How to process recycled gypsum

Abstract

The present invention relates to a method for treating gypsum containing a siliconate, the method comprising: (i) stirring an aqueous gypsum suspension with a solution of an alkali metal hydroxide and / or alkaline earth metal hydroxide having a pH of 10 or greater, and separating a solid component from the aqueous gypsum suspension. Treatment with the alkali metal and / or alkaline earth metal hydroxide significantly reduces the deleterious effects of the siliconate on subsequent foams prepared using the treated gypsum. The invention further relates to methods for preparing foamed gypsum compositions and gypsum boards constructed by such methods, gypsum and gypsum boards prepared accordingly, and the use of alkali metal and / or alkaline earth metal hydroxide solutions to deactivate siliconates in gypsum recycle.

Description

[Technical Field]

[0001] The present invention relates to a method for treating gypsum containing siliconates, particularly siliconates containing Si-H bonds, comprising the steps of (i) stirring an aqueous gypsum suspension with a solution of alkali metal hydroxide and / or alkaline earth metal hydroxide having a pH of 8 or greater, and separating the solid components from the suspension. The invention further relates to a method for preparing a foamed gypsum composition and gypsum boards constructed by such methods, gypsum treated accordingly, and gypsum boards containing the treated gypsum, as well as the use of an alkali metal and / or alkaline earth metal hydroxide solution to deactivate hydrogenmethylsiliconate in gypsum recycle. [Background technology]

[0002] A building material that is readily and cheaply available today is the so-called FGD gypsum, which means gypsum obtained from the flue gas desulfurization of coal-fired power plants. FGD gypsum is obtained from the combustion of coal by reacting the exhaust gases from the combustion with a calcium oxide or calcium carbonate suspension, whereby the sulfur dioxide (SO2) contained in the flue gases is first converted in the presence of oxygen into sulfur trioxide (SO3) and then bound as calcium sulfate (CaSO4).

[0003] However, as many governments have decided to phase out coal-fired power generation in the coming years, FGD gypsum is expected to become an increasingly scarce raw material. To ensure that the existing demand for gypsum building materials can still be met, recycling of gypsum products will play an increasingly important role.

[0004] One problem faced when recycling demolition gypsum products is that many gypsum products, such as plasterboard, are treated with or contain additives that impart specific functionality to the finished product. However, these additives remain when the demolition gypsum product is recycled simply by crushing. The resulting recycled "raw" material has altered properties compared to quarried gypsum or synthetic gypsum, such as FGD gypsum. For example, plasterboard is often treated with water-repellent additives to reduce the material's water absorption capacity. Such treatments are described, for example, in German Patent No. 10 220 659 (A1), in which a mixture of "H-siloxane" (i.e., an organosiloxane containing silicon-bonded hydrogen atoms) and starch ether is added to the plaster body during production. Such H-siloxanes have a foam-destroying effect, leading to variations in building materials and board properties during the manufacturing process of boards (made from foamed gypsum compositions for weight reduction and better insulation), thus providing a revolutionary effect on the use of recycled gypsum to create new gypsum boards.

[0005] Therefore, there is a need to provide a gypsum recycled product made from a recycled gypsum product containing a siliconate, particularly a siliconate containing an Si-H bond, or a recycled gypsum product produced by including such a siliconate. Such a gypsum recycled product should be as comparable as possible to conventional natural gypsum or FGD gypsum in terms of its foam-forming properties. It is of utmost importance to be able to process the gypsum recycled product into a product with properties as consistent as possible with non-recycled natural gypsum or FGD gypsum.

[0006] The present invention addresses this need. Summary of the Invention

[0007] In the research underlying this application, it was surprisingly discovered that the antifoaming effect of siliconates contained in recycled gypsum or gypsum treated with such siliconates can be minimized by treating the gypsum with a solution of alkali metal and / or alkaline earth metal hydroxide, particularly calcium hydroxide. Such treatment is relatively easy to achieve by stirring the gypsum in such a solution for a sufficient period of time. Without being bound by theory, the inventors believe that the Si-H bonds and possibly also easily cleavable Si-C bonds contained in the siliconate are converted to Si-OH by the action of the hydroxide, thereby increasing the solubility of the silicon component in the alkaline solution. In this way, the siliconate can either be dissolved in the alkaline solution and separated from the gypsum, or at least chemically modified to such an extent that it no longer prevents the reuse of such recycled gypsum for the production of new building materials.

[0008] Thus, according to a first aspect, the present invention relates to a method for treating gypsum recycle, the method comprising the steps of: (i) stirring an aqueous gypsum suspension with a solution of an alkali metal hydroxide or alkaline earth metal hydroxide or a mixture thereof having a pH of 8 or greater, thereby producing a mixed solution; and (ii) separating a solid component from the mixed solution.

[0009] In the process according to the invention, the separation of the solid gypsum component from the mixed solution is advantageously carried out by filtration and, if necessary, by pressing. Gypsum is obtained as a solid product of the process.

[0010] Prior to being treated by the method of the present invention, the gypsum may be advantageously crushed and sieved to remove excess fibers, such as paper and / or glass fibers, as well as metal parts, e.g., screws, or other physically separable debris.

[0011] The alkali metal and / or alkaline earth metal hydroxide used in the present method can be any alkali metal or alkaline earth metal hydroxide commercially available in large quantities, but magnesium hydroxide, sodium hydroxide, potassium hydroxide, and calcium hydroxide are preferred due to cost considerations. Calcium hydroxide is most preferred because it contains the same cation as gypsum and therefore provides an adequate calcium ion concentration in solution to prevent gypsum dissolution.

[0012] With regard to the alkali metal and / or alkaline earth metal hydroxide, it is not critical whether it is used as is or whether the hydroxide is first produced by reacting an oxide, such as calcium oxide (CaO), with water and then mixed with the aqueous gypsum suspension. The reaction of the alkali metal and / or alkaline earth metal oxide in water to produce the hydroxide may be more preferable, for example in the case of calcium oxide, since the reaction to form the hydroxide is exothermic. The released heat can be advantageously used to accelerate the deactivation reaction of the siliconate.

[0013] When using an alkali metal and / or alkaline earth metal hydroxide solution, a higher pH has a favorable effect on the reaction rate of the siliconate. Therefore, in the context of the present invention, it is preferred that the alkali metal and / or alkaline earth metal hydroxide solution have a pH of at least 9, more preferably at least 10. It is particularly preferred that the alkali metal and / or alkaline earth metal hydroxide solution is a saturated solution, especially a calcium hydroxide saturated solution, since this can maximize the reaction rate of the siliconate and minimize the dissolution of gypsum in the mixture. More preferably, the pH of the alkali metal or alkaline earth metal solution is less than 12, and particularly preferably less than 11.5. When the pH of the hydroxide solution is higher than about 11.5, foam efficiency appears to decrease. The term foam efficiency relates to the foam volume and foam stability of various samples when used in stucco treated by the method of the present invention but made from gypsum with different pH values.

[0014] There are no relevant limitations regarding the concentration of siliconate in the gypsum treated by the method of the present invention. The concentration of such siliconate is typically in the range of 0.1 to 10% by weight, and in most cases is about 0.5 to 5% by weight or less, for example, in the range of 0.1 to 1% by weight. Preferably, at least a portion of the siliconate contained is hydrogenmethyl siliconate. In this regard, it should be noted that the terms "siliconate having Si-H bonds" and H-siliconate in this application are used interchangeably and have the same meaning.

[0015] The siliconate contained in the gypsum treated / recycled by the method according to the invention may be an oil or a resin or a mixture thereof. In most cases, the siliconate has the formula: H a SiR b O (4-a-b / 2) [In the formula, R is a monovalent, optionally halogen-substituted C-C 15 represents a hydrocarbon group, a hydrocarbonoxy group, or a hydroxyl group, a represents 0, 1, 2 or 3, particularly 0.01 to 1.2, preferably 0.1 to 1; b represents 0, 1, 2 or 3, particularly 1.0 to 2.0, preferably 1.4 to 1.8; However, there is at least one Si-bonded hydrogen atom per molecule. It is a siliconate having a Si-H bond. 15 Examples of hydrocarbon groups are alkyl groups, for example in the form of methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl. Preferred radicals R are unsubstituted C1-C6 alkyl groups, and the H-siloxanes usually preferably have at least a methyl group or are present in the form of hydrogenmethylsiliconate.

[0016] The stirring in the context of the described method does not require any special conditions. However, a higher pH value of the alkali metal and / or alkaline earth metal hydroxide solution and / or a higher temperature will accelerate the inactivation of the siliconate. On the other hand, to make the method economically viable and ensure that most of the gypsum does not dissolve in the stirred mixture, the stirring is preferably carried out at room temperature or at an elevated temperature above room temperature up to 100°C, preferably between room temperature and 60°C.

[0017] The stirring time is also not subject to any relevant limitations and can be selected according to the amount of siliconate contained in the starting material and the stirring temperature.Times of 5 to 180 minutes, preferably 15 to 120 minutes, and more preferably 30 to 120 minutes are considered suitable in terms of favorable economic efficiency of the method.As a general rule, the shorter the time required to achieve sufficient siliconate deactivation, the better the process and economic efficiency.

[0018] The solid product obtained by the separation step is essentially gypsum. For further processing, it may be useful to subsequently dry this gypsum (e.g., for storage) and / or thermally convert it to a stucco consisting primarily of calcium sulfate hemihydrate (CaSO 1 / 2H O) and, to a lesser extent, calcium sulfate anhydrite (CaSO anhydrite). Therefore, the solid component separated from the present method is generally preferably subjected to a drying and / or calcination or heat treatment step. According to one embodiment, the solid component is treated so that the resulting product is composed of at least 80% by weight, more preferably at least 90% by weight, calcium sulfate hemihydrate, excluding non-calcium sulfate impurities. For conversion to hemihydrate, the solid product can be, for example, dried and calcined at about 160°C for about 2 hours. According to another embodiment, the solid component is treated so that the resulting calcium sulfate product is composed of at least 80% by weight, more preferably at least 90% by weight, calcium sulfate anhydrite. The treatment temperature is higher than that for producing the hemihydrate, for example in the range of 200 to 1000°C.

[0019] Advantageously, the alkali metal and / or alkaline earth metal hydroxide solution used in the treatment of siliconate-containing gypsum can be reused after separation of the solid components. The recycled alkali metal and / or alkaline earth metal hydroxide solution can be added to another batch of gypsum and fed to another treatment according to the method of the present invention. If the pH value of the solution drops significantly during treatment (i.e., to a value of 0.2 or more), additional alkali metal and / or alkaline earth metal hydroxide can be added for regeneration.

[0020] Another aspect of the invention relates to gypsum obtainable or produced by the above process.

[0021] Yet another aspect of the present invention relates to a method for preparing a foamed gypsum composition, comprising adding a foaming agent to a stucco slurry and generating foam from the composition, or generating foam from a foaming agent and water and adding the foam to stucco or a stucco slurry (a suspension of calcium sulfate in water), thereby obtaining a foamed gypsum composition, wherein the stucco is produced by calcining the gypsum treated by the above method. Thus, the method comprises at least three steps: treating the gypsum as described above, calcining the treated gypsum, and subjecting the resulting stucco to a process for producing a foamed gypsum composition. This procedure avoids the adverse effects of reduced foam stability observed when recycled gypsum containing siliconates, or gypsum that has been in contact with such siliconates, is processed into foam. It is irrelevant whether the foamed gypsum composition is prepared by foaming a mixture of all ingredients in water, or whether an aqueous foam is pre-prepared from foaming agent and water and then mixed with stucco and water or a stucco slurry, thus producing the foamed gypsum composition.

[0022] The foamed stucco composition prepared by the present method preferably has a viscosity of 300 to 800 kg / m after the composition has hardened and dried.3 in the range of 550-750 kg / m 3 The ratio of solids to gas components is such that a bulk density of the foamed gypsum composition is obtained in the range of

[0023] According to a preferred embodiment, the foamed calcium sulfate slurry is (continuously) cast, as known in the art, to form a planar substrate, such as plasterboard. The plasterboard is preferably surrounded by a liner made of paper or veil, which is typically a primarily inorganic fiber mat. For this purpose, the stucco slurry is cast onto the liner, and the cast stucco slurry is preferably covered with another liner. After casting, the planar substrate is preferably subjected to a curing step and, optionally, a drying step, to obtain a foamed gypsum composition.

[0024] Another aspect of the present invention is a gypsum board, preferably in the form of a gypsum plasterboard, produced by the above method.

[0025] Finally, another aspect of the present invention relates to the use of an alkali metal and / or alkaline earth metal hydroxide solution having a pH of 8 or greater for the deactivation of siliconates in gypsum recycle, wherein a gypsum suspension is stirred with the alkali metal and / or alkaline earth metal hydroxide solution. The solution used in this application is preferably a saturated calcium hydroxide solution.

[0026] Gypsum recycle differs from conventional gypsum, i.e., FGD gypsum or quarry gypsum, in that it contains components typically found in processed gypsum products. In accordance with the present invention, gypsum recycle is a gypsum product that contains or is obtained by reaction with a siliconate.

[0027] The present invention will be further described with reference to some examples of embodiments, which are intended to be only illustrative of the present invention and should not be construed as limiting the scope of protection of the present invention in any way. [Brief explanation of the drawings]

[0028] [Figure 1] The results in Table 1 are presented graphically. The left axis shows the maximum volume of the foam-stucco mixture, shown in black. The right axis shows the time elapsed from foam collapse until 50 mL and 100 mL of water were discharged, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0029] Example 1 Gypsum samples containing fixed amounts of H-siloxane of 0.5 and 1.0 wt % relative to the amount of gypsum were prepared by adding each amount of H-siloxane to the gypsum.

[0030] A saturated solution of calcium hydroxide was prepared by stirring such an amount of calcium hydroxide in water that a bottom residue remained after stirring. The mixture thus obtained was filtered and added to a gypsum sample with or without added H-siloxane. The mixture thus obtained was stirred at a temperature of 50-60°C for 120 minutes. The solid material was then filtered off, and the calcium hydroxide solution was removed from the residue. From this material, setting gypsum (stucco) was obtained by drying and calcining at 160°C for about 2 hours.

[0031] For the foam stability test, 1 g samples of the resulting stucco, each stucco not treated with saturated calcium hydroxide solution, and stucco made from conventional gypsum were mixed with a foam made from a specified amount of foaming agent and water. The volume of the mixture of foam and stucco was measured directly after mixing; that is, it represents the maximum volume of each sample. For comparison, a sample without added stucco was prepared (reference), i.e., the sample contained only foam. For each sample, the elapsed time until 50 mL and 100 mL of water were expelled from the foam due to foam collapse was measured.

[0032] The results of these tests are shown in Table 1 below: [Table 1]

[0033] Figure 1 graphically illustrates the results of Table 1. The left axis indicates the maximum volume of the mixture of foam and each stucco, shown in black. The right axis indicates the time elapsed from foam collapse until 50 mL and 100 mL of water were drained, respectively. This refers to the patterned columns of the chart. The numbers on the x-axis refer to the concentration of siloxane contained in each sample, while the designations "Treated" and "Untreated" refer to the fact that stucco treated according to the present invention or untreated stucco was used.

[0034] As can be seen from the table above and Figure 1, the presence of H-siloxane significantly reduces foam stability. The untreated sample containing 1 wt. % H-siloxane showed no further increase in volume after the addition of foam. The added foam collapsed instantly, with the only volume increase occurring due to the addition of the water used to form the foam. In all cases where the gypsum was previously treated with a saturated calcium hydroxide solution, improved foam stability was observed. This improvement can be seen by the longer elapsed time until a given amount of foam collapsed and the larger maximum foam volume at the start of the measurement.

[0035] Example 2 Gypsum treated with calcium hydroxide containing 0.5% by weight and 1% by weight of H-siloxane, and untreated gypsum were each calcined to obtain stucco. Samples containing 2.5% by weight of foam were prepared from the resulting stucco using the amounts of stucco and water shown in Table 2. After hardening and drying, the bulk density of the sample was measured. The results of these tests are shown in Table 2 below.

[0036] [Table 2]

[0037] As is evident from Table 2, the bulk densities of the samples treated with calcium hydroxide before calcining the gypsum were significantly lower than those obtained with the untreated samples and were much closer to the bulk density of the gypsum benchmark.

Claims

1. 1. A method for treating gypsum containing siliconates, said treating comprising: (i) stirring an aqueous gypsum suspension with a solution of an alkali metal or alkaline earth metal hydroxide, or a mixture thereof, having a pH of 8 or greater, thereby producing a mixed solution; (ii) separating the solid component from the mixed solution; The method comprising:

2. 2. The method of claim 1, wherein calcium hydroxide is used as the alkaline earth metal hydroxide.

3. 3. The method according to claim 1 or 2, wherein the solution of alkali metal hydroxide or mixture of alkaline earth metal hydroxides has a pH of at least 9, preferably at least 10, more preferably is an alkaline earth metal hydroxide saturated solution, or said alkali metal hydroxide or alkaline earth metal hydroxide solution has a pH of less than 12, preferably less than 11.

5.

4. 4. The method of claim 3, wherein the saturated alkaline earth metal hydroxide solution is a calcium hydroxide solution prepared by adding calcium hydroxide or calcium oxide to water.

5. 5. The method according to any one of claims 1 to 4, wherein the stirring is carried out at or above room temperature and at an elevated temperature up to 60°C.

6. The method according to any one of claims 1 to 5, wherein the stirring is carried out for a period of time from 5 to 180 minutes, preferably from 15 to 120 minutes, more preferably from 30 to 120 minutes.

7. The method according to any one of claims 1 to 6, further comprising a drying step and / or a calcination step.

8. 10. The method of claim 1, wherein the solution of alkali metal or alkaline earth metal hydroxide or a mixture thereof is used for further processing of the gypsum batch after separation of the solid component, wherein further alkali metal or alkaline earth metal hydroxide or a mixture thereof is optionally added for regeneration.

9. 10. A method for preparing a foamed gypsum composition, comprising adding a foaming agent to a stucco slurry and generating foam from the composition, or generating foam from the foaming agent and water and adding the foam to stucco or a stucco slurry to obtain a foamed stucco composition, wherein the stucco is produced by calcining gypsum treated by the method of any one of claims 1-8.

10. The foamed gypsum composition has a melting point of 300 to 800 kg / m ３ in the range of 550 to 750 kg / m ３ 10. The method of claim 9, wherein the dry bulk density is in the range of 0.1 to 0.

5.

11. 11. The method of claim 9 or 10, further comprising the steps of casting a foamed stucco composition to form a planar substrate and allowing the stucco composition to harden to obtain the foamed gypsum composition.

12. 12. The method of claim 11, wherein the foamed calcium sulfate slurry is cast onto a liner and the cast calcium sulfate slurry is preferably covered with another liner.

13. Gypsum produced by the method according to any one of claims 1 to 8.

14. A gypsum board manufactured by the method according to any one of claims 9 to 12.

15. 1. Use of a solution of an alkali metal or alkaline earth metal hydroxide or a mixture thereof, preferably in the form of a saturated calcium hydroxide solution, having a pH of 10 or more, for the deactivation of siliconates, preferably siliconates having Si—H bonds, in gypsum recycle, wherein the gypsum is stirred with the alkali metal or alkaline earth metal hydroxide solution or the mixture thereof.

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