Patent Publication Number: US-2011067525-A1

Title: Apparatus and methods for mercury and precious metal recovery

Description:
TECHNICAL FIELD 
     This invention relates to apparatus and methods for mercury and precious metal recovery. 
     BACKGROUND 
     Mercury contamination of soils and water bodies due to industrial waste dumping and mining activities is a serious environmental and health concern. For example, the ability of mercury to amalgamate with gold and silver has been exploited in placer mining operations to recover these precious metals from host materials. The crude devices used in placer mining operations lose much of the mercury to tailings and this has resulted in significant mercury contamination of all historic gold and silver mining sites on the planet. 
     Smaller fractions of precious metals are also lost to tailings in placer mining operations for a number of reasons. Pit run material is typically fed into a sluice with little classification and propelled from one end of the sluice to the other with vast quantities of water that wash fine particulate precious metals to tailings. Cleanup of the surfaces of the sluice where the precious metal is collected is typically infrequent but for the first one or two metres of sluice length that accounts for much of the loss of precious metals. Large quantities of magnetic black sand in sluice concentrates impede its separation from fine particulate precious metals. 
     Few efforts have been made to remediate mercury contaminated sites. Contaminated soils and mine tailings have occasionally been excavated and moved to designated remote sites. However, moving contaminated soils and tailings to remote sites is expensive and adds the environmental cost of transportation emissions. Merely moving the problem also does not solve the basic problem of safe storage of the mercury, nor addresses the possibility of recovering fine particulate precious metals from the soils and tailings that would offset some of the remediation costs. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to mercury recovery system. The mercury recovery system includes a granulometric separator for separating a feed by particle size; a centrifugal concentrator in flow communication with the granulometric separator for isolating and concentrating an elemental mercury-containing fraction of the feed; and an accumulator tank for collecting elemental mercury from the elemental mercury-containing fraction. The accumulator tank has an inlet in flow communication with the centrifugal concentrator; an outlet; a plurality of baffles defining a serpentine flow path to slow the flow of the elemental mercury-containing fraction from the inlet to the outlet; and a mercury accumulation area for collecting elemental mercury settling from the elemental mercury-containing fraction. 
     Another aspect of the present invention relates to a method of recovering mercury. The method of recovering mercury includes the steps of
     (a) granulometrically separating feed from a source;   (b) centrifuging a fraction of the granulometrically separated feed to obtain an elemental mercury fraction; and   (c) channeling the elemental mercury fraction through a plurality of baffles defining a serpentine flow path to slow the flow of the elementary mercury fraction to facilitate gravitational accumulation and recovery of elementary mercury.   

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In drawings which show non-limiting embodiments of the invention: 
         FIG. 1  is a left side view of a mercury and precious metal recovery system according to one embodiment of the present invention; 
         FIG. 2  is a right side view of the embodiment of shown in  FIG. 1 ; 
         FIG. 3  is a top view of the forward portion of the embodiment shown in  FIG. 1 ; 
         FIG. 4  is a top view of the rear portion of the embodiment shown in  FIG. 1 ; 
         FIG. 5  is a rear view of the embodiment shown in  FIG. 1 ; 
         FIG. 6  is a plan view of the embodiment shown in  FIG. 1 ; 
         FIG. 7  is a side cross-sectional view of a mercury accumulator tank according to one embodiment of the invention; 
         FIG. 8  is a side cross-sectional view of a mercury accumulator tank according to another embodiment of the invention; and 
         FIG. 9  is a flowchart of a mercury and precious metal recovery system according to another embodiment of the invention. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     The mercury and precious metal recovery system of the present invention involves granulometrically separating feed particles into at least two fractions of predetermined different sizes, and centrifuging feed particles below the predetermined size to obtain an elemental mercury fraction and a heavy metal concentrate fraction that includes free precious metal and precious metal amalgamated to mercury. The elementary mercury fraction is channeled through a baffled accumulator tank to slow its flow and facilitate gravitational settling and recovery of elementary mercury. An optional step of magnetically separating magnetic material after granulometric separation, and only processing the diamagnetic fraction at the centrifuging step may be included. While the following disclosure focuses on mercury and precious metals recovery from contaminated soils, the present invention may also be used for general purpose precious metal exploration and testing of both placer properties and feed streams in modern precious metal hard rock operations. 
       FIGS. 1 to 6  show a mercury and precious metal recovery system  10  according to an example embodiment of the invention. System  10  is a trailer-mounted pilot plant capable of processing an approximate volume of 5 metric tons per hour of dry or semi-dry feed. A person skilled in the art would appreciate that system  10  could be scaled up to a larger, higher throughput stationary plant in other embodiments of the invention. 
     System  10  is mounted on a trailer  2  for hitching to a truck or other suitable vehicle for ease of transportation between remediation sites. Trailer  2  has a built-in office/lab  3  for on-site analysis of results. While system  10  does not include a source of wash water, both placer and hard rock operations are inevitably close to water or have access to sources of water. 
     System  10  includes a bin  4  for receiving feed particles from a source (not shown) for remediation. Feed particles may be dry or semi-dry. Feed particles may be any finely crushed igneous, metamorphic, or sedimentary rock, including from mine tailings, industrial wastes, dredge spoils, alluvium and eluvium. 
     Below bin  4  and in gravity free flow communication for receiving feed particles from an outlet of bin  4  is a grizzly feeder  6 . The top part of grizzly feeder  6  has parallel bars spaced apart by about 7.5 cm. 
     The bars break up some oversize particles and prevent other oversize particles from passing through the bars. The parallel bars extend and slant downward toward the rear of trailer  2 . The slant of grizzly feeder  6  and the further flow of feed particles from bin  4  cause oversize feed particles which do not break up or pass through grizzly feeder  6  to slide off onto the ground or into a waste receptacle (or slide off onto first vibratory screen  8  then onto the ground or into a waste receptacle) at the rear of trailer  2 . 
     A first vibratory screen  8  below grizzly feeder  6  receives and screens feed particles which pass through grizzly feeder  6 . First vibratory screen  8  also slants downward toward the rear of trailer  2 . Feed particles which do not pass through first vibratory screen  8  therefore slide off onto the ground or into a waste receptacle at the rear of trailer  2 . The pore size of first vibratory screen  8  may, for example, be about 12 mm. First vibratory screen  8  is mounted by springs  12  onto frame  14 . Frame  14  is rigidly fixed to trailer  2 . First vibratory screen  8  is a rectangular deck-type screen in the illustrated embodiment, but may be any suitable known vibratory screen in alternative embodiments. 
     A hopper  16  below first vibratory screen  8  receives feed particles that pass through first vibratory screen  8 . Hopper  16  funnels the feed particles to an auger  18  (not shown in  FIG. 1  but shown in  FIG. 6 ) extending across the bottom of hopper  16  to conveyor  20 . In other embodiments the auger may be absent. Conveyor  20  is preferably textured to better grip the feed particles loaded on to it by auger  18 . Conveyor  20  inclines from auger  18  up to a first inlet  22  of a housing  24 . Housing  24  contains a second vibratory screen  26 . The pore size of second vibratory screen  26  may, for example, be about 6 mm. Second vibratory screen  26  is a 76 cm diameter circular high frequency screen in the illustrated embodiment. In other embodiments, the second vibratory screen may be other sizes, shapes and pore sizes suitable, for example, for obtaining fines. 
     System  10  may include a water pump (not shown) for pumping wash water from a nearby source to second vibratory screen  26 . Wash water may be provided at a rate of about 80 US gallons/minute, for example. In other embodiments the rate of wash water may be greater or less than 80 US gallons/minute. 
     Feed particles passing through second vibratory screen  26  are optionally output to a magnetic separator  30  in flow communication with an outlet of housing  24 . Magnetic separator  30  may be employed when the feed material is believed to contain magnetic particles. Magnetic separator  30  separates the magnetic fraction from the diamagnetic (i.e., non-magnetic) fraction. Magnetic separator  30  may be a high gauss magnetic wet drum. The magnetic fraction is discharged to storage or disposal. Magnetic separator  30  is in flow communication with a vertical tank pump  32  operative to pump the diamagnetic fraction to a centrifugal concentrator  34 . The inventors have discovered through testing that removal of the magnetic fraction resulted in higher downstream recovery of fine particulate gold, amalgam and elementary mercury. 
     Centrifugal concentrator  34  may be of the type generally described in Knelson (U.S. Pat. No. 5,368,541), the substance of which is incorporated herein by reference. Centrifugal concentrator  34  may be any type of centrifuge that exploits differences in specific gravity and liquid/solid properties to separate the diamagnetic fraction (or fraction passing through the second vibratory screen) into an elementary mercury fraction, a heavy solid fraction and a light solid fraction (i.e. effluent). The centrifugal force applied by centrifugal concentrator  34  may be 60 to 80 G, for example. Centrifugal concentrator  34  may be operated at a back pressure of 5 to 10 psi, for example. A water pump (not shown) may provide fluidisation water to centrifugal concentrator  34 . Fluidisation water may be provided at a rate of about 20 to 40 US gallons/minute, for example. The water pump may be the same pump providing wash water to second vibratory screen  26 . 
     During centrifugation, the elementary mercury fraction is collected in a liquid mercury launder and then in an accumulator tank. Using a simple accumulator tank has resulted in loss of elemental mercury in the outfall of the tank. In order to prevent such loss, at least a portion of the interior of the accumulator tank according to the invention provides an extended, serpentine flow path to provide more time for, and thereby enhance, gravitational settling of the elementary mercury from the elementary mercury-containing fraction. 
       FIG. 7  shows an accumulator tank according to one embodiment of the invention. An inlet of the accumulator tank is in fluid communication with and receives the liquid mercury fraction from the liquid mercury launder of the centrifugal concentrator. The accumulator tank may be integrally constructed with, or separately constructed from, the centrifugal concentrator. The accumulator tank may have a capacity, for example, of about 1.5 L. The accumulator tank includes one or more baffles to divert and slow the flow (shown by arrows) of elemental mercury-containing fraction through the accumulator tank to allow more time for the elemental mercury to settle at the bottom of the accumulator tank. The bottom of the accumulator tank is inclined downwardly toward a mercury drain/outlet valve so that the elementary mercury settles in a mercury accumulation area adjacent the valve. The settled elementary mercury is then recovered by discharge through the valve. Outfall water leaves the accumulator tank through the outlet. The illustrated embodiment has three baffles. At least some of the baffles may be attached to the lid of the mercury accumulator tank. The elemental mercury-containing fraction from centrifugal concentrator  34  is channeled under the first baffle, over the second baffle, and under the third baffle. Other embodiments may have a lesser or greater number of baffles in similar or different configurations. 
       FIG. 8  shows a Y-shaped accumulator tank according to another embodiment of the invention. The cross-sectional diameter of the tank may be 6 inches, for example. A first upper arm of the tank is in fluid communication with and receives, at an inlet at an end portion of the first upper arm, the elementary mercury-containing fraction from the liquid mercury launder of the centrifugal concentrator. A second upper arm of the accumulator tank comprises a plurality of baffles and, at an end portion of the second upper arm, an outfall water outlet. Elemental mercury settles at a mercury accumulation area at the bottom leg of the Y and is recovered by discharge through a mercury drain/outlet valve at an end portion of the bottom leg. 
     To further prevent elemental mercury loss, a water-filled safety tank (not shown) collects outfall water from the accumulator tank to recover any elemental mercury not collected by the mercury accumulator tank. For example, puddling or concentration of elemental mercury in spots in the ground being excavated for treatment may result in a surge of elemental mercury that does not completely settle in the mercury accumulator tank. The safety tank may have a capacity, for example, of about 200 L and may be positioned on the ground beside system  10 . The drop in elevation from the mercury accumulator tank to the safety tank, which would otherwise increase the velocity of the mercury-containing water (and inhibit settling of mercury), is offset by the large size of the safety tank. 
     The heavy solid fraction which accumulates in the heavy solid fraction launder of centrifugal concentrator  34  consists of gangue, free or liberated precious metal, and mercury amalgamated precious metal. Recovery of mercury and precious metal from this fraction may be performed using known processing methods such as tumbling, amalgamation, acidification, leaching, and zinc precipitation. 
     Example 
     Field tests were conducted on dried tailings of a former aggregate plant and deltaic bottom sediments of the Bear River, Calif. Test head materials were formed into a 50% slurry. The potable water used to make the slurry was analyzed for contaminants by BSK laboratories in Fresno, Calif. The slurried test head materials were divided into 18 containers. (Nine composite head samples were collected by isometric sampling of each of the 18 containers, placed into nine acid washed glass jars, packed in ice, and assayed within 24 hours for head sample mercury content by the United States Geological Survey (USGS) lab in Menlo Park, Calif., as described further below.) 
     The slurried head materials were granulometrically separated with wash water via one or more circular screens. Material passing through the screen(s) was directed to a centrifugal concentrator operating at a back pressure of 6 psi, fluidisation rate of 40 gpm, and spinning at 60-80 Gs. Magnetic separation was not performed prior to centrifugal concentration because the test material was not believed to contain any significant magnetic fraction. Heavy metals of specific gravity of ≧3 kg/cm 3  (namely the liquid mercury fraction (i.e., elemental mercury) and the heavy solid fraction (e.g. gold, gold/mercury amalgam)) were spun to the outer edges and jacket of the concentrator. The liquid mercury fraction was collected in a liquid mercury launder and then a Y-shaped mercury accumulator tank as described above. The elemental mercury recovered from the accumulator tank was collected and weighed. The gold (and any gold/mercury amalgam) was collected in a heavy solid fraction launder, flushed, and collected in a tote. 
     Effluent materials from the centrifugal concentrator with a specific gravity of &lt;3 kg/cm 3  (namely the light solid fraction) were immediately isometrically sampled three times at the outflow of the centrifugal concentrator and placed into acid washed glass containers as tail samples. In addition, three water quality samples of the effluent water were analyzed using the same water quality constituents as monthly water samples from the Bear River. The three tail samples and three water quality samples were placed on ice and assayed within 24 hours by the USGS labs in Menlo Park for mercury content, as described below. 
     The nine head samples were composted into three head samples. The resultant three head samples, and the three tail samples of the effluent, from each test were assayed for total mercury and methyl mercury using methods equivalent to United States Environmental Protection Agency (EPA) methods 1631 and 1630 for total mercury and methyl mercury, respectively. Reactive mercury analysis was conducted on the samples and was based on a 15-minute digestion with a strong reducing agent (SnCL 2 ). The bulk density of each sample was recorded so that percentage water content of the head material could be used in the mass balance equations to obtain a dry weight of the material processed. 
     Table 1 below sets out averaged results from two representative tests. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Elemental 
                   
                   
               
               
                   
                   
                 Elemental 
                 Hg 
                 Elemental 
               
               
                   
                 Test material 
                 Hg 
                 Recovered 
                 Hg 
                 Methyl Hg 
               
               
                 Test 
                 (kg, dry 
                 Recovered 
                 (ug/g, dry 
                 Recovered 
                 in Effluent 
               
               
                 No. 
                 weight) 
                 (g) 
                 weight) 
                 (%)* 
                 (%*) 
               
               
                   
               
             
            
               
                 1 
                 108 
                 0.3297 
                 3.06 
                 93.7 
                 6.3 
               
               
                 2 
                 288 
                 0.3154 
                 1.10 
                 93.5 
                 6.5 
               
               
                   
               
               
                 *Percentage calculated by dividing the amount of elemental Hg extracted or methyl Hg in the effluent by the amount of Hg associated with fine particles of silt and clay in the head samples according to the USGS assays. 
               
            
           
         
       
     
     As indicated in Table 1, approximately 94% of mercury in the test material was recovered in the form of elemental mercury extracted and collected in the mercury accumulator tank. The remaining mercury in the test material was found to be in the form of dissolved methyl mercury in the effluent from the centrifugal concentrator. 
     This invention may be modified in a number of ways within the spirit of the invention. For example:
         housing  24  may have an inlet for receiving wet feed or slurry from a dewatering centrifuge  28  as shown in  FIG. 6 , or from some other wet feed or slurry source (as in the Example), as an alternative to receiving dry or semi-dry feed from conveyor  20 ;   the first and second vibratory screens may be replaced by a single vibratory screen, or three or more vibratory screens;   the vibratory screen(s) may be substituted with any other suitable, known granulometric separation system; and   a mercury and precious metal recovery system  10  according to another embodiment of the present invention is shown in  FIG. 9 , which is similar to the embodiment shown in  FIGS. 1 to 6  but includes: a secondary concentrator for the heavy solid fraction; further processing of the heavy solid fraction; and recycling of water from the light solid fraction.       

     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.