METALS AND THEIR ORES. 53 



weight of coal is consumed for each ton of ore, is the only furnace operation required 

 to obtain the metallic copper in a precipitated form known as cejnetit copper. An im- 

 portant item of cost in wet processes is the metallic iron employed to separate the me- 

 tallic copper from its solutions. The same amount of iron is required to precipitate a 

 ton of copper, whether extracted from a poor or a rich ore ; but as for the smelting of 

 the latter much less fuel is required, it follows that rich ores are generally treated by 

 smelting rather than in the wet way, any saving of fuel in the latter being more than 

 compensated for by the cost of iron. No general rule, however, can be laid down to 

 determine what grade of ore can be more profitably treated by one method or the 

 other, inasmuch as circumstances of locality, affecting the cost of fuel and the price of 

 iron, must in each case be taken into account. 



The various other wet methods of copper extraction may be divided into two classes : 

 those in which the previously oxidized ore is treated with hydrochloric or sulphuric 

 acid to dissolve the oxide of copper, and those in which sulphuretted ore, generally 

 after a preliminary roasting, is calcined with an admixture of sea-salt or sulphate of 

 soda, by which the copper is converted into chloride or into sulphate. All of these 

 methods, when properly applied, effect a pretty thorough extraction of the copper ; but 

 the cost of the reagents which have to be added to every charge of ore precludes alto- 

 gether the use of some of these methods, except in certain favored localities, and ren- 

 ders them in almost all cases, it is believed, less economical than the present one with 

 the Hunt and Douglass bath, for which the following advantages are claimed : 



I. It is a general method adapted to all compounds of copper, while that by calcina- 

 tion with salt is only applicable to sulphuretted ores. 



II. It does not require the addition of reagents, such as acids, salt, or sulphate of 

 soda, to each charge of ore, since in the regular course of the operation the solvent 

 required for the treatment of the ore is constantly reproduced. 



III. The bath employed being neutral, certain impurities of the ores, such as arsenic, 

 which passes into solution and contaminates the product in the wet processes, remain 

 undissolved, so that a purer copper is obtained. 



IV. There is no unnecessary waste or consumption of metallic iron. 



Ores reached by this process. First, may be included the various sulphuretted ores, 

 as copper pyrites (often mixed with iron pyrites) and the variegated and vitreous sul- 

 phurets, all of which are readily oxidized by calcination. Second, are the oxidized 

 compounds of copper, such as the red and black oxides, the green and blue carbonates, 

 and salts, like the oxy-chloride and silicates like chrysocolla. Third, are the deposits 

 of native or metallic copper, which in almost all instances are most advantageously 

 treated by mechanical means. The presence of carbonate of lime or magnesia is ob- 

 jectionable, since it decomposes the proto-chloride of copper, and thus indirectly pre- 

 cipitates the iron from the bath. The action of oxides of lead and zinc, which come 

 from the roasting of blende and galena when these are present in the ore, produces a 

 similar effect. When not too abundant, the effect of all these substances may be cor- 

 rected by careful roasting. 



