Coal suspension flowability improvers
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to compositions for improving the flowability of aqueous coal suspensions, and a method for the use of such compositions. 
     2. Statement of the Related Art 
     Flowable aqueous coal suspension suitable for being transported through a pipeline must have viscosities of less than 1,000 cP at shear velocities between 10 and 200 s -1 . At the same time, economic factors require a suspended coal solids content which is as high as possible. The coal contents of aqueous additive-free coal suspensions (depending on the properties of the types of coal employed) amount to a maximum to 55% by weight, since higher coal proportions result in viscosity increases and eventually in the loss of flowability. It has been known that additives containing surface active agents (tensides) and/or polymers cause the viscosity to be reduced and the flowability to be improved, thus making possible higher coal contents in the suspension. 
     U.S. Pat. No. 4,282,006 discloses aqueous coal slurries having coal contents of at least 60% by weight which contain an additive composition consisting of an anionic surfactant (e.g. naphthalene sulfonates, one example of which is &#34;Lomar D&#34;) and an electrolyte (e.g. NaOH). The viscosities of the sus-pensions at 60 rpm are between 300 and 4,000 cP (aqueous Black Mesa mine coal slurry) and between 200 and 2,400 cP (aqueous West Virginia mine coal slurry). 
     Published Japanese patent application No. 56/57,891 describes aqueous coal slurries having coal contents of 60% by weight which contain maleic acid/acrylic acid copolymers in a concentration of 1% by weight as an additive. At an average copolymer molecular weight of 4,000, the viscosity is 1,800 cP. 
     Published European patent application No. 109,740 discloses aqueous coal suspensions having coal contents between 30 and 90% by weight, which contain propylene oxide/ethylene oxide block copolymer condensates of aliphatic or aromatic alcohols. The suspensions containing the polymers (independently of the coal properties) have lower viscosities than those suspensions wherein the block polymer has been replaced by sodium or calcium lignin sulfonates. 
     Published Japanese patent application No. 58/122,991 describes a mixture of glycerol-ethylene oxide/propylene oxide block polymer ether and tenside (e.g. sodium naphthalene sulphonate) which is added to coal suspensions having coal contents of from 50 to 80% by weight. The stability-life of the obtained suspensions was more than 4 weeks. 
     U.S. Pat. No. 4,441,888 describes the viscosity reduction of coal slurries using maleic acid/olefin copolymer salts, particularly the sodium salt of a maleic acid/C 3-12  olefin copolymer. The specifically named copolymers are pentene sodium maleate and diisobutylene sodium maleate. 
     SUMMARY OF THE INVENTION 
     This invention affords compositions for improving the flowability of aqueous coal suspensions (slurries) having at least a 60% by weight coal solids content, as well as methods for their use. The viscosities of the coal slurries achieved using the compositions of this invention are below 1,000 cP, thus affording sufficient flowability. 
     The compositions of this invention comprise (preferably consist essentially of, most preferably consist of): 
     (a) at least one viscosity regulator, which may be an anionic such as a naphthalene sulfonate or lignin sulfonate and/or a nonionic such as an ethylene oxide/propylene oxide block polymer; 
     (b) optionally, at least one polysaccharide stabilizer; and 
     (c) at least one maleic acid/acrylic acid copolymer having a maleic:acrylic ratio of 0.5-2:1 and a specific viscosity of about 0.1 to 5 mPa.s (as determined in a 1% aqueous solution with a pH of 8 at 25° C.) 
     Component (b) may be present in up to 0.3% by weight, based on the coal content of the suspension, and the weight ratio of components (a+b) to component (c) is about 0.5-2:1. 
     The compositions of this invention may be added to a coal slurry at any time and in any manner, in an amount at least effective to improve the flowability of the slurry so that an at least 60% coal solids content slurry is transportable by pipeline, or the like. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, or defining ingredient parameters used herein are to be understood as modified in all instances by the term &#34;about&#34;. 
     More specifically, the compositions of this invention are preferably added to aqueous coal suspensions in concentrations of 0.1 to 2% by weight, most preferably 0.3 to 1.2% by weight, each based on the amount of the coal solids. The amounts of the individual components are preferably selected so that the ratio by weight of the sum of components (a) and (b) to component (c) is about 1:1. 
     The ethylene oxide/propylene oxide block polymers useful in the compositions as nonionic viscosity regulators have molecular weights between 1,000 and 20,000, those having molecular weights between 10,000 and 16,000 being preferred. The ethylene oxide content is at least 50% by weight, and preferably at least 70% by weight. The hydrophilic part of the block polymers consists of at least about 100 ethylene oxide units. Particularly suitable is a block copolymer marketed by BASF Wyandotte Corporation, Parsippany, N.J., under the trademark &#34;Pluronic&#34; F 108 which has the following characteristic data as stated in the U.S.P.N.F.: Molecular weight: 14,600 as found by hydroxyl end point determination; HLB: 27.0; ethylene oxide content: 80% by weight; the hydrophilic part consists of about 260 ethylene oxide units (i.e., there are about 54 propylene oxide units). 
     As anionic viscosity regulators there may be used any of the known alkali, alkaline earth and/or ammonium salts of naphthalene and/or lignin sulfonic acids. Preferred are an ammonium naphthalene sulfonate commercially available under the trademark &#34;Lomar&#34; D from Diamond Shamrock Corporation, Morristown, N.J., and an ammonium lignin sulfonate marketed by Lignin-Chemie, Germany, under the trademark &#34;Hansa&#34; AM. 
     The maleic acid/acrylic acid copolymers of the flowing-aid composition are preferably employed in the form of their alkali, alkaline earth and/or ammonium salts, and most preferably their sodium salts. They are obtained in a known manner by free radical-initiated copolymerization of acrylic acid with maleic acid or the inner anhydrides thereof, the ratio by weight of the maleic acid/acrylic acid being 0.5-2:1, preferably 0.5-1:1. The polymerization temperatures are between 50° C. and 200° C., preferably between 100° C. and 150° C. The number average molecular weight of the copolymers is characterized by their viscosity in aqueous solution. The maleic acid/acrylic acid copolymers generally, and their salts and especially sodium salts have specific viscosities of between 0.1 and 5 mPa.s, preferably between 0.1 and 0.5 mPa.s, and most preferably between 0.17 and 0.5 mPa.s in a 1% aqueous solution at a pH of 8 and 25° C. The characteristic data of the preferred maleic acid/acrylic acid copolymers are summarized in the following Table. 
     
                       TABLE 1                                                     
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Sodium salts of maleic acid/acrylic acid copolymers                       
Maleic acid/Acrylic                                                       
               Maleic acid/Acrylic                                        
                             Viscosity*                                   
acid copolymer acid (weight ratio)                                        
                             (mPa · s)                           
______________________________________                                    
M/A Copolymer 11-1                                                        
               1:1           0.178                                        
M/A Copolymer 11-2                                                        
               1:1           0.491                                        
M/A Copolymer 12-1                                                        
               1:2           0.220                                        
M/A Copolymer 12-2                                                        
               1:2           0.393                                        
______________________________________                                    
 *As determined in a 1% aqueous solution at pH 8 and 25° C.        
 
    
     In order to prevent sedimentation of the aqueous coal suspensions during a possible storage it may be advantageous to add polysaccharides such as xanthan, guar gum and/or hydroxypropyl guar gum in concentrations of up to 0.3% by weight--based on the coal content--as stabilizers to the suspensions. 
     The viscosities of the aqueous coal suspensions containing the additives which were prepared by adding the invention composition into a stirred coal/water mixture were determined using the rotation viscosimeter &#34;Rheomat&#34; 30 (cylindrical measuring body) a trademark of the Contraves Company, at a shear rate of 40 s -1  or using the &#34;Epprecht&#34; rotation viscosimeter (anchor-shaped measuring body), at a shear rate of 200 s -1 . As will be apparent from Tables 2 and 3, aqueous coal suspensions having coal solids contents of at least 64% by weight and containing anionic or nonionic viscosity regulators have viscosities in the case of naphthalene and/or lignin sulfonates (as determined with the Rheomat 30) of at least 800 cP or viscosities (as determined with the Epprecht viscosimeter) of at least 2,100 cP. For the ethylene oxide/propylene oxide block polymer a viscosity of 870 cP was measured. If the viscosity regulators are replaced by maleic acid/acrylic acid copolymers, the aqueous coal suspensions are no longer flowable. 
     Surprisingly it has now been found that flowing-aid compositions containing per se known anionic viscosity regulators based on at least one of naphthalene sulfonates and/or lignin sulfonates and/or nonionic viscosity regulators based on at least one of ethylene oxide/propylene oxide block polymers and optionally per se known stabilizers from the group comprising the polysaccharides in combination with maleic acid/acrylic acid copolymers cause a synergistic enhancement of the flowability of stable aqueous coal suspensions to be effected. As will be apparent from Table 2, aqueous coal suspensions having a coal content of 64% by weight which contain ammonium naphthalene sulfonate (&#34;Lomar&#34; D) or ammonium lignin sulfonate (&#34;Hansa&#34; AM) in combination with sodium maleic acid/acrylic acid copolymer in a ratio by weight of 1:1 and in a total concentration of 1% by weight, based on the weight of the coal solids, have viscosities between 600 and 750 mPa.s. In contrast thereto, coal suspensions containing only one component of the flowing-aid composition have much higher viscosities. In the case of the ammonium naphthalene sulfonate the viscosity is 800 mPa.s, and in the case of the ammonium lignin sulfonate the viscosity is 930 mPa.s. In the case of the copolymer the suspension is no longer flowable. The viscosity is reduced to the highest extent in the presence of flowing-aid compositions containing nonionic viscosity regulators in combination with maleic acid/acrylic acid copolymers. 
     The synergistic enhancement of the flowability which allows the coal content in the suspension to be increased and improves the economic efficiency in the transportation and subsequent direct processing of the coal has been observed with several coal types having different dispersibilities (cf. Tables 2 and 3). The Russian coal mentioned in Table 2 has a particle size of less than 200 μm, a heating value of from 6,500 to 7,000 kcal and an ash content of about 10%, and has very unfavorable dispersing properties. Coal types having a lower ash content such as the Ruhr fat coal of Table 3, having an ash content of 6.8% (particle size: 90% larger than 200 μm) have more favorable dispersing properties. 
     Independently of the properties of the kinds of coal the synergistic effects of the inventive compositions with respect to the reduction of the viscosity are striking for all of the coal suspensions. Coal slurries comprising additive compositions containing anionic and/or nonionic viscosity regulators in combination with maleic acid/acrylic acid copolymers in a ratio by weight of 0.5-2:1, and preferably of 1:1, have a viscosity which is as low as 50% less than that of coal slurries containing only one component--the viscosity regulator or the maleic acid/acrylic acid copolymer. 
     Coal slurries containing this invention&#39;s compositions are particularly well suited to continuous transportation via pipelines at low cost. Furthermore, such coal slurries may be directly combusted without prior dehydration due to lower pollutant emissions. In coal gasification using aqueous slurries, dust explosions as caused by dry finely ground coal particles are avoided. 
     
                                           TABLE 2                                 
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Viscosities of aqueous coal suspensions of Russian coal, determined by    
using a &#34;Rheomat&#34;                                                         
30 viscometer at 25° C. (shear rate: 40 s.sup.-1); coal content:   
64% by weight; additive                                                   
concentration 1% by weight, based on coal.                                
                               Reduction of Viscosity                     
Composition                                                               
       Ingredient Weight Ratio of                                         
                          Viscosity                                       
                               against using only                         
(a)    (b)        (a):(b) (mPa · s)                              
                               ingredient (a) - %                         
__________________________________________________________________________
Lomar D                   800                                             
Hansa AM                  930                                             
Pluronic F 108            870                                             
       M/A copolymer 11-1 *                                               
       M/A copolymer 11-2 *                                               
       M/A copolymer 12-1 *                                               
       M/A copolymer 12-2 *                                               
Lomar D                                                                   
       M/A copolymer 11-1                                                 
                  1:1     750  6                                          
Lomar D                                                                   
       M/A copolymer 11-2                                                 
                  1:1     660  18.5                                       
Hansa AM                                                                  
       M/A copolymer 11-2                                                 
                  1:1     730  21.5                                       
Hansa AM                                                                  
       M/A copolymer 12-1                                                 
                  1:1     600  35.5                                       
Hansa AM                                                                  
       M/A copolymer 12-2                                                 
                  1:1     620  33                                         
Hansa AM                                                                  
       M/A copolymer 12-2                                                 
                  1:2     650  30                                         
Pluronic F 108                                                            
       M/A copolymer 11-2                                                 
                  1:1     430  50                                         
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 *not flowable                                                            
 
    
     
                                           TABLE 3                                 
__________________________________________________________________________
Viscosities of aqueous coal suspensions of Ruhr fat coal, determined by   
using an &#34;Epprecht&#34;                                                       
viscosimeter (shear rate: 200 s.sup.-1); additive concentration 1% by     
weight, based on coal.                                                    
                          Coal       Reduction of Viscosity               
Composition                                                               
       Ingredient Weight Ratio of                                         
                          Content                                         
                                Viscosity                                 
                                     against using only                   
(a)    (b)        (a):(b) (% by wt.)                                      
                                (mPa · s)                        
                                     ingredient (a) - %                   
__________________________________________________________________________
Lomar D                   70    2,100                                     
Lomar D                   71    3,000                                     
       M/A copolymer 11-1 70    *                                         
       M/A copolymer 11-2 70    *                                         
Lomar D                                                                   
       M/A copolymer 11-2                                                 
                  1:1     70    1,700                                     
                                     19                                   
Lomar D                                                                   
       M/A copolymer 11-2                                                 
                  1:2     70    1,950                                     
                                      7                                   
Lomar D                                                                   
       M/A copolymer 11-1                                                 
                  1:1     70    1,900                                     
                                     10                                   
Lomar D                                                                   
       M/A copolymer 11-2                                                 
                  1:1     71    2,200                                     
                                     27                                   
__________________________________________________________________________
 *not flowable