Apparatus for granulating sodium percarbonate

Provided is a new granulator for preparing granules from a powdery material by agglomerating it through a kneading action, comprising a receptacle and two screws having screw blades disposed within the receptacle in parallel alignment to ensure intermeshing of the screw blades. The novel granulator consists of a feeding zone, a kneading zone divided into an upstream section and a downstream section, and a breaking zone. In the feeding zone, the blades on the two screws are forwardly conveying blades. In the upstream section of the kneading zone, on one screw a backwardly conveying blade is first provided and a forwardly conveying blade follows, and on the other screw, a forwardly conveying blade comes first and a backwardly conveying blade follows, said alternate arrangement occurring at least once in each screw. In the downstream section of the kneading zone, the blades on both of the screws are backwardly conveying screws. In the breaking zone, a plurality of small projections are provided on the peripheral surfaces of the screw shafts. The material transferred to the kneading zone from the feeding zone gradually advances while repeating a forward movement and a backward movement in an 8-figured pattern whereby it is agglomerated. The agglomerates are then broken by the projection in the breaking zone into granules of a suitable size.

This invention relates to an apparatus for continuously granulating a 
powder by kneading and agglomerating it in the wet state and more 
specifically, to a granulator especially suitable for granulating powdery 
sodium percarbonate. 
Sodium percarbonate can be easily obtained by reacting sodium carbonate and 
hydrogen peroxide, and is useful as a bleaching agent, etc. Handling of 
powdery sodium percarbonate is inconvenient because it irritates the 
mucous membrane of the nose upon scattering. Moreover, since a finely 
divided powder of sodium percarbonate tends to cake on long-term storage, 
it is extremely difficult to keep it non-sticky. Furthermore, sodium 
percarbonate is unstable to moisture, and moisture tends to reduce the 
content of active oxygen of sodium percarbonate. It is desirable therefore 
to reduce the area of contact of sodium percarbonate powder with moisture. 
For these reasons, sodium percarbonate is generally converted to granules. 
Generally, sodium percarbonate has previously been pelletized by means of a 
pelletizer (a manufacture of Fuji-Powder) in which it is kneaded with 
suitable additives such as a binder and a stabilizer and water and then 
the kneaded mixture is extruded through a perforated plate, or by means of 
a granulator of the Henschel mixer type. 
Granules having a relatively uniform shape can be obtained by the aforesaid 
method comprising extrusion of the kneaded mixture through the perforated 
plate. But the granules are hard, and because of a restriction on the pore 
size of the perforated plate, the resulting granules have a large particle 
diameter and a slow speed of dissolution in water. Furthermore, the pores 
of the porous plate tend to be blocked up, and the porous plate must be 
exchanged incessantly. These drawbacks make this method unsuitable for 
continuous operation. The granulator of the Henschel mixer type has the 
advantage that granulation is performed within a short period of time, and 
the heat of friction is little. However, the granules obtained are soft 
and brittle, and tend to be crushed during transportation. Furthermore, 
the granules have a low bulk density. 
Various apparatus for continuously kneading a material have been known 
which comprise a receptacle and two intermeshing rotating screws provided 
therein. When sodium percarbonate is granulated by these known apparatus, 
it is impossible to obtain particles which fully meet these requirements 
for the particle size, hardness and speed of dissolution of these 
particles. 
It is an object of this invention therefore to remove the various defects 
of the prior art, and to provide an apparatus and a method which can 
convert powdery sodium percarbonate continuously in one step without any 
need for pre-treatment into granular sodium percarbonate having a fully 
satisfactory particle size, hardness, bulk density and speed of 
dissolution in water. 
The object of the invention is achieved in accordance with this invention 
by a granulator comprising a receptacle and a pair of intermeshing 
rotating screws provided therein in parallel alignment, one end of the 
receptacle having a hopper for charging a material, the other end thereof 
having a downwardly opening discharge port and the top surface thereof 
having a cover; wherein 
the inside of said receptacle is composed of a feeding zone, a kneading 
zone and a breaking zone, 
the feeding zone has forwardly conveying screw blades on both of the two 
screws, 
the kneading zone is comprised of an upstream section and a downstream 
section, said upstream section having an alternate arrangement of a pair 
of a backwardly conveying screw blade on one screw shaft and a forwardly 
conveying screw blade on the other in opposition to each other and a pair 
of a forwardly conveying screw blade on said one screw shaft and a 
backwardly conveying screw blade on said other screw shaft in opposition 
to each other, said alternation of the forwardly and backwardly conveying 
screw blades occurring at least once in each of said screw shafts, and 
either one of the forwardly and backwardly conveying screw blade in each 
pair being discontinuous for meshing with the other screw blade in said 
pair, and said downstream section having backwardly conveying screw blades 
on both of the two screw shafts, 
the blades are aligned with the same pitch in each of the feeding zone, the 
upstream section of the kneading zone and the downstream section thereof, 
the breaking zone is of an open structure and has a plurality of 
projections provided on both of the two screw shafts for breaking 
agglomerated masses, and 
that portion of the cover of said receptable which corresponds to the 
feeding zone and the upstream section of the kneading zone is a fixed 
cover and that portion of the cover of the receptacle which corresponds to 
the downstream section of the kneading zone is a movable cover. 
The object of the invention is also achieved in accordance with this 
invention by a method for granulating sodium percarbonate, which comprises 
feeding powdery sodium percarbonate, an additive such as a binder or a 
stabilizer and water into the granulator through said hopper, kneading and 
agglomerating the powdery sodium percarbonate therein, and making the 
particle size of the resulting agglomerated masses by a size uniforming 
machine equipped with knife cutters rotating at high speeds. 
In the granulator of this invention, each screw blade generally has a 
trapezoidal shape in its cross-section taken at right angles to the 
advancing direction of the thread of the screw. In another embodiment of 
the invention, the screw blades of the feeding zone and the downstream 
section of the kneading zone have a large thickness and a trapezoidal 
cross-section, both of the forwardly and backwardly conveying screw blades 
in the upstream section of the kneading zone are formed as thin twisted 
blades, and thin fragmentary ribbon-like blades are provided between the 
discontinuous backwardly conveying screw blades. The height of the 
ribbon-like blades is lower than that of the other blades because this 
permits effective mixing of the materials.

Referring to FIGS. 1 to 3, the reference numeral 1 represents a receptacle 
of the granulator; 2, a screw shaft; 3, a forwardly conveying screw blade; 
4, a backwardly conveying screw blade; 4', a discontinuous backwardly 
conveying screw blade; 3', a forwardly conveying screw blade in opposition 
to the blade 4'; 5, a meshing or discontinuous section; 6, a projection; 
7, a hopper; 8, a discharge port; 9, a fixed cover; 10, a movable cover; 
and 11, 12 and 13, inlets or outlets for cooling water. 
The inside of the granulator of the invention is composed of a feeding zone 
I, a kneading zone II and a breaking zone III as shown in the drawings. In 
the feeding zone I, forwardly conveying screw blades 3 are formed on both 
of the two screws so as to deliver the material to the kneading zone. The 
kneading zone II is comprised of an upstream section II.sub.1 and a 
downstream section II.sub.2. In the upstream section II.sub.1, forwardly 
conveying screw blades 3' and backwardly conveying screw blades 4' are 
alternately provided on one of the screws in such a manner that a 
forwardly conveying blade 3' is first formed and is followed by a 
backwardly conveying blade 4' and this sequence is repeated. On the other 
screw, the same alternate alignment of screw blades occurs except that it 
starts with a backwardly conveying screw 4'. According to this blade 
arrangement, any forwardly conveying blade 3' on one screw faces a 
backwardly conveying blade 4' on the other, and any backwardly conveying 
blade 4' on one screw faces a forwardly conveying blade 3' on the other. 
In the illustrated embodiment, the backwardly conveying screw blades 4' 
are discontinuous blades. In the downstream section II.sub.2, the screw 
blades on both of the two screws are adapted to perform backward 
conveyance. 
By making the screw blades in the kneading zone II in a special structure 
and aligning them in a special arrangement as described hereinabove, the 
materials charged make an 8-figured motion in the kneading zone II, and 
consequently, are well kneaded and agglomerated. 
In the breaking zone III, a plurality of projections 6 are secured to the 
two screw shafts so that agglomerated masses sent from the kneading zone 
II are broken by these projections 6. 
In the granulator of the type shown in FIGS. 1 to 3, any of the screw 
blades provided in the feeding zone and the upstream and downstream 
sections of the kneading zone is trapezoidal in its crosssection taken at 
right angles to the advancing direction of the thread of the screws. 
In the modified embodiment shown in FIGS. 4 and 5, the screw blades in the 
feeding zone and the downstream section of the kneading zone has the same 
trapezoidal sectional shape as in FIGS. 1 to 3, but both the forwardly and 
backwardly conveying screw blades in the upstream section of the kneading 
zone are thin twisted blades. Specifically, the thin twisted fragmentary 
ribbon-like blades 4" are provided between the discontinuous backwardly 
conveying screw blades 4'. 
The screw blades are aligned with the same pitch in each of the feeding 
zone, the upstream section of the kneading zone, and the downstream zone 
thereof. Preferably, the pitch between the blades in the kneading zone is 
larger than that between the blades in the feeding zone. 
In the granulator of the invention, the screw blades in the feeding zone 
are forwardly conveying blades, and the kneading zone include forwardly 
conveying blades, discontinuous backwardly conveying blades, and 
backwardly conveying blades. By arranging these blades in the manner 
described hereinabove, a material transferred from the feeding zone I is 
sufficiently mixed and kneaded into agglomerates by the forward conveying 
action of the forwardly conveying blades and the returning action of the 
backwardly conveying blades. The agglomerates are sent to the breaking 
zone III by a proper balance between the feeding power of the feeding zone 
and the backwardly conveying power of the backwardly conveying blades in 
the downstream section of the kneading zone II. Usually, the agglomerates 
sent to the breaking zone III have some thixotropy. In the breaking zone, 
the agglomerated masses are broken and the thixotropy of the material is 
removed. For sufficient performance of this operation, it is desirable to 
cause the agglomerates to reside for a certain period of time in the 
breaking zone so as to impart a kind of agitation thereto. This operation 
can be effectively performed according to a preferred embodiment by 
forming those projections which are located in the upstream portion of the 
breaking zone near the downstream section II.sub.2 of the kneading zone in 
a slightly angled vane shape to permit slight backward conveyance of the 
agglomerated product, or by providing projections 6 densely in the 
upstream portion of the breaking zone and sparsely in its downstream 
portion. The structure of the breaking zone, however, is not limited to 
the embodiment specifically illustrated above, and can be determined as 
desired depending upon the state of the material transferred from the 
kneading zone. For example, when the material has undergone a relatively 
weak kneading force in the kneading zone, the projections may be rod-like 
projections adapted to mainly perform a breaking action. When the material 
has experienced a strong kneading force, the projections may be vane-like 
projections adapted to have the function of removing thixotropy. 
In the granulator of this invention, a cover adapted for up-and-down 
movement is provided at that part of the granulator which corresponds to 
the downstream section II.sub.2 of the kneading zone. The cover is 
slightly opened at the downstream side of the section II.sub.2 so that it 
does not cover all of the backward conveying screw blades in the section 
II.sub.2. When it is desired to obtained agglomerates having a relatively 
high bulk density with a strong kneading force exerted in the kneading 
zone, it is sufficient only to lower the movable cover slightly. On the 
other hand, when it is desired to obtain agglomerates having a low bulk 
density with a relatively weak kneading force exerted in the kneading 
zone, the movable cover is slightly raised. Furthermore, because of the 
open structure of the cover at the downstream side of the section 
II.sub.2, agglomerated means can be disintegrated to some extent. 
When it is necessary to cool the granulator of the present invention, 
cooling water may be passed through a jacket provided externally of the 
receptacle of the granulator and a preset pipe within the screw shafts. 
The conditions for agglomerating powdery sodium percarbonate by the 
granulator of this invention cannot be generalized because they may differ 
depending upon the size and capacity of the granulator, the particle size 
and water content of the starting powdery sodium percarbonate, the type of 
the additive, etc. For example, the conditions are: the diameter of each 
screw blade 270 mm; the total length of the receptacle 2400 mm; the 
rotating speed of the screws 40 to 80 rpm; the amount of the material to 
be fed 700 to 2000 kg/hr. The amounts of the additive and water are 
determined depending upon the type of the additive and the water content 
of the starting sodium percarbonate so that the water content of the 
resulting granules will be about 10 to 16%. 
The agglomerates produced by the granulator of this invention are 
discharged from the discharge port 8, subjected to size adjustment by a 
size uniforming machine equipped with a knife cutter adapted for 
high-speed rotation, and dried to form a final product. The size 
uniforming machine is comprised of a rotating shaft equipped with 8 to 12 
double vane-like knife cutters, a cover having a hopper for charging the 
granules, and a cylindrical vertical bottomless receptacle having a height 
of 25 to 150 mm. The rotating shaft is enclosed within the cylindrical 
vertical bottomless receptacle with a slight clearance being provided 
between the tips of the knife cutters and the inner wall of the 
receptacle, and the knife cutters are rotated at a high speed of 1000 to 
4000 rpm. If the number of knife cutters is the same, the average particle 
diameter of the granules becomes smaller as the rotating speed of the 
cutters is higher and the length of the cylindrical receptacle is larger. 
For example, if the height of the cylindrical receptacle is 25 mm, the 
number of the knife cutters is eight, and the rotating speed of the knife 
cutters is 4000 rpm and the same number of knife cutters and the same 
rotating speed are used, the resulting particles have an average particle 
diameter of 610 microns. If the height of the cylindrical receptacle is 
150 mm, the resulting particles have an average particle diameter of 430 
microns. Furthermore, if the height of the cylindrical receptacle and the 
rotating speed remain the same, the average particle diameter of the 
particles tend to become smaller as the number of the knife cutters is 
increased. For example, if the rotating speed is 4000 rpm and the height 
of the cylindrical receptacle is 150 mm, the use of 12 knife cutters gives 
particles having an average particle diameter of 340 microns. 
The bulk density, hardness and speed of dissolution of the particles 
granulated by the process of this invention are mainly affected by the 
state of kneading in the granulator, and the particle size of the 
particles is determined by the conditions for operating the size adjusting 
machine. 
EXAMPLE 
A granulator of the same structure as shown in FIGS. 1 to 3 was used. 
Sodium percarbonate having a particle diameter of 50 to 100 microns and a 
water content of 8 to 10% was charged continuously at a rate of 20 kg/hr 
from the hopper 7, and a 15% aqueous solution of sodium meta-silicate as a 
binder was fed at a rate of 2 liters/hr. The rotating speed of the screws 
was adjusted to 80 rpm, and the height of the movable cover, to 6 mm. The 
sodium percarbonate was kneaded in the granulator, and passed through 
cooling water in a jacket of the granulator receptacle and a pipe within 
the screws shafts to obtain agglomerates. The resulting agglomerates were 
then subjected to size adjustment by a cylindrical size-adjusting machine 
having a length of 150 mm and equipped with eight knife cutters rotating 
at 4000 rpm. The granules were then dried to obtain a final product. The 
properties of the product are shown in Table 1. 
TABLE 1 
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Speed of dissolution 
Particle diameter distribution 
in water (*) Strength of particles (*2) 
20 mesh 
20-80 
80 mesh 
Particle 
1 2 5 32 mesh 
32-60 
60 mesh 
on mesh 
under 
diameter 
min. 
min. 
min. 
Bulk 
on mesh 
under 
(%) (%) (%) (.mu.) 
(%) 
(%) (%) 
density 
(%) (%) (%) 
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10 80 10 450 50 80 100 
0.75 
-10 +8 +2 
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(*1) The speed of dissolution in water of the particles was determined as 
follows: One liter of pure water was put in a 1liter beaker, and 5g of th 
sample was added. A stirring blade having a height of 40mm and a width of 
25mm was rotated at a speed of 250 rpm to agitate the water for a 
predetermined period of time while maintaining the water temperature at 
25.degree. C. .+-. 1.degree. C. The solution was sampled every 
predetermined time, and the concentration of hydrogen peroxide was 
measured by titration with 1/10N KMnO.sub.4. The speed of dissolution in 
water of the particles was determined from the results obtained. 
(*2) The strength of particles was determined as follows: A 32mesh screen 
and a 60mesh screen were each set on a shaking screen. One hundred grams 
of the sample was put into each of the screen, and shaken for 1 hour at 
450 rpm. The strength of the particles was expressed by a increase or 
decrease of the sample on each screen which resulted from shaking. 
COMATIVE EXAMPLE 
A vertical Henschel mixer-type granulator was used Twenty kilograms of the 
same sodium percarbonate powder as used in the Example was introduced into 
the granulator, and 2 liters of a 15% aqueous solution of sodium 
meta-silicate as a binder was added. They were kneaded for 3 minutes to 
obtain agglomerates. The agglomerates were subjected to size adjustment by 
the same size uniforming machine as used in the Example and dried to 
obtain a final product. The properties of the product are shown in Table 
2. It is seen that although the speed of dissolution is very fast, the 
particles obtained have a low bulk density and a weak strength. The 
product was therefore not entirely satisfactory. 
TABLE 2 
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Speed of dissolution 
Particle size distribution 
in water Strength of particles 
20 mesh 
20-80 
80 mesh 
Particle 
1 2 5 32 mesh 
32-60 
60 mesh 
on mesh 
under 
diameter 
min. 
min. 
min. 
Bulk 
on mesh 
under 
(%) (%) (%) (.mu.) 
(%) 
(%) (%) 
density 
(%) (%) (%) 
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5 75 20 380 70 90 -- 0.65 
-20 +10 +10 
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