GB1576749A - Carbon black pelleter - Google Patents

Description

(54) CARBON BLACK PELLETER (71) We, PHILLIPS PETROLEUM COMPANY, a corporation organised and existing under the laws of the State of Delaware, United States of America, of Bartlesville, Oklahoma, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the pelletizing of carbon black. More specifically, the present invention relates to a carbon black pelletizer. Particularly the invention relates to the injection of pelleting fluid into a pelletizer for wet pelleting carbon black. In another aspect this invention relates to a carbon black pelleter with a pin shaft rotatably arranged in a housing.

Carbon black, which is made by the pyrolytic decomposition of hydro-carbons, is collected from a carbon black-containing smoke by filtration methods. This procedure produces carbon black in a flocculent form. In order to enhance the ease of handling of such carbon black, both in the packaging and in the ultimate use of such carbon blacks, it has been the practice to pelletize the carbon blacks.

Various kinds of pelletizers and various methods of pelletizing carbon black are known in the art. One very successfully employed pelletizer comprises a cylindrical housing and a pin shaft rotationally arranged in said housing. Carbon black and pelletizing liquid are introduced by means of openings in this housing into the space between the shaft and the housing. The pinned shaft is rotated whereby wet carbon black pellets are formed.

It has also been proposed to provide a carbon black pelleter with a hollow shaft and with open ended conduit-like pins communicating with the hollow interior of the shaft. This has been proposed for the injection of fluid material at high speeds toward the housing wall in order to prevent a buildup of carbon black cake. However, the construction of such a pelleter is difficult and the costs for such a pelleter are correspondingly high. Furthermore, the replacement of such hollow pins in fluid communication with a hollow shaft is rather cumbersome. Furthermore, the hollow pins arranged over the entire length of the shaft cannot be used for the injection of the entire quantity of the pelleting liquid because it is important for the pelleting process that the flocculent carbon black and the pelleting liquid be present in the proper relative amounts right at the beginning of the pelleting operation or in other words, at the upstream end of the pelleter.

It would, therefore, be desirable to have a carbon black pelleter available in which the pelleting liquid can be injected in a manner to achieve a fast and very efficient mixing of the dry flocculent carbon black and the injected liquid, and in which this injection and mixing can be achieved in the upstream portion of the carbon black pelleter.

In the operation of standard carbon black pelletizers for wet pelleting carbon black and a pelleting solution in a roughly 1:1 weight ratio, some problems arise. One of these problems is that it would be desirable to utilize more sturdy pins than heretofore possible. Such sturdy pins could better resist to the forces and abrasion occurring in the pelleters. A further problem in carbon black pelleters is the fact that the wet carbon black mass is subjected to a strong mechanical action from the pins at or near the internal surface of the housing of the pelleter. The carbon black mass at or near the shaft "sees" a much slower pin motion.

It is believed that it would be desirable to subject the entire wet carbon black mass to approximately the same mechanical action in the pelleter in order to produce uniform pellets.

The main problem in making pellets of carbon black particularly in such pelleting apparatus thus is the pellet size distribution, as well as the pellet shape. Ideally the produced pellets would be all spheres of the same diameter. This cannot be achieved in a commercially feasible apparatus or process.

However, it would be desirable to have a pelleter available that can produce pellets with a very uniform, i.e. quite narrow, pellet size distribution.

In accordance with the present invention a carbon black pelleter is provided having a particularly even and closeiy defined pin distribution such that the carbon black being pelleted is subjected to a smooth and constant pin action throughout the pelleting process.

In accordance with this invention a carbon black pelleter is provided which comprises a housing having a cylindrically shaped internal surface, a shaft coaxially and rotatably arranged within said housing, a plurality of pins arranged on said shaft extending radially outwardly from said shaft into close proximity with said internal surface, feed and exit apertures to the housing for feeding thereto ahd withdrawing therefrom, respectively, carbon black to be pelleted and carbon black pellets produced in said housing by agitation of the feed carbon black by said pins upon rotation of said shaft and in the presence of a pelleting liquid, and a feed line for introducing said pelleting liquid into said housing, wherein the distribution of the pins on said shaft is such that over the area of the shaft covered by said pins the pin density is substantially constant and is such that, on the development of an imaginary cylinder drawn about said shaft and having a radius of 0 5x(R1+R2), where R1 is the radius of the shaft and R2 is the radius of the housing, the radius R of the largest circle that can be drawn on said development without encompassing the trace of a pin (as hereinafter defined) is in the range 0.58r to 0.8r, where r is the minimum distance between the pin traces, and wherein no more than 3 /" of the pins are located at the same axial location and that no more than 10% of the pins at the same azimuthal location, such locations being as hereinafter defined.

In order to make the description of this invention more readily understandable, terms that will be used in this description will first be defined. These terms are mostly connected to the description of the pin distribution along a cylinder. Each mathematical point on a given cylinder can be described and is described by the axial position, e.g. in inches from the begining of the cylinder, and the azimuthal position, e.g.

in degrees starting at any reference line at 0 and counting to 3600. These two coordinates, namely the axial position and the azimuthal position, are generally sufficient to describe the important pin geometry of the present invention since the description will relate the position of the pins with respect to each other on a given cylinder so that the third of the cylindrical coordinates, namely the radial position, will generally be constant or the same for all the pins.

Whenever in the following relative positions between pins are described. these relative positions are intended to refer to the mathematical centres of these pins. Since the actual distance between two pins is smallest on the shaft and largest at the free pin ends, the pin geometry in the following is in some instances described with respect to an imaginary cylinder which is located coaxially both to the shaft and to the housing at the same distance from the shaft and from the housing. This imaginary cylinder therefore has a radius of 0.5x(R1+R2), R1 is the radius of the shaft, whereas R2 is the radius of the housing. The term 'trace of a pin' refers to the intersection of the centre axis of the pin and the imaginary cylinder.

The distance between pin centres is referred to in the following as the distance between pins and is measured on the development of the imaginary cylinder as above described, i.e. with said imaginary cylinder rolled into a plane.

The term 'same axial location' of pins refers to the fact that the centres of these pins are located within a ring on the surface of the shaft having an axial length of 1.5 pin diameters. Similarly, the term 'same azimuthal location' refers to the fact that the centres of these pins are arranged along the surface of the shaft within a longitudinal straight strip of 2 pin diameters width. An important feature of this invention consists in the fact that only very small percentages of the pins as the defined above have the same axial position (the pins are not arranged in discs) and that only very few of the pins have the same azimuthal position (the pins are not arranged in combs).

The characteristic feature of this invention is, however, the fact that the pin density, i.e. the numbers of pins per unit area, is substantially constant all over the pinned shaft and is such that the radius R of the largest circle that can be drawn on the development of said imaginary cylinder without encompassing at least one pin trace, is within the range 0.58r to 0.8r, where r is the minimum distance between the pin traces. The circle of radius R can also be defined as the maximum empty circle (i.e.

containing no pin trace centre) admissible; any larger circle will contain at least one pin trace centre. Every set of values r and R within this definition defines the minimum and maximum spacing of the pins. The radius r of a circle that can be drawn around every pin trace without encompassing another pin trace therein is preferably in the range of 1/4(R1+R2) and 1/6(R1+R2). This relationship defines the minimum pin centre distance as a function of the diameter of the imaginary cylinder defined above. The minimum pin distance preferably is 6 to 12 pin diameters. This relates the minimum pin distance to the pin diameter.

In a preferred embodiment of this pelleter, the shaft is provided with from 80 to 200 pins. These pins preferably are each situated at a different axial position and the distance between axially adjacent pins is smaller than the diameter of the pins. This axial distance between axially adjacent pins can be in the range of 0.5 to 0.9 pin diameters. It has to be emphasized that axially adjacent pins are two pins that are closest together in the axial direction only and are not necessarily those pins that are located closest together.

The presently preferred pelleter is one that has the pins arranged in such a manner that one or more of the following absolute ranges apply to the pin geometry. The pin density across the entire imaginary cylinder is in the range of 1/50 to 1/20 pins/sq. in. The radius of the imaginary cylinder is 0.4 foot to 1.5 feet. The axial length of the shaft that is provided with pins is 30 to 60 inches. The shaft diameter is 0.6 foot to 2 feet. The diameter of the internal surface of the housing is 1 to 4 feet.

The pins being attached to the shaft so that the pin density is approximately constant all over the shaft are preferably arranged in a geometrical pattern that can be characterized as a defective helix. The pins are arranged along the shaft in at least one such defective helix. The defective helix is defined negatively as compared to a normal or non-defective helix. When the pins are arranged on a normal helix, the ratio of the axial distance of two axially adjacent pins to the angular or azimuthal distance of these pins is constant. This can be expressed by the formula 360" P= . t a in which the formula p is the pitch of the helix, a is the azimuthal angular distance in degrees between adjacent pins along the helix, and t is the axial distance of two axially adjacent pins on the helix. In normal helix, the value p is a constant. In accordance with the preferred embodiment of this invention, however, the pins are arranged on a defective helix that is a line winding around the shaft in approximately a helical configuration with the important exception, however, that the pitch p as defined by the formula above is not a constant value along this line but changes several times along this defective helix. For the preferred defective helix, all axially adjacent pins have the same axial distance t between each other, and the azimuthal angular distance a, between axially adjacent pins, changes a plurality of times along the defective helix. Advantageously, this change of the aximuthal angular distance a, along the defective helix, is a periodical change. This periodical change in a particularly preferred variation is such that the azimuthal distance a, between axially adjacent pins, changes periodically between valuers along the defective helix. Thus the azimuthal angular distance a, along the defective helix, has a first value al for a first number of pins, then has a second value a2 for a second number of pins, then has a first value al for a third number of pins which is the same as the first number of pins, etc.

Preferably the difference between al and a2 is a small fraction of 360 , e.g., 1/10 to 1/20 of 360". A particular example for the change of the azimuthal distance of axially adjacent pins would be that the azimuthal distance for three consecutive pins is 90" followed by a distance of 112" 30' for one pin, which sequence thereafter is periodically repeated.

The ratio of the inner diameter D of the housing of the pelleter to the outer diameter d of the shaft to which the pins are attached in accordance with another preferred aspect of this invention is within the range of 1.3 to 2. This means that the shaft diameter is rather large and the space in which the actual pelleting occurs is an annular space -left between the shaft and the housing. This particular configuration allows the use of short- and sturdy pins and the diameter of these pins can be considerably reduced as compared to pelleters having the same throughput but having a very small diameter shaft as compared to the internal diameter of the housing. Furthermore, the carbon black mass and the carbon black pellets, as well as the pelleting liquid, are subjected to a very uniform pelleting action.

The internal diameter D of the housing in a further embodiment of the invention is related to the length L of the portion of the shaft that is provided with pins. The ratio D/L preferably is in the range of 0.5:1 to 2:1.

Furthermore, the ratio of the length of the pins to the diameter of the pins preferably is within the range of 5 to 30. This ratio most preferably has a value of 10.

The horizontally arranged carbon black pelleter is provided with a carbon black inlet at the upstream end thereof and with an outlet for the pellets at the downstream end thereof. Furthermore, the carbon black pelleter can be provided with a continuous flight at the upstream end of the shaft underneath the inlet of the carbon black such as to move the carbon black introduced into the housing towards the pelleting section or the pin section of the shaft following this continuous screw or flight in axial direction. The pitch of such a screw is generally several times larger than the average pitch of the defective helix as defined above.

The invention will be further described with reference to the accompanying drawings in which: Figs. 1 and 2 show two cross-sections through a pelleter in accordance with this invention; Fig. 3 shows a detail of the pelleter; Fig. 4 shows a diagram illustrating an example of the pin arrangement, and Fig. 5 shows another diagram to illustrate the pin arrangement on the shaft when rolled into a plane.

Referring to the drawings 'Fig. 1 shows a longitudinal cross-section through a pelleter in accordance with this invention and Fig. 2 shows a cross-section through the pelleter shown in Fig. 1 along lines 2-2. Within housing I, closed by an upstream end plate 2 and a downstream end plate 3, a shaft 4 is rotatably arranged. Several pins 5 are welded to the shaft 4. An inlet 6 for the introduction of flocculent carbon black and outlet 7 for the removal of carbon black pellets are provided for. The outlet 7 is obtained by cutting out a segment of large azimuthal extension from the circular housing. A motor 8 is provided which can rotate the pin shaft 4. At the upstream end of the pin shaft 4 and underneath the inlet 6, the pin shaft 4 is provided for with a continuous flight or screw 9.

The shaft 4 essentially consists of a hollow cylinder 41 closed on both ends by end plates 42, of which only the downstream end plate is shown in the drawing. The end plates in turn are connected to rods 43 and 44. These rods are rotatably arranged in bearings 21 and 31. Rod 43 has an axial channel 46 to permit the throughput of pelleting liquid. At the end facing into the hollow cylinder 41, the rod 43 is provided with a thin tube 48 communicating with the channel 46. This thin tube 48 in turn is connected to a first plate 52. This first plate 52 and a second plate 54 are arranged perpendicular to the longitudinal axis of the pelleter. The distance between the two plates is small and in order of 2". The two plates 52 and 54 are arranged inside of the hollow cylinder 41 in a fluid-tight manner such as to define a chamber 53 between them. The thin pipe 48 connects the channel 46 and the chamber 53. In the area between the two plates 52 and 54, four holes 55 are drilled into the cylinder 41 at locations where no pins 5 are arranged. Rod 43 and channel 46 are connected via a fluid-tight joint 62 to a source of pelleting fluid 64.

The pelleter housing 1 is supported by two supports 10 and 11.

The pins 5 which are arranged along the hollow cylinder 41 along a defective helix, as will be explained in detail later, have chisel-shaped edges at their free ends that during the rotation of the shaft 4 are moved along the housing 1 at a small distance of, e.g., 1/4" to 1/8" from said housing like knives. The individual pins 5 are welded to the cylinder 41. As mentioned several times above, the pins 5 are arranged along the shaft 4, more particularly along the outside of the hollow cylinder 41, in the pattern of a defective helix. To explain this in more detail, the pins have been numbered 1 through 109. These numbers are shown in the drawing in Figs. 1, 2 and 4. Every pin has the same axial distance from that pin that is axially adjacent to this pin on the defective helix. It has to be emphasized that this axially adjacent pin is not the closest pin.

Thus the axial distance between pins 1 and 2, between pins 2 and 3, between pins 3 and 4, etc., is always 3/8". The azimuthal angular distance between those adjacent pins along the defective helix is not the same for all the pins. As can be seen particularly from Figs. 2 and 4, the first three distances between pins 1, 2, 3 and 4 are all over 90".

However, the azimuthal distance between pin 4 and pin 5 is 112 30'. After pin 5 three pins (6, 7 and 8) follow that have 90" distance.

Then again the azimuthal distance between pin 8 and pin 9 is 112"30' as can be seen from Figs. 2 and 4. The consequence of this arrangement of pins along a defective helix can also be seen in Fig. 1. Most of the pins shown in that figure are 15 pin numbers apart. In case of pins 49 and 68, as well as 17 and 36 and 81 and 100, however, the pins are 19 pins apart. It has to be emphasized here that these pins are not axially adjacent pins in the sense defined above, since several revolutions of the defective helix are between pins 34 and 49, for instance.

Fig. 4 shows the pin location in a defective helix rolled quasi into a plane. A non-defective or normal helix would consist of a row of points connected by a straight line. In Fig. 4 the pin locations are not on a straight line but along a series of segments of a straight line. This deviation of the pin arrangement from the ideal helix is referred to as a defective helix.

In Fig. 5 the actual pattern of the pins on the shaft rolled into a plane is shown. The numbers of the pin traces are the same as those in the other figures. As can be seen from this Fig. 5, the axially adjacent pins are consecutively numbered. Thus pin 16, for instance, is axially adjacent to pin 15 and to pin 17, whereas neither pin 15 nor pin 17 is the closest pin to pin 16. The trace diagram shown in Fig. 5 is that of the imaginary cylinder that is located at the same distance from the hollow shaft 4 and from the housing 1.

During the operation of the pelleter, flocculent carbon black is introduced via inlet 6 into the annular space between shaft 4 and housing 1. The rotation of flight 9 advances this black towards the pelleting section of the shaft 4 which is provided with the pins 5. A pelleting liquid is injected into the same annular space between the shaft 4 and the housing 1 via the four openings 55 in the hollow cylinder 41 that are arranged 90" azimuthally apart and at the same axial location. The pelleting liquid can be plain water or it can be water containing such additives as HNO3, molasses lignosulfonate, or oil-water emulsion, etc. The rotation of the shaft 4 causes the mixture of carbon black and pelleting liquid to be moved to the left in Fig. 1 from the upstream location to the downstream location, and during this movement wet carbon black pellets are formed. These wet carbon black pellets are withdrawn via the outlet 7. The wet carbon black pellets are further processed by such as drying and are thereafter ready for packaging and shipment.

In the following a specific example for the dimensions of a carbon black pelleter in accordance with this invention is given: Length of the hollow cylinder 41: 6ft. lOin.

External diameter d of the shaft 4: 2ft. Oin.

Wall thickness of the hollow cylinder 41: 1/2in.

Axial length of the zone of the screw of flight 9: 2ft. 3in.

Pitch of the screw flight: 6in.

Internal diameter D of the housing 1: 3ft. 1/8in.

Diameter of the pins 5: 5/8 in.

Distance between the pin end and the housing: 1/4--1/8 in.

Pin length: 5-7/8in.

Axial distances between axially adjacent pins on shaft surface: 3/8in.

The shaft 4 with the pins 5 is rotated in this pelletizer for normal operations at about 200--450 rpm. The throughput of pelletizer of this size will be about 4000 lbs/hr. of carbon black or about 8000 Ibs./hr.

of wet carbon black pellets.

The invention will be still more fully understood from the following example.

This example is given to show the influence of the pins being arranged along a defective helix rather than on an undisturbed or perfect helix on the pellet size and size distribution.

EXAMPLE In a lab size pelleter having a shape as shown in Figs. 1, 2 and 3, flocculent carbon black and water containing 1% molasses was pelletized in a weight ratio of about 1:1. The shaft was rotated at about 400 rpm. The resulting pellets were dried and analyzed for the pellet size distribution in accordance with ASTM method 1511.

As a comparative run, the same quantity of carbon black and water was used to produce pellets in a pelleter that distinguished from the pelleter shown in Figs. 1, 2 and 3 in the fact that the pins 5 were not arranged along a defective helix (see particularly Fig. 4) but were arranged along a non-defective helix so that both the axial distance between the pins and the azimuthal angular distance between adjacent pins were the same for the entire helix. The axial distance between adjacent pins was the same as for the defective helix, namely 3/8 inch, and the azimuthal distance between adjacent pins was 90". The pellets produced in this pelleter were also dried and analyzed for their pellet size distribution in accordance with ASTM method 1511. The results of these test methods are shown in the following table.

TABLE Pin geometry Pellet Size Distribution Wt. % Wt. % (ASTM D 1511) Standard helix Defective helix On sieve 10 17.4 7.6 On sieve 18 58.6 16.4 On sieve 35 12.0 64.6 On sieve 60 4.0 4.2 On sieve 120 3.4 3.8 In pan 4.6 3.4 Total wt. 0/,, 100.0 100.0 In -18+60 range 16 68.8 The above-shown results indicate that pelleting carbon black with a pelleter having the pins arranged on a defective helix as defined resulted in 68.8 wt. /n of dry carbon black pellets in the desired -18+60 sieve range. Opposite thereto the pellets made in a pelleter having the pins arranged on a standard helix resulted in only 16 wt. /n of dry pellets within the desired sieve range mentioned above. 76 Wt. % of the carbon black produced in this pelleter with a standard helix distribution of the pins was larger than the desired sieve size range mentioned. The results also show that the pellets made by the pelleter having the defective helix distribution of the pins are smaller than pellets made in a pelleter differing from the pelleter of this invention only in having a pin geometry being a standard helix. A pellet size range of -18+60 sieve refers to pellets that pass through as 18-mesh sieve but are retained on a 60-mesh sieve. Larger sieve numbers, as usual, refer to finer openings in the sieve than smaller sieve numbers.

WHAT WE CLAIM IS: 1. A carbon black pelleter which comprises a housing having a cylindrically shaped internal surface, a shaft coaxially and rotatably arranged within said housing, a plurality of pins arranged on said shaft extending radially outwardly from said shaft into close proximity with said internal surface, feed and exit apertures to the housing for feeding thereto and withdrawing therefrom, respectively, carbon black to be pelleted and carbon black pellets produced in said housing by agitation of the feed carbon black by said pins upon rotation of said shaft and in the presence of a pelleting liquid, and a feed line for introducing said pelleting liquid into said housing, wherein the distribution of the pins on said shaft is such that over the area of the shaft covered by said pins the pin density is substantially constant and is such that, on the development of an imaginary cylinder drawn about said shaft and having a radius of 0.5x(R1+R2) where R1 is the radius of the shaft and R2 is the radius of the housing, the radius R of the largest circle that can be drawn on said development without encompassing the trace of a pin (hereinbefore defined) is in the range 0.58r to 0.8r where r is the minimum distance between the pin traces, and wherein no more than 3% of the pins are located at the same axial locations and that no more than 10% of the pins are located at the same azimuthal location, such locations being as hereinbefore defined.

2. A pelleter according to claim 1, wherein the pins are arranged over the surface of the shaft on at least one defective helix, as hereinbefore defined.

3. A pelleter according to claim 2, wherein, in said defective helix, the axial distance t, between all the axially adjacent (as hereinbefore defined) pins is constant and the azimuthal angular distance, a, between axially adjacent pins, changes a plurality of times along the defective helix.

4. A pelleter according to claim 3, wherein said change is a periodic change.

5. A pelleter according to claim 4, wherein the azimuthal angular distance a between axially adjacent pins is constant for a first consecutive number of axially adjacent pins and is constant for a second consecutive number of axially adjacent pins, the two constants being different, said numbers and constants repeating sequentially along the length of the defective helix.

6. A pelleter according to claim 5, wherein said first consecutive number of pins is four, spaced angularly from each other at 900, and said second consecutive number is two spaced angularly from each other at 112.50.

7. A pelleter according to any one of the preceding claims, wherein the axial distance between the centres of adjacent pins is less than their respective diameters.

8. A pelleter according to claim 7, wherein said axial distance is from 0.5 to 0.9 times the pin diameter.

9. A pelleter according to any one of the preceding claims, wherein the ratio of the diameter, D of said cylindrical surface to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. TABLE Pin geometry Pellet Size Distribution Wt. % Wt. % (ASTM D 1511) Standard helix Defective helix On sieve 10 17.4 7.6 On sieve 18 58.6 16.4 On sieve 35 12.0 64.6 On sieve 60 4.0 4.2 On sieve 120 3.4 3.8 In pan 4.6 3.4 Total wt. 0/,, 100.0 100.0 In -18+60 range 16 68.8 The above-shown results indicate that pelleting carbon black with a pelleter having the pins arranged on a defective helix as defined resulted in 68.8 wt. /n of dry carbon black pellets in the desired -18+60 sieve range. Opposite thereto the pellets made in a pelleter having the pins arranged on a standard helix resulted in only 16 wt. /n of dry pellets within the desired sieve range mentioned above. 76 Wt. % of the carbon black produced in this pelleter with a standard helix distribution of the pins was larger than the desired sieve size range mentioned. The results also show that the pellets made by the pelleter having the defective helix distribution of the pins are smaller than pellets made in a pelleter differing from the pelleter of this invention only in having a pin geometry being a standard helix. A pellet size range of -18+60 sieve refers to pellets that pass through as 18-mesh sieve but are retained on a 60-mesh sieve. Larger sieve numbers, as usual, refer to finer openings in the sieve than smaller sieve numbers. WHAT WE CLAIM IS:

1. A carbon black pelleter which comprises a housing having a cylindrically shaped internal surface, a shaft coaxially and rotatably arranged within said housing, a plurality of pins arranged on said shaft extending radially outwardly from said shaft into close proximity with said internal surface, feed and exit apertures to the housing for feeding thereto and withdrawing therefrom, respectively, carbon black to be pelleted and carbon black pellets produced in said housing by agitation of the feed carbon black by said pins upon rotation of said shaft and in the presence of a pelleting liquid, and a feed line for introducing said pelleting liquid into said housing, wherein the distribution of the pins on said shaft is such that over the area of the shaft covered by said pins the pin density is substantially constant and is such that, on the development of an imaginary cylinder drawn about said shaft and having a radius of 0.5x(R1+R2) where R1 is the radius of the shaft and R2 is the radius of the housing, the radius R of the largest circle that can be drawn on said development without encompassing the trace of a pin (hereinbefore defined) is in the range 0.58r to 0.8r where r is the minimum distance between the pin traces, and wherein no more than 3% of the pins are located at the same axial locations and that no more than 10% of the pins are located at the same azimuthal location, such locations being as hereinbefore defined.

2. A pelleter according to claim 1, wherein the pins are arranged over the surface of the shaft on at least one defective helix, as hereinbefore defined.

3. A pelleter according to claim 2, wherein, in said defective helix, the axial distance t, between all the axially adjacent (as hereinbefore defined) pins is constant and the azimuthal angular distance, a, between axially adjacent pins, changes a plurality of times along the defective helix.

4. A pelleter according to claim 3, wherein said change is a periodic change.

5. A pelleter according to claim 4, wherein the azimuthal angular distance a between axially adjacent pins is constant for a first consecutive number of axially adjacent pins and is constant for a second consecutive number of axially adjacent pins, the two constants being different, said numbers and constants repeating sequentially along the length of the defective helix.

6. A pelleter according to claim 5, wherein said first consecutive number of pins is four, spaced angularly from each other at 900, and said second consecutive number is two spaced angularly from each other at 112.50.

7. A pelleter according to any one of the preceding claims, wherein the axial distance between the centres of adjacent pins is less than their respective diameters.

8. A pelleter according to claim 7, wherein said axial distance is from 0.5 to 0.9 times the pin diameter.

9. A pelleter according to any one of the preceding claims, wherein the ratio of the diameter, D of said cylindrical surface to

the external diameter, d, of the shaft is from 1.3:1 to 2:1.

10. A pelleter according to any one of the preceding claims, wherein the ratio of the diameter D, of said cylindrical surface to the axial length L of that part of the shaft which carries said pins from 0.5:1 to 2:1.

Il. A pelleter according to any one of the preceding claims, wherein the pins are arranged on said shaft in a distribution defined by the traces of the pins on an imaginary cylinder coaxial with said internal surface and having a radius equal to 0.5x(R1+R2) where R2 is the radius of said internal surface and R1 is the external radius of the shaft such that, when said imaginary cylinder is developed into a plane the radius, r, of the largest circle which can be drawn about each pin trace without encompassing another pin trace is from l/4x(R1+R2) to 1/6 x(R1+R2) where R1 and R2 are as above defined.

12. A pelleter according to any one of the preceding claims, wherein the pins are arranged over said shaft at a density of from 1/50 to 1/20 pin per square inch.

13. A pelleter according to any one of the preceding claims, wherein the shaft is hollow and has a plurality of openings therein spaced around the shaft intermediate its ends at a location which is free from said pins, and wherein the feed line for said pelleting liquid is located within the shaft in communication with said openings for the feeding of the pelleting liquid into the space between the shaft and the housing.

14. A pelleter according to claim 13, wherein the interior of the shaft is provided with two parallel plates extending thereacross and located on opposite sides of said openings, thereby to form within said shaft a fluid tight chamber having said openings in the peripheral wall thereof, and wherein the feed line for the pelleting liquid is connected to feed said liquid into the chamber.

15. A pelleter according to claim 13 or 14, wherein the feed and exit apertures are located in said housing at opposite ends thereof and spaced along the length of the shaft, and wherein the upstream end of the shaft adjacent the feed aperture is provided with a helical flight operable to convey carbon black to be pelleted from the feed aperture to the exit aperture, the pins on said shaft being confined to that portion of the shaft which is downstream of said flight and downstream of said openings therein for the pelleting liquid.

16. A pelleter according to claim 15, wherein the ratio of the diameter D of the cylindrical surface of the housing to the axial length L of that part of the shaft which carries said pins in the range 0.7:1 to 1:1.

17. A pelleter according to claim 1, substantially as hereinbefore described with reference to the accompanying drawings.