Method for producing a carbonate bonded, compacted article

Abstract

The method for producing a carbonate bonded, compacted article comprises the steps of providing a particulate material which comprises a particulate carbonatable binder; compacting the particulate material to form a compact having pores which contain pore water; and carbonating said compact to produce carbonates including calcium carbonates thus transforming the compact into said carbonate bonded, compacted article. The carbonatable binder contains fluoride. In order to limit leaching of fluoride from the carbonated article a fluoride adsorbing, magnesium-bearing solid compound, which comprises surface hydroxyl groups bound to magnesium atoms, is provided in the compact. At least one magnesium-bearing additive is distributed in the particulate material prior to the carbonating step. This additive has a magnesium content of at least 30 wt.% MgO and comprises the fluoride adsorbing, magnesium-bearing solid compound and / or produces this fluoride adsorbing, magnesium-bearing solid compound in the particulate material by reaction in said pore water.

Description

[0001] "Method for producing a carbonate bonded, compacted article"

[0002] The present invention relates to a method for producing a carbonate bonded, compacted article, which method comprises the steps of providing a particulate material which comprises a particulate carbonatable binder composed of particles having a sieve size smaller than 500 pm; compacting the particulate material to form a compact having pores which contain pore water; and carbonating said compact for a predetermined period of time with a gas which contains carbon dioxide to produce carbonates including calcium carbonates thus transforming the compact into said carbonate bonded, compacted article. The particulate carbonatable binder contains calcium oxide and silicon dioxide and has a CaO / SiO2 ratio higher than 1.1 and a pH, measured after a contact time of 24 hours in accordance with DIN EN 12457- 4:2003-01 , of at least 10.5. The carbonatable binder also contains fluoride, i.e. fluoride ions, so that the carbonate bonded, compacted article has a fluoride leaching value which can be measured in accordance with DIN EN 12457- 4:2003-01.

[0003] There are different industrial production processes which produce carbonatable materials as by-products. These by-products are for example fly ashes, bottom ashes (in particular municipal waste incineration bottom ashes) and slags generated during the production of phosphorus or during the production of ferrous or non-ferrous metals, such as zinc, copper and lead and iron or steel. Also dust from air filters for example from steel furnaces is carbonatable in particular when it contains calcium oxides and has a sufficiently high pH value. Some of these by-products can be used in different applications. Blast furnace slags can be used for example in road construction and also in the production of cement. Some slags, such as common steel slags (for example LD slags) which have a high neutralizing value can for example also be used as a soil conditioning agent. Other materials, such as bottom ashes and stainless steel slags contain however considerable amounts of heavy metals which are problematic in view of their leaching behaviour.

[0004] In order to limit the impact, both economic and environmental, of these domestic and industrial waste materials, attempts have been made more and more to develop methods of processing these materials, i.e. methods for converting these waste materials into economically valuable materials. A large quantity of these waste materials is alkaline and comprises carbonatable phases, such as calcium oxides and / or hydroxides. Other phases, which contain for example calcium silicates and calcium magnesium silicates may also be carbonatable. It is known that when using a fine fraction of these carbonatable waste materials as a fine carbonatable binder either alone or in combination with a coarser aggregate fraction, the carbonation of the fine binder fraction makes it possible to obtain materials having good mechanical qualities. Moreover, carbonation may also help in preventing leaching of pollutants such as heavy metals which may be contained in these carbonatable binders.

[0005] One major group of carbonatable materials are iron and steel making slags. These slag include for example EAF slags, BOF (especially LD) slags, AOD slags, VOD slags, ladle slags and tundish slags. All of these slags usually contain fluoride. Fluoride can be added in the form of fluorspar (CaF2) to the liquid slag present on the top of the molten iron or steel to control the viscosity and the melting point of the liquid slag. Phosphorous slag, which is in particular generated in the process of yellow phosphorus production, also has a high pH and fluoride content.

[0006] Stainless steel slags are a particular group of slags which contain not only fluoride but also relatively large amounts of heavy metals such as chromium and often also nickel and molybdenum. As disclosed in EP-B- 0837043, EP-B-1055647 and in EP-B-1146022 the leaching problems of stainless steel slags can be solved by crushing the steel slags, removing the valuable stainless steel particles therefrom and by applying the different fractions of the remaining crushed slags in bounded applications, for example as fine or coarse aggregate in concrete or asphalt. The leaching problems of not only the heavy metals but also of fluoride were solved by this method. This method is used successfully in practise to recycle the coarser fraction of stainless steel slag as aggregate for producing concrete or asphalt.

[0007] A more problematic fraction of stainless steel slag are the fines thereof, more particularly the fine fraction of the crushed steel slags which have a particle size smaller than 0.5 mm. This fine fraction has a considerably higher gamma dicalcium silicate (y-C2S) content than the coarser fractions. As a result thereof, this fine fraction has high water absorption properties and is thus not suited for being used in concrete or asphalt applications. It can however be used as carbonatable binder for producing carbonate bonded articles, such as blocks, bricks and even larger construction materials.

[0008] WO-A-2009 / 133120 discloses a method wherein particulate carbonatable materials, in particular the fines of crushed stainless steel slags which have a size of between 0 and 0.5 mm, are used as carbonatable binder for producing high quality, carbonate bonded, compacted articles. In this known method the particulate material is first compacted with a relatively high compaction pressure of between 5 and 65 MPa, and the obtained compact is subsequently carbonated under a relatively high temperature and pressure. In this way, carbonated compacts with a high compressive strength can be produced. By controlling the porosity and the intrinsic permeability of the compacts, and by carbonating for several hours (more particularly for 18 hours at an increased pressure and temperature), compressive strengths of between 26 and 66 MPa were obtained with a 0 - 500 pm fine stainless steel slag. Leaching of different heavy metals has been measured in accordance with DIN EN 12457-4, but no mention is made of any leaching of fluoride.

[0009] WO 2020 / 260568 discloses an alternative carbonation method wherein high strength carbonate bonded compacted articles can be produced without having to carbonate the compacts under a high pressure and temperature, i.e. without requiring the use of a high pressure carbonation chamber. In this known method, the compacted particulate carbonatable material is first carbonated with a low partial carbon dioxide pressure in the carbonation gas after which the article is further carbonated with a high partial carbon dioxide pressure. Also in this patent publication no mention is made of any fluoride leaching notwithstanding the fact that stainless steel slag was used as binder and also as coarser aggregate fraction.

[0010] KR 101 351 613 B1 discloses a method for reducing fluoride leaching from steel slag, in particular from powdery steel slag. The powdery steel slag is mixed with 5 to 20% of an ammonium chloride solution in order to promote the elution of calcium ions from the slag surface. The obtained mixture is put in a reactor and flue gas containing about 20% carbon dioxide is passed for 2 hours through the loose mass of powdery steel slag. The steel slag particles are thus uniformly carbonated and a calcium carbonate coating is produced on the surface of the slag particles. This calcium carbonate coating was found to reduce leaching of fluoride out of the slag particles.

[0011] A drawback of this method is that it is not suitable for reducing fluoride leaching out of carbonated steel slag compacts. When carbonating steel slag compacts, the steel slag particles will not be carbonated uniformly. Especially when the pore water is enriched in calcium ions due to the increased elution of calcium ions by the presence of ammonium chloride most of the carbon dioxide gas will react already at the surface of the compact. Calcium ions which are consumed during the carbonation reaction are quickly replenished by diffusion of calcium ions from the centre of the compact towards the surface thereof. The calcium carbonates produced at the surface of the compacts will hamper the penetration of the carbon dioxide into the compact and thus the carbonation of the centre parts of the compact.

[0012] Due to the non-uniform carbonation of steel slag compacts, the method disclosed in KR 101 351 613 B1 is not suited to obviate the fluoride leaching problems of such compacts. In the centre of the compacts, the steel slag particles will not be sufficiently carbonated. At the surface, the steel slag particles will be carbonated to a much larger extent. The present inventors have however found that when steel slag compacts are strongly carbonated in order to achieve a high compressive strength, the fluoride leaching value is not reduced but is instead substantially increased. The presence of ammonium chloride cannot prevent this increased leaching of fluoride ions as it does not immobilize fluoride ions. The ammonium chloride only increases the leaching of calcium ions in the pore water to thereby enhance the carbonation reaction. The present inventors have thusfound that when the carbonatable binder which contains fluoride is used for producing carbonate bonded, compacted articles, leaching of fluoride is not reduced by the carbonation process but becomes even more problematic. The fluoride leaching value of the carbonate bonded, compacted article was indeed found to be considerably higher than that of the particulate material prior to the carbonation step. Leaching limits are imposed by different governments for the leaching of fluoride and for different heavy metals. The Walloon region of Belgium has for example set an upper limit for the leaching of fluoride of 5.0 mg / L or 50 mg / kg dry matter (the liquid / solid ratio applied in the leaching test is 10 / 1). Fluoride leaching is measured in accordance with DIN 38414-S4 (EN-12457-4) with the analytical method ISO10359-1 (1992) DIN38405-19-88. In the Walloon region of Belgium, the upper limit for chromium (Cr (VI)) leaching is 0.1 mg / L (1.0 mg / kg dm), for molybdenum (Mo) leaching 0.15 mg / L (1.5 mg / kg dm) and for nickel (Ni) leaching 0.2 mg / L (2.0 mg / kg dm). These leaching values have all to be measured in accordance with the same EN-12457-4 standard and with the analytical method ISO11083 (1994) / calorimetry for Cr(VI), DIN3806-22 (1988) ICP / AAS method for Mo, and ISO8288 (19996) DIN38406-06-08-85 / DIN3806- 22 (1988) for Ni. For steel slags, the upper limit for the leaching of V (vanadium) is not yet imposed in so many countries, but is for example 0.18 mg / L in Germany and 1 .0 mg / kg dm (dry matter) in the Netherlands. The present inventors have found that when using stainless steel making slag fines as carbonatable binder, the upper limit of 50 mg / kg dm for the fluoride leaching is often exceeded after the carbonation step. This may possibly be explained as follows.

[0013] In steel slag fluoride is contained in particular in the mineral phases cuspidine (Ca4Si2O7F2) and fluorite (CaF2). In the article of Remus Ion lacobescu et al. (2014) “Stabilisation and Microstructural Modification of Stainless Steel Converter Slag by Addition of Aluminium Rich By-Product” the mineral phases of the fines of the AOD stainless steel converter slag which was used in the described experiments contained for example 21.2 wt.% of cuspidine and 0.3 wt.% of fluorite. Based on a Kspvalue (solubility product) of 3.9 10-11for CaF2(= [Ca2+] . [F]2) a saturated solution of CaF2in water would contain about 8 mg / L (= 0.68 mM) of fluoride (= F). Such a CaF2solution would thus exceed the fluoride leaching limit of the Walloon region. The pore water of steel slags contains however a relatively high amount of calcium ions since it is a saturated solution of Ca(OH)2. Based on a Kspvalue of 5.02 10-6for Ca(OH)2, the pore water saturated with Ca(OH)2and having a pH of 12.5 would have a calcium concentration of about 200 mg / L (= 5.0 mM). As a result of such a high calcium concentration in the pore water, the dissolution of CaF2in the pore water is considerably suppressed to a value of less than 0.1 mM of fluoride (= 1.9 mg F / L). This also appears from Figure 10 of the article “Fluorine-containing Mineral Phases in Ironmaking and Steelmaking Slags and Their Solubilities in Aqueous Solution” by Ryo Inoue et al. (2002).

[0014] As to the fluoride contained in cuspidine, it appears from Figure 14 of this same article that when shaking a dispersion of cuspidine powder in water, cuspidine releases fluoride but at higher pH values the release of fluoride is considerably reduced. At pH 12, for example, only about 1 to 2 ppm F was released. Consequently, the leaching of fluoride from alkaline carbonatable materials, such as ironmaking and steelmaking slags, is quite low as a result of the high pH of these materials and their high calcium content. When the pH of the pore water is lowered, for example during the carbonation step, more fluoride may leach out of the cuspidine.

[0015] Carbonation of the carbonatable binder however not only lowers the pH of the particulate material but especially also lowers the calcium concentration in the pore water. Based on a Kspvalue of 3.3 10-9for CaCOs (= [Ca2+] . [CO32]), a saturated calcium carbonate solution would contain indeed only 2.3 mg / L (= 0.057 mM) of calcium ions. With such a low calcium concentration in the pore water, much more CaF2will dissolve resulting, based on the Kspvalue of CaF2, in a fluoride concentration of about 16 mg / L (= 0.8 mM), which is considerably higher than the fluorine leaching limit of 5 mg / L. In their article “Experimental Study on the Dissolution Behavior of Calcium Fluoride” Suchandra Sar et al. (2020) also described that for efficient removal of fluoride from primary and secondary raw materials for zinc extraction, the COs2- concentration in the solution should be kept high enough so that the solubility product for CaCOs is exceeded before the one for CaF2is reached. In this way, the available carbonate ions in the system limit the concentration of calcium ions by precipitation of CaCOs, which enhances the dissolution of CaF2.

[0016] A problem of the prior art carbonation methods for producing carbonate bonded, compacted articles with a fluoride containing carbonatable binder is therefore that by the carbonation process required to produce such carbonate bonded, compacted articles, in particular carbonate bonded, compacted articles having a compressive strength of at least 10 MPa, leaching of fluoride is considerably increased, in particular to such an extent that existing fluoride leaching limits may be exceeded.

[0017] Another problem of the prior art carbonation methods is that when the carbonatable binder contains vanadium, leaching of vanadium is also increased by the carbonation process. This effect on the leaching of vanadium has been described by Andre van Zomeren et al. (2011) in their article “Changes in mineralogical and leaching properties of converter steel slag resulting from accelerated carbonation at low CO2 pressure”.

[0018] An object of the present invention is to provide a new method for producing carbonate bonded, compacted articles with a fluoride containing particulate carbonatable binder which enables to limit first of all the fluoride leaching value of the carbonated articles.

[0019] To this end the method of the present invention is characterised in that a fluoride adsorbing, magnesium-bearing solid compound, which comprises surface hydroxyl groups bound to magnesium atoms, is provided in said compact by distributing at least one magnesium-bearing additive in said particulate material prior to said carbonating step, which additive has a magnesium content, expressed as oxide and by dry weight, of at least 30 wt.% MgO and comprises said fluoride adsorbing, magnesium-bearing solid compound and / or produces said fluoride adsorbing, magnesium-bearing solid compound in said particulate material by reaction in said pore water.

[0020] In accordance with the present invention it was found that fluoride ions adhere quite strongly to the surface hydroxyl groups of the fluoride adsorbing, magnesium-bearing solid compound. The fluoride ions can also be adsorbed onto the fluoride adsorbing, magnesium-bearing solid compound by substitution of hydroxyl groups by fluoride. Adsorption of fluoride onto the fluoride adsorbing, magnesium-bearing solid compound may thus be chemically and / or physically. In this way, at least a portion of the fluoride ions which are released in the pore water during the carbonation step from mineral phases such as fluorite and possibly also cuspidine can be bound by the surface hydroxyl groups in such a manner that they are not released in the fluoride leaching test described in DIN EN 12457-4. The fluoride adsorbing, magnesium bearing solid compound may be added as such, as said magnesium-bearing additive, to the particulate material or a magnesium-bearing additive may be used which produces the fluoride adsorbing, magnesium-bearing solid compound in the pore water. The present inventors also found that, although the carbonatable binder may have a quite high fluoride content, and a quite large amount of carbonates are produced during the carbonation step, only a relatively small amount of the magnesium-bearing additive is needed, in particular an amount of magnesium-bearing additive which does not disturb the carbonation process. This may be due to the fact that by the carbonatable binder itself other fluoride binding phases may be produced during the carbonation step. The present inventors have found for example that when the carbonatable binder comprises periclase, periclase is not carbonated or only to a small extent but it is hydrated during the carbonation step producing brucite which can immobilize fluoride as described for example in the article of Liu Xiaoji et al. (2012) “Adsorption and co-precipitation behavior of fluoride onto Mg- bearing minerals in Si-AI-Mg mineral system at hyperalkaline conditions”. In addition or instead of brucite hydrated magnesium carbonates may be formed as a result of the carbonation step. Fluoride may also be bound by coprecipitation in these hydroxide containing magnesium compounds (by substitution of hydroxyl groups by fluoride) and / or in particular also in magnesium / calcium carbonates (magnesian calcite) wherein carbonate groups may be substituted by fluoride.

[0021] In an embodiment of the method according to the present invention, said at least one magnesium-bearing additive comprises a magnesium-aluminium hydrotalcite, which magnesium-aluminium hydrotalcite adsorbs in particular at least a portion of the fluoride which is leached out of the particulate carbonatable binder during said carbonating step.

[0022] Hydrotalcite is a layered double layer hydroxide having for example the formula Mg6Al2CO3(OH)i6.4H2O. The carbonate ions of hydrotalcite are loosely bound between the layers and can easily be exchanged by other ions such a fluoride ions. Although the affinity of hydrotalcite to multivalent interlayer anions such as COs2-is higher than for monovalent anions such as F, and notwithstanding the fact that during the carbonation step carbonates are produced in the pore water, hydrotalcite was found to be able to adsorb fluoride released from the carbonatable binder during the carbonation step and to bound the fluoride sufficiently strongly so that it does not leach out of the carbonated article during the leaching test in accordance with DIN EN 12457-4:2003-01.

[0023] In an embodiment of the method according to the present invention, or according to the preceding embodiment, said at least one magnesium-bearing additive comprises magnesia, doloma or magnesia-doloma, preferably magnesia or magnesia-doloma having a magnesium content of at least 50 wt% MgO, preferably at least 60 wt.% MgO.

[0024] Magnesia, doloma and magnesia-doloma have a limited solubility in water. They are able to adsorb fluoride and to bound fluoride sufficiently strongly so that it does not leach out of the carbonated article during the leaching test in accordance with DIN EN 12457-4:2003-01. Magnesia, doloma and magnesia-doloma may be hydrated in the pore water to form Mg(OH)2and / or hydrated magnesium carbonates such as nesquehonite (Mg5(HCO3)(OH).2H2O) and hydromagnesite (Mg5(CO3)4(OH)2.4H2O). The present inventors have found that even dead-burned magnesia (periclase) may be hydrated in the pore water during the carbonation step. The adsorption of fluoride can thus at least partially be due to an isomorphic substitution of hydroxyl groups by fluoride so that the fluoride is strongly bound to the at least partially hydrated magnesia, doloma or magnesia-doloma.

[0025] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said at least one magnesium-bearing additive has a D5o sieve size value smaller than 15.0 pm, preferably smaller than 10.0 pm and more preferably smaller than 5.0 pm.

[0026] When at least a portion of the magnesium-bearing additive does not dissolve in the pore water and fluoride is adsorbed chemically and / or physically onto the surface of the magnesium-bearing additive, or onto the surface of the fluoride adsorbing compound produced by the magnesium- bearing additive by reaction with the pore water, the adsorption capacity of fluoride can be enhanced by the reduced particle size. A smaller D5o sieve size value of the magnesium-bearing additive may also increase the reactivity thereof and / or may also increase the release of magnesium ions by this additive in the pore water.

[0027] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said at least one magnesium-bearing additive comprises magnesia, doloma or magnesia- doloma, which magnesia, doloma or magnesia-doloma has a citric acid reactivity of less than 900 sec, preferably of less than 600 sec, and more preferably of less than 300 sec. Preferably, the citric acid reactivity is more than 60 sec.

[0028] As a result of the higher citric acid reactivity, the magnesia, doloma or magnesia-doloma can be hydrated more easily and quickly in the pore water thus providing more hydroxyl groups which are available for binding fluoride, in particular for being exchanged with fluoride. The present inventors have found that a citric acid reactivity of less than 60 sec is however less effective. Possibly, this may be due to some further reaction of the magnesia, doloma or magnesia-doloma during the carbonation step producing for example carbonate groups which immobilize less fluoride than surface hydroxyl groups.

[0029] In an alternative embodiment of the method according to the present invention, said at least one magnesium-bearing additive comprises magnesia, doloma or magnesia-doloma, which magnesia, doloma or magnesia- doloma has a citric acid reactivity of more than 900 sec, the at least one magnesium-bearing additive comprising spent magnesia, doloma or magnesia- doloma refractory material, in particular spent refractory material recycled from ironmaking or steelmaking furnaces The present inventors have found that, notwithstanding their low reactivity, spent refractory magnesia, doloma or magnesia-doloma can be used as said magnesium-bearing additive. Although these materials are dead-burned, the present inventors found they still can be hydrated by reaction with the pore water during the carbonation step and were thus also effective for adsorbing fluoride. Especially in this embodiment, the spent refractory material has a D5o sieve size value smaller than 15.0 pm, preferably smaller than 10.0 pm and more preferably smaller than 5.0 pm to provide a larger surface area for the hydration reaction and for the binding of fluoride.

[0030] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said at least one magnesium-bearing additive comprises hydrated magnesia, hydrated doloma or hydrated magnesia-doloma, said at least one magnesium-bearing additive preferably comprising hydrated magnesia (i.e. brucite).

[0031] An advantage of this embodiment is that the magnesia, doloma or magnesia-doloma is already hydrated so that the hydroxyl groups are readily available, already from the start of the carbonation step, to immobilize fluoride which is released in the pore water as a result of the carbonation process.

[0032] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said at least one magnesium-bearing additive is at least partially dissolved in said pore water to provide magnesium ions in said pore water, at least a portion of the magnesium ions which are dissolved in said pore water precipitating by reaction with at least hydroxyl ions present in the pore water to produce in particular brucite and / or hydrated magnesium carbonates such as nesquehonite and hydromagnesite as said fluoride adsorbing, magnesium-bearing solid compound. Fluoride ions can be adsorbed physically onto the hydroxyl groups of these magnesium compounds or they can be adsorbed chemically onto these compounds by substitution of some of the hydroxyl groups. Preferably, said brucite and / or said hydrated magnesium carbonates are co-precipitated with at least a portion of the fluoride which is leached out of the particulate carbonatable binder during said carbonating step. Preferably, said magnesium ions are at least partially co-precipitated with calcium and carbonate ions present in said pore water to produce magnesian calcite during said carbonating step, which magnesian calcite is co-precipitated with at least a portion of the fluoride which is leached out of the particulate carbonatable binder during said carbonating step. In the present specification magnesian calcite is understood to comprise also other forms of calcium magnesium carbonates, especially also magnesian aragonite, or combinations or magnesian aragonite and magnesian calcite.

[0033] The magnesium-bearing additive may be water-soluble. By water-soluble salt or additive is meant in the present specification a salt or an additive which has a solubility in water at 20°C of at least 10 g / 100 ml, i.e. at 20°C a saturated solution of the salt or additive in 100 ml of water contains at least 10 g of the salt or additive. However, the magnesium-bearing additive may also have a lower solubility in water. In this embodiment, part of the magnesium- bearing additive still dissolves however in the pore water under the conditions prevailing therein, especially during the carbonation step. Preferably, the citric acid reactivity of the magnesium-bearing additive is so high that more magnesium ions are released in the pore water, the citric acid reactivity being preferably, as described hereabove, less than 900 sec, more preferably less than 600 sec, and most preferably less than 300 sec.

[0034] The solubility of magnesium hydroxide is strongly dependent upon the pH under alkaline conditions. For example when the pH of the pore water is equal to about 12.5, and decreases for example during the carbonation step to pH 10, the solubility of magnesium increases from 5.6 1 O'6mM to 0.56 mM (based on the solubility product Kspof Mg(OH)2of 5.6 10-12). At pH 9.5, the solubility of magnesium would be about 5.6 mM or 134 mg / L. During the carbonation step the pH may even be lower enabling even a higher magnesium concentration in the pore water. When a sufficiently high amount of magnesium is present in the pore water, magnesian calcite can be produced instead of or in addition to calcite during the carbonation step. An important advantage of magnesian calcite is that fluoride may co-precipitate and may thus be immobilised in the magnesian calcite. As appears from the article of Yasushi Kitano and Minoru Okumura (1973) “Coprecipitation of fluoride with calcium carbonate” fluoride does not coprecipitate with pure calcite. However, in the presence of magnesium ions fluoride coprecipitates with calcite to produce magnesian calcite. Calcite crystal in solutions containing magnesium ions tends to concentrate magnesium at its surface. Fluoride may associate easily with magnesium concentrated at the surface of calcite to form ion pairs, which may explain why the amount of fluoride coprecipitated with calcite increases with increasing concentration of magnesium ions in the solution. Reference can also be made of the article of Minoru Okumura at al. (1983) “Incorporation of fluoride ions into calcite - Effect of organic materials and magnesium ions in a parent solution”. According to Figure 4 of this article, 200 to 300 mg / L magnesium in the parent solution may already result in about 5 mole percent of magnesium carbonate in the magnesian calcite. Such high magnesium concentration can be achieved in the pore water at a pH which is somewhat lower than 9.5, in particular at a pH of between 9.0 and 9.5 or lower. Such pH values can easily be obtained during the carbonation step. According to Figure 3 of this article, 5 mole percent of magnesium carbonate in the magnesian calcite would already enable to immobilise about 1000 ppm of fluoride in the magnesian calcite, for a fluoride concentration of 1 ppm in the solution and even more for higher fluoride concentrations and / or for higher magnesium contents.

[0035] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, the gas used to carbonate the compact has a partial carbon dioxide pressure of at least 0.10 bar, preferably at least 0.20 bars, more preferably at least 0.30 bars and most preferably at least 0.40 bars, the partial carbon dioxide pressure in said gas being preferably lower than 5.0 bars, more preferably lower than 2.0 bars and more preferably lower than 1.0 bar. In the present specification the unit bar refers to the absolute pressure bara.

[0036] Using a partial CO2 pressure higher than these lower limits is advantageous in view of obtaining a lower pH in the pore water during the carbonation step. Since, compared to calcium hydroxide, magnesium hydroxide has a lower solubility product Ksp, less magnesium is dissolved in the alkaline pore water. At a lower pH value, more magnesium is in solution so that more calcium magnesium carbonates (in particular magnesian calcite Cai-o.ssMgo- 0.15CO3) is produced. As more fluoride can co-precipitate in magnesian calcite (i.e. than in pure calcite), more fluoride is bound in these newly formed calcium magnesium carbonate phases which contributes to a reduction of the fluoride leaching value. Moreover, a higher acidity of the pore water may contribute to an enhanced hydration of magnesium oxide, which may be either contained in the magnesium-bearing additive, such as magnesia, doloma and magnesia- doloma, and / or in the carbonatable binder itself. In this respect, it was found that the fines of ironmaking and steelmaking slags contain an amount of periclase (i.e. free lime or dead-burned magnesium oxide) which is, notwithstanding its low reactivity, hydrated during the carbonation step. A small amount of the hydrated magnesium oxide may be carbonated as such, in particular to produce hydrated magnesium carbonates, or co-carbonated with calcium hydroxide, and with a small amount of fluoride, to produce magnesian calcite, but the major part of the hydrated magnesium oxide is not carbonated and forms brucite which is an effective adsorbent for fluoride. Also phases such as Mg2CI(OH)3.4H2O may be formed which also immobilize fluoride. A partial carbon dioxide pressure lower than the upper limits is advantageous to avoid having to use high pressure carbonation chambers. Moreover, less magnesium oxide / hydroxide is carbonated leaving more magnesium oxide / hydroxide to adsorb fluoride.

[0037] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said compact is carbonated until said carbonate bonded, compacted article has a pH, measured after a contact time of 24 hours in accordance with DIN EN 12457- 4:2003-01 , lower than 11.0, preferably lower than 10.8 and more preferably lower than 10.6, the pH of the carbonate bonded, compacted article being preferably higher than 9.0 and more preferably higher than 9.5.

[0038] A lower pH after the carbonation step is correlated with a higher degree of carbonation. As explained hereabove, this may be advantageous to enhance the hydration of less reactive magnesium oxide, such as periclase or hard or dead-burned magnesia, doloma or magnesia-doloma, for example recycled refractory magnesia, doloma or magnesia-doloma materials, to produce brucite and / or hydrated magnesium carbonates which can adsorb fluoride physically or chemically, in particular by co-precipitation. Moreover, more magnesium may co-precipitate with calcium, and with fluoride, to produce fluoride containing calcium magnesium carbonates. The pH is however still sufficiently high after the carbonation step so that less magnesium oxide / hydroxide is carbonated leaving more magnesium oxide / hydroxide to adsorb fluoride.

[0039] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said at least one magnesium-bearing additive comprises at least one water-soluble magnesium salt, which is preferably selected from the group consisting of magnesium chloride, magnesium nitrate, magnesium sulphate and magnesium citrate and which is more preferably selected from the group consisting of magnesium chloride, magnesium nitrate, and magnesium citrate, said at least one additive comprising preferably magnesium chloride.

[0040] Water-soluble magnesium salts were found to be more effective for immobilizing fluoride in the pore water of the compact during the carbonation step. Due to the high pH value of the particulate material, a portion of the dissolved magnesium ions immediately precipitates in the form of magnesium hydroxide (brucite). The pH of the particulate material thus decreases and further hydroxide is released which precipitates with the dissolved magnesium ions in the form of magnesium hydroxide. Also during the carbonation step itself, further hydroxide is released which may precipitate as magnesium hydroxide and / or hydrated magnesium carbonates in the presence of the relatively large amount of magnesium ions provided in the pore water by the water-soluble salt. As appears from Table 1 of the article “Adsorption and co-precipitation behavior of fluoride onto Mg-bearing minerals in Si-AI-Mg mineral system at hyperalkaline conditions” of Liu Xiaoh et al. (2012), an amount of fluoride can be adsorbed onto the produced brucite but the fluoride removal from the solution was more than twice as high when the fluoride ions were allowed to co-precipitate with the brucite. Fluoride ions can also co-precipitate with hydrated magnesium carbonates. Another possible explanation of the enhanced immobilization of fluoride is that the precipitated magnesium hydroxide, and / or the precipitated hydrated magnesium carbonates, may have a quite high specific surface area, in particular a larger BET specific surface area, than commercial magnesium hydroxide powder so that a larger surface area is available for fluoride adsorption. Reference can be made for example to CN107324753A wherein, in the Examples, Mg(OH)2is produced from a MgCI2solution by precipitation with Ca(OH)2. The precipitated Mg(OH)2had in these examples a high BET specific surface area, measured by nitrogen absorption manometry, namely a BET specific surface area of between 52 m2 / g and 78 m2 / g.

[0041] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, a predetermined amount of carbonates is produced during said carbonating step, and said at least one additive is distributed in said particulate material to add an amount of magnesium, expressed as oxide, of at least 2.0 wt.%, preferably of at least 3.0 wt.% and more preferably of at least 4.0 wt.% of said predetermined amount of carbonates to said particulate material.

[0042] Even when the carbonatable binder comprises phases such as periclase, these phases appear not to be carbonated, or only to a limited extent, during the carbonation step so that when no magnesium-bearing additive is added, only a limited amount of magnesian calcites are produced. Reference can be made for example to the article of Rafael M. Santos et al. (2013) “Accelerated mineral carbonation of stainless steel slags for CO2 storage and waste valorisation: effect of process parameters on geochemical properties”. According to Figure 5 of this article, the AOD slag used in the described experiment had a pH of about 11.7 and contained a relatively high amount of periclase, namely 7%, of which about 10% was carbonated in the thin-film carbonation process, i.e. in total about 0.7% of MgO was carbonated in the carbonatable stainless steel slag by the thin-film carbonation process. The CC stainless steel slag (continuous casting steel slag), which had a higher pH of about 12.3, contained even more periclase, namely 11%, but due to the high pH of the CC slag, the periclase was not carbonated at all. According to Figure 7, about 20% of the carbonates which are produced after 26 hours of carbonation in the AOD slag was magnesian calcite.

[0043] In the present embodiment of the method according to the invention, the magnesium-bearing additive adds an amount of magnesium, expressed as MgO, which is at least 2.0 wt.% of the produced carbonates. In this way, more magnesian calcite is produced or the magnesian calcite which is produced contains more MgO so that more fluoride can be immobilized. Preferably, the magnesian-bearing additive is water soluble so that most of the magnesium contained in this additive will be dissolved in the pore water and may co-precipitate with calcite to form magnesian calcite.

[0044] The magnesium-bearing additive is preferably a salt which lowers the pH of the pore solution so that the solubility of magnesium is increased and more magnesium calcite can be produced and / or magnesian calcite which contains more magnesium and which is thus more effective for immobilizing fluoride.

[0045] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, an amount of carbon dioxide is taken up by said particulate carbonatable binder during said carbonating step to produce said carbonates, said amount of carbon dioxide being at least 6 % by dry weight, preferably at least 8 % by dry weight of said particulate carbonatable binder.

[0046] The more carbon dioxide is taken up, the more the carbonatable binder is carbonated so that the carbonate bonded article has a higher strength. Moreover, in case of a less reactive magnesium-bearing additive, a more pronounced carbonation may increase the hydration thereof and thus its fluoride adsorption capacity. More brucite and hydrated magnesium carbonates can thus be produced which are effective for immobilizing fluoride. Moreover, the higher the carbon dioxide uptake, the lower the pH of the carbonate bonded article and the more magnesian calcite it may contain due to the higher solubility of magnesium in the less alkaline pore water.

[0047] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said at least one magnesium-bearing additive is distributed in said particulate material to add an amount of magnesium, expressed as oxide, of at least 0.50 wt.%, preferably of at least 0.75 wt.% and more preferably of at least 1 .00 wt.% to said particulate material.

[0048] It has been found that such amounts of magnesium-bearing additive are effective to decrease the fluoride leaching value of the carbonate bonded article. In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said carbonatable binder comprises at least 4000 mg / kg, in particular at least 6000 mg / kg of fluoride. Preferably, said carbonatable binder comprises less than 75 000 mg / kg, more preferably less than 50 000 mg / kg of fluoride (F ).

[0049] Such high fluoride content may result in quite high fluoride leaching values in the carbonated article in case most of the fluoride in not effectively immobilized in the carbonate bonded article.

[0050] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, when starting said carbonating step, said compact has a moisture content of between 3.0 and 25 wt.%, and preferably between 5.0 and 20 wt.%.

[0051] Such relatively low moisture contents enable an effective carbonation of the carbonatable binder whilst ensuring that the pH of the carbonate bonded article does not drop too low, in particular not below 9.0 or even not below 9.5.

[0052] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said particulate carbonatable binder comprises carbonatable slag from a metal production process, slag from the production of phosphorus, bottom ash and / or fly ash, the particulate carbonatable binder preferably comprises steel slag, in particular stainless steel slag.

[0053] The viscosity and melting point of these slags is often lowered by the addition of fluorspar so that the carbonatable binder materials may all contain leachable fluoride. They may moreover contain vanadium. The method according to the present invention was found to be able to reduce also the leaching of vanadium.

[0054] In an embodiment of the method according to the present invention, or according to any one of the preceding embodiments, said particulate carbonatable binder is at least partially crystalline and comprises cuspidine and / or calcium fluoride. The method according to the present invention is especially suited to solve the leaching problems that arise when carbonating a carbonatable binder which comprises cuspidine and / or fluoride.

[0055] Other particularities and advantages of the invention will become apparent from the following more detailed description of some particular embodiments.

[0056] The present invention generally relates to a method for producing a carbonate bonded, compacted article by compacting and carbonating a particulate material which comprises a particulate carbonatable binder.

[0057] The expression “particulate material” refers to any material which consists of loose particles. These particles may be of different sizes so that the expression “particulate material” not only embraces coarse or fine granulates but also very fine granulates, in particular powders.

[0058] The particulate material may consist of one particulate material which is carbonatable, i.e. of the carbonatable binder, or it may consist of a mixture of at least the carbonatable binder and at least one further particulate material which may be carbonatable or not. A particulate material which is not carbonatable comprises for example natural sand, such as sea sand or river sand.

[0059] The particulate carbonatable binder is composed of particles having a sieve size smaller than 500 pm. It contains calcium oxide and silicon dioxide and has a CaO / SiO2 ratio (wt.% CaO over wt.% SiC ) higher than 1.1 and a pH, measured after a contact time of 24 hours in accordance with DIN EN 12457-4:2003-01 , of at least 10.5, preferably of at least 11.0 and more preferably of at least 11.5 or even of at least 12.0. Such a carbonatable binder often contains fluoride. As the method according to the present invention is concerned with the problem of reducing leaching of fluoride from the carbonate bonded, compacted article, use is made in the method according to the invention of such a fluoride containing binder.

[0060] If the particulate material contains one or more further particulate materials, these materials may either be carbonatable or not and may consist even of the same material as the carbonatable binder. The difference with the carbonatable binder is then that the particles of this further particulate material have a sieve size larger than 500 gm. These particles can thus also be carbonated, and may even release fluoride, but are not or less effective to function as binder in view of their relatively small surface area.

[0061] The granulometry of the particulate material is preferably selected to achieve a higher packing density, or in other words a smaller total porosity, since in this way a higher compressive strength can be obtained. The compressive strength of the compact before the carbonation step, i.e. the green strength of the compact, as well as the compressive strength of the carbonated compact is determined in accordance with the European standard EN 12390- 3:2009.

[0062] The particulate carbonatable binder comprises preferably a byproduct or a waste product. It may contain different crystalline and amorphous phases and preferably contains at least one alkaline earth metal silicate phase, in particular crystalline dicalcium silicate.

[0063] The particulate carbonatable binder preferably also comprises free calcium oxide (free lime) and / or calcium hydroxide (portlandite), the total amount of free calcium oxide and free calcium hydroxide being preferably at least 1% by dry weight, more preferably at least 2% by dry weight. It may also contain free magnesium oxide and / or free magnesium hydroxide. These oxides and hydroxides may be in an amorphous and / or in a crystalline form, in particular in the form of portlandite (Ca(OH)2), free lime (CaO), brucite (Mg(OH)2) and in the form of periclase (MgO). They may also be present in amorphous or crystalline forms comprising magnesium and calcium, in varying ratios, and oxygen. Initially, as they are often produced under high temperatures, the freshly produced carbonatable binder usually contains no hydroxides but only oxides, the hydroxides being formed upon aging (weathering) of the carbonatable material or during the carbonation step. As the air also contains a small amount of carbon dioxide, upon aging of the carbonatable material a portion of the hydroxides may be further transformed into carbonates (by natural carbonation), which may include carbonates which are still partially hydrated.

[0064] A wide variety of carbonatable materials is suitable for being used as binder in accordance with the method according to the present invention. Suitable carbonatable materials are for example bottom ashes, more particularly bottom ashes produced during the incineration of waste, in particular of municipal waste (i.e. municipal waste incineration bottom ashes). Also fly ashes can be carbonated, in particular non-coal fly ashes and moreover filter dust from a steel furnace, in particular from an electric arc furnace (EAF filter dust). Most preferred carbonatable materials are however slag materials resulting from metal production processes (production of iron, steel, stainless steel and production of non-ferrous metals such as copper and zinc) and from the production of phosphorus. The used carbonatable binder is preferably a non- hydraulic, or substantially non-hydraulic material. Since a non-hydraulic material cannot provide as such a settable matrix by reaction with water (in particular by CSH formation), a solid article can still be produced by carbonation of this material.

[0065] The slag material is preferably a steel making slag, more preferably a stainless steel making slag. Steel making slags may be converter slags (such as LD, AOD and VOD slags) or electric arc furnace slags (EAF slags). Steel making slags may also be ladle slags or tundish slags. Common steel making slags do not contain or only small amounts of heavy metals such as chromium and nickel and therefore do not present leaching problems as stainless steel slags do. However, they may contain relatively high amounts of vanadium. Stainless steel slags generally contain more than 3000 mg / kg chromium and usually even more than 5000 mg / kg chromium. They may also contain nickel, more particularly more than 300 mg / kg, in particular more than 400 mg / kg and often even more than 500 mg / kg nickel. By carbonating these carbonatable slags, leaching of these heavy metals can be reduced or even prevented. The leaching of vanadium was however found to increase by the carbonation process.

[0066] Steel slags, and in particular stainless steel slags, are usually crushed to produce a granular material from which the metal fraction can be recycled. The coarser fraction of the crushed stainless steel slag can be used as coarse or fine aggregate in concrete of asphalt. The finer fraction, in particular the 0-500 pm fraction, has however too high water absorption properties so that it is not suitable, as such, for these applications. The finer fraction, i.e. the so- called fines, contains indeed a larger amount of gamma dicalcium silicate (y- C2S) which is produced during the solidification of the liquid slag when a portion of the beta dicalcium silicates (P-C2S) is further transformed in the gamma polymorph. Due to the resulting expansion, cracks are formed and a so-called falling slag is produced which has high water absorption properties. This stainless steel slag material, which contains in particular at least 3% by dry weight, more particularly at least 5% by dry weight and even more particularly at least 7% by dry weight of y-C2S is preferably used as carbonatable binder in the method of the present invention.

[0067] In the method of the present invention, the particulate material is first compacted to produce compacts. This can be done by applying the material in a mould and vibrating and / or compressing the material therein. In order to obtain compacts which have a certain green strength, the particulate material comprises water. The compacts thus have pores which contain pore water. The presence of pore water is also essential for the next step. After having compacted the particulate material, the produced compact is carbonated by means of a gas which contains carbon dioxide thereby producing carbonates including calcium carbonates which transform the compact in the carbonate bonded, compacted article. When starting the carbonating step, the compacts have preferably a moisture content of between 3.0 and 25 wt.%, and more preferably between 5.0 and 20 wt.%.

[0068] The compacts are preferably made in a mould and are preferably removed from the mould before being placed in a carbonation chamber for being carbonated. This chamber may be an autoclave wherein relatively high gas pressures are applied. In the method according to the present invention the carbonation step is however preferably carried out with lower gas pressures. The partial carbon dioxide pressure in the carbonation gas is preferably lower than 5.0 bars, more preferably lower than 2.0 bars and most preferably lower than 1.0 bars. The carbonation step can in particular be carried out at atmospheric pressure. A less expensive climate chamber can thus be used as carbonation chamber. For an effective carbonation, the partial carbon dioxide pressure in the carbonation gas is preferably at least 0.10 bar, more preferably at least 0.20 bars, most preferably at least 0.30 bars and even more preferably at least 0.40 bars.

[0069] The compacts are preferably carbonated for 16 to 32 hours, more preferably for 18 to 28 hours and most preferably for 20 to 24 hours. In practice, a total carbonation time of at most 24 hours is preferred so that each day a new load of compacts can be carbonated.

[0070] The compacts are preferably carbonated until the produced carbonate bonded, compacted articles has a pH, measured after a contact time of 24 hours in accordance with DIN EN 12457-4:2003-01 , lower than 11.0, preferably lower than 10.8 and more preferably lower than 10.6. The pH of the carbonate bonded, compacted article is however preferably higher than 9.0 and more preferably higher than 9.5.

[0071] During the carbonating step, an amount of carbon dioxide is taken up by the particulate carbonatable binder to produce carbonates. The carbonation step is preferably continued until the amount of carbon dioxide which is taken up by the particulate carbonatable binder is at least 6 % by dry weight, preferably at least 8 % by dry weight of the particulate carbonatable binder. This amount of carbon dioxide, i.e. the carbonate content expressed as carbon dioxide, can easily be determined by a simultaneous thermal analysis (STA, DIN 51004, DIN 51006 and DIN 51007).

[0072] The carbonatable binder contains fluoride F, in particular at least 4000 mg / kg or even at least 6000 mg / kg of fluoride (expressed as F). The carbonatable binder is preferably at least partially crystalline and comprises cuspidine (Ca4Si2O?(F, OH)2) and / or calcium fluoride (CaF2).

[0073] The fines of stainless steel slag are the most problematic fines. They contain heavy metals such as chromium, vanadium, nickel and molybdenum and moreover also fluoride. The fines of stainless steel slag, formed by the particles which have a sieve size smaller than 0.5 mm, are very suitable for being used as carbonatable binder. The coarser sand fraction of the crushed stainless steel slag, having for example a particle size of between 0.5 and 2.0 mm, can then be used as fine aggregate, in combination with the stainless steel slag binder, to form the particulate material. The further description of the invention will therefore be given with reference to the use of this stainless steel slag binder although also other particulate carbonatable binders can be used. The working principles of the invention are then the same or at least similar.

[0074] Properties of air cooled stainless steel slag fines

[0075] Stainless steel slag was solidified by the method as described in EP 3 122 909 B1 to minimise the formation of fines. During the production of stainless steel slag aggregates from this solidified steel slag over a period of nine months, the composition of the freshly produced fines, having a particle size of between 0 and 0.5 mm, has been analysed weekly. The volume percent passing through different filters has been determined by dry sieving. Before the fines were analysed, they were already hydrated and carbonated to some extent as a result of the production process. During the cooling process, water was for example sprayed onto the solidified steel slag and in order to remove the fines and metal fractions from the crushed solidified steel slag wet processes were used.

[0076] The fines are mainly applicable as binder for the production of carbonate bonded, compacted articles such as blocks, bricks, pavers, etc. by an accelerated carbonation process. It is important to know how much carbon dioxide the fines can take up. The maximum carbonate content which can be achieved in these fines has therefore been determined by carbonating each time a sample of 10 g by the following test method.

[0077] The steel slag sample is dried at 110°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 cup is placed in an autoclave which is flushed with 100% CO2 gas and brought with 100% CO2 gas to a pressure of 0.5 barg (= 1.5 bara). 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 950°C is due to the decomposition of carbonates liberating carbon dioxide. This weight loss (due to the release of CO2 by the decomposition of carbonates), divided by the initial weight of the dried sample and multiplied by 100, is considered as the maximum carbonate content of the carbonated particulate steel slag material expressed in percent by dry weight of carbon dioxide. This weight loss is converted in percent by weight of CaOeq (by multiplying the weight loss by the factor 56 / 44). The weight loss between 350 and 550°C is also measured. This weight loss is considered as the amount of water released by the decomposition of hydroxides and is converted in percent by weight of CaOeq (by multiplying this weight loss by the factor 56 / 18). As brucite and hydrated magnesium carbonates also decompose at temperatures below 550°C, these are considered as hydroxides and not as carbonates.

[0078] The average, minimum and maximum values of these measurements are indicated in Table 1. Moreover, the total amounts of periclase and brucite, both converted to CaOeq, of the stainless steel slag fines before carbonation are also indicated in Table 1 .

[0079] Table 1 : Particles size distribution values and the periclase / brucite content (wt.% CaOeq) of the powdery stainless steel slag fines, and the carbonate content (wt.% CaOeq) and hydroxide content (wt.% CaOeq) of the carbonated powdery stainless steel slag fines.

[0080] In Table 2 the average amounts of the major mineral phases of the steel slag fines, expressed in percent by weight of the crystalline phases are indicated. The steel slag fines contained about 25% of amorphous phases and 75% of crystalline phases.

[0081] Table 2: Average amounts of the major mineral phases of the stainless steel slag fines expressed as percent by weight of the total amount of crystalline phases.

[0082] The minor phases included, apart from other phases, brucite (0.19 wt.%), dolomite (0.37 wt.%), lime (0.08 wt.%), portlandite (0.51 wt.%).

[0083] From these data it appears that, even when carbonating with 100% carbon dioxide gas at a pressure of 1.5 bars, not all the hydroxides are completely carbonated. The remaining hydroxides are magnesium hydroxides, including brucite and also partially hydrated magnesium carbonates such as nesquehonite and hydromagnesite. These hydroxides and hydrated magnesium carbonates are at least partially produced by hydration and partial carbonation of periclase during the carbonation process. Prior to the carbonation process, only a very small amount of brucite was formed. Since the amount of remaining hydroxides is larger than the amount of hydroxides which can be produced by the periclase and brucite present in the steel slag fines prior to carbonation, it appears that further magnesium hydroxides are produced by the carbonation of part of the merwinite, bredigite and akermanite phases (see for example Figure 5 of Rafael M. Santos et al. 2013). Moreover, partially carbonated magnesium hydroxides, i.e. hydrated magnesium carbonates such as nesquehonite and hydromagnesite are also produced during the carbonation process. These hydrated magnesium carbonates are not only dehydroxylated at a temperature lower than 550°C (namely at a temperature between 250 and 350°C) but they are also decarbonated as such a lower temperature. Their presence thus contributes substantially to the hydroxide content of the carbonated steel slag as determined by the above-described test method, in particular as the release of carbon dioxide by the decarbonation causes a larger decrease of the mass of the test sample than the release of water by the dihydroxylation (carbon dioxide having a larger molecular weight than water). The chemical composition, expressed as oxides, of the stainless steel slag fines may vary to some extent. Usually, they contain about 40-60% CaO, 20-30% SiC>2, about 9-10% MgO and further minor amount of i.a. AI2O3, Mn2Os, Cr2O3, Fe2O3, NiO and Mo203. The basicity, i.e. the CaO / SiO2weight ratio, is higher than 1.1 and usually lower than 2.5, in particular lower than 2.1 .

[0084] The stainless steel slag fines also contain an amount of fluoride. Based on 14 total analyses of these fines over a period of 14 years, the average fluoride content of the fines was equal to 16150 mg / kg, the maximum was 25400 mg / kg and the minimum was 4 220 mg / kg. During the same years, the fluoride leaching values of the non-carbonated stainless steel slag fines have also been measured in accordance with EN 12457-S4. The average fluoride leaching value of the fines was equal to 3.33 mg / L (= 33.3 mg / kg), the maximum was 4.75 mg / L and the minimum was 2.29 mg / L.

[0085] According to the chemical formula of cuspidine (Ca8(Si2O7)2F4), cuspidine contains about 10.4% of fluoride. For an average cuspidine content of 8.46% of the crystalline phases, the stainless steel slag fines would contain, on average, about 6 000 mg / kg F bound in cuspidine. A large part of the fluoride, i.e. on average 10 000 mg / kg, is thus contained in other phases such as fluorite phases (CaF2), which can release fluoride more easily than cuspidine during the carbonation process.

[0086] As appears from the above measurements, stainless steel slag fines as such meet the leaching limits for fluoride. However, the present inventors have now found that when using the fines as carbonatable binder, leaching of fluoride is considerably increased.

[0087] Fluoride adsorbing, magnesium-bearing solid compound

[0088] In accordance with the present invention, fluoride leaching from the carbonated binder is reduced by providing a fluoride adsorbing, magnesium- bearing solid compound in the compact. This is done by distributing at least one magnesium-bearing additive in the particulate material prior to the carbonating step. This additive has a magnesium content of at least 30% by dry weight of MgO. In this way, an effective amount of magnesium can be added to the particulate material without having to add a large amount of magnesium-bearing additive. The additive may either be the fluoride adsorbing, magnesium-bearing solid compound itself or it may produce the fluoride adsorbing, magnesium- bearing solid compound in the pore water contained in the compact which is being carbonated.

[0089] The carbonatable binder can already contain as such magnesium-bearing phases which can adsorb fluoride either as such or after reaction in the pore water. Merwinite and especially bredigite may release and produce for example Mg(OH)2during the carbonation process, but the MgO content of these mineral phases is much less than 30 wt.% so that only a small amount of brucite (Mg(OH)2) is formed and these mineral phases are moreover not added, as additive, to the particulate material. Periclase can also be contained in the carbonatable binder. As described hereabove, periclase was found to hydrate during the carbonation step to produce brucite. However, periclase which is already present in the carbonatable binder as one of the mineral phases contained therein, is not distributed as magnesium-bearing additive in the particulate material. It appears that a relatively large amount of brucite and other magnesium hydroxide containing compounds such hydrated magnesium or calcium magnesium carbonates can be produced in this way in the particulate material and can thus immobilise a relatively large part of the fluoride which is released from the carbonatable binder during the carbonating step. However, the present inventors have found that this is not sufficient to limit the fluoride leaching to a sufficient extent. They have found indeed that, after the carbonating step, fluoride leaching of the carbonatable binder, in particular of the stainless steel slag fines, increases considerably. For stainless steel slag fines for example from fluoride leaching values comprised between 3 and 4 mg / L to fluoride leaching values of upto more than 30 mg / L, measured in accordance with DIN EN 12457-4:2003-01. This means that upto 300 mg / kg fluoride may additionally leach out of the carbonated binder so that more of the fluoride contained in the carbonated binder needs to be immobilised therein.

[0090] In a first embodiment of the method according to the present invention, this can be achieved by means of a magnesium-bearing additive which comprises a magnesium-aluminium hydrotalcite. Hydrotalcites are layered double hydroxides which are insoluble in water and display a variety of stoichiometries caused by the different arrangement of the stacking of the layers, ordering of the metal ions, as well as the arrangement of anions and water molecules, in the interlayer galleries. They consist of three layers, namely of two outer layers having a positive charge and an inner intermediate layer which contains water molecules and replaceable anions. Mineral hydrotalcite Mg6Al2(OH)i6CO3.4H2O is the best-known representative of layered double hydroxides (LDHs) and is the preferred hydrotalcite used in the experiments described hereinafter. The COs2’ group can however be easily exchanged by other anions. The magnesium-aluminium hydrotalcite which can be used in the method according to the present invention has the general formula Mg2+vxAI3+x(OH2)x+(Anx / n.yH2O)x-, wherein some divalent magnesium ions are isomorphically replace by trivalent aluminium ions. Originally, electroneutral hydroxide layers thus obtain a positive charge, which is compensated by the charge of anions An-located in the interlayer together with the molecules of crystalline water. The value of x usually lies in the range of between 0.20 and 0.33. The affinity of hydrotalcite to multivalent interlayer anions is higher than for monovalent anions, and the affinity for some ions has been reported to decrease in the order COs2-> SC2' > F’ > HPO42’ > Cl’ > NOs’. The higher affinity for COs2’ than for F’ could be detrimental during carbonation but the test results have demonstrated that hydrotalcite could effectively capture fluoride released for the carbonatable binder upon carbonation thereof.

[0091] In a further embodiment of the method according to the present invention, fluoride released during the carbonating step can be immobilised by means of at least one magnesium-bearing additive which comprises magnesia, doloma or magnesia-doloma, and which has a magnesium content, expressed as MgO, of at least 30 wt.%. The additive preferably comprises magnesia or magnesia-doloma which has a magnesium content, expressed as MgO, of at least 50 wt% MgO, preferably at least 60 wt.% MgO.

[0092] Magnesia is a compound which comprises MgO but no CaO. Doloma is a compound which contains an equimolar amount of MgO and CaO, and can be represented by the formula Cao.5Mgo.5O. Magnesia-doloma contains more moles of MgO than of CaO and can be represented by the formula Cai-xMgxO, wherein x is larger than 0.5. The magnesia, doloma or magnesia-doloma additive is not soluble in water. By water-soluble salt or additive is meant in the present specification a salt or an additive which has a solubility in water at 20°C of at least 10 g / 100 ml, i.e. at 20°C a saturated solution of the salt or additive in 100 ml of water contains at least 10 g of the salt or additive. Fluoride may thus adsorb onto the surface of these magnesium oxides but adsorbs much better when at least some of the oxide groups are hydrated to for surface hydroxyl groups. Since periclase, i.e. dead-burned magnesia, was found to be hydrated during the carbonating step, it appears that also the magnesia, doloma or magnesia-doloma additives are hydrated during the carbonating step to produce, at least on their surfaces, hydroxyl groups.

[0093] In order to increase the fluoride adsorbing capacity, the magnesium-bearing additive has a D5o sieve size value smaller than 15.0 pm, preferably smaller than 10.0 pm and more preferably smaller than 5.0 pm. The D5O 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. Alternatively, the particle size distribution can be determined by laser technology, for example by means of a Horiba 250 analyser, with the material being dispersed in isopropanol by ultrasonic treatment.

[0094] Although even dead-burned magnesia appeared to be hydrated during the carbonating step, the magnesia, doloma or magnesia-doloma, has preferably a citric acid reactivity of less than 900 sec, preferably of less than 600 sec, and more preferably of less than 300 sec. In this way, the magnesia, doloma or magnesia-doloma may be hydrated under less severe carbonating conditions and may thus be hydrated earlier to capture the release fluoride ions more quickly. The magnesia, doloma or magnesia-doloma may however have a lower citric acid reactivity (more than 900 sec) and may in particular also contain spent magnesia, doloma or magnesia-doloma refractory material, in particular spent refractory material recycled from ironmaking or steelmaking furnaces. Even such a quite unreactive material appeared to be hydrated at least partially during the carbonating step and appeared to be effective for immobilizing fluoride released during the carbonating step.

[0095] On the other hand, the magnesium-bearing additive may comprise as such already hydrated magnesia, hydrated doloma or hydrated magnesia-doloma, so that it does not need to be hydrated any more.

[0096] The citric acid reactivity can be determined by means of the following test. An 0.40 N citric acid solution is prepared and a slurry of 2.0 g of powdered MgO in 100 ml of the 0.4 N citric acid solution is shaken at 30°C, with phenolphthalein as indicator, until the color changes from white to pink. The time needed to completely neutralize the acid is reported as the citric acid reactivity.

[0097] In a preferred embodiment, the magnesium-bearing additive is at least partially dissolved in the pore water contained in the compact to provide magnesium ions in this pore water which may precipitate in the form of magnesium hydroxide. During the carbonating step the dissolved magnesium ions may also precipitate in the form of magnesium carbonate or hydrated magnesium carbonate and may also at least partially co-precipitate with calcium and carbonate ions present in the pore water to produce magnesian calcite. The hydroxides, hydrated carbonates and the magnesian calcite are found to coprecipitate with fluoride which is leached out of the particulate carbonatable binder during the carbonating step.

[0098] To maximise this effect, the magnesium-bearing additive preferably comprises a water-soluble magnesium salt, such as magnesium chloride, magnesium nitrate, magnesium sulphate and magnesium citrate. Preferably, a water-soluble magnesium salt different from magnesium sulphate is used as additive since the addition of magnesium sulphate may produce a calcium sulphate precipitate which may hamper the carbonation process. The additive most preferably comprises magnesium chloride, optionally in combination with an amount of a citrate containing compound, preferably an alkali metal citrate, such as sodium citrate. According to Figure 3 of the article of Minoru Okumura et al. (1983), the presence of citrate ions in the water would indeed considerably increase the amount of fluoride which can be immobilised in the magnesian calcite. When bringing the water-soluble magnesium salt in contact with the pore water which is quite alkaline as a result of the high pH of the carbonatable binder, a portion of the magnesium ions of the magnesium salt will immediately precipitate in the form of magnesium hydroxide. This may be pure Mg(OH)2 but the magnesium hydroxide may be for example also in the form of Mg2CI(OH)3.4H2O. During the carbonating step, hydrated magnesium and / or calcium magnesium carbonates may be formed but most of the water-soluble magnesium ions will be incorporated in magnesian calcite.

[0099] Preferably, the magnesium-bearing additive, in particular the water-soluble magnesium salt, which is added to the particulate material contains such an amount of magnesium, expressed as MgO, that this amount comprises at least 2.0 wt.%, preferably of at least 3.0 wt.% and more preferably of at least 4.0 wt.% of the amount of carbonates that are produced during the carbonating step. The amount of carbonates is the actual amount of calcite or magnesian calcite. When these amounts of magnesium are coprecipitated in magnesian calcite, they are effective to achieve a mole percent MgCO3in the magnesian calcite of at least about 5 mole percent to at least about 10 mole percent. According to the properties of the stainless steel slag fines described hereabove, the maximum amount of carbonates that are produced in the carbonation test in these fines is on average equal to 12.32 wt.% of CaOeq, i.e. about 22 wt.% of CaCO3equivalent. The above percentages would thus amount to at least 0.44 wt.% MgO, preferably at least 0.66 wt.% MgO and more preferably at least 0.88 wt.% MgO of the amount of carbonatable binder.

[0100] In case the magnesian calcite contains 1000 mg / kg fluoride, this would correspond to an amount of about 220 mg / kg fluoride immobilised in the carbonated binder. The magnesian calcite may contain however higher amounts of fluoride, for example 2000 mg / kg fluoride so that about 440 mg / kg fluoride could be immobilised in the carbonated binder. This amount of fluoride is in the same order of magnitude than the 300 mg / kg fluoride which was found to leach out of the binder after is has been carbonated. A further amount of fluoride is moreover immobilised by being adhered to the magnesium hydroxide and the hydrated magnesium carbonates which are precipitated when the water-soluble magnesium salt is brought in contact with the alkaline pore water and carbonated.

[0101] The precipitated magnesium hydroxide and hydrated magnesium carbonates are effective to adsorb a large amount of fluoride due to their high BET specific surface area. Brucite has for example a layered structure whilst hydromagnesite has a lamellar structure, both having a large BET specific surface area. This BET specific surface areas is 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.

[0102] The water-soluble salt can be distributed as a dry particulate material in the particulate material. However, it can also be first dissolved in water which is added to the particulate material in order to achieve the desired moisture content. A combination of these two alternatives is also possible. Moreover, it is clear that the above described magnesium-bearing additives can be used either separately or in combination with one another.

[0103] Experimental test results

[0104] The different experiments were done with stainless steel slag fines, having a particle size of 0 to 0.5 mm, and in some experiments additionally with a fine stainless steel slag sand fraction, having a particle size of 0.5 to 2.0 mm. These two stainless steel slag fractions were obtained as follows.

[0105] A stainless steel slag material was crushed to a particle size of between 0 and 35 mm and was separated in a 10 to 35 mm fraction and a 0 to 10 mm fraction. The 0 to 10 mm fraction was separated in a 0 to 2 mm fraction and in a 2 to 10 mm fraction. From the 0 to 2 mm fraction, the steel particles were removed and the fraction was separated in a fine sand fraction of 0.5 to 2 mm and in a fraction of fines of 0 to 0.5 mm. These fines are a carbonatable, particulate material which is used to produce carbonate bonded, compacted articles. These articles can be made entirely of these fines but preferably other aggregate fractions are added such as natural sand or a steel slag sand fraction. This steel slag sand fraction can also be carbonatated but much less than the fines due to the much smaller specific surface area so that the carbonatable fines are required to achieve the required strength. The compressive strength of the carbonate bonded, compacted articles is to be determined in accordance with the European standard EN 12390-3:2009. For the articles produced in accordance with the present invention, this compressive strength is preferably higher than 10 MPa, more preferably higher than 15 MPa and most preferably higher than 20 MPa. The compressive strength which can be achieved is usually lower than 60 MPa.

[0106] Examples 1 to 14

[0107] General test procedure

[0108] The stainless steel slag fines F and the fine stainless steel slag sand fraction S were dried under inert N2atmosphere at a temperature of 105°C. After drying the fines were deagglomerated using a disk mill set at the smallest gap (0.1 mm).

[0109] The different mixtures were made and all mixtures were moisturized with 10% water at least 2 hours before compacting. Compaction was carried out in a cylindrical mould of 23 mm diameter with a compaction force of 15 MPa. The height of the cylinders was approximately 46 mm.

[0110] Carbonation was carried out in a ventilated climate chamber, under the following conditions: 1 barg CO2 (total pressure 2 bar, 50% CO2), 40°C, 24 hours. The compressive strength was measured in accordance with the European standard EN 12390-3:2009. After compressive strength testing the material was further reduced in size with a jaw crusher to < 4 mm.

[0111] The batch leaching test conform EN 12457-2 was carried out on the size reduced material (< 4 mm), with analysis of fluoride in the eluate by liquid ion chromatography with conductometric and / or UV detection of fluoride. This test method only differed from EN 12457-4:2003-01 in that the material was reduced in size to less than 4 mm instead of only to less than 10 mm. Magnesium-bearing additives

[0112] A number of different magnesium-bearing additives were tested to examine their effectivity in reducing fluoride leaching from carbonate bonded, compacted articles made by the above described procedure and containing 49 wt.% stainless steel slag fines (0 - 0.5 mm), 49 wt.% stainless steel slag sand fraction (0.5 - 2.0 mm) and 2 wt. of the magnesium-bearing additive. These additives, as well as their main properties, are indicated in Table 3. Table 3: Magnesium-bearing additives, their content and their properties including the specific surface area (BET in m2 / g), the citric acid reactivity (sec.), the D5o value (pm) and the mean particle size distribution (PSD in pm).

[0113] Additives A1 to A5 are commercial magnesium oxides. They are produced by calcining magnesium carbonate. As this phase decomposes, the escape of CO2 gas initially creates porous structures made up of agglomerates of many MgO microcrystals. These MgO crystals undergo sintering, increasing their average size and reducing the internal porosity of the material. Therefore, the specific surface area and hydraulic reactivity of the resulting MgO depends strongly on the calcination conditions, i.e. calcination temperature and residence time. MgO reacts with water and diluted acids, and its reactivity (rate and degree of reaction) depends considerably on the physical properties and purity of the material. MgO reactivity increases by reducing its particle size and, consequently, increasing its specific surface area. MgO surface area and particle size are both controlled by the production conditions (raw material type and purity, calcination temperature, and residence time during calcination).

[0114] In general 4 types of MgO are produced: light-burned or caustic- calcined MgO (calcined at 700-1000°C), with the highest reactivity and greatest specific surface area; hard-burned MgO (calcined at 1000-1500°C), with lower reactivity and specific surface area than the light-burned MgO; dead-burned MgO or periclase (calcined at 1400-2000°C), with the lowest specific surface area, making them almost unreactive; fused MgO (calcined at 2800-3000°C) with the lowest reactivity. Dead-burned MgO, formed above 1500°C, is used for refractories and is nearly unreactive.

[0115] The present inventors have however found that periclase which is present in stainless steel slag and which is also dead-burned in view of the high temperatures in the stainless steel slag furnace can be hydrated when carbonating the stainless steel slag fines. Also refractory magnesia and doloma, or magnesia doloma, can be hydrated during the carbonation process when these refractory materials have a sufficiently small particle size.

[0116] Additive A6 is an MgO nano-powder. Based on the BET specific surface area the mean PSD has been calculated by the equation: dBET=6 / (SBET X p), wherein dBET is the mean particle size, SBET is the BET surface and p the theoretical density.

[0117] A7 is commercial Mg(OH)2 and A8 is hydrotalcite having the formula Mg6Al2CO3(OH)i6.4H2O.

[0118] A9 to A11 are spent EAF magnesia refractory materials, namely a fine 0 to 10 mm fraction thereof which have been milled to different degrees of fineness. A12 is also a spent EAF refractory material, namely recycled larger EAF magnesia stones which have been milled to a very fine particle size.

[0119] A13 and A14 are spent AOD doloma refractory materials which have been milled to a different degree.

[0120] Experimental results

[0121] Different mixtures were made with the stainless steel slag fines F, the stainless steel slag sand fraction S and the different additives. As described in the general procedure carbonate bonded compacts were made with the different mixtures and the compressive strength and the fluoride leaching was determined. The test results indicated in Table 4 are average values of 3 measurements.

[0122] Table 4: Composition of the different mixtures, compressive strength (MPa), pH after carbonation and fluoride leaching value (mg / L)

[0123] Before carbonation, the stainless steel slag fines F, optionally mixed with the stainless steel slag sand fraction S, had a pH of around 12.45.

[0124] Based on the solubility product Kspof Ca(OH)2it can be calculated that a saturated solution of Ca(OH)2in water has a pH of about 12.33. It thus appears that before carbonation, the pH of the mixture is mainly determined by the portlandite, i.e. hydrated lime. After carbonation, the pH was equal to about 10.4. Based on the solubility product Kspof Mg(OH)2it can be calculated that a saturated solution of Mg(OH)2in water has a pH of about 10.35. It thus appears that after carbonation, the pH of the mixture is mainly determined by the brucite, i.e. hydrated periclase. This calculation indicates again that during the carbonation step periclase, or any other MgO, is hydrated and that at least some of the hydrated MgO remains in the carbonated article. This is advantageous since hydrated MgO was found to be able to adsorb fluoride more effectively than non-hydrated MgO. This is still the case when the hydrated MgO is partially carbonated to produce hydrated magnesium carbonates. Moreover, some of the hydrated MgO was found to be cocarbonated with CaO to produce magnesian calcite wherein a relatively large amount of fluoride may be co-precipitated.

[0125] In the test results indicated in Table 4 it can be seen that carbonated stainless steel slag fines release much more fluoride than noncarbonated fines. The leaching value increased from 43 mg / kg (= 4.3 mg / L) to 370 mg / kg so that more than 330 mg / kg fluoride needs additionally to be immobilised by phases which are newly formed during the carbonation step.

[0126] Examples Ex.1 to Ex. 6 show that MgO is able to immobilise fluoride. Additive A1 gave the best results although its specific surface area was the lowest. This indicates that the reactivity of the additive is also important. By the carbonation of the additive, the specific surface area may increase. Moreover, some of the Mg(OH)2may dissolve and may co-precipitate with fluoride, both as brucite or hydrated magnesium carbonates and as magnesian calcite. Additive A6, i.e. the MgO nano-powder had a very high BET surface area but was not the best MgO powder for reducing fluoride leaching. Moreover, MgO nano-powder is quite expensive.

[0127] Mg(OH)2of Ex.7 also appears to work well, especially in view of the fact that it has a lower MgO content so that more Mg(OH)2should be added. The hydrotalcite of Ex.8 is also able to adsorb fluoride but relatively large amounts will have to be used to reduce the fluoride leaching value to a sufficiently large extent.

[0128] Also the refractory magnesia and doloma of examples Ex.9 to Ex.14 seems to be effective notwithstanding the fact that the refractory magnesia and doloma are dead-burned. Especially also doloma appeared to be relatively effective in view of its low MgO content (a 50 mole % CaO / 50 mole % MgO composition only contains about 41 wt.% of MgO). This may be due to the fact that doloma, and also magnesia-doloma, hydrates better and more quickly than magnesia. This appears for example from the article “Improving hydration resistance of magnesia-doloma refractories by iron oxide addition” by F. Kashaninia et al. (2011 ). According to this article, the hydration resistance of the magnesia-doloma refractories improved by decreasing the CaO content, because CaO is much more prone to hydration compared to MgO.

[0129] The refractory magnesia of examples Ex.9 to Ex.12 is less reactive than the commercial MgO used in examples Ex.1 to Ex.6. For such less reactive magnesia, it appears that better fluoride adsorption properties can be obtained when milling the magnesia material more finely. When milling it for example to a D5o value of about 2 pm, as in Ex.12, also quite effective fluoride adsorption properties were achieved. Milling the material more finely as such does not provide a much larger BET specific surface area but the larger surface area enables to hydrate more MgO during the carbonation step which, as such, was found by the present inventors to increase the BET specific surface area to a considerable extent, especially when the material contains CaO which is carbonatable. They found for example that when carbonating stainless steel slag fines having a D5o sieve size of about 25 pm , the BET specific surface thereof increased considerably, for example from about 2 m2 / g to a BET specific surface area of higher than 40 m2 / g.

[0130] Examples 15 to 33

[0131] General test procedure

[0132] The general test procedure was the same as for Examples 1 to 14. The same stainless steel slag fines F were used and the same fine stainless steel slag sand fraction S. The compacts produced had the same dimensions but were produced with a somewhat lower compaction pressure of 10 MPa.

[0133] Carbonation was carried out in three different manners:

[0134] - CM1 : atmospheric pressure, 50% CO2, 40°C;

[0135] - CM2: atmospheric pressure, 50% CO2, 60°C; and

[0136] - CM3: atmospheric pressure, 20% CO2, 60°C.

[0137] Magnesium-bearing additives

[0138] The following magnesium-bearing additives were used.

[0139] Table 5: Magnesium-bearing additives, their content and their properties including the specific surface area (BET in m2 / g), the citric acid reactivity (sec.), the D50 value (pm) and the mean particle size distribution (PSD in pm).

[0140] The magnesia additive A15 corresponds to additive A12 but has a slightly larger PSD. The doloma additive A16 corresponds to additive A13 but has been milled to a smaller PSD.

[0141] Experimental results

[0142] Different mixtures were made with the stainless steel slag fines F, the stainless steel slag sand fraction S and the different additives. As described in the general procedure carbonate bonded compacts were made with the different mixtures, under the different carbonation conditions, and the compressive strength and the fluoride leaching was determined. The test results indicated in Table 6 are average values of 3 measurements.

[0143] Table 6: Composition of the different mixtures, compressive strength (MPa), pH after carbonation and fluoride leaching value (mg / L)

[0144]

[0145] Compared to comparative Example C.4 the fluoride leaching of comparative Example C.5 is somewhat lower. This is due to the less severe carbonation conditions. The fluoride leaching values of comparative Examples C.6 and C.7 are even lower as the degree of carbonation was even somewhat lower.

[0146] When comparing Example Ex.1 with Examples Ex.15 to Ex.21 it can be seen that by increasing the amount of MgO additive, the fluoride leaching value could be reduced to a low value, in particular to a value lower than 5 mg / L (= 50 mg / kg). This is especially the case when applying less severe carbonation conditions as in Examples Ex.17-18 and Ex.20-21 so that less fluoride is released. The compressive strength of the carbonated articles was reduced to some extent, but it was still sufficiently high. The lower compressive strength may be due to the fact that most of the MgO particles are only slightly hydrated and carbonated.

[0147] When comparing Example Ex.12 with Examples Ex.22 to Ex.27 it can be seen that by increasing the amount of finely milled spent magnesia refractory additive, the fluoride leaching value could be reduced to a low value. It appears however that stronger carbonation conditions are required to lower the fluoride leaching to a value below 5 mg / L. These stronger carbonation conditions may be required in order to enable to hydrate the dead-burned magnesia. The compressive strength of the carbonated articles was also reduced to some extent.

[0148] When comparing Example Ex.13 with Examples Ex.28 to Ex.33 it can be seen that by increasing the amount of finely milled spent doloma refractory additive, the fluoride leaching value could be reduced. In order to achieve a fluoride leaching value of less than 5 mg / L a relatively high amount of doloma is required and relatively strong carbonation conditions. Apparently, the doloma particles need to be carbonated to liberate the magnesium oxide contained therein. An advantage of the use of doloma is that the compressive strength of the carbonated articles is substantially not negatively affected by the doloma particles, which indicates that those particles, and especially the calcium oxide contained therein, contribute to the carbonation reaction.

[0149] In addition to the leaching test in accordance with EN 12457-2 on carbonated articles with size reduction to less than 4 mm, the fluoride leaching value of some of the carbonated articles have also been tested in accordance with DIN EN 12457-4:2003-01 , with carbonated articles reduced to a size of less than 10 mm. These test results were comparable to the test results obtained in accordance with EN 12457-2:2003-01.

[0150] Leaching results Apart from the leaching of fluoride, also the leaching values of other elements have been measured according to EN 12457-2, with analysis of all relevant parameters. The results for fluoride for the 3 additives are given in Table 6 and the leaching concentrations off all elements in Table 7. For the following elements all values remained below the reporting limits: Cd (<1 pig / L); Co (<5 pig / L); Cu (<5 pig / L); Ni (<5 pig / L); As, Pb (<10 pig / L); Sb, Se, Sn (<20 pg / L) and Al (<25 pg / L). These elements are therefore not included in the tables. Table 7: Leaching values of the carbonate bonded, compacted articles.

[0151] The leaching of chromium is considerably suppressed with the addition of the MgO additive A1 in case of the carbonation conditions CM1 and CM2. All the three different additives considerably reduced the leaching of vanadium.

[0152] Examples 34 to 43

[0153] General test procedure

[0154] Compared to the previous examples, other fresh batches of stainless steel slag fines F and of the fine stainless steel slag sand fraction S taken from the daily production were used in these examples. The fines F and the sand fractions S were dried at a temperature of 110°C. After drying the fines were sieved by means of a 500 pm sieve. In some examples, natural sand NS having a D5o sieve size of about 250 pm was additionally used.

[0155] The different mixtures were made and all mixtures were moisturized with 10% by dry weight of water at least 2 hours before compacting. Compaction was carried out in a cylindrical mould with a compaction force of 4 MPa. About 800 g of the wet mixture was applied in the cylindrical mould. The compacts were removed from the mould and the moulded compacts were then carbonated. Two identical compacts were made from each mixture, one for measuring the compressive strength and a second one for measuring the fluoride leaching value in accordance with DIN EN 12457-4:2003-01.

[0156] Carbonation was carried out in a climate chamber which kept at atmospheric pressure and controlled at 40°C, 50% CO2 and 75-80% relative humidity. Carbonation was carried out for 24 or 48 hours.

[0157] Magnesium-bearing additives

[0158] The following magnesium-bearing additives were used:

[0159] - MgO; and

[0160] - MgCl2.

[0161] The magnesium chloride was either dissolved in the water (MgCl2_D) used to produce the mixture and / or it was mixed as a solid powder (MgCl2_S) in the mixture. 2% by dry weight of MgCh could be dissolved in the 10% by dry weight of water which was used to make the mixtures. 20 wt.% of MgCh was thus dissolved in the water.

[0162] The magnesium oxide was not soluble in water.

[0163] Experimental results Two different mixtures were made with the stainless steel slag fines F, the stainless steel slag sand fraction S and the natural sand NS. A first mixture M1 contained only the stainless steel slag fines F. A second mixture M2 contained the stainless steel slag fines F, the stainless steel slag 0-0.5 mm sand fraction S and the natural sand NS in a 50 / 30 / 20 ratio. To these mixtures, one or more of the additives were added. As described in the general procedure carbonate bonded compacts were made with the different mixtures and the fluoride leaching was determined. The test results indicated in Table 8 are each based on the measurement of one carbonated compact, which was however much larger than the compacts made in the preceding examples.

[0164] Table 8: Composition of the different mixtures and fluoride leaching values

[0165] In the experiments with Batch 2 the stainless steel slag fines leached out more fluoride. This may be due to the fluoride content of these fines.

[0166] Examples Ex.34, 37, 39 and 42 show that the fluoride leaching value of the different mixtures M1 and M2 could be reduced to a value of about 5 mg / L or less by means of only 2 wt% of MgCh (based on the dry weight of the mixture) dissolved in the water used to make the mixtures. These results are better than the results obtained with the different MgO additives used in examples Ex.1 to Ex.6.

[0167] The MgO additives may hydrate in the pore water, especially during the carbonating step during which the pH of the pore water was found to be reduced for example from a pH value of about 12.5 to a pH value of less than 8.2. The inventors have indeed cut cylindrical compacts made of mixture M1 and freshly carbonated for 3 hours at 40°C with 50% CO2 in two pieces, and have made the carbonation front visible with phenolphthalein. The core of the compacts still had a pH of between 8.2 and 12.5 whilst the outer rim part had a pH of less than 8.2. At such low pH values, the MgO, even when dead-burned, was found to hydrate. The MgO is thus an alkaline substance:

[0168] MgO + H2O Mg(OH)2.

[0169] At lower pH values, especially at pH values of less than 10.33, the produced magnesium hydroxide may dissolve in the pore water thus releasing hydroxyl ions in the pore water.

[0170] The water soluble magnesium salt, on the other hand, such as for example the MgCl2, was found to be an acidic substance, especially at pH values higher than 10.33, which reduces the pH of the pore water by the following precipitation reaction of magnesium hydroxide (and / or compounds such as Mg2CI(OH)3.4H2O):

[0171] MgCI2+ 2OH- Mg(OH)2+ 2Ch.

[0172] When adding magnesium chloride, an amount of hydroxide is precipitated so that the pH decreases down to a pH value which is dependent on the amount of magnesium ions in solution. Due to the pH decrease, further CaO (Ca(OH)2) is released from the stainless steel slag fines.

[0173] The following tests were done with additions of 2 and 5% of MgCh to the M1 and M2 mixtures without carbonating step.

[0174] 100 g of the M1 or the M2 mixture was added to 1 L of water. 2 or 5 g of MgCh was added and the mixture was stirred for 2 or 24 hours, after which the pH of the liquid was measured.

[0175] Table 9: Decrease of pH by the addition of MgCI2.

[0176] It can be seen that by adding MgCI2the pH of the water is lowered. The more MgCI2is added, the lower the pH due to the higher magnesium concentration in the solution (based on the Kspvalue of Mg(OH)2, a higher amount of magnesium in the solution corresponds to a lower amount of hydroxyl ions in the solution).

[0177] The MgCI2may have two effects on the immobilisation of fluoride. When adding MgCI2, the pH quickly drops so that fluoride is released and a Mg(OH)2precipitate is formed. This magnesium hydroxide has a high specific surface area. Moreover, when fluoride co-precipitates with the magnesium hydroxide much more fluoride can be immobilised compared to the situation wherein the fluoride has to be adsorbed onto magnesium hydroxide added as such in the form of a solid powder. This appears for example from examples Ex.37 and 42 wherein a higher reduction of the fluoride leaching could be obtained with 2% MgCI2(= 0.85% MgOeq.) than with 2% Mg(OH)2(= 1.23% MgOeq.).

[0178] A second effect of the use of the water soluble MgCI2is that the pore water has a higher magnesium concentration so that more magnesian calcite will be formed, which has moreover a higher magnesium content, so that more fluoride can be immobilised by being co-precipitated with the magnesian calcite. The present inventors did some additional tests which prove that also at the end of the carbonation process, the pore water still has a high magnesium concentration. They measured indeed the pH of stainless steel slag fines, carbonated in the same way as examples Ex.34 to 43, which have been mixed with different amounts of magnesium chloride and also measured the magnesium and calcium leaching values thereof in accordance with DIN EN 12457-4:2003-01.

[0179] Table 10: pH values, magnesium and calcium leaching values(mg / L) and mole % of Mg on the total molar Ca and Mg content in the extraction liquid of different stainless steel slag fines - magnesium chloride mixtures after 48 hours carbonation.

[0180] The pH of the carbonated stainless steel slag fines is lower when magnesium chloride has been added thereto. The pH of the carbonated stainless steel slag fines to which no magnesium chloride has been added corresponds substantially to the pH of a saturated Mg(OH)2solution. Since the pH of the carbonated stainless steel slag fines to which magnesium chloride has been added is lower, the water should contain more dissolved magnesium. This also appears from the leaching values of magnesium from the carbonated stainless steel slag fines. During the entire carbonation process, magnesium ions are thus available in solution to form magnesian calcite wherein fluoride can be effectively co-precipitated. The mole percent of magnesium carbonate in this magnesian calcite appears to be somewhat higher than 10%. According to Figure 3 of the article of Minoru Okumura et al. (1983) such magnesian calcite is quite effective to immobilise fluoride.

[0181] As appears from examples Ex.35 and Ex.38 more fluoride can be immobilised by adding, apart from dissolved MgCl2, an additional amount of solid MgCh. When more and more magnesium is precipitated in the form of magnesium hydroxide, and / or in the form of hydrated magnesium carbonates, more MgCl2 will dissolve in the pore water. During the carbonation reaction, there will thus always be a sufficiently high magnesium concentration in the pore water to produce magnesian calcites and magnesium hydroxide (brucite) and hydrated magnesium carbonates.

[0182] Example Ex.40 shows that even when adding only solid MgCl2 a large fraction of the fluoride can be immobilised. It is possible to add a relatively large amount of MgCl2 to immobilise a large part of the fluoride. When considering examples Ex.41 and Ex.43, the use of MgO in addition to MgCl2 appears to be less effective than the use of a larger amount of MgCh. Again, water soluble MgCh appears to be more effective than MgO.

Claims

CLAIMS1. A method for producing a carbonate bonded, compacted article, which method comprises the steps of:- providing a particulate material which comprises a particulate carbonatable binder composed of particles having a sieve size smaller than 500 pm, which particulate carbonatable binder contains fluoride and calcium oxide and silicon dioxide, and has a CaO / SiO2 ratio higher than 1.1 and a pH, measured after a contact time of 24 hours in accordance with DIN EN 12457-4:2003-01 , of at least 10.5, preferably of at least 11.0 and more preferably of at least 11 .5;- compacting the particulate material to form a compact having pores which contain pore water; and- carbonating said compact for a predetermined period of time with a gas which contains carbon dioxide to produce carbonates including calcium carbonates thus transforming the compact into said carbonate bonded, compacted article, which carbonate bonded, compacted article has a fluoride leaching value measured in accordance with DIN EN 12457- 4:2003-01 , characterised in that a fluoride adsorbing, magnesium-bearing solid compound, which comprises surface hydroxyl groups bound to magnesium atoms, is provided in said compact by distributing at least one magnesium-bearing additive in said particulate material prior to said carbonating step, which additive has a magnesium content, expressed as oxide and by dry weight, of at least 30 wt.% MgO and comprises said fluoride adsorbing, magnesium-bearing solid compound and / or produces said fluoride adsorbing, magnesium-bearing solid compound in said particulate material by reaction in said pore water.

2. The method according to claim 1 , wherein said at least one magnesium-bearing additive comprises a magnesium-aluminium hydrotalcite.

3. The method according to claim 1 or 2, wherein said at least one magnesium-bearing additive comprises magnesia, doloma or magnesia- doloma, preferably magnesia or magnesia-doloma having a magnesium content of at least 50 wt% MgO, preferably at least 60 wt.% MgO.

4. The method according to any one of the claims 1 to 3, wherein said at least one magnesium-bearing additive has a D5o sieve size value smaller than 15.0 gm, preferably smaller than 10.0 pm and more preferably smaller than 5.0 pm.

5. The method according to any one of the claims 1 to 4, wherein said at least one magnesium-bearing additive comprises magnesia, doloma or magnesia-doloma, which magnesia, doloma or magnesia-doloma has a citric acid reactivity of less than 900 sec, preferably of less than 600 sec, and more preferably of less than 300 sec.

6. The method according to any one of the claims 1 to 5, wherein said at least one additive comprises magnesia, doloma or magnesia- doloma, which magnesia, doloma or magnesia-doloma has a citric acid reactivity of more than 60 sec.

7. The method according to any one of the claims 1 to 6, wherein said at least one magnesium-bearing additive comprises spent magnesia, doloma or magnesia-doloma refractory material, in particular spent refractory material recycled from ironmaking or steelmaking furnaces.

8. The method according to any one of the claims 1 to 7, wherein said at least one magnesium-bearing additive comprises hydrated magnesia, hydrated doloma or hydrated magnesia-doloma, said at least one magnesium-bearing additive preferably comprising hydrated magnesia.

9. The method according to any one of the claims 1 to 8, wherein said at least one magnesium-bearing additive is at least partially dissolved in said pore water to provide magnesium ions in said pore water, at least a portion of the magnesium ions which are dissolved in said pore water precipitating by reaction with at least hydroxyl ions present in the pore water to produce in particular brucite and / or hydrated magnesium carbonates such as nesquehonite and hydromagnesite as said fluoride adsorbing, magnesium- bearing solid compound.

10. The method according to claim 9, wherein said brucite and / or said hydrated magnesium carbonates are co-precipitated with at least a portion of the fluoride which is leached out of the particulate carbonatable binder during said carbonating step.

11. The method according to claim 9 or 10, wherein said magnesium ions are at least partially co-precipitated with calcium and carbonate ions present in said pore water to produce magnesian calcite during said carbonating step, which magnesian calcite is co-precipitated with at least a portion of the fluoride which is leached out of the particulate carbonatable binder during said carbonating step.

12. The method according to any one of the claims 1 to 11 , wherein said gas has a partial carbon dioxide pressure of at least 0.10 bar, preferably at least 0.20 bars, more preferably at least 0.30 bars and most preferably at least 0.40 bars, the partial carbon dioxide pressure in said gas being preferably lower than 5.0 bars, more preferably lower than 2.0 bars and more preferably lower than 1 .0 bar.

13. The method according to any one of the claims 1 to 12, wherein said compact is carbonated until said carbonate bonded, compacted article has a pH, measured after a contact time of 24 hours in accordance with DIN EN 12457-4:2003-01 , lower than 11.0, preferably lower than 10.8 and more preferably lower than 10.6, the pH of the carbonate bonded, compacted article being preferably higher than 9.0 and more preferably higher than 9.5.

14. The method according to any one of the claims 1 to 13, wherein said at least one magnesium-bearing additive comprises at least one water-soluble magnesium salt, which is preferably selected from the group consisting of magnesium chloride, magnesium nitrate, magnesium sulphate and magnesium citrate and which is more preferably selected from the group consisting of magnesium chloride, magnesium nitrate, and magnesium citrate, said at least one additive comprising preferably magnesium chloride.

15. The method according to any one of the claims 1 to 14, wherein, during said carbonating step, a predetermined amount of carbonates is produced, said at least one additive being distributed in said particulate material to add an amount of magnesium, expressed as oxide, of at least 2.0 wt.%, preferably of at least 3.0 wt.% and more preferably of at least 4.0 wt.% of said predetermined amount of carbonates to said particulate material.

16. The method according to any one of the claims 1 to 15, wherein, during said carbonating step, an amount of carbon dioxide is taken upby said particulate carbonatable binder to produce said carbonates, said amount of carbon dioxide being at least 6 % by dry weight, preferably at least 8 % by dry weight of said particulate carbonatable binder.

17. The method according to any one of the claims 1 to 16, wherein said at least one additive is distributed in said particulate material to add an amount of magnesium, expressed as oxide, of at least 0.50 wt.%, preferably of at least 0.75 wt.% and more preferably of at least 1 .00 wt.% to said particulate material.

18. The method according to any one of the claims 1 to 17, wherein said at least one magnesium-bearing additive is distributed in said particulate material to add an amount of magnesium, expressed as oxide, of less than 10.0 wt.%, preferably of less than 8.0 wt.% and more preferably of less than 6.0 wt.% to said particulate material.

19. The method according to any one of the claims 1 to 18, wherein said carbonatable binder comprises at least 4000 mg / kg, in particular at least 6000 mg / kg of fluoride.

20. The method according to any one of the claims 1 to 19, wherein, when starting said carbonating step, said compact has a moisture content of between 3.0 and 25 wt.%, and preferably between 5.0 and 20 wt.%.

21. A method according to any one of the claims 1 to 20, characterised in that said particulate carbonatable binder comprises carbonatable slag from a metal production process, slag from the production of phosphorus, bottom ash and / or fly ash, the particulate carbonatable binder preferably comprises steel slag, in particular stainless steel slag.

22. A method according to any one of the claims 1 to 21 , characterised in that said particulate carbonatable binder is at least partially crystalline and comprises cuspidine and / or calcium fluoride.

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