Asbestos

Chrysotile            : Mg3Si2O5(OH)4
Refractive Index   : 1.53-1.56
Specific Gravity   : 2.5-2.6
Mohs Hardness    : 2.5-4

Asbestos is a generic term applied to six minerals that occur in nature as strong, flexible, heat-resistant fibers. Nearly all (>98%) commercial asbestos is the mineral chrysotile. Chrysotile is differentiated from the other five asbestos minerals by its tubular serpentine rather than ribbon-like amphibole structure, its generally greater fiber flexibility and strength, its lower heat resistance, its greater surface area and positive surface charge, its lower refractive index, and its greater susceptibility to decomposition by strong acids. Of the ampibole asbestos minerals, only two − amosite, a magnesium iron silicate, and crocidolite, a sodium iron silicate − are produced in commercial quantities.
 

The other three − anthophyllite asbestos, tremolite asbestos, and actinolite asbestos − are rare asbestiform varieties of the nonasbestiform prismatic minerals anthophyllite, tremolite, and actinolite. It is the rarity of these three asbestiform varieties that has precluded the assignment of distinct mineral
names and unduly caused the prismatic analogues to become identified with asbestos. The nonasbestos forms of chrysotile, amosite, and crocidolite are sufficiently common to have earned separate mineral identities as antigorite, cummingtonite-grunnerite, and riebeckite, respectively. Asbestos ore is typically processsed in highly automated operations designed to minimize worker exposure to mineral dust. The ore is crushed, dried, screened, milled, and air separated to produce a variety of grades.
 

TYPES

The two major world producers, the former Soviet Union and Canada, designate several major grades of asbestos, with further subdivisions within each grade. Grades are based on fiber length, strength, color, and purity, plus intended application. The following grades are based on Canadian standards.
Spinning fiber − The cleanest and longest fibers, to >12mm, are reserved for producing woven asbestos textiles.
 

Asbestos cement fiber − This is the longest fiber grade that is <12mm. Paper/shingle fiber − This is essentially <5 mm (-4 mesh) fiber, with shingle fiber being generally shorter than paper fiber.

 

Shorts/floats 

These are the shortest fibers, with most shorts or all floats <2mm (-10 mesh).

 

Crudes

This is crushed ore containing staple fibers >10mm. Crudes are sold to customers who process them into fibers for their own purposes.

 

USES

Approximately 3.5 million metric tons of asbestos are produced annually. Major producers are the former Soviet Union (60%) and Canada (17%). Production and use in the United States is very minor due to health and liability concerns, although California hosts a short fiber chrysotile deposit considered to be the largest single mineral ore body in the world. Major asbestos applications worldwide are asbestos cement, friction products, roofing, insulation, flooring, plastics, and gaskets.

 

Asbestos cement

In asbestos cement pipe asbestos provides good drainage and high green strength during manufacture, plus high pipe tensile strength, impact strength, heat resistance, and alkali resistance. In asbestos cement sheets it provides high flexural stregth as well.

 

Friction products

Paper and shingle fibers are used in molded clutch plates and disk brake pads, while short and float fibers are used in brake linings. Clutch plates are also made from open-weave asbestos cloth impregnated with resin. In all cases, asbestos is used for its durability, heat and moisture resistance, low thermal conductivity, and high strength. Roofing − Short, float, and shingle fiber are used in asphalt shingles and roofing felts and in asphalt-based roof coatings to provide dimensional stability and flexibility, to enhance crack resistance and weatherability, and to control rheology (coatings).

 

Insulating products 

Textiles for heat-resistant protective clothing are woven from spinning fiber, but most asbestos insulation products are in the form of paper, paperboard, millboard, and mat from paper-grade fiber.
Asbestos provides flexibility, dimensional stability, tear resistance, heat resistance, chemical resistance, moisture resistance, low thermal conductivity, and high electrical resistivity. Products include pipe wrap, thermal insulation in appliances, and electrical and heat insulation in electronics.
Flooring − Short fiber is used in vinyl tile to provide flexibility, resilience, durability, fire resistance, and dimensional stability. Short fiber is also coated with rubber latex and formed into paper used as backing for vinyl sheet flooring.

 

Plastics

Abrasion-free asbestos is used to thicken and reinforce thermosets, providing heat, tear, and electrical resistance, low heat deformation, high strength, and stiffness. Short and float fibers are used as fillers; mat, felt, paper and cloth are impregnated with resin to form laminates. Gaskets − Abrasion-free asbestos cement- and paper-grade fibers are used in rubber-based gaskets and packing to provide resilience, plus resistance to heat, tear, and chemical attack. Densified latex-asbestos paper is also used to make gaskets.

 

Other uses

Short and float fibers are used in textured paints, drywall joint cements, caulking compounds, automotive undercoatings, and asphalt paving mixes for high traffic areas.