HRP920446A2 - TREATMENT OF PETROL COCKS FOR INHIBITING COX INFLUENCE - Google Patents

Abstract

A method for treating high sulfur petroleum coke in which portions of petroleum coke are contacted with a compound containing alkali or alkaline earth metal selected from the group consisting of sodium, potassium, cesium and magnesium at an elevated temperature above that at which the alkali compound or alkaline earth metal begins to react with carbon but is below the temperature at which the coke particles begin to inflate in the absence of the compound. The coke particles are maintained at elevated temperature for a sufficient period of time to allow the reaction to take place and to allow the reaction products to penetrate the particles and to form an alkali or alkaline earth metal deposit over the entire mass of the particles; and then the coke particles treated like this are cooled.

Description

The technical field to which the invention belongs

The invention is from the field of coal article production technology.

Technical problem

The present invention relates to articles of coal and graphite, particularly to electric furnace electrodes, and to a process for producing such electrodes of improved quality using high sulfur cokes. More specifically, the Invention relates to a process for treating calcined petroleum cokes with a swelling inhibitor before introducing the coke into the carbon mix. In an important aspect, the invention relates to a coal filler or aggregate containing discrete particles of calcined petroleum coke having a high sulfur content and having a swelling inhibitor distributed throughout the mass of particles, wherein the swelling inhibitor serves to reduce or eliminate blowing coke during production and using graphite and coal products.

State of the art

A common practice in the manufacture of both coal and graphite electric furnace electrodes is to use calcined petroleum coke (ie raw petroleum coke that has been heated to a temperature above about 1200ºC) as a filler or aggregate material and to mix this filler or aggregate with such with a carbon binder such as resin. The mixture is formed into an electrode shape, either by casting or extrusion, and then fired at an elevated temperature sufficient to carbonize the binder (eg around 800ºC). In those cases where a graphitized electrode is required, the baked electrode is further heated to temperatures of at least around 2800ºC.

Petroleum coke particles tend to "puff", that is, to expand and even break up when heated at temperatures above about 1500ºC, if they contain more than about 0.2% by weight. sulfur. Electrodes made from such coke lose density and strength and sometimes tear longitudinally when heated at these high temperatures. As indicated, graphite electrodes are normally heated to at least 2800ºC during their manufacturing process. Carbon electrodes, which are not graphitized during the arbitrary process, reach temperatures between about 2000ºC and 2500ºC during their use in silicon or phosphorus furnaces.

Blowing is accompanied by the release of sulfur from its bond with carbon within the coke particles. If the sulfur-containing vapors cannot escape from the particles or the electrode quickly enough, they create an internal pressure that again increases the volume of the particles and can cause the electrode to break.

The conventional remedy for swelling was to add such an inhibitor as iron oxide or another metal compound to the coke-resin mixture before the electrodes were formed. It has been shown, for example, that about 2 wt. percentage of iron oxide can be effective in reducing coke swelling. Some cokes that have a higher tendency to swell or start swelling at a lower temperature cannot be properly controlled with iron oxide.

Various attempts have been made to provide other improved processes to overcome the above and other disadvantages of the prior art. For example, in the U.S. Pat. But. 2,814,076 from J.W. Gartland dated November 26, 1957, described an improved process for the manufacture of such graphite articles as electrodes for electric furnaces in which a compound of Group I of the Periodic Table, particularly sodium carbonate, is used as a swelling inhibitor. Sodium carbonate can be added to the article by impregnating the article after baking with sodium carbonate solution or by adding a swelling inhibitor directly to the coke-resin mixture. Although adding sodium carbonate to the coke-resin mixture is more convenient than adding it to the baked article, this process produces a finished electrode of poor quality, i.e. of lower density and strength.

Another problem encountered when blowing inhibitor is added directly to the coke-resin mix is that the sodium carbonate reacts with the acidic extrusion aids that may be used in the mix. Unfortunately, this reaction often causes extrusion problems that lead to a weak electrode structure.

Another approach to solving the problem of coke blowing in the production of carbon and graphite electrodes is described in U.S. Pat. Pat. But. 3,506,745 published by L.H. Juel et al 14 April 1970 In this approach, high sulfur coke particles are treated prior to their introduction into the carbon mix by contacting the coke particles with a swelling inhibitor and heating the particles in a substantially non-oxidizing atmosphere at temperatures above about 1400ºC, and also above at which the coke begins to swell in the absence of a swelling inhibitor and preferably above 2000ºC. The blowing inhibitor can be introduced by dusting inhibitor powders onto the granular petroleum coke, or an aqueous slurry containing the inhibitor can be made and then sprayed onto the coke before heating the coke particles to blowing temperatures. The coke particles are then cooled to ambient temperatures and mixed with a resin binder to form a conventional carbon mix. The blowing inhibitor combines with the sulfur and volatilizes when the coke is heated at and above the blowing temperatures. The problem with this approach is that the process requires heating the coke particles to temperatures significantly higher than those typically used during a conventional calcination process. That's why this treatment can only be done with a process that is different from ordinary practices for calcination, which consumes more energy and requires more expensive equipment.

Description of the solution to the technical problem with implementation examples and a list and brief description of the drawings

The present invention is directed to an improved process for treating high sulfur petroleum coke with a blowing inhibitor prior to feeding the coke into the carbon mix. In its broadest sense, the improved process comprises contacting particles of high sulfur petroleum coke with a compound containing an alkali or alkaline earth metal selected from the group consisting of sodium, calcium, potassium and magnesium, at an elevated temperature above that at which the compound of alkaline or alkaline earth metal begins to react with carbon but is below the temperature at which coke particles begin to swell in the absence of the compound; maintaining the coke particles at an elevated temperature for a sufficient period of time to allow the reaction to take place and to allow the reaction products to penetrate the particles and form a deposit, containing an alkali or alkaline earth metal, throughout the mass of the particles; and then cooling the treated coke brushes.

The process of the present invention is preferably carried out at an elevated temperature between about 1200ºC and 1400ºC. However, temperatures as low as 750ºC have been found to be adequate to promote the necessary reaction between the swelling inhibitor and the coke particles and can be used.

The swelling inhibitor used in the process of the present invention may be an alkali or alkaline earth metal salt and preferably sodium carbonate. The inhibitor may be mixed with the petroleum coke particles before or after heating during the normal calcination process, and may be incorporated with the coke particles in the form of dry, granular powders or as a solution containing the inhibitor that can be sprayed into particles. The inhibitor is used in larger amounts of about 0.2 wt.% of the coke.

In a preferred embodiment of the present invention, an improved process for treating petroleum coke (its particles) with a high sulfur content comprises: calcination of petroleum coke particles with a high sulfur content;

adding sodium carbonate to the calcined fraction of coke at an elevated temperature above about 1200ºC but below the temperature at which the particles of coke will begin to swell in the absence of sodium carbonate;

maintaining the calcined particles of coke and sodium carbonate at an elevated temperature for a sufficient period of time to allow the sodium carbonate to react with the coke and to allow the resulting sodium to penetrate the particles and deposit sodium throughout the mass of particles; and

cooling of thus treated coke particles.

In another aspect of the invention there is provided a coal filler or aggregate for use in the manufacture of carbon or graphite articles comprising discrete particles of petroleum coke having a high sulfur content and having a swelling inhibitor and which is distributed throughout the mass of particles. The anti-bloat agent comprises a water-dissolved alkali or alkaline earth metal compound selected from the group consisting of sodium, potassium, calcium, and magnesium; the average amount of metal in the particles is greater than about 0.15 wt. percentage.

On the attached drawings:

Figure 1 is a schematic elevation view of a calciner modified to carry out the process of the present invention;

Figure 2 is an enlarged cross-sectional view of the modified part of the device shown in Figure 1;

Fig. 3 is a sectional view of a modified calciner taken along line 3-3 of Fig. 2;

Fig. 4 is a schematic elevational view of a calciner according to another embodiment of the present invention;

Figure 5 is an enlarged side elevational view of the device shown in Figure 4;

Figure 6 is a graph showing the swelling rates of petroleum coke treated with a conventional inhibitor and the same coke treated according to the present invention;

Figures 7, 8 and 9 are graphs showing the blowdowns of several different types of petroleum coke according to the present invention;

Figure 10a is a photomicrograph taken with a scanning electron microscope (SEM) at 200X magnification and shows a surface near the edge of an inner plane made by grinding 1.27 cm particles of coke treated according to the present invention;

Figure 10b is a photomicrograph of the same surface shown in Figure 10a, but shows a map of elemental sodium obtained by Energy Dispersive X-Ray Analysis (EDX) also at 200X magnification.

Figure 10c is a photomicrograph of the EDX spectrum of the same surface shown in Figures 10a and 10b;

Figure 11a is a photomicrograph taken with a scanning electron microscope (SEM) at 45X magnification and shows a second surface, closer to the center of the same inner plane shown in Figures 10a and 10b;

Figure 11b is a photomicrograph of the same surface shown in Figure 11a, but showing an X-ray map of elemental sodium obtained by EDX analysis, also at 45X magnification.

Figure 11c is a photograph of the EDX spectrum of the same surface shown in Figures 11a and 11b;

Figure 12a is a photomicrograph taken with SEM at 50X magnification and shows the third surface of the same inner plane shown in Figure 10a and 10b;

Figure 12b is a photomicrograph of the same surface shown in Figure 12a but showing an X-ray map of elemental sodium obtained by EDX analysis, at the same 50X magnification;

Figure 12c is a photograph of the EDX spectrum of the same surface shown in Figures 12a and 12b;

Figure 13a is a photomicrograph taken with SEM at 200X magnification and shows the fourth surface of the same inner plane shown in Figures 10a and 10b;

Figure 13b is a photomicrograph of the same plane shown in Figure 13a but showing an X-ray map of elemental sodium obtained by EDX analysis, at the same magnification of 200X;

Figure 13c is a photograph of the EDX spectrum of the same surface shown in Figures 12a and 12b;

Figure 14a is a photomicrograph taken with SEM at 15X magnification and shows both an internal plane made by grinding a 0.635 cm treated coke particle according to the present invention and also shows the original surface exposed by grinding;

Figure 14b is a photomicrograph of the same surface shown in Figure 14a but showing an X-ray map of elemental sodium obtained by EDX analysis, at the same 15X magnification;

Figure 14c is a photograph of the EDX spectrum of the same surface shown in Figures 14a and 14b;

Figure 15a is a photomicrograph taken with SEM at 15X magnification of the same surfaces shown in Figure 14a, but taken after leaching the particles with water;

Figure 15b is a photomicrograph of the same surface shown in Figure 14a, but shows an X-ray map of elemental sodium obtained by EDX analysis, at the same 15X magnification;

Figure 15c is a photograph of the EDX spectrum of the same surfaces shown in Figures 15a and 15b;

Petroleum coke is produced by burning heavy oil residues, as is known from the prior art. Crude petroleum coke, i.e., petroleum coke that has not been calcined, typically has a volatile matter content of between about 6 and about 14 percent. The volatile substance is typically separated by heating raw petroleum coke in a calciner at temperatures between about 1200ºC and about 1400ºC. Calcining temperatures as high as 1500ºC can occasionally be used. The content of volatile substances in the coke after calcination is usually less than about 1 mass percent. Crude petroleum coke is usually reduced to a particle size of 10.16 cm or less prior to calcination.

For the purposes of the present invention, the starting coke material can be either petroleum coke or petroleum coke that has been calcined by conventional processes. In any case, the petroleum cokes to which the invention specifically relates are the so-called "high sulfur" petroleum cokes which usually contain more than about 0.7 wt. % sulfur. These high-sulfur petroleum cokes typically cannot be adequately controlled by blow-inhibiting procedures currently known in the art. Although these cokes cost less, their use in the production of carbon or graphite articles is either limited or requires modified, more expensive process technology.

Sulfur is released from its chemical bond with carbon when petroleum coke is heated to temperatures higher than about 1500ºC, and in many cases to at least about 1600ºC, which is higher than ordinary calcination temperatures. If this release of sulfur is not inhibited or the sulfur is not chemically bound within the coke structure, then the rapid escape of sulfur-containing vapors creates an internal pressure in the coke particles that tends to spread the particles, sometimes resulting in their tearing or the tearing of articles made from them. . This phenomenon is called inflation.

According to the present invention it has been found that swelling of the formed carbon or graphite article can be significantly reduced or eliminated by treating the petroleum coke particles with some alkali or alkaline earth metal compound, such as sodium or calcium carbonate at temperatures well above the temperatures at which the coke begins to swell , before the introduction of coke particles into the carbon mix. From the literature, “Efectt of Sodium Carbonate upon Gasification of Carbon and Production of Producer Gas” by D. A. Fox et al, Industrial and Engineering Chemistry, Vol. 23, No. 3, March 1931, it is known that an alkali compound (eg, sodium carbonate) can be effectively reduced with carbon in a reactor at high temperature so that alkali metal vapors and carbon monoxide are produced. According to the invention it has unexpectedly been found that if an alkali or alkaline earth metal compound is allowed to be in contact with petroleum coke particles for a sufficiently long period of time, eg about 1 minute or more, while maintaining a temperature above that at which this reduction reaction occurs, eg .about 750ºC in the case of sodium carbonate then the alkali or alkaline earth metal thus produced will penetrate and form a deposit containing the alkali or alkaline earth metal in the entire mass of coke particles and not only in its pores. A dwell time of 30 seconds has been found to be effective in suppressing bloat in the laboratory. In the operational production tests, the residence time at the reaction temperature was longer than one minute.

It has been known for some time that sodium carbonate, when used as an inhibitor in the conventional way, when added to the coke-resin mix, forces the product to have a lower density and lower strength compared to the same product made with a conventional blowing inhibitor, i.e. with iron oxide. We have found that sodium carbonate when used as a swell inhibitor in accordance with the present invention caused no loss of either density or strength in the product and gave a product equal to that produced using iron oxide as a swell inhibitor.

Since the inhibiting agent is deposited within the coke particles, it does not come into contact with the resin during processing of the coal mix and does not interfere with any extrusion aids such as fatty acids.

Although alkali or alkaline earth metal compounds can be placed in contact with petroleum coke particles either before or after heating the coke particles at the necessary temperatures to carry out the reaction,, it is very convenient to add the inhibitor compound to the coke particles in the form of a dry granular powder since the particles are coke heated to calcination temperatures between about 1200ºC and about 1400ºC. In actual practice, a dry granular powder of the inhibitor compound is added to the calcined coke fractions at the discharge end of the calciner. It is also possible for the inhibitor compound to be added to raw coke in the form of a dry powder or for the coke to be sprayed with a solution containing the inhibitor before calcination.

A compound of alkaline or alkaline earth metal, for example sodium carbonate, is mixed with petroleum coke particles in larger quantities of about 0.2 mass percent. Preferably, the inhibitor is used in amounts varying from about 0.5 to about 2.5 weight percent of the coke.

Figures 1-3 of the drawings show a typical rotary type apparatus that has been modified to carry out the improved process of the present invention. As shown, the calciner includes an elongated cylindrical rotary calciner 10 having an inlet end 12 and an outlet end 14. The inlet end 12 of the calciner 10 is mounted for rotation within a stationary coke inlet chamber 16 having a vertical drain or stack. 18 for exhaust gases from inside the calciner. The outlet end 14 of the calciner 10 is similarly mounted for rotation within a stationary coke discharge chamber 20 which includes a conventional clinker box 22 positioned vertically below the chamber 20.

Particles of raw petroleum coke 24 are fed into the calciner by means of a horizontal conveyor 26 and are charged down the coke slag 28 into the inlet end 12 of the rotary kiln 10 for calcination. As shown in the drawing, the furnace 10 is inclined at a small angle along the longitudinal axis from its inlet end 12 downwards toward its outlet end 14. Thus, when the coke particles 24 enter the furnace 10, they are forced by gravity so that they are slightly they move along the length of the furnace 10 when it rotates until they reach the exit end 14 from where they are emptied into the chamber 20.

Some fuel, such as natural gas, is burned at the hot end of the furnace and the combustion gas passes through the furnace 10 in the opposite direction to the flow of the coke particles 24. The hot combustion gases heat the coke particles 24 and force the volatiles contained therein to evaporate. and burn.

The hot particles of calcined coke 24 fall from the chamber 20 into the clinker box 22, from where they flow over the refractory block 30 (Figure 2). The block 30 is located at the bottom of the rectangular outlet opening 32 located in the stationary head 34 of the cooler 36.

An elongated, cylindrical, rotary cooler 36 is positioned below the discharge chamber 20. The cooler 36 has an inlet end 38 which is mounted for rotation about the stationary head 34 of the clinker box 22. The outlet end 40 of the cooler 36 is mounted for rotation within the stationary coke discharge chamber. 42.

The elongated, cylindrical cooler 36 is also inclined downwardly at a slight angle from its inlet end 28 toward its outlet end 40. As shown in Figure 2, hot particles of calcined coke 24 collect in a body at the bottom of the clinker box 22 and for the refractory block 30 and eventually spill over the edge of the block 30 and fall into the inlet end 38 of the rotary cooler 36. The coke particles are then forced by gravity and rotation of the cooler to move slowly downward along the cooler 36 until they reach the outlet end 40 from which the particles enter and are collected in the coke feed chamber 42.

Although some calciners may use indirect cooling, eg through a steel jacket cooler 36, almost all calciners quench the hot, calcined coke directly by spraying it with water. This direct spraying reduces the temperature of the hot coke particles immediately after they leave the clinker box 22. It is typical, in order to achieve this purpose, to provide a series of nozzles directly below the outlet opening 32 of the clinker box 22.

As shown in Figure 2, a conventional calciner can be modified to carry out the process of the present invention by inserting a hot zone 44 within the inlet end 38 of the cooler 36. The hot zone is formed according to the present invention by locating a circular refractory ring 46 at a predetermined distance downstream from the outlet. clinker box 32 and by moving the quench water spray nozzles 56 downstream of the refractory ring 46. As shown, the ring 46 is mounted against a refractory lining 45 which is placed near the inner cylindrical side walls of the cooler 36. The refractory retaining ring 46 increases the depth of the coke layer in the hot zone 44, thus increasing the coke retention time. The temperature of the coke particles 24 when they enter the hot zone 44 is somewhat reduced by the process reaction but remains above 1100ºC.

A dry, granular sodium carbonate powder 48 is charged into the hot zone 44 through a hopper 50. The hopper 50 has an elongated, tubular stem 52 that extends through the side wall 34 of the clinker box 22 and deposits the powder on top of a bed of hot calcined coke particles 24 at the bottom of the hot zone 44. As best shown in Figure 3, the powder is mixed with the coke particles 24 by the rolling action that occurs inside the rotary cooler 36. The powdered calcium carbonate melts upon contact with the hot coke particles 24 and reacts with the coke according to the following endothermic reaction:

Na2CO3(l) + 2C(s) = 2Na(g) + 3CO(g)

H = 415.4 kJ/mol ----------- at 1330ºC

(l), (s) and (g) refer to the physical state of the reactants and mean liquid solid or gaseous. The elemental sodium produced in the above reaction penetrates the coke particles and is distributed through the mass of coke particles so that modified coke containing sulfur and sodium is created.

After treatment with sodium carbonate powders in the hot zone 44 for a satisfactory period of time, the hot particles of calcined coke 24 eventually flow over the refractory ring 46 into the cooling compartment 53 of the cooler 36.

In this modified version of the cooler 36, the pipe 54 which carries the extinguishing water towards the series of nozzles 56 at its outer end is mounted in the usual way inside the lower part of the side wall 34 of the clinker box 22 but in this case the pipe 54 is longer so that it extends completely through the hot zone 44 and into the cooling compartment 53. Thus, water is sprayed from the nozzles 56 directly onto the hot coke particles as they leave the hot particle quenching zone 44, thereby significantly reducing their temperature.

Quenched or cooled particles of the treated calcined coke are then discharged from the chamber 42 and the moving conveyor 58 conveys the particles of coke towards the storage surface. The steam produced in the cooler from the extinguishing water is separated from the cooler together with some air by the fan 62 and blown into the atmosphere. The steam/air mixture passes through the dust collector 60, where the coke dust is captured so as to prevent air heating.

Figures 4 and 5 show a calciner designed specifically for use in treating petroleum coke according to the present invention. This calciner is equipped with a holding chamber comprising a separate reactor section 68. This reactor vessel is located downstream of the calciner and upstream of the cooler and may be designed for long retention times. Particles of calcined coke are charged from the discharge chamber 20 towards the reactor vessel 68, where they are treated with dry granular powders of an alkali or alkaline earth metal compound, for example, sodium carbonate, which is simultaneously charged through the inlet 70. After treatment, the hot coke particles exit through the outlet 72 into the reactor vessel 68 and enter the inlet end 38 of the rotary cooler.

It can be seen from the above that the process of the present invention can be practiced either in existing capacities using a conventional calciner or in a new facility using a calciner provided with a special reactor according to the present invention.

An important advantage achieved by adding an inhibitor, eg sodium carbonate to the particles of calcined petroleum coke in a special reaction vessel located at the discharge end of the calcining furnace is that the gas does not flow through this vessel, and therefore there is practically no possibility of the inhibitor being separated and released into the atmosphere.

A number of laboratory experiments were carried out to determine the amount of sodium carbonate required in the present process for effective blow suppression and also the minimum residence time in the case of four different petroleum cokes having different sulfur content. In these experiments, one kilogram of calcined coke particles are placed in an open-top graphite container and inserted into a muffle furnace preheated to about 1200ºC. When the coke temperature (measured by bimetallic) reaches 1200ºC, the furnace door is opened and a predetermined amount of sodium carbonate eg 0.4%, 0.8%, 1.2%, 1.6%, etc., is dripped onto the coke surface using a long graphite tool. The coke sample is then soaked for a short time. At the appointed time, the graphite container is removed from the furnace and the coke is extinguished by spraying water on it and is split at the same time. The time required to reduce the coke temperature to between 300ºC and 500ºC varies from about 30 seconds to about 90 seconds.

The specified reaction time of the experiments was counted from the moment of dropping the inhibitor onto the coke until the moment when quenching with water is started. The slaked coke is allowed to cool to room temperature without further spraying with water. The cooled coke samples are then tested for swelling, ie the irreversible expansion that occurs in sulfur-containing cokes when heated to between about 1600ºC and 2200ºC.

The swelling is measured on a sample prepared from coke and placed in a dilatometer assembly (a device for measuring expansion) which is made of low-expansion graphite. The assembly, containing the sample, is placed in a tube furnace and heated at 450ºC for one hour to 2400ºC. After the temperature reaches 1000ºC, the differential expansion of the sample above that of the graphite container is recorded at 15-minute intervals.

Several different values can be derived from this measurement, ie (1) total expansion over the temperature interval; (2) inflation rate per unit time as a function of temperature (3) temperature at which the inflation rate reaches a maximum.

Figures 6 to 9 show the relationships between the highest swelling rate and the amount of inhibitor used, the swelling rate unit in these figures is 10-4 m/m per 15 minutes at a heating rate of 450ºC per hour. The temperature at which the swelling rate of these particular cokes reaches its highest value was about 1750ºC.

Figure 6 is a graph showing the relationship between the maximum inflation rate as determined in the above experiment and the amount of inhibitor used. Curve A shows this relationship in the case of spiked coke, coke D1, which contains 1.05 wt. % sulfur and using different amounts of sodium carbonate as an inhibitor. A swelling rate of about 10 is the desired limit for processing coke into graphite electrodes using modern graphitization processes. It can be seen from curve A that the allowable inflation rate is achieved with only one mass percentage of sodium carbonate inhibitor.

For comparison, the same experiment described above was repeated with the same spiked coke having the same sulfur content but using the conventional inhibitor, iron oxide. Curve B in Figure 6 shows the results of this experiment. It can be seen that the suppression of swelling in the case of the conventional inhibitor is far less than that obtained with the same coke treated with sodium carbonate according to the present invention. Iron oxide, even when used at twice the conventional concentration (4 mass % instead of 2 mass %) did not produce a comparable reduction in the swelling of this particular coke. The same type of experimental test was performed on petroleum coke of normal quality, coke E1, which contains 1.3 wt. % sulfur. In this test, the coke is treated according to the process of the present invention, using sodium carbonate as an inhibitor and a residence time of about one minute. The results of this test are shown by the curve in Figure 7. It can be seen that an adequate reduction in the inflation rate is achieved when only about 0.6 wt.% sodium carbonate inhibitor is used.

A similar experimental test was conducted on another calcined petroleum spiked coke, coke F1, containing about 1.3 wt.% sulfur using sodium carbonate as an inhibitor and a residence time of about one minute. The results of this test are shown by the curve in Figure 8. it can be seen that this particular coke required about 1.3 wt.% sodium carbonate inhibitor, to suppress the swelling below the permissible level.

The second experimental test was performed on another spiked coke coke G1, which contains 1.1 wt.% sulfur again using sodium carbonate as an inhibitor and a residence time of about one minute. The results of this test are shown by the curve in Figure 9. It can be seen that in this case about 1.2 wt.% of sodium carbonate was needed as an inhibitor in order to suppress the swelling below the permissible swelling limit. The same type of coke, coke G1, required about 1.6 wt.% sodium carbonate as an inhibitor, when its sulfur content was increased to about 1.25 wt.% sulfur.

A number of large-scale experimental trials have also been carried out using a modified calciner substantially as shown in Figures 1-3, in which several hundred tonnes of three different normal and spiked cokes, containing about one mass percent or more of sulfur, were calcined and treated by the method of the present invention. In these tests, approximately one mass % of sodium carbonate powder less than 800 micrometers in size was added to calcined coke in a hot zone constructed within the inlet end of the cooling drum, while at temperatures between 1200ºC and 1300ºC and over a period of at least one minute. The calcined and treated coke is then cooled and samples are collected and subjected to the same type of testing as described above to determine the swelling rate. The swelling of these particular cokes was found to be significantly reduced for rapid longitudinal graphitization. It was also unexpectedly found that the present process substantially reduced the amount of chemicals, eg, chlorides, sulfates, etc., which are normally released into the atmosphere in the off-gas of the cooler during calcination. Further, since the process also eliminates the acidity of the exit gas, the potential for equipment corrosion is substantially reduced.

Graphite electrodes for an electric furnace having a diameter of 50.8 cm and a length of 243.84 cm were made using one of the spiked cokes, which has a high sulfur content, which was calcined and treated in the experimental tests described above, the calcined and treated coke was used as an aggregate or filler and mixed is with a resin binder and the usual extrusion aids, so that a carbon mix is formed. The mix is then squeezed out, baked at around 800ºC and then graphitized at temperatures around 3000ºC. There were no processing problems during extrusion and firing and there was no evidence of inflation problems. The electrodes were later tested experimentally in a steel arc furnace and had performance comparable to electrodes made from more expensive spiked cokes, a premium grade with low swelling.

Particles of coke of standard quality, coke E1, containing an average of 1.28 percent sulfur, were treated according to this invention with different amounts of sodium carbonate varying from 0.25% to 1 percent. The treated particles were then tested, using routine analytical procedures, for sulfur, sodium and ash content, and were tested for swelling. The results are summarized in Table 1. The data show (1) that the addition of 0.55% sodium carbonate reduced coke swelling to an acceptable level, while 0.25% did not; (2) that the sodium content of the coke was proportional to the amount of sodium carbonate added during treatment, within experimental error, and (3) that a content of 0.18%, corresponding to the amount of 0.55% added Na2CO3, reduced the swelling of this particular coke to an acceptable level level, while 0.12% sodium in coke was not enough.

Table 1

Sample %Added Rate %Ash %On u

But. Na2CO3 blowing in coke coke

Control 0 62.0

1 1 0 1.88 0.36

2 0.85 2.3 1.22 0.26

3 0.7 8.7 1.0 0.24

4 0.55 11.3 0.76 0.28

5 0.26 41.0 0.68 0.12

Sodium penetration into the body of particles treated according to this invention was examined with a scanning electron microscope using the energy dispersive X-ray (SEM-EDX) method. The particles are mounted in epoxy resin and ground to a medium level to expose the inner plane and also to leave a natural porous surface.

Figures 10a - 13a in conclusion show a series of photomicrographs taken at various magnifications (i.e. 200x, 45x, 50x, and 200x), and show SEM images of three internal plane surfaces produced by grinding 0.635 cm coke particles. The surface shown in Figure 10a is near the center of the plane and the surface shown in Figure 12a is in the center of the ground plane. The fourth plane shown in Figure 13a is also near the center of the plane, similar to the surface shown in Figure 11a.

The position and distribution of sodium in the inner plane are shown in microphotographs in Figures 10b - 13b inclusive. Photomicrographs were produced at the same magnification as indicated above by EDX analysis for sodium using a scanning electron microscope.

It can be seen from the fairly uniform distribution of bright spots throughout the photomicrographs, each representing a different surface in the same internal plane of the coke particles, that sodium actually penetrates deep within each particle treated according to the process of the present invention and that the distribution of sodium throughout the mass of each of individual coke particles essentially uniform. The concentration of sodium can vary from one particle to another, but within an individual particle the concentration is essentially uniform. It should be clear that the sodium produced by the reaction between sodium carbonate and coke forms, after diffusion into the mass of coke particles, a compound that is not dissolved in water and is not reactive with water, and that sodium is present as a compound containing sodium, and not as elemental sodium. The exact composition of the sodium-containing compound is not clearly understood at this time.

A series of graphs of energy spectra taken on the basic internal surfaces of each zone of coke particles examined in these tests is shown in Figures 10c - 13d inclusive. It can be seen from the graph that the intensity of two peaks dominates the energy spectrum and that these peaks are located in the same two positions that correspond to sodium and sulfur, thus confirming the presence of these two elements in a fraction of hair. Further, since a maximum for sodium occurs in each graph representing a different particle zone, it can be inferred that sodium is actually deposited substantially uniformly throughout the body mass of coke particles treated according to the present invention.

Further study of sodium penetration and its solubility after reaction with coke was performed with coke particles F1, dimensions 0.3048 cm by 0.635 cm, which were treated with 20% sodium carbonate at about 1200ºC according to the present invention. One of these treated particles was mounted and milled to wear out both the inner plane and the original porous surface. The fragment was examined with the same SEM-EDX procedures as the fragment shown in Figures 10a - 13a. After testing, the piece was leached with water to remove any compounds that were dissolved in the water and then tested again using the same techniques. Figures 14a, 14b, and 14c show tests before leaching, while Figures 15a, 15b, and 15c show tests after leaching. Figure 14b demonstrates that the sodium was distributed essentially uniformly on the basal interior plane and also essentially uniformly, but at a much higher concentration, on the exposed original surface of the porous volume. Figure 15b shows that after leaching, the penetration and distribution of sodium on the inner plane remain essentially unchanged, but the concentration of sodium on the original porous surface decreases to approximately the same level as on the inner plane, and its distribution is essentially uniform.

The dissolved sodium observed in the above study is believed to be a product of the interaction between sodium and coke, while the sodium dissolved in water found only on the original surface but not within the particle body is unreacted sodium carbonate.

Analyzes of the aqueous extract using standard analytical procedures confirmed the presence of sodium carbonate. The presence of unreacted sodium carbonate on the surface of the treated particles indicates that, under the same reaction conditions, the reaction between sodium carbonate and coke was not completed. Thus, the present invention provides an improved process for treating calcined petroleum coke in order to reduce or eliminate swelling in which the coke particles are heated in the presence of an alkaline or alkaline earth compound, preferably sodium carbonate, at a temperature above about 750ºC and preferably between about 1200ºC and 1400ºC. The inhibitor should be kept in contact with the coke particles for a sufficiently long period of time, eg one minute or more, to allow the inhibitor to react with the carbon and to allow the reaction products to penetrate deep into the mass of coke particles. Although it is possible to add the inhibitor directly to the raw coke before heating or calcining, it is preferable to add the inhibitor immediately after the coke particles are discharged from the calciner. This avoids possible environmental pollution problems and also achieves the advantage of reducing the acidity of the exit gas, as explained above.

The present invention further provides a process for producing carbon and graphite electrodes, such as electric furnace electrodes, in which treated coke is mixed with a conventional resin binder to form a carbon mix which is then shaped and extruded, baked to carbonize the binder and, if desired, it is graffitied. The main advantage offered by this improved process is that the manufacturer of carbon and graphite articles can now use cheaper high sulfur petroleum cokes and still produce high quality electrodes.

The best way to use the invention in the economy known to the Applicant

The invention is best used to make electrodes for electric furnaces using coke particles, which contain a high sulfur content, treated as detailed in the descriptive part of this application. In this way, very significant savings are achieved, since cheaper coke is used to make the electrodes, and the produced electrodes are of the same quality as those produced from premium quality petroleum coke, according to the procedures from the earlier state of the art.

Claims (30)

1. Process for treating petroleum coke particles with a high sulfur content, characterized by the fact that it includes: the reaction of said petroleum coke particles in the absence of a binder with a compound containing an alkali metal from the group containing sodium and potassium at an elevated temperature above that at which said compound begins to react with carbon but below the temperature at which said particles begin to swell in the absence of said connection; maintaining said coke particles and said compound at said elevated temperature for a sufficient period of time to allow the reaction to take place and to allow the reaction products to penetrate said coke particles and to form a deposit containing sodium or potassium throughout the mass of said particles; and cooling of thus treated coke particles. 2. Process according to Claim 1, characterized in that said coke particles are raw petroleum coke. 3. Process according to Claim 1, characterized in that said coke particles are calcined coke. 4. Process according to Claim 1, characterized in that said alkali metal compound is mixed with said coke particles before the reaction of said alkali metal compound with said coke particles at said elevated temperature. 5. The method according to Claim 1, characterized in that the aqueous solution containing said alkali metal compound is sprayed onto said coke particles before the reaction of said alkali metal compound and said coke particles at said elevated temperature. 6. The method according to Claim 1, characterized in that said coke particles are heated to a temperature of at least about 750ºC before the reaction of said coke particles with said coke particles with said alkali metal compound at said elevated temperature. 7. Process according to Claim 1, characterized in that said elevated temperature is between 1200ºC and 1400ºC, wherein said alkali metal compound is in the form of dry, granular powders. 8. The method according to Claim 1, characterized in that said alkali metal compound is sodium carbonate. 9. The method according to Claim 1, characterized in that said alkali metal compound is potassium carbonate. 10. Process for treating coke particles with a high sulfur content, characterized by the fact that it includes: calcination of said petroleum coke particles; adding sodium carbonate to the calcined coke particles at a temperature above about 1200ºC but below the temperature at which said coke particles begin to swell in the absence of said sodium carbonate; maintaining said calcined coke particles and said sodium carbonate at said temperature for a sufficient period of time to allow said sodium carbonate to react with the carbon and to allow the reaction products to penetrate said coke particles and to form a deposit containing sodium throughout said mass coke particle; and cooling of calcined coke particles treated in this way. 11. The method according to Claim 10, characterized in that said sodium carbonate is added to said calcined coke particles in the form of dry granulated powders. 12. The process according to Claim 10, characterized in that said sodium carbonate powders are added to said calcined coke particles at a temperature in the interval between 1200ºC to about 1400ºC. 13. The method according to Claim 12, indicated by the fact that the mentioned sodium carbonate powders are added to the mentioned pieces of calcined coke in larger amounts than 0.2 wt. percentage of calcined coke particles. 14. The method according to Claim 11, characterized in that said calcined coke particles and said sodium carbonate powders are maintained at said temperature for at least about 30 seconds. 15. A process for calcining raw petroleum coke particles wherein said coke particles are passed through a rotary calciner having a discharge end while said particles are heated to calcining temperatures, comprising an improvement in which said calcined coke particles are treated to inhibit swelling of said calcined coke, characterized by adding sodium carbonate in the form of a dry, granular powder to the particles of calcined coke in a hot zone associated with said discharge end of said calcining furnace at a temperature in the interval between 1200ºC and 1400ºC; and maintaining said particles of calcined coke and said sodium carbonate at said temperature while in said hot zone for a sufficient period of time to allow said sodium carbonate to react with the carbon and to allow the reaction product to penetrate said particles of calcined coke and to form a deposit which contains sodium in the entire mass of said calcined coke particles. 16. The method according to Claim 15, characterized in that said sodium carbonate powder is added to said particles of calcined coke in amounts varying from about 0.2 to about 2.5 wt.% of the particles of calcined coke. 17. The method according to Claim 16, characterized in that said sodium carbonate powder is added to said particles of calcined coke in amounts varying from about 0.5 to 2.5 wt.% of the particles of calcined coke. 18. The method according to Claim 15, characterized in that said particles of calcined coke and said sodium carbonate are maintained at said temperature for at least about 1 minute. 19. A process for the production of a carbon article from petroleum coke with a high sulfur content, characterized by the fact that it includes: reacting particles of said petroleum coke with a compound containing an alkali metal selected from the group containing sodium and potassium at an elevated temperature above that at which said compound begins to react with carbon but below the temperature at which said particles of coke begin to swell in the absence of said compound; maintaining said coke particles and said compound at an elevated temperature for a sufficient period of time to allow the reaction to take place and to allow the reaction products to penetrate said coke particles and to form a deposit containing sodium or potassium throughout the mass of said particles; cooling thus treated coke particles; mixing cooled coke particles with a resin binder; forming the mixture into an article of the desired shape; and firing the molded article at a temperature sufficient to carbonize the binder. 20. The method according to Claim 19, characterized in that said baked article is heated at a sufficiently high temperature to graphitize said article. 21. The method according to Claim 19, characterized in that said elevated temperature is between 1200ºC and 1400ºC and said alkali metal compound is in the form of dry, granular powders. 22. A process for the production of a carbon electrode from petroleum coke with a high sulfur content, characterized in that it includes in combination: calcination of said petroleum coke particles; adding sodium carbonate in the form of a dry, granular powder to the calcined coke particles at a temperature above about 1200ºC but below the temperature at which said calcined coke particles begin to swell in the absence of said sodium carbonate; maintaining said particles of calcined coke and said sodium carbonate at said temperature for a sufficient period of time to allow the sodium carbonate to react with the carbon and to allow the reaction products to penetrate said particles of calcined coke and to form a deposit containing sodium throughout the mass of said particles calcined coke; cooling of calcined coke particles treated in this way; mixing cooled particles of calcined coke with a resin binding agent; forming the mixture into the desired shape of said electrode; and baking the shaped electrode at an elevated temperature sufficient to carbonize the binder. 23. The method according to Claim 22, characterized in that said shaped electrode is heated at elevated temperatures above 2800ºC to graphitize said electrode. 24. Carbon filler for use in the production of carbon electrodes comprising discrete particles of petroleum coke having a high sulfur content and having an anti-swelling agent distributed throughout the mass of said particles, characterized in that said anti-swelling agent comprises a deposit containing sodium or potassium which is distributed in the entire mass of said particles, whereby the average amount of inhibitor in said particles is greater than about 0.15 wt. percentages. 25. Carbon filler according to Claim 24, characterized in that said deposit inhibiting agent is sodium containing. 26. Carbon filler according to Claim 24, characterized in that the sulfur content is greater than 0.7 wt.%. 27. Apparatus for treating raw petroleum coke particles, characterized by the fact that it includes in combination: an elongated cylindrical calcining furnace having an inlet end and an outlet end; an inlet chamber and a discharge chamber, said calciner having its inlet end mounted for rotation in said inlet chamber and its outlet end mounted for rotation in said discharge chamber; an elongated, cylindrical cooler having an inlet end and an outlet end; means defining a rotary chamber in communication with said discharge chamber for collecting and retaining calcined coke particles when they are discharged from the outlet end of said calcining furnace; means defining a hot zone in communication with said retention chamber and said inlet end of said cooling drum; means for introducing a dry, granular blowing inhibitor into said reaction chamber in contact with said calcined coke particles; and a coke delivery chamber for collecting cooled calcined coke particles at the outlet end of said cooler, wherein said cooler has its inlet end mounted for rotation in said retention chamber and has its outlet end mounted for rotation with said delivery chamber. 28. Apparatus according to Claim 27, characterized in that said retention chamber is a clinker box mounted below said discharge chamber and having an exit opening and where said zone is inserted within said inlet end of said cooler. 29. Apparatus according to Claim 28, characterized in that said zone is formed by means of a refractory ring mounted in said inlet end of said cooler and is spaced at a predetermined distance from said outlet opening of said clinker box. 30. Apparatus according to Claim 27, characterized in that said retention chamber incorporates said hot zone in a separate reaction vessel mounted below said discharge chamber.