Sand producing method and apparatus

In an apparatus and method for producing sand from stone, the stone is first crushed in a centrifugal crusher and then filtered through a screen. The stone retained on the screen is further crushed by a vibration mill. The stone passing through the screen and the stone crushed by the vibration mill are combined and then are filtered through a separater to separate out the minute particules. Measurements of various flow rates and particle sizes are made in order to control the rotational speed of the vibration mill and the rate at which stone is supplied to the screen.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
The present invention relates to a method for producing sand having a 
predetermined granular size by crushing stone and to an apparatus used in 
the method. 
Due to a recent deficiency of natural sands such as river sand, sea sand, 
mountain sand and the like to be used as fine aggregates, there is a trend 
that demands for sand obtained by crushing rock or stone are increased. 
In general, a rod mill has been used to obtain the sand, and the sand of 
the predetermined size has been obtained by adjusting the feed rate of the 
material to be crushed, a water feed rate, and a rod numbers contained in 
the rod mill. A rapid adjustment of such parameters is required to obtain 
the sand having a predetermined granular size. 
This adjustment requires a great deal of experience and skill of an 
operator. Furthermore, there is danger in making the adjustments, since 
the operator must handle rods having a heavy weight while temporarily 
stopping the operation of the rod mill. Thus, the adjustment operation is 
difficult to carry out and is low in productivity. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for producing 
with ease sand having predetermined granular size, without the 
aforementioned skillful and difficult adjustment work, and an apparatus 
therefore. 
The present invention relates to a method and an apparatus for producing 
crushed sand having a predetermined granular size, wherein the steps of 
crushing through a centrifugal crusher stone which is the material of the 
sand, screening the crushed stone and crushing the rough stone obtained 
over the screen, and removing minute particles from a mixture of crushed 
stone obtained by the vibration mill and by passing through a screen. The 
present invention also relates to a sand producing method and an apparatus 
therefore in which inputted into a computer in a control board are 
respective fines modulus of the roughly crushed material crushed by the 
centrifugal crusher, the material on the screen to be fed into the 
vibration mill and material from which the minute particle are removed, 
respective weights of the material on the screen and below the screen, a 
volume ratio and a predetermined granular size of the sand. Such inputted 
values are judged by the computer to output a suitable signal, thereby 
controlling a rotational speed of the vibration mill and the feed rate of 
the material to the screen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the invention will now be described in detail 
with reference to the accompanying drawings. FIG. 1 shows a method and 
apparatus according to the present invention for producing sand in the 
case of removing minute particles from crushed stone in a wet condition. 
Stone which is the material of sand is fed from an upper portion of a 
centrifugal crusher 1. The crushed stone is picked up from a lower portion 
of the centrifugal crusher 1 and is delivered by a belt conveyor 2 to a 
continuous screen 3 to be screened. The rough lumps of stone on the screen 
are again fed into a vibration mill 5 by a belt conveyor 4 and further 
crushed into fine granules. The fine granules are mixed with the crushed 
stone which has been screened through the continuous screen 3. The mixture 
of the crushed stone contains very fine particles, therefore, the mixture 
is introduced into a classifier 6. In the classifier 6, the mixture of the 
crushed stone is mixed with water fed therein and the granules of the 
crushed stone having a relative rough size is scraped upwardly by a 
inclined screw rotating in the classifier 6, so that moisture contained 
therein is reduced as much as possible and the granules are conveyed by a 
belt conveyor 7 for storage and become a final product, i.e. sand. The 
minute particles contained in the crushed stone are discharged from the 
classifier 6 in a contaminated state mixed with water and are processed by 
a waste water treating plant 8. The aforementioned method involved the 
classifier 6 using water. The produced sand contains water and the waste 
water treating plant is costly. Also, the separation of the minute 
particles contained in the waste water and the treatment thereof require a 
relative large number of steps. Therefore, it is possible to use a dry 
type system as shown in FIG. 2. 
The crushed stone screened by the continuous screen 3 and discharged from 
the vibration mill 5 in FIG. 1 are raised to a higher level by a bucket 
elevator 21 or the like shown in FIG. 2 and are fed into a louver 22 shown 
in FIG. 2 where the stone is classified into sand and minute particles by 
air flows. The sand falls through the louver 22 and is stored in a sand 
tank 28. The air flow containing the minute particles is introduced into a 
preduster 23 where the rough particles entrained in the air flow are 
removed. Then, the air is introduced into a cyclone 24 and is discharged 
to the outside through a bag-filter 25 and a pipe 26. The rough particles 
separated from the air flow in the preduster 23 fall into the sand tank 28 
whereas the minute particles separated in the cyclone 24 and the bag 
filter 25 are collected into a filler tank 29 by a conveyor 27. 
FIG. 3 shows a detail of a structure of the centrifugal crusher 1 in FIG. 
1. A rotor 32 fixed to a bearing 33 is rotated high speed around a center 
axis of the casing 31 of the crusher 1 by a belt 34 driven by an external 
motor (not shown). The stone fed from a supply port 35 fixed to the upper 
side of the casing in the central portion of the upper side of the rotor 
32 is thrown by a centrifugal force caused by the rotation of the rotor 32 
and collides against the crushed stone 36 accumulating on horizontal 
bottom 31A of the casing to be crushed and accumulated thereon. The 
crushed stone is fed out by displacing a control plate 37 downwardly or 
discharging the stone through holes formed in the bottom 31A. 
The vibration mill shown in FIG. 1 is of the well known type generally used 
as a super minute particle crusher and is such that a shaft to which an 
unbalance weight is fixed is rotated at a high speed so that a crushing 
body as a whole is vibrated at a high speed whereby rods incorporated in 
the crushing body impart collision, frictional crushing action and 
shearing action to the material to thereby crush the material. 
The aforementioned method and apparatus according to the invention are 
controlled by a computer control box to obtain sand having a predetermined 
granular size. The method and apparatus according to the invention are 
characterized by the following system. 
The respective weights of the material to be fed to the continuous screen 3 
and the material to be fed to the vibration mill 5 are measured by scales 
9A and 9B provided to the conveyors. Also, these materials on the 
conveyors and the product on the conveyor 7 are picked up by samplers 10A, 
10B and 10C respectively, to thereby measure the fineness modulus. These 
measured values are inputted into the control board 11 and are judged by a 
computer belonging to the control board 11. Based upon the judgement, 
suitable control signals are fed from the control board 11 to drive 
control sections of the belt conveyor 2 and the vibration mill 5, 
respectively, to thereby adjust the material feed rate to the continuous 
screen 3 and the rotational speed of the vibration mill 5. 
The adjustment will be undermentioned in great detail. Assuming that F is 
the fineness modulus of the material crushed by the vibration mill 5 and 
F.sub.1 is the fineness modulus of the material at which the vibration 
mill is driven to meet a predetermined fineness modulus. When F&gt;F.sub.1, 
the drive motor of the vibration mill is accelerated and inversely when 
F&lt;F.sub.1, the vibration mill is adjusted to be decelerated. 
The amount of the material to be fed to the vibration mill 5 is set to be a 
constant amount Q.sub.1. Assuming that Q is the amount of the material fed 
to the screen 3 and x is the ratio of the amount Q.sub.1 of the material 
on the screen to the amount Q. Such adjustment is carried out so that the 
amount Q of the material fed to the continuous screen 3 is increased when 
x=Q.sub.1 /Q is greater than a predetermined value, and the amount Q of 
material fed to the continuous screen 3 is conversely decreased when x is 
smaller than said value. In view of the case where the material is changed 
or the screening effect is not sufficiently attained due to clogging of 
the continuous screen, the weight ratio between the material on and below 
the screen is inputted into the computer. An error signal is generated in 
advance in the case where there is a fear that the sand to be a product 
would have a granular size becoming out of an allowable range of the 
predetermined granular size on the basis of the ratio. When the error 
signal is generated, the materials are newly sampled by the samplers to 
input into the computer the fineness modulus of the respective materials 
and the weight ratio between the materials on and below the screen. Thus, 
the rotational speed of the vibration mill 5 and the amount of the 
material to the continuous screen 3 by the belt conveyor 2 are controlled. 
The apparatus is, however, continuously driven without being stopped 
during above operations. 
According to the present invention, it is possible to continuously product 
having a desired granular size in safety by means of the computer of the 
control board. 
Furthermore, it is possible to obtain an extremely low cost and high 
quality product as a whole, since a part of the material is fed to the 
vibration mill which is expensive like the rod mill but almost all of the 
material is crushed by the centrifugal crusher which is of low cost both 
for the machine and for the running costs.