Method and apparatus for melting gel-like substances

A method and apparatus for converting gel-like substances from a gel state to a sol state in which the melting speed can be accurately controlled over a wide range and the apparatus can be easily cleaned. A grid-type heater constructed of rotatable parallel heating pipes is disposed at the bottom of a melting tank. Rotation of the pipes which simultaneously heating them imparts flowability to a heat transfer boundary formed on the side of the gel-like substance. The melting speed is controlled by controlling the speed of rotation of the pipes.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for converting a 
substance which exhibits a sol-gel change from a gel state to a sol state 
depending upon temperature such as an emulsion or an emulsified product 
used in manufacturing photographic material. 
During the manufacture of photographic material it is necessary to cool 
gel-like substances such as photographic emulsions, matting agents or 
emulsified products after preparation, store them at low temperatures, and 
melt them as required when they are to be coated on a support. Two primary 
methods for melting such gel-like substances have been used in the past. 
One of these is a so-called batchwise method such as disclosed in Japanese 
Patent Publication Nos. 9495/69 and 1738/76 while the other is a melting 
method based on continuous control as disclosed in Japanese Laid-Open 
Patent Publication No. 34713/76 and U.S. Pat. No. 3,847,616. 
The batchwise method is disadvantageous in that the gel-like substance 
tends to undergo localized overheating within a container, a long period 
of time is required for it to be completely melted, and the amount of the 
final product is small. Hence, this method is not feasible for use with 
gel-like substances such as photographic emulsions which undergo 
qualitative degradation with the passage of time. 
With the latter melting method, the gel-like substance is placed on a 
grid-type heater in the shape of, for example, convolutions provided at 
the bottom portion of a melting tank, a heating medium is introduced into 
the grid, and the gel-like substance is continuously melted. Since this 
method permits the melting operation to be carried out continuously, the 
gel-like substance can be treated in the same vessel within a shorter 
period of time than with the batchwise method. 
The melting method, however, has the following disadvantages. 
(i) The melting speed must be adjusted by varying the flow rate, 
temperature or pressure of the heating medium introduced into the grid. 
Hence the melting speed cannot be markedly increased because there are 
limits to the permissible ranges of these parameters. For example, if the 
temperature is raised to too high a level, the properties of the gel-like 
substance are adversely affected. The melt-treating time is therefore 
difficult to shorten. 
(ii) For the same reasons, the margin of control of the melting speed is 
narrow, and its control is difficult. 
(iii) Since the heater is fixed to the bottom of the tank, it is 
troublesome to wash the tank, especially the undersurface of the heater, 
and a long period of time is required for a complete washing of the tank. 
Accordingly, it is an object of the invention to provide a method for 
melting a gel-like substance which eliminates the aforesaid disadvantages 
of conventional melting methods and in which the speed of melting the 
gel-like substance is greatly increased, the margin of control of the 
melting speed is increased, and the melting tank can be easily washed. 
Another object of the invention is to provide an apparatus for practicing 
the aforesaid method. 
SUMMARY OF THE INVENTION 
The inventors have investigated the cause of the aforesaid disadvantages, 
especially (i) and (ii), and have as a result ascertained that there is a 
marked difference in the thickness of a heat transfer boundary film at the 
tube wall between a heat transfer medium flowing through a tube of a 
grid-type heater and a gel-like substance and that the ratio of the heat 
transfer coefficient of the heat transfer boundary film on the side of the 
heat transfer medium (warm water) to that on the side of the gel-like 
substance (photographic emulsion) may range from about 10 to about 20. It 
has been found, in view of this situation, that an increase of the melting 
speed of the gel-like substance can be achieved by minimizing the 
thickness of the heat transfer boundary film on the side of the gel-like 
substance. This discovery has led to the present invention. 
Accordingly, the aforesaid objects of the invention are achieved by a 
method for melting a gel-like substance which includes feeding the 
gel-like substances to a heating surface provided at the bottom of the 
inside of a melting tank, heating the gel-like substance at the heating 
surface to convert it to a sol, and withdrawing the sol from the bottom of 
the melting tank wherein, during the melting of the gel-like substance, 
the heating surface is rotated to impart flowability to the heat transfer 
boundary film on the side of the gel-like substance. The invention also 
encompasses an apparatus for practicing this method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 which shows the overall structure of an apparatus for 
melting a photographic emulsion as a gel-like substance constructed 
according to a preferred embodiment of the invention, reference numeral 1 
designates a melting tank with a closed interior, 2 the photographic 
emulsion to be melted, 3 heating pipes, 4 a coupling, 5 a motor, 6 a 
tubular line through which the photographic emulsion converted to sol by 
melting is withdrawn, 7 a stock tank, 8 a conduit for drawing off the 
photographic emulsion, 9 a liquid feed pump, 10 a thermometer, 11 a 
constant temperature bath using a stock of warm water as a heating medium, 
and 12 a liquid feed pump for circulating warm water. 
As shown, in the apparatus of the invention, a predetermined amount of 
crushed gel-like photographic emulsion is fed into the tank 1 upon the 
surface of the heating pipes 3 provided at the lower portion of the tank 
1. Warm water is passed through the heating pipes 3 from the constant 
temperature bath 11 by means of pump 12 thereby beating the pipes. 
Simultaneously, the motor 5 rotates the pipes 3 about their center. The 
photographic emulsion converted to a sol by melting is taken out through 
the tubular line 6 and temporarily stored in the tank 7. After its flow 
rate has been adjusted to a constant value, the photographic emulsion is 
transferred to a coating station through the line 8 by the pump 9. 
As shown in FIG. 2, the heating pipes 3 are aligned parallel to each other 
with small gaps therebetween. To one end of each of these pipes are fixed 
a gear 13, a coupling 4 and a liquid feed pipe 14 leading to the constant 
temperature bath 11 while a coupling 4 and a liquid discharge pipe 15 
which leads to the constant temperature bath 11 are fixed to the other 
end. At that part of each heating pipe 3 where it passes through the tank 
1, a sealing bearing 16 such as a bearing fashioned of hard Teflon 
(registered Trademark) is fitted as shown in FIG. 3. The inside of the 
tank 1 is completely closed. The individual heating pipes 3 are 
mechanically rotatingly connected to each other the gears 13. The gears 13 
are connected to the motor 5 via a reduction gear 17. When the motor 5 
rotates, the heating pipes 3 are simultaneously rotated, for instance at 
speeds of up to 50 rpm. 
Hot water at 50.degree. to 60.degree. C. is introduced into the heating 
pipes 3 and heat from the warm water is transferred to the gel-like 
substance 2 through the walls of the heating pipes 3. Desirably, the 
heating pipes 3 are made of a thin metallic pipe having a high thermal 
conductivity. The diameter, length, and other properties of the heating 
pipes are properly determined depending upon the amount of gel-like 
substance to be treated. 
A characteristic feature of the invention is that, in melting the 
photographic emulsion 2, the heating pipes 3 whose walls have been heated 
with warm water are rotated about their centers. This feature brings 
provides for the following operation and brings about the following noted 
results. 
(i) When the heating pipes 3 are rotated, the photographic emulsion 2 in 
contact with the heating pipes 3 is rendered flowable and the thickness of 
the heat transfer boundary film on the side of the photographic emulsion 
is reduced. Accordingly, the coefficient of heat transfer increases and 
the attainable speed of melting the photographic emulsion 2 is markedly 
increased. 
(ii) The increase in the speed of melting the photographic emulsion 2 
increases as the speed of rotation of the heating pipes is increased, as 
shown in FIG. 4. This becomes more pronounced as the viscosity of the 
melted photographic emulsions is lowered. Thus, the speed of melting the 
photographic emulsion 2 can be controlled by controlling the speed of 
rotation of the heating pipes 3. As a result, the margin of control of the 
melting speed is broadened compared with a conventional method which 
depends only upon the parameters noted above for controlling the heating 
medium. 
(iii) When the inside of the melting tank 1 is washed during maintenance of 
the apparatus, the entire heating surface can be washed by simply feeding 
wash water from the side of the tank 1 while the heating pipes 3 are 
rotated. The efficiency of the cleaning operation is therefore greatly 
increased. 
While the present invention has been described hereinabove with reference 
to a preferred embodiment in which a photographic emulsion is melted, it 
should be understood that the invention is not limited to the aforesaid 
embodiment and that various changes and modifications are possible within 
the spirit and scope of the invention as described in the specification 
and in the appended claims. 
For example, the gel-like substance to be melted is not limited to 
photographic emulsions and the invention is applicable to all gel-like 
substances which can be converted to a sol. The heating medium may also be 
steam under reduced pressure or another known medium. Moreover, the shape, 
number and the like of the heating pipes forming the heating surface may 
be changed as required. 
As described in detail hereinabove, the present invention makes it possible 
to increase the speed of melting the gel-like substance and to control the 
melting speed by adjusting the speed of rotation of the heating surface. 
Moreover, the operation of washing the melting tank can be simplified and 
the washing time can be greatly reduced.