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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 VIBROSCREEN 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.

Method and results

Single-deck Kason vibratory Screeners are used for the removal of insoluble solids from press liquor. Although many carbon steel machines are in operation stainless steel is preferable particularly if the process demands do not require continuous 24-hour service. Selection of screen opening size will be based upon the type of press used and general requirements of the process, but it will always be finer than 60-mesh (opening 0.0092") and screens as fine as 120-mesh (opening 0.0046") have been successfully used without any tendency to "blind". Because of the many variables it is impossible to state the exact % insoluble solids left in the press liquor. To cite one example, a plant reduces insoluble solids content from 15 to 2% solids dry basis when the Kason is equipped with an 80-mesh screen (opening 0.00701"). Moisture content of the meal will depend upon particle size distribution of the solids, oil content of the fish being handled, but an average plant figure is 25% solids and 75% liquid at the Kason discharge. The unit capacity will depend upon all of the variables cited above and rates may vary from 40 to 90 IGPM.


1. The Kason vibrating screen is preferred for this application because absence of blinding eliminates the need for operator attendance.
2. The Kason's method of mounting stainless steel screen mesh eliminates wire "fluttering" and a completely taut screen is maintained even when feed liquor runs close to the boiling point. Screen life is exceptionally good and some users have reported using one screen for as long as three years.
3. The unique three-dimensional motion produced by the Kason allows for the use of much finer screens than that possible on conventional inclined vibrating machines. The result is much higher recovery of press solids which can be of great value in many reduction plants.
4. The Kason's high capacity is obtained with the use of a 1 HP motor.


  • Type 316 stainless steel at all points of material contact preferred but carbon steel may be satisfactory in some installations.
  • Stainless steel screen cloth from 60- to 120-mesh as dictated by process requirement.
Kason Application Data No: 12