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Purpose of classification

The purpose of classification is twofold:

  • - to hinder the apparition of fines;
  • - to eliminate existing fines.

The separate decanting of clarified liquor automatically leads to an increase in the crystal content of the magma. The crystal surface available for growth is increased and will absorb the liquor’s supersaturation to the detriment of both nucleation and the apparition of fines. The solid-phase content should preferably reach 20% in volume but should not exceed this value in order to avoid too much attrition in the draft tube.

The decanting of clarified liquor also includes the undesirable fines, which can then be:

  • - dissolved by heating the liquor;
  • - separated in a centrifugal screw decanter with horizontal axis. The sludge obtained is reinjected into the supply. However, this solution is expensive in terms of investment.

Theory of CHC without attrition

If we assume that the residence time is identical for both the crystals and the solution, this implies that the slurry decanting must be performed in an isokinetic manner (the crystals move at the same velocity as the liquor). Therefore, the balance equation is:

For dimensions greater than 100 nm and below 2 mm, growth often depends less on the crystals’ size. Therefore, the balance equation is easily integrated and we obtain:

This result can be obtained in the laboratory in a crystallizer of several liters. However, according to Mersmann [MER 01], some precautions are indispensable in order to avoid attrition:

  • 0.5 W/kg of suspension is the upper limit of the agitation power density =
  • ? .

“density” (mass concentration) mT < 50kg.m-3

Volume fraction of crystals: 9T < 0.02

Residence time < 5,000 s

The origin ordinate e and the slope of the straight line Ln(n) = f(L) provide no and G. We may deduce:

NT = J noexp (-GiTjdL = noGT (number of crystals per cubic meter

of suspension)

The nucleation rate J per unit of time and per cubic meter of suspension is:

Nyvlt attempted to measure J by direct observation of the apparition of the first seeds, but this particularly dubious method was abandoned.

Note that, in the absence of attrition, the expression of J according to the derivatives taken for L = 0 is universal, irrespective of the n variation law as a function of L.

 
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