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Design Report

Steelmaking

Picture Courtesy of Richard Furrer of Door County Forgeworks

AdvisorDr. Howard

Team MembersAngelica Moore

Devin RoweThomas Simpson

Justin Twohy

DateMay 1, 2014

Table of Contents

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Executive Summary 3

Introduction 3

Proposed Design 4

Environmental Context 7

Global and Societal Context 8

Conclusions and Acknowledgements 8

Future Work 9

References 10

Appendices 11

Executive Summary

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The Steelmaking team is to produce, from any iron oxide, ten pounds of steel with under 0.8 wt% C, ten pounds of steel/cast iron with 1.5-2.5 wt% C and ten pounds of cast iron with greater than 3.0 wt% C for the use of a future Swordmaking design team and the MET Department. To accomplish this the iron oxide hematite (Fe2O3) will be reduced with carbon by layering crushed hematite pellets with coke in a crucible and raising the temperature of the crucibles contents, in a portable caster, above the Fe-C reduction temperature for an extended time period. The amount of carbon can then be adjusted by introducing more carbon during the smelting process or removing carbon through a form of decarburization.

IntroductionSteelmaking is the process of producing steel from ferrous materials, if naturally formed

iron oxides such as hematite and magnetite are used to make the steel the process is known as smelting. Typically when making steel, impurities such as silicon, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, and chromium are added to make different types of steels. These different types, or grades as they are referred to professionally, can have their properties altered further by the use of various heat treatments. This is the main reason that steel is the most widely used material in the world its properties can be adjusted to for such a wide array of uses from medical implants to building skyscrapers.

Since its discovery, there are now two major ways to make steel, through an Electric Arc Furnace where electrical energy is used to reduce ferrous material, and burning furnace steelmaking where the energy from the exothermic reactions of the burning fuels and the material themselves are used to reduce ferrous material.

The Steelmaking team shall be focusing on a type of burning furnace steelmaking known as crucible steelmaking to create a basic grade of steel known as the 10xx series or plane carbon steel. Crucible steelmaking is when steel is made in a crucible by either melting scrap ferrous materials or reducing iron oxides. The customers for the steelmaking team will be the MET Department and future Swordmaking teams. Because the best swords are composed of two or more grades of carbon steel, the Steelmaking team is to produce three ten pound batches of steel/cast iron. One batch must be below 0.8 wt% C meaning it will be defined as plain carbon steel, unless it’s less than .002 wt% C then it’s considered commercially pure iron. Another batch must contain 1.5-2.5 wt% C, where the up to about 2.1 wt% is defined as high carbon steel and greater than 2.1wt% is defined as cast iron. Making the last batch, that must contain greater than 3.0 wt% C, cast iron as well. This becomes the team’s main problem, determining the carbon content and classifying the metal. To make the steel/cast iron, the Steelmaking team may use any source of iron oxide. The source that has been chosen to be used are the hematite pellets that have been used in the past.

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Proposed DesignHematite is comprised of oxygen and iron atoms. To obtain metallic iron, hematite must

be sent through a direct reduction process to remove the oxygen from the mineral. To separate oxygen from the iron a stronger bonding element then iron must be present to pull off the oxygen. The two element most commonly used today are hydrogen and carbon.

The team chose carbon as the reducer element because it is easily obtained and used in the form of coke. Carbon begins to reduce iron around 850-900o Celsius which makes it ideal for use in the portable caster to reduce the hematite, see Appendix D for more information. To aid in removal of the oxygen, the hematite must be comminuted. Reducing the size of the hematite solids will increase the surface area exposing more oxygen atoms to be removed by the carbon. The pellets were crushed in a roller mill twice with the rollers first set to a spacing of ¼ inches then 1/16 inches. However, the process of reducing iron isn’t as simple as some believe. Since carbon monoxide is the primary compound that chemically strips oxygen from iron, the entire smelting process must be kept at an oxygen deficient state to promote the burning of carbon to produce CO not CO2. This can be accomplished in two way, either by putting a lid on the crucible during smelting or by creating a flux layer. The lid/flux are used to let CO2 out of the crucible while keeping as much O2 from the atmosphere out as possible.

The idea behind the lid approach is that once the crucible is sealed, oxygen from the atmosphere can no longer get in. This forces the carbon to bond with what little oxygen was in the crucible to begin with and the oxygen present in the iron oxide. But because the gas is expanding the lid will be forced open allowing more oxygen to enter as carbon dioxide escapes.

Melting a flux such as silicon dioxide will create a liquid barrier that will perform the same function as the lid but will also bond to impurities in the raw material, and once the iron and flux has solidified the flux layer can be broken off. The drawback being that if the wrong composition is used, not only will the flux not work, it could have adverse effects on the crucible, steel or both.

To cut back on expenses the flux was chosen over the lid approach. The flux used was composed of one part SiO2, one part CaO and .53 parts Al2O3. The resulting mix should have a melting point around 1250o Celsius, a basicity of 1 and being largely made of silica, will have minimal effect on the clay comprising the crucible, see Appendix G for details. An oxygen sensor was intended to be used to monitor the O2 levels inside the furnace but because it was introduced too long after the caster had been on the resulting thermal shock fractured the zirconia tube yielding one and most likely inaccurate data point, see Appendix C for further information regarding the sensor.

If everything is done correctly the following reaction series will take place to produce metallic iron:

● Hot air and coke:

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2 C + O2 → 2 CO (1)

● Reduction of iron oxide with Carbon monoxide (CO).3 Fe2O3 + CO → 2 Fe3O4 + CO2 (2)Fe3O4 + CO → 3 FeO + CO2 (3)

FeO + CO → Fe + CO2 (4)

Oxide choicePure hematite pellets were used because of their lack of contaminants. These pellets and

others were examined using XRD (X-ray diffraction) analysis to determine their composition. These results can be found in appendix A and illustrate that the iron biscuits contained wustite, hematite, quartz, nickel, morimotoite, and portlandite whereas the iron pellets that were used contained pure hematite.

Smelting methodThe choices on what method to smelt the hematite pellets with came down to the

bloomery furnace and the portable caster because both were pre-existing tools on campus.Both need repairs before use, but upon later inspection it was determined that the bloomery would, besides its foundation, have to be completely rebuilt. Both methods appear to be equally reliable and safe if proper procedures are followed. The bloomery was far more traditional in its use but this factor was not critical for the design goal. So as the matrix in Table 1 below shows, the Caster was the preferred method.

The lack of refractory materials to repair the portable caster caused a massive delay last semester but, after a meeting with a previous steelmaking team member, it was decided that damages to the caster were minor enough that repairs were not necessary.

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Crucible SelectionAs can be seen in Figure 1, the material to use for the crucible came down to sintered

alumina and clay bonded graphite. The sintered alumina crucible would eliminate excess carbon caused by the graphite crucible but may have a difficult time dealing with the thermal shock. The graphite crucible, on the other hand, has been used before but could add excess carbon to the steel/cast iron making a decarburization process necessary.

Figure 1. Diagram of the Crucible Selection Process.

The minimum dimensions desired for this crucible can be seen in Figure 2. These dimensions were based on the limitations of the size of the portable caster and the ability to hold enough raw material to yield ten pounds of the finished metal. The calculations used can be seen in Appendix B.

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Figure 2. Dimensions of the Crucible to Hold 10 lb of Material. (Source: Shanghai Ceramics Corp)

Because of concerns with time and budget the decision was made to use a clay bonded graphite crucible. The one purchased has a brim filled volume of 4 L, which was the approximate size determined to be necessary. We were later made aware that the sublimation of carbon from the graphite crucible will cause the crucible to slowly dissolve so a second crucible was purchased just in case.

Analysis methodsOnce the metals have been poured a few test can be done to determine if the metal have

reached the desired wt% C or if they need to be adjusted. The first test will be to use metallography, this entails that a portion of the metal from 1-3 areas of the single mass will be cut, mounted, polished and etched. Contrast analysis will then be used to determine an area percentage of carbon from which a wt% can be determined. This methods won’t be entirely accurate but it will yield results that can be consider very close. In addition to metallography the MET departments Leco carbon analyzer may be used if it is hooked up soon enough. What this machine does is it vaporizes a sample then using inferred sensors determines the amount of CO2 given off then back calculates the samples wt% C. If this device isn’t ready in time the team may due an Atomic Absorption Spectroscopy test to determine the Concentration of carbon in the metal. The drawback to this method is that a Carbon cathode lamp, costing about $375, will need to be purchased.

Design procedure For the first batch, the greater than 3.0 wt% C cast iron was attempted by using 3250

grams of the crushed hematite layered with about 1100 grams of coke. This was about 200 grams more than needed to fully reduce the iron and in this excess it is estimated to have enough carbon to hit the desire range. The mix was then covered with about a half inch of flux, then a dusting of more coke to keep the flux layer from being blown away during the run. During the run the

2cm

2cm

V=4L 26cm

18.6cm

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temperature as held between about 1300o Celsius for nearly 6 hours, so the flux layer could fully liquefy. This was believed to be more than adequate since iron begins reduces with carbon at around 900o Celsius and 1200o C is what is typically used in industry (refer to the Ellingham diagram information in Appendix D). Because of the extra coke used, the total volume of the material exceeded amount the crucible could hold so batch was made in halves, reducing the second half on top of the first. Unfortunately the team couldn’t reach the desired temperature of around 1500oC to pour the metal, believing that the choice of wood for the preheater wasn’t burning hot enough. Following the failed pouring it was decided that the crucible would last another run to pour so it was broken to free the reduced iron.

Batch 2, was an attempt at the less than 0.8 wt% C. For this mix only 900 grams of coke was layer with the hematite, this being just a few grams shy of what was calculated necessary. In the teams’ calculations it was assumed that the coke was 80% C so using less than what was calculated would account for any error made by this assumption. The rest of the process was carried out in the same manner as the first batch. This time using oak charcoal in the preheater, the contents of the crucible was hot enough to pour. The product was poured into a cinder block that was lined with kerosene soaked casting sand. When cooled the block was broken to free the metal.

The final batch was the teams attempt at the 1.5-2.5 wt% C. In this mix a hundred grams of extra coke was used, this judged to enough extra carbon to push the metal into the desired carbon content. Following the same procedure done for the previous batch, the desired temperature was achieved after some trial an error with the fuel mix. Unfortunately a whole had developed in the crucible and was not noticed until the team tried to pour, so only half of the metal was successfully poured, the rest being on the bottom of the caster. This metal was later recovered with a chisel and a hammer.

ResultsApproximately 11 pounds of metal was produced from Batch 1. It has a heavily pearlitic

microstructure, see Figure 3, and general observations identified coke particles embedded in the metal along with being quite porous.

Batch 2 produced about 7.3 pounds of metal. It appears to be much less porous than the first batch, in its microstructure the pearlite is less densely packed and some ferrite grains were seen, see Figure 4.

Batch 3 produced around 7.7 pounds of iron product. General observation varied greatly with this batch because of the circumstances involving the recovery. Its microstructure, shows a lot of pearlite along with cementite forming along the grain boundaries, see Figure 5.

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Environmental ContextThe fire hazard presented during the making of the steel/cast iron is going to be high

because of the temperatures needed to melt the iron in addition to the sparks that occasionally escape containment. The high temperature at which the portable caster will be run is going to be a danger for burns. Preventative measures will be taken to reduce these risks such as acquiring a burn permit, using the proper PPE (personal protective equipment). The portable caster was moved away from buildings with exterior wood finishes and placed on top of a paved surface.

Historically, metallurgical coke sprang from the ravaged forests of Europe. The use of charcoal in iron making had all deleted the primeval forests making wood difficult to acquire and expensive where it was found; the attempt to use coal in this industry began. Coal was, however, rich in contaminants such as sulfur and phosphorus and choked the furnace as it burned. This problem remained until coke was developed by a combined effort by brewers and a Mr. Abraham Darby, thus lessening deforestation and increasing the effects of coal mining and the pollutants from coke production on steelmaking (1).

Figure 3. Microstructure of Batch 1 at 100X Figure 4. Microstructure of Batch 2 at 100X

Figure 5. Microstructure of Batch 3 at 100X

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The introduction of the EAF increased the amount of recycled materials used in steelmaking, ergo, less iron ore was mined per quantity of steel produced. It also increased the amount of electricity used per quantity of steel compared to blast furnaces (about 1 kW/kg steel produced) which in turn increases the quantity of coal mined to produce that electricity (2). On the other hand, EAFs also lower the amount of coal (converted to metallurgical coke) used. BOFs (basic oxygen furnaces) require 770 kg of coal to produce one ton of steel vs the 150 kg needed for EAFs (2).

Global and Societal ContextLike copper, bronze, and iron before it, steel revolutionized the world (1). From the

Ulfberht to the simple plow, a weapon or tool that did not break as easily meant life, or death and the rise and fall of civilizations. The Bessemer process allowed steel to once again shape the world; steel became the life’s blood of urbanization and architectural achievement. The Electric Arc Furnace (EAF) caped this chain of developments by allowing another use for recycled materials and reducing the use of iron ore. In essence; advances in steel allowed for sharper, stronger tools and weapons, the growth of cities and the development of skyscrapers, and cleaning up scrap yards across America. This design project took a modern look at the ancient art of crucible steelmaking, and while something of this scale did not involve large scales or imposing weaponry, the whisper of discovery, of nascent possibility, still whispered into existence.

Conclusion and AcknowledgementsThe steelmaking team is to produce 30 pounds of a steel/iron products of various carbon

contents by weight for the use of future steelmaking design teams. To achieve this, the portable caster will first be used to reduce crushed hematite pellets with coke in a clay bonded graphite crucible. Flux will be used on the crucible to allow for the egress of CO and to keep oxygen out of the steel/cast iron. Coke was chosen as the reducing agent as carbon is very high in coke is a reducing agent at higher temperatures. The reduced iron will then be alloyed with carbon in the form of coke or white cast iron that was produced by past steelmaking teams. Decarburizing will most likely need to take place for the lower carbon iron products. These ingots will be poured into a mold lined with petrobond in ten pound quantities. These ingots will be homogenized by working and then analyzed for carbon content and properties using metallography, Leco Carbon analyzer along with other possible methods,. So far, the pellets have been crushed in a roller mill less than 1/16 of an inch, and a test run was performed, where the iron was reduced but as we attempted to increase the temperature to melt the iron, the propane tanks lost pressure. This prevented the caster from getting hot enough to melt and pour the iron. An oxygen sensor was created and upon testing the sensor the nichrome wire used on the outside melted and the zirconia tube cracked.

A special thanks to the following people and departments:

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Brett Carlson and Mitchell Kramer, former Steelmaking team, for advice on last steelmaking team’s process.

Dr. Crawford for guidance in composing the design report.Dr. Howard for advising the team.Dr. West for the suggestion and use of the radiation pyrometer.Jeff Marshall for the assistance in the location of various materials and equipment.Russel Lingenfelter for running composition analysis.The Met Departments for supplying materials and equipment

Future WorkThe original schedule, refer to Figure 3, was quite ambitious. In it, all steels/iron products

would be created and finished by the end of 2013 Fall Semester with all analysis, testing, and refining of the process (if necessary) taking place the following semester.

Figure 3. Original Gantt Chart- Green Represents Tasks to be Completed

The updated schedule can be seen in Figure 4 below. As of now all preparations have been completed and one batch of metal is nearly complete.

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(1/14) (1/21) (1/28) (2/4) (2/11) (2/18) (2/25) (3/4) (3/18) (3/25) (4/1) (4/8) (4/15) (4/22) (4/29) (5/6)Fill propane tanksResearch/order O2 sensor componetsConstruct O2 sensorTest O2 SensorResearch/ Order CrucibleResearch/Order Temp. DeviceFind/Order Bar MoldRepair Portable Caster (P.C.)Trouble Shoot Pre-heater (P.C.)Smelt .8% steelSmelt 1.5-2.5% Cast IronSmelt 3.0% Cast IronAnalyze Iron product/steelDesign FairSemester Report

Figure 4. Updated Gantt Chart- Red Represents Tasks Completed and Green Represents Tasks to be Completed.

All that remains is to finish the smelting the different steel/cast iron, begin analysis on them, adjust the carbon content if necessary and prepare for the design along with the semester report.

References

(1) Raymond, Robert. Out of the Fiery Furnace: The Impact of Metals on the History of Mankind. 1st. University Park and London: The Pennsylvania State University Press, 1986. 150-152_184-186. Print.

(2) "Coal and Steel Statistics." World Coal. World Coal Association, n.d. Web. 26 Nov 2013.

(3) "Ellingham Diagram." Wikipedia. Wikimedia Foundation, 13 Dec. 2013. Web. 15 Dec. (4) Gaskell, David R. Introduction to the Thermodynamics of Materials. New York:

Taylor & Francis, 2008. Print.(5) KNAW, Proceedings, 19 I, 1917, Amsterdam, 1917, pp. 175-188(6) Gilchrist, J.D., Extraction Metallurgy 3rd Edition.(7) Levin, Ernest M., Robbins, Carlr R. and McMurdie, Howard F., Phase Diagrams for Ceramists,

Columbus, The American Ceramic Society.

Images

FURRER , RICHARD. Blacksmith. 2012. Photograph. Door County Forgeworks Web. 26 Nov 2013. <http://www.doorcountyforgeworks.com/Steel_Making.html>.

Appendices

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A. XRD Analysis of Potential Iron Sources

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B. Crucible Size Calculations

C. Oxygen Sensor Design and Calculations

The Steelmaking Team of 2013/14 will need to be able to monitor the oxygen levels inside the furnace during each use to make sure the iron oxide is reducing properly and to make sure the heating element isn’t being given more oxygen then it needs. To monitor the oxygen inside the furnace an oxygen sensor will be made. The following sections will discuss methods of making a sensor and the ideas behind how it is used.

Design

An oxygen sensor will be used to monitor the oxygen levels inside the furnace. The design for this sensor will either be a modified automotive oxygen sensor or a closed end zirconia tube filled with Chrome/chrome oxide with platinum contacts.

Round up to 4 L just in case we need the extra space

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Figure 1 shows a cross section of an automotive planar zirconia oxygen sensor with some degree of detail. Exhaust gas enters the front of the sensor and contacts the outer electrode, while the inner surface of the sensor is in contact with the atmosphere, which serves as the reference gas. The difference in oxygen potentials on the outer and inner electrode creates a voltage that may be measured.

Figure 1. Cross section of an automotive oxygen sensor. (Source: www.Delphi.com)

Because these sensors were not designed to operate at steel making furnace temperatures, the sensor needs to be modified. While the crucial components such as the electrodes and electrolyte, made of platinum and zirconia, can withstand the temperatures needed to reduce and melt iron, the casing and wire leads cannot. These parts need to be redesigned.

The case metal is stainless steel so it needs to be removed and the wire leads which are traditionally made from copper are to be replaced with platinum wires. Figure 2 is a detailed image of a standard planar zirconia oxygen sensor before modifications. Figure 3 is an illustration after it has had its case removed and wire leads have been replaced.

Figure2. Unmodified Oxygen sensor. (Source: www.boschautoparts.com)

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As a final adjustment to the automotive sensor’s design, the area referred to in Figure 1 that contains the atmospheric oxygen will be filled with powdered chromium and chrome oxide as shown in Figure 4. This last modification will lower the partial pressure on the reference side of the cell to near what would be expected in the furnace. Doing so will cause any change in oxygen potential more significant, giving a more accurate measurement. The oxide will then have a wire lead, replacing the wire that was attached to the sensor’s inner electrode. Figure 4 shows the sensor after modifications.

The cost of materials are approximately $10/centimeter for platinum wire, $7/OZ for chrome oxide, and $.40/gram for chrome powder.

An alternate design is still a possibility at this point. That being, instead of modifying an automotive sensor, the team may choose to construct an entirely new sensor from a one-end-closed zirconia tube. The design will ultimately end up being the same as the one illustrated in Figure 4, but may require less work to make while ending up to be more expensive. If this route is taken platinum contacts will need to be added to the outside of the tube. And like with the last design chrome/chrome oxide will need to be placed inside the tube with a platinum wire leading out.

Figure 3. Modified Oxygen sensor. (Source:www.boschautoparts.com)

Figure 4: Finished Oxygen sensor.

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The approximate cost of materials are $1014 for 25x25x.001 mm platinum foil or $386 for 20x20 mm platinum mesh with a nominal aperture of .12mm, and $212 for a single closed end zirconia tube with an outer diameter of 6.35mm wall thickness of .78mm and length of 320mm.

Theory

Change in Gibbs energy across the zirconia wall is given by the integration of the Fundamental Equation at constant temperature

(1)

To give

(2)

Where

is the pressure inside the furnace

is the pressure fixed by T is the absolute temperature (K)

R= the gas constant

The Nernst Equation relates the Gibbs energy change to the electrical potential.

(3)Where

n=4, the number of equivalents per mole of .

F= Faraday’s constant= .

The pressure solved for furnace oxygen pressure.

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(4)

ApplicationThe zirconia tube method was chosen and applied but was irreversible damaged from thermal

shock. One data point was collected before failure but is most likely inaccurate as the chrome/chrome-oxide matrix wasn’t up to the temperature that the gas leaving the caster was at before it failed. But Table one will show the results.

Table 1: O2 Sensor dataVoltage (mV) Temperature (K) Fixed Partial Pressure (p2, atm) Calculated Partial pressure (p1, atm)

23 1373 10-15 4.6 x 10-16

D. Ellingham Diagram Information

Ellingham diagrams were first constructed by Harold Ellingham, a British physical chemist in 1944. Ellingham diagrams can tell a large amount of information related to metallurgy. They are constructed by plotting the Gibbs energy versus temperature of reactions with one mole of oxygen. Ellingham diagrams can tell under what conditions ore can be reduced to its metal. We can use this to determine what will be the better reducer of our hematite pellets and at what temperatures the reduction will start. Gibbs energy (ΔG) of a reaction is a measure of the thermodynamic driving force that makes a reaction occur. Negative values of ΔG indicate that a reaction can occur spontaneously, while positive values indicate that external inputs need to happen for the reaction to occur. The equation for Gibbs energy is.

ΔG = ΔH - TΔS (1)

Where ΔH is the enthalpy, T is the temperature and ΔS is the entropy. Enthalpy is a measure of the total energy of a thermodynamic system. If the enthalpy is negative for the reaction, the reaction will give off energy, whereas if it is positive it will need energy. Entropy is a measure of disorder for a thermodynamic system, solids being in an ordered state to gasses which are a highly disordered state. To construct an Ellingham diagram, ΔG is plotted versus temperature. ΔS and ΔH are mostly constant with temperature except for phase changes, so ΔG can be plotted using straight lines. The slope of the line is the entropy and the y-intercept is the enthalpy, with the slope of the line changing with phase changes. The diagram is made with ΔG=0 at the top of the diagram and increasing negative numbers as you go down the diagram as most metal oxides have a negative ΔG value. The oxides found towards the top of the diagram are the so called “noble” metals such

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as gold, silver, platinum, etc. Their oxides are unstable and weak and are reduced easily, while the oxides found towards the bottom of the diagram are stable and much harder to reduce. Because of this, any metal found below the oxide line of a metal may be used to reduce that oxide to its metal. For example 4/3 Al + O2 = 2/3 Al2O3 is below SI + O2 = SiO2, therefore aluminum can be used to reduce silicon oxide to silicon. Ellingham diagrams can also be used to tell what partial pressure of O2 is in equilibrium with the metal and its oxide at a given temperature. This can be seen with the scale on the right hand side labeled PO2. To find the equilibrium pressure, find the temperature of interest for the oxide and place a point on its line. Then line up a straightedge with the point labeled “O” on the left hand side of the diagram and the point marked on the line for the oxide, and draw a line through to the PO2 scale. This gives the partial pressure of oxygen that is in equilibrium with the oxide. This partial pressure of oxygen can tell us whether the oxide will be reduced or oxidized. If the partial pressure is below the equilibrium value, the metal will be reduced whereas if it is higher it will be oxidized. The use of an Ellingham diagram will help to reduce the hematite pellets to iron. From the diagram it can be seen that carbon will be a better reducer of hematite at the temperature we will be working with. If the temperatures were lower that we were working with below about 700°C, CO would be a better reducer. Based off this the following equation was determined to reduce the hematite pellets to iron.

Fe2O3 + 3C = 2Fe + 3CO (2)

This equation is most favorable for the temperatures we are going to be working at because the gibbs energy for the formation of CO is more negative at higher temperatures, that is the slope of the 2C + O2= 2CO line is a downward slope across the diagram. To perform the reduction the addition of coke to the hematite pellets will be used. The coke will be used as the carbon source for the reduction of the hematite pellets.

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Figure 1. Oxides Ellingham Diagram

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E. Fe-O-C Diagram Information

Figure 1 below is an iron-carbon-oxygen phase diagram. It illustrates the relationship between these three elements in alloys. As we are using hematite (Fe2O3) pellets and reducing with carbon to form CO according to the reactions below

Reduction of iron oxide with Carbon monoxide (CO).3 Fe2O3 + CO → 2 Fe3O4 + CO2 (2)Fe3O4 + CO → 3 FeO + CO2 (3)

FeO + CO → Fe + CO2 (4)

Figure 1. Fe-O-C Diagram

As can be seen from this figure, a larger amount of oxygen makes the above reactions more difficult and time consuming. This is why oxygen should be kept out of the steel/iron product as much as is possible while still letting the carbon gasses escape.

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F. Heat Transfer Equations for the Portable Caster

Introduction

This project is a model of heat flow in the South Dakota School of Mines Steelmaking Design team’s portable caster. It was found through this model that the interior of the crucible will reach equilibrium temperature after 161 seconds.

Concept

By using centered difference numerical methods and steady-state one-dimensional systems heat flows can be modeled with the help of excel. Using conduction as the method of heat transfer and assuming Newtonian heating we can find the time needed for the crucible’s charge to reach the temperature at which iron is reduced.

The general equation for the conduction of heat in a solid free of heat sources can be expressed as (1).

∇∗k∇T=ρ∗∂ (C pT )

∂T

(1)

The more common form of (1) is written for conductivity independent of position and with heat capacity independent of temperature:

∂T∂ t

=α ∇2 T=α [ ∂2T∂ x2 + ∂2T

∂ y2 +∂2T∂ z2 ]

(2)

When temperature is not a function of time but only on position equation (2) becomes:

∇2T=∂2 T∂ x2 + ∂2T

∂ y2 +∂2 T∂ z2 =0

(3)

This equation applies to steady-state conduction. Equation (3) is referred to as the Laplace equation.

Since we assumed steady-state one-dimensional system for the crucible and caster system, we can also assume that the system is an infinite flat plate. With these assumptions equation (3) then reduces to:

d2Td x2 =0

(4)

In our system the boundary conditions are

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B .C .1at x=0 ,T=1200 (5)

B .C .2at x=10 , T=1200 (6)

Using these conditions we can simplify equation (4) to:

T−T f

T i−T f= x

L

(7)

The heat flux through the solid can be described as:

q=−k∗DTdx

=k /L(T 1−T 2)

(8)

Results

Using excel and centered difference numerical methods using equation (4) as our deferential equation it was found that the time needed for the charge to reach reduction temperatures was about 161 seconds.

G. Flux Design

As explain in the report flux will melt to form a liquid barrier, called slag, allowing the gas produced through reduction to pass through because of a difference in densities and by the same notion preventing the introduction of outside gases. But it is still not that simple, slags are comprised of three types oxides which behave differently thus influencing the reduction of the metal. Acidic oxides accept the oxygen anion, which may help in the reducing of the iron oxide. Basic oxides on the other hand generate the oxygen anion, hindering the process. The last type of oxides are amphoteric oxides, these behave basic in the presence of acidic oxides and acidic with basic mixes. This brings up the notion that any number of these oxides or combinations of them may be effective as slag for the reduction of iron oxides.

So which one(s) to use? This question will be answered by adhering to the parameters set forth by the team’s advisor.

DesignFor the flux to work properly, the following condition must be met:

Flux must melt below 1350oC, the lower the better Have a basicity of about 1. Must be made of material that will not damage the crucible.

The first step to creating a flux it to determine what material to make it from. Since one of the conditions is to make it from materials that won’t damage the crucible, whatever the crucible is made

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from should be one of the components. The Crucible is made from clay bonded graphite, were silica is major component of clay, and so SiO2 will be the starting point.

Pure silica will not satisfy the flux conditions of melting below 1350oC or basicity of one, so another component is needed. Silica is acidic so a basic material is need to achieve a basicity of about 1. Our team’s advisor referred me to a table for basicity calculations, Figure 1 below, to help find a basic material.

After cross referencing what chemicals the Department had in stock, CaO was the only choice. The Cao that was available was hydrated and after drying for 4hours at 600oC it was determined that it was about 83% CaO by weight. This will become important later when determining the amount needed.

A basicity of about 1 is required of this flux mix where basicity I, is determined by equation 1 below.

(EQ 1)So to attain the require basicity the flux needs to be comprised of nearly equal parts by weight acidic and basic oxides. This however, introduces the final dilemma. An equal mix of silica and lime melts at about 1400oC, which is higher than the desired melting temp put forth by this design task. To solve this problem another oxide must be introduced, an amphoteric oxide since it will not affect the basicity.

With another look at Table 1 there are a number of intermediate or amphoteric oxides to choose from but few are available at the amounts needed for this project. Supplies limited the list to Fe2O3 and Al2O3. A check of a couple of tertiary phase diagrams showed that either was capable of dropping the melting temperature of the flux to below 1350oC but Al2O3’s mix was far easier to manage.

From CaO-SiO2-Al2O3 phase diagram, Figure 2b below, we can see that there is a small area where the melting temperature is about 1265oC.

Figure 1. Table 4 from J. D. Gilchrist’s Extraction Metallurgy 3rd Edition

26

(A) (B)

This area is at a ratio of 1 : 1 : .53 (SiO2: CaO: Al2O3).

With this the flux will meet all or the requirements, the mix should not damage the crucible due to their similar composition, the Melting temperature is 1265oC about 90o below the required 1350oC and

a basicity of 1:

Figure 2. A) CaO-SiO2-Al2O3 tertiary phase diagram from Phase Diagrams for Ceramists. B) Close up or the area of interest.