Patent Publication Number: US-2005132921-A1

Title: Explosive for rock breaking

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a Continuation, under the provisions of 35 U.S.C. 120 and 365(c), of International Application No. PCT/ZA02/00210, filed Dec. 17, 2002, which designates the US. 
    
    
     BACKGROUND OF THE INVENTION  
      This invention relates generally to the breaking of rock and more particularly is concerned with a propellant which can be used for the breaking of rock and with a method of manufacturing a propellant of this type.  
      As used herein the word “rock” includes rock, ore, coal, concrete and any similar hard mass, whether above or underground, which is difficult to break or fracture. It is to be understood that “rock” is to be interpreted broadly.  
      A number of techniques have been developed for the breaking of rock using non-explosive means. These include a carbon dioxide gas pressurisation method (referred to as the Cardox method), the use of gas injectors (the Sunburst technique), hydrofracturing and various methods by which cartridges containing energetic substances pressurise the walls or base of a sealed drill hole to produce penetrating cone fractures (known as PCF).  
      These techniques may be an order of magnitude more efficient than conventional blasting in that they require approximately {fraction (1/10)} of the energy to break a given amount of rock compared to conventional blasting using high explosives. The lower energy reduces the resulting quantity of fly rock and air blast and to an extent allows the rockbreaking operation to proceed on a continuous basis as opposed to the batch-type situation, which prevails with conventional blasting.  
      Most non-explosive rock breaking techniques rely on the generation of high gas pressures to initiate a tensile fracture at the bottom of a relatively short drill hole.  
      When the propellant is initiated, combustion products are produced. Depending on the combustion process the combustion products may be toxic or harmful to the health of persons in the vicinity thereof. Another factor which is to be taken into account is that incomplete or imperfect combustion may lead to a reduction in the amount of energy generated by the combustion process. Related to this aspect is the burning rate of the propellant. If the propellant burns too slowly then the rate at which energy is released is reduced and the ability of the propellant to initiate rock fracture is also reduced.  
      On the other hand if the propellant burns too fast then it may burn at a temperature which is so high that the propellant effectively functions as an explosive and, for safety reasons, the propellant must then be classified as an explosive. Classification in this way places severe restrictions on the storage, handling and use of the propellant.  
     SUMMARY OF INVENTION  
      The invention provides a propellant composition which includes a propellant material and an oxygen source material.  
      The propellant composition may be in particulate form and each particle in the particulate composition may include propellant material and oxygen source material.  
      Each particle may include a granule of propellant material which is coated, preferably only partly, with the oxygen source material.  
      In a different form of the invention each particle is a mixture at least of the propellant material and the oxygen source material.  
      The propellant material may be of any appropriate type which is known in the art and preferably is nitrocellulose which may be provided, as indicated, in granular form.  
      The oxygen source material may be of any suitable kind and for example may be ammonium nitrate.  
      Alternatively the propellant composition is in liquid form. A liquid has the advantage, over particulate material, that it is essentially non-compressible. Consequently, when initiated, a greater expansive force is generated than for a particular material. The liquid may include nitrocellulose as propellant and, optionally, nitro glycerine as an enhancer. The liquid propellant may be a single, double or compound base type. The oxygen source material may be ammonium nitrate or any similar material.  
      The invention also provides a method of manufacturing a propellant composition which includes the step of mixing a granular propellant material with an oxygen source material thereby to coat the propellant granules, at least partly, with a layer of oxygen source material.  
      The oxygen source material may be ammonium nitrate or any equivalent material. The oxygen source material may be coated onto the propellant granules using any appropriate technique and preferably use is made of a technique which is similar to that use for cladding diamond particles with metal as developed by companies such as De Beers Industrial Diamonds or Boart.  
      It is also possible to waterproof the propellant using any appropriate technique which, for example, coats the propellant granules with a waterproof or water resistant film or layer. Use may for example be made of a film, eg. of nitrocellulose, which is coated onto the granules using an appropriate technique such as chemical vapour deposition.  
      In a different form of the invention there is provided a method of manufacturing a propellant composition which includes the step of mixing a particulate propellant material with an oxygen source material thereby to form particles wherein each particle is a mixture of the propellant and the oxygen source material.  
      The invention also provides rock breaking apparatus which includes a cartridge which forms an enclosure and a propellant composition of the aforementioned kind inside the enclosure.  
      The cartridge may be made from a malleable material which in this specification includes a material which is capable of plastic deformation, without rupturing, by at least a predetermined extent eg. of the order of 10% or more.  
      As used herein “propellant” includes a material such as a propellant, blasting agent, gas-evolving substance, explosive or similar means which, once initiated, generates high pressure jet material typically at least partly in gaseous form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is further described by way of examples with reference to the accompanying drawings in which:  
       FIG. 1  schematically illustrates a method of manufacturing a propellant composition according to the invention,  
       FIG. 2  is an enlarged cross sectional view of a particle of a propellant composition produced by the manufacturing method shown in  FIG. 1 ,  
       FIG. 3  is a view similar to  FIG. 2  of a different form of a propellant particle, and  
       FIG. 4  illustrates a cartridge which makes use of a propellant composition according to the invention for breaking rock.  
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  of the accompanying drawings illustrates, somewhat schematically, a method according to the invention for manufacturing a propellant composition.  
      Propellant particles  10  such as nitrocellulose, in granular form, are supplied to a mixer  12 . The propellant particles are suitably small, for example with an effective diameter of between 0.2 mm to 1 mm.  
      An oxygen source material is held in a container  14  and is supplied at a desired rate, relatively to the supply rate of the propellant, to the mixer  12 . The oxygen source material may vary according to requirement and for example is ammonium nitrate. The oxygen source material is in liquid form.  
      The mixer  12  is mechanically agitated to ensure thorough mixing of the propellant with the oxygen source material. During the mixing process the various particles of the granular propellant are coated to a greater or lesser extent with the oxygen source material. This gives rise to composite particles.  
       FIG. 2  illustrates in cross section a typical composite particle  16  which includes a granular core  18  of propellant, ie. nitrocellulose, and an outer coating  20  of oxygen source material, ie. ammonium nitrate, which extends over an outer surface of the core. The coating  20  preferably does not cover the entire core  18  and leaves at least a small section  22  of the core exposed. The thickness of the coating and the extent of the coating determine the amount of oxygen source material which is available, in relation to the amount of the core propellant.  
      As an alternative to the aforegoing the ammonium nitrate may be coated onto the propellant particles or grains by using techniques which are analogous or similar to the techniques developed by De Beers Industrial Diamonds and Boart, and other companies, for the cladding of diamond particles with metal.  
      The composite propellant/oxygen source material, in granular form, is output to a cartridge filling device  26 .  
      When a propellant such as nitrocellulose is ignited, in the absence of insufficient oxygen, the resulting combustion process gives rise to carbon monoxide, nitrates (NO 3 ) and nitrogen dioxide. These are undesirable and toxic by-products. An object of the invention in this respect is to make available additional oxygen by means of the source material  20  to allow the combustion process to proceed more fully so that the quantity of undesirable combustion products produced by the burning process is reduced.  
      By increasing the quantity of available oxygen on a micro basis, ie. on a particle by particle basis, it is possible to produce combustion products such as water, carbon dioxide and nitrogen which predominate over the aforementioned undesirable by-products.  
      It is also highly desirable to prevent the propellant from reacting with water moisture which may be present in the atmosphere. The propellant particles may be waterproofed using any appropriate technique although a preferred route to be followed, in this connection, is to apply a film or coating of nitrocellulose lacquer to the propellant particles using chemical vapour deposition or other appropriate techniques.  
       FIG. 3  illustrates an alternative form of the invention wherein a composite propellant particle  30  is produced by the mixer  12  and wherein the particle includes propellant particles  32  and oxygen source material particles  34  which are held in a matrix  36 . The matrix  36  may be any appropriate binder which is combustible or of an explosive nature and which is introduced from a source  38  during the mixing process. Again the intention is to bring into close proximity with each other particles of oxygen source material and propellant to ensure that when the burning process takes place adequate oxygen is available to effectively complete the combustion process.  
      The situation should be contrasted with what prevails when ammonium nitrate in particulate form is mixed with granular propellant. If the particles are not intimately associated with each other in the manner shown in  FIG. 2  or  FIG. 3  then the mixture is capable of separating or stratifying, depending on the transport or storage conditions, into distinct pockets of propellant and oxygen source material.  
      A further benefit which arises from the intimate mixture arrangements shown in  FIGS. 2 and 3  is that the oxygen source material acts as a deflagratory agent, ie. burn agent, which increases the burn rate of the propellant. This increases the energy release rate which results from the burning propellant. Despite the increase in the energy release rate the ignition temperature is reduced. The propellant thus becomes capable of exhibiting properties, from the energy release point of view, which are associated with substances normally classified as explosives. It is believed however that the propellant will still accurately be classifiable as a propellant and not as an explosive despite its enhanced properties.  
      In a variation of the invention the propellant composition is provided in liquid form and comprises a mixture of a liquid propellant eg. nitrocellulose, and oxygen source material in liquid form, eg. ammonium nitrate. The composition may include an enhancer, eg. nitro glycerine, and may thus be of a double base type, or have a compound base.  
       FIG. 4  illustrates one manner in which the propellant which is produced by the method shown in  FIG. 1 , can be used. A hole  50  is drilled into a rock mass  52  from a face  54  using conventional drilling equipment, not shown.  
      A cartridge  56  is loaded into the hole. The cartridge has a flat base  58  which engages closely with an end of the hole, and a generally cylindrical side wall  60 .  
      The cartridge forms an enclosure for a propellant material  64  which is produced by the method described in connection with  FIG. 1  and which is loaded into the cartridge during the step  26  under factory conditions. An initiator  66 , of any appropriate type, is loaded into the cartridge preferably on site.  
      Control wires  68  from the initiator to a control unit, not shown, which is used in a known manner for initiating the blasting process.  
      Stemming  70  is placed into the hole  50  from the rock face covering the cartridge to a desired extent and is tamped in position.  
      The propellant  64  is ignited by the firing the initiator  66  and when this happens, high pressure jet material is released as a consequence of the combustion process. The cartridge is designed to contain the expanding high pressure jet material and is allowed to deform outwardly, without rupturing, so that the cartridge is forced into sealing contact with an opposing surface of the wall of the hole  50 .  
      The high pressure jet material is initially confined by the cartridge and the cartridge is allowed to fracture at a desired point or region which means that the force which is released by the combusting propellant is then directed onto a chosen surface of the wall of the hole in order to fracture the rock.  
      The use of a propellant according to the invention in a rock breaking process of the type shown in  FIG. 4  means that the capability of the propellant to release energy is increased while the production of harmful by-products is reduced. As noted, if the propellant is correctly formulated, the resulting product can still accurately be classified as a propellant and not as an explosive.  
      As has been indicated the propellant composition can be provided in liquid form. The liquid is, for all practical purposes, incompressible and thus, when initiated, produces a pressure wave which can have a more rapidly rising leading edge than the particulate composition.