JPS595527B2 - Manufacturing method of dihydrate gypsum crystals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention produces coarse and thick gypsum trihydrate, which is suitable as a raw material for calcined gypsum for gypsum boards, from gypsum hemihydrate containing adhering acid content and inclusion acid content, which is produced as a by-product from a chemical factory. It is about the method.

When hydrating gypsum hemihydrate to obtain gypsum trihydrate crystals, if the gypsum hemihydrate is simply made into a slurry in hot water, only fine acicular trihydrate crystals with little utility value will be obtained. For example, gypsum hemihydrate, which is separated and washed from a mixed acid mother liquor consisting of sulfuric acid, phosphoric acid, etc. produced as a by-product in a wet phosphoric acid production factory, has extremely low levels of solid-soluble impurities and organic substances that would interfere with hydration, and is slurried as it is in hot water. However, despite its high purity, only fine acicular trihydrate can be obtained which has little utility value.

In order to obtain coarse trihydrate crystals that have utility value by hydrating gypsum hemihydrate, it is necessary to select crystal growth conditions that suppress nucleation of trihydrate as much as possible and promote only nucleation growth.
In reality, it is often sufficient to adjust the initial hydration reaction rate to be slow.

This adjustment method includes ■) adjusting the water usage rate by adjusting the temperature in the transition region from hemihydrate gypsum to trihydrate gypsum;
2) A method of causing a third substance to coexist during hydration of gypsum hemihydrate to provide a medium crystal effect, 3) A method of coexisting with gypsum trihydrate seed crystals,
4) A method that is a compromise between the above three methods may be mentioned.

Here, the method focusing on 1) is disadvantageous in terms of energy cost, and the method focusing on 2) is disadvantageous in terms of cost because it is difficult to treat wastewater containing the third substance, and furthermore, the method focusing on 2) is disadvantageous in terms of cost. Attention must be paid to the effect of the third substance remaining in the plaster of Paris on the physical properties of the plaster of Paris.

As a result of studying the modeding effect in method 2), the present inventors found that the influence of the pH during the water utilization reaction is large, and furthermore, if the pH is adjusted appropriately, there is no need to use a special moderating agent. We found that it is possible to adjust the hydration rate to match the crystal growth conditions.

In other words, by adding alkali immediately after adding water to hemihydrate gypsum to quickly adjust the pH from neutral to weakly alkaline, the hydration rate is slow, nucleation is small, and coarse trihydrate is removed. I found out that it is easy to obtain.

The reason for this delay in hydration due to the addition of alkali is not clear, but it is due to changes in the ζ (zeta) potential of gypsum hemihydrate and the influence of salts generated by neutralization. This is thought to be mainly due to the precipitation of poorly soluble phosphates on the surface of gypsum hemihydrate.

On the other hand, in the case of gypsum hemihydrate, which is produced as a by-product during the production of phosphoric acid, diluted adhering acid content (acid content attached to the surface of gypsum hemihydrate) and inclusion acid content (acid content present inside the gypsum hemihydrate crystal) If you try to hydrate it by simply adding water, you will only get fine needle-like crystals because hydration occurs rapidly in the acidic region. considered to be a thing.

When adding the alkali mentioned above, gypsum hemihydrate, which is a by-product of the wet phosphoric acid production process, has a large variation in adhering acid content and inclusion acid content depending on the operating conditions, so it is necessary to simply slurry it and continuously add an alkali substance. Crystal growth is still insufficient just by using water.

According to the studies conducted by the present inventors, the pH at the initial stage of the water utilization reaction is such that in acidic and strongly alkaline regions, the water utilization reaction rate is too high, resulting in fine crystals, while in the alkaline region, the water utilization rate is too slow, making it economical. Therefore, it is necessary to maintain the soil in a neutral to slightly alkaline range, and for this purpose, neutralization of adhering acids must be completed quickly within the induction period before the water utilization reaction starts. I found that this is essential.

However, when using water in one stage, taking into account the acid content fluctuations mentioned above and the accompanying local pH fluctuations, it is necessary to set the pH in order to completely complete neutralization within the induction period. Since the value needs to be high on the strongly alkaline side,
The hydration rate at the initial stage of the hydration reaction fluctuates irregularly, sometimes becoming very small and sometimes very fast, and only fine crystals could be obtained within an economical time.

In order to solve this problem, the present invention quickly neutralizes the adhering acid content, which has a particularly large fluctuation rate, in a weak alkaline region, and then performs the water utilization reaction and inclusion acid content in the weak alkaline region from neutralization in the coexistence of seed crystals. It takes a two-step reaction process to complete the neutralization reaction between the water and residual alkali, and because the pH setting value in the second stage can be set lower to the neutral side, water can be used at an appropriate rate within an economical time. Even without a special crystallizing agent, coarse platy trihydrate can be obtained in the presence of seed crystals.

That is, the present invention is capable of inducing the initiation of a hydration reaction when adding an alkali component approximately equivalent to the total acid content to gypsum hemihydrate containing attached acid content and inclusion acid content to cause a hydration reaction in warm water. After completing the neutralization reaction of a considerable amount of adhering acid and alkali in the alkaline region within the period, a hydration reaction and a reaction between the included acid and residual alkali occur at around neutral pH in the coexistence of seed crystals. This method is characterized by a two-step reaction process that completes the summation reaction.

In the present invention, a neutralization tank is provided during the induction period until the hydration reaction of gypsum hemihydrate begins, and after the adhering acid content is neutralized with alkali, it is transferred to the original hydration tank. Since the hydration reaction has not yet started, there is no need to consider the influence of the stirring conditions on the crystal shape, so sufficient mixing and stirring may be performed to avoid local non-uniformity in pH.

In addition, the residence time of the slurry in the neutralization tank varies depending on the type of alkali used, but these are determined based on the hemihydrate gypsum water rate curve in the weakly alkaline region, and the water rate is almost zero. It is necessary to set it within the period.

Further, the optimum pH range in this neutralization step varies depending on the type of alkali used, but in the case of a strong base, the range becomes narrower.

The gypsum hemihydrate slurry that has been neutralized in this manner and leaves the neutralization tank undergoes a water utilization reaction and a neutralization reaction for included acids in a neutral region in the hydration tank in the presence of seed crystals.

Such seeding may be carried out by self-circulation of the slurry, as is known in the art.

Normally, the time required for the water utilization reaction is about 2 hours, and even if the produced water gypsum is separated without washing, it is neutral and contains almost no free alkali content or free acid content. It is obtained as a coarse rhombohedral plate crystal with a thickness of about 200 μm, width of 100 μm, and thickness of about 50 μm.

Furthermore, most of the separated P solution can be used again in the neutralization tank.

The alkaline component for pH adjustment used in the present invention may be any alkaline component that neutralizes acid components, but it reacts with attached acids and included acids to form a completely inert, slightly soluble salt. C
a(OH)2 and CaO are preferred, and alkaline components such as NaOH, NH4, and OH are also effective in terms of coarsening.

The present invention has energy advantages and has the advantage that there is no need to use a modifier that requires consideration of the effect on the physical properties when used as calcined gypsum, and has little impact on the physical properties of calcined gypsum. It is of course possible to simultaneously use known crystallizing agents (for example, alkali metal sulfates).

Next, an embodiment of the present invention will be described using an example of using wet phosphoric acid by-product gypsum hemihydrate, using the reaction process diagram shown in FIG.

In this figure, 1 is a neutralization tank, 2 is a first hydration tank, 3 is a third hydration tank, 4 is an aging layer, 5 is a classifier, 6 is a p filter tank, 7 is aqua regia, and 8 is a half-water tank. Water gypsum, 9 is alkaline content (milk of lime), 10 is slurry circulation line, 11 is dihydrate gypsum removal line,
12 indicates a trihydrate separation P liquid supply line.

Raw material gypsum hemihydrate from line 8, aqua regia for slurrying from line 7, alkaline content (milk of lime) for neutralizing acid content in gypsum from line 9, gypsum trihydrate separation P solution described later. are introduced into the neutralization tank 1 from the line 12, and by adjusting the amount of each, the slurry concentration in the tank is 10~10.
The pH is set to 50%, preferably 20 to 30%, and pH 7 to 12, preferably 8 to 11.

Slurry concentrations outside this range are actually possible, but
If it exceeds 50%, the viscosity becomes high and operation becomes extremely difficult.

Furthermore, at pH<7, poorly soluble phosphate is not formed sufficiently, so the water use delay effect does not appear and the rate of water use slows down, resulting in the generation of microcrystals6. Also, at pH>12, poorly soluble phosphate There is a phenomenon in which the salt formation conditions change and the hydration rate suddenly becomes slow, and the amount of alkali added exceeds the total acid content, so it is necessary to add an acid content for pH adjustment, which is not economically appropriate. isn't it.

It is convenient to adjust the pH using a pH meter.

The temperature of the slurry is 30 to 90°C, preferably 5°C by steam heating.
Maintain at 0-70°C.

At temperatures below 30°C, hydration occurs rapidly, making it impossible to convert coarse crystals into trihydrate, which may result in rapid solidification and inoperability.

On the other hand, temperatures above 90°C are not practical because the rate of water use is extremely slow.

The slurry residence time in the neutralization tank is within the induction period of the water utilization reaction of gypsum hemihydrate, and is set to 5 to 20 minutes in the case of gypsum hemihydrate, a by-product of phosphoric acid.

The slurry whose adhering acid content has been neutralized in neutralization tank 1 is transferred to the second hydration tank 2.
line 1 as a seed crystal from the second hydration tank 3, which will be described later.
The slurry is mixed with the slurry that is being circulated from zero.

In the first hydration tank 2 and the second hydration tank 3, the slurry concentration is 15.
~40%, preferably 30-35%, temperature 30-90°
G, L< is adjusted to 50 to 70°C, pH 6 to 8, preferably 6 to 7, and the residence time is about 1 hour each, and the hydration reaction and the reaction between the included acid content and the residual alkali content are performed. complete.

About 10 to 300% of the trihydrate slurry generated in the second hydration tank 3 is sent as seed crystals from the neutralization tank 1 and circulated to the first hydration tank 2, and the rest is sent to the ripening tank 4. The first and second hydration tanks are kept under the same conditions for about 1 hour to ensure uniform crystal formation through ripening.

The trihydrate gypsum slurry that has been matured in the aging tank 4 is transferred to the separator 5.
Coarse and thick trihydrate crystals are obtained from line 11 by filtration.

The separated filtrate is stored in a p-liquid tank 6, and a part or all of it is sent to the neutralization tank 1 through a line 12, where it is used to slurry the raw material gypsum hemihydrate.

As described above, according to the present invention, gypsum hemihydrate, a by-product whose acid content fluctuates rapidly, can be converted into calcined gypsum by simply carrying out a neutralization process within the induction period before the start of the water utilization reaction. This makes it possible to obtain coarse and thick trihydrate gypsum without using an expensive crystal modifier that affects physical properties and poses problems in post-treatment of waste liquid.

Next, examples of the present invention will be shown.

Example 1 Gypsum hemihydrate, a by-product of the wet phosphoric acid production process, was converted to trihydrate according to the process shown in FIG.

First, in the neutralization tank 1, a hemihydrate gypsum cake (adhered moisture 12 %) to 10
Inject at a rate of 8 kg/hour and adjust the pH to 8-11 with lime milk.
After adjusting for about 10 minutes, the circulating slug IJ
The slurry was sent to the first and second hydration tanks 2 and 3 containing -1501/hour, maintained at about 60°C by steam heating, and the slurry concentration was 30%.
The mixture was kept at pH 7 to 7.5 for about 3 hours, and then kept in an aging tank for about 1 hour, and the resulting trihydrate slurry was separated using a centrifuge.

The obtained gypsum trihydrate cake was heated for 107 hours (adhered moisture 9
%) and consisted of coarse rhombohedral plate crystals measuring 200μ in length x 100μ in width x 50μ in thickness (second
Figures, photos).

Of the 250 l/hour of separated filtrate, 701/hour was discharged to the outside of the system.

Comparative Example 1 Same conditions as Example 1 except that gypsum hemihydrate, aqua regia, alkali, etc. were directly introduced into the first hydration tank adjusted to pH 7 to 7.5 without the neutralization tank in Example 1. The crystals obtained with a hydration rate of dog were small, thin rhombohedral plate crystals measuring 50 μm long, 20 μm wide x 5 μm thick, as shown in the photograph in Figure 3.

Comparative Example 2 The pH of the first hydration tank was directly adjusted without the neutralization tank in Example 1.
Almost the same operation as in Example 1 was performed, except that the experiment was conducted with the value adjusted to 8 to 9.

In this case, hydration is often not completed even after staying for about 4 hours (see the photo in Figure 4), and the crystals obtained are heterogeneous and relatively small rhombohedral plate crystals (see the photo in Figure 5). ).

In either case, the rate of water use varied widely and irregularly, making continuous operation virtually impossible.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing an outline of the method of the present invention,
FIG. 2 is a photograph showing the state of trihydrate gypsum crystals obtained in the present invention, and FIGS. 3, 4, and 5 are photographs showing the state of trihydrate gypsum crystals obtained in a comparative example.

Claims (1)

[Claims] 1. When obtaining gypsum trihydrate by adding an alkali component approximately equivalent to the total acid content to gypsum hemihydrate containing attached acid content and included acid content and causing a hydration reaction in warm water, induction of the initiation of the hydration reaction is performed. After completing the neutralization reaction of the adhering acid content and a considerable amount of alkali content in the alkaline area within the period, a hydration reaction and a neutralization reaction of the included acid content and residual alkali content are carried out in the presence of seed crystals in another tank. A method for producing trihydrate crystals characterized by comprising a two-step reaction in which the following steps are completed.