[10.0] Traditional Materials

v1.3.0 / chapter 10 of 15 / 01 jun 16 / greg goebel

* Chemistry is particularly evident in the materials used to build the structures and machines around us. This chapter discusses the production of the most important metals, as well as glass, ceramics, concrete, and other construction materials.

steel mill

[10.2] IRON & STEEL


* Well before the beginning of recorded history, humans were making use of a wide range of materials. Originally, materials were obtained pretty much as nature provided them: humans used wood, grass, and leaves; animal hides and bone; stones and clay bricks for construction; and tools chipped out of flint or obsidian. Some stone tools were elaborately chipped out and carefully shaped.

At first, the only metals that could be used were those that could be dug up in a pure or "native" form -- copper, gold, silver. Native copper isn't that common, sometimes being found in meteorites, but by 3500 BCE, humans managed to figure out how to smelt it out of the common material copper pyrite (CuFeS2) using a fairly ordinary fire. Copper could be used for pots and the like, but gold and silver were really only applicable to luxury items and coinage.

The discovery of copper-tin bronze, presumably by accident, was a big step forward in terms of materials sophistication: it was one of the first materials used by humans that nature did not provide directly. Although better metals would be found, it still remains in use in statues and bells. Brass, a copper-zinc alloy, is still in widespread use for pipe fittings and the like, since it is corrosion-resistant. Of course, pure copper is commonly used for electrical wiring.

* The modern process for smelting copper is much the same as it was centuries ago, though much more streamlined. Smelting starts by powdering copper ore and separating the copper-rich component. The actual smelting process involves three stages:

The roasting is done in a "fluidized bed" roaster, in which a strong updraft lifts the particles as they are roasted, or in a "sintering machine", in which the particles are heated to a level where they stick together.

The smelting process releases air pollutants, and the smelter generally has flues hundreds of meters high to disperse the pollutants. The sulfur oxides tended to produce acid rain, and old-time copper smelter towns were blighted areas where nothing grew. These days, along with scrubbers and baghouses, smelters use a catalytic converter system, a tall cylinder containing vanadium pentoxide (V2O5), to convert the sulfur dioxides into sulfur trioxide, which is combined with water to produce sulfuric acid. The sulfuric acid is sold on the market -- a reflection of the optimistic proverb that pollution is nothing more than a misplaced resource.


[10.2] IRON & STEEL

* Bronze was relegated to statues and the like after it was generally replaced by a more impressive material, iron, that required high-temperature smelting. Iron is almost unarguably the most useful metal, partly because it is so common: it is the fourth most common element in the Earth's crust.

Iron may be found in a relatively pure form in some meteorites, but it is much more generally found in the form of "hematite (Fe2O3)" -- which has a blood-red color and has been long used as a pigment. It is also found in other compounds, including "iron pyrite (FeS2)", known as "fool's gold" for its goldlike appearance, but the primary source for industrial purposes is hematite.

Iron recovered from meteorites has been found in prehistoric tools, but it wasn't until somebody in the Middle East figured out how to smelt hematite ore into iron, sometime around 3000 BCE, that iron went into gradually widening use. The "Iron Age" really didn't begin for another millennium or two, however, since smelting iron is more difficult than smelting, say, copper, and it's harder to get good results. Iron didn't become the prevalent metal until about 1000 BCE.

Iron smelting is based on the reaction:

   3Fe2O3  +  11C  -->  2Fe3C  +  9CO

    Fe2O3  +   3C  -->  2Fe    +  3CO

The CO -- carbon monoxide -- escapes as a gas, and what is left over is a mixture of Fe3C, AKA "cementite", and pure iron. The reaction requires high temperatures, which is one of the big reasons that smelting iron was relatively troublesome. It was also troublesome to get a product with predictable properties, the results tending to be either too soft or too brittle.

It was of course not known at the outset that the major factor in coming up with useful iron was the proportion of carbon in the iron. Pure iron is soft; iron with as much carbon as it can hold -- about 4% or 5% by weight -- is very brittle. Carbon hardens the metal by acting as a sort of atomic "grit", jamming up the iron crystal lattice, preventing the iron atoms from shifting around, but the proportion of carbon is critical. Get the amount of carbon just right -- below 2% or 1% -- and the result is an ideal iron-carbon alloy, not too soft and not too brittle, known as "steel".

The original manufacturing technique for iron was based on a "bloomery", a furnace in which iron ore was mixed with burning charcoal, with the furnace fed by a bellows to increase the heat. A bloomery could get hot enough to smelt iron but not completely melt it; the metal collected in the bottom of the furnace as a spongy mass or "bloom". The bloom would be yanked out of the furnace and hammered while it was still hot, to drive out impurities; the bloom would then be heated again, hammered again, then heated again, and so on, until reasonably pure "wrought iron" was produced. It was, obviously, a very hot, dirty, and laborious process.

There were various ways of converting the wrought iron to steel, most prominently heating it in a crucible for several days to drive the carbon out. It was more of an art than science. Ironsmiths also learned to "quench-harden" steels, heating them up and then cooling them abruptly in water or oil, a process that adjusted the crystalline structure of the metal, making it much harder. In any case, wrought iron was difficult enough to produce, steel was just that much more difficult, and so it was only suitable for small-scale items like swords.

* By the time of the Renaissance, iron production had moved on to "blast furnaces", which had forced drafts (generated by bellows driven by waterwheels) that could honestly melt iron. By the 18th century, the process had been improved to permit large-scale production of iron, needed for cannon, cannonballs, and -- as the process matured -- other large items. A 30-meter-long bridge made of iron was erected in England in 1779, showing that iron production had come of age.

The output of a blast furnace was high-carbon "pig iron", the name said to have been derived from the fact that the molten metal was often poured into a sand trough that was flanked by smaller troughs used to form ingots: the arrangement looked like piglets around a sow. In the late 18th century, a new process was invented to convert the pig iron into pure iron, known as "puddling", in which the pig iron was heated in a furnace for a long period of time and mixed with rust -- iron oxide -- which combined with the carbon to be driven off as carbon monoxide.

Puddling was a small-scale process, and conversion to steel remained small scale as well, stuck in the crucible process. In hindsight, it all seemed a bit absurd -- why get rid of the carbon out of pig iron, and then reintroduce it to make steel? Couldn't there be some way to convert the pig iron straight to steel? And, preferably, do it in volume?

In 1855, an English engineer named Henry Bessemer (1813:1898) came up with a process to do all that, using a huge pivoting metal jar lined with firebrick and driven by blasts of air that drove the carbon out of the pig iron. The "Bessemer converter" allowed the mass production of steel for the first time. At first, the process only worked with high-quality Swedish ores, but it was found that if limestone were added to the melt, it would mix with impurities, rising to the top to form a layer of slag that could be poured off the top of the melt. Other additives were devised to ensure a predictable product. Techniques were developed to determine the carbon content of the melt and figure out when the batch was "done".

Steel production skyrocketed. Wrought iron, in the literal sense, ceased to exist, since the Bessemer process could be controlled to yield a wide range of carbon proportions, including nearly none. Improved steel-making processes were introduced over the next century.

In modern times, simple smelting of iron is done with a "blast furnace", which is a huge hot furnace supplied with ore, limestone, and coke, in a ratio of about 6 units of ore + 1 unit of limestone + 2 units of coke. The blast furnace is driven by blasts of air driven by huge fans, with the air preheated to high temperature by a set of "stoves" ringing the furnace and then driven through an encircling duct system called a "bustle". Since the process is run continuously and the blast furnace is under pressure, an airlock system has to be used to feed in new materials.

The molten mix or "charge" resides in the bottom of the furnace, known as the "hearth". The carbon in the coke combines with the oxygen in the iron ore, to be released as carbon monoxide. The limestone flux combines with impurities to form slag. There are two ports in the hearth, the upper one being used to drain slag off into a "slag pot", also known as a "slag thimble", and the lower one being used to drain the smelted iron off into a rail car with a heavy duty tank that looks like a big American-style football, and is known as a "Pugh car".

Modern steelmaking is performed by the "basic oxygen process", which involves a furnace into which a tube known as a "lance" is inserted from the top, with pure oxygen forced through it in a high pressure blast to churn through the molten pig iron. Using pure oxygen reduces the time needed to produce the steel from hours to less than an hour, of course reducing energy costs as well.

Once the steel is ready, it is poured out through a "ladle". Once upon a time, the ladle then dumped the steel into molds, but now the name of ladle is misleading, since it's more of a water-cooled funnel that dumps the steel through what looks like a playground slide into a "continuous casting machine". In a continuous casting machine, the hot steel flows through a copper molding channel as an unbroken ribbon of metal, to eventually be cut into slabs of steel, with a solid crust but hot interior, about 15 centimeters thick, 1.8 to 3 meters wide, and 6 meters long. The molding channel is vibrated and coated with a glassy lubricant to keep the steel from sticking. The slabs are hauled off to a "rolling mill", a long building in which the slabs are drawn into wires, bars, or sheet metal.

Modern steelmaking includes production of a wide range of different steel alloys:

* Smelting of iron ore is not as profitable as it could be, because recycling is an established order as far as iron and steel are concerned and that helps keep prices down. Much of the steel we now purchase comes from scrap, being processed through what are called "minimills", though they may not be all that tiny. There are no blast furnaces or basic oxygen process furnaces at a minimill, just an "electric arc" furnace. Scrap is sorted and dumped by batch into such a furnace, which is covered by a heavy lid through which carbon electrodes the diameter of a plate protrude. The electrodes are driven by a big power transformer, generating arcs that melt down the scrap in about 15 to 20 minutes. The output then goes through a continuous-casting machine and into a rolling mill.

Recycling scrap requires less investment in capital equipment, and is more energy-efficient than making steel from ore. While steel production used to be restricted to regions where ore was not far away, scrap is everywhere, and as a result, so are minimills. Minimills tend to specialize in the steel products they make, for example turning out only girders and concrete reinforcing bar. Minimills are not cost-effective for production of specialized steels at this time, due to the lack of control over the composition of scrap, and such materials are still obtained from raw ore.



* By the time steel was on the way to becoming a mass-market metal, competitors were beginning to emerge. After Woehler managed to isolate aluminum in the 1820s, people were very taken with the metal, which seemed very much like silver but was, by all evidence, much less rare. Alumina -- aluminum oxide (Al2O3) -- is very common, and these days is used as sandpaper grit and similar applications. Rubies and sapphires are large alumina crystals with various impurities; they can be synthesized by fusing alumina crystals along with the proper impurities in a high-temperature torch. The bonds in alumina were so strong, however, that extracting the pure metal was very difficult. The only method known for decades involving heating alumina with sodium or some other reactive element -- a procedure that was difficult and nasty. Aluminum was more expensive than silver in that timeframe.

In the 1880s, a young American chemistry student named Charles M. Hall (1863:1914) was told by one of his professors that a fortune awaited anyone who could figure out a reasonable way of refining aluminum. By that time, electricity was coming into widespread commercial use, and Hall decided that an electrical refining process might do the trick.

Electrolytic separation had been known for decades, of course, but aluminum couldn't be separated as a solute in water. Hall figured out a scheme in which cryolite (Na3AlF6), an aluminum salt, was melted along with bauxite -- alumina ore -- in a vessel with a carbon lining that acted as a cathode. A carbon anode was dipped into the vessel, with a high electric current passed between the two electrodes. The process extracted pure aluminum from the alumina, with the refined metal flowing to the bottom of the vessel to be drained off. The reaction at the carbon anode produced CO2 and ate away the anode, which had to be replaced at intervals.

Hall-Heroult process

As often happens in the history of technology, once the pieces were in place to allow a scheme to work, it would be discovered in several places at once. While Hall was working on his experiments, a French student named Paul Louis Toissant Heroult (1863:1914) was working along the same lines -- the only major difference between the two was that Heroult used several electrodes, while Hall used one -- and they both filed patents at almost the same time in 1886. After a period of troublesome patent litigation, the two men came to a sensible agreement, with Hall retaining rights to the process in the US and Heroult retaining rights in Europe. (By other coincidences, Hall was born and died about half a year after Heroult.)

The "Hall-Heroult" (or "Heroult-Hall") process made aluminum drastically cheaper, but the pure metal was relatively soft and still not as cheap as steel; given the large amounts of electric power required to refine aluminum, it was unlikely it would ever compete with steel on a level basis. Aluminum production didn't take off until the next century.

In 1906, a German researcher named Alfred Wilm was tinkering with aluminum-copper alloys. His early experiments didn't seem to produce a harder aluminum, so he tried adding traces of magnesium and manganese as well, since they were used to produce harder steel alloys. That didn't seem to make any difference either, until the alloy was allowed to sit for four days -- and then it proved to be three times harder than it had been at the outset. Wilm had discovered what is now called "age hardening", as well as "Duraluminum" alloy, which would become the basic metal for aircraft, providing high strength at a third of the weight of steel, and much higher resistance to corrosion than steel. Aluminum had finally come of age. Improved alloys were developed; in modern times, strong but lightweight lithium-aluminum alloys are sometimes used in aerospace applications.

Incidentally, like iron, aluminum is heavily recycled -- it's much cheaper in terms of power consumption to melt down scrap aluminum than refine it from alumina using the Hall-Heroult process.

* Magnesium had been known for well longer than aluminum, but through the 19th century it was little more than an additive to steel. It didn't become a useful metal in its own right until well into the 20th century. Magnesium is a common element in sea water, and processes were developed in which calcium oxide (CaO) was mixed in tanks of brine, resulting in a precipitate of magnesium hydroxide -- Mg(OH)2. The magnesium hydroxide was then converted to magnesium chloride by reaction with hydrochloric acid, with the magnesium metal extracted from magnesium chloride by an electrolytic process.

Magnesium has an even higher strength-to-weight ratio than aluminum, making it a good material for use in aircraft, where its expense is less of a problem. Magnesium is also used in flares and incendiaries, since it burns very hot and white. Incidentally, aluminum also burns, but it has to be converted to a fine powder to do so effectively, since once it starts burning it forms an oxide layer that blocks further combustion. Powdered aluminum is a component in both some explosives and in solid rocket fuels.

* Magnesium has been generally displaced in its use in aircraft by an even lighter metal, titanium, that is also highly resistant to heat. Titanium is a common element, found generally as titanium dioxide (TiO2), but refining it is particularly troublesome. Traditionally, titanium ore is treated using chemical reduction, using an approach developed by William J. Kroll (1889:1973) in the late 1930s and introduced on an industrial scale in the 1950s. It is a two-stage process:

The Kroll process is troublesome and expensive, and even Kroll recognized the process left a lot to be desired. He predicted in the early 1950s that it would be replaced by an electrolytic process in the mid-1960s. That didn't happen; traditional electrolytic processes -- involving the dissolution of titanium dioxide in an electrolyte -- flatly didn't work. However, in the early 1990s, British researchers were tinkering with titanium dioxide and found out, by accident and much to their surprise, that could be electrolyzed without being dissolved. Promising work is underway to scale up the process.

It now seems at least possible that titanium will eventually be as cheap and common as aluminum, which leads to a vision of household rolls of titanium foil -- assuming there's any use for such a thing. It's not such a preposterous idea; certainly any chemist from the 1850s who saw a modern roll of aluminum foil would be completely astounded. However, even assuming a relatively low-cost production process, titanium has its quirks -- for example, it has to be welded in a dry, inert-gas atmosphere, or the welds are dangerously weak. Incidentally, when the US first started using titanium technology, most of the ore was purchased from the USSR. Since titanium was too expensive for civilian uses, the Soviets had to be aware that it was being largely used to build American weapons that could be used against them.



* Although many different alloys have been developed with a wide range of properties, sometimes it is useful to build a metal part of two metals -- one metal making up the structure of the part, another providing a surface plating to improve appearance, increased durability, or provide corrosion resistance. The plating is deposited on the part using an electrical process. The most common metals deposited by "electroplating" include zinc, cadmium, chromium, copper, gold, nickel, silver, and tin. Tableware is often silver-plated, while automotive parts are often chrome-plated.


Parts to be electroplated are wired as a cathode to the negative terminal of a direct-current source providing about one to six volts, and then dipped into a "bath", or solution of a salt of the metal to be plated on the part. The positive terminal is connected to an anode plate, often made of the plating metal, which is also inserted into the bath. The part gradually builds up a plating layer. Even parts made of nonconductive materials can be electroplated by covering them with a layer of conductive material, such as graphite. To get an even coating, the part has to be very clean, while the voltage passed through the bath and the temperature of the bath have to be strictly controlled.

Some metals, particularly chromium, have poor "throwing power", or what might be more easily said to be poor covering power. Chromium will plate heavily on projections but will not plate well in crevices, or on parts remote from the anode.



* Other popular materials include glass, ceramics, bricks, concrete, and blacktop. Glass is essentially based on silica, or silicon dioxide (SiO2). The silica is melted and then cooled quickly, forming a transparent irregular "amorphous" solid, lacking an orderly crystal structure. Glasses with different properties can be fabricated using different additives. The table below gives some ordinary glasses, along with the percentages of their constituents:


                                     SiO2 CaO Na2O B2O3  Al2O3  K2O  MgO

   window (soda-lime glass)           72   11  13    -     0.3  3.8   -
   cookware (aluminosilicate glass)   55   15   -    -    20      -  10
   pyrex (borosilicate glass)         76    3   5   13     2    0.5   -
   hardened optical glass             69   12   6    0.3   -   12     -

Trace elements can also be added for tinting, for example:


   calcium fluoride (CaF2)        milky white
   copper(I) oxide (Cu2O)         red, green, or blue
   cobalt(II) oxide (CoO)         blue
   uranium compounds              yellow or green
   iron(II) compounds             green
   manganese(IV) oxide (MnO2)     violet
   tin(IV) oxide (SnO2)           opaque
   selenium in suspension         red

Incidentally, there is a popular story that glass is a slow-flowing liquid, and that this is supposedly demonstrated by the fact that glass in medieval cathedrals is thicker on the bottom than it is on the top. Actually it was made that way, and it's just as often thicker at the top than it is at the bottom. The flow rate of glass is too slow to be noticed even over millennia.

* Ordinary ceramics are made of of a number of different types of clays, which are "silicates", based on rings of a silicon-based tetrahedral unit cell, with the rings arranged in sheets. They are formed by the weathering of granite. Due to the "inner surfaces" formed by rings of the silicate tetrahedrons, they absorb water and when wet they can be easily formed into pottery. Firing in a kiln drives the water out of the structure, making it stiff and no longer absorbent.

The ceramic structure consists of tiny crystals of silicates suspended in a glassy matrix. If clays are fired along with feldspar (an "aluminosilicate", [KAlSi3O8]n), the result is a glassy ceramic called "porcelain" or (with a bit of calcium phosphate / CaPO4 added) "china". Production of fine ceramics also involves a sophisticated knowledge of pigments or "glazings"

Industrial ceramics are different beasts from the materials used to make pottery, being made of materials such as silicon carbide (SiC), boron nitride (BN), silicon nitride (Si3N4), and so on. They are selected when high heat resistance, electrical insulation, or resistance to chemical attack is needed.

* Bricks are really much the same technology as pottery. Ancient methods of brickmaking remain in service in underdeveloped parts of the world: clay is dug up with pick and shovel; kneaded by bare feet in a wood tub; pressed into wood frames to dry in the sun; and then fired in a wood-burning kiln to give the final product.

The modern industrialized method of brickmaking is actually little more than automating the same steps. Clay is dug up, or "won" as the industry puts it, with backhoes and other heavy machinery, to be dumped into grinding mills and turned into a dry powder. A mixer unit blends the powder with water, with the resulting material usually drawn out as a long ribbon that is sliced into bricks by a fine wire, though sometimes the output is still dumped into wooden forms.

The damp or "green" bricks are loaded onto carts and then drawn through a kiln in the form of a long tunnel with a staged series of temperature zones. The bricks are dried for a day or two at low heat, fired for two or three days at high heat, then allowed to cool off for a few days. Bricks tend to be nonstandardized, varying between locales not merely because of the variation in clays available, but because of local differences in taste.

* Concrete is clearly a ceramic, but given its widespread use, it's clearly in a class by itself. It has also been around for a long time, the material having been used to build the dome of the Pantheon in Rome. The Roman formula was misplaced during the Dark Ages, to be reinvented in Britain in the early 19th century. People tend to confuse "concrete" and "cement", but they're not the same thing. Cement is just a sort of glue, made of dry limestone and clay; mix it with water and sand, and the result is "mortar" or "grout", used to set bricks and tiles. Add crushed stone or "aggregate" to mortar, and that's concrete.

Producing the cement for concrete is a big business, requiring use of a "cement kiln", which is a long steel tube about 3 meters in diameter or more, and with a length of 30 to 150 meters. The kiln is lined with firebrick to allow it to be heated to high temperature, and is set at a slight angle, with clay and limestone dumped into the high end. The tube is rotated at the rate of once every minute or two, with the hot materials gradually flowing down to the low end.

The output is in the form of dried-out "clinkers", which are then crushed into cement powder in rotating bins full of steel balls. The kiln features exhaust flues and smokestacks, with baghouses to trap dust. As mentioned, converting the limestone into lime releases carbon dioxide in large quantities, and cement production is a significant source of CO2 emissions.

The cement kiln may be one of the most interesting items in a concrete plant, but it is generally dwarfed by the bins for raw materials and silos for finished product. The raw materials are fed into a "ready-mix" or "transit-mix" truck, more generally known as a concrete truck, with a spinning top-shaped drum on the rear. The drum is spun rapidly at first to mix the product -- a corkscrew blade lining the drum does the mixing -- and then turned more slowly to keep the mix from settling. The drum is spun in the reverse direction to pour out the concrete at the work site.

Concrete plants are necessarily local affairs, since concrete sets in about an hour and a half. A driver whose truck breaks down with a full load of concrete is in big trouble, since the end result will be a drum full of solid concrete. Once properly poured, concrete has to set or "cure". This is not a drying process as such, being more an irreversible chemical reaction that generates a matrix of the material with water bound into it. The reaction releases heat, and massive concrete structures sometimes featured temporary refrigeration systems to help cure the concrete more rapidly.

Concrete may also include reinforcing steel mesh or bars ("rebar"). These days, plastic reinforcement is sometimes used as well -- plastic's relatively expensive, but it resists corrosion and so is better suited for, say, traffic bridges in northern climates where roads are salted during the winter.

* This is a fairly simplistic description of concrete manufacture and use. It is possible to tweak the mix to obtain a range of properties; for example, encouraging the distribution of tiny air bubbles in the concrete makes it more durable, since cracks won't propagate as far. Another trick to harden concrete is to add powdered, purified quartz along with a variable mix of other ingredients, creating a more expensive but much tougher concrete.

Intriguingly, a trick was introduced in recent years to add carbon fibers to the concrete. The proportion of fibers is very small, only about 1%, but they make the concrete electrically conductive. Passing a current through the concrete will heat it up, melting snowfall on the roadway. The conductive concrete costs about four and a half times more than ordinary concrete, but that doesn't cycle in life-cycle costs, such as the use of salt to de-ice roads, and the damage caused to roads and cars by salt. Another property of conductive concrete is that the compression of the concrete as a vehicle passes over it compresses the fibers, giving a greater electrical conductivity per cross-sectional area, and allowing the concrete to monitor and weigh traffic.

* Blacktop is sometimes referred to as "asphalt" or "macadam", though purists insist that all these terms mean slightly different things; the pros tend to call it "hot mix". The term "blacktop" is generally a safe bet, in any case meaning a material composed of aggregate stone mixed with hot "bitumen", or tar from the "bottoms" of the petroleum refinery. Blacktop is dumped in a hot state on a roadway, to spread out, cool, and harden.

A blacktop plant is a bit like a concrete plant, with various bins (or one internally segregated bin) containing different grades of aggregate. The aggregate is heated and dried before being stored into the bins. The stones are dumped into a truck along with bitumen and mixed for delivery.