Ceramic filter

A ceramic filter comprising a support layer having an inner surface for forming at least one filter passage, and a filter layer coated on the inner surface of the support layer and having an inner surface functioning as a filter surface, the filter layer having a thickness of 10 to 40 microns from the filter surface, a cumulative intrusion volume of all pores in the filter layer being 0.2 cc/g or less, a cumulative intrusion volume of pores having pore diameters of 0.1 to 3.0 microns being 0.1 cc/g or more, and a cumulative intrusion volume (H) of the pores ranging within a pore diameter width (W) of 0.1 microns around a center pore diameter (PD) which is a pore diameter in case of a half of a cumulative intrusion volume (IV) at a pore diameter of 0.1 microns being 50% or more of a cumulative intrusion volume of all pores in the filter layer.

BACKGROUND OF THE INVENTION 
This invention relates to an improved ceramic filter for use in 
ultrafiltration or precision-filtration. 
A conventional filter for ultrafiltration or precision-filtration has a 
separation layer such as a synthetic organic film such as cellulose 
acetate, PVA and nylon; or a filter cloth coated with a filtration 
auxiliary such as diatomaceous earth. Such a separation layer is used as a 
full forced-flow filter. 
The synthetic film has a poor strength so that the pressure difference must 
be 20 kgf or less. It cannot be used for a long time at a temperature of 
50.degree. C. or more. It cannot be used for the purpose of filtering an 
acid solution. Back washing is not effective in case of the conventional 
forced filtering. 
Recently, ceramic filters have been proposed because they have a good 
strength, heat resistance or chemical resistance and can be easily formed 
in a pipe shape. Also, as cross-flow filtration is possible, back washing 
is effective. However the conventional ceramic filters do not have all of 
the desired strength, filtration rate and accuracy characteristics. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide a ceramic filter which can 
easily have all of desired strength, filtration and accuracy 
characteristics. 
According to this invention, a ceramic filter includes a filter layer 
having a thickness of 10 to 40 micron meters (hereinafter called merely 
microns) from a filter surface. When it is measured by a mercury 
porous-meter, the cumulative intrusion volume of all pores in the filter 
layer is 0.2 cc/g or less. The cumulative intrusion volume of the pores 
having pore diameters of 0.1 to 3.0 microns is 0.1 cc/g or more. The 
cumulative intrusion volume (H) of the pores ranging within the pore 
diameter width (w) of 0.1 microns around a center pore diameter (PD), 
which is a pore diameter in case of a half (IV/2) of a cumulative 
intrusion volume (IV) at a pore diameter of 0.1 microns is 50% or more of 
a cumulative intrusion volume of all pores. 
A ceramic filter of this invention has not only a relatively small 
cumulative intrusion volume of all pores so that an excellent strength can 
be obtained, but also a relatively large cumulative intrusion volume of 
pores having a reasonable performance so that the filtration rate is large 
enough for use in ultrafiltration or precision-filtration. In addition, as 
a majority of pores range within a pore diameter width (w) of 0.1 microns, 
filtration is accurate and precise. 
If a thickness of the filter layer is less than 10 microns, a high strength 
can not be obtained, and nonuniformity can not be avoided. If it is more 
than 40 microns, filtration performance is decreased, and the filter layer 
is sometimes broken away due to heat expansion. 
If the cumulative intrusion volume of all pores is more than 0.2 cc/g, the 
desired strength can not be easily obtained. If the cumulative intrusion 
volume of the pores having a pore diameter of 0.1 to 3.0 microns is less 
than 0.1 cc/g, the filtration rate is decreased.

DESCRIPTION OF THE EXAMPLES 
A binder is mixed with alumina powders having a high purity and a particle 
size of 10 to 30 microns thereby to make a mixture. After this mixture is 
formed in the shape of a pipe having an outer diameter of 19 mm, an inner 
diameter of 15 mm and a thickness of 2 mm, such a shaped body is heated in 
order to remove the binder. Next, a suspension containing a high purity of 
alumina powders having a particle size of 2 to 10 microns is coated on an 
inner surface of the shaped body and then dried whereby an intermediate 
filter layer is formed. Again, a suspension containing a high purity of 
alumina powders having a particle size of 0.4 to 1 microns is coated on 
the intermediate filter layer and then dried whereby an inner filter layer 
is formed on the intermediate filter layer. Finally, the shaped body and 
the two filter layers are heated at a temperature of 1500.degree. C. so 
that a pipe-like ceramic filter can be obtained. 
The intermediate filter layer having a thickness of 30 microns and the 
inner filter layer having a thickness of 10 microns function as a filter 
layer while the pipe-like body having a thickness of 2 mm functions as a 
support layer. 
FIGS. 1 to 5 show relationships between cumulative intrusion volume and 
pore diameter of each filter layer in the ceramic filter of this invention 
and other conventional ceramic filters of the comparative examples 1 to 3. 
The cumulative intrusion volumes are measured by a well-known mercury 
porous-meter. 
Table 1 shows the following items of the example of this invention and the 
comparative examples 1 to 3 shown in FIG. 1: 
(a) Cumulative intrusion volume of all pores in each filter layer; 
(b) Cumulative intrusion volume of pores having a pore diameter of 0.1 to 
0.3 microns; and 
(c) Percentage of cumulative intrusion volume of pores ranging within a 
pore diameter width (W) of 0.1 microns around a center pore diameter (PD) 
which is the pore diameter when the cumulative intrusion volume is a half 
of the cumulative intrusion volume (IV) at a pore diameter of 0.1 microns. 
In FIGS. 1 to 5, the curves showing the pore diameter and cumulative 
intrusion volume relationships have the largest inclination angle at each 
center pore diameter where the largest cumulative intrusion volume can be 
obtained. 
Referring to Table 1 and FIG. 2 showing the example of this invention, the 
cumulative intrusion volume of all pores is 0.18 cc/g. The cumulative 
intrusion volume of the pores having a pore diameter of 0.1 to 3.0 microns 
is 0.17 cc/g. The center pore diameter (PD) is 0.2 microns. The cumulative 
intrusion volume (H) of the pores ranging within a pore diameter width (W) 
of 0.1 microns is 0.12 cc/g, the percentage of which is 66.7%. This 
cumulative intrusion volume (H) is larger than a half (IV/2) of the 
cumulative intrusion volume (IV) at a pore diameter of 0.1 microns. 
Referring to Table 1 and FIG. 3 showing the comparative example 1, the 
cumulative intrusion volume of all pores is 0.23 cc/g. The cumulative 
intrusion volume of the pores having a pore diameter of 0.1 to 3.0 microns 
is 0.20 cc/g. The center pore diameter (PD) is 1.1 microns. The cumulative 
intrusion volume (H) of the pores ranging within a pore diameter width (W) 
of 0.1 microns has a percentage of 42.7%. In other words, this cumulative 
intrusion volume (H) is smaller than a half (IV/2) of the cumulative 
intrusion volume (IV) at a pore diameter of 0.1 microns. 
Referring to Table 1 and FIG. 4 showing the comparative example 2, the 
cumulative intrusion volume of all pores is 0.21 cc/g. The cumulative 
intrusion volume of the pores having a pore diameter of 0.1 to 3.0 microns 
is 0.19 cc/g. The center pore diameter (PD) is 1.2 microns. The cumulative 
intrusion volume (H) of the pores ranging within a pore diameter width (W) 
of 0.1 microns has a percentage of 20.4%. In other words, this cumulative 
intrusion volume (H) is smaller than a half (IV/2) of the cumulative 
intrusion volume (IV) at a pore diameter of 0.1 microns. 
Referring to Table 1 and FIG. 5 showing the comparative example 3, the 
cumulative intrusion volume of all pores is 0.17 cc/g. The cumulative 
intrusion volume of the pores having a pore diameter of 0.1 to 3.0 microns 
is 0.05 cc/g. The center pore diameter (PD) is 3.8 microns. The cumulative 
intrusion volume (H) of the pores ranging within a pore diameter width (W) 
of 0.1 microns has a percentage of 3.0%. In other words, this cumulative 
intrusion volume (H) is smaller than a half (IV/2) of the cumulative 
intrusion volume (IV) at a pore diameter of 0.1 microns. 
FIG. 6 shows a filter apparatus equipped with a ceramic filter according to 
this invention. A tank 1 contains a starting liquid to be filtered. A 
pipe-like ceramic filter 3 is set in a filter container 2. The tank 1 is 
connected to the filter container 2 through a line 6 including a pump 4 
and a flow meter 5. The starting liquid flows through the ceramic filter 3 
in the filter container 2 so as to be filtered. The filtrate is supplied 
through a float valve 7 to a predermined place out of the filter container 
2 while the enriched liquid flows back to the tank 1. 
This application incorporates by reference the disclosure of co-pending 
U.S. application Ser. No. 087,290, filed Aug. 20, 1987, now abandoned, 
which discloses construction details of a filter unit using a ceramic 
filter such as that of the present invention, as well as a back wash 
system for cleaning the filter during use. This application also 
incorporates by reference the disclosures of copending U.S. applications 
Ser. No. 087,351, filed Aug. 20, 1987, now U.S. Pat. No. 4,839,488, and 
Ser. No. 087,357, filed Aug. 20, 1987, now abandoned, which disclose 
further specific uses for the present ceramic filter to purify a 
dielectric fluid used in electric discharge engraving and a reaction 
mixture used in the manufacture of esters, respectively. 
The filter container 2 is equipped with a cylinder means 8 in which a 
piston 9 is arranged so as to be actuated by an air actuator 10. The oil 
11 is disposed in a sealed condition between the piston 9 and the air 
actuator 10. If the air actuator 10 actuates the piston 9, then the 
ceramic filter 3 is back-washed by the filtered liquid remaining in the 
filter container 2. 
Table 2 and Table 3 show test results of the example of this invention and 
the three comparative examples 1 to 3 as above-stated each of which is set 
in the filter apparatus of FIG. 6. 
In Table 2, a starting liquid to be filtered is aged mash for sake, a 
Japanese alcoholic drink. Table 2 shows the number of yeast bacteria and 
Lactobacillus homohiochii or Lactobacillus heterohiochii (hereinafter 
called hiochii bacteria) which is a kind of lactic bacteria, contained in 
the aged mash for sake before filtration, filtration rate when filtered; 
and the number of yeast bacteria and hiochii bacteria leaked through the 
ceramic filter 3 after filtered. 
In Table 3, the liquid to be filtered is aged mash for soy sauce, a dark 
brown liquid made from soybeans. Table 3 shows the volume of the aged mash 
for soy sauce dropped on an agar in a Petri dish before filtration; 
filtration rate when filtered; and the number of colony and bacteria 
contained in the soy sauce dropped on an agar in a Petri dish after 
filtration. 
In Table 2 and Table 3, a reference example is a filter having a filter 
cloth coated with diatomaceous earth. The reference example is tested in 
the same manner. 
Although in the shown embodiment a ceramic filter has only one filter 
passage, plural filter passages can be formed in a support layer, for 
example, in parallel to each other. 
TABLE 1 
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Percentage of cumulative 
Cumulative intrusion 
intrusion volume of 
Cumulative intrusion 
volume of pores having 
Center pore 
pores ranging within a 
volume of a pore diameter of 
diameter 
pore diameter width 
all pores (cc/g) 
0.1-3.0 microns (cc/g) 
(microns) 
of 0.1 microns (%) 
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Comparative 
1 0.23 0.20 1.1 42.7 
Examples 
2 0.21 0.19 1.2 20.4 
3 0.17 0.05 3.8 3.0 
Example of 
0.18 0.17 0.2 66.7 
this invention 
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TABLE 2 
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Filtration 
Leakage of Leakage of 
rate yeast hiochii bacteria 
(m.sup.3 /Hm.sup.2) 
(number/ml) 
(number/ml) 
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liquid to be filtered 
-- (580) (400) 
Reference example 100 100 
Comparative 0.6 95 145 
example 1 
Comparative 0.5 110 170 
example 2 
Comparative 0.3 250 350 
example 3 
Example of this 
1.0 0 55 
invention 
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TABLE 3 
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The 
Filtration 
Liquid per 
The number 
rate one plate number of of bacteria 
(m.sup.3 /Hm.sup.2) 
(1) colony per 1 ml 
______________________________________ 
liquid to be 
-- 20 35 37000 
filtered 50 51 21000 
100 135 28000 
Reference 
3.0 20 29 1200 
example 50 58 1500 
100 121 1600 
Comparative 
0.4 20 25 17000 
example 1 50 50 12000 
100 128 12000 
Comparative 
0.3 20 30 15000 
example 2 50 55 12000 
100 132 13000 
Comparative 
0.1 20 32 32000 
example 3 50 63 20000 
100 145 25000 
Example of 
0.8 20 0 less than 10 
this invention 50 0 less than 10 
100 0 less than 10 
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