Method of making concrete sand

Abstract: A method of making concrete sand is provided in which a mixture of medium stones (3), material sand (4), and water (5) is agitated in the inner milling chamber (2) of a first cylindrical drum (1) by rotation of the first cylindrical drum (1). The medium stones (3) are maintained piling to a height equal to 1/4 to 1/3 the inner diameter of the first cylindrical drum (1). The number of rotation of the first cylindrical drum (1) is decreased as the inner diameter of the same increases. The supply of the water (5) is controlled for increase and decrease in precise or approximate proportion to the feeding of the material sand (4). The medium stones (3) are prepared by a novel manner from rubble stones (24) using a second cylindrical drum.

Description: The present invention relates to a method of making medium stones in use for a production of concrete sand according to the preamble of claim 1. A rod mill is a well-known apparatus for making concrete sand, which has a plurality of metal rods rotatably mounted in the milling chamber of a cylindrical drum. In action, as the cylindrical drum rotates, the metal rods crush down material sand to grains of sand by application of impact force. The hardness of the metal rods is far greater than that of the material sand and the impact force applied by the metal rods becomes high. Also, the action of the metal rods resides in crushing of the sand with little grinding effect and no abrasive action to gradually grind down the surface of each material sand grain will be ensured. Hence, the resultant grain sand produced with such a rod mill is low in quality for use as a finely sized aggregate material for making concrete. On the other hand, natural river sand, mountain sand, sea sand, and land sand are more preferable for use as aggregate sand. Among them, the river sand which is high in the rigidity and almost spherical in the grain shape, exhibits the most desirable quality. It was found through experiment that the percentage of round shaped grains of common water rinsed river sand (calculated through dividing the mass of a unit volume by an absolute dryness specific Weight) conforming to JIS A5004 was 57% to 59% while the same of crushed sand produced with a known rod mill was about 53%. Also, grains of the crushed sand are not round but angular and flat in the shape and their surface is not smooth and may have cracks. Those disadvantages will affect the properties of liquid concrete including workability and fluidity. The applicant of this specification has invented a novel method of making crushed sand (which was filed at U.S. Patent Office as Patent No.4995561 or EP 0384004 A2). In more detail, the method is illustrated in Fig.9 in which material sand 4 is continuously fed together with water 5 into an inner milling chamber 2 of a cylindrical drum 1 which is loaded with a pile of medium stones 3...3 and milled by means of the rotation of the cylindrical drum 1 actuated by a drive means to milled grains of sand 6 which is in turn unloaded from an exit 7 together with used water. Through the milling action together with water and medium stones, material sand will be ground under a condition similar to that in which river sand is produced from rubble stones by the action of nature. More specifically, the material sand is ground at the surface by degrees so that impurities on its surface are removed and the shape of each grain becomes round. The resultant sand will thus be as good as natural river sand, having a quality for use as an aggregate material material which is not angular nor flat in shape and smooth at the surface having less sign of crack. However, the concrete sand making method invented by the applicant still has a drawback to be overcome. For grinding the material sand 4 at optimum conditions, it is essential that the medium stones 3...3 which act as abrasives to the material sand 4 are round in shape, like pebbles or ballast stones which are rolled to round shapes in river water. To have such naturally existing round stones or river pebbles in bulk is not easy and requires a tough, troublesome labor and time. Since natural round pebbles are limited in the amount, they will hardly be applicable. Also, the medium stones 3 are worn partially through the milling action to particles which are then mixed up with the milled sand 6. If the particles contain unfavorable substances, they will chemically react with alkali components of a cement thus resulting in an expansion and causing the resultant concrete solid of the aggregate sand 6 to be fractured. In addition, steel bars when used for reinforcement of concrete will suffer from corrosion. When the medium stones 3 exhibit an improper absolute dryness specific weight and are low in the roundness, their pressing or grinding force to the material sand 4 remains less than a desired level ensuring no optimum abrasive action. Furthermore, the medium stones 3 become low in the rigidity and hardness thus resulting in fragmentation to bits and pieces. As understood, it is more preferable to use artificially cracked rubbles rather than naturally ground pebbles in view of less labor or time and no limitation to bulk. The cracked rubbles are however not round but angular in shape and will thus be unsuited for carrying out an abrasive action to material sand. It is not possible to produce fine grains of sand when such artificially cracked rubbles are used directly with no preparatory action of proper rounding. The action of proper rounding of cracked, angular rubbles is now a target to be tackled. The inventor of the present invention has found through a series of experimental achievements that important factors for producing fine grains of concrete sand include the properties, shape, and size of each medium stone. Hence, the present invention is directed towards an improved method of making concrete sand, in which the foregoing factors are provided at optimum level for achievement of the purpose. To comply with the object of the invention, the invention is characterized by the features of the single claim. According to the invention, the medium stones are prepared using a medium stone producing apparatus which comprises a second cylindrical drum having at interior a milling chamber, a drive device for rotation of the second cylindrical drum, an opening provided in one side wall of the second cylindrical drum for serving as both a feeding inlet and a discharge outlet, a spiral blade mounted about the opening to the inner side of the side wall of the second cylindrical drum for feeding material stones towards the center of the milling chamber during the forward rotation of the second cylindrical drum and discharging the same from the milling chamber through the opening during the reverse rotation, and a dust collector mounted to the other side wall of the second cylindrical drum. Also, the medium stones are prepared by examining material stones of rubbles using an alkali-silica reaction test, selecting desired material stones which are determined acceptable by the test and more than 2.65 in the absolute dryness specific weight and 5 to 150 mm in the size, feeding the selected material stones into the inner space of a second cylindrical drum, grinding the material stones by means of rotation of the second cylindrical for a given period of time, dividing the ground material stones into a particular number of groups according to their size, and determining a desired combination of the material stones of different size for use through selecting from the groups. According to the method of the present invention, medium stones are prepared with the medium stone producing apparatus, prior to making of concrete sand, in which rubble stones are fed into the inner milling chamber of the second cylindrical drum through the opening provided in the side wall of the same and agitated for grinding. During the grinding, the second cylindrical drum rotates in a right direction and the spiral blade mounted about the opening on the inner side of the side wall of the same presses the rubble stones towards the center of the milling chamber. More particularly, the grinding of the rubble stones will be encouraged by not only the parallel ribs on the inner side of the second cylindrical drum but also the spiral blade on the side wall, thus preventing any jam of the rubble stones at the opening end of the milling chamber. Also, the dust collector mounted to the other side wall of the second cylindrical drum performs a sucking action of dust generated during the grinding so that no dust can remain affecting adversely. The grinding action is carried out for a given duration of time, whereby the rubble stones which are angular in shape will be turned to round shape. Upon completion of the grinding action, the second cylindrical drum is switched over to rotate in the reverse direction by the drive device. Hence, the rubble stones (now, medium stones) are moved by the spiral blade along the inner side of the side wall towards the opening and unloaded from the second cylindrical drum through the opening. The spiral blade permits a systematic unloading movement of the medium stones and also, allows both the feeding and the discharging actions to be executed through one single opening provided in the side wall of the second cylindrical drum. The other side wall of the second cylindrical drum can thus be utilized for installation of the dust collector. The medium stones are also selectively used through examining their acceptable properties by a common alkali-silica reaction test and their fragments resulting from worn-off during the grinding will not react with alkali components of a cement when mixed in the aggregate sand thus causing no unfavorable effect, e.g. expansion, to a finished concrete solid. The medium stones are selected of more than 2.65 in the absolute dryness specific weight and 5 to 150 mm in the overall size so that their pressing or abrasive force to the material sand can be appropriate during the grinding in water. As the result, the medium stones can contribute to production of the concrete sand of desired quality and will be prevented from fragmentation to bits. The medium stones ground for a given time are divided into groups depending on their size. Accordingly, an optimum combination of the medium stones of different size can be determined for providing most favorable conditions during the grinding action thus producing a desired quality of concrete sand. Fig.1 is a front view of an apparatus of producing medium stones; Fig.2 is a cross sectional view of a primary part of the medium stone producing apparatus showing one end of the interior of a second cylindrical drum; Fig.3 is a longitudinal cross sectional view of the same end of the interior of the second cylindrical drum of the medium stone producing apparatus; Fig.4 is a cross sectional view of a primary part of the medium stone producing apparatus showing the other end of the interior of the second cylindrical drum; Fig.5 is a longitudinal cross sectional view of the other end of the interior of the second cylindrical drum of the medium stone producing apparatus; Fig.6 is a cross sectional view showing the production of medium stones; Fig.7 is a schematic front view of a rubble stone prior to grinding to a medium stone; Fig.8 is a schematic front view of a medium stone after grinding; Fig.9 is a longitudinal cross sectional view of an apparatus for making concrete sand; One preferred embodiment of the present invention will be described. An apparatus is provided for producing medium stones which are used in a concrete sand making apparatus, shown in Fig.9, for making fine grains of sand 6. The medium stone producing apparatus 10 illustrated in Figs.1 and 2 has a second cylindrical drum 11 of which interior forms a milling chamber and a drive device 12 for actuating the second cylindrical drum 11. The drive device 12 comprises a base 13, four rollers 15...15 mounted for rotation by brackets 14...14 on the upper surface of the base 13, a motor 16 mounted to end side end of the upper surface of the base 13, a drive sprocket 18 fixedly mounted to a rotating shaft 17 of the motor 16, an idler sprocket 19 fixedly mounted to a center region of an axially extending outer wall of the second cylindrical drum 11, and a chain 20 mounted between the two sprockets 18 and 19. In action, the rotation of the motor 16 is transmitted through the chain 20 to the second cylindrical drum 11 which thus rotates on the four rollers 15...15. The two sprockets 18, 19 may be toothed wheels or V-belt pulleys. The chain 20 may be a timing belt or V-belt. The rollers 15...15 are not limited to four and may be more than four (for example, eight). The second cylindrical drum 11 has an opening 21 provided in the center of one side wall 11a thereof which serves as both a feeding inlet and an outlet for rubbles or material stones, as shown in Figs.1 to 3. A small-diameter tube 22 is arranged about the opening 21 extending outwardly from the side wall 11a. Also, a spiral blade 23 is mounted about the opening 21 to the inner side of the side wall 11a. When the second cylindrical drum 11 rotates in a right direction with rubble stones 24...24 being loaded in its milling chamber, the spiral blade 23 performs an action for moving the rubble stones 24...24 towards the center of the chamber (or the left) as best shown in Fig.3. When it rotates in a reverse direction, the spiral blade 23 presses the rubble stones 24...24 towards the opening 21 for unloading. In addition, a chute 25 is provided directly beneath the opening 21 (or the tube 22) for downward transfer of the unloaded stones, as shown in Fig.6. As shown in Figs.1, 4, and 5, the other side wall 11b of the second cylindrical drum 11 has at inner side a plurality of radially extending mixing ribs 26...26 and at center a dust outlet 27 provided with a net filter. The dust outlet 27 is communicated at outer end with a dust collector 28. In action, dust 29 generated during the milling action is blown out from the dust outlet 27 of the side wall 11b to the dust collector 28, as shown in Fig.5. A flange 30 is provided about the outer edge of the side wall 11b of the second cylindrical drum 11, as shown in Fig.1, and at lower side end sandwiched for movement by a pair of sub-rollers 31, 31 which are in turn mounted by two sub-brackets respectively to the base 13. A single sub-roller having a circumferentially extending groove at center for supporting the flange 20 may be used in place of the two sub-rollers 31, 31. Also, any other applicable arrangement for supporting the flange 30 will be possible. The inner surface of the second cylindrical drum 11 is entirely protected with a rubber sheet 33 which acts as an impact relief cushion and also, provided with a plurality of equally spaced parallel mixing ribs 34...34 extending lengthwisely of the drum 11, as shown in Figs.2 to 5. The medium stone producing apparatus 10 is adapted for rounding the rubble stones 24...24 which are directly supplied from a quarrying plant. It should be noted that the rubble stones 24 are of a type which clears an alkali-silica reaction test. Such an alkali-silica reaction test will be conducted according to "the (chemical) method of testing alkali-silica reaction of aggregate" stipulated in Appendix 7 of JIS A5308 (1989) or "the mortar bar method" depicted in Appendix 8 of the same. In addition, the rubble stones 24 are more than 2.65 in the absolute dryness specific weight and 5 to 150 mm in the size. In operation, the rubble stones 24...24 are loaded through the opening 21 (or the tube 22) of the side wall 11a of the second cylindrical drum 11 into the inner milling chamber and then, the drive device 12 is actuated to rotate the second cylindrical drum 11. As shown in Fig.11, the rotation of the second cylindrical drum 11 causes a rolling action of the rubble stones 24...24 and resultant powders of dust 29 are blown out to the dust collector 28. The grinding action is accelerated for more efficient and positive movement by the spiral blade 23 on the side wall 11a, the radially extending mixing ribs 26 on the other side wall 11b, and the parallel mixing ribs 34. Simultaneously, the rubber sheet 33 bonded to the inner surface of the second cylindrical drum 11 relieves impact stresses exerted onto the rubble stones 24...24. Those movements simulate the natural actions of river water in which angular rubbles turn to pebbles and gravel stones and will even be carried out in a short period of time. After the grinding action through a given time, the drive device 12 is switched over to rotate the second cylindrical drum 11 in the reverse direction. As the second cylindrical drum 11 rotates in the reverse direction, the rubble stones 24...24 move along the spiral blade 23 of the side wall 11a towards the opening 21 (or the tube 22) prior to being discharge out from the opening 21 to the chute 25 for downward transfer. During the grinding action, the rubble stones 24...24 which have been angular at surface as illustrated in Fig.7 are rounded off to a shape shown in Fig.8 or turned to medium stones 3..3. Then, the finished medium stones are divided by a screen classifier into groups depending on their size. More specifically, a first group contains a size ranging from 80 mm to 100 mm, a second group from 60 mm to 80 mm, a third group from 40 mm to 60 mm, a fourth group from 20 mm to 40 mm, and a fifth group from 13 mm to 20 mm. A desired number of the medium stones to be used for milling are selected from the groups in order to have a proper combination of the medium stones of different sizes for producing appreciable sized grains of concrete sand. Preferably, the combination of the medium stones is consisted of 5% the first group stones, 12.5% the second group, 20% the third group, 27.5% the fourth group, and 35% the fifth group. As five different size groups of the medium stones are prepared, concrete sand of any grain size can be produced with the use of an optimum combination of the medium stones of different size. The medium stones 3 prepared by the foregoing manner are placed in a pile in the first cylindrical drum 1 of the concrete sand making apparatus shown in Fig.9 prior to being mixed with the material sand 4 and the water 5. The pile of the medium stones 3 comes up to a height equivalent to 1/4 to 1/3 the inner diameter L of the first cylindrical drum 1. More particularly. the top of the pile should be in the hatching area denoted by M in Fig.10. When the medium stones 3 are worn to the lower limit (1/4 of L) of the range M during the milling action produced by the rotation of the first cylindrical drum 1, a new supply of the medium stones is needed, on condition that the top of the pile is not higher than the upper limit (1/3 of L) of the range M and remains between 1/3 and 1/4 of L. An excessive supply of the medium stones may result in a declination in the milling action because the movement of the medium stones in the drum is limited and disturbed by their own mass. On the other hand, a shortage of the medium stones may cause their pressing force to be unevenly exerted onto the material sand or result in less loading thus diminishing the grinding effect. The material sand 4 is preferably less than 5 mm in the grain diameter of sea sand, mountain sand, dust produced in quarries, or the like.

Claims: Method of making medium stones in use for a production of concrete sand by means of a medium stone producing apparatus comprising: a cylindrical drum having its inside section designed for a milling chamber; a drive device for rotating said cylindrical drum; an opening, provided in one wall end of said cylindrical drum which serves not only as a feeding inlet but also as a discharge outlet; a spiral blade being mounted to run from an inner face of said one wall end of said cylindrical drum to said opening; and a dust collector being mounted in another wall end of said cylindrical drum; said method of making medium stones in use for the production of concrete sand being characterized by the steps of: selecting a plurality of material stones which are judged to be harmless according to an alkali-silica reaction test, 2,65 or more for absolute dryness specific weight and 5 to 150 mm for size; feeding a plurality of selected material stones into said milling chamber; milling a plurality of selected material stones while forcing them to be pushed into a central side of said milling chamber by means of said spiral blade, when said drive device permits said cylindrical drum to take a positive rotation; and discharging selected material stones from said opening, when said milling process begins to round off said selected material stones, by means of rotating said cylindrical drum in a reverse turn; thereby obtaining finished material stones which have been rounded off and different in size from one another.

Inventors: OOGAWARA YUKIMI
Applicants: NAKAYA JITSUGYO CO LTD, NAKAYA JITSUGYO CO., LTD.
Application year: 1992
Main classification: B02C17
IPC codes: B02C23, B02C17