Title: Magnesium oxide particle aggregate
Abstract: The present invention is related to a magnesium oxide particle aggregate with the requirement of a first inflection point diameter is more than 0.30×10-6 to 0.60×10-6 m, a particle void volume is 0.50×10-3 to 0.90×10-3 m3·kg-1, and a micropore volume is 0.04×10-3 to 0.11×10-3 m3·kg-1 in the cumulative intrusion volume curve of said particle by having controlled particle aggregation structure so that the solid phase-solid phase reaction between magnesium oxide and the SiO2 film on the surface can be appropriately controlled.
Patent Number: 6,899,768 Issued on 05/31/2005 to Toutsuka, et al.
| Inventors:
|
Toutsuka; Atsuo (Ako, JP);
Hiratsu; Yutaka (Ako, JP)
|
| Assignee:
|
Tateho Chemical Industries Co., Ltd. (JP)
|
| Appl. No.:
|
415168 |
| Filed:
|
October 25, 2001 |
| PCT Filed:
|
October 25, 2001
|
| PCT NO:
|
PCT/JP01/09354
|
| 371 Date:
|
April 24, 2003
|
| 102(e) Date:
|
April 24, 2003
|
| PCT PUB.NO.:
|
WO02/34965 |
| PCT PUB. Date:
|
May 2, 2002 |
Foreign Application Priority Data
| Oct 25, 2000[JP] | 2000-325274 |
| Current U.S. Class: |
148/27; 148/113; 423/583; 423/635; 501/108; 502/34 |
| Intern'l Class: |
B23K 035/24 |
| Field of Search: |
148/27,113
423/583,635
501/108
502/340
|
References Cited [Referenced By]
U.S. Patent Documents
| 3930906 | Jan., 1976 | Irie et al.
| |
| Foreign Patent Documents |
| 03115585 | May., 1991 | JP.
| |
| 10046259 | Feb., 1998 | JP.
| |
| 11181525 | Jul., 1999 | JP.
| |
Primary Examiner: Sheehan; John P.
Attorney, Agent or Firm: Muserlian, Lucas and Mercanti
Parent Case Text
This application is a 371 of PCT/JP01/09354 filed Oct. 25, 2001.
Claims
1. A magnesium oxide particle aggregate, characterized in that a first inflection
point diameter is more than 0.30×10
-6 to 0.60×10
-6 m,
a particle void volume is 0.50×10
-3 to 0.90×10
-3 m
3·kg
-1,
and a micropore volume is 0.04×10
-3 to 0.11×10
-3 m
3·kg
-1
in the cumulative intrusion volume curve of said particle aggregate.
2. A magnesium oxide particle aggregate, characterized in that an interparticle
void volume is 0.80×10
-3 to less than 1.40×10
-3 m·kg
-1,
a particle void volume is 0.50×10
-3 to 0.90×10
-3 m
3·kg
-1,
and a micropore volume is 0.04×10
-3 to 0.11×10
-3 m
3·kg
-1
in the cumulative intrusion volume curve of said particle aggregate.
Description
FIELD OF THE INVENTION
The present invention relates to a magnesium oxide particle aggregate having
a controlled particle aggregation structure. More particularly, the present invention
relates to a magnesium oxide particle aggregate used as an annealing separator
to form a forsterite film which imparts excellent insulation properties and magnetic
properties to a grain-oriented magnetic steel sheet.
BACKGROUND ART
Grain-oriented magnetic steel sheets used in transformers or generators
are generally produced by a process in which silicon steel containing about 3%
of Si is hot-rolled, subsequently cold-rolled so as to have a final sheet thickness,
and then subjected to decarburization annealing (primary recrystallization annealing),
followed by finishing annealing. In this process, for imparting insulation properties
to a magnetic steel sheet, after the decarburization annealing and before the final
finishing annealing, a slurry containing magnesium oxide is applied to a surface
of the steel sheet and then dried, and wound into a coil form. Si contained in
the silicon steel sheet reacts with oxygen during the decarburization annealing
to form an SiO
2 film on the surface of the steel sheet. SiO
2 in
the film then reacts with magnesium oxide in the slurry during the finishing annealing
to form a forsterite (Mg
2SiO
4) film having excellent insulation
properties on the surface of the steel sheet. The forsterite film is considered
to impart not only insulation properties but also a tension to the surface thereof
due to the difference in the coefficient of thermal expansion between the forsterite
film and the steel sheet, thus lowering core loss of the grain-oriented magnetic
steel sheet to improve the magnetic properties.
Therefore, the forsterite film plays an extremely important role in the
production of grain-oriented magnetic steel sheets, and hence the properties of
magnesium oxide forming the forsterite film directly affect the magnetic properties
thereof. For this reason, a number of inventions have been made with respect to
the magnesium oxide used as an annealing separator, especially having a controlled
citric acid activity (CAA) between magnesium oxide particles and citric acid, and,
for example, Japanese Prov. Patent Publication Nos. 58331/1980, 33138/1994, and
158558/1999 have been disclosed.
However, CAA merely simulates empirically the reactivity in the solid phase-solid
phase reaction between SiO
2 and magnesium oxide whose reaction actually
proceeds on the surface of the magnetic steel sheet, based on the solid phase-liquid
phase reaction between magnesium oxide and citric acid. Further, magnesium oxide
is often present in the form of particle aggregate in which several powder particles
bind together and agglomerate, and therefore CAA cannot appropriately evaluate
the forsterite formation reaction.
On the other hand, Japanese Prov. Patent Publication No. 46259/1998 discloses
an invention made in respect of the fact that the film quality varies depending
on the state of the pores in magnesium oxide. In this invention, the pore volume
is restricted using a constant-capacity gas adsorption method, however, in the
gas adsorption method, it determines the amount of gas molecules which adsorb onto
the pore surfaces present on the particle surfaces. Therefore, only pores as very
small as, for example, 0.1×10
-6 m or less can be measured and thus,
it is considered that this method is difficult to apply to the particle aggregation
structure having a size of about 1×10
-5 to 1×10
-6 m
observed in the actual magnesium oxide particle aggregates, and hence cannot appropriately
evaluate the forsterite formation reaction.
In view of this, the present inventors have found indices which can appropriately
evaluate the forsterite formation reactivity of a magnesium oxide particle aggregate,
and have completed an invention of a magnesium oxide particle aggregate having
a particle aggregation structure specified using the indices (Japanese Patent Application
No. 2000-132370). Namely, in the cumulative intrusion volume curve of the magnesium
oxide particle aggregate, if these are controlled in a range where a first inflection
point diameter is 0.30×10
-6 m or less, an interparticle void volume
is 1.40×10
-3 to 2.20×10
-3 m
3·kg
-1,
and a particle void volume is 0.55×10
-3 to 0.80×10
-3
m
3·kg
-1, it is possible to form forsterite at
a satisfactory rate on the surface of an magnetic steel sheet.
However, the present inventors have found that, with respect to the range
considered to be unsuitable for the forsterite formation in the above earlier patent
application filed by the present inventors, for example, a range in which the first
inflection point diameter exceeds 0.30×10
-6 m, there is a possibility
that excellent forsterite formation is achieved by further strictly controlling
the particle aggregation structure of the magnesium oxide particle aggregate.
An object of the present invention is to provide a magnesium oxide particle aggregate
having a particle aggregation structure further controlled so that the forsterite
formation rate can be appropriately controlled.
In addition, another object of the present invention is to provide an annealing
separator for grain-oriented magnetic steel sheet, using the magnesium oxide particle
aggregate of the present invention, and to provide a grain-oriented magnetic steel
sheet obtainable by a treatment using the annealing separator for grain-oriented
magnetic steel sheet of the present invention.
DISCLOSURE OF THE INVENTION
Namely, the present invention is a magnesium oxide particle aggregate characterized
in that a first inflection point diameter is more than 0.30×10
-6 to
0.60×10
-6 m, a particle void volume is 0.50×10
-3 to
0.90×10
-3 m
3·kg
-1, and a micropore volume
is 0.04×10
-3 to 0.11×10
-3 m
3·kg
-1
in the cumulative intrusion volume curve of the particle.
In addition, the present invention is a magnesium oxide particle aggregate characterized
in that an interparticle void volume in the cumulative intrusion volume curve of
the particle is 0.80×10
-3 to less than 1.40×10
-3 m
3·kg
-1,
a particle void volume is 0.50×10
-3 to 0.90×10
-3 m
3·kg
-1,
and a micropore volume is 0.04×10
-3 to 0.11×10
-3 m
3·kg
-1
in the cumulative intrusion volume curve of the particle.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows examples of cumulative intrusion volume curves of particle aggregates
comprised mainly of magnesium oxide, determined from a pore distribution measurement
by mercury porosimetry.
FIG. 2 shows a relationship between a forsterite formation rate, a first inflection
point diameter, and a particle void volume with respect to various magnesium oxide
particle aggregates.
FIG. 3 shows a relationship between a forsterite formation rate, an interparticle
void volume, and a particle void volume with respect to various magnesium oxide
particle aggregates.
FIG. 4 shows a relationship between a micropore volume and a forsterite formation
rate with respect to the particle aggregate having a first inflection point diameter
of more than 0.30×10
-6 to 0.60×10
-6 m and a particle
void volume of 0.50×10
-3 to 0.90×10
-3 m
3·kg
-1.
FIG. 5 shows a relationship between a micropore volume and a forsterite formation
rate with respect to a particle aggregate having an interparticle void volume of
0.80×10
-3 to less than 1.40×10
-3 m
3·kg
-1
and a particle void volume of 0.50×10
-3 to 0.90×10
-3 m
3·kg
-1.
FIG. 6 shows temperature and time conditions suitable for controlling the reaction
so that the first inflection point diameter becomes more than 0.30×10
-6
to 0.60×10
-6 m when magnesium hydroxide is prepared by reacting
an aqueous solution of magnesium chloride with calcium hydroxide.
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, a cumulative intrusion volume curve of particles refers
to a curve which shows a relationship between a pore diameter and a cumulative
pore volume determined from a pore distribution measurement by mercury porosimetry,
and FIG. 1 shows examples of cumulative intrusion volume curves of two types of
magnesium oxide particle aggregates having different particle aggregation structures.
The first inflection point is the inflection point at the largest pore diameter
among inflection points at which the cumulative intrusion volume curve suddenly
rises, and it is indicated by a solid circle in the figure. The first inflection
point diameter refers to a pore diameter at the first inflection point. The interparticle
void volume refers to a cumulative pore volume at the first inflection point. The
particle void volume is represented by a volume value obtained by subtracting the
cumulative pore volume at the first inflection point from the total pore volume
represented by the cumulative pore volume at a pore diameter of 0.003×10
-6
m. The micropore volume refers to a cumulative pore volume of micropores
smaller than 0.05×10
-6 m, and is represented by a volume value
obtained by subtracting the cumulative pore volume at 0.05×10
-6 m
from the total pore volume and falls in the region indicated by arrows near the
right-hand ordinate in FIG.
1.
The present inventors have made studies on a solid phase-solid phase reaction
between magnesium oxide and SiO
2, which reaction proceeds on a surface
of a grain-oriented magnetic steel sheet, and, as a result, they have found that
the first inflection point diameter in the cumulative intrusion volume curve as
determined from a pore distribution measurement by mercury porosimetry, a particle
void volume, and a interparticle void volume as well as a micropore volume can
be used as indices for properly indicating the structure of a magnesium oxide particle
aggregate. Based on the above finding, these indices are controlled so as to fall
in respective appropriate ranges to create a magnesium oxide particle aggregate
which can appropriately control a forsterite formation on a surface of a grain-oriented
magnetic steel sheet.
The pore distribution measurement by mercury porosimetry for obtaining indices
indicating a particle aggregation structure was conducted by the following method.
In the measurements of pore distribution of porous solid materials, the method
of mercury porosimetry is well known as an analysis method for obtaining pore distribution
data of powder and data about a particle aggregation structure.
As a mercury porosimeter, AutoPore 9410, manufactured by Micromeritics GbmH,
is
used. Measurement cell for powdery sample having a cell capacity of 5×10
-6
m
3 and a stem capacity of 0.38×10
-6 m
3 is
used. A sample to be measured is preliminarily treated using a 330 mesh standard
sieve (JIS-R8801-87) so as to have substantially uniform particle diameters and
then precisely weighed in the mass range of from 0.10×10
-3 to 0.13×10
-3
kg, and placed in the measurement cell. The measurement cell is set in the
porosimeter, and then the inside of the cell is maintained in a reduced pressure
state at 50 μHg (6.67 Pa) or less for 20 minutes. Next, mercury is charged
into the measurement cell until the pressure in the cell becomes 1.5 psia (10,342
Pa). Then, the mercury is pressed under a pressure in the range of from 2 psia
(13,790 Pa) to 60,000 psia (413.7 MPa) to measure pore distribution. As the mercury
for the measurement, a special grade mercury reagent having a purity of 99.5 mass
% or higher is used, and the density of the mercury used is 13.5335×10
3 kg·m
-3.
The mercury pressing pressure is converted to a pore diameter using the following
formula (I)(Washburn's equation).
wherein D: pore diameter (m);
- P: mercury pressing pressure (Pa);
- γ: surface tension of mercury {485 dyne·cm-1 (0.485
Pa·m)}; and
- φ: contact angle of mercury (130°=2.26893 rad).
When pressing mercury into a particle aggregate, mercury first penetrates into
the voids between the particles. In this instance, as the mercury pressing pressure
increases, that is, the pore diameter determined from the mercury pressing pressure
decreases, the cumulative pore volume increases with a substantially constant gradient.
After all voids between the particles are filled with mercury, mercury starts penetrating
into the voids in the particles. A great number of voids having substantially the
same size are present in the particles, and the sum of the voids in the particles
(the sum of the pore volumes) is large. Therefore, when the penetration of mercury
is changed from through the voids between the particles to through the voids in
the particles, the cumulative pore volume drastically increases even as the mercury
pressing pressure slightly increases. This can be seen in the cumulative intrusion
volume curves of FIG.
1.
A first inflection point diameter, a particle void volume, an interparticle void
volume, and a micropore volume are individually determined from the cumulative
intrusion volume curve as follows.
In the cumulative intrusion volume curves of FIG. 1, the cumulative pore volume
on the ordinate indicates a cumulative value of the pore volume in the particle
aggregate per unit mass of the sample determined from larger pore diameters successively.
The inflection point is a point at which a cumulative intrusion volume curve suddenly
rises. The number of inflection point is not necessarily one, and, depending on
the sample to be measured, as can be seen in curve B in FIG. 1, there is the case
where a plurality of inflection points are present, but the inflection point at
the largest pore diameter is taken as the first inflection point. The first inflection
point diameter is the pore diameter at the first inflection point. The interparticle
void volume is a void volume between the aggregate particles, and it is represented
by the cumulative pore volume at the first inflection point. The particle void
volume is a void volume which is present in the particles and smaller than the
diameter of the aggregate particles, and it is represented by a volume value obtained
by subtracting the cumulative pore volume at the first inflection point from the
total pore volume. The micropore volume is represented by a volume value obtained
by subtracting the cumulative pore volume at a pore diameter of 0.05×10
-6
m from the total pore volume. The total pore volume is a cumulative pore
volume at a pore diameter of 0.003×10
-6 m. This is because the
particle structure is changed due to the mercury pressing pressure in the pore
distribution measurement by mercury porosimetry, and therefore, the measurement
error can be lowered by using the cumulative pore volume at the maximum mercury
pressing pressure as a total pore volume.
Next, magnesium oxide particle aggregates having different particle aggregation
structures, in which a first inflection point diameter, a particle void volume,
a interparticle void volume, and a micropore volume in a cumulative intrusion volume
curve are individually different, were prepared to examine the reaction rates of
the solid phase reactions between the individual magnesium oxide particle aggregates
and SiO
2. The results are shown in Table 1.
| |
TABLE 1 |
| |
|
| |
First in- |
|
|
|
Forsterite |
| |
flection |
Particle |
Interparticle |
Micropore |
forma- |
| |
point |
void volume |
void volume |
volume |
tion |
| |
diameter |
*10-3 |
*10-3 |
*10-3 |
rate |
| Unit |
*10-6 m |
m3 · kg-1 |
m3 · kg-1 |
m3 · kg-1 |
% |
| |
| A |
0.17 |
0.77 |
2.16 |
0.04 |
91.3 |
| B |
0.18 |
0.88 |
1.31 |
0.02 |
86.2 |
| C |
0.17 |
0.83 |
2.44 |
0.08 |
84.7 |
| D |
0.23 |
0.65 |
1.68 |
0.05 |
91.9 |
| E |
0.23 |
0.66 |
1.35 |
0.03 |
89.7 |
| F |
0.24 |
0.65 |
2.33 |
0.01 |
86.5 |
| G |
0.21 |
0.85 |
1.81 |
0.03 |
89.5 |
| H |
0.22 |
0.58 |
1.43 |
0.01 |
90.5 |
| I |
0.25 |
0.56 |
2.12 |
0.01 |
90.1 |
| J |
0.22 |
0.52 |
1.59 |
0.01 |
86.8 |
| K |
0.27 |
0.74 |
1.46 |
0.05 |
91.8 |
| L |
0.26 |
0.49 |
1.31 |
0.02 |
81.2 |
| M |
0.27 |
0.53 |
2.27 |
0.05 |
82.1 |
| N |
0.45 |
0.71 |
1.51 |
0.03 |
87.2 |
| O |
0.43 |
0.72 |
1.28 |
0.01 |
84.9 |
| P |
0.44 |
0.72 |
2.29 |
0.02 |
83.6 |
| Q |
0.43 |
0.91 |
1.62 |
0.02 |
81.0 |
| R |
0.42 |
0.48 |
1.45 |
0.01 |
78.8 |
| S |
0.59 |
0.52 |
2.25 |
0.01 |
77.0 |
| T |
0.61 |
0.48 |
1.34 |
0 |
75.5 |
| U |
0.88 |
0.84 |
1.27 |
0 |
77.7 |
| V |
0.90 |
0.85 |
2.32 |
0.03 |
75.6 |
| a |
0.38 |
0.61 |
0.85 |
0.08 |
92.5 |
| b |
0.55 |
0.82 |
1.29 |
0.10 |
91.9 |
| c |
0.42 |
0.72 |
1.88 |
0.05 |
90.7 |
| d |
0.28 |
0.66 |
0.89 |
0.05 |
90.6 |
| e |
0.67 |
0.58 |
1.45 |
0.06 |
87.6 |
| f |
0.95 |
0.77 |
1.89 |
0.10 |
79.3 |
| g |
0.41 |
0.54 |
1.24 |
0.02 |
85.6 |
| h |
0.35 |
0.72 |
1.42 |
0.13 |
87.3 |
| i |
0.45 |
0.45 |
1.37 |
0.07 |
85.4 |
| j |
0.39 |
0.94 |
1.28 |
0.09 |
86.1 |
| k |
0.24 |
0.62 |
0.75 |
0.01 |
82.3 |
| l |
0.45 |
0.71 |
2.33 |
0.03 |
84.6 |
| m |
0.54 |
0.82 |
0.87 |
0.18 |
83.1 |
| |
In the solid phase-solid phase reaction between magnesium oxide and SiO
2,
these were directly reacted with each other to form forsterite. Namely, each magnesium
oxide particle aggregate having an individual particle aggregation structure and
amorphous SiO
2 were mixed in a molar ratio of 2:1 to form a mixture,
and it was press-molded under a pressure of 50 MPa into pellets having a diameter
of 15×10
-3 m and a height of 15×10
-3 m. Then, the
molded articles were calcined in a nitrogen gas atmosphere at 1,200° C. for
4 hours. With respect to the thus obtained sintered product, an X-ray diffraction
analysis was conducted to quantitatively determine a forsterite formation rate
in the sintered product.
With respect to the thus measured samples, a relationships between a forsterite
formation rate, a first inflection point diameter, a particle void volume, a interparticle
void volume, and a micropore volume in a cumulative intrusion volume curve are
shown in FIGS. 2 to
5. In FIGS. 2 and 3, forsterite formation rate values
are indicated by classifying into three levels, i.e., 90% or more, more than 80
to less than 90%, and less than 80%. In the figures, area a sectioned by a dotted
line corresponds to the range defined in the above-mentioned earlier patent application
filed by the present inventors, and area b sectioned by a solid line corresponds
to the range defined in the present invention.
The first inflection point diameter indicates the size of the largest particle
structure in the aggregate particles, and, the smaller the first inflection point
diameter, the larger the number of contact points between the magnesium oxide particles
and SiO
2, or the higher the activity. Therefore, the first inflection
point diameter is preferably 0.30×10
-6 m or less, which corresponds
to the range in the earlier patent application filed by the present inventors,
and, when the first inflection point diameter exceeds 0.30×10
-6 m,
the number of contact points between the magnesium oxide particles and SiO
2
generally lacks, making it difficult to form forsterite at a satisfactory rate.
However, as can be seen in FIG. 2, there is a specific region such that forsterite
can be formed at a rate of 90% or higher at a first inflection point diameter of
0.60×10
-6 m or less even when the first inflection point diameter
exceeds 0.30×10
-6 m. This region corresponds to the case where
the particle void volume falls in the range of 0.50×10
-3 to 0.90×10
-3
m
3 kg
-1 (FIG. 2) and the micropore volume falls in
the range of 0.04×10
-3 to 0.11×10
-3 m
3·kg
-1
(FIG.
4). The reason for this is presumed that the highly active magnesium
oxide particle surfaces which are partially present on the particles make up for
the lack of the contact points in this region. With respect to the forsterite formation,
a value of 90% or more was taken as a satisfactory value for the forsterite formation
rate. When the forsterite formation rate can satisfy such a reference value, a
forsterite film having excellent adhesion to the surface of a magnetic steel sheet
can be formed.
On the other hand, the interparticle void volume is an index which indirectly
indicates the form of the aggregate particles, and, when the interparticle void
volume falls in an appropriate range, the contact points between the magnesium
oxide particles and SiO
2 can be appropriately controlled. Therefore,
when the interparticle void volume is less than 1.40×10
-3 m
3·kg
-1,
which corresponds to the range in the earlier patent application filed by the present
inventors, the number of contact points between the magnesium oxide particles and
SiO
2 generally lacks, making it difficult to form forsterite at a satisfactory
rate. However, as can be seen in FIG. 3, there is a region such that forsterite
can be formed at a rate of 90% or higher at an interparticle void volume of 0.80×10
-3
m
3·kg
1 or more even when the interparticle void
volume is less than 1.40×10
-3 m
3·kg
-1.
This region corresponds to the case where both the particle void volume and the
micropore volume fall in the above respective ranges shown in FIGS. 3 and 5.
In other words, even when the magnesium oxide particle aggregate has a first
inflection
point diameter and an interparticle void volume falling in the respective ranges
which are not suitable for the forsterite formation in the earlier patent application
filed by the present inventors, forsterite is stably formed at a rate of 90% or
higher as long as both the particle void volume and the micropore volume fall in
the respective appropriate ranges.
Namely, the particle void volume is an index indicating the density of the
aggregate particle. When the particle void volume is less than 0.50×10
-3
m
3·kg
-1, the number of contact points lacks, and,
when the particle void volume exceeds 0.90×10
-3 m
3·kg
-1,
the probability of contact between the magnesium oxide particle aggregates becomes
too large, and hence magnesium oxide undergoes a reaction therebetween before forming
forsterite and is then inactivated. Thus, in any cases, forsterite is not formed
at a satisfactory rate.
Further, the micropore volume is critical. The smaller the particles, the
larger the curvature of the particle surface or the higher the surface energy,
then it makes the activity higher. Therefore, it is considered that the particle
structure having micropores formed therein is a highly reactive particle structure.
However, when such a highly active particle structure has a micropore volume of
more than 0.11×10
-3 m
3·kg
-1, the probability
of contact between the highly active particle structures becomes too large, and
hence the highly active particle structures undergo a reaction therebetween before
contributing to the formation of forsterite, so that forsterite is not formed at
a satisfactory rate. On the other hand, when the highly active particle structure
has a micropore volume of less than 0.04×10
-3 m
3·kg
-1,
the lack of the contact points cannot be made up for, so that forsterite cannot
be formed at a satisfactory rate.
From the above results, when a requirement that the first inflection point diameter
be more than 0.30×10
-6 to 0.60×10
-6 m or the interparticle
void volume be 0.80×10
-3 to less than 1.40×10
-3 m
3·kg
-1,
the particle void volume be 0.50×10
-3 to 0.90×10
-3 m
3·kg
-1,
and the micropore volume be 0.04×10
-3 to 0.11×10
-3 m
3
kg
-1 is satisfied, forsterite can be formed from the magnesium
oxide particle aggregate and SiO
2 stably at a rate of 90% or higher.
Next, a magnesium oxide particle aggregate having a first inflection point
diameter, an interparticle void volume, a particle void volume, and a micropore
volume each being in the above-mentioned range can be prepared as follows. It is
noted that the preparation method described below is merely one example, and a
magnesium oxide particle aggregate having the particle aggregation structure defined
in the present invention can be prepared by other methods.
The magnesium oxide particle aggregate can be prepared as follows. For example,
calcium hydroxide is added to an aqueous solution of magnesium chloride as a raw
material to form magnesium hydroxide, and then the magnesium hydroxide is subjected
to filtration by means of a filter press, and dehydrated and dried, and then calcined
using a rotary kiln, followed by grinding.
The magnesium oxide particle aggregate can be prepared by various methods, for
example, a method in which an alkaline aqueous solution, such as an aqueous solution
of calcium hydroxide, sodium hydroxide, or potassium hydroxide, is reacted with
a magnesium chloride-containing aqueous solution, such as bittern, brackish water,
or sea water, to obtain magnesium hydroxide, and the magnesium hydroxide is calcined
to obtain a magnesium oxide particle aggregate; a method in which magnesite is
calcined to obtain a magnesium oxide particle aggregate; a method (Aman method)
in which a magnesium oxide particle aggregate is directly obtained from a magnesium
chloride-containing aqueous solution; and a method in which magnesium oxide obtained
by the above method is subjected to hydration to form magnesium hydroxide, followed
by calcination, to obtain a magnesium oxide particle aggregate.
The first inflection point diameter and particle void volume of the magnesium
oxide particle aggregate can be adjusted by controlling the particle structure
of magnesium hydroxide which is a precursor of magnesium oxide, the interparticle
void volume can be adjusted by controlling the conditions for grinding magnesium
oxide obtained by calcination, and the particle structure having micropores formed
therein can be obtained by mixing particle aggregates having different calcination
degrees prepared by calcining magnesium hydroxide particles having a specific size
or larger (BET specific surface area of 15×10
3 m
2·kg
-1
or less).
The first inflection point diameter and particle void volume of the magnesium
oxide particle aggregate are adjusted by controlling the particle structure of
magnesium hydroxide which is a precursor of magnesium oxide. Namely, a calcium
hydroxide slurry is added to a magnesium chloride solution so that the resultant
magnesium hydroxide concentration becomes a predetermined value, and the resultant
mixture is stirred to effect a reaction at a predetermined temperature for a predetermined
time, and then, the reaction mixture is subjected to filtration by means of a filter
press, and washed with water and dried to form magnesium hydroxide.
For adjusting the first inflection point diameter to be more than 0.30×10
-6
to 0.60×10
-6 m, the reaction temperature and reaction time
for the magnesium hydroxide formation are controlled. Namely, as shown in FIG.
6, magnesium hydroxide is formed by a reaction under conditions such that the reaction
temperature (T, ° C.) and the reaction time (t, hr) satisfy the relationship
represented by the following formula (II).
Further, it is more preferred that the reaction temperature (T, ° C.)
and the reaction time (t, hr) satisfy the relationship represented by the following
formula (III).
The particle void volume is adjusted by controlling the magnesium hydroxide concentration
after the reaction. Namely, the ratio between the magnesium chloride solution and
the calcium hydroxide slurry mixed is adjusted so that the magnesium hydroxide
concentration after the reaction becomes 0.2 to 4.5 mol·kg
-1, preferably
0.5 to 3 mol·kg
-1.
The micropore volume is adjusted by controlling the calcination conditions (temperature×time)
for the magnesium hydroxide formed, and magnesium oxide particle aggregates having
different calcination degrees are prepared and mixed with each other. In this case,
the calcination temperature is 750 to 1,250° C., and the calcination time
is 0.2 to 5 hours. In the mixing, with respect to each of the magnesium oxide particle
aggregates obtained by calcination under specific calcination conditions, a micropore
volume is measured using a mercury porosimetry curve and a mixing ratio is determined
by making calculation, and then a mixture having a specific micropore volume is obtained.
For obtaining a particle structure having micropores formed therein, it is required
to form by calcination magnesium hydroxide particles having a specific size or
larger such that the BET specific surface area is 15×10
3 m
2·kg
-1
or less. The crystal of magnesium hydroxide is of trigonal crystal system,
and generally has a hexagonal plate form. In the present invention, the magnesium
hydroxide particles may be either single crystalline or polycrystalline, and the
form of the particles is not limited to the hexagonal plate form, but the size
of the magnesium hydroxide particles indicated by the BET specific surface area
is important. The reason for this resides in that, when the BET specific surface
area of the magnesium hydroxide particles exceeds 15×10
3 m
2·kg
-1,
it is difficult to obtain a particle structure having micropores required. Namely,
when magnesium hydroxide changes to magnesium oxide, a volume shrinkage of 50%
or more occurs, and therefore, in the smaller magnesium hydroxide particles, the
magnesium oxide particles formed moves due to deformation caused by the shrinkage
to form relatively large magnesium oxide particles, thus making it difficult to
form small magnesium oxide particles needed to form micropores in a specific range.
The interparticle void volume is adjusted by controlling the grinding of the
calcined magnesium oxide. For example, when grinding using a hammer mill-type grinder
at a power of 5.5 kW having a classifier, the hammer rotational frequency is preferably
2,800 rpm or less.
As a grinder, a hammer mill-type grinder, a high-speed rotating mill-type grinder,
a jet mill-type grinder, a roller mill-type grinder, or a ball mill-type grinder
can be used. The optimal conditions of the grinder for obtaining the interparticle
void volume which falls in the range defined in the present invention vary depending
on the system and ability (power) of the grinder used, but too strong grinding
increases the interparticle void volume and too weak grinding lowers the interparticle
void volume. In the jet mill-type grinder in which the impact energy applied during
grinding is large, the impact energy may lower the particle void volume, and therefore
the operation of the grinder of this type needs to be controlled under conditions
suitable for the apparatus. Further, a classifier is not necessarily used, but
the use of a classifier makes it possible to control the grinding conditions more flexibly.
In the reaction of magnesium hydroxide, a flocculant can be added for promoting
the aggregation reaction, and a flocculation preventing agent can be added for
preventing the aggregation reaction from proceeding to an excess extent. Examples
of flocculants include aluminum sulfate, polyaluminum chloride, iron sulfate, and
polyacrylamide, and preferred are polyaluminum chloride and anionic polyacrylamide.
The flocculant can be added in an amount of 1 to 1,000 ppm, preferably 5 to 500
ppm, more preferably 10 to 100 ppm, based on the total mass of the magnesium chloride
solution and the calcium hydroxide slurry. It is not preferred to add a flocculant
in an excess amount since a particle aggregate having too high a density such that
the particle void volume is less than 0.50×10
-3 m
3·kg
-1
is disadvantageously formed.
On the other hand, as a flocculation preventing agent, sodium silicate, sodium
polyphosphate, sodium hexametaphosphate, a nonionic surfactant, or an anionic surfactant
can be added, and preferred are sodium silicate, sodium hexametaphosphate, and
nonionic surfactants. The flocculation preventing agent can be added in an amount
of 1 to 1,000 ppm, preferably 5 to 500 ppm, more preferably 10 to 100 ppm, based
on the total mass of the magnesium chloride solution and the calcium hydroxide
slurry. It is not preferred to add a flocculation preventing agent in an excess
amount since a particle aggregate having such a low density that the particle void
volume is more than 0.90×10
-3 m
3·kg
-1 is
disadvantageously formed.
A calcium hydroxide slurry is added to a magnesium chloride solution, and then
the stirring was conducted at a stirring rate of 350 to 450 rpm. The stirring does
not largely affect the particle structure, but the interparticle void volume can
be increased by stirring at a high speed and at a high shear rate by means of,
for example, a homogenizer during the reaction or can be lowered by almost no stirring.
Next, using the thus obtained magnesium oxide, an annealing separator for grain-oriented
magnetic steel sheet and a grain-oriented magnetic steel sheet are produced as follows.
A grain-oriented magnetic steel sheet is produced as follows: a silicon steel
slab
having an Si content of 2.5 to 4.5% is hot-rolled and washed with an acid, and
then subjected to cold rolling or twice cold rolling including intermediate annealing
so that the resultant sheet has a predetermined thickness. Then, the cold-rolled
coil is subjected to recrystallization annealing, which also effects decarburization,
in a wet hydrogen gas atmosphere at 700 to 900° C. to form an oxide film comprised
mainly of silica (SiO
2) on the surface of the steel sheet. An aqueous
slurry obtained by uniformly dispersing in water the magnesium oxide particle aggregate
having the particle aggregation structure of the present invention prepared by
the above method is continuously applied onto the resultant steel sheet using a
roll coater or a spray, and dried at about 300° C. The thus treated steel
sheet coil is subjected to final finishing annealing, for example, at 1,200°
C. for 20 hours to form forsterite (Mg
2SiO
4) on the surface
of the steel sheet, and the forsterite imparts a tension to the surface of the
steel sheet along with the insulating film to improve the core loss of grain-oriented
magnetic steel sheet.
As described in, for example, Japanese Prov. Patent Publication No. 101059/1994,
for facilitating the forsterite film formation, a known reaction accelerator, inhibitor
auxiliary, or tension-imparting insulating film additive can be added to the annealing separator.
EXAMPLES
Next, the present invention will be described with reference to the following Examples.
Example 1
A calcium hydroxide slurry was added to a magnesium chloride solution having a
concentration of 2.0 mol·kg
-1 so that the magnesium hydroxide concentration
after reaction became 1.2 mol·kg
-1, and the resultant mixture was
subjected to reaction in an autoclave at 150° C. for 3 hours to prepare magnesium
hydroxide having a BET specific surface area of 8.2×10
3 m
2·kg
-1.
The magnesium hydroxide prepared was calcined using a rotary kiln individually
at temperatures of 800° C., 950° C., and 1,050° C. for one hour,
and then ground by means of an impact grinder to produce three types of magnesium
oxide particle aggregates having different calcination degrees. Then, the three
types of magnesium oxide particle aggregates produced were mixed together in a
mixing ratio of 30:40:30 to obtain a magnesium oxide particle aggregate having
a particle aggregation form which falls in the range defined in the present invention.
Example 2
Magnesite was calcined using a rotary kiln at 1,100° C. for one hour
to prepare magnesium oxide having a BET specific surface area of 5.2×10
3
m
2·kg
-1. The magnesium oxide prepared was added
to water so that the slurry concentration became 2 mol·kg
-1 to
effect a reaction at 90° C. for 2 hours, preparing magnesium hydroxide having
a BET specific surface area of 7.5×10
3 m
2·kg
-1.
Then, the magnesium hydroxide prepared was calcined using a rotary kiln at 980°
C. individually for 0.2 hour, 0.5 hour, 0.8 hour, and 2 hours, and then ground
by means of an impact grinder to produce magnesium oxide particle aggregates having
different calcination degrees. Then, the four types of magnesium oxide particle
aggregates produced were mixed together in a mixing ratio of 25:30:15:30 to obtain
a magnesium oxide particle aggregate in Example 2 having a particle aggregation
form which falls in the range defined in the present invention.
Example 3
A slaked lime slurry was added to bittern so that the magnesium hydroxide concentration
after reaction became 1.2 mol·kg
-1, and the resultant mixture was
stirred at 600 rpm to effect a reaction at 80° C. for 2 hours. Then, the reaction
mixture was subjected to filtration by means of a filter press, and washed with
water and dried, and the resultant magnesium hydroxide was calcined using a rotary
kiln at 900° C. for one hour to prepare magnesium oxide having a BET specific
surface area of 20.6×10
3 m
2·kg
-1. The
magnesium oxide prepared was added to water so that the slurry concentration became
3 mol·kg
-1, and then calcium chloride was added thereto in an amount
of 2 mol %, based on the mole of the magnesium oxide, and the resultant mixture
was subjected to reaction at 80° C. for 2 hours to prepare magnesium hydroxide
having a BET specific surface area of 11.0×10
3 m
2·kg
-1.
Next, the magnesium hydroxide was calcined using a muffle furnace at a furnace
temperature of 1,200° C. individually for calcination times of 2 hours, 3
hours, and 4 hours, and then ground by means of an impact grinder to produce magnesium
oxide particle aggregates having different calcination degrees. Then, the three
types of magnesium oxide particle aggregates produced were mixed together in a
mixing ratio of 25:40:35 to obtain a magnesium oxide particle aggregate in Example
3 having a particle aggregation form which falls in the range defined in the present invention.
Comparative Example 1
A slaked lime slurry was added to bittern so that the magnesium hydroxide concentration
after reaction became 2 mol·kg
-1, and the resultant mixture was
stirred at 600 rpm to effect a reaction at 80° C. for 2 hours. Then, the reaction
mixture was subjected to filtration by means of a filter press, and washed with
water and dried, and the resultant magnesium hydroxide was calcined using a rotary
kiln at 890° C. for one hour to prepare a magnesium oxide particle aggregate.
Then, the particle aggregate prepared was ground by means of an impact grinder
to produce a magnesium oxide particle aggregate in Comparative Example 1 having
a specific particle aggregation structure in the earlier patent application filed
by the present inventors (Japanese Patent Application No. 2000-132370).
Comparative Examples 2 to 4
The magnesium oxide particle aggregates obtained in Example 1 by calcining magnesium
hydroxide using a rotary kiln individually at temperatures of 800° C., 950°
C., and 1,050° C. for one hour, and then grinding the calcined product by
means of an impact grinder were not mixed together but individually used.
Comparative Example 5
Bittern and slaked lime were reacted with each other at 40° C. for 10
hours to form magnesium hydroxide, and then the magnesium hydroxide was calcined
by means of a rotary kiln at 1,050° C. The thus produced magnesium oxide particles
are not controlled with respect to the particle aggregation structure as conducted
in the present invention, and they are magnesium oxide for annealing separator
used for general magnetic steel sheets.
Comparative Example 6
Slaked lime was added to sea water so that the magnesium hydroxide concentration
after reaction became 0.05 mol·kg
-1 to effect a reaction at 50°
C. for 20 hours, thus forming magnesium hydroxide. 5 Hours before completion of
the reaction, anionic polyacrylamide was added in an amount of 200 ppm, and then
the reaction mixture after completion of the reaction was subjected to filtration
by means of a filter press and dried. Then, the resultant magnesium hydroxide was
calcined by means of a rotary kiln at 950° C. to prepare magnesium oxide particles.
The thus prepared particles are not controlled with respect to the particle aggregation
structure as conducted in the present invention, and they are magnesium oxide used
in an application other than annealing separator.
Table 2 shows the measurement values for particle aggregation structures of
the particles or particle aggregates in Examples 1 to 3 and Comparative Examples
1 to 6. As can be seen from this table, in each of Examples 1 to 3 in which the
particle aggregate was produced while controlling the particle aggregation structure,
the requirement of the present invention that the first inflection point diameter
be more than 0.30×10
-6 to 0.60 ×10
-6 m or the interparticle
void volume be 0.80×10
-3 to less than 1.40×10
-3 m
3·kg
-1,
the particle void volume be 0.50×10
-3 to 0.90×10
-3 m
3·kg
-1,
and the micropore volume be 0.04×10
-3 to 0.11×10
-3 m
-3·kg
-1
is satisfied. In Comparative Example 1, the first inflection point diameter
is 0.30×10
-6 m or less, the interparticle void volume is 1.40×10
-3
to 2.20×10
-3 m
3·kg
-1 and the particle
void volume is 0.55×10 to 0.80×10
-3 m
3·kg
-1,
namely, the particle aggregate structure falls in the range defined in the earlier
patent application filed by the present inventors. On the other hand, in each of
Comparative Examples 2 to 4, the first inflection point diameter, the interparticle
void volume, and the particle void volume fall in the respective ranges defined
in the present invention, but the micropore volume in Comparative Example 2 exceeds
the upper limit of the range defined in the present invention, and the micropore
volume in each of Comparative Examples 3 and 4 is less than the lower limit. Further,
in each of Comparative Examples 5 and 6, the particle aggregation structure is
not controlled, and hence, the interparticle void volume in Comparative Example
5 and the first inflection point diameter in Comparative Example 6 fall outside
of the respective ranges defined in the present invention, and almost no micropores
are present in the particles.
| TABLE 2 |
| Measurement values for particle aggregation structures |
| |
First |
|
|
|
| |
inflection |
| |
point |
Interparticle |
Particle |
Micropore |
| |
diameter |
void volume |
void volume |
volume |
| Unit |
*10-6 m |
*10-3 m3 · kg-1 |
*10-3 m3 · kg-1 |
*10-3 m3 · kg-1 |
| Example 1 |
0.38 |
1.26 |
0.74 |
0.09 |
| Example 2 |
0.41 |
1.19 |
0.69 |
0.06 |
| Example 3 |
0.29 |
1.13 |
0.66 |
0.10 |
| Com- |
0.28 |
1.45 |
0.69 |
0.01 |
| parative |
| Example 1 |
| Com- |
0.32 |
1.18 |
0.75 |
0.25 |
| parative |
| Example 2 |
| Com- |
0.35 |
1.25 |
0.77 |
0.02 |
| parative |
| Example 3 |
| Com- |
0.55 |
1.38 |
0.72 |
0.01 |
| parative |
| Example 4 |
| Com- |
0.35 |
2.63 |
0.84 |
0.00 |
| parative |
| Example 5 |
| Com- |
1.17 |
0.52 |
0.62 |
0.00 |
| parative |
| Example 6 |
Next, with respect to the above magnesium oxide particle aggregates or powder
particles, the behavior of formation of a forsterite film was examined. It is presumed
that the formation of forsterite proceeds according to the solid phase reaction:
2MgO+SiO
2→Mg
2 SiO
4 Therefore, the magnesium
oxide powder in each of Examples and Comparative Examples and SiO
2 were
mixed in a molar ratio of 2:1 to form a mixture, and the mixture was shaped under
a pressure of 50 MPa to obtain a shaped article having a diameter of 1.5×10
-3
in and a height of 15×10
-3 m, and then the shaped article
was calcined in a nitrogen gas atmosphere at 1,2000° C. for 4 hours. This
calcination temperature corresponds to the temperature of the finishing annealing
in which SiO
2 is reacted with a slurry containing magnesium oxide on
the grain-oriented magnetic steel sheet. With respect to the thus obtained sintered
product, an X
