CA2007711C - Method for producing high opacifying kaolin pigment - Google Patents

Abstract

A method of producing a high opacity paper filler pigment which comprises subjecting an iron-stained titania-containing kaolin to beneficiation by froth flotation or high intensity magnetic separation;
recovering the froth or magnet rejects, dewatering and drying the rejects to produce a kaolin enriched in titania, and calcining the same at a temperature in the range of about 1500° to about 2200°F.

Description

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Method for Producing High OQacifying Kaolin Pigment Background of Invention This invention relates generally to calcined clay products. hlore specifically, it relates to a cal-cined kaolin clay pig ment and method of manufacture of same. The pig ment prod uced by the method of the inven-tion has very high opacifying properties when incorporated as a filler in paper products.
In the course of manufacturiwg paper and simi-lar products, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. The use of appropriate such fillers vastly improves the opacity and printability of certain types of light weight papers such as newsprint. This aspect of use of calcined kaolin clay pig ments is discussed in some detail, for example, in Fanselow and Jacobs, U.S. Patent No. 3,586,523. Other aspects of the presently preferred commercial methods for manufacturing calcined kaolin pig ments for use particular-ly as fillers in paper manufacture, are also set forth in the said Fanselow et al patent, as well as in additional United States patents such as McConnell et al, U.S.
Patent No. 4,381,948.
The Fanselow et al and McConnell et al patents are indeed representative of the methodolog y now widely employed in the kaolin industry in order to produce cal-cined kaolin clay pig ments for use in paper manufacturing.
Study of these patents will show that the objective of same, as is customary in the art, is to produce a fine particle size calcined kaolin clay pigment of relatively ~~~r.

very high brightness, beginning with a crude kaolin which has a relatively very low brightness. Indeed , a preferred crude feed material for use in processes such as are dis-closed in the Fanselow and McConnell patents, is a highly discolored , so-called "gray" kaolin, which is referred to in the Fanselow patent as a "hard sedimentary kaolin clay." Thus, the gray crude which is used in the example of Fanselow has an initial brightness of 78, where the figure cited refers to the so-called GE scale. Procedures for measuring brightness as set forth in this application, and as is generally recognized in the industry, are in accord with TAPPI procedure T646os75. As a result of the beneficiation treatment set forth in the Fanselow et al patent, these brightnesses are considerably increased indeed to a very high whiteness. Claim 2 of said Fanselow et al patent thus recites a GE brightness within the range of 92~ to 950. Similarly, the McConnell et al patent describes a resultant pigment having a brightness of at least 93 as being the final output product from practice of the beneficiation methods set forth therein. A cal-cined kaolin pig meet substantially produced in accordance with the McConnell et al patent is available commercially from Anglo-American Clays Corporation, under the trademark ALPHATEX~.
It is of particular interest pertinent to the present invention to observe that in the McConnell patent, it is emphasized that the crude used to produce the high brightness pigments preferably includes not more than 2~
by weight of titanium expressed as Ti02. A principal reason for this is that clay minerals occurring in nature, including kaolin clays, frequently contain their discolor-ing contaminants in the form of iron and/or titanium-based impurities. The quantities of the titaniferous impurities in sedimentary kaolins of Georgia are significant and are commonly present as iron oxide-stained titanium oxides.

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Irrespective of whether ~calcining is used, it has commonly been considered in the kaolin industry that it is para-mount to refine the crude kaolins to bring the brig ht-ness characteristics of the resultant product to a level acceptable for various applications such as paper coating or as mentioned, even for ffilling. Among the techniques which have been used in the past to remove the discoloring impurities, are the use of hydrosulfites for converting at least part of the iron-based impurities to soluble form, which may then be extracted from the clay. A further method which has come into increasing use in the kaolin industry involves the use of high intensity magnetic separtion as described, for example, in such patents as Marston, U.S. Patent No. 3,627,678. This method is also useful in removing titaniferous impurities in that although titania when pure has little magnetic attract-ability, the iron-stained titania which forms the basis (as mentioned) for the bulk of discolorants, may often be quite effectively removed by imposition of such a high intensity magnetic field.
One of the further, very effective methods for removing titaniferous impurities including iron oxide-stained titanium oxides, is the froth flotation technique.
Generally according to this method , an aqueous suspension or slurry of the clay is formed, the pH of the slurry is raised to an alkaline value and a collector agent is added. The slurry is then conditioned by agitating for a short period. A frothing agent if necessary is added to the conditioned slurry, after which air is passed throng h the slurry in a froth flotation cell to effect separation oC the impurities from the mineral.
Further details regarding the use of froth flotation techniques for removing titanium-based impuri-ties from kaolins may be found at numerous places in the prior art, including for example U.S. Patents Nos.

2tlfl~'~'1~.

3,450,257 to E.K. Cund y, 4,518,491 to B.M. Bilimoria, and U.S. Patent No. 4,090,688 to Alan Nott. In the procedures set forth in these patents, the iron-stained titania "contaminants" are separated with the froth. These separated materials, because of their very high titania content and high degree of discoloration, have in the past simply been termed "rejects," and as such discarded or used for some purpose unrelated to being pig ments for paper manufacture. What therefore is to be especially appreciated, is that the discoloring iron-stained titania-based impurities removed from the crude kaolin by froth flotation (or even where such materials have been removed by magnetic separation), have heretofore been regarded as essentially useless material having little or no economic value for proper manufacture. The view in short, has in the past been that the object of beneficiation of kaolins was simply stated to remove these "contaminants," to thereby brighten the output product from which these con-taminants had been removed.
Both the brig htness characteristics of the given kaolin and the opacifying properties of same when incorpo-rated as a filler in paper, may be quantitatively related to a property of the filler identified as the "scattering coefficient S". The said parameter, i.e., the scattering coefficient S of a given filler pigment, is a property well-known and extensively utilized in the paper technolo-g y art, and has been the subject of numerous technical papers and the like. The early exposition of such mea-surements was made by Kubelka and Munk, and is reported in Z. Tech Physik 12:539 (1931). Further citations to the applicable measurement techniques and detailed defini-tions of the said scattering coefficient are set forth at numerous places in t-he patent and technical literature.
Reference may usefully be had in this connection, e.g. to U.S. Patents Nos. 4,026,726 and 4,028,173. In addition i ~ f to the citations set forth in these patents, reference may further be had to Pulp and Paper Science Technolog y, Vol. 2 "Paper", Chapter 3, by H.C. Schwalbe (McGraw-Hill Book Company, New York).
Summary of Invention In accordance with the present invention, and contrary to prior experience and practice in the kaolin industry, it has unexpectedly been found that a pig ment possessing highly useful properties, especially with respect to opacifying light weight paper such as newsprint, may be produced by calcining the very high titania content "rejects" yielded by practice of conventional froth flota-tion beneficiation. Other "rejects" having similar hig h content of iron-stained titania, as for example those yielded from high intensity magnetic separation (H.I.t4.S.), may also be used in the practice of the invention.
According to a preferred method of practicing the present invention, an iron-stained titania-containing kaolin is subjected to froth flotation, after which froth "rejects" comprising kaolin enriched in said iron-stained titania are recovered, then subjected to dewatering and drying, to thereby produce a kaolin enriched in titania.
This intermediate is calcined at a temperature in the range of about 1500 to 2200°E to yield the finished prod-uct. The froth "rejects" are not otherwise beneficiated prior to calcining, but are used in their as-is form as recovered from the flotation process. The starting mate-rial subjected to the froth flotation is typically a naturally occurring kaolin crude having a titania content of from about 1 to 2 weight ~, and the enriched kaolin yielded from the froth flotation process has a titania content of from about 2 to about 15 weight $ and prefer-ably has a titania content of at least 5 weight ~. Simi-lar considerations apply where H.I.M.S. is the source of the rejects.

~ I 2t~~'~11 ~~then a pigment produced in accordance with the present invention is utilized in the filling of paper, it imparts a brig htness in the range of about 60 to 90, but yields subtantially higher opacifying properties than a prior art calcined kaolin not enriched in titania. Yet more preferably, the brightness of the pig ment resulting from the invention is in the range of about 60 to 80. To be particularly noted is that brightness values as indicated have typically been considered undesirable for use as fillers; but in accordance with the present inven-tion, these fillers have been found to produce very hig h opacifying properties, thereby vastly improving the use and printability of such thin, "low grade" papers such as newsprint.
In a further aspect of the invention, it is found that for a given opacity, the retention in the paper web is higher for the present pig ments than with prior art pigments , by virtue partially of the relatively large particle size yielded as a result of practice of the inven-tion.
Brief Description of Drawing The Figure herein is a graph depicting scatter at 10~ loading as a function of mean calcined particle size for a series of pig ment samples, some of which are prepared in accordance with the invention, and others being controls .
Description of Preferred Embodiment The practice of the present invention will now be illustrated by Examples, which are deemed illustrative of the invention and not delimitative thereof.
Example In this Example , a series of samples , as set forth in Tables 1 and 2 below, were subjected to calcina-tion in a laboratory muffle furnace for a period of 35 minutes at calcination temperatures as indicated in ~O~'~'11 the same table. The samples identified as "froth" were obtained as the discards from a plant flotation process, which process was substantially in accord with the procedure set forth in the aforementioned U.S. Patent No.
4,518,491. The froth yielded as overflow from the flota-tion process was subjected to dewatering and drying prior to being subjected to calcination at the schedule indi-cated. Ti02 content for each sample was determined after calcination. The products yielded from the calcination treatment were pulveri zed to a particle si ze of 99$
by weight finer than 325 mesh screen, and the final output product was first measured for brightness using TAPPI procedure T646os75 as previously indicated.
The pigment products yielded. by this Example were subjected to tests to determine the pig ment scatter-ing coefficient and absorption of the pigment when same was used as a filler in paper at a 10~ loading factor by weight. More specifically, the pig ment was tested at 10~
loading in Storafite (trademark of Nova Scotia Forest Industries) bleached sulphite pulp beaten to a CSF
(Canadian Standard Freeness) of 300-350. Deionized water was used in the sheet formation, tog ether with 20 lbs/ton alum, and Percol 292 (trademark of Allied Colloids) as a retention aid.
This 10$ value was derived by nominally loading the material at 5, 10, and 15~, obtaining the sheet scatter for the three sets of sheets, fitting the points to a straight line by a least square analysis, and calcu-lating the 10$ level by the equation generated.
As a control, the aforementioned Alphatex~
product of Anglo-American Clays Corporation was used at 10~ loading under otherwise identical conditions. Table 1 sets forth scatter levels achieved at 10% filler level for the samples. All scattering data are normalized by comparison to the scattering coefficient of samples of -$_ the aforementioned Alphatex~. Ideally in a study of the present nature, the same batch of beaten pulp should be used throughout. As this is not practical, the method adopted was to fill one set of sheets in each series of tests using the same Alphatex~ from series to series.
Statistically, the Alphatex~ filled samples at 10$ filler had a scattering coefficient of 680 cm2/gram, and in each series in which Alphatex~ differed from 680, the scatter-ing coefficients of the experimental samples were accord-ingly adjusted proportionally to the adjustments which the Alphatex~-containing paper required to bring its value to 680 cm2/gram. This procedure, which was used in subse-quent examples, is from time to time referred to in the specification as "normalizing" the scattering coeffi-c Tents.
In the case of the data in Table 2, the filler level was varied in the manner indicated. The data in Table 1 all reflects a filling level of 10$. Opacity was determined by TAPPI Procedure T519 om-86. The absorp-tion coefficient is derived from the Kubelka and Munk analysis, op. cit.
Table 1 Pigment Physical and Optical Properties Normalized Absorption Ti02 Calcination Pig meet Scatter Coefficient Sample Conc. Temperature Brightness (10~) (100$) Froth 7.7~ 1975°F 88.0 676 cm2/gm 77.7 cm2/gm Froth 7.7 1650 72.6 660 183.3 HiopaqueTMl.O NA 87.0 594 34.4 Alphatex 1.4 1975 92.5 G80 13.0 20 0 77 1 ~
_9_ Table 2 Filler Loading Required to Give Equal Sheet Opacity Sheet $ Filler Sheet Opacity Pigment Loading Brightness @ 60 gsm Alphatex 3.0 85.4$ 78.1 Froth (1975°) 2.2 33.6 ?8.1 Froth (1650°) 2.0 81.3 78.1 HiopaqueT'~ 4.2 84.1 78.1 As is seen, the froth product was in the case of the first two samples used directly. Also, as con-trols two commercial calcined kaolin products were used and evaluated, namely HiOpaqueM hick is a product of Engelhard Corporation and the aforementioned Alphatex~
product.
Table 2 is of particular interest in showing the percentage of filler that is required with the differing samples in order to produce the opacity level 78.1, which is held constant at the right-hand column. It is of great significance to note here that in order to thus achieve the same opacity as can be obtained with the product of the invention, it is necessary to typically use S0~ more content of Alphatex~ which is, however, a much higher cost product. The brightness of the products of the present invention are seen to be relatively low, but this is not considered in any way a detriment for the application of the said products to the area where they find most value, namely in filling so-called low cost thin papers, such as newsprint. In these instances, it is especially the opacity which is of interest, and the products of this invention yield outstanding results in that respect as can be seen from the appended data.
Example II
In the Figure herein, scatter at 10$ loading has been plotted against mean calcined particle size '.

-lo-for a series of samples, some of which are prepared as in the invention, and others being prior art controls (as identified in the graph). The showing illustrates that for any given particle size, scatter is much hig her for the iron-stained titania enriched products of the invention.
While the present invention has been set forth in terms of specific embodiments thereof, it will be un-derstood in view of the instant disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching . According ly, the invention is to be broadly construed and limited only by the scope and spirit of the claims now. appended hereto.

Claims (14)

1. A method of producing a high opacifying pigment which comprises subjecting an iron-stained titania-containing kaolin to froth flotation, recovering froth "rejects" comprising kaolin enriched in said iron-stained titania, said enriched kaolin containing said titania in the amount of 2 weight percent to 15 weight percent, dewatering and drying said froth "rejects" and calcining said "rejects" at a temperature in the range of about 1500° to about 2200°F.

2. The method according to claim 1 in which the froth "rejects" are not further beneficiated prior to calcining.

3. The method according to claim 1 in which the starting material is a naturally occurring kaolin having a titania content of from about 1 to 2 weight % and said enriched kaolin has a titania content of above 2 to about 15 weight %.

4. The method according to claim 3 in which said enriched kaolin has a titania content of at least 3 weight %.

5. A pigment comprising a calcined product of froth "rejects"
containing kaolin enriched in iron-stained titania, said enriched kaolin containing said titania in the amount of 2 weight percent to 15 weight percent, and being derived from a kaolin froth flotation, said pigment imparting to paper when used as a filter a brightness in the range of about 60-90 and having higher opacifying properties than calcined kaolin not enriched in titania.

6. The pigment as set forth in claim 5 in which brightness is in the range of about 60-80.

7. A paper sheet containing a filler comprising a calcined product of froth "rejects" containing kaolin enriched in iron-stained titania and derived from a kaolin froth flotation process, said enriched kaolin containing said titania in the amount of 2 weight percent to 15 weight percent, and said paper sheet having a brightness in the range of about 60-90 and higher opacity as compared with employing a filler comprising calcined kaolin not enriched in titania.

8. The paper sheet as set forth in claim 7 in which brightness is in the range of about 60-80.

9. A method of producing a high opacifying kaolin paper filling pigment, which comprises beneficiating an iron-stained titania-containing kaolin crude including at least 1% by weight titania, by separating therefrom a kaolin "reject" fraction containing from 2 to 15 weight % of tita niferous discoloring contaminants;
recovering said "reject" fraction and dewatering and drying the same to produce a kaolin enriched in titania; and calcining the said "reject" fraction at a temperature in the range of about 1500° to about 2200°F.

10. The method according to claim 9, in which the "reject" fraction is not otherwise beneficiated prior to calcining.

11. The method according to claim 9, in which said "reject" kaolin fraction has a titania content of at least 3 weight %.

12. The method according to claim 9, in which said "reject" kaolin fraction has a titania content of at least 5 weight %.

13. A method in accordance with claim 9, in which said kaolin crude is beneficiated by high intensity magnetic separation.

14. A method in accordance with claim 9, in which said kaolin crude is beneficiated by a froth flotation.