PHOSPHORUS FERTILIZERS 121 



means of forged steel balls, the presence of a few other 

 particles of steel does not make much difference. 



The blocks of basic slag are usually first of all broken up 

 by hand to eliminate the larger pieces of steel. A common 

 stone breaker, consisting of two jaws meeting in a "V" 

 shape, which move to and fro, is sometimes used for breaking 

 up slag, but owing to the lumps of iron contained it is not 

 very satisfactory. The resulting broken-up slag is then 

 generally ground in a ball mill, in which a draught of air, 

 caused by a fan or chimney, draws the fine powder away 

 through sieves. The sieve used is a hundred-mesh sieve, 

 containing 10,000 holes to the square inch, through which 

 all the material should pass. In some cases coarser sieves are 

 employed, so that only 80 % or 90 % of the finished product 

 passes through the hundred-mesh sieve. The finished product 

 may contain many small particles of iron, probably owing to 

 some failure in the sieves of the ball mill. These particles 

 of iron are extremely objectionable, as they block up the drill 

 or distributor when the basic slag is applied to the field, and 

 every effort should be made to reduce these metallic impurities 

 as far as possible. 



The ball mill used for grinding usually contains a large 

 number of balls, which move round over steps, as in Fig. 7, 

 p. 137. Basic slag, like most by-products, has a composition 

 which has little relationship to the actual needs of the crop 

 to which it may be applied as a fertilizer ; there is }^et a 

 great deal of work to be done to discover all the points of 

 value in basic slag. From time to time the method of manu- 

 facture is altered, so that empirical guides to the general 

 nature of the composition of slag have no permanent value. 

 When basic slags were first used, a test was devised by Wagner 

 to extract the basic slag with a solution of ammonium 

 citrate, but owing to the changes in manufacture this was sub- 

 sequently altered to a test with citric acid. This method was 

 originally designed to detect admixture with rock phosphate, 

 but has been used to measure the solubility of basic slag for a 

 long time. Collins and Hall showed that there was a distinct 

 correlation between the citric solubility and the amount of 



