Abstract:
A process for the production of alumina from aluminum trihydrate in which the trihydrate is dried and pre-heated, after which the remaining free and chemically-bonded water is removed when the trihydrate is converted by calcination to form hot calcined alumina which is cooled to about 50-100° C. in one or more stages. In at least one stage the hot calcined alumina is cooled in a countercurrent cyclone

Description:
[0001]     This invention relates to a process for the production of alumina from aluminum trihydrate in which the trihydrate is dried and pre-heated, after which the remaining free and chemically-bonded water is removed when the trihydrate is converted by calcination to alumina and finally cooled to about 50-100° C. in two stages, where the calcined alumina through at least one stage is suspended in a gas and a cyclone that is utilized in the process.  
       BACKGROUND OF THE INVENTION  
       [0002]     The expression aluminium hydrate is used in trade and industry in reference to aluminum hydroxides. Various types of hydroxides are known, but the most well-defined crystalline forms are trihydrates, Al 2 (OH) 3 , gibbsite (α-Al 2 (OH) 3 ), bayerite (β-Al 2 (OH) 3 ), and nordstrandite.  
         [0003]     The Bayer Process—an economical method of producing aluminium oxide—was discovered by an Austrian chemist Karl Bayer and patented in 1887.  
         [0004]     The process dissolves the aluminium component of bauxite ore in sodium hydroxide (caustic soda); removes impurities from the solution; and precipitates alumina trihydrate which is then calcined to aluminium oxide. The final stage in the production of alumina by the Bayer process is the calcination of aluminium trihydrate: 
 
Al 2 (OH) 3 +energy→Al 2 O 3+3 H 2 O 
 
         [0005]     Calcined alumina is normally produced from gibbsite, and this trihydrate is converted by thermal decomposition to α-alumina, α-Al 2 O 3  (corundum).  
         [0006]     Calcination has traditionally been carried out in a rotary kiln so that both dehydration and recrystallisation take place continually in the kiln. The finished calcined product can then be partially cooled in a planetary cooler and this may be followed by cooling in a unit with a water-cooled fluid bed.  
         [0007]     It is also known that calcination can take place in a fluid bed and in such an installation the primary cooling of the material will normally take place in a multiple-stage cyclone cooler with three to five stages, where the primary cooling can be followed by a secondary cooling stage, for instance in a water-cooled fluid bed.  
         [0008]     Since about 1980 the use of a GSC—Gas Suspension Calciner—has been well known in the industry for the calcination of aluminium hydrate (see for example GB Patent no. 2097903).  FIG. 1  illustrates an installation of this type.  FIG. 1  also shows the traditional method of cooling the alumina through the use of a plurality of cooling cyclones. Four cyclones, set forth as  15   a ,  15   b ,  15   c  and  15   d , are depicted. Typically, the cyclones utilized will include a vertically situated upper cylindrical portion. The gas stream enters the upper cylindrical portion tangentially in a generally horizontal direction. The hollow housing will further include a lower portion. The tangential entry of the gas stream into the hollow upper housing converts the linear gas flow into a downwardly spiraling, rotating vortex in a generally helical direction relative to a vertical central axis of the cyclone. The swirling action of the vortex causes the particles in the gas stream within the vortex to be moved to an outer region by the centrifugal force exerted by the swirling action of the vortex. There is an interaction of vortex flow which results in a continuous upward cleaned gas flow toward a cleaned gas outlet mounted generally at the top of the upper cylindrical housing portion. Typically, the cleaned gas outlet is a tube or pipe mounted in the center of the upper cylindrical portion. Thus as the vortex moves downwardly within the lower housing portion particles within the gas stream are moved outwardly toward the inside housing wall of the cyclone and downwardly through an exit at the lower portion of the housing.  
         [0009]     In the production of alumina it is important that the breakdown of the alumina particles should as far as possible be minimized, since the breakdown of the particles leads to finer pulverization of the particles, which thus causes the development of dust and the resulting problems in handling the material.  
         [0010]     It has been seen that particle breakdown is relatively great during the heat exchange process in multiple-stage cyclone coolers, and thus necessitates the production of stronger and larger-grained particles of hydrate, entailing further investment costs which have limited the cost-effectiveness of the use of fluid beds and GSC, which otherwise prove to be the most compact installations, require the least energy and are the most economical to install. It would be advantageous, therefore, to reduce the number of cyclone coolers utilized in the production of alumina.  
       DESCRIPTION OF THE INVENTION  
       [0011]     The invention according to this application demonstrates a process for the production of alumina where the cooling of the calcined alumina in at least one stage is carried out in a countercurrent cyclone in which the cold gas creates a spirally-formed stream from the gas inlet to the gas outlet, in that the cooling gas is introduced tangentially into the countercurrent cyclone through the outer casing of the cyclone and is discharged through an opening close to the horizontal axis of the cyclone. The process is notable in that the countercurrent cyclone, as described in more detail below, will replace at least two, and at times all, of the cooling cyclones used in the prior art process.  
         [0012]     In the present invention, the hot alumina is fed into the countercurrent cyclone close to and parallel with its horizontal axis, but radially displaced in relation to the axis, and is discharged through the base cone of the countercurrent cyclone.  
         [0013]     A countercurrent cyclone of this type can wholly or partially replace a multiple-stage cyclone having three or more cyclone stages with the result that particle breakdown is considerably reduced while the full effect of the heat transfer is maintained and at the same time a better separation of the gas from the material is achieved. Furthermore, as a result of the very simple design of the countercurrent cyclone, it will be more economical to build an installation with a countercurrent cyclone.  
         [0014]     A type of countercurrent cyclone suitable for this application is described in two Danish patents, No. 160586 and No. 161786. A countercurrent cyclone of this type has proved to give a minimal particle breakdown and extremely good heat transfer. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  illustrates a prior art installation for the production of alumina.  
         [0016]      FIG. 2  illustrates an installation for carrying out a process according to the invention.  
         [0017]      FIG. 3  illustrates another embodiment of an installation for carrying out a process according to the invention.  
         [0018]      FIG. 4  is a partial side cut-away view of a “countercurrent” cyclone used in the process of the present invention.  
         [0019]      FIG. 5  is a full side view, with the gas inlet being shown in relief, of a “countercurrent” cyclone used in the process of the present invention.  
         [0020]      FIG. 6  is a top view of a “countercurrent” cyclone used in the process of the present invention.  
         [0021]      FIG. 7  is a full side view of a conical “countercurrent” cyclone used in the process of the present invention.  
         [0022]      FIG. 8  is a full side view of a standard “countercurrent” cyclone used in the process of the present invention, with the material inlet and gas outlet being shown. 
     
    
       [0023]     In the drawings, similar numerals refer to similar elements.  
       DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION  
       [0024]      FIG. 1  illustrates a prior art installation for the production of alumina. Aluminum trihydrate is introduced at  1  into a drying unit  2 . The drying unit  2  may by way of example be a flash drier. As the hydrate is dried and heated up it will become suspended in a gas and will be led via duct  3  to the first pre-heating cyclone  4 . The gas in which the hydrate is suspended is hot process gas from second stage pre-heating cyclone  9  which is introduced via duct  10 .  
         [0025]     In the pre-heating cyclone the dried and pre-heated material is separated from the gas and the gas is led via a duct  5  to be cleaned in a unit  6  which by way of example may be an electrofilter. The material is led via duct  7  into a hot gas flow (0-100° C. cooler than the temperature the material is subject to in calciner  12 ) and is then led in suspension through duct  8  to a further pre-heating cyclone  9 , in which the material is separated from the gas and led via duct  11  to the calciner  12 . The gas is led via duct  10  to the drying unit as previously stated.  
         [0026]     Calciner  12  in this example is a GSC—a Gas Suspension Calciner—in which the material will remain for a few seconds. In calciner  12  the material is heated to approximately 900-1500° C., and preferably to between approximately 1100-1200° C., by means of the burning of gaseous or liquid fuel, which is introduced at  15 , and the material is then led further in suspension with carrier gas introduced into the calciner via duct  16   a  to the separating cyclone  13  where the material is separated from the carrier gas and led via duct  14  to the cooler. The separated gas is led via duct  8  first to a pre-heating cyclone  9  and then to drying unit  2 .  
         [0027]     In this prior art system the primary cooling unit consists of a multiple-stage cyclone cooler with four stages, illustrated as cyclones  15   a ,  15   b ,  15   c  and  15   d , with riser pipes  16   b ,  16   c  and  16   d . The material is introduced into the first cyclone  15   a  via a gas duct  16   b  and in the cyclone the material is separated from the gas and led via duct  17   a  to gas duct  16   c . The gas, which has a temperature of approximately 600-800° C., is led via duct  16   a  to the base of the calciner  12 .  
         [0028]     The material which is suspended in the gas in gas duct  16   c  is led to cyclone  15   b . From there the gas is led via duct  16   b  and the separated material is transported via duct  17   b  to a gas duct  16   d , where it is again suspended in a gas and led to the third cyclone  15   c.    
         [0029]     From cyclone  15   c  the material is led via duct  17   c  to a gas duct  18 , through which cold air is transported to the primary cooling unit and through this duct the material is carried in suspension to the last cyclone  15   d , after which the gas is led to the third cyclone via duct  16   d  and the material is led via duct  17   d  to the secondary cooling unit  20 . A convenient type of secondary cooling unit  20  is the fluid bed type to which cold gas is led via duct  22  and where the finished product is discharged for storage or for further processing via duct  21 .  
         [0030]      FIG. 2  illustrates an installation in which the flows between the drying unit  2 , the pre-heating cyclones  4  and  9  and between the calciner  12  and the separating cyclone  13  are identical with the flows described and illustrated in  FIG. 1 .  
         [0031]     Referring now to  FIGS. 2 and 3 , the material which is separated from the gas in separating cyclone  13  is led via duct  14  into a gas flow of pre-heated gas in which the material is suspended, and this suspension is then introduced into cyclone  15   a  via duct  26 . In cyclone  15   a  the material is separated from the gas and the material is led via duct  17   a  into a gas flow in which the material is suspended and the suspension is thereafter introduced into a countercurrent cyclone  25  via duct  23 . Duct  23  leads the suspension of gas and material into the central section of the countercurrent cyclone  25  close to the horizontal axis A-A′ of the cyclone. In the cyclone the material is cooled to a temperature of between about 180° and 300° C. The material is driven outwards by centrifugal force towards the cyclone casing, is led out through the base of the cyclone and transferred via duct  27  to a secondary cooling unit  20 , in which the material is cooled to a temperature of between about 50° and 100° C.  
         [0032]     The hot alumina material is introduced into the cyclone parallel with, but radially dispersed from, the horizontal axis A-A′ of the countercurrent cyclone and in such a manner that immediately on introduction the hot alumina possesses a tangential velocity component in relation to the axis of the countercurrent cyclone, and that this velocity component rotates in the same direction around the axis as the spirally-formed cold gas flow and thereafter the material is transported for a time by said cold gas flow.  
         [0033]     Cold gas for cooling the material is introduced into the countercurrent cyclone through duct  24 . This duct  24  directs the cold gas flow tangentially into the cyclone and the gas forms a spiral flow inwards toward the cyclone axis A-A′, where there is a discharge outlet for the gas now pre-heated by heat transfer with the hot material. The pre-heated gas is then led forward via duct  26  to cyclone  15   a.    
         [0034]      FIG. 3  illustrates an installation which is equipped with a possible means of directing part of the hydrate, that normally would all be directed to calciner  12 , via a bypass to first cooling stage  30 . An adjustable amount of the material which leaves the pre-heating cyclone  4  is sent directly via duct  28  to first cooling stage  30 . The remainder of the material which leaves cyclone  4  is led via duct  7  across to gas duct  8 , where it is brought into suspension and led to the calciner  12  via pre-heating cyclone  9 .  FIG. 3  also illustrates directly sending the preheated gas directly from countercurrent cyclone  25  to calciner  12  via duct  32 , rather than sending the gas first through preliminary cooler  15   a , as is the case in the embodiment of  FIG. 2 .  
         [0035]     The gas which is led away from the pre-heating cyclone  4  is sent to be cleaned, for instance in an electrofilter. The dust which settles out in cleaning unit  6  can then be led to the cooling system via duct  29 , which leads the dust to material duct  7 .  
         [0036]     In the installation of  FIG. 3 , the calcined material passes, unlike the installations illustrated in  FIGS. 1 and 2 , via duct  14  to a first cooling unit  30 . In this case the first cooling unit  30  is of the fluid bed type and material reaches it via ducts  14  and  7 , while the cooled material leaves the unit via duct  33  and pre-heated gas is led via duct  31  to duct  10 , through which the heated gas is led further to the drying unit  2 . The cooled material is led via duct  33  into a gas duct  23 , where the material is brought into suspension and led into the countercurrent cyclone  25  close to its horizontal axis. The material is then driven outwards by centrifugal force towards the cyclone casing, and is led out through the base of the cyclone via duct  35 .  
         [0037]     Cold gas for cooling the material is introduced into the countercurrent cyclone through duct  24 . This duct  24  directs the cold gas flow tangentially into the cyclone and the gas forms a spiral flow inwards toward the cyclone axis, where there is a discharge outlet for the now pre-heated gas. The pre-heated gas is then led forward via duct  32  to the calciner  12 .  
         [0038]     The term “counter current” cyclone (hereafter alternatively “CCC”) as utilized herein is exemplified by cyclone  60  as illustrated in  FIGS. 4-8 . Referring to  FIG. 4 , a cyclone separator generally designated as  60  includes a hollow housing of a tubular or barrel configuration generally designated as  41  for receiving a flowing cooling gas stream moving upwardly in the direction of arrows  42  through an inlet duct  43 . The hollow housing  41  includes an upper generally cylindrical portion  44  which is mounted in cooperation with the inlet duct  43  for receiving the gas stream which enters the upper cylindrical portion  44  tangentially and moving initially in an upper direction. The hollow housing  41  further includes a lower, frustro-conical portion  45  which is mounted under the upper cylindrical portion  44  by any suitable means. The upwardly directed, tangential entry of the gas stream into the upper portion  44  converts the linear gas flow into a laterally spiraling, rotating vortex rotating in a generally helical direction about horizontal axis A-A′. Housing  41  may be entirely cylindrical if desired, the principal structural requirement being that the housing  41  receive the gas stream entering duct  43  and cooperate therewith to convert such gas stream into a vortex moving in the direction shown by arrows  46 . Particles to be cooled will enter the cyclone through a side wall  47  via inlet  48  located close to horizontal axis A-A′ of cyclone  60  and essentially near the center or inner region  49  of vortex  46 . The swirling action of vortex  46  causes the particles in the gas stream within vortex  46  to be moved to an outer vortex region  51  close to side wall  55  of cyclone  60 . The movement of the particles or particulate in vortex  46  into outer region  51  thereof is caused by the centrifugal force exerted by the swirling action of vortex  46 . The segregation of particles into outer vortex region  51  leaves gas substantially freed of particles within the inner region  49  of vortex  46 .  
         [0039]     As indicated, in traditional cyclones material enters the cyclone tangentially into the vertical wall of the cyclone and therefore first encounters the outermost portion of the swirling vortex. In the CCC utilized in the present invention, cooling gas enters cyclone  60  tangentially but in an upward direction through gas inlet  43 . Thus, cooling gas swirls in a generally helical direction relative to a horizontal central axis of the cyclone. The hot material is delivered into the center of swirling vortex  46  of the cooling gas and therefore will have to work its way to outer vortex region  51  resulting in the material spending more time, relative to a traditional cyclone, in contact with the cooling gas. Depending on the velocity of the gas, cooled material above a predetermined weight will fall by gravity through material exit  52 . Gas heated from contact with the hot material and any entrained material too light to fall by gravity through the swirling air within the cyclone will exit through gas outlet  53  which is located close to horizontal axis A-A′ in sidewall  54  (shown in  FIG. 5 ) of cyclone  60  opposite sidewall  47 .  
         [0040]      FIGS. 5 and 6  illustrate modifications to the cyclone used in the present invention. Baffle plate  55  (shown in relief) is mounted horizontally and is positioned so that material entering cyclone  60  will strike baffle plate  55  and will be more evenly distributed throughout the interior of cyclone  60 . In addition,  FIG. 6  depicts movable gas velocity adjustment plate  75  which is situated within gas inlet  43 . The plate acts like an adjustable venturi, regulating at that site the size of the gas opening (not shown) within gas inlet  43 , and serves to give the operator the ability to regulate the velocity of cooling gas entering the cyclone. Movable plate  75  serves to adjust the direction of air flow in the interior of cyclone  60 .  
         [0041]     Tests were run utilizing both a primarily “conical” shaped CCC and a “standard” shaped CCC. The conical CCC of the present invention, as depicted in  FIG. 7 , has conical shaped sidewalls  61 ,  62  on opposite lateral sides of the cyclone  60 . The conical side walls each include an upper portion  61  above the material inlet  48  and gas outlet  53  and a lower portion  62  below the inlet  48  and outlet  53 .  
         [0042]     The standard CCC of the present invention, as depicted in  FIG. 8 , has upper standard shaped sidewalls  71  located above material inlet  48  and gas outlet  53  and lower standard shaped sidewalls  72  and lower conical shaped sidewalls  73 . Obviously, in the “standard” CCC depicted in  FIG. 8 , there is a larger ratio of vertical sidewalls to conical sidewall then in the conical CCC depicted in  FIG. 7 .  
         [0043]     The tests discussed below were made utilizing conical and standard CCCs with and without an interior heat insulating lining (not depicted). Such a lining can be positioned in the cone section of the CCC by spot welding or can be fitted therein friction tight.  
       EXAMPLES  
       [0044]     Test runs were conducted utilizing various sizes and configurations of the CCC of the present invention.  
         [0045]     The parameters tested were as follows: 
        a. Separation efficiency, calculated as  
       x   =       M   product       M   feed           
 
 x-cyclone separation efficiency 
        M  product =amount of collected material in the bottom of the cyclone, kg     M  feed =amount of material fed to the cyclone, kg    
 
         [0049]     In cyclones utilized in alumina processes high (preferably above 65%) separation efficiencies are desirable. 
        b. Heat capacity ratio (“H”), defined as the ratio of the heat capacity flow for the material relative to the gas. If H&lt;1, gas is in excess; if H&gt;1, material is in excess, according to the following formula:  
       H   =         m     mat   ⁢               ⁢           ⁢     Cp   mat           M   gas     ⁢           ⁢     Cp   gas             
        H=heat capacity ratio; m  mat =mass flow of inlet material, kg/s;     M  gas =mass flow of inlet gas, kg/s;     Cp  mat =material average heat capacity from t  mat,out  to t  mat,in , kcal/kg/K;     Cp  gas =gas average heat capacity from t  gas,in  to t  gas,out , kcal/kg/K;     t=temperature;  mat =material;  gas =gas;      in,out =refers to, respectively, inlet and outlet temperatures of the counter current cyclone. 
        c. Thermal efficiency—the measure of the actual heat exchange. In the ideal case the figure is 100%, and if no heat at all is exchanged the figure will be 0%.  
         H   ≤     1   ⁢     :     ⁢           ⁢   ∅       =         t     mat   ,   in       -     t     mat   ,   out             t     gas   ,   in       -     t     gas   ,   out               
         H   ≥     1   ⁢     :     ⁢           ⁢   ∅       =         t     gas   ,   out       -     t     gas   ,   in             t     mat   ,   in       -     t     gas   ,   in               
       
 
         [0058]     Thermal efficiencies in cyclones above 55% are desired in cyclones utilized in alumina processes.  
         [0059]     The values set forth in the TABLE were observed for each of the cyclones tested:  
                                                             TABLE                       Parameter Cyclone       Standard   Standard   Conical           Type   Unit   w/lining   wo/lining   wo/lining   Conical w/lining                                Cyclone Size, Nom.   mm   495   1500   1500   1500       Dia.       Capacity   MTPD        50-116    32-120    51-110       Heat Capacity Flow       0.7-1.6   0.53-1.34   0.50-1.29   0.45-1.21       Ratio       Thermal Efficiency   %   57-77   72.3-93.1   72.3-90.8   66.3-91.3       Separation Efficiency   %   90-97   93.9-99.7   93.2-99.7   86.3-94.7       Inlet Air Velocity   m/sec   24.0-25.2   15.5-34.0   25.0-26.7       Particle Breakdown       21 micron   %   0.4   0.4-0.6   0.2-1.0   0.7-2.6       45 micron   %   2.0   0.3-2.4   1.5-3.3   1.6-2.6                  
 
         [0060]     The tests indicate that, in general, the CCCs of the present invention showed results which were, for the most part, better than what would be expected to be realized in cyclones utilized in alumina processes.  
         [0061]     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or from practice of the invention disclosed. It is intended that the specification be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.