Patent Publication Number: US-9839915-B2

Title: Impact grinding plant for the communition of ore

Description:
FIELD OF THE INVENTION 
     There is provided an impact grinding plant. In particular, there is provided an impact grinding plant for the communition of ore. 
     DESCRIPTION OF THE RELATED ART 
     Communition of ore in the mining industry is most commonly performed by tumbling mills. These are typically in the form of ball mills, semi-autogenous grinding mills, or autogenous mills. A ball mill is a cylindrical vessel charged with metal grinding balls. Ore is introduced to the mill as it rotates. The balls and ore particles collide with each other to cause size reduction. This motion can be characterized as collision with breakage induced primarily by impact or as rolling with breakage induced primarily by crushing and attrition. Autogenous mills typically have larger diameters than ball mills, causing the material to be dropped from a greater height. Size reduction in an autogenous mill occurs due to direct collisions between ore particles. A semi autogenous mill, often referred to as a SAG mill, operates in a manner similar to an autogenous mill, except that some metal balls are introduced to assist the grinding process. The existing technology used for tumbling has changed little in recent years, other than, for example, increases in the size of the mills. 
     An alternate grinding technology starting to grow in popularity is the use of high pressure grinding rolls, often referred to as HPGRs. HPGR mills consist of two rollers of the same dimensions, rotating against each other with the same rotational velocity. Bulk material is ground as it is fed between the two rollers which are pressed against the material by springs or hydraulic cylinders. These mills may not have yet demonstrated significant improvement over tumbling mills. 
     A number of other grinding technologies exist such as tower mills or ISA mills. However these are typically specialized for grinding specific materials, high fineness or small volumes, and may not be suited for the large volumes necessary for industrial mineral processing facilities. 
     Communition, or size reduction of particulate material is an extremely energy intensive process. Some estimates suggest that communition consumes 3-6% of all electricity generated worldwide. Much of this energy is expended in mining and mineral processing. Grinding, or the process of reducing particles to a size small enough to perform common mineral processing functions, is one of the most important communition processes. Unfortunately the energy efficiency of grinding remains low. Tumbling mills, the most common method of grinding in mineral processing typically only apply 25% of consumed energy to break particles. The remainder is lost as heat, noise or friction. It is therefore highly desirable to increase the energy efficiency of grinding. 
     Grinding typically occurs in two modes. In impact grinding, a moving particle impacts a hard surface and breaks into smaller particles. In abrasive grinding, the motion of moving particles in contact with each other causes them to break apart. Impact grinding is more efficient due to greater incidence of first impact breakage. First impact breakage refers to particles breaking on initial impact, rather than after repeated impacts. In abrasive grinding, particles typically require multiple impacts before particles break, which typically consumes more energy. 
     In tumbling mills, particle impact velocities have been increased in recent years by increasing the diameters of mills, particularly the diameters of semi-autogenous grinding (SAG) mills. A larger diameter causes particles to fall from a greater height, undergoing longer acceleration and therefore impacting at a higher velocity. However tumbling mills are possibly approaching the maximum diameter that can be practically fabricated, moved and installed. Indeed, the largest mills may now reach twelve meters in diameter, a size comparable to a four story apartment building. The technical challenges in constructing even larger mills, moving them, installing them and putting them into rotating operation are significant. Fabricating, transporting and installing large tumbling mills is difficult and requires a significant amount of specialized transport and construction equipment. 
     Charge participation can increase grinding efficiency and it refers to the percentage of material that is impacted with each rotation of the mill. Tumbling mills may not achieve full charge participation as some percentage of material is not lifted, or insufficiently lifted during each rotation. 
     Tumbling mills may also suffer from overgrinding, which is the term for particles that are ground smaller than desired before exiting the mill. Overgrinding can occur in tumbling mills due to the physical mechanics of tumbling, and reduces the energy efficiency of the mill. Also, a component of energy in tumbling mills is required to overcoming interparticle rotational friction and abrasive grinding. 
     Tumbling mills may further require replaceable mill liners to protect the mill wall from the impact of the particles and balls. These must be regularly replaced, requiring the mill to intermittently stop operation. 
     Lastly, it may be difficult if not impossible to protect tumbling mills from chemical corrosion when processing corrosive ore. 
     There is accordingly a need for a more cost-effective and efficient means of grinding ore. 
     BRIEF SUMMARY OF INVENTION 
     There is provided an impact grinding plant disclosed herein that overcomes the above disadvantages. It is an object herein to improve grinding efficiency, with further goals to reduce initial capital cost while improving constructability and ease of operation. 
     There is accordingly provided an impact grinding assembly. The assembly includes an impact plate upon which unground material operatively impacts. The assembly includes a vibratory mechanism operatively connected to the impact plate. The vibratory mechanism causes the impact plate to vibrate. The material so impacted thus moves away from the impact plate thereafter. 
     There is further provided an impact grinding plant. The plant includes a plurality of the impact grinding assemblies as set out above. Each assembly further includes a conveyor for elevating the unground material. Each assembly also includes a separation assembly for separating out ground material and operatively returning still unground material to the conveyor for impacting with its corresponding impact plate again. The plant further has a feeder assembly which selectively conveys unground material to respective ones of the conveyors of the impact grinding assemblies. 
     There is also provided an impact grinding assembly that includes a pair of conveyors for conveying at least partially unground material. Each conveyor has a lower portion and an upper portion which is spaced-apart above its lower portion. The assembly includes a pair of vibrating impact plates aligning below the upper portions of respective ones of the conveyors to receive the at least partially unground material. A first one of the impact plates operatively directs material thereon to the lower portion of a second one of the conveyors. A second one of the impact plates operatively directs material thereon to the lower portion of a first one of the conveyors. 
     There is yet further provided a method of impact grinding. The method includes elevating unground material via a conveyor. The method includes dropping the material onto a vibrating impact surface such that 15 to 25% of the material is ground to a particle size no greater than a desired particle size. The method includes separating out the material that has the particle size no greater than the desired particle size. The method includes returning the rest of the material back to the conveyor to be dropped again. 
     There is yet also provided an impact grinding assembly which includes a means for conveying unground material to a drop zone. The assembly includes a means for impact grinding the unground material. The assembly includes a means for separating out fully ground material from the material so impacted and returning the rest of the material back to the means for conveying. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be more readily understood from the following description of preferred embodiments thereof given, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic flow chart of an impact grinding plant according to a first aspect, the impact grinding plant including a feeder assembly and a plurality of impact grinding assemblies; 
         FIG. 2  is a perspective view of the impact grinding plant of  FIG. 1 ; 
         FIG. 3  is a top plan view of the impact grinding plant of  FIG. 2 ; 
         FIG. 4  is a side elevation view of the impact grinding plant of  FIG. 2 ; 
         FIG. 5  is a perspective view of one of the impact grinding assemblies of the impact grinding plant of  FIG. 2 , together with the feeder assembly shown in fragment; 
         FIG. 6  is a top perspective view of a vibratory pan feeder for the impact grinding assembly of  FIG. 5 , the vibratory pan feeder including an impact plate; 
         FIG. 7  is a side perspective view of a conveyor shown in fragment, a drop chute partially shown in fragment to reveal its interior, a vibratory grizzly feeder having an impact plate, and a separation assembly for the impact grinding assembly of  FIG. 5 ; 
         FIG. 8  is a top perspective view of the vibratory grizzly feeder and impact plate of  FIG. 7 ; and 
         FIG. 9  is a perspective view similar to  FIG. 5  of an impact grinding plant according to a second aspect, the plant including an impact grinding assembly and a feeder assembly which is shown in part. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings and first to  FIG. 1 , there is shown an impact grinding plant  20  for grinding at least partially unground material, in this example ore  22  as seen in  FIG. 7 . In this case, the plant performs communition of the ore to reduce the particle size of crushed ore sufficiently so that other mining processes can subsequently be used to separate the valuable ore from the gangue, or the commercially valueless material. 
     Referring to  FIGS. 1 and 2 , the plant  20  includes a feeder circuit or assembly  24 . The feeder assembly includes a moveable cart  25  in this example. The cart includes a feeder conveyor  26  that draws material, such as a crushed ore, from an external source such as a stockpile. The conveyor has a lower end  28  upon which unground material may be deposited, as seen by arrows  29  and  32  in  FIG. 1 . The conveyor has an upper end  30  which is spaced-apart above its lower end. The conveyor  26  includes a belt conveyor in this example, though this is not strictly required. The conveyor extends in a generally diagonal direction from its lower end to its upper end in this example. 
     The conveyors shown in  FIGS. 1 to 8  as herein described are in the form of endless loop belt conveyors in this example. Belts conveyors per se, including their various parts and functionings, are well known to those skilled in the art and therefore will be not be described in further detail. Also, belt conveyors are not strictly required and other types of conveyors can be used. 
     As seen in  FIG. 2 , the moveable cart  25  includes a chute  34  which is in communication with the upper end  30  of conveyor  26 . The chute is generally in the shape of a hollow, rectangular prism which is rectangular in cross-section in this example. The chute  34  is shaped to downwardly direct unground material, conveyed upwards by the conveyor, to an outlet  36 , as shown by arrow  38 . The conveyor  26  may be selectively activated to feed unground material through the chute. 
     The cart  25  is linearly moveable along an elongate structure, in this example along conveyor rails  40 . Moveable carts, chutes and rails per se, including their various parts and functionings, are well known to those skilled in the art and therefore will not be described in further detail. 
     As seen in  FIG. 2 , the plant  20  includes a plurality of impact grinding circuits or assemblies  42 ,  44 , and  46 . The impact grinding assemblies extend in a generally elongate manner in this example and are aligned in parallel with each other in this example. The feeder assembly  24  is adjacent to and aligns substantially perpendicular to the impact grinding assemblies  42 ,  44  and  46  in this example. Cart  25  is moveable in a direction transverse to the impact grinding assemblies for selectively feeding unground material in an alternating manner to respective ones of the impact grinding assemblies at different times. 
     Referring to  FIG. 5 , impact grinding assembly  42  has a proximal end  48  adjacent to the feeder assembly  24  and a distal end  50  which is spaced-apart from the proximal end. Chute  34  is positioned to direct unground material to respective ones of the impact grinding assemblies at their proximal ends  48 . The proximal ends of the assemblies  42 ,  44  and  46  align with each other and the distal ends  50  of the assemblies  42 ,  44  and  46  also align with each other in this example. 
     Each of the impact grinding assemblies  42 ,  44  and  46  is substantially similar in parts and functionings. Thus, only impact grinding assembly  42  will be discussed in detail with the understanding that the other of the impact grinding assemblies  44  and  46  are substantially the same. 
     The impact grinding assembly  42  includes a first or send conveyor  52  for elevating unground material, as shown by arrow  53  in  FIG. 5 . The send conveyor includes an elevating section, in this example an inclined section  60  having a lower end  54  and an upper end  56  which is spaced-apart above its lower end. The lower end  54  corresponds to a lower portion of the conveyor and the upper end  56  corresponds to an upper portion of the conveyor. The inclined section of the conveyor  52  extends diagonally from its lower end to its upper end  56  in this example. The send conveyor  52  also has a substantially horizontal section  58  in this example that extends from proximal end  48  of the assembly  42  to lower end  54  of the inclined section  60  of the conveyor. The horizontal section of the conveyor may receive unground material from the feeder assembly  24 . 
     The impact grinding assembly  42  has a first drop zone  62  within which at least partially unground material drops, as shown by arrow  64  in  FIGS. 1 and 5 . The drop zone extends in a substantially vertical direction in this example. 
     The impact grinding assembly  42  has a first drop chute  66  which extends around the drop zone. The chute is substantially in the shape of a rectangular prism which is rectangular in cross-section in this example. The chute has an inlet  68  positioned to receive unground material from the upper end  56  of the inclined portion  60  of conveyor  52 . The chute  66  has an outlet  70  at a lower end  71  thereof which is spaced-apart below its inlet. 
     Referring to  FIGS. 5 and 6 , the impact grinding assembly  42  includes a first impact surface, in this example in the form of a vibrating impact plate  72  upon which the at least partially unground material operatively impacts after falling from the upper end  56  of conveyor  52 . The impact plate aligns below the upper end of the conveyor and the conveyor is thus positioned to drop the at least partially unground material onto the impact plate. The conveyor  52  may be referred to as a means for conveying at least partially unground material to a drop zone. The chute  66  and drop zone  62  extend from the upper end of the inclined section of the conveyor to the impact plate. 
     Referring to  FIG. 6 , the assembly  42  also includes a receptacle  74  of which the impact plate  72  forms a first portion or half  76  thereof. The receptacle is hollow, with an open top and is generally rectangular in shape in this example. The receptacle  74  includes a second portion or half  78  connected to and extending from its first half. 
     Still referring to  FIG. 6 , the assembly  42  includes a vibratory pan feeder  80  in this example of which the receptacle  74  and impact plate  72  are parts. As seen in  FIG. 6 , the feeder has a closed end  82  adjacent to the impact plate and adjacent to a position where the at least partially unground material is received. The feeder also has an open end  84  which is spaced-apart from its closed end. The feeder  80  further includes a pair of spaced-apart side walls  77  and  79  that extend from end  82  to end  84  thereof. 
     The impact plate  72  and vibratory feeder  80  extend in a substantially horizontal direction in this example, with the closed end  82  of the feeder  80  being only slightly elevated relative to open end  84  of the feeder to enable material adjacent to end  82  to move via the feeder&#39;s vibration and gravity towards end  84 . The feeder  80  includes a wear liner  81  that extends from end  82  to  84  in this example. Referring to  FIG. 6 , outwardly-extending flanges  83  of the feeder extend from respective side walls  77  and  79  thereof. The flanges connect to the lower end  71  of the chute  66 , seen in  FIG. 5 . Flanges  83  connect to the receptacle  74  via a plurality of springs  86  seen in  FIG. 6 . 
     The conveyors, chutes, feeders, separations assemblies and the like as described herein are supported by conventional framing, as generally shown by numeral  89  in  FIG. 2 , in this example in this form of metal frames and trusses. 
     Referring to  FIG. 6 , the feeder  80  includes a vibratory mechanism  88  operatively connected to the receptacle  74  and thus to the impact plate  72 , which causes the impact plate to vibrate. As seen in  FIG. 2 , the vibratory mechanism in this example includes a motor  90  mounted to framing  89  of the impacting grinding assembly  42 . The vibratory mechanism  88  includes an unbalanced shaft  92 , in this example, coupled to the motor  90  via an endless belt  95 . Housing  93  of the unbalanced shaft is connected to the receptacle  74 . The rotation of shaft  92  by motor  90  causes housing  93 , and thus feeder  80  to selectively vibrate. 
     The vibratory mechanism  88  causes a bed of material  94  to form above the impact plate  72  and onto which further at least partially unground material  96  impacts. The term “operatively impacts” is used herein to refer to the fact that falling material may collide with the impact plate  72  merely indirectly, because the falling material may directly impact the bed of material  94 , which in turn may be abutting the impact plate. In this case, the assembly  42  includes a control system  91 , seen in  FIG. 6 , operatively in communication with motor  90  and which sends signals thereto maintain the bed of previously dropped particles by controlling the feeder&#39;s vibration rate. A portion of the dropped material breaks upon impact. In this embodiment, assembly  42  includes a plurality of mill balls, in this example metal balls  97 , as seen in  FIGS. 6 and 7 , that move along conveyor  52 , seen in  FIG. 5 , and drop onto the bed of material  94 , further creating at least partially ground material. Balls  97  may assist in the breakage of particles. Additional abrasive grinding may occur due to the vibrating action of the feeder. 
     Selectively adjusting the extent of vibration of feeder  80  enables the thickness of the bed of material  94  to be tailored as desired. Material  99  and balls  97  within the feeder are thereafter moved away from the impact plate  72 , as shown by arrow  98  in  FIG. 6 , also based at least in part on the extent to which the feeder  80  is vibrated. 
     The impact grinding assembly  42  includes a second or return conveyor  100  for elevating material exiting from feeder  80 , as shown by arrow  102  in  FIG. 3 . The return conveyor  100  includes an elevating section, in this example an inclined section  110 . The inclined section of the conveyor  100  includes a lower end  104  and an upper end  106  which is spaced-apart above its lower end. The lower end  104  corresponds to a lower portion of the conveyor and the upper end  106  corresponds to an upper portion of the conveyor. The inclined section  110  in this example extends diagonally from its lower end  104  to its upper end  106 . 
     Lower end  104  of the inclined section of the conveyor  100  aligns with and is adjacent to the open end  84  of the feeder  80  for receiving material therefrom. Lower end  104  substantially aligns with impact plate  72  seen in  FIG. 6 . Impact plate  72  thus operatively directs material thereon to the lower end  104  of conveyor  100 . The vibrator energy of feeder  80  conveys material at a constant rate to conveyor  100  seen in  FIG. 4 . 
     The return conveyor  100  has a substantially horizontal section  108  in this example that extends from distal end  50  of the assembly  42  to the lower end  104  of the inclined section  110  of the conveyor. The horizontal section of the conveyor receives material exiting from the feeder  80  in this example. The inclined sections  60  and  110  of conveyors  52  and  100 , respectively, are inclined in opposite directions in this example, in a criss-crossing arrangement. 
     As seen in  FIG. 5 , the lower end  104  of the inclined section  110  of the conveyor  100  aligns with and is spaced-apart below the upper end  56  of the inclined section of the conveyor  52 . Upper end  106  aligns with and is spaced-apart above the lower end  54  of the inclined section  60  of conveyor  52 . The lower ends  54  and  104  of the inclined sections of conveyors  52  and  100 , respectively, substantially align within the same horizontal plane and are at substantially the same elevation in this example. The upper ends  56  and  106  of the inclined sections of the conveyors  52  and  100 , respectively, also substantially within the same horizontal plane and are at substantially the same elevation in this example. 
     The material, now consisting of a mixture of unground material, metal balls, intermediate-sized material, and ground material, is conveyed by the return conveyor  100 , in a direction opposite to the direction of movement of send conveyor  52  in this example, as shown by arrow  111  in  FIG. 5 . The ground material has a particle size no greater than a desired particle size. 
     The impact grinding assembly  42  includes a second drop zone  112  within which at least partially unground material drops, as shown by arrow  114  in  FIGS. 1, 5 and 7 . The drop zone extends in a substantially vertical direction in this example. The assembly  42  further includes a second drop chute  116  which extends around drop zone  112 . The chute is substantially in the shape of a rectangular prism which is rectangular in cross-section in this example. The chute has an inlet  118  positioned to receive at least partially ground material from the upper end  106  of the inclined section of the conveyor  100 . The chute  116  has an outlet  120  at a lower end  121  thereof which is spaced-apart below its inlet. 
     Referring to  FIGS. 7 and 8 , the impact grinding assembly  42  includes a second impact surface, in this example in the form of a vibrating impact plate  122 . Upon reaching end  106  of the inclined portion  110  of conveyor  100 , material falls through the chute  116  to operatively impact the impact plate. The impact plate  122  aligns below the upper end of the inclined section of the conveyor and the conveyor is thus positioned to drop the at least partially unground material onto the impact plate. Impact plate  122  extends in a substantially horizontal direction in this example and is substantially level with the lower end  54  of the inclined section  60  of conveyor  52  seen in  FIG. 5 . As seen in  FIGS. 2 and 3 , lower end  54  substantially aligns with impact plate  122 . Referring to  FIG. 7 , the chute  116  and drop zone  112  extend from the upper end  106  of the inclined section of the conveyor to the impact plate  122 . 
     Referring to  FIG. 4 , the drop zone  62  and impact plate  72  seen in  FIG. 6  and the drop zone  112  and impact plate  122  seen in  FIG. 7 , respectively, may be referred to individually or collectively as a means for impact grinding at least partially unground material. 
     As seen in  FIG. 8 , the assembly  42  includes a receptacle  124  of which the impact plate  122  forms a first portion or half  126  thereof. The receptacle is generally rectangular in shape in this example and includes a second portion or half  128  connected to and extends from its first half. The second portion of the receptacle comprises a material-separation grid  129 . The material-separation grid is thus positioned adjacent to impact plate  122 . 
     The assembly  42  includes a vibratory feeder, in this example a vibratory grizzly feeder  130  of which the receptacle  124 , impact plate  122  and material-separation grid  129  are parts. The impact plate includes a wear liner  123  which extends overtop thereof in this example. Feeder  130  may be referred to as a combined impact surface and grizzly feeder. 
     As seen in  FIG. 8 , the feeder has a closed end  132  adjacent to the impact plate and adjacent to a position where the at least partially unground material is received. The feeder has an open end  134  which is spaced-apart from its closed end. As seen in  FIG. 8 , the feeder  130  has a pair of spaced-apart side walls  133  and  135  that extend from end  132  to end  134 . The impact plate  122  and vibratory feeder extend in a substantially horizontal direction in this example, with the closed end  132  of the feeder  130  being only slightly elevated relative to open end  134  of the feeder to enable material adjacent to end  132  to move via the feeder&#39;s vibration and gravity towards end  134 . Outwardly-extending flanges  136  of the feeder extend from walls  133  and  135 , respectively. In this example, the flanges connect to the lower end  121  of the chute  116 , seen in  FIG. 7 . Flanges  136  connect to the receptacle  124  via a plurality of springs  138  seen in  FIG. 8 . 
     Current grizzly feeders may devote the majority of their surface area to grizzly rods or bars, which are used to separate large and small pieces of material, aided by vibratory motion. By contrast, a significant area of feeder  130  is devoted to provide a bed and impact area  137 , corresponding to half  126  of receptacle  124  and within which is impact plate  122  for falling material. This area is substantially rectangular in shape, bounded by closed end  132  and parts of side walls  133  and  135 , of a distance approximating the feeder&#39;s width. A second adjacent area  139 , corresponding to half  128  of receptacle, which is similar in size to area  137 , is devoted to grizzly bars and material separation, as described below. Feeder  130  therefore may perform three functions simultaneously, namely, providing an impact surface for falling material, feeding material forward through vibratory motion, and size-separating material through the material-separation grid  129 . 
     The feeder  130  includes a vibratory mechanism  140  operatively connected to the receptacle  124  and thus the impact plate  122 , which causes the impact plate to vibrate. As seen in  FIG. 3 , the vibratory mechanism in this example includes a motor  142  mounted to framing  144  of the impacting grinding assembly  42 . Referring back to  FIG. 8 , the vibratory mechanism  140  includes an unbalanced shaft  146 , in this example, coupled to the motor  142  via an endless belt  148 . Housing  149  of unbalanced shaft  146  is connected to the receptacle  124 . The rotation of shaft  146  by motor  142  causes housing  149 , and thus feeder  130  to selectively vibrate. 
     Referring to  FIG. 7 , the vibratory mechanism  140  causes a bed of material  150  to form above the impact plate  122  and onto which further at least partially unground material  152  and metal balls  97  operatively impact. Selectively adjusting the extent of vibration of feeder  130  enables the thickness of the bed of material  150  to be tailored as desired. In this case, the assembly  42  includes a control system  141 , seen in  FIG. 8 , operatively in communication with motor  142 , which sends signals thereto to maintain the bed of previously dropped particles by controlling the feeder&#39;s vibration rate. A portion of the dropped material breaks upon impact. Balls  97  may assist in the breakage of particles. Additional abrasive grinding may occur due to the vibrating action of the feeder. 
     Material and balls within the feeder thereafter are moved away from the impact plate  122  thereafter, as shown by arrow  154  in  FIG. 7 , also based at least in part on the extent to which the feeder  130  is vibrated. 
     As best seen in  FIG. 7 , the impact grinding assembly  42  includes a material separation assembly  158  for separating out ground material and returning still unground material to conveyor  52  for impacting in the drop zones  62  and  112  again, as seen in  FIG. 4 . This assembly  158  may be referred to as a means for separating out fully ground material from the material so impacted and returning the rest of the material back to the means for conveying. Referring back to  FIG. 8 , the material separation assembly is interposed between impact plate  122  and horizontal section  58  of conveyor  52  seen in  FIG. 2 . 
     As seen in  FIG. 7 , the material separation assembly  158  includes the material-separation grid  129  of the grizzly feeder  130 . The grid comprises a plurality of elongate, spaced-apart bars  160  in this example that extend from end  134  of the feeder  130  towards end  132  of the feeder. The bars are spaced-apart so as to inhibit metal balls  97  and oversized material  162  from passing therethrough. Thus, material exiting chute  116  crosses a number of horizontal slots  131  positioned between the grids, seen in  FIG. 8 , of the feeder  130  and the size of the slots is selected such that the metal balls and large unground particles cannot pass therethrough. 
     The balls and large unground particles are conveyed via the vibration of the grizzly feeder and gravity away from grid  129 . The vibrator energy of feeder  130  conveys material at a constant rate to conveyor  52  seen in  FIG. 4  in this example. This material is returned to the conveyors  52  and  100  for impacting within the drop zones  62  and  112  again, as seen in  FIG. 4 . The metal balls are thus separated from the at least partially ground material and returned to conveyor  52  via the material-separation grid  129  as seen in  FIG. 7 . In this manner, this material fraction of balls and oversized material is returned to the grinding circuit for further grinding. 
     Referring back to  FIG. 7 , the material-separation grid  129  allows at least partially ground material  166  to be removed from the conveyors via gravity, as shown by arrow  168 , where further size separation occurs. The at least partially ground material  166  comprises intermediate sized material  170  and fully ground material  172 . 
     The material separation assembly  158  includes a product screen  174  positioned below the material-separation grid  129  and through which the fully ground material  172  passes. Openings in the product screen are thus sized to allow fully ground material to pass through. The assembly  158  includes a water spray header  176 , seen in  FIG. 1 , through which water  178  sprays to assist in product separation. Referring to  FIG. 7 , the impact grinding assembly  42  includes a slurry sump  180  positioned within a recess  181  of the ground in this example, as seen in  FIG. 4 , below the material separation assembly  158 . The assembly  42  also includes a slurry pump  182  operatively connected to the sump. The water  178  and fully ground material  172  form a slurry  184  that collects within the slurry sump. The slurry is pumped to the next stage in the mineral processing facility (not shown) by the slurry pump  182  via a conduit, in this example a single slurry pipe  183  seen in  FIG. 2 , to which each of the assemblies  42 ,  44  and  46  is in communication. Water  178  also provides make up water to replace water as it leaves the circuit as slurry. 
     Referring to  FIG. 7 , according to one aspect, the percentage by mass flow of material (balls and large rocks) passing over the grizzly feeder  130  to continue circulation on conveyor  52  is 50-90% according to one preferred aspect, 75-85% according to a further preferred aspect, and 80% according to yet a further preferred operating point, in one example. Conveyor  100  may thus be positioned to cause 15-25% of the material operatively colliding with impact plate  122  to be fully ground so as to pass through the material-separation assembly  158 , with the impact grinding assembly  42  operatively conveying the rest of the material back to the conveyor  52 . 
     As seen  FIGS. 5 and 7 , the impact grinding assembly  42  further includes an intermediate conveyor  186  having a horizontal section  185  positioned adjacent to screen  174 . The conveyor includes an elevating section, in this example an inclined section  187 . Referring to  FIG. 5 , the inclined section of conveyor  186  has a lower end  188  and an upper end  190  which is spaced-apart above its lower end. The lower end  188  corresponds to a lower portion of the conveyor and upper end  190  corresponds to an upper portion of the conveyor. The horizontal section  185  of the conveyor extends to lower end  190 . The elevating section  187  of the conveyor  186  extends diagonally, in this example, from its lower end to its upper end. Lower end  188  aligns with and is positioned adjacent to the lower end  54  of the inclined section  60  of conveyor  52 . The upper end  190  of the inclined section of the conveyor  186  is positioned adjacent to the proximal end  48  of the assembly  42 . 
     As seen in  FIG. 7 , the conveyor  186  receives, adjacent to its lower end, intermediate-sized material  170  which does not pass through the screen, as seen by arrows  189  and  191 , and elevates this material upwards, as shown by arrow  194  in  FIG. 5 . 
     Still referring to  FIG. 5 , the assembly  42  further includes a drop chute  192  through which conveyor  186  drops the intermediate-sized material, as shown by arrow  196 . The drop chute directs material back onto conveyor  52 . In this manner this material fraction is directed back to the conveyors  52  and  100  of the grinding assembly  42  for further grinding. 
     In summary and referring to  FIG. 2 , once material to be ground is introduced to the assembly  42 , it will continue in an endless circuit comprising conveyor  52 , chute  66 , vibratory pan feeder  80 , conveyor  100 , chute  116 , grizzly feeder  130 , and back to conveyor  52 . Particles have no means of leaving the assembly other than ultimately having their size reduced through one or more falling impacts in the drop zones. Once a particle&#39;s size is reduced sufficiently to pass through the slots in the grizzly feeder  130 , and then the product screen  174  seen in  FIG. 7 , the fully ground material exits the assembly  42 . The system&#39;s final product is slurry  184 , seen in  FIG. 1 , containing fully ground product and which may be pumped to the next stage in the mineral processing facility. The moveable cart  25  seen in  FIG. 2  alternates from feeding one grinding assembly to the next, feeding the overall plant at a rate that matches the discharge of fully ground product. 
     The metal balls  97  may be optionally used to aid the grinding process and are not strictly required. If metal balls are utilized, they may be manually introduced to conveyors  52  and  100  before the assembly  42  is started according to one example. They may then remain within the assembly  42  indefinitely or until they are replaced due to wear. 
     Unground material and metal balls may represent 80% to 90% of the mass circulating in the assembly  42 , according to one example. As the lift distance and fall distance of this material fraction are nearly the same, conveying energy is efficiently converted to material breakage. As the intermediate portion of material may only 10%-20% percent of the circulating mass according to one example, the additional energy requirements of the intermediate recirculation conveyor may not significantly impact the overall system efficiency. 
     Wear liners  81  and  123  on the feeders  80  and  130 , seen in  FIGS. 6 and 8 , may require occasional replacement, as may belts on the conveyors, as shown by belt  55  for conveyor  52  in  FIG. 2 . In these circumstances only one of the three or more assemblies  42 ,  44  and  46  needs to be shut down at a given time, allowing the grinding plant  20  to otherwise continue in operation. 
     The control systems  91  and  141 , shown by way of example only in  FIGS. 6 and 8 , ensure that the mass of material fed into the plant  20  does not exceed the mass of material leaving the plant. This thereby inhibits a buildup of material within the impact grinding assemblies  42 ,  44  and  46 . The control systems may also proportion make up water addition with rate of production of fully ground product to ensure a consistent and pumpable slurry discharging from the system. 
     Many advantages may result from the structure of the present invention. The plant  20  and assemblies described herein may provide a number of means of increasing grinding efficiency compared to prior known grinding systems. For example, the present invention may increase the velocity of impact of materials during impact grinding. The present invention may achieve this end and overcome the size limitation of tumblers by elevating material linearly using low friction conveyors. With this innovation, greater particle drop heights may be readily achieved. This may result in correspondingly higher impact velocities, higher frequency of first impact breakage and thus higher grinding efficiency. To provide further efficient grinding, large particles and grinding balls (90% of the weight in a semi-autogenous grinding mill) may fall a distance substantially similar to the height lifted, maximizing energy efficiency thereby. 
     Also, the plant  20  and associated assemblies described herein may increase charge participation, compared to conventional grinding systems, by providing one hundred percent charge participation. This is because all material is fully elevated and impacted during each passage through the grinding assemblies. This may increase the energy efficiency of the present invention, in comparison to tumbling mills. 
     Furthermore, the plant  20  and associated assemblies described herein maximize expended energy on impact breakage in lieu of abrasive breakage. This is because, in comparison to tumbling mills, the present invention primarily devotes consumed energy to raising and dropping particles due to its physical configuration. Therefore most grinding occurs using impact breakage. As impact breakage is more energy efficient than abrasive breakage, this may further increase the energy efficiency of the present invention, when compared to tumbling mills. 
     Further energy savings result by the plant  20  and associated assemblies described herein reducing overgrinding. This is achieved by passing all material repeatedly through sizing and screening stages which remove adequately ground material from the circuit before it can be overground, or excessively ground. 
     Tumbling mills may not able to take full advantage of the above set out improvements due to limitations resulting from their rotational geometry. The present invention may not experience these limitations and may therefore offers greater grinding efficiency. 
     Yet further energy savings may be provided by the plant and associated assemblies described herein by reducing or inhibiting slurry pooling. Slurry pooling occurs when excessive liquid builds up in a tumbling mill which lessens the impact received by of dropped particles. In the current invention, the potential for efficiency reduction due to slurry pooling is reduced and/or essentially eliminated as the grinding chamber is not sufficiently enclosed to contain a buildup of liquid. 
     In the present invention, according to one aspect, the plant is configured so that particles impact a bed of previously dropped material. This may both maximize grinding efficiency and prevent rapid and destructive wear to the metal surface of the liner/impact plate(s). The present invention may thus require significantly fewer liners. These liners may be less costly and easier to replace because they can be replaced without requiring a full shutdown of the grinding operation. This may thereby enable the plant to process a relatively large grinding volume. The present invention may further offer a higher operational availability than tumbling mills. 
     Also, in the current invention, high capacity belt conveyors are utilized, which may have much higher capacities than bucket elevators for example. 
     A further advantage of this invention is that it is an assembly of common and readily available equipment components. Belt conveyors, vibratory feeders, grizzly feeders and vibrating screens are very common equipment in the mineral processing industry and readily available from a large number of suppliers. This may lower initial capital outlay, lead to faster fabrication and lead to more rapid construction in comparison to tumbling mills. The use of common components may further facilitate ongoing operation and maintenance. 
     For most applications, the belt conveyors may commonly be fabricated with a carbon steel structure with rubber or elastomeric belts. Feeders and screens may also commonly have carbon steel frames. Liners may be of abrasion resistant metals or energy absorbing elastomerics such as rubber. The present invention can also readily be adapted to processing corrosive materials. 
     A further advantage of this invention is that it can be readily adapted for use with corrosive ores. In this circumstance conveyor belts and liners would be selected for chemical compatibility with the material to be ground. 
       FIG. 9  shows an impact grinding plant  20 . 1  including at least one impact grinding assembly  42 . 1  according to a second aspect. Like parts have like numbers and functions as the plant  20  and assemblies  42 ,  44  and  46  shown in  FIGS. 1 to 8  with the addition of the designation “. 1 ”. Plant  20 . 1  and assembly  42 . 1  are substantially the same as plant  20  and assembly  42  shown in  FIGS. 1 to 8  with the following exceptions. 
     Conveyors  52 . 1 ,  100 . 1 , and  186 . 1  are flexible sidewall conveyors instead of belt conveyors. Flexible sidewall conveyors are belt conveyors with side walls and intermediate slates that allow them to convey uphill. Each conveyor comprises a plurality of receptacles that are hollow, open topped and substantially in the shape of rectangular prisms in this example, as shown by receptacle  198 . The elevating sections  60 . 1 ,  110 . 1  and  187 . 1  of the conveyors  52 . 1 ,  100 . 1  and  186 , respectively, extend in substantially vertical directions in this example. The conveyors  52 . 1 ,  100 . 1 ,  186 . 1  further include further horizontally-extending section  200 ,  202 , and  204 , respectively, which extend from their elevating sections to chutes  66 . 1 ,  116 . 1  and  192 . 1 , respectively. 
     It will be appreciated that many further variations are possible within the scope of the invention described herein. For example, while send conveyors  52  and return conveyors  100  are shown, in another embodiment a looping and/or bending single conveyor, single chute, single drop zone, single feeder and separation assembly may be used. In this case, the conveyor may convey material to the drop zone, receive oversized material from the feeder, and again raise further unground material to be dropped within the same drop zone once more. 
     The conveyor described in  FIGS. 1 to 8  includes belt conveyors. In the alternative, these conveyors may be in the form of tube conveyors, or sandwich conveyors, for example. A sandwich conveyor has a similar configuration to a belt conveyor but has a second belt on top of the material which allows a steeper angle. 
     Each of the assemblies  42 ,  44  and  46  may alternatively comprise of two, four, six or more send conveyors  52  arranged in series with one another and two, four, six or more return conveyors  100  operating in series with one another. Alternatively, each of the assemblies  42 ,  44  and  44  may be arranged themselves in series with each other, instead of in parallel with each other. 
     The feeders described herein may alternatively consist of belt feeders or apron feeders, for example. 
     Conveyor  26  of the feeder assembly  24  seen in  FIG. 2  may alternatively be replaced with a plurality of individual conveyors from the material source for each of the impact grinding assemblies  42 ,  44  and  46 . 
     Material discharging from the top of the product screen  174 , seen in  FIG. 7 , may be alternatively directed to an unrelated grinding circuit with equipment suited to pebble size particles. 
     Slurry  184 , comprising completely ground product, may be pumped separately to subsequent stages in the mineral processing circuit. 
     It will be understood by someone skilled in the art that many of the details provided above are by way of example only and are not intended to limit the scope of the invention which is to be determined with reference to at least the following claims.