JPH0679274A - Method for recovering waste slicing water - Google Patents

Abstract

PURPOSE:To maintain the filtering capacity of a membrane separator with the small number of cleanings without decreasing the recovery rate of the effective component in a method for recovering filtered water obtained by separating and removing suspended matter in the waste water by keeping the waste water to be supplied to the separator below its clouding point. CONSTITUTION:The waste slicing water is treated by a membrane separator to recover the filtered water obtained by separating and removing suspended matter in the waste water. In this case, the waste water to be supplied to the separator is kept below its clouding point. A surfactant is deposited at the clouding point to cause clouding. The waste water contains <= about 5% of the water-soluble cutting oil, rust inhibitor, surfactant, antifoaming agent, etc. Since the concn. is low, the property of the waste water is not affected by the concn. but affected by the presence of solubilized matter. Since the waste slicing water is kept below its clouding point, filing is suppressed, and the stability in filtering capacity is remarkably improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for recovering slicing waste water. More specifically, the present invention relates to an improvement in a method for collecting slicing wastewater in which the wastewater discharged from the slicing step of the semiconductor wafer manufacturing process is applied to a membrane separation device and the filtered water is reused as cutting water.

[0002]

2. Description of the Related Art In the process of manufacturing a silicon wafer, there is a process called a slicing process in which a single crystal ingot is cut to a predetermined thickness with a diamond blade. In this step, cutting water containing 1 to 5% of water-soluble cutting oil is used in order to impart lubricity at the time of cutting the silicon ingot, remove heat generated, and extend the life of the blade.

Cutting drainage (hereinafter referred to as slicing drainage)
Since it is suspended water containing silicon fine particles, some treatment is necessary, but as mentioned above, it contains expensive water-soluble cutting oil, so the economic loss due to disposable is very large, and it is more difficult than conventional filter cloths. The particles of diatomaceous earth are pre-coated to separate and remove fine particles of silicon,
Used repeatedly.

Recently, a collection method using a membrane separation device using a microfiltration membrane or an ultrafiltration membrane has been proposed.

[0005]

However, in the method of precoating diatomaceous earth on a filter cloth and filtering, silicon fine particles of 0.5 micron or less leak, so that clear filtered water cannot be obtained. In addition to the drawbacks, there is a problem that the filter cloth is clogged and the filtering ability is lowered, so that the filter cloth must be washed manually.

Further, a method of filtering with a membrane separator using a microfiltration membrane or an ultrafiltration membrane gives clear filtered water,
Although it has advantages such as easy operation management, the fouling is large and the filtration capacity gradually decreases, so that it is necessary to wash the membrane frequently. The membrane is washed with a chemical, for example, with an alkali, but in that case, a large amount of turbid washing waste liquid is generated.

After washing, it is necessary to rinse with pure water to prevent the chemical components from contaminating the filtered water.
Since the rinse wastewater cannot be returned to the wastewater, there is a problem that a special treatment device or treatment means is required like the cleaning wastewater. This problem cannot be solved even when the membrane is removed and washed outside the system of the membrane separation device, for example, when the filtration capacity is lowered.

[0008]

On the other hand, the inventors of the present invention have earnestly studied the method of operating a membrane separation device for maintaining stable filtration performance of a microfiltration membrane and an ultrafiltration membrane, and as a result, the membrane separation device has been investigated. The present invention has been found that by keeping the temperature of the slicing wastewater to be supplied to the cloud point below the cloud point, fouling is suppressed and the stability of the filtration capacity is significantly improved.

That is, according to the present invention, in a method for collecting slicing wastewater to a membrane separator to recover filtered water from which suspended substances in the wastewater are separated and removed, the temperature of the slicing wastewater supplied to the membrane separator is below a cloud point. It is a method for collecting slicing wastewater, which is characterized in that Here, the cloud point is a temperature at which a surfactant precipitates to cause turbidity.

The slicing wastewater to which the present invention is applied contains a water-soluble cutting oil, specifically, a rust preventive agent such as a synthetic lubricant, an inorganic salt and amines, a surfactant, an antifoaming agent and the like and has a cloud point. It is drainage, and the cloud point is usually a low concentration of water-soluble cutting oil of 5% or less, so it does not change much depending on the concentration, but it actually changes because of the presence of solubilizing substances. However, it is difficult to confirm because the slicing wastewater is turbid, and it is necessary to dilute the water-soluble cutting oil to a predetermined concentration for confirmation.

The membrane used in the present invention is a microfiltration membrane or an ultrafiltration membrane. Although the material and structure of the membrane are not particularly limited, since the pH of the slicing wastewater is generally about 8 to 9, the material of the membrane is required to be alkali resistant, and the pore diameter of the membrane is 0.5 micron or less. Due to the presence of silicon fine particles, it is 0.5 micron or less, preferably 0.2 micron or less.

The temperature of the slicing wastewater supplied to the membrane separator is controlled by installing a cooler in the storage tank of the slicing wastewater or by providing a heat exchanger in the supply pipe of the membrane separator. In a membrane separator equipped with a backwashing mechanism, it is usually necessary to keep the concentration of water-soluble cutting oil in the cutting water constant, so filtered water is used as backwashing water. It is important to keep the temperature of the backwash water below the cloud point in order to prevent clogging of the membrane due to washing.

Further, the operation system of the membrane separation device must be a cross flow system (circulation filtration system) in order to keep the filtration capacity stable, and the linear velocity during circulation is 0.5 to 3 m / sec, preferably 1 0.5 to 2.5 m / sec.

[0014]

EXAMPLES Examples and comparative examples of the present invention will be given below, but the present invention is not limited thereto.

[0015]

Example 1 Using a membrane separator equipped with a hollow fiber polyacrylonitrile ultrafiltration membrane (manufactured by Asahi Kasei Kogyo Co., Ltd., molecular weight cutoff 13000), the slicing wastewater having the average properties shown in Table 1 was introduced at an inlet pressure of 1. 5 Kg / cm ² , outlet pressure 0.5 Kg / cm ² , linear velocity 2.2 m / sec, temperature 21-
Concentration factor 10 times (Cutting water recovery rate 90
%). The cloud point is 25 when measured by diluting the water-soluble cutting oil to a predetermined concentration (3%).
It was ℃.

Every 30 minutes, the membrane separator is operated while performing filtration while alternately changing the direction of flow through the ultrafiltration membrane (reversing the inlet and the outlet), and then using 1.0 k of filtered water.
This was repeated by backwashing for 30 seconds at a pressure of g / cm ² . Initial filtration capacity is 92 liters / m ² Hr, 1
It was 73 liters / m ² Hr after a month. The average properties of the filtered water during this period are shown in Table 1.

[0017]

Comparative Example 1 The filtration capacity after operating for 1 month in the same manner as in Example 1 except that the membrane separator was operated at 28 to 30 ° C. was 37 liter / m ² Hr / 95 liter / m ² Hr at the beginning of the operation. m ² Hr, which was a decrease of about 60%. Table 1 shows the average properties of the filtered water during this period.

[0018]

[Table 1]

[0019]

Industrial Applicability According to the present invention, the temperature of the slicing waste water applied to the membrane separation device is kept below the cloud point to maintain the filtration capacity of the membrane separation device with a small number of washings without lowering the recovery rate of the active ingredient. Can be, efficient and economical.

Claims (1)

[Claims] 1. A slicing wastewater is applied to a membrane separation device,
A method for recovering slicing wastewater, characterized in that the temperature of the wastewater supplied to the membrane separation device is maintained at a cloud point or lower in the method for collecting filtered water from which suspended substances in the wastewater are separated and removed.