SYSTEM AND METHOD

Background

A gravitational classifier apparatus, system and method for separating mixtures of materials are described and, more particularly, a gravitational classifier apparatus, system and method for use in separating granular mined materials into its constituents for recovering valuable material, such as valuable metals.

In a conventional mining process, iron bearing rock is crushed and processed and then separated in flotation tanks. About 75% of the iron in the rocks is captured in the flotation tanks capture. The remaining 25% of the iron and all of the silica sand, plus other minor components, comprise an effluent sludge from the flotation tanks. The sludge held in tailings dams. The materials mixtures in the tailings dams will vary by geography, but generally iron fines represent about 25%, silica sand is about 50% and the remaining 25% is lighter materials, including clay and other miscellaneous heavier materials.

The separation of mixtures of mining materials to collect valuable metals tends to be expensive or inaccurate in certain applications. Conventionally, the process of separating mining materials comprises a mechanical method and apparatus for recovering selected constituents of the mined material from a path of movement. The desired constituents are separated from the other constituents also moving along the path of movement. The separating apparatus typically comprises an inclined, vibrating conveyor and, optionally, screening, wherein the desirable materials are collected off the end of the conveyor. This process relies on gravity separation to collect the heavier components, such as gold and other precious metals.

For the foregoing reasons, there is a need for an apparatus that can separate the constituents of mixtures of mining materials to recover valuable material. Ideally, the apparatus will cause the mixture of materials to be economically processed in a continuous manner at high throughputs and in large volumes while recovering the valuable materials. Summary

An apparatus is provided for separating a mixture of solid materials including mining materials into constituent parts. The apparatus comprises a substantially planar endless belt having a proximal feed end for receiving a slurry of the materials, an elevated distal discharge end, and sides extending between and interconnecting the feed end and the discharge end. A conveyor assembly rotates the endless belt longitudinally upwardly at an incline from the feed end to the discharge end. Means are provided for vibrating the belt. An overhead spray array includes a plurality of nozzles for spraying fluid under pressure downwardly onto the slurry as the slurry travels on the conveyor belt for agitating the mixture of materials and influencing the constituents of the mixture to separate based on specific gravity. The material constituents having a higher specific gravity are urged along the conveyor to the discharge end for collection and the material constituents having lower specific gravity are urged off the sides of the conveyor belt for collection under the action of the fluid spray. The apparatus may further comprise a feed tank for receiving a fluid and the mixture of materials to be separated for forming the slurry of the mixture of materials. The fluid may be water.

In one aspect, the vibrating means may comprise a vibrating tray for supporting the belt. In an embodiment, the vibrating means comprises an electric motor, an eccentric driven by the electric motor, and a connector extending from the eccentric to the tray. The tray can have a central crown such that the belt surface slopes downwardly from an apex along a central longitudinal axis of the belt. The downward slope may be at an angle of about 8 degrees relative to the horizontal.

In another aspect, the separating apparatus may further comprise a lower spray array for spraying fluid in a generally upward direction against an underside of the belt. The overhead spray array and lower spray array can be plumbed to provide for independent operation. In one embodiment, the separating apparatus further comprises one or more of a needle valve, a pressure reducing valve, an isolating valve, or any combination thereof for regulation of the spray nozzles.

In a further aspect, the overhead spray array includes an array of overhead spray bars longitudinally spaced above the belt and extending across the width of the belt, and wherein a plurality of nozzles are provided at spaced intervals along the length of the spray bars. The overhead spray array can be plumbed to provide for independent operation of each overhead spray bar. At least some of the nozzles may be directed approximately perpendicular to, or at a selected angle relative to, the belt surface for adjusting the direction of fluid discharge from the nozzles.

In an embodiment, the conveyor assembly includes a variable speed motor providing for variable speed of rotation of the belt. A predetermined length of the endless belt adjacent the feed end may be planar.

In another embodiment of the separating apparatus, the surface of the belt has a textured surface for impeding a flow of the materials along the belt under the action of the fluid sprays as the endless belt moves up the incline. In an aspect, the textured surface comprises a plurality of discrete recesses arranged in longitudinal and transverse rows in the surface of the belt.

A method is also provided for separating a mixture of solid materials including mining materials into constituent parts. The method comprises the steps of feeding a slurry of the materials on to a substantially planar endless belt having a proximal feed end for receiving a slurry of the materials, an elevated distal discharge end, and sides extending between and interconnecting the feed end and the discharge end. The endless belt is rotated upwardly at an incline between a first position and a second position, wherein the second position is elevated with respect to the first position, while vibrating the belt. Fluid is sprayed under pressure downwardly onto the slurry as the slurry travels on the conveyor belt for agitating the mixture of materials and influencing the constituents of the mixture to separate based on specific gravity. Collecting of the material constituents having a higher specific gravity urged along the conveyor occurs at the discharge end. Collecting occurs for the material constituents having lower specific gravity urged off the sides of the conveyor belt under the action of the fluid sprays.

In one aspect, the separating method further comprises the step of maintaining at least a portion of the endless belt adjacent the feed end planar and free of fluid sprays. The sprays may be directed at an angle relative to the belt surface.

In another embodiment, the separating method further comprises the step of providing at least a portion of the endless base section with a textured surface for impeding a flow of the materials along the belt under that action of the fluid sprays as the endless belt is moved up the incline.

In another embodiment, the separating method further comprises the step of directing sprays upwards against an underside of the belt via a lower spray array. In a further aspect, the step of rotating the belt comprises rotating the belt at variable speeds depending on characteristics of the slurry.

Brief Description Of The Drawings

For a more complete understanding of a gravitational classifier apparatus, system and method for separating mixtures of materials, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings:

FIG. 1 is a side elevation view of an embodiment of an apparatus for use in separating mixtures of materials including minerals into fractional components.

FIG. 2 is a top plan view of the separating apparatus as shown in FIG. 1.

FIG. 3 is a top plan view of the separating apparatus as shown in FIG. 2 showing two water flow headers.

FIG. 4 is a top plan view of the separating apparatus as shown in FIG. 3 showing an embodiment of a spray bar arrangement.

FIG. 5 is a schematic top plan view of the separating apparatus as shown in FIG. 4 showing a spray bar configuration.

FIG. 6 is an end elevation view of the separating apparatus as shown in FIG. 1. FIG. 7 is a schematic elevation view of an embodiment of a nozzle arrangement along a spray bar for use with the separating apparatus as shown in FIG. 1.

FIG. 8 is a schematic side elevation view of the separating apparatus as shown in

FIG. 1.

FIG. 9 is a schematic side elevation view of the separating apparatus as shown in FIG. 8 showing an embodiment of an electrical system.

FIG. 10 is a schematic side elevation view of the separating apparatus as shown in FIG. 8 showing an embodiment of a hydraulic system.

FIG. 11 is a transverse cross-section of the separating apparatus as shown in FIG.

1

FIG. 12 is a schematic view of a portion of a conveyor belt showing a pattern on a top surface for use with the separating apparatus as shown in FIG. 1

FIG. 13 is a transverse cross-section of the conveyor belt as shown in FIG. 12. FIG. 14 is a close-up top plan view of a section of the conveyor belt as shown in FIG. 12.

Description

Certain terminology is used herein for convenience only and is not to be taken as a limiting. For example, words such as "upper," "lower," "left," "right," "horizontal," "vertical," "upward," "downward," "top" and "bottom" merely describe the configurations shown in the Figures. Indeed, the components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise. The words "interior" and "exterior" refer to directions toward and away from, respectively, the geometric center of the core and designated parts thereof. The terminology includes the words specifically mentioned above, derivatives thereof and words of similar import.

As used herein, the term "mined material” is intended to include, but is not limited to, mean metalliferous material and non-metalliferous material. Iron- containing and copper-containing ores are examples of metalliferous material. Coal is an example of anon-metalliferous material. Coal is an example of a non- metalliferous material. The term "mined material” is further intended to include, but is not limited to, (a) run-of-mine material and (b) run-of-mine material that has been subjected to size reduction, for example primary crushing or similar size reduction and optionally other crushing and grinding steps, after the material has been mined and prior to being sorted, and (c) run-of-mine material that has been subjected to size classification without size reduction after the material has been mined and prior to being sorted. The "mined material" includes mined material that is in stockpiles.

The mined material may be any mined material that contains valuable material, such as valuable metals, particularly heavier metals such as gold. Examples of valuable materials are valuable metals in minerals, such as minerals that comprise metal oxides or metal sulfides. Specific examples of valuable materials that contain metal oxides are iron ores and nickel laterite ores. Specific examples of valuable materials that contain metal sulfides are copper-containing ores. Other examples of valuable materials are salt and coal.

It is also noted that the invention is not confined to copper-containing ores and to copper as the valuable material to be recovered. In general terms, the embodiments of the separating apparatus and method provide for separation of any mined materials which exhibit characteristics or properties that enable separation of high and low grade fragments.

Referring now to the drawings, wherein like reference numerals correspond to same or similar elements throughout the several views, an embodiment of a gravitational classifier apparatus and system is shown in FIGs.

1 and 2 and generally designated at 20. The separating apparatus 20 comprises an inclined rotating endless belt 22 for separating a slurry of a mixture of materials, including mined materials. The endless belt 22 runs on a vibrating tray 24 base for supporting the belt 22 and to further distribute the materials. The tray base 24 is crowned in the middle so that materials separated from the mixture are urged to fall all of both side edges of the belt 22 (FIG. 6 and 11).

Generally, the separating apparatus 20 may be configured in any manner using any type of frame 26 or structure for supporting at least a portion of the endless belt 22 at an inclined angle while allowing rotation of the belt. A fluid spray system or array 30 comprising a plurality of overhead spray nozzles 32 is also supported by the frame 26. Vibration of the endless belt 22 and the tray base 24 provides a mechanical basis for material separation. The spray system 30 and the nozzles 32 provide continuous mixing of the material mixture as it moves with the belt 22 along the tray base 24, thereby creating an environment for separation of the mineral mixture. The belt 22 speed, rate of vibration and fluid pressure in the spray system 30, as well as the composition of a feed slurry, are all parameters which can be used to calibrate the separating apparatus 20 for each material mixture.

An embodiment of a method for gravitational classification for separating a mixture of mined materials, comprises feeding a slurry of, for example, mined materials onto the endless belt 22, vibrating the tray base 24 and conveyor belt 22 as the slurry is conveyed along with the belt 22, directing a plurality of fluid sprays onto the slurry via the overhead array 30 of nozzles 32, and collecting the separated materials off the sides of the belt and at the discharge end of the belt 22. An optional additional step comprises directing a plurality of fluid spray nozzles 34 upwardly against an underside of the belt 22 via a lower pipe header 36.

The endless belt 22 has an upper surface 23 and a lower surface 25. The upper surface 23 receives the mineral mixture slurry as feed from a hopper 38 mounted on the frame 26. As shown in FIGs. 12-14, the upper surface 23 of the endless belt 22 has a diamond-shaped pattern embossed into a Vulcan rubber material forming discrete recesses to capture and separate certain materials having a higher specific gravity. In general, the recesses in the pattern is configured to retain the heavier material from the feed mixture. The fine particles of the heavier mineral materials are captured by the textured surface of the belt 22 during the separation process and carried to the discharge end of the upper run of the belt. Spray nozzles 40 at the discharge end of the belt spray the material off of the belt 22 and into a receiving tank 42 for collection. In one embodiment, the embossed pattern includes contiguous diamond shaped indentations including triangular indentations that result in many small peaks and pits that capture the fine minerals. The indentations have a depth as measured perpendicularly from the upper surface of the belt and space between the indentations. It is understood that different sized indentations may be used depending on the material being separated and the desired sizes of the minerals classified.

An upper run 44, or reach, of the endless belt 22 carrying the mineral mixture slurry is crowned at an angle of 8 degrees relative to the horizontal (FIG. 11). A portion 48 of the belt 22 at the lower feed end, about ten feet (3 m), may be flat in cross-section to allow time to establish the stratification process for the constituents of the material mixture. The upper run 44 of the belt has a length of about 72 feet (22 meters). It is understood that example dimensions for the separating apparatus are set out, but are intended to give a sense of scale for exemplary purposes, and are not intended to limit the scope of the description. The endless belt 22 moves in an ascending manner up an incline from a lower end to an upper end of the separating apparatus 20. The speed and incline angle of the belt 22 are variable and controlled to provide time for the separation process to occur and are adjustable so that separation of materials can be optimized. The endless belt 22 is about 8 feet (2.4 m) wide.

The vibrating tray base 24 (FIGs. 6 and 11) supports the endless belt 22. The tray base 24 is shaped so that there is a center crown to the transverse cross-sectional profile of the belt 22 (FIG. 11). This arrangement creates a slope of about 8 degrees slope from the longitudinally central axis of the belt 22 to each side edge. This allows the classified material to fall off both sides of the belt 22 into troughs 50 for collection, thereby doubling the capacity of the system. Vibration is provided by a mechanical eccentric system 62, including a counterweight 52 and a motor with continuous variable speed control over the range of operation. The tray base 24 and the counterweight 52 are mounted on springs 54 consisting of groups of flexible hardwood staves. The staves are approximately 3 feet (1 meter) long and mounted vertically along each side of the length of both components. The vibrating tray base 24 is formed stainless steel to ensure structural integrity in the face of the continuous vibration. The vibration of the endless belt 22 is established and maintained by the eccentric system 62 connecting the belt and the counterweight 52. Both of these elements are sprung using the vertical hardwood stave spring assemblies 54. In this embodiment, two 10 HP motors with variable speed drives control the rate of vibration.

As shown in FIGs. 1 and 2, the endless belt 22 is positioned on an upper end roller 56 and a lower end roller 58 for continuous rotation on the isolated vibrating frame 26. In this embodiment, one of the rollers 56, 58 is also a drive roller which is coupled to a variable speed motor to drive the roller and move the endless belt 22. Any suitable drive mechanism may be used and the present invention is not limited to any particular drive component. In this embodiment, the belt 22 is driven by a 3-phase, 10 hp motors that is controlled by a variable speed drive. Preferably, the endless belt 22 is moved at a speed in the range of about 150 to about 300 feet per minute. However, such speed will be dependent at least in part on the angle of incline of the belt 22 and on the application for which the separating apparatus 20 is being used. Support rollers 60 are positioned along the sides of the belt to keep it centered from end to end.

The fluid spray system 30 comprises an array of overhead nozzles 32 for spraying fluid downwards onto the slurry as it travels along the belt 22. A lower array of nozzles 34 spray fluid upwards against the lower reach 46 of the belt 22.

The upper array and lower array of spray nozzles 32, 34 are arranged in combination to assist in promoting separation of minerals on the upper surface 23 of the belt 22. A water distribution system provides fluid to the arrays of spray nozzles. The water distribution system comprises three subsystems or headers. A first header 70 feeds 760 overhead spray nozzles 32 positioned longitudinally along the length of the belt 22 and maintains a continuous environment for gravitational separation of the mixture along the upper run 44 of the belt. A second header 72 feeds spray nozzles 34 located under the belt 22 along the length of the tray 24 to provide lift lubrication for the movement of the belt for reducing resistance to movement. A third header 74 provides water to nozzles (not shown) in the feed hopper 38 to create a proper slurry mixture for the material fed onto the moving belt 22. As described above, an additional set of spray nozzles 40 may be provided at the end of the separating apparatus 20 for washing material, typically gold, entrained on the belt 22. A fluid pump or supply header provides a constant pressure of a fluid to the water distribution system. Manual valves provide a constant flow to each of the subsystems.

Referring to FIGs. 3-5, an upper portion of the frame 26 supports the first header 70 along the edge of the endless belt 22. Overhead spray bars 76 in fluid communication with the first header 70 extend transversely from the header relative to the direction of the belt 22 and span the entire width of the belt. In one embodiment, the spray bars 76 are equidistant from one another. The spray bars 76 carry a plurality of spaced nozzles 32 configured to direct jets of water downwardly onto the slurry being carried on the upper reach 44 of the endless belt 22. The array of nozzles 32 are positioned spaced apart transversely, and generally substantially perpendicular to, the length of the belt 22 along the length of each spray bar 76 extending from one side to the other side of the belt 22. The number of nozzles 32 per spray bar 76 will depend on the width of the belt, however there will typically be up to six nozzles 32 per spray bar 76. The nozzles are about 200 mm from the upper surface 23 of the belt 22. The nozzles 32 are configured so that they provide complete coverage across the width of the endless belt 22.

The second header 72 of the water distribution system feeds spray bars having nozzles 34 which direct a plurality of jets of water, for example two to five jets of water, upwardly against the lower surface 25 of the belt 22 across its entire width.

The water pressure at each nozzle 32 is set so that each zone covered by a spray bar 76 along the length of the belt experiences the appropriate material separation conditions. The result is that materials having a lighter specific gravity are continuously urged and exit off the sides of the belt 22 into troughs 50. The heavier material is captured by the profile of the textured upper surface 23 of the belt

22 and is discharged off the end of the belt.

FIG. 7 shows a schematic view of a typical spray bar 76 comprising an array of nozzles 32 projecting downwardly from the spray bar 76. Water is pumped to the spray bar 76 and the nozzles 32 through the first manifold 70. The water is then expelled out through the nozzles 32 in a stream which strikes the material mixture. Any one nozzle 32 acts on material on a certain proportion of the width of the belt 22. Each of the spray bars 76 is fitted with a manual valve to establish the appropriate pressure to the corresponding nozzles 32. The pressure of the sprays promotes sufficient agitation of the aggregate to keep the material from settling on the belt. The manifold 70 pressure is adjusted depending on the material mixture being separated. Each bank of nozzles 32 has a constant dynamic flow to enable constant mixing of the slurry on the belt.

The direction or angle of the nozzles 32 relative to the belt 22 is arranged to direct the stream of water projected from each nozzle 32 so as to allow agitation of the material. The nozzles 32 can be strategically rotated to optimize the separation action along the belt 22. Generally, the nozzles 32 should be angled so that the streams of water emitted are substantially parallel or strike the upper surface

23 of the belt 22 at a shallow angle. The angle of each nozzle 32 within the spray bar 76 array, or of all the nozzles along the spray bar, may be selectively varied in order to alter the angle with which the stream of water impinges the flow of material along the belt 22. Adjusting the discharge attitude of the nozzles 32 effects the angle of impact of the water on the material.

Means are provided for varying the flow rate of fluid, for example liquid, through each bank of nozzles 32. This is done using manual valves on each lateral branch of nozzles on a spray bar 76.

The water distribution system is supplied by a constant pressure system so that when each system is set there is no need for further changes. The settings of each subsystem are set and remain fixed for the duration of the run.

Once the system is set for the characteristics of the material being processed, the control parameters for the system are only the belt speed and the vibration of the belt. The other characteristics of the system and method are set only once for each type of material, including the nozzles 32 on the surface of the belt 22 which are positioned as required to maintain the mixing process. The nozzles in the inlet hopper 38 establish the liquidity of the flow of material onto the belt 22 for optimum spreading over the upper surface 32. The lower surface 25 of the belt 22 orifices 34 are set to provide the lift required to facilitate smooth operation. The nozzles 40 at the end of the belt 22 are set to clean the entrained material into the gathering system. The result is a low pressure operation that keeps the material from settling on the belt 22.

The nominal water requirement is proportional to the volume of the material being processed. A water management system may be provided, including a filtration system to filter and recycle the recovered water. With the water filtration system incorporated, water usage can be minimized by recycling the water that comes off the belt. About half of the incident water will be available to be recycled through the filter system.

In operation, the separating apparatus 20 creates and maintains a continuous environment for gravitational classification of a mixture of materials along the upper reach 44 of the belt 22. Initially, mined material in the inlet hopper 38 is conditioned with water using the hydraulic spray system in a regulated manner to control the consistency of the slurry feed. The slurry comprising the mined material or tailings of appropriate consistency is then fed from the bottom of the hopper 38 onto the upper surface 23 at the feed end of the belt 22. In one embodiment, the feed hopper 38 distributes the slurry substantially over the entire width of the belt 22. The hopper 38 supplies the slurry at a rate of approximately 10 cubic yards per minute.

Once on the belt 22, the slurry mixture moves with the belt. Heavy materials begin to settle almost immediately into the indentations in the embossed upper surface 23 of the belt 22 and the material begins to stratify according to specific gravity, experiencing the conditions for gravitational classification. As the slurry is conveyed along with the belt 22, the nozzles 32 extending from the upper spray bars 76 prevent the slurry from settling, which promotes a stratified mixture of particles on the belt 22. The flat portion at the beginning of the belt 22 and the array of 670 nozzles 32 over the length of the belt 22 creates the conditions so that as the material reaches the crowned section of the belt 22 the stratified material begins to fall off the sides of the belt into the gathering troughs 50 according to their specific gravity. Each nozzle 32 is directionally focused so that the effect is to maintain the turbulence in the material so that the stratified material sloughs off the side edges of the belt according to its specific gravity. Relatively heavier and lighter materials separate, and the heavier materials continue along with the belt 22. The lighter lower specific gravity materials tend to rise to the surface of the mixture, where they may remain until they are urged to fall off the edges of the belt 22. As described hereinabove, the separating apparatus 20 physically translates a vibration into the slurry moving down the belt 22 causing the mixture to become stratified. The vibration causes the stratification of the different sized constituents or particles of the feed mixture.

The separating apparatus 20 relies on specific gravity of the respective constituents of the material mixture to provide the forces required to cause particle separation. The material of lower specific gravity is moved or washed laterally with respect to the direction of movement of the belt 22 and ejected with the flow of fluid from the spray nozzles 32 off the outer side edges of the belt. The material of higher specific gravity continues its descent downwardly on the belt 22 through the nozzles 32 and is deposited off the end of the belt 22.

The end of the belt 22 is sprayed by a set of water nozzles 40 located at or adjacent the end of the belt to wash the material that is captured in the surface of the belt during the separation process as the belt goes over the end of the last conveyor roll into the gathering tank 42 for the heavier material that stays on the belt.

It is understood that the dimensions and positioning of the various components of the separating apparatus 20 can be adapted to suit the throughput and in accordance with the characteristics of the feed material. For example, the separating apparatus 20 may comprise a belt 22 having a various widths and length. The speed of the belt 22 will be varied to give maximum efficiency for the instant feed material conditions. Typically, the separating apparatus 20 will be configured to provide for various suitable belt speeds.

The number and spacing of the upper overhead spray bars 76 and the lower spray bars will also depend on the application. Their spacing provides a reasonable coverage of the belt. Similarly, the number and spacing of the nozzles 32 on each spray bar 76 are selected to as to provide complete coverage across the width of the belt. This number and spacing will depend on the type of nozzle used, the direction of the nozzles, water pressure, and distance between the spray bar and the belt. In one embodiment, the upper spray bars are positioned approximately 200 mm above the upper reach of the belt. The lower spray bars are positioned approximately 200 mm below the upper reach of the belt. Water is supplied to the upper and lower spray headers 70, 72, 74 at approximately adjustable rates and pressures.

The following non-limiting examples demonstrate the operation and output of the gravitational classifier apparatus, system and method for separating mixtures of materials. In this example, note that the specific gravity of iron is 5.3, the specific gravity of quartz sand is 3.5, and the specific gravity of other miscellaneous materials is about 2.0

Table 1. Recovery of Iron fines m ³ /min Tons/m ³ Tons/min Min/day Tons/day Tons/year

Feed rate 10

Fe recovered 2.5 5.3 13.3 1440 19,080 6,487,200

Feed rate 8

Fe recovered 2 5.3 10.6 1440 15,264 5,189,760

Feed rate 6

Fe recovered 1.5 5.3 8 1440 11,448 3,892,320

Table 2. Recovery of quartz sand m ³ /min Tons/m ³ Tons/min Min/day Tons/day

Tons/year

Feed rate 10

Sand recovered 5 3.5 17.5 1440 25,200

8,568,000

Feed rate 8

Fe recovered 4 3.5 14.0 1440 20,160

6,854,400

Feed rate 6

Fe recovered 3 3.5 11 1440 15,120

5,140,800

Although the gravitational classifier apparatus, system and method have been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that I do not intend to limit the description to the embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the apparatus, system and method, particularly in light of the foregoing teachings. For example, the apparatus, system and method are suitable for separating a broad range of mixtures of materials and not just mining materials. Accordingly, I intend to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the apparatus, system and method as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.