A method and apparatus to produce precipitated calcium carbonate The invention described in this application consist of a method and an apparatus to produce precipitated calcium carbonate (PCC) or a composite of it with some another material in a continuous process.

In this method carbonation takes place by mixing calcium hydroxide and carbon dioxide gas in a pressurized reactor. Both of these are dissolved into water phase, in which the crystallization takes place.

In addition this innovation describes an apparatus and the principle according which carbonation is made.

Precipitated calcium carbonate has been used for centuries among others in paper industry, but the share of it among paper minerals started to increase significantly only in 1990's. Today PCC is used worldwide in paper industry about 6 million tons yearly.

Altogether paper industry is using minerals for paper filling and coating about 30 million tons yearly. Using minerals one can change the structure of a paper to improve for example printability and optical properties of it. It is a well known fact that mineral particles have better light scattering than wood fibers does have. Therefore they decrease transparency of paper, i.e. improves opacity, which is an important property among others for printing and writing papers. Coating pigments on the other hand even out topography of paper, increase smoothness of it and one can adjust ink setting speed of paper. There are several other properties that can be improved by paper minerals, from which decrease of production costs is not at all the least, price ratio between paper minerals and fibers is about 1/6

All the traditional paper minerals are produced either by grinding and/or classification, which means that impact in particle size, size distribution and morphology is only limited. For example kaolin is just taken out from the ground, suspended to water and then purified from side stones and impurities with different classification processes. Finally the purified kaolin is classified to fractions such that the fine end is used for paper coating and the coarse end for paper filling. In some cases the coarse fractions are further refined by so called delamination grinding, which means that in addition to particle size reduction, the aspect ratio is

increased (= length/thickness of the particles), i.e. the particles are more platy. It is typical for kaolin that the particles are packages of platy particles, which are separated in this delamiantion grinding.

Ground calcium carbonate (GCC) is according to its name ground lime stone. It is not possible to influence on particle shape in this process, but of course particle size can be decreased by increasing grinding. On the other hand grinding energy need increases exponentially along with the particle size, i.e. the finer particles one want to make, the more grinding energy is needed. Particle size distribution can be influenced by coarse material circulation after grinding, which means that after grinding this material is classified and the coarse end is circulated back to grinding and the fine end is collected as product. In practice one has to dilute the material for classification, which means that the product has to be thickened after classification, which inevitably increases production costs.

It is logical to think that by adjusting particle shape, size and size distribution of paper minerals one can achieve optimal properties for each paper or board grade. As stated earlier, the possibilities to adjust these properties with the conventional paper minerals are limited, but in precipitation process these adjustment possibilities are almost unlimited. The production philosophy of precipitate calcium carbonate is essentially different to conventional paper minerals, where production is based on "from top to bottom". PCC production is based on "from bottom to top", which means that PCC crystals are grown up form crystal nucleus, grinding is based on existing bigger particle crushing.

The disadvantage of PCC to GCC is the higher production costs. The essential cost factor in PCC production is the lime burning, where calcium carbonate is broken down to calcium oxide and carbon dioxide in about 1000 ⁰ C:

CaCO ₃ + energy -> CaO + CO ₂ Before precipitation calcium oxide is slaked to calcium hydroxide:

CaO + H ₂ O -> Ca(OH) ₂ + energy This reaction is highly exothermal, i.e. heat releasing. Calcium carbonate precipitation, carbonation takes place according to the following reaction equation:

Ca(OH) ₂ + CO ₂ ^CaCO ₃ + H ₂ O

This reaction is slightly exothermal, which has to be taken into account when planning the crystallization conditions.

In the known precipitation processes the needed carbon dioxide is taken from a power plant's exhaust gases or form a recovery kiln of a pulp mill, in which cases the carbon dioxide content can easily vary between 10 - 25 %. The sizes of reactors, pipes and pumps are determined according to the carbon dioxide content, in other words, the lower the CO ₂ content, the higher the dimensions of those. Naturally the higher the dimensions of these process elements are, the higher are the investment and production costs. In addition PCC processing in the conventional plants take place batch wise, which further increases investment and production costs in form of several intermediate and storage tanks. Both carbon dioxide and calcium hydroxide solubility to water has direct impact on the speed of the carbonation process. It is known that carbon dioxide dissolves to water more easily if the pressure of the vessel is increased, temperature decreased and the gas is broken down to small bubbles, i.e. the gas/water interface area is as high as possible. In a large batch reactor pressure increase is not economically possible and small carbon dioxide gas bubbles generation assumes effective mixers and thus high energy input, which means high production costs. It is essential to take into account that because of hydrostatic counter pressure of the vessel, the gas, also the inert portion of it, has to be injected to the reactor with a compressor or a blower, which can handle the counter pressure at least 0,1 MPa.

As a summary one can thus say that the conventional PCC carbonation assumes high investments and production costs.

In this innovation PCC production takes place continuous way in a very small reaction chamber and 100 % conversion of the reaction is reached in a surprisingly short time, « 5 seconds. A small reactor is easily pressurized up to 2 MPa, high shear speed and ultra sound are breaking down the CO ₂ bubbles to micro bubbles. Together these factors improve mass transfer from gas phase to water phase, CO ₂ solubility to water. The very same factors improve also mass transfer from solid phase to water phase, solubility of calcium hydroxide to water. Very small reactor size (50 liters in comparison to 6 x 60 m ³ ) naturally decreases investment and production costs.

In this invention the CO ₂ gas is broken down to micro bubbles with high shear speed and ultra sound, from which it under high pressure dissolves to water significantly faster than under atmospheric pressure and from bigger bubbles, i.e. mass transfer from gas phase to water phase is dramatically increased. The pressure of the reaction chamber is typically 0,3 - 2,0 MPa, but is not limited to this. The circumferential velocity of the rotor of the reactor is more than 50 m/s,

which is high enough to produce together with the holes and/or slits in the stator and rotor ultra sound, which effectively break down the CO ₂ bubbles. The shear speed is determined with ratio of the speed difference between rotor and stator and the distance between the two: Y = dv / dy

In this case the shear speed is:

Y = 50/ 0,005 1/s = 100 000 1/s

Similarly the solubility of calcium hydroxide is increased significantly by the high shear speed and ultra sound. This device efficiently breaks down the agglomerates of Ca(OH) ₂ and therefore mass transfer from solid phase to water phase takes place significantly faster than without these phenomenas.

The actual carbonation reaction takes place in water phase.

The principle of the carbonation can be seen from figure 1. Slaked lime, calcium hydroxide (1) is introduced with the help of Mohno pump (2) or similar to "Cavitron", which is high shear speed, high pressure and ultra sound reaction chamber (3). Carbonation pressure is adjusted by the valve after reaction chamber (4), typically 0,3 - 2,0 MPa. Pressurized carbon dioxide gas (5) is introduced either to reaction chamber or to the flow of calcium hydroxide, into the pressure side of the pump. The pressure of the calcium hydroxide and carbon dioxide has to exceed the pressure of the reaction chamber. To the reaction chamber or to the calcium hydroxide flow before the pump or after it can be dosed chemicals (6) to adjust particle shape and size of PCC.

Calcium hydroxide flow (1) is adjusted such that 100 % conversion is achieved with one run through the "Cavitron". In practice another "Cavitron" is arranged in series with the first one or there is a post carbonation vessel (7) to secure the 100 % conversion.

Carbon dioxide can be diluted by air, nitrogen or some another inert gas (8), which of course means that the speed of carbonation will slow down and additionally also the pressure of the gas combination has to be increased to compensate the counter pressure, which means that one has to use compressor to inject the gas to the system, the pressure of condensed carbon dioxide is enough as such. This alternative illustrates the situation that one wants to use for example exhaust gas as a source of CO ₂ .

The best source of CO ₂ for this technology is condensed carbon dioxide. One can assume that in the future, when power plants may be forced to capture CO ₂ from their exhaust cases, the price of condensed CO ₂ will decrease significantly. In this process the condensed CO2 has to be vaporized before injecting to the system. In bio fuel production there will be significant amounts of high pressure carbon dioxide gases available, which could be used in PCC production as such. For example the bio diesel production will generate up to 300 000 tons CO ₂ per each 150 000 ton of bio fuel.

Along with calcium hydroxide suspension one can dose to the reactor for example fibers, starch, CMC, synthetic fibers and so on to produce composites of PCC and those other materials.

In figure 2 there is a picture of the stator and rotor of the reaction chamber. The rotation speed of the device used in this work is 12 000 1/min, which gives circumferential velocity of about 50 m/s. The rings (9) of the rotor and stator are overlapping such that the suspension of calcium carbonate is flowing through the slits or holes of the rings or between the rings. The gap between the rings is about 0.5 mm, but it is not limited to this dimension, it can be higher or lower. The flow is pulsating, because the channels through the rings are open only when the holes or slits are in the same position. Because of this and high circumferential velocity the device generates ultra sound effect. The material is flowing also between the rings, which gives high shear speed. The volume of the reaction chamber of this device is only about 50 ml. By increasing this volume one can increase the productivity of the reactor. This can be done by increasing the diameter of the reactor, which means that the rotation speed of it can be decreased to keep the circumferential velocity constant.

In figure 3 there is a picture of the laboratory unit to produce PCC continuously. Already with this little device one can produce PCC 300 kg per day. With a reactor having 50 I reaction chamber one can produce same amount, about 100 000 t/a, as a commercial on site PCC plant. In a conventional plant like that about 3 - 5 batch reactors are needed, about 50 m ³ each, to produce the same amount.

The target of this innovation is to reduce investment and production costs of PCC by utilizing the foreseeable supply of condensed carbon dioxide form power pants and high pressure gas from bio fuel production.

This target is reached by using instead of conventional batch reactor a continuous reactor and by considerably speeding up the carbonation process, which is done by having an effect of high shear speed, typically > 20 000 1/s, high pressure and

ultra sound on calcium hydroxide (solid phase), on carbon dioxide (gas phase) and on water (liquid phase). This increases significantly mass transfer from solid phase (Ca(OH) ₂ ) to liquid phase and from gas phase (CO ₂ ) to liquid phase, which makes the carbonation process surprisingly fast. Continuous and extremely fast carbonation decreases the size of reactors, piping, pumps and all the other process devices. Condensed and/or high pressure carbon dioxide gas eliminates the need of compressors use. These altogether reduce investments and production costs of a plant

What is characteristic for this innovation is more precisely described in the patent claims later on.

In the patent Fl 105179 B the key idea to make PCC is to break down the calcium hydroxide slurry into tiny droplets in carbon dioxide phase by high energy intensity pin mill. In other words the carbon dioxide is the continuous phase in the reactor and calcium hydroxide is discontinuous phase. These phases are in atmospheric pressure. This patent is essentially different to the invention described in this application. Firstly in this application both calcium hydroxide and carbon dioxide are broken down into very small units in water phase, which increases mass transfer from solid phase to liquid phase and from gas phase to liquid phase, which allows the actual carbonation reaction to proceed very fast. Secondly in this application the reactor is pressurized, which increases carbon dioxide solubility to water and increases carbonation speed. In the description of the patent Fl 105179 B there is four reactors in series and an end pointing reactor at the end, which means that reaching the 100 % conversion of the reaction can easily take minutes though the delay time at one reactor can be significantly shorter. Obviously the slow conversion in the patent in question is due to slow shear speed inside the reactor, 1000 - 10 000 1/s, because the distance between the pins can easily be 5 cm and the distance between the rings about 0,5 cm. In addition the reactor is in atmospheric pressure and there is no ultra sound to improve mass transfer both for the calcium hydroxide and carbon dioxide. There is no evidence in the patent description of Fl 105179 B that the method has an impact with the known chemicals on particle shape, size and size distribution, which actually would be beneficial for paper industry minerals as discussed earlier in this patent application.

The method of making PCC in the patent application WO9936361 Al is essentially the same as in the patent Fl 105179 B, i.e. the calcium hydroxide suspension is dispersed into gas phase. The target though is not fast carbonation but powder PCC. Therefore this method uses high volume of gas. The weaknesses of this method are the same as in the paten Fl 105179 B.

The method of PCC carbonation in patent application WO2006005793 A1is also essentially the same as in patent Fl 105179 B and therefore the weaknesses are the same. In this application the target is to make nano size PCC by selecting optimal carbonation temperature. The following examples are given to illustrate the invention without limiting the scope of protection. The examples were executed with the devise in figure 3.

Example 1

According to the method described above the carbonation was made in the following conditions: - Carbonation temperature 40 ⁰ C

- Pressure of the reaction chamber 0,7 MPa

- Shear sped in the reaction chamber 50 000 1/s

- CO ₂ flow 25 l/min (NTP)

- Conversion 77 % In the figure 4 there are PCC crystals that are produced according to the method described in this application. The crystals are relatively big rhombohedral discrete or agglomerated particles, which are perhaps not ideal for paper making, but particle size and shape can be significantly changed with the reaction conditions as can be seen later on.

Example 2

Next carbonation was made in the following conditions:

- Carbonation temperature 15 ⁰ C

- Pressure of the reaction chamber 0,7 MPa - Shear speed in the reaction chamber 50 000 1/s

- CO ₂ flow 11 l/min (NTP)

- Addition of known chemicals A and B

- Conversion 83 %

In the figure 5 there are the PCC crystals of this carbonation. Similarly as in the first example, the conversion is not complete and one needs another reactor in series or the carbonation is completed in a storage tank.

Example 3

One can impact on PCC crystal shape in addition to carbonation conditions and chemical additions by seeding, which gives an impulse for the desired particle shape and size.

- Carbonation temperature 27 ⁰ C - Pressure of the reaction chamber 0,7 MPa

- Shear speed in the reaction chamber 50 000 1/s

- CO ₂ flow 11,5 1/min

- Seeding C

- Conversion 100 % In the figure 6 there are the PCC crystals of this example. As can be seen from the picture, the morphology of the PCC particles can be adjusted with this method, which is important for paper industry applications. Full conversion is achieved, which means that there are no need for any post treatments..

By crystallization conditions, adding known chemical and by seeding one can almost infinitely adjust the properties of produced PCC crystals.