Kaolinite; china clay; bolus alba; porcelain clay; aluminum silicate hydroxide; Kaopectate; aluminum silicate (hydrated); aluminum silicate dihydrate
Kaolin is a hydrous aluminosilicate mineral that is found in large natural deposits of kaolinite in Georgia, South Carolina, and Texas (156). A typical kaolin contains 38.5% by weight aluminum oxide, 45.5% silicon dioxide, 13.9% water, and 1.5% titanium dioxide, with small amounts of calcium, magnesium, and iron oxides. A single crystal consists of a layer of silicon dioxide that is covalently bonded to a layer of aluminum oxide. When the clay is processed by centrifugal classification, it can be separated into fractions consisting of stacks of the hexagonal plates (< 2 mm in diameter and particles greater than 2 mm in diameter consisting of stacks of the hexagonal plates). Kaolin, as mined, contains other minerals including quartz, muscovite, and altered feldspars (157). The purification process removes much of the crystalline silica, so that commercial products typically contain less than 3% crystalline silica and the respirable dust contains less than 1%. On the other hand, if kaolin is calcined, some of it may be converted to cristobalite.
Color: white to yellowish or grayish powder (149)
Domestic production of kaolin was estimated at 8.6 million tons in 1988 (158). More than 80% was produced in Georgia. A major use of kaolin is as a filter and a pigment in the manufacture of coated paper. Kaolin is also used as an extender and pigment in paints, in ceramics, rubber, thermosetting resins, and adhesives.
7.3.3 Workplace Methods The recommended methods for determining workplace exposures to kaolin are NIOSH Method #0500 for total dust and #0600 for respirable dust (31).
The health effects of exposure to kaolin dust by inhalation have not been adequately studied. Historically, reports of respiratory diseases among workers exposed to kaolin were attributed to possible contamination of the kaolin by crystalline silica. Before 1991, the ACGIH TLV for kaolin was that for a nuisance dust, namely, 10 mg/m TWA. However, in 1991, ACGIH reviewed the available information and issued a notice of intended change (157). The ACGIH cited a number of case reports and epidemiological studies of workers who were exposed to kaolin during mining and processing in Georgia. For the most part, Georgia kaolinite contains little or no crystalline silica. In workers who were exposed to kaolin dust during the milling and bagging of kaolin, there was an increased prevalence of pneumoconiosis. The prevalence of pneumoconiosis was correlated with both the intensity and duration of exposure. Pneumoconiosis incidence was not increased in open pit miners, who were exposed to significantly lower dust concentrations than workers involved in milling, bagging, and loading. As with many epidemiological studies of the effects of respiratory particulate matter, quantitative data on past exposure for these workers were of poor quality, but exposure levels in the past were unquestionably very high.
There are no reports to suggest that workers exposed to kaolin free silica have a history of malignant respiratory diseases. The carcinogenic potential of kaolin has not been systematically studied in either experimental animals or exposed workers, however. Based on the available evidence that kaolin induces pneumoconiosis.
7.5 Standards, Regulations, or Guidelines
NIOSH has a recommended exposure limit of 10 mg/m (total); 5 mg/m (resp); OSHA has a standard of 15 mg/m3 (total); 5 mg/m3 (resp) (149). The ACGIH TLV standard is 2 mg/m3 for matter containing no asbestos and <1% crystalline silica in the respirable fraction (157). It may be carcinogenic for humans but cannot be assessed conclusively because of lack of data. Pneumoconisis is the critical effect (155).
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