Patent Publication Number: US-6342190-B1

Title: Process for increasing recovery of precious metals in an ore processing operation

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an improvement in the methods used to separate desired minerals from the naturally occurring ore in which they are found. More specifically, to a method of separating these minerals which does not require the addition of chemical agents to the process in order to stimulate separation reactions, many of which are harmful to the environment and all of which add unnecessary cost to the production of these minerals. 
     In the past, miners and mining companies have struggled to find and perfect a method of separating desired minerals, most commonly precious metals such as gold and silver, from the body of the ore with which they are associated. These recovery problems are compounded due to the fact that these ores are refractory in nature, that is to say that they do not respond well to the heating and melting techniques that are employed with other types of ores. 
     The refractory nature of these ores is a result of two different possible chemical compositions contained therein. The first of these is the presence of sulfide minerals within the ore that are chemically associated with the precious metal. This association is difficult to break and requires that the sulfides be decomposed prior to the recovery. The decomposition of the sulfides is usually accomplished by pressure oxidizing the ore at highly elevated temperatures and pressures and under acidic conditions which will oxidize the sulfides and make the precious metal much easier to recover. 
     The second circumstance that will cause an ore to react in a refractory manner is the presence of organic carbon within its chemical structure. This creates a problem because of the method that is used to recover the precious metal contained in the ore. The recovery is most commonly accomplished by the introduction of cyanide into the ore which leaches out the precious metal and forms a cyanide complex containing the metal and the cyanide. This cyanide complex can then be absorbed by activated carbon from which the precious metal is later recovered. The presence of organic carbon in the ore is a problem because it will compete in the precious metal absorption process with the supplied activated carbon. This works to rob precious metal from the recovery process which limits its effectiveness. 
     This condition is not responsive to the cyanide method that is effective with sulfides and so requires the application of a different process to the ore to recover the precious metal. This is commonly accomplished by subjecting the ore to a chlorine containing compound prior to the recovery process. The addition of chlorine does solve the organic carbon problem but is not effective when the ore is also refractory due to the presence of sulfides and is also very expensive due to the added cost of the chlorine and the necessary additional steps needed to process the byproducts. 
     The prior art has attempted to address these problems by providing a single step method of dealing with refractory problems due to both the presence of sulfides and organic carbon. Most notably, in U.S. Pat. No. 5,536,480 issued to Simmons, a precious metal recovery method is disclosed in which a pressurized oxidation mechanism is employed to treat ores that are refractory due to the presence of organic carbon. This process is claimed to reduce the ability of the organic carbon to rob the precious metal from the recovery process and to also substantially oxidize any sulfides contained in the ore. However, this process requires the creation of a highly pressurized and oxidized environment which greatly increases the costs involved in the recovery of precious metal from refractory ores. 
     From the forgoing discussion it can be seen that it would be advantageous to provide a method of recovering precious metals such as gold and silver from common and refractory ores. Additionally, that it would be advantageous to provide such a method that would be in a single step as effective in precious metal recovery from ores that are refractory due to the presence of sulfides as with ores that are refractory due to the presence of organic carbon. Further, to provide such a method that does not require the addition of chemical agents or a highly pressurized environment to accomplish the recovery of precious metals from ores. 
     SUMMARY OF THE INVENTION 
     It is the primary objective of the present invention to provide a method of initiating the process of removing the values from wastes such as ore or mine tailings typically obtained from or stored around mine sites. 
     It is an additional objective of the present invention to provide such a method of material separation which does not require the use of additional chemicals to the ore or mine tailings to initiate this separation process. 
     It is a further objective of the present invention to provide such a method of mineral separation that is less expensive, simpler and more effective to operate than present methods. 
     It is a still further object of the present invention to provide such a method of mineral separation that produces the desired results in shorter periods of time then those that are currently available and that requires little maintenance during normal operation. 
     These objectives are accomplished by the use of an ore separation operation that begins with a conveyor system which feeds the ore into a specially designed primary grinding mill. The primary grinding mill grinds the ore into a very fine powder, at least half of which is 100 mesh (0.0059″ or 0.150 mm) or smaller. This powder is then blown by the primary grinding mill through a discharge duct and into a primary sizing baghouse which separates the smaller particles (100 mesh or smaller) from the larger oversize particles. The oversized particles are channeled from this point through an oversized duct to the secondary grinding mill where they are ground again and blown to the secondary sizing baghouse. The smaller particles go to the final ore sizing baghouse and the oversize ore particles are sent back to the storage silo to go through the ore grinding process again. The grinding process is repeated until all the ore is the proper size. 
     Once the ore has been ground to the proper size (whether in the primary or secondary grinding mill), it is channeled into the final ore sizing baghouse where the ore particles are separated from the air stream and sent to the blower mill which, in turn, blows them into the air mixing chamber of the ore-roasting oven. In the air mixing chamber, the ore powder is mixed with a precise amount of air (the exact amount being determined by the chemical properties of the ore being processed) and then blown into the ore-roasting oven. Within the ore-roasting oven, the ore is flash heated to a temperature that exceeds 300 degrees Fahrenheit which ignites the powdered ore mixture and some of the combustible chemicals contained in the ore. This ignition process initiates a pyrolysis reaction with the other chemicals in the ore and begins the separation process that is the primary function of the invention. 
     When these processes are completed, the roasted ore particles are channeled into a primary quench chamber where they are quickly cooled with a water spray. At this point, the extremely fast changes in the ore particles&#39; temperature coalesces and cracks them, which separates the mineral or minerals from the remainder of the undesirable material contained therein. Once these processes are complete, all but the very smallest of the ore particles drop to the bottom of the primary quench chamber where they, and the cooling water, are removed by a slurry pump to the ore concentration units which separate the values from the wastes. The lightest of the particles in the primary quench chamber are transferred to the secondary quench chamber where they undergo a sequence of processes and reactions that are very similar to those that occur within the primary quench chamber. 
     Finally, after being separated from the ore, both the ore and water that are used in the separation process are transferred to cleaning units contained within the body of the invention and are purified before either their release into the environment or prior to their being recycled for further use. These processes ensure that the use of the present invention conforms to all Environmental Protection Agency and all other governmental regulations, and that the use of the invention will have the least possible degree of detrimental impact upon the natural environment. 
     For a better understanding of the present invention, reference should be made to the drawings and the description in which there are illustrated and described preferred embodiments of the present invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of the present invention illustrating the entirety of the machinery involved in the ore separation process. 
     FIG. 2 is a front elevation view of the primary grinding mill component of the present invention. 
     FIG. 3 is a side elevation view of the primary grinding mill component of the present invention detailing as illustrated in FIG. 2, 
     FIG. 4 is a front elevation cut-away view of the primary grinding mill component of the present invention as shown in FIG. 3 taken along line  3 — 3  and detailing the interior workings of the grinding mill. 
     FIG. 5 is a side elevation view of the ore-roasting oven and quench chamber components of the present invention, showing their orientation to one another within the body of the invention. 
     FIG. 6 is a side elevation of the roasting oven component of the present invention of FIG.  5  and illustrates the manner in which it is constructed. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and more specifically to FIG.1, the ore separation process  10  is a process in which mine tailings and/or raw ore particles  104  are fed into an ore load point  12  by the use of a front end loader or other material transport device. From the load point  12 , the ore particles  104  are transported to the ore storage silos  16  by a belt conveyor device  14 . Typically, the present invention is equipped with a plurality of the storage devices  16  which hold enough ore particles  104  to allow the invention to operate uninterrupted for a thirty-two (32) hour period (FIG. 1 shows only one (1) storage silo  16  for the purposes of illustrative simplicity) before the storage devices  16  need attendance. 
     From the storage devices  16 , the ore particles  104  fall into the primary grinding mill  18  which is located directly beneath the device  16 . The primary grinding mill  18  functions to grinding the ore particles  104  to a fine power, 50% of which should be at least 100 mesh (0.0059″ or 0.150 mm) or smaller. Once the grinding process has been completed, the ore particle  104  powder is blown by the primary grinding mill  18  through the discharge duct  20  and into the primary ore sizing baghouse  22  which functions to separate the smaller ore particles  104  (100 mesh or smaller) from the larger ore particles  104 . Once this separation process is complete, the larger particles of ore  104  are sent through the oversize duct  24  to the secondary grinding mill  26  which repeats the grinding process. The ground-up ore particles  104  from the secondary grinding mill  26  are then passed through a discharge duct  20  to the secondary ore sizing baghouse  27 , which again separates the small particles from the large. The larger ore particles  104  are transported back to storage silo  16  via the oversize ore conveyor  42  to reenter the primary grinding mill  18  where the grinding process is repeated to obtain the necessary particle size. 
     Once the ore particles  104  have been ground to the proper size, they are passed from the primary and secondary baghouses,  22  and  27 , through the discharge ducts  20  and then to the final ore sizing baghouse  28 . The final ore sizing baghouse  28  functions to separate the solid particles from the air stream and then directs them into the blower mill  30 . Additionally, the primary and secondary grinding mill,  18  and  26 , and the final ore sizing baghouse  28 , are all commonly vented to the outside air through the vent ducts  54  and the stack  50 . This ensures that any pressure that is built up in these systems can be vented and will not create any problems with the flow of ore particles  104  through the invention during the grinding and separation processes. 
     These small particles of ore  104  that are separated in the primary and secondary baghouses,  22  and  27 , proceed through the final ore sizing baghouse  28  to the blower mill  30 . The blower mill  30  blows the ore particles  104  into the air mixing chamber  32  which further mixes the ore particles  104  powder with a specific amount of air and it is then directed into the ore roasting oven  108  which is a large cylindrical tube that is supported by the burner frame  100 . Within the ore-roasting oven  108 , the ore  104  is heated to a temperature that exceeds 300 degree Fahrenheit and any combustible chemicals, such as sulfur, that are contained in the ore particles  104  also ignite and enhance the pyrolysis reaction with the other naturally occurring chemicals contained within the ore particles  104 . The oreroasting oven  108  may be mounted at any angle from vertical to horizontal, depending only upon the chemistry of the ore particle  104  principally being processed. This is the beginning of the mineral separation process that is the subject of the present invention. 
     After leaving the ore-roasting oven  108 , the ore particles  104  enter the primary quench chamber  38  where it is quickly cooled by passing through the water spray  62 . The water spray  62  is provided within the quench chambers,  38  and  56 , by means of the water in pipe  58  which passes into the interior of the quench chambers,  38  and  56 , to where it is equipped with a plurality of water spray nozzles  60 . The spray nozzles  60  direct a fine spray of cool water  62  into the path of the heated ore particles  104  which provides the necessary cooling. At this point, the extremely fast change in the ore particle&#39;s  104  temperature coalesces and cracks it which separates the mineral or minerals from the remainder of the undesirable material contained in the ore particles  104 . This cracking process is at the heart of the purposes and function of the present invention and is an effective step in the separation process of the precious metal from the ore. 
     After the cooling and separation processes, all but the very lightest particles drop to the bottom of the primary quench chamber  38  from where they flow into the slurry pump  40  located beneath the primary quench chamber  38 . The slurry pump  40  transfers the ore particles  104  and the remaining cooling water to the ore concentration units (not shown). The air stream that is used to move the ore particles  104  to this point, continues through the present invention to the electrostatic precipitator and scrubber  46  which begin the air purification process. From the electrostatic precipitator and scrubber  46 , the airflow proceeds through the carbon air filter  48  for final filtering and odor removal. The exhaust air, now having no odors or solid particles, exits the present invention through the exhaust stack  52 . 
     The smallest of particles leave the primary quench chamber  38 , are transferred by means of the transfer tubes  44  to the secondary quench chamber  56 . These remaining ore particles  104  are washed from the air in the secondary quench chamber  56  and removed by the slurry pump  40  located directly beneath it. The slurry, or the ore particles  104  and water, from the primary and secondary quench,  38  and  56 , is pumped to a magnetic separator (not shown) where the wastes (magnetics) and the values (non-magnetics) are separated. The values are routed through a cyclone and then sent to a refinery for final separation and refining as is well known in the art. The wastes are also routed through a cyclone to separate out the remaining water and then are stored for later disposal. 
     The manner of construction of the primary grinding mill  18  is further detailed in FIGS. 2,  3 , and  4  (the construction of the secondary grinding  26  and blower mill  30  is nearly identical to that of the primary grinding mill  18  with the exception that the secondary mill  26  and blower mill  30  have a different method of ore particle  104  introduction). Ore particles  104  are introduced into the primary grinding mill  18  through the ore intake. During the operation of the primary grinding mill  18 , the ore particles  104  are fed into the open top of the intake housing by the weight of the stored material in the ore storage silo  16  which are stored above the grinding mill  18 . 
     The primary grinding mill  18  is made up of the mill housing  88  which is divided into the upper mill housing  92  and the lower mill housing  94 . These two halves of the grinding mill  18  can be easily separated to gain access to the interior  70  of the primary grinding mill  18  for repairs and maintenance of the interior components. The mill housing  88  is held in place by the use of the triangularly shaped mill base  98 , which additionally provides the point of attachment for the primary electric drive motor  96 . The primary electric drive motor  96  provides the necessary rotational force to drive the primary grinding mill  18  through the primary drive belt  102 , which runs between the primary electric drive motor  96  and the mill shaft  86 . 
     The exterior of the primary grinding mill  18  is also equipped with a variable air intake  82  which is the component of the primary grinding mill  18  which allows for the introduction of additional air into the primary grinding mill  18  during the ore particle  104  grinding process. The introduction of air into the mill interior  70  through the variable air intake  82  is critical to the grinding process because, by opening it and allowing more air to enter the mill interior  70 , the operator can vary the size of the ground ore particles  104  that the primary grinding mill  18  is putting out. That is to say, the introduction of more air into the primary grinding mill  18  decreases the amount of time that the ore particles  104  remain in the primary grinding mill  18  which in turn increases the size of the ore particles  104  that exit the primary grinding mill  18 . Conversely, decreasing the amount of air entering the mill interior  70  through the variable air intake  82 , increases the amount of time that the ore particles  104  remain in the primary grinding mill  18  and, therefore, the longer stay within the mill  18  produces a finer size in the ore particles  104 . 
     The interior components of the primary grinding mill  18  and its manner of operation are further detailed in FIG.  4 . Once the ore particles  104  enter the mill interior  70 , they are immediately and forcefully struck by one of the plurality of the spinning impeller blades  72  that are attached to the outer surface of the flywheel  84 . During the operation of the primary grinding mill  18 , the flywheel  84  is spinning at a high rate of speed which is provided by the primary electric drive motor  96  as previously described. 
     The ore particles  104  being struck by the spinning impeller blades  72  accomplishes two separate functions. First, the high rate of speed at which the impeller blades  72  are spinning creates enough of an impact to begin breaking up the ore particles  104 . Second, the high-speed impact accelerates the ore particles  104  to an extremely high velocity towards the anvil plate  74  which is located at the end of the mill housing  88 . The impact of the ore particles  104  with the anvil plate  74  serves to further break up the ore particles  104  into the smaller sized pieces that are necessary for the invention to perform its primary separation function. 
     After striking the anvil plate  74 , the ore is then channeled by the air flow within the primary grinding mill  18  through the reroute tubes  76  which take off from either side of the anvil area. The reroute tubes  76  direct the flow of ore particles  104  and air back into the center area of the mill interior  70  where, due to the airflow created by the rotation of the impeller plates  72 , they travel out towards the interior wall  78  of the primary grinding mill  18 . At this point, the ore particles  104  are again struck by the impeller blades  72 , which drives them into the interior wall  78 . The interior walls  78  are constructed of a hard face material that is formed in a rough and uneven manner so that when the ore particles  104  strike the walls  78  they bounce in a random fashion that promotes their further breakdown into the desired size. Due to the design of the primary grinding mill  18 , the ore particles  104  that enter it are impacted (by the impeller plates  72  and against the anvil plate  74  and the interior walls  78 ) a great number of times pulverizing them into a very fine powder-like substance prior to its exiting the primary grinding mill  18 . 
     Once the ore particles  104  have been fractured within the primary grinding mill  18 , they exit through the outlet opening  80  located at the top of the mill housing  88  and adjacent to the point at which the ore particles  104  enter the mill interior  70 . The outlet opening  80  is sized to the displacement of the primary grinding mill  18 , which may vary depending upon the mineral characteristics that are being ground. From the primary grinding mill  18 , the ore particles  104  are channeled to the other componets of the invention through the exhaust duct  90  which extends upward over the outlet opening  80 . 
     The manner of construction and the operation of the ore-roasting oven  108  component of the present invention are further detailed in FIGS. 5 and 6. The ore-roasting oven  106  is situated just down stream (in reference to the body of the invention) from the blower mill  30  which feeds the ore particles  104  and the air stream into the ore mixing chamber  32 . The ore mixing chamber  32  is positioned at the base of the burner frame  100  which is a relatively tall structure typically built of I-beams and which also provides the means of support for the ore-roasting oven  108 . The ore-roasting oven  108  channels the roasted ore particles  104  to the transfer tubes  44  which directs the flow into the primary quench chamber  38  where a coalesce reaction and further particle breakdown takes place. 
     The ore-roasting oven  108  is made up of the ore mixing chamber  32  and the four burners  106 . The ore particles  104  enter the ore-mixing chamber  32  from the blower mill  30  by means of the blower discharge duct  20 . Once the ore particles  104  enter the air mixing chamber  32 , it may be mixed with an additional supply of air which aids in the roasting process once the mixture reaches the roasting oven  108 . From the ore mixing chamber  32 , the ore particles  104  pass through the burners  106  which contain the propane injectors  112  which provide the fuel that produces the necessary heat to flash roast the ore  104 . Once the ore particles  104  have passed through the burners  106  and have been heated to the appropriate temperature, they pass into the body of the roasting oven  108  where they are forced through the transfer tubes  44  which transfers them to the primary quench chamber  38  as described above. Finally, the ore roasting oven  108  also has a screw conveyor to the blower motor  110  which allows for the transference of large ore particles  104  that may reach the oven  108  to the ore grinding cycle at the blower mill  30  which ensures that such particles will be properly processed by the present invention. 
     Although the present invention has been described in considerable detail with references to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.