Low abrasion calcined kaolin pigments are currently used primarily, but not exclusively, as fillers and coating pigments for paper and paperboard products. This represents one of the largest uses of any industrial mineral. Manufacturing facilities produce vast quantities of low abrasion calcined kaolin pigments. The pigments are supplied as dry powders and as aqueous slurries. Use of these pigments for filling paper significantly reduced the consumption by the industry of other opacifiers such as precipitated silicas, alumina trihydrate and synthetic silicates.
Long prior to the invention of low abrasion calcined kaolin pigments it was known that raw (uncalcined) kaolin clay undergoes profound physical and chemical changes when it is heated and that such changes vary with the source of the clay as well as with temperature and calcination conditions. Thus, when heated to about 1350.degree. F. an endothermic reaction occurs. Essentially all of the water of hydration associated with the uncalcined kaolin crystals is eliminated and an essentially amorphous (as measured by X-ray diffraction) material referred to as "metakaolin" results. Generally, the clay darkens significantly during the conversion to metakaolin. During the transformation to metakaolin, the abrasiveness of the material increases, even if the kaolin undergoing calcination is of high purity.
If the kaolin is heated under conditions in excess of those required to result in conversion to metakaolin, further significant changes in phases and properties occur when calcination conditions are sufficiently severe to cause the material to undergo an exothermic reaction (which typically occurs at about 1850.degree. F. and varies from clay to clay). It is now conventional to refer to such material as "fully calcined kaolin".
A remarkable change in the brightness of the kaolin takes place when it is calcined to undergo the exothermic reaction. Thus, crude of the type containing well-crystallized kaolin which initially has a G.E. brightness of 80% may decrease to about 75% when calcined to metakaolin, and a brightness of 90% or higher when heated through the exotherm. For this reason it is conventional to control the operations of kaolin calciners to produce calcined kaolin of desired brightness. A corollary to the usual increase in brightness associated with full calcination is an undesirable increase in abrasivity.
Another change in the physical properties of kaolin that occurs as a result of calcination of powdered raw clay is the remarkable increase in opacifying properties, manifest by increase in light scatter of the clay (increased opacification).
Inspite of the abundance of kaolin crudes capable of providing bright, visually white fully calcined kaolin powders with the potential of opacifying paper products at a considerably lower cost than would be possible using high purity alumina or titania pigments, it took many years before the needs of the paper market could be satisfied by available calcined kaolin products because of the highly abrasive character of the fully calcined kaolin and the low brightness combined with abrasivity of the metakaolin form of the calcined kaolin.
The long-sought means to provide high brightness low abrasion calcined kaolin opacifying pigments awaited the discovery that a specific type of sedimentary kaolin clay crude would provide high brightness "white" calcined clay pigments with abrasivity sufficiently low to permit use by the paper industry. See U.S. Pat. No. 3,586,523, Fanselow et al (1969), the teachings of which are incorporated herein by cross-reference. The crudes, known as "hard" kaolins were of Tertiary origin and were characterized by containing extremely fine particles (e.g.., average particle size below 1/2 micron). When examined by X-ray these clays appeared to be less well crystallized than so-called "soft" kaolins. Generally, the iron content of these ultrafine Tertiary clays was of the order of about 0.7% to 0.9% by weight (expressed as Fe.sub.2 O.sub.3) and in many cases the uncalcined clay had a distinctly grey color, hence the designation "grey kaolin".
When calcined in powder form these unique sedimentary clays aggregated to produce coarser particles possessing remarkably high opacifying power, but without the high abrasion generally characteristic of earlier calcined kaolins. The crudes found to be useful in the production of high brightness low abrasion calcined kaolin pigments were frequently lower in G.E. brightness, e.g., about 85%, than quality crudes used to provide high brightness uncalcined pigments. Nevertheless, when calcined, products with G.E. brightness of at least 90% were produced. When used as filler pigments in paper, the low abrasion calcined kaolin fillers also provided desired print-through resistance and color at acceptable burst levels.
Continued efforts to improve the quality of low abrasion calcined kaolin pigments and to reduce the manufacturing costs have been ongoing. For example, in U.S. Pat. No. 4,381,948, McConnell et al, the proposal was made to employ especially fine raw clay (100% by weight less than 1 micron) as feed to the calciner to improve opacification of the pigment when used as a paper filler. As in the case of Fanselow et al, the emphasis was on providing a high brightness (at least 93% G.E. in this case) and a "white" color.
This as well as other efforts retained the original expressed intent of the inventors in the '523 patent to maintain product brightness (at least about 90% G.E.) and whiteness. In fact, the low abrasion calcined kaolin pigments presently widely used by the paper industry have a G.E. brightness of about 93%.
To the best of our knowledge, processing schemes presently commercially used to produce low abrasion calcined kaolin pigments invariably involve preliminary upgrading of fine particle size crude kaolin, dispersion in water to form a pulp, and removal of coarse particles (so-called "grit") followed by fractionation of the degritted pulp to recover the desired ultrafine particle size fraction, generally at least 100% by weight finer than 2 microns and at least 90% finer than 1 micron. In the case of McConnell et al, supra, fractionation was carried out to recover an ultrafine fraction which was 100% by weight finer than 1 micron. Fractionation, heretofore considered essential, has been followed optionally by removal of colored impurities and/or bleaching, drying (usually by means of a spray dryer), followed by pulverization, calcining and repulverization. With regard to the crudes, these have been of the ultrafine Tertiary kaolins of the type described in the '523 patent, with the general proviso that crude brightness be sufficiently high, e.g., no lower than about 80%, a restriction inherently restricting the content of colored impurities (notably ferruginous and titaniferous) to relatively low values. Typically, Fe.sub.2 O.sub.3 content was approximately 1% by weight, e.g., 0.85-1.10%. See, for example, U.S. Pat. No. 4,381,948 McConnell et al which states "said crude preferably includes not more than 0.5% in total by weight of glass-forming metal oxides, such as potassium, sodium, magnesium, and calcium oxides, and not more than 1.5% by weight of iron, expressed as Fe.sub.2 O.sub.3, nor more than 2% by weight of titanium, expressed as TiO.sub.2 ". An illustration example of McConnell et al utilized a crude identified as a "hard kaolin" and reported to analyze 45% SiO.sub.2 ; 37% Al.sub.2 O.sub.3, 0.93% Fe.sub.2 O.sub.3, 1.6% TiO.sub.2 ; 0.15% CaO; 0.08% MgO, 0.10% K.sub.2 O, Na.sub.2 O% 0.07%, the balance (14%) "consisting principally of water together with a small amount of organic matter". This was consistent with the conventional desire to provide high brightness products, e.g., 91.5-93.5%.
The invention described in the '523 patent spawned the development of the vast scale industrial production of low abrasion calcined kaolin clay pigments and the exploitation of the pigments in industrial paper filling and coating. The preferred use of the calcined kaolin pigments as originally envisioned by the inventors in the '523 patent was to fill newsprint paper, a low basis weight, low brightness (e.g., 50-60% G.E. brightness) paper. Inspite of the fact that the newsprint industry was the segment of the paper industry initially targeted for use of low abrasion calcined clay fillers, the demand of the newsprint industry has had essentially no impact on the current vast market for the pigments for the reason that the cost of such pigments did not result in a cost-benefit to the manufacturer of newsprint (or other papers made from mechanical pulps). Thus, the major use of low abrasion calcined kaolin pigments has been to coat and/or fill higher quality (value added) papers.
Historically, newsprint and groundwood specialities have been produced using a blend of conventional stone groundwood as the primary furnish component (70-90%) in combination with the required amount of chemical pulp (typically 10-30%) to achieve acceptable paper machine and printing press runnability. Recent developments in mechanical pulping technology in conjunction with market demands for improved paper quality have produced a vast array of new mechanical pulps that are well suited for use in newsprint and groundwood specialty papers. Thermomechanical pulp (TMP), chemi-thermomechanical (CTMP), bleached chemi-thermomechanical (BCTMP), pressurized groundwood (PGW), refiner mechanical pulp (RMP), and chemirefiner pulp (CRMP) may be blended in various combinations or with conventional chemical pulps to produce fiber furnishes specifically designed for producing a given grade of paper. The blend ratio of the fiber components is usually determined by the best compromise of their scattering/strength relationships. For example, kraft pulp has high strength, but low scattering, while stone groundwood has good scattering but low strength.
Several types of pigments are commercially available to manufacturers of newsprint and groundwood specialties for use as fillers. Kaolin-based pigments, both hydrous and calcined, sodium-alumino-silicates, precipitated silicas, urea-formaldehyde condensates and tri-hydrated aluminas are all used as filler pigments in mechanical pulp-containing papers. The amount of filler that may be used is largely grade specific. The following table lists typical values and readily illustrate the differences in grade requirements and filler usage of various mechanical pulp-containing papers.
______________________________________ Basis Weight % % Filler % - (g/m.sup.2) Brightness Content Mech . Pulp ______________________________________ Newsprint 44-49 55-60 0-4 up to 100 Directory/ 29-40 55-60 0-4 60-85 Catalogue Hi-Brite 52-90 60-66 0-4 70-90 Grades SC.sup.(1) - "C" 40-80 60-66 .sup. 0-8.sup.(2) 70-90 SC.sup.(1) - "B" 40-80 64-68 .sup. 9-17.sup.(2) 60-80 SC.sup.(1) - "A" 40-80 66-72 .sup. 18-30.sup.(2) 60-70 ______________________________________ .sup.(1) SC supercalendered grades .sup.(2) Typically hydrous kaolins are the primary filler component
The use of a retention aid or retention aid system is required when filler pigments are utilized in the production of mechanical pulp-containing papers. Once again, paper manufacturers have a broad spectrum of products to choose from. Retention aid systems currently being used can be broken down into three classifications: single component systems, dual component systems, and triple component systems. Within each classification, there are several system options available to the papermaker. The following table lists the three broad classifications and some of the more commonly used systems within each classification.