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Ferrosilicon productiion and submerged arc furnace

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Ferrosilicon productiion and submerged arc furnace

  1. 1. Ferrosilicon and submerged arc furnace A simple preview
  2. 2. Course preview The aim of this course is to present the basic principles for the production of ferrosilicon alloys processes ,and a simple preview for submerged arc furnaces. It is attended for Engineers working on this field ,and students or metallurgists maybe find answers for their questions, because at the end of this course you will figure out that you have a preview for this manufacture and the main problems and solutions, Metallurgist/ Nasser Harby
  3. 3. First Module Introduction What Is ferroalloys? Fig (1) Ferroalloy is an alloy of iron with some element other than carbon. Ferroalloy is used to physically introduce or "carry" that element into molten metal, usually during steel manufacture. the main task of the ferroalloys industry is the primary recovery (reduction) of needed metals from natural minerals. Historically, the ferroalloys production technology used in the 19th century was developed for blast furnaces (high-carbon ferromanganese, low-grade ferrosilicon), as at those times it was the main route for cast iron processing. However, in a blast furnace it is not possible to produce ferroalloys with elements that have a higher affinity for oxygen or with low carbon content..
  4. 4. This led to the development of ferroalloys to be manufactured (smelted) in electric furnaces at the beginning of the 20th century . Ferroalloys have been developed to improve the properties of steels and alloys by introducing specific alloying elements in desirable quantities in the most feasible technical and economic way. Ferroalloys are namely alloys of one or more alloying elements with iron, employed to add chemical elements into molten metal. Not a single steel grade is produced without ferroalloys
  5. 5. Ferroalloys classification • First classification :-Ferroalloys are usually classified in two groups: bulk (major) ferroalloys (produced in large quantities) • minor ferroalloys (produced in smaller quantities, but of a high importance). Bulk ferroalloys are used in • steelmaking and steel or iron foundries exclusively, whereas the use of special ferroalloys is far more varied. About 85% to 90% of all ferroalloys are used in steelmaking; the remaining ferroalloys are used for nonferrous alloys (e.g., those that are nickel or titanium based) and by the chemicals industry.
  6. 6. •Second Classification as Slag and Slag-Less Processes • For the slag-less process, ferroalloys are smelted when the amount of slag formed is small (3% to 5% by weight of the metal), such as in crystalline silicon smelting, and ferrosilicon (FeSi) and ferrosilicochrome (FeSiCr) processing. This slag forms from small quantities of oxides, ores, and concentrates in the coke ash that were not reduced during the heat. • Slag processes, on the other hand, are accompanied by the formation of a considerable amount of slag. The amount of the slag can be 120% to 150% of metal mass smelting of high-carbon ferromanganese (HC FeMn) and ferrosilicomanganese (SiMn) and 200% to 350% for low-carbon ferrochrome (LC FeCr).
  7. 7. •Third Classification of Ferroalloy Processes by Technological Features •Continuous and Batch Processes • Ferroalloy processes are divided into continuous and periodic. Continuous processes are characterized by • continuous loading of the charge and periodic (or continuous) slag and ferroalloy tapping. The charge is in the furnace at a certain level throughout the process. The electrodes are immersed in a charge continuously. The • furnaces used for these processes usually have high power (>16 MVA) and the reducing agents are carbon • materials (coke, char, charcoal, anthracite coal). • Batch processes use a certain amount of charge material for the same heat. The charge loaded into the furnace is • completely melt, leading to the reduction of the elements. The products are released periodically (metal and • slag tapping), most often at the same time.
  8. 8. • Fourth classification using Flux and Fluxless Processes • Fifth Classification of Ferroalloy Processes by Reductant Type :- • Reduction by Carbon (Carbothermal Processes) • Reduction by Silicon (Silicothermal Processes) • Reduction by Aluminum (Aluminothermal Processes)
  9. 9. Silicon applications 1- Deoxidation and alloying of steel and cast iron 2-alloying of other metals especially aluminum 3-Raw materials at chemical industry 4-Raw material for semiconductor industry
  10. 10. Figure 2 Examples of usage of silicon and ferrosilicon alloys.
  11. 11. What is Submerged arc furnace ? The smelting of ferroalloys is commonly performed in electric arc furnaces., To create the required high temperature and low oxygen environment, carbon electrodes are inserted into a mixture of ore, flux, and carbon reductant in an electric arc furnace. The low-voltage, high-current arcs at the electrode tips create a zone of high temperature and low oxygen potential. No oxygen is blown into the furnace for the purpose of raising the temperature. Radiation from the arc zone impinges directly on the feed material, allowing the efficient transfer of energy from the arc to the feed material. As the arc is considerably shorter than that of an open-arc furnace, the bulk of the energy is transferred to the feed material by resistance (Joule) heating due to the flow of electric current through the furnace contents.
  12. 12. Figure 3. Typical submerged arc furnace design. cooling of the molten metal.
  13. 13. Oxygen from the metal oxide combines with the carbon in the feed material to form carbon monoxide, liquid metal, and slag. In submerged arc processes, there is novisible arc as the electrodes are immersed through the charge in the furnace so we called it Submerged arc furnaces A next photograph of a furnace in operation showing the electrodes passing into the furnace charge
  14. 14. FIGURE 4 The furnace in operation showing the electrodes passing into the furnace charge.
  15. 15. Module 2 Raw materials The right choice of raw materials is perhaps the most important factor for obtaining good furnace operation the first step towards improvements is to reduce the variations. Frequent changes in the mix order make it difficult for the metallurgists to interpret measurements and observations from the furnace. We also know that choice of electrical setpoint and carbon. Balance may depend on the type of raw materials used. Frequent changes in the mix order therefore reduce the possibility for furnace optimisation. The next step is to choose the best raw material for the actual process, taking both technical parameters and price into account.
  16. 16. The reactants The reactants consist of metallic ores (ferrous oxides, silicon oxides, manganese oxides, chrome oxides, etc.) and a carbon-source reducing agent, usually in the form of coke,charcoal, high- and low-volatility coal, or wood chips. Limestone may also be added as a flux material. Raw materials are crushed, sized, and, in some cases, dried, and then conveyed to a mix house for weighing and blending.Conveyors, buckets, skip hoists, or cars transport the processed material to hoppers above the furnace The mix is then gravity-fed through a feed chute either continuously or intermittently, as needed. At high temperatures in the reaction zone, the carbon source reacts with metal oxides to form carbon monoxide and to reduce the ores to base metal.
  17. 17. • Silicon source For silicon and ferrosilicon production both quartz and quartzite are used as siliconoxide source . The ore body containing the mineral quartz and smaller amounts of gangue is currently referred to as quartzite The process requirements are related to size and strength, the thermal strength is so important as it affect the gas flow in the charge if to much fine quartz is present. Undersized materials is normally unwanted and may be sieved and removed before fed to the furnace. The normal size of quartz varies from 10 to 150 mm while in practically for good reduction process the better size is between 35 to 100 mm.
  18. 18. Quartz chemical analysis • Table (1) shows chemical analysis for two kinds of quartz A and B we need to know Quartz Chemical Analysis Sio2 Al2O3 Fe2O3 TiO3 S A 98.8 0.5 0.8 0.02 0.05 B 97.7 2.0 1.0 0.1 0.2
  19. 19. Carbon source • The choice of carbon reduction material is based on the following three main factors: process, product, and environmental considerations. The material must have the properties needed to achieve a high Si yield in the furnace and it must meet the product specifications. This means that the conditions for impurity elements in the raw materials (as, e.g., phosphorus) must be met. • Reactivity of carbon reducers to SiO and degree of conversion on the SiC carbide are directly affecting the silicon recovery as well as the efficiency of the ferrosilicon process. In addition to economic considerations should take into account of a number of mutually exclusive conditions resulting from the silica reduction process and technological conditions: Cfix content and the stoichiometry of the chemical reactions, quality requirements on the chemical composition of the ferrosilicon: Al, Ti, P, granulometric composition, other technological and organizational conditions.
  20. 20. Reactivity One of the most important parameter and it could defined as the ability of a carbon source to react with the SiO-gas from the crater according to the reaction : Sio + 2 C = SiC + CO According to sintef test this done by moving a gas through standard amount of of graded carbon material . Every type of carbon has a different reactivity number Low number(ml Sio) indicates high reactivity in the carbon.
  21. 21. Reduction material quality • Both size and reactivity of the carbon used will affect the process performance. • The size of carbon vary from 1mm to 30 mm (recommended size for coal 3 to 15 mm,and for semi coke 4 to 18 mm) • we have metallurgical size, we should keep watch at charging process. The specific size is increased by decreasing the size of the material. Due to the high gas velocity of the furnace the smaller particles may be carried into the gas outlet. This may lead to losing control with the amount of carbon to the process ,the critical minimum size for charcoal particles can be up to 1.5 mm to avoid such problem
  22. 22. Table (2)
  23. 23. The iron source • Metallic iron as scrap or iron oxide ore are used as the iron source for • making ferrosilicon • While iron scrap reduce the need for carbon and electrical energy, • iron oxide may be easier to control and better suited for automatic • raw material handling equipment .
  24. 24. Module 3 Ferrosilicon production • Charging process • The raw material is transported from the weighting system by conveyors elevator • Some plants use chutes system to charge the furnace. • Other plants or factories use another dose system at charging the oven(furnace) they are using bucket car as a machine, the driver of the machine used to full the furnace with charge around the electrodes and to the center • For low capacity furnaces may the workers feed or charge the furnace with shovels with their own hands. • The charge around electrodes usually higher a little than the center of the furnace. The charging material should be near the electrodes about 40 cm as it is the active area to heat the charge .
  25. 25. Fig (5) Using chutes at the top of the furnace for charging process
  26. 26. Due to the slag forming the size of the oven shrink or decrease so the distance between the electrodes to outside the diameter decrease and collapse of the charge time increase. Some opinion At the charging process - average cycle time- we have collapse-accumulation. The time of this cycle should be natural. we load the charge evenly, every hour about 5.0-5.2 tons of mix raw materials Other opinion at charging ,the main thing is not to push the mixture into the furnace, but to wait until the mixture itself begins to collapse into the furnace under its own weight..
  27. 27. It is important to start the tapping of metal from the furnace at the time of maximum accumulation. Before the next collapse begins. That is, at maximum current strength. In this case there will be no obstacles to the tapping of metal from the furnace . At control room the operator on the computer screen has information about the weight of the charge in the hopper above the furnace, It is necessary to record this weight. stabilizing the load will positively affect the stability of the sintered electrode and this reduced the risk of mechanical breakage of the electrodes.
  28. 28. Fig (6) Charging by machine car
  29. 29. Fig (7 ) Ready for stoking process
  30. 30. Stoking after charging:- after charging we use stoking machine to organize the charge around electrode 40 cm nearby and to push it through the furnace center ,may be also it be useful at unreacted charge or crust deal with .
  31. 31. FERROSILICON SMELTING TECHNOLOGY Basic Principles of Operation ferrosilicon are produced carbothermally in electric arc furnaces. Although the production is relatively simple in principle, the industrial process, which needs to be safe, environmentally sound, and effective, is much more complicated. As a high temperature is needed for production, silicon is produced in electric melting furnaces. This is the heart of the smelting plant. As shown in Figure (8), the plant consists of the following production units in addition to the furnace: 1. The mix unit where the raw materials are weighed and mixed before they enter the raw material silos. 2. The electrical system giving energy to the furnace 3. The off-gas units that cool down and remove the off-gas from the furnace 4. The processing units where produced liquid silicon is refined cast and crushed 5. The electric energy is fed into the furnace through three electrodes, which are slowly consumed during production.
  32. 32. Fig (8) The schematic of the silicon smelting plant process. (From Schei et al., 1998.)
  33. 33. Reactions in the Furnace Electrical energy is fed into the process through electrodes. In AC furnaces for silicon and ferrosilicon smelting, the electric current flows from one electrode, through a gas cavity, down into the liquid bath, and up the next electrode. The gas cavity is formed around the electrodes’ tip, and there an electric arc is ignited. The arc itself is a high-temperature plasma, up to 20000 K, and is the source of heating in the arc furnace, increasing the temperature of the charge materials to 1800 to 2000C The materials that now sink down through the furnace into the high temperature area will consist of SiO2, C, SiC, and some Si. Pure silica melts at 1723C forming a viscous liquid. In the high temperature area (~2000C), The mixture of SiO2, SiC, and condensate behave as a bridge, stopping the further descent of raw materials.
  34. 34. FIGURE (9) Overview of zones in the furnace around electrode
  35. 35. When there is overpressure, internal avalanches and gas channels may be formed, leading to high off -gas temperatures and high SiO content. The temperature of the top of the charge could be as high as 1300C, but it is very dependent on how much SiO will be condensed to Si and SiO2 also be of importance for the rate of the reactions that take place. The SiO condensation can take place close to the crater walls where the estimated temperature is 2000oC. Of note, two cavities, one for the arc crater and one built up by condensate, can be found in even larger furnaces or furnaces with other temperature distributions Below this region it will therefore develop a void around the electrode. In the bottom of this void, there will be a liquid silicon pool containing SiC particles. From the bridge of liquid SiO2, drops of SiO2 will fall down in the pool and react with SiC to form liquid Si and SiO gas. Heat is generated from electric current forming an arc between the electrode and the liquid silicon pool. In the cavity, a slight overpressure of 1 to 1.04 atm is measured
  36. 36. Smelting Reactions • The production of silicon can be described by the overall reaction: SiO2+2C=Si+2CO (1) • Reactions in the furnace are, however, more complex and generally schematically classified into taking place in either the hot (T>1811ºC), lower part of the furnace or the less hot (T<1512ºC) upper part of the furnace(1). Hot zone reactions involve the production of silicon metal, in addition to SiO and CO gases according to: 2SiO2(s,l) +SiC(s)=3SiO(g)+CO(g) (2) SiO2(s,l)+Si(l)=2SiO(g) (3) SiC(s)+SiO(g)=2Si(l)+CO(g) (4) • SiO gas is carried by CO gas and recovered in the upper part of furnace through condensation or reactions forming silicon carbide: • SiO(g)+2C(s)=SiC(s)+CO(g) (5) • 2SiO(g)=SiO2(s,l)+Si(l) (6)
  37. 37. The reduction of iron oxide to metallic iron takes place in the upper furnace zone, primarily through reactions with CO and volatiles produced in the lower part of the furnace. Carbon-saturated iron droplets in the furnace may also dissolve silicon through reactions with SiO gas according to: SiO(g)+C=Si+CO(g) (7)
  38. 38. Materials demand and energy Typical Materials and Energy Demand for Smelting of Ferrosilicon in Closed Furnaces (per one basis ton of alloy) FeSi Alloy FeSi20 FeSi25 FeSi45 FeSi65 FeSi75Al1 Quartzite, kg 370 552 931 1568 1930 Iron chips, kg 810 780 658 343 250 Coke, kg 200 280 438 720 845 Electrode paste, kg 10 8 16 43.3 54 Electric energy, MWh/t 2.1 2.7 4.8 7.4 8.8 Silicon yield, % 94 e 95 97 e 98.5 98 e 99 92 e 94 91 e 93
  39. 39. Signs of good silicon/ferrosilicon furnace operation progress are:- (1)Uniformly charge flow in all areas, without zones of over-sintered charge ; (2)A deep position of electrodes; (3)Positive gas pressure; (4)Optimal temperature on the charge top (for ferrosilicon this could be below 500 to 600C); (5)Hydrogen content of the off-gas <5%, oxygen <1%; and (6) A constant off-gas flow rate. Too-high gas pressure is often due to a lack of reducing agent and the formation of large quantities of SiO.
  40. 40. Tapping process Now we are ready to open the taphole to receive the molten metal, metal is tapped periodically from the metal tap holes into a refractory lined launder, which discharges into a ladle. The tap hole blocks are water-cooled copper elements into which the refractory of the tapping channel is set. This configuration extends the campaign life of the tapping channel. The metal tap holes are opened and closed by means of tap hole drill and clay gun, mounted on rails above the tap holes. Also tapping process maybe manual Opening by steel rods and closing tap hole by clay manual Slag often with metal or at the end of the heat and ,with small quantity . • Tapping can be continues or discontinuous the real difference is moderate
  41. 41. Manual tapping process video
  42. 42. CASTING PROCSESS After the tapping process ,we have a ladle full with molten metal-of ferrosilicon- now we are reach the casting process . Several ways of cast ferrosilicon are in use today the most casting principles are :- Casting into beds made of silicon materials fine Casting into iron moulds Layer casting Granulation on water
  43. 43. Figure (11) Rotating casting machine
  44. 44. Rotating casting machine video
  45. 45. Figure (12) :-Casting by winch
  46. 46. Segregation Phenomena Although the liquid Si/FeSi has a homogeneous distribution of trace elements, they will segregate (segregation happened because of the difference at the density between the the alloy elements) during solidification due to their partition ratios (segregation coefficients) between solid and liquid phases. The Solubility of the elements is in most cases lower in the solid phase than in the liquid phase. Segregation phenomena happens basically at (45 or 65 ) % FeSi . Note :- Original charge in FeSi smelting furnaces, by briquetting, pelletizing, or remelting in induction furnaces.some technologies have been developed and implemented for briquetting ferrosilicon fines.
  47. 47. Module 4 Submerged arc furnace • The design of a submerged arc furnace depends on its application. In principle, it is a dished vessel (Figure brick lined, as with the arc furnace. Fig (14) But there the similarity ends: the dish and the roof are axially fixed, although the roof, together with the electrodes, may rotate. The furnace is charged through ports in the roof, and molten metal and slag flow from the furnace continuously from the bottom of the furnace. The electrodes are of the Soderberg type, formed in situ by pouring a mixture of pitch and tar plus anthracite into a steel tubular shell. The process is carried out several meters above the furnace, and as the electrode is lowered, it bakes, so driving off the volatile binding. By the time it enters the furnace it is a solid mass. Electrodes capable of carrying very high currents up to 120 kA, can be produced in this way.
  48. 48. • Furnace Parameters Furnace Parameters Parameter Options Overall shape Circular or rectangular Power supply AC or DC Arc type Open, submerged, shielded, brush, resistance (no arc) Number of electrodes ( AC furnaces ) Single phase: 2 Three phase: 3, 6 (three pairs), 6 (2,3) Number of cathodes ( in DC furnaces) 1 , 2 Roof Gas-tight (hermetic), semiclosed, open Sidewall cooling Natural convection, forced air convection, water film cooling, water spray cooling, water-cooled copper elements
  49. 49. Fig (18) Types of furnace arc
  50. 50. Smelting ferrosilicon furnace Examples of the Furnace Parameters for Smelting Ferrosilicon Parameters Furnace A Furnace B Furnace C Furnace power, MVA 27 40 80 Hearth depth, mm 2900 3500 5000 Hearth diameter, mm 6800 8700 11600 Electrodes diameter, mm 1400 1500 1900 Electrodes current, kA 85 103 172 Power factor 0.93 0.91 0.89 Electrical efficiency 0.91 0.915 0.89
  51. 51. Parameters of ferroalloys furnace design
  52. 52. Electrical design Traditionally, smelting furnaces have been three-phase AC furnaces, although DC furnaces are increasing in popularity. The most common furnace type for ferrosilicon (FeSi), ferromanganese is a circular AC furnace with three electrodes arranged in a triangular configuration. Electric current is introduced into the furnace by three electrodes. The electrodes receive power from a single three phase transformer or three single-phase transformers. The design of an AC submerged arc furnace is based on the electrical power required to produce the required tonnage of ferroalloy
  53. 53. The required tonnage of alloy per year is decided based on the available ore and other economic factors. The specific energy requirement (SER) of the process (in kWh/ton) is determined by the process and the ore characteristics and is usually obtained by calculating a mass and energy balance. When the SER is known, and the required annual alloy production is known, the power (MW) rating of the furnace is determined by multiplying the kWh per ton by the number of tons per hour. Once the power (MW) rating of the furnace is known, the other parameters may be determined.
  54. 54. A number of empirical methods have been developed to describe the relationship between the various parameters such as electrode diameter and furnace resistance. The famous one is developed by Westly (1974) a different approach to sizing furnaces and determining favorable electrical operating conditions. This method finds greatest acceptance in slagless (flux-less) processes such as silicon metal and FeSi production. Based on the performance of operating furnaces, he showed that Iel =C 3 P2/3 (1) • Where Iel is the electrode current in kA, C3 is the factor appropriate to the particular smelting process, and P is the total power (VA) rating of the three-phase furnace in MVA. For example, for FeSi 75% ,typical Value of C3 is approximately 10.5. The optimum electrode current for a 30 MVA furnace producing a 75% Si product would be thus Iel = 10.5 ( 30)2/3 = 101.4 kA. From equation (1), the electrode-to-bath (phase) voltage in kV for the same case phase voltage would be Vph = (30)1/3/(3*10.5) =0.0986 kV = 98.6V.
  55. 55. Fig (20) ampere and voltage
  56. 56. AC Furnaces The incoming voltage is reduced to that required by the AC furnace by means of a single three-phase, step- down transformer or three single-phase, step-down transformers. Voltage changes on the secondary side of the transformers are made using on-load tap changers AC furnaces may be circular or rectangular. Most AC furnaces are powered from a three-phase electrical supply. The most common furnace type for ferrochrome, ferromanganese, ferrosilicon, and silicon metal is a circular AC furnace with three electrodes arranged in a triangular configuration. current in an AC furnace passes through an electrode, through the arc, and into the conductive material in the bath, predominantly the molten metal, from where it passes into the other electrodes to close the circuit. Different arc configurations and operating modes are possible, based on the electrical properties of the feed material and slag
  57. 57. DC furnaces DC furnaces are increasing in popularity for some processes. A single carbon electrode similar to those used in AC furnaces forms the cathode. Due to the high current density, the cathode is usually a prebaked graphite electrode. Soderberg (self-baked) electrodes are not used. The anode is embedded into the lining of the hearth. A DC furnace usually has a relatively long open arc, operating in a similar manner to a steelmaking electrical arc furnace. A DC arc furnace is powered by a single transformer connected to the utility which powers the 33 MV bus, in turn transferring power to the furnace power supply and auxiliary loads. A vacuum circuit breaker is used to feed power to two rectifiers through a furnace transformer. The rectifiers produce the resulting DC voltage. Reactors ensure low ripple content in the DC current, providing arc stability and reducing harmonic levels.
  58. 58. Compression of AC & DC arc furnace Furnace type AC DC Basic system Based rectangular (3 or 6 electrodes in-line) Based circular Based circular furnaces (single or twin electrode) Process type Open design Semi-closed design Closed design Open design Semi-closed design Closed design Pre-baked/graphite electrodes Soderberge or self-baking electrodes Prebaked electrode Number of electrodes Single phase: 2 Three phase: 3 , 6 (three pairs), 6 (2,3) 1 , 2
  59. 59. Figure (19) :- Roof section of a DC smelter
  60. 60. Mechanical design The shell holds the refractory lining together and provides a certain degree of resistance to thermal expansion of the crucible. The furnace crucible is designed to limit the heat losses from the furnace, and maintain a safe refractory and shell temperature. to achieve the balance, the shell may be cooled. Water and air are the most common cooling media, although oil may also be used. The use of oil carries a fire risk, however. The shell floor plates are supported on a set of steel rail sections or I-beams resting on the foundation, which consists of concrete plinths spaced at intervals to allow forced air cooling of the floor plates.
  61. 61. Fig (20) Shell, rotating mechanism, and tap hole fume extraction ducting of a silicon metal furnace.
  62. 62. Electrode spacing hand drawing
  63. 63. Fig (21) Open submerged arc furnace
  64. 64. Fig (22) Flexible water-cooled copper conductors at the electrodes.
  65. 65. Fig (22) Ordinary steel ring
  66. 66. Fig (23) Semi closed roof on a FeCr furnace.
  67. 67. fig (24) Closed roof on a FeCr furnace
  68. 68. Fig (25) The shell of a DC furnace
  69. 69. Electrodes The electrode column provides the path through which electrical power is transferred from the switchgear to the material in the furnace. Current passes into the furnace from the transformers through bus bars or water-cooled bus tubes into a number of copper contact shoes placed around the circumference of the electrode. The contact shoes are clamped onto the electrode by means of a pressure ring designed to force the contact shoes against the electrode in order to provide good electrical contact for the transfer of current. The electrodes extend through the roof of the furnace and are submerged in the raw material inside the furnace . The electrode column includes a supporting structure for the electrode and a clamping mechanism. The clamping mechanism may be hydraulically or pneumatically powered or it may be connected to a winch. During operation, electrodes may periodically be moved up or down by a few centimeters during operation to assist in the control of the furnace by increasing or decreasing the bath resistance, an activity termed regulating. This is achieved by raising or lowering the clamp assembly.
  70. 70. FIGURE (22) Zonal structure of the “liquid” paste; III, coking zone; IV, backed electrode (with four subzones); 1, electrode; 2, contact shoe; 3, shield; 4, electrode shell
  71. 71. Typical Properties of Soderberg Electrode Paste Typical Properties of Soderberg Electrode Paste Property Value Unbaked Paste Density 1600 kg/m 3 Volatile content 12 e 16 % Softening point 50 e 80 C Melting point 100 C Ash < 8.5 % Thermal conductivity 2.5 W/mK Baked Paste (baked at 1000) Density > 1350 kg/m 3 Electrical resistivity (room temperature) 70 mU m Electrical resistivity (1000 c) 40 mU m Thermal conductivity 8 W/mK Crushing strength 20 MPa Bending strength 3 e 5 MPa Maximum current density 7 A/cm 2
  72. 72. As the electrode is carbonaceous, it is consumed in the furnace. To compensate for this erosion of the electrode, it is fed into the furnace by a process called slipping. During this routine activity, one electrode clamp is released and the electrode, fully gripped by the other clamp, and is moved up or down by a precise amount, slipping it through the open clamp. Slipping is performed at regular intervals. The operating resistance of the furnace is determined by the furnace contents as well as the height of the electrode tip in the furnace. In a submerged arc furnace, the reaction zone tends to form near the electrode tip. With a short electrode, the reaction zone will be higher in the furnace than when the electrode is long. Likewise, moving the electrode up or down will cause short-term variations in conductivity, but these conductivity variations tend to disappear as the reaction zone reestablishes itself around the new electrode tip position. The optimal electrode type for any application is determined by requirements such as the required current density, the electrode diameter, and capital and maintenance costs. The electrical conductivity and diameter of an electrode determine its current carrying capacity.
  73. 73. Furnace Lining The material of construction of the lining varies from process to process and is determined by considerations such as the operating temperature of the process and the chemistry of the contents of the furnace. Refractories are sophisticated technical ceramics that are tailored to suit the demands of high temperatures, thermal cycling, mechanical stresses, and chemical attack imposed by smelting processes. Although linings are, to a certain extent, consumable items with a finite life, the choice of an appropriate lining and careful attention to installation details is an important aspect of furnace construction due to the cost of the refractory material combined with the cost of the plant downtime required to reline a furnace and the potentially catastrophic nature of a lining failure.
  74. 74. The most common constituents of refractory bricks and monolithic are magnesia (periclase), alumina (corundum), silica and chromite, in some cases in combination (e.g., chromia-magnesia bricks). Silicon carbide and carbon are also used. The materials used in various parts of the lining are chosen for their thermal properties as well as their resistance to thermal shock, resistance to wear, their mechanical strength, or their resistance to chemical attack by the processing. Lining wear is one of the major causes of furnace failure. These furnaces employ a carbon lining in contact with the molten metal on the hearth and sidewalls, with various grades of aluminosilicate bricks used to form an insulating lining between the carbon and the shell. In the slag zone, the lining may include graphite tiles bonded to the shell to provide a thermally conductive layer between the bricks and the shell, with carbon bricks and aluminosilicate bricks on the hot face.
  75. 75. Fig (18) cross section at furnace lining
  76. 76. During lining process inside the furnace
  77. 77. Furnace control The quality of the product and the life of the furnace crucible are enhanced when stable conditions are maintained within the furnace. A furnace operating in a stable manner exhibits very small variations in metal and slag temperature, electrode current, electrode resistance, and electrode immersion. In addition, the electrode currents and voltages are balanced. This leads to a more consistent quality of the product, consistent electrode quality, and a reduction in the number of electrode breaks Industrial process control systems are installed to automate the operation of the furnace and ancillary equipment. The plant is fitted with appropriate instrumentation to monitor and control the equipment. This instrumentation includes flow meters , thermocouples, level sensors, pressure sensors, limit switches, and comprehensive instrumentation of the electrical operating parameters of the furnace and its auxiliaries.
  78. 78. The output from these sensors is captured by a supervisory control and data acquisition (SCADA) system, which displays the state of each system on a computer screen in the control room and stores the data on a historian system, from where trends may be extracted. A programmable logic controller (PLC) is used to automate routine functions, such as feeding batches of raw materials, starting pumps, and shutting down systems. The PLC is also programmed to raise alarms when conditions exceed predetermined limits and to automatically initiate corrective actions such as shutting down a pump if the bearing temperature is too high or tripping the furnace electrical supply if the emergency stop button is pressed.
  79. 79. FIGURE (20) Typical screen for a PLC-based furnace control module Furnaco Status: On Furnaco Mode: Remote Primary Mode: MW General Furnaco Information Line Currents (A) Apparent Power (MVA) Power (MW) Power Factor Star / Delta Switch Current Imbalance (%) Power Setpoint (MW) Primary Current (A) Secondary Current (kA) Secondary Voltage (V) Load (MVA) Tap Position Current Setpoints (kA) 18.8 15.2 0.81 Star 8 15.0 45.0 110.4 99.2 112.9 Transformor Information Electrode Information Accumulative Power (MWh) Phase I 110 34.3 186 6.4 10 99 30.3 192 5.8 11 112 34.9 188 6.6 10 Phase 2 Phase 3 Current (kA) Resistance (mO) Power (MW) Position (mm) E1 E2 E3 62.2 1.23 4.8 547 54.5 1.54 4.6 630 55.6 Previous Tap Previous Shift Previous Day 375.36 100.40 349.18 This Tap This Shift Today 1.89 5.9 315 62.24 101.34 305.66 1000 1000 1000 200 200 200 Tap Furnaco Image Secondary Mode: Current
  80. 80. FIGURE (21) Typical screen for a PLC-based furnace control module
  81. 81. FIGURE (22) Typical screen for a PLC-based furnace control module
  82. 82. Module 5 Maximizing Furnace Production& Reducing Cost Operation at maximum power (at constant MWh/t) is the first step to ensure that the furnace throughput is maximized. As can be seen from Figure (23), the maximum power is obtained by operating as close as possible to the limits and at as high a resistance as possible. The maximum power obtainable from the power supply is therefore achieved when operating at operating point (OE) in Figure (24) (resistance = 1,20 mQ). At this point, the Power input to the furnace is maximized within the maximum tap position, maximum MVA, and maximum electrode-current limits. The electrically optimum resistance at this point does not usually correspond to the optimum resistance for the process as a whole.
  83. 83. FIGURE 23. Characteristic curves for a submerged-arc furnace
  84. 84. The operating point of a furnace can be identified on these curves, giving an indication of steps to be taken in order to maximize the power input while remaining within the constraints. The optimum resistance is generally lower, resulting in operation at, say, point OP in figure (24) (resistance = 0,95 mQ). A number of factors govern the selection of the optimum resistance for a process,, when it is shown that there is generally no merit in increasing the resistance above the electrically optimum resistance.
  85. 85. Power maximization can be achieved only by balanced operation of a furnace. in which the three electrode circuits exhibit similar impedances (maximum power transfer occurs when a three-phase load is balanced). From an operational point of view, this means that the furnace should be balanced at some optimum resistance. The transformers can then be tapped up to maximize power input without reaching the MVA or electrode-current constraints of one phase before those of the others. This type of operation ensures maximum power dissipation in each electrode, which is essential for maximum production. Maximizing furnace availability will also assist in maximizing production. Unstable and poorly controlled operation of a furnace often results in down-lime, and frequent switching out of a furnace aggravates this condition. The effective management of a furnace at a balanced resistance setpoint and within the operational constraints is therefore conducive to the stable and controlled operation required to maximize throughput.
  86. 86. Reducing Costs A reduction in the furnace MWh/t (which improves the energy efficiency) is the objective central to reductions in the marginal unit cost of production. Improvements of this nature have the additional benefit of increasing the production of a furnace at a particular power input. Experience has shown that, for a given process, the maintenance of electrode penetration at some predetermined maximum has a number of economic advantages. Electrodes penetrating evenly at this predetermined maximum level will reduce energy losses from the top of the furnace burden, thus reducing the cost .
  87. 87. From an operational point of view, this implies a balanced operation at the corresponding minimum MWh/t electrode-to-bath resistance (point OL in Figure 24, with resistance = 0,67 mQ). Deep, balanced, and consistent penetration results in the development of regions of high power density beneath the electrodes, and in optimal heat transfer from the hot furnace gases to the burden. These conditions usually boost the reduction of the oxides that require higher temperatures and larger amounts of energy. In some cases, these reduced materials are required to be of a high grade in the product from the furnace (e.g. a high percentage of silicon in the product of a Ferro-silica-chromium or Ferro-silicomanganese process).
  88. 88. Module 6 Some operations problems Electrode breakage :- Hard electrode breaks may be caused by a number of factors: 1. Thermal stresses or thermal shock, where the surface of the electrode is exposed to a sudden change of temperature, which creates a severe temperature gradient through the thickness of the electrode. This may lead to cracking and spalling of the electrode. 2. Underbaking, where the paste is inadequately baked and the electrical current carrying capacity of the electrode as well as its mechanical strength is inadequate. 3. Overbaking, where the paste dries out too much during baking, leading to a brittle electrode 4. Unbalanced lateral mechanical forces on the electrode due to asymmetric feed piles 5. Paste segregation, where the anthracite particles and the binder separate before baking. This leads to a mechanically weak and brittle electrode, as there is nothing holding the anthracite particles together. Note:- So you have to notice slipping rate and shutdown time
  89. 89. Fig (24)Electrode Breakage due to paste segregations
  90. 90. Deposits in the furnace:- Formation of the deposit makes difficulty in ferrosilicon production process ,there are three main types of the deposits :- Incompletely converted charge materials (slagging) at the bottom Sic with Si at the bottom Sintered charge materials in the upper layers of the furnace (Crust ) If the furnace produce slag ,this is because the heating is incomplete to avoid slagging in the silicon furnace It is necessary to control the ratio between heating and the of the charge flow to inner zone . Si-Sic deposits can be removed by access of Sio2 in the charge Normally the charge can be broken down around the electrode during the stoking operation, and a ring shaped opening is formed around the electrode when the furnace is open 30-40 cm from the electrode Stoking and charging is easy and the gas distribution become uniform .
  91. 91. Tapping problems The tapping operation has two main problems Clogging of the tapping channel Several method are available to remove the obstacles such as penetrating with powerful mechanical equipment, burning with graphite electrode or blowing with oxygen . Gassing or blowing through the taphole Gassing happened during normal tapping as silicon evaporates from the molten metal ,gassing through the taphole require some overpressure in the inner zone of the furnace and an open channel from the overpressure through the tabhole and may this because of charging before tapping process, gassing is often related to lower electrode ,or lower carbon content Metallurgists should also notice lump size of the raw materials ,state of the taphole and silicon content of the produced alloy .
  92. 92. Bounce Websites furnace/?referralCode=2F584884ACA389531C06 My channel on rumble References Production of high Si alloys Anders Schei; Johan Kr Tuset; Halvard Tveit 1988 edition • Michael Gasik (Eds.)-Handbook of Ferroalloys. Theory and Technology-Butterworth Heinemann (2013) edition