EP0113474B1 - Regenerator for carburizing salt baths and process for making it - Google Patents

Description

The invention relates to regeneration agents for salt baths for carburizing iron and steel parts in the form of polymeric organic compounds and a process for their production.

Salt baths for carburizing iron and steel parts in hardening technology generally consist of a mixture of alkali cyanide as an effective carburizing substance, barium chloride as a carrier melt and alkali carbonate. They are operated at a temperature of 800 to 950 ° C. At this temperature, carbon preferably diffuses into the surface of the workpieces suspended in the melt for about 1 to 5 hours. Subsequent quenching of the carburized workpieces results in a high hardness and high wear resistance. During the operation of the salt bath, however, there is a gradual oxidation of the cyanide by atmospheric oxygen to carbonate, which is ineffective for the carburizing process.

Because the baths become inactive as a result, the original bath composition had to be restored from time to time by adding cyanide or salt mixtures containing cyanide. For each regeneration, part of the salt had to be removed from the bath and discarded as highly toxic old salt. However, this mode of operation has the disadvantage that highly toxic waste salts have to be disposed of and that toxic cyanide has to be stored as a regenerating agent. It has therefore already been proposed (DE-C-2 310 815) to use polymeric triazine compounds, polymeric hydrocyanic acid and polymeric carboxamides (DE-C-2 409 285) for the regeneration of salt baths for the nitriding of workpieces. The advantage of these additives is that they are non-toxic. In principle, they can also be used for carbonic salt baths. When these compounds are added, the carbonate in the salt bath crucible, which is formed by oxidation and is ineffective for carbonization, is converted back into carbon-active cyanide. With this mode of operation, there is no need to exhaust waste salts and store cyanide.

In practice, however, the abovementioned regeneration agents are predominantly used only for the regeneration of nitriding salt baths at temperatures around 580 ° C. When these substances are used to regenerate carbonic acid salt baths at 800 to 950 ° C, there are a number of disadvantages which have hitherto prevented their use in technology.

For example, when using melon or polymeric urea as a regeneration agent in carbonation baths at 800 to 950 ° C, only small amounts of the carbonizing cyanide arise, but large amounts of cyanate, which only partially and slowly breaks down into cyanide and is undesirable in carbonating salt baths because it has a negative effect on carbonization influences and leads to edge oxidation on the steel. In addition, due to the decomposition of the cyanate at the high temperatures of the carbonation baths, the salt bath is foamed to a great extent, which can lead to the melt overflowing. Furthermore, the implementation of these regenerants with the melt is extremely violent. Melon has the gross formula [C ₆ H ₃ N ₉ ] _x , as for example from J. appl. Chem. 9, June 1959, pages 340-344.

When using the known polymeric hydrocyanic acid (azulmic acid), the above difficulties occur to a lesser extent, but other problems arise for this. In addition to cyanide, azulmic acid forms considerable amounts of carbon in the reaction with carbonate, which creates a dense bath cover that makes regeneration difficult. The cyanide yield is therefore unsatisfactory. In addition, the production of the polymeric hydrocyanic acid requires extensive safety measures and a large outlay on equipment, since the highly toxic hydrogen cyanide must be assumed.

It was therefore an object of the present invention to find regeneration agents for salt baths for carburizing iron and steel parts in the form of polymeric organic compounds which bring about a virtually complete conversion of carbonate to cyanide, do not cause the salt bath to foam, do not form any carbon residues and are safe to produce .

This object was achieved in that the polymeric organic compounds have a gross composition [C ₆ H _" Ny] _z , where x = 3-5, y = 5-8 and z = 10 to 10,000, and by reacting about 6 moles of formaldehyde with 3 moles of dicyandiamide or 6 moles of cyanamide or 2 moles of melamine or corresponding mixtures of these compounds at 300 to 600 ° C. and subsequent pyrolysis treatment of the resinous condensation products at the same temperature Form included.

Compounds of the composition [C _a H ₃ N _{7] z 'are} preferably used, where z = 10 to 10,000. Polymers with z between 100 and 1,000 have proven particularly useful. However, the determination of z is very difficult. It is therefore possible that compounds with other z values can also be used well.

These polymeric compounds react with the carbonate in the carbonation salt bath in a calm reaction and with very good yield to cyanide, with neither carbon residues nor disruptive cyanate components.

The polymeric regeneration agents according to the invention thus enable problem-free regeneration of carburizing salt baths. The good carbonation effect of these baths is not affected by the regeneration.

In the preparation of the regeneration agents, the formaldehyde is preferably used in the form of solid paraformaldehyde and the pyrolysis is carried out in a nitrogen atmosphere.

The gross composition of the regenerant thus obtained can be changed by varying the pyrolysis conditions (temperature and pyrolysis time) and by varying the mixing ratio of the starting materials. The reaction and pyrolysis are preferably carried out at 400 to 500 °, for a period of 10 to 60 minutes. The addition of the formaldehyde can be varied in the range from 5 to 7 moles.

The reaction of 2 moles of paraformaldehyde with 1 mole of dicyandiamide at a pyrolysis temperature of 400 ° C. and a pyrolysis time of 30 minutes has proven to be particularly suitable. In this process, the resinous-sticky white dicyandiamide-formaldehyde condensation products known per se form firstly with elimination of water (see R. Wegler and H. Herlinger in Houben-Weyl, Volume 14/2, Macromolecular Substances II, p. 382 ff, Stgt . 1963), which react further at the pyrolysis temperatures with elimination of water vapor, ammonia, urotropin and small amounts of hydrocyanic acid to give a black polymer of the composition [C ₆ H ₃ N ₇ ] _z , z = 100-1000.

This polymer is a deep black substance which is practically insoluble in the usual solvents. It has no optically recognizable melting point and decomposes slowly and exothermically in air at temperatures above 610 ° C.

The material properties do not allow a clear structure determination with the usual analytical methods. However, the gross composition and the material properties mentioned, in particular the insolubility and the black color, and the elimination of ammonia, urotropin, water and traces of hydrocyanic acid during the preparation, show that the compounds according to the invention must have a completely different structure than the white ones which occur as intermediates and water-soluble formaldehyde resins.

The following table shows the advantages of the regeneration agent according to the invention compared to the substances melon and polymeric hydrocyanic acid used in nitrating salt baths. To this end, the same weight amounts of regenerating agent are added to commercially available carbonic salt baths which have been in operation for a long time and which still contained 9.0% cyanide and 0% cyanate. The increase in cyanide and cyanate values was measured 5 minutes after the regeneration agent was added.

The marginal oxidation that can be observed metallographically is directly related to the cyanate content of the bath.

The following examples are intended to describe in more detail the preparation of the regenerants according to the invention.

• 1. Man vermischt 4,2 kg Dicyandiamid mit 3,0 kg Paraformaldehyd möglichst innig. Dieses Gemisch wird in einem eisernen Tiegel unter Stickstoffbeschleierung langsam auf 400 °C erhitzt. Es tritt Reaktion unter Abspaltung von Wasserdampf ein. Im Wasserdampf sind geringe Mengen Ammoniak, Urotropin und Blausäure enthalten. Die zunächst weiße, harzartige, aufgeblähte Reaktionsmasse verfärbt sich mit steigender Temperatur zunehmend dunkler und wird bei Erreichen von 400 °C schwarz, spröde und fest. Sie wird mechanisch im Tiegel zerstoßen und noch 20 Minuten bei 450 °C getempert. Dieses Material besitzt die Formel [C₆H_x,N_y]_z, x=3-5, y = 5-8, z = 10-10 000. Bei der Reaktion werden 3,6 kg Wasserdampf frei. Die Ausbeute beträgt- 60 % der Theorie.
• 2. Technisch erfolgt die Herstellung kontinuierlich in einem mit Stickstoff gespülten elektrisch auf 400-450 °C beheizten Reaktorrohr. Das Gemisch von Dicyandiamid und Paraformaldehyd im MolVerhältnis 1 : 2 wird automatisch über eine Schleuse in den Reaktor eingetragen. Es reagiert dort nach Erreichen der angegebenen Temperatur unter Abspaltung von 2 Mol Wasserdampf und geringen Mengen NH₃, HCN und Urotropin zu dem Polymerisat [C₆H₃N₇]_z, z = 10-10 000. Dieses Produkt wird durch eine bewegte Welle im Reaktor gebrochen und über eine zweite Schleuse pulverförmig ausgetragen.

Smaller amounts of the regeneration agent according to the invention can be produced as follows:

• 1. Mix 4.2 kg of dicyandiamide with 3.0 kg of paraformaldehyde as intimately as possible. This mixture is slowly heated to 400 ° C in an iron crucible with nitrogen blanketing. A reaction occurs with the elimination of water vapor. Small amounts of ammonia, urotropin and hydrocyanic acid are contained in the water vapor. The initially white, resin-like, inflated reaction mass turns increasingly darker with increasing temperature and becomes black, brittle and solid when 400 ° C is reached. It is mechanically crushed in the crucible and annealed at 450 ° C for a further 20 minutes. This material has the formula [C ₆ H _{x, Ny} ] _z , x = 3-5, y = 5-8, z = 10-10,000. The reaction releases 3.6 kg of water vapor. The yield is 60% of theory.
• 2. Technically, the production takes place continuously in a nitrogen-purged reactor tube heated electrically to 400-450 ° C. The mixture of dicyandiamide and paraformaldehyde in a molar ratio of 1: 2 is automatically introduced into the reactor via a lock. After the specified temperature has been reached, 2 moles of water vapor and small amounts of NH ₃ , HCN and urotropin are reacted to give the polymer [C ₆ H ₃ N ₇ ] _z , z = 10-10,000. This product is caused by a moving wave broken in the reactor and discharged in powder form via a second lock.

The reaction gas is passed through dust chambers to separate entrained solid particles and then into a torch.

The analysis of the compounds produced by the above methods is very difficult, in particular in the determination of z. The values of x vary between 3 and 5 and the values of y between 5 and 8, depending on the manufacturing conditions. The values of z have to be estimated more or less.

• In einem Tiegelofen werden 100 kg eines Kohlungssalzes der Zusammensetzung 40 % BaCl₂, 50 % Na₂C0₃ und 10 % NaCN aufgeschmolzen und auf.930 °C erhitzt. Infolge der Aufkohlung der eingebrachten Chargen und durch Luftoxidation sinkt der Cyanidgehalt dieses Bades stündlich und kontinuierlich um etwa 0,15 %. Bei normaler Betriebsweise würde dieses Bad nach 24 Stunden nur noch 6,4 % NaCN enthalten und keine optimalen Kohlungsergebnisse mehr liefern.

The following example shows the use of the regeneration agent according to the invention:

• 100 kg of a carbonic salt of the composition 40% BaCl ₂ , 50% Na ₂ CO ₃ and 10% NaCN are melted in a crucible furnace and heated to 930 ° C. As a result of the carburization of the introduced batches and due to air oxidation, the cyanide content of this bath drops continuously and continuously by approximately 0.15%. Under normal operating conditions, this bath would only contain 6.4% NaCN after 24 hours and would no longer provide optimal coaling results.

This drop in the cyanide content is prevented by adding 150 g / h of the regenerating agent according to the invention, since the carbonate formed during the oxidation of NaCN is converted back to carbon-active NaCN by the regenerating agent. In this way, the optimal cyanide content of 10% NaCN can be constantly maintained.

Claims (6)

1. Regeneration agents for cyanide-containing salt baths for the carburization of iron and steel members in the form of polymeric organic compounds, characterised in that the polymeric organic compounds have an empirical formula [C₆H_xN_y]_z, wherein x = 3-5, y = ₅-8 and z = 10-10 000, and are produced by reaction of about 6 mol of formaldehyde with 3 mol of dicyandiamide or 6 mol of cyanamide or 2 mol of melamine or corresponding mixtures of these compounds at from 300 to 6000C and subsequent pyrolysis treatment of the resin-like condensation products at the same temperature.

2. Regeneration agents according to Claim 1, characterised in that the polymeric organic compounds have the formula [C₆H₃N_7],, wherein z = from 10 to 10 000.

3. Regeneration agents according to Claims 1 and 2, characterised in that z lies between 100 and 1 000.

4. A process for the production of regeneration agents according to Claim 1, characterised in that the formaldehyde is used in the form of paraformaldehyde.

5. A process for the production of regeneration agents according to Claims 1 and 4, characterised in that the pyrolysis treatment is carried out for 10 to 60 minutes.

6. A process for the production of regeneration agents according to Claims 1, 4 and 5, characterised in that the reaction and the pyrolysis are carried out at temperatures of from 400 to 500 °C.