Process and apparatus for refining sand

Abstract

A process for refining sand for use as frac sand includes the steps of passing the sand through a first fines separation stage to remove fine particles of contaminant from the sand, reducing the water content of the sand (such as to less than 20%), passing the sand into an attrition scrubber unit containing moving blades to delaminate clay and other contaminants from the sand grains, passing the sand from the attrition scrubber unit through a second fines separation stage to separate fine contaminants from the sand, and dewatering the resulting sand product in a further dewatering stage.

Claims

1. A process for refining sand for use as frac sand, the method comprising the steps of: passing the sand through a first fines separation stage to remove fine particles of contaminant from the sand; reducing the water content of the sand to less than 20%; passing the sand into an attrition scrubber unit containing moving blades to delaminate clay and other contaminants from the sand; passing the sand from the attrition scrubber unit through a second fines separation stage to separate fine contaminants from the sand; dewatering the resulting sand product in a further dewatering stage; and controlling the water content of the sand upstream of the attrition scrubber unit such that the sand entering the attrition scrubber unit has a water content of between 20% and 25%; wherein the water content of the sand is controlled by determining the water content of the sand and adding water to the sand to achieve the required water content, and wherein the water content of the sand is determined by monitoring the torque demand of the attrition scrubber unit.

2. The process of claim 1, wherein at least one of the first and second fines separating stages are carried out in a respective hydro-cyclone unit, and wherein the hydro-cyclone unit comprises multiple cyclones arranged in parallel to one another.

3. The process of claim 1, wherein the water content of the sand downstream of the first fines separation stage is reduced by a first dewatering screen.

4. The process of claim 1, wherein the further dewatering stage is carried out on a second dewatering screen.

5. The process of claim 1, comprising a first step of grading the sand to remove oversize material from the sand on a vibratory screen having an apertured deck upstream of the first fines separation stage.

6. The process of claim 5, wherein the step of grading the sand removes material having a particle size of greater than 2 mm.

7. The process of claim 1, wherein the sand is passes into a counter flow classification unit upstream of the second fines separation stage, wherein an upwards flow of water separates particles smaller than 200 μm from the sand, the sand product settling in the bottom of the counter flow classification unit, the sand being removed from the bottom of the counter flow classification unit before being passed into the further dewatering stage.

8. An apparatus for refining sand for use as frac sand, said apparatus comprising: an elongate chassis; a first fines separating device mounted on the chassis for separating fine material from the sand; a first dewatering screen mounted on the chassis adjacent to and downstream of the first fines separating device and configured to receive sand therefrom; an attrition scrubber unit mounted on the chassis adjacent to and downstream of the first dewatering screen, the attrition scrubber unit configured to receive sand from the first dewatering screen, the attrition scrubber unit containing a plurality of moving blades adapted to delaminate clay and other contaminants from the sand as the sand passes through the attrition scrubber unit; a second fines separating device; a second dewatering screen provided on a second end of the chassis, opposite a first end thereof, for receiving the sand from the second fines separating device and operable to dewater the sand product; and a control device that is operable to control the water content of the sand upstream of the attrition scrubber unit such that the sand entering the attrition scrubber unit has a water content of between 20% and 25%; wherein the control device is operable to determine the water content of the sand downstream of the first dewatering screen and add water to the sand as required to achieve the water content of between 20% and 25%, wherein the control device determines the water content of the sand by monitoring torque applied the attrition scrubber unit.

9. The apparatus of claim 8, wherein the first fines separating device comprises a first hydro-cyclone unit.

10. The apparatus of claim 9, wherein the second fines separating device comprises a second hydro-cyclone unit, and wherein at least one of the first and second hydro-cyclone units comprises multiple cyclones having a common inlet manifold and a common outlet manifold.

11. The apparatus of claim 10, wherein the second fines separating device further comprises a classifier unit mounted on the chassis for receiving sand from the second hydro-cyclone unit for further separating fine contaminants from the sand.

12. The apparatus of claim 11, wherein the classifier unit comprises a counter flow classification unit comprising at least one tank, wherein the at least one tank is configured to receive the sand downstream of the second hydro-cyclone unit, wherein the at least one tank is configured to receive an upward flow of water passed into a lower region of the at least one tank at a predetermined velocity to thereby lift fine particles up into an overflow weir of the tank, and wherein a lower region of the tank is configured to collect settled sand product.

13. The apparatus of claim 8, further comprising a grading screen mounted on a first end of the chassis, upstream of the first fines separating device.

14. The apparatus of claim 8, wherein the chassis is separable into two or more elongate sections to allow the apparatus to be containerized for transportation to the site.

15. The apparatus of claim 8, wherein the first and second dewatering screens, the first and second fines separating devices, and the attrition scrubber unit comprise respective stages mounted on and spread along the length of the chassis in an arrangement that minimizes the distance the sand is moved between each stage.

16. The apparatus of claim 8, wherein the first and second dewatering screens, the first and second fines separating devices, and the attrition scrubber unit are located at substantially the same height along the length of the chassis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A frac sand refining apparatus and method will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a perspective view of an apparatus for refining frac sand in accordance with an embodiment of the present invention;

(3) FIG. 2 is a side view of the apparatus of FIG. 1;

(4) FIG. 3 is a plan view of the apparatus of FIG. 1;

(5) FIG. 4 is a longitudinal sectional view through the apparatus of FIG. 1;

(6) FIG. 5 is a detailed plan view of the attrition cell cluster of the apparatus of FIG. 1; and

(7) FIG. 6 is a longitudinal sectional view through one bank of cells of the attrition cell cluster of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) As illustrated in the drawings, a frac sand refining apparatus in accordance with an embodiment of the present invention includes an elongate chassis 2 upon which the various stages of the refining apparatus are mounted. The chassis 2 may be separable into two or more sections to allow the apparatus to be containerised for transportation to the site. The components of the various stages of the apparatus are mounted on and are spread along the length of the chassis 2, minimising the distance the sand has to be moved between each stage. Furthermore, the various stages of the apparatus are located at substantially the same height along the length of the chassis 2, reducing pumping loads compared to prior art plants, where sand and water typically have to be pumped to considerable height as they are transferred between different stages of the refining operation.

(9) In a first stage, material is loaded onto a lower end of an upwardly inclined feed conveyor 4 provided at a first end of the chassis 2, including a belt conveyor, and is delivered onto an inclined vibrating grading screen 10 located on the first end of the chassis 2 beneath the feed conveyor 4. The feed material is screened by the grading screen 10 into an oversize (waste product), passing off a lower end of the grading screen 10 to be delivered into a hopper or skip or onto a belt conveyor located beneath the feed conveyor 4 at the first end of the chassis 2. The undersize material (i.e. the sand product) passes through apertures in the deck of the grading screen 10 to be collected beneath the deck of the screen in a sump. Typically the cut point of this first stage may be approximately 2 mm (i.e. the screen deck having 2 mm apertures) to enable 250 tons per hour of material to pass through the 2 mm apertures of the screen deck, whereby the oversize material having a particle size greater than 2 mm is removed from the sand product. It is envisaged that decks having a smaller aperture size may be used where a smaller grain size if required. In one embodiment the screen deck may be 1.8 meters wide and 8 meters in length.

(10) In a second stage, the product from the grading screen 10 is pumped from the sump of the grading screen 10, preferably using a centrifugal slurry pump and rubber lined pipework, to a first set of hydro-cyclones 20. The first set of hydro-cyclones 20 perform a separation process, typically removing very fine particles of sand and contaminants (e.g. clay & organics), as well as dewatering the product. The cut point of the second stage may be in the region of 63-75 μm. The hydro-cyclones 20 will also increase the concentration (i.e. reduce the water content) of the product by removing water as well as contaminants to facilitate the next stage of the process. Typical the underflow of the first set of the hydro-cyclones 20 is in the region of 1000 g/l. As shown in the drawings, the first set of hydro-cyclones 20 may include four cyclone arranged in adjacent pairs, fed from a common manifold and delivering an underflow and overflow to respective common outlet manifolds.

(11) The third stage of the process is a dewatering stage, wherein the product from the first set of hydro-cyclones 20 is delivered onto a first vibratory dewatering screen 30 mounted on the frame 2 adjacent the first set of hydro-cyclones 20. The first set of hydro-cyclones 20 are mounted above a feed end of the first dewatering screen 30 such that the underflow of the hydro-cyclones 20 may be delivered onto the dewatering screen 30 by gravity.

(12) A linear reciprocating force is applied to the first dewatering screen 30, via a pair of counter rotating eccentric masses as is known in the art, at a desired stroke and at a set frequency. This reciprocating motion of the screen 30 effectively shakes the excess water from the product, through small holes in the deck of the screen, reducing the moisture content of the product down to about 15%.

(13) In a fourth stage, the product (sand) from the first dewatering screen 30 is then discharged from a discharge end of the screen and is subsequently gravity fed (or alternatively pumped) into an attrition cell cluster 40 mounted on the frame 2 adjacent and downstream of the first dewatering screen 30. The attrition cell cluster 40 includes a plurality of attrition scrubber cells (up to eight) arranged in series, each containing rotating blades which force sand grains against each other, resulting in intense scrubbing, polishing and disintegration of the sand, delaminating clay, graphite and other contaminants from the sand grains.

(14) FIGS. 5 and 6 illustrate the attrition cell cluster 40 in more detail. As can be seen from FIG. 5, the attrition cell cluster includes eight cells 42 arranged in two banks of four, each cell 42 having a drive motor 44 mounted at an upper end and containing a vertically extending drive shaft 46 having a plurality of blades or vanes 48 mounted thereon. The drive motors 44 rotate the drive shafts 46 and thus move the blades 48 through the sand slurry contained within each cell 42. Openings 49 are provided between the cells 42 in each bank at alternating between upper and lower ends of the adjacent cells so that sand must pass through all of the cells 42 of each bank of cells in series, preferably passing vertically though each cell between the openings 49.

(15) The water content of the product entering the attrition cell cluster 40 is carefully controlled to obtain a water content of 20% to 25% (adding water to the product to achieve the desired water content) to ensure optimum operation of the attrition cell cluster. This may be achieved by monitoring the torque load applied to the blades 48 by the drive motor 44 of the upstream most cell 42 of the attrition cell cluster 40 and adding water as necessary to achieve the optimum water content, resulting in maximum attrition of the sand. The attrition cell cluster 40 will remove all surface contamination off the sand grains. The water content may be controlled by means of a PLC (programmable logic controller), monitoring the torque of the motor 44 and controlling a motorised or pneumatically operated valve to add water into the cell 42 as required.

(16) The product discharged from the attrition cell cluster 40 is fed into a sump or tank 42 mounted in the frame adjacent and downstream of the attrition cell cluster 40. Fresh water is added to the sand in the sump 42 to achieve the correct concentration for a subsequent pumping process (typically 350 g/l). A centrifugal slurry pump may be then used to feed the product into a second set of hydro-cyclones 50.

(17) The second set of hydro-cyclones 50 perform another separation process in a fifth stage of the refining process, removing the very fine material (clay and other contaminants) separated from the sand grains in the attrition process. As shown in the drawings, the second set of hydro-cyclones 50 may also include an arrangement of four cyclones arranged in pairs, fed from a common manifold and delivering material to common outlet manifolds above and below the cyclones.

(18) In a sixth stage, the underflow of the second set of hydro-cyclones 50 is fed into two (or more) counter flow classification units (CFCUs) 60A,60B upon which the second set of hydro-cyclones 50 are mounted. As shown in the drawings, each pair of hydro-cyclones 50 of the second set of hydro-cyclones 50 may feed a separate CFCU, the CFCUs 60A,60B being mounted side by side on the chassis 2 to maximise classification of the product without increasing the width of the apparatus. The CFCUs 60A,60B may separate particles smaller than 100 um or 200 um from the product, the cut point possibly varying from customer to customer to suit customer requirements.

(19) In each CFCU 60A,60B the product is passed into a tank, wherein an upward flow of water is pumped into a lower region of the tank at a predetermined velocity (typically 8-10 mm/s) to lift finer particles up into an overflow weir of the tank. Larger particles will 20 want to settle in the tank, sinking to the bottom of the tank.

(20) Using pressure transducers, the ‘teeter bed’ inside the tank of each CFCU can be determined, then, using this signal, a PLC can control the operation of an outlet valve, such as a modulating outlet pinch valve, at the bottom of each tank to maintain the level of this teeter bed. Using these controls a consistent level of separation can be performed. The overflow from each CFCU tank, including water contaminated with clay and other fine materials separated from the sand product, may be piped to a further process, such as a further cyclone separator and or dewatering screen, to allow the water to be recycled.

(21) The underflow (product) of the tank of each CFCU is transferred to a seventh stage, including a further dewatering screen 70, for removing excess water from the sand product, which may then be discharged onto a stockpile conveyor.

(22) The invention is not limited to the embodiment(s) described herein but can be amended or modified without departing from the scope of the present invention. Therefore, it will be appreciated that changes and modifications to the specifically described embodiments may be carried out without departing from the principles of the present invention, which is intended to be limited only by the scope of the appended claims as interpreted according to the principles of patent law including the doctrine of equivalents.