GB835659A - Crystallisation apparatus and method of operating the same - Google Patents

Abstract

Apparatus for obtaining uniform granular crystals from saturated solutions comprises an evaporating vessel for the partial evaporation in vacuo of the solvent, the vessel having: a conical bottom; a vapour outlet adapted for connection to vacuum means; a bottom outlet for crystals; an internal funnel-shaped attachment extending downwardly into the conical bottom with its top edge located above the intended liquid level in the vessel and not <PICT:0835659/III/1> touching the vessel wall; a system for circulating the liquor from closely below the intended liquid level to a location above the mouth of the funnel so that the solution can flow thereinto; means for injecting air or other gases into the circulating system to raise the solution in the manner of an air-lift pump, and means for introducing the solution to be treated into the apparatus. The preheated solution is preferably introduced into the circulating system in a proportion sufficient to raise the temperature of the circulating liquor by 1 DEG or 2 DEG C. In the apparatus shown air introduced through inlet 4 causes circulation of the liquor to be crystallized in evaporator 1. Vacuum exerted at 10 concentrates and cools the liquor discharged at 12 into funnel 5 to produce a supersaturated solution which passes through cylindrical extension 6 at the bottom of which it meets partially formed crystals and deposits its excess solute thereon. The larger crystals sink and are removed through outlet 8 whilst smaller crystals are maintained in suspension by the circulating liquor. Fresh warm liquor is supplied through inlet 15 to restore the temperature of the liquor circulating through pipe 2 which is returned to the funnel 5 at 12. Excess liquor is removed through line 16. The amount of air, injected through valve 14, required to effect circulation is kept to a minimum by making the ratio H : h lie between 10 : 1 and 20 : 1. The air is preferably warmed to the saturation temperature of the circulating liquor before admission. In the preferred mode of operation the temperature drop in the evaporator is only 1 DEG or 2 DEG C. In a modification a number of evaporators are arranged in series, the discharge pipe 16 of one evaporator being connected with the inlet pipe 15 of the next. In operating this embodiment it is preferred that the temperature drop between the first and last evaporators is 4 DEG to 7 DEG C. and the rate of transfer of liquor sufficient for the incoming liquor to raise the temperature of the circulating liquor by 1 DEG or 2 DEG C.