SOLUTION OF PHOSPHATIC SLAGS IQ7 



may be obtained by determining its degree of solubility in one of 

 the menstrua named. 



1 68. Separation by Sifting. The relative availability of a slag, 

 as in the case of a mineral phosphate, is determined very largely 

 by the percentage of fine material it contains. Sieves of varying 

 apertures are used to determine this percentage. A one-half 

 millimeter or a one-quarter millimeter circular aperture is best, 

 and the percentage of the total material passing through is deter- 

 mined. A method used in Germany consists in sifting the slag in 

 a sieve 20 centimeters in diameter, the meshes of which are from 

 0.14 to 0.17 millimeter square and which measure diagonally from 

 0.22 to 0.24 millimeter. 



169. Solution of Phosphatic Slags. Sulfuric acid has been 

 found to be an excellent solvent for basic slags preparatory to the 

 determination of total phosphoric acid. There is, however, no 

 unanimity of opinion concerning the best method or means of 

 solution. Aqua regia and nitric acid are objected to because they 

 may convert any phosphorus in combination with the iron into 

 phosphoric acid and thus increase the quantity present. 48 But 

 iron phosphid is seldom found in slags, and therefore this objec- 

 tion is not always tenable. Sulfuric acid has also been deemed 

 objectionable because the gypsum separated is likely to carry 

 with it some of the other substances to be determined. 



Hydrochloric acid is also excluded by some from the list of sol- 

 vents because it dissolves so many of the foreign elements in the 

 slag and thus tends to complicate the subsequent determinations, 

 especially of magnesia. Besides, a hydrochloric acid solution is 

 not suited to the use of the citrate method formerly much em- 

 ployed in the determination of total phosphoric acid. When 

 hydrochloric acid is used, moreover, the dissolved silica must be 

 removed and thus the time required for making a phosphoric 

 acid determination is much increased. 



If the sample be sufficiently fine the occlusion of undissolved 

 phosphate particles by the gypsum formed when sulfuric acid is 

 used is not to be feared, and the disturbance of volume by the 

 gypsum is nearly constant and can be allowed for. When 



48 von Reis, Zeitschrift fur angewandte Chemie, 1888, 1 : 354. 



