RECARBURANT AGENTS FOR INDUCTION FURNACE IN SMELTING

Transcription

1 RECARBURANT AGENTS FOR INDUCTION FURNACE IN SMELTING 1. Aims. 2. Smeltings. 3. Types of ovens in smelting. 4. Types of recarburants. 5. Influence of constituents in carbonaceus materials. 6. Factors of influence in recarburation. 7. Selection of recarburants for the smelting. 8. Graphit pellets and coke of calcined petroleum. 9. Pelletization. 10. Practical tests. 11. Conclusions. 1.- AIMS. - Revising materials habitually used on the recarburation of the bath to obtain smeltings. - Concepts to keep in mind at the moment of selection of a recarburant depending on the process. - Let you know the latest developments for the revaluation of fines of coke and graphit (pellets of coke of petroleum and pellets of graphit). 2.- SMELTINGS. The steels and smeltings are alloys of Iron - Carbon. According to the diagram (Fe-C): Steels: Up too1.76 % Carbon. Smeltings: From 1.76 % to 6.67 % Carbon. In the industrial practice, it is only used the smelting between 2.8 % and 3.9 % Carbon. The smeltings are classified primarily by the type of graphit: lamellar, vermicular, nodular (malleable) and spheroidal, as well as by their mechanical characteristics and matrix. Page 1 of 19

2 Characteristics: They have got good quality to be used on machines (thanks to the graphit which acts as a lubricant, good resistance to heat, to oxidation, similar mechanical characteristics to certain steels in the case of spheroidals, etc. Applications: They are used among other applications to make pieces for automotion machinery, ornamental pieces, etc. Mechanical properties: - Traction resistance: It varies from 10 Mpa for a lamellar iron casting and 150 Mpa for a spheroidal ADI. - Plasticity: they vary from the practically zero of a lamellar casting up to the 20% of an ironing spheroidal smelting which has gone through a termical treatment of annealing. - Hardness: it varies from 130 HB in an iron spheroidal smelting up to 321 HB in a pearlitic spheroidal smelting, but it can reach up to HRc with a termic treatment of templer and drawback. 3.- TYPES OF OVENS FOR SMELTING. The most usual types of ovens employed are: Cupola furnace (of cold air, hot air and gas). Coreless induction furnace (of frequency of net, middle or high frequency). Electric oven of electrodes. Channel induction furnace. Rotary kiln (oxigas). CUPOLA FURNACE: Except for the gas oven, the fusion is made by heating provoked by burning coke on the metallic charge through the blowing of air (hot or cold). In this case, the coke makes the fusion, the reheating and the recarburation (depending on the composition of charge and march - oxidant or reducing). It can be considered a metallurgist oven. The metallic charge (above all the steel), is limited by the calorific power of coke, although it can be charged with some more steel in the case of hot air. CORELESS INDUCTION FURNACE: The metal melts thanks to the induction made by a coil on the metal to melt, which acts as the primary of a transformer being the metallic charge the secondary one. It is the most used type of oven to obtain smeltings nowadays. The metallic charge can be completely synthetic and it can be melted even without base of bath in the case of ovens of middle and high frequency. The recarburation can be made with any recarburant agent and the only difficulty appears in the shaking of the bath, which is bigger when the frequency is lower. That s why an oven of frequency of net 50 Hz will produce a better shaking than another one which works at 250 Hz frequency. Page 2 of 19

3 ELECTRIC OVEN WITH ELECTRODES: The fusion is made by an electric arc (it has three electrodes of graphit submerged in the charge or in the metallic bath). Its more habitual usage is for the smelting of steels and the obtention of iron alloys. CHANNEL INDUCTION FURNACE: The functioning is the same that the induction oven without nucleus, except that the coil is located in a channel through which the liquid iron circulates. The low level of movement in the bath makes this kind of ovens be used only for keeping the temperature of the liquid metal; furthermore, if buzza and stopper are added, it can be employed for the direct teeming. ROTARY KILN (OXIGÁS): The fusion is made by heating of the refractory with a burner that heats it. Due to problems in the process itself, which tends to decarburate, the recarburation must be made by a recarburant injection with nozzle, for this reason it is recommendable to realize a duplex process. The widespread duplex process is a practical process when the fusion oven does not guarantee a definite composition and/or it is not able to give steadiness of analysis, or that there is certain difficulty to adjust the composition to the previously specified one. The metal melts in an oven (fe. Cupola furnace, rotary) and afterwards it is taken to another oven, normally a coreless induction furnace, taking place in the latter the adjusts in compositions, among them the recarburation. 4.- TYPES OF RECARBURANTS. Industrially, the materials of charge of oven which can obtain a smelting are: - Lingot. - Steel scrap. - Returns (CASTING). - Iron alloys. - Recarburants. The recarburants are products capable of incorporating carbon to the iron in liquid state. There are a lot of types of materials with the possibility to be used as recarburants, but the most usual are: - Cokes (of petroleum, metallurgical, of acetylene, of pitch). - Natural graphit. - Synthetic graphits (graphit of electrodes, graphitized cokes). - Calcined hard coal. Some of the most used recarburant agents in smelting are commented below. Page 3 of 19

4 Coke of calcined petrol: It s obtained from the calcination, termical treatment between and º C, of green coke - solid material which consists of carbon mainly, volatiles and humidity - which is produced during the refinement process of fuel. The green coke has a high grade of harmful elements for the smelting. (Chart 1). FIXED CARBON % SULPHUR 0,3 2,5 % VANADIUM PPM VOLATILE 9 13 % ASHES 0,1 0,8 % SILICON 0,01 0,08 % SOLUBLE SALTS 0,2 0,8 % CALCIUM 2,5 500 PPM SODIUM 2,5 100 PPM WATER 2 4 % Chart 1: Chemical analysis typical of green coke. With the calcination it is obtained an increase in the Carbon content as well as a reduction of volatiles and humidity. The values of the remaining elements (sulphur, nytrogen.) will depend on the employed raw material, different origins/qualities of the fuel are the explanation in the differences of analysis in the cokes of calcined petroleum. Natural Graphit: It comes from the mine and its usage as recarburant has been located mainly in USA. Its employment has not spread due to the high content in ashes, which increases scums, because of its low content in Carbon and the variations in quality. Graphit of electrodes: The product which is used as recarburant for the smelting is the byproduct of the manufacturing of Graphit of Electrodes- which is generated during the works of cutting, turning, etc - which treated, classified and controlled correctly is one of the best materials to be used in the production of smeltings. To obtain electrodes of graphit it is habitually used coke of pin and resine of tar which, once comformed, are submitted to a first termical treatment- baking- to 1700ºC followed by a 2 nd termical treatment - graphitation- between 2800 and 3000 º C. With it, impurities such as volatiles, ashes, humidity, sulphur, nitrogen and hydrogen are eliminated. It s essential that the raw materials are of good quality in order to make sure that an exact balance between chemical, electrical, mechanical and termical properties is kept through the manufacturing process. When temperature is increased, the product is recrystallized depending on energy, making the atoms migrate within their own structure. When the structure is transformed in crystalline hexagonal similar to that of the natural graphit, it s called production of synthetic graphit. Page 4 of 19

5 Graphitized coke: Another recarburant agent which can be included within the category of synthetic graphits is the graphitized coke. It is obtained from coke of petroleum submitted to similar temperatures to those of graphitation in the production of electrodes of graphit. During this termical treatment, it is produced as in the previous case, a purification effect eliminating volatiles, ashes, sulphur, nitrogen and hydrogen. Type of Recarburant GRAPHITIZED GRAPHIT OF ELECTRODES NATURAL CRYSTALINE GRAPHIT OF CALCINED PETROL OF CALCINED PETROLEUM (low S) METALLURGICAL ACETYLENE PITCH Carbon (%) Humidity (%) Ashes (%) Volatiles (%) N (%) S (%) Chart 2: Chemical analysis typical in the recarburants. 5.- INFLUENCES IN THE CONSTITUENTS OF CARBONACEOUS MATERIALS. Carbon: Incorporationn: %. It s presented in two allotropic forms. The amorphous form (coke of calcined petroleum) only incorporates Carbon, but it does not influence in the metallurgical quality. The crystaline form does influence in the metallurgic quality, reducing the trend to tempering and to subcooling, that s why we talk about an improvement in the metallurgic quality. Ashes: In general, they make difficult the yield of Carbon, they generate more scums and reduce the yield of the refractory. Volatiles: They generate gases and smokes during the fusion and potentially they can provoke blowings in the pieces. Page 5 of 19

6 Humidity: It can provoke an explosion when it is incorporated to the liquid bath. It is a source of hydrogen that can provoke blowings in the pieces. Sulphur: It adds the 100 % of the content in the recarburant agent. In spheroidal smelting: : It is recommendable a content between and % in the base oven (before treating with Mg). According to some authors, it is advisable a minimum content to improve the metallurgic trend. In lamellar casting: It is advisable a content between and % in the base oven. The sulphur gets involved in the inoculation, below %. It is possible to find cementite in the pieces (depending on passivity). There are specific inoculants that contain sulphur. Nitrogen: Incorporation: 50 %. In general, the lower the better because of the defects that its presence brings, but we cannot forget that in lamellar smelting, the usually ordered pieces have a quality that varies from 220 Mpa to 300 Mpa, and to obtain the matrix, hardness and mechanical, we need a pearlitic matrix of fine grain, and to get it, a content in Nitrogen a piece between the 50 and 1000 PPM, has shown to be of help. 6.- FACTORS OF INFLUENCE IN RECARBURATION. It can be briefly explained the development of the recarburation following the process of dissolution. The main motor of this process is the gradient of concentration between the limit of the phase carbonaceus-smelting process and the interior of the bath of fusion. The more important the gradient, the more accelerated is the process of recarburation. From this, it can be deduced a relation of direct dependency between the speed of recarburation and the behaviour related to the solubility of the carbonaceous products. One of the most important facts for the smelter is the speed of dissolution and the yield of incorporation, since they influence directly in the productivity and in the costs. They influence in general in: The purity of the product. The size of particle. Agitation of bath. Type of oven. Form of charge. Temperature of bath. Content of ashes. Structure of recarburant (Isotropic, anisotropic). Page 6 of 19

7 Purity of product: In general, the higher the purity, the higher content in Carbon and that s why the content in ashes is lower, and the yield is better. When the content in ashes of the product is very high, the speed of dissolution is reduced exponently, due to the formation of a layer of ashes in the periphery of the grain which avoids the transfer from Carbon to the bath. Size of particle: For identical granulometries, the solubility of the different recarburants commented sets up a difference from one quality to the other since, as it is explained below, there is a dependency of behaviour with regards to the solubility of recarburant and its structure. The best granulometry of the recarburant is depends on the addition method. If the addition of the recarburant is made with the charge, there is no need of particular granulometries since there is plenty of time for the dissolution of Carbon. On the contrary, when the incorporation of Carbon is effected as a final correction/adjustment in the surface of the bath, the granulometry plays an important role if the carburant is too fine, floats on the surface of the bath, gets burned, the air of aspiration blows it away or if it gets mixed with the scums and it is taken out when the scum is skimmed off, and if the size of grain is too gross, the depth of penetration and consequently, the surface of reaction bath/carbon is too small, the recarburation is two slow and its efficiency is reduced. For a coreless induction furnace, the granulometries between 0.5 and 8 mm. are usually employed without any problem. For the graphit of electrodes, the best obtained yield is with a granulometry from 3 to 5 mm. And for the submerged nozzle of blowing in cupola furnace: 0 1 mm. / 0-3 mm. Agitation of bath: The higher the best. The best agitation is obtained with the ovens of frequency of net, but it worsens when we use ovens of Hz. Bath composition: The dissolution of Carbon is reduced in the bath due to the presence of elements, such as the Silicon, which has a higher affinity with the metal than the affinity showed by the Carbon. If you want to obtain a maximum yield, it is necessary to add the product of recarburation before adding the Silicon. It is recommended that before the recarburation, the Silicon will not be higher than this : from 1,5 to 1,8 %. The maximum level of Carbon admissible in dissolution depends on temperature, on the level of Carbon and on the level of the other elements present in the following formula: %C max. = * T(ºC) * Si (%) * P (%) * Mo (%) S (%) Page 7 of 19

8 Type of oven: ELECTRICAL OVEN FOF ARCH: The addition of carburant is made in the solid load, but if it is necessary to make a correction, we habitually make it through blowing in the bath or in spoon using a porous plug. CORELESS INDUCTION FURNACE: Recuperation between %, depending on the form of load and the way of operating, fullfilling of oven or reloads. For example, the addition of recarburant to º C gives us a yield of 85 % but if we elevate the temperature to 1525 º C, the yield is increased to %. CHANNEL INDUCTION FURNACE: Generally, little corrections are only made between 0.1 and 0.3% in order to keep a good level of nucleation, obtaining yields between %. CUPOLA FURNACE: Recarburant through blowing can be added between 1 or 2 %, obtaining a recuperation between %. ROTARY KILN: In general, it is employed the submersion nozzle and granulated recarburant, obtaining recuperations between %. Notes: The product of recarburation which is added in the blowing proceeding must have a good solubility. Therefore, it will be used only graphit of electrodes or products of analogous quality such us the graphitized coke, if you want to reach a high yield in Carbon. In blowing, the yield in Carbon is determined by the quality of the product in recarburation, the gas carrier and even the handling of nozzle. To blow, it is used compressed air, inert gases or natural gas. The best results have been obtained due to its reducer character. Form of load: For coreless induction furnaces of medium and high frequency, which have the capacity to start in cold, it is recommendable to introduce regular charges in the following order: Steel Recarburant The rest of steel Iron alloys Own returns and lingot For coreless induction furnaces of frequency of net, very used in recharges because it is necessary to maintain a foot of bath, it is recommendable to add first the graphit and submerge it with the scrap of steel, followed by the iron alloys and the returns. For channel induction furnaces: in general they are not used in fusion, but if an addition of graphit is required, it can be made through spear of immersion, keeping always in mind that it can damage the refractory one. For ovens of arch: In general the coal addition is made in the cold load following the order: Recarburant Steel (covering the recarburant) Own returns and lingot Iron alloys If it is required a posterior addition of recarburant, it can be made through nozzle of immersion. Page 8 of 19

9 Temperature: The higher the best (look at formula for maximum C). Content of ashes: The speed of dissolution is inversally proportional to the content of ashes. A recarburant of low content in ashes will be incorporated very quickly (the bend of time of incorporation will show an exponential form), on the contrary, for a recarburant of high content in ashes the same bend has a lineal form, which corresponds to a slower recarburation. Another disadvantage is that you must take into account during the employment of products with a high content of ashes in the recarburation, the negtive influence on the duration of refractory coating: The oxides contained in the ashes have an fundent effect. Structure of the recarburant (Isotropic, Anisotropic): There is dependency of the behaviour on the solubility of carburant and its structure. In general, we talk about crystaline structures, more or less defined, and about structures clearly amorphous. Graphits of hexagonal crystaline structures and other similar Carbons with important anisothopy are the recarburants with the highest speed of dissolution in the bath. The petroleum coke and the metallurgical coke are in the following according to their behaviour of anisotropy/isotropy. 7.- RECARBURANT SELECTION FOR THE SMELTING OBTAINED IN ELECTRICAL OVENS. REQUIRED CHARACTERISTICS (IN GENERAL): It must have an easy introduction in the bath. It must have a quick solubility. It must add seed crystal (good nucleation of bath). It must have the best yield possible. It must have good homogeneity. It must present steadiness in the analyses. REQUIRED CHARACTERISTICS (WITH REGARDS TO CHEMICAL COMPOSITION): Nitrogen: Required depending on application. Humidity, Hydrogen: The least content possible. Carbon: The highest possible. Volatile: The lowest possible. Ashes: The lowest possible. Sulphur: required depending on application, high in lamellar, low in spheroidal. Page 9 of 19

10 TYPE OF RECARBURANT GRAPHIT OF ELECTRODES CRISTALINE GRAPHIT (NATURAL) OF CALCINED PETROLEUM METALLURGIST PITCH INCORPORATION SCUM TREND TO THE TEMPERING OF THE BATH AVAILIABILITY VERY GOOD LITTLE LOW SCARCE BAD MUCH LOW HIGH GOOD LITTLE HIGH HIGH VERY BAD MUCH HIGH GOOD BAD ENOUGH HIGH GOOD Chart 3: Usual characteristics. 8.- GRAPHIT PELLETS AND OF CALCINED PETROLEUM. During all the manufacturing process, and particularly during the mechanized of electrodes of graphit, dust of graphit is obtained. It is also obtained dust of other recarburants- among them the coke of calcined petrol-, during the process of moulding -molturation, classification, and packing before the commercialization as recarburants. They are habitually considered as fines the fraction below 0.5 mm. This dust has two types of problems related to their usage in recarburation, one environmental and the other with regards to the yield. Among all the smelters is known the problem of using this size of grain in this way (as such), and that the yield takes a dive as long as some means as the nozzle of injection or similar are not used. It is easy to think that it would be useful to look for a formula which allows the revaluation and reusing of the product. Another justification is that the availiability of graphit of electrodes is ocasionally scarce, that s why the motivation to revaluate and reuse is high following a process, the pelletization - that transforms a problematic byproduct in a first quality product. MATERIAL RECEPTION GRINDING MILLING FINES CLASSIFICATION MIXING PACKING Diagram 1: Process of production of graphit and coke of calcined coke for recarburation. Page 10 of 19

11 9.- CONCEPT OF PELLETIZATION. The pelletization is defined as the production of uniform bodies by means of agglomeration from dustyaterials, granulars or cakes that have sometimes particles of very different sizes. The final products are pellets and the size of these grains is almost constant, however, the precision does not correspond with the precision of tablets and other agglomerated. The pellets can be, depending on the production of process, espherical, cubic or cylindric, with a particle diametre that varies between 0,5 and 50 mm. The limitations of the technic avoid pelleting very hard products and/or abrasive in an economical way, due to fretting /wear that the matrixes used in the pressing phase suffer, that s why products such as metallurgist coke are not often pelletized. The pelletization process has several stages: FINES OF PETROLEUM FINES OF GRAPHIT OF ELECTRODES FINES OF PETROLEUM MATERIALS RECEPTION PREPARATION AND CONDITIONING DOSIFICATION AND MIXING PRESSING IN PELLETS DRYING AND COOLING PACKING: (PELLETS OF GRAPHIT OR OF CALCINED PETROLEUM Diagram 2: Process of production and packing of graphit pellets and coke of calcined petroleum. Preparation and conditioning: In order to obtain good results with the pelletization, the material to pelletize is often conditioned before being pelletized. Some products are grinded, mixed with other components and liquids, heated by direct addition of steam or indirect heating up to the ideal temperature of the process. Page 11 of 19

12 Dosification and pressing by extrusion: The paste obtained is passed through a holed matrix, that in our case it can have a hole diametre between 1 and 40 mm. The compaction is made in the channels (perforations) of pressing that are located in the matrix. (Look at picture 1). Picture 1: Extrusion through the press (detail). Drying and cooling: For this type of application, it is recommendable the using of the dryers/coolers of line, since these products require precise controls about the drying and cooling parameters. The controls of humidity and temperature by areas as well as the reversible flow of air, allow a uniform drying of the product. Product: The final product in our case has a cylindric form of 4 mm of diameter and a variable length of about 8 mm. 4 mm diam. Aprox. 8 mm The used binder is an organic product, and the final product only contains between 0 5 and 1 % of this binder, obtaining practically the same chemical specification in pellets than in fines which are pelletized, according to the specifications explained below. Page 12 of 19

13 PELLETS SPECIFICATIONS OF CHEMICAL COMPOSITION: GRAPHIT OF PETROLEUM Carbon: > 98 % > 98 % Hydrogen: < 0.05 % (500 ppm) < 0.05 % (500 ppm) Nitrogen: < 0.05 % (500 ppm) < 0.1 % (1000 ppm) Humidity: < 0.5 % < 0.5 % Ashes: < 1 % < 1 % Volatiles: < 1 % < 1 % Sulphur: < 0.05 % < 0.1 % 10.- PRACTICAL TEST. COMMENTS A group of practical tests is made to compare the behaviour of: Graphitized coke. Graphit of electrodes. Coke of calcined petroleum. Pellets of petroleum coke. CHARACTERISTICS OF THE TESTS Fusion equipment: The tests have always been realized in coreless induction furnace of 5000 Kw, with 8 Tm. of capacity, that works to 250 Hz (middle frequency). Analytical: Analysis of Carbon realized with LECO C-S. Analysis of the spectrum of elements: spectrometer of optical emision. Test 1: The test has been realized for the following materials: Graphitized coke Graphit of electrodes Coke of petroleum Pellets of graphit of electrodes Pellets of coke of petroleum - The oven has been loaded in all the cases with around Kg. - The first sample has been taken out in a temperature superior to 1410 º C to determine the % of Carbon and Silicon Kg have always been added in order to increase the equivalent Carbon in %. Page 13 of 19

14 - Once added, the energy has been increased to rise the temperature to 1515º C 1530º C. - The own agitation of the medium in the oven makes easier the incorporation of the recarburant in the iron mass. - In an automatical way, when the final designed temperature is reached, the oven stops applying power minutes after beginning to apply power we take out a sample, to make the analysis of C and Si, and thus determinate the yield. Test 2: The test has been realized for the following materials: Graphitized coke Graphit of electrodes Coke of petroleum Pellets of graphit of electrodes Pellets of coke of petroleum - The oven has been loaded in all the cases with around Kg. - The first sample has been taken out on a temperature superior to 1410 º C to determine the % of Carbon and Silicon Kg have always been added to increase the equivalent Carbon in %, but differently form test 1, we try the equivalent to be below 4.3 %. - Once added, the energy has been increased to rise the temperature in the oven to 1515º C 1530º C. - The own agitation of the medium in the oven makes easier the incorporation of the recarburant in the iron mass. - Automatically, when the final designed temperature is reached, the oven stops applying power minutes after the beginning of application of power we take out a sample to make the analysis of C and Si and thus determine the yield. Test 3: The test has been realized for the following materials: Graphitized coke Graphit of electrodes Coke of petroleum Pellets of graphit of electrodes Pellets of coke of petroleum - The oven has been loaded in all cases with around Kg. - The first sample has been taken out with a temperature superior to 1410 º C to determine the % of Carbon and Silicon. - 7 Kg have always been added to increase the equivalent Carbon in %, but differently form test 1, we try the equivalent to be below 4.3 %. - Once added, the energy has been increased in order to rise the temperature 1515º C 1530º C. - The own agitation of medium in the oven facilitates the incorporation of recarburant in the iron mass. - Automatically, when the designed final temperature is reached, the oven stops applying power. - 5 minutes after the beginning of the application of power, we take a sample out to make the analysis of C and Si and thus determine the yield. Page 14 of 19

15 Comments to test 3: 1) Since this test is about a final adjustment in the composition before teeming the oven, it is not realized for the following materials: Petroleum coke Pellets of petroleum coke 2) The additions has always been made once the scum has already been removed (skimmed off) from the oven, in order not to interfere in the incorporation of recarburant in the medium CHARACTERISTICS OF THE EMPLOYED MATERIALS CHEMICAL ANALYSIS GRAPHITIZED GRAPHIT OF ELECTRODES PELLETS OF GRAPHIT OF ELECTRODES PETROLEUM CARBON > ASHES < VOLATILES < NITROGEN < HUMIDITY < PELLETS OF PETROLEUM OF SULPHUR < GRANULUS mm mm Diámetro 4 mm mm Diámetro 4 The previous chart gathers the values provided by the manufacturers of the materials used in the practical test. Moreover, in the case of graphitized coke and graphit of electrodes, the following values were found in the analysis realized by an external laboratory. Graphit of Electrodes Graphitized Coke Nitrogen ( % ) Sulphur ( % ) < 0.02 < 0.02 Volatiles ( % ) Ashes ( % ) Aluminium ( % ) Zinc ( % ) Lead ( % ) < 0.01 < 0.01 Page 15 of 19

16 RESULTS TYPE GRAPHIT T Kg. C Si Kg. Time Time C Ceq Ceq Incr. beginning oven oven oven addition end (min.) h.final 1 2 Ceq Yield Graphit of electrodes ,45 2, ,87 4,13 4,55 0,42 84,34 Graphit of electrodes ,38 1, ,79 4,03 4,44 0,41 83,97 Graphit of electrodes ,31 2, ,64 4,01 4,34 0,33 92,58 Graphit of electrodes ,35 2, ,67 4,03 4,35 0,32 91,02 Graphit of electrodes ,85 2, ,94 4,57 4,66 0,09 95,14 Graphit of electrodes ,82 2, ,91 4,53 4,62 0,09 95,79 Petroleum coke ,42 2, ,82 4,10 4,50 0,40 80,96 Petroleum coke ,34 1, ,74 4,00 4,40 0,40 81,76 Petroleum coke ,36 1, ,67 3,98 4,29 0,31 88,69 Petroleum coke ,40 1, ,71 3,98 4,29 0,31 87,75 Pellets of graphit of electrodes ,41 1, ,84 4,06 4,49 0,43 89,10 Pellets of graphit of electrodes ,37 2, ,81 4,05 4,49 0,44 91,71 Pellets of graphit of electrodes ,36 1, ,69 4,02 4,35 0,33 95,88 Pellets of graphit of electrodes ,32 1, ,66 3,97 4,31 0,34 95,39 Pellets of graphit of electrodes ,86 2, ,94 4,57 4,65 0,08 81,71 Pellets of graphit of electrodes ,84 2, ,92 4,54 4,62 0,08 82,40 Pellets of petroleum coke ,42 1, ,84 4,05 4,47 0,42 86,07 Pellets of petroleum coke ,46 1, ,88 4,08 4,50 0,42 85,85 Pellets of petroleum coke ,34 1, ,66 4,00 4,32 0,32 89,42 Pellets of petroleum coke ,30 1, ,61 3,93 4,24 0,31 89,38 Graphitized coke ,42 1, ,83 4,06 4,47 0,41 83,72 Graphitized coke ,38 1, ,80 4,01 4,43 0,42 84,17 Graphitized coke ,31 1, ,62 3,93 4,24 0,31 88,18 Graphitized coke ,41 1, ,72 4,04 4,35 0,31 87,80 Graphitized coke ,82 2, ,91 4,52 4,61 0,09 91,99 Graphitized coke ,86 2, ,95 4,57 4,66 0,09 93,21 Yields: Test 1 Test 2 Test 3 Increased yield Test 1 regarding 2 Graphit of electrodes 84,15 91,80 95,46 7,65 Petroleum coke 81,36 88,22 6,86 Pellets of graphit of electrodes 90,40 95,64 82,06 5,23 Pellets of petroleum coke 85,96 89,40 3,45 Graphitized coke 83,95 87,99 92,60 4,04 Page 16 of 19

17 Yield by test and material: Rdto 100,00 95,00 90,00 85,00 80,00 75,00 70,00 Graphit of electrodes Petroleum coke Pellets of graphit o... Pellets of petrole.. Graphitized coke Serie1 Serie2 Serie3 From the previous graphic it can be observed what follows: a) Comparing the graphit of electrodes with the pellets of graphit of electrodes in test 1 and 2, the obtained yield with the pellets of graphit in both tests, is higher than the obtained one with the graphit of electrodes. b) In the same way, comparing the petroleum coke with the pellets of petroleum coke in test 1 and 2, the obtained yield in both test is higher in the pellets of petroleum coke than the obtained one in the petroleum coke. c) When comparing the yield of graphit of electrodes to the pellets of graphit one in the final adjustment (test 3) we have as a result a lower yield in the graphit pellets than in the graphit of electrodes (contrarily to the obtained in test 1 and 2). d) When comparing the graphit of electrodes to the graphitized coke, there is a very little difference in the yield of both in test 1, but this difference is clearly noticed in test 2 and 3. e) The lower the increase of equivalent Carbon, the higher is the obtained yield, except for the case of pellets of graphit where it is clear that they are non advisable for final adjustments of oven. Page 17 of 19

18 Development Yield vs. Increase Ceq Development Yield vs.increase Ceq Yield Yield ,42 0,32 0 Ceq ,4 0,31 0 Ceq pellets of petroleum coke Coke of petroleum Development Yield vs. Increase Ceq Yield ,42 0,31 0,09 Ceq Graphitized coke Development Yield vs. Increase Ceq Development Yield vs. Increase Ceq Yield Yield ,42 0,33 0,09 Ceq 70 0,44 0,34 0,08 Ceq Graphit of electrodes Pellets of graphit of electrodes Page 18 of 19

19 11.- CONCLUSIONS ON THE TESTS. The pellets (of graphit and of petroleum coke) have a similar behaviour or a bit better than the own products from which they come from (graghit of electrodes and coke of calcined petroleum), making more interesting its use in the recarburation in oven for the production of smeltings, added with the metallic load. For the final adjustment the behaviour is different, producing a descent of yield without a clear and evident justification. Since we can consider that the agitation of the medium, not only in the addition of pellets but also in the addition of graphit and petroleum coke, has been the same (it has been added to similar weights and temperatures in oven, applying the same power), the difference in yield could be explained by the own specific surface and the bigger weight that the agglomerates (pellets) have with regards to the product from which they came. They are wrapped in medium just before dissociating, releasing the fines that they have in an aggregated way. When these fines are released, they are already trapped in the bath, and show a bigger specific surface, that s why their incorporation can be much easier. Nevertheless, this does not happen when pellets of graphit are used as a final correction in oven. It can be explained that when we make this corrective addition to the composition, the oven is already full and for this reason, when the maximum power is applied in order to rise the temperature (to similar values than the previous ones), it is obtained a little agitation of medium (almost all the incorporation of graphit in the medium is by the migration of the atoms of Carbon by the gradient of concentration) and if the particles are very fine, they can have been removed from the system by the own aspiration of the oven before having been integrated to the medium. This could explain what happens with the pellets in the final adjustment: when they are ripped, the fine particles remain in the bath surface, and in its migration, a lot of particles can be eliminated from the medium by aspiration. It can also be noticed an important difference between the graphit of electrodes and the graphitized coke, being both synthetic graphit. In all the tested cases, the graphit of electrodes has a higher yield than the graphitized coke although, being higher the volatiles and ashes in the graphit of electrodes used, it could be expected a worse yield in it. If we check the technical characteristics of the materials, it is observed that the granulometry of the graphitized coke used in the test is smaller than the size of grain of the graphit of electrodes; for this reason, we can consider that the granulometry of the used product has a great influence too. Since we could observe how the yields vary in relation to the variation of the equivalent Carbon, we can think that it is better to make little additions of recarburant during all the loading of oven and final correction than to make an only massive addition. Raul Biel Pedro Bayés March 2004 GRAFITOS BARCO, S.A. would like to thank the authors of the present essay for the work done. Page 19 of 19