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

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.