WO2011120093A1

WO2011120093A1 - Recovering metals from pickle liquor

Info

Publication number: WO2011120093A1
Application number: PCT/AU2011/000368
Authority: WO
Inventors: Andrew Tong; Maritza Valencia-Bejarano
Original assignee: Intec Ltd
Priority date: 2010-03-30
Filing date: 2011-03-30
Publication date: 2011-10-06
Also published as: AU2011202421A1

Abstract

A process for recovering zinc from pickle liquor is disclosed. The process comprises: (i) passing the liquor to an iron recovery stage in which iron present in the liquor is separated from the liquor; (ii) passing the iron-depleted liquor to an electrolysis stage in which zinc present in the liquor is electro-recovered; and (iii) passing the zinc-depleted liquor to a separate acid regeneration stage in which the pickle liquor is regenerated. This allows for recovery of ferric oxide and zinc, both saleable products. When the regenerated acid is recycled to the galvanizing process, the present process approaches zero waste.

Description

Recovering Metals from Pickle Liquor

Technical Field

A process is disclosed for the treatment of pickle liquor, especially spent pickle liquor from the galvanising industry, for the recovery of the metals contained in the liquor. A primary application of the process disclosed herein is in relation to the recovery of zinc, and iron, although other metals may be recovered.

Background Art

In the galvanising industry, prior to immersion in a galvanising bath, low alloy, alloy and high alloy surfaces are prepared by pickling in hydrochloric, sulphuric, or a mixture of nitric and hydrofluoric acids. Three kinds of waste streams are generated during pickling: waste water from rinsing, waste water from fume absorbers and spent pickle acid.

Technologies for spent pickle liquor treatment include:

• Side-stream mechanical filtering, acid recovery and internal recycling;

• HCI regeneration by a fluidised bed spray roasting process;

• Separation and reuse of free acid fraction (e.g. H2SO crystallisation or HCI evaporation); and

· Recovery of spent mixed pickle liquor by solvent extraction.

However, when both zinc and iron are present in the spent pickle liquor at elevated concentrations, each of the existing on-site treatment options listed above become problematic. For example, iron and zinc cannot be simultaneously recovered while acid is regenerated, and high concentrations of zinc (> 10 g/L) make the spent acid difficult to re-use or recycle back to the process.

Often spent pickle liquor containing both zinc and iron is treated by co- precipitation of oxides/hydroxides under alkaline conditions. The resulting filter cake is immobilised then sent to landfill, and fresh acid is purchased to replenish the pickle baths. Aside from the environmental risks associated with this technique, in part due to rising costs for landfill, users of this technique also face cost pressures,

30/03/1 1 GB 1404586 discloses a process for recovering sulphuric acid. However, in this process, the sulphuric acid is regenerated from pickle and leach liquors at the anode of an electrolysis cell (i.e, in the cell itself).

The above references to the background art do not constitute an admission that such art forms a part of the common and/or general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the process disclosed herein.

Summary of the Disclosure

A process is disclosed for recovering zinc from pickle liquor. The pickle liquor may be spent and may typically^' come from a galvanising process (e.g. from a pre- galvanising cleaning stage of the galvanising process).

In a broad sense the process comprises:

(a) iron separation;

(b) electro-recovery of zinc as zinc metal; and

(c) in a separate stage, regeneration of acid in the pickle liquor.

Separation of the iron allows for electro-recovery of the zinc. The iron can be precipitated and recovered as ferric oxide which may either represent a saleable product or at least clean landfill. The electro-recovered zinc metal can be sold, or re-used in the galvanising process..

Separation of the acid generation stage also allows for economic recovery (and subsequent reuse of the zinc) in the electro-recovery stage (i.e. without the interference of acid generation).

The regenerated acid can be recycled to the galvanising process (e.g. to the pre- galvanising cleaning stage of the galvanising process). Potentially the present process thus approaches zero waste and is also able to be operated in a closed loop mode.

In this regard, when the pickle liquor comprises hydrochloric acid, and when it is regenerated by the addition of sulphuric acid, the process can be configured such that the sulphate anion in the liquor (resulting from addition of sulphuric acid) can be precipitated as a product. For example, when a calcium-based alkali precipitation agent (e.g. limestone) is added to the solution to precipitate the iron as ferric oxide, the calcium still in solution can precipitate with the sulphate anion as a calcium sulphate product, which may itself be saleable.

In a more specific form, the process can comprise the following steps:

(i) passing the liquor to an iron recovery stage in which iron present in the liquor is separated from the liquor;

(ii) passing the iron-depleted liquor to an electrolysis stage in which zinc present in the liquor is electro-recovered; and

(iii) passing the zinc-depleted liquor to a separate acid regeneration stage in which the pickle liquor is regenerated.

To produce the iron-depleted liquor in step (i), iron present in the liquor is advantageously oxidized to the ferric state. In the ferric state, the iron can then be precipitated and separated from the solution.

In one embodiment, the iron can be oxidized to the ferric state through the addition of chlorine gas to the solution. In this embodiment, when chlorine gas has subsequently been generated in the electrolysis stage (ii), that gas can be fed back to the solution in stage (i) to participate in oxidation of the iron. In this embodiment, a sealed reactor to which the chlorine gas is fed can be employed for iron oxidation. The generation and use of chlorine gas thus facilitates the use of "by-products" in the process, to help the process approach zero waste and to remain closed loop.

In circumstances where the amount of chlorine gas generated within the process is insufficient to oxidize all ferrous iron in solution, oxygen/air can be used to supplement the shortfall in chlorine oxidant (i.e. both chlorine and oxygen/air can be employed in the step (i) oxidation of iron). Further, because pickle liquor usually (though not exclusively) comprises hydrochloric acid, this acid can conveniently participate in and be consumed by the addition of the oxygen/air.

In one embodiment the ferric oxide can be precipitated by adding a calcium- based alkali precipitation agent to the solution. For example, a readily available, low cost agent is limestone, although lime, hydrated lime, slaked lime, etc can be employed, In one embodiment the ferric oxide precipitate can be separated and recovered from the liquor in a separation stage subsequent to the precipitation stage. For example, the separation stage may comprise a thickener unit, followed by a filtration stage.

30/03/11 In one embodiment, in step (ii) the zinc is electroplated on a cathode of an electrolytic cell. The cathode may comprise titanium, or a material which does not form a chemical bond with the electroplated zinc (e.g. an inert material that facilitates subsequent separation of the zinc by simple physical separation). Numerous examples of suitable dimensionally stable electrodes are known, and can be applied for this purpose. Alternatively, non-conventional electrodes such as fluidised beds, zinc starter sheets, etc can also be adapted for the process.

In one embodiment, the process comprises a number of parallel electrolytic cells. This enables one or more cells to be taken offline to enable cathode removal, stripping and zinc recovery. The zinc recovered from the cathodes can be cast into ingots for sale of the zinc, or crushed and recycled back to the galvanising process for use therein.

In one embodiment, in step (iii), the zinc-depleted liquor can be regenerated by adding sulphuric acid (e.g. up to a concentration of 98%) to the solution in the acid regeneration stage. This also adds sulphate anions to the solution. In this embodiment, the final concentration of the hydrochloric acid produced can be adjusted by adjusting the sulphuric acid addition. The ultimate acid strength selected can relate to the requirements of the galvaniser (e.g. when recycling acid back into the pickle baths).

In this embodiment, the sulphate anions can be precipitated, and the precipitate can be separated and recovered from the liquor in a separation stage subsequent to precipitation. For example, in stage (i), when the ferric oxide has been precipitated by adding a calcium-based alkali precipitation agent to the solution, the sulphate anion resulting from sulphuric acid addition can form a precipitate with the calcium still in solution in the acid regeneration stage. The resultant calcium sulphate precipitate can be separated (e.g. by filtration) and recovered as a saleable product.

The regenerated acid from step (iii) can be recycled to the galvanising process for use therein. For example, to close the process loop, the regenerated liquor can be recycled to the pre-galvanising cleaning stage of the galvanising process (e.g. from whence the pickle liquor originally came).

3003 11 Brief Description of the Drawings

Notwithstanding any other forms which may fall within the scope of the process as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic process flow diagram showing integrated operating circuits to process spent pickle liquor and recover zinc, etc therefrom;

Figure 2 depicts the iron precipitation circuit of Figure 1 in greater detail, the circuit comprising a series of reactors followed by a solid-liquid separation stage;

Figure 3 depicts the zinc electrowinning circuit of Figure 1 in greater detail, the circuit comprising a series of electrowinning cells and associated storage tanks;

Figure 4 depicts the acid regeneration circuit of Figure 1 in greater detail, the circuit comprising a series of reactors followed by a solid-liquid separation stage; and

Figure 5 depicts the correlation of total iron, ferrous and ferric content vs. the reduction-oxidation potential (Eh).

Detailed Description of Specific Embodiments

As shown schematically in Figure 1 , a process to treat and recycle hydrochloric acid spent pickle liquor containing zinc, iron and hydrochloric acid consists of three main circuits: iron separation (oxidation and precipitation/removal), followed by zinc electrowinning, and finally acid regeneration.

The chemistry for the three main circuits can be summarised as follows:

Iron Oxidation: 2FeCI₂^ + Cl₂^ — » 2FeC\ _aqj

4FeCl₂„ + 0_2(g) + 4HCI_W → 4FeCI_3ft) + 2H₂O_w

Iron Precipitation: 2FeCI_3(V; + 3CaC0_3W → Fe₂0_3w + 3CaCl_2W + 3C0_2W

Zinc Electrowinning: ZnCI^ — » Zn_w + Cl₂^

Acid Regeneration: H₂S0_4W +

→ CaSO₄.aH₂0 _Ji, + 2H ½ It was noted that similar but alternative reagents can be employed to achieve a required effect/outcome (e.g. substitution of limestone, etc).

Iron Oxidation & Precipitation

In the process schematically depicted by Figure 1 , and as shown in more detail in Figure 2, hydrochloric acid spent pickle liquor (stream 101 ) is heated to 70 - 90°C and is fed to the iron oxidation and precipitation circuit (100). This stage comprises 2-4 reactors (120), where ferrous cations are oxidised into ferric cations using chlorine gas generated in the electrowinning cell (stream 202) and, as necessary, air (stream 103).

In this regard, where the amount of chlorine gas generated within the process is insufficient to oxidize all ferrous iron in solution, oxygen/air can be used to supplement any shortfall in oxidant. This is shown by the dotted oxygen/air line in Figure 2. The pickle liquor usually, though not exclusively, comprises hydrochloric acid, and this is conveniently consumed when adding oxygen/air for the ferrous oxidation reaction, as shown in Reaction 2.

Figure 2 shows a configuration in which four such reactors ( 120) are employed. Spent pickle liquor (101) is fed to Reactor 1 , optionally with oxygen/air (103), to commence oxidation of the ferrous cations into ferric cations. Chlorine gas (202) from the electrowinning cell is fed to Reactor 2, to continue oxidation of the ferrous cations into ferric cations. The ferric cations are precipitated as an iron oxide (hematite or goethite) product by the addition of limestone (CaC0 ) (stream 102) to Reactor 3.

A system of recycle streams, namely, internal recycle (see dotted lines between Reactors) and underflow recycle is used to increase the selectivity of the iron precipitation reaction and improve the filtration characteristics of the product. In this regard, an internal recycle is employed between Reactor 4 and Reactor 3 to maximise the precipitation and recovery of iron as ferric oxide. The precipitation reaction temperature is controlled to (i.e. maintained) between 50-1 10°C and the solution pH between 1 -5. An optimal pH for precipitation of the ferric oxide (hematite) is in the range 3-4, and an optimal temperature is between 80-90°C.

30/03/11 Iron Separation

In the process schematically depicted by Figure 1 , and as shown in more detail in Figure 2, after passing through the oxidation and precipitation Reactors 1 -4, the solution ( 105) is passed to a solid-liquid separation system. As shown in Figure 2, the solid-liquid separation system comprises a thickener ( 109) to which a flocculant ( 1 10) is added. The solid-liquid separation system further comprises a filtration unit ( 1 1 1 ). In this embodiment, the filtration unit comprises a belt filter, although any known equipment suitable for separating solids and liquid may be used.

More specifically, the slurry from final Reactor 4 overflow (stream 105) is passed to the thickener (109) to increase solids density to 30% w/w by the addition of the flocculant.

The clear overflow (108) from the thickener (109) is sent to the zinc electrowinning circuit (200) whereas the underflow slurry ( 107) is sent to the filtration unit (1 1 1 ). The filtered iron oxide product ( 1 12) is able to be sold as feed material to the pigmentation industry or cement industry, or alternatively used as an inert ground cover for landfill sites.

Zinc Recovery

As shown schematically in Figure 1 , and in greater detail in Figure 3, liquor overflow (stream 108) from the iron precipitation circuit is fed to the electrowinning cells via a feed tank (210), where high-grade metallic zinc product is electro-won onto titanium cathodes. The electrowinning cell is operated between 2-5 Volts and at a current density of 50-1000 A/m², and more typically at 100-300 A/m². Under these conditions chlorine gas is generated (stream 202) and is fed to the iron precipitation circuit (e.g. to Reactor 2) to oxidise ferrous cations incoming with the pickle liquor.

Zinc sheets are separated from the Ti cathodes using a stripping mechanism, The zinc from the cathodes can be cast into ingots for sale, or crushed and recycled directly into the galvanising process. The spent liquor (stream 203) is now advanced to the acid regeneration circuit.

30/03/11 Acid Regeneration

Referring to Figures 1 and 4, hydrochloric acid regeneration is achieved by reacting concentrated (98%) sulphuric acid (stream 301 ) with the calcium rich, zinc- poor liquor exiting the electrowinning cell (stream 203), as shown in Reaction 5. The stream (203) still comprises a portion of chloride ions in solution from the spent pickle liquor, together with the added calcium.

As best shown in Figure 4, the acid regeneration circuit comprises one to four reactors, and typically two reactors as shown (305), as well as a solid-liquid separation unit (306) comprising a press and plate filtration unit (310). Alternatively, any suitable equipment for separating solids and liquid may be used. The calcium rich, zinc-poor liquor (203), which still comprises a portion of chloride ions in solution from the spent pickle liquor together with the added calcium, is fed to Reactor 1 , where it is contacted with an internally recycled underflow stream from Reactor 2. The concentrated sulphuric acid (301 ) is fed to Reactor 2, where it contacts liquor advancing from Reactor 1.

Under the processing conditions specified herein, calcium sulphate precipitates and hydrochloric acid is formed. Heat generated from the dilution of the sulphuric acid is used to control the form and quality of the calcium sulphate. In this regard, the system is normally operated above 90°C to ensure a highly crystalline and filterable calcium sulphate product is made. The slurry is pumped from the reactors directly to the press and plate filter (310) to recover high purity calcium sulphate (303) and a regenerated hydrochloric acid solution (304).

The separated calcium sulphate (303) can then be sold as a domestic or commercial soil modifier, or used in the building industry as a filler material, The concentration of the final hydrochloric acid (304) can be adjusted to an optimum level for the given pickling plant to enable direct recycling of a "fresh" pickle liquor, by adjusting the amount of sulphuric acid added to the system.

Examples

Examples of the process are now provided. These examples are provided to validate the process, and are not intended in any way as a limitation to the process.

30/03/11 Example 1 (Typical Spent Pickle Liquor)

Samples of spent pickle liquor were collected from commercial galvanising businesses in, Australia. The results of analytical studies are shown in Table 1 . Importantly, the dominant iron species was ferrous and not ferric cations.

Table 1 : Spent Pickle Liquor Analysis

This data indicated that the samples were typical representative samples from the galvanising industry.

Example 2 (Oxidation of Ferrous Chloride with Chlorine Gas)

A total of 12 m³ of pickle liquor was treated through a demonstration plant. The plant was operated continuously for 240 hrs, and the nominal feed rate of pickle liquor was 50 L/hr. Chlorine was produced in an electrolysis cell, and passed through the iron oxidation reactor using a series of pumps and gas diffuser apparatus. The oxidation of ferrous to ferric cations was measured every 5 hrs by withdrawing sample from the reactor. Correlation of total iron, ferrous and ferric content versus Eh (reduction- oxidation potential) is shown in Figure 5.

The results, as presented in Figure 5, indicate that it is feasible to oxidise ferrous to ferric using chlorine gas.

30/03/11 Exaraple 3 (Iron Oxidation & Precipitation)

A sample of spent pickle liquor from a commercial galvanising operation was treated in a continuous pilot plant. The major components of the liquor were iron (91 g/L), zinc (45 g/L) and hydrochloric acid (10 g/L). All of the iron in the pickle liquor was shown to be in the ferrous state.

A total of 85 L of liquor was treated through the plant at a steady feed rate of 1.8 L hr for 47 hours. The plant consisted of three parts: firstly a stirred tank for oxidation of ferrous chloride with air; secondly a contacting column for oxidation of remaining ferrous chloride using chlorine gas; and thirdly a series of tanks for precipitating iron from the solution with limestone.

Samples were obtained every two hours and analysed for total iron and ferrous content. The average results for the entire run are given in Table 2. Overall 88% of the incoming total iron entering the plant was oxidised from ferrous into ferric and then selectively recovered from the solution by precipitation.

Table 2: Summary of Oxidation/Precipitation Results

The results indicated that a bulk of the iron was oxidised and the chlorine gas was taken up in solution.

Example 4 (Precipitation of Iron Oxide with Limestone)

Once the iron had been oxidised in Example 2, the liquor was forwarded to precipitation reactors where limestone was added to precipitate ferric oxides. Control over reagent addition was maintained using an automated control-loop connected to pH probes. The pH was maintained at 3-3.5, and the temperature was 80-85 °C. The solids were separated from the iron-free liquor using a thickener followed by a belt filter. Flocculant was added to the slurry to assist with thickening. Over 100 kg of product was produced, and the elemental ICP assay for 80 hr composites is shown in Table 3. Table 3: Results of ICP Analysis

Metal Unit Sample 1 Sample 2 Sample 3

Ag ppm <0,5 <0.5 <0.5

Al % 0.20 0.23 0.21

As ppm <5 <5 <5

Ba ppm 48 48 48

Bi ppm 21 21 18

C % 0.20 0.25 0.14

Ca % 0.09 0.09 0.09

Cd ppm <0.2 <0.2 <0.2

CI ppm 4620 4635 4852

Co ppm 14 13 12

Cr ppm 368 376 361

Cu ppm 9 ^• 10 20

Fe % 63.83 66.25 66.21

Hg ppm <3 <3 <3

K % <0.01 <0.01 <0.01

La ppm 89 92 91

Mg % 0.69 0.78 0.72

Mn ppm 189 195 193

Mo ppm 31 32 31

Na % 0.17 0.04 0.01

Ni ppm 160 148 137

P % 0.03 0.03 0.03

Pb ppm 3 19 239 192

Sb ppm <5 <5 <5

Sc ppm < 1 <1 <1 -

Sr ppm 1 <1 <l

Ti % 0.01 0.01 0.01

Tl ppm <2 <2 <2

V ppm 32 34 32

W ppm <5 <5 <5

Zn ppm 28973 26669 23671

Zr ppm <1 . <1 <l

The iron grades of mid-60 % indicates very pure grades of iron oxides, with the only major "contaminant" being zinc. Example 5 (Pilot Zinc Electro-Recovery)

A portion of the liquor treated in Example 3, was forward to an electrowinning cell where zinc metal was plated at the cathode and chlorine gas was generated at the anode. The cell consisted of a titanium cathode measuring 0.2 m² per side, and two titanium mesh anodes measuring 0.18 m². The total volume of the cell was 100 L, and the feed rate was set at 3 L/hr. Plating was conducted at a cathode current density of 270 A/m². The l iquor fed to the cell contained 38-40 g/L zinc, while the liquor exiting the cell only contained 10- 12 g/L zinc, hence the zinc strip produced under these conditions was ~ 30 g/L.

After plating for 3 1 hours a relatively smooth zinc cathode was recovered from the cell. By comparing the weight of the recovered zinc with the applied current, a plating efficiency of 60% was determined. The low efficiency was a result of iron being present in the liquor. This was noted not to effect the commercial viability of the process.

Chlorine gas was collected from the top of the cell and was recycled to a contacting column for absorption by ferrous containing pickle liquor.

Example 6 (Demonstration Electrowinning)

To demonstrate the commercial viability of the promising results produced in the pilot plant, the iron-free liquor obtained in Example 4 was forwarded to a demonstration-scale electrowinning cell where zinc metal was plated at the cathode and chlorine gas was generated at the anode. The cell consisted of eight titanium cathodes measuring 1 m² per side, and nine titanium mesh anodes measuring 1 m². The total volume of the cell was 750 L, and the feed rate was set at 200 L/hr. Plating was conducted at a cathode current density of 200-250 A m², and the cycle time for the cell was 6-24 hours. The liquor fed to the cell contained 15 g/L zinc, while the liquor exiting the cell only contained 2-5 g/L, hence the zinc strip produced under these conditions was ~ 10 g/L.

The zinc products from three cycles are compared in Table 4. The electrical efficiency of the cell is measured by weight of zinc produced divided by the theoretical weight of zinc plated, relative to the applied current.

30/03/11 Table 4: Zinc Production

These results demonstrated better electrical efficiency at a shorter electrowinning time.

Chlorine gas was collected from the top of the cell and was recycled to a contacting column for absorption by ferrous containing pickle liquor, as explained in Examples 2 and 4 above. Example 7 (Demonstration Hydrochloric Acid and Gypsum Production)

A total of 12 m³ of liquor, collected from Example 6, was treated in a demonstration-scale hydrochloric regeneration circuit. Sulphuric acid was added to the liquor, resulting in the production of hydrochloric acid and gypsum according to the reaction:

H₂S0₄ + CaCl₂→ 2HC1 + CaS0_4(s)

The slurry was then filtered using a three stage water wash counter-current belt filter. 150 kg of dry calcium sulphate was produced, and the filtrate contained 12- 14% hydrochloric acid. An XRF elemental analysis of the solids is given in Table 5.

Table 5: Results of XRF Analysis of Lead, Zinc and

Iron In the Calcium Sulphate Product

The data revealed that there was very little contamination (e.g. by zinc, lead or iron) in the calcium sulphate product.

3003/11 Whilst a specific process embodiment has been described, it should be appreciated that the process may be embodied in other forms.

For example, whilst typically the pickle liquor comes from a galvanising process, there is no reason why the pickle liquor cannot be sourced from other processes. Further, whilst typically the process is used to recover zinc and iron, there is no reason why metals additional to zinc and iron cannot be selectively recovered from the liquor.

In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but riot to preclude the presence or addition of further features in various embodiments of the process as disclosed herein.

30/03/11

Claims

1. A process for recovering zinc from pickle liquor comprising the steps:

2. A process as claimed in claim 1 , wherein in step (i), to produce the iron-depleted liquor, iron present in the liquor is oxidized to ferric oxide through the addition of chlorine gas to the solution, with the ferric oxide then being precipitated and separated from the solution.

3. A process as claimed in claim 2, wherein the pickle liquor comprises hydrochloric acid and chlorine gas is generated in the electrolysis stage (ii) and is fed back to the solution in stage (i).

4. A process as claimed in any one of the preceding claims, wherein in step (i), to produce the iron-depleted liquor, iron present in the liquor is oxidized to ferric oxide through the addition of oxygen/air to the solution, with the ferric oxide then being precipitated and separated from the solution.

5. A process as claimed in any one of claims 2 to 4, wherein the ferric oxide is precipitated by adding a calcium-based alkali precipitation agent to the solution.

6. A process as claimed in any one of claims 2 to 5, wherein the ferric oxide precipitate is separated and recovered from the liquor in a separation stage subsequent to the precipitation stage.

7. A process as claimed in any one of the preceding claims, -wherein in step (ii) the zinc is electroplated on a cathode of an electrolytic cell.

30/03/11

8. A process as claimed in claim 7, wherein the cathode is periodically recovered and the zinc is recycled to a galvanising process for use therein,

9. A process as claimed in any one of the preceding claims, wherein in step (iii) the zinc-depleted liquor is regenerated by adding sulphuric acid to the solution in the acid regeneration stage.

10. A process as claimed in claim 9, wherein in the acid regeneration stage the sulphate anions from the sulphuric acid are precipitated, with the precipitate being separated and recovered from the liquor in a separation stage subsequent to precipitation.

11. A process as claimed in claim 10, wherein when the ferric oxide has been precipitated by adding a calcium-based alkali precipitation agent to the solution in stage (i), the sulphate anions of the sulphuric acid form a precipitate with the calcium still in solution in the acid regeneration stage.

12. A process as claimed in any one of the preceding claims, wherein in step (iii) the zinc-depleted liquor is regenerated for recycling to a galvanising process for use therein.

13. A process as claimed in claim 12, wherein the regenerated liquor is recycled to a pre-galvanising cleaning stage of the galvanising process.

14. A process as claimed in any one of the preceding claims, wherein the pickle liquor from which the zinc is recovered is spent pickle liquor from a galvanising process.

15. A process as claimed in claim 14, wherein the spent pickle liquor comes from a pre- galvanising cleaning stage of the galvanising process.

3003 11