DE102010053164A1 - Grinding solid particles in a stirred ball mill comprises providing a solid particle suspension in the stirred ball mill and promoting through the stirred ball mill at intervals, by alternating promoted solid particle suspension - Google Patents

Abstract

Grinding solid particles in a stirred ball mill comprises providing a solid particle suspension in the stirred ball mill and promoting the solid particle suspension through the stirred ball mill, at intervals, by alternating the promoted suspension (interval A) or non promoted suspension (interval B).

Description

Field of the invention

The invention relates to a method for improved in terms of quality and flexibility continuous Rührwerksmahlung of solid particles, in particular of titanium dioxide.

Technological background of the invention

An agitator mill consists in principle of a container which partially with spherical grinding media of z. As ceramic, steel, glass or specially prepared sand is filled and in which, for example, a shaft with several, arranged in stages discs rotates. The millbase suspension is subjected to shear, pressure and impact forces, which cause dispersion and deagglomeration or comminution of the comminuted particles. At the mill outlet, the grinding media are separated from the millbase suspension. In principle, an agitator mill can be operated batchwise or continuously. Agitator mills as such are known and are commonly used in continuous operation for the comminution or deagglomeration of solid particles, in particular titanium dioxide particles (eg. US 5,356,470 ).

In the case of agitator grinding, the desired fineness of grinding can be influenced by the type, size, density and quantity of the grinding media, by the shaft speed, the density of the suspension and by the throughput. In a continuous process, a batch can also be pumped several times through the mill, either in the so-called passages or circulating mode. Passage operation means that the grinding stock was completely pumped through the mill before re-feeding. During circulation, the millbase suspension is pumped continuously.

In batch mulling, all solid particles of the suspension remain the same length in the grinding chamber, resulting in a relatively narrow particle size distribution depending on the starting material. However, batch processes in practice have the disadvantage that the mills overheat in a short time and after each batch completely emptied and the grinding media (for example, sand) must be refilled again. This procedure is very costly and time consuming.

In the continuous process, however, the millbase suspension flowing continuously through the mill during the grinding process in the agitator mill is subject to a mixing process which causes portions of the suspension to remain in the grinding space for different lengths of time. The result for the particles is a residence time distribution and thus also a stress distribution.

In the production of titanium dioxide pigments, the absolute particle size and the particle size distribution have a decisive influence on the optical properties of the finished pigment, for example on the whitening power (TS) and the gloss. Coarse fractions impair the gloss and too fine fractions as well as a too broad particle size distribution reduce the lightening power. It is desirable to have a very narrow particle size distribution in the range of 0.2 to 0.4 microns. Usually titanium dioxide base body particles are ground before the final coating with inorganic and / or organic compounds so that they have the smallest possible particle size distribution.

Generally, in a single pass through the mill (a "pass"), the residence time distribution is most unfavorable and a relatively broad particle size distribution is achieved. With an increase in the number of passages or cycles, an improvement, ie a narrowing of the residence time distribution is achieved, and thus a narrower particle size distribution, with multi-pass milling being more favorable than circulation grinding with the same number of circuits and passages. However, the Mehrpassagen- like the Kreislaufmahlung additional equipment costs such as additional piping, stacking containers, optionally further mills and with the same overall throughput especially an increased individual throughput through the mill. In particular, in vertical, open 1000-liter sand mills (for example, type PM1000, Draiswerke GmbH, with Ottawa sand 20/30 and 30/40 as grinding media), however, a throughput of about 7 m ³ / h can hardly be realized because no ideal flow is formed in the mill, but rather channeling takes place. As a result, after a certain start-up time, the grinding media, the so-called "spitting", will rise.

Task and brief description of the invention

The invention is therefore based on the object to provide a method for continuous Rührwerksmahlung, with the help of which a narrower particle size distribution can be achieved.

The object is achieved by a method for grinding solid particles in a stirred mill, characterized in that a) a solid particle suspension is provided and b) the solid particle suspension is conveyed through the agitator mill at intervals by alternately conveying the suspension (interval A) or not is promoted (interval B).

Further advantageous embodiments of the invention are specified in the subclaims.

The invention relates to a simple and flexible method for operating agitator mills, with the aid of which solid particle grinding with narrow particle size distributions can be produced. In particular, titanium dioxide pigments with improved optical properties such as whitening power, spectral characteristics and gloss can be produced by the process according to the invention.

Description of the invention

The invention is based on the recognition that a multi-pass milling compared to a Kreislaufmahlung and especially compared to a "normal" Einpassagemahlung - provides a more favorable residence time distribution for the particles. The residence time distribution directly affects the particle size distribution of the milled particles. A narrower residence time distribution leads to a narrower particle size distribution. These relationships are discussed in the article "Agitator Bead Mills for Dispersing and Comminution - Applications for Ceramic Processing" (cfi ceramic forum international / Ber. DKG 86 (2009), p. E23 to E28) shown.

The residence time distribution is usually represented as a function of the de-dimensioned time θ, where θ = t / τ with the time t and the average residence time in the mill τ. The 8th and 9 of the mentioned article show the residence time distribution E (θ) in a typical agitator mill for single and multi-pass grinding as well as for cycle grinding.

The inventive method includes a Einpassagen grinding method in which the mill inflow is interrupted at regular intervals (intervals). By interrupting the flow is surprisingly a cheaper d. H. a narrower residence time distribution. Thus, the inventive method combines the advantages of Einpassagenverfahrens (no additional space and no investment costs because of additional equipment) with those of the multi-pass method (tighter residence time distribution).

By means of the interval mode of operation according to the invention, the floating of the grinding bodies is reduced in vertical, open sand mills, since in the conveying pauses the grinding bodies can flow back into the lower part of the mill. As a result, a more homogeneous distribution of the grinding media in the grinding chamber is achieved over the entire grinding period. Since increased wear on the internals occurs in a sand mill typically in the upper and in the lower part of the mill, where the grinding media compacts, the procedure according to the invention can also lead to reduced or at least even wear.

In the context of the invention, interval A is understood to be the period during which the suspension is conveyed through the mill. Interval B is the period during which the delivery is interrupted, that is, for example, the delivery pump is switched off. In a particular embodiment of the invention, the interval times A and B are each the same length.

In an alternative embodiment of the invention, the interval time B is approximately twice to four times the interval time A.

The dimensioning of the interval times is not fixed in principle, but must be within the framework that the interval time B must not be too short, so that a significant interruption of the flow occurs. On the other hand, the interval time A may not be so long that the residence times approach the state of Einpassagemahlung. Preferably, the interval times A and B are each about 10 to 100 seconds.

Preferably, the suspension contains a dispersing agent, for example sodium hexametaphosphate.

Examples

The invention will be explained with reference to the following examples; However, this is not to be understood as limiting the invention.

residence

The residence time distribution when carrying out the process according to the invention in comparison to Einpassagenmahlung was determined by means of impact marking with KCl solution in deionized water. 10 ml of a highly concentrated KCl solution (330 g / l KCl) were added by pulse to the demineralized water by means of a syringe at the inlet of the sand mill. At the exit of the sand mill, the KCl concentration was continuously measured over the resistivity of the suspension.

The residence time distribution was determined for the following operating modes: continuous single-pass milling (reference) and interval grinding according to the invention (A / B in seconds) with 15/15, 30/30, 60/60, 120/120, 30/90, 60/180 (Tab. 1, 1 to 3 ). The total throughput (intervals A + B) was 10 l / h.

From the measured data, the mean residence time τ within the mill and the variance σ ^{2 of} the mean residence time for the different operating modes were determined (Table 1). Table 1 Interval A Interval B average residence time τ Variance σ ² 15s 15 s 382 s 28355 S. Figure 1 30 s 30 s 401 s 33256 60 s 60 s 434 s 32217 s. FIG. 2 120 s 120 s 458 s 35716 30 s 90 s 499 s 35783 s. FIG. 3 60 s 180 s 478 s 38816 Continuous (reference) 405 s 46067 s. FIGS. 1, 2, 3

The variance σ ² can be considered as a measure of the width of the residence time distribution: smaller values mean a narrower distribution.

The 1 to 3 each show the residence time density distribution E (θ) as a function of the de-dimensioned time θ for 3 different operating modes (see Table 1), the bold curve representing the respective operating mode according to the invention and the thin curve the reference.

The 1 to 3 show that the Intervallmahlverfahren invention leads to a much narrower residence time distribution compared to the reference Einpassagenmahlverfahren.

Pigment properties (whitening power TS)

The invention was further realized by way of example with an aqueous suspension of titanium dioxide base material from the chloride process. The suspension had a TiO 2 concentration of 500 g / l and contained 0.1% by weight of sodium hexametaphosphate as a dispersant. The pH was 11. The suspension was conveyed through a vertical sand mill (type PM5, Draiswerke GmbH).

• a) Als Referenz wurde die Suspension in einer Passage mit einem Durchsatz von 5 kg TiO2 pro Stunde gemahlen. Anschließend wurde weitere Suspension erfindungsgemäß jeweils in folgenden Intervallen (A/B in Sekunden) gemahlen: 10/10, 30/30, 120/120, wobei der Durchsatz in Intervall A jeweils 10 kg TiO2 pro Stunde betrug (Tab. 2).
• b) Als Referenz wurde die Suspension in einer Passage mit einem Durchsatz von 10 kg TiO2 pro Stunde gemahlen. Anschließend wurde weitere Suspension erfindungsgemäß jeweils in folgenden Intervallen (A/B in Sekunden) gemahlen: 10/10, 30/30, 120/120, wobei der Durchsatz in Intervall A jeweils 20 kg TiO2 pro Stunde betrug. Schließlich wurde weitere Suspension erfindungsgemäß in folgenden Intervallen (A/B in Sekunden) gemahlen: 10/30, wobei der Durchsatz in Intervall A 40 kg TiO2 pro Stunde betrug (Tab. 3).

Tabelle 2 Intervall A Intervall B Intervall A Intervall B Gesamtdurchsatz TS Durchsatz Durchsatz Zeit (s) Zeit (s) (kg/h) (kg/h) (kg/h) .. 10 0 10 10 5 104,3 10 0 30 30 5 104,7 10 0 120 120 5 104,1 Referenz 5 103,8 Tabelle 3 Intervall A Intervall B Intervall A Intervall B Gesamtdurchsatz TS Durchsatz Durchsatz Zeit (s) Zeit (s) (kg/h) (kg/h) (kg/h) 20 0 10 10 10 104,4 20 0 30 30 10 104,7 20 0 120 120 10 104,1 40 0 10 30 10 104,6 Referenz 10 102,8 To produce titanium dioxide pigment patterns, the milling was carried out as follows:

• a) For reference, the suspension was ground in a passage at a rate of 5 kg TiO 2 per hour. Subsequently, further suspension according to the invention was ground in each case at the following intervals (A / B in seconds): 10/10, 30/30, 120/120, the throughput in interval A being 10 kg TiO 2 per hour (Table 2).
• b) For reference, the suspension was ground in a passage at a rate of 10 kg TiO 2 per hour. Subsequently, according to the invention, further suspension was in each case ground at the following intervals (A / B in seconds): 10/10, 30/30, 120/120, the throughput in interval A being 20 kg TiO 2 per hour. Finally, further suspension according to the invention was milled in the following intervals (A / B in seconds): 10/30, the throughput in interval A being 40 kg TiO 2 per hour (Table 3).

Table 2 Interval A Interval B Interval A Interval B Total throughput TS throughput throughput Time (s) Time (s) (Kg / h) (Kg / h) (Kg / h) .. 10 0 10 10 5 104.3 10 0 30 30 5 104.7 10 0 120 120 5 104.1 reference 5 103.8 Table 3 Interval A Interval B Interval A Interval B Total throughput TS throughput throughput Time (s) Time (s) (Kg / h) (Kg / h) (Kg / h) 20 0 10 10 10 104.4 20 0 30 30 10 104.7 20 0 120 120 10 104.1 40 0 10 30 10 104.6 reference 10 102.8

Subsequently, the titanium dioxide particles were coated in a customary manner with 0.1% by weight of SiO ₂ and 3% by weight of Al ₂ O _3, based on the total pigment, and ground in a jet mill with the addition of 0.25% by weight of trimethylolpropane (TMP). The whitening power (TS) was determined according to the following procedure on the titanium dioxide pigment particles treated in this way:
The whitening power of the pigment is reduced after incorporation into a black paste DIN 53165 determined at 17% pigment volume concentration. The gray paste pasted on an Automatic Muller is applied to a white Morest card. With a HunterLab Colorimeter PD-9000, the reflectance values of the layer in the wet state are determined. The derived TS value is based on an internal standard.

The test results (Tab. 2, 3) show that the interval grinding method according to the invention leads to a significantly increased whitening power (TS) of the titanium dioxide pigment in comparison with the reference single pass milling method.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 5356470 [0002]

Cited non-patent literature

• "Agitator Bead Mills for Dispersing and Comminution - Applications for Ceramic Processing" (cfi ceramic forum international / Ber. DKG 86 (2009), p. E23 to E28) [0012]
• DIN 53165 [0029]

Claims (6)

Process for the milling of solid particles in an agitator mill, characterized in that a) a solid particle suspension is provided and b) the solid particle suspension is conveyed through the agitator mill at intervals by alternately conveying the suspension (interval A) or not conveying it (interval B) , A method according to claim 1, characterized in that the solid particles are titanium dioxide. A method according to claim 1 or 2, characterized in that the interval times A and B are the same. A method according to claim 1 or 2, characterized in that the interval time B is twice to four times the interval time A. Method according to one or more of claims 1 to 4, characterized in that the interval times A and B each amount to about 10 to 100 seconds. Method according to one or more of claims 1 to 5, characterized in that the solid particle suspension contains a dispersant, preferably hexametaphosphate.

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