WO1999051524A1

WO1999051524A1 - Process for producing finely divided slaked calcium oxide

Info

Publication number: WO1999051524A1
Application number: PCT/FI1998/000305
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: AU6834198A

Abstract

The invention concerns a process for the preparation of finely divided slaked lime. According to the process, slaked lime is taken to an impact mill (5) having a high shear rate together with excess water, whereafter it is slaked with water in said impact mill, whereby an aqueous suspension containing calcium hydroxide is obtained. According to the invention, calcium oxide is slaked in the impact mill (5) which is operated such that the speed difference between the rotors is in excess of 50 m/s, whereby calcium hydroxide is obtained having a degree of hydration after the impact mill of over 80 %, said calcium hydroxide being essentially slaked within 5 minutes. After slaking, the aqueous suspension containing calcium hydroxide is left to settle in a settling vessel (7) for 1 to 20 min, the product is separated and the settling part of the aqueous suspension is again taken through the impact mill (5). By means of the invention, a product is obtained which is well suited for the preparation of precipitated calcium carbonate pigments.

Description

Process for producing finely divided slaked calcium oxide

The present invention relates to a process according to the preamble of claim 1 for rapid slaking of lime and for the preparation of finely divided slaked lime.

According to such a process calcium oxide is added to water under stirring, whereby the burnt calcium oxide is wet milled and slaked by taking the calcium oxide into an impact mill having a high shear rate together with excess water. As a result, an aqueous suspension containing calcium hydroxide is obtained.

Lime slaking is age-old prior art. Many kinds of apparatuses and techniques have been proposed for this process where calcium oxide is turned into calcium hydroxide. Typically (Kirk-Othmer/Encyclopedia of Chemical Technology 1985) the slaking of lime is performed using slowly rotating kneaders whereby water is slowly added in small amounts. At the same time the large particles contained in the burnt lime are disintegrated and the lime is gradually milled. Lime slaking generates quite a lot of heat and normally it evaporates a part of the added slaking water. Typically, carefully slaked lime is dry and has a particle size of e.g. 60 % < 10 microns, and the variation range of the particle size is between 5 and 10 microns.

Calcium hydroxide is used for a number of purposes. If it is merely used as a cheap base, a water neutralizer, for removing the hardness of water, or as a completely water-soluble chemical reagent, e.g. CaCl₂, calcium formiate, etc., the particle size of the slaked lime is not a major issue.

Modern commercial lime slaking apparatuses use approximately twice the amount of water bound during hydration, the rest of the water evaporates and a dry slaked calcium hydroxide powder is obtained. The typical slaking time is approximately 1 hour.

This commercial lime obtained by a conventional process has been used everywhere for 2 the preparation of precipitated calcium carbonate and it has apparently been considered obvious that the base material is suitable as such and that no other alternatives are available.

It has been found that when slaked lime is used for the preparation for finely divided pigments, e.g. precipitated calcium carbonate (PCC), the particle size of the calcium hydroxide is of great significance. As has been stated in our previous patent application (cf. FI Patent Application No. 950411), calcium hydroxide having a particle size of less than 3 μm should be used for the preparation of PCC used as pigment in paper. Such a particle size is achievable e.g. by means of an impact mill.

It has now unexpectedly been found in connection with the present invention that a particularly advantageous result is obtained if the calcium oxide is slaked and wet milled in an impact mill, whereby a difference in circumferential speed of more than 50 m/s prevails. Hereby calcium hydroxide can be produced having a degree of hydration which right after the impact mill is more than 80 % and which is essentially totally slaked in 5 minutes. In this context, the difference in circumferential speed refers to a difference in the rotation speed of grinding rings of the impact mill arranged next to each other and rotating in opposite directions. During milling, particles of different sizes are produced, and the rough part is separated in a sorting section and returned to the impact mill. As a result, finely divided calcium hydroxide is obtained having a particle size which to 99 % is below 3 μm.

It has further been found in connection with the invention that particles having a size of less than 3 μm are apparently composed of a large number of smaller particles still, or can at least be broken into such particles in an economical way. Thus, according to a preferable embodiment of the invention, slaked lime is provided by a new multistep process, whereby during the first step, i.e. about 0J seconds, the lime can be slaked to approximately 80 %. The rest of the slaking is achieved by leaving the lime to settle or by mixing it as an aqueous slurry and by passing the as yet not slaked rough part which is deposited onto the bottom at least once more through the lime slaking apparatus described below.

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

The invention offers considerable benefits. Thus, the wet milling opens new particle and crystal surfaces which are highly reactive, whereby a slaking time of 90 % in less than 1 min is achieved. In less than 5 minutes slaking has taken place and the calcium hydroxide can be used in a further process e.g. for the preparation of PCC (precipitated calcium carbonate).

In the following, the invention is examined in more detail by means of a detailed description and the annexed drawing. The figure illustrates the principle of the apparatus used in the invention.

The apparatus according to the invention comprises a feed hopper 1 for burnt calcium oxide, from which calcium oxide is fed to a metering device 3 by means of a belt conveyor 2. From the metering device calcium oxide is fed into the mill hopper 6 of the impact mill 5 together with water from the water storage tank 4. The lime is slaked with water by feeding the lime slurry into said rapidly rotating impact mill 5 having rotor rings rotating in opposite directions, whereby the differences in circumferential speed may be as high as 200 m/s, typically, however, about 100 m/s. Such an apparatus typically has 3 to 7 rings of which 2 to 4 turn on the same axis into the same direction while every other ring rotates into the opposite direction on a different axis. The calcium oxide particle to be slaked has to travel along a path which accelerates in many directions and often collides with the rotors of the mixer, and it only takes < 0J s to follow the path. In typical full-scale operation the impact force imposed on the material flow is, for example, 47 kW, while the radial shear force is about 19 kW and in the intermediate gas a turbulence of 100 kW is generated when the apparatus has a no-load power of 40 kW.

In this process the hydrated particle is continuously shelled and unhydrated, free oxide surface is exposed from the core of the particle, said surface as such reacting very rapidly with water. As it is further known how much heat is released in the hydration of calcium oxide, it is evident that the rapid hydration achieved by impact milling is further accelerated, because the released heat and the great difference in temperature between the particle surface and the particle core further shell hydrated calcium hydroxide off the oxide surface.

The above-described shelling mechanism produces, conceptually speaking but above all, verified by measurements, calcium hydroxide having a very small particle size.

Calcium oxide is fed into the impact mill together with excess water whereby an aqueous slurry is advantageously formed having a dry matter content of approximately 5 to 40 %. The term "excess water" refers to the fact that there is more water than the stoichiometric amount required for slaking the burnt lime.

The obtained aqueous slurry which comprises an aqueous suspension of slaked lime is allowed to settle for at least a while in the settling vessel 7. Typically, an aqueous suspension containing calcium hydroxide is allowed to settle for 1 to 60 min, advantageously about 5 to 20 min. The slaking can be ensured by 5 min of final mixing. Then an extremely finely divided product is obtained whereby more than 99 % of its particles are smaller than 3 μm and the maximum of the particle size distribution is between 0J and 0.5 μm.

As the solubility of slaked lime in water only is about 2 g/1, the size of the calcium hydroxide particles here means the size of an insoluble solid particle in the aqueous suspension expressed by its approximative average diameter.

As stated above, a particle size distribution emerges in the product during slaking. The rough and heavy unslaked or nondegraded particles are recycled back to the impact mill. In practice, this is implemented by allowing the particles to deposit in the settling vessel

7 from which they are removed at its lower end and returned to the impact mill 5 via the conduit 8. The sorting which takes place in the vessel 7 can be performed by controlling the stream flow rate, i.e. by altering the flow rate of the bottom fraction, the sorted bottom fraction can be recycled together with the feed material through the impact mill.

The product can be used for the preparation of precipitated calcium carbonate pigments.

Hydrated calcium oxide produced by the method of the invention and the obtained product are well suited for producing precipitated PCC particles, independent of whether the production method comprises traditional precipitation using flue gases or other gases rich in carbon dioxide, or whether a product obtained from soluble carbonates by a causticizing reaction is concerned. The product can also be used for the preparation of calcium carbonate pigment when the source of carbonate ions comprises a soluble carbonate, such as ammonium, sodium or potassium carbonate.

The following non-limiting working example is provided by way of illustrating the invention:

Example

Slaked lime having a size of < 20 mm was impact milled in an apparatus driven by 2 x 20 kW electric motors, the circumferential velocity at the outermost ring being 45.4 m/s and at the innermost ring 38.5 m/s, and the feed rate being 750 kg/h for CaO and 6000 kg/h for water. Said mixture was passed through the impact mill described above, whereafter it was allowed to settle and the rest was allowed to react for 5 min. After this run, when the lime slurry left the mixer, its degree of hydration was 85 % and after 5 min of storage in a normal storage tank it was 100 %. Thus, the burnt lime was wet milled and slaked at the same time.

Ca(OH)₂ was obtained having a dry matter content of 12.5 % and 99 % of the particles had a size of less than 3 microns. A very typical particle size was from 0J to 1 microns. Several dozens of test runs were carried out with different mixing speeds and intermediate settling sortings.

After one single run particle sizes ranging from OJ to 4 microns were obtained, 0.1 being the predominant one. After four runs, a particle size distribution ranging from 0.1 to 0.5 microns was obtained, the degree of hydration being 100 %.

Claims

1. Process for preparing finely divided slaked lime, according to which process

- calcium oxide is conducted to an impact mill having a high shear rate together with an excess of water and the calcium oxide is slaked with water in said impact mill to produce an aqueous suspension containing calcium hydroxide, c h a r a c t e r i z e d by

- slaking the calcium oxide in an impact mill operated in such a way that the speed difference between the rotors is in excess of 50 m/s in order to produce calcium hydroxide having a degree of hydration after the impact mill of more than 80 %, said calcium hydroxide being essentially completely slaked within 5 minutes,

- allowing the aqueous suspension containing calcium hydroxide to settle, and

- passing the settling part of the aqueous suspension through the impact mill a second time.

2. The process according to claim 1, wherein the aqueous suspension containing calcium hydroxide is allowed to settle for 1 to 20 min.

3. The process according to claim 2, wherein the dry matter content of the aqueous suspension left to settle is set at 5 to 40 weight-%.

4. The process according to any of the claims 1 to 3, wherein the settling part of the aqueous suspension is reconducted through the impact mill by altering the flow rate of the bottom fraction of the sorting.

5. The process according to any one of the preceding claims, wherein the product is used for preparing precipitated calcium carbonate pigments.

6. The process according to any one of the preceding claims, wherein the product is used for preparing calcium carbonate pigments when the carbonate source comprises flue gas 8 or another gas rich in carbon dioxide or pure carbon dioxide.

7. The process according to any one of the claims 1 to 5, wherein the product is used for preparing calcium carbonate pigments when the source of carbonate ions comprises a soluble carbonate, such as ammonium, sodium or potassium carbonate.

8. An extremely finely divided slaked lime, characterized in that it is prepared by a process according to any one of the claims 1 to 7.

9. The lime according to claim 8, wherein its particle size distribution is essentially smaller than 0.3 ╬╝m.