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| United States Patent |
4,623,436 |
| Umehara |
November 18, 1986 |
Method and apparatus for removing impurities from liquids
Abstract
In a method and apparatus for removing impurities from a liquid, the liquid
containing impurities is supplied to an electrolytic bath at a pressure higher
than atmospheric pressure. Electrolysis is performed by applying a voltage to
electrodes consisting of a metal which easily dissolves upon electrolysis. The
liquid is then exposed to atmospheric pressure, and is treated in a flotation
separation tank. Fine bubbles formed during the treatment of the liquid in the
flotation separation tank attach to the impurities flocculated in the liquid by
electrolysis. The flotation separation of the flocculated impurities is
performed very efficiently, and high-purity water can be recovered. The
apparatus comprises an electrolytic bath consisting of a pressure vessel, in
which electrodes of a metal which easily dissolves upon electrolysis are housed,
and a constant current source for applying a voltage to said electrodes so as to
obtain a predetermined current density in a liquid contained in said
electrolytic bath, wherein impurities in the liquid are caused to flocculate by
a hydroxide of the metal.
| Inventors: |
Umehara; Yoshio (Hoya, JP) |
| Assignee: |
Showakoki Co., Ltd. (Tokyo, JP) |
| Appl. No.: |
817690 |
| Filed: |
January 10, 1986 |
Foreign Application Priority Data
|
Jan 10, 1985[JP] |
60-2413 |
|
Apr 24, 1985[JP] |
60-88399 |
|
Sep 27, 1985[JP] |
60-214208 |
| Current U.S. Class: |
205/743; 204/194; 204/277;
205/757; 210/748 |
| Intern'l Class: |
C02F 001/46 |
| Field of Search: |
204/194,131,149,305,306,277 210/748
|
References Cited [Referenced
By]
U.S. Patent Documents
| 3444062 |
May., 1969 |
Felici et al. |
204/131. |
| 3664951 |
May., 1972 |
Armstrong |
204/748. |
| 3673065 |
Jun., 1972 |
Anderson |
210/748. |
| 3888751 |
Jun., 1975 |
Minegishi |
210/748. |
| 3933606 |
Jan., 1976 |
Harms |
210/748. |
| 4071447 |
Jan., 1978 |
Ramirez |
210/748. |
| 4101409 |
Jul., 1978 |
Austin |
204/149. |
| 4120765 |
Oct., 1978 |
King |
204/149. |
| 4149953 |
Apr., 1979 |
Rojo |
204/149. |
| 4205966 |
Jun., 1980 |
Horikawa |
210/748. |
| 4224148 |
Sep., 1980 |
Lindmann et al. |
210/748. |
| Foreign Patent Documents |
| 3031773 |
Feb., 1982 |
DE |
210/748. |
Primary
Examiner: Demers; Arthur P.
Attorney, Agent or Firm: Finnegan,
Henderson, Farabow, Garrett & Dunner
Claims
I claim:
1. A method of removing impurities from a liquid,
comprising the steps of:
forming electrodes using a metal which easily
dissolves upon electrolysis and produces a hydroxide;
installing said
electrodes in a pressure vessel;
continuously supplying a liquid
containing impurities from an inlet of said pressure vessel at a pressure higher
than atmospheric pressure;
continuously supplying the liquid from an
outlet of said pressure vessel to an inlet of a tank at atmospheric pressure;
continuously discharging the liquid from an outlet of said tank;
performing electrolysis by applying a voltage to said electrodes while
the liquid is passed through said pressure vessel, thereby causing the
impurities to flocculate; and
passing the liquid through the pressure
vessel and the tank, wherein the then produced fine bubbles attached to the
flocculated impurities to rise to the surface and seperate the impurities.
2. A method according to claim 1, wherein the impurities in the liquid
are silicic acid ions and colloidal silicic acid in water, and the electrodes
consist of a material selected from the group consisting of aluminum, zinc and
magnesium.
3. A method according to claim 1, wherein the polarity of
said electrodes is inverted at predetermined time intervals.
4. A method
according to claim 2, wherein the polarity of said electrodes is inverted at
predetermined time intervals, and a voltage is applied between said electrodes
so as to provide a current density of not less than 1 A/dm.sup.2.
5. A
method according to any one of claims 1 to 4, wherein an electrolytic bath to be
used is sequentially changed when the voltage has exceeded a predetermined
value.
6. An apparatus comprising an electrolytic bath consisting of a
pressure vessel, in which electrodes of a metal which easily dissolves upon
electrolysis are housed, and a constant current source for applying a voltage to
said electrodes so as to obtain a predetermined current density in a liquid
contained in said electrolytic bath, wherein impurities in the liquid are caused
to flocculate by a hydroxide of the metal.
7. An apparatus according to
claim 6, further comprising a polarity inversion unit for inverting the polarity
of said electrodes at predetermined time intervals.
8. An apparatus
according to claim 6 or 7, further comprising a plurality of electrolytic baths
provided with said electrodes therein, a voltmeter for measuring the voltage,
and a switching unit for switching among said plurality of electrolytic baths
the one bath to another bath to be used when a read on said voltmeter for said
one bath has exceeded a predetermined value.
9. An apparatus according
to claim 6 or 7, wherein said electrodes consist of a material selected from the
group consisting of aluminum, zinc and magnesium.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for removing
impurities such as suspended or dissolved substances from a liquid.
2.
Description of the Prior Art
Ultra-pure water has recently come to be
used in the manufacture of electronic appliance parts, medical appliance parts,
and high-purity chemical products.
Such water, however, is only
available at high cost. If a large amount of high-purity water is used,
manufacturing costs are increased. In view of this, it has been proposed to
remove impurities from used water so that it can be reused.
Even if the
water is not reused, it must be purified to comply with water quality standards
provided by the Water Pollution Preventive Act.
In addition, if
untreated water such as underground water or river water can be purified to a
satisfactory degree, the high product costs including purchasing of high-purity
water can be reduced.
When dust or the like from the grinding of wafers
such as silicon or gallium arsenide wafers in the semiconductor process becomes
mixed in with water as an impurity, the dust is dispersed as colloidal
particles.
In this case, the dust is suspended in the water and cannot
be removed by normal filtering.
In reverse osmosis or ultrafiltration
which is used in the manufacture of the ultra-pure water required in large
quantities in the electronics industry and in particular in the manufacture of
VLSIs, the substances most responsible for deposition on the filtering membrane
or permeation membrane and degradation of filtering efficiency are believed to
be silicic acid ions (HSiO.sub.3.sup.- or SiO.sub.3.sup.2-) dissolved in
untreated water and the colloidal silicic acid suspended therein. The silicic
acid colloidally suspended in the water is known to be converted upon reaction
with water into orthosilicic acid, metasilicic acid, or metabisilicic acid to
metatetra silicic acid. All of these substances can be dissolved in neutral
water in only trace amounts, and are scarcely converted into silicic acid ions
(e.g., SiO.sub.3.sup.2- or SiO.sub.4.sup.2-). it is difficult to remove
colloidal silicic acid with a method using an anion exchange resin. In addition,
that method can provide only a low adsorption rate of silicic acid ions and
requires several passes of the untreated water through the ion exchange resin
layer for the removal of ions. Still worse, other substances or ions such as
Cl.sup.-, SO.sub.4.sup.2-, or CO.sub.3.sup.2- are preferentially removed and the
pH of the water is increased to cause some troubles.
In order to remove
solids from suspension, the suspended solids must be flocculated prior to
filtration. This also applies to solutions containing a solute such as an
organic substance.
Electrolysis is known as a first conventional method
of flocculating impurities. In electrolysis, sulfuric acid or the like is added
to a liquid in order to enhance the electrical conductivity of the liquid. Metal
plates which cannot easily be corroded by electrolysis, e.g., plates obtained by
coating lead with titanium, are used as electrodes.
A substance, when
suspended in water, is normally charged. Therefore, the suspended substance is
electrically neutralized and flocculated by the electrodes. A substance
flocculated in this manner floats in fine bubbles of hydrogen or oxygen, and the
floating flocculated substance can then be separated by filtration.
In
the case of electrolysis, the electrolysis of a liquid is performed at
atmospheric pressure using electrodes consisting of a soluble metal.
Japanese Patent Publication No. 59-5032 discloses a discontinuous
method. In this method, a liquid contained in a pressure-resistant container is
subjected to electrolysis under pressure. After performing electrolysis for a
predetermined period of time, the pressure in the container is returned to
normal pressure. Fine bubbles formed upon this pressure change attach to the
floc and cause it to rise to the liquid surface. The floc can then be removed.
As a second conventional method of flocculating impurities, a large
amount of a substance having a high degree of ionization such as a strong
electrolyte, a strong acid, or a strong base is added to a liquid.
In
that method, suspended substances are electrically neutralized, flocculated and
then settled by the ion generated in electrolytic dissociation.
As a
third conventional method of flocculating impurities, a base is added to a
liquid after adding a water soluble metal salt. In this method, a metal
hydroxide produced by neutralization between the metal salt and the base adsorbs
impurities and precipitate them by flocculation. Therefore, the third method is
effective for liquids containing as an impurity a soluble substance less charged
such as organic substance as well as suspended substance.
In the
conventional method wherein a liquid is electrolyzed at atmospheric pressure, a
large amount of hydrogen gas is produced at the cathode. The amount of hydrogen
gas produced is calculated by electrochemical equivalency to be 417.8 ml/hour
per 1 A of current at 20.degree. C. and 1 atm. Hydrogen gas dissolves by 0.0182
ml per 1 ml of water at 20.degree. C. and 1 atm. In practice, however, most fine
bubbles of hydrogen gas immediately join together and rise as large bubbles
having a diameter of more than about 0.5 mm. For this reason, only a small
proportion of the bubbles attach to impurities flocculated by the hydroxide and
contribute to the flotation seperation. Assuming that a large tank having a
sectional area of 0.05 m.sup.2 is used for 1L/min of liquid, about 95% of the
total impurities in the liquid can be separated from the liquid surface, about
5% remains deposited on the tank bottom, and about 0.01% remains dissolved or
suspended in the liquid unrecovered. Unless re-filtered through a filtering
material having a mesh of about 0.5 .mu.m, pure water cannot be obtained and the
consumption of filtering material becomes considerable.
With the second
method, the added substance remains in the liquid at a high concentration as an
additional impurity. Therefore, even if the suspended substance is completely
separated, the treated water cannot be used for intended purposes without
further treatments.
In the third method, the concentration of metal salt
as a separation additive is low. However, further ions are produced in the
liquid upon neutralization. Again, with the third method, the treated water
cannot be used directly for intended purposes.
In order to allow the
repeated use of pure water, ions produced in each purification process must be
removed before reusing the water. Thus, a large amount of ion exchange resin
must be used, and costs are increased and the advantage of reusing pure water is
not realized.
A large quantity of ions also remains unfiltered in the
first and second methods. In addition, the solid residue separated by
flocculation contains large quantities of electrolytes or metal hydroxides. For
this reason, melting or recrystallization of silicon wafers grinding dust cannot
be performed, and the solid residue cannot be reused effectively, the residue
being almost of no utility.
Furthermore, the known method of
electroyzing a liquid under pressure is a discontinuous process as described
above, and a continuous treatment method is preferable.
OBJECTS AND
SUMMARY OF THE INVENTION
In view of the foregoing it is an object of the
invention to provide a method and apparatus for removing impurities from a
liquid, in which impurities in a liquid having a low electrical conductivity can
be flocculated without decreasing the purity of the liquid, and attachment of
other substances to the impurities can be prevented.
According to the
present invention, there is provided a method of removing impurities from a
liquid, comprising the steps of:
forming electrodes using a metal which
easily dissolves upon electrolysis and produces a hydroxide;
installing
the electrodes in a pressure vessel;
continuously supplying a liquid
containing impurities from an inlet of the pressure vessel at a pressure higher
than atmospheric pressure;
continuously discharging the liquid from an
outlet of the pressure vessel to an inlet of a tank at atmospheric pressure;
continuously exhausting the liquid from an outlet of the tank;
performing electrolysis by applying a voltage to the electrodes while
the liquid is passed through the pressure vessel, thereby allowing the
impurities to flocculate; and
passing the liquid through the pressure
vessel and the tank, wherein the then produced fine bubbles attach to the
flocculated impurities to rise to the surface and seperate the impurities.
Preferably, the impurities in the liquid consist of silicic acid ions
and colloidal silicic acid, the electrodes consist of one of aluminum, zinc, and
magnesium, and the voltage is applied between the electrodes to provide a
current density of 1 A/dm.sup.2 or more. More preferably, the polarity of the
electrodes is inverted at predetermined time intervals. Still more preferably,
the electrolytic bath to be used is sequentially changed.
There is also
provided an apparatus for removing impurities from a liquid, which comprises an
electrolytic bath consisting of a pressure vessel, in which electrodes of a
metal which easily dissolves upon electrolysis are housed, and a constant
current source for applying a voltage to said electrodes so as to obtain a
predetermined current density in a liquid contained in said electrolytic bath,
wherein impurities in the liquid are caused to flocculate by a hydroxide of the
metal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic
diagram showing an embodiment of the present invention;
FIG. 2 is a
block diagram showing connection of electrolytic baths;
FIG. 3 is a
perspective view of an electrode block;
FIG. 4A is a longitudinal
sectional view of the electrolytic bath shown in FIG. 1;
FIG. 4B is a
cross-sectional view of the electrolytic bath shown in FIG. 1 taken along the
line A--A in FIG. 4A;
FIGS. 5A and 5B are longitudinal sectional views
of the flotation separation tank shown in FIG. 1;
FIGS. 6 to 8 are
graphs showing the relationship between the amount of floc and pressure during
electrolysis in Examples 1 to 3 and Comparative Example 1; and
FIG. 9 is
a graph showing the relationship between current density and decrease of silicic
acid.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The
present invention will be described with reference to the accompanying drawings.
Waste water containing impurities from an exhaust source is stored in a
waste water tank 1, as shown in FIG. 1. The waste water is supplied to an
electrolytic bath 3 (to be described later) by a high capacity fixed delivery
pump 2 at a pressure higher than atmospheric pressure and is subjected to
electrolysis. The electrolyzed waste water is released into the air through a
pressure control valve 4 and is supplied through an inlet 6 into a flotation
separation tank 5 (to be described later) at atmospheric pressure. Hydrogen gas
produced and dissolved in the waste water upon electrolysis forms fine bubbles
and rise in the tank 5. Impurities in the waste water in the electrolytic bath 3
flocculate and produce the flock. Hydrogen gas produced and not dissolved in the
waste water in the electrolytic bath 3 upon electrolysis forms fine bubbles and
attaches to the floc. The floc in the electrolytic bath 3 is produced by the
flocculation of metal hydroxide formed by dissolution of the electrode metal in
electrolysis. Most of the floc rise together with the fine bubbles in the
flotation separation tank 5, is scraped off by a scraper 33, and is discharged
into a floating residue receiver 8. Water from which the floc has been separated
is passed through double walls 24 and supplied to a recovery tank 10 through a
pipeline 9. The floc, to which fine bubbles are not attached, gradually settles,
collects at the bottom of the flotation separation tank 5, and is discharged.
The outer housing of the electrolytic bath 3 is a pressure vessel 7 and
consists of a synthetic resin with satisfactory insulating and sealing
properties or a metal having an inner surface lined with such a synthetic resin.
As shown in FIG. 4A, the electrolytic bath 3 has water inlet 11 and water outlet
12.
A number of electrode plates 13 and 14 are arranged opposing each
other inside the electrolytic bath 3, as shown in FIG. 3. The electrode plates
13 and 14 consist of a metal which easily dissolves upon electrolysis, such as
copper, iron, zinc, nickel, or aluminum. The plates have a thickness of 1 to 5
mm.
Separating plates 15 are clamped between adjacent electrode plates
13 and 14. The separating plates 15 consist of a synthetic resin having
satisfactory insulating properties such as a polyvinyl chloride. Each separating
plate 15 has a thickness of 1 to 5 mm, a width of 5 mm or less, and a length of
100 mm or less.
More specifically, the electrode plates 13 and 14 are
fixed through the separating plates 15 (of a predetermined thickness) at a gap
between the electrodes so as to provide sufficient insulation and maintain a
predetermined electrical conductivity. Lead wires 16, consisting of copper or
the like, are connected to the respective electrode plates 13 and 14.
In
this manner, an electrode block 17 is constituted by the electrode plates 13 and
14, the separating plates 15, and the lead wires 16. As shown in FIGS. 4A and
4B, the electrode block 17 is housed in the pressure vessel 7. The separating
plates 15 can be replaced with square or round rods having a side or diameter of
1 to 5 mm. Insulating packing materials 18 and 19 are packed between the
electrode block 17 and the pressure vessel 7.
A constant current device
(not shown) is connected to the lead wires 16. A voltage corresponding to the
electrical conductivity of waste water is applied to the electrode plates 13 and
14 to obtain a predetermined current density.
The waste water forced
into the electrolytic bath 3 from the inlet 11 is electrolyzed while passing
through the gap between the electrode plates 13 and 14 at a predetermined speed,
and discharged from the outlet 12. Upon electrolysis, the anode side metal of
the electrode plates 13 and 14 dissolves to form a metal hydroxide. The metal
hydroxide attaches to and flocculates impurities in the waste water such as
suspended and dissolved substances to form the floc.
The floc and
precipitated metal hydroxide partially attach to the anode side surfaces of the
electrode plates 13 and 14.
When the current used for electrolysis is a
DC current, the attachment of the floc and metal hydroxide occurs on either the
electrode plates 13 or the electrode plates 14. Therefore, the path of waste
water is narrowed and the electrolysis area is reduced with time. As a result,
the current density abnormally increases and results in insufficient
electrolysis. Finally, the separation of solids from the liquid becomes
impossible.
When the current used for electrolysis is a DC current, only
the anodes are dissolved and consumed, even if both the electrode plates 13 and
14 consist of the same metal. As a result, the plate lifetime during which
electrolysis can be performed is reduced to half that obtained when both the
electrode plates 13 and 14 are uniformly consumed.
In view of this
problem, a polarity inversion device (not shown) is preferably interposed
between the DC power source (not shown) and the electrolytic bath 3, so that the
polarities of the electrode plates 13 and 14 are inverted at predetermined time
intervals. With this arrangement, upon polarity inversion an anode to which the
floc and the like are attached is switched to serve as a cathode, and it
produces hydrogen gas. As a result, the impurites deposited on the cathode
(which was an anode before polarity inversion) are removed. Thus, deposits of
the floc and the like will not build up exclusively on a single type of
electrode plate.
Even with this arrangement, the electrode plates 13 and
14 are finally consumed and cannot be used any longer. Before this point is
reached, however, the gaps between the electrode plates 13 and 14 increase and
the electrolysis areas of the electrode plates 13 and 14 gradually decrease.
Thus, in the embodiment adopting a constant current system, a voltage applied
between the electrode plates 13 and 14 is higher in this state.
As shown
in FIG. 2, a plurality of electrolytic baths (3a and 3b in the illustration) are
arranged. A voltmeter (not shown) is connected to the electrolysis power source
system. When the measurement obtained with this voltmeter exceeds a
predetermined value, those of solenoid valves 21a and 21b at the inlet of the
electrolytic baths 3a and 3b which have been open are closed, and those which
have been closed are opened. When the solenoid valves 21a and 21b are switched,
power supply to the electrolytic baths 3a and 3b is also switched. When the
electrolytic baths 3a and 3b are automatically switched in this manner, the
electrode block 17 for the electrolytic baths 3a and 3b, which are not in
operation, can be replaced with a new one as needed.
The treated water
flowing from the outlets of the electrolytic baths 3a and 3b is supplied to the
flotation separation tank 5 arranged above the electrolytic baths 3a and 3b.
As shown in FIGS. 5A and 5B, the flotation separation tank 5 has a
substantially conical bottom 23 and double walls 24 on each side. The inlet 6 of
the flotation separation tank 5 projects upward from the bottom 23 and is
located substantially at the center of the flotation separation tank 5. An
outlet 26 of the flotation separation tank 5 communicates with the gap between
the double walls 24 through small holes 27.
A scraper 33 is arranged
above the flotation separation tank 5. Rubber or urethane foam scraper fins 31
of the scraper 33 may be moved by a chain 32 at a rate of several centimeters
per minute. A plurality of floating residue receivers 8 having cloth or paper
bags 34 are arranged near the side surfaces of the flotation separation tank 5.
The floating residue receivers 8 communicate with the waste water tank 1.
The floc is supplied to the flotation separation tank 5 from the inlet 6
together with the treated water and hydrogen gas. Most of the floc then rises to
the surface of the water in the flotation separation tank 5 and forms a floc
layer. The floc layer is scraped off by the scraper 33 and is dumped into the
floating residue receiver 8.
The floc dumped into the floating residue
receiver 8 is collected by the bag 34 and the filtered water is returned to the
waste water tank 1. A concentrated solid residue accumulates in the bag 34. When
the weight of the solid residue in the bag 34 reaches a predetermined value, the
floating residue receiver 8 is automatically replaced with an adjacent floating
residue receiver (not shown).
A portion of the floc, to which an
insufficient amount of hydrogen gas is attached, settles in the flotation
separation tank 5. The bottom 23 is opened for a very short period of time with
a predetermined time interval, and the floc settled on the bottom 23 is supplied
to the floating residue receiver 8 together with the floating floc.
Some
of the floc rise between the double walls 24 together with the treated water and
are passed through the outlet 26 via the small holes 27.
The treated
water passed through the outlet 26 is stored in the recovery tank 10 and is then
supplied to an ultrafilter (not shown) by a pump 41. The floc not removed at the
flotation separation tank 5 normally has a size of about 1 .mu.m or more. For
this reason, the ultrafilter uses a filtering material with a mesh of 0.5 .mu.m.
This filtering material is automatically replaced.
EXAMPLES 1-4 AND
COMPARATIVE EXAMBLE 1
In the electrode block 17 shown in FIG. 3, the
electrode plates 13 and 14 were constituted by copper plates having a width of
5.5 cm, a length of 2.0 cm, and a thickness of 2.0 mm such that the total
electrolysis area of the opposing pairs was 16 dm.sup.2. The separating plates
15 were polyvinyl chloride plates having a thickness of 2.0 mm. The electrode
block was fixed in the pressure vessel to provide an electrolytic bath.
Waste water (specific conductivity=102 .mu.S/cm), in which silicon fine
particles having an average diameter of 0.3 .mu.m were dispersed and suspended
at a concentration of 0.3 g/l , was treated by 100 l for each of the Examples
and Comparative Example. The waste water was supplied to the electrolytic bath
by a fixed delivery pump at a rate of 4 l /min at a predetermined pressure, and
electrolysis was performed at a current density of 1 A/dm.sup.2. The waste water
supplied to the electrolytic bath under pressure was continuously supplied to
the flotation separation tank 5 at atmospheric pressure illustrated in FIG. 1
through the pressure control valve. The amount of flocculated floc was measured
in accordance with the following classification:
I: Floc floating on the
surface of the flotation separation tank
II: Floc deposited on the
bottom of the flotation separation tank
III: Floc remaining in the
recovered water.
For classification I, the amount of floc scraped off by
the scraper and dumped into the floating residue receiver was dried at
50.degree. C. and measured.
For classification II, the valve at the
bottom of the tank was opened 2 hours after electrolysis was completed. After
the floc was dumped into another container together with about 5 l of water, the
mixture was suction-filtered through a qualitative filter (Toyo Roshi K.K.),
dried at 50.degree. C. and measured.
For classification III, the floc
was filtered under pressure using a filtering material with a mesh of 0.45
.mu.m, dried at 50.degree. C. and measured.
The pressure for feeding the
waste water into the electrolytic bath was atmospheric pressure for a
Comparative Example and was as indicated for four Examples. The results
(measured amounts in % by weight of the flocs in relation to the total amount of
the flocs) are shown in Table 1 below and in FIGS. 6 to 8.
TABLE 1
______________________________________
Amount of Floc
Pressure
Voltage (wt %)
No. (Kg/cm.sup.2)
(V) I II III
______________________________________
Example 1 2 20 97.4 2.595 0.005
2 3 18 98.6 1.496 0.004
3 4 16 98.7 1.398 0.004
4 5 14 99.1 0.999 0.002
Comparative
1 1 24 95.0 4.990 0.01
Example
______________________________________
As can be seen from the above results, when compared with
electrolysis at atmospheric pressure, the electrolysis under pressure has the
following advantages:
(1) The amount of floc floating in the flotation
separation tank is increased.
(2) Since the amount of floc mixed in with
the recovered water is small, the consumption of industrial filtering material
is small.
(3) When the pressure is increased, the voltage necessary for
a ccnstant current of 1 A/dm.sup.2 is decreased.
EXAMPLES 5 AND 6
Using 100 l of waste water for each Example, the electrolysis and
flotation separation tank treatment were performed following the same procedures
as in Example 2 except that the current densities were 0.90 A/dm.sup.2 and 0.85
A/dm.sup.2 The results are shown in Table 2 below.
TABLE 2
______________________________________
Current
Amount of Floc
Density
(wt %)
No. (A/dm.sup.2)
I II III
______________________________________
Example 5 0.90 98.8 2.193
0.007
6 0.85 97.9 2.091
0.009
______________________________________
As can be seen from the above results, when the electrolysis is
performed under pressure, the current density can be decreased compared with the
electrolysis at atmospheric pressure. This is because the diameter of bubbles of
the hydrogen gas formed as fine bubbles is reduced as the pressure is increased,
reduction of cathode area due to the bubbles is decreased, and loss of
electrolysis efficiency is decreased.
During studies in establishing the
present invention, the following facts were found. As shown in Table 3, when
industrial waste water containing SS (suspended substance) at a ratio of 300 mg
per 1 l of normal city tap water was subjected to electrolysis, filtered, and
then deionized by an ion exchange resin, only a small portion of the dissolved
silica in the industrial waste water was accounted for by silica ions. It was
theoretically surmised that silica ions are adsorbed and removed by an anion
exchange resin and colloidal suspended silica attaches to the metal hydroxide
produced at the anode. The initial total amount of silica (79 mg/l) was reduced
to 0.01-0.005 mg/l.
Based on this finding, the present inventor
conducted Experiments 1-3 described below.
As a result, it has been
found that among suspended colloidal silica and silicic ion in tap water or
underground water, silicic ion is perfectly adsorbed and removed through
conventional cation exchange resin, while most part of suspended colloidal
silica is removed by electrolysis process for a predetermined period of time at
a constant water flow rate and at a predetermined temperature (usually at
ambient temperature) with an appropriate electric current density, wherein the
anode is made of a metal which is water soluble and forms immediately its
hydroxide when the water is weak alkaline having a pH of 9-7, and the cathode is
made of a metal identical with that of anode or other metal, or carbon.
TABLE 3
______________________________________
Comparison of Compositions Before and
After Recovery of Waste Water
After Before
No. Detection Item
Unit Recovery Recovery
______________________________________
1 pH 7.2 4.65
2 Electrical .mu.S/cm 0.5-2.0 226
conductivity
3 Residual silica
mg/l 0.01-0.005
79
4 Total cations
mg/l <0.3 124.2
5 Ammonium ions
mg/l <0.05 --
6 Sodium ions mg/l 0.08-0.03 16.5
7 Potassium ions
mg/l Not detected
2.1
8 Calcium ions mg 0.4-0.1 50.0
CaCO.sub.3 /l
9 Magnesium ions
mg 0.08-0.03 4.2
CaCO.sub.3 /l
10 Iron mg/l 0.10-0.02 10.5
11 Manganese mg/l 0.05-0.02 2.3
12 Other anions mg/l Not detected
Ni 0.4
13 SS mg/l Not detected
300
______________________________________
Measurements were performed by the atomic absorption spectroscopy
for sodium and potassium ions, and in accordance with JIS K0101 for the others.
EXPERIMENT 1
Underground water having an electrical conductivity
of 373 .mu.Scm was treated. Copper, nickel, iron, zinc, and aluminum plates
having a thickness of 1 to 1.5 mm were cut into electrode plates having a width
of 10 cm and a length of 20 cm. The electrode plates were combined as shown in
Table 4. The gap between the adjacent electrodes was 5 mm. Three metal plates
serving as cathodes and two metal plates serving as anodes were alternately
opposed to each other such that the total electolysis area on the anode side was
4 dm.sup.2.
Electrolysis by the constant current system was performed,
flowing water at a rate of 1 l per minute under the conditions shown in Table 4.
The treated water was filtered by a qualitative filter (Toyo Roshi K. K.), and
silicic acid in the filtered water was measured by the atomic absorption
spectroscopy. The results are shown in Table 4 below.
TABLE 4
______________________________________
Electrode Silicic Acid
Metal Current
Measured
Cathode Voltage Density
Value Reduction
No. Anode (V) (A/dm.sup.2)
(ppm) (%)
______________________________________
Untreated Water
-- -- 29.6 --
1 Cu Zn 10 1 10.0 66.2
2 Cu Al 10 1 11.6 60.8
3 Cu Fe 10 1 19.7 33.4
4 Fe Cu 10 1 22.3 24.6
5 Cu Ni 10 1 19.7 33.4
Untreated Water
-- -- 32.6 --
6 Cu Zn 20 2 4.7 85.5
7 Cu Al 20 2 2.8 91.4
8 Cu Fe 20 2 10.9 66.5
9 Fe Cu 18 2 18.4 43.5
10 Cu Ni 20 2 15.0 54.0
______________________________________
The above results indicate that zinc and aluminum can be used as
an electrode metal for effective removal of silicic acid.
In order to
determine the effects of reducing the gap between the electrodes, the following
Experiment 2 was performed.
EXPERIMENT 2
Untreated water having
an electrical conductivity of 150 .mu.Scm was used. Five zinc or aluminum
plates, each having a thickness of 1 mm were placed at a gap of 2 mm such that
cathodes and anodes were alternately opposed and the total electrolysis area was
4 dm.sup.2. The water was passed through the gap between the electrodes at a
rate of 1 l per minute, and the electrolysis was performed at indicated current
densities. The amount of silicic acid in the filtered water was measured
following the same procedures as in Experiment 1. The results are shown in Table
5.
TABLE 5
______________________________________
Silicic Acid
Current Measured
Electrode voltage Density Value Reduction
No. Metal (V) (A/dm.sup.2)
(ppm) (%)
______________________________________
Untreated -- -- 29.3 --
Water
1 Zn 12 1 13.0 55.5
2 Al 20 1 10.9 63.0
3 Zn 30 2 5.3 81.9
4 Al 39.5 2 4.3 85.3
______________________________________
It was found that particularly when aluminum plates were used as
electrode plates, an oxide or hydroxide film which formed on the anodes blocked
the path of the water and the voltage increased abruptly. That is, part of the
flocculated substance or metal hydroxide attaches to the anodes when the
polarities of the current used for electrolysis are fixed. Therefore, the
treated water path is narrowed and the electrolysis area is reduced, resulting
in insufficient electrolysis.
In order to find a means to prevent such a
problem when the electrode gap is reduced, the following experiment 3 was
performed as below.
EXPERIMENT 3
Untreated water having an
electrical conductivity of 150 .mu.Scm was used. Zinc or aluminum plates having
a thickness of 1 mm or magnesium plates having a thickness of 3.5 mm were used
as electrodes, and the electrolysis was performed following the same procedures
as in Experiment 2. The effects obtained when the polarities of the current were
inverted was determined. The results are shown in Table 6.
TABLE 6
______________________________________
Polarity Silicic Acid
Elec- Inversion Vol- Current
Measured
Reduc-
trode (once/ tage Density
Value tion
No. Metal 5 sec) (V) (A/dm.sup.2)
(ppm) (%)
______________________________________
Untreated Water
-- -- 40.7 --
1 Zn Not 12 1 14.2 65.2
performed
2 Al Not 30 0.5 25.2 38.1
performed
3 Zn performed 30 1.5 9.6 76.4
4 Al performed 20 0.6 24.0 41.0
5 Mg performed 8 0.5 16.9 52.1
6 Mg performed 14 1 8.8 75.1
7 Mg performed 30 2.0 1.8 95.0
______________________________________
In this experiment, when polarity inversion was performed, no
attachment of hydroxide and the like to the electrodes was observed, and the
voltage did not increase.
The results obtained in Experiments 1 to 3 are
summarized in FIG. 9.
EXAMPLES 1-4 AND COMPARATIVE EXAMPLES AND 2 and 3
Untreated water having an electrical conductivity of 157 .mu.Scm was
used. Zinc or aluminum electrodes having a thickness of 1 mm were used at an
electrode gap of 2 mm such that the electrolysis area was 16 dm.sup.2. The
electrolysis was performed at a pressure of 3 kg/cm.sup.2 by the pressurizing
method of the present invention. Polarity inversion (once per 5 sec) was
performed while the electrolysis was performed at a rate of 4 l/min. The amount
of silicic acid in the treated water was measured as in Experiment 1. The
results are shown in Table 7.
TABLE 7
______________________________________
Silicic Acid
Elec- Vol- Current
Measured
Reduc-
trode tage Density
Value tion
No. Metal (V) (A/dm.sup.2)
(ppm) (%)
______________________________________
Untreated Water
-- -- 38.5 --
Example 1 Zn 8 0.5 29.2 24.2
2 Al 16 0.5 24.4 36.6
3 Zn 16 1 15.7 59.2
4 Al 20 1 16.6 56.8
Compara-
2 Ni 30 0.5 36.2 6.0
tive 3 Ni 30 1 30.8 20.1
Example
______________________________________
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