JP4051857B2 - Method for producing finely powdered calcium carbonate - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing finely powdered calcium carbonate.
[0002]
[Prior art]
Fine powder calcium carbonate having a particle size in the range of submicron to several tens of μm is useful as a filler for paper, plastic, rubber and the like, and is also used as a material for ink and paint. It is also used as a desulfurization / dechlorination agent.
[0003]
This type of finely powdered calcium carbonate is conventionally produced by a liquid phase precipitation method. In the liquid phase precipitation method, first, limestone CaCO ₃ is calcined to obtain quick lime CaO, this quick lime is digested to obtain slaked lime Ca (OH) _2, and CO ₂ is blown into the dilute solution to make CaCO ₃ again. It is. Although this method has already been established technically, it requires a multi-step process and a large-scale manufacturing facility, and there are various difficulties in controlling the product granularity. For example, when trying to produce a very fine powder, there is also a problem that the precipitation operation takes a long time. Therefore, it is desirable to be able to produce finely powdered calcium carbonate in a manner that is easy to control with few steps and simple equipment.
[0004]
One of the inventors, together with a collaborator, invented a method for producing fine powdered slaked lime using a fluidized bed and has already disclosed it (Japanese Patent Laid-Open No. 9-268012). In the method, an inert substance such as silica sand having a particle size range of 100 to 1500 μm and a narrow particle size distribution is fluidized with a heated gas to form a fluidized bed having a constant temperature of 30 to 300 ° C. The slurry consists of supplying a slurry in which slaked lime powder having a particle size of 50 μm or less is dispersed in water, and finely powdered slaked lime is obtained by drying the slurry.
[0005]
Through subsequent research, the inventors have found that when slaked lime slurry using this fluidized bed is dried, when a gas containing CO ₂ is used for fluidized bed formation, the slaked lime reacts quickly with CO ₂ in the gas. Knew that it would be a fine powder of calcium carbonate.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide a new and simple method for producing finely powdered calcium carbonate using a fluidized bed by utilizing the new knowledge obtained by the inventors.
[0007]
[Means for Solving the Problems]
In the method for producing finely powdered calcium carbonate of the present invention, a heated gas containing CO ₂ is supplied, and fluid particles are formed by fluidizing inert medium particles by the gas flow. A slurry in which calcium oxide is dispersed in water is supplied, and water in the water slurry is evaporated and dried on the surface of the medium particles. At the same time, calcium hydroxide and CO ₂ in the gas are reacted to form calcium carbonate. the dry powder of calcium carbonate that has left from the surface to carry away put the flow of gas, Ri Do from be recovered by solid-gas separating means, the difference between the temperature of the fluidized bed, the saturation temperature of the gas leaving the fluidized bed It is the manufacturing method implemented by selecting reaction conditions so that the "saturation approach degree" defined as may become the range of 10-83 degreeC .
[0008]
DETAILED DESCRIPTION OF THE INVENTION
As the medium particles of the inert substance that forms the fluidized bed, those that are selected from silica sand, river sand, alumina particles, zirconia particles, glass beads, and limestone particles and that are less likely to wear due to collision during flow are preferable. The fluidized bed is advantageously a jet type fluidized bed that does not use a gas dispersion plate, but of course, a normal fluidized bed having a gas dispersion plate may be used.
[0009]
The fluidized gas temperature and supply rate, water slurry temperature and supply rate are adjusted so that the temperature in the fluidized bed is appropriate for the dew point or saturation temperature of the gas flowing out of the fluidized bed. It is recommended to do. This difference between the temperature in the fluidized bed and the gas saturation temperature is referred to as “approach to saturation temperature”.
[0010]
In the method of the present invention, when a slurry containing calcium hydroxide is supplied to the fluidized bed, the slurry adheres to the surface of the vigorously flowing medium particles and is uniformly dispersed in the layer. For this reason, the drying of the slurry and the reaction with CO _{2 in} the gas occur simultaneously on the surface of the medium particles having a large area as a whole, and calcium carbonate is generated in a short time.
[0011]
Under such a mechanism, what is the difference between the temperature in the fluidized bed and the gas dew point, that is, the degree of saturation approach is important. When the degree of saturation approach is small, the drying rate of the slurry in the fluidized bed becomes slow, so that the above-mentioned fine powdery CaCO ₃ is likely to be produced, and the residence time in the layer of fine particle calcium hydroxide is also long. Therefore, the production of calcium carbonate is realized with high efficiency. However, if the temperature of the fluidized bed is too low, the evaporation of moisture becomes insufficient, and the slurry adhering to the surface of the media particles acts like a binder to fix the media particles, making it difficult to maintain the fluidized bed. Become. From this balance, the saturation approach degree naturally has a preferable range.
[0012]
The produced calcium carbonate rides on the gas flow, leaves the fluidized bed, and is separated and recovered by a solid / gas separation device such as a bag filter or a cyclone + bag filter.
[0013]
Factors that determine the conversion rate of supplied slaked lime into calcium carbonate include the above-mentioned saturation approach, the particle size of calcium hydroxide in the slurry, the reaction temperature, that is, the temperature in the fluidized bed, the concentration of carbon dioxide in the gas And the average residence time of the gas in the fluidized bed.
[0014]
【Example】
An experimental apparatus having the configuration shown in FIG. 1 was assembled. The main part of the apparatus is a reactor (9) in which a fluidized bed of medium particles is formed, and this reactor is a PPSB (powder particle spouted fluidized bed reactor) type. The inner diameter of the reactor is 104 mm, the height is 1 m, the gas inlet has an inner diameter of 28 mm, and the cone angle is 45 degrees.
[0015]
A gas mixture for forming a fluidized bed was prepared by mixing CO ₂ from a carbon dioxide gas cylinder (7) with air and heated to a predetermined temperature by a preheater (8). The air is pressurized by the compressor (1), passes through the oil filter (2) and the valve (3), is then dried by the silica gel column (5) and enters the reactor. The flow rate of air was measured with an orifice (4), and the flow rate of CO ₂ was measured with a rotameter (6).
[0016]
The slaked lime water slurry was stored in the slurry tank (11). The reactor was fed through a tube pump (12). The temperature was controlled by the PID controller (13). Glass particles having a diameter of 850 μm were used as the inert substance particles forming the fluidized bed.
[0017]
A quicklime powder having a diameter of 5 μm was used as a raw material. The product calcium carbonate from the reactor was collected with a bag filter (10). Make the following items constant as the operating conditions of the reactor,
Water to quicklime ratio 24.6
Gas superficial velocity 75cm / s
Inlet temperature 130 ° C
Static layer height 88mm
Conversion rate from Ca (OH) ₂ to CaCO ₃ by selecting the CO ₂ concentration in the gas as 2.7% or 5.4% by volume and changing the “saturation proximity” in the range of 10 to 83 Was measured.
[0018]
The result is as shown in the graph of FIG. 2, and the conversion rate increases as the concentration in the gas increases and the degree of saturation approach decreases. The particle size of the obtained calcium carbonate was in the range of 1.23-1.88 μm, and the specific surface area was in the range of 15-25 m ² / g. Conversion is 87%. This finely powdered calcium carbonate had characteristics required for an absorbent for desulfurization / dechlorination.
[0019]
【The invention's effect】
According to the present invention, calcium carbonate powder having a very fine particle size can be produced using a fluidized bed reactor under operating conditions that are easy to control. The conversion rate from Ca (OH) ₂ to CaCO ₃ can be improved by selecting the conditions, for example, by increasing the CO ₂ concentration. According to this method, a fine powder calcium carbonate can be easily produced with few steps without requiring a large-scale apparatus unlike the liquid phase precipitation method.
[Brief description of the drawings]
FIG. 1 is a conceptual longitudinal sectional view showing a configuration of an experimental apparatus in which a manufacturing method of the present invention is implemented.
FIG. 2 is a graph showing the relationship between the “saturation approach degree” and the conversion rate, which is data of an experiment performed using the apparatus of FIG. 1;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Compressor 2 Oil filter 3 Valve 4 Orifice 5 Silica gel tower 6 Rotor meter 7 Carbon dioxide gas cylinder 8 Preheater 9 Powder particle spouted fluidized bed reactor 10 Bag filter 11 Slurry tank 12 Tube pump 13 PID controller

Claims (3)

A method for producing finely powdered calcium carbonate , comprising supplying a heated gas containing CO ₂ , fluidizing inert medium particles by a gas flow to form a fluidized bed, and A slurry in which calcium is dispersed in water is supplied, and moisture in the water slurry is evaporated and dried on the surface of the medium particles. At the same time, calcium hydroxide reacts with CO ₂ in the gas to form calcium carbonate. the dry powder leaving the calcium carbonate from the carry away put the flow of gas, Ri Do from be recovered by solid-gas separation means, and the temperature of the fluidized layer, as the difference between the saturation temperature of the gas leaving the fluidized bed A production method which is carried out by selecting reaction conditions so that the defined “saturation approach” is in the range of 10 to 83 ° C. The production method according to claim 1, wherein the medium particles of the inert substance forming the fluidized bed are selected from silica sand, river sand, alumina particles, zirconia particles, glass beads and limestone particles. The manufacturing method of Claim 1 which employ | adopted the jet type fluidized bed which does not use a gas dispersion plate as a fluidized bed.

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