GB2173781A

GB2173781A - Water slurry of gypsum or chalk

Info

Publication number: GB2173781A
Application number: GB08608889A
Authority: GB
Inventors: Axel Ingemar Thebrin; Pia Christina Thebrin
Original assignee: Berol Kemi AB
Current assignee: Nouryon Surface Chemistry AB
Priority date: 1985-04-11
Filing date: 1986-04-11
Publication date: 1986-10-22
Also published as: SE8501773D0; DE3612087A1; GB8608889D0; GB2173781B; DK161586A; DK161586D0; FI861530A0; FI861530A

Abstract

A slurry of gypsum or chalk in water containing a dispersing agent in the form of a surface active organic sulphate or sulphonate, and a complexing agent in the form of an inorganic phosphate or an organic amine. The slurry has low viscosity at the desired content of gypsum or chalk.

Description

SPECIFICATION Water slurry of gypsum or chalk The present invention relates to a water slurry of gypsum or chalk containing an anionic surface active agent as a dispersing agent, as well as an inorganic phosphate or organic amine compound as a complexing agent.

Aqueous slurries of fine-particuiate gypsum, CaSO4. 2H2O, or chalk, CaCO3, have been used as paper coating agents, within the building industry and as fillers for many applications. In order to obtain a homogeneous dispersion, carboxymethyl cellulose (CMC) is usually added as a dispersing agent. However, water slurries based on gypsum or chalk have found limited use because it is difficult to produce a slurry having a sufficiently high gypsum or chalk content and, at the same time, low viscosity and high stability.

Normally, a dry content of about 65-80% by weight is desired in such slurries.

It has now proved to be possible to reduce the viscosity of an aqueous slurry of gypsum or chalk at a given dry content of usually about 50% and, in some useful embodiments, of 70% or more, as compared to the viscosities obtained with a corresponding slurry but with CMC as a dispersing agent. According to the invention, the slurry contains, besides fine-particulate gypsum or chalk and water, a dispersing agent in the form of a surface active orgnic sulphonate or sulphate in an amount of 0.2-3, preferably 0.4-1.5% by weight, based on the weight of the composition, and a complexing agent in the form of an inorganic phosphate or an organic amine compound in an amount of 0.1-3, preferably 0.2-1.5% by weight, based upon the weight of the composition.The reason for this surprising viscosity reduction is not known, but one theory is that divalent cations occurring in the slurry serve as a coupling member between anionic sites on the gypsum or chalk particles and the anionic compound. In this manner, a bimolecular layer is formed, whereby an electrostatic dispersion of the particles is made possible. A slurry of gypsum or chalk always contains a certain amount of Ca2+ ions because gypsum and chalk is sparingly water-soluble. Due to the ability of the complexing agent to form complex with Ca2 it is believed according to this theory that the Ca2+ function as a coupling member is facilitated. In many cases, the amount of Ca2+ is insufficient in relation to the total amount of cations in the aqueous phase, for which reason in such cases further bi- or polyvalent cations should be introduced into the slurry.Preferably, this is done by adding the anionic compound in the form of a divalent salt, preferably Ca2+. Suitable anionic surface active sulphonates and sulphates according to the invention are, for example, alpha-olefin sulphonate derived from alpha-olefins having 822 carbon atoms, alkyl sulphate having 822, preferably 12-18 carbon atoms, and alkyl benzene sulphonate having 10-30, preferably 12-24 carbon atoms. They may be present in the form of a salt and/or as an acid, salts with divalent cations, such as calcium and magnesium, being preferred.

Examples of suitable inorganic complexing phosphates according to the invention are conventional complexing phosphates, such as tetrasodium pyrodiphosphate, sodium hexametaphosphate, trisodium phosphate, sodium hydrogen pyrophosphate, sodium tripolyphosphate, and corresponding potassium compounds. Of these compounds tetrasodium pyrophosphate, sodium hexametaphosphate, and trisodium phosphate are preferred.

The complex forming organic amine compounds comprise any conventional complexing agent of this group and especially those amine compounds having at least one primary or secondary nitrogen and at least one hydroxyl group, such as monoethanolamine, diethanolamine, hydroxypiperazine, and aminoethyl ethanolamine. Hydroxypiperazine and aminoethyl ethanolamine are preferred.

Gypsum in finely divided form is obtained in large quantities in the production of phosphoric acid from the phosphate-containing minerals via recrystallisation. Chalk suitable in the preparation is precipitated chalk but chalk of other qualities may also be used. The final selection of qualities of gypsum and chalk is of course ruled by the intended use of the slurry. The particle size and the particle size distribution affect materially the viscosity of the slurry. The average particle size suitably is below 100 um, preferably below 25 um. A particle size distribution having a high proportion of fines facilitates close packing of the particles and also facilitates the preparation of slurries having high filler contents at suitable viscosities. A suitable milling technique to obtain the desired particle size distribution is described in international patent application No. 83/04046.

Finely divided material may also be achieved by milling the aqueous slurry itself. The amount of gypsum and/or chalk in the slurry is normally about 65-85% by weight.

In addition to the said surface active anionic compounds and complexing agents, the slurry may contain conventional additives, such as bactericides, defoaming agents, pH-controlling agents, and stabilizers. In some cases, the stabilisation obtained with the dispersing compound alone may be further improved by the addition of supplementing stabilisers. Such stabilisers are primarily clays, such as bentonite coated with a hydrophobing agent. Other useful stabilisers are cellulose ethers, such as CMC, methyl cellulose and ethyl hydroxy cellulose. The above-mentioned cellulose ethers also have a useful water-retaining ability. The amount of stabilisers preferably is 0.02-0.5, preferably 0.05-0.3% of the weight of the slurry.

The slurry of the invention may also besides gypsum and chalk contain additional particulate material, such as talc and clays. Such additional particulate material can be added when formulating a slurry of the invention. This method,is suitable in the case the amount of additional particulate material is low. Another method is to prepare a particular aqueous slurry containing the additional particulate material and mix this slurry with a slurryofthe invention.

The invention will be further illustrated by the following Example.

Example To 100 parts of finely divided gypsum (runs 1-19 and references A-D) or chalk (runs 20 and 21 and references E and F) having a water content of 25% by weight and a particle size of 1-25 um, an anionic surface active compound and a complexing agent in the form of an inorganic phosphate or an organic amine compound as well as sodium hydroxide and additional water were added, such that pH 9 and the dry contents stated in the Table below were obtained. The mixture was rapidly agitated so that a dispersion was obtained. The viscosity of the dispersion was determined by means of a Brookfield RLV at 30 and 60 rpm.

Stability was determined visually after 24 hours according to a scale of 1-2, 1 meaning that no phase separation has occurred and 2 that two distinct phases could be observed. The following results were obtained.

TABLE Run Anionicsurface g/kg par- Complexing agent g/kg par- Drycon- Viscosity Stability active compound ticulate ticulate tent%by 30 rpm 60 rpm material material weight 1 Ca-dodecyl- 10 Na4-pyrophosphate 5 67.0 240 160 1 benzene sulphonate 2 " 10 " 3 69.7 300 140 1 3 " 10 " 2 71.8 420 390 1 4 " 10 " 1 72.0 560 560 1 5 " 7 Na6-metaphosphate 6 73.4 350 300 1 6 " 7 Na4-pyrophosphate 6 73.5 800 750 1 7 " 11 Na6-metaphosphate 2 72.8 920 800 1 8 " 11 Na4-pyrophosphate 2 75.0 580 400 1 9 " 7 Na6-metaphosphate 3 75.0 280 250 1 10 " 11 K4-pyrophosphate 2 72.1 590 410 1 11 Dodecylbenzene 5 Na4-pyrophosphate 1 70.9 1240 900 1 sulphonic acid 12 " 5 ,, 1 72.0 830 790 1 13 Ca--C,4 t6 10 " 3 70.2 380 240 1 olefin sulphonate 14 Ca-C11G-alkyl 10 " 3 70.1 380 260 1 sulphate 15 Dodecylbenzene 5 Monoethanolamine 5 72.1 600 450 1 sulphonic acid 16 " 5 Diethanolamine 5 70.1 1000 800 1 17 " 5 Hydroxyethyl 5 70.2 100 100 1 piperazine 18 " 5 Aminoethyl 5 70.1 50 50 1 ethanolamine 19 " 4 ,, 4 70.0 400 300 1 20 " 3 ,, 2 73.2 350 250 1 21 " 3 Na6-metaphosphate 2 73.0 390 300 1 Reference Additive A Carboxymethyl 15 72.0 1300 1500 2 cellulose B Ca-calcium bensene 15 72.1 3100 1800 2 C Na4-pyrophosphate 15 72.0 5000 5000 2 D Na6-metaphosphate 15 72.0 5000 5000 2 E ,, 3 73.0 2000 2000 2 F Aminoethyl 3 73.0 2100 2000 2 ethanolamine

Claims

1. A water slurry of gypsum or chalk comprising as dispersing agent 0.2 to 3% by weight of a surface active anionic organic sulphonate or sulphate and, as complexing agent, 0.1 to 3% by weight of an inorganic phosphate or organic amine compound.

2. A slurry according to claim 1 comprising 0.4 to 1.5% by weight of the surface active anionic organic compound.

3. A slurry according to claim 1 or claim 2 comprising 0.2 to 1.5% by weight of the inorganic phosphate or organic amine compound.

4. A slurry according to any one of claims 1 to 3 wherein the surface active anionic organic compound is alkyl benzene sulphonic acid or the calcium salt thereof.

5. A slurry according to any one of claims 1 to 4 wherein the complexing agent is tetrasodium pyrophosphate, sodium hexametaphosphate or trisodium phosphate.

6. A slurry according to any of claims 1 to 5 wherein the complexing agent is an organic amine compound having at least one primary or secondary nitrogen and at least one hydroxyl group.

7. A slurry according to claim 6 wherein the organic amine compound is aminoethyl ethanolamine or hydroxyethyl piperazine.

8. A slurry according to any one of claims 1 to 7 comprising, as stabiliser, a clay or a cellulose ether in an amount of 0.02 to 0.5% by weight.

9. A slurry according to any one of claims 1 to 8 wherein the average particle size is below 1 00um.

10. A slurry according to claim 9 wherein the average particle size is below 25um.

11. A slurry according to any one of claims 1 to 10 comprising from 65 to 85% total wieght of gypsum and chalk.

12. A slurry according to claim 1 and substantially as hereinbefore described with reference to the Example.

13. A process for producing a slurry according to any one of claims 1 to 12 comprising bringing into association water, gypsum or chalk, a surface active anionic organic sulphonate or sulphate and an inorganic phosphate or organic amine compound.

14. A process according to claim 13 and substantially as hereinbefore described with reference to the Example.

15. A process for producing coated paper comprising applying a slurry according to any of claims 1 to 12 to paper.

16. Use of a slurry according to any one of claims 1 to 12 in the coating of paper.