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iron  present  in  the  rock.  A series  of  secondary  reactions  set  in 
whereby  the  free  phosphoric  acid  combines  with  the  unacidulated 
phosphate  to  form  di-calcium  or  citrate  soluble  phosphate.  If 
the  reaction  has  been  incomplete  and  a large  amount  of  tri- 
calcium phosphate  is  left  the  acid  phosphate  unites  with  this, 
again  forming  reverted  phosphate. 
2Ca3Po08  +2H3P04-f- 1 2H20=3CaoH2P208.4Ho0 
CaH4P208+Ca3P208=2Ca2H2P208 
The  tendency  is  to  under-acidulate  and  to  leave  a small  amount 
of  the  rock  unattached,  for  by  so  doing  one  is  assured  that  the 
acid  will  be  used  completely  and  no  free  acid  will  be  left  to  dam- 
age the  crop  for  which  it  is  used.  In  these  islands  where  water 
soluble  phosphate  is  called  for,  reversion  has  to  be  guarded 
against  much  more  carefully  than  in  the  states  where  water 
soluble  and  citrate  soluble  phosphates  are  considered  of  equal 
value. 
A concentrated  superphosphate  or  “double  superphosphate”  is 
made  in  Europe  where  low  grade  rock  is  plentiful  and  labor 
cheap.  This  material  contains  about  44%  phosphoric  anhydrid 
and  is  quite  free  from  gypsum  and  other  impurities.  The 
method  of  manufacture  is  similar  to  that  used  in  the  production 
of  ordinary  superphosphate  and  is  somewhat  as  follows : ' 
The  rock  is  first  treated  with  sufficient  dilute  acid  to  form 
phosphoric  acid  and  to  precipitate  the  gypsum.  The  dilute  phos- 
phoric acid  is  filtered  off  into  lead  pans  where  it  is  concentrated 
to  about  450  Be.  During  the  concentration  of  the  acid  the  solu- 
tion is  constantly  agitated  to  prevent  the  gypsum  which  has  been 
held  in  solution  from  forming  a scale  on  the  bottom  of  the  pans 
and  the  steam  pipes  allowing  them  to  overheat  and  melt.  This 
450  Be  acid  which  contains  about  45%  of  phosphoric  acid  is  al- 
lowed to  act  on  ground  phosphate  rock  in  the  proper  proportion 
to  form  mono-calcium  phosphate. 
Ca3P,08+3H2S04+6H.0=3  ( CaS04.2H,0 ) +2HoP04 
Ca3P2Os+ 4H3P04-f  6H20=3  ( CaH4P208+2H20 ) 
As  the  reaction  is  not  as  energetic  as  with  surphuric  acid  and 
there  is  much  more  water  to  be  evaporated  the  double-super- 
phosphate usually  has  to  be  dried  artificially  and  disintegrated 
before  it  is  readv  for  use. 
