FI70200B - FRAMSTAELLNING AV ARSENSYRA - Google Patents

Description

70200

This invention relates to a process for the preparation of arsenic acid by reacting arsenic trioxide with water and oxygen in a closed reactor at an elevated temperature in the presence of a catalyst.

Arsenic acid is generally understood to mean the compound H 2 AsO 2, sometimes represented by the formula H 3 Agg4.1 / 21 ^ 0 and also known as ortho-arsenic acid. meta-arsenic acid HAs03 and pyro-arsenic acid H4As20? (also referred to as arsenic pentoxide hydrate As 2 O 2 - * 2H 2 O) are other forms of arsenic acid, but both are soluble in water to form ortho-arsenic acid. The term "arsenic acid" as used herein refers primarily to ortho-arsenic acid, but also includes other arsenic acids where appropriate.

Arsenic acid has long been used as a pesticide in a variety of forms and applications, including, for example, use as insecticides, herbicides and rodenticides, and as a component of certain wood preservatives. It is also used in the manufacture of arsenates, in the manufacture of glass, as a leaf remover under certain conditions, as a cotton desiccant and as a soil sterilizer.

Many methods have been proposed for the industrial production of arsenic acid, and the vast majority of these methods involve the oxidation of arsenic trioxide As 2 O 3 (also known as crude arsenic, white arsenic, arsenic acid, arsenic (3) oxide, arsenic oxide, arsenic anhydride, and simply "arsenic"). Oxidizing agents proposed for this purpose include, for example, chlorine (see U.S. Patents 1,415,323, 1,447,937, and 1,515,079), sodium or potassium chlorate (see U.S. Patents 1,677,257 and 1,699,823), air or oxygen (see GB Patent 445,468, JP Patent 94024) 1931 - Chem.Abs. 27, 1456; and Khimiko Farmatsevticheskaya Promshlennost, 1933, 193-194 - 2 70200

Chem.Abs. 28, 428) and hydrogen peroxide (see SU Patent 510430 (1976) - Chem. Abs. 85, 35175 t). Oxidation by electrolysis has also been proposed (see SU Patent (1940) - Chem.

Abs. 38, 5736 and JP Patent 9178 (1960) Chem.Abs. 55, 20732).

However, the process, which has become common commercial use, involves oxidation with nitric acid. Various catalysts or auxiliary oxidants have been proposed for use in the nitric acid process, including, for example, oxygen (see GB Patent 255,522), hydrochloric acid (see U.S. Patent 1,493,798 and SU Patent 4,5279 (1935) - Chem.Abs. 32, 3100 ), air and water (see U.S. Patent 1,615,193) and air (see Chem.Abs. 55, 11776). The catalyst most commonly used in modern plants for the production of arsenic acid by the nitric acid process is, for example, iodine in the form of sodium or potassium iodide (see U.S. Patent 1,974,747) or potassium iodate (see Chem.Abs. 36, 3091).

The basic reaction which takes place in the preparation of arsenic acid by the nitric acid method can be represented in a simple form as follows: Catalyst I Aso0-, + 2HNOo + 2H „0 -> 2HoAs0. + NO + NO 2 3 3 2 3 4 2

The process is generally carried out by continuously feeding arsenic trioxide, nitric acid and a catalyst (e.g. potassium iodide) to a heated reactor at normal pressure, discharging the gaseous nitrogen oxides formed and draining the arsenic acid formed.

Several problems are encountered in performing this process on a large scale. One of these problems is excessive foaming, which tends to occur as a result of the large volume of gaseous nitrogen oxides formed, and if foaming increases too much, there is a risk of overboiling or discharging. Such a hazard occurs especially when large amounts of one or both of the unreacted feedstocks accumulate in the reactor, which may occur if there is a blockage of the feed or other irregular feed of a reagent or if the temperature of the reaction mixture drops too low for some other reason.

In any case, in addition to the problems caused by excessive foaming, the reaction is difficult to control: a considerable amount of heat is released in the early stages of the process, while heat must be fed in in the later stages. The reactor must be heated before the reagents are added - often allowed to warm overnight - as there is a risk of slurry and consequent subsequent foaming and overboiling, which occurs if unreacted starting materials initially accumulate as a result of too low an initial temperature. On the other hand, due to its exothermic nature, the process cannot be easily terminated prematurely if, for example, it appears to be escaping control. Besides, there is no simple indication of when the reaction is complete.

Various measures have been proposed to add and mix reagents in a controlled manner to overcome such difficulties (see, e.g., U.S. Patents 1,603,308 and 2,165,944), but difficulties still exist even in modern factories and are somewhat exacerbated by the current need to use low quality arsenic trioxide as starting material.

Another serious disadvantage of the conventional nitric acid method is the need to use large amounts of nitric acid: it must be fed a stoichiometric amount relative to arsenic trioxide according to Equation I above; this means that 2 moles of nitric acid are required per mole of arsenic trioxide. This means, for example, that the production of an 8 tonne batch of 80% arsenic acid requires about 5 tons of commercial nitric acid, which is reduced to nitrogen oxides during the process. These nitrogen oxides must be regenerated by reoxidation to nitric acid for both economic, health and safety reasons.

4 70200

All plants that produce arsenic acid by the nitric acid method therefore include an apparatus for recovering nitric acid. This can be, for example, a series of reaction towers connected directly to the main reactor, through which the gaseous nitrogen oxides from the main reactor are reacted with water and oxygen, and the last tower in which the gases are washed with sodium hydroxide to remove any residual nitrogen oxides. (Details of examples of nitric acid recovery apparatus are given in U.S. Patent 2,165,957 and JP Patent 79,016943). Due to the very large volumes of nitric oxides that develop during the process, the nitric acid recovery device forms a significant part of the entire arsenic acid production plant and contributes significantly to the overall cost of the arsenic acid obtained.

In addition, although the recovered nitric acid is recycled, a total loss of nitric acid of up to 10% tends to occur. Thus, for example, a nitric acid loss of about 500 kg would be expected in the production of an 8 tonne batch of arsenic acid. This loss adds to the cost of the final product, both due to the need to replace lost nitric acid and due to the need to wash the gases with sodium hydroxide to neutralize nitrogen oxides that do not oxidize back to nitric acid.

Despite all the problems mentioned above, the nitric acid method has remained a widely accepted method for the production of arsenic acid on an industrial scale for many years.

This invention relates to a process for the preparation of arsenic acid by reacting arsenic trioxide with water and oxygen in a closed reactor at elevated temperature in the presence of a catalyst, characterized in that the catalyst consists of nitric acid together with iodine or iodine compound, the amount of nitric acid does not exceed 5% by weight and based on the weight of arsenic trioxide and that the reaction is carried out at a pressure of 3 to 10 bar.

Surprisingly, it has been found that with the process of the present invention it is possible to overcome - in an industrial scale process - many of the disadvantages of the conventional 5,200,200 industrial process of arsenic acid by the acid process. Thus, in the process of the present invention, foaming and the associated problems resulting from the development of gaseous nitrogen oxides are avoided, as is the need for an expensive nitric acid recovery device. Another important advantage of this process is that the need for large stoichiometric amounts of nitric acid is avoided.

The present invention is thus considerably more economical than a conventional process.

In the process of this invention, arsenic trioxide is oxidized with water and oxygen in the presence of a catalyst, and the overall reaction can be represented in a simple form as follows: Catalyst used in this process consists of nitric acid and iodine or iodine compound. The iodine compound can be any compound that forms iodide ions, for example, potassium iodide, hydroiodic acid or iodate. However, it is preferred to use iodine (X2): if, for example, potassium iodide were used, the arsenic acid obtained at the end of the process would contain a small amount of potassium nitrate which has been found difficult to remove.

It has been found that, at least in some cases, the reaction rate can be adjusted to some extent by varying the amount of catalyst used or the amount of any catalyst component.

It is believed that when a nitric acid / iodine catalyst is used in the process according to the invention, the reaction actually proceeds with a complex series of reactions, which can be represented by the following simplified reaction scheme: 70200

In In Out i i t

IIa As2 ° 3 + 2I2 + 5H2 ° -> 2H3AsO4 + 4HI

Hb 4HI + 2H2O

He 2HN0 ^ "+ ~ 02 ^ t

In

It is observed that the sum of Equations Ila, Ilb and Ile is Equation II above.

It is also believed that the complex reaction cycle involves, at one stage, an equilibrium, which is presented as follows:

HO

III HAsO2 + 12 - - 2 · -> 4I ~ + H3AsO4

In order for the reaction cycle to proceed as desired, as shown by Equations IIa, Hb and Ile above, it is necessary that the equilibrium represented by Equation III be driven to the right. It has been found that this can be accomplished at a relatively high acidic pH, but that a low pH favors the left side of the indicated equilibrium. Therefore, it is important for the process of the invention that the amount of nitric acid present is not such as to prevent the above balance from advancing to the right.

In a conventional industrial process for the production of arsenic acid by the nitric acid method, a very large amount of nitric acid is used, as previously mentioned, and therefore could not proceed through the reaction scheme of Equations IIa, IIb and Ile above because the equilibrium shown in Equation III would drift to the left. In fact, it is believed that the conventional nitric acid process using a conventional catalyst, potassium iodide, proceeds - in contrast to this process - through a reaction cycle involving iodate ion and which is presented in a simplified form as follows: HNO_ IV Γ --- ^ io ~ K as2o3 \ H2 ° l “+ H ^ AsO.

34 In this reaction cycle, iodide is regenerated and is therefore a catalyst, but nitric acid is consumed by reduction to nitrogen oxides, as shown in Equation I. In contrast, in this process, both iodine or iodide and nitric acid act as catalysts.

It can be seen from Equations Ha and Hb that there is a direct stoichiometric relationship between the amount of iodine or iodide and the amount of nitric acid that needs to be added. Thus, in theory, only a minimal, catalytic amount of nitric acid needs to be used sufficiently to initiate the reaction, since it is then continuously generated in situ, as shown by Equations Hb and Ile above. However, it has been found that if too little nitric acid is used, the reaction ends prematurely but can be restarted by adding more nitric acid. This is thought to be due to the slow disappearance of nitric acid by reaction with impurities in the arsenic trioxide used and / or by reduction to nitrous oxide N

It is therefore preferred to use nitric acid in excess of the stoichiometric amount of iodine and iodide. However, from the foregoing, it can be judged that too much excess nitric acid should not be used, otherwise the pH of the reaction mixture will become too low. As mentioned above, the amount of nitric acid does not exceed 5% by weight and is preferably 0.1 to 3% and especially 1 to 3% by weight, calculated as HNO 2 and based on the amount of arsenic trioxide used. Typically, about 20 to 60 liters of commercial nitric acid can be used per ton of arsenic trioxide.

The nitric acid used in this process may be fed in the form of any suitable strength nitric acid according to what is available. For example, it can be fed as fuming nitric acid, ordinary commercial nitric acid (strength about 62% by weight) or any nitric acid solution up to only about 1% strength (by weight). It is only necessary to change the amount of water added to the reactor according to the dilution of nitric acid used. A special advantage of this process is that it is not necessary to use large amounts of nitric acid, unlike in conventional processes carried out by the nitric acid method, in which approximately stoichiometric amounts of nitric acid have to be used with respect to arsenic trioxide.

The amount of iodine or iodine compound used is generally not critical and may be similar to the amount used in a conventional nitric acid method relative to the amount of arsenic trioxide. However, it has been found that the reaction rate and hence the degree of exotherm and reaction time can be easily controlled and very satisfactorily by the amount of iodine or iodine compound used, as the reaction shown in Equation Ha proves to be the slowest of the three reactions in the reaction scheme: lower iodine or iodine is less exothermic. The amount of iodine or iodine compound need not exceed 0.1% by weight based on the weight of arsenic trioxide and may be as small as 0.01% by weight. Amounts of 0.02 to 0.05% by weight based on the weight of arsenic trioxide have been found to be suitable: typically about 0.3 kg of potassium iodide or about 0.5 kg of iodine per tonne of arsenic trioxide can be used.

The process according to the invention is carried out in a closed reactor at a pressure of 3 to 10 bar, preferably 4 to 8 bar, and in particular without discharging anything from the reactor during the process, although oxygen and possibly other reagents can be fed intermittently or continuously. It has been found that this process for carrying out the process of the present invention makes it possible to use considerably smaller amounts of nitric acid, even only about 1% by weight, based on the amount of arsenic trioxide used or even less.

The process may conveniently be carried out by first feeding non-gaseous reagents into the reactor. Air or, preferably, oxygen can then be supplied to the desired pressure, optionally initially with ventilation to remove nitrogen from the atmosphere, which helps to improve the reaction rate. Preferably, the oxygen and the liquid mixture of other reagents are stirred or otherwise mixed in a closed vessel, because good mixing increases the amount of gas-liquid contact and thus the reaction rate. The amount of gas-liquid contact can also be increased, for example, by using a wide, low vessel that provides a large gas-liquid interface.

As discussed above, oxygen is used under pressure because it has the advantage of forcing more oxygen into solution, and also because the reaction step shown in Equation Ile has been found to be highly dependent on pressure.

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As the reaction progresses, oxygen is consumed and, as a result, the pressure in the reactor decreases. The reaction continues as long as sufficient oxygen and other reagents are present, but the reaction rate slows as the pressure decreases. Therefore, it is preferable to feed oxygen to the reactor continuously or intermittently throughout the process to maintain or return the pressure to the desired value. This can be done automatically, for example, by means of a valve which opens by injecting oxygen whenever the pressure in the reactor drops to a predetermined value or by adjusting the amount of oxygen supplied to match the rate at which it is consumed.

It can be seen from the reaction scheme that one mole of oxygen is consumed per mole of arsenic trioxide. Thus, 1 ton of arsenic nitric oxide requires about 1.2 x 10 liters of oxygen (measured at standard temperature and pressure), which corresponds to about 1400 liters of liquid oxygen, although naturally excess oxygen is naturally present in the reactor.

The process is preferably carried out at a temperature of at least 80 ° C, preferably at least 85 ° C and at most 150 ° C, especially in the temperature range 95-150 ° C. It has been found that if the temperature is initially too low, the reaction may be reluctant to begin, and that if the temperature is too high, other reactions prevent the reaction cycle shown by Equations Ha and Ile from occurring. It is preferred to initially heat the reaction mixture to 100-120 ° C, after which the temperature rises to about 140-150 ° C as a result of the exotherm of the reaction. It is then preferred to cool the reactor so that the temperature drops back to 100-120 ° C, which can then be maintained until the reaction is complete. In general, the exotherm of the reaction is sufficient to maintain this temperature, but heating may sometimes be necessary in the later stages of the reaction.

11 70200

During the process, the progress of the reaction can be adjusted to some extent by varying the pressure and the degree of stirring, since the rate of oxygen dissolution turns out to be a slow rate-determining step as well as the reaction step shown in Equation Ha previously described. The degree of exotherm can also be adjusted in this way during the process, and it may be advantageous to include a thermally controlled oxygen supply valve to bypass the pressure controlled valve and restrict or shut off the oxygen supply if the reaction mixture temperature reaches a predetermined value, e.g. 150 ° C. again as the temperature drops.

Completion of the reaction is indicated by a rapid increase in the rate of oxygen consumption followed by a sudden cessation of consumption. A rapid increase in the rate of consumption is likely to occur because the final reaction is represented by Equation Ile and at this stage of the process the slow reaction represented by Equation IIa is complete.

When the reaction is complete, the pressure can be released and this can generally be done safely without the risk of overboiling, since the temperature of the reaction mixture is normally below its boiling point at normal pressure. The gas to be released is mainly oxygen, but when a catalyst containing nitric acid is used, it contains some nitrogen oxides and it would therefore be advantageous to pass the gas through a scrubber, which may simply be a filled tower through which the gas is passed down and released into the air. the hydroxide solution is passed countercurrently. The type and size of scrubber and / or exhaust pipe required may be dictated by local health and safety regulations, although low levels of nitrogen oxide emissions should be considered.

The content of the reactor at this stage is mainly arsenic acid and water, although it also contains a small amount of nitric acid and iodine. Nitric acid can be removed by adding a small amount of arsenic trioxide and / or preferably blowing it off by passing 70,200 air or oxygen bubbles through the mixture while maintaining it at about 80-90 ° C. Iodine is generally removed simultaneously, as can be seen from the change in color of the reaction mixture depending on the presence of colored impurities. In any case, the presence or absence of iodine can be easily confirmed simply by shaking the sample with chloroform: the presence of iodine gives a blue color.

The arsenic acid can then be filtered to remove any residual excess arsenic trioxide and any other impurities. Its concentration can be determined from its specific gravity and it can be diluted with water to the required strength. Experiments have shown that the nitrate content of the arsenic acid obtained is generally less than about 0.02% by weight, although it can be reduced even more by blowing air or oxygen through the mixture for a long time.

This process can be used not only in the case of pure arsenic trioxide, but also in the case of commercially low-grade arsenic trioxide. The use of a poor quality starting material may require the use of slightly higher amounts of catalyst, e.g. iodine and nitric acid than would otherwise be necessary and may also require the use of a reaction temperature closer to 120 ° C than 100 ° C and a slightly longer reaction, but poor quality material can be used successfully in this process. without the disadvantages described above which tend to occur when used in a conventional nitric acid process. The use of a poor quality material can lead to some kind of residue in the resulting arsenic acid, but this can usually be filtered off or, depending on the intended use of the arsenic acid, may even be left in the final product. Alternatively, the poor quality material may be first purified, for example, by mixing with water and nitric acid or another acid (e.g. hydrochloric acid) depending on the type of impurity present, and heating for a short time and then filtering. It can then be reacted under conditions similar to those used for the pure starting material.

13 70200

The process of the invention has numerous advantages over the conventional process for producing arsenic acid by the nitric acid process. Problems such as overboiling, blowouts and other foaming problems are substantially avoided even when low quality starting materials are used. The need for an expensive nitric acid recovery plant is eliminated, resulting in savings in the amount of sodium hydroxide needed to scrub the exhaust gases.

In addition, the need to use large volumes of nitric acid is avoided, resulting in economic savings, savings in storage needs and reduced health and safety risks. In fact, when a nitric acid / iodine catalyst is used, the total amount of nitric acid used in this process (about 7.5 kg per tonne of arsenic trioxide) is substantially less than the amount of nitric acid lost in the conventional process (by conversion to nitrous oxide and otherwise) (which can increase to about 10% of total consumption). corresponding to a loss of about 80 kg per tonne of arsenic acid). Thus, not only is the need to use large amounts of nitric acid avoided, but the total net consumption of nitric acid is also significantly reduced. If desired, the net consumption can be further reduced by incorporating a small nitric acid recovery device in the equipment to recover the residual nitric acid blown out of the arsenic acid as nitrogen oxides, but any such device naturally needs to be on a much smaller scale than used in a conventional process. (In this respect, it is pertinent to note that under UK law, a conventional process can be registered as both an arsenic acid production process and a nitric acid production process, whereas a process according to the invention does not need to be registered as a nitric acid production process even with a small nitric acid recovery plant).

70200

The process according to the invention is described in more detail by means of the following examples:

Example 1 500 g of arsenic trioxide (fairly pure), 325 ml of water, 4 ml of fuming nitric acid (density 1.5 g / ml) and 150 mg of potassium iodide were charged to a pressure vessel. Oxygen was then supplied at a pressure of about 3.5 bar, the vessel was vented to remove most of the atmospheric nitrogen, and oxygen was again supplied at a pressure of about 6.9 bar. The reaction mixture was then slowly heated to about 100 ° C with stirring, causing the pressure to rise to about 8.6 bar. At about 100 ° C, the reaction started and the temperature of the reaction mixture rose to about 145 ° C as a result of the exothermic nature of the reaction, while the pressure decreased due to oxygen consumption. Additional oxygen was fed each time the pressure dropped to about 6.9 bar, to bring the pressure back to about 8.6 bar, and after the first exothermic rise, the temperature was maintained at about 100 ° C with the necessary cooling. Completion of the reaction was indicated by a sudden cessation of oxygen consumption after about 1 hour.

The vessel was then vented through a scrubber (due to small amounts of oxygen-mixed nitrogen oxides). No foaming was observed during draining. A small amount of arsenic trioxide was then added to the reaction mixture and air was blown through it for about an hour while maintaining the temperature at about 70-80 ° C to remove a small amount of nitrogen. The original potassium iodide, now present as iodine, was also removed during this procedure, which manifested itself as a change in color of the product from light tan to colorless. Some water was also lost during this procedure. The product was then filtered to remove a small excess of arsenic trioxide.

The product was arsenic acid with a concentration of about 83-85% by weight and a specific gravity of 2.0-2.14. Water was added to dilute arsenic acid to 80% by weight. A yield of about 820 g was obtained from 80% arsenic acid. This corresponds to a catch of about 91.5 g of the theoretical catch.

A nitrate experiment based on the color reaction of α-naphthylamine showed a nitrate content of arsenic acid of less than 0.02% by weight.

Example 2 200 g of commercially available arsenic trioxide (reported as impurities Fe 0.5%, Pb 0.5%, Cu 0.5%, Sb 1-1.5%, total 2.5-3% or 3.5-4% assuming that the impurities are present as oxides, dark gray in color compared to white, pure arsenic trioxide, but similar in density and particle size to the pure sample), 130 ml of water, 3-4 ml of fuming nitric acid (see Example 1) and 50 mg of potassium iodide were charged.

The procedure of Example 1 was followed with the changes that the temperature was maintained at about 120 ° C instead of 100 ° C and that the pressure was allowed to vary between about 6.9-11.0 bar instead of 6.9-8.6 bar.

Arsenic acid with a strength of 80% by weight was obtained after a reaction time of about 4 hours. The product was covered with fine black scum and also left a gray-black residue on the bottom of the reactor. The black slag sank to the bottom in the water and its color suggested that it could be elemental arsenic or antimony, which was confirmed by subsequent analysis. The total weight of the impurities corresponded to about 3-4% by weight based on arsenic trioxide.

This example shows that this process works quite satisfactorily with impure commercially available arsenic trioxide, although the amount of catalyst may need to be slightly increased compared to the amount required for a corresponding amount of fairly pure arsenic trioxide. (Other experiments showed that the amounts of nitric acid and potassium iodide given above were the minimum amounts required to give a satisfactory reaction with this particular arsenic trioxide sample and that reduction of either 70 70200 or both of these reagents produced only a slow reaction or no reaction at all. interfering with the iodine / nitric acid double-catalytic cycle).

Example 3 200 g of commercially available arsenic trioxide of the type used in Example 2 was purified as follows; enough water was added to form a thin slurry, then 3 mL of fuming nitric acid was added, after which the mixture was heated to 70 ° C for 30 minutes, filtered, washed with water to remove residual nitrates, and finally filtered again.

The procedure of Example 2 was repeated using arsenic trioxide thus purified except that only a temperature of about 100 ° C was found to be sufficient.

80% by weight of arsenic acid was obtained only after a reaction time of 1-2 hours. A small amount of a very fine black residue was obtained, but represented less than 0.01% of arsenic trioxide and less than 0.006% of arsenic acid and does not need to be separated for certain uses of arsenic acid.

This example shows that the simple purification of the impure arsenic trioxide carried out at the beginning makes it possible to proceed the reaction much faster and more easily with similar results to those obtained with fairly pure arsenic trioxide. (Other experiments showed that the amount of catalyst could be calculated in this case).

Example 4 Using the apparatus schematically shown in the accompanying drawing, 5,000 parts of arsenic trioxide, 1.6 parts of potassium iodide (or 2.5 parts of iodine) and 3,000 parts of water were mixed into a slurry in a premix tank 10 equipped with a mixer 12, a solids inlet 14 and with a water supply port 16. The slurry is then transferred via line 18 to a pressure reactor 20 with a jacket 70200 and a capacity of 8000 liters. The transfer is performed by pulling the gas outlet pipe 22 under vacuum with valves 24, 25 and 26 closed and valves 28 and 30 open. The contents of the reactor 20 are then heated through the reactor jacket to about 85 ° C while being stirred by the stirrer 32. With valve 26 open and valves 24, 25, 28 and 30 closed, oxygen is fed to the reactor through line 33 until a pressure of about 1.4 bar prevails inside the reactor. 62% nitric acid is then added through line 35 with valve 25 open and more oxygen is supplied at a pressure of about 8.3 bar. The reaction starts and is allowed to proceed for 2.5-6 hours while maintaining the pressure between about 4.1-8.3 bar by adding more oxygen intermittently or continuously, but otherwise keeping all five valves 24, 25, 26, 28 and 30 closed while maintaining a temperature of 100-150 ° C in the reactor by circulating cooling water through the reactor jacket. Upon completion of the reaction, the pressure in the reactor 20 is released by opening the valve 24, which causes the gases to flow through the tube 34 through the base scrubber 36 upward to neutralize the nitrogen oxides and then through the tube 38 to an exhaust pipe (not shown). Meanwhile, air is blown into the reactor 20 through the tube 44 and the valve 46 and through its liquid content to remove residual nitric acid while heating the reactor by passing steam through its jacket. Finally, the liquid content of the reactor, namely arsenic acid and water, is cooled by passing water through the reactor jacket and then discharged through line 40 by opening valve 42.

Claims (6)

70200 Process for the preparation of arsenic acid by reacting arsenic trioxide with water and oxygen in a closed reactor at elevated temperature in the presence of a catalyst, characterized in that the catalyst consists of nitric acid together with iodine or iodine compound, nitric acid not exceeding 5% by weight based on HNO trioxide and that the reaction is carried out at a pressure of 3 to 10 bar. Process according to Claim 1, characterized in that the amount of nitric acid is 0.1 to 3% by weight and in particular 1 to 3% by weight. Process according to Claim 1 or 2, characterized in that nitric acid is used in an amount which exceeds 0.5 moles per gram atom of iodine in the iodine or iodine compound. Process according to any one of Claims 1 to 3, characterized in that iodine or an iodine compound is used in an amount of 0.01 to 0.1% by weight and preferably 0.02 to 0.05% by weight, based on the weight of arsenic trioxide. Process according to any one of Claims 1 to 4, characterized in that the reaction is carried out at a pressure of 4 to 8 bar. Process according to any one of Claims 1 to 5, characterized in that the reaction is carried out at a temperature of at least 80 ° C, preferably at least 85 ° C, in particular between 95 and 150 ° C.