containing ammoniacal ammonium carbonate zinc solution, the method comprising the steps of: i. Contacting the aqueous impurity-containing zinc ammoniacal ammonium carbonate solution with an organic solution of a zinc extractant, such that a portion of the zinc and the impurities from the aqueous impurity-containing ammoniacal ammonium carbonate zinc solution producing an impurity-depleted zinc-depleted aqueous ammoniacal ammonium carbonate solution and an impurity-enriched zinc-enriched organic solution of a zinc extractant; ii. Separating the impurity-enriched zinc-enriched organic solution of a zinc extractant and impurity-depleted zinc-depleted aqueous solution; iii. Contacting the impurity-enriched zinc-enriched organic solution with an impurity scrub solution to reduce the impurities thereby producing an impurity-depleted zinc-enriched organic solution of a zinc extractant and an impurity-enriched solution; iv. Contacting the impurity-depleted zinc-enriched organic solution with an acidic solution producing a zinc-enriched aqueous acid solution and an impurity-depleted zinc-depleted organic solution of a zinc extractant; and v. Recovering zinc from the zinc-enriched aqueous acid solution

14. The method according to claim 11, wherein the impurity scrub solution preferably has a pH in the range 0 to 5.

15. The method according to claim 11, wherein the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia is controlled to between 0.05 and 2.0

16. The method according to claim 10, wherein the aqueous ammoniacal ammonium carbonate zinc solution contains both free ammonia and impurities, where the method more specifically comprises both the steps of: Contacting the zinc-enriched ammonia-enriched organic solution with a ammonia scrub solution to remove ammonia thereby producing a zinc-enriched ammonia-depleted organic solution of a zinc extractant and an ammonia-enriched solution; and contacting the impurity-enriched zinc-enriched organic solution with an impurity scrub solution to remove the impurities thereby producing an impurity-depleted zinc-enriched organic solution of a zinc extractant and an impurity-enriched solution.

17. The method according to claim 1, wherein the molar ratio of ammonia to zinc in the aqueous solution is between 3 and 20.

18. The method according to claim 1, wherein the zinc extractant is selected from the group: liquid organophosphorus extractant, liquid oxime extractant, carboxylic acid extractant and combinations thereof.

19. The method according to claim 1, wherein the zinc extractant is Bis(2-ethylhexyl) hydrogen phosphate.

20. The method according to claim 1, wherein the method further comprises the step of: adding a phase modifier to the organic solution of a zinc extractant prior to the step of: contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-enriched organic solution of a zinc extractant

21. The method according to claim 1, wherein the organic solution of a zinc extractant, comprises a zinc extractant and a diluent.

22. The method according to claim 1, wherein the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 2.0.

23. The method according to claim 1, wherein the molar ratio of ammonia to zinc to carbonate in the aqueous ammoniacal ammonium carbonate zinc solution is about 4:1:1.

24. The method according to claim 1; wherein at least 10% of the zinc species present in the organic solution following the step of: Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-enriched organic solution of a zinc extractant, is in the form of ZnRHCC>3, where R is the extractant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0148] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

[0149] FIG. 1 is a flow diagram representing the method of the embodiment, in the context of an aqueous ammoniacal zinc solution produced by the ammoniacal leaching of a zinc carbonate ore;

[0150] FIG. 2 is a graphical representation of the loading isotherms for Zn and CO.sub.3 ions from the high carbonate solution of Example 1;

[0151] FIG. 3 is a graphical representation of the loading isotherms for Zn and CO.sub.3 ions from the low carbonate solution of Example 2;

[0152] FIG. 4 is a graphical representation of the extraction isotherms for 10, 40 and 60 vol % D2EHPA (bottom to top) from solutions where CO3:Zn>=1 in the trials of Example 3;

[0153] FIG. 5 is a graphical representation of the Zn/D2EHPA ratios for isotherms of FIG. 4; and

[0154] FIG. 6 is a graphical representation of the zinc extraction as a function of equilibrium pH after 1, 2 and 3 contacts in the trials of Example 4.

DESCRIPTION OF EMBODIMENTS

[0155] A zinc ore 20 is leached 10 in an ammoniacal ammonium carbonate solution 62 to produce a slurry 27 comprising an aqueous zinc ammoniacal ammonium carbonate solution containing impurities and a zinc-depleted solid, the slurry is subjected to solid-liquid separation 11, to produce a solid waste 41, which is disposed of, and an impurity-containing zinc ammoniacal ammonium carbonate solution 30. The aqueous impurity-containing zinc ammoniacal ammonium carbonate solution 30 is then introduced into a solvent extraction loading stage 12, in which the aqueous impurity-containing zinc ammoniacal ammonium carbonate solution 30 is contacted with an organic solution of a zinc extractant, in the form of D2EHPA, dissolved in an aliphatic 43. The zinc, some impurities and some ammonia are transferred into the organic phase, thereby producing a zinc-depleted impurity-depleted ammonia-depleted aqueous ammoniacal ammonium carbonate solution 60 and a zinc-enriched impurity-enriched ammonia-enriched organic solution of a zinc extractant 33 which are then separated. The zinc-depleted impurity-depleted ammonia-depleted aqueous ammoniacal ammonium carbonate solution 60 is largely recycled back to the leach with a small volume 31 used to absorb the carbon dioxide 59 evolved during the zinc strip 15 forming a carbonic acid solution 32.

[0156] The impurity-enriched ammonia-enriched zinc-enriched organic solution of a zinc extractant 22 proceeds to an ammonia scrub extraction 13. In the ammonia scrub stage 13 the impurity-enriched ammonia-enriched zinc-enriched organic solution of a zinc extractant 33 is contacted with an acidic solution of carbonic acid 32, thereby producing an impurity-enriched ammonia-depleted zinc-enriched organic solution 47 and an ammoniacal ammonium carbonate solution 61, which is combined with the raffinate 60 from the SX loading stage 12 to form an ammoniacal ammonium carbonate solution 62 which is recycled back to the leach stage 10. The impurity-enriched ammonia-depleted zinc-enriched organic solution 47 is then contacted with an acidic aqueous solution containing zinc 56 in an impurity scrub stage 14 forming an acidic aqueous solution containing zinc and impurities 58, which can be disposed of and an impurity-depleted ammonia-depleted zinc-enriched organic solution of a zinc extractant 49. The impurity-depleted ammonia-depleted zinc-enriched organic solution of a zinc extractant 49 proceeds to a zinc strip stage 15 where it is contacted with a strongly acidic solution 51 producing a zinc-enriched acid aqueous phase 52, an impurity-depleted ammonia-depleted zinc-depleted organic solution of a zinc extractant 43 and carbon dioxide 59. The zinc-enriched acid aqueous phase 52 proceeds to zinc electrowinning 16 where zinc cathodes 53 are produced. The impurity-depleted ammonia-depleted zinc-depleted organic solution of a zinc extractant 43 is recycled back to the SX loading stage 12. The carbon dioxide 59 is absorbed into an aqueous solution 31 and the resultant solution 32 used in the ammonia scrub 13.

EXAMPLE 1

[0157] A zinc silicate ore was leached with an ammoniacal ammonium carbonate solution consisting of 25 g/L free ammonia and 8 g/L of ammonium carbonate. After 24 h of leaching, the solution was separated by filtration, the zinc tenor of this solution was 4.67 g/L. An organic phase comprising 100 mL of D2EHPA diluted to 1.00 L with aliphatic kerosene (Recosol V80), giving a solution containing 0.303 M D2EHPA, was made up.

[0158] A small volume of the aqueous zinc solution was shaken with an aliquot of the organic D2EHPA solution for several minutes. The phases were allowed to separate and the aqueous phase drained off and analysed for zinc and carbonate ions using standard methods. The masses of zinc and carbonate ions transferred into the organic phase was calculated as the difference between the feed solution (4.67 g/L) and the solution after contact with the organic phase.

[0159] A fresh aliquot of aqueous zinc solution was then contacted with the same aliquot of organic D2EHPA solution and the cycle repeated until there was little further extraction of zinc from the aqueous solution.

[0160] The data points and best-fit Langmuir isotherms for zinc and carbonate ions resulting from this work are shown in FIG. 2. The carbonate data and isotherms are triangles and dashed line respectively.

[0161] The Langmuir isotherm indicates that the maximum zinc loading in the organic solution is 19.04 g/L, (0.291 M) and the maximum carbonate loading is 17.53 g/L (0.292 M). Thus the molar ratio of CO.sub.3:Zn in the organic phase is 0.292/0.291=1.003. Since the D2EHPA concentration was 0.303 M the molar ratio of R:Zn:CO.sub.3=0.303:0.291:0.292=1.04:1:1 or 1:1:1.

EXAMPLE 2

[0162] The leaching run in Example 1 was repeated using 1 g/L of ammonium carbonate, this being the only difference between the two runs. The solution tenor for zinc of 4.60 g/L is very close to that from Example 1, evidently, the level of carbonate ions in the leach solution has little effect on the extent of leaching from this ore. The extraction process and analyses were also repeated to give the isotherms shown in FIG. 3.

[0163] The isotherms for zinc and carbonate ions are shown in FIG. 2. The maximum loadings of zinc and carbonate ions were 10.203 g/L (0.156 M) and 0.491 g/L (0.00818 M) respectively. This leads to a molar ratio of Zn:CO.sub.3 of 0.156/0.00818=19.1. The D2EHPA concentration of 0.303 M leads to the molar ratio of R:Zn=0.303:0.156=1.94:1.

[0164] Simple comparison with Example 1 shows that reducing the molar ratio of carbonate ions to zinc ions in the aqueous feed solution from 1.18 to 0.15 reduced the maximum extraction of zinc from 19.04 g/L to 10.20 g/L, a very substantial reduction. Clearly, the presence of carbonate ions during the extraction have a very significant effect on zinc extraction.

[0165] Without wishing to be bound by theory, the low level of ammonium carbonate in the aqueous solution (1.0 g/L, 0.010 M) is clearly insufficient to enable the transfer of more than 6.4% (0.010/0.156 *100) of the zinc into the organic phase as the proposed ZnRHCO.sub.3 complex. The remaining 93.6% of the zinc will be extracted as the well-known ZnR.sub.2 complex. On this basis, the average molar ratio of D2EHPA to Zn can be calculated to be (6.4*1+93.6*2)/100=193.6/100=1.936. This is extremely close to the experimental value of 1.94 derived from the isotherm.

EXAMPLE 3

[0166] A zinc bearing ammoniacal ammonium carbonate solution was made by leaching a dolomite ((Ca,Mg)CO.sub.3) hosted smithsonite (ZnCO.sub.3) ore in an ammoniacal solution for 24 h. After this time the solution was filtered to eliminate solids. The resultant solution contained 19 g/L of zinc, 30 g/L of total ammonia (i.e. ammonia+ammonium) and 29 g of carbonate ions. This gives a carbonate to zinc molar ratio of 1.04.

[0167] Two organic solutions were prepared with either 40 or 60 vol % D2EHPA in addition to 5 vol % TBP, the balance being kerosene. The aqueous and organic solutions were shaken together at different volume ratios, allowed to settle and the aqueous solution analysed for zinc. The zinc concentration in the organic solution was calculated by difference.

[0168] FIG. 4 show the extraction isotherms for 40 and 60 vol % D2EHPA along with the isotherm for 10 vol % in high carbonate solution from Example 1. The experimental data are shown as points, the lines show a Langmuir isotherm calculated from the data.

[0169] The composition of the species present in the organic solution at saturation can be determined from the maxima of the Langmuir isotherms. The molar ratio of zinc to D2EHPA in the organic phase can be calculated. 40 vol % D2EHPA is 1.21 M, 83 g/L Zn is 1.27 M, thus the molar ratio Zn/D2EHPA is 1.05. Similarly, in the 60 vol % D2EHPA run, the ratio is 1.02. Accordingly, it appears that the zinc species extracted from ammoniacal ammonium carbonate can be summarised as ZnR, which is significantly different to the ZnR.sub.2 species extracted from other aqueous media.

[0170] As would be recognised by those skilled in the art, the Langmuir isotherm can also be used to determine the Zn/D2EHPA ratio for any level of loading of the organic. This data is shown in FIG. 5 for all three isotherms from FIG. 3.

[0171] The Zn/D2EHPA ratio for the individual data points shown in FIG. 4 ranges from 0.09 to 1.05. In the former case, only one in every eleven D2EHPA molecules present in the organic is attached to a zinc ion, the remaining ten are available to complex with other zinc ions as they are extracted in subsequent stages. This is made clear by the increasing Zn/D2EHPA ratio as the quantity of zinc in the organic phase increases with increasing numbers of contacts with the leach solution.

[0172] Those skilled in the art will recognise that the most economic and efficient utilisation of the D2EHPA will be where the maximum number of D2EHPA molecules are associated with zinc ions, i.e. at the upper end of the D2EHPA saturation where the zinc concentration in organic as a percentage of the maximum possible zinc concentration is close to 100% and the Zn/D2EHPA ratio is highest. It would be possible, but economically poor to operate at much less than 50% saturation where the Zn/D2EHPA ratio is <0.5.

[0173] Without wishing to be bound by theory, it is clear that when the D2EHPA is not at maximum saturation there will be D2EHPA molecules in the organic phase which are not associated with the zinc ions. Under such circumstances the Zn/D2EHPA ratio must always be <1 and at the lowest saturations the amount of free D2EHPA present will be highest and the Zn/D2EHPA ratio lowest.

[0174] Without wishing to be bound by theory, it is believed that the species extracted from ammoniacal ammonium carbonate solutions also contains a hydrogen carbonate. The reason is twofold. Firstly, the valence of zinc is unchanged during extraction and a univalent anion is required to balance the charge in both aqueous and organic solutions. In the aqueous solution the only anion present at any useful concentration is carbonate. Secondly, between pH 6.2 and 10.2 the carbonate is predominantly present as hydrogen carbonate (bicarbonate), HCO.sub.3.sup.. Thus it would seem that the zinc-bearing species present in the organic solution after extraction from ammoniacal ammonium carbonate is RZnHCO.sub.3.

[0175] It is well known by those skilled in the art that using the concentration of D2EHPA used to extract zinc from aqueous acid sulphate solutions has an upper practical limit of around 40 vol %. Above this level, the viscosity of the organic solution is too high for use in an industrial operation. Even at this concentration, the solvent extraction circuit is typically operated at elevated temperature to reduce viscosity problems. The increased viscosity has been attributed to formation of polymers of the ZnR.sub.2 complexes (Z. Kolarik and R. Grimm, J.Inorg.Nucl.Chem., 1976, 38, 1721-1727). The D2EHPA molecules attach to opposite sides of a square plane surrounding the zinc effectively forming a much longer chain, which a structural formula of R-Zn-R. The longer chains more readily become entangled raising the viscosity to the point where it is too high for industrial use.

[0176] A recent publication (Phys. Probl. Miner. Proc. 50(1), 2014, 311-325) examines zinc recovery from a 60 g/L Zn solution using 36 vol % D2EHPA. They note that using 36 vol % D2EHPA was unsatisfactory to use Due to high viscosity and that as a consequence, the concentration in further experiments has been limited to 18% (vol.).

[0177] The present system showed no apparent viscosity problems, even when using 60 vol % D2EHPA as the extractant. If the putative complex RZnHCO.sub.3 is present, then it will be notably shorter than R-Zn-R as there is only a single organic chain on the complex. This is less likely to become polymerised than the R-Zn-R complex and therefore not have a substantially higher viscosity than the starting organic solution.

EXAMPLE 4

[0178] A large volume of zinc bearing aqueous solution was prepared by leaching 24 kg of 35% Zn ore in 300 L of 35 g/L free ammonia solution to give a zinc tenor of 16.7 g/L. The ore also contained leachable lead and cadmium. Fresh aqueous at a starting pH of 10.91 was contacted at a 1:1 volume ratio with fresh organic (40 vol % D2EHPA+10 vol % TBP in kerosene), the organic removed, the aqueous sampled and replaced with fresh organic. This was repeated three times. FIG. 4 shows the zinc extraction as a function of equilibrium pH after 1, 2 and 3 contacts.

[0179] Despite the high starting free ammonia in the PLS, it is clear that three contacts were sufficient to reduce the pH to a level where the metals extraction was high.

[0180] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.