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How does one go about separating virgin metals from common ores? This is the job of an extractive metallurgist. An ore is a rock that contains metal-bearing minerals in sufficient quantity to make it economical to mine and process. Taconite, for example, is an iron ore that comes from the Lake Superior District of the United States. It may contain the mineral hematite (Fe O ) or magnetite (Fe O ). Bauxite, an ore of aluminum,_{2 3 3 4} is rock containing hydrated aluminum oxide. The first step in processing ore is to

mechanically enrich the content of the mineral of interest. This is known as beneficiation or mineral dressing.

The ore is typically crushed and pulverized in a series of mills until the desired particle size is obtained. We are now ready to begin enriching the ore by separating and removing the gangue (the junk rock, clay, sand, etc.) from the mineral of interest. There are many ways to do this depending on the nature of the ore. Most make use of the difference in densities between the gangue and the good stuff.

Simple washing may be used to remove clay, dirt, etc. from the ore. Heavy media separation consists of dumping the pulverized ore into a container filled with a fluid that has a specific gravity between that of the trash rock and that of the mineral of interest. One will float and the other will sink thereby becoming separated. Floatation separation uses a liquid with a chemical additive known as a collector to separate out the junk. The collector has a strong affinity for either the mineral of interest or the gangue, and for air. The collector is added to a liquid suspension of ore particles and

the mixture and individual bubbles will become attached to the particles coated with the collector causing them to rise to the surface. The froth (with one type of particles) on top is periodically decanted off thus separating it from the other class of particles still in suspension.

Magnetic separation is used to separate out magnetite from gangue in some iron ores. The pulverized ore is passed over a rotating drum that is magnetized. The

ferromagnetic magnetite particles will cling to the drum while the non-magnetite gangue falls off. The magnetite particles are scraped off the rotating drum after being carried away from the gangue particles.

There are many other methods of mineral dressing. Most ores require the use of a combination of methods before the ore is enriched enough for further processing. The final step in mineral dressing is often an agglomeration process. In order to process the ore through the various separating methods, it may have been necessary to pulverize the starting material into a powder. This powder made it easier to separate the gangue from the mineral of interest, but it may not be suitable for the next step in our quest for virgin metal.

In the case of iron, the next step is the blast furnace. A blast furnace (which we'll discuss in detail shortly) utilizes a flow of hot gases in contact with the furnace charge to cause certain chemical reactions to occur. The powdered product from some mineral dressing operations is too fine to use as blast furnace feed and so must be

agglomerated by pressing into brignettes, mixing the powder with a fuel such as coke and then sintering the mixture together by exposing it to a flame, or by pelletizing. Pelletizing is done by loading a balling mill with the powdered ore and a binder such as bentonite (a type of clay). A solid fuel is sometimes added to improve sintering. As the drum rotates, the particles will grow larger due to a "snowball" effect. Once the particles have reached the desired size (typically 1/2" - 1" in diameter), they will be taken from the mill and sintered in a continuous furnace to improve hardness and strength. These pellets are sufficiently large to let the gases in a blast furnace flow readily in between them, but not so large as to reduce the effective surface area exposed to the gases.

Thus far we have produced an ore rich in the metal-bearing mineral of interest, but no virgin metal. In the rest of this chapter we'll examine the processes used to extract the virgin metal from the ore, how the metal is refined, and how the metal is alloyed into a useful engineering material. We'll limit our discussion to the two classes of metal most commonly used in the Oil Patch: steels and nickel base alloys. We'll look at how steel is made first.

STEEL

1. The Blast Furnace - The enriched iron ore (often in the shape of pellets) that we obtained from mineral dressing is transformed into metallic iron in a blast furnace. A blast furnace is essentially a big refractory-lined pot up to 10 stories tall. The process begins by charging the top of the furnace with a mixture of iron ore, coke, and limestone. A continuous "blast" of hot, preheated air enters near the bottom of the furnace and reacts with the coke to form carbon

monoxide gas. The carbon monoxide then combines with oxygen from the iron oxides in the ore to form carbon dioxide gas thus reducing the ore to metallic iron. The limestone is added as a flux. It facilitates the removal of impurities in the iron ore by making them more fusible and combining with them to form a slag on top of the molten iron. The molten iron will collect at the bottom of the furnace where its will eventually be tapped into a ladle.

The metallic iron produced in a blast furnace contains substantial amounts of impurities such as carbon, sulfur, phosphorous, and many others. These impurities make the iron very brittle and useless without further processing.

2. The Porcine Beginnings - Steels are direct descendants of pigs. Pigs, besides being the domesticated quadrupeds responsible for bacon and the covers on footballs, are the ingots that result from pouring the molten iron produced in the furnace into small, block-shaped molds. The iron produced in the blast furnace, as a consequence, is referred to as pig iron. The steel making process consists of refining pig iron or ferrous scrap in order to remove undesirable elements and then adding the required alloying elements in the correct proportion. There are three basic types of steel making processes: open-hearth, basic oxygen, and electric-arc furnace. Most high quality steels are made by the basic oxygen or electric furnace process.

A. Open-Hearth Process - A schematic of an open-hearth furnace is shown in Figure 1. It consists of a large, shallow basin lined with refractory material, an arched roof lined with fire-brick, gas or oil burners, and regenerators called checkers for preheating the air used for combustion.

Figure 1: Open Hearth Process

The furnace is charged with scrap, limestone, solid pig iron, and sometimes iron ore (used in conjunction with the limestone flux). The charge, laying on the "open" hearth, is swept by flames from the burners. When this solid charge begins to melt, molten pig iron/scrap may be added. Oxygen is frequently introduced by lances through the roof.

The charge is refined through several mechanisms. Impurities such as manganese, silicon, and phosphorus are oxidized and float to the top of the molten metal and become part of the slag. Carbon will be oxidized to CO. Sulfur and other impurities will combine with limestone in forming the slag. The refining time ranges from 4 to 10 hours. When the molten metal is sufficiently pure, the furnace is tapped from the bottom into a ladle. The molten metal in the ladle may be treated with deoxidizers and alloying elements.

The open-hearth process can make a wide variety of steels in 100-500 ton melts. The open hearth process is seldom used now because of the long refining time and the generally dirty steel it produces.

B. Basic Oxygen Process (BOP) - Figure 2 is an illustration of a basic oxygen furnace. It is essentially a big, refractory lined pot that is charged with molten pig iron and scrap. The furnace is on trunnions so it can be tilted in order to facilitate the charging process.

A water-cooled oxygen lance is brought down through the top opening in the furnace and high-velocity stream of oxygen is directed onto the charge. This causes the rapid oxidation of carbon, manganese, and silicon in the charge: reactions which give off heat and help to melt the scrap and aid in the refining process. At the same time that the oxygen is being blown in, burnt lime and fluorspar are being added. These act as fluxes

Figure 2: Basic Oxygen Process

and form a slag which floats to the top of the molten metal. The impurities in the metal are removed by oxidation and/or

interaction with the slag. With the BOP process, heats as large as 300 tons can be made in less than an hour and having quality comparable to open-hearth steel. The molten metal is tapped into a ladle where deoxidizers and alloying elements are added.

C. Electric-Arc Furnace Process - An electric-arc furnace (see Figure 3) is a refractory lined pot with three carbon or graphite electrodes entering through the roof. The furnace is charged with solid scrap and pig iron. The electrodes are brought down to a point near the charge and an arc is established which causes the charge to melt. When the charge is roughly 70% melted, iron ore, burnt lime, and other ingredients are added to form a slag. Impurities are removed through interaction with the slag which floats on top of the metal.

In comparison with the BOP and open-hearth process the electric-arc furnace process has the advantage of being able to start with a solid charge. The process permits extremely close control over temperature, composition, and refining conditions for each heat making it particularly useful for specialty steels. A heat may vary form a few hundred pounds to over 200 tons. It can take 3 to 8 hours to produce one heat depending on the type of steel. Alloying elements and deoxidizers are added once the molten metal is tapped from the bottom of the furnace into a ladle.

Figure 3: Electric Arc Furnace Process

3. Deoxidation - In the three types of steel making processes that we have described, one of the primary reactions taking place is the combination of oxygen and carbon to form a gas. If the oxygen available for this reaction is not removed prior to the molten steel solidifying, then gaseous products will continue to evolve as the metal freezes in the ingot mold and may form blowholes, porosity, etc. in the solidified steel. There are many applications where these cavities cannot be tolerated because of their effect on the steel's mechanical properties. Steels destined for use in these critical applications will be deoxidized. The amount of oxygen removed will determine the type of steel. Deoxidizers such as aluminum or ferro-silicon are added to the ladle or the ingot mold to control the amount of available oxygen by combining with it to form non-metallic oxides or silicates. If no gas is evolved during solidification, the steel is termed killed. Steels having increasing amounts of gas evolution are referred to as semi-killed, capped, or rimmed.

A. Killed Steels - Killed steels are sufficiently deoxidized so that very little gas evolves during solidification. The top surface of the ingot freezes first. As the rest of the ingot solidifies, the metal shrinks and forms a large cavity or pipe in the center near the top. The portion of the ingot containing the pipe will later be cropped off. A killed steel has the most uniform chemistry and properties.

B. Semi-killed Steels - These have an intermediate level of deoxidation. Enough oxygen is retained so that there will be a

sufficient amount of gas evolution during solidification to replace the pipe by an approximately equal volume of deep-seated blowholes.

C. Rimmed Steels - Rimmed steels are only slightly deoxidized. In rimmed steels, the evolution of gas is sufficient to keep the top of the ingot a liquid until a side and bottom rim of substantial thickness has solidified. The gas evolution is a result of the interaction between carbon and oxygen in the molten steel at the boundary between the solidifying rim and the remaining molten metal. This causes the decarburization of the rim which, in turn, makes it very ductile. The center portion of the ingot, which solidifies last, will generally have a great deal of segregation.

The low carbon surface layer of a rimmed steel makes it ideal for cold forming applications and where surface is of major importance. Rimmed steels contain less than 0.25% carbon and 0.60% manganese. The rimming action is very slow or

nonexistent in steels above these levels.

D. Capped Steels - These steels are the same as rimmed steels, except that the rimming action was stopped before it was complete. The amount of gas entrapped in the solidifying metal causes it to raise to the top of the mold. When the rising metal comes in contact with the heavy metal cap on top of the mold, the rimming action is cut short. Rimming may also be stopped by the addition of more deoxidizers.

Capped steels have a thin, low carbon rim with the remainder of the ingot cross-section having a uniformity typical of semi-killed steels. This gives capped steels good cold forming

characteristics and uniform properties.

4. Vacuum Treating - Molten steel contains other gases besides oxygen such as nitrogen and hydrogen. These entrapped gases can be sources of non-metallic inclusions, porosity, metal embrittlement, and a host of other evils. Vacuum degassing will frequently be specified for critical steel applications (those that require the greatest internal

soundness, structural uniformity, or some other property that could be adversely affected by uncontrolled amounts of dissolved gases). Vacuum degassing is used in conjunction with deoxidizers. The deoxidizers are generally added late in the vacuum cycle. Degassing may take place in a vacuum furnace as the steel components are being melted (i.e. vacuum induction melting), in a ladle or as a ladle is

or in a vacuum furnace in which the steel is being remelted (i.e. vacuum are remelting).

A. Vacuum Induction Melting (VIM) - An induction furnace consists of a crucible surrounded by an electric coil. The crucible is charged with scrap, pure iron, and alloying elements in

proportions correct for the chemistry of the desired steel. VIM has the crucible in a vacuum chamber. An alternating current passing through the coil induces currents in the crucible's charge. These induced currents generate heat and cause the charge to melt. The molten metal is thoroughly mixed by

electromagnetic forces resulting from the induced currents. This insures good chemical homogeneity as well as exposing more and more molten metal to the surface where the vacuum draws off the dissolved gases.

B. Stream Degassing - In this process a ladle full of molten steel is placed on top of a vacuum chamber that contains an empty ladle directly underneath the full one. The molten metal is tapped off the bottom of the full ladle through a spout that is connected to an opening in the chamber. As the metal stream enters the vacuum, the low pressure in the chamber causes the stream to break up into droplets. This increases the surface area of the molten metal exposed to the vacuum facilitating the removal of gases. This teeming (pouring) practice is often referred to as "ladle-to-ladle".

C. Continuous Circulation Degassing - This process is illustrated in Figure 4. A ladle full of molten metal is placed beneath a

vacuum chamber. The chamber is lowered so that two

refractory tubes are immersed in the liquid metal. Inert gas is bubbled into one of the tubes and this creates a density differential between the two tubes. This allows atmospheric pressure to force the molten metal up one tube into the vacuum chamber, where the vacuum draws off the gases, and down the other tube back into the ladle. This is also known as the

Figure 4: Continuous Circulation Degassing

D. Ladle Degassing - In this process, the ladle is placed inside a vacuum chamber. The molten metal is usually stirred in order to expose the maximum amount of steel to the vacuum where the gases can be drawn off. Stirring is accomplished by electrical induction or by bubbling argon up through the bottom of the ladle.

E. Vacuum Arc Remelting (VAR) - The VAR process utilizes a consumable electrode that is cast out of the steel to be refined. This electrode is remelted inside a vacuum furnace (see Figure 5) by creating an electrical arc between the top of the electrode and a water-cooled copper mold at the bottom. As the arc causes the tip of the electrode to melt, metal droplets are

transferred down through the vacuum to the bottom of the mold. Here they form a molten pool. The bottom portion of the molten pool that is in contact with the water-cooled mold will start to solidify and form an ingot. This ingot will continue to grow as more and more metal is transferred across the arc into the molten pool on top of the ingot and then solidifies.

The VAR process results in a greatly reduced gas content in the steel. Some of the more volatile impurities may also be

removed. This virtually eliminates porosity and can significantly improve the cleanliness of the steel. The toughness and ductility are thus optimized. Another benefit of the VAR process is the elimination of the gross chemical segregation found in

conventionally cast ingots. Segregation refers to the non- uniform distribution of alloying elements, impurities, or phases during solidification. The center section of a conventionally cast ingot is much richer in low melting temperature impurities and

Figure 5: Vacuum Arc Remelting

mold walls because the center is last to freeze. In the VAR process, the metal droplets become well mixed in the molten pool on top of the forming ingot and quickly solidify. This increases homogeneity and limits the time available for

segregation to occur. It also eliminates shrinkage cavities found in conventional ingots. Shrinkage cavities are the voids that occur in the center of a conventionally cast ingot that result from the fact that the solidified metal occupies less volume that does molten metal. The properties of a VAR steel are thus more uniform that a conventionally processed steel.

F. Vacuum Arc Degassing (VAD) - This degassing process utilizes a vacuum chamber that has electrodes extending into the chamber through the top somewhat like an electric furnace. Molten metal from the steel making furnace is tapped into a chrome magnesite slag-lined ladle. Burnt lime, fluorspar

bauxite, and grain aluminum are added to form a slag. The ladle