The Iron or Nail Method Principle: Sufficient soda is added to flux the silica, and an excess to give a fluid alkaline slag in which the FeS formed by the iron introduced and the sulphur in the pyrites may dissolve. The sulphur present acts to a certain extent as a reducer. One gram FeS2, is capable of reducing 8 gms. Pb.
Therefore if a button of 15 gms. is required, this would be obtained if about 2 gms. FeS2 were present, that is, if 30 gms. of ore are taken, the ore must contain 2 gms. FeS2, or about 6% FeS2. If 50 gms. ore, about 4%. When less than this of FeS2 is present, charcoal or argol is added as a reducer. If more is present, nails or iron wire is added. A preliminary assay will give the necessary information.
Borax is added to flux any FeO that may be formed by oxidation of FeS.
Apparatus and Reagents.—As before, with the addition of 4 nails or loops of wire.
Details of the Assay.—Weigh out in duplicate the following charge, the ore being sampled and crushed as before.
Note.—Red lead is used in preference to litharge on account of its higher oxidising power.
As a preliminary, mix charge and fuse one charge, using no charcoal. Examine the resulting button, noting its weight. If too small a button is obtained, add sufficient reducer to bring down the required lead; if too large, add another nail or wire loop.
The assay proper is conducted as before, the fire being kept at a dull red for 5 to 10 minutes, and then at a bright red for another 20 minutes. The nails or wire are washed and removed and the charge poured, and the remainder of the operation and the calculations are conducted as usual.
Accuracy.—From data available it seems that this method gives good results when applied to ores containing less than about 20% FeS2. Only a certain amount of FeS can be dissolved in the slag, and any rise above this saturation point results in the formation of a rcgulus in addition to the lead button. The addition of more iron only partially remedies this evil, so that the scope of the method is limited.
(β) Gold Ores containing over 20% FeS2.
The ‘Roast’ Assay.
Method.—The ore is carefully roasted, ferric oxide being formed. The roasted ore is then treated as a basic ore.
Apparatus. Reagents, etc.—As before, with the addition of roasting appliances.
Retails of the Assay.—Weigh out two charges of 30 gms. ore each. Transfer to 10 cm. roasting dishes and carefully roast ‘dead,’ as before described (see Roasting). In this process loss by careless stirring must be avoided. This roasting will take from 40 to 60 minutes for 30 gms. ore.
Flux each charge of roasted ore with the charge given for basic ores, conducting the operation as there described.
The student is advised to assay the same ore by the Nail method, and compare the results.
Accuracy.—For the ores considered this is the most accurate of all methods given in the various text-books. Though not so quick as the Nail method, the results are higher and more reliable. Clean buttons are obtained, and therefore clean cupellation, though in the case of the nail assay but little loss is experienced if an impure button is treated at the outset with a high temperature in the muffle furnace.
Note.—These and other sulphide ores under certain conditions are quickly and accurately treated by scorification, the roasting being performed in the scorifier (see Silver Sulphide). Some assayers use the Nail Method on all sulphide ores. Any regulus obtained should be scorified with the lead button.