Patent Publication Number: US-7591946-B2

Title: Dual zone feedwell for a thickener

Description:
FIELD OF THE INVENTION 
   The present invention relates to separation devices for liquid suspensions and pulps and in particular to a feedwell for use in such devices. It has been developed primarily for use in thickeners and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use. 
   BACKGROUND OF THE INVENTION 
   The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should not be construed as an admission that such art is widely known or forms part of common general knowledge in the field. 
   Known separation devices of this type typically include a thickening tank and a feedwell disposed generally within the tank to contain feed slurry. The feedwell is comprised of a single chamber having an inlet for receiving feed slurry and at least one restricted outlet for directing the slurry into the thickening tank. The feedwell is configured to reduce the turbulence of the incoming feed stream, to allow reagents to be mixed into the slurry, and to allow for reaction between reagents and the feed material before the treated slurry is discharged into the thickening tank. The configuration of the feedwell also promotes even distribution of slurry flowing from its outlet into the thickening tank. 
   Known extraction devices do not optimise the mixing and distribution of liquid entering the thickening tank from the feedwell outlet. They suffer from back-mixing of liquid from the thickening tank into the feedwell as well as short-circuiting of liquid flow from the feedwell itself. Another shortcoming, in some circumstances, is that insufficient residence time is provided to allow for optimum reaction of flocculants and reagents. Similarly, the residence time is often insufficient to allow for staged addition of reagents, requiring additional upstream tanks to be added, in turn increasing cost and space requirements. 
   It is an object of the invention to overcome or ameliorate one or, more of the deficiencies of the prior art, or at least to provide a useful alternative. 
   DISCLOSURE OF THE INVENTION 
   According to a first aspect of the invention, there is provided a feedwell for a separation device, said feedwell including at least two adjacent upstream and downstream substantially concentric chambers, said chambers being in sequential unidirectional fluid flow relationship such that fluid entering said feedwell flows sequentially from said upstream chamber into said downstream chamber for subsequent discharge into the separation device. 
   Preferably, the inlet to the downstream chamber is located centrally above the floor of the upstream chamber. 
   In a particularly preferred form, the upstream chamber is cylindrical and the downstream chamber is concentrically frusto-conical with the sidewalls of the downstream chamber diverging downwardly and outwardly from its inlet. 
   Preferably also, the feedwell includes a concentrically nested deflection cone forming the floor of the downstream chamber and defining an annular outlet from the is downstream chamber. More preferably, the annular outlet has an area substantially less than the mean cross-sectional area of the downstream chamber partially to restrict the flow of liquid from the feedwell. 
   Preferably, the upstream chamber includes at least one agitator for mixing flocculant into the liquid suspension. Preferably, the separation device is a thickener. 
   According to a second aspect of the invention, there is provided a separation device for a liquid suspension including a feedwell as defined above having at least two adjacent upstream and downstream substantially concentric chambers, said feedwell being centrally located upstream of a third chamber defining a thickening tank, said three chambers being in sequential unidirectional fluid flow relationship. 
   In a preferred form, the third chamber includes a circumferential overflow launder for decanting excess overflow liquor from the thickening tank to a discharge pipe. 
   Preferably, a rake assembly is centrally rotatably mounted within the thickening tank. Preferably, the rake assembly includes a plurality of radially extending rake arms  each fixedly connected at one end to a central hub. More preferably, a plurality of rake blades is fixedly connected to each arm with each blade extending axially into the tank such that as the rake rotates, the blades sweep through the fluid in the tank. 
   According to a third aspect, the invention provides a method for separating liquid suspensions or pulps in a thickener, said method including the steps of: 
   providing a thickening tank adapted to facilitate settling; 
   providing a feedwell having an upstream chamber and a downstream chamber, disposed such that fluid entering said feedwell flows sequentially from said upstream chamber into said downstream chamber and then into said thickening tank; 
   directing a liquid suspension or pulp into said upstream chamber; 
   allowing said liquid suspension or pulp to flow from said upstream chamber into said downstream chamber; and 
   allowing said liquid suspension or pulp to flow from said downstream chamber into said thickening tank to facilitate separation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
       FIG. 1  is a plan view of a thickener for a liquid suspension according to the invention; 
       FIG. 2 , is an offset sectional view of the thickener, taken on line  2 - 2  of  FIG. 1 ; and 
       FIG. 3  is a pictorial perspective view of a feedwell according to the invention. 
   

   PREFERRED EMBODIMENTS OF THE INVENTION 
   A preferred application of the invention is in the fields of mineral processing, separation and extraction, whereby finely ground ore is suspended as pulp in a suitable liquid medium such as water at a consistency which permits flow, and settlement in quiescent conditions. The pulp is precipitated from the suspension by a combination of chemical and mechanical processes. Initially, coagulant is added and mixed into the suspension in a first chamber, followed by the addition of flocculant in a second chamber. The suspension is then carefully mixed to facilitate the clumping together of solid particles, eventually forming larger denser particles that are precipitated out of suspension. 
   Referring to the drawings, the extraction device  1  includes two adjacent upstream and downstream concentric chambers  2  and  3  defining a feedwell  4 . The feedwell is centrally located upstream of a third chamber in the form of a thickening tank  5 . All three chambers are in sequential unidirectional fluid flow relationship such that feed liquid flows firstly from chamber  2  to chamber  3  and then to the tank  5 . 
   In this embodiment, the first chamber  2  is right-cylindrical in shape and includes two agitators  6  for mixing reagents into the liquid suspension and for preventing solids from settling onto the floor of the chamber. Chamber  3  is located centrally within chamber  2  and is frusto-conical in shape, with its sidewall  7  diverging downwardly and outwardly from inlet  8 . The inlet is located centrally above the floor  9  of chamber  2 . 
   A deflection cone  10  is located internally of chamber  3  and forms its floor. The space between the chamber sidewall  7  and the deflection cone  10  defines a restricted annular outlet  11  from chamber  3  into the tank  5 . This outlet  11  has an area substantially less than the mean cross-sectional area of chamber  3  to restrict the flow of liquid between feedwell  4  and tank  5 , thereby promoting smooth, non-turbulent flow and homogenous distribution of liquid within the tank. 
   The tank  5  is substantially right-cylindrical and in this embodiment is formed of reinforced concrete. The tank includes a downwardly sloping flusto-conical floor  12  for directing precipitated pulp towards an underflow withdrawal pipe.  13 . The tank also includes an inwardly directed circumferential overflow launder  14  for decanting dilute overflow liquor from the thickening tank into a discharge pipe  15 . 
   A rake  16  is centrally rotatably mounted within the tank  5 . The rake includes a plurality of radially extending rake arms  17 , each supporting a plurality of fixedly connected rake blades  18 . The blades extend axially into the tank such that as the rake  16  rotates, the blades sweep through the fluid in the tank. This sweeping assists in the release of water from the slurry in the bed, thereby increasing its density. The rake is mounted on a centrally located drive-shaft  19  driven by a motor  20  and an associated  gearbox (not shown). 
   In use, the fluid level  21  of tank  5  is located above the level of outlet  11  from chamber  3 . The feed liquid enters chamber  2  through an inlet  22  and reagents are added according to a predetermined dosage rate. The volume of slurry in chamber  2  gradually increases, thereby providing sufficient residence time for reaction of the reagents until they overflow into the inner chamber  3 . The slurry entering chamber  3  may be dosed with a different reagent to that used in chamber  2 , or it may be dosed with more of the same reagent, thus allowing for staged addition. It then flows downwardly under gravity towards the deflection cone  10 , which gradually disperses the liquid radially outwardly through the outlet  11  into the surrounding tank. 
   In the tank, the precipitate in the incoming feed stream settles to form a relatively dense bed of thickened pulp  23  that displaces an upper layer of relatively dilute liquid  24  towards the top of the tank. The thickened pulp  23  is drawn off from below through the underflow withdrawal pipe  13 , while the dilute liquor is progressively drawn off through the overflow launder. 
   In another feedwell embodiment (not shown), the first chamber is frusto-conical, with its sidewalls diverging radially inwardly from its floor to an outlet at its peak. This first chamber is located centrally within a second right-cylindrical chamber and fluid leaves this second chamber through an annular outlet defined by the space between the respective chamber walls. 
   It will be appreciated that the illustrated device provides improved mixing and distribution of fluid entering the thickening tank from the feedwell outlet. The device also reduces the incidence of back-mixing of fluid from the thickening tank into the feedwell, allows for staged addition of reagents, as well as reducing short-circuiting of fluid flow from the feedwell itself. The concentric configuration of the device also reduces the space that it occupies within the tank. In all these respects, the invention represents a practical and commercially significant improvement over the prior art. 
   Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.