Clarification of Press Liquor in Fish Reduction Plants

In conventional fish reduction plants, meal, oil and soluble solids are recovered from whole fish or trimmings by a process of cooking and mechanical pressing, followed by a series of separation steps. Press cake is fed directly to rotary dryers to produce fish meal, but separated liquor from the press may contain up to 20% insoluble solids. The industry uses two methods of separating solids prior to further processing of liquor or sending it to waste. Horizontal centrifuges have found wide application but in recent years Kason Separators have been used because of low initial cost and minimum maintenance requirement. Selection will depend upon the needs of each process since it is known that a centrifuge will produce a product with less water thus improving steam economy at the dryer and will in some instances yield higher recovery of solids. This may be particularly important if the plant operates principally on non-oily fish and cannot justify the installation of an evaporation plant to recover soluble solids. When oily fish is processed the partially clarified press liquor is centrifuged in a 3-phase machine that separately discharges oil, stick water and fine insoluble solids that have passed the first stage centrifuge or screen. Centrifuges used at this stage employ nozzles for solids discharge with an opening of approximately 60 mesh and preliminary clarification serves a double function by removing solids larger than this size which could cause blockage problems.