Patent Publication Number: US-8118956-B2

Title: Manufacture of pyrotechnic time delay compositions

Description:
THIS INVENTION relates, broadly, to the manufacture of pyrotechnic time delay compositions, of the type used, for example, in delay elements employed for the initiation of explosives. More particularly, the invention relates to a method of manufacturing such compositions, and to such compositions when made in accordance with the method. 
     According to the invention, broadly, there is provided a method of manufacturing a pyrotechnic time delay composition, the method including
         admixing together a solid oxidizer, a solid fuel and water to form an aqueous slurry;   transforming the slurry into droplets; and   gas-drying the droplets to form particles comprising the oxidizer and the fuel, with the particles constituting a pyrotechnic delay composition.       

     A surfactant may be present during the admixing of the oxidizer, the fuel and the water to form the slurry. It is expected that routine experimentation can be employed to select desirable or appropriate surfactants, and the proportions thereof to be used, to facilitate formation of an aqueous slurry of suitable consistency for the intended atomisation and gas-drying. An example of a suitable surfactant is a wetting agent such as an acrylic ester, a styrene polymer, and/or an acrylic copolymer. The wetting agent, when present, may be used in proportions amounting to 0.25-4% by mass of the slurry. Another example of a suitable surfactant is a rheology modifier or a thickening agent such as polyethylene glycol, carboxymethyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidone and powdered smectite clay. The rheology modifier, when present, may be used in proportions of 0.25-4% by mass of the slurry. 
     The admixing of the oxidizer, the fuel and the water may be by using high-shear mixing techniques, such as those used in the paint industry for the high-shear mixing of paints. In this regard a Silverson Abramix high-shear mixer (obtainable in South Africa from Stewart and Brierly (Pty) Limited of 71 2 nd  Street, Booysens, Johannesburg) has been found to be suitable for use on a laboratory scale. 
     More or less conventional oxidizers may be employed such as, for example, red lead and/or barium sulphate, in particulate form. The oxidizer may comprise 24-54%, by mass, of the slurry. More typically, the oxidizer may comprise 30-50%, by mass, of the slurry. More or less conventional fuels such as silicon, zinc and/or magnesium, in particulate form, may be employed. The fuel may comprise 5-50%, by mass, of the slurry. More typically, the fuel may comprise 7-40%, by mass, of the slurry. The proportion of water in the slurry may be 30-70% by mass. More typically, the proportion of water in the slurry may be 40-50% by mass. 
     Transforming the slurry into the droplets may include atomizing the slurry. The method may include spray-drying the slurry, thereby to achieve the atomization of the slurry into the droplets and the gas-drying of the slurry droplets. The atomization may include pumping the slurry at suitably high pressure, eg in the range of 500-2500 kPa, through an orifice in a nozzle to achieve atomising of the slurry, the orifice typically being circular and having a diameter selected to achieve such atomising. Instead, the atomization may include pumping the slurry at a low pressure, e.g. at a pressure of 10-100 kPa, through an orifice in a so-called two fluid nozzle, the orifice typically being circular and being selected to suit the drying capacity required for the slurry in question. A so-called two fluid nozzle is designed so that the additional introduction of compressed air achieves the desired atomization. The size of the orifice in the nozzle is determined by the desired spray pattern and the slurry viscosity; however, typically, it has a diameter of 1.5 mm or 2 mm. The pressure of the compressed air used to achieve the desired atomization is dependent on the viscosity of the slurry; however, typically the compressed air pressure can be around 200-300 kPa, to maximize particle size. Higher compressed air pressures will result in smaller particle sizes. Instead, the atomization may include allowing the slurry to impinge on a rotating disc whereby high-velocity centrifugal forces generated by the rotating disc are used to form droplets of the slurry in the gas stream. In each case the atomization may be effected in the presence of a heated gas stream. Thus, when an orifice is used to form droplets as hereinbefore described, the droplets exiting the orifice will contact the heated gas stream, thereby to be dried by the heated gas. When the droplets are formed by means of a rotating disc as hereinbefore described, the droplets formed by the rotating disc will contact the heated gas stream, thereby to be dried by the heated gas. 
     The gas may thus be at an elevated temperature. The gas may be air, preferably heated air. 
     In each case, the atomisation will thus act to form the droplets in the heated air stream, with the heated air serving to dry the droplets. In each case, the atomization may be affected in a chamber having an air inlet and an air outlet. The heated air may then, for example, have an inlet temperature of 190° C. to 240° C., typically about 210° C. The air will typically have an outlet temperature of 110° C. The stream of hot air will thus pass through the chamber, e.g. lengthwise along the interior of a cylindrical chamber, acting to dry the particles while it is cooled down. In each case, it is expected that the water in the droplets will evaporate rapidly over a period of 1-40 seconds, to form more or less spherical particles comprising the oxidizer and the fuel more or less homogeneously mixed and dried, and having a moisture content of at most 1% by mass, typically 0.1%-0.8%. Such particles are suitable for use as a pyrotechnic delay composition. 
     Introducing the droplets into the stream of air may be either co-current or counter-current, as desired, to obtain acceptable air/droplet contact times and drying times. 
     The invention extends also to a pyrotechnic time delay composition, when manufactured by the method as hereinbefore described. 
     It is expected that substantial advantages of the invention will be that it avoids the environmental difficulties associated with organic solvents employed in the admixing, while avoiding or reducing any need for classification of product particles to eliminate or reduce the proportion of undersize and/or oversize particles. Furthermore, aqueous slurries are expected to be sufficiently safe to permit the use of high-shear mixers for slurry formation, leading to quick and efficient slurry production. 
    
    
     
       The invention will now be described, by way of non-limiting illustrative example, with reference to the following Examples and the following schematic drawing, in which the single FIGURE shows a diagrammatic side elevation, in more or less block-diagram format, of an installation for carrying out the method of the present invention. 
     
    
    
     In the drawing, the installation is generally designated by the reference numeral  10  and comprises a spray-drying chamber  12 . The chamber  12  is shown provided with a slurry feed line  14  terminating in a centrally positioned, upwardly directed two fluid spray nozzle  16  having a 1.5 mm or 2 mm diameter orifice. The chamber  12  has a cylindrical upper portion and a downwardly tapering conical lower portion which terminates in a solids outlet line  18 . An air feed line  20  is shown feeding tangentially in to the top of the cylindrical upper portion of the chamber  12 . The chamber  12  will typically be fitted with an explosion relief panel, to relieve any pressure generated should an ignition occur in the chamber, thereby limiting any damage to the chamber only. Such an explosion relief panel will typically be designed to release pressure at 10 kPa when the chamber has a design pressure of 60 kPa. 
     The chamber  12  has an air outlet line  22  shown feeding successively through a powder separation cyclone  24 , a bag filter  26  and a pair of ancillary filters  28 ,  30  to the atmosphere. In turn, the air feed line  20  is shown feeding successively through a pre-filter  32 , a fan  34 , a heater  36  and a filter  38 . 
     EXAMPLE 1 
     An aqueous slurry for a pyrotechnic time delay composition was prepared having the following composition in terms of solids on a dry basis: 
     
       
         
           
               
               
             
               
                   
               
               
                   
                 Proportion 
               
               
                 Constituent 
                 (% by mass) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 red lead particles (d50 approx. 3 μm) (oxidizer) 
                 38.25 
               
               
                 barium sulphate particles (d50 approx. 3 μm) (oxidizer) 
                 54.25 
               
               
                 silicon particles (d50 approx. 3 μm) (fuel) 
                 7 
               
               
                 smectite clay particles (BENTONE ®EW) (rheology 
                 0.5 
               
               
                 modifier/thickener) 
                   
               
               
                 Total 
                 100 
               
               
                   
               
            
           
         
       
     
     All four the dry particulate constituents were homogeneously mixed with water to form a slurry in which the water formed 50% by mass, with the solids thus forming 50%. The BENTONE® EW smectite clay particles were obtained from Carst &amp; Walker (Pty) Limited of Zenith House, 12 Sherborne Road, Parktown, Johannesburg, South Africa. The slurry was pumped, at a low pressure of 10-100 kPa, along the feed line  14  and through the orifice of the nozzle  16  (together with compressed air), thereby being atomized and thus formed into droplets, while low pressure air at a temperature of 210° C. was fed into the chamber  12  via the filters  32  and  38  and via the heater  36 , by the fan  34 , to dry the droplets. Spray-drying thus took place in the chamber  12  to form more or less spherical dried particles of more or less homogeneous composition. These particles had a moisture content of about 0.1% by mass and remained in the chamber  12  for a period of 1-40 seconds. The dried particles were collected from the solids outlet line  18 , while the drying air, which issued from the chamber  12  at 80° C. via outlet line  22 , was cleaned by the cyclone  24  and filters  26 ,  28  and  30 , dried particles being collected from the cyclone outlet line  40  and dried fines being collected from the outlet line  42  of the bag filter  26 . 
     The dried product from the line  18  was found to comprise acceptably low proportions of both oversize and undersize particles which could be used, without additional classifying, as a pyrotechnic time delay composition in the manufacture of pyrotechnic time delay elements. The method was found to be safe, quick, efficient and pollution-free. 
     EXAMPLE 2 
     
       
         
           
               
               
             
               
                   
               
               
                   
                 Proportion 
               
               
                 Constituent 
                 (% by mass) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 barium sulphate particles (d50 approx. 3 μm) (oxidizer) 
                 54.75 
               
               
                 silicon particles (d50 approx. 3 μm) (fuel) 
                 44.75 
               
               
                 smectite clay particles (BENTONE ®EW) (rheology 
                 0.5 
               
               
                 modifier/thickener) 
                   
               
               
                 Total 
                 100 
               
               
                   
               
            
           
         
       
     
     All three the dry particulate constituents were homogeneously mixed with water to form a slurry in which the water formed 50% by mass, with the solids thus forming 50%. The slurry was pumped, at a low pressure of 10-100 kPa, along the feed line  14  and through the orifice of the nozzle  16  (together with compressed air) thereby being atomized and thus formed into droplets, while low pressure air at a temperature of 210° C. was fed into the chamber  12  via the filters  32  and  38  and via the heater  36 , by the fan  34 , to dry the droplets. Spray-drying thus took place in the chamber  12  to form more or less spherical dried particles of more or less homogeneous composition. These particles had a moisture content of about 0.1% by mass and remained in the chamber  12  for a period of 1-40 seconds. The dried particles were collected from the solids outlet line  18 , while the drying air, which issued from the chamber  12  at 80° C. via outlet line  22 , was cleaned by the cyclone  24  and filters  26 ,  28  and  30 , dried particles being collected from the cyclone outlet line  40  and dried fines being collected from the outlet line  42  of the bag filter  26 . 
     As was the case in Example 1, the dried product from the line  18  was found to comprise acceptably low proportions of both oversize and undersize particles which could be used, without additional classifying, as a pyrotechnic delay composition in the manufacture of pyrotechnic time delay elements. As before the method was found to be safe, quick, efficient and pollution-free. 
     Conventionally, in pyrotechnic time delay compositions, an oxidizer such as red lead is used to impart sensitivity to the composition, particularly to compositions having a slow burning rate, e.g. about 210 ms/mm. It has thus unexpectedly been found that, by employing the method according to the invention to manufacture a pyrotechnic time delay composition, it is possible to eliminate the use of red lead, which is desirable due to the hazardous nature of red lead, while still obtaining acceptable burning rates. 
     Furthermore, it is important that the surfactant used is such that little or no gas is generated by the surfactant when the composition burns. Gas generated by the burning surfactant could lead to malfunctioning of a delay element incorporating the composition.