For technical purposes it is rarely necessary to determine all the constituents present in a slag. The lead or copper smelter wishes to know the lead or copper contents of his slags; also their silver and gold contents (if any), and, as a rule, the silica, ferrous oxide, and lime present. Certain other constituents, such as sulphur, potassium, sodium, barium, magnesium, and aluminium, rarely require estimation. In certain cases the oxides of manganese and zinc are important constituents. It is difficult to lay down a scheme suitable to all cases and at the same time rapid in operation. The chemist must frequently construct his own scheme according to the number of substances to be determined. The methods here laid down, though only partial, are capable of contraction or expansion according as few or many determinations are to be made, and in making such alterations due precautions must be taken that any important steps in the treatment are not omitted when certain substances in a complete scheme are not determined. For instance, although Cu, Pb, Bi, etc. are not to be estimated, and the only determinations required are those of SiO2, FeO. and CaO. the student must not omit the precipitation of the Cu. Pb, and Bi before proceeding to determine the FeO, etc. Whatever modification be adopted, the effect of any omissions or additions must be carefully considered, and where any doubt exists, the results should be checked by a complete analysis, aided by a thorough qualitative examination of all precipitates obtained. The following estimations will here be considered in detail:
Pb, Cu, Ag and Au, SiO2, FeO, and CaO.
To expedite matters, the samples of slag should be chilled when taken (consult text-books on Assaying), as in this state they are readily broken up by acid. The estimation of Pb. SiO2, FeO. and CaO will be taken first, then that of the Cu, Ag, and Au.
Estimation of Pb and. SiO2.—Weigh out 2 gms. of the slag, finely ground in an agate mortar. Transfer to a 200 c.c. conical beaker. Add to the beaker, stirring continuously, 30 c.cs. 5E. HCl, 10 drops HNO3, and 10 drops H2SO4. Evaporate to dryness. Take up with 30 c.cs. E. HCl. Filter (through paper) and wash well, keeping most of the precipitate in the beaker. The precipitate contains SiO2, PbSO4 (and BaSO4). Reserve the solution. To the precipitate in the beaker add 10 c.cs. hot 5E. NH4C2H3O2 Boil for a few minutes and pass the solution through the filter paper into a clean beaker. Repeat the solution till no more Pb is dissolved (test). Wash the contents of the beaker on to the paper and wash well with hot water.
Dry, ignite, and weigh the precipitate. Note the weight. Test the precipitate for Ba (by flame coloration), and if present, fuse with about 5 times its bulk of Na2CO3K2CO3 over the blast. Take up in hot water; boil and filter till all soluble sulphates are washed through. The SiO2 and BaCO3 remain on the filter. Dissolve out the BaCO3 with HCl. Filter, wash well, and precipitate the Ba as BaSO4 in the usual manner. Deduct the BaSO4 thus found from the weight of the precipitate previously found; and calculate the percentage of SiO2.
Determine the lead in the acetate solution by Alexander’s Molybdate method. (For a modified ‘ Fire ’ method consult Furman, p. 308.)
Estimation of FeO.—Pass H2S through the filtrate from the SiO2, PbSO4, and BaSO4. Filter. Boil off the H2S. To the filtrate add NH4HO till alkaline. Boil, filter, and wash once. Redissolve the precipitate in a little HCl. Reprecipitate as before. Filter, and combine the filtrates, reserving them for the lime estimation.
The Fe in the precipitate may be estimated by solution in H2SO4 and titration of part of the solution with K2Mn2O8. Calculate from this the percentage of FeO.
Estimation of CaO.—Evaporate the combined filtrate from the iron to a bulk of about 100 c.cs. If not distinctly alkaline, make it so with NH4HO. Precipitate the lime as usual with (NH4)2C2O4. Filter through a Gooch crucible. Wash well. Dissolve the precipitate, and estimate the CaO volumetrically by K2Mn2O8 (standardised on a pure calcium salt).
Estimation of Silver and Gold.—This is best done by fire-assay. As only small quantities of these metals are present, 4 lots of 1 A.T. (30 gms.) each should be taken, and the buttons combined by scorification. In the Assaying Section of this book the student will find suitable charges.
The Estimation of Copper—The following method, given by C. F. Shelby in the N.Y. Eng. and Min. Journ., June 16, 1900, has been found by the author to give rapid and accurate results. The Cu is easily and completely separated in the presence of large amounts of SiO2 and FeO, the complete estimation taking from 60 to 100 minutes.
Dissolve 5 gms. finely powdered slag in a 100 c.c. casserole or porcelain dish with cover, by moistening with a few c.cs. water 10 c.cs. strong HCl and 1 c.c. strong HNO3. Heat till all the gelatinous SiO2 is thoroughly dehydrated. Add 10 to 15 c.cs. strong HCl. Heat to boiling. Dilute and filter, washing first by decantation and then on the paper. To the filtrate, now about 150 c.cs. in bulk, gradually add a saturated solution of Na2S2O3,10H2O till the yellow colour of the solution just disappears. Fe2Cl6 is now reduced to FeCl2. In the solution place a coiled strip of sheet zinc, 2.5 c.m. x 5 c.m. Nascent H and S combine to form H2S, and in about 30 to 60 seconds all the Cu is precipitated as Cu2S. Filter and wash well after all the Zn has dissolved.
The Cu2S may now be dissolved in HNO3, and estimated volumetrically by KCN, or colorimetrically, or the Cu2S may be dissolved, and the Cu estimated volumetrically by iodine and sodium thiosulphate.
Note.—The Student is referred to the Engineering and Mining Journal, N. Y., October 18th, 1902, and February 21st, 1903, for a letter and article by Thorn Smith, giving details of results of slag analyses and suggestions for improvement of methods. He cannot do better than read these references and benefit by them; they show what care is necessary in obtaining accurate results, and will do much to further the degree of accuracy obtainable in such work, and, at the same time, they clearly lay down the standard of excellence at which the student should aim.