Process for melamine purification

Abstract

A process for the purification of a melamine melt (5) containing melamine and by-products, comprising the steps of: (a1) In quenching of said melamine melt; (a2) decomposition of by-products with alkali, providing an alkaline aqueous solution of melamine (26); (b) stripping of said alkaline aqueous solution of melamine (26); (c) crystallization of melamine with a first alkali-containing solution (6b) and separation of solid melamine (7) from a mother liquor (8); (d) treatment of said mother liquor, providing a waste water stream (11) containing carbonates; (e) decomposition of at least part of the carbonates contained in said waste water stream (11) into carbon dioxide and alkali, providing a second alkali-containing aqueous solution (30); (f) recycle of at least part of said alkali-containing aqueous solution (30) to at least one of said steps (a1), (a2) and (c).

Claims

1. A method of revamping of a melamine purification section of a high-pressure melamine plant comprising: a quencher, receiving a melamine melt containing melamine and by-products; a decomposer, wherein at least part of said by-products are decomposed by means of alkali, providing an alkaline aqueous solution of melamine; a stripper fed with said alkaline aqueous solution of melamine and with a stripping medium, providing a stripped melamine solution; a crystallizer fed with said stripped melamine solution and a first alkali-containing aqueous solution, wherein melamine is separated from a melamine mother liquor; a treatment unit for at least part of said melamine mother liquor, providing an aqueous solution containing carbonates; the method being characterized in that: a decomposition unit is installed downstream of said treatment unit in order to at least partially decompose the carbonates contained in said aqueous solution into carbon dioxide and alkali, providing a second alkali-containing aqueous solution and a water stream; one or more flow lines are installed in order to at least partially recycle said second alkali-containing aqueous solution to at least one of the quencher, the decomposer and the crystallizer.

2. The method according to claim 1, said plant comprising a quencher of the melamine melt operating with water or with an aqueous solution of ammonia, and the method comprising the replacement of said quencher with a quencher operating with an aqueous solution of soluble hydroxides of the alkali metals.

3. The method according to claim 1, wherein said decomposition unit is an electrolytic cell.

4. The method according to claim 1, wherein one or more flow lines are installed in order to at least partially recycle said water stream to said melamine plant.

5. The method according to claim 2, wherein said soluble hydroxides are of sodium or potassium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified scheme of a melamine purification process according to the invention.

(2) FIGS. 2 to 5 are block schemes of melamine purification processes according to various embodiments of the invention.

(3) FIG. 6 is a schematic representation of the operation of an alkaline electrolytic cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) FIG. 1 shows a simplified block scheme of a melamine purification section 1.

(5) Said section 1 comprises a first block 2 essentially including a quencher, a decomposer, a stripper and a crystallizer; a treatment unit 3 and an electrolytic cell 4.

(6) Said block 2 receives a first input stream 5 and a second input stream 6. Said first stream 5 is a melamine melt produced from the high pressure synthesis section (not shown) of a melamine plant and said second stream 6 is an aqueous solution of sodium hydroxide. A portion 30 of said second input stream 6 is provided by the electrolytic cell 4.

(7) As shown in the example of FIG. 2, said second stream 6 consists of an input stream 6a to the quencher and an input stream 6b to the crystallizer. More in particular, a portion 30a of the stream 6a and a portion 30b of the stream 6b are provided by the electrolytic cell 4.

(8) Said block 2 provides solid melamine 7 and a mother liquor 8 containing for example poly-condensates, OATs and NaOH.

(9) Said solid melamine 7 is collected and exported from the purification section and said mother liquor 8 is at least partially subjected to a suitable treatment in the treatment unit 3. Said unit 3 is also fed with an ammonia solution 9, which is preferably provided by the stripper contained in the first block 2, and provides an ammonium carbonate stream 10 and an aqueous solution 11 containing sodium carbonates and bicarbonates.

(10) Said stream 10 containing ammonium carbonate is exported and said solution 11 is fed to the electrolytic cell 4. Said electrolytic cell 4 also receives a stream of fresh water 12 and provides an aqueous solution of sodium hydroxide 30 and a desalinated water stream 13. Said solution 30 is recycled back to the first block 2 and said desalinated water stream 13 is at least partially discharged from the purification section 1.

(11) FIG. 2 shows in greater detail the process of FIG. 1.

(12) According to FIG. 2, the melamine purification section 1 essentially comprises a quencher 21, a decomposer 22, a stripper 23 and a crystallizer 24 (which are part of the block 2 of FIG. 1), the treatment unit 3 and the electrolytic cell 4.

(13) The above mentioned melamine melt 5 is fed to the quencher 21 together with the aqueous solution 6a of sodium hydroxide. Said melamine melt 5 contains melamine, unconverted urea, dissolved ammonia and a number of by-products. Said by-products essentially comprise OATs and poly-condensates.

(14) Inside the quencher 21, the melamine, the unconverted urea and the by-products are dissolved, thus providing a first alkaline aqueous solution of melamine 25 containing by-products.

(15) Said solution 25 is subsequently sent to the decomposer 22, wherein the poly-condensates are at least partially hydrolysed into melamine and OATs thanks to the presence of alkali (NaOH) in said solution 25, thus providing a second alkaline aqueous solution of melamine 26.

(16) Said solution 26 is introduced into the stripper 23, wherein ammonia is stripped out thus providing an aqueous solution of ammonia 27 and a stripped solution 28. Steam 29 is generally used as stripping medium.

(17) According to the example of the figure, said aqueous solution of ammonia 27 is split into two portions; a first portion 27a is sent to the treatment unit 3 and a second portion 27b is exported from the purification section 1.

(18) The stripped solution 28 is further purified by e.g. filtration and clarification with activated carbon and subsequently subjected to crystallization within the crystallizer 24, wherein melamine crystals 7 are separated from the melamine mother liquor 8. Said crystallizer 24 is also fed with an aqueous solution 6b containing sodium hydroxide. According to the example of the figure, said solution 6b is formed by a portion 30b of the alkali-containing solution provided from the electrolytic cell 4 and by a portion 6c of make-up. According to even preferred embodiments, said solution 6b is entirely formed by the portion 30b of the alkali-containing solution from the cell 4, while the portion 6c of make-up is not needed.

(19) A first portion 8a of said mother liquor is mixed with an aqueous solution 30a of sodium hydroxide provided from the electrolytic cell 4 to form the input stream 6a to the quencher 21, as better explained below.

(20) A second portion 8b of said mother liquor is subjected to a high temperature and high pressure treatment in the unit 3 wherein by-products contained in the mother liquor 8 are hydrolysed into CO2 and NH3, providing the sodium (bi)carbonates-containing aqueous solution 11 and the ammonium carbonate solution 10. More in detail, a part of the so obtained CO2 reacts in the unit 3 with the sodium hydroxide contained in the liquor 8 to form sodium carbonates (Na.sub.2CO.sub.3) and sodium bicarbonates (NaHCO.sub.3). Part of the CO2 also reacts with NH3 to form ammonium carbonate ((NH.sub.4).sub.2CO.sub.3). Said NH3 is both provided by the by-product hydrolysis and by the portion 27a of the aqueous solution of ammonia.

(21) The sodium (bi)carbonates-containing aqueous solution 11 is then provided to the alkaline electrolytic cell 4, which is further fed with a stream of fresh water 12.

(22) Within said cell 4, sodium carbonates and bicarbonates contained in the solution 11 are decomposed into CO2 and NaOH to form the aqueous solution of sodium hydroxide 30 and the desalinated water stream 13.

(23) Said solution 30 is partly recycled back to the quencher 21 and partly to the crystallizer 24, while said desalinated water stream 13 is at least partially discharged from the purification section 1.

(24) FIG. 2 illustrates an embodiment wherein the solution 30 is split into two portions 30a and 30b: the portion 30a is directly recycled to the quencher 21 upon mixing with the first portion 8a of the mother liquor thus forming the input stream 6a, and the portion 30b is directly recycled to the crystallizer 24 upon mixing with an optional alkali solution 6c of make-up thus forming the input stream 6b. Further embodiments are illustrated in the FIGS. 3 to 5.

(25) FIG. 3 illustrates an embodiment wherein the sodium hydroxide solution 30 is concentrated in a proper concentration unit 31, which provides a concentrated solution 32. Said solution 32 is split into a first portion 32a and a second portion 32b; the first portion 32a is mixed with the portion 8a of the mother liquor and recycled to the quencher 21 and the second portion 32b is mixed with an optional alkali solution 6c of make-up and recycled to the crystallizer 24. Said concentration unit 31 releases a steam flow 33 which can be recycled to a plant steam system.

(26) FIG. 4 illustrates an embodiment wherein the desalinated water stream 13 leaving the electrolytic cell 4 is split into three portions: a first portion 13a is recycled back to the quencher 21, a second portion 13b is joint with fresh water 12 to provide stream 13d which is recycled to the electrolytic cell 4, a third portion 13c is discharged.

(27) FIG. 5 illustrates an embodiment which is a combination of FIGS. 3 and 4, wherein the sodium hydroxide solution 30 is concentrated in the unit 31 before being recycled to the quencher 21 and the crystallizer 24 and the desalinated water stream 13 is split into three portions 13a, 13b, 13c as in the process of FIG. 4.

(28) FIG. 6 schematically shows an alkaline electrolytic cell 4, which comprises an anodic compartment 41 and a cathodic compartment 42. Said compartments are separated by a diaphragm 43.

(29) The aqueous solution 11 containing sodium carbonates and bicarbonates is fed to the anodic compartment 41 of the cell 4, while the stream of fresh water 12 is fed to the cathodic compartment 42. The global reactions taking place within the cell 4 are the following:
Na.sub.2CO.sub.3(aq)+2H.sub.2O(I).fwdarw.2NaOH(aq)+CO.sub.2(g)+½O.sub.2(g)+H.sub.2(g)
2NaHCO.sub.3(aq)+2H.sub.2O(I).fwdarw.2NaOH(aq)+2CO.sub.2(g)+O.sub.2(g)+2H.sub.2(g)

(30) Oxygen and carbon dioxide are released from the anodic compartment 41 as stream 34; cations (i.e. H.sub.3O.sup.+ and Na.sup.+) migrate to the cathodic compartment 42 forming sodium hydroxide and hydrogen, the former being separated as stream 30 and the latter being extracted as stream 35.

(31) As a result, the aqueous solution of sodium hydroxide 30 is provided by said cathodic compartment 42, while the desalinated water stream 13 is provided by said anodic compartments 41.