Abstract:
A method of calcining green petroleum coke which includes separating the green coke having a particle size of between 0.1 mm and 50 mm into undersized and oversized fractions, pelletizing the undersized fraction with a binder to form pelletized coke, combining the oversized fraction and the pelletized coke to form a feed mixture, and calcining the feed mixture to form calcined coke. The method includes the addition of a pulverization step wherein all the green coke is pulverized before pelletization and the pellets are then calcined to produce a pelletized calcined coke product.

Description:
[0001]    The present invention is directed to a method for calcining green petroleum coke and more particularly for calcining green petroleum coke utilizing a shaft calciner which will reduce product dusting problems or with a rotary kiln calciner to significantly improve recovery and produce a more consistent and homogeneous product. 
         [0002]    The value of green petroleum coke used to produce calcined petroleum coke for use in the aluminum industry and other industries which use calcined petroleum coke has been growing. This is being driven by increased demand from these growing end-user industries and a diminishing supply of suitable quality green petroleum coke from the oil refining industry. The total global production of green petroleum coke has been increasing but much of the new, incremental production is lower in quality with higher contaminant levels such as sulfur, vanadium and nickel. Calcined petroleum used by the aluminum industry and other industries requires higher quality green petroleum coke with lower contaminant levels and a favorable structure for the end-use application. 
         [0003]    As the value of green petroleum coke suitable for calcining increases due to global supply/demand imbalances, it becomes more feasible and desirable to add additional processing steps which can further enhance the value of the green petroleum coke and the calcined coke product. The additional cost of adding such processing steps can be justified if it allows a broader range of green petroleum cokes to be used or improves the quality of the calcined coke product or improves the recovery of calcined coke in the transition from green petroleum coke to calcined petroleum coke or all of the above. The addition of processing steps such as screening, grinding and pelletization/agglomeration or briquetting are all examples of things that can be done to enhance the overall value or improve the utility value of the calcined coke product produced. It is the addition of these extra processing steps that form the basis of this invention and patent application 
         [0004]    The present invention therefore utilizes a combination of pelletization, agglomeration or briquetting technologies in combination with screening and milling/grinding technologies to eliminate dusting problems in shaft calcining. A combination of this technology can also significantly improve the ability to use a wider range of green petroleum coke raw materials to make calcined petroleum coke and significantly improve calcined coke quality by making more dense pellets or briquettes or improve the recovery of calcined coke from the green coke starting product in both shaft calcining technology and rotary kiln calcining technology. 
       SUMMARY OF THE INVENTION 
       [0005]    A method in accordance with the present invention for calcining green petroleum coke includes separating green coke having a particle size between about 0.1 mm and 50 mm into undersized and oversized fractions. More particularly, the undersized fractions may have a particle size of less than 3 mm and the oversized fraction may have a particle size of greater than 3 mm. These particle sizes are given as an example only. Any particle size could be chosen as the delineation point between the undersize and over-size coke fractions. 
         [0006]    Thereafter, the undersized fraction is pelletized with a binder to form pelletized coke or briquetted to form briquettes. Pelletization is considered preferable because it is advantageous to produce particles with a spherical shape. 
         [0007]    More particularly, the binder may include a coal tar pitch with a softening point of 90-120° and further the undersized fraction may be heated to a temperature of about 150° before pelletizing. Many other binders could be used in principal however including petroleum pitch, lignin, polyvinyl alcohol etc. Any organic based material that can act as a glue to bind fine particles could be used and many other industries use a wide variety of different binders. 
         [0008]    Thereafter, the oversized fraction and the pelletized coke or briquetted coke are combined to form a feed mixture, which is thereafter calcined in a shaft calciner or a rotary kiln calciner. 
         [0009]    The binder may be utilized in an amount of between about 0 to about 15% by weight of the pellet or briquette. 
         [0010]    Alternatively, a method in accordance with the present invention may also include milling green petroleum coke to a particle size of about 2 mm or less and thereafter pelletizing or briquetting the milled coke with a binder to form pelletized or briquetted coke. The particle size of 2 mm is given as an example. It may be advantageous to mill to a finer or coarser particle size. 
         [0011]    As hereinabove noted, the binder may include a coal tar pitch having a softening point of about 90-120° and the milled coke may be heated to a temperature of about 150° before pelletization. 
         [0012]    Preferably, the pellet size is between about 2 mm and 25 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The advantages and features of the present invention will be better understood by the following description when considered in conjunction with the accompanying drawings in which: 
           [0014]      FIG. 1  is a block diagram of an embodiment of the present invention utilizing a screen for separating green coke before pelletization and calcining; and 
           [0015]      FIG. 2  is a block diagram of another embodiment of the present invention utilizing a pulverizer before preparation of green coke before pelletization and calcination thereof. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Rotary kilns and shaft kilns have been used successfully to produce calcined coke for the aluminum industry such as, for example, the production of electrodes useful for molten salt electrolysis of aluminum oxide to aluminum set forth in U.S. Pat. No. 7,141,149 to Edwards, et al. 
         [0017]    The primary goals of calcining green coke are to: 
         [0018]    1. Remove volatile matter (VM); 
         [0019]    2. Densify the structure to avoid shrinkage of coke during anode baking; and 
         [0020]    3. Transform the structure into an electrically conductive form of carbon. 
         [0021]    Rotary kilns are large diameter, sloped refractory lined steel-shelled cylinders which rotate during operation. Green coke is fed continuously in one end and calcined coke is discharged from the other end at 1200-1300° C. The coke bed loading in the kiln is low (7-10% of the cross-sectional area) and heat is transferred to the coke bed predominantly by radiative and convective heat transfer from the counter-current gas stream and refractory lining. 40-50% of the VM is combusted inside the kiln and the rest is combusted in the pyroscrubber upstream of the kiln. The VM combusted in the kiln provides most of the heat for calcination but natural gas, fuel oil and/or pure oxygen can be added to provide additional heat. 
         [0022]    In a rotary kiln, approximately 10% of the finest particle size green coke becomes entrained in the flue gas stream and blows out the back end or feed end of the kiln. From there, it travels to a pyro-scrubber and is combusted completely producing a large volume of waste-heat. This waste heat is typically recovered in the form of waste-heat energy. As a result of VM loss and coke fines loss, the typical recovery of calcined coke in a rotary kiln is around 77-80%. In other words, for every 1 ton of dry green coke fed to the kiln, 0.77-0.80 tons of calcined coke product is produced. 
         [0023]    A shaft kiln, or calciner, has multiple vertical refractory shafts surrounded by flue walls. The green coke is fed into the top and travels down through the shafts and exits through a water cooled jacket at the bottom. The movement of coke is controlled by opening a slide gate or rotary valve at the bottom of each shaft to discharge a small amount of coke. The discharge is intermittent (˜every 20 minutes) and green coke is added to the top to maintain the feed. 
         [0024]    The VM in a shaft furnace travels up through the coke bed and enters flue wall cavities at the top of the furnace. It is mixed with air at this point and then drawn down through a set of horizontally oriented flues. VM is combusted inside the flue walls and heat is conducted to the coke indirectly from the flue walls in an analogous manner to heat transfer in an anode bake furnace. 
         [0025]    There is no large volume, counter-current gas flow inside a shaft calciner so there is very little loss of fine particle size green coke. As a result, the recovery in a shaft calciner is much higher than a rotary kiln, typically around 85-89%. So for every 1 ton of dry green coke fed to a shaft calciner, 0.85-0.89 tons of calcined coke product is produced. Unfortunately, the very fine particle size green coke fed to the furnace stays with the product after calcination and creates dusting problems. Once the calcined coke product is handled, the fine calcined coke particles attached to the surface of larger particles are dislodged and they create dusting problems for the end-user of the product. 
         [0026]    The dusting problem created by shaft calciners coke can be solved by the present invention by eliminating fine green coke introduced into the kiln. 
         [0027]    With reference to  FIG. 1  a source of green petroleum coke  12  with a range of particle sizes from ˜&lt;0.1 mm to &gt;25 mm is first separated into two size ranges using industrial scale, particle size separation equipment such as vibrating screen decks  16 . 
         [0028]    The two size ranges can be referred to as “undersize” and “oversize” fractions or “fines” and “coarse” fractions. In this example, the coke is sized at a particle size of 3 mm so that all the −3 mm particle size petroleum coke is separated from the bulk petroleum coke using a series of mechanical, vibrating screens. When the particle size separation is completed, the coke will be separated into two different particle size range piles. The −3 mm pile is hereafter referred to as the undersize fraction and the +3 mm pile is hereafter referred to as the oversize fraction. 
         [0029]    Green petroleum coke from the undersize pile is fed to an industrial scale pelletizing or agglomerating machine  20 . There are many different types of pelletizing and agglomerating equipment available and the present invention covers the application of all such equipment including briquetting machines (not shown). The basic concept of the present invention is to take green petroleum coke fines or calcined petroleum coke fines and build larger particles through the application of pelletizing, agglomeration or briquetting technology. 
         [0030]    In this first embodiment, a rotary drum pelletizer or granulator  20  of the type made by the Eirich Company is used to make spherical pellets of green petroleum coke fines. The petroleum coke fines are first dried to remove moisture and then fed to a pelletizer. A small amount of binder in the weight range of 0-15% is then added to the pelletizer and mixed with the coke fines to impart sufficient strength to the spherical pellets formed inside the pelletizer. 
         [0031]    In this embodiment, a low softening point coal tar pitch is used as a binder. The coal tar pitch has a softening point of ˜90-120° C. and the coal tar pitch and coke fines are heated to a temperature of 150° and then mixed together in the pelletizer  20 . An addition rate of 5 weight % coal tar pitch is used. Spherical pellets are formed inside the pelletizer and the process is conducted on a continuous basis. The pellets range in size from 2 mm up to 25 mm. They are continuously discharged from the mixer and then allowed to cool to room temperature. The coal tar pitch binder solidifies as it cools and provides the pellets with sufficient mechanical strength for subsequent handing and processing. 
         [0032]    The green coke pellets can be fed directly to a coke calcining kiln  24  (rotary or shaft) or blended with the oversize coke particles and then fed to the calcining furnace to produce calcined coke  30 . Any coke calcining furnace or kiln  24  can be used including a shaft calciner, rotary kiln calciner or a rotary hearth calciner. The application of this technology works best with a shaft calciner since a shaft calciner has no moving parts and therefore does not damage or disrupt the green coke pellets. It also has a very slow heating rate and this is important for proper densification of the green coke pellets. The aim of the present invention is to make dense, low porosity calcined coke pellets and this is best achieved by heating the green coke/coal tar pitch very slowly so that structural rearrangement of the carbon molecules can occur without the disruptive release of volatile matter. A slow heat up rate also ensures more uniform shrinkage of the green coke pellets when the volatile matter is released. 
         [0033]    That is not to say that a rotary kiln calciner cannot be used but a rotary kiln and rotary hearth calciner use faster heating rates and the coke undergoes mechanical tumbling and movement which can potentially disrupt the physical structure and integrity of the pellets. When a shaft calciner is used to calcine green petroleum coke, the average volatile matter content of the green coke fed to the furnace must be controlled within a narrow range to (a typical range is 10-11%) to avoid operational problems with the furnace. This same strategy must be adopted when feeding green petroleum coke pellets made with a coal tar pitch binder. The average volatile matter content of the feed mixture containing green coke pellets, oversize coke and even calcined coke must be controlled to the target volatile matter content. When high volatile matter cokes are used in the green coke blend fed to the furnace, varying amounts of calcined coke are added to the feed mix to reduce the average volatile matter content of the feed. 
         [0034]    When green petroleum coke is separated into an undersize and oversize fraction as described above, pelletized and then calcined, the calcined coke pellets are both dense and mechanically strong and this makes them ideal for use in the production of anodes used for the electrolytic production of aluminum. The spherical shape of the pellets improves the packing density of calcined coke particles used to make an anode and this in turn helps improve anode density. 
         [0035]    The pelletization of the green coke fines eliminates one of the main disadvantages of shaft calcining which is production of a dusty, calcined coke product. This is problematic with a shaft calciner because there is no mechanism to remove fine green coke inside the shaft calciner. All the fine green coke in the feed to the furnace ends up as fine dust in the calcined coke product. This is quite different to a rotary kiln where most of the fine particle size green coke becomes entrained in the flue gas stream and exits the kiln counter-current to the green coke feed. The entrained coke fines are then combusted in a pyroscrubber or incinerator downstream of the kiln. In many modern plants, the waste heat generated through this combustion is recovered in the form of energy. hi a shaft calciner, there is no high volume, high velocity counter-current gas flow to entrain coke fines so they stay with the product loosely agglomerated or attached to the surface of larger calcined coke particles. 
         [0036]    The embodiment above represents one quite specific application of the present invention. The concept of using pelletization or any other form of agglomerating or briquetting green coke fines to make large pellets can be applied to any type of green petroleum coke with any chemical and physical composition. The calcined coke pellets produced during the calcination step can then be used in any application including, but not limited to, anode and aluminum production, titanium dioxide production, carbon raiser applications in metallurgical foundries, graphite electrode manufacture etc. Basically, any existing application which uses calcined petroleum coke could benefit from the present invention. 
         [0037]    In an analogous manner to the above, any type of binding agent can be used to impart sufficient mechanical strength to the pellets or briquettes. Coal tar pitch is given as an example but it could be any organic based binder. Inorganic binders containing elements such as sodium, calcium or silicon are not suitable because they will contaminate the calcined coke product making it unsuitable use. Examples of other organic type binders include petroleum pitch, lignite, cellulose material and polymers such as PVA. 
         [0038]    A second embodiment of the present invention is illustrated in  FIG. 2  with common steps being identified by common reference numbers shown in  FIG. 1 . In this second embodiment, an additional process step is added whereby all the green petroleum coke is first ground or milled to produce a fine particle size product. A wide range of industrial scale crushing and milling/grinding equipment  32  can be used to pulverize the green petroleum coke to a finer particle size. There are several potential advantages to adding this pulverizing step before pelletizing the green petroleum coke fines as follows:
       1) It ensures a more consistent particle size feed to the pelletizing equipment. This will ultimately lead to better control of pellet size, density and mechanical strength.   2) It provides a well-controlled way to mix and blend together green petroleum cokes with different properties. This could include cokes with different chemical, physical and structural properties.   3) It provides an excellent means for controlling the average volatile matter content of the pelletized product through the addition of small amounts of calcined coke.       
 
         [0042]    The addition of a pulverizing step prior to pelletization of green petroleum coke fines may dramatically change the way green petroleum coke is used to make calcined petroleum coke. Coarse particle size green petroleum coke which is typically beneficial and desirable for calcination would no longer be important. It would also significantly improve the ability to use a wide range of green petroleum cokes to make calcined coke product with a specific and desirable set of properties. For example, the aluminum industry typically prefers to use green petroleum coke with a sponge structure to make anodes. Petroleum coke with a shot structure is less desirable due to generally higher impurity levels, a harder, more abrasion resistant structure and a higher coefficient of thermal expansion. 
         [0043]    If all the green coke is pulverized first, cokes with a wide range of properties can be blended together to produce green petroleum coke pellets which can then be calcined to produce a consistent quality, pelletized calcined coke product with good bulk and apparent density and targeted chemical and thermal expansion properties. For example, a mixture of shot coke and sponge coke could be pelletized to produce a calcined coke product with more desirable thermal expansion properties than a mixture containing 100% shot coke. The second example therefore seeks to broaden the application of pelletization and briquetting technology to something with much greater product potential. It will provide the industry with a much more flexible technology package for utilizing different quality green petroleum cokes to produce a consistent quality calcined coke with the properties desired by the end user. 
         [0044]    Although there has been hereinabove described a specific pelletization and calcination of green coke in accordance with the present invention for the purpose of illustrating the manner in which the invention may be used to advantage, it should be appreciated that the invention is not limited thereto. That is, the present invention may suitably comprise, consist of, or consist essentially of the recited elements. Further, the invention illustratively disclosed herein suitably may be practiced in the absence of any element which is not specifically disclosed herein. Accordingly, any and all modifications, variations or equivalent arrangements which may occur to those skilled in the art, should be considered to be within the scope of the present invention as defined in the appended claims.