The particular smelting process employed depends on the chemical constitution of the ore: that is, whether it is a sulphide, an oxide or native copper. With the sulphide ore the first operation is roasting, in order to oxidize the sulphur and iron present, and to volatilize the arsenic, bismuth and antimony. The roasted ore is then reduced in a blast furnace with anthracite coal, the product of this state being known as "matte." This 'is again roasted and again passed through the blast furnace, the prod uct being the impure copper known as 'black copper." Oxide ore is reduced by mixing it with sulphide and then smelting it for matte, as explained above. Native-copper ore is smelted with anthrkcite and coke in a reverberatory furnace, with or without the addition of lime+ stone flux. The product is crude metallic cop+ per, which is often tapped directly into the refining furnace.
The metallic copper of various forms as it comes from the reducing furnaces contains as impurities some if not all of the following named minerals: iron, nickel, cobalt, lead, zinc, tin, arsenic, antimony, bismuth, sulphur, sele nium and tellurium, besides a percentage of sil ver and gold. The object of the refining process is to remove to the utmost practicable limit all of these substances, and to turn out the metallic copper at least 99.8 per cent pure. The process of fire-refining is essentially an oxidizing proel ess. It is however, conducted at a degree of heat so high that such 'metallic impurities as arsenic, antimony and bismuth are volatilized. The other metals present r are oxidized into dross and removed with the slag. The cuprous oxide (C20) held in solution in the metal, the only oxide of copper which is stable at the melting point of copper, is gradually reduced until only about one-half of 1 per cent re mains, and this is left in the metal 'to make certain 'that no other metallic impurities are present except in the form of hartnless oxides. The furnace used in fire-refining is of the re verberatory type. The more impure the metal the longer the treatment and the smaller the quantity which can be worked st one time. The "charge' of solid copper, in the form of cakes, is placed in the furnace basin by hand or by mechanical appliances, and the first or melting fire is set in the fire-box. If the copper is relatively pure it melts quickly; if impure "black" copper, it is much slower and may be haiteited with an air blast: When the melting is complete the fire is drawn and a new, oxidiz, ins fire built. As the slag forms it is constantly skimmed away and the clear metal exposed, Cnprous oxide is formed on the clear surf ace and is dissolved in the molten metal.. The metallic impurities which have 'a stronger affint ity for oxygen than has copper take up oxygen from the cuprous oxide and rise to the surface as slag. This operation is forwarded by forcing air into the molten metal through iron pipes dipping well down below the surface. These
pipes melt off in the great heat, but the iron is oxidized and rises to form part of the slag.
When the slag ceases to form, the melted copper retains about 6 per cent of cuprous oxide in solution. This proportion of cuprous oxide is reduced by the process of *poling,* which con sists of forcing poles of green timber, butt end first, into the mass of melted copper, and feed ing them in as they burn away. A state of ebullition is excited by the vapors arising from the scorching wood: the carbonic oxide and hydrogen set free seize upon the oxygen of the cuprous oxide and the charcoal aids in the reduction. This operation is continued until a microscopic examination determines that the cuprous oxide is reduced to the correct propor tion— from .5 to .7 per cent If the poling is carried too far the other impurities give up their oxygen and resume their metallic state, and the entire mass of the copper has to be reoxidized to the 6 per cent content, and the poling done over again. When the copper has reached the proper constitution it is cast into ingots, ingot bars, wire-bars or cakes.
The electrolytic method of refining copper is generally preferred in the United States, and nearly three-fourths of the country's product is thus refilled. This process is essentially electro plating. The anodes are of cast copper, about three feet square and one inch thick and weigh ing 475 pounds each. The cathodes are of pure electro-deposited copper, a little longer and wider than the anodes, and about one-sixteenth of an inch thick The anodes are placed in the tank .at from three to five inches apart i• the longer distance for the more impure metal. A cathode is hung between each of two anodes and one at each end of the tank. The tanks are built of timber and lined with lead. They are two inches wider than the cathodes, allowing an inch of free electrolyte at each side. The tanks are usually about 10 feet long and hold from 2A to 30 pairs of anodes and cathodes, con nected usually in multiple. Some few of the largest works, however, connect them in series. The electrolyte is a solution of copper sulphate in the proportion of 12 per cent, and acidulated with about 9 per cent of sulphuric acid. The copper content is held up to at least 3 per cent to prevent plating with arsenic where it is pre sented in the anode. The free sulphuric acid in the bath is limited to 12 per cent to avoid de composition. The temperature of the electrolyte is held between 105° and 140° F. by means of steam A continuous circulation of the electrolyte is necessary to prevent irregular deposition. The current ranges from 12 to 34 amperes per square foot of anode. The lower figures are adopted where arsenic is present. The voltage varies in different plants from 60 to 180. The time required to deposit the pure metal from the anode is about 20 days.