Abstract:
A method and installation for forming and maintaining a slurry are disclosed. The method withdraws a slurry from a reservoir and directs it at a heap of solid material from which the slurry is drained into the reservoir, the slurry containing additional solid material from the heap to increase its specific gravity. The volume of slurry in the reservoir and its specific gravity are controlled. The installation comprises a support having a support surface which slopes to permit slurry to drain therefrom A reservoir receives slurry draining from the support and has a liquid feed. A pump withdraws the slurry from the reservoir and pumps it to a spray nozzle for spraying slurry at a heap of material on the support.

Description:
FIELD OF THE INVENTION 
     THIS INVENTION relates to slurries. More particularly, the invention relates to a method and to an installation for forming and maintaining, for consumption, a stock of slurry. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a method of forming and maintaining for consumption a stock of slurry derived from a liquid and from a particulate solid material, the method including the steps of: 
     withdrawing a slurry of the liquid and the solid material from a stock of the slurry in a reservoir; 
     directing a stream of the liquid or of the withdrawn slurry at a bulk supply of the solid material in the form of a heap of the solid material, and on to the solid material of the heap; 
     allowing the stream directed at the heap to drain under gravity from the heap into the reservoir, the stream, while draining from the heap, entraining therein solid material from the heap to increase the specific gravity of the stream and to carry solid material from the heap into the stock of slurry in the reservoir to increase the specific gravity thereof; and 
     feeding liquid to the reservoir to maintain the volume of the slurry in the reservoir, the method including controlling both the volume of slurry in the reservoir and the specific gravity of the slurry in the reservoir, the feeding of the liquid into the reservoir being initiated, or having its feed rate increased, in response to decreases in the volume of the slurry in the reservoir associated with withdrawal of slurry from the reservoir for consumption thereof, and the directing of the stream at the heap of solid material being initiated, or having its flow rate increased, in response to decreases in the specific gravity of the slurry in the reservoir associated with feeding the liquid to the reservoir, thereby both to maintain the volume of slurry in the reservoir at a desired value, and to maintain the specific gravity of the slurry in the reservoir at a desired value, the feeding of liquid to the reservoir being reduced in rate or discontinued when the desired slurry volume is regained, and the directing of the stream at the heap being reduced in rate or discontinued when the desired specific gravity value is regained. 
     It will be appreciated that increases and decreases to the specific gravity of the slurry are associated respectively with increases and decreases to the solids content thereof. 
     While it is in principle possible to carry out the method by directing a stream of the liquid at and on to the solid material of the heap, to form a slurry having a solids content and specific gravity greater than the solids content and specific gravity respectively of the slurry in the reservoir, to permit drainage of a slurry from the heap into the reservoir which increases the solids content and specific gravity of the slurry in the reservoir, it is preferable to employ slurry withdrawn from the reservoir for this purpose. The stream directed at the heap may thus be a stream of the slurry withdrawn from the reservoir. 
     The method of the invention contemplates that withdrawal of slurry from the reservoir for consumption thereof may be continuous or intermittent, and that the liquid feed to the reservoir, which may be continuous or intermittent, will be at a rate such that the volume of slurry in the reservoir is maintained at the desired value when slurry is withdrawn for consumption at any rate up to the maximum intended rate. Similarly, the withdrawal of the stream of slurry from the reservoir to direct at the heap, which withdrawal may also be continuous or intermittent, will in turn be at a rate such that the specific gravity of the slurry in the reservoir is maintained at the desired value when the slurry is withdrawn for consumption at any rate up to the maximum intended withdrawal rate for consumption. Thus, the desired volume of the slurry, at least on average and with no more than acceptable departures from the desired value, can be maintained in the reservoir while the specific gravity, ie the density or solids content, of the slurry is likewise maintained, at least on average and with no more than acceptable departures from the desired value. In a particular embodiment of the invention, when the stream directed at the heap is a stream of slurry withdrawn from the reservoir, the liquid feed to the reservoir may be intermittent, being at a fixed rate no less than the maximum rate at which the slurry is withdrawn from the reservoir for consumption, the stream directed at the heap being withdrawn from the reservoir intermittently and at a fixed rate. 
     In a further particular embodiment of the invention the liquid may be water, the solid material being particulate or powdered limestone, or possibly dolomite. In other words, the liquid may be water, the solid material being selected from the group consisting of limestone, dolomite and mixtures thereof. 
     The controlling of the volume of slurry in the reservoir may be automatic, being by controlling the depth of slurry in the reservoir by means of a valve, the valve being selected from the group consisting of ultra-violet (UV)-controlled valves and, preferably, float-controlled valves and the valve controlling the liquid feed to the reservoir. Similarly, controlling the specific gravity of the slurry in the reservoir may be automatic, for example by using one or more load cells on which the reservoir is supported, to measure the mass of the reservoir, or by using one or more pressure cells in the interior of the reservoir to measure the static pressure in the reservoir, said cell or cells controlling the withdrawal of the stream of liquid from the reservoir which is directed at the heap. In other words, the controlling of the specific gravity of the slurry in the reservoir may be automatic, being by using a cell selected from load cells which measure the mass of the reservoir and pressure cells which measure static pressure in the reservoir, to control the directing of the stream at the heap. The liquid feed to the reservoir or the slurry stream withdrawal from the reservoir may be provided with on/off flow control, and/or they may incorporate variation of their flow rates in response to the magnitude of any departure of the slurry volume or specific gravity from the respective desired values thereof. 
     Indeed, instead of using load cells or pressure cells, the reservoir can in principle float in a body of water, the depth at which the reservoir floats in the body of water being related to the mass of the reservoir and hence to the specific gravity of the slurry therein, changes in this depth being used to control the directing of the stream at the heap, although it is expected that use of load cells or pressure cells will be preferred. 
     The method may involve agitation of the slurry in the reservoir to promote maintenance of the solid material in suspension in the liquid and to resist its settling out from the liquid, and the method may include replenishing the bulk supply of particulate material in the heap, as and when required, e.g. intermittently by means of vehicles loaded therewith Withdrawal of slurry from the reservoir, either as a stream to be directed at the heap, or for consumption thereof, may be by pumping it, and directing the stream of slurry at the heap may be by spraying it, e.g. from one or more spray nozzles at the ends of respective hoses, which may be hand-held. In particular, the method may include the step of agitating the slurry to promote maintenance of the solid in suspension in the liquid, withdrawal of slurry from the reservoir being by pumping, directing the stream at the heap being by spraying it from one or more hand-held nozzles, and feeding the liquid to the reservoir being by directing it at the heap. 
     If desired, the method may make provision for the removal of stones and/or grit from the slurry, between the reservoir and the hose or hoses, e.g. by passing the slurry upwardly through an inverted-conical tank, the slurry from the reservoir being fed into the tank at a low level where its diameter is small, and the slurry issuing from the tank at a high level where its diameter is large, the tank shape and size and the slurry flow rate being selected to promote retention of stones and grit in the tank, while an overflow from the tank takes the place of slurry containing particles of acceptably small size, and stones and grit are removed, e.g. intermittently from the lower parts of the tank; and slurry can, if desired, be recirculated from the top of the tank to its bottom, to achieve upward flow rates in the tank which keep small slurry particles in suspension. 
     The feeding of the liquid to the reservoir to maintain the volume of slurry in the reservoir may be directly from a liquid supply into the reservoir, or it may be indirect, the liquid being fed from the liquid supply into the stream of slurry which is directed at the bulk supply, so that the liquid enters the reservoir as part of the slurry draining into the reservoir 
     According to another aspect of the invention there is provided an installation for forming and maintaining, for consumption, a stock of slurry derived from a liquid and from a bulk supply of particulate solid material, the installation including; 
     a support having an upwardly facing support surface for supporting a heap of particulate solid material, the support surface sloping so that it is inclined to the horizontal to permit a slurry of liquid and particulate solid material to drain under gravity therefrom; 
     a reservoir arranged to receive slurry draining under gravity from the support surface of the support and to hold a stock of slurry; 
     a liquid feed for feeding liquid to the reservoir; 
     a slurry withdrawal device for withdrawing slurry from a stock of slurry in the reservoir; 
     a spray device for spraying a liquid or slurry at a heap of particulate material supported on the support and on to the particulate material of the heap; 
     a volume control device for controlling the volume of the stock of slurry in the reservoir and operative, in response to a decrease in said volume below a desired value, to initiate, or increase the rate of, the feeding of liquid by the liquid feed to the reservoir; and 
     a specific gravity control device for controlling the specific gravity of the stock of slurry in the reservoir and operative, in response to a decrease in said specific gravity below a desired value, to initiate, or increase the rate of, the spraying of the liquid or slurry at the heap, 
     the volume control device being operative to discontinue, or to reduce the rate of feeding of, liquid to the reservoir, when the desired volume is regained and the specific gravity control device being operative to discontinue, or to reduce the rate of spraying of, the liquid or slurry at the heap, when the desired specific gravity is regained. 
     The support may be a cementitious, e.g. concrete, slab resting on the ground and having a shaped support surface for supporting the heap, the support surface having, for example, two flat portions which slope and converge downwardly to meet at a line of intersection therebetween, which line in turn slopes downwardly towards a drainage point where the support surface drains into the reservoir. The slab may have a pair of ramps, respectively sloping upwardly from ground level to peripheral edges of the flat portions of the support surface, to facilitate the tipping of particulate material from load vehicles on to the support surface. In a particular embodiment of the installation, the support may thus be a slab of cementitious material having a shaped support surface for supporting the heap, the support surface sloping downwardly to a drainage point and the slab being provided with a vehicle ramp leading upwardly from ground level to a peripheral edge of the slab, which edge is raised above ground level, the reservoir being a completely mixed tank located below the level of the drainage point and provided with a stirrer in its interior. 
     The reservoir may in turn be in the form of an open-topped completely mixed tank, located in a pit below ground level, at a position where the support surface can drain under gravity into the open top of the tank, and having a stirrer in its interior. 
     The slurry withdrawal device may be a pump. The pump for withdrawal of slurry for consumption may simply feed into a pipe or similar flow line; and there may be a further similar pump for withdrawal of a stream of slurry to be directed at the heap, which further pump may feed Into one or more hoses, each optionally provided with a spray nozzle at its free end. Thus, the spray device may comprise one or more hoses, each of which has a free end provided with a spray nozzle. 
     The volume control device may be in the form of a liquid level control means and may thus be an ultra-violet (UV)-controlled valve or preferably a float-controlled valve controlling feeding of liquid from the liquid feed, which may be a pipe, into the reservoir, the valve conveniently being a shut-off or on-off valve arranged to open fully and permit feeding of liquid at a fixed rate into the reservoir if the level of slurry in the reservoir decreases below a desired value, equivalent to the desired slurry volume in the tank, and to shut off liquid flow from the pipe when said desired level is regained. Accordingly, the volume control device may be a shut-off valve selected from the group consisting of ultra-violet (UV)-controlled valves and float-controlled valves, the shut-off valve being arranged to open if the level of slurry in the reservoir decreases below a desired value and to close when said desired value is regained. 
     The specific gravity control device may be one or more load cells on which the tank is supported in the pit, arranged to control withdrawal of the stream of slurry from the tank by automatically starting the associated pump, or increasing its pumping rate, when the mass of the tank and the desired volume of slurry in the tank contents decreases below a predetermined value corresponding to the desired specific gravity, and arranged automatically to discontinue the pumping, or reduce its rate, when the predetermined value is regained Thus, generally, the specific gravity control device may comprise at least one cell selected from the group consisting of load cells on which the reservoir is supported for sensing the mass of the reservoir, and pressure cells in the interior of the reservoir for sensing static pressure in the reservoir, the cell being operatively connected to a shut-off valve for supplying liquid or slurry to the spray device. 
     In this regard it will be appreciated that spraying slurry on to the heap and allowing it to drain under gravity into the tank while entraining particulate solid material from the heap, will increase the concentration of solid material in the slurry in the tank and hence will increase the mass of the slurry and hence will increase the total mass of the tank and its contents when filled with the desired volume of slurry. Correspondingly, feeding liquid into the tank will dilute the slurry and reduce the mass of the slurry and hence reduce the total mass of the tank and its contents when filled with slurry to the desired volume or level. Withdrawal of slurry from the tank for consumption leads to feeding liquid into the tank and to a decrease in specific gravity of the slurry, which in turn leads to spraying of slurry on to the heap and washing solid material from the heap into the tank, thereby causing a compensatory increase in specific gravity. Thus, both the volume or level of slurry in the tank, and its specific gravity, can automatically be maintained. 
     Instead of comprising one or more load cells, the specific gravity control device may comprise one or more pressure cells in the tank, e.g. at or near its bottom, arranged to control the withdrawal of the stream of slurry from the tank by automatically starting the associated pump, or increasing its pumping rate, when the pressure sensed by the pressure cells decreases below a predetermined value corresponding to the desired specific gravity, the pressure cells being arranged automatically to discontinue the pumping, or to reduce its rate, when the sensed pressure regains its predetermined value. In this regard it will be appreciated that the pressure exerted by the slurry, at a particular depth, is directly related to the specific gravity of the slurry which in turn is, directly related to the solids concentration in the slurry. The pressure cell or cells are thus functionally equivalent to the load cell or cells, the cell or cells in question in each case being, for example, electrically connected to a switch or pump flow control means for switching the associated pump on or off or for controlling its pumping rate, which pump is thus conveniently electrically driven. In particular, the specific gravity control device may comprise at least one cell selected from the group consisting of load cells on which the reservoir is supported for sensing the mass of the reservoir, and pressure cells in the interior of the reservoir for sensing static pressure in the reservoir, the cell being operatively connected to a shut-off valve for supplying liquid or slurry to the spray device. 
     The pump for withdrawing the slurry stream from the tank, and the pump for withdrawing slurry for consumption from the tank, may each be submersible pumps suspended in slurry in the tank, or, conveniently, may each be outside the tank having inlet pipes projecting downwardly into the slurry in the tank, in each case extending downwardly via the open top of the tank into the slurry, so that the mass of or pressure in the tank is essentially unaffected thereby, there being no direct physical connection between said pumps (or the their inlet pipes), and the tank walls of floor, particularly when load cells are employed. If desired, the inlet pipes to the pumps which project downwardly from the pipes into the slurry, may be replaced by flexible pipes or hoses connected to the tank, in which case it may be preferable to replace the load cells with pressure cells which function as described above. 
     If desired, the installation may be in the form of a portable apparatus. Thus, the reservoir and the associated slurry withdrawal means, volume control means and separate gravity control means may be of a size which permits them to be taken from site to site, where they are installed, on a vehicle such as a truck or lorry, and the support may, instead of being a concrete slab, be a container, such as a portable shipping container, having an open top to permit loading thereof, for example, with particulate material by means of a front end loader, and having a closeable slot at an end thereof from which drainage of slurry can take place if the container is mounted at an angle with the slot lowermost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example, with reference to the accompanying diagrammatic drawings, in which: 
     FIG. 1 is a schematic plan view of an installation according to the invention; and 
     FIG. 2 is a schematic side elevation of the installation of FIG.  1 . 
    
    
     In FIGS. 1 and 2 of the drawings the same reference numerals are used to designate the same parts, unless otherwise specified, and, in the drawings an installation for forming and maintaining a stock of slurry, in the form of a limestone or calcium carbonate slurry is generally designated by reference numeral  10 . 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The installation  10  comprises a concrete slab, generally designated  12 , shown in FIG. 2 to be supporting a heap  14  of limestone powder. The slab  12  has a flat lower surface (not shown) resting on the ground surface and an upwardly facing support surface supporting the heap  14 , the support surface being made up of two flat portions  16  (see in particular FIG.  1 ). The portions  16  shape and converge downwardly from uppermost ridges  18  to meet at a line of intersection  20  therebetween. The ridges  18  meet at a position  22  which is at the uppermost end of the line of intersection  20 , the line of intersection sloping downwardly from an upper end thereof at the position  22 , to a lower end thereof at  24 , at the level of the ground  26 . The line of intersection  20  defines the bottom of a drainage channel formed in the support surface of the slab  12  by and between the portions  16  which drains, downwardly from position  22  to position  24 , position  24  forming a drainage point where the slab  12  finally drains. 
     The slab has a pair of ramps  28  sloping upwardly from the level of the ground  26  respectively to the ridges  18 . The ridges  18  are horizontal and the ramps  28  are flat and slope at an angle of 13° to the horizontal, upwardly from ground level to their respective ridges  18 , which are 1.14 m above ground level and are each 12.02 m long. The line of intersection  20  is 13.05 m long. 
     The portions  16  of the support surface of the slab  12  each have a short edge  30  sloping downwardly from an upper end thereof, at a position  31  which is 0.35 m above the ground, towards the line of intersection  20 , which they meet at the position  24  Each portion  16  further has an edge  32  sloping upwardly from the associated position  31  to the adjacent end of the associated ridge  18 . Each edge  32  has a 0.3 m high wall (broken lines at  33  in FIG. 2) extending along its length. The ground level periphery of the slab  12  is made up of the lower edges  34  of the ramps  28 , the lower edge  36  of a triangular face  37  on the slab  12  whose corners are at the adjacent ends of the edges  34  and the position  22 , and edges  38  of quadrangular faces  39  of the slab  12 . The edges  32  of the faces  39  are at an included acute angle A of 7.1° to the edges  38  of said faces  39 . Each face  39  has its corners respectively at opposite ends of its edge  38 , at the associated position  31  and at the end of its edge  32  remote from the position  31 . While the face  37  in fact slopes upwardly and inwardly from its edge  36  to the position  22 , the faces  39  are vertical, the end of each ridge  18  remote from the position  22  being directly above the associated edge  38 . The ramps  28  are elongate rectangular in outline having end edges joining the edges  18 ,  34  thereof, the end edges being 5.06 m in length, the edge  36  in turn being 6.97 m long and each edge  32  being 12.09 m long, while each edge  38  is 16.93 m long. 
     Adjacent the edges  30  of the portions  16  of the upwardly facing support surface of the slab  12  is a pit  40  in the ground  26  (the pit  40  not being shown in FIG.  1 ). In the illustrated pit an open topped tank  42  is mounted on a pair of load cells  44 . The tank  42  contains a stirrer  46  (not shown in FIG. 1) and a valve, such as the illustrated float controlled inlet valve  48 , is suspended in the top of the tank  42  at the end of a water supply line  50 . A drainage line  52  is shown feeding from the lower end  24  of the line  20  of intersection  20  of the slab  12  into the tank  42 , the line  52  being defined by a pipe  54  (not shown in FIG.  1 ). 
     A flow line defined by a pipe  56  extending downwardly into the tank  42  via its open top leads to a pump  58  which feeds into a hose  60  having a spray nozzle at its end remote from the pump  58 . Similarly a flow line defined by a pipe  64  extending downwardly into the tank  42  via its open top leads to a pump  66  which feeds Into a pipe  68  leading to a container/aeration tank  70  of an acid water neutralization plant, the tank  70  being shown feeding via a line  72  into a thickener  74 . 
     In a variation of the slab  12  (which is not drawn to scale in FIGS.  1  and  2 ), the dimensions are somewhat different. Thus, in the variation the ridges  18  are 12.03 m long, the line of intersection  20  being 13.09 m long, while the ramps  28  remain inclined at 13° to the horizontal. The ridges  18  are 1.5-1.6 m above the flat bottom of the slab and the position  31  is 0.4-0.5 m above the bottom of the slab. Edge  32  of face  39  is 6.41 m in length and edge  38  is 7.63 m in length, angle A being 8.5° Face  37  has its edge  36  1.83 m long and the short edges of the ramps  28  interconnecting the ends of the edges  34  and the ends of the ridges  18 , are 1.33 m in length. In use it is intended that this variation of the slab be partially embedded in the ground, with its flat lower surface horizontal and 1.07 m below ground level, and its ridges  18  horizontal and 0.3 m above ground level. Edges  32  of faces  39  will in this case be provided with walls having horizontal upper edges which project 0.5 m above ground level; and the top of the tank  42  will be sufficiently sunken below ground level to permit drainage from position  24  on the slab into the open top of the tank  42 . In a yet further variation, if it is desired, for example, to have the tank  42  on and above the ground to avoid the need for a pit  40  (see FIG. 2) either of the slabs  12  described above can be raised above ground level on a platform or plinth, in which case loading of the slab  12  can, instead of being by means of a tipper truck as described hereunder for FIGS. 1 and 2, be by means of a front end loader. Finally, with regard to the slab  12 , it need not be entirely solid, and its lower surface need not be flat, the slab, for example having downwardly facing cavities or indentations to effect a saving of concrete. 
     In accordance with the method of the present invention, and with reference to FIGS. 1 and 2 of the drawings, at start-up, one or more loads of calcium carbonate may be loaded on to the upwardly facing support surface constituted by the portions  16  of the upper surface of the slab  12 . This is conveniently carried out by tipper trucks, which can reverse up either of the ramps  28  prior to tipping, so as to tip on to the portions  16 , to form the heap  14  (FIG. 2) of calcium carbonate there The tank  42  can, at the same time, be at least partially filled with water, from the water supply line or pipe 
     Then, the pump  58  is used to pump water from the tank  42  along the pipe  56  and hose  80  to the nozzle  62 , and the nozzle  52  is directed by hand at and on to the calcium carbonate of the heap  14 , above or adjacent the position  24  at the lower end of the channel defined by the line of intersection  20  between the portions  16 . A slurry of calcium carbonate is formed, which is washed towards said position  24 , the slurry then draining in the direction of flow line  52  along the pipe  54  (FIG. 2) into the open top of the tank  42  In this way a starting charge or load of slurry can be charged into the tank  42 , up to a desired level or depth, which can be determined by the float-controlled valve  48  which admits water into the tank  42  from the pipe  50 . 
     Thereafter, after start-up, slurry for consumption is pumped by pump  66  along fines  64  and  68  and line  72  in series to the conditioner/aeration tank  70  and thickener  74 , as required. As soon as such pumping for consumption takes place, the float-controlled valve  48  will open, to admit further water into the tank  42  from flow line  50 . The load cells  44  on which the tank  42  is mounted measure the mass of the tank  42  and it slurry contents. In this regard it is to be noted that the pipe  54 , the stirrer  46  which is employed to keep the contents of the tank  42  agitated and to resist settling of solids from the slurry, and the pipes  50 ,  56  and  64  and the float-controlled valve  48 , are not connected to the tank  42 , but are merely suspended to project downwardly into its interior, into the slurry in the tank  42 . The thus do not contribute to the mass of the tank  42  and its contents, as measured by the load cells  44 . 
     As slurry for consumption is withdrawn by pump  66  and water is admitted from pipe  50  via float-controlled valve  48 , the specific gravity of the slurry in the tank  42  progressively decreases, until it falls below a desired level (unless it was already below the desired specific gravity value at start-up). If the specific gravity is, or as soon as its falls below this desired level, the load cells  44  automatically cause operation of the pump  58  to spray slurry from the nozzle  62  on to the heap  14 . This washes additional calcium carbonate from the heap into the tank  42  along line  52 , thereby raising the specific gravity of the slurry in the tank  42 , until it exceeds said desired level, at which stage the load cells  44  switch off the pump  58 . 
     From time to time the calcium carbonate on the heap  14  is replenished from tipper trucks, and in this fashion a full charge of slurry of the appropriate specific gravity is maintained in the tank  42 , for consumption as and when desired, in the conditioning/aeration tank  70  and thickener  74 . 
     The method as described above will involve intervention by an operator from time to time to tip calcium carbonate on to the slab  12 , and to direct the hand-held hose  60  and nozzle  62  at appropriate parts of the heap  14 . Naturally, however, if increased automation is desired, several hoses  60  may be provided with nozzles  60  which are either directed in fixed attitudes and in fixed positions at different parts of the heap  14 , or which are moved automatically and mechanically when the pump  58  is operative, to lay down a desired spray pattern on the calcium carbonate of the heap  14 . Thus the calcium carbonate can be progressively entrained in the circulating slurry, and is washed into the tank  42  in a fashion such that, if the calcium carbonate of the heap  14  is not replenished, it will eventually all be washed into the tank  42 . 
     It should be noted that the capacity of pump  58 , and the capacity of the water supply along pipe  50 , should be selected so that the slurry level in the tank  42  can be maintained at the desired level even if the pump  66  is operated continuously, and so that the solids concentration and specific gravity of the slurry can also be maintained at the desired level, in the event of such continuous pump operation. 
     It is an advantage of the invention that it provides a method and installation for forming and maintaining a stock of slurry in the tank  42 , for example a water/calcium carbonate slurry, in a easily applied automated or semi-automated fashion.