Gypsum powder for mold and preparation method

Abstract

The invention discloses gypsum powder for a mold and a preparation method of the gypsum powder. The gypsum powder is prepared from 85 to 95 parts of synthetic gypsum powder, 5 to 10 parts of calcium sulfate whiskers, 0.025 to 0.05 part of poly (diallyldimethylammonium chloride), 2 to 5 parts of quicklime powder, 1 to 3 parts of diatomite, 0.05 to 0.1 part of a water reducing agent, 0.02 to 0.04 part of a defoaming agent and 0.5 to 1.0 part of an active exciting agent. Wherein the synthetic gypsum powder is prepared from household garbage incineration fly ash, waste acid and waste salt through co-treatment-wet synthesis, the fly ash is subjected to acid leaching to obtain refined calcium water, the waste salt is subjected to catalytic pyrolysis to obtain refined salt water, the refined calcium water and the refined salt water are subjected to precipitation according to the ratio of Ca < 2 + >: SO4 < 2-> = 1-1.3, washing, two-step drying, aging and grinding are performed, and the synthetic gypsum powder is obtained. The calcium sulfate whiskers are improved in dispersity after being subjected to PDADMAC surface modification, the calcium sulfate whiskers cooperate with the additive, so that the wet bending resistance of the product is larger than or equal to 2.7 MPa in 2 h, the dry bending resistance of the product is larger than or equal to 6.0 MPa at 45 DEG C, the initial setting time is larger than or equal to 7 min, the final setting time is smaller than or equal to 30 min, and meanwhile recycling of fly ash, waste acid and waste salt is achieved.

Description

Technical Field

[0001] This invention relates to a plaster powder for molds and its preparation method. Background Technology

[0002] Fly ash from municipal solid waste incineration is a type of hazardous waste generated during the incineration process. It is rich in calcium and traditionally treated primarily through landfill. However, landfilling not only occupies significant land resources but also poses potential environmental risks. To utilize fly ash for resource recovery, existing technologies include chemical stabilization, cement kiln co-processing, and acid leaching extraction and separation. While chemical stabilization and cement kiln co-processing can solidify harmful substances in fly ash, they cannot effectively recover calcium resources. Acid leaching extraction and separation can extract calcium, but it generates large amounts of calcium-containing wastewater, which is difficult to treat. Furthermore, large quantities of waste acids (such as waste hydrochloric acid) and waste salts (such as waste sodium sulfate) generated in industrial production also face disposal challenges, necessitating the development of co-resource utilization pathways.

[0003] On the other hand, the demand for gypsum powder for molds in industries such as ceramics is enormous, and traditional products mainly rely on natural gypsum powder or desulfurized gypsum powder. Natural gypsum powder is expensive to mine, and resources are increasingly depleted; desulfurized gypsum powder, as a byproduct of coal-fired power plants, has a complex composition and high impurity content, leading to large fluctuations in product quality and making it difficult to meet the requirements of high-end mold applications. While existing chemical synthesis gypsum powder technology can produce high-purity products, it requires specific temperature and pressure conditions, making the process complex and requiring large equipment investments. Therefore, there is an urgent need to develop alternatives that are simpler, cheaper, and offer more stable quality.

[0004] To date, there have been no reports on the co-production of mold plaster powder using fly ash, waste acid, and waste salt from municipal solid waste incineration. If the co-processing and resource utilization of these three wastes could be achieved, it would not only address the environmental risks of fly ash, waste acid, and waste salt, but also reduce the production cost of mold plaster powder, which would be of great significance for promoting the development of a circular economy. Summary of the Invention

[0005] The present invention provides a plaster powder for molds and a preparation method thereof to solve the problems existing in the prior art.

[0006] The technical solutions adopted in this invention are as follows:

[0007] A type of plaster powder for molds, characterized in that: the raw materials of the plaster powder comprise the following components in parts by weight:

[0008] 85-95 parts of synthetic gypsum powder;

[0009] 5-10 parts of calcium sulfate whiskers;

[0010] 0.025~0.05 parts of polydiallyldimethylammonium chloride;

[0011] 2-5 parts quicklime powder;

[0012] 1-3 parts diatomaceous earth;

[0013] Water-reducing agent: 0.05~0.1 parts;

[0014] Defoamer 0.02~0.04 parts;

[0015] 0.5-1.0 parts of activity activator;

[0016] The synthetic gypsum powder is prepared by wet synthesis using fly ash from municipal solid waste incineration, waste acid, and waste salt as raw materials, after co-processing.

[0017] Furthermore, the aspect ratio of the calcium sulfate whiskers is ≥20.

[0018] Furthermore, the solid content of the polydiallyl dimethyl ammonium chloride is ≥40%.

[0019] Furthermore, the water-reducing agent is a powdered polycarboxylate water-reducing agent; the defoamer is one or two of organosilicon defoamers and polyether defoamers; and the active activator is potassium alum.

[0020] Furthermore, the co-processing technology includes:

[0021] The fly ash from municipal solid waste incineration is first leached with waste acid to obtain an acid leachate, which is then subjected to denitrification, dephosphorization, defluorination, oxidation, and gravimetric removal to obtain refined calcium water.

[0022] Waste salt is purified by catalytic pyrolysis, salt dissolution, denitrification, phosphorus removal, fluoride removal, oxidation, and heavy metal removal to obtain refined brine.

[0023] Refined calcium water and refined salt water are reacted in equal volumes at a calcium ion to sulfate ion molar ratio of 1 to 1.3. After stirring and allowing to stand, a gypsum mixture is obtained.

[0024] The gypsum mixture is washed, filtered, dried, aged, and ground to obtain synthetic gypsum powder.

[0025] Furthermore, the waste acid is waste hydrochloric acid with a mass fraction of 5% to 10%; the waste salt is waste sodium sulfate or a mixture of waste sodium sulfate and sodium chloride.

[0026] Furthermore, the calcium ion concentration in the refined calcium water is 40 g / L to 55 g / L; the sulfate ion concentration in the refined salt water is 90 g / L to 120 g / L.

[0027] Furthermore, the denitrification agent used in the co-processing is sodium hypochlorite;

[0028] The phosphorus and fluoride removal agent is calcium hydroxide;

[0029] The oxidant used in the oxidation process is one or a combination of hydrogen peroxide and ozone.

[0030] Furthermore, the catalytic pyrolysis is low-oxygen pyrolysis (oxygen concentration < 5%), and the pyrolysis temperature is 300℃~550℃.

[0031] Furthermore, when the refined calcium water reacts with the refined salt water, the stirring speed is 100r / min~250r / min, the reaction temperature is 20℃~55℃, the reaction time is 1h~2h, and the standing time is 1h~2h.

[0032] Furthermore, the gypsum mixture is dried in two stages: the first drying temperature is 45℃~55℃, and the time is 2.5h~4h; the second drying temperature is 110℃~130℃, and the time is 2h~3h.

[0033] This invention also discloses a method for preparing plaster powder for molds, characterized by comprising:

[0034] (1) According to the weight ratio of calcium sulfate whiskers: polydiallyldimethylammonium chloride = 1: 0.005, polydiallyldimethylammonium chloride was prepared into an aqueous solution with a mass fraction of 0.5%, calcium sulfate whiskers were added, stirred thoroughly, filtered and dried to obtain modified whiskers;

[0035] (2) The modified whiskers, synthetic gypsum powder, quicklime powder, diatomaceous earth and active activator are mixed in a dry powder mixer to obtain mixture A;

[0036] (3) Mix the water-reducing agent and the defoamer in a dry powder mixer to obtain mixture B;

[0037] (4) First, place mixture A in a dry powder mixer, add mixture B while stirring, and continue mixing to obtain chemical gypsum powder for molds.

[0038] The present invention has the following beneficial effects:

[0039] (1) The mold gypsum powder of the present invention is made of synthetic gypsum powder as raw material, plus calcium sulfate whiskers, polydiallyl dimethyl ammonium chloride, quicklime powder, diatomaceous earth, water reducing agent, defoamer and active activator. The particle size distribution and synergistic effect of each component are fully utilized, which solves the problem that the mold gypsum powder products produced by chemical by-product gypsum have many impurities and the product quality is difficult to control.

[0040] (2) The synthetic gypsum powder in the mold gypsum powder of the present invention is a product obtained by co-processing fly ash, waste acid and waste salt, realizing the resource utilization of fly ash, waste acid and waste salt, and the obtained synthetic gypsum powder is hemihydrate gypsum, which can be used as the main material of mold gypsum powder, and the product quality is stable.

[0041] (3) The mold gypsum powder of the present invention innovatively adds calcium sulfate whiskers and polydiallyldimethylammonium chloride to the synthetic gypsum powder. The hydration process of synthetic gypsum powder involves the reaction of hemihydrate calcium sulfate with water to form dihydrate calcium sulfate. During hydration, dihydrate calcium sulfate crystals interlock to form a network structure, which is the basis for the hardening of gypsum powder. Calcium sulfate whiskers are fibrous single crystals of calcium sulfate. As a homologous material of synthetic gypsum powder, they fully utilize their fibrous and ultrafine inorganic filler advantages without affecting the hydration process of hemihydrate gypsum, thus enhancing the strength of gypsum powder. However, after adding water to gypsum powder, calcium sulfate whiskers have strong hydrophilicity and slightly poor dispersibility. Polydiallyldimethylammonium chloride is a strong cationic electrolyte. The positive charge generated by electrolysis in solution can strongly adsorb the negative charge on the surface of calcium sulfate whiskers. The surface coating greatly reduces the hydrophilicity of the whiskers, resulting in better dispersibility in the system and significantly reducing the amount of water used in the mold gypsum powder process. Furthermore, polydiallyldimethylammonium chloride has good mechanical stability and is more stable during the mixing and stirring of gypsum powder. The addition of both can achieve effects that cannot be achieved by using synthetic gypsum powder alone.

[0042] (4) The addition of quicklime powder and diatomaceous earth to the gypsum powder for molds in this invention can improve the workability of gypsum, increase the adhesion of the slurry, and further increase the strength; the addition of polycarboxylate superplasticizer can release the encapsulated free water, reduce the frictional resistance between particles, increase the flow performance of the system, regulate the initial setting time of gypsum powder, and prevent the gypsum powder from setting too quickly; the addition of defoamer can make the surface of the gypsum powder molded product smooth and reduce the presence of pores, making the whole more compact; quicklime powder itself can provide an alkaline environment to play an alkaline activating role in the system, and together with the acidic activator potassium alum, the two work together to play the role of a composite activator, and the effect is better than that of a single acidic or alkaline activator. Each component plays its own unique advantages, so that the prepared gypsum powder can achieve a wet flexural strength of more than 2.7 MPa at 2h, a dry flexural strength of more than 6.0 MPa at 45℃, an initial setting time of more than 7 min, and a final setting time of less than 30 min. Detailed Implementation

[0043] In this embodiment and the comparative example, the raw materials used all meet the following specifications: the aspect ratio of calcium sulfate whiskers is ≥20, and the solid content of polydiallyl dimethyl ammonium chloride is ≥40%.

[0044] The water-reducing agent is a powdered polycarboxylate water-reducing agent, and the active activator is potassium alum.

[0045] The waste acid is waste hydrochloric acid with a mass fraction of 5% to 10%. Sodium hypochlorite is used as the denitrification agent, and calcium hydroxide is used as the dephosphorization and defluorination agent.

[0046] All performance tests were conducted in accordance with QB / T 1640-2015 "Test Method for Physical Properties of Gypsum Powder for Ceramic Molds". The test indicators included initial setting time, final setting time, 2-hour wet flexural strength, 45℃ dry flexural strength and molding appearance.

[0047] Example 1

[0048] Preparation of synthetic gypsum powder:

[0049] Take fly ash from municipal solid waste incineration, and then take 8% waste hydrochloric acid by mass. Dilute the waste hydrochloric acid to a hydrogen ion molar concentration of about 1 mol / L, and then use the diluted acid to carry out an acid leaching reaction to obtain an acid leaching solution.

[0050] Sodium hypochlorite was added to the acid leaching solution to complete the denitrification treatment, then calcium hydroxide was added to remove phosphorus and fluorine, and finally hydrogen peroxide was added to oxidize and remove heavy metals. After filtration, a refined calcium solution with a calcium ion concentration of 48 g / L was obtained.

[0051] A mixed salt with a sodium sulfate mass fraction greater than 90% and a sodium chloride mass fraction less than 10% was catalytically pyrolyzed at 450℃ under an oxygen-deficient environment (oxygen concentration < 5%). The pyrolysis products were dissolved in salt and then sequentially passed through sodium hypochlorite for denitrification, calcium hydroxide for phosphorus and fluoride removal, and hydrogen peroxide for heavy removal. The filtered product yielded a refined brine with a sulfate ion concentration of 105 g / L.

[0052] Equal volumes of refined calcium water and refined salt water were mixed at a molar ratio of calcium ions to sulfate ions of 1.1. Refined calcium water was slowly added dropwise to the refined salt water. The stirring speed was controlled at 180 r / min, the reaction temperature at 35℃, and the reaction time at 1.5 h. After the reaction was completed, the mixture was allowed to stand for 1.5 h to obtain a gypsum mixture.

[0053] The gypsum mixture was washed three times in countercurrent and filtered to obtain wet gypsum. It was first dried at 50°C for 3 hours and then dried at 120°C for 2.5 hours. The dried product was aged for 1.5 days under natural sealed conditions and then ball-milled to a particle size of 80 μm to obtain synthetic gypsum powder (hemihydrate gypsum).

[0054] Preparation of plaster powder for molds:

[0055] Five parts of calcium sulfate whiskers were weighed, and 0.025 parts of polydiallyldimethylammonium chloride were weighed according to the weight ratio of calcium sulfate whiskers to polydiallyldimethylammonium chloride 1:0.005. The mixture was prepared into a 0.5% (w / w) aqueous solution. After adding the calcium sulfate whiskers, the mixture was stirred thoroughly at 100 r / min for 1 h. After filtration, the mixture was dried at 65 °C for 3 h to obtain the modified whiskers.

[0056] The modified whiskers were mixed with 85 parts of synthetic gypsum powder, 2 parts of quicklime powder, 1 part of diatomaceous earth, and 0.5 parts of potassium alum in a dry powder mixer and mixed for 12 minutes to obtain mixture A.

[0057] Take 0.05 parts of powdered polycarboxylate superplasticizer and 0.02 parts of organosilicon defoamer, and mix them in a dry powder mixer for 6 minutes to obtain mixture B;

[0058] Place mixture A in a dry powder mixer, and while mixing, add mixture B within 45 seconds. Continue mixing for 12 minutes to obtain plaster powder for molds.

[0059] Example 2

[0060] The preparation process of synthetic gypsum powder is consistent with that of Example 1, with only some parameters adjusted: the mass fraction of waste hydrochloric acid is 10%, the hydrogen ion molar concentration during acid leaching is 5 mol / L, the calcium ion concentration of refined calcium water is 55 g / L, the catalytic pyrolysis temperature of waste salt (waste sodium sulfate is used in this example) is 550℃, the sulfate ion concentration of refined brine is 120 g / L, the reaction stirring speed is 250 r / min, the reaction temperature is 55℃, the first drying temperature is 55℃, the drying time is 4 h, and the second drying temperature is 130℃, the drying time is 3 h. In this example, ozone is used for oxidation and weight removal.

[0061] Preparation of plaster powder for molds: Weigh 10 parts of calcium sulfate whiskers, and weigh 0.05 parts of polydiallyldimethylammonium chloride according to the weight ratio of calcium sulfate whiskers to polydiallyldimethylammonium chloride 1:0.005. Prepare an aqueous solution according to the same method as in Example 1, and mix, stir, filter, and dry to obtain modified whiskers. Put the modified whiskers into a dry powder mixer with 95 parts of synthetic plaster powder, 5 parts of quicklime powder, 3 parts of diatomaceous earth, and 1.0 part of potassium alum, and mix for 15 minutes to obtain mixture A. Take 0.1 parts of powdered polycarboxylate superplasticizer and 0.04 parts of polyether defoamer, and mix in a dry powder mixer for 8 minutes to obtain mixture B. Place mixture A in a dry powder mixer, and add mixture B within 1 minute while stirring, and continue mixing for 15 minutes to obtain plaster powder for molds.

[0062] Comparative Example 1

[0063] The raw material formula does not include calcium sulfate whiskers and polydiallyl dimethyl ammonium chloride. The remaining raw materials and their proportions are as follows:

[0064] The preparation process of the synthetic gypsum powder is completely consistent with that of Example 1, consisting of 85 parts synthetic gypsum powder, 2 parts quicklime powder, 1 part diatomaceous earth, 0.05 parts powdered polycarboxylate superplasticizer, 0.02 parts organosilicon defoamer, and 0.5 parts potassium alum.

[0065] Preparation of plaster powder for molds: Synthetic plaster powder, quicklime powder, diatomaceous earth and potassium alum are directly put into a dry powder mixer and mixed for 12 minutes to obtain mixture A. Subsequently, it is mixed with mixture B according to the method of Example 1 to finally obtain the product.

[0066] Comparative Example 2

[0067] The raw material ratio does not contain water-reducing agent, and the remaining raw materials and proportions are the same as in Example 1. The preparation process of the synthetic gypsum powder is the same as in Example 1.

[0068] Preparation of gypsum powder for molds: Modified whiskers were prepared according to the method in Example 1. The modified whiskers were mixed with synthetic gypsum powder, quicklime powder, diatomaceous earth and potassium alum to obtain mixture A. The preparation step of mixture B was omitted. 0.02 parts of organosilicon defoamer were directly added to mixture A and mixed for 12 minutes to obtain the product.

[0069] Comparative Example 3

[0070] The raw material ratio does not include defoamer, and the remaining raw materials and proportions are the same as in Example 1; the preparation process of the synthetic gypsum powder is the same as in Example 1;

[0071] Preparation of gypsum powder for molds: Modified whiskers were prepared according to the method in Example 1. The modified whiskers were mixed with synthetic gypsum powder, quicklime powder, diatomaceous earth and potassium alum to obtain mixture A. The preparation step of mixture B was omitted. 0.05 parts of powdered polycarboxylate superplasticizer were directly added to mixture A and mixed for 12 minutes to obtain the product.

[0072] Performance test results

[0073] Testing items Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Initial setting time / min 10 9 8 5 9 Final setting time / min 24 20 18 16 20 2h wet flexural strength / MPa 2.9 3.0 2.3 2.8 2.9 45℃ dry flexural strength / MPa 6.3 6.4 5.4 6.0 5.8 Appearance after molding Smooth surface Smooth surface Smooth surface Smooth surface The surface has pores

[0074] Results Analysis

[0075] Examples 1 and 2 were prepared according to the complete technical solution of the present invention. The initial setting time of the products was ≥7 min, the final setting time was ≤30 min, the wet flexural strength at 2 h was ≥2.7 MPa, and the dry flexural strength at 45℃ was ≥6.0 MPa. The molded appearance was smooth, which fully met the requirements of β gypsum powder in QB / T1639-2014 "Gypsum Powder for Ceramic Molds". This proves that the technical solution of the present invention has reliable feasibility.

[0076] Comparative Example 1: Due to the absence of calcium sulfate whiskers and polydiallyldimethylammonium chloride, the wet flexural strength at 2 hours and the dry flexural strength at 45°C both failed to meet the standard requirements. This indicates that the synergistic effect of these two components (the reinforcing effect of calcium sulfate whiskers and the effect of polydiallyldimethylammonium chloride in improving whisker dispersibility) is the key to improving the strength of gypsum powder.

[0077] Comparative Example 2, without the addition of water-reducing agent, had an initial setting time of only 5 minutes, which did not meet the standard of ≥7 minutes, demonstrating the regulatory effect of water-reducing agent on the setting time of gypsum powder;

[0078] Comparative Example 3: No defoamer was added. After molding, the product surface had pores and the 45℃ dry flexural strength did not meet the standard. This shows that the defoamer can effectively optimize the product molding quality and improve the structural density.

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A type of plaster powder for molds, characterized in that: The raw materials for the gypsum powder include the following components in parts by weight: 85-95 parts of synthetic gypsum powder; 5-10 parts of calcium sulfate whiskers; 0.025~0.05 parts of polydiallyldimethylammonium chloride; 2-5 parts quicklime powder; 1-3 parts diatomaceous earth; Water-reducing agent: 0.05~0.1 parts; Defoamer 0.02~0.04 parts; 0.5-1.0 parts of activity activator; The synthetic gypsum powder is prepared by wet synthesis using fly ash from municipal solid waste incineration, waste acid, and waste salt as raw materials, after co-processing.

2. The plaster powder for molds as described in claim 1, characterized in that: The aspect ratio of the calcium sulfate whiskers is ≥20.

3. The plaster powder for molds as described in claim 1, characterized in that: The solid content of the polydiallyl dimethyl ammonium chloride is ≥40%.

4. The plaster powder for molds as described in claim 1, characterized in that: The water-reducing agent is a powdered polycarboxylate water-reducing agent; the defoamer is one or two of organosilicon defoamers and polyether defoamers; and the active activator is potassium alum.

5. The plaster powder for molds as described in claim 1, characterized in that: The co-processing technology includes: The fly ash from municipal solid waste incineration is first leached with waste acid to obtain an acid leachate, which is then subjected to denitrification, dephosphorization, defluorination, oxidation, and gravimetric removal to obtain refined calcium water. Waste salt is purified by catalytic pyrolysis, salt dissolution, denitrification, phosphorus removal, fluoride removal, oxidation, and heavy metal removal to obtain refined brine. Refined calcium water and refined salt water are reacted in equal volumes at a calcium ion to sulfate ion molar ratio of 1 to 1.

3. After stirring and allowing to stand, a gypsum mixture is obtained. The gypsum mixture is washed, filtered, dried, aged, and ground to obtain synthetic gypsum powder.

6. The plaster powder for molds as described in claim 5, characterized in that: The waste acid is waste hydrochloric acid with a mass fraction of 5% to 10%; the waste salt is waste sodium sulfate or a mixture of waste sodium sulfate and sodium chloride.

7. The plaster powder for molds as described in claim 5, characterized in that: The calcium ion concentration in the refined calcium water is 40 g / L to 55 g / L; the sulfate ion concentration in the refined salt water is 90 g / L to 120 g / L.

8. The plaster powder for molds as described in claim 5, characterized in that: Sodium hypochlorite is the denitrification agent used in the co-processing procedure; The phosphorus and fluoride removal agent is calcium hydroxide; The oxidant used in the oxidation process is one or a combination of hydrogen peroxide and ozone.

9. The plaster powder for molds as described in claim 5, characterized in that: When calcium water reacts with salt water, the stirring speed is 100r / min~250r / min, the reaction temperature is 20℃~55℃, the reaction time is 1h~2h, and the standing time is 1h~2h.

10. The plaster powder for molds as described in claim 5, characterized in that: The gypsum mixture is dried in two stages: the first drying temperature is 45℃~55℃, and the time is 2.5h~4h; the second drying temperature is 110℃~130℃, and the time is 2h~3h.

11. A method for preparing plaster powder for molds as described in any one of claims 1-10, characterized in that, include: (1) According to the weight ratio of calcium sulfate whiskers: polydiallyldimethylammonium chloride = 1: 0.005, polydiallyldimethylammonium chloride was prepared into an aqueous solution with a mass fraction of 0.5%, calcium sulfate whiskers were added, stirred thoroughly, filtered and dried to obtain modified whiskers; (2) The modified whiskers, synthetic gypsum powder, quicklime powder, diatomaceous earth and active activator are mixed in a dry powder mixer to obtain mixture A; (3) Mix the water-reducing agent and the defoamer in a dry powder mixer to obtain mixture B; (4) First, place mixture A in a dry powder mixer, add mixture B while stirring, and continue mixing to obtain chemical gypsum powder for molds.