FI72334B - GIPSSLAM SOM AER PUMPBART VID HOEG FASTAEMNESHALT - Google Patents

Description

1 72334

Gypsum slurry pumpable at high solids content

The invention relates to a gypsum slurry pumpable at a high solids content.

In Paper and Wood No. 9 (1976) p. It is known from the published article 558-570 that conventional dispersants, polyphosphate and Na polyacrylate, do not disperse gypsum as well as other pigments used in paper coating. Carboxymethylcellulose (CMC), on the other hand, has been shown to be useful for this purpose, according to the article. In order to achieve the best dispersion result, CMC of up to 3% must be used for fine gypsum and about 2% for even coarser gypsum. A similar dispersion method is disclosed in Finnish patent application 793162, according to which gypsum is ground only after dispersion.

The use of a polyacrylate dispersant in the dispersion of gypsum in combination with a pulp derivative which provides steric stability is known from Finnish patent application 750387. The same application discloses Significance of EDTA when used with the foregoing.

The cost of gypsum slurry increases considerably with these known dispersion methods due to the high amount of CMC and the high unit cost of CMC. Therefore, it would be highly desirable to find cheaper dispersion methods for this purpose. The properties of the gypsum slurry prepared according to the above-mentioned Finnish patent application 793162 as a paper coating pigment are surprisingly poor, especially with regard to the poor brightness obtained by the paper 2 72334. Namely, it has been found that when using gypsum pigment slurry prepared in this way as a paper coating agent, the brightness of the coated paper remains approximately as low as when using kaolin, although the brightness of the gypsum itself far exceeds the brightness of kaolin. A further problem is that the heating of the sludge during grinding causes a strong increase in viscosity, so that grinding becomes difficult and the product obtained is very thixotropic and difficult to handle with conventional sludge treatment equipment.

The present invention relates to gypsum slurry in which said disadvantages have been largely eliminated in a simple manner. Indeed, it has surprisingly been found that by using depolymerized starch or a derivative thereof in place of CMC, with the exception of at least two different polymers, the first being a polysaccharide derivative, preferably CMC, and the latter a polysaccharide or a second polysaccharide-derived with a much smaller amount of dispersant.

The gypsum slurry according to the invention can be prepared by dispersing gypsum ground to the desired particle size and shape with the dispersants according to the claims. Grinding of the gypsum can then take place either wet or dry. Another way is to make a flexible gypsum slurry according to the invention from coarse gypsum and grind it using wet grinding to the desired final fineness depending on the purpose of the slurry. The application can be, for example, a paper filler or coating pigment, a paint or glue filler or a component of a mortar or leveling compound.

The effect of the polyphosphates and polyacrylates commonly used in the dispersion of pigments and fillers is based on the fact that they provide a negative charge on the particle to be dispersed. The larger the

3 72334 charge, the more the particles tend to repel each other due to their electric charges of the same brand and the more stable the dispersion is.

CMC is only slightly anionic. Its effect as a dispersant is believed to be due to its protective layer, which prevents the dispersed particles from colliding and flocculating (steric stabilization).

When CMC was used in a known manner as a dispersant in the grinding of gypsum, at least 1.5% of CMC of the amount of gypsum was required in order to be able to achieve a grinding result of 85% below 2 [mu] m at a high solids content.

As there was no certainty about the functionality of CMC used in grinding as a water retention agent for coating paste, for which purpose it is commonly used, studies were initiated to find a cheaper alternative. The experiments showed, as can be seen from Finnish application 810311, that using starches, for example phosphoric acid ester made from oxidized potato starch alone or with CMC, gave ground gypsum with a lower viscosity than using CMC alone. However, the unground phosphogypsum used in the experiments was more easily dispersed into a pumpable slurry by means of CMC, which in this case required a maximum of 0.5%.

As the experiments continued, it was now surprisingly found that in addition to this small amount of CMC, in addition to starches, several other polymeric compounds could be used to obtain the desired slurry with cheaper additives or lower dosage amounts or a better preserved state than starches. Such other polymeric compounds included: - dextrin - dextran (polysaccharide produced by polymerization of sugar) - polyvinyl alcohol 4 72334 - polyalkylene glycol (= polyalkylene oxide) and - carboxymethylcellulose with a different degree of polymerisation from the CMC mentioned above.

A combination of two different CMCs was found to work / especially at elevated temperatures.

The gypsum slurry according to the invention is obtained by using polymers 0.2-2% by weight of gypsum. More can, of course, be added, then preferably after grinding the gypsum, for example when making a paper coating paste from gypsum, to which these substances may otherwise be added. In addition to the additives according to the invention, a latex binder which may be required in the paste can then also be added.

In the polymer combination according to the invention, the ratio of polymers can be varied in the range of approximately 0.2: 1 to 1: 0.2, although e.g. the relative proportion of dextran may be lower.

The invention is further illustrated by the following examples. The viscosity measurement has been performed at the optimum pH for each combination. This pH was found to be highly variable depending on the quality of the starting gypsum and was in the range of 7-10.5 in various example cases.

Example 1

Approximately 80% of the gypsum ground to a particle size of less than 2 microns, with a moisture level adjusted to 31%, was dispersed using 0.27% CMC (average degree of polymerization about 200) and 0.08% dextrin1 from the dry gypsum amount and 0.1% polyacrylate dispersant and 0.15% sodium hexa-metaphosphate. A gypsum slurry with a viscosity of 2300 cP was obtained. The same experiment was repeated without dextrin, but using CMC 0.35% of the amount of gypsum. The viscosity was then about 9000 cP.

II

= medium viscosity commercial quality 5 72334

Example 2

The dispersion test according to Example 1 was performed using 0.27% CMC with an average degree of polymerization of about 200 and 0.08% CMC with an average degree of polymerization of about 300. The viscosity at pH 8.6 was found to be 1900 cP.

Example 3

Various finely ground gypsums were dispersed using 0.2% sodium hexametaphosphate and 0.1% polyacrylate dispersant in the amount of dry gypsum. Increasing amounts of CMC were added to the dispersant and viscosities were measured. The average polymerization rates of the CMC grades used in the experiments were about 200 for CMC 1 and about 300 for CMC 2. The results are shown in Table 1.

table 1

Dispersion test results of two finely ground gypsum samples from different gypsum samples (Brookfield 100 rpm; cP) CMC amount Gypsum 1, solids 54% Gypsum 2, solids 61%

% gypsum (approx. 85% <2 μm), 30 ° C (approx. 85% <2 μm), 40-45 ° C

by weight CMC χ CMC1 / CMC2 = 4: 1 CMC 1 CMC1 / CMC2 = 4: 1 O 7100 7100 9400 9400 0.1 1900 1800 6000 4300 0.2 1900 1600 5000 4500 0.3 2200 1300 5000 5000 0.4 2300 1500 6500 5400 0.5 2800 1600 6600 5300

Example 4 At 54% solids, gypsum was ground to 85% less than 2 microns. This gypsum was dispersed using increasing amounts of CMC (average degree of polymerization about 200) as a dispersant or first 0.15% of the amount of gypsum dextran (about 2 x 10 4) and then increasing amounts of CMC. The dispersion result shown in Figure 1 was obtained. Increasing the proportion of dextran worsened the dispersion result.

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Example 5

The gypsum was ground in a mill equipped with micromills using 1.5% CMC (average degree of polymerization about 200) as dispersants and 0.2% polyacrylate dispersant at the highest possible solids content of about 85% to a fineness of less than 2 .mu.m. The solids content was then 68%.

At the same solids content and the same particle size, gypsum was ground using 0.2% polyacrylate dispersant and, in addition, the following polymer combinations according to the invention at different grinding times.

a) 0.5% CM3 (average degree of polymerization approx. 200) and 0.2% dextrin (medium viscosity grade), b) 0.5% CMC and 0.5% polyethylene glycol (M approx. 400), c) 0.5 % CMC and 0.5% polyvinyl alcohol (average molecular weight 500-600).

Grinding was performed well as desired, although the amount of dispersant was less than half in case a) and two-fifths in cases b) and c) compared to that used in the comparative experiment.

Example 6

Paper coating pastes were prepared from the gypsum slurries of Example 4 by adding 9 parts by weight of styrene-butadiene latex (calculated as dry matter) per 100 parts of dry coating pigment and 0.5 parts of CMC (average degree of polymerization) to improve the water retention of the pastes. the addition of polyacrylate dispersant required to achieve this, which was 0.1 part when using the gypsum slurries according to the invention, 0.3 parts when using the reference slurry (stability was tested by measuring the viscosity of pastes stored at 45 ° C for 1 day).

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In addition, a comparison paste was made from the most commonly used coating kaolin (0.3 parts by weight of polyacrylate dispersant, 9 parts of latex, 0.5 parts of CMC with an average degree of polymerization of about 200 and 0.5% of CMC with a degree of polymerization of more than 200 ).

Pastes were tested for coating wood-containing paper with a Dixon-branded test coater using the blade method. The results shown in Table 2 were obtained (results interpolated to correspond to a coating amount of about 10 2 g / m 2).

Table 2

Results of a paper coating test (description of gypsum: see example 4)

Feature Comparative Kaolin gypsum gypsum according to the invention a) b) c)

Opacity,% 89.4 89.6 90.2 90.5 90.1

Brightness,% 71.0 72.1 71.8 72.2 71.2

Gloss (Hunter 75 °) 40 40 44 38 52

Wet M16 (lowest reading best) 22 13 12 20 64 Color absorption (K&N) 20 21 21 24 16

Surface strength (1 g / t) 2.0 1.9 1.6 1.6 1.0

The gypsum according to the invention achieved better brightness than kaolin, but worse with the reference gypsum. The gypsum according to the invention obtained all other properties better or almost at the same level as in the reference gypsum, with the exception of the surface strengths of gypsum b) and c), which also clearly exceeded the corresponding value of kaolin.

Claims (10)

A high solids pumpable gypsum slurry consisting essentially of gypsum, water and dispersants, characterized in that the dispersant contains at least two different polymeric compounds other than depolymerized starch or its derivatives, the first being a polysaccharide derivative, the second a polysaccharide derivative, the second a polysaccharide derivative -niglycol (polyalkylene oxide) or a different polysaccharide derivative than the first polysaccharide derivative. Gypsum slurry according to Claim 1, characterized in that the first polymer is a cellulose derivative. Gypsum slurry according to claim 1, characterized in that the first polymer is carboxymethylcellulose having a degree of polymerization of about 200 to 500. Gypsum slurry according to one of Claims 1 to 3, characterized in that the second polymer is dextrin. Gypsum slurry according to one of Claims 1 to 3, characterized in that the second polymer is dextran. Gypsum slurry according to one of Claims 1 to 3, characterized in that the second polymer is a cellulose derivative having a substantially higher degree of polymerization than the first polymer. Gypsum slurry according to Claim 6, characterized in that the second polymer is carboxymethylcellulose. Gypsum slurry according to one of the preceding claims, characterized in that it additionally contains conventional dispersants, such as, for example, polyphosphates and / or polyacrylates. Gypsum slurry according to any one of the preceding claims, characterized in that it contains a total of 0.5-1.2% by weight of gypsum of said polymeric compounds. Gypsum slurry according to one of the preceding claims, characterized in that the weight ratio of both polymers is from about 0.2: 1 to 1: 0.2. 10 _ - τ ~ 72334