Chemistry: molecular biology and microbiology – Measuring or testing process involving enzymes or... – Involving viable micro-organism
Reexamination Certificate
2000-05-26
2002-04-30
Lankford, Jr., Leon B. (Department: 1754)
Chemistry: molecular biology and microbiology
Measuring or testing process involving enzymes or...
Involving viable micro-organism
C435S029000, C435S262000, C435S262500, C423S658500, C423S001000, C075S330000
Reexamination Certificate
active
06379919
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to microbiological processes for the oxidative pretreatment of refractory gold and base metal ores and concentrates, and, more specifically, to novel bacterial cultures and inoculums strongly resistant to thiocyanate toxicity which are useful in such processes. The invention further relates to methods of developing and using these resistant bacterial strains.
Precious metals are found throughout the world as an ore within the Earth's crust on the crust surface and dispersed in bodies of water. The precious metal is nearly always in an unrefined state intimately associated with other minerals such as sulfur in the form of arsenopyrite or pyrite. To extract the metal, ore must be processed to remove contaminating minerals such as sulfur, carbon and iron. A commonly used processing technique is cyanidation which involves, quite simply, leaching the ore with cyanide. The cyanide leaches the ore, releasing the precious metal from its association with the gangue minerals. Released metal leaches into a liquid phase from which it can be recovered.
Gold ores are categorized into two types—free milling and refractory—depending on their refractoriness to cyanidation treatment. Free milling ores generally have a low sulfur content and are most often processed by simple gravity techniques or direct cyanidation. Refractory ores, having a higher sulfur content, are difficult to process due to a high excess content of metallic sulfides, such as pyrite, arsenopyrite and other matter, and require more complex extraction methods. One of the most common of such measures is oxidation.
Oxidation of refractory ores involves a pretreatment step in which the ore is subjected to well-known roasting or pressure-oxidation techniques, typically in conjunction with a pre-concentration process. Increasingly, biooxidation is being used as the pretreatment of choice in substitution of these other more traditional oxidation processes. In biooxidation, the metal sulfides in ore are oxidized in a microbial pretreatment step, prior to the cyanidation step. Specifically, the bacteria oxidize both iron and sulfur under acidic conditions. Oxidation of iron sulfide particles causes the solubilization of iron as ferric ion and sulfide as sulfate ion. This liberates the encapsulated precious metal and makes it amenable to a leaching agent, such as cyanide. The precious metal is subsequently recovered from the oxidized materials by cyanidation, carbon-in-leach of thiosulfate leaching processes.
The adaptation of bacteria in the biooxidation process to recover precious metals from refractory ores has been previously described in a number of variations. For example, one method involves oxidizing multi metallic sulfide ores using a combination of chemical/biological leaching process and at least three different types of bacteria (U.S. Pat. No. 4,987,081). Bacterial cultures of
Thiobacillus thiooxidans, Thiobacillus ferrooxidans
and
Leptospirillum ferrooxidans
are first adapted to high dissolved arsenic concentrations and low pH by subjecting the cultures in a solution containing dissolved arsenic to successive incremental concentrations of arsenic while operating in a continuous mode.
Another process involves the biological oxidation of sulfide in sulfide-containing gold ore followed by cyanide leaching (U.S. Pat. No. 5,006,320). This method involves a further processing step for aerating microorganisms during the oxidation step followed by a subsequent extraction of the metal value from the biooxidized ore.
Biooxidation is not limited to the treatment of gold ores. A related method for producing nickel from sulfide ore involves oxidation by heap leaching (Canadian Patent No. 2,155,050). According to this method, nickel ore, which contains a substantial amount of iron, is subjected to a biological oxidation step and separated from iron into an eluate solution. Nickel is removed from the solution by solvent extraction or by use of an ion exchange resin and subsequent electrowinning of the ferronickel.
Metals can also be recovered from refractory sulfide ores by first separating the crushed ore in to a fines and a coarse fraction (U.S. Pat. No. 5,573,575). A heap is formed from the coarse fraction and a concentrate is produced from the fines. The concentrate is then added to the heap for biooxidation.
Alternatively, biooxidation of sulfides in the mineral ores may be done by forming particulates. A heap of particulates is formed and a leaching solution is circulated within the heap (U.S. Pat. No. 5,246,486). A variation on this technique involves polymer agglomeration to aid in the removal of particulates from the metal ore (U.S. Pat. No. 5,332,559).
Metals can also be recovered from a refractory sulfide ore by first separating the clays and fines from the crushed ore, and forming a heap from the crushed refractory ore (U.S. Pat. No. 5,431,717). If there is a sufficient amount of precious metal in the separated clays and fines, these materials are further processed.
Methods for the biooxidation of refractory carbonaceous or carbonaceous-sulfidic ore material using a specific carbon-deactivating microbial consortium have also been used with varying degrees of success (U.S. Pat. No. 5,244,493).
Preg-robbing by carbon and carbon-containing compounds is also a major problem interfering with efficient recovering of metals from refractory ores. One process to overcome this problem uses leaching with a thiosulfate lixiviant to selectively remove the metal (U.S. Pat. No. 5,354,359). This process involves contacting particulates containing precious metal and preg-robbing carbonaceous components with a thiosulfate lixiviant solution forming stable precious metal thiosulfate complexes. The lixiviant solution is recovered after it has had time to become loaded with the metal in the ore material.
Leaching has also been used to remove copper from copper sulfide-containing ore (U.S. Pat. No. 4,571,387). According to this process, ore is ground and mixed with an aqueous acid-leaching medium containing sulfide-oxidizing bacteria, a bacterial nutrient and a catalytic amount of silver. Carbon dioxide and oxygen are provided as well as a bacterial compatible acid. The basic leaching process has been enhanced to increase the leaching rate of a mineral when the mineral is characterized by the tendency to form a reaction product layer during leaching (U.S. Pat. No. 4,343,773). A particulate modifier such as carbon is mixed with the mineral before leaching and selectively alters the characteristics of the reaction product layer.
Prior to incurring the substantial costs inherent in scaling up to biooxidize a particular ore, the ore under consideration typically is batch tested to determine if it is suitable for biooxidation. However, conventional testing procedures can take as long as six months to complete due to the time needed for adaptation of the bacteria and the lag phase between inoculation and the onset of oxidation.
A number of intermediate oxidation products of sulfur including elemental sulfur, polymeric sulfur, sulfite, thiosulfate and polythionates are generated during biooxidation, particularly if oxidation is incomplete. Many of these compounds will react with cyanide in the gold extraction process to form thiocyanate, which is a major cyanide consumer. If a cyanide destruction process is not incorporated as part of the treatment plant, the thiocyanate is discharged with the process tailings. In conventional gold extraction processes, tailings solution containing thiocyanate is often recycled to the process. However, thiocyanate is toxic to the bacteria employed in biooxidation at relatively low levels of between 5 ppm and 25 ppm.
Thus, in systems utilizing microbial biooxidation as the method for oxidizing the ore, tailings solution cannot currently be recycled upstream of the biooxidation process.
In addition, the need for high quality water in biooxidation places further constraints on the process, both in terms of the requirement for large quantities of fresh wate
Lankford , Jr. Leon B.
Patton & Boggs LLP
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