Process for removing ink from wastepaper

Wastepaper is treated by a ink-removing process in which an aqueous wastepaper pulp slurry is introduced into a treating vessel, air is blown into the wastepaper pulp slurry preferably through a number of air-blowing orifices formed on a peripheral surface of at least one horizontal cylinder located in the bottom portion of the vessel to absorb ink particles floating in the wastepaper pulp slurry by air bubbles ejected from the orifices and rising toward the wastepaper pulp slurry surface. The resultant ink-absorbed froth is then removed. In this process, air is blown in an amount satisfying the relationship (I) EQU G/L.gtoreq.5.0 (I) wherein L is the volume (m.sup.3) of the wastepaper pulp slurry in the treating vessel and G is the total volume (Nm.sup.3) of the air bubbles blown into the vessel, to promote the contact of the air bubbles with the ink particles in the wastepaper pulp slurry.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a process and apparatus for removing ink 
from wastepaper. More particularly, the present invention relates to a 
process and apparatus for removing ink from wastepaper at a high 
ink-removing efficiency to obtain a high yield of paper pulp having an 
enhanced brightness. 
(2) Description of the Related Art 
It is well known to remove ink from wastepaper by preparing an aqueous 
slurry of wastepaper pulp treated with a chemical, for example, a 
carbon-removing agent or bleaching agent, in a treating vessel, by blowing 
air from the bottom portion of the vessel into the wastepaper pulp slurry 
in the treating vessel so as to cause the resultant air bubbles to absorb 
ink particles floating in the wastepaper pulp slurry while the bubbles 
rise to the surface of the wastepaper pulp slurry, and then by removing 
the resultant ink-absorbed froth from the wastepaper pulp slurry. 
In the above-mentioned ink-removing process, to enhance the ink-removing 
efficiency, it is important to cause a uniform flow of a number of fine 
air bubbles into the wastepaper pulp slurry, to enhance the chances of 
contact of the ink particles with the fine air bubbles. 
For this purpose, various approaches have been made. For example, U.S. Pat. 
Nos. 4,186,094 and 4,347,128 disclose one such approach in which an eddy 
stream of a wastepaper pulp slurry is formed in a treating vessel, an 
additional amount of the wastepaper pulp slurry is supplied into the 
vessel at angles tangential to the eddy stream so as to flow the 
wastepaper pulp slurry in the eddy stream, and air bubbles are blown into 
the eddy stream of the wastepaper pulp slurry. However, the ink-removing 
effect of the above-mentioned method is unsatisfactory. 
Japanese Unexamined Pat. Publication No. 50-25804 published on March 18, 
1975 for Escher Wyss GbmH discloses another approach in which air is blown 
through a number of holes perforated on a plate fixed in the bottom 
portion of a treating vessel. 
DE-OS No. 2,409,235 published on Oct. 23, 1975, for Escher Wyss GmbH, 
discloses still another approach in which air was blown through a number 
of holes formed on a periphery of an air-blowing pipe fixed in the bottom 
portion of a treating vessel. 
However, the holes on the plate are sometimes blocked by dusts in the 
wastepaper pulp slurry and the resultant ink-removing effect is 
unsatisfactory. 
It is generally believed that the brightness of the ink-removed wastepaper 
pulp is increased with an increase in the ratio (G/L) of the entire volume 
(G) of the air bubbles in the treating vessel to the volume (L) of the 
wastepaper pulp slurry in the treating vessel. However, it is also 
believed that the increase in the brightness of the ink-removed wastepaper 
pulp reaches saturation, at a certain point, even if the ratio (G/L) is 
further increased. That is, an excessive increase of the entire volume (G) 
of the air bubbles is not effective for enhancing the brightness of the 
resultant pulp and is not economical. Therefore, in the conventional 
ink-removing process, the ratio (G/L) is limited to a relatively small 
value, that is, 1.5 or more but less than 5.0. 
Furthermore, in the conventional ink-removing processes and apparatuses, it 
is taught that it is very difficult and uneconomical to increase the ratio 
G/L to 5.0 or more. 
In the other approach, the wastepaper pulp slurry which has been treated 
with the air bubbles is further washed one or more times with fresh water. 
This washing procedure results in a decrease in the yield of the resultant 
ink-removed pulp and also in the contamination of environmental water by 
the waste water discharged from the washing procedure. 
Under the above-mentioned circumstances, it is desired by the industry that 
a new process and apparatus be provided that will effectively remove ink 
at a high efficiency and with a high yield of ink-removed pulp having an 
enhanced brightness. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process and apparatus 
for removing ink from wastepaper at a high ink-removing efficiency. 
Another object of the present invention is to provide a process and 
apparatus for removing ink from wastepaper, which is effective for 
recovering wastepaper pulp having an enhanced brightness. 
The above-mentioned objects can be attained by the process and apparatus of 
the present invention. 
The process of the present invention for removing ink from wastepaper 
comprises the steps of, introducing an aqueous slurry of wastepaper pulp 
into a treating vessel; blowing a number of air bubbles into the 
waste-paper pulp slurry in the treating vessel to cause the air bubbles to 
absorb ink particles from the wastepaper pulp slurry while rising to the 
surface of the wastepaper pulp slurry; and removing the resultant 
ink-absorbed froth from the wastepaper pulp slurry, which process is 
characterized in that, in the air-blowing step, air is blown in an amount 
satisfying the relationship: 
EQU G/L.gtoreq.5.0 
wherein L represents the volume in m.sup.3 of the wastepaper pulp slurry in 
the treating vessel and G represents the total volume in Nm.sup.3 of the 
air bubbles blown into the wastepaper pulp slurry. 
In the process of the present invention, it is preferable that the air 
bubbles are blown into the wastepaper pulp slurry through a number of 
orifices formed on a peripheral surface of at least one horizontal 
cylinder located in the bottom portion of the treating vessel. Also, it is 
preferable that the horizontal cylinder is rotated around a horizontal 
axis thereof. 
The apparatus of the present invention for removing ink from wastepaper 
comprises, a treating vessel for containing an aqueous slurry of 
wastepaper pulp, having an entrance and an exit for the wastepaper pulp 
slurry; at least one air-blowing horizontal cylinder located in the bottom 
portion of the vessel, having a number of air-blowing orifices formed on 
the peripheral surface of the cylinder and connected to an air-supply 
source, the horizontal cylinder being rotatable around the horizontal axis 
of the cylinder; and means, for removing resultant froth from the 
wastepaper pulp slurry, located above the surface of the wastepaper pulp 
slurry in the vessel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the process of the present invention, wastepaper is converted into an 
aqueous wastepaper pulp slurry by usual procedures and then introduced 
into a treating vessel, preferably in the form of a vertical drum. Air is 
then blown into the wastepaper pulp slurry in the vessel. It is preferable 
that air is blown through a number of air-blowing orifices formed on a 
peripheral surface of at least one horizontal air blowing cylinder located 
in the bottom portion of the treating vessel, preferably while the 
horizontal cylinder is rotated around the horizontal axis of the cylinder. 
The blown air is converted to a number of fine air bubbles, to absorb ink 
particles floating in the wastepaper pulp slurry while rising from the 
orifices to the surface of the wastepaper pulp slurry in the treating 
vessel. The ink-absorbed bubbles form a froth on the surface of the 
wastepaper pulp slurry, and this froth is then removed from that surface. 
Referring to FIGS. 1, 2, and 3, a treating vessel (floatater) 1 is provided 
with at least one air-blowing horizontal cylinder 2 located in the bottom 
portion la of the vessel 1. The cylinder 2 has a number of air-blowing 
orifices 3 and is connected to a supply source of air (not shown in the 
drawings). The cylinder 2 is rotatable around a horizontal shaft 4 
extending along the horizontal axis of the cylinder 2. 
A wastepaper pulp slurry 5 containing ink particles is introduced from a 
supply source (not shown) into the vessel 1 through an entrance trough 5, 
flows through the vessel 1, and is then discharged therefrom through an 
exit trough 6 to the outside of the vessel 1. When the vessel 1 is filled 
with the wastepaper pulp slurry S, air is blown from a supply source (not 
shown) into the wastepaper pulp slurry S through the horizontal 
air-blowing cylinder 2 and the air-blowing orifices 3, while the 
horizontal cylinder 2 is rotated. This procedure causes the creation of a 
number of air bubbles 7 distributed throughout the wastepaper pulp slurry 
S in the vessel 1. The air bubbles 7 float to the surface 8 of the 
wastepaper pulp slurry S and the ink particles floating in the wastepaper 
pulp slurry S are absorbed on the interfaces between the air bubbles 7 and 
the medium liquid of the water paper pulp slurry S. The ink-absorbed air 
bubbles 7 reach the surface 8 of the wastepaper pulp slurry and form a 
froth 9 on the surface of the wastepaper pulp slurry S. The ink-absorbed 
froth 9 is removed from the wastepaper pulp slurry S in the vessel 1 by 
means of a froth-removing device 10. The froth-removing device 10 is 
provided with a plurality of blades 11 rotating around a shaft 12. When 
the blades 11 are rotated, the froth 9 on the wastepaper pulp slurry 
surface 8 is collected and discharged into a froth-discharging trough 13. 
The collected froth 9 is treated with a water shower 14 and is allowed to 
flow into an air-liquid separator 15 to separate the removed 
ink-containing liquid from the air. The separated air is discharged into 
the outside atmosphere through a pump 16. The separated ink-containing 
liquid 17 is collected in a vessel 18 and is then discharged to the 
outside of the apparatus or, if necessary, is supplied into a secondary 
ink-removing apparatus (secondary floatater) (not shown). 
It is important that, in the process and apparatus of the present 
invention, air is introduced into the wastepaper pulp slurry S in an 
amount satisfying the relationship (I). 
EQU G/L.gtoreq.5.0 (I) 
wherein L represents the volume in m.sup.3 of the wastepaper pulp slurry S 
in the treating vessel 1 and G represents the total volume in Nm.sup.3 of 
the air bubbles 7 blown into the wastepaper pulp slurry S in the treating 
vessel 1. The ratio G/L is preferably 10 or more. It was found that the 
increase of the ratio G/L to 5.0 or more is very effective for enhancing 
the brightness of the resultant pulp, and when the ratio G/L is less than 
5.0, the brightness of the resultant pulp and the ink-removing efficiency 
are unsatisfactory. 
In order to introduce air in the above-mentioned large ratio (G/L) into the 
wastepaper pulp slurry, it is preferable to rotate the air-blowing 
horizontal cylinder 2 having a number of air-blowing orifices 3 to agitate 
the wastepaper pulp slurry 5 and to convert the blown air into small 
bubbles 7. 
In the process of the present invention, it is preferable that the 
wastepaper pulp slurry S in the treating vessel 1 be agitated. For this 
purpose, the orifices 3 project from the peripheral surface 2a of the 
horizontal cylinder 2 as shown in FIGS. 4 and 5. Each orifice 3 is in the 
form of a short pipe 3b having a hollow 3a through which air is ejected. 
The horizontal cylinder 2 may have a number of agitating paddles 19, as 
shown in FIG. 2, extending outward from the peripheral surface 2a of the 
cylinder 2. 
When the horizontal cylinder 2 is rotated at a predetermined rotating 
peripheral speed, preferably from 6 to 11 m/sec, the projected orifices 3 
or paddles 19 agitate the wastepaper pulp slurry S so as to accelerate the 
contact of the air bubbles 7 with the ink particles and to promote the 
rising speed of the air bubbles 7 through the wastepaper pulp slurry S. 
Another way of promoting the contact of the air bubbles 7 with the ink 
particles is to make the size of the air bubbles 7 as small as possible, 
i.e., to a point at which the rising speed of the air-bubbles 7 becomes 
excessively slow. 
The size of the air bubbles 7 can be controlled to a desired value by 
adjusting, respectively, the inside diameter 3a of the air-bubbling 
orifices 3, the length of the projected air-bubbling orifices 3b, the 
ratio of the total cross-sectional area of the air-bubbling orifice 3 to 
the entire peripheral surface area 2a of the horizontal cylinder 2, the 
rotating peripheral speed of the horizontal cylinder 2, and the blow rate 
and blowing pressure of air per unit area of the cross-section of each 
orifice 3, to appropriate values. 
For example, it is preferable that, in the air-blowing step, the 
air-blowing rate satisfy the relationship (II): 
EQU 0.5.ltoreq.g/A.ltoreq.2.0 
wherein g represents an air-blowing rate in Nm.sup.3 /min through all the 
air-bubbling orifices 3 and A represents the area in m.sup.2 of the 
horizontal cross-section of the treating vessel 1. 
A more preferable relationship is 
EQU 1.0.ltoreq.g/A.ltoreq.2.0 
If the ratio g/A is less than 0.5, it can become difficult to bring the 
ratio to G/L 5 or more. Also, a ratio g/A of more than 2.0 sometimes 
causes the size of the resultant air bubbles 7 to become excessively 
large, resulting in an undesirable bumping phenomenon occurring in the 
wastepaper pulp slurry S. 
When the ratio g/A is kept within the range of from 0.5 to 2.0, it is 
possible to keep the ratio G/L at the level of 5.0 or more, without 
causing bumping in the wastepaper pulp slurry S. 
It is preferable that the inside diameter of the air-bubbling orifices 3 be 
in the range of from 5 to 40 mm, more preferably from 15 to 35 mm. The 
inside diameter 3a of the orifices 3 should be determined by taking into 
consideration the rotation speed of the horizontal cylinder 2, the level 
of the wastepaper pulp slurry S in the vessel 1, the concentration of 
waste-paper pulp in the slurry S, and the other operational conditions. 
The length 3b of the orifices 3 extending from the peripheral surface 2a 
of the horizontal cylinder 2 is perferably in the range of from 2.5 to 20 
mm, more preferably from 3 to 10 mm. These projected orifices 3 are highly 
effective for promoting contact between the ink particles and the air 
bubbles 7 in the wastepaper pulp slurry S. 
In the process of the present invention, it is preferable that the 
wastepaper pulp slurry S in the treating vessel 1 flow downward in 
countercurrent to the rising flow of the air bubbles 7 blown from each 
horizontal cylinder 2 and the flow upward concurrently with the rising 
flow of the air bubbles 7 blown from the horizontal cylinder 2. When air 
is blown from two or more horizontal cylinders 2, the combination of the 
above-mentioned downward and upward flows of the wastepaper pulp slurry S 
may be carried out twice or more. For the downward and upward flows, in 
the apparatus of the present invention, the treating vessel 1 is 
preferably divided into at least two compartments connected to each other 
by at least one vertical partition. 
Referring to FIGS. 1, 2 and 3, vertical partitions 21 and 22 are inserted 
into the treating vessel 1. Each of the vertical partitions 21 and 22 is 
located above each of the horizontal cylinders 2 and extended vertically 
from above the surface 8 of the wastepaper pulp slurry S in the vessel 1 
to a point near to the corresponding horizontal cylinder 2, through the 
waste paper pulp slurry surface 8. 
Another vertical partition 23 is located between two adjacent horizontal 
cylinder 2 and extended vertically from the bottom 24 of the treating 
vessel 1 to a point near to the surface 8 of the wastepaper pulp slurry S 
in the vessel 1. 
Referring to FIG. 2, the space inside the treating vessel 1 is divided into 
four compartments A, B, C and D by the vertical partitions 21 and 22 and 
the other partition 23. 
The compartments A and B, and C and D are respectively connected to each 
other at the bottom portion of the vessel 1 below the lower ends of the 
partitions 21 and 22 and around the horizontal cylinder 2. Also, the 
compartments B and C are connected to each other at the upper portion of 
the vessel 1 above the upper end of the other partition 23. 
The wastepaper pulp slurry S introduced into the treating vessel 1 through 
the entrance trough 5 flows downward through the compartment A in 
countercurrent to the rising flow of the air bubbles 7, then upward 
through the compartment B concurrently with the rising flow of the air 
bubbles 7. The wastepaper pulp slurry S flows downward through the 
compartment C and then upward through the compartment D in the same manner 
as mentioned where, and finally flows out through the exit trough 6 to the 
outside of the vessel 1. While the wastepaper pulp slurry S travels 
through the compartments A to D, the ink particles in the slurry S are 
absorbed by the air bubbles 7. The resultant ink-absorbed froth 9 is 
accumulated on the surface 8 of the wastepaper pulp slurry S and is 
discharged by the froth-removing device 10 in the afore-mentioned manner. 
The abovementioned downward and upward flow of the wastepaper pulp slurry 
S is highly effective for promoting the contact of the ink particles with 
the air bubbles 7 and for enhancing the ink-removing efficiency. 
The embodiment of the apparatus indicated in FIGS. 1, 2, and 3, is provided 
with two horizontal cylinders 2, and the treating vessel 1 is divided into 
four compartments. However, the apparatus of the present invention may 
contain only one horizontal cylinder 2 and two compartments, or three or 
more horizontal cylinders 2 and six or more compartments. 
The rising speed of the ink-absorbed air bubbles 7 is variable depending 
upon the concentration and temperature of the wastepaper pulp slurry S. 
The lower the concentration and the higher the temperature of the 
wastepaper pulp slurry S, the higher the ink-removing efficiency. The 
wastepaper pulp slurry S has a preferable concentration of from 0.5 to 
1.5% and a temperature of from 10.degree. C. to 50.degree. C. 
In the process of the present invention, the effect of the severe agitation 
of the wastepaper pulp slurry by rotating the horizontal cylinder 2 having 
air-blowing orifices 3 is synergetic with the effect of the air-blowing in 
the large ratio G/L of 5.0 or more, to promote the contact of the air 
bubbles 7 with the ink particles in the wastepaper pulp slurry S and to 
enhance the ink-removing efficiency and the brightness of the resultant 
pulp. 
SPECIFIC EXAMPLE 
The present invention will be further illustrated by the following example 
which is representative and does not restrict the scope of the present 
invention in any way. 
EXAMPLE 
Wastepaper was defiberized to provide an aqueous pulp slurry and treated 
with an ink-removing agent and bleaching agent. The resultant aqueous 
wastepaper pulp slurry had a dry concentration of pulp of 1.1%. 
The bleached wastepaper pulp slurry was introduced at a supply rate (l) of 
26 l/min, 40 l/min, 80 l/min, or 160 l/min into a treating vessel as shown 
in FIGS. 1, 2, and 3 having a cross-sectional area A of 0.4 m.sup.2 and a 
capacity of 800l. The treating vessel contained therein two air-blowing 
cylinders having a length of 1 m and an inside diameter of 78 mm and 
provided with 200 air-blowing orifices having a length of 5 mm and an 
inside diameter of 11.4 mm. The distance between the surface of the 
wastepaper pulp slurry in the vessel and the horizontal axis of the 
horizontal cylinders was 1.5 m. 
Air was blown at a supply rate of 0.15 to 1.21 Nm.sup.3 /min into the 
wastepaper pulp slurry at a temperature of 30.degree. C. 
When the operation was carried out at a ratio G/L of 5.0 and at a ratio g/A 
of 0.38, an increase in the brightness of the resultant pulp over that of 
the bleached wastepaper pulp was 9.0%. When the operation was effected at 
a ratio G/L of 5.0 at a ratio g/A of 1.16, the increase in brightness of 
the resultant pulp was 7.3%. 
At a ratio G/L of 10 and at a ratio of g/A of 0.38, the increase in 
brightness of the resultant pulp was 12.2%. At a ratio G/L of 10 and at a 
ratio of g/A of 1.16, the increase in brightness of the resultant pulp was 
9.8%. 
Referring to FIG. 6, the increase (%) in brightness of the resultant pulp 
is satisfactory at the ratio G/L of 5.0 or more. 
FIG. 6 also indicates that an excessively large ratio g/A of 3.08 is not 
preferable for enhancing the brightness of the resultant pulp even at the 
ratio G/L of 5.0 or more.