SECTION 1. ASBESTOS MATERIALS IN MODERN TECHNOLOGY

Transcription

1 SECTION 1. ASBESTOS MATERIALS IN MODERN TECHNOLOGY THE GEOLOGY, OCCURRENCES, AND MAJOR USES OF ABESTOS N. W. Hendry Canadian Johns-Manville Co. Limitpd, Asbestos, Quebec Although the mineral asbestos has been known and used for at least 2000 years, it is truly amazing how few people really understand its properties and many important uses today. New uses for asbestos are the life blood of an industry whose annual production is estimated at over 400 million dollars and the estimated 1000 uses of the mineral are growing year by year. Geology The word asbestos is a broad term embracing a number of fibrous mineral silicates that differ in chemical composition. They may be divided into two mineral groups : (1) Pyroxenes-chrysotile ; (2) Amphiboles-crocidolite, amosite, tremolite, actinolite, and anthophyllite. The geology of asbestos deposits provides a fascinating study of differences and similarities. Their formation requires a common geological principle of the right host rock, basic structural features, and ideal conditions of temperature and pressure. Consider the most important type, chrysotile, which amounts to over 90 per cent of total asbestos output. The most valuable deposits occur in serpentine, formed by hydrothermal alteration of ultrabasic rocks such as dunite, peridotite, and pyroxenite. Their history can be briefly stated. First, emplacement and subsequent alteration of an ultrabasic rock mass below the then-existing earth s surface took place ; structural deform a t ion followed including folding, faulting, and shearing processes. The serpentine shattered in specific areas, providing fractures for hot waters to enter. Subsequently these waters dissolved the serpentine (hydrous magnesium silicate) over a considerable period of time, followed by a gradual drop in temperature and pressure until the solution became supersaturated. Fibrous crystals of magnesium silicate began to grow within the solutionfilled channelways, maintained in their open state by tremendous hydrostatic pressure. These crystals, which we refer to as chrysotile asbestos, commenced their growth on both walls of the fracture and sprouted inward toward the center. The width of the fracture determined the maximum length of the fiber. This is the most common type of chrysotile asbestos deposit. However, there are variations: (a) Slip fiber; (b) mass fiber; and (c) cross fiber in dolomitic limestone. 12

2 Hendry : Geology, Occurrences, and Major Uses of Asbestos 13 These deposits also produce commercial grades of asbestos fiber although certain fiber properties are different. Slip fiber deposits result from extreme deformation of the serpentine. The highly shattered rock, saturated with hot waters, was subjected to continual shearing action during crystallization, and fiber growth took place parallel to the fracture faces rather than normal to them. Mass fiber deposits, such as the Coalinga, California type, reveal extreme intergranular and faulting processes in the serpentine host rock. Fibers occur as small white leathery platelets or clumps within the sheared rock. It is apparent that hot waters permeated the sheared mass and that crystal growth occurred throughout the rock rather than along fractures or shearing planes. The third type of occurrence in dolomitic limestone is quite common, and the process of formation is similar to that in the normal cross-fiber deposit. The basic difference is that the host rock is of sedimentary origin, rather than igneous, but fortunately the all-important magnesium was present in the dolomite for the hot waters to dissolve, forming supersaturated solutions in contact fissures. This was followed by the crystallization of asbestos fibers normal to the vein walls. The second important varities of asbestos are crocidolite (hydrous iron sodium silicate) and amosite (hydrous iron magnesium silicate). Although they are slightly dissimilar in chemical composition, they have a common origin and occur in similar rocks. The host rocks, generally referred to as banded ironstones, are of sedimentary origin. The zones within the strata containing the crocidolite veins are soda rich in contrast to the amosite zones, but it is generally conceded that crystallization took place in situ under conditions of moderate pressure and temperature in the presence of moisture from the surrounding rocks. This then can be described as a metamorphic process with a recrystallization in fibrous form as contrasted to crystal growth of chrysotile in supersaturated magnesium solutions. The last group of asbestos minerals including tremolite, actinolite, and anthophyllite was formed in quite a different manner. These three minerals are similar in many respects in that they occur in lenticular forms of mass fiber, and are common to schistose igneous and metamorphic rocks rich in hornblende or pyroxene. Also, tremolite and actinolite are often found in impure limestone and dolomite that has undergone recrystallization. Common accessory minerals include talc, mica, and chlorite. Now, a word about the properties of these six asbestiform minerals. The difference in properties has a most significant bearing on the use and commercial value of each variety. These relative properties are illustrated in TABLE 1.

3 Flexibility Length Texture Tensile strength Acid resistance Spinnability Resistance to heat Chrysotile Very flexible short to 3 harsh to silky very high fairly soluble very good good TABLE 1 PROPERTIES OF ASBESTOS FIBER Crocidolite Amosite Tremolite fair to good good brittle short to 3" 1/4" to 6" short to 10% harsh or soft coarse but pliable harsh to soft very high fair weak very good fairly resistant fair fair poor poor fair to good Actinolite brittle short to long harsh very weak highly resistant poor Ant hophyllite g flexible tt % brittle to short Z harsh to 2 soft * 0 9) weak b. 8 Y 0 n, poor very good

4 Hendry : Geology, Occurrences, and Major Uses of Asbestos 15 TABLE 2 WORLD PRODUCTION OF ASBESTOS BY COUNTRIES* Country NORTH AMERICA Canada (sales) United States (sold or used by producers) SOUTH AMERICA Bolivia (exports) Brazil EUROPE Austria Bulgariat Finland France Italy U. S. S. R. Yugoslavia ASIA Chinat Cyprus India Japan Korea Philippines Taiwan Turkey! AFRICA Bechuanaland Kenya Mozambique Southern Rhodesia South Africa Swaziland Egypt OCEANIA Australia t New Zealand WORLD TOTAL (ESTIMATE) Production (Short Tons)? 1,272,016 66,606 1,338, ,201 26,455 63,418 1,212, ,323, ,200 22, , , , ,173 33, ,379 14, ,320 3,220,756 *From Mineral Trade Notes Vol. 59, No. 2, International Statistics Division of International Activities.?Metric tons from Mineral Trade Notes table converted to short tons. :Estimate.

5 16 Annals New York Academy of Sciences TABLE 3 TOTAL PRODlJCTIoN OF TYPES OF ASBESTOS (ESTIMATED) [ Production Per Cent Chrysotile 2,995,456 Crocidolite 112,700 Amosite 77,600 Ant hop hyllite Tremolite 35,000-3,220, The properties of flexibility, tensile strength, spinnability, absorption, filtration, and resistance to heat place chrysotile asbestos in a class by itself and clearly define it as a mineral of unparalleled properties. Occurrence Asbestos probably occurs in nearly every country in the world. However, the difference between occurrences and commercial deposits is a most important one. Therefore this section will deal primarily with economic concentrations. To set the stage for world occurrences, it is pertinent to first look at world production of asbestos (TABLES 2 and 3). It is pertinent to examine the various continents to determine the locations of the most important asbestos deposits. In terms of total production of all types of asbestos, North America is presently the leader, followed in sequence by Europe, Africa, Asia, Australia, and South America. No~tli Amckn Canada has long been the world s number one producer, with much of this country s output - all of which is chrysotile - coming from eight companies operating in the Eastern Townships of the Province of Quebec. Here, all mines are located on a discontinuous serpentinite belt extending some 60 miles in length. In addition, important deposits are being worked in northern British Columbia and northeastern Newfoundland. Until recent months, northern Ontario was a significant producer, and even now other occurrences are undergoing development in this province. Potential ore bodies are reported in the Yukon, N.W.T., and Ungava region of northern Quebec. The United States is becoming increasingly important as a chrysotile producer. For many years Vermont and Arizona were the only chrysotileproducing states, although anthophyllite has been mined in Georgia and the Carolinas. However, California has entered the ranks of chrysotile producers, with mining operations located east of Sacramento and near

6 Hendry: Geology, Occurrences, and Major Uses of Asbestos 17 Coalinga. The latter deposits are quite unique and could achieve considerable importance as short fiber producers in the future. Europe This continent is now close to North America in terms of total output, and this is due almost entirely to rapid expansion of Russian production. Russia is the number two producer, following close behind Canada, and, like Canada, all output is chrysotile. Indications are that Russia will shortly replace Canada as the world s leading asbestos producer. The main deposits are located on the eastern slopes of the Ural Mountains with the town of Asbest being the main center. (This is 40 miles east of Sverdlovsk, a town perhaps better known because of its connection with the U-2 spy plane and the Gary Powers incident.) Five mills are in operation on an asbestos zone some 16 miles long. A major ore body, located near Aktovrak on the Mongolian border, is scheduled for production this year. Other deposits are known to exist, but little information is available. Elsewhere in Europe, Italy continues as a producer of tremolite fiber and also slip fiber chrysotile, with deposits at Balangero near Turin and on the island of Corsica. France, Cyprus, Yugoslavia, and Finland are all small, but long-standing producers. Africa Asbestos output from this continent comes essentially from three countries - Southern Rhodesia, Swaziland, and the Republic of South Africa. Southern Rhodesia is the world s third largest producer of chrysotile, with the principal deposits located in the Shabani and Mashaba areas, respectively 100 and 150 miles east of Bulawayo. Numerous small deposits of chrysotile are located elsewhere in the country, all east of the Great Dyke. The Republic of South Africa is important due to the production of crocidolite (or blue asbestos) and amosite, plus minor amounts of chrysotile. Crocidolite is obtained primarily from Cape Province, in a belt of iron formation some 250 miles long in the eastern part of the province from Prieska to Kuruman; some crocidolite is also obtained from the Transvaal west of Pietersburg, and it is from this same belt that amosite is obtained with Penge being the center for amosite mining. Chrysotile comprises only 15 per cent of South African asbestos output and is obtained from deposits in the Barberton area east of Johannesburg. Elsewhere in Africa an important deposit of chrysotile occurs in Swaziland and there is a smaller operation in Bechuanaland.

7 18 Annals New York Academy of Sciences Asbestos Textiles Asbestos Cement Products Friction Materials &Gaskets Asbestos Paper Floor Tile Paints, Roof Coatings, Caulks, etc. Plastics Miscellaneous A s ia China has reportedly been expanding output in recent years, but little factual information is available, other than the fact that the main deposits are located in Szechwan Province. Japan has a small producer located on North Island, and very minor production is reported from India and Korea. Australia Crocidolite is produced in Western Australia, making this country the world s number two producer of this type of fiber. A very limited amount of chrysotile is obtained from northeast New South Wales. South America Asbestos occurrences are known in most countries on this continent, but only Brazil is currently a producer and a very minor one indeed. Major Uses of Asbpstos Asbestos is termed the magic mineral, the mineral of unparalleled properties, : the mineral of a thousand uses. These terms are most suitable since asbestos is a major constituent in such a wide variety of products, a fact that we tend to take for granted. In many cases, the absence of asbestos from a specific product could radically change its form and thereby render it useless for the requirement involved. TABLE 4 lists major-use categories of asbestos fiber with an approximation of the tons of asbestos and the dollar value of each category. Although the figures shown include all types of asbestos, 93 per cent of the total can be considered to be chrysotile. However, it is necessary in the broad picture to consider the major uses of each type of asbestos. TABLE 4 MAJOR USES OF ASBESTOS 1 Short Tons Approx. Value 1 Dollars 66,000 26,400,000 2,190, ,500, , ,000 11,100,000 19,800, ,000 13,200,000 85,000 3,740,000 21, , ,700 19,670,000 3,220, ,334,000

8 Hendry: Geology, Occurrences, and Major Uses of Asbestos 19 (1 1 Tremolite and actinolite can be considered as the least important of the asbestos minerals from an economic standpoint. When found in a pure state, they find limited utilization in the chemical industry as filter mediums and as inexpensive fillers in a limited assortment of manufactured products. Tremolite is processed and repurified by acid treatment for special uses in the filtration field. (2) Anthophyllite, because of its low strength fibers, finds limited usage in products in which reinforcement is required, such as asbestes cement, floor tile, etc. However, it is interesting to note that an expansion of usage has been developed for this mineral, principally as a filler or as a partial replacement for the higher strength chrysotile fibers. Anthophyllite is used in the chemical industry as a filler in the rubber and plastic industries and in various adhesives and cements. (3) Amosite, which represents approximately 2.5 per cent of total asbestos usage, has a unique place among the asbestos minerals and is used in end products in at least 20 countries in the world. For example, approximately 22,000 tons of amosite is consumed annually in the U.S. The principal use of amosite is in the insulation field in the form of pipe and boiler coverings, bulkhead lining in ships, 85 per cent magnesia insulation, etc. (4) Crocidolite, which accounts for approximately 3.5 per cent of total asbestos usage, is used chiefly in the manufacture of asbestos cement products. In addition, this mineral is used in the manufacture of acid-resistant filters, packings, insulations, and certain types of lagging. Approximately 17,000 tons of crocidolite were consumed in manufactured products in the U.S. in Finally, the mineral chrysotile must be examined in more detail from a usage standpoint in view of its widespread importance throughout the world. As shown in TABLE 4, the asbestos textile industry consumes approximately 66,000 tons of long fiber annually. The spinning and weaving of asbestos is done in the manufacture of safety clothing, curtains, lagging cloth, woven brake linings, clutch facings, and many other articles. These products serve both the electrical insulation industry and the construction industry. The chrysotile asbestos content of these products can vary from 80 to 100 per cent, It can be stated that chrysotile textile fibers have been improved by a minimum of 50 per cent by a reduction of the fines or dust content over the past five years. Also, manufacturing processes have been improved materially during the same period, including humidity and dust control. The asbestos cement industry is the principal consumer of chrysotile asbestos both from a tonnage and dollar standpoint. Most countries of the world have one or more asbestos cement operations used in the manufacture of pipes, flat and corrugated sheets, shingles, etc. The asbestos cement

9 20 Annals New York Academy of Sciences manufacturing process combines medium long to medium short fibers with cement, silica, and water, thoroughly mixed to produce the maximum open state of the fibers. The solids are drawn from the slurry on a fabric belt and deposited in successive layers on a rotating mandril. The chrysotile fibers serve the function of tiny reinforcing rods in the finished product. Depending on the product, the proportion by weight of asbestos to cement may vary from 15 to 90 per cent. A wide variety of chrysotile grades are used in the manufacture of friction materials and packings. Chrysotile asbestos is used in friction materials for the following basic reasons, and serves in the following ways : (1) Because of its heat stability; (2) as a reinforcing agent; (3) as a filler; (4 ) as a regulator or inhibitor of the resin flow in molding; (5) as a dispersing agent for the metal chips and other particles during formation; and (6) because it is less abrasive than other heat stable fillers in the same low price range. There are five major types of friction materials that contain chrysotile, namely, woven, dry mix, sheeter, profile calender, and extruded. Basically, the composition of friction materials includes binder, metal chips, friction particles, chrysotile asbestos, filler, and solvent. The asbestos content may vary from 30 to 80 per cent of the total composition. Most of the excellent friction materials in use today would not be possible without the contribution of chrysotile. The relationship between packings and friction materials is that packings are developed to produce the least amount of friction between surfaces whereas friction materials require high friction without too much wear. Packings may contain from 40 to 75 per cent of chrysotile and are essential for the strength, toughness, resiliency, durability, and heat resistance of the final product. A great variety of paper products are made from chrysotile, including millboard, roofing felts, pipe covering, fine quality electrical papers, insulating papers, asbestos-latex flooring felt, and many others. Asbestos papers fill the need for a paper that hits the inorganic properties of chrysotile, such as heat resistance, chemical inertness, and electrical and insulating properties. The asbestos content in most papers in this category is 80 to 90 per cent. The manufacture of asbestos papers involves mixing in water to form a slurry, followed by laminating on cylinder molds or Fourdrinier machines. Although the floor tile industry is the second largest user of chrysotile asbestos, it is far down the list in dollar value. This is due to the fact that only very short chrysotile fibers, which serve as fillers and reinforcement media, are used in the manufacture of floor tile. The floor tile formulation contains from 10 to 30 per cent chrysotile together with vinyl resin or as-

10 Hendry: Geology, Occurrences, and Major Uses of Asbestos 21 phalt together with various other fillers. The ingredients are mixed, pressed into a blanket, calendered, and cut to size. The use of short chrysotile fibers in the paint, roof coating (aluminum and blackline) caulks, sealants, and joint filler industries has been developed to a high degree in the U.S. and Canada. In these uses chrysotile serves the purpose of an inexpensive filler as well as a reinforcer. Chrysotile is an important component in a wide range of plastic products, including cold molded, thermoplastic, and thermosetting plastics. Usually, the very short grades of chrysotile are used to impart toughtness, increase hardness, retard the burning rate and reduce molding costs, and control the flow of the material under pressure. Finally, the miscellaneous uses of chrysotile cover a wide field including sprayed insulation, asphalt paving and curbing, welding rod coatings, filter mediums, chlorine cell diaphragms, acetylene cylinder packings, and many others. A clcno loled gm rn.ts The author gratefully acknowledges the permission of Canadian Johns- Manville Co., Ltd. to publish this paper. Also, valuable suggestions and a critical review of the paper were provided by H. K. Conn and D. J. Straw of the Canadian Johns-Manville staff. Refwences 1. MUNRO, R. C. & K. M. REIM Pacific Southwest Minerals Conference Am. Inst. Mining Eng. 2. BADOLLET, M. S Encycl. Chem. Tech. 3. BADOLLET, M. S A Symposium of.4rtirles Can. Inst. Mining Metal. 4. MONTPETIT, L Asbestos in the construction industry. Can. Inst. Mining Metal. Bull., Sept. Discussion of the Papcr G. w. H. SCHEPERS (Biweaic (If Labntwtol-ies, Dept. of Health, Washington, D.C.) : This review of the natural and industrial ecology of asbestos was most fascinating. Obviously the growing exploitation of this natural, useful, but also dangerous fibrous mineral has presented industry with major medical challenges. Industry should, however, be commended for the excellent safeguards which it has introduced. This has conserved many lives. I believe, however, that it should also be stressed that the medical or health problem originating through exposure to asbestos dust may come about equally through minor uses of asbestos. In this paper emphasis was logically placed on major uses. However, there are thousands of individuals who are exposed to asbestos dust on account of participation in minor asbestos industries. The major asbestos industries have the technical personnel and knowledge that generally ensure reasonable protection for their

11 22 Annals New York Academy of Sciences workers. In the minor industries, hazardous exposures to asbestos may occur without the hygiene problem being recognized. A good example is the carpenter who uses asbestos-reinforced plaster board and asbestosimpregnated corrugated roofing materials. These men often spend long hours machine-sawing such boards and seldom take any precautions against excessive inhalation of the asbestos dust, having long been assured that the dust generated in sawing plaster or gypsum board is harmless. One of the varieties of gypsum, however, has tremolite asbestos as a filler or reinforcing agent. As I have demonstrated experimentally (Archives of Industrial Health, 1955 ), prolonged exposure to such admixtures of gypsum and asbestiform minerals can induce pulmonary asbestosis. In this respect the dilution of asbestos fibers by the gypsum does not modify the action of the asbestos component. When gypsum is mixed with quartz dust, the potential injurious action of the latter on living tissue is reduced. This was proven experimentally by myself and colleagues. (Archives of Industrial Health, 1955). The trouble with these minor asbestos industries is that the health problems they create are seldom recognized for what they are. Since the practicing physician is not likely to recognize the etiologic role of an asbestiform ingredient of dust in nonasbestos industries, his diagnosis is likely to be one of idiopathic lung pathology. This not only introduces a diagnostic error for the individual victim, but also leads to faulty epidemiology. It would be greatly advantageous if a detailed directory of asbestos uses were more generally available, or if industrial products carried appropriate labelling, as do drugs.