This application claims priority from New Zealand Application No. 768604 the contents of which are included by reference.

TECHNICAL FIELD

Described herein is a process to extract dissolved metals and an apparatus thereof. More specifically, a method for the recovery or treatment of a liquid that contains dissolved iron, and another dissolved metal that utilises electrochemical oxidation prior to electrochemical reduction to selectively separate the iron and the other metal within this liquid. These processes can be conducted within a single reactor or split across multiple reactors.

BACKGROUND ART

In 2018 the annual global production of zinc exceeded 13 million tons. The majority of this zinc is used in the galvanising industry for galvanising steel, which protects it against corrosion. Hot-dip galvanising coats steel parts with zinc by immersing the steel in a molten zinc (96.5-99% purity) bath at approximately 450°C. The molten zinc bonds strongly to the surface of steel, by reacting with the iron to form an iron-zinc alloy, with a composition which is iron rich in its interior and almost pure zinc at the outer surface. This film is usually between 35-250 pm thick that can resist abrasion and deformation to a considerable degree.

The performance of the coating process depends largely on the pre-treatment or cleaning of the steel prior to the hot dipping step. In order to obtain an uniform and rapid alloying reaction with the steel by the molten zinc, it is essential that the steel is free of any oxides and surface contaminants (for example, oils, grease, paints). The first pre-treatment step (degreasing step) is to remove any surface contaminants such as dirt, grease, oil and paint from the steel. This is done by dipping the steel into either a hot alkali solution or a dilute acid. Next, the surface of the steel is treated by pickling in an acid (e.g. hydrochloric or sulphuric acid) bath. This pickling process dissolves any iron oxides (mill-scale or rust) from the steel surface (pickling step).

For steel with an existing complete or partial galvanised coating, this coating is removed by dipping the steel in an acid (e.g. hydrochloric or sulphuric acid) to dissolve the old zinc layer (stripping step). This optional stripping step occurs prior to the pickling step, and generates a waste acid containing both dissolved zinc and iron known as spent stripping liquor In some cases, the spent pickling acid is re-used as stripping acid.

After each pre-treatment step (degreasing, stripping, pickling), the steel parts are rinsed in water.

Each of the pre-treatment steps generate waste streams: wastewater from rinsing, wastewater from fume absorbers, spent stripping acid and spent pickling acid. Some galvanising plants keep stripping and pickling acids and the corresponding spent acid stream separate. Others mix these acids or use a single acid bath for both stripping and pickling steps, and thus only produce a single spent acid wate stream.

When both zinc and iron are present in the spent acid waste stream, existing treatment options become problematic. For example, iron and zinc cannot be separately recovered by electrolysis or chemical precipitation. High concentrations of zinc (20-200 g/L), and iron (100- 200 g/L) make the spent acids difficult to re-use or recycle back to the galvanizing process.

Current treatment processes involve neutralisation and precipitation of the spent acid waste. The metal sludge formed in the precipitation treatment stage must be sent to landfill. Disposing of metals in landfills has huge economic implications. It is expensive and represents a lost opportunity to recover valuable materials. Furthermore, increasingly stringent environmental regulations are increasing the costs of this disposal practice and thus there is an increasing amount of research into methods for recycling and treating this waste. The difficulty underlying the development of recovery processes is associated with the complexity of galvanizing effluents.

By far the most valuable material within the waste is the zinc (NZ $4,000/tonne). For a recycling process to be economically viable, zinc must be recovered in a high value form. As above, directly electrowinning zinc from the waste is ineffective due to the large amount of iron also dissolved in the spent acid. This is because iron both reduces zinc purity and provides sites that the undesirable hydrogen evolution reaction can occur on. When the hydrogen evolution reaction occurs in parallel with zinc deposition, the process has very low energy efficiency. Iron will interfere producing similar effects with other transitional metals when depositing potential near or above iron’s deposition potential (-0.44 V).

A traditional method for disposing of spent acids from galvanising plants is by neutralisation with an alkali for example sodium hydroxide (NaOH) or calcium carbonate (CaCOs). This causes all dissolved metals (mostly zinc and iron) to be precipitated from solution, which are then placed into landfill. As environmental regulations become stricter the cost of this practice is increasing and driving interest in alternative recycling strategies. Many different separation techniques have been reported in literature for the recovery of materials from spent acid.

WO 2011/20093 describes a process for recovering zinc from spent pickle liquor by first oxidising the Fe ²⁺ to Fe ³⁺ using the chemical oxidant chlorine gas and hydrogen peroxide (H2O2), then electroplating the zinc from the iron depleted liquor. Chlorine gas is a powerful oxidant and is effective at oxidising iron but it has several disadvantages. Fresh H2SO4 is required to run the process, the consumption of such material is an additional cost in this process. Furthermore, chlorine gas has a corrosive nature (stainless steel cannot be used and any pipework transporting the gas must be manufactured from carbon steel or an inert plastic) and the use of chlorine gas is highly dangerous. Any process that utilises this gas must have significant inbuilt safety features to reduce risk to operators. Thus, would vastly increase the capital costs of any process designed that utilises chlorine gas. Finally, the use of chlorine to oxidise iron in the spent pickling liquor has poor energy efficiency as the chlorine must be first made electrochemically at high potentials (>1.36 V) before being used to oxidise the iron (II) at a much lower potential (0.77 V).

US 3,622,478 describes a process for regenerating Fe ³⁺ from Fe ²⁺ by electrochemical oxidation of Fe ²⁺ at the anode, followed by returning the regenerated products to the pickling tank. The purpose of this oxidation is to increase the pickling speed of the acid.

WO 2000/026440 describes a process in which copper from a spent copper pickling solution is deposited on the cathode and iron is re-oxidised at the anode before being fed back in to the pickling solution to adjust a predetermined redox potential.

US 6,210,0558 describes a process to maintain the Fe ³⁺ concentration of a stainless steel pickling liquor at a predetermined concentration by electrochemically oxidising Fe ²⁺ formed during the pickling process.

US 4,591 ,489 discloses a process for the selective removal of ferric ions from an aqueous solution containing ferric and other metal ions. They also utilise liquid-liquid (solvent) extraction to remove the Fe ³⁺ from the acid. However, the issues with solvent extraction are that it is an expensive technique, difficult to operate, and the solvents used are often toxic.

US 4,148,700 discloses a method for purifying the liquor of a galvanising process plant after contamination using an electrolytic cell.

WO 1995/023880 discloses the treatment electrolyte solutions. In particular, the treatment of spent acid with an electrolytic cell. However, as above for US 4,148,700 where the spent acid is placed into a cathodic chamber and iron and zinc are deposited as metal composites, the leftover NaCI solution is placed in the anodic chamber, where chloride evolution takes place at the anode. The use of chlorine to oxidise iron in the spent acid is not energy efficient due to the minimum electrical potential (determined by thermodynamics of chlorine evolution being more positive than that of iron (II) oxidation).

EP 0935005 discloses a process for treating metallic dust, mostly oxidised waste, in particular galvanising dust and/or steelworks smoke. The metal dust containing 50-70% zinc and 0.5- 0.8% iron is leached into solution with an acid. However, inefficient oxidation agents such as oxygen, compressed air, hydrogen peroxide, and chlorine are utilised.

From the above, it can be seen that there is a requirement for an efficient and cost-effective process to recycle and treat galvaniser’s spent acid and/or at least provides the industry with a useful alternative. Further aspects and advantages of the methods, apparatus and uses thereof - will become apparent from the ensuing description that is given by way of example only.

SUMMARY

Described herein is a process to extract dissolved metals and optionally recover hydrochloric acid and an apparatus thereof. More specifically, a method for the recovery or treatment of a liquid that contains dissolved iron, and another dissolved metal that utilises electrochemical oxidation prior to electrochemical reduction to selectively separate the iron and the other metal within this liquid. These processes can be conducted within a single reactor or split across multiple reactors. When split across multiple reactors, hydrogen and chlorine can be generated and then reacted to form hydrochloric acid.

The present invention provides a method of processing a feed solution which is an acidic metal solution containing iron and at least one plateable metal selected from the list consisting of zinc, copper, nickel and cobalt, using at least one anode chamber in fluid connection with at least one cathode chamber, such that a first anode chamber and a first cathode chamber which are separated by a permeable separator form an electrochemical cell, where the permeable separator is an anion exchange membrane, the method includes the following steps:

1. measuring the Fe ²⁺ concentration in the feed solution;

2. carrying out an oxidative electrolysis step where Fe ²⁺ is oxidised to Fe ³⁺ forming a primary solution in the first anode chamber, where the Fe ²⁺ concentration in the primary solution is at or below a predetermined concentration X;

3. neutralise the excess acid by raising the pH to between 1 and 1.5 then precipitate the Fe ³⁺ present in the primary solution using an alkali to produce a secondary solution with a dissolved iron concentration at or below X;

4. determine the concentration of at least one of the at least one plateable metal in the secondary solution; and 5. electrochemically reduce at least one of the at least one plateable metal ions in the secondary solution to plated metal in the first cathode chamber to produce a spent solution.

In a preferred form X is at most 15 g/L and in a highly preferred form X is at most 5 g/L.

Preferably the acid in the feed solution is hydrochloric acid or sulphuric acid. In a highly preferred form the feed solution is a spent hydrochloric acid galvanising pickle or spent hydrochloric acid stripping solution.

Preferably the Fe ²⁺ concentration is determined by UV/visual spectroscopy. In a highly preferred form where the at least one plateable metal is selected from the list consisting of zinc, nickel and cobalt then the Fe ²⁺ concentration is determined by absorbance at 950nm. In a still more preferred form the absorbance at 950nm is determined in line and/or real time.

Preferably the concentration of Fe ²⁺ in the secondary solution is no greater than 15 g/L and in a more preferred form no greater than 5 g/L.

Preferably the alkali used in step 3 is sodium hydroxide (NaOH), potassium hydroxide (KOH) or Calcium Carbonate (CaCOs). Preferably the alkali is sodium hydroxide and the molar ratio of NaOH:Fe used in step 3 is between 2.1 :1 and 3.5:1. Preferably the molar ratio of NaOH:Fe is between 2.5:1 and 3.1 :1.

Preferably the anode chamber or anode chambers include electrodes of a form selected from the list consisting of carbon felt, carbon sheet, and carbon rod.

Preferably the permeable separator is an anion exchange membrane selected from the list consisting of Fumasep FAP-375-PP (a polypropylene Fluorinated anion-exchange membrane - thickness 70 - 80 pm made by FUMATECH BWT GmbH), Fumasep FAP-450 a nonreinforced Fluorinated Anion Exchange Membrane (AEM) made by FUMATECH BWT GmbH) and Fumasep FAB-PK-130 (a polyketone reinforced Anion Exchange Membrane (AEM) made by FUMATECH BWT GmbH).

Preferably there is a further step after step 5 where the spent solution is processed to recover hydrogen chloride or hydrochloric acid.

In a further aspect there is provided a process to extract a metal and recover an acid including the following steps in order:

1. passing the liquid containing the dissolved metals to an electrolysis stage where electrochemical oxidation of Fe ²⁺ to Fe ³⁺ occurs at an anode;

2. precipitation of Fe ³⁺ as iron (III) oxides/hydroxides through the addition of an alkali;

3. passing the iron depleted liquor into an electrochemical cell wherein the desired non- iron metals are deposited in a metallic form onto the cathode and, wherein the iron oxidation and the other metal deposition reactions occur in the same reactor; and optionally

4. the iron oxidation and non-iron metal deposition can be conducted in separate reactors and coupled with hydrogen and chlorine or hydrogen and oxygen evolution reactions respectively, herein hydrogen and chlorine gas can then be reacted together to form hydrogen chloride and absorbed into water to produce hydrochloric acid, or hydrogen and chlorine ions are combined electrochemically in an at least three chamber cell configuration.

In a still further aspect there is provided an apparatus to extract zinc from a feed solution based on the method as substantially described above.

Advantages of the above include a process that utilises anion exchange membranes which prevents the undesirable transport of cation metals from the cathodic chamber. Even low concentrations of iron will interfere greatly with the deposition of many other transition metals at the cathode, particularly zinc. The process can selectively separate and recover the valuable zinc , copper, nickel or cobalt and iron from the spent acid. Furthermore, the only effluent from the process is a salt water (NaCI or KCI) solution which can be further neutralised and processed at minimal cost before being discharged as trade waste with minimal environmental impact and cost. Alternatively, this salt water can be processed in a conventional chlor-alkali process to regenerate the NaOH or KOH used in the process and produce further hydrochloric acid. The process is approximately 38% more energy efficient than prior art processes hence requiring less voltage to drive the electrochemical reactions. It can recover the zinc and iron from the spent acid at high purity which allows these to be sold rather than been landfilled.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the present invention is described in detail below with reference to the accompanying drawings, in which:

Figure 1 is a flowchart of the process;

Figure 2 is a schematic view of the system;

Figure 3 is a graph showing the absorbance at 950nm for solutions with various concentrations of Fe ^2+; and

Figure 4 is a schematic view of an electrochemical cell with Fe oxidation and plateable metal plating occurring concurrently. DEFINITONS

About or Approximately and grammatical variations thereof: means within +/- 20% for a value or within 10° for an angle.

Substantially or grammatical variations thereof: means within +/- 10% of a value or within 5° of an angle.

HER: refers to the hydrogen evolution reaction.

Transitional metal: is any of the set of metallic elements occupying a central block (Groups IVB-VIII, IB, and I IB, or 4-12) in the periodic table, e.g. iron, manganese, chromium, and copper.

Heavy metal: is a metal that even in low concentrations are toxic and that have a density of above 5 g cm ³ when in their metallic form. wt % or grammatical variations thereof refers to weight percent of the chemical of interest in the solution.

Molar ratio where a molar ratio of ccp is given as A:B then A and B are numbers, and A is the number of moles of species a and p is the number of moles of species B, for example a molar ratio of NaOH:Fe of 2.5:1 would have 2.5 moles of NaOH for every 1 mole of Fe.

Spent Acid: This term is meant to include any used acid solution from industry that contains dissolved metal the acid may have been used to strip or pickle metal prior to downstream processing such as painting, plating, galvanising, other surface treatment, etc. or for inspection (e.g. aircraft components, machine components). For example it is common to pickle in hydrochloric or sulphuric acid in the galvanising industry,

Electric Arc Furnace Dust: a solid waste generated in the collection of particulate material during the steelmaking process in electric arc furnace

DETAILED DESCRIPTION

As noted above, described herein is a process to extract dissolved metals and optionally recover hydrochloric acid and an apparatus thereof. More specifically, a method for the recovery or treatment of an acidic liquid that contains dissolved iron, and other dissolved metals that utilises electrochemical oxidation followed by precipitation to reduce the iron concentration within this liquid followed by (or concurrent with) electrochemical reduction to extract remaining metals. These processes can be conducted within a single reactor or split across multiple reactors. When split across multiple reactors, hydrogen and chlorine can generated and then reacted and dissolved to form hydrochloric acid. It should be appreciated by those skilled in the art that this invention may be primarily utilised to recycle a zinc and iron contaminated spent acid produced by the galvanized steel industry or a solution prepared specifically for the recovery of metals. However, this should not be seen as limiting as conceivably this invention may also be applied to the treatment of other metal contaminated liquid wastes and metal extraction processes that contain both iron and another transitional metal.

Referring to Figure 1 a preferred method of processing a spent acid (this could be hydrochloric acid or sulphuric acid both are commonly used) solution, a feed solution (1), to produce Zn with an optional acid recovery step is shown as a flowchart where:

Step 1 . Is determine the Fe ²⁺ concentration in a feed solution (1) and proceed to step 2;

Step 2. Is electrochemically oxidise the Fe ²⁺ to Fe ³⁺ in the feed solution (1) to form a primary solution (3) with a concentration of Fe ²⁺ below a predetermined value X, once the concentration of Fe ²⁺ is below the predetermined value X, then carry out step 3;

Step 3. Is an iron precipitation and removal step used to create a secondary solution (5) which is low in Fe prior to step 4;

Step 4. Is the determination of the Zn ²⁺ in the secondary solution (5) prior to step 5;

Step 5. Is the Zn ²⁺ to Zn electrochemical reduction step that creates Zn and a spent solution (7); and

Step 6. Is an optional step that processes the spent solution (7) to produce HCI (where the initial spent acid is hydrochloric acid).

Referring to Figure 2 this method is carried out in a processing system (10) which includes at least one cathode chamber (12) which is separated from, but in fluid connection with, at least one anode chamber (14) by a permeable separator (16). The feed solution (1) is processed in one anode chamber (14) and the secondary solution (5) is processed in one cathode chamber (12), with the permeable separator (16) allowing charge to transfer whilst minimising the migration of the Fe ions to the cathode chamber (12) as this results in contamination of the recovered zinc.

The processing system (10) further includes a precipitation unit (18) and a separation unit (20) which are used to carry out step 3 (iron precipitation and removal step), in some cases the precipitation unit (18) and the separation unit (20) are the same physical piece of equipment.

Looking at the method in more detail, and referring to Fig. 1 or Fig. 2 as required, and linking each step to the processing system (10) we note that:

In Step 1 the concentration of Fe ²⁺ in the feed solution (1) is determined to allow the charge transfer required to achieve the target Fe ²⁺ concentration (X) to be calculated. The target concentration X is no higher than 15g/L of Fe ²⁺ and preferably less than 5 g/L of Fe ²⁺ . The concentration of Fe ²⁺ in the feed solution can be determined by a variety of methods including titration and UV/Visual Spectroscopy. It has been found that at 950nm there is an essentially linear relationship between mass per litre and absorbance at this wavelength over the 0 to 60 gram per litre (g/L) range (see Fig. 3) and this is essentially maintained until the concentration exceeds 150 g/L. As some feed solutions (1) can exceed this value it may be necessary to include two optical sensors each with a different optical path lengths through the feed solution (1) to allow these higher Fe ²⁺ concentration feed solutions (1) to be measured. The latter allows inline or essentially real time measurement of the Fe ²⁺ concentration in a solution (1 ,3,5).

In Step 2 the concentration of Fe ²⁺ determined in step 1 is used to calculate the charge required to lower the Fe ²⁺ concentration to below 15 g/L, preferably below 5 g/L. Once the charge required is determined the time to process the feed solution (1) at a predetermined current density for step 2 can be determined. Then the Fe ²⁺ in the feed solution (1) in the anode chamber (14) is electrochemically oxidised to Fe ³⁺ . As the Fe ²⁺ in the anode chamber (14) is oxidised the feed solution (1) is converted to a primary solution (3), where the primary solution (3) has a predetermined concentration (X) of Fe ²⁺ remaining, the rest has been oxidised to Fe ³⁺ . The concentration of Fe ²⁺ in the anode chamber (14) can be determined in line by:

(i) the 950nm absorbance of the contents are measured and the current Fe ²⁺ concentration of the anode chamber (14) contents determined;

(ii) the Oxidation Reduction Potential (ORP) can be determined by a standard probe and this used to determine the fraction of Fe ²⁺ oxidised to Fe ³⁺ . Care needs to be taken with this measurement technique the ORP increases rapidly near the endpoint and also rapidly increases if chlorine generation commences;

(iii) alternative known methods or titration.

Method (ii) detects a step change once essentially all (less than 15 g/L) of the Fe ²⁺ has been oxidised. The preferred measurement is obtained by measuring the absorbance at 950nm. Referring to Figure 2 the anode chamber (14) is shown with a test loop (21) including a sensor (22), the contents of the anode chamber (14) can be circulated through this anode test loop (21) where the sensor measures the ORP (Oxidation Reduction Potential) or 950nm absorbance in real time. For high concentration feed solutions (1) a second sub-branch with a shorter optical path length could be present to allow absorbance to be measured. Once the ORP end point has been reached, or the concentration is determined to be below X then the primary solution (3) is transferred to the precipitation unit (18) and step 3 undertaken;

In step 3 an alkali, for example KOH or NaOH, is added to first neutralise any excess acid, (in this case raise the pH to between 1 and 2, preferably between 1 and 1.5) then precipitate the iron, the addition of the alkali precipitates the Fe ³⁺ and a proportion of the Fe ²⁺ . As the primary solution (3) has a low pH, normally below 2, the pH can be used to determine how complete the reaction is. Unfortunately, the pH quickly plateaus as the alkali is added, and though it could be used to determine a precipitation endpoint it is easy to overshoot and precipitate the Zn ²⁺ at a high pH. This said careful pH control can be used. Preferably the iron concentration in the primary solution is determined and, for NaOH (Sodium Hydroxide), a molar ratio of NaOH:Fe between 2.1 :1 and 3.5:1 is used to determine the NaOH addition required after excess acid is neutralised. It may be possible to use absorbance to determine the Fe ³⁺ or potentially monitor the Fe ²⁺ 950nm absorbance as the precipitation of Fe ³⁺ appears to reduce the Fe ²⁺ concentration however these are untested. If the NaOH:Fe ratio method is used then the process is likely to be a batch process with the required amount of NaOH (or other alkali) added and the mixture stirred to increase the rate of reaction. Once the reaction is complete the precipitate/ suspended solids are removed, this may be by simply allowing the precipitate to settle or by processing with a separation unit which can be a filter press, a hydro cyclone, a centrifuge, any other suitable solid liquid separation device or a combination of these. The precipitation unit (18 and separation unit (20) can as indicated be the same piece of equipment or separate units. The resultant low iron (below 15 g/L and preferably below 5 g/L, normally confirmed by absorbance at 950nm) solution is the secondary solution (5) which then undergoes step 4.

In step 4 the Zn ²⁺ concentration of the secondary solution (5) is determined, for example by taking a sample of the secondary solution (5) for analysis by known means (for example Inductively Coupled Plasma mass spectrometry, Microwave- pl as ma Atomic Emission Spectroscopy, flame atomic adsorption spectroscopy or a known titration method). The Zn ²⁺ concentration in the secondary solution (5) is used to determine the charge required to plate out the zinc present in the volume of secondary solution (5) transferred to the cathode chamber (12) for the electrochemical reduction undertaken in step 5.

In step 5 the secondary solution (5) undergoes electrochemical reduction to plate out the zinc present in the volume of secondary solution (5) within the cathode chamber (12). Once the predetermined amount of Zn ²⁺ has been reduced to Zn it leaves a spent solution (7) which is high in NaCI and this can optionally proceed to step 6.

In optional step 6 the spent solution is processed, for example it could be used for the Chor- Alkali process to produce NaOH and HOI, the NaOH used in step 3 and the HOI returned to make up more pickle or more stripping acid.

As it is preferred to have the anode chamber (14) and cathode chamber (12) forming an operational electrochemical cell from the start, the cathode chamber (12) is initially filled with a ZnCh solution which is a substitute secondary solution (5) until this becomes available.

The operating voltage or overpotential for optimum electrical efficiency is expected to range from 1.75 to 3.5 volts. More likely the cell will operate at between 2-3 volts. In this way, a large current density can be achieved at high current efficiencies. It should be noted that the tradeoff for increasing cell voltage is a reduction in electrical efficiency.

The permeable separator (16) is an ion exchange membrane, and it would be normal to use a cation exchange membrane as this uses protons (H ⁺ ) as the charge carrier, unfortunately this has been found to create two problems:

1. Due to the high overpotential required for the evolution of hydrogen on the surface of zinc, zinc deposition will occur preferentially unless the solution is very acidic (< pH = 0.2). If a cation exchange membrane is used, protons are the primary charge carrier across the membrane (from anode to cathode), and during operation the cathodic chamber becomes increasingly acidic until it reaches the point that significant hydrogen evolution starts to occur. When using an anion exchange membrane as the permeable separator (16) the primary charge carriers are chloride ions transferred from the cathode chamber (12) to the anode chamber (14) which removes this undesirable side effect.

2. To conduct the electrochemical oxidation of Fe ²⁺ and zinc deposition in the same electrochemical cell (25) (see Fig. 2) requires effective separation of the anode chamber (14) and cathode chamber (12) by the permeable separator (16) as shown. This permeable separator (16) should limit the transport of iron from the anode chamber (14) to the cathode chamber (12), without imposing a high ohmic resistance to the cell (25). Iron transport across the permeable membrane (16) will contaminate the metal deposit at the cathode. Preventing this is essential when the zinc deposition reaction is been undertaken.

Surprisingly it has been found that using an anion exchange membrane as the permeable separator (16) reduces the Fe ²⁺ transport from the anode chamber (14) to the cathode chamber (12) giving a plated zinc purity of over 95%, and in most cases over 99%.

A series of tests were carried out using a variety of permeable separators (16) that were cation exchange membranes and anion exchange membranes and the results are shown in table 1 .

TABLE 1.

Using an anion exchange membrane as the permeable separator between fluidly linked anode chambers (14) and cathode chambers (12) forming an electrochemical cell reduces or eliminates the transfer of Fe to the cathode improving the purity of the Zn obtained and avoiding excess hydrogen generation.

The membranes identified above are commercial products identified by trade marks, please refer to Table 2 for the membrane, who owns the trade mark, and the type of membrane it is.

TABLE 2.

Dissolved metals are often recovered from solution by using electrochemical reduction. Pure metals are never recovered using electrochemical oxidation. Thus, it is counterintuitive that to efficiently recover some metals such as zinc from liquids containing iron, the liquid must be first electrochemically oxidised. It should be noted that the prior art does not teach or suggest a process to extract zinc from waste galvanising acid by firstly using electrochemical oxidation prior to the deposition step. Furthermore, the prior art also does not suggest a process wherein coupled iron oxidation and non-iron metal deposition reactions occur in the same reactor. Prior art also does not suggest a process split into two reactors wherein the iron oxidation is couple with hydrogen evolution and zinc deposition is coupled with chlorine or oxygen evolution. A significant issue that has been overcome is utilising an anion exchange membrane to prevent iron being transported across the membrane into the cathode chamber.

As Fe ²⁺ is soluble over a large pH range, this may co-precipitate with zinc from solution during traditional treatment processes. The inventors have found that electrochemically oxidising Fe ²⁺ to Fe ³⁺ followed by alkali precipitation results in superior removal of iron whilst leaving the Zinc in solution.

The above method has been found to be effective for separating zinc and iron from the waste hydrochloric acid used to clean steel prior to galvanizing. This has been achieved by combining an unconventional electrochemical process with a selective precipitation process.

Further, where specific integers are mentioned herein which have known equivalents in the art to which the embodiments relate, such known equivalents are deemed to be incorporated herein as if individually set forth.

Though an electrochemical cell consisting of a single anode chamber (14) and a single cathode chamber (12) are described multiple chambers (12,14) can be used, with partial oxidation or reduction of the chamber (12,14) contents occurring in each of them. For example, if on testing the secondary solution was found to be high in Fe ²⁺ it could be sent to an additional anode chamber (14) for additional oxidation to Fe ³⁺ prior to returning to the precipitation unit (18).

Though described with specific reference to acidic zinc/iron solutions the method is expected to work with the following metals in an acid (hydrochloric acid and/or sulphuric acid) solution. and iron: In this case the insoluble hydroxides/oxides of copper are formed at similar pH values to zinc hydroxide/oxides and as such the precipitation of Fe (III) hydroxide/oxides occurs without reducing the copper concentration significantly. The Fe2+ determination by absorbance at 950nm cannot be used to confirm Fe ²⁺ is below 15 g/L in hydrochloric acid solutions due to the interference by chloro- complexes of copper. The resultant electrochemical cell will have a different overvoltage but this is easy to determine and compensate for as such it is expected to behave similarly to the Zn/Fe cell.

Cobalt and Iron: In this case the insoluble hydroxides/oxides of cobalt (II) are formed at similar pH values to zinc hydroxide/oxides and as such the precipitation of Fe (III) hydroxide/oxides occurs without reducing the cobalt concentration significantly. Cobalt (III) hydroxides/oxides precipitate at slightly lower pH than Fe (III). Co(ll) ions can be electrochemically oxidised to Co(lll) at potentials higher than that required to oxidise Fe(ll) so care is needed to avoid excessively high anode potentials to prevent cobalt removal as Co(lll) oxides/hydroxides. Cobalt does not interfere with the Fe ²⁺ determination by absorbance at 950nm.

Nickel and Iron: Nickel (II) hydroxides/oxide form at pH values similar to zinc hydroxides/oxides. Ni(lll) also forms hydroxides/oxides at pH > 4 provided that the Oxidation- Reduction Potential of the solution is kept below 1 V vs Standard Hydrogen Electrode. Some nickel complexes adsorb at around 800nm so the Fe ²⁺ determination by absorbance at 950nm may require more care, but it is expected to work.

EXAMPLES

Source of Feed Solution

The feed solution (1) referred to below was sourced from galvanisers located in Christchurch, New Zealand. Some of initial properties of the feed solution (1) that were observed/measured was a pH below zero, often below -0.5, it was a dark green, and it contained >300g/L of total dissolved solids. The feed solution (1) was based on hydrochloric acid.

The iron and zinc concentrations in the feed solution from this source are given in Table 3.

Table 3.

EXAMPLE 1

Electrochemical oxidation of Fe ²⁺ in Feed Solution (1)

To facilitate the separation of iron and zinc from the feed solution (1), Fe ³⁺ is selectively precipitated from the feed solution (1). As the raw feed solution contains Fe ²⁺ , this must be oxidised to Fe ³⁺ prior to the precipitation step. This can be efficiently achieved both oxidising the Fe ²⁺ to Fe ³⁺ at an anode in an electrochemical cell.

In one configuration, the iron (II) oxidation and zinc electrode deposition were performed in the same reactor (Figure 2). When operating this cell with a cation exchange membrane, protons and Fe ions are transported from the anode to the cathode chamber. When operating this cell with an anion exchange membrane, chloride ions are transported from the cathode to the anode chamber. The anion exchange membrane configuration is surprisingly preferred as Fe in the cathode chamber contaminates the zinc deposit and promotes the hydrogen evolution reaction at the cathode.

Membrane comparison and impact on ion-cross over

It has been found that even at low concentrations iron can interfere greatly with zinc deposition. In the single cell electrochemical cell (25) (Figure 2), the choice of membrane strongly influences the transport of iron form the anode chamber (14) (which contains feed solution (1) and thus high concentrations of iron) to the cathode where zinc is recovered by electrochemical deposition.

An example is given wherein the cathode was a graphite sheet (area=6cm ² ) and the catholyte was initially an iron-depleted feed solution (1) (100 g/L zinc at pH=3.5) (the secondary solution (5)). The anolyte was 50 mL of spent acid (100g/L Zn, 80 g/L Fe, pH<-0.5) and was oxidised at a graphite felt anode (3mm thick, 6 cm2 in area). The anode to cathode separation was 10mm and the electrodes were separated by a Nation 115 membrane (area=6cm ² ). UV/Vis measurements on the catholyte were performed while a current of -1.2 A was applied to the cathode. Oxidation of the iron (II) in the anode simultaneously occurred with the zinc deposition at the cathode (see Fig. 4).

Precipitation of oxides from primary solutions

Currently, most feed solution (1) sourced from the galvanising industry is disposed I treated by directly neutralising the feed solution (1) with NaOH or CaCOs until all of the dissolved metals have precipitated from solution. A key step of the present invention is that electrochemically oxidised feed solution (1) (the primary solution (3)) can be partially neutralised to precipitate the Fe ³⁺ (it also appears to remove a proportion of the Fe ²⁺ ) from the primary solution (3) without also precipitating zinc from this solution. In comparison, if a feed solution (1) is directly partially or completely neutralised, iron and zinc oxides or hydroxides precipitate together. Selectively removal of iron from the feed solution (1) is critical to enable the later step of electrodeposition of metallic zinc. Thus neutralization I precipitation experiments were therefore performed on the feed solution (1) and the primary solution (3).

Small additions of 50 wt% NaOH to the feed solution (1 ) cause the pH to rise quickly to approx. 5. This results in simultaneous precipitation of iron and zinc oxides or hydroxides from solution. Once all the metals have been precipitated, pH rapidly rises to 12.5 and zinc in the precipitate begins to re-dissolve back into the liquor. This shows that the separation of iron and zinc (by precipitation) cannot be achieved without the use of large quantities of NaOH.

This is a different case for primary solutions (3) where the Fe ²⁺ has been electrochemically oxidised to Fe ³⁺ . In this case, addition of NaOH does not immediately change the pH to 5, but instead it stays at approx. 2 while the Fe concentration drops continuously. This difference is caused by the differing solubility of Fe ²⁺ vs Fe ³⁺ over these pH ranges. Unlike the case of feed solution (1), the Zn concentration in the primary solution (3) remains near constant. This behaviour occurs until essentially all of the iron is precipitated at approximately 30 vol% NaOH (corresponding to pH ~ 2). After this point the pH rapidly rises precipitating the zinc oxides or hydroxides which do not appear to readily redissolve.

This clearly demonstrates that by oxidising Fe ²⁺ to Fe ³⁺ first, an essentially iron-free zinc solution can be separated by selective precipitation, though the NaOH addition needs to be carefully controlled as described elsewhere herein to avoid an overshot.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope of the description herein.

Preparation of Feed Solution from other Sources

Providing the feed solution (1) contains dissolved metals including iron then the method can be used, for example:

1. Electric arc furnace dust: Electric Arc Furnace (EAF) dust is high in iron and contains amounts of other metals. Table 4. Has a typical metal composition for an electric arc furnace dust in wt%.

Table 4.

As can be seen this EAF dust is high in zinc, iron and calcium and it is known to dissolve this in hydrochloric acid, normally hot, to produce an acid solution. This acid solution of EAF dust can be used as the feed solution (1).

2. Electronic waste: Electronic waste contains zinc, copper, iron, tin, lead and nickel. It is possible to dissolve a proportion of these in hydrochloric and/or sulphuric acid, though aqua regia (nitric acid combined with hydrochloric acid) is often used as this also takes the precious metals (gold/silver) into solution. If aqua regia is used then it may be necessary to pretreat this acidic solution to form the feed solution (1).