DE2012709C3 - Process for crushing calcium carbonate mineral - Google Patents

Description

The invention relates to a further development of the method for comminuting or grinding Calcium carbonate mineral according to patent 1 507 514. z. B. marbled calcite, chalk or calcite marble do not have a surface crystal structure, are obtained in large quantities and for many purposes, for example as fillers or pigments used for paints, ceramics, plastics and paper.

It is known to grind calcium carbonate minerals in a ball mill after previous coarse comminution in order to reduce the partial size of the substance. However, the conventionally obtained end product has a brightness value of about 85%, measured in percent of the light reflection at a wavelength of light of 458 millimicrons, and contains about 25 to about 35 percent by weight of particles that are smaller than 2 microns in spherical diameter and less than 1 percent by weight of particles smaller than 0.5 microns in spherical diameter. Although the product is suitable for various of the above purposes, it is not suitable for use as a pigment in paper coatings because it is normally not sufficiently white and does not consist of sufficiently fine particles. For example, the gloss value of a sheet of paper which has been provided with a coating agent using such a product as a pigment, using a conventional adhesive system, has 10 TAPPI units. In general, calcium carbonate is therefore obtained as a pigment for paper coating by chemical precipitation, which leads to a much lighter and finer product.
The method of the parent application for comminuting calcium carbonate mineral consists of grinding an aqueous suspension of the mineral containing a dispersing agent with which a grinding material is admixed. It is characterized in that the weight of the calcium carbonate mineral is at least 25% of the weight of the aqueous suspension (excluding the grinding material), that the grinding material also has a particle size on the order of 1.2 cm to 0.15 mm, and that the grinding material is added to the calcium carbonate mineral to be crushed in a weight ratio of 2: 1 to 5: 1.

With the method in particular improvements in the optical properties of z. B. Chalk achieved, however, the special conditions are

such that it is not always possible to get optimal results.

With the present process it is now possible to use calcium carbonate minerals, such as. B. chalk, rational to grind, whereby a product is obtained that is finer and compared to products of the known Grinding methods has an improved brightness and at the same time a desired low Share of binders, such as starch or casein, has to keep the whiting chalk on the paper surface to be noted, also a low oil absorption and rheological properties of common commercial products of chalk pigments.

The method according to the invention is characterized by the following work stages:

a) Formation of an aqueous suspension with a content of at least 25 percent by weight of the Calcium carbonate minerals;

b) Grinding the suspension together with a certain grinding aid consisting of particles

with a particle size of 150 microns to 0.63 cm and their proportion as high is that the volume of the grinding aid material is at least half the volume of the Suspension corresponds, and where the suspension

is married until at least 183.75 Kilowatt hours of energy per ton of dry calcium carbonate mineral are consumed.

The calcium carbonate mineral should preferably not contain particles larger than No. 8 BS sieve (2.00 mm) are especially not particles larger than BS No. 52 (300 microns) sieve. If necessary, the mineral is pre-crushed prior to treatment with the method according to the invention,

z. B. by coarse grinding.

The product obtained according to the invention generally contains at least 70 percent by weight Particles that are smaller than 2 microns corresponding to a sphere diameter and at least! 5 Gef> 5 weight percent of particles smaller than 0.5 microns are corresponding to a ball diameter. A sheet of paper that has been treated with a coating agent is what the product according to the invention

contains as pigment and a common binder, has a gloss value of at least 25 TAPPI units.

The aqueous suspension of the calcium carbonate mineral preferably contains a water-soluble, non-glue-like dispersant. In this case the suspension can be up to about 75 percent by weight preferably at least 40 percent by weight of calcium carbonate contain. Dispersants used with preference are water-soluble salts with macromolecular salts Anions and with an average molecular weight of 100 to 10,000. Particularly water-soluble salts with organic polymeric anions with an average Molecular weight from 100 to 10,000. For example sodium, potassium and ammonium salts of polyacrylic acids and polymethacrylic acid. Other suitable dispersants with macromolecular anions and a molecular weight of 100 to 10,000 are water-soluble salts of polyphosphoric acids, Polysilicic acids or water-soluble dinaphthyl methanesulfonates, lignosulfonates and alginates. the The required amount of such dispersants is generally between 0.05 and 0.60 percent by weight, calculated on the dry matter of the calcium carbonate mineral. Unless the suspension is not a dispersant or less than 0.05 percent by weight, it can contain up to about 40 percent by weight Contain calcium carbonate mineral.

The milling material should consist of particles which have a particle size of 150 microns up to 0.6 cm in diameter, preferably 500 microns up to 2 mm in diameter. In order to achieve good results, it is important that the particle size is in the specified range. The proportion of the auxiliary grinding material should be so high that the volume corresponds to at least approximately half the volume of the aqueous suspension of the calcium carbonate mineral. The volume should preferably not be more than IV ₂ times the volume of the suspension of the mineral. Suitable grinding materials are silica sand, e.g. B. Ottawa sand, a calcined clay, grains of flint, a ceramic material, or a glass or grains of synthetic polymeric material.

The grinding of the aqueous suspension should be carried out until it is at least in it 183.75 kilowatt hours of energy per ton of dry calcium carbonate mineral are used up. In general, the amount of energy consumed will not exceed 367.50 kilowatt hours or not exceed 735 kilowatt hours. The grinding can be done in any suitable device, preferably a sand mill.

example 1

Raw chalk was first coarsely ground to remove flint contamination. The grinding was carried out in an aqueous suspension for 15 minutes with a content of 70 percent by weight of dry chalk and 0.25 percent by weight of a sodium polyacrylate as a dispersant, calculated on the weight of the dry chalk. Then the Suspension sieved to remove all particles above 53 microns.

A sample of the flint-free suspension containing 9 kg of dry chalk, corresponding to a dry chalk concentration of 70 weight percent, was placed in a sand mill along with 20.8 kg of Ottawa sand a particle size of 500 microns to 2 now brought. The volume of the aqueous chalk suspension was 7.3 liters, that of the Ottawa Sands was 8 liters, i.e. that is, the volume ratio is 0.92 and the weight ratio Ottawa sand to chalk is 2.3: 1.

ίο The mixture of suspension and Ottawa sand was ground in the sand mill until the required amount of energy was consumed, then the sand was sifted off. The suspension was flocculated with a polyacrylamide, filtered and the Chalk dried in an oven at 80 ° C.

The weight indication of the chalk consisting of particles less than 2 microns in size and less than 0.5 microns in spherical diameter was measured. To determine the optical properties (gloss) of a sheet of paper which had been "treated" with a coating agent containing the chalk as pigment, a suitable base paper was used with the coating agent in an application device as described in US Pat. No. 3,416,943 is, overdone. The gloss of the treated paper was measured by the TAPPI standard no. T 480 ts-65 method, expressed in TAPPI gloss units in comparison to a pure glass with a refractive index of 1540, measured in the sodium D line. The base paper had a weight of 81 g / m ² , the coating agent consisted of 100 parts by weight of the pigment, 10 parts by weight of Viscosol 310 starch, 10 parts by weight of Dow 636 latex and so much water that a solids content of 63% by weight was achieved. The coating composition also contained 0.3 percent by weight of a sodium polyacrylal as a dispersant, calculated on the weight of the calcium carbonate. The coated paper sheets were conditioned at 20 ° C. and 65% relative humidity and then calendered in 10 passes at 85 ° C. and a line pressure of 44.5 kg per centimeter in the longitudinal direction. the circumferential speed of the calender rolls * was 36 m / minute. Finally, the calendered paper sheets were conditioned again at 20 ^° C. and 65% relative humidity.

The grinding was carried out at a number of different energies, the results are summarized in Table I:

Table I.

Energy expenditure

(kWh / ton
Dry chalk)

21.4

47.8
114.0
176.4
227.9
338.1

Weight percent

<2μ

32
42
49
65
80
92
100

size shine Weight (TAPPI- percent Units) <0.5μ 12.0 2 13.8 3 15.5 6th 20.0 18th 27.0 20th 31.5 27 34.0 37

continuation

Particle size Weight shine Energy expenditure percent Weight <0.5μ (TAPPI- (kWh / ton percent 49 Units) Dry chalk) <2μ 65 36.5 470.4 100 77 37.5 624.8 100 86 37.6 793.8 100 37.6 882.0 100

Example 2

18 kg of Italian calcite marble in the form of Lumps about 0.6 to 1.2 cm in size were placed in a rubber-lined ball mill designed for about Half of it was filled with flint stones 5 to 7 cm in diameter, a total of 7 hours 5 minutes grind dry. The mill product was screened through No. 16 BS sieve. The visual product became further grinding according to the invention using a silica sand from Leighton Buzzard, Bedfordshire, England, as a grinding aid, made up of essentially spherical particles of a size accordingly No. 16 BS sieve to No. 30 BS sieve. The grinding took place in a sand mill. The ground material consisted of 5 kg of dry calcite marble, 10 kg of sand, 2.138 l of water and 0.3 percent by weight of an ammonium polyacrylate, calculated on the dry weight of the marble. The ratio of sand volume to aqueous marble suspension was 0.94: 1 and the solids content of the marble suspension 70 percent by weight, calculated on the dry weight of the marble. The weight ratio the grinding aid (sand) to the calcium carbonate mineral was 2: 1. Three specimens of calcite marble became different in this way for a long time, with different amounts of energy being consumed in the suspensions.

Each of the three samples were sifted through No. 300 BS sieve and dried in a conventional spray dryer. The weight fractions of each sample composed of particles smaller than 2 microns and smaller than 0.5 microns corresponding to a spherical diameter were measured.

Each sample was combined into a coating agent consisting of 50 parts by weight of dry marble, 50 parts by weight of dry kaolin having such average particle size such that 80 percent by weight of particles were less than 2 microns, 7 '/ ₂ parts by weight Viscosol strength and I ¹ I ₂ Parts by weight of a styrene-butadiene rubber latex. The agent contained so much water that a total solids content of 63 percent by weight was reached. In addition, 0.3 percent by weight ammonium polyacrylate (based on the weight of the dry kaolin) was in the water to deflocculate the kaolin. A base paper with a weight of 81 g / cm ² was coated with 10 g / m ^{2 of} the agent using a laboratory application device as described in US Pat. No. 3,416,943. The coated paper sheets were conditioned at 20 ° C. and 65% relative humidity and then calendered in 10 passes at 85 ° C. and a line pressure of 113 kg per 0.3 m, the peripheral speed of the calender roll being 36 m / min. Finally, the paper sheets were treated again at 20 ° C. and 65% relative humidity.

The gloss of the paper samples was measured by the method of TAPPI standard No. T 480 ts-65. the Results are summarized in Table II:

Table II

Energy expenditure (kWh / ton

dry. Calcite marble)

136.7
183.8
257.3

Weight percent

Particle size

Weight percent

0.5 µ

63
72
87

14th
19th

25th

shine

(TAPPI units)

36.2
43.0
43.8

Example 3

Marbled Calrit ais Flintshire, Wales, consisting of from up to 15 cm large lumps, was washed in a stone crusher down to a particle size of Ground 0.6 to 1.2 cm and dried. Any visible discolored particles were made by hand removed and the residue in a ball mill, half filled with balls 5 to 7.6 cm in diameter was filled, dry-wedded. Each batch consisted of 18 kg of dry calcite that took 2 hours and Milled for 5 minutes and then screened through No. 52 BS sieve.

Four batches of the cdcite ground in this way were then further processed in accordance with the invention in a sand mill grind, using a Leighton-Buzzard silica sand as a grinding aid having a particle size of No. 16 to No. 30 BS sieve was used. Each batch consisted of 5.0kg dry calcite, 10.0kg Sand, 2.138 liters of water and 0.3 percent by weight of an ammonium polyacrylate, calculated on the weight of dry calcite. The weight ratio of the grinding sand to the calcium carbonate was 2: 1,

the volume ratio of the grinding sand to the aqueous calcium carbonate suspension 0.94: 1. The four Batches were mixed for different lengths of time, so that the energy consumption for each sample was different.

Subsequent to the grinding, the samples were screened through No. 300 BS sieve and in a conventional one Spray dryer dried. The weight percentages of the particles less than 2 microns and smaller measured as 0.5 microns and the gloss value of a paper coated with a coating agent containing the marbled calcite and the same kaolin as in Example 2, was coated, as in Example 2 measured. The results are summarized in Table III:

Table III

energy
consumption Particle size Weight shine (kWh / ton
dry- Weight percent marbled percent <0.5μ (TAPPI- Calcite) <2μ ^ 16 Units) 121.3 67 22nd 36.0 169.1 78 27 44.5 230.8 93 35 46.9 316.1 99 47.6

Example 4

Three lots of flint-free chalk and Wiltshire, England, with a particle size such that 31 percent by weight consisted of particles <2 microns, were ground according to the invention in a sand mill without the addition of a dispersant. A silica sand from Leighton Buzzard with a particle size through BS sieve No. 16 to No. 30 was used as a grinding aid. Each batch consisted of 2.9 kg of chalk, 12 kg of sand and 7.94 l of water. The solids content of the aqueous chalk suspension was 26.8 percent by weight, the volume ratio of sand to aqueous suspension was 0.51: 1, and the weight ratio of sand to calcium carbonate mineral was 4.1: 1. The three parties were married for different lengths of time, which resulted in a different amount of energy consumption.

After milling was complete, the samples were screened through No. 300 BS sieve and the

The suspensions are converted into a stiff paste by centrifugation. The centrifuge cake was made with 0.3 percent by weight of an ammonium polyacrylate, calculated on the weight of dry chalk, mixed and in dried in a conventional spray dryer. The weight amounts of particles smaller than 2 microns and less than 0.5 microns were determined, as well as the gloss value of a paper as in Example 2. Die Results are summarized in Table IV:

Table IV

Particle size Weight shine energy percent consumption Weight 0.5 µ (TAPPI- (kWh / ton percent 9 Units) Dry chalk) 2 μ 25th 34.1 73.5 55 42 43.9 213.2 89 46.5 441.0 100

409633 / Π5

Claims (4)

Patent claims: 1. Method of crushing calcium carbonate mineral by grinding a dispersing agent containing aqueous suspension of the mineral, which is mixed with a grinding material, wherein the aqueous suspension with at least 25 percent by weight calcium carbonate is formed and the grinding material is the calcium carbonate mineral is added in a weight ratio of 2: 1 to 5: 1, according to Patent 1507 514, characterized in that that the grinding material consists of particles with a size of 150 microns to 0.63 cm and their proportion is so large that their volume is at least half the volume corresponds to the aqueous suspension of the calcium carbonate mineral, the aqueous suspension is married until at least 183.75 kilowatt hours (250 horsepower hours) are consumed per tonne of dry calcium carbonate mineral. 2. The method according to claim 1, characterized in that the grinding material has a particle size from 500 microns to 2 mm in diameter. 3. The method according to claim 1 and 2, characterized in that the aqueous suspension 40 to 75 percent by weight of calcium carbonate mineral and a water-soluble, non-glue-forming dispersant contains, which consists of a salt with a macromolecular anion and an average Molecular weight from 100 to 10,000. 4. The method according to claim 3, characterized in that the dispersant consists of a substance the group of the sodium, potassium and ammonium salts of polyacrylic acids and polymethacrylic acids consists.