More particularly, this invention concerns a simple and unexpensive process to separately recover melamine and some important by-products deriving from the melamine synthesis reaction or its subsequent treatments, as well as from solutions or dispersions containing melamine and said by-products formed in the course of melamine purification cycle. In fact, it is known that in almost all melamine processes starting from urea, both catalytic or without catalyst, the raw reaction product is dissolved in water, then submitted to one or more purification steps, finally the product having the requested purity level, is separated from the solution by crystallisation.
During the melamine water solution treatment, temperature is kept at levels higher than 130° C., (preferably 160 to 170° C.) in order to take advantage of the higher melamine solubility and, consequently, minimising the water volume to be handled.
Higher temperatures are not desired because of more water steam entraining in the effluent—so-called off-gas essentially consisting of NH3 and CO2, more equipment corrosions, as well as more hydrolysis melamine losses. The resulting product, consisting of high purity degree melamine (>99.8% grade), is eventually separated from the solution by cooling and subsequent crystallisation at a temperature of 40-50° C. In these conditions most of melamine precipitates in crystalline form and is separated by filtration or centrifugation or any suitable conventional means. Aqueous solution, after high purity melamine separation contains, besides melamine, Oxyaminotriazines (OAT) which include both amelide and ameline having the following formulas: OAT besides being melamine synthesis reaction intermediate products, are also formed in the aqueous solution because of melamine hydrolysis. Therefore OAT are always present in the aqueous solution from which melamine is recovered.
OAT are products much less water soluble than melamine, but its water solubility increases by some orders of magnitude when pH increases, whilst melamine solubility remains practically unchanged within the pH range of 7 to 14.
Such a different behaviour makes possible the crystallisation of melamine alone when a melamine and OAT-containing solution at a pH higher than 7, preferably higher than 11, is cooled.
The high pH value is obtained by adding to the solution an alkaline compound such as ammonia or sodium hydroxide.
The crystallisation mother liquor recovered after the precipitation and separation of melamine contains therefore in a solution the entire starting amount of OAT plus the residual melamine corresponding to the melamine solubility at the crystallisation conditions.
For instance, by operating the crystallisation at 40° C. and a pH higher than 11, the resulting mother liquor contains the following amounts of melamine and OAT:
melamine0.7/1wt %OAT0.3-0.5wt %.In order to obtain a high melamine yield from the manufacturing process it is necessary to recycle to the process the entire amount of such mother liquor or a portion thereof. However before recycling the mother liquor it is necessary to remove OAT, otherwise OAT accumulate in the recycling solution up to the saturation point and then precipitate together with melamine polluting same.
To maintain the cycle equilibrium it is necessary to remove from the mother liquor the same amount of OAT formed both in the reaction zone and in the aqueous solution by melamine hydrolysis.
The above operation is accomplished by acidifying the mother liquor, obtained after melamine crystals removal, with a suitable acid, to the pH value of 7. Typically the acidification agent is CO2 in order to avoid the introduction of any foreign substance in the process fluids.
At pH 7 OAT are practically insoluble in water and therefore they precipitate completely; on the contrary, melamine, due to the fact that its solubility, within the range of pH 7 to 14, is independent on pH, does not precipitates. Therefore, once the OAT are separated, the mother liquor can be partially or totally recycled to the process obtaining the partial or total recovery of the melamine content.
The separation of precipitated OAT is difficult to be carried out due to the colloidal nature of OAT precipitate.
As a matter of fact, OAT suspended in the acidified mother liquor, are hardly separated by settling, even if the operation is carried out in a high velocity centrifugal equipment. Also filtration operations are very difficult and in practice can not be used in that the filtration cake colloidal nature is such that filtration surface is rapidly blocked. The only system allowing the separation is the filtration assisted by filtration aids like, for instance infusorial earth (dicalite or similar products). Also in this case, however, filtration is not completely satisfactory and the filter life cycle (including filtration aid loading—filtration—cake removal—washing) typically does not operate for more than 4 hours. In addition OAT are obtained as a mixture with a foreign material (the filtration aid) that makes difficult their separation.
Then it results that the only industrially applicable operation to separate OAT (i.e. filtration in presence of a filtration aid) is very expensive in that:                it requires high investment costs because of operating complexity as well as the required high filtering surface;        it requires much manpower to carry out the filtration cycle which is repeated every 4 hours;        it consumes a big amount of filtration aid (5 to 30 wt % based on the separated OAT)        it allows only a partial melamine recovery, because being an imperfect filtration, it is necessary to purge a substantial amount of filtrate to avoid the OAT accumulation in the water cycle,        it recovers an OAT/filtration aid mixed panel, which is practically useless.        
Because of the above problems, processes have been studied and operated implying the total decomposition to CO2 and NH3 of the organic products contained in the mother liquor. Said processes, besides of decomposing OAT, they destroy also the melamine (that may be profitably recovered) and consume a significant amount of energy and require large investment costs.
This invention intends to obviate to all the abovementioned drawbacks by means of a process that allows to recover from the melamine crystallisation mother liquor both the residual dissolved melamine and the OAT.