Increase in fines content and adhesion behavior in the pneumatic conveying of CB.

This is the first of a two-part series. Part two will appear in the August issue.

In the production of rubber compounds in the mixing room, carbon blacks are predominantly used as active fillers. Since their proportion in a finished rubber blend is on average about 30%, large amounts of carbon black have to be conveyed every day. An important requirement from the point of view of the rubber processor is therefore fast and trouble-free conveying of the carbon black, in order to ensure constant and efficient incorporation into the blend.

Carbon blacks are generally stored in silos on the processor's premises and are conveyed by means of pneumatic conveyors into day containers or directly into a weighing hopper above the mixing units. These pneumatic conveyors are tailored to the specific requirements of the fillers to be transported in the rubber mixing room. Compared with alternative transport systems, the pneumatic ones have the following advantages (ref. 1):

* Very substantial freedom from dust pollution;

* gentle product handling;

* minimum maintenance work;

* conveying substantially without residue;

* high operational safety owing to few moving machine parts;

* superiority of planning through free choice of conveying routes; and

* compactness.

The conveyor systems most commonly used today are the so-called dilute phase and dense phase systems. The major differences result from the air velocities and solid-to-air ratio. The older system of dilute phase conveying operates at high air velocities of 20-40 m/s and solid-to-air ratios of about five kg of carbon black per kg of air. The more modern system of dense phase conveying is characterized by low air velocities of 2-10 m/s and very high solid-to-air ratios of about 20 kg carbon black per kg of air. According to the diagram by Heep and Winkhardt (ref. 2) of state for the transport of disperse bulk materials in pipelines, dilute phase conveying operates in an aerodynamic state and dense phase systems operate in a state of plug flow. The latter state is feasible in the case of carbon black conveying only through the use of bypass pipes which introduce secondary air into the conveyor pipe at specific intervals as a function of pressure, so that blockage of the system is prevented. The "plugs" are divided up and "moved on."

A major problem is the blockage behavior in conveyor pipes. Adhesion to the pipe walls can cause problems here. Furthermore, parts of the adhering material may become detached from the wall and lead to weighing errors and, depending on the compression state, to undispersed carbon black in the rubber compound and to contamination between different raw materials. In addition, the air velocity increases owing to the declining free cross-section of the pipeline.

For simpler handling and better incorporation, the carbon black itself is, as a rule, delivered in the form of pellets. It is easy to imagine that, owing to the mechanical stress in the conveyor pipe, the pellets are destroyed to a greater or lesser extent. So-called fines (ref. 1) form. An important carbon black parameter, which leads to different fines depending on the conveying process, is the strength of the pellets. High amounts of fines may lead to problems in the mixing process. On the one hand, the metering times in the mixers increase owing to the increasing amount of fines, causing the cost-efficiency to suffer, and, on the other hand, the mixing process itself may also be influenced. The latter may occur in the form of dispersion problems through the compaction of the fine material and/or fluctuations in the degree of filling which arise from the longer metering time in conjunction with suction effects (aspiration effect) (ref. 1). Furthermore, recent, so far unpublished research results show that the incorporation of the carbon black into the polymer is influenced as a function of the fines, of the viscosity and of the degree of branching of the polymer.

Problems during the transport of the carbon black in pneumatic conveyor systems occur at high fine contents, in the form of conveying instabilities. The fine material favors the formation of excessively large plugs, which results in a temporary pressure increase in the pipe. The pressure continues to increase until the plug is broken up again, resulting in acceleration of the plug. These effects are evident from the pressure fluctuations in the pipe system. A further problem, which may occur in pipelines, is the adhesion of carbon black to the pipe material. This causes the pipe to become blocked in the course of time. The pipe cross-section decreases and the air velocity increases. Consequently, all above mentioned effects are enhanced. High fine contents furthermore promote the segregation by particle size in storage silos and intermediate containers. This may also cause operating faults. Any effects on the adhesion behavior due to the carbon black property may be associated with the carbon black parameters of specific surface area and structure as well as the pipeline material used. The extent to which the individual sizes have any effect is unclear to date.

Such problems are being investigated in the Mini Derucom research project supported by the European Commission and forming part of the Brite Euram III program. Buhler and Degussa-Huls were integrated into the project as further partners in order, among other things, to be able to examine the specific question of the increase in the fines content and the adhesion behavior of carbon black in pneumatic conveyor systems. In the investigations described below, the effect of conveying various carbon black types was investigated. The increase in the fines content and the adhesion behavior were examined as a function of the conveyor pipe material and the conveyor condition.

Experimental materials

Carbon blacks are produced by incomplete combustion or by thermal cleavage of hydrocarbons under strictly defined conditions on the industrial scale (ref. 3). A general classification of the industrial carbon blacks, which is based on the reinforcing effect of rubber articles, is as follows:

* Hard carbon blacks are highly reinforcing, finely divided carbon blacks with a primary particle diameter of 10-30 nm, which are used in particularly highly stressed rubber articles, e.g., tire treads.

* Soft carbon blacks are less highly reinforcing industrial carbon blacks with mean primary particle diameters of 40-60 nm. These are used in applications in which the heat build-up plays a decisive role, for example in tire carcasses and extruded articles, such as tubes, in which the dimensional stability and strength are important.

* Inactive carbon black has scarcely any reinforcing properties. The primary particle diameter is larger than 60 nm. However, these carbon black types permit high degrees of filling and play a very major role in determining the processing behavior and the performance of extruded articles.

The most important parameters for characterizing carbon black are the specific surface area and the carbon black structure. The specific surface area is associated with the primary particle diameter, i.e., the smaller the particle diameters, the greater is the specific surface area. The carbon black structure, on the other hand, indicates the number of primary particles contained in an aggregate and their degree of branching.

During the process for the formation of carbon blacks, a first phase involves the formation of highly viscous droplets or solid particles, which build up through coalescence of three-dimensional branched clusters which are referred to as aggregates. These are the smallest, stable and independent units of the finished carbon black. Highly structured carbon blacks exhibit strong concatenation or branching, whereas this aggregation is not very pronounced in carbon blacks having low structure. The aggregates in turn combine to form loose agglomerates, which are bound by means of weak Van der Waals interactions. The void between the aggregates and agglomerates, usually measured as the volume of dibutylphthalate (DBP) absorbed by a predetermined amount of carbon black, describes the terminus "structure" of carbon blacks.

The specific surface area of an industrial carbon black arises primarily from the particle geometry. A widely used method for determining the specific surface area is the adsorption of cetyl-trimethyl-ammonium-bromide (CTAB). The CTAB adsorption most closely resembles the determination of the geometrical surface area, i.e., the surface area without pores, which correlates with the surface area available to the rubber. The CTAB adsorption therefore permits conclusions about the performance characteristics of the carbon black in a rubber compound.

Figure 1 shows, on the left, a single carbon black aggregate under a scanning electron microscope. A highly structured carbon black with a high degree of branching can be seen in the center, and a low-structure carbon black with a low degree of branching in the right micrograph.

[ILLUSTRATION OMITTED]

Carbon blacks are available as powder or pellets. In the case of the pelleted carbon blacks, a distinction is made in mm between dry-, wet- and oil-beaded carbon blacks. Pelleted carbon blacks have substantial advantages over the powder carbon blacks from the ecological and economic points of view and in terms of performance characteristics:

* Little dust pollution;

* good flow behavior;

* good silo storage properties and meterability;

* high bulk density;

* little space required for storage and transport;

* lower transport and storage costs; and

* faster incorporation and wetting.

Since predominantly wet-beaded carbon blacks are used in the rubber industry, this study is also based only on such carbon blacks. In the case of pelleted carbon blacks, the pellet hardness plays a decisive role. The pellet hardness must, on the one hand, be sufficiently high for the granules not to be destroyed during conveying, transportation and silo storage, but, on the other hand, must be as low as possible in order to avoid adversely affecting the dispersibility. Methods for determining the total pellet hardness, the individual pellet hardness, the fines and the pellet abrasion are available for characterizing the pellets.

It is to be assumed that the specific surface area and the structure constitute decisive carbon black parameters in the adhesion behavior in conveyor pipes. The choice of carbon blacks used for the investigation was therefore made in each case in such a way that one of the stated parameters was kept as constant as possible.

The term fines in the conveying process is understood below to mean all particles smaller than 125 [micro]m.

Experimental setup

The tests were carried out at Buhler. A flexibly adjustable, pneumatic conveyor system with the following structure was available there: Two pipelines installed in parallel and with the same geometry make it possible to perform experiments with both a steel pipe and conveying pipe and with a pipe provided by a rubber inliner. The different conveying states can be set up in both systems. The corresponding conveyor pipe is controllable by means of a diverter. The length of the conveyor pipeline is 67 m and the diameter of the pipe is 100 mm at the entrance and 110 mm at the exit. The material must flow through ten horizontal bends and a vertical one with an angle of 90 [degrees] in each case. Figure 2 shows the experimental installation.

[ILLUSTRATION OMITTED]

Two pipelines laid parallel to one another are shown; the steel pipe and the pipe with the rubber inliner. A bypass pipe, which is connected to the conveying pipe via a nonreturn valve at intervals of about 1 m, is laid for each conveying pipe. This installation is necessary for performing dense phase conveying tests, in order to feed air from the outside into the conveying pipe, this air ensuring, as described above, plug flow of the carbon black to be conveyed.

The pressure is built up by means of a compressor. The air flow rate is set by means of a system of Laval nozzles which, individually or in groups, generate a constant air mass flow which, in a wide range, is independent of the back-pressure in the system and hence of the product mass flow. With such a system, specific conveying states, such as dense phase or dilute phase conveying, can be set up (ref. 4). Owing to the arrangement of receiving container and sending vessel, it is possible with this system to convey the same batch of material again after each circulation.

Test procedure

Test plan

At the beginning of each test, the pneumatic conveyor had to be filled with about 500 kg of carbon black, which corresponds to about a 1 [m.sup.3] flexible intermediate bulk container (FIBC). The weight of the FIBC depends on the bulk density or the carbon black type. The carbon black passed from the FIBC via the receiving container through the action of gravity into the sending vessel. Each time the carbon black was released from the receiving container into the sending vessel, samples were taken to document the change in the fines starting from the unconveyed state and after each circulation. In addition, it is also possible at this point to fill the material back into FIBCs through a branch. Each carbon black type was conveyed from the sending vessel through the pipe back into the receiving container. This process was repeated between four and 10 times per carbon black type, pipe material and conveyor state set up. Depending on the resulting fines and on the adhesion state, conveying became unstable and was therefore terminated. Table 2 shows the partially factorial test plan.

Table 2 - test plan for determining fines and adhesion behavior

Carbon   Dense        Dilute       Dense phase      Dilute phase
black    phase        phase        conveying,       conveying,
type     conveying,   conveying,   rubber inliner   rubber inliner
         steel pipe   steel pipe

N 772    Test 1                                     Test 5
N 650    Test 2       Test 3                        Test 6
N 234                 Test 4       Test 8           Test 7

First, tests 1 and 2 have been carried out in the dense phase conveying state. The conveying pipe consists of steel. In the next test configuration for 3 and 4, conveying was changed to dilute phase conveying and the pipeline material was retained in order to minimize the cost of the change of the conveying state. For tests 5, 6 and 7, the conveying pipe was changed from steel to rubber. The dilute phase conveying state has been retained. Test 8 once again required a change of the method of conveying from dilute phase to dense phase conveying. Owing to the relatively high cost of a test, it was not possible to implement all theoretically possible settings in table 2. Consequently, the tests in which knowledge from experiments already performed indicated that there were unlikely to be any strong adhesions or high levels of fines formation were omitted. Furthermore, tests where major difficulties are known to occur, for example in the case of dilute phase conveying in the steel pipe, were not carried out.

To check for any adhesion in the pipeline, the 90 [degrees] bend at the corner furthest away from the sending vessel was unscrewed after 3, 5 and 10 rounds, and the layer thickness on the inner surface was photographed.

The pipeline was cleaned before the respective next test by means of corn grits.

Sampling of the carbon black to be tested

Particle size determination by sieving is one of the oldest and most widely used test methods for fine-particled and coarse-particled bulk materials. The increase in the fines content in carbon black conveying can be determined by this method of mechanical process engineering, and hence the increase after each circulation can be recorded.

The disadvantage of this method is frequently inaccurate sampling since samples can generally be taken only at one point, and therefore a representative cross-section of the population is not necessarily obtained. The difficulty in the case of these samples was to obtain a cross section as good as possible from the large amount of sample from an FIBC. Sampling from a material stream has, in principle, some advantages. However, it must be ensured that the total cross-section of the stream is included. The sampling intervals should therefore be chosen at the same time interval and at the same point over the total material stream (ref. 5).

For determining the fines in the tests described here, a sample was taken after a circulation by pneumatic conveying when releasing the carbon black downwards between receiving container and sending vessel, with .the aid of an apparatus. This apparatus consists of a flap, which can be adjusted by means of a lever so that the carbon black is directed via a diverter into a sample vessel. If this lever is operated at constant time intervals during the circulation process, so that the sample container is uniformly filled, a representative cross-section of the population is obtained.

Test parameters

The conveying parameters, such as the air velocity and the solid-to-air ratio, have been kept constant during the tests. This should ensure exact comparability of the tests with one another and rule out further influences of the parameters which are not varied. Figure 3 shows the air velocity in the empty pipe (empty pipe velocity) for all tests performed, as a function of four conveyor circulations.

[GRAPH OMITTED]

The set empty pipe velocity is about 25 m/s in dilute phase conveying compared with 7 m/s in dense phase conveying. The solid-to-air ratio as a function of four conveyed circulations is shown in figure 4.

[GRAPH OMITTED]

Figure 4 shows the solid-to-air ratios in the various tests. In the case of dense phase conveying, the solid-to-air ratio varies within certain limits owing to the pressure-dependent secondary air feed. In the case of dilute phase conveying, the solid-to-air ratio is virtually constant in all tests, owing to the low ratio between carbon black and air.

Table 1 - analytical data of the carbon black types

Carbon      CTAB          DBP         Mean     Fines (as       Bulk
black    adsorption    absorption    pellet    delivered)    density
 type    ([m.sup.2]/     (ml/g)     hardness      (%)          (kg/
             g)                       (g)                   [m.sup.3])

N 772        33            65          31          4.4         530
N 650        36           122          17          1.9         360
N 234        119          125          18          3.2         325

References

(1.) Freitag, K-.H.: "Economic aspects in the pneumatic conveying of carbon black," Schuttgut (Bulk material), 1, 1995, pp. 115-117.

(2.) Heep D., Winkhardt G. and Nijman G: "Use and optimization of a dense-stream carbon black conveying system in a tire factory," Schuttgut (Bulk material), 1, 1995, pp. 103-114.

(3.) "What is carbon black," Information publication from Degussa-Huls AG.

(4.) Hauser, G., "Pneumatic conveying in the Department for Mechanical Engineering and Apparatus Technology in Weihenstephan," Schuttgut (Bulk material), 1, 1995, pp. 145-147.

(5.) Nowak, R.H.T., "Quality assurance of bulk materials," Schuttgut (Bulk material), 1, 1995, pp. 135-138.

(6.) Rumpf, H., "Principles and methods of agglomeration," Chemie-Ingenieur-Technik (Chemistry-Engineering-Technology) No. 3/1958, pp. 144-158.

H. Keuter and A. Limper, University of Paderborn; A. Wehmeier and T. Riedemann, Degussa-Huls; and K.-H. Freitag, Buhler