WO2020118455A1

WO2020118455A1 - Process for the recovery of zinc and associated value metals from various materials

Info

Publication number: WO2020118455A1
Application number: PCT/CA2019/051819
Authority: WO
Inventors: Bryn Harris; Carl White
Original assignee: 9203-5468 Quebec Inc. Dba Nmr360
Priority date: 2018-12-14
Filing date: 2019-12-16
Publication date: 2020-06-18

Abstract

It is disclosed a chemical process for recovery of silver, copper, lead, cadmium, zinc and iron from a zinc and iron bearing feed material including electric arc furnace dust and spent pickle liquor. In the process, a feed solution prepared from the feed material is initially subjected to a series of sequential steps of silver, copper, and lead/cadmium precipitations by addition of metallic copper, iron and zinc to filtrates obtained from each step. A filtrate obtained from the zinc/cadmium precipitation comprises ferrous chloride and zinc chloride. Iron and zinc are recovered from the filtrate by two alternative paths. A first path employs iron oxidation/hydrolysis and zinc hydroxide precipitation to obtain ferric oxide and zinc hydroxide from the filtrate. The second path employs solvent extraction method to form zinc chloride and ferric chloride with quaternary ammine extracts from the filtrate.

Description

PROCESS FOR THE RECOVERY OF ZINC AND ASSOCIATED VALUE METALS

FROM VARIOUS MATERIALS

Cross-Reference to Related Applications

[0001] The present patent application claims the benefits of priority of U.S. Provisional Patent Application No. 62/779,567 entitled“PROCESS FOR THE RECOVERY OF ZINC AND ASSOCIATED VALUE METALS FROM VARIOUS MATERIALS”, and filed at the United States Patent and Trademark Office on December 14, 2018, the content of which is incorporated herein by reference.

Field of the Invention

[0002] The present invention relates generally to chemical processes for the recovery of zinc, iron and associated value metals, such as but not limited to, silver, copper, lead and cadmium, in a marketable form from various feedstocks, and particularly from electric arc furnace dust (EAFD) and SPL/ZPL (steel plant spent pickle and zinc pickle liquors).

Background of the Invention

[0003] A number of pyrometallurgical and hydrometallurgical techniques have been developed for recovering zinc and associated metals such as silver, copper, lead, cadmium, and iron from various primary and intermediate zinc-bearing sources, especially from steel plant EAFD (Electric Arc Furnace Dust). One such hydrometallurgical technique involves leaching the ore with a lixiviant that promotes dissolution of one or more metals into the leaching solution. Various compounds have been used individually as leaching agents in the lixiviant, for instance, sulphuric acid, hydrochloric acid, ferric chloride, ferric sulphate, cupric chloride, ammonium chloride, sodium hydroxide and magnesium chloride. Recently, there has been renewed interest in the area of chloride-based leaching processes.

[0004] The most common technique currently practiced is pyrometallurgical, wherein the dust is smelted in a Waelz Kiln to fume off an impure zinc oxide. This zinc oxide invariably contains the other volatile materials in the EAFD, notably lead, cadmium and halogens. All of these are detrimental to directly processing the zinc oxide in an electrolytic zinc refinery, such that secondary processing of the zinc oxide is usually required.

[0005] Further drawbacks of the Waelz Kiln Process are that zinc recoveries are generally in the range of 70%, and that a toxic slag is produced which requires a secure landfill. Occasionally, when the impurity content is low, this slag can be suitable for use as road aggregate. There is no commercial process currently in operation for the re-processing of the slag.

[0006] Numerous hy drometallurgical processes have also been attempted, but the problem with such processes has been their inability to generate an iron-product suitable either for sale or for return to the steel plant. R.O. McElroy in U.S. Patent no. 5,336,297, issued to Terra Gaia on August 9, 1994, and in a concurrent publication, R.O. McElroy and M. McClaren, entitled “Processing of Electric Arc Furnace Dust Via Chloride Hydrometallurgy”, published in Hydrometallurgy’94, Proceedings of an International Conference by IMM and SCI, Chapman and Hall publishers, July 1994, pp. 993-1010, describes a process wherein EAFD is leached in ferric chloride solution, and the iron precipitated as hematite in an autoclave. Zinc is recovered from the iron-free filtrate by solvent extraction with a solvating extractant, wherein the zinc is stripped with a dilute sodium chloride solution and subsequently recovered by electrowinning. Lead is recovered by cooling crystallisation, which is possible in this flowsheet, since the overall chloride concentration is kept below 5M, mitigating against the formation of complex lead-chloro anions. No methodology for the recovery of any silver, copper or cadmium is provided, nor is it mentioned how calcium, magnesium and manganese, some or all of which are often present in most zinc feedstocks, are dealt with, other than that these may be used as background to increase the overall chloride concentration.

[0007] Ho In Lee et al, in U.S. Patent no. 6,338,748, describe a hy drometallurgical process for the recovery of zinc metal from electric arc furnace dust. In this process, in order to selectively leach zinc and minimize iron dissolution by simultaneously precipitating it as FeOOH (goethite) and Fe203 (hematite), hot acid leaching is carried out with the aqueous solution containing 37-74 g/L HC1 (i.e. 1-2M) and 104-270 g/L ZnCk. The leach solution is purified by zinc oxide addition and then with activated carbon prior to electrowinning of zinc. The purpose of adding the zinc oxide is to raise the pH of the solution in order that residual iron and other impurities will be selectively precipitated.

[0008] This process wherein the dust itself is used to neutralise the iron must necessarily result in a low recovery of zinc, since if the residue were to be re-leached, there would be no outlet for the iron. Such a process also would not allow the effective recovery of minor metals such as silver and copper.

[0009] Further purification is proposed in the process by the addition of zinc powder to recover lead, copper and cadmium. This must be only as a polishing step, since most of these metals would have been removed either during iron precipitation or by activated carbon. Certainly, it is not a primary recovery step.

[0010] W.P.C. Duyvesteyn andM.C. Jha, in US Patent 4,572,771, describe a process wherein a zinc-bearing residue, and especially EAF dust, is leached with hydrochloric acid to provide a pregnant liquor containing zinc, iron, and lead. The iron and other impurities such as lead are selectively removed from the pregnant liquor by precipitation with lime, and the zinc extracted from the purified liquor by solvent extraction. The zinc is subsequently stripped from the organic solvent using water or hydrochloric acid to provide a zinc chloride solution which is then fed to an electrowinning cell for the production of marketable electrolytic zinc. Chlorine gas from zinc electrowinning is burned with hydrogen to reconstitute the hydrochloric acid for recycle to the leaching stage.

[0011] C. Allen et al. in U.S. Patent no. 6,395,242 Bl, issued on May 28, 2002, describe a process for the recoveiy of zinc oxide from complex sulphide ores and concentrates using chloride processing. In this process, the feed material is leached with hydrochloric acid and oxygen, the iron is precipitated oxidatively with magnesium oxide, and the silver, copper, lead, cadmium and cobalt are recovered as a single mixed product using zinc dust, and finally zinc oxide precipitated by magnesium oxide. The resultant magnesium chloride solution undergoes pyrohydrolysis to regenerate the magnesium oxide and recycle the hydrochloric acid. This necessarily generates a relatively dilute acid (<18%), thus limiting the concentration of zinc that can be taken into solution, and having a serious negative impact on the water balance. Zinc chloride is very soluble (1800 g/L or 27M at 20°C), such that 18% acid (~6M) is dilute in comparison, approximately half of the concentration that could be achieved, and therefore does not take advantage of the high solubility of zinc.

[0012] Using zinc dust to precipitate all of the silver, copper, lead, cadmium and cobalt is expensive, since zinc dust is a premium reagent. Normally, there are other impurities in such complex feeds as aluminium, calcium and manganese. The process described by Allen et al. does not address these metals, all of which will report to the final magnesium chloride liquor. Calcium chloride, in particular, does not decompose during pyrohydrolysis, and will therefore both debase the magnesium oxide product, as will aluminium and manganese, as well as incurring significant chloride losses, and nor will it allow any bleed for calcium. [0013] Spent zinc pickle liquor (ZPL) is the waste product containing zinc and ferrous chlorides from steel plant galvanizing pickling lines. At the present time, there is no method for treating this, and it is commonly disposed of by deep well injection.

[0014] Steel plant spent pickle liquor (SPL) is waste material containing predominantly ferrous chloride. Current methodology for treating this is via spray or fluid bed roasting, which is costly and is only able recover acid at a maximum strength of 18%, rather than concentrated acid at 33-35% strength.

[0015] In light of the foregoing, it would be advantageous to be able to not only separate, but also to selectively recover all the valuable components found in EAFD and similar feeds, such as, but not limited to, silver, copper, lead, cadmium and magnesium in a generally useful and pure form, together with concentrated (33-36% HC1) hydrochloric acid for recycle within the flowsheet.

[0016] Furthermore, it would be additionally advantageous to combine such a process with treating spent galvanizing zinc pickle liquor, for which there is no existing method of recovering the contained zinc, and furthermore to combine this with treating this spent steel plant pickling liquor.

Summary of the Invention

[0017] The shortcomings of the prior art are generally mitigated by the process has described herein, for treating zinc-bearing intermediates or residues, in particular from steel plant toxic wastes such as electric arc furnace dust (EAFD) and spent pickle liquors.

[0018] According to a first aspect, it is disclosed a process for recovering zinc, iron and optionally associated value metals comprising at least one of silver, copper, lead or cadmium, from a feed material comprising zinc, lead and silver. The process comprising:

i. providing a feed liquor from the feed material;

ii. treating the feed liquor with metallic copper to precipitate metallic silver, and filtering off the metallic silver to form a first filtrate;

iii. treating the first filtrate from silver recovery with metallic iron to precipitate the copper, and filtering off the copper to form a second filtrate; iv. treating the second filtrate from copper recovery with metallic zinc to recover lead and any cadmium if present, and filtering off the lead-cadmium metal to form a third filtrate;

v. oxidising the third filtrate from lead recovery in presence of hydrochloric acid (HC1) to convert ferrous iron to ferric iron and form as such a ferric solution;

vi. hydrolysing the ferric solution to cause precipitation of iron oxide, and aluminum oxide if present, and recover hydrochloric acid (HC1);

vii. treating a bleed from hydrolysis to recover zinc hydroxide and form a fourth filtrate; viii. treating the fourth filtrate from zinc precipitation via pyrohydrolysis to recover hydrochloric acid (HC1) and magnesium oxide or magnesia (MgO); and ix. optionally, recycling at least in part said HC1 and/or magnesia.

[0019] According to a second aspect, the process for recovering zinc, iron and optionally associated value metals comprising at least one of silver, copper, lead or cadmium, from a feed material comprising zinc, lead and silver, comprises:

i. providing a feed liquor from the feed material;

iii. treating the first filtrate from silver recovery with metallic iron to precipitate the copper, and filtering off the copper to form a second filtrate;

iv. treating the second filtrate from copper recovery with metallic zinc to recover lead and any cadmium if present, and filtering off the lead-cadmium metal to form a third filtrate;

v. reacting the third filtrate from lead-cadmium recovery step iv) with a quaternary amine to form an extractant comprising a zinc-amine complex and a raffinate comprising ferrous chloride;

vi. stripping the extractant with water to generate a zinc chloride solution; and vii. oxidising the raffinate to form ferric chloride solution; or

viii. alternatively, pyrohydrolysing the raffinate to form iron oxide or hematite (Fe203) and recovering the hydrochloric acid. [0020] The process as described herein, allows treating zinc-bearing intermediates or residues, in particular from steel plant toxic wastes such as electric arc furnace dust (EAFD) and spent pickle liquors. Such materials may contain, in addition to iron and zinc, silver, copper, lead, cadmium and magnesium, all of which can be recovered in a marketable form, thereby maximising the value of the feed material. As discussed in the Background section, no such process for achieving recovery of all of these metals currently exists.

[0021] According to a preferred embodiment, the feed material may be combined with other steel plant wastes, in particular spent zinc pickle liquor and/or spent pickle liquor. It is a further aspect of the invention that in combination, EAFD and ZPL, or EAFD and ZPL/SPL significantly improves the water balance of the process.

[0022] It is an aspect of the invention to provide a single overall process which permits the recovery of iron, zinc and any other metals in a form of economic value, and to generate within the process its own reagents for internal recycle.

[0023] In a particular embodiment, the process combines EAFD and ZPL more efficiently than treating either on their own. In a further embodiment, also including SPL offers a much improved and efficient method for recovering a useful iron product and concentrated hydrochloric acid, and improves the overall water balance of the circuit

[0024] Other and further aspects and advantages of the present invention will be better understood upon the reading of the illustrative embodiments about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

Brief Description of the Drawings

[0025] The above and other aspects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which:

[0026] Figure 1 is a flowchart illustrating the process according to a preferred embodiment;

[0027] Figure 2 is a flowchart illustrating the process according to another preferred embodiment; and

[0028] Figure 3 is a flowchart illustrating the process according to yet another preferred embodiment. Detailed Description of the Preferred Embodiment

[0029] A novel process will be described hereinafter. Although the invention is described in terms of specific illustrative embodiments, it is to be understood that the embodiments described herein are by way of example only and that the scope of the invention is not intended to be limited thereby.

[0030] The terminology used herein is in accordance with definitions set out below.

[0031] As used herein % or wt.% means weight % unless otherwise indicated. When used herein % refers to weight % as compared to the total weight percent of the phase or composition that is being discussed.

[0032] By "about", it is meant that the value of weight % (wt.%), time, pH, volume or temperature can vary within a certain range depending on the margin of error of the method or device used to evaluate such weight %, time, pH, volume or temperature. A margin of error of 10% is generally accepted.

[0033] The description which follows, and the embodiments described therein are provided by way of illustration of an example of particular embodiments of principles and aspects of the present invention. These examples are provided for the purposes of explanation and not of limitation, of those principles of the invention. In the description that follows, like parts and/or steps are marked throughout the specification and the drawing with the same respective reference numerals.

[0034] A process for recovering zinc, iron and optionally associated value metals comprising at least one of silver, copper, lead or cadmium, from a feed material comprising zinc, lead and silver, is disclosed.

[0035] The process first comprises: (i) providing a feed liquor from the feed material. To do so, the process may further comprise before (i), the step of leaching the feed material with an acidic aqueous solution to obtain the feed liquor. The feed material is preferably derived from electric arc furnace dust, or from a complex lead-zinc-copper-silver ore or concentrate.

[0036] According to a preferred embodiment, the process may further comprise the step of adding a spent pickle liquor to the provided feed liquor. Preferably, the spent pickle liquor is a zinc pickle liquor (ZPL) to provide additional zinc, iron and a portion of the chloride to the feed material; a steel plant spent pickle liquor (SPL) to provide a portion of a chloride leaching medium; or a mixture thereof. [0037] The process also comprises: (ii) treating the feed liquor with metallic copper to precipitate metallic silver, and filtering off the metallic silver to form a first filtrate. Preferably, in step (ii), a free acid solution is present with a concentration of about 1 to 2 g/L HC1. More preferably, the pH of the free acid solution is reduced by the addition of recycled zinc hydroxide, or the addition of recycled magnesia. According to another preferred embodiment, in step (ii), a portion of copper is recycled to recover silver. The process according to the present invention therefore allows to recover silver as one value metal.

[0038] The process also comprises: (iii) treating the first filtrate from silver recovery with metallic iron to precipitate the copper, and filtering off the copper to form a second filtrate. The copper can be reused or recycled in the previous step of silver recovery.

[0039] The process further comprises: (iv) treating the second filtrate from copper recovery with metallic zinc to recover lead and any cadmium if present, and filtering off the lead- cadmium metal to form a third filtrate. Preferably, the residence time for the recovery of lead and cadmium is from about 1 to about 10 minutes, more preferably the residence time is about 5 minutes. According to another preferred embodiment, the step (iv) is carried out in the absence of air, and more preferably using a rotating inclined reactor having a bottom inlet and a top outlet for circulating the solution containing the second filtrate. The rotating inclined reactor may be then configured for having the second filtrate circulating from the bottom to the top of the rotating inclined reactor. According to another preferred embodiment, the reactor has a rotation speed from about 1 to about 20 rpm, more preferably from about 5 to about 10 rpm. In step (iv), the inclined reactor may have an inclination slope of from about 1° to about 20°, more preferably from about 5° to about 10°. The process according to the present invention therefore allows to recover lead and any cadmium if present, as value metals.

[0040] Now, the third filtrate obtained from the lead and cadmium recovery step (iv), can be processed according to two different paths to recover iron, aluminum if present, and zinc.

[0041] According to a preferred embodiment, the first path of the process comprises:

(v) oxidising the third filtrate from lead recovery in presence of hydrochloric acid (HC1) to convert ferrous iron to ferric iron and form as such a ferric solution;

(vi) hydrolysing the ferric solution to cause precipitation of iron oxide, and aluminum oxide if present, and recover hydrochloric acid (HC1);

(vii) treating a bleed from hydrolysis to recover zinc hydroxide and form a fourth filtrate; (viii) treating the fourth filtrate from zinc precipitation via pyrohydrolysis to recover hydrochloric acid (HC1) and magnesium oxide or magnesia (MgO); and

(ix) optionally, recycling at least in part said HC1 and/or magnesia.

[0042] Preferably, the step (vi) comprises raising a temperature up to about 170-200°C, more preferably up to about 180-190°C, in order to effect the hydrolysis of ferric iron and precipitation of hematite.

[0043] Preferably, in step vii), the bleed comprises predominantly zinc chloride and magnesium chloride, the hydrolysis step then comprising treating the bleed with magnesia to precipitate zinc hydroxide. The hydrolysis may be performed at a temperature of about ambient up to 100°C, more preferably from 60 to 65°C, and at a pH of about 6.0 to 8.5, more preferably of about 6.5 to 7.0 Typically, the precipitated zinc hydroxide is pure zinc hydroxide. Preferably, the process then further comprises the step of heating the pure zinc hydroxide at a temperature from 150 to 300°C, preferably from 200-250°C, to generate pure zinc oxide. Also, a solution may be obtained when the zinc hydroxide precipitates, this solution typically comprising pure magnesium chloride. The process then comprises in step (viii) pyrohydrolysing the pure magnesium chloride to recover pure magnesia (MgO), and hydrochloric acid, both of which being optionally recycled in the process circuit, wherein at least a portion of the hydrochloric acid may be recycled into the process, and at least a portion of the pure magnesia maybe recycled into the zinc precipitation step of the process.

[0044] According to another preferred embodiment, the second path of the process for recovering iron and zinc comprises, after step iv) disclosed above:

x. reacting the third filtrate from lead-cadmium recovery step with a quaternary amine to form an extractant comprising a zinc-amine complex and a raffinate comprising ferrous chloride;

xi. stripping the extractant with water to generate a zinc chloride solution; and xii. oxidising the raffinate to form ferric chloride solution; or alternatively

xiii. pyrohydrolysing the raffinate to form iron oxide or hematite (Fe2C>3) and recovering the hydrochloric acid.

[0045] Preferably, after oxidising in step xii), the process further comprises reacting the oxidised raffinate with a second quaternary amine, and stripping with water to generate a ferric chloride solution. [0046] Preferably, the process further comprises crystallising the raffinate from zinc extraction to recover ferrous chloride crystals.

[0047] The embodiments of the present invention shall be more clearly understood with reference to the following detailed description taken in conjunction with the accompanying drawings, Figures 1, 2 and 3.

[0048] In this particular description, the feed has been taken to be a leach solution derived from a zinc and iron-bearing feed containing recoverable amounts of zinc, iron, silver, copper, lead cadmium and magnesium.

[0049] Referring to Figure 1, there is shown a schematic representation of a process in accordance with one embodiment of the invention. Broadly speaking, the process involves the recovery of iron material, zinc material, a copper material, a silver material, a lead-cadmium material and a magnesium material, and where ZPL and SPL are included, of concentrated hydrochloric acid.

[0050] The feed solution 10 is derived from the processing a zinc and iron-bearing material, such as, but not limited to EAFD or a complex zinc-lead-silver-copper concentrate or ore. Typically, such solutions are very concentrated, this being an advantage of a chloride-based processes, with a total chloride concentration of 10-12M. It is understood that more dilute solutions may be processed, but the preferred concentration is the range 10-12M total chloride concentration. Optionally, this solution is combined with spent zinc pickle liquor or steel plant pickle liquor 11. This is an advantage of the process, especially if the plant producing the EAFD, for example, also generates the waste pickle liquor.

[0051] The spent pickle liquor and/or spent zinc pickle liquor 11 may be added as an additional source of zinc, iron and/or hydrochloric acid, and in particular, in place of any fresh water that might be needed to adjust the solids loading in the leach.

[0052] The solutions undergo pH adjustment 12 with recycled zinc hydroxide 13 from the zinc precipitation stage. Alternatively, recycled MgO (45, Figure 2) may be used. This stage is required in order to reduce the free acid content of the solution to <10 g/L HC1, and preferably to <1 g/L HC1 in order that the subsequent metathesis stage(s) may operate efficiently.

[0053] Control may be effected by a pH reading, in the range 0.5 to 2.0, but it should be understood that in concentrated chloride solutions, pH reading does not have its normal meaning, since it is significantly affected by the high activity of the proton caused by the high concentration of chloride ions. It is noted that even with calibration, probes from different manufacturers give different readings in such solutions.

[0054] The solution is treated in one, two or three sequential metathesis (otherwise known as double decomposition or exchange) stages 15, 21 and 27, depending upon the metals present in the solution. In the present example, three stages are employed. This is in direct contrast to the process used by Allen et al. referred to above, where a single stage with expensive zinc dust was used to effect a bulk removal all of these metals.

[0055] The first stage 15 uses recycled copper 16 to selectively recover metallic silver 19. The copper used is recycled from the next metathesis step 21. Alternatively, should there be a convenient source of scrap copper available, then this may be used as an effective way of re processing such copper. In this silver stage, any ferric iron present is also reduced to ferrous, and additional copper taken into solution. This is a novel aspect of the current process in that internal reagents are self-generating, in this case copper, and which allows for a very pure metallic silver product to be generated, in contrast, for example, to the process of Allen et al. described above.

[0056] The reaction may be effected in any convenient reactor, such as a stirred tank vessel.

[0057] The first metathesis slurry 17 proceeds to solid-liquid separation 18, which may be effected by any convenient means, such as, but not limited to, flocculation and thickening, filter press or vacuum belt filter. A small vacuum pan filter is the preferred equipment. The solids 19 are pure metallic silver, which may be washed and cast into bars.

[0058] The filtrate from the first stage proceeds to the second metathesis stage 21, which employs metallic iron 22, either in powder form, or as a metal plate. In this stage, the iron replaces the copper, and further reduces any remaining ferric iron.

[0059] The sponge copper metal slurry 23 proceeds to solid-liquid separation 24, which may be effected by any convenient means, such as, but not limited to, flocculation and thickening, filter press or vacuum belt filter. A small vacuum pan filter is the preferred equipment. The solids 25 are pure metallic copper, which may be washed and cast into bars.

[0060] A bleed 16 of the copper is recycled as a slurry to the silver metathesis reactor 15, thereby acting as its own reagent. The iron powder used may be conveniently sourced from a steel mill, especially the one that is the source of the original EAFD or pickle liquor, or alternatively, it may be generated within the process by direct reduction of the hematite 42 (Figure 2) generated within the process. [0061] In the third stage 27, the filtrate 26 from the second stage is treated with zinc powder 28 in order to effect the recovery of lead and cadmium 31. In this stage, it is essential to ensure that there is no air present in the reactor, since air promotes the rapid re-leaching of the cadmium in particular. In conjunction with ensuring that no air is present, it has also been determined that a very short residence time is essential, again to prevent the rapid re-dissolution cadmium in particular, but also the lead. Whilst this reaction is well-known and straightforward in sulphate media, such as an electrolytic zinc plant, it does not normally proceed in a chloride medium due to the fact that the cadmium, in particular, but also the lead, will immediately re dissolve. This is because of the high concentration of chloride making these metals veiy reactive.

[0062] Hence, in order to achieve complete removal of both lead and cadmium, ideally the residence time will be between one and ten minutes, preferably between one and five minutes. Advantageously, a specially-designed, slowly rotating, inclined reactor may be used. The slope of the reactor will be 5-30°, ideally 10-15°, and preferably 10°. Feed solution is introduced at the low end of the reactor and discharges at the top, whereas zinc powder is added at the top and the resultant lead-cadmium solids discharge at the bottom. The reactor rotates at between one and ten rpm, preferably at 5 rpm. The combination of slope and passage of the solution up the reactor may ensure that no air is present.

[0063] The sponge lead-cadmium metal slurry 29 proceeds to solid-liquid separation 30, which may be effected by any convenient means, such as, but not limited to, flocculation and thickening, filter press or vacuum belt filter. Preferably, a small vacuum pan filter may be used. The solids 31 are a mixture of metallic lead and metallic cadmium, which may be further processed for separate recovery of the two metals, or sold as-is to a lead smelter, for instance.

[0064] In operating the metathesis circuits in this novel manner, not only are individual metals recovered in high purity, namely silver and copper, but the consumption of expensive zinc dust is greatly reduced compared alternative processes, especially that described by Allen et al.

[0065] The chemical reactions of the metathesis processes are shown as follows in reactions (3) to (8). These reactions may be carried out at any temperature in the range 25-110°C, but optimally in the range 40-70°C, and most preferably in the range 50-60°C.

2AgCl + Cu 2Ag + CuCb (3)

2FeCl₃ + Cu 2FeCb + CuCh (4)

CuCh + Fe Cu + FeCb (5) 2FeCb + Fe 3FeCh (6)

PbCh + Zn Pb + ZnCb (7)

CdCh + Zn Cd + ZnCb (8)

[0066] The filtrate 32 from the third metathesis stage contains predominantly a mixture of ferrous and zinc chlorides, together with some magnesium chloride. Any calcium present is removed by the addition of sulphuric acid. Small amounts of aluminium may also be present, especially if EAFD was the original feed.

[0067] Turning now to Figure 2, it is apparent from reactions (4) and (6) that some or all of the iron is reduced to its ferrous state. Preferably, oxidation of the iron is effected in the filtrate 32 prior to its removal as hematite by hydrolysis and acid regeneration. It is one aspect of the current invention that both oxidation 34 and hydrolysis/acid recovery 38 will be accomplished using the methodology of a previous invention by the current inventors, as described in U.S. Patent no. 9,889,421 B2, Process For the Recovery of Metals and Hydrochloric Acid, granted February 13, 2018, herein incorporated by reference.

[0068] In this embodiment, the filtrate 32 from lead/cadmium recoveiy (Figure 1), which is primarily a mixture of zinc and ferrous chlorides at this point, is oxidised 34 by any method familiar to those skilled in the art, but specifically by the method of the above-referenced US Patent 9,889,421 B2. If necessary, sufficient HC1 36 is added to preclude the formation of hematite in-situ. An oxidant 35, in this case air or oxygen, is added, but it is understood that any other suitable oxidant may be used.

[0069] The oxidised solution undergoes hydrolysis and acid recovery 38 at a temperature of 160-200°C, and preferably in the range 170-190°C, by the methodology of the above- referenced US Patent. Ferric iron and any other trivalent ions present, such as aluminium, form their respective oxides, namely hematite and alumina, and evolve hydrogen chloride vapour 39. This HC1 vapour may be used as-is in a leach circuit, of EAFD for example, or condensed to concentrated hydrochloric acid (30-35%), which may be sold.

[0070] The hydrolysis reactions are shown as follows for iron and aluminium. Zinc chloride does not hydrolyse, and remains liquid under these conditions.

2FeCl3 + 3H₂0 Fe₂0₃ + 6HC1 (9)

2AlCb + 3H₂0 AkCb + 6HC1 (10)

[0071] The hematite slurry 40 undergoes solid-liquid separation in a hot filter 41. Any suitable filter may be used, but a Nutsche-type or heat resistant filter press are preferred. The solids 42 are predominantly a pure, crystalline form of hematite, together with a small amount of alumina if there was any aluminium in the feed solution. These solids may be sold, or returned to the steel plant.

[0072] The liquid 43 from the filter is recycled to the hydrolyser 38, with a bleed being taken for further processing 44. The bleed is initially diluted, ideally to give a zinc concentration in the range of 150-250 g/L, preferably 200-220 g/L. Recycled magnesia 45 is added to remove any last traces of iron and other impurities, such as manganese, which may be present.

[0073] The impurities removal slurry 46 proceeds to solid-liquid separation 47, which may be effected by any convenient means, such as, but not limited to, flocculation and thickening, filter press or vacuum belt filter. The solids 48 are returned to the feed circuit.

[0074] The filtrate 49 is predominantly zinc chloride/magnesium chloride, and is then treated with recycled magnesia 45 to effect precipitation 50 of zinc hydroxide. This may be carried out in any suitable reactor, preferably a stirred tank reactor. The temperature may be from ambient to 100°C, preferably from 60-65°C. The pH is from 6.0-8.5, preferably from 6.5-7.0. The precipitation may be carried out in one or two stages, with a single stage being shown in Figure 2 for convenience.

[0075] The chemical reaction involved is:

ZnCk + Mg(OH)₂ Zn(OH)₂ + MgCk (15)

[0076] Solid-liquid separation 52 of the precipitation slurry 51 may be effected by any convenient means, such as, but not limited to, flocculation and thickening, filter press or vacuum belt filter. The solids 53 are pure zinc hydroxide, which are calcined 54 to generate pure zinc oxide 55. The temperature of calcination is low, from 150-300°C, preferably from 200-250°C.

[0077] The solution 56 is pure magnesium chloride, which may be treated by conventional pyrohydrolysis 57 to recover hydrochloric acid 59 for recycle or sale, and pure magnesia (MgO). Part of the magnesia 45 is recycled to the impurity and zinc precipitation stages, and the balance 58, equivalent to fresh input from the feed, is suitable for sale.

[0078] The chemical reaction involved is:

MgCk + H2O MgO + 2HC1 (16)

[0079] Turning now to Figure 3, there is shown an alternative embodiment for treating the lead- cadmium filtrate 32 as obtained above in view of Figure 1. In this embodiment, both zinc and iron are separated and recovered via solvent extraction (SX), although equally ion exchange (IX) could be employed using similar reagents. The choice of SX or IX is dependent on the capacity of the plant. For small operations, IX is preferred, whereas for large plants, SX is preferred.

[0080] The nature of the circuit shown in Figure 1 is such that all of the iron is present in its ferrous state, and this affords a convenient method of separation of zinc via an organic extraction mechanism. Ordinarily, when iron is present, there is at least some of the oxidised ferric form, which will normally co-extract with any organic extraction reagent. However, in this case, there is both a concentrated chloride component, and the iron is present as ferrous. In chloride solutions, zinc forms a strong anionic chloro complex, which means it can be readily extracted with basic extractant such as a quaternary amine.

[0081] The lead-cadmium filtrate proceeds to solvent extraction of zinc 60 using a quaternary amine reagent 61. The filtrate is a mixture of predominantly zinc along with ferrous chlorides. The amine extractant, an organic compound, preferentially extracts zinc from the solution, the zinc combining with the amine. This is an immiscible organic solution, which is then separated from the aqueous solution, which is now ferrous chloride. By recontacting the zinc-loaded amine with water 62, the zinc then may pass back into the aqueous (water) solution, leading to a solution of zinc chloride. There are several such reagents available commercially, such as, but not limited to N,N-dioctyl-l-octanamine, also known as Alamine 336. Since the zinc is extracted as a complex anion, it is readily stripped from the loaded organic using water 62, generating a concentrated, pure zinc chloride solution 63. This solution may be sold as-is, or preferentially may be treated with magnesia as described in the circuit of Figure 2 above to generate pure zinc oxide.

[0082] The raffinate 64 from zinc solvent extraction, is essentially pure ferrous chloride solution, and proceeds to iron oxidation 65. This may be effected by any method familiar to those skilled in the art. An oxidant 66 is added, which may be, but not limited to, air, oxygen, hydrogen peroxide or chlorine. If necessary, additional HC1 67 may be added to prevent premature precipitation of hematite.

[0083] The solution 68 from oxidation is now ferric chloride together with impurities such as magnesium and manganese. As with zinc, the ferric iron forms an anionic chloro complex, less strong than that of zinc, which makes it amenable to solvent extraction 69 with a basic quaternary amine extractant 70. The same reagent as used for zinc is preferentially preferred, but there are several suitable commercial reagents which may also be used.

[0084] The loaded organic can be stripped with water 71 , to yield a pure ferric chloride solution 72. However, because the iron complex is less strong than that of zinc, a small amount of dilute HC1 may be needed to ensure complete stripping. Typically, concentrations up to 150 g/L iron can be achieved. The solution generated is suitable for sale as an additive in water treatment plants, or it may be further processed to recover hydrochloric acid and hematite in the manner described in Figure 2 and referenced as earlier as US Patent 9,889,421 B2, the content of which is enclosed herewith by reference.

[0085] Alternative embodiments for treating the zinc solvent extraction raffinate 64 will now be described, but are not shown in Figures. As noted, this raffinate is predominantly ferrous chloride, which may be directly crystallised from solution. Such crystals may be sold, for example, as an additive to water treatment plants.

[0086] Another embodiment would see the ferrous chloride solution 68 undergo direct pyrohydrolysis, either by spray roasting or a fluid bed, using conventional steel industiy technology. In this manner, the HC1 is recovered and may be sold or reused in the flowsheet. A hematite product is also obtained, the purity of which will depend upon the analysis of the ferrous chloride solution 68.

[0087] The process will now be illustrated by way of several examples. These examples are provided for illustration of certain embodiments of the invention, and are not intended as limitations thereof.

Example 1

[0088] Leach solution generated from the leaching of EAFD was processed through a 3-stage continuous metathesis (double decomposition) circuit In the first stage, copper was used to recover silver and reduce ferric iron to ferrous, in the second stage, iron was used to recover the copper, most of which was recycled to the silver stage, and in the third stage, zinc was used to recover lead and cadmium. The results of a 5-day run are summarised in Table 1.

[0089] Table 1. Results of Continuous Metathesis Run

[0090] The results clearly demonstrate the efficiency of the metathesis circuit in removing and recovering silver, copper, lead and cadmium.

Example 2

[0091] A sample of leach solution, after removal of all impurities via the process describe in Figure 2, was precipitated using lime slurry. The zinc hydroxide produced was filtered off and well-washed, dried and analysed. The sample showed only calcium as an impurity at a concentration of 0.07%, showing that a purity for zinc oxide of >99.9% could be achieved.

[0092] This demonstrates that the process is capable of producing pure zinc oxide. In practice, however, magnesia would be used instead of lime, since calcium chloride does not hydrolyse and decompose, whereas magnesium does.

Example 3

[0093] A solution with a total chloride concentration of 8.5M, containing 2 g/L zinc and various base and light metal impurities, including ferrous iron, copper, nickel, cobalt and magnesium up to a total concentration of 30 g/L, was treated with a strong base quaternary amine ion exchange resin. Complete recovery of the zinc was achieved with none of the other metals being recovered. Stripping of the resin with water yielded a solution of 8.5 g/L zinc, showing a greater than fourfold factor for increase in concentration.

[0094] The purpose of this experiment was to demonstrate that even low levels of zinc can be completely recovered in chloride media with quaternary amine extractants, even at low concentrations, from complex solutions, and that water is a very efficient stripping agent.

[0095] While illustrative and presently preferred embodiments of the invention have been described in detail hereinabove, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.

Claims

1. A process for recovering zinc, iron and optionally associated value from a feed material comprising zinc, lead and silver, the process comprising:

1 providing a feed liquor from the feed material;

2. The process according to claim 1, further comprising before i), leaching the feed material with an acidic aqueous solution to obtain the feed liquor.

3. The process according to claim 1 or 2, wherein the feed material is derived from electric arc furnace dust, or from a complex lead-zinc-copper-silver ore or concentrate.

4. The process according to any one of claims 1 to 3, further comprising adding a spent pickle liquor to the provided feed liquor.

5. The process according to claim 4, wherein the spent pickle liquor is:

a zinc pickle liquor (ZPL) to provide additional zinc, iron and a portion of the chloride to the feed material; a steel plant spent pickle liquor (SPL) to provide a portion of a chloride leaching medium; or

a mixture thereof.

6. The process according to any one of claims 1 to 5, wherein in step (ii), a free acid solution is present with a concentration of about 1 to 2 g/L HC1.

7. The process according to claim 6, wherein the pH of the free acid solution is reduced by the addition of recycled zinc hydroxide, or the addition of recycled magnesia.

8. The process according to any one of claims 1 to 7, in step (ii), a portion of copper is recycled to recover silver.

9. The process according to any one of claims 1 to 8, wherein in step (iv), a residence time for the recovery of lead and cadmium is from about 1 to about 10 minutes.

10. The process of claim 9, wherein the residence time is about 5 minutes.

11. The process according to any one of claims 1 to 10, wherein the step (iv) is carried out in the absence of air.

12. The process according to any one of claims 1 to 11, wherein step (iv) is performed using a rotating inclined reactor, and wherein the rotating inclined reactor is configured for having the second filtrate circulating from a bottom end to a top end of the rotating inclined reactor.

13. The process according to claim 12, wherein during step (iv), the reactor has a rotation speed from about 1 to about 20 rpm.

14. The process according to claim 13, wherein the rotation speed of the reactor is from about 5 to about 10 rpm.

15. The process according to any one of claims 12 to 14, wherein during step (iv), the inclined reactor has an inclination slope of from about 1° to about 20°.

16. The process according to claim 15, wherein the inclination slope is from about 5° to about 10°.

17. The process according to any one of claims 1 to 16, wherein the step (vi) comprising raising a temperature up to about 170-200°C, in order to effect the hydrolysis of ferric iron and precipitation of hematite.

18. The process according to claim 17, wherein the temperature is raised up to about 180-

19. The process according to any one of claims 1 to 18, wherein in step vii) the bleed comprises predominantly zinc chloride and magnesium chloride, the hydrolysis step comprising treating the bleed with magnesia to precipitate zinc hydroxide.

20. The process of claim 19, wherein the hydrolysis is performed at a temperature of about ambient up to 100°C, at a pH of about 6.0 to 8.5.

21. The process of claim 20, wherein the temperature and pH of the hydrolysis step are respectively from 60 to 65°C and from 6.5 to 7.0.

22. The process of any one of claims 19 to 21, wherein precipitated zinc hydroxide is pure zinc hydroxide, the process further comprising the step of heating the pure zinc hydroxide at a temperature from 150 to 300°C to generate pure zinc oxide.

23. The process of any one of claims 19 to 22, wherein a solution is obtained when the zinc hydroxide precipitates, the solution comprising pure magnesium chloride, the process further comprising pyrohydrolysing the pure magnesium chloride to recover pure magnesia (MgO), and hydrochloric acid.

24. The process of claim 23, wherein at least a portion of the hydrochloric acid is recycled into the process, and at least a portion of the pure magnesia is recycled into the zinc precipitation step of the process.

25. A process for recovering zinc, iron and optionally associated value metals from a feed material comprising zinc, lead and silver, the process comprising:

i. providing a feed liquor from the feed material;

v. reacting the third filtrate from lead-cadmium recovery step iv) with a quaternary amine to form an extractant comprising a zinc-amine complex and a raffinate comprising ferrous chloride; vi. stripping the extractant with water to generate a zinc chloride solution; and vii. oxidising the raffinate to form ferric chloride solution; or

viii. alternatively, pyrohydrolysing the raffinate to form iron oxide or hematite (Fe203) and recovering the hydrochloric acid.

26. The process of claim 25, wherein after oxidising in step vii) comprises reacting the oxidised raffinate with a second quaternary amine, and stripping with water to generate a ferric chloride solution.

27. The process from claim 25, wherein the process further comprises crystallising the raffinate from zinc extraction to recover ferrous chloride crystals.

28. The process according to any one of claim 25 to 27, further comprising before i), leaching the feed material with an acidic aqueous solution to obtain the feed liquor.

29. The process according to any one of claim 25 to 28, wherein the feed material is derived from electric arc furnace dust, or from a complex lead-zinc-copper-silver ore or concentrate.

30. The process according to any one of claim 25 to 29, further comprising adding a spent pickle liquor to the provided feed liquor.

31. The process according to claim 30, wherein the spent pickle liquor is:

a zinc pickle liquors (ZPL) to provide additional zinc, iron and a portion of the chloride to the feed material;

a steel plant spent pickle liquor (SPL) to provide a portion of a chloride leaching medium; or

a mixture thereof.

32. The process according to any one of claims 25 to 31, wherein in step (ii), a free acid solution is present with a concentration of about 1 to 2 g/L HC1.

33. The process according to claim 32, wherein the pH of the free acid solution is reduced by the addition of recycled zinc hydroxide, or the addition of recycled magnesia.

34. The process according to any one of claims 25 to 33, in step (ii), a portion of copper is recycled to recover silver.

35. The process according to any one of claims 25 to 34, wherein in step (iv), a residence time for the recovery of lead and cadmium is from about 1 to about 10 minutes.

36. The process according to claim 35, wherein the residence time is about 5 minutes.

37. The process according to any one of claims 25 to 36, wherein step (iv) is carried out in the absence of air.

38. The process according to any one of claims 25 to 37, wherein step (iv) is performed using a rotating inclined reactor, and wherein the rotating inclined reactor is configured for having the second filtrate circulating from a bottom end to a top end of the rotating inclined reactor.

39. The process according to claim 38, wherein during step (iv), the reactor has a rotation speed from about 1 to about 20 rpm.

40. The process according to claim 39, wherein the rotation speed of the reactor is from about 5 to about 10 rpm.

41. The process according to any one of claims 38 to 40, wherein during step (iv), the inclined reactor has an inclination slope of from about 1° to about 20°.

42. The process according to claim 41, wherein the inclination slope is from about 5° to about 10°.