Primary tailings

The mass flowrate of tail gas is known from the overall mass balance, but the tail-gas preheater balance can only be performed if the inlet temperature of the tail gas is known, This must be calculated from the specified design conditions for the absorber, and from the heat exchange at the primary tail-gas warmer. Tail gas leaves the absorber at 10°C (design specification) and it passes through the primary tail-gas warmer where it exchanges heat with the secondary air (for the product-acid bleaching column). The air enters at 232°C and leaves at 80°C. Assuming the tail gas has approximately the same heat capacity as air (a realistic assumption), then ... 

The second exchange in the primary tail-gas warmer is with boiler-feed water. This water enters at 103°C and leaves at 96°C. 

The coarse rapidly settling solids in the primary separation vessel are kept in motion by mechanical rakes at the bottom of the vessel and are drawn off from the bottom as a slurry (primary tailings). The tailings from the primary and secondary flotation processes are combined and transported to a tailings pond. Some of the supernatant water from the pond can be recycled into the process the remaining fine solids and water undergo a slow consolidation into sludge. 

Oversized particles rejected by the classifier (the primary tailings) consist of varying proportions of unbumt limestone, hydrated quicklime and gritty hydrate. They are generally processed in one of three ways. 

Weak Acid. Stainless steels (SS) have exceUent corrosion resistance to weak nitric acid and are the primary materials of constmction for a weak acid process. Low carbon stainless steels are preferred because of their resistance to corrosion at weld points. However, higher grade materials of constmction are required for certain sections of the weak acid process. These are limited to high temperature areas around the gau2e (ca 900°G) and to places in which contact with hot Hquid nitric acid is likely to be experienced (the cooler condenser and tail gas preheater). 

The reactions of the radicals (whether primary, secondary, solvent-derived, etc.) with monomer may not be entirely regio- or ehemoseleetive. Reactions, such as head addition, abstraction or aromatic substitution, often compete with tail... 

NMR methods can be applied to give quantitative determination of initiator-derived and other end groups and provide a wealth of information on the polymerization process. They provide a chemical probe of the detailed initiation mechanism and a greater understanding of polymer properties. The main advantage of NMR methods over alternative techniques for initiator residue detection is that NMR signals (in particular nC NMR) are extremely sensitive to the structural environment of the initiator residue. This means that functionality formed by tail addition, head addition, transfer to initiator or primary radical termination, and various initiator-derived byproducts can be distinguished. 

---