JPH0699149B2 - Method for producing wet powdery calcium carbonate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a light calcium carbonate for neutralization in neutral papermaking, which is a combination of pulp fiber and AKD or ASA sizing agent, cationic starch, polyacrylic amide, etc. The present invention relates to a manufacturing method.

[Conventional background and problems]

In recent years, due to problems such as storability of paper and closed drainage, the papermaking method for paper is shifting from acidic papermaking to neutral papermaking.

In neutral papermaking, it is possible to use calcium carbonate, which is abundantly produced in Japan, as a filler. At first, inexpensive heavy calcium carbonate was paid attention to and used. However, this heavy calcium carbonate has a high degree of wire abrasion in the papermaking wire and cannot sufficiently satisfy the required quality (whiteness, opacity, smoothness, ink receptivity, etc.) of the paper. The use of light calcium carbonate, which has drawbacks and is therefore excellent in these properties, has come to be considered in part.

Light calcium carbonate has been manufactured in Japan for a long time,
Spindle-shaped ones are generally used for the filling of neutral paper, and they give excellent paper quality due to their properties such as uniform particle size distribution, high whiteness, bulkiness and high oil absorption.

However, since this light calcium carbonate is a synthetic product, its price is higher than that of talc which has been conventionally used in acidic papermaking or the above-mentioned heavy calcium carbonate, and introduction to neutral papermaking is a stepping stone. there were.

In view of the current situation as described above, the present inventors have earnestly studied to produce low-priced light calcium carbonate for neutral papermaking, and have completed the present invention.

[Problems to be solved by the invention]

Conventionally, light calcium carbonate is supplied to a paper mill in one of the following three forms.

A) Slurry product having a concentration of 15 to 30% ... The compounded slurry is transported as it is or after being concentrated by a truck.

B) 50-65% high-concentration slurry product: The compound slurry is mechanically dehydrated, then made into a high-concentration slurry using a dispersant, and transported by a tank truck or liquid container bag.

C) Dry powder: The compound slurry is mechanically dehydrated, dried and then crushed, then placed in a paper bag or container bag for supply.

However, it is impossible to maintain the price comparable to that of conventional talc, because the above-mentioned product form and manufacturing process cause various cost increases and adverse effects.

That is, in (a), since the slurry has a low concentration, the transportation cost is greatly increased, and the transportation range is limited to an extremely short distance. For this reason, there is no choice but to construct a facility for producing light calcium carbonate in the vicinity of a paper manufacturing company, and it is impossible to supply it to many paper manufacturing companies.

In (a), the cost is increased due to the use of an expensive dispersant, and the use of a dispersant which is a strong anion increases the basic unit of chemicals in the papermaking process, which adversely affects the paper quality. Further, transportation of high-concentration slurry requires the use of tank trucks or liquid containers, which increases transportation costs.

In c), the cost will be drastically increased due to the equipment, heavy oil, labor costs, etc. required for the drying process.

The present inventors previously solved the cost increase due to the causes as described above, and for the purpose of providing an inexpensive light calcium carbonate, after centrifugal dehydration of the compound slurry, dehydrated cake-like without drying or forming a high-concentration slurry. We considered using it as it was. However, since the light calcium carbonate slurry by the conventional compounding method has a small particle size, the dewatering property by the centrifuge is not good, and since the cake has a high content of water and the cake is lumpy, if it is filled in a paper bag or a container bag and left to stand. There is a problem in that the solidification causes a failure of extraction or a failure of reslurrying in water for the user.

[Means for solving problems]

After that, through intensive studies, it was confirmed that when light calcium carbonate is produced under specific compounding conditions, it is possible to obtain wet calcium carbonate having a low water content and good centrifugal dewaterability without impairing the quality during paper filling. I found it.

That is, in the compounding step, [first step] when carbon dioxide gas is passed through the lime milk, the carbon dioxide gas concentration is 25% by volume or more, the gas flow rate is 20 to 100 Nl / min · kg calcium hydroxide, and the reaction tower outlet When the gas gauge pressure is 1.0 kg / cm ² or more, p
Pass gas until H is 8 or less. Liquid temperature of lime milk is 30
The temperature is not particularly limited as long as it is at least ° C, and the lime milk concentration may be at least 5% by weight, but in order to improve the dehydration efficiency, it is preferably as high as possible. If the carbon dioxide flow rate is 20 Nl / min · kg calcium hydroxide or less, or the carbon dioxide concentration is 25% by volume or less or the gas gauge pressure at the reaction tower outlet is 1.0 kg / cm ² or less, spindle-shaped calcium carbonate with good dispersibility is obtained. It cannot be obtained and becomes non-uniform and has a large amount of aggregation, which makes it difficult to obtain a uniform secondary agglomerate in the second step. Further, when the carbon dioxide gas flow rate is 100 Nl / min · kg or more of calcium hydroxide, the gas absorption efficiency is extremely poor, and the economy is poor. By the first step, it was found that it is possible to obtain a spindle-shaped light calcium carbonate having a primary particle diameter of about 1 μm and almost no aggregation.

[Second Step] Lime milk is added to the combined slurry obtained in the first step in a molar ratio of 0.
It is added in the range of 2 to 2 mol, and then gas is passed until the carbon dioxide gas flow rate is 40 Nl / min.kg calcium hydroxide or less and the pH is 8 or less. When the added mol of lime milk is 0.2 mol or less, the amount of lime milk is small to form a uniform secondary aggregate, and the partially dispersed calcium carbonate remains. When the added molar ratio of lime milk is 2 mol or more, the added amount is too large, so that part of the added lime milk does not work as a coagulation effect and free dispersed calcium carbonate is generated. Further, when the carbon dioxide gas flow rate is 40 Nl / min · kg or more of slaked lime, new nuclei are generated by the added lime milk, resulting in a very nonuniform agglomerated particle size. Lime milk to be used may be 5% by weight or more, but it is preferable that the concentration is as high as possible. By the second step, it is possible to obtain light calcium carbonate having a uniform aggregated particle size of 2 μm to 5 μm. The size of the aggregated particle size depends on the added lime milk molar ratio, and the larger the added molar ratio, the larger the particle size.

The slurry obtained in the second step can be sufficiently made into a wet powdery product having a water content of 35% or less, which is the object of the present invention, by performing centrifugal dehydration as it is. However, when the particle size is further increased to reduce the dehydration property and the water content, lime milk is added again to the calcium carbonate slurry obtained in the second step, and carbon dioxide gas is passed in the same manner to obtain an aggregate particle size of 3 μ to 10 μm.
It is possible to produce light calcium carbonate having a uniform particle size of μ. The second step may be performed twice or more in order to obtain agglomerated light calcium carbonate having a desired agglomerated particle size.

[Third step] The spindle-shaped aggregated light calcium carbonate slurry obtained by the first step and the second step described above is subjected to centrifugal dehydration at a speed of 3000 m / min or more perimeter to give an extremely low water content ( Wet base 35% by weight or less) Wet powdery calcium carbonate can be obtained.

〔The invention's effect〕

(Cost effect) The light calcium carbonate obtained by the present invention can be supplied to a papermaking user after centrifugal dehydration without being dried or slurried, and can be supplied to a papermaking user. It enables a huge cost reduction.

(Other effects) The dewatered light calcium carbonate from the centrifuge prepared by the method according to the present invention has an extremely low water content and is in the form of a powdery powder, and is completely solidified even when filled in a container bag and stored for a long time. It maintains its fluidity and is easily discharged from the container bag. Further, when reslurrying is performed on the paper making side of the user, the dispersion in water is better than dry powder, and there is little scattering at the time of addition, which is extremely easy to handle. Regarding the paper quality during papermaking, it has a uniform three-dimensional agglomerate structure, so it has excellent whiteness and opacity, and is superior in paper strength to conventional light calcium carbonate. Is.

The invention is further described by the following examples.

Example 1 [First Step] 10 m ³ of lime milk having a concentration of 10% by weight was filled in a reaction tower, and a gas having a carbon dioxide gas concentration of 30 to 35% discharged from a lime furnace was filled into the reactor every minute.
Carbonation reaction was carried out by passing 40 Nm ^{3 at} a gas gauge pressure of 1.3 kg / cm ² at the outlet of the reaction tower. 32 minutes after the start of compounding, the slurry pH was 7.8.
I stopped compounding next door. The particle size distribution of the obtained calcium carbonate was measured by the sedimentation light transmission method, and the average particle size was
It was 1.2μ. Observation with an electron microscope revealed that the calcium carbonate had a uniform spindle shape and almost no aggregation.

Centrifuge dehydration of this slurry at a peripheral speed of 3500 m / min for 10 minutes,
The cake formed on the cloth was scraped and discharged by an automatic scraper. The obtained cake had a water content of 38% by weight (wet base), and did not appear to be powdery, but was lumpy.

This product was filled in a container bag, and a constant load was applied from above to leave it for a long period of time, but it was solidified during standing and could not be discharged from the container bag.

[Second Step] 3 m ³ of lime milk with a concentration of 10% by weight was added to 7 m ^{3 of the} calcium carbonate slurry obtained in the first step to obtain a carbon dioxide gas concentration of 30
Carbonation was carried out by passing ~ 35% of gas at 9 Nm ³ per minute, and after 42 minutes, the pH reached 7.0 and the compounding was stopped. The average particle size of the obtained calcium carbonate was 2.5μ. Observation with an electron microscope revealed uniform three-dimensional aggregated particles.

The obtained slurry was subjected to centrifugal dehydration at a peripheral speed of 3500 m / min for 10 minutes, and the cake was scraped and discharged. The cake thus obtained had a water content of 32% by weight (wet base) and was powdery and fluid. This product was filled in a container bug and left for a long time under a constant load, but it did not solidify and maintained its original fluidity, and could be easily discharged from the container bag.

Example 2 To 7 m ^{3 of} the slurry obtained by the same method as in Example 1 and the first step, 3 m ³ of lime milk having a concentration of 10 wt% was added, and a gas having a carbon dioxide gas concentration of 30 to 35% was added every minute. Carbonation was carried out with 9 Nm ³ and, after 39 minutes, the compounding was stopped at pH 7.2. The obtained calcium carbonate had an average particle diameter of 4.1 μm, was observed by an electron microscope to be uniform three-dimensional aggregated particles, and neither coarse particles nor primary dispersed particles were observed.

The resulting slurry was centrifugally dehydrated at a peripheral speed of 3300 m / min for 10 minutes, and after dehydration, scraped and discharged. Moisture content of the resulting cake
It was 27% by weight (wet base) and was powdery and fluid. The long-term storability in the container bag was good and no solidification was observed.

(Comparative Example 1) Under the same conditions as in the first step of Example 1, only the gas flow rate was changed per minute.
Carbonation was carried out at 15 Nm ³ and, after 85 minutes, the compounding was stopped at pH 7.9. The average particle diameter of the obtained calcium carbonate was 1.9 μm, and by observation with an electron microscope, many aggregated particles were observed in addition to the primary dispersed particles.

Comparative Example 2 An additive compound was added to the calcium carbonate slurry obtained in Comparative Example 1 in the same manner as in the second step of Example 1. The obtained calcium carbonate had an average particle size of 3.0 μm, and by observation with an electron microscope, extremely uneven secondary agglomerated particles having a particle size of 2 μm to 6 μm were observed.

(Comparative Example 3) In the same operation as in the second step of Example 1, carbonation was carried out at a gas flow rate of 15 Nm ^{3 /} min, and the compounding was stopped after 23 minutes at pH = 7.1. The obtained calcium carbonate had an average particle diameter of 2.3 μm, and a large number of primary dispersed particles were observed by observation with an electron microscope.

Claims (4)

[Claims] 1. A carbon dioxide concentration of 25% by volume or more and a carbon dioxide flow rate.
20-100 Nl / min · kg calcium hydroxide, carbon dioxide is introduced into lime milk at a gauge pressure of the gas at the outlet of the reaction tower of 1.0 kg / cm ² or more to produce spindle-shaped light calcium carbonate slurry No. 1
In the step and the calcium carbonate slurry obtained in the first step, lime milk is added in an amount of 0.2 to 2 in a molar ratio, and carbon dioxide gas having a carbon dioxide concentration of 25% by volume or more is supplied at a gas flow rate of 40 Nl / min · kg calcium hydroxide or less. And a second step for producing an agglomerated light calcium carbonate slurry having a uniform particle size,
A method for producing wet powdery calcium carbonate, comprising a third step of performing centrifugal dehydration of the obtained agglomerated light calcium carbonate slurry after performing the step a desired number of times to remove water in the cake. 2. The wet powdery calcium carbonate according to claim 1, wherein the spindle-shaped calcium carbonate obtained in the first step has a spindle shape with an average particle size of 2 μm or less and good dispersibility. Production method. 3. The method for producing a wet powdery calcium carbonate according to claim 1, wherein the agglomerated light calcium carbonate obtained in the second step has an agglomerated particle size in the range of 2 to 5 μm. 4. The wet powdery calcium carbonate according to claim 1, wherein in the third step, centrifugal dehydration is performed at a peripheral speed of 3000 m / min or more to reduce the water content in the cake to 35% or less. Manufacturing method.