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US-12617719-B2 - Synthetic gypsum and gypsum boards produced therefrom

US12617719B2US 12617719 B2US12617719 B2US 12617719B2US-12617719-B2

Abstract

This invention discloses a synthetic gypsum and gypsum boards produced therefrom. Limestone (Calcium Carbonate), slaked lime (calcium hydroxide), water, and sulfuric acid are mixed, and alpha hemihydrate gypsum is optionally added as crystal seed to produce synthetic gypsum. The synthetic gypsum is then used to make gypsum boards. The gypsum board produced according to this invention contains at least 10% alpha hemihydrate gypsum.

Inventors

  • Feng-Shuo HSU
  • Chih-Yuan Hou

Assignees

  • UNIVERSAL CEMENT CORPORATION

Dates

Publication Date
20260505
Application Date
20230417

Claims (17)

  1. 1 . A process of producing synthetic gypsum, the process comprising: step 1 which comprises mixing Calcium Carbonate (CaCO 3 ) and one of Calcium Hydroxide (Ca(OH) 2 ) and Calcium Oxide (CaO) in a predetermined ratio, and adding water to form a suspension with a predetermined percentage of solid content; and step 2 which comprises adding sulfuric acid under normal pressure, and stirring the suspension and the added sulfuric acid, wherein during step 2, a temperature of the suspension rises, gas is generated, and gypsum is precipitated, and wherein synthetic gypsum is obtained after drying, wherein the synthetic gypsum, in a composition analysis according to ASTM-C471M standards, comprises: free water less than 10%, CaSO 4 ·2H 2 O over 95%, particle size over 150 μm, and pH 5˜8, wherein in step 1, a predetermined amount of alpha-hemihydrate gypsum is added to the suspension as a crystal seed of dihydrate gypsum, and wherein the synthetic gypsum, in the composition analysis according to ASTM-C471M standards, further comprises: alpha-hemihydrate gypsum 20%˜80%.
  2. 2 . The process according to claim 1 , wherein in step 1, the predetermined ratio of Calcium Carbonate (CaCO 3 ) and the one of Calcium Hydroxide (Ca(OH) 2 ) and Calcium Oxide (CaO) is in a range of 2:1˜8.
  3. 3 . The process according to claim 1 , wherein in step 1, the predetermined percentage of solid content of the suspension after said adding water is in a range of 20%˜40%.
  4. 4 . The process according to claim 1 , wherein the predetermined amount of alpha-hemihydrate gypsum added is in a range of 0.3%˜5%.
  5. 5 . The process according to claim 1 , wherein in step 2, the sulfuric acid added is a 20%˜60% sulfuric acid solution, in a volume equivalent to 1˜1.5 times a molar amount of Calcium.
  6. 6 . The process according to claim 1 , wherein in step 2, after the sulfuric acid is added, a solution including the suspension and the added sulfuric acid is further adjusted to a pH of 3˜5 with dilute sulfuric acid.
  7. 7 . The process according to claim 1 , wherein an entirety of the process is carried out under normal pressure self-exothermic heat, and requires no external heating.
  8. 8 . A process of producing synthetic gypsum, the process comprising: step 1 which comprises mixing Calcium Carbonate (CaCO 3 ) and one of Calcium Hydroxide (Ca(OH) 2 ) and Calcium Oxide (CaO) in a ratio of 2:1˜8, with water to form a suspension with 20%˜40% solid content; step 2 which comprises adding a 20%˜60% sulfuric acid solution at normal pressure in a volume equivalent to 1˜1.5 times a molar amount of Calcium, and stirring and mixing the sulfuric acid solution with the suspension, wherein, during step 2, a temperature of a mixture of the sulfuric acid solution and the suspension continues to rise and gas is produced, until gas production stops, to obtain an intermediate product; and step 3 in which: the intermediate product is centrifuged to be dehydrated, the dehydrated intermediate product is mixed and stirred with water and a dilute sulfuric acid solution to obtain a product of pH of 3˜5, the product of the pH of 3˜5 is allowed to settle and precipitate, the settled and precipitated product is centrifuged to be dehydrated, and then washed with water, and the washed product is dried to produce synthetic gypsum.
  9. 9 . The process according to claim 8 , wherein in step 1, an amount of alpha-hemihydrate gypsum in a range of 0.3%˜5% is added to the suspension as a crystal seed for dihydrate gypsum.
  10. 10 . The process according to claim 9 , wherein the synthetic gypsum, in a composition analysis according to ASTM-C471M standards, comprises: free water less than 10%, CaSO 4 ·2H 2 O over 95%, particle size over 150 μm, pH 5˜8, and alpha-hemihydrate gypsum 20-80%.
  11. 11 . A process of producing a gypsum board, comprising: step 1 which comprises mixing a predetermined ratio of Calcium Carbonate (CaCO 3 ) and one of Calcium Hydroxide (Ca(OH) 2 ) and Calcium Oxide (CaO), and adding water to form a suspension with a predetermined concentration of solid; step 2 which comprises adding a sulfuric acid solution of a predetermined concentration under normal pressure and in a volume equivalent to a predetermined multiple of a molar amount of Calcium, and stirring the sulfuric acid solution with the suspension, wherein, during step 2, a temperature of a mixture of the sulfuric acid solution and the suspension continues to rise and gas is generated, until the gas production stops, to obtain an intermediate product; step 3 in which: the intermediate product is centrifuged to be dehydrated, the dehydrated intermediate product is mixed with water and a dilute sulfuric acid solution to obtain a product of pH of 3˜5, the product of the pH of 3˜5 is allowed to settle, the settled product is centrifuged to be dehydrated, and then washed with water, and the washed product is dried to produce synthetic gypsum; step 4 which comprises: drying, grinding, and calcining the synthetic gypsum to produce 10˜50% alpha-hemihydrate calcium sulfate plaster powder, and then pulverizing and storing the alpha-hemihydrate calcium sulfate plaster powder; step 5 which comprises forming a gypsum slurry by mixing the alpha-hemihydrate calcium sulfate plaster powder, water, and auxiliary materials; and step 6 in which: the gypsum slurry is injected into a paper-lined mold, with a top or face paper and a bottom or back paper, to form continuous wet boards, and the wet boards are hardened, transferred, cut, dried, and trimmed to produce gypsum boards.
  12. 12 . The process according to claim 11 , wherein in step 1, a predetermined amount of alpha-hemihydrate gypsum is added to the suspension as a crystal seed of dihydrate gypsum.
  13. 13 . The process according to claim 12 , wherein in step 1, the predetermined amount of alpha-hemihydrate gypsum is 0.3%˜5%.
  14. 14 . The process according to claim 12 , wherein the gypsum board, in a composition analysis according to ASTM-C471M standards, comprises at least 10% of alpha-hemihydrate calcium sulfate.
  15. 15 . A process of producing a gypsum board, comprising: step 1 which comprises mixing Calcium Carbonate (CaCO 3 ) with one of calcium hydroxide (Ca(OH) 2 ) and calcium oxide (CaO) in a ratio of 2:1˜8, and adding water to form a suspension with a solid concentration of 20%˜40%; step 2 which comprises adding a 20%˜60% sulfuric acid solution at normal pressure in a volume equivalent to 1˜1.5 times a molar amount of calcium, and stirring and mixing the sulfuric acid solution with the suspension, wherein, during step 2, a temperature of a mixture of the sulfuric acid solution and the suspension rises and gas is generated, until gas production stops, to obtain an intermediate product; step 3 in which: the intermediate product is centrifuged to be dehydrated, the dehydrated intermediate product is mixed and stirred with water and a dilute sulfuric acid solution to obtain a product of pH of 3˜5, the product of the pH of 3˜5 is allowed to settle and precipitate, the settled and precipitated product is centrifuged to be dehydrated, and then washed with water, and the washed product is dried to produce synthetic gypsum; step 4 in which the synthetic gypsum is dried, ground, and calcined to produce a plaster powder containing 10˜50% alpha-hemihydrate calcium sulfate, which is then pulverized and stored; step 5 which comprises mixing the plaster powder, water, and auxiliary materials to form a gypsum slurry; and step 6 in which: the gypsum slurry is injected into a paper-lined mold, with a top or face paper and a bottom or back paper, to form continuous wet boards, and the wet boards are hardened, transferred, cut, dried, and trimmed to produce gypsum boards.
  16. 16 . The process according to claim 15 , wherein in step 1, an amount of alpha-hemihydrate gypsum in a range of 0.3%˜5% is added in the suspension as a crystal seed of dihydrate gypsum.
  17. 17 . The process according to claim 16 , wherein the gypsum board, in a composition analysis according to ASTM-C471M standards, comprises: at least 10% of alpha-hemihydrate calcium sulfate.

Description

BACKGROUND Technical Field The present invention relates to a synthetic gypsum and a gypsum board made from the synthetic gypsum, particularly a gypsum board containing alpha-hemihydrate gypsum. Description of Related Art Traditional gypsum boards do not contain alpha-hemihydrate gypsum, and their strength is relatively low. To meet the requirements of modern construction, the development of a new generation of high-strength gypsum boards has always been the focus of efforts by gypsum board manufacturers. DETAILED DESCRIPTION OF THE INVENTION If gypsum (CaSO4·2H2O) is simply produced by reacting limestone (CaCO3) with sulfuric acid (H2SO4), the resulting gypsum tends to recrystallize on the surface of the original limestone to form a layer of gypsum with low solubility. When the gypsum completely covers the original limestone core, the reaction will gradually stop at a temperature of about 45 degrees Celsius. To continue the reaction between the core limestone and sulfuric acid, an external heat source must be used so that the mixture can be stirred at higher temperatures to break the encapsulating gypsum layer in order to continue the reaction until completion. Taking room temperature at 25 degrees Celsius as an example: as the ratio of slaked lime increases, the process temperature also increases. For example, when limestone and slaked lime are mixed in a ratio of 2:1, the process temperature can be raised to above 45 degrees Celsius; when limestone and slaked lime are mixed in a ratio of 2:8, the process temperature can reach near 80 degrees Celsius. Therefore, the present invention uses a mixture of slaked lime and limestone as raw materials to produce synthetic gypsum under normal pressure and chemical internal heat, neither external pressure nor external heat source is needed, which can achieve energy-saving and carbon reduction. The present invention further discloses manufacture of high-strength gypsum boards containing alpha-hemihydrate gypsum from the disclosed synthetic gypsum. The gypsum boards manufactured by the present invention contain more than 10% alpha-hemihydrate calcium sulfate. The chemical reactions related to the present invention are described as follows: Calcium Sulfate Dihydrate (CaSO4·2H2O), commonly known as gypsum or dihydrate gypsum, which loses 1.5 crystalline water molecules and forms hemihydrate gypsum (CaSO4·½ H2O) upon heating: CaSO4·2H2O→CaSO4·½H2O+3/2H2O Hemihydrate gypsum can be further divided into alpha-hemihydrate and beta-hemihydrate: Heating dihydrate gypsum to around 97° C. leads to the formation of alpha-hemihydrate gypsum, which has a regular rhombic crystal shape. Heating dihydrate gypsum to around 45° C. leads to the formation of beta-hemihydrate gypsum, which has a loose and porous solid structure. Calcium Carbonate (CaCO3) reacts with Sulfuric acid and water to produce dihydrate gypsum and carbon dioxide: CaCO3(aq)+H2⁢SO4(aq)+H2⁢O(l)→"\[Rule]"CaSO4·2⁢ H2⁢O↓+CO2(g)↑Δ⁢ H=-306.98⁢ kJ/mol Calcium hydroxide (Ca(OH)2) reacts with sulfuric acid to produce dihydrate gypsum: Ca⁡(OH)2⁢(aq)+H2⁢SO4(aq)→"\[Rule]"CaSO4·2⁢ H2⁢O↓Δ⁢ H=-428.59⁢ kJ/mol The present invention has found that gypsum boards containing alpha-hemihydrate gypsum have significantly increased physical strength compared to traditional gypsum boards that do not contain alpha-hemihydrate gypsum. Please refer to Table 1: Comparison of Physical Properties. TABLE 1Comparison of Physical Properties.Tested ItemsThe present disclosedTraditional GypsumGypsum Board (containingBoards (withoutalpha-hemihydratecontaining alpha-gypsum over 10%)hemihydrate gypsum)Thickness   15 mm  15 mmLongitudinal bending970 N910 Nbreaking loadTransverse bending580 N310 Nbreaking loadImpact resistancehammer height 800 mm,hammer height 800 mm,diameter of the depressiondiameter of the depressionless than 20 mm, and noless than 25 mm, and nocrack through the back.crack through the back.Nailed side surface load825 N750 N Table 1 shows a comparison of the physical properties between the gypsum board of the present invention and conventional gypsum board, which includes at least the following advantages: (1) The longitudinal bending breaking load of the present invention gypsum board is 970 N, which is better than the conventional gypsum board at 910 N.(2) The transverse bending breaking load of the present invention gypsum board is 580 N, which is better than the conventional gypsum board at 310 N.(3) The present invention gypsum board has better impact resistance, with a dent diameter below 20 mm, compared to the conventional gypsum board with a dent diameter below 25 mm.(4) The Nailed side surface load of the present invention gypsum board is 825 N, which is better than the conventional gypsum board at 750 N. The present invention mixes limestone (Calcium Carbonate) and slaked lime (calcium hydroxide) in a specific ratio, and adds a predetermined amount of alpha-hemihydrate gypsum as a crystal seed for dihydrate calcium sulfate