CHILEAN COPPER

Despite today’s low prices, reflecting a persistent oversupply, the copper indus try remains active in Chile. Since the country possesses more than 25% of the world’s copper, it is not surprising that it has again assumed the position of the world’s leading producer, a position which it lost to the U.S. toward the end of the last century. Chile now produces 15% of the world’s output. The country’s copper mining industry is shared between two major state-owned enterprises and a number of important private mining companies. The state-owned e nterprises are codelco-chile and enami. Of the several companies belonging to th e private sector, the Anglo American Corp. operates the Empresa Minera de Mantos Blancos, one of the major mines in the country not controlled by the state.

The relative importance of Mantos Blancos in Chile’s copper industry can be seen from this table of the main producers in 1985:

Company Ownership Tonnes of fine copper

Codelco-Chile Stated owned 1,077,000

Enami Stated owned 102,000

Disputada &

El Soldado Exxon 74,000

Mantos Blancos Anglo American Corp. 1,323,000

For half a century after the discovery of the Mantos Blancos deposit, development was hampered by the peculiar characteristics of the copper which made diffic ult the usual methods of metallurgical treatment. The main oxide copper mineral type in the area is atacamite, a chloride mineral which, although typical of the arid, rainless Atacama desert, is not commonly found in other places.

Furthermore the copper deposits at Mantos Blancos are different from the typic al porphyry copper deposits common to the rest of the region. Fairly large masse s of ore, with sharply defined mineralogical and grade boundaries, are distribut ed in complexes brought about by block faulting which has displaced the blocks b oth laterally and vertically in relation to one another. This has necessitated m ining in a series of open pits rather than in a single large pit as is usually d one in the extraction of porphyry coppers.

Although mining has proved to be complicated, by far the most difficult problem faced was the development of a treatment route for the chloride containing ata camite ores. Following several years of intensive testwork, a unique leach/preci pitation process was developed, and a plant incorporating this technology was co mpleted late in 1960. The oxide ore (60% atacamite and 40% chrysocolla) is crush ed and then leached in a series of vats with a solution containing sulphuric aci d to which common salt has been added to produce a 50% mixture of sulphuric and hydrochloric acids. Copper is precipitated from the leach solution as a relative ly pure cuprous chloride by counter-current contact with gaseous sulphur dioxide in steam-heated brick-lined towers. After the cleaning and washing, the crystals are pelletized, together with coke and limestone, and are then smelted in drum -type rotary furnaces. The resulting calcium chloride slag is discarded and the molten copper is fire-refined by blowing and poling, before being cast into 50-k g ingots. The final copper product is 99.96% pure and has an excellent electrical conductivity of 100.5%-101.5% by international standards.

As mining at Mantos Blancos went deeper and deeper into the copper oxide ores, sulphides began to appear in large and irregular masses. Waste- to-ore ratios increased in the open pits and this led to the institution of underground mining and the use of a variety of fresh technical methods, including sub-level caving, room and pillar, as well as sub-level open stoping. At present the whol e underg round operation is on a cut-and-fill basis with post pillars developed in three 80-m lifts in the thicker parts of the orebody.

Research continues. The sulphide flotation plant was commissioned in April, 1981, and its capacity was extended from 3,000 tonnes per day to 8,000 tonnes per day towards the end of 1985. A unique feature of the plant is the sulphidization and flotation of the oxide atacamite from the flotation tailings, producing an oxide concentrate which is treated by acid leaching. In the desert environment, water conservation is a critical factor, and an unusual aspect of the plant is t he filtration of the tailings on two large belt filters, prior to disposal. Wate r recovered as filtrate is recycled to the plant; and this also overcomes the pr oblems associated with disposing of waste water onto a ground surface containing soluble salts.

With the rest of Chile’s copper industry, Mantos Blancos has had to adapt to changing economic conditions — the low market prices for copper and , too, diminu tion in the grades of the ores being mined. Innovations in techniques have becom e necessary — essential — to cut costs and to maintain the viability of produc tion. Numbers of technical improvements have been put into operation and have en abled Mantos Blancos to maintain a competitive position in a depressed copper ma rket by reducing production costs to 52 cents (US) per lb in 1986 from 88 cents per lb in 1980. * Condensed from an article in Optima, Vol 34, No 4.