Ammonium  Nitrate-­‐Based  Explosives  Overview  

 J.H.  Heck  /  B.D.  Northup  

Op3muck,  LLC  April  1,  2017  

Agenda  

•  Basic  Ammonium  Nitrate  Chemistry  •  Ammonium  Nitrate  Prills  •  ANFO  •  Ammonium  Nitrate-­‐Based  Emulsion  Explosives  •  ANFO  /  Emulsion  Blends  •  Gassed  Emulsions  •  ANFO/Emulsion  Blend  Addi3ves  •  Applica3ons  •  General  Guidelines  •  Discussion  

Basic  Ammonium  Nitrate  Chemistry  

•  Ammonium  Nitrate  is  created  by  the  reac3on  of  ammonia  and  nitric  acid  

       NH3  +  HNO3  (concentrated)  →  NH4NO3  +  H2O  

Ammonium  Nitrate  Prills  

•  Ammonium  nitrate  blas3ng  prills  are  crystallized  spheres  of  solid  AN  coated  with  surfactants  and  clay  –  Par3cle  size  0.83  to  2.3  mm  –  Porosi3es  8%  to  12%    –  Densi3es  1.3  to  1.5  grams  per  cubic  cen3meter  

 

Prill tower ~ 200 feet tall

spray nozzles

droplets of supersaturated AN solution

crystallized AN

updraft of warm air

Explosive-grade prills are made in a prill tower in which a hot, supersaturate AN liquid (4-percent water)—along with other additives to achieve porosity—is dropped from spray nozzles at a height of 100 to 200 feet against an updraft of warm air. Droplets of the AN solution crystallize as they fall; the longer the droplets are suspended, the larger the prill diameters. The crystallized AN particles are then completely dried and coated with surfactants and clay to minimize porosity and protect the surface from absorbing water in preparation for fuel absorption. Porosities range from 8 to 12 percent, whereas solid-grain densities range from 1.3 to 1.5 g/cm3. Particle sizes range from 0.83 to 2.3 mm in diameter. By comparison to explosive-grade prills, agricultural-grade prills are less porous (3- to 5-percent porosity) and far more dense.

Ammonium Nitrate Prill Manufacture

ANFO  

•  ANFO  is  a  mixture  of  94%  ammonium  nitrate  prills  and  6%  fuel  oil  –  The  oil  is  absorbed  into  the  prill,  crea3ng  in3mate  contact  between  oxidizer  and  fuel,  allowing  reliable  detona3on.  

–  This  mixture  gives  the  op3mum  oxygen  balance  for  detona3on.  AN  alone  has  too  many  oxygen  atoms.    The  carbon  based  fuel  supplies  carbon  and  hydrogen  atoms  to  balance  the  equa3on.  

                                         NH4NO3  +  CXHY  →  N2  +H2O  +  CO2  –  Some  CO  (carbon  monoxide)  and  NOX  (nitrous  oxides)  are  produced  as  well  but  the  mixture  is  designed  to  minimize  these  undesirable  compounds          

Ammonium  Nitrate-­‐Based  Emulsion  Explosives  

•  Blas3ng  emulsions,  like  ANFO,  are  composed  of  an  oxidizer  salt  and  oil  and  have  other  minor  compounds  to  allow  the  emulsion  to  form  and  remain  stable.  

•  They  are  liquids  of  varying  viscosi3es,  depending  on  the  applica3on.  

•  Unlike  ANFO,  they  do  not  dissolve  in  water  and  become  desensi3zed  

•  Because  they  contain  water,  they  have  less  energy  per  unit  mass  than  ANFO  (~680  vs  880  calories  per  gram)  

Emulsions Emulsions Emulsions are “water-in-oil” mixes that were developed in the early 1960’s to improve the performance of water gels. They amount to hot solutions of oxidizer salts (ammonium nitrates [AN], calcium nitrates [CN] or sodium nitrates [SN]) mixed with oil and an emulsifying agent. The oil phase usually consists of diesel fuel and/or mineral oil that includes micro- balloons as sensitizers.

The oxidizer solution is broken up into small, micron- sized droplets, which form a discontinuous phase within the continuous oil phase.

The small size of the liquid-nitrated salt particles provides a large surface area-to-volume ratio that amounts to more fuel being placed in intimate contact with the oxidizer. This, in turn, allows for a very fast detonation rate and a powerful explosive.

Packaged in plastic tubes or bulk-loaded from trucks, emulsions exhibit properties as follows:

•  Bulk density = 1.15 - 1.45 g/cm3 •  Detonation velocity = 14,500 - 18,500 ft/sec •  Detonation pressure = 100 - 120 kbar.

5 µ

=

liquid oxidizer (AN) surrounded by liquid fuels

and sensitizers

ANFO/Emulsion  Blends  

ANFO  is  oben  mixed  with  varying  percentages  of  emulsion  explosive  to  vary  the  detona3on,  handling    and  water  resistance  proper3es.  •  Ammonium  nitrate  prills  have  a  rela3vely  high  density  (~1.5  

g/cc)  but  because  they  are  spherical,  there  is  ~  45%  void  space  in  a  borehole  column  of  ANFO.    The  bulk  loaded  density  of  ANFO  is  thus  ~  0.82  g/cc.  

•  The  addi3on  of  a  liquid  emulsion  explosive  fills  in  the  void  spaces  between  the  prills,  adding  more  pounds  and  energy  to  a  given  volume  of  borehole.    This  mixture  does  not  have  as  much  energy  per  unit  weight  as  ANFO  alone  but  increases  the  energy  per  foot  of  borehole.  

ANFO/Emulsion  Blends  •  When  the  volume  of  emulsion  fills  all  the  void  space  (~45%),  

the  less  energe3c  emulsion  pushes  the  prills  farther  apart  and  the  energy  per  unit  volume  begins  to  decrease.  

•  Although  the  energy  per  unit  mass  decreases  with  higher  emulsion  percentages,  the  detona3on  velocity  increases  up  to  ~70%  emulsion,  then  begins  to  decrease.  

ANFO/Emulsion  Blends  (cont’d)  •  The  op3mum  blend  percentage  for  a  given  site  is  based  on  a  

number  of  factors,  including:  –  ANFO  and  emulsion  pricing  –  Water  resistance  required  –  Available  loading  equipment  –  Drilling  cost  –  Ini3a3on  cost  

•  Low  percentages  (10-­‐15%)  can  be  cost  effec3ve  in  dry  holes  •  If  some  water  resistance  is  required,  blends  of  25-­‐30%  are  

effec3ve.  •  For  very  wet  holes,  emulsion  percentages  of  50%  and  higher  

are  required.  •  The  higher  the  percentage  of  emulsion  in  the  blend,  the  less  

energy  is  available  per  unit  weight.  

ANFO/Emulsion  Blends  (cont’d)  •  At  blend  percentages  from  0%  (all  ANFO)  to  ~40%,  augured  

delivery  systems  are  typically  used.  •  Blend  percentages  from  40%  to  50%  are  not  normally  used  

because  of  sensi3vity  and  handling  problems  (the  mixture  is  very  viscous).  

•  Blend  percentages  of  50%  to  70%  can  be  augured  successfully.  

•  Blend  percentages  above  70%  are  pumped.  •  In  standing  water  applica3ons,  ANFO  alone  cannot  be  used  as  

it  dissolves.  •  Even  though  they  are  heavier  than  water,  augured  blends  can  

“bridge  over”  in  standing  water,  crea3ng  pockets  of  water  in  the  borehole  and  hindering  or  stopping  the  detona3on.  

ANFO/Emulsion  Blend  ProperCes  

ANFO/Emulsion  Blend  ProperCes  (cont’d)  

Gassed  Emulsions  •  Chemically  sensi3zed  (“Gassed”)  emulsions  do  not  use  ANFO.  •  They  are  manufactured  in  a  plant  and  sensi3zed  in  the  field  by  the  

addi3on  of  chemical  sensi3zers  as  they  are  loaded  in  the  borehole.  •  The  density  can  be  varied  by  the  addi3on  of  chemicals  that  react  

together  and  produce  very  3ny  gas  bubbles  in  the  explosive  column.    In-­‐hole  densi3es  typically  vary  from  ~1.25  g/cc  at  the  bojom  of  the  hole  to  ~0.9  g/cc  at  the  base  of  the  stemming.  

•  The  addi3on  of  gassing  agents  in  the  field  must  be  closely  controlled  to  ensure  product  consistency  throughout  the  blast.  

•  Since  the  product  expands  with  3me,  3me  consuming  quality  checks  must  be  taken  throughout  the  process.  

•  Gassed  emulsion  usage  assumes  that  more  energy  is  needed  at  the  bojom  than  the  top  of  the  hole.    This  assump3on  is  not  valid  if  the  burdens  are  consistent  along  the  borehole..  

Gassed  Emulsion  ProperCes  

ANFO/Emulsion  Blend  AddiCves  •  A  number  of  less  expensive  energe3c  material  addi3ves  have  been  tried  

over  the  years  to  reduce  blas3ng  costs  without  adversely  affec3ng  performance.  –  Coal  –  Corn  –  Rice  Hulls  –  Aluminum  –  Metal  Filings  –  Wood  Chips  –  Styrofoam  –  Sawdust  

•  Each  of  these  materials  either  hindered  proper  detona3on  or  had  storage  or  handling  problems  

•  In  recent  years,  the  use  of  OS5,  a  specialized  oil  shale  addi3ve  without  these  drawbacks  has  shown  excellent  results  when  mixed  with  both  ANFO  and  augured  ANFO/emulsion  blends  –  Energy  =  2190  cal/g  –  Reduces  poten3al  for  NOX  gasses  –  About  ⅓  cost  of  ANFO  per  pound  

ApplicaCons  The  selec3on  of  the  best  product  for  any  given  rock  forma3on  is  based  on  a  number  of  factors:  •  Rock  characteris3cs  •  Amount  of  water  present  •  Loading  equipment  available  •  Product  pricing  •  Site  loading/crushing  equipment  

General  Guidelines  There  are  too  many  variables  to  make  firm  recommenda3ons  on  the  op3mum  product  for  any  given  rock  forma3on.  However,  some  general  guidelines  are  useful:  •  In  dry  holes,  in  most  rock  types,  ANFO  is  the  most  cost  effec3ve  

choice.    The  addi3on  of  low  percentages  of  emulsion  can  boost  the  available  energy  per  unit  borehole  length  at  a  low  addi3onal  cost.  

•  Where  there  is  occasional  water  or  water  in  the  bojoms  of  the  boreholes,  dewatering  allows  the  use  of  a  bojom  load  of  a  low  percentage  blend  and  a  column  load  of  ANFO  from  there  up.  

•  Another  op3on  for  the  above  case  is  to  keep  a  supply  of  waterproof  bags  on  the  blend  truck  and  load  smaller  diameter  bags  with  a  waterproof  emulsion  blend  un3l  the  explosive  column  is  out  of  the  water.    The  remainder  of  the  borehole  can  then  be  loaded  with  ANFO.    In  a  6  ½  inch  borehole,  a  1.3  g/cc  blend  in  a  5  inch  bag  gives  ~  the  same  pounds  per  foot  as  the  full  column  diameter  of  ANFO.  

General  Guidelines  (cont’d)  •  In  holes  that  are  slowly  making  water,  dewatering  the  holes  and  

loading  a  water  resistant  blend  (above  25%)  is  recommended.  •  For  loca3ons  where  dewatering  is  imprac3cal  or  impossible,  a  

pumpable  emulsion  blend  can  be  pumped  in  the  bojom  of  the  hole,  displacing  the  water  without  danger  of  the  explosive  column  “bridging  over”.  

•  As  a  general  rule,  the  more  energy  the  explosive  has  per  unit  of  weight,  the  more  cost  effec3ve  it  is.    A  mine  is  buying  energy,  not  pounds,  to  break  rock.    It  takes  ~1.3  pounds  of  a  straight  emulsion  or  ~1.1  pounds  of  a  40%  blend  to  provide  the  same  energy  as  1  pound  of  ANFO.    A  less  energe3c  but  more  dense  explosive  may  be  desirable  if  there  are  high  drilling  or  ini3a3ng/priming  costs  or  water  condi3ons  demand  it.  

Energy  Equivalents  per  Pound  •  20%  Blend    835  cal/g  1.05  #  emulsion  =  1.0  #  ANFO  •  25%  Blend    830  cal/g  1.06  #  emulsion  =  1.0  #  ANFO  •  30%  Blend    820  cal/g  1.07  #  emulsion  =  1.0  #  ANFO  •  40%  Blend    800  cal/g  1.10  #  emulsion  =  1.0  #  ANFO  •  50%  Blend    780  cal/g  1.13  #  emulsion  =  1.0  #  ANFO  •  60%  Blend    760  cal/g  1.16  #  emulsion  =  1.0  #  ANFO  •  70%  Blend    740  cal/g  1.19  #  emulsion  =  1.0  #  ANFO  •  80%  Blend    720  cal/g  1.22  #  emulsion  =  1.0  #  ANFO  •  100%  Blend    680  cal/g  1.29  #  emulsion  =  1.0  #  ANFO  

Delivery  Systems  •  ANFO  

–  Paper  bags  –  Auger  truck  –  Cement  mixer  

•  10%-­‐70%  emulsion  blends  –  Auger  truck  

•  70%-­‐100%  emulsion  blends  –  Pump  Truck  

•  Chemically  sensi3zed  (“gassed”)  emulsions  –  Specialized  pump  truck  

Acknowledgement  •  Physical  and  chemical  property  data  were  taken  from  the  

DYNO  Explosives  Engineer’s  Guide  in  the  DYNO  iPhone  app.  •  Other  manufacturer’s  literature  show  that  their  equivalent  

products  have  similar  physical  and  chemical  proper3es.  

Discussion