AU2007213629A1

AU2007213629A1 - A rake arm supporting mechanism for a thickener

Info

Publication number: AU2007213629A1
Application number: AU2007213629A
Authority: AU
Inventors: Richard Triglavcanin
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2006-02-10
Filing date: 2007-02-09
Publication date: 2007-08-16
Also published as: PE20071123A1; FI20060124A0; WO2007090924A1; FI118209B; CA2638062A1; ZA200806346B

Description

WO 2007/090924 PCT/FI2007/000031 1 A RAKE ARM SUPPORTING MECHANISM FOR A THICKENER This invention relates to an apparatus for clarifying and/or thickening a solids containing solution. Furthermore the invention relates to a rake arm supporting mechanism for a thickener. The clarifying and thickening apparatus comprises a tank, into which the slurry to be clarified is fed via a feedwell, which is located in the centre of the clarification tank, generally in its upper section. When the term clarification is used later in the specification of the invention, it is used however to refer also to thickening, since the methods and apparatus described can usually be applied for both purposes. Thickeners are known as a means of separating suspended pulps. They are commonly used in the field of mineral processing and extraction, as well as other industries such as paper production. The diameter of the clarification tank could be ten times larger than that of the feedwell. In addition to the slurry a flocculant used in clarification is fed into the feedwell. The flocculant is often mixed into the slurry while it is being fed into the feedwell. The clarification or thickening tank may be equipped with a rake operating near the bottom, or function without one. The rake is mounted on a centrally located drive shaft, driven by a motor and an associated gearbox. The clarified liquid, or overflow, is removed from the clarification tank as an overflow and the thickened slurry, or underflow, is removed from the central part of the tank bottom, to where it has been moved by the rake arms. The bottom of a thickening tank is usually inclined, i.e. sloping down towards the centre, whereby the removal of solids is made easier. The number of arms in the rake can be varied depending on the design solution. Some of the rake arms may be the length of the tank radius, others may be shorter. Vertical plates are fixed to the rake arms, and they move the slurry from the edges towards the centre. The plates extend axially into' the tank such that as the rake rotates, the plates sweep through the fluid in the tank. This sweeping shears the pulp particles, WO 2007/090924 PCT/FI2007/000031 2 releasing trapped water, which rises to the surface through the sheared material, allowing the solids to settle into a more dense bed of thickened pulp. This process is referred to in the art as "dewatering". The sweeping also rakes the settled solids to the centre of the tank from where they are discharged. Plates are fixed either perpendicularly to the rake arm or for example at an angle of 30 degrees to it. In contrast to its intended purpose, however, this rake in practice tends to push the whole or part of the settled material around with it, thus preventing it from performing its functions of assisting in the dewatering process and raking the settled solids to the central discharge point. This condition is referred to in the art as "donutting" and impedes the formation of the desired bed of relatively uniform, thickened pulp. Usually donuts start from the centre and grow outwards. That is when a pulp stabilisation mechanism is needed for preventing the rotating of the thickened pulp. It is near the drive shaft and in the centre of tank where pulp stabilisation mechanism is required the most. To be most effective the pulp stabilisation mechanism also needs to be at maximum depth into the bed to have maximum contact with the pulp bed. These problems bring challenges to the designing of the rake mechanism. Usually there are arranged struts from the drive shaft to the half way point of the rake arm in order to give vertical support between the drive shaft and the rake arms of the rake. In order to prevent "donutting" there is needed space around the drive shaft. When using the pulp stabilisation mechanism for the pulp donuts in the thickener there should be space near the drive shaft, meaning that the rake arms should be supported without struts or with small struts near the drive shaft. According to the US 4751002 the support of the rake arms in a thickener is implemented so that the sets of separate support cables and separate torque cables extend between rotating strusses and rake arms. Cable torque has pivoting rake arms and the cable supports are attached to some support member. According to this invention it is difficult to know position / angle of the WO 2007/090924 PCT/FI2007/000031 3 arms. Some bed slump issues can also occur as higher angle bed is achieved when rake arm is tilted upwards. The purpose of the present invention is to provide a new kind of apparatus for clarifying and/or thickening a solids-containing solution and to introduce a new way to support rake arms and a rake of a thickener. Furthermore the object is to provide a more effective way to support the rake mechanism, which leaves the space for pulp stabilization mechanism around the drive shaft and could be fitted into thickeners having a small floor slope. The essential features of the invention become evident from the appended claims. The invention relates to an apparatus for clarifying and/or thickening a solids containing solution, whereby the apparatus comprises a clarification and/or settling tank, which is equipped with a feedwell for the slurry to be settled, a discharge launder for the clarified solution or overflow, and discharge ports for removing the thickened slurry and a rake rotating on its drive shaft, which is for moving the underflow from the edges of the tank to the centre, where said rake has at least four rake arms reaching the edge of the tank in a radial direction, when at least two of the rake arms are vertically supported by at least one vertical supporting member arranged on the rake arm, which vertical supporting member is connected to at least two rake arms, so that the rake arm slope 22 could be as small as possible, at least 1:4 or smaller. There are many advantages for the invention. When using the rake supporting mechanism according to the invention, the thickeners having smaller rake arm slope than 1:4 could be used advantageously. When the tank floor slope gets smaller, there is much more vertical support needed in the rake arms in order the rake arms to be parallel with the tank floor. The invention also relates to a method for supporting a rake in an apparatus for clarifying and/or thickening a solids containing solution, whereby the apparatus comprises a clarification and/or settling tank, which contains a feedwell, which settles the slurry, when the clarified solution or overflow is removed by a discharge launder, and when the thickened slurry is removed by discharge ports and a rake rotates on its drive WO 2007/090924 PCT/FI2007/000031 4 shaft, which rake moves the underflow from the edges of the tank to the centre, where said rake has at least four rake arms reaching the edge of the tank in a radial direction, when at least one vertical supporting member arranged on at least two of the rake arms is connecting the supported rake arms to at least two rake arms, which gives a vertical support to the rake, and when the rake arm slope could be as small as possible, at least 1:4 or smaller. The rake arm supporting system of the invention is self supporting, torque is applied via the drive shaft. Advantageously no extra supporting system is needed. In the following the preferred embodiments are described in more details with reference to the accompanying drawings, where Fig. 1 is a side view illustration of a preferred embodiment of the invention Fig. 2 is a side view illustration from Fig 1 Fig. 3 is a three-dimensional illustration of a preferred embodiment of the invention Figures 1 and 2 show an apparatus 1 for clarifying and/or thickening a solids containing solution such as finely ground ore suspended as pulp in a suitable liquid medium, such as water. Figure 3 is a three-dimensional illustration of a preferred embodiment of the invention. The apparatus 1 comprises a clarification and/or settling tank 2 for containing liquid pulp 4, which tank has right cylindrical sidewalls 7 and centrally downwardly sloping floor 8. Tank floor slope means how much the floor 8 slopes towards the drive shaft 9, which is centrally rotatably mounted within the tank 2. Tank floor slope is the same than rake arm slope 22, slope could be for example less than 1:4, like 1:6. An apparatus 1 is for recycling some of the dilute liquid (liquor) overflow from the top of the thickener tank 2 back to the feedwell 3 in order to dilute the inflowing pulp. A feedwell 3 is for the slurry 4 to be settled. The feedwell 3 is located centrally in the tank 2, which feedwell is supplied via a feed line. The feedwell has a restricted bottom opening in fluid communication with the tank 2, and through which a liquid pulp in the feedwell can flow into the tank. There are WO 2007/090924 PCT/FI2007/000031 5 discharge ports for the settled solids or underflow in the centre of the settling space where the thickened pulp 4 is drawn off. An overflow launder 6 for the clarified solution 20 or overflow surrounds the tank and channels the excess overflow liquor to a discharge pipe. A flocculant agent is added and mixed into the suspension. In normal operation the flocculation takes place in the feedwell, whereby a bed of thickened pulp settles to displace an upper layer of relatively dilute liquid towards the top of the tank. The thickened slurry is removed from below through discharge ports 5 like underflow withdrawal pipes. An apparatus 1 according to the invention contains a rake 10 rotating on its drive shaft 9, which rake moves the underflow from the edges of the tank to the centre, where said rake has at least four rake arms 11, 12, 13 and 14 reaching the edge of the tank in a radial direction. Each arm 11, 12, 13 and 14 supports an array of blades 21 extending axially into the tank 2. In use the rake rotates and the blades 21 sweep through the fluid to shear the pulp particles, allowing the release of trapped water, which rises to the surface through the sheared material. This, in turn, allows the solids to settle into a more dense bed of thickened pulp. The thickened pulp displaces an upper layer of liquid 20, which is drawn off through an overflow launder 6, extending circumferentially around the tank. The thickened pulp is drawn off below through discharge ports 5 included at the base of the tank. The rake 10 according to the invention has at least four rake arms 11, 12, 13 and 14 reaching the edge of the tank 2 in a radial direction. According to this example of invention two of the rake arms 11, 12 are vertically supported by one vertical supporting member 15 arranged on the rake arm, which vertical supporting member 15 is connected to other two rake arms 13,14, so that the rake arm slope 22 could be as small as possible, at least 1:4 or smaller. A vertical supporting member 15, such as steel rod or pipe being welded or attached by fastening elements on the rake arm is connected to the rake arms 11, 12, 13 or 14 by tying element 16 or 23 made of steel for example. In figures the tying element 23 connects the vertical supporting member 15 with the rake WO 2007/090924 PCT/FI2007/000031 6 arms 11 and 12 and the tying element 16 connects the vertical supporting member 15 with the rake arms 13 and 14. A rake 10 consists of four rake arms 11, 12, 13 and 14, when there are two shorter rake arms 11, 12 having the same length and two longer rake arms 13, 14 having the same length. The shorter rake arms 11, 12 have the length half of the longer rake arms 13, 14. A vertical supporting member 15 is arranged on the end point 17 of the shorter rake arm 11, 12, which is opposite to the drive shaft 9. The vertical supporting member 15 is connected to the longer rake arm 13, 14 on the point, which is half away of the radius 18 of the tank. The angle 19 between adjacent rake arms is 90 degrees. By changing the length of the vertical supporting member 15, the length of the tying element changes and it depends on the tank floor slope needed. The vertical supporting members 15 provide the angle for the tying elements that has enough vertical component to hold up the rake arms instead of all coming from the rake arm angle. At least one of the rake arms 11, 12, 13 and 14 is connected to the drive shaft 9 by a support element 24, which support elements 24 take so little space in the tank, that it leaves the space for pulp stabilization mechanism around the drive shaft 9. While the invention has been described with reference to its preferred embodiments, it is to be understood that modifications and variations will occur to those skilled in the art. Such modifications and variations are intended to fall within the scope of the appended claims.

Claims

1. An apparatus (1) for clarifying and/or thickening a solids-containing solution, whereby the apparatus comprises a clarification and/or settling tank (2), which is equipped with a feedwell (3) for the slurry to be settled, a discharge launder (6) for the clarified solution or overflow, and discharge ports (5) for removing the thickened slurry and a rake (10) rotating on its drive shaft (9), which rake is for moving the underflow from the edges of the tank to the centre, where said rake has at least four rake arms (11, 12, 13, 14) reaching the edge of the tank in a radial direction, characterised in that, at least two of the rake arms (11, 12) are vertically supported by at least one vertical supporting member (15) arranged on the rake arm, which vertical supporting member (15) is connected to at least two rake arms (13,14), so that the rake arm slope (22) could be as small as possible, at least 1:4 or smaller.

2. An apparatus according to claim 1, characterised in that, a vertical supporting member (15) is connected to the rake arm (11, 12, 13, 14) by at least one tying element (16, 23).

3. An apparatus according to claims 1 or 2, characterised in that, a rake (10) consists of four rake arms, when there are two shorter rake arms (11, 12) having the same length and two longer rake arms (13, 14) having the same length.

4. An apparatus according to claim 3, characterised in that, the shorter rake arms (11, 12) have the length half of the longer rake arms (13, 14).

5. An apparatus according to claims 1, 2 or 3, characterised in that, a vertical supporting member (15) is arranged on the end point (17) of the shorter rake arm (11, 12), which is opposite to the drive shaft (9). WO 2007/090924 PCT/FI2007/000031 8

6. An apparatus according to any of the preceding claims, characterised in that, the vertical supporting member (15) is connected to the longer rake arm (13, 14) by tying element (16) on the point, which is half away of the radius (18) of the tank (2).

7. An apparatus according to any of preceding claims, characterised in that, the angle (19) between adjacent rake arms is 90 degrees.

8. An apparatus according to claim 1 or 2, characterised in that, the rake arms are of same length.

9. An apparatus according to claim 1, characterised in that, at least one of the rake arms (11, 12, 13 14) is connected to the drive shaft (9) by a support element (24).

10.An apparatus according to claim 2, characterised in that, the tying element (16, 23) is a steel rod or pipe.

11.An apparatus according to claim 1, characterised in that, the vertical supporting member (15) is made of steel.

12.A method for supporting a rake (10) in an apparatus (1) for clarifying and/or thickening a solids-containing solution, whereby the apparatus comprises a clarification and/or settling tank (2), which contains a feedwell (3), which settles the slurry, when the clarified solution or overflow is removed by a discharge launder (6), and when the thickened slurry is removed by discharge ports (5) and a rake (10) rotates on its drive shaft (9), which rake moves the underflow from the edges of the tank to the centre, where said rake has at least four rake arms (11, 12, 13, 14) reaching the edge of the tank in a radial direction, characterised in that, at least one vertical supporting member (15) arranged on at least two of the rake arms (11, 12) is connecting the supported rake arms to at WO 2007/090924 PCT/FI2007/000031 9 least two rake arms (13,14), which gives a vertical support to the rake (10), and when the rake arm slope (22) could be as small as possible, at least 1:4 or smaller.