| Claims
[1] An apparatus for recycling disposed slurryproduced in a manufacturing process of silicon wafers, comprising: a first heater for heating the disposed slurry at a temperature ranging from 6O 0 C to a boiling point; a first centrifugal separator for rotating the disposed slurry heated by the first heater, at a rotation speed ranging from 1200rpm to 1500rpm in order to separate into a solid matter and a first liquid matter via centrifugation; a second heater for heating the first liquid matter separated by the first centrifugal separator, at a temperature ranging from 5O 0 C to a boiling point; and a second centrifugal separator for rotating the first liquid matter heated by the second heater, at a rotation speed of at least 2800rpm in order to separate into sawdust and a second liquid matter via centrifugation.
[2] The apparatus according to claim 1, wherein the first heater heats the disposed slurry at a temperature ranging from 6O 0 C to 90 0 C.
[3] The apparatus according to claim 1 or 2, further comprising a return line for returning the second liquid matter separated by the second centrifugal separator, to the second centrifugal separator, so that the second centrifugal separator repeats thecentrifugation at least twice.
[4] The apparatus according to claim 3, wherein the return line feeds the second liquid matter to the second heater so that the second liquid matter is heated in the second heater before entering the second centrifugal separator.
[5] The apparatus according to claim 3, further comprising a third heater on the return line, the third heater heating the second liquid matter at a temperature ranging from 5O 0 C to a boiling point.
[6] The apparatus according to claim 3, wherein the second centrifugal separator rotates at a uniform rotation speed or an increasing rotation speed while repeating the centrifugation.
[7] The apparatus according to claim 3, further comprising a recycling unit for mixing the solid matter which is separated by the first centrifugal separator, and the second liquid matter which is separated by the second centrifugal separator, thereby producing recycled slurry.
[8] The apparatus according to claim 1 or 2, further comprising a recycling unit for mixing the solid matter which is separated by the first centrifugal separator, and the second liquid matter which is separated by the second centrifugal separator, thereby producing recycled slurry. |
Description
APPARATUS FOR RECYCLING THE DISPOSED SLURRY PRODUCED IN THE MANUFACTURING PROCESS OF THE
SILICON WAFER
Technical Field
[1] The present invention relates to an apparatus for recycling disposed slurry produced in a manufacturing process of silicon wafers, and more particularly, to an apparatus for recycling disposed slurry by effectively separating and recovering abrasive and cutting oil from the disposed slurry. Background Art
[2] As information technologies and semiconductor industries have made rapid progress, the demands for single crystalline silicon wafers have been explosively increasing. As is well known in the art, single crystalline silicon wafers are generally manufactured by slicing a single crystalline silicon ingot with an abrasive-coated wire saw while supplying cutting oil. Then, the wafers are polished using a polisher. Approximately 20% to 30% of the original single crystalline silicon ingot is lost as sawdust in this process.
[3] In the manufacturing process of single crystalline silicon wafers, by-products such as abrasive (e.g., silicon carbide, aluminum oxide and silicon dioxide), sawdust and cutting oil are removed from the silicon wafers through cleaning. Accordingly, the abrasive and the silicon sawdust, which are dispersed in the cutting oil, exist in disposed slurry that is produced in the manufacturing process of the silicon wafers.
[4] Such disposed slurry, which is produced in the manufacturing process of the silicon wafers, is classified as special industrial waste. The disposed slurry cannot be simply burned up by fire since it contains the sawdust and the cutting oil. The disposed slurry cannot also be buried under the ground because the cutting oil would cause serious soil pollution. Accordingly, the disposed slurry produced from the manufacturing process of silicon wafers is solidified with cement, and then the solidified slurry is buried under the ground.
[5] However, such a conventional treatment is inadequate from environmental, economical and temporal points of views. Accordingly, in order to overcome the conventional treatment of solidifying and burying the disposed slurry, a method of separating and recovering abrasive, sawdust and cutting oil from disposed slurry was introduced.
[6] In this kind of method for recycling disposed slurry, a centrifuge based recycling method is widely used rather than a method for recycling slurry through solvent
extraction. The cutting oil can be easily separated from the slurry since the slurry is easily dissolved by water or oil. However, the cutting oil includes emulsifying additive, which is easily transformed in a drying or distillation process. When the cutting oil is reused, the transformed emulsifying additive causes the abrasive to deposit rather than to disperse in the cutting oil.
[7] The centrifuge based recycling method is carried out by two steps: In the first cen- trifugation step, abrasive is recovered by separating the slurry into a solid matter which mainly includes abrasive, and a first liquid matter, which mainly includes sawdust and cutting oil. In the second centrifugation step, the cutting oil is recovered by separating the first liquid matter, which is obtained from the first centrifugation step, into a second liquid matter, which mainly includes the cutting oil, and the sawdust.
[8] In the conventional centrifuge based recycling method using the two centrifugation steps, cutting oil is added into the disposed slurry prior to the first centrifugation in order to reduce the viscosity of the disposed slurry. The disposed slurry has very high viscosity because it contains a large amount of Si sawdust. Due to the large amount of Si sawdust, it is impossible to centrifuge the disposed slurry without adding the cutting oil into the disposed slurry. Generally, the cutting oil added into the disposed slurry prior to the first centrifugation step is recovered from the disposed slurry in the second centrifugation step.
[9] According to conventional technologies, together with the addition of the cutting oil, the disposed slurry is heated at a normal temperature or a temperature slightly higher than the normal temperature, for example, about 30 0 C ± 15 0 C prior to the first centrifugation step. However, this targets only on smoothly mixing the disposed slurry with the cutting oil. That is, the heating has been regarded only as a supplementary way for improving the efficiency of adding the cutting oil.
[10] The conventional method for recycling slurry by centrifuging the slurry after adding the cutting oil has the following shortcomings.
[11] At first, addition the cutting oil inevitably increases the amount of slurry to be processed. This lengthens process time, raises processing cost, and requires a large- sized apparatus. It has been found that the addition of the cutting oil increases the amount of slurry to be processed about 4 to 5 times.
[12] Also, adding the cutting oil requires an additional device for returning the cutting oil recovered from the second centrifugation step and another additional device for keeping the mixing ratio of the disposed slurry and the cutting oil constant. Those devices make a slurry recycling apparatus complicated. Disclosure of Invention Technical Problem
[13] The present invention has been made to solve the foregoing problems with the prior art, and therefore an object of the present invention is to provide an apparatus for recycling disposed slurry which can raise the recycling efficiency of disposed slurry, as an alternative to the prior art in which cutting oil is added for the purpose of dilution.
[14] Another object of the present invention is to provide an apparatus for recycling disposed slurry which can minimize process time and process cost and make a simple and compact structure.
[15] A further object of the present invention is to provide an apparatus for recycling disposed slurry which can have a simple structure that requires minimum installation cost. Technical Solution
[16] According to an aspect of the invention for realizing the object, the invention provides an apparatus for recycling disposed slurry producedin a manufacturing process of silicon wafers.The apparatus includes a first heater for heating the disposed slurryat a temperature ranging from 6O 0 C to a boiling point; a first centrifugal separator for rotating the disposed slurry, heated by the first heater, at a rotation speed ranging from 1200rpm to 1500rpm in order to separate into a solid matter and a first liquid matter via centrifugation; a second heater for heating the first liquid matter, separated by the first centrifugal separator, at a temperature ranging from 5O 0 C to a boiling point; and a second centrifugal separator for rotating the first liquid matter, heated by the second heater, at a rotation speed of at least 2800rpm in order to separate into sawdust and a second liquid matter via centrifugation.
[17] Preferably, the first heater heats the disposed slurry at a temperature ranging from
6O 0 C to 9O 0 C.
[18] Preferably, the apparatus can further include a return line for returning the second liquid matter, separated by the second centrifugal separator, to the second centrifugal separator so that the second centrifugal separator repeats the centrifugation at least twice.
[19] Preferably, the return line can feed the second liquid matter to the second heater so that the second liquid matter is heated in the second heater before entering the second centrifugal separator, or the apparatus can further include a third heater on the return line, the third heater heating the second liquid matter at a temperature ranging from 50 0 C to a boiling point.
[20] Preferably, the second centrifugal separator rotates at a uniform rotation speed or an increasing rotation speed while repeating the centrifugation.
[21] Preferably, the apparatus can further include a recycling unit for mixing the solid matter, which is separated by the first centrifugal separator, and the second liquid
matter, which is separated by the second centrifugal separator, thereby producing recycled slurry.
Advantageous Effects
[22] According to the present invention as set forth above, the apparatus for recycling disposed slurry achievesexcellent recycling efficiency by adjusting viscosity through the heating of disposed slurry and controlling the rotation speed of the centrifugal separators, instead of adding cutting oil for the purpose of dilution. [23] The present invention does not increase the amount of slurry to be processed, and thus can minimize process time and process cost, and make a simple and compact structure. [24] Furthermore, the apparatus for recycling disposed slurry of the present invention can have a simple structure that requires minimum installation cost. That is, the present invention can make the apparatus for recycling disposed slurry have high efficiency of recycling the disposed slurry using a simple operation principle and structure.
Brief Description of the Drawings [25] FIG. 1 is a diagram illustrating a process for separating abrasive, sawdust and cutting oil from disposed slurry and recycling according to an exemplary embodiment of the present invention; [26] FIG. 2 is a conceptual view illustrating an apparatus for recycling disposed slurry, which carries out the process shown in FIG. 1; [27] FIG. 3 is a reference view illustrating an equation for calculating the processing capacity of a centrifugal separator; [28] FIG. 4 is a graph showing changes in the oil content of a solid matter which is separated through a first centrifugation, according to variations in temperature and rotation speed; [29] FIG. 5 is a graph showing changes in the sawdust (Si) content of a solid matter which is separated through a first centrifugation, according to variations in temperature and rotation speed; [30] FIG. 6 is a graph showing changes in the abrasive (SiC) content of a solid matter which is separated through a first centrifugation, according to variations in temperature and rotation speed; [31] FIG. 7 is a graph showing changes in the density of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed lOOOrpm; [32] FIG. 8 is a graph showing changes in the particle size of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed lOOOrpm;
[33] FIG. 9 is a graph showing changes in the density of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed 1200rpm; [34] FIG. 10 is a graph showing changes in the particle size of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed 1200rpm; [35] FIG. 11 is a graph showing changes in the density of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed 1500rpm; [36] FIG. 12 is a graph showing changes in the particle size of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed 1500rpm; [37] FIG. 13 is a graph showing changes in the oil content of a second liquid matter which is separated through a second centrifugation, according to variations in temperature and rotation speed; [38] FIG. 14 is a graph showing changes in the sawdust (Si) content of a second liquid matter which is separated through a second centrifugation, according to variations in temperature and rotation speed; [39] FIG 15 is a graph showing changes in the abrasive (SiC) content of a second liquid matter which is separated through a second centrifugation, according to variations in temperature and rotation speed;and [40] FIG. 16 is a graph showing changes in the density of a second liquid matter which is separated through a second centrifugation, according to variations in temperature and the number of second centrifugation.
Best Mode for Carrying Out the Invention
[41] The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown. [42] FIG. 1 is a diagram illustrating a process for separating and recycling abrasive, sawdust and cutting oil from disposed slurry according to an exemplary embodiment of the present invention. [43] In order to recycle disposed slurry by separating and recovering abrasive and cutting oil therefrom, two centrifugation steps are carried out. [44] The first centrifugation step is performed to separate the disposed slurry into a solid matter which mainly includes abrasive made of SiC, and a first liquid matter, which mainly includes cutting oil. Then, the second centrifugation step is carried out at a high speed to remove fine Si sawdust from the first liquid matter, thereby recycling cutting
oil, which has a density similar to that of new oil, such as 0.89g/cc.
[45] FIG. 2 is a conceptual view illustrating an apparatus for recycling disposed slurry which carries out the process shown in FIG. 1.
[46] Disposed slurry, which has entered a tank lorry or a drum 1, is fed to a storage tank
3 by a pump P. Since the concentration of abrasiveis changed according to wire saw equipment and recovery conditions, such as recycling number, storage period and ambient air temperature, the slurry in the storage tank 3 is stirred to be uniform.
[47] The storage tank 3 heats the slurryat a predetermined temperature to lower the viscosity thereof. Heating is performed so that the temperature is maintained at a predetermined range, preferably, from 6O 0 C to a boiling point, and more preferably, from 60 0 C to 90 0 C.
[48] This means that the storage tank 3 acts as a first heater. As an alternative, the first heater can be embodied separately from the storage tank 3.
[49] The heated slurry is introduced into a first centrifugal separator 5. The first centrifugal separator5 is run at a rotation speed ranging from 1200rpm to 1500rpm, thereby separating the slurry into a solid matter which mainly includes SiC abrasive, and a first liquid matter, which mainly includes Si sawdust and cutting oil.
[50] If the disposed slurry is introduced into the first centrifugal separator 5 at a temperature lower than 6O 0 C or the centrifugal separator5 is run at a rotation speed lower than 1200rpm, the process efficiency of the first centrifugal separator excessively drops, thereby it may be impossible to run a second centrifugal separator 11. That is, as the efficiency of the first centrifugation is lowered, a large amount of abrasive enters the second centrifugal separator, thereby causing, for example, a breakdown of the second centrifugal separator that is running a high speed.
[51] The amount of the disposed slurryto be processed is not increased since cutting oil is not added to the slurry. Therefore, it does not increase process time, process cost or apparatus size.
[52] After the separation step performedin the first centrifugal separator 5, the solid matter is fed to a storage tank 15 via a storage tank 7, andthe first liquid matteris fed to a storage tank 9.
[53] The storage tank 9 heats the first liquid matter, which is centrifuged by the first centrifugal separator 5, at a temperature ranging from 5O 0 C to a boiling point in order to lower the viscosity thereof.
[54] This means the storage tank 9 acts as a second heater. As an alternative, the second heater can be embodied separately from the storage tank 9.
[55] The heated first liquid matter is introduced into the second centrifugal separator
1 l.The second centrifugal separatorl 1 is run at a high speed of 2800rpm or more, thereby separating the first liquid matter again into sawdust and a second liquid matter.
Here, the sawdust mainly includes Si sawdust and the second liquid matter mainly includes cutting oil. [56] Subsequent to the separation step in the second centrifugal separator 11, the second liquid matter is fed to a second storage tank 13. [57] Then, the second liquid matter is returned from the storage tank 13 to the second centrifugal separator 11. Prior to the returning, the second storage tank 13 heats the second liquid matterat a temperature ranging from 50 0 C to a boiling point in order to lower the viscosity thereof. [58] This means that the storage tank 13 acts as a third heater. As an alternative, the third heater can be embodied separately from the storage tank 13. It is also possible to return cutting oil to an upstream point of the second heater so that the second heater heats cutting oil. [59] These procedures of returning and centrifugation are performed once or more.Preferably, the second centrifugal separator 11 repeats the centrifugation twice or more. [60] The rotation speed of the second centrifugal separator 11 is preferably increased or maintained the same while the centrifugation step is being repeated. [61] After the second centrifugal separator 11 repeats the centrifugation step for a preset number, the centrifuged liquid matter is fed through the storage tank 13 to a recycling tank 15. [62] The recycling tank 15 mixes cutting oil which is fed from the storage tank 13, with abrasive which is introduced from the storage tank 7, thereby producing recycled slurry .Accordingly, the storage tank 15 acts as a recycling unit. [63] FIG. 3 is a reference view illustrating an equation for calculating the processing capacity of a centrifugal separator. [64] In a centrifugal field, a centrifugal force, a buoyant force, and a frictional force are applied to particles as shown in Eq. 1. [65] m-^ dt = F e - F 1 * - F d
= ( | ) π α 3
p ω 2
R - (|- ) π α 3
p 0
ω 2
R - - ( 1)
[66] The particles are fine grains having a several μ diameter. Since a liquid where the particle floats has very high viscosity, it may assume that Stake's Law is applied for the frictional force.
c M = Uμ_ ... (2)
Re apv
[68] If Eq.2 is substituted for Eq.1, Eq.3 can be obtained.
[69] m *L = (l) π α 3 p GU 2 R - (4)π Q 3 P 0 OJ 2 R - 6iμQ, •■• (3)
[70] Since all forces applied to the particle in the centrifugal field of the high viscosity liquid keep equilibrium, the acceleration become zero. Therefore, the left term of Eq.3 may become zero and thus an equation for velocity may be expressed as Eq.4.
[71]
2a(ρ — P 0 )W 2 R v - (4)
9μ [72] In Eq.4, v can be expressed as dRldt because it denotes a radial velocity from the center. Therefore, it may be expressed as Eq.5. [73] dR _ r 2a(p — Po] ^ w 2 R
]dt (5)
R 9μ
[74] If Eq.5 is integrated, Eq.6 is obtained.
[75]
R ^ = 2a 2 (p-p Q )wH _ ^ ln(
[76] If Eq.6 is simplified for t, Eq.7 is obtained.
[77]
t = tk • ■ • (7)
2α 2 (p- p Q )w 2
[78] A time required for moving a fine grain from a starting point R to a predetermined radius R can be calculated through Eq.7.
[79] The linear velocity of the raw sludge dl/dt may be expressed as below Eq.8.
[80]
AL = R. = Q •■■ (R) dt A n(R 2 -R 2 )
[81] Therefore, the retention time τ of the raw sludge in the centrifugal separator can be calculated by Eq.9.
= L π (R? - Rf )L r ,
~ dT. Q i y J { dt }
[83] In Eq. 7, if the retention time of Eq. 9 is applied into Eq. 7 as the time required for centrifugation, Eq. 10 can be obtained. [84]
π {n 1 - Jt 0 )L _ K 0 _ _ _ QQ λ
Q 2α 2 (p - p o )zυ 2
[85] In the left term of Eq. 10, terms R , R and L denote the volume of centrifugal separator as shown in Fig. 2, and Q is the velocity of liquid flowing into the centrifugal separator, which denotes the processing capacity of the centrifugal separator.
[86] If Eq. 10 is simplified based on the capacity of the centrifugal separator, it can be expressed as below Eq. 11.
[87]
Q _ ( 11 )
[88] In Eq. 11, R , R and L are terms related to the volume of the centrifugal separator, and t, a, p,
P o and μ are terms related to the properties of particles and liquid, ω is a term related to the driving condition of the centrifugal separator.
[89] The processing capacity of a centrifugal separator can increase by enlarging the volume of the centrifugal separator or accelerating the rotation speed of the centrifugal separator. Among the methods for increasing the processing capacity with the volume of the centrifugal separator and driving conditions fixed, the most effective method for increasing the processing capacity is to reduce the viscosity of liquid.
[90] The viscosity of the liquid may be reduced by adding solvent into the liquid or by increasing the temperature of the liquid.
[91] If the viscosity of the liquid is reduced by adding the solvent into the liquid, the apparent viscosity can be reduced and the processing capacity of the centrifugal separator can increase. However, the dilution is not matched with the object of the centrifugation since the object of the centrifugation is the concentration of the particles. Furthermore, since the liquid has to be diluted with a solvent before centrifuging the liquid, the volume of the disposed sludge increases. Consequently, the total processing
time for centrifuging the disposed sludge increases. [92] On the contrary, a method of increasing a temperature can reduce the viscosity of the liquid without increasing the volume of the disposed sludge. Therefore, the method of increasing the temperature has higher efficiency compared to the method of adding the solvent. [93] Therefore, in the present invention, the viscosity of the disposed sludge is reduced by increasing the temperature of the sludge. [94] Furthermore, the present invention has also been focused on a method of running the centrifugal separatorat the optimum rotation speed in order to enlargethe processing capacity for the slurry. [95] Such a method of reducing the viscosity of the disposed sludge is matched with the object of the present invention for increasing the recycling efficiency. [96] When a solid matter, which mainly includes abrasive, and a second liquid matter which mainly includes cutting oil, are recovered from disposed slurry and mixed together, it is preferred that the density of the solid matter is 2.04g/cc or more, the particle size of the solid matter is 4.2D or more, and the density of the second liquid matter is 0.93g/cc or less. [97] If the density and the particle size of the solid matter are smaller than these values,
SiC is not sufficiently separated and a large amount of sawdust and cutting oil are mixed into the solid matter. In this case, cutting is not performed properly, thereby causing wafer warping, which is hazardous in the semiconductor industry where precision is the key factor. [98] In addition, if the density of the second liquid matter is greater than these values,wafer qualities (e.g., surface roughness, surface waviness and flatness of wafers) are affected. Then, it is necessary to additionally mix new oil (density:
0.89g/cc), which increase cost. [99] The composition, density and particle size of the solid matter are measured with the temperature of disposed slurry and the rotation speed of the first centrifugal separator changed. [100] Tables 1 to 3 report the experimental results, which are obtained by running the first centrifugal separator at rotation speeds lOOOrpm, 1200rpm and 1500 rpm, respectively. [101] Table 1
[102] [103] Table 2
[104] [105] Table 3
[106] [107] FIGS. 4 to 6 are graphs showing changes in the oil, sawdust (Si) and abrasive (SiC) contents of a solid matter which is separated through a first centrifugation, according to variations in temperature and rotation speed.
[108] As shown in FIGS. 4 to 6, at the rotation speed lOOOrpm, it is seen that increasing temperature does not greatly influence SiC content change. However, at 1200rpm and 1500rpm, a sharp rise is observed at 6O 0 C, followed by a smooth rise.
[109] According to repetitive experiments, SiC recycling is most efficient in a temperature range from 6O 0 C to 9O 0 C. While a slight rise in content is observed in a temperature range higher than 9O 0 C, it is not efficient when the cost of recovered SiC is compared with the cost of energy for maintaining the temperature.
[110] Likewise, the SiC recovering result is most efficient at the rotation speed ranging from 1200rpm to 1500rpm and increase in content is rarely observable in a rotation speed range greater than 1500rpm. Accordingly, the rotation speed ranging from 1200rpm to 1500rpm is regarded optimal when the cost of recovered SiC is compared with the cost of energy for raising the rotation speed.
[I l l] FIGS. 7 and 8 are graphs showing changes in the density and the particle size of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed lOOOrpm.
[112] As shown in FIGS. 7 and 8, at the rotation speed lOOOrpm, it is impossible to reach target values of a solid matter (target density: 2.04g/cc, target particle size: 4.2D), even if temperature is raised. Accordingly, the rotation speed lOOOrpm is determined unsuitable for the rotation speed of the first centrifugal separator.
[113] FIGS. 9 and 10 are graphs showing changes in the density and the particle size of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed 1200rpm.
[114] As shown in FIGS. 9 and 10, it is shown that the target density and the target particle size of the solid matter can be obtained from 6O 0 C at the rotation speed 1200rpm.
[115] FIGS. 11 and 12 are graphs showing changes in the density and the particle size of a solid matter which is separated through a first centrifugation, according to variations in temperature at a rotation speed 1500rpm.
[116] Similar to FIGS. 9 and 10, it is shown that the target density and the target particle size of the solid matter can be obtained from 6O 0 C.
[117] Next, the composition of a second liquid matter which is separated by the second centrifugal separator, is analyzed while changing the temperature of a first liquid matter which is separated by the first centrifugal separator, and the rotation speed of the second centrifugal separator.
[118] Tables 4 to 7 report the experimental results, which are obtained by running the second centrifugal separator at rotation speeds 2600rpm, 2800rpm, 3000rpm and 3200rpm, respectively.
[119] Table 4
[120] [121]
[122] [123]
[124] [125]
[126] [127] FIGS. 13 to 15 are graphs showing changes in the oil, sawdust (Si) and abrasive (SiC) contents of a second liquid matter which is separated through a second cen- trifugation, according to variations in temperature and rotation speed.
[128] As shown in FIGS. 13 to 15, the recovery ratio of cutting oil is excellent in a condition of 5O 0 C or more and 2800rpm and more. [129] Table 8 is a graph showing changes in the density of a second liquid matter which is separated through a second centrifugation, according to variations in temperature and the number of repeating centrifugation by the second centrifugal separator.
[130] Table 8
[131] [132] FIG. 16 is a graph showing changes in the density of a second liquid matter which is separated through a second centrifugation, according to variations in temperature and the number of second centrifugation.
[133] According to the experimental results shown in FIG. 16, the density of the second liquid matter shows a large difference from the target density at a normal temperature, but is close to the target density at 5O 0 CAs the centrifugation is repeated, the density of the second liquid matter drops to a value below the target density.
[134] It is also seen that the recovery efficiency of cutting oil can be raised further by increasing rotation speed rather than keeping it uniform while the centrifugation is being repeated.
[135] Collectively analyzing the above experimental results, if disposed slurry having a temperature below 6O 0 C enters the first centrifugal separatoror the first centrifugation is performed at a rotation speed less than 1200rpm, the efficiency of the first centrifugation drops, the second centrifugal separator cannot separate cutting oil with a reusable reference density from the second liquid matter, and pipeline clogging is caused. Accordingly, this condition is rarely applicable to mass production.
[136] On the other hand, if disposed slurry has a temperature of 9O 0 C or more and the rotation speed of the first centrifugal separatoris 1500rpm or more, it is seen that process costs for maintaining temperature and rotation speed, rise sharply while the separation efficiency does not have a significant change. Accordingly, it is concluded that a more preferable temperature range of disposed slurry is from 6O 0 C to 9O 0 C and a more preferable rotation speed of the first centrifugal separator is from 1200rpm to 1500rpm.
[137] Furthermore, if disposed slurryhaving a temperature below 5O 0 C enters the second centrifugal separator or the second centrifugation is performed at a rotation speedless than 2800rpm, the efficiency of the second centrifugation drops and thus cutting oil which is separated through a the second centrifugation, cannot have a desired value (density: 0.93g/cc or less) that is available for sawing.