| 摘要 |
1 The process to recover both melamine and oxyaminotriazines (OAT) from the mother liquor of melamine crystallisation comprising the following steps: a) addition to a melamine and OAT containing aqueous solution at high temperature of an alkaline agent until the solution pH reaches a value equal or higher than 11; b) crystallisation of the most part of melamine by cooling the solution resulting from step a); c) separation and recovery of the precipitated melamine; d) acidification of the resulting solution (mother liquor), after precipitated melamine separation, in order to reach a pH of about 7 in consequence of which practically all the dissolved OAT are precipitated; e) the suspension obtained in d) above, is subject to filtration according to tangential filtration technique obtaining a clear permeate which contains as a solution all the melamine not precipitated in step b) and a retentate comprising of OAT dispersion; f) OAT deprived mother liquor from e) (the permeate) is recycled to raw melamine solution preparation step; in such a way almost the total amount of melamine, not precipitated in step b), is recovered; g) OAT are recovered from step e) retentate by using any conventional separation technique. 2 The process according to claim 1, characterised in that the alkaline agent solution treatment according to stage a) takes place at a temperature higher than 130 degree C. and preferably in the range of 160 to 180 degree C. 3 The process according to claim 1, characterised in that in order to crystallise melamine according to step b) the solution is cooled down to a temperature in the range of 40 to 50 degree C. 4 The process according to claims 1 to 3, characterised in that the pH of the stage b) has value in the range of 6.5 to 7.5. 5 The process according to claims 1 to 4, characterised in that the surface filtering material useful for tangential filtration is selected among the material resisting to pressure and chemical agents. 6 The process according to claims 1 to 5, characterised in that the surface filtering material useful for tangential filtration is selected among alumina, silico-alumina, zirconia, titania, boria, zeolites, thoria and mixture thereof. 7 The process according to claim 6, characterised in that the surface filtering material useful for tangential filtration according step e) consists of high purity alumina. 8 The process according to claims 1 to 6, characterised in that the surface filtering material useful for tangential filtration exhibits a porosity (i.e. the average diameter of passing through meates) lower than 5 micron. 9 The process according to claim 8, characterised in that the surface filtering material useful for tangential filtration exibits a porosity lower than 0.2 micrometres. 10 The process according to claim 8, characterised in that the surface filtering material useful for tangential filtration exibits a porosity in the range of 30 to 100 nanometres.
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