Evaluation of recycled galvanised steel as a resource for Zn production

The high reduction potential of zinc has enabled its extensive use in the steel industry as a protective galvanising coating. Galvanising, which consumes approximately 50-60% of the global production of zinc, significantly increases the life span of steel products and contributes towards addressing the costly global problem of corrosion. While there does not appear to be a readily available and agreed-upon annual quantity of galvanised steel produced, it has been estimated by the authors to be 320-390 Mt/a (equating to 15-20% of the annual steel production). Due to the various methods employed to galvanise steel and different steel shapes which are coated, there is no clarity on the average percentage of zinc which galvanised steel is comprised of. However, a zinc content of between 1.5-2.5% appears to correlate with the estimated annual galvanised steel production. The reprocessing of secondary zinc streams is becoming increasingly important due to legislation changes and increased consumer demand for recycled and low carbon metals. Within the steel industry this will include the recycling of galvanised scrap steel, using primary process routes, as well as the re-processing of waste streams, such as steel dusts, using additional technologies such as the Waelz kiln. Zinc associated with galvanised scrap steel would exclusively report to these dusts, and thus represents a key resource for recycled zinc. Although there are two main steel process routes preceding the galvanising process, the focus here is on the Electric Arc Furnace (EAF) process route due to its ability to incorporate high levels of secondary (zinc containing) steel feeds, and the dust produced typically contains economically recoverable levels of zinc. EAF dust production is estimated between 5-10 Mt/a at a zinc content of 10-36%. Failure to recover this zinc would represent a loss of 0.5-3.6 Mt Zn/a to waste. The Waelz kiln is the most prominent technology utilised to recover Zn from EAF dust. However, this process requires additional energy, and thus the associated carbon footprint needs to be included in the evaluation of the potential of Waelz oxide as a secondary Zn resource. To this end, model analysis of a simplified Waelz kiln using the HSC simulation software shows that the carbon footprint associated with the processing of EAF dust ranges from 4-12 kg CO2/kg Zn dust at 10 wt.% Zn in the dust to 1-4 kg CO2/kg Zn at 35 wt.% Zn in the dust.


Introduction
Zinc's most significant property is its high reduction potential.This property has enabled its extensive use in the steel industry as a protective galvanised coating.While the growth in demand for steel as a ubiquitous engineering and construction material is due to increased urbanisation and population growth, an additional demand relates to the costly global problem of material loss due to corrosion.The addition of zinc in the form of galvanising contributes towards addressing corrosion and thus significantly increases the life span of steel products.As a result, approximately 50-60% of the global production of zinc (13 Mt/a) is used to galvanise steel [1] which equates to 6.5-7.8Mt/a.
The reprocessing of secondary zinc streams is becoming increasingly important, due to legislation changes and increased consumer demand for recycled and low carbon metals [2][3][4][5].As zinc's primary usage is in galvanised steel, the recycling of this product and the associated by-product and waste streams represents the biggest opportunity for zinc recovery.The recycling of galvanised scrap steel would occur using primary production process routes while the re-processing of waste streams, such as steel dusts, would occur using additional technologies (e.g.Waelz kiln).During reprocessing, zinc associated with galvanised scrap steel almost exclusively reports to these dusts, and thus they represent a key resource for recycled zinc.Although there are other secondary zinc streams, such as the neutral leach residue from the dominant Roast-Leach-Electrowinning (RLE) primary process route, these are not considered in the present study which is focussed on recycling of Zn used in galvanised steel.
The current global recycled content of zinc is approximately 25% [6].While this may appear low, it is due to zinc's primary function in extending the lifespan of products.Therefore, a large portion of Zn historically used in galvanising steel remains in service and is thus not yet available for reprocessing and recycling.
Recovering the zinc from secondary resources requires additional energy, typically supplied to a pyrometallurgical process through the addition of coke (carbon) as a reductant or through the addition of electrical energy.Given the relatively large recycling rate of steel (± 85%) [7], the carbon footprint of recycling technologies warrants investigation.This additional carbon footprint needs to be determined to more comprehensively assess the metrics with which the "greenness", "sustainability" and "circularity" of metals are evaluated.
The objective of this desktop study was therefore to address gaps in available literature with respect to quantifying potential zinc secondary resources and the additional carbon (CO2) footprint associated with returning this secondary zinc back into primary production.To this end, a preliminary model analysis of a simplified Waelz kiln using simulation software is presented to provide this data.

Methodology
The methodology for this desktop study involved a literature review of the technologies associated with galvanised steel production using a combination of journal papers and open access reports from the steel industry.As galvanised steel is seen as the major source for recycled Zn, it will be the steel type primarily considered in terms of demand, supply, and associated technologies and processing routes (EAF and Waelz kiln).Based on the collected information, an estimate of the quantity (Mt/a) of galvanised steel produced was made.Similarly, the quantity (Mt/a) and zinc content (%) associated with the production of galvanised steel was also estimated using extrapolated data from reports.It should be noted that many of the values presented have a wide range due to conflicting pieces of readily available information, resulting in uncertainty.Finally, the Waelz kiln was reviewed as a technology for the recycling of electric arc furnace (EAF) dust, and the CO2 footprint (kg CO2/kg Zn) was estimated using a high-level Waelz kiln model with Metso's HSC simulation software [8] in combination with carbon footprint associated with different types of energy sources available from literature.

Global Steel and Galvanised Steel Production and Key Technologies
3.1.1.Overview: Steel and galvanised steel.Although there are over 3 500 different grades of steel, each with their own physical and chemical properties, steel is principally an iron alloy which is commonly combined with other elements to improve the properties of steel, such as carbon (<2%), manganese (<1%), silicon, phosphorus sulphur and oxygen [9].There are many different standards which are used to classify the grades and types of steel.Common classifications include carbon steel, alloy steel (low and high alloy steel), stainless steel (>10% chromium) and tool steel, each of which can be further divided into grades.
The embodied energy and the carbon footprint per tonne of steel produced is reported to be low compared to other metals [10]; however, the sheer quantity of crude steel produced globally (> 1950 Mt in 2021) results in high annual energy consumption and carbon emissions [11].Although, the steel industry has reduced the energy intensity of its processes, with the average energy needed to produce one tonne of steel decreasing by 60% since the 1960s and the carbon footprint (per tonne of steel) decreasing by 16% in the last 10 years, the industry still accounts for approximately 7-8% of total global carbon emissions [11,12].
Corrosion is an underestimated and costly global problem.It is estimated that it costs the global economy US $2.5-3 trillion annually (in the U.S.A. it accounts for ± 3-3.5% of GDP) as a result of corroded steel infrastructure and buildings [13,14].This is not only an economic cost, but also adds to the demand for primary resources and therefore has environmental implications [15,16].Corrosion prevention and management strategies are therefore of vital importance to maximise resource efficiency.
One method to protect steel from corrosion is the addition of a thin zinc coating on top of the steel, through galvanisation.The most common method is hot-dip galvanising (HDG), although other processes such as electroplating can also be applied [17,18].The versatility of galvanised steel has allowed its application in construction, telecommunications, automotive, agriculture and energy (solar and wind) projects [18].
Stainless steel is also often used due to its corrosion resistance properties.However, stainless steel usually costs 4-5 times more than galvanised steel.Consequently, the demand for galvanised steel is highest in certain industries such as construction, automotive, and home-appliances [19].As the lifespan of steel products is measured in decades, longer if galvanising finishing treatments are applied (Figure 1), there is a significant time delay between production and end of service life at which recycling would occur.Although this optimises the in-service value of the material, it does result in the sustained primary production of steel.Steel currently has an average of 37% recycled content [11].Therefore, both primary and secondary production methods of steel production will continue to be prevalent in the industry.

Steel production processing routes and technologies.
There are two main process routes for steel, namely the blast furnace-basic oxygen furnace (BF-BOF) and the electric arc furnace (EAF), which account for 70-72% and 27-29% of global steel production, respectively (Figure 2), with variations and combinations of technologies in existence [11].The key difference between the two routes is the incorporation of coke as a raw material within the BF-BOF process (Figure 3).While BF-BOF primarily uses iron ore and coke as a feed and has more limited option of incorporating recycled steel, the EAF process typically uses recycled steel as its primary feed and electricity as its energy source [11].The large carbon footprint associated with steel production therefore primarily originates from the BF-BOF's use of coke as a raw material, while for EAF the carbon footprint would be dependent on the local source of electricity.
In the BF-BOF process, the iron ores need to be first reduced to iron (pig iron / hot metal) in the BF, and this is followed by its conversion to steel within the BOF.The steel is then cast and rolled into various shapes (tubes, sheets, rods etc.) [11].Within the EAF process, electricity is used to melt recycled steel, while additives are used to adjust the steel to the required chemical composition.Oxygen injection can also be used to supplement the electrical energy supply to the EAF.The downstream process steps are similar to those of the BF-BOF process [11].While the BF-BOF process can use up to 30% recycled steel as a feed, 100% of the EAF processes feed can use recycled steel.Although steel can be infinitely recycled as its properties are preserved during reprocessing, the increasing demand will require the continued usage of both BF-BOF and EAF routes due to need to process fresh iron ore for the foreseeable future [11].
With reference to Zn recycling, the EAF process route can incorporate high levels of secondary, zinc containing steel feeds, and the dust produced typically contains economically recoverable levels of zinc.

Blast Furnace Electric Arc Furnace Open Hearth Furnace Other
The Waelz kiln is the most prominent technology utilised to subsequently treat EAF dust.EAF dust production is estimated between 5-10 Mt/a, at a zinc content of 10-36%.The recycling of this dust is and should continue to be a priority as failure to recover this zinc would represent a loss of 0.5-3.6Mt Zn/a, or up to 60% of the Zn consumed in galvanising.
Other technologies include the open-hearth furnace (OHF), otherwise known as reverberatory furnace, which accounts for approximately 0.4% of crude steel production, and the usage of which is in decline.As of 2012, there were only 7 plants that use reverberatory furnaces (of which OHF is the main type) left globally [22].Since then, 6 OHFs in Mariupol, Ukraine, closed in 2015 [23] and 2 further OHFs in Vyksa, Russia, were closed in 2018 [24], with any remaining furnaces likely to be to be located in Ukraine, India, or Uzbekistan [24][25][26].The reason for this decline is that the OHF is uneconomical due to its excessively energy-intensive and long process times, in addition to other environmental concerns [11].Another known technology is the induction furnace (IF); however, this technology was banned in China in 2017 for producing environmentally unfriendly and sub-standard quality steel products [27,28], and it is unclear whether any IF facility remains in operation.

Regional distribution of steel technologies.
With regards to the regional contribution that each of these technologies make to global crude steel production, Asia accounts for the combined majority (72%), and China represents 74% of Asia's production (i.e.China represents 53% of global production of crude steel).With respect to global BF-BOF and EAF steel production, Asia represents 81% and 48% respectively, with China representing 82% (China produces 67% of world BF-BOF steel) and 41% (China produces 20% of world EAF steel) of Asia's production [21].Figure 4 summarises the global contribution that the BF-BOF and EAF technologies make to global crude steel production and is divided by region.Asia represents 48% of global EAF steel production with China representing 41% (i.e.China produces 20% of world EAF steel) of Asia's production.North America and Europe produce a combined total of 28% of world EAF production (compared with 12% of BF-BOF production), which highlights the focus of the Western world on secondary (recycling) rather than primary production.

Galvanised Steel.
There does not appear to be a readily available and agreed-upon annual production quantity of galvanised steel.Multiple methods were therefore used to arrive at a consistent value.Firstly, a recent market forecast report placed the global production as 337.3 Mt/a [29] in 2017.Secondly, another source reported that China consistently produced ± 65 Mt/a of HDG sheets during the 2017-2020 period [30].Assuming that sheeting represents ± 40-41% [31] of the global galvanised steel finished product market, this places China's total HDG steel production at ± 160 Mt/a.It should be noted that this would be an under-estimate as other galvanising techniques are not included in this value.Given China's dominance within the zinc industry (± 50% market share) as well as within the steel industry (53%), a similar dominance (± 50%) is expected within the galvanising steel market, thereby conservatively placing the global annual production close to 320 Mt/a.A third attempt to corroborate these values and to establish a more recent (2022) value was made using values found within one of the market forecast reports [29].The Compound Annual Growth Rate (CAGR) (Formula 1 in Table 1) was restructured (Formula 2 in Table 1) using Vi=Vf -D, and applied.The annual production in 2022 was then back-estimated as 388.2 Mt/a respectively.The values used within the calculations can be found in the Table 2.
Thus, the three methods appear to produce relatively consistent values ranging from 320-390 Mt/a between 2017 and 2022 (equating to 15-20% of the annual steel production).As the zinc used in Electric Arc Furnace galvanisation only represents approximately 6.5-7.8Mt/a, the values remain the same irrespective of whether it is assumed that the above values are inclusive or exclusive of the zinc coating.These same reports, forecasting the global galvanised steel market, project the galvanised steel market to grow by over 80 Mt between 2022 and 2027, primarily due to demand from China and India in the pre-engineered buildings sectors [29,31,32].The United States of America and Japan also represent large consumers of galvanised steel [19].In addition to the uncertainty surrounding the size of the galvanised steel market, there is no average percentage of zinc which galvanised steel is comprised of.This is due to the various methods used to coat steel with zinc, the different shapes of steel which are coated (sheets, tubes etc.) as well as the numerous standard categories which each specify minimum thicknesses of zinc coating.For instance HDG sheets produced from cold rolled steel have an average thickness of 0.25-0.2mm which sheets made form hot rolled steel are typically in the 2-3 mm thickness range [33].It has been claimed that zinc usage is up to 5-7% 2 of the mass of coated steel [34] (assuming no zinc losses nor wastage this equates to only 80-130 Mt/a galvanised steel production), however in contrast, zinc content of between 1.5-2.5% 3 (realistically averaging between 1.6-2% if taking into account up to 10% Zn loss) would correlate with the reported galvanised steel production quantities of 320-390 Mt /a.The exact density of the ungalvanised steel (7.75-8.05g/cm 3 ) and whether the zinc coating is included or excluded, appears to make very little difference to the overall Zn content (%).The most significant factor is the thickness of the steel to be galvanised and secondly is the thickness of the Zn coating (Table 3).Based on these numbers, assuming a zinc content of between 1.5-2.5% in galvanised steel, for the modelling of zinc content within recycled steel processed within EAF furnaces, does not seem unreasonable.

Quantifying production of waste from galvanised steel production
Both the BF-BOF and EAF processes produce wastes (Figure 3).These wastes are primarily in the form of slag (90% w/w), dust and sludge and cumulatively represent 400 kg (BF-BOF) and 200 kg (EAF) worth of co-products per tonne of steel produced 4 [35].
As can be seen from the above numbers, BF-BOF dust represents a significantly higher contribution (2-4 times) to the quantity of steel dust compared to EAF dust.Due to the chemical nature of these dusts, they are regarded as hazardous and must be chemically / thermally treated prior to disposing in landfills, which is expensive [41].Alternative methods for (economically) processing these BOF dusts are therefore important areas of research.In addition, there needs to be increased focus on producing "quality" waste which would be better suited for further processing [42].Currently the economic threshold for the recycling of EAF dust is determined by the zinc content being higher than 15% [41,42].EAF dusts containing less than this threshold of zinc are currently sent to landfill, which in the EU reportedly makes up two thirds of their EAF dust production [41].The reason for the low Zn content in the EU in unknown and may simply be as a result of the types of scrap steel available for recycling (i.e., non-galvanised steel).However, the costs of disposal are no longer negligible and continue to rise with increasing legal responsibilities being placed on producers of hazardous material [42,43].
Given that 30-40% of the steel produced by the steel industry is from recycled scrap steel, which may not necessarily be galvanised steel scrap, the composition of EAF dust depends strongly on which types of steel scraps which are processed in the EAF [44].Only when galvanised steel scraps are included in the feed material to EAF will it result in the presence of zinc in the EAF dust.For this study only EAF dust which results from the processing of galvanised steel is considered.
Iron and zinc oxides represent the major constituents in EAF dust.Depending on the source of galvanised scrap, which is increasingly introduced into EAFs, the content of zinc in EAF dust can vary from 10-36% (Table 4) [42,44,45].The most common range, however, is between 20-30%5 , while the Fe content can range from 20-30% [40].In comparison, the zinc content of BOF dust can vary between 0.1-3.2%while Fe ranges from 30-85% [36].The low Zn content in BOF dusts have made it difficult to economically recover the zinc, however, the presence of Zn and other deleterious metals still presents a hazard which needs to be treated prior to the disposal of the BOF dust.The exact percentages of EAF dust and BOF dust which are recycled is unknown.While some authors have reported that 75% of the EAF dust produced is treated using Waelz kilns [40], others claim that over 50% of EAF dust is landfilled [46].In the absence of other information this range (50-75% recycling of EAF dust) will therefore be assumed (see Figure 6).If there is no recycling of EAF dust this equates to a maximum loss of Zn of 3.5 Mt/a, which is the maximum height of the dotted area on the far-left bar of Figure 6.However, realistically the maximum zinc loss would be closer to 1.5-2 Mt/a (11-15% of annual Zn production), which is represented in yellow in Figure 6.If 50% was recycled it would represent a maximum loss of about 1.75 Mt/a, while realistically being closer to 0.75-1 Mt/a (5.5-7.5% of annual Zn production).This corresponds to another author's estimation of annual Zn loss to landfilled EAF dust [46].Lastly at 75% recycling there would be a maximum loss of ±1 Mt/a, and realistically 0.4-0.5 Mt/a (3-4% of annual Zn production).In the absence of additional information, BOF dust is assumed to have limited or no recycling.Therefore, it is assumed that the zinc in BOF dust is lost to landfill (±0.5 Mt Zn/a lost via BOFD).

Review of Waelz kiln for EAF dust recycling and Carbon Footprint
Although some hydrometallurgical processing routes have been proposed and demonstrated, only High Temperature Metals Recovery (HTMR) processes, which are pyrometallurgical, have been commercially successful [42].The following pyrometallurgical processes are or have been in operation to treat EAF dust: Rotary kiln (Waelz kiln), Rotary Hearth Furnace (RHF), Multi-stage furnace (PRIMUS), and the Electric Smelting Reduction Furnace (ESRF) [47].80-90% of the EAF dust which is recycled (approximately 2-2.1 Mt) is treated using the Waelz kiln, or a derivate of this technology, as it is currently deemed the most efficient processing route [42,44,48].

Waelz kiln.
In principle, the Waelz kiln process involves the carbothermic reduction of zinc compounds (typically ZnO), followed by the volatilisation of metallic zinc and the subsequent reoxidation of metallic zinc to ZnO.The selective volatilization of certain metals in the zinc waste (EAF dust or leaching residue) results in a more concentrated final zinc oxide product which can then be returned to traditional zinc processing routes such as the RLE for further zinc recovery.
The following reactions (3(8) represent the key reactions occurring in the Waelz kiln, assuming that the Zn in the feed (EAF dust) is primarily present in the form of ZnO.Further details regarding the processing of sulphide feeds and additional operations of the Waelz kiln can be found through the following references: [44,[49][50][51][52].The first stage is the evaporation of moisture at temperatures of approximately 600⁰C.
2  () →  2  () (3) This is followed by the reduction and immediate volatilization of Zn after which the zinc vapour is re-oxidised and removed as fume.
The Waelz kiln is a type of rotary kiln / furnace, that typically consists of the kiln and auxiliary equipment such as a mixer, a fan, and a fume-collecting system (settlers).The Waelz kilns, for zinc operations, can reach a 1250⁰C, however the temperature is difficult to measure and varies over the length of the kiln.The largest kilns require 18-22 kW of power (motors etc.).The fume-collecting equipment consists of different types of settlers, namely cyclones, baghouses, and Electrostatic Precipitators (ESP).As the dust particles are heavier than the zinc fume, a simple settler is often sufficient [50].The zinc enriched product is referred to as Waelz oxide with the by-product being the Waelz slag.Waste gas, dust and Waelz oxide (fume) leave at the upper (feed) end of the kiln, while the slag leaves from the lower end of the kiln.
Like many pyrometallurgical processes the Waelz kiln allows for greater operational flexibility in terms of the feed streams.The Waelz kiln's ability to treat ZnO (zinc oxide) and ZnFe2O4 (ferrites) has added to its greater success over hydrometallurgical processes [42].It is also unnecessary to control the roasting conditions as at the operating conditions of the Waelz kiln ferrites, ferrates, sweet roast and sulphate roast are all easily reduced [50].
The Waelz process can process material with low zinc concentrations, however, the higher amounts of inert material result in significant increase in energy consumption.As a result, recycling companies will charge producers a higher price for the processing of low zinc dusts.A minimum threshold of 15% Zn is typically required to achieve financial feasibility [42].
Unfortunately, like many pyrometallurgical processes, the Waelz kiln requires a high Capital (CAPEX) investment and operating (OPEX) expenditure.It benefits from scaling, which means that larger operations are more economically viable than small scale.This poses a challenge for processing operations which would normally incur high transportation costs due to their remote locations [42].
Another disadvantage of the Waelz process is that it leaves Waelz slag as a residue which contains oxides of iron and calcium [42].This Waelz slag is not typically of a quality that makes it saleable or useful [47] and is often landfilled.To address this shortcoming, significant research has been put into other technologies such as MINTEK's Enviroplas process [53].
The most commonly used options for reduction of the Waelz kiln feed streams are metallurgical coke and coke breeze [50].Coke usage can range from as low as 10-12% [50] to treat calamine waste, to 25-40% [44] of the feed (assumed on a dry basis), with the average being 30% [50].The moisture content of the coke can range from 15-20% [50].The incoming air is assumed to be at STP and consist of 78-79% nitrogen and 20-21% oxygen.The flowrate of air into the Waelz kiln is reported to be ±3000-3500 m 3 /h (35-50 cu.ft/s) [50].The main heat losses are in the waste gas stream.The exit gas compositions have been reported as 18-28% CO2, 0-3% O2 and 0-2% CO [50].

Carbon Footprint and reduction options.
The zinc industry's average carbon footprint, across all processing routes, appears to range from 2.5-4 kg CO2 eq/ kg Zn, however, this can vary widely reaching as much as 6 kg CO2 eq/ kg Zn in extreme cases [54][55][56][57].Although the embodied energy and the carbon footprint per tonne of steel produced is considered low compared to other metals, the enormous quantity of steel produced results in significant energy usage and CO2 footprint considerations.
While the average energy consumption of steel is 21.31 GJ/t crude steel, the technology choice plays a significant factor.While the BF-BOF route requires 24.43 GJ/t, the EAF only requires 10.04 GJ/t, which is less than half of that associated with BF-BOF [58][59][60].As coke is a significant feed source for BF-BOF route, the associated CO2 footprint is also significantly higher than that of EAF.EAF has an average global carbon footprint of 0.67 kg CO2/kg steel, as opposed to 2.32 kg CO2/kg BF-BOF steel.It is unclear, which exact energy mix was used for this calculation, however; 10.04 GJ/t equates to 2789 kWh/t which in turn results in a carbon footprint of ±0.24 kg CO2/kWh, which is significantly lower than coal while being higher than any of the usual low carbon green sources of energy (wind, solar, hydro).It shows, however, that the CO2 footprint for EAF steel can be further reduced if other low carbon green sources of energy such as those listed are used for the electricity generation [59,61,62].
With respect to the EAF, the reported energy (electricity) requirement is only 440 kWh/t EAF steel produced (1.61 GJ/t EAF steel) [66], although this directly equates to approximately 0.36 kg CO2/kg EAF steel (assuming 820 g CO2/kWh using coal as an electricity source).This is significantly less than the above reported values and it is assumed that there is additional auxiliary equipment which has been taken into account to form the above significantly larger energy requirement and CO2 footprint.As EAF processes are reported to have a total CO2 footprint of 0.67 kg CO2/kg EAF steel, the values in Table 5 were adjusted to account for this discrepancy by normalising them with reference to coal.As can be seen in Table 5, green energy options (solar, wind, hydropower) may result in the footprint decreasing by up to 95%.However, it should be noted that these values do not take into account the footprint of any auxiliary equipment (energy storage) which may also need to be installed when changing energy sources.The CO2 footprint associated with other energy sources for EAF steel have been calculated by ratios using the reported CO2 footprint of EAF steel (using coal-based electricity) as a basis.

HSC Modelling.
A simplified Waelz kiln model was created using Metso's HSC simulation software package.Three different Zn wt.% (10%, 26%, 35%) were used to create simplified EAF dust compositions.Each of these Zn wt.% were used to investigate the carbon footprint associated with the Waelz kiln (see Table 6).For each of the 3 chosen wt.% which span the expected Zn content within EAF dusts (Table 4), the mineralogy was varied to determine whether the presence of ferrites significantly impacted the carbon footprint.The following assumptions and ranges were used.Inlet temperatures of EAF dust, coke, flux and air were at 25⁰C.Exit temperatures of the off gas and Waelz oxide was 600⁰C while the Waelz slag was 1300⁰C.Flux (SiO2) was added as 20% of the EAF dust feed (wt.%) and the coke (assumed pure carbon) was varied between 10 and 40% of the EAF dust feed (wt.%).The moisture content of the EAF dust was 15%.The exit gas composition was maintained at approximately 15-25% CO2, 0.5-1% O2 and <2% CO which is in line with reported values [50].100% of the water was assumed to have evaporated and reported to the off-gas stream, 99% of Zn was assumed to have reacted and volatilised, while 100% of Fe was assumed to have reacted to form FeO.An unknown factor is the extent to which the carbon reacts and therefore the extent to which it reports to the Waelz Slag.From literature carbon content in the slag typically ranges from 10-20% and this range was thus used [49].The same Zn and Fe wt.% were maintained while it was assumed that 90% of Fe was present as ferrites (ZnFe2O3), with the balance as ZnO and Fe3O4.
As can be seen in Figure 9, the carbon footprint associated with processing of EAF dust in a Waelz kiln appears to range from 4-12 kg CO2/kg Zn dust (10 wt.% Zn in EAF dust) to 1-4 kg CO2/kg Zn (35 wt.% Zn in EAF dust).In this simple model, the presence of ferrites appeared to have no significant impact on the carbon demand and thus carbon footprint.

Conclusions
The addition of zinc in the form of galvanising contributes towards addressing corrosion and thus significantly increases the life span of steel products, with approximately 50-60% of the global production of zinc being used to galvanise steel.Almost all processes produce waste, and the reprocessing of secondary zinc streams is becoming increasingly important, due to legislation changes and increased consumer demand for recycled and low carbon metals.However, the recovering of zinc from these secondary resources requires additional energy which needs to be quantified.
The objective of this desktop study was to address gaps in available literature with respect to quantifying potential zinc secondary resources and the additional carbon (CO2) footprint associated with returning this secondary zinc back into primary production.To this end the authors have estimated the annual galvanised steel production between 2017 and 2022 ranges from 320-390 Mt/a (equating to 15-20% of the annual steel production).
The application of zinc for galvanising can be considered dispersive in nature due to the low quantities of zinc coating required per ton of steel.This makes economically recycling zinc from these sources challenging.For modelling purposes, it has been determined that a zinc content of 1.5-2.5 % for galvanised steel is reasonable.However, it is acknowledged that these values will vary widely depending on the type of steel being recycled.
Although there are two main process routes for steel making, the EAF process route is predominantly discussed with reference to zinc due to its ability to incorporate high levels of secondary (zinc containing) steel feeds and the dust produced typically contains economically recoverable levels of zinc (15 wt.%).EAF dust production is estimated between 5-10 Mt/annum and at a zinc content of 10-36%.If this zinc is not recovered it represents a loss of 0.5-3.6Mt/annum or up to 60% of primary consumption for galvanising.Due to conflicting information the recycling rate of EAF dust appears to range from 50-75%.While the CO2 footprint for EAF steel is reported as 0.67 kg CO2/kg EAF steel and can therefore be significantly reduced (up to 95%) if low carbon green sources of energy (wind, solar, hydro) are used for the electricity generation which is required for the EAF operation.
The Waelz kiln is the most prominent technology utilised to treat EAF dust and further extract the zinc, in the form of Waelz oxide, which can subsequently be incorporated into primary zinc processing routes such as roast-leach-electrowinning (RLE).Unlike the EAF, the Waelz kiln is reliant on coke as a reducing agent.This represents a significant contributing factor to the carbon footprint associated with the recycling of secondary zinc streams.
Model analysis of a simplified Waelz kiln using the HSC simulation software was presented with the carbon footprint primarily dependent on the wt.% of Zn in the EAF dust, the ratio of coke to EAF dust, as well as the extent of reaction of the coke within the kiln.The carbon footprint associated with processing of EAF dust in a Waelz kiln appears to range from 4-12 kg CO2/kg Zn dust (10 wt.% Zn in EAF dust) to 1-4 kg CO2/kg Zn (35 wt.% Zn in EAF dust).The carbon footprint associated with the Waelz kiln could be lowered if the zinc content is kept at the highest possible level through selective processing of high zinc secondary waste streams in EAF and through an increased efficient utilisation of coal which would reduce the feed rate of carbon per tonne EAF dust.
A future study to investigate the carbon footprint associated with the primary zinc processing routes will allow for a comparison between the primary and secondary routes.However, it is anticipated that the carbon footprint associated with primary Zn processing routes such as RLE will be significantly lower than the of recovery of Zn through the Waelz kiln.It is therefore anticipated that while the reprocessing of zinc dust is important from an environmental and material efficiency perspective, the industry may be unable to achieve the consumers' demand for recycled "green" zinc at a lower carbon footprint than primary zinc.

Acknowledgements
The authors would like to acknowledge the following funders for supporting this research: the South African Minerals to Metals Research Institute (SAMMRI), the South African National Research Foundation (NRF) through its SARChI Chair in Minerals Beneficiation (Grant UID 64829) and the South African Department of Science and Innovation (DSI) as well as input through discussions with the International Zinc Association (IZA).

Figure 1 .
Figure 1.Life expectancy of galvanised steel based on thickness of zinc coating (µm) [20].Service life = time to 5% rusting of steel surface.Grey area = typical range of coating thickness.

Figure 3 .
Figure 3. Process flow diagram for the incorporation of secondary zinc streams (EAF dust) back into primary zinc production.Dotted lines represent feasible, but less commonly incorporated feeds.

Figure 4 .
Figure 4. Global distribution of crude steel using the BF-BOF, EAF, OHF and other process routes in 2019 [21].C.I.S = Commonwealth of Independent States.

Figure 6 .
Figure 6.Potential Zn loss (tonnes) using estimated global EAF dust generation in 2022.Dotted area represents the maximum range of Zn loss, yellow colour represents the realistic range with blue representing the minimum expected loss.Left bar assumes no recycling of EAF dust; middle bar assumes 50% recycling rate; right bar assumes 75% recycling rate [40,46].

Figure 9 .
Figure 9. Carbon footprint (kg CO2/kg Zn in EAF dust) ranges associated with different concentrations of Zn in EAF dust.Dotted line represents minimum viable zinc content.

Table 3 .
[33]mated Zinc content of Galvanised Steel at different coating thicknesses (µm) and different steel thicknesses (assuming Zn coating included in steel thickness and steel density of 7.75 g/cm 3 )[33].

Table 5 .
Estimated CO2 footprint of EAF steel production based on different energy sources.

Table 6 .
EAF dust compositions used as feed to the Waelz kiln.Zn was assumed to be 100% present as ZnO and Fe as Fe3O4 i.e. no ferrites (ZnFe2O3) were assumed.
a b