Innovations

August 2001

Phelps Dodge Morenci Has Converted All Copper Production to Mine-for-Leach

Copper Applications in Mining & Metallurgy

By William H. Dresher, Ph.D., P. E.

History of the Site | Unique Mineralogy | Clean and Cost-Effective | References

SX/EW and other process improvements provide cost savings and significant environmental benefits.

In early 2001, the Phelps Dodge Mining Company's Morenci, Arizona operations were completely converted from a conventional mining, milling, smelting and refining operation to a system whereby ore will be mined exclusively for leaching. The new "mine-for-leach" process uses bacteria to extract copper from the ore and solvent extraction/electrowinning (SX/EW) to recover copper from the resulting leach liquor.

Phelps Dodge has ceased all recovery of copper from sulfide mineral concentrates by means of conventional smelting operations at its Morenci, Arizona operations. Transition to a new mine-for-leach process is, in fact, a natural step, since the company had previously abandoned smelting at this site in 1984 and has since been shipping its copper concentrates to its smelter in Chino, New Mexico. All copper now shipped from the Morenci site is in the form of 99.99% pure copper cathode.

Total conversion of the Morenci operation to mine-for-leach provides numerous improvements in energy conservation and environmental impact compared with previous operations at the site. (One very important benefit: the operation emits no sulfur dioxide to the atmosphere.) In addition, Phelps Dodge anticipates a 10% cost-of-production savings, between $0.07 and 0.09/pound of copper, as a result of the new processing regime.

While Morenci will not be the world's first mine-for-leach operation, at 75,000 t/d (tons per day), it will be the largest by far. Similar operations that preceded Phelps Dodge's entry include Cerro Verde (1993 - 15,000 t/d) Zaldivar (1992 - ~20,000 t/d), both in Perú, and Quebrada Blanca (1993 - 17,300 t/d) in Chile. Interestingly, the predecessor company to Phelps Dodge at the Morenci site mined-for-leach as early as 1885.

Phelps Dodge is hardly a stranger to SX/EW, having first used the environmentally friendly process on a large scale in 1987. In 1985, the company initiated large-scale leaching of oxide ores and waste dumps using sulfuric acid and, two years later, using SX/EW to recover the copper. The ore currently being leached is a portion of the sulfide ore now being mined. In addition, four major sulfide waste dumps from previous operations are now being leached. These dumps collectively contain over one billion tons of low-grade copper ore. (A description of the SX/EW process can be found here).

Prior to the current expansion, the Morenci site contained three SX plants feeding two EW tank houses. In 1998, this installation accounted for a total annual output of approximately 551 million pounds (250,000 mt) of copper. Copper in the low-grade dumps was previously considered waste and would have been unrecoverable by the conventional smelting process. Thus, the new recovery process not only creates more high purity copper for human use, it also removes an uneconomic waste from the environment.

History of the Site

The Morenci operations of Phelps Dodge Mining Company traces its origins back to 1865 when a U.S. Army patrol, in pursuit of Indians who had stolen some horses, noted strong copper mineralization on the surface. Five years later, a group of ranchers from Silver City, New Mexico came to the area and claimed areas over the copper mineralization, hoping to find gold. These areas were later to give rise to the towns of Metcalf and Morenci. (Metcalf was named for brothers James and Robert, two of the ranchers; Morenci took its name after the Michigan hometown of a financier, William Church.) Church raised fifty thousand dollars from the Phelps Dodge Corporation in New York and formed the Detroit Mining Company, with he and Phelps Dodge as joint owners of the Morenci property. In 1886, Church sold his half of the company to Phelps Dodge, which built the first copper concentrator in Arizona. In 1882, the Metcalf brothers sold their interests in the Metcalf property to the Arizona Copper Company of Edinburgh, Scotland, which built a concentrator, leach plant and smelter at Clifton, Arizona.

A depression in the price of copper in 1892 threatened both the Morenci and Metcalf operations. The Detroit Mining Company shut down operations, but the Arizona Copper Company was able to continue because, in 1885, they developed a unique leaching operation that could economically treat tailings from the Clifton concentrator. The leaching operation was further extended to mined oxide-type copper ores. These ores consisted of copper oxide, carbonate, silicate and sulfates from the Metcalf mine. Ten tons of sulfuric acid per day was provided from an acid plant at the Clifton smelter. Copper was separated from the leach solution using a process known as cementation, whereby the solution is contacted with scrap iron, causing the copper in solution to be exchanged for iron while the copper plates onto the scrap. Copper was recovered from the resulting cement copper by feeding it into the smelter together with the sulfide concentrates. By using this combination of smelting and leaching, the company was able to survive the economic depression.

In 1921, Phelps Dodge purchased the assets of the Arizona Copper Company and both the Detroit Mining Company and Arizona Copper Company, along with their names, were consolidated under the Phelps Dodge banner. From that time until the mid-1980s, processing of ore from the Morenci and Metcalf mines was confined to the concentration and smelting of sulfide ores. The Morenci and Metcalf pits were consolidated in 1981. In 1986, 15% of the operation was sold to Sumitomo Metal Mining Company, and the Phelps Dodge Morenci Company, the present operator of the property, was formed.

Unique Mineralogy

Leaching was possible in 1885 - and remains so today - due to the unique mineralogy of the Metcalf orebody, which contains both oxide and sulfide copper mineralization. The main sulfide minerals are chalcocite (copper), and pyrite (iron) with small amounts of molybdenite (molybdenum), sphalarite (zinc) and galena (lead). The weathered, or oxidized, minerals are goethite (iron), hematite (iron), jarosite (iron), chrysocolla (copper) and malachite (copper). In addition, there are minor amounts of the copper minerals tenorite, cuprite, brochantite and azurite. Early mine-for-leach was confined to high-grade concentrates of oxidized copper; namely, malachite, azurite and tenorite. Today's mine-for-leach will include both oxide and sulfide ore minerals.

The conversion to mine-for-leach maintains the property's annual production rate of approximately 800 million pounds (360,000 mt) of copper per year. It includes expansion of the mine's current crushing and conveying system, installation of two mobile stackers to disperse crushed ore evenly on two leach piles, expansion of existing solution extraction facilities and construction of a new electrowinning tankhouse. The Morenci concentrator, formerly used to process sulfide ores for smelting, has been placed on standby status. The Metcalf crushing facilities will continue to process 75,000 tons of ore daily for the expanded leach operation. Eliminating the need to grind the ore results in a considerable energy savings compared with the former operations.

In the leaching process tales place as an acidic solution percolates through two large piles of crushed ore, dissolving copper from the ore as it does so. The ore in these piles is crushed to 80% minus 1-1/2 inch (40 mm). Fines from the crushing operation, which are leached along with the ore, are agglomerated to nominally ½ inch (12 mm) using an acidic solution. The copper-laden solution (pregnant leach liquor) is recovered from the bottom of the piles and stripped of its copper in a solvent extraction process. Copper is exchanged for hydrogen in the solvent using an organic extractant, and the resulting acid solution, now stripped of copper, is returned to the leach pile for further leaching. Copper is stripped from the organic extractant using depleted electrolyte. This copper-bearing aqueous stream is then pumped to the tank house where high purity copper is recovered as by electowinning. Since copper is exchanged for hydrogen at the cathode, the electrolyte becomes acidified as it is depleted of copper. This depleted electrolyte is then pumped to the SX plant, where it represents the primary source of acid used in the process.

Sulfide ores are difficult to leach satisfactorily. Thus, the process used at Morenci calls upon bacteria (that are native to the site) to oxidize the sulfide minerals chalcocite and pyrite in the ore, forming ferric sulfate and sulfuric acid. The ferric sulfate, in turn, oxidizes the copper minerals and enables them to be leached by the acid. Air is blown into the piles to facilitate the oxidation process. The leach piles are constructed in 22-ft (7-m) high layers called lifts. Air will be injected in every second lift. When completed, the lifts will be stacked to a height of 400 ft (122 m).

Clean and Cost-Effective

Not only does the Phelps Dodge mine-for-leach operation represent a major step in strengthening the company's competitiveness, it also demonstrates one innovative way that copper can be produced - economically and with minimal environmental impact - on a very large scale. No two orebodies are alike, of course, and the economic, technical and environmental success Phelphs Dodge is enjoying at Morenci may not be achievable elsewhere. Whether the Morenci operation ultimately proves to beunique or whether it ushers in a new millennium of copper production remains to be seen. For the moment, Morenci is producing as much copper as it has in all of its colorful past, and it is using less energy and leaving less copper behind in waste dumps. That will do, for now.

References:

This paper was written with the assistance of John D. Marsden, Vice President, Technology, Phelps Dodge Corporation.

Historical facts were drawn from:

Conger, W. C., "History of Clifton-Morenci District," in History of Mining in Arizona, Vol. I, J. M. Canty and M. N. Greeley, editors, Mining Foundation of the Southwest, Tucson, Arizona, 1987, pp 99 - 128.

Granger, B. H., Arizona Place Names , The University of Arizona Press, Tucson, Arizona, 1960. p 169.

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