HYDROTHERMAL TREATMENT OF DOLOMITE / LIMESTONE DUST AND FLOUR WITH STEAM IN AN AUTOCLAVE

Description

[0001] The invention relates to a method for treating rock mixtures.

[0002] When burning rock mixtures such as raw dolomite or limestone, magnesium carbonate and / or calcium carbonate are converted into their respective oxides. These magnesium and / or calcium oxides then react with water in a slaking step to form the corresponding hydroxide compounds. A variety of methods have been developed for slaking these burned rock mixtures.

[0003] However, during the firing of the rock mixtures, particularly in rotary kilns, filter dust is produced which, in addition to magnesium carbonate and / or calcium carbonate particles, contains a high proportion of overburned or sintered magnesium oxide and / or calcium oxide particles. Overburned or sintered magnesium oxide and / or calcium oxide particles from these rock mixtures are characterized by the fact that they were exposed to particularly high temperatures during the firing process. This resulted in a significant densification of the magnesium oxide and / or calcium oxide particles due to the sintering or melting of components of the rock mixtures. This densification causes overburned or sintered magnesium oxide and / or calcium oxide particles to exhibit longer reaction times or even to be unreactive to water.

[0004] Filter dusts containing rock mixtures with a high proportion of overburned or sintered magnesium oxide and / or calcium oxide particles cannot be quenched using existing hydration methods for quenching burnt rock mixtures because they exhibit insufficient reactivity with water. With all currently available quenching methods, significant amounts of unslaked magnesium oxide and / or calcium oxide particles always remain.

[0005] The unreacted components of the aggregate mixtures can cause significant problems when filter dust is used in the construction industry. Because the filter dust still contains considerable amounts of unreacted magnesium oxide and / or calcium oxide particles, these can react slowly with atmospheric water over a period of years. This reaction causes the filter dust particles to swell within the building material, leading to cracks and weak points. In the worst-case scenario, such cracks can cause walls to collapse. Consequently, filter dust from combustion processes is not used in the construction industry and is instead disposed of or landfilled.

[0006] A process that could reliably and efficiently quench rock mixtures containing overburned or sintered magnesium oxide and / or calcium oxide particles would enable the use of quenched filter dusts as building materials in the construction industry.

[0007] When overburned or sintered magnesium oxide and / or calcium oxide particles are mentioned here or elsewhere, this also includes calcium oxide magnesium oxide particles.

[0008] The currently used methods for quenching burnt rock mixtures containing magnesium oxide and / or calcium oxide do not meet these requirements.

[0009] German patent DE 1 571 323 A1 describes a process for slaking dolomitic lime with liquid water under pressure in an autoclave. However, the aim of this process is to produce dead-burned, refractory masses, which are subsequently obtained from the hydroxide compounds by sintering. Therefore, the process is not intended for slaking filter dusts with a high proportion of overburned or sintered magnesium oxide and / or calcium oxide particles.

[0010] FR 2 692 253 A1 addresses the problem of incomplete hydration of sulfur- and lime-containing ash. In a continuous process, the ash is mixed with cold water at below 50 °C and then reacts with water in an autoclave at temperatures of 110–170 °C for 20–40 minutes. The hydration process described in FR 2 692 253 A1 is not intended for slaking rock mixtures containing overburned or sintered magnesium oxide and / or calcium oxide particles.

[0011] US 2009 / 283016 A1 describes a process for treating cement kiln dust with a hydration step, a dehydration step and a carbonization step with CO2 gas in a liquid bed reactor.

[0012] US Patent 4,407,677 describes a composition for the manufacture of a cement block containing aggregate, cement, and cement kiln dust.

[0013] None of the hydration processes described in the prior art offer a solution for quenching rock mixtures comprising magnesium oxide and / or calcium oxide that have a high overburned or sintered content, such as filter dusts.

[0014] The present invention therefore aims to provide a method for treating a rock mixture comprising magnesium oxide and / or calcium oxide. In particular, the invention aims to provide a method for treating a rock mixture with a high proportion of overburned or sintered magnesium oxide and / or calcium oxide particles.

[0015] Furthermore, the invention aims to provide a method for treating filter dusts.

[0016] The invention also aims to enable the use of filter dust as a filler in the construction industry by treating the rock mixture with a high proportion of overburned or sintered magnesium oxide and / or calcium oxide particles.

[0017] These tasks are accomplished by the method described in claim 1.

[0018] Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general concept of the invention.

[0019] The inventive method for treating a rock mixture comprising at least magnesium oxide and / or calcium oxide is characterized in that the rock mixture is treated with steam at a pressure greater than 1.01 bar. According to a preferred embodiment of the invention, the steam is pressurized steam.

[0020] Surprisingly, it has been found that treating a rock mixture comprising at least magnesium oxide and / or calcium oxide with steam, in particular with pressurized steam, at a pressure greater than 1.01 bar, makes it possible to convert the rock mixture comprising at least magnesium oxide and / or calcium oxide with a high proportion of overburned or sintered rock particles to a proportion of at least > 80%, in particular to a proportion of at least > 90%, into the corresponding magnesium and / or calcium hydroxide compounds.

[0021] Without committing to a specific scientific theory, it appears that steam, particularly pressurized steam, forced into a reactor at a pressure greater than 1.01 bar, is able to diffuse into or fracture the sintered or overburned rock particles, thereby reacting with the internal magnesium oxide and / or calcium oxide components. In this way, the steam manages to induce a reaction in the otherwise relatively unreactive overburned or sintered rock particles due to their high density. Experimental studies have shown that for the successful reaction of rock mixtures containing at least magnesium oxide and / or calcium oxide with a high proportion of overburned or sintered material, the steam, especially pressurized steam, must be generated before entering the reactor.The steam, especially pressurized steam, is then forced into a reactor under overpressure and reacts there with the rock mixture at a pressure greater than 1.01 bar.

[0022] The use of steam, particularly pressurized steam, for the reaction of the rock mixture comprising at least magnesium oxide and / or calcium oxide has the advantage over other hydration processes that the introduction of steam, especially pressurized steam, ensures better control of the reaction. The moisture content can be better controlled, and the process economy improves because less mass needs to be moved, especially compared to hydration processes using liquid water.

[0023] When pressurized steam is mentioned here or elsewhere, it refers to steam that exists at a temperature and pressure corresponding to the saturated steam curve of water. Pressurized steam is obtained by heating water to its boiling point and then heating it further by the amount of heat of vaporization. The boiling point of water shifts depending on the pressure, so a higher temperature must be chosen at higher pressures to obtain pressurized steam.

[0024] The specific temperature-pressure relationship of saturated steam is also recorded in the following table, where T is the temperature in °C and p is the pressure in bar: T [°C] p [bar] 100 1.01 110 1.04 120 1.98 130 2.70 140 3.61 150 4.76 160 6.18 170 7.91 180 10.02 190 12.54 200 15.54 210 19.06 220 23.18 230 27.95 240 33.45 250 39.74 260 46.90 270 55.00 280 64.13 290 74.38 300 85.84

[0025] Deviations of the exact temperature-pressure equilibrium of saturated steam of approximately ± 2 °C are acceptable according to the invention for pressurized steam.

[0026] According to a preferred embodiment of the invention, the method according to the invention comprises at least the following steps: a. Placing the rock mixture in an autoclave; b. Introducing steam, in particular pressurized steam, into the autoclave; c. Treating the rock mixture with the steam, in particular pressurized steam, at a pressure greater than 1.01 bar.

[0027] These process steps ensure that the rock mixture is reacted with steam, especially pressurized steam, which has already been generated, at a pressure greater than 1.01 bar.

[0028] The use of an autoclave as a reactor for the conversion of the rock mixture comprising at least magnesium oxide and / or calcium oxide enables the efficient conversion of overburned or sintered rock particles.

[0029] When an autoclave is mentioned here or elsewhere, it refers to a pressure vessel that is essentially gas-tight and sealable. The autoclave is suitable for the thermal treatment of materials under pressure. Typically, an autoclave according to the invention includes a pressure relief valve, in particular a pressure-limiting valve or an overpressure valve. Such a pressure relief valve protects the autoclave against excessive pressure, thus preventing damage. Autoclaves commonly used for curing calcium silicate bricks, for example, are very well suited as reactors for the process according to the invention.

[0030] The temperature of the steam, particularly the pressurized steam, upon entering the autoclave is of great importance for the efficient conversion of the overburned or sintered portion of the rock mixture comprising magnesium oxide and / or calcium oxide. According to a preferred embodiment of the invention, the steam, particularly the pressurized steam, has a temperature of at least 120 °C upon entering the autoclave. Preferably, the steam, particularly the pressurized steam, has a temperature of 120 °C to 220 °C upon entering the autoclave, more preferably 140 °C to 210 °C, and most preferably 160 °C to 200 °C.

[0031] In addition to the temperature of the steam, its vapor pressure upon entering the autoclave is also significant. For pressurized steam, the vapor pressure and temperature must be coordinated according to the saturated steam curve. According to a preferred embodiment of the invention, the steam, particularly the pressurized steam, has a vapor pressure of 2 to 25 bar upon entering the autoclave, preferably 5 to 20 bar, more preferably 10 to 18 bar, and most preferably 12 to 16 bar.

[0032] According to a preferred embodiment of the inventive method, the rock mixture experiences essentially no additional heat input from an external heat source during treatment with steam, in particular with pressurized steam. Therefore, no further heat source is required to ensure efficient and complete quenching of the rock mixture.

[0033] A special feature of the process according to the invention is the efficient quenching of rock mixtures comprising magnesium oxide and / or calcium oxide, which have a high proportion of overburned or sintered magnesium oxide and / or calcium oxide particles.

[0034] When a distinction is made here or elsewhere between normal and overfired or sintered components of the rock mixture containing magnesium oxide and / or calcium oxide, this distinction refers to the reactivity of the magnesium oxide and / or calcium oxide particles with water. The normally fired components are significantly more reactive towards water than the overfired or sintered components. A suitable method for measuring the reactivity of calcium oxide with water is described in DIN EN 459-2:2010-12, section 6.4.4 ( "For quicklime, lime putty, dolomite lime and hydrated dolomite lime"The process is described in section 6.4.4 of DIN EN 459-2:2010-12. The procedure can be applied analogously to rock mixtures containing magnesium oxide. In this process, a cake of lime, lime putty, dolomite lime, or hydrated dolomite lime is produced by applying a slurry of the material to suction plates and then placing it on dry plates in a heating oven. After treatment, the cakes are inspected for expansion cracks.

[0035] The overburned or sintered magnesium oxide and / or calcium oxide particles in the rock mixture according to the inventive method are characterized in that cakes produced from the particles according to the standard DIN EN 459-2:2010-12, section 6.4.4, do not pass the test of the standard DIN EN 459-2:2010-12, section 6.4.4.

[0036] The normally fired magnesium oxide and / or calcium oxide particles in the rock mixture are characterized by the fact that cakes produced from the particles according to DIN EN 459-2:2010-12, section 6.4.4, pass the test of DIN EN 459-2:2010-12, section 6.4.4. Examples of normally fired rock mixtures according to the inventive method include soft, medium, and hard-burned lime.

[0037] Rock mixtures used in the process according to the invention have a proportion of magnesium oxide and / or calcium oxide particles that do not pass a test according to DIN EN 459-2:2010-12, section 6.4.4, of 0.1 to 20 wt.%, in particular of 1 to 15 wt.%, of 2 to 11 wt.% or of 3 to 8.5 wt.%, based on the rock mixture.

[0038] In principle, rock mixtures with a lower or higher proportion of overburned or sintered rock particles can also be used in the process according to the invention. However, the process according to the invention is particularly suitable for rock mixtures with the overburned or sintered components described above, since overburned or sintered components are slaked particularly efficiently at these proportions. Furthermore, rock mixtures with these proportions of overburned or sintered rock particles cannot be converted into the corresponding hydroxide compounds by other treatment processes using water.

[0039] According to a preferred embodiment, the inventive method transforms rock mixtures which, due to the burnt or sintered components in the rock mixture, do not pass a test according to DIN EN 459-2:2010-12, section 6.4.4 before carrying out the inventive method, into rock mixtures which do pass such a test according to DIN EN 459-2:2010-12, section 6.4.4.

[0040] Filter dust generated during rotary kiln combustion processes typically contains overburned or sintered components in the area described above. Therefore, the method according to the invention is particularly well suited for quenching filter dust, especially filter dust obtained from a rotary kiln combustion process. According to a preferred embodiment of the invention, the rock mixture comprises filter dust. According to a further preferred embodiment of the invention, the rock mixture is filter dust.

[0041] In addition to magnesium oxide and / or calcium oxide particles, filter dusts often contain large quantities of magnesium carbonate and / or calcium carbonate particles. The carbonate particles do not interfere with the treatment according to the inventive process. A rock mixture suitable for use in the inventive process can consist largely of magnesium carbonate and / or calcium carbonate particles. For example, filter dusts from rotary kiln combustion processes typically contain a proportion of magnesium carbonate and / or calcium carbonate particles in the range of 40 to 90 wt.%, particularly 60 to 75 wt.%, based on the total composition of the rock mixture. However, rock mixtures with fewer or no carbonate particles are also suitable for the inventive process.

[0042] The process according to the invention is suitable for quenching rock mixtures with a wide range of possible grain sizes. According to one embodiment of the invention, possible average grain sizes are between 100 and 300 µm, preferably between 50 and 150 µm, and particularly preferably between 90 and 110 µm. In a particularly preferred embodiment of the process according to the invention, filter dust with an average grain size of 80 to 120 µm is used.

[0043] A preferred embodiment of the invention is characterized in that the rock mixture, comprising magnesium oxide and / or calcium oxide, includes at least burnt dolomite and / or burnt lime. According to a further preferred embodiment of the invention, the rock mixture also includes dolomite and / or lime.

[0044] Many different objects and products are suitable as a base for treating the rock mixture. In a preferred embodiment of the inventive method, the rock mixture is treated on a sheet with a rim, preferably on a sheet with a rim and a perforated base. Such a base provides a large surface area for the steam, especially pressurized steam, and the quenching reaction is particularly efficient. If a sheet with a rim and perforated base is used, the steam can also reach the rock mixture through the partially open underside of the base, thus quenching it particularly efficiently.

[0045] According to a further embodiment of the invention, the rock mixture swells during the process by at least 10 vol.%, preferably at least 25 vol.%, more preferably at least 50 vol.%, most preferably at least 100 vol.%, based on the volume of the rock before quenching. This occurs through the efficient absorption of water during the quenching reaction.

[0046] When the autoclave is opened after the quenching reaction is complete, the rock mixture usually still contains residual moisture. According to one embodiment of the invention, this residual moisture is in the range of 0 to 15 wt.%, preferably in the range of 2 to 10 wt.%.

[0047] According to a preferred embodiment of the method according to the invention, the process comprises, following the treatment of the rock mixture with steam, in particular pressurized steam, a further drying step d. This drying step d. serves to reduce the residual moisture of the rock mixture. The drying step d. can preferably take place in a separate oven.

[0048] Following the optional drying step d., the rock mixture preferably has a residual moisture content of less than 3 wt.%, more preferably less than 2 wt.%, and most preferably less than 1 wt.%. According to a particularly preferred embodiment of the invention, the rock mixture is completely dry after drying step d. and can be used directly for possible applications.

[0049] A wide range of reaction times is suitable for the quenching reaction of the process according to the invention. In principle, shorter reaction times can be selected if the proportion of overburned or sintered material in the rock mixture is lower. In the course of experimental studies, reaction times in the range of 20 minutes to 30 hours have proven advantageous. Reaction times in the range of 2 to 24 hours, 4 to 12 hours, or 5 to 8 hours are particularly preferred.

[0050] The rock mixtures used in the process can contain many other components besides magnesium oxide and / or calcium oxide. Rock mixtures containing magnesium carbonate and / or calcium carbonate particles in addition to magnesium oxide and / or calcium oxide are particularly suitable. Magnesium carbonate and / or calcium carbonate particles can also be the main component of the rock mixture. Other components that may be contained in a rock mixture suitable for the process according to the invention include, among others, calcium silicates, magnesium silicates, aluminum oxide, iron oxide, silicon dioxide, cement paste, clay, sand, halite (sodium chloride), sulfur, sulfur trioxide, anhydrite (calcium sulfate), minerals, or mixtures thereof.Silicon dioxide can occur in many different phases and structures, including calcium, magnesium, and / or aluminum components, particularly calcium silicates and / or magnesium silicates. Aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃) can also exist in a variety of different phases and structures. For example, iron oxide can occur as srebrodolskite or iowaiite.

[0051] The use of a rock mixture obtainable from the inventive process as a filler is further described. The rock mixture obtainable from the inventive process can be used in a variety of ways. Preferred uses of the rock mixture as a filler are in the construction industry, the asphalt industry, cement-bound materials, mortar, ready-mixed plaster, floors, bituminous roofing membranes, calcium silicate bricks, plastics, or rubbers. In general, the rock mixture can be used as a filler wherever dolomite and / or calcium hydroxide are typically used. The rock mixture can also be used in applications where it is not used as a filler. Here, too, those application areas where dolomite and / or calcium hydroxide are typically used are particularly suitable.

[0052] The use of pressurized steam for slaking rock mixtures comprising magnesium oxide and / or calcium oxide is further described. Preferably, the pressurized steam is used for slaking rock mixtures comprising magnesium oxide and / or calcium oxide according to the process according to the invention.

[0053] A building material obtainable from the inventive method is also described. This building material is characterized by high stability and durability. It does not react with water contained in the air, even over extended periods. Therefore, no cracks form in the material, even over long periods. According to a preferred embodiment of the invention, the building material is characterized by being obtainable from fine dust using the inventive method.

[0054] The invention is explained in more detail below with reference to exemplary embodiments, which serve solely to illustrate the invention and are not intended to limit it. Example 1

[0055] A filter dust from a rotary kiln process was used as the rock mixture.

[0056] The composition of the rock mixture (rock mixture A1) was determined by X-ray crystal structure analysis (XRD) prior to use in the inventive method (Table 1). Table 1: Composition of rock mixture A1 determined by X-ray crystal structure analysis; quantities in wt.% Salt dolomite Calcite Periclase Brucite Quicklime Anhydrite Halite Srebrodolsk it formula CaMg (CO 3 ) 2 CaCO3 MgO Mg(OH) 2 CaO CaSO4 NaCl Ca 2 FeO 5 A1 68,49 11,93 5,76 0,00 2,43 2,48 3,11 5,38

[0057] The rock mixture was then treated with pressurized steam at different pressures and for different durations (Table 2): Table 2: Reaction conditions for the treatment of rock mixture A1 Experiment number Pressure of the water vapor temperature of the water vapor Duration of the experiment B1 2 bar 120 °C 4h B2 4 bar 143 °C 4h B3 8 bar 170 °C 4h B4 16 bar 203 °C 4h B5 2 bar 120 °C 12 h B6 4 bar 143 °C 12 h B7 8 bar 170 °C 12 h B8 16 bar 203 °C 12 h

[0058] The temperature of the water vapor was always chosen to be dependent on the pressure, ensuring that it was pressurized water vapor. The corresponding pressure-temperature relationships can be found in the saturated vapor curve of a phase diagram of water.

[0059] The formation of brucite (Mg(OH) 2 ) in the rock mixture A1 was determined by loss on ignition analysis (temperature gradient from 30 to 950 °C, loss at 436 °C) (Table 3): Table 3: Formation of brucite (Mg(OH)₂) in the rock mixture after treatment with pressurized steam according to the reaction conditions specified in Table 2; amounts in wt.% experiment Amount of brucite [wt.%] A1 0,00 B1 10,97 B2 14,21 B3 13,27 B4 14,76 B5 15,50 B6 14,60 B7 15,96 B8 15,15

[0060] As can be seen from Table 3, the process according to the invention is ideally suited for the effective formation of brucite (Mg(OH)₂) even from otherwise unreactive filter dust from a rotary kiln process. The treatment of the filter dust is generally more effective at higher pressure, particularly when a reaction time of 4 hours is selected (see results from Examples B4 with B1). The process according to the invention effectively converts magnesium oxide from rock mixtures with a high proportion of overburned or sintered magnesium oxide particles into magnesium hydroxide (brucite).

[0061] It was further observed that the physical properties of the rock mixture changed, in some cases noticeably, after treatment with pressurized steam. The powder density of the rock mixture was reduced, while the specific surface area increased and the particles of the rock mixture were finer than before treatment. All these physical changes are advantageous for the use of the rock mixture as a filler.

[0062] The inventive method allows oxide components in filter dusts to be efficiently quenched, so that only traces of oxides remain in the filter dusts. Example 2

[0063] As a further experiment, the influence of autoclaved dolomitic filter dusts on the properties of calcium silicate bricks was investigated.

[0064] Three different calcium silicate bricks were produced for this purpose. The first calcium silicate brick, C1, consisted of quartz sand, hydrated quicklime, and untreated dolomitic filter dust. The second and third calcium silicate bricks, C2 and C3, each consisted of quartz sand, hydrated quicklime, and dolomitic filter dust treated according to the inventive process. The dolomitic filter dust had been treated for 12 hours at 4 bar (see Table 3, Reaction Conditions B6).

[0065] The calcium silicate bricks were manufactured as follows: First, quartz sand (approximately 90% by weight) and binder (dolomitic filter dust and quicklime; approximately 10% by weight) were mixed with water until a moisture content of about 5% was reached. The moist mixture was then stored in a drying oven at 70°C for two hours to hydrate the quicklime. No hydration of the dolomitic filter dust was observed.

[0066] The mixture was then homogenized in a second mixer, weighed (500 g each) and then compressed in a special compression mold into a cuboid prism (length approx. 160 mm; width approx. 40 mm; height approx. 40 mm; volume approx. 256 mm³) under a compression force of 12,000 kg.

[0067] The resulting prism was then placed directly into a preheated autoclave and treated with pressurized steam at 16 bar (approx. 203 °C) for four hours. The physical properties of the hardened calcium silicate brick obtained in this way were measured after cooling overnight (results in Table 4). Table 4: Composition and physical properties of calcium silicate bricks C1 to C3 Calcium silicate brick C1 C2 C3 Quartz sand [wt.%] 92,7 92,0 82,8 Quicklime [wt.%] 7,3 6,4 8,7 Untreated filter dust [wt.%] 0,7 - - Treated filter dust [wt.%] - 1,6 9,2 Bulk density [g / cm³< ] 1,75 1,87 1,85 Flexural strength [N / mm²] 2,8 4,0 4,1 Compressive strength [N / mm²] 15,7 17,8 19,6

[0068] The C1 calcium silicate brick exhibited blisters and fractures, which was also reflected in its less favorable physical properties. In contrast, the C2 and C3 calcium silicate bricks were fracture-free and displayed good flexural and compressive strength.

[0069] Calcium silicate bricks containing filter dust treated according to the inventive method exhibit good physical properties. The inventive method thus allows filter dust to be used in various applications and eliminates the need for costly disposal.

Claims

1. Method for treating a rock mixture comprising at least magnesium oxide and / or calcium oxide, characterized in that the rock mixture is steam treated at a pressure greater than 1.01 bar, the method comprising at least the following steps: a. introducing the rock mixture into an autoclave; b. introducing steam into the autoclave; c. treatment of the rock mixture with the steam at a pressure greater than 1.01 bar, and the rock mixture having a proportion of magnesium oxide and / or calcium oxide particles which do not pass a test according to DIN EN 459-2:2010-12, section 6.4.4, of 0.1 to 20 wt.% based on the rock mixture.

2. Method according to claim 1, characterized in that the steam is superheated steam.

3. Method according to one or more of the preceding claims, characterized in that the steam entering the autoclave has a temperature of at least 120°C, in particular a temperature of 120°C to 220°C, 140°C to 210°C or 160°C to 200°C.

4. Method according to one or more of the preceding claims, characterized in that the proportion of magnesium oxide and / or calcium oxide particles which do not pass a test according to DIN EN 459-2:2010-12, section 6.4.4, is from 1 to 15 wt.%, from 2 to 11 wt.% or from 3 to 8.5 wt.%, based on the rock mixture.

5. Method according to one or more of the preceding claims, characterized in that the rock mixture comprising at least magnesium oxide and / or calcium oxide is a filter dust, in particular a filter dust from a rotary kiln process.

6. Method according to one or more of the preceding claims, characterized in that the rock mixture comprises at least burnt dolomite and / or burnt lime.

7. Method according to one or more of the preceding claims, characterized in that during the method, the rock mixture swells by at least 10 vol.%, in particular by at least 25 vol.%, by at least 50 vol.% or by at least 100 vol.%, based on the volume of the rock before slaking.

8. Method according to one or more of the preceding claims, characterized in that after the treatment in step c., the rock mixture has a residual moisture content of 0 to 15 wt.%, in particular of 2 to 10 wt.%.

9. Method according to one or more of the preceding claims, characterized in that the rock mixture contains calcium silicates, magnesium silicates, aluminum oxide, iron oxide, silicon dioxide, cement stone, clay, sand, halite (sodium chloride), sulfur, sulfur trioxide, anhydrite (calcium sulfate), minerals, or mixtures thereof.

10. Method according to one or more of the preceding claims, characterized in that the steam entering the autoclave has a steam pressure of 1.5 to 25 bar, in particular 2 to 20 bar, 4 to 18 bar, 6 to 17 bar or 12 to 16 bar.

11. Method according to one or more of the preceding claims, characterized in that the method comprises a drying step d.

12. Method according to claim 11, characterized in that after the drying step d., the rock mixture has a residual moisture content of less than 3 wt.%, in particular less than 2 wt.% or less than 1 wt.%.

13. Method according to one or more of the preceding claims, characterized in that the steam treatment of the rock mixture takes 2 to 24 hours, in particular 4 to 12 hours.

14. Method according to one or more of the preceding claims, characterized in that the treatment of the rock mixture takes place on a tray which has a rim, in particular on a tray which has a rim and a sieve base.

15. Method according to one or more of the preceding claims, characterized in that during the steam treatment, the rock mixture undergoes substantially no additional heat exposure from an external heat source.