Patent Document

CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Application No. 60/408,770 filed Sep. 6, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to furnaces for the heat treatment of powders and granular materials.  
         BACKGROUND OF THE INVENTION  
         [0003]    The production of graphitized carbon has been practiced since the end of the 19 th  century and marks the early stages of the industrial revolution. The production of graphite from so-called carbon electrodes had traditionally been carried out in the Acheson furnace in which the electrodes are typically placed in a transverse orientation to the flow of the electrical current, and surrounded by a resistor medium, thereby causing the current to pass alternately through tiers of electrodes and resistor media, the latter being typically metallurgical or petroleum coke. The Acheson process is of such ancient vintage and so well known as not to require any further description. However, it is worth noting that the process is a batch process, not particularly energy efficient and relatively labor intensive.  
           [0004]    The LWG process, although very old, is less well known and has been practiced on a commercial scale only recently. This process is carried out by arranging the carbon electrodes in a continuous column with an electrical connection at each end of the column. See U.S. Pat. No. 1,029,121 issued to Heroult and U.S. Pat. No. 4,015,068, issued to Vohler. In the LWG process, the electrodes themselves form the dominant path for the heating current, with one or both of the ends of the column subjected to a mechanical or hydrostatic pressure source in order to keep the connection tight under expansion or contraction of the column during the heating cycle. A packing medium of granular coke is used for insulation, however, Vohler does not use a packing medium, but discloses a metal shell with a felt liner as insulation.  
           [0005]    Carbon electrodes consist of the essentially amorphous carbon from petroleum coke which has been calcined, ground, classified by size, mixed with a binder, and bound in a matrix of amorphous carbon derived from the binder after baking at temperatures of approximately 700.degree. C. to 1100.degree. C. in a baking furnace. Graphite electrodes are produced from the carbon forms by placing them in an Acheson furnace and in recent years in a Lengthwise Graphitization (LWG) type furnace and heating them to a temperature between 2500.degree. C. to 3000.degree. C., which converts the amorphous form of carbon to the crystalline graphite, and vaporizes most of the impurities present in the original carbon, including most metals and sulfur compounds. Graphite, compared to amorphous carbon has much higher electrical and thermal conductivity, lower coefficient of thermal expansion (CTE), superior ductility and vastly superior thermal shock resistance at the operating temperatures of the electric arc steel furnace. These physical properties are uniquely valuable in the electric furnace with its need for high electrical currents, and the need to resist the mechanical and thermal shock suffered by the electrodes from the falling scrap, fluctuations in metal and electrode level, and generally high thermal stresses. Consequently, graphite is universally used as an electrode in the electric arc melting of steel.  
           [0006]    The LWG process has many advantages over the Acheson process. The energy efficiency is much higher, as the material is heated directly instead of indirectly, and the cycle time for the process is much faster taking typically less than 12 hours as compared to 60 to 120 hours for the Acheson process. U.S. Pat. No. 4,394,766 issued to Karagoz describes an LWG furnace where “ . . . the current is applied, heating the column of electrodes rapidly by the Joule effect to the required graphitization temperature, usually from 2400.degree.-2800.degree. C., sometimes as high as 3000.degree. C., taking approximately 4 to 12 hours, until the graphitization process is completed. The power is turned off, the furnace moved to a cooling station and the electrodes allowed to cool. When the electrodes have reached approximately 1500.degree. C.-1700.degree. C., the furnace is moved to the dump and re-load station and the transporter is replaced by a chute car with ducts leading from the dumping gates to the hoppers below. The electrodes are unloaded by a grab (stock extractor), the insulation medium is dumped at a weighted average temperature of from 700.degree. to 1100.degree. C. into the hoppers, and the furnace loaded with another electrode string and insulation charge. The chute car is removed and the furnace is transported back to the firing station.” There is no heat exchange between the carbon electrodes and the graphite electrodes in this process resulting in significant energy inefficiency.  
           [0007]    U.S. Pat. No. 5,229,225 issued to Karagoz, et al., also shows an improved LWG furnace of modular design comprising roughly semi-circular shell coupled by expansion joints in which a pressure seal is held in place by deadweights. The shell is formed of corrugated steel panels with cast-able ceramic lining. The panel design allows more freedom for thermal expansion in the transverse direction while accommodating longitudinal expansion by freedom to slide over its support cradle. The corrugated panel design also enhances faster cooling of the furnace after off-fire by improved heat transfer in comparison to a simple steel plate structure. The corrugated structure has a higher surface area than a simple plate, which gives more radiative cooling and turbulent air movement giving more convective cooling. Again however, there is no energy exchange between starting materials and finished product resulting in significant energy loss.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention comprises, in one form thereof, a continuous processing apparatus for high temperature thermal treatment of granular materials. The apparatus includes a vertical conveyor means with an internal feed mechanism for transporting granular feedstock upward, an external export means for taking reacted product downward wherein said internal feed heats said granular feedstock by absorbing heat from the product flowing downward through said export means, and a heating means disposed around a top portion of said vertical conveyor means and said external export means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be better understood by reference to the following description of the embodiments of the invention in conjunction with the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a cross-sectional view showing the structure of the vertical conveyor furnace the present invention;  
         [0011]    [0011]FIG. 2 is a cross-sectional view of the furnace of FIG. 1;  
         [0012]    [0012]FIG. 2 a  is a cross-sectional view of the thermal processor of FIG. 1;  
         [0013]    [0013]FIG. 3 is a schematic of the thermal processor of FIG. 2 a;    
         [0014]    [0014]FIG. 4 is a graph of the temperature profile of Example 1; and  
         [0015]    [0015]FIG. 5 is a graph of the temperature profile of Example 2. 
     
    
       [0016]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate the preferred embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION  
       [0017]    Referring to FIGS. 1 and 2, there is shown the vertical conveyor furnace of the present invention. The vertical conveyor furnace  10  includes an input unit  100 , a heating unit  200 , and an output unit  300 .  
         [0018]    The input unit  100  includes a raw material hopper  104 , a raw material feed tube  108 , a raw feed conveyor screw  110 , a heater intake tube  112 , a vertical conveyor screw  114 , and a delivery cone  116 . The raw material hopper  104  is isolated from ambient gasses by intake valve  102  which allows raw material  122  to be added to the raw material hopper  104  followed by a purge cycle using process-neutral gasses such as N 2  and Ar or mixtures thereof. The raw material hopper  104  includes a raw material level sensor  106 . The raw feed conveyor screw  110  is situated within the raw material feed tube  108 , which is in communication with the raw material hopper  104 . The raw feed conveyor screw  110  is driven by a motor at one end to transport the raw material  122  from the raw material hopper  104  to the heater intake tube  112 . The heater intake tube  112  is in communication with the raw material feed tube  108  and encloses the vertical conveyor screw  114 . The heater intake tube  112  is sealed at the lower end to prevent material from falling through. The vertical conveyor screw  114  is driven at one end to transport preprocess material  124  in the upward direction into the delivery cone  116 . The delivery cone  116  includes a pre-furnace portion  118  and a furnace portion  120 . The pre-furnace portion  118  is somewhat cooler than the furnace portion  120  and therefore may be made of a metal such as steel. The furnace portion  120  may require high temperature resistant materials such as graphite for high temperature processes. The furnace portion  120  has an increasing diameter in the upward direction in order to reduce the outward pressure on the walls of the delivery cone  116 .  
         [0019]    The heating unit  200  may be one of many types of furnaces, for example resistance heaters, natural gas heaters, and induction furnaces. By way of example, an induction furnace is described here and shown in the drawings. The heating unit  200  includes a processing chamber  202 , an inner liner  204 , an outer liner  206 , insulation  208 , a structural heater wall  210 , and a heater cover  212  in sealing engagement with the heater wall  210 . The inner liner  204  and the outer liner  206  act as the suseptors in the induction heater and are preferably made of graphite. The insulation  208  may be a material such as carbon black insulation material sold under the trademark THERMAX N991 (ULTRAPURE) owned by Cancarb Limited Corporation Canada. Induction coils  214  surround the heater wall  210  in proximity to the processing chamber  202 . Cooling coils  216  surround the remainder of the heater wall  210  and carry a coolant such as water. A coil retaining wall  218  secures the induction coils  214  and the cooling coils  216 . A pyrometer port  220  and the exhaust ports  222   a  and  222   b  penetrate the heater cover  212  and the insulation  208  into the processing chamber  202 .  
         [0020]    The output unit  300  includes a cooling wall  302 , an output passage  304  formed between the cooling wall  302  and the heater intake tube  112 , an output tube  306 , an output conveyor screw  308 , an output isolation chamber  310 , and a product hopper  316 . The cooling wall  302  is preferably made of steel and is cooled by a coolant such as water. The cooling wall  302  includes a flange at the top end to support the heating unit  200 . The output tube  306  is in fluid communication with the cooling wall  302  and is cooled by a coolant such as water. The output conveyor screw  308  is driven by a motor at one end to transport the cooled processed material  320  to the output isolation chamber  310 . The output isolation chamber  310  includes a chamber input valve  312  and a chamber output valve  314 . The chamber output valve  314  normally isolates the cooled processed material  320  and the product  322  from the ambient gasses in the product hopper  316 . The chamber input valve  312  closes when the chamber output valve  314  opens so that the cooled processed material  320  remains isolated from the ambient gasses.  
         [0021]    In use, granular material is fed in a feedstock hopper ( 104 ) and the feedstock reserve ( 122 ) may be purged with a desired gas or gas mixture by flowing such gas through the intake valve ( 102 ) after purging the hopper. The feedstock is then fed into raw feed tube ( 108 ) and advanced by the raw feed conveyor screw ( 110 ) into the heater intake tube ( 112 ) of the thermal processor ( 226 ). The vertical conveyor screw ( 114 ) advances the feedstock up the heater intake tube into the delivery tube ( 116 ) which is made of a high temperature compatible material such as graphite or ceramic. The delivery tube may be conical with an increasing radius it projects further into the heating section of the apparatus. The granular feedstock is pushed through the heater intake tube into the heating section, and out of the top of the intake tube falling into the output means that gravitationally feeds the thermally treated feedstock into the cooling portion of the output passage ( 304 ) downward to means for transporting the thermally treated feedstock away from the thermal treatment apparatus.  
         [0022]    The feedstock material when thermally treated may produce unwanted gas by products. The present method anticipates this concern and allows for venting of unwanted gases by streaming relatively inert gases over the head of the feed stock in the heating section ( 324 ,  326 ). The treated feedstock may be transported from the thermal treatment unit by a water cooled conveyor means so that the heat build up is minimized. The feedstock can then be further cooled in an isolation chamber  310  and allowed to cool to a desired temperature limit and then released to a final product hopper  316 .  
         [0023]    In instances where the method is employed to thermally treat coke powder or granules, the feedstock is held in the intake hopper, purged and filled with nitrogen. After the purge and nitrogen flow, the hopper is sealed and the intake hopper is opened and material is advanced through the raw feed conveyor by the screw. The conveyor may have a flow of inert gas introduced to insure against seal imperfections. The screw in the raw feed conveyor is designed to compact the feed material flow to insure some degree of compaction of the feedstock into the vertical conveyor. The vertical conveyor takes the feedstock into the heating section of the equipment where the coke is heated to 900 to 2500 degrees centigrade for a relatively short period of time to graphitize the feedstock into a finished product. Typical time periods can be as short as a few minutes to complete the thermal conversion process.  
         [0024]    An advantage of this method is that heat can be transferred from the treated product stream both in the insulated portion ( 330 ) and the heated portion ( 328 ) to the feedstock in the feed tube ( 108 ). This heat transfer increases the efficiency of the treatment process. Furthermore, the fact that the system is a continuous treatment process provides a significant increase in energy efficiency and lowers setup and labor cost relative to the state of the art batch processes.  
         [0025]    The following examples comprising numeric models of the present invention specifically show the method and apparatus in use in order to thermally process milled coke and crushed coke.  
                             TABLE 1                       General Parameters for the Examples                                    Production rate   150-kg/hr           Process Atmosphere   N 2             Process temperature   &gt;2500° C.           Output temperature of product 322    &lt;150° C.           Vertical conveyor screw 114 length   2-m           Constant Diameter upflow cross-sectional area   0.033-m 2             Height of cooling wall 302   2.5-m           Constant Diameter downflow cross-sectional area   0.469-m 2             Insulated product stream 330 length   1.1-m           Process chamber 202 height   0.4-m           Process chamber 202 diameter   0.8-m                      
 
       EXAMPLE 1 
       [0026]    [0026]                             TABLE 2                       Example 1 Material Parameters                                    Stock material 122   Crushed Coke           Stock material 122 bulk density   1020-kg/m 3             Max particle size   10-mm                        
         [0027]    Referring to FIG. 4 and Tables 1 and 2, Example 1 is illustrated. The maximum upflowing coke, bulk upflowing coke, and the bulk downflowing coke temperatures are plotted as a function of height. The vertical conveyor screw  114  terminates at 2.0-m, at which the temperature is predicted to be no more than 500° C. The bulk downflowing coke, after it has passed the processing surface  224  is greater than 2500° C. The upflowing coke profile demonstrates how efficient the invention is in heat recovery. The upflowing coke has been heated to at least 1000° C. by the downflowing coke before the upflowing coke reaches the actively heated processing chamber  202 . This equates to about a 40% heat exchanger effectiveness between the upflowing coke and the downflowing coke.  
       EXAMPLE 2 
       [0028]    [0028]                             TABLE 3                       Example 2 Material Parameters                                    Stock material 122   Milled Coke           Stock material 122 bulk density   670-kg/m 3             Max particle size   20-μm to 40-μm                        
         [0029]    Referring to FIG. 5 and Tables 1 and 3, Example 2 is illustrated. The milled coke is known to have a lower thermal conductivity than the crushed coke, and this is demonstrated in FIG. 5. The upflowing coke preheats to approximately 900° C. Therefore, more energy is required to bring the bulk upflow temperature to the required degree in the processing chamber  202  than in Example 1. Further, the downflowing coke does not cool down as readily and thus more work is required of the cooling output tube  306  to cool the processed material  318 .  
         [0030]    While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the scope of the invention.  
         [0031]    Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.

Technology Category: c