Examination of Hardhead.—In the smelting of tin ores a quantity of speise, known as "hardhead," is produced. It is essentially an arsenide of iron, carrying a considerable quantity of tin. Much of this last is present in the form of small buttons of metal distributed through the mass. The buttons can be seen on careful inspection, and become evident on powdering.
In assaying the substance, a variation in the usual method of sampling is required, because of the quantity of metal present which cannot be powdered. After powdering as finely as possible, the coarse particles are sifted off and weighed. The weight of the powder is also taken. The method of working is best illustrated by an example. A sample of hardhead weighed 155.1 grams, and gave 21.0 grams of coarse particles, equivalent to 13.5 per cent. of the whole. The fine portion weighed 134 grams, which is equivalent to 86.5 per cent.
Thirteen and a half grams of the coarse material were dissolved in aqua regia, and diluted with water to 1 litre. Ten c.c. of this contain 0.135 gram of the metallic portion, which is the amount contained in 1 gram of the original hardhead. If, in a determination, 1 gram of the substance is wanted, weigh up 0.865 gram of the powdered portion, and add to it 10 c.c. of the solution. It will be seen that these together make up 1 gram of the original sample. The solution of the metallic portion must be saved until the analysis is finished.
Tin and Copper.—Weigh up the portion of the powdered stuff equivalent to 1 gram of the sample. Transfer to a flask, and cover with 10 c.c. of the solution of the metallic portion and 10 c.c. of aqua regia. Boil gently till oxidation is complete and the nitric acid for the greater part driven off. Dilute to 100 c.c. with water, and pass sulphuretted hydrogen for some time. Filter, wash with hot water, and rinse through the funnel back into the flask. Digest with yellow sodium sulphide until only a light, flocculent, black precipitate is left. Filter this off, wash with hot water, dry, calcine, treat with a little nitric acid, ignite, and weigh as copper oxide, CuO. The weight multiplied by 0.7983 gives the weight of copper.
The filtrate containing the tin is rendered acid with hydrochloric acid, and filtered. The precipitate is rinsed into a half-pint beaker, covered with 20 c.c. of hydrochloric acid, and boiled down to about 20 c.c. The solution is filtered off from the sulphur and sulphide of arsenic, which, after washing with hot water, is transferred to a flask labelled "arsenic." A strip of sheet zinc (2 in. by 1 in.) is placed in the solution. The evolution of hydrogen should be brisk. In five or ten minutes decant off a few c.c. of the liquid, and test with sulphuretted hydrogen for tin. If no yellowish precipitate is formed, decant off the rest of the liquid, and wash the precipitated metal with hot water two or three times by decantation. The metal should be in a lump; if there are any floating particles they must be made to sink by compression with a glass rod. Transfer the washed metal to an evaporating dish 3 or 4 in. across, and cover with a few c.c. of hot water. Add nitric acid drop by drop till the tin is completely attacked. Evaporate nearly to dryness, and add a drop or two more of nitric acid and 20 c.c. of water. Boil and filter. Wash with hot water, dry, ignite, and weigh as stannic oxide, SnO2. Calculate to metallic tin by multiplying by 0.7867.[76]
The filtrate from the first treatment with sulphuretted hydrogen will probably no longer smell of the gas. Warm and pass the gas for a few minutes longer. Filter off any precipitate of sulphide of arsenic, and transfer it to the flask for "arsenic." Boil the filtrate (ignoring any signs of a further precipitation of arsenic) with a few c.c. of nitric acid, and separate the iron as basic acetate. Wash; reserve the filtrate for cobalt.
Iron.—Rinse back the "basic acetate," precipitate into the flask, add ammonia, dilute with water to about 100 c.c., and pass sulphuretted hydrogen for a few minutes. Filter, and wash with hot water. Collect the filtrate in the flask labelled "arsenic." Boil the precipitate with dilute sulphuric acid, filter, and titrate the filtrate with the permanganate of potassium solution after boiling off the sulphuretted hydrogen. Report the result as iron. The sulphuric acid will not effect complete solution, a light black residue will remain, chiefly sulphur; this must be rinsed into the filtrate from the acetate separation. It contains cobalt.
Cobalt.—The filtrate from the acetate separation will have a pink colour. Render it ammoniacal and pass sulphuretted hydrogen. Collect the precipitate on a filter, dry, and ignite. Dissolve in hydrochloric acid, and evaporate nearly to dryness with an excess of nitric acid. Dilute with 10 or 20 c.c. of water and add potash solution in slight excess. Add acetic acid until the solution is acid and the precipitate is quite dissolved. Add 20 or 30 c.c. of a strong solution of potassium nitrite, and determine the cobalt, as described on pp. 254, 256. Boil the filtrate from the cobalt, precipitate with hydrochloric acid, render ammoniacal, and test for zinc, nickel, and manganese.
The remainder of the tin will be contained in the flask labelled "arsenic." Acidify with hydrochloric acid and filter. Rinse into a beaker, and evaporate to a small bulk with 10 c.c. of nitric acid. Dilute and filter. Dry the precipitate, consisting of stannic arsenate (2SnO2.As2O5), ignite, and weigh. Calculate the tin it contains by multiplying by 0.4453, and add to that already found.
Arsenic.—This is determined in a separate portion. Weigh up a portion of the powder equivalent to 1 gram of the hardhead, place in a pint flask, and boil with 10 c.c. of nitric acid. When action has ceased add 10 c.c. of the solution of the metallic portion and then hydrochloric acid (a few drops at a time) till solution is complete. Warm gently in dissolving, but do not boil. Dilute to about 100 c.c., render alkaline with ammonia, and add 20 c.c. of yellow ammonium sulphide. Digest at a gentle heat for about thirty minutes, filter, and wash. Add 50 c.c. of magnesia mixture, shake well, allow to stand for an hour, filter, and wash with dilute ammonia. The precipitate is dissolved and then titrated with uranium acetate, or it is evaporated with nitric acid, ignited, and weighed as pyrarsenate of magnesia. Calculate the result to arsenic, As.