Abstract:
An apparatus for separating a mineral from a slurry of mineral and impurities, including a fluid vessel having a first, open end and a second end and a feed well disposed near the first end. The feed well has a first, open end, for receiving the slurry, and a second end. At least one first member is received through the first ends of the vessel and the feed well for providing aerated water, creating a froth in the feed well including substantially the mineral. The mineral froth substantially separates from the impurities and floats out of the feed well towards the first end of the vessel, and a collection unit receives the mineral froth. The impurities and any remaining mineral fall toward the second end of the vessel. A measurement unit is placed within the vessel for measuring at least one of density and pressure of the fluid in the vessel. A related process includes introducing the slurry into the first, open end of the feed well, providing aerated water to the feed well and the vessel in a direction from the first ends to the second ends, respectively, creating a froth in the feed well including substantially the mineral, substantially separating the mineral froth from the impurities, collecting the mineral froth, and allowing the impurities and any un-separated mineral to fall towards the second end of the vessel. Further, the process includes measuring at least one of density and pressure of the fluid in the vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Provisional Patent Application No. 60/582,862, filed on Jun. 28, 2004, in the U.S. Patent Trademark Office, under the same title as above, and of Provisional Patent Application No. 60/583,606, filed on Jun. 30, 2004, also under the same title, the disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is directed to the recovery of minerals such as phosphate and, more particularly, to a column flotation cell and related method to enhance phosphate recovery. 
   2. Description of the Related Art 
   Presently, phosphate is recovered from a sand-clay mixture that is mined from mineral deposits. Traditionally, phosphate is mixed with a collector, suspended in water and urged to the surface of an aeration tank called a flotation cell. 
   For example, as shown in  FIG. 1  herein, U.S. Pat. No. 4,735,709 describes a flotation system  10  with means for introducing a gaseous medium such as air, to facilitate flotation. The system  10  generally includes a flotation vessel  12 , and two air sparger systems  14  and  16  for introducing a gaseous medium or air into the vessel  12 . 
   The vessel  12  is formed as an upright elongated cylinder having a vertical wall  18  and a bottom wall  20 . The vessel  12  is typically open at an upper end  22 . A substantially horizontally-disposed constriction plate  24  is located within the vessel  12 , spaced above the bottom wall  20 , to separate the vessel  12  into a flotation compartment  26  above the constriction plate  24  and a distribution compartment  28  below the constriction plate  24 . The constriction plate  24  has a plurality of orifices  30  to permit passage of aerated water from the distribution compartment  28  to the flotation compartment  26 . 
   A feed well  32  is supported within the upper end  22  of the flotation compartment  26  by a base  33 . A feed tube  34  from an external source of aqueous slurry (not shown) delivers a controlled quantity of the aqueous slurry to the feed well  32 . The feed well  32  has an overflow baffle  36  to distribute the aqueous slurry throughout the flotation compartment  26 . The feed well of this conventional design is located under the water level  11  in the vessel  12 . 
   Air bubbles are introduced into the bottom of the fluid vessel  12  by flowing aerated water through the air sparger  14  and into a manifold  38  exiting into the vessel  12  via the orifices  30 . The air bubbles aerate the slurry in the vessel  12 . 
   The slurry entering vessel  12  contains phosphate, impurities and a collector. The use and types of collectors are well known in the art. An example of a typical collector used in the art is a hydrocarbon such as tall oil. See, e.g., U.S. Pat. No. 6,178,383. The phosphate suspended in the aqueous slurry adheres to the rising air bubbles and collects at the upper end of the flotation compartment  26  as a froth. 
   A launder  44  is provided at the upper end  22  of the vessel  12 , atop the cylinder wall  18 . The launder  44  generally includes a circular inner wall  45 , a relatively higher outer wall  47  and a bottom wall  49  that form a trough  51  to receive the froth, which overflows from the flotation compartment  26 . The froth overflows into the trough  51  when the froth inside the flotation compartment  26  rises and spills over the top of the lower circular inner wall  45 . An outlet  46  is provided in the outer wall  47 , near the bottom wall  49 , to convey the overflowing phosphate-laden froth from the launder  44  to further processing or storage. 
   The impurities including sand and clay contained within the slurry along with any residual phosphate that is not captured by the levitating air bubbles percolates downwardly through the aqueous slurry by gravity. An opening  48  is formed through the center of the constriction plate  24  into which the impurities pass through. An outlet  50  extends from the opening  48  through the bottom wall  20  of the cylinder  12 . The outlet  50  allows removal of the impurities from the vessel  12 . 
   The orifices  30  can “choke” over a period of time because the velocity of the air bubbles moving through the orifices  30  is not high enough to prevent the downwardly percolating impurities including sand from plugging the orifices  30 . The result of the choking is that aerated water will not be able to enter and circulate through the vessel  12 , which results in poor separation of the phosphate from the impurities. 
   Further, the system  10  generally exists an as alkaline environment, which can allow algae growth. Algae growth is promoted near the orifices  30  because the levitating air bubbles create low-turbulence areas near the orifices  30 . Algae will attach and grow at these low-turbulence areas such that over time the orifices  30  will get sealed off, preventing the even dispersion of aerated water throughout the vessel  12 . 
   U.S. Pat. No. 4,735,709 also describes the use of a separate air sparger system  16  that discharges aerated water above the constriction plate  24  into the vessel  12  via orifices  41  in pipes  40 . However, these orifices  41  can also choke over a period of time as the impurities from the slurry percolate downward in the vessel  12  for the same reasons as described above. 
   The choking of the orifices  30  and  41  can not only prevent the even aeration of the vessel  12 , but also require maintenance involving the cleaning or redrilling of the orifices  30  and  41  in order to un-choke or un-plug them. The phosphate separation process, therefore, has to be suspended for maintenance and cannot be carried on as a continuous process. A continual need for maintenance introduces down time and maintenance costs into the separation process, which results in reduced recovery of phosphate and high cost of operation. 
   As also known in the art, the above-described column flotation cell can require significant capital expenditures to build, depending upon the size, component parts, etc. The system is also known to require a significant amount of energy to thrust aerated water from the bottom to the top of the column flotation cell. 
   Thus, although the prior art described above has generally been widely used for the purposes of recovering minerals such as phosphate from impurities, it still does not disclose or teach a column flotation cell and a method of use that reduces capital costs, lowers energy consumption, prevents substantial choking of the column and allows easier maintenance. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an aspect of the present invention to provide an apparatus and process for separating minerals such as phosphate from impurities using a column flotation cell that substantially eliminates choking. 
   It is another aspect of the present invention to provide an apparatus and process for recovering phosphate using a column flotation cell that is easier to maintain. 
   It is another aspect of the present invention to provide an apparatus and process for recovering phosphate using a column flotation cell that utilizes a cell density control process wherein the density within the column is monitored to regulate the discharge of impurities from the bottom of the column influenced by the amount of incoming slurry. 
   It is also an aspect of the present invention to provide an apparatus and process for recovering phosphate using a column flotation cell receiving aerated water via down pipes to substantially eliminate choking and improve dispersion of air into the cell. 
   It is a further aspect of the present invention to provide an apparatus and process for enhanced recovery of phosphate that uses less energy per yield. 
   Finally, it is an aspect of the present invention to provide an apparatus for recovering phosphate having a substantially compact design to reduce capital cost for installation and maintenance. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a perspective, partial cross-sectional view of a prior art column floatation cell. 
       FIG. 2  is a schematic view of a separation system according to an embodiment of the present invention, including a pair of interconnected column floatation cells. 
       FIG. 3  is a side, elevational view of a column used with the column flotation cell of the present invention. 
       FIG. 4  is a left side, elevational view of a launder used with the column flotation cell of the present invention. 
       FIG. 5  is a front, elevational view of the launder shown in  FIG. 4 . 
       FIG. 6  is a side, elevational view of one column flotation cell according to an embodiment of the present invention. 
       FIG. 7  is a top plan view of a column flotation cell according to an embodiment of the present invention. 
       FIG. 8  is a side cross-sectional view of a feed well according to an embodiment of the present invention. 
       FIG. 9  is a schematic view of the cell density control process according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Various embodiments of the present invention will now be described with reference to the drawings. In this description, certain dimensions are used to assist in understanding the structure of the invention. Of course, one of ordinary skill in the art may vary the dimensions without departing from the invention. As a result, it is not intended that the invention be limited by any particular dimensions. 
     FIG. 2  is a schematic view of a separation system  60  according to an embodiment of the present invention. The system  60  includes generally a pair of interconnected column floatation cells, i.e., a primary cell  62  and a secondary cell  64 , each having similar constructions, as described below. 
   The two cells  62  and  64  are placed at different heights and are interconnected for a staged separation of a feed slurry having phosphate, impurities such as sand and clay, and one or more known collectors such as tall oil. The incoming feed slurry is referred to herein as “A”. 
   Although in this embodiment two cells are described and shown, other numbers of cells, including one, may be used. Use of multiple staged cells achieves a higher phosphate recovery from A. 
   Into the primary cell  62  there extends a feed tube  66  extending from a conventional slurry source  68 . An outlet  108  is formed at a bottom  102 , described below, of the primary cell  62  and is connected to a pipe  72 , which leads to another feed tube  74 . 
   A water line  70  having a pair of eductors  92  supplies aerated water and one or more known frothers such as polyglycol into the cells  62  and  64 , respectively. Eductors  92  are aspirators that introduce air into the water coming through the water line  70 . Valves  96  control the amount of water passing to the eductors  92 . Air is introduced into the water entering the eductors  92  via valves  98  to generate aerated water. Therefore, both the amount of water and air passing through the eductors  92  are adjustable. 
   Down pipes  90  are connected to the eductors  92  and thrust the aerated water into the primary and secondary cells  62  and  64 . The down pipes  90  are preferably clamped to a baffle  160  located near the top of each cell, as described below, and terminate near the bottom  102  of both cells  62  and  64  (see  FIG. 6 ). Down pipes  90  are also preferably clamped to sides  144  of feed wells  140  in each of the cells  62  and  64 . The down pipes  90  terminate near the bottom  142  of the feed wells  140  within both cells  62  and  64  (see  FIG. 8 ). 
   The feed slurry A entering cell  62  via feed tube  66  pours into the feed well  140 , where the separation process begins. Phosphate froth is generated due to the air bubbles rising in the feed well  140  as described below. In the first step of separation, the feed well  140  separates much of the impurities contained in A from the phosphate to create phosphate froth substantially free from impurities. This phosphate froth is referred to herein as “C” and the separated impurities are referred to herein as “B”. 
   C is thrust upward out of the feed well  140  and into a column  100  of cell  62  as the phosphate froth rises towards a top  146  of the feed well  140  continuing to a top  106  of the column  100  (see  FIGS. 6 and 8 ). The rise of the phosphate froth C is promoted by the thrust of the rising air bubbles and the hydrophobicity of the collector in the phosphate froth C. The hydrophobicity of the phosphate froth C renders it amenable to flotation by attachment to the rising air bubbles. The impurities B along with any un-separated phosphate fall toward the bottom  102  of the column  100 . The separation process is completed in the column  100  where rising air bubbles from the bottom  102  of the column  100  generate an upward thrust of air, referred to herein as “air hold-up”. C then spills out of the top  106  of the column  100  into the launder  120  as the top  106  of the column  100  is lower than a top  124  of the launder  120 . (See discussion below regarding  FIG. 6 ). 
   The launder  120  of the primary, upper cell  62  has an outlet  128 , which draws off the separated phosphate froth C and sends it via an outflow pipe  80  for further processing or storage, as in the prior art described above. 
   The impurities B and the remaining un-separated phosphates at the bottom  102  of the column  100 , exit cell  62  via the outlet  108 . A pinch valve  94  controls movement of the impurities B and the remaining un-separated phosphates through the outlet  108 . The column  100  of the secondary, lower cell  64  also has an outlet  108  controlled by a pinch valve  94 . The feed tube  74  introduces the impurities B and the remaining un-separated phosphates, herein referred to as A′, into the feed well  140  of the secondary cell  64  for staged separation. Therefore, A′ collected from cell  62  is separated again in cell  64  allowing for improved recovery of phosphate. 
   The outlet  128  of the launder  120  of the lower, secondary cell  64  allows removal of the separated phosphate froth C via an outflow pipe  82  in a manner similar to that described above for further processing or storage. The outlet  108  on the secondary cell  64  allows the impurities B such as sand and clay particles that accumulate along with any unrecovered phosphate near the bottom  102  of the column  100  to be drawn off via a pipe  79 . These impurities B and any unrecovered phosphate can either be disposed of or directed to another staged cell for further processing in order to recover additional phosphate. 
     FIG. 3  illustrates in greater detail the column  100  of the column flotation cells  62  and  64 . A central axis is shown by “Y”. The column  100  includes the solid bottom  102  and a substantially columnar sidewall  104 , which is open at the top  106 . The diameter of the sidewall  104  increases from the bottom  102  to the top  106  of the column  100 . Due to the differences in diameter, there are formed a lower columnar portion  110 , a middle beveled portion  112 , and an upper columnar portion  114 . The bottom  102  includes the outlet  108 , which allows the removal of A′ from the bottom  102  of the column  100  in cell  62 . 
   In a preferred embodiment of the present invention, the height of the column  100  is about 6 feet. The external diameter at the top  106  of the column  100  is preferably about 10 feet. Both the external diameter towards the top of the column and the height of the column  100  may be in the range of about 3-40 feet. The ratio of the external diameter of the top  106  of the column  100  to the height of the column  100  is in the range of about 0.5-2.0. More preferably the ratio is in the range of about 0.6-1.33. Therefore, unlike conventional columns that have a height that is much greater than the width, the present invention can use the reverse configuration, i.e., a height that is less than the diameter of the column. This configuration allows the system  60  of the present invention to be more compact, requiring less energy to thrust water from the bottom  102  of the column  100  via down pipes  90 . 
     FIG. 4  is a left side elevational view, and  FIG. 5  is a front elevational view, of the launder  120  used with the cells  62  and  64 . The launder  120 , like the column  100 , is generally cylindrical, having a bottom  122 , a top  124  and a sidewall  126 . The bottom  122  is angled relative to the top  124 . 
   The diameter of the sidewall  126  increases from the bottom  122  to the top  124  of the launder  120 . Due to this difference in diameter, there is formed a lower angled portion  132 , a middle beveled portion  134 , and an upper columnar portion  136 . A trough  138  ( FIGS. 6 and 7 ) is formed between the upper portions of the column  100  and launder  120  for collecting the phosphate froth C. The lower angled portion  132  is angled (side view  FIG. 4 ) and curved ( FIG. 5 ) toward the outlet  128  to allow the collected phosphate froth C to run therealong via gravity, as in the prior art launder  44  described above. The pipe  128  extends from the sidewall  126  near the bottom  122  of the launder  120  to allow removal of the phosphate froth C from the launder. 
     FIG. 6  shows the primary cell  62 , which is exemplary of the structure of the cell  64 , as noted above. The cell  62  generally includes the column  100 , the launder  120 , the feed well  140 , plates  150 , support pipes  154 , and baffles  160 ,  170 , and  188 . Each of these components will now be described in greater detail. 
   The column  100  receives the launder  120  towards the top  106  of the column  100  and is preferably welded to the launder  120 . The top  106  of the column  100  is lower than the top  124  of the launder  120  (see  FIGS. 2 and 6 ). Due to this difference in height, the phosphate froth C can run over the top  106  of the sidewall  104  of the column  100  and into the launder  120 , i.e., into the trough  138  formed between the column  100  and the launder  120 , where the phosphate froth C moves by gravity to the bottom  122  of the launder  120 , to be removed via the outlet  128 . 
   The angling of the launder bottom  122  (see  FIG. 4 ) aids in the movement of the phosphate froth C by gravity. That is, the phosphate froth is relatively tacky due to its hydrophobicity, which causes water to be “shed” from the phosphate froth frustrating any natural flow of the material. The sloped bottom of the launder  120  substantially eliminates this problem and allows for the collection of the phosphate froth near the pipe  128 . 
   The feed well  140  receives the incoming slurry A via feed tube  66 . The rate of the incoming slurry is controlled by a regulating device  118  such as a variable speed pump, a conveyor belt or a pinch valve. The feed well  140  of the present invention discharges over the top  146 . In addition, the feed well  140  of the present invention is at an adjustable height below the operating water line  148 . (See  FIG. 8 ). 
   The feed well  140 , having a bottom wall  142 , a sidewall  144 , and an open top  146  is disposed towards the center of the column  100 . The feed well  140  is secured by using four equally radially-spaced plates  150  thereon with holes to allow bolts  152  to attach to the support pipes  154 . The plates  150  allow the placement of the feed well  140  at an adjustable height within the column  100  with respect to the support pipes  154 . 
   Down pipes  90  are introduced into the column  100  and the feed well  140  for the purpose of introducing aerated water into the cells  62  and  64  and the feed well  140 . (See  FIGS. 2 ,  6  and  8 ). These down pipes  90  enter the column  100  and the feed well  140  from the top and extend about 2-3 pipe diameters or approximately 3 inches from the bottom  102  of the column  100  and the bottom  142  of the feed well  140 , respectively. 
   According to a preferred embodiment of the present invention, five down pipes  90  are introduced directly into the column  100 . The down pipes  90  are fixedly attached by clamping (not shown) to the baffles  160  of the column  100 . Extension rods  162  extend from the bottom of the down pipes  90  attaching to horizontally disposed metal discs  190  that rest on top of a wear plate  158 . The wear plate  158  rests on the bottom  102  of the column  100 . 
   Down pipes  90 , e.g., two, can also be introduced directly into the feed well  140 . These down pipes  90  are clamped (not shown) to the sides  144  of the feed well  140  and rest on a wear plate  159  at the bottom  142  of the feed well  140 , in a similar manner as described above. See  FIG. 8 . 
   The wear plates  158  and  159  at the bottom  102  of the column  100  and at the bottom  142  of the feed well  140 , respectively, protect the bottoms  102  and  142  from excessive wear. For example, without the plate  158 , the bottom  102  of the column  100  would have to be replaced or consistently maintained due to excessive abrasion caused by large quantities of sand and other abrasive impurities from the slurry moving across the bottom  102  of the column  100 . 
   The wear plates  158  and  159  may be made of any material such as reinforced steel. Cladded wear plates may also be used as they provide the abrasion resistance that approaches ceramics. The wear plates  158  and  159 , therefore, serve the function of both support (of the down pipes  90 ) and protection of the bottoms  102  and  142 . 
   It is preferred that the down pipes  90  be fixedly attached, e.g., clamped, to the baffles  160  in the column  100  in order to ensure even dispersion of the incoming aerated water throughout the entire column  100  and the feed well  140  via the down pipes  90 , by substantially reducing any movement of the down pipes  90  due to turbulence. 
   The down pipes  90  are also spaced such that maximum distribution of aerated water is allowed in the feed well  140  and the column  100 . Generally, maximum distribution of aerated water may be achieved by the equidistant spatial placement of all down pipes  90  in the column  100  and the feed well  140 . 
   The feed well baffle  188  aids in the separation of impurities such as sand from the phosphate which spills out of the feed well  140  into the column  100 . The feed well baffle  188  is attached to the feed well  140  such that as the impurities B spill out of the feed well  140 , B moves across the feed well baffle  188  before spilling into the column  100 . The feed well baffle  188  redirects the flow and constricts the flow area reducing turbulence in column  100 . This constricting enhances coalescence of the un-separated phosphate in B and acts to separate any suspended impurities flowing with the rising air bubbles. The separated impurities fall downwardly and settle at the bottom  102  of the column  100 . 
   As shown in  FIG. 7 , four equally radially-spaced braces  182  extend between a pipe support  180  (for receiving the feed tube  66 ) and the first baffle  160 , four equally radially-spaced braces  184  extend between the first baffle  160  and the second baffle  170 , and four equally radially-spaced braces  186  extend between the second baffle  170  and the top  106  of the sidewall  104  of the column  100 . 
   With particular reference to  FIG. 7  there is seen, from the inside to the outside, the following components: the pipe support  180 , the feed well  140 , the feed well baffle  188 , the first baffle  160 , the second baffle  170 , the top  106  of the sidewall  104  of the column  100  and finally the top  124  of the sidewall  126  of the launder  120 . 
   Baffles  160  and  170  aid in the further reduction of turbulence in column  100  and separation of impurities B from the phosphate C. C then spills out of the top  106  of the column  100  into the launder  120  for collection. It is possible to have one or more baffles depending on the dimensions of the column. 
   In order to maintain the column  100  level, a level control  164  is used. A level control  164  ensures that the phosphate froth C spilling into the launder  120  spills out evenly, reducing the spilling of any of the other contents of the column  100  into the launder  120 . The level control  164  may be any conventional level control device and may be clamped to the top  106  of the column  100 . (See  FIGS. 5 and 6 ). 
   A computerized process controller or programmable logic controller, e.g. an Allen Bradley model AB Micrologic 1000 PLC control interface  200 , herein referred to as “PLC”, is used for a density control process  210 , described below. (See  FIGS. 6 and 9 ). The PLC  200  is coupled to a bubble tube  130 , the pinch valve  94  and the regulating device  118 . 
     FIG. 8  is a side cross-sectional view of the feed well  140 . Two down pipes  90  are introduced directly into the feed well  140 . See also  FIGS. 2 and 6 . The down pipes  90  extending into the feed well may be of smaller diameter compared to the down pipes  90  introduced directly into the column  100 . 
   The shape of the feed well  140  is shown to be generally columnar. (See  FIGS. 2 ,  6 , and  8 ). However, in alternate embodiments, the feed well  140  may be cubical, or conical as in U.S. Pat. No. 4,735,709, or of another shape. The shape of the feed well  140  does not have a significant impact on the separation process so long as the down pipes  90  entering the feed well allow for sufficient aeration of the incoming slurry. 
   In regard to operation of the separation system  60 , reference is made particularly to  FIGS. 2 ,  6 , and  8 . Slurry A is fed from the slurry source  68  into the feed well  140 . The incoming slurry A contains premixed collector such as tall oil, as discussed above. 
   The down pipes  90  introduce aerated water into the feed well  140  aerating the feed slurry A. The column  100  is filled with aerated water via the down pipes  90 . Feed slurry A filling the feed well  140  and begins frothing as the air bubbles released from the aerated water move from the bottom  142  of the feed well  140  in an upward flow to the top  146  of the feed well  140 . The aeration of A produces phosphate froth C. This upward flow of the air bubbles provides the air holdup. The air holdup carries the attached hydrophobic phosphate particles C to the top  146  of the feed well  140  and to the top  106  of the column  100 . 
   The impurities B including sand spill out from the feed well  140  over the feed well baffle  188  and baffles  160  and  170 . As discussed above, the feed well baffle  188  functions to substantially reduce the turbulence from the impurities and un-separated phosphate flowing outward from the feed well  140  to yield substantially impurity-free phosphate froth C. Substantially most of the separation of the phosphate from the impurities such as sand and clay occurs in the feed well  140 . 
   The remainder of the separation is completed inside the column  100 , which also receives down pipes  90  that introduce aerated water at the bottom  102  of the column  100 . C spills out of the top  106  of the column  100  due to the air holdup mentioned above to be received by the trough  138  formed between the column and the launder  120 , where C moves by gravity to the bottom  122  of the launder  120 , to be removed, when desired, via the outlet  128  of the launder  120  of the primary cell  62 . It is possible to use additional baffles towards the top of the column to further aid in removing any remaining impurities from C. The impurities separated from the phosphate froth C collect at the bottom  102  of the column  100  to form a sand bed “W” containing clay and other separated impurities as well. (See  FIG. 6 ). The bottom of sand bed W is a plug or choked condition which can be controlled by varying the number and the spacing of the down pipes  90  in the column  100 . The number of down pipes  90  that may be placed within the column  100  is directly related to the average particle size of the settling impurities forming the sand bed W at the bottom  102  of the column  100 . 
   The down pipes  90  provide the air holdup mentioned above along with even dispersion of air throughout the column  100  and the feed well  140 . The down pipes  90  can accomplish these important functions without choking. This is possible because aerated water is thrust down the down pipes  90 . The aerated water discharges at the bottom  102  of the column  100  inside the sand bed W or at the bottom  142  of the feed well  140 . 
   The aerated water releases the air bubbles within the sand bed W. The sand bed W breaks up the air bubbles into a multitude of small air bubbles that rise through the sand bed substantially uniformly. The sand bed W also slows down the turbulence caused by the aerated water being released via the down pipes  90  at the bottom  102  of the column  100 . As the aerated water is introduced into the bottom  102  of the column  100  via the down pipes  90 , the air bubbles naturally pass through the sand bed by taking the path of least resistance. No constriction plate is used or required, unlike the prior art discussed above. Since, there is no such plate with orifices used in the present invention, there is no choking. 
   It is possible for the down pipes  90  to choke, however, this problem can be avoided by maintaining a high pressure of aerated water being thrust into the down pipes  90  by adjusting the position of the eductors  92  with respect to the bottom  102  of the column  100  (See  FIG. 6 ). Aerated water exiting at high pressure from the bottom of the down pipes  90  will prevent any possibilities of choking of the down pipes  90  because of the pressure differential created at the bottom of the down pipes  90  where the aerated water is released. Therefore, the incoming aerated water via the down pipes  90  has to have sufficient air pressure in order to overcome the weight of the sand bed W. 
   The air bubbles being released at the bottom of a column of water, as in the prior art ( FIG. 1 ), achieve maximum rise velocity within 2-3 feet. As the air bubbles move towards the top of a water column formed inside the column  100 , they get larger and larger because the pressure inside the column decreases towards the top  106  of the column  100 . This expansion and rise velocity creates some turbulence or a slight back eddy, which is a low-pressure zone behind these rising air bubbles. The impurities B, including fine particles of sand within the column, actually ‘tail gate’ this low-pressure zone behind the air bubbles. The result, therefore, is that it is possible to obtain a cleaner phosphate froth C at the top of the column  106  by hindering the rise of the air bubbles. 
   As the separation process moves forward, it is possible to have five virtual zones, shown in  FIG. 6 , within the column  100 , based on the type of materials found in each zone separated by physical and chemical properties. 
   Zone “R” is found towards the bottom of the column and is the zone containing settling impurities B such as sand and clay forming the sand bed W. Zone “R” is the zone where aerated water from the down pipes  90  enters the bottom  102  of the column  100 . Since, the sand bed acts as a dispersion plate by breaking up the air bubbles being released from the aerated water into smaller air bubbles, and also evenly dispersing the air bubbles throughout the column  100 , it is important to have a substantial quantity of sand in Zone “R” to ensure the even dispersion of the air bubbles throughout the column  100 . 
   The next zone is Zone “S”, which is located above Zone “R”. Zone “S” is a substantially homogeneous slurry zone. Zone “S” contains a higher percentage of un-separated phosphate particles. There is sufficient stability to allow for density measurements within Zone “S”, as discussed below. 
   The next zone is Zone “T”, which is located above Zone “S”. Zone “T” is the contact zone, where the rising air bubbles within the column  100  interact with B spilled over from the top  146  of the feed well  140 . Zone “T” is generally located above the top  146  of the feed well  140 . Zone “T” generally is a higher density zone compared to zones “U” and “V”, discussed below. Zone “T” contains lighter components of impurities B such as clay particles, sticks, etc. that were separated from the incoming feed slurry A. The bottom of Zone “T” is where the air bubbles exit the sand bed W. The air bubbles rise rapidly in this zone but are hindered in their upward movement through the column because of the settling sand from the top  106  of the column  100 . This hindrance is more prominent towards the bottom Zone “T” where the air bubbles are first being released. The middle through the top of Zone “T” is therefore fairly turbulent. However, this provides a good environment to increase the contact probability of any un-separated hydrophobic phosphate particles to an air bubble to rise upward to become part of the phosphate froth C. 
   Zone “U” is located above Zone “T” and is the next zone moving towards the top  106  of the column  100 . Zone “U” primarily contains clear water, rising air bubbles, and some phosphate froth rising upwards by the help of the air holdup discussed above. 
   Finally, there is Zone “V”, which is located above Zone “U” and is found on the very top  106  of the column  100 . Zone “V” primarily contains phosphate froth C substantially free from impurities, spilling out the top  106  of the column  100  into the launder  120 . 
   As the slurry A continues pouring into the feed well  140  via feed tube  66 , the environment within the cell  62  keeps changing depending on the quantity and quality of the feed slurry A being introduced. Over a period of time, the cell  62  may tend to have one or a combination of the following problems due to the fluctuations in the feed slurry A: too much sand is deposited in the sand bed, too much or too little slurry feed A enters the cell  62 , the particle size of the impurities including sand is too small increasing the total surface area of the impurities within the column  100  requiring more water for effective separation, etc. If one or a combination of these problems occur, the separation process will have to be suspended until the contents within the cell can reach equilibrium to continue with effective separation of the phosphate froth from the impurities. 
   In order to avoid down time and to avoid the suspension of the separation process, the density control process  210  can be used, as shown in  FIG. 9 . The PLC  200  is coupled to the pinch valve  94  and regulating device  118 . The PLC  200  is also coupled to the bubble tube  130  to measure the density of the contents within the column  100 . Any suitable density measuring device may be used instead of the bubble tube  130 . 
   In a preferred embodiment, as shown in  FIG. 9 , the density control process  210  entails measuring the density of the contents within the column towards the top of Zone “S” for the reasons described above by using the bubble tube  130 . One or multiple density measurements may be taken so long as the density is measured in a zone or zones where stable measurements can be taken, e.g., the bottom or middle of Zone “S”. 
   The PLC  200  reads the density measurement by the bubble tube  130  and compares it to a stored manually adjustable set point  202  inputted into the PLC. Then based on the comparison between the density measurement and the set point  202 , the PLC  200  controls the opening or closing of the pinch valve  94  and the regulating device  118 . The pinch valve  94  controls the flow of the impurities exiting the cell  62  via outlet  108 . The regulating device  118  controls the flow of the incoming slurry via feed tube  66  within the cell  62 . The density control process  210  repeats continuously maintaining the density within the cell  62  at the set point  202 . 
   For example, when the slurry feed A comes into the cell  62 , the density of the cell  62  automatically starts to increase. If the PLC  200  detects that the measured density is greater than the set point  202 , the PLC  200  controls the opening of the pinch valve  94  allowing the discharge of the impurities including the sand from the bottom  102  of the column  100 . The PLC  200  also controls the regulating device  118  to suspend the entry of the incoming slurry feed A. The PLC  200  thus controls the opening and closing of the pinch valve  94  and the regulating device  118  to lower the density within the cell  62  to the set point  202 . 
   Similarly, if the PLC  200  detects that the density inside the cell  62  has fallen below the set point  202 , the PLC  200  controls the regulating device  118  to allow entry of the incoming slurry feed A. The PLC  200  will also control the closing of the pinch valve  94  to suspend the discharge of the impurities from the bottom  102  of the column  100 , allowing the density of the cell  62  to increase to the set point  202 . The set point may be adjusted depending on temperature, pressure, type of slurry feed being introduced, etc., by manually adjusting the set point  202  in the PLC  200 . The density control process  210  therefore takes into account the overall environment of the cell  62 . 
   The cell  62  is no longer dependent on the manual control of the incoming feed via feed tube  66  based on visual monitoring of the changes within the cell  62 . In fact, the density control process  210  by the PLC  200  reacts to changes within the cell  62  and keeps the recovery rate of phosphate substantially stable. In a preferred embodiment of the present invention, the set point  202  can be preset in the PLC  200  in the range of about 1-1.5 specific gravity of the contents within the cell  62  as compared to the specific gravity of water. 
   Although it is preferred to measure density towards the top of the Zone “S” as discussed above, it is possible to measure density at various locations within the column  100  so long as stable measurements can be made. For example, another measurement of density could be taken towards to top of Zone “U” for obtaining a differential density measurement. 
   The advantages of the density control process include but are not limited to maintaining equilibrium within the cell  62 , allowing a continuous separation process, eliminating the need for regular maintenance of the cell  62 , and, most importantly, increasing phosphate recovery and efficiency by minimizing fluctuations within the cell  62  caused by the type of incoming slurry feed A, temperature, pressure etc. 
   As an alternative, the same process control can be achieved by measuring a pressure differential in any two stable zone. Based on the comparison between pressure differential and the set point pressure differential, the pinch valve  94  and regulating device  118  can be controlled in the same manner, and achieve the same advantages, as described above. 
   As can be seen from above, the present invention provides an apparatus and a process for recovering phosphate using a column flotation cell that substantially reduces the requirement for regular maintenance of the cell  62  because of the use of down pipes that do not choke and the density control process described above. 
   The invention also prevents substantial choking of constriction plates because no such plates are required for the dispersion of air throughout the column. The down pipes  90  do not choke due to sufficiently pressurized aerated water that thrusts out of the bottom of the down pipes  90 . Further, the sand bed in Zone “R” eliminates the need for a constriction plate to disperse air. 
   This invention also allows agitation of feed and water in the bottom of the column which promotes better dispersion of air into the cell than the prior art apparatus because the sand bed in Zone “R” naturally breaks down air bubbles into a multitude of smaller air bubbles, while at the same time dispersing the air bubbles evenly as they rise towards the top of the column. 
   Because the invention utilizes cell density control or pressure differential control there is improved recovery of the phosphate. 
   Further, this columnar floatation cell is characterized by a substantially compact design, relative to the prior art, which reduces capital cost for installation and reduces maintenance costs: smaller columns are easier and cheaper to install and are easier to operate and maintain. 
   Although the above description provides an example of recovering phosphate from impurities, other minerals may be separated from impurities using the same apparatus and method as described above. 
   The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.