High temperature heap bioleaching process

Details High temperature heap bioleaching process High temperature heap bioleaching process

2002-02-28

2004-10-12

Andrews, Melvyn (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Free metal or alloy reductant contains magnesium

C075S743000, C423S027000, C423SDIG001

Reexamination Certificate

active

06802888

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the extraction of copper from hypogenic copper sulfide bearing ores and concentrates.
2. Background
Hypogenic copper sulfides are an economically important sources of copper. Hypogenic deposits are formed by ascending solutions carrying high levels of metal ions at fairly high temperatures (up to 500° C.). As these solutions cool, metal sulfides are deposited as crystallized ore minerals as the solutions move up toward the Earth's surface. As a result, hypogenic deposits are characterized by metal sulfide bearing veins or irregular masses formed within fractures in the country rock. Within these hypogenic deposits, a variety of hypogenic copper sulfides may be found depending on the chemical composition of the ascending solution. Some of hypogenic copper sulfides found in hypogenic deposits are chalcopyrite, bornite, enargite, tetrahedrite, and tennatite. Hypogenic copper sulfides are also sometimes referred to as primary enriched copper sulfide minerals. The ascending solutions may also eventually reach the surface and appear as hot springs. In these situations, the solutions generally become diluted with ground water and thus have lower metal ion levels. As a result, the metal ions in these hot springs typically precipitate out as metal sulfate salts over time. In addition, the copper sulfide minerals that are formed above the water table may become altered over time by oxidation to sulfates by the circulation of air, water, and bacteria. These soluble metal salts are subsequently carried away in solution by the downward moving ground water. As the ground water moves to the oxygen deficient lower levels a secondary enrichment can take place. The copper-bearing solutions react with the existing chalcopyrite and other hypogenic sulfides such as bornite, enargite, tetrahedrite, and tennatite to form new copper sulfide minerals. The new minerals formed by the descending solutions are sometimes called supergenic or secondarily enriched copper sulfide minerals. The supergenic copper sulfides—or secondary enriched copper sulfides as they are sometimes referred to—are higher in copper and are characterized by the minerals covellite and chalcocite. They are also more readily oxidizeable copper sulfide minerals than the hypogenic copper sulfide minerals. These supergenic copper sulfide minerals are generally located below the oxidized zone and the water table and above the lower grade of primary sulfide ore.
Chalcopyrite is economically the most important hypogenic copper sulfide mineral, as well as the most economically important source of copper overall. Presently, smelting technology remains the primary technology for recovering copper from chalcopyrite. Smelting chalcopyrite, however, has a number of drawbacks. These include sulfur dioxide gas emissions which are environmentally unacceptable, large production of sulfuric acid even though there presently exist only a limited market for sulfuric acid in most areas, and expense. As a result, alternative methods for recovering copper from chalcopyrite, as well as other hypogenic copper sulfides, that are more environmentally friendly and less expensive have been sought for a number of years.
A number of alternatives that have been investigated for recovering copper from chalcopyrite and its ores have included hydrometallurgical processes. Hydrometallurgical processes have long been used to recover copper from oxide ores. These processes typically involve sulfuric acid leaching of the oxide ore, copper separation from the pregnant leach liquor by solvent extraction techniques and recovery of metallic copper from the strip liquor by electrowinning. These techniques have not only demonstrated an ability to recover copper at a competitive cost advantage over most smelting processes, but the electrowon copper produced in such processes is also now fully competitive in terms of quality with electrorefined copper produced by the known smelting and refining techniques. Presently, however, a commercially viable hydrometallurgical process for the recovery of copper from chalcopyrite, and other commercially important hypogenic copper sulfide minerals, has remained elusive despite extensive research efforts to develop such a process. The development of a hydrometallurgical process for the direct leaching of chalcopyrite either by chemical or biological means has been continuously sought for more than twenty years.
The direct leaching of chalcopyrite and other hypogenic copper sulfide minerals in sulfuric acid solution poses a variety of problems. At temperatures below the melting point of sulfur (approximately 118° C.), the rate of copper dissolution has, to date been uneconomically slow. At temperatures above the melting point of sulfur the chalcopyrite and other hypogenic copper sulfide minerals are passivated by what is believed to be a layer of elemental sulfur which forms over the unreacted sulfide particles. This again renders the extraction of copper uneconomical by this process. Other leaching systems that have been studied over the years for the extraction of copper from chalcopyrite on laboratory or pilot scale include systems employing concentrated solutions of ferric chloride or ammoniacal ammonium as lixiviants.
Efforts to bioleach chalcopyrite and other hypogenic copper sulfides on a commercial scale have also proven unsuccessful to date. Hypogenic copper sulfides such as chalcopyrite are notoriously difficult to bioleach even though bioleaching is now used as the principal production approach to extract copper from supergenic copper sulfide minerals such as chalcocite and covellite at several mining operations around the world.
Stirred tank and heap biooxidation processes that have employed mesophiles, such as
Thiobacillus ferrooxidans
, the most commonly used microorganism for biooxidizing sulfide minerals, have largely been unsuccessful due to the slow leach kinetics of chalcopyrite and other hypogenic copper sulfides. The slow leach kinetics and incomplete biooxidation of chalcopyrite and other hypogenic copper sulfides are often attributed to the formation of an inhibiting or passivation layer that forms on the surface of these copper sulfides as they oxidize. A number of different additives have been used in an attempt to increase the dissolution of copper from chalcopyrite, presumably by disrupting the passivating layer. These additives include metal salts such as Ag
2
SO
4
, Bi(NO
3
), graphite, and other sulfide minerals. Any biohydrometallurgical process for treating hypogenic copper sulfides such as chalcopyrite, therefore, will have to address the problem of this surface layer. Studies of the problem have led to several theories concerning the nature of the inhibiting layer.
One theory is that a jarosite coating forms on the surface of hypogenic copper sulfides as they are leached. Jarosite is formed in the presence of sulfate and ferric iron, in environments in which the pH increases to above about 1.8. However, high concentrations of jarosite constituent molecules (sulfate, ferric iron, ammonium or potassium) will lead to jarosite formation at lower pH. The presence of jarosite in analysis of bioleached chalcopyrite supports this theory. However, experiments performed by the present inventors that show slow leaching even at low constituent molecule concentration and low pH, as well as reports in the literature, contradict this theory.
Another theory is that elemental sulfur produced during bioleaching forms a thick blanket that excludes bacteria and chemical oxidants from the surface of the hypogenic copper sulfide minerals. The detection of large amounts of sulfur in bioleached chalcopyrite supports this theory. In addition, many electron micrographs have shown a thick sulfur coating on leached chalcopyrite. This theory, however, does not adequately explain why other metal sulfides that also form sulfur when leached do not leach as slow as chalcopyrite.
A third theory proposes that the inhibition is caused by the formation of

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