Concentration of Phosphate.
A number of experiments have been carried out in order to determine to what extent francolite may be concentrated by light crushing and screening. The clue to the problem is to be found in a comparison of the grain-sizes of the chief components of the phosphorite, viz. of quartz and albite, and of francolite. The average diameter of the quartz and albite is 0·5–0·7 mm., while the francolite crystals do not exceed 0·06 mm. in length. With gentle crushing or rubbing by such means as passing between rollers and subsequent screening through 90-mesh and 200-mesh sieves, a notable concentration of francolite results. Thus, when the original phosphorite containing 12.5% P2O5 was crushed and screened through 200-mesh, 23% of the crushed material was passed. A quantitative analysis of this product indicated 24% P2O5. Investigation of the material remaining on the 200-mesh screen proved that further careful crushing yielded more concentrate with approximately the same P2O5 content. Similar operations were carried out using a 90-mesh sieve, and in this case 31% of the material passed through, and this was found to have a P2O5 content of 21%. Crushing tests on a specimen that had been naturally baked (P.4187) by overlying igneous rocks appeared to indicate greater friability than in the case of the unbaked material. Therefore it is suggested that initial roasting of the phosphorite might facilitate a more complete separation of the mineral particles on crushing, resulting in better concentration of francolite.
Thus it is evident that the arenaceous phosphorite of the Milburn-Clarendon areas might, by careful crushing and screening, supply a product with at least a content of 80% of francolite (28% P2O5). However, it is of economic importance to remember that the phosphate mineral is francolite with a maximum P2O5 content of 35%, whereas in the case of Nauru Island or Ocean
Island phosphatic material, the maximum P2O5 value obtainable would be about 39–40%, while an analysis of a typical guano from Christmas Island shows 39–44% P2O5. On the other hand, when apatite is the raw material used, such as that from the Kukisvumchorr or Yukspor mines of Northern Russia, the P2O5 percentage reaches 41–42%.