Patent Publication Number: US-6663030-B2

Title: Replaceable grate device for maximizing the throughput of solid material in ore mills

Description:
FIELD OF THE INVENTION 
     This invention broadly relates to grate devices for ore grinding or ore comminuting machines in commercial mining operations. More specifically, this invention is directed to an improved system for replacing and fastening grates to the rotatable interior of an ore grinding or ore comminuting machine. 
     BACKGROUND OF THE INVENTION 
     Commercial mining operations require the use of ore comminuters or grinders, which reduce the size of large ore fragments for further refining. Several types of ore comminuters or grinders may be used, one of which takes the form of a large cylindrical rotatable shell that is rotated on a substantially horizontal axis and is driven by a very powerful motor through conventional reduction gearing. With this type of grinder, ore is introduced into one end of the drum through an inlet, and after reduction or comminution, the reduced ore is discharged through an outlet in the opposite end. 
     Within the drum, the charge of ore fragments rests at the bottom of the rotating drum. As the drum rotates, part of the ore charge is carried upwardly along the contoured inner surface of the drum until the carried fragments drop from the drum surface due to gravity, tumbling back onto the ore charge and breaking the fragments. This continuous process reduces the size of the fragments until they become small enough to pass through apertures in grate segments. These fragments are then discharged from the mill. 
     Individual grate segments are typically mounted on pulp lifters within the interior cylindrical surface of the rotatable drum. The individual grate segments are composed of a series of apertures with a predetermined size. Apertures in grate segments generally will range from 0.25″ to 4″(0.6-10.2 cm) depending on the specific application, with 1″ to 3″(2.5-7.6 cm) apertures being typical for most industry applications. An ore fragment must pass through at least one of the apertures before it is able to exit the mill. The grate segments are cast from alloys that are optimized to increase the wear rate while avoiding breakage caused by impacts from the ore charge and grinding media. 
     To maximize economic efficiency, ore comminuting mills of this type generally operate continuously, 24 hours a day. The ores being comminuted are highly abrasive. Therefore the continuous process wears the grate segments down over a period of time, depending on the type of ore and application. When grate apertures become worn, ore fragments that exit the mill may become larger than desired. When this happens, the grate segments must be replaced. It is desirable to replace the series of individual grate segments as quickly as possible because down time of the ore comminuting mill adversely affects the economic efficiency of the process. 
     Even when grate segments are not worn, it may be desirable to replace grates segments containing apertures of one size with grate segments containing apertures of a different size. This is especially important when the same ore grinder is used for variable ore types. 
     The process of replacing the series of individual grate segments presents certain problems that are not immediately evident. For example, the mere size of the equipment presents practical difficulties. A typical mill can measure 15 feet (4.6 m) long and over 28 feet (8.5 m) in diameter. Individual grate segments are commonly 2.5 to 4 inches (6.4-10.2 cm) thick, may be up to 4 feet (1.2 m) high and 6 feet (1.8) wide, and typically weigh up to several thousand pounds. 
     Individual grate segments that line the drum of the mill are conventionally fastened to pulp lifters on the cylindrical shell by transverse mounting bores that extend from the grinding surface to the mounting surface of the cylindrical drum. Each pulp lifter may include two such mounting bores. The cylindrical shell has the same number of mounting bores that are similarly spaced, permitting the mounting bores of the segments to be positioned in alignment. Once aligned, bolts are passed from the inside of the shell through the grate segments and the aligned mounting bores in the shell. 
     This type of segment fastening system works quite well in installing the individual grate segments. However, the bolt heads may be exposed at least partially to the comminution process, and by the time the grate segments require replacement, the bolt heads may be severely deformed. The continuous bombardment of fragments usually causes peening of the casting immediately around the bolt head, which may occlude the head and reduce its accessibility for removal. 
     Furthermore, there is at least some minimum space necessary between the sides and ends of adjacent grate segments to permit installation. During the ore comminution process, ore fines tend to fill up these spaces and are compacted in place. The grate segments may also be peened onto each other. This results in significant difficulty in removing the compacted grate segments when replacement is necessary. Even if the external nuts or the mounting bolts are removed relatively easily, this does not release the individual grate segments because of such compacting. Further, the bolts themselves have significant shear forces placed on them during the ore comminution process, often causing deformation to the point that they become skewed and tightly lodged within the interior of the rotating drum. The force necessary to remove a particular grate segment often requires the use of a crane and heavy hammering equipment. 
     Another approach that may be used instead of or in addition to forcibly hammering the bolts or grate segments is torch cutting the worn material from within the rotating drum. If the bolt head can be reached effectively by torch cutting, the bolt may be removed, thereby facilitating segment removal. 
     As will be appreciated, the conventional fastening of liner segments results in difficult grate segment removal when replacement is necessary, and this in turn causes significant mill down time. 
     The present invention is directed to a grate segment fastening system that can make grate segment installation and removal easier and less time consuming. 
     SUMMARY OF THE INVENTION 
     The present grate segment fastening system comprises a grate segment, and a grate holder. The grate segment is designed to work in conjunction with corresponding sides of the grate holder segments. For example, the grate segment generally may be configured with tabs on one end and mounting bores on the other end. The grate holder segments can be anchored onto existing pulp lifter sections on the interior of an ore grinder. The grate holder segments are typically anchored with threaded bolts or similar fastening means. In any case, the holder segments contain anchor bores that are configured to align with corresponding bores on the existing holder segments. 
     For assembly, in one example the grate segment is placed between adjacent holder segments so that the tabs interlock with recessed portions on one side of the grate holder. Once the tabs are securely in place, the grate is rotated until the mounting bores on the grate come into alignment with mounting bores on the other holder segment. Once the grate is in place, elongated members, such as bolts or pins, may be inserted through the mounting bores on the grate and into the mounting bores on the other grate holder segment. After the bolts or pins are inserted, protective plug members may be inserted over the bolts or pins to protect them from wear caused by the internal operation of the mill. 
     Preferably, the grate and grate holders are cast from a ferrous alloy such as pearlitic steel or martensetic white iron. In any case a suitable material can be chosen to maximize the wear-resistant characteristic of the grate holder while avoiding breakage due to brittleness. 
     In a typical milling scenario, the present invention would be used with existing mills to enable milling personnel to replace grate segments with less effort than present systems require. For example, preferred embodiments of the present invention only require the removal of two elongated members from the mounting bores on the grate. Then, the mounting end of the grate segment will rotate upward and away from the interior wall of the mill. At this point, the tabs on the grate easily can be slid outward from the recesses on the grate holder, thereby releasing the grate from the mill structure. 
     The grate holder segments then can be inspected visually to determine whether they need replacing. If they do not require replacement, a new grate segment easily can be installed by sliding its tabs into the recesses in the grate holder and aligning the mounting bores on the grate and grate holder. Elongated members are then inserted into the mounting bores, thereby locking the new grate in place. The holder can typically last through the effective wear life of at least two grate segments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of the grate segment fastening system according to the present invention; 
     FIG. 2 is a top view of the grate segment; 
     FIG. 3 is front view of the grate segment; 
     FIG. 4 is a side view of the grate segment; 
     FIG. 5 is a schematic bottom perspective view of the grate segment; 
     FIG. 6 is a schematic perspective view of the grate holder; 
     FIG. 7 is a top view of the grate holder; 
     FIG. 8 is a side view of the grate holder; 
     FIG. 9 is a perspective view of the threaded member; 
     FIG. 10 is a perspective view of the protective plug member. 
     FIG. 11 is a side elevational view of an ore grinding mill for reducing the size of ore fragments. 
     FIG. 12 is cross-sectional view of a ore grinding mill. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With initial reference to FIG. 1, a grate segment fastening system is shown with its component parts. The grate  1  is disposed between two similarly configured grate holders  2  and  3 . Threaded members  4  and  5  are shown near mounting bores  6  and  7  that are used to fasten the grate  1  to grate holder  3 . Protective plug members  8  and  9  are shown near the threaded members  4  and  5 . In an alternative embodiment, the protective plug members  8  and  9  will cover corresponding threaded members  4  and  5 . For reference, FIG. 11 generally shows an ore grinding mill  101  employing the inventive grate assembly. Mill  101  includes a hollow cylindrical drum or shell  107  having an inlet end  105  and an outlet end  106 . 
     The cylindrical drum  107  is arranged for rotation about a substantially horizontal axis in suitable bearings  109 ,  118  by a drive of conventional construction in a housing  108 . A inlet  102  communicating with the axial inlet receives ore fragments  104  from a conveyor  119 . The comminuted material leaves the mill  107  through the outlet  103 . 
     Cylindrical drum  107  is made up of a plurality of cylindrical sections  110 - 112 , each of which in turn is assembled from a set of cylindrical quadrants by bolts extending through axial flanges. For example, section  112  consists of quadrants  113   a ,  113   b , and  113   c  (one quadrant is not shown) which are secured together circumferentially by a plurality of bolts passing through radially extending, axially aligned flanges  114 ,  115 . The cylindrical sections  111 ,  112  are secured together axially by a plurality of bolts passing through circumferential flanges  116 ,  117  extending radially from the periphery of each side. Cylindrical sections  111 ,  112  are secured in an identical manner, as are the inlet end  105  and the outlet end  106  to the cylindrical sections  112 ,  110 , respectively. 
     FIG. 12 generally shows a cross-sectional view of an ore grinding mill  101  with grates  1  and  1 ′ in their operating environment. The inlet  102  and outlet  103  of the ore grinding mill  101  are also shown for reference. 
     FIG. 2 shows grate  1  which illustrates the top surface  28 . The grate  1  is typically cast from a ferrous alloy such as pearlitic steel or martensetic white iron. In any case a suitable material should be chosen to maximize the wear-resistant characteristic of the grate  1  while avoiding breakage due to brittleness. 
     With continued reference to FIG. 2, the grate  1  defines an axis end  21  and a circumferential end  22 , which is located opposite the axis end  21 . The grate  1  is further defined by a second lateral surface  23  and a first lateral surface  24 , which is located opposite the second lateral surface  23 . Tabs  25  and  26  extend outwardly from the second lateral surface  23  as shown. The grate  1  may have one tab  25 , or a plurality of tabs, as shown in FIG.  2 . The grate  1  is also formed with two or more throughput apertures  27 , all of which extend through the body of grate  1  from the top surface  28 . The throughput apertures are typically elongated, extending generally from the second lateral surface  23  to the first lateral surface  24 . Generally, throughput apertures that are located near the axis end  21  are shorter than throughput apertures located near the circumferential end  22 . Each throughput aperture has a width, measured from the side of the throughput aperture nearest the axial end to the side of the throughput aperture nearest the circumferential end. The width of the elongated throughput aperture  27  can vary considerably, depending on the type of application. For example, with some applications, it may be desirable to reduce ore fragments to 0.25″(0.635 cm) diameters. In other applications, it may be desirable to reduce the ore fragments only to 4″(10.16 cm) diameters. Many applications require grates with throughput apertures that are between 1″ to 3″(2.54-7.62 cm). With this system, grates  1  with throughput apertures  27  of a certain width can easily be replaced with grates  1  with throughput apertures  27  of a different width. 
     With reference to FIG. 3, a front view of grate  1  is shown, with second lateral surface  23  and first lateral surface  24 . Grate  1  has a thickness  31 , which can be uniform or varying. In most applications, the thickness  31  must be at least 2″ (5.08 cm), while a thickness of at least 3 41  (7.62 cm) is preferred for most applications. A raised mounting portion  32  is located near the first lateral surface  24 . The raised mounting portion  32  has a mounting thickness  33  greater than thickness  31  and can extend from the axis end  21  to the circumferential end  22 , although it is possible to interrupt the mounting portion at a location between the axis end  21  and the circumferential end  22 . Alternatively, the raised mounting portion  32  may be set back from either or both the axis end  21  and the circumferential end  22 . The mounting thickness can be uniform, or varying, however it is generally preferred that the mounting thickness  33  is at least two times greater than the thickness  31 . 
     With reference to FIG. 4, a side view of grate  1  is shown, with the axis end  21  and the circumferential end  22 . Mounting bores  41  and  42  are shown. The mounting bores pass through the raised mounting portion  32  and must be of suitable size for a threaded member  4  of considerable strength to pass through. For example, mounting bores can range in diameter from 0.5″(1.27 cm) to over 3″(7.62 cm), depending on the application. In an alternative embodiment a recessed portion  43 ,  44  is disposed integrally with the mounting bore  41  so that a protective plug member  9  can be inserted into the recessed portion  43 . In a preferred embodiment, the material around the mounting bores  41  and  42  may project outward as shown on the first lateral surface  24  in FIG.  2 . This configuration, illustrated in FIG. 2 as support area  29  and support area  30 , adds additional structural support. 
     Turning now to FIG. 5, the bottom surface  51  of grate  1  is shown with the first lateral surface  24  and the second lateral surface  23 . The axis end  21  and the circumferential end  22  are also shown along with tabs  25  and  26 , which extend outward from the second lateral surface  23 . As described above, the mounting thickness  33  is shown to be greater than the thickness  31 . 
     In FIG. 6, the grate holder  2  is shown with the pulp lifter contact surface  61  and the mounting surface  62 . The grate holder  2  is typically cast from a ferrous alloy such as pearlitic steel or martensetic white iron. In any case a suitable material must be chosen to maximize the wear-resistant characteristic of the grate holder  2  while avoiding breakage due to brittleness. Anchor bores  63  and  64  are shown extending through the pulp lifter contact surface  61 . The anchor bores  63  and  64  must be of suitable size and configuration to allow a fastening member, such as a bolt, to pass through the anchor bores  63  and  64  and affix the grate holder  2  to an existing pulp lifter located on the internal surface of a mill&#39;s rotating drum. Mounting bores  67  and  68  are shown extending through the mounting surface  62 . Mounting bores  67  and  68  are alignable with mounting bores  41  and  42  on the grate  1  to facilitate the grate  1  installation process. Preferably, the mounting bores  41  and  42  on the grate are larger in diameter than the mounting bores  67  and  68  on the grate holder  2  for easier location. 
     Grate holder  2  may also be formed with recessed portions  65  and  66  located on the opposite side of the mounting surface  62 . The recessed portions  65  and  66  are generally configured so tab  26  and tab  25  may fit within recessed portions  65  and  66 . In an alternative embodiment, recessed portions  65  and  66  are configured to lock tab portions  25  and  26  in place. Additionally, the area around mounting bore  67  and mounting bore  68  on the mounting surface side  62 , may also be recessed to align with support area  29  and support area  30  shown on the grate  1  in FIG.  2 . This configuration can add structural support and minimize the wear and structural degradation of the tab  25 , tab  26 , support area  29  and support area  30 . 
     With reference to FIG. 7, grate holder  2  is shown with top surface  71 , located on the opposite side of the pulp lifter contact surface  61 . Anchor bore  63  and anchor bore  64  are shown extending through the grate holder  2 . Slot surface  74  is shown on the opposite side of the mounting surface  62 . In an alternative embodiment, alignment portions  75  and  76  are shown as recessed areas on slot surface  74 . The alignment portions  75  and  76  are not necessarily recessed as much as the recessed portions  65  and  66 . Rather, tabs  25  and  26  should preferably be able to fit within recessed portions  66  and  65  and be securely disposed between alignment portions  75  and  76  and the surface of the pulp lifter on the interior of the mill. Guide portions  77  and guide portions  78  are shown on mounting surface  62 . The guide portions  77  and  78  are generally configured to align with support area  29  and support area  30  on the grate  1 . This configuration eases installation and may reduce structural degradation of the mounting bores  67  and  68 . 
     With reference to FIG. 8, grate holder  2  is shown, with axial side  72 , circumferential side  73 , top surface  71 , and pulp lifter contact surface  61 . Mounting surface  62  is also shown with mounting bores  67  and  68 . Mounting bores  67  and  68 , as shown, extend through grate holder  2 . In an alternative embodiment, mounting bores  67  and  68  may extend only partially through grate holder  2 . 
     With reference to FIG. 9, a threaded member  4  is shown. The threaded member  4  is alignable with the mounting bore  41  or mounting bore  42  of the grate  1  and mounting bore  67  or mounting bore  68  of the grate holder. While mounting bores  67 ,  68 ,  41 , and  42  are generally circular in transverse configuration, the threaded member  4  also has a generally circular configuration. In the installation of the grate segment fastening system shown in FIG. 1, the grate  1  is fastened to the grate holder  3  by threaded members  4  and  5 . The threaded member  4  is generally elongated with a threaded section  91  and a head section  92 . 
     To protect the head section  92  and the threaded section  91  of the threaded member  4  from the internal environment of the mill, protective plug member  9  may be used. Protective plug member  9  is shown in FIG.  10  and may be inserted into recessed portion  43  or  44  of grate  1  to protect threaded member  4 , and specifically the head section  92  of threaded member  4 . The cross sectional configuration of protective plug member  9  corresponds to the recessed portions  43  and  44  as shown on FIG.  4 . However, it is dimensioned to be slightly greater than recessed portions  43  and  44  to allow for a secure force fit. 
     In the preferred embodiment, protective plug member  9  is injection molded from urethane or other suitable polymer. It defines a plurality of coaxial barbed segments  100 , each of which is angled to facilitate insertion into the recessed portion  43 . As configured, and being slightly oversized relative to the recessed portion  43 , protective plug member  9  initially can be placed into the recessed portion  43  and then pounded into place until its top surface is even with the surface of the grate  1 . 
     As grinding takes place, grate  1  and grate holders  2  and  3  are continuously worn away, as is protective plug member  9 . However, protective plug member  9  remains firmly in place to protect the threaded head section  92  of the threaded member  4 . When the grate  1  has worn to the point that protective plug member  9  no longer exists, it is generally time for replacement of the grate  1 . 
     Protective plug member  9  can be formed from different materials and take different structural forms. Generally, the protective plug member  9  will take the same transverse configuration as recessed portion  43  or  44 , and will fill the cross section of recessed portion  43  to protect the threaded member  4 . The protective plug member  9  desirably will include features for improving its retention in the recessed portion  43  during the ore grinding process. 
     When replacement of a grate  1  is necessary, the protective plug members  8  and  9  must be removed, if they are still in place, and then the threaded members  4  and  5  must be removed. At this point, a wrench may be applied to the head section  92  of the threaded members  4  and  5 . The grate is then released from the grate holder  3  and the tabs  25  and  26  can be removed from the recessed portions  65  and  66  of the grate holder  2 . Grate holders  2  and  3  can be inspected, e.g. visually, to determine whether they have experienced enough structural degradation from the internal milling operations to merit replacement. For example, if extensive wear appears to the top surface  71 , the slot surface  74  or the mounting surface  62 , the grate holders  2  and  3  should be replaced. If too much wear occurs, the grate holders  2  and  3  may not be able to effectively hold the grate  1  in place. 
     The above specification provides a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention is not limited by the above description but is defined in the claims hereinafter appended.