 The lower @-@ grade ores are reduced in blast furnaces while the higher @-@ grade ores are reduced in reverberatory furnaces . 
 The lower @-@ grade ores are reduced in blast furnaces while the higher @-@ grade ores are reduced in reverberatory furnaces . 
The lower @-@ grade ores r reduced in blast furnaces while the higher @-@ grade ores are reduced in reverberatory furnaces .