226 
IOWA ACADEMY OF SCIENCE 
coal may be removed. Moreover, impurities that are attached 
to the coal in quantities sufficient to make the piece of coal and 
impurities together more than ten per cent heavier than coal 
can be separated from the coal, in which case the good coal at- 
tached to the impurity is lost in the operation. Considerable ex- 
perimentation is necessary to determine the special treatment 
required for each variety of coal. In some plants the run-of- 
mine coal is washed, in other plants the screenings only are 
washed. When the run-of-mine coal is washed, it is necessary, 
to get the best results, to crush and size the coal, and, as a gen- 
eral rule, the smaller the ratio of reduction of the pieces of coal 
the more completely the impurities will be removed by washing. 
The only coal that has been washed at the plant at Lakonta 
is screenings. These screenings have been obtained from mines 
which are being operated at no great distance from the plant. 
The largest tonnage, at present, is being furnished by the Con- 
solidation Coal Company at Buxton. Some screenings are be- 
ing taken from mines owned by the company which is operating 
the washing plant, and part of the screenings of the other mines 
adjacent to the Chicago & North Western Railway Company iu 
this vicinity are being purchased by contract and washed at the 
plant. In the process of washing at the Lakonta plant, the 
screenings are elevated and then passed over jigs which are verti- 
cal in operation. These jigs are inclined and have one-fourth 
inch and one-half inch perforations. When the jigs are in opera- 
tion 800 gallons of water a minute is used in each jig. The 
washed coal passes over the head of the jig, whereas the impuri- 
ties, being heavier than the coal, seggregate at the bottom of the 
jig, and are removed through gates that open readily. The 
washed coal is next passed over two revolving screens which 
separate the coal into three sizes, one of which is below five- 
eighths inch in diameter, another between five-eighths inch and 
one and a half inch, and a third larger than one and a half 
inch. The coal is then elevated and sprayed over perforated 
screens to remove fine material adhering to the surfaces of the 
coal. After spraying, the clean coal is carried into bins where 
it is ready for shipment. 
The effect of washing is well shown by comparing the coals 
before and after treatment. The unwashed screenings have been 
shown by analyses to have a sulphur, ash, and refuse content of 
from 25 to 35 per cent. During washing about 27 per cent of 
