As part of my programme of self-improvement regarding the science behind the economics of the (mainly) 19th century copper industry I have been reading Percy’s Metallurgy, 1861, the classic work on metallurgical processes in use at the time.
An ongoing exchange between me and a colleague in Chile who is assiduously documenting the country’s lost smelting heritage encouraged me to make a note of any sources that would be helpful to his research.
While going through Percy’s Metallurgy I transcribed some of the contents regarding the types of ore that were extracted from Chilean mines and the particular form of smelting that was conducted there. Chilean ores and copper products in the form of blister copper and sometimes regulus were a major source of raw materials for the Welsh smelting industry in the mid-19th century.
Extracts on Chilean ores
1. Native copper.
A considerable quantity of ore is imported into Swansea from Chili under the name of “copper sand” or “copper barilla;” it consists of from 60 to 85 percent. of metallic copper intermixed with quartz.
Native copper is generally remarkable for its toughness. Mr. Morgan, of the Hafod Works, informed me that the toughest copper he had ever seen was a piece of native copper from Chili, about three-eighths of an inch in thickness: it was bent backwards and forwards forty-eight times before breaking.
7. Purple copper
3 Cu2S+F2S3.–When pure it contains 55.53 per cent. of copper. It generally occurs massive and disseminated, and but very seldom crystallized. 2nd Class No. 3. From Coquimbo, Chili contains 60.80 % copper.
8. Copper-pyrites, or yellow copper-ore
Cu2S+Fe2S, or as it was formerly expressed, CuS+FeS.–When pure, it contains 34.81 per cent. of copper. It is the most abundant ore of copper. It is largely imported from Cornwall, Devonshire, Cuba, and South America.
It is esentially [sic] a hydrated oxychloride of copper, of the formula CuCl+3 CuO, combined with different proportions of water. It occurs in Chili, and other parts of the West Coast of South America. The following analyses of crystallized mineral from Copiapó were made by Field:-
Protoxide of copper………….70.74…..70.48
These results lead to the formula 2(CuCl+3 CuO)+9 HO. A considerable quantity of this interesting mineral has been imported into Swansea, where I have seen it in the ore-yards.
Description of copper smelting in Chile
The smelting was effected in reverberatory furnaces with coal from this country [Wales/England].
Fusion for regulus.–One charge consists of 70 quintals (over 3 tons English), and is composed as follows:-
|Ore||Locality||Quintals||Average % copper|
|Carbonates and oxychloride (green and dark brown)||Caldera||12||12|
|Iron fluxes from various parts of||Coquimbo||14||8|
|Carbonates and oxychlorides; hard to be fused; containing a good deal of “Tofo” (chiefly carbonate of lime)||2||8|
|Sulphides (yellow)||Various places||6||8|
Each furnace smelts four charges in 24 hours. The regulus contains about 60 per cent. of copper; and the slag, which is “sharp” and brittle, is said rarely to contain more than 1 per cent. copper.
2. Roasting for spongy regulus.–A charge of 4 tons is roasted during about 8 hours; the time from charge to charge, inclusive of charging, tapping etc., is 10 hours. Out of 20 pigs of the spongy regulus (metal) about 6 or 8 have “bottoms.” The metal is not allowed to become too spongy, as in that state it would become mixed with too large a quantity of sand, which it is stated would retard the next roasting. The slag contains 9 per cent. of copper
3. Roasting for blister copper.–The charge consists of sufficient spongy regulus and “bottoms” to yield from 4 to 5 tons of blister-copper. The time required to work off the charge is from 16 to 18 hours. This operation of roasting is conducted as follows:-The charge is first allowed to sweat down, which requires about 6 hours, when the air-holes are luted and the temperature raised, so that the whole charge may be perfectly melted in 1 hour. The front door is now taken down and any slag that may have accumulated is skimmed off, after which the charge is allowed to set by opening the side door and air-holes. As soon as it is quite set—or before, if the copper can easily be seen by striking back the “rigole” (regulus) floating on the surface of the bath, with a skimming rabble—the side-door is luted and the temperature gradually increased until the whole charge is in a state of fusion. During this period the two air-holes are left open, but too much air must not be admitted, as the charge would be thereby prevented from melting. If the furnace has had proper attention, and the charge is “working” (i.e. it appears to boil), it continues in this state from 30-40 minutes. The “working” ceases first in those parts near the air-holes, and soon afterwards in ever part of the furnace, the surface becoming covered with a thick yellow coat called “cream,” from which small blisters about the size of a pea are thrown up. The blister-copper is now tapped into sand moulds. After the second fusion, or before the charge begins to work, any slag covering the face of the bath should be skimmed off.
Three furnaces are used. In an establishment of 9 furnaces, 6 smelting-furnaces will keep the 2 “roasters” continually going; but this depends on the per centage of copper in ores used.