Stalls  are  simply  spaces  for  calcination,  surrounded  by
walls,  a  common  size  being  9  feet  long,  7  feet  wide,  and  6  feet
high.  They  are  generally  built  back  to  back,  and  a  row  of
several  stalls  is  placed  in  a  line.  The  walls  contain  central
flues  which  communicate  with  a  chimney,  or  the  flues  may
conduct  the  gases  to  a  sulphuric  acid  chamber.  The  charging


COPPER.
301


and  arrangement  are  similar  to  that  of  heaps,  but  the  heat
is  better  concentrated,  and  the  duration  of  the  operation
shorter.
Kilns  are  used  for  ores  poor  in  sulphur.  They  resemble
ordinary  shaft  furnaces,  but  the  zone  of  combustion  is  greatly
extended,  since  the  temperature  required  is  very  moderate.
The  Gerstenhoffer  calciner  (Figs.  120,  120A)  is  a  rectangular
kiln  20  feet  high  and  5  feet  across,  in  which  are  arranged
20  rows  of  triangular  fire-clay  bars,  2  feet  6  inches  long.
_

T  TT


iy  T  T  T  T  T-.
-J


Fig.  12a


Fig.  120A.
The  ore  is  ground  fine,  and  the  supply,  regulated  by  feed
rollers,  passes  through  three  narrow  channels,  and  falls  on
the  flat  surface  of  the  bars,  then  gradually  descends  as  fresh
ore  is  added,  being  oxidised  in  its  descent,  and  is  finally
removed  at  the  bottom.  Air  is  admitted  by  a  series  of
apertures  in  the  front  of  the  furnace,  and  the  combustion
of  the  sulphur  maintains  the  necessary  temperature.  If
rich  ores  were  treated  in  this  kiln,  they,  being  more  easily
fusible,  would  clot  together  on  the  bars  and  choke  the
draught,  as  well  as  defeat  the  object  of  the  calcination.
302


PRINCIPLES  OF  METALLURGY.
