FI103402B - Process and apparatus for producing precipitated calcium carbonate - Google Patents

Description

103402

Process and equipment for the production of precipitated calcium carbonate

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

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

Precipitated calcium carbonate (PCC) has become increasingly common, especially in the coating and filling of paper. The same PCC material is also used as a filler for plastics and as a carrier for many drugs and color pigments. Typical carrier media are the heat-reactive diazo compounds used in, for example, plotter papers and prior telefax papers.

Precipitated calcium carbonate is most often prepared by flue gas carbonation of calcium hydroxide slurry. It is also known to employ ammonium, potassium or sodium carbonate in a so-called. via causticization reaction. The causticization reaction is the most common regeneration of chemicals used in traditional cellulose production. In the so-called sulphate-cellulose process, lime circulates between causticization and the kiln and. ; the resulting PCC material will not be recovered.

20: · US Patent Application Serial No. 942815, filed by Glatfelter Co., discloses that reagents, soda, and lime milk are brought into contact by mixing them in more than one reagent stream and finally allowing the mixture to react for 1 to 8 hours. in the range of 27-60 ° C. This law produces about 1.2 microns of 25 so-called. rosettes or PCC crystal clusters at the time. The process, or equipment, is a · · · * ··· mixing tank equipped with a mixer with a reported peripheral speed of 6.6. m / s. The resulting PCC slurry is separated from the reaction liquor (NaOH) by filtration.

• * · '...' The filter cake is slurried again in water and neutralized with CO 2.

. U.S. Pat. No. 5,342,600 discloses that PCC can advantageously be prepared by causticizing when the calcium hydroxide reagent is first sieved on a 40 to 70 micron sieve and then mixed in a high shear mixer prior to reaction with 2 103402 alkali metal carbonates.

According to U.S. Patent No. 5,075,093, the calcium hydroxide slurry used is reacted prior to reaction with alkali carbonate by shear decomposition such that the viscosity of the slaked lime milk is substantially increased to a certain value. This implies that the fines increase and / or form particulate aggregates.

Many patents in which PCC is prepared by causticization add a crystallization retardant chemical, such as sugar, etc., to provide fine crystals.

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It has long been known for a long time that very fine PCC, which has been used mainly for medical purposes and in pharmacopoeia, has been obtained by reacting CaCl2 and (NH4) 2CO3. 0.05-0.5 micron PCC particles are easily obtained. It can be said that said process is expensive.

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WO 96/23728 deals with the same subject matter, but here the details of the process have been substantially improved, and the equipment has been developed to meet the requirements of industrial realization in many, many details.

Most of the process of this invention that we have developed is done. : things quite unlike many previously published patents and in connection with the development of this process / I ', a new theory of crystallization and precipitation has been learned which is fully utilized.

• · · * · «25 In the course of processing the process, many technical details that require process implementation have been learned.

It is an essential feature of the present invention that: 30 - The lime is quenched in a high-speed impact mixer, which produces very fine quenched lime.

103402 n - Slaked lime and soda solution are supplied in a constant molar ratio to a similar high speed impact mixer with a residence time of <1 second.

- The reaction mixture is allowed to react for a period of time without stirring.

5 - The growth of the crystals is stopped by a high speed impact mixer.

- Filter the crystals by washing them in the filter with a detergent containing neutralizing agent.

More specifically, the method according to the invention is essentially characterized in what is stated in the characterizing part of claim 1.

The process is based on three process units: (1) a quench unit, (2) a reaction and maturity unit, and (3) a filtration, carbon dioxide treatment and net unit.

The apparatus therefore consists of a lime-quenching mixer, a high-speed pin mill or a disintegrator mixer with peripheral speeds of up to 200 m / s; a non-agitated quenching tank in which the lime is extinguished in a relatively short time, about 5 to 20 minutes; '; 20 of the same mixer as above with a contact of slaked lime and soda solution. : happening; a non-stirred maturing vessel; and from the blender to stop the growth of crystals from the blender and filtration described above, and returning the precursor to a separate pumping vessel, from where it is pumped after forming the filter cake to a pumping station, from where it is pumped again after filtration.

The apparatus according to the invention is characterized by what is stated in the characterizing part 11 of the claims.

♦ · ♦ · · • ♦ «

In the following, the invention will be further explored with reference to the accompanying drawings, in which '· · ·' in Figure 1 compares the mixing intensity of a deintegrator mixer and a normal tank mixer along its path element volume, and 4103402 in Figure 2.

A particular idea of the solution of the present invention is that all steps aim at homogeneous material transfer and crystallization conditions, whereby all things, such as material transfer and diffusion and degree of grinding, crystal breaking and initially lime slaking, result in conditions where all particles are treated with the same power, , and time = homogeneous process.

As the process develops, it has surprisingly been found that the so-called. a critical particle size distribution of 10 that is so narrow at all stages that the small particles no longer dissolve and the large ones grow, but the suspension particles all grow very close to the same rate.

In a maturation vessel following mixing of the reagent, where only homogeneous diffusion is allowed to act (continuous), after a certain nucleation step, the viscosity begins to rise sharply in a given layer, again lowering to a lower level after 5-20 min, from which time cutting mixing and filtration. The term cleavage mixture is a specific observation of the present invention. Crystals stop and slow down when they get particularly sharp. ; a set of impact and shear forces that apparently damage crystal growth centers such as; . '. 20 crystal sharp heads or other parts, whereupon growth stops. At the same time, the size of the crystals is homogenized, so that uneven growth cannot be started when all have the same surface energy.

«• · · • t · · i; <: Both the lime maturing mat and causticizing mattresses are essential *: important in their structure for continuous process availability. The walls of each vessel 25, in particular at the point where the liquid-gas interface is in and around, are susceptible to clogging with continuous PCC and / or lime.

• «* • * · •.,. In the solution we have developed, both containers are equipped with upper diaper coolers, '' *, which are provided by a continuous condensation of water vapor and thus a continuous stream of water, which washes the tricky dirty interface to keep it clean.

Particularly advantageous is this troublesome site obtained when using a polyethylene-plastic coating or especially fluorinated polymers.

As a special feature of the process, it can be mentioned that it is preferred, but not obligatory, to neutralize the PCC precipitate in the filtration and filter, for example with carbon dioxide and / and phosphoric acid.

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It is also typical of the process that only the CaO feed is externally controlled, the soda and water supply is cascaded, and the Ca (OH) 2 and PCC maturation tanks preferably have an overflow to save many control circuits and pumps.

10 Example:

The calcined lime is fed to the pulp obtained by the impact mixer 1, which is driven by 2 x 20 kW motors and has a peripheral speed of 46 m / s.

The material is fed to the impact mixer at a rate of 0.2 kg / s CaO and 1.17 kg / s water.

The temperature of the extinguishing water is 30 ° C. When leached, the temperature of the lime slurry is 77-78 ° C. 30% of the pulp stream containing particles larger than the pulverized material is returned from the shredder 1 following the separator 2 to the downstream shredder 3.

. 20 With the impact mixer 1, the ground and slaked lime was allowed to "ripen" in the standing tank 4; five to ten minutes, when the temperature rose 1-2 degrees and the lime was found to be: • almost 100% extinguished.

The baking soda was mixed and dissolved in water to form a 20% solution and pumped simultaneously with the slaked lime slurry through causticization using the identical impact mixer 5 described above.

• ♦ • ·: It was lowered directly from the agitator to the reactor vessel 4 where the PCC matured.

After about 5-10 min, vigorous nucleation started and the viscosity of the slurry increased to about 1000 cP. After 10 min, the viscosity again decreased to about 200 cP.

'' For 5 minutes from this event, the slurry was pumped into a shear mixer 6, from which it was run once ':' '30 and immediately filtered. The final clear NaOH solution contained PCC particles -: ... · 2 mg / liter which separated within about 1 day and clarified the rest of the solution at a depth of 0.5 m.

103402 6

In filter 7, the cake was displaced with water and washed with carbonated water to effect complete neutralization. Subsequently, it is preferable to pump a small amount of phosphoric acid into either the filter 7 or / and the subsequent slurry mixer 8. Reference numeral 9 denotes the heat exchanger and reference numeral 10 the pumping vessel to which the precursor 5 can be returned.

If the PCC particles are neutralized with phosphoric acid without the use of any chelating agent or the like, a needle-like calcium phosphate crystal is formed on the surface of the particle, which resembles the appearance of a maritime mine lighter. For this reason, for example, U.S. Pat. No. 5,043,017 proposes the addition of phosphoric acid together with a complexing agent to form PCC. However, no reference is made in the reference to what this measure seeks to achieve. As stated above, many impurities deprive the crystal of its correct form, and so it does. We have found this experimentally. However, according to the present invention, it has surprisingly been found that if the phosphoric acid is suitably added before the final soaking time, the reaction between the phosphoric acid and the PCC particle surface begins and continues substantially in the slurry blender, no needle-like calcium phosphate crystals are formed. By doing so, no tricky and expensive auxiliary chemicals are needed at all.

.'20 •: The particles obtained were 0.1 microns in size per biscuit and had a standstill time of 0.2 ·: · microns, and very much longer standstill 0.4 microns. When the slurry was allowed to stand for about 40 min prior to cleavage, elongated crystals of 0.2 x 0.8 microns were obtained instead of round crystals.

25 * · • · ** The resulting lye solution contained soda and lye as follows: t «I • * · • ·

NaOH Na3CO3 g / liter 21 samples 92.8 20.5 30 "95.6 18.9" 95.0 17.3 Filtration "92.8 18.9" 90.0 16.5 7 103402 "78.4 16 , 2 Injection "16.8 3.8 8.4 2.7 5" 5.6 2.7 Washing H20 + C02 "0 2.7 Detailed functional solutions have been developed which are characterized by the present invention and the process and apparatus thereof The general principle, 10 here defined as the principle of homogeneity of processing, is that mixes are those in which the intensity distribution of the mixing is as narrow as possible and so-called maturation tanks are completely unmixed so that they do not disperse under material transfer conditions; crystal growth simultaneously.

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There is already a new and essential element in process control. At a particular point in Figure 2, a densified zone is drawn inside the causticization maturation vessel 4. This zone represents the point where nucleation or formation of PCC gel begins. In this case, as noted above, the viscosity increased rapidly to drop again significantly to a lower value of 20. This difference in time that the ripening crystals are in said container when the viscosity is reduced and when the pulp enters the shear mixer 6 is an essential control and control of the process.

According to our invention, we can adjust the crystal size obtained in a controlled manner in this homogeneous process by adjusting either the overflow height, the dimensions of the container or any process engineering parameter. so that, after a desired time, the PCC pulp maturing from a decrease in the viscosity of the gel is subjected to a crystallization cut-off mixture.

• * • · · • ··. During this time, the final size of the crystals and, to a certain extent, the shape of the crystals '' 30 can be fine-tuned when allowed to grow to a sufficiently large size, whereupon the crystal shape begins to slowly needle like circular crystals.

8 103402

Example of crystal size control: Dimensions in micrometers.

Time after gel viscosity calculation: 5 19 min <0.1, 22.5 min 0.2, 24 min 0.2 x 0.4, 29 min 0.4 x 1

One dimension indicates that the crystals are spherical and the A x B dimension indicates that the crystals have grown needle-like.

10 The times in the example are indicative, since they depend strictly on the type of temperature, especially on the lectivity of lime. The reported lime grain size of 80% <3 microns substantially reduces the effect of reactivity of different lime grades on crystal growth time.

Examining the previous example, it is noted that when sufficient crystals were allowed to grow, the shape factor also began to shift out of the round shape. According to the invention, the crystals can be grown round by passing the crystal mass through a shear mix and allowing the crystals to grow slowly thereafter. This can be done several times.

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Claims (12)

A process for producing precipitated calcium carbonate produced by causticization, which comprises homogeneous process steps and filtration, which homogeneous process steps 5 comprise: - quenching of the quay in an impact mixer: 80% <3 microns - contacting the quenched quench with soda in a similar impact mixer, - ripening of the reaction mixture in a mixed state, - interrupting the growth of the crystals in the impact mixer or in a corresponding device, providing a homogeneous residence time and impact energy, and - separating the crystalline and the formed lute by filtration, characterized by - the growth of the crystals is interrupted by a homogeneous impact mixture, when the viscosity of the standing and maturing mass after gel formation has decreased, and the crystal size is subsequently adjusted. 2. A method according to claim 1, characterized in that as a mixer, a multi-stage impact mixer is used in which two multi-stage mixers on the inner peripheries rotate in different directions, which cut and cut the material intended for machining in such a way that: the material is very likely to have the same impact and cutting energy. Method according to claim 1 or 2, characterized in that the ripening of the growing crystalline cells takes place in a container in which they are not mixed. * I I • · · • · · 25 Method according to any of claims 1-3, characterized in that the growth of the crystal is stopped by means of the aforementioned impact mixer. • »· • · • · · • * *! Method according to any of claims 1-4, characterized in that a blow mixer is used, which operates with peripheral velocity differences of 40 - 200 m / s. Method according to any one of claims 1-5, characterized in that a quenching container and a causticizing container for the quay are used, each of which is at least one; At its upper part is provided with a cooling jacket for effecting a purifying drainage of internal condensation water. Process according to any one of claims 1-6, characterized in that the static inner parts of the device are treated or coated with a material which poorly moisturizes the water and / or oil, such as fluoridized polymer, polyethylene, polypropylene or silicon-based polymer. . Process according to any of claims 1-7, characterized in that 10 process devices are used which are connected to each other by means of as few pipes as possible and that a flow rate of above 2.5 m / s is used in the pipes to prevent contamination and precipitation. . Method according to any of the preceding claims, characterized in that the mass intended for ripening is run through the interrupting mixture a number of different times and in between the crystals are allowed to grow, obtaining larger, almost spherical crystals. Process according to any of the preceding claims, characterized in that the precipitated calcium carbonate is stabilized with phosphates and / or phosphoric acid in such a way that in the acidic environment in the slurry made preferably by means of a percussion mixer. the finished filtered product is added a necessary amount of phosphoric acid and / or phosphates. 11. Apparatus for the production of precipitated calcium carbonate, characterized in that it comprises a quenching quenching unit, a reaction and ripening unit, and a • · • · · \. filtration, carbon dioxide processing and slurry unit, the apparatus comprising: • a · a lime extinguisher mixer consisting of a quick-acting tapping mill or a · · · • · · ′; disintegration mixer with peripheral speeds of up to 200 m / s; . - a quenching tank which is not mixed in which the quench goes out for a relatively short period of time, - a mixer in which contact between the quenched quench and the soda solution occurs, - a ripening vessel which is not mixed, - a power mixer, which interrupts the growth of crystals, and - a filtration unit. Device according to claim 11, characterized in that it comprises a pump vessel. to which the pre-filtrate can be recovered. • · · «« «1