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The major quality requirements of coal for Sopnge iron production are
 1) Non-cooking characteristics
 2) Low ash content
 3) Low sulphur content
 4) Good reactivity
 5) High ash fusion temperature
 6) Medium volatile matter
   
                 
a) Non-cooking character
Non-cooking coal is required in sponge iron production, since  the  cooking  reaction leads to formation of  rings inside the kiln. Practically permissible caking index is 3 maximum for rotory kiln operations.
 
b) Low ash content
High ash  leads  to under utilization of  the  kiln volume available  for reduction  it  also  increases the consumption of fixed carbon  since  additional energy  is consumed  in heating  the inert ash  mass to reaction temperature. An ash content 30 percent maximum is the practicable limit for normal operation.
c) Low sulphur content
To control the sulphur in sponge iron the sulphur in coal should be one percent maximum.
d) Good reactivity
An  important step  in solid  state reduction  is  the  boundward reaction, whereby carbondioxide generated from burning of coal  reacts with  coal  to regenerate  carbon monoxide  generated  from  the  coal, therefore, should be possess sufficient "reactivity" so  that  the  bounded reaction proceeds Bituminous  and sub-bituminous  coal exhibits  very good  suitable for production of sponge iron.
e) High ash fusion temperature
Coals with low ash fusion temperature leads to sticky and consequent unstable  kiln  operation by forming accretion inside the  kiln, and  reducing  the  reduction  rate  by  forming  of  slag  layer on surface of  the ore. Hence coals with ash fusion temperature above 1150°C are desirable for  rotory kiln operation. Ash temperature  is governed by its chemistry. Coal with ash of  high  sulphuric trioxide equivalent low silica  ratio and high alkali  ratio has lower eutectic temperature and hence is not desirable.
f) Medium volatile matter
Coals with optimum volatile content are required for coal based rotory kiln process. Coal with very low volatile matter result in very low reactivity of char and thus give rise to poorly metallised product. Very high volatile coals, though they yield highly reactive char, are also not preferred. Generally for rotory kiln process volatile matter should be in the range of 28 to 32 percent.
         
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