Am  jour.  Pharm.j    Pharmaceutical  Use  of  Filter  Press. 
February,  1917-  ' 
73 
monly  worthless.  The  centrifuge  is  wasteful ;  it  splashes  the  nitrate 
about  the  outer  jacket  and,  unless  the  center  of  gravity  is  carefully 
maintained  in  the  shaft,  will  vibrate,  shifting  the  "cake  "  and  upset- 
ting the  filtration. 
In  the  filter  press  process  the  liquid  to  be  filtered  is  pumped 
through  the  filtering  medium.  The  rate  of  flow  can  be  easily  con- 
trolled and  the  amount  of  pressure  exerted  adjusted  to  the  particular 
conditions.  Loss  of  material  due  to  slopping  or  evaporation  may 
be  reduced  to  a  minimum  and,  once  started,  the  press  may  usually 
be  left  to  itself  to  complete  the  operation.  The  method  has  the 
disadvantages  of  requiring  expert  handling,  the  absorbed  liquid  is 
likely  to  be  large,  small  quantities  cannot  economically  be  filtered 
through  the  press,  and  the  cleaning  of  the  apparatus  is  a  matter  of 
some  labor. 
The  press  used  in  the  laboratory  of  the  Standard  Chemical  Com- 
pany is  a  small  Sperry  machine  of  six  chambers.  The  liquid  is  fed 
into  the  press  by  a  single-plunger,  belt  driven  pump  equipped  with 
an  air  chamber  and  safety  tube.    The  modus  operandi  is  as  follows : 
The  liquid  to  be  filtered  is  prepared  by  proper  aging,  with  or 
without  the  addition  of  filtering  agent  according  to  conditions,  and  is 
then  filled  into  barrels  which  are  connected  with  the  pump.  A 
quantity  of  a  filtering  agent  is  now  put  into  the  liquid  and  thoroughly 
stirred  in  so  that  it  may  enter  the  press  gradually  with  the  liquid. 
This  is  an  important  detail  and,  if  not  observed,  the  press  will  clog 
when  resinous  or  mucilaginous  precipitates  are  present. 
The  liquid  is  sucked  into  the  pump  through  an  iron  pipe  con- 
nected with  a  rubber  tube  of  convenient  size  which  is  lowered  into 
the  barrel.  The  open  end  of  this  tube  should  be  protected  by  a 
piece  of  sieve  rolled  into  a  cylinder  and  fitted  so  as  to  form  a  pro- 
longation of  the  tube.  This  precaution  prevents  the  entrance  of 
coarse  particles  which  might  lodge  in  the  pump  valves  and  interfere 
with  their  proper  action. 
The  pump  should  be  selected  with  care.  We  consider  a  motor- 
driven  pump  preferable  to  a  steam-driven  pump  for  general  pharma- 
ceutical use  for  the  reason  that  the  steam  power  available  in  the 
average  pharmaceutical  house  will  not  be  adequate  to  overcome 
a  back  pressure  of  one  hundred  twenty  pounds  in  the  press,  a 
pressure  often  encountered.  The  number  of  cylinders  desirable  in- 
creases with  the  size  of  the  press  and  the  filtering  surface  to  be  fed. 
A  three-cylinder  pump  will  deliver  a  steady  stream  of  liquid  prac- 
