WO1999051525A1

WO1999051525A1 - Process and apparatus for producing precipitated calcium carbonate

Info

Publication number: WO1999051525A1
Application number: PCT/FI1998/000306
Authority: WO
Inventors: Pentti Virtanen
Original assignee: Fp-Pigments Oy
Priority date: 1998-04-07
Filing date: 1998-04-07
Publication date: 1999-10-14
Also published as: AU6834298A

Abstract

A process and an apparatus for preparing precipitated calcium carbonate produced by causticizing. The process consists of homogeneous process steps and filtration. The homogeneous process steps comprise lime slaking in an impact mixer, whereby 80 % of the slaked calcium hydroxide particles are smaller than 3 microns, contacting the slaked lime and soda in an impact mixer, maturing the rection mixture in an unmixed state, interrupting crystal growth in an impact mixer or a corresponding apparatus providing homogeneous residence time and impact energy, and separating the crystals and the formed lye by filtration. According to the invention, crystal growth is interrupted by homogeneous impact mixing, when the viscosity after the gelling of the settling and maturing mass has decreased, and the time interval thereafter is used to control crystal size. The apparatus comprises three units, namely a slaking unit, a reactor and maturing unit, and a filtration and elutriation unit.

Description

Process and apparatus for producing precipitated calcium carbonate

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

The invention further relates to an apparatus for preparing precipitated calcium carbonate.

Precipitated calcium carbonate (PCC) has become increasingly common above all in the coating and filling of paper. The same PCC material is also used as a filler in plastics and as a carrier agent for many drugs and colour pigments. A typical field of carrier use comprises heat-reacting diazo compounds used in, e.g. plotter paper and earlier telefax paper.

Precipitated calcium carbonate is usually prepared by carbonating a calcium hydroxide slurry using flue gases. A process is also known where ammonium, potassium or sodium carbonate is passed through a so called causticizing reaction. A causticizing reaction is the most common method of chemical regeneration in traditional cellulose production. In the so called sulphate cellulose process lime circulates between the causticizing process and the burning oven and the PCC material obtained is not recovered.

The US company Glatfelter Co has applied for the patent FI 942815 where a characterizing feature comprises contacting the reagents, soda and lime white with each other by mixing them in more than one reagent streams and by then allowing the mixture to react for 1 to 8 hours within the temperature range from 27 to 60 °C. In this manner, so called rosettes or PCC crystal clusters having a size of about 1.2 microns are obtained. As the process or the apparatus a mixing tank equipped with a mixer reportedly having a rotor speed of 6.6 m/s is used. The PCC slurry obtained is separated from the lye (NaOH) generated during the reaction by filtration. The filter cake is again elutriated in water and neutralized with CO₂. US Patent Specification No. 5,342,600 states that PCC can be advantageously prepared by causticizing once the calcium hydroxide used as reagent has first been screened with a 40 to 70 micron screen and thereafter mixed in a mixer having a high shear force prior to the reaction with an alkali metal carbonate.

According to US Patent Specification No. 5,075,093, the calcium hydroxide slurry used is taken to a shearing disintegration with alkali carbonate prior to the reaction, such that the viscosity of the slaked lime white is essentially increased to a given value. This means that the amount of fine material is increased and/or that catenary particle clusters are formed.

In many patent specifications where PCC is produced by causticizing, a chemical which slows down crystallization is added, such as sugar etc., in order to obtain fine crystals.

It has long been generally known that extremely fine PCC, which has mainly been used for medical purposes and in the pharmacopoeia, has been obtained by allowing CaCl₂ and (NH₄)₂CO₃ to react with one another. Hereby 0.05 to 0.5 micron PCC particles have easily been obtained. It may be stated that said process is a costly one.

The subject matter of Patent Specification No. WO 96/23728 is the same but in the present document details of the process have been essentially improved and clarified and the apparatus has been developed to meet the requirements for industrial implementation as regards certain, numerous details.

In the process of the present invention developed by us most things are carried out in a completely different manner from a number of previously published patents, and during the development of the process new things about the theory of crystallization and precipitation have been observed and learnt, and this knowledge is fully exploited.

Further in connection with the process development many demanding technical details relating to the apparatuses required for implementing the process have become clear. The most essential features of the present invention are the following:

- The lime is slaked in a rapid impact mixer where extremely finely divided slaked lime is formed.

- The slaked lime and the soda solution are taken in equal molar ratios to a similar rapid impact mixer wherein the dwell time is < 1 s.

- The reaction mixture is allowed to react for a certain time without mixing.

- The growth of the crystals is stopped by the rapid impact mixer.

- The crystals are filtered by washing them in a filter using a wash liquor containing neutralizer.

In more detail the process according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.

The process is based on three process units: 1) a slaking unit, 2) a reaction and maturing unit, and 3) a unit for filtering, carbon dioxide treatment and elutriation.

Thus, the apparatus is composed of a lime slaking mixer which is a rapid-running pin mill or a disintegrating mixer having circumferential speeds of up to 200 m/s; a slaking container which is not mixed and in which the lime is slaked within a relatively short time, i.e. about 5 to 20 min; a similar mixer as that described above in which the slaked lime and the soda solution are contacted; a maturing vessel which is not mixed; and a power mixer as described above for interrupting crystal growth, and filtration; and returning the primary filtrate to a separate pumping vessel from which it is pumped to a pumping vessel after filter cake formation, from where it is then returned to filtration by pumping after filter cake formation. The apparatus according to the invention is characterized by what is stated in the characterizing part of claim 11.

In the following, the invention is examined in more detail by means of the annexed drawings in which

Fig. 1 compares the mixing intensities of a disintegrating mixer and a normal container mixer and its distribution in the path of the volume element, and

Fig. 2 shows the general flow chart of the process according to the invention.

The special idea behind the solution according to the present invention is that homogeneous mass transport and crystallization conditions are aimed at during each step, whereby everything, such as mass transport and diffusion and degree of grinding, breaking of the crystals and, at the beginning, lime slaking, is caused by conditions where the same power, diffusion and time is applied to all particles, i.e. the process is homogeneous.

When developing the process it has unexpectedly been found that a so called critical particle size distribution has been reached which is so narrow in all phases that small particles are no longer dissolved and large particles no longer increase in size but all particles of the suspension grow very close to the same velocity.

In the maturing vessel after the mixing of the reagents, where only homogeneous diffusion is allowed to be active (continuous), after a certain nucleation phase in a certain layer the viscosity begins to rise steeply merely to decrease again 5 to 20 min later to a lower level, from which moment the dwell time may be determined after which the crystals must fairly rapidly be taken to interruption mixing and filtration. The term "interruption mixing" covers special observations made in connection with the present invention. The growth of the crystals ends and is decelerated when they are subjected to a particularly violent impact and shear forces in series, which would seem to damage the growth centres of the crystals, such as the sharp last-to-grow ends or the like of crystals, whereby growth comes to an end. At the same time crystal size has been homogenized, whereby no uneven growth can begin to occur as each one has the same surface energy.

The structure of both the maturing vessel for lime slaking and the maturing vessel for causticizing is of essential importance in order for the process to be constantly in use. The walls of both vessels, particularly at the interface between liquid and gaseous atmosphere and near it, are easily blocked because PCC and/or lime constantly adheres to them.

In the present solution both vessels are furnished with jacket cooling means at the top part, said means causing aqueous vapour to condensate on a continuous basis, resulting in an uninterrupted stream of water flowing downwards, said stream washing the problematic interface which has a tendency to gather dirt and keeping it clean.

Said problem spot is implemented in a particularly advantageous way if a plastic coat of polyethylene is used, or, in particular, fluorinated polymers.

To cite a special feature of the process it may be mentioned that it is preferred but not obligatory to neutralize the PCC precipitate with, e.g. carbon dioxide or/and phosphoric acid during filtration and in the filter.

It is also characteristic of the process that only the feed of CaO is externally controlled, the feed of soda and water is based on cascade control, and advantageously, there is an overflow from the Ca(OH)₂ and PCC maturing tanks, whereby savings are incurred in that numerous control circuits and pumps are not needed.

Example:

Burnt lime is taken to an impact mixer 1 driven by 2 x 20 kW motors and having circumferential speeds of 46 m/s.

The material is fed into the impact mixer at a speed of 0.2 kg/s of CaO and 1.17 kg/s of water. The temperature of the slaking water is 30 °C. When leaving the mixer, the temperature of the lime slurry is 77 to 78 °C. 30 % of the mass flow, containing particles larger than those in the refined material, is returned from the separator 2 after the impact mill 1 back into the impact mill as the bottom fraction 3 of the separator.

Lime milled and slaked with the impact mixer 1 was allowed to "mature" in the settling tank 4 for five to ten minutes, whereby the temperature was increased by 1 to 2 degrees and the lime was found to have been slaked to almost 100 %.

Soda was mixed and dissolved in water to obtain a 20 % solution and it was pumped, together with the slaked lime slurry, to the causticizing step via an impact mixer 5 identical to the one described above. From the mixer it was let straight into a reactor container 4 where the maturing of the PCC took place. After about 5 to 10 min, an intensive nucleation started and the viscosity of the slurry was increased to nearly 1000 cP. After 10 minutes, the viscosity was again decreased to about 200 cP. 5 minutes later, the slurry was pumped into an interrupting mixer 6 through which it was passed once, whereafter it was immediately filtered. The obtained clear NaOH solution contained 2 mg/1 of PCC particles, and these were separated in about 24 hours, resulting in a solution which was clear to a depth of 0.5 m.

In the filter 7 the cake was displaced with water and washed with water which contained carbon dioxide, thereby obtaining final neutralization. Next, it is of advantage to pump a small amount of phosphoric acid either into the filter 7 or/and the elutriating mixer 8 arranged after it. Reference numeral 9 indicates a heat exchanger and reference numeral 10 a pumping vessel into which the primary filtrate can be returned.

If the PCC particles are neutralized with phosphoric acid without using any chelating agent or the like, acicular calcium phosphate crystals are formed on the particle surface, resembling the detonator claws of a sea mine. For this reason it has been suggested, for instance in US Patent Specification No. 5,043,017, that phosphoric acid be added, together with a complexing agent, into the formation of PCC. However, the cited reference does not in any way indicate what effect is sought by this measure. As is stated above, many impurities prevent the crystal from having its correct shape, and this is what will happen even in the case of the cited reference. We have verified this by experiments. However, it has surprisingly been found in the present invention that if the phosphoric acid is suitably added before the final elutriation and if the reaction between the phosphoric acid and the surface of the PCC particle begins and is continued in the power mixer essentially used for elutriation, no acicular calcium phosphate crystals are formed, but instead, the surface is covered with a thin phosphate coat. When proceeding in this manner, chemical aids, which are both difficult to use and costly, are not needed at all.

The obtained particles had an average size of 0.1 microns and, after a longer settling time, 0.2 microns, and after a very much longer settling time 0.4 microns. When the slurry was left to settle for about 40 min before interruption, longitudinal crystals were obtained instead of spherical ones, their dimensions being 0.2 x 0.8 microns.

The lye solution obtained contained the following amounts of soda and lye:

NaOH Na₂CO₃ g i

2 1 sample 92.8 20.5

2 1 sample 95.6 18.9

2 1 sample 95.0 17.3 filtration

2 1 sample 92.8 18.9

2 1 sample 90.0 16.5

2 1 sample 78.4 16.2 displacement

2 1 sample 16.8 3.8

2 1 sample 8.4 2.7

2 1 sample 5.6 2.7 wash H,O + CO,

2 1 sample 0 2.7

In the present invention and in the process and apparatus implemented in accordance with the invention, detailed functional solutions have been developed, which are characterized by a general principle which is here defined as the principle of the homogeneity of processing. This means that such mixings are used where the intensity distribution of the mixing is as narrow as possible and the so called maturing tanks are totally unmixed, whereby there is no divergence in said tanks under mass transport conditions, and instead, an only temperature-dependent diffusion homogeneously controls the growth of all crystals at the same time.

A new and essential feature for process control will be clear from the above. At a point in Fig. 2 a densely dotted zone is marked inside the maturing tank 4 for causticizing. This zone marks the part where the formation of PCC gel, i.e. nucleation, is initiated. Hereby, as was stated above, the viscosity rose rapidly in order to then fall again to a considerably lower value. This time difference during which the maturing crystals are in said tank, whereby the viscosity is decreased and the mass ends up in the interrupting mixer 6, constitutes an essential control variable in the process.

According to our invention we can control the obtained crystal size in a controlled manner in this homogeneous process by controlling either the height of the overflow, the dimensions of the tank, or any process technical parameter such that after a desired time after the decrease in the viscosity of the gel the maturing PCC mass is taken to a mixing step for interrupting crystallization.

By means of this time the final size of the crystals can be controlled with great precision and to a certain extent even the shape of the crystals, when they are allowed to grow until they reach a sufficiently large size whereby the crystals slowly begin to assume an acicular shape instead of the spherical crystals normally obtained in this process.

Example of crystal size control: dimensions in micrometres.

Time after the decrease in the viscosity of the gel:

19 min < 0.1, 22.5 min 0.2, 24 min 0.2 x 0.4, 29 min 0.4 x 1 One measure indicates that the crystals are spherical whereas A x B dimensions indicate that they have assumed an acicular shape.

The times given in the example are of an indicative nature only because, to be accurate, they are dependent on the temperature, and particularly on the reactivity of the lime. The given lime particle size of 80 % < 3 microns essentially reduces the effect of the reactivity of different kinds of lime on the growth time of the crystals.

When examining the above example it is found that when the crystals are allowed to grow sufficiently the form factor begins to change into a direction other than a spherical shape. According to the invention crystals can be grown spherical by passing the crystal mass through interrupting mixing and by thereafter letting the crystals slowly grow. This procedure can be repeated several times.

Claims

Claims:

1. A process for preparing precipitated calcium carbonate produced by causticizing, which process consists of homogeneous process steps and filtration, said homogeneous process steps being :

- lime slaking in an impact mixer: 80 % < 3 microns

- contacting the slaked lime and soda in a similar impact mixer,

- allowing the reaction mixture to mature in an unmixed condition,

- interrupting crystal growth in an impact mixer or a similar apparatus providing homogeneous residence time and impact energy, and

- separating the crystals and the lye formed by filtration, c h a r a c t e r i z e d by

- interrupting the growth of the crystals by means of a homogeneous impact mixer when the viscosity of the settling and maturing mass after gelling has decreased and the crystal size is controlled by the time interval thereafter.

2. The process according to claim 1, wherein a multi-stage impact mixer is used as mixer, said impact mixer comprising two multi-stage mixing means rotating in different directions on concentrically arranged rotors, which mixing means will beat and cut the processed material so that all material passing through with high probability will receive the same energy of impact and shear.

3. The process according to claim 1 or 2, wherein the maturing of the growing crystals takes place in a tank in which they are not mixed.

4. The process according to any one of the claims 1 to 3, wherein the growth of the crystals is interrupted with the above-mentioned impact mixer.

5. The process according to any one of the claims 1 to 4, wherein an impact mixer is used operating with circumferential speed differences of 40 to 200 m/s.

6. The process according to any one of the claims 1 to 5, which comprises using a lime slaking and a causticizing container, both of which are provided with a cooling jacket at least in the top portion thereof in order to achieve a cleansing internal flow of condensation water.

7. The process according to any one of the claims 1 to 6, wherein at least the static inner parts of the apparatus are treated or coated with a material not easily wetted by water and/or oil, such as with a fluorinated polymer, polyethylene, polypropylene and a silicone-based polymer.

8. The process according to any one of the claims 1 to 7, which comprises using process equipment which are interconnected with as little piping as possible and that a flow rate of more than 2.5 m s is employed in the pipes for preventing fouling and precipitation.

9. The process according to any one of the preceding claims, wherein the maturing mass is run through an interrupting mixing several different times and inbetween the crystals are allowed to grow so as to produce bigger, almost spherical crystals.

10. The process according to any one of the preceding claims, wherein the precipitated calcium carbonate is stabilized with phosphates and/or phosphoric acid to endure an acid environment by adding the necessary amount of phosphoric acid and/or phosphates to the elutriation of the finished, filtrated product, said elutriation preferably being performed by means of an impact mixer.

11. An apparatus for preparing precipitated calcium carbonate, c h a r a c t e r i z e d in that it contains a slaking unit for lime, a reaction and maturing unit and a unit for filtering, carbon dioxide treatment and elutriation, the apparatus comprising

- a lime slaking mixer (1) which is a rapid-running pin mill or a disintegrating mixer having circumferential speeds of up to 200 m/s; - a slaking tank (2) which is not mixed and in which lime is totally slaked within a relatively short time, - a mixer (5) in which the contacting of the slaked lime and the soda solution takes place,

- a maturing vessel (4) which is not mixed,

- a power mixer (6) for interrupting crystal growth, and

- a filtering unit (7).

12. The apparatus according to claim 11, which comprises a pumping vessel (10) to which primary filtrate can be returned.