DE2660425C2 - Process for the production of a paper filler and / or coating pigment - Google Patents

Description

The invention relates to a process for the production of a paper filler and / or coating pigment, wherein dolomite is converted ^{into a mixture of calcium oxide and magnesium oxide by heating to about 1000 to 1100 0} C, the mixture obtained is suspended in water and practically the all calcium hydroxide is converted into the calcium carbonate.

A method of the type mentioned is known from DE-OS 19 54 205. Calcined dolomite is quenched in water and carbon dioxide is added to the quenched material, the carbon dioxide only being added until the Ca (OH) ₂ content of the solids has been converted into the carbonate. The known method works with a dilution which corresponds to a ratio of 3.5 parts of water: 1 part of oxide. Under these conditions, a pigment is obtained which essentially consists of magnesium hydroxide containing up to 5% calcium.

The object of the present invention is to provide a method for producing a paper filler and / or to specify coating pigment, in which a process product is obtained, which is made of fine-grained predominantly platelet-shaped basic calcium-magnesium carbonates exists and can be carried out without digestion chemicals and special reactants is. To solve this problem it is proposed that the calcium and / or magnesium oxide mixture in a weight ratio of at least 20 parts of water: 1 part of oxide is suspended in water, which in hydroxides formed in the suspension by introducing carbon dioxide into basic calcium magnesium carbonate be converted back, and that by the addition of a dispersant after the concentration of the Suspension a stable suspension with a high solids content is produced.

It has been shown that a precipitated fine-grained, predominantly platelet-shaped basic calcium magnesium carbonate, as it is obtainable by the process according to the invention, excellent as Mass additive and / or as a coating pigment for the manufacture! on glossy papers. The one with kaolin or satin white coated papers are surpassed in terms of their gloss and print quality.

Further advantages of using the precipitated basic calcium magnesium carbonate are the improved opacity and, at the same time, the higher whiteness of the treated paper. The fineness of the calcium-magnesium carbonate is 100% <1 μm, its specific surface area is in the order of 20 m ² / g. It has a whiteness of> 95%. The shape factor is a power of ten greater than that of kaolin, which is special

ίο is due to platelet shape

Carbon dioxide-containing waste gases from dolomite calcination can be used for the recarbonization, so that no special supply of carbonic acid is required. It goes without saying, however, that pure CO ₂ or a carbon dioxide-nitrogen mixture of other origins can also be used for the fumigation. The calcination of the dolomite is advantageously carried out in a rotary kiln.

The invention is illustrated below with the aid of 2 exemplary embodiments and a comparative example explained in more detail.

example 1

Dolomite is heated to about 1100 ° C. in a rotary kiln, whereby a CaO-MgO mixture is obtained. This oxide mixture is extinguished with wasber in a weight ratio of about 1 × 20 and at the same time intensive mixing with the cleaned exhaust gases produced in the rotary kiln to form a basic one Calcium magnesium carbonate recarbonised. The suspension then containing approximately 8% solids concentrated on a centrifuge or concentrated on passage through a filter press. After adding a Dispersant a stable suspension is obtained.

Example 2

A CaO-MgO mixture obtained by burning dolomite is after addition of 5% CaO turned off. The re-carbonization of CaO and MgO

takes place during pumping with a circulating pump by adding CO ₂ , with particularly good absorption taking place. After the suspension has been concentrated using a filter press, a stable suspension is obtained by adding dispersants.

In both examples, the basic calcium-magnesium-carbonate predominantly falls Part in a plate-like structure.

Comparative example

For comparison with the prior art, a comparative experiment was carried out, a pigment according to DE-OS 19 54 205 was produced analogously to Example 1, page 6, as follows:
2000 g of dolomite were extinguished in 6000 ml of water at 80 ° C. with stirring. Another 1000 ml of cold water was then added in portions to the reaction vessel during quenching to control the severity of the reaction. The slurry solids were left at 60 ° C for 24 hours with stirring

M) hydrated and then sieved through a sieve with a mesh size of 0.074 mm to remove coarse particles, whereupon the slurry volume was adjusted to 9 liters with water.
Using a laboratory turbine mixer, the

b5 CA (OH) 2 portion of the solids with a mixture of 20 volume% CO2 and 80 volume% air at a rate of 1 volume of gas per volume Slurry converted into the carbonate per minute.

The progress of carbonization of Ca (OHJj was monitored by measuring the pH value and taking samples for X-ray diffraction at the same time. at a pH value of 10.6 shows the Ron .genbeiffungsdiagram, that no more Ca (OH) 2 was present (sample A). All of the calcium was in the form of Calcium carbonate.

For comparison, according to the invention A method of producing a pigment according to claim, wherein the oxide mixture in the weight ratio suspended from 20 parts of water to 1 part of oxide in water and carbon dioxide in the suspension for as long was introduced until the magnesium carbonate (hydromagnesite) had completely formed. From Sample B taken from the suspension (pH 7.6)

ι ο

showed characteristic according to the X-ray diffraction diagram Hydromagnesite and calcium carbonate peaks. The diagram also showed that no Magnesium hydroxide was more present in sample B.

This was followed by material from samples A and B for scanning with the scanning electron microscope prepared. For this purpose, part of the sample was diluted with ethanol and dried on glass. There were Recordings of each sample taken at a scale of 7500: 1. Sample B (product according to the invention) exhibited particles with a platelet-shaped structure. The sample A consisted of finely divided particles whose longest dimension was in the range of less than 2 μίτι, As in the description of DE-OS 19 54 205 on page 5.2. Abs "is required.

1 sheet of drawings

Claims (1)

Claim: Process for the production of a paper filler and / or coating pigment, where dolomite is converted ^{into a mixture of calcium oxide and magnesium oxide by heating to about 1000 to 1100 °} C., the mixture obtained is suspended in water and practically all of the calcium hydroxide is converted into calcium carbonate by introducing carbon dioxide is characterized in that the calcium and magnesium oxide mixture is suspended in water in a weight ratio of at least 20 parts of water to 1 part of oxide, the hydroxides formed in the suspension are converted back into basic calcium-magnesium carbonate by introducing carbon dioxide,
and that by adding a dispersing agent after the suspension has been concentrated, a stable suspension with a high solids content is produced.