Agglomerates, mixing time

Studies on the kinetics of carbon black dispersion in various rubbers have been reported using a Brabender mixer fitted with cam-type rotors [110]. Dispersion rating, determined by visual inspection of photomicrographs, was found to depend strongly on mixing time. For an SBR emulsion, it was observed that there was an initial delay period where the carbon black agglomerates were thought to be fractured and incorporated into the rubber. Subsequently, the process of dispersion continued for a considerable time thereafter. 

Time. An example of the breakup of agglomerates of solid oxidizer as a function of mixing time is given in Figure 4. 

Figure 4. Effect of mixing time on breakup of agglomerates...

The dispersion of agglomerates is strongly dependent on mixing time, rotor speed, temperature, and rotor blade geometry [3], Figure 3.28 [6,4] shows the fraction of undispersed carbon black as a function of time in a Banbury mixer at 77 rpm and 100°C. The broken line in the figure represents the fraction of particles smaller than 500 nm. 

We note that the volume fraction of fluid with agglomerates of this size begins with zero and passes through a maximum at a mixing time r/r = 1 and then drops exponentially. Similarly, all fractions at k > 1 will exhibit the same type of behavior, but with the maxima at increasingly longer mixing times. 

The quality requirements of dispersive mixing generally require that the fraction of agglomerate above a critical size be below a certain set value. For carbon black dispersion in rubber, for example, generally the requirement is that the fraction of agglomerates above 10 pm be less than 1%. In terms of distribution function the mixing time needed to meet such a... 

From the preceding equations, we can also calculate the discreet agglomerate size distribution at any mixing time. The total initial number of agglomerates is... 

Considering the agglomerate size to be on the order of the Kolmogoroff scale (or some other appropriate length scale), the micro-mixing time for molecular diffusion is given by... 

The results of the electrical resistivity and the agglomerate area ratio as a function of SME for polyamide 6 composites filled with 5 wt% Nanocyl NC7000 prepared under different melt mixing conditions are summarized in Figure 7.8. To vary the SME, different rotation speeds and mixing times were used. The lowest volume resistivity value was found at 2000 Q cm for the composites mixed at 50 rpm and 15 min corresponding to a SME of 0.5 kW h kg ... 

Solution preparation is a complex problem and not very well understood. Recent experiments by Chang and Darby [1] and Oliver and Bakhtiyarov [2] indicate that the drag reduction effectiveness of partially hydrolyzed polyacrylamides increases with mixing time. At Arizona State University we have obtained similar results especially when the solvent is deionized water. On the other hand freshly mixed poly (ethylene oxide) (PEO) solutions give the most drag reduction. Impurities and ions present in v/ater also affect the polymers. In addition agglomerates of molecules may be present in the solution or can be formed by the flow. These solution difficulties are present in both aqueous and nonaqueous solutions. 

Retention time for an industrial drum agglomerator has been reported to be 1 to 4 minutes [12 13]. As such, time was fixed at 3 minutes for the first set of experiments when moisture and acid strength were being investigated. Then to examine the effect of time, it was varied in the range of 3 to 6 minutes. Due to the experimental setup used, shorter mixing times were not obtainable in a safe and reproducible manner. 

The effects of moisture content, acid concentration, drum rotation speed and mixing time on the agglomerate size distribution of the nickel laterite were studied. In the agglomerate size distribution figures, the legends have the form of A B C. A represents the moisture content in wt while B represents the acid solution concentration (g/L). Finally, C represents special conditions for example NC is for drum critical speeds and min is for mixing times in minutes. 

The effects of moisture content and acid concentration on the size of the nickel laterite agglomerates were studied first. Mixing time and drum rotation speed were keep constant at... 

Since agglomeration is often used to control the amount of fines, mixing time had minimal effect with the wt% -1.7 mm by <1% for all times. This indicates that shorter mixing times for this ore may be effective for agglomeration. A short mixing time would result in a shorter agglomeration drum in practice and reduce the necessary capital. 

Figure 8. ASDs of agglomerates produced with different mixing times at 20.4% moisture, 102...

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