Method for treating materials containing iron and zinc

Abstract

A method for processing a raw material containing zinc ferrite, comprising the steps of: B. partially reducing the raw material using a hydrogen-containing reducing gas to form a reduced material, wherein step B is carried out at less than 1000 degrees Celsius using a reducing gas containing at least 0.25% (volume percent) and up to 70% (volume percent) hydrogen in a carrier gas.

Description

[Technical field]

[0001] The present invention relates to a method for treating iron / zinc materials containing zinc ferrite, for example iron / zinc dust such as electric arc furnace dust, which includes a controlled reduction step. Electric Arc Furnace Dust (EAFD) is produced in large quantities in the steel industry, with approximately 20 kg per tonne of steel produced each year, meaning that more than 8 million tonnes of EAFD are produced each year. EAFD contains iron and zinc, as well as other valuable metals such as cobalt, copper, chromium and cadmium. [Background technology]

[0002] The composition of EAFD varies, but tends to contain, by weight, 10%-50% zinc, 20%-50% iron, 3%-20% calcium, up to 4% chromium, up to 6% manganese, and up to 5% magnesium. Some of the zinc is present in the form of readily available ZnO, but a significant proportion is present as zinc ferrite (ZnFe2O4), from which zinc extraction is difficult. Direct dissolution of zinc ferrite will carry the zinc and a significant proportion of the iron into solution, further complicating the separation process to extract the zinc.

[0003] EAFD is generally particulate matter less than 100 microns, with only 10%-15% often being greater than 10 microns. Because a significant proportion of EAFD (greater than 85%) is less than 10 microns, there is a high risk of entrainment with the moving gas stream, which can cause problems in handling and processing. Because of entrainment problems and other issues associated with fine feed materials, pelletization of EAFD (often with carbon) is common and is suggested as a necessity in (US Pat. No. 5,399,433). Such pelletization creates additional costs and can make some EAFD processes uneconomical in the long term. Carbon is often used as part of the pellet binder, which can increase CO2 emissions during processing of pelletized EAFD.

[0004] If the EAFD is pelletized, although pelletization is known to improve heat transfer in some cases, there is an increased risk that diffusion effects will limit the processing step and require a fractionation step through the pellets. Such diffusion problems can be partially or completely mitigated by carefully determining the pellet size and / or pellet composition to maintain porosity. The risk then is that in order to maintain the structural integrity of the porous pellet, it will become brittle and prone to mechanical (or thermomechanical) collapse during processing.

[0005] The most common method of recovering zinc from EAFD using pelletization with carbon is called the Weltz process. The Weltz process uses carbon as a reducing agent to produce zinc metal, which is formed as a vapor at the temperatures used (1000°C to 1500°C). The zinc vapor thus produced then reacts with an air / oxygen stream to form zinc oxide, which is then leached to produce a zinc-rich liquid from which the zinc is recovered.

[0006] The Wertz process and many other recovery processes first pelletize the EAFD with carbon and a binder (see U.S. Patent No. 5,399,633 (Sumitomo Heavy Industries), U.S. Patent No. 5,399,633 (Ban), and U.S. Patent No. 5,499,633). Such processes then inject a carbon-rich reducing gas (e.g., carbon monoxide), methane, or the like to reduce the metals present. Although pelletization reduces dust migration problems, the carbon used in such processes produces significant amounts of carbon dioxide.

[0007] A further problem with the Weltz process and processes based thereon is the production of zinc vapor, which is often cooled below the boiling point of zinc and can cause the zinc to condense and solidify before reacting with oxygen or air. If the zinc condenses or solidifies, it can impede or restrict flow from the treatment vessel, creating the need for mechanical removal. Zinc vapor is also believed to shorten the useful life of furnace linings.

[0008] To mitigate zinc concentration, US Pat. No. 5,399,633 uses a lead rain condenser, but such a method is only practical if sufficient lead is present in the feed material being treated.

[0009] There are many scientific studies and patents on the treatment of EAFD, but most of them use synthetic EAFD made of zinc ferrite and zinc oxide. Unfortunately, real EAFD does not behave like such synthetic EAFD, and laboratory-scale results obtained from synthetic EAFD experiments are not functional. For example, real EAFD contains up to 5 wt% halides and 1 wt% carbon. One of the documents that considers the difference between real EAFD and synthetic EAFD is (Non-Patent Document 1).

[0010] Many of the existing methods for EAFD processing result in the formation of elemental iron and iron oxides that dissolve during the acid leaching process. Such iron in the leachate is problematic because it increases the difficulty and / or cost associated with zinc recovery. Due to the problems posed by acid leaching, some researchers believe that the use of acid leaching, particularly with sulfuric acid, is not commercially viable. See, for example, (Non-Patent Document 2).

[0011] (Non-Patent Document 3) state that although alkaline leaching is more costly than acid leaching, acid leaching has not been widely adopted because of the undesirable high iron levels in solution. They state that another disadvantage of the acid leaching process is the simultaneous leaching of iron and zinc [5]. Among the various zinc forms in EAFD [3,6], ZnFe2O4 is extremely difficult to dissolve under normal acid leaching conditions. Thus, zinc recovery from ZnFe2O4 requires harsh conditions during leaching, including high temperature, high pressure, and high acid concentration

[12] . However, such a process involves large amounts of iron leaching and high energy costs. They suggest selecting an optimal EAFD with an iron / zinc ratio and using sulfuric acid with a pH above 4.5 to minimize iron dissolution. Such a process selects EAFD with low zinc ferrite levels, as they have high levels of ZnO that are easily leached under conditions that result in low iron dissolution.

[0012] (Non-Patent Document 4) discusses mechanochemical reduction of zinc ferrite with iron followed by an alkaline leaching step to minimize iron in the leachate. This requires grinding the zinc ferrite in a ball mill for 6 hours before leaching the resulting material with 6M NaOH. It is unclear whether the same results would be obtained from EAFD due to impurities.

[0013] (Non-Patent Document 2) found that using a hydrogen / nitrogen reducing gas with 50% hydrogen in the EAFD and caustic leaching would result in 100% successful recovery of zinc. The caustic leaching step is necessary because after the reduction step, very high levels of iron are present in acid-dissolved form. Sulfuric acid (acid leaching) results in a leachate with high iron concentrations, which increases the cost and complexity associated with recovering the zinc present. They further state that if the reducing gas has less than 50% hydrogen, zinc recovery drops to 50% when the reducing gas is about 10% hydrogen. With an alkaline leaching step and high hydrogen levels, the process can recover virtually all of the zinc from the EAFD. As previously mentioned, the use of an alkaline leaching step is more costly than an acid leaching step, but avoids the dissolution of iron in the leachate. To obtain good results in such a process, the hydrogen content in the reducing gas needs to be about 50%. Such high levels of hydrogen pose a significant risk of fire / explosion and increased costs.

[0014] Other industrial processes also produce materials containing zinc ferrite, including the mining and refining of zinc ores, where the zinc is typically recovered using alkaline or acid leaching solutions that are often heated to temperatures in excess of 100 degrees Celsius. Such harsh leaching processes often result in highly corrosive waste solutions that require recovery and disposal, which can sometimes be expensive and / or environmentally damaging.

[0015] The discussion of prior art throughout this specification is not an admission that such prior art is widely known or forms part of the general common knowledge in the art.

[0016] The present invention provides an EAFD processing method that reduces zinc ferrite without producing large amounts of acid-dissolved iron or zinc vapor, or provides a beneficial option to the consumer. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] International Publication No. 2000 / 039351A1 [Patent Document 2] U.S. Pat. No. 4,612,041 [Patent Document 3] U.S. Pat. No. 3,262,771 [Patent Document 4] Polish Patent No. 406290 [Patent Document 5] China Patent Application Publication No. 109457123 [Non-patent literature]

[0018] [Non-Patent Document 1] Lee Fui Tong & Peter Hayes (2006), “Mechanisms of the reduction of zinc ferrites in H2 / N2 gas mixtures,” Mineral Processing and Extractive Metallurgy Review, 28:2, 127-157, DOI:10.1080 / 08827500601012878 [Non-Patent Document 2] J. Antrekowitsch, H. Antrekowitsch, “Hydrometallurgically recovering zinc from electric arc furnace dusts,” JOM.53(2001)26-28. https: / / doi.org / 10.1007 / s11837-001-0008-9 [Non-Patent Document 3] Lee, Han-Saem & Park, Da & Hwang, Yuhoon & Ha, Jong & Shin, Hyung (2019), “Toward high recovery and selective leaching of zinc from electric arc furnace dust with different physicochemical properties,” Environmental Engineering Research. 25. 10.4491 / eer.2019.132 [Non-Patent Document 4] Zhang, C., Zhuang, L., Wang, J., Bai, J., & Yuan, W.,(2016), “Extraction of zinc from zinc ferrites by alkaline leaching: enhancing recovery by mechanochemical reduction with metallic iron,” Journal of the Southern African Institute of Mining and Metallurgy, 116(12), 1111-1114.https: / / dx.doi.org / 10.17159 / 2411-9717 / 2016 / v116n12a3J Summary of the Invention

[0019] The present invention relates to a method for treating a raw material containing zinc ferrite, the method comprising the steps of: B. partially reducing the raw material with a hydrogen-containing reducing gas to form a reduced material; Step B is carried out at or below 1000 degrees Celsius using a reducing gas containing at least 0.25% (volume percent) hydrogen in a carrier gas.

[0020] Step B is preferably carried out at a temperature of 800 degrees Celsius or less. Preferably, the hydrogen in the reducing gas is at most 70 volume percent. In a highly preferred form, the hydrogen in the reducing gas is at most 60 volume percent.

[0021] The raw material is preferably a particulate material that is not pelletized prior to processing. Preferably the raw material is from the steel or iron making industry, or the zinc mining and / or refining industry. Preferably the waste material is from a furnace. In a highly preferred form the raw material is electric arc furnace dust (EAFD).

[0022] In another preferred form, the raw material is from the zinc industry. In a highly preferred form, the raw material is zinc mining and / or refining waste. Preferably, step B is carried out between 500°C and 750°C. In a highly preferred embodiment, the temperature of step B is at most 700°C. In a further preferred embodiment, the temperature of step B is at most 650°C. In a further preferred embodiment, the temperature is at least 650°C. Preferably, the temperature of step B is at most a temperature selected from the group consisting of 550°C, 600°C, 650°C, and 700°C. Preferably, the temperature of step B is at least a temperature selected from the group consisting of 550°C, 600°C, and 650°C.

[0023] The method preferably includes a step C, in which step C is cooling the reduced material in an inert or reducing gas environment to a temperature below 350 degrees Celsius. The hydrogen content of the reducing gas is preferably between 0.25 and 10 volume percent.

[0024] Step B is preferably preceded by step A, in which the raw material is analyzed to determine the total zinc and / or zinc ferrite concentration. In a highly preferred form, the total zinc content is determined by X-ray fluorescence (XRF) or standard chemical analysis, and the zinc ferrite fraction by X-ray diffraction.

[0025] In a highly preferred form, step A determines the proposed process conditions to be used in step B, including reduction time (Rt), reduction temperature (RT), and reduction gas flow rate (GF). Preferably, step A involves small scale experiments to confirm that the proposed process conditions result in less than 10% mass loss, and preferably the mass loss is no more than (2+ / -0.5)%. In a more preferred form, step A confirms that any wustite and / or metallic iron produced under the proposed process conditions is less than a predetermined value.

[0026] In a preferred embodiment, the reducing gas also contains moisture. The moisture is added at a predetermined time after the start of step B. The predetermined time is preferably before the temperature of step B reaches 650 degrees Celsius. In a more preferred embodiment, the temperature is selected from the list consisting of 500 degrees Celsius, 550 degrees Celsius, 600 degrees Celsius, and 650 degrees Celsius. In another preferred embodiment, the predetermined time occurs when the temperature is within a range selected from the list consisting of 500 degrees Celsius to 550 degrees Celsius, 550 degrees Celsius to 600 degrees Celsius, 600 degrees Celsius to 650 degrees Celsius, 500 degrees Celsius to 600 degrees Celsius, and 500 degrees Celsius to 650 degrees Celsius. In a highly preferred embodiment, the water to hydrogen ratio (H2O:H2) on a molar basis is between 20:1 and 1:5. The water to hydrogen ratio is preferably between 1:2 and 2:5. In a highly preferred form, the water to hydrogen ratio is between 5:1 and 1:1.

[0027] In another form, the reducing gas further comprises carbon monoxide such that the ratio of H to H0 to CO is 2 to (1-10) to (0-10) at a concentration between 0.25 and 15 volume percent in the carrier gas.

[0028] The carrier gas is preferably a neutral carrier gas selected from the list consisting of nitrogen, helium, and argon. In another embodiment, the neutral carrier gas is a combination of one or more gases selected from the list consisting of nitrogen, helium, and argon. The neutral carrier gas is preferably an inert gas.

[0029] Step C is preferably followed by step D, in which case step D is an acid leaching step. The acid leaching step preferably uses an acid leach comprising one or more acids selected from the list consisting of sulphuric acid, phosphoric acid, hydrochloric acid, hydrofluoric acid and organic acids having less than 15 carbon atoms. In a highly preferred form, step D is carried out at a pH of less than 3, preferably at a pH of 2. The acid leach used comprises sulphuric acid. In a highly preferred form, the acid leach is sulphuric acid.

[0030] Step B is preferably monitored and / or controlled by directly or indirectly measuring the extent of reduction by determining the form of iron present and / or by determining the composition of the reducing gas in the furnace and / or the effluent gas stream, preferably by determining the composition of the effluent gas stream using a thermal conductivity detector.

[0031] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0032] [Figure 1] FIG. 1 is a free chart of the method. [Diagram 2] FIG. 2 is a H2 / H2O phase equilibrium diagram of iron forms at various temperatures. [Diagram 3] FIG. 3 is a CO / CO2 phase equilibrium diagram of the iron form at various temperatures. [Figure 4] FIG. 4 is a TCD (thermal conductivity detector) graph comparing various water to hydrogen ratios with synthetic electric arc furnace dust. [Diagram 5] FIG. 5 is a TCD (Thermal Conductivity Detector) graph comparing actual electric arc furnace dust (EAFD) to synthetic electric arc furnace dust (EAFD). [Figure 6] FIG. 6 is a TCD (Thermal Conductivity Detector) graph comparing synthetic electric arc furnace dust (EAFD) with actual electric arc furnace dust (EAFD). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] (definition) EAFD: Electric Arc Furnace Dust Franklinite: Some literature uses the term franklinite to refer to ZnFe2O4 (zinc ferrite) present in EAFD due to morphological similarities. In many EAFDs (and mineral forms), franklinite contains ZnFe2O4 and ZnMnFeO4, so in this specification, when franklinite is used, it contains zinc ferrite and may also contain ZnMnFeO4.

[0034] Magnetite: Fe3O4 Sulfuric acid: Includes oleum. Water: This term includes water in any liquid form, such as water vapor (steam) and liquid water as determined by local environmental conditions.

[0035] Wustite: A nonstoichiometric iron oxide (Fe 0.947 O), and both Fe(II) and Fe(III) are present in the oxide. It is sometimes written as FeO. Zincite: Essentially zinc oxide (ZnO).

[0036] The materials treated by the present method may be complex if chemical formulas are used and may simply refer to the predominant species present. For clarity of terminology, the term "about" is typically used when a parameter is within + / - 15%, and the term "approximately" is used when a parameter is within + / - 10%, unless otherwise specified. (First embodiment of the invention) Although the method is described with reference to EAFD, it is believed to be applicable to other zinc-rich dusts from steel and iron making processes, for example dusts from basic oxygen furnaces where ZnMnFeO4 is present. It is also believed that the method can be used in any process or industry that produces materials containing zinc ferrite, for example for the recovery of materials obtained from the mining and refining of zinc ore.

[0037] Referring to FIG. 1, the core steps of a method for processing a raw material (1), most often EAFD, are shown in flow chart form. In this modification, the steps are performed in the following order:

[0038] A. Analyze raw material (1). B. A hydrogen-containing reducing gas (2) is used to partially reduce the raw material (1) to form a reduced material (3).

[0039] C. Cool the reduced material (3). D. The reduced material (3) is acid leached. Step A is optional and is not present in all variations and is currently used to determine optimal processing variables. Steps C and D are preferred steps, but are also optional.

[0040] Step A is the analysis of the raw material (1) to determine the amount of zinc and zinc ferrite present so that process parameters can be determined. Such parameters include the reduction temperature (RT), reduction time (Rt), hydrogen concentration (HC) of the reduction gas (2), and flow rate (GF) of the reduced material (3). The total zinc content is currently determined by X-ray fluorescence (XRF) or standard chemical analysis, and the zinc ferrite fraction by X-ray diffraction.

[0041] Once step A is complete, the reduction conditions for step B can be determined. The objective is to reduce the zinc ferrite to magnetite and zinc oxide and avoid the formation of wustite, iron, or zinc, which can be problematic in the reduction furnace since wustite and iron dissolve in acid and zinc melts at around 420 degrees Celsius.

[0042] First, a small-scale experimental reduction of the sample is performed based on the analysis from step A. The small-scale experimental results and / or equilibrium calculations are used to determine the reduction time (Rt), reduction temperature (RT), and reduction gas flow rate (GF) for the reduction of a larger volume of sample performed in step B. This will most likely be determined by setting conditions to ensure that the mass loss during the reduction of the sample is less than 2% (2% + / - 0.5%).

[0043] As a result, the equilibrium / maximum reduction is determined and the results from step A allow for optimal selection of the hydrogen concentration in the material being reduced (3). In most cases, the reduction temperature (RT) and reduction gas flow rate (GF) of step B are predetermined, and the reduction time (Rt) is set by small-scale experiments. As a result, the batch furnace can be maintained at the optimum temperature and reduction gas flow rate (GF) that are preset to fix the synchronization of the EAFD to a minimum. As a result, for the continuous furnace, the temperature and reduction gas flow rate (GF) can be optimally set, and the moving speed of the EAFD through the furnace can be adjusted to provide the optimum reduction time (RT).

[0044] Step B is a process for partially reducing the zinc ferrite in the EAFD to obtain magnetite and zinc oxide without forming significant amounts of wustite, metallic iron, or metallic zinc. This process is carried out in a furnace of known type and configuration. To minimize the formation of wustite, metallic iron, or metallic zinc, the mass loss during this process will be controlled to about 2% (2% + / - 0.5%) or less. This is accomplished by using a reducing gas (2) that includes hydrogen in a carrier gas such as nitrogen or argon at a temperature below about 800 degrees Celsius. Such processing often occurs between 500 degrees Celsius and 750 degrees Celsius, with the optimum temperature believed to be between 650 degrees Celsius and 750 degrees Celsius.

[0045] The risk of fire or explosion will determine the maximum hydrogen concentration (HC) in the reducing gas (2). Furthermore, the maximum hydrogen concentration (HC) in the reducing gas (2) will vary depending on the process equipment and local safety requirements.

[0046] Based on attempts to date, the reducing gas (2) is expected to be a neutral carrier gas such as nitrogen containing 0.25 to 10 volume percent hydrogen (although concentrations of up to 70% hydrogen are believed to be maximum, with less than 66% being optimal).

[0047] Table 1 shows typical mass losses for EAFD test reductions using 5% (volume percent) hydrogen in nitrogen reduction gas (2).

[0048] [Table 1]

[0049] As shown in the table above, the mass loss of EAFD of this sample is less than about 2% when the reduction temperature (RT) is less than 650 degrees Celsius and the reduction time (Rt) is about 1 hour using a reduction gas (2) containing 5 volume percent hydrogen in a neutral carrier gas (nitrogen). The mass loss is approximately linear with time, so the effect of extending the time can be determined. Increasing the reduction temperature (RT) also changes the mass loss, but it is not linear because the reaction kinetics are more sensitive to temperature changes.

[0050] A mass loss of about 2% has been found to reduce franklinite to zinc oxide and magnetite with low risk of reducing magnetite to undesirable wustite and iron, but mass losses of more than 2% are feasible without forming metallic iron, metallic zinc, or wustite.

[0051] Table 2 below shows the XRD analysis results of the reduced material (3) at various reduction temperatures (RT) and reduction times (Rt).

[0052] [Table 2]

[0053] As shown in the table above, the amount of metallic iron and wustite formed is determined by the reduction gas flow rate (GF), reduction time (Rt), and reduction temperature (RT). Further results based on the total zinc and zinc ferrite fractions performed in step A are needed to determine the optimal parameters.

[0054] To keep the amount of metallic iron and wustite in the reduced material (3) low, the optimum reduction temperature (RT) appears to be between 500°C and 800°C, probably below about 700°C.

[0055] Example 1 shows a theoretical thermodynamic analysis of the results of hydrogen reduction gas (2). If such calculations are to be confirmed by EAFD testing to minimize wustite formation and provide sufficient reduction of zinc ferrite to zinc oxide, a molar excess of H2 is required. With a reduction temperature of 400 degrees Celsius, the theoretical analysis shows that the desired results are achieved using a reduction gas (2) with a 3 molar excess of hydrogen.

[0056] Once step B is complete and the desired level of reduction has been achieved, step C is carried out, which is the cooling of the reduced material (3) under reducing or neutral conditions to a temperature below 350 degrees Celsius or even lower. When the reduced material is cooled in air to a temperature above 400 degrees Celsius, ZnFe2O4 is rapidly formed.

[0057] After cooling of the reduced material (3), an optional acid leaching step, step D, is carried out. It is expected that this step will use sulfuric acid at a pH of about 3. (Best Mode for Carrying Out the Invention) In a further variation of the method, step A is also optional (but preferred), but the reducing gas (2) contains a molar ratio of H2O to H2 of 1:2 to 2:5, i.e., 2 molar hydrogen to 1 molar water to 5 molar hydrogen to 2 molar water. As a result, the reducing effect of the reducing gas (2) is changed. A water to hydrogen ratio in the range of 20:1 to 1:5 has been found to have the desired properties.

[0058] The use of H2 / H2O (see Figure 2) results in a very different phase equilibrium diagram compared to when CO / CO2 is used (see Figure 3). The CO / CO2 system favors reduction to iron at equilibrium. The H2 / H2O reducing gas (2) favors the formation of magnetite at equilibrium. That is, the equilibrium trends are different depending on whether the reducing gas contains hydrogen or carbon monoxide.

[0059] It has been found that the use of hydrogen and water in a neutral carrier gas (likely nitrogen) as the reducing gas (2) provides a means of adjusting the reducing power of the reducing gas (2). By controlling the water vapor present, it is possible to limit the potential reducing power of the reducing gas (2) and reduce the formation of undesirable species. The water vapor concentration can be measured for the reducing gas (2) entering the furnace and the spent reducing gas (2) leaving the furnace, and by controlling the moisture content of the reducing gas (2), the reducing power of the reducing gas (2) can be controlled. Example 2 shows a theoretical thermodynamic analysis of the results of a water / hydrogen reducing gas (2). If EAFD testing is confirmed to minimize wustite formation, the temperature should be kept below 650 degrees Celsius.

[0060] In a further variation, small amounts of carbon monoxide may be added to the reduction gas (2). At concentrations between 0.25% and 15% (volume percent) in the neutral carrier gas, a ratio of H2 to H2O to CO of 2 to (1-10) to (0-10) would allow for further control of the reduction reaction that occurs. Of course, the use of CO is undesirable due to environmental concerns and is an odorless, toxic gas that is difficult to handle, but it does improve the reaction kinetics and therefore reduce the required energy input.

[0061] In some variations, steps A and / or D are not present. In the absence of step A, step B is controlled by measuring the iron form present and / or by measuring the degree of reduction directly or indirectly, by measuring the composition of the reducing gas (2) in the furnace or by using a thermal conductivity detector (TCD) on the exhaust gas stream. Step D is not performed if the reduced material (3) is stored or sent elsewhere for further processing.

[0062] The reaction of step B is monitored and / or controlled, irrespective of the presence of other processes, by directly or indirectly measuring the degree of reduction by measuring the iron form present and / or by measuring the composition of the reducing gas (2) in the furnace and / or by using a thermal conductivity detector (TCD) on the exhaust gas stream.

[0063] In some variations, there is a hydrogen recycle loop that extracts hydrogen from the exhaust gas. The exhaust gas is treated so that hydrogen is added to the neutral carrier gas or to the hydrogen-containing reducing gas (2).

[0064] In some variations, the composition of the reducing gas (2) in step B is purposefully changed over time within previously given parameters. Such purposeful change in the composition of the reducing gas (2) is likely to be achieved by adding hydrogen or water at a predetermined time after the start of step B but before the subsequent step is started. Such a delay of a predetermined time is likely to be implemented when the method is carried out as a continuous process. For example, water may be added after a portion of step B to allow for a rapid initial reaction with the water added to mediate the end point of the reaction. Such a delay of a predetermined time is likely to be determined before the temperature of step B reaches a particular value or falls within a particular range of values. Such a delay of a predetermined time may occur before the temperature reaches 650 degrees Celsius, 600 degrees Celsius, 550 degrees Celsius, or 500 degrees Celsius, or may occur if the temperature is within a predetermined range, for example, 600 degrees Celsius to 650 degrees Celsius, 550 degrees Celsius to 600 degrees Celsius, or 500 degrees Celsius to 550 degrees Celsius.

[0065] Although sample runs have been limited to 900°C due to furnace limitations, thermodynamic simulations predict that the process will be successful up to about 1000°C. The method works successfully at temperatures up to about 1000°C (+ / -10%), which we believe is not necessarily optimal, and is supported by thermodynamic simulations such that about 1000°C appears to be a practical upper limit. Processing below 800°C is particularly advantageous, with 800°C and below being the optimal range, and 500°C to 750°C and 550°C to 650°C appear to offer additional benefits such as controllability. (Example) A prophetic example is given below. Example 1 Thermodynamic predictions for various reactants in hydrogen reducing gas without added moisture at 400°C to 700°C are provided below (FactSage calculations using Equilib-Web).

[0066] Zinc ferrite reduction with hydrogen uses 1 / 3 the amount of H2 per zinc ferrite.

[0067] [ka]

[0068] The reaction of 1 mole of ZnFe2O4 with 1 / 3 mole of H2 (i.e., no excess) is: Table 3: Reduction of 1 mol ZnFe2O4 with 1 / 3 mol H2 at various temperatures (1 atm)

[0069] [Table 3]

[0070] To reach 95% with stoichiometric H2, approximately 700 degrees Celsius is required. The same calculation was performed with a 3-fold excess of H2. Table 4: Reduction of 1 mol ZnFe2O4 with 1 mol H2 at various temperatures (1 atm)

[0071] [Table 4]

[0072] (Conclusion) Thermodynamically, excess H2 is required for operation below 700°C (with a 3-fold excess, equilibrium conversion can reach 100% at 400°C). As long as H2 recycling is implemented, there is no problem. It has also been shown to be beneficial in limiting FeO (wüstite) formation, as long as temperatures are kept below 650°C. Example 2 Thermodynamic predictions for various reactants at 600 degrees Celsius in water and hydrogen reducing gas are provided below (FactSage calculations using Equilib-Web).

[0073] The reaction is as follows:

[0074] [ka]

[0075] An equilibrium calculation with 1 mole of ZnFe2O4 and 1 mole of H2 (i.e., 3-fold excess) at 600 degrees Celsius is shown below. Table 5: Reduction of 1 mole of ZnFe2O4 at 600 degrees Celsius (1 atm) with various H2 to ZnFe2O4 molar ratios

[0076] [Table 5]

[0077] Thermodynamics show that reduction of iron oxide to FeO occurs at 600 degrees Celsius at molar ratios greater than 1:1. Steam (water vapor) can limit further reduction of iron oxide. Calculations at a 6-fold excess (H2:ZnFe2O4 molar ratio = 2:1) show this.

[0078] Table 6: Reduction of ZnFe2O4 with 2M H2 at 600 degrees Celsius (1 atm) with various H2O / H2 molar ratios

[0079] [Table 6]

[0080] The addition of small amounts of steam (1:5 = 20% water vapor in H2) eliminates the reduction to FeO. The addition of 6 moles of steam (water vapor) to 1 mole of H2 begins to limit the reduction of ZnFe2O4.

[0081] Table 7: Reduction of ZnFe2O4 with 10 mol H2 at 600 degrees Celsius (1 atm) with various H2O / H2 molar ratios

[0082] [Table 7]

[0083] **At 7.5 moles H2O to 1 mole H2, zinc ferrite begins to become non-reducible. At 8 to 1, zinc ferrite no longer reduces. Tables 6 and 7 show that the addition of water vapor can prevent over-reduction of zinc ferrite by promoting the formation of magnetite (Fe3O4). Example 3 The programmed reduction of synthetic EAFD without water and with water to hydrogen ratios of 1.6:1 and 2.1:1 is shown in FIG.

[0084] In this example, 50 mg of synthetic EAFD was heated in 5% H2 in flowing argon from 100°C to 850°C with a temperature ramp of 20°C / min. The exiting gas was monitored using a thermal conductivity detector, which effectively measures hydrogen in the exiting gas stream indicating reduction, where higher numbers indicate more reduction has occurred.

[0085] In FIG. 4, line SNW is the synthetic EAFD with no added water, line SD1 has a water to hydrogen ratio of 1.6:1 (1.6 moles of water to 1 mole of hydrogen), and line SD2 has a water to hydrogen ratio of 2.1:1. (Effect of water to hydrogen ratio) As can be seen by comparing SNW with SD1 or SD2, the addition of water slows the reaction rate and the reaction is initiated at a lower temperature, but counterintuitively, increasing the water to hydrogen ratio from 1.6:1 to 2.1:1 increases the reaction rate and the reaction is initiated at a lower temperature. Example 4 (Real EAFD vs. Synthetic EAFD) The experiments carried out in Example 3 were carried out on two real EAFDs and a synthetic EAFD with the same zinc content, and the results are shown in Figure 5. The water to hydrogen ratio was 1.6 to 1.

[0086] The lines shown in Figure 5 are as follows: SNW: Synthetic soot without added water SD1: Synthetic EAFD with a water to hydrogen ratio of 1.6:1 RD1: Actual EAFD sample 1 with a water to hydrogen ratio of 1.6 to 1 RD2: Actual EAFD sample 2 with a water to hydrogen ratio of 1.6 to 1 As shown in the figure, the synthetic dust reaction rate appears to occur in two stages with the highest reaction rate occurring at about 820°C, whereas the actual EAFD (based on two actual samples considered to be representative) does not show such an obvious two-stage process, with the highest reaction rate shown in the range of 700°C and 720°C. This indicates that the reaction characteristics of the synthetic EAFD do not match the actual EAFD. Based on FIG. 5, optimizing an actual EAFD processing plant using the synthetic EAFD would be designed at about 820°C, resulting in a much lower reaction rate and much higher energy usage than using the actual EAFD in the optimization test. Based on such results, the experiments using the synthetic EAFD cannot be used to determine the reduction conditions for the actual EAFD with any confidence. Example 5 The programmed reduction of synthetic EAFD and real EAFD without water is shown in Figure 6 .

[0087] In this example, 50 mg of synthetic and actual EAFD were heated in 5% H2 in flowing argon from 100°C to 850°C with a temperature ramp of 20°C / min. The exiting gas was monitored using a thermal conductivity detector, which effectively measures hydrogen in the exiting gas stream indicating reduction, where higher numbers indicate more reduction has occurred.

[0088] The lines shown in Figure 6 are as follows: SNW: Synthetic soot, no moisture RNW: Real soot, no moisture As can be seen, the reaction peak of the synthetic EAFD is lower than that of the real EAFD, with the reaction occurring at a higher temperature (approximately 720 degrees Celsius). The reaction peak of the real dust occurs at approximately 690 degrees Celsius, with the reaction also being approximately 40% higher. Interestingly, the reaction peak of the real EAFD is narrower than that of the synthetic dust, thus allowing for specific optimization of reaction conditions. Again, based on these results, the experiments with the synthetic EAFD cannot be used to determine the reducing conditions for the real EAFD with any confidence. [Explanation of symbols]

[0089] 1: Raw materials (EAFD, BOFD) 2: Reducing gas 3: Reduced material (material after partial reduction of the raw materials using a hydrogen-containing reducing gas) RT: Reduction temperature Rt: Reduction time HC: Hydrogen concentration in reducing gas HF: Flow rate of reducing gas

Claims

1. A method for processing a raw material containing zinc ferrite, comprising the following steps: The following step is B. A step of partially reducing the raw materials using a hydrogen-containing reducing gas to form a reduced material. The method wherein the reducing gas contains water.

2. The method according to claim 1, wherein step B is carried out at a temperature of 1000 degrees Celsius or less.

3. The method according to claim 1, wherein the raw material is particulate matter that has not been pelletized prior to the treatment.

4. The method according to claim 1, wherein the raw material is electric arc furnace dust (EAFD).

5. The method according to claim 1, wherein step B is carried out between 500 degrees Celsius and 750 degrees Celsius.

6. A method according to claim 1, wherein step C is a step of cooling the reduced substance to a temperature of 350 degrees Celsius or less in an inert environment or a reducing gas environment.

7. The method according to claim 1, wherein the hydrogen content of the reducing gas is between 0.25 volume percent and 10 volume percent.

8. The method according to claim 1, wherein step A precedes step B, and in step A, the raw materials are analyzed to determine the total zinc and / or zinc ferrite concentration. Step A is a method for determining proposed processing conditions used in Step B, including reduction time (Rt), reduction temperature (RT), and reduction gas flow rate (GF).

9. The method according to claim 1, wherein the water is added to the reducing gas at a predetermined time after the start of step B.

10. In the method according to claim 9, the predetermined time is a) reached before the temperature reaches a value selected from the list consisting of 500 degrees Celsius, 550 degrees Celsius, 600 degrees Celsius, and 650 degrees Celsius, or b) A method for reaching a temperature when the value is within a range selected from the list consisting of 500°C to 550°C, 550°C to 600°C, 600°C to 650°C, 500°C to 600°C, 550°C to 650°C, and 500°C to 650°C.

11. The method according to claim 1, wherein step D is a step after step C, and step D is an acid leaching step.

12. The method according to claim 11, wherein the acid leaching step is an acid leaching method comprising one or more acids selected from the list consisting of sulfuric acid, phosphoric acid, hydrochloric acid, hydrofluoric acid, and organic acids having less than 15 carbon atoms.

13. The method according to claim 11, wherein step D is carried out at a pH of less than 3.

14. The method according to claim 13, wherein step D is carried out at pH 2.

15. The method according to claim 12, wherein the acid leaching used contains sulfuric acid.

16. The method according to claim 1, wherein step B is monitored and / or controlled by one or more methods selected from a list of directly or indirectly measuring the degree of reduction by determining the form of iron present, determining the composition of the reducing gas in the furnace, and determining the composition of the effluent gas flow.

17. A method according to any one of claims 1 to 16, wherein the water-to-hydrogen ratio (H₂O:H₂) on a molar basis is between 1:20 and 7:

1.

18. A method according to any one of claims 1 to 16, wherein the water-to-hydrogen ratio (H₂O:H₂) on a molar basis is between 1:20 and 5:

1.

19. A method according to any one of claims 1 to 16, wherein the water-to-hydrogen ratio is between 1:2 and 2:

5.

20. A method according to any one of claims 1 to 16, wherein zinc ferrite is reduced to magnetite and zinc oxide, and the formation of wustite, iron, or zinc is substantially avoided.

21. A method according to any one of claims 1 to 16, wherein the mass loss in step B is controlled to be about 2% (2% ± 0.5%) or less.

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