Abstract:
In a process for beneficiating phosphate rock a slurry is provided having 30% to 70% by weight of a liquid phase and having a solid phase comprising clay, sand, and phosphate rock. In the process, the slurry is exposed to ultrasonic energy released from a sonotrode located within the slurry. The slurry may be exposed to the ultrasonic energy for less than 10 seconds. The ultrasonic energy may be produced by a piezoceramic transducer to have a resonance frequency within the range of from 16 kHz to 100 kHz. The ultrasonic energy may have an intensity within the range of from 0.0001 W/cm 3  to about 1000 W/cm 3 . The ultrasonic energy may create cavitational forces within the slurry. After exposure to ultrasonic energy, clay and sand are separated from the phosphate rock, perhaps using an air flotation process and a cycloning process.

Description:
This application is based on and claims priority to U.S. Provisional Application 60/620,721, filed Oct. 22, 2004, which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
     FIG. 1  is a schematic diagram outlining the treatment of phosphate ore after it is mined. The phosphate ore retrieved from the ground is in the form of “matrix” which includes phosphate pebbles sand and clay. After mining, the matrix is pumped in pumping station  3  from a dragline  1 . The matrix is pumped into washing equipment  5 , which produces pebble product, waste clay and small particles. The small particles are sent to sizing equipment  7 , and then subsequently to flotation equipment  9 . 
     FIG. 2  is a schematic diagram of the washing equipment shown in  FIG. 1 . From the pumping section  3 , the matrix is supplied to a receiving section  501 , which receives and decreases the velocity of the incoming matrix. The matrix is then sent to scalping screens  503  (“trommel screens”). The function of the scalping screens  503  is to scalp out particles that are greater than 1 inch in diameter (+1 inch). The particles that are smaller than 1 inch in diameter (−1 inch) go into a matrix tank  505 . The +1 inch material goes into a mudball slicer  507 . The mudballs enter the mudball slicer  507  in a relatively dry state and are sliced using high-pressure water. The water breaks up the mudballs, without necessarily creating a slurry. After breaking up the mudballs, the material is sent through another screen (not shown) to perform the +1 inch, −1 inch separation shown. The +1 inch particles form a waste stream. The −1 inch particles are sent to the matrix tank  505 . 
   In the matrix tank  505  water is added. A portion of the fine clay floats during this operation. The floating clay from the matrix tank is sent to de-sliming. The remaining particles are sent to log washers  509 . In the log washers, shafts with paddles thereon rotate in a tank causing the incoming material to be ground such that smaller clay particles are broken down. The incoming feed to the log washers is a slurry, perhaps containing 30% solids. These solids are particles having a diameter of less than 1 inch, and include phosphate particles, sand particles and clay particles. The log washers perform grinding and scrubbing on the incoming materials due to inter-particle friction caused by the movement of the paddles attached to the rotating shaft. 
   From the log washers  509 , the material is sent to screens  511 , which separate out a phosphate pebble product having a diameter larger than 1 mm. This phosphate pebble product is a phosphate concentrate that can be subsequently used without further processing. The particles smaller than 1 mm do not have a sufficiently high phosphate content for further processing. The particles less than 1 mm in diameter include sand and phosphate particles, which are about the same size and weight, thus making difficult other separation techniques. 
   These smaller particles are coated with clay and are sent to a de-sliming to remove clay.  FIG. 3  is a schematic view of the desliming process. In  FIG. 3 , hydrocyclones are used to separate finer and coarser particles. The finer particles exit over the top of the cyclone and contain clay. The finer particles are sent to waste clays. The coarser particles are considered clean feed. The coarser particles exit from the bottom of the cyclones and are sent to sizing. 
     FIG. 4  is a schematic view of a sizing process. In  FIG. 4 , the particles are sent to a series of sizers. The sizers include a fine sizer  701 , a coarse sizer  703  and an ultra coarse sizer  705 . From the various sizers, the particles are sent to separate storage tanks before being supplied to flotation  9 . The flotation process must run continuously, and one purpose of the three-storage tanks is to provide a buffer to compensate for any flow problems occurring before or during sizing. The fine, coarse and ultra coarse particles are collectively referred to as “feed” material. 
     FIG. 5  is a schematic view of the flotation process  9  shown in  FIG. 1 . After the feed is de-slimed and sized, the fine, coarse and ultra coarse particles are separately floated. After sizing, the particles are stored in water. The first step in flotation is to remove this water in a dewatering cyclone  901 . Water is removed such that the feed is perhaps 70% solids. In the dewatering cyclone  901 , fine clay particles exit as an overflow stream (not shown). Throughout the treatment process described above, clay removal is important because steps subsequent to dewatering employ chemicals, and the clay acts as a diluent for these chemicals. With less clay, smaller amounts of chemicals are required, thereby reducing operating costs. From the dewatering cyclone  901 , the particles are sent to a conditioning process  903 . During conditioning, reagents are added to the feed, which is substantially free from clay after the dewatering cyclone. The pH is increased, perhaps to about 9. For example, a 70% solution of soda ash may be used to increase the pH. Also during conditioning, a fatty acid/tall oil reagent is added. Due to the surface chemistry, the reagent coats the phosphate particles. The reagent does not coat the sand particles. After conditioning  903 , the coated particles are sent to a rougher flotation process  905 . 
   The coated phosphate particles are hydrophobic. In the rougher process  905 , air is bubbled through a flotation column or other flotation machine. The coated phosphate particles float to the top of the column or other flotation machine because of the incoming air. The phosphate particles, which float off the top of the column, are collected and sent to acid scrubbing  907 . The sand particles are not coated and do not float. The sand particles exit from the bottom of the rougher process  905 . 
   The hydrophobic phosphate particles, along with some fine sand particles, are sent to an acid scrubbing  907 , where an acid, such as sulfuric acid, removes the fatty acid/tall oil mixture coating the phosphate particles. After scrubbing, the particles are sent to a cleaner flotation process  911  where an amine solution is used. The amine solution causes the sand to float off the top of the column leaving behind the substantially clean phosphate concentrate product. 
   Although the foregoing process works well, there are many steps, and it is expensive to run. Various attempts have been made to improve the process. For example, Jacobs Engineering Group, “New Technology for Clay Removal,” Publication No. 02-138-177 (Florida Institute of Phosphate Research, 2001) proposed to use a vibrating ramp to separate mudballs. An ultrasonic generator caused vibrations in the ramp. However, there was no direct contact between the ultrasonic waves and the material. It was not possible to deliver enough energy to separate. 
   SUMMARY OF THE INVENTION 
   To address these and other concerns, the inventors propose a system that directly supplies ultrasonic energy to an impure phosphate medium. The ultrasonic energy can be supplied by placing an ultrasonic waveguide or sonotrode in direct contact with a slurry stream of phosphate material. 
   The inventors suggest that using high energy ultrasonic waves causes cavitation bubbles to be formed in the phosphate slurry. The ultrasonic waves are a series of compressions in rarefactions which occur thousands of times per second. The ultrasonic waves compress and expand water molecules in the slurry causing some of the water molecules to vaporize. These bubbles of water vapor, along with bubbles of entrained gases, such as air, are believed to grow to a size between 1 and 10 microns in diameter. With repeated compressions and rarefactions, the temperature in the bubbles is believed to approach 5000° C., and the pressure in the bubbles is believed to approach 2000 atmospheres. After this increase in energy, the bubbles collapse during a compression cycle releasing sheer energy waves. With inter-particle collisions and particle collisions with the conduit, the phosphate matrix breaks apart. Clay becomes dislodged from the phosphate particles. Unlike the Jacobs vibration system, particles can be effectively be broken apart. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a schematic diagram outlining the treatment of phosphate ore after it is mined; 
       FIG. 2  is a schematic diagram of washing equipment shown in  FIG. 1 ; 
       FIG. 3  is a schematic view of de-sliming equipment represented in  FIG. 1 ; 
       FIG. 4  is a schematic view of sizing equipment represented in  FIG. 1 ; 
       FIG. 5  is a schematic view of flotation equipment represented in  FIG. 1 ; 
       FIG. 6  is a side sectional view of an ultrasonic flow cell; 
       FIG. 7  is a partially removed side view of ultrasonic equipment within a slurry flow pipe; and 
       FIG. 8  is an end view of the equipment shown in  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
   Ultrasonic energy can be directly supplied to the  FIG. 1  system at many different places, as long as there is a slurry of phosphate material that can accommodate an ultrasonic waveguide therein. Where the ultrasonic energy is supplied depends on where the efficiency of the system can be most efficiently increased. 
   Although many locations are possible, there are several preferred locations for the ultrasonic equipment. First, the ultrasonic equipment may be used before the receiving section  501  (see  FIG. 2 ) of the washer  5 . As this location, the ultrasonic energy can be used to break up the matrix such that substantially all of the particles have a diameter less than 1 inch. In this case, the mudball slicer  507  may be unnecessary. 
   A second possible location for the ultrasonic equipment is in series with or instead of the mudball slicer  507 . The material coming from the scalping screens  503  would be slurried and sent through a conduit having one or more ultrasonic waveguides therein. After treatment with ultrasonic energy, screens which could be used to separate out any remaining particles having a diameter greater than 1 inch. Particles having a diameter of less than 1 inch would be sent to the matrix tank  505 . 
   A third possible location for the ultrasonic equipment is to enhance or replace the log washers  509 . The steam existing the matrix tank  505  is a slurry. One or more ultrasonic waveguides can be placed in the conduit carrying this slurry to break apart the particles and detach clay from the phosphates. If the ultrasonic equipment sufficiently treats the slurry from the matrix tank  505 , the log washers  509  could be eliminated. Otherwise, the log washers  509  could be used in series with the ultrasonic equipment. 
   A fourth possible location for the ultrasonic equipment is before the flotation equipment  9 . The ultrasonic equipment may be placed between the dewatering cyclone  901  (see  FIG. 5 ) and the storage tanks for the fine, coarse and ultra coarse particles. At this location, the ultrasonic equipment would remove clay from the particles, thereby decreasing the amounts of chemicals required for conditioning  903 , acid scrubbing  907  and cleaner flotation  911 . The clay, which is separated from the phosphate particles by this ultrasonic equipment, would exit the dewatering cyclone as an overflow stream. This clay would not be supplied to the conditioning process  903 . 
     FIG. 6  is a side sectional view of an ultrasonic flow cell. The ultrasonic flow cell is a device that delivers ultrasonic energy to a slurry of fine particles. As such, the ultrasonic flow cell can be used at the fourth location, before flotation  9 . At this point in the process, the particles have diameter of less than one millimeter. The flow cell would be connected in the pumping line between the fine, coarse or ultra coarse storage tank and the dewatering section  901 . Reference numeral  601  represents an inlet from a storage tank. The slurry is passed upwardly through an inner sleeve past an ultrasonic waveguide (or “sonotrode”)  603 . At the top of the flow cell, the slurry changes direction around an inter chamber wall  605 . The slurry flows downwardly to an outlet  607 . 
   A casing, comprising an outer wall  609 , the inner wall  605 , the inlet  601  and the outlet  607 , may be formed of a single piece of material or from different sections. The casing can be constructed of stainless steel, which has good reflective properties. With stainless steel, the energy waves within the flow cell are reflected back into the slurry rather than being absorbed. Other materials, such as plastic and glass, may also be used. However, plastic may absorb a substantial portion of the energy waves. Both plastic and glass may not be robust enough to withstand the processing of the sand and clay in the phosphate feed over an extending period of time. 
   There are two passes through the flow cell, an upward pass and a downward pass. The two passes increase the residence time. The downward pass also controls the flow to reduce turbulence at the top of the cell. The downward pass allows an even distribution of ultrasonic waves throughout the medium. Most of the separation is achieved in the first, inner pass, where the slurry is in direct contact with the sonotrode  603 . 
   The sonotrode  603  can have various configurations. Ultrasonic waves are emitted from all parts of the sonotrode, including the bottom tip. The classic radial sonotrode emits ultrasonic waves radially outwards through the surrounding conduit. The sonotrode can be made of titanium, stainless steel, aluminum, hastalloy (chemical resistant), a niobium alloy (heat resistant) or any other suitable material. Titanium is a preferred material for the sonotrode. 
   Outside of the casing is the remainder of the ultrasonic equipment. The sonotrode  603  is the only part of the ultrasonic equipment that interacts with the slurry. A generator  611  (for power supply and power control), a piezo ceramic transducer  613  and a booster  615  supply ultrasonic vibration to the sonotrode  603 . AC current is supplied to the transducer  613  from the generator  611 . The generator may receive a 480 volt input signal and produce a 60 hertz AC current. In the transducer  613 , piezo ceramic crystals are supplied with the AC current. The AC current changes the polarity of the crystals, causing expansion and contraction, thus producing an ultrasonic vibration which is amplified by the sonotrode  603 . The transducer  613  is connected to the sonotrode  603  through an anti-vibrational flange  617 , which limits energy lost via vibration from the flow cell to the other equipment. 
   The booster  615  amplifies/intensifies the ultrasonic waves or reduces the amplitude of the waves. The amplitude of the waves should correspond to the length of the sonotrode  603 . If the amplitude is too high, then decoupling occurs, which limits the energy transferred to the slurry medium. The booster controls the amplification thereby controlling the amount of energy released from the sonotrode. 
   The main resonance frequency is in part determined by the vibration frequency of the piezo ceramic crystals. The resonance frequency can vary between 16 kilohertz to 100 kilohertz. A 20 kilohertz frequency has been used with success. Changes in temperature and pressure within the system cause changes in the frequency. Therefore, the system must be monitored to track the resonance frequency in order to operate at maximum output power. Otherwise, the efficiency could drop significantly. The piezo ceramic transducer scans 2 kilohertz on either side of the main resonance frequency, for a total bandwidth of approximately 4 kilohertz. The wavelength of the ultrasonic signal is directly proportional to the length of the sonotrode  603 . 
     FIG. 7  is a partially removed side view of ultrasonic equipment within a slurry flow pipe.  FIG. 8  is an end view of the equipment shown in  FIG. 7 . When the particles are larger, they may not easily flow through the flow cell shown in  FIG. 6 . In this case, the ultrasonic equipment may instead be added to a pipe such that the ultrasonic waveguide  803  extends perpendicular to the direction of flow, instead of parallel to the direction of flow, as shown in  FIG. 6 . The embodiment shown in  FIGS. 7 and 8  may be used for the first through third locations of the ultrasonic equipment. 
   The pipe  805  shown in  FIGS. 7 and 8  may be an existing pipe within the processing facility. For example, the pipe  805  may be a 20 inch pipe between the scalping screens  503  and the mudball slicer  507 . Pipe  805  may carry a slurry of “matrix” from the field to the plant. A hole can be drilled in the existing pipe  805  to insert the sonotrode  803 . The anti-vibrational flange is mounted in the hole. The electrical equipment, including the booster  615 , the piezo ceramic transducer  613  and the AC generator will remain outside of the pipe. 
   It is important that the power delivered to the slurry be sufficient to separate the material. The power is rated based on the cross-sectional area of the conduit and/or based on the throughput volume. To increase the power, the signal to the sonotrode  803  can be amplified. If sufficient power cannot be obtained using a single sonotrode  803 , additional sonotrodes can be used. The additional sonotrodes can be separated circumferentially around the pipe and/or separated through the length of the pipe. The United Kingdom Patent Application No. 9825349.5, filed on Nov. 20, 1998, which is hereby incorporated by reference, describes various configurations for the sonotrodes. 
   It should be apparent that the sonotrode  803  shown in  FIGS. 7 and 8  has a different configuration from the sonotrode  603  shown in  FIG. 6 . Various sonotrode configurations are possible. The sonotrode  803  shown in  FIGS. 7 and 8  has teeth, which increase the surface area and the intensity of the ultrasonic waves. The teeth also alter the flow through the pipe, creating a vortex recirculation effect. This increases the residence time of the medium in the vicinity of the sonotrode  803 . The teeth further create turbulence in the medium allowing for increased inter-particle collisions and particles collisions with the sonotrode  803 . 
   The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.