]MEW  PROCESS  OF  MANUFACTURING  SODA,  ETC. 
123 
will  vary  with  its  reducing  properties.  In  England  they  use 
ordinary  coal. 
The  amount  of  oxyd  of  iron  must  be  such  as  will  combine  with 
all  the  sulphur  of  the  sulphate  of  soda  to  form  SFe.  For  9  of 
the  pure  and  dry  sulphate,  not  less  than  5  parts  of  the  pure  and 
dry  oxyd  of  iron  are  required ;  a  small  excess  of  oxyd  of  iron  is 
advantageous.  If  the  oxyd  contains  lime  it  should  be  removed 
by  treating  with  hydrochloric  acid  and  washing  ;  for  the  lime 
would  give  rise  to  CaS,  then  CaO  SO3,  and  then  again  CaS,  in- 
creasing unnecessarily  the  volume  of  material  under  manipula- 
tion, and  causing  a  loss  of  carbon  and  heat.  The  carbon  should 
not  be  in  excess,  as  it  favors  the  formation  of  sulphuret  of  sodium, 
and  because  also  of  this  excess  remaining  with  the  sulphuret  of 
iron,  will  afterwards  afford,  in  the  roasting  of  the  latter,  some 
sulphurous  acid  mixed  with  the  carbonic  acid.  The  proportion 
of  carbon  should  hence  be  diminished  until  there  is  a  minute 
proportion  of  the  sulphate  of  soda  left  undecomposed  in  the 
blocks  of  crude  ferruginous  soda. 
The  quantity  of  the  mixture  that  may  be  put  into  the  calcin- 
ing furnace  at  one  time  will  depend  of  course  on  its  size ;  but 
the  amount  may  be  full  twice  as  large  as  in  the  Leblanc  process, 
since  the  ferruginous  soda  works  more  easily  than  the  ordinary 
soda. 
For  calcination,  the  furnace  may  be  similar  to  that  for  the 
calcareous  soda  ;  but  to  economise  heat,  there  had  better  be  two 
or  three  stories,  the  lowest  nearest  the  fire.  The  furnace  then 
holds  three  charges  at  once,  which  are  moved  downward  in  suc- 
cession, another  being  added  above  when  one  is  taken  out  below. 
The  treatment  in  the  furnace  is  like  that  for  the  crude  calcare- 
ous soda,  and  the  phenomena  are  nearly  the  same.  The  whole 
softens,  becoming  pasty,  and  the  fluid  as  the  action  goes  on  dis- 
engages a  yellow  flame ;  then  the  action,  which  has  been  very 
bright,  diminishes  as  the  flames  become  less  abundant,  and  when 
the  mass  is  homogeneous,  it  is  finished.  It  is  then  removed  im- 
mediately from  the  furnace,  being  run  while  still  red  into  a 
wagon  on  wheels  in  which  it  cools  and  solidifies,  having  been 
partly  covered  for  security  from  contact  with  the  air.  When 
cold,  it  is  a  block  in  the  form  of  a  parallelopiped,  blackish  in 
color  and  more  or  less  porous,  very  hard  and  of  considerable  den- 
