Acid leaching studies of chrysotile asbestos from mines in the Coalinga region of California and from Quebec and British Columbia

Our plain-English summary of Acid leaching studies of chrysotile asbestos from mines in the Coalinga region of California and from Quebec and British Columbia, by A. Morgan (The Annals of Occupational Hygiene, 1997).

The study

• Author: A. Morgan
• Journal: The Annals of Occupational Hygiene
• Published: 1997;41(3):249-68
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The dissolution of magnesium (Mg) and silicon (Si) from various samples of chrysotile asbestos was measured in N HCl at 25 degrees C. Nine samples were used, five from Canada and four from the Coalinga deposit in California. With milled samples from Quebec, the fraction of Mg dissolving was linearly related to the square root of the leaching time until at least 65% had dissolved. With a hand-picked sample of ore from Quebec, the sample from British Columbia and all the Californian samples, the Mg leaching patterns were sigmoid. The leaching patterns for Si were sigmoid in shape for all the materials tested. Mean Mg dissolution rates were calculated for each leaching period. Considerable differences were observed between samples from the different mining regions and also between hand-picked and milled samples from the same mine. Initially, Mg dissolved more rapidly from milled Quebec chrysotiles than from the Coalinga samples. This difference is due in part to the rapid dissolution of non-structural brucite, present in all the samples from Quebec but not in those from California. An additional cause is greater damage to the fibre surfaces resulting from the milling to which the less readily-opened fibres, typical of the Quebec mining area, were subjected. Once this readily-available Mg had dissolved, there was little difference in leaching rates between milled and unmilled samples from the different regions. When the fraction of Mg dissolving is plotted against that of Si, all the materials follow a similar pattern, suggesting that the dissolution of Si (as silica) is the rate-controlling step in the dissolution of Mg.

In plain English

This laboratory study measured how fast magnesium and silicon dissolved from nine chrysotile asbestos samples in 1 N hydrochloric acid at 25°C. No people or animals were studied. Five samples were Canadian: the UICC B reference blend, three Jeffrey mine (Quebec) samples and one Cassiar mine (British Columbia) sample. Four came from Coalinga, California. UICC B and the two milled Jeffrey samples lost magnesium fast at first. Their initial rates exceeded 100% per hour, and 20 to 23% of their magnesium dissolved in this rapid phase. Coalinga samples started at 30 to 81% per hour, losing only a few percent in that phase. A hand-picked, unmilled Jeffrey sample started at only 6% per hour. A plateau of generally 1 to 2.5% per hour followed. By the time half the magnesium had gone, rates were below 1% per hour for most samples. Atomic force microscope images showed damaged surfaces on milled fibres. The author concluded that silicon dissolution controlled magnesium loss, and that true fibril dissolution rates for Quebec and Coalinga chrysotile would probably not be markedly different.

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 (in vitro) acid-leaching experiment on chrysotile asbestos samples
• Who was studied: Nine chrysotile samples; no people or animals. Five were Canadian: UICC B reference sample (a blend from seven Quebec mines and one British Columbia mine), a hand-picked Jeffrey mine sample, milled Jeffrey samples Johns-Manville 4-D and Jeffrey 4T-30, and a Cassiar mine (British Columbia) sample. Four were Californian, all from the Union Carbide mine in the New Idria range (Coalinga): RG-144, COF-25, RG-244 (COF-25 treated to give a silica coating), and NIOSH/IITRI CH-29 (knife-milled 'Calidria' chrysotile).
• Exposure: About 20 mg of each chrysotile sample was leached in 25 ml of 1 N hydrochloric acid at 25°C for set periods, the shortest being 2 minutes. Magnesium, silicon and iron in the solution were measured.
• Main results:
  • For UICC B and the milled Quebec samples, magnesium loss followed a straight line against the square root of leaching time until at least 60% of the magnesium had dissolved. Correlation coefficients were all greater than 0.99. Y-axis intercepts were 8.0% (UICC B), 2.6% (J-M 4-D) and 2.2% (Jeffrey 4T-30).
  • The magnesium leaching patterns for the Californian (Calidria) samples were all sigmoid (S-shaped).
  • Silicon leaching patterns were sigmoid for all samples, whatever the mining region, with y-axis intercepts all less than 0.5%.
  • UICC B and the milled Quebec samples had initial magnesium leaching rates above 100% per hour. Table 3 gives UICC B 301% per hour with 23% of magnesium dissolved in the rapid phase, Jeffrey JM 4-D 156% per hour with 20%, and Jeffrey 4T-30 179% per hour with 22%. The prose says 'more than 20%', but the table shows 20 to 23%.
  • The Coalinga samples had initial magnesium leaching rates of 30 to 81% per hour. Only a few percent of their magnesium dissolved in the rapid phase (Table 3: 0.5% to 6%).
  • The hand-picked (unmilled) Jeffrey sample had an initial rate of only 6% per hour, with about 0.2% of its magnesium removed in the rapid phase.
  • The Cassiar sample had a low initial rate (16% per hour) and thereafter appeared to have the lowest magnesium dissolution rate of all samples.
  • In all samples, the initial fast phase was followed by a plateau phase in which leaching rates declined only slowly and were generally 1 to 2.5% per hour.
  • After about 30% of the magnesium had been removed, leaching rates declined more rapidly. By the time 50% had been removed, rates were below 1% per hour for most samples.
  • When magnesium loss was plotted against silicon loss, all samples except Cassiar followed a similar pattern. The author interpreted this as showing that silicon (silica) dissolution controls the rate of magnesium removal.
  • Atomic force microscope images showed broken and damaged fibrils on the surface of the milled Jeffrey 4T-30 sample, compared with the hand-picked Jeffrey sample.
  • Magnesium dissolution rates were unrelated to the samples' specific surface areas.
• The authors' conclusion: The author concluded that commercial chrysotiles differ so much physically that it cannot be stated categorically that magnesium dissolves faster from one than another. The author also concluded that the results suggest true fibril dissolution rates for Quebec and Coalinga chrysotile would not be markedly different.
• Limitations the authors state:
  • Differences in how ore is processed into commercial products obscured the effects of fibril length and diameter on dissolution.
  • Comparing intrinsic rates would ideally need undamaged samples in the fibrillar state, which the author considered unlikely to be obtainable for Quebec chrysotile.
  • Refluxing in acid did not dissolve all the silica, so silicon loss was expressed against the ideal formula rather than the measured content.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Cassiar mine, British Columbia
• Jeffrey mine, Asbestos, Quebec
• Santa Rita mine, Joaquin Ridge near Coalinga, California

All asbestos mines

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Chrysotile, Quebec Chrysotile, Brucite.

Related studies

• Acid leaching studies of neutron-irradiated chyrsotile asbestos — A. Morgan, R.J. Talbot (1997)
• Respiratory health in chrysotile asbestos miners in British Columbia: a longitudinal study — D.A. Enarson, V. Embree, et al. (1988)
• Lung mineral fibers of former miners and millers from Thetford-Mines and asbestos regions: a comparative study of fiber concentration and dimension — A. Nayebzadeh, A. Dufresne, et al. (2001)
• The lung biological activity of American attapulgite — R. Bégin, S. Massé, et al. (1987)
• Characterisation of palygorskite specimens from different geological locales for health hazard evaluation — R.P. Nolan, A.M. Langer, et al. (1991)
• Environmental pleural plaques in residents of a Quebec chrysotile mining town — A. Churg, L. DePaoli (1988)

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