Method for producing cement clinker using stainless steel slag

Abstract

In a method for producing cement clinker, raw materials including at least one stainless steel slag material are calcined and combusted to produce cement clinker. Since cement clinker is only allowed to contain a limited amount of hexavalent chromium, the chromium content of the stainless steel slag material needs to be limited. This is achieved by solidifying the liquid steel slag so that the liquid steel slag contains a sufficiently low amount of fines. These fines and / or the fines produced when crushing the coarser fraction of the steel slag to recover the stainless steel are used as powdered stainless steel slag material to produce cement clinker. It has been found that these fines actually contain significantly less chromium than the sand and coarser aggregate fractions produced from the solidified stainless steel slag.10 By partially replacing traditional lime sources with stainless steel slag material, the carbon dioxide emissions and energy requirements of the cement kiln can be reduced. Furthermore, the powdered stainless steel slag material does not need to be pulverized and melts quickly in the rotary kiln.

Description

[Technical field]

[0001] The present invention relates to a method for producing cement clinker, comprising calcining and combusting raw materials including at least one steel slag material to produce cement clinker, in particular Portland cement clinker. [Background technology]

[0002] Portland cement clinker is made up of lime or calcium oxide (CaO), silica (SiO 2 ), Alumina (Al 2 O 3 ) and iron (Fe 2 O 3 ). Portland cement clinker is made starting from raw meal produced from raw materials such as limestone, chalk, shells and shale or calcareous rock as lime sources, sand, old bottles and claystone or argillite as silica sources, bauxite as alumina sources, recycled aluminium and clay, and iron sources such as clay, iron ore, scrap iron and fly ash. These raw materials are crushed, milled and mixed to produce the raw meal. In the dry process, the raw materials are fed in a dry state to the cement kiln, while in the wet process, a wet paste is made and fed to the cement kiln. In the cement kiln, the raw materials are heated to calcine carbonate materials and combust the materials to produce cement phases. These cement phases are made up of calcium aluminate (Ca 3 Al 2 O 6 Or C 3 A), tetracalcium aluminate (Ca 4 Al 2 Fe 2 O 10 Or C 4 AF), belite or dicalcium silicate (Ca 2 SiO 4 Or C 2 S) and alite or tricalcium silicate (Ca 3 SiO 5 Or C 3S) is included. 2 The cement clinker is rapidly cooled to avoid or reduce the formation of gamma-dicalcium silicate, since S has no hydraulic properties, and to obtain a fine-grained structure that enhances the reactivity of the cement. The cement clinker is then mixed with gypsum and finely ground. Different types of Portland cement, for example as defined in the European standard CEN / EN197-1 (2011), can be produced from the cement clinker by adding different cement additives.

[0003] The production of cement clinker requires a lot of energy and is a major source of carbon dioxide emissions, not only as a result of fuel combustion, but also due to the release of carbon dioxide from the calcination of carbonates contained in the raw materials. In order to reduce the energy requirements and carbon dioxide emissions, it has already been proposed to use steel slag materials as one of the raw materials. New steel slag produced at high temperatures in steel furnaces is in fact carbonate-free. This new steel slag contains silicates instead of carbonates. Since the calcination reaction of calcium carbonate is very endothermic, by replacing part of the traditional lime source with steel slag, energy can be saved and the CO2 emissions from fuel combustion can be reduced. 2 Furthermore, no carbon dioxide is released by the steel slag during the firing process since the steel slag contains no or very little carbonates.

[0004] US Patent No. 5,399,633 discloses the use of coarsely crushed steel slag material in the production of Portland cement clinker. To increase the residence time of the steel slag material in the kiln, the coarse steel slag material is fed at a feed location in the center of the kiln, rather than at the combustion end. However, due to the very large size of the steel slag particles, a 10% steel slag feed in the kiln caused significant variations in the amount of hydraulic tricalcium silicate and tricalcium aluminate phases in the cement clinker, necessitating the mixing of the crushed clinker material to achieve a more uniform composition. Furthermore, a drawback of large steel slag particles is that they cannot be preheated in the preheater or precalciner. The preheater actually includes a cyclone through which the finely crushed raw material passes before reaching the kiln. The combustion gases leaving the kiln flow upwards against the raw material through the cyclone, preheating it. Preheaters often include a section where additional fuel is burned to precalcine the calcareous raw materials, and therefore, when the steel slag material is fed into the kiln at a feed location in the center of the kiln, the steel slag particles are not heated very efficiently.

[0005] In the method disclosed in the patent application JP 2003-233636 A, granular steel slag is also used as a raw material for producing cement clinker. However, in this case, the coarse steel slag material is finely ground because it reacts more readily with other elements and chemical compounds contained in the raw mix. Furthermore, due to its greater surface area to volume ratio, the finely ground steel slag is more thermally reactive when added to the kiln compared to the coarser steel slag material received from the steel slag supplier. In order to minimize any possibility of excessive wear or damage to the raw mill that may occur when only the steel slag is milled in the raw mill separately from the other raw materials, the coarse steel slag material is first mixed with the other raw materials to form a raw mix that is fed to the raw mill. A drawback of this prior art method is that energy, time and additional grinding capacity are required to finely grind the steel slag material, causing additional wear to the raw mill.

[0006] In both prior art methods, the steel slag material is not described as a stainless steel slag material, and therefore the steel slag supplied as raw material is a granular steel slag material, in particular a general steel slag material. Patent document 2 further describes the steel slag material as having a very high iron content, indicating that it is a general steel slag material.

[0007] The use of steel slag to produce cement is also disclosed in US Pat. No. 5,399,633. This steel slag also contains a large amount of iron, i.e. 19.41% Fe 2 O 3 It is clearly not stainless steel slag as it does not contain chromium. The fine fraction of steel slag, having a particle size of less than 0.6 mm, was added to the raw materials in an amount of 4% to 6.5% by weight, and the coarser fraction of steel slag was added to the cement kiln in an amount of 5% to 35% by weight. Due to its high iron content, steel slag was used specifically to add iron oxide to the cement raw materials.

[0008] Unlike stainless steel slag, conventional steel slag does not contain heavy metals such as chromium and is therefore a less problematic waste product. Fine powder of conventional steel slag or finely ground conventional steel slag can be used, for example, as a soil conditioner, whereas fine powder of stainless steel slag must be landfilled under controlled conditions. Stainless steel slag has not been used practically as a raw material for the production of cement clinker, since cement is only allowed to contain a limited amount of soluble chromium upon hydration.

[0009] However, the use of stainless steel slag material for producing cement clinker is described in US Pat. No. 5,399,633. The steel slag material described in this patent indeed contains an average Cr content of 1.16 wt.%, which corresponds to a Cr content of 7936 mg / kg. 2 O 3content and a low free lime content averaging only 0.5% by weight. To avoid costly comminution or pulverization steps, the steel slag material is crushed only to reduce its particle size to a value below 51 mm. The crushed steel slag is charged into the kiln at its feed end so that the crushed steel slag passes through the entire kiln. Steel slag is used to achieve significant energy savings and reduce the emission of volatile substances such as carbon dioxide. Since the steel slag material actually melts at a lower temperature, there is no need to use energy to crush and pulverize the steel slag. Furthermore, since the steel slag is already heat treated in the steel furnace, most of the volatile substances are already removed.

[0010] However, as disclosed in US Pat. No. 5,399,633, coarsely crushed stainless steel slag material is a valuable secondary raw material that can be used to replace natural aggregates in civil or building construction, and more particularly in concrete or asphalt. In such applications, the chromium contained in stainless steel slag aggregate is found to be bound, so that it does not cause leaching problems. Furthermore, due to the low free lime content, stainless steel slag is very easily weatherable, and swelling problems are avoided. In contrast, although common steel slag does not contain chromium, it has a much higher free lime content, making common steel slag less suitable for civil construction.

[0011] In the examples described in Patent Document 4, stainless steel slag materials were used in amounts of 5% and 10% by weight in the raw materials for producing cement clinker. 3 S, C 2 S, C 3 A and C 4A cement clinker containing AF is obtained. The chemical composition of the resulting clinker material is described in the patent, but the chromium content is not mentioned. However, based on the chromium content of the stainless steel slag, its addition increases the chromium content of the cement clinker by 397 mg / kg for a 5% addition and 794 mg / kg for a 10% addition. Most of the chromium contained in the stainless steel slag is trivalent chromium, which is practically insoluble in water. However, during the cement processing in the kiln at temperatures between 1400°C and 1500°C, chromium III is partially oxidized to chromium VI. The degree of chromium oxidation varies from kiln to kiln. In practice, for example, about 15% of chromium III can be oxidized to chromium VI in the kiln.

[0012] Hexavalent chromium is known to be carcinogenic, mutagenic and a dermatitis inducer. According to the EU Directive on chromates (2003 / 53 / EC), cement is not allowed to contain more than 2 ppm of soluble chromium (VI) upon hydration. Soluble chromium should be measured according to the STN EN 196-10 standard (Non-Patent Document 2). The correlation between the content of water-soluble chromium VI in cement and allergic skin reactions has already been described in the evaluation of ferrous sulfate by the Scientific Advisory Committee to Examine the Toxicity and Ecotoxicity of Chemical Compounds (CSTE / 93 / 12 / COM) in assessing its mutagenic potential and safety with regard to reproductive toxicity.

[0013] Ferrous sulfate is the most commonly used reducing agent in cement. In practice, the reduction of chromium VI in cement can be achieved by adding 0.35% by weight of ferrous sulfate to the cement. As soon as the cement is hydrated, the ferrous sulfate reduces the water-soluble chromium VI to the less soluble chromium III. According to Christina Laskowski in her paper, "Ferrous Sulfate Heptahydrate (FeSO4)," 4 7H 2The maximum recommended dosage of ferrous sulfate is 0.5 wt.%. The addition of this amount of ferrous sulfate to a cement containing 21.8 ppm chromium VI reduced the amount of soluble chromium VI upon hydration to less than 0.1 ppm. The cement admixture requirements were more fully met, but the cement properties were affected by the ferrous sulfate. The addition of 0.5 wt.% ferrous sulfate retarded the initial setting times of slag and Portland cements, but accelerated the initial setting for the higher strength cement classes. The final setting times were delayed for all tested cement types except CEM I 52,5 R Portland cement. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] US Patent Application Publication No. 2004 / 0157181 [Patent Document 2] U.S. Patent No. 6,491,751 [Patent Document 3] China Patent Application Publication No. 107540250 [Patent Document 4] European Patent No. 0739318 [Patent Document 5] European Patent No. 1055647 [Non-patent literature]

[0015] [Non-Patent Document 1] Iron(II)-sulfate as concrete admixture for chromium (VI) reduction [Non-Patent Document 2] "Methods of testing cement" - Part 10: "Determination of the water-soluble chromium (VI) content of cement" Summary of the Invention [Problem to be solved by the invention]

[0016] The object of the present invention is to provide a new method for producing cement clinker that can use stainless steel slag as one of the raw materials, which allows to reduce the chromium content of the cement clinker without the need to use a higher amount of reducing agent with the same amount of stainless steel slag material fed to the cement kiln. [Means for solving the problem]

[0017] Therefore, the method according to the invention comprises: providing a stainless steel slag comprising trivalent chromium and a non-metallic slag phase, the stainless steel slag having been solidified starting from a liquid stainless steel slag to produce a solidified stainless steel slag comprising slag particles having a sieve size smaller than 0.5 mm in an amount of less than 70% by weight, preferably less than 60% by weight, more preferably less than 50% by weight, and most preferably less than 40% by weight, based on the non-metallic slag phase, the non-metallic slag phase of the stainless steel slag comprising at least 40% by weight of a crystalline non-metallic slag phase; D less than 0.5mm 90 A powdered steel slag fraction having a particle size distribution with a sieve size value of greater than 5.0 mm, preferably greater than 6.0 mm, more preferably greater than 8.0 mm is obtained from the solidified stainless steel slag and / or 90 separating from a particulate material having a first particle size distribution having a sieve size value and produced by reducing the particle size of the solidified stainless steel slag; using said powdered steel slag fraction as said steel slag material for producing said cement clinker; The present invention is characterized by comprising:

[0018] According to the present invention, it has been found that the powdered steel slag fraction of solidified stainless steel slag or coarsely crushed solidified stainless steel slag contains significantly less chromium than the coarser sand and aggregate fractions of the solidified stainless steel slag. When the solidified stainless steel slag is crushed and pulverized into a powdered material in its entirety, the powder has a certain overall chromium content equal to the average chromium content of the stainless steel slag. However, according to the present invention, the powdered steel slag fraction can be separated from the stainless steel slag in the solidified state and / or after reducing the grain size of the solidified stainless steel slag, for example by crushing, and still be sufficiently coarse, in particular with a D of greater than 5.0 mm. 90 It is necessary to produce a granular material having a sieve size value. By separating the powdered steel slag fraction from the solidified stainless steel slag and / or from the coarsely crushed stainless steel slag, it is surprisingly found that the powdered steel slag fraction has a significantly lower chromium content than the remaining coarser fraction or fractions of the stainless steel slag. Furthermore, this powdered stainless steel slag fraction already has a small particle size by itself and therefore does not need to be pulverized for use as a raw material fed into a cement kiln.

[0019] When the stainless steel slag is solidified, the solidified slag must be cooled sufficiently quickly to avoid complete dusting of the steel slag, otherwise the fine stainless steel slag fraction will not have a reduced chromium content. The solidified stainless steel slag should contain less than 70% by weight of slag particles smaller than 0.5 mm, i.e. passing through a sieve with a square opening of 0.5 mm x 0.5 mm. This weight percentage should be determined based on the weight of the non-metallic slag phase, i.e. without including any steel contained in the stainless steel slag material. Preferably, the solidified stainless steel slag should contain even less of such small stainless steel slag particles, i.e. less than 60% by weight, preferably less than 50% by weight, more preferably less than 40% by weight, based on the weight of the non-metallic slag phase in the stainless steel slag and in the small particles, respectively.

[0020] Furthermore, the liquid slag should be solidified in such a way that the non-metallic slag phase contains at least 40% by weight of crystalline non-metallic slag phase. Therefore, the liquid slag cannot be solidified by quenching or granulation (with water) since this would result in a predominantly amorphous slag. Although this has yet to be proven, it appears that the formation of a crystalline phase is essential to achieve the chromium content difference between the powdered steel slag fraction and the coarser steel slag fraction.

[0021] The coarser fraction or fractions of the solidified stainless steel slag can be used as fine aggregate (sand) or coarse aggregate for producing concrete or asphalt, where leaching of the chromium contained in the stainless steel slag aggregate is prevented. Since the powdered steel slag fraction has a lower chromium content, it can be used in larger quantities as raw material for producing cement clinker, in order to limit the carbon dioxide emissions and energy requirements of the clinker production process. Preferably, the raw material used for the production of cement clinker contains at least 1% by weight, more preferably at least 2% by weight, of said steel slag material. Due to the small particle size of the powdered stainless steel slag fraction, it can be fed at any desired location in the rotary kiln, in particular at the feed location at the hot end or in the middle of the kiln. In the case of a wet process, the powdered slag material is preferably fed to the rotary kiln at one or more of these locations, otherwise it would absorb too much water from the wet paste. For dry process kilns equipped with a preheater, the powdered slag material can also be fed into the rotary kiln at a feed location in the center of the kiln or at the heated end, or at the feed end, but the powdered slag material is preferably mixed with the raw materials or fed to the preheater along with other components of the raw materials so that the powdered slag material is also preheated by the gases exiting the kiln.

[0022] In an embodiment of the method according to the invention, the method comprises the steps of reducing the particle size of the solidified stainless steel slag to produce the granular material and separating at least a portion of the powdered steel slag fraction from the granular material, preferably before any carbonation of the non-metallic slag phase contained therein or before the non-metallic slag phase contained in said portion of the powdered steel slag fraction absorbs up to 3.0 wt. %, preferably up to 2.0 wt. %, more preferably up to 1.0 wt. % carbon dioxide due to carbonation of the non-metallic slag phase.

[0023] The advantage of this embodiment is that the granular material from which the powdered steel slag fraction has been removed is a valuable aggregate material for use in concrete or asphalt. Moreover, by reducing the grain size of the solidified stainless steel slag, more valuable stainless steel can be recovered from the solidified stainless steel slag. At the same time, more powdered steel slag fraction is released, which can be used as raw material for producing cement clinker in the method of the present invention. The non-metallic steel slag phase contained in the granular material from which the powdered steel slag fraction has been removed is preferably not carbonated or only slightly carbonated. This means that the non-metallic steel slag phase contained in the part of the powdered steel slag material separated from the granular material can absorb up to 3.0% by weight, preferably up to 2.0% by weight, more preferably up to 1.0% by weight of carbon dioxide due to carbonation of said non-metallic slag phase. The absorption of carbon dioxide may have taken place before and / or after the grain size reduction of the solidified stainless steel slag material to produce the granular material. The absorption of carbon dioxide may have taken place by reaction with carbon dioxide contained in the atmosphere, i.e. by natural weathering / aging. Carbonation is preferably avoided or limited prior to separating the powdered stainless steel slag fraction from the granular material, as it can cause fine steel slag particles to agglomerate to form larger particles, such that the fine steel slag particles with a lower chromium content can no longer be effectively separated from the coarser slag particles with a higher chromium content.

[0024] Slag particles having a sieve size smaller than 0.5 mm can remain in the solidified stainless steel slag used in the size reduction step to produce the granular material. In this way, the slag particles do not have to be treated separately from the fine particles produced in reducing the size of the solidified stainless steel slag. This embodiment is particularly advantageous when the solidified stainless steel slag contains only a limited amount of fine particles, for example less than 50% or less than 40% by weight.

[0025] Alternatively, a portion of the powdered steel slag fraction may already be separated from the solidified stainless steel slag before its particle size is reduced to produce the granular material. In this case, this portion of the powdered steel slag fraction is preferably separated from the solidified stainless steel slag before any carbonation of the non-metallic slag phase contained therein or before the non-metallic slag phase contained in the further portion of the powdered steel slag fraction absorbs up to 3.0% by weight, preferably up to 2.0% by weight, more preferably up to 1.0% by weight of carbon dioxide due to carbonation of the non-metallic slag phase. In this case, too, carbonation is preferably avoided or limited before separation of the powdered stainless steel slag fraction from the solidified stainless steel slag, since carbonation can cause fine steel slag particles to agglomerate to form larger particles, whereby the fine steel slag particles with a lower chromium content can no longer be efficiently separated from the coarser slag particles with a higher chromium content. This embodiment, in which a portion of the powdered steel slag fraction is separated from the already solidified stainless steel slag, is particularly advantageous when the solidified stainless steel slag contains a higher amount of fine particles, for example more than 50% or more than 60% by weight. A portion of the powdered steel slag fraction can already be removed from the solidified stainless steel slag before the latter is further crushed and subjected to screening and steel recovery operations. Thus, more material can be processed using the same equipment. Moreover, since the powdered stainless steel slag fraction thus recovered can be separated from the dried stainless steel slag, no further drying is necessary before introducing the powdered stainless steel slag fraction into the cement clinker plant or in the cement clinker plant itself.

[0026] In an embodiment of the method according to the invention or according to any of the above embodiments, the grain size of the solidified stainless steel slag is reduced by crushing the solidified stainless steel slag to produce the granular material having the first grain size distribution, which may in particular be produced by a crushing and sieving operation to re-crush oversized particles.

[0027] The term crushing means to squeeze or pound into small pieces. Crushing stainless steel slag reduces the particle size by breaking up the steel slag, while grinding steel slag reduces the steel slag particles to powder by friction. Compared to grinding, crushing exerts less friction on the particles, and therefore produces less fines by friction in the crushing operation. Much of the fines produced by the crushing operation are the result of the breakdown of weaker parts of the stainless steel slag particles. Just like the fines of the solidified stainless steel slag itself, these weaker parts are stronger and have been found to contain less chromium than the larger stainless steel particles. The weaker parts may be due to them being partially broken down by the transformation of β-dicalcium silicate to γ-dicalcium silicate, which leads to the expansion of the dicalcium silicate phase and the falling or disintegration of the slag, rendering the slag unusable as an aggregate. The inventors have found that this fallen fraction of the slag contains substantially less chromium than the strong aggregate fraction of the stainless steel slag and can therefore be used in greater quantities in the production of cement clinker. A portion of the fallen dust fraction of the stainless steel slag is already present in the stainless steel slag in the solidified state, but a further portion is produced or released when reducing the particle size of the solidified stainless steel slag, in particular by crushing it. A portion of the powdered steel slag fraction can already be separated from the stainless steel slag in the solidified state, and a further portion from the crushed stainless steel slag or solidified stainless steel slag can first be crushed to produce a granular material containing the fines already present in the stainless steel slag in the solidified state, and then the powdered steel slag fraction can be separated from this granular material. Both parts of the powdered steel slag fraction are preferably either non-carbonated or contain up to 3.0% by weight, preferably up to 2.0% by weight, more preferably up to 1.0% by weight, of carbon dioxide in the form of carbonates, based on these non-metallic slag phases.The amount of carbon dioxide can be determined by TGA analysis in which the powdered steel slag fraction is first heated to 110°C to remove any moisture and determine the dry weight, the powdered steel slag fraction is further heated to 550°C to remove hydroxides and then to 950°C to decompose carbonates and remove the carbon dioxide produced. The percentage of carbon dioxide absorption is the amount of carbon dioxide divided by the dry weight of the steel slag fraction multiplied by one hundred. Preferably, the particle size of the solidified stainless steel slag is reduced to produce said granular material by crushing the solidified stainless steel slag in at least two successive steps, a first crushing step to produce a coarser granular material, a portion of the coarser granular material being removed from the coarser granular material and a second crushing step to produce a further portion of the granular material.

[0028] The advantage of this preferred embodiment is that it produces less fines, especially those with higher chromium content, and also less fine aggregate (sand) which is less valuable than the coarser aggregates.

[0029] Preferably, after the first crushing step, the first stainless steel rich fraction is separated from said coarser particulate material and then said portion of the particulate material is removed from the coarser particulate material.

[0030] Stainless steel is a valuable material that can be reused in the stainless steel production process, especially in the EAF (Electric Arc Furnace).

[0031] In an embodiment of the method according to the invention or according to any of the above embodiments, the granular material is sieved to a size D of less than 4 mm, preferably less than 3 mm, more preferably less than 2 mm. 90 At least a portion of the powdered steel slag fraction is separated from the fine fraction by dividing the powdered steel slag fraction into at least two different fractions, the fine fraction having a sieve size value and a coarser fraction having a sieve size value that is coarser than the fine fraction, the coarser fraction may contain more than 5000 mg / kg chromium, particularly more than 5500 mg / kg chromium, more particularly more than 6000 mg / kg chromium.

[0032] The fine fraction contains a relatively large amount of the powdered steel slag fraction, which remains when the sand fraction is separated from the fine fraction.

[0033] Preferably, the second stainless steel rich fraction is separated from the coarser fraction.

[0034] The more uniform size of the particles contained in the coarser fraction allows additional stainless steel to be recovered from this fraction, particularly by gravity separation techniques such as jigs.

[0035] At least a portion of said fine fraction is preferably mixed with water to produce an aqueous mixture, the aqueous mixture is separated into a sand fraction and an aqueous dispersion, in particular by a dewatering classifier such as a dewatering screw, and at least a portion of said powdered steel slag fraction is removed from said aqueous dispersion, in particular filtered from said aqueous dispersion. Preferably, at least a portion of said powdered steel slag fraction is filtered from said aqueous dispersion leaving a filtrate, the filtrate is stored in at least one reservoir, a precipitate comprising stainless steel slag particles is produced in the reservoir, and at least a portion of said precipitate is removed from the reservoir to produce a further portion of said powdered steel slag fraction.

[0036] Prior to separating the aqueous mixture into the sand fraction and the aqueous dispersion, a third stainless steel rich fraction is preferably separated from the aqueous mixture by gravity separation techniques, the third stainless steel rich fraction is crushed, preferably by a ball mill crusher, stainless steel is recovered from the crushed third stainless steel rich fraction, and the remaining crushed slag material is recycled to the aqueous mixture.

[0037] The more uniform size of the particles contained in the fine fraction allows additional stainless steel to be recovered from the fine fraction, particularly by gravity separation techniques such as a hydrosizer. The small size of the particles in the stainless steel rich fraction results in a relatively large amount of slag material still adhering to the stainless steel particles. In this embodiment, the stainless steel particles are washed by subjecting them to a crushing operation, preferably in a ball mill crusher.

[0038] In an embodiment of the method according to the invention or according to any of the above embodiments, the non-metallic slag phase of said powdered steel slag fraction comprises less than 5000 mg / kg chromium, preferably less than 4500 mg / kg, more preferably less than 4000 mg / kg chromium. Since the powdered steel slag fraction is separated from the stainless steel slag, said powdered steel slag fraction comprises chromium, in particular more than 1000 mg / kg chromium.

[0039] The lower chromium content allows for more powdered stainless steel slag material to be used as a raw material for making cement clinker without the need to add more reducing agent to the cement, or the lower chromium content allows for less reducing agent, such as ferrous sulfate, to be added to the cement to reduce its hexavalent chromium content.

[0040] In an embodiment of the method according to the invention or according to any of the above embodiments, the non-metallic slag phase of the solidified stainless steel slag contains more than 5000 mg / kg, in particular more than 5500 mg / kg, more in particular more than 6000 mg / kg chromium.

[0041] Thus, solidified stainless steel slag is a secondary material produced in the production of stainless steels, particularly stainless steels containing at least 10.0 wt. % chromium.

[0042] In an embodiment of the method according to the invention or according to any of the above embodiments, the powdered steel slag fraction contains at least 3% by weight, preferably at least 5% by weight, more preferably at least 7% by weight of γ-dicalcium silicate (γ-C 2 The steel slag contains a crystalline non-metallic steel slag phase consisting of S or calico-olivine.

[0043] In the method according to the invention, the powdered steel slag fraction is not a finely divided fraction, but is separated from the solidified stainless steel slag and / or from a relatively coarse granular stainless steel slag material obtained by reducing the grain size of the solidified stainless steel slag. The powdered steel slag fraction is therefore a fraction of β-C 2 S to γ-C 2 The powdered material contains a large amount of particles produced by the decomposition of steel slag as a result of its conversion to S. Thus, the powdered material has a higher γ-C content than the coarser fraction of steel slag. 2 S content. Thus, the mineral composition of the powdered material and also its basicity are different from those of the remaining coarser parts of the stainless steel slag material, which may be a possible explanation for the lower chromium content of the powdered material.

[0044] The present invention also relates to a method for producing a cement clinker, comprising calcining and combusting raw materials comprising at least one steel slag material to produce said cement clinker, and using the obtained powdered steel slag fraction as defined herein above as said steel slag material.

[0045] Since the powdered stainless steel slag fraction has been obtained by the process steps defined herein above, said powdered stainless steel slag fraction has a reduced chromium content compared to the other stainless steel slag fraction or fractions, and therefore a larger amount of this powdered stainless steel slag fraction can be used to limit the carbon dioxide emissions and energy requirements of the clinker production process.

[0046] The powdered stainless steel slag fraction can be used as freshly produced, but it is also possible that the powdered stainless steel slag fraction is initially stored, for example in a pile in the open. During storage, the powdered stainless steel slag fraction can be carbonated to some extent by natural carbonation. This has only a limited impact on the carbon dioxide emissions of the cement kiln, since the amount of carbonate thus formed is only limited. The carbonation of the powdered stainless steel slag fraction can cause the particles of the powdered stainless steel slag fraction to agglomerate. However, in the cement kiln, the particles fall back apart quickly during the firing process. It is also possible to grind the agglomerated stainless steel slag fraction, and since the grains or chunks of the agglomerated stainless steel slag fraction are not difficult to grind, the fraction does not cause much wear on the raw material mill. If the powdered stainless steel slag fraction is to be added to the paste of a wet process cement kiln, the fraction can be carbonated, preferably after granulation into larger grains, to reduce its water absorption. In this way, the water requirement of the wet paste is not increased or is increased to a lesser extent by the addition of the carbonated powdered stainless steel slag fraction.

[0047] Preferably, the steel slag material is a stainless steel slag material, the non-metallic slag phase of which contains less than 5000 mg / kg chromium, preferably less than 4500 mg / kg, more preferably less than 4000 mg / kg chromium, and the stainless steel slag material contains a crystalline non-metallic steel slag phase of at least 3 wt.%, preferably at least 5 wt.%, more preferably at least 7 wt.% gamma dicalcium silicate.

[0048] The present invention also relates to the use of the powdered steel slag fraction obtained as defined herein above as a raw material for producing cement clinker.

[0049] Other advantages and details of the invention will become apparent from the following detailed description of some particular embodiments of the method for producing cement clinker according to the invention, which is given by way of example only and is not intended to limit the scope of the invention. The reference signs used in the detailed description relate to the attached drawings. [Brief description of the drawings]

[0050] [Figure 1] FIG. 1 is a schematic diagram of an in-line calciner kiln. [Figure 2A] FIG. 2 is one of two parts of a flow diagram of a particular embodiment of a method for producing a powdered stainless steel slag fraction that can be used in a method for producing cement clinker according to the present invention. [Figure 2B] FIG. 2 is one of two parts of a flow diagram of a particular embodiment of a method for producing a powdered stainless steel slag fraction that can be used in a method for producing cement clinker according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] In this description and claims, unless otherwise stated, percentages or percentages by weight refer to percentages by dry weight.

[0052] The term "slag phase" refers to a non-metallic slag phase, which may be amorphous or crystalline. Thus, the term "slag phase" does not encompass a steel phase or steel particles. Such steel particles may consist of separate steel particles that may be embedded in the non-metallic slag phase or may be coated or covered by the slag phase.

[0053] The chromium content is Cr 2 O 3Unless expressly indicated to relate to the amount in mg Cr / kg dry weight, in particular mg / kg of dry weight of non-metallic slag phases. Metallic phases / particles are therefore first removed from the steel slag. The amount of chromium is determined using the acid HNO 3 as described in the Compendium on the Sampling and Analysis of Waste and Soil Samples CMA / 2 / II / A.3 and CMA / 2 / I / B.1 (both published in the Belgian Official Journal / Moniteur belge on 11 January 2021). 3 , HCl, and HBF 4 It is determined by ICP after complete decomposition of the non-metallic slag phase with a mixture of ICP and ICP-OES.

[0054] The present invention relates to a method for producing cement clinker, in particular Portland cement clinker. After the production and cooling of the cement clinker, a certain amount of gypsum (CaSO 4 2H 2 O or CSH 2 ) is added to the cement clinker and the mixture is finely ground. The cement clinker thus obtained generally contains 25% to 50% by weight of C. 3 S, 20% to 45% by weight of C 2 S, 5% to 12% by weight of C 3 A, 6% to 12% by weight of C 4 AF and 2wt% to 10wt% CŜH 2 The cement clinker mixed with the gypsum preferably contains less than 5% by weight MgO.

[0055] After mining, crushing, grinding, and homogenization of the raw materials, the first step in cement production is the calcination of calcium carbonate, followed by reaction of the resulting calcium oxide with silica, alumina, and ferrous oxide at high temperatures to form clinker. The clinker is then crushed or ground with gypsum and other ingredients to produce cement. Natural calcareous deposits such as limestone, marl, or chalk are the source of calcium carbonate. Silica, iron oxide, and alumina are found in various ores and minerals. Several types of waste materials can also be used as partial replacements for natural raw materials.

[0056] The cement industry is an energy intensive industry, with energy typically accounting for around 40% of production costs. A variety of traditional fossil and waste-derived fuels can be used to supply the thermal energy demand required for the process. Essentially, the nature of the clinker combustion process itself allows for the use of waste materials as raw materials and / or fuels. Clinker combustion takes place in rotary kilns that may be part of a wet or dry long kiln system, a semi-wet or semi-dry grate preheater (Lepol) kiln system, a dry suspension preheater kiln system, or a preheater / calciner kiln system.

[0057] In the method according to the invention, at least one powdered stainless steel slag material is used as one of the raw materials for producing cement clinker. In fact, common steel slags such as LD slag are already used in the production of cement clinker. However, a drawback of LD slag is its high free lime content. Since the free lime in LD slag is completely burned, the free lime does not melt or dissolve easily in the kiln and therefore may end up in the cement clinker unreacted. The free lime in the cement clinker needs to be closely monitored to ensure the quality of the cement. Excessive free lime indeed leads to undesirable effects such as volume expansion, increased setting time or reduced strength. LD slag also needs to be finely ground because the slag contains silicates that have high hardness and are difficult to grind, which increases the grinding costs.

[0058] Compared to common steel slag, stainless steel slag has a lower free lime content. Stainless steel slag is produced in furnaces used to produce stainless steel, i.e. steel containing iron and at least chromium, in particular steel containing at least 10% by weight chromium. In many cases, stainless steel slag also contains nickel and / or molybdenum. Stainless steel slag is produced in particular in EAF (electric arc furnace), AOD (argon oxygen decarburization) furnace, VOD (vacuum oxygen decarburization) furnace, ladle (ladle slag) or tundish (used for continuous casting of molten steel). Due to its high heavy metal content, stainless steel slag fines is a much more problematic waste, since common steel slag fines can be used, for example, as a soil conditioner. Also, due to its high water absorption, stainless steel slag fines cannot be used in asphalt or concrete. Therefore, there is a need to find new applications for stainless steel slag fines that would allow them to be used as a secondary raw material.

[0059] In the method according to the invention, the powdered stainless steel slag material is used as a secondary raw material for producing cement clinker. The cement clinker is preferably produced in a dry process rotary kiln, since dry process kilns require less energy and therefore produce less carbon dioxide compared to wet or semi-wet process kilns. The kiln is preferably equipped with a preheater, in which the raw materials are heated by heat recycled from the kiln or from the clinker cooler. The preheater may be a preheater / calciner, if part of the fuel is burned in this preheater / calciner.

[0060] In practice, there are various types of cement clinker installations, including calciner units, namely In-Line Calciners (ILC) and Separate Line Calciners (SLC). More specifically, the kilns may be so-called preheater kilns, air through calciners, in-line calciners, in-line calciners with combustion chambers, and air separate calciners. Such installations are well known to those skilled in the art, so in-line calciners are briefly described below, purely as an example.

[0061] As shown diagrammatically in FIG. 1, the in-line calciner comprises a number of cyclones, in this case five cyclones 1-5 connected by four riser ducts 6-9, and a calciner unit 10, which together form a heat-exchange cyclone tower 11. An induced draft fan (ID fan) in the exhaust pipe 12 creates an upward gas flow in the cyclone tower 11. Raw clinker meal is typically introduced in the riser duct 6 between the second cyclone 2 and the first cyclone 1. After passing through the cyclone tower 11, the heated and at least partially calcined (decarbonated) raw clinker meal is directed through a kiln feed pipe 13 to the feed end 14 of a rotary drum kiln 15. The raw clinker meal passes through the calciner unit 10, in which temperatures of 870° C.-900° C. are created, so that the calcination degree of the material introduced into the rotary drum kiln 15 is 90%-95%. The material is then forced through the interior of the kiln 15 by a combination of the kiln's tilt and its rotational movement. The rotating drum kiln 15 has a burner at its end through which air is fed into the kiln, as shown by arrow 19, creating a flame. As the material approaches the flame, its temperature increases and clinkerisation occurs. On exiting the kiln the material is discharged onto a grate cooler 16 where it is cooled by heat exchange with secondary air to produce cement clinker.

[0062] To heat the calciner unit 10 and generate an ascending gas flow therein, the combustion gases generated by the combustion in the rotary drum kiln 15, i.e., kiln gases having a temperature of about 1000° C., are led to the bottom of the calciner unit 10 through the kiln riser duct 17. Further, tertiary air heated to a temperature of about 750° C. to about 900° C. in the grate-type cooler 16 is further led to the calciner unit 10 through the tertiary air duct 18. This tertiary air enters the calciner unit 10 tangentially, generating a gentle vortex, ensuring effective mixing of the fuel, raw materials and gases. The temperature in the calciner unit 10 is further controlled by the burners to which the fuel and primary air are supplied. Thus, the air for fuel combustion in the calciner unit consists of the kiln gases, the tertiary air from the cooler, and the fuel-carrying / atomizing air, which is supplied through the calciner burners in the calciner unit as indicated by the arrows 20. About 55% to about 60% of the fuel is supplied to the calciner burners, and the remaining amount of fuel is supplied to the kiln burners.

[0063] The powdered stainless steel slag fraction used in the method according to the invention can be added to the raw material and therefore also fed into the heat exchange cyclone tower 11 via the riser duct 6. However, it is also possible to introduce the powdered stainless steel slag fraction directly into the rotary kiln 15 via its feed end 14, since the powdered stainless steel slag fraction does not need to be calcined, since it contains no or only small amounts of carbonates.

[0064] In the case of a wet process kiln, the raw materials are fed into the rotary kiln in the form of a wet paste at the feed end. The powdered stainless steel slag fraction can also be introduced into the rotary kiln at this feed end. Since the powdered stainless steel slag fraction used in the method according to the invention has a very high water absorption, it is preferably not necessary to add the powdered stainless steel slag fraction to the wet paste, since the powdered stainless steel slag fraction increases the amount of water required to produce the wet paste. In the case of a wet process kiln, the powdered stainless steel slag fraction is preferably introduced into the kiln at a feed position in the center of the kiln, located in the firing zone. A burner is provided at this position, through which additional fuel is introduced into the kiln. Since the powdered stainless steel slag fraction has a relatively low melting temperature, it melts rapidly at this position. It is therefore possible to prevent the powdered stainless steel slag fraction from leaving the kiln via the flow of combustion gases leaving the kiln. Alternatively or additionally, the powdered stainless steel slag fraction may be introduced into the rotary kiln at its combustion end, so that the stainless steel slag has less time to react and mix with the other constituent elements of the cement clinker.

[0065] In the method according to the invention, the powdered stainless steel slag fraction used as raw material for producing cement clinker is produced in such a way that it has a lower chromium content than the steel slag from which it is produced and in such a way that it does not require an additional milling step. The powdered stainless steel slag fraction is produced starting from air-cooled stainless steel slag which, apart from any stainless steel particles contained therein, contains a crystalline steel slag phase which contains a chromium-containing phase and usually also an amorphous steel slag phase. The powdered stainless steel slag fraction is not obtained by milling solidified stainless steel slag and the powdered stainless steel slag fraction has a D of less than 0.5 mm. 90 The solidified stainless steel slag is separated to have a particle size distribution of sieve size values.

[0066] D 90 The sieve size value is the particle size at 90% by volume cumulative passing. The particle size distribution can be tested using a sieve with square openings according to ASTM D6913 / D6913M-17. The volume percentage of a particle can be calculated, inter alia, by dividing the weight of the particle passing through the sieve by the average density of the material forming the particle.

[0067] The air-cooled stainless steel slag is produced by pouring liquid stainless steel slag into a slag yard and allowing it to solidify. The cooling of the solidified stainless steel slag is preferably accelerated, in particular by spraying water onto the solidified steel slag. Compared to the granulation method, such an air-cooling method results in a slower solidification of the liquid steel slag. In this way, the solidified steel slag is not completely amorphous but contains a crystalline slag phase, usually, but not necessarily, contains an amorphous slag phase. The non-metallic phase of the solidified stainless steel slag should contain at least 40% by weight, preferably at least 50% by weight, of the crystalline non-metallic slag phase, i.e., the mineral phase. The non-metallic phase of the solidified stainless steel slag may further contain an amorphous non-metallic slag phase, in particular at least 20% by weight, more particularly at least 25% by weight, of the amorphous non-metallic slag phase.

[0068] A portion of the powdered stainless steel material used in the manufacture of cement clinker may be separated from the solidified stainless steel slag, which in fact already contains a certain amount of particles having a sieve size smaller than 0.5 mm itself. After removing these smaller particles or removing a portion of them, or without removing such smaller particles from the solidified stainless steel slag, the grain size of the solidified stainless steel slag is preferably reduced to a size still larger than D 5.0 ​​mm. 90 A particulate material is produced having a first particle size distribution having sieve size values.

[0069] The cooling of the steel slag should be controlled so that the amount of particles with a sieve size smaller than 0.5 mm, i.e. the amount of fines, is less than 70% by weight, preferably less than 60% by weight, more preferably less than 50% by weight, most preferably less than 40% by weight, based on the non-metallic slag phases. In other words, the amount of non-metallic slag phases contained in these slag fines should be less than 70% by weight of the total amount of non-metallic slag phases in the total amount of solidified steel slag. This amount depends not only on the chemical structure of the slag, but also in particular on the cooling regime of the liquid steel slag. In order to keep the amount of fines below these maximum values, for example the techniques disclosed in EP 3122909 can be applied.

[0070] A preferred embodiment of the process for producing a powdered steel slag fraction of stainless steel slag that can be used in the production of cement clinker and has a reduced chromium content is shown in Figure 2. Liquid stainless steel slag from the various ladles and furnaces of the stainless steel production plant is poured into a cooling pit in the slag yard, where the liquid stainless steel slag is air cooled, i.e. slowly solidified and cooled. During the cooling process, water may be sprayed onto the solidified slag in order to accelerate the cooling process somewhat, especially at the end. Since the solidification is relatively slow, the slag does not solidify almost completely into the amorphous phase, but instead solidifies mostly into the crystalline phase. One of the mineral phases of stainless steel slag is dicalcium silicate (C 2 As crystalline dicalcium silicate cools, it undergoes several polymorphic phases: α, which has a hexagonal crystal structure; αH', which has an orthorhombic crystal structure; αL' having an orthorhombic crystal structure, β having a monoclinic crystal structure, γ, which has an orthorhombic crystal structure.

[0071] In pure dicalcium silicate under laboratory conditions, the transformation from αL'-dicalcium silicate to β-dicalcium silicate occurs at 675°C, followed by the transformation from β-dicalcium silicate to γ-dicalcium silicate at 490°C. The transformation from β-dicalcium silicate to γ-dicalcium silicate is accompanied by a 12% increase in volume due to its different crystal structure, which leads to the destruction of the dicalcium silicate phase. This leads to the pulverization of a fraction of the slag, producing fine powders. This transformation also leads to microcracks in the pulverized particles, which may explain why the fine powders thus produced are capable of absorbing and retaining large amounts of water. This water absorption property makes these fine powders very unsuitable for most uses in construction. However, according to the present invention, it has been found that these fine powders are suitable for use as raw material in the production of cement clinker. These fine powders in fact have a lower chromium content than the coarser fractions of stainless steel slags, and it has been found that these fine powders do not need to be pulverized because they already have a sufficiently small particle size. In order to limit the generation of fines during the cooling process, the cooling process can be accelerated, especially by spraying water onto the solidified steel slag, so that the steel slag can be cooled more rapidly, especially from temperatures above 500° C., and there can be less conversion of β-dicalcium silicate to γ-dicalcium silicate.

[0072] In the process shown in Figure 2, solidified stainless steel slag 31 is stored in a slag bunker 32. The steel slag 31 is fed into a hopper 33 which contains a grate to stop any oversized slag pieces 34, in this particular case larger than 300mm. As the oversized pieces could damage the crusher used later in the process, these oversized pieces 34 are removed for later special processing, e.g. breaking with a hammer and removing the large metal pieces, before feeding again through the hopper 33.

[0073] Slag particles 35 smaller than 300 mm drop through a hopper 34 onto a conveyor belt and are sent to a first metal sorting cabin 36 where an operator removes large metal pieces 37 from the slag particles 35 on the conveyor belt. In a next step, not shown in FIG. 2A, the steel slag particles 35 can be sieved to remove fines having a particle size of 0.5 mm or less, or a part of them. These fines are dry or relatively dry, so they do not need to be dried before they can be fed into the cement clinker plant. 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 sent to a first sieve 42 along a first metal separation magnetic belt 40 which removes metal particles 41 from the crushed steel slag particles 39. The slag particles 39 then pass through a first sieve 42 which separates the slag material 39 into three fractions: particles larger than 35mm 43, particles between 10mm and 35mm 44, and particles smaller than 10mm 45. The fraction of particles larger than 35mm 43 is sent by a second conveyor belt to a second metal sorting cabin 46 where more metal pieces 47 are removed. The particles larger than 35mm 43 are then returned to the first crusher 38. The fraction of particles between 10mm and 35mm 44 enters a second crusher 48 which produces further crushed steel slag material 49 which is fed to a second sieve 50. The crushed steel slag material 49 is further separated in a second sieve 50 into three fractions, a fraction of particles larger than 20 mm 51, a fraction of particles smaller than 10 mm 52, and a fraction of particles between 10 mm and 20 mm 53. The fraction of larger particles between 10 mm and 20 mm 53 is passed by a third conveyor belt through a second metal separating magnetic belt 54 where more metal 55 is removed and returned to the second crusher 48. The fraction 51 having particles larger than 20 mm passes through a third metal separating magnetic belt 56 where more metal 57 is removed and stored in a box 58. This fraction 51 is a valuable coarse aggregate used 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 mm to 10 mm granular material 59, which is stored in a bunker 60.

[0074] As shown in FIG. 2B, the granular slag material 59 is fed to a third sieve 61 which is a 2 mm sieve that separates the 0 mm to 10 mm stainless steel slag granular material 59 particles into a fine fraction 62 of particles smaller than 2 mm and a coarser fraction 63 of particles between 2 mm and 10 mm.

[0075] The coarser fraction 63 is fed to a wet jigging device or jig 64 where metal particles 65 are removed from the coarser fraction 63 between 2 mm and 10 mm. 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 where the stainless steel slag particles are subjected to a jigging action so that gravity separates the heavier metal particles 65 from the lighter slag particles 66. The lighter slag particle fraction 66 passes through a fourth metal separation 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 finer aggregate valuable for use in building and road construction. The metal-rich fraction 68 is milled to recover the metal particles it contains. The remaining milled demetallized fraction is a filler fraction that can be used, for example, as a filler in concrete or asphalt.

[0076] The third sieve 61 is preferably a wet sieve that is fed with water so that the fine fraction 62 is mixed with water to produce an aqueous mixture 70. This aqueous mixture is also produced using water 71 from a jig 64 that contains fine slag material that has been washed away from the coarser slag fraction 63.

[0077] The aqueous mixture 70 is fed to a hydrosizer 72. The operating principle of the hydrosizer 72 is also described in the article "Gravity Separation: Old Techniques / New Methods" by Andrew Falconer. In the hydrosizer 72, a stainless steel rich fraction 73 is separated from the aqueous mixture 70. The stainless steel rich fraction 73 is fed to a first gravity separation screw 74, from which the overflow 75 is added back to the aqueous mixture 70 and the underflow 76 is fed to a bowl mill crusher 77, where the slag material adhering to the stainless steel particles is removed from the slag material. The crushed material 78 leaving the bowl mill crusher 77 is fed to a second gravity separation screw 79. The overflow 80 of this second gravity separation screw 79 is added back to the aqueous mixture 70 and the underflow is the valuable fine stainless steel fraction 81.

[0078] The aqueous mixture 70, including the slag fractions 75 and 80 removed from the stainless steel rich fraction 73, is fed to a dewatering classifier, in particular a dewatering screw 82, where as underflow a sand fraction 83, in particular a sand fraction between 0.5 mm and 2 mm, is removed from the aqueous mixture 70. This sand fraction 83 can be used as fine aggregate to produce concrete or asphalt. The overflow leaving the dewatering screw 82 is an aqueous dispersion 84 of fine slag particles in water.

[0079] The aqueous dispersion 84 is fed to a hydrocyclone 85 for removing fine slag particles from the aqueous dispersion 84. 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 a disc filter 87 to remove fine slag particles forming a powdered 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 to a water reservoir 92 and the underflow 93 is treated using the disc filter 87. Thus, further fine slag particles removed by the thickener 90 also end up in the powdered steel slag fraction 88. The filtrate 94 from the disc filter 87 is also fed to the reservoir 92.

[0080] The purified water contained in the reservoir 92 can be reused to water the wet sieve 61 and the jig 64. In the reservoir 92, additional fine stainless steel slag material settles to the bottom forming a sediment 95. From time to time, this sediment can be removed from the reservoir 92 and added to the powdered stainless steel slag fraction 88.

[0081] Experimental Data The following experimental data shows that the powdered stainless steel slag fraction 88 produced in the method described with reference to Figures 2A and 2B is suitable for use as a raw material in the production of cement clinker.

[0082] average particle size First, the average particle size distribution was measured for a large number of samples of a portion of the powdered stainless steel slag fraction 88 produced by the disc filter 87. The results are shown in Table 1.

[0083] [Table 1]

[0084] Based on the average particle size distribution of these samples, D 90 The sieve size value is approximately equal to 0.18 mm, and D for the sample with the smallest particle size (largest passing) 90 The sieve size value is approximately equal to 0.125 mm, and for the sample with the largest particle size (smallest passing) D 90 The sieve size value is equal to about 0.4 mm. The powdered steel slag fraction 88 is therefore composed of small particles and does not require any further comminution in order to melt easily in the cement kiln. Moreover, the particle size is so small that in the case of a dry process it can even be added directly to the raw material fed to the preheater / calciner.

[0085] chemical composition As an example, a sample of the powdered stainless steel slag fraction 88 produced by the disk filter 87 and a sample of the precipitate 95 obtained from the reservoir 92 were treated with a mixture of acids, in particular HNO 3 , HCl and HBF 4 After complete digestion using a combination of the following, the mixture was analyzed by ICP. The following results were obtained:

[0086] [Table 2]

[0087] The powdered steel slag fraction produced by the disc filter further contained, in mg / kg dry weight, <120 Br, 17200 F, 220 Cl, <200 K, 2560 Na, 6090 Ti, 15 Cu, 245 Ni, 17 Zn, 215 Ba, 9.8 Co, 8980 Mn, 60.2 Mo, 11.2 Se, 152 V, 1860 S and 1380 B. The precipitate further contained less than 110 Br, 15,000 F, 310 Cl, 370 K, 4,440 Na, 5,010 Ti, 15 Cu, 161 Ni, 26 Zn, 236 Ba, 7.3 Co, 5,820 Mn, 38.4 Mo, less than 10.0 Se, 119 V, and 1,710 S.

[0088] Both materials have high calcium and silicon contents, and it can be seen that the calcium to silicon ratio is similar to the calcium / silicon ratio in cement clinker. Aluminum is also a required element in cement clinker. Chromium is undesirable and the amount of soluble chromium after hydration of the cement produced from the cement clinker must be kept below 2 ppm. This can be achieved by limiting the amount of powdered stainless steel slag fraction used as a raw material for the production of the cement clinker, as well as adding a sufficient amount of a reducing agent to the cement. Non-limiting examples of suitable reducing agents include ferrous sulfate (FeSO 4 ), especially ferrous sulfate heptahydrate (FeSO 4 7H 2 O), stannous chloride (SnCl 2 ), stannous sulfate (SnSO 4 ), stannous oxide (SnO), stannous hydroxide (Sn(OH) 2 ), manganese tin sulfate, iron sulfide (FeS), and / or ferrous chloride (FeCl 2 ), especially ferrous chloride tetrahydrate (FeCl 2 4H 2 O), and combinations thereof. A preferred reducing agent is FeSO 4 7H 2 O, SnSO 4 , SnCl 2 , SnO, Sn(OH) 2 , and combinations thereof.

[0089] If not stored / aged, the piled powdered steel slag fraction collected by the disc filter contains only limited amounts of carbonates. Over a period of one year, the carbonate content of this fresh powdered steel slag fraction was measured by TGA. The fresh powdered steel slag fraction contained an average of 1.6 wt.% CO 2The precipitate 95 collected from the reservoir 92 contained a maximum value of 2.5% by weight and a minimum value of 0.4% by weight. These carbonates are formed during the processing of the stainless steel slag, but partly after the separation of the powdered steel slag fraction from the granular steel slag material. The precipitate collected from the reservoir was found to contain more carbonates. The water used in the plant is in fact very alkaline due to the contact with the stainless steel slag particles and therefore very easily absorbs carbon dioxide from the environment. This carbon dioxide precipitates in the form of fine calcium carbonate particles and is therefore found to be contained in the precipitate 95 collected from the reservoir 92 in addition to the stainless steel slag particles. In several samples, TGA analysis showed that CO 2 The amount of CO released was measured at 3% to 7% by weight. 2 %, which corresponds to a calcium carbonate content of about 7% to about 16% by weight. Since only about 4% to about 9% by weight of CaO is bound in these carbonates, most of the CaO is lost to CO when used in the production of cement clinker. 2 It is still contained in other compounds such as silicates that do not release the

[0090] The maximum amount of these powdered steel slag fractions that can be used as raw material for producing cement clinker is limited by the MgO content. In cement clinker, the MgO content must in fact be lower than 5% by weight. However, the maximum amount is still then about 30% by weight. In such amounts, Na 2 O eq , Cl, SO 3 Other parameters of the cement clinker, such as DoS (degree of sulfation), etc., are not limiting factors. The only limiting parameter is the chromium content.

[0091] Reduction in chromium content Over a period of 10 years, powdered stainless steel slag fraction 88 was produced by the method described hereinabove with reference to Figures 2A and 2B. However, during that period, precipitate 95 formed in the reservoir was not collected and therefore not added to powdered steel slag fraction 88.

[0092] The liquid stainless steel slag used to produce the solidified stainless steel slag originated from the EAF and AOD of a stainless steel production plant. The liquid slag was slowly solidified and then cooled more rapidly by spraying water on the solidified stainless steel slag. In this way, the production of fines in the slag pit was limited. These fines were also treated by the method shown in Figures 2A and 2B, thus resulting in a powdered stainless steel slag fraction 88. This powdered stainless steel slag fraction 88 was, on average, about 30% of the total amount of the coarse aggregate fraction 51 and the finer aggregate fraction 66 of the sand fraction 83 and the powdered stainless steel slag fraction 88. The amount of fines produced in the slag pit was not measured, but it can be estimated to be about 70% of the amount of the powdered stainless steel slag fraction 88, i.e., about 20% of the total amount of non-metallic slag phases contained in the solidified stainless steel slag 31.

[0093] The steel slag fraction was piled and stored in a box. The 2 mm-10 mm fraction 66 was sieved into two fractions, namely the 2 mm-6 mm aggregate fraction and the 6 mm-10 mm aggregate fraction. Table 3 shows the results of different analyses of the chromium content in the different fractions produced over a period of 10 years. The analyses were carried out immediately after the production of the different slag fractions and in different production years. Thus, these results cover the production of stainless steel slag over a period of 10 years. During these years, different types of stainless steel were produced by the steelworks. Furthermore, slag was produced in different furnaces, in particular EAF and AOD furnaces.

[0094] [Table 3]

[0095] According to these analyses, the fine (sand) and coarser aggregate fractions of stainless steel slag all have roughly the same chromium content, on average, while the powdered (0 mm - 0.5 mm) fraction has, on average, a much lower chromium content, i.e., only about half the chromium content of the aggregate fraction. Since the chromium content of stainless steel slag appears to be a limiting factor in the use of stainless steel slag as a raw material for producing cement clinker, by using the powdered stainless steel slag fraction instead of the aggregate fraction, approximately twice the amount of stainless steel slag can be used. Furthermore, the powdered stainless steel slag fraction melts more rapidly in the cement kiln and does not need to be finely ground to be suitable for feeding the preheater / calciner.

[0096] Several stainless steel slag fractions collected over several years were further analyzed by XRD analysis to analyze their crystalline phase. The various stainless steel slag fractions contained, on average, about 35% by weight of amorphous slag phase and about 65% by weight of crystalline phase. Some powdered stainless steel slag fractions had a somewhat higher content of amorphous phase, which resulted in less chromium, but this cannot explain the much lower chromium content of the powdered steel slag fractions.

[0097] However, the XRD results showed a large difference in the olivine (γ-dicalcium silicate) content. 2 The S content in the crystalline slag phase varied from 6.2% to 16.5%, with an average of 11.3%. 2 The S content in the crystalline slag phase varied from 1.6% to 3.4%, with an average of 2.6%. Chromium is not accumulated in the dicalcium silicate, but in the γ-C 2 The S content can be an indicator of the basicity of steel slag. In fact, the higher the basicity of steel slag, the higher the γ-C content. 2 Generally, chromium tends to form a spinel phase, i.e., MgO(Al 2 O 3 , Fe 2 O3 , Cr 2 O 3 ) phase. These spinel phases may contain more chromium the lower the basicity of the steel slag. This may be one of the reasons why the powdered stainless steel slag fraction has a significantly lower chromium content than the aggregate fraction.

[0098] Chromium Magnesium Content Determined by XRD Chromium-magnesium is a spinel phase with a high chromium content. Most of the chromium in stainless steel slags is contained in this chromium-magnesium phase. Over a period of one year, 88 freshly produced powdered steel slag fractions were analyzed for chromium-magnesium content approximately twice a week. In total, 120 samples were analyzed. The average chromium-magnesium content was equal to 3.21% of the crystalline slag phase, the minimum was equal to 1.1% and the maximum was equal to 5.4%.

[0099] Eight samples of the demetallized filler fraction obtained by fine grinding and demetallization of the metal-rich fraction 68 were further analyzed by XRD. The average chromium-magnesium content of these samples was equal to 10.00%, the minimum was equal to 9.2% and the maximum was equal to 12.1% of the crystalline slag phase. Thus, the chromium-magnesium content of this finely ground and demetallized 2 mm to 10 mm slag fraction was more than three times higher than the chromium-magnesium content of the powdered steel slag fraction 88.

[0100] γ-C 2 In terms of S content, the powdered stainless steel slag fraction 88 also had a much higher γ-C content than the demetallized filler fraction. 2 The S content, on average, was 8.01% compared to 1.17% for the filler fraction.

[0101] Estimated Cr(VI) content of cement clinker composition The powdered stainless steel slag fraction is free or substantially free of hexavalent chromium, i.e. Cr(VI). However, during the clinker formation process at high temperatures, some of the Cr(III) present in the powdered stainless steel slag fraction is converted to Cr(VI). The degree of oxidation depends on the cement kiln and the calcination / clinker formation process. This depends, for example, on the gases present in the kiln, in particular their oxygen content.

[0102] In the article "Removal of soluble Cr(VI) in Cements by Ferrous Sulfate Monohydrate, solid lignin and other Materials" by Emin Erdem et al., Ceramisc Slikaty, March 2011, three different cement types were tested: CEM I 42.5R with 190.4 mg / kg total chromium, CEM II / AP 42.5N with 125.2 mg / kg total chromium, and CEM IV / B (P) 32.5N with 56.3 mg / kg total chromium. Total chromium includes both Cr(III) and Cr(VI). Soluble chromium, i.e. Cr(VI), measured according to the STN EN 196-10 standard, must be lower than 2 ppm. For the three cement types, the soluble chromium content was 17 ppm, 11 ppm, and 4.5 ppm, respectively. To reduce the soluble chromium content to 2 ppm, it was necessary to add 0.16%, 0.08%, and 0.04% by weight of ferrous sulfate heptahydrate, respectively, amounts which are substantially directly proportional to the total chromium present in the cement.

[0103] Addition of 5 wt.% of powdered stainless steel slag fraction with a total Cr content of 3000 ppm increases the Cr content of the cement by about 150 ppm. The Cr content of CEM I 42.5R cement is therefore almost doubled, and therefore the soluble chromium content and the amount of reducing agent required are also almost doubled. It can therefore be estimated that a total amount of about 0.30 wt.% of ferrous sulfate heptahydrate is sufficient to maintain the soluble Cr(VI) content below 2 ppm as determined according to STN EN 196-10. This amount is well below the maximum recommended dosage of 0.5 wt.% mentioned by Christina Laskowski in her publication referenced herein above.

[0104] By replacing 5% of the calcium carbonate in the raw clinker feedstock with the powdered stainless steel slag fraction, it can be calculated that a reduction in carbon dioxide emissions caused by calcination of about 6% to about 7.5%, depending on the composition of the raw feedstock, can be achieved. In addition, energy savings of about 5% are achieved, which is accompanied by a further reduction in carbon dioxide emissions generated by the fuel.

[0105] A higher amount of powdered stainless steel slag is also possible by adding a larger amount of reducing agent or by, for example, selecting the source of the stainless steel slag. Some stainless steels contain, for example, only about 12% chromium, while others contain 17%-18% chromium, so that the slag floating on the molten stainless steel also contains less chromium. The chromium content of the stainless steel slag can also be reduced by, for example, adding metallic aluminum to the liquid slag floating on the liquid steel, which chemically reduces the chromium and reduces the chromium content of the slag. The addition of aluminum in stainless steel furnaces, as described in EP 3901289, can also reduce the formation of fines in the solidified steel slag. Furthermore, a higher amount of aluminum in the powdered stainless steel fraction increases the value of this fraction as a raw material for cement clinker production, since the cement clinker must also contain alumina.

Claims

1. 1. A method for producing cement clinker, comprising calcining and combusting raw materials including at least one steel slag material to produce the cement clinker, the method comprising: providing a stainless steel slag (31) containing trivalent chromium and a non-metallic slag phase, the stainless steel slag (31) having been solidified starting from a liquid stainless steel slag to produce a solidified stainless steel slag (31) containing slag particles having a sieve size of less than 0.5 mm in an amount of less than 70% by weight based on the non-metallic slag phase, the non-metallic slag phase of the stainless steel slag (31) comprising at least 40% by weight of a crystalline non-metallic slag phase; D less than 0.5 mm ９０ A powdered steel slag fraction (88) having a particle size distribution with a sieve size value of greater than 5.0 mm is obtained from the solidified stainless steel slag (31) and / or ９０ separating from the granular material (59) having a first particle size distribution with a sieve size value and produced by reducing the particle size of the solidified stainless steel slag (31); using the powdered steel slag fraction (88) as the steel slag material for producing the cement clinker; A method comprising:

2. 2. The method of claim 1, comprising reducing the particle size of the solidified stainless steel slag (31) to produce the granular material (59) and separating at least a portion of the powdered steel slag fraction (88) from the granular material (59).

3. 3. The method of claim 2, wherein the portion of the powdered steel slag fraction (88) is separated from the granular material (59) before any carbonation of the non-metallic slag phase contained therein or before the non-metallic slag phase contained in the portion of the powdered steel slag fraction (88) absorbs up to 3.0 wt. % carbon dioxide due to carbonation of the non-metallic slag phase.

4. 4. The method according to claim 2 or 3, characterized in that slag particles having a sieve size smaller than 0.5 mm remain in the solidified stainless steel slag (31) used to produce the granular material (59) in the size reduction step.

5. 4. The method according to claim 2 or 3, characterized in that a further part of the powdered steel slag fraction is separated from the solidified stainless steel slag (31) before reducing the particle size thereof to produce the granular material (59), in particular before any carbonation of the non-metallic slag phase contained therein, or before the non-metallic slag phase contained in said further part of the powdered steel slag fraction absorbs up to 3.0 wt. % carbon dioxide due to carbonation of the non-metallic slag phase.

6. 3. The method of claim 2, further comprising reducing the particle size of the solidified stainless steel slag (31) by crushing the solidified stainless steel slag (31) to produce the granular material (59) having the first particle size distribution.

7. 7. The method of claim 6, wherein the particle size of the solidified stainless steel slag (31) is reduced to produce the granular material (59) by crushing the solidified stainless steel slag (31) in at least two successive steps, comprising a first crushing step to produce a coarser granular material (39), removing a portion (45) of the granular material (59) from the coarser granular material (39), and then subjecting the coarser granular material (39) to a second crushing step to produce a further portion (52) of the granular material (59).

8. 8. The method of claim 7, further comprising removing said portion (45) of said granular material (59) from said coarser granular material (39) after said first crushing step and separating a first stainless steel-rich fraction (41) from said coarser granular material (39).

9. The granular material (59) is sieved to remove particles of D smaller than 4 mm. ９０ 3. The method according to claim 2, characterized in that the powdered steel slag fraction (88) is separated into at least two different fractions (62, 63) including a fine fraction (62) having a sieve size value and a coarser fraction (63) coarser than the fine fraction (62), and at least a portion of the powdered steel slag fraction (88) is separated from the fine fraction (62).

10. 10. A method according to claim 8 or 9, characterized in that a second stainless steel rich fraction (65) is separated from the coarser fraction (63).

11. 10. The method according to claim 8 or 9, characterized in that at least a portion of the fine fraction (62) is mixed with water to produce an aqueous mixture (70), and the aqueous mixture (70) is separated into a sand fraction (83) and an aqueous dispersion (84), in particular by a dewatering classifier (82), and at least a portion of the powdered steel slag fraction (88) is removed from the aqueous dispersion (84).

12. 12. The method of claim 11, further comprising separating a third stainless steel-rich fraction (73) from the aqueous mixture (70) by gravity separation techniques, followed by separating the aqueous mixture (70) into the sand fraction (83) and the aqueous dispersion (84), crushing the third stainless steel-rich fraction (73), recovering stainless steel (81) from the crushed third stainless steel-rich fraction (79), and recycling the remaining crushed slag material (80) to the aqueous mixture (70).

13. 12. The method of claim 11, further comprising filtering at least a portion of the powdered steel slag fraction (88) from the aqueous dispersion (84) to leave a filtrate (94), storing the filtrate (94) in at least one reservoir (92), forming a precipitate (95) comprising stainless steel slag particles in the reservoir (92), and removing at least a portion of the precipitate (95) from the reservoir (92) to form a further portion of the powdered steel slag fraction (88).

14. 2. The method of claim 1, wherein the non-metallic slag phase of the powdered steel slag fraction (88) contains less than 5000 mg / kg of chromium.

15. 2. The method according to claim 1, characterized in that the non-metallic slag phase of the solidified stainless steel slag (31) contains more than 5000 mg / kg, in particular more than 5500 mg / kg, and more in particular more than 6000 mg / kg of chromium.

16. The method of claim 1, wherein the powdered steel slag fraction (88) comprises a crystalline non-metallic steel slag phase consisting of at least 3% by weight of gamma dicalcium silicate.

17. 10. The method of claim 1, wherein the raw material comprises at least 1% by weight of the steel slag material.

18. 2. The method of claim 1, wherein at least a portion of the powdered steel slag fraction is separated from the solidified stainless steel slag (31) before any carbonation of the non-metallic slag phase contained therein or before the non-metallic slag phase contained in said portion of the powdered steel slag fraction absorbs up to 3.0 wt. % carbon dioxide due to carbonation of the non-metallic slag phase.

19. 10. A method for producing cement clinker, characterized in that raw materials comprising at least one steel slag material are calcined and combusted to produce said cement clinker, and a powdered steel slag fraction (88) obtained as defined in claim 1 is used as said steel slag material.

20. 10. Use of the powdered steel slag fraction obtained as defined in claim 1 as a raw material for producing cement clinker.

21. 21. The method according to claim 19 or the use according to claim 20, wherein the steel slag material is a stainless steel slag material, the stainless steel slag material containing less than 5000 mg / kg chromium and comprising a crystalline non-metallic steel slag phase consisting of at least 3 wt. % gamma dicalcium silicate.

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