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    The Ellingham Diagram: How to Use it in Heat-Treat-Process

    Atmosphere Troubleshooting

    Harold Johann Thomas Ellingham (1897-1975) was a British physical chemist and is best

    known for the diagrams named after him that plot the change in standard free energy with

    respect to temperature for reactions like the formation of oxides, sulfides and chlorine ofvarious elements, such as:

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    2x/yM + O22 x/yMxOy

    The oxide plot tends to be the most common and will be highlighted here as its applicability

    to heat-treating processes is the most relevant. This phenomenon was known before

    Ellinghams time, but Ellingham demonstrated it more clearly and made it more accessible to

    industry as a tool. His diagram and its variants help to select the best reducing agent for

    various ores in the extractive-metal process.

    Ellingham discovered that by normalizing the thermodynamic functions to a given reaction

    with one mole of oxygen he was able to compare the temperature stability of many different

    oxides on the same diagram. In particular, and this reaction is critical to metal reduction

    systems that use carbon dioxide, he could show graphically that carbon becomes a stronger

    reducing agent as the temperature increases. The reduction of metal oxides with carbon (or

    carbon monoxide) to form the free, reduced metals is of immense industrial importance (blast

    furnace reduction of iron ores), and Ellingham diagrams show the lowest temperature at

    which the reaction will occur for each metal.

    Lets restate and go stepwise through the Ellingham diagram to set up and make clearer how

    metal heat treaters can use it. We can adapt the diagram from its original use as a higher-

    temperature extractive-metallurgy tool to one where we can predict the effects of protective

    atmospheres and common atmosphere impurities and their impact on heat-treated product.

    See Fig. 1 for the classic Ellingham diagram that would typically be used for heat-treatingatmosphere processes.

    Background

    An Ellingham diagram is a plot of DG (change in Gibbs free energy) versus temperature,

    which, for our purposes, would be a temperature in a continuous furnace by zone or in a

    batch furnace by time in the cycle. The Ellingham diagram shown is for metals reacting to

    form oxides.

    Since any explanation of how to calculate and plot an Ellingham diagram is more about the

    mechanics of the derivation process and less about how to use the tool, which is the intent of

    this article, the concept is covered very briefly here.

    Enthalpy (DH) and entropy (DS) are essentially constant with temperature. Unless a phase

    change occurs, the free energy (DG) versus temperature plot can be drawn as a series of

    straight lines, where DS is the slope and DH is the y-intercept.

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    DG = DHT DS

    The slopes of those plotted lines change when any of the materials involved melt (M) or

    vaporize/boil (B). In a heat-treat system this is (for the most part) irrelevant, except in the

    brazing and sintering processes where those phase changes (melting) can indeed occur.

    The free energy of formation is negative for most metal oxides, which means the reaction can

    proceed without further influence. Therefore, the diagram is drawn with DG=0 at the top of

    the diagram, and the values of DG shown are all negative numbers. Temperatures where

    either the metal or oxide melt (M) or vaporize (B) are marked on the diagram.

    Note that the majority of the plots for metals slope upward because both the metal and the

    oxide exist as condensed solid or liquid phases. The oxygen partial pressure is taken as 1

    atmosphere, and all of the reactions are normalized then plotted to represent consumption of

    one mole of O2.

    Interestingly, there are two plots that do not look like all of the others.

    C + O2=> CO2

    Carbon, a solid, reacts with one mole of oxygen and produces one mole of carbon dioxide

    (CO2), which results in little change of entropy an almost horizontal plot. The other has a

    distinct negative slope:

    2C + O2=> 2CO

    In this reaction, a solid once more reacts with one mole of gas but produces two moles of gas

    carbon monoxide (CO). This causes a substantial increase in entropy, and the plot has a

    distinct negative slope.

    The severe negative slope of this carbon reaction results in an increasingly more powerful

    reducing agent as temperature increases. For example, at 1500C (2732F), the carbon

    reaction crosses under that for silicon dioxide (SiO2), resulting in the potential ability to

    reduce the highly stable compound of SiO2to silicon under oxygen-starved conditions.

    Note on the diagram there is a scale on the right and along the bottom of the graph illustrating

    PO2(atm). PO2 is plotted showing partial-pressure values from 1 to 10-100. It will be shown

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    how to use this scale in the next section, along with the scales immediately to the right of this

    one showing the relationship of PH2/H2Oand PCO.CO2to the various metals and their oxides as a

    function of temperature. These partial-pressure values and their associated ratios can be

    plotted on the Ellingham diagram and can be obtained by sampling the process atmosphere at

    various temperatures, or at specific times (temperatures) in a cycle, using familiar and readily

    available atmosphere analytical tools (oxygen, hydrogen, dewpoint/moisture, CO and CO2).

    Ellingham Diagram for Metals Processing

    When plotting whether a given metal system will oxidize, reduce or remain as an oxide or

    pure metal, the Ellingham diagram does not indicate the quantitative rate of the reaction, only

    the probability of it occurring based on a given set of conditions. One can make the

    assumption that the reaction will qualitatively occur more rapidly as temperature increases or

    as the conditions for reducing or oxidation deviate farther from neutral-atmosphere

    conditions.

    We can use the diagram to determine the relative ease by which a metal can be oxidized or an

    oxide can be reduced. Metals plotted high up on the diagram are easier to reduce (noble

    metals) than those plotted lower on the diagram, which naturally tend to exist in very stable

    oxide forms. For example, Ag, or silver, is very hard to oxidize, while Ca, or calcium, doesnot naturally exist in its elemental form, indicating a very stable oxide form. The metal/metal

    oxide plots also interrelate to each other. A metal plotted below another metal can reduce the

    oxide of the one plotted higher up the diagram. Hence titanium, not the oxide, can reduce the

    oxide of chromium, which is plotted higher on the diagram.

    We can also use the diagram to determine the following at a given temperature:

    The ratio of hydrogen to water/dewpoint (PH2/H2O) that can reduce a metal oxide to

    metal or prevent a metal from oxidizing

    The ratio of CO to CO2(PCO/CO2) that can reduce a metal oxide to metal or prevent a

    metal from oxidizing

    The partial pressure of oxygen that will be in equilibrium with a metal oxide

    Determining the Equilibrium Partial Pressure of Oxygen

    For a PO2higher than the equilibrium value at a given temperature, the metal will be oxidized.

    Conversely, for an oxygen partial pressure that is lower than the equilibrium value at a giventemperature, that metal will be reduced.

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    Use the scale or nomograph to determine the equilibrium PO2by the following method. Using

    a straightedge and knowing the temperature or series of temperatures you wish to use (and

    the metal/metal-oxide system), put the one side of the straightedge on the upper left-hand

    corner of the diagram that is labeled O (near the Ag to silver-oxide plot). Next, position the

    straightedge from that anchor point to the temperature point where the metal in question

    intersects that temperature value. Now, continue across the straightedge plot to the value on

    the PO2scale. This is the equilibrium partial pressure of oxygen. For this diagram, it is

    expressed in atmospheres. Once again, any oxygen partial pressure that is lower than the one

    derived will cause metallic reduction. Any partial pressure above will cause oxidation.

    For a real-life example, find the equilibrium partial pressure of oxygen for chromium at a

    temperature of 1300C (2372F). Put the straightedge on the O dot in the upper left hand

    corner of the diagram. Find the chromium plot and where it intersects the temperature of

    1300C. While anchoring the one end of the straightedge on the O dot, move the other end

    of the straightedge so that it runs through that intersection point and continue down the

    straightedge to the PO2scale. Read off the value, which should be 10-16expressed in

    atmospheres. This is an extremely small amount of oxygen than can be present in this high-

    temperature system before the deleterious effects of oxygen will adversely oxidize the

    metallic chromium.

    The effect of chromium oxidation by even the smallest amounts of oxygen can be countered

    by using a strong atmosphere reducing agent such as hydrogen. Lets go next to determining

    the equilibrium ratio (PH2/H2O) or the hydrogen-to-water (dewpoint) ratio.

    Determining the Equilibrium PH2/H2ORatio

    For a PH2/H2O, or hydrogen-to-water ratio, that is higher than the equilibrium valuerelatively

    more hydrogen to waterat a given temperature, the metal will be reduced. For a PH2/H2Othat

    is lower than the equilibrium value (relatively less hydrogen to water) at a given temperature,

    the metal will be oxidized.

    Using essentially the same straightedge method that we used above to plot the equilibrium

    partial pressure of oxygen, we now plot from the H dot on the left side of the diagram

    through our desired metal and desired temperature and now read from the P H2/H2Oplot on the

    right side of the diagram.

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    A real-life example would be to find the equilibrium hydrogen-to-water ratio for chromium at

    a temperature of 1300C. Locate the straightedge left side on the H dot, pass through the

    chromium equation where it intersects at 1300C and read off the equilibrium hydrogen -to-

    water ratio value on the PH2/H2Oscale. The value should be between 102and 103. This

    represents a value of between 100 and 1,000 to 1 ratio of hydrogen needed to water or

    dewpoint level. For this exercise, we can estimate the hydrogen-to-water ratio to be 400 to 1.

    This is a high level of hydrogen that is necessary to counter the oxidizing effects of water or

    dewpoint on chromium, hence the reason why most stainless steel (chromium present as an

    alloy) high-temperature processing is usually done in very high levels of hydrogen often

    100%. At the processing temperature of 1300F, in the presence of hydrogen, oxygen species

    will convert to water and become a part of whatever background moisture or dewpoint that

    might be in the process-atmosphere system. Enough hydrogen must be present in the

    atmosphere system to counter oxidation of chromium. With too low of a hydrogen ratio to a

    given moisture level, chromium oxidation will occur. With no hydrogen in such an

    atmosphere system (vacuum-furnace processes excluded), it is literally impossible to exclude

    enough oxygen from the system to prevent chromium oxidation.

    Determining the Equilibrium PCO/CO2Ratio

    The PCO/CO2, or CO/CO2ratio, is used with more frequency in extractive-metallurgy systemsfor all metals to determine whether this reaction will reduce or oxidize a given metal system

    at a given temperature. In heat-treated metals, the ratio is usually used for carbon-bearing iron

    alloys to determine whether the metal will decarburize or carburize. It is the backbone of

    carburizing and hardening and all of its derivative processes.

    The topic of carburization and its derivative processes is reserved for articles far beyond the

    space allotted and the scope of this article on the Ellingham diagram. There is far more to

    consider than merely plotting the equilibrium value for the CO/CO2ratio. In these systems,

    we are often interested in depth of the carbon in the iron, activities, diffusion rates, time at

    temperature, degree of surface oxidation and its removal, carbon concentration profiles,

    avoiding intergranular oxidation (IGO), etc. to name but a few variables. However, we can

    get a rough idea of what the equilibrium ratio should be so that conditions can be set to

    promote deliberate decarburization in the case of electrical steels. In the case of carburization

    of steel, we can at least see what equilibrium ratio we must roughly exceed to cause the

    carburization process to occur.

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    Summary

    Harold Ellinghams work made clearer and more accessible, in an easy-to-understand

    graphical format, the well-known concepts of standard free energy with respect to

    temperature for reactions like the formation of oxides, sulfides and chlorides. In the case of

    his metal and metal-oxide diagram, by normalizing the thermodynamic functions to reaction

    with one mole of oxygen, Ellingham was able to compare the temperature stability of many

    different oxides on the same diagram. The result was his relatively easy-to-use Ellingham

    Diagram to determine reduction and oxidation conditions without resorting to painstaking

    efforts to interrelate all metal systems free-energy-based calculations from scratch. The

    addition of the original nomograph tool that plots, in a continuous scale, the values of PO2for

    ease of calculation by the user is the work of F. D. Richardson and Jeffes.

    Since CO/CO2and H2/H2O ratios are often used in conjunction with determining equilibrium

    partial pressure of oxygen, L.S. Darken and R.W. Gurry added their calculations as

    nomograph scales for these additional ratios. These diagrams have subsequently been referred

    to as Ellingham, Ellingham-Richardson, Darken and Gurry, or modified Ellingham diagrams.

    http://www.industrialheating.com/Articles/Feature_Article/BNP_GUID_9-5-

    2006_A_10000000000001026454

    http://www.industrialheating.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000001026454http://www.industrialheating.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000001026454http://www.industrialheating.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000001026454http://www.industrialheating.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000001026454http://www.industrialheating.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000001026454
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    Diagram Kellog

    Secara termodinamika pemanggangan oksidasi terhadap logam sulfide dapat diatur sehingga

    didapat : logam oksida, logam sulfat, logam, dll.

    Untuk menentukan kondisi yang tepat bagi pembentuk produk, perlu ditinjau hubungan

    kesetimbangan system : logam, belerang, oksigen.

    Reaksi pemanggangan oksidasi dibagi menjadi 3 (tiga) kelompok :

    Reaksi utama :

    2 MS + 3O2= = = = 2 MO + 2SO2 (1)

    MS + 2O2= = = = = MSO4 (2)

    MS + O2= = = = = = M + SO2 (3)

    Reaksi Samping, Fase Gas

    2 S + 2O2= = = = = = 2 SO2. (4)

    2SO2+ O2= = = = = = 2SO3. (5)

    Reaksi Samping, Fase Padat

    4MSO4= = = = = = 2MO.MSO4+ 2SO2+ O2. (6)

    2MO.MSO4= = = = 4MO + 2SO2+ O2 (7)

    2MO = = = = = 2 M + O2... (8)

    Catatan :

    a.

    Tidak semua senyawa padat dapat terbentuk, missal oksida basa (mo.mso4). Ada

    beberapa logam yang sangat tidak stabil. Hal ini diabaikan.

    b. Beberapa logam mungkin dapat membentuk lebih dari satu jenis oksida basa

    (mox.mso4), x disini bervariasi.

    c. Bila logam mempunyai lebih dari satu macam valensi, dapat terbentuk lebih banyak

    lagi jenis senyawa padat.

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    Masing-masing reaksi kesetimbangan dapat diperoleh suatu persamaan termodinamika

    Misal reaksi (1) pada kesetimbangan :

    K1 =

    Bila semua fasa terkondensasi (padat) berada dalam keadaan standard, maka harga activity

    (a) = 1

    K1 =

    jika harga ini dilogaritmakan, maka didapat persamaan :

    log k1 = 2 log p. so23 log p . o2

    pada temperatur tertentu harga k dapat diperoleh dari persamaan :

    g0 = - 4. 575 t log k

    g0 = harga energi bebas standart (lihat

    table)

    t = temperatur

    log k1 = , jadi log k dapat dihitung

    A2MO.P

    2SO2

    A2MS.P

    3O2

    P3O2

    P2SO2

    4.575 T

    G0

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    maka dari reaksi (1)(8) dapat ditulis persamaan :

    2 ms + 3 o2 = = = 2 mo + 2 so2 (1)

    ms + 2 o2 = = = mso4.. (2)

    ms + o2= = = m + so2.. (3)

    2 s + 2o2= = = 2 so2. (4)

    2 so2+ o2= = = 2 so3 (5)

    4 mso4= = = 2 mo.mso4+ 2 so2+ o2 (6)

    2 mo.mso4= = = 4 mo + 2 so2+ o2..(7)

    2 mo = = = 2 m + o2. (8)

    (1) log k1 = 2 log p.so23 log p.o2

    (2) log k2 = - 2 log p. o2

    (3) log k3 = log p. so2lo p. o2

    (4) log k4 = 2 log p. so22 log p.s2log p. o2

    (5) log k5 = 2 log p. so32 log p. so2log p.o2

    (6) log k6 = 2 log p. so2+ log p. o2

    (7) log k7 = 2 log p. so2+ log p. o2

    (8) log k8 = log p. o2

    dari reaksi :

    a.

    terlihat bahwa komposis gas campuran ditentukan oleh tekanan parsial, dua

    diantaranya adalah komponen gas-gas dalam system.

    b. untuk komposisi gas tertentu, maka komposisi fasa padat yang stabil akan tertentu

    pula, oki daerah masing-masing komposisi padat yang stabil dapat digambarkan

    diagramnya dengan tekanan parsial dua komponen gas sebagai koordinat (log p. so 2

    dan log p. o2).

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    c. pada diagram kellog persamaan kesetimbangan (1) (3) dan (6) (8) merupakan

    garis lurus, sebagai pembatas stabilitas komponen padat sesuai reaksi yang

    bersangkutan.

    untuk persamaan (4) dan (5) masing-masing mewakili reaksi pembentukan so2, so3

    untuk temperature tertentu, hubungan antara log p. so2dan p. o2juga tergantung harga

    p. s2 atau p. so3.bila p. so2 dan p. o2 membesar, maka p. so3 juga akan membesar.

    harga p.s2akan membesar bila p.so2besar & p.o2kecil.

    d. bila titik a berada pada daerah ms merupakan keadaan awal dengan p.so2 dan p.o2

    tertentu, sedangkan p.o2diperbesar p.so2tetap. maka akan didapat komposis mo. jika

    p.o2diperbesar lagi akan didapatkan mo.mso4

    catatan : p.s2 = 1 atm (pers.4)

    p.so3 = 1 atm (pers.5)

    (5)

    (6)

    (2)(4)

    (1)

    (3)(8)

    MO.SO4

    MSO4

    +

    0

    -

    0 +-

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    Pyrometallurgy

    Pirometalurgi adalah Suatu proses ekstraksi metal dengan memakai energi panas. Suhu yang

    dicapai ada yang hanya 50o250o C (proses Mond untuk pemurnian nikel), tetapi ada yang

    mencapai 2.000o C (proses pembuatan paduan baja). Umum dipakai hanya berkisar 500o 1.600o C ; pada suhu tersebut kebanyakan metal atau paduan metal sudah dalam fase cair

    bahkan kadang-kadang dalam fase gas.

    Umpan yang baik adalah konsentrat dengan kadar metal yang tinggi agar dapat mengurangi

    pemakaian energi panas. Penghematan energi panas dapat juga dilakukan dengan memilih

    dan memanfaatkan reaksi kimia eksotermik (exothermic).

    Sumber energi panas dapat berasal dari :

    1. Energi kimia (chemical energy = reaksi kimia eksotermik).

    2.

    Bahan bakar (hydrocarbon fuels) : kokas, gas dan minyak bumi.3.

    Energi listrik.

    4. Energi terselubung/tersembunyi (conserved energy = sensible heat), panas buangan

    dipakai untuk pemanasan awal (preheating process).

    Peralatan yang umumnya dipakai adalah :

    Tanur tiup (blast furnace)

    Blast Furnace

    http://www.anakunhas.com/2011/11/penjelasan-pirometalurgi-pyrometallurgy.htmlhttp://www.anakunhas.com/2011/11/penjelasan-pirometalurgi-pyrometallurgy.html
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    Reverberatory furnace.

    Sedangkan untuk pemurnian Pirometalurgi dipakai :

    Pierce-Smith converter.

    -

    Bessemer converter.

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    -

    Kaldo cenverter.

    -

    Open hearth furnace.

    http://www.anakunhas.com/2011/11/penjelasan-pirometalurgi-pyrometallurgy.html

    http://www.anakunhas.com/2011/11/penjelasan-pirometalurgi-pyrometallurgy.htmlhttp://www.anakunhas.com/2011/11/penjelasan-pirometalurgi-pyrometallurgy.htmlhttp://www.anakunhas.com/2011/11/penjelasan-pirometalurgi-pyrometallurgy.html