Continuous separation of fines from fibers in a wedge-shaped vessel

The continuous removal of fines from a pulp suspension can be achieved by a particle elutriation process using, for example, a wedge-shaped vessel with an inclined tube at one vertical end wall that serves as an inlet for the suspension. Near the other vertical end, the suspension flow is split into a bottom stream containing the recovered pulp and a top stream containing the separated fines. The effect of operating conditions on both the hydrodynamic behavior and the separation efficiency was investigated. There are two critical limits for the operating flow rates of the feed suspension: below a minimum flow rate, fibers settle near the bottom exit and above a maximum flow rate, fibers escape in the top stream. These limiting flow rates do not show a strong dependence on operating conditions, except for a maximum inlet consistency above which the pulp bed expands into the outlet section, thus preventing fractionation. The particle concentration in the top stream was measured with a spectrophotometer, from which the separation efficiency was determined as a function of the inlet and outlet flow rates and the inlet consistency. The separation efficiency increases with increasing split ratio (i.e., the ratio of top to bottom streams) and with decreasing consistency of the feed suspension. Fiber loss in the top stream is about 1% of the fibers entering the vessel.