0.100 - 0.200mm Cuts with Sieve Bends

The pilot scale performances of a 0.150mm aperture conventional and a 0.150mm aperture pressure sieve bend were assessed.   The slimes contamination level of the pressure sieve bend product was around ½ of that of the conventional unit.  This, however, was achieved at reduced feed rates but required no spray or dilution water.   To be comparable, a conventional sieve bend in repulp configuration would be needed, ie two units in series with sprays and intermediate primary oversize solids repulping.   Simplified calculations indicate the sieve bend requirements to process the same 200m³/h throughput of a 380mm classifying cyclone cutting at around 0.100mm are:

· 1 off 4.7m wide conventional sieve bend unit;

· 2 off 4.7m wide pressure sieve bend units using 21m³/h less water than conventional unit.

To achieve lower slimes contamination levels, the sieve bend requirements for this would be:

· 2 off 4.7m wide conventional sieve bend units in repulp configuration;

· 2 off 4.7m wide pressure sieve bend units using 60m³/h less water than conventional unit.

The conventional sieve bend tests indicated:

· Cut points in the range 0.092 - 0.165mm were achieved;

· Without vibration or spray water, the cut point (S₅₀) was around 0.100mm, and did not appear affected by reducing the flow rate by a factor of 2;

· For both flow rates tested, use of the vibration unit at 30Hz had only a small effect on both the cut point (S₅₀) and slimes carryover (C) values;

· The use of both vibration (30 Hz) and spray water (4.5m³/h) increased the cut point (S₅₀) to around the aperture (0.150mm) and reduced slimes carryover (C) values significantly.  These results may be explained in terms of the removal of slimes (material less than 0.25% of aperture, ie minus 0.038mm);

· Without vibration and spray water addition, cut points around 67% of the aperture were obtained.  With the use of vibration and spray water, cut points around 85 - 100% of aperture were obtained;

· Generally, sharper separations and higher slimes carryover (C) values were obtained with higher feed flows.

For the pressure sieve bend tests:

· The pressure sieve bend unit was tested at feed % solids (8.0 - 9.5%) that tended to be lower than the conventional sieve bend (10.8-16.5%);

· Cut points (S₅₀) in the very narrow range 0.132 - 0.148mm were achieved.  This is again within the target range (0.100 - 0.200mm);

· Cut points (S₅₀) were in the range 88% - 99% of the aperture (0.150mm);

· For flow rates at 120% and 100% of nominal manufacturer's recommendation, very sharp (high α values) separations were obtained.  For the 80% flow rate setting, an α value more akin to conventional sieve bends was obtained;

· Lower slimes carryover (C) values were obtained than with the conventional sieve bends;

· On a manufacturer's recommendation basis, the feed rate to the pressure sieve bend is around 40% of that for a conventional sieve bend of equivalent area;

· To achieve separations with similar slimes carryover (C) values as a pressure sieve bend, simulations indicated that two conventional sieve bends in a repulp configuration would be required.  A mass balance indicated that the pressure sieve bend unit would require a reduced spray and repulp water requirement of around 2m³/h per tph dry sieve bend feed solids.

Further test work is recommended to:

· Validate the simulations of the conventional repulp configuration;

· Determine the performance of the pressure sieve bend unit at much higher feed rates;

· Address the general lack of understanding of how sieve bends work and how to improve them, particularly their operational variations and unit capacities.