421 
But  as  the  chamber  acid  is  only  about  65%  pure  there  would 
be  required  .632X100-^-65  or  .97  pounds  of  the  dilute 
acid  for  each  pound  of  calcium-phosphate.  As  the  Ocean 
Island  phosphate  contains  about  83%  it  would  require 
80.5  pounds  of  .acid  for  each  100  pounds  of  rock 
to  just  combine  with  the  phosphate  alone.  But  in  addition  to 
this,  calculations  have  to  be  made  for  the  calcium  carbonate,  iron 
and  aluminium  phosphates,  magnesium  and  ever}-  other  in- 
gredient which  combines  with  sulphuric  acid,  for  the  acid  does 
not  show  any  particular  preference  for  calcium  phosphate  and 
enough  has  to  be  added  to  satisfy  them  all. 
ACIDULATOR. 
After  having  computed  the  acid  necessary,'  the  materials  are 
brought  to  the  mixer  or  acidulator,  as  it  is  known.  This  is  a 
large  cast  iron  or  lead-lined  pan  set  upon  rollers  so  that  it  may 
be  revolved  in  one  direction  while  a set  of  iron  paddles  or  arms 
revolve  within  it  in  the  opposite  direction,  similar  to  the  action 
of  an  ice  cream  freezer.  These  pans  are  made  in  a variety  of 
sizes  and  shapes,  holding  from  one  to  five  tons,  depending  upon 
the  material  used  and  the  capacity  of  the  plant.  The  idea,  of 
course,  with  all  makes  is  to  produce  a thorough  and  uniform  mix- 
ing quickly.  The  acidulator  is  set  over  a vat  or  pit  into  which 
the  mixture  is  dropped,  leaving  the  pan  ready  for  an- 
other charge.  This  operation  is  repeated  until  the  pit  contains 
the  daily  output  of  fifty  or  a hundred  tons  or  whatever  it  may  be. 
REACTION  IN  PIT. 
As  soon  as  the  rock  and  acid  come  together  in  the  mixer  the 
reaction  commences  and  after  three  or  four  minutes  is  dropped 
in  the  form  of  a thin  mud  into  the  pit  where  the  reaction  con- 
tinues to  completion.  Carbon  dioxide  and  hydroflouric  acid  are 
given  off  together  with  the  excess  water  which  begins  to  come 
off  as  the  temperature  of  reaction  rises.  The  highest  tempera- 
ture reached  is  about  no°  C.  and  seldom  over  120°,  if  the  proper 
acid  has  been  added.  Gradually  the  mass  begins  to  solidify  as 
it  cools  and  in  so  doing  catches  the  bubbles  of  gas  that  are  on 
their  way  to  the  surface  until  finally  there  is  a single  cake  of 
porous  acid  phosphate  filling  the  pit.  This  acid  phosphate  is  then 
dug  out  with  pick  and  shovel  when  it  has  cooled  enough  and 
carried  to  the  storage  bins  where  it  is  allowed  to  stand  for  a 
month  or  more  to  become  dry  and  to  season. 
REVERSION. 
It  is  during  this  storage  period  that  reversion  takes  place  in 
improperly  made  acid  phosphate  or  if  there  is  a large  amount  of 
