Method for processing iron and zinc containing materials

Abstract

A method for processing a source material for a zinc-containing ferrite, comprising the steps of: B. partially reducing the source material with a hydrogen-containing reducing gas to form a reduced material; wherein step B is carried out at a temperature below 1000 DEG C using a reducing gas containing at least 0.25% (by volume) and up to 70% (by volume) hydrogen in the carrier gas.

Description

Technical Field

[0001] This invention relates to a method for processing zinc-containing ferrite iron / zinc materials, such as iron / zinc dust like electric arc furnace dust, the method comprising a controlled reduction step. Background Technology

[0002] The steel industry generates a significant amount of electric arc furnace dust (EAFD), approximately 20 kg per ton of steel per year. This translates to over 8 million tons of EAFD annually. EAFD contains iron and zinc, as well as other valuable metals such as cobalt, copper, chromium, and cadmium.

[0003] The composition of EAFD varies; however, by weight, it typically contains 10% to 50% zinc, 20% to 50% iron, 3% to 20% calcium, up to 4% chromium, up to 6% manganese, and up to 5% magnesium. Some zinc exists in the readily available form of ZnO; however, a significant portion exists as zinc ferrite (ZnFe₂O₄), from which extraction is difficult. The process of dissolving zinc ferrite directly introduces zinc and a considerable portion of iron into the solution, further complicating the separation process for zinc extraction.

[0004] EAFD (Extractable Electronic Feed) is a particulate material, typically smaller than 100 micrometers, and often contains only 10% to 15% particles larger than 10 micrometers. Since a significant portion (greater than 85%) of EAFD is smaller than 10 micrometers, the risk of entrainment in the moving airflow is high, posing challenges to handling and processing. Carbon is often used to granulate EAFD to reduce entrainment and other problems associated with finer feed, and WO2000039351A1 indicates this is necessary. This granulation is an additional cost step that may render some EAFD processes uneconomical in the long run. Carbon is often used as part of the particle binder, which can increase CO2 emissions during granulated EAFD processing, posing an environmental challenge.

[0005] If EAFD is granulated, any processing step may be subject to diffusion effects, increasing the risk of fractionation processing through the particles. Although granulation has been found to improve heat transfer in some cases, this diffusion problem can be partially or completely mitigated by carefully classifying particles and / or particle composition to maintain porosity. The risk is that, in order to maintain the structural integrity of high-porosity particles, they may become brittle and undergo mechanical (or thermomechanical) decomposition during processing.

[0006] The most common method for recovering zinc from EAFD is granulation using carbon, known as the Wiltz process. The Wiltz process uses carbon as a reducing agent to generate zinc metal, which is then vaporized at the operating temperature (1000°C to 1500°C). The generated zinc vapor then reacts with an air / oxygen stream to form zinc oxide, which is then filtered to form a zinc-rich solution from which zinc is recovered.

[0007] The Wiltz process, along with many other recycling processes, first utilizes carbon and a binder to granulate the EAFD (External Energy Filtration Facility), see, for example, US 4,612,041 (Sumitomo Heavy Industries, Ltd.) and US 3,262,771 (Banner) and PL406290. The process then involves injecting a carbon-rich reducing gas, such as carbon monoxide, methane, or a similar gas, to reduce the metal present. Granulation reduces dust movement problems, but the carbon generated in the process produces significant amounts of carbon dioxide.

[0008] Another problem with the Wiltz process and those processes based on it is that zinc vapor is generated before reacting with oxygen or air. This zinc vapor can be cooled, typically below the boiling point of zinc, leading to condensation and potential solidification of the zinc. If the zinc condenses or solidifies, it can block or restrict the flow of fluid from the processing vessel and requires mechanical removal. Zinc vapor is also believed to shorten the life of the furnace lining.

[0009] To slow down zinc condensation, CN109457123 includes a lead rain condenser, which is only feasible if there is sufficient lead in the feed material being processed.

[0010] Numerous scientific studies and patents relate to the processing of EAFD, and many of these studies and patents utilize synthetic EAFD composed of zinc ferrite and zinc oxide. Unfortunately, real EAFD does not behave as well as these synthetic EAFDs, and obtaining laboratory-scale results from synthetic EAFD experiments is generally unreliable. For example, real EAFD can contain up to 5% halides and 1% carbon (by weight). One paper considering the differences between real and synthetic EAFD is Lee Fui Tong and Peter Hayes (2006), “Reduction Mechanism of Zinc Ferrite in H2 / N2 Gas Mixtures,” *Mineral Processing and Extractive Metallurgy Review*, 28:2, 127-157, DOI:10.1080 / 08827500601012878.

[0011] Many existing methods for processing EAFD (Electric Arc Furnace Dust) result in the dissolution of elemental iron and iron oxide during the acid leaching step. This iron in the leachate is problematic because it increases the difficulty and / or cost of zinc recovery. Given the problems arising from acid leaching, some researchers consider its use commercially unfeasible, particularly with sulfuric acid. See, for example, J. Antrekowitsch, H. Antrekowitsch, "Recovery of Zinc from Electric Arc Furnace Dust by Hydrometallurgical Method," JOM.53 (2001) 26-28. https: / / doi.org / 10.1007 / s11837-001-0008-9 .

[0012] Lee, Han-Saem & Park, Da & Hwang, Yuhoon & Ha, Jong & Shin, Hyung. (2019). "For the recovery and selective leaching of zinc from electric arc furnace dust with different physicochemical properties". Environmental Engineering Research 25.10.4491 / eer.2019.132. This study indicates that alkaline leaching is more expensive than acid leaching; however, acid leaching introduces a higher iron content into the solution, which is undesirable, and therefore, acid leaching has not been widely adopted. Acid leaching indicates that "another disadvantage of acid leaching is the simultaneous leaching of Fe and Zn [5]. Among the different forms of Zn in EAFD [3,6], ZnFe2O4 exhibits strong resistance to dissolution under normal acid leaching conditions. Therefore, the recovery of zinc from ZnFe2O4 requires more stringent conditions, including high temperature, high pressure, and high acid concentration during leaching

[12] . However, these methods are affected by the large amount of Fe leaching and high energy costs." These methods suggest selecting the optimal EAFD by the Fe / Zn ratio and using sulfuric acid with a pH greater than 4.5 to minimize Fe dissolution. The process selects EAFDs with low zinc ferrite content because they have high ZnO content that is easily leached under conditions that cause low iron solubility.

[0013] Zhang, C., Zhuang, L., Wang, J., Bai, J., & Yuan, W. (2016). Extraction of Zinc from Zinc Ferrite by Alkali Leaching: Improving Recovery by Mechanochemical Reduction of Metallic Iron. Journal of the South African Institute of Mining and Metallurgy, 116(12), 1111-1114. https: / / dx.doi.org / 10.17159 / 2411-9717 / 2016 / v116n12a3J The mechanochemical reduction of zinc using iron was discussed, followed by an alkaline leaching step to minimize iron in the leachate. This requires grinding zinc ferrite in a ball mill for 6 hours, followed by leaching with 6M NaOH to produce the material. It is uncertain whether the same results can be obtained from EAFD due to the presence of impurities.

[0014] Antrekowitsch, H. Antrekowitsch, "Recovery of Zinc from Electric Arc Furnace Dust by Hydrometallurgical Method", JOM.53 (2001) 26-28. https: / / doi.org / 10.1007 / s11837-001-0008-9 It was found that 100% of zinc could be successfully recovered from EAFD using a hydrogen / oxygen reducing gas with 50% hydrogen, with caustic leaching. A caustic leaching step is necessary because of the high iron content present in acid-soluble form after the reduction step. Sulfuric acid leaching produces a leachate contaminated with high iron content, which increases the cost and complexity of recovering any zinc present. They also noted that zinc recovery decreased when the reducing gas had less than 50% hydrogen, dropping to 50% when the reducing gas had approximately 10% hydrogen. Through the caustic leaching step and high hydrogen content, the process was able to recover virtually all zinc from EAFD. As mentioned earlier, the cost of using the caustic leaching step is higher than the acid leaching step; however, this avoids iron dissolving into the leachate. For this process, the hydrogen content of the reducing gas needs to be approximately 50% to achieve good results. This high hydrogen content poses a significant fire / explosion risk and increases costs.

[0015] Other industrial processes that produce zinc-containing ferrite materials include zinc mining and refining, where zinc is typically recovered using alkaline or acidic leaching solutions heated to above 100°C. These corrosive leaching processes often generate highly corrosive wastewater that requires recovery or treatment, which is usually expensive or environmentally harmful, and sometimes both.

[0016] Any discussion of prior art in the specification does not constitute an admission that such prior art is well known or constitutes part of common general knowledge in the field.

[0017] This invention provides a method for processing EAFD, which reduces zinc ferrite without generating large amounts of acid-soluble iron or zinc vapor, or provides a useful option for consumers. Summary of the Invention

[0018] This invention provides a method for processing zinc-containing ferrite source materials, the method comprising the following steps in sequence:

[0019] B. Using hydrogen-containing reducing gas to partially reduce the source material to form reducing material;

[0020] Step B is carried out at or below 1000°C using a reducing gas containing at least 0.25% (by volume) hydrogen in the carrier gas.

[0021] Preferably, step B is performed at or below 800°C.

[0022] Preferably, the hydrogen content in the reducing gas does not exceed 70% (by volume). In a highly preferred embodiment, the hydrogen content in the reducing gas does not exceed 60% (by volume).

[0023] Preferably, the source material is granular material that has not been granulated before processing.

[0024] Preferably, the source material originates from the steel or iron manufacturing industry, or the zinc mining and / or refining industry. Preferably, the source material originates from a furnace. In a highly preferred embodiment, the source material is electric arc furnace dust (EAFD).

[0025] In one preferred alternative, the source material is derived from the zinc industry. In a highly preferred alternative, the source material is zinc mining and / or refining waste.

[0026] Preferably, step B is performed between 500°C and 750°C. In a highly preferred embodiment, the temperature in step B does not exceed 700°C. In another preferred embodiment, the temperature in step B does not exceed 650°C. In yet another preferred embodiment, the temperature is at least 650°C. Preferably, the temperature in step B does not exceed a temperature selected from the group consisting of 550°C, 600°C, 650°C, and 700°C. Preferably, the temperature in step B is not lower than a temperature selected from the group consisting of 550°C, 600°C, and 650°C.

[0027] Preferably, the method includes step C, wherein step C cools the reducing material to a temperature not exceeding 350°C in an inert gas or reducing gas environment.

[0028] Preferably, the hydrogen content of the reducing gas is between 0.25% and 10% (by volume).

[0029] Preferably, step A precedes step B, wherein in step A, the source material is analyzed to determine the total zinc and / or zinc ferrite concentration. In a highly preferred form, currently, the total zinc is determined by X-ray fluorescence (XRF) or standard chemical analysis, and the zinc ferrite fraction is determined by X-ray diffraction.

[0030] In a highly preferred embodiment, step A determines the proposed processing conditions to be used in step B, including reduction time (Rt), reduction temperature (RT), and reducing gas flow rate (GF). Preferably, step A includes laboratory testing to confirm that the proposed processing conditions produce a mass loss of less than 10%, and more preferably, no more than (2 + / - 0.5)% mass loss. In another preferred embodiment, step A confirms that the ferroalloy and / or metallic iron produced under any processing conditions are below predetermined values.

[0031] In a preferred embodiment, the reducing gas further includes water. Preferably, water is added at a predetermined time after the start of step B. Preferably, this predetermined time is before the temperature in step B reaches 650°C. In another preferred embodiment, the temperature is selected from a list consisting of 500°C, 550°C, 600°C, and 650°C. In an alternative preferred embodiment, the predetermined time occurs when the temperature is within a temperature range selected from a list consisting of 500°C to 550°C, 550°C to 600°C, 600°C to 650°C, 500°C to 600°C, and 500°C to 650°C. In a highly preferred embodiment, the water-to-hydrogen ratio (H₂O:H₂), in molar quantities, is between 20:1 and 1:5. Preferably, the water-to-hydrogen ratio is between 1:2 and 2:5. In a highly preferred embodiment, the water-to-hydrogen ratio is between 5:1 and 1:1.

[0032] In alternative forms, the reducing gas also includes carbon monoxide, such that when the concentration in the carrier gas is between 0.25% and 15% (by volume), the molar ratio of H2:H2O:CO is 2:(1 to 10):(0 to 10).

[0033] Preferably, the carrier gas is a neutral carrier gas selected from a list consisting of nitrogen, helium, and argon. Alternatively, the carrier gas is a combination of one or more gases selected from a list consisting of nitrogen, helium, and argon. Preferably, the neutral carrier gas is an inert gas.

[0034] Preferably, step C is followed by step D, where step D is an acid leaching step. Preferably, the acid leaching step uses an acid leaching solution comprising one or more acids selected from the list of sulfuric acid, phosphoric acid, hydrochloric acid, and hydrofluoric acid, as well as an organic acid having fewer than 15 carbon atoms. In a highly preferred form, step D is carried out at a pH less than 3, and preferably at a pH of 2. Preferably, the acid leaching solution used contains sulfuric acid. In a highly preferred form, the acid leaching solution is sulfuric acid.

[0035] Preferably, step B is monitored and / or controlled by: directly or indirectly measuring the degree of reduction by determining the form of iron present, and / or determining the composition of the reducing gas in the furnace and the composition of the outlet gas flow. Preferably, step B is monitored and / or controlled by determining the composition of the outlet gas using a thermal conductivity detector. Attached Figure Description

[0036] By way of example only, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings, wherein:

[0037] Figure 1 It is a flowchart of the method;

[0038] Figure 2 It is a phase equilibrium diagram of H2 / H2O with different iron forms at different temperatures;

[0039] Figure 3 It is a CO / CO2 phase equilibrium diagram with different iron forms at different temperatures;

[0040] Figure 4 This is a thermal conductivity detector (TCD) image comparing different water-to-hydrogen ratios with synthetic electric arc furnace dust (EAFD).

[0041] Figure 5 It is a thermal conductivity detector (TCD) image comparing real electric arc furnace dust (EAFD) with synthetic electric arc furnace dust (EAFD); and

[0042] Figure 6 This is a thermal conductivity detector (TCD) image comparing synthetic electric arc furnace dust (EAFD) with real electric arc furnace dust (EAFD). Detailed Implementation

[0043] definition

[0044] EAFD: Electric Arc Furnace Dust.

[0045] Zinc-iron ore: Due to morphological similarity, some references use the term zinc-iron ore to refer to ZnFe2O4 (zinc ferrite) present in EAFD. In many EAFDs (and mineral forms), zinc-iron ore contains both ZnFe2O4 and ZnMnFeO4, such as the zinc-iron ore used herein, which contains zinc ferrite and may contain ZnMnFeO4.

[0046] Magnetite: Fe3O4

[0047] Sulfuric acid: This is intended to include fuming sulfuric acid.

[0048] Water: This term is intended to include all fluid forms of water as determined by local environmental conditions, such as water vapor (steam) and liquid water.

[0049] Iron ore: sometimes referred to as non-stoichiometric iron oxide (Fe2+) 0.947 Fe(II) and Fe(III) all exist in oxides. Sometimes it is also written as FeO.

[0050] Zinc oxide: is essentially zinc oxide (ZnO).

[0051] Because the materials processed by this method are complex and involve chemical formulas, this may simply refer to the dominant varieties that exist.

[0052] For clarity, unless otherwise stated, the term "about" is generally used when the parameter is within + / - 15%, and "approximately" is used when the parameter is within + / - 10%.

[0053] First embodiment

[0054] Although described in conjunction with EAFD, this method is considered applicable to other zinc-rich dusts in steelmaking or ironmaking processes, such as dust from basic oxygen furnaces containing ZnFe2O4. It is also thought that this method can be used to recover zinc in any process or industry that produces zinc-containing ferrite materials, such as zinc mining and refining materials.

[0055] refer to Figure 1 The core steps of the method for processing source material (1) are most likely EAFD, which is shown in the form of a flowchart.

[0056] For this variant, the steps are as follows:

[0057] A. Analyze the source material (1);

[0058] B. Use hydrogen-containing reducing gas (2) to partially reduce the source material (1) to form reducing material (3);

[0059] C. Cooling and reducing raw materials (3); and

[0060] D. Acid leaching to reduce raw materials (3).

[0061] Step A is optional and not present in all variations; it is used to determine the optimal processing variable. Steps C and D are preferred but also optional.

[0062] Step A involves analyzing the source material (1) to determine the content of zinc and zinc ferrite present, so that processing parameters can be determined. These parameters include reduction temperature (RT), reduction time (Rt), hydrogen concentration (HC) of the reducing gas (2), and flow rate (GF) of the reducing gas (3). Currently, total zinc is determined by X-ray fluorescence (XRF) or standard chemical analysis, and the zinc ferrite fraction is determined by X-ray diffraction.

[0063] Once step A is completed, the reduction conditions for step B can be determined. The aim is to reduce zinc ferrite to magnetite and zinc oxide while avoiding the formation of argentite, iron, or zinc. Argentite and iron are soluble in acids, and zinc melts at approximately 420°C, which could cause problems in the reduction furnace.

[0064] Initially, a laboratory-scale reduction of the sample will be performed based on the analysis from step A. The reduction time (Rt), reduction temperature (RT), and reducing gas flow rate (GF) for the bulk material reduction performed in step B will be determined using the laboratory-scale results and / or equilibrium calculations. Most likely, this will be determined by setting conditions to ensure that the mass loss during sample reduction is less than approximately 2% (2% + / - 0.5%).

[0065] Since the equilibrium / maximum reduction is thus determined, the result of step A allows for optimal selection of the hydrogen concentration (HC) in the reducing gas (3).

[0066] In most cases, the reduction temperature (RT) and reducing gas flow rate (GF) in step B are predetermined, and the reduction time (Rt) is set by laboratory testing. This ensures that the batch furnace is maintained at the predetermined optimal temperature, and the reducing gas flow rate (GF) is fixed to minimize EADF entrainment. For continuous furnaces, optimized settings of the temperature and reducing gas flow rate (GF) are allowed, and the EADF transfer rate can be adjusted via the continuous furnace to provide the optimal reduction time (RT).

[0067] Step B involves partially reducing the zinc ferrite in the EAFD to magnetite and zinc oxide without forming significant amounts of aragonite, metallic iron, or metallic zinc. This process is carried out in a furnace of known type and configuration. To minimize the formation of aragonite, metallic iron, or metallic zinc, the mass loss in this step is likely controlled to no more than about 2% (2% + / - 0.5%). This is achieved by using a reducing gas (2) comprising a carrier gas, such as nitrogen or hydrogen in argon, at a temperature below about 800°C. Most likely, the process will be carried out between 500°C and 750°C, with the optimal temperature considered to be between 650°C and 750°C.

[0068] The risk of fire or explosion may determine the maximum hydrogen concentration (HC) in the reducing gas (2). It is also possible that the maximum hydrogen concentration (HC) in the reducing gas (2) will vary depending on the processing equipment and safety requirements at the site.

[0069] Based on tests to date, the reducing gas (2) is expected to be in a neutral carrier gas, such as hydrogen, containing 0.25% to 10% hydrogen (by volume), although a hydrogen concentration of up to 70% is considered the upper limit, and below 66% is considered optimal.

[0070] Table 1 below shows typical mass loss results for the trial reduction of EAFD using 5% (volume%) hydrogen in reducing gas (2).

[0071]

[0072] Table 1

[0073] It can be seen that if the reduction temperature (RT) is below 650°C and the reduction time (Rt) is approximately 1 hour, using a reducing gas of 5% (by volume) hydrogen in a neutral carrier gas (nitrogen) (2), the mass loss of the EAFD sample is less than approximately 2%. The mass loss is roughly linear with time, therefore the effect of increasing the time can be determined. Since the reaction kinetics are more sensitive to temperature changes, increasing the reduction temperature (RT) also alters the mass loss, although not in a linear fashion.

[0074] It has been found that a mass loss of about 2% can reduce zirconia to zinc oxide and magnetite, with a low risk of reducing magnetite to unwanted aragonite or iron. However, a mass loss or zinc loss greater than 2% is acceptable without the formation of metallic iron, metallic zinc, or aragonite.

[0075] Table 2 below shows the XRD analysis results of the reducing material (3) under different reduction temperatures (RT) and reduction times (Rt).

[0076]

[0077] Table 2

[0078] It can be seen that the amounts of metallic iron and hematite formed are determined by the reducing gas flow rate (GF), reduction time (Rt), and reduction temperature (RT). Based on the total zinc and zinc ferrite fraction analysis performed in step A, more results are needed to determine the optimal parameters.

[0079] As mentioned above, the optimal reduction temperature (RT) is considered to be in the range of 500°C to 800°C, and most likely below about 700°C to keep the metallic iron and ferromagnetic ore in the reducing material (3) at a low level.

[0080] Theoretical thermodynamic analysis of reduced hydrogen (2) is provided in Example 1. If these calculations are verified by EAFD experiments, then a molar excess of H2 is required to minimize the formation of ferrite and affect the complete reduction of zinc ferrite to zinc oxide. Theoretical analysis shows that a reduction temperature of 400°C can achieve the desired result for reducing gas (2) containing 3 molar excess hydrogen.

[0081] Once step B is completed and the desired degree of reduction is achieved, step C is performed. Step C involves cooling the reducing material (3) under reducing or neutral conditions until the temperature of the reducing material is below 350°C, possibly even lower. If the reducing material is cooled in air at a temperature greater than 400°C, ZnFe2O4 forms rapidly.

[0082] After cooling the reduced raw material (3), an optional step D, namely the acid leaching step, is performed. It is expected that sulfuric acid with a pH less than 3 will be used.

[0083] Best practice

[0084] In another variation of the method, step A is also optional, though preferred, and the reducing gas (2) comprises water with a H2O:H2 molar ratio of 1:2 to 2:5, i.e., 2 moles of hydrogen:1 part of water to 5 moles of hydrogen:2 parts of water. This alters the reducing effect of the reducing gas (2). It has been found that a water-hydrogen ratio range of 20:1 to 1:5 has the desired properties.

[0085] Using H2 / H2O generates a completely different phase equilibrium diagram; see [link / reference]. Figure 2 When using CO / CO2, see [reference needed]. Figure 3 When equilibrium is reached, the CO / CO2 system favors the reduction to iron. When equilibrium is reached, the H2 / H2O reducing gas (2) favors the formation of magnetite. The iron equilibrium will tend to differ depending on whether the reducing gas contains hydrogen or carbon monoxide.

[0086] It has been found that using hydrogen and water in a neutral carrier gas (most likely nitrogen) as a reducing gas (2) provides a method for regulating the reducing power of the reducing gas (2). By controlling the presence of water vapor, the reducing potential of the reducing gas (2) to reduce the formation of unintended varieties can be limited. The water vapor concentrations of the incoming reducing gas (2) and the outgoing reducing gas (2) can be measured, and the reducing power of the reducing gas (2) can be controlled by controlling the water content of the reducing gas (2). Example 2 shows the theoretical thermodynamic analysis of the water-hydrogen reduction of the gas (2). If these are verified by EAFD experiments, then to minimize the formation of ferroalloys, the temperature needs to be maintained below 650°C.

[0087] In another variation, a small amount of carbon monoxide can be added to the reducing gas (2). It is thought that an H2:H2O:CO ratio of 2:(1 to 10):(0 to 10) with a concentration between 0.25% and 15% (by volume) in the neutral carrier gas would allow for additional control over the reduction reaction. Of course, the use of CO is undesirable for environmental reasons, and CO is a difficult-to-handle odorless and toxic gas, but it can improve reaction kinetics and thus reduce the required energy input.

[0088] In some variations, steps A and / or D are absent. If step A is absent, step B is controlled by directly or indirectly measuring the degree of reduction, determining the composition of the reducing gas (2) in the furnace, and using a thermal conductivity detector (TCD) on the outlet gas stream. If the reducing gas (3) is stored or sent off-site for further processing, step D may be omitted.

[0089] Regardless of the presence of other steps, the degree of reduction can be measured directly or indirectly by determining the form of iron present, and / or by determining the composition of the reducing gas (2) in the furnace, and / or by using a thermal conductivity detector (TCD) on the outlet gas flow to monitor and / or control the reaction in step B.

[0090] In some variations, there is a hydrogen recirculation loop that extracts hydrogen from the exhaust gas. The exhaust gas is processed, and the hydrogen returned from the process is used to add to a neutral carrier gas or a hydrogen-containing reducing gas (2).

[0091] In some variations, the composition of the reducing gas (2) in step B is intentionally altered over time, remaining within the previously given parameters. This intentional alteration of the composition of the reducing gas (2) after the start of step B is most likely achieved by adding hydrogen or water at a predetermined time, but before subsequent steps. This predetermined time delay is most likely to occur when the method is implemented as a continuous process. For example, water may be partially added in step B to allow for a rapid initial reaction, while the addition of water is used to regulate the reaction endpoint. This predetermined time delay is most likely determined before the temperature in step B reaches a certain value or within a certain range of values. This predetermined time delay is expected to be before the temperature reaches 650°C, 600°C, 550°C, or 500°C, and most likely when the temperature is within a predetermined range, for example, 600°C to 650°C, 550°C to 600°C, or 500°C to 550°C.

[0092] Thermodynamic simulations predicted successful operation of the process up to approximately 1000°C, although sample runs were limited to 900°C due to furnace limitations. We believe the method will operate successfully up to approximately (+ / -10%) 1000°C, but not necessarily optimally, and this is supported by the thermodynamic simulations; therefore, approximately 1000°C appears to be the practical limit. Processing below 800°C offers significant advantages; therefore, 800°C or below is believed to be the optimal range, with the ranges of 500°C to 750°C and 550°C to 650°C providing additional benefits such as controllability.

[0093] Example:

[0094] Theoretical Implementation Examples:

[0095] Example 1

[0096] For reducing gases without added water, the following are thermodynamic predictions for different reducing agents between 400 °C and 700 °C (calculated using FactSage from Equilib-Web):

[0097] The reduction of zinc ferrite using hydrogen gas requires 1 / 3 H2 per zinc ferrite.

[0098] 3ZnFe2O4+H2→3ZnO+2Fe3O4+H2O(α)

[0099] Reacting 1 mole of ZnFe₂O₄ with 1 / 3 mole of H₂ (i.e., not in excess) yields:

[0100] Table 3:

[0101] At different temperatures, 1 mole of ZnFe2O4 was reduced using 1 / 3 mole of H2 (1 atm).

[0102] Temperature / ℃ 400 500 550 600 650 700 ZnO / molar 0.53 0.75 0.83 0.89 0.93 0.95 <![CDATA[ZnFe2O4 / mole]]> 0.47 0.25 0.17 0.11 0.07 0.05 <![CDATA[Fe3O4 / mole]]> 0.35 0.50 0.55 0.59 0.62 0.63 FeO / molar 0 0 0 0 0 0 <![CDATA[H2 / mole]]> 0.16 0.08 0.06 0.04 0.02 0.02 Zn / molar (steam) <![CDATA[7x10 -11 ]]> <![CDATA[5x10 -9 ]]> <![CDATA[3x10 -9 ]]> <![CDATA[1.2x10 -7 ]]> <![CDATA[4x10 -7 ]]> <![CDATA[1x10 -6 ]]>

[0103] To achieve 95% of the stoichiometric amount of H2, approximately 700°C is required.

[0104] Perform the same calculation using an excess of 3x H2:

[0105] Table 4:

[0106] Reduction of 1 mole of ZnFe2O4 by 1 mole of H2 (1 atm.) at different temperatures

[0107]

[0108]

[0109] in conclusion

[0110] According to thermodynamics, operation at 700°C requires an excess of H2 (with an excess of 3x, the equilibrium transition can reach 100% at 400°C). This can be achieved as long as H2 recovery is possible. This also suggests that keeping the temperature below 650°C may be useful in limiting the formation of FeO (hemispherite).

[0111] Example 2

[0112] Thermodynamic predictions for the reduction of gases by water and hydrogen are given below for different reducing agents at 600℃ (calculated using FactSage from Equilib-Web):

[0113] The reaction is:

[0114] 3ZnFe2O4+H2→3ZnO+2Fe3O4+H2O(β)

[0115] The equilibrium calculation results using 1 mole of ZnFe2O4 and 1 mole of H2 (i.e., excess 3x) at 600℃ are given as follows:

[0116] Table 5:

[0117] Reduction of 1 mole of ZnFe2O4 at 600℃ using different H2:ZnFe2O4 molar ratios (1 atm)

[0118]

[0119] Thermodynamics shows that at 600℃, when the molar ratio is greater than 1:1, iron oxides will be reduced to FeO. Steam (water vapor) can limit further reduction of iron oxides. Calculations using an excess of 6x (H2:ZnFe2O4 molar ratio = 2:1) demonstrate this effect.

[0120] Table 6: Reduction of 1 mole of ZnFe2O4 by 2 moles of H2 with different H2O:H2 molar ratios (1 atm) at 600℃

[0121]

[0122]

[0123] Add a small amount of steam (1.5 = 20% H2 steam) to eliminate the reduction of FeO. Add more than 6 moles of steam (water vapor): 1 mole of H2 to begin limiting the reduction of ZnFe2O4.

[0124] Table 7:

[0125] 1 mole of ZnFe2O4 was reduced by 10 moles of H2 with different H2O:H2 molar ratios (1 atm) at 600℃.

[0126]

[0127] **At a ratio of 7.5 mol H₂O: 1 mol H₂, zinc ferrite begins to stop reducing. At an 8:1 ratio, no zinc ferrite is reduced.**

[0128] Tables 6 and 7 show that the addition of water vapor can prevent the over-reduction of zinc ferrite by facilitating the formation of magnetite (Fe3O4).

[0129] Example 3

[0130] Figure 4 The procedures for synthesizing EAFD without water and with water-to-hydrogen ratios of 1.6:1 and 2.1:1 are shown.

[0131] In this embodiment, 50 mg of the synthesized EAFD was heated from 100°C to 850°C in a nitrogen stream of 5% H2, with a temperature rise of 20°C / min. The outlet gas was monitored using a thermal conductivity detector. The detector effectively measures hydrogen in the outlet gas stream indicating reduction; a higher number indicates a more vigorous reduction.

[0132] exist Figure 4In the diagram, line SNW represents the synthesis of EAFD without the addition of water, line SD1 represents a water-to-hydrogen ratio of 1.6:1 (1.6 moles of water to 1 mole of hydrogen), and line SD2 represents a water-to-hydrogen ratio of 2.1:1.

[0133] The effect of water-hydrogen ratio:

[0134] It can be seen that by comparing SNW with SD1 or SD2, adding water slows down the reaction rate and causes the reaction to begin at a lower temperature. However, counterintuitively, increasing the water-to-hydrogen ratio from 1.6:1 to 2.1:1 speeds up the reaction rate and causes the reaction to begin at a lower temperature.

[0135] Example 4

[0136] Real EAFD vs. Synthetic EAFD

[0137] In Example 3, experiments were repeatedly conducted using two real EAFDs and one synthetic EAFD with the same zinc content, and the results are as follows. Figure 5 As shown. The water-to-hydrogen ratio is 1.6:1.

[0138] refer to Figure 5 The lines shown are:

[0139] SNW Synthetic EAFD without added water

[0140] Synthesis of EAFD from SD1 with a water-to-hydrogen ratio of 1.6:1

[0141] RD1 real EAFD sample 1 with a water-to-hydrogen ratio of 1.6:1 and

[0142] RD2 Real EAFD Sample 2 with a water-to-hydrogen ratio of 1.6:1

[0143] It can be seen that the reaction rate of the synthetic dust appears to occur in two stages, with the highest reaction rate occurring at approximately 820℃. However, the real EAFD (based on two actual samples considered representative) does not exhibit this obvious two-stage process and has a peak reaction rate in the range of 700℃ to 720℃. This means that the reaction characteristics of the synthetic EAFD are not suitable for the real EAFD. Figure 5 Using synthetic EAFD results to optimize a real EAFD process may result in a design temperature of approximately 820°C, which significantly reduces the reaction rate and increases energy consumption compared to using real EAFD in optimization experiments. Based on these results, experiments using synthetic EAFD cannot be used to determine reduction conditions for a real EAFD process with any reliability.

[0144] Example 5

[0145] Figure 6The procedure for reproducing synthetic EAFD without the use of water and real EAFD is shown.

[0146] In this embodiment, 50 mg of synthetic EAFD and real EAFD were heated from 100°C to 850°C in a nitrogen stream of 5% H2, with a temperature rise of 20°C / min. The outlet gas was monitored using a thermal conductivity detector. The detector effectively measures hydrogen in the outlet gas stream indicating reduction; a higher number indicates a more vigorous reduction.

[0147] exist Figure 6 In the middle, the lines shown are:

[0148] SNW - Synthetic dust, anhydrous; and

[0149] RNW - Real Dust, No Water.

[0150] As can be seen, compared to real EAFD, the reaction peak of synthetic EAFD is lower and occurs at a higher temperature (-720℃). The reaction peak of real dust occurs at approximately 690℃ and is about 40% higher. Interestingly, the real EAFD peak is much narrower than the synthetic peak, which may allow for specific optimization of reaction conditions. Again, based on these results, experiments using synthetic EAFD cannot be used to determine reduction conditions for a real EAFD process with any reliability.

[0151] KEY

[0152] Source materials (EAFD, BOFD);

[0153] Reducing gas;

[0154] Reduced raw materials (materials after partial reduction using hydrogen-containing reducing gas);

[0155] RT reduction temperature

[0156] Rt Restore Time

[0157] Hydrogen concentration of HC reducing gas

[0158] HF reducing gas flow rate

Claims

1. A method for processing a source material containing zinc ferrite, comprising the following steps: B. partially reducing the source material using a hydrogen-containing reducing gas to form a reduced material; Wherein step B is carried out at 1000°C or below using a reducing gas containing at least 0.25% (by volume) and up to 70% (by volume) hydrogen in a carrier gas.

2. The method according to claim 1, wherein: Step B is performed at 800°C or below.

3. The method according to claim 1 or claim 2, wherein: The source material is a granular material that has not been granulated before processing.

4. The method according to any one of claims 1 to 3, wherein: The source material is derived from the steel or iron manufacturing industry, or the zinc mining and / or refining industry.

5. The method according to claim 4, wherein: The source material comes from a furnace.

6. The method according to any one of claims 1 to 5, wherein: The source material is electric arc furnace dust (EAFD).

7. The method according to any one of claims 1 to 5, wherein: The source material comes from the zinc industry.

8. The method according to claim 7, wherein: The source material is zinc mining and / or refining waste.

9. The method according to any one of claims 1 to 8, wherein: Step B is carried out between 500°C and 750°C.

10. The method according to claim 9, wherein: The temperature in step B does not exceed a temperature selected from the group consisting of 550°C, 600°C, 650°C, and 700°C.

11. The method according to claim 10, wherein: The temperature in step B is not lower than a temperature selected from the group consisting of 550°C, 600°C, and 650°C.

12. The method according to any one of claims 1 to 11, wherein: The method comprises step C, wherein step C cools the reduced material to a temperature not higher than 350° C. in an inert gas or reducing gas environment.

13. The method according to any one of claims 1 to 12, wherein: The hydrogen content of the reducing gas is between 0.25% and 10% by volume.

14. The method according to any one of claims 1 to 13, wherein: Step A precedes step B, wherein, in step A, the source material is analyzed to determine total zinc and / or zinc ferrite concentration.

15. The method according to claim 14, wherein: The total zinc is determined by X-ray fluorescence (XRF) or standard chemical analysis, and the zinc ferrite fraction is determined by X-ray diffraction.

16. A method according to any one of claims 14 or 15, wherein: Step A determines the proposed processing conditions to be used in Step B, including the reduction time (Rt), the reduction temperature (RT), and the reducing gas flow rate (GF).

17. The method according to any one of claims 14 to 16, wherein: Step A involves laboratory testing to confirm that the proposed processing conditions produce less than 10% mass loss.

18. The method according to claim 17, wherein: The mass loss does not exceed (2+ / -0.5)%.

19. The method according to any one of claims 14 to 18, wherein: Step A confirms that the wüstlerite and / or metallic iron produced under any of the proposed processing conditions is below a predetermined value.

20. The method according to any one of claims 1 to 19, wherein: The reducing gas also includes water.

21. The method according to claim 20, wherein: After step B starts, the water is added to the reducing gas at a predetermined time.

22. The method according to claim 21, wherein: The predetermined time is reached before the temperature reaches a value selected from the list consisting of 500°C, 550°C, 600°C and 650°C.

23. The method according to claim 21, wherein: The predetermined time is reached when the temperature has a value 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.

24. The method according to any one of claims 20 to 23, wherein: The water to hydrogen ratio (H2O:H2), on a molar basis, is between 20:1 and 1:

5.

25. The method according to claim 24, wherein: The water-to-hydrogen ratio is between 5:1 and 1:

1.

26. The method according to any one of claims 20 to 25, wherein: The reducing gas also includes carbon monoxide such that when the concentration in the carrier gas is between 0.25% and 15% (by volume), the molar ratio of H2:H2O:CO is 2:(1 to 10):(0 to 10).

27. The method according to any one of claims 1 to 26, wherein: The carrier gas is a neutral carrier gas selected from the list consisting of nitrogen, helium and argon.

28. The method according to any one of claims 1 to 26, wherein: The carrier gas is a combination of one or more gases selected from the list consisting of nitrogen, helium and argon.

29. The method according to any one of claims 1 to 28, wherein: Step C is followed by step D, wherein step D is an acid leaching step.

30. The method of claim 29, wherein: The acid leaching step uses an acid leaching solution including one or more acids selected from the list consisting of sulfuric acid, phosphoric acid, hydrochloric acid, hydrofluoric acid, and an organic acid having less than 15 carbon atoms.

31. The method according to claim 29 or 30, wherein: Step D is carried out at a pH less than 3.

32. The method according to claim 31, wherein: Step D is carried out at pH 2.

33. The method according to any one of claims 30 to 32, wherein: The pickling solution used contains sulfuric acid.

34. The method according to any one of claims 1 to 33, wherein: Step B is monitored and / or controlled by one or more methods selected from the list consisting of: measuring the degree of reduction directly or indirectly by determining the form of iron present, determining the composition of the reducing gas in the furnace and determining the composition of the outlet gas stream.

35. The method of claim 34, wherein: Step B is monitored and / or controlled by determining the composition of the outlet gas using a thermal conductivity detector (TCD).

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