20 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 



mechanical condition. It is usually kept for at least one month be- 

 fore shipping, but adverse weather conditions may delay shipment 

 considerably longer and even seriously affect the quality of the final 

 product. The production of acid phosphate by the "open-dump" 

 method is impracticable in the vicinity of towns or in a rich farming 

 country unless phosphate rock very low in fluorine compounds is used, 

 for the fumes given off during the process are not only so obnoxious 

 as to constitute a nuisance, but are quite injurious to both animal 

 and vegetable life. At points where these fumes do no harm, how- 

 ever, and where the climate is not too cold, an excellent product is 

 obtained by this method at less cost and with less danger than by 

 the "den" system. 



THE DRYING OF ACID PHOSPHATE. 



Acid phosphate which is carefully made, especially that produced 

 by the "den" system, seldom requires any subsequent drying. It is 

 customary abroad, however, to dry superphosphates artificially, par- 

 ticularly when they contain an excess of phosphoric acid or are in a 

 poor mechanical condition due to improper mixing. There are two 

 general methods employed in drying acid phosphate. The first con- 

 sists of the application of artificial heat and the second of adding 

 some material to take up or combine with the water or free phos- 

 phoric acid present. 



In Europe a number of machines for artificially drying acid phos- 

 phate have been patented. Among the most efficient of these are 

 the dryers of Lutjens and of Moller and Pfeiffer. 1 In both- of these 

 machines the disintegrated acid phosphate is submitted to the action 

 of a current of hot air under pressure. No direct heat can be used 

 in drying acid phosphate because of the tendency of the material to 

 revert at high temperatures. 



The second method of drying acid phosphate is often practiced in 

 this country when the material is too sticky or wet (due to faulty 

 manipulation) to be uniformly spread on the field or mixed with 

 other fertilizer ingredients. Such a condition when due to an excess 

 of phosphoric acid can be frequently remedied by mixing the sticky 

 mass with small percentages of phosphate rock or limestone. If the 

 condition is due to the presence of a large proportion of iron and 

 aluminum, the addition of finely ground peat or calcined gypsum 

 will dry the material. In expelling the water from acid phosphate 

 by artificial heating the value of the fuel consumed must be added 

 to the cost of production, but no matter how the drying is done it 

 entails additional handling, which is always expensive and should be 

 avoided. 



i Fritsch, Manufacture of Chemical Manures, pp. 123-129 (1911). 



