DESCRIPTION METHOD OF AND APPARATUS FOR SORTING MINERALS TECHNICAL FIELD [0001] The present invention relates to a method of sorting minerals into a plurality of levels according to sizes thereof and an apparatus used for this. BACKGROUND ART [0002] The minerals are crushed by a centrifugal crusher and the like into crushed stones, sorted according to the sizes thereof, and supplied on demand as a base course material, aggregate concrete and the like. [0003] The applicant of the present application has developed the apparatus disclosed in Japanese Patent Application Laid-Open No. 2003-175367 as the apparatus for sorting. The apparatus for sorting is to sort materials to be sorted (crushed stones) blown off by a blast of an air blow device by utilizing different blow distances according to the sizes (weights) thereof. This is referred to as separation by air force. Two screw conveyors for conveying the minerals are installed with different distances from an air blower under a space in which the crushed stones are blown of f, and the fallen 1 crushed stones are collected on the conveyor to be divided into two sizes according to the blow distance. [0004] Also, as another conventional means of sorting the minerals, means of dropping the minerals of a variety of sizes at once on the sorting screens having different sizes of sieve mesh (mesh sizes) vertically arranged in a descending order of the size of the sieve mesh, and allowing the minerals having a size smaller than the sieve mesh to fall on the screen below, thereby sequentially screening them to sort into a plurality of sizes is also adopted. [0005] However, when the minerals of a variety of sizes are f allen at once on the same position, the minerals having the size smaller than the sieve mesh roll on the screen so as to be entangled with the minerals larger than the sieve mesh, so that it is required to use a long and large area sorting screen as the sorting screen for screening such that the minerals roll for a long distance to ensure the sorting. That is to say, such sorting apparatus is extremely large. [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-175367 DISCLOSURE OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0006] The present invention is to solve the problem that the 2 blown minerals (crushed stones) are not efficiently sorted by compact means. MEANS FOR SOLVING THE PROBLEM [0007] A method of sorting according to the present invention is the method of sorting minerals including air blow means for separating introduced minerals by air force and sorting means for receiving fallen minerals, sorting the minerals of a predetermined size or larger, and allowing the minerals of a predetermined size or smaller to pass therethrough, wherein including blowing off the minerals of a smaller diameter farther by the air blow means, allowing the minerals of sizes of a relatively limited certain range to fall on the sorting means, and sorting the minerals of sizes of the certain range by each sorting means. An apparatus for sorting the minerals according to the present invention is the apparatus for sorting minerals for sorting sizes of the minerals by separating the minerals to be sorted by air force, including a dust collection device for collecting dust from a space to separate the minerals by air force, wherein a sorting screen is arranged below the space to separate the minerals by air force. Also, the apparatus for sorting the minerals according to the present invention is the apparatus for sorting minerals for sorting sizes of the minerals by separating the minerals to be sorted by air force, including a dust collection device 3 for collecting dust from a space to separate the minerals by air force, wherein a sorting screen is arranged below the space to separate the minerals by air force, and an end of the sorting screen is located at a position to which the minerals to be sorted are blown off. Further, the apparatus for sorting the minerals according to the present invention is the apparatus wherein a collection opening without the sorting screen is formed on a portion deeper from the end of the sorting screen. Further, the apparatus for sorting theminerals according to the present invention is the apparatus wherein an intermediate damper for dividing the minerals is provided on the sorting screen on a position to which the minerals to be sorted are blown off. EFFECT OF THE INVENTION [00081 The method of sorting the minerals and the apparatus for sorting used for this according to the present invention may obtain a following effect by the above-described configuration. <a> The minerals of substantially the same size out of the minerals already separated according to the sizes by being blown off by the blast are fallen on a surface of a certain range of the sorting means, so that the minerals do not roll for a long distance by being entangled with larger minerals when passing through the sorting screen and the minerals pass the mesh (sieve mesh) smoothly, and the large (long) sized sorting screen is 4 not required, therefore, the sorting apparatus itself may be made compact. <b> By including the dust collection device, it is possible to collect the dust separately from the sorting screen, so that sorting operation of another mineral becomes significantly easy. That is to say, the fine minerals do not attach to the sieve mesh (mesh) of the sorting screen, and an opening of the sieve mesh is not narrowed, and this does not interfere with the sorting operation. <c> Since the minerals of different sizes fall on other positions of the sorting means, the minerals of the large size do not fall and damage of a fine screen is reduced. <d> Since the end of each of the sorting screens is located at a position to which the materials to be sorted are blown of f, the materials to be sorted of a small size, which are blown farther, directly fall on the sorting screen having the sieve mesh of the small size with which the minerals are to be sorted, so that a case in which the minerals hit many times the sorting screen having the larger mesh size located above and smooth falling is prevented does not occur. <e> By providing the collection opening without the sorting screen on a portion deeper than the sorting screen, the fine materials do not pass through the sorting screen, so that ability of the screen is further improved. <f> By providing the intermediate damper for dividing the minerals on the sorting screen on a position to which the minerals of the size to be sorted are blown off, a dividing 5 position may be adjusted according to a blow condition of the minerals. BEST MODE FOR CARRYING OUT THE INVENTION [0009] The present invention is to separate minerals by air force and allow the minerals to fall on a surface of a certain range of sorting means, thereby allowing the minerals to smoothly pass through a sorting screen to realize compact selecting means. [First Embodiment] [0010] <1> Housing Fig. 1 shows a first embodiment of the present invention, and a housing frame 1 of an entire apparatus is a screener A in which side walls of a vertically intermediate portion are connected by upper and lower accordions la such that oscillation of a vibrator and the like is transmitted thereto. The screener A, which is the sorting means, has a plurality of sorting screens 11 to be described inside thereof, and the sorting screens 11 oscillate by the oscillation of the vibrator transmitted thereto. A blow opening 2 is opened on a side of an upper portion of the housing frame 1. A blast of a blow fan 3, which is air blow means, is sent from the blow opening 2 to a mineral sorting space 4 of the housing frame 1. A mineral slot 5 is opened in the vicinity of the blow opening 2, immediately above. 6 [00111 <2> Dust Collection Device A dust collection device 9 is installed on the upper portion of the housing frame 1, on an opposite side of the blow opening 2. The dust collection device 9 has a dust collection opening 10 toward the mineral sorting space 4, and this may collect minute minerals by suction force of the dust collection device 9. [0012] <3> Sorting Screen The sorting screens 11, which are the sorting means, each having a number of sieve meshes, are arranged in a stack under the mineral sorting space 4 of the housing frame 1. The sorting screens 11 are such that the size of the sieve mesh (mesh sizes) thereof becomes smaller according to a classification of sorting from the uppermost screen toward the lowermost screen. A partition wall 14 is provided on a side end of the blow opening 2 of each of the sorting screens 11, and a separate exhaust 15 is formed for each of the sorting screens 11. [0013] A method of sorting the minerals using the above-described sorting apparatus is hereinafter sequentially described. <4> Blow-Off Operation Minerals 12 are introduced into the apparatus from the 7 above-described mineral slot 5. The fallen minerals 12 are blown off by the blast from the blow fan 3. The minerals 12, which are larger and heavier, are not blown far. On the other hand, the minerals 12, which are smaller and lighter, are blown farther by the blast. That is to say, the minerals 12 are separated according to the sizes of a certain range by air force, and this is referred to as separation by air force in the present invention. (0014] <5> Sorting Operation The sorting screens 11 are net-like members and vertically stacked in a descending order of the size of the sieve mesh for screening, and the minerals 12 of the size smaller than the size of the sieve mesh pass through the screen 11 to be fallen further below. In Fig. 1, four screens 11a to lld are arranged in an order of the size of the sieve mesh thereof such that the screen ild having the size of 20 mm first, then the screens llc and llb having the sizes of 13 mm and 5 mm, respectively, and further the lowermost screen 1la having the size of 3 mm. Smallest and lightest minerals 12a are blown farthest to pass through the sieve meshes of all the sorting screens lld to 1la and are fallen below to be exhausted from an exhaust 15a. Minerals 12b, which are larger than the minerals 12a in size (classification), pass through the screens lld, llc and llb to be fallen on the screen 1la and roll on the screen 1la 8 to be exhausted from an exhaust 15b. Similarly, minerals 12c and 12d, which are further larger in size (classification) are fallen on the screens l1c and lid, respectively, to be exhausted from exhausts 15c and 15d of those classifications. Minerals 12e of the largest size do not pass through the screens 11a to d at all, and roll on the screen d to be exhausted from an exhaust 15e. The present invention prevents the minerals from being fallen so as to be entangled with larger minerals, by setting positions on which the minerals 12 of a certain range fall within a certain range on the surface of the sorting screen 11, by combining the separation of the minerals according to the sizes thereof by air force and the sorting screens 11, thereby significantly improving sorting ability by the sorting screens 11. That is to say, the sorting by air force and the sorting screens are not simply combined, but the both are combined for smoothly sorting by the sorting screens to achieve a special effect to make the length and the width of the sorting screens 11 compact. [0015] <6> Dust Collection Operation The minerals in a state of extremely fine dust are collected from the mineral sorting space 4 through the dust collection opening 10 by operation of the dust collection device 9. 9 In this manner, in the sorting apparatus in Fig. 1, the minerals 12 are sorted into six sizes. Although the illustrated embodiment is provided with a plurality of sorting screens 11, when using the dust collection device 9 and a collection opening 17 together, the minerals 12 may be sorted into f our sizes even when there is a unique sorting screen 11. That is to say, a plurality of sorting screens 11 are not necessarily required. [Second Embodiment] [0016] Fig. 2 shows a second embodiment of the present invention, and the collection opening 17 without the sorting screen for collecting the minerals, which fall below the same, is provided on the deepest portion of the housing frame 1 away from the blow opening 2. A rear damper 19 capable of changing an inclination angle is attached to the uppermost end of a partition wall 18, which forms the collection opening 17. When there is a slight difference in blow distance according to the kind of the minerals, it is possible to more correctly sort from the smaller minerals 12 by changing an angle of the rear damper 19 according to the blow distance. The minerals 12 of the small size, which are blown farthest, are exhausted from the collection opening 17 and collected. In this manner, the minerals 12 of which diameter is the smallest are collected without being passed through the sorting 10 screen, so that prevention of the sorting is eliminated as much as possible, thereby improving sorting efficiency. A damper 21 capable of inclining to the collection opening 17 side is provided in the middle of the partition wall 18. When exhausting the minerals 12, which are blown farther than the partition wall 18, to the exhaust 15a side and mixing the minerals 12 with the minerals 12a, which pass through the sorting screen lla, the damper 21 is inclined to the collection opening 17 side to exhaust the minerals 12 to the exhaust 15a side. [Third Embodiment] [0017] Fig. 3 shows a third embodiment of the present invention, and the end of each of the sorting screens 11 on the side apart from the blow opening 2 is located at the longest reach position of the minerals 12, which should fall thereon to be sorted. The blow distance of the minerals 12 differs according to the size of the minerals 12 together with a performance of the blow fan 3, and this is obtained in advance as experimental data. By this experiment, each of the sorting screens 11a to d may determine blow distances of minerals 12a to d of sizes collected by screening using the sorting screens 11a to d. Each of the sorting screens 1la to d in the third embodiment preferably extends a tip end thereof on the side away from the blow opening 2 correctly to the distance (longest reach position) obtained by the experiment, but does not extend to 11 a position to which the minerals 12 of smaller size are blown off. Accordingly, the blown minerals 12 do not pass through the screen 11 with which the minerals of a larger size are sorted and most of them substantially correctly fall on the screen 11 with which the minerals are to be sorted. [0018] In the sorting apparatus in Figs. 1 and 2, showing the above-described first and second embodiments, there is a case in which the minerals having a diameter smaller than the size of the sieve mesh hit the net of the sorting screen having a larger sieve mesh and are flipped in front. When flipped, this remains on the screen together with the minerals of a larger size, and this does not smoothly pass through the screen. In the embodiment shown in Fig. 3, the minerals 12 do not fall onto the screen 11 for sorting a larger size, and directly fall on the screen 11 for screening the size. Therefore, the minerals 12 of a smaller size are not prevented from being fallen by the screen 11 having a larger sieve mesh, and the minerals smoothly pass through the screen 11. [Fourth Embodiment] [0019] Fig. 4 shows a fourth embodiment of the present invention, and the tip end of the sorting screen 11 is located at a position to which the minerals 12 of the size to be screened are blown off as in the third embodiment, and an intermediate damper 20 12 is attached to the tip end of each of the screens 11 such that the angle of inclination may be changed. When the blow distance slightly changes according to the kind of the minerals 12, a boundary position of the sorting may be adjusted according to the position to which the minerals are blown off by changing the angle of the intermediate damper 20, thereby enabling more correct sorting. Also, in the embodiment in Fig. 4, the collection opening 17 without the sorting screen is provided on a portion deeper than the sorting screen 11. Such an intermediate damper 20 is available in the first and second embodiments, and the intermediate damper 20 is provided not on the tip end of each of the sorting screens 11 but in the middle of the sorting screen 11 at a position to which the minerals 12 to be sorted are blown off. BRIEF DESCRIPTION OF THE DRAWINGS [0020] FIG. 1 is an illustration diagram of a first embodiment of a sorting apparatus according to the present invention. FIG. 2 is an illustration diagram of a second embodiment in which an exhaust is provided on a deepest portion. FIG. 3 is an illustration diagram of a third embodiment in which an end of a sorting screen is located at a position where minerals to be sorted are blown off. FIG. 4 is an illustration diagram of a fourth embodiment in which the end of the sorting screen is located at the position 13 where the minerals to be sorted are blown off, and an intermediate damper is provided on the end thereof. EXPLANATION OF REFERENCE NUMERALS [0021] A: Screener 1: Housing 2: Blow opening 3: Blow fan 4: Sorting space 5: Slot 9: Dust collection device 11: Sorting screen 12: Mineral 15: Exhaust 17: Collection opening 14