Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-02-24
2001-08-07
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S416000, C106S436000, C106S447000, C106S464000, C106S465000, C106S468000, C106S469000, C106S471000
Reexamination Certificate
active
06270564
ABSTRACT:
The invention relates to a process for the bleaching of mineral or synthetic pigments.
Pigments are prepared either from natural minerals or synthetically. Mineral pigments include kaolin, calcium carbonate, chalk, marble, talc, bentonite and natural gypsum. Synthetic pigments include synthetic gypsum, TiO
2
and precipitated calcium carbonate. In certain uses, for example in the coating of paper and as fillers, pigments are desired to have a maximal degree of brightness. For this purpose, there have been developed numerous processes intended for enhancing brightness.
Australian patent application AU-A-70277/96 discloses for kaolin a purification and bleaching process wherein oxidative ozone is used for bleaching kaolin from organic coloring impurities. U.S. Pat. No. 5,397,754 proposes as other oxidative agents oxygen and hydrogen peroxide, as well as sodium hypochlorite or lithium hypochlorite. According to U.S. Pat. No. 3,589,922, it is also possible to use potasium permanganate, zinc sulfate, oxalic acid, sodium dithionite and sulfur dioxide for the bleaching. From other publications there is also known the possibility of using chlorine, hydrazine and sodium silicate for the bleaching. It has been observed that the result of the bleaching of kaolin further improves if after an oxidative bleaching there is carried out a reducing bleaching, in which sodium dithionite, i.e. hydrosulfite, is commonly used. A process of this type is disclosed in U.S. Pat. No. 4,935,391.
Corresponding bleaching processes have also been disclosed for other pigments. In EP patent publication 291271, hydrogen peroxide is used on calcium carbonate, chalk and marble. The use of ozone for bleaching chalk has been known at least since 1970. In certain patent publications the use of ozone has been proposed for the bleaching of natural gypsum and synthetic gypsum. Ozone has proved to be an effective bleaching chemical, but it has the disadvantage of toxicity, which requires a high gas-tightness of the apparatus, and also a high price. Chlorine-containing bleaching agents also have their known disadvantages.
The bleaching result can be affected by means of temperature and pressure. For example, FI patent publication 97972 discloses a process for the bleaching of chalk by using hydrogen peroxide. It has been stated that this process works at a temperature of 95-150° C. When the operation takes place at temperatures this high, the bleaching must take place under pressure. Elevated pressure improves the bleaching effect but lowers the financial profitability.
Many pigments suitable for the coating of paper and for other uses involve the problem of a low degree of brightness. In many uses, brightness higher than at present would be desirable. For example, modern printed products are required to have as high a brightness as possible. State-of-the-art processes are not capable of bleaching pigments sufficiently, or the processes are too expensive. Thus there is a clear need in the art for a bleaching process by which the brightness of pigments could be increased and which would not have the prior art disadvantages.
It has now been observed, surprisingly, that a bleaching chemical, peracetic acid (PAA), used in chlorine-free bleaching of pulp works excellently even in small doses in the bleaching of pigments also. The observation is especially surprising, since by the use of peracetic acid a significant improvement of brightness was achieved also with pigments which did not contain any organic oxidizable components. Very high brightness values were achieved with peracetic acid, as shown by the results of the experiments described hereinafter.
Thus, according to the invention a process has been achieved for the bleaching of pigment, the process being characterized in that the pigment to be bleached is contacted with a solution which contains peracetic acid. The pigment to be bleached may be, for example, a mineral pigment such as kaolin, calcium carbonate, chalk, marble, talc, bentonite and natural gypsum, or a synthetic pigment such as synthetic gypsum, TiO
2
or precipitated calcium carbonate.
Peracetic acid is a chemical which forms when acetic acid and hydrogen peroxide react in the presence of a catalyst. There are various products commercially available, for example a 38-percent distilled peracetic acid (dPAA) and an equilibrium mixture of peracetic acid (ePAA). In spite of differences in composition, the various peracetic acid products are effective in the bleaching of pigments. The peracetic acid used for bleaching may also contain stabilizing agents. When reacting, peracetic acid releases nascent active oxygen, which is highly reactive. The acetic acid which is released upon the reaction of peracetic acid is also capable of binding cations when forming various acetate compounds.
The peracetic acid is preferably dosed into an aqueous slurry of the pigment, the slurry being stirred simultaneously. The dry matter content of the slurry is preferably approx. 30-80% by weight.
Typically the stirring time is 15 min, but it may vary. Stirring is important also for the reason that during stirring gaseous substances can escape from the slurry. At this stage the slurry may slightly froth or bubble. The dosing and stirring of peracetic acid may be carried out at room temperature or at an elevated temperature. Judging from the bleaching results, bleaching with peracetic acid works at low temperatures and does not require an elevated temperature. The bleaching result can be improved even considerably through the raising of the temperature. Also, the process does not require higher pressures, but an increasing of the pressure probably improves the bleaching result.
The dosing of peracetic acid for pigment is preferably approx. 0.05-5 kg/metric ton of dry pigment, indicated as a 100-percent peracetic acid. The most advantageous range is approx. 1-2 kg/metric ton. Peracetic acid works in bleaching within a very wide pH range. The peracetic acid addition lowers the pH value of the slurry to be bleached. In general it is not necessary to control the acidity of the slurry. In the experiments, good bleaching results were obtained when the pH value ranged from 2 to 12. If pigments which contain calcium carbonate are bleached, it is possible to compensate for the lowering of the pH by adding to the slurry being bleached substances which react in an alkaline manner.
On an industrial scale the bleaching may be carried out as a batch process or a continuous process.
It is preferable to carry out the bleaching on a material comminuted to the final fineness. Also coarser material, such as crude gypsum, can be bleached with good results (Example 2). In this case the achieved increase in brightness is retained at least in part also during the comminution of the material, as is seen from the results of Example 8.
After an oxidative bleaching carried out using peracetic acid, a reducing bleaching can be carried out using a reducing agent such as a dithionite, e.g. sodium dithionite.
The bleached pigment is in general used in slurry form, but it may, when necessary, be separated from the slurry by a procedure known per se, for example by filtration and possibly by drying.
The invention is described below in greater detail with the help of Examples 1-10. In the examples, the bleaching experiments were carried out on a pigment in slurry form. All of the experiments were carried out using the same procedure, and only the solids content of the slurry varied in some experiments.
Carrying out of the Experiments
In the experiments, a slurry was used which contained 667 g of pigment and 333 g of water, and possibly also chemicals required for the dispersing of the pigment particles. The slurry was poured into a vessel having a diameter of 120 mm and a height of 110 mm. There was placed in the vessel an anchor-like stirrer, which was rotated using a stirring motor at a rate of 200 revolutions/min.
The peracetic acid was added in the course of approx. one minute to the slurry being stirred, and the slurry was stirred thereafter for 15 minutes to
Bell Mark L.
DiVerdi Michael J.
Greenberg & Traurig, LLP
Kemira Chemicals Oy
Manak Joseph M.
LandOfFree
Process for the bleaching of pigments does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for the bleaching of pigments, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the bleaching of pigments will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2472591