Note: Descriptions are shown in the official language in which they were submitted.
~!lJ9~S~L~5
The present invention relates to a process for the
elimination by flotation of impurities which are in the form of
solid particles contained in a liquid, in which process a stream
of air is injected into a flow of the liquid in question in a flo-
tation cell, in such a manner as to form air bubbles which aredistributed in the liquid, are fixed on the impurities, and rise
to the surface so as to form a foam which is charged with these
impurities and which is discharged from the surface of the liquid.
A flotation process of the kind described above is
10 known for the removal of ink from paper pulp prepared from old
papers, according to which air is introduced directly into the
bottom of the cell by suction with the aid of a propeller which
also serves for the intimate mixing and distribution of the fibrous
suspension in the cell.
The admission of air directly into the pulp undoubtedly
makes it possible to regulate the air pressure and therefore the
diameter of the bubbles in dependence on the volume of air, which
in turn is adjustable within certain limits. Although an increase
of the volume of air makes it possible to raise the pressure and
consequently increase th diameter of the air bubbles, by this
known process it is on the other hand impossible to reduce the
diameter of the bubbles while increasing the flow of air, which in
certain cases may be necessary in order to effect the separation of
certain impurities~
It has in fact surprisingly been found that it was
possible to effect selective separation of the impurities by 10ta-
kion, by controlling the dimension of the bubbles and the volume~
of air independently of one another. Thus, for example in the case
of the de-inking of defibred, pretreated paper pulp prepared from
old papers, it is now possible to obtain diferent bubble dimensions
-
1 for the flotation of the above inks, for the flotation of fillers
such as kaolin, or for the flotation of fibres, and thus to proceed
to their selective separation.
The present invention has in particular as its object
a process permitting the adjustment of the dimensions of bubbles
independently of the volume of air mixed with the water which is
to be purified.
To this end, according to the invention, before in-
jecting the air and the liquid into the flotation cell, this
liquid is circulated in at least one mixing chamber in the form of
a thin layer, preferably 2.5 - 4 mm thick, while the air is simul-
taneously admitted into this layer of liquid transversely of the
path of the latter and along a portion of this path.
In cases where the liquid is a suspension of fibres,
such as paper pulp, it has in addition been found that the forced
passage of the pulp in the form of a thin layer in a narrow chamber
and its encounter with divided air injected transversely onto the
fine layer of liquid charged with suspended fibres give rise to
an effect of rubbing and cleaning these fibres.
According to one characteristic of the process of the
invention the thickness of the layer of liquid in the chamber and
consequently the speed of passage of the liquid in the chamber are -`
regulated.
In the case of the de-inking of pulp made from old
papers, it has been found that the volume of air mixed with the
pulp and the speed of passage of the pulp in the mixing chamber
have an effect on the whiteness of the pulp, and that by adjust-
ing them it is possible to obtain differences of whiteness
ranging from 57 to 66.5 Scan.
These adjustments make it possible to achieve effective
-- 2
purification with minimum rejection.
According to another characteristic of the process, the
length of the path of admission of air into the liquid is regulated.
It is thus possible for the amount of air to be accu-- -
rately proportioned in relation to the layer of liquid passing
through the mixiny chamber.
Yet another characteristic of the process consists in
entraining the foams by a ~low of air into a hopper and recovering
the air for recycling.
In the case of flotation with a plurality of cells, a
mixture of air and liquid coming in each case from a mixing chamber
is admitted into each cell. It is thus possible to control the
dimensions of the bubbles differently from one cell to another, in
order to recover the impurities selectively
When the liquid contains fibres, in order to avoid
maximum loss of the latter, it is possible for the foams coming
from other cells to be recycled to the liquid to be purified which
passes into the first flotation cell.
The present invention also relates to an installation
for the application of the process.
According to the invention the installation also com-
prises, upstream of the flotation cell, at least one narrow mixing
chamber capable of allowing the passage of a thin layer of liquid
to be purified and provided with means capable of admitting the
flow of air transversely of the layer of liquid over a path in the
chamber
According to one characteristic the mixing chamber is
bounded by two parallel walls at a distance from one another, so
as to allow the passage of liquid to be purified
Accordiny to another characteristic the mi~ing chamber
-- 3
s~
is provided with a means of regulating the distance between the
parallel walls.
Other features and details of the invention will be
seen from the description of an installation given with reference
to the drawings accompanying the present specification and showing,
solely by way of example, one embodiment of the invention which is
suitable in particular for the de-inking of pulp prepared from old
papers.
Figure 1 is a diagrammatical view in elevation of a
multi-storey installation;
Figure 2 is a horizontal section on the line II-II in
Figure 1;
Figure 3 shows on a larger scala a view in longitudinal
vertical section of a mixing chamber;
Figure 4 is a view in section of the line IV-IV in
Figure 3;
Figure 5 shows on a larger scale and in vertical section
; a first form of construction of a mixing chamber;
Figure 6 is a view in section on the line VI-VI in
Figure 5, and
Figure 7 is a diagram showing whiteness curves.
In these drawings the same reference numerals designate
identical elements.
The installation comprises four flotation cells 1,`'2,
3, 4. These cells are annular and are axially superimposed so as
ko constitute ~he four stages of a tower having a plurality of
floors 5. Obviously this tower could have a number of stages
difEersnt from four.
The defibred, chemically pretreated paper pulp in the
form of a fibrous suspension containing a foaming agent is delivered
5~S
by means of a pump 6 t'nrough a pipe 7 to the inlet of the cell 1,
first passing through a mixing chamber 8. This chamber has a nar-
row passage in which the fibrous suspension circulates in the form
of a thin layer, while at the same time air is admitted into it
transversely of the path of this layer of paste or along a portion
thereof through a pipe 9 connected to an air pressure source (not
shown), with the interposition of a valve 10.
The mixing chamber 8 is in communication with the
annular cell 1 through a pipe 11 which leads tangentially into the
cylindrical wall of the cell near its floor 5, so that the fibrous
suspension is caused to turn about the central pipe 12 of the annu-
lar cell. The bubbles created in the suspension are fixed on the
impurities and cause them to rise to the surface in the form of
; foams. The latter are entrained out of the cell through the out-
let 18 provided in the pipe 12,ppartially through the rotational
, movement of the suspension and partially by a current of air provi-
ded above them. A vertical plate 50 disposed above the level of
the pulp guides the foams to the outlet. The foams are collected
in a hopper 13 surrounding the pipe 12 and received in a reject
tank 37 filled with water. The flow of air for the discharge of
the foams is introduced into the upper portion of the cell through
a pip~ 14 connected to a suction device 15 disposed above a frusto-
conical axial pipe 16 for the recycling of the foam discharge air.
This pipe is disposed concentrically in the central foam discharge
pipe 12 and is common to all the superimposed cells. The foam dis-
charge air is admitted into the cell through the opening 17 and
leaves it through the outlet 18, entraining the foams, which through
the action of gravity fall into the hopper 13 while the air escapes
through the pipe 16, The latter extends into the adjacent cell
o~ming a cone narrowing in the upward direction and disposed
facing the wider pipe for the recycling of the foam discharge air
of this adjacent cell and of the other cells 3, 4. The parts
constituting the pipe 16 form a cyclone, thus imparting a centri-
fugal velocity to the foam, permitting the separation of the air
which is drawn into this pipe by the action of the section device
15, whence it is returned to the pipe 14 for reuse for the dis-
c'narge of foams.
The pulp partially cleaned in the cell 1 leaves th~
latter through a pipe 19 provided with a pump 20 enabliny the
pressure to be increased. The pipe 19 penetrates into the mixing
chamber of the next cell 4 situated in the last stage of the tower,
and the process applied is identical to that utilised in the first
cell, and so on in the other cells. The latter are fed success-
ively with the pulp or water containing fibres of an increasing
degree of purity, while the foams cGllected in these other cells
are in turn recycled to the lower cell.
The foams containing the rejected material from the
first cell and other cells can of course either be recylced or
discharged at each stage for treatment in a separate device,
depending on the type of product for which the pulp is intended.
The installation also comprises a cover 40 closing the
cell of the last stage; an inspection window 41 permi~ting rapid
discharge for cleaning purposes; an emptying valve 42 for each
cell; a flap 46 in the foam transporting air pipe; a floor 43 in
; 25 the pipe 12 for receiving recycled rejected materials; a pipe 44
for the discharge of this rejected material in cell 1, and a pipe
45 or the discharge o~ the accepted pulp in the cell 2 treating
the most highly purified pulp.
E'igures 3, 4, and 5, 6 illustrate respectively two
~orms of construction of the mixing chamber
In the form of construction shown in Figures 3 and 4
the mixing chamber has a rectangular section comprising two para-1-
1~ plane walls 21, 22 mounted in a parallelepipedic body 23, and
forming between them a channel in communication with the pulp inlet
and outlet pipes 7 and 11. The wall 21 is movable parallel to the
wall 22 and is operated by means of a threaded rod 24 guided in a
tapped hole 26 provided in a sleeve 27 engaged in the upper adja-
cent wall of the parallelepipedic body permitting by means of a
wheel 25 the adjustment o~ the distance between the parallel walls,
1~ in order to determine the thickness o-f the layer of pulp to be
subjected to the action of the jet of air. The wall 21 is provided
with a diaphragm ensuring the tightness of the chamber, while the
wall 22 is a porous material, in the particular case considered
being of fritted glass permitting the passage of the air jets onto
15 the layer of pulp passing through the chamber, It is advantageous
for the mixing chamber also to be provided with a plate 28 making
it possible to close off part of the surface of the passages pro--
vided in the fritted plate so as to regulate the air flow admission
sur~ace.
; 20 In the embodiment shown in Figures 5 and 6 the mixing
chamber 8 is conical in shape and has two parallel conical walls
29, 30 spaced apart from one another to form the passage for~ the
pulp through them. The pulp is fed into the chamber through a
noz71e 31 leading tangentially into the chamber through the wall
25 29 in order to impart to the pulp a rotational movement while
receiving the air injected through the channels 47 provided in the
inner wall 30, which are in communication with an air inlet 32
connected to a compressed air source (not shown)O The inner cone
30 is adapted to slide axially in -the outer cone 2~ so as to adjust
30 the space between these walls and consequently the thickness of the
d~
layer of pulp. For this purpose the inner cone, which may be
rotatable, is controlled by a tube 32 which at the same time forms
the air supply pipe. A conical obturator 33 disposed inside the
cone 30 is mounted on two rods 34 and 35 sliding in end walls of
5 the cone 30, and makes it possible, when desired, to close off
a part of the holes of the air distributor cone, in order in this
way to adjust the surface admittiny the flow of air into the
mixing chamber. The adjustment of the air flow independently of
the speed and rate of flow of the pulp makes it possible to create
10 in the cells a type of bubble suitable for the flotation of inks,
for the flotation of kaolin, or for the flotation of fibres) and
thus to achieve selective separation of these components.
The invention which has been described relates to an
installation for the de-inking of paper pulp. It could be used
15 for other applications, for example for the purification of ores
or for the purification of white water by flotation of fibresO
Laboratory tests have made it possible to establish a diagram,
which in Figure 7 shows pulp whiteness curves 48 and 49 plotted
against the ratio of air volume to pulp volume and against the
20 speed of the pulp in the mixing chamber. On the abscissa is
indicated the ratio of litre of air to litre of pulp, and on the
ordinate the whiteness of the pulp in degrees Scan. The curve 48
shows a pulp whiteness for a pulp speed of 4O23 metres per second,
while the curve 49 shows a pulp whiteness for a pulp speed of 6.35
25 metres per secondO
By adjustment of the air-pulp mixture and of the speed
of the pulp in the mixing chamber it is possible to obtain difer-
ences of whiteness ranging from 57 to 66.5 Scan. The tests were
carried out with old papers of the "magazine" type, containing
30 about 50% of the mechanical pulp having an original whiteness of
-- 8 --
~5~5~
the unprinted edges of 66.5 Scan.
sy applying the process of the invention the whiteness
of the printed parts containing about 2% of ink can be increased
from 46 to 66.5 Scan, with a retention of 20 minutes. After 2.5
minutes retention the whiteness has already passed from 46 Scan
to 64.5 Scan. The best present conventional process made it
possible with the same paper to obtain 62.3 Scan after 20 minutes
retention and about 54 Scan after 2.5 minutes.
It is obvious that the invention i~ not limited exclu-
sively to the embodiments illustrated and that many modifications
can be made to the form, arrangement, and constitution of certain
of the elements used in its performance, without depar~ing from
the scope of the present invention, provided that these modifica-
tions are not in contradiction with any of the following claims.
