FI65552C - FRAMSTAELLNINGSFOERFARANDE FOER MINERALFYLLAEMNEN GENOM VAOTFINFOERDELNING - Google Patents

Abstract

1. Process for the manufacture of mineral fillers, especially natural calcium carbonates, by wet grinding, whereby 70-80% or more of the particles are < 1 mu m but a maximum of 15% of the particles are < 0.2 mu m, characterised in that the grinding is carried out without dispersant and at a solids concentration of less than 60% by weight, in that the ground suspension is filtered and the resulting filter cake which contains about 20% water is either dried or, by the addition of a dispersant, converted to a suspension of lower viscosity.

Description

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Method for the production of mineral aggregates by wet grinding - Framställningsförfarande för mineralfyllämnen genom vät-finfördelning

The present invention relates to a process for the preparation of wet-milled mineral fillers, in particular natural calcium carbonates, in which 70 to 80 * or more of the particles are <1, but not more than 15% of the particles are <0.2.

Wet road grinding processes for minerals, especially natural non-calcium carbonates, are derived from two methods known for a long time. In a smaller method, the aqueous suspension is reduced in a horizontal ball mill and the coarsest particles are separated by centrifugation. This process, which is used with several minerals (calcium carbonate and kaolin), is described in detail, for example, in U.S. Pat. No. 2,323,550. The final product is obtained as a fine centrifugation fraction. The aqueous suspension is then filtered and the product is dried by known methods.

Dispersants do not have to be used in this method of ball milling because the degree of grinding obtained is not very high and because there is no need to operate at elevated dryer concentrations that would differ during centrifugation during the decomposition step. The concentration of the ground product in this case is less than 60%.

The disadvantage of this method is the limited fineness, since, for example, natural calcium carbonate can only be used economically for products in which 40 to 50% by weight of the particles are less than 2 [mu] m and 20 to 25% by weight of the particles are less than 1 [mu] m. However, at this fineness, such calcium carbonates cannot be used in significant amounts in paper coating.

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When comminution should be used at higher dry matter concentrations above 60%, and when subsequent classification of the particles is to be avoided, dispersants are used, for example salts of polyphosphoric acid, polysilicic acid, polyacrylic or polymethacrylic acid or products of alkylaryl sulfonates, lignosulfonates, alginates.

These substances reduce the viscosity of aqueous solutions and facilitate the dispersion of solids. A similar method has already been described in GB-PS 1,215,576.

In the newer method, wet-road grinding is carried out in microspheres or bead or sand grinders, the axes of which are generally vertical and in which the diameter of the grinding pieces is in the order of 0.5 to 5 mm, in contrast to the ball mills used in the previous method. 80 mm.

By using such grinders, it is possible to achieve higher degrees of fineness than ball mills. Thus, it is possible to achieve a fineness in which 95% by weight of the particles are less than 2 μm, 70-80% by weight of the particles are less than 1 μm and 25-35% by weight of the particles are less than 0.2 μm. The viscosities of the aqueous solutions are then such that dispersants must be used to reduce the viscosity. The disadvantage of this method is that while the percentage of particles smaller than 1 μm increases, so does the percentage of very fine particles, i.e. particles smaller than 0.2 μm (25-35%). The use of their finest particles is detrimental to chalk paper and colors (see DE-OS 28 08 425). French patent application 78,307 23, filed October 30, 1997, describes the coating of paper in which natural calcium carbonate is used as the sole pigment and in which high concentrations are used in the coating process.

German patent application P 29 43 652.0-43 describes mineral fillers in which the ratio% <1 μm R% <0.2 μm is greater than 3.5 and preferably between 4 and 10. In this case, it should be noted that those particles smaller than 0.2 pm, the percentage should be as small as possible and that the percentage of particles smaller than 1 pm should be as large as possible.

Furthermore, FR-A 2,347,417 * DE-A 2,715,721 discloses a method for producing an ultrafine lime slurry slurry which is useful as a filler and which does not contain hard abrasive particles.

The purpose of the above publication is to create improved fillers for polymer-based pulps, such as plastic and rubber pulps, for example polyvinyl chloride pulps, and to create pulps for coating paper or the like containing fillers containing finely ground lime slurry with at least 95% by weight of particles. a diameter of not more than 5 μm and not less than 40% by weight, and preferably 70% by weight, the particles have a ball equivalent diameter of not more than 2 μm and not more than 0.5% by weight of the particles have a ball equivalent diameter of more than 10 μm, preferably 5 μm (page 9, paragraph 2).

The solution to this problem is that the lime slurry is ground in an aqueous suspension with a grinding agent consisting essentially of calcium carbonate, which in that form has a compressive strength which is higher than that of the slurry.

Finally, FR-A 2,272,154 discloses a method for improving the optical properties of synthetic Na-Al silicates used in paper. According to the pearl mill principle, these Na-Al silicates are ground using 1-2 mm glass balls at a dry matter content of 10-40.

The use of the method is described only for synthetic minerals. In the case of synthetic Na-Al silicates, it is not a question of actual fine grinding, but of the reduction of aggregate parts in the finished product. This naturally achieves a slightly better fineness, which when used in paper gives better optical properties.

It is an object of the present invention to provide a process for the preparation of mineral fillers, in particular natural calcium carbonate, containing as many particles as possible of less than 1 [mu] m (70-80% and above) and as few as possible of the most fine-grained particles of less than 0.2 [mu] m. (up to 15 S).

It is a further object of the invention to be able to prepare suspensions in which the dry matter concentrations are very high, i. Up to 80%, which is a great advantage when the paper is coated by a method, the so-called HCS method, in which the dry matter concentrations are high.

These objects are achieved according to the invention by grinding without a dispersant and with a dry matter concentration of less than 60% by weight, filtering the comminuted suspension and either drying the filter cake containing about 20% water or converting it to a lower viscosity suspension by adding a dispersant.

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According to the invention, it has been found that the dry matter concentration of the comminution step strongly influences the formation of very fine or finest particles (less than 0.2 μm) in a comminution device equipped with microparticles. The lower the dry matter concentration of the natural calcium carbonate suspension, the higher the ratio of% of particles <1 nm% of particles <0.2 pm This rule applies to all mineral fillers, for example natural alkaline earth carbonates, regardless of whether it is chalk, size chalk, marble or dolomite . This condition is indicated in Table I below.

Table I

Grinding of natural limestone

Dry matter concentration% <χ concentration% of particles% of particles R - in comminution_ <1 μι »_ <0.2 μm_% <0.2 μm 76% 73.5 28 2.62 72% 73 26 2.81 70% 75 24 3.12 65% 72.5 18 4.01 55% 73 15 4.86 40% 74 9 8.22 30% 72.5 7 10.35

The table shows that with the percentage of particles smaller than 1 μm remaining almost constant (between 72.5 and 74%), the lower the percentage of particles smaller than 0.2 μm. suspension concentration. The ratio S 65552% <1 pm% <0.2 pm increases from 2r62 to 10.35. In order to obtain aqueous suspensions of natural calcium carbonate containing 72 to 76% or more dry matter, as required in the chalk paper industry, and having a fineness of less than about 1 to about 70% of the particles, a dispersant (e.g., the early salt type of polyacrylic acids) must be used. , in order to obtain powdered calcium carbonate, most of this dispersant is wasted.

According to the invention, it has been found that, in contrast to conventional methods, it is possible and advantageous to carry out comminution without a dispersant, for example according to the following embodiment:

Working Example:

Of natural calcium carbonate (chalk or limestone), pre-ground, i.e. containing, for example, 1-10% by weight of particles larger than a screen size of 325 (mesh size 43 μm), an aqueous suspension is prepared. The degree of fineness of the pre-comminuted material is not very important, as it is sufficient that the time or power of the subsequent comminution step be adjusted accordingly.

The dry matter concentration of the suspension to be comminuted is chosen so that the viscosity of the suspension, without a dispersant, allows a good flow of the substance in a ball-operated comminution device so that the desired fineness is obtained with the final product. Select a dry matter concentration that is lower the higher the desired ratio 7 65552% <1 pm R »-% <0.2 μτη

It can thus be between 25 and 60% dry matter. This suspension is comminuted without a dispersant in a microparticulate comminution device with discontinuous or continuous operation, in a single flow-through, or preferably by discontinuous circulation, until the desired fineness is obtained.

One feature of the invention is thus to provide comminution without the use of a dispersant but with a sufficiently low viscosity, i. with a low dry matter concentration, in which case small diameter grinding pieces can be used in the comminuting device, which is advantageous from the point of view of grinding yield. For example, balls with an average diameter of 0.4 to 3 mm are used. Balls made of glass, Ottawa hleka, Buzzard-Leighton sand, or alumina or zirconia balls can be used. The disadvantage in this case, however, is that it is difficult to avoid catching the grinding balls with the final product. According to the invention, this difficulty is avoided by calculating the dry matter concentration of the suspension, i. the viscosity is reduced.

After the calcium carbonate suspension is comminuted without a dispersant, the product is filtered through a filter press, tube filter, fabric filter, slit filter or vacuum filter to give a filter cake or filtrate paste containing about 20% water, which would be impossible to use when dispersing.

The resulting filter cake can then be either dried or converted to a lower viscosity suspension by the addition of a dispersant. In the former case, the drying of β 65552 takes place in the usual way. This gives a powdered product which does not contain a dispersant, which is an important advantage in terms of manufacturing costs. In the latter case, when it is desired to obtain the final product to be delivered in liquid form, it is sufficient to obtain a liquid having the desired viscosity. This operation is carried out in a continuous mixer, screw conveyor or other processing device.

It has been found that in carrying out the invention, less dispersant is required for a particular product than if it were added in the comminution step. The saving of the dispersant is of the order of 20%.

Example 1 of PR-A-2,347,417 = DE-A 2,715,721 starts from a dry matter content of 43.7% by weight and no dispersant is used. However, it cannot be learned by one of ordinary skill in the art in accordance with the invention for technical processing based on three characteristics. Rather, it is a pure coincidence in this publication that 43.7% by weight of dry matter is used in the example, which is less than 60%, and that no dispersant is mentioned. On page 5 of the French publication, lines 20 to 33 of the description of the method, it is mentioned that in wet milling, a dispersant is usually added in order to grind at a high dry matter content. These are probably two important characteristics of a functioning wet mill. Accordingly, the ordinary person skilled in the art thus performs wet milling with a high dry matter content using dispersants.

This publication does not give the average person any indication that wet milling should be consciously performed without dispersants and at a low dry matter content, and that products with special properties can be obtained, as described in the attached application.

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Also, FR-A 2.272.154 does not teach one of ordinary skill in the art that wet milling must be performed consciously without dispersants and at a low dry matter content in order to achieve the favorable properties set forth in the accompanying application.

The advantages obtained according to the invention can be summarized as follows: 1. Products are obtained in which the ratio% <1 μm% <0.2 μm is very high, in particular more than 3.5, i.e. for example, 85% of the particles are smaller than 1 μm 10% or less of the particles are smaller than 0.2 μm, which has not been possible with the hitherto known methods for natural calcium carbonate.

2. Powdered products are obtained without dispersants.

3. Concentrated suspensions are obtained with a concentration of 75 to 80% by weight or more and containing a minimal amount of dispersant.

Claims (3)

65552 ίο A process for the preparation of wet-milled mineral fillers, in particular natural calcium carbonates, in which 70 to 80% or more of the particles are <1 μm but not more than 15% of the particles are <0.2 μm, characterized in that the grinding is carried out without a dispersant and with a dry matter concentration of less than 60% by weight - that the comminuted suspension is filtered and the filter cake containing about 20% water is either dried or converted to a lower viscosity suspension by the addition of a dispersant. Method according to Claim 1, characterized in that grinding bodies with a small diameter, preferably 0.4 to 3 mm, are used. Process according to Claim 1 or 2, characterized in that suspensions are prepared which contain 80% or more of dry matter.