Patent Publication Number: US-2010116091-A1

Title: System and Method for Leaching a Metal from a Base Mineral Rock

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to systems and methods employed in the mining industry to extract certain metals from their original natural mineral state and, more specifically, to systems and methods that employ a leaching process. 
     In nature, the leaching of metals from a mineral state occurs over time through the action of water, air, temperature and other environmental forces. In the mining industry, this natural process is accelerated to reduce the time required to obtain the metal and increase the amount of metal recovered. Leaching operations, therefore, involve the use of chemicals to extract the metal from the mineral. Depending on such factors as the type of mineral, the metal to be extracted, the concentration of the metal in the mineral, the presence of other metals, and the chemicals used, the leaching process might take as little as a few hours or might take days and even months. 
     Leaching operations in the gold mining industry take place in large open tanks into which a slurry is pumped that contains the mineral along with a cyanide solution and reagents. The slurry is then agitated by propeller blades for the amount of time required for 90 to 95% of the gold contained within the mineral to leach into the slurry. The amount of time that the slurry is retained and agitated in the tank varies depending on such factors as the type of mineral, the granularity of the ground mineral, the chemicals used and the type of agitation employed. On average, however, the amount of time required using the current technology takes an average of 12 hours and the extraction of gold in the best of cases only reaches 90 to 95% of that available in the mineral. A need exists, therefore, for a system and method that reduces the processing time yet increases the yield. 
     SUMMARY OF THE INVENTION 
     A system and method for leaching gold, silver or other metals from a base mineral rock includes a closed reactor vessel into which a slurry containing the ground base mineral rock, a reagent, and an attacking agent is fed. The rock is preferably ground so that at least 90% of the rock is under 80 mesh in size. The attacking agent may have a concentration in the range of 3 to 10 kg/m3 of attacking solution and may be a cyanide solution. 
     The interior space of the vessel is then pressurized via an oxygen supply to a pressure P. The slurry—which may be in a range of 20% to 50% solids by weight and have a pH in the range of 10.5 to 11.5—is then agitated under pressure P and within the closed reactor vessel for a time “t”. The temperature of the slurry may be in the range of 20° to 30° C. 
     Agitation preferably occurs by way of re-circulating the oxygen within the vessel and through the slurry. The agitator preferably includes at least one blower or compressor and re-circulating piping in communication with the slurry. Jets may also be employed to enhance the aeration effects within the slurry. 
     The required pressure P and time “t” are effective for obtaining a metal recovery of up to 98% of the total metal available in the base mineral rock. Tests demonstrate that a time “t” in a range of 8 to 14 minutes and a pressure P of 10 to 50 psi is effective for obtaining the desired recovery amount (depending on the concentration of attacking agent). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a pressure leaching reactor configured for use in a batch processing operation. A slurry containing a ground base mineral rock, an attacking agent and a reagent is fed into a closed pressure vessel. The vessel is then closed and oxygen is introduced to pressurize the vessel. While under pressure, the slurry is constantly agitated by a blower or compressor that re-circulates the oxygen and passes the oxygen through the slurry. 
         FIG. 2  is a view of the pressure leaching reactor configured for use in a continuous processing operation. The slurry is agitated under pressure by blowers or compressors that re-circulate the oxygen through the slurry via jets. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments will now be described in reference to the drawings and the following element numbers: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 10 
                 System for leaching metal 
               
               
                 12 
                 Slurry source/tank 
               
               
                 14 
                 Oxygen source 
               
               
                 20 
                 Reactor 
               
               
                 22 
                 Upper portion 
               
               
                 24 
                 Top end 
               
               
                 26 
                 Lower portion 
               
               
                 27 
                 Baffles 
               
               
                 28 
                 Bottom end 
               
               
                 30 
                 Slurry inlet 
               
               
                 32 
                 Oxygen inlet 
               
               
                 34 
                 Pressure gauge 
               
               
                 36 
                 Outlet 
               
               
                 38 
                 Discharge nozzle 
               
               
                 40 
                 Sight glass 
               
               
                 42 
                 Level float 
               
               
                 50 
                 Piping arrangement 
               
               
                 52 
                 Slurry pump 
               
               
                 60 
                 Agitator 
               
               
                 62 
                 Blower or compressor 
               
               
                 64 
                 Re-circulating piping 
               
               
                 66 
                 Inlet end 
               
               
                 68 
                 Outlet end 
               
               
                 70 
                 Jet 
               
               
                   
               
            
           
         
       
     
     A system and method for leaching a metal from a base mineral rock employs an attacking agent mixed with the ground base mineral rock and oxygen that re-circulates through and agitates this slurry mix while the slurry is under a pressure above ambient pressure. Referring now to  FIG. 1 , a system  10  adapted for batch processing includes a reactor  20 , a piping arrangement  50  and an agitator  60 . Reactor  20  is preferably a vertical pressure vessel having a tubular upper portion  22  with top end  24  and a conical-shaped lower portion  26  with bottom end  28 . A slurry inlet  30  with valve and an oxygen inlet  32  with valve and optional pressure gauge  34  are located on upper portion  32 . Each valve on inlet  30  and  32  may be manually or automatically controlled. Oxygen inlet  32  is in communication with an oxygen source  14  that provides pure or substantially pure oxygen to reactor  20 . An outlet  36  with valve is located at bottom end  28 . 
     Piping arrangement  50  interconnects reactor  20  to a slurry source  12  and provides a means for transporting the processed slurry to downstream storage or processes (not shown) for further processing or analysis. The slurry entering reactor  20  contains a ground base mineral rock, water, an attacking agent and a reagent in the quantities and proportions required to obtain a slurry of pumping consistency. In the case of gold leaching, the slurry contains the ground base mineral, water, sodium cyanide, and calcium hydroxide. The proportion of the ingredients will vary accordingly to the type of mineral or the requirements of the operation. In a preferred embodiment, the slurry is about 40% solids by weight. In other preferred embodiments, the concentration of the slurry is in a range of 20% to 50% solids by weight. 
     The slurry is fed into reactor  20  through slurry inlet  30  until a desired volume of slurry is contained within the reactor  20 . The volume and level of slurry fed into reactor  20  may be controlled manually and monitored by a sight glass  40  or controlled and monitored automatically via control means such as a level control float valve  42  (see  FIG. 2 ). Slurry inlet  34  is then closed and oxygen inlet  36  is opened. Pure oxygen or substantially pure oxygen is introduced into reactor  20  until a predetermined pressure P is obtained. In a preferred embodiment, the pressure P is about 40 psi. In other preferred embodiments, pressure P is in a range of 20 to 50 psi. 
     Agitator  60  is preferably a forced air system that includes a blower or compressor  62  located at top end  24  and a re-circulating piping  64  having an inlet end  66  in communication with the blower or compressor  62  and an outlet end  68  in communication with a bottom portion of the slurry. Agitator  60  re-circulates the oxygen, taking the oxygen residing above the slurry level and forcing it up through the slurry. By continuously re-circulating the oxygen, agitator  60  continuously agitates the slurry for a required amount of residence time. The oxygen supply  14  is preferably maintained at the required pressure P to replenish any oxygen consumed. 
     When the required amount of time is accomplished, blower or compressor  62  is stopped and any oxygen remaining in the reactor  20  may be removed to storage (not shown) or discarded to atmosphere. When the pressure P in reactor  20  is substantially equal to atmospheric pressure, outlet  36  may be opened to empty reactor  20 . The processed slurry may be transported via piping arrangement  60  for storage, further treatment or analysis. 
     Referring now to  FIG. 2 , an alternate embodiment of system  10  includes a reactor  20  adapted for continuous processing. Reactor  20  is preferably a horizontal pressure vessel and may include baffles  27  to isolate portions of the slurry and ensure proper agitation of each portion. Piping arrangement  50  includes a slurry feed pump  52  in communication with slurry inlet  30 . Jets  70  may be located at the outlet end  68  of the re-circulating piping  64 . 
     Similar to the batch operation (see  FIG. 1 ), the ground base rock mineral, the reagents and the cyanide solution are prepared in the required proportions to obtain a slurry of pumping consistency. The slurry is pumped to reactor  20  at the required rate and at the required pressure while pure or substantially pure oxygen is inserted into reactor  20  until the pressure P reaches the required pressure. Blower or compressor  62  re-circulates the oxygen to maintain the slurry in agitation during the required time. The oxygen supply should be kept at the required pressure to replenish any oxygen consumed. The required residence time of the slurry in reactor  20  may be controlled by a discharge nozzle  38  at outlet  36 . 
     A person of ordinary skill in the art would recognize that reactor  20  may be of different types and sizes to accommodate a batch operation, like in a pilot plant or laboratory, or for a continuous operation, like in a commercial treatment plant. Regardless of type or size, reactor  20  must have the capability to keep the slurry mixture under pressure and in an agitated state for the required time to allow for the grade of dilution of the metal required by the process. 
     Tests using system  10  and carried out by the inventor on ores from or near Zaruma, Ecuador, achieved recoveries of over 98% with leaching times ranging from 8 to 14 minutes, depending on the cyanide concentration and oxygen pressure. These results were consistent over the 100 tests carried out by the inventor under the conditions described below. Tests carried out by the inventor on this same type of ore using open air tanks and conventional extraction technology demonstrated that, on average, it took 16 hours to obtain at 90% extraction result. 
     The conditions under which the tests using system  10  were conducted are as follows:
         Test quantity: . . . 14 kg of solids per sample   Material grind size: . . . 90% under 80 mesh   Cyanide concentration . . . Varied from 3 kg/m 3  to 10 kg/m 3  of attacking solution   pH of the slurry . . . 10.5 to 11.5   Pressure in the reactor: . . . Varied from 10 psi to 50 psi   Slurry consistency: . . . Varied from 20% to 50% solids by weight   Oxygen used . . . Industrial type   Duration of tests: . . . From 5 minutes to 120 minutes of pressure agitation   Temperature of slurry . . . Ambient, ranging from 20° to 30° C.
 
Leaching time was found to be proportional to cyanide concentration and oxygen pressure. Lab analysis of the clear liquid from the slurry was made using an atomic absorption spectrometer, with lab analysis of the solids being made by fire assay.
       

     Due to the prototype construction of reactor  20  for use in the above tests, any remaining oxygen was discarded to atmosphere. The consumption of oxygen, therefore, will need to be tested at future. The inventor plans to carry out further tests to find the more efficient parameters of operation for various ore characteristics. A person of ordinary skill in the art would recognize that different types of ores may require, for example, a different attacking agent or reagent, different concentrations of the various ingredients, and different reside times and pressures. 
     The tests using system  10  suggest that system  10  significantly reduces the leaching time and significantly increases the yield (and extraction rate) in comparison to current leaching technologies. System  10  permits a smaller footprint and reduced power consumption of a leaching section in an extraction plant, allows for a coarser grind than the 200 mesh commonly used in milling operations, and reduces the flow time through and stocks of gold in the extraction plant at any given time. For example, because of the reduced residence time and increased extraction rate, gold could be at the foundry within hours rather than days of leaving the mill. Furthermore, the reduced time to leach the gold does not permit leaching of the copper typically present in the ore. Copper tends to cause problems, such as fouling activated carbon, further downstream in the gold recovery process. Last, the density of the processed slurry does not affect the recuperation of the gold. 
     While a system and method for leaching metal has been described with a certain degree of particularity, many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. It is understood that the system and method is not limited to the embodiments set forth herein for purposes of exemplification, but is to be limited only by the scope of the attached claims, including the full range of equivalency to which each element thereof is entitled.