Title:
THICKENER
United States Patent 3685654


Abstract:
A thickener for the concentration of a solids-liquid suspension includes a vessel or tank into which such a suspension is continuously introduced and a rotating rake assembly for exerting mechanical pressure on the solids-sediment layer settling in the bottom of the vessel. The rake assembly is carried by a main shaft which undergoes vertical movement automatically in response to the hydraulic pressure in a fluid supply line feeding a main hydraulic motor drivingly coupled to the main shaft. The variation in the supply line pressure is transferred to a hydraulic chamber of a double-acting cylinder housing a piston which is mechanically coupled to the main shaft for conjoint vertical movement therewith. Air contained within a pneumatic chamber of the double-acting cylinder upwardly of the piston provides a buffer action for such automatic adjustment of the vertical position of the main shaft. Movement of the main shaft also controls an adjustable flow control valve in the fluid supply line to reduce hunting and the fluid discharge line from the main drive motor includes a second hydraulic motor drivingly coupled to a positive displacement pump for affecting the continuous discharge of the solids-sediment from the thickener.



Inventors:
Maclellan, James A. (Manitoba, CA)
Wilson, John E. (Manitoba, CA)
Application Number:
05/126840
Publication Date:
08/22/1972
Filing Date:
03/22/1971
Assignee:
GORDON SHERRITT MINES LTD.
Primary Class:
Other Classes:
210/528, 210/531
International Classes:
B01D21/00; (IPC1-7): B01D21/06; B01D21/20
Field of Search:
210/97,528,531
View Patent Images:
US Patent References:
2750038Material handling mechanism1956-06-12Hardinge
2588115Sedimentation device1952-03-04Hines
2360817Sedimentation apparatus1944-10-17Scott
2291836Rake lifting device in thickeners1942-08-04Scott
2265046Thickener1941-12-02Sabin



Foreign References:
GB772668A1957-04-17
Primary Examiner:
Feldman, Peter
Claims:
What we claim as new and desire to protect by Letters Patent of the United States is

1. A thickener for the concentration of a solids-liquid suspension by the removal of liquid therefrom and which comprises: a vessel into which such a solids-liquid suspension is introduced for downward settling of solids to form a solids-sediment layer within said vessel; a sediment discharge outlet in said vessel for the removal of solids-sediment therefrom; a liquid discharge outlet for the removal of supernatant liquid from said vessel; a rotary rake assembly carried by a shaft journalled for conjoint rotation therewith within said vessel for raking solids within the solids-sediment layer towards the sediment discharge outlet, and for upward and downward movement within said vessel; a fluid-operated motor drivingly engaging said shaft to cause rotation thereof on operation of said motor; a fluid supply line leading to said motor and adapted to supply operating fluid thereto at a fluid pressure responsive to the load applied to said motor; a fluid operated cylinder including a piston mechanically coupled to said rake assembly for causing upward and downward movement thereof; a fluid pressure transfer line operatively interconnecting said fluid supply line and said fluid-operated cylinder to urge said piston thereof and in turn said rake assembly upwardly in response to the fluid pressure within said fluid supply line; a buffer operatively associated with said fluid-operated cylinder and adapted to apply a downward force against said piston of said fluid-operated cylinder on upward movement of said piston; and a fluid discharge line extending from said fluid-operated motor for the discharge of operating fluid therefrom; whereby on operation of said fluid-operated motor for rotation of the rake assembly by the supply of pressurized fluid thereto through said fluid supply line, said rake assembly is moved upwardly and downwardly in response to the upward force exerted on said piston by changes in the fluid pressure within said fluid pressure transfer line in balance with the downward force exerted on said piston by said buffer, whereby the position of said rake assembly is automatically adjusted, during operation of said thickener, to maintain the load on said fluid-operated motor within a predetermined range.

2. A thickener as claimed in claim 1 in which said piston is mechanically coupled to said shaft of said rake assembly to cause axial bodily movement of that assembly within said vessel on variation of the load on said fluid-operated motor.

3. A thickener as claimed in claim 2 in which said fluid supply line extends to said fluid-operated motor from a constant pressure source of operating fluid and includes an adjustable flow control valve.

4. A thickener as claimed in claim 3 in which said adjustable flow control valve is mechanically coupled to said shaft of said rake assembly so as to allow a progressively increasing flow of operating fluid from said constant pressure source to said fluid-operated motor on upward movement of said rake assembly.

5. A thickener as claimed in claim 2 which also comprises a positive displacement solids-sediment pump coupled to said sediment discharge outlet for withdrawing solids-sediment from said vessel and in which said fluid discharge line includes a second fluid-operated motor for operation by the flow of fluid through said fluid discharge line and drivingly connected to said solids-sediment pump.

6. A thickener as claimed in claim 4 which also comprises a positive displacement solids-sediment pump coupled to said sediment discharge outlet for withdrawing solids-sediment from said vessel and in which said fluid discharge line includes a second fluid-operated motor for operation by the flow of fluid through said fluid discharge line and drivingly connected to said solids-sediment pump.

7. A thickener as claimed in claim 2 in which said fluid-operated cylinder is a double-acting cylinder and in which said buffer comprises a mass of air contained within said cylinder upwardly of said piston whereby, on conjoint upward movement of said rake assembly and said piston, said mass of air is increasingly compressed to apply a progressively increasing downward force on said piston.

8. A thickener as claimed in claim 7 in which said fluid-operated motor is a hydraulic motor and in which said fluid-operated cylinder is a hybrid pneumatic-hydraulic cylinder.

9. A thickener as claimed in claim 4 in which said constant pressure source is a pressure-compensated hydraulic pump for the supply of hydraulic fluid under pressure through said fluid supply line to said adjustable flow control valve, in which said fluid-operated motor is a hydraulic motor, in which said fluid-operated cylinder is a hybrid pneumatic-hydraulic cylinder, and in which said buffer comprises a mass of air contained within said hybrid cylinder whereby, on conjoint upward movement of said rake assembly and said piston of said hybrid cylinder, said mass of air is increasingly compressed to apply a progressively increasing downward force on said piston to resist such upward movement thereof while said adjustable flow control valve is progressively opened to allow the progressively increasing flow of hydraulic fluid to said hydraulic motor.

10. A thickener as claimed in claim 6 in which both said fluid-operated motors are hydraulic motors, in which said constant pressure source is a pressure-compensated hydraulic pump for the supply of hydraulic fluid under pressure through said fluid supply line to said adjustable flow control valve, in which said fluid-operated cylinder is a hybrid pneumatic-hydraulic cylinder, and in which said buffer comprises a mass of air contained within said hybrid cylinder whereby, on conjoint upward movement of said rake assembly and said piston of said hybrid cylinder, said mass of air is increasingly compressed to apply a progressively increasing downward force on said piston to resist such upward movement thereof, said adjustable flow valve is progressively opened to allow the progressively increasing flow of hydraulic fluid to said fluid supply line, and the rotational speed of said second hydraulic motor and in turn that of said solids-sediment pump vary in response to the rate of flow of hydraulic fluid through said hydraulic motor engaging said shaft.

Description:
The present invention relates generally to thickeners used for the continuous concentration of solids-liquid suspensions as obtained, for example, from froth flotation units as used in ore processing operations. More particularly, the invention relates to a thickener of the known type in which a rotary rake assembly is provided for exerting mechanical pressure on the solids-sediment layer formed in the bottom of such a thickener during its operation.

It is well known that, in thickeners of the aforementioned type, the action of the rake assembly is to accelerate the rate of exudation of liquid from the settled solids-sediment and so to provide a higher degree of liquid removal or solids concentration.

It is also known that difficulties arise in practice during the operation of thickeners of the aforementioned type since the solids-sediment layer does not consistently maintain its optimum thickness or consistency. For example, if such a layer becomes too thin, the rake assembly will apply insufficient pressure thereto while, if the layer becomes too thick, overloading and even failure of the rake drive system can result.

It is also known to provide control devices to permit manual adjustment of the vertical position of the rake assembly in a thickener of the type in question but such manual control systems suffer from the serious disadvantage that they call for constant attention and adjustment if anything approaching optimum efficiency is to be attained at all times.

It has also heretofore been proposed to incorporate automatic alarm systems in the rake drive systems of thickeners of the considered type but all such alarm systems have primarily been intended to alert the operators of the occurrence of potentially dangerous operating conditions, in distinction to ensuring optimum thickener operating performance on a continuous basis.

It has also been proposed to provide a thickener of the type including a rake assembly with axially pivoted rake arms so that the angular positions of such arms will vary automatically in response to the load exerted on such arms as a result of their engagement with the settled solids-sediment layer. Although such an arrangement provides some degree of automatic control in the operation of the thickener, it principally has been intended to serve as a safety factor in reducing the risk of damage to the rake assembly and its drive system in the event of an overload. It will readily be understood that, in such previously proposed constructions, there must inevitably be a compromise between increasing the weight of the rake arms to ensure adequate downward pressure and reducing such weight to guarantee upward pivoting of the arms in the event of an overload condition.

It is a principal object of the present invention to provide an automatic control system for a thickener of the aforementioned type and including a rake assembly for exerting mechanical pressure on a solids-sediment layer settling in the bottom of the thickener vessel.

Another object of the invention is to provide a thickener of the aforementioned type and incorporating an automatic control system whereby the vertical position of the rake assembly of the thickener is automatically adjusted in response to the load exerted on that assembly.

Still another object of this invention is the provision of an automatic control system for the aforementioned purpose and which is effective for maintaining the load exerted on the rake assembly and consequently on the main drive motor of the thickener within a predetermined range.

Other objects of the invention will become apparent as the description herein proceeds.

The present invention involves the use of a fluid-operated motor for rotating the rake assembly in a thickener of the type already considered herein. The invention provides an automatic control system in which the variation of the pressure in a fluid supply line leading to the main drive motor of the rake assembly is used to control the vertical position of the rake assembly and consequently the load exerted on that assembly due to its engagement with a settled solids-sediment layer within the vessel of the thickener.

In its broadest scope, the present invention provides a thickener for the concentration of a solids-liquid suspension by the removal of liquid therefrom which comprises a vessel into which such a solids-liquid suspension is introduced for downward settling of solids to form a solids-sediment layer within said vessel; a sediment discharge outlet in said vessel for the removal of solids-sediment therefrom; a liquid discharge outlet for the removal of supernatant liquid from said vessel; a rotary rake assembly carried by a shaft journalled for conjoint rotation therewith within said vessel for raking solids within the solids-sediment layer towards the sediment discharge outlet, and for upward and downward movement within said vessel; a fluid-operated motor drivingly engaging said shaft to cause rotation thereof on operation of said motor; a fluid supply line leading to said motor and adapted to supply operating fluid thereto at a fluid pressure responsive to the load applied to said motor; a fluid-operated cylinder including a piston mechanically coupled to said rake assembly for causing upward and downward movement thereof; a fluid pressure transfer line operatively connecting said fluid supply line and said fluid-operated cylinder to urge said piston thereof and in turn said rake assembly upwardly in response to the fluid pressure within said fluid supply line; a buffer operatively associated with said fluid-operated cylinder and adapted to apply a downward force against said piston of said fluid-operated cylinder on upward movement of said piston; and a fluid discharge line extending from said fluid-operated motor for the discharge of operating fluid therefrom; whereby, on operation of said fluid-operated motor for rotation of the rake assembly by the supply of pressurized fluid thereto through said fluid supply line, said rake assembly is moved upwardly and downwardly in response to the upward force exerted on said piston by the fluid pressure within said fluid pressure transfer line in balance with the downward force exerted on said piston by said buffer, whereby the position of said rake assembly is automatically adjusted, during operation of said thickener, to maintain the load on said fluid-operated motor within a predetermined range.

Although the piston of the fluid-operated cylinder can be mechanically coupled directly to the rake assembly so as to cause pivotal elevation or lowering of the rake arms of that assembly in response to variations in the load on the main drive motor, such piston is usefully mechanically coupled to the rake assembly through the main shaft so as to cause conjoint vertical axial movement of that shaft and the rake assembly on variation of the load. With the latter construction, automatic control is obtained without causing variation of the angular configuration of the conical top surface of the solids-sediment layer. Such change in the top surface configuration may, under certain circumstances, lead to undesirable changes in the operating characteristics of the thickener.

In accordance with a particularly useful feature of this invention, the fluid supply line leading to the fluid-operated main drive motor of the control system provided in accordance with the invention includes an adjustable flow control valve which is automatically controlled in response to upward and downward movement of the rake assembly so as to allow a greater flow of operating fluid through that valve as the load on the rake assembly increases and that assembly moves upwardly, and vice versa. This arrangement is particularly advantageous in reducing undesired hunting movements.

Other features and advantages of the invention and the manner in which they can be attained will become apparent as the description herein proceeds.

The invention will now be described merely by way of illustration with reference to the accompanying drawings, in which:

FIG. 1 is a somewhat schematic illustration, partly in section, of one particularly useful embodiment of a thickener in accordance with invention; and

FIG. 2 is an enlarged illustration of a valve control for use in the thickener of FIG. 1.

The thickener generally indicated at 10 in the accompanying drawings comprises a large cylindrical vessel 12 shown as being formed of metal but equally possibly formed of wood, concrete or other material. The vessel 12 includes a peripheral wall 11 and, generally centrally in its base 13, it is formed with a discharge outlet 14 terminating in a conduit 15 which is shown schematically at 16 as extending to a diaphragm pump generally indicated at 17 and including a flexible diaphragm 18 driven by an actuating arm 19. The diaphragm 18 defines a pump chamber 20 into which sediment is supplied from the line 16. The sediment is discharged from the pump 17 through conduit 22 from which it can be recycled to the vessel 12 through conduit 25 under the control of valve 26 or from which it can pass to a subsequent treatment stage, such as a filtration stage, through conduit 23 under the control of a valve 24. The pump 17 is, of course, provided with appropriate inlet and outlet valves. The invention is, however, in no way restricted to the use of a diaphragm pump and other types of positive displacement pumps are equally suitable for use in discharging the sediment from the vessel 12.

The vessel 12 is also provided, as is conventional in thickener tanks, with an inwardly and downwardly sloping frustoconical floor 28 which is shown as being formed of concrete. It will be understood, however, that such a sloping floor can be provided by manufacturing the base 13 of the vessel 12 to such a desired configuration.

Peripherally around the upper edge of the side wall 11 of the vessel 12, there is provided an annular overflow trough 29 for receiving supernatant liquid from the vessel 12, the liquid then being discharged from the trough 29 through line 30.

Within the vessel 12, there is provided a rake assembly generally indicated at 32 and including a number of radially extending arms 33 each carrying fingers 34 which apply mechanical pressure against the solids-sediment layer 35 collecting in the bottom of the vessel 12. The action of the fingers 34 is to rake settled solids towards the discharge outlet 14. The rake assembly 32 is carried by an axially upwardly extending shaft 36 journaled in a bearing 38 which is in turn mounted on a diametrical bridge 40 provided across the top of the vessel 12.

A solids-liquid suspension feed conduit 42 terminates below the normal liquid level within the vessel 12 for the introduction of a non-concentrated solids-liquid suspension thereinto. The discharge end of the feed conduit 42 terminates in a partially immersed cylindrical feed well 43 provided generally centrally of the vessel 12.

To the extent that it has so far been described herein, the thickener 10 shown in the accompanying drawing is essentially one of a well known type. The manner in which the thickener 10 differs from those previously known will now be described in more detail. First, it should be noted that the shaft 36 carrying the rake assembly 32 is journalled in the aforementioned bearing 38 for rotation and for limited vertical axial movement for a reason which will be more readily understood as the description herein proceeds.

Upwardly of the bearing 38, there is keyed to the shaft 36 a driven gear 46 meshed with a driving gear 47 keyed in turn to a motor shaft 48 of a hydraulically operated fixed displacement motor generally indicated at 50. The driving gear 47 has a greater axial thickness than the driven gear 46 to ensure engagement of such gears throughout the full range of vertical movement of the gear 46 with the shaft 36.

The hydraulic motor 50 is supplied with hydraulic fluid under pressure through a fluid supply line 52 extending to the motor 50 from an adjustable flow control valve generally indicated at 53, hydraulic fluid being supplied to the valve 53 from a constant pressure output hydraulic pump 55 through a line 56. The pump 55 is shown in the accompanying drawings as receiving hydraulic fluid from a fluid reservoir 57 through a line 58.

In the particular embodiment illustrated, hydraulic fluid is discharged from the motor 50 through a fluid discharge line 59 to a second hydraulic motor 60 from which the fluid is finally returned through a line 61 to the reservoir 57. The hydraulic motor 60 is drivingly coupled to the diaphragm pump 17 as indicated schematically by the broken line 62 so that the rate of discharge of solids-sediment from the vessel 12 by the pump 17 is directly proportional to the rate of hydraulic fluid discharge through the motor 60.

The aforementioned main shaft 36 carrying the rake assembly 32 extends upwardly from the driven gear 46 and is coupled at its upper end to a piston 63 of a hybrid pneumatic hydraulic cylinder 64. The piston 63 divides the cylinder 64 into an upper pneumatic cylinder chamber 65 and a lower hydraulic cylinder chamber 66. The pneumatic chamber 65 is connected by an air line 67 to an accumulator 68 maintained through line 69 at a constant predetermined pressure. A metering check valve 70 in the line 67 prevents the escape of air from the pneumatic cylinder chamber 65.

The hydraulic cylinder chamber 66 of the cylinder 64 is connected by a fluid pressure transfer line 72 directly to the aforementioned fluid supply line 52 extending from the adjustable flow valve 53 to the hydraulic motor 50.

Upwardly of the driven gear 46 but below the cylinder 64 the drive shaft 36 is mechanically coupled as indicated schematically by the broken line 74 to the aforementioned adjustable flow valve 53 so that, on upward movement of the shaft 36 carrying the rake assembly 32, the valve 53 is opened to allow a greater flow of hydraulic fluid from the hydraulic pump 55 through line 56 to the line 52 leading to the hydraulic motor 50.

One suitable construction for such mechanical coupling between the shaft 36 and the vaLve 53 will now be briefly described with reference to FIG. 2 of the drawings. In the particular construction illustrated, the shaft 36 extends through a bearing generally indicated at 76 and including an inner race 77 retained in position on the shaft 36 by spring washers 78 received in spaced apart peripheral recesses formed in the shaft 36. The outer race 79 of the bearing 76 is provided with a pair of spaced apart, radially extending lugs 80, one of which can be seen in FIG. 2 and between which a lever arm shown fragmentarily at 83 is pivotally mounted for movement in a vertical plane about a horizontal pivot pin 84 supported in horizontal slots 97 in lugs 80.

At its opposite end, the lever arm 83 is formed with a cam surface 85 and is pivotally mounted at 86 between a pair of upright members 87 secured by bolts 88 to a frame member shown fragmentarily at 89. The frame member 89 is secured in any appropriate manner (not shown) on the fixed structure of the thickener 10, for example, to the diametrical bridge 40 (FIG. 1).

The cam surface 85 of the lever arm 83 engages a cam roller follower 90 pivotally mounted on the outer end of a valve rod 91 extending from the valve 53 which may, for instance, be a spool valve of any suitable adjustable flow type. The valve 53 is provided with a mounting flange 92 bolted at 93 to an upstanding bracket 94 secured in turn by bolts 95 to the aforementioned frame member 89. It will be seen that the shaft 36 is free to rotate within the bearing 76 but that, when the shaft is moved upwardly or downwardly by the piston 63 (FIG. 1) in a manner yet to be explained and as indicated by the double-headed arrow A, the bearing 76 undergoes conjoint vertical movement therewith. Such movement is transmitted through the lever arm 83 to cause pivotal movement of the cam surface 85 and such movement of the cam surface 85 then causes retraction or extension of the valve rod 91 as indicated by the double-headed arrow B. As will be described hereinafter in greater detail, such movement of the valve rod 91 serves to increase or decrease the flow of hydraulic fluid through the valve 53 into the line 52 leading to the motor 50.

Having completed the description of the thickener 10 shown in the drawing, its manner of operation will now be described. As already explained, a dilute solids-liquid suspension, for example, as obtained from a froth flotation mineral concentration unit, is continuously fed into the vessel 12 through the feed conduit 42. Within the vessel 12, the solids contained within such a suspension gradually settle to form the highly concentrated solids-sediment bottom layer 35 and a supernatant layer of generally clear liquid which overflows into the trough 29 for discharge through the line 30. In practice, the suspension within the vessel 12 will generally form into a number of intermediate zones or layers but such layers are omitted from, the the accompanying drawing for the sake of clarity. The solids-sediment layer 35 is withdrawn from the vessel 12 through lines 15 and 16 by the pump 17 and discharged through line 23 and recycled to the vessel 12 through line 25 in any desired relative proportions.

During operation of the thickener 10, the fingers 34 of the rake arms 33 which have a "picket fence" type structure rake settled solids inwardly towards discharge outlet 14 while applying pressure to the solids-sediment layer 35 to increase the rate of exudation of the liquid material from that layer, resulting in a lower liquid content for the discharged sediment than would be the case if reliance were placed solely on exudation caused by the gravitational forces acting within the layer 35.

It will be understood that, in practice, the thickness and density of the solids-sediment layer 35 will depend on numerous factors such as, for example, the solids-liquid ratio of the feed suspension, the detention time of the suspension within the vessel 12, the sediment discharge rate and the discharge-recycle ratio. As is well known, the major cause of operational failure of thickeners including rake mechanisms of the type in question is due to the application of an excessive load against the rake arms 33. Additionally, the position of each of the rake arms 33 should at all times be such as to apply the optimum pressure against the solids-sediment layer 35.

As already explained, the present invention significantly reduces the risk of overloading of the rake assembly and its associated drive mechanism as well as providing a much more effective mechanical working function for that rake assembly by providing an automatic means for adjusting the vertical position of the rake assembly relative to the solids-sediment layer 35 in response to the load applied to the rake assembly by the layer 35.

In the particular embodiment shown in the accompanying drawing, variations in the load on the rake assembly and consequently on the shaft 48 of the hydraulic motor 50 cause a change in the fluid pressure in the fluid supply line 52, i.e., an increase in load on the rake results in an increase of fluid pressure. This pressure change is transmitted through the fluid pressure transfer line 72 to the hydraulic cylinder chamber 66 of the cylinder 64. As a result, the piston 63 and, in turn, the main shaft 36 and the rake assembly 32 are moved vertically until the hydraulic pressure in the hydraulic cylinder chamber 66 is again balanced by the pneumatic pressure in the pneumatic cylinder chamber 65.

For example, if the load on the rake arm assembly 32 increases due to an increase in the thickness of the solids-sediment layer 35, the rotational speed of the drive motor 50 will tend to decrease and the pressure in the fluid supply line 52 will then consequently increase. The resulting increased pressure in the hydraulic chamber 66 will cause upward movement of the piston 63 until such pressure in the chamber 66 is again balanced by the pressure of the air compressed in the pneumatic chamber 65. Such upward movement of the piston 63 causes conjoint upward movement of the rake assembly 32 which in turn will serve to reduce the load on that assembly 32 which in turn will serve to reduce the load on that assembly and allow the pressure in the supply line 52 to fall to a lower value. It will now be understood that such a structure provides an automatic load compensating mechanism for the rake assembly and its associated drive system.

It will also be understood that the air supply system comprising the lines 67 and 69, the accumulator 68 and the metering check valve 70 is operative to maintain a minimum pressure within the pneumatic cylinder chamber 65. Such a pressurized air supply system will generally be necessary in practice in view of the inevitable loss of air pressure from the chamber 65, for example, around the piston 63.

In accordance with another particularly useful feature of this invention, the vertical movement of the shaft 36 resulting from the changes in pressure in the fluid supply line 52 is additionally operative to cause adjustment of the rate of fluid flow through the flow control valve 53. By the provision of such an additional control system, the tendency for the system to hunt is significantly reduced. Reconsidering the aforementioned example of the rake assembly rising due to an increase in the load on that assembly, it will be noted that such upward movement of the shaft 36 will serve to permit a greater fluid flow rate through the valve 53 so reducing the pressure drop across that valve. Such automatic adjustment of the valve 53, therefore, causes a pressure increase in the fluid supply line 52 and in the hydraulic cylinder chamber 66 as the rake assembly 32 is rising to reduce the load thereon.

In accordance with yet another feature of this invention, further operating advantages and a more rapid return of the solids-sediment layer thickness to its optimum operational value are obtained by driving the solids-sediment discharge pump 17 by the hydraulic motor 60. With this particular arrangement, the rate of solids-sediment discharge from the vessel 12 is directly responsive to the rate of fluid flow through the drive motor 50.

In a typical operation of the thickener 10 shown in the accompanying drawing, the several components of that thickener were adjusted to maintain the rake assembly vertically stationary with a fluid pressure in the fluid supply line 52 of 100 psig and with the air check valve 70 set to open at a pressure of 100 psig. The hydraulic pump 55 provided a fluid pressure in the line 56 of 1,500 psig.

In a further typical operation of the thickener as a storage thickener, the several components were adjusted to allow the rake assembly to rise vertically for two days. The fluid pressure in the fluid supply line 52 was maintained at 1,000 psig and the air check valve 70 was opened to atmosphere, i.e., 0 psig. The hydraulic pump 55 provided its normal pressure of 1,500 psig. At the end of 2 days at normal feed rates to the thickener, the rake assembly had risen vertically to the limit of its travel.

The air check valve 70 was then adjusted to 120 psig. Valve 24 was opened and valve 26 closed. The thickener then discharged 48 hours of normal feed in 8 hours at a very high density.

Control valve 53 was particularly useful under these conditions in that the opening of valve 53 due to the raising of shaft 36 permitted an initially high rake speed and a very rapid initial discharge, which gradually tapered off as the rakes approached the more dense permanent bed material.

A further surprising advantage of the system in driving motors 50 and 60 in series was that if the discharge density was tending to become too great for pump 20, the back pressure increased on motor 50 causing it to stall and the rakes to lift, thus momentarily interrupting the flow of settled solids pulp to pump 20.

Motor 50 had a pressure regulated bypass and the blow-by of fluid to motor 50 while in the stalled position was sufficient to keep motor 60 continually turning. With a more positive motor in position 50 one might have to supply a spring loaded bypass around motor 50 to fully utilize this feature.

Although the invention has been described herein with particular reference to the specific embodiment illustrated in the accompanying drawings, it will be understood that numerous variations therein may be made without departing from its scope. Merely by way of illustration, it may be noted that the buffer system provided by the pneumatic cylinder chamber 65 could, for example, be replaced by a suitable compression spring mechanism. Furthermore, it is equally possible in accordance with this invention, to utilize the various described control system features both individually and in different combinations, the invention being in no way restricted to the specific combination actually described herein.