FR2683539A1 - Process for grinding in aqueous suspension in the presence of a grinding agent partly neutralised with magnesium - Google Patents

Abstract

The grinding process employs, as grinding agent, a polymer and/or copolymer with a specific viscosity of between 0.3 and 0.8 and obtained by treating with the aid of one or a number of polar solvents a polymer and/or copolymer which results from the polymerisation and/or copolymerisation of acrylic and/or vinyl monomers containing acidic groups in which the active acidic sites are partly or completely neutralised to a degree of neutralisation of the active acidic sites with a neutralising agent supplying the magnesium ion ranging from 40% to 60%, inclusive, and with a neutralising agent containing the sodium ion ranging up to 60%. Aqueous suspensions obtained by the said process and their applications to the field of the bulk filler for coating of paper, of paint and of plastics.

Description

AQUEOUS SUSPENSION GRINDING PROCESS IN THE PRESENCE OF A
GRINDING AGENT NEUTRALIZED IN PART BY MAGNESIUM.

The invention relates to a process for grinding an aqueous medium of coarse mineral materials in the presence of a water-soluble grinding agent whose active acid sites are partially or completely neutralized by a neutralization agent containing the magnesium ion, so that the percentage of neutralization of the active acid sites by the magnesium ion varies between 40% and 60%, limits included, and by a neutralization agent having the sodium ion that can neutralize up to 60% of the active acid sites of said agent. Said process leads to aqueous suspensions of inorganic materials whose viscosity remains stable over time even if the suspensions are not maintained with stirring, thus ensuring great ease of handling and application.This process is particularly applicable to grinding an aqueous suspension of calcium carbonate having a solids content of at least 70% by weight, of which at least 90% of the constituent particles after grinding have a size of less than two microns, while at least 60% between them have a dimension less than one micron. In addition, this invention relates to aqueous suspensions of inorganic particles obtained by said process as well as their applications.

For a long time now, it has been known to use mineral substances such as calcium carbonates and titanium dioxide for the preparation of industrial products intended for the fields of paints, paper coating, fillers for rubbers and synthetic resins , or others.

However, since these mineral substances do not have a natural lamellar or laminated structure facilitating disintegration, as is the case with certain substances such as the aluminum silicates usually known as kaolin, the skilled person , in order to be able to use them in the pigmentary field, must transform them by grinding into an aqueous suspension of great fineness whose constitutive grains have a smaller dimension possible, that is to say less than a few microns.

By the number of publications made in this field, the specialized literature reveals the importance and the complexity of the grinding in an aqueous medium of mineral substances to obtain a partially refined quality allowing a pigment application. C1 is thus that, in the particular case of the coating of papers, it is well known that the coating mass formed of inorganic pigments such as kaolin, calcium carbonate and titanium dioxide put in suspension in the water. water also contains binders and dispersants, as well as other adjuvants such as thickeners and coloring agents. Or, it is desirable that such a suspension has a low viscosity and stable during the storage period for facilitate handling and application, as well as the highest mineral content possible to reduce the amount of water handled. Such an ideal suspension, combining all these fundamental qualities, would solve for the skilled person the well-known problems of grinding, storage, transport from the place of production to the place of application, finally transfer by pump when of its use.

Thus, until now, those skilled in the art know the use of grinding agents and / or aqueous dispersion of inorganic particles consisting of polymers and / or acrylic copolymers totally or partially neutralized with various neutralization agents (patents
FR2539137, FR2531444, FR2603042).

Those skilled in the art also know the use of a grinding agent and / or dispersion consisting of the fraction of alkaline acrylic polymers and / or copolymers totally neutralized by a single cation, and whose specific viscosity is between 0.3 and 0.8 (patent FR 2488814), or partially neutralized by a single cation such as sodium, potassium, lithium or ammonium (EP0127388;
EP0185458).

These various types of grinding and / or dispersion agents of the prior art as well as other treatment or grinding processes known to those skilled in the art (EP0278880; FR2642415) make it possible to obtain aqueous suspensions refined mineral particles, considered until now as stable over time, but for which the user has some handling difficulties because it is necessary to store these suspensions several days or weeks without agitation and is thus faced with the increase the viscosity of the unstirred suspension.

In fact, in the prior art, the viscosity measurement of the aqueous suspens ion of mineral particles was carried out using a Brookfield type RVT viscometer at a temperature of 200 ° C. and at a speed of rotation of 100 revolutions per minute. a / at the outlet of the mill, b / after eight days of rest in the bottle and after prior agitation of the suspension left to rest.

These two parameters did not make it possible to highlight "the increase of the viscosity" of the mineral aqueous suspension harmful for the user having to handle suspensions stored for several days without agitation, and characterized by a Brookfield viscosity before stirring greater than 2000 mPa.s for a measurement made at 10 revolutions per minute.

Faced with these drawbacks, the Applicant then found that the aqueous mineral suspensions obtained according to the prior art had high Brookfield viscosities if the measurement was carried out without prior stirring of the suspension, which is generally the case in which the user of these suspensions must face.

On the strength of this observation, the Applicant has continued his research and has developed a grinding process which, surprisingly, makes it possible to carry out aqueous mineral suspensions meeting the aforementioned quality criteria, by the use of a water-soluble agent of grinding and / or dispersion whose active acid sites are partially or completely neutralized by a neutralization agent containing the magnesium ion, such that the percentage of neutralization of the active acid sites by the magnesium ion varies between 40% and 60% %, limits included, and by a neutralizing agent having the sodium ion that can neutralize up to 60% of the active acid sites of said agent.

Surprisingly, the objects of the invention are obtained when the percentage of neutralization of the acidic active sites of the polymer by the neutralization agent containing the magnesium ion is between 40 and 60% inclusive, while the percentage of neutralization active acid sites of the polymer with a neutralizing agent containing the sodium ion is less than or equal to 60%.

Thus, one of the aims of the invention is to provide a grinding process in aqueous suspension of mineral particles.

These suspensions, intended for pigment applications in which at least 90% of the particles have a size of less than two microns and of which at least 60% of them have a dimension of less than one micron, according to the invention, have a low viscosity and stable over time, even without stirring, namely a viscosity after eight days at rest of the same order as that obtained at the end of the mill at the end of grinding.

Another object of the invention relates to the use of these aqueous mineral suspensions refined in the field of mass loading and coating of paper and in the field of paints and polymeric materials after, optionally, removal of the aqueous phase .

These aims are achieved by the use in the grinding process of an agent obtained by treatment, using one or more solvents, of a polymer and / or copolymer, which results from the polymerization and / or copolymerization of acidic acrylic and / or vinyl monomers according to methods known to those skilled in the art and whose active acid sites are partially or completely neutralized with a neutralization rate of the active acid sites by a neutralizing agent having the magnesium ion ranging from 40% to 60% inclusive and by a neutralization agent containing the sodium ion of up to 60% .These polymers and / or acrylic copolymers are obtained according to the known methods of radical polymerization, in presence of polymerization regulators such as, for example, organic compounds based on hydroxylamine and in the presence of polymerization initiators such as peroxides and persalts, for example water oxygenated, persulfate, sodium hypophosphite, hypophosphorous acid or the like, of at least one of the following monomers and / or comonomers: acrylic acid and / or methacrylic acid, itaconic acid, crotonic acid, fumaric acid, maleic anhydride, or isocrotonic acid aconitic, mesaconic, sinapic, undecylenic, angelic, canellic, hydroxyacrylic, in acid or partially neutralized form, acrolein, acrylamide, acrylonitrile, esters of acrylic and methacrylic acids, and in particular dimethylaminoethyl methacrylate, vinylpyrrolidone, vinylcaprolactam, ethylene, propylene, isobutylene, diisobutylene, vinyl acetate, styrene, alphamethylstyrene, methylvinylketone, in a polymerization medium which may be water, methanol, methanol , propanol, isopropanol, butanols, or mixtures thereof, or else dimethylformamide, dimethylsulfoxide, tetrahydrofuran, acetone, methylethyl ylketone, ethyl acetate, butyl acetate, hexane, heptane, benzene, toluene, xylene, mercaptoethanol, tert-dodecylmercaptan, thioglycolic acid and esters, n-dodecylmercaptan acetic acid, tartaric acid, lactic acid, citric acid, gluconic acid, glucoheptonic acid, 2-mercaptopropionic acid, thiodiethanol, halogenated solvents such as carbon tetrachloride, chloroform, methylene chloride, monopropylene glycol ethers, diethylene glycol or their mixture.

The acidic polymerisate thus obtained is optionally distilled and then neutralized partially or totally in a proportion of 40% to 60% with a neutralization agent containing the magnesium ion, and in an amount of less than or equal to 60% with a neutralizing agent containing the sodium ion.

The solution of the acid polymer or so neutralized is then treated according to static or dynamic processes known to those skilled in the art, by one or more polar solvents belonging in particular to the group consisting of methanol, methanol, propanol, isopropanol. , butanols, acetone, tetrahydrofuran, thus producing a two-phase separation - the least dense phase, comprising the major fraction of the polar solvent and the polymer fraction and / or low molecular weight acrylic copolymer, - the phase the densest aqueous composition comprising the fraction of polymer and / or acrylic copolymer of higher molecular weight.

The desirable fraction consisting of acrylic polymers and / or copolymers whose specific viscosity is between 0.3 and 0.8 and which are used in the process according to the invention is collected.

This specific viscosity of the acrylic polymers and / or copolymers, symbolized by the letter "g7", is determined in the following manner.

A sample of the solution of the fraction of the polymers and / or copolymers collected is taken so as to obtain a solution corresponding to 2.5 g of dry polymer and to 50 ml of a 60 g / l sodium chloride solution. Then measured with a capillary viscosimeter of Baume constant equal to 0.000105 placed in a thermostatically controlled bath at 250C the flow time of a given volume of the aforementioned solution containing the polymer and / or acrylic copolymer, as well as the time flow of the same volume of aqueous sodium chloride solution without said polymer and / or copolymer.

It is then possible to define the viscosity "" thanks to the following relation:
(flow time of (flow time of
the polymer solution) - the NaCl solution)
flow time of the NaCl solution
The capillary tube is generally chosen such that the flow time of the NaCl solution, free from polymer and / or copolymer, is from about 90 to 100 seconds, thus giving very good specific viscosity measurements. precision.

The temperature at which the selection process of the acidic or alkaline acrylic polymer and / or copolymer fraction is carried out is not critical in itself since it influences the only partition coefficient. It is between 100 and 800C, and preferably between 200C and 600C.

On the other hand, the ratio of the quantities of dilution water and polar solvents is preponderant, since it directly influences the separation. Similarly, when the separation is carried out continuously by means of a centrifugal extractor, that is to say by a dynamic process, the ratios of the extracted fractions are dependent on the centrifugation conditions.

 It is also possible and desirable in some cases to further refine the selection of the fraction of acidic or alkaline acrylic polymers and / or copolymers by treating again the denser aqueous phase previously collected, using a new amount of solvent. polar, which may be different from that or those initially used, or which may still be a mixture of polar solvents.

The liquid phase obtained after treatment can be subjected to distillation to remove the solvent (s) used for the treatment.

The aqueous phase containing the fraction of acrylic polymers and / or copolymers with a specific viscosity of between 0.3 and 0.8 corresponding to a weight average molecular weight (Mw) of between 1000 and 10,000 can be used in this form as grinding agent for mineral substances to porphyrise; but it can also be treated by any known means, to remove the aqueous phase and isolate the polymer and / or acrylic copolymer in the form of a fine powder that can be used in this other form as a grinding agent.

In practice, according to the invention, the operation of grinding the mineral substance is to refine in very fine particles said substance with a grinding body in an aqueous medium containing the grinding agent. With the aid of the latter, an aqueous suspension of the mineral substance to be milled, the grains of which have an initial size of at most 50 microns, is formed in such a quantity that the concentration of dry matter of said suspension is at least 70% by weight.

To the suspension of the mineral substance to be ground is added the grinding body of particle size advantageously between 0.20 millimeters and 4 millimeters. The grinding body is generally in the form of particles of materials as diverse as silicon oxide, aluminum oxide, zirconium oxide or their mixtures as well as synthetic resins of high hardness, steels , or others.

An example of composition of such grinding bodies is given by the patent FR2203681 which describes grinding elements formed from 30 to 70% by weight of zirconium oxide, 0.1 to 5% of aluminum oxide and from 5 to 20% by weight. % silicon oxide. The grinding body is preferably added to the suspension in an amount such that the ratio by weight between this grinding material and the mineral substance to be ground is at least 2/1, this ratio preferably being between the limits 3 / 1 and 5/1.

The mixture of the suspension and the grinding body is then subjected to the mechanical stirring action, such as that which occurs in a conventional microelement mill.

The grinding agent is also introduced into the mixture formed by the aqueous suspension of mineral substances and by the grinding body in a proportion of 0.2 to 2% by weight of the dried fraction of said polymers relative to the dry mass of the mineral substance to refine.

The time required to obtain an excellent fineness of the mineral substance after grinding varies according to the nature and the quantity of the mineral substances to porphyrise and according to the stirring mode used and the temperature of the medium during the grinding operation.

The mineral substances to be refined according to the process of the invention can be of very diverse origins such as calcium carbonate and dolomites, kaolin, titanium dioxide, that is to say all the mineral substances which must be crushed to be used in applications as diverse as the coating of papers, the pigmentation of paints and coatings, the load of synthetic rubbers or resins, the matting of synthetic textiles, or others.

Thus, the application of the grinding agent whose active acid sites are partially or completely neutralized, with 40 to 60% inclusive limits of a neutralization agent having the magnesium ion and with up to 60% of a neutralization agent having sodium ion, allows, according to the invention, the transformation by grinding aqueous suspension of high concentration of dry matter of coarse mineral substances into very fine particles, at least 90% of them have a size always less than two microns and at least 60% of which have a size less than one micron, and obtain a suspension of very fine mineral substances whose viscosity is low and stable over time.

The scope and interest of the invention will be better understood by the following examples.

Example 1
This example relates to the preparation of a slurry of coarse calcium carbonate subjected to grinding to refine it into a microparticulate suspension. For this purpose, a suspension of coarse calcium carbonate was prepared from a natural calcium carbonate of average diameter 50 microns, using - for the test n01 illustrating the prior art, the densest fraction of sodium polyacrylate resulting from an isopropanol fractionation of a sodium polyacrylate of specific viscosity 0.525 obtained by radical polymerization and total neutralization (100% of the active acid sites) with sodium hydroxide, - for Test No. 2 also illustrating the prior art, the densest fraction of a mixed polyacrylate of sodium and calcium resulting from an isopropanol fractionation of a mixed polyacrylate of sodium and calcium of specific viscosity 0.525 obtained by radical polymerization and total neutralization with sodium hydroxide and lime in a ratio corresponding to a neutralization of the active acid sites by 70% by sodium - 30% by calcium, for test No. 3 still illustrating the prior art, the densest fraction of a sodium polyacrylate resulting from an isopropanol fractionation of a sodium polyacrylate of specific viscosity 0.525 obtained by radical polymerization and partial neutralization (66% of the active acid sites) with sodium hydroxide, - for test No. 4 illustrating the invention, the densest fraction of a mixed polyacrylate of sodium and magnesium resulting from a isopropanol fractionation of a mixed polyacrylate of sodium and magnesium of specific viscosity 0.525 obtained by total neutralization (100% of the active acid sites) with sodium hydroxide and magnesia in a ratio corresponding to a neutralization of the active acid sites at 50% by sodium - 50% by magnesium.

For each test, an aqueous suspension of calcium carbonate from the Orgon deposit (France) having a particle size of less than 10 microns was prepared.

The aqueous suspension has a dry matter concentration of 76% by weight relative to the total mass.

The grinding agent is introduced into this suspension in the amounts indicated in the table below, expressed in percent by dry weight relative to the mass of dry calcium carbonate to be ground.

The suspension circulates in a mill of the Dyno-Mill type with a fixed cylinder and a rotating impeller whose grinding body consists of corundum balls of diameter ranging from 0.6 millimeters to 1.0 millimeters.

The total volume occupied by the grinding body is 1150 cubic centimeters while its mass is 2900 g.

The grinding chamber has a volume of 1400 cubic centimeters.

The circumferential speed of the mill is 10 meters per second.

The suspension of calcium carbonate is recycled at the rate of 18 liters per hour.

The output of the Dyno-Mill mill is equipped with a 200 micron mesh separator to separate the suspension resulting from grinding and the grinding body.

The temperature during each grinding test is maintained at approximately 600 ° C.

At the end of milling (To), a sample of the pigment suspension is recovered from a flask in which 80% of the particles have a size of less than one micron, and the viscosity is measured using a Brookfield type RVT viscometer. , at a temperature of 200C and a rotational speed of 10 revolutions per minute with the appropriate mobile.

After a rest period of 8 days in the flask, the viscosity of the suspension is measured by introducing into the unstirred flask of the appropriate mobile of the viscometer.
Brookfield type RVT, at a temperature of 200C and a rotation speed of 10 revolutions per minute (viscosity AVAG = before stirring).

All experimental results are reported in Table 1.

TABLE 1

Viscosity <SEP> Brookfield <SEP> of <SEP><SEP> suspension
<tb> AGENT <SEP> FROM <SEP> MILLING <SEP> USED
<tb><SEP> assay (at <SEP> 20 C <SEP> at <SEP> mPa.s)
<tb> No. <SEP> Consumption
<tb> NEUTRALIZATION
<tb> in <SEP> agent <SEP> To <SEP> 8 <SEP> days
<tb> Rate <SEP> of <SEP> groups <SEP> Cation <SEP> in <SEP>% <SEP> sec / sec <SEP> AVAG
neutralized <SEP> acids <SEP> neutralizing <SEP> 10 <SEP> RPM <SEP> 10 <SEP> RPM
<tb> 1 <SEP> 100 <SEP> Na <SEP> 0.86 <SEP> 1400 <SEP> 8290
<tb> Art
<tb> 2 <SEP> 70/30 <SEP> Na / Ca <SEP> 1.05 <SEP> 1250 <SEP> 2390
<tb> previous <SEP> 3 <SEP> 66 <SEP> Na <SEP> 1.25 <SEQ> 1500 <SEQ> 6750
<tb> Invention <SEP> 4 <SEP> 50/50 <SEP> Na / Mg <SEP> 0.98 <SEP> 1040 <SEP> 1040
<tb> AVAG: Viscosity measurement before stirring the suspension
From Table 1, it can be seen that only the suspension of calcium carbonate ground by the process according to the invention (test No. 4) has a Brookfield viscosity measured at 10 revolutions per minute after 8 days of rest and without much lower initial agitation. at 2,000 mPa.s while all the tests of the examples carried out according to a method of the prior art give viscosities higher than 2,000 mPa.s.

Thus, this reading of Table 1 makes it possible to observe the increase in viscosity in the suspensions of ground calcium carbonate according to the processes of the prior art, whereas the grinding method according to the invention makes it possible to obtain the stability of the viscosity of the calcium carbonate suspensions even before any agitation.

Example 2
A second group of tests (tests Nos. 5 to 12) concerns the preparation and grinding of the aqueous suspension of the same calcium carbonate at the same concentration of dry matter in the presence of the densest fraction obtained by fractionation at the same time. isopropanol of the same polyacrylic acid of specific viscosity 0.525 neutralized by varying magnesium levels.

These tests were performed according to the same experimental criteria as those of Example 1.

All experimental results are reported in Table 2.

TABLE 2

Viscosity <SEP> Brookfield <SEP> of <SEP><SEP> suspension
<tb> AGENT <SEP> FROM <SEP> MILLING <SEP> USED
<tb><SEP> assay (at <SEP> 20 C <SEP> at <SEP> mPa.s)
<tb> No. <SEP> Consumption
<tb> NEUTRALIZATION
<tb> in <SEP> agent <SEP> To <SEP> 8 <SEP> days
<tb> Rate <SEP> of <SEP> groups <SEP> Cation <SEP> in <SEP>% <SEP> sec / sec <SEP> AVAG
neutralized <SEP> acids <SEP> neutralizing <SEP> 10 <SEP> RPM <SEP> 10 <SEP> RPM
<tb> 5 <SEP> 70/30 <SEP> Na / Mg <SEP> 1.05 <SEP> 1330 <SEP> 3590
<tb> 6 <SEP> 60/40 <SEP> Na / Mg <SEP> 1.03 <SEP> 1200 <SEP> 1900
<tb> 7 <SEP> 55/45 <SEP> Na / Mg <SEP> 0.9 <SEP> 1210 <SEP> 1510
<tb> 8 <SEP> 50/50 <SEP> Na / Mg <SEP> 0.98 <SEP> 1040 <SEP> 1040
<tb> Invention <SEP> 9 <SEP> 45/55 <SEP> Na / Mg <SEP> 1.02 <SEP> 1250 <SEP> 1190
<tb> 10 <SEP> 40/60 <SEP> Na / Mg <SEP> 1.09 <SEP> 1390 <SEP> 1635
<tb> 11 <SEP> 30/70 <SEP> Na / Mg <SEP> 1.16 <SEP> 1160 <SEP> 1730
<tb> Invention <SEP> 12 <SEP> 50/45 <SEP> Na / Mg <SEP> 1.08 <SEP> 1420 <SEP> 1900
<tb> AVAG: Viscosity measurement before stirring the suspension
The reading of Table 2 makes it possible to observe the exceptional rheological results of the invention, as well as to note that the process according to the invention makes it possible to cross a break-even point in practical tests.

Indeed, two criteria are taken into account by the user, the economic aspect and the rheological aspect.

An overdose of the grinding agent results in an additional cost in the production of refined calcium carbonate suspensions.

Thus, the consumption of grinding agent and / or dispersion in the grinding operation according to the invention must not exceed 1.1% of dry dispersing agent relative to the dry weight of calcium carbonate in order to obtain a suspension of which 80% of the particles are less than 1 micron and 76% dry matter concentration. Tests n06 and n010 delimit the limits of the invention.

As for the rheological aspect, the user having to handle concentrated suspensions stored for several days without agitation wishes to have suspensions whose viscosity is almost identical to that of suspensions leaving the mill and always less than 2000 mPa.s. It is this exceptional result which is obtained for the first time by the process according to the invention (tests Nos. 6 to 10 and 12, and in particular the tests 7 to 9).

Example 3
This example (tests n013-14) concerns the preparation and grinding of the aqueous suspension of the same calcium carbonate at the same concentration of dry matter in the presence of the densest fraction obtained by fractionation with isopropanol of the same polyacrylic acid of specific viscosity 0.525 neutralized with different neutralizing agents containing polyvalent ions other than magnesium and such as calcium.

Thus, the grinding of test No. 13 is carried out using the same fraction of the above polyacrylic acid and neutralized with sodium hydroxide and lime in a ratio corresponding to a neutralization of the active acid sites at a rate of 70%. by the sodium - 30% by the calcium, while the test n014 concerns the same fraction of the same polyacrylic acid neutralized with sodium hydroxide and lime in a ratio corresponding to a neutralization of the active acid sites at a rate of 55% by sodium - 45% by calcium.

These tests were performed according to the same experimental criteria as those of Example 1.

All results are shown in Table 3.
TABLE 3

Viscosity <SEP> Brookfield <SEP> of <SEP><SEP> suspension
<tb> AGENT <SEP> FROM <SEP> MILLING <SEP> USED
<tb><SEP> assay (at <SEP> 20 C <SEP> at <SEP> mPa.s)
<tb> No. <SEP> Consumption
<tb> NEUTRALIZATION
<tb> in <SEP> agent <SEP> To <SEP> 8 <SEP> days
<tb> Rate <SEP> of <SEP> groups <SEP> Cation <SEP> in <SEP>% <SEP> sec / sec <SEP> AVAG
neutralized <SEP> acids <SEP> neutralizing <SEP> 10 <SEP> RPM <SEP> 10 <SEP> RPM
<tb> 13 <SEP> 70/30 <SEP> Na / Ca <SEP> 1.05 <SEP> 1250 <SEP> 2390
<tb> 14 <SEP> 55/45 <SEP> Na / Ca <SEP> 1.3 <SEP> 1500 <SEP> 3300
<tb> 7 <SEP> 55/45 <SEP> Na / Mg <SEP> 0.9 <SEP> 1210 <SEP> 1510
<tb> AVAG: Viscosity measurement before stirring the suspension
The reading of Table 3 shows that the use of another polyvalent neutralizing cation such as calcium, even at different levels, does not result in a slurry of refined calcium carbonate Brookfield viscosity measured at 10 revolutions per minute after 8 days of rest and without previous agitation lower than 2000 mPa.s (tests n013 and 14). Thus, the use of other neutralizing agents having a polyvalent function other than magnesium does not make it possible to obtain viscosity suspensions.
Brookfield after 8 days at rest below 2000 mpa.s for a measurement performed at 10 revolutions per minute.

Example 4
This example (tests No. 15-17) concerns the preparation and milling of the aqueous suspension of the same calcium carbonate at the same concentration of dry matter in the presence of the densest fraction obtained by fractionation with isopropanol of the same polyacrylic acid. of specific viscosity 0.525 neutralized to 50% by the magnesium ion and to 50% by different monovalent neutralization agents.

These different monovalent neutralization agents are ammonium for test n015, lithium for test n016, potassium for test n017.

These tests were performed according to the same experimental criteria as those of Example 1.

All results are shown in Table 4.

TABLE 4

Viscosity <SEP> Brookfield <SEP> of <SEP><SEP> suspension
<tb> AGENT <SEP> FROM <SEP> MILLING <SEP> USED
<tb><SEP> assay (at <SEP> 20 C <SEP> at <SEP> mPa.s)
<tb> No. <SEP> Consumption
<tb> NEUTRALIZATION
<tb> in <SEP> agent <SEP> To <SEP> 8 <SEP> days
<tb> Rate <SEP> of <SEP> groups <SEP> Cation <SEP> in <SEP>% <SEP> sec / sec <SEP> AVAG
neutralized <SEP> acids <SEP> neutralizing <SEP> 10 <SEP> RPM <SEP> 10 <SEP> RPM
<tb> 15 <SEP> 55/45 <SEP> NH4 / Mg <SEP> Impossibility <SEP> to obtain <SEP> a <SEP> granulometry
<tb> equivalent <SEP> to <SEP> 80% <SEP><<SEP> 1 <SEP> um
<tb> 16 <SEP> 50/50 <SEP> Li / Mg <SEP> 0.89 <SEP> 1620 <SEP> 3800
<tb> 17 <SEP> 50/50 <SEP> K / Mg <SEP> 1.11 <SEP> 1800 <SEP> 2700
<tb> AVAG: Viscosity measurement before stirring the suspension
Reading Table 4 shows that we can not advantageously replace the sodium ion with another monovalent ion.

Claims (10)

1) Method of grinding in aqueous suspension of coarse mineral materials intended for pigmentary applications consisting of preparing an aqueous suspension of these materials, to introduce a grinding agent consisting of acrylic and / or vinyl polymers and / or copolymers comprising partially acidic groups or completely neutralized resulting from treatment with one or more polar solvents by static or dynamic methods of acrylic and / or vinyl polymers and selection of the desired molecular weight fraction, to add to the suspension a grinding body and to subject the mixture thus carried out with a grinding action, characterized in that said active acid sites of said grinding agent are totally or partially neutralized by a neutralization agent containing the magnesium ion, so that the percentage of neutralization of the active acid sites by 1 magnesium ion varies between 40 and 60%, included terminals, and by a neutralizing agent having the sodium ion that can neutralize up to 60% of the active acid sites of said agent. 2) method for grinding in aqueous suspension of coarse mineral materials according to claim 1, characterized in that said active acid sites of said grinding agent are partially neutralized by a neutralization agent containing the magnesium ion, so that the percentage neutralization of the active acid sites by the magnesium ion varies between 40 and 60%, limits included, and by a neutralizing agent having the sodium ion can neutralize up to 60% of the active acid sites of said agent. 3) method for grinding in aqueous suspension of coarse mineral materials according to claim 1, characterized in that said active acid sites of said grinding agent are completely neutralized by a neutralization agent containing the magnesium ion, so that the percentage neutralization of the active acid sites by the magnesium ion varies between 40 and 60%, limits included, and a neutralizing agent having the sodium ion neutralizing the 60% to 40% of the remaining active acid sites. 4) method for grinding in aqueous suspension of coarse mineral materials according to claim 1, characterized in that said active acid sites of said grinding agent are completely neutralized with at least one neutralization agent containing the magnesium ion, so that the The percentage of neutralization of the active acid sites by the magnesium ion varies between 45 and 55%, limits included, and by a neutralizing agent having the sodium ion neutralizing the 55% to 45% of the remaining active acid sites. 5) Process for grinding in aqueous suspension of coarse mineral materials according to claims 1 to 4, characterized in that said grinding agent has a specific viscosity of between 0.3 and 0.8. 6) Process for grinding in aqueous suspension of coarse mineral materials according to any one of claims 1 to 5, characterized in that the grinding agent is introduced into the suspension in a proportion of 0.2% to 2% by weight. dry weight of said polymer relative to the dry weight of the mineral substance to be ground. 7) Process for grinding aqueous suspension of coarse mineral materials according to any one of claims 1 to 6, characterized in that the aqueous suspension of mineral material to be grinded comprises 70% by weight of solids. 8) An aqueous suspension of mineral materials refined by means of the grinding method according to any one of claims 1 to 7, characterized in that at least 90% of the particles of the ground substance have a size of less than two microns and 60 at least some of them are less than one micron in size. 9) An aqueous suspension of refined mineral materials according to claim 8, characterized in that the viscosity Brookfield of said suspension measured without agitation after at least 8 days of storage does not exceed 2000 mPa.s at 10 revolutions per minute. 10) Application of the aqueous suspension of refined mineral materials according to claims 9 and 10 to the field of mass loading, coating of paper, paint and plastic.