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WO2025040239A1 - Procédé de recyclage de scories d'aciérie en tant que sorbant pour la capture de polluants acides - Google Patents

Procédé de recyclage de scories d'aciérie en tant que sorbant pour la capture de polluants acides Download PDF

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Publication number
WO2025040239A1
WO2025040239A1 PCT/EP2023/072854 EP2023072854W WO2025040239A1 WO 2025040239 A1 WO2025040239 A1 WO 2025040239A1 EP 2023072854 W EP2023072854 W EP 2023072854W WO 2025040239 A1 WO2025040239 A1 WO 2025040239A1
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Prior art keywords
steel slag
particulate
slag material
carbonated
flue gas
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English (en)
Inventor
Dirk Van Mechelen
Frédérique BOUILLOT
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Orbix Solutions
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Orbix Solutions
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Priority to PCT/EP2023/072854 priority Critical patent/WO2025040239A1/fr
Publication of WO2025040239A1 publication Critical patent/WO2025040239A1/fr
Pending legal-status Critical Current
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
    • C22B7/04—Working-up slag
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/38—Removing components of undefined structure
    • B01D53/40—Acidic components
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/81—Solid phase processes
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B1/00—Preliminary treatment of ores or scrap
    • C22B1/005—Preliminary treatment of scrap
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B1/00—Preliminary treatment of ores or scrap
    • C22B1/14—Agglomerating; Briquetting; Binding; Granulating
    • C22B1/16—Sintering; Agglomerating
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B1/00—Preliminary treatment of ores or scrap
    • C22B1/14—Agglomerating; Briquetting; Binding; Granulating
    • C22B1/24—Binding; Briquetting ; Granulating
    • C22B1/242—Binding; Briquetting ; Granulating with binders
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B1/00—Preliminary treatment of ores or scrap
    • C22B1/14—Agglomerating; Briquetting; Binding; Granulating
    • C22B1/24—Binding; Briquetting ; Granulating
    • C22B1/248—Binding; Briquetting ; Granulating of metal scrap or alloys
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/40—Alkaline earth metal or magnesium compounds
    • B01D2251/402—Alkaline earth metal or magnesium compounds of magnesium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/40—Alkaline earth metal or magnesium compounds
    • B01D2251/404—Alkaline earth metal or magnesium compounds of calcium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/20—Halogens or halogen compounds
    • B01D2257/204—Inorganic halogen compounds
    • B01D2257/2045—Hydrochloric acid
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/20—Halogens or halogen compounds
    • B01D2257/204—Inorganic halogen compounds
    • B01D2257/2047—Hydrofluoric acid
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/30—Sulfur compounds
    • B01D2257/302—Sulfur oxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/40—Nitrogen compounds
    • B01D2257/404—Nitrogen oxides other than dinitrogen oxide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/02—Other waste gases
    • B01D2258/0233—Other waste gases from cement factories
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2258/00—Sources of waste gases
    • B01D2258/02—Other waste gases
    • B01D2258/0283—Flue gases

Definitions

  • the present invention relates to a method for recycling steel slag generated in a steelmaking process in a steel mill as sorbent for capturing acid pollutants comprising HCI, HF, SO X and/or NO X from a flue gas, in particular by a dry process.
  • the BOF slags include in particular LD slags which are obtained as byproduct in the production of steel by the Linz-Donawitz process.
  • Stainless steel-making slags are obtained during the production of stainless steel and contain chromium, and optionally also other alloying elements such as nickel and molybdenum.
  • Stainless steel making slags include electric arc furnace slags (EAF slags), argon oxygen decarburization slags (AOD slags), vacuum oxygen decarburation slags (VOD slags), ladle metallurgy slags (LM slags) and slags produced in a tundish (used for the continuous casting of the molten steel).
  • EAF slags electric arc furnace slags
  • AOD slags argon oxygen decarburization slags
  • VOD slags vacuum oxygen decarburation slags
  • LM slags lad
  • steel slags include common steel slags as well as stainless steel slags.
  • Common steel slags comprise electric arc furnace slags (EAF slags), basic oxygen furnace slags (BOF slags), LD slags and ladle metallurgy slags (LM slags).
  • Stainless steel slags comprise electric arc furnace slags (EAF slags), argon oxygen decarburization slags (AOD slags), vacuum oxygen decarburation slags (VOD slags), ladle metallurgy slags (LM slags) and slags produced in a tundish.
  • One of such method consists in the use of steel slag as sorbent for capturing acid pollutants from a flue gas.
  • the use of LD slags for desulfurizing flue gasses is already disclosed in a number of publications, in particular in a number of patent publications such as US 4 191 731. Most of these processes are however wet processes.
  • WO 2009/039393 discloses for example a dry sorbent process for purifying flue gas wherein use is made of stainless steel slag as sorbent.
  • stainless steel slag was mixed with cement in a 90/10 ratio. 30% water was added to this mixture and the mixture was pelletized in a mixer to produce pelletized particles having an average diameter of 2 to 6 mm.
  • a packed bed reactor was made with the pelletized particles and flue gas from a coal-burning furnace was led through this filter. The carbon dioxide content of the flue gas was reduced by about 31%.
  • WO 2009/039393 teaches indeed to remove the surface of the agglomerated solid material particles which contains the calcium carbonates and sulphates by surface grinding and screening in order to enable to reuse the solid material in the reactor bed.
  • the used agglomerated solid material pellets could be ground and then mixed with new binder component to reform solid material pellets.
  • a drawback of this known technique is that by the carbonation reaction the pores of the agglomerated material are quickly obstructed by the produced carbonates so that the reaction with the sulphur dioxide is limited to the outer layer of the agglomerated solid material pellets.
  • a further disadvantage is that the removal efficiency of nitrogen oxides is quite limited. The purification process appears to be mainly due to the chemical reactions of carbon dioxide and sulphur dioxide with reactive calcium oxide containing phases of the steel slag material.
  • An object of the present invention is to provide a new method for recycling steel slag as sorbent for capturing acid pollutants which enables to capture nitrogen oxides with a higher efficiency, in particular in dry sorbent processes including dry sorbent injection processes and dry sorbent filter bed processes.
  • the method according to the present invention is characterised in that said steel slag comprises non-metallic slag phases, at least 40 wt.% of which are crystalline non-metallic slag phases, including at least crystalline calcium silicate and calcium magnesium silicate phases, and in that the method comprises the steps of: - producing a particulate steel slag material from said steel slag, which particulate steel slag material consists of steel slag particles and has a first particle size distribution with a D 5 o sieve size value which is smaller than 70 pm and a first specific surface area, which is in particular smaller than 5 m 2 /g;
  • the first and second specific surface areas are specific surface areas determined by the BET method in accordance with ISO 9277:2022(E) by nitrogen absorption manometry, in particular after vacuum degassing at 190°C for at least two hours.
  • the steel slag comprises non- metallic slag phases, which non-metallic slag phases should consist, in their totality, for at least 40 wt.% of crystalline non-metallic slag phases.
  • the steel slag from which all of the metal, i.e. iron or steel, has been removed so that only then non- metallic slag phases remain should consist for at least 40 wt.% of crystalline non-metallic slag phases and hence for less than 60 wt.% of amorphous non-metallic slag phases.
  • the steel slag is a silicate slag, the crystalline non-metallic slag phases of which include at least crystalline calcium silicate and calcium magnesium silicate phases.
  • Such calcium and calcium magnesium silicate phases are reactive towards carbon dioxide.
  • calcium oxide may be extracted from these phases and can react with carbon dioxide to produce calcium carbonates on the surfaces of the slag particles.
  • the steel slag should be carbonated in the form of a fine particulate steel slag material which has a first particle size distribution with a D 5 o sieve size value which is smaller than 70 pm.
  • Such a fine particulate steel slag material can be produced by milling the steel slag and/or by separating such a fine fraction from the remaining coarser fractions of the steel slag. In both cases, the solidified steel slag is preferably first crushed.
  • the fine particulate steel slag material can be carbonated to a higher degree than the coarser steel slag fractions, i.e. more calcium oxide can be extracted from the steel slag particles and converted into carbonates on the surfaces thereof. It has also been found that the BET specific surface area of such a fine particulate steel slag material could be increased substantially by the carbonation step to a value of 5 m 2 /g and even much higher, for example to a value higher than 10, 15 or 20 m 2 /g. It was moreover found that this increase of specific surface is correlated with an increased capture level of the gaseous pollutants, not only of the acid pollutants but especially also of the nitrogen oxides.
  • the produced carbonates may thus not only react with the acid pollutants but they appear to provide an increased surface area suitable for adsorbing gaseous pollutant including nitrogen oxides. Moreover, and presumably even more importantly, by the extraction/leaching of calcium oxide from the silicate phases pores are produced in the calcium depleted outer zones of the steel slag material, in particular nanopores, wherein the gaseous pollutants such as sulphur oxides but especially also nitrogen oxides may be physically captured.
  • the gas which comprises carbon dioxide and which is used to carbonate the particulate steel slag material is free of HCI or contains less than 10 mg/Nm 3 HCI, is free of HF or contains less than 1 mg/Nm 3 HF, is free of SO2 or contains less than 50 mg/Nm 3 SO X and is free of NO X or contains less than 200 mg/Nm 3 NO X .
  • An advantage of this embodiment is that in the carbonated particulate steel slag material the reactive calcium oxides have mainly only reacted with carbon dioxide to produce carbonates. Other compounds such as for example chlorides, sulphites and sulphates are thus not or substantially not produced.
  • the steel slag sorbent has a higher content of carbonates which still may react with acid pollutants such as HCI, HF and SO X present in the flue gas.
  • acid pollutants such as HCI, HF and SO X present in the flue gas.
  • the additional surface area created by the carbonation process does not get occupied by and remains preferably free of acid pollutants such as in particular SO X and NO X .
  • the carbonated particulate steel slag material may thus retain its absorption properties to a maximum extent.
  • the carbon dioxide containing gas used for carbonating the particulate steel slag material may be pure carbon dioxide, or may be composed with pure carbon dioxide. It may however also be a flue gas preferably from processes which mainly produce carbon dioxide, such as for example flue gas from a kiln, in particular from a lime or a cement kiln.
  • the flue gas is preferably not a flue gas produced by an incinerator, in particular a waste or a coal incinerator.
  • said second specific surface area i.e. the specific surface area of the carbonated particulate steel slag material, at least a portion of which is used as sorbent, is higher than 10 m 2 /g, preferably higher than 15 m 2 /g and more preferably higher than 20 m 2 /g.
  • the second specific surface area will usually be lower than 70 m 2 /g.
  • said particulate steel slag material is carbonated until it has a carbonate content, expressed in percent by dry weight of carbon dioxide, of at least 2.0 wt.%, preferably of at least 4.0 wt.%, more preferably of at least 6.0 wt.% and most preferably of at least 8.0 wt.%.
  • said particulate steel slag material is carbonated until it has a carbonate content, expressed in percent by dry weight of carbon dioxide, of at least 50%, preferably of at least 70% and more preferably of at least 90% of its maximum carbonate content, expressed in percent by dry weight of carbon dioxide, in particular of its maximum carbonate content which is obtained by carbonating a sample of the particulate steel slag material having a moisture content of 15 wt.% (percent by dry weight) for 24 hours at a temperature of 23°C with 100% carbon dioxide gas at a pressure of 0.5 barg.
  • the D 5 o sieve size value of said first particle size distribution i.e. the particle size distribution of the particulate steel slag material which is carbonated, is smaller than 60 pm, preferably smaller than 50 pm, more preferably smaller than 40 pm and most preferably smaller than 30 pm. Preferably, it is larger than 1.0 pm.
  • a finer granulometry of the particulate steel slag material corresponds to a higher outer surface area, in particular a higher Blaine value. It appears that the carbonates are mainly produced during the carbonation step onto the outer surface of the particles of the particulate steel slag material so that when the total surface area of the particles of the particulate steel slag material is higher, more carbonates can be produced during the carbonation step.
  • the particulate steel slag material is uniformly carbonated.
  • a uniformly carbonated particulate steel slag material is meant a carbonated particulate steel slag of which substantially all the particles of a size greater than the Dw sieve size of the particulate steel slag, measured before carbonation, are carbonated.
  • Steel slag has only a limited content of calcium compounds which are available for reaction with carbon dioxide. Consequently, by carbonating the particular steel slag material uniformly, it is possible to produce a maximum amount of carbonates and also a maximum amount of pores on the surfaces of the particles of the particulate steel slag material.
  • the gas which comprises carbon dioxide and which is used to carbonate the particulate steel slag material has a carbon dioxide content of less than 5.0 vol.%, preferably of less than 3.0 vol.%, more preferably of less than 1 .0 vol.%, said gas being in particular atmospheric air. Usually, it will contain at least 0.03 vol.% of carbon dioxide.
  • a gas having a relatively low carbon dioxide concentration is advantageous for avoiding crust formation on the outer surface of the mass of particulate steel slag material which is being carbonated.
  • a crust is formed when carbonates are produced in the pores between the particles of the particulate steel slag material adhering these particles to one another and hampering further penetration of the carbon dioxide containing gas in the mass of particulate steel slag material.
  • a gas which has a relatively low carbon dioxide concentration it is thus easier to obtain a uniform carbonation of the particulate steel slag material. It has been found that carbonation of the particulate steel slag material with atmospheric air, i.e. natural carbonation, enables to carbonate large heaps of the carbonated still slag material relatively uniformly throughout the entire heap.
  • a lower carbon dioxide concentration in the carbonating gas takes a longer time to carbonate the steel slag material but, since less pores are closed of by an accelerated carbonation on the surface of the material, a larger mass of the particulate steel slag material can be carbonated more uniformly.
  • the particulate steel slag material is carbonated upto 90% or more of its maximum carbonation degree, the particulate steel slag material is quite uniformly carbonated when carbonating with a gas having a relatively low carbon dioxide content, in particular when carbonating with atmospheric air.
  • the gas which comprises carbon dioxide and which is used to carbonate the particulate steel slag material has a carbon dioxide content of more than 5.0 vol.%, preferably of more than 10.0 vol.%, and more preferably of more than 15.0 vol.%.
  • the maximum carbon dioxide content is 100 vol.%.
  • This embodiment enables to carbonate the particulate steel slag material more quickly.
  • the particulate steel slag material is arranged in a heap or in a bed during the carbonation step.
  • the carbonated particulate steel slag material is preferably a bulk material.
  • the porosity of the heap or the bed of the particulate steel slag material is not reduced so that the carbon dioxide containing gas can penetrate more easily into the heap or the bed and the tendency to form a crust on the surface of the heap or bed by the carbonation process can be avoided or at least reduced.
  • the heap can be either arranged onto a surface or in a container, for example in a bag which is preferably made of a gas permeable material.
  • the particles of said particulate steel slag material are loosely stacked in said heap or in said bed.
  • no external pressure is exerted onto the heap or bed of particulate steel slag material.
  • the heap or bed of particulate steel slag material is preferably not compacted by vibrations.
  • the first particle size distribution of the particulate steel slag material has a D 6 o sieve size value and a D sieve size value, with the ratio of the Deo sieve size value over the D sieve size value, i.e. the grain size uniformity coefficient U, being smaller than 15, preferably smaller than 14, more preferably smaller than 13 and most preferably smaller than 12. Smaller grain size uniformity coefficients, such as grains size uniformity coefficients smaller than 10 or 8, or even smaller than 6, are even more preferred. The uniformity coefficient will usually be larger than 2.
  • the porosity of the heap or bed of particulate steel slag material is partially determined by the grain size uniformity coefficient II, which is the ratio of the Deo sieve size value over the D sieve size value (D 6 o/Dw). This is especially the case when the particulate steel slag material is compacted to some extent, such as for example when it is compacted under its own weight in a larger heap.
  • the porosity n is equal to 0.255(1 +0.83 u ).
  • the porosity of the heap or bed of particulate steel slag material would thus be larger than 27%, even when the heap or bed has been compacted under its own weight. Since only a limited amount of carbonates are produced in the carbonated particulate steel slag material, the pores of the particulate steel slag will not be filled completely with carbonates during the carbonation step. Lower grain size uniformity coefficients are preferred since the minimum porosity of the particulate steel slag material is then even greater, for example greater than 28% when the grain size uniformity coefficient is equal to 12, or greater than 30% when the grain size uniformity coefficient is equal to 9. A smaller grain size uniformity coefficient can be obtained by taking a narrower grain size fraction from the steel slag material.
  • said particulate steel slag material is stirred for one or more times during said carbonation step.
  • An advantage of such flue gas cleaning process is that it does not require the installation of additional filters in the flue gas cleaning system. Moreover, there is no need to periodically replace the sorbent material in the packed bed filter.
  • the sorbent can simply be injected and can be collected by the existing dust filters, in particular by bag filters. There is a very effective contact between the injected sorbent particles and the flue gas. If the contact time needs to be increased, the collected sorbent can be reinjected into the flow of flue gas.
  • the advantage of using a fraction of the steel slag with such a high gamma-dicalcium silicate content is that the conversion of beta-dicalcium silicate to gamma-dicalcium silicate during the solidification and cooling of the steel slag is accompanied by an increase of 12% in volume of the dicalcium silicate phases, as a result of which the dicalcium silicate phase will break up.
  • the steel slag fraction which contains such a high amount of gamma-dicalcium silicate is thus automatically pulverized and forms the so-called fines of the solidified steel slag. As such this fraction does not require further milling, or less further milling, to be used as particulate steel slag material in the method of the present invention.
  • this fraction may be somewhat higher, i.e. the CaO/SiO2 ratio, so that more calcium is available for the carbonation reaction.
  • the carbonated particulate steel slag material is mixed after said contacting step with a binder, and the mixture of carbonated particulate steel slag material and binder is hardened to produce a shape retaining mass, said binder being in particular a hydraulic binder which is cured in the presence of water, a particulate carbonatable material, in particular a steel slag material, which is cured with carbon dioxide or a bituminous binder.
  • the particulate steel slag material may especially be the powdery steel slag material which remains after removing the coarse and fine (sand) fractions from the crushed steel slag. As such, due to its high water absorption properties, this powdery steel slag material is not suited for being used in bound applications such as in concrete or asphalt. However, the particulate steel slag material has been carbonated in the method according to the present invention prior to being used as sorbent and also, if not fully carbonated, it can be further carbonated during the contacting step with the flue gas by the carbon dioxide contained therein.
  • a carbonatable particulate material which can be cured by means of carbon dioxide should be added as binder since the particulate steel slag material which is used as sorbent can no longer be carbonated.
  • shape retaining mass is meant a moulded article or any mass which may be produced in situ and which has a compressive strength of at least 2 MPa, measured in accordance with the Belgian standard NBN B 15-220: 1990.
  • Figures 1A and 1 B are two parts of a flow diagram of a particular embodiment of a method for producing a powdery stainless steel slag fraction a particulate steel slag material which can be used as sorbent after being carbonated to increase its BET specific surface area to a value higher than 5 m 2 /g;
  • Figure 2 schematically illustrates an installation with a packed filter bed for cleaning flue gas produced in a brick baking kiln
  • Figure 3 schematically illustrates an installation for cleaning flue gas produced in a waste incinerator with generation of energy in the form of electricity and heat by injection of the sorbent in the stream of flue gas;
  • Figure 4 is a graph showing the relationship between the percentage of particles smaller than 63 pm in the powdery stainless steel slag fraction produced by the method illustrated in Figures 1 A and 1 B and the maximum carbon dioxide uptake by this powdery stainless steel slag fraction;
  • Figure 5 is a graph showing the relationship between the amount of gamma dicalcium silicate in the powdery stainless steel slag fraction produced by the method illustrated in Figures 1 A and 1 B and the percentage of particles smaller than 63 pm in this powdery stainless steel slag fraction;
  • Figure 6 is a graph showing the relationship between the amount of carbonates, expressed in percent of carbon dioxide, contained after carbonation in the powdery stainless steel slag fraction produced by the method illustrated in Figures 1A and 1 B and the BET specific surface area of this carbonated stainless steel slag material;
  • Figure 7 is a graph showing the relationship between the BET specific surface area of the carbonated powdery steel slag fraction and the capture level of NO X , SO2 and HCI by this carbonated particulate steel slag material from a flue gas composition.
  • percent, % or wt.% refers to percent by dry weight, unless indicated otherwise.
  • slag phases refers to non-metallic slag phases, which may be amorphous or crystalline.
  • the expression “slag phases” therefore does not embrace metal phases or metal particles.
  • Such metal particles may be embedded in the non-metallic slag phases or may consist of separate steel particles which may be coated or covered with slag phases.
  • the chromium content is expressed in mg Cr/kg dry weight, in particular in mg/kg by dry weight of non-metallic slag phases, unless it is explicitly indicated that it relates to the amount in Cr 2 O3.
  • the metal phases/particles have thus been first removed from the steel slag.
  • the amount of chromium is determined by ICP after complete destruction of the non-metallic slag phases with a mixture of the acids HNO3, HCI and HBF 4 as described in the compendium for taking samples and for analysis of waste materials and soil samples CMA/2/II/A.3 and CMA/2/I/B.1 (both published in the Belgian Official Gazette/Moniteur beige of January 11 , 2021 ).
  • the ICP analysis is in particular an ICP-OES analysis.
  • the present invention relates in general to a method for recycling steel slag generated in a steelmaking process in a steel mill.
  • the steel slag is recycled as sorbent for capturing acid pollutants from a flue gas, such as HCI, HF, SO 2 , SO3, NO 2 and NO.
  • a flue gas such as HCI, HF, SO 2 , SO3, NO 2 and NO.
  • the sorbent which is brought in contact with the flue gas is thus free of water or contains a small amount of water which is evaporated from the sorbent upon being heated by the flue gas.
  • Wet processes, wherein the sorbent is applied in a liquid through which the flue gas is passed are not included.
  • dry sorbent processes thus also embrace a so-called semi-wet processes.
  • the characterizing feature of a dry process is that the sorbent forms a dry product after having been contacted with the flue gas, which dry product has in particular a moisture content of at most 5 wt.%.
  • the steel slag may be a slag material generated in a steel mill during the production of carbon steel, in particular EAF slag, BOF or LD slag and LM slag, but is preferably a slag material produced during the production of stainless steel, in particular EAF slag, AOD slag, VOD slag, LM or tundish slag.
  • Steel slag material is commonly generated during the production of steel, and forms a protective layer on the steel bath, thereby protecting the molten steel against oxidation.
  • the slag material also purifies the steel by absorbing oxides and impurities.
  • steel slags also contain steel particles of different sizes, which can be recycled especially after having crushed and optionally milled the solidified steel slag.
  • liquid steel arrives in this way onto the floor of the steel mill.
  • some fine refractory material also arrives onto the floor when dismantling the refractory lining of the furnaces and ladles within the steel mill.
  • a waste material is thus collected which comprises steel slag mixed with a smaller amount of other materials, for example about 5 to 10 wt.% of other materials such as spent refractory materials.
  • the waste material collected from the floor of the steel mill may be further mixed with a fine fraction of refractory material, for example a 1 to 10 mm fraction, which is separated from the coarser fraction of the dismantled refractory linings.
  • a fine fraction of refractory material for example a 1 to 10 mm fraction, which is separated from the coarser fraction of the dismantled refractory linings.
  • steel slag material can be used in the method according to the present invention but also the mixed waste material since it still contains the steel slag material.
  • the mixed waste material preferably consists for more than 50 wt.%, preferably for more than 60 wt.% of steel slag material.
  • the parameters of the steel slag material described or claimed in the present description and claims for example the percent by weight of crystalline non-metallic slag phases contained in the steel slag, only relate to the steel slag material contained in the mixture, i.e. excluding other materials which may be present in the mixture.
  • the main steel slag by-product generated in the steel mill is the steel slag obtained by solidifying the liquid steel slag collected in the slag pots. Upon appropriate cooling of the liquid steel slag and crushing thereof, most of this steel slag product can be recycled as construction aggregate, including coarse and fine aggregates.
  • the fine aggregates are in practice free of fines, in particular free of particles smaller than 0.5 mm. When producing these aggregates, the fines thus remain.
  • These fines consist in particular of a 0 - 0.5 mm fraction, which is a by-product that can nowadays mainly only be used as carbonatable binder for producing artificial stone materials. There is thus a need for finding new uses for these fines.
  • the steel slag fines are not only continuously produced in large amounts but there are also historic mono-landfill sites filled over years or even decades with steel slag fines. There are especially several mono-landfill sites filled with stainless steel slag fines since stainless steel slag fines have too high heavy metal contents for being applied as soil conditioner, in contrast to for example LD slags.
  • the steel slag used in the method according to the present invention has to comprise non-metallic slag phases, at least 40 wt.% of which are crystalline non- metallic slag phases.
  • the maximum amount of crystalline non-metallic slag phases is 100 wt.% of the non-metallic slag phases.
  • the steel slag is not a rapidly quenched steel slag, such as granulated steel slag, since such steel slag contains too much amorphous slag phases, i.e. more than 60 wt.% of amorphous slag phases.
  • the crystalline non- metallic slag phase have to include at least crystalline calcium silicate and calcium magnesium silicate phases.
  • Calcium silicate phases are phases which contain at least CaO and SiC>2.
  • Examples of crystalline calcium silicate phases are mono- and dicalcium silicates (CaSiOs, Ca2SiO4), including gamma and beta dicalcium silicate, and cuspidine (Ca4Si2O7F 2 ).
  • Calcium magnesium silicate phases are phases which contain at least CaO, MgO and SiO 2 .
  • Examples of crystalline calcium magnesium silicate phases are bredigite (Ca?Mg(SiO4)4), merwinite ( CasMg(SiO4)2) and akermanite (Ca2MgSi2O?).
  • a particulate steel slag material is produced from the steel slag.
  • This particulate steel slag material should have a first particle size distribution with a D 5 o sieve size value which is smaller than 70 pm.
  • the particulate steel slag material has in particular a first specific surface area which is lower than 5 m 2 /g.
  • the D 5 O sieve size value is the particle size at 50% by volume cumulative passing.
  • the particle size distribution can be tested according to ASTM D6913/D6913M- 17 using sieves having square openings.
  • the volume percent of the particles can in particular be calculated by dividing the weight of the particles passing through the sieve by the average density of the material forming the particles.
  • the particle size distribution can be determined by laser technology.
  • the specific surface area are BET specific surface areas determined by the BET method in accordance with ISO 9277:2022(E). This is done by nitrogen absorption manometry after vacuum degassing at 190°C for at least two hours to remove any free moisture from the material.
  • the small particle size can for example be obtained by milling the steel slag.
  • the steel slag By milling the steel slag to this particle size, most of the steel particles/inclusions are liberated and can be separated from the non-metallic slag phases.
  • the waste material collected from the floor of the steel mill, and optionally mixed with the fine spent refractory material, can be milled in this way to recover the steel fraction contained therein.
  • the steel slag produced by solidifying the liquid steel slag collected in the slag pots is however first of all used to produce valuable aggregate fractions. This is especially the case for stainless steel slags since, compared to common steel slags, they contain less free lime which can moreover be neutralised sufficiently to avoid any swelling problems when using the aggregate fractions as construction material, in particular as aggregates in concrete or asphalt. The remaining fines, or some even finer fractions separated therefrom, can then be used as such in the method according to the present invention or after some additional milling.
  • the air-cooled steel slag is generally produced by pouring liquid steel slag in a slag yard and by allowing it to solidify.
  • Cooling of the solidified steel slag is preferably accelerated, in particular by spraying water onto the solidified steel slag. Compared to a granulation method, such an air-cooling method results in a slower solidification of the liquid steel slag.
  • the solidified steel slag is not entirely amorphous but comprises crystalline slag phases and usually, but not necessarily, amorphous slag phases.
  • the non-metallic phases of the solidified steel slag should comprise at least 40 wt.%, preferably at least 50 wt.% of crystalline non-metallic slag phases, i.e. mineral phases. They may also comprises amorphous non-metallic slag phases, in particular at least 15 wt.%, more particularly at least 20 wt.% of amorphous non-metallic slag phases.
  • Part of the steel slag material which is used in the method according to the invention to produce the particulate steel slag material may be separated from the steel slag as solidified.
  • the steel slag as solidified contains indeed already as such an amount of particles which have a sieve size which is smaller than 0.5 mm.
  • the liquid steel slag from the different ladles and furnaces of a steel production plant is poured by means of the slag pots in a slag yard into cooling pits wherein the liquid steel slag is air-cooled, i.e. left to slowly solidify and cool.
  • these fines can be converted into a sorbent for capturing acid pollutants from a flue gas in particular by simply carbonating these fines.
  • the cooling process can be accelerated, in particular by spraying water onto the solidified steel slag, so that the steel slag is cooled down more quickly, in particular as from a temperature higher than 500°C, and less p-dicalcium silicate can transform into y-dicalcium silicate.
  • the solidified steel slag 31 is stored in a slag bunker 32.
  • the steel slag 31 is fed into a hopper 33 which comprises a grid for stopping all oversized slag pieces 34, in this particular case those bigger than 300 mm.
  • oversized pieces could damage the crushers used in the later process, these oversized pieces 34 are removed for later particular treatment, such as breaking with hammers and extraction of large metal fragments before being fed again through the hopper 33.
  • the slag particles 35 smaller than 300 mm fall through the hopper 34 onto a conveyor belt and are transported to a first metal handpicking cabin 36 wherein operators remove large metal pieces 37 from the slag particles 35 on the conveyor belt.
  • the steel slag particles 35 can be sieved to remove the fines, having a particle size which is smaller than 0.5 mm or smaller, or a portion thereof. These fines can be used in the method according to the present invention as particulate steel slag material.
  • the steel slag particles 35, or the remaining steel slag particles, are crushed in a first crusher 38 to produce a crushed slag material 39 which is transported along a first metal separating magnetic belt 40, removing metal particles 41 from the crushed steel slag particles 39, to a first sieve 42.
  • the slag particles 39 then pass through the first sieve 42 which separates them into three fractions: particles 43 bigger than 35 mm, particles 44 between 10 and 35 mm and particles 45 smaller than 10 mm.
  • the fraction of particles 43 bigger than 35 mm is taken by a second conveyor belt through a second metal handpicking cabin 46 where more metal pieces 47 are removed.
  • the particles 43 bigger than 35 mm are then put back into the first crusher 38.
  • the fraction of particles 44 between 10 and 35 mm goes into a second crusher 48 to produce a further crushed steel slag material 49 which is fed into a second sieve 50.
  • the further crushed steel slag material 49 is separated in the second sieve 50 in three fractions: a fraction 51 of particles bigger than 20 mm, a fraction 52 of particles smaller than 10 mm and a fraction 53 of particles between 10 and 20 mm.
  • the fraction 53 of particles between 10 and 20 mm is taken by a third conveyor belt through a second metal separating magnetic belt 54, where more metal 55 is removed, and back into the second crusher 48.
  • the fraction 51 with particles bigger than 20 mm is passed through a third metal separating magnetic belt 56, where more metal 57 is removed, and is stored in a box 58.
  • This fraction 51 is a valuable coarse aggregate for use in building and road construction.
  • the fraction 45 of particles smaller than 10 mm from the first sieve 42, and the fraction 52 of particles smaller than 10 mm from the second sieve 50 are combined to form a 0-10 mm granular material 59 which is stored in a bunker 60.
  • the granular slag material 59 is supplied to a third sieve 61 , which is a 2 mm sieve separating the particles of the 0-10 mm steel slag granular material 59 into a fine fraction 62 of particles smaller than 2 mm and a coarser fraction 63 of particles between 2 and 10 mm.
  • the coarser fraction 63 is fed into a wet jigging apparatus or jig 64, wherein metal particles 65 are removed from the coarser 2-10 mm fraction 63.
  • the jig is a wet jigging apparatus or jig 64, wherein metal particles 65 are removed from the coarser 2-10 mm fraction 63.
  • the jig 64 is preferably an In Line Pressure jig as described in the article “Gravity Separation: Old Techniques/New Methods” by Andrew Falconer in Physical Separation in Science and Engineering, 2003, Vol. 12, No. 1 , pp. 31 -48.
  • the jig 64 is filled with water wherein the steel slag particles are subjected to a jigging action so that the heavier metal particles
  • the fraction 66 of lighter slag particles is passed through a fourth metal separating magnetic belt 67, where a metal rich fraction 68 is removed, and the slag particle fraction 66 is stored in a box 69.
  • This fraction 66 is a valuable finer aggregate for use in building and road construction.
  • the metal rich fraction 68 is preferably milled or ground by a grinder 96 and is then fed to a metal separator 97 wherein the steel particles 99 are separated from the finely milled steel slag fraction 98.
  • the third sieve 61 is preferably a wet sieve wherein water is supplied.
  • the fine fraction 62 is therefore mixed with water to produce an aqueous mixture 70.
  • This aqueous mixture is also produced with water 71 from the jig 64 containing fine slag material which has been washed off from the coarser slag fraction 63.
  • the aqueous mixture 70 is fed into a hydrosizer 72.
  • the working principle of the hydrosizer 72 is also described in the article “Gravity Separation: Old Techniques/New Methods” by Andrew Falconer.
  • a steel rich fraction 73 is separated from the aqueous mixture 70.
  • This steel rich fraction 73 is fed into a first gravity separation screw 74 from which the overflow 75 is added again to the aqueous mixture 70 whilst the underflow 76 is fed into a ball mill crusher 77 wherein slag material adhering to the steel particles is removed therefrom.
  • the crushed material 78 leaving the ball mill crusher 77 is fed into a second gravity separation screw 79.
  • the overflow 80 of this second gravity separation screw 79 is added again to the aqueous mixture 70 whilst the underflow is a valuable fine steel fraction 81 .
  • the aqueous mixture 70 including the slag fractions 75 and 80 removed from the steel rich fraction 73, is fed to a dewatering classifier, in particular a dewatering screw 82, wherein, as an underflow, a sand fraction 83 is removed from the aqueous mixture 70, in particular a 0.5-2 mm sand fraction.
  • This sand fraction 83 can be used as fine aggregate for producing concrete or asphalt.
  • the overflow leaving the dewatering screw 82 is an aqueous dispersion 84 of fine slag particles in water.
  • this aqueous dispersion 84 is fed into a hydrocyclone 85.
  • the working principle of the hydrosizer 72 is described in the article Gravity Separation: Old Techniques/New Methods” by Andrew Falconer.
  • the underflow 86 of the hydrocyclone 85 is filtered by means of disk filters 87 to remove the fine slag particles forming a powdery steel slag fraction 88.
  • the overflow 89 of the hydrocyclone 85 is further treated in a thickener 90, the overflow 91 of which is pumped into a water reservoir 92 whilst the underflow 93 is treated with the disk filters 87.
  • the further fine slag particles which are removed by means of the thickener 90 thus also arrive in the powdery steel slag fraction 88.
  • the filtrate 94 leaving the disk filters 87 is also supplied to the reservoir 92.
  • the purified water contained in the reservoir 92 can be reused for supplying water to the wet sieve 61 and to the jig 64. In the reservoir 92 further fine steel slag material will settle to the bottom to produce a precipitate 95. From time to time this precipitate can be removed from the reservoir 92 and can be added to the powdery steel slag fraction 88.
  • a particulate steel slag material is produced from the steel slag.
  • this can be done by simply milling the steel slag, for example the waste material collected from the floor of the steel mill, and removing the metal fraction from the milled steel slag, or the steel slag can be crushed, the coarser fractions and the metal fractions can be removed so that a powdery steel slag fraction remains.
  • This powdery steel slag fraction can be further milled or it can be divided in finer and coarser fractions. It is also possible to mill one or more of the coarser steel slag fractions to produce a powdery steel slag fraction.
  • the metal rich fraction 68 described hereabove is preferably further milled so that the finely milled slag phases, i.e. the finely milled steel slag fraction 98, are removed from the steel particles 99 contained in the metal rich fraction 68.
  • the finely milled steel slag fraction 98 has for example a D 90 particle size of about 80 pm, a D 5 o particle size of about 25 pm and a Dw particle size of about 2 pm. Prior to carbonation, this finely milled steel slag fraction, which has this particular particle size distribution, had a BET specific surface area of 2.4 m 2 /g.
  • the milled slag phases 98 absorb less water than the powdery steel slag fraction 88 and can thus be used as filler, for example in concrete or asphalt. However, they can also be applied as particulate steel slag material in the method according to the present invention.
  • the particulate steel slag material produced from the steel slag is that it should be a fine material, i.e. a particulate material which has a particle size distribution with a D 5 o sieve size value which is smaller than 70 pm.
  • a particulate material has a relatively low first specific surface area, in particular a first specific surface area determined by the BET method in accordance with ISO 9277:2022(E) by nitrogen absorption manometry after vacuum degassing at 190°C for at least two hours which is smaller than 5 m 2 /g.
  • the D[3,2] particle size (in nm), i.e. the surface mean particle size (which is the average diameter of a sphere having the same surface area as the particles), can be calculated from the BET specific surface ABET (in m 2 /g) and the density p (in g/cm 3 ) of the material by the following equation:
  • the density of the steel slag is equal to 3.28 g/cm 3
  • the present inventors have found that there is a correlation between the BET specific surface area and the degree of carbonation. Due to the high temperature in the steel furnace, fresh steel slag does not comprise carbonates. It comprises a small amount of free lime and periclase and larger amounts of calcium silicate and calcium magnesium silicate phases which are reactive towards carbon dioxide.
  • the maximum CO2 uptake was equal to about 10 wt.%, corresponding to about 12.7 wt.% of CaO, which is only a fraction (about 22%) of the total amount of CaO and MgO present in the stainless steel slag.
  • the maximum CO2 uptake was equal to about 18 wt.%, corresponding to about 23 wt.% of CaO, which is more by also still only a fraction (about 42%) of the total amount of CaO and MgO present in the BOF steel slag.
  • the BOF slag contained more free lime than the stainless steel slag, namely 2.1 wt.% versus 0 wt.%.
  • stainless steel slag is preferably used since common steel slag contain more free lime so that more of the carbon dioxide reacts in the common steel slag with free lime without producing pores in the calcium and calcium magnesium silicate phases.
  • the particulate steel slag material Before starting the carbonation step, the particulate steel slag material has a particle size distribution with a D 5 o sieve size value which is smaller than 70 pm. Preferably this D 5 o sieve size value is even smaller, in particular smaller than 60 pm, preferably smaller than 50 pm, more preferably smaller than 40 pm and most preferably smaller than 30 pm. It has been found that the finer the particulate steel slag material at the start of the carbonation process, the higher the maximum amount of carbonates which can be produced and thus the higher the BET specific surface area which can be achieved.
  • the particulate steel slag material is preferably carbonated until it has a BET specific surface area which is higher than 10 m 2 /g, preferably higher than 15 m 2 /g and more preferably higher than 20 m 2 /g.
  • the particulate steel slag material is preferably carbonated until it has a carbonate content, expressed in percent by dry weight of carbon dioxide, of at least 2.0 wt.%, preferably of at least 4.0 wt.%, more preferably of at least 6.0 wt.% and most preferably of at least 8.0 wt.%.
  • This carbonate content can easily be determined by a simultaneous thermal analysis (STA, DIN 51004, DIN 51006 and DIN 51007).
  • the maximum carbonate content of the particulate steel slag is first determined by analysing at least one sample of the particulate steel slag material and the particulate steel slag material is then carbonated until at least 50%, preferably at least 70% and more preferably at least 90% of this maximum carbonate content is achieved.
  • the maximum carbonate content corresponds for example to the maximum carbonate content which is obtained by the following test.
  • the steel slag sample is dried at 1 10°C until there is no longer any weight loss.
  • the sample is then sieved over a 250 pm sieve.
  • 10 g of the sample are placed in a cup without compacting the sample and 1 .5 g of water is added and mixed with the steel slag.
  • the autoclave is kept for 24 hours at a temperature of about 23°C.
  • An STA analysis from 0-1000°C is carried out on the carbonated sample.
  • the weight loss between 550°C and 850°C is due to the decomposition of carbonates liberating carbon dioxide. This weight loss, divided by the initial weight of the dried sample and multiplied by 100, is the maximum carbonate content of the carbonated particulate steel slag material expressed in percent by dry weight of carbon dioxide. When carbonating the particulate steel slag material the degree of carbonation can be determined by the same test method.
  • the steel slag material has only a limited content of calcium compounds which are available for reaction with carbon dioxide. Consequently, by carbonating the particular steel slag material uniformly, it is possible to produce a maximum amount of carbonates and also a maximum amount of pores on the surfaces of the particles of the particulate steel slag material.
  • a carbonating gas which has a relatively low carbon dioxide content, in particular a carbon dioxide content of less than 5.0 vol.%, preferably of less than 3.0 vol.% and more preferably of less than 1.0 vol.%.
  • the carbonating gas may be in particular atmospheric air, which has a carbon dioxide content of about 0.04 vol.%.
  • the outer pores of the mass of particulate steel slag are filled more quickly with carbonates so that carbonation of the inner parts of the mass of particulate steel slag is hampered and a uniform carbonation is more difficult to achieve.
  • the present inventors have found that upon natural carbonation, i.e. carbonation under atmospheric conditions, large heaps (piles) of particulate steel slag material can be carbonated quite uniformly over a number of years down to the centre of the heap. Although the particulate steel slag material is loosely stacked in the heap, it is compacted under its own weight in the lower parts of the heap.
  • the particulate steel slag material can also be carbonated in smaller heaps, for example in masses contained in big bags which are permeable for the carbonating gas.
  • the particulate steel slag material contained in such smaller heaps, in particular in big bags, can be carbonated within a few days with a carbonating gas containing less than 5 vol.% of carbon dioxide.
  • the particulate steel slag material can however also be carbonated with a carbonating gas containing more than 5 vol.% of carbon dioxide. It can then be loosely stacked in a heap or in a bed.
  • a heap can be defined as having a height higher than its width whilst a bed can be defined as having a height which is smaller than its width.
  • Both a heap and a bed can be elongated, in which case the width has to be measured in a direction perpendicular to its longitudinal direction.
  • the carbonating gas preferably has a carbon dioxide content of more than 10.0 vol.%, and more preferably of more than 15.0 vol.%.
  • the mass of particulate steel slag material is preferably stirred for one or more times during said carbonation step.
  • Stirring has to be understood here in its broadest meaning. It also includes any type of mixing the particulate steel slag material contained in the heap or bed. Stirring can in particular also be achieved by blowing the carbonating at such a flow rate through the particulate steel slag material that the steel slag particles are agitated. They can be agitated to such an extent that agglomeration of the particulate steel slag material is avoided or at least reduced. Agglomeration of the particles of the steel slag material can be avoided in particular by keeping them in a fluidized bed.
  • the particles of the particulate steel slag material will agglomerate during the carbonation step.
  • a granular material will thus be obtained which contains grains of carbonated particulate steel slag material wherein the steel slag particles are bonded to one another by means of the carbonates produced on the surface of these particles. It is also possible to granulate the particulate steel slag material before being carbonated. In this way, the particles of the steel slag material are also agglomerated during the carbonation step to produce a granular material which contains grains of carbonated steel slag material wherein the steel slag particles are bonded to one another by means of the carbonates produced on the surface of these particles.
  • the granulated particulate steel slag material is preferably carbonated by means of a carbonating gas which has a relatively low carbon dioxide content, in particular a carbon dioxide content of less than 5.0 vol.%, preferably of less than 3.0 vol.% and more preferably of less than 1 .0 vol.%.
  • a carbonating gas which has a relatively low carbon dioxide content, in particular a carbon dioxide content of less than 5.0 vol.%, preferably of less than 3.0 vol.% and more preferably of less than 1 .0 vol.%.
  • the present inventors have found that when producing a granular material which contains grains of the carbonated particulate steel slag material an even higher BET specific surface area can be achieved, namely within the pores formed in between the steel slag particles.
  • the thus obtained granular material can be used as such as sorbent for capturing acid pollutants.
  • the granular material is too coarse, the grains thereof can however be reduced in size. Preferably, they are not milled but crushed. In this way, the BET specific surface area could be maintained maximally after the crushing step. Finely milling the carbonated material, on the other hand, may reduce the BET specific surface of the carbonated material.
  • the carbonating gas used to carbonate the particulate steel slag material may contain an amount of acid pollutants but preferably only in a limited amount.
  • it is preferably is free of HCI or contains less than 10 mg/Nm 3 HCI, it is free of SO2 or contains less than 50 mg/Nm 3 SO X and it is free of NO X or contains less than 200 mg/Nm 3 NO X .
  • a clean gas can thus be used or a flue gas provided it contains only a limited amount of acid pollutants.
  • Acid pollutants such as HCI and SO X will indeed diminish the formation of carbonates, by forming instead chlorides and sulphites/sulphates, whereas NO X present in the carbonating gas will already occupy the micropores formed in the carbonated steel slag material.
  • a flue gas is contacted with the carbonated particulate steel slag material or with at least a fraction thereof.
  • the flue gas comprises acid pollutants such as HCI, HF, SO X and NO X .
  • the flue gas may be generated by different types of processes, for example in a kiln, such as a lime or a cement kiln or a ceramic tile kiln. More heavily polluted flue gasses are however produced in incinerators, such as in a municipal waste incinerator.
  • the existing flue gas treatment processes can be classified in three categories, namely in wet flue gas cleaning processes, semi-dry flue gas cleaning processes and dry flue gas cleaning processes.
  • a semi-dry or a dry flue gas cleaning process is preferably applied since they require lower investments than a wet process and since the carbonated steel slag material used in the method according to the present invention is less effective in a wet flue gas cleaning process.
  • the dry and the seme-dry flue gas cleaning processes are referred to as dry sorbent processes since in both processes the sorbent which has reacted is a dry product.
  • the sorbent which is brought in contact with the flue gas is either a dry product or a wet product, the wet product drying however when being in contact with the hot flue gas.
  • the carbonated particulate steel slag material can be arranged in a packed filter bed through which the flue gas is guided during the contacting step.
  • the carbonated particulate steel slag material is preferably in the form of a granular material which contains grains of agglomerated particulate steel slag material. Since the grains of the agglomerated carbonated steel slag material are coarser than the particles of steel slag material contained in these grains, the granular carbonated steel slag material has larger pores and is thus more permeable for allowing the flue gas to flow through the packed filter bed than the initial particulate steel slag material.
  • An advantage of the packed filter bed is that the residence time of the flue gas in the filter can be increased by providing a larger filter bed.
  • the carbonated steel slag material can also be contacted with the flue gas until it has adsorbed the desired amount of acid pollutants.
  • the packed filter be can then be renewed but it is also possible to renew the carbonated steel slag material continuously, by using a so-called moving filter bed, which is a packed filter bed to which new sorbent material (carbonated steel slag material) is fed on the top and used sorbent material is removed at the bottom.
  • FIG. 2 An example of a flue gas cleaning installation having a moving packed filter bed is illustrated schematically in Figure 2.
  • the flue gas 100 is generated in a kiln 101 for burning bricks, tiles, pottery or the like.
  • the hot flue gas 100 can be led through a heat recovery system 102 wherein the flue gas is cooled down to a temperature of for example 160 to 270°C.
  • the flue gas is then led through a filter 103 containing a packed filter bed 104 composed of sorbent material 105, i.e. of the carbonated particulate steel slag material 105.
  • the filter bed 104 illustrated in Figure 2 is a moving filter bed 104.
  • the sorbent material 105 is continuously fed from a funnel 106 onto the top of the filter bed 104.
  • sorbent material 105 which has captured acid pollutants is continuously collected and transported by an endless screw 107 into a container 108 for the used sorbent material 105.
  • a baffle structure 109 is preferably provided in order to create open spaces within the bed of sorbent material 105. Through these open spaces, the flue gas 100 can spread more easily and thus more uniformly within the bed of sorbent material 105. In case the residence time of the sorbent material 105 is too short, it is possible to recycle a portion of the used sorbent material 105 to the funnel 106.
  • the flow of flue gas 100 through the flue gas cleaning installation is created by drawing the flue gas 100 through the filter 103 and blowing it into the stack 11 1 by means of a fan 110.
  • the carbonated particulate steel slag material can be injected in a flow of the flue gas.
  • An example of a flue gas cleaning installation wherein the sorbent material 105 is injected in a flow of the flue gas 100 is illustrated schematically in Figure 3.
  • the flue gas 100 is generated in an incinerator 1 12 for incinerating waste 1 13, for example municipal waste.
  • the installation may thus be a MSWI (Municipal Solid Waste Incinerator).
  • the incinerator 1 12 may comprise a moving grate 1 14 carrying the burning waste 1 13 and transporting it towards the container 115 for collecting the bottom ashes 1 16.
  • the hot flue gas 100 produced by the burning waste 1 13 rises and passes through a boiler 1 17.
  • steam can be generated for driving a turbine (not illustrated), in particular a steam turbine of an electricity generator (not illustrated).
  • a flow of flue gas 100 is created by means of the fan 1 10 which is provided upstream the stack 11 1.
  • a reactor 118 can be provided wherein for example water can be sprayed onto the flue gas 100 to condition the flue gas 100, i.e. to control its temperature and its moisture content.
  • some of the dust particles contained in the flue gas 100 may be removed from the flue gas 100 and may be collected as boiler ash 1 19 in a boiler ash container 120. Downstream the optional reactor 118 a fabric filter 121 , preferably a so-called baghouse filter, is arranged to remove the remaining dust particles from the flue gas 100. These dust particles are collected as fly ash 122 in a fly ash container 123.
  • the carbonated particulate steel slag material can be injected as dry sorbent material in different locations in the flow of flue gas 100. It can first of all be injected in the top part of the incinerator 1 12, via an injector 124. At this location, the flue gas 100 still has a very high temperature.
  • An advantage of injecting the sorbent material 105 at this location in the flow of flue gas 100 is that a maximum contact time between the sorbent material 105 and the flue gas 100 can be achieved.
  • the sorbent material 105 contains some moisture, it is quickly dried in the flow of flue gas 100 and at the same time the moisture content of the flue gas is increased.
  • the increased moisture content of the flue gas may increase the reactivity of the sorbent material 105 with the acid pollutants.
  • a dryer sorbent material 105 was more effective for capturing acid pollutants from the flue gas.
  • the sorbent material 105 can also be injected at other locations in the flow of flue gas 100, for example in the boiler 1 17, in the reactor 118 or at a location, indicated by arrow 125, in between the reactor 1 18 and the fabric filter 121 .
  • An advantage of the use of a fabric filter 121 is that the dust contained in the flue gas 100 is collected on the fabric filter 121. This dust also contains the injected sorbent material 105. Since a layer of dust is formed onto the fabric filter 121 before it drops from the filter 121 , the contact time between the flue gas and the sorbent material is considerably increased by the presence of the fabric filter 121 .
  • the mixture of the usual fly ash and the used sorbent material is collected in the fly ash container 123. A portion of this mixture may however be reinjected upstream the fabric filter 121 , by means of the injector 126, in the flow of flue gas 100 in order to enable the sorbent material 105 to capture a larger amount of acid pollutants.
  • the carbonated particulate steel slag material When the carbonated particulate steel slag material is to be injected in the flow of flue gas, it preferably has a particle size distribution with a D 90 sieve size value which is smaller than 2000 pm, preferably smaller than 1000 pm, more preferably smaller than 500 pm and most preferably smaller than 250 pm.
  • the smaller the particle size of the steel slag particles the better they will be carried along with the flow of the flue gas.
  • relatively coarse particles/grains are present in the carbonated particulate steel slag material, it is preferably injected above the incinerator. The heavier grains which are not carried along with the flow of flue gas then fall into the incinerator, where they may capture acid pollutants, and arrive in the bottom ashes.
  • the D 90 sieve size value of the carbonated steel slag material is preferably larger than 50 pm and more preferably larger than 75 pm.
  • the filter can remove the carbonated particulate steel slag material more effectively from the flow of flue gas.
  • the carbonated particulate steel slag material will cause less clogging problems of the filter, especially in case the flue gas has a relatively high moisture content. It has indeed been found that when calcium chloride is formed in the sorbent material, the higher hydrates of calcium chloride become sticky, which is especially a problem in case of high calcium chloride concentrations in small, light sorbent particles.
  • the carbonated particulate steel slag material which is used as said sorbent is preferably dried after the carbonation step, but before being brought in contact with the flue gas, to a moisture content which is smaller than 10 wt.%, preferably smaller than 7 wt.% and more preferably smaller than 4 wt.%. Higher moisture contents were found to reduce the capture level of acid pollutants.
  • the carbonated particulate steel slag material may thus be used to clean flue gas having a lower temperature since it does not need to be dried (as is the case in a semi-wet process). Since the carbonation step requires the presence of water in the particulate steel slag material, preferably more than 10 wt.%, a drying step is required to achieve the preferred moisture contents.
  • the moisture content of the carbonated particulate steel slag material which is brought in contact with the flue gas is higher or equal to 0 wt.%.
  • the moisture content of the carbonated particulate steel slag material is determined by keeping a sample of 10 g of the material for 24 hours at a temperature of 110°C, by measuring the weight loss and by calculating this weight loss as a percentage of the initial weight.
  • Table 1 Particles size distribution values, calcite content, maximum carbon dioxide absorption, total maximum carbon dioxide absorption and gamma dicalcium silicate content of the powdery stainless steel slag 88.
  • the total amount of CO2 is the sum of the calcite content, expressed in percent by dry weight of CO2, and the maximum CO2 absorption.
  • the gamma C2S content (y-C2S) is the percent by weight of gamma C2S on the total amount of crystalline slag phases. This content was determined by XRD.
  • the present inventors have found that the finer the particle size distribution, the higher the maximum carbonate content (total CO2) of the particulate steel slag.
  • This positive correlation can be seen in the data of the weekly analysis results shown in Figure 4.
  • This Figure shows the correlation between the volume percent of particles smaller than 63 pm and the maximum carbonate content, expressed in percent by dry weight of CO2.
  • a same correlation was found between the volume percent of particles smaller than 45 pm or smaller than 125 pm, and the maximum carbonate content.
  • the smaller particle size is corresponds with a larger surface area, such as a larger Blaine surface area, which is available for the formation of carbonates by reaction with carbon dioxide.
  • a smaller particle size can be achieved by milling the particulate steel slag.
  • the carbonate content was measured by STA and expressed as percent by dry weight of CO2 and the BET specific surface area was measured.
  • the two samples of the finely milled steel slag fraction 98 had the smallest carbonate contents and BET values, namely two times a carbonate content of 0.16 wt.% of CO2, and BET values of 1 .7 and 2.4 m 2 /g. These BET values were relatively high due to the very fine particle size of this finely milled steel slag fraction 98, having in particular a D50 sieve size value of 25.9 pm.
  • Figure 6 shows the relationship between the carbonate content of the particulate steel slag material and the BET specific surface area thereof.
  • the curve fitting through these data is an exponential curve.
  • the increase in BET specific surface area is rather limited. This may be due to the fact that first free lime (including portlandite) may be carbonated, which does not produce additional porosity in the calcium and calcium magnesium silicate phases.
  • the granular material was then finely milled to achieve a fine material having a D10 sieve size value of 0.9 pm, a D 5 o sieve size value of 6.4 pm and a D 90 sieve size value of 81.9 pm.
  • the BET specific surface area decreased by the fine milling process, namely to a BET specific surface area of 8.6 m 2 /g.
  • such a fine milling process may destroy the pore structure of the carbonated particulate steel slag material or the carbonate structures produced on the surfaces of the steel slag particles in the pores of the granular material.
  • a coarse crushing process may however be used to reduce the grain/particle size of the granular material produced by the carbonation process. Effect of the increased BET specific surface area on the capture levels of acid pollutants in a flue gas composition
  • Packed filter beds were made, each with 200 g of one of the dried slag materials, and were placed in a reactor.
  • Three gas mixtures were made, namely a first gas mixture with air containing 1000 mg/m 3 NO2 gas, a second gas mixture with air containing 1000 mg/m 3 of SO2 gas and a third gas mixture with nitrogen gas containing 2000 mg/m 3 HCI gas. These gasses were led simultaneously, each at 2 Nl/min, through the packed filter bed contained in the reactor and this for 60 minutes.
  • the reactor was heated to about 80°C with an external electric heating mantle. During the test, the reactor was further heated. At the end of the test, the temperature in the reactor has increased to about 105°C.
  • Slag B had the highest BET specific surface area. According to an XRD analysis, its crystalline phases comprised 20.3% CaCOs. On average, the 200 g samples captured during the 60 minutes test a total amount of about 0.45 g of the acid pollutants. Slag B had an average HCI capture level which was about the same as the average HCI capture level of Sorbacal® SP. Its average SO2 capture level was however even somewhat higher. Surprisingly, its NO X capture level was substantially higher, and was about the same as its SO2 capture level. This can be explained by the fact that the carbonated Slag B contains micropores wherein the NO2 can be captured since, in contrast to SO2 and HCI, NO2 does substantially not react with carbonates.
  • the carbonated particulate steel slag material used in the method according to the present invention is not only effective based on its carbonate content but especially also by its (micro)pores and its increased BET specific surface area.
  • the pores in the silicate phases were found to be able to capture even NO X .
  • the SO2 gas mixture containing 1000 mg/m 3 SO2, was passed for 60 minutes at a flow rate of 5 Nl/min through the 200 g filter bed in the reactor heated at the start of the test to about 80°C.
  • the SO2 content of the gas was measured in a continuous manner by means of an ecom-J2KNpro flue gas analyser. During the first 15 minutes 100% of the SO2 was captured by the filter bed. During the next 45 minutes, the capture level linearly decreased to about 80%.
  • the 200 g sample of Slag C thus captured about 0.28 g of SO2.
  • the same test was done with Slag C to which 8% of water was added. As from the start of the test the SO2 capture level decreased substantially linearly from 100% to about 70%.
  • the first test with the combination of the three gas mixtures was also repeated with Slag C to which 8% of water was added instead of using Slag C which has been dried.
  • the moisture content of the slag dropped.
  • the filter bed had a moisture content of 2.4%.
  • the test was only done once and a SO2 capture level of 54% was achieved, a HCI capture level of 90% and an NO X capture level of 75%. These three values were thus lower due to the moisture content. It thus appears that the water fills a portion of the smaller pores and reduces the surface area of the slag which is available for adsorption and/or reaction of the acid pollutants with the carbonated slag material.
  • the particulate steel slag material was taken from the powdery steel slag fraction 88 as produced from stainless steel slag by the method as described with reference to Figures 1 A and 1 B.
  • a fine fraction was sieved out by means of a 63 pm sieve.
  • the fine fraction was dried completely at 105°C. Water was added to a moisture content of 15%.
  • the humid steel slag fraction was spread in a thin layer/bed and carbonated with >80% CO2 gas for 24 hours at 40°C. The steel slag fraction was thus completely carbonated.
  • the particles of the steel slag fraction did not agglomerate together.
  • the total amount of carbonates in the carbonated steel slag fraction was equal to about 12.5 wt.% CO2.
  • Adsorption of HCI and SO 2 was measured by means of a HCI gas mixture containing 1200 mg HCI/Nm 3 in N2 and an SO2 gas mixture containing 3363 mg SC /Nm 3 in N2.
  • As sorbent use was made of 50 mg samples of the carbonated fine steel slag fraction and 30 mg samples of Sorbacal®SP. The difference in sample size between both types of samples and isothermal exposure is due to the packing density of the sample in the crucible, which is much lower for Sorbacal®SP.
  • the carbonated particulate steel slag could adsorb about 1 .5 to 2% of the acid pollutants.
  • adsorption percentages upto about 3% could be obtained for SO2 with increasing reaction times. It appeared that after a quick adsorption of SO2 a slower increase in SO2 capture took place due to the required reaction between the carbonates and the SO2. For HCI an initial quick adsorption was not observed and the HCI capture appeared to be mainly due to a chemical reaction with the carbonates.

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Abstract

La présente invention concerne un procédé de recyclage de scories d'aciérie, produites dans le cadre d'un processus de fabrication d'acier dans une aciérie, en tant que sorbant pour la capture de polluants acides, dont le HCl, le HF, les SOx et/ou les NOx, présents dans un gaz de combustion. Afin d'augmenter l'efficacité de capture des oxydes d'azote, les scories d'aciérie doivent comprendre du silicate de calcium cristallin et des phases de silicate de calcium et de magnésium. De plus, le procédé de production du sorbant doit comprendre l'étape de production, à partir desdites scories d'aciérie, d'un matériau particulaire à base de scories d'aciérie qui présente une première distribution de la taille des particules dont la valeur de la taille de tamis D50 est inférieure à 70 µm et une première surface spécifique ; et l'étape consistant à augmenter la première surface spécifique du matériau particulaire à base de scories d'aciérie jusqu'à une seconde surface spécifique qui est supérieure à 5 m²/g par carbonatation du matériau particulaire à base de scories d'aciérie grâce à un gaz comprenant du dioxyde de carbone. Le gaz de combustion est ensuite mis en contact avec au moins une partie du matériau particulaire carbonaté à base de scories d'aciérie formant ledit sorbant pour capturer lesdits polluants acides dans le matériau particulaire carbonaté à base de scories d'aciérie.
PCT/EP2023/072854 2023-08-18 2023-08-18 Procédé de recyclage de scories d'aciérie en tant que sorbant pour la capture de polluants acides Pending WO2025040239A1 (fr)

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