Recycling potential of secondary zinc materials

Circularity is a decisive corner stone of a sustainable use of raw materials. This has been recognized at global level and is pushed forward specifically in Europe and e.g., Japan. Based on intensive desk top research and interviews with experts from the zinc industry, information on secondary raw materials for zinc recovery was compiled. This paper comprises of an overview of zinc bearing wastes, residues, and by-products occurring in all stages of zinc mining, production, as well as first-use and end-use production. These materials are quantified and characterized, and ways of recycling are described. As a result, potentials for an increase of zinc recycling rates are identified. Mine tailings are a meaningful resource for zinc recovery. Industrial wastes occurring in zinc smelters, such as Jarosite, Goethite, and their likes are in the scope of research projects aiming at recycling zinc and other valuable metals. Residues, ashes, drosses, slags that are produced in first use operation of the zinc value chain, such as galvanizing, zinc die casting, or brass plants are recycled almost completely in Europe and in many other regions in the world. The biggest potential for increasing recycling rates for zinc still lies in steel mill dusts. Also, the majority of zinc die cast parts seem to end up in waste incineration and dump sites. This low end-of-life zinc recycling rate bears room for additional recycling opportunities via an improvement of the intelligent collection and sorting of waste and other innovative recycling processes utilizing residues from waste-to-energy plants.


Introduction
Over the past years, the importance of circularity has increasingly been acknowledged at European and at global level.In Europe, the EU New Green Deal [1] that was agreed on in December 2019, initiated and even more increased focus on the transition to a circular, European economy.Beyond Europe, China, Russia, India, Brazil, Indonesia, and other developing countries mark the regions in which the rapidly rising demand for metals is expected to occur.Metals are a major part of the materials that are needed to meet the demands caused by increasing world population and rising living standards as well as by changes in use patterns associated with greening the economy by e.g., renewable energy and emobility.The OECD projects the global metals use to more than double from 8 Gt in 2011 to 20 Gt in 2060 [2].
Increasing circularity of zinc in practical terms requires quantitative information on materials that are available for recycling, their chemical and physical characteristics, and technologies available for recycling.In an extensive desk top study combined with interviews with experts from the zinc industry, such information was compiled and may now serve as information hub for various questions and projects Since the use of zinc is linked to the industrialization of countries around the world, it is possible to draw a parallel between the global growth of zinc production and the economic growth of industrialized countries.Today, it seems that the refined zinc production level is in the range of 13 to 14 million tonnes (Mt) per year (Figure 1 [3]).That number includes the zinc production from minerals (primary or mined zinc) and the zinc production from steel dust reprocessing loops (refined secondary zinc) but doesn´t take into account direct zinc recycling loops from zinc industrial residues (directly re-used secondary zinc).Direct zinc remelting provided another 4 to 5 Mt of secondary zinc products per year to the market in between 2010 and 2019 [4].

Resources and Reserves
Mineral deposits can be classified as: 1) Mineral resources that are potentially valuable, and for which reasonable prospects exist for eventual economic extraction, or 2) Mineral reserves that are valuable and legally, economically, and technically feasible to extract.These two concepts can be further subdivided into different categories from proven ore reserve to probable inferred mineral resource.
Reserves and resources are not fixed once and for all.This notion of evolution is often misunderstood, and data published in the media is often misinterpreted as a fixed limit.This applies to many natural resources, including zinc.Of course, there is an ultimate limit, which would be related to the full exploitation of geologically feasible zinc ores at a very high theoretical cost.
In his report on the geological long-term availability of zinc, Pirard, University of Liege [5], concludes on zinc mining indicators: • Accessible crustal content of zinc: 198,000,000 Mt • Extractable global resources: 63,000 Mt • Proven and probable reserves: 250 Mt These tonnages are complemented by 247 Mt of zinc being in use today as anthropogenic stock with most of it becoming available for recycling one day (see Figure 2).

Zinc losses in the Extraction and in the Minerals Processing
Non-ferrous metal ores usually are polymetallic ores.Zinc in ores is typically associated with other metals of value, such as lead, copper, iron, silver, gold, indium, germanium, and sometimes gallium.Sometimes, zinc is mined as a by-product from ore bodies that are mined for their silver content in the first place.Other metals typically found in zinc ores are arsenic and cadmium.
A typical loss of 2.5% during ore mining is quoted, this is called the mine cut-off.This cut-off depends on many different factors such as the type of ore, the conformation of the deposit, the average zinc content and the technical means of processing the ore.Total zinc losses during mining for 2015 are estimated at about 350 kt Zn/year.These losses stay behind in mines and are challenging to recover.
Losses during mineral processing (e.g., flotation) have been estimated at 15% on average, with a range between 10-30%.These losses depend on the type of ore, the type of ore processing facility and other technical and economic factors.The price of zinc compared to other non-ferrous metals is also an element to be considered, in particular in processing polymetallic minerals.15% zinc losses during mineral processing for 2015 are estimated at an absolute volume of about 1,900 kt Zn/year.These zinc losses report to the mine tailings after mineral processing.To recover part of these losses, tailings can be reprocessed in some cases.Reprocessing tailings is not always easy to implement.However, new paradigm shifts may influence tailings reclamation decisions.These include new environmental regulations, improved zinc recovery capacity, recovery of other metals that have recently increased in value and changes in mineral technology.New Century Resources' zinc tailings treatment in Australia is a good example.The tailings pond at the Century mine was converted into the 14 th largest known zinc deposit in the world -77 Mt of tailings, containing 3% recoverable zinc with a homogenous grade distribution, gave a deposit of 2.3 Mt of zinc.They have ramped up tailings throughput through the existing infrastructure.Another tailing reprocessing facility, in Poland, processes tailings from the Boleslaw mine (closed 1996), Olkusz mine (closed 2001) and Pomorzany mine (closed 2020) with an average Zn% of 1.0% and maximum Zn% of 1.5%.After reprocessing, the tailings have an average Zn% of 0.05% so the facility achieves a recovery of approx.95%.Recovery can technically be as high as close to 100% in theory, but it is often not economically viable.

Zinc losses in Refined Zinc Production
The production of refined zinc is divided among various industrial groups operating in different countries around the world.Often, ore-producing countries are also producers of refined zinc so have integrated their mining and refining activities.Zinc losses in the main refining routes roast-leachelectrowinning (RLE), imperial smelting furnace (ISF) and solvent extraction-electrowinning (SX-EW) have been studied with a focus on residue production rates and related zinc losses.RLE is a hydrometallurgical process and currently the most widely used process in the world for refining zinc ores and crude zinc oxides (CZO) to metallic refined zinc with high purity.It accounts for approximately over 90% of the volume of refined zinc produced.The RLE process is very efficient for treating zinc ores as it has a very high yield and produces high purity special high grade (SHG) zinc (min.99.995%).However, the RLE process is limited to a feed with approx.20% CZO originating from recycled electric arc furnace (EAF) dust, and even lower than 20% if no extra CZO washing and/or direct leaching capacity is available.In the RLE process, zinc losses to residues are estimated to be between 2% and 6%.These losses are mainly concentrated in iron residues of the type Jarosite, Goethite or Hematite, depending on the used residue precipitation process.An average of 3% for these zinc losses is assumed in this paper, which for 2015 is estimated at about 345 kt Zn/year.
The precipitation of iron as Jarosite by hydrolysis generates sulphuric acid which must be neutralised in order to achieve near complete iron precipitation.Calcine from the roaster is added as neutralising agent, amongst others.Under the hydrolysis stage conditions, less than 5% of the zinc ferrite content in calcine is dissolved and 95% is retained in the Jarosite waste.This explains the presence of zinc in Jarosite and generates a limit in the amount zinc which is recovered in the leaching step, which can only be recovered by further treatment of Jarosite, for example in a Waelz Kiln.The more Fe is present in the concentrate feed to the roaster, the more Zn is lost to Jarosite.Jarosite contains 4.5% Zn on average, but can vary between 1.5 and 13% depending on the feed composition and leaching process conditions.
ISF is a pyrometallurgical process and is still used but its relative importance has been decreasing for the last 20 years.It accounts for approximately 7% of the volume of refined zinc produced.This process is generally considered to be less advantageous for treating zinc ores than the RLE process because the zinc product from this process has more and higher impurities.However, ISF is capable of processing zinc-lead ores and more secondary raw materials from the EAF dust reprocessing loop.It can produce zinc and lead at the same time.ISF is also able to process some secondary raw materials from the RLE process and it can treat more complex ores, polymetallic ores and other waste materials.In the ISF process, zinc losses occur to the slag which can have a zinc content of 6-10%.These zinc losses are estimated at about 42 kt Zn/year.
The SX-EW process has been developed more recently and its importance has been increasing for the last 10 years.The main advantage of the SX-EW process is that it can treat secondary materials such as CZO from the EAF dust processing loop without the need of washing it first to remove the halides.The use of CZO as raw material can be up to 100% which is much higher than in the RLE process; it can be used to produce SHG zinc with a 100% recycled content, depending on CZO availability in the market.Total zinc losses to SX-EW residues in 2015 are estimated at 18 kt Zn/year.At present, its importance is still low, but it will probably increase in the next few years depending on the need to treat secondary raw materials.China was planning to build three more SX-EW process plants between 2021 and 2023: Yongxin nonferrous phase II in Guangxi, Tenglong Recycling in Hebei and Yuguang Zinc in Henan.

Table 1: Estimated amount of zinc lost in residues from zinc smelting processes in 2015
Table 1 gives an overview of estimated total global zinc losses in refined zinc production in 2015.The lost zinc units are split by origin of the feed material, being either primary mined zinc or secondary crude zinc oxide (CZO).The total estimated losses for 2015 were 415 kt of zinc.Some zinc form residues (including slags) from refined zinc production is recovered in recycling processes.Recycling of zinc residues is apparent today, in particular in China but also in other regions where these residues cannot be landfilled anymore.The following chapter will explain how zinc can be recovered from residues and shows that it is done on a smaller scale at some smelting operations.

Zinc recovery from secondary raw materials from refined zinc production
Reprocessing of residues from zinc refining (RLE and SX-EW) by hydrometallurgical processes was first used for the stabilization of Cd-and As-containing residues, to make them chemically inactive to allow for safe landfilling or use it road construction.The Jarofix process is operated by some smelting operations to stabilize residues (Table 2).More recent processes are being developed to recover zinc, precious metals and other non-ferrous metal.Smelting residues such as Jarosite can still contain valuable amounts of silver, indium, lead and zinc, of which silver and indium together often represent more than 50% of the total value.Next to an economic incentive, new environmental regulations are a large driver for the development of new recovery and recycling technologies for smelter residues.
Existing process technologies to treat zinc residues from smelting and refining operations are the Ausmelt process which is a top submerged lancing (TSL) furnace (from Metso Outotec), ISASMELT which is a TSL furnace as well (Glencore technology), the Waelz process and the imperial smelting process (ISP).Table 2 gives an overview of zinc and lead producing companies operating such technologies for recycling of residues [8].Other more experimental processes or technologies under development are the HIsarna process (from Tatasteel [9]) and the Jarogain process (from VTT Technical Research Center & Aalto University [10]).
In a TSL furnace, zinc is fumed out of the liquid phase in the furnace.This liquid phase is typically a lead or copper smelt.So, next to reprocessing secondary zinc residues, fumed zinc collected in zincrich dust can also be a by-product from lead or copper (alloy) production in a TSL furnace.

Zinc losses in zinc first use production
When zinc was first used, more than 150 years ago, the applications were initially related to objects made of solid zinc or zinc alloy such as household and industrial objects.Zinc sheets for roofing also appeared quite quickly.Then batch galvanizing was developed.The emergence of zinc alloys Zamak and brass allowed for a wider application range for zinc.During the last decades, continuous galvanizing has been an important growth engine for zinc.Other applications have also emerged such as zinc powders in oxide or metal form and zinc wires for thermal spraying.More recently, new applications are emerging such as rechargeable zinc batteries.
The distribution of zinc applications therefore takes into account its versatility of use.Typical uses of refined zinc, which account for more than 90% by volume of all metallic zinc uses, are galvanizing, die-casting, brass and production of zinc oxide powders.Other uses of zinc with minor volumes are production of zinc dust, zinc wire, fertilizer nutrients and zinc sheet.There are other applications of zinc that are produced in small volumes for niche markets.

Zinc losses in continuous steel sheet galvanizing
Continuous galvanizing is the largest first use of zinc and accounts for 35% of total zinc consumption (primary and secondary zinc).Continuous galvanizing allows steel sheets to be coated with zinc or zinc alloys in a continuous process.There are three technologies currently used: 1) Hot-dip galvanizing (Sendzimir process) by dipping the sheet in a molten zinc (alloy) bath; 2) Electro-galvanizing by depositing zinc to the steel surface by means of an electric current from a generally aqueous bath in which zinc ions and, possibly, other metals (Ni, Fe, Co) are present; 3) Vapour phase galvanizing in which the steel sheet surface is put in contact with a zinc vapor flux (Physical Vapour Deposition (PVD) and Jet Vapour Deposition (JVD) processes).With the evolution of technologies, it appears that, of these three technologies, hot-dip galvanizing has become the predominant technology for the automotive market, for the construction market and for other markets.
In continuous hot-dip galvanizing, different bath compositions exist and are detailed in Table 3.The zinc feed into these processes is often alloyed SHG zinc.Alloying elements are typically aluminium, magnesium, silicon, lanthanum and/or cerium in varying concentrations.During the process, the zinc bath is saturated with iron due to the reaction between the liquid zinc bath and the iron in the steel.Traces of other elements from the steel can be found in the coating, such as manganese or chromium.
In continuous hot-dip galvanizing, top drosses and skimmings are formed in two ways: 1) Due to the reaction between the bath and the steel; and 2) Due to the oxidation of the bath alloy on the surface and during the wiping.When the aluminium content in the zinc bath is below 0.135%, bottom drosses are formed.The composition of the top skimmings and bottom drosses vary with the zinc bath composition.
The estimated global production of continuously galvanized steel in 2015 has been estimated at 160 Mt.It is estimated that in 2015 about 5.7 Mt of zinc was used for continuous galvanizing globally.About 635 kt of zinc per year was lost to galvanizing drosses, galvanizing top skimmings, and drosses and ashes generated in zinc alloying & casting plants.Because of the very high zinc content, residues from continuous galvanizing are almost fully recycled into new secondary zinc products.

Zinc losses in batch galvanizing
Batch galvanizing is the second largest application for zinc.This represents about 25% of total zinc consumption in first use.Batch galvanizing is usually done by hot-dipping of steel parts in a bath of molten zinc.The batch galvanizing process typically exists out of the following process steps: cleaning, pickling, fluxing, galvanizing, and cooling.With some exceptions, the zinc bath composition is generally >99% zinc.The other elements are either small amounts of intentionally added elements (Bi, Ni, Sn, Al, Pb) or small amounts of associated elements (Fe, Pb, Cd, Cu).
Residues produced in batch galvanizing are of two types: bottom drosses and surface ashes or top ashes.Bottom drosses are formed as a result of the reaction between the liquid zinc and the iron in the workpieces, the steel in the tank and the steel in the working tools (suspension hooks).Bottom drosses contain >95% zinc, the remainder is mainly Fe and Pb.Surface ashes or top ashes are produced due to an exothermic reaction between the activating flux, the zinc, the iron and the oxygen from the air.The total composition of top ashes is a mix of solidified droplets of metallic zinc from the bath and the oxidechloride powders (like ZnO, ZnxCly(OH)z, NaCl, KCl, …).Top ashes contain >80% zinc, of which about 50% is in the metallic form.
It is estimated that in 2015 about 4.4 Mt of zinc was used for batch galvanizing globally.About 1.15 Mt of zinc per year was lost to galvanizing drosses, galvanizing ashes, and top skimmings generated in zinc alloying & casting plants.Because of the very high zinc content, residues from batch galvanizing are almost fully recycled into new secondary zinc products.

Zinc losses in zinc die casting
Zinc alloys for die casting (called Zamak) appeared between 1926 and 1929, when it was developed and patented by the New Jersey Zinc Company.Foundries needed a cheap, hard and resistant material easy to mould and mechanize.The aim was to obtain parts with lower price, higher precision, better finishing and higher quality than silver, which gets dirty and cannot be painted.Zamak alloys are cast in special hot-chamber casting machines.The hot-chamber casting machine allows for the manufacturing of pieces at very high precision and very high speed, so with a high productivity.Zamak alloys are very pure and have low levels of impurity elements Fe, Pb, Cd and Sn.Zamak alloys are generally produced from SHG zinc (min.99.995% purity), pure aluminium, copper, and magnesium.Recycled "non-used cast Zamak" or new scrap coming directly from the casting process are also used.Those scrap pieces need to be very clean and without additional metallic coating or plating.In some cases, top skimmings from continuous galvanizing are used to produce Zamak.This secondary raw material requires reprocessing in order to lower the iron content, lower the iron-aluminium dross particles, eliminate the zinc and aluminium oxide particles and to precisely adjust the alloy composition according to international standards before it can be used as alloy for die casting.Aluminum is added to improve strength and fluidity, and Al% in Zamak alloys is typically 4.0% which is close to the Zn-Al eutectic composition.
Copper makes the alloy stronger and harder; Cu% varies by alloy, and is 0.0% for Zamak alloy #3; 0.9% for #5 and 3.0% for #2.Magnesium is added as grain refiner and Mg% is typically 0.05% for highpressure die cast alloys, but can be higher for zinc alloys for gravity casting.Zinc alloys for casting are specified in international standards such as EN 1774 (1997) and ASTM B86 (2022).
Global volumes of Zamak produced and used are difficult to estimate because the market fir zinc die castings is volatile and can change rapidly depending on the economic situation in a region or in the world.Zamak alloys are used in a wide variety of applications.In addition, the market represents an important part of total zinc first use (about 15%) and zinc die casting alloys are used in many countries by die cast companies of various sizes.The applications of Zamak are very versatile: buildings, appliances, clothing, electronics, electric, toys, automotive and various other applications.It is assumed that the global use of zinc for producing zinc die casting alloys is about 2 Mt of zinc per year.
For zinc die casting alloys, the big question revolves around end-of-life (EoL) recycling.Zinc die cast parts are generally small parts that are dispersed in many applications.Very often, they end up in municipal solid waste (MSW) going to landfills or incinerators.In addition, the equipment in which these parts are used is often imported or exported.Recycling at the EoL is therefore very difficult to implement and the estimation of the EoL recycling rate is complicated to evaluate.Europe is a region where the recovery of these metals may be well organized through effective waste collection and sorting schemes.In the rest of the world, it is likely to see much lower EoL recycling rates for post-consumer zinc die cast parts.It is estimated that about 475 kt of zinc is recycled per year from EoL zinc die cast scrap, the remaining 1.525 Mt of zinc pet year is either abandoned in place or lost to landfilling sites and waste incinerators.

Zinc losses in brass production
The brass market is a large market in volume and represents about 8 Mt of brass per year.Typical brass products are tube, rolled brass sheet, wire, powders and castings.The zinc content in brass is between 32% and 45% (with an assumed average of 38%).It can also contain a few percent of lead, and the remainder is copper.Lead is considered necessary in brass because it gives better machinability to the alloys, so the brass industry is often considered as an attractive outlet for lead-containing secondary zinc from recycling operations.In recent years however, the use of lead in brass has also been questioned by brass users because of environmental concerns.
In 2016, the amount of primary brass produced was about 4.4 Mt.For producing this amount of primary brass, both primary and secondary zinc was required and estimated at 38% x 4.4 Mt = 1.7 Mt of zinc.The total zinc losses to residues generated during melting and alloying of primary brass, plus residues generated during manufacturing of primary brass products, are estimated at 100 kt of zinc per year in total.Similar to EoL zinc die castings, meaningful amounts of brass scrap are lost in the post-consumer phase.Figure 3 shows copper alloys (brass) losses during the stages of collection and separation stages of post-consumer scraps [11].The figure shows that about 44% of post-consumer brass scrap is lost through dissipation or landfilling, caused by collection and separation inefficiencies.This 44% represents about 1,917 kt x 38% = 730 kt of zinc losses per year.The remaining 1,909 kt of copper alloys (brass) which is collected and sorted per year is recycled either through direct remelting into new brass, or treatment in a copper refinery to recover and refine the copper.Before the copper reaches the copper refinery for treatment, the zinc has been typically removed from the brass scrap, e.g., in a top submerged lancing (TSL) furnace in which it is fumed out.The so-called copper furnace dust from this TSL furnace, which is very high in zinc content, can be treated in a zinc refinery (e.g., in an ISF) to recover the zinc and produce refined zinc out of it.

Zinc losses in zinc sheet production
The manufacturing of rolled zinc is ancient and dates back to the beginning of the 19 th century when the methods of extraction, purification and shaping of zinc began to be industrialized.Zinc sheet can be used to make roofs and facades of very different styles, often with very beautiful aesthetic views.The manufacturing method of rolled zinc consists of melting cathodes or ingots of SHG quality zinc, followed by adding and alloying with titanium, copper and aluminium in proportions generally lower than 1%.The melting and alloying yield is about 97%, so about 3% of residues are produced with a composition of pure SHG zinc or alloyed zinc.Generally, these residues are in the form of ashes which consists of oxides and metal.Once the zinc is alloyed, the liquid alloy is cast in a continuous or semicontinuous slab casting machine.The resulting slab is then rolled in several stages and coiled to obtain coils of zinc sheet.These coils are then cut to width.Coils can be sold as such, or they can be coated with different organic or inorganic coatings.Part of the produced material is for manufacturing of gutters, tubes, elements, and various downpipes for rain-and stormwater drainage.
Rolled zinc is mainly produced in Europe (France, Germany, Netherlands, Spain).There are also producers in Peru, the USA and China, and probably very small producers elsewhere in the world.Rolled zinc production is estimated between 250 kt and 290 kt per year.Rolled zinc is assumed to have the best end-of-life (EoL) recycling rates amongst other zinc applications.In Europe this rate is typically >95%.The recycling route exist of collection of scrap by roofing and building demolition companies, followed by sorting at metal recyclers, followed by remelting and casting at specialized recycling companies.The recycling market is estimated between 100 kt and 200 kt of zinc per year in Europe.Zinc losses to landfills are estimated to be nil.
Collected EoL rolled zinc ("roofing scrap") is typically recycled into alloys for the brass industry because it can contain small amounts of lead, tin and cadmium.Roofing zinc scrap can be very old, dating back more than 50 years when it was installed.At that time, rolled zinc was made with an alloy of zinc, lead and cadmium.Also, solders used in roofing installation are typically lead and/or tin alloys.

Recovery of zinc from secondary raw materials from first use production
It has been mentioned already that residues from galvanizing are almost fully recycled into new secondary zinc products, because of their very high zinc content.These secondary zinc products are typically zinc oxide powders, metallic zinc powders and metallic zinc dust.The manufacturing of zinc powders or zinc dust is carried out in different processes.Some processes manufacture only zinc powders and others only zinc dust, some only metallic zinc and some only zinc oxide.However, some processes can produce both metallic and oxidic type of products.Thanks to the processes used, often based on preferential evaporation of zinc at high temperature, the production of zinc powders or zinc dusts allows an efficient recycling of secondary material flows from galvanizing.

Zinc recovery from secondary raw materials with the French process
The French process (FP) or indirect process is the simplest and most widely used process for making zinc oxide (ZnO) powders.Zinc is melted in a graphite crucible and vaporized at temperatures typically around or above 1,000°C.The high temperature drives the separation of zinc and iron in iron-zinc intermetallics, often without the help of reducing agents such as carbon.The zinc vapor reacts with the oxygen in the air to produce ZnO, accompanied by a drop in temperature and bright luminescence.The feed is generally bottom dross from batch galvanizing process but can also be a primary feed such as special high grade SHG or high grade (HG) zinc.This process is often used to treat this type of secondary raw materials because bottom drosses contain associated elements such as iron, lead, tin, and nickel.These elements are not very volatile around 1,000°C and therefore remain, for a large part, at the bottom of the crucible.This makes possible to purify secondary materials in a very simple way.Depending on process parameters, a wide variety of zinc powder grades can be produced, see Table 4 [12].

Zinc recovery from secondary raw materials with the American process
The American process (AP) or direct process is a more complicated process able to treat complex oxidized residues such as galvanizing top ashes, after mechanical separation of the metallic parts from the ashes.The zinc oxide in the residues is reduced via carbon sources in a rotary drum furnace at high temperature (about 1,200°C).When the zinc is reduced into metal, the zinc evaporates, and the fume is re-oxidized to make purer ZnO powders.This process is able to purify the zinc from different associated elements.Effectively, some associated elements are not reduced (Al, Mg) and stay in the oxide fraction.Other associated elements are reduced but the resulting metals are not volatile (Fe, Sn) so stay in the slag phase.It must be mentioned that the separation is not perfect because the oxides of some metals are also volatile at the process temperature and report to the ZnO fraction in the gas stream.Also, dust with other associated elements can be entrained in the ZnO fraction in the gas phase.Without addition of carbon, the American process can serve as a clinker process to remove halides (Cl, F) from material streams.

Zinc recovery from secondary raw materials with other processes
The Larvik process allows the treatment of different zinc materials to produce powders.The materials are melted in an electric resistance furnace equipped with various baffles.This furnace is topped by a distillation column which allows a certain purification to be achieved.The vapor is then condensed into zinc oxide powder or zinc metal dust depending on the atmosphere.The feed is usually SHG zinc ingots, HG zinc ingots and zinc drosses from the batch galvanizing process.Zinc dusts obtained by condensation are generally round in shape.
The Norzinco process (or New Jersey process) is also a process that uses an electric melting furnace topped by a distillation column.The electric furnace is heated by induction.Like the Larvik process, it is possible to produce oxidized zinc powders and zinc metal dusts.The feed for this type of furnace also consists of primary zinc (SHG and HG) and/or secondary zinc.
In 2015, the production of zinc oxide powders and zinc metal powders & dust was estimated at 1.28 Mt per year and 300 kt per year respectively, which represents a total of 1.58 Mt per year.Raw materials used for these products were 546 kt zinc in secondary raw materials (35%) and 1,034 kt zinc in primary raw materials (65%).

Zinc recovery from galvanized steel scrap
Steel is primarily produced via: 1) The blast furnace (BF) / basic oxygen furnace (BOF) route, or 2) The electrical arc furnace (EAF) route.Other technologies exist such as the open hearth furnace (OHF), or are under development (e.g.gas direct reduced iron (GDRI) or coal direct reduced iron (CDRI)).The BF/BOF route represents about 70% and the EAF route represents about 25% of the total steel production volume globally.These two technology routes are the most used processes with about 95% of the total steel production volume.The ratio between EAF and BF/BOF varies as a function of time and region.In developing regions, BF/BOF production is predominant because the available of steel scrap is typically low and EAF technology is not well developed.Figure 4 shows a strong growth of EAF production technology in North America and, to a lesser extend, in Europe in the last 3 to 4 decades [13].In the longer term, absolute steel volumes produced through the EAF route will increase in almost all regions, and probably, the relative volumes will also increase.
New and old, non-galvanized and galvanized steel scraps are all recycled via the BF/BOF and EAF routes.The total scrap ratio in the BF/BOF process ranges between 5% to 35%, where scrap is used as coolant for the converter in the BOF process.The scrap ratio in the EAF process can be close to 100%, although direct reduced iron (DRI) can be used in EAF processes as well to substitute scrap, depending on availability of steel scrap in the marketplace.Zinc which is contained in the coating layer of galvanized scrap does not report to the new steel produced in an EAF.The operating temperatures of an EAF (generally >1,500°C) are much higher than the evaporation temperature of zinc (907°C).Zinc is therefore reporting to the steel dust residue, that is collected in filters.
Blast furnaces and basic oxygen furnaces also produce steel dust which is collected in filters.The typical zinc content in the 3 types of steel dust is very different: • Blast Furnace (BF) flue dust consists of iron-oxide particles, oxides, carbon, and lime.It is generated by the blast furnace during smelting and collected in the gas exhaust and cleaning system (e.g., bag house filters).Those residues consist of mainly iron and 0-3% zinc on average.• Basic Oxygen Furnace (BOF) flue dust consists of iron-oxide particles generated by the basic oxygen furnace during steel making and is collected in the gas exhaust and cleaning system (e.g., bag house filter).It typically consists of mainly iron oxide and 0.5-6% zinc on average.• Electric Arc Furnace (EAF) flue dust consists of iron oxide particles generated in the electric arc furnace during steel making.It is collected in the gas exhaust and cleaning system (e.g., bag house filter).Most of the dust is iron oxide with 10-40% zinc and trace amounts of lead, cadmium, chromium and arsenic.Figure 5 show the absolute weights and relative proportions of residues produced by BF/BOF and EAF processes [14].Slags are the residues produced in larger quantities (150 kg to 300 kg per ton of steel produced).Dusts and sludges are generally produced in smaller quantities (20 kg to 30 kg per ton Based on the estimates described above, a calculation of the quantities of zinc ending up in steel dusts per year can be made.The amount of crude steel produced in 2015 was 1,620 Mt per year.For each route, an estimate of the produced zinc volumes in the steel dusts can be assumed: • BF (Blast Furnace): 340 kt zinc per year in produced BF dust globally • BOF (Basic Oxygen Furnace): 1,200 kt zinc per year in produced BOF dust globally • EAF (Electric Arc Furnace): 2,050 kt zinc per year in produced EAF dust globally The zinc concentration in BF and BOF dusts is too low to be economically processed in classical recycling processes for zinc.Consequently, at the time being, a very low amount of zinc is recycled and recovered from BF and BOF dusts.This quantity is below 10% of the total zinc present in BF and BOF dusts.An exemption is a BF process operated DK Recycling in Germany.This special BF process is able to process a variety of secondary raw materials, including a significant amount of BOF dusts.The unique BF process is able to accept up to 38% of zinc in the feed materials, which is extremely high compared to the feed limitations of classical BF processes which is typically up to 0.1% of zinc.Also, relatively small amounts of BOF dusts are used in the cement industry as BOF dust provides a source of Fe in cement.This is considered as a loss of zinc and not a use of zinc as the zinc has no meaningful function in cement apart from the fact that zinc in concrete is a corrosion inhibitor to reinforcing steel.
More constringent regulation for landfilling of BF and BOF residues can stimulate and increase the recycling of those materials.In some regions, like in some developed Asian countries where land area is scarce, innovative, or classical processes such as the rotary hearth furnace (RHF) and the electric smelting reduction furnace (ESRF) have been developed to recover both iron and zinc from these steel dusts.EAF dusts from these regions are recycled in these two processes.
The zinc concentration in EAF dusts is often high enough to be economically processed in classical recycling processes for zinc.There are many possible treatment technologies for EAF dust.Most of these treatments are pyro-metallurgical, but there are also hydro-metallurgical process options.However, the Waelz process is still by far the most used process and still representing, in volume, 85% of the market in 2015.Not all EAF dusts are recycled, a meaningful amount is still landfilled, even though EAF dust is considered a hazardous waste.It is mentioned above that 2,050 kt of zinc in EAF dusts was available for recycling in 2015.The actual recycled amount of zinc in 2015 from EAF dusts was a fraction of this, being 890 kt of zinc or 42% of all zinc in EAF being available for recycling.Consequently, 1,160 kt of zinc was lost to landfill sites.
Pyrometallurgical processes are often used for recycling of steel dusts because they rely on the ability to reduce (at about 1,200°C) and vaporize the zinc metal at a relatively low temperature (the zinc boiling point is 907°C).Within this group of processes, some recover only zinc (such as the Waelz process) while other processes recover both zinc and iron.Within this last group, there are processes that recover iron in the form of DRI (e.g., RHF) and processes that recover iron in liquid form (e.g., ESRF).Table 5 Figure 5: Absolute weights and relative proportions of residues produced by BF/BOF and EAF processes presents the main commercial and experimental pyrometallurgical processes that are currently used in the world [15].Hydrometallurgical processes for recycling of steel dusts have disadvantages linked to the intensive use of water, and advantages such as the possibility of recovering valuable metals present in the steel dusts (copper, nickel, etc.).
Once zinc is separated from the other elements, the resulting material, which is called crude zinc oxide (CZO), contains typically 50-70% zinc.If CZO is produced with the Waelz process, it is typically called Waelz oxide (WOX).CZO or WOX is generally further processed by existing hydrometallurgical processes to produce special high grade (SHG) zinc (min.99.995%).Other process for treating CZO or WOX can be used as well, like the pyro-metallurgical imperial smelting process (ISP) or the solvent extraction & electrowinning (SX-EW) process.Other processes like the ZincOx process purify CZO or WOX to manufacture marketable zinc oxide powders directly, thus avoiding the energy spent (3,100-3,500 of electricity) to produce metallic refined zinc metal.

Zinc recovery from municipal solid waste
According to the International Solid Waste Association (ISWA), the total volume of municipal solid waste (MSW) generated worldwide is about 2,000 Mt per year.It is estimated that about 260 Mt of this waste is incinerated.Many countries in Europe, North America and part of Asia incinerate a higher proportion of their waste.Incineration, composting and recycling have become increasingly important over time.At the time being, the MSW incinerated in Europe is estimated at 70 Mt per year.
The incineration of municipal wastes is often conducted at around 850°C for normal waste (1,100°C for hazardous waste).Figure 6 shows the incineration process with the formation of incinerator fly ashes (IFA) and incinerator bottom ashes (IBA) [16].In this figure, the (potential) recovery of metals and energy is also mentioned.In some incinerators, IFAs are treated in order to sort out certain elements like metals.At 850°C, zinc is melted and partially oxidized.Part of it can be vaporized and/or oxide particles can be carried to the filter.Figure 7 shows the repartition of the metals between the IFA and the IBA [17].
It is estimated that an average of 200 kt of zinc in IBA and 400 kt of zinc in IFA is available in Europe for recycling from waste incinerator residues, however most of these zinc volumes are currently not recovered in zinc recovery processes.If all MSW produced in the world would be incinerated, there is a potential source of 1.8 Mt of zinc to be recovered per year.

Conclusions
Zinc losses occur in the refined zinc production chain, in zinc first use production processes and at the end-of-life phase of products that contain zinc.Some of these zinc losses are recycled, in particular in case when the collection and sorting of zinc scrap zinc waste is possible, when zinc scraps or wastes contain a high amount of zinc and low concentrations of other impurities, and when the recycling of zinc is economical.
The largest recycling loops for zinc are the recycling and recovery of zinc from batch galvanizing residues (1,150 kt Zn/year); pre-and post-consumer galvanized steel scrap (890 kt Zn/year); brass scrap (874 kt Zn/year); continuous galvanizing residues (635 kt Zn/year) and end-of-life zinc die casting scrap (475 kt Zn/year), all with reference to the year 2015.Recycling of rolled zinc (for roofing and building) has the highest end-of-life recycling rate of >95%.Recycling of manufacturing or prompt scrap from galvanizing (galvanizing residues) has a very high recycling rate also due to the very high zinc content in these residues and relatively low concentration of impurities in these pre-consumer scrap materials, which makes it economically viable to recycle them in commercial grade secondary zinc products.
At the other hand, many opportunities still exist to increase zinc recycling and reduce losses of zinc in abandoned products, to landfilling sites and to waste incinerators.Various zinc containing waste materials are not yet recycled and zinc is not recovered because the collection and sorting is a challenge, or recycling is not economical.Examples of opportunities to increase recycled zinc volumes are losses of zinc in the form of tailing losses from mineral processing (up to 1,900 kt Zn/year); not collected endof-life zinc die cast parts (up to 1,525 kt Zn/year); landfilled BF and BOF steel dusts (up to 1,540 kt Zn/year) including BOF dust now used in cement production; landfilled EAF steel dusts (up to 1,160 kt Zn/year); not collected end-of-life brass parts (up to 730 kt Zn/year); not recycled bottom and fly ashes from municipal solid waste incinerators in Europe (up to 600 kt Zn/year) and residue losses from zinc smelting and refining (up to 415 kt Zn/year), all with reference to the year 2015.

Figure 2 :
Figure 2: Evaluation of the Reserves and the resources of zinc

Figure 3 :
Figure 3: Copper alloy losses during the stages of collection and separation of post-consumer scraps from EoL phase

Figure 4 :
Figure 4: Variability of the EAF process share as a function of producing regions

Figure 7 :
Figure 7: IBA and IFA volume fractions.Repartition of the metals between IBA and IFA

Table 3 :
Bath compositions used generally in the hot dip continuous galvanizing lines, the balance is zinc and trace elements

Table 4 :
Zinc oxide powder commercialized grades