Copper occurs native in large quantities, especially in the Lake Superior district; in this state it is generally pure. More frequently it is found in combination. The ores of copper may be classed as oxides and sulphides. The most abundant oxidised ores are the carbonates, malachite and chessylite; the silicates, as also the red and black oxides, occur less abundantly. All these yield their copper in solution on boiling with hydrochloric acid.
The sulphides are more abundant. Copper pyrites (or yellow ore), erubescite (or purple ore), and chalcocite (or grey ore) are the most important. Iron pyrites generally carries copper and is frequently associated with the above-mentioned minerals. These are all attacked by nitric acid. They nearly all contain a small quantity of organic matter, and frequently considerable quantities of lead, zinc, silver, gold, arsenic, bismuth, &c.
The copper ores are often concentrated on the mine before being sent into the market, either by smelting, when the product is a regulus or matte, or by a wet method of extraction, yielding cement copper or precipitate. A regulus is a sulphide of copper and iron, carrying from 30 to 40 per cent. of copper. A precipitate, which is generally in the form of powder, consists mainly of metallic copper. Either regulus or precipitate may be readily dissolved in nitric acid.
Copper forms two classes of salts, cuprous and cupric. The former are pale coloured and of little importance to the assayer. They are easily and completely converted into cupric by oxidising agents. Cupric compounds are generally green or blue, and are soluble in ammonia, forming deep blue solutions.
DRY ASSAY.
That, for copper, next after those for gold and silver, holds a more important position than any other dry assay. The sale of copper ores has been regulated almost solely in the past by assays made on the Cornish method. It is not pretended that this method gives the actual content of copper, but it gives the purchaser an idea of the quantity and quality of the metal that can be got by smelting. The process is itself one of smelting on a small scale. As might be expected, however, the assay produce and the smelting produce are not the same, there being a smaller loss of copper in the smelting. The method has worked very well, but when applied to the purchase of low class ores (from which the whole of the copper is extracted by wet methods) it is unsatisfactory. The following table, which embodies the results of several years' experience with copper assays, shows the loss of copper on ores of varying produce.
The figures in the fourth column show how rapidly the proportion of copper lost increases as the percentage of copper in the ore falls below 30 per cent. For material with more than 30 per cent. the proportion lost is in inverse proportion to the copper present.
LOSS OF COPPER.
-----------------+------------+-----------+------------------ Copper present. Dry Assay. Margin. Loss on 100 Parts of Copper.
-----------------+------------+-----------+------------------ Per cent. Per cent. Per cent. 100 98 2.0 2.0 95 92-1/2 2.5 2.6 90 87-3/8 2.6 2.9 85 82-3/8 2.6 3.0 80 77-3/8 2.6 3.2 75 72-3/8 2.6 3.5 70 67-1/2 2.5 3.6 65 62-1/2 2.5 3.8 60 57-5/8 2.4 4.0 55 52-3/4 2.3 4.2 50 47-3/4 2.2 4.4 45 43 2.0 4.5 40 38-1/8 1.8 4.6 35 33-1/4 1.7 4.8 30 28-1/2 1.50 5.0 25 23-1/2 1.50 6.0 20 18-1/2 1.56 7.8 18 16-1/2 1.53 8.5 16 14-1/2 1.48 9.3 14 12-5/8 1.40 10.0 12 10-5/8 1.37 11.4 10 8-3/4 1.28 12.8 8 6-7/8 1.14 14.3 6 5 1.05 17.5 5 4 1.00 20.0 4 3 1.00 25.0 3.75 2-3/4 0.97 26.0 3.50 2-9/16 0.94 27.0 3.25 2-5/16 0.91 28.0 3.00 2-1/8 0.87 29.0 2.75 1-15/16 0.82 30.0 2.50 1-3/4 0.77 31.0 2.25 1-1/2 0.72 32.0 2.00 1-5/16 0.66 33.0 -----------------+------------+-----------+------------------
The wet assay being known, the dry assay can be calculated with the help of the above table by deducting the amount in the column headed "margin"
opposite the corresponding percentage. For example, if the wet assay gives a produce of 17.12 per cent., there should be deducted 1.5; the dry assay would then be 15.62, or, since the fractions are always expressed in eighths, 15-5/8. With impure ores, containing from 25 to 50 per cent. of copper, the differences may be perhaps 1/4 greater.
Wet methods are gradually replacing the dry assay, and it is probable that in the future they will supersede it; for stock-taking, and the various determinations required in smelting works and on mines, they are generally adopted, because they give the actual copper contents, and since it is obvious that a knowledge of this is more valuable to the miner and smelter. Moreover, the working of the dry method has been monopolised by a small ring of assayers, with the double result of exciting outside jealousy and, worse still, of retarding the development and improvement of the process.
The principal stages of the dry assay are: (1) the concentration of the copper in a regulus; (2) the separation of the sulphur by calcining; (3) the reduction of the copper by fusion; and (4) the refining of the metal obtained.
The whole of these operations are not necessary with all copper material. Ores are worked through all the stages; with mattes, the preliminary fusion for regulus is omitted; precipitates are simply fused for coarse copper, and refined; and blister or bar coppers are refined, or, if very pure, subjected merely to washing.
The quantity of ore generally taken is 400 grains, and is known as "a full trial"; but for rich material, containing more than 50 per cent. of copper, "a half trial," or 200 grains, is used.
~Fusion for Regulus.~--The ore (either with or without a previous imperfect roasting to get rid of any excess of sulphur) is mixed with borax, glass, lime, and fluor spar; and, in some cases, with nitre, or iron pyrites, according to the quality of the ore. The mixture is placed in a large Cornish crucible, and heated as uniformly as possible in the wind furnace, gradually raising the temperature so as to melt down the charge in from 15 to 20 minutes. The crucible is removed and its contents poured into an iron mould. When the slag is solid, it is taken up with tweezers and quenched in water. The regulus is easily detached from the slag. It should be convex above and easily broken, have a reddish brown colour, and contain from 40 to 60 per cent. of copper. A regulus with more than this is "too fine," and with less "too coarse." A regulus which is too fine is round, compact, hard, and of a dark bluish grey on the freshly broken surface. A coarse regulus is flat and coarse grained, and more nearly resembles sulphide of iron in fracture and colour.
If an assay yields a regulus "too coarse," a fresh determination is made with more nitre added, or the roasting is carried further. With low class ores a somewhat coarse regulus is an advantage. If, on the other hand, the regulus is too fine, less nitre or less roasting is the remedy. With grey copper ores and the oxidised ores, iron pyrites is added.
~Calcining the Regulus.~--It is powdered in an iron mortar and transferred to a small Cornish crucible, or (if the roasting is to be done in the muffle) to a roasting dish or scorifier. The calcining is carried out at a dull red heat, which is gradually increased. The charge requires constant stirring at first to prevent clotting, but towards the end it becomes sandy and requires less attention. If the temperature during calcination has been too low sulphates are formed, which are again reduced to sulphides in the subsequent fusion. To prevent this the roasted regulus is recalcined at a higher temperature, after being rubbed up with a little anthracite. The roasted substance must not smell of burning sulphur when hot. It is practically a mixture of the oxides of copper and iron.
~Fusion for Coarse Copper.~--The calcined regulus is mixed with a flux consisting of borax and carbonate of soda, with more or less tartar according to its weight. Some "assayers" use both tartar and nitre, the former of course being in excess. The charge is returned to the crucible in which it was calcined, and is melted down at a high temperature, and, as soon as tranquil, poured. When solid it is quenched and the button of metal separated.
The slag is black and glassy. The small quantity of copper which it retains is recovered by a subsequent "cleaning," together with the slags from the next operation.
The button of "coarse copper" obtained must be free from a coating of regulus. It will vary somewhat in appearance according to the nature and quantity of the impurities.
~Refining the Coarse Copper.~--The same crucible is put back in the furnace, deep down and under the crevice between the two bricks. When it has attained the temperature of the furnace the coarse copper is dropped into it and the furnace closed. The copper will melt almost at once with a dull surface, which after a time clears, showing an "eye." Some refining flux is then shot in from the scoop (fig. 48), and, when the assay is again fluid, it is poured. When cold the button of metal is separated.
[Illustration: FIG. 48.]
The button of "fine" copper is flat or pitted on its upper surface, and is coated with a thin orange film; it must have the appearance of good copper. If it is covered with a red or purple film, it is overdone or "burnt." If, on the other hand, it has a rough, dull appearance, it is not sufficiently refined. Assays that have been "burnt" are rejected.
Those not sufficiently fine are treated as "coarse copper," and again put through the refining operation.
~Cleaning the Slags.~--These are roughly powdered and re-fused with tartar, etc., as in the fusion for coarse copper. The button of metal got is separated (if big enough refined) and weighed.
The details of the process are slightly varied by different assayers: the following will be good practice for the student.
~Determination of Copper in Copper Pyrites.~--Powder, dry, and weigh up 20 grams of the ore. Mix with 20 grams each of powdered lime and fluor, 15 grams each of powdered glass and borax, and 5 or 10 grams of nitre.
Transfer to a large Cornish crucible and fuse under a loose cover at a high temperature for from 15 to 20 minutes. When fluid and tranquil pour into a mould. When the slag has solidified, but whilst still hot, quench by dipping two or three times in cold water. Avoid leaving it in the water so long that it does not dry after removal. When cold separate the button, or perhaps buttons, of regulus by crumbling the slag between the fingers. See that the slag is free from regulus. It should be light coloured when cold and very fluid when hot. Reject the slag.
Powder the regulus in a mortar and transfer to a small crucible.
Calcine, with occasional stirring, until no odour of sulphurous oxide can be detected. Shake back into the mortar, rub up with about 1 gram of powdered anthracite, and re-calcine for 10 minutes longer.
Mix the calcined regulus with 10 grams of tartar, 20 grams of soda, and 3 grams of borax; and replace in the crucible used for calcining. Fuse at a bright red heat for 10 or 15 minutes. Pour, when tranquil.
As soon as solid, quench in water, separate the button of copper, and save the slag.
To refine the copper a very hot fire is wanted, and the fuel should not be too low down in the furnace. Place the crucible well down in the fire and in the middle of the furnace. The same crucible is used, or, if a new one is taken, it must be glazed with a little borax. When the crucible is at a good red heat, above the fusing point of copper, drop the button of copper into it, and close the furnace. Watch through the crevice, and, as soon as the button has melted and appears clear showing an eye, shoot in 10 grams of refining flux, close the furnace, and, in a few minutes, pour; then separate the button of copper. Add the slag to that from the coarse copper fusion, and powder. Mix with 5 grams of tartar, 0.5 gram of powdered charcoal, and 2 grams of soda. Fuse in the same crucible, and, when tranquil, pour; quench, and pick out the prills of metal.
If the copper thus got from the slags is coarse looking and large in amount, it must be refined; but, if small in quantity, it may be taken as four-fifths copper. The combined results multiplied by five give the percentage of copper.
The refining flux is made by mixing 3 parts (by measure) of powdered nitre, 2-12 of tartar, and 1 of salt. Put in a large crucible, and stir with a red-hot iron until action has ceased. This operation should be carried out in a well-ventilated spot.
For pure ores in which the copper is present, either as metal or oxide, and free from sulphur, arsenic, &c., the concentration of the copper in a regulus may be omitted, and the metal obtained in a pure state by a single fusion.[50] It is necessary to get a fluid neutral slag with the addition of as small an amount of flux as possible. The fusion should be made at a high temperature, so as not to occupy more than from 20 to 25 minutes. Thirty grams of ore is taken for a charge, mixed with 20 grams of cream of tartar, and 10 grams each of dried borax and soda. If the gangue of the ore is basic, carrying much oxide of iron or lime, silica is added, in quantity not exceeding 10 grams. If, on the other hand, the gangue is mainly quartz, oxide of iron up to 7 grams must be added.
_Example._--Twenty grams of copper pyrites, known to contain 27.6 per cent. of copper, gave by the method first described 5.22 grams of copper, equalling 26-1/8 per cent. Another sample of 20 grams of the same ore, calcined, fused with 40 grams of nitre, and washed to ensure the removal of arsenic and sulphur, and treated according to the second method, gave a button weighing 5.27 grams, equalling 26-3/8 per cent.
The ore contained a considerable quantity of lead. Lead renders the assay more difficult, since after calcination it remains as lead sulphate, and in the fusion for coarse copper reappears as a regulus on the button.
~The Estimation of Moisture.~--The Cornish dry assayer very seldom makes a moisture determination. He dries the samples by placing the papers containing them on the iron plate of the furnace.
It is well known that by buying the copper contents of pyrites by Cornish assay, burning off the sulphur, and converting the copper into precipitate, a large excess is obtained.
NOTES ON THE VALUATION OF COPPER ORES.
Closely bound up with the practice of dry copper assaying is that of valuing a parcel of copper ore. The methods by which the valuation is made have been described by Mr. Westmoreland,[51] and are briefly as follows:--The produce of the parcel is settled by two assayers, one acting for the buyer, the other for the seller; with the help, in case of non-agreement, of a third, or referee, whose decision is final. The dry assayers who do this are in most cases helped, and sometimes, perhaps, controlled, by wet assays made for one or both of the parties in the transaction.
In the case of "ticketing," the parcels are purchased by the smelters by tender, and the value of any particular parcel is calculated from the average price paid, as follows:--The "standard," or absolute value of each ton of fine copper in the ore, is the price the smelters have paid for it, plus the returning charges or cost of smelting the quantity of ore in which it is contained. The value of any particular parcel of ore is that of the quantity of fine copper it contains, calculated on this standard, minus the returning charges. The ton consists of 21 cwts., and it is assumed that the "settled" produce is the actual yield of the ore.
If at a ticketing in Cornwall 985 tons of ore containing 63.3 tons of fine copper (by dry assay) brought 2591 12s., the standard would be 83 15s. This is calculated as follows:--The returning charge is fixed at 55s. per ton of ore. This on 985 tons will amount to 2708 15s.
Add this to the actual price paid, and there is got 5300 as the value of the fine copper present. The weight of copper in these 985 tons being 63.3 tons, the standard is 5300/63.3, or 83 15s. (nearly).
The value of a parcel of 150 tons of a 6 per cent. ore on the same standard would be arrived at as follows:--The 150 tons at 6 per cent.
would contain 9 tons (1506/100) of fine copper. This, at 83 15s.
per ton, would give 753 15s. From this must be deducted the returning charges on 150 tons of ore at 55s. per ton, or 412 10s. This leaves 341 5s. as the value of the parcel.
At Swansea the returning charge is less than in Cornwall, and varies with the quality of the ore. This appears equitable, since in smelting there are some costs which are dependent simply on the number of tons treated, and others which increase with the richness. The returning charge then is made up of two parts, one fixed at so much (12s. 2d.) per ton of ore treated, and the other so much (3s. 9d.) per unit of metal in the ore. In this way the returning charge on a ton of ore of 8-3/4 produce would be (12s. 2d.)+(8-3/4(3s. 9d.)), or 2 5s.
If, for example, Chili bars, containing 96 per cent. of copper, bring 50 per ton, the standard is 71 9s. 4d. It is got at in this way.
The returning charge on a 96 per cent. ore is (12s. 2d.)+(96(3s.
9d.)), or 18 12s. 2d. This added to 50 gives 68 12s. 2d., and this multiplied by 100 and divided by 96 (100 tons of the bars will contain 96 tons of fine copper) will give 71 9s. 4d.