Smelting Flotation Concentrate

Smelting Flotation Concentrate

In the November issue of Teniente Topics, the monthly publication of the Braden Copper Co., Chile, a member of the staff briefly outlines the development of the smelter from 1909 to the present time. Metallurgical difficulties have been many, but were overcome, in spite of being 6000 miles from the base of supplies. The plant now treats 350 tons of concentrate daily, yielding 60 tons of copper, during which operation 60 tons of coke and 10 tons of fuel-oil are burned, employing 350 men and 1500 hp. This quantity of concentrate is recovered from 4000 tons of ore crushed per day. The concentrate consists of 19% copper, 17% silica, 23% iron, 2% lime, 8% alumina, and 28% sulphur. It is sandy and slimy, and contains 20% water. Of the 350 tons of concentrate, about 215 tons is dumped from V-shaped steel cars into bins, which supply the nodulizing kilns. This concentrate is then fed to conveyor-belts, thence into kilns, heated by oil-burners to a temperature of 1750°F. In the kilns, the sandy concentrate is quickly heated by the burning of the oil, and also by the combustion of a part of the sulphur content, to a sticky consistence, in which state the rolling motion tends to ball it into nodules of varying size. The kilns are sloped an inch per foot toward the discharge-end, out of which the red-hot nodules pour onto an endless chain of cast-iron pans, which convey the product to hoppers ready to charge into the blast-furnaces. The nodules have about the same chemical content as the original concentrate, except that the proportion of sulphur has been reduced from 28 to 18%, and, of course, the moisture has been evaporated.

A by-product of the nodulizers is flue-dust, that is, a small proportion of the concentrate blown out by the draft in the kilns and caught in dust-chambers, removed, and hauled to the bins for re-treatment.

Another 50 tons of the original concentrate is sent to bins that discharge to the sinter-plant, of four units. Each unit is a concrete box. 4 ft. wide by 50 ft. long. In place of a top there is a cast-iron grate similar to that of a stationary boiler, but with smaller air holes. An exhaust-fan is connected to the box, creating a strong down-draft of air through the grate. A 4-in. layer of raw concentrate is spread on the grate with an inch layer of saw-dust ignited with kerosene or gasoline torches, after the fan has been started. The saw-dust starts the combustion of the sulphur in the concentrate. This then continues to roast for an hour, when the sulphur is reduced to 12% and the loose layers are reduced to a hard cake. The cake is broken into pieces six to eight inches in diameter and raked into cars that go to the blast-furnaces.

The remaining 85 tons of concentrate received daily is discharged into bins, thence fed by conveyors into cars directly to the blast¬furnaces; this amount being smelted raw.

The two blast-furnaces are 25 and 30 ft. long, respectively, 4 ft. wide, and 9 ft. deep, with hollow-steel water-jackets. The furnaces are fed with a charge consisting of varying proportions of nodulized, sintered, and raw concentrates, together with converter-slag (containing 60% iron) as a flux, and coke as fuel. The proportion of coke to concentrates averages about 15%, and is dependent directly on the amounts of raw and nodulized concentrates. This mixture gradually sinks in the furnace, becoming hotter and continually melting, until in the bottom it is liquid at a temperature of 2500°F., and runs into the settler. The matte, containing 45% copper, 30% iron, and 25% sulphur, remains in the settler until removed through a hole near the bottom and poured into the converters through a brick-lined launder.

The converters are of the Pierce-Smith basic-lined type. Each consists of a horizontal cylindrical sheet-steel shell 25 ft. long by 10 ft. diameter, inside of which is a lining 18 in. thick of magnesite brick. This material is not attacked by the chemical reactions in the converter, and consequently lasts for a long time, unless allowed to over-heat. The cylindrical converter-shell rests on heavy rollers, and can be revolved around its axis so as to empty its contents through a hole in the side when necessary. The converter is pierced by a horizontal row of blast-pipes through the sheet-steel and lining, for the entrance of compressed air. These holes are in a line parallel with the axis of the cylinder somewhat below the centre-line, and point down toward the bottom of the converter. A large hole in the top receives the charge of matte, and serves as a chimney for the escape of gases.

When ready to receive a charge, the converter is revolved until the mouth is under the end of the matte-launder leading from the settler mentioned; this position places the tuyeres at about the centre¬line of the cylinder. A stream of matte is run by gravity into the mouth, until the converter is filled almost to the level of the tuyeres. There is also added a small amount of quartz. Compressed air at 10 to 12 lb. pressure is then forced through the tuyeres and the converter is revolved until the tuyeres are submerged about 12 in. under liquid matte.

The elimination of the iron and sulphur leaves practically pure copper as the only remaining constituent of the matte; after 12 hours of alternate blowing-in air and pouring off slag a bath remains of 25 to 30 tons of molten copper. This goes into ladle-cars and is hauled to a receiver, which is simply a huge brick-lined kettle capable of lifting and pouring its contents into a series of moving cast-iron molds.

The copper solidifies, is removed, and carried to a platform to be loaded on cars for shipment. This final product is known as ‘blister’ copper, on account of large blisters or bubbles of gas formed on the surface of the bars while cooling. The bars run 99.5% copper, and average 220 pounds in weight.