FI103965B - Precipitated calcium carbonate multicore particle cluster and process for its preparation - Google Patents

Description

103965

The present invention relates to a precipitated calcium carbonate product according to the preamble of claim 1. The present invention relates to a precipitated calcium carbonate product.

The invention also relates to a process according to the preamble of claim 4 for the preparation of such a calcium carbonate product.

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Precipitated calcium carbonate has recently gained great interest, especially since -90, as a filler and coating material for paper and plastics. A large number of patents have been published in this field because precipitated calcium carbonate (PCC) has proven to be a versatile paper filler and coating pigment. PCC provides good opacity and lightness, and protects acidic papers from brittleness and yellowing due to aging.

The main forms of PCC are calcite, aragonite and vaterite, the former of which is closest to, like this cube, vaterite is amorphous, spherical and arangonite oblong, '. : 20 even needle-shaped, with basic crystal shape. Intermediate crystals of crystals 1 are also known as rosettes and / or scalenohedric crystals. These are mainly shaped like a ::: rose flower.

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Particularly fine-grained PCC is suitable for paper filling and coating. The preparation of this • 25 is described in, e.g. WO Application Publication 96/23728, which discloses how causticizing reaction can produce very small, mostly spherical PCC particles, 9 in the range 0.2 to 0.4 micrometers. The solution described in U.S. Patent No. 4,367,207 solved * * · - '· ·. A fine PCC material is prepared by maintaining the temperature of the reaction medium 9 below 18 ° C.

The prior art has also sought to provide finely divided crystals and / or their flocs by the use of auxiliary chemicals. By flocculating the crystals, the retention of PCC can be improved in the manufacture of 30 2 103965 paper. Thus, U.S. Patent No. 5,332,564 teaches how to make fine so-called pigment by adding a small amount of sugar to the lime water. The use of sugar has been known to inhibit crystallization, mainly in the manufacture of concrete.

One of the major problems of PCC in the manufacture of paper is its reactivity and degradation under acidic conditions, such as in the circulating water of a papermaking machine, because stronger acids than carbonic acid are always present. To overcome this problem, the PCC particles have been treated by reacting them to a degree with e.g. phosphoric acid 10 and / or known derivatives thereof. Such inventions are described, for example, in U.S. Patent Nos. 4,219,590 and 4,927,618. In addition, fine particles generally have the problem that they are difficult to filter because they are easily clogged by the pores of the filters.

As mentioned above, very small PCC particles of almost the same size can be produced by the method described in WO 96/23728. It has been surprisingly found in the present invention that these can be joined together into regular multicore clusters, which greatly facilitates their separation from reaction suspensions. The clusters of the invention are spherical and contain a plurality of interconnected spherical calcium carbonate particles having a particle size of about 40 to 400 nm. PCC aggregates consisting of smaller · *,. *,: Particles are known in the art. Thus, U.S. Patent Application No. 944355 describes PCC agglomerates * * * V · 'in the size range of about 1.5 to 3.5 µm. These agglomerates, in turn, consist of prismatic crystals of 1 to 1.8 µm. The particles of the invention are of various shapes and are considerably smaller in size.

The particles of the precipitated calcium carbonate according to the invention are formed into multi-nucleated clusters by setting the Z potential of the crystals to between -1 ... 20 mV. This is produced in a chamber filter by adjusting the pH of the crystal slurry to 6.5 ... 9.5, most preferably to 7.5 ... 9.3. Thereafter, the crystal slurry is preferably mixed in the impact mixer such that the perimeter velocity differences on the slurry are in the order of 50 to 200 m / s. The use of a high power mixer in the preparation of PCC is generally known from U.S. Patent No. 3,103,965, 5,342,600, but prior art does not suggest setting the Z potential to the above level and does not address the difference in peripheral speed of the alloy. magnitude.

More specifically, the calcium carbonate products of the invention are characterized in what is set forth in the characterizing part of claim 1.

The method according to the invention, in turn, is characterized by what is stated in the characterizing part of claim 4.

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The invention provides considerable advantages. Thus, as already mentioned above, separation of calcium carbonate crystals in the form of a cluster or jam from the suspension by filtration is easier and cheaper than conventional fine crystals. In addition, the clusters of the invention have abundant internal reflective surfaces which improve the opacity of the pigment and filler. In this context, it can be mentioned that paper coating has previously been proposed for use in e.g. small polystyrene foam beads because of their high micron content inside the particle. '. reflective surfaces. The same phenomenon is based on e.g. use of calcined kaolin and structured pigments. By these structured pigments are meant pigments, I I I <:, I 20, which have been implanted on the surface of the same or other material in different particle sizes. Thus, ,,; | ' the diffused reflection of the light is increased, whereby the probability that the light beam collides with the reflected surface increases. The present invention provides a calcium carbonate end having a plurality of internal reflecting surfaces, as mentioned above, obtained substantially without the use of chemicals. The clusters remain together under the influence of van der Waals forces. Because chemicals are not used, the product can flexibly reshape, for example, in the context of supercalendering, and is ideal for paper coating. · * For support, coating and filling.

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The invention will now be examined in greater detail in the accompanying drawings and in the drawings. 30 detailed explanation.

Figures 1a to la show the basic structure of a lattice structure consisting of 18 PCC particles, wherein in Figure 1a the particles are connected by separate fluid bridges, in Figure Ib 4 the 103965 bridge network and in Figure le there is a fluid-filled capillary space between the particles. Figure 2 shows an electron microscope image of a grid of PCC particles.

The theory underlying the invention will now be described briefly with reference to the preparation of precipitated calcium carbonate:

One object of the invention is to provide, for example, a calcium carbonate-containing solid mixture (dispersion) in which the particles remain separated at still high solids concentrations of up to 70% and high viscosity values (about 200 cP). Particle traction 10 and thrust control contribute to this goal as well as the removal of capillary force between the particles. The attraction between the particles is represented by e.g. van der Waals force which increases as the distance between the particles decreases and the diameter of the particles decreases. Thus, at a particle diameter of 0.1 µm, the van der Waals force is about 1,000,000 N / mm 2. As the particle distance increases, the van der 15 Waals force decreases rapidly and is only about 1,000 N / mm 2 at 100 µm diameter. The particle thrust is represented by the Z or zeta potential, which is the electrical voltage difference between the ionic field of the particle and the medium. Fields with the same sign of the particles produce a removal force. Capillary force occurs when air is blown between particles.

When no particle surface modifying excipient is used, the pH dependence of the particle's Z potential can be readily and reproducibly determined. Particularly repeatable is · · · \ · · 'when caC is precipitated from almost pure soda and almost pure calcined or slaked lime.

It has been found that the Z-potential of PCC with calcium carbonate varies as a function of pH between -25 and -1 mV. At pH 8.2 ... 8.4, the Z potential is at its lowest (about -1 ... -5 mV) and thus the particle's distancing force is also at its lowest.

According to the invention, the precipitated calcium carbonate particles are brought into the action range of a van der Waals force by producing particles having a diameter of <0.1 µm and adjusting the pH of the dispersion containing PCC particles to 6.5 ... 9.5 5 103965 to minimize zeta potential.

The resulting PCC cluster is shaped by strong turbulence.

5 Kolmogorov vortex-scale theory, the length that is the effective vortex diameter in a turbulent milieu or equivalent is: L ~ (n / exp3E) exp (1/4), 10 where n = kinematic viscosity L = tubulence vortex diameter

The previous equation states that L is the distance considered as the diameter of the turbulence vortex, which smaller pieces or particles cannot be broken or modified by the shear force generated by the turbulence. L is inversely proportional to 1/4 power of local mixing power (E) kW / m3. Similarly, the diameter of said multi-core groups to be formed is proportional to both this and the function of pH:

Dmy - F [F (pH), L]

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, 20., 'Where Dmy is the total diameter of the multicore cluster.

Based on the foregoing, the invention proceeds as follows: The mixture of Ca (OH) 2 and the Na 2 CO 3 solution are mixed homogeneously. This creates a Ca2 + ion cloud around the Ca · OH · particle closest to the diffusion zone. In the solution, Na2CO3 is Na + and CO332 'ions. Ca2 + combines with C032 'to form CaCO3, which binds 8 µg H2O to form a gel-like state. The CaCO3 molecules combine to form crystal embryos and. '·: The water is released and the mixture begins to sag. If the temperature at this stage is less than 42 ° C, the crystal · · * · · '30 is calcite, while if the temperature is above 42 ° C, the crystal element is aragonite. Small, nanocrystalline CaCO 3 mass balls are formed in the cloud surrounding Ca (OH) 2 and continue to grow and rearrange until Ca (OH) 2 is mainly used. 6,103,965 spherical particles consisting of a CaCO3 nanocrystalline disorganized mass are obtained.

The density of the arangonite crystal is 2.71 and that of the calcite crystal is 2.93. The CaCO3 nanocrystalline mass contains crystalline embryos in a non-rigid state and must have a density lower than that of the resulting organized crystal, i.e., at a temperature greater than 42 ° C, a density less than 2.71 and a density less than 2.93 .

The suspension of calcium carbonate crystals resulting from the preparation of PCC is adjusted to a pH of 6.5 to 9.5, preferably 7.5 to 9.3, particularly preferably about 7.9 to 9.2, The potential of Z-10 is reduced to -1 ... -20 mV, preferably about -1 ... -5 mV, particularly preferably -1 ... -3. The same object can be achieved by adding, for example, suitable additives, dispersing aids, such as poly electrolytes, to the suspension. Examples of these are polyacrylates, polyacrylamides and lignosulfonates. These excipients change the pH range at which the crystals have a minimum Z potential of 15 at -1 ... -20 mV. Therefore, it is advisable to adjust the pH value only after the addition of the dispersion excipients. Typically, a preferred pH is between 6 and 10 if excipients are used.

The pH adjustment can be carried out directly in connection with the preparation of PCC, e.g. by removing PCC slurry from the reactor, but preferably by filtration.

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: · The resulting multicore spheres are separated by strong turbulence before * · * •. * · As they reach a "critical size" where they form a uniform crystal. Loose spheres form a homogeneous mixture in which all spheres have a mass-to-surface ratio • · · * * * 25, whereby their interaction ceases and the spheres retain their size and shape .

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Thus, after adjusting the pH (or additive additive), the suspension is agitated with a 1 L blender so that it has a peripheral velocity of 50-200 m / s. With this · · · • · *; In this way, any larger particles which may have formed in the suspension are disintegrated:. '·: Agglomerates into smaller multicore PCC clusters that can be used in English' ·. · 30 uses the term Multiple Heart PCC, abbreviated as MHPCC.

Figure 2 shows an electron microscope image of these clusters. As shown in the figure 7103965, the clusters are obtuse and contain van der Waals connected PCC particles (typically about 5 to 8000 pieces) having a particle size of 40 to 400 nm, preferably about 40 to 200 nm, most preferably ca. 80-120 nm. The size of the clusters is about 0.1 to 10 µm, typically about 0.2 to 9 µm. The cluster-forming particles are substantially spherical and of uniform size, preferably with a deviation of less than 10%.

Also, the clusters as such are spherical or substantially spherical. They are elastic because they are held together by the forces of van der Waals and not by additives, so that they can easily be shaped into the desired shape upon paper finishing at the top or surface of the paper. In addition, the clusters include an open channel network, the configuration of which is illustrated in Figure 1.

The structures (particles and clusters, respectively) visible in the electron microscope image are referred to as "spherical" because it is from regular geometric, three-dimensional shapes that approximates the configuration of the structure.

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According to a preferred embodiment of the invention, the interrelated particles in the chamber filter are washed with washing water containing a suitable acid: (e.g. carbon dioxide is dissolved in the washing water), whereby the pH of the filter cake is preferably reduced to 8.4 ± 0.5 , with the Z potential dropping to -1 ... -2 mV and the particles 20 adhering to each other by van der Waals forces. The carbon dioxide reduces the pH value of the filter cake and at the same time lowering the pressure in the filtrate obtained by filtering the gas developed "··· 'in the cake to release the gas displaces the liquid between the particles. Event" * "shown in Figure 1, there is shown a kyllästymistilat randomly compressed agglomerates acetate The figure on the right (Fig. 1c) shows the situation before washing, when the liquid t

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25 fills the interparticle capillary spaces. By washing with carbon dioxide-containing washing liquid and «· ·« liberating carbon dioxide, the liquid is removed between the particles (Fig. Ib),; whereupon separate liquid bridges are left and a porous structure is obtained (Fig. 1a).

• · * «· *; # ['A continuous mat of particles is formed. After filtration, the grapes are processed in a shuffle with strong turbulence. As a function of the intensity of the turbulence, the grapes become spherical and of a size similar to the intensity of the turbulence. Thus, when the contents of the chamber filter are subsequently emptied and subjected to intense gas turbulence, regular clusters according to the invention are obtained.

When producing the above clusters or grapes in a chamber filter, the amount of solids requires that the particles be less than 0.1 µm apart. It has been found in connection with the invention 5 that the disclosed jam-like structures are preferably achieved by adjusting the solids content of the PCC slurry to substantially more than 40%, preferably from about 50 to 60%, and passing, as mentioned above, washing water dissolved in carbon dioxide for adjusting the pH to 7.5 to 9.5, preferably from about 7.9 to 9.2. The particles will then adhere to each other.

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In the context of the invention, it has been found that the size of the MHPCC particles to be formed can be effectively controlled by the ratio of shear rate to Z potential such that lowering the Z potential produces the same particle size with a smaller peripheral velocity difference or .

The invention uses a device for disintegrating agglomerates in which the rotating peripheries have a plate-like collision surface which, under the effect of the force produced (after the collision), flows the liquid and suspension from the wings of the inner agitator. 20 concentric outer periphery to the mixing blades, whereby the mixture is exposed as it moves; greater forces from one wing to another as a result of differences in speed and / or direction of rotation of the successive vanes. The apparatus is used herein to be supplied with liquid and • · »'* slurry at all times with a lower volume flow capacity than is removed. In such a ... device, the plates are either radial or slightly inclined.

• · ·, ·; ·. 25 The ability of a percussion mixer to perform uniform mixing and shearing I I »« as a result of shocks between a solid and a fixed impact surface is likely to be incomparably more uniform in terms of reasons than other types of mixer. Typically, more than 95% of the material flow receives nearly 100% of the maximum impact energy and only 5% to 10% of the material receives less than 60% of the maximum impact energy. When i · “'30 this dispersion corresponding to the turbulence intensity of the mixing is many times greater in all, even Dissolver-type high-shear mixers. Due to the above reasons, the re-dispersion of the formed aggregates and flocs in the .

Since the suspension from the preparation of PCC is quite alkaline, the pH is practically neutralized by the suspension. Typically, hap-5 sons are used for neutralization. These include various inorganic acids, for example mineral acids such as sulfuric, nitric and hydrochloric acids. Phosphoric acid has been found to be preferred. Organic acids such as formic, acetic and propionic acids or sulfonic acids may also be used.

However, as stated above, the small discrete PCC particles formed in accordance with the preferred embodiment of the invention are neutralized with carbon dioxide. Particularly preferably, this is done in a chamber filter, in which the carbon dioxide gas is introduced into the wash water just before the filter. Mineral acid (e.g., phosphoric acid) can also be added to the filter.

It has been found that surface treatment of the grapes with phosphoric acid produces a calcium phosphate layer on their surface. The turbulence of the agitator causes the calcium phosphate molecules to settle at the surface.

The invention makes it possible to prepare PCC clusters from precipitated calcium carbonate produced, for example, by causticization reaction. As reagents, calcium hydroxide and sodium carbonate or sodium hydroxide and carbon dioxide gas may be used. For the preparation of PCC, we refer to the process described in WO Application Publication 96/23728.

The following examples are intended to illustrate the invention:

Example 1 * • · 9 • · · · · 10 103965

The distance between the opposing rotary mixing rings was 0.005 m. The residence time in the device was only <0.1 seconds between the rings. The temperature was room temperature, i.e., about 20 ° C.

The viscosity of the MHPCC particles (as measured by a Brookfield viscometer) dropped from the initial (PCC 0.1 micron) value at 1000 cP to 200 cP at 50% dry matter content in each case. The MHPCC particles produced in this case had an average size of 0.8 to 1.2 microns.

The filtration rate increased by 2 to 3 times the filtrate rate and the washing water drainage rate through the filtrate cake, respectively, without the described treatment.

Such MHPCC particles having a diameter of about 0.8 to 1.2 microns were obtained, while the individual forming particles were about 0.1 microns in size.

At the same time, the phosphoric acid neutralized milieu formed a thin layer of calcium phosphate on the surface of the MHPCC particle, but not on its core. This was observed when MHPCC was treated with weak acids that were stronger than carbonic acid e.g. acetic acid. Acetic acid is always present, for example, in the circulation water of a paper machine and in similar waters.

Example 2 '!.' The resulting MHPCC treated with dilute phosphoric acid to bring the pH to • · * • · · * · * '8.2 was contacted with a circulating paper machine at pH 4.8. The contact time was 2 h and the mixture was gently stirred at 35 ° C.

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The product was dried and its weight compared to its original weight, whereby it was found that the weight of the · · v '· "' had been reduced by about 6 to 7%. The particle size and shape of the product as measured by electron microscopy.

From the above experimental data, it was concluded that the product had been corroded from the inside and had formed larger cavities than were originally present. This fact clearly has the effect of improving the opacifying effect of the pigment.

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Claims (15)

1. Precipitated calcium carbonate crystals, characterized by taking the form of spherical clusters having a plurality of cores, containing a plurality of spherical calcium carbonate particles which adhere to each other and exhibit a particle size of about 40 ... 400 nm. 2. Crystals according to claim 1, characterized in that the clusters have a size of about 0.1 - 10 µm and the calcium carbonate particles have a size of about 40 ... 200 nm, preferably about 40 ... 120 nm. 10 Crystals according to claim 1 or 2, characterized in that they are in the form of a suspension having a pH of 6.5 ... 9.5, preferably 7.5 ... 9.3. A process for preparing calcium carbonate crystals, according to which process 15. The calcium carbonate crystals are produced by precipitation from suitable starting materials, obtaining a suspension containing crystals, and. , - the crystal line is separated into a chamber filter, characterized by • i · · - the Z-potential of the crystals is installed in the suspension in the chamber filter to a value of • t · · · 20 -1 .. -20 mV, after which • · «·:: j - the suspension is mixed by means of a high-power mixer in such a way that the circular velocity differences directed at this task range to 50 - 200 m / s. Method according to Claim 4, characterized in that the pH of the suspension is installed to a value of 6.5 ... 9.5, preferably about 7.5 ... approx. 9.3. • · · • «I • · · Ml Process according to claim 4 or 5, characterized in that the dispersant is added. In fermentation aids in the suspension, the pH value of the suspension is adjusted so that * ·: *: the Z potential of the crystals rises to -1 ...- 20 mV. 30 Process according to claim 6, characterized in that a polyelectrolyte is used as a dispersing aid. 8. A method as claimed in Claims 4 to 7, characterized by the manufacture of precipitated calcium carbonate particles having the form of substantially spherical clusters having a plurality of nuclei and having a size of about 0.1 ... 10 µm. Method according to claim 8, characterized in that the size of the clusters intended for formation is controlled by the ratio of the cutting speed to the Z potential in such a way that by lowering the Z potential the same particle size is obtained with a smaller circular velocity or in a corresponding manner. larger particle size by constantly holding the Z potential and by decreasing the circle velocity. Process according to claim 4 or 5, characterized in that inorganic or organic acids are used for the adjustment of the pH. 15 11. Process according to claim 10, characterized in that carbonic or phosphoric acid is used for the adjustment of the pH. Process according to any of claims 4, 5, 10 or 11, characterized in that the pH of the suspension is set to the desired value in connection with the washing in the chamber filter. IN | : ';'; Process according to claim 12, characterized in that the pH of the suspension is set to a desired value by passing carbon dioxide gas into the wash water in connection with the filtration. * · * • · · • · «* • ·« · '· 25 Method according to any of claims 4-13, characterized in that the PCC is; formed by precipitation by a caustic reaction. • · · »M • · • * • · ·. Method according to any of claims 4-14, characterized in that the PCC is • ·; prepared by precipitation from lime with carbon dioxide gas.