METHOD FOR CALCIUM SULFATE REMOVAL FROM CALCIUM SULFATE-CONTAINING CONSTRUCTION WASTE

Description

[0001] The invention relates to a process for removing calcium sulfate and optionally recovering calcium sulfate from calcium sulfate-containing construction waste, in which the calcium sulfate phases (such as gypsum, anhydrite, bassanite, AfM phases, ettringite, and thaumasite) are converted into calcium carbonate. This process makes it possible to process gypsum-containing building materials and residues in such a way that their sulfate content can be reduced below the specified limits, so that they can subsequently be used in recycled building materials.

[0002] As a building material in walls, ceilings, and floors, gypsum (CaSO₄·2H₂O) already makes up to 10% of the building fabric in Germany. After demolition, these substances cause significant problems in recycled building materials (RC materials), as undesirable secondary effects can occur. Therefore, they are subject to very strict regulations (DIN 4226-100) for reuse. For example, RC materials of type 1 (concrete aggregate / crushed concrete sand) may contain only 0.2% gypsum by mass, and RC materials of type 2 (construction site aggregate / crushed construction site sand) only 0.5% by mass. If these regulations are not met, the construction waste must be landfilled. In Germany alone, approximately 5 million tons of fine construction waste fractions with grain sizes smaller than 2 mm are generated annually, which, due to the gypsum issue, end up directly in landfills.Based on chemical analyses of construction debris and model walls, it is currently assumed that an average gypsum content of 3.6% by mass is realistic in buildings and therefore also in construction debris. However, in individual cases, gypsum content of up to 10% by mass of the building material used has been found. Even the disposal of demolition material, especially with regard to aerated concrete (sometimes also called gas concrete), is becoming increasingly difficult, as since the beginning of 2016 it may no longer be disposed of with conventional construction debris, but must be disposed of as Class I waste due to its high sulfate leaching levels.

[0003] Furthermore, other mineral residues also suffer from sulfate contamination, for example steel slag or paper ash. This limits their use in building materials to a very limited extent.

[0004] Gypsum is inherently a very recyclable building material; however, gypsum-containing construction waste has so far only been reused in very limited quantities. The purity of the material plays a crucial role here. Precise pre-sorting of the material or targeted deconstruction makes it possible to separate gypsum material from bricks or similar masonry.

[0005] In the case of construction waste such as aerated concrete, plasterboard, mortar or concrete, or in mineral residues in which the sulfates are not present as adhesions but chemically bound in mineral phases (e.g. in ettringite or AfM phases), it is not yet possible to separate them out and thus produce a sulfate-free material.

[0006] The technique of removing plaster using ammonium carbonate is known from art restoration. Here, plaster is removed from fresco surfaces using ammonium carbonate cloths. Such a method is disclosed, for example, in EP 0 189 866 A1.

[0007] DE 198 12 262 A1 describes processes for the treatment of dilute acid, which is produced in the manufacture of titanium dioxide, whereby high-quality gypsum precipitates.

[0008] JP 2001 000947 A, in turn, concerns the treatment of gypsum boards consisting of calcium sulfate and paper with ammonia and carbon dioxide from composting plants.

[0009] Also known is the so-called Merseburg process for obtaining high-purity ammonium sulfate, which serves as a fertilizer, and the production of calcite from ammonia water, gypsum residues or FGD gypsum, and carbon dioxide. Based on the prior art, the invention is thus based on the objective of providing a process for sulfate removal and, if necessary, recovery of calcium sulfate from mineral residues, in particular construction waste, wherein the sulfate content can be reduced below predetermined limits so that the construction waste can be used in recycled building materials. This objective is achieved according to the invention by the process according to claim 1.

[0010] According to the invention, a method for removing calcium sulfate from calcium sulfate-containing residue is proposed, which comprises the following steps: a) Action of ammonium carbonate and / or ammonium bicarbonate on the calcium sulfate-containing residue in an aqueous medium, such that the calcium sulfate is converted into calcium carbonate by an exchange reaction, and b) separation of the aqueous medium, wherein the aqueous medium contains ammonium sulfate.

[0011] The residual material used as a starting material is a building material selected from concrete, bricks, tiles, clinker bricks, mortar residues, tiles, glass blocks, ceramics, gypsum-containing construction waste and / or sand.

[0012] This method for calcium sulfate removal is based on the fundamental principle of converting the calcium sulfate (CaSO₄) contained in construction waste into calcium carbonate (CaCO₃) under the influence of ammonium carbonate and / or ammonium bicarbonate, leaving ammonium sulfate in the aqueous medium. Although it was known from the prior art to remove gypsum from fresco surfaces using ammonium carbonate cloths, it was completely surprising that this was also possible with the considerably more complex and less homogeneous construction waste, especially since the gypsum primarily adheres to the surface of frescoes. It was particularly surprising that the method according to the invention can also be carried out with ammonium bicarbonate.

[0013] Furthermore, the process according to the invention is designed as a recycling process in which calcium sulfate is recovered from the aqueous medium containing ammonium sulfate.

[0014] An ammonium bicarbonate solution can either be obtained commercially or produced easily by acidifying an ammonium carbonate solution with CO2 or by introducing CO2 into an ammoniacal solution.

[0015] For the purposes of the present invention, "calcium sulfate-containing construction waste" means any construction waste containing calcium sulfate-containing mineral phases, such as gypsum, anhydrite, bassanite, AfM phases, ettringite and thaumasite.

[0016] Calcium sulfate-containing construction waste comprises mineral waste and building materials, selected from concrete, bricks, clinker bricks, mortar residues, tiles, glass blocks, ceramics, or sand. In particular, it may include concrete, mortar, aerated concrete, and / or crushed gypsum building materials, as well as mineral raw materials in which the sulfates are chemically bound within the mineral phases.

[0017] In some embodiments, the calcium sulfate-containing construction waste has or consists of sieve fractions with a particle size of 2 mm or less. It has been found that sulfate removal from construction waste using the inventive method is most effective in the sieve fractions smaller than 2 mm.

[0018] In some embodiments, ammonium carbonate and / or ammonium bicarbonate is used in at least a stoichiometric amount, based on the calcium sulfate content of the construction waste. For this purpose, the sulfate content of the gypsum-containing construction waste can be determined in a preliminary step using a known method to establish the stoichiometric amount of ammonium carbonate or ammonium bicarbonate. To ensure the most complete possible exchange of sulfate to carbonate ions, it is advantageous to use an excess of ammonium carbonate or ammonium bicarbonate relative to the stoichiometric amount. The upper limit is not critical; it is advantageously chosen to ensure that, on the one hand, no unnecessary ammonium carbonate or ammonium bicarbonate is used, and on the other hand, that at least nearly complete conversion is guaranteed.

[0019] In the process according to the invention, an aqueous medium is used. An aqueous medium is understood to be, in particular, a medium containing water in which ammonium carbonate or ammonium bicarbonate can be dissolved in the amounts required for the process according to the invention. Water, for example distilled water, is used as a particularly suitable aqueous medium.

[0020] The amount of aqueous medium can be chosen so that, firstly, ammonium carbonate and ammonium bicarbonate can be dissolved in sufficient quantities and, secondly, the construction debris is adequately covered.

[0021] In step a) of the process according to the invention, the ammonium carbonate or ammonium bicarbonate reacts with the calcium sulfate-containing construction waste, this reaction being carried out in an aqueous medium, in particular water. The order in which the calcium sulfate-containing construction waste, the ammonium carbonate or ammonium bicarbonate, and the aqueous medium are combined is not critical. The order can therefore be chosen arbitrarily.

[0022] In one embodiment of the invention, the reaction in step a) can take place over a period of approximately one hour to several days. In other embodiments of the invention, the reaction can take place over a period of approximately one hour to approximately five days. In yet another embodiment, the reaction can take place over a period of approximately 12 hours to approximately three days. In a further embodiment, the reaction can take place over a period of approximately two days to approximately five days. In yet another embodiment, the reaction can take place over a period of approximately 24 hours to approximately 48 hours. During this period, a complete conversion of calcium sulfate to calcium carbonate occurs. In some embodiments of the invention, the reaction can take place at room temperature.

[0023] To facilitate this complete conversion, it is advantageous if, in step a), the components contained in the aqueous medium can be brought into contact and mixed by movement. This can be achieved, for example, by conventional stirring or shaking, as is done in laboratories and technical plants.

[0024] In step b), the aqueous medium containing the dissolved substances, in particular the dissolved ammonium sulfate formed from the sulfate present in the construction waste, is separated. In some embodiments, this separation in step b) can be carried out by filtration. This yields a filtrate containing the ammonium sulfate. If necessary, the remaining filter residue can be dried after the aqueous medium has been separated.

[0025] The aqueous medium containing ammonium sulfate, separated for example by filtration, is used to produce high-purity calcium sulfate with the aid of calcium carriers, particularly calcium carbonate, lime, or portlandite. Based on current knowledge, a gypsum shortage is expected in a few years. For instance, it is anticipated that the gypsum recovered from flue gas desulfurization will cease to be a viable source in the not-too-distant future, as hard coal and lignite mining will likely decline. Therefore, recovering gypsum from the aqueous medium containing ammonium sulfate can unlock another gypsum source that will likely be needed in the future.Furthermore, it should be noted that this additional recovery step of gypsum from the ammonium sulfate-containing aqueous medium makes the application of the inventive process to calcium sulfate-containing construction waste such as gypsum plasterboard or aerated concrete particularly economically viable.

[0026] In addition, some of the aqueous medium containing ammonium sulfate can be directly reused to produce fertilizers, for example using the Merseburg process.

[0027] In one embodiment, gypsum recovery can be achieved by adding one of the aforementioned calcium carriers to the ammonium sulfate-containing aqueous medium and increasing the temperature, for example, to a temperature of approximately 58 °C to approximately 100 °C. During this process, gypsum precipitates from the solution, and the remaining ammonium carbonate decomposes back into the gases ammonia and carbon dioxide. These gases can be collected in an aqueous solution and fed back into the process in step a). Alternatively, sulfuric acid can be added to the ammonium sulfate-containing aqueous medium containing the calcium carrier to drive off CO₂ and convert the calcium content of the calcium carrier into gypsum. This yields a pure ammonium sulfate solution without the thermal treatment described above.

[0028] Furthermore, additional treatment (acidification) of the solution can ensure that the precipitated calcium sulfate meets the quality criterion of < 1.2 wt% CaCO3 recommended by Eurogypsum.

[0029] The present invention makes it possible to process calcium sulfate-containing construction and residual materials in such a way that the sulfate content can be reduced below the specified limits. This is a fundamental requirement for construction waste to be used in recycled building materials in the first place. The process according to the invention therefore conserves landfill space, reduces landfill costs, and increases resource efficiency.

[0030] In particular, it is advantageous to use an ammoniacal solution, i.e., an ammonium carbonate and / or ammonium bicarbonate solution, to effect the exchange reaction between the sulfate and the carbonate ion without dissolving calcium from the material structure.

[0031] The technical application areas of the process according to the invention lie in the recycling and processing industry, since large quantities of sulfate-containing construction waste and mineral residues can be processed using the method described above.

[0032] The treated residues can then be used in recycled construction materials. Since ammonium sulfate is obtained through the process, it can be used as a fertilizer. Alternatively, the resulting ammonium sulfate solution can be treated with a calcium carrier, particularly calcite, to produce pure calcium sulfate. This allows the recycling loop for gypsum-containing mineral residues and construction materials to be closed.

[0033] The invention will now be explained with reference to figures and exemplary embodiments, without limiting the general concept of the invention. Figure 1an X-ray diffraction diagram of an untreated plaster sample, such as the one used as starting material in Examples 4 and 5. Figure 2 shows an X-ray diffraction diagram of a gypsum sample treated with ammonium carbonate, as obtained in Example 4. Figure 3 shows an X-ray diffraction diagram of a sample treated with ammonium bicarbonate, as obtained in Example 5. Examples Example 1:

[0034] 20 g of mortar < 2 mm were weighed out and mixed with 10 g of ammonium carbonate, then both were transferred to a shaker flask. This was filled with 800 ml of water and mounted on a shaking platform. The shaking period lasted one day. Example 2:

[0035] 15 g of aerated concrete rubble < 2 mm were weighed out and mixed with 0.6 g of ammonium carbonate, and both were transferred to a shaker flask. This was filled with 800 ml of water and attached to a shaker plate. The shaking period was five days. Example 3:

[0036] 15 g of aerated concrete rubble < 2 mm were weighed out and mixed with 0.6 g of ammonium bicarbonate, and both were transferred to a shaker flask. This was filled with 800 ml of water and attached to a shaker plate. The shaking period was five days. Example 4:

[0037] Twenty grams of crushed gypsum building material were weighed out and mixed with 15 grams of ammonium carbonate. Both were transferred to a shaker flask and mounted on a shaker plate. The flask was filled with 800 ml of distilled water and shaken for 6 hours. Example 5:

[0038] Twenty grams of crushed gypsum building material were weighed out and mixed with 300 grams of ammonium bicarbonate. Both were transferred to a shake flask and mounted on a shaker plate. The flask was filled with 800 ml of distilled water and shaken for one day.

[0039] In Figure 1 Figure 1 shows an X-ray diffraction pattern of an untreated gypsum sample, such as those used as starting material in Examples 4 and 5. It is clearly evident that this sample contains a high proportion of gypsum. Such a sample was treated with ammonium carbonate (see Figure 2). . Figure 2 , example 4) as well as ammonium bicarbonate (see Figure 3 , example 5).

[0040] It was demonstrated that the gypsum in the construction waste could be almost completely converted into calcite by an exchange reaction, both when an ammonium carbonate and an ammonium bicarbonate solution were used.

[0041] Naturally, the invention is not limited to the embodiments illustrated in the figures and examples. The preceding description is therefore not to be considered limiting but rather explanatory. The following claims are to be understood as meaning that a named feature is present in at least one embodiment of the invention. This does not preclude the presence of further features. Features from different embodiments of the invention can be combined at any time to obtain further embodiments of the invention.

Claims

1. Recycling circulation method for mineral waste materials containing gypsum, comprising the steps: a) allowing ammonium carbonate and / or ammonium bicarbonate to act on the waste material in an aqueous medium so that the calcium sulfate is converted into calcium carbonate, and b) separating the aqueous medium, the aqueous medium containing ammonium sulfate, and c) recovering calcium sulfate from the aqueous medium containing ammonium sulfate, characterized in that the waste material is a construction material that is selected from concrete, building bricks, bricks or tiles, clinker bricks, mortar waste, tiles, glass bricks, ceramics, construction waste containing gypsum, and / or sand.

2. Method according to claim 1, characterized in that the waste material contains or consists of sieve fractions having a grain size smaller than or equal to about 2 mm.

3. Method according to any one of claims 1 or 2, characterized in that the ammonium carbonate and / or ammonium bicarbonate is used in at least stoichiometric quantity relative to the calcium sulfate content of the construction waste.

4. Method according to any one of claims 1 to 3, characterized in that water is used as an aqueous medium.

5. Method according to any one of claims 1 to 4, characterized in that the action in step a) takes place over a period from about 1 hour to about 5 days or from about 12 hours to about 3 days or from about 2 days to about 5 days or from about 24 hours to about 48 hours.

6. Method according to any one of claims 1 to 5, wherein the separation in step b) is carried out by filtration.

7. Method according to any one of claims 1 to 6, characterized in that the recovery is carried out by adding a calcium carrier to the aqueous medium containing ammonium sulfate and by bringing the resulting mixture to a temperature from about 58°C to about 100°C.

8. Method according to any one of claims 1 to 6, characterized in that the recovery is carried out by adding a calcium carrier to the aqueous medium containing ammonium sulfate and by adding sulfuric acid to the resulting mixture.

9. Method according to any one of claims 7 or 8, characterized in that calcite, quicklime or free lime, portlandite and / or a mixture of at least two of these compounds is used as the calcium carrier.