Pin-Type Mixers

Polyurethane foam Natural oil polyol, water, and surfactants were weighed into a 1 quart metal cup and premixed for 15 seconds at 1800 rpm using a pin type mixer. The catalyst was added and the mixture stirred an additional 15 seconds at 1800 rpm. The polyisocyanate was then added and the mixture stirred at 2400 rpm for 3 seconds and immediately transferred to a 15 x 15 x 10 wooden box lined with a polyethylene bag. The buns were allowed to cure overnight before testing. 

B.3 Turbulent mixers (Lddige- and Eirich-type mixers, pin mixers)... 

Flight width and flight clearance were 13.5 and 0.12 mm, respectively, in all sections of the screw. The extruder had a smooth-bore feed section. Three rings of pin mixers were evenly positioned in the middle portion of the metering section of the screw. All pins in a single ring had the same axial position as the type shown by Fig. 8.25. The specific rotational flow rate for the metering section was calculated at 5.0 kg/(h rpm). The compression ratio was 2.7. 

Although the full history of this screw is unknown, the mixing pins appeared to have been added to the screw after the screw failed to mix and homogenize the extrudate. Pin mixers have been retrofitted to screws for many years as a way to improve the distributive mixing for an extruder. Here the pin mixers would distribute the solid polymer fragments evenly in the extrudate, hut the mixers lacked the ability to trap and disperse them. Pin mixers were clearly the wrong type of mixer to use for this application. Instead, a dispersive mixer such as a Maddock-style mixer would be preferred see Section 8.5.1. 

As an extension to the above study, several types of dispersive and distributive mixing elements (26) were evaluated using a single-screw extruder (27) compared with the Maddock mixing element used in the previous study. Elements evaluated included dispersive elements such as straight and tapered Maddock, pineapple and Twente mixers, and distributive elements such as pin and gear... 

Further improvements in the quality of mixing obtained can be brought about by modifying the screw or by the use of special mixing sections, these usually being located in the metering section of the screw. Typical examples of such sections are pins, barriers, Barr type mixing sections and cavity transfer mixers. 

In outline, reprocessing was based on disassembly of the fuel assemblies to individual pins, single pin cropping, dissolution in nitric acid and operation of a Purex-type flowsheet in a chemical separation plant equipped with pulsed mixer settlers. The modified dissolver cell and solvent extraction plant were recommissioned late in 1979 and were tested in reprocessing of the final batches of DFR fuel. The new head-end facility, to disassemble the PFR fuel assemblies and to crop the pins, and the new waste treatment facilities were commissioned in 1980 reprocessing of the first batches of PFR fuel began later that year. 

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