Chemistry: molecular biology and microbiology – Process of utilizing an enzyme or micro-organism to destroy...
Reexamination Certificate
1998-03-02
2001-03-27
Redding, David A. (Department: 1744)
Chemistry: molecular biology and microbiology
Process of utilizing an enzyme or micro-organism to destroy...
C075S710000, C075S712000, C075S743000, C075S744000, C075S728000
Reexamination Certificate
active
06207443
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed generally to bioleaching of sulfidic ores and specifically to heap and vat bioleaching of sulfidic ores.
BACKGROUND OF THE INVENTION
A major source of many metals, particularly copper and gold, is sulfidic ores. In sulfidic ores, the metals are either present as or immobilized by stable metal sulfides, which are frequently nonreactive or slow reacting with lixiviants such as cyanide, ferric ion or sulfuric acid. To promote the dissolution of the metals in a lixiviant, the elements compounded with the metal (e.g., sulfide sulfur) are first be oxidized. In one approach, oxidation of the sulfide sulfur is induced by organisms, such as
Thiobacillus Ferrooxidans
and
Thiobacillus Thiooxidans
(commonly referred to as biooxidation or bioleaching).
Although biooxidation can be performed in a continuous stirred tank reactor, a common technique is to perform biooxidation in a heap. Compared to biooxidation in a continuous stirred reactor, heap biooxidation generally has lower capital and operating costs but a longer residence time and lower overall oxidation rate for the sulfide sulfur in the feed material.
In designing a heap biooxidation process, there are a number of considerations. First, it is desirable to have a relatively high heap permeability and porosity. Fine material can decrease heap permeability and porosity and result in channeling. Channeling can cause a portion of the material in the heap to have a reduced contact with the lixiviant, thereby limiting the degree of biooxidation of the material. Second, it is desirable that the residence time of the feed material in the heap (i.e., the time required for an acceptable degree of biooxidation) be as low as possible. Existing heap leaching processes typically have residence times of the heap on the pad of 12 months or more for an acceptable degree of biooxidation to occur.
SUMMARY OF THE INVENTION
These and other objectives are addressed by the process of the present invention. The process includes the steps of:
(a) biooxidizing a first portion of a feed material containing metal sulfides to form a biooxidized fraction;
(b) combining the biooxidized fraction and a second portion of the feed material to form a combined feed material; and
(c) thereafter biooxidizing the combined feed material. The metal in the metal sulfides can be copper, gold, silver, nickel, zinc, arsenic, antimony, and mixtures thereof. As will be appreciated, precious metals, such as gold, generally are not compounded with sulfide sulfur but are rendered immobile in the lixiviant by close association with metal sulfides, especially pyrite and arsenopyrite.
Because the biooxidized fraction includes large active cultures of organisms, such as
Thiobacillus Ferrooxidans; Thiobacillus Thiooxidans; Thiobacillus Organoparus; Thiobacillus Acidphilus; Sulfobacillus Thermosulfidooxidans; Sulfolobus Acidocaldarius, Sulfolobus BC; Sulfolobus Solfataricus; Acidanus Brierley; Leptospirillum Ferrooxidans
; and the like for oxidizing the sulfide sulfur and other elements in the feed material, the combination of the biooxidized fraction and the second portion of the feed material (which typically has not been biooxidized) “jump starts” the biooxidation of the second portion. In other words, the time required to substantially complete biooxidation of the second portion is significantly reduced relative to existing heap leaching processes, thereby reducing heap pad area and capital and operating costs.
The biooxidation in step (a) can be performed in a continuous stirred reactor or on a heap. A continuous stirred reactor is preferred because of the relatively rapid rate of biooxidation in such reactors and the high concentration of microbes on the biooxidized residue. After inoculation of the slurried portion of the feed material, the continuous stirred reactor preferably is sparged with oxygen and supplied with suitable nutrients for the microbes to foster biooxidation.
Typically, the second portion of the feed material has not been biooxidized. In one embodiment, the second portion is coarsely sized while the biooxidized fraction (i.e., the first portion) is finely sized. The biooxidized fraction typically has a P
80
size preferably ranging from about 5 to about 200 microns and more preferably from about 10 to about 200 microns while the second fraction has a P
80
size in excess of that of the biooxidized fraction.
The combining step can be performed in a number of ways. For example, the biooxidized fraction can be agglomerated with the second portion of the feed material. “Agglomeration” refers to the formation of particles into a ball (i.e., an agglomerate), with or without the use of a binder. Alternatively, the biooxidized fraction can be placed on the conveyor belts along with the second portion of the feed material and be carried by the belts to the heap.
In another embodiment, the process includes the step of floating a portion of the feed material to form a tailing fraction and a concentrate fraction. The first portion of the feed material includes the concentrate fraction. A substantial portion of the fine material is discarded in the tailing fraction so that the porosity and permeability of the heap remains unaffected by the fine size of the relatively small quantity of concentrate fraction (which is incorporated into the heap after partial or complete biooxidation of the concentrate fraction). Commonly, the first portion of the feed material constitutes no more than about 15 wt % of the feed material while the concentrate fraction constitutes no more than about 30 wt % of the first portion (i.e., no more than about 4.5 wt % of the feed material). Accordingly, the tailing fraction constitutes at least about 70 wt % of the first portion.
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Placer Dome Inc.
Redding David A.
Ross P.C. Sheridan
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