Abstract:
Methods and apparatus are disclosed for recovering metals from metal-containing support materials such as mineral ores. In one embodiment, the metal may be separated from crushed support material or ore in a bioleaching lagoon by the action of hydrocarbon-utilizing bacteria under anaerobic conditions. The bioleached material is then pumped into a precipitation lagoon where hydrocarbon-utilizing bacteria oxidize the metals under aerobic conditions. In another embodiment, metals may be directly biooxidized from a heap of the metal-containing support material having a hydrocarbon/oxygen injection system embedded therein. A water sprinkler system may be used to wet the heap while the hydrocarbon/oxygen injection system stimulates the growth of hydrocarbon-utilizing bacteria. The resulting effluent solution may be pumped or gravity fed to an aerobic precipitation lagoon where aerobic hydrocarbon-utilizing bacteria are used to precipitate or otherwise deposit the metals onto a deposition material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/308,211, filed Jul. 27, 2001, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the recovery of metals from metal-containing support materials, and more particularly relates to a method and apparatus for recovering metals from support materials with hydrocarbon-utilizing bacteria. 
     BACKGROUND INFORMATION 
     Precious metal ores such as gold ores can be categorized as either free milling or refractory. Free milling ores are those that can be processed by simple gravity techniques or direct cyanidation. Refractory ores, on the other hand, are not amenable to conventional cyanidation treatment. Such ores are often refractory because of their excessive content of metallic sulfides such as pyrite or organic carbonaceous matter. A large number of refractory ores consist of ores with a precious metal such as gold occluded in iron sulfide particles. The iron sulfide particles consist principally of pyrite and arsenopyrite. If gold or other precious metals remain occluded within the sulfide host, even after grinding, then the sulfides must be oxidized to liberate the encapsulated precious metal values and make them amenable to a leaching agent. 
     Conventional biological methods have focused on the recovery of precious metals using sulfur-oxidizing bacteria. A conventional process includes the steps of distributing a concentrate of refractory sulfide minerals on top of a heap of material, biooxidizing the concentrate of refractory sulfide minerals, leaching precious metal values from the biooxidized refractory sulfide minerals with a lixiviant, and recovering precious metal values from the lixiviant. 
     Problems exist using sulfur-oxidizing organisms in bioleaching processes. These problems include nutrient access, air access, carbon dioxide access, the generation of sulfuric acid from reactions of the sulfur-oxidizing bacteria, and the generation of heat during the exothermic biooxidation reactions which can kill growing bacteria. Ores that are low in sulfide or pyrite, or ores that are high in acid consuming materials such as calcium carbonate or other carbonates, may also be problematic during heap biooxidation processes. The acid generated by these low pyrite ores is insufficient to maintain the low pH and high iron concentration needed for bacteria growth. 
     The bioremediation of various pollutants using butane-utilizing bacteria is disclosed in U.S. Pat. Nos. 5,888,396, 6,051,130, 6,110,372, 6,156,203, 6,210,579, 6,244,346 and 6,245,235, which are herein incorporated by reference. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, metals such as precious metals are recovered from metal-containing support materials such as mineral ores. The support material is contacted with a solution containing a hydrocarbon which stimulates the growth of hydrocarbon-utilizing bacteria. The hydrocarbon may comprise one or more alkanes, such as butane, methane, ethane and/or propane, or other types of hydrocarbons. 
     Gold, silver, platinum, copper, zinc, nickel, uranium, palladium and the like may be recovered using the present invention. One embodiment of the present invention provides for the treatment of metal-containing support materials in the form of slurries contained in lagoons, tanks or other vessels. Another embodiment of the present invention provides a bioleaching technique, which initially uses hydrocarbon-utilizing bacteria under anaerobic conditions to pretreat ore-containing materials for subsequent biooxidation using hydrocarbon-utilizing bacteria under aerobic conditions. The process can be used to biooxidize metal-containing support materials such as precious metal-bearing refractory sulfide ores. A further embodiment of the present invention provides a heap bioleaching process. In one embodiment of this process, ores that are low in sulfide minerals, or ores that are high in acid consuming materials such as calcium carbonate, may be treated. 
     In addition to precious metal-bearing sulfide minerals, there are many other sulfide ores that can be treated using the present process, such as copper ores, zinc ores, nickel ores and uranium ores. Biooxidation with hydrocarbon-utilizing bacteria can be used to cause the dissolution of metal values such as copper, zinc, nickel and uranium from concentrates of these ores. 
     An aspect of the present invention is to provide a method of recovering a metal from a metal-containing support material. The method includes contacting the support material with a hydrocarbon to stimulate the growth of hydrocarbon-utilizing bacteria, and recovering the metal from the support material. 
     Another aspect of the present invention is to provide a system for recovering metal from a metal-containing support material. The system includes means for contacting the support material with a hydrocarbon to stimulate the growth of hydrocarbon-utilizing bacteria, and means for recovering the metal from the support material. 
     A further aspect of the present invention is to provide a system for metal recovery from a support material, wherein the system includes a source of hydrocarbon, a hydrocarbon injection system in communication with the hydrocarbon source and the support material, and a deposition material upon which the metal is deposited. 
     These and other aspects of the present invention will be more apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the use of anaerobic and aerobic hydrocarbon-utilizing bacteria to bioleach and precipitate (biooxidize) metals from a metal-containing support material. 
         FIG. 2  is a schematic diagram illustrating the use of aerobic hydrocarbon-utilizing bacteria in a heap to biooxidize and precipitate metals from a metal-containing support material. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the present invention, hydrocarbon-utilizing bacteria are used to liberate metals from metal-containing support materials such as mineral ores. The process may be used to biooxidize metals from ore-containing material using hydrocarbons under aerobic conditions only. Alternatively, the process may use anaerobic and aerobic processes to pretreat and biooxidize metals from ore-containing materials. The ore type and metal composition may determine which process would yield the most favorable metal recover. Under anaerobic conditions, the hydrocarbon may serve as an electron donor and carbon source while sulfate originating from the ore may serve as a final electron acceptor. Other electron acceptors may be used, such as nitrate, iron or carbon dioxide. Subsequently, aerobic hydrocarbon-utilizing organisms and their operative enzymes may be used to precipitate metals from solution, which may then be recovered. 
     In accordance with an embodiment of the present invention, a hydrocarbon such as butane may be utilized to drive a treatment process anaerobic, thereby encouraging the growth of anaerobic microorganisms capable of reducing sulfur-containing compounds. Under anaerobic conditions, sulfate and elemental sulfur may serve as electron acceptors while the hydrocarbon substrate is oxidized. The anaerobic processes may include, for example, desulfurization, sulfur respiration and dissimilatory sulfate reduction. A hydrocarbon such as butane may be used to enhance anaerobic microbiological processes thereby liberating precious metals from recalcitrant sulfide ore bodies. Subsequently, aerobic hydrocarbon-utilizing bacteria may be used to precipitate (biooxidize) metals from solution, which may then be recovered. 
     The metal-containing support material may include sulfide-containing mineral ores, such as precious metal-containing ores, copper ores, zinc ores, nickel ores and uranium ores. The sulfide-containing minerals and ore material may be, for example, coarsely or finely ground ore. The support material may also include lava rock, gravel, sand deposits or any other geologic materials. The recovered metals may include gold, silver, platinum, palladium, copper, zinc, nickel and uranium or any other metal or precious metal. 
     The hydrocarbon may comprise one or more alkanes, alkenes, alkynes, poly(alkene)s, poly(alkyne)s, aromatic hydrocarbons, aromatic hydrocarbon polymers or aliphatic hydrocarbons. The hydrocarbons preferably comprise at least one alkane such as butane, methane, ethane or propane. In a preferred embodiment, the hydrocarbon comprises butane which may serve as an electron donor under aerobic or anaerobic conditions. The high solubility of butane facilitates dispersion of the hydrocarbon food source throughout the metal-containing support material. Furthermore, the high solubility of butane may accelerate the transformation of aerobic conditions to anaerobic by initially stimulating the growth of aerobic butane-utilizing microorganisms in the presence of oxygen to produce carbon dioxide. As the oxygen is depleted and anaerobic conditions prevail, butane or another hydrocarbon may serve as an electron donor to enhance anaerobic microbiological processes that will aid in the leaching of metals from the metal-containing support material. 
     In accordance with a preferred embodiment, butane is the most prevalent compound of the hydrocarbon substrate on a weight percent basis, and typically comprises at least about 10 weight percent of the hydrocarbon substrate. The other constituents of the hydrocarbon substrate may include other alkanes or other hydrocarbons, as well as inert gases such as nitrogen, helium or argon. The hydrocarbon substrate preferably comprises at least about 50 weight percent butane. More preferably, the hydrocarbon substrate comprises at least about 90 weight percent butane. In a particular embodiment, the hydrocarbon substrate comprises at least about 99 weight percent n-butane. The butane may contain straight (n-butane) and/or branched chained compounds such as iso-butane. 
     Suitable hydrocarbon-utilizing bacteria may include the following Groups (in addition to fungi, algae, protozoa, rotifers and other aerobic and anaerobic microbial populations found in decaying materials):
         Group 1: The Spirochetes   Group 2: Aerobic/Microaerophilic, motile, helical/vibroid, gram-negative bacteria   Group 3: Nonmotile (or rarely motile), gram-negative bacteria   Group 4: Gram-negative aerobic/microaerophilic rods and  cocci      Group 5: Facultatively anaerobic gram-negative rods   Group 6: Gram-negative, anaerobic, straight, curved, and helical bacteria   Group 7: Dissimilatory sulfate- or sulfur-reducing bacteria   Group 8: Anaerobic gram-negative  cocci      Group 10: Anoxygenic phototrophic bacteria   Group 11: Oxygenic phototrophic bacteria   Group 12: Aerobic chemolithotrophic bacteria and associated organisms   Group 13: Budding and/or appendaged bacteria   Group 14: Sheathed bacteria   Group 15: Nonphotosynthetic, nonfruiting gliding bacteria   Group 16: The fruiting, gliding bacteria and the  Myxobacteria      Group 17: Gram-positive  cocci      Group 18: Endospore-forming gram-positive rods and  cocci      Group 19: Regular, nonsporing, gram-positive rods   Group 20: Irregular, nonsporing, gram-positive rods   Group 21: The  mycobacteria      Groups 22-29: The  actinomycetes      Group 22:  Nocardioform actinomycetes      Group 23: Genera with multiocular  sporangia      Group 24:  Actinoplanetes      Group 25:  Streptomycetes  and related genera   Group 26:  Maduromycetes      Group 27:  Thermomonospora  and related genera   Group 28:  Thermoactinomycetes      Group 29: Genus  Glycomyces , Genus  Kitasatospira  and Genus  Saccharothrix      Group 30: The Mycoplasmas—cell wall-less bacteria   Group 31: The Methanogens   Group 32: Archaeal sulfate reducers   Group 33: Extremely halophilic,  archaeobacteria  ( halobacteria )   Group 34: Cell wall-less  archaeobacteria      Group 35: Extremely thermophilic and hyperthermophilic S 0 -metabolizers.       

     In addition, suitable bacteria may include facultative anaerobes and microaerophilic anaerobes, which are capable of surviving at low levels of oxygen. These bacteria do not require strict anaerobic conditions such as the obligate anaerobes. Acidophilic, alkaliphilic, anaerobe, anoxygenic, autotrophic, chemolithotrophic, chemoorganotroph, chemotroph, halophilic, methanogenic, neutrophilic, phototroph, saprophytic, thermoacidophilic, and thermophilic bacteria may be used. Hydrocarbon and oxygen injection may encourage the growth of other microorganisms such as fungi, protozoa and algae that may be beneficial to the metal recovery process. The injected oxygen may be in the form of air (e.g., dry air comprising 20.9 percent oxygen), a gas stream with varying concentrations of oxygen, substantially pure oxygen, or the like. 
     Recovery of the metal involves the removal of at least a portion of the metal contained in or on the metal-containing support material. For example, from about one percent to substantially all of the metal contained in the support material may be recovered. Recovery may be achieved using various techniques such as heaps, slurries, precipitation lagoons and bioreactors. During the treatment process, metals may be deposited on a metal deposition material comprising, for example, a polymer, felt, rubber, metallic or natural fiber material that is porous or non-porous. The deposition material may be provided in sheet form or in other forms that provide increased surface area such as spheres and other geometric shapes. 
       FIG. 1  schematically illustrates an anaerobic and aerobic metal recovery system  10  in accordance with an embodiment of the present invention. A metal-containing support material such as low grade ore  12  is fed to a rock crusher  14 . Crushed ore  16  from the rock crusher  14  is fed to a bioleaching lagoon  18  lined with a membrane  19  and equipped with mixers  20  and  21 . A source of hydrocarbon  22  such as butane is connected to hydrocarbon injectors  24  in the bioleaching lagoon  18 . 
     After treatment in the bioleaching lagoon, the material is pumped  26  to a precipitation lagoon  28  equipped with mixers  30  and  31 . A hydrocarbon/oxygen source  32  is connected to injectors  34  in the precipitation lagoon  28 . A membrane  36  lines the precipitation lagoon  28 . After treatment in the lagoon  28 , liquid  38  comprising water and the support material is removed from the precipitation lagoon  28 . Metal deposited on the membrane liner  36  may be recovered from the precipitation lagoon  28  at suitable intervals. 
     In the embodiment shown in  FIG. 1 , the first phase of the metal recovery process occurs in the bioleaching lagoon  18  under anaerobic conditions. Within the lagoon  18 , the metal-containing support material is contacted with the hydrocarbon to accelerate the transformation of aerobic conditions to anaerobic conditions. This is accomplished by initially accelerating the activity of aerobic hydrocarbon-utilizing bacteria in the presence of oxygen present in the lagoon  18  in order to produce carbon dioxide. Under the resultant anaerobic conditions, the hydrocarbon will serve as an electron donor, thereby accelerating anaerobic microbiological treatment processes. In the bioleaching lagoon, the crushed ore  16  is pretreated for subsequent recovery in the precipitation lagoon. The second phase of the metal recovery process occurs in the precipitation lagoon  28 , where the aerobic cycle with air injection may be used to accelerate metal precipitation. 
       FIG. 2  schematically illustrates an aerobic metal recovery system  40  in accordance with another embodiment of the present invention. A heap  42  comprising the metal-containing support material is subjected to water spray by a sprinkler system  44 . A hydrocarbon/oxygen supply  46  is connected to injectors  48  in the heap  42 . An effluent trench  50  under the heap  42  carries effluent to a precipitation lagoon  52  equipped with mixers  54  and  55 . Alternatively, the effluent could be pumped to the lagoon  52 . Another hydrocarbon/oxygen supply  56  is connected to injectors  58  in the lagoon  52 . A membrane liner  60  lines the lagoon  52 . After treatment in the lagoon  52 , liquid  62  comprising water and the support material is removed from the lagoon  52 . Metal deposited on the membrane liner  60  may be recovered from the lagoon  52  at suitable intervals. 
     In the embodiment shown in  FIG. 2 , the heap  42  may comprise ore deposits. The piping  48  through which the hydrocarbon/oxygen mixture or hydrocarbon alone is delivered may be operated under steady or intermittent pulses. The sprinkler system  44  flushes the oxidized metal values from the heap  42  and creates an effluent solution, which flows to the precipitation lagoon  52 . In the precipitation lagoon  52 , the hydrocarbon-utilizing bacteria and injected oxygen deposit the metal values onto the membrane deposition material  60  for recovery. 
     Based on the molecular weight of specific metals, the different metal precipitate out of solution at differing time intervals, thereby providing the opportunity to replace the membrane liners during successive depositional events. Alternatively, electrolysis methods may be employed to further separate the precipitating metals. The metals may then be easily assayed and further refined using conventional techniques. 
     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention.