The invention relates to a method for preparing hydroxylamine in a reaction zone in a continuous process, comprising i) optionally directly introducing nitric acid comprising < 0.1 ppm Mo into the reaction zone and ii) wherein the nitric acid is transported, stored, transferred in vessels of which the walls consists of steel consisting essentially of Mo free and preferably carbon free steel and iii) the reaction zone is carried out in vessels of which the walls consists of steel consisting essentially of Mo free and preferably carbon free steel.

An important application of hydroxylammonium salts is in the preparation of oximes from ketones or aldehydes, in particular the preparation of cyclohexanone oxime from cyclohexanone. For this preparation method of an oxime, a cyclic process is known wherein an aqueous acid-buffered reaction medium is kept in circulation via a hydroxylammonium salt synthesis zone and an oxime synthesis zone. The reaction medium is acid-buffered by means of for instance phosphoric acid and/or sulphuric acid and the buffer salts derived from these acids, for instance alkali and/or ammonium salts. In the hydroxylammonium salt synthesis zone, nitrate ions or nitrogen oxides, are converted with hydrogen to hydroxylamine. The hydroxylamine reacts with free buffer acid to produce the corresponding hydroxylammonium salt, which is subsequently transferred to the oxime synthesis zone where it reacts with a ketone to the corresponding oxime with release of acid. After separation of the oxime from the reaction medium the reaction medium is recycled to the hydroxylammonium salt synthesis zone and fresh nitrate ions or nitrogen oxides are added, to the reaction medium.

In the case where the hydroxylammonium salt synthesis starts from a solution of phosphoric acid and nitrate the above-mentioned chemical reactions are represented as follows:

Reaction 1 ) Preparation of the hydroxylammonium in the hydroxylammonium salt synthesis zone:

2 H3PO4 + N0 ₃ ^" + 3 H ₂ -» ΝΗ3ΟΙ-Γ + 2 H2PO4 ^" + 2 H ₂ 0 Reaction 2) Preparation of the oxime in the oxime synthesis zone:

Reaction 3) Supply of HN0 ₃ to make up the depletion of the source of nitrate ions after removal of the oxime formed:

H3PO4 + H2PO4 ^" + HNO3 + 3 H ₂ 0 -» 2 H3PO4 + N0 ₃ ^" + 3 H ₂ 0

The first reaction is catalyzed heterogeneously. Preferably, the catalyst is present as finely divided solids as a disperse phase in a liquid reaction mixture.

The resulting mixture of the first reaction is an aqueous inorganic process liquid comprising a suspension of solid catalyst particles in a

hydroxylammonium salt solution.

Before this aqueous inorganic process liquid is transported to the oxime synthesis zone (reaction 2), the solid catalyst particles are preferably separated from the aqueous inorganic process liquid. After filtration the inorganic process liquid is a hydroxylammonium salt solution filtrate.

CN101058410, CN1547552, CN101218171 and CN1418809 describe a number of general methods for preparing hydroxylammonium salts.

Another process (as described in US5364609) makes use of an additional step where ammonia is combusted to form NO, N0 ₂ and water which are introduced into a reactor and together form nitric acid. The nitric acid is then reacted with hydrogen to form hydroxylammonium. During the hydroxylammonium formation N ₂ 0 and N ₂ are formed as gaseous by products and in addition ammonia is formed, that is dissolved in the aqueous inorganic process solution. The ammonia in turn reacts with NO and N0 ₂ from the combustion process and forms N ₂ gas (Piria reaction).

It is also possible in such a process to add nitric acid directly (instead of being formed in situ), however this can lead to a detrimental build up of ammonia as there is no dissolved NO and N0 ₂ to react with. Therefore there is a limit as to how much nitric acid can be added directly without the presence of NO and N0 ₂ in a single time period.

The limit is resultant concentration of ammonia which needs to be low to avoid crystallisation.

It is however desirable to be able to add nitric acid directly to increase the production of hydroxylammonium and to give time to shut of the ammonia combustion unit to allow for example maintenance. A problem with adding nitric acid directly is that it is a liquid and not a gas and tends to dissolve the steel containers it is transported in and used in. Most steel contains some molybdenum. It is known that molybdenum is detrimental to the selectivity of the hydroxylammonium reaction, for example 1 ppm of Mo in the reaction medium can lead to a more than 2 % reduction in hydroxylammonium selectivity.

Selectivity is defined as the molar ratio of hydroxylammonium production over proton (H ⁺ ) consumption where one hydroxylammonium needs two protons (H ⁺ ). In order to obtain a 100 % conversion the 'hydroxylammonium selectivity' as used herein, (the selectivity towards the production of hydroxylammonium) is defined as follows: molar ratio of twice the amount of hydroxylammonium produced in the reaction zone divided by the amount of H ⁺ consumed in the reaction zone. A low selectivity means that more byproducts are generated which is not desirable.

US4340575 discloses hydroxylammonium salts manufactured by a process comprising the catalytic reduction of nitric oxide with hydrogen in a dilute aqueous mineral acid in the presence of a suspended platinum catalyst at an elevated temperature, wherein the reaction is carried out in vessels of which the walls consist of conventional copper-free molybdenum-containing austenitic chromium-nickel steels which contain from 16 to 28 % by weight (wt%) of chromium, from 20 to 50 % by weight of nickel, from 1 to 4 % by weight of molybdenum and at most 0.1 % by weight of carbon and which in addition contain an amount of titanium which is at least 5 times the amount of carbon but is not more than 1.0 % by weight, or an amount of niobium or tantalum which is at least 8 times the amount of carbon but is not more than 1 .5 % by weight.

EP1901994 discloses a process and equipment for the continuous production of hydroxylammonium by reduction of nitrate ions or nitrogen oxides with hydrogen in the presence of a catalyst whereby directly supplying nitric acid instead of producing nitric acid is also possible. EP1451 100 also discloses that the

hydroxylammonium synthesis zone may be enriched with nitrate ions by the addition of nitric acid or by the absorption of nitrous gases in the aqueous medium to form nitric acid in situ.

It is also know in the art to try and remove Mo from the reaction zone.

US4062927 describes a method to remove dissolved molybdenum from nitrate/nitrogen monoxide solution by co-precipitation with a complex iron- ammonium phosphate.

US7399885 describes a method to remove dissolved molybdenum from acid buffer solution (after pre-treatment) by selective adsorption on a certain resin/polymer.

US3767758 describes that Mo lowers the selectivity towards hydroxylamine and results in the production of more ammonia.

Therefore, it is an aim of the invention to provide a process that shows improved selectivity and yet allows the direct addition of nitric acid as required.

Therefore according to the invention there is provided a method for preparing hydroxylammonium in a reaction zone in a continuous process, comprising steps:

I) combustion of ammonia to form NO, N0 ₂ and water;

II) introducing the NO, N0 ₂ and water from step I) into a NO, N0 ₂ absorption zone to form nitric acid and reducing the nitric acid with hydrogen in the reaction zone thereby forming hydroxylammonium; and/or

III) directly introducing nitric acid comprising < 0.1 ppm Mo into the reaction zone and reducing the nitric acid with hydrogen thereby forming aqueous

hydroxylammonium;,

wherein the nitric acid is transported, stored, transferred in vessels and pipes comprising steel;

wherein the reaction is carried out in vessels of which the walls of the vessels and connecting pipes comprise steel;

wherein said steel comprises 0 to 0.08 wt% C and 0 to 0.03 wt% Mo.

Preferably said steel consists essentially of a steel selected from the group consisting of: quench annealed steel A comprising 0 to 0.08 wt% C,

0 to 2.0 wt% Mn, 0 to 2.0 wt% Si, 0 to 0.045 wt% P, 0 to 0.03 wt% S,

17 to 21 wt% Cr, 0 to 0.03 wt% Mo, 8.0 to 13 wt% Ni;

low carbon steel B comprising 0 to 0.03 wt% C, 0 to 2.0 wt% Mn, 0 to 1.0 wt% Si, 0 to 0.045 wt% P, 0 to 0.03 wt% S, 17 to 21 wt% Cr,

0 to 0.03 wt% Mo, 8.0 to 13.0 wt% Ni;

stabilised Steel C comprising 0 to 0.08 wt% C, 0 to 2.0 wt% Mn, 0 to 2.0 wt% Si, 0 to 0.045 wt% P, 0 to 0.04 wt% S, 17 to 21 wt% Cr, 0 to 0.03 wt% Mo, 9.0 to13.0 wt% Ni and either Ti (minimum: 5 times wt% C to maximum 0.8 wt% C) or Nb + Ta (minimum: 8 times wt% C to maximum: 1.1 wt% C).

Preferably the resultant aqueous hydroxylammonium comprises < 3 ppm Mo, more preferably < 2.5 ppm Mo, most preferably < 2.0 ppm Mo and especially <1 .5 ppm Mo. 3 ppm of Mo in aqueous hydroxylammonium is equivalent to 0.0003 wt%.

The NO, N0 ₂ absorption zone is usually in a different vessel to the vessel where the reaction zone is but may also be in the same vessel.

During normal production preferably the ratio of nitric acid produced in step II) to the nitric acid added in step III) is in the range of from 100:0 to 10:90.

If the ammonia combustion unit is stopped during production, for example for maintenance of the ammonia combustion unit, then the ratio of nitric acid produced in step II to the nitric acid added in step III would be 0:100.

Preferably the ammonia combustion unit is stopped for no more than

100 hours.

Step III (without step II) is preferably carried out for no more than 100 hours continuously. If Step III (without step II) is carried out for more than 100 hours continuously the ammonia concentration becomes too high and there is a risk that salts in the inorganic process liquid will crystallise and the resultant precipitate may block the pipes, valves, filters, heat exchangers etc.

Preferably step III (without step II) is carried out for not more than 80 hours continuously and most preferably for not more than 60 hours continuously.

Preferably there is a period of at least 100 hours between carrying out step III (without step II) continuously and the next time step III is carried out without step II)

continuously.

For 100 hours of normal operation the ratio of nitric acid produced in step II to the nitric acid added in step III is preferably 100:0 to 20:80. A range of steels are well known for the manufacture of vessels, pipes and reactors for the preparation of hydroxylammonium. This includes steels known as 304, 316, 304L or 316L. Different grades have differing levels of

molybdenum and other metals. Furthermore the level of carbon and other elements can also impact on corrosion resistance.

The carbon ranges are 0.08 wt% maximum for grades 304 and 316 and 0.030 wt% maximum for the 304L and 316L grades.

All other element ranges are essentially the same (for example the nickel range for 304 is 8.00 to 10.50 wt% and for 304L is 8.00 to 12.00 wt%).

The lower carbon 'variants' (316L) were established as alternatives to the 'standards' (316) carbon range grade to overcome the risk of intercrystalline corrosion (weld decay), which was identified as a problem in the early days of the application of these steels. This can result if the steel is held in a temperature range 450 to 850°C for periods of several minutes, depending on the temperature and subsequently exposed to aggressive corrosive environments. Corrosion then takes place next to grain boundaries.

Preferably the carbon level of the steel is 0 to 0.03 wt% C. If the carbon level is below 0.030 wt% then this intercrystalline corrosion does not take place following exposure to these temperatures, especially for the sort of times normally experienced in the heat affected zone of welds in 'thick' sections of steel. Low carbon types may also be easier to weld than the standard carbon types.

Steel may also be annealed. Annealing is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and then cooling. In the cases of steel this process is performed by substantially heating the material (generally until glowing) for a while and allowing it to cool. Annealing does not reduce the carbon content but makes the distribution of element more homogenous and therefore improves corrosion resistance.

Even more preferably in the present invention austenite steel is used.

Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1 ,000 K (1 ,340 °F); other alloys of steel have different eutectoid temperatures. The catalyst applied in the preparation of the hydroxylammonium salt mostly consists of a metal from the platinum metal group, for instance Pd or (Pd + Pt) as active component on a carrier material such as for instance carbon. The catalyst may be activated by the presence of one or more catalyst activators. The catalyst activator may be an element from the group comprising Cu, Ag, Cd, Hg, Ga, In, Ti, Ge, Sn, Pb, As, Sb and Bi. Most preferably the catalyst activator is Ge. Compounds containing the elements in question may also be used as catalyst activators, for example oxides, nitrates, phosphates, sulphates, halogenides, and acetates. The elements or their compounds can be directly applied to the catalyst as described in US3767758 or they can be added to the reaction medium.

The catalyst employed in the preparation of the hydroxylammonium salt solution (reaction 1 ) preferably comprises a precious metal on a support, preferably platinum (Pt), palladium (Pd), or a combination of palladium and platinum on a support.

The Pd:Pt weight ratio may vary, although in general the preference is for pure Pd. The pure Pd may contain some Pt impurities. Preferably the Pd comprises less than 25 wt% Pt, more preferably less than 5 wt%, even preferably less than 2 wt% and especially less than 1 wt% Pt.

Preferably, the support comprises carbon (e.g. graphite, carbon black, or activated carbon ) or alumina support, more preferably graphite or activated carbon. The catalyst employed in the hydroxylammonium salt synthesis zone preferably comprises between 1 to 25 wt%, more preferably between 5 to 15 wt% of the precious metal, relative total weight of support plus catalyst.

Generally, the catalyst is present in a hydroxylammonium salt synthesis zone in an amount of 0.05 to 25 wt%, preferably in an amount of 0.2 to 15 wt%, more preferably in an amount of 0.5 to 5 wt% relative to the total inorganic process liquid weight in the hydroxylammonium salt synthesis zone.

The catalyst particles have in general an average size of between 1 and 150 μηη, more preferably between 5 and 100 μηη, more usually between 5 and 60 μηη and most preferably between 5 and 40 μηη. By "average particle size" is meant that 50 vol% of the particles are larger than the specified diameter.

Preferably, the catalyst activator is present in an amount of between 0.01 and 100 mg/g catalyst, preferably between 0.05 and 50 mg/g catalyst, more preferably between 0.1 and 10 mg/g catalyst, most preferably between 1 and 7 mg/g catalyst. The production unit for preparing hydroxylammonium may be any suitable reactor, for instance a reactor with a mechanical stirrer or a column, most preferably a bubble column. An example of a suitable bubble column is described in NL6908934.

A typical reactor configuration for preparing aqueous hydroxylammonium salt solution and for separating solid catalyst particles is a hydrogenation bubble column reactor with a cooling section. The reactor configuration usually comprises a number of sets of gas separation vessels and filtration vessels.

The inorganic process liquid is transferred from the bubble column reactor via a gas separation vessel to a filtration vessel comprising a filter for the first filtration step.

From the filtration vessel some (usually around 3 to 10 %) of the hydroxylammonium salt solution filtrate is then transferred into a filtrate vessel and from there to a filtrate buffer vessel and from there is transferred to the oxime synthesis zone. The remaining (usually around 97 to 90 %) hydroxylammonium salt solution filtrate is transferred back to the hydrogenation column and is cooled in the cooling section. The pipe between filtration vessel and cooling section may also contain cooling means.

The invention will be further elucidated by means of the following examples without being limited thereto.

Example 1

In a hydroxylamine phosphate oxime plant working according to DSM HPO ^® technology, that was operated in a continuous manner, an inorganic process liquid comprising hydroxylammonium salt solution was prepared. The average particle size of the catalyst particles (10 wt % palladium / activated carbon) was approximately 15 μηι.

In an ammonia combustion plant NO, N0 ₂ and water was prepared and then introduced into an absorption zone to form nitric acid which is then transferred to a reaction zone and then the nitric acid was reduced with hydrogen thereby forming hydroxylammonium.

After 200 hours, food grade nitric acid (60-65% concentrated) was directly introduced into the reaction zone and reduced with hydrogen thereby forming aqueous hydroxylammonium. The food grade nitric acid was transported, stored, transferred in vessels and pipes and the reaction is carried out in vessels of which the walls of the vessels and pipes comprise steel consisting essentially of a low carbon steel B comprising 0 to 0.03 wt% C and 0 to 0.03 wt% Mo.

The measurement of Mo concentration in the food grade nitric acid and in the resultant hydroxylammonium was carried out using atomic absorption spectroscopy using a "Thermo Scientific iCAP6500 (ICP-AES) spectrometer or a Perkin Elmer DRC-e (ICP-MS) spectrometer.

Nitric acid or hydroxylammonium solutions may be diluted with water in order to prevent crystallization at lower temperatures in the spectrometers.

The Mo content of the feedstock solutions such as the food grade nitric acid (HN0 ₃ ) and H ₃ P0 ₄ was measured and found to be less than 0.1 ppm.

The Mo content of the resultant aqueous inorganic process liquid containing hydroxylammonium was measured and found to be < 1 ppm.

Comparative Example 2:

In comparative example 2 example 1 was repeated except for using vessels of which the walls of the vessels and pipes comprise steel consisting essentially of 316 steel with > 0.03 wt% Mo.

The Mo content of the resultant aqueous inorganic process liquid containing hydroxylammonium was measured and found to have built up to 3 to 4 ppm Mo and lower hydroxylammonium selectivity was observed.