EP2376707A1

EP2376707A1 - Talc- and/or chlorite-based compositions and use thereof in controlling pollutants in papermaking industry processes

Info

Publication number: EP2376707A1
Application number: EP08876126A
Authority: EP
Inventors: Mario Mondonico; Marco Leusciatti; Piergiovanni Ercoli Malacari
Original assignee: IMI Fabi SpA
Current assignee: IMI Fabi SpA
Priority date: 2008-12-15
Filing date: 2008-12-15
Publication date: 2011-10-19
Also published as: WO2010070682A1; WO2010070682A8

Abstract

The present invention relates to talc and/or chlorite-based compositions which are used in papermaking processes, as well as to the use of such compositions, in solid form or in the form of aqueous dispersions, for removing stickies from the aqueous cellulose suspensions employed in such processes.

Description

TALC- AND/OR CHLORITE-BASED COMPOSITIONS AND USE THEREOF IN CONTROLLING POLLUTANTS IN PAPERMAKING INDUSTRY PROCESSES Field of the invention

The present invention relates to talc- and/or chlorite-based compositions and their use in papermaking industry processes. Prior art

Paper is made from aqueous suspensions containing cellulose fibers which, floating on the free surface of the suspension, form a felt; this is then transformed into the final sheet by means of calendering and drying processes. In addition to cellulose fibers, the initial suspensions normally contain intentionally added additives, or impurities known in the field as "stickies". The latter may be, for example, resin particles extracted from wood fiber, if new raw material is used; or particles of glue, ink, latex or the like if (as increasingly more common) recycled paper is used. Larger size stickies (generally known in the field as "macro- stickies") may be removed by means of mechanical procedures. On the other hand, small-sized stickies ("micro-stickies") may not be removed from the aqueous suspension by means of mechanical procedures.

The micro-stickies in the suspension may aggregate forming larger size particles, which may cause serious problems during the subsequent steps of manufacturing paper sheets, soiling the wires and the felts used in these productions, creating deposits and therefore stains on the sheet, or in the case of adhesion to the calendering rollers, even causing the breakage of the sheet.

Micro-stickies may be removed, or at least made inert, by adding chemical additives or appropriate enzymes to the aqueous suspension. The first may however give rise to environmental impact issues, and may interfere with the chemism of the downstream processes. The enzymes, instead, have the drawbacks of having selective efficiency on the various types of stickies (thus not ensuring good removal results with all types of micro-stickies), of being slow-acting and having sensitive efficiency with respect to variations of parameters, such as pH and temperature. An alternative method of removing micro-stickies is by using mineral powders. It is known in the field to add fine powders of some minerals to the aqueous suspensions, normally phyllosilicates which have the capability of absorbing impurities on their organophilic surfaces, thus segregating them and making them inert. The mineral powder which has absorbed micro-stickies may then be fixed onto the surface of the cellulose fibers and remain in the final paper sheet, without compromising its features.

Suitable minerals for the purpose are micronized talc and chlorites. Talc and chlorites are minerals with similar chemical-physical features, and are often associated in ore deposits; both minerals, and mixtures thereof in any proportion, are suitable for the purposes of the present invention. For simplicity, in some cases in the present description, reference will be made to talc only, but the term is to be understood broadly speaking as defined above, i.e. meaning talc only, talc in association with chlorites, or even chlorites alone. Talc powders of size smaller than 40 micrometers (μm) are generally used as starting material for producing micronized talc. These powders are introduced into high pressure and high speed air flows, and by making the flows from opposite directions collide with one another, a breakage due to particle impacts is obtained to form powders of smaller size. It is further possible to use other techniques, such as ball-tube mills, hammer mills, etc. This process is named "dry micronization" in the field; powders of a few micrometers in size are obtained in this way.

Despite the fact that micronized talc is widely used for the removal of micro- stickies, its efficacy is not optimal; tests carried out by the inventors demonstrated that in order to halve the amount of micro-stickies in a suspension in this way, it is needed an amount of talc of approximately 2% by weight (b.w.) with respect to the weight of cellulose.

Summary of the invention

It is the object of the present invention to provide the use of talc and/or chlorite- based compositions for removing stickies from aqueous suspensions for making paper, which have a higher removal efficiency than the currently employed talc and/or chlorite forms. Another object of the invention is to provide compositions comprising talc and/or chlorite for the removal of stickies from aqueous suspensions for paper making.

These and other objects are obtained according to the present invention which in a first aspect relates to the use, in a papermaking process, of talc and/or chlorites in the form of particles having an average diameter smaller than 5 μm obtained by wet grinding of the mineral, or of compositions containing them.

According to a convention commonly accepted in the field, "average diameter" means a value indicated as D₅₀, corresponding to the value according to which 50% b.w. of the particles have a diameter smaller than the indicated value; another value commonly indicated for briefly representing the size distribution of powder particles is value D₉₈, i.e. the diameter value such that the size of 98% b.w. of the particles is smaller than such a diameter. Since talc particles are not spherical, diameter in this case means the equivalent diameter, i.e. the diameter of a sphere of equal weight

(and equal density) of the particle actually measured by using a sedimentation technique.

The use of additives made by wet grinding of minerals other than talc (e.g. kaolin used for glossing) in papermaking processes is known; the wet grinding of talc for use as additive in thermoplastic polymers for improving some mechanical features thereof is also known from Patent US 6,348,536 B1. On the other hand, the use of wet ground talc in the papermaking industry, specifically for removing stickies from the aqueous suspensions used for manufacturing cellulose fiber felt, was not previously known. The inventors found out that the talc prepared in this way has a considerably higher stickies removal efficiency than the micronized talc used heretofore for the same purpose. Brief description of the drawings

Further features and advantages of the present invention will be more apparent from the following detailed description by way of non-limitative example and illustrated in the accompanying drawings, in which:

- figure 1 diagrammatically shows a talc wet grinding system; - figure 2 shows a graph illustrating stickies removal test results of three samples of compositions prepared according to the invention; and - figure 3 shows a graph illustrating stickies removal test results of a composition according to the invention and of two additives of the known art. Detailed description of the invention The invention requires that talc be wet ground. Other minerals may be further associated, mainly carbonates and/or quartz. A raw material with various degrees of purity may be used as starting material, from rocks having a talc and chlorite content higher than 90%, to "soap-rock" containing from about 55 to 90% b.w. of talc and chlorite, with the remaining part comprising other minerals, such as carbonates or quartz which are considered impurities. Said raw material may be as extracted from the mine or may be "pre-processed", i.e. obtained from mined material, e. g. following concentration by means of flotation.

The raw material is first mechanically ground, to obtain powders of maximum size preferably smaller than 50 μm. These powders are then dispersed in water; the dispersion may contain up to about 45% b.w. of powders, and typically their weight content is equal to at least 25%, because at lower contents the product yield per grinding cycle is low, and therefore the process becomes economically not convenient; preferably, the dispersion contains at least 35% b.w. of said powders. It is also possible to further increase the charge of talc powders in the dispersion, thus reaching percentages of about 55% b.w., raising pH by adding NaOH or KOH, or by adding additives known in the field, such as, for example, the commercial products Lumiten^® PT or Polisalz^® S from BASF, wetting additives (e.g. surfactants), in percentages by weight lower than 1% of the total dispersion.

The dispersion thus obtained is then ground by an appropriate apparatus, e.g. a ceramic ball mill.

Figure 1 diagrammatically shows a possible talc wet grinding system. System 10 consists of a tank 11 , in which talc and water are introduced in the desired proportions; a pump 12, which allows to recirculate the dispersion in the system; a heat exchanger 13, which avoids the dispersion from overheating upon grinding; and the actual grinding apparatus 14, which may be a ball mill (generally ceramic balls). The four components are connected to one another in a closed circuit in which the dispersion moves in the direction of the arrows; the tank 11 is preferably provided with a mixer (e.g. a helical mixer, not shown in the figure) run for the entire duration of the process in order to ensure the homogeneity of the dispersion. The heat exchanger 13 is optional and may not be present or excluded from the processing fluid recirculation by means of a by-pass line (not shown), e.g. if the grinding is short and thus the generated amounts of heat are not excessive.

Grinding is extended by circulating the dispersion in system 10, performing one or more cycles, until the size of talc particles is reduced to an average diameter smaller than 5 μm, and typically in the range of 0.5-3 μm. The reaching of these dimensions may be monitored by taking a sample of the dispersion at predetermined intervals of time and by analyzing the distribution of the size of talc particles, for example by using a SediGraph 5120 instrument from Micromeritics Instrument Corp., and by using the procedure described in ISO 13317-3:2001 standard. The wet ground talc may be used in the aqueous dispersion form, as obtained with the above-described process; such dispersions are commonly known as "slurries" in the field.

Alternatively, water may be removed from the dispersion. It is however preferable not to proceed with a complete drying of the talc; the term "complete drying" in the field does not mean reaching a zero water content; talc is considered completely dried when its humidity content is lower than 0.5% b.w. A certain residual degree of humidity in the product is desirable because it ensures an easier re- dispersion thereof in water, feature which is useful for preparing papermaking suspensions. The humidity further confers a lower dust content to the product, which improves the features of hygiene both in the departments where the powder is made and where it is used; a complete drying would thus represent a waste of energy which is not justified by the required result. For the purposes of the invention, the ground talc is dried to a residual degree of humidity comprised between 2 and 8% b.w. of the final product; the residual humidity is monitored during the drying process by taking small amounts of the product and subjecting them to a weight loss test in thermobaiance at 105 ⁰C which, leading to the complete drying, determines the amount of water present in the sample. Various known methods may be used to dry the talc, such as, for example, drying in ventilated ovens, under fixed or fluidized bed conditions, or by atomizing, also called "spray drying", i.e. by atomizing the dispersion in a (generally cylindrical) drying chamber, in which a counterflow of hot air is made on dispersion micro-drops.

In a preferred embodiment of the invention, the wet ground talc is used to produce a composition comprising, as second component, a smectite, preferably a bentonite. For the purposes of the invention, an amount of smectite is added to the talc so that its percentage by weight is in the range of 0.1%-5%, and preferably of 1 %-3%, of the talc/smectite mixture.

The inventors found out that the addition of smectite powders to the talc improves the features of the composition, making it easier to be stored and used.

Specifically, when the talc/smectite mixture is used in the form of slurry, the smectite has the effect of stabilizing the dispersion of the powders, thus preventing the sedimentation thereof; this effect is particularly advantageous because it avoids the formation of muds on the bottom of the storage or transportation containers, which would then be difficult to be re-distributed to obtain a homogenous dispersion.

The solid talc/smectite mixtures are preferably used in the form of micro- granules; the presence of smectite has the effect of considerably reducing the tendency of micro-granules to form powders as compared to talc alone, thus protecting the users from inhalation; furthermore, it makes the granules more tough, preserving the shape and therefore the fluidity thereof.

The talc/smectite mixtures, both in the form of slurries and in the form of micro- granules, may be obtained using the same previously described talc wet grinding process, adding the desired amount of smectite to the solid load initially fed in the grinding apparatus. The sum of the weights of talc and smectite may be in the range of 25%-65%, and preferably of 38%-45%, of the total weight of the dispersion. In case of production of micro-granules, at the end of the step of grinding, the slurry is dried to maintain a degree of humidity of the final powder in the range of about 2-8% b.w. of the product. Even in this case, the presence of residual humidity ensures the advantage of re-dispersion speed in water already mentioned for talc alone.

When minerals which are not particularly pure are used for the production of talc/smectite mixtures, all the previously mentioned amounts and proportions by weight must be recalculated to be related to the pure mineral.

Chemical components known in the formulation of additives for papermaking industry, such as cationizing components (e.g. quaternary amines), anionizing components (such as the Lumiten^® PT or Polisalz^® S products from BASF) or strong bases (e.g. NaOH) may be added to either talc used alone or the mixtures with smectite. These components may be used to adapt the chemism of the product for its subsequent use for making paper and/or to decrease the viscosity thereof during grinding. None of these components pose problems from the environmental point of view, as well as talc and smectites which are the main components of the composition according to the invention. The invention will be further illustrated by means of the following examples.

EXAMPLE 1

This example refers to the production of a dispersion comprising talc according to the invention.

A grinding system of the type diagrammatically illustrated in figure 1 is used; the tank 11 is provided with a helical mixer for continuously homogenizing the dispersion. The grinding apparatus 14 is an AHM 200 model agitated bead mill from Hosokawa Alpine AG (Augsburg, Germany), loaded with zirconium oxide beads having a diameter in the range of 0.8-1 mm; the beads occupy about 80% of the internal volume of the grinding chamber, and are actuated by a rotor shaft. 75 kg of water and 50 kg of HM4 talc produced by the applicant are introduced into the tank 11 ; the HM4 talc is in the form of powders having a maximum size smaller than 40 μm, D₅o = 7.5 μm and D₉₈ = 30 μm, produced from a talc of Chinese origin with purity higher than 95%.

The dispersion is circulated in the system at a flow speed of 6 l/min by means of pump 12, and the grinding proceeds for 20 minutes; the energy consumed in the process is also monitored, this being an important monitoring parameter. The grinding is then interrupted, a fraction of the dispersion is collected and its pH and viscosity features at 22 °C are measured, using a Brookfield DV-I+ Viscometer with impeller number 2, both at a rotation speed of 10 rpm and of 100 rpm. Another dispersion fraction is collected and dried, and the powder thus obtained is characterized to measure its average diameter (D₅₀), by means of the aforementioned SediGraph 5120 instrument, and the specific surface thereof, by means of a BET gauge. The test results are shown in Table 1. EXAMPLE 2 This example relates to the production of a dispersion comprising talc and smectite according to the invention.

The test in example 1 is repeated under the same conditions as described above, with the only difference that 1 kg of bentonite B 3378 from Sigma Aldrich is introduced in the tank 11 , in addition to 75 kg of water and 50 kg of HM4 talc. A dispersion fraction and a powder fraction are characterized as described in example 1 ; the test results are shown in Table 1. EXAMPLE 3

The test in example 2 is repeated, in this case continuing the grinding for 35 minutes. A dispersion fraction and a powder fraction are characterized as described in example 1 ; the test results are shown in Table 1. EXAMPLE 4

The test in example 2 is repeated, in this case continuing the grinding for 60 minutes. A dispersion fraction and a powder fraction are characterized as described in example 1 (in this case, viscosity at 100 rpm could not be measured, being too high for the reading scale of the instrument); the results are shown in Table 1. Table 1

EXAMPLE 5

In this example, the stickies removal features from papermaking aqueous suspensions using the dispersions of the invention are evaluated. A series of aqueous suspensions of de-inked cellulose (from recycled paper) taken from a paper mill is prepared; each suspension contains 5% b.w. of cellulose. Apart, compositions of the invention are produced in the form of aqueous dispersions, by diluting the fractions of liquid dispersions prepared in examples 1 , 2, 3 and 4 with water to obtain a concentration of 10% b.w. of solids. Different volumes of the dispersions according to the invention are added to each cellulose aqueous suspension, to obtain suspensions which respectively contain 0.5%, 1.0%, 1.5% and 2.0% b.w. of solids of the composition of the invention with respect to the weight of cellulose (a series is produced for each of the dispersions in examples 1-4).

The cellulose suspensions thus produced are stirred by means of a helical stirrer for 1 minute and then left to rest for 5 minutes; at the end of the procedure, the number of particles of stickies present in each suspension is evaluated; Allen's hemocytometer method is used, which includes counting the number of particles in a known volume of liquid deposited in a specific cell under optical microscope; the number of particles is then normalized to 1 cubic centimeter. The same method is used to evaluate the number of particles present in a comparative sample, consisting in the cellulose suspension alone. The results of the measurements carried out on samples with the dispersion of the invention and on the comparative sample (0%) are shown in Table 2. The test results are also provided in the form of a graph in figure 2.

Table 2

EXAMPLE 6

In this example, the features of stickies removal from cellulose aqueous suspensions of a composition of the invention are compared with similar features of two forms of talc of the known art.

A new series of aqueous suspensions of de-inked cellulose taken from a paper mill is prepared, as described in example 5. Apart, a dry micronized talc dispersion, named dispersion T hereinafter, is prepared (using a mill known in the field as "jet mill") without further additions; and a micronized talc dispersion, hereinafter named Tc, is prepared, to which a solution of amine resin is added as cationizing additive; the talc used for preparing both dispersions T and Tc has D50 = 3.2 μm and Dgs = 14 μm. Cellulose suspensions are filled with variable amounts (equal to those in example 5) of the dispersions T and Tc, and with variable amounts of the dispersion prepared in example 3. The procedure of stirring, resting and measuring the number of stickies described in example 5 is repeated on the suspensions thus obtained, as well as on a suspension of cellulose only. The results are shown in Table 3 and in the form of a graph in figure 3. Table 3

EXAMPLE 7

This example refers to a drying test of a composition sample of the invention.

A fraction of the aqueous dispersion prepared as shown in example 3 is subjected to drying, by means of the spray-drying technique. The dispersion, having an initial density of 1.312 g/l and a water content of 62.5% b.w., is fed at 25 ⁰C into an ATM-6 laboratory atomizer from Sacmi lmola S. C. (Imola (BO), Italy) which atomizes it into a treatment tower, in which air at about 350 ⁰C is introduced; a slight vacuum of 10 hectopascals (hPa) is maintained in the tower to segregate the powders inside. The residual humidity of the product obtained from the process, which at the tower outlet has a temperature of 60 ⁰C and appears in micro-granular form, is measured by means of thermo-gravimetric analysis and results equal to 6% b.w. of the sample.

The micro-granular product is subjected to a vibrating sieve test to define the grain size distribution; the results are shown in Table 4.

Table 4

This micro-granular product has a low dust content and a high fluidity, despite the residual humidity; furthermore, it is easily dispersible in water under stirring. EXAMPLE 8

In this example, the features of stickies removal from papermaking aqueous suspensions by using a dry composition of the invention are evaluated.

A sample of talc produced as described in example 1 is dried with the procedure described in example 7, until a degree of residual humidity of 3% is obtained.

The talc thus produced is tested as shown in the procedure of example 5, i.e. by adding it to a series of aqueous suspensions, each containing 5% b.w. of de-inked cellulose from recycled paper; the granular talc is added until suspensions are obtained, which contain respectively 0.5%, 1.0%, 1.5% and 2.0% b.w. of solid with respect to the weight of cellulose. The stickies removal test results are shown in Table 5. EXAMPLE 9

The test in example 8 is repeated, using this time a sample of the talc/bentonite composition produced as described in example 4. The test results are shown in Table 5.

Table 5

As apparent from the data in table 3 and the observation of figure 3, a talc dispersion made according to the invention (sample of example 3) has considerably higher stickies removal features than the two dispersions (T and Tc) normally used in the field, produced according to known methods. The data in table 5, relating to the use of compositions dried before use, show stickies removal values comparable to those of the dispersion of example 3. The comparison of data in tables 2 and 3 further shows that even the least effective sample made according to the invention (sample of example 2) has stickies removal features either similar to (with low charge by weight of the inventive composition as compared to the cellulose to be treated) or higher than (with charge equal to or higher than 1 % b.w. as compared to the cellulose to be treated) the two comparative samples T and Tc.

Claims

1. Use of talc and/or chlorites in the form of particles having an average equivalent diameter smaller than 5 μm obtained by wet grinding of the mineral in a papermaking process.

2. Use according to claim 1 for removing micro-stickies in said process.

3. Use according to claim 1 , wherein cationizing compounds, anionizing compounds or strong bases are added to the talc and/or chlorites.

4. A process for grinding talc and/or chlorites for a use according to claim 1, comprising the steps of: - obtaining a raw material containing at least 55% by weight of talc and/or chlorites;

- grinding said raw material for obtaining powders of the same;

- dispersing an amount of said powders in water so that the resulting dispersion has a mineral powder content of up to 65% by weight, and preferably not lower than 25% by weight;

- subjecting said dispersion to wet grinding in a ball mill to reduce the size of the powder particles to an average diameter smaller than 5 μm and preferably in the range of 0.5-3 μm.

5. A process according to claim 4, wherein an amount of smectite is added to the dispersion of raw material to be subjected to wet grinding so that its percentage by weight is in the range of 0.1 %-5% of the total weight of smectite, talc and/or chlorites.

6. A process according to claim 4 or 5, wherein the wet grinding product is subjected to a partial dehydration process to obtain a residual water content in the range of 2-8% by weight of the final product.

7. A process according to claim 4 or 5, wherein the wet grinding product is subjected to a dehydration process to obtain a residual water content lower than 0.5% by weight of the final product.

8. A composition comprising talc and/or chlorites in the form of particles having an average diameter smaller than 5 μm obtained by wet grinding the mineral and a smectite.

9. A composition according to claim 8, wherein the percentage by weight of smectite in the smectite mixture with talc and/or chlorites is in the range of 0.1 %- 5%.

10. A composition according to claim 9, wherein said percentage by weight is in the range of 1%-3%.

11. A composition according to claim 8, wherein said smectite is a bentonite.

12. A composition according to claim 8, further comprising cationizing compounds, anionizing compounds or strong bases.

13. Use of a composition according to claim 8 in a papermaking process.

14. Use according to claim 13, wherein said composition is used in the form of micro-granules.

15. Use according to claim 14, wherein said micro-granules contain from 2 to 8% by weight of humidity.