WO2024046656A1

WO2024046656A1 - Process for heavy metal removal from iron- and steelmaking flue dust

Info

Publication number: WO2024046656A1
Application number: PCT/EP2023/070414
Authority: WO
Inventors: Petrus Cornelis Jozef Beentjes; Yanping Xiao
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2022-09-01
Filing date: 2023-07-24
Publication date: 2024-03-07
Anticipated expiration: 2025-03-01
Also published as: JP2025534576A; KR20250051715A; EP4581175A1

Abstract

The invention relates to a process for the selective heavy metal removal from iron- and/or steelmaking flue dust, the process comprising the steps of: preparing a feedstock (FS) by blending or mixing a chloride precursor material (CPM) and ironmaking and/or steelmaking flue dust comprising heavy metals (ISFD), the heavy metals being at least Pb and Zn and optionally also Cd; in a first reaction step in a first reactor reacting the CPM with the ISFD by thermal treatment of the FS at a temperature in a range of 700°C to 950°C removing at least 70 wt.% of Pb from the ISFD; in a subsequent second reaction step in a second reactor further reacting the CPM with the ISFD by thermal treatment of the feedstock FS at a temperature in a range of 850°C to 1200°C; and obtaining a solid material after the second reaction step. The invention also relates to a plant implementing the process of this invention.

Description

PROCESS FOR HEAVY METAL REMOVAL FROM IRON- AND STEELMAKING FLUE DUST

FIELD OF THE INVENTION

The invention relates to a process for the selective heavy metal removal from iron- and/or steelmaking flue dust by chlorination and evaporation of heavy metals present in the flue dust. The invention also relates to a plant implementing the process of this invention.

BACKGROUND TO THE INVENTION

Blast furnace (BF) sludge is a by-product that is generated when the flue gas formed during the production of pig iron in a blast furnace is purified in a wet scrubber and the dried sludge is called flue gas dust or flue dust. The mineralogical composition of BF sludge usually reflects that of the charge that is used for the blast furnace ironmaking. Due to the very high temperatures in the ironmaking process, the BF sludge contains volatile, non-ferrous heavy metals and metalloids such as zinc, lead, cadmium and mercury that mix with and condense onto solid particles dragged along by the process gas in the upper, colder part of the furnace. The zinc in the sludge predominantly originates from the internal recycling of flue dust from the iron- and steelmaking operations including recycled coated steel scrap for example in an EAF or BOF. Too high a zinc content (commonly in the form of ZnO or ZnS compounds), typically more than about 2 wt.%, in the final BF sludge precludes on-site recycling by directly refeeding the sludge back after sintermaking into the BF furnace. This is because vaporized metal zinc readily condenses onto the upper furnace walls which negatively impacts furnace operation. The heavy metals such as lead, cadmium and mercury predominantly originate from the substantial amounts of iron ores, coke and coal used in a BF operation. The recycling of the sludge or flue dust into the ironmaking process recovering the iron and carbon present is very limited. This is because the ores used is also the source of the naturally occurring radioactive nuclide lead-210. The accumulation of lead-210, but also of other toxic elements, in the blast furnace sludge and subsequently in the flue dust seriously limits the re-use thereof in the ironmaking process. For that reason the heavy metal enriched blast furnace sludge and blast furnace flue dust are partially stored carefully in sedimentation ponds or landfills. Therefore, a suitable recycling process that may remedy at least part of any pollution of the environment and allow recovery of the iron and the carbon lost by landfilling of the blast furnace sludge and flue dust is highly sought-after.

The rotary hearth furnace (RHF) is known to be an effective process for zinc removal from blast furnace (BF) flue dust or sludge, and has been installed in several steel plants world-wide, mostly in Asia. In the RHF process, the zinc is removed via a high temperature pyrometallurgical route and zinc is enriched in the RHF flue dust as a mixture with other heavy metals and harmful elements, which requires further processing for zinc extraction and final waste disposal. It requires operating temperature of higher than 1250°C. A RHF plant requires high initial investment and a high production capacity to be economical feasible. The RHF has been developed by several industrial companies, however it remains very challenging to recycle the secondary flue dust which contains Zn, Pb and other harmful elements.

The Waelz-process is a method of recovering zinc and other low boiling point metals, e.g. lead and cadmium, from metallurgical waste, most typically Electric Arc Furnace (EAF) flue dust, using a rotary kiln. The process consists of treating zinc containing material, in which zinc can be in the form of zinc oxide, zinc silicate, zinc ferrite, zinc sulphide together with a carbon containing reductant, within a rotary kiln typically at about 1200°C to 1300°C. The kiln feed material comprising the zinc, fluxes and reducing agent (i.e. coke) is typically pelletized before addition to the rotary kiln. The chemical process involves the reduction of zinc compounds to elemental zinc having a boiling point of 907°C which volatilises and oxidises in the vapour phase to zinc oxide. The zinc oxide is collected from the kiln outlet exhaust by filters, electrostatic precipitators, settling chambers, etc. The Waelz-process is commonly carried out with oxygen enriched air which is fed in counter-current to the treated material. Increased use of galvanised steel has resulted in increased levels of zinc in steel scrap which in turn leads to higher levels of zinc in EAF flue dust. Currently the Waelz-process is the preferred or most widely used industrial scale process for zinc recovery from EAF flue dust. However, the recycling of blast furnace sludge by means of the Waelz-process is not economically attractive as the material does not contain enough Zn.

Patent document W02022/172495-A1 discloses a zinc recovery method from electric arc steelmaking furnace dust containing zinc and iron, the dust is placed in a rotating cylindrical kiln base body of an indirect-heating rotary kiln, and in a single heat treatment at about 950-1000°C in the kiln body so that zinc contained in the dust is vaporized, the vaporized zinc together with any other low-melting components are guided to a treatment device through an exhaust pipe provided in a discharge part of the rotary kiln and the zinc is said to be recovered. In addition, a residue resulting from the treatment in the kiln body is guided from a residue outlet provided in the discharge part of the rotary kiln to a burner device and mixed with air and carbon so that they are blown into the electric furnace and burned. The disclosed zinc recovery method is a single heat-treatment method and does not address the presence of other heavy metals present in the electric arc furnace steelmaking dust. Patent document W02019/043261-A1 discloses a process for the purification of waste materials or industrial by-products comprising chlorine, notably bypass dusts from cement production, the process comprising the steps of a) preparing a composition (C) by blending or mixing waste materials or industrial by-products comprising chlorine (B) with one or more materials comprising heavy metals (HM), b) reacting (B) and (HM) by a single thermal treatment of (C), c) separating evaporated heavy metal chloride compounds (HMCC), d) obtaining a solid material after the thermal treatment step, wherein the heavy metals (HM) are one or more from the following set of elements: Zn, Pb, Hg, Cu, Cd, Tl, In, Sn, N i, Co, the single thermal treatment is carried out at a temperature of 200-1500°C, and most preferably between 600-700°C, and under a non-oxidizing atmosphere, the materials comprising heavy metals (HM) and the waste materials or industrial by-products comprising chlorine (B) being mixed or blended in the presence of water, with 2-50% by mass, more preferably 10-20% by mass, of water being present in the total composition (C), and the ratio of the materials comprising heavy metals (HM) and the waste materials or industrial by-products comprising chlorine (B) is chosen so that the chlorine content of the composition (C) is between 100-150%, most preferably between 100-110%, of the amount being necessary for a stoichiometric conversion of the heavy metals (HM) in the materials comprising heavy metals (HM) into chlorides, or the ratio of the materials comprising heavy metals (HM) and the waste materials or industrial byproducts comprising chlorine (B) is chosen so that the chlorine content of the composition (C) is between 80-100%, most preferably between 90-95%, of the amount being necessary for a stoichiometric conversion of the zinc in the materials comprising heavy metals (HM) into chlorides. The ZnCI₂ is evaporated by performing the thermal treatment at 600-680°C. In an embodiment the thermal treatment is performed in combination with the Waelz-process.

European patent document EP-3333272-A1 discloses a wet-chemical process for selectively reducing the amounts of heavy metals comprising Zn from a metallurgical plant waste product containing Fe comprising the steps of: selectively leaching Zn by mixing the waste product with a leaching solution comprising ammonia and an ammonium salt with a pH in the range of 8-12 into a reaction mixture; and controlling the pH of the reaction mixture and keeping the pH in the range of 8-12; separating the reaction mixture into a leaching filtrate and a leached solid residue; and recovering ZnO from the leaching filtrate.

Patent document US-6,083,295 discloses a method of processing finely divided material incorporating metal based constituents, the method comprising the steps of: forming the finely divided material into pellets; drying the pellets; heating the pellets in a first rotary kiln at a temperature between 900-1200°C, preferably 1050-1200°C, and residence time sufficient to sinter the pellets by reducing these and to drive off volatile first constituents, predominantly lead oxide and any chlorides, from the pellets; removing any material in a finely divided form from the sintered pellets; and heating the sintered pellets in a second rotary kiln together with anthracite and some dolomite fines, in a reducing atmosphere, whereby one or more second constituents in the pellets, predominantly zinc oxides, are reduced at temperatures of about 1080-1100°C to a volatile form and driven off, leaving one or more reduced third constituents. In this step the zinc oxide becomes reduced to metallic zinc and the atmosphere in the kiln is kept sufficiently oxidising for the metallic zinc to again oxidise and is then carried away with waste gases and is then collected from the waste gases. The zinc oxide is mentioned to have a high degree of purity.

Patent document US-5,547,490 discloses a method of removing lead and zinc from foundry dust material having components containing lead in the form of alkalichloride complexes, zinc and iron, the method comprising the steps of feeding the material to be treated to a lead-processing furnace and heating the material at a temperature of 900-1100°C, preferably 1000-1100°C, in the lead-processing furnace only until vaporization of said lead- containing components for generating a zinc bearing residual material which is freed from the chloridic fraction of said lead-containing components being removed from the lead-processing furnace by means of a scavenging gas flow and the scavenging gas flow which is charged with the lead-containing components being cooled down and filtered, and next heating the zinc- bearing residual material together with coal in a zinc-processing furnace under reducing conditions at temperatures in a range of 1100-1400°C, preferably 1150-1350°C, for reducing the zinc oxide with the formation of zinc metal vapours, the zinc vapours being removed from the zinc-processing furnace by means of a scavenging gas flow such that an oxygen-bearing furnace atmosphere is present. Here, the CO which rises out of the coal is oxidised to give CO₂ and the zinc metal becomes zinc oxide again. The scavenging gas flow being cooled down and filtered.

Patent document US-6,132,488 discloses a treating method of recovering zinc in the metal state from a waste containing the zinc in the oxide state, lead, chlorine, fluorine, and water, the method comprising: a mixing process of mixing the waste and a reducing material together to obtain a to-be-treated mixed material; a chlorine recovery process of recovering the chlorine and the water by heating the to-be-treated mixed material at a temperature in a range of 40-600°C; a lead recovery process of recovering the fluorine and the lead by heating the to- be-treated mixed material under a vacuum state at a temperature in a range of 200-600°C; a zinc recovery process of reducing and vaporizing the zinc to recover metallic zinc by heating the to-be-treated mixed material at a temperature in a range of 600-1100°C, with the vacuum state maintained; and a residue recovery process of recovering a residue of the to-be-treated mixed material by compression-moulding the residue into a briquette, with the vacuum state maintained. The degree of vacuum state in each of the processes is 0.001-20 Torr.

There is a demand for an improved process enabling the heavy metal removal from iron- and/or steelmaking flue dust, viz. flue dust originating from for example a blast furnace (BF) operation.

DESCRIPTION OF THE INVENTION

As will be appreciated herein, for any description of compositions or preferred compositions, all references to percentages are by weight percent unless otherwise indicated.

It is an object of the invention to provide an improved process enabling the selective heavy metal removal from iron- and/or steelmaking by-products, in particular dust, sludges or filter cakes (collectively referred to as “flue dust”), that are obtained from gas scrubbing of iron- and steelworks off-gas and originating from a blast furnace operation, a direct reduced iron making process, an electric arc furnace operation, a reducing electrical furnace, a Hlsarna- type ironmaking process, or a basic oxygen steelmaking operation.

These and other objects and further advantages are met or exceeded by the process according to claim 1 , the use according to each of claims 11 and 12, and the plant according to claim 13, and with preferred embodiments set out in the dependent claims.

In order to achieve these objects, the present invention proposes, in a first aspect, a process for the selective heavy metal removal from iron- and/or steelmaking flue dust, the process comprising, in that order, the steps of:

(i) preparing a feedstock (FS) by blending or mixing a chloride precursor material (CPM) 16 with ironmaking and/or steelmaking flue dust comprising heavy metals (ISFD) 15, the heavy metals being at least Pb and Zn and optionally also Cd;

(ii) in a first reaction step in a first reactor 1 reacting the CPM with the ISFD by thermal treatment of the FS at a temperature in a range of 700°C to 950°C reducing the Pb content of the ISFD by removing at least 70 wt.% of Pb present in the ISFD by chlorination and evaporation of the Pb and removing it (i.e. the chlorinated Pb) from the first reactor 1 via the off-gas formed; (iii) in a subsequent second reaction step in a second reactor 2 further reacting the CPM 16 with the ISFD 15 by thermal treatment at a temperature in a range of 850°C to 1200°C of the feedstock FS outputted from the first reactor step in the first reactor 1 and inputted into the second reactor 2, and reducing at least the Zn content of the ISFD by removing the Zn present in the ISFD by chlorination and evaporation of the Zn and removing it (i.e. the chlorinated Zn) from the second reactor 2 via the off-gas formed; and

(iv) obtaining a secondary solid material 8 substantially free from at least Pb, Zn and Cd and removing it from the second reactor 2 after the second reaction step. The secondary solid material 8 comprising high fractions of metallic iron, some remaining carbon, nonreduced FeOx (hematite and magnetite) and inert gangue minerals, and has very low levels of non-ferrous heavy metals, in particular lead, zinc, and cadmium, now removed from the ISFD 15.

This process achieves the effect of selectively removing non-ferrous heavy metals with a very high efficiency from the ISFD 15, in particular the zinc, lead, and also cadmium. After the second reaction step the remaining secondary solid material 8 comprises high fractions of metallic iron, some remaining carbon, non-reduced FeOx and inert gangue minerals, and can be re-used in an ironmaking operation, for example in a blast furnace operation.

By separating the removal of the heavy metals from the ISFD 15 into at least two separate reaction steps in separate reactors the off-gasses from each reaction step can be treated individually and separately and creating two separate fractions or portions of flue dusts, namely of lead-enriched flue dust fraction 5 from the first reaction step and a zinc-enriched flue dust fraction 7 from the second reaction step. This zinc-enriched flue dust fraction 7 is substantially free of lead, and thus also substantially free from lead-210, and thereby enhancing the value and useability of this flue dust fraction 7. As most of the lead is concentrated into a single flue dust fraction 5, the total amount of lead-containing flue dust is significantly reduced and thus contributing to a significant reduction of by-products from an ironmaking or steelmaking operation that have to be considered environmental unfriendly or even environmentally hazardous. As a consequence more than about 60%, typically more than about 70%, and in the best examples more than about 80%, of the ISFD 15 following a treatment in accordance with this invention remains to have commercial value, either as zinc-enriched flue dust 7 from the second reaction or as the secondary solid material 8 after the second reaction, and does not need to be discarded via landfilling or otherwise. This is a significant improvement compared to a single thermal treatment process of BF flue dust. The process enables the advantageous processing of flue dust obtained from gas scrubbing of iron- and steelworks off-gas originating from a wide range of operations including a blast furnace (BF) operation, a direct reduced iron (DRI) making process, a reducing electrical furnace (REF) operation, an electric arc furnace (EAF) operation, a Hlsarna-type ironmaking process, and a basic oxygen steelmaking operation (BOF, BOS, or BOP).

The chloride precursor material 16 is a compound or substance susceptible of being transformed into chloride. The transformation may take place by heating the chloride precursor material 16 thereby providing chloride for selective chlorination and evaporation of the heavy metals from the ISFD 15. Examples of chloride precursor material 16 include polyvinyl chloride (PVC), waste comprising PVC, chlorinated rubber or other chlorinated polymers, FeCI₂ or FeCI₃, CaCI₂. In a preferred embodiment the chloride precursor material 16 is FeCI₂ (iron(ll)chloride), in particular FeCI₂ obtained as a by-product from a steel pickling operation.

Preferably the ISFD 15 and the CPM 16 can be pre-mixed to form a feedstock (FS) prior to the first reaction step.

When the ISFD 15 and the CPM 16 are pre-mixed prior to feeding into a reactor for the first reaction step, e.g. into a rotary kiln, said ISFD and CPM may be compacted by (micro-) granulation or pelletizing to form the FS. Various binders can be used, e.g. about 0.3-0.6 wt.% bentonite with a small amount of water. The pellet diameter may vary depending on the reactor capacity and the residence time inside the reactor and would typically be in a range of about 5 to 20 mm in diameter. An advantage of granulation or pelletising is that in an industrial process the composition of the ISFD 15 is not constant over time and the amount of added CPM 16 can be tailored per batch of FS and stored till use. This creates considerable flexibility in the blend recipe. The use of pellets also limits the occurrence of heavy metal containing dust in a rotary kiln process and on the shop floor. In an embodiment the granules or pellets are formed in an apparatus 3 comprises a granulation apparatus, a pelletising apparatus or a pelletising plant.

When separately fed into a first reactor 1 , e.g. a rotary kiln, the feeding can still occur simultaneously or consecutively, and furthermore at the same entry point of the rotary kiln, or at different entry points of the rotary kiln.

The first reaction step in the first reactor 1 removing at least substantial parts of the heavy metals including particularly lead and cadmium from the ISFD by chlorination and evaporation can be performed under an oxidizing atmosphere or a non-oxidizing atmosphere. In a preferred embodiment the first reaction step is performed under a non-oxidizing atmosphere.

The second reaction step in the second reactor 2 removing at least substantial parts of the heavy metal zinc from the ISFD by chlorination and evaporation can be performed under an oxidizing atmosphere or a non-oxidizing atmosphere. In a preferred embodiment the second reaction step is performed under a non-oxidizing atmosphere.

In the field of iron- and steelmaking metallurgy a non-oxidizing atmosphere requires the oxygen partial pressure to be as low as feasible. In an embodiment the oxygen partial pressure of the non-oxidizing atmosphere is less than 1x10^A(-8) atm when measured at 800°C, preferably less than 1x10^A(-10) atm when measured at 800°C, and more preferably less than 1x10^A(-12) atm when measured at 800°C. The non-oxidizing atmosphere may consists of nitrogen, noble gases, CO₂, CO, H₂, or mixtures of any of these.

A non-oxidising atmosphere is preferred so as to avoid the formation of ZnO which evaporates only at a temperatures well above 1200°C and its presence lowers the yield of the zinc removal in the process according to the invention. ZnO has a melting point of about 1974°C and a boiling point of about 2360°C. In a non-oxidizing atmosphere the carbon is used mainly as a reducing agent and the remaining non-oxidized carbon can be utilized as a source for hot metal carburization, for instance subsequently in an REF.

In an embodiment of the process during the first reaction step at least 70% of the lead is removed from the ISFD 15 by evaporating PbCI₂ at temperatures from 700°C to 950°C, preferably from 800°C to 900°C, and more preferably 800°C to 890°C. In this temperature range substantial parts of the lead, and also other heavy elements like cadmium and mercury, are removed from the ISFD 15, and with controlling to upper-limit temperature of the first reaction the removal of zinc, either following chlorination and subsequent evaporation or evaporation as metallic element, is shifted as much as feasible to the second reaction step in the second reactor 2. Metallic zinc evaporates at about 905°C.

In an embodiment of the process during the first reaction step in the first reactor 1 at least 80 wt.% of the lead is removed by chlorination and evaporation from the ISFD 15, and preferably at least 90 wt.%, and more preferably at least 95 wt.%, thus compared the Pb-content at the start of the first reaction step and outputted at the end of the first reaction step. Inevitable due to the process temperatures applied during the first reaction step also some zinc is removed from the ISFD 15, however, a substantial part of the zinc is removed only in the second reaction step in the second reactor 2.

In an embodiment of the process during the subsequent second reaction step in the second reactor 2 at least the remaining zinc is removed from the ISFD 15 by evaporating ZnCI₂ at temperatures from about 850°C to 1200°C. In an embodiment the upper-limit of the temperature does not exceed 1100°C. In an embodiment the temperature is at least 900°C, and more preferably at least 950°C. These operating temperatures are substantially lower compared to those applied in a Waelz-process and thereby providing costs advantages e.g. in fuel savings. Having lower operating temperatures also limits the issues related to accretion well-known from the Waelz-process.

It has been found that in the process according to the invention most of the zinc, preferably more than about 50 wt.%, and preferably more than about 60 wt.%, present in the ISFD, i.e. the Zn-content in the ISFD at the start of the first reaction step, is being removed by chlorinated and evaporated during the second reaction step in the second reactor 2. In total more than 85 wt.%, and in the best examples more than 95 wt.%, of the zinc present in the ISFD 15 is removed by chlorination and evaporation from the ISFD 15 when processed according to the invention.

In an embodiment of the process each of the first reaction step and second reaction step comprising further the step of off-gas cleaning the off-gasses as is known in the art created during each reaction step using off-gas treatment systems 4,6 connected with the reactors. As both reaction steps are separated from each other, the off-gas treatment can be tailored to the composition and temperature of the off-gas and the subsequently formed flue dust have different compositions that are handled separately.

As most of the lead is being removed during the first reaction step this enables the formation of a lead-enriched flue dust 5 originating from the off-gas treatment as in known in the art, e.g. gas scrubbing, following the first reaction step. This lead-enriched flue dust fraction 5 is very distinct from the zinc-enriched flue dust 7 originating from the off-gas treatment, e.g. gas scrubbing, of the second reaction step. This zinc-enriched flue dust 7 is substantially free of cadmium and lead, and thus also substantially free from lead-210. This difference significantly increases the value of the zinc-enriched flue dust 7 and is a valuable source material for processes known in the art to recover metallic zinc from this flue dust, e.g. in a subsequent hydrometallurgical process or an electrolytic conversion process. This contrary to the processes known from the prior art, e.g. patent document W02019/043261-A1 , where the ISFD is being treated in a single thermal treatment such that following the subsequent off-gas treatment the resultant secondary flue dust fraction is enriched with evaporated zinc but also with substantial amounts of the lead, cadmium and other non-ferrous heavy metals, thereby creating a significantly larger amount of environmentally unfriendly by-product that has to be carefully treated and landfilled against considerable costs.

In an embodiment of the process the ratio of ironmaking and/or steelmaking flue dust comprising heavy metals (ISFD) 15 and the chloride precursor material (CPM) 16 being chosen so that the chloride content of the feed stock (FS) is between 100% and 150%, preferably between 100% and 130%, and more preferably between 100% and 110%, of the amount necessary for a stoichiometric conversion of the heavy metals in the ISFD into chlorides.

Each of the first reaction step and the second reaction step are performed in separate reactors, viz. in a first reactor 1 and a second reactor 2 respectively, where the reaction conditions are regulated including the temperature and reaction atmosphere, and thereby making also the control and consistent handling of the generated off-gas of each reactor 1 ,2 easier. In an embodiment each reactor 1 ,2 is a fluidised bed reactor, a rotary heart furnace, a traveling strand, or any other gas-tight reactor enabling good temperature and off-gas control.

In a preferred embodiment each reactor 1 ,2 is a rotary kiln. Typically, a rotary kiln has a cylindrical shape, the length of the cylinder being much greater than its width. The kiln rotates around a rotation axis which is most often inclined allowing the raw materials to be pyroprocessed in the kiln to travel downwards through the kiln under the effect of gravity. The rotary kiln may comprise a burner assembly at its lower end for the combustion of fuel so as to generate the heat necessary for pyro-processing; however, also indirect heating or electrical heating can be applied. The flue gases or off-gases, along with any volatile compounds are generated in the rotary kiln and then evacuated from the kiln at its upper end following which the gasses are captured and processed in an off-gas treatment system 4,6.

After the first reaction step the feedstock (FS) having a reduced amount of non-ferrous heavy metals, notably lead and cadmium, can be cooled prior to feeding it into a second reactor 2 for the subsequent second reaction step further reacting the CPM 16 with the ISFD 15 by thermal treatment of said feed stock at a temperature in a range of 850-1200°C removing substantially all of the remaining zinc.

In a preferred embodiment the reactors 1 ,2 for the first reaction step and the second reaction step are positioned in-line to enable a continuous feed of the feedstock from the first reactor 1 into the second reactor 2, and thereby avoiding loss of thermal energy in said feed stock.

Operating a rotary kiln reactor at high temperatures, in particular at temperatures above about 1100°C, creates various operational issues such as the accretion of ferrous components like Ca-ferrites and Fe-silicates on the surface walls and sintering of for example pellets to each other. For that reason the temperature in the second reaction step preferably does not exceed about 1100°C. Pyro-metallurgical processes in rotary kilns are prone to build-ups and accumulation of particles on the inner wall of the rotary kiln, thereby forming "rings" of accumulated particles, so called "kiln rings". Such kiln rings can drastically limit the production capacity of the rotary kiln and lead to tedious cleaning operation where the production process has to be shutdown. Kiln rings hold up materials from moving down the rotary kiln in normal conditions, by reducing the cross area of the rotary kiln. Furthermore, the accumulation of particles on the inner wall of the rotary kiln lowers heat transfer. Periodic shutdown operations to clean and/or to remove kiln rings result in lost production time.

An advantage of separating the heavy metal removal from the ISFD 15 into at least two separate reaction steps, namely in a relative low temperature first reaction step and a higher temperature second reaction step, and preferably positioned in-line of each other, is that due to the relative low operational temperatures accretion is not any issue within the first reaction step and can be controlled within the second reaction step by controlling the maximum operating temperature applied. This offers substantial costs advantages and the avoidance of operation issues related to accretion.

The solid residue or secondary solid material 8 obtained after the second reaction step and removed from the second reactor 2 comprises high fractions of metallic iron, some remaining carbon, non-reduced FeO_x and inert gangue minerals, and has very low levels of nonferrous heavy metals, and can be re-used in an ironmaking operation as a ferrous raw materials and/or carbon resource, preferably either via direct injection or after agglomeration, for example as pellets or briquetted as HBI (hot briquetted for a DRI).

In a second aspect of the invention it relates to a plant implementing the process for the selective heavy metal removal from iron- and/or steelmaking flue dust 15 according to this invention, the plant comprising an apparatus 3 for preparing a feedstock by blending or mixing the CPM 16 and ISFD 15, and in an embodiment said apparatus 3 comprises a granulation apparatus, a pelletising apparatus or a pelletising plant; a first reactor 1 configured for the first reaction step and the first reactor 1 being equipped with or connected to an off-gas treatment system 4; and a second reactor 2 configured for the second reaction step and the second reactor being equipped with or connected to an off-gas treatment system 6.

Preferably the first reactor 1 and the second reactor 2 are positioned in-line to facilitate a continuous operation.

It relates further to an integrated ironmaking industrial complex or integrated ironmaking plant and to a method of operating such a complex or plant, comprising at least one operation selected from the group comprising a blast furnace (BF) operation 13, a direct reduced iron (DRI) making operation 12, a reducing electrical furnace (REF) operation 10, an electric arc furnace (EAF) operation 10, a Hlsarna-type ironmaking process 14, and a basic oxygen steelmaking operation 11 (BOF, BOS, or BOP); the operation(s) including off-gas generation and being equipped with an off-gas treatment system to capture the flue dust 15 comprising the heavy metals, the heavy metals being at least lead, zinc and optionally also cadmium (ISFD); the flue dust 15 (ISFD) comprising the heavy metals being blended or mixed in appropriate equipment 3 with a chloride precursor material (CPM) 16 to prepare a feedstock (FD); and subjecting said feedstock (FD) to a thermal treatment in a first reaction step in a first reactor 1 , and a subsequent second reaction step in a second reactor 2 as herein described and claimed; the obtained secondary solid material 8 after the second reaction step and removed from the second reactor 2 being re-used in the ironmaking operation 10,12,13,14 as ferrous raw materials and/or carbon resource, either via direct injection or after agglomeration, for example as pellets or briquettes; and wherein preferably the flue gas or off-gas of the second reaction step in the second reactor 2 is treated in a cleaning step using an off-gas treatment system 6 such that a solid residue 7 is obtained which may be re-used in a hydrometallurgical process or electrolytic process or other zinc recovery process to recover the zinc metal or zinc compounds.

In a preferred embodiment of the integrated ironmaking industrial plant the chloride precursor material 16 comprises at least FeCI₂, and preferably FeCI₂ originating from a steel pickling operation.

The invention is also embodied in the use of or method of use of the obtained secondary solid material 8 after the second reaction step in an ironmaking operation as a ferrous raw materials and/or carbon resource, preferably either via direct injection or after agglomeration, for example as pellets or briquettes.

The invention is also embodied in the use of or method of use of the flue dust of the second reaction step in the second reactor 2 such that a zinc-enriched solid residue 7 is obtained in a hydrometallurgical process or an electrolytic conversion process and is converted into metallic zinc or other valuable zinc resources. DETAILED DESCRIPTION OF THE FIGURE

The invention will now be explained by means of the following, non-limiting figure.

Fig. 1 shows schematically the process flow of the process according to the invention. The ironmaking and/or steelmaking flue dust 15 comprising heavy metals (ISFD), notably at least zinc, lead and optionally also cadmium, and originating from one or more operations including a blast furnace (BF) operation 13, a direct reduced iron (DRI) making process 12, a reducing electrical furnace (REF) operation 10, an electric arc furnace (EAF) operation 10, a Hlsarna-type ironmaking process 14, and a basic oxygen steelmaking operation (BOF) 11 , is mixed or blended in an apparatus 3 with a chloride precursor material (CPM) 16, for example FeCI₂, to prepare a feedstock (FD), for example in the form of pellets.

The feedstock is feed into a first reactor 1 (in this embodiment a rotary kiln) to react in a first reaction step the CPM 16 with the ISFD 15 by thermal treatment of said feedstock at a temperature in a range of 700°C to 950°C, and with preferred narrower temperature ranges, removing at least 70 wt.% of lead present from the ISFD by chlorination and evaporation of PbCI₂ via the off-gas formed. The formed off-gas comprising the PbCI₂ during the first reaction step is cleaned in a cleaning step using an off-gas treatment system 4 such that a solid lead- enriched residue or flue dust 5 is obtained having a very high fraction of lead and other heavy metals like cadmium and mercury together with some zinc. This highly heavy-metal enriched solid residue or flue dust 5 has no immediate commercial value and is carefully stored in a landfill against high costs. However, the total amount of heavy-metal enriched solid residue or flue dust 5 is still significantly smaller compared to the outputted solid residue originating from for example the process disclosed in WO2019/043261-A1 where the ISFD is being treated in a single thermal treatment such that following off-gas treatment the resultant flue dust or solid residue is enriched with the evaporated zinc but also with substantial higher amounts of the lead, cadmium and other toxic non-ferrous heavy metals originally present in the ISFD. The high contamination levels of lead and other toxic non-ferrous heavy metals renders such solid residue unsuitable for re-use in the zinc recovery industry and it has to be discarded via landfilling or otherwise.

In accordance with the invention, next the feedstock outputted from the first reaction step is subjected to a subsequent second reaction step in a second reactor 2 (in this embodiment a rotary kiln) positioned in-line to the first reactor to further react the CPM 16 with the ISFD 15 by thermal treatment of the feedstock at a temperature in a range of 850°C to 1200°C, and with preferred narrower temperature ranges, removing substantially most of the zinc present in the ISFD by chlorination and evaporation of ZnCI₂. The off-gas of the second reaction is cleaned in a cleaning step using another off-gas treatment system 6 such that a zinc-enriched solid residue or flue dust 7 is obtained having a very high weight fraction of zinc compounds. The zinc-enriched solid residue or flue dust 7 is of high purity as the presence of other heavy non-ferrous metals in particular lead, cadmium and mercury is very low as most of these have already concentrated in the lead-enriched solid residue or flue dust 5 originating from the first reaction step. The zinc-enriched solid residue or flue dust 7 has a high added value and can be readily re-used in a hydrometallurgical process (not shown) or electrolytic process (not shown) or another zinc recovery process as known in the art of zinc recovery to recover the zinc metal or zinc compounds. The output of the second reaction is so-called a secondary solid material 8 having very low levels of non-ferrous heavy metals and having high fractions of metallic iron, some remaining carbon, non-reduced FeO_x and inert gangue minerals, and can be readily re-used in an ironmaking operation 10, 12, 13, or 14 as a ferrous raw materials and/or carbon resource, either via direct injection or after agglomeration, for example as pellets or briquettes, and thereby avoiding the disadvantageous accumulation of lead and other toxic heavy metals in an ironmaking operation.

The invention will now be illustrated with reference to non-limiting embodiments according to the invention.

EXAMPLE

Blast furnace flue dust origination from an industrial ironmaking operation and for which the composition (dry mass) is listed in Table 1 has been pelletized into a feedstock of micropellets having a diameter in a range of 3 to 5 mm using about 0.5 wt.% bentonite as a binder material and about 4.6 wt.% FeCI₂ as chloride precursor material originating as by-product from steel pickling. The composition of the micro-pellets is listed also in Table 1. All compositions listed in Table 1 have been determined using thermogravimetric analysis (TGA) and inductively coupled plasma (ICP) techniques well known to the person skilled in the art.

A small batch of 15 gram of these pellets has been heat-treated on a laboratory scale in a quartz glass tube under a non-oxidizing flowing inert nitrogen atmosphere in a first reaction step for 1 hour at 800°C for chlorination and evaporation of at least the lead and cadmium and some of the zinc. The flue dust from this first reaction step is Pb-rich with about 4.54 wt.% of lead and 15.43 wt.% of Cl as listed in Table 1. The flue dust is also enriched with Cd. Given the presence of high levels of toxic heavy metals, this flue dust as by-product of the first reaction step has no commercial value in an iron- and/or steelmaking operation. The micro-pellets or solid residue (or feedstock) after this first reaction step have a very low lead content of about 0.01 wt.% and still about 2.14 wt.% zinc remaining compared to the 2.73 wt.% zinc in the original BF flue dust. The output of the first reaction step is used as input for a subsequent second reaction step under a non-oxidizing flowing inert nitrogen atmosphere by holding said solid material for 1 hour at 1000°C for chlorination and evaporation of the zinc and remaining other heavy metals. The flue-dust from this second reaction step is Zn-rich with about 17.26 wt.% of zinc and has a very low lead content of about 0.02 wt.% as listed in Table 1 , and may form a valuable byproduct for use in the zinc recovery industry. The secondary solid residue 8 obtained after the second reaction step is both very low in zinc and lead content of respectively 0.04 wt.% and 0.01 wt.%, and is also free from Cd. Whereas the original BF flue dust has about 2.73 wt.% zinc and about 0.54 wt.% lead. Due to its high iron content, high carbon content and very low zinc and lead contents the solid residue obtained after the second reaction step can be re-used in an ironmaking process.

The calculated mass balance of the experiments in accordance with the invention is: Residual solid mass after 800°C: -86%,

Gas produced at 800°C: -13%,

Residual solid mass after 1000°C: -72%,

Gas produced at 1000°C: -25%.

Where previously the BF flue dust due to its composition had to be discarded, using the process according to the invention at least two useable by-products are extracted from this BF flue dust, namely the flue dust 7 from the second reaction step and the secondary solid residue 8 obtained after the second reaction step, and thereby very significantly reducing the total amounts of ironmaking by-products that have to be discarded.

Table 1. Compositions of input and output materials at various stages of the chlorination process in accordance with the invention. Compositions are in wt.%.

Table 1 . - continued

The above-discussion is intended to be merely illustrative of the present process and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Accordingly, the specification and drawing are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawing, the disclosure, and the appended claims. The mere fact that certain measures are recited in different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the appended claims.

List of reference numbers:

1 . first reactor;

2. second reactor; 3. blender/mixer;

4. off gas treatment system connected to the first reactor;

5. lead-enriched flue dust;

6. off gas treatment system connected to the second reactor; 7. zinc-enriched flue dust;

8. secondary solid material;

10. reducing electrical furnace operation;

11 . basic oxygen steelmaking operation;

12. direct reduced iron making process; 13. blast furnace operation;

14. Hlsarna-type ironmaking process;

15. ironmaking and/or steelmaking flue dust;

16. chloride precursor material;

Claims

1. Process for the selective heavy metal removal from iron- and/or steelmaking flue dust, the process comprising the steps of: preparing a feedstock (FS) by blending or mixing a chloride precursor material (CPM) and ironmaking and/or steelmaking flue dust comprising heavy metals (ISFD), the heavy metals being at least Pb and Zn and optionally also Cd; in a first reaction step in a first reactor (1) reacting the CPM with the ISFD by thermal treatment of the FS at a temperature in a range of 700°C to 950°C removing at least 70 wt.% of Pb from the ISFD by chlorination and evaporation of the Pb and removing it from the first reactor (1) via the off-gas formed; in a subsequent second reaction step in a second reactor (2) further reacting the CPM with the ISFD by thermal treatment of the feedstock FS at a temperature in a range of 850°C to 1200°C removing the Zn from the ISFD by chlorination and evaporation of the Zn and removing it from the second reactor (2) via the off-gas formed; and obtaining and removing a secondary solid material (8) after the second reaction step from the second reactor (2).

2. Process according to claim 1 , wherein during the first reaction step at least 70% of the Pb is removed from the ISFD by evaporating PbCI₂ at temperatures from 700°C to 950°C, preferably from 800°C to 900°C, under a non-oxidizing atmosphere.

3. Process according to claim 1 or 2, wherein during the first reaction step at least 80 wt.% of the Pb is removed from the ISFD, and preferably at least 90 wt.%.

4. Process according to any one of claims 1 to 3, wherein during the second reaction step the zinc is removed from the ISFD by evaporating ZnCI₂ at temperatures from 850°C to 1200°C, preferably from 900°C to 1100°C, under a non-oxidizing atmosphere.

5. Process according to any one of claims 1 to 4, wherein off-gasses generated in the first reaction step in the first reactor 1 are treated in an off-gas treatment system (4) to obtain lead-enriched flue dust (5).

6. Process according to any one of claims 1 to 5, wherein off-gasses generated in the second reaction step in the second reactor 2 are treated in an off-gas treatment system (6) to obtain zinc-enriched flue dust (7).

7. Process according to any one of claims 1 to 6, wherein the ratio of ironmaking and/or steelmaking flue dust comprising ISFD and CPM being chosen so that the chloride content of the FS is between 100% and 150%, preferably between 100% and 130%, of the amount necessary for a stoichiometric conversion of the heavy metals in the ISFD into chlorides.

8. Process according the any one of claims 1 to 7, wherein the chloride precursor material (CPM) is selected from the group comprising polyvinyl chloride (PVC), waste comprising PVC, chlorinated rubber or other chlorinated polymers, FeCI₂, FeCI₃, and CaCI₂.

9. Process according the any one of claims 1 to 8, wherein the chloride precursor material (CPM) is FeCI₂.

10. Process according to any one of claims 1 to 9, wherein each of the first reaction step and the second reaction step are performed in a rotary kiln.

11. Use of the obtained secondary solid material 8 after the second reaction step according to any one of claims 1 to 10 in an ironmaking operation as a ferrous raw materials and/or carbon resource.

12. Use of zinc-enriched flue dust from the second reaction in the second reactor according to any one of claims 1 to 10 in a hydrometallurgical process or electrolytic process to recover the zinc metal or zinc compounds.

13. A plant for implementing the process for the selective heavy metal removal from iron- and/or steelmaking flue dust according to any one of claims 1 to 10, the plant comprising an apparatus (3) for preparing a feedstock FS by blending or mixing the CPM (16) and ISFD (15); a first reactor (1) configured for the first reaction step and the first reactor (1 ) being equipped with an off-gas treatment system (4); and a second reactor (2) configured for the second reaction step and the second reactor (2) being equipped with an off-gas treatment system (6). A plant according to claim 13, wherein the first reactor (1) and the second reactor (2) are positioned in-line, and preferably each of the first reactor (1) and the second reactor (2) are a rotary kiln. A plant according to any one of claims 13 and 14, comprising

- at least one operation selected from the group comprising a blast furnace operation (13), a direct reduced iron making operation (12), a reducing electrical furnace operation (10), an electric arc furnace operation (10), a Hlsarna-type ironmaking process (14), and a basic oxygen steelmaking operation (11);

- the operation(s) including off-gas generation and being equipped with an off-gas treatment system to capture the flue dust comprising heavy metals, the heavy metals being at least lead, zinc and optionally also cadmium (ISFD);

- an apparatus (3), first reactor (1) and second reactor (2) according to claim 13 or 14;

- the ISFD being blended or mixed in apparatus (3) with a chloride precursor material (CPM) to prepare a feedstock (FD);

- subjecting said feedstock (FD) to a thermal treatment in a first reaction step in the first reactor (1), and a subsequent second reaction step in the second reactor (2) according to any one of claims 1 to 10; and

- the obtained secondary solid material (8) after the second reaction step and removed from the second reactor (2) is re-used in an ironmaking operation (10,12,13,14) as ferrous containing raw material via direct injection or after agglomeration.