MATERIALS

Energy and materials were developed together in early civilizations, beyond the use of fire for cooling and heating. In the courtyard of the stepped pyramid at Sakahra, built near Cairo, Egypt, in about 2600 B.C.E., there are a series of carved relief panels in stone, showing that the ancient Egyptians had mastered the smelting and working of metals with heat energy (fire), as well as many other technical skills. Glass melting was discovered at least 10,000 years ago, and fired ceramics (pottery) even much earlier.

Solid materials are essential to the production and transmission of energy; in the next section gives examples of materials used in these processes. The article then focuses on the large energy requirements needed to produce the metals, ceramics, glasses, and electronic materials (silicon and germanium) that our technological civilization demands, and finishes with an overview of the environmental problems encountered in trying to satisfy this vast demand for materials.

MATERIALS IN ENERGY PRODUCTION, TRANSMISSION, AND STORAGE

Metals are of overwhelming importance in these applications. Steel, which is iron containing carbon and many different metallic additions, is still the most used metal. Aluminum is increasingly used in many applications because of its light weight and resistance to chemical attack. Copper is required for transmission lines, wiring, and generators because of its high electrical conductivity. There are many specialized uses of other metals: manganese, vanadium, and molybdenum as alloying elements to improve strength and chemical durability of steel; uranium and plutonium in nuclear reactors; silver in electrical contacts; and tungsten as filaments in lamps.

Ceramics, including concrete, are useful especially in structures, reactors, as refractories in combustion of fuels, and as nuclear fuel. Porcelain insulators on transmission lines are an example of a specialized application of ceramics.

Electronic materials are needed for computers and control devices; purified silicon is the basic material for these applications. In addition silica glass (SiO₂) is an insulator, aluminum an electrical conductor, and polymers are reactive materials for patterning in these devices. Control of every step of energy production and transmission is now completely dependent on electronics.

ENERGY USE: REFERENCES AND METHODS

The energy use data in the tables come from three Battelle-Columbus reports and an article by H. H. Kellogg on "Energy Considerations in Metals Production in the Encyclopedia of Materials Science and Engineering." The Battelle-Columbus reports have detailed descriptions of the processing of all the materials listed in the tables, with methods from mining through separation to purification, and cost estimates at each processing step. There is a remarkable amount of valuable information on each material in these reports. The section on metals also relied heavily on the article by Kellogg.

The estimates for energy use can be separated into the following components: where F is the heating values of fuels used, E is the fuel equivalent of electrical energy (from the U.S. average fuel equivalent of 11.1(10)⁶ joules per kilowatt hour), S is the fuel equivalent of supplies and chemical reagents consumed in the processing, and B is the fuel equivalent of by products and surplus steam. The units of energy use are given in joules per kilogram of final material produced in the tables; the conversion to English units is: divide J/kg by 1.16 to get Btu/ton (British thermal units per 2,000 pounds).

The energy equivalent of one barrel (159 liters) of crude petroleum is about 6.6(10)⁹ joules. Table 1 shows that the production of a metric ton (1,000 kg or 2,200 pounds) of steel requires about four barrels of oil; a ton of aluminum requires about forty barrels

of oil, and a ton of gold more than ten thousand barrels of oil. Table 2 shows the energy needed to produce metals for alloying with iron to produce steel.

ENERGY FOR MINING

The amount of energy required for mining the ores and minerals needed to make materials depends on their depth in the ground, and processing and separation methods. Ore and minerals lying near the surface need only be excavated by shovel or dredge, and thus require low energy expenditure (10¹¹ J/kg). Fine grinding (0.1 mm) requires up to 3(10)¹¹ J/kg. Loading, elevation out of a mine, and transporting the ore can require up to 5(10)¹¹ J/kg.

If the ore consists of separate grains containing the desired material, it can be separated from undesired minerals by physical methods such as flotation, sedimentation, or magnetic separation. For metals this step can lead to 80 to 95 percent concentration of the value of the ore. Ceramic raw materials such as sand and clay can often be found pure enough in nature so that no concentration is needed.

If the desired material is not in separate grains, chemical treatment of the ore is required for metals, and for purification of ceramics.

ENERGY USE FOR PROCESSING OF METALS

Table 1 shows estimated values of energy to produce a kilogram of reasonably pure metals, or for metal useful for practical applications. The range of a factor of more than 2,000 between steel and gold depends on the concentration in ore, the chemical processing needed, and amount of technology development for the particular metal. Steel production has been developed during about three millennia, and iron ore is highly concentrated and can be reacted directly to steel

alloys. Gold is widely dispersed in low concentrations, and requires intensive chemical treatment of ores.

Table 3 shows the total consumption of some metals in the United States.

If the metallic compound in the ore can be selectively leached by acid or base without dissolving much of the remaining ore, then the energy requirement is only about 10¹² J/kg. Examples are leaching of oxide ores of copper, zinc, or uranium with sulfuric acid.

Much more energy is needed if the entire rock matrix must be chemically dissolved to free the metallic compounds. Examples are the separation of aluminum oxide (Al₂O₃) from bauxite ore by dissolution by strong base at elevated temperature and pressure (Bayer process), with an energy requirement of about 8(10)¹² J/kg, and smelting of nickel ores by heating to produce a molten nickel-iron alloy and oxide slag, with energy up to (10)¹³ J/kg needed. These high energy requirements result from the fuels needed to heat furnaces or reactors to the high temperatures of these chemical reactions, and for the energy equivalent to make the chemical reagents employed, such as acids, bases and iron alloys.

The grade of metallic ore is the percentage of metal (native and chemically combined) in the ore. For example, high grade iron ore can contain up to 65 percent iron, whereas usual gold ores contain less than 0.001 percent gold. The weight of ore that must be processed is inversely proportional to the grade of the ore. For example, about 7(10)¹³ J/kg are required for mining and concentration of hard rock ore containing 0.6 percent to copper, and 4(10)¹⁶ J/kg are needed to mine and concentrate a gold ore containing 0.001 percent gold. Thus, the grade of ore is a major factor in energy consumption.

High grade ores are used first, and are already substantially depleted for most metals. With time one might expect the energy use in Table 1 to increase. Improved technology can offset this increase somewhat; belt conveyors in mines and computer control of grinding are examples. Ordinary rocks contain small quantities (parts per million by weight) of many different metals, and have been suggested as a source for rare metals and those with depleted ores. For example, many rocks contain about 0.01 percent copper; it would require about one thousand times the energy to recover copper from these ores as from presently-available ores (Table 1). The large energy demand precludes the use of these rocks for producing copper, because of the low price of copper. Gold, however, is more valuable, and would justify a larger energy input.

In the concentrated ores most metals are in chemical compounds, as oxides or sulfides. Reducing these compounds to the metallic state in the final stage in producing metal can be accomplished by chemical processes or electrolysis. Two examples of chemical reduction are
Copper sulfide

Steel blast furnace

where 2Fe₂O₃ is hematite coke ore. Because the oxides and sulfides of many metals are stable, their chemical reduction is difficult, and they are reduced to metal by electrolysis (electrowinning); examples are zinc, aluminum, and magnesium.

The electrolytic processing of concentrated ore to form the metal depends on the specific chemical properties of the metallic compound. To produce aluminum about 2 to 6 percent of purified aluminum oxide is dissolved in cryolite (sodium alumino-fluoride, Na₃AlF₆) at about 960°C. The reduction of the alumina occurs at a carbon (graphite) anode:

Magnesium is reduced from a mixture of magnesium, calcium, and sodium chlorides. Electrolysis from aqueous solution is also possible: zinc, copper, and manganese dissolved as sulfates in water can be reduced electrolytically from aqueous solution.

Process energies are found by subtracting energies for mining and concentrating from the values in Table 3. The free energies of formation of the metal oxides are a measure of the total (theoretical) energy required to reduce the metal from the oxide. The ratio of the actual process energy to the free energy of formation is a rough measure of the efficiency of the reduction process. The free energies of formation are a measure of the chemical stabilities of the oxides; stable oxides such as aluminum, magnesium, and titanium intrinsically require more energy for reduction than from less stable oxides (or sulfides) of copper, lead, and nickel.

The most efficient processes in Table 1 are for steel and aluminum, mainly because these metals are produced in large amounts, and much technological development has been lavished on them. Magnesium and titanium require chloride intermediates, decreasing their efficiencies of production; lead, copper, and nickel require extra processing to remove unwanted impurities. Sulfide ores produce sulfur dioxide (SO₂), a pollutant, which must be removed from smokestack gases. For example, in copper production the removal of SO₂ and its conversion to sulfuric acid adds up to 8(10)¹² J/kg of additional process energy consumption. In aluminum production disposal of waste cryolite must be controlled because of possible fluoride contamination.

As global warming develops, the formation of large quantities of carbon dioxide (see Equations 1 and 3) may become a problem in metals production. There is no simple or inexpensive way to reduce these emissions. Switching to electrolysis processing of steel, or a different electrode reaction for aluminum, would involve unacceptably large energy use and cost. Chemical absorption of carbon dioxide may be necessary in a wide variety of chemical and energy-producing processes, at enormous cost.

The scale of production also influences efficiency. Small-scale batch processing for metals such as titanium, tungsten, and zirconium leads to higher energy use and costs.

RECYCLING OF METALS

Reuse of waste metals generated from metal fabrication and from discarded products (scrap) can save large amounts of energy, particularly for metals that have high energy use in production, such as aluminum. The low fractions of energy used to produce metals from scrap for aluminum, certain sources of copper, and nickel show the value of recycling these metals.

The purity of the scrap mainly determines the fraction of energy needed to produce metal from it, and the value of recycling. Clean copper scrap need only be remelted and cast to form recycled copper; if the copper is contaminated with organic materials and other metals, more complex separation processes are needed that are similar to production from ores. It is easier to remelt the steel of a car driven in Arizona compared to one rusted by the road salt in snowy areas. Scrap that is produced as a by-product of metal processing can be easily recycled, and it can be collected from relatively few locations. There has been a strong effort to educate both householders and industrial users to separate scrap and return it to waste collectors, leading to a supply of reasonably separated scrap.

Despite the efforts of many communities to encourage recycling, there is still a large amount of metal that is not recycled. Only an estimated 30 percent of aluminum is recycled, as compared with up to 50 percent for precious metals. Landfills contain large amounts of metals, especially large use metals (Table 3) such as iron, aluminum, and copper, and more metals continue to accumulate in landfills. As the cost of disposal increases and ores of metals such as copper become of lower grade, it may be economically feasible to "mine" landfills for metals. Development of new technologies for treatment and separation of waste materials is needed to make this mining economical.

ENERGY USE IN PRODUCING CERAMICS

Traditionally ceramic raw materials have been dug out of the ground and used with little or no treatment or purification. Sand, fireclay, talc, and gypsum are examples. The energy expenditure for producing these materials is therefore small. Some of these materials can be found naturally in high purity. Silica sands (SiO₂) with less than 100 ppm (parts per million by weight) of impurities are known, and some clay deposits are nearly pure kaolin. Minerals such as feldspar, kyanite, and kaolinite (clay) can be purified by washing or solution treatments at near ambient temperatures, with low energy expenditure.

Many ceramic products require firing at high temperatures, and the fuels required to reach and sustain these temperatures are major factors in the energy consumed to make these products. Portland cement is made by firing a mixture of compounds, mainly carbonates, sulfates, and silicates to form the desired calcium silicate products. The firing is done in a rotary furnace or kiln, so that a fraction of the raw materials become liquid. As the resultant calcium silicates cool, they go through a large volume change that causes the cement particles to break into smaller sizes.

Concrete is made from a mixture of about equal parts of sand, gravel, and cement, plus some added water to give a mixture that flows. The low energy expenditures to make these raw materials mean that concrete is a material that requires very low energy; the only additional energy is a small amount for transport and for mixing the constituents. Concrete requires about one-third the energy expenditure for steel, and one thirtieth that for aluminum (Table 1). In Western Europe concrete has replaced metals and wood in many applications, because forests are depleted and energy costs are higher than in the United States. Examples are in building; American homes still use a wood frame, but in Western Europe almost all homes are made from concrete or stone. Electrical transmission poles in the United States are made of wood or aluminum, and in most of the rest of the world these poles are made of concrete because of their lower energy requirements. Considerable energy savings could occur by substituting concrete for metals in a variety of applications. Concrete has excellent compressive strength but is weak in tension or bending. By reinforcing concrete with steel bars the concrete building or structure has good strength in bending and tension as well as compression.

Other applications of ceramics require clay, either raw or purified, sand, and feldspar. Brick, porcelain, and white wares are made from these raw materials; the main expenditure in making these products is in firing the mixtures of powders to a dense solid. Ordinary brick made from fire-clay requires a small amount of energy; even refractory brick for high temperatures and chemical durability, made partly from purified oxides such as alumina or chrome ore, requires only about the same energy to make as an equivalent weight of steel.

Glass for containers is made continuously in a large tank or furnace. Raw materials (sand, soda-ash, limestone) are fed in at one end of a gas-fired furnace. The molten glass slowly passes through the furnace at high temperature (1,200°C-1,300°C) to homogenize it and remove bubbles. At the exit end the molten glass is fed into special molds in two stages to make containers of desired sizes and shapes. Flat glass (windows) is also made in a continuous furnace; a glass layer leaving the furnace is spread onto a bath of molten tin (float glass) to provide smooth surfaces. Lamp bulbs are made from a continuous furnace; a ribbon of glass is fed to blowers that blow the bulbs into a mold (ribbon machine). All these processes are continuous with large furnaces for melting, and so are energy efficient, using about 20(10)¹² J/kg, a factor more the ten lower than the energy required to make an equivalent weight of aluminum. Nevertheless, aluminum containers have replaced glass for many purposes, because aluminum is easier to handle, and harder to break. Polymer (plastic) containers are also popular because of low cost, chemical durability, and ease of handling. As energy costs increase, aluminum containers will become less attractive than glass; the raw material (petroleum) for polymers may also become more expensive, leading to a return of glass as the primary container material.

For many specialized uses glass is made in small batches, so the energy costs are much higher than for the continuous furnaces. Special processes, such as for drawing fibers, casting optical components, and making laser glass, require highly purified or controlled raw materials, leading to much higher energy requirements than for continuously made glass.

Many ceramic applications are high value and small volume, so energy expenditure is high. Ferroelectric magnets, electronic substrates, electro-optics, abrasives such as silicon carbide and diamond, are examples. Diamond is found naturally, and made synthetically by the General Electric Company at high pressure and temperature. Synthetic diamonds for abrasives require less energy to make than the value in Table 4; nevertheless, the market is carefully divided between natural and synthetic diamonds.

Large quantities of uranium oxide are required for nuclear reactor fuel. The uranium ore must be carefully purified and processed to desired shapes, causing high energy expenditure.

Single crystals of synthetic quartz are made by crystallization from aqueous solution at temperatures and pressures well above ambient. The crystallization is slow and carefully controlled, so energy costs are high.

The energies for producing some gases are listed in Table 5 for comparison with those for other materials.

RECYCLING OF CERAMICS

Bulk ceramics such as building materials, porcelain, and concrete are not recycled, because of the low energy required to make them and the difficulty of collecting, transporting, and reforming them into useful shapes. Some glass is recycled in the form of "cullet," which is waste glass. The amount of cullet in a glass furnace is rigidly controlled, because the final product of the furnace must have just the right viscosity for the automatic machinery (container mold, tin bath, or ribbon machine) that forms the glass. Glass manufacturers are unwilling to build tanks to accept waste glass, because its variable composition leads to uncontrollable variations in the viscosity of the glass. Viscosities of silicate glasses are highly sensitive to impurities, especially water and alkali (sodium and potassium) compounds.

ELECTRONIC MATERIALS

Silicon wafers are the basis for electronic circuits. The silicon must be highly purified, then grown as a single crystal containing a small amount (a few parts per million) of additions to give either negative carriers (electrons from phosphorous or arsenic) or positive carriers (holes, from boron or aluminum). These processes require temperatures above the melting point of silicon (1,414°C) and careful control of several processing steps. The energy expenditure for making silicon for wafers (chips) is about the same as that for germanium, given in Table 4. The energy of about 2,500(10)¹² J/kg is greater than that required to make such a valuable metal as silver, or zirconium, which is strongly bonded in compounds, because of the highly complex processing and high purity required for the semiconductors. Subsequent processing of silicon wafers to form devices on the wafers for practical use is highly specialized, carefully controlled, and expensive in cost and energy.

ENVIRONMENTAL CONCERNS

There is great interest in the more energy efficient production of materials to reduce costs and environmental damage. The products of materials production receiving the greatest attention are sulfur dioxide, fluorides, and carbon dioxide. The production of energy from fossil fuels, especially coal and oil, leads to production of sulfur dioxide, which causes much damage locally and at long distances. It leads to respiratory problems and damage to plants, especially trees, and can acidify soils and lakes, damaging them for growing plants and animals. Sulfur dioxide can be scrubbed from flue glass at considerable expense, but much of it is still discharged into the atmosphere. Sulfur dioxide is a by-product in much of the materials production discussed here; sulfide ores (copper) when oxidized produce sulfur dioxide (Equation 2), and some raw materials for cement contain sulfates.

Fluorides are used in many materials processes, and can poison the environment when they are discarded. Examples are cryolite (sodium aluminofluoride, Na₃AlF₆) used to dissolve aluminum oxides for electrolysis, and hydrofluoric acid (HF) used in etching lamp bulbs and semi-conducting circuits. Today lamp bulbs are etched much less than they used to be to reduce fluoride disposal; not much has been done to reduce the amount of cryolite for aluminum production.

Some heavy metals and semi-metals are quite toxic (chromium, lead, and antimony) and expensive care is needed to prevent them from being dispersed in the environment. Lead in gasoline and paint has been

almost completely eliminated; its use in storage batteries has resisted efforts to find a suitable substitute.

The discharge of carbon dioxide from combustion of fuels from vehicles, and from processes such as steelmaking, cement production, and much other materials production has increased the concentration of carbon dioxide in the atmosphere. Some computer models demonstrate that this increase is responsible for an increase in the mean temperature of the surface of the Earth, and there are numerous predictions of further temperature increases as more carbon dioxide is discharged into the atmosphere. There are other claims that this result is not proven. Reduction of carbon dioxide emissions is highly difficult and expensive. If the connection between carbon dioxide emissions and global warming is proven more conclusively and a carbon dioxide reduction plan is instituted, materials industries will feel a great impact because they consume about 20 percent of all industrial energy.

Reduction of overall energy use is one solution to the above problems. It requires money, technical advances, political power, and courage; some reduction has been achieved, but much more is needed to reduce emissions of gases. One solution being advanced is use of processes to produce energy that do not emit gases. Hydropower has been exploited about as fully as possible, and supplies only a small fraction of total energy needs. Other sources such as wind and solar power are still much too expensive.

One energy source that first appeared to be highly attractive was nuclear power. The problem with nuclear power is that some costs were hidden in its initial development. Especially pernicious is the disposal of uranium oxide fuel after it has become depleted. It can be reprocessed, but at considerable expense, and the product plutonium can be used for weapons. In the United States the plan is to bury depleted uranium from reactors, but many persons are not convinced that burial is safe. Much work has been done on encapsulation of radioactive waste in glass; the problem of reactor waste remains.

SUMMARY

The energy required to produce materials varies widely, gold requires more than two thousand times the energy to produce the same weight of steel, and diamonds two hundred thousand times the energy required to make ordinary brick. Factors in energy use are the quality (concentration) of ore, the complexity of processing, and the technological development of processing.

Some recycling of metals occurs; much more is possible, and substitution of materials requiring less energy for those requiring more has much potential.

Reduction of environmental pollution requires lower energy use and new technology to decrease emission of gases such as sulfur dioxide and carbon dioxide, and to prevent toxic fluoride, heavy metal, and radioactive wastes from discharging into the environment.

Robert H. Doremus

See also: Building Design; Climatic Effects; Drilling for Gas and Oil.

BIBLIOGRAPHY

Battelle-Columbus Laboratories, Energy Use Patterns in Metallurgical and Nonmetallic Mineral Processing. (1975). High Priority Commodities, PB 245 759; (1975). Intermediate Priority Commodities PB 246 357; (1976). Low-Priority Commodities, PB 261 150.

Doremus, R. H. (1994). Glass Science. New York: Wiley.

Fine, H. A., and Geiger, G. H. (1993). Handbook on Material and Energy Balance Calculations in Metallurgical Processes. Warrendale, PA: TMS.

Kaplan, R. S., and Ness, H. (1986). "Recycling of Metals: Technology." In Encyclopedia of Materials Science and Engineering, ed. M. B. Bever. Cambridge, MA: MIT Press.

Kellogg, H. H. (1986). "Energy Considerations in Metals Production." In Encyclopedia of Materials Science and Engineering, ed. M. B. Bever. Cambridge, MA: MIT Press.

Kingery, W. D.; Bowen, H. K.; and Uhlmann, D. R. (1976). Introduction to Ceramics. New York: Wiley.

Mayer, J. W., and Lau, S. S. (1990). Electronic Materials Science. New York: Macmillan.

Shackelford, J. F. (1997). Introduction to Materials Science for Engineers. New York: Macmillan.