Recycled granules

Figure 1 represents, in a schematic way, a continuous operation of plastics processing, such as injection moulding, incorporating a recycling/granulation step. 

Multiple re-use of recycled granules makes for a continuous presence of an increasing amount of degraded molecules, and means that visible defects may occur much earlier in moulded parts, e.g., brittleness or emission of gases during processing. 

Polyurethane is pulverized to iacrease its bulk density, mixed with 30—80% of a thermoplastic mol ding material, gelled, and then granulated to give coated urethane foam particles 0.1 to 0.15 mm in size (48). The particle bulk density is three times that of the polyurethane, while the volume is 15% less. This material may be injection molded or extmsion molded into products (49). Other technologies for recycling polyurethanes have also been reported. 

The urea produced is normally either prilled or granulated. In some countries there is a market for Hquid urea—ammonium nitrate solutions (32% N). In this case, a partial-recycle stripping process is the best and cheapest system. The unconverted NH coming from the stripped urea solution and the reactor off-gas is neutralized with nitric acid. The ammonium nitrate solution formed and the urea solution from the stripper bottom are mixed, resulting in a 32—35 wt % solution. This system drastically reduces investment costs as evaporation, finishing (priQ or granulation), and wastewater treatment are not required. 

Pan granulation of ammonium nitrate also has been developed commercially (16,17). Essentially water-free melt (<0.5%moisture) is sprayed onto a cascading bed of fines (usually cmshed recycle) ia a tilted, rotatiag pan. Production of 18—20 t/h is common. Product made ia this manner is said to require ao conditioning. The pan is an excellent size classifier, making it easy to produce granules of any desired size from 1 to 11 mm diameter. 

The turbo-tray dryer can handle materials from thick slurries [1 million (N s)/m (100,000 cP) and over] to fine powders. It is not suitable for fibrous materials which mat or for doughy or tacky materials. Thin slurries can often be handled by recycle of dry product. Filter-press cakes are granulated before feeding. Thixotropic materials are red directly from a rotary filter by scoring the cake as it leaves the drum. Pastes can be extruded onto the top shelf and subjected to a hot blast of air to make them firm and free-ffowing after one revolution. 

Significant changes in the moisture content in the drum due to recycle fluctuations (recycle of diy granules in fertilizer granulation) [Zhang et al., Control of Pai ticulate Processes TV (1995)]. 

When fines are recycled, as in iron ore, sinter feed, or fertilizer drum granulation, they are rapidly granulated and removed from the... 

Continuous fluid-bed granulators are used in the fertilizer and detergent industries. For fertilizer apphcations, near-size grannies are recycled to control the granule size distribution. Dust is not recycled directly, but first remelted or slurried in the liquid feed. 

From the third reactor the polymer is then run into a devolatilising ( stripping ) vessel in the form of thin strands. At a temperature of 225°C the solvent, residual monomer and some very low molecular weight polymers are removed, condensed and recycled. The polymer is then fed to extruder units, extruded as filaments, granulated, lubricated and stored to await dispatch. 

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