Method for producing carbonate-bound compacts

Abstract

A method for producing a carbonate-bonded compact includes providing a particulate carbonatable material, compressing the particulate material to form a compact, and carbonating the compact. Carbonation of the compact is initiated at a low carbon dioxide partial pressure in a carbonation gas of less than 0.5 bar and continued for at least one hour, followed by carbonation of the compact at a high carbon dioxide partial pressure in a carbonation gas of greater than 0.5 bar for at least eight hours. The two-stage carbonation at low and high carbon dioxide partial pressures allows for a higher compressive strength of the carbonated compact to be achieved within a given carbonation time, particularly within a carbonation time of about 24 hours, thereby enabling new compacts to be carbonated daily.

Description

[Technical field]

[0001] The present invention relates to a method for producing a carbonate bonded compact comprising the steps of: providing a particulate carbonatable material; compressing the particulate material to form a compressed body; and carbonating the compressed body with a gas containing carbon dioxide for a predetermined period of time to produce carbonate, thereby converting the compressed body to a carbonate bonded compact. [Background technology]

[0002] There are various industrial manufacturing processes that produce carbonatable materials as by-products. These by-products are, for example, fly ash, bottom ash (especially municipal waste incineration bottom ash) and slags produced during the production of phosphorus or ferrous or non-ferrous alloys such as zinc, copper, and lead and iron or steel. Dust from air filters, for example from steel furnaces, can also be carbonated, especially if it contains calcium oxide. Some of these by-products can be used for 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 (e.g. LD slags), which have a high neutralization value, can also be used, for example, as soil conditioners. However, other materials, such as bottom ash and stainless steel slags, contain significant amounts of heavy metals that are problematic in terms of their leaching behavior.

[0003] In order to limit both the economic and environmental impact of these domestic and industrial waste materials, there are increasing attempts to develop methods for treating these materials, i.e., methods for converting these waste materials into economically valuable materials. Many of these waste materials are alkaline and contain carbonatable substances, such as calcium oxide and / or hydroxide, and magnesium oxide and / or hydroxide. Other substances, such as calcium silicate, contained in the waste materials, may also be carbonatable. It is known that the carbonation of these substances makes it possible to obtain materials with good mechanical qualities. Furthermore, carbonation may also help to prevent the leaching of pollutants, such as heavy metals.

[0004] For example, WO-A-2007 / 096671 proposes to carry out accelerated carbonation of waste in a rotating drum in order to produce secondary granules that can serve as construction materials. A similar process is proposed in WO-A-2009 / 024826 for waste from metal extraction or processing. In the materials obtained by these processes, the carbonation of the lime present in the waste forms a lime matrix, ensuring both a reduced leaching of the heavy metals contained in the waste and an increased mechanical strength.

[0005] Stainless steel slags are a particular group of slags that contain relatively large amounts of chromium and often also nickel and / or molybdenum. As disclosed in EP-B-0837043, EP-B-1055647, and EP-B-1146022, the leaching problem of stainless steel slags can be solved by grinding the steel slags, removing the valuable stainless steel particles therefrom, and applying different fractions of the remaining ground slag in limited applications, for example as fine or coarse aggregates in concrete or asphalt. However, due to their higher gamma dicalcium silicate (γ-C2S) content, the finer fractions (0-0.5 mm) of these ground steel slags have high water absorption properties and are therefore not suitable for use in concrete or asphalt applications.

[0006] In order to reduce the high water absorption of the fine powders removed from the coarser fractions of ground stainless steel slags, so that these coarse fractions as well as the fine powders can be used in concrete or asphalt, WO2009 / 090219 proposes agglomerating these fine powders under relatively low pressure, followed by carbonation. In this way, aggregates with low water absorption properties and the necessary strength for use in concrete or asphalt could be produced.

[0007] Another carbonation method for producing more valuable construction materials starting from particulate carbonatable material, in particular fine powder of crushed stainless steel slag with a size of 0-0.5 mm, is disclosed in WO-A-2009 / 133120. In this method, the particulate material is first compressed at a relatively high compaction pressure of 5-65 MPa and the resulting compact is subsequently carbonated at relatively high temperature and pressure. In this way, a carbonated compact having a relatively high compressive strength can be produced. By controlling the porosity and intrinsic permeability of the compact and by carbonating for several hours (more specifically 18 hours at high pressure and temperature), a carbonation strength of 182 kg / cm3 or more can be achieved. 2 Compressive strengths of 26-66 MPa were obtained using fine stainless steel slag fractions of 0-500 μm compressed at a compaction pressure of (=17.8 MPa). The disadvantage of this prior art method is that, despite the fact that relatively small blocks were carbonated (62×62×32 mm and 120×55×46 mm), high gas pressures, i.e. more than 5 bar, are required, which makes the process very expensive.

[0008] US2017 / 0073270 discloses a carbonation method in which carbonate-bound building blocks are produced starting from a mixture of slag sand and steel slag binder. In the example "Sample Building Product 1", the steel slag binder is a mixture of EAF slag and BOF slag with very high basicity (39.08 wt.% CaO vs. 12.47 wt.% SiO2), so that the steel slag binder contains a very large amount of free lime that can easily react with carbon dioxide to produce the required carbonates. The mixture contained about 62 wt.% steel slag binder and about 38 wt.% slag sand. The mixture was first compacted at a high compaction pressure of 12 MPa, after which the compact was dried and carbonated with carbon dioxide kept at a constant pressure of 1.5 bar for 24 hours. The carbon dioxide gas was heated to 22°C before being placed in the carbonation chamber.

[0009] After the carbonation process, the carbonated blocks were further hydrated for 35 days to increase their compressive strength. Despite the high compression pressure, large amount of binder, very long carbonation time, and relatively large amount of carbonate produced (6.6% CO2 uptake), the compressive strength was only 22.8 MPa. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] WO-A-2007 / 096671 [Patent Document 2] WO-A-2009 / 024826 [Patent Document 3] EP-B-0837043 [Patent Document 4] EP-B-1055647 [Patent Document 5] EP-B-1146022 [Patent Document 6] WO2009 / 090219 [Patent Document 7] WO-A-2009 / 133120 [Patent Document 8] US2017 / 0073270 Summary of the Invention

[0011] The object of the present invention is to provide a method for producing a carbonation solution having a predetermined carbonation time. In The object of the present invention is to provide a novel method for producing carbonate-bonded compacts, which allows obtaining higher compressive strength of the compacted carbonated bodies.

[0012] For this purpose, the method of the invention is characterized in that carbonation of the compressed body is initiated and then continued for at least 1 hour at a low carbon dioxide partial pressure in the gas, after which carbonation of the compressed body is continued for at least 8 hours at a high carbon dioxide partial pressure in the gas, said low carbon dioxide partial pressure being less than 0.5 bar and said high carbon dioxide partial pressure being equal to or greater than 0.5 bar.

[0013] Carbon dioxide partial pressure is defined as the partial pressure of carbon dioxide in the gas used to carbonate the compressed body. If this gas consists entirely of carbon dioxide, the partial pressure of carbon dioxide is equal to the total pressure of this gas. In practice, however, the gas is usually a mixture of several gases. In particular, the gas may consist of carbon dioxide-rich air. The partial pressure of carbon dioxide in such a gas is then determined by the total pressure of this gas and the volume percentage of carbon dioxide contained in this gas. The pressures shown are absolute pressures.

[0014] The inventors have found that by first carbonating at a low carbon dioxide partial pressure and then carbonating at a higher carbon dioxide partial pressure, a higher compressive strength can be achieved within the same period, particularly within a carbonation time of 24 hours. It is known that a higher carbon dioxide partial pressure, i.e. a higher carbon dioxide concentration in the carbonation chamber and / or a higher gas pressure in the carbonation chamber, accelerates the carbonation process and thus produces more carbonate. As shown, for example, in FIG. 7 of EP2276714B, a higher carbonate content further correlates with a higher compressive strength of the carbonate. According to the present invention, when the compact is first carbonated at a low carbon dioxide partial pressure and then carbonated at a higher carbon dioxide partial pressure, the compressive strength of the carbonate is surprisingly higher. Experiments have shown that, for example, fine stainless steel slag binder with a particle size of 0-500 μm and a moisture content varying between 5-11% makes it possible to produce carbonates with compressive strengths of 46-54 MPa, considerably higher than the abovementioned compressive strengths of about 22 MPa obtained in US 2017 / 0073270, by compressing the slag binder at a relatively small pressure of 4 MPa and carbonating the compressate first for 2 hours at a carbon dioxide partial pressure of 0.4 bar (40 vol.% CO2 at atmospheric pressure) and subsequently for 18 hours at a carbon dioxide partial pressure of 0.8 bar (80 vol.% CO2 at atmospheric pressure). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] In one embodiment of the method according to the invention, after starting the carbonation of the compressed body, the carbonation is continued for at least 1.5 hours at said low carbon dioxide partial pressure in the gas. In a further embodiment of the method according to the invention, after starting the carbonation of the compressed body, the carbonation is continued for less than 16 hours, preferably less than 12 hours, more preferably less than 8 hours at said low carbon dioxide partial pressure in the gas. Carbonation of the compressed body having a low carbon dioxide partial pressure in the gas is therefore preferably carried out for 1 to 16 hours, preferably 1.5 to 12 hours, more preferably 1.5 to 8 hours, most preferably 1.5 to 6 hours after starting the carbonation.

[0016] The advantage of these embodiments is that sufficient time remains in the carbonation period to produce more carbonate with the gas having a high carbon dioxide partial pressure, while improving the strength of the carbonated compact.

[0017] In one embodiment of the method according to the invention, the carbonation of the compressed mass is characterised in that it continues at said high carbon dioxide partial pressure in the gas for at least 12 hours, preferably at least 16 hours.

[0018] In one embodiment of the method according to the invention, the gas with low carbon dioxide partial pressure is at a pressure below 5 bar, preferably below 3 bar, more preferably below 2 bar, most preferably below 1.5 bar, said pressure being preferably equal to or higher than atmospheric pressure. In one embodiment of the method according to the invention, the gas with high carbon dioxide partial pressure is at a pressure below 5 bar, preferably below 3 bar, more preferably below 2 bar, said pressure being preferably equal to or higher than atmospheric pressure.

[0019] In this specification, gas pressure or partial gas pressure is absolute pressure. The relatively low pressures shown in these embodiments allow carbonation to be carried out in a climate chamber that does not need to withstand high pressures, which is advantageous in that preferably no partial vacuum needs to be created. Thus, large volume production is possible without excessively high investments in the necessary carbonation equipment.

[0020] In one embodiment of the method according to the invention, said low carbon dioxide partial pressure is less than 0.45 bar.

[0021] The lower the carbon dioxide partial pressure, the slower the carbonation rate. In accordance with the present invention, it has been found that stronger carbonates can be obtained if the compact is initially carbonated at a slower carbonation rate.

[0022] In one embodiment of the method according to the invention, said low carbon dioxide partial pressure is greater than 0.05 bar, preferably greater than 0.1 bar, more preferably greater than 0.15 bar.

[0023] An advantage of this embodiment is that by using such carbon dioxide partial pressures during the initial stages of the carbonation process, high compressive strengths can be achieved while reducing the time required for the initial low pressure carbonation stage.

[0024] In one embodiment of the process according to the invention, the high carbon dioxide partial pressure is greater than 0.6 bar, preferably greater than 0.7 bar, more preferably greater than 0.75 bar.

[0025] The advantage of this embodiment is that the higher the partial pressure of carbon dioxide in the gas during the second stage of the carbonation process, the faster the carbonate will be produced and the required compressive strength will be achieved, in other words, the duration of the carbonation process can be shortened or a higher compressive strength can be achieved within the same carbonation period.

[0026] In one embodiment of the method according to the invention, the predetermined period during which the compressed mass is carbonated comprises less than 32 hours, particularly less than 28 hours, more particularly less than or equal to 24 hours.

[0027] Such short carbonation times allow high production capacities to be achieved, in particular it is possible to produce fresh amounts of carbonate every day without the need to arrange night shift work.

[0028] In one embodiment of the method according to the invention, the predetermined period during which the compressed mass is carbonated comprises more than 16 hours, particularly more than 18 hours, more particularly more than 20 hours.

[0029] Such a carbonation period makes it possible to achieve relatively high compressive strengths when the compact is carbonated according to the present invention, i.e., when the compact is carbonated at low carbon dioxide partial pressure followed by high carbon dioxide partial pressure.

[0030] In one embodiment of the method according to the invention, the carbonation of the compacted mass is initiated with the gas having a temperature below 50°C, preferably below 45°C, more preferably below 40°C.

[0031] The advantage of such a lower initial temperature is that the carbonation rate is slower and higher compressive strengths can be achieved.

[0032] In one embodiment of the method according to the invention, carbonation of the compacted mass is initiated with the gas having a temperature above 20°C, preferably above 25°C, more preferably above 35°C.

[0033] By using such high temperatures, carbonation of the compact begins sooner and therefore a sufficiently high compressive strength can be achieved in a short period of time.

[0034] In one embodiment of the process according to the invention, during carbonation of the compressed mass the temperature of the gas is increased to a temperature above 50° C., preferably above 55° C., more preferably above 60° C. Preferably the temperature of the gas is maintained below 95° C., more preferably below 90° C.

[0035] Due to the exothermic hydration / carbonation reaction, the temperature automatically increases during the carbonation process. Higher temperatures, especially during the second stage of the carbonation process, further accelerate the carbonation process, however, without adversely affecting the compressive strength that can be achieved.

[0036] In one embodiment of the method according to the invention, the particulate material comprises carbonatable slag from a metal production process, slag from phosphorus production, bottom ash and / or fly ash, preferably the particulate material comprises steelmaking slag, in particular stainless steel slag.

[0037] In one embodiment of the method according to the invention, the particulate material compressed to form said compact has a moisture content, by dry weight, of at least 1%, preferably at least 3%, more preferably at least 5%.

[0038] The minimum amount of water required for the carbonation step is in particular at least 1% by dry weight, while a higher water content is advantageous for achieving a higher green strength of the compacts.

[0039] Other features and advantages of the invention will become apparent from the following more detailed description of some specific embodiments.

[0040] The present invention relates generally to a method for producing a carbonate bond compact by compressing and carbonating a particulate carbonatable material.

[0041] The expressions "particulate material" or "granular material" refer to any material that consists of discrete particles. These particles may be of different sizes, such that the expression "particulate material" encompasses not only coarse or fine grains, but also very fine grains, in particular powders.

[0042] The particulate material may consist of one particulate material capable of carbonation, or may consist of a mixture of at least one first particulate material capable of carbonation and at least one second particulate material that may or may not be capable of carbonation. Particulate materials that are not capable of carbonation include natural sands, such as sea sand or river sand, for example.

[0043] The particle size distribution of the particulate material, or of the mixture of the first and second particulate materials, is preferably selected to achieve a higher packing density, or in other words a smaller total porosity, since this allows a higher compressive strength to be obtained. The compressive strength of the compact before the carbonation step, i.e. the green strength of the compact, and the compressive strength of the carbonated compact are determined in accordance with European Standard EN12390-3:2009.

[0044] The carbonatable particulate material, i.e. in the case of the carbonatable material as a whole or the above-mentioned mixture of the first specific material and the second specific material, if the first particulate material and / or the second particulate material are carbonatable, preferably comprises a by-product or waste. The carbonatable particulate material has in particular a pH of at least 8.3 and comprises at least one source of alkaline earth metal, in particular a source of calcium. The pH of the carbonatable material is defined as the pH of the demineralized water in which the particulate material is soaked for 18 hours at a liquid / solid ratio of 4.5. The carbonatable material may contain different crystalline and amorphous phases, and preferably contains at least one alkaline earth metal silicate phase, in particular crystalline dicalcium silicate.

[0045] The carbonatable particulate material also preferably contains calcium oxide and / or calcium hydroxide, the total amount of calcium oxide and calcium hydroxide being preferably at least 1% by dry weight, more preferably at least 2% by dry weight. It may also contain magnesium oxide and / or magnesium hydroxide. These oxides and hydroxides may be in amorphous and / or crystalline form, in particular in the form of portlandite (Ca(OH)2), free lime (CaO), brucite (Mg(OH)2) and periclase (MgO). They may also be in amorphous or crystalline form, containing various ratios of magnesium and calcium, as well as oxygen. Initially, as they are often produced under high temperatures, freshly produced carbonatable materials usually contain no hydroxides, only oxides, with hydroxides being formed during the aging (weathering) of the carbonatable material or during the carbonation process. As air also contains small amounts of carbon dioxide, some of the hydroxides are further converted (by natural carbonation) to carbonates during the aging of the carbonatable material.

[0046] A wide variety of carbonatable materials are suitable to be treated according to the method of the present invention. A suitable carbonatable material is, for example, bottom ash, more particularly bottom ash produced during the incineration of waste, particularly municipal waste (i.e. municipal waste incineration bottom ash). Fly ash, particularly non-coal fly ash, can also be carbonated, as well as filter dust from steel furnaces, particularly electric arc furnaces (EAF filter dust). However, the most preferred carbonatable materials are slag materials resulting from metal production processes (production of pig iron, steel, stainless steel, and non-ferrous metals such as copper and zinc) and from the production of phosphorus. The carbonatable materials used are preferably non-hydraulic or substantially non-hydraulic materials. Since non-hydraulic materials cannot produce such a sedimentary matrix by reaction with water (particularly by CSH formation), solid objects can still be produced by carbonation of this material.

[0047] The slag material may be blast furnace slag, but is preferably steel slag, more preferably stainless steel production slag. The steel slag may be converter slag (such as LD slag) or electric arc furnace slag (EAF slag). Common steel slags do not contain heavy metals such as chromium and nickel, or contain only small amounts of them, and therefore do not have the leaching problems that occur with stainless steel slags. Stainless steel slags generally contain more than 3000 mg / kg chromium, usually even more than 5000 mg / kg chromium. They may also contain nickel, more specifically more than 300 mg / kg, especially more than 400 mg / kg, and often more than 500 mg / kg nickel. By carbonating these carbonatable slags, the leaching of these heavy metals can be reduced or even prevented.

[0048] Steelmaking slags, especially stainless steel slags, are usually crushed to produce granular materials from which the metal fraction can be recycled. The coarse fraction of crushed stainless steel slags can be used as coarse or fine aggregates in asphalt concrete. The fine fraction, especially the 0-500 μm fraction, is however not suitable for these applications due to its too high water absorption properties. The fine fraction, i.e. the so-called fine powder, actually contains a large amount of gamma dicalcium silicate (γ-C2S) which is generated during the solidification of the liquid slag when part of the beta dicalcium silicate (β-C2S) is further converted in the gamma polymorph. Due to the resulting expansion, cracks are formed and so-called falling slags with high water absorption properties are generated. This stainless steel slag material, which in particular contains at least 3% by dry weight, more particularly at least 5% by dry weight, even more particularly at least 7% by dry weight of gamma dicalcium silicate, is preferably used as the granular material or as one of the granular materials in the method of the invention.

[0049] In the method of the present invention, the particulate carbonatable material is first compressed to produce a compacted body. This can be done by placing the material in a mold and vibrating or pressurizing the material therein. After compressing the carbonatable material, the produced compacted body is carbonated with a gas containing carbon dioxide, thereby producing carbonate that converts the compacted body into a carbonate-bonded compact.

[0050] The compact is removed from the mould and placed in a carbonation chamber. This chamber may be an autoclave where a relatively high gas pressure is applied. However, in the method according to the invention, the carbonation step is preferably carried out at a lower gas pressure, in particular at a total gas pressure of less than 5 bar, preferably less than 3 bar, more preferably less than 2 bar, most preferably less than 1.5 bar. The carbonation step may in particular be carried out at atmospheric pressure. A cheaper climatic chamber may therefore be used as the carbonation chamber.

[0051] The compressed mass is preferably carbonated for a period of 16 to 32 hours, more preferably 18 to 28 hours, most preferably 20 to 24 hours. In practice, a total carbonation time of up to 24 hours is preferred so that a fresh amount of compressed mass can be carbonated each day.

[0052] The compressive strength of the carbonated compact depends on a number of factors, such as the nature and size distribution of the particulate material, the degree of compaction (i.e., porosity), and the amount of carbonate produced in the carbonated compact by the carbonation reaction. More carbonate can be produced at higher temperatures and higher carbon dioxide partial pressures in the same period of time.

[0053] However, in accordance with the present invention, it has been found that higher compressive strengths can be achieved not by producing as much carbonate as possible within a given carbonation period, but by first carbonate the compact more slowly at a low carbon dioxide partial pressure in the carbonation chamber during an initial stage of the carbonation process, and then carbonate the compact more quickly at a higher carbon dioxide partial pressure during the next stage of the carbonation process.

[0054] In the method according to the invention, carbonation of the compressed mass is initiated and then carbonation of the compressed mass is continued for at least 1 hour at a low carbon dioxide partial pressure in the carbonation chamber and thereafter for at least 8 hours at a high carbon dioxide partial pressure in the carbonation chamber, the low carbon dioxide partial pressure being less than 0.5 bar, while the high carbon dioxide partial pressure is equal to or greater than 0.5 bar.

[0055] Carbonation of the compressed body begins from the moment the compressed body comes into contact with a gas in the carbonation chamber that has a higher carbon dioxide content than the ambient air. The next stage of the carbonation process begins when the compressed body subsequently comes into contact with a gas that provides a low carbon dioxide partial pressure in the carbonation chamber. Usually, the compressed body is first placed in the carbonation chamber after the carbon dioxide partial pressure has been provided, in particular by providing carbon dioxide gas in the carbonation chamber. During the carbonation stage at low carbon dioxide partial pressure, the carbon dioxide content or the corresponding partial pressure can be monitored and additional carbon dioxide can be provided to the carbonation chamber to maintain the carbon dioxide partial pressure within the required range. Carbon dioxide is consumed during the carbonation process and should usually be replenished in the carbonation chamber. To start the carbonation stage at a high carbon dioxide partial pressure, additional carbon dioxide can be provided to the carbonation chamber to increase the carbon dioxide partial pressure in the carbonation chamber.

[0056] In the above embodiment, the pressure in the carbonation chamber can be held constant and the carbon dioxide partial pressure is altered by varying the carbon dioxide content in the gas contained in the carbonation chamber.

[0057] The gas in the carbonation chamber can also be recycled to a first vessel containing gas with a low carbon dioxide content, followed by a second vessel containing gas with a higher carbon dioxide content. In this way, loss of carbon dioxide to the atmosphere can be avoided or reduced when the carbonation chamber is opened and the carbonated compact is removed from the carbonation chamber. Alternatively, at the end of the carbonation process, the carbon dioxide content in the carbonation chamber can be first lowered before the carbonated compact is removed from the carbonation chamber.

[0058] The gas with low carbon dioxide content may be flue gas. If at atmospheric pressure the carbon dioxide partial pressure in the flue gas is not high enough for the first stage of the carbonation process, the pressure of the flue gas can be increased. If a carbon dioxide partial pressure of 0.4 bar is required and the flue gas contains 20% by volume of carbon dioxide, the flue gas should be brought to a pressure of about 2 bar in the carbonation chamber.

[0059] Also provided is a continuous carbonation system in which the compressed body is conducted through a first zone in which a low carbon dioxide partial pressure is maintained, followed by a second zone in which a high carbon dioxide partial pressure is maintained. Such a system can be performed especially where the various zones are at atmospheric pressure to avoid gas escaping the system.

[0060] Instead of or in addition to adjusting the carbon dioxide partial pressure by adjusting the carbon dioxide content of the gas used to carbonate the compressed body, it is also possible to increase the pressure of that gas if a higher carbon dioxide partial pressure is required. This can be obtained automatically in the carbonation chamber when the carbonation chamber is sealed by adding additional carbon dioxide gas to increase the partial pressure from a low carbon dioxide partial pressure to a high carbon dioxide partial pressure. If the first stage of the carbonation process is carried out at atmospheric pressure, the second stage of the carbonation process can be carried out at about 0.5 bar or 1.5 bar by adding carbon dioxide pressure.

[0061] The first stage of the carbonation process, i.e. carbonation of the compressed mass at low carbon dioxide partial pressure, takes at least 1 hour, preferably at least 1.5 hours. Preferably, this first stage is stopped by increasing the carbon dioxide partial pressure, especially in the carbonation chamber, within less than 16 hours, preferably within less than 8 hours, more preferably within less than 6 hours. Thereafter, the second stage of the carbonation process, i.e. carbonation of the compressed mass at high carbon dioxide partial pressure, takes at least 8 hours, preferably at least 12 hours, more preferably at least 16 hours.

[0062] According to the present invention, the low carbon dioxide partial pressure applied in the carbonation chamber during the first carbonation stage is less than 0.5 bar, preferably less than 0.45 bar. It has been found that such a low carbon dioxide partial pressure results in a relatively slow carbonation of the compact, which produces carbonates that impart greater strength to the carbonated compact, even when the carbonation is accelerated in the next carbonation stage by providing a higher carbon dioxide partial pressure in the carbonation chamber.

[0063] In order to produce the required strength-increasing carbonate during the first stage of the carbonation process, the low carbon dioxide partial pressure provided during that first carbonation stage in the carbonation chamber is preferably above 0.05 bar, more preferably above 0.1 bar, most preferably above 0.15 bar. The low carbonation partial pressure is therefore preferably comprised between 0.05 and 0.5 bar, preferably between 0.1 and 0.45 bar, most preferably between 0.15 and 0.4 bar.

[0064] During the second stage of the carbonation process, in order to produce the required amount of carbonate, the high carbon dioxide partial pressure provided during that second carbonation stage in the carbonation chamber is more than 0.5 bar, preferably more than 0.6 bar, more preferably more than 0.7 bar, most preferably more than 0.75 bar. The high carbonation partial pressure is preferably less than 5 bar, more preferably less than 3 bar, even more preferably less than 2 bar, most preferably less than 1.5 bar. Thus, the high carbonation partial pressure is preferably comprised between 0.5 and 5 bar, preferably between 0.6 and 3 bar, most preferably between 0.7 and 2 bar.

[0065] The temperature of the carbonation gas also affects the carbonation rate: higher temperatures accelerate carbonation, while lower temperatures result in slower carbonation.

[0066] In the first carbonation stage, the carbonation of the compressed body is preferably started with a carbonation gas having a temperature below 50° C., preferably below 45° C., more preferably below 40° C., since the carbonation process must be sufficiently slow. In order to start the carbonation process sufficiently quickly, the carbonation of the compressed body is preferably started with a carbonation gas having a temperature above 20° C., preferably above 25° C., more preferably above 35° C. During the second carbonation stage, the temperature of the carbonation gas is preferably increased or can be increased to a temperature above 50° C., preferably above 55° C., more preferably above 60° C. Preferably, this temperature should be maintained below 95° C. or at least 10° C. below the boiling point of water at the pressure prevailing in the carbonation chamber, in order to avoid boiling of the water contained in the compressed body. EXAMPLES

[0067] Example 1 Stainless steel slag material was ground to a particle size of 0-35 mm and separated into a 10-35 mm fraction and a 0-10 mm fraction. The 0-10 mm fraction was separated into a 0-2 mm fraction and a 2-10 mm fraction.

[0068] Steel particles were removed from the 0-2 mm fraction and the fraction was separated into a 0.5-2 mm coarse sand fraction and a 0-0.5 mm fine sand fraction.

[0069] The fine sand fraction was dried and its moisture content (expressed in % by weight) was adjusted to the values ​​shown in Table 1. The compacts were made with 100% fine sand fraction at a compression pressure of 0.4 MPa. Carbonation was carried out at atmospheric pressure, i.e. at a pressure of about 1 bar absolute. The gas used to carbonate the compacts is a mixture of gases obtained by enriching air with CO2. The CO2 content of the gas used to carbonate the compacts (enriched air) is given in volume percent. Since the total pressure of the gas used to carbonate the compacts is equal to the sum of the partial pressures of the different gases contained in this gas (mainly CO2, N2 and O2 in this case), the partial pressure of carbon dioxide in this gas can be easily determined and is equal to the volume percentage of CO2 in the gas multiplied by the total pressure of the gas. In this example, the total pressure of the gas used to carbonate the compressed mass was equal to about 1 bar, so that when this gas contained 40% CO by volume (first carbonation stage), the partial pressure of carbon dioxide in this gas was equal to about 0.4 bar, while when this gas contained 80% CO by volume (second carbonation stage), the partial pressure of carbon dioxide in this gas was equal to about 0.8 bar.

[0070] TIFF0007633948000001.tif42167

[0071] Due to the relatively high reactivity of the fine sand fraction, high compressive strengths were obtained after a total carbonation time of 20 hours. These compressive strengths were much higher than those obtained in the example "Sample Building Product 1" of US2017 / 0073270, where a reactive steel slag binder was also used and with a similar moisture content, but on the other hand a much higher compression pressure of 12 MPa was applied, despite the fact that this should normally result in a higher compressive strength due to the reduced porosity, the resulting carbonated building block only had a compressive strength of 22.8 MPa.

[0072] Example 2 In this example, the same 0-0.5 mm fine sand fraction used in Example 1 was used along with a 0.5-2 mm sand fraction and a 2-6 mm fraction sieved from the 2-10 mm fraction.

[0073] TIFF0007633948000002.tif70164

[0074] It can be seen that the two-stage carbonation process resulted in a higher compressive strength than the comparative example, despite the fact that pure CO2 gas was used at a higher pressure of 1.5 bar and for a longer period of time, i.e., 24 hours instead of just 20 hours.

[0075] Example 3 In this example, the same 0-0.5 mm fine sand fraction used in Example 1 was used along with 0.5-2 mm sand fraction.

[0076] TIFF0007633948000003.tif76161

[0077] It can be seen that the two-stage carbonation process resulted in a higher compressive strength than the comparative example, despite the fact that pure CO2 gas was used at a higher pressure of 1.5 bar and for a longer period of time, i.e., 24 hours instead of just 22 hours.

Claims

1. 1. A method for producing a carbonate bonded compact comprising the steps of: - providing a particulate carbonatable material; - compressing the particulate carbonatable material to form a compressed body; - carbonating said compact with a gas containing carbon dioxide for a predetermined period of time to produce carbonate, thereby converting said compact into said carbonate-bound compact; Including, carbonation of the compressed body is commenced and then continued for at least 1 hour and less than 16 hours at a first carbon dioxide partial pressure in the gas, after which carbonation of the compressed body is continued for at least 8 hours at a second carbon dioxide partial pressure in the gas, the first carbon dioxide partial pressure being less than 0.5 bar and the second carbon dioxide partial pressure being equal to or greater than 0.5 bar; A method comprising:

2. 2. The method of claim 1, wherein after initiating carbonation of the compressed body, the carbonation of the compressed body is continued for at least 1.5 hours at the first carbon dioxide partial pressure in the gas.

3. 3. The method of claim 1 or 2, characterized in that after initiating the carbonation of the compressed body, the carbonation of the compressed body is continued for less than 12 hours at the first carbon dioxide partial pressure in the gas.

4. The method of claim 3, characterized in that after initiating carbonation of the compressed body, carbonation of the compressed body is continued for less than 8 hours at the first carbon dioxide partial pressure in the gas.

5. 5. The method according to any one of claims 1 to 4, characterized in that the carbonation of the compressed mass is continued for at least 12 hours at the second carbon dioxide partial pressure in the gas.

6. The method according to any one of claims 1 to 5, characterized in that the gas having the first carbon dioxide partial pressure is less than 5 bar.

7. The method according to any one of claims 1 to 6, characterized in that the gas having the second carbon dioxide partial pressure is less than 5 bar.

8. 8. The method according to claim 1, wherein the first carbon dioxide partial pressure is less than 0.45 bar.

9. 9. The method according to any one of claims 1 to 8, characterized in that the first carbon dioxide partial pressure is greater than 0.05 bar.

10. 10. The method according to any one of claims 1 to 9, characterized in that the second carbon dioxide partial pressure is greater than 0.6 bar.

11. The method of claim 10, wherein the second carbon dioxide partial pressure is greater than 0.7 bar.

12. The method according to any one of claims 1 to 11, characterized in that said predetermined period of time comprises less than 32 hours.

13. The method according to any one of claims 1 to 12, characterized in that the predetermined period of time comprises more than 16 hours.

14. A method according to any one of the preceding claims, characterized in that the carbonation of the compressed mass is initiated with the gas having a temperature below 50°C.

15. A method according to any one of the preceding claims, characterized in that the carbonation of the compressed mass is initiated with the gas having a temperature that is above 20°C.

16. A method according to any one of the preceding claims, characterized in that during carbonation of the compressed mass the temperature of the gas is increased to a temperature which is above 50°C.

17. 17. A method according to any one of claims 1 to 16, characterised in that the particulate carbonatable material comprises carbonatable slag from metal production processes, slag from phosphorus production, bottom ash and / or fly ash.

18. The method of claim 17, wherein the particulate carbonatable material comprises stainless steel slag.

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