48 CIRCULAR 3 6 9, U. S. DEPARTMENT OF AGRICULTURE 



CHOICE OF FUMIGANT 



Chloropicrin and hydrocyanic acid are the two fumigants that can 

 be used successfully for mill fumigation. Hydrocyanic acid is more 

 popular and gives more consistent results. 



HYDROCYANIC ACID GAS FOR MILL FUMIGATION 



In general, a dosage of 8 ounces of liquid hydrocyanic acid or its 

 equivalent per 1,000 cubic feet of space has been found most satis- 

 factory for flour-mill fumigation. All the methods that have been 

 described for generating this gas may be used with good results. 

 For the miller who does his own fumigating, the pot or barrel 

 method is safer and cheaper, and therefore more desirable, although 

 the hot gas produced by this method is likely to escape from the 

 building faster than the cold gas produced by other methods. 



Professional fumigators generally use liquid hydrocyanic acid. An 

 initial dosage of 4 ounces per 1,000 cubic feet of space, followed 

 by two dosages of 2 ounces at intervals of 2 hours, is sometimes used, 

 though it is doubtful whether the results are any better than when 

 the entire dosage is applied at one time. 



A combination of the liquid hydrocyanic acid and the calcium 

 cyanide (40 to 50 percent Ca(CN) 2 ) methods has also been used 

 with success. An initial high concentration is obtained from the 

 liquid hydrocyanic acid, and this is maintained for from 4 to 6 

 hours by the gas that is slowly evolved from the calcium cyanide. 

 Four ounces of liquid hydrocyanic acid and 1 pound of calcium cyan- 

 ide per 1,000 cubic feet of space produce approximately the same 

 quantity of gas as 8 ounces of liquid hydrocyanic acid, and this is a 

 good combination to use. The calcium cyanide is applied first, and 

 after all the fumigators have left and closed the building the liquid 

 hydrocyanic acid is forced in through the piping system from the 

 loading platform. 



Distribution of the Fumigant 



In general the gas should be equally distributed on all floors of a 

 mill. If, however, the floors are not well separated one from the 

 other on account of belt openings, poorly fitting doors, floors, ele- 

 vator openings, etc., the gas will tend to concentrate on the top floors; 

 hence the dosage on the lower floors should be somewhat heavier. 

 Floors that contain more machinery than others should also receive 

 a heavier dosage. 



Length of Exposure 



Since it is difficult to hold a killing concentration of the gas in the 

 average mill for more than a few hours, an exposure of from 18 

 to 20 hours is all that is practical. 



CHLOROPICRIN AS A MILL FUMIGANT 



Under favorable conditions chloropicrin at the rate of 1 pound 

 per 1,000 cubic feet of space is a satisfactory flour-mill funiigant. 

 It should be used only in a tight building and when the temperature 

 is above 70° F. 



