Charles B. Kenahan US.Bureau of Mines Washington, D.C.
Solid waste
resources out of place l h e constant increase in per capita generation of solid waste, stimulated by production growth and coupled with a rapidly increasing population concentrated in urban areas, is responsible for the nation’s present environmental crisis. Increased demand and the increased pro+ duction to meet it are the basic causes of increased pollution. This is confirmed by the fact that the real output of goods and services in the U.S. has grown as much since 1950 as it grew in the entire period from the landing of the Pilgrims up to 1950. To add fuel to the fire-or waste to the pile-consider that a similar growth period is predicted between 1970 and 1980, which can easily be translated into more junk cars, cans, bottles, plastics, fly ash, and paper products. If the present production of solid waste is not managed, what of the future? Solid waste
The terminology and characteristics of this solid waste that is causing such a furor must be examined. Where does it come from? How much of it is there? Where is its final resting place? And, what is being done about it?
Solid waste falls into three major source categories. The first is urban refuse, which includes domestic, commercial, municipal, and industrial waste products; the second category contains the mineral waste which results from mining and mineral processing operations; and the 594 Environmental Science & Technology
last, agricultural waste, includes farming, animal, and crop waste. A further breakdown of urban refuse shows that the nation generates about 400 million tons each year. This includes 60 billion cans, 36 billion bottles, 58 million tons of paper and pa’per products, 4 million tons of plastics, over 1 million abandoned automobiles, mountains of demolition debris, 180 million tires, and countless millions of tons of refrigerators, stoves, TV sets, and like items. The cost to collect and dispose of urban solid waste alone is about $6 billion annually. Where does it all go? About half is burned in some manner, and the other half is buried in landfills and dumps, with the values it contains lost forever. The second category, mineral waste, is larger-about 1.7 billion tons each year. The production of 1 ton of copper results in about 500 tons of waste earth and rock. Additionally, a past accumulation of about 23 billion tons of mineral waste is scattered across the nation. The final category, agricultural waste, is even more awesome-over 2 billion tons annually-including farming, slaughterhouse, and animal waste. An average-sized steer generates about 10 tons of solid waste each year. Furthermore, over 100 lb of solid waste daily is generated for every man, woman, and child in the country. By 1980, this is expected to increase to 150 lb per day. The importance of secondary metals-which represent the only
growing metal resource-can best be assessed by comparing the gross production of major metals with quantities reclaimed from secondary sources. According to production estimates (on an annual basis), over 50% of the lead, 40% of the copper, 45% of the iron and steel, and 25% of the zinc and aluminum made available for new products last year were derived from secondary sources. Equally important are the estimated quantities of these metals accumulating in the “in-use” channels of the economy. Nearly 40 million tons of copper, over 3.5 million tons of lead, and about 4 million tons of zinc are presently in this category, which represents a constantly growing man-made mine of future raw materials. These figures are indeed impressive, but the amounts of metals still being wasted are equally impressive. Annually discarded in municipal dumps are 11 million tons of ferrous metals and over 1 million tons of nonferrous metals, including copper, aluminum, tin, lead, and zinc. An estimated 12 million junk cars still remain to be reclaimed from auto graveyards across the nation. In addition, automotive scrappage has now reached a rate that can provide over 10 million tons of ferrous and a half million ton of nonferrous metals annually. Nearly 400,000 tons of aluminum was used for manufacturing cans, lids, and caps in 1970; only a small percentage of this was reclaimed. Thrown away each year in city dumps is 25,000 tons of tin in tin-coated steel cans. which is
feature
A pioneer in secondary met; the U.S. Bureau of Mines posscsscs LIK recnnical to reclaim solid waste effect
equivalent to the amount of tin salvaged from all other secondary sources. These are just a few opportunities.
The Department of the Interior’s Bureau of Mines has always considered waste products and scrap generated by the mineral and metals industry and the consuming public as potential resources. In the business of reclaiming values from metal and mineral-based by-products for over 30 years, the bureau has been a pioneer in the field of secondary metals recovery and solid waste research. The authority and responsibility for con-
ducting research on separation, recovery, and recycling of metal, mineral, and energy-based by-products, however generited, are inherent in the Organic Act of May 16, 1910, as amended in 1913, which established the Bureau of Mines. Congress assigned a major role to the Department of the Interior in the Solid Waste Disposal Act of 1965, and more recently in the Resource Recovery Act of 1970. Under the original Solid Waste Act, the Department of the Interior was authorized $32.3 million over a 3-year period for research relating to metal and mineral waste. Only $11 million was appropriated to the Bu-
Screening. Pilot plant separates iron and fin cans f r o m incinerator residues
reau of Mines under this law. The Resource Recovery Act of 1970 authorizes funding of $8.75 million for fiscal year (PY) 1971, $20 million for FY 1972, and $22.5 million for FY 1973 to the Department of the Interior. The bureau’s Metallurgy Research Activity, equipped for metal and mineral waste problems, includes the Divisions of Metallurgy and Solid Wastes with 777 full-time employees at eight research centers across the country. About half are professionally trained in metallurgy, chemical engineering, chemistry and physics, and mineral engineering. The bureau’s coal and petroleum research activities also deal with solid waste problems relating to energy recovery. The bureau has already laid valuable groundwork in several solid waste research areas, including urban refuse, junk cars, mining and processing waste, and industrial waste.products. For over a year the bureau has operated a pilot plant at the College Park (Md.) Metallurgy Research Center which separates and recovers, in much the same way that minerals are separated from their ores, the major metal and mineral values contained in municipal incineratoi residues. The process, which employs conventional mineral engineering equipment such as magnetic separation, screening, grinding, and shredding procedures, separates the ferrous and nonferrous metals and glass from the burned refuse on a continuous hasis (see left). The plant, which can process one-half Volume 5, Number 7, July 1971 595
Resources. Ten to twelve dollars worth of recoverable products is found in each ton o f municipal incinerator residue
ton of residues per hour, is sophisticated enough to separate glass into clear and colored components. Based on the engineering data developed from the pilot-plant. operation, estimates for capital and operating costs for a 1000-ton-per-day plant show a cost of about $2 per ton of residue. Each ton of residue processed through the plant yields 700 Ib of iron, 40 Ib of nonferrous metals including aluminum, copper, lead, tin, and zinc, small amounts of silver, and a half ton of glass. The total value is $10 to $12 per ton of residues. The remainder is finely powdered ash, which makes excellent fill material and also has agricultural nutrient value. The values contained in 1 ton of municipal incinerator residues are depicted in the photograph above. An air classification system to re-
cover the metal and mineral values from raw, unburned refuse is also heing investigated. The bureau has developed an experimental, simple, horizontal air classifier with a feed rate of over 1 ton per hour, which produces a concentrate from shredded municipal refuse assaying 65% metal and 85% paper, with high metal and paper recovery. Potential uses for the paper and plastics are also being examined. Significant progress has also been made in developing processes for refining ferrous and nonferrous products reclaimed from refuse. High-purity zinc and an aluminum alloy having utility in the secondary aluminum industry have been produced in the laboratory. Tin cans and other iron products can be used for precipitating copper from waste copper dump leach solutions. which is an established
Glass wool. Dolomite fused with glass waste produces an excellent mineral wool 596 Environmental Science &Technology
practice, or used as feed material for steelmaking after removing deleterious contaminants. The bureau has been successful in producing standard bricks from various grades of glass recovered from refuse. The bricks, made by dry pressing or extruding mixes of 70% glass and 30% clay and firing at 1000°C, meet or exceed ASTM specifications for severe weathering, FBS brick. Another use for glass from refuse is mineral wool production. By fusing glass with about 50% dolomite and blowing, an excellent grade of colorless glass wool, having a bulk density of 2.5 Ib per ft3 and an average fiber diameter of 6 p, is produced. The fibers are shorter and softer than typical commercial slag wools tested (see below left). Another development that could significantly affect the disposal or utilization of urban solid waste is a process recently developed by the bureau’s Pittsburgh Coal Research Center for converting garbage and waste paper into crude petroleum. In the process, the combustible material from refuse, including garbage, is hydrogenated by reaction with carbon monoxide and steam at 500 psi. The resulting product is a heavy paraffiic oil, that is low in sulfur. It is estimated that each ton of dry refuse would yield over two barrels of this crude petroleum. The same process has been applied to wood by-products, sewage sludge, and animal manure to yield a crude oil. Based on current generated domestic refuse and animal manures,
this represents a potential equivalent to 2 billion barrels of oil annually. Other research has demonstrated that organic fractions of wastes can be pyrolyzed thermally (destructive distillation in the absence of oxygen) to yield liquid and gaseous hydrocarbons, tar, and valuable chemicals (see helow). Worn-out rubber tires, for example, are a nationwide waste product that can he disposed of by pyrolysis. About 180 million tires containing nearly 2 million tons of rubber are scrapped yearly. Tire disposal by burning causes serious air pollution by the billowing, acrid, black smoke produced. Disposal in landfills or open dumps is less than desirable because tires are difficult to compact and are essentially not biodegradable. Bureau research on pyrolysis of tires has shown that 1500 ft3 of high-quality gas (800 to 1200 Btu per ft3) and 140 gallons of hydrocarbon liquid oil can be produced per ton of tires treated. The Firestone Tire and Ruhher Co. has constructed a pilot plant to prove out the technical and economic feasibility of the bureau-diveloped process. In other research, the pyrolysis
process has been applied successfully on a laboratory scale to urban refuse, wood bark and sawdust, plastics, and discarded battery cases to produce a variety of useful products. The bureau is also investigating vortex incineration to solve problems encountered in conventional furnaces. The vortex incinerator is more compact, has a lower capital cost, and greater potential for cleaner effluents than the conventional grate-type incinerator commonly used to burn refuse. A pilot-scale vortex unit, incinerating combustible solids ranging from highmoisture-content sludge to industrial and domestic refuse, is currently under evaluation at the bureau’s Coal Research Center in Pittsburgh. Bureau of Mines researchers at the Twin Cities Metallurgy Research Center in Minneapolis, Minn., have found another novel use for the mounting avalanche of trash from this affluent society. The refuse, traditionally posing a costly disposal problem to every municipality, has proved to be an effective reductant for converting nonmagnetic taconite ore (a waste product of iron ore mining) into high-qual-
ity magnetic iron ore in a reductionroasting process developed by the bureau. The average composition of raw refuse includes about 8 % metallics, 42% paper, and 22% other combustible rubbish such as wood, rags, ruhber, grass, leaves, and plastics. These supply both fuel and metal for the reduction process. The nonmagnetic taconite ore, containing about 30% iron, is roasted with the refuse in a large rotary kiln furnace at 1000°C. The final recovered product analyzes 70% iron-a high-grade iron ore suitable for smelting in the blast furnace. Junk cars
Another problem to which the bureau’s Salt Lake City, Utah, research center has been devoting a considerable research effort is jupk cars. Although discarded auto hulks constitute only a small fraction of the waste disposal problem in terms of tonnage, they are higher in metal values than most waste materials. As a result of recent research by the bureau, practical and economic methods have been developed for dismantling junk automobiles to produce
Destructive distillation. Organic fractions of municipal wastes can be pyrolyzed thermally to yield valuable by-products Volume 5 , Number 7, July 1971 591
Clean air VS. pollution. Smokeless junk car incinerator fops open-air burning
high-quality scrap. All components of 15 scrap automobiles procured from auto-wrecking yards, scrap processors, and insurance salvage firms-cars manufactured between 1954 and 1965were dismantled, separated into various components, and analyzed. Alternative means and methods of stripping and dismantling the cars were employed to determine the fastest and most practical technique. Derived from the information obtained, a representative junk automobile weighing 3600 lh could yield approximately 2500 lb of steel, 500 Ib of cast iron, 32 Ib of copper, 54 Ib of zinc, 51 Ib of aluminum, and 20 Ib of lead. The remaining 400 Ib consisted of nonmetallics. The bureau conducted time and motion studies on the 15 scrap cars using various dismantling procedures (cutting torches, hand-stripping, and cutoff saws) and found that a typical vehicle could be economically burned 598 Envirmmental Science Bi Technology
in a smokeless incinerator and handdismantled, and the steel could be baled into a high-quality bundle containing less than 0.1 % copper. A cost evaluation study showed that such an operation could provide an annual return rate on investment of 25%. In a cooperative effort with the Wasatch Metal and Salvage Co. (Salt Lake City, Utah), the bureau developed, constructed, and is presently operating a practical, smokeless junk car incinerator (above). It is relatively inexpensive and can efficiently process as many as 80 cars in an eighthour period. Also, the combustion gases are smokeless and meet or exceed most clean air standards. The new incinerator’s principle attraction is its $22,000 construction cost (roughly one-tenth the cost of smokeless models now commercially available) and a relatively low operating cost of about $2 per car. The incinerator has stimulated wide interest among
scrap processors whose open-air burning practices are being increasingly restricted. At least nine scrap car processors are constructing auto incinerators which are based on the bureau design. Bureau engineers are also working on the problem of upgrading the nonmetallic and nonferrous rejects from junk car shredding operations. Such rejects are currently being generated at the rate of about 1 million tons annually. Aluminum, copper, zinc, and lead constitute over 30% of this reject. For the most part, these valuable nonferrous metals are being wasted because no practical method has been devised to separate and recover them. Recently, the bureau’s Salt Lake City research center developed an air separation method that yields a concentrate of the metallic constituents from this residue. This air classifier has a 1,6-ton-per-bour capacity for shredded residue. The concentrate, which is 84% metal, contains 91 % of the nonferrous content of the residue. This metallic concentrate can be refined into some of its constituent parts by physical concentration methods and pyrometallurgical processing. Other bureau research aimed at developing a continuous process for steelmaking in the electric furnace promises to have an impact on automotive scrap consumption that makes an ideal feed for such a process. Test results to date indicate that the steel produced will be suitable for teeming into ingots and rolling into a variety of finished structural products. A relatively simple technique was developed at the Twin Cities, Minn., research center, for recovering copper from starters, generators, armatures, and similar high-copper automotive components. This process may offer the solution to a troublesome and time-consuming problem for scrap processors. In this process, copper-containing scrap is dipped in a molten salt (calcium chloride) and agitated briefly. The bath does not affect the iron and steel scrap but quickly melts the copper, even in inaccessible small boles and crevices. The molten copper collects in the bottom of the vessel and can easily be tapped off. About 99% of the copper can be reclaimed in this manner. The process is economical because the salt is cheap and can be reused. Bureau research on utilization of auto scrap as a reductant for convert-
ing currently nonexploitable, nonmagnetic taconite ores to a commercially attractive iron resource has been given much attention. In this process, the nonmagnetic taconites, rejected as waste during mining and beneficiation of the magnetic taconites and analyzing about 30% iron, are mixed with unburned auto scrap. This material is heated in a rotary kiln at 1000°C to reduce simultaneously the nonmagnetic oxide iron in the ore to the magnetic form and oxidize the scrap iron to the magnetic oxide. The final product is a high-grade ore that analyzes about 70% iron. Tin cans, borings and turnings, and other low-grade ferrous scrap are also ideal raw materials for this process. Mineral waste
The bureau has also been active in reclaiming values from mining, metallurgical, chemical, and industrial processing operations. This work not only includes salvage and reuse, but also stabilizing nonusahle mineral waste. A large-scale effort his heen made to stabilize the waste tailing piles from mining operations that have no mineral or utilization values (see below). These wastes are often air, water, and land pollution sources. Successful chemical and vegetative techniques have been demonstrated on copper and uranium-mill tailing piles.
Thirty-four acres of uranium leach plant residues, located on the Navajo Indian Reservation in Arizona, has heen effectively stabilized against wind erosion using a low-cost chemical method developed by bureau scientists. In Durango, Colo., another 13acre plot of waste uranium tailings was stabilized by a vegetative cover under bureau supervision. The procedure has been so successful that a stalk of wheat grown on the waste pile, where formerly nothing would germinate, won second prize in a local flower and garden show. Engineers have also demonstrated that mineral waste such as copper and gold mill tailings, coal washer rejects, and power plant bottom and fly ash can he converted into useful finished products. The bureau also initiated a modest contract and grant program under authority of the Solid Waste Disposal Act of 1965. Under the contract and grant program, the University of Utah made significant progress producing crystallized glass and ceramic tile and pipe from copper tailings, fabricating a promising refractory and tile from asbestos tailing waste, and producing high-quality ferrites from mill scale. West Virginia University, under a bureau grant, developed a process producing rock wool insulation from coal
ash slag, a waste product from coalfired central power plants. Commercially competitive structural concrete blocks also have been fabricated from the power plant fly ash. Stanford University researchers, under another bureau grant, demonstrated the technical and economic feasibility of producing steam-cured, calcium-silicate bricks from California gold mine waste. A preliminary report indicates that the calcium silicate bricks can he produced from the tailings and delivered to the market areas at costs below the existing lowest quoted selling price of standard clay bricks. Research conducted hy the IIT Research Institute, Chicago, Ill., on red mud has shown that this waste material can he fabricated into a variety of potentially useful commercial products. Red mud is a slimy, high-iron content reject resulting from processing bauxite to produce alumina for making aluminum. Ceramic articles such as tile pipe, wall and floor tile, and lightweight building blocks have been fabricated from the mud which otherwise poses an acute disposal and impounding problem. Processes have heen developed by bureau scientists to convert asbestos mining waste, phosphorus furnace slags, and mine and mill tailings into raw materials for manufacture of wall tile and bricks.
Stabilization. Planted and nonplunfed areas show chemical-vegetative stabilization o f copper failings after one year Volume 5, Number I, July 1971 599
industrial waste
In the industrial waste area, the bureau has developed several promising methods to wnvert waste materials into useful resources. A unique method for reclaiming valuable cobalt from sintered carbide scrap was developed at the research center in Rolla, Mo. The process involves the use of molten zinc, which forms an alloy with the cobalt hinder. The zinc is then recovered by distillation techniques. Over 99% of the tungsten carbide and over 98% of the cobalt can be recovered and reused. Virtually all of the zinc is recovered and can be reused to reclaim additional scrap. The process yields products that are directly reusuable without further treatment. Evaluating the products made from recycled material shows that there is no difference between the properties of those made from used material and those made from virgin sources. The treatment is unique in that virtually all components of the process are recovered and can he reused. The technique is believed to be a breakthrough in the recycling of cemented carbides, which is a $250 million business nationally. The Wendt-Sonis United Greenfield Division of TRW Inc. is presently testing the bureau process on a pilot-size scale.
Chades B. Kenahan, chief o f the bureau’s Division of Solid Wastes, received his B.S. in chemistry from King’s College ( P a . ) .Since loining the Bureau of Mines in 1953, Mr. Kenahan has conducted metallurgical research in elecfroplating, corrosion, elecfrowinning, and electrorefining of metals, and mineral and metal solid waste problems. Mr. Kenahan was appointed to his present position in 1970. 600 Environmental Science & Technology
Other research has focused on developing methods that may alleviate serious water pollution problems for the nearly 20,000 metal-plating and coating facilities across the nation. Electroplating and metal-finishing wastes are significant stream pollutants-either directly, owing to their content of toxic and corrosive materials such as cyanide, acids, and metals, or indirectly, owing to the deleterious effect of these components on sewage treatment systems. Bureau scientists have shown that reducing an organic-cyanide electroplating waste with formaldehyde will cause metallic copper and silver to coprecipitate while destroying all of the poisonous cyanides. Another method, which is even more promising, employs two plating waste solutions to recover metals and chemicals and leaves behind a harmless liquid free of toxic metal ions and hazardous cyanides. Bureau researchers have developed processes for recovering expensive metals from cuttings and grindings left over when “superalloy” jet engine parts are machined. Such scrap, containing nickel, cobalt, molybdenum, and chromium, has a metal content worth nearly $IC00 per ton. It is regularly sold to overseas markets for far less, because of the high cost of seperating and recovering the metals by methods now available in the US. Remelting the material for reuse as alloy is impractical because oil, rags, abrasive grit, and other contaminants are mixed with the alloy, and its alloying metals content is not uniform. In the bureau process, the scrap is cleaned by screening, burning, and magnetic separation, and dissolved in hot acid. Molybdenum, cobalt, chromium, and nickel are removed from the hot acid solution in successive operations by solvent extraction, followed by selective precipitation of the valuable metals. The bureau is working on recovering and reusing waste materials that are being ejected in the stack gases generated by the minerals and metal industries during smelting and chemical processing. Good progress is being made by removing these waste materials in a form that will permit their recycling and reuse. For example, fluorine liberated in processing phosphate rock to phosphoric acid has been converted to synthetic fluorite or calcium fluoride, which is a raw material in short supply for steel and aluminum manufacturing.
There has been exceptional success in developing a process for removing sulfur dioxide from copper and other base-metal smelter gases. The process not only effectively removes all of the sulfur dioxide from the gases but converts the sulfur dioxide to elemental sulfur that can be stored for indefinite periods or easily shipped over long disfances to consumer points. Currently, a major copper producer is constructing a pilot plant at a smelter in Arizona to test the process and obtain detailed operating and cost figures preparatory to design and construction of a full-sized facility. These are some of the uses the bureau has found for solid wastes. Several of the processes developed have already attracted wide interest by the industrial sector and probably will be adopted. It is not inconceivable that the present-day mine tailing dumps, municipal landfills, and junk car graveyards may he looked upon in the future as “manmade mines” for minerals whose natural ores have been depleted or remain in deposits that can be mined only at greater cost than required for recycling waste. The Bureau of Mines considers solid waste as resources out of place-and is simply trying to put it hack where it belongs.
Additional Reading
Appell, H. R., Wender, I., Miller, R. D., “Conversion of Urban Refuse to Oil,” Bureau of Mines Technical Progress Rept. 25, May 1970. Chindgren, C. J., Dean, K. C.. Sterner, J. W., ”Construction and Testing of a Junk Auto Incinerator,” Bureau of Mines Technical Progress Rept. 21. February 1970. CseNenyak, F. J.. Kenahan, C. B., “Bureau of Mines Research and Ac. complishments in Utilization of Solid Wastes,” Information Circ. 8460, March 1970. George, L. C.. Cochran, A. A., “Re. covery of Metals From Electroplating Wastes by the Waste.Plus-Waste Method.” Bureau of Mines Technical Progress Rept. 27, August 1970. IIT Research Institute. “Continuous Physical Beneficiation .of Metals and Minerals Contained in Municipal Incinerator Residues,” Proceedings of the Second Mineral Waste Utiliza. tion Symposium, March 18-19, 1970, Chicago, 111. IIT Research Institute, “Recovery of Metallurgical Values from Industrial Wastes,’’ ibid. Sanner. W. S . , Ortuglio, C., Walters, J. G., Wolfson. D. E., “Conversion of Municipal and Industrial Refuse Into Useful Materials by Pyrolysis,” Bureau of Mines Report of Investigations 7428, August 1970.
