CA1125456A

CA1125456A - Pigment treatment method

Info

Publication number: CA1125456A
Application number: CA338,625A
Authority: CA
Inventors: Magnus B. Kvant
Original assignee: Supra AB
Current assignee: Supra AB
Priority date: 1978-10-30
Filing date: 1979-10-29
Publication date: 1982-06-15
Also published as: FR2440386A1; IT1128747B; JPS5835937B2; FI66416B; DE2943205A1; SE417104B; GB2032901A; FR2440386B1; NL7907976A; GB2032901B; ES8102059A1; ES485483A0; JPS5560024A; IT7950519A0; FI793163A; SE7811216L

Abstract

ABSTRACT OF THE DISCLOSURE

Methods of treating calcium sulphate (gypsum) so that it can be used as a pigment or filler agent are disclosed.
The method includes treatment of the calcium sulphate with an oxidizing bleaching agent, such as chlorine, hypochlorites, hypochlorous acid, chlorine dioxide, and various peroxides.
Preferably the treatment is carried out with the calcium sulphate being in the form of an aqueous suspension, and the calcium sulphate slurry is subjected to washing and fraction-ation in order to obtain a definite particle size distribution.

Description

1~2S4$~

FIELD OF THE INVENTION
The present invention relates to a method of treating pigMent and/or filler agent. More particularly, the present invention relates to a method of treating calcium sulphate so that it can be used as a pigment or filler agent.
Applicants copending application Serial No. 338,826 discloses related subject matter.
BACKGROUND OF THE INVENTION
Calcium sulphate or gypsum is a natural material, which is found in a number of countries, such as those in the Mediterranean area. Its usual form is the dihydrate, having the composition of CaSO4 2H20. Gypsum, however, also exists as a waste product from the manufacture of phosphoric acid, where it often is called "chemical gypsum". The term "calcium sulphate"
in connection with the present invention is to be understood to include calcium sulphate in all its forms, i.e. both the sulphate per se as well as its dihydrates and semihydrates.
It has previously been proposed to use gypsum as pigment in coating agents for paper. This proposal has not been successfully realized in practice, however, apparently because the special prere~uisites which must prevail for obtainingla good result have not been understood.
An object of the present invention is to provide a method of pretreating calcium sulphate so as to render it suitable for use as a pigment and/or filler agent.

~"
~ ,;,, .

*' ~ S4~6 SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a method for producing a pigment or filler agent for use in coating compositions from calcium sulphate comprising purifying said calcium sulphate by fractionation so as to remove undesired particles therefrom, and subsequently bleaching an aqueous suspension of said purified calcium sulphate having a dry content of up to 60% by weight with an oxidizing, bleaching agent selected from the group consisting of chlorine, hypo-chlorous acid, hypochlorite, and mixtures thereof, wherein saidbleaching step is carried out at a pH of from about 1.5 to 6.
Good results are obtained when the dry content in the slurry amounts to a maximum of 60 per cent by weight, and pre-ferably a maximum of 50 per cent by weight.
In a preferred embodiment of this invention when the bleaching is carried out with chlorine, hypochlorous acid or hypochlorite, it has been found to be essential that the pH
value in the slurry be held to from about 1.5 to 6, preferably from about 2 to 4, and most preferably about 3. In another embodiment, bleaching can be carried out for a period of from about 5 minutes to 1 hour, preferably from about 20 to 30 minutes.
In another embodiment of this invention, when the oxidizing bleaching agent is chlorine or chlorine compounds, it ; is important to add specified amounts thereof in order to obtain a maximum beneficial effect. It has thus been found by experi-mentation that amounts af from 0.1 to 1% and preferably from about 0.3 to 0.5% active chlorine should be employed, calculated on the amount of clacium sulphate.

In another embodiment of the present invention, the calcium sulphate is purified by fractionation, preferably in a cyclone, either prior or subsequent to the bleaching step, in order to remove undesired particles.
In another embodiment, subsequent to the bleaching step the calcium sulphate must be washed with water, for example on a filter or in a column, in order to remove chlorine residue, chlorides and oxidizing compounds.

BRIEF DESCRIPTION OF THE DRAWING
The invention may be more fully understood by referring to the drawing, which is a block diagram of a method in accordance with the present inverltion.

DETAILED DESCRIPTION ~
The invéntion is described in greater detail in the following example thereof.
~' .
~:
EXAMPLE
In this example waste gypsum from the manufacture of phosphoric acid was used as the raw material. The raw material was suspended in water in a container, 1, to a dry content of about 15 per cent by weight, and the pH value of the slurry was adjusted to a value below 3. This value is a critical one, becaase at higher pH yalues discoloring impurities will adhere to the gypsum particles. Gypsum of the type used in this embodiment includes as impurities, besides organic material, heavily discolored gypsum, calcium fluoride and silicates from previous processes. When the pH value for the slurry exceeds 3, acidification is preferably carried out by means of sulphuric acid.

~2S~

The resultant slurry is then fractionated in a hydro-cyclone, 2, whereby a fraction having a particle size of from about 25 to 100~ was recovered. If the particle size is thus below 25~ it has been found that~ dewatering in a subsequent process step would be rendered substantially more difficult. The fraction containing particles with a size above 100~ includes dark-colored particles, partially consisting of tourmaline. However, the rock phosphate which constitutes the starting material for the manufacture of phosphoric acid exists in many different qualities, and the impurities therein can therefore be of various types.
The fractionation was carried out in two steps, 3 and .
4, respectively. In the first step, 3, the coarser fraction was removed in l1ne 19, and in the second step, 4, the finer fraction was removed in line 20~, and in èach case the fractions so removed we~e combined and carried away in line 21 from the~process as waste. By carrying out the separation in this manner, it is possible to operate independently of the~operation conditions in the phosphoric acid factory, from which the gypsum is collected. In any event, the resultant slurry had a dry content of about 45 per cent by weight.
The recovered gypsum fraction was then bleached in a continuous bleaching plant, 5, by the addition of chlorine water through line 6. The dry content dropped to about 20 per cent by weight as a result of this addition of chlorine water. After addition of the chlorine water the pH value was a*justed to 3, and the bleaching was carried out by stirring in two tanks connected in series in order to extend ~12S4~i6 the stay-time. The amount of added chlorine was 0.5 per cent by weight, calculated on the basis of dry gypsum. After the bleaching step, the slurry was thickened in a sedimentation thickener to a dry content of about 35 per cent by weight.
In a subsequent washing vessel, 7, the gypsum was washed in order to remove foaming agents, chlorides, organic material and other impurities. The washing was carried out continuously with water entering through line 8, and flowing countercurrent to the slurry in vertical pipes, and with the gypsum being taken out by a screw at the lever portion of vessel 7. During this washing step thickening to about 45 per cent by weight dry content was simultaneously obtained.
Filtering of the slurry resulting from the washing step was then carried out on rotary vacuum filters, 9, onto ; which water was~simultaneously sprayed in order to primarily remove chlorides residue. From the filter, 9, a clean, bleached product having a dry content of~about 75 per cent by weight ; was obtalned. The effluents from the washing vessel, 7, and~the filters, 9, are removed through lines 10 and 11, Z ~ ~respect;ively, and can be combined and removed through line 22.
The main constituents in a coating composition, irrespec-tiye of whether it is to be used for coating paper and card-board or as paint, are pigment, binding agent and a liquid j:
phase. It is generally necessary to also add a dispersing agent in order to obtain good distribution of the pigment.
The viscosity of the composition can thereby often be simul-taneously lowered so that a higher dry content can be obtained without a concomitant deterioratlon in coatability.

4~6 For coating paper, cardboard or the like, kaolin, titanium dioxide or chalk have conventionally been employed as pigment.
It has also been proposed, however, to use gypsum. Gypsum has not been used in practice, however, probably because it has been considered that gypsum requires a substantially greater amount of dispersing agent.
Binding agents previously used for coating compositions include water-soluble substances such as starch and casein, or dispersions of different polymers in water such as, for example, polyacrylate, polymethacrylate and polyvinyl alcohol.
It is also known, however, to use certain cellulose derivatives, such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose and carboxymethyl cellulose.
A greater number of materials are available for use as dispersing agents. The most commonly used dispersing agents, beyond all comparison, are~different types of phos-~hates, particularly sodium hexametaphosphate. This dispersing agent, however, like several others, has certain disadvantages.
` For example, the binding force of the binding agent used ,~ ~
is often reduced. Furthermore, when these dispersing agents are to be used for the coating of paper which is to be printed in offset print, they can in certain cases give rise to :
emulsification of the printing ink in the offset water, and an undesired toning of the print may thus result.
In view thereof, in the method of the present invention a dispersing agent is used which itself acts substantially as a binding agent, viz. a polysaccharide substituted with carboxyl groups.

It has in this connection been found to be decisive that the gypsum be beaten in the presence of a polysaccharide substituted with carboxyl groups.
In a particularly advantageous embodiment thereof, such beating is carried out in the presence of water.
As a result of experimentation which has been carried out in connection with the development of the present in-vention, it was discovered that particularly good results were obtained with beating of the yypsum in the presence of carboxymethyl cellulose, usually called CMC, and particularly with CMC having a mean molecular weight of from about 10,000 to 500,000, preferably frcm about 20,000 to 150,000. When CMC is thus used, it is added in amounts ranging from about 0.5 to 3 per cent by weight, calculated on the amount of dry calcium sulphate.
It has also been found in this connection that the particle size of the calcium sulphate is essential. When, for example, a coating composition is to be prepared for use in preparing a mat paper, the gypsum must have a particle ;
size distribution such that about 30 per cent has a size smaller than about 2~. For coating compositions to be used in preparing a glazed paper, 95 per aent by weight of the material should have a particle size smaller than about

-2~. When, however, pigment for a filler agent is involved, the particle size of the calcium sulphate must be such that about 50 per cent by weight of the material has a particle size smaller than about 5~.
The particle size distributions discussed above were determined by means of an Andreasen sedimentation apparatus, which is marketed in Sweden by the enterprise Kebo Grave l~S4S6, under the reference number 111,904. Test samples are prepared by weighing about 10 g gypsum slip, to which 2 ml of a 1-%
CMC solution is added, whereafter the slip is mixed and stirred with about 40 ml water. The sample is transferred to the sedimentation apparatus, and amounts of 10 ml are taken out with the pipette according to the folIowing schedule:

1st sample 0 minutes 2nd sample after 60 minutes 3rd sample after 120 minutes 4th sample after 1080 minutes 5th sample after 1560 minutes .
According to another preferred embodiment a binding agent `:
such as latex or starch ~lS employed. Addition in such case is preferably carried out after the beating step.
- As mentioned above, a high dry content is of great lmportance for the preparation of an effective economic coating agent. By selecting the binding agents/dispersing agents used in accordance with this inventioin, dry contents for the coating agent as high as 60 to 80 per cent by weight can be obtained in connection with the preparation of a mat p~per. For such coating agents used to produce a glazed paper, dry contents of from about 55 to 80 per cent by weight were obtained.
In accordance with yet another embodiment hereof, the beating is preferably carried out at increased temperatures, but preferably at temperatures below about 130C. With reference again to the drawing, an example of the manufacture of the coating agent is as follows: The clean, bleached product li;~S45~i resulting from the above example was supplied to dispersion equipment, 12, while 1.6 per cent by weight of carboxymethyl cellulose (CMC), calculated on the basis of dry gypsum, was simultaneously added to the dispersion equipment, 12, through line 13. The CMC was added in the form of a l4 %
solution. Thereafter the pH value of the solution was raised to 12 by the addition of sodium hydroxide, because in cases such as this one the gypsum can include small amounts of phosphoric acid, which is set free during the beating step.
In addition, conventional glaze-raising agents, antifoaming agents, optical bleaching agents, coloring substances and preservatives can be added to this mixture, as may be desired.
The various components are continuously supplied to the ~ mixing vessel so as to obtain a flow of about 4 m5/h. Dispersion ; is brought about by means of a rapidly rotating stirrer.
From the dispersion~equipment~, 12, the dispersion is continuously trarsferred to a vacuum deaerator, 14, in order to remove entrained air. After the deaeration, the dispersion was beaten in a pearl beater, 15, at a temperature of about S0C, in a single step, cooled in cooling equipment, 16, ~`~ and then screened in a self-cleaning screen, 17. The final product removed through line 18 was a storage-durable slurry with a dry content of 71%. The product can be pumped and delivered in tanks ~r barrels. The invention is not restricted to the embodiments described, but can be varied within the scope of the inventive concept.

.

_ ... . , . , . . , , ~ ,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for producing a pigment or filler agent for use in coating compositions from calcium sulphate comprising purifying said calcium sulphate by fractionation so as to remove undesired particles therefrom, and subsequently bleaching an aqueous suspension of said purified calcium sulphate having a dry content of up to 60% by weight with an oxidizing, bleaching agent selected from the group consisting of chlorine, hydro-chlorous acid, hypochlorite, and mixtures thereof, wherein said bleaching step is carried out at a pH of from about 1.5 to 6.

2. The method of claim 1, wherein said aqueous suspension has a dry content of up to about 50 per cent by weight.

3. The method of claim 1, wherein said bleaching step is carried out at a pH of between about 2 and 4.

4. The method of claim 3, wherein said bleaching step is carried out at a pH of about 3.

5. The method of claim 3 or 4 in which said bleaching step is carried out for a period of between about 5 minutes and 1 hour.

6. The method of claims 3 or 4, wherein said bleaching step is carried out for a period of between about 20 and 30 minutes.

7. The method of claim 1, wherein said oxidizing bleaching agent comprises a chlorine compound and is added in an amount of from between about 0.1 and 1 per cent active chlorine calculated on the basis of said calcium sulphate.

8. The method of claim 1, wherein said fractionation is carried out by means of a cyclone.

9. The method of claim 1, wherein said fractionation is carried out prior to said treating step.

10. The method of claim 1, wherein said fractionation step i is carried out subsequent to said treating step.

11. The method of claim 1, including washing said bleached calcium sulphate with water in order to remove residual chlorine, chlorides or oxidized compounds therefrom.

12. The method of claim 11, wherein said washing step is carried out on a filter.

13. The method of claim 11, wherein said washing step is carried out in a column.

14. The method of claim 1, wherein said calcium sulphate is fractionated to a particle size of between about 25 and 100 µm.