Improved methods for recycling zinc (Zn)

Abstract

A method for recycling zinc (Zn) includes the steps of providing a feed composition, adding the feed composition to a rotary furnace, heating the added feed composition to produce a first liquid molten metal phase and a first skim dross, adding aluminum (Al) to the first liquid molten metal phase to form a second skim dross and a second liquid molten metal phase, adding at least one flux to the second liquid molten metal phase followed by at least one separation step of removing the second liquid molten metal phase from the rotary furnace, casting the second liquid molten metal phase or adding the removed second liquid molten metal phase to a casting furnace, and casting the second liquid molten metal phase from the casting furnace. The method further includes removing the second supernatant dross from the rotary kiln, subjecting the removed second supernatant dross to at least one grinding step and at least one classification step to separate at least one zinc fraction and at least one zinc oxide fraction from the second supernatant dross, and using the at least one zinc fraction to contribute to providing the feed composition.

Description

[Technical field]

[0001] The present invention relates to a method for recycling zinc (Zn) from secondary feedstock, also known as recyclable materials, optionally combined with a primary source, i.e. special high-grade zinc (SHG zinc), by pyrometallurgy. Recyclable materials may be, for example, by-products, waste materials, and end-of-life materials. [Background technology]

[0002] Economic and human progress (or development) has always been closely linked to the control and production of raw materials. At the same time, the continuous growth of the global economy and world population increases the demand for natural resources such as fossil fuels, metals and minerals. This not only raises issues such as the depletion of natural resources, but also environmental and climate impacts. Circular thinking is an approach that can help understand how a material such as zinc (Zn) finds its way through mining, manufacturing, product life and reuse, but it also leads to effective measures for optimization and changes in a holistic approach, for example more resource-efficient ways to produce, use and then reuse zinc. Indeed, zinc recycling has a key role to play in terms of securing resources for the maintenance and expansion of technologies and infrastructures in the future.

[0003] Annual global production of zinc (Zn) exceeds 13 million tonnes. Over 50% of this amount is used for plating, such as hot-dip galvanizing of steel, with the remainder primarily used in the production of brass, zinc alloys, semi-finished products and zinc compounds such as zinc oxide and zinc sulphate.

[0004] Zinc undergoes a complex life cycle, from its mining as an ore, through its refining and use in society, to its eventual recovery and reuse in end-of-life products. Based on the current trend where economic benefits are (seem to be) increasingly intimate with ecological and sustainability benefits, emphasis is being placed on technologies for a more efficient use and high-grade reuse of zinc. Metal recycling processes, specifically metal recycling processes of a pyrometallurgical nature, are aimed, among other things, at the recovery of zinc (Zn) from recyclable materials. These zinc metal recycling processes, and more generally zinc metal production processes, typically include at least one, and usually several, pyrometallurgical process steps, during which both zinc (Zn) and zinc oxide occur in the liquid molten zinc metal phase, and one or more supernatant slag phases are formed. These supernatant slag phases include various ash, metal oxides (such as zinc oxide (ZnO)) and intermetallic compounds (e.g. Fe-Al intermetallic compounds), which, by gravity, separate to the top of the molten zinc metal phase and typically float as light phases. These one or more supernatant slag phases are typically removed from the process as separate streams to obtain a high quality molten zinc metal phase of the desired purity. However, upon removal, these supernatant slag phases, which are by-products of zinc metal production, still contain large amounts of captured and desired zinc (Zn) to be reused. For economic reasons, it is often desirable to further extract zinc (Zn) as efficiently as possible from the removed supernatant slag phases prior to disposal of the residue for use elsewhere in low value applications.

[0005] Known techniques for removing, i.e. recovering, zinc (Zn) from the supernatant slag phase are known in the art and are specifically described, for example, in US Pat. No. 5,399,633 and US Pat. No. 5,499,623.

[0006] It is apparent from the above that there is a need for improved methods for recycling zinc (Zn) that are capable of achieving a higher final yield of zinc (Zn) combined with higher (energy) efficiency, higher throughput, no landfill waste, more environmentally friendly and a lower carbon footprint. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Chinese Patent No. 102423803(A) [Patent Document 2] International Publication WO2013 / 056348(A1) Summary of the Invention

[0008] The inventors have surprisingly found that it is possible to obtain a process for reusing (recycling) zinc (Zn) which meets the above-mentioned needs.

[0009] Thus, a method for recycling zinc (Zn) is provided herein, the method comprising the steps of: a) providing a feed composition comprising zinc (Zn) and further comprising iron (Fe); b) adding a feed composition to a rotary kiln; c) heating the added feed composition in a rotating rotary kiln to a temperature of at least 420° C. to produce a first liquid molten metal phase and a first skim dross floating on top of the first liquid molten metal phase under the influence of gravity; d) adding aluminum (Al) to the first liquid molten metal phase in the presence of the first supernatant dross, the iron (Fe) present reacts at least partially with the added aluminum (Al) to form at least one intermetallic compound and under the influence of gravity a second supernatant dross is formed, floating on top of the second liquid molten metal phase; e) adding at least one flux to the second liquid molten metal phase in the presence of the second supernatant dross, followed by at least one separation step in which the second liquid molten metal phase is at least partially removed from the rotary furnace; f) adding the removed second liquid molten metal phase to at least one mold for casting the second liquid molten metal phase or adding the removed second liquid molten metal phase to a casting furnace, the second liquid molten metal phase being maintained at a temperature of at least 400° C. in the casting furnace; g) casting the second liquid molten metal phase from the casting furnace into at least one mold; the method further comprising: h) removing the second supernatant dross from the rotary kiln; i) subjecting the removed second supernatant dross to at least one grinding step and at least one classification step in order to separate at least one zinc fraction and at least one zinc oxide fraction from the second supernatant dross; j) adding at least one zinc fraction obtained in step i) to step a) so as to contribute to providing a feed composition in step a); Includes. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The term "comprising" as used in the claims should not be construed as being limited to the meaning mentioned below, nor should such terms exclude other elements or steps. The term should be construed as embodying the presence of the expressly defined features, integers, steps or ingredients, but without excluding the presence or addition of one or more other features, integers, steps or ingredients, or groups thereof. Thus, the scope of an expression such as "steps A and B" should not be limited to a process consisting of only steps A and B. This means that, in the context of the present invention, the only relevant steps of the process are A and B. Thus, the terms "comprising" and "including" encompass the more restrictive terms "consisting essentially of" and "consisting of".

[0011] Within the scope of the present invention, the term "optionally" means that a described event or circumstance may or may not occur, and that the description includes instances when the event or circumstance occurs and instances when it does not occur.

[0012] In this specification, unless otherwise stated, the amounts of metals and oxides are given according to typical practice in pyrometallurgy. The presence of each metal is expressed in its total presence, regardless of whether the metal is present in its elemental form (oxidation state = 0) or in any chemically combined form, typically in an oxidized form (oxidation state > 0). For metals that can be relatively easily reduced to their elemental form and that may occur as molten metal in a pyrometallurgy process, it is somewhat common to express their presence in terms of elemental metal form, where most of these metals may actually exist in oxidized form, even when the composition of the slag is given. For this reason, the composition of slags, such as those obtained in the process according to the invention, specifies the contents of Fe, Zn, Pb, Cu, Sb, Bi, Ni, Cr as elemental metals. Base metals are more difficult to reduce under non-ferrous pyrometallurgy conditions and occur mostly in oxidized form. These metals are then typically expressed in terms of their most common oxidation form. Thus, in slag or dross compositions, the Si, Ca, Al, and Na contents are typically expressed as SiO2, CaO, Al2O3, and Na2O, respectively.

[0013] As mentioned above, according to step a) of the method according to the invention a feed composition is provided, the feed composition comprising zinc (Zn) and further comprising iron (Fe).

[0014] Within the context of the present invention, a feed composition as detailed above is provided such that the feed composition comprises zinc (Zn) and further comprises iron (Fe). The zinc (Zn) may be contained in the feed composition in the form of a zinc metal alloy. The iron (Fe) may be contained in the feed composition in the form of an iron metal alloy.

[0015] Within the scope of the present invention, such compositions as detailed above may be provided, i.e. composed, of one or more secondary feed materials, also known as recyclable materials, optionally combined with one or more primary sources, i.e. very pure zinc (SHG zinc) having a zinc content of, for example, 99.995%, according to a given recipe for the presence of zinc (Zn) combined with one or more non-zinc metal components such as iron (Fe) and aluminum (Al) and the respective amounts present in a given feed composition. These secondary feed materials, also known as recyclable materials, may come from a wide variety of sources and may thereby include a wide variety of compounds. Recyclable materials may be, for example, by-products, waste materials, and end-of-life materials.

[0016] Furthermore, secondary feedstocks are known to have a (significantly) lower carbon footprint compared to primary feedstocks. Zinc (Zn) recovery from secondary feedstocks has been a key operation for many years. Recycling of post-consumer zinc (Zn) has become a significant contributing factor in the industry due to the continued demand for Zn and the declining availability of high-quality virgin metal ores.

[0017] Non-limiting examples of secondary feed materials also include supernatant slag phases comprising zinc (Zn) and zinc oxide (ZnO). Such slag phases may originate from external zinc plants, zinc alloy plants, zinc recycling plants and zinc alloy recycling plants. Within the scope of the present invention, in order to separate at least one external zinc fraction and at least one external zinc oxide fraction from these external supernatant slag phases, these are preferably first subjected to at least one grinding step and at least one classification step, or in the case of external supernatant slag phases already consisting of granular material with small particles, at least one classification step, which may eliminate the need for at least one grinding step. The addition of excessive proportions of zinc oxide (ZnO) to the feed composition for use in the method according to the invention should preferably be avoided, since the presence of zinc oxide (ZnO) in the feed composition directly leads to a decrease in the (energy) efficiency and a decrease in the yield for the entire method for recycling zinc (Zn) according to the invention.

[0018] In one embodiment of the present invention, the at least one external supernatant slag phase is subjected to at least one grinding step and at least one classification step by adding the at least one external supernatant slag phase to a rotating perforated drum such as that described in US Patent No. 8,245,962. In such a rotating perforated drum, the at least one external supernatant slag phase is first subjected to at least one grinding step, and the at least one external supernatant slag phase is subsequently systematically broken down into granular materials of different particle sizes, in particular using grinding means selected from teeth, blades, spines or any combination thereof. The resulting granular material is subjected during or between the at least one grinding step to at least one classification step by sieve-based separation in the presence of air countercurrent in the rotating perforated drum, i.e. gravity separation based on air countercurrent, in order to separate the at least one external zinc oxide fraction from the at least one external zinc fraction. The outer zinc oxide fraction is generally characterized by a lower density than the outer zinc fraction, and the lighter outer zinc oxide fraction is therefore separated from and removed from the heavier outer zinc fraction. Preferably, the rotating perforated drum further comprises at least one magnetic conveyor belt for partially removing at least free iron (Fe) from at least one outer supernatant slag phase.

[0019] In another embodiment of the invention, the at least one supernatant slag phase is subjected to at least one classification step by adding at least one external supernatant slag phase to an apparatus such as that described in WO 2021 / 074528 A1, in particular when the at least one external supernatant slag phase is already composed of granular material with small particles. In such an apparatus, the at least one external supernatant slag phase is subjected to at least one classification step by air separation, i.e. gravity separation, to separate at least one external zinc oxide fraction from at least one external zinc fraction, which is generally less dense than the external zinc fraction, and which is therefore separated and removed from the heavier external fraction. Preferably, the apparatus further comprises at least one magnetic drum for partially removing at least free iron (Fe) from these external supernatant slag phases.

[0020] In one embodiment of the present invention, the method detailed above comprises a step carried out before the first step, i.e. step a) of the method, in which the subsequent secondary feed materials are classified based on their source, their chemical composition with respect to the presence of zinc (Zn) combined with one or more non-zinc metallic components such as iron (Fe) and aluminum (Al), and the final target alloy for which these secondary feed materials are used. As detailed above, such subsequent secondary feed materials are first subjected to at least one grinding step and at least one classification step, in particular to separate at least one external zinc fraction and at least one external zinc oxide fraction from the external supernatant slag phase comprising zinc (Zn) and zinc oxide (ZnO), when these subsequent secondary feed materials are external supernatant slag phases, or to at least one classification step, in case of an external supernatant slag phase already consisting of granular material with small particles and optionally other metals such as free iron (Fe), which may eliminate the need for at least one grinding step.

[0021] In general, those skilled in the art are familiar with available analytical techniques for determining the chemical composition of secondary feed materials for the presence of zinc (Zn) and other non-zinc metallic components, non-limiting examples of such analytical techniques are organoleptic techniques and optical emission spectroscopy (OES).

[0022] In terms of the amount of zinc (Zn) included in the feed composition relative to the total weight of the feed composition, zinc (Zn) can be present in any amount ranging from 0.10 to 99.99 weight percent.

[0023] Preferably, zinc (Zn) is present in an amount of at least 0.10 wt.-%, preferably at least 1.00 wt.-%, preferably at least 5.00 wt.-%, preferably at least 10.00 wt.-%, preferably at least 20.00 wt.-%, preferably at least 30.00 wt.-%, preferably at least 40.00 wt.-%, preferably at least 50.00 wt.-%, preferably at least 60.00 wt.-%, preferably at least 70.00 wt.-%, preferably at least 80.00 wt.-%, preferably at least 90.00 wt.-%, preferably at least 93.00 wt.-%, preferably at least 95.00 wt.-%, preferably at least 97.50 wt.-%, preferably at least 98.00 wt.-%, based on the total weight of the feed composition.

[0024] Regarding the amount of iron (Fe) contained in the feed composition, such amount of iron (Fe) is in principle not limited, since the skilled person can select the amount of iron (Fe) contained in the feed composition so as to ensure the proper implementation of the method according to the invention. In general, iron (Fe) is present in an amount ranging from 0.001 to 7.000% by weight, preferably ranging from 0.001 to 5.000% by weight, preferably ranging from 0.001 to 3.000% by weight, preferably ranging from 0.001 to 2.000% by weight, preferably ranging from 0.001 to 1.000% by weight, relative to the total weight of the feed composition.

[0025] In one embodiment of the method according to the invention, the feed composition consists essentially of zinc (Zn) in an amount ranging from 93.00 to 99.99 wt. % and iron (Fe) in an amount ranging from 0.001 to 7.000 wt. %, based on the total weight of the feed composition.

[0026] Within the scope of the present invention, the feed composition may contain smaller amounts of other metals in addition to zinc (Zn) and iron (Fe). Non-limiting examples of such other metals are aluminum (Al), silicon (Si), nickel (Ni), lead (Pb), magnesium (Mg), manganese (Mn), cesium (Ce), tin (Sn), copper (Cu), lanthanum (La), antimony (Sb), arsenic (As), bismuth (Bi), germanium (Ge), tellurium (Te), cobalt (Co), selenium (Se), thallium (Tl), gallium (Ga), silver (Ag), gold (Au), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os), and iridium (Ir). The feed composition may also contain elements that are not considered metals, such as sulfur (S), carbon (C), and oxygen (O).

[0027] As mentioned, according to step b) of the process according to the invention, the feed composition detailed above is added to a rotary kiln.

[0028] Within the context of the present invention, the feed composition detailed above can be added to the rotary kiln as a whole, or alternatively, the individual components that make up the feed composition can be added separately to the rotary kiln to form the feed composition therein.

[0029] Generally, known rotary kilns used in the field of pyrometallurgy and known to those skilled in the art can be used. Non-limiting examples of rotary kilns are described, for example, in particular in Canadian Patent CA1336135_C and International Publication WO2013 / 056348 (A1).

[0030] Within the context of the present invention, a rotary kiln is intended to denote a kiln having a chamber, such as a cylindrical chamber, which is coated with a suitable refractory material, such as refractory concrete, and which is heated from the inside. The chamber can be arranged with its axis generally horizontal, and a mechanism is provided for tilting the kiln on the horizontal axis. The rotary kiln is equipped with a rotating and tilting function and can also be closed. Heating is performed by burning natural gas (with oxygen) in a double-pass flame directly in the kiln, a plasma torch or an electric arc. Preferably, heating is performed by burning natural gas (with oxygen) in a double-pass flame directly in the kiln. Furthermore, the rotary kiln comprises an exhaust pipe or chimney, at least one oxygen burner, a door stop, a kiln entrance and a hinged door. The theoretical capacity of the rotary kiln is generally in the range of 30 to 50 tons.

[0031] As mentioned, according to step c) of the method according to the invention, the added feed composition as detailed above is heated to a temperature of at least 420° C. in a rotary furnace during rotation in order to produce a first liquid molten metal phase and a first supernatant dross which, under the influence of gravity, floats on top of the first liquid molten metal phase.

[0032] Preferably, the added feed composition detailed above is heated in a rotary furnace during rotation to a temperature of at least 450°C, preferably at least 500°C, preferably at least 550°C, preferably at least 600°C.

[0033] Furthermore, the upper temperature limits for heating in the rotary furnace while rotating the added feed composition detailed above are 900°C or less, or 880°C or less, or 860°C or less, or 850°C or less.

[0034] In a preferred embodiment of step c) of the method according to the invention, the added feed composition, as detailed above, is heated in a rotary furnace during rotation to a temperature in the range of 420-900°C, or in the range of 450-900°C, or in the range of 500-880°C, or in the range of 550-860°C, or in the range of 600-850°C.

[0035] Within the context of the present invention, "first skim dross" is intended to denote the often lighter, pasty material that forms as a result of the process and that usually separates from the denser first liquid molten metal phase under the influence of gravity and usually floats on top of said first liquid molten metal phase. Thus, the first skim dross can usually be mechanically scraped or removed from the underlying first liquid molten metal phase. The first skim dross is usually rich in various undesirable components such as ash and metal oxides (such as zinc oxide (ZnO)) that are still trapped in and combined with the large amount of the first liquid molten metal phase containing zinc (Zn) to be recycled.

[0036] Within the context of the present invention, the term "first liquid molten metal phase" is intended to denote the denser liquid molten zinc metal phase that forms as a result of the process carried out and that separates, usually under the influence of gravity, from the first supernatant dross detailed above, and that mainly comprises zinc (Zn) and small amounts of undesirable components such as ash and metal oxides, such as zinc oxide (ZnO), since these undesirable components are here at least partially contained in the first supernatant dross.

[0037] Within the scope of the present invention, a person skilled in the art will provide, according to standard practice, a rotary furnace with a suitable rotation speed to heat the added feed composition to produce a first liquid molten metal phase as detailed above and a first supernatant dross floating on top of the first liquid molten metal phase under the influence of gravity as detailed above.

[0038] Within the scope of the present invention, a person skilled in the art can adjust the direction of rotation, the rotation speed, the at least partial tilt, the heating degree of the rotary kiln while heating the added feed composition as detailed above, so as to achieve optimal separation in said rotary kiln between the first liquid molten metal phase as detailed above and the first supernatant dross as detailed above.

[0039] As mentioned, according to step d) of the method according to the invention, aluminum (Al) is added to the first liquid molten metal phase in the presence of a first supernatant dross, and the iron (Fe) present partially reacts with the added aluminum (Al) to form at least one intermetallic compound, resulting in the formation of a second supernatant dross which floats on top of the second liquid molten metal phase under the effect of gravity.

[0040] Within the context of step d) of the method according to the invention, it is therefore understood that the aluminum (Al) added to the first liquid molten metal phase detailed above may be present in the form of an aluminum metal alloy.

[0041] In order to produce a marketable zinc product, the iron (Fe) present must be at least partially removed (depending on the specifications present and used for the desired zinc metal alloy to be obtained), preferably completely removed. Iron (Fe) is a metal that can form an intermetallic compound with aluminum (Al). Thus, the iron (Fe) present in the first liquid molten metal phase detailed above can be reacted at least partially, preferably completely, with the added aluminum (Al) to form at least one intermetallic compound, such as Fe2Al5, and a second supernatant dross is formed that floats at the top of the second liquid molten metal phase under the influence of gravity, and the formed second supernatant dross contains at least one intermetallic compound.

[0042] Within the context of the present invention, "secondary dross" is intended to denote a lighter, often pasty, material that forms as a result of the process and that typically separates from the denser second liquid molten metal phase under the influence of gravity and typically floats on top of said second liquid molten metal phase. The second dross can therefore typically be mechanically scraped or removed from the underlying second liquid molten metal phase. The second dross is typically rich in various undesirable components, such as ash and metal oxides (such as zinc oxide (ZnO)), as well as the first dross detailed above, and at least one further intermetallic compound (such as Fe-Al intermetallic compound) formed by at least partial reaction of iron (Fe) present in the first liquid molten metal phase with the addition of aluminum (Al). Furthermore, the second dross contains a large amount of the second liquid molten metal phase, including zinc (Zn), which is still trapped therein and is recycled.

[0043] Within the context of the present invention, the term "second liquid molten metal phase" is intended to denote a denser liquid molten zinc metal phase that forms as a result of the process carried out and that separates, usually under the influence of gravity, from the second supernatant dross detailed above, this second liquid molten metal phase mainly comprising zinc (Zn) optionally in the presence of other non-zinc metallic components, as well as ash and small amounts of undesirable components such as metal oxides (such as zinc oxide (ZnO)) and iron (Fe), since these undesirable components are here at least partially contained in the second supernatant dross.

[0044] In one embodiment of step d) of the method according to the invention, aluminium (Al), optionally contained in the aluminium metal alloy, is added to the first liquid molten metal phase in at least the stoichiometric amount required to react with the amount of iron (Fe) present in the feed composition as detailed above, or preferably in a stoichiometric amount of at least 2%, more preferably in a stoichiometric amount of at least 5%, even more preferably in a stoichiometric amount of at least 10%, such as 20%.

[0045] In one embodiment of step d) of the method according to the invention, aluminium (Al), optionally contained in the aluminium metal alloy, is added to the first liquid molten metal phase in a content of not more than 200% of the stoichiometric amount required to react with the amount of iron (Fe) present, preferably not more than 150% of the stoichiometric amount, more preferably not more than a maximum of 125% of the stoichiometric amount, even more preferably not more than 120% of the stoichiometric amount, even more preferably not more than 115% of the stoichiometric amount.

[0046] The inventors have found that the above-mentioned amounts of aluminum (Al) optionally included in the aluminum metal alloy are sufficient to obtain acceptable removal of iron (Fe) present in the first liquid molten metal phase and to obtain a desired target concentration of iron (Fe) in the second liquid molten metal phase, such as a target concentration of 0.02 wt. % (200 ppm) or less.

[0047] As mentioned, according to step e) of the method according to the invention, at least one flux is added to the second liquid molten metal phase in the presence of the second supernatant dross, followed by at least one separation step in which the second liquid molten metal phase is at least partially removed from the rotary furnace.

[0048] Non-limiting examples of fluxes suitable for use in step e) of the method according to the invention are ZnS, ZnCl2, NH4Cl, (NH4)2ZnCl4, hydrates thereof or mixtures thereof.

[0049] In a preferred embodiment of step e) of the method according to the invention, the at least one flux is selected from the group consisting of ZnS, ZnCl2, NH4Cl, (NH4)2ZnCl4, hydrates and mixtures thereof.

[0050] Within the context of the present invention, the addition of at least one flux as detailed above performs one or more functions, such as in particular (further) purifying the second liquid molten metal phase as detailed above of undesirable components and chemical impurities contained therein, further melting the second supernatant dross as detailed above so that the second liquid molten metal phase containing recycled zinc (Zn) trapped in the second supernatant dross can be returned to the underlying second liquid molten metal phase, and reducing the amount of zinc oxide (ZnO).

[0051] The inventors have found that by adding at least one flux, in particular selected from the group consisting of ZnS, ZnCl2, NH4Cl, (NH4)2ZnCl4, hydrates and mixtures thereof, to the second liquid molten metal phase in the presence of the second supernatant dross, a partial amount of the second liquid molten metal phase containing recycled zinc (Zn) captured in the second supernatant dross can be successfully returned to the underlying second liquid molten metal phase without further contamination of the second liquid molten metal phase with, for example, metal oxides and / or intermetallic compounds contained in the second supernatant dross.

[0052] After adding at least one flux as detailed above to the second liquid molten metal phase, the added flux is contacted with the second liquid molten metal phase and the second supernatant dross in the rotary kiln during rotation, particularly in the inclined position of the rotary kiln, for a given time, such as 15 to 30 minutes. For good results, the axis of the rotary kiln may form an angle of, for example, 15 to 20 degrees with respect to the horizontal.

[0053] The addition of at least one flux as detailed above to the second liquid molten metal phase in the presence of the second skimmed dross as detailed above, and the contact of the added flux with the second liquid molten metal phase and the second skimmed dross for a certain time in the rotary kiln during rotation, is followed by a segregation step in which the second liquid molten metal phase is at least partially removed from the rotary kiln. The second liquid molten metal phase accumulated under the second skimmed dross can be separated (specifically poured) from the rotary kiln in one or more cycles, and the second skimmed dross, being in a more solid form compared to the second liquid molten metal phase, remains in the rotary kiln when pouring the second liquid molten metal phase. The separation of the second liquid molten metal phase from the rotary kiln during at least one separation step can be carried out via the rotary kiln inlet.

[0054] After at least partial removal of the second liquid molten metal phase from the rotary furnace by at least one separation step, the door of the rotary furnace is kept closed (while the rotary furnace rotates) and the rotary furnace is provided with an inert gas atmosphere to reduce or prevent exothermic reactions in the rotary furnace with the formation of undesirable zinc oxide from the desired recycled zinc (Zn). Non-limiting examples of inert gases that can be used to achieve such an inert gas atmosphere are nitrogen gas and argon gas.

[0055] It is considered advantageous that after the second liquid molten metal phase detailed above has been at least partially removed from the rotary furnace during one separation step, the rotary furnace is rotated again for a certain time, because the second supernatant dross detailed above is still trapped and is humidified by the large amount of second liquid molten metal phase containing zinc (Zn) to be reused.

[0056] In a preferred embodiment of step e) of the method according to the invention, the second liquid molten metal phase as detailed above is removed from the rotary furnace in two or more separation steps, such as two or three separation steps.

[0057] The inventors have found that step e) of the method according to the invention, i.e. the step comprising the addition of at least one flux as detailed above followed by at least one separation step, preferably followed by two or more separation steps, allows the maximum amount of the second liquid molten metal phase as detailed above to be removed from the rotary furnace, in combination with a favourable ratio of zinc (Zn) yield to energy consumed leading to an increased sustainability of the method according to the invention.

[0058] As mentioned, according to one embodiment of step f) of the method according to the invention, the second liquid molten metal phase removed from the rotary furnace as detailed above is added to at least one mold for casting said second liquid molten metal phase.

[0059] Non-limiting examples of suitable molds are ingot molds and block molds.

[0060] The second liquid molten metal phase thus removed, as detailed above, is cast into at least one mold and then cooled to become solid.

[0061] According to an alternative embodiment of step f) of the method according to the invention, the second liquid molten metal phase removed from the rotary furnace as detailed above is added to a casting furnace, in which the second liquid molten phase is maintained at a temperature of at least 400°C.

[0062] Preferably, the added second liquid molten metal phase as detailed above is maintained in the casting furnace at a temperature of at least 405°C, preferably at a temperature of at least 410°C, preferably at a temperature of at least 415°C.

[0063] It is further understood that the upper temperature limit of the added second liquid molten metal phase detailed above maintained in the casting furnace is 900°C or less, preferably 550°C or less, preferably 500°C or less, preferably 450°C or less, preferably 430°C or less.

[0064] In a preferred embodiment of step f) of the method according to the invention, the second liquid molten metal phase, added as detailed above, is maintained in the casting furnace at a temperature in the range of 415-430°C.

[0065] Generally, known casting furnaces such as those used in the pyrometallurgical art may be utilized and are known to those skilled in the art.

[0066] The second liquid molten metal phase detailed above is added to the casting furnace in the liquid molten state, thereby avoiding any loss of thermal energy, any metal oxidation reactions of the zinc (Zn) contained in the second liquid molten metal phase, and any cooling of the casting furnace if this is already contained in a fraction of the second liquid molten metal phase, before cooling and solidification of said metal phase.

[0067] In one embodiment of step f) of the method according to the invention, further non-zinc metal components can be added to the added second liquid molten metal phase detailed above in the casting furnace so as to produce in the casting furnace a final zinc metal alloy containing recycled zinc (Zn) having a desired chemical composition, which can then be cast from the casting furnace into at least one mold and cooled to become solid.

[0068] In general, those skilled in the art are familiar with available analytical techniques for determining the chemical composition of metallic phases and / or metal alloys with respect to the presence of zinc (Zn) and other non-zinc metallic components. A non-limiting example of such an analytical technique is optical emission spectroscopy (OES).

[0069] As a non-limiting example, copper (Cu) can be added to the added second liquid molten metal phase detailed above in a casting furnace to produce a final metal alloy containing recycled zinc (Zn) having an amount of copper (Cu) in the range of 0.001-5 wt. % based on the total amount of the final zinc metal alloy containing recycled zinc (Zn), which can then be cast from the casting furnace into at least one mold and cooled to become solid.

[0070] As another non-limiting example, the added second liquid molten metal phase in the casting furnace, as detailed above, may already contain an amount of molten aluminum (Al) when added to the first liquid molten metal phase in the presence of the first supernatant dross, as detailed above, in particular in an amount greater than the stoichiometric amount of aluminum (Al) required for the aluminum (Al) optionally contained in the aluminum metal alloy to react with the amount of iron (Fe) present in the first liquid molten metal phase in one embodiment of step d) of the method according to the invention, as detailed above. Thus, for example, a final zinc metal alloy containing recycled zinc (Zn) has an amount of copper (Cu) in the range of 0.50-3 wt.% and an amount of aluminum (Al) in the range of 2-8 wt.%, relative to the total weight of the final zinc metal alloy containing recycled (Zn), which is then cast from the casting furnace into at least one mold and cooled to become solid.

[0071] As mentioned, according to step g) of the method according to the invention, the second liquid molten metal phase as detailed above is added from a casting furnace to at least one mold in order to cast said metal phase in said at least one mold.

[0072] Non-limiting examples of suitable molds are ingot molds and block molds.

[0073] The second liquid molten metal phase, as detailed above, is removed from the casting furnace and cast into at least one mold, after which it is cooled to become solid.

[0074] As mentioned, according to step h) of the process according to the invention, the second supernatant dross as detailed above is removed from the rotary kiln.

[0075] The removal of the above-specified second supernatant dross from the rotary kiln can be carried out via an inlet of the rotary kiln, which was also utilized in step e) of the method according to the invention for at least partially removing the above-specified second liquid molten metal phase from the rotary kiln during at least one separation step.

[0076] The second supernatant dross thus removed is then cooled to solidify as detailed above.

[0077] As mentioned, according to step i) of the method according to the invention, the removed second supernatant dross detailed above is subjected to at least one grinding step and at least one classification step for separating at least one zinc fraction and at least one zinc oxide fraction from the second supernatant dross.

[0078] Within the context of step i) of the method according to the invention, the at least one zinc fraction may be in the form of a zinc metal alloy.

[0079] In one embodiment of step i) of the method according to the invention, the at least one grinding step may be performed by grinding means selected from teeth, blades, spines or any combination thereof. The subsequent second supernatant dross as detailed above is systematically broken down into granular materials of different particle sizes.

[0080] In one embodiment of step i) of the method according to the invention, the at least one classification step may be carried out by sieve-based separation in the presence of a countercurrent air flow, i.e. gravity separation based on a countercurrent air flow.

[0081] In one embodiment of the present invention, as referred to in step i) of the method according to the present invention, the at least one grinding step as detailed above and the at least one classification step as detailed above can be carried out by adding the second skim dross as detailed above to a rotating perforated drum, such as that described in US Pat. No. 8,245,962 (B2). In such a rotating perforated drum, the removed second skim dross as detailed above is first subjected to at least one grinding step, and the second skim dross is then systematically broken down into granular materials of different particle sizes, in particular using grinding means selected from teeth, blades, spines or any combination thereof. The resulting granular materials are subjected during or between the at least one grinding step to at least one classification step by sieve-based separation in the presence of air countercurrent in the rotating perforated drum, i.e. gravity separation based on air countercurrent, to separate at least one zinc oxide fraction and at least one zinc fraction. The zinc oxide fraction is generally characterized by a lower density than the zinc fraction, and the lighter zinc oxide fraction is therefore separated and removed from the heavier zinc fraction.

[0082] In a preferred embodiment, the rotating perforated drum detailed above further comprises one or more magnetic means for removing, i.e. extracting, free metallic iron (Fe), if present in the second supernatant dross.

[0083] As mentioned, according to step j) of the process according to the invention, at least one zinc fraction as detailed above obtained in step i) is added to step a) so as to contribute to the provision of the feed composition as detailed above in step a).

[0084] The inventors have found that the method according to the invention allows maximum extraction and reuse of zinc (Zn) in the form of at least one zinc fraction, by further subjecting the second supernatant dross, after removal from the rotary kiln, to at least one grinding step and at least one classification step as detailed above, in order to separate at least one zinc fraction and at least one zinc oxide fraction from the second supernatant dross. The zinc (Zn) thus recovered then serves to contribute to providing the feed composition in step a), which directly allows to obtain a higher final yield of zinc (Zn) in combination with a higher (energy) efficiency, a higher throughput, no landfill waste and a lower carbon footprint.

[0085] The inventors have further found that the process of the invention, comprising steps a) to j) as a whole, makes it possible to achieve a higher final yield of zinc (Zn) in combination with a higher (energy) efficiency, a higher throughput, no landfill waste, a higher environmental friendliness and a lower carbon footprint.

[0086] The at least one zinc oxide fraction detailed above and obtained in step i) of the process according to the invention can often further be applied in other low value applications, such as for example as an activator for rubber vulcanization or as an additive or filler for plastics, ceramics, glass and cement.

Claims

1. 1. A method for recycling zinc (Zn), the method comprising: a) providing a feed composition, the feed composition comprising zinc (Zn) in an amount of 40.00 wt. % or more and iron (Fe) in an amount of 0.001 to 7.000 wt. %, based on a total weight of the feed composition; b) adding the feed composition to a rotary kiln; c) heating the added feed composition in the rotary kiln during rotating motion to a temperature of at least 420° C. to produce a first liquid molten metal phase and a first skim dross floating on top of the first liquid molten metal phase under the influence of gravity; d) adding aluminum (Al) to the first liquid molten metal phase in the presence of the first supernatant dross, the iron (Fe) present reacts at least partially with the added aluminum (Al) to form at least one intermetallic compound and under the influence of gravity a second supernatant dross is formed floating on top of the second liquid molten metal phase; e) adding at least one flux to the second liquid molten metal phase in the presence of the second supernatant dross, followed by at least one separation step in which the second liquid molten metal phase is at least partially removed from the rotary furnace; f) adding the removed second liquid molten metal phase to at least one mold for casting the second liquid molten metal phase or adding the removed second liquid molten metal phase to a casting furnace, the second liquid molten metal phase being maintained at a temperature of at least 400° C. in the casting furnace; g) casting the second liquid molten metal phase from the casting furnace into at least one mold; Including, The method further comprises: h) removing the second supernatant dross from the rotary furnace; i) subjecting the removed second supernatant dross to at least one grinding step and at least one classification step in order to separate at least one zinc fraction and at least one zinc oxide fraction from the second supernatant dross; j) adding said at least one zinc fraction obtained in step i) to step a) to contribute to providing said feed composition in step a); The method of claim 1, further comprising:

2. The method of claim 1 , wherein the feed composition is provided from one or more secondary feed materials.

3. 3. The method of claim 2, wherein the one or more secondary feed materials are first subjected to at least one grinding step and at least one classification step, or to at least one classification step, to separate the at least one external zinc fraction and the at least one external zinc oxide fraction.

4. The method according to any one of claims 1 to 3, wherein the feed composition comprises zinc (Zn) in an amount of 50.00 wt.% or more, based on the total weight of the feed composition.

5. 2. The method of claim 1, wherein the feed composition comprises iron (Fe) in an amount ranging from 0.001 to 5.000 wt. %, based on the total weight of the feed composition.

6. 2. The method of claim 1, wherein in step c) the added feed composition is heated in the rotary furnace during rotating motion to a temperature in the range of 420-900°C, or in the range of 450-900°C, or in the range of 500-880°C, or in the range of 550-860°C, or in the range of 600-850°C.

7. 2. The method of claim 1, wherein in step d) aluminum (Al) is added to the first liquid molten metal phase in an amount equal to or greater than the stoichiometric amount required to react with the amount of iron (Fe) present in the feed composition.

8. 2. The method according to claim 1, wherein in step d) aluminum (Al) is added to the first liquid molten metal phase in a content of up to 200% of the stoichiometric amount required to react with the amount of iron (Fe) present.

9. In step e), the at least one flux is ZnS, ZnCl ２ , N.H. ４ Cl, (NH ４ ) ２ ZnCl ４ , hydrates and mixtures thereof.

10. 2. The method according to claim 1, wherein in step e) the second liquid molten metal phase is removed from the rotary furnace in two or three separate steps.

11. 2. The method of claim 1, wherein in step f) the added second liquid molten metal phase is maintained at a temperature of at least 405°C in the casting furnace.

12. 2. The method of claim 1, wherein in step f) the added second liquid molten metal phase is maintained at a temperature of 900° C. or less in the casting furnace.

13. 10. The method of claim 1, wherein in step f) additional non-zinc metallic components are added to the second liquid molten metal phase added to the casting furnace.

14. 2. The method of claim 1, wherein in step i), the at least one grinding step is performed using grinding means selected from teeth, blades, spines, or any combination thereof.

15. 2. The method according to claim 1, wherein in step i) the at least one classification step is carried out by separation on a sieve in the presence of a countercurrent of air.

16. 15. The method of claim 14, wherein step i) is carried out in a rotating perforated drum.

17. 17. The method of claim 16, wherein said rotating perforated drum is provided with one or more magnetic means for removing metallic iron (Fe).