3Cu + 8HNO3 = 3Cu(NO3)2 + 4H2O + 2NO.
What volume of nitric acid will be required to dissolve 1 gram of copper?
LEAD.
The chief ore of lead is galena, a sulphide of lead, common in most mining districts, and frequently associated with blende and copper-pyrites. It always carries more or less silver; so that in the assay of the ore a silver determination is always necessary. Carbonate (cerussite), sulphate (anglesite), and phosphate (pyromorphite) of lead also occur as ores, but in much smaller quantities.
Lead ores are easily concentrated (owing to their high specific gravity, &c.) by mechanical operations, so that the mineral matter sent to the smelter is comparatively pure.
Lead is readily soluble in dilute nitric acid. The addition of sulphuric acid to this solution throws down heavy, white, and insoluble lead sulphate.
Galena is soluble in hot hydrochloric acid, sulphuretted hydrogen being evolved; but the action is retarded by the separation of the sparingly soluble lead chloride. If a rod of zinc is placed in this solution, metallic lead is precipitated on it as a spongy mass, the lead chloride being decomposed as fast as it is formed. The opening up of the ore is thus easily effected, the sulphur going off as sulphuretted hydrogen, and the lead remaining in a form easily soluble in dilute nitric acid. Galena itself is readily attacked by nitric acid, part of the lead going into solution, and the rest remaining as insoluble lead sulphate. The sulphate is due to the oxidation of the sulphur by nitric acid; its amount will vary with the quantity and concentration of the acid used. Sulphate of lead is soluble in solutions of ammonium or sodium acetate; or it may be converted into carbonate by boiling with carbonate of soda. The carbonate, after washing off the sulphate of soda, dissolves easily in nitric acid. The precipitation of lead from acid solutions with sulphuric acid, and the solubility of the precipitate in ammonium acetate, distinguishes it from all other metals. The addition of potassium chromate to the acetate solution reprecipitates the lead as a yellow chromate.
DRY ASSAY.
The dry assay of lead is largely used, but it is only applicable to rich or concentrated ores, and even with these only gives approximate results. Both lead and lead sulphide are sensibly volatile at a moderately-high temperature; hence it is necessary to obtain a slag which is easily fusible. As a reducing agent iron is almost always used, and this is added either in the form of an iron rod, or the crucible itself is made of this metal. The flux used is carbonate of soda.
When a clay crucible is used, the method of working is as follows:—Weigh up 25 grams of the dry and powdered ore, mix with an equal weight of "soda" and 2 grams of tartar; place in a crucible (E. Battersea round), and then insert a piece of iron rod about half an inch in diameter, and of such a length that it will just allow the crucible to be covered. The rod should be pushed down so as to touch the bottom of the crucible, and the mixture should be covered with a sprinkling of borax. Place in a furnace heated to, but not above, redness, and cover the crucible. In about twenty minutes the charge will be fused: the fusion is complete when bubbles of gas are no longer being evolved; and then, but not till then, the iron is withdrawn, any adhering buttons of lead being washed off by dipping the rod a few times in the slag. Cover the crucible, leave it for a minute or two, and then pour. Detach the slag, when cold, by hammering. The weight of the button multiplied by 4 gives the percentage. The commoner errors of students in working the process are too high a temperature and too quick a withdrawal.