FI69155B - FOERFARANDE FOER FRAMSTAELLNING AV GIPSSLAM - Google Patents

Description

69155

Method of making gypsum slurry

The invention relates to a process for the preparation of gypsum slurry which can be pumped at a high solids content. For example, such a slurry can be ground into a paper coating pigment or filler.

In Paper and Wood Journal 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, 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, carboxymethylcellulose of up to 3% must be used for fine gypsum quality and about 2% for even coarser gypsum quality. Such a dispersion method is also disclosed in Finnish Patent Application No. 793162.

The cost of gypsum slurry increases considerably with this known dispersion method due to the high amount of carboxymethylcellulose and the high unit cost of carboxymethylcellulose. Therefore, it would be highly desirable to find cheaper dispersants for this purpose. Furthermore, the use of carboxymethylcellulose has the disadvantage of a large amount of water entering the gypsum, since aqueous solutions of carboxymethylcellulose contain up to 15% by weight of active ingredient.

It is therefore an object of the present invention to provide a process for preparing gypsum slurry by dispersing water, finely divided gypsum and at least one dispersant to give 69155 a very thick but still pumpable gypsum slurry which can be further ground by wet milling equipment from paper fillers or coating pigments.

It has now surprisingly been found that a very thick but still pumpable and uniform gypsum slurry can be prepared using depolymerized starch or a derivative thereof as a dispersant. Preferably, the acid is enzymatically or oxidatively cleaved starch, optionally etherified or esterified, for example phosphoric acid ester (starch phosphate). In addition, it has been found that a mixture of cleaved starch or a derivative thereof and carboxymethylcellulose or sodium hexametaphosphate gives a better result than would be expected from the separate action of these substances.

The starch depolymer or derivative thereof is usually added in an amount of about 0.5-3% and preferably 1-1.5% by weight of the gypsum. When carboxymethylcellulose is used concomitantly, the weight ratio of depolymerized starch or derivative thereof to carboxymethylcellulose is preferably about 1-2: 1.

If it is desired to use starch as a binder in the coating paste, the above-mentioned amounts of starch sufficient for dispersion can be exceeded and the amount of starch required as binder can be added before grinding or in multi-stage grinding between grinding steps or, if the mill design allows, directly to the mill. In this way, it is possible to prepare a starch-containing coating paste with a higher solids content than when the binder is added after grinding. The high solids content of 3 691 55, in turn, leads to more economical drying of the coating and is an advantage e.g. to provide a better gloss to the coated paper.

Thus, it has surprisingly been found in experiments that de-polymerized starches or their derivatives, which are considerably cheaper than carboxymethylcellulose, gave almost as good a dispersing result, often even clearly better. In addition, mixtures of depolymerized starches and carboxymethylcellulose achieved lower viscosities than both components alone.

Phosphoric acid ester of depolymerized starch has previously been used as a binder for coating paper with semi-crystalline calcium sulfate (CaSO 4/2 1 0 0) (FR Patents Nos. 2,343,082 and 2,370,124). However, the present invention relates to gypsum,

CaSO 2 and 2 starch phosphate are used as dispersants and not as binders.

Particularly useful are those grades of starch in which the molecular weight of the native starch has been reduced so that the starch can be made into a solution containing more than 20% solids. Such a solution can be added to the wet gypsum obtained by filtration after the cleaning steps (washing, grading, foaming, neutralization and / or bleaching) without reducing the solids content of the gypsum slurry. Such grades of starch include starches modified from natural starch by acid treatment, enzymatically or by means of oxidants and / or products obtained by substituting them (e.g. by esterification or etherification). Such a substituted starch is, for example, a phosphoric acid ester of depolymerized starch. Such starches are prepared e.g. binders for the paper industry.

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They can also be prepared separately for the purpose of the invention, in which case it is advantageous to produce smaller molecular qualities than the previous ones.

The invention will be described in more detail below by means of examples and with reference to the accompanying drawings, in which Figures 1 and 2 show the viscosity of gypsum slurry as a function of dispersant and its amount when using different dispersants or combinations thereof.

Example 1

For dispersant experiments related to the preparation of paper coating pigment, gypsum was ground in an aqueous slurry in a mill equipped with micro-refiner bodies exceptionally without any dispersants.

This gave a gypsum slurry with a solids content of 57% and a viscosity of 4500 cP (Brookfield, 100 rpm).

Gypsum slurry was dispersed using increasing amounts of carboxymethylcellulose (average degree of polymerization about 200) and oxidized hot water soluble potato starches. A 20% solution of the latter was prepared by raising the temperature from 5 ° C / min to 90 ° C and continuing the heating for another 15 minutes.

The results are shown in Figure 1. A 1: 1 mixture of oxidized starch (average molecular weight about 200,000) and carboxymethylcellulose gave a better result than either alone. Starch gave a slightly better result than carboxymethylcellulose. Separately, the effect of lower molecular weight starch (average molecular weight below 100,000) was tested. The result was better than the previous starch grade and was practically the same as shown in the bottom curve of Figure 1.

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Example 2

For dispersant experiments, an aqueous slurry was prepared from purified phosphogypsum without the use of any dispersants and ground by grinding the slurry in a mill equipped with microminer pieces. The result was a 47% solids slurry with a viscosity of 2400 cP (Brookfield, 60 rpm). As determined by the sedimentation method, about 60% by weight of the gypsum particles had a particle size of less than 2.

This gypsum pigment slurry was used to test the effect of different dispersants on the viscosity of the slurry. The polyacrylate dispersant (trade name Dispex N 40) commonly used in the dispersion of coating pigments reduced the viscosity of the slurry only to a very small extent. Thus, 1.5% (active ingredient based on dry gypsum) reduced the viscosity to only 1500 cP. With carboxymethylcellulose (average degree of polymerization 200), a viscosity of 400 cP was achieved with the same active ingredient dosage. Using the starch phosphate of the invention as a dispersant (trade name Nylgum A 85), a viscosity of 80 cP was obtained. Complete results for carboxymethylcellulose and starch phosphate are shown in Figure 2.

Example 3

From the purified, dispersant-free phospho-gypsum, a slurry in which 80% by weight of the gypsum had a particle size of less than 2 [mu] m was ground in a mill equipped with microminer pieces at the highest possible solids content. Gypsum was dispersed using carboxymethylcellulose, starch phosphate, and a mixture thereof. The results shown in Table 1 were obtained.

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table 1

Effect of carboxymethylcellulose, starch phosphate and their mixture on the viscosity and state of gypsum pigment slurry

Dispersion Amount Viscosity, cP Slurry% (Brookfield) state 50 rpm 100 rpm - 12,000 6,900 near-uniform dispersion

Carboxymethyl 1,6,900 4,100 uniform cellulose dispersion Starch phosphate 1,200,200 1,100 floccutate slurry

Carb.methylcellulose li 45Q 45Q uniform Starch phosphate 1 (dispersion

Carboxymethyl 2,400,700 uniform cellulose dispersion Starch phosphate 2,850,850 flocculated slurry Using starch phosphate gave a lower viscosity than carboxymethylcellulose. The lowest viscosity was obtained with a mixture of starch phosphate and carboxymethylcellulose.

When using starch phosphate, gypsum slurry formed flocculated in the opposite direction as when using carboxymethylcellulose. Using a mixture of carboxymethylcellulose and starch phosphate, flocculation could be avoided.

The above examples are from experiments in which the dispersible gypsum was pre-ground. In practice, the dispersion has to be carried out before grinding, otherwise it is not possible to make pumpable 69155 gypsum slurries with a solids content at the level required for paper coating pigments, i.e. at least 65%, preferably above 70%.

Example 4

Purified phosphogypsum was dispersed at 68% solids using carboxymethylcellulose and starch. The dispersion result was evaluated not only by viscosity measurements but also by the so-called juoksevuustestillä. The latter experiment was carried out by filling the ring (diameter 33 mm, height 31 mm) with the dispersed gypsum slurry, which was lifted perpendicularly upwards from the substrate. The average width of the cake formed by the gypsum slurry was measured. The large width therefore means good fluidity. The test results are shown in Table 2.

Table 2

Results of dispersion experiments of unground phosphogypsum

Dispersion- Amount Viscosity, cP Spreading agent (Brookfield 100 measure rpm) mm

Carboxymethyl 1.5% 1700 136 cellulose Starch phosphate 1.5% 1400 55 A 2: 1 mixture of starch phosphate 1.5% 1300 138 and carboxymethylcellulose In this case, too, the Brookfield viscometer measured starch phosphate better than carboxymethylcellulose. However, the disadvantage is the low spread rate, which indicates poor handling. Using the mixture, this deficiency was eliminated.

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Example 5

An experiment similar to the previous example was also performed with a second unmilled gypsum sample at a solids content of 68%. Carboxymethylcellulose and an oxidized potato starch grade (average molecular weight about 100,000) supplied as a ready-to-use solution containing stabilizers were used for dispersion. The results are shown in Table 3.

Table 3

Results of dispersion experiments of unground phosphogypsum

Dispersion Amount Viscosity, cP Dispersant (Brookfield 100 measure r / min) mm

Carboxymethyl-1.5% 280 147 cellulose

Carboxymethyl 1.5% 290 143 cellulose + oxygen starch. 2: 1

Carboxymethyl 1.5% 240 145 cellulose + oxygen starch. 1: 2

Oxidizing starch 1.5% 700 87

Based on Examples 4-5, it is found that at least two-thirds of the carboxymethylcellulose can be replaced by starch without compromising its flow properties.

When using starches, the preservation of the slurry must be ensured in a known manner by means of preservatives.

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Claims (8)

691 55 A process for preparing a high solids pumpable gypsum slurry by mixing water, gypsum and one or more dispersants, characterized in that up to 3% by weight of gypsum of depolymerized starch or its derivative is added as a dispersant. Process according to Claim 1, characterized in that starch which is enzymatically or oxidatively modified and optionally substituted by an acid is added as depolymerized starch or a derivative thereof. Process according to Claim 1 or 2, characterized in that esterified or etherified depolymerized starch is added as the starch derivative. Process according to one of the preceding claims, characterized in that starch phosphate is added as a derivative of the depolymerized starch. Process according to one of the preceding claims, characterized in that carboxymethylcellulose is added in addition to the depolymerized starch or a derivative thereof. Process according to one of the preceding claims, characterized in that sodium hexametaphosphate is added in addition to the depolymerized starch or a derivative thereof. Process according to one of the preceding claims, characterized in that at least about 0.5% by weight of the gypsum is added to the depolymerized starch or its derivative. 10,691 55 Process according to one of the preceding claims, characterized in that depolymerized starch or a derivative thereof and carboxymethylcellulose are added in a weight ratio of about 1-2: 1. 69155