Permanganate-containing pellets and method of manufacture
RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending applications Ser. No. 188,419, filed Apr. 29, 1988, now U.S. Pat. No. 4,961,751, and Ser. No. 279,941, filed Dec. 5, 1988, now abandoned.&#34; 
    
    
     FIELD OF INVENTION 
     This invention relates to the use of water-soluble permanganate salts as oxidizing agents. More particularly, the invention relates to the incorporation of permanganates in carriers to facilitate controlled release of the permanganate. 
     BACKGROUND OF INVENTION 
     Potassium permanganate and other bleaching or oxidizing agents have been encapsulated or adsorbed on support materials or embedded in formed bodies. See, for example, U.S. Pat. Nos. 3,535,262, 4,279,764, 4,460,490, 4,665,782, 4,657,784, and 4,711,748. German Patent 2,311,964 describes the preparation of a product for decontaminating radioactive waste containing manganese dioxide (MnO 2 ) in Plaster of Paris (gypsum). A slurry is formed from manganese sulfate (MnSO 4 ) and potassium permanganate (KMnO 4 ) and gypsum which is cast into blocks. The MnSO 4  and KMnO 4  react in the slurry to form the MnO 2 , which is dispersed throughout the gypsum mass and removed radionuclides by adsorption. 
     U.S. Pat. No. 4,740,213 describes the impregnation of pumice granules or other absorbent material with an aqueous solution of a bleaching agent such as sodium hypochlorite. Permanganate bleaching agents are not specifically disclosed. The liquid bleaching agent is held in the capillary passages of the natural pumice. In use, the liquid bleaching agent is transferred from the interior of the granules to a substrate contacted by the granules, such as denim fabrics being bleached by contact with the granules in a tumbling operation. 
     SUMMARY OF INVENTION 
     The method of this invention for manufacturing permanganate-containing pellets employs a hydratable, self-curing mineral cement such as gypsum to form the body or matrix of the pellets. A water-soluble permanganate in the form of solid particles is distributed throughout the body of the pellets. The permanganate can be released by attrition of the surface of the pellets in certain applications such as the tumble bleaching of moist denim fabrics, or for other purposes it can be released by slow extraction. In either case, the permanganate is gradually released. For denim fabric bleaching, for example, pellets can be used for a plurality of bleaching cycles. Where the pellets are immersed in water, the permanganate will gradually solubilize and be released by aqueous extraction. 
     In the preferred method of manufacturing the pellets, water-soluble permanganate salt and the hydratable, self-curing cement are mixed with water to form a moldable mix. The mixture is formed into the pellets, and the pellets are cured to integrated bodies by hydration of the cement. The hydration removes most of the free water, leaving the interior of the pellets in substantially dry condition. When the permanganate is mixed with the cement in the form of finely divided permanganate particles, it will remain in particulate form during the mixing, forming, and curing of the pellets. If the permanganate is mixed in a concentrated aqueous solution, or if part of the permanganate becomes solubilized during the mixing, it will be reformed into solid particles by removal of the water through hydration. In certain cases, if free water remains within the pellets, they can be subjected to drying to complete the preparation of the pellets. 
    
    
     DETAILED DESCRIPTION 
     The pellets of this invention are especially suitable for use with potassium permanganate (KMnO 4 ) and sodium permanganate (NaMnO 4 ). However, they can be advantageously used with other water-soluble permanganate salts. The permanganate salt is preferably added to the cement formulation in solid form. For example, KMnO 4  can be added as a finely-divided particulate solid, whereas in the case of sodium permanganate addition as a solution may be preferred. With liquid addition and the use of a hydrable self-curing cement, even though the permanganate is added as an aqueous solution, the water-binding action of the cement can leave most of the permanganate as highly dispersed solid particles. The particles are fine to mlcroscopic depending on the method of addition. 
     Even though some bleaching agents other than potassium or sodium permanganate, i.e., sodium hypochlorite and sodium dichlorocyanurate dihydrate, function as active ingredients in the artificial frosting stones, their performance is at a much lower level than those of permanganate-containing formulations. The preferred choices for high-intensity frosting are combinations of sodium or potassium permanganate in either gypsum or magnesia cements. Where low bleaching intensities are desired (such as in chemically enhanced stone washing), combinations of Na or K permanganate with Portland cement (preferably white cement) can be used. 
     The pellets of this invention can be prepared with abradable surfaces. More specifically, they comprise cemented aggregates of mineral particles with the permanganate embedded therein in particulate form. The pellets can be formed from low moisture mixes, which may be a paste or thick slurry, which can be formed into the shaped bodies by processes, such as extrusion, molding, agglomeration, etc. 
     A preferred major component of the stones&#39; matrix material is a self-curing inorganic cement. Gypsum (Plaster of Paris) is particularly desirable. Hydratable gypsum may be used in a similar form as for preparing gypsum wallboard. When mixed with a small amount of water the gypsum will hydrate and set to an integrated solid body. By premixing the hydratable gypsum powder with the particulate bleaching agent, adding a small amount of water to form a thick paste, the artificial stones can be formed with the agent particles dispersed therethrough essentially in encapsulated or embedded form. Even though the porosity of the artificial stones is limited, the bleaching agent can be progressively released either by surface abrasion or prolonged extraction in water. 
     Depending on the method of aggregation chosen, various commercial forms of gypsum may be used. Unformulated gypsum, in the hemihydrate form, is a rapid setting material, allowing only a very limited time for forming into pellets. Specifically when using extrusion as the aggregation method, the hydration of gypsum is accelerated by the addition of permanganate. The setting rate can be controlled by addition of one or more decelerants, to allow time to mix and form the material into pellets prior to setting. Commercially available slow-set gypsums are usually retarded by addition of an organic component, e.g., citric acid or hydrolyzed protein, which are attacked by the oxidizing agent. The retardants used for this process should be inorganics such as H 3  PO 4 , NaH 2  PO 4 , Ca(H 2  PO 4 ) 2 , Na 2  B 4  O 7 , etc. Elevated temperature and pressure are also accelerants of gypsum setting, so a very dry mixture, which will generate heat and pressure when being worked, should be avoided. 
     Other self-curing cements include the family of magnesia cements, viz., magnesium oxychloride and magnesium oxysulfate. These cements are also referred to a &#34;Sorel&#34; cements. Further usable cements also include Portland cement (white Portland cement is especially desirable because of its low iron content), Pozzolan cement, calcium aluminate cement, and related cements. 
     A representative formulation of the pellets are set out below. 
     
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General Formulas for Permanganate-Containing                              
Pellets Formed from Self-Curing Cements                                   
Ingredients   Wt. % Range                                                 
                         Preferred Wt. %                                  
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Cement*       70-99.5    85-90                                            
Bleaching agent                                                           
              0.5-30     10-15                                            
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 *May contain additives                                                   
 
    
     For effective use as frosting agents, the bleach-containing solidified cements are formed into suitable lump or pellet form. The size and form can influence the bleaching pattern obtainable in the frosting step. Given comparable tumbling times, the regularity and uniformity of the bleach effect increases with decreasing stone size. Conversely, the larger the pellet, the more spotty and irregular the bleached areas become. Preparation of stones of various sizes can be achieved in a number of ways. For example, the bleach-containing cement paste can be poured into molds of a variety of shapes and sizes. For example, large slabs of 0.5 to 1.5 inches thickness can be formed, and then cut into rectangular or square pieces of 1&#34; to 1.5&#34; side length, or any other desirable dimension. Alternatively, the slabs can be mechanically crushed to give irregular shaped lumps, with desirable size ranges to be separated out by a classifier. As another procedure, the cement paste can be poured directly into individual molds of the desired shape and size. For agglomeration by molding, the water content of the paste should be slightly higher to make it pourable) than for the aggregation methods described below. Pellets suitable for frosting or garments can also be made by extrusion, disk pelletization, briquetting, tabletting, or other methods familiar to those skilled in the art. 
     For example, 60 to 95 parts of a slow setting gypsum material (preferred 80 to 90 parts) are mixed with 5 to 15 parts of KMnO 4  and 0 to 25 parts of a thickener (preferred 0 to 10 parts), and water sufficient to form a stiff dough. This dough can then be formed into pellets by any method familiar to those skilled in the art; for example, by extrusion, or by rolling between textured rolls, or by pelletization, etc. Once formed, the pellets are self-drying and self-hardening due to the rehydration and setting of the gypsum. The amount of KMnO 4  used is an added control of bleaching intensity, along with tumbling time, and weight ratio or garments to pellets selected during the &#34;frosting&#34; step of this process. 
     This invention is further illustrated by the following examples. 
     EXAMPLE I 
     A measured quantity of crystalline or powdered potassium permanganate is dry mixed with a predetermined amount of filler. After a homogeneous blend is obtained, a predetermined quantity of binder plus the proper amount of water is worked in the mixture so that an extrudable mass is obtained. This, in most cases, represents a still powdery but slightly cohesive material. The mass is then extruded to form 1/4&#34; to 1/2&#34; diameter rounds of about 3/4&#34; to 11/2&#34; in length. The sizes and shapes of the product are selected for convenience and maximum production rate. Diameters of about 1/16&#34; or even less or of 1&#34; or more are possible. Instead of rounds, other geometrical shapes such as triangular, rectangular, or stars can be used. After extrusion, the product is cured at either ambient or elevated temperature 60°-110° C.). Curing at higher temperatures produces products of higher hardness and with slower release characterization. 
     The extruded product, containing about 10% KMnO 4  or about 12% K 2  MnO 4 ) is tumbled with damp denim garments for a period of 5 to 30 minutes. The weight ratio between the quantity of frosting agent and dry garment weight may range from 4 to 0.1, depending on the degree of bleaching desired. In the course of the tumbling operation the extruded pellets are abraded, being finally reduced to a powder. In this manner, the garments make large number of contacts with the permanganate-containing extrudates of various sizes, whereby each contact produces localized bleaching action. 
     After completion of the frosting step, the garments are treated with a reducing agent--commonly sodium metabisulfite--to remove the brown stains of manganese dioxide. 
     EXAMPLE II 
     89 lb gypsum was mixed with 1 lb Ca(H 2  PO 4 ) 2  (to retard hydration) and 10 lb KMnO 4  crystals, forming a uniform dry blend. Water was added to this blend in a high shear mixer to form a wet dough, which was then extruded through a die plate having 1/2&#34; square holes. The soft pellets formed were fed onto a moving belt to set. 
     About 20 lb of water was used in forming this dough. As the gypsum hydrates, it uses about 15 lb of the water present CaSO 4  1/2 H 2  O+1.5 H 2  O→CaSO 4  2 H 2  O), the heat of hydration causing vaporization of part of the remaining water. Some free water apparently remained in a highly dispersed form. A hard, dry plaster pellet containing KMnO 4  crystals was obtained. 
     EXAMPLE III 
     260 g of slow setting gypsum was intimately mixed with 72 ml of a commercial 40% solution of sodium permanganate and 30 ml of water. The resulting deep purple paste was transferred into plastic molds of about 3.5 ml volume each. The mass began to stiffen after about 20 minutes and was set after 45 minutes, at which point the gypsum castings were removed from their molds. The black cherry colored pieces contained 10.3% sodium permanganate in a highly dispersed form. 
     A frosting test with this product 50 g frosting agent with 60 g blue denim tumbled for 30 minutes) showed high intensity, high contrast bleaching. 
     EXAMPLES IV to IX 
     Additional stone formulations and test results are summarized in Table A. 
     
                                           TABLE A                                 
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     Quantity &amp;                                                           
               Quantity &amp; Kind                                            
Example                                                                   
     Kind of   of Bleaching                                               
                       Water                                              
                            Thickening                                    
                                  Set       Results of                    
No.  Cement Used                                                          
               Agent Used                                                 
                       Used Time  Time                                    
                                      Hardness                            
                                            Frosting Test                 
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IV   Magnesia cement                                                      
               25 g KMnO.sub.4                                            
                       --   1 hr  2.5 hr                                  
                                      hard  low intensity                 
     50 g MgO + 120 ml                                                    
               (solid)                      bleaching                     
     saturated MgCl.sub.2                                                 
     solution                                                             
V    Magnesia cement                                                      
               45 mL = 63 g                                               
                       58 mL                                              
                            1 hr  2.5 hr                                  
                                      hard  high intensity                
     50 g MgO + 66 g                                                      
               40% NaMnO.sub.4              bleaching                     
     MgCl.sub.2.6 H.sub.2 O                                               
VI   260 g Portland                                                       
               28.9 g KMnO.sub.4                                          
                       84 mL                                              
                            1.5 hr                                        
                                    6 hr                                  
                                      very hard                           
                                            very low intensity            
     Cement (white)                                                       
               (as solid)                   bleaching                     
VII  260 g Portland                                                       
               52 mL = 72.8 g                                             
                       50 mL                                              
                            10 min                                        
                                  1.5 hr                                  
                                      very hard                           
                                            moderate intensity            
     Cement (white)                                                       
               40% NaMnO.sub.4              bleaching                     
VIII 260 g Portland                                                       
               107.7 g NaOCl                                              
                       none 25 min                                        
                                    1 hr                                  
                                      very hard                           
                                            low intensity                 
     Cement (white)                                                       
               solution                     bleaching                     
               (17% active                                                
               chlorine)                                                  
IX   260 g Portland                                                       
               28.9 g sodium                                              
                       125 mL                                             
                            40 min                                        
                                  1.5 hr                                  
                                      rough,                              
                                            low intensity                 
     Cement (white)                                                       
               dicyanurate            crumbly                             
                                            bleaching                     
               dihydrate              surface                             
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