Claims
- 1. A method of using calcium carbonate particles having an acid-resistant coating as a filler in the production of paper at a pH of 4.0-7.0, the particles having been produced by mixing a slurry of calcium carbonate particles simultaneously with the solution of a zinc compound and a solution of a silica-containing substance at a temperature of 70.degree.-95.degree. C. and a pH of 8-11.
- 2. The method according to claim 1 wherein the silica containing compound is sodium water glass.
- 3. The method according to claim 1 wherein the zinc compound is zinc chloride or zinc oxide.
- 4. The method according to claim 1 wherein the temperature is 85.degree.-95.degree. C.
- 5. The method of claim 1 wherein the pH is 9-10.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 8802708 |
Jul 1988 |
SEX |
|
BACKGROUND
This application is a divisional, of application Ser. No. 07/382,318, filed July 20, 1989 pending.
The present invention relates to a process for the preparation of an acid-resistant coating on filler particles for e.g. paper by mixing slurried calcium carbonate particles simultaneously with a solution of a zinc compound and a solution of a silica-containing substance. The invention also relates to such coated particles, and to the use thereof as a papermaking filler.
Fillers in the form of finely divided particulate or plate-shaped inorganic materials are in general use in the manufacture of, for example, paper, paint, printing ink and rubber. In this connection, calcium carbonate is in common use and has the advantages of being in ample supply and cheap. However, its low acid-resistance is a disadvantage, especially in the papermaking industry. Thus, calcium carbonate, when used as a filler in paper, is inconvenient in so far as its reacts with alum which is also used in papermaking. Alum which is aluminium sulphate (Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O), produces aluminium hydroxide and sulphuric acid when added to the "wet" papermaking system. The flocculation of aluminium hydroxide is capable of collecting and retaining the filler and also part of the resin-alum system for paper sizing. During manufacture, the alum flocks in the paper web are removed, and the sulphuric acid is increasingly concentrated in the white water, in spite of the buffer action of the system. The presence of calcium carbonate in a system of pH 4 will quickly consume part of the sulphuric acid, thereby forming calcium sulphate and free Ca.sup.2+ ions. The presence of these ions in the recycled white water causes a considerable reduction of the sizing effect in the papermaking pulp, and this in turn necessitates the use of large amounts of alum to maintain stable sizing conditions.
A variety of techniques to modify calcium carbonate by surface treatment to achieve higher acid-resistance are disclosed in literature.
GB 1,295,264 discloses particulate surface-treated calcium carbonate adapted to be used as a filler and treated with an aqueous solution of, inter alia, water glass to coat the particles with a shell of, inter alia, calcium silicate, whereby it is possible to reduce solubility by at least 50% in an alum solution at a pH of about 5.5. However, the stability of this shell is not entirely satisfactory since it may break loose or be dissolved out upon further working of the paper product.
DE-A 3,014,620 discloses surface-modified calcium carbonate particles that have been treated with a solution containing specific cations of similar size, inter alia zinc ions, whereby the calcium ions in the crystal lattice are replaced by, inter alia, zinc ions, and this results in an improved acid-resistance and higher stability than with the first-mentioned technique. Nevertheless, a certain amount of alum is still precipitated in papermaking, for which reason the acid-resistance should be improved further.
SE-A 415,028 discloses the modification of inorganic fillers, for example calcium carbonate, by organosilane modification and metal silicate treatment of the filler.
It has now been found that a modification with a silica-containing compound combined with a modification of a zinc-containing compound imparts to calcium carbonate to be used as a filler an acid-resistance significantly higher than can be obtained with prior art technique, if the combination is conducted such that both additions are made at the same time.
By testing different variants of stepwise addition and simultaneous addition it was found that stepwise additions give but an insignificant improvement of the acid resistance in relation to the individual effects, whereas the simultaneous addition causes a synergistic effect resulting in a surprisingly significant improvement of the acid-resistance.
The present invention thus relates to a process having the characteristic features stated in the appended claims, for the preparation of a surface-modified filler of calcium carbonate acid-resistant at pH 4.5-7, which makes it useful as a filler in paper produced by acid technique and treated with alum upon sizing.
According to the invention, a slurry of CaCO.sub.3 filler particles is mixed simultaneously with a solution of a zinc compound and a solution of a silica-containing substance which preferably is sodium water glass. The zinc compound preferably is zinc chloride or zinc oxide. The two solutions are added at a temperature of 70.degree.-95.degree. C., preferably 85.degree.-95.degree. C., especially preferred 90.degree. C. Furthermore, the addition is carried out pH 8-11, preferably 9-10, especially preferred 9.2-9.4. The added amounts of coating agent, i.e. of a silica-containing substance, preferably water glass, and of a zinc compound, preferably zinc chloride, are 300-400 parts by weight SiO.sub.2 and 40-60 parts by weight Zn. The SiO.sub.2 :Zn ratio is 7.5:1 to 6.6:1, which corresponds to pH 8-11.
The simultaneous addition implies that the two substances are added without any intervening treatment step, the addition being initiated in principle at the same time or with as little a time lag as possible and conducted at substantially the same flow rate.
To carry the process according to the invention into effect, one proceeds as follows. A slurry of calcium carbonate particles (PCC--Precipitated Calcium Carbonate, milled chalk or other CaCO.sub.3 filler) in deionised water is added simultaneously with a diluted solution of ZnCl.sub.2 or ZnO and with a diluted Na water glass solution containing SiO.sub.2 and vigorously stirred. The addition is carried out at a tempeture of 70.degree.-95.degree. C. By optionally adding 0.1 M H.sub.2 SO.sub.4, the pH is adjusted during the process to 8-11. After filtration, washing and drying, the surface-modified particles can be used as filler.
The acid-resistance (AR--Acid-Resistance) is measured in the following manner. A 4% PCC slurry is admixed for 10-20 min, preferably about 15 min, and under intense agitation with a 10% or 20% solution of alum (Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O) in an amount maintaining a given pH (4.5-5.5) measured by means of a pH electrode. The amount of alum consumed can be illustrated as a function of the time (see FIG. 1 which will be described in more detail hereinafter).
The following Examples are given to illustrate the invention. In these Examples, all parts and percentages are by weight, unless otherwise indicated.
US Referenced Citations (1)
| Number |
Name |
Date |
Kind |
| 4167423 |
Williams |
Sep 1989 |
|
Divisions (1)
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Number |
Date |
Country |
| Parent |
382318 |
Jul 1989 |
|
Patent Grant
5037477
