Patent Publication Number: US-4545797-A

Title: Process for manufacturing porous slag

Description:
FIELD OF THE INVENTION 
     This invention relates to the treatment of glassy slag. More particularly it relates to the treatment of glassy slag from a coal gasification operation to increase the porosity thereof. 
     BACKGROUND OF THE INVENTION 
     As is well known to those skilled in the art, glassy slag may be a by-product of various industrial operations including those wherein coal ds burned with production of molten by-products which are generally referred to as slag. The composition of these inert glassy slags may vary depending on whether they are derived from metallurgical, petroleum, or coal industries, although they are generally characterized by a low surface area (in terms of square meters per gram of slag) whether as large particles or as small particles. 
     It is an object of this invention to provide a process for improving the properties of such slags. Other objects will be apparent to those skilled in the art. 
     STATEMENT OF THE INVENTION 
     In accordance with certain of its aspects, this invention is directed to a process for treating an inert glassy slag having a low surface area which comprises 
     leaching said inert glassy slag having a low surface area in aqueous medium in the presence of (i) base having a pK b  of about 2-6 or (ii) acid having a pK a  of about &lt;1-5 thereby forming a leached slag of increased surface area; and 
     recovering said leached slag of increased surface area. 
     In accordance with certain of its other aspects, this invention is directed to a leached slag of increased surface area prepared by the process which comprises 
     leaching an inert glassy slag having a low surface area in the presence of (i) base having a pK b  of about 2-6 or (ii) acid having a pK a  of about &lt;1-5 thereby forming a leached slag of increased surface area; and 
     recovering said leached slag of increased surface area. 
     DESCRIPTION OF THE INVENTION 
     The charge, inert glassy slags, which may be treated by the process of this invention include heterogeneous mixtures derived as by-products from metallurgical operations, from petroleum operations, from the combustion (e.g. gasification) of coal, etc. These slags represent unwanted components of the charge materials admitted to these operations: and their composition may vary over a wide range. They typically contain so-called acidic oxide components, most commonly silica, but also including alumina, chromia, molybdena, etc. as well as so-called basic oxide components including metals of Group IA of the Periodic Table typically sodium, or metals of Group IIA of the Periodic Table, most commonly calcium or magnesium. In some instances the predominant species present in the slag may be considered to be a calcium silicate. Other species may be present including iron sulfides or oxides of manganese, copper, titanium, vanadium, etc. 
     These slags are characterized by high melting points, usually above 1200° C., high thermal stability, high chemical inertness, and by a very low surface area whether as a particle (say 2 mm to 10 cm in largest dimension) or as a fine powder (say 2 mm down to micron sizes in largest dimension). Generally the surface area may be 0.3-0.9 square meters per gram of slag charge. For example, a typical slag of particle size of 3 mm may have a typical surface area of only 0.5 square meters per gram. When carbon is present in the charge slag, the measured surface area will reflect this fact-i.e. it will appear to be high. It will however be found that if the carbon be removed by calcining, the actual area of the slag is very low--typically 0.5 square meters per gram. 
     A typical glassy slag may be that obtained as a by-product of gasification of Illinois No. 6 Coal (a low grade coal) by the Texaco Coal Gasification Process as described for example in U.S. Pat. No. 2,871,114 of Texaco. This coal may contain 8 w % water and may be characterized, on a dry basis, as follows: 
     
                       TABLE                                                       
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Illinois No. 6 Coal (dry basis) Analysis                                  
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Property           Value                                                  
______________________________________                                    
Density (g/cc)     1.27                                                   
Surface Area (sq. m/g)                                                    
                   51                                                     
Ash (w %)          9.2                                                    
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Components         w %                                                    
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Carbon             70.2                                                   
Sodium             0.01                                                   
Magnesium          0.05                                                   
Calcium            0.38                                                   
Silica             4.40                                                   
Alumina            1.60                                                   
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     The glassy slag obtained by the gasification of the above charge may typically be characterized as follows: 
     
                       TABLE                                                       
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Property           Value                                                  
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Water (w %)        2                                                      
Density (g/cc)     2.51                                                   
Surface Area (sq. m/g)                                                    
                   0.5                                                    
Particle Size (average)                                                   
                   3 mm                                                   
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Components (dry basis)                                                    
                   w %                                                    
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Carbon             7.92                                                   
Sodium             2.20                                                   
Magnesium          0.97                                                   
Calcium            3.50                                                   
Silica             42.85                                                  
Alumina            20.77                                                  
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     It is preferred that the glassy slag, to be admitted to the leaching operation of this invention, contain not more than about 1 w % carbon. Accordingly, if the charge slag contains carbon above this content (as is the case with the above illustrated slag), it is preferred to subJect it to calcination at 500°-600° C., say 600° C. for 1-3 hours, say 3 hours in an oxidizing atmosphere. Although it may be possible to use oxygen or oxygen-enriched air as the oxidizing atmosphere, it is preferred to utilize air. 
     During calcination under these conditions, the carbon content of the charge slag is lowered from an initial content of 1-10 w %, say about 7 w % down to a final carbon content of less than about 1 w %, typically 0.1-1 w %, say 0.8 w %. The other properties of the so-oxidized slag are found to be essentially the same as those of the glassy charge slag. 
     The slag containing less than about 1 w % carbon, preferably the calcined slag, is subjected to leaching in aqueous medium. If the average particle size of the slag is not within the range of 0.1-3 mm, it may be reduced, prior to leaching, as by grinding in a hammer mill, or a ball mill, or a rod mill, followed by sieving to obtain desired fractions. Preferably substantially 100% of the slag should have a particle size in the range of 0.1-3 mm. 
     Leaching is preferably effected in aqueous medium in the presence of (i) a base preferably an inorganic base having a pK b  of about 2-6, preferably about 3.5 typified by sodium hydroxide, potassium hydroxide, sodium carbonate, trisodium phosphate sodium metasilicate, calcium hydroxide, etc. or a combination thereof or (ii) an acid, preferably an inorganic acid having a pK a  of about &lt;1-5, preferably about 3.2 typified by hydrofluoric acid, hydrochloric acid, sulfuric acid, phosphoric acid, chlorosulfonic acid, nitric acid, or a combination thereof. 
     One preferred agent is a 20-35 w %, say 30 w % aqueous solution of sodium hydroxide. A more preferred leaching agent is a 6-16.3 w %, say 6 wt % aqueous solution of hydrogen fluoride. 
     Leaching may be effected at 20° C.-80° C., say 25° C. for 10-60 minutes, say 15 minutes. Preferably 100 parts by weight of solid are immersed in 750-1200 parts, say 1000 parts by weight of aqueous leaching agent: and the mix is agitated for the requisite period. 
     During leaching, a small portion of the glassy slag is dissolved in the solution. Control of the conditions leaching, including temperature, time, amount and concentration of leaching agent, and degree of agitation will insure that the product leached glassy slag has the desired high surface area. 
     The leached slag is particularly characterized by a substantially increased porosity and surface area. The original glassy slag which has essentially zero porosity is now found to be highly porous; and the surface area which was originally essentially non-existent is now found to be 1-20, say 13.5 square meters per gram for a product slag having a particle size of 0.1-3 mm, say less than 0.19 mm. 
     The product so-prepared may be used in building materials, in ceramics manufacture, in the cement industry, and as a catalyst support. In a typical instance, 100 parts of 0.1-3 mm leached coarse glassy slag may be immersed in 0.5-4 w %, say 1.5 w % aqueous solution of chloroplatinic acid at 25° C.-80° C., say 25° C. for 1-2 hours, say 2 hours followed by drying at 110° C.-120° C., say 120° C. for 1-16 hours, say 8 hours, followed by reduction in hydrogen at 200° C.-500° C., say 500° C., for 1-2 hours, say 1 hour. The so prepared supported catalyst may be used in the hydrogenation of butene to butane, benzene to cyclohexane, etc. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIGS. 1-9 are photographs taken from a scanning electron microscope-(2000×) wherein the longest dimension of each photograph represents a distance of fifty microns--of the products described in detail in Examples I-II. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Practice of the process of this invention will be apparent to those skilled in the art from the following wherein as elsewhere in this specification, all parts are parts by weight. 
     EXAMPLES I-II 
     In this series of Examples, the charge slag was obtained from a coal gasification unit wherein Illinois No. 6 Coal (a low rank coal) had been gasified according to the process of U.S. Pat. No. 2,871,114. The slag was screened to form a fine slag (20%) and a coarse slag (80%). The properties of each were as follows: 
     
                       TABLE                                                       
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Fine Slag                                                                 
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Property            Value                                                 
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Particle Size (microns)                                                   
                    10-250                                                
Surface area (sq. meters/g)                                               
                    51.3                                                  
Bulk Density (g/cc) 0.59                                                  
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Composition         w %                                                   
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Organic Carbon      27                                                    
Silica              45.6                                                  
Alumina             17.3                                                  
Iron oxides         18.0                                                  
Calcium oxide       4.4                                                   
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                       TABLE                                                       
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Coarse Slag                                                               
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Property            Value                                                 
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Particle Size (microns)                                                   
                    250-3000                                              
Surface area (sq. meters/g)                                               
                    0.5                                                   
Bulk Density (g/cc) 1.33                                                  
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Composition         w %                                                   
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Organic Carbon      7.9                                                   
Silica              46.8                                                  
Alumina             16.8                                                  
Iron oxides         17.0                                                  
Calcium oxide       3.92                                                  
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     Each of these materials was dried at 110° C. for 2 hours. 
     In Example I, the coarse glassy slag was examined under a scanning electron microscope. FIG. 1 is a photograph taken in the scanning electron microscope of the charge slag. This material appeared to be extremely smooth and in fact it had a low surface area (only 0.5 sq. meters/gram). 
     The coarse glassy slag was calcined at 600° C. for 3 hours in air and, after cooling to room temperature of 25° C., was examined. The photograph FIG. 2 shows that the product was essentially as smooth as the uncalcined slag. The surface area of the calcined slag was 0.5 sq. meters per gram. 
     The cooled calcined coarse slag was immersed at 80° C. for 2 hours in a 30 w % aqueous solution of sodium hydroxide and allowed to stand for two days at room temperature. The leached materials were washed with water several times. FIGS. 3 and 4 are scanning electron microscope photographs of different portions of the slag. As may be readily observed, the slag is no longer smooth but is characterized by a highly irregular surface and by the presence of a large number of interstices. The surface area of this material is found to be 2.6 square meters per gram. 
     In Example II, the charge slag was the fine slag described above. It was treated in manner identical to the treatment for the coarse slag of Example I except that it was treated with aqueous caustic for 10 minutes (rather than 2 hours). 
     From FIG. 5, it may be observed that the surface of the dried fine slag has a fine texture. The measured surface area is 51.3 square meters per gram. The high surface area was due to the presence of undesired carbonaceous material (representing more than 30 w % of the fine slag). 
     From FIG. 6, it may be observed that the texture of the calcined fine slag has a spherical glassy smooth surface. The carbon which contributed the high surface area has been oxidized and removed. The measured surface area is 2.3 square meter per gram. 
     From FIG. 7, it may be observed that the leached calcined fine slag has an increase in surface area (the grains were etched and rough surface was generated). The measured surface area is 4.3 square meters per gram. 
     EXAMPLES III-XI 
     In this series of Examples, the surface area of calcined coarse glassy slag was measured. Portions of this slag were ground in a ball mill and then sieved to produce aliquots having a particle size of (i) 100% on 30 mesh, (ii) 100% on 60 mesh, (iii) 100% on 80 mesh and (iv) 100% through 80 mesh. A portion of each aliquot was leached at room temperature for 15 minutes with 6 w % aqueous hydrogen fluoride. The surface area of the unleached and the leached aliquots was measured.______________________________________                  Surface Area                  Sq. meters/gramExample       Leached   Particle Size                            Unleached Leached______________________________________III    --        0.125   inch 0.5IV     --        30      mesh 0.3V      Y         30      mesh           1.3VI     --        60      mesh 0.7VII    Y         60      mesh           1.4VIII   --        80      mesh 1.2IX     Y         80      mesh           3.9X      --        -80     mesh 0.9XI     Y         -80     mesh           13.5______________________________________ Y means leached -- means unleached 
     From the above table, the following may be determined: 
     (i) as the unleached material is reduced in particle size, the surface area varies erratically; 
     (ii) as the leached material is reduced in particle size, the surface area desirably increases; and 
     (iii) it is possible to increase the surface area of the leached slag (when compared to the unleached slag) by a factor of 13.5/0.9 or 15 times!. The particle size, shape, and porosity after leaching are noticeably different from the starting material. 
     FIGS. 8 and 9 show the scanning electron microscope picture of the calcined coarse slag (Example X) and the leached slag (Example XI) respectively. It may be observed that the leached calcined slag FIG. 9, Example XI has an increased porosity, roundness of the grains (due to the etching at the edges), and smaller particles size than in the original material of FIG. 8, Example X. 
     The portion of the glassy slag dissolved in the hydrofluoric solution (Example XI) has the following composition:______________________________________Element             (W %)______________________________________Si                  0.32Na                  0.22Al                  0.21Fe                  0.14Ca                  0.056K                   0.017Zr                  32     (ppm)Ti                  27     (ppm)Mg                  26     (ppm)Mn                  4      (ppm)Cr                  3      (ppm)Ni                  2      (ppm)V                   2      (ppm)Co                  &lt;2     (ppm)Mo                  &lt;2     (ppm)Zn                  &lt;2     (ppm)Cu                  &lt;1     (ppm)______________________________________ 
     Results comparable to the above may be obtained if the slag is derived from: 
     
                       TABLE                                                       
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Example          Slag                                                     
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XII              Anthracite Coal                                          
XIII             Bituminous Coal                                          
XIV              Sub-bituminous Coal                                      
XV               Lignite                                                  
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     Results comparable to the above may be obtained if the leaching agent is an aqueous solution of:______________________________________Example           Agent______________________________________XV                Nitric AcidXVI               Hydrochloric AcidXVII              Phosphoric AcidXVIII             Sodium Carbonate______________________________________ 
     Although this invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made which clearly fall within the scope of this invention.