S5003145

30

Copper

Before sulphate roasting, suit able ores could be easily proceaaed when they contalned bicuminous materials. There are many examplea such aa those found in the distrlct of Hansfeld-Eisleben-He 1 lstedt, sltuated at the Southern edge of the Harz ln central Germany, or the cupiferous marls of Hesse and Ualdeck which contain fossll plant materiał (wood, Jet, grass, nutshells with imprints of plants and even insects) and are rich ln copper (8-20X and averaging 13%) and which also contain silver (up to 1 kg te^-1).

There are also the cupiferous 'Mergelschiefer' ores of the northern foot-hills of the Riesengebirge (Lower Silesia) which are already degraded by the atmosphere and roast very easily producing copper granules (globulisa-tion) which the miners cali 'Graupen' (grain) because of the similarity to grains of wheat. Thls class of ores was sometlmes decomposed in the atmosphere and could be concentrated by washing. According to the old writer F L Cancrinus (1767), who described the deposits of Hesse and Ualdeck, there was at Goddelsheim and at Niederense green or brownish 'Graupen* of couvellite (CU2S) capable of producing 25 tonnes of copper per year lying just beneath the turf.

Roasting in heaps first drove off the moisture and broke down the carbonates and sulphides, completing the breakdown already started by expo-sure to alr. The combustibles naturally present in the ore burned easily and reduced the residual oxides and sulphides, forming globules of metal which grew in varying degrees according to the duration of the roasting.

One must not forget that the sulphur in the sulphides is also combustible and that if they are placed in heaps surrounded by refractory Stones one has only to start the combustion and the reaction proceeds until almost all the sulphur is removed. Up to 600*C the reaction is:

Cu₂S + 20₂    ---> 2CuO + S0₂

and at 800*-850*C:

2Cu₂S + 2CuO ---> 4Cu + S0₂

The copper oxides are then reduced in the appropriate furnace. If the ore contains too much pyrites it is obviously necessary to concentrate the copper and iron sulphides (matte) after the partial oxidisation of the iron. Some of this iron is removed in the slag. If there is arsenie or antlmony in the ore one sometlmes obtains a lighter matte than the sulphidic matte, known generally as speiss (containing high melting point iron arsenides and antlmonidea). The operation is repeated several times in the roasting furnacee, the mattes becoming increasingly richer, to obtain morę than 20X of copper. In practlce morę than eight and sometlmes up to fifteen or six-teen sępa ratę roasting operations have been known to take place (Liege 1778).

Lead and Sllver

Ulth lead ores roasting in air gives a mixture of oxides and sulphates with a varylng residua of sulphides, depending on the temperaturę. When the temperaturę is quite Iow (500-600*C) then the product has morę sulphlde than oxide, but accordlngly if the temperaturę is raised (800^,-900*C), then the formatlon of the oxlde predomlnat es. To moderate the heat, faggots of wood, peal or dry wood were added. First there ls the formatlon of a yellowiah crust which covers the mass of the ore which must be stlrred and mlxed with the untouched galena, producing new sulphates and oxides which react with the galena to glve the oxy-sulphlde. This is known as the

'Schlicks', which is ready to be reduced by carbon. If there is silica in the gangue, or if lt has been deliberately added. lead silicates are foraed which csn be reduced by limestone:

Pb 810^ ♦ CaO —> Ca SiC>₃ + PbO 2PbO ♦ C--->    2 Pb ♦ CO,

If the gangue contains barytes (bariua sulphate) as weIX as silica (which ls the case ln certain regions such as in Ceilhe, departaent-Herault, see ^Flg- 1 ). barium oxide is foraed which facllltates the desulphuring of the galena. It ls thus a useful substance ln the treataent of lead sulphate, actlng as a very necessary flux which will improve the fluldlty of the shaft furnace slags, especlally if aixed with fluorspar (CaF,).

If one adds iron ore in the fora of an oxide or liaonlte to the lead ore, they fora a matte consisting of the sulphides of lead, copper (if present ln the ore), and iron, together with a slag principally of iron slllcate. In order for the air to be able to circulate through the con-tents of the furnace, one aust adjust the openings in the bottoa of the furnace interior, such as is shown in representations of several furnaces depicted on the lead plaques found at Lascours (Ceilhe, Herault). Morę than a score of these circular cast lead seals were found in 1936 by Dr J Brunei, and depict ore-roasting furnaces and other apparatus.

As most of the lead veins lying to the west of the upper Orb have barytic gangues, the origin of this netalliferous distrlct is to be explained not only by the galena, but also by the stibnite (Sb S^), grey copper, misDickel and bournonite (CuPbSbS^). As well as leaa. antiaony (especlally from silver) was extracted.

The massif is formed of Hercynian microgranites (Mont Faulat) giving rhyolithic veins towards the edges. Towards the north the 'Grand Mourgls' of basalt with veins of argentiferous galena and stibnite was considered by the German geologist Trautmann as the 'Harz of the South of France-

The local archaeologist Robert Gourdiolle considered that ninIng and metallurgy started in the area towards the end of the second century BC, or the beginning of the first century BC, which does not aean that earlier prospectors did not collect the native silver found in the upper veins of the lead-antimony deposits. A sacred place was established durlng the first century BC on the site of an ancient roasting area at Lascours (Mange-Homme, see Fig. 1 ). The roasted products were washed in the River Rabasse and sent as concentrates to the smelter, the site of which has not yet been found. The Processing, traces of which have been found, took place at an early stage in the process, which was temporarily abandoned and then restored at the beginning of the first century AD. The site of Hange-Hoaae has a slag heap in which ¹ ingot s¹ of waste metal, piat es, and slags have been found, which M Boyer of the Societe de la Viellle Montagne, Vlvlez, has analysed (Table 1). These saaples could be identlfied as matte (saaple 1) or speiss, where the sulphur had been eliainated by roasting, leavlng arsenie and antimony in the product (saaples 2, 3 or 4). Nlckel, which is resistant to oxidisation, was possibly concentrated in these speiss after roasting. Analyses of the three speiss saaples froa Hange-Hoaae (2, 3 and 4 in Table 1) can be compared with those given by Witter froa Guschau (Kr Sorau) produced by treating fahlerz, and those given by Werner (1976) ln Table 2.