JP2010105886A - Method for manufacturing gypsum - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing high-purity α gypsum. <P>SOLUTION: The method for manufacturing α gypsum having high purity (for example, the purity of ≥99.5 wt.%) comprises the steps of: using high-purity calcium hydroxide slurry and a sulfate radical-containing aqueous solution as raw materials to synthesize high-purity β gypsum; crystallizing the synthesized high-purity β gypsum by lowering the temperature thereof to obtain gypsum dihydrate and purify the gypsum dihydrate; removing fine particles from the slurry containing the crystallized gypsum dihydrate; adding a crystal habit modifier to the fine particle-removed slurry; and pressurizing/heating the crystal habit modifier-added slurry. It is preferable that the purity of the calcium hydroxide in solid materials contained in the high-purity calcium hydroxide slurry is ≥98 wt.% and the sulfate radical-containing aqueous solution is an ammonium sulfate aqueous solution. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing α-gypsum. More specifically, after synthesizing β-gypsum by a reaction under heating, dihydrate gypsum is precipitated by lowering the temperature in the system, and after separating the fine particles from the slurry, the dihydrate gypsum contains a crystallizing agent. The present invention relates to a method for producing high-purity α-gypsum by heating and pressurizing a solvent.

  As a manufacturing method of industrial alpha gypsum, a method of manufacturing naturally occurring dihydrate gypsum or industrially produced dihydrate gypsum through steps such as semi-hydration, concentration, drying, and pulverization is common. It is.

Industrial dihydrate gypsum production methods include flue gas desulfurization gypsum and by-product gypsum in the phosphoric acid production process, all of which are synthesized by the reaction of sulfate radical and calcium source. As shown.
Ca (OH) ₂ + H ₂ SO ₄ → CaSO ₄ + 2H ₂ O

Since this reaction is usually carried out using water as a solvent, the gypsum obtained is dihydrate gypsum (CaSO ₄ · 2H ₂ O), and in order to obtain α gypsum, a step of semi-hydrating the dihydrate gypsum is necessary. It becomes.

There are methods such as a calcination method, a pressurized steam method, and a pressurized solution method for semi-hydration, and the production reaction is shown as follows.
CaSO ₄ · 2H ₂ O → CaSO ₄ · 1 / 2H ₂ O + 3 / 2H ₂ O

 The α gypsum obtained by the above reaction is usually sent to a drying process to remove water, evaporates the water, and then undergoes a pulverization step for particle size adjustment to become a product α gypsum.

  As a method for producing high-purity α-gypsum, for example, gypsum slurry by-produced in the phosphoric acid production process is converted to α-gypsum using saturated steam at 135 ° C., and then the gypsum coarse particles are placed in the lower zone of the autoclave. A method for producing high-purity α-gypsum by collecting and recovering, and solid-liquid separation of the coarse particle-containing slurry has been proposed (Patent Document 1).

In addition, as a method for obtaining dihydrate gypsum from the reaction between sulfate radical and calcium source, for example, calcium hydroxide slurry and ammonium sulfate aqueous solution are subjected to metathesis reaction at 110 ° C. to synthesize β gypsum, and ammonia in the system is distilled. After removing by step, by adding sulfuric acid and keeping the pH in the system weakly acidic, the temperature is lowered to precipitate dihydrate gypsum to obtain dihydrate gypsum having a high whiteness and a coarse particle size (Patent Document 2).
JP-A-1-249636 Japanese Patent Laid-Open No. 48-97796

However, in the method disclosed in Patent Document 1, since dihydrate gypsum used as a raw material is a byproduct of the phosphoric acid production process, it contains many impurities, and the gypsum purity after pregelatinization is 98.5 wt. It has improved only to%. Further, these impurities contain P ₂ O ₅ and fluorine compounds, which adversely affect physical properties such as gypsum strength and water content.

  In the method disclosed in Patent Document 2, coarse dihydrate gypsum crystals are obtained, but there is no description about gypsum purity. In addition, since the fine particles contained in the gypsum are not separated and the impurities derived from calcium hydroxide are not removed, even if the α-ized reaction is performed using this dihydrate gypsum as a raw material, the progress of the α-ized reaction It is presumed that this will cause inhibition.

  As described above, as a method for producing high-purity α-gypsum, natural dihydrate gypsum or dihydrate gypsum by-produced in various chemical industries is used as a raw material and heated and pressurized in an aqueous solvent for production. The method is common. However, these dihydrate gypsum contains a lot of naturally-derived impurities and uses low-purity calcium sources and sulfate radicals, so the purity of dihydrate gypsum itself is low. It is difficult to produce alpha gypsum. Furthermore, in the presence of α gypsum and water of less than 80 ° C., dihydration of α gypsum is promoted, and it is difficult to purify α gypsum itself.

  Therefore, in the present invention, in the method of producing α gypsum via β gypsum / dihydrate gypsum, high purity α gypsum is produced through a process of purifying an intermediate (β gypsum / dihydrate gypsum). It is an object of the present invention to provide a method for performing the above.

  As a result of intensive studies on the above, as a raw material, calcium hydroxide slurry and an aqueous solution containing sulfate radicals are reacted under heating to synthesize high-purity β-gypsum, and the system temperature is lowered to cause crystallization. It is possible to produce high-purity α-gypsum by purifying gypsum and removing fine particles from the precipitated dihydrate gypsum and then adding a crystallizing agent to pressurize and heat the dihydrate gypsum slurry. I found it possible.

In order to achieve the above object, the present invention adopts the following configuration. That is,
(1) A step of synthesizing β-gypsum from a calcium hydroxide slurry and an aqueous solution containing a sulfate group by a metathesis reaction under heating, a step of adding an aqueous solution containing a sulfate group to a slurry containing β-gypsum, and β-gypsum The step of lowering the temperature of the slurry to dissolve β gypsum to precipitate dihydrate gypsum, the step of removing fine particles from the slurry containing the precipitated dihydrate gypsum, and the dihydrate gypsum to a solvent containing a crystallizing agent A method for producing high-purity α-gypsum characterized by including a step of heating and pressurizing in the interior.
(2) The method for producing α-gypsum according to (1), wherein the calcium hydroxide slurry is produced by a hydration reaction of calcium oxide.
(3) The method for producing α-gypsum according to (2), wherein the average particle diameter (median diameter) of the solid contained in the calcium hydroxide slurry is 20 μm or less.
(4) The method for producing α gypsum according to (2) or (3), wherein the purity of calcium hydroxide in the solid contained in the calcium hydroxide slurry is 98% by weight or more.
(5) The method for producing α-gypsum according to any one of (1) to (4), wherein the aqueous solution containing a sulfate group used for the metathesis reaction is an ammonium sulfate aqueous solution.
(6) The method for producing α-gypsum according to (5), wherein the ammonium sulfate concentration of the aqueous ammonium sulfate solution is 35 to 45% by weight.
(7) The step of synthesizing β gypsum is a step of subjecting excess calcium hydroxide slurry and ammonium sulfate aqueous solution to a metathesis reaction under heating, a step of removing ammonia by-produced in the metathesis reaction by distillation, a gypsum slurry after removal of ammonia The method for producing α-gypsum according to any one of (1) to (6), wherein the α-gypsum is produced through a process comprising a step of adding an aqueous solution containing a sulfate group to adjust the pH.
(8) The α gypsum according to (7), wherein the metathesis reaction between the calcium hydroxide slurry and the aqueous ammonium sulfate solution is performed within a molar ratio of 1.02 to 1.1 (calcium hydroxide / sulfate radical). Manufacturing method.
(9) The method for producing α gypsum according to (7) or (8), wherein the temperature of the metathesis reaction between the calcium hydroxide slurry and the aqueous ammonium sulfate solution is performed in the range of 90 to 120 ° C.
(10) The method for producing α-gypsum according to any one of (7) to (9), wherein the temperature for distilling off ammonia produced as a by-product in the metathesis reaction is in the range of 100 to 120 ° C.
(11) In the step of adding an aqueous solution containing sulfate radicals to adjust the pH of the β gypsum slurry, the pH of the slurry is adjusted to a range of 2.0 to 6.5, (7) The manufacturing method of alpha gypsum in any one of-(10).
(12) The method for producing α gypsum according to any one of (7) to (11), wherein the aqueous solution containing sulfate radicals added to adjust the pH of the β gypsum slurry is sulfuric acid.
(13) Any one of (1) to (12), wherein the temperature of β-gypsum slurry is lowered and the temperature at which β-gypsum is dissolved and dihydrate gypsum is precipitated is in the range of 80 to 95 ° C. The manufacturing method of alpha gypsum as described in above.
(14) The temperature after the β-gypsum slurry is decreased, and the solvent after the α-ization reaction is added to the aqueous solvent in the step of dissolving β-gypsum to precipitate dihydrate gypsum. The manufacturing method of alpha gypsum in any one of 13).
(15) The classification point (limit particle diameter) of the fine particles to be removed from the slurry containing the precipitated dihydrate gypsum is in the range of 5 to 20 μm, according to any one of (1) to (14) Of manufacturing α-gypsum.
(16) The iron content in the dihydrate gypsum is reduced by 50% by weight or more by removing fine particles from the slurry containing the precipitated dihydrate gypsum. The manufacturing method of alpha gypsum as described in any one of.
(17) Any one of (1) to (16), wherein the dihydrate gypsum to be heated and pressurized in a solvent containing a crystallizing agent contains 20 wt% or more of dihydrate gypsum particles having a particle size of 150 μm or more. The manufacturing method of alpha gypsum as described in any one of.
(18) The method for producing α-gypsum according to any one of (1) to (17), wherein dihydrate gypsum is heated in a range of 105 to 130 ° C. in a solvent containing a crystallizing agent.
(19) The method for producing α-gypsum according to any one of (1) to (18), wherein dihydrate gypsum is pressurized to a range of 50 to 300 kPa in a solvent containing a crystallizing agent.
(20) The method for producing α-gypsum according to any one of (1) to (19), wherein the purity is 99.5% by weight or more.
(21) Purity is 99.8 weight% or more, The manufacturing method of alpha gypsum in any one of Claim (1)-(20) characterized by the above-mentioned.

  By this invention, it becomes possible to manufacture alpha gypsum with very high purity. In addition, α-gypsum produced according to the present invention has features such as very low impurity content, high strength, low expansion rate, high whiteness, and very low water content. -It can be used as a mold material such as a dental model or a raw material for dental investment.

  The present invention uses a calcium hydroxide slurry and an aqueous solution containing sulfate radicals as raw materials, and reacts under heating to synthesize high-purity β-gypsum, crystallizing it at a reduced temperature to dihydrate and use the gypsum This is a method for producing high-purity α-gypsum by removing fine particles from purified and precipitated dihydrate gypsum and then adding a crystallizing agent to heat and pressurize the dihydrate gypsum slurry.

  The raw material in this invention is demonstrated below.

  The calcium hydroxide slurry is preferably produced by a hydration reaction of calcium oxide, and calcium oxide having a purity of 95% by weight or more is preferably used. The hydration reaction condition is preferably a water / calcium oxide weight ratio of 1 to 30, and the hydration water temperature or hydration temperature is 85 to 100 ° C., and the mixture is sufficiently stirred and mixed. It is preferable.

  In order to increase the reactivity of the calcium hydroxide slurry generated by the hydration reaction, it is preferable to separate solids and coarse particles of the calcium hydroxide slurry. As long as the coarse particles can be separated according to the particle size, any method such as sieve separation, gravity separation, and centrifugal separation may be used. It is preferable to use a centrifuge for the separation. These separation methods may be used in combination.

  The average particle size (median diameter) of the solid contained in the calcium hydroxide slurry is preferably 20 μm or less, and more preferably 15 μm or less. If the average particle size becomes higher than the above, the amount of solids with β-gypsum only on the surface and unreacted calcium hydroxide remaining inside will increase during the metathesis reaction, and the dihydration (crystallization) reaction will occur. Inhibits and decreases the purity of the α-gypsum produced.

  In addition, although it does not specifically limit about the average particle diameter of the solid substance contained in a slurry, and the method of measuring the average particle diameter of gypsum (dihydrate gypsum and alpha gypsum), For example, from the laser particle size measuring apparatus or the photograph by SEM A method of directly measuring the particle size is preferably used. In the present invention, the particle size distribution is measured using a laser particle size measuring device, and the median value is taken as the average particle size.

  After carrying out the hydration reaction using high-purity calcium oxide, the purity of calcium hydroxide in the solid matter contained in the calcium hydroxide slurry is improved by separating solids and coarse particles, so finally It is possible to improve the purity of the α-gypsum produced. The calcium hydroxide purity in the solid contained in the calcium hydroxide slurry is preferably 98% by weight or more, and more preferably 99% by weight or more. When the calcium hydroxide purity is lower than the above, the dihydration (crystallization) reaction is inhibited by impurities and the purity of the α-gypsum produced tends to be reduced.

  As an aqueous solution containing a sulfate radical, it is preferable to use sulfuric acid or an ammonium sulfate aqueous solution, and a method of dissolving ammonium sulfate generated as a by-product in the caprolactam production process in water at a temperature of 20 to 40 ° C is preferable.

  The concentration of ammonium sulfate used for the metathesis reaction is preferably 35 to 45% by weight. If the ammonium sulfate concentration is lower than the above, the progress of the metathesis reaction rate will be slow, and the slurry concentration of the gypsum produced will be thin, so that it is necessary to enlarge the apparatus, which is disadvantageous in terms of cost. If the ammonium sulfate concentration exceeds the above, the saturated solubility of ammonium sulfate will be exceeded, and solid ammonium sulfate may remain in the gypsum produced during the metathesis reaction.

  The method for adding the aqueous solution containing a sulfate group is not particularly limited, but a method of introducing the solution into a metathesis reaction tank after performing heat exchange by contacting with the vapor of an ammonia distillation column in the subsequent step is preferable.

  Details of the step of synthesizing β gypsum in the present invention will be described below.

  The reaction between the calcium hydroxide slurry and the aqueous solution containing sulfate radicals must be carried out under heating, and is preferably carried out at a temperature in the range of 90 to 120 ° C. When the reaction temperature is higher than the above, the reaction rate is fast, and the β gypsum particles become fine, and a part of the β gypsum is dehydrated. When the reaction temperature is lower than the above, the reaction does not proceed sufficiently, the remaining amount of unreacted calcium hydroxide increases, and the α-gypsum purity tends to decrease.

  The molar ratio between the calcium hydroxide slurry and the sulfate radical is preferably in the range of 1.02 to 1.1 (calcium hydroxide / sulfate radical). If the molar ratio is lower than the above, sulfate radicals remain in the gypsum slurry that is produced, and this results in reaction inhibition and apparatus corrosion in the subsequent crystallization / alpha conversion step. When the molar ratio is higher than the above, the residual amount of unreacted calcium hydroxide is increased, which tends to cause an increase in the amount of sulfuric acid used in the neutralization step and a decrease in α-gypsum purity.

  Since the reaction solution containing β gypsum contains ammonia as a by-product, after removing ammonia by distillation, neutralize unreacted calcium hydroxide contained in β gypsum slurry with sulfate radical (gypsumization) ) To add an aqueous solution containing sulfate radicals. The method for distilling the reaction solution containing β gypsum is not particularly limited, but it is preferable to remove ammonia using a rectifying column, and it is more preferable to remove ammonia using a perforated plate type rectifying column. . The distillation temperature is preferably in the range of 100 to 120 ° C. If the distillation temperature is higher than the above, the gypsum tends to be dehydrated. When the distillation temperature is lower than the above, ammonia in the system cannot be completely removed, and ammonium sulfate is generated when a solution containing a sulfate group is added, which tends to cause a decrease in the purity of gypsum.

  The aqueous solution containing sulfate radicals added to adjust the pH of the β gypsum slurry is not particularly defined, but sulfuric acid is preferably used, and dilute sulfuric acid having a concentration of 5 to 15% by weight is more preferably used. Moreover, in order to complete the neutralization reaction of the unreacted calcium hydroxide, it is preferable to adjust the pH of the β gypsum slurry to a range of 2.0 to 6.5. If the β gypsum slurry PH becomes higher than the above, the remaining amount of unreacted calcium hydroxide increases, leading to a decrease in α gypsum purity. If the β gypsum slurry PH becomes lower than the above, the amount of sulfate radicals remaining in the subsequent process increases, so care must be taken because the effect of the crystallizing agent on the corrosion of the apparatus and the α conversion reaction tends to be hindered.

  Details of the step of synthesizing dihydrate gypsum in the present invention will be described below.

  After adjusting the pH of the β gypsum slurry, the temperature in the system is lowered, and the β gypsum is once dissolved and precipitated as dihydrate gypsum. The pressure for precipitating dihydrate gypsum is preferably normal pressure, and the temperature is preferably in the range of 80 to 95 ° C. When the precipitation temperature is higher than the above, the β gypsum state becomes stable, so that the amount of dihydrate gypsum deposited is extremely reduced. When the precipitation temperature is lower than the above, since the dihydrate gypsum precipitation rate is too high and fine dihydrate gypsum crystals are likely to precipitate, coarse dihydrate gypsum crystals tend not to be obtained. It becomes difficult to obtain coarse α-gypsum crystals in the reaction.

  In the precipitation of the dihydrate gypsum, the temperature may be continuously changed in a single crystallization reaction tank or may be sent to a plurality of crystallization reaction tanks having different temperatures. When feeding to a plurality of crystallization reaction tanks, the crystallization temperature in the subsequent crystallization reaction tank is preferably lower than that in the previous crystallization reaction tank. In any crystallization reaction tank, as β gypsum dissolves, unreacted calcium hydroxide present in β gypsum dissolves, so PH increases. In order to prevent equipment corrosion in the pregelatinization reaction and to neutralize the pH of the resulting α gypsum, the pH of the slurry in the subsequent crystallization reaction tank is adjusted by adjusting the amount of dilute sulfuric acid added to the previous crystallization reaction. It is preferable to make it higher than the tank.

  The solvent generated when the water content in the slurry is reduced by using a concentrator before drying the α gypsum in the post-treatment α-formation reaction contains a small amount of a crystallizing agent that controls crystal growth. . By adding this solvent to the aqueous solvent in the step of precipitating the dihydrate gypsum and controlling the crystal growth of the dihydrate gypsum with the crystallizing agent, it is possible to produce dihydrate gypsum with a coarse particle size. Further, by using the solvent in a circulating manner, it is possible to reduce the amount of water used. Further, since this solvent is at a high temperature, the amount of energy used can also be reduced.

Most of impurities contained in the precipitated dihydrate gypsum are oxides derived from calcium hydroxide as a raw material, such as Fe ₂ O ₃ , MgO, and SiO ₂ . Since these impurities have a fine particle size and cause a decrease in gypsum purity and a pregelatinization reaction, it is necessary to separate the dihydrate gypsum slurry into fine and coarse particles using a separator. As a method for separating fine particles, a format in which fine particles and coarse particles are separated using a centrifuge is preferable, and a method in which fine particles and coarse particles are separated using a cylindrical centrifuge is more preferable.

  The classification point (limit particle diameter) when separating coarse particles and fine particles is preferably in the range of 5 to 20 μm, and more preferably in the range of 5 to 15 μm. When the limit particle diameter is larger than the above, the amount of dihydrate gypsum discharged to the fine particle side increases, and the yield of the product deteriorates. If the critical particle size is smaller than the above, impurities cannot be removed sufficiently.

  By removing the impurities in the dihydrate gypsum by the fine particle separation described above, the purity of the gypsum can be improved. The iron content, which is an impurity in typical dihydrate gypsum, is preferably removed by 50% by weight or more, and more preferably by 60% by weight or more. If the iron removal rate is lower than the above, the purity of gypsum is deteriorated and the pregelatinization reaction is inhibited.

  In addition, about the iron content in gypsum, it measures by the method described below, and the measuring method is as follows.

1 g of a sample is put into a beaker 100 ml, and 10 ml of hydrochloric acid (1 + 1) and 40 ml of water are added and dissolved by heating. After cooling, the whole amount is put into a 100 ml volumetric flask, water is added to the marked line, 20 ml is taken from this solution, 1 ml of 10% by weight aqueous hydroxylamine hydrochloride, 10 ml of 20% by weight aqueous ammonium acetate, and 1% by weight Add 0.2 ml of α, α′-bipyridyl methanol solution to a 100 ml volumetric flask and add water up to the marked line. Absorbance is measured using a spectrophotometer, and iron content is obtained from a calibration curve prepared using an analytical iron standard solution for atomic absorption. (Hereinafter, this measurement method is referred to as dipyridyl method)
Details of the step of synthesizing α gypsum in the present invention will be described below.

  If dihydrate gypsum having a large particle size is used as a raw material for the α-izing reaction, crystals of α gypsum having a large particle size can be obtained. Since the α gypsum having a large particle size is easy to grind and adjust the particle size, it is possible to have various particle size distributions according to the intended use. As dihydrate gypsum to be heated and pressurized in a solvent containing a crystallizing agent, it is preferable to use those containing 20 wt% or more of dihydrate gypsum particles having a particle diameter of 150 μm or more, and those containing 35 wt% or more are used. More preferred. When the content of dihydric gypsum particles having a particle size of 150 μm or more is lower than the above, the generation rate of α-gypsum crystal nuclei increases, but the growth rate of crystals decreases, so the particle size of the generated α-gypsum decreases. There is a tendency.

  There is no particular limitation on the method of α-izing in the solvent containing the crystallizing agent, but it is preferable to add the crystallizing agent in an aqueous solvent and to gelatinize the dihydrate gypsum by using the pressurized solution method. . Further, about 1 to 10% by weight of α-gypsum may be added to a solvent containing a crystallization agent as a seed crystal. The reaction temperature is preferably in the range of 105 to 130 ° C. When the reaction temperature is higher than the above, the dehydration reaction of α gypsum proceeds and the purity of α gypsum is reduced. When the reaction temperature is lower than the above, the dihydric gypsum pregelatinization rate becomes extremely slow. The reaction pressure is preferably in the range of 50 to 300 kPa. When the reaction pressure becomes lower than the above, the dihydric gypsum pregelatinization rate becomes extremely slow. When the reaction pressure exceeds the above, the temperature required for the reaction exceeds 130 ° C., the dehydration of α gypsum proceeds, and the purity of α gypsum is reduced.

  When carrying out the alpha conversion of dihydrate gypsum, it is necessary to adjust the direction of crystal growth and to add a crystallization agent in order to promote the alpha conversion. The substance to be added as a crystallizing agent is not particularly limited as long as it is a substance having a crystallizing effect on α-gypsum, but it is preferable to use a sodium salt of succinic acid or citric acid, and disodium succinate is used. It is more preferable. The addition amount of the crystallizing agent is preferably in the range of 0.01 to 0.5% by weight with respect to the dihydrate gypsum slurry. If the addition amount of the crystallizing agent is lower than the above, the alpha conversion of dihydrate gypsum is not promoted, and the amount of β gypsum produced increases. If the addition amount of the crystallization agent exceeds the above, dehydration of dihydrate gypsum proceeds.

  The generated α gypsum slurry reacts with the water in the system when the temperature decreases, and dihydration proceeds. Therefore, after reducing the moisture in the slurry using a concentrator, etc., the powder is then used using a dryer. It is desirable to use α-gypsum. As a drying method of α-gypsum, it is preferable to use a dryer by a heating drying method such as a hot air receiving type, a conduction receiving type, a radiation receiving type, etc., and a multi-stage disk dryer using high-pressure steam as a heating medium is used. Is more preferable.

  The α gypsum thus obtained is synthesized through a purification process such as distillation, neutralization, crystallization, and impurity separation, so that the impurity content is extremely low, and the purity is 99.5% by weight or more, and high purity. Depending on conditions such as using raw materials, it is possible to synthesize α-gypsum with a purity of 99.8% by weight or more.

  In addition, about the purity measuring method of (alpha) gypsum, it measures by the method based on "(1) purity" as described in JIS-K8963 (1994) "calcium sulfate dihydrate", and the measuring method is as follows. is there.

Add 0.5 g of sample to 100 ml of beaker, add 5 ml of hydrochloric acid (2 + 1) and 30 ml of water, and dissolve by heating. After cooling, the whole amount is put into a 250 ml volumetric flask, and water is added up to the marked line. 25 ml is taken from this solution, neutralized with potassium hydroxide solution (100 g / L), and 12 ml of potassium hydroxide solution (100 / L) and 75 ml of water are added. Titrate with 0.01 mol / L EDTA solution using HSSN diluted powder as indicator.
Hereinafter, an embodiment of the present invention will be described.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited by these Examples.

Example 1
Calcium oxide (CaO content 97.8% by weight) was hydrated at a water / calcium oxide weight ratio of 7 and a hydration temperature of 90 ° C. The calcium hydroxide slurry produced by hydration was centrifuged with a decanter (PTM-300 manufactured by Sakai Kogyo, centrifugal force 1300G, dam height 3.5 mm), and the coarse calcium hydroxide slurry was extracted out of the system. The calcium hydroxide purity in the solid substance contained in the obtained calcium hydroxide slurry is measured by a method based on “(1) Purity” described in JIS-K8575 (1994) “Calcium hydroxide (reagent)”. As a result, it was 99.2% by weight. Moreover, it was 12.7 micrometers when the average particle diameter (median diameter) of the solid substance contained in a calcium hydroxide slurry was measured with the laser particle size measuring apparatus (LA-920 type | mold by Horiba Ltd.).

  Subsequently, water was added to ammonium sulfate (added ammonia to caprolactam sulfate, separated from caprolactam aqueous solution and ammonium sulfate aqueous solution and then crystallized with a magma type crystallizer) to prepare an ammonium sulfate solution having a concentration of 40% by weight. The calcium hydroxide slurry was reacted at a molar ratio of 1.05 (calcium hydroxide / ammonium sulfate) at a temperature of 100 ° C. The produced reaction solution containing β gypsum was distilled using a perforated plate distillation column having a column bottom temperature of 110 ° C. to remove ammonia in the solution. To the reaction solution containing β-gypsum after distillation of ammonia, a solution obtained by flushing a later-described α-gypsum slurry with hot water was added, and the unreacted calcium hydroxide was added for 0.7 hours at a temperature of 99 ° C. and a pH of 2.5. The sum reaction was carried out. PH adjustment at the time of neutralization was carried out by adding 10% by weight sulfuric acid as needed.

  In order to precipitate dihydrate gypsum from this β-gypsum slurry, crystallization was carried out for 2 hours at a temperature of 94 ° C. and a pH of 5.5, and then the gypsum slurry was fed into another reaction tank to obtain a condition 1 at a temperature of 90 ° C. and a pH of 7.0. Crystallized for 5 hours to precipitate dihydrate gypsum. PH adjustment at the time of crystallization was carried out by adding 10% by weight of sulfuric acid as needed. From the obtained dihydrate gypsum slurry, fine particles having a particle size of 10 μm or less were removed using a cylindrical centrifuge (manufactured by TEK). Water was added to the dihydrate gypsum slurry from which the fine particles had been removed, and the dihydrate gypsum slurry concentration was adjusted to 50% by weight. The iron content in the dihydrate gypsum before removal of the fine particles was measured by the dipyridyl method and found to be 45 ppm, and the iron content in the dihydrate gypsum after the removal was 20 ppm. Further, the content of particles having a particle diameter of 150 μm or more contained in dihydrate gypsum was measured using a JIS standard sieve and found to be 57.6% by weight.

  Using the dihydrate gypsum slurry after the concentration adjustment as a raw material, the pregelatinization reaction was carried out under the conditions of a reaction temperature of 120 ° C., a reaction pressure of 200 kPa, and a medium crystal (disodium succinate) addition amount of 0.1% by weight. The obtained α-gypsum slurry is concentrated to a slurry concentration of 75% by hot water flushing using a vertical cylindrical concentrator (manufactured by Kotobuki Techlex), and then supplied to a two-stage disk dryer (manufactured by Tamagawa Machinery). And dried. The temperature of α gypsum discharged from the dryer was 110 ° C. It was 143 micrometers when the average particle diameter (median diameter) of (alpha) gypsum after drying was measured with the laser particle size measuring apparatus (LA-920 type | mold by Horiba, Ltd.). This α-gypsum was pulverized using a pin mill (M-5 type, manufactured by Nara Machinery Co., Ltd.) and then put into a silo for storage.

  The purity of α-gypsum taken out from the silo was measured by a method based on “(1) purity” described in “Calcium sulfate dihydrate” in JIS-K8963 (1994), and it was 99.9% by weight. It was.

Example 2
Β gypsum, dihydrate gypsum, and α gypsum were synthesized in the same manner as in Example 1 except that a liquid obtained by flushing α gypsum slurry with hot water was not added to the reaction solution containing β gypsum after ammonia distillation. When the content of particles having a particle size of 150 μm or more contained in the purified dihydrate gypsum (intermediate) was measured using a JIS standard sieve, it was 27.7% by weight. When the particle size of the α-gypsum after drying was measured, it was 121 μm. The purity of α-gypsum taken out from the silo was measured by a method based on “(1) purity” described in “Calcium sulfate dihydrate” in JIS-K8963 (1994), and found to be 99.7% by weight. It was.

Comparative Example 1
Calcium oxide (CaO content 97.8% by weight) was hydrated at a water / calcium oxide weight ratio of 7 and a hydration temperature of 90 ° C. The calcium hydroxide slurry produced by hydration was centrifuged with a decanter (PTM-300 manufactured by Sakai Kogyo, centrifugal force 1300G, dam height 3.5 mm), and the coarse calcium hydroxide slurry was extracted out of the system. The calcium hydroxide purity in the solid substance contained in the obtained calcium hydroxide slurry is measured by a method based on “(1) Purity” described in JIS-K8575 (1994) “Calcium hydroxide (reagent)”. As a result, it was 99.2% by weight. Moreover, it was 12.7 micrometers when the average particle diameter (median diameter) of the solid substance contained in a calcium hydroxide slurry was measured with the laser particle size measuring apparatus (LA-920 type | mold by Horiba Ltd.).

  Subsequently, water was added to ammonium sulfate (added ammonia to caprolactam sulfate, separated from caprolactam aqueous solution and ammonium sulfate aqueous solution and then crystallized with a magma type crystallizer) to prepare an ammonium sulfate solution having a concentration of 40% by weight. The calcium hydroxide slurry was reacted at a molar ratio of 1.05 (calcium hydroxide / ammonium sulfate) at a temperature of 100 ° C. The produced reaction solution containing β-gypsum was distilled at 110 ° C. using a flask to remove ammonia in the solution. The reaction solution containing β gypsum after ammonia distillation was subjected to a neutralization reaction of unreacted calcium hydroxide under the conditions of a temperature of 99 ° C. and a pH of 2.5. PH adjustment at the time of neutralization was carried out by adding 10% by weight sulfuric acid as needed.

  In order to precipitate dihydrate gypsum from this β-gypsum slurry, crystallization was carried out for 3 hours under the conditions of a temperature of 90 ° C. and a pH of 5.5, followed by crystallization in the same flask under the conditions of a temperature of 85 ° C. and a pH of 7.0 for 0.5 hours. And dihydrate gypsum was deposited. PH adjustment at the time of crystallization was carried out by adding 10% by weight of sulfuric acid as needed. Water was added without removing fine particles in the dihydrate gypsum slurry, and the slurry concentration of the dihydrate gypsum was adjusted to 50% by weight.

  Using the dihydrate gypsum slurry after the concentration adjustment as a raw material, the pregelatinization reaction was carried out under the conditions of a reaction temperature of 120 ° C., a reaction pressure of 200 kPa, and a medium crystal (disodium succinate) addition amount of 0.1% by weight. The α-gypsum produced was filtered without cooling and dried for 2 hours in a drier set at a temperature of 110 ° C. The average particle diameter (median diameter) of the dried α-gypsum was measured with a laser particle size measuring device (LA-920 model, manufactured by Horiba, Ltd.), and it was 122 μm.

  The purity of α-gypsum after drying was measured by a method based on “(1) purity” described in JIS-K8963 (1994) “Calcium sulfate dihydrate”, and was 99.1% by weight. Since the fine particles were not removed, high-purity α-gypsum could not be obtained.

Comparative Example 2
Calcium hydroxide slurry in which water is added to calcium hydroxide (Ca (OH) ₂ content 95.4% by weight), ammonium sulfate solution having a concentration of 40% by weight (adding ammonia to caprolactam sulfate, caprolactam aqueous solution and ammonium sulfate aqueous solution After being separated, the product crystallized with a magma crystallizer was adjusted by adding water to a molar ratio of 1.05 (calcium hydroxide / ammonium sulfate) at a temperature of 30 ° C. The reaction solution containing the produced dihydrate gypsum was subjected to solid-liquid separation by filtration to remove aqueous ammonia. Water is again added to the dihydrate gypsum after separation, and the pH is adjusted to 2.5 by adding 10% by weight of sulfuric acid, and then unreacted water at a temperature of 30 ° C. and a pH of 5.5 for 0.7 hours. A neutralization reaction of calcium oxide was performed. PH adjustment at the time of neutralization was carried out by adding 10% by weight sulfuric acid as needed. When the iron content in dihydrate gypsum was measured by the dipyridyl method, it was 316 ppm. Further, the content of particles having a particle diameter of 150 μm or more contained in dihydrate gypsum was measured by using a JIS standard sieve and found to be 0% by weight.

  After adding water to the obtained reaction solution containing dihydrate gypsum and adjusting the slurry concentration to 50% by weight, the reaction temperature is 120 ° C., the reaction pressure is 200 kPa, and the amount of the crystal modifier (disodium succinate) is added. The pregelatinization was performed under the condition of 0.1% by weight. The α-gypsum produced was filtered without cooling and dried for 2 hours in a drier set at a temperature of 110 ° C. It was 30 micrometers when the average particle diameter (median diameter) of (alpha) gypsum after drying was measured with the laser particle size measuring apparatus (LA-920 type | mold by Horiba Ltd.).

  The purity of α-gypsum after drying was measured by a method based on “(1) purity” described in JIS-K8963 (1994) “Calcium sulfate dihydrate”, and was 92.9% by weight. High purity α-gypsum could not be obtained.

  The α gypsum produced according to the present invention has a very low content of impurities and can be kneaded with a small amount of water, so that a high-strength cured body is obtained and the expansion rate upon curing is small. Because of its characteristics, it can be used as a raw material for mold materials such as ceramics, precious metals, and dental models, and dental investment materials.

Claims (21)

A step of synthesizing β-gypsum by heating a metathesis reaction between a calcium hydroxide slurry and an aqueous solution containing sulfate radicals, a step of adding an aqueous solution containing sulfate radicals to a slurry containing β-gypsum, and the temperature of the slurry containing β-gypsum The step of dissolving β gypsum to precipitate dihydrate gypsum, the step of removing fine particles from the slurry containing the precipitated dihydrate gypsum, and heating the dihydrate gypsum in a solvent containing a crystallizing agent -The manufacturing method of the high purity alpha gypsum characterized by including the process to pressurize. The method for producing α-gypsum according to claim 1, wherein the calcium hydroxide slurry is produced by a hydration reaction of calcium oxide. The method for producing α gypsum according to claim 2, wherein the average particle size (median diameter) of the solid contained in the calcium hydroxide slurry is 20 µm or less. The method for producing α-gypsum according to claim 2 or 3, wherein the purity of calcium hydroxide in the solid substance contained in the calcium hydroxide slurry is 98% by weight or more. The method for producing α-gypsum according to any one of claims 1 to 4, wherein the aqueous solution containing a sulfate group used for the metathesis reaction is an aqueous ammonium sulfate solution. The method for producing α-gypsum according to claim 5, wherein the ammonium sulfate concentration of the aqueous ammonium sulfate solution is 35 to 45% by weight. The process of synthesizing β gypsum is a process in which excess calcium hydroxide slurry and ammonium sulfate aqueous solution undergo metathesis reaction under heating, a process of removing ammonia by-produced in the metathesis reaction by distillation, and a pH of gypsum slurry after ammonia removal. It manufactures through the process including the process of adding the aqueous solution containing a sulfate radical in order to adjust, The manufacturing method of alpha gypsum in any one of Claims 1-6 characterized by the above-mentioned. The method for producing α-gypsum according to claim 7, wherein the metathesis reaction between the calcium hydroxide slurry and the aqueous ammonium sulfate solution is carried out in a molar ratio of 1.02 to 1.1 (calcium hydroxide / sulfate radical). . The method for producing α-gypsum according to claim 7 or 8, wherein the temperature of the metathesis reaction between the calcium hydroxide slurry and the aqueous ammonium sulfate solution is in the range of 90 to 120 ° C. The method for producing α-gypsum according to any one of claims 7 to 9, wherein the temperature for distilling off ammonia produced as a by-product in the metathesis reaction is in the range of 100 to 120 ° C. The step of adding an aqueous solution containing a sulfate group to adjust the pH of the β gypsum slurry, wherein the pH of the slurry is adjusted to a range of 2.0 to 6.5. The manufacturing method of alpha gypsum in any one. The method for producing an α gypsum according to any one of claims 7 to 11, wherein the aqueous solution containing a sulfate group added to adjust the pH of the β gypsum slurry is sulfuric acid. The temperature according to any one of claims 1 to 12, wherein the temperature of β-gypsum slurry is lowered and the temperature at which β-gypsum is dissolved to precipitate dihydrate gypsum is in the range of 80 to 95 ° C. A method for producing gypsum. The temperature after the β gypsum slurry is lowered, and the solvent after the α-ization reaction is added to the aqueous solvent in the step of dissolving β gypsum to precipitate dihydrate gypsum. The manufacturing method of alpha gypsum as described in any one of. The alpha gypsum production according to any one of claims 1 to 14, wherein the classification point (limit particle diameter) of the fine particles to be removed from the slurry containing the precipitated dihydrate gypsum is in the range of 5 to 20 µm. Method. The production of α gypsum according to any one of claims 1 to 15, wherein the iron content in the dihydrate gypsum is reduced by 50% by weight or more by removing fine particles from the slurry containing the precipitated dihydrate gypsum. Method. The α gypsum according to any one of claims 1 to 16, wherein the dihydrate gypsum heated and pressurized in a solvent containing a crystallizing agent contains 20 wt% or more of dihydrate gypsum particles having a particle size of 150 µm or more. Manufacturing method. The method for producing α-gypsum according to any one of claims 1 to 17, wherein dihydrate gypsum is heated in a range of 105 to 130 ° C in a solvent containing a crystallizing agent. The method for producing α-gypsum according to any one of claims 1 to 18, wherein the dihydrate gypsum is pressurized in a range of 50 to 300 kPa in a solvent containing a crystallizing agent. The method for producing α-gypsum according to any one of claims 1 to 19, wherein the purity is 99.5% by weight or more. Purity is 99.8 weight% or more, The manufacturing method of alpha gypsum in any one of Claims 1-20 characterized by the above-mentioned.