Settling Characteristics of Various Inorganic Dusts

Our plain-English summary of Settling Characteristics of Various Inorganic Dusts, by L. Berkelhamer II (1940).

The study

• Author: L. Berkelhamer II
• Published: Journal of Industrial Hygiene and Toxicology, Volume 22 No7. September 1940
• Type of article: Journal article
• Our copy: from Johns-Manville's own medical library. The article is still under copyright, so we describe it here rather than post it.

In plain English

This 1940 laboratory study did not involve asbestos. It measured how quickly fine mineral dusts settled out of air. The author ground more than a dozen materials, including quartz, calcined gypsum, limestone, magnetite, montmorillonite, kaolin and sericite, and tested them alone, mixed with quartz, or both. Each dust was blown into a Leitz tyndallometer, a light-scattering instrument, and read every minute for 45 to 60 minutes. Two impure quartz samples settled most slowly of all, followed by sericite and then pure quartz. Mixtures of calcined gypsum and quartz settled much faster than quartz mixed with any other material, and limestone came closest. Heavy magnetite settled at about the same rate as lighter minerals. Adding montmorillonite to quartz did not appreciably change how fast it settled. A shale and limestone mixture settled more slowly than either part alone. Electrical charge on the particles did not explain every result. The author concluded that the usual laws of particle settling did not seem to apply to all mineral dusts, especially mixtures. The author also noted that moisture uptake and other physical properties had not been fully evaluated.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment comparing the settling rates of mineral dusts in still air, measured with a tyndallometer (light-scattering instrument); no human or animal subjects in this study
• Who was studied: Samples of mineral dusts tested in the laboratory of the Trudeau Foundation, some alone, some mixed with pure quartz, and some both ways. The materials were quartz (pure and impure), calcined gypsum (two lots), sand, shale, limestone, 'low limestone', portland cement, foundry dust, crude sulfide ore, magnetite, montmorillonite, metallic aluminum, kaolin and sericite. No asbestos was tested.
• Exposure: Hand-ground dust particles, about 3 microns or smaller, blown into the roughly 1000 cc tyndallometer chamber. Readings were taken every minute for 45 to 60 minutes. Not an exposure study of people or animals.
• Main results:
  • Two varieties of impure quartz (containing undetermined impurities) settled more slowly than any other mineral tested. Sericite was next, then pure quartz.
  • All mixtures of calcined gypsum with quartz settled much more rapidly than quartz mixed with any other material investigated.
  • Limestone was the only added material that came close to calcined gypsum in speeding the settling of quartz.
  • The speeding-up effect of calcined gypsum on quartz settling was considerably more marked with one gypsum sample than with another of slightly different composition.
  • Magnetite, a heavy mineral, settled at about the same rate as lighter materials, so specific gravity appeared to matter little.
  • Adding montmorillonite to quartz did not appreciably affect the settling rate.
  • Mixtures of metallic aluminum with quartz settled somewhat faster than pure quartz. The effect was never as marked as that of limestone or calcined gypsum.
  • A mixture of shale and limestone settled more slowly than either component alone.
  • Opposite electrostatic charges could help explain the rapid settling of quartz and calcined gypsum mixtures. They could not explain the rapid settling of quartz and limestone mixtures, because both of those components carried a negative charge.
• The authors' conclusion: The author concluded that no single factor accounted for settling rates. The author also concluded that the laws then used to describe how fine particles settle do not appear to apply to all mineral dusts, especially mixed dusts.
• Limitations the authors state:
  • The possible influence of a material's capacity to absorb moisture was not fully evaluated.
  • Other physical properties were not considered, such as particle surface, refractive index, light absorption and polarization. These may have influenced the tyndallometer readings.
  • Although all materials were ground the same way, the proportion of the smallest particles varied between samples.
  • A very fine silica smoke could not be tested in the apparatus because it clumped together excessively.

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