CHAPTER VII.

ASSAY OF COPPER ORES.

In the assay of copper ores by the dry way, all minerals containing that metal are divided into four classes. Class 1.-Comprises substances containing neither sulphur, arsenic, or any foreign metals but iron.

Class 2.-Comprises ores, &c., containing sulphur, but no other metal than iron.

Class 3.-Comprising the sulphurets which contain other metals than iron, as arsenic, antimony, lead, &c. Class 4.-Various alloys.

Assay of the substances contained in the First Class.

Rich Ores.-The assay of the ores of this class is comparatively facile, and the results obtained are very

exact.

The assay is conducted by the aid of black flux, as follows: The powdered ore must be well mixed with about 3 parts of ordinary black flux, and placed in a crucible, (which it ought not to fill more than twothirds), and covered with a thin layer of black flux, and heated gradually in an ordinary furnace, for about a quarter of an hour; a cover is then placed on the pot, and the heat increased to bright redness, which will be sufficient to melt the copper. A quarter of an hour, at the full heat, is ample, and no fear need be entertained

that metal will be lost by excess of heat, for copper is perfectly fixed at the above temperature.

When the crucible is cold, it must be broken; and if the assay has succeeded, a button of copper will be obtained, which ought not to adhere to the crucible, and which separates readily from the slag.

Poor Ores, &c.—The slags produced by the treatment of copper ores in the large way, can be assayed after the plan just described, when they are rich; but when they contain no more than about 2 per cent. of copper, that method will not succeed; because, the small quantity of copper they contain would remain in the flux, either combined or disseminated in very small grains. In this case, if the slag be attackable by acids, the assay must be made by the wet way; if they be unacted on, or nearly so, by acids, their assay must be conducted in a charcoal crucible, in the same manner as iron, and at the same temperature. Sometimes it may be made without the addition of any flux; nevertheless, it is always better to add 15 to 20 per cent. of chalk.

Those which are acted on by acids generally contain much oxide of iron, and assimilate very much in their character to finery slags.

In case the dry assay is made as above stated, the copper will be found alloyed with much iron, from which it must be separated in the wet way.

To estimate the per centage of copper by the humid method, the substance must be finely pulverized and acted on by boiling aqua regia, (made in the proportion of 3 parts hydrochloric acid, and 1 part nitric acid); or still better by hydrochloric acid alone, if it be attackable by that agent. After all action has ceased, evaporate

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nearly to dryness; moisten the residue with hydrochloric acid, dilute with water, and filter, in order to separate silica and other insoluble substances.

The copper has now to be separated by precipitation, which is most readily effected by placing in the liquid a well cleansed plate of iron, and leaving it there until all the copper is thrown down.

This precipitation takes place either when the solution is hot or cold; in the latter case, it is requisite to leave it for at least twenty-four hours; in the former more especially; if a boiling heat be given, it takes place at most in an hour or two. It must be borne in mind, however, that in both cases the solution must be kept decidedly acid, otherwise an insoluble sub-salt of iron will be thrown down with the copper, and much interfere with the exactness of the result. Either hydrochloric or dilute sulphuric acid will serve for the acidification of the liquid.

When all the precautions just pointed out have been taken, the precipitated copper is in thin light scales, of a fine red colour, and perfectly pure. It must be washed rapidly in a large quantity of water, until the last washing comes off perfectly tasteless, then dried at a very gentle heat (212o F.) and weighed. If it be dried at a stronger heat, the finely divided metal has a great tendency to combine with oxygen and form an oxide, which of course would be much heavier than its contained metal; therefore the result would be a greater per centage than the substance actually contained.

In case the greatest possible exactitude be wished for, it is better to precipitate the copper by means of sulphuretted hydrogen than iron. This re-agent detects

the smallest traces, and hence it is desirable to employ it when very poor substances are to be examined. It is better, when this gas is employed, to dissolve the ore or slag in hydrochloric acid alone, if possible, and the solution must contain an excess of acid.

The precipitate must be collected, washed, and dried. It must then be perfectly roasted in a small platinum or porcelain capsule, and weighed as deutoxide of copper, every 40 parts of which correspond to 32 parts pure copper.

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Assay of ores, &c. of the Second Class.

The substances belonging to this class are either sulphurets or sulphates.

The larger

Sulphates.-The sulphates of copper give very pure copper with a reducing flux, and a slag containing a double sulphuret of the alkali and copper. the proportion of flux employed, so is the quantity of sulphuret produced, and a correspondingly small quantity of metallic copper reduced. But if the sulphate of copper be fused with an alkaline flux, which contains only the exact quantity of the reducing agent, to reduce its contained oxide of copper, then all the metal is reduced, and the slag contains only sulphate and no sulphuret.

Thus, it has been ascertained, that the neutral anhydrous sulphate of copper gives: firstly, 27 per cent. of copper with 3 parts of black flux (about two-thirds only of that which it contains), and a black crystalline slag, containing much sulphur. Secondly, with 2 parts of the same flux, 37 per cent. of copper, and a clean grey slag, containing very little sulphur. When but one

part of black flux is employed, there is not a sufficiency of carbon to reduce the whole of the oxide, and only about 12 to 14 per cent. of metallic copper is produced, which is enveloped in a red, vitreous, opaque slag, composed of protoxide of copper and alkali, and above which, is a layer of fused sulphate of potash, which is crystalline and colourless.

It may thus be seen, that in order to extract the whole of the copper from a sulphate in this manner, that the proportion of reducing flux which will give the maximum result, and which also will produce a slag containing but little sulphur, must be arrived at by guess-work.

The sulphates of copper, however, are completely decomposed by heat; and the easiest and best method of assaying them, is to expose them to a white heat in a platinum crucible until nothing more is given off. The residue is oxide, which can be fused with three parts of black flux, when all the copper contained in it is exactly and readily separated in the metallic state.

The compounds of oxide of copper with sulphuric acid may be assayed by fusion in an earthen crucible with from 1 to 2 parts of carbonate of soda; the fused matter must be poured into an ingot-mould, pulverized, and re-fused in the same crucible, after mixture with its own weight of black flux. By fusion with carbonate of soda, the sulphate of copper is decomposed, and sulphate of soda and a compound of the oxide of copper and alkali formed, which is afterwards completely reduced by black flux, without the possibility of the formation of a sulphuret of copper.

Sulphurets.-Copper, combined with sulphur, is